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

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Abstract
The dissociative sticking S 0 for CH4, C2H6, and their deuterated analogs has been measured on Pt(110) as a function of surface temperature T s under thermal ‘‘bulb’’ conditions at low gas pressures (T g =300 K). Large increases in S 0 with T s are observed for all species, in partial qualitative agreement with previous measurements. S 0 for CH4 has also been measured at a high Ar background pressure (T g ≊T s ) 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 S 0 with T s for C2H6 and C2D6 exhibits concave curvature at low T s , 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.