HYDROGENATION AND DEHYDROGENATION OF PROPYLENE ON PT(111) STUDIED BY SUM-FREQUENCY GENERATION FROM UHV TO ATMOSPHERIC-PRESSURE

The reactions of adsorbed propylene on Pt(lll) in the presence and abs ence of hydrogen have been studied over 13 orders of magnitude in pres sure using infrared-visible sum frequency generation. In the presence of near ambient pressures of hydrogen and propylene at 295 K, propylen e hydrogenation was observed to proceed from a physisorbed intermediat e (pi-bonded propylene) through a 2-propyl species (Pt-CH(CH3)(2)) to propane, Under UHV conditions in the absence of hydrogen, di-sigma-bon ded propylene was found to dehydrogenate to propylidyne just below roo m temperature and then to vinylmethylidyne (M=CCHCH2) at 450 K on the May to graphite formation.