Electrochemical Nitric Oxide Reduction on Metal Surfaces
Electrocatalytic denitrification is a promising technology for removing NO x species (NO 3 − , NO 2 − and NO). For NO x electroreduction (NO x RR), there is a desire for understanding the catalytic parameters that control the product distribution. Here, we elucidate selectivity and activity of catalyst for NO x RR. At low potential we classify metals by the binding of * NO versus * H. Analogous to classifying CO 2 reduction by * CO vs * H, Cu is able to bind * NO while not binding * H giving rise to a selective NH 3 formation. Besides being selective, Cu is active for the reaction found by an activity-volcano. For metals that does not bind NO the reaction stops at NO, similar to CO 2 -to-CO. At potential above 0.3 V vs RHE, we speculate a low barrier for N coupling with NO causing N 2 O formation. The work provide a clear strategy for selectivity and aims to inspire future research on NO x RR.
Introduction<!>Electrochemical reduction of NO<!>Results and Discussion<!>NORR under Low Potential<!>NORR towards N 2 O under High Potential<!>Conclusions<!>Computational Details
<p>Nitrogen can be transformed through biological and physical processes within the nitrogen cycle to form a broad spectrum of inorganic compounds, including ammonia (NH 3 ), hydrazine (N 2 H 4 ), hydroxylamine (NH 2 OH), nitrite (NO 2 − ), nitrate (NO 3 − ), nitric oxide (NO), and nitrous oxide (N 2 O). 1,2 While many of these processes are essential to life, largescale commercial food production has led to over-fertilization and consequent accumulation of nitrate and nitrite ions in groundwater and in agricultural runoff streams. Both nitrate and nitrite ions are a source of pollution in groundwater, lakes, and coastal water, causing serious health problems, such as methemoglobinemia and cancer, when ingested in high concentrations. [3][4][5] As a result, groundwater treatments to reduce the concentration of these ions have been developed and have become an important environmental consideration. 6,7 Meanwhile, the artificial N 2 fixation towards NH 3 is produced via the Haber-Bosch process. In the Haber-Bosch process, NH 3 is produced in a reaction between H 2 and N 2 at high pressure and temperature. As a consequence, the Haber-Bosch process depends on the use of gaseous H 2 which is commonly made from fossil fuel steam methane reforming (SMR).</p><p>By contrast, the electrochemical conversion of NO x (x = 1, 2, and 3) to NH 3 involves only H + and electrons from electricity that can be from sustainable sources, like wind and solar energy.</p><p>Electrochemical denitrification is a promising strategy for the removal of NO x species from waste-water and constitutes a plausible alternative to traditional denitrification methods, such as biological and physical separations processes, due to its environmental compatibility, energy efficiency, safety, product selectivity, and potential for use in smaller-scale devices. [6][7][8][9][10][11][12][13][14][15] Nitrate and nitrite electroreduction can produce a variety of products, such as N 2 , N 2 O, NH 3 , and NH 2 OH. 7,9,16,17 The reduction of NO is an essential environmental reaction, since it determines the performance of waste-water treatment catalysts for NO 3 − , NO 2 − and NO removal. 6,7 Investigating electrochemical NO reduction (NORR) is a first step toward understanding the fundamentals of electrocatalytic NO 3 − /NO 2 − reduction and developing improved electrochemical denitrification or even sufficient NH 3 production technologies. 6,7,9,[18][19][20] In addition, the study of NO electrochemistry is of fundamental scientific interest because observations and developed methods can be straightforwardly applied to other environmentally friendly electrocatalytic reduction chemistries. Indeed, as we will show, the electrochemistry of NO has numerous analogies to CO 2 /CO electroreduction, which has been reported before. 21,22 In this work, we study the potential dependence of the electrocatalytic reduction of NO among a group of metals via density functional theory (DFT) simulations. Electrochemical NO reduction is complicated reaction, and in this work, we aim to understand the reaction and capture the main features of the reaction, which can enable us to find promising catalysts without knowing all the reactions details, such as kinetics. The ideal scenario of all electrocatalysts is that a catalyst can conduct the reaction at low overpotential and with acceptable rates and faradaic efficiency. For NORR at catalyst potential, just above HER, NH 3 formation has been reported with fairly high Faradaic efficiency. 