(268b) First Principles Simulation of Intermediate Steps in Nitrate Reduction On Metal Catalysts

Nitrate is a pollutant of concern present in drinking water sources. Heterogeneous catalysis with H2 as reductant is one promising approach for its removal. Experiments have identified Pd and Pd/Cu bimetallics as potential reduction catalysts. Experiments also indicate that NO is the key intermediate from which the desired product N2, the undesired product NH3, as well as the intermediate N2O are formed. To interpret experiment and aid in catalyst design, we use supercell density functional theory calculations to characterize intermediate steps in the catalytic transformation of NO to products. In this work we examine the effect of surface termination and adsorbate coverage on the stability of adsorbates and the mechanism of reduction. Our first-principles atomistic calculations are used to investigate the adsorption of relevant intermediate species (NO, H, O, N, N2O, H2O, NH, NH2, NH3, OH) involved in nitrate reduction on Pd surfaces at low coverage and also at high coverage in the case of Pd(111). The different Pd surface terminations explored include (111), (100), (110), and (211). We find that NO binds more strongly than the other adsorbates, suggesting that high coverage NO is in fact a key intermediate. We discuss implications of our results for the interpretation of experiments.