Dissociation of methane and ethane on Pt(110): Evidence for a direct mechanism under thermal conditions

Abstract

The dissociative sticking S0 for CH4, C 2H6, and their deuterated analogs has been measured on Pt(110) as a function of surface temperature Ts under thermal "bulb" conditions at low gas pressures (Tg = 300 K). Large increases in S0 with Ts are observed for all species, in partial qualitative agreement with previous measurements. S0 for CH4 has also been measured at a high Ar background pressure (T g ≈ Ts) and shows a much faster increase with T s than the low pressure case. This is interpreted as evidence that the dissociation mechanism for CH4 under these bulb conditions is direct rather than precursor mediated, a conclusion which is in conflict with previous interpretations. In addition, the increase in S0 with T s for C2H6 and C2D6 exhibits concave curvature at low Ts, i.e., is non-Arrhenius, and this is difficult to reconcile with standard precursor models. Both experimental results, however, are shown to be in qualitative accord with a previously developed dynamical model for direct dissociation, as long as the dissociation limit is endothermic. © 1994 American Institute of Physics.