14,23,25,31 However, at these potentials a relatively low rate is obtained, simply due to the slow HER. In addition,</p><p>Increasing the overpotential to speed up the reaction, is not possible since this leads to a significant increase in H 2 evolution and as a result the Faradaic efficiency quick drops. 14,26,31 In conclusion, the scenario that we try to cover in our selective scheme is the case where selective NH 3 formation from NORR is obtained with acceptable rate with a normal surface area catalyst. We note that rates will depend on the number of active sites; if a high surface area catalyst is used, sufficient rates could in principle be obtained prior to HER with acceptable Faradaic efficiency. Here, we classify metals for the electrochemical reduction of Our working hypothesis is that the binding energies of relevant intermediates will determine trends in catalytic properties of each metal catalyst. This is under the assumption that (a) the interaction between metals and the electrochemical environment is similar; (b) the trends in low-coverage regime capture the most important features of the catalyst performance; (c) the classification is assumed to be within the major products.</p><p>(</p><!><p>The electrochemical conversion of NO 3 − under reduction condition is complex, involving multiple reactions, products and stable intermediates (e.g., ammonia, nitrite, hydroxylamine, nitric oxide, nitrous oxide) spanning the many nitrogen oxidation states (from -III up to +V).</p><p>With referring to the reversible hydrogen electrode potential (RHE), the reduction products would be produced following the reactions below:</p><p>As for the NO 2 − reduction process:</p><p>Electrochemical NO reduction reaction has been studied under a variety of reaction conditions, including reactions on different metal electrodes, [23][24][25][26] and varying NO concentrations. 23,27 Overall, there are four different products depending on potential observed during NORR process:</p><p>Except the reactions mentioned above, the competitive hydrogen evolution reaction (HER) becomes pronounced when potential goes below 0 V vs RHE:</p><p>Thus, below 0 V (vs RHE), the observed Faradaic efficiencies, or percent of electrons used in NORR, can be greatly impacted. All the reactions described above from NO 3 − /NO 2 − /NO, illustrate that adsorbed nitric oxide ( * NO) as divergent center, may control the electrocatalytic selectivity towards N 2 O, N 2 , NH 2 OH and NH 3 as illustrated in Fig. 2. As a result, the affinity to NO on metals will play an essential role in their reduction selectivity performance.</p><!><p>Fig. 1(a) shows for Pt that there are two major reaction regions for NO reduction:</p><p>• at low potential (< 0.3 V vs RHE) where both H 2 and NH 3 are observed, and at the potential below 0 V vs RHE, H 2 becomes the main product.</p><p>• at high potential (0.</p><!><p>To conduct electrocatalytic NO x reduction on catalysts, NO x species are expected to be adsorbed on the surfaces. In addition, the competition from HER also has to be avoided, especially under low potential region. and dominant H 2 production with more negative potential applied to these catalysts. This can be seen in some literature. 25,26,31 Taking Ni as an example, few observations of NH 3 has been seen at very low overpotential, before HER. 31 This is in the region where Ni can have * H underpotential deposited ( * H upd ) but is not working as sufficient HER catalyst. It is general observation that NH 3 can be formed as the main product at low overpotential when * H is present just prior to efficient HER for the catalyst.</p><p>For the metals above the horizontal line like Ag and Au, it is unfavourable to bind * NO, which limits NO reduction. Indeed, on Ag and Au, NH 3 formation has been reported in limited current. 14,31 This is at potentials where Ag and Au have very low coverage of * H,</p><p>just prior or at the potential for H 2 evolution. At more negative potentials, H 2 becomes the main product when more negative potential is applied. 