THE CHEMISTRY OF METHYL-IODIDE ON MO(100) - FORMATION AND REACTION OFADSORBED METHYL SPECIES

The adsorption of methyl iodide is studied on Mo(100) where ultraviole t photoelectron spectroscopy shows that the methyl iodide thermally de composes to form methyl species on warming to similar to 200 K. This s elf-hydrogenates to yield methane which desorbs at similar to 230 K an d predosing the surface with deuterium forms essentially only CH3D ind icating that the methyl species reacts with adsorbed hydrogen and that there is little H-D exchange. This methane desorption temperature is identical to that found when methylene species are grafted on the surf ace by adsorbing methylene iodide indicating that methyl hydrogenation to methane is the rate-limiting step. This temperature is substantial ly lower than that at which methyl and methylene species yield methane from clean and oxygen-covered Mo(100) when formed by other routes. It is proposed that this is due to the effect of relatively large iodine atoms adsorbed in the four-fold hollow site affecting neighboring (ei ther atop or bridge) sites. (C) 1998 Elsevier Science B.V.