Reactivity of Unsaturated Linear C6Hydrocarbons on Pd(111) and H(D)/Pd(111)

The adsorption and reactivity of 1-hexene, 1,5-hexadiene, 1,3-hexadiene, and 1,3,5-hexatriene on clean Pd(111) and hydrogen (deuterium)-saturated/Pd(111) were investigated using temperature-programmed reaction spectroscopy. The low-temperature adsorption configuration for the linear C6hydrocarbons is proposed to be a weakly π-bonded species. The adsorbed molecules first desorb molecularly with a fraction converting to a more tightly bonded half-hydrogenated state, which either β-hydride eliminates to release the alkene or dehydrogenates completely to adsorbed carbon and hydrogen. Dehydrocyclization to benzene is observed on this surface, whereas it does not occur on Pd(100). Cyclization of 1,3,5-hexatriene to benzene occurs at temperatures as low as 333 K on Pd(111). Although hydrogenation of 1,3-hexadiene adsorbed on clean Pd(111) and of 1,3-hexadiene and 1,5-hexadiene adsorbed on H/Pd(111) to hexene was observed, and hexatriene was hydrogenated to hexadiene and hexene, hydrogenation of the alkenes to hexane was not observed for any of the unsaturated species. H–D exchange into all the adsorbed alkenes was observed. The exchange reaction is proposed to occur through the reversible C–H bond formation in a half-hydrogenated intermediate.