text:
Co Oxide Core - Silica Shell Constructs for Artificial Photosynthesis
A critical step for assembling a robust integrated artificial photosystem is the efficient coupling of the sites for carbon dioxide reduction and water oxidation across a physical barrier that separates the products of the two half reactions. The need for a separating membrane is particularly important for CO2 reduction by 6 or more electrons to a liquid fuel if extensive back and cross reactions are to be avoided. A silica wall of a few nanometer depth may possess the desired properties because it transmits protons fast on the time scale of catalytic turnover, yet is impermeable to small molecules. We have developed a solvothermal method to obtain crystalline Co3O4 nanotubes for a range of sizes, from 500 nanometer outer diameter, 200 nm inner diameter and tens of micrometer length, to nanotubes with 10 nm outer and 6 nm inner diameter. Using a visible light sensitization system in close to neutral aqueous solution, the Co3O4 nanotubes were found to exhibit high water oxidation activity. A cylindrical shell of a few nanometer thickness was cast around the Co3O4 nanotube by a modified version of a hydrothermal synthesis method originally introduced by Stoeber. Attaching a visible light chromophore and a reduction catalyst on the outside silica surface opens up an approach for separating the light absorber and fuel forming catalysis from the water oxidation chemistry on the inside of the tube. In order to accomplish efficient directional electron transport between the water oxidation catalyst and the chromophore across the silica wall, the materials chemistry for embedding rectifying molecular wires into the nanometer-thin silica layer has been developed. The wire molecules (oligo para phenylene vinylene) are covalently attached to the Co3O4 surface and extend across the silica layer. Directionality for charge flow is imposed by proper alignment of HOMO and LUMO of the wire molecules with catalyst and chromophore electronic levels. To demonstrate the concept, spherical Co3O4 catalyst core/SiO2 shell particles were used. FT-Raman, infrared and UV-Vis spectra confirmed the presence of the embedded wires. Nanosecond absorption spectroscopy revealed very efficient transfer of charge from the chromophore to the wire molecules at a small overpotential of 150 mV by monitoring the characteristic transient hole absorption at 600 nm. Similar constructs using the nanotube geometry will be presented. The latter are suitable for demonstrating visible light-driven water oxidation catalysis under separation of O2 by the silica wall and as well as evaluation of the charge transport efficiency by transient spectroscopy. This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, Division of Chemical, Geological and Biosciences of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
CO2 Direct Conversion to Organic Materials with Light and Water by AlGaN/GaN Photo-Electrode
Deguchi1, Satoshi
Yotsuhashi1, Hiroshi
Hashiba1, Yuji
Zenitani1, Reiko
Hinogami1, Yuka
Yamada1, Kazuhiro
As the seriousness of environmental issue has been grown up, reduction of carbon dioxide (CO2) becomes crucial worldwide. In addition to the attempts for reducing the CO2 consumption, it is important to construct a system for direct CO2 reduction. To reduce the amount of CO2 and to utilize CO2 effectively, so-called artificial photo-synthesis has been intensively investigated. However, several problems persist, such as low efficiency, poor stability, and the need for sacrificial materials or external power input. On the other hand, it was reported that gallium nitride (GaN) can be used as photo-catalyst for water splitting in stead of titanium oxide (TiO2). GaN has an advantage for photo-catalytic reaction because of its wide band gap and low affinity. The research area of nitride semiconductor has been expanded beyond its potential application to highly efficient optical and power devices for energy saving. In the recent study, we have succeeded in realizing direct CO2 conversion with water and UV light illumination using GaN photo-electrode. Here, we report that the CO2 conversion can be improved by replacing the photo-electrode from GaN to AlGaN/GaN film from the viewpoints of both photo current and selectivity of reaction products. The nitride semiconductor films, which were Si-doped n-type gallium nitride (GaN) and intrinsic aluminum gallium nitride (AlGaN), were grown on (0001) sapphire substrate by metalorganic vapor-phase epitaxy (MOVPE). And nickel oxide (NiO) particles were appended to the surface of the nitride semiconductor film as co-catalysts. For counter electrode, a copper (Cu) or indium (In) plate was chosen. In the case of GaN photo-electrode and Cu counter electrode, the generation of formic acid (HCOOH) from CO2 and H2O with 8.8% Faradic efficiency was confirmed. For AlGaN/GaN photo-electrode, both photo current and Faradic efficiency of HCOOH were increased in spite of the decrease of light absorption of long-wavelength side. It is considered that the separation of electron-hole pair exited by light irradiation becomes efficient due to the internal electric field induced in the AlGaN layer. By changing the counter electrode from Cu to In with keeping AlGaN/GaN photo-electrode, the selectivity for HCOOH was enhanced in which its Faradic efficiency of HCOOH reached at ~ 68%.
Vanadium-Doped In and Sn Sulphides: Photocatalysts Able to Use the Whole Visible Light Spectrum
Lucena1, Fernando
Fresno1, Pablo
Palacios2, Yohanna
Seminovski2, Perla
Wahnon2, Jose C.
Using photocatalysis for energy applications depends, more than for environmental purposes or selective chemical synthesis, on converting as much of the solar spectrum as possible; the best photocatalyst, titania, is far from this. Many efforts are pursued to use better that spectrum in photocatalysis, by doping titania or using other materials (mainly oxides, nitrides and sulphides) to obtain a lower bandgap, even if this means decreasing the chemical potential of the electron-hole pairs. Here we introduce an alternative scheme, using an idea recently proposed for photovoltaics: the intermediate band (IB) materials [1]. It consists in introducing in the gap of a semiconductor an intermediate level which, acting like a stepstone, allows an electron jumping from the valence band to the conduction band in two steps, each one absorbing one sub-bandgap photon. For this the IB must be partially filled, to allow both sub-bandgap transitions to proceed at comparable rates; must be made of delocalized states to minimize nonradiative recombination; and should not communicate electronically with the outer world. For photovoltaic use the optimum efficiency so achievable, over 1.5 times that given by a normal semiconductor, is obtained with an overall bandgap around 2.0 eV (which would be near-optimal also for water phtosplitting). Note that this scheme differs from the doping principle usually considered in photocatalysis, which just tries to decrease the bandgap; its aim is to keep the full bandgap chemical potential but using also lower energy photons. In the past we have proposed several IB materials based on extensively doping known semiconductors with light transition metals, checking first of all with quantum calculations that the desired IB structure results [2]. Subsequently we have synthesized in powder form two of them: the thiospinel In2S3 and the layered compound SnS2 (having bandgaps of 2.0 and 2.2 eV respectively) where the octahedral cation is substituted at a ≈10% level with vanadium, and we have verified that this substitution introduces in the absorption spectrum the sub-bandgap features predicted by the calculations [3]. With these materials we have verified, using a simple reaction (formic acid oxidation), that the photocatalytic spectral response is indeed extended to longer wavelengths, being able to use even 700 nm photons, without largely degrading the response for above-bandgap photons (i.e. strong recombination is not induced) [3b, 4]. These materials are thus promising for efficient photoevolution of hydrogen from water; work on this is being pursued, the results of which will be presented. [1] A. Luque, A. Martà Phys. Rev. Lett. 78, 1977, 5014. [2] a) P. Palacios et al., Phys. Rev. B 73 (2007) 085206; ibid. Thin Solid Films 515 (2007) 6280; ibid. Phys. Rev. Lett. 101 (2008) 046403. [3] a) R. Lucena et al.: Chem. Maters. 20 (2008) 5125. b) P. Wahnón et al. PCCP, in press (DOI: 10.1039/c1cp22664a). [4] R. Lucena et al., submitted.
Durable Photoelectrochemical Water Splitting on p-GaInP2 via Surface Nitridation
Wang1, Adam
Welch1, John
High efficient III-V semiconductors have been successfully applied for direct photoelectrochemical (PEC) water splitting systems. Monolithic p-GaInP2/n/p-GaAs PEC-PV tandem cell device demonstrated a 12.4% solar-to-hydrogen (STH) conversion efficiency [1], however the lifetime of the device was a challenge due to the corrosion of the material. This study addresses the corrosion issue of the materials and the stability of the top layer. Therefore, thin films of p-GaInP2 were tested 24h in different solutions (including 3M H2SO4 for comparison) at 1 sun. Different analytical techniques (including SEM, EDX, XPS and ICP) were used to analyze the surfaces and the tested solutions. Samples tested in sulfuric acid experienced extensive corrosion and surface Ga was selectively dissolved, identified both from the corroded surface and tested solutions. The samples tested in ammonium-bearing solutions experienced significant less corrosion, almost identical to that of an as-received one. SEM and EDX investigation also confirmed the significant reduced corrosion with samples tested in ammonium-bearing solutions. In other words, ammonium showed inhibitive effect for p-GaInP2, which is very promising to meet US DOE's 2013 goal of 1000h durability at 8% STH efficiency.
Doped Hematite Nano-particles for Bottom-up Solar Water-splitting Photoanode Preparation
Cornuz1, Morgan
Stefik1, Kevin
Sivula1, Michael
Hematite has been identified as a promising material for solar water splitting in a so-called tandem-cell configuration where, given its low bandgap (2.1 eV), it absorbs up to 16% of terrestrial solar illumination. The two main challenges when using metal oxides like hematite are the poor overall charge transport and the very short minority carrier diffusion length. The former can be overcome with proper electronic doping and the latter can be addressed through precise nanostructuring of the material. We have previously developed an attractive way to prepare photoactive hematite films using a simple solution-based colloid technique, but the precise doping of intrinsic iron oxide nanoparticles has remained an ongoing challenge. So far, dopant integration in hematite colloids has been possible only when heating the films to a substantially high temperature of 800 °C, causing drastic changes in morphology and growth of the particles size (not to mention the requirement for temperature resistant substrates). Herein, we discuss the challenges of preparing doped iron oxide nanoparticles and present a method to facilitate dopant integration directly during particle fabrication in order to avoid the high temperature annealing step. We further show these particles employed in highly scalable processes for the preparation of efficient hematite photoanodes for solar water splitting. The synthesis route was chosen to be water-free to avoid iron hydroxide formation further reducing the need for high temperature annealing. Iron oxide nanoparticles are found to be crystalline and properly doped as-synthesized with a fine control over diameter and size distribution. The film preparation consists in printing the solution on a conductive substrate while controlling the necking of the particle, the thickness and the porosity in order to achieve efficient solar energy to hydrogen conversion. 1. Sivula, K.; Le Formal, F. & Grätzel, M. (2011), 'Solar Water Splitting: Progress Using Hematite (alpha-Fe2O3) Photoelectrodes', ChemSusChem 4(4), 432-449. 2. Sivula, K.; Zboril, R.; Le Formal, F.; Robert, R.; Weidenkaff, A.; Tucek, J.; Frydrych, J. & Grätzel, M. (2010), 'Photoelectrochemical Water Splitting with Mesoporous Hematite Prepared by a Solution-Based Colloidal Approach', Journal of the American Chemical Society 132(21), 7436-7444.
Molecular Materials for Solar Energy Applications
Molecular framework materials containing light-harvesting and catalytically competent molecules have been rationally designed. These materials act as an excellent light-harvesting system by combining intra-framework energy migration and interfacial electron transfer quenching. We have also demonstrated that framework materials built from catalytic components are active catalysts in a range of reactions that are relevant to solar energy utilization, including water oxidation, carbon dioxide and proton reduction, and photocatalytic organic transfromations. Our work illustrates the potential of combining photosensitizers, molecular catalysts, and framework structures in developing highly active heterogeneous catalysts for solar energy utilization.
Nanostructured Photocatalytic Metal Oxides with High Porosity Prepared by Low Energy Ion Irradiation
De Temmerman1, Matthew
Baldwin2, Russel
Doerner2, Laurent
Marot3, Richard
van de Sanden1 4.
Efficient generation of fuels through direct conversion of solar energy requires the development of suitable photo-electrode materials. Amongst the properties that such materials should possess are: strong visible light absorption, high surface area and low cost. Nano-structured metal oxides with high aspect ratios (such as nanowires) can be produced by a variety of techniques included wet-chemistry and plasma-based methods. It has recently been reported that irradiation of metal surfaces at elevated temperatures by low-energy helium ions (energy below the ion damage threshold) leads to the formation of a fibreform nano-structure, with filament diameter below 20nm [1]. To date, this effect has mainly been studied in the context of nuclear fusion where it is potentially detrimental for the lifetime of the tungsten plasma-facing materials. The helium-induced nano-structure does however present very interesting properties for a photocatalytic surface. The very high level of porosity (up to 90%) indeed leads to significant levels of light absorption across the whole solar spectrum, with measured values of up to 98% of the total solar light [2]. The nano-structure is easily turned into a stoichiometric oxide by oxidation in an oxygen-rich environment. In this contribution, we will describe the formation of nano-structured tungsten and molybdenum trioxide surfaces by low energy helium irradiation and subsequent oxidation. The formation kinetics of the helium-induced nano-structure depends strongly on the surface temperature during plasma exposure and the ion energy (in the range 10-50eV). In addition, the size and porosity of the structure depend on the surface temperature. Thermal oxidation has been used to produce stoichiometric WO3 and MoO3 oxides, the structure of which has been studied by high-resolution secondary electron microscopy and X-Ray diffraction while the optical absorption is measured by a spectrophotometer. Results of the photocatalytic activity of such nano-structured oxides towards water splitting will be presented. References: [1] M.J. Baldwin, et al, Nucl. Fusion, 48 (2008) 035001 [2] S. Kajita et al, Applied Physics Express, 3 (2010) 085204
Nanostructured Materials as Efficient Oxygen Evolution Catalysts
Jiao1, Venkata Bharat Ram
Boppana1, Seif
Solar energy harvesting is an important technological challenge, considering that the energy of sunlight that strikes the earth's surface in an hour is sufficient to meet our energy demands for a year. Moreover, an economic and mobile energy storage media that does not significantly affect the current energy infrastructure is necessary to offset the diffuse and intermittent nature of sunlight. These challenges could be resolved by generating transportable solar fuels (like hydrogen or methanol) from abundant sources, e.g. H2O and CO2, utilizing sunlight as the primary energy source. Multiple approaches including photoelectrochemical and photocatalytic methods have been proposed and investigated in the past decades. Irrespective of the approach that is pursued, oxygen evolution from water is the critical reaction, because water is the only cheap, clean and abundant source that is capable of completing the redox cycle for producing either hydrogen (from H2O) or carbonaceous fuels (from CO2) on a terawatt scale. Thus, an effective catalyst for oxygen evolution via water oxidation is the key to accomplish the challenge of efficient solar energy harvesting. Here, we will demonstrate that the morphology and crystal structure have negligible effect on the photocatalytic properties of MnO2 based oxygen evolution catalysts, while the turnover rate is proportional to its surface area (i.e. Mn sites available on the surface). In order to testify the hypothesis, a wide range of manganese oxides with various morphologies and polymorphs are synthesized and their structures are well characterized. The as-synthesized catalysts, such as α-MnO2 nanotubes, α-MnO2 nanowires, and β-MnO2 nanowires, exhibit excellent activities in water oxidation driven by visible light. By calculating the TOFs per surface Mn site, the rates for all the different catalysts are similar (~0.001-0.0005 per second per surface Mn), indicating the negligible morphology and crystal structure effects. Based on this finding, one should expect that the highest activity would be obtained from the manganese oxide with the highest surface area. To further enhance the turnover frequencies (TOFs) that limited by surface area, surface active site with a higher TOF rate compared with Mn4+ is required. Along this direction, we introduce K+ doped MnO2 catalysts into oxygen evolution reaction. By doping MnO2 with K+, we create Mn3+ sites on the surface of mixed manganese oxides. Our preliminary data show that more than one order higher oxygen evolution rates per surface Mn were observed. In order to explore the origin of the enhancement in oxygen evolution activity, detailed structural characterizations have been performed and the results indicate that Mn3+ sites generated by K+ doping may be responsible for the high TOFs.
500-h Stability for Hydrogen Generation by Photoelectrolysis of Water Using GaN
Ohara1, Daisuke
Uchida1, Tomoe
Hayashi1, Momoko
Deura1, Kazuhiro
We confirmed that GaN photocatalyst produced hydrogen continuously for 500 hours without etching of GaN layers after the electric charge of 722 coulombs. The energy conversion efficiency from light to hydrogen (H2+1/2O2=H2O, ΔG=-237.13 kJ/mol) was up to 0.72% in spite of no extra bias applied to this photoelectrolysis system. Hydrogen is environmentally clean energy because it becomes only water after combustion. As a method of hydrogen generation, photoelectrolysis of water is promising. Various oxides have been used as photocatalysts so far. We found that GaN is a desirable material to split water by photoelectrolysis without any bias [1]. Moreover, we can control bandgap by changing group-III content of InGaN, which can absorb not only ultraviolet light but also visible one. Although long-time stability have been studied in a few oxides such as Cu2O and InNiTaO4 [2,3], we have already revealed that deposition of NiO co-catalyst on the GaN layer significantly prevents etching during the photoelectrolysis. In this study, we investigated stability of the GaN photocatalyst with NiO for 500 hours. We used a 3-μm-thick GaN layer on a sapphire substrate grown by metalorganic vapor-phase epitaxy and NiO was deposited on the GaN surface. This GaN working electrode connected to a Pt counterelectrode was dipped into a NaOH solution with the concentration of 1 mol/L and no bias was applied to this system. This GaN electrode was irradiated by light from a 300-W Xe lamp with the energy density of 100 mW/cm2. We performed 10-hour experiments for 50 times. Hydrogen was produced continuously for 500 hours, the total amount of which was 82.2 mL, and its evolution rate was 0.164 mL/h. This total time was longer than that for InNiTaO4 [3], and the evolution rate were superior to those for Cu2O [2], that is, GaN is more adequate for photocatalyst than any other materials. The energy conversion efficiency from light to hydrogen was 0.72% at the maximum. In fact, surface state after the experiment had no clear change compared with that before the experiment by a Nomarski microscope, i.e., the GaN layer still survives. This GaN generated the electric charge of 722 C, which was calculated by the integration of photocurrent for 500 hours. In conclusion, we confirmed that the GaN photocatalyst with NiO shows excellent stability. 1) M. Ono, K. Ohkawa et al., J. Chem. Phys. 126, 054708 (2007). 2) M. Hara, K. Domen et al., Chem. Commun. 357, (1998). 3) Z. Zou, H. Arakawa et al., Chem. Nature 625, (2011).
Structure and Dynamics of III-V Electrode/Water Interfaces for Photoelectrochemical Hydrogen Production
Wood1, Tadashi
Ogitsu1, Woon-Ih
Choi1, Eric
Hydrogen production using photoelectrochemical water splitting represents an attractive approach to realizing a clean, sustainable energy infrastructure. Unfortunately, finding a semiconductor electrode that offers high solar conversion efficiency while remaining stable under operating conditions has been difficult. In large part, this can be traced to a poor understanding of the complex chemistry active at the electrode-electrolyte interface. In order to understanding the reactive states precursory to photoexcitation, hydrogen evolution, and photocorrosion, we have studied the structure, stability, and chemical activity of III-V semiconductor electrodes in the presence of an electrolyte using a combination of first-principles molecular dynamics simulations and model density-functional calculations. We find that a local bond-topological model is able to capture much of the basic chemistry and structural motifs of the surfaces, allowing efficient study of complex surface morphologies built using simpler models and providing a map by which local structural features might be identified experimentally. Our results point to the particular importance of oxygen-containing surface adsorbates in determining the available reaction pathways for photocorrosion and water dissociation. Explicit modeling of water molecules at the interface reveals the importance of hydrogen bonding in determining the surface structure, which has additional implications for surface reactivity.
Water Splitting on Transition Metal Oxynitrides
In recent years, visible-light-driven hydrogen evolution from water using a photocatalyst has attracted significant attention as a potential means of supplying hydrogen from renewable resources. For efficient solar energy conversion, a photocatalyst that permits light absorption at wavelengths longer than 600 nm (band gap smaller than 2 eV) is highly desirable. Assuming overall water splitting with a quantum yield of unity, the theoretical potential of photocatalysts with an absorption edge of 600 and 650 nm can achieve, respectively, 16.2 and 20.6% solar energy conversion. Thus, the development of "600-nm-class photocatalysts†is an important mission for solar energy conversion. Oxynitrides of LaTiO2N, Ta3N5, and BaTaO2N are active photocatalysts that have band gaps of 2.1, 2.0, 1.8 eV, respectively. Our group has attempted to improve the photocatalytic activities by several ways including a new precursor route, surface modification by cocatalysts that act as efficient gas evolution sites, forming a solid solution with a wide gap metal oxide, and so on. For example, single-crystalline LaTiO2N particles with mesoporous and microporous architecture can be prepared by heating well-grown La2Ti2O7 particles synthesized by a flux method. The thus-prepared LaTiO2N modified with a cobalt oxide cocatalyst photocatalytically generates oxygen from water with an apparent quantum yield of nearly 30% at 440 nm. In this presentation, recent development of oxynitride-type photocatalysts, which have an absorption band longer than 600 nm, will be given.
Photoelectrochemical Synchrotron Studies on Metal Oxides for Solar Hydrogen Applications
Braun1, Debajeet
Bora1 2, Edwin
Constable2, Zhi
Liu3, Jinghua
Guo3, Kevin
Sivula4, Thomas
Graule1, Michael
Close inspection of the pre-edge in oxygen NEXAFS spectra of titanium oxynitride photocatalysts with 20 nm particle size reveals an additional eg resonance in the valence band that went unnoticed in previous TiO2 anion doping studies. The spectral weight of this Ti(3d)-O(2p) hybridized state with respect to and located between the readily established t2g and eg resonances scales with the photocatalytic decomposition power, suggesting that this resonance bears co-responsibility for the photocatalytic performance of titanium oxynitrides at visible light wavelengths. Anodic oxidation of hematite (α-Fe2O3) nanoparticulate films at 600 mV in KOH electrolyte forms a species at the surface which causes a new transition in the upper Hubbard band between the regions of the hybridized Fe(3d)-O(2p) and Fe(4sp)-O(2p) states, as evidenced by oxygen NEXAFS spectra. This transition, not known for pristine α-Fe2O3, is at about the same x-ray energy at which pristine Si doped Si:Fe2O3 has such transition. These states coincide with the onset of an oxidative dark current wave as observed in the cyclic voltamogram. Electrochemical oxidation at 200 mV does not form such an extra NEXAFS feature. A 100 nm thick pulsed laser deposited blue, non-stoichiometric WO3-δ film grows on TiO2 (110) in [220] direction. Oxidative treatment at 400°C turns the film color from blue to yellow and improves the film quality considerably, as shown by improvement of the Kiessig oscillations in the x-ray reflectometry curves. Detailed analysis of resonant valence band photoemission spectra of the as-prepared non-stoichiometric film and oxidized yellow film suggests that a transition near the Fermi energy originates from the non-stoichiometry, i.e. oxygen deficiency and insofar poses electronic defect states which partially can be eliminated by heat treatment in oxygen. We will also present very recent unpublished in-situ electrochemical studies on WO3 and Fe2O3 with ambient pressure XPS and liquid electrolyte NEXAFS spectroscopy. 1)A. Braun, K.K. Akurati, G. Fortunato, F.A. Reifler, A. Ritter, A.S. Harvey, A. Vital, T. Graule, J. Phys. Chem. C 2010, 114, 516–519. 2)D.K. Bora, A.Braun , S.Erat, R.Löhnert, A.K. Ariffin, R.Manzke, K.Sivula, J. Töpfer, T.Graule, M.Grätzel, E.Constable, J. Phys. Chem. C 2011, 115, 5619–5625. 3)A. Braun , S. Erat, X. Zhang, Q. Chen, F. Aksoy, R. Löhnert, Z. Liu, T. Graule, S.S. Mao, J. Phys. Chem. C 2011, 115, 16411–16417.
High Efficiency Solar Water Splitting with Hematite-ETA Host/Guest Heterostructure
Brillet1, Morgan
Stefik1, Kaupo
Kukli2, Kevin
Sivula1, Markku
Leskela2, Michael
Graetzel1, Nicolas
Since the seminal demonstration of water dissociation using a semiconductor absorbing photons to generate electron-hole pairs (1), hematite as a photoanode has been a material of choice. It's abundance and relatively low cost, its good light absorption capabilities (bandgap of 2.1 eV) and chemical stability are the most noticeable advantages of α -Fe2O3. However, this material exhibits a number of challenges, including its less than ideal electrical vs. optical properties. Indeed, its hole diffusion length is extremely small (2 to 5 nm) (2) as compared to its light penetration depth (α–1 = 118 nm at λ = 550 nm) (3), giving photo-generated charges low probability to reach the semiconductor/liquid interface and therefore participate to water oxidation. The resulting poor external quantum efficiency (EQE) close to the band edge has been identified to be one of the major limitations in context of a tandem device (4). To answer this problem, we present here progresses towards a high efficiency host-guest heterostructure5 that allows the carrier generation close to the surface where a high EQE extremely thin absorber (ETA) is conformaly coated on a tridimensional transparent conductive oxide (3D-TCO) (6). High EQE has been achieved through the improvement of atomic layer deposited (ALD) films by doping the hematite and passivating both the substrate and electrolyte interfaces. (1) Boddy, P. J Electrochem Soc 1968. (2) Kennedy, J.; Frese, K., Jr J Electrochem Soc 1978, 125, 709. (3) Balberg, I.; Pinch, H. Journal of Magnetism and Magnetic Materials 1978, 7, 12–15. (4) Brillet, J.; Cornuz, M.; Le Formal, F.; Yum, J.-H.; Graetzel, M.; Sivula, K. J Mater Res 2010, 25, 17–24. (5) Sivula, K.; Le Formal, F.; Graetzel, M. Chem Mater 2009, 21, 2862–2867. (6) Lin, Y.; Zhou, S.; Sheehan, S. W.; Wang, D. J. Am. Chem. Soc., 2011, 133, pp 2398–2401
How to Boost the Efficiency of CO2 Photoreduction by Using Bimetallic Catalyst Nanoparticles
Andreu1, Andres
Parra1, Cristian
Fabrega1, Maria
Ibanez2, Raquel
Nafria1, Andreu
Cabot1 2, Joan R.
Morante1 2.
In this work, we will report on the use of metallic and multimetallic nanoparticles as additives for modifying the surface reactivity of the TiO2 nanoparticles, which have conveniently modified in order to increase the CO2 reduction capability under photo catalytic conditions. For improving these photocatalytic processes, not only must be generated electron-hole pairs, but the electronic transfer must be efficient enough, which requires multi-electronic redox processes, together with the condition that that the oxidation and reduction must occur simultaneously. In order to overcome these problems, the use of the multimetallic particles can be followed for increasing the photocatalyst efficiency. In this contribution, special attention is paid to the silver and silver-copper bimetallic nanoparticles, which reached competitive efficiencies compared to other noble metal bimetallic nanoparticles, such as platinum and platinum-copper systems. The structural and functional characterization as well as the use and reliability of these kind of additive will be presented and discussed considering the viability of efficiency increase and the tune of the production of different sub products for fuel synthesis.
Highly Efficient CoPi-Catalyzed Thin Film BiVO4 Photoanodes
Abdi1, Roel
van de Krol1.
BiVO4 is considered to be a promising photoanode material for solar water splitting applications. The monoclinic phase has a bandgap of ~2.4 eV, which corresponds to an optical absorption edge at ~520 nm. However, the quantum efficiencies reported so far do not exceed 55% [1-3], and the main performance-limiting factors are still not clear. In this work, photoelectrochemical characterization of spray-deposited, dense films of BiVO4 results in a quantum efficiency of up to 95% at low light intensities. By comparing front- and back-side illumination, electron transport was found to be the main rate-limiting factor under these conditions [2]. This is further confirmed by a time-resolved microwave conductivity study that reveals a carrier mobility value of ~3 x 10-3 cm2/Vs, which is 2 – 3 orders of magnitude lower than is typically observed for metal oxide photoanodes. Intriguingly, Mott-Schottky analysis of BiVO4 films consistently indicates very high donor densities, which is difficult to reconcile with the high quantum efficiencies and poor electron transport properties. We will present the results of a recent electrochemical impedance spectroscopy study that was carried out to elucidate these contradictory observations. Integrating the quantum efficiencies over the solar spectrum leads to a predicted AM1.5 current density of 3.6 mA/cm2. However, the observed photocurrent density under simulated AM1.5 illumination is much lower than the predicted value (~0.4 mA/cm2) due to extensive carrier recombination. Under this condition, poor water oxidation kinetics (hole transfer) become rate-limiting, as evidenced by a significant increase of photocurrent density when hydrogen peroxide is added as a hole scavenger. Based on these insights, we have modified the sample with cobalt-phosphate as a water oxidation catalyst [4] to address the poor hole transfer, and tungsten as a donor-type dopant to enhance electron transport. This has resulted in an AM1.5 photocurrent density of 2 mA/cm2 at 1.23 VRHE. While this is lower than the state-of-the-art photocurrent of 2.8 mA/cm2 recently reported by Luo et al., our results have been obtained for a five-times thinner compact film of only 200 nm and a low cost cobalt phosphate as the co-catalyst. In addition, the amount of light scattering in these spray-deposited films is negligible, which is an important advantage since these films would ultimately have to be incorporated in a tandem device structure to achieve overall water splitting. References [1] K. Sayama et al., J. Phys. Chem. B, 110 (2006) 11352 [2] Y. Liang, T. Tsubota, L.P.A. Mooij, and R. van de Krol, J. Phys. Chem. C 115 (2011) 17594 [3] W. Luo et al., Energy Environ. Sci., 4 (2011) 4046 [4] M. Kanan and D. Nocera, Science, 321 (2008) 1072
Photocatalytic Hydrogen Production by Utilizing Solar Energy Roles of Cocatalysts in Photocatalysis
This lecture presents the photocatalytic hydrogen production by utilizing solar energy, mainly discussing three types of reactions including water splitting, biomass reforming and industrial waste reforming. Hydrogen molecule is an energy carrier for solar energy storage, and is ideal for clean energy process, and the hydrogen production utilizing solar energy could eventually realize the so called Hydrogen Economy and as a means for solar energy powering the world. The bottleneck to produce hydrogen through photocatalysis is the developing of high active photocatalysts which have been extensively investigated and explored during last several decades. This talk will focus on the role of cocatalysts in photocatalysis. The crucial roles of dual co-catalysts respectively for both half reactions, oxidation and reduction are highlighted. Three typical catalysts, Pt/TiO2, Pt-PdS/CdS and CoPi/BiVO4 are studied for methanol reforming (as a representative reaction of biomass reforming), H2S reforming and water splitting reaction respectively. Our research shows that the cocatalyst is necessary for hydrogen evolution or/and oxygen evolution, and the coloading of dual co-catalysts are absolutely necessary for improving the photocatalytic activity by reducing the recombination of photogenerated electrons and holes and reducing the activation energy. It is found that the oxidation half reaction is the most difficult part of the photocatalytic hydrogen production, particularly from water splitting reaction, and the cocatalyst to reduce the activation energy for oxygen evolution from water splitting essentially acts as the same role as the electrochemical catalysts to lower the overpotential. By well designing and loading the dual cocatalysts, Pt and PdS on CdS, using Na2S+Na2SO3 (H2S dissolved in NaOH) as sacrificial reagent, we can achieve a quantum efficiency of the artificial photosynthesis high as 93%. In situ photoelectrochemical measurements, photoluminescence spectroscopy and high resolution transmission electron microscopy characterizations indicate that the exceptionally high quantum efficiency can be attributable to the vital factors including mainly the spatially separated PdS and Pt as the oxidation and reduction active sites respectively; the efficient utilization of the electrons at the shallow trap states of CdS for photocatalytic reactions; and the formation of atomic heterojunctions between the co-catalyst PdS and CdS as these factors could effectively prohibit the recombination of photogenerated electrons and holes and favor the full utilization of the photogenerated electrons.
