A kinetic study of vinyl acetate synthesis over Pd-based catalysts: kinetics of vinyl acetate synthesis over Pd–Au/SiO 2 and Pd/SiO 2 catalysts

Abstract

Kinetic studies of vinyl acetate (VA) synthesis were carried out for Pd (1.0 wt%)/SiO 2 and Pd (1.0 wt%)–Au (0.5 wt%)/SiO 2 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 O 2 is believed to be the rate-determining step. This step is suppressed by adsorbed CO/C 2H 4 and gives rise to a negative reaction order with respect to CO/C 2H 4 and a positive order with respect to O 2. 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/C 2H 4; in particular, the order with respect to C 2H 4 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 PdAu 5 and PdAu 6. The surface properties of Pd versus Pd–Au catalysts were explored by CO/C 2D 4-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 C 2D 4 to Pd in the Pd–Au alloy also varied. As a result, the coverage of CO and C 2H 4 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.