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(111) in the presence and absence of hydrogen have been studied over 13 orders of magnitude in pressure using infrared−visible sum frequency generation. In the presence of near ambient pressures of hydrogen and propylene at 295 K, propylene hydrogenation was observed to proceed from a physisorbed intermediate (π-bonded propylene) through a 2-propyl species (Pt-CH(CH3)2) to propane. Under UHV conditions in the absence of hydrogen, di-σ-bonded propylene was found to dehydrogenate to propylidyne just below room temperature and then to vinylmethylidyne (M⋮CCHCH2) at 450 K on the way to graphite formation.