Byeong Jun Cha,Ji Yoon Choi,Soo Hyun Kim,Shufang Zhao,Sher Ali Khan,Beomgyun Jeong,Young Dok Kim
In Situ Spectroscopic Studies of NH3 Oxidation of Fe-Oxide/Al2O3
Simple temperature-regulated chemical vapor deposition was used to disperse iron oxide nanoparticles on porous Al2O3 to create an Fe-oxide/Al2O3 structure for catalytic NH3 oxidation. The Fe-oxide/Al2O3 achieved nearly 100% removal of NH3, with N2 as a major reaction product at temperatures above 400 °C and negligible NOx emissions at all experimental temperatures. The results of a combination of in situ diffuse reflectance infrared Fourier-transform spectroscopy and near-ambient pressure–near-edge X-ray absorption fine structure spectroscopy suggest a N2H4-mediated oxidation mechanism of NH3 to N2 via the Mars–van Krevelen pathway on the Fe-oxide/Al2O3 surface. As a catalytic adsorbent—an energy-efficient approach to reducing NH3 levels in living environments via adsorption and thermal treatment of NH3—no harmful NOx emissions were produced during the thermal treatment of the NH3-adsorbed Fe-oxide/Al2O3 surface, while NH3 molecularly desorbed from the surface. A system with dual catalytic filters of Fe-oxide/Al2O3 was designed to fully oxidize this desorbed NH3 to N2 in a clean and energy-efficient manner.
In Situ Spectroscopic Studies of NH3 Oxidation of Fe-Oxide/Al2O3

Simple temperature-regulated chemical vapor deposition was used to disperse iron oxide nanoparticles on porous Al2O3 to create an Fe-oxide/Al2O3 structure for catalytic NH3 oxidation. The Fe-oxide/Al2O3 achieved nearly 100% removal of NH3, with N2 as a major reaction product at temperatures above 400 °C and negligible NOx emissions at all experimental temperatures. The results of a combination of in situ diffuse reflectance infrared Fourier-transform spectroscopy and near-ambient pressure–near-edge X-ray absorption fine structure spectroscopy suggest a N2H4-mediated oxidation mechanism of NH3 to N2 via the Mars–van Krevelen pathway on the Fe-oxide/Al2O3 surface. As a catalytic adsorbent—an energy-efficient approach to reducing NH3 levels in living environments via adsorption and thermal treatment of NH3—no harmful NOx emissions were produced during the thermal treatment of the NH3-adsorbed Fe-oxide/Al2O3 surface, while NH3 molecularly desorbed from the surface. A system with dual catalytic filters of Fe-oxide/Al2O3 was designed to fully oxidize this desorbed NH3 to N2 in a clean and energy-efficient manner.

Ammonia (NH3) is an indispensable substance in the modern industry, as it is not only essential for the production of fertilizers, plastics, and useful chemicals but also useful in purifying waste water or storing hydrogen for fuel applications. However, because NH3 is caustic and hazardous, NH3 can cause serious health and environmental problems when released to the atmosphere. In addition, noxious odors caused by NH3 in living environments, particularly near agricultural areas, have become a major social issue. To reduce NH3 emissions, various strategies such as scrubbing, condensation, biofiltration, thermal oxidation, adsorption, and catalytic oxidation can be applied at the NH3 emission sources. Catalytic oxidation is a promising technique for field applications; ideally, catalytic filters for NH3 oxidation can work permanently without the need for exchanges or regeneration. A representative example is selective catalytic oxidation of NH3, in which NH3 is used to reduce nitrogen oxide (NOx) generated by fossil-fuel combustion in power plants or diesel vehicles, and its slip to the atmosphere can be reduced by this selective catalytic oxidation technology, which normally operates under an NH3/NOx ratio of approximately 0.9–0.95 and a high O2 concentration. Nanoparticles of platinum-group metals or earth-abundant transition metals such as Ti, Fe, Mn, V, and Cu can be used for this application at 250–500 °C. Platinum-group metals have a high NH3 conversion rate at low temperature in general, but significant NOx emission is a major limitation for their application. Nanostructures of Ti, Mn, V, and Cu oxides can show high NH3 conversion at low temperature with low NOx emission, but their low thermal stability has been considered as a limitation. The operating temperature of these catalysts can be reached easily when fossil-fuel combustion is taking place nearby, and little external thermal energy is therefore required to increase catalyst temperature. However, to remove NH3 from a living environment, catalytic technology with improved energy efficiency and reactivity in the absence of NOx