Simulation of benzene formation from acetylene on palladium and oxygen-covered palladium surfaces

Acetylene cyclotrimerization catalyzed by palladium has been shown to proceed via the initial rapid formation of a C₄H₄ species. This can either react with adsorbed acetylene to form benzene or alternatively, as has been recently suggested, it can dimerize forming cyclooctatetraene which decomposes to yield benzene. Trimerization is sufficiently fast that reaction is desorption rather than surface reaction rate limited so that the nature of the ensembles to benzene formation can be simulated assuming that acetylene molecules adsorb randomly on the surface. Calculating the number of properly oriented triads on Pd(111) correctly predicts the benzene yield observed in temperature-programmed desorption as a function of initial acetylene coverage for both Pd(111) and oxygen-covered Pd(111) and suggests that cyclooctatetraene formation, at best, provides only a minor route to the synthesis of benzene. The nature of the acetylene environment around each reactively formed benzene can also be interrogated and these results suggest that the majority (∼ 90%) of the reactively formed benzene is sterically crowded on the surface but that this crowding is relieved by the conversion of acetylene to vinylidene species between ∼ 200 and 300 K. Finally, the structure sensitivity of benzene formation found in temperature-programmed desorption experiments is addressed.