Thermal Chemistry of Iodomethane on Ni(110). 1. Clean and Hydrogen-Predosed Surfaces

Abstract

The thermal chemistry of iodomethane on Ni(110) single-crystal surfaces was studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy. The activation of the C−I bond occurs by 150 K, and produces methyl surface moieties. Hydrogen desorption is always seen, in one peak at 350 K at low coverages and in additional lower temperature regimes above a 1.0 langmuir CH3I exposure. Methane is also produced, in an initial peak at 274 K for 1.0 langmuir, and mainly in a second feature at 238 K on saturated surfaces. Additional experiments with CD3I and with coadsorbed hydrogen or deuterium indicate that the hydrogenation step of methyl to methane is controlled by the population of surface hydrogen, which originates either from decomposition of some of the methyl intermediates and/or from preadsorption. Interestingly, and in contrast to what is observed on Ni(100) or Ni(111) surfaces, some production of heavier hydrocarbons is seen on this Ni(110) surface at about 188 K at high CH3I coverages. A mechanism is proposed where an initial dehydrogenation of some of the adsorbed methyl to methylene surface species is followed by a rate-limiting methylene migratory insertion into a metal−alkyl (methyl) bond to yield a heavier alkyl (ethyl) intermediate. Facile subsequent β-hydride and reductive eliminations (the latter with surface hydrogen) account for the formation of the alkenes and alkanes seen in the TPD experiments, respectively.