Artificial Photosynthesis - Use of a Ferroelectric Photocatalyst
Dunn1, Matt
There has been a growing interest in developing systems that are suitable for the conversion of atmospheric CO2 into hydrocarbons that can used as a fuel using sunlight. This is a process that mimics photosynthesis and has, to date, mostly eluded mankind at a level that could effectively limit our dependence on other fuel sources. In this work we focus on using a ferroelectric material that exhibits a reduction potential of the photoexcited electron around -3V v's NHE. This highly reducing electron enables a series of reaction schemes to be utilised in the reduction of CO2 that have not been possible using semiconductors that have a highly oxidising hole. The photocatalytic fixation of CO2 to hydrocarbons was performed using LiNbO3. 0.126 cm2 of powdered LiNbO3 was irradiated with 64.2mW/cm2 light from a mercury lamp with 10ml of water under 30% CO2/air. The LiNbO3was held on a platen above the water to give a gas-solid catalytic reaction. We use GC-MS to determine that formic acid and formaldehyde were present in the water after irradiation for 6 hours. The products where produced at rates of 7.7mmol and 1.1 micro mol /hour. This rate of production enabled the calculation of the efficiency of the catalyst. This was calculated to be ca 2% which compared to 0.17% for TiO2 under the same conditions. The efficiency was calculated from a ratio of light available for reaction and the energy contained in the chemical products. MgO doped LiNbO3 was also tested and found to have an energy conversion efficiency of 0.72%. We explain the differences in the performance for the efficiency between the doped and undoped materials in terms of the majority carriers in the system. The high efficiency of the LiNbO3 system, when compared to TiO2, is explained in terms of the ferroelectric nature of the catalyst. A ferroelectric will effectively separate the reduction and oxidation processes on its surface. This is due to the formation of an internal self developed p-n junction as a result of the spontaneous polarisation due to ion movement in the crystal lattice. This spatial selectivity reduces the chance for back reactions and enables the reaction to move towards completion. A second reason is that the reduction potential of a photoexcited electron is sufficiently high to directly reduce CO2 to CO2+-. This enables a previously unavailable reaction path to occur. The third and final reason is that the high surface charge density of LiNbO3 means that chemisorbed molecules may exhibit anomalous band structures. For example there is evidence that CO2 is not a linear molecule on the surface of a ferroelectric, this bending of the molecule structure would more readily enable the injection of extraction of electrons. In summary we show that LiNbO3 is an attractive material to act as a catalyst for artificial photosynthesis due to inherent materials properties associated with the band locations and ferroelectric properties.
Copper Tungstate (CuWO4)-Based Materials for Photoelectrochemical Hydrogen Production
Gaillard1, Yuancheng
Chang1, Artur
Braun1 2, Alexander
DeAngelis1, Jess
Three decades after the demonstration of photoelectrochemical (PEC) water splitting by Fujishima and Honda with TiO2, intensive research is still ongoing to identify a suitable semiconductor to be integrated in an efficient, cost effective, and durable PEC system. Among all candidates, transition metal oxides are still drawing lots of attention as they offer good resistance to corrosion and are inexpensive to produce. The major issue to solve remains their poor optical absorption, which restrains their solar-to-hydrogen efficiency to only a few percent. Numerous attempts have been made to reduce the band gap of transition metal oxides, mainly via incorporation of foreign elements such as nitrogen. Unfortunately, this incorporation method usually leads to an increase in structural defects and poor PEC performances. Thus, it appears that the best research strategy is to focus on metal oxides that already have appropriate optical absorption properties and then fine-tune, if necessary, their material properties such as bulk charge transport and catalytic surface activity. With an optical band gap of 2.2 eV, copper tungstate (CuWO4) should be considered as a potential candidate for PEC hydrogen production. As of today, most publications on CuWO4 report on its fundamental properties but only a handful on its potential as a PEC water splitting material. In the present communication, we report on the synthesis of CuWO4 thin film via low-cost processes for PEC hydrogen production. Our preliminary study indicated that copper tungstate material synthesized using co-sputtering methods at 275°C were amorphous and did not show any photo-response in 0.33M H3PO4 electrolyte under AM1.5G simulated illumination. However, a major improvement in PEC performance was observed after a post-deposition treatment performed at 500°C in pure argon for 8 hours. Indeed, a photocurrent density of approximately 0.4 mA.cm-2 at 1.6 V vs. SCE was measured on annealed CuWO4 samples. Subsequent X-ray diffraction analysis indicated a clear transformation of as-deposited amorphous thin films into a triclinic CuWO4 structure after the annealing step. Electrochemical impedance spectroscopy (EIS) finally revealed that polycrystalline n-type CuWO4 possessed a much lower flat-band potential (-0.35 V vs. SCE) than WO3 (+0.15 V vs. SCE) and ideal surface band-edges that straddle both the hydrogen and oxygen evolution reactions. However, it appeared that the main Achilles' heel of CuWO4 is its bulk charge transfer resistance (approx. 2000 ohm.cm-2, measured by EIS). Current research efforts are focused on resolving this issue by adding dopants and/or conductive carbon nano-tubes into the CuWO4 matrix.
Photoelectrochemical Investigation and Electronic Structure of a New Class of p-type Semiconductor Materials for Photocathode in Photoelectrochemical Cell
Joshi1, Paul
The direct conversion of solar energy to chemical fuels using semiconductor electrodes has been a topic of intense research for several decades. While a plethora of research has been directed at highly efficient n-type semiconductor photoanodes, a much smaller number of p-type semiconducting oxide materials are currently known. Recently, our research efforts have sought to investigate the reduction of the bandgap sizes of early transition-metal oxides via the incorporation of transition metals with d10 electron configurations, that is, specifically Cu+ and Ag+. An extension of these investigations into the photcatalytically-relevant niobates and tantalates has recently been shown to yield a promising class of new semiconductors based on mixed Cu+/Ta5+ and Cu+/Nb5+ metal oxides. While many alkali-metal niobates and tantalates are reported to be highly active photocatalysts, the related copper (I) niobates and copper (I) tantalates have not been well-explored yet owing to the difficulties in their high-purity preparation and structural characterization. This paper presents a detailed investigation of the photoelectrochemistry and electronic structure of this new class of p-type semiconductor materials for photocathode in photoelectrochemical cell (PEC). Photoelectochemical measurements carried out under visible light show a photocathodic current that is characteristic of p-type semiconducting behavior of the copper niobates and copper tantalates working electrode. Electronic strcuture calculations of copper niobate shows that its conduction band consists of Nb (d0) and O (p) orbitals, whereas its valence band is comprised of primarily Cu (d10) orbitals. Similarly, electronic structure calculations of copper tantalate shows that its conduction band consists of Ta (d0) and O (p) orbitals, whereas its valence band is comprise of primarily Cu (d10) orbitals. The niobate and tantalate films were deposited onto fluorine-doped tin oxide (FTO) glass using the doctor blade technique and characterized by X-ray diffraction (XRD), UV-visible spectroscopy and SEM. These new class of p-type semiconductor materials can be used for CO2 reduction to generate chemicals.
Photoelectrochemical Water Oxidation Using Heterojunction Oxide Semiconductor/Electrocatalyst Electrodes
Jeon1, Sung
Choi2, Hye
Jeong3, Hyunwoong
Park1 2 3.
Mimicking photosynthesis has been intensively studied primarily for generating carbon footprint-free fuels. Such artificial photosynthesis consists of three key reactions: water oxidation, proton transfer through membranes and wired electron transfer, and electrochemical reduction of carbon dioxide. Various semiconductor photoelectrodes have been studied on their photoelectrochemical water oxidation yet most of them suffer from inactivity for visible light, high charge recombination upon visible light irradiation, and photocorrosion. Very recently, cobalt and nickel-based electrocatalysts have been reported to have high electrocatalytic effect for water oxidation and maintain their catalytic effect even when loaded on semiconductor electrodes. We have found that such effect is highly dependent on the kind of semiconductors. For systematic investigation, we have tested six semiconductor electrodes with different bandgaps and energy levels for their photoelectrochemical water oxidation without or with the electrtocatalysts. In this presentation, the photoelectrochemical characterization of and gaseous oxygen evolution from the semiconductor electrodes will be discussed.
Durability of Platinum-based Catalysts in Fuel Cell Membranes
Jia1, Siming
Dong1, Takuya
Hasegawa2, Jiping
Ye3, Reinhold
The durability of proton exchange membrane (PEM) fuel cells is a challenging issue for their long term operational use. Mechanical degradation of the catalyst coated membrane (CCM), which consists of one PEM coated with two platinum-based catalyst layers on both sides, represents a common failure mode that limits the lifetime of the cell. We apply novel thin film characterization techniques to investigate the effect of foreign cations and chloride contamination on the mechanical durability of CCMs under the simulated fuel cell operational environment. The results show that the fracture resistance of contaminated CCMs was significantly deteriorated. In addition, the loss of catalyst materials from CCMs with the existence of byproduct hydrogen peroxide (H2O2) is characterized by electron microscopy. Accordingly, the deterioration in fracture resistance of the CCMs indicates that the byproduct H2O2 may be implicated in the acceleration of mechanical degradation of CCMs, which is detrimental for the overall durability of a fuel cell.
Water Electrolysis with Different Sized Co3O4 Films Prepared by a Paste Coating Method
Hyo Sang
Jeon1 2, Byoung Koun
Min1 2.
In electrolysis, the obstacle would be an anodic reaction for water oxidation where substantial energy loss occurs mainly due to the large overpotential. Numerous electrocatalysts have been applied to reduce the overpotential in the water oxidation including expensive precious metals (e.g. Ir, Ru, and Rh). In this study, we demonstrate successful fabrication of different sized Co3O4 electrocatalyst films at low temperature (50 oC) on a stainless steel substrate by a paste coating method using Nafion as a binder. The structural characteristics of Co3O4 films were investigated by SEM, XRD and BET. We also investigated the electrochemical characteristics of the films with different particle sizes using classical electrochemical analysis. Based on the cyclic voltammetric measurements, the number of active sites of the films was estimated to 57.8, 86.1 and 95.7 mC cm-2 for the films with particle size of 145.9, 63.3 and 36.5nm, respectively. The highest hydrogen production rate was measured to 39.6 ml/h for the film with particle size of 38.9 nm. The details will be discussed in the presentation.
U3.4 Transferrred to U4.2
Bi-functional Nanoceria Catalyst for Cyanosilylation of Aldehydes
Wang1, Xiang
Fei1, Lu
Wang2, Wai-Ning
Mei2, Chin Li
Cyanosilylation of carbonyl compounds with trialkylsilyl cyanide is an important reaction for producing cyanohydrins derivatives, which can be further transformed into valuable chemicals with functional groups including α-hydroxy acids and β-hydroxyamines. In this study, we report our use of nanoceria with high density of exposed subsurface coordinatively unsaturated cerium atom sites (cuc sites) for heterogeneous catalytic cyanosilylation of aldehydes. These catalytically active centers were produced by exploiting the nanosize effect of nanoceria to efficiently remove the superficial oxygen atoms using a low-pressure activation process. Compared to its inert bulk counterpart, nanoceria was illustrated to be a highly active catalyst for the cyanosilylation of various aromatic and aliphatic aldehydes with trimethylsilyl cyanide (TMSCN) with 99% yield within three hours of reaction time at room temperature. Experimental results showed that the activation of TMSCN and the release of cyanide group were likely initiated by the electron transfer from the electron-rich oxygen atoms on the nanoceria surface (Lewis base sites) to the silicon atom of TMSCN. The coordinatively unsaturated cerium sites on the surface or subsurface of nanoceria (Lewis acid sites) were likely accounted for the activation of the carbonyl group in the aldehyde. Density functional theory calculations were employed to determine the partial charge density mapping of the defective nanoceria surface. These findings were applied to explain the bifunctional chemical property of defective nanoceria in the catalytic cyanosilylation reactions.
AuxRh100-x Thin Film Alloyed Catalysts Synthesized by Pulsed Laser Deposition
Imbeault1, David
Reyter1, Sebastien
Garbarino1, Lionel
Roue1, Daniel
The methanol oxidation reaction (MOR) is of interest due to its relevance for direct methanol fuel cells. In acidic solution, platinum is the most active anode catalyst for the oxidation of methanol but it is easily poisoned by strongly adsorbing intermediates like CO and oxygenated species. The MOR is easier in alkaline solution since the presence of OH- species favours the electro-oxidation of methanol and reaction intermediates adsorbed at the surface of the catalyst. Also, the use of an alkaline medium considerably widens the range of elements to choose from. Recently, Pd-, Au-, and Ni-based material catalysts have been studied for the MOR in alkaline solutions. Amongst them, gold is particularly interesting because of its resistance to poisoning. In the present work, AuxRh100-x/C thin films have been synthesized by crossed beam pulsed laser deposition (CBPLD), using pure Au and Rh metal targets ablated simultaneously by two UV laser beams. The deposition was performed in vacuum chamber with a 100 mTorr He background pressure and the films' composition was modified by varying the fluence of the laser beams. The physicochemical characterization of the thin films was performed by X-ray diffraction (XRD), energy-dispersive X-ray fluorescence (EDX) and X-ray photoelectron spectroscopy (XPS). The analyses confirm that single-phase face centered cubic AuxRh100-x alloys are obtained over the whole range of composition from x = 0 to 100, despite the fact that the phase diagram for bulk Au-Rh displays a large immiscibility gap. The electrocatalytic activity of AuxRh100-x alloys was assessed by cyclic voltammetry in 1M NaOH with and without methanol. The results revealed improved kinetics for the anodic oxidation of methanol which point towards the presence of a synergetic activity between the two metals on both Au25Rh75 and Au50Rh50. Indeed,the onset potential for methanol oxidation is shifted by ca. 500 mV towards less positive potentials in comparison with pure Au electrode while methanol oxidation currents remained significant over a wide range of potential values as compared to quickly deactivated pure Rh electrode.
Oxygen Reduction Reaction Electrocatalytic Activity of SAD-Pt/GLAD-Cr Nanorods
Khudhayer1, Nancy
Kariuki2, Deborah
Myers2, Ali
Shaikh3, Tansel
Nanorod arrays of chromium (Cr) were grown on glassy carbon (GC) electrodes by a dc magnetron sputtering glancing angle deposition (GLAD) technique. The Cr nanorods were used as low-cost, high surface area, metallic supports for a conformal layer of Pt thin film catalyst, as a potential low-loading electrocatalyst for the oxygen reduction reaction (ORR) in polymer electrolyte membrane (PEM) fuel cells. A conformal coating of Pt on Cr nanorods was achieved using a dc magnetron sputtering small angle deposition (SAD) technique, which included a relatively small deposition angle of θ = 30o with a substrate rotation of 2 rpm. The small-angle incident flux during SAD allowed Pt atoms to reach the sidewalls and bottoms of the nanorod supports and led to a more conformal Pt film coating compared to a conventional normal incident angle deposition. The ORR activity of SAD-Pt(30o)/GLAD-Cr electrodes was investigated using cyclic volatemmetry (CV) and rotating-disk electrode (RDE) techniques in a 0.1 M HClO4 solution at room temperature. A reference sample consisting of GLAD Cr nanorods coated with a Pt thin film deposited at normal incidence (θ = 0o) was prepared and compared with the SAD-Pt(30o)/GLAD-Cr nanorods. All the electrodes had the same Pt loading of 0.04 mg/cm2, which was measured by quartz crystal microbalance (QCM). The thickness of the SAD Pt thin film on the GLAD Cr nanorods was estimated to be approximately 3 nm. The CV results show that the SAD-Pt(30o)/GLAD-Cr nanorod array electrode has an oxide reduction peak potential of 0.78 V, which is 50 mV more positive than the oxide reduction peak observed for Pt(0o)/GLAD-Cr nanorods. The Pt/GLAD-Cr nanorods also show a higher onset potential for oxide formation (≥ 0.86 V) as compared to Pt alone (~0.8 V), which may be indicative of an electronic interaction between the Pt thin film and the Cr nanorods. Compared to Pt(0o)/GLAD-Cr electrode, the SAD-Pt(30o)/GLAD-Cr nanorods exhibit higher electrochemically-active surface area (ECSA) and enhanced stability against loss of ECSA during potential cycling in acidic electrolyte. Specific ORR activities, determined using the RDE technique, were found to be higher for the SAD-Pt(30o)/GLAD-Cr electrode compared to that of the Pt(0o)/GLAD-Cr electrode. The improved activity might be attributed to a better Pt conformality, especially at the sidewalls of the nanorods, and a preferential exposure of certain crystal facets. Moreover, the ORR specific activities of the SAD-Pt(30o)/GLAD-Cr electrodes were also compared with the literature values for conventional Pt/C, solid GLAD Pt nanorods, and nano-structured thin film Pt (3M NSTF Pt) electrocatalysts.
Surfactant Removal of Colloidal Nanoparticles for Catalytic Applications
Li1 2, Chao
Wang2, Dusan
Tripokovic2, Shouheng
Sun1, Nenad
Markovic2, Vojislav
We carried out a systematic investigation of the effect of surfactant removal on the catalytic performance of colloidal nanoparticles (NPs). Pt NPs have been synthesized using oleylamine as surfactant. The as-synthesized Pt NPs were subject to different treatments in order to remove the surfactant. The surface cleanness of treated and untreated samples was characterized in an electrochemical cell for the catalytic activity of oxygen reduction reaction (ORR). The electrochemical studies were further correlated to thermogravimetric analysis (TGA) and infrared adsorption spectroscopy (IRAS) to unravel the surface cleanness after the treatments. The results show that thermal annealing is the most effective way to clean the surface in our system. Acetic acid wash is second to that; in which further potential cycling is necessary to further clean the catalyst. UV-ozone treatment also partially removed the surfactant, but is less effective than thermal annealing and acid wash. TMAOH (tetramethylammonium hydroxide) method did not improve the catalytic activity in our system. The findings clearly prove that the electrocatalytic activity of as-synthesized Pt NPs was highly dependant on the surface cleanness after surfactant removal, which may be important for fuel cell catalysis.
Cr Incorporated Mesoporous Ceria by One-pot and Impregnation Procedures for Dehydrogenation of Propane
Aktas1, Sena
Yasyerli1, Gulsen
Chromium based catalysts are known to show good performance in dehydrogenation of alkanes to alkenes, which is an important step in the production of petrochemicals. To eliminate thermodynamic limitations in such dehydrogenation reactions, oxidative dehydrogenation processes were proposed. Commercialization of such oxidative processes requires development of new selective catalysts. Ceria, with its good redox properties and highly mobile capping oxygen atoms on its surface, is considered as a very good oxidation catalyst. In the present study, Cr incorporated mesoporous ceria catalysts were synthesized following one-pot hydrothermal synthesis (Cr-MCe) and impregnation routes (Cr@MCe) and the catalytic performances of these materials were tested both in oxidative and non-oxidative dehydrogenation of propane to produce propylene. In the synthesis of these mesoporous materials, cerium nitrate tetrahydrate and cetyl-trimethylammonium bromide were used as the Ce source and the structure directing surfactant, respectively. Synthesis was achieved at 60oC with a solution pH value being in the range of 9-10. Produced materials were calcined at 600oC. Cr-MCe and Cr@MCe contained 6% and 7% Cr, respectively. Analysis results proved that the incorporated Cr was well dispersed within the mesoporous ceria, in both of these materials. Mesoporous ceria and Cr@MCe contained pores in the range of 5-15 nm and the material prepared by the one-pot procedure (Cr-MeCe) mostly in the range of 10-20 nm. Both of these materials had surface area values of about 50 m2/g. Nitrogen adsorption-desorption analysis showed that a mesoporous structure was obtained with interconnected pores. XRD analysis also indicated a mesoporous structure with short range order and also formation of ceria crystals. Propane dehydrogenation test results obtained in a differential fixed bed reactor showed excellent propylene selectivity values of about 99% with both of these catalysts. However, avtivity of the material prepared by the one-pot procedure (Cr-MCe) was higher than the activity of the impregnated material (Cr@MCe). Propane fractional conversion obtained with Cr-MCe was about 0.06 at 550oC, at a space time of about 0.4 s.g.cm-3. In the oxidative dehydrogenation, much higher propane conversion values were obtained with lower propylene selectivity. Performance of Cr@MCe was better in the oxidative process, giving 51% propylene selectivity at a propane conversion of 20%, at 550oC. Results indicated potential advantages of mesoporous ceria incorporated Cr catalysts in propylene production from propane. Acknowledgement: Gazi University Research Fund.
Study of Co-assembled Conducting Polymers for Enhanced Ethanol Electro-oxidation Reaction
Le1, Hiroyuki
Yoshikawa1, Masato
Saito1, Eiichi
Among many renewable energy sources, ethanol stands out as one of the most promising candidate since it allows the generation of electricity via a direct ethanol fuel cell (DEFC) with high energy efficiency, non-toxicity, natural availability, and greenhouse gas free. However, to realize the future of DEFC, current challenges such as slow reaction kinetics of ethanol oxidation, fuel cell fabrication costs, and low durability of catalysts need to be overcome. To date, researchers have successfully attempted to explore the use of metal alloys and metal oxides as catalysts for the ethanol oxidation process. For instance, Pt alloys containing tin (Sn), tin oxides, or other metals, have significantly enhanced both catalytic capacity and stability. These enhancements have been attributed to Sn or SnOx, which provide a surface rich in oxygen-containing species capable of removing an adsorbed intermediate product, -COads, by facilitating its complete oxidation to CO2. Taking into understanding these alloy effects, our approach makes use of the organic species as the support for the conventional Pt nanoparticles, rather than metal oxides and alloys. In this method, it is imperative that the organic species is not only electroactive but also conductive, hence conducting polymers are promising candidates. Herein, we particularly investigated the effect of polyaniline (PANI) and polypyrrole (PPY) in their native and co-assembled forms not as matrix but as a top-layer of Pt nanoparticles decorated on multi-walled carbon nanotubes(Pt/MWCNTs) on EOR. The co-assembled conducting PANI-PPY deposited Pt/MWCNTs was successfully synthesized and demonstrated significant enhancement of the electro-catalytic activity and stability toward ethanol oxidation reaction as revealed by electrochemical characterizations. The presented results indicate that in the co-assembled form, PANI and PPY retained their own superior effects on the enhancement of stability and catalytic activity via intermediate species removals and ethanol adsorption, respectively. This preliminary result reveals a new strategy for the use of conducting polymers as potential catalyst supports due to its facile fabrication and functionalization, cost effectiveness and environmentally friendly in comparision to alloys and metal oxides; factors which are necessary for the practical application of direct ethanol fuel cell in the near future. References: [1] Hoa, L. Q.; Yoshikawa, H.; Saito, M.; Tamiya, E. J. Mater. Chem. 2011, 21, 4068–4070. [2] Hoa, L. Q.; Vestergaard, M. C.; Yoshikawa, H.; Saito, M.; Tamiya, E. Electrochem. Commun. 2011, 13, 746–749.
Hydrogen Evolution from Water Using Semiconductor Nanoparticle/Graphene Composite Photocatalysts without Noble Metals
Tremendous efforts have been focused on developing efficient and inexpensive methods to achieve solar-driven water splitting. However, in most photocatalytic hydrogen-generating systems the yields of H2 are quite modest and a noble metal is still used as a co-catalyst. Graphene as an excellent supporting matrix for photocatalyst particles can efficiently suppress the growth of semiconductor particles as well as facilitate the transfer of the photogenerated electrons to the surface of photocatalysts, which could enhance the efficiency of the photocleavage of water. Herein, semiconductor nanoparticle/graphene composite photocatalysts containing semiconductor CdS or TiO2 nanoparticles are fabricated by one-pot solution methods and their structures are characterized. The photocatalytic hydrogen-generating capabilities of the composite photocatalysts are investigated in the presence of sacrificial reagent and compared with those of the same semiconductor materials with platinum as a co-catalyst under the same conditions. The results obtained by the measurements of time-resolved emission spectra, photocurrent generated response and electrochemical impedance spectra revealed that graphene attached to semiconductor surfaces can efficiently accept and transport electrons from the excited semiconductor, suppressing charge recombination and improving interfacial charge transfer processes. The semiconductor nanoparticle/ graphene photocatalysts displayed higher activity for photocatalytic hydrogen evolution, which can be compared with the hydrogen-generating efficiency of systems containing the well-known Pt co-catalyst. This work provides an inexpensive means of harnessing solar energy to achieve highly efficient hydrogen evolution without noble metal.
Facile Synthesis of Gold Nanorice Enclosed by High-index Facets and Its Application for CO Oxidation
Xia1, Yiqun
Zheng2, Jie
Zeng1, Yanyun
Ma1, Christine
Moran1, Taekyung
We recently developed a facile method for generating Au nanorice enclosed by high-index facets in high yields by using a Au(I)-tetra(ethylene glycol) complex as the precursor. The key to the success of this synthesis is to introduce a small amount of silver ions to mediate the growth. Both electron microscopy and diffraction analyses indicate that the Au nanorice has a penta-twinned structure and enclosed by {611} facets on the side surface and {111} facets at the two ends. The Au nanorice with high-index facets showed much higher catalytic activity than the conventional, multiply twinned nanoparticles enclosed by {111} facets for CO oxidation.
In-situ X-Ray Studies of Electrocatalytic Activity and Stability of Perovskite Oxide Thin Films
Seo Hyoung
Chang1, R.
Subbaraman1, Kee-Chul
Chang1, June Hyuk
Lee2, N.
Danilovic3, D.
Meyers4, M.
Kareev4, J.
Tchakhalian4, D.
Fong1, M.
Highland1, P.
Baldo1, V.
Stamenkovic1, J.
Freeland2, J.
Eastman1, N.
Understanding and improving the efficiency of water dissociation and formation processes are prerequisites to realizing the next generation of hydrogen-based energy storage and conversion systems. Searching for novel complex perovskite oxides (ABO3-type) that exhibit bi-functionality and site-selective reactivity is a promising route to overcoming current limits of existing electrocatalysts. Despite extensive recent interest, there is still a lack of fundamental understanding of electrochemical reaction mechanisms on complex oxide surfaces and interfaces. One reason is that there are few experimental tools that can directly probe time-dependent phenomena coupled to external electric potential in the complex environments associated with electrocatalysis in aqueous solutions. To investigate the stability and electrochemical activity of perovskite oxides under electrocatalytic conditions, we performed in-situ synchrotron x-ray studies at the Advanced Photon Source, combining structural, spectroscopic, and electrochemical characterization. We examined epitaxial SrRuO3 thin films grown on conducting Nb-doped SrTiO3 (001) substrates. We found that SrRuO3 exhibits high oxygen evolution reaction activity, but the surface is not structurally stable under relevant electrochemical conditions. Surprisingly, the c-axis (out-of-plane) lattice parameter of pseudocubic SrRuO3 was observed to change with increasing potential in an alkaline solution. This may indicate that dissolution/precipitation of Ru4+ is occurring under electrochemical potential, which is consistent with spectroscopic measurements. By using an in-situ x-ray approach, we are determining the stability of active sites in complex perovskite oxides during water dissociation and formation processes. The results of our studies are providing needed insight into the discovery of new stable and active electrocatalysts, created by atomic level design rather than traditional trial-error. Work supported by the U. S. Department of Energy under Contract No. DE-AC02-06CH11357 (BES-DMSE).
Preparation and Characterization of Platinum/Ceria-based Catalyts for Methanol Electro-oxidation: An Electrochemical and EIS Approach
Menendez2 1, Carlos
Cabrera1, Ana-Rita
Different composite materials have been used with platinum in methanol oxidation catalysis. Recently, cerium oxide/platinum mixtures are being tested to prepare catalysts in alkaline and even in acid medium. The presence of ceria shows an increase in the anodic current for methanol and a shift towards more negative potentials of the onset in the anodic peak. Chronoamperometric responses and EIS data can be used to test the prepared catalyts with methanol in KOH and sulfuric acid solutions.
Co-Mg Incorporated Mesoporous Alumina with Ordered Pore Structure for Steam Reforming of Ethanol to Produce Hydrogen Rich Syngas
Gunduz1, Timur
Ethanol is a highly promising bio-resource for on-board hydrogen production. Hydrogen rich syngas can be produced by steam reforming of ethanol at relatively low temperatures. Reverse water gas shift reaction, ethanol dehydration, cracking and coke formation may strongly influence product distributions and H2 yield. C2H5OH + 3 H2O ↔ 6H2 + 2CO2 Cobalt based catalysts have been considered to have good potential in reforming reactions. Catalyst deactivation due to coke formation and transport limitations were reported to be much less in mesoporous catalyst supports than conventional microporous materials. However, silicate structured mesoporous materials (like MCM-41) were not quite stable at the steam reforming conditions. Mesoporous alumina was considered as a much more stable catalyst support [1]. In the present study, quite stable Co and Mg incorporated mesoporous alumina (Co-Mg-MA) catalysts with narrow pore size distributions were synthesized following a one-pot hydrothermal procedure, using Pluronic P123 as the structure directing template. Synthesized materials were then calcined and reduced in H2 at 650oC. XRD, nitrogen adsorption-desorption and TEM analysis indicated formation of ordered mesopore structure of the synthesized materials, which had surface area and average pore diameter values of about 380 m2/g and 3.8 nm, respectively. Co and Mg were successfully incorporated and well distributed in the ordered mesopore structure of alumina, with Co/Al and Mg/Al ratios of 0.17 and 0.22, respectively. Ethanol reforming reactions performed in a fixed bed flow reactor in the 500-600oC range indicated very high activity and stability of the synthesized catalysts, with quite high H2 yield. Complete conversion of ethanol was achieved at a space time of 0.18 s.g.cm-3 at 550oC, with H2 yield values of about 5.1 per mole of ethanol reacted. This was about 85% of the maximum possible yield of six. The main carbon containing compound in the product stream was CO2. Presence of some methane and ethylene (about 6% each) in the product stream indicated occurance of some cracking and dehydration reactions. Presence of some CO was mainly due to thermodynamic limitations of water gas shift reaction. Further increase of temperature eliminated ethylene formation, however caused some decrease in CO2/CO ratio in the product stream. Another set of experiments performed with some CaO addition to the reactor, for in-situ capture of CO2, caused further improvements in hydrogen yield and eliminated both CO2 and CO. Results obtained in this work demonstrated that the Co-Mg incorporated mesoporous alumina catalysts can be successfully synthesized by a one-pot procedure and these catalysts perform very well and show stable activity in steam reforming of ethanol to produce H2 rich syngas. [1] Q. Yuan et al., J.Am.Chem.Soc. 130 (2008) 3465-72. Acknowledgement: TUBITAK Grant 111M338.
Direct Evidence for Adsorption and Reduction of Diols on Reduced TiO2(110)
Acharya1, Xiao
Lin1, Zhenjun
Li1, Bruce
Kay1, Zhenrong
Zhang1 2, Zdenek
TiO2 has become one of the model oxides for a variety of applications including photodecomposition of organic pollutants, solar cells and photoinduced water splitting. Here we study propane-1,3-diol and ethane-1,2-diol molecules on partially reduced TiO2(110) using high-resolution variable temperature scanning tunneling microscopy (STM). STM images recorded before and after the adsorption of molecules on Ti rows at low temperatures (≤ 250 K) show that the diols adsorbed initially on Ti rows. Equilibrium between molecularly-bound diol molecules and dissociated diolate and bridging hydroxyl group is observed at temperatures as low as ~ 120 K indicating only a small barrier for OH bound dissociation. Upon annealing, the Ti bound molecules diffuse along the Ti rows and dissociate irreversibly on bridging oxygen vacancies (VO's) via scission of one of the O-H groups. The split-off hydrogen binds to the neighboring bridging oxygen (Ob) and forms hydroxyl species. At room temperature, the hindered rotation of diolate species across the bridging oxygen row is observed. Further annealing of TiO2(110) to ~450 K leads to the formation of ethoxide/peroxide intermediates. The final desorption products observed in the temperature program desorption are ethylene/propylene and water. These results show a complete sequence of elemental steps involved in the reductions of glycols to alkenes at oxygen vacancies on TiO2(110). This work was supported by the U.S. Department of Energy Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences. The experiments were performed at Environmental Molecular Science Laboratory at Pacific Northwest National Laboratory.
