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The thermal chemistry of diiodomethane on Ni(1 1 0) single-crystal surfaces was studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Diiodomethane was chosen as a precursor for the formation of methylene surface species. I 3d and C 1s XPS data indicated that, indeed, adsorbed diiodomethane undergoes the C–I bond dissociations needed for that transformation, and detection of iodomethane production in TPD experiments pointed to the stepwise nature of those reactions. Significant amounts of methane are produced from further thermal activation of the chemisorbed methylene groups. This involves surface hydrogen, both coadsorbed from background gases and produced by dehydrogenation of some of the adsorbed diiodomethane, and can be induced at temperatures as low as about 160 K, right after the C–I bond breaking steps. Unique to this system is the detection of significant amounts, up to 10% of the total CH2I2 adsorbed, of heavier hydrocarbons, including ethene, ethane, propene, propane, and butene. Deuterium labeling experiments were used to provide support for a mechanism where the initial hydrogenation of some adsorbed methylene to methyl moieties is followed by a rate-limiting methylene insertion step to yield ethyl intermediates. Facile subsequent b-hydride elimination and reductive elimination with coadsorbed hydrogen account for the formation of ethene and ethane, respectively, while a second and third methylene insertions lead to C3 and C4 production. Based on the final product distribution, the methylene insertion was estimated to be approximately 20 times slower than the following hydrogenation–dehydrogenation reactions. Normal kinetic isotope effects were observed for most of the hydrogenation and dehydrogenation reactions involved. � 2003 Elsevier B.V. All rights reserved. |
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