Molecular beam study of the H2–D2 exchange reaction on stepped platinum crystal surfaces: Dependence on reactant angle of incidence

The exchange reaction H2+D2→2HD was studied as a function of the angle of incidence (polar and azimuthal) of the beam of reactants on two stepped platinum surfaces, the Pt(S) -[6(111) × (111)] and Pt(S) -[5(111) × (111)]. The reaction probability was found to depend markedly on the direction of approach of the reactants to the step structure. The production of HD is highest when the reactants strike the open side of the step structure, decreasing by approximately a factor of 2 when the inner corner of the step is shadowed. These variations are intimately related to the geometry of the stepped surfaces and cannot be attributed to an activation energy barrier for adsorption. The observed dependence can be accounted for in a simple model, assigning different activities for H–H bond breaking to the step and terrace sites. For the best fit with the experimental data we find that the activity of a step site is seven times that of a terrace site at normal incidence.