LEED analysis of acetylene and ethylene chemisorption on the Pt(111) surface: Evidence for ethylidyne formation

Abstract

The stable surface species formed from the chemisorption of acetylene (C₂H₂) or ethylene (C₂H₄) on the Pt(111) surface (T∼300–350 K) has been studied by a low‐energy electron diffraction intensity analysis. High resolution electron energy loss spectra reported by Ibach et al. have been interpreted by comparison to infrared data on relevant model compounds. The surface species most consistent with these studies is ethylidyne ( C–CH₃). The species is coordinated to a threefold surface site with the C–C axis normal to the surface within an uncertainty of ∼15°. A saturated C–C bond length of 1.50±0.05 Å and three equivalent Pt–C bond lengths of 2.00±0.05 Å are determined by the LEED analysis and are consistent with the reported structures of ethylidyne in organometallic clusters. The ethylidyne group forms readily upon exposure of C₂H₄ to the Pt(111) surface at T∼300 K with the loss of one hydrogen atom per ethylene. The complete conversion of C₂H₂ to ethylidyne requires the presence of hydrogen atoms and proceeds rapidly at T∼350 K. By comparison to reported reaction mechanisms on related transition metal clusters it seems likely that vinylidene ( C=CH₂) is an intermediate in both reactions.