Atomic Layer Deposition of Iridium Islands as a High Surface Area Oxidation Catalyst Layer for Solar Water Splitting
Esfandyarpour1, Yi Wei
Chen2, Paul
C. McIntyre2.
Nanoscale materials are of great interest for applications in energy storage and energy conversion. In research on solar-driven water splitting as a method of fuel synthesis, ultrathin and high surface area oxidation catalyst layers can greatly improve the efficiency of photoelectrolysis. In the present research, nano-size metallic iridium islands were grown by atomic layer deposition (ALD) from iridium [Ir(acac)3] and oxygen on very thin Al2O3 –coated TiO2 layers. The Ir films had low resistivity and low impurity content and good adhesion to the substrate. The average film thickness depended linearly on the number of deposition cycles. The development of the Ir surface morphology on SiO2, TiO2 and Al2O3 surfaces was studied by atomic force microscopy (AFM). Iridium film roughness evolution with increasing film thickness also was studied by AFM. The effective deposition of the polycrystalline and islanded ALD-Ir film on the Al2O3 surface is much greater than that TiO2 and SiO2/Si substrates for a given number of ALD cycles. Our results show that high-quality iridium films can be grown by ALD but the results are strongly dependent on the substrate surface layer. Water splitting anode structures consisting of ALD-grown Ir/Al2O3/TiO2 stacks on SiO2/p+silicon substrates were synthesized and investigated by cyclic voltammetry in the dark. The p+Si substrate is protected from oxidation by the ~2 nm thick ALD-grown titanium dioxide layer, which is highly uniform in thickness. The surface of the TiO2 is coated by ~ 0.5 nm of ALD-Al2O3. Several different thicknesses of ALD-Ir were investigated as a known water oxidation catalyst. Dark cyclic voltammetry measurements confirmed low-to-moderate overpotentials for water oxidation over a wide range of pH and facile electron transport between the anode and aqueous electrolytes.
Nanoporous Platinum Thin Films with Controlled Porosity for Catalytic Applications
Sang Hoon
Kim1, Jeong Beom
Choi1, Do Hyung
Kim1, Hyun Young
Jung2, Yung Joon
Jung2, Heon Phil
Ha1, Ji Young
We report on the fabrication of nanoporous platinum (Pt) thin films with controlled porosity. First we prepared 150 nm of Pt-Si binary alloy thin films by sputter depositing Pt and Si at the same time with pre-determined composition, ranging between Pt0.9Si0.1 and Pt0.1Si0.9. Then we obtained nanoporous Pt thin films by dealloying process to selectively etch Si out of the films. Those porous Pt thin films showed increasing average pore size from 4 to 22 nm and the specific surface area from 10 to 50 m2/g with decreasing ratio of Pt to Si. Pore size and composition of the nanoporous films were measured by Scanning Electron Microscopy, Auger Electron Spectroscopy, Rutherford Back Scattering, and X-ray Diffraction. Specific surface area was measured by hydrogen ion adsorption and desportion in 0.5M H2SO4 solution using Cyclic Voltammetry. Next, we evaluated the catalytic activity of our nanoporous Pt films using methanol electrooxidation. The catalytic efficiency and reliability were well correlated with the specific surface area of the nanoporous Pt films. The relationship between the structure and the catalytic activity of the nanoporous Pt film will be discussed.
Gold Clusters on Nb-doped SrTiO3: Effects of Metal-insulator Transition on Heterogeneous Au Nanocatalysis
Zhou1, Yuan Ping
Feng1, Chun
Zhang1 2.
Doping induced metal-insulator transition (MIT) in transition-metal (TM) oxides has been the topic of continued interest outside the field of catalysis chemistry. In this paper, via ab initio calculations (GGA+U), we show that the Nb-doping induced MIT in SrTiO3 causes a dimensionality crossover of supported Au clusters, and at the same time, greatly enhances the stability and catalytic activity of these clusters. Underlying the predicted high catalytic activity of Au clusters towards the CO oxidation is the MIT induced interaction between O2 antibonding 2Ï€* orbital and Au conduction bands, leading to the population of electrons from Au to the antibonding orbital and the activation of the O2 molecule. We expect these results to provide a new methodology for the control of catalytic performance of TM-oxide supported Au nanoclusters.
The Role of Support Acid Properties in the Hydrodeoxygenation of m-Cresol over Pt Catalysts
Foster1, Phuong
Do1, Raul
Thermal degradation of biomass to pyrolysis oils is a potential route for the production of renewable transportation fuels. However, biomass pyrolysis oil is primarily composed of phenols, furans, and other hydrogen-deficient oxygenated compounds that have undesirable fuel properties. Further catalytic upgrading to remove oxygen functional groups is necessary to make these biomass-derived liquids compatible with petroleum-based fuels. We have investigated bifunctional catalytic materials for the hydrodeoxygenation (HDO) of m-cresol that can remove oxygen via hydrogenation and dehydration. A high hydrogenation rate is necessary to for a high rate of HDO, but complete hydrogenation of unsaturated C=C bonds requires additional hydrogen and is thus less desirable. Selective catalysts for oxygen removal must catalyze dehydration reactions quickly relative to the rate of hydrogenation to reduce unnecessary hydrogen consumption. The reactions of m-cresol and its products from HDO over Pt/γ-Al2O3 have been studied to determine important reaction intermediates and pathways. Pt/γ-Al2O3 was prepared by incipient wetness impregnation to produce uniform Pt nanoparticles 2-4 nm in size. m-Cresol was converted to toluene with high selectivity over this catalyst at 533 K and 0.5 atm H2. It is suggested that toluene is formed by the hydrogenation of m-cresol to an unsaturated alcohol intermediate followed by dehydration. Methylcyclohexane was primarily formed through a separate pathway: saturation of m-cresol to 3-methylcyclohexanol, dehydration to methylcyclohexene, and hydrogenation to methylcyclohexane. The surface chemistry of the catalyst support was modified to improve the reaction rate. Reaction rates were found to increase with increasing surface acidity. Modification of the γ-Al2O3 with fluorine was used to enhance the Brønsted acidity of the catalyst; treatment with potassium decreased the surface acidity. Pt/SiO2 was able to convert m-cresol more quickly than Pt/γ-Al2O3, with higher selectivity to toluene at high conversion. Dehydration of the fully hydrogenated m-cresol (3-methylcyclohexanol) proceeds slowly on the SiO2 surface, but weakly acidic hydroxyl groups can dehydrate unsaturated alcohol intermediates to toluene. The stability and deactivation of Pt-based catalysts with time-on-stream was also studied. This study provides insight into the role of catalyst surface chemistry during hydrodeoxygenation of phenolic compounds. This knowledge can be extended to the future design of catalysts for processing biomass to transportation fuels.
Seedless and Templateless Growth of Porous Platinum on Substrates by Chemical Reduction of a Pure Platinum Precursor by Polyol Vapor
Swee Jen
Cho1, Jianyong
Porous metals with good adhesion to substrates are important in practical application, such as in catalysis and sensors. The conventional methods include the immobilization of pre-prepared metal nanoparticles on substrates by sintering and seed-growth of metal nanostructures on substrates. But those methods are time consuming and not cost-effective. Here, we report a facile and scalable method to deposit catalytic porous platinum on substrates by chemical reduction of a pure Pt precursor without solvent with ethylene glycol (EG) vapor at a relative low temperature. No seed and template are needed in this process. This process includes two steps. The first step is to from a thin layer of H2PtCl6 on a substrate such as fluorine-doped tin oxide (FTO) by coating its solution. The precursor layer dries by post-coating annealing. The second step is to chemically reduce the H2PtCl6 layer into metallic Pt with ethylene glycol vapor at 160 oC. The conversion completes in a few minutes. The Pt nanoparticles generated during the chemical reduction agglomerate together into a continuous porous Pt structure and they deposit onto the substrate immediately after the generation, so that they can form a porous structure with good adhesion to substrates. The Pt nanostructures cannot be removed from the substrates by adhesive tape peeling or ultrasonication. This porous Pt deposited by this method exhibited good electrochemical catalysis in oxidation of methanol.
The Effect of Metal Catalysts in the Electrocatalytic Activity of Nitrogen-Doped Carbon Nanotube Cups
Tang1, Yong
Zhao1, Alexander
Nitrogen-doped carbon nanomaterials have been attracting a lot of attentions in recent years because, with the doping of nitrogen, they have further tailored electric and electrochemical properties. One important application lies in the fuel cells area where they can be great candidates for substituting the expensive Pt-based catalysts for oxygen reduction reaction (ORR). However, the origin of such ORR electrocatalytic activity - whether it's from the nitrogen doping or from a metal-nitrogen-carbon complex - is still somewhat ambiguous. Here we use chemical vapor deposition (CVD) method to synthesize nitrogen-doped carbon nanotube cups (NCNCs), adopting two different metal catalysts: ferrocene and nickelocene. With all other parameters remained the same, both catalysts produced NCNCs with similar stacked-cups structure, and the resulting nitrogen concentration. However, electrochemical testing for ORR revealed that Fe based NCNCs had better performance compared to Ni based NCNCs in terms of both the onset potential and ORR current density. NCNCs synthesized from Ni catalyst only showed catalytic activity comparable to non-doped multi-walled carbon nanotubes (MWNTs). Such different performances proved that both an active metal catalyst (Fe) and nitrogen doping is essential for improved ORR catalytic activity. Furthermore, rotating ring-disk electrode voltammetry and rotating disk electrode voltammetry were conducted to study the ORR mechanism, showing that NCNCs from both Fe and Ni catalysts processed a complex mix of two electron transfer and four electron transfer process. Although not having a perfect four electron transfer ORR process, NCNCs showed better stability compared to Pt/C catalyst. To sum up, compared to NCNCs synthesized from Ni catalyst, NCNCs synthesized from Fe catalyst demonstrated improved ORR catalytic property, owing to the effect of both Fe catalyst metal and nitrogen doping. Such understanding is of great importance to the future design of non-precious-metal catalysts for low-cost fuel cells and other applications.
Template Synthesis of Three-Dimensionally Ordered Macro-/Mesoporous Metals
Xu1, Chengwei
Zhang1, Hui
Yang1, Jianfeng
Chen1, Yushan
Three-dimensionally ordered macro-/mesoporous (3DOM/m) metals (such as Pt, Ni) were fabricated by a dual-templating synthesis approach employing a combination of both colloidal crystal templating (hard-templating) and lyotropic liquid crystal templating (soft-templating) techniques. The precursors, consisting of metal salts and nonionic surfactant Brij 56, were infiltrated into the void spaces of the poly(methyl methacrylate) (PMMA) colloidal crystals. Subsequent chemical reduction and removal of the tempaltes (PMMA and Brij 56) produced 3DOM/m metals. The macropore walls of the prepared 3DOM/m metals exhibit a well-defined mesoporous structure with narrow pore size distribution. Details on the fabrication and characterization of these materials are presented including scanning electron microscopy, transmission electron microscopy, electron diffraction, powder X-ray diffraction, nitrogen adsorption, and electrocatalytic activity studies.
Synthesis and Characterization of Rare Earth Doped Gallium Oxynitrides for Photoelectrochemical Electrodes
Acevedo1, Kiran
Dasari1, Ratnakar
Rare earth doped Gallium oxynitrides are of interest as electrode catalyst materials for the photoinduced electrolysis of water to hydrogen and oxygen; in addition to many other interesting applications. This photoelectrochemical process has great potential as a clean and efficient alternative for the succesful implementation in the hydrogen economy. Rare earth doped Gallium nitrides are synthesized and then annealed in pure oxygen to produce the oxynitride. Samples are characterized before and after the annealing to establish a comparison base. The analyses performed on the samples are: X Ray Diffraction, Scanning Electron Microscopy, Hall Effect measurement and X Ray Photoelectron Spectroscopy.
Preparation of Zinc Oxide/Metal Oxide Hybrid Nano-aggregates and Their Use for Enegy Materials
Jae Young
Kim1, Ji Chan
Park2, Hyunjoon
We have synthesized two distinct zinc oxide based hybrid nanostructures, ZnO/CuO, and ZnO/Co3O4 with controlled morphology. First, thin CuO branches anchored on ZnO spheres were generated by oxidation of ZnO/Cu2O hetero-aggregates. The resulting ZnO/CuO core-branch nanoparticles exhibited the highest catalytic activities among conventional heterogeneous catalysts in [3+2] azide-alkyne cycloaddition reactions by an assistance of ultrasound. The ZnO/CuO hybrid system represented a good example for the bifuctional catalysts with well-defined morphology of each component. Second, ZnO/Co3O4 nanostructures were generated by oxidation of ZnO/Co(OH)2 hetero-aggregates. The ZnO part of resulting ZnO/Co3O4 hybrid nanoparticles was removed by HF or HCl treatment. By HF treatment, the ZnO core was removed and the Co3O4 part was converted to CoF2 structure. Consequently, CoF2 hollow-aggregates were yielded. When ZnO/Co3O4 nanoparticles were treated with HCl, the resulting structure was Co3O4 hollow-aggregates. These structures could be employed for anode materials of lithium ion battery. In addition, we synthesized ZnO/NiO hybrid nanostructures by similar synthetic method as other structures. This synthetic scheme can be applied to other zinc oxide based hybrid nanostructures, for instance, ZnO/V2O5 or TiO2 structures for high capacity electrode material or catalyst of photocatalytic reactions.
Photoelectrochemical Properties of the p-Cu2O Films Electrodeposited on the FTO Transparent Conductive Glass
Hsu1, Ching-An
Huang2, Yi-Ting
Tsai1, Yu-Wei
The p-Cu2O film has potential for splitting water for hydrogen generation. In this research, the Cu2O films were electrodeposited on the FTO transparent conductive glass. The photoelectrochemical properties and stability of the film were investigated. A conventional cell of 3-electrodes was used for the photoelectrochemical testing. The water solution for the experiments consists of 0.5 M Na2SO4. All tested samples are (111) prefer-orientated. A halogen lamp of 150 W was used as the light source for the experiment of photoelectrochemical properties. Structure and composition of the film were detected by using XRD, FE-SEM and EDS. Experimental results show that Cu2O has strong photoelectrochemical response. In the XPS investigation, surface layer of the deposited Cu2O was further oxidized to CuO during drying and handling of samples in the ambient atmosphere. After cyclic voltammetry (CV) test in the range of 0 to -1 V, Cu(OH)2 layer was deposited on the surface, according to the XPS experimental results. The fact reveals that Cu2O was reduced to pure copper in the cathodic reaction and further oxidized to Cu(OH)2 in the anodic reaction. The effect of catalyst by adding NiSO4 was also tested. The responding current increased with increasing the content of additive. The results were also analyzed and reported.
Carbon-supported PtNi Nanoparticles Synthesized by Ultrasound for Oxygen Reduction Reaction in Fuel Cells
Lee1, Ji-Hoon
Jang2, Young-Uk
Kwon1 2.
Polymer electrolyte membrane fuel cells were remarkable candidate as promising power energy with high efficiency and cleanliness for application in transportation and in portable electronic devices. In order to commercialize the fuel cells, the several key challenges have to resolve. One of the key challenges for energy generation is to develop the cathode electrocatalyst for oxygen reduction. Previously, we reported that the ultrasound could reduce volatile metal precursors into nanoparticle electrocatalyst without stabilizer and capping reagent. This process is one of novel approach to obtain core-shell-like structured electrocatalyst in fuel cells. In this study, PtNi nanoparticle was prepared with Pt(acac)2 (acac = acetylacetonate) and Ni(hfac)2xH2O (hfac = hexafluoroacetylacetonate) by ultrasound-assisted polyol reduction method. Structural and morphological properties of electrocatalyst were characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy-Energy Dispersive X-ray (SEM-EDX), Transmission Electron Microscopy (TEM). Based on physical characterization results, the particle size was in the range of 3~4 nm and Pt-rich composition was estimated. The electrocatalytic activity of catalyst for oxygen reduction reaction was measured by linear sweep voltammetry (LSV) with rotating disk electrode (RDE) which showed superior electrocatalytic activity than commercial Pt/C (E-TEK).
First Principles Calculations of Fischer-Tropsch Processes Catalyzed by Nitrogenase Enzymes
Varley1, Lars
Grabow1 2, Jens
Nørskov1 3.
The nitrogenase enzyme system of the bacteria Azotobacter vinelandii, which is used in nature to catalyze ammonia synthesis, recently has been found to catalyze the efficient conversion of carbon monoxide (CO) into hydrocarbons under ambient temperature and pressure.[1] These findings indicate that nitrogenase enzymes could inspire more efficient catalysts for electrochemical CO and CO2 reduction to liquid fuels. The nitrogenase variants, in which vanadium substitutes the molybdenum in the active site of the enzyme, show distinct features in their reaction pathways to hydrocarbon production. To compare and contrast the catalytic properties of these nitrogenase enzymes, we perform first-principles calculations to map out the reaction pathways for both nitrogen fixation and for the reduction of CO to higher-order hydrocarbons. We discuss the trends and differences between the two enzymes and detail the relevant chemical species and rate-limiting steps involved in the reactions. By utilizing this information, we predict the electrochemical conditions necessary for the catalytic reduction of CO into fuels by the nitrogenase active sites, analogous to a Fischer-Tropsch process requiring less extreme conditions. This work was supported by the Global Climate and Energy Project (GCEP) at Stanford University. [1] Y. Hu, C.C. Lee, M.W. Ribbe, Science 333, 753 (2011).
In situ Spectroscopic Characterization of Some LaNi1-xCoxO3 Perovskite Catalysts Active for CH4 Reforming Reactions
Caballero1, Rosa
Pereñiguez1, Victor
Gonzalez-delaCruz2, Fatima
Ternero2, Juan
The recent discoveries of natural gas (as shale gas and others) have renewed the interest in the reforming of methane. As an endothermic reaction, high temperatures are required, being the coke formation and the deactivation of catalysts the main problems for industrial practices. Although nickel or cobalt as active phases are the best candidates, these metals present also some disadvantages: the Ni produces an important coke deposit; while the Co is less resistant to oxidation under reaction conditions [1]. We have focused the study to bimetallic Ni-Co systems supported on La2O3 prepared by reduction of a LaNi1-xCoxO3 perovskite. Experimental The LaNi1-xCoxO3 perovskites were prepared by the spray pyrolysis method [2], using a solution of La(NO3)3 and Ni(NO3)2/Co(NO3)2, which is passed through two on-line furnaces at 250C and 600C respectively, producing a powder that was later calcined in air at 600C. The physicochemical state of the powders was characterized by means of SEM, XRD, TPR, XPS, etc. The measurements of the catalytic performance in the DRM/SRM reactions were accomplished using an atmospheric flow reactor. The feed gas was a methane/carbon dioxide (or water)/helium mixture with a space velocity of 300000 ml/hg. XAS spectra were collected in transmission mode at the BM25 station of the ESRF (Grenoble, France), while the Ambient Pressure Photoemission Spectroscopy (APPES) experiments were performed at beam line U49/2-PGM1 at BESSY II (Berlin, Germany). Results/Discussion The nickel phase in the reduced LaNiO3 and LaNi0.5Co0.5O3 samples, analyzed by operando XAS, evolves from a mixture of Ni3+ and Ni2+ to metallic Ni0 under both, hydrogen reduction treatment and DRM reaction. This behavior contrasts with the partial oxidation of nickel observed under SRM reaction [3]. The results obtained by in situ XAS for Co phase for DRM and SRM in the LaCoO3 and LaNi0.5Co0.5O3 reveal that in both cases the cobalt phase is partially oxidized, remaining visibly less oxidized in the LaNi0.5Co0.5O3 that in the monometallic catalysts. These results could be explained considering the formation of a NiCo bimetallic alloy after hydrogen reduction of the original Ni-Co perovskite. The APPES spectra obtained for the LaNi0.5Co0.5O3 sample submitted to a hydrogen reduction treatment agree with the XAS results. The differences observed in the spectra obtained for the LaNi0.5Co0.5O3 sample during a treatment with H2 at 600C with two different photon energies (200 and 600 eV, related respectively with the surface and bulk composition of the metallic particles), allow us to propose a structure for the bimetallic particles, where the metals are arranged as a "pseudo core-shell†Ni@Co. References. 1. K. Takanabe, K. Nagaoka, K. Nariai, K. Aika; J. Catal., 232 (2005) 268. 2. E. López-Navarrete, M. Ocaña; J. Europ. Cer. Soc., 22 (2002) 353-359. 3. R. PereñÃguez, V.M. González-DelaCruz, J.P. Holgado, A. Caballero; Appl. Catal. B: Env., 93 (2010) 346-353.
H2 Dissociation by Ti Catalysts in Al - Initial Stages of Alanate H-storage Reactions
Al-Mahboob1, Altaf
Karim2, James
Muckerman2, Peter
Surface and near surface alloying of catalytically inactive materials with atoms of a highly active species is a promising route toward engineering future catalyst systems. Here we will discuss the application of this concept to accelerating solid-state reactions involved in hydrogen storage processes, focusing on the role of Ti dopants in accelerating the hydrogenation kinetics of alanate hydrogen storage materials. Combined scanning tunneling microscopy experiments and ab-initio calculations for a well-defined model system, Al(111):Ti, provide direct evidence that atomic Ti catalysts near the Al surface are active in dissociating H2 adsorbed from the gas phase, a key initial step in the hydrogenation of alanates. Samples with controlled placement of Ti near the Al(111) surface – ad-Ti on the surface, alloyed into the surface, or buried in subsurface layers – were used to test experimentally the activity of different Ti species in dissociating H2 and spilling over atomic H. Ad-Ti and Ti alloyed into the Al surface layer bind the dissociation products (H) strongly, thus hindering their spillover to the Al surface and suppressing the formation of Al-hydrides. The binding energy is moderated for Ti catalyst atoms buried in subsurface sites. While still dissociating H2, subsurface Ti readily allows the spillover of H and reaction with Al to form Al-hydrides. Ab-initio calculations rationalize this competition between lowering the transition state energy in H2 dissociation (i.e., enhancing catalytic activity) and lowering the binding energy of the products (i.e., preventing catalyst poisoning). Our findings suggest that conditions favoring the retention of Ti in shallow subsurface Al sites are important to enhancing the hydrogenation kinetics of doped alanate hydrogen storage materials.
U3.39 Transferred to U6.0
Remarkable Enhancement in Performance of Ultrathin Hematite Photoanode for Water Splitting by an Oxide Underlayer
Mathews1 2, Takashi
Hisatomi2, Hen
Dotan3, Morgan
Stefik2, Avner
Rothschild3, Subodh
Mhaisalkar1, Michael
The photoelectrolysis of water using semiconductors is a very attractive yet challenging approach towards storing solar energy within the chemical bonds of H2. This necessitates the utilization of materials that are stable, have good optical absorption and are abundant in nature. Within this context, hematite (Fe2O3) which has a bandgap of ~2eV, is of great interest and have been widely explored. Hematite photoanodes for photoelectrochemical (PEC) water splitting are often fabricated as extremely-thin films to minimize charge recombination due to the short diffusion lengths of photoexcited carriers. However the performance of such ultrathin electrodes is limited because of electronic and structural interactions with the underlying conductive substrate. Here we demonstrate that oxide layers, particularly Nb2O5 and TiO2, deposited beneath the hematite layer by atomic layer deposition improved photocurrents in ultrathin hematite films. Hematite layers of 9 nm and 16 nm layer thickness gave an absorbed photon to current efficiency as high as 40% at 400 nm at +1.43 V vs. RHE, one of the highest ever reported. Photoelectrochemical analyses using H2O2 and [FeII(CN)6]4-/[FeIII(CN)6]3- as a hole scavenger and a reversible redox couple, respectively, revealed that the Nb2O5 underlayer increased the charge separation efficiency and the photovoltage of ultrathin hematite films by suppressing electron back injection. These findings pave the way towards designing more effective host-guest-type nanocomposites based on rough scaffolds and ultrathin hematite films.
Investigation of the Nanostructure and Properties of Ni/Gd-Doped Ceria Catalysts for Fuel Reforming
Cavendish1, Peter
Heterogeneous catalysis and nanoscience are important in solid oxide fuel cell (SOFC) anodes because the device is fuel-flexible with the ability to run directly on fuels other than hydrogen. Internal fuel reformation directly on the anode of the fuel cell could greatly reduce the cost and complexity of the device and highlight the benefits of fuel flexibility. However, many problems arise with current anode catalysts including susceptibility of nickel (Ni) to carbon formation leading to reduced performance, especially when using higher hydrocarbon fuels. Ceria (CeO2) has been shown to interact strongly with Ni, affecting its structural and electronic properties. Active oxygen species in ceria arise during redox processes and lattice oxygen can react with surface carbon to prevent deactivation. The use of doped ceria is beneficial because it exhibits mixed ionic-electronic conductivity (MIEC) in reducing environments and thus improves the activity of the anode and ceria doped with gadolinium (Gd) is one promising choice. Doping Ni metal into ceria can create structures that improve the dispersion of Ni particles on the catalyst surface for reforming and offer improved coke resistance. In the present work, Ni and Gd doped ceria nanocatalysts are created at different concentrations through the use of a spray drying technique. Several compositions are tested for internal reforming through partial oxidation (POX) and steam reforming (SR) reactions with methane and ethane. Performance is measured in a catalytic reactor and catalyst structure-activity relationships are determined through the use of transmission electron microscopy (TEM). Different structures including solid solutions, segregated phases, and metal surface nucleation are investigated. The potential of Ni and Gd doped ceria materials as reforming catalysts for POX and SR is determined based on these findings.
Low-Temperature Chemovoltaic Effect in Catalytic Metal-Semiconductor Nanostructures
Dasari1, Mohammad
Hashemian1, Eduard
Observations of chemically induced hot electron flow over Schottky barriers in catalytic planar nanostructures by these and other authors provides interesting possibilities for electrolyte-free conversion of chemical energy into electricity and ultra-fast chemical sensor applications. This phenomenon became known as the chemovoltaic effect. It is associated with breakage of the adiabatic Born-Oppenheimer approximation in exoergic heterogeneous surface reactions, and therefore implies a direct involvement of energetic electron states in catalytic processes on nanostructured surfaces. An efficient hot electron harvester features a continuous nanofilm of a chemically stable metal, whose chemoexcitation is of interest, deposited on a planar semiconductor substrate to form a wide area Schottky junction cathode. Thickness if the metal nanofilm has to be smaller than hot electron mean free path in this metal, usually <20nm. Thermal stability and electrical continuity of the nanocathode are critical for the potential practical applications. This work makes advantage of the nonthermal nature of diabatic chemicurrents and explores the low-temperature (70-160oC) chemovoltaic effect of hydrogen to water oxidation on catalytic Pt/GaP/Sn, Pt/SiC/Ag and Rh/SiC/Ag Schottky nanostructures. The chemicurrent is also studied at high pressures of the oxyhydrogen mixture diluted with nitrogen, all the way to atmospheric pressures. Separation of the nonadiabatic component to the total generated current is performed with an accurate method based on resistive nanofilm heating. Most recent chemovoltaics advances by other groups will also be overviewed in brief.
Photochemical Reduction of CO2 Using Cu-based Delafossite-type Materials
Underwood1, Jonathan
Lekse2, Christopher
Matranga2, James
Primarily studied for use as transparent conducting oxides (TCOs), the delafossite family of materials consists of metal oxides that are promising for the photocatalytic reduction of CO2 due to their stability in aqueous solution, low toxicity and high carrier mobility. These materials are of the form ABO2 where A is a +1 metal ion, and B is a +3 metal ion, for example CuGaO2 and AgFeO2. The problem with using these materials for CO2 reduction by photocatalysis using visible light is that they have large optical band-gaps in the UV absorption range (3.7 eV in CuGaO2). However, these materials have much smaller fundamental band-gaps (1.5 eV in CuGaO2) that are disallowed due to inversion symmetry effects. In previous research we have shown both computationally and experimentally that doping or alloying with a secondary metal ion in the B site adds enough stress on the crystalline system to break inversion symmetry. This leads to a material with a smaller band-gap than its parent structure. For example, we measured pure CuGaO2 to have an absorption feature at 3.7 eV which shifted to about 1.5 eV in our synthesized CuGa0.95Fe0.05O2 structure. Motivated by our previous results, we explore a variety of doped delafossites of the form CuB1(1-x)B2(x)O2 using computational high-throughput methods combined with experimental synthesis and characterization. Additionally, previous research has shown that the B-site ion used determines if the low-temperature structure of the delafossite is rhombohedral (R-3m) or hexagonal (P63/mmc). To simplify the synthesis, when determining doped materials we will choose two B-site ions that prefer the same structure as differing B-site elements tend to phase-separate. We hope to expand our knowledge of the nature of doped delafossites by creating statistically significant models of these structures. By measuring absorption spectra, conduction band edge potentials, band gaps, density of states, and relative energies, we aim to find an optimum delafossite material for CO2 reduction by photocatalysis using visible light. In the research presented here, we show both our experimental and computational results of our search thus far including high-throughput search techniques, electronic structure data, and CO2 reduction results.
High Surface Area Macroporous Transparent Conducting Oxide Electrodes
Forman1, Zhebo
Chen1, Thomas
The ability to control the interfacial area and light trapping characteristics of structured electrodes provides great benefits in modern optoelectronic devices. Textured, high surface area electrodes (HSEs) combine large interfacial areas and increased loading of active material with a light trapping morphology – key parameters for boosting device performance. HSEs with tunable pore sizes on the nanometer to micron scale facilitate diffusion through the structure and serve as a broadly applicable platform for functionalization in many fields including electrochemical sensors, catalysis, (super)capacitors, batteries, photovoltaics and photoelectrochemistry, particularly if the HSE consists of a transparent conducting oxide (TCO). In this talk we describe the development of optically transparent, electrochemically stable, physically robust HSEs with tunable roughness factors (RFs) from approximately 20-1000. We present a bottom-up approach to future device design and a low-cost, scalable synthetic route for production of transparent HSEs of several TCO compositions.
Methane Combustion Using CeO2-CuO Fibers Catalysts
Berutti1, Raquel
Reolon1, Annelise
Alves1, Carlos
The interest in the study and application of nanoparticles has been growing in recent years, mainly due to its unique physical and chemical properties that make them significantly different from their usual microstructure. Nanostructured particles have better sinterability and a high catalytic activity due to the high surface area and surface properties. On this context, the electrospinning technique has been recognized as a versatile and effective method for producing fibers with very small diameters and high surface-volume ratio. The morphology and properties of fibers depend on the characteristics of the polymer and process parameters used, for example, average molecular weight of polymer, solvent, viscosity and conductivity of the solutions, applied electric field strength and distance from the collector. The use of three-way catalysts is an accepted method to control greenhouse gas emissions. These catalysts are generally formed by the support, stabilizers, promoters metal and transition metals, the most used metals of the platinum group. The use of cerium as a promoter is usually related to its ability to store oxygen and structural aspects such as the property of increasing the dispersion of metals and slow change of phase of the stabilizing support. On the other hand, the metal copper was explored as a possible replacement for palladium and platinum in the reduction of NO by CO. Despite the importance of oxidation of CO on Cu, the reaction is not elucidated because changes in oxidation state when the reaction conditions are changed. In this work, fibers of cerium oxide doped with copper were obtained from an acetate solution of cerium nitrate, copper and polyvinyl (PVB). This solution went through the process of electrospinning to give rise to nanostructured fibers. After heat treatment, cerium oxide fibers were obtained. These fibers were characterized structurally by scanning electron microscopy (SEM), had their specific surface area determined by BET, were subjected to thermogravimetric test to determine their thermal decomposition and were analyzed by X-ray diffraction. The catalytic activity was assessed by the amount of O2 consumed and CO and CO2 formed for the combustion of methane and air. SEM images show fibers oriented randomly in the substrate. TEM images show that the diameter of the fibers is approximately 100 nm and the size of its crystallites are around 17nm. The catalytic activity of the fibers was significant. In the absence of catalysts until the temperature of 600°C there was no combustion reaction of methane and air mixture for the gas flow rates used. In the presence of the catalyst, the combustion reaction started around 550°C, with the consumption of methane and oxygen and the formation of CO and/or CO2. In the presence of the catalyst there was no emission of NO and NOx gases.
