The chemistry of iodomethane on MoAl alloy thin films formed on dehydroxylated alumina: Formation and reaction of surface methyl species

Abstract

The surface chemistry of iodomethane is studied in ultrahigh vacuum using X-ray and Auger spectroscopies and temperature-programmed desorption, on a MoAl alloy film formed by reacting molybdenum hexacarbonyl with dehydroxylated alumina. The alloy is grown by reacting Mo(CO) 6 with a thin alumina film on a molybdenum substrate at 700 K and heating to 1500 K. A portion of the iodomethane dissociates following adsorption at ∼150 K. Heating to ∼220 K desorbs molecular iodomethane from the surface leaving adsorbed methyl species and iodine, where the iodine appears to adsorb preferentially on the aluminum. The resulting methyl species can either decompose to deposit carbon and evolve hydrogen, hydrogenate to yield methane or oligomerize yielding predominantly ethylene and propylene, and a small amount of ethane is formed. Both methyl and ethyl radicals are found to desorb from the surface suggesting that the ethylene is formed by methylene insertion into the methyl surface bond to form an ethyl intermediate, which forms ethylene by β-hydride elimination, or hydrogenates to yield ethane. Propylene is likely to form by further methylene insertion into the ethyl-surface bond.