Aqueous N2O Reduction with H2 Over Pd-Based Catalyst: Mechanistic Insights From Experiment and Simulation
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.
KeywordsNitrous oxide reduction Hydrogen Palladium Aqueous Density functional theory
This work was supported by the National Science Foundation with Division of Chemistry Grant CHE 07-18078 and with WaterCAMPWS STC Agreement Number CTS-0120978. The Notre Dame Center for Research Computing is acknowledged for computational facilities.
- 1.United Nations Environmental Program: Ozone Secretariat. In The Montreal Protocol on Substances that Deplete the Ozone Layer: as either adjusted and/or amended in London 1990, Copenhagen 1992, Vienna 1995, Montreal 1997, Beijing 1999; 2000Google Scholar
- 3.United States Environmental Protection Agency. ‘2010 US Greenhouse Gas Inventory Report: Inventory of U.S. Greenhouse Gas Emissions and Sinks, 1990–2008′. 2010, U.S. EPA # 430-R-10-006Google Scholar
- 23.Chaplin B, Reinhard M, Schneider W, Schueth C, Shapley J, Strathmann T, Werth C (2012) Environ Sci Technol 46:3655–3670Google Scholar
- 31.Kang Dae-Bok (2007) Bull Korean Chem Soc 28(12):2369Google Scholar
- 33.Zhang R (2011) Elucidation of nitrate reduction mechanisms on a pd/in bimetallic catalyst using isotope labeled nitrogen species. PhD Thesis, University of Illinois at Urbana, ChampaignGoogle Scholar
- 34.Perdew JP, Chevary JA, Vosko SH, Jackson KA, Pederson MR, Singh DJ, Fiolhais C (1992) Phys Rev B 46(11):6671Google Scholar
- 35.Blochl (1994) Phys Rev B 50(24):17953Google Scholar
- 36.Kresse G, Furthmuller J (1996) Phys Rev B 54(16):11169Google Scholar
- 37.Kresse G, Joubert D (1999) Phys Rev B 59(3):1758Google Scholar
- 40.Boudart M, Djega-Mariadassou G (1984) In Kinetics of heterogeneous catalytic reactions. Princeton University Press, New Jersey, pp 213Google Scholar