The blocking of the step-mediated indirect channel to hydrogen dissociation by oxygen on Pt(5 3 3)

The initial dissociative sticking probability, S0, of H2 on clean and oxygen modified Pt(5 3 3) was measured as a function of incident kinetic energy, Ei, surface temperature, Ts, and coverage, θH. At low oxygen coverages, where oxygen adsorption leads to oxygen atoms decorating step sites, the indirect channel to hydrogen dissociation operating in the energy regime Ei<150 meV, is blocked. This blocking is shown to be complete when the (1 0 0) step sites are saturated by oxygen at a coverage of θO=0.12. For this oxygen decorated surface, a second indirect channel, mediated by an accommodated physisorbed precursor on the clean surface at Ei<30 meV, remains intact, as does the higher energy direct dissociation channel characteristic of the (1 1 1) terraces. A similar weak Ts dependence is exhibited for the very low energy indirect channel on the clean and oxygen modified surface. The low energy (physisorbed precursor) indirect channel, and the direct channel, are blocked only as oxygen is atomically adsorbed on the (1 1 1) terraces. The θH dependence measured at various energies on the clean and oxygen decorated surface at θO=0.12 also indicate the blocking of the step-mediated indirect channel which is characterised by a weak θH dependence. These results are consistent with the conclusion that the indirect channel to hydrogen dissociation operating in the energy regime Ei<150 meV, suggested to involve an unaccommodated precursor, is mediated by step sites. The importance of the poisoning of step sites in reactions involving hydrogen dissociation from a thermalised isotropic gas at 300 K is also considered.