SELECTIVITY AMONG DEHYDROGENATION STEPS FOR ALKYL-GROUPS ON METAL-SURFACES - COMPARISON BETWEEN NICKEL AND PLATINUM

The thermal chemistry of ethyl and neopentyl iodides on Pt(111) surfac es was investigated by temperature-programmed desorption and reflectio n-absorption infrared (RAIRS) spectroscopies. The analysis using RAIRS of the isotopic composition of the ethylidyne formed from adsorption of CH3CD2I at different temperatures provided a reasonable estimate fo r the difference in activation energies between alpha- and beta-hydrid e elimination steps from alkyl groups adsorbed on Pt(lll). A study of the reactivity of neopentyl groups on the same surface using selective deuterium labeling yielded additional information on the relative rat es of alpha- versus gamma-hydride eliminations. It was determined that C-H bond-scission steps at the alpha and gamma positions display comp arable rates and that both are several orders of magnitude slower than dehydrogenation at the beta carbon. A comparison is also presented he re to similar chemistry on nickel substrates, where dehydrogenation re actions are usually much faster and where alpha-elimination dominates over the gamma counterpart. The implications of these results to catal ysis are discussed in the text.