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 O-2 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 O-2. However, the reaction mechanism was significantly modified by the addition of An 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 An. By TEM-EDS, Au surface enrichment was detected for the Pd-Au alloy, and the interaction between An 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. (c) 2005 Elsevier Inc. All rights reserved.