14 Similar phenomena is reported on Ag and Au for CO 2 reduction where limited methane/methanol is observed. 32,33 These observation indicates that there exists local interesting potential range for NH 3 or CH 4 formation at limited rates on Ag/Au for NORR or CO 2 reduction respectively. However, experiments for NORR also have shown that no NO adsorbate layer on Au is formed and also little formation of NH 3 is observed. 25 In addition, no drastic changes in voltammograms for Ag in Ar and NO was also observed, supporting the limited NORR activity on Ag. 31 Here, only Cu metal binds * NO while not having * H upd leading to NH 3 conversion.</p><p>The analysis above, shows that NO x (NO 3 − , NO 2 − and NO) reduction on the metals to the left of the vertical line in Fig. 3 is greatly inhibited by HER, leading to low NO x product faradic efficiency. For the metals above the horizontal line in Fig. 3 much stronger than * H (-0.2 eV). 35 Although NO x or CO binds stronger at the catalyst in both cases, the availability of * H and hence kinetics for HER is so fast that NO x RR and CORR is not observed. This correspond to major differences in reaction prefactor for the competing HER and NO x RR/CORR, resulting in H 2 as primary product. However, the product distribution might change in the different setup, such as gas diffusion electrode (GDE) where NO can have higher chemical potential (pressure). 31 In rotating disk electrode (RDE)/aqueous setup, there is a limitation of the NO solubility (0.0056g/100ml at 20 °C).</p><p>Due to different setup, GDE and RDE change the availability of * H relative to the reactant * NO on surfaces which again is similar to the situation in CORR where different products distribution is reported in RDE and GDE setups. 36,37  formation among all metals investigated here, and its catalytic activity can be still enhanced by strengthening its * N adsorption, since it is still not on the top of the volcano.</p><p>Following the scaling relation between * N and * H and the scaling relation between * N and * NO shown Fig. S6, the product selectivity (vertical dashed lines) can be also indicated in Fig. 4. When ∆G * N is smaller than -1.5 eV, ∆G * H becomes negative and H 2 will be the main product. For metals with ∆G * N larger than -0.83 eV, the adsorption of * NO is unfavorable, leading to the formation of NO. Cu has ∆G * N between -1.5 eV and -0.83 eV, leading to high selectivity for NH 3 production in the process of NORR, as also seen in Fig. This computational analysis explains that the experimental results with high faradaic efficiency (> 90%) for NO x (NO 3 − , NO 2 − and NO) reduction to NH 3 have been achieved</p><p>on Cu-based catalysts. 10,11,26,[38][39][40][41][42] More interestingly, enhanced catalytic activity has also been observed when Cu is alloyed with strong * N adsorption metals, like Ni and Rh. 11,[39][40][41] This might be due to the stabilization of * NOH intermediate, getting close to the top of the volcano shown in Fig. 4(a) but still ∆G * N in the region of -1.5 eV to -0.83 eV. Fig. 4 shows that there is a very little window to find both selective and active catalyst for the NO reduction in the volcano. Specifically, it hints that we cannot find a metal catalyst that is better than Cu, since it will then become selective toward HER or it will not adsorb NO. At medium potentials (around 0 -0.3 V vs RHE), the HER is thermodynamically impossible and NH 3 is reported as the major product 25 . With limited access to adsorbed * H, much less current density is observed experimentally for the metals. 16,18,25 Still, much higher current density at medium potential is reported on Cu for NO x (NO 3 − , NO 2 − and NO) reduction under the same experimental conditions, compared to other transition metals.</p><!><p>At high potential region, N 2 O formation is observed. For this reaction to occur, N-N coupling is required.</p><!><p>In</p><!><p>The computational analysis was carried out using the grid-based projector-augmented wave (GPAW) method, a DFT code based on a projected augmented wave (all-electron frozen core approximation) method integrated with the atomic simulation environment (ASE). [43][44][45] The revised Perdew-Burke-Ernzerhof (RPBE) functional was used as an exchange-correlation functional. 46 The wavefunctions were represented on a uniform real-spaced grid with 0.18</p>