is needed. In many living environments, external energy should be applied to increase the catalyst temperature, and it is not energy-efficient to maintain catalytic operating temperatures, particularly when NH3 is not continuously emitted or is at low concentrations. Incorporation of catalytic nanoparticles into porous adsorbent materials with a high surface area is an effective option that can take advantages of both catalysts and adsorbents, reducing energy costs. At room temperature (RT), NH3 can be adsorbed on the surface of these materials, and during the intermittent thermal oxidation process to regenerate the adsorption sites, adsorbed NH3 can be converted into nonharmful N2 with the help of catalytic particles. It is important to minimize the molecular desorption of NH3 and emissions of harmful NOx gases during this process. In the present work, simple temperature-regulated chemical vapor deposition (tr-CVD) was used to deposit catalytically active iron oxide (Fe-oxide) nanoparticles on mesoporous alumina (Al2O3) beads in a highly dispersed manner, and this structure was used for NH3 removal. The Al2O3-supported Fe-oxide nanoparticles (Fe-oxide/Al2O3) were able to oxidize NH3 to N2 through a hydrazine (N2H4)-mediated pathway with no harmful gas emission including NH3 and NOx at temperatures above 400 °C. When Fe-oxide/Al2O3 was used as catalytic adsorbent, molecular desorption of NH3 occurred during the thermal treatment of NH3-covered surfaces. To eliminate NH3 emissions at a minimal energy cost, a system with two interconnected catalytic filters was designed. By heating the rear side of the catalytic filter prior to the thermal treatment of the front-side filter, emissions of NH3 to the atmosphere were fully prevented. From the fundamental point of view, the present study provides novel information on the mechanism responsible for NH3 oxidation of an Fe-oxide catalyst. Previous studies determined N2 selectivity indirectly by measuring the removed NH3 that was not converted into NOx, whereas in the present work, the amount of generated N2 was measured directly, which enabled consideration of reaction intermediates on the surface during the reaction. In addition, the present work combined in situ diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) with near-ambient pressure–near-edge X-ray absorption fine structure spectroscopy (NAP-NEXAFS), which allowed us to elucidate both structural changes in reactants during the catalytic reaction and the catalyst itself. Using this combination, N2H4-mediated oxidation of NH3 to N2 via the Mars–van Krevelen pathway could be suggested. This study shows that NAP-NEXAFS can not only be applied to studies of model catalyst structures but also used for unveiling practical catalyst structures under operating conditions. From an application point of view, the system of dual catalytic filters designed in the present study can solve problems that arise when catalytic adsorbent materials are used to remove NH3, specifically the re-emission of NH3 during thermal treatment of NH3-covered surfaces.

Fe-oxide nanoparticles were deposited on Al2O3 using temperature-regulated chemical vapor deposition (tr-CVD). The tr-CVD is a simple one-pot approach to incorporate small metal oxide nanoparticles into a porous substrate via diffusion of metal precursor throughout the porous network and its subsequent oxidation to metal oxide species. This method can be more environmentally friendly than liquid phase methods because it does not require any organic solvent. In addition, this method can take advantage of using vapor, which enables efficient incorporation of small nanoparticles into not only on the external surface of the substrate but also into its small internal meso- or micropores. This technique can be applied to various other substrates including SiO2, zeolite, and TiO2. Inside the tr-CVD chamber, the Fe precursor ferrocene, Fe(Cp)2, (Aldrich) and Al2O3 beads (bead size: 1 mm; mean pore size: 11.6 nm, Sasol) were placed separately. In the first step, the Fe precursors evaporated and diffused into the entire porous network of Al2O3 at 60 °C over 2 h. In the second step, the diffused ferrocene molecules oxidized into Fe-oxide nanoparticles at 200 °C for 12 h. In the present work, 2.5 g of ferrocene and 10 g of Al2O3 beads were used to prepare the Al2O3-supported Fe-oxide nanoparticles. This structure was then annealed at 750 °C for 2 h in dry air (30 mL/min), and the prepared sample was labeled Fe-oxide/Al2O3. Details of the tr-CVD are provided in the Supporting Information (Figure S1).

