SURFACE-CHEMISTRY OF METHYL-GROUPS ADSORBED ON PT(111)

The surface chemistry of adsorbed methyl groups on Pt(111), generated from the gas-phase pyrolysis of azo-methane, has been studied using temperature-programmed desorption (TPD) in conjunction with reflection-absorption IR spectroscopy (RAIRS). Using this method of direct adsorption from the gas phase, the surface chemistry of adsorbed methyl groups has been explored at higher coverages and lower temperatures than can be accessed using methyl iodide as a methyl group precursor. The surface chemistry of adsorbed methyl groups is characterized by a competition between hydrogenation, to produce methane, and dehydrogenation which ultimately leads to the production of surface carbon. The reactive partitioning between these two pathways is found to be sensitive to the initial coverage of both hydrogen and methyl with larger coverages favouring hydrogenation. Isotopic-labelling experiments also reveal that the kinetics of methyl group hydrogenation exhibit a non-linear dependence upon the initial hydrogen pre-coverage, with the appearance of a new, previously unreported methane desorption state below 200 K. Limited hydrogen exchange between the hydrocarbon species produced as the result of methyl group dehydrogenation and surface hydrogen is observed. Experiments carried out using both protonated and deuteriated methyl groups indicate that, consistent with previous studies, almost no kinetic isotope effect exists in the hydrogenation or dehydrogenation of adsorbed methyl groups.