Particle size effects in vinyl acetate synthesis over Pd/SiO2

Pd catalysts with varying particle size supported on a high-surface-area (600 m2/g) SiO2 were prepared by wet impregnation. The kinetics of vinyl acetate (VA) synthesis at atmospheric pressure was investigated in a fix-bed microreactor. With transmission electron microscopy (TEM), the average diameter of the Pd particles was estimated to be 4.0±0.2 and 2.5±0.1nm over the reduced Pd(5.0 wt%)/SiO2 and Pd(1.0 wt%)/SiO2 catalysts, respectively; limited sintering of the Pd particles during the reaction was observed for both catalysts. X-ray diffraction (XRD) showed primarily metallic Pd crystallites on the reduced catalyst and the formation of Pd carbide (PdCx) on the reacted catalyst. The Pd kinetics shows that the apparent activation energy increases with a decrease in the Pd particle size, varying from 17.0 kJ/mol over Pd(5.0 wt%)/SiO2 to 39.0