Electronic Surface Structure of GaInP2 Thin Films Used for Photoelectrochemical Water Splitting
George1, Michael
Weir1, Stefan
Krause1, Ich
Tran1, Kimberly
Horsley1, Monika
Blum1, Lothar
Weinhardt1, Clemens
Heske1, Todd
Deutsch2, John
Turner2, Tadashi
Ogitsu3, Brandon
Wood3, Regan
Wilks4, Marcus
Baer1 4 5, Wanli
To derive the electronic surface structure of GaInP2 thin films used for photoelectrochemical water splitting, we have employed a suite of experimental techniques, in particular X-ray and UV photoelectron spectroscopy (XPS and UPS), inverse photoemission spectroscopy (IPES), and synchrotron-based X-ray emission (XES) and absorption (XAS) spectroscopy. Experiments were performed both in the lab at UNLV, as well as at Beamline 8.0 of the Advanced Light Source (ALS), Lawrence Berkeley National Laboratory. Using a step-wise surface cleaning procedure with very-low-energy Ar+ ions ("ion-stimulated desorptionâ€), we have removed surface adsorbates from an air-exposed GaInP2 film and determined the electronic surface structure after each step. We used XPS to carefully monitor the chemical changes at the GaInP2 surface as a function of Ar+ ion treatment time. A decrease of the intensities of the carbon and oxygen adsorbate lines can be observed and we find that most of the adsorbates can be removed before additional treatment leads to undesired changes in the surface stoichiometry. After this optimal treatment time, we can derive the electronic surface band gap, the position of the valence band maximum and conduction band minimum with respect to the Fermi level, and the work function directly from the experiments. Based on this data, a comprehensive picture of the electronic structure of the GaInP2 surface can be painted. The electronic structure will be discussed in view of the performance of GaInP2 thin films for solar water splitting.
Metal-semiconductor Hetero-nanostructures for Plasmon-enhanced Visible-light Photocatalysis
Xue1, Shaowen
Cao1, Jun
We present a new strategy to synthesize hetero-structures consisting of noble metal (gold or silver) nanocrystals and various visible-light-active metal oxide semiconductor nanostructures such as N-doped TiO2, Fe2O3, BiVO4 etc. In the synthetic approach, metal nanoparticles are grown in-situ on metal oxide surfaces through a photoinduced seed-mediation method. These new hetero-nanostructure exhibit high photocatalytic efficiencies for both dye degradation and water splitting under visible-light irradiation. Further mechanistic studies reveal that the surface plasmon resonance (SPR) of the conjugated metal nanocrystals contribute significantly to the photocatalytic activities through SPR-induced light harvesting and charge transfer as well as plasmon-enhanced charge separation.
Passivation of Photoelectrochemical Water Splitting Electrodes Based on III-V Compound Semiconductors via Surface Nitridation
Deutsch1, Adam
Welch1 6, Avery
Lindeman3, Marcus
Baer2 4 5, Lothar
Weinhardt2, Michael
Weir2, Kyle
George2, Clemens
Heske2, John
Photoelectrochemical (PEC) production of hydrogen fuel, based on water electrolysis through light absorption in one integrated system, has the potential for high solar-to-hydrogen yields. A tandem PEC semiconductor configuration consisting of a photovoltaic p/n-GaAs bottom cell and a p-GaInP2 top cell has demonstrated (unbiased) solar-to-hydrogen conversion efficiency of 12.4% but is prone to corrosion. III-V nitride semiconductors, however, have exhibited excellent durability in a PEC environment in the past. Thus surface nitridation of p-GaInP2 was investigated as a means of reducing corrosion in the aqueous PEC electrolyte. Thin films of p-GaInP2 grown by metal organic chemical vapor deposition (MOCVD) on a p-GaAs substrate were nitrided by bombardment with low-energy N2+ ions at room temperature. The nitrided sample surfaces were investigated by optical and electron microscopy and x-ray photoelectron spectroscopy (XPS). Respective PEC devices were characterized by incident photon-to-current efficiency (IPCE), illuminated open-circuit potential (OCP), and chopped-light voltammetry (J-V). The results were compared to those of (pristine) control samples of p-GaInP2 to determine the effect of the N2+ surface treatment. Control and nitrided samples were tested for durability by applying a constant current of -8mA/cm2 in an aqueous solution of 3M H2SO4 for long durations under AM1.5G illumination. Three-electrode J-V was collected following durability testing to assess photoelectrode performance. After the durability tests, the respective electrolytes were analyzed by inductively coupled plasma mass-spectrometry (ICP-MS) to determine concentrations of indium and gallium in solution to quantify semiconductor corrosion. Optical profilometry was also used to measure the volume of material lost from the surface due to corrosion during operation. Initial XPS results indicated that the surfaces of nitrided p-GaInP2 samples are composed in part of GaN and interstitial N2. Some of the nitrided samples entirely resisted corrosion over 26 hours of testing, where similarly tested untreated samples experienced material loss from their surfaces of around 1 μm in depth. Surface nitridation by ion bombardment could hence be an effective passivation treatment to ensure durability for a highly efficient PEC material, ultimately yielding a viable device capable of converting sunlight and water to a benign solar fuel.
Quantum Confined MoS2 Nanoparticles for Solar Energy Conversion and Storage
Chen1, Thomas
In order to address the tandem challenge of solar energy conversion and energy storage, new approaches are necessary to develop materials that can efficiently drive chemical transformations. By leveraging the effects of quantum confinement in concert with enhanced catalysis at the nanoscale, we can engineer material properties that can lead to superior device performance. In this study, we demonstrate that molybdenum disulfide (MoS2) is one such material which can be enhanced for photoelectrochemical water splitting when nanostructured. In bulk form, MoS2 is a poor catalyst and possesses a bandgap too small (~1.2 eV) to drive efficient water splitting. However, we show that nanostructured architectures of MoS2 can drive excellent kinetics for the hydrogen evolution reaction while remaining stable for thousands of stability cycles. Furthermore, we present multiple synthetic routes to achieving low temperature synthesis of MoS2 nanoparticles that display bandgap enlargement effects. Photoelectrochemical flatband measurements reveal that these nanoparticles have conduction and valence band edges that properly straddle the redox potentials for water splitting. Lastly, we present the development of high surface area, transparent conducting supports that will be necessary for complete device integration of nanoscaled materials that bridge the gap between efficient solar absorption and charge transport.
In-situ Single Particle Plasmon Photocatalysis
Baldi1 2, Jennifer
Metallic nanoparticles exhibit strong light scattering and absorption, due to the collective oscillation of their surface electrons. These localized surface plasmon resonances (LSPR) are strongly dependent on the nanoparticles shape, size, dielectric properties and electron density. For most commonly used metals (Au and Ag) LSPR occur in the visible part of the spectrum and are therefore easily detected by standard optical spectroscopy. Nanoparticles of Au and Ag (typically < 100 nm) are being extensively studied as co-catalysts in many photoactive materials for light harvesting and solar fuel generation processes. Their role is both to separate the electrons from the holes, minimizing charge recombination, and to enhance the photoresponse of the semiconducting material at energies corresponding to the LSPR. Given their extremely small sizes, however, most studies of the catalytic activity of these nanoparticles rely on ensemble properties and are often unable to unveil the details of their structure-function interplay. Here we study the catalytic activity of individual silver nanoparticles, used in conjunction to a TiO2 photocatalyst, by monitoring the LSPR shifts induced by the addition or removal of electrons during a redox reaction. Well-dispersed Ag-core TiO2-shell nanostructures are prepared by a two-step colloidal synthesis. The evolution of the scattering spectra of individual particles during irradiation with UV light is monitored in-situ and real-time in an inverted dark-field optical microscope. The light excitation generates an electron-hole pair in the TiO2 shell with subsequent electron transfer to the Ag core, while the holes are scavenged by the solvent. The optical spectra are then correlated with electron microscopy images of the individual nanoparticles. During charging of the nanoparticles, we find a strong blue shift of the plasmon resonance, with magnitudes that can be correlated with the particle geometry. Such results agree with ensemble absorption measurements, which exhibit a blue shift from 430 to 415 nm upon UV irradiation. Our results not only exhibit the potential for plasmonic nanoparticles to enhance charge separation in existing photocatalysts, but also may enable new insight into charge-separation mechanisms in multi-electron redox reactions.
Materials for Energy Conversion at Solid-Liquid Interfaces
Wang1, Dusan
Strmcnik1, Dennis
van der Vliet1, Nenad
Markovic1, Vojislav
The ultimate goal in electrocatalysis is focused on the ability to control critical properties of electrified solid-liquid interfaces. The complexity of these interfaces offers a vast source of systems for fundamental studies. Particularly, materials that are employed as electrocatalysts are one of the key components in those electrochemical systems. Consequently, the majority of research efforts are placed on the design and synthesis of catalysts aiming to improve their efficiency. It has been found that properties such as surface structure, surface and subsurface composition influence the electronic properties that in turn have distinguished roles in determining functional properties of electrocatalysts [1,2]. In this report the material-by-design-approach, will be used to demonstrate how the knowledge obtained from well-defined surfaces can be employed to create tailor-made nanomaterials with advanced catalytic properties. It will be demonstrated that multimetallic systems can provide exceptional benefits by bringing together, in a controlled manner, highly diverse constituents that can alter and tune both catalytic activity and durability of the catalyst [3]. In addition to solid materials, it is of paramount importance to emphasize the role of the liquid phase, which also influences the overall properties of electrified interface. Molecular species from the employed electrolyte and the nature of their interaction with the catalyst surface can be also used in tuning the catalytic performance. More recently, it was demonstrated that molecular patterning is a powerful tool that can be utilized to induce selectivity of the electrocatalyst. In each case, the same level of catalytic enhancement has been established for tailored well-defined surfaces and corresponding nanoscale materials. All of this demonstrates the great potential of the materials-by-the-design-approach in addressing challenging issues in the areas of energy conversion, fuel production and energy storage. References: [1] V.R. Stamenkovic, B. Fowler, B.S. Mun, G. Wang, P.N. Ross, C.A. Lucas, N.M. Markovic, Science 315 (2007) 493-497. [2] V.R. Stamenkovic, B.S. Mun, M. Arenz, K.J.J. Mayrhofer, C. A. Lucas, G. Wang, P.N. Ross, N.M. Markovic, Nature Materials 6 (2007) 241. [3] C.Wang, D.van derVliet, K.L. More, N.J. Zaluzec, S. Peng, S. Sun, H. Daimon, G. Wang, J. Greeley, J.Pearson, A.P. Paulikas, G. Karapetrov, D. Strmcnik, N.M.Markovic, and V.R.Stamenkovic, Nano Letters, 11(2011)919-928.
Synthesis of Ultrathin FePtPd Nanowires and Their Catalysis for Methanol Oxidation Reaction
Guo1, Sen
Zhang1, Xiaolian
Sun1, Shouheng
We report a facile synthesis of ultrathin (2.5 nm) trimetallic FePtPd alloy nanowires (NWs) with tunable compositions and controlled length (less than 100 nm). The NWs were made by thermal decomposition of Fe(CO)5 and sequential reduction of Pt(acac)2 (acac = acetylacetonate) and Pd(acac)2 at temperatures from 160 oC to 240 oC. These FePtPd NWs showed composition-dependent catalytic activity and stability for methanol oxidation reaction. Among FePtPd, FePt NWs as well as Pd, Pt, and PtPd nanoparticles (NPs) studied in the 0.2 M methanol and 0.1 M HClO4 solution, the Fe28Pt38Pd34 NWs showed the highest activity with their mass current density reaching 488.7 mA/mg Pt and peak potential for methanol oxidation decreased to 0.614 V from 0.665 V (Pt NP catalyst). The NW catalysts were also more stable than the NP ones with the Fe28Pt38Pd34 NWs retaining the highest mass current density (98.1 mA/mg Pt) after 2 h i-t test at 0.4 V. These trimetallic NWs are a promising new class of catalyst for methanol oxidation reaction and for direct methanol fuel cell applications.
Electrochemical Reactions on Oriented Pt(100-x)Zr(x) Films
Hays1, Poyan
Bahrami1, Michael
Errico1, James
Kulleck1, Daniel
Konopka1, Adam
Kisor1, Harold
Pt-based electrode materials used in hydrogen-air fuel cells comprise ~ 34% of the cell cost, a key barrier to their use. To reduce cost and increase performance, we have examined reduced Pt-content Pt(100-x)TM(x)VM(y) (TM = Co, Ni, and VM = Ti, V, Zr) films, as candidates for cell electrodes. We've shown that (111) Pt(66)Co(24)Zr(10) surfaces exhibit significantly enhanced ORR currents, over 15X (1500%) larger than (111) Pt films measured in the same cell [1, 2, 3]. We will present results of microstructural (SEM), crystallographic (XRD), film composition and surface chemistry (SEM/EDS, XPS), and electrochemical (multi-electrode (ME) half-cell) measurements, for thin films in the series Pt(100-x)Zr(x), 0 < x < 35 (At. %). XRD and SEM/EDS results show the films are single-phase, with (111) crystallographic orientation, and 40-50 nm average grain sizes. For solid solutions, with x < 10%, Pt-Zr exhibits a Cu3Au fcc symmetry. XRD results show that the unit cell volume tracks well with features in the Pt-Zr phase diagram. For x ≤ 10 %, the change in volume with x obeys Vegard's law. The Pt-Zr (111) films are stable in 0.1 M perchloric acid, and XPS shows that the surface (after EChem) has Pt and Zr atoms present in the upper-most layers. ME half-cell measurements show that films are active for the hydrogen-evolution-reaction (HER), hydrogen-oxidation-reaction (HOR), and the oxygen-reduction-reaction (ORR). The composition dependence of the Pt-active area is not strong. However, the active areas are ~ 17% greater than Pt (111) surfaces. Polarization scans to negative potentials show no evidence for hydride formation. Similar to the Pt-Co-Zr series but with a smaller increase, at x ~ 10 %, the ORR kinetic currents at 0.9 V (vs. NHE) for Pt(90)Zr(10) are 65% larger than those observed on the Pt (111) surfaces. We propose that this behavior is related to the intra-alloy electron transfer between Pt and Zr, specifically, that hole states present in the top of the nearly-filled Pt 5d9 band are partially filled by transfer of spectral weight from Zr 4d2 states. Evidence for the role of e- - e- correlations will be discussed. References: [1] C. C. Hays, J. G. Kulleck, B. E. Haines, and S.R. Narayan, ECS Transactions 25, 619 (2009). [2] C. C. Hays, J. G. Kulleck, B. E. Haines, A. Kisor, and S.R. Narayan, Abstract B1-0875O, ECS 218, Las Vegas, NV 2010. [3] C. C. Hays, M. A. Johnson, P. Bahrami, J. G. Kulleck, D. A. Konopka, A. Kisor, and H. F. Greer, Abstract 1006, ECS 220, Boston, MA 2011. Acknowledgements: The research in this presentation was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. We acknowledge support from the Department of Energy (DE-PS36-08GO98101). We thank our collaborators at Argonne National Laboratory, Dr. Nenad Markovic and Dr. Voja Stamenkovic, and Dr. Mark Debe and Dr. Radoslav Atanasoski of the 3M Corporation.
Enhanced Oxygen Reduction Activity of Pd by Strong Metal and Metal Oxide (Mn3O4 and CeO2) Interactions
Jin1, Dae Jong
You1, Kang Hee
Lee1, Chanho
Fuel cell is desirable candidate for future power generation system when the fuel cell overcomes crucial problems such as Cost and Durability problem. To decrease the cost of the fuel cell, numerous researchers developed many kind of catalysts using relatively inexpensive metal. Pd is considered as one of candidate material to replace of Pt despite its lower activity toward oxygen reduction reaction (ORR) than platinum due to its most comparable properties with Pt. Moreover, strong metal-support (metal oxide) interaction (SMSI) has been regarded as one of possible approaches for enhancing activity of catalyst. In this sense, improvement of catalytic activity via SMSI has been reported not only on chemical reactions but also on electro-catalysis reactions. In this study, we investigated an effect of strong metal-metal oxide interaction between nano-sized metal oxide and Pd on the ORR activity. Manganese oxide and cerium oxide was selected to make strong interaction because their redox properties. The manganese oxide and cerium oxide nanoparticles were dispersed on Ketjen Black via impregnation and calcination. Subsequently, Pd was dispersed on the metal-oxide/carbon composite via impregnation and reduced at 500°C under H2 atmosphere(Pd/MeOx-C). Both Pd/MeOx-C type catalysts showed higher onset potential and ORR activity than the Pd/C catalyst, which attributed to the strong interaction between Pd and metal oxide.
Improved Electrocatalytic Performance of Carbon Supported Pt-(Cu, Fe) Nanoparticles as Positive Electrode for All-vanadium Redox Flow Battery
Flox1, Javier
Rubio-Garcia1, Marcel
Skoumal1, Teresa
Andreu1, Juan Ramón
Morante1 2.
The development of energy storage technologies having a great potential to be scaled-up is highly demanded by the energy sector. Many researchers have proposed the all-vanadium redox flow battery (VRB) as a promising grid energy storage system for renewable energy due to its low cost, modular configuration, flexible operation, fast power response, high energy efficiency, long service lifetime and environmental compatibility. However, this technology shows some drawbacks, such as a relatively poor energy-to-volume ratio, with a power-to-weight ratio of ca. 25-35 Wh kg-1, which is quite low if compared to other rechargable battery types. Within this framework and taking into account that the electrochemical kinetic limitations of VRB is found in the positive side ([VO2]+/[VO]2+ redox couple), we have focused on the development of novel electrodes with high catalytic activity towards the [VO2]+/[VO]2+ redox reaction. These novel electrodes have been prepared on carbon-supported Pt-based electrodes in combination with iron or cupper metals (e.g. Fe-Pt /C and Cu-Pt/ C electrodes) in order to assess the role of both. These electrodes have demonstrated higher electrocatalytic activities than the simplex Pt catalyst/C towards the positive reaction of VRB. Different stoichiometric series of (Fe-Pt) and (Cu-Pt) nanoparticles were prepared in order to comprehensive elucidation of the optimal stoichiometry to achieve the best electrocatalytic performance of the bimetallic nanoparticles on carbon-supported. The physical and electrochemical properties of the as-prepared electrocatalysts are investigated by High- resolution transmission electron microscopy and energy diffraction x-ray, cyclic voltammetry, spectroscopy impedance electrochemical, chronoamperometry and chronopotentiometry. The carbon-supported electrodes containing (Cu-Pt or Fe-Pt) nanoparticles have shown a very positive synergistic contribution of both metals, which is attributed to the Pt-OH, Fe-OH and Cu-OH bonds that favor the adsorption of a large amount of vanadium ions. As a result, the electron and oxygen tranfer processes involved in the [VO2]+/[VO]2+ redox reaction become accelerated at the decorated electrodes open wide possibility for the electrode improvement.
Meso-structured Thin Film Pt-based Catalysts for the Oxygen Reduction Reaction
Gorlin1, Jakob
Kibsgaard1, Thomas
One of the major challenges toward commercialization of proton exchange membrane fuel cells (PEMFCs) is the development of active and stable cathode catalysts [1]. The current industry standard for PEMFC cathodes consists of Pt nanoparticles supported onto high surface area carbon (Pt/C) [2]. Pt/C has high specific and mass activities but limited stability under operating conditions [1]. One morphology that is known to exhibit excellent stability and high per-site specific activity is flat Pt, as found for bulk Pt samples [3]. However, Pt utilization in bulk Pt catalyst is low, leading to low mass activities and greater costs [2]. In an effort to combine the stability and per-site activity of bulk Pt and mass activity of Pt/C nanoparticles, we directed Pt morphology at the nano-scale and synthesized an extended 3D mesoporous Pt network with double gyroid (DG) structure. The high surface area DG Pt shows better specific activity for the oxygen reduction reaction (ORR) than commercial Pt/C, as measured by the intrinsic kinetic current density per mass and per surface area. An accelerated stability test performed by potential cycling up to 10,000 cycles demonstrates that DG Pt thin film catalyst maintains over 80% of the initial electrochemical surface area (ECSA), while Pt/C nanoparticulate catalyst loses over half of its ECSA. Consequently, the meso-structured Pt film successfully combines excellent durability and high turnover frequency (TOF) of bulk Pt with the high surface area and mass activity of Pt/C nanoparticles. To further improve activity of meso-structured catalyst, we synthesized DG alloy catalysts and evaluated their ORR activity and stability. References: [1] W. Vielstich, H. A. Gasteiger, A. Lamm, & H. Yokokawa. Handbook of Fuel Cells: Fundamentals, Technology, and Applications. (John Wiley & Sons Ltd, 2009). [2] Gasteiger, H. A., Kocha, S. S., Sompalli, B. & Wagner, F. T. Appl. Catal., B 56, 9-35, (2005). [3] Komanicky, V. et al. J. Electrochem. Soc. 153, B446-B451, (2006).
Role of Surface Oxide Layer during CO2 Reduction at Copper Electrodes
Tsai1, Joel
Bugayong1, John
Flake1, Greg
We have compared the rates of CO formation on Cu and Cu oxide surfaces during the electrochemical reduction of CO2. Rates are measured in aqueous KHCO3 electrolyte saturated with continuously flowing CO2 using a two-compartment cell with Nafion membrane separator and three-electrode configuration. Copper foil electrodes are used "as received†after cleaning using either HCl etching or H3PO4 electropolishing. Copper oxide electrodes are prepared by electrochemical deposition onto Cu foils from CuSO4 and lactic acid bath. The CO2 reduction behavior of the metallic Cu electrodes is similar to previous literature reports. Hydrogen formation is the main reaction at potentials less cathodic than –1.4 V(SCE). At -1.4 V(SCE) the formation of CO becomes significant, and CH4 is eventually observed at cathodic potentials beyond –1.6 V(SCE). The behavior of the Cu oxide electrodes shows a complex transient response. At constant potential (–1.4 V(SCE)), there is a large, transient cathodic current, accompanied by a color change of the electrode. The integrated current during this transient correlates with the thickness of the electrochemically deposited film, and indicates that most of the film is being reduced at this potential. After the initial oxide reduction transient, the current density stabilizes and the formation rates of H2 and CO show a more slowly varying transient behavior. The H2 formation rate increases with time on stream, and begins to approach the rate observed on freshly cleaned Cu foil. The magnitude of the difference below the rate on Cu metal correlates with the thickness of the initial oxide layer. In contrast, the CO formation rate is at least one order of magnitude higher on the (reduced) Cu oxide samples than on Cu foil at the same potential. The CO formation rate increases with the thickness of the initial oxide layer, but decays significantly with time on stream. These results are consistent with a model in which the initial Cu oxide layer is rapidly reduced, creating a mixture of highly dispersed Cu clusters (which continue to evolve with time) and residual Cu oxide clusters (which continue to decay). Some fraction of the underlying Cu foil substrate may also become exposed during the film reduction process. Hydrogen formation occurs primarily at Cu metal surfaces, and is inhibited by the presence of larger amounts of Cu oxide (cf. the thickness dependence noted above) but becomes more facile as the Cu oxide layer becomes more fully reduced (cf. time-on-stream results). In contrast, the CO formation rate is enhanced on the Cu oxide clusters, or possibly on highly dispersed Cu clusters that are formed by the Cu oxide reduction process.
Early Transition Metal Oxide Electrocatalysts for the Oxygen Reduction Reaction (ORR)
Khalifah1 2, Bingfei
Cao1 2, Shujie
Hu1, Radoslav
The noble metal electrocatalysts commonly used in hydrogen fuel cells have severe limitations in their cost and availability, and also require undesirably high overpotentials to operate at useful current densities. We have chosen to search for potential replacements among complex oxides of early transition metals (Ti, Nb, Ta) which are likely to strongly resist dissolution in acidic conditions. Although these oxides are typically wide band gap semiconductors, starting compositions and synthetic conditions have been chosen to produce reduced transition metal oxides which are black in color indicating high carrier concentrations and the potential for good electronic conductivity. The catalytic performance of these compounds (in both acidic and basic media) has been measured, and the dependence of activity on structure, physical properties, and electron counts will be discussed for this general family of materials.
Integrated Aligned Carbon Nanotube Based Electrodes with High Efficiency and Stability for Proton Exchange Membrane Fuel Cells
Chen1, Daniel
Carbon nanotubes (CNT) have shown promising characteristics as alternative catalyst support material for proton exchange membrane (PEM) fuel cell. Previous studies on CNT based electrode have suggested that CNT support with high surface area and chemical stability could enhance the electrocatalyst's activity and stability by their active interaction. However, CNTs are reportedly difficult to coat with metal particles on their inert surface without surface oxidation treatment. In addition, an additional carbon black based gas diffusion layer (GDL) is still necessary for CNT supported catalyst thus limiting its overall effectiveness. In this work, a complex Pt/CNT based electrode has been fabricated by in-situ growth of a dense CNT layer on plain carbon paper using thermal chemical vapour deposition (CVD) technique followed by direct sputter deposition of Pt nanoparticles onto the CNT layer. Furthermore, by modifying the CNT catalyst, aligned CNTs perpendicular to the substrate has been obtained. The in-situ grown aligned CNT layer on carbon paper showed tunable diameter and density under different growth processes. Transmission electron microscopy (TEM) micrographs showed a very good dispersion of Pt nanodots on the surface of the CNTs. Raman spectroscopy demonstrated an ID/IG of 0.42, revealing a high crystalline of the aligned CNTs. The maximum power density based on Pt loading is 14.7 W/mg Pt for the Pt/CNT-based electrode, compared to 12.4 W/mg Pt for the Pt/VXC72R-based electrode under a very low 0.04 mg/cm^2 Pt loading. In-situ characterization techniques such as cyclic voltammetry, electrochemical impedance spectroscopy and accelerated degradation tests (ADT) have been carried out to fully evaluate the Pt/aligned CNT based electrode. Results confirmed that the Pt/CNT catalyst prepared by the combined fabrication method can yield higher Pt utilization as well as superior electrochemical stability, demonstrating that an optimum aligned CNT layer is able to provide extremely high surface area and porosity which can serve both as the GDL and catalyst layer simultaneously. Moreover, the reduced charge transfer and mass transport resistance of the electrode further suggest that this integrated CNT GDL and catalyst layer has an intrinsic structural merit, where the high mass transport was attributed to the unique aligned structure of CNTs resulting in reduced water flooding, higher conductivity and ease of airflow pathway. ADT showed that the aligned CNTs excellent corrosion resistance properties, most probably due to the high crystallinity of the ACNTs.
Highly Corrosion Resistant Platinum - Niobium Oxide - Carbon Nanotube Electrodes for the Oxygen Reduction in PEM Fuel Cells
Mitlin1, Li
Zhang1, Liya
Wang3, Chris M. B.
Holt1, Beniamin
Zahiri1, Kourosh
Malekc2, Titichai
Navessin2, Michael H.
Nanocomposite materials consisting of platinum deposited on carbon nanotubes are emerging electrocatalysts for the oxygen reduction reaction in PEM fuel cells. However, these materials albeit showing promising electrocatalytic activities suffer from unacceptable rates of corrosion during service. This study demonstrates an effective strategy for creating highly corrosion-resistant electrocatalysts utilizing metal oxide coated carbon nanotubes as a support for Pt. The electrode geometry consisted of a three-dimensional array of multi-walled carbon nanotubes grown directly on Inconel and conformally covered by a bilayer of Pt/niobium oxide. The activities of these hybrid carbon-metal oxide materials are on par with commercially available carbon-supported Pt catalysts. We show that a sub-nanometer interlayer of NbO2 provides effective protection from electrode corrosion. After 10,000 cyclic voltammetry cycles in the 0.5 V to 1.4 V range, the loss of electrochemical surface area, reduction of the half-wave potential, and the loss of specific activity of the NbO2 supported Pt were 10.8%, 8 mV and 10.3%, respectively. Under the same conditions, the catalytic layers with Pt directly deposited onto carbon nanotubes had a loss of electrochemical area, reduction of half-wave potential and loss of specific activity of 47.3 %, 65 mV and 65.8%, respectively. The improved corrosion resistance is supported by microstructural observations of both electrodes in their post-cycled state. First principles calculations at the density functional theory level were performed to gain further insight into changes in wetting properties, stability and electronic structure introduced by the insertion of the thin NbO2 film.
Novel Pd-Pt Bimetallic Nanocrystals for Catalytic Applications
We are developing novel Pd-Pt bimetallic catalysts that have much less platinum content but drastically enhanced activity and durability for ORR. By focusing on a number of well-defined, highly tunable, and precisely controllable systems, we have addressed the following issues: i) how does the facet or arrangement of atoms on the surface affect the ORR activity; ii) how do the thickness and morphology of the Pt overlayer affect the coupling between the Pd and the Ptm; and iii) how does the morphology of a bimetallic electrocatalyst contribute to the enhancement in stability by altering the sintering or ripening kinetics. In this talk, I will focus on our recent progress in the design and synthsis of these novel bimettalic catalysts.
Green Synthesis for Green Energy: Core-shell Metal Nanocatalysts Synthesized Using Ethanol for Applications in Fuel Cells and Water Electrolyzers
Wang1, Yu-Chi
Hsieh1, Yu
Zhang1, Rui
Advances in nanotechnology have demonstrated the important roles of particle size, shape, and component distribution in determining nanocatalysts' performance for applications in renewable energies. For example, low-Pt-content, active, and durable metal nanocatalysts are required for commercialization of fuel cells and for lowering the cost of hydrogen generation through water electrolysis. Enhanced catalytic performances were found on several core-shell type nanoparticles with narrow particle size distribution[1-3]. There is a great need for methods that can produce them in high quality at low cost for large scale commercialization. This presentation will discuss the synthesis methods based on ethanol as the solvent and reducing agent to fabricate core-shell nanocatalysts without any capping agents. One example is Ru(core)-Pt(shell) nanoparticles supported on carbon black and nanotubes with or without functional groups. Besides structural characterization of the nanocatalysts, in-depth discussion will be given on how to evaluate catalysts' performance for hydrogen evolution reaction (HER) and hydrogen oxidation reaction (HOR) on catalyzed gas diffusion electrodes and in the membrane electrode assembly tests in water electrolyzers and fuel cells, respectively. The results revealed that the HER-HOR on Pt in acids can be considered as barrier-less, i.e., the reaction rate is determined solely by the frequency factor, A, in the Arrhenius equation, k = A exp(-Ea/RT) = A, when the activation barrier, Ea = 0. Thus, HER and HOR are completely reversible and their current densities are determined by the number of collisions between reactants and Pt surface, as well as the electronic and protonic resistances in the circuit. Therefore, gaining high effective Pt surface area with as little Pt as possible and reducing mass transport limitation of the reactants are the main routes to enhance catalysts' performance. To achieve these goals, the synthesis conditions were fine tuned to control the density of nucleation sites on various carbon supports, to enhance crystallographic ordering during particle growth, and to form suitable Pt shell structure. Functional groups were introduced to assist synthesis process or modifying the hydrophilicity of the catalysts. Other examples, such as, Pd(core)-Pt(shell) nanoparticles for oxygen reduction reaction and Ir/IrOx nanoparticles for oxygen evolution reaction may be briefly discussed. (1) Wang, J. X.; Brankovic, S. R.; Zhu, Y.; Hanson, J. C.; Adzic, R. R. Journal of the Electrochemical Society 2003, 150, A1108. (2) Adzic, R. R.; Zhang, J.; Sasaki, K.; Vukmirovic, M. B.; Shao, M.; Wang, J. X.; Nilekar, A. U.; Mavrikakis, M.; Valerio, J. A.; Uribe, F. Topics in Catalysis 2007, 46, 249. (3) Wang, J. X.; Inada, H.; Wu, L. J.; Zhu, Y. M.; Choi, Y. M.; Liu, P.; Zhou, W. P.; Adzic, R. R. Journal of the American Chemical Society 2009, 131, 17298.
