Acetoxylation of ethylene to vinyl acetate (VA) was used to investigate the mechanism of the promotional effect of gold (Au) in a palladium (Pd)-Au alloy catalyst. The enhanced rates of VA formation for low Pd coverages relative to high Pd coverages on Au single-crystal surfaces demonstrate that the critical reaction site for VA synthesis consists of two noncontiguous, suitably spaced, Pd monomers. The role of Au is to isolate single Pd sites that facilitate the coupling of critical surface species to product, while inhibiting the formation of undesirable reaction by-products.

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The formation of Pd carbide (PdC x ) during the synthesis of vinyl acetate (VA) was investigated over Pd/SiO 2 catalysts with two different Pd particle sizes, as well as over a Pd-Au/SiO 2 mixed-metal catalyst. XRD data show that PdC x was produced in the pure Pd catalysts after reaction based on the downshift of the Pd (111) and (200) XRD features. The smaller Pd particles showed greater resistance to the formation of PdC x . The XRD and XPS data are consistent with formation of a PdC x species at the surface of the Pd-Au catalyst, however, the primary contributor to the downshift of the Pd(111) feature subsequent to reaction in the mixedmetal catalyst is believed to arise from reaction-induced alloying of Au with Pd. The alloying of Au with Pd is apparently very effective in preventing PdC x formation in Pd-based catalysts for VA synthesis.

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