SIMULATION STUDIES OF THE DISSOCIATION OF H2 ON STEPPED SURFACES

We have simulated the dissociation reaction of H-2 on stepped ideal honeycomb, square and hexagonal surfaces with a simple kinetic model based on the idea proposed by Poelsema and Comsa [Scattering of Thermal Energy Atoms from Disordered Surfaces, Springer Tracts in Modern Physics 115 (Springer, Berlin, 1989)] that dissociation on a stepped Pt(111) surface occurs exclusively on step sites. The observed sticking coefficient s(H-2) of H-2 consists of two competing channels: (1) physisorbed molecules incident on terrace sites diffusing to the step, and (2) direct dissociation of molecules incident on or near a step site. The results support this picture qualitatively and indicate that the behaviour of the system can be separated depending on surface temperature into three domains in each of which a different mechanism dominates. (1) T < T1: a geometric effect, i.e. the cluster size distribution of vacant adsorption sites on the surface, (2) T1 < T < T2: a geometric effect combined with nearest neighbour interactions due to adsorbed H atoms, and (3) T > T2: direct adsorption near a step site. Of the three surfaces structures studied the honeycomb structure corresponds most closely to the observed H2 dissociation.