Aqueous N2O Reduction with H2 Over Pd-Based Catalyst: Mechanistic Insights From Experiment and Simulation
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Nitrous Oxide (N2O), an ozone depleting greenhouse gas, is an observed intermediate in aqueous nitrate/nitrite reduction mediated by both natural microbial and synthetic laboratory catalysts. Because of our interest in catalytic nitrate/nitrite remediation, we have endeavored to develop a detailed concordant experimental/theoretical picture of N2O reduction with H2 over a Pd catalyst in an aqueous environment. We use batch experiments in H2 excess and limiting conditions to examine the reduction kinetics. We use density functional theory (DFT) to model the elementary steps in N2O reduction on model Pd(100), Pd(110), Pd(111) and Pd(211) facets and including the influence of adsorbed O, H, and of H2O. Both experiments and theory agree that hydrogen is necessary for removal of adsorbed oxygen from the catalyst surface. The dissociation of N2O to N2(g) and O(ads) is facile and in the absence of H proceeds until the catalyst is O-covered. Water itself is proposed to facilitate the hydrogenation of surface O by transferring absorbed hydrogen to Pd-absorbed O and OH. We measure an apparent activation energy of 41.4 kJ/mol (0.43 eV) for N2O reduction in the presence of excess H2, a value that is within 0.1 eV of the barriers determined theoretically.