Semi-Empirical Calculations on the Stability and Reactivity of NHx Species on Metal Surfaces

Abstract

The semi-empirical method of interacting bonds was used to elucidate the mechanism of oscillation phenomena in the NO + H2 reaction on metal surfaces. Basic single-crystal planes of Pt, Rh, Ir, Fe, Ru, and Re were examined with respect to the stability of adsorbed NH n species (n = 0, 1, 2, 3); to the reactivity of NH n (n = 0, 1, 2) species toward adsorbed hydrogen atoms; and to the possibility of combination reactions between two NH or two NH2 species resulting in the formation of gaseous N2 molecules. All studied surfaces were found to form readily stable NH species. The principal difference between Pt, Rh, and Ir single-crystal planes exhibiting reaction rate oscillations, and Fe, Ru, and Re surfaces, which do not show an oscillatory behavior, is that the combination reaction of NH species can easily proceed in the former case, whereas this reaction is not allowed thermodynamically in the latter. This result is consistent with an earlier suggested model that attributes the oscillatory surface wave propagation to the intermediate formation of NH species. Stable NH2 species can be formed on Ru, Re, and Fe surfaces, whereas the noble metal surfaces of Pt, Rh, and Ir can only form weakly stable NH2 species at the very edge of their existence region. The combination reaction between two NH2 species is endothermic in all cases.