Mapping Thermodynamics and Kinetics of Oxygen Vacancies in Fuel Cell Electrolytes on the Nanoscale
Kumar1, Mike
Biegalski1, Anna
Morozovska3, Francesco
Ciucci2, Stephen
Jesse1, Sergei
One of the critical steps in the SOFC and Li-air battery operation leading to large overpotentials and charge-discharge hysteresis is the kinetics of the oxygen oxidation reaction (ORR). It is important thus to explore the mechanisms that control this reaction which remain elusive, largely due to the lack of experimental techniques capable of probing ORR on the nanoscale. Oxygen vacancies play a significant role in determining the functionality of electro-resistive devices, non-volatile memories based on resistive switching and solid oxide fuel cells. Traditionally, the study of the role of oxygen vacancies in these processes is limited by high activation temperature and macroscopic measurement techniques. Here, we demonstrate spatially resolved local probing of the thermodynamics and kinetics involving the generation and diffusion of oxygen vacancies by utilizing chemical expansivity of these oxides upon application of concentrated electric fields. Using Band excitation Electrochemical strain microscopy (ESM), a strongly confined electric field at tip is used to drive the oxygen vacancies in these oxide materials and the resulting localized electrochemical strain is detected. A high frequency periodic bias is applied on the oxide material and the PFM tip acts as a probe of the local displacement arising due to migration of oxygen vacancies. Mapping the loop opening as a function of the final bias allows establishment of the onset and kinetics of the diffusion process. Signal relaxation measurements enable us to locally characterize the diffusion dynamics of the vacancies. Correlated mapping of the local oxygen vacancy movement and diffusivity has been achieved with a resolution of ~ 30 nm. The mapping of vacancies is shown on purely ionic oxides (Yttrium stabilized zirconia and Samarium doped Ceria). Systematic mapping of ORR/OER activity on bare and Pt-functionalized yttria-stabilized zirconia (YSZ) surfaces is demonstrated. This approach allows directly visualization of ORR\OER activation process at the triple-phase boundary. The electrical field-dependence of ionic mobility is explored to determine the critical bias required for the onset of electrochemical transformation, potentially allowing to deconvolute reaction and diffusion processes in the fuel cell system on a local scale. The results of ESM performed at elevated temperatures and controlled oxygen environments on solid electrolytes will also be discussed. Acknowledgement This material is based upon work supported by research division of Materials Science and Engineering, Office of basic energy sciences, DOE.
Sustainable Synthesis of Nitrogen and Sulfur Doped Carbon Aerogels and Their Application as Electrocatalysts in the Oxygen Reduction Reaction
Wohlgemuth1, Robin
White1, Maria-Magdalena
Titirici1, Markus
Heteroatom modification of carbon based materials is becoming increasingly important within the context of greener technologies, as it provides a useful tool to moderate physical and chemical properties for specific applications. Concerning the oxygen reduction reaction in fuel cells, scientists seek to find sustainable alternatives to the current state of the art platinum based catalysts. We present a one pot, hydrothermal synthesis of nitrogen and sulfur dual doped carbon aerogels, based on our previously published glucose and albumin system.[1] This template-free approach gives rise to controlled morphologies with surface areas from 190 m2 g-1 to 320 m2 g-1. Two additives, thienyl–cysteine (TC) and 2-thiophene carbaldehyde (TCA), were used for sulfur incorporation, giving rise to distinct morphologies and varying doping levels of sulfur. Nitrogen doping levels of ~ 5 wt% and sulfur doping levels of 1 wt% (using TCA) to 4 wt% (using TC) could be obtained. A thermal pyrolysis step was used to further tune the aerogel conductivity and heteroatom binding states. By comparing solely nitrogen-doped with nitrogen/sulfur doped carbon aerogels, we show that the presence of sulfur improves the overall electrocatalytic activity of the carbon material in both basic and acidic media. Compared to commercially available Vulcan carbon, our materials exhibit superior catalytic performance in 0.1 M HClO4. Carbon materials are known for their poor ORR catalytic activity in acidic media, which are the standard working conditions of commercial proton exchange membrane fuel cells.[2] These findings are hence expected to be relevant to future research concerning the improvement of non-precious ORR catalyst. 1. White, R.J., et al., A sustainable synthesis of nitrogen-doped carbon aerogels. Green Chemistry, 2011. 2. Zhang, J., PEM fuel cell electrocatalysts and catalyst layers: fundamentals and applications. 1 ed. Vol. 53. 2008: Springer. 1137.
Quantum Mechanical Evaluation of Solid Oxide Fuel Cell Materials
Solid oxide fuel cells (SOFCs) offer the potential for distributed power sources that are both very efficient and nonpolluting, as well as providing fuel flexibility not readily available with other fuel cell types. However, the very high temperatures required for operation leads to premature degradation of the fuel cell materials. Therefore it is critical to find materials that can operate at lower temperatures without sacrificing the electronic and ionic conductivity crucial to efficient operation. Various observables that are key metrics for determining the utility of a fuel cell material can be accurately calculated from quantum mechanics. Our focus is on SOFC cathode materials, often considered the limiting factor in reducing the high operating temperatures of current SOFCs. Porous electrodes can be readily synthesized for SOFCs such that gas transport is facile. If oxide ion diffusion and charge transport through the cathode could be enhanced, along with rapid dissociative adsorption of dioxygen on the cathode surface, lower temperatures could be used, which would facilitate wider deployment of these fuel cells for clean and efficient electricity production. Here we present predictions from first principles quantum mechanics calculations of key properties associated with some standard perovskite-based materials (LaSrMO3), as well as possible variants, and a promising new cathode material (Sr2Fe1.5Mo0.5O6). These properties include oxygen vacancy formation energies, oxide ion diffusivity, and electronic conductivity. Trends in properties can be explained by simple quantum mechanical analyses, which allows new materials design principles to be extracted. These design principles should prove useful in future SOFC cathode materials optimization.
Ab initio Modeling of Ru-based Inorganic Catalysts for Water Oxidation
Piccinin1, Stefano
Water oxidation is one of the main bottlenecks in the conversion and storage of solar energy into chemical fuels. Natural and artificial catalysts formed by multicenter metal-oxide cores embedded in molecular complexes promote this reaction, but their working mechanisms remain largely controversial or unknown. Among these, a tetraruthenium-polyoxometalate complex is one of the most efficient and stable artificial catalysts reported so far. Using density functional theory calculations combined with metadynamics we identify the mechanism of the O-O bond formation and determine the origin of its efficiency [1,2]. We demonstrate that the catalyst is activated by the formation of a Ru-oxo moiety, which undergoes a nucleophilic attack from a solvent water molecule. The low overpotential stems from the optimal distribution of the reaction free energy among the key intermediates, in analogy with the best heterogeneous metal-oxide catalysts for water oxidation. We also show that the mechanism promoted by multicenter metal-oxo cores does not necessarily involve electronic processes in which all Ru atoms change their oxidation state. Instead, the minimum overpotential can be achieved also by reaction pathways in which a single metal site within the tetraruthenium core promotes a multi-electron process, in agreement with the experimental evidence that single center catalysts can efficiently catalyze the oxidation of water. The correlations we establish between the mechanism of reaction, thermodynamic efficiency, and local structure of the active sites provide useful guidelines for the rational design of superior catalysts. [1] S. Piccinin and S. Fabris, PCCP 13, 7666 (2011) [2] S. Piccinin, A. Sartorel, M. Bonchio and S. Fabris, submitted (2011)
First-Principles Calculation Study on the Intrinsic Defects in TaON
Chen1, Lin-Wang
As a compound between the tantalum oxide Ta2O5 and tantalum nitride Ta3N5, the tantalum oxynitride TaON has a smaller band gap than Ta2O5, and better chemical stability than Ta3N5 in aqueous solution, thus is an efficient material for visible-light-driven water splitting. Despite the experimental progress in the photocatalytic performance of TaON, it is currently not well understood what intrinsic defects contribute to the observed n-type conductivity. For transition metal oxides and nitrides, usually the anion (O or N) vacancies and cation interstitials play an important role in determining its intrinsic n-type conductivity. Previously the defects related to the anion vacancies or reduced Ta were assumed as the origin of the observed n-type behavior. Using the first-principles calculation, we show that TaON has different defect properties from the binary oxides and nitrides: (i) instead of O or N vacancies or Ta interstitials, the O_N antisite (N atom replaced by O) is the dominant defect, which determines its intrinsic n-type conductivity and the p-type doping difficulty; (ii) the O_N antisite has a shallower donor level than O or N vacancies, with a delocalized defect state wave function composed mainly of the Ta 5d orbitals, which gives rise to better electronic conductivity in the oxynitride than in the oxide and nitride. We also analyzed the phase stability of TaON in the chemical potential space and reveal that the oxidation of TaON is easy when the oxygen partial pressure is high, thus a relatively O poor condition is required to synthesize stoichiometric TaON samples. This work is supported by the Joint Center of Artificial Photosynthesis and the BES/SC of the U.S. Department of Energy under the contract No. DE-AC02- 05CH11231, and the computation is performed using the NERSC and NCCS supercomputers.
On the Activity and Selectivity of Syngas Conversion Processes
Studt1, Frank
Abild-Pedersen1, Jens
Norskov1 2.
Syngas, a mixture of CO, CO2 and H2, can be converted to bulk chemicals like methanol and transportation fuels like higher hydrocarbons or higher alcohols. Typically syngas is produced by steam reforming of natural gas, but other feedstocks like biomass can be used as well representing an interesting route to sustainable fuels. We investigate the catalytic conversion of syngas to methane and methanol on transition-metal surfaces using density functional theory(DFT) calculations. Based on scaling relations of adsorption energies[1] and transition-states[2,3] on transition-metal surfaces we were able to describe both, methane and methanol formation in terms of the carbon and oxygen binding energy of the transition-metal in question. The combination of these scaling relations with a microkinetic model leads to activity volcanos for methane and methanol formation as a function of carbon and oxygen binding energies.[4,5] Having established volcanos for the activity of these two competing reactions allows for the determination of the parent selectivity. Importantly, to map out the activity and selectivity as a function of only two parameters allows for fast computational screening for new leads of improved catalysts for syngas conversion processes. [1] F. Abild-Pedersen, J. Greeley, F. Studt, J. Rossmeisl, T. R. Munter, P. G. Moses, E. Skúlason, T. Bligaard, J. K. Nørskov, Phys. Rev. Lett. 2007, 99, 016105. [2] T. Bligaard, J. K. Nørskov, S. Dahl, J. Matthiesen, C. H. Christensen, J. Sehested, J. Catal. 2004, 224, 206. [3] S. Wang, B. Temel, J. Shen, G. Jones, L. C. Grabow, F. Studt, T. Bligaard, F. Abild-Pedersen, C. H. Christensen, J. K. Nørskov, Catal. Lett. 2011, 141, 370. [4] M. P. Andersson, T. Bligaard, A. Kustov, K. E. Larsen, J. Greeley, T. Johannessen, C. H. Christensen, J. K. Nørskov, J. Catal. 2006, 239, 501. [5] J. K. Nørskov, F. Abild-Pedersen, F. Studt, T. Bligaard, Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 937.
Catalytic Mechanism of Ultra-small Ceria Nanoparticles for Water Splitting through DFT Investigation
Huang1, Matthew
Experiments have shown the application of cerium dioxide and ceria-based materials as photocatalyst for water splitting to generate hydrogen or oxygen. Current understanding of such process is based on solar-energy-induced electron transfer between multivalent Ce ions and water. However, the exact reaction mechanism on the atomic scale is still unclear. On the other hand, ultra-small ceria nanoparticles (CNPs) with diameters less than 10 nm exhibit very high surface/volume ratio, which strongly suggests its enhanced catalytic potential and unique catalytic behavior. By means of density functional theory calculation, we first identified the structure of ultra-small CNPs in aqueous solution of any pH measurable range. It was found that the surface of these ultra-small CNPs is completely terminated by negatively charged hydroxyl groups. The possible scenario of catalytic water splitting reaction on the atomic scale between water and OH-terminated surface of ultra-small CNPs was then systematically investigated. The results not only elucidate the possible catalytic mechanism of ultra-small CNPs involving detailed reaction process on the atomic scale in contrast to ambiguous understandings based on band structure modification and related requirements on electron transfer, but also provide guidance on designing efficient photocatalytic materials and tailoring their catalytic properties.
Catalysis for Sustainable Energy
Essentially all sustainable energy systems rely on the energy influx from the sun. In order to store solar energy it is most conveniently transformed into a chemical form, a fuel. The key to provide an efficient transformation of energy to a chemical form is the availability of suitable catalysts, and we will need to find new catalysts for a number of processes if we are to successfully synthesize fuels from sunlight. Insight into the way the catalysts work at the molecular may prove essential to speed up the discovery process. The lecture will discuss some of the challenges to catalyst discovery, the associated challenges to science as well as some approaches to molecular level catalyst design. Specific examples will include the (photo-)electrochemical oxygen evolution and hydrogen evolution reactions, carbon dioxide reduction, and biomass transformation reactions.
A First-Principles Study of Stable Pd-Pt Nanoalloy Configurations for Use as Catalysts in the Hydrogen Evolution Reaction
Tan1, Kewu
Due to size effect, nanoparticles exhibit different catalytic properties from surface catalysts and can be tuned via alloying. Using first-principles methods, we demonstrate both the effects of size and alloying on the hydrogen evolution reaction (HER), which plays an important role in technologies such as electrochemistry, hydrogen fuel cells and water splitting. Using an atomistic Cluster Expansion Hamiltonian derived from density functional theory (DFT) calculations, we first identify the groundstate configurations of a 1nm-sized Pd-Pt cubo-octahedron versus composition. The predicted groundstates exhibit core-shell configurations with Pt in the core and are weakly magnetic. We next study the adsorption energies of hydrogen on these predicted groundstate nanoclusters and predict their catalytic activities for the HER via microkinetic modeling and/or Monte Carlo simulations. The microkinetic model [1] is extended to account for multiple adsorption sites. Despite composing of (111) and (100) facets, the nanocluster's adsorption energetics differ from that of perfect (111) and (100) surfaces, resulting in higher hydrogen coverage for pure Pt. By increasing the composition of Pd, the nanocluster's catalytic property is tuned as the adsorption energy at the bridge sites on (100) facets increases while those on (111) hollow sites decreases. The methodology shown here is general and may be applied to other catalytic reaction systems. [1] : J. Greeley et. al., ChemPhysChem 7, 1032 (2006).
Modeling of Hydrogen Evolution Reactions on III-V Semiconductor Surfaces
Woon Ih
Choi1, Brandon
Wood1, Eric
Schwegler1, Tadashi
III-V Semiconductors are promising photoelectrode materials in photoelectrochemical cell due to its good carrier mobility and tunable band gap. Hydrogen gas evolution on the cathode surface involves charge transfer reactions such as Volmer-Heyrovsky reaction. Within the framework of first-principles molecular dynamics, however, realistic simulation of charge transfer reactions still remains challenging. Recently Santos et al. explained electrocatalysis for hydrogen evolution reactions (HER) on various metals. They get parameters of Anderson-Newns model using DFT results with the consideration of solvent represented as generalized solvent coordinate in Marcus theory. To examine III-V surface, we additionally consider excited electrons which actually drive photocatalytic HER. We also show how surface oxide and catalytic atoms change adiabatic free energy surface (FES) of HER. As overall wisdom drawn from our modeling, we will mention what is important component of good catalyst for HER.
Beyond Band Engineering: Desigining New Catalysts by Computational Approach
There are stringent electronic requirements for materials that can absorb sunlight and efficiently convert it into electricity or other forms of useful energy, such as chemical energy stored in hydrogen. As naturally occurring materials do not fulfill all these criteria, these electronic requirements in materials are usually achieved by a band-engineering approach. In conventional band engineering approach, the electronic structures of the host materials are modified by selective doping or alloying. For example, WO3 has been studied for as a photocatalyst, but has enjoyed limited success by itself mostly because of two reasons: (i) higher band gap, namely 2.6 eV, compared to 2.0 eV band gap required to split water by sunlight and (ii) the conduction band minimum lies below the H2 reduction potential, hence unable to produce H2 by splitting water. Doping in WO3 has not improved its' performance significantly. Here, instead of following a conventional band engineering approach by alloying WO3 with other impurities and try "trial and error†method, a mineral search method can be performed. To do this, first we will look for minerals which would contain W+6-oxide. It is well known that double tungstates of mono- and trivalent metals with composition of MR[WO_4 ]_2 have structural polymorphism, and several reference structures can be found for them. For its specific compositions, it can be argued that higher energy levels of Cu 3d would help to uplift the VBM and may push the CBM up. In addition, it has been shown that Bi can contribute anti-bonding s-bands on top of the valence band. It can be then argued that CuBiW2O4 would be a preferable photocatalyst to split water to produce hydrogen. To date, the exact crystal structure of CuBiW2O8 is unknown. We will use density functional theory (DFT) to determine its crystal structures and study its electronic properties. We will demonstrate a systematic strategy to design and achieve new catalysts for energy conversion.
Controlled Synthesis of Nanostructured Catalysts
Dai1 2.
Catalysis is critically important to energy production and to meeting the environmental quality mission of promoting the development and utilization of clean, efficient, and reliable energy resources. It is essential to understand the relationships between the atomic and nanoscale structure of metal nanoparticles and catalyst supports and the crucial role these play in promoting or altering catalytic pathways. The key focus of this talk lies in the controlled synthesis of metallic catalysts with unique metal-support interactions and nanostructured nonmetallic catalysts for heterogeneous catalysis. Critical issues and emerging science and technology in heterogeneous catalysis will be discussed in context of controlled synthesis. Acknowledgments: This work was conducted at the Oak Ridge National Laboratory and supported by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy, under contract No. DE-AC05-00OR22725 with UT-Battelle, LLC.
Heterogeneous Catalysts with Novel Nanoarchitectures and Improved Selectivities
Lee1, Jibong
Joo1, Zhihuan
Weng1, Yadong
Yin1, Francisco
Several examples from ongoing work in our laboratory on the use of self-assembly to prepare heterogeneous catalysts with novel architectures will be discussed in this presentation. In one case, catalysts consisting of dispersed platinum metal nanoparticles with narrow size distributions and well-defined shapes were prepared and tested for the selective promotion of carbon-carbon double-bond cis-trans isomerization reactions in olefins. It was shown that the selective formation of the cis isomer could be controlled by using tetrahedral particles with exposed (111) facets. In a second example, catalysts based on small platinum nanoparticles of well-defined sizes were made by using dendrimers as scaffolding structures. The organic framework in that case can provide new fuctionality, including chirality as a way to introduce enantioselectivity. The third example involves the control of metal nanoparticle sintering by covering those with a layer of mesoporous silica grown on top. The final case to be discussed is one where yolk@shell metal-semiconductor constructs are being developed for increase stability in oxidation and photocatalytic applications.
Transition Metal Nanocrystals for Catalytic Energy Conversion
Doan-Nguyen1, Matteo
Cargnello3, Thomas
Gordon2, Paolo
Fornasiero3, Christopher
Murray1 2.
Transition metals in the d8 family such as Ni, Pd, and Pt have been synthesized with great control in size and shape. Using a high-temperature chemical synthetic method, spherical nanocrystals ranging from 4-12 nm have been synthesized for heterogeneous catalysis testing. The tunability in nanocrystal size allows for controlled catalytic study of model reactions such as CO oxidation, CO hydrogenation and methane oxidation. The monodispersity of the particles allow for a systematic correlation between size and catalytic activity. There is consistent size dependence of CO oxidation for each of the metal systems on CeO2 support as indicated by the lower temperatures for full conversion. With higher surface-area-to-volume ratio, the smallest particles were the most catalytically active as indicated by the lower activation energy calculated from catalytic testing under differential conditions. The same trend was observed for CO hydrogenation amongst the d8 metals on CeO2 support. This trend was inverted for the oxidation of methane. This current study aims to elucidate the size dependence of catalytic activity in model systems with supported uniform nanocrystals.
Ni-based Bimetallic Catalysts for Acetylene Semi-hydrogenation
Spanjers1, Subhra
Jana1, Jacob
Held1, Michael
Janik1, Robert
The removal of trace acetylene in ethylene streams destined for polyethylene production is a high-volume industrial catalytic-based process that must demonstrate high selective for alkyne hydrogenation over alkene hydrogenation. The current industrial catalyst is a Pd-Ag alloy; Ag dilutes Pd ensembles that are unselective toward acetylene/ethylene hydrogenation. The inclusion of Ag substantially reduces activity but increases selectivity to acetylene semi-hydrogenation. Recent research efforts to replace Pd-Ag catalysts with base-metal catalysts demonstrated Ni-Zn alloy catalysts had comparable activity and selectivity [1]. Additionally, the computational aspects of this work demonstrated the activity and selectivity were highly sensitive to the composition of the Ni-Zn alloy catalyst. We report on the synthesis and characterization of Ni-Zn catalysts that exhibit a higher selectivity than Pd-Ag for acetylene semi-hydrogenation in excess ethylene. Ni nanoparticles (NPs) with an average diameter of 3, 6 and 10 nm synthesized by a high-temperature colloidal method were converted into Ni-Zn NPs by the injection of diethylzinc into hot solvent containing Ni NPs. Wide-angle XRD demonstrates Ni NPs – regardless of their initial size – incorporated Zn in a 1:1 atomic ratio with respect to Ni. The atomic composition of the Ni-Zn sample could not be altered by changing the amount of diethylzinc addition. The Ni-Zn NPs were subsequently adsorbed from solution on the surface of mesoporous SBA-15 silica and further characterized. X-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) were used to determine the oxidation state and local coordination environment of the metal atoms in Ni-Zn NPs. XANES results demonstrate reduced Ni and Ni-Zn NPs form during colloidal syntheses with no required reductive pretreatment of the supported NPs. This is an inherent advantage of the colloidal route compared to Ni-Zn catalysts prepared by traditional simultaneous or co-impregnation synthetic routes. The latter catalysts must be reduced and the disparate thermal stability of Ni and Zn lead to catalysts with uncontrollable Ni:Zn ratios. Ni-Zn NPs are much more selective than Ni NPs for acetylene semi-hydrogenation which is in agreement with previous experimental and theoretical results [1]. DFT-based reaction energy calculations for acetylene and ethylene hydrogenation were compared over Ni and Ni-Zn surfaces to determine the origin of the experimentally-observed selectivity enhancement on the bimetallic catalyst. A sequential hydrogenation mechanism from acetylene to ethane is not sufficient to capture the selectivity enhancement over Ni-Zn compared with Ni. Additional in-situ characterization of the Ni and Ni-Zn catalysts was conducted to rectify the apparent contradictions between the DFT calculations and experimental observations. 1. F. Studt et al., Science 320 (2008) 1320-1322
Towards a Molecular Level Understanding of Fischer-Tropsch Synthesis on Cobalt Nanoparticles
Lewis1, April
Jewell1, Georgios
Kyriakou1, Charles
The development of sustainable energy technologies, including the production of synthetic fuels, is of global importance. In this respect, Fischer-Tropsch synthesis (FTS) has recently gained increased attention as it involves the formation of hydrocarbons via the catalytic conversion of syngas (CO and H2), which can be derived from renewable sources. FTS is often performed using cobalt-based catalysts that are greatly affected by the adsorption state of reactants, as well as nanoparticle shape and size. Here we have used low-temperature scanning tunneling microscopy (LT-STM) to study the interaction of syngas with well-defined cobalt nanoparticles controllably grown onto copper {111}, an inert metal for FTS. Hydrogen adsorbs dissociatively on the cobalt surfaces, and we have observed 3 unique coverage-dependent phases. We demonstrate that these phases can resolve crystal packing ambiguities of the underlying cobalt nanoparticles, a question that has been debated in the literature. Simultaneous exposure of the cobalt to H2 and CO results in segregated islands of the adsorbates on the nanoparticle surface at 80 K, and we propose that atomic H blocks CO adsorption, causing the build-up of CO at the nanoparticle step edges. With increasing CO coverage, a two-dimensional phase compression of H by CO is observed, providing the first direct visualization of this long proposed phenomenon in a catalytically relevant system. Finally, our data suggest that FTS reactivity must be dominated by the available interface length between the two adsorbates, and thus, be subject to unforeseen kinetic restraints as a function of particle size.
Nanostructured Catalysts for Energy Conversion Applications
Wang1 2.
Nanostructured catalysts are playing increasing roles in energy conversion ranging from renewable biomass conversion to fuel cell application. Here two examples will be given on our recent work on; 1) multifunctional nanostructured ZrxZnxOy catalysts for direct conversion of bio-ethanol to isobutene, and 2) stabilized Pt nanoparticles at triple junction points of Pt-ITO-graphene for PEM fuel cells. Specifically, nanosized ZnxZryOz mixed oxides with balanced acidic/basic sites were synthesized for direct and high-yield conversion of bio-ethanol to isobutene (∼83%). In the case of electrocatalysts, we find that Pt nanoparticles are stabilized at Pt-ITO-graphene triple junction points. The new electrocatalyst exhibits an extremely high durability and high activity for oxygen reduction reaction (ORR).
Porous Metal Oxides as Catalysts
Suib1, Boxun
Hu1, Chung-Hao
Our research involves the synthesis, characterization and catalytic activity of porous transition metal oxide materials. Both microporous and mesoporous materials have been prepared. A variety of single and mixed metal oxides systems have been studied. In situ characterization of these materials has been done using Fourier transform infrared, X-ray powder diffraction and temperature programmed desorption methods. Novel mixed metal catalysts have been used in Fischer Tropsch catalytic reactions in order to make oxygenates and olefins. Selective catalytic oxidations have also been studied. Some research in the area of biomass conversion has also been done. The above areas will be discussed as regards the role of various catalytically active sites in these systems.
In-situ Visualization of Catalysts at Atomic Scale under Operation Conditions by Environmental Transmission Electron Microscopy (ETEM)
Currently the strong focus on energy producing and environmental protecting technologies relies on the advancement of new functional nanomaterials, especially catalysts. Characterization of the state and properties of such a nanocatalyst as well as of the catalyst's activation, deactivation and poisoning demands detailed time resolved atomic-scale insights while in operation conditions [1,2]. In particular, visualization of solid heterogeneous catalysts and their structural evolution in situ under reaction environments are crucial to obtain detailed knowledge about the relationship between the nanostructure of the catalyst/support system and the actual reaction mechanism, because generally there is no evidence that the dynamic state of the materials can be truly inferred from postmortem examinations of the catalyst materials. To our advantage, atomic-scale transmission electron microscopy (TEM), markedly advanced by utilizing hardware correctors compensating aberrations present in electromagnetic lenses [3], has become a powerful and indispensable tool for characterizing nanomaterials and provides this unique ability to image size, shape, as well as surface and interface structures of individual nanocatalysts [4]. The implementation of differential pumping apertures in an aberration corrected environmental TEM (ETEM) enables to maintain these high-resolution imaging and analytical capabilities, while confining a gas environment in the close vicinity of the catalyst specimen [5]. Atomic-scale imaging in ETEM has opened up a unique possibility to monitor heterogeneous catalysts during exposure to reactive gas environments and/or elevated temperatures. Catalytic nanomaterials can be observed at work - time resolved at atomic resolution [6]. Application of environmental TEM (ETEM) for in situ studies of gas-solid interactions makes it an essential complement to theoretical approaches as well as to the arsenal of established spectroscopic techniques that average information over length scales considerably larger than the characteristic dimensions of the nanostructures themselves. Recent applications of gold and platinum nanocatalysts are provided [7-9] that exploit atomic-resolution ETEM for understanding the role of gas–surface interactions in nanostructured catalysts in their functional state. [1] E. D. Boyes, P. L. Gai. Ultramicroscopy 67, 219 (1997). [2] P. L. Hansen et al. Science 295, 2053 (2002). [3] M. Haider et al. Nature 392, 768 (1998). [4] C. Kisielowski et al. Angewandte Chemie Int. Ed. 49 (15), 2708 (2010). [5] T. W. Hansen et al. Mater. Sci. Technol. 26, 1338 (2010). [6] J.R. Jinschek, S Helveg. Micron (2011), submitted. [7] K. Yoshida et al. Microsc. Microanal. 17 (2), 1706 (2011). [8] H. Yoshida, et al. Appl. Phys. Express 4, 065001 (2011). [9] T. Uchiyama et al. Angew. Chem. Int. Ed. 50, 1 (2011).
Dehydrogenation of Cyclohexene on Size Selected Subnanometer Cobalt Clusters: Improved Catalytic Performance via Structural Fluxionality of Cluster-assembled Nanostructures
Vajda1 3, Sungsik
Lee2, Marcel
di Vece3, Byeongdu
Lee2, Soenke
Seifert2, Randall
The elucidation of the size/composition/shape/structure and function correlation, the effect of support along with the determination of the nature of the catalytic particles under reaction conditions are instrumental for addressing fundamental aspects of catalysis on the way to the design of new catalytic materials. The experimental studies are based on 1) chemically uniform support fabrication, 2) size-selected cluster deposition and 3) in situ synchrotron X-ray characterization of clusters under working conditions, combined with mass spectroscopy analysis of the reaction products.(1) Subnanometer size-selcted cobalt clusters were deposited on TiO2, ZnO, Al2O3 and MgO. While the activity and selectivity of TiO2, ZnO and Al2O3 supported clusters were comparable, the MgO-supported clusters were up to 3 times more active.(2) In situ grazing incidence small X-ray scattering and X-ray absorption data reveal that a formation of a fluxional ~2-3 nm structure and the change of the oxidation state of Co clusters is responsible for the dramatic increase in activity. The results indicate low-temperature activation of molecular oxygen on sub-nm Co clusters and their nanometer size assembled structures. (1) S. Lee, B. Lee, S. Seifert, S. Vajda and R. E. Winans, Nucl. Instr. and Meth. A, 649. 200-203 (2011) (2) S. Lee, M. Di Vece, B. Lee, S. Seifert, R. E. Winans, S. Vajda Phys. Chem. Chem. Phys., submitted
Relative Resistance of Pt, Pt3Co and Pt4Ni Films to Sulfur Poisoning: A Raman Spectroelectrochemical Study
Pomfret1, Jeremy
The alloying of Pt with iron-group metals has been studied for the promotion of catalytic reactions and alleviation of catalyst poisoning. The use of in situ optical methods to identify molecular species on electrocatalyst surfaces is critical to optimizing and developing catalysts. In the present work, benzenethiol (BT) adsorption/desorption at Pt metal, Pt3Co and Pt4Ni alloy films under electrochemical conditions are investigated with in situ Raman spectroscopy and cyclic voltammetry (CV). When adsorbed to a surface, BT exhibits an angle-specific Raman spectrum that features an easily-tracked, strong peak at 1572 cm-1. This peak is evaluated over potentials of 200-1300 mV (vs SHE). Complete desorption of BT occurs at 1300 mV of positive-going sweeps for all three film compositions. During negative-going sweeps, the BT peak reappears on Pt3Co films at 850 mV, which is 50 mV lower than on Pt and Pt4Ni. Furthermore, at 200 mV, the recovered BT signal on Pt3Co is only ~25% as intense as when first deposited. The Pt and Pt4Ni films both recover 55-65% of their signal; however, the normalized BT signal is ~20% stronger on Pt than Pt4Ni during a second positive-going sweep. These results provide spectroscopic evidence that Pt3Co and Pt4Ni alloys are more easily cleaned after BT poisoning than Pt metal. None of the three film compositions tested display a sulfate spectral feature between 1000 and 1200 cm-1, indicating an absence of residual S species after BT desorbs. Features in the low-frequency region of the Raman spectra further confirm that the metal–S bond is broken during electrooxidative desorption of BT. A more complete picture of potential-dependent BT-metal interactions is obtained when Raman spectroelectrochemical data are compared to CV stripping data. BT can be oxidatively removed from electrocatalysts surfaces by CV cycling between 200 mV and 1300 mV. Pt4Ni requires the least number of CV cleaning cycles to recover maximum electrocatalytic activity, followed by Pt3Co. The maximum extent of electrocatalytic activity recovered is highest for Pt3Co. The ease of cleaning, based on the trends from the electrochemical measurements, corroborates the conclusion of the Raman spectroelectrochemical study that Pt3Co is the most S-resistant of the electrocatalyst compositions investigated.