The loading of Fe on Al2O3 was measured using inductively coupled plasma-optical emission spectroscopy (ICP-OES, PerkinElmer AVIO 550Max, Varian). Dispersion of Fe-oxide on a mesoporous network of Al2O3 was confirmed by energy dispersive spectroscopy and scanning electron microscopy (SEM, JEOL, JSM-7100F). The Fe-oxide/Al2O3 beads were cut in half, and cross-sections were used for this analysis. The geometrical structure of the Fe-oxide particles was analyzed by high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM, JEOL, JEM ARM 200F). The surface area and pore size of the Fe-oxide/Al2O3 were estimated using the Brunauer–Emmett–Teller (BET) and Barret–Joyner–Halenda (BJH) methods, respectively, based on N2 adsorption isotherm analysis (3Flex, Micromerities). The chemical structure of Fe-oxide on Al2O3 was analyzed by X-ray photoelectron spectroscopy (XPS). Mg K-alpha radiation (1253.6 eV) was used as an X-ray source, and the kinetic energy of the photoelectrons was measured with a concentric hemispherical analyzer (PHOIBOS 150 1D-DLD, SPECS, Germany).

The catalytic activity of Fe-oxide/Al2O3 for NH3 oxidation was evaluated using a flow-type reactor connected to an online gas chromatograph (GC, Hewlett Packard, HP 6890). The GC was equipped with two capillary columns, each of which was connected to a thermal conductivity detector with a detection limit of approximately 400 ppm. A CP-Volamine column (Agilent Technologies, 30 m × 32 μm) was used to separate NH3, H2O, and nitrous oxide (N2O) from the gas mixture, while a CP-Molseive 5A column (Agilent Technologies, 25 m × 30 μm) was used for N2, nitric oxide (NO), and nitrogen dioxide (NO2). To measure activity with respect to reactor temperature, 4.0 g of Fe-oxide/Al2O3 (or bare Al2O3) was first heated to 750 °C for 2 h in dry air. The temperature was decreased to 450 °C, and from this point, the temperature was decreased to 200 °C at −1 °C/min. Simultaneously, a He-balanced gas mixture containing 2% NH3 and 6% O2 (total 50 mL/min) was introduced into to the reactor. The gas from the reactor outlet was analyzed by the GC periodically during this process. The catalytic stability of Fe-oxide/Al2O3 (or bare Al2O3) at 450 °C was also tested under the same catalyst-annealing and gas-flow conditions. These measurements were also taken under 30 and 60% of relative humidity (RH) conditions. For the adsorption and thermal treatment experiment of NH3, 50 mL/min gas mixture containing 2% NH3 and 6% O2 balanced with He was introduced into the reactor along with 4 g of the catalyst at RT for NH3 adsorption. After the concentration of NH3 at the reactor outlet reached 2%, indicating adsorption saturation, the flow of NH3 was stopped while maintaining O2 and He flow until the weakly adsorbed NH3 or gaseous NH3 in the reactor was completely purged out. When NH3 was no longer detected by the GC, a temperature-programmed oxidation experiment was performed by increasing the reactor temperature from RT to 500 °C at a ramping rate of +1 °C/min under the same gas-flow condition as used in the purging stage. The concentration of the desorbed species from the catalyst surface was analyzed with respect to temperature. This adsorption and thermal treatment process was repeated four times without replacing the catalyst. Adsorption and thermal oxidation of NH3 using a system of dual catalytic filters was allowed to proceed in the following order (Figure 5 provides a schematic of the system): first, 2 g of Fe-oxide/Al2O3 was placed in each reactor (reactor 1 and reactor 2), after which 50 mL/min He-balanced gas mixture containing 2% NH3 and 6% O2 was injected into reactor 1 at RT. At this time, the gas mixture passing through reactor 1 was directed to the online GC without passing to reactor 2. Second, once the catalyst in reactor 1 was saturated with NH3, the NH3 flow was stopped while maintaining the flow of He and O2, and the temperature of reactor 2 was rapidly increased to 400 °C. Third, when the temperature of reactor 2 reached 400 °C, the temperature of reactor 1 was increased from RT to 400 °C at a ramping rate of +1 °C/min to desorb or oxidize the NH3 adsorbed on the Fe-oxide/Al2O3 in reactor 1. In this step, the gas mixture that passed through reactor 1 was directed to reactor 2 before the online GC. Fourth, with the increasing temperature of reactor 1, the concentration of N2 or NH3 was analyzed by the online GC. Finally, steps 1 through 4 were repeated without replacing the catalyst. Further details of experimental setup and their procedures are provided in the Supporting Information (Figure S2).

