Carbon-carbon coupling of methyl groups on Pt(111)

Abstract

Although methyl radicals are widely postulated to be important intermediates in many catalytic processes involving hydrocarbon species, relatively little is known about their reaction kinetics on metal surfaces. In order to more fully understand the surface chemistry of methyl groups we have developed a source of gas-phase methyl radicals based on the pyrolysis of azomethane. The surface chemistry of methyl groups adsorbed on Pt(111) has been studied using TPD and reflection-adsorption infrared spectroscopy (RAIRS). Methyl radicals can be dosed directly onto the surface at 150 K and are bound in a C 3v geometry. Methyl groups become thermally activated above 230 K, reacting to produce a hydrogenation product (methane) and various dehydrogenation products. The nature of these dehydrogenation products depends upon the initial methyl coverage. At low coverages CH x ( x 3) species are predominant while at higher methyl coverages is evidence of carbon-carbon coupling reactions to produce higher order hydrocarbons one of which has been identified as ethylidyne (CCH 3) Ethylidyne is also observed as a reaction product on Pt(111), at 150 K, for large methyl radical exposures.