On the Hierarchy of Nanoscaling Effects on the Electrocatalytic Activity of TiO2 Supported Gold Nanoparticles
Reinecke1, Hirohito
Ogasawara2, Frank
Abild-Pedersen2, Lin
Li1, Anders
Nilsson2, Thomas
It is known that atomically flat, Au 111 surfaces are inactive for gas phase CO oxidation. Several decades ago, it was found that when gold is nanoscaled to less than 5 nm in diameter on certain metal oxide supports, it becomes one of the most active CO oxidation catalysts. Gold has also been shown to be electrocatalytically active for CO oxidation. The physical origins are widely debated. Possible explanations are: 1. Size dependent coordination number effects; 2. Support directed coordination number effects; 3. Gold lattice strain; 4. Support electronic promotion; 5. Gold surface oxidation; 6. Support activation of oxygen. It is our objective to establish a nanoscaling factor hierarchy from this list by measuring the physical, electronic, and electrocatalytic factors for the highly active Au on TiO2 system. Our approach entails the design of a well defined, flat TiO2 support and the deposition of Au nanoparticles using a contamination free, size controlled e-beam technique. This design approach is well suited for determination of size, shape and lattice strain that we measure by Scanning Electron Microscopy and Transmission Electron Microscopy. Additionally, we measure the electronic structure using the 5d subshell sensitive hard x-ray valence band photoelectron spectroscopy to help disentangle the previous nanoscaling effects. There is a trend in the valence band electronic structure that we attribute to a surface coordination number effect. Finally, we measure the CO electrocatalytic oxidation activity versus Au nanoparticle size on TiO2 in both base (0.1 M KOH) and acid (0.1 M H2SO4). The results show that in both base and in acid, there is an optimum in activity with size at 5 nm in diameter. It is hypothesized that the optimum in activity is related to an optimal average coordination number at 5 nm that is able to bind OH- not too strongly or weakly.
Structural Studies of (GaN)1-x(ZnO)x Semiconductors for Solar Water Splitting
Reinert1 2, James
Ciston4, Fulya
Dogan6, Derek
Middlemiss7, Katharine
Page3, Thomas
Proffen5, Ashfia
Huq5, Clare
Grey7, Peter
Khalifah1 2.
The use of semiconductors to split water into H2 fuel and O2 gas is a promising technology for renewable producing chemical fuels that can be stored and transported. The best stable semiconductor discovered thus far to have activity for visible-light-driven solar water splitting is (GaN)1-x(ZnO)x, but even for this material the maximum quantum efficiency with visible light is still only about 6%. The average and local crystal structure of this compound has been investigated using a broad range of tools (x-ray and neutron powder diffraction and pair distribution function studies, 14N and 71Ga solid state nuclear magnetic resonance, scanning and transmission electron microscopy). All of these techniques suggest the presence of an intergrowth defect that is prevalent for Zn-rich but not Zn-poor compositions, and which is present in sufficient quantities to appear in bulk measurements (including powder diffraction patterns). The role that this defect may play in influencing the optical and charge transport properties of this phase will be discussed.
Palladium Nanoshells on HOPG Surfaces for Oxygen Reduction Reaction
Arroyo-Ramirez1, Diego
Rodriguez2, Wilfredo
Otano2, Carlos
Carbon-supported nanomaterials can be done with different techniques or methods. The composition and morphology of the nanomaterials determines its future application. The direct methanol fuel cell (DMFC) cathode has the problem of requiring high cost catalysts and its degradation due to methanol crossover through the membrane. Also, the deposition methods for the catalysts are complex and time consuming. To solve these drawbacks is necessary to find a catalyst with high methanol tolerance and simple methodology for its deposition. Palladium (Pd) has resulted in active catalysts for the oxygen reduction reaction with high methanol tolerance. The research focus is the development of palladium nanostructures on carbon support for oxygen reduction reaction (ORR) using sputtering and electrospinning techniques . The synthesis of palladium thin films and nanoshells was carried out with dc-magnetron sputtering and electrospinning techniques on highly ordered pyrolytic graphite (HOPG) surface. Electrospun polymer fibers mats of poly(ethylene) oxide (PEO) were used as templates for the Pd shell nanostructures formation. The Pd thin films and nanoshells thicknesses were between 25 nm to 95 nm of Pd. The Pd nanoshells were characterized by electrochemical and surface analysis techniques. Scanning electron microscopy and energy-dispersive X-ray fluorescence spectroscopy (SEM/EDS) were used to study the morphology and composition of the Pd nanoshells. Electrocatalytic activity towards ORR and methanol tolerance in oxygen saturated 0.5 M H2SO4 solution was done. Palladium nanoshells have activity for the oxygen reduction reaction and present higher methanol tolerance than platinum catalysts. This research presents a novel approach for the synthesis of electrocatalysts for fuel cells.
Multimetallic Nanoparticles as Highly Efficient Catalysts for Electro-oxidation of Methanol/Formic Acid
Zhang1, Shaojun
Guo1, Huiyuan
Zhu1, Dong
Su2, Shouheng
Low-temperature fuel cells based on methanol/formic acid have attracted growing attention as a promising power source due to the high energy density and the convenient storage, transport of the small organic molecules. However, nanoparticle (NP) catalysts studied to date for methanol/formic acid oxidation reaction (MOR/FAOR) are prone to be severely poisoned by CO or other intermediate species during the catalytic process. Here we report a reliable chemical synthesis of catalysts based on high-quality FePt bimetallic NPs and MFePt (M= Co, Ni, Pd etc.) trimetallic NPs. With exquisitely tailoring on the composition and structure, these multimetallc NP catalysts exhibit exciting enhancement in activity and CO-tolerance for MOR/FAOR compared to traditional Pt NP catalysts. The present design and synthesis provide a robust approach to practical catalysts for MOR/FAOR.
Photoelectrochemical Hydrogen Production from Water Using Modified CuInS2 Electrodes
Ikeda1, SunMin
Lee1, Yasunari
Otsuka1, Takashi
Harada1, Michio
Photoelectrochemical water splitting has become an attractive approach for hydrogen (H2) production in view of energy and environmental issues. Since the first report of a TiO2 thin-film photoelectrode, a variety of semiconductor electrodes and devices have been investigated. To date, performances with conversion efficiency as high as 10% have been demonstrated for electrodes based on stacked III-V semiconductors prepared by the MOCVD technique. However, these electrodes have limited corrosion resistance in aqueous electrolytes and are expensive for practical applications. On the other hand, the well-studied transition metal oxides are corrosion-resistant and inexpensive, but conversion efficiencies for these electrodes are not sufficiently high yet due to the lack of optical and photoelectrochemical properties required for realizing high photocurrents and H2 evolution rates. Chalcopyrite p-type semiconductors such as CuInSe2, CuGaSe2, CuInS2 and their mixed crystals are used as absorber layers in thin film solar cells. Due to their high absorption coefficient, 1-2-μm thick layers are enough to absorb the most part of the incident solar radiation. A wide range of band gap values (1.0-2.4 eV) can be obtained by changing the In/Ga and/or Se/S ratios. These specific properties are also attractive for the use of a photocathode for H2 production. Although there have been a few reports in which photoelectrochemical properties of the series of chalcopyrite families for H2 production were discussed, there have been little work on efficient H2 production. In the present study, therefore, we attempted to fabricate a CuInS2-based photocathode for efficient H2 production. Polycrystalline CuInS2 films were fabricated by sulfurization of electrodeposited Cu and In metallic precursor films. Structural analyses revealed that the CuInS2 film formed compact agglomerates of crystallites with grain sizes of ca. 0.5-1.5 μm. Photoelectrochemical characterization revealed that the film was p-type with a flat band potential of 0.3-0.4 V (vs Ag/AgCl at pH), which is suitable for water reduction but cannot be for water oxidation. Upon loading Pt deposits, the film worked as a H2 liberation electrode under cathodic polarization. Moreover, by introduction of n-type thin layers such as CdS and ZnS on the CuInS2 surface before the Pt loading, appreciable improvements of H2 liberation efficiency were achieved: for the CdS modified sample, spectral response data showed incident photon to current efficiency as high as 20% at wavelengths ranging from ca. 500 to 750 nm.
The Effective Vertical Growth and Enhanced Photocatalysis of ZnO Nanowire Arrays on Ag Nanosheets
Cheng Wei
Chang1, Fang Xian
Lu2, Ta Jen
ZnO/Ag belonging to a semiconductor–metal composite possesses versatile material characteristics such as the introduction of the charge transfer and the suppression of electron–hole recombination. Great efforts thereby have been devoted for preparation of ZnO/Ag heterostructures with various morphologies such as clusters (0D) and dendrites (1D). In this study, polygon Ag nanosheet capable of high surface–to–volume ratio and large flat plane was used, which is able to support a stable atmosphere of vertical growth for high–density ZnO nanowires (NWs) and hence provide more ZnO/Ag junctions. Besides, we demonstrated the ZnO/Ag heterostructure in the presence of ZnO NWs and Ag nanosheets by means of a two–step growth process. Ag nanosheets were fabricated by utilizing polyol reduction and CN- etching, and then the seed–mediates method was employed in order to deposit ZnO NWs robustly on Ag nanosheets for the anisotropic growth. The experimental results show that the single crystalline Ag nanosheets with the thickness of 20–30 nm enlarge its area along 〈110〉 and ⅓〈422〉. ZnO NWs arrays are vertically assembled on both sides of Ag nanosheets (111) facets. Meanwhile, these interfaces of ZnO/Ag heterostructures can boost the electron–hole pairs separation, so the enhancement of the photocatalytic activity can be achieved by observing the representative pollutant concentrations.
U9.9 Transferred to U6.1
Hybrid Polymer-Metal Surfaces for Electrochemical CO2 Reduction
Abram1, Kendra
Kuhl2, Etosha
Cave3, Thomas
Alternative energy sources are being extensively explored such as wind and solar energy. Due to their intermittency, it will be difficult for grids to handle a large percentage of their energy coming from these sources. This energy could be stored in chemical bonds by converting CO2 and water to fuels and oxygen, creating a carbon neutral cycle. Heterogeneous catalysts studied for the CO2 reduction reaction in aqueous environments consist mainly of metals, which produce varied product distributions but all require large overpotentials.1 There has also been some work done exploring chemical modification of metal surfaces.2-4 This study is focused on lowering overpotentials required for and tuning the product distribution of electrochemical CO2 reduction on metals. A custom electrolysis cell was designed with a large working electrode area to electrolyte volume ratio to increase sensitivity for detection of liquid products. Electrolysis experiments were run potentiostatically for 1 hour. Gas Chromatography (GC) and Nuclear Magnetic Resonance (NMR) were used to detect gas and liquid products, respectively. Several potentials were chosen for each catalyst tested to explore current densities ranging from about 0.5 to 15 mA/cm2. Pt metal was explored, and CO2 reduction products were detected in small quantities as early as -0.6V vs. RHE. Consistent with literature, H2 was by far the dominant product with formate (<0.5%) as a minor product.1 However, CO gas, methane, and methanol were also detected in small amounts (<0.5%). The platinum surface was then modified with a thin film of polyaniline (PANi) in an effort to modify Pt reactivity. The PANi was electrodeposited in sulfuric acid using cycling voltammetry. The PANi-Pt catalysts show a small but noticeable increase in efficiency for formate and CO production and a decrease in methanol and methane production compared to the pure Pt metal catalyst. The mechanism for this change in activity is being explored. ACKNOWLEDGMENT The authors would like to thank Chevron, the Stanford Graduate Fellowship, and the National Science Foundation for student funding and the Global Climate and Energy Project (GCEP) for project funding. REFERENCES 1. Hori et al. Electrochimica Acta, 39, 1833-1839, 1994. 2. K. Ogura et al. J. Electrochem. Soc., 145, 3801-3809, 1998. 3. B. Aurian-Blajeni et al. J. Electroanal. Chem., 149, 291-293, 1983. 4. R. Aydin et al. J. Electroanal. Chem., 535, 107-112, 2002.
Pd Oxidation State Effect on Nitrogen Doped TiO2 Nanoparticles for Photocatalytic Production of Hydrogen
Wu1, Che-Pu
Hsu1, Min-Ping
Lin3, Yu-Cheng
Cho2, Geza
Toth4, Krisztian
Kordas4, Yang-Fang
Chen2, Wei-Fang
The discovery of photoelectrochemical splitting of water on titanium dioxide (TiO2) by Fujishima and Honda in 1972 has initiated a considerable boom of semiconductor-based photocatalyst research. TiO2 is probably the most promising photocatalyst being environmentally friendly, with low cost, good photocatalytic activity and excellent photostability. However, the large band gap of TiO2 (~3.2 eV for anatase TiO2 and ~3.0 eV for rutile TiO2) restricts its applications in visible light region. In recent researches, nitrogen doping can decrease the band gap of n-type TiO2 due to the mixing of N 2p states with O 2p states. Decoration of TiO2 with metals and metal oxides such as Pt, Au, Pd, PdO, Ni, and Ag, has been found to enhance the photocatalytic properties. In these cases, low cost Pd-based catalysts are more suitable for industrial application as compared with Pt-based catalysts (approximately 20%-25% of that of Pt metal). In this study, we are combining the efforts of Pd-based nanoparticle decoration with nitrogen doped TiO2 (N-TiO2) synthesis in order to develop novel and efficient photocatalytic materials that are easy to produce in industrial quantities. First, TiO2 are annealed at 600 °C for 12 hours in ammonia to achieve N-doped TiO2. Palladium precursor was dissolved in acetone and mixed with N-TiO2 by ultrasonic agitation. After evaporating the solvent, the samples were calcined in air at 300 °C for 2 hours, and then reduced in 15%H2-85%Ar flow at 500 °C for different time. The microstructure, morphology, size and chemical composition of various N-TiO2-Pd series nanoparticles synthesized and used in the work were characterized by using synchrotron radiation X-ray diffraction, transmission and scanning electron microscopy as well as X-ray photoelectron spectroscopy. When we applied N-TiO2-Pd photocatalyst in the water ethanol mixture (molar ratio ~ 3:1), the hydrogen evolution rates were as high as 73,400 μmol/g.h under the UV-B radiation (Sankyo Denki, G8T5E UV-B lamps, 8W × 6 piece). Our study confirms that optimal Pd/PdO nanoparticles decorated on N-TiO2 surface increase the photocatalytic activity. Furthermore, the ideal reduction time for N-TiO2-Pd series photocatalysts is ~15 min. The results of the study display that N-TiO2-Pd photocatalysts are suitable for the generation of renewable energy .
Study on Optimal Active Sites of Cu Nanoparticle/ZnO Nanowire Catalysts for Methanol Steam Reforming Process
Yeon Ho
Im1, Sanggon
Kim1, Chanseok
You1, Yeonghyo
Cu/ZnO based catalysts have been commonly used to achieve for hydrogen production from methanol steam reforming under relatively low operating temperature. According to previous works so far, it has been known that Cu morphology in the conventional Cu/ZnO bulk catalysts synthesized by co-precipitation methods is one of the main issues related to the active sites toward high performance reforming. The advanced methods to control Cu morphology have been investigated as introducing the additive metals or varying process conditions of precipitation or calcination. In this work, we synthesized novel Cu nanoparticle/ZnO nanowire catalysts and investigated their performance with the variation of Cu morphology according to the changes of Cu deposition and reduction time. Our results showed clearly that the morphology of Cu nanoparticle on single crystal ZnO nanowire play a great role in the methanol steam reforming processes. In addition, the morphology changes of Cu during chemical reaction were investigated systematically. Finally, we will discuss the optimal morphology of Cu nanoparticle on ZnO nanowire toward best catalytic performance.
High Performance Ru85Se15 Cathode Catalysts for PEM Fuel Cell Applications
Zheng1, Xuan
Cheng1 2, Ting-Chu
Jao3 4, Fang-Bor
Weng3 4, Ay
Su3 4, Yu-Chun
Chiang3 4.
The Ru85Se15 nanoparticles supported on commercial Vulcan XC-72R or multi-wall carbon nanotubes (MWCNTs) were synthesized by microwave assisted polyol method with different solution pH. The Ru85Se15 nanoparticles on citric acid (CA)-treated supports prepared at pH=7 exhibited the most uniform particle distribution, higher degree of graphitization on supports, and four-electron ORR mechanism. Using lower loading of 0.138 mg Ru cm-2, the maximum power densities (Pmax) for the Ru85Se15/CA-MWCNTs and Ru85Se15/CA-XC72R were 380 mW cm-2 at 1430 mA cm-2 and 336 mW cm-2 at 1230 mA cm-2, respectively, with oxygen, while 166 mW cm-2 at 710 mA cm-2 and 126 mW cm-2 at 510 mA cm-2, respectively, with air. The Pmax of 103 mW cm-2 with air for the Ru85Se15/CA-MWCNTs could be retained (38% loss), while 46 mW cm-2 for Ru85Se15/CA-XC72R (64% loss) upon 6000 cycles. The four-electron ORR mechanism and highly graphitized MWCNTs might be responsible for the high performance and durability of Ru85Se15/CA-MWCNTs.
Ag Composite Materials for Intermediate Temperature Solid Oxide Fuel Cell
Song1, Seung-Bok
Lee1, Jong-Won
Lee1, Tak-Hyoung
Lim1, Seok-Joo
Park1, Dong-Ryul
The Solid Oxide Fuel Cell (SOFC) which employs a ceramic electrolyte represents the most efficient and clean way to generate electricity by electrochemical reaction between fuel and oxygen at high temperature in the range of 800-1000 oC. In addition, SOFCs have many advantages such as multi-fuel capability and simplicity of system design. However, the degradation of component performance and cell lifetime problems occur due to high operation temperature. To solve these problems, the lowering operation temperature have been achieved by using a thin film electrolyte on an anode supported cell and high conductivity oxide ion conductors such as the doped LaGaO3 and scandia doped zirconia. While the electrolyte resistance decreases, cathode polarization limits the performance of SOFCs at low temperature (below 800 oC). LSCF(La0.6Sr0.4Co0.2Fe0.8O3) is the most candidate cathode. However, it exhibits insufficient electrocatalytic activity at intermediate temperature operation. The addition of noble metals to LSCF is known to improve the physicochemical properties. The purpose of this study is to improve power density of anode-supported flat-tube SOFCs at intermediate temperature such as 600-750 oC by using silver infiltration method on LSCF cathode. Because silver metal has good catalytic activity and high electrical conductivity, it improves the catalytic activity for oxygen reduction. The solution containing Ag was infiltrated into the LSCF cathode. Also Ag-glass composite materials was studied as the interconnect for intermediate temperature. The anode-supported flat-tube SOFC with Ag composite interconnect was manufactured and showed a good performance in a temperature range of 600-750 oC.
Redox-active Metal-organic Framework Supported Palladium Nanoparticles for CO Oxidation: In situ Generated Active Species during Catalytic Reaction
Sang Hoon
Joo1, Hoi Ri
MOF-based catalysis has recently emerged as a burgeoning subfield of heterogeneous catalysis. The catalytic functions of MOFs are endowed by active metal sites and/or reactive organic groups that constitute the frameworks of MOFs. In addition, the incorporation of catalytic metal nanoparticles (NPs) into the cavities of MOFs can also impart catalytic function. Herein we will present the preparation of redox active MOFs supported Pd nanocatalysts via the redox-couple-driven method, and their catalytic applications toward CO oxidation. We prepared Pd NPs on the redox-active Ni-carboxylate-based MOF (Pd@ra-MOF) for CO oxidation. The Pd@ra-MOF was found to be highly active catalyst for CO oxidation. Importantly, we found that the catalytically more active composite, PdO-NiO/C, was generated in situ by thermal transformation of Pd@ra-MOF during CO oxidation reaction, which is stable with repeated reaction runs. We believe that this work provides a new avenue to the MOF-based metal catalysts and can be extended to other MOFs that are constructed with redox active species.
Deconstructing Charge Transport in Thin Film Tantalum Nitride and Oxynitride Photoanodes
Pinaud1, Thomas
Hydrogen production via photoelectrochemical water splitting is a promising energy storage avenue to circumvent the intermittency of solar insolation. No known semiconductor satisfies all material requirements for high efficiency operation and viable options for the oxygen evolving photoanode are particularly sparse. With band edges straddling the water redox potentials, absorption in the visible-light region, and facile synthetic routes, tantalum nitride and oxynitride are attractive photoanode candidates. However, their performance is potentially hindered by several charge transport limitations, including poor (i) bulk charge transport, (ii) charge transport across grain boundaries, and (iii) charge transfer across the interface at the back contact. The primary goal of this work is to understand which of these mechanisms limits overall performance. To answer this question, emphasis is placed on designing an electrode architecture to isolate the losses associated with each aspect of charge transport. Thin films (10 nm – 500 nm) of tantalum nitride (Eg = 2.1 eV) were synthesized by oxidation of tantalum metal in air above 500°C and subsequent nitridation in ammonia above 800°C. Synthesis of a pure tantalum oxynitride (Eg = 2.4 eV) phase is challenging but can be accomplished by nitridation in humid ammonia under the right conditions. Characterization of the photoelectrode morphology by scanning electron microscopy and crystallinity by x-ray diffraction will be presented to support our conclusions. The photoactivity of the films as a function of thickness was studied and relevant metrics such as the absorbed-photon-to-current efficiency will be reported. The relationship between film thickness and electrode performance is complicated by several competing effects that govern charge transport. These effects include minimizing the distance photogenerated charge carriers must travel to be collected, changing the degree of crystallinity, and varying the surface texturing which results in a different electrochemically active surface area. Our results show that the variation in crystal grain size is slight with a minimal impact on performance, while changes in film morphology lead to drastically different photocurrents. Minority charge carrier (hole) transport is believed to be limiting in these tantalum based material systems. Continuing efforts will focus on designing an advanced architecture to maximize the optical absorption while maintaining the optimal charge transport benefits of the very thin film geometry.
Electrocatalytic Reduction of CO2 on Copper Surfaces
Kuhl2, Etosha
Cave3, David
Abram1, Thomas
The development of a cost effective process for the electrochemical reduction of CO2 to fuels and chemicals could enable a shift to a sustainable energy economy. Coupled to a renewable energy source such as wind or solar, such a process could generate carbon neutral fuels or fine chemicals that are conventionally produced from petroleum. The key to developing such a process is the discovery of a catalyst capable of performing the conversion at a low overpotential selectively to the desired product. To gain insight into the factors important to designing better catalysts, we began by studying metallic copper. It is well known that copper is capable of catalyzing the electroreduction of CO2 into hydrocarbons with high current efficiency. To further our understanding of CO2 reduction on copper, we employed an experimental method capable of accurate current and voltage measurement coupled with sensitive product detection. Using a custom electrochemical cell with a large electrode area and small electrolyte volume led to higher concentrations of minor liquid phase products. We used onstream gas chromatography to detect gas phase products and ex-situ liquid phase product analysis with NMR to measure the product distribution at a range of potentials. Our results are in good agreement with past studies which have reported methane, ethylene, CO, formate, ethanol, 1-propanol, allyl alcohol, acetaldehyde, propionaldehyde, acetate, and methanol as products. In addition to the reported products we also detected several minor products that have not been reported before to our knowledge: ethylene glycol, glycolaldehyde, hydroxyacetone, acetone, and glyoxal. This expanded knowledge of the products of CO2 electroreduction on copper and their voltage dependence leads us to consider a possible mechanistic pathway based on the enol tautomers of the multicarbon products being the active species on the electrode surface. Hopefully, a better understanding of CO2 electroreduction on copper can lead to the development of new and better catalysts for this important reaction.
Scalable Wet Chemical Synthesis of Nanostructured Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production
Benck1, Zhebo
Chen1, Thomas
The electrochemical hydrogen evolution reaction (HER), 2H+ + 2e- → H2, is critical to the production of hydrogen fuel via water splitting. The best known HER catalysts are precious metals such as platinum. In order to make electrochemical hydrogen fuel synthesis economically feasible, highly active and stable HER electrocatalysts composed of inexpensive and abundant materials must be developed. Several studies have shown that molybdenum sulfide (MoS2) has potential as a low cost, active HER catalyst, but previous techniques for synthesizing MoS2 catalysts have required ultra-high vacuum or high temperature processing steps, which make these procedures expensive and incompatible with some substrates (1, 2). In order to develop a practical molybdenum sulfide catalyst, further efforts to create scalable synthetic techniques are required. In this study, we developed a simple wet chemical synthesis for a nanostructured molybdenum sulfide catalyst. Unlike previous synthesis techniques, this method requires no high temperature treatment or separate sulfidization step, yields a high density of catalytically active sites, and enables catalyst deposition onto many substrates. To synthesize the catalyst, precursor salts containing molybdenum and sulfur ions were combined in an aqueous acid to create nanoparticles, which were collected via centrifugation. The particles were then deposited onto a glassy carbon disk substrate via drop casting to create a catalyst film. Physical and chemical characterization revealed that the catalyst is an amorphous molybdenum sulfide material. Electrochemical polarization curves collected in a rotating disk electrode configuration showed that the molybdenum sulfide catalyst has good HER activity compared to other non-precious metal catalysts. Electrochemical capacitance measurements were used to determine the catalyst surface area and calculate a lower bound turn over frequency. Finally, electrochemical stability tests indicated that the catalyst degrades after extensive reductive cycling, but some activity was recovered after refreshing the electrolyte. This room temperature, wet chemical synthetic technique successfully created a nanostructured molybdenum sulfide catalyst film with high activity for the HER. Developing strategies to improve the catalyst stability has the potential to further increase the performance of this material. 1. T. F. Jaramillo, K. P. Jorgensen, J. Bonde, J. H. Nielsen, S. Horch and I. Chorkendorff, Science, 317, 100 (2007). 2. Z. Chen, D. Cummins, B. N. Reinecke, E. Clark, M. K. Sunkara and T. F. Jaramillo, Nano Letters, 11, 4168 (2011).
Growth of Monolayer Molybdenum Disulfide on Copper: Access to a Monolayer Direct Bandgap Semiconductor without Exfoliation
Sun1, Wenhao
Lu1, Daeho
Kim1, Quan
Ma1, Chen
Wang1, John
Mann1, Ludwig
MoS2 is a semiconducting material consisting of sulfur-molybenum-sulfur tripledecker layers loosely bound by van der Waals interactions. Through its bandgap of 1.9 eV at the monolayer limit and band alignment, it is a material with great potential for photocatalytic water splitting. Single layer MoS2 can be exfoliated mechanically similar to graphene. While this method is simple, it is hard to control and not amendable to mass production of thin films. Solution-based processes have been proposed and may provide a scalable source of a mixture of single and multilayer material. Here we show an alternative avenue for the fabrication of MoS2 monolayers: growth of MoS2 on a sulfur-preloaded copper surface. In contrast to all other methods, this route has the potential of providing exclusively monolayer material, as the sulfur source is only available until the substrate is covered. Practically, this approach is related to the growth of graphene monolayers on copper or ruthenium films, where segregation of carbon to the surface is employed in aggregating a carbonaceous layer that transforms into graphene under the correct conditions. Small MoS2 triangles of a few nanometers in size have been grown previously on gold in a dilute H2S athmosphere. Here we show significantly larger patches, tens of nanometers in size. In contrast to gold, copper forms a multitude of sulfur surface coverages and also readily absorbs sulfur into the bulk. Thus, we can preload the substrate with a specific amount sulfur using an easy to handle liquid precursor, benzenethiol. In previous work we have shown that heating to below 400K removes the phenyl group of benzenethiol reliably from copper leaving sulfur coverages behind. STM imaging shows that MoS2 flakes can grow across substrate step edge which is a critical requirement for large size CVD growth on a metal substrate. The flakes exhibit a Morie pattern due to lattice mismatch between the MoS2 and the Cu substrate; this can be used to locate the position of underlying Cu substrate at a atomic resolution.
U9.24 Transferred to U4.1
Reversible Formation of Palladium Carbide on Palladium Nanoparticles under CO Oxidation Conditions
Balmes1, Andrea
Resta1, Didier
Wermeille1, Roberto
Felici1, Maria
Messing2, Knut
Deppert2, Zhi
Liu3, Michael
Grass3, Hendrik
Bluhm5, Sara
Blomberg4, Johan
Gustafson4, Rasmus
Westerstrom7, Jesper
Andersen4, Edvin
Lundgren4, Richard
van Rijn6, Joost
The structure and chemical composition of Pd nano particles (15 and 35 nm) exposed to pure CO and mixtures of CO and O2 at elevated temperatures have been studied in situ by a combination of X-ray Diffraction and X-ray Photoelectron Spectroscopy in pressures ranging from ultra high vacuum to 10 mbar. In this talk, the X-ray Diffraction results will be presented. Our investigation shows that under CO exposure in a flow reactor, the lattice parameter of the nanoparticles change from the nominal parameter of palladium to a larger lattice parameter. This lattice parameter change is also observed when the gas flow is composed of CO-rich CO/O2 mixtures (with respect to stoechiometric ratio for CO2 production). The lattice parameter decreases back to nominal Pd value when the CO/O2 ratio is under stoechiometric. This lattice parameter change, which is reversible, is consistent with carbon dissolving into the Pd particles forming PdCx [1,2]. This phenomenon demonstrates that dissociation of CO on Palladium is possible, and that the excess carbon readily dissolves into the lattice. This result contrasts with the results obtained on single crystals, where CO dissociation was not observed,[3] and is an argument in favor of the existence of a material gap. 1 M. Maciejewski and A. Baiker J. Phys. Chem. 98, 285-290 (1994) 2 N. Seriani, J. Harl, F. Mittendorfer, G. Kresse J. Chem. Phys. 131, 054701 (2009) 3 V. V. Kaichev, I. P. Prosvirin, V. I. Bukhtiyarov, H. Unterhalt, G. Rupprechter, H-J. Freund J. Phys. Chem. B 107, 3522-3527 (2003)
CoTiO3 as a Catalyst for Oxygen Evolution and Solar Water Splitting
Seitz1, Thomas
Solar energy provides an abundant potential source of renewable clean energy provided there exists an efficient method of energy storage. Photocatalytic water splitting can be used to store solar energy in the form of chemical bonds, particularly those of hydrogen (H2) which can then be used as a fuel. However, the water splitting reaction is severely limited by the high overpotential costs of the water oxidation half reaction to produce oxygen (O2). Furthermore, there exists a need for a non-precious metal catalyst to drive the oxygen evolution reaction (OER) at reasonable overpotentials to make the overall water splitting process more economical. Focusing on non-precious transition metal oxides as potential catalysts, we have identified CoTiO3 as a novel catalyst for OER. Attempting to combine the stability of TiO2 with the high OER activity of cobalt oxide materials, this material presents itself as promising catalyst candidate. We began this study by characterizing the structure, morphology, oxidation state, and OER activity of CoTiO3 in the absence of light. Results show varying effects on the crystallinity and morphology of this material depending on synthesis conditions. Overall, it was determined that this material catalyzes the oxygen evolution reaction with overpotentials nearly comparable to those of RuO2 and IrO2, the best known precious metal catalysts for this reaction. Photospectral characterization of this material also revealed synthesis-dependent differences in its optical properties. Limited amounts of photocurrent were measured from this material with the potential to be optimized given appropriate nanostructuring.