Catalytic oxidation mechanism of NH3 on Fe-oxide/Al2O3 surfaces was studied using in situ DRIFTS and NAP-NEXAFS spectroscopy. For DRIFTS, a Fourier-transform infrared (FT-IR) spectrometer (Thermo Fisher Scientific, Nicolet iS10, USA) combined with a praying mantis DRIFTS accessory (Harrick Scientific, USA) was used. In 10 mg of mechanically ground Fe-oxide/Al2O3 diluted with 190 mg KBr, 50 mL/min He-balanced gas mixture containing 2% NH3 and 6% O2 was flowed, and FT-IR spectra were obtained at 30, 250, 350, and 450 °C. The spectrum of each temperature was obtained using a fresh catalyst sample. The infrared spectrum in the presence of bare Al2O3 at 450 °C was obtained for comparison. NAP-NEXAFS analysis on Fe-oxide/Al2O3 was carried out in the 8A2 AP-XPS beam line constructed at the Pohang Accelerator Laboratory (PAL). A 1.2 mg sample of ground Fe-oxide/Al2O3 powder in ethanol was drop-casted onto a 1 × 1 cm2 silicon wafer for analysis. Using a back-filling method and laser heating, atmospheric conditions and the sample temperature were controlled. Fe L edge spectra at 450 °C or ultrahigh vacuum (UHV) conditions at RT were obtained using the total electron yield mode. Residual gas analysis was conducted in parallel to determine whether the reaction atmosphere was maintained during surface analysis. The obtained Fe L edge spectra were deconvoluted using mixed functions of Gaussian and Lorentzian. For each component, peak width was fixed while its position was adjusted within 0.1 eV (Figure S7 and Table S1).

The tr-CVD-prepared Fe-oxide/Al2O3 beads, with an Fe content of 5.39 wt % as determined by ICP-OES analysis (Table 1), exhibited a yellowish color due to the incorporated Fe-oxide on Al2O3 (Figure 1a). In an elemental Fe mapping image of the cross-sectional plane of an Fe-oxide/Al2O3 bead (Figure 1b), the Fe is distributed evenly throughout the entire bead of Al2O3 with porous networks. In XRD analysis of Fe-oxide/Al2O3, diffraction peaks related to Fe-oxide were not visible, which also implies high dispersion of Fe-oxide on Al2O3 (Figure S3). Inspecting the structure of the Fe-oxide more closely with HAADF STEM imagery (Figure 1c), round protrusions were observed, the mean size of which was estimated to be ∼1 nm (see particle size distribution plot in the inset of Figure 1c). The protrusions were identified as the particles of Fe compounds from the line EDS analysis (Figure 1d). The pore structure of Al2O3 was not significantly altered upon deposition of Fe-oxide nanoparticles, based on N2-adsorption and desorption isotherm analysis. Both Al2O3 and Fe-oxide/Al2O3 showed typical isotherm plots of mesoporous materials (Figure 1e). Mono- and multilayer adsorption of N2 takes place at low pressures, while at high pressures, capillary condensation and evaporation take place in mesopores, indicated by the hysteresis loop in the plot. Only a slight decrease in the quantity adsorbed was found upon the deposition of Fe-oxide, which is related to the decrease of BET surface area of Al2O3 from 160.9 to 139.2 m2/g after the Fe-oxide deposition (Table 1). There was no notable change in the pore-size distribution plot (inset, Figure 1e) upon Fe-oxide deposition except for a slight decrease in total pore volume. Accordingly, the average pore size estimated by the BJH method (approximately 12 nm) remained almost constant after Fe-oxide deposition on Al2O3 (Table 1). The decrease in pore volume upon Fe-oxide deposition was likely due to the demolition of some Al2O3 pores upon annealing in the presence of Fe-oxide due to the strong interaction between Al2O3 and Fe-oxide, rather than due to the blocking effect of mesopores by Fe-oxide nanoparticles, which are much smaller than the average pore size of Al2O3. The chemical structure of the Al2O3-supported Fe-oxide nanoparticles was studied using XPS. In Figure 1f, the Fe 2p3/2 spectrum is shown with a peak deconvolution based on the Gupta and Sen (GS) method, which considers various possible factors related to the peak broadening of Fe 2p (e.g., electrostatic interactions, L-S coupling between unpaired 3d electrons in a photoionized Fe cation and its 2p hole, and crystal field interactions). Using the GS method, six independent components—prepeak, surface peak, and multiple peaks 1 to 4 ranging from 708.5 to 713.8 eV—were required to fit an Fe 2p3/2 spectrum of a single phase of Fe-oxide, and each phase of Fe-oxide had unique ratios among peaks 1–4. In our Fe-oxide/Al2O3, ratios among peaks 1–4 were 22, 35.89, 19.14, and 22.97%, respectively, which did not perfectly match any of single phase of Fe-oxide, implying that our Fe-oxide nanoparticles were instead a mixture of various Fe-oxide phases (e.g., FeO, Fe2O3, Fe3O4, FeOOH, ...). Raman spectroscopy analysis of the Fe-oxide/Al2O3 could also indicate the coexistence of various phases of Fe-oxide (Figure S4). Although the additional small component at 707 eV can be attributed to either Fe carbide or metallic Fe, its relative composition was less than 10%. In addition, in the X-ray absorption spectroscopic analysis (Figure 3b), which generally has higher informative depth than XPS, these components are not clearly visible, indicating that they contributed only marginally to overall the composition, and the majority of the Fe in our catalyst sample was in oxidized forms.

