Mechanistic insights in the catalytic synthesis of vinyl acetate on palladium and gold/palladium alloy surfaces

The reaction pathways for the synthesis of vinyl acetate monomer (VAM) are explored on model palladium and gold-palladium alloy single crystal catalysts by combining experiments carried out in ultrahigh vacuum together with density functional theory calculations and Monte Carlo simulations. Previous work by Goodman has shown that both pure palladium and gold-palladium alloys catalyze VAM formation at high pressures, thereby paving the way for fundamental studies of the pathways for this reaction. The coverages of the reactants and products on the surface were found to play an important role in controlling both the reaction pathways and the selectivity. The high coverages on the catalyst under reaction conditions favor bond-forming reactions while inhibiting bond-breaking reactions. On Pd(111), the reaction is initiated by the coupling of ethylene and surface acetate species to form an acetoxyethyl-palladium intermediate, a bond-forming reaction. The high coverages also act to control the selectivity since VAM is stabilized on the crowded surface. The gold in model Au/Pd(111) and Au/Pd(100) alloys gold preferentially segregates to the surface. In the case of Au/Pd(111) alloys, there is a slightly repulsive interaction between the gold and palladium atoms, resulting in a larger proportion of isolated palladium sites than would be expected if they were randomly distributed, while the longer-range interactions on Au/Pd(100) lead to the formation of ordered surface structures and the existence of isolated palladium sites for gold coverages greater than 0.5 ML. Higher coverages of Au on the Au/Pd(111) and Au/Pd(100) alloys decrease the population of bridging Pd sites and thus increase Pd site isolation. This eliminates the larger Pd ensembles that that lead to the decomposition of VAM and ethylene thus increasing the reaction selectivity and weakens the adsorption of ethylene and acetate which enhances the rate of reaction. Higher coverages of Au, however, also suppress the activation of O₂ which decrease the rate of acid deprotonation thus resulting in optimal Au/Pd compositions.