The stable surface species formed from the chemisorption of acetylene (C2H2) or ethylene (C2H4) on the Pt(111) surface (Tsim;300ndash;350 K) has been studied by a lowhyphen;energy electron diffraction intensity analysis. High resolution electron energy loss spectra reported by Ibachetal. have been interpreted by comparison to infrared data on relevant model compounds. The surface species most consistent with these studies is ethylidyne (thinsp;Cndash;CH3). The species is coordinated to a threefold surface site with the Cndash;C axis normal to the surface within an uncertainty of sim;15deg;. A saturated Cndash;C bond length of 1.50plusmn;0.05 Aring; and three equivalent Ptndash;C bond lengths of 2.00plusmn;0.05 Aring; 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 C2H4to the Pt(111) surface atTsim;300 K with the loss of one hydrogen atom per ethylene. The complete conversion of C2H2to ethylidyne requires the presence of hydrogen atoms and proceeds rapidly atTsim;350 K. By comparison to reported reaction mechanisms on related transition metal clusters it seems likely that vinylidene (thinsp;C=CH2) is an intermediate in both reactions.
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