NH3 removal and N2 selectivity with respect to temperature under dry and humid (30% and 60% RH) conditions obtained from Fe-oxide/Al2O3 are displayed in Figure 2. The parameters obtained from bare Al2O3 at 450 °C under dry conditions are also included for comparison. Under dry conditions, 90% or greater NH3 removal was observed above 400 °C, after which it decreased gradually with temperature, reaching approximately 10% from 250 °C. N2 selectivity under dry conditions also decreased with reactor temperature. Nitrogen oxide gases (NO, NO2, or N2O) were not detected over the entire temperature range under our experimental conditions. That is, the majority of the removed NH3 that was not converted to gas phase N2 could reside on the surface of Fe-oxide/Al2O3 in reaction intermediate states or adsorbed forms. When bare Al2O3 was used, NH3 removal and N2 selectivity at 450 °C were approximately 29 and 58%, respectively, which are much lower than those from Fe-oxide/Al2O3. The results show that Fe-oxide deposition on Al2O3 can enhance not only the removal of NH3 but its selectivity to N2 as well. In comparison with dry conditions, NH3 removal under a 30% humid condition was similar at temperatures above 420 °C, but it became slightly lower below this temperature. N2 selectivity was affected only marginally by humidity at temperatures above 350 °C, but below this temperature, the presence of humidity reduced N2 selectivity. When humidity increased from 30 to 60%, NH3 removal did not decrease further but actually increased in a broad range of temperature, while N2 selectivity seemed to decrease slightly in the range of 320–400 °C. It can be speculated that H2O molecules can compete with NH3 molecules for adsorption sites, while they can also help conversion of NH3 to surface residual nitrogen compounds. The former role of H2O can induce reduction of NH3 removal; however, from a certain level of humidity, the later pathway can become more activated, leading to the improved NH3 removal and the reduced N2 selectivity. Based the observations shown in Figure 2, Fe-oxide/Al2O3 can effectively oxidize NH3 with high N2 selectivity and negligible nitrogen oxide gas emissions at temperatures above approximately 400 °C. The high catalytic activity and selectivity to N2 could also be maintained for at least 7 h under both dry and humid conditions at 450 °C (Figure S5).

In situ DRIFTS analysis was used to reveal the NH3 oxidation mechanism of Fe-oxide/Al2O3. Figure 3a shows infrared spectra obtained from Fe-oxide/Al2O3 at RT and 250, 350, and 450 °C under the same atmospheric conditions as those in the activity test (2% NH3 and 6% O2, He-balanced, 50 mL/min). An infrared spectrum obtained from bare Al2O3 at 450 °C under the same atmospheric condition is also displayed. Each spectrum in Figure 3a was obtained after 60 min of reactant gas flow to a fresh sample at each temperature until the spectral shape no longer changed significantly with time. The infrared spectrum obtained before flowing the reactant gas was used for a background in each temperature, and each dotted line represents the spectrum obtained immediately after reactant gas flow. Spectral changes with reactant gas flow time in each reaction condition are provided in Figure S6. The IR signal assignments of the features observed in DRIFTS analysis are summarized in Table 2. At RT, the two infrared absorption peaks at 1260 and 1615 cm–1 can be attributed to symmetric and asymmetric N–H bending modes of physiosorbed NH3, respectively. Most of the NH3 molecules appeared to adsorb on Fe-oxide/Al2O3 surfaces, maintaining their structure at RT. With the increase in reaction temperature to 250 °C, noticeable changes in spectral shapes were observed; a broad signal appeared between 1100 and 1500 cm–1, and a more distinct peak could be seen at 1586 cm–1. Due to the broadness of the infrared signal, it is difficult to interpret the origin of the absorbance increase at 1100–1500 cm–1. However, given that the peak at 1586 cm–1 is typical of N–H scissoring of amino −NH2) groups and that other various bending modes of N–H from −NH2 generally fall into the 1100–1500 cm–1 range, it is likely that NH3 dissociatively adsorbs as −NH2 on an