THE CHEMISTRY OF SIMPLE ALKYL SPECIES ON PT(111) GENERATED BY HYPERTHERMAL COLLISIONS

The identification of the primary dissociation products of hypertherma l collisions of methane and ethane with a Pt(lll) surface at similar t o 150 K and the study of their subsequent thermal evolution has been a fforded by the use of reflection-absorption infrared spectroscopy, tem perature programmed desorption, and Auger electron spectroscopy. In bo th cases, the primary dissociation products have been identified as an adsorbed alkyl moiety (methyl, CH3, and ethyl, C2H5, respectively) an d an adsorbed hydrogen atom. The thermal treatment of both alkyl adlay ers ultimately results in the formation of the ethylidyne (=CCH3) moie ty at temperatures between 300 and 400 K. At similar to 500 K, this sp ecies itself thermally decomposes to yield adsorbed carbon atoms and g aseous hydrogen. While this behavior may not be unexpected for the C-2 ethyl fragment, the formation of the C-2 ethylidyne fragment from the C-1 methyl fragment can only be explained through the occurrence of c arbon-carbon coupling reactions. Such reactions have only recently bee n observed on the Pt(lll) surface. The C-C coupling reactions associat ed with the formation of ethylidyne from methyl show considerable depe ndence upon the surface coverage of the methyl fragment and are accomp anied by the evolution of both gaseous hydrogen and methane. The latte r results from recombinative desorption of methyl moieties with adsorb ed hydrogen atoms. (C) 1996 American Vacuum Society.