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) surfaces was investigated by temperature-programmed desorption and reflection-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 for the difference in activation energies between alpha- and beta-hydride 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 rates of alpha- versus gamma-hydride eliminations. It was determined that C-H bond-scission steps at the alpha and gamma positions display comparable rates and that both are several orders of magnitude slower than dehydrogenation at the beta carbon. A comparison is also presented here to similar chemistry on nickel substrates, where dehydrogenation reactions are usually much faster and where alpha-elimination dominates over the gamma counterpart. The implications of these results to catalysis are discussed in the text.