Fe-oxide/Al2O3 surface at 250 °C. An imine group (=N—H) can also exist from the dissociative adsorption of NH3 on the catalyst surface, as its bending modes generally appear at 1100–1500 cm–1. It is important to note that formation of nitrogen oxide compounds (NO, NO2, or N2O) did not likely form at 250 °C, not only because they were not detected as gaseous forms (Figure 2), but also because their related signals were not clearly resolved in the infrared spectrum. For example, in most cases that use Fe-oxide catalysts, a nitroxyl group (−HNO) is formed as a reaction intermediate in the formation of the nitrogen oxide compounds. It generally shows distinctive peaks of N–O stretching near 1480 and 1529 cm–1, and these were not clearly observed in our case. In addition, adsorbed NO or NO2 typically produce an infrared peak of N–O stretching at 1800–1900 cm–1, while adsorbed N2O shows an infrared peak from N–N stretching at 2100–2200 cm–1, and these were also not clearly observed in our experiments. Formation of nitrate (NO3–) is possible but less likely, as its N–O stretching vibration generally shows a sharp peak at 1300–1400 cm–1. The wiggling fine structures with fixed interval in 1400–1800 cm–1 are attributed to the rotational modes of water vapor, probably formed by the NH3 oxidation reaction. When increasing the temperature to 350 °C, the infrared signals at 250 °C increased in intensity, with no other prominent qualitative change, and at 450 °C, clear information on the NH3 oxidation mechanism was obtained; in addition to the absorbance increase at 1100–1500 and 1586 cm–1, new features were observed at 1190, 1311, 1540, and 1661 cm–1. The former three features at 1190, 1311, and 1540 cm–1 are most likely attributable to N–N stretching, NH2 wagging, and NH2 scissoring modes of adsorbed hydrazine (N2H4), respectively. The appearance of these features implies that −NH2 species generated from the dissociative adsorption of NH3 can form N–N bonds and dimerize into N2H4 on an Fe-oxide/Al2O3 surface, which is a key intermediate for the oxidation of NH3 to N2 in many transition metal oxide catalysts. From comparing IR spectra of as-prepared and hydrazine-adsorbed Fe-oxide/Al2O3, the N2H4 formation during the NH3 oxidation at high temperature could be also supported (Figure S8). The band appearing at 1661 cm–1 can be most likely assigned to the symmetric N–H bending mode of ammonium ions (NH4+), formation of which can be also supported by the concomitant absorbance increase near 1450 cm–1 attributed to its asymmetric N-H bending. Formation of NH4+ can be also involved in N2H4 formation; at high temperatures, NH3 can strongly adsorb on the surface, forming NH4+, and NH4+ reacts with active oxygen species to form −NH2 and H2O, after which −NH2 can be converted to N2H4. From the DRIFTS analysis, we speculated that −NH2 species can be actively formed at low temperatures by dissociative NH3 adsorption on the catalyst, but the formation of N–N bonds between two −NH2 species can be facilitated at high temperatures, which could increase N2 selectivity. The dissociatively adsorbed NH3, NH4+, and N2H4 structures on the surface of Fe-oxide/Al2O3 at elevated temperature can account for the missing portion of N2 selectivity, since the only gas product we could detect at a significant level during the NH3 oxidation was N2. For bare Al2O3 without Fe-oxide, a prominent peak at 1405 cm–1 attributable to NO3– was evident at 450 °C without indication of N2H4 formation. From activity measurements (Figure 2), bare Al2O3 could convert NH3 to N2 with 58% of N2 selectivity at 450 °C under dry conditions. Based on these results, bare Al2O3 seems to catalyze oxidation of NH3 to N2 in a different mechanism from Fe-oxide/Al2O3. An internal SCR mechanism where NO is formed by NH3 oxidation, which then react with other NH3 to produce N2, or −HNO intermediated mechanism is possible. Both of these pathways can induce surface poisoning by accumulation of nitrogen oxide species like NO3–, which can reduce N2 selectivity. The DRIFTS results overall suggest that Fe-oxide nanoparticles suppress the formation of NO3– from NH3 and simultaneously facilitate conversion of NH3 to N2 via the N2H4-mediated pathway.

