The Formation of PdC_x over Pd-Based Catalysts in Vapor-Phase Vinyl Acetate Synthesis: Does a Pd–Au Alloy Catalyst Resist Carbide Formation?

Abstract

The formation of Pd carbide (PdC _x ) during the synthesis of vinyl acetate (VA) was investigated over Pd/SiO₂ catalysts with two different Pd particle sizes, as well as over a Pd–Au/SiO₂ 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 mixed-metal 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.