ORIGINAL PAPER Aqueous N 2 O Reduction with H 2 Over Pd-Based Catalyst: Mechanistic Insights From Experiment and Simulation Dorrell C. McCalman • Kathleen H. Kelley • Charles J. Werth • John R. Shapley • William F. Schneider Published online: 4 May 2012 Ó Springer Science+Business Media, LLC 2012 Abstract Nitrous Oxide (N 2 O), 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 N 2 O reduction with H 2 over a Pd catalyst in an aqueous environment. We use batch experiments in H 2 excess and limiting conditions to examine the reduction kinetics. We use density functional theory (DFT) to model the elementary steps in N 2 O reduction on model Pd(100), Pd(110), Pd(111) and Pd(211) facets and including the influence of adsorbed O, H, and of H 2 O. Both experiments and theory agree that hydrogen is necessary for removal of adsorbed oxygen from the catalyst surface. The dissociation of N 2 O to N 2 (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 N 2 O reduction in the presence of excess H 2 ,a value that is within 0.1 eV of the barriers determined theoretically. Keywords Nitrous oxide reduction  Hydrogen  Palladium  Aqueous  Density functional theory 1 Introduction Nitrous oxide is currently the most important ozone- depleting emission that is not severely constrained by international agreements [1–3]. It is also a potent greenhouse gas, with a global warming potential (GWP) of 310 equiv- alents of CO 2 [3], and is the third-most-important contributor to global warming, after CO 2 and methane. The nitrous oxide levels in the atmosphere are the highest since the last glacial epoch [4, 5], with an 18% increase since 1750 that is pri- marily attributed to anthropogenic causes [3, 4]. N 2 O þ H 2 ! N 2 þ H 2 O ð1Þ Catalytic remediation of N 2 O as shown in Eq. (1) has been extensively studied, but much of the previous work has focused on heterogeneous gas-phase chemistry at elevated temperatures or pressures [6–12]. N 2 O also occurs in water, both naturally, and more importantly as a reaction intermediate during denitrification of water and wastewater containing nitrate and/or nitrite [3, 12–23]. D. C. McCalman  W. F. Schneider (&) Department of Chemical and Biomolecular Engineering and Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, IN 46556, USA e-mail: wschneider@nd.edu D. C. McCalman  W. F. Schneider Center of Advanced Materials for the Purification of Water with Systems, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA K. H. Kelley  J. R. Shapley (&) Department of Chemistry and Center of Advanced Materials for the Purification of Water with Systems, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA e-mail: shapley@illinois.edu C. J. Werth Department of Civil and Environmental Engineering, and Center of Advanced Materials for the Purification of Water with Systems, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA 123 Top Catal (2012) 55:300–312 DOI 10.1007/s11244-012-9795-1