While in situ DRIFTS analysis was used to study how the structure of reactant molecules changes on the surface during the reaction, NAP-NEXAFS analysis was employed to investigate which change the structure of catalyst itself undergoes during the reaction. Figure 3b shows the Fe L edge spectra from Fe-oxide/Al2O3 under UHV conditions at RT or various atmospheric conditions at 450 °C. The Fe Ledge spectrum under UHV at RT has two main features at 705.8 and 707.6 eV that are attributable to Fe’s oxidation states of 2+ and 3+, respectively. In an O2 atmosphere (0.3 mbar) at 450 °C, the ratio of Fe2+ to Fe3+ was similar to that in UHV conditions at RT. In an NH3 atmosphere (0.03 mbar), the ratio of Fe2+ to Fe3+ increased dramatically, along with the appearance of a shoulder at approximately 705 eV attributable to metallic Fe. Lattice oxygen atoms of Fe-oxide were consumed by their reaction with NH3 at the surface, contributing to a lower Fe3+ ratio. When O2 was added here, Fe3+ became dominant again, producing a spectral shape that at first glance appears to be similar to that in the O2-only condition. However, when superimposed with the spectrum under O2 only, it is clear that the ratio of Fe2+ to Fe3+ was higher in the presence of NH3 (inset of Figure 3b), indicating that some lattice oxygens of Fe-oxide are being reduced by NH3 oxidation. From Figure 3c, showing the composition of the Fe state estimated from the peak deconvolution of Fe L edge spectra in Figure 3b, the increase of Fe2+ composition upon adding NH3 to the O2 atmosphere could be more clearly seen. These results suggest that an Fe-oxide surface does not simply provide adsorption sites for NH3 and O2 without structural change but plays a more active role, based on a Mars–van Krevelen mechanism; lattice oxygen of Fe-oxide is used for the oxidation of adsorbed NH3 to −NH2, resulting in the reduction of Fe-oxide and the formation of product molecules such as H2O, which later desorb from the surface. The concomitantly generated oxygen-vacancy sites are replenished by dissociative adsorption of gaseous O2. Combining the DRIFTS and NAP-NEXAFS results, dissociative adsorption of NH3 forming −NH2 species took place, the Fe-oxide deposited on Al2O3 could facilitate this oxidation process of NH3 to −NH2 by its partial reduction, and the generated surface −NH2 species could further dimerize to form N2H4 species, desorbing as N2 gas.

For environments where NH3 flows noncontinuously or its concentration is low, it is not necessary to maintain high catalyst operating temperatures to decompose NH3, and using a catalytic adsorbent can be a superior strategy with an improved energy efficiency. NH3 can be adsorbed onto a catalytic adsorbent material at RT, and when all adsorption sites are saturated, adsorbed NH3 can be thermally oxidized into less- or nonharmful gases; in this way, thermal energy consumption for NH3 decomposition could be reduced. We repeated a cycle of NH3 adsorption at RT and subsequent heating to 500 °C four times consecutively without changing the catalyst. Figure 4a depicts the change in NH3 concentration at the reactor outlet as a function of NH3 flow time at RT in the presence of Fe-oxide/Al2O3 in each cycle. The gray line, which represents the change in NH3 concentration without the catalyst, shows that NH3 concentrations reached 2% within 20 min. However, it took much longer (approximately 100 min) to reach 2% in the presence of the catalyst due to NH3 adsorption on the Fe-oxide/Al2O3. This breakthrough curve was almost identical in all four cycles, indicating that the adsorption capacity of Fe-oxide/Al2O3 recovered through heating without significant deactivation. Figure 4b,c shows changes of NH3 and N2 concentrations at the reactor outlet as a function of reactor temperature during the heating after NH3 adsorption in each cycle. NH3 and N2 were the only gaseous products detected during the heating process; no other nitrogen oxide compounds were observed. NH3 was generated over a temperature range of 50 to 300 °C, indicating that the adsorbed NH3 molecules were desorbed in their molecular forms in this temperature range (Figure 4b), while N2 was generated from 150 to 400 °C as a result of NH3 oxidation on the Fe-oxide/Al2O3 surface. The generation profiles for NH3 and N2 were almost unchanged in all cycles of the thermal treatment process, i.e., the adsorption capacity of Fe-oxide/Al2O3 could be recovered via thermal treatment up to 500 °C. In our experiment, the purging step between NH3 adsorption and thermal treatments could remove not only the remaining NH3 gas in the reactor but also the weakly adsorbed NH3 on the catalyst surface. As they could not be separated easily, it is difficult to determine the exact amount of NH3 adsorbed on the sample at the beginning of the heating process, limiting our ability to estimate the selectivity of NH3 or N2 from the adsorbed NH3. It is possible that some of adsorbed NH3 was not desorbed from the surface but remained in oxidized form after thermal oxidation. This suggests that they had either had low surface coverage, which did not affect the total adsorption capacity of the catalyst, or they accumulated on the surface sites irrespective of NH3 adsorption. Despite the strong recovery of NH3 adsorption capacity of Fe-oxide/Al2O3, molecular desorption of NH3 during thermal treatment still poses a challenge as it corresponds to secondary emissions of the original pollutant (NH3) to the atmosphere.

