Probing Surface Chemistry Under Catalytic Conditions: Olefin Hydrogenation, Cyclization and Functionalization

摘要

The specific goal of this work was to understanding the catalytic reactions pathways for the synthesis of vinyl acetate over Pd, Au and PdAu alloys. A combination of both experimental methods (X-ray and Auger spectroscopies), low-energy ion scattering (LEIS), low-energy electron diffraction (LEED) and theory (Density Functional Theory (DFT) calculations and Monte Carlo methods under various different reactions) were used to track the surface chemistry and the influence of alloying. The surface intermediates involved in the various reactions were characterized using reflectionabsorption infrared spectroscopy and LEED to identify the nature of the surface species and temperature-programmed desorption (TPD) to follow the decomposition pathways and measure heats of adsorption. These results along with those from density functional theoretical calculations were used determine the kinetics for elementary steps. The results from this work showed that the reaction proceeds via the Samanos mechanism over Pd surfaces whereby the ethylene directly couples with acetate to form an acetoxyethyl intermediate that subsequently undergoes a beta-hydride elimination to form the vinyl acetate monomer. The presence of Au was found to modify the adsorption energies and surface coverages of important surface intermediates including acetate, ethylidyne and ethylene which ultimately influences the critical C-H activation and coupling steps. By controlling the surface alloy composition or structure one can begin to control the steps that control the rate and even the mechanism.