Palladium‐Catalyzed Acetylene Cyclotrimerization: From Ultrahigh Vacuum to High‐Pressure Catalysis

The surface reactions involved in the palladium‐catalyzed formation of benzene from acetylene are discussed as an example of the way in which such strategies can be used to understand catalytic reaction pathways in detail. This provides an ideal system for surface science/catalysis studies since benzene is formed both in ultrahigh vacuum in temperature‐programmed desorption and under high‐pressure, catalytic conditions. It is found that benzene is synthesized on clean Pd(111) by an initial fast reaction between two adsorbed acetylene molecules to yield a tilted C 4 H 4 metallocyclic intermediate. This reacts with a third acetylene to form benzene, and the hexagonal (111) surface of palladium acts as a template for the reaction. Under catalytic conditions, however, it is found that the surface is covered by a relatively unreactive vinylidene layer. Organometallic studies show that acetylene and vinylidene can react to form a C 4 H 4 intermediate analogous to that found to form from acetylene on a palladium surface. This suggests an alternative possible reaction pathway under catalytic conditions in which acetylene and vinylidene react to eventually form benzene. This proposal is confirmed using NMR analysis of a 13 C‐labeled vinylidene‐covered surface. This pathway also successfully rationalizes the observed change in activity as a function of reaction time found using supported palladium catalysts and the observed first‐order reaction kinetics.