Metal-Catalyzed Hydrocarbon Conversion Reactions†

Palladium-catalyzed acetylene trimerization and the metathesis of olefins catalyzed by molybdenum and its oxides are investigated in ultrahigh vacuum and at high pressures. Benzene is formed on Pd(111) by the reaction between adsorbed acetylene and a surface C4 metallocycle. The resulting benzene evolves in temperature-programmed desorption in two distinct states. The low-temperature state is proposed, following investigations of the desorption kinetics of benzene from Pd(111), to be due to tilted benzene formed on a sterically crowded surface, and the high-temperature state, to be due to flat-lying benzene. The low steady-state benzene formation rate found at high pressures (∼1 atm) is suggested to be due to blocking of the surface by the formation of vinylidene species. Olefin metathesis is found to proceed in two different regimes: one below ∼650 K, which mimics supported molybdena catalysts and where MoO3 is the best catalyst, and another region above this temperature, where the reaction proceeds with a high activation energy (∼60 kcal/mol) and the most effective catalyst is MoO2. The latter kinetics resemble those found for metallic molybdenum, where the reaction is proposed to proceed by a mechanism through which alkenes dissociate and recombine on the surface. This reaction is found to proceed in the presence of a thick carbonaceous layer. The addition of hydrogen is found to increase the rate of both metathesis and cyclotrimerization even though neither of these reactions involves hydrogen directly. This is proposed to be due to the titration of carbonaceous species from the surface. Extension of these ideas to ethylene hydrogenation, a reaction that does involve hydrogen, is successful in rationalizing the kinetics behavior found in that case.