Disentangling ensemble, electronic and coverage effects on alloy catalysts: Vinyl acetate synthesis on Au/Pd(1 1 1)

The activation energies for the reaction of gas-phase ethylene with adsorbed acetate species on Au/Pd(1 1 1) alloys are measured as a function of gold coverage in the alloy to probe the elementary steps in gold-palladium alloy catalyzed vinyl acetate monomer (VAM) synthesis. The activation energy decreases with increasing gold coverage in the alloy up to a gold coverage of ∼0.25 ML and then increases. The reaction exhibits a kinetic isotope effect when using C₂D₄ for gold coverages below ∼0.25 ML, and no isotope effect for alloys with larger gold coverages. This is ascribed to a change in rate-limiting step for the reaction, which is postulated to proceed via an initial coupling between ethylene and adsorbed acetate species to form an acetoxyethyl intermediate, which decomposes via a β-hydride elimination reaction to form VAM. For low gold coverages, the second step is rate limiting, while at higher coverage, the coupling step becomes rate limiting. This allows the reaction energies for each of the steps to be obtained as a function of gold coverage, where it is found that both energies decrease with increasing gold coverage, causing an increase in overall reaction rate. The activation energy for β-hydride elimination decreases more rapidly than that for the coupling step. The contributions to the energy change are estimated, and it is found that the effect of coverage on the reaction energies is comparable to the more commonly considered ensemble and electronic effects.