Ethylidyne formation from ethylene over Pt(111): A mechanistic study from first-principle calculations

The conversion of ethylene to ethylidyne on Pt(111) has been studied using density functional periodic slab model calculations. Similar to our recent investigation of this reaction on Pd(111), we considered the following three mechanisms: (M1) ethylene → vinyl → ethylidene → ethylidyne; (M2) ethylene → vinyl → vinylidene → ethylidyne; (M3) ethylene → ethyl → ethylidene → ethylidyne. We systematically compared three coverages of the adsorbate, 1/3, 1/4, and 1/9. Our calculations show that the typical barriers of hydrogenation-dehydrogenation reactions on Pt(111), 19-92 kJ mol^-1, are slightly lower than those on Pd(111), 25-120 kJ mol ^-1. The barriers of direct 1,2-H shift reactions are much higher, above 160 kJ mol^-1. The surface coverage notably affects the relative barriers of the reactions, by up to 30 kJ mol^-1. Mechanisms M1 and M2 are expected to be competitive. As the barriers of the three elementary steps of mechanism M3 are lower or comparable to the rate-limiting barriers of the other two mechanisms, M3 could be operative when a sufficient concentration of surface hydrogen is present. However, at such conditions one expects the formation of ethane rather than that of ethylidene. On the basis of our calculated vibrational frequencies and reaction barriers, we suggest that an intermediate identified in recent vibrational spectroscopic studies of the title reaction is possibly not ethylidene but perhaps vinyl.