The Effect of Hydrogen Coadsorption on the Thermal Chemistry of Methyl Iodide on Ni(100) Surfaces

Abstract

A detailed investigation of the surface chemistry of methyl iodide with preadsorbed hydrogen on Ni( 100) is reported here. TPD data indicate that, in general, the presence of hydrogen on the surface induces a yield increase in methane formation and a reduction of the extent to which methyl species decompose as compared with the case where the methyl iodide is adsorbed by itself on a clean surface. Furthermore, two very different methane desorption regimes are observed at 150 and 220 K. By using both isotopically labelled methyl iodide and deuterium (D2 + CH3I and H2 + CD3I) it was determined that while the high-temperature methane is formed via the reductive elimination of surface methyl species produced by decomposition of methyl iodide with coadsorbed hydrogen atoms, the lower temperature methane TPD peak may be the result of a direct attack of the surface hydrogen on adsorbed methyl iodide molecules in a concerted fashion instead. The TPD data also indicate that neither methylene nor methylidyne intermediates form during the decomposition of methyl iodide on nickel below 200 K, and experiments with CH2I2 clearly show that methylene species can be easily hydrogenated to methane in the presence of D2 coadsorption at quite low temperatures. Finally, no H-D exchange between methyl species and coadsorbed hydrogen (H2) was observed.