Ethylidyne formation from ethylene over Pd(111): Alternative routes from a density functional study

Recently we presented a computational study on the conversion of ethylene to ethylidyne over Pd(111) via a plausible three-step mechanism, ethylene → vinyl → ethylidene → ethylidyne. Here, using essentially the same periodic slab model density functional approach, we investigate two further possible routes, ethylene → vinyl → vinylidene → ethylidyne and ethylene → ethyl → ethylidene → ethylidyne. We systematically compared three coverages of the adsorbate, 1/3, 1/4, and 1/9. We show that the reaction pathway via vinylidene is also feasible on Pd(111). One is not able to judge solely from the potential energy landscape whether the route via ethylidene or that via vinylidene dominates the formation of ethylidyne; at low coverages, our results tend to favor slightly the latter mechanism. The mechanism via ethyl could be operative when a sufficient concentration of surface hydrogen is present. It features the lowest activation barrier for the rate-limiting second step, 81-88 kJ mol^-1, but the activation energy for ethyl hydrogenation to ethane, 51 kJ mol^-1, is still much lower; this suggests that ethyl should preferentially convert to ethane rather than to ethylidene.