Reactivity and Selectivity in the Au/Pd(111) Alloy-Catalyzed Vinyl Acetate Synthesis

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The rates of reaction of acetate species adsorbed on a range of Au/Pd(111) alloys with gas-phase ethylene to form vinyl acetate monomer (VAM) were explored by monitoring the time dependence of the adsorbate coverages using infrared spectroscopy. It was found that the acetate species react directly to form VAM since the decrease in the coverage of acetate species correlates directly with the rate of VAM formation. The VAM was retained on the surface, in accord with the stronger binding of VAM on Au/Pd(111) alloys than on the metal as found in previous surface science studies. In addition, the formation of ethylidyne species from ethylene, previously found on Pd(111), was suppressed on alloys for gold coverages ≥0.37 monolayers. A substantial increase in the rate of VAM formation was found with increasing gold coverage in the alloy. In addition to the strengthening of the binding of VAM on the alloy, the binding of the reactants decreased with increasing gold content in the alloy, leading to an overall increase in the exothermicity of the reaction. This is expected to lead to a decrease in reaction activation energies, rationalizing the observed increase in reactions rate. However, it has also been found previously that the reactant coverages influence the elementary step activation energies on Pd(111). Increasing the gold coverage in the alloy also decreases the coverages of the reactants and may therefore also influence the VAM formation activity.