U9.28 Transferred to U7.5
The Implementation of Three-dimensional Inverse Opal Transparent Conducting Oxide Electrodes in the Photoelectrochemical Oxidation of Water Using WO3 and Fe2O3
Moir1, Navid
Soheilnia1, Abdinoor
Jelle1, Geoffrey
Since the first report of the photocatalytic splitting of water in 1972(1), many attempts have been made to increase the efficiency of the reaction, as the oxidation/reduction of water is kinetically very difficult, and thermodynamically unfavourable.(2) Examples include doping of semiconductors, band-gap engineering, nanostructures, heterostructures, surface plasmon resonance enhancements, and many more.(3) Despite these efforts, however, the development of a single semiconductor material that is suitably robust, cheap and efficient, and that absorbs a large portion of the solar spectrum, has been elusive. One of the reasons for this difficulty is the short charge diffusion length of many of these semiconductor materials. As layered materials increase in thickness, the amount of light they absorb increases as well. Unfortunately, this also means that there is a greater path length that the photoinduced charge carrier must travel in order to reach the electrode, resulting in increased recombination and grain boundary losses. One method of circumventing this issue is to construct a three-dimensional transparent conducting oxide (TCO) inverse opal electrode.(4) By depositing the water oxidation catalyst on the surface of this transparent three-dimensional structure, one can maintain a high light absorption by the catalyst while improving the charge collection efficiency of the material. We report our findings on the application of antimony-doped tin oxide (ATO) inverse opal architectures for the photoelectrochemical oxidation of water using WO3 and Fe2O3 catalysts. Polystyrene spheres were synthesized using a free radical polymerization method, and used as templates in the formation of ATO inverse opals using an ATO nanocrystalline solution precursor, followed by calcination at high temperatures. The WO3 and Fe2O3 materials were then deposited using sol precursors, and the materials were tested using linear sweep voltammetry in acidic and basic electrolyte solutions. A significant improvement in activity was observed for the catalysts on the ATO inverse opal electrodes when compared to the respective inverse opal catalysts without ATO and the planar thin film morphologies. Further studies will be required, however, on the optical properties of these materials, in order to avoid confusion over the source of the enhancement in the activity.(5) References 1. Fujishima, A., and Honda, K. (1972) Nature. 238: 37-38. 2. Walter et al. (2010) Chemical Reviews. 110: 6446-6473. 3. Barreca et al. (2011) Advanced Functional Materials. 21: 2611-2623. 4. Yang et al. (2011) ACS Advanced Materials and Interfaces. 3: 1101-1108. 5. Chen et al. (2011) ACS Nano. 5: 4310-4318.
Local Probing of Activation Energy of Ionic Transport
Jesse1, Nina
Balke1, Nancy
Dudney1, Sergiy
Kalnaus1, Claus
Daniel1, Sergei
The functionality of energy storage and generation systems like Li-ion batteries or fuel cells is not only based on but also limited by the flow of ions through the device. To understand device limitations and to draw a roadmap to optimize device properties, the ionic flow has to be studied on relevant length scales of grain sizes, structural defects, and local inhomogeneities, i.e. over tens of nanometers. Knowledge of the interplay between the ionic flow, material properties, and microstructure can be used to optimize the device properties, for example to maximize energy density, increase charging/discharging rates, and improve cycling life for Li-ion batteries for applications in electric vehicles and aerospace. Until recently, existing solid-state electrochemical methods were limited to a spatial resolution of ~10 um or greater, well above the characteristic size of grains and sub-granular defects. Our development of Electrochemical Strain Microscopy (ESM) has reduced this resolution limit to length scales down to 100 - 10 nm which allows studies of the local Li-ion flow in electrode materials and across interfaces. Here, we present how ESM can be used to measure ionic transport properties and extract spatially resolved maps of the activation energy in LiCoO2 thin film cathodes for Li-ion batteries. The ionic transport is measured using the coupling between strain and material volume and temperature-dependent ESM measurements allow to extract the activation energy for ionic transport on the nanoscale which will be correlated with the microstructure of the cathode film. Theoretical calculations are shown to support the experimental data and to give insight into the signal generating mechanism. Research was sponsored as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Research was conducted at the Center for Nanophase Materials Sciences at Oak Ridge National Laboratory, which is sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy. Part of this research was sponsored by the Vehicle Technologies Program for the Office of Energy Efficiency and Renewable Energy.
Catalyzing the Oxygen Reduction Reaction (ORR) with Thin Pt Films on Ru
Jackson1, Peter C. K.
Vesborg1, Thomas
Platinum is known to be one of the best catalysts for the oxygen reduction reaction (ORR). Improvements over the catalytic activity of Pt have come by modifying the adsorption strength of ORR reaction intermediates to be slightly weaker than pure Pt, to approach the optimum binding energy. This has been demonstrated with Pt alloy catalysts such as Pt3Y, Pt3Ni, and other transition metal alloys. In order to be of practical use in a fuel cell, the catalysts are prepared as nanoparticles to take advantage of the much higher surface area. The drawback in using nanoparticles, however, is the high number of under-coordinated sites that bind oxygen too strongly, reducing the catalytic activity. This follows the general correlation that the lower the coordination number of a surface catalytic site, the stronger the adsorption bond. It has been shown that thin metal overlayers can exhibit properties different from the metal in its bulk form, creating the potential for higher ORR activity and greater tolerance to surface variations. The purpose of this study is to investigate thin films of Pt on Ru supports. We synthesized these structures by a variety of methods including reduction of Pt onto carbon supported Ru nanoparticles, sequential reduction of Ru and Pt by polyol solvents, and electrodeposition. They have been characterized to verify composition and structure and tested for ORR activity. We will discuss the relationship between different structures and their catalytic activities.
Nanocatalyst for Selective Hydrogenation of Alkyne to Alkene
Kwon1, Elena
Shevchenko1, Galyna
Krylova3, Emilio
Bunel2, Christopher
Marshall2, Julius
Jellinek2, Sumer
We have optimized the synthesis of Pt and Co-Pt alloy nanoparticles (NPs) and investigated their catalytic properties. We have carried out room temperature hydrogenation of octyne as model reactions and ex-situ characterization of the products. We found that our NPs were highly efficient as catalyst compared to conventional Pt particles deposited on carbon or chemically synthesized Pt NPs. We investigated the effect of ligands adsorbed onto the surface of NPs on the catalytic reactions. Despite the common perception about the "bad†influence of surface ligands on the catalytic properties of NPs, we see that certain types of ligands can improve selectivity significantly without decreasing the catalytic activity. In hydrogenation of octyne, we observed that primary amines significantly improved the selectivity for alkene. In the best conditions, we were able to control the selectivity for alkene from zero to > 90% with the conversion yield fixed at around 100%. Note that the blank experiment with amines without NPs ruled out the possibility of direct effect of amines on this reaction. To investigate the effect of amines on the selective hydrogenation, we carried out various experiment to study the relative binding strength of alkyne, alkene, and amines. To investigate the catalytic functionality of NPs mentioned above, DFT computations on the adsorption of various ligands, including molecular and atomic hydrogen, on the surfaces of pure and Co-doped Pt clusters were carried out and the result was compared with the experimental data. Our computations indicate that in the case of Pt/Co nanocatalysts atomic hydrogen binds preferentially to Pt. The energetics of this binding is, however, substantially affected (reduced) by the presence of Co. Work is progress on evaluation of the adsorption characteristics of other ligands, including C4H6 as a prototype of a highly unsaturated hydrocarbon. The goal of this study is to develop the next generation nanocatalyst with high selectivity for a desired degree of hydrogenation.
Electrochemical Reduction of CO2 on Modified Gold Surfaces
Cave2, Kendra
Kuhl3, David
Abram1, Thomas
The efficient electrochemical reduction of CO2 to fuels could be a viable means to store electricity generated by renewable technologies such as solar cells or wind turbines. Almost all metals have the ability to electrochemically reduce carbon dioxide at low temperatures, however, most do so with low current efficiencies for carbon based fuels or at high overpotentials [1]. Gold has previously been shown to produce carbon monoxide with faradaic efficiencies around 90%, as well as formate with ~1% faradaic efficiency [2]. CO2 reduction, as with many electrochemical reactions, is often dependent upon the electrode surface structure and preparation. Thus, this presentation will focus on enhancement of the activity and product selectivity of CO2 reduction on gold by changing the topology of a polycrystalline gold surface. In this study, the surfaces of gold foils were roughened and then tested for the electrochemical reduction of carbon dioxide. Testing was performed in a 3-electrode, 2-compartment compression cell separated by an anion exchange membrane. Gas product analysis was achieved by a gas chromatograph, liquid products by NMR. The gold foils were characterized by their roughness. A variety of methods were employed: surface roughness was calculated with charge transfer measurements and electrochemical activity was studied using cyclic voltammetry in a 3-electrode electrochemical compression cell. CO2 reduction activities were measured with a continuous flow of CO2 in a CO2 saturated 0.1M potassium bicarbonate solution at 23°C. An anion exchange membrane was used to prevent liquid products from being oxidized at the counter electrode which consisted of a platinum foil. A Ag/AgCl reference electrode was used during experimentation. Potentials were adjusted post experimentation to the reversible hydrogen electrode (RHE). Potentials were also adjusted 100% for uncompensated resistance. Hydrogen, carbon monoxide and methanol were the main products formed from the electrochemical reduction of CO2 on gold. This paper will describe our efforts to modify the surface of gold and how such modifications translate to differences in activity and selectivity for CO2 reduction. We will also discuss the physical and chemical properties of the surface that could give rise to such changes. REFERENCES [1] M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe and T. Sakata, Journal of The Electrochemical Society 137 (1990) 1772. [2] Y. Hori, H. Wakebe, T. Tsukamoto and O. Koga, Electrochimica Acta 39 (1994) 1833.
Translating an Active Bi-functional Thin-film MnOx Catalyst for Oxygen Reduction and Water Oxidation to a Fuel Cell Environment
Ng1, Yelena
Gorlin1, Thomas
The world's energy demand has been rapidly increasing over the past few decades; however, traditional sources of energy such as oil and coal are running out and will not be able to keep pace with the demand. One solution to this problem is the development of regenerative fuel cells which are able to use renewable electricity (e.g. wind and solar) to split water and form H2, and when necessary, reverse operation and use the H2 to produce electricity.1 However, current regenerative fuel cells use Pt and Pt/Ir catalyst which are very expensive; hence there is a need to develop alternative oxygen electrocatalysts comprising abundant materials. Manganese oxide (MnOx) electrocatalysts are potential candidates for reversible oxygen catalysis in a regenerative fuel cell due to its high activity, low toxicity and low cost.2 Previously, we have synthesized thin films of nanostrucutred MnOx via electrodeposition, and the activity for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is comparable to that of the best known precious metal catalysts such as Pt, Ir and Ru.3 However, the MnOx films are deposited onto a glassy carbon (GC) disk and in their current form, are not suitable for use in a regenerative fuel cell. Hence the goal is to translate this active catalyst to a fuel cell environment. The high temperature calcination involved in the synthesis procedure necessitates a support that is heat-resistant. GC particles are appropriate due to their high temperature resistance, high corrosion resistance which is needed due to the harsh alkaline testing environment, and high conductivity to enhance electron transport to and away from the catalyst surface. MnOx was deposited onto GC particles via an impregnation technique followed by calcination, which resulted in a nanostructured surface dominated by Mn2O3 as determined from SEM and XPS analysis. Electrochemical testing in a rotating disk electrode setup revealed that the ORR activity is similar to the MnOx thin films, while the OER activity is only slightly lower. However, the lower OER activity can be overcome via Ni or Co doping. The synthesis procedure was extended to form NiOx-GC and CoOx-GC particles, which exhibited lower ORR activity but much higher OER activity. The active MnOx-GC particles were then loaded onto carbon paper to form a gas diffusion electrode which can be used in an actual fuel cell. References 1. K.A. Burke, International Energy Conversion Engineering Conference. 2003, AIAA 2003-5939 2. J.O.M. Bockris, Int. J. Hydrogen Energ. 1999, 24, 15 3. Y. Gorlin, T.F. Jaramillo, J. Am. Chem. Soc. 2010, 132, 13612
Role of Pt Nanoparticles in Photoreactions on TiO2 Photoelectrodes
Woo Jin
An1, Wei-Ning
Wang1, Balavinayagam
Ramalingam2, Somik
Mukherjee2, Shubhra
Gangopadhyay2, Pratim
Titanium dioxide (TiO2) has been widely used as a photocatalytic material for solar energy applications. To deposit nanostructured metal oxide films with controlled morphologies, an aerosol-chemical vapor deposition (ACVD) method was developed.1 The system is a simple, one-step process operating at ambient pressure. It has been noted that besides intrinsic properties, the morphology of metal oxide films plays a significant role in determining the efficiency of solar energy applications. Along with the film morphology, surface modification by noble metal contacts can improve electrochemical properties of metal oxide films. The size and number density of noble metal particles and the distance between noble metal particles play a significant role in determining the efficiency of solar energy applications. For this study, the room temperature RF (radio frequency) magnetron sputtering method2 was employed to deposit monodispersed nano-sized platinum (Pt) metals on the columnar TiO2 films. Uniform nanoparticle size could be achieved from 0.5 to 3 nm having a high particle density (>10E12 cm−2) by varying deposition time with constant pressure and power intensity. As-synthesized Pt-TiO2 films were used as photoanodes for splitting water and for photocatalytic conversion of CO2. Pt nanoparticles deposited on the TiO2 film for 20s produced the highest photocurrent and maximized the energy conversion efficiency. The Pt particles, with a size of approximately 1 nm, reduced electron-hole recombination rates. However, as the size of Pt particles increased, more trapping sites for photogenerated electron-hole pairs caused the decreased photoreaction. REFERENCES 1. An, W.-J., Thimsen, E., and Biswas, P. Aerosol-Chemical Vapor Deposition Method for Synthesis of Nanostructured Metal Oxide Thin Films with Controlled Morphology. J. Phys. Chem. Lett. 1, 249-253 (2010). 2. Yun, M., Maruf Hossain, D.W.M., Misra, V., and Gangopadhyay, S. Sub-2 nm Size-Tunable High-Density Pt Nanoparticle Embedded Nonvolatile Memory. IEEE Electron Device Lett. 30, 1362-1364 (2009).
Metal Oxides as Catalysts Promoters for Methanol Oxidation
Kolla1, Chuilin
Lai1, Srujan
Mishra2, Mark
Miller3, Phil
Ahrenkiel2, Rajesh
Shende3, Alevtina
Direct Methanol Fuel Cells (DMFC) are being considered as commercial alternatives to hydrogen/air system in portable and automotive industries. However, high activation energy of methanol oxidation requires high anode catalyst loadings when compared to hydrogen oxidation in PEMFC. While Pt-Ru based materials are well known as anode catalysts for methanol oxidation, promotion of these catalysts with non-noble metal oxides [1, 2] is considered as one of the ways to make DMFC viable. Some metal oxides are known to show electrochemical catalytic activity because of metal ability to switch between different valences [3]. These metal oxides are also known to exhibit metal-like conductivity when partially filled d- and f-bands are available. In the present work, the catalytic activity of metal oxides such as CeO2, TiO2, MoO2, and NiCoO4 dispersed on carbon aerogel was studied in the reaction of methanol oxidation. These oxides were also studied in combination with Pt-Ru/C. A modified sol-gel Pechini method with glycine as complexing agent was used to deposit 10-13 wt. % of metal oxide nanoparticles within carbon aerogel or commercial catalyst structure. These catalyst powders were annealed in air at 350oC and in N2 at 6000C and 9000C and characterized using XRD, HRTEM, H2-chemisorption, BET, RDE cyclic voltammetry, and chronoamperometry, as a function of catalyst loading and annealing temperature. The crystalline structure of 2-5nm metal oxide nanoparticles derived from HRTEM was confirmed by XRD. The total surface area of the CA-metal oxide powders decreased from 800m2/g to 600m2/g in presence of metal oxide. The surface area of the commercial catalyst without and with 10wt.% of metal oxide was about 150-200m2/g. Cyclic voltammetry of metal oxides in 0.1M HClO4 containing 0.1M-3.0M CH3OH demonstrated catalytic activity towards methanol oxidation. A significant synergic effect was observed when ceria and titania were combined with the state-of-the-art PtRu/C Tanaka catalyst. Sintering of metal oxides on Pr-Ru/C at 600oC resulted in a lower activity towards methanol oxidation. Further studies on complex metal oxide nanoparticles, optimization of catalyst structure, and catalytic activity in DMFC will be presented. Reference (1) M. Aulice Scibioh, Soo-Kil Kim, Tae-Hoon Lim, Seong-Ahn Hong, Heung Yong Ha, Pt-metal oxide Anode Electrocatalysts for Direct Methanol Fuel Cells, ECS Transactions, 6 (13) 93-110 (2007). (2) Roderick E. Fuentes, Brenda L. Garci, John W. Weidner, Effect of Titanium Dioxide Supports on the Activity of Pt-Ru toward Electrochemical Oxidation of Methanol, Journal of The Electrochemical Society, 158 (5) B461-B466 (2011). (3) B. Viswanathan, Ch. Venkateswara Rao, U. V. Varadaraju, On the search for non-noble metal based electrodes for oxygen reduction reaction, Energy and Fuel, 43-101(2006).
Examination of the Relative Catalytic Activities of Undoped and Cobalt-Doped Barium Cerate-Zirconates towards Methanol Partial Oxidation
Suresh1, Barry
Carter1, Benjamin
Doped barium cerate-zirconates [Ba(Ce,Zr)O3] have been widely investigated as high-temperature protonic conductors. The solid solution of barium cerate and barium zirconate enables an optimum balance between the relatively high protonic conductivity characteristic of the cerate and the chemical stability against CO2 characteristic of the zirconate. The material has been used as an electro-ceramic in various applications such as solid-oxide fuel cell electrolytes and hydrogen-separation membranes. However, the catalytic properties of the material have remained largely neglected in the literature. Recently, Suresh et al. [1] reported the catalytic activity of Ba(Ce,Zr,Co)O3-δ towards hydrogen generation from CH3OH partial oxidation, which was attributed to the presence of Co in the material. The aim of the present study is to examine that assumption and, in the process, identify the inherent catalytic activity of undoped Ba(Ce,Zr)O3. Any observable catalytic activity in the undoped material would open a new avenue in the investigation of doped BaCeO3, BaZrO3 and BaCeZrO3, which has thus far been restricted to the field of electrochemistry. Powders of undoped BaCe0.25Zr0.75O3 and BaCe0.25Zr0.75-xCoxO3-δ were synthesized using solid-state reaction and characterized using X-ray Diffraction (XRD) and BET. The materials were then subjected to CH3OH partial oxidation in a packed-bed reactor at different temperatures and O2:CH3OH ratios. A gas chromatograph was used to analyze the reaction products on a dry basis in order to quantify the catalytic activity of the materials. [1] Suresh et al., Journal of Materials Science, 45 (2010) 3215
Sonochemical Synthesis of Carbon Supported Pt Containing Bimetallic Nanoparticles and Their Electrochemical Applications
Jang1, Eunjik
Lee2, Young-Uk
Kwon1 2 3.
We report on the preparation of carbon supported Pt containing bimetallic nanoparticles (NPs) with electrocatalytic activity for fuel cell applications. To form the bimetallic NPs, we chose 3d transition metals such as Co and Fe. In order to synthesize the NPs, sonochemical syntheses method was used. Ultrasound irradiation into carbon support, Pt(acac)2 and Fe(acac)3 or Co(acac)2 dispersed polyol solution could generate a reducing condition of the precursors which resulted in the formation of bimetallic NPs on carbon support. The structures of the nanoparticles were characterized by XRD, XPS, IR, SEM-EDS, HRTEM and STEM-HAADF. The NPs show narrow size distribution with averaging size of about 2 nm and no macroscopic phase segregation. Electrocatalytic oxygen reduction reaction (ORR) behavior of the materials was measured by rotating disk electrode (RDE) technique and compared with commercial Pt/C (E-TEK, 20 wt% and TKK, 37.7 wt%). As a result, we obtained an enhanced electrocatalytic ORR activity than commercial Pt/C.
Mechanisms of CO2 to CO Conversion at Electron-rich Surfaces of 12CaO.7Al2O3
Torrisi1, Yoshitake
Toda2, Sung-Wng
Kim2, Alexander
Shluger1, Hideo
Hosono2, Peter
Low cost conversion of gas phase CO2 into CO is an important step in artificial fuel synthesis. Here we investigate theoretically a possibility and mechanisms of such conversion at surfaces of the stoichiometric and oxygen-deficient subnanoporous complex oxide 12CaO.7Al2O3 (C12A7). C12A7 is formed by a positively charged framework consisting of 12 cages per cubic unit cell and compensated by extra-framework oxygen anions. Thermal treatment of C12A7 in reducing conditions results in formation of a so-called electride state, in which extra-framework anions are replaced with electrons. Recent experimental [1] and theoretical [2] studies indicate that intact bulk-like cages are present at or near the C12A7 surface. The results of our density functional calculations suggest that the near surface cages are capable of trapping and releasing electrons and oxide species. In the case of oxygen deficient C12A7, adsorption of a CO2 molecule near an empty cage can result in decomposition of the molecule, so as an oxygen atom is trapped in the cage and the CO fragment is physisorbed at the surface with the overall energy gain of over 1.2 eV per molecule. We have investigated several pathways of such decomposition and found that interstitialcy-like oxygen in-diffusion process has the lowest activation energy of 0.8 eV, which suggests a possibility of "cold†CO2 to CO conversion at such surfaces. [1] Y. Toda, Y. Kubota, M. Hirano, H. Hirayama, H. Hosono, ACS Nano, 5, pp 1907–1914 (2011). [2] P. V. Sushko, A. L. Shluger, Y. Toda, M. Hirano, H. Hosono, Proc. Royal Soc. A 467, 2066-2083 (2011).
In situ Nanoscale Observation of Photocatalysts under Visible and UV Irradiation
Miller1, Peter
Inorganic photocatalysts are currently being intensely studied for their potential use for the production of fuels from H2O and CO2. Designing new efficient photocatalysts requires an increased understanding of the link between catalyst microstructure and activity. Transmission electron microscopy (TEM) is a well established and powerful technique for studying the structure of materials at the nanoscale. However, the environment of the TEM, namely vacuum, room temperature, and total darkness, makes it difficult to correlate the structures observed with experimentally determined activities of the catalysts studied. Thus, environmental TEM (ETEM) is sometimes used to more closely mimic the conditions experienced by a material in use. However, while gaseous environments and variable temperatures are common to ETEM work, illumination of the sample by visible, ultraviolet, and infrared light is much less common. A critical experimental condition is therefore usually absent from current TEM studies of photocatalysts. We have installed a variable wavelength light source to irradiate the sample area of an ETEM column. The current design consists of a broadband light source with filters, optical fibers with a vacuum feedthrough, and a manipulator to precisely position the fiber tip with respect to the TEM sample in the microscope. This apparatus is able to illuminate a photocatalyst sample inside the TEM column with up to 3mW/cm2/nm, from 300-800nm, which is several times the average spectral irradiance of the sun on the Earth's surface over the same range. Intensities above 1mW/cm2/nm are possible down to 200nm. We are using this new capability to study the structure of titania based nanostructured catalysts, such as nanotubes, nanowires and high surface area powders. We are also working on synthesis and characterization of these same materials functionalized with metal nanoparticles.
Using Nanostructure to Predict Surface Energy in Platinum Fuel Cell Catalysts
Redmond3, Brian
Setzler3, Amir
Mazaheripour1, Pavol
Juhas2, Simon
Billinge2, Thomas
With their high surface-to-volume ratio, ultra-small platinum nanoparticles exhibit extremely desirable catalytic behavior in low-temperature proton exchange membrane fuel cells (PEMFCs). However, catalyst degradation is a critical issue that limits the lifetime of PEMFCs (1, 2), and these nanoparticles are no exception. Better understanding of each degradation process will allow for better mitigation techniques and the development of new materials. One of the main processes contributing to electrochemically active surface area (ECSA) loss of catalysts is nano-scale Ostwald ripening, a process driven by surface energy differences (1-3). A reliable method of determining these surface energies involves measuring the size-dependent bond strain of the nanoparticles, since this data can be correlated to a surface energy constant (6, 7). However, at the scale of ultra-small nanoparticles—less than 3 nm in diameter—2 major problems appear: the increased local disorder from a high percentage of surface atoms enfeebles traditional XRD analysis techniques, and the emergence of non-bulk properties questions the validity of a surface energy 'constant'. Here we utilize the pair distribution function (PDF) to analyze high energy x-ray powder diffraction data for carbon supported nanoparticles of varying diameters. The PDF method has accurately extracted structural information in other materials of this diameter scale (8). We report our values for size-dependent internal bond strain and show evidence favoring a size-dependent surface energy for nanoparticles in the sub-3 nanometer regime.
In-situ Environmental TEM study on Cu, Au-Cu, and Pt-Cu Alloy Nanoparticles Supported on SiO2 under Reduction Conditions
Diaz Rivas1, Eric
Stach1, Jeff
Miller2, Wu
It has been shown that Cu and Cu alloy nanoparticles supported on inactive ceramics are broadly used for many applications such as selective hydrogenation, oxidation, and methanol formation. The catalytic activity of such supported nanoparticles is related to their particle size, morphology, and electronic properties. It is important to understand the interaction between metals in both Au-Cu and Pt-Cu alloy nanoparticles, as well as the interaction between Cu and the inactive ceramic support. Here we use in-situ environmental transmission electron microscopy to characterize the SiO2 supported Cu and Cu alloy nanoparticles and to study the oxidation state of the reduced Cu and Cu alloy nanoparticles. We show that in-situ electron energy loss spectroscopy is a powerful technique to study the valence state and bond formation on these supported metal catalyst.
Porous Nickel Nanospheres with Tunable Structures through the Aerosol Assisted Route
Sohn1, Qiangfeng
Xiao1, Yunfeng
Porous metallic nanostructures with tunable morphologies have intrigued us in the general synthesis of functional materials owing to their wide variety of applications in catalytic, optical, electronic, magnetic, sensing, photocatalyst, energy storage devices and bio-device ranging from energy storage to drug-delivery carriers. In this context, various synthetic approaches (e.g. template-directed and non-template method) have been intensively explored to construct porous nanostructures in various morphologies (e.g., hollow and yolk-shell) in convenient and efficient ways. Nevertheless, there has not been reported for the fabrication of porous non-oxide metal with tunable structure through the aerosol pyrolysis. Herein, we present a generalized approach to construct 3D porous nanoarchitecture (porous nickel) with tunable structure through aerosol process. Morphological regulation was achieved by 1) control of thermal behavior of precursor's ligands and inorganic salt and by 2) control of the solubility of precursor. Controlling the thermal behavior of precursor was performed by employing various inorganic salts and organic ligands with different thermal degradation behavior. The solubility of precursor was controlled by changing the ratio of inorganic salt to organic ligands or by addition of acids with different pKa to the precursor solution. After analysis on the morphology, crystalline, and pore structure of porous nickel, the effect of morphological difference on the catalytic property (propylene hydrogenation reaction) was evaluated where porous nickel with different morphologies were employed as catalysts. Catalytic tests exhibit that the catalytic activity of nickel depends on the structural factors (surface area, crystallinity (grain size) and morphology), morphology (hollow/non-hollow) and elemental compositions (nickel/carbon amount) of catalyst. Since this synthetic process can be generalized by employing different chemicals (e.g., metal salt and organic ligand) and aerosol process condition, various species of porous metallic nanosphere can be prepared. The porous metals prepared by current approach are also applicable to broad spectrum of practices. For example, such porous metals can be used as hydrogen storage since they have capability to store and release the hydrogen at enhanced kinetics. We believe that this new strategy would pave the noble way for the fabrication of high performance catalysts in facile and cheaper way.
CuPt Alloy Nanoplates as Novel Oxygen Reduction Reaction Electrocatalysts
Larsen1, K.
Neyerlin1, Svitlana
Pylypenko1, Shyam
Kocha1, Bryan
CuPt alloys have demonstrated improved intrinsic activity as ORR (oxygen reduction reaction) electrocatalysts compared to conventional nanoparticle-based Pt materials. Electrocatalysts possessing shape anisotropy, such as large-area planar surfaces and 1-d nanowires, have also demonstrated enhanced intrinsic activity. In this work, CuPt nanoplates are presented as a novel ORR electrocatalyst with thorough materials and electrochemical characterization. Detailed experimental synthesis and characterization methods will be presented.
Nanocatalyst Structure as a Template to Define the Chirality of Nascent Single-walled Carbon Nanotubes
Gomez-Gualdron1 2, Perla
Balbuena2 1.
The outstanding electronic and optical properties of single-walled carbon nanotubes (SWCNT) make them a promising candidate to lead the creation of a new generation of small, fast and energy-efficient electronic devices. Remarkably, these properties can be controlled, provided that control on structural features of the nanotubes (i.e. chirality) is achieved. However, current synthesis processes do not possess such level of sophistication. Chemical vapor deposition (CVD) is widely regarded as the most viable option for industrial-scale nanotube production. In this process, nanotubes grow on metallic nanoparticles that catalytically decompose the carbon-containing precursor gas, and act as a support of the growing nanotubes. Recently, it has been proposed that since the nanoparticle and the nanotube are in contact during the growth process, it is possible to control the nanotube chirality by controlling the nanocatalyst structure, through a template effect. Here we use computational tools such as density functional theory (DFT) and classical reactive molecular dynamics (RMD) to investigate whether a structural relation between the nanocatalyst and the nanotube at different stages of growth can be detected. Firstly, DFT is used to optimize a set of nanotube caps and nanoparticles revealing that nanoparticles adopt a particular geometry depending on the chirality, but also showing the likelihood of an inverse template effect on an unsupported nanoparticle. Also, DFT optimizations are used to study the impact of the nanoparticle surface structure on the nucleation of carbon structures using model surfaces Co(211) and Co(321), where the preference for armchair (ac) or zigzag (zz) structures is demonstrated to depend on the surface structure. Secondly, RMD is used to simulate the growth of nanotubes (at 1000 K) on supported nanoparticles of several sizes at various nanoparticle/substrate interaction strengths, where the latter is shown to alter the dynamic and structural behavior of the nanoparticle, thus demonstrating the effect of changing the support. On the other hand, the arrangement of the nascent nanotube –and related nascent carbon structures – is found to be continuously influenced by the underlying surface structure of the nanoparticle supporting the template effect hypothesis. However, the effectiveness of such template effect is found to depend on the interaction with the support, and on the nanoparticle size, which are shown to affect factors such as atom mobility, and site occupation among others. At weak interactions, defect annealing is facilitated, but a cooperative inverse/direct template effect is observed. At strong interactions, the inverse template effect is eliminated, but detriment of defect annealing hinders the structural matching between nanotube and nanoparticle. Our results suggest that controlling the nanotube structure via a template effect is plausible, provided that the right synthesis conditions are found.
Microstructure of Low-Pt-loaded Catalysts Dispersed via a Dry One-step Process onto Corrosion-resistant Supports
Roller1 2, M.
