Structure and reactivity of propylene on clean and hydrogen-covered Pd(1 1 1)

The adsorption of propylene on clean and hydrogen-covered Pd(1 1 1) has been studied by using temperature-programmed desorption (TPD) and reflection-absorption infrared spectroscopy. Propylene adsorbs in a di-σ configuration on clean Pd(1 1 1). At high exposures, it forms second and subsequent layers. Upon heating, propylene desorbs molecularly at ∼200 and 280 K and, in contrast to ethylene adsorbed on Pd(1 1 1), it undergoes significant dehydrogenation. Two major products of dehydrogenation were identified: propylidyne and η1-allyl species. While the formation of η1-allyl species was found to be favored, both are observed to co-exist on the surface. The pre-adsorption of hydrogen induces the appearance of some π-bonded propylene. The conversion to this weaker state of adsorption induced by hydrogen is not complete, due to steric effects caused by the methyl group. Formation of propane is observed by reaction of hydrogen with propylene in TPD experiments, and the activation energy is measured to be 3.4 ± 0.4 kcal/mol.