(267b) Mechanistic Insights Into the Reduction of Nitric Oxide by Hydrogen On Platinum Catalysts

Nitric oxide (NO) is a major pollutant gas that has many deleterious effects. In order to limit its release into the atmosphere, increasingly stringent environmental regulations have been enacted, stimulating the development of more efficient methods to abate NO emitted from combustion processes. Three-way catalysts (TWC) in automobiles and the selective catalytic reduction (SCR) process in stationary plants are two common methods for catalytic removal of NO.^[1]  Although these methods are widely applied, the use of a very expensive metal, Rh, in TWC and the environmental and economic drawbacks associated with ammonia (NH₃) in SCR motivate the search for improved NO removal processes. The reduction of NO by H₂ over transition metal catalysts is one promising alternative method. Pt catalysts show high activity, but the low temperature selectivity towards the desired product, N₂, is low.^[2-3] Nitrous oxide (N₂O) and NH₃ are the two undesired side-products, due to their negative environmental impacts.^[4]

Here we use periodic, self-consistent Density Functional Theory (DFT) calculations to investigate the mechanism of NO reduction by H₂ on Pt(100). We consider direct NO dissociation as well as hydrogen-assisted pathways, where NO is hydrogenated prior to cleaving the N-O bond. The minimum energy pathway for NO activation is identified by comparing the potential energy surfaces of direct and hydrogen-assisted N-O dissociation pathways. The energetics leading to the formation of competing products is studied to understand the reaction selectivity. Furthermore, a surface with substantial NO coverage is simulated to understand the effect of NO coverage on the reaction mechanism. We find that the dominant N-O activation pathway changes with surface coverage on Pt(100). Finally, results on Pt(100) and Pt(111) are compared to elucidate the structure sensitivity of NO reduction by H₂.  

[1]        R. M. Heck, R. J. Farrauto, S. T. Gulati, Catalytic air pollution control: commercial technology, 2nd ed., Wiley-Interscience, New York, 2002.

[2]        H. G. Stenger, J. S. Hepburn, Energy & Fuels 1987, 1, 412.

[3]        P. Granger, F. Dhainaut, S. Pietrzik, P. Malfoy, A. S. Mamede, L. Leclercq, G. Leclercq, Topics in Catalysis 2006, 39, 65.

[4]        T. P. Kobylinski, B. W. Taylor, Journal of Catalysis 1974, 33, 376.