A system with dual catalytic filters was designed to minimize NH3 emissions to the atmosphere during thermal treatment after NH3 adsorption (Figure 5). An additional reactor (reactor 2 in Figure 5) containing Fe-oxide/Al2O3, the temperature of which could be controlled independently, was installed at the outlet of the existing catalytic reactor (reactor 1 in Figure 5). Our intention was that only the Fe-oxide/Al2O3 in reactor 1 was used for NH3 adsorption at RT, while reactor 2 oxidizes NH3, emitted from the thermal treatment process of reactor 1, into N2. When the surface of the catalyst in reactor 1 was about to be saturated with NH3 at RT, the temperature of reactor 2 was rapidly increased, and when the temperature of reactor 1 was increased gradually, NH3 molecules desorbed from the catalyst surface of the reactor 1 passed through the sufficiently hot reactor 2 to be converted into N2. Using this process, not only the emission of NH3 to the atmosphere but also the energy consumed by the operation of the device for catalytic conversion of NH3 to N2 can be reduced. Figure 6a–c shows the experimental results from the dual catalytic filters, experimental details of which are provided in the experimental section. For comparison, experimental results from the single catalytic filter with same total catalyst amount (Figure 4) are displayed together in Figure 6. Because 2 g of catalyst was placed in each reactor in the system of dual catalytic filters, which is the half amount used in a single reactor, it took about half the time to reach NH3 adsorption saturation (50 min) with the dual system compared to the single reactor. In the subsequent thermal treatment process, NH3 was desorbed from 60 to 300 °C in the single reactor. However, in the dual reactors, in which the temperature of reactor 2 was maintained at 400 °C during the thermal treatment process of reactor 1, NH3 was not detected at any temperature (Figure 6b), indicating that NH3 desorbed from the catalyst in reactor 1 was totally removed upon passing to reactor 2. Although there was some NH3 slippage when 2% of NH3 flowed to catalyst at 400 °C (Figure 2), the concentration of NH3 desorbed from catalyst in reactor 1 was much lower, so 400 °C was sufficient for reactor 2 to totally remove NH3. Using the single reactor, N2 generation started from approximately 150 °C, whereas it started at 60 °C (temperature of reactor 1) using the dual reactors, which is similar to the NH3 desorption temperature in the single reactor. In addition, the shape of the N2 concentration-change profile with the dual reactors was similar to that of NH3 with the single reactor. Considering that the half amount of NH3 was adsorbed in the system of dual filters as compared to the single reactor system, it can be stoichiometrically estimated that almost all the desorbed NH3 in this temperature window was converted into N2. In the repeated adsorption and thermal oxidation experiment, NH3 was effectively converted to N2 without a decrease in catalytic conversion performance. Using the dual catalytic filters of Fe-oxide/Al2O3, NH3 can be efficiently decomposed to N2 without slippage or oxidation to other gaseous compounds.

Catalytically active Fe-oxide nanoparticles (<2 nm) were deposited on commercial porous Al2O3 beads using tr-CVD and subsequent annealing, resulting in high dispersion of thermally stable Fe-oxide species throughout the entire porous substrate. Based on XPS, Raman, and NEXAFS analyses, the Fe-oxide nanoparticles appeared to exist in a mixture of 2+ and 3+ oxidation states. The Fe-oxide/Al2O3 was able to oxidize NH3 to N2 with no harmful gas emissions, including NH3 and NOx, at temperatures above 400 °C. Combined analysis of in situ DRIFTS and NAP-NEXAFS suggests a Mars–van Krevelen-type surface NH3 oxidation mechanism for Fe-oxide/Al2O3; the lattice oxygen of Fe-oxide was used to oxidize adsorbed NH3 to −NH2, which dimerized into N2H4 intermediate species and then desorbed as N2. NH3 was emitted from the surface between RT and 300 °C during thermal treatment of the Fe-oxide/Al2O3 surface precovered with NH3 at RT. A system with dual catalytic filters of Fe-oxide/Al2O3—two interconnected catalytic reactors with independent temperature control—was designed to fully oxidize this desorbed NH3 to N2. By heating the rear side reactor to approximately 400 °C prior to thermal treatment of an NH3-covered surface in the front side of the reactor, the desorbed NH3 can be oxidized to N2 via passing the rear-side catalysts. Using this approach, NH3 can be removed without emissions to the atmosphere with efficient energy consumption.