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 ei ther react with adsorbed acetylene to form benzene or alternatively, a s has been recently suggested, it can dimerize forming cyclooctatetrae ne which decomposes to yield benzene. Trimerization is sufficiently fa st that reaction is desorption rather than surface reaction rate limit ed so that the nature of the ensembles to benzene formation can be sim ulated assuming that acetylene molecules adsorb randomly on the surfac e. Calculating the number of properly oriented triads on Pd(111) corre ctly predicts the benzene yield observed in temperature-programmed des orption as a function of initial acetylene coverage for both Pd(111) a nd oxygen-covered Pd(111) and suggests that cyclooctatetraene formatio n, at best, provides only a minor route to the synthesis of benzene. T he nature of the acetylene environment around each reactively formed b enzene can also be interrogated and these results suggest that the maj ority (similar to 90%) of the reactively formed benzene is sterically crowded on the surface but that this crowding is relieved by the conve rsion of acetylene to vinylidene species between similar to 200 and 30 0 K. Finally, the structure sensitivity of benzene formation found in temperature-programmed desorption experiments is addressed.