Arellano-Jimenez1, Radenka
Maric1 2, C. Barry
Carbon-supported Pt electrocatalysts used in a PEMFC catalyst layer are arguably the most significant component affecting cost, performance, robustness and durability of the membrane electrode assembly (MEA). Conventional MEAs currently in use are based on finely dispersed Pt nanoparticles supported on carbon black and dispersed as ink. Corrosion of the carbon support leads to poor durability and unacceptable lifetimes. During ink fabrication, the colloidal solution of carbon/Pt and ionomer self-organizes into phase-segregated regions with interpenetrating percolating phases for the transport of electrons, protons, and gases. The process of microstructure formation depends on the type of catalyst support, the type and amount of ionomer added, the type of dispersion medium used during ink preparation, and the fabrication conditions. Limitations to this approach have been observed in the past few years, suggesting that a new approach will be required to meet the targets set for successful commercialization. For catalyst layers, the main objective is to obtain the highest current density with respect to the desired electrochemical reactions using a minimum amount of the Pt catalyst (DOE target for 2010: 0.29 gPt/kW). This requires a large active surface area with appropriately engineered microstructure, optimal orientation of the Pt crystal facets, small kinetic barriers to bulk transport and interfacial transfer of protons, electrons and reactant gases, and proper management of product water and waste heat. In order to address these challenges, our research is focused on the fabrication of thin, low Pt loaded catalysts by Reactive Spray Deposition Technique (RSDT) [1]. This one-step direct catalyst coated membrane (CCM) process enables a decoupling of all three catalyst layer components (Pt, carbon, and nafion). The ability to introduce components separately into the hot-dry process stream allows for flexibility in manufacturing hereto unavailable via wet processing techniques. Observations on the RSDT catalysts will be presented. The thermal history of the forming catalyst particle is expected to affect the crystal structure of the formed catalyst. Particle size, distribution, and dispersion on carbon support ranges from adatoms to highly crystalline particles due to sublimation and subsequent coarsening in the process stream. Further work has been done to disperse the catalyst onto supports other than Vulcan XC-72R that show more promising durability. In this work the microstructure and oxygen reduction reactivity will be examined on a highly graphitized carbon support from Cabot as well as on Ebonex® which contains sub-oxides of titanium (Magneli phases). Catalyst dispersion, electrochemical activity and electrode formation is discussed. [1] R. Maric, J. Roller, and R. Neagu, J. Thermal Spray Techn., 20(4) (2011) 698.
Gold Catalysis Helping a Sustainable Future
Hutchings1, Peter
Recently, there has been an explosion of interest in gold as a catalyst [1] and gold catalysis is now a major topic for both heterogeneous and homogeneous catalysis worldwide. This presentation will explore the latest developments using supported gold and gold palladium nanoparticles as heterogeneous catalysts. This will include recent studies on the nature of the active site for CO oxidation [2] where recent aberration-corrected microscopy has revealed that the active species may involve only 7-10 gold atoms, and the direct synthesis of hydrogen peroxide [3] and the oxidation of toluene [4]. References [1] M. Haruta, Nature 437, (2005) 1098. [2] A. A. Herzing, C. J. Kiely, A. F. Carley, P. Landon and G. J. Hutchings Science 321 (2008) 1331. [3] J.K. Edwards, B.E. Solsona, E Ntainjua N, A.F. Carley, A. Herzing, C.J. Kiely and G. J. Hutchings, Science 323 (2009) 1037. [4] L. Kesavan, R. Tiruvalam, M. H. Ab Rahim, M. I. bin Saiman, D. I. Enache, R. L. Jenkins, N. Dimitratos, J. A. Lopez-Sanchez, S. H. Taylor, D. W. Knight, C. J. Kiely, G. J. Science, 331 (2011) 195.
Catalysts with Superior Catalytic Properties: Nanoporous Gold as a Platform for a Catalytic Building-block Design
Wittstock1 2, Andre
Wichmann2, Kristian
Frank2, Monika
Biener1, Juergen
Biener1, Andreas
Rosenauer2, Marcus
Corrosion-derived skeletal nanomaterials such as Raney nickel have been employed in industrial catalysis for the last 70 years. Based on recent developments in the field of material sciences a variety of novel skeletal bulk-nanomaterials evolved. In fact, one of the rising stars in catalysis, gold, usually known for its inertness, can be prepared as a nanoporous bulk material showing astonishing catalytic properties for low temperature oxidation of CO or selective coupling of alcohols such as methanol. However, the full potential of the material might not have been fully developed. We show that the concept of corrosion-derived nanoporous gold materials can be even extended and used as the very basis for a catalytic building block design. Pursuing this approach, we prepared catalysts for applications in gas phase catalysis by functionalizing the surface of nanoporous gold with oxides in a controlled and reproducible way using state of the art gas phase deposition techniques such as atomic layer deposition as well as bench top techniques such as impregnation and calcination. The derived material is coined by a three dimensional sponge-like structure of gold ligaments and pores in the range of about 30 nm densely decorated with oxide nanoparticle. In particular, in the case of a titania coated material, the activity of the material could be enhanced by factor of more than hundred. Concomitantly, the metal oxide stabilizes the nanostructure up to 600°C, opening the door for high temperature applications.
Catalytic Hydrogen Production from Methanol and Formic Acid Reactions over Efficient and Stable Gold Clusters on Cerium Oxide
Yi1, Howard
Saltsburg1, Maria
Clean and efficient hydrogen production through harnessing chemical energy from methanol or formic acid is of interest because of the potential of hydrogen to be the practical and industry-usable energy carrier of the future. Here we investigated the efficacy and practical application of a new generation catalyst, namely gold modified different crystal morphology of ceria (nanorods and nanocubes), for methanol steam reforming and decomposition of formic acid, two processes that convert raw organic compounds into hydrogen. First, ceria nanorods were found to have strong interactions with reactants compared to ceria nanocubes. Also, ceria nanorods facilitate gold dispersion and affect gold oxidation states on their surfaces. Second, kinetics studies over parent samples and sodium cyanide leaching samples revealed that only dispersed gold species is active for both reactions. Furthermore, catalytic performance tests through the addition of CO and H2O excluded the occurrence of water gas-shift reaction (WGS), which explained the excellent CO2 selectivities over gold-ceria. Third, temperature programmed reactions were applied to determine the reaction intermediates, allowing us to elucidate the reaction pathway. For the decomposition of formic acid, the dehydrogenation pathway was dominant. For methanol reaction, the different reaction pathways between Pd, Pt- doped ceria and Au- doped ceria for methanol reactions were established. Decomposition-WGS sequence was identified for IB metals (Pt, Pd) modified ceria, while the methyl formate pathway is unique to IB metal (Au) doped ceria. This particular finding is significant because it provided the necessary insight into how to rank order and select the precious metals for their practical application in PEM fuel cells. In summary, the findings here demonstrated that gold-ceria is a breakthrough in the efforts to generate energy cleanly and efficiently for scalable fuel cells.
The Structure and Chemical Activity of 2-D Gold Islands on Single-layer Graphene/Ru(0001)
Liu1, Ye
Xu2, Zihao
Zhou1, Qinlin
Guo3, Zhen
Yan1, Yunxi
Yao1, Lynmarie
Semidey-Flecha2, D. Wayne
Single-layer graphene supported on transition metals provides a unique substrate for synthesizing metal nanostructures due to the high crystallographic quality, thermal stability, and chemical inertness of the graphene. Contrary to its formation of three-dimensional (3-D) nanoclusters on graphene supported on a SiO2 substrate, Au forms two-dimensional (2-D) islands on graphene moiré/Ru(0001). These Au islands maintain their 2-D structures up to 1 monolayer (ML) equivalent of Au dosage and are stable at room temperature. Our scanning tunneling microscopic study further shows that the 2-D Au islands are most likely two layers high, and conform to the graphene moiré in the lateral direction. Spin- and angle-resolved photoemission studies indicate even though these Au islands are largely electronically isolated, a weak through-graphene coupling exists between the Au islands and the Ru(0001) substrate. The structure for these 2-D Au islands and the corresponding electronic band structures are proposed based on DFT calculations. Parallel studies using polarization modulation infrared reflection absorption spectroscopic (PM-IRAS) and high resolution electron energy loss spectroscopic (HREELS) indicate that CO adsorbs on these 2-D gold islands at 85 K with a characteristic CO stretching feature at 2095 cm-1 for a saturation coverage. Temperature programmed desorption study showed that these nanostructured Au has catalytic activity toward CO oxidation. These electron-rich, weakly coupled 2-D Au islands provide a unique platform to study the intrinsic catalytic activity of low-dimensional Au nanostructures.
Thermal and Chemical Stability of Magnesium Aluminate Spinel Supported Pt Nanoparticles
Liu1, Weizhen
Li1, Charles
Peden1, Yong
Supported Pt catalysts are widely used in the petroleum industry in reforming and hydrotreating processes and in the automotive exhaust gases cleaning. A key issue is the sintering of supported Pt nanoparticles over long periods at elevated temperature, which decreases the exposed surface area of Pt, resulting in a decrease of the catalytic activity. Much effort has focused on stabilizing the Pt nanoparticles by alloying with other metals of higher melting point, or by encapsulating the Pt nanoparticles in thermally stable and chemically inert oxide shells, or by anchoring effect based on a "strong metal-support interactionâ€. A fundamental knowledge on what kind of surface structure can stabilize Pt nanoparticles is still lacking. In this presentation, we discuss a spinel supported Pt catalyst with superior thermal and chemical stability. Supported Pt catalyst (1 wt.%) was prepared by wetness impregnation the support in Pt precursor solution. After aging at 800 oC in static air for 1 week, the aged samples still showed small Pt nanoparticles of 1-3 nm in TEM image. The Pt dispersion was maintained at 15.9%. The average mass specific activity of Pt for the oxidative dehydrogenation of methanol at low temperature (80 oC) for the aged samples retained at 80% of the fresh sample. The fundamental mechanisms of particle size stabilization against sintering in these materials will be discussed through a combination of high resolution TEM study and theoretical modeling of how the metal particles interact with different surfaces of the supports. Such knowledge is essential for guiding the design of other sinter-resistant supported noble and non-noble metal catalysts.
Strong Metal-support Interaction between Gold Nanoparticles and ZnO Nanorods
Mou1 2.
The catalytic activities of supported gold nanocatalysts are sensitive not only to the particle size but also to the nature of the support. The reducibility, acidity/basicity or the availability of defect sites can significantly affect the interaction between gold nanoparticles and the support. However, the intrinsic catalytic mechanism of supported gold catalysts has not been understood yet. Gold possesses high ionization potential and electron affinity, it is a poor electron donor. Till now, there is still no evidence of the strong-metal support interaction (SMSI) of supported gold nanoparticle as similar to group VIII metal supported on the TiO2. ZnO, as a typical n-type semiconductor, has similar properties as TiO2. In this work, we report for the first time evidences of strong metal-support interaction (SMSI) in ZnO nanorod supported gold nanocatalyst based on the results of structural characterization and spectroscopic studies. The results manifest the encapsulation of gold nanoparticles by ZnO and affect electron transfer between them. The CO oxidation over the Au/ZnO-nanorod is further investigated to exemplify the significance of SMSI effect. These structural and chemical studies allow us to examine the nature of interaction between gold and the support and shed new light on the catalytic mechanisms of supported gold nanoparticles involved in heterogeneous catalysis. SMSI similar to that of the TiO2 supported Pt nanoparticles is found for the first time in the Au/ZnO-nanorod system. However, the pretreatment atmosphere leading to SMSI is quite different: in the Au/ZnO-nanorod system, oxygen was needed while in the Pt(Pd)/TiO2 system, hydrogen pre-treatment was used. Various characterizations including HRTEM, XANES, EXAFS, XPS, in situ DRIFT, CL and EPR are employed to manifest the encapsulation of gold by ZnO and the charge transfer between gold nanoparticles and the ZnO support. A possible mechanism of the SMSI between gold and the ZnO support is proposed.
Effects of Composition and Size of PtxSn1-x/Mg(Al)O on the Catalytic Dehydrogentaion of Light Alkanes
Peng1, Alexis
Catalytic dehydrogenation of C2-C4 alkanes to produce valuable alkenes and hydrogen is of considerable interest for the petrochemical industry. Supported Pt and Pt-Sn bimetallic nanoparticles are effective for the thermal dehydrogenation of such light alkanes and have been investigated extensively. The addition of Sn to Pt can significantly improve the selectivity to alkenes and the catalyst durability by suppressing methane formation and coke deposition. It has been reported in many studies that the catalytic properties of Pt-Sn bimetallics can be altered by varying the Sn content in Pt-Sn particles. Our recent study has also revealed a size effect in coke formation on Pt particles. However, the Pt and Pt-Sn catalysts prepared using conventional impregnation methods are not uniform in composition/size. In this talk we present the results of an investigation of light alkane dehydrogenation carried out using PtxSn1-x particles prepared with uniform composition (0.5 ≤ x ≤ 1) and size (2 nm ≤ D ≤ 6 nm). PtxSn1-x alloy nanoparticles were synthesized by reducing platinum acetylacetonate and tin acetylacetonate precursors in organic solvents and then dispersed onto a calcined hydrotalcite (Mg(Al)O) support. Their particle size and composition were finely tuned by adjusting the synthetic parameters including capping reagents and reaction temperature. The activity, selectivity, and coke formation were investigated for PtxSn1-x/Mg(Al)O catalysts as functions of the particle size and Sn content. We will show that alkane dehydrogenation activity and selectivity depend on both the size and composition of the PtxSn1-x particles, as does the extent of catalyst deactivation by coke formation.
Understanding the Evolution of NiRu Bimetallic Nanoparticles for Partial Oxidation of Methane Using In-Situ Environmental TEM
Chenna1, Peter
Bimetallic catalysts are an effective way of combining the superior catalytic performance of noble metals with less expensive metals. A major challenge for bimetallic catalysts lies in understanding atomic level structures that form between the two metal species. Moreover, the metal distribution in an as prepared catalyst may significantly vary on exposure to reacting gas conditions. To elucidate the relationship between nanoscale structure and catalytic properties we employ in-situ environmental transmission electron microscopy (ETEM) to follow the nanoscale structural and chemical changes taking place in the bimetallic catalyst under reacting gas conditions. In-situ ETEM studies were performed to follow the evolution of NiRu bimetallic particles supported on SiO2 under reacting gas conditions relevant to partial oxidation of methane. Catalytic activity measurements in parallel with in-situ ETEM studies were performed on NiRu catalysts to understand the effect of nanostructural changes on the performance of the catalyst. From the in-situ experiments we observed that an initially uniform NiRu nanoparticle in the presence of H2 at 400oC phase separated to form core-shell structures, with a mixed oxide shell and a Ru metal core in the presence the reactant gases CH4 and O2 (in 2:1 ratio). The oxide shell predominantly contained NiO along with some RuO2. This oxide layer reduced back to metal and intermixing took place when the gas composition was switched to pure CH4. To facilitate the interpretation of the results, similar experiments were performed on monometallic Ru and Ni catalysts. A detailed discussion on the mechanism of oxidation and reduction behavior in NiRu bimetallic nanoparticles will be presented and correlated with catalytic performance for partial oxidation of methane.
Anisotropic ZnO Nanostructures for Energy Applications
Bacsa1, Eva
Castillejos1 2, Antonio
Guerrero-Ruiz2, Sonia
Carabineiro3, Bruno
Machado1 3, Figueiredo
Jose3, Faria
Joaquim3, Nicolas
Reuge4, Caussat
Brigitte4, Khaja
Zakeeruddin5, Michael
Graetzel5, Philippe
Bulk oxides are usually robust and stable systems with well-defined crystallographic structures. At the nanoscale, these compounds can exhibit unique physical and chemical properties due to their limited size and a high density of defect sites such as edges, corners and point defects. As for other materials, the process of size reduction is expected to dictate structural, transport and chemical properties. We developed a large scale synthesis process for nanocrystalline ZnO spheres and tetrapods production by chemical vapor synthesis using zinc metal as precursor and air as oxidant. The chemical vapor synthesis of ZnO tetrapods (ZnOT) from zinc metal will be described using a combination of experiments and fluid dynamics modeling.[1] These materials can find applications in the field of energy, for example for dye sensitive solar cells or as catalyst for CO removal in fuel cells. We will show why the anisotropic ZnOT have increased efficiencies for application in dye sensitized solar cells when compared to ZnO nanospheres.[2] Similarly, the anisotropy of the tetrapods is at the origin of the higher performances for the CO oxidation reaction of Au/ZnOT catalysts, compared to gold catalysts deposited on commercial spherical nano-ZnO.[3,4] The epitaxy of gold nanoparticles could explain the high activity obtained. [1] N. Reuge, R. Bacsa, P. Serp, B. Caussat J. Phys. Chem. C 2009, 113 (46), 19845-19852. [2] R.R. Bacsa, J. Dexpert-Ghys, M. Verelst, A. Falqui, B. Machado, W.S. Bacsa, P. Chen, S.M. Zakeeruddin, M. Graetzel, P. Serp Adv. Func. Mater. 19, 2009, 875-886. [3] S.A.C. Carabineiro, B.F. Machado, R.R. Bacsa, P. Serp, G. Drazić, J.L. Faria, J.L. Figueiredo J. Catal. 2010, 273, 191-198. [4] E. Castillejos, R. Bacsa, A. Guerrero-Ruiz, I. RodrÃguez-Ramos, L. Datas, P. Serp, Nanoscale 2011, 3 (3), 929-932.
Cu2+ Ion Enhanced Synthesis and Catalytic Activity of Nanostructured CO3O4
Feng1, Xiaolin
Cobalt oxide (Co3O4) is the most active catalyst among all the transitional metal oxides for the oxidation of hydrocarbons, which is important for energy conversion and emission control. Conventional method supports Co3O4 nanoparticles (NPs) on porous substrates to maximize their catalytic surface areas, but they suffer from problems such as the agglomeration of NPs and chemical reactions between the catalyst and the porous substrates. In contrast, one-dimensional (1-D) nanostructured Co3O4 supported on metal meshes is a promising alternative catalytic geometry, which not only avoids the above issues associated with the supported NPs, but also benefits from large surface-to-volume ratio, small pressure drop, good heat transfer and ease of surface modification of the 1-D nanocatalysts. However, it remains a challenge to controllably grow nanostructured Co3O4 on metal meshes with high coverage density and good repeatability. Herein, we report an improved ammonia-evaporation-induced synthesis method to grow nanostructured Co3O4 on stainless steel (SS) meshes, by introducing Cu2+ ions to facilitate the nucleation and growth process of Co(OH)2. Firstly, the morphology of as-grown Co3O4 on SS meshes evolves from nanowire (NW) bundles, to microwires (MW) and finally to surface coated MWs. The characterizations of the final products illustrate that the main phase is Co3O4, with some Cu in the core region, which means the surface of all the morphologies is mainly covered by Co3O4. Secondly, the mass loading density greatly increases with the presence of Cu2+ ions. The sample with cobalt to copper atomic ratio of 8:2 in the initial solution has the maximum mass loading and largest surface area. Finally, the catalytic activities of those Co3O4 supported on SS meshes with different amounts of Cu2+ ions were tested for the methane oxidation. With smaller grain size and larger surface area, all the Cu-containing Co3O4 samples show higher catalytic activity than the pure Co3O4, and the methane conversion percentage over the optimized Cu-containing Co3O4 sample is about 50% higher than that over the pure Co3O4 NWs in a temperature range of 300 to 400oC. We believe that these results not only demonstrate an elegant method to control the morphologies of 1-D metal oxides synthesized by solution phase methods, but also facilitate the use of 1-D metal oxide nanocatalysts supported on metal mesh as an economical, scalable and yet efficient catalytic structure for hydrocarbon oxidation reactions and other catalytic applications.
Monodispersed Nickel Phosphide Nanoparticles with Tunable Ni/P Content as Noble-Metal Free Nanocatalysts for H2 Activation
Carenco1 3, Antonio
Leyva-Perez2, Patricia
Concepcion2, Cedric
Boissiere1, Nicolas
Mezailles3, Avelino
Corma2, Clement
H2-based fuel technologies are highly beneficial for reducing the carbon footprint of human activities. Efficient catalysts for water-splitting and H2 evolution reactions are thus actively sought. In particular, large-scale development of these green technologies will require noble-metal free catalysts, robust towards contaminants such as CO and sulfur. Nickel-based nanocatalysts, such as the Raney catalysts, have shown promising activities for H2 activation in the past. However, their use was hampered by the lack of control concerning their preparation (batch effect) and their sensitivity towards sulfur poisoning. To overcome this limitation, we have modified monodispersed Ni(0) nanoparticles with white phosphorus (P4), a cheap and ton-scale available source of phosphorus, to produce a family of sulfur-resistant nickel phosphide nanoparticles: pure Ni2P nanoparticles, core-shell Ni2P/Ni and amorphous Ni-P alloys. The colloidal synthesis was designed at the gram scale and performed in soft conditions (a few hours, T<220°C), allowing an excellent control of the nanoparticles size, composition and surface species. This made them a suitable model for the fine understanding of the catalysis at the molecular scale, as they were systematically compared with Ni(0) nanoparticles (same size and surface species). A model reaction was chosen to probe the H2 activation performances of our catalysts: the industrially-relevant alkyne hydrogenation reaction. Here, we report the strikingly high selectivity of the Ni2P nanocatalysts compared with the Ni(0) one for the selective formation of alkenes.[4] Cis-alkene with limited steric hindrance were obtained with a high conversion (>90%) and a high selectivity (>90%) for a range of substrates. Through dedicated mechanistic studies, including isotope labeling and in situ FTIR spectroscopy, this behavior was shown to take roots into the nature and abundances of active sites for H2 activation on the catalyst. In particular, the Ni2P surface was found to store less active hydrogen species, leading to a better controlled hydrogenation reaction. The phosphorus content was then adjusted to Ni/P=3 and an intermediate behavior was observed, confirming the paramount role of the phosphide moiety on the controlled formation of surface hydrides. Altogether, this model study highlighted the exciting performances of nickel phosphide nanoparticles as a cheap alternative to Pt-based catalysts for the development of smart nanocatalysts in H2-based energy technologies. [1] Carenco S, Boissière C, Nicole L, Sanchez C, Le Floch P, Mézailles N, Chem. Mater. 2010, 22, 1340 [2] Carenco S, Resa I, Le Goff X, Le Floch P, Mézailles N, Chem. Commun. 2008, 2568. [3] Carenco S, Le Goff X, Shi J, Roiban L, Ersen O, Boissière C, Sanchez C, Mézailles N, Chem. Mater. 2011, 23, 2270. [4] Carenco S., Leyva-Pérez A., Concepciòn P., Boissière C., Mézailles N., Sanchez C., Corma A., Submitted Paper.
Shape and Composition Tuned Bimetallic CoCu Nanoparticle Catalysts for CO2/H2 Fischer-Tropsch (F-T) Synthesis
Alayoglu1 2, Simon
Beaumont1 2, Colin
Specht1 2, Gabor
Somorjai1 2.
Truly bimetallic Co(1-x)Cux (x=0.2-0.8) nanoparticles were synthesized by using amine functional solvents and acetylacetonate precursor salts of metals at elevated temperatures. Spherical nanoparticles could be adjusted in the size range between 3 nm and 10 nm by exploiting various amine functional solvents. 10 nm Co80Cu20 nanoparticles with octahedral shapes were obtained from nitrate precursor salts of metals under otherwise identical synthetic conditions. Nanoparticles were characterized by employing an array of in-situ and ex-situ techniques. Powder X-ray diffraction showed that a poorly-crytalline metal oxide phase co-exists with the nanocrystalline bimetallic alloy phase for all the composition range studied. Scanning transmission electron microscopy with energy dispersive spectroscopy and electron energy loss spectroscopy revealed the formation of truly bimetallic nanoparticles with metallic Cu rich cores and mostly oxidized Co rich shells. Ambient pressure X-ray photoelectron spectroscopy exhibited Cu segregation to surfaces corresponding to mean free path of 300 eV photoelectrons under O2, which illustrated the surface dynamics of bimetallic CoCu nanoparticles under redox atmospheres. Additionally nanoparticle colloids resulted in Cu segregation to surfaces and the formation of hollow nanostructures at room temperature when lightly washed and stored in chloroform. Control experiments put forward a combined action of excess amine, chlorine and oxygen on the differential diffusion of Cu into Co and the leaching of Cu. Well-characterized nanoparticles were supported in MCF-17 SiO2 to evaluate their catalytic reactivity for the Co2/H2 F-T synthesis. At 300 degree Celcius and 5.5 bar, bimetallic nanoparticles performed more selectively toward methane than the monometallic Co and Cu nanoparticles.
Structure of Nanosized PtAu Alloy Catalysts by Resonant High-energy XRD
Currently, catalysis research is focusing on nanosized alloy particles. Such particles, however, show a great deal of structural (1, 2) and chemical order-disorder effects (3) that influence their catalytic activity very substantially. To understand and gain control over the latter a good knowledge of the former is needed. In the talk we will introduce a recent development in resonant high-energy x-ray diffraction (4) as a tool for characterization the atomic arrangement in alloy nanoparticles both with an excellent spatial resolution and chemical specificity. Results from recent experiments on carbon supported 5 nm PtxAu1-x (x=0, 0.2, 0.4, 0.5, 0.77, 1.0) alloy particles will be presented. Questions like i) are really Pt and Au alloying at the nanoscale, ii) is there bond-length contraction with diminishing particle size and alloying, and iii) how those structural characteristics relate to the PtxAu1-x nanoparticle's catalytic performance will be addressed. References: 1. V. Petkov et al "Periodicity and atomic ordering in nanosized particles of crystals" J. Phys. Chem. C 112 (2008) 8907. 2. V. Petkov, "3D structure of Nanosized catalysts by High-energy X-ray Diffraction", Synchr. Rad. News 22 (2009) 29. 3. Jin Luo et al "Carbon-Supported AuPt Nanoparticles with Different Bimetallic Compositionsâ€, Chem. Mater., 17 (2005) 3086. 4. V. Petkov and S. Shastri, "Element-Specific View of the Atomic Ordering in Materials of Limited Structural Coherence by High-Energy Resonant X-Ray Diffraction and Atomic Pair Distribution Functions Analysis: A Study of PtPd Nanosized Catalystsâ€, Phys. Rev. B 81 (2010) 165428.
``One-Pot'' Soft-templated Synthesis of Phosphorylated Mesoporous Carbon Heterogeneous Catalysts with Tailored Surface Acidity
Fulvio1, Richard
Mayes1, John
Bauer1, Xiqing
Wang4, Shannon
Mahurin1, Gabriel
Veith2, Sheng
Dai1 3.
Phosphorylated mesoporous carbons with homogeneous distributions of phosphate groups were prepared by a "one-pot†soft-template method using mixtures of phosphoric acid with hydrochloric, or with nitric acids in the presence of Pluronic F127 triblock copolymer. Carbons with distinct adsorption, structural and surface acidity properties were prepared by adjusting the various ratios of phosphoric acid used in these mixtures. Nitrogen adsorption at -196°C showed that mesoporous carbons exhibit specific surface areas between 300-600m2/g, and 6 to 13nm mesopore diameters from the calculated pore size distributions (PSDs). Both structural ordering of the mesopores and the final phosphate contents were strongly dependent on the ratios of H3PO4 in the synthesis gels, as shown by transmission electron microscopy (TEM), X-ray photoelectron (XPS) and energy dispersive X-ray spectroscopy (EDS). The number of surface acid sites determined from temperature programmed desorption of ammonia (NH3-TPD) were in the range of 0.3-1.5mmol/g while the active surface areas are estimated to comprise 5-54% of the total surface areas. Finally, the conversion temperatures for the isopropanol dehydration were lowered by as much as 100°C by transitioning to the catalysts with the higher numbers and strength of acid sites, and higher active surface areas (ASA). In addition to the good thermal and chemical stabilities of soft-templated mesoporous carbons, these results show that our phosphorylated materials have potential for the large scale application as heterogeneous acid catalysts. Acknowledgements: Work supported as part of Fluid Interface Reactions, Structures and Transport (FIRST) Center, Energy Frontier Research Center, U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.
Supported Ni Catalyst Made by Electroless Ni-B Plating for Diesel Autothermal Reforming
Hong1 2, Zetao
Xia2, Wei
Wang2, Zhaolin
An on-board fuel reformer has been designed to convert diesel fuel oil into hydrogen-rich reformates in a fixed bed catalytic reactor, known as auxiliary power units (APUs). The reformates comprising primarily of H2, CO2 and CO can directly power solid oxide fuel cell (SOFC). In an APU system, deactivation of catalyst due to the presence of heavy and branched hydrocarbons, aromatics and sulfur (H2S and organic sulfides or thiols) in the fuel is the major obstacle to the commercialization of diesel fuel reformer. An innovative Ni-based catalyst was developed in this work with the aim of tackling these deactivation issues. The catalyst was prepared through a two-step procedure: firstly, nickel boron (Ni-B) alloy particles (~5 nm) were deposited on an activated carbon substrate (C), secondly, the Ni-B/C powder was mixed with a metallo-organic gel of cerium(III) and Gd(III) ions, and then the mixture was subjected to calcination. The resulting Gd-doped ceria (GDC) supported Ni catalyst displayed excellent resistance to carbon deposition and sulfur poisoning in the autothermal reforming of a proxy fuel comprising of 75%-dodecane, 25%-tetralin and 50ppm 3-methyl-benzothiophene. A conversion of the hydrocarbons >95% and selectivity of H2 > 60% underwent no change after a125-h duration of assessment. The catalyst down loaded from the reactor showed no any coke deposition and sintering of Ni namo particles. This paper also provides fundamental interpretation for the unique performance of this catalytic system.
Shape-controlled Synthesis of Silver and Palladium Nanocrystals Using Beta-Cyclodextrin
Berhault1, Hafedh
Kochkar2, Abdelhamid
Introduction Shape control of metallic nanocrystals led to well-defined morphologies exhibiting preferential exposition of crystallographic planes and allowing the establishment of structure-reactivity relationships for structure-sensitive reactions like selective hydrogenations. In this respect, shape-controlled colloidal synthesis can be monitored through the selective adsorption of surfactants like cetyltrimethylammonium bromide (CTAB) on specific crystallographic facets leading to well-defined objects. An important issue for improving colloidal techniques is to develop "greener†approaches. The replacement of alkyltrimethylammonium halides by biocompatible agents is required. Therefore, beta-cyclodextrins, nontoxic oligosaccharides, have been for the first time herein used for the preparation of well-defined Ag and Pd NPs. After deposition onto TiO2, the Pd/TiO2 catalysts were tested in the hydrogenation (HYD) of cinnamaldehyde to evaluate their catalytic properties. Materials and Methods Ag or Pd seeds were first prepared by mixing aqueous solutions of AgNO3 or Na2PdCl4 with sodium citrate. NaBH4 was then added to reduce the Ag or Pd salt. Second, Ag and Pd growth solutions were obtained by mixing aqueous solutions of beta-CD, AgNO3 or Na2PdCl4 with ascorbic acid. Different beta-CD/Ag(Pd) molar ratios were used. Finally, 100 µL of Pd or Ag seeds were injected to initiate growth. Pd NPs were then impregnated onto a TiO2 support. Results and Discussion In the case of Ag, well-defined morphologies were obtained by tuning the beta-CD/Ag molar ratio. At a molar ratio of 50, different morphologies (icosahedra, nanodisks, ..) were found. Increasing the molar ratio to 150 led to the selective formation of icosahedra. The main role of beta-CD is to restrain particle growth favoring the evolution of seeds into icosahedra. In the case of Pd, Pd dendrites were formed at beta-CD/Pd = 20 while increasing this ratio to 100 led to the selective formation of multipods. This is related to an oriented attachment mechanism. Primary particles formed from an heterogeneous nucleation are first attached to the growing seeds while surface diffusion was then restrained by beta-CD leading to dendrites exposing {111} facets. After deposition onto TiO2, Pd/TiO2 catalysts were evaluated in the cinnamaldehyde HYD. Activity results showed that beta-CD does not interfere with the hydrogenation properties. Selectivity results were close to those expected from theoretical calculations validating in real experimental conditions the influence of {111} facets on catalytic properties. Conclusion "Greener†synthesis strategies based on the use of cyclodextrins have been developed for the shape-control of Ag or Pd NPs leading to well-defined morphologies for Ag or to dendrites or multipods for Pd. After deposition onto TiO2, evaluation of catalytic properties allows to validate in real conditions theoretical predictions about this structure-sensitive reaction.