The Role of NHn Species in Oscillation Phenomena in the NO + H2 Reaction on Noble Metal Surfaces: Semi-Empirical Calculations
A. R. Cholach1, N. N. Bulgakov1 and B. E. Nieuwenhuys2
1G. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences,
Pr. Akademika Lavrentyeva 5, Novosibirsk 630090 (Russia), E-mail: cholach@catalysis.nsk.su
2Leiden Institute of Chemistry, P. O. Box 9502, 2300 RA Leiden (The Netherlands)
Abstract
The semi-empirical Method of Interacting Bonds was used in the present work to clarify the mechanism of the title process. Various single crystal planes of Pt, Rh, Ir, Fe, and Re were examined with respect to the stability of the adsorbed NHn species (n = 0, 1, 2, 3); to the reactivity of NHn (n = 0, 1, 2) species towards adsorbed hydrogen atoms; and to the possibility of proceeding the combination reactions between two NH or two NH2 particles resulting in the formation of gaseous H2 and N2 molecules. All the surfaces studied were found to form readily the stable NH species. The principal difference between Pt, Rh, Ir single crystal planes, on which the reaction exhibits rate oscillations, and Fe, Re surfaces, which do not show an oscillatory behaviour, is that the combination reaction of NH species can easily proceed in the former case, but this reaction is not allowed thermodynamically on the latter surfaces. This result is consistent with an earlier suggested mechanism for the oscillatory behaviour that attributes the surface wave propagation to the intermediate formation of NH species. Stable NH2 species can be formed on Re and Fe surfaces, whereas the noble metal surfaces can form weakly stable NH2 particles at the very edge of their existence region. The combination reaction between two NH2 species is endothermic in all cases.
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