A kinetic study of vinyl acetate synthesis over Pd-based catalysts: kinetics of vinyl acetate synthesis over Pd–Au/SiO2 and Pd/SiO2 catalysts

Kinetic studies of vinyl acetate (VA) synthesis were carried out for Pd (1.0 wt%)/SiO2 and Pd (1.0 wt%)–Au (0.5 wt%)/SiO2 catalysts, with highly dispersed metal particles (Pd and Pd–Au) characterized by X-ray diffraction and transmission electron microscopy–energy dispersive spectroscopy (TEM-EDS). The kinetics of the related CO oxidation reaction were also explored. The kinetics of VA synthesis and CO oxidation reactions proceed via a Langmuir–Hinshelwood mechanism. For Pd-only catalysts, dissociative adsorption of O2 is believed to be the rate-determining step. This step is suppressed by adsorbed CO/C2H4 and gives rise to a negative reaction order with respect to CO/C2H4 and a positive order with respect to O2. However, the reaction mechanism was significantly modified by the addition of Au to Pd, as indicated by the change in the reaction order with respect to CO/C2H4; in particular, the order with respect to C2H4 becomes positive. The modification of the mechanism for the Pd–Au catalyst correlates with the reduction in the concentration of Pd–Pd ensembles upon alloying with Au. By TEM-EDS, Au surface enrichment was detected for the Pd–Au alloy, and the interaction between Au and Pd leads to a formation of more active Pd ensembles, such as PdAu5 and PdAu6. The surface properties of Pd versus Pd–Au catalysts were explored by CO/C2D4-TPD on thick films of Pd and Pd–Au. These studies indicate that the adsorption sites on Pd are significantly modified by Au; concurrently, the bonding of CO and C2D4 to Pd in the Pd–Au alloy also varied. As a result, the coverage of CO and C2H4 on the Pd–Au surface decreased markedly. The enhanced capacity of the Pd–Au surface for oxygen and/or the enhanced mobility of adsorbed oxygen under the reaction conditions are likely responsible for the unusually high reactivity of Pd–Au alloy catalysts for VA synthesis.