The formation of PdCx over Pd-based catalysts in vapor-phase vinyl acetate synthesis:: does a Pd-Au alloy catalyst resist carbide formation?

The formation of Pd carbide (PdCx) 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 PdCx 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 PdCx. The XRD and XPS data are consistent with formation of a PdCx 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 PdCx formation in Pd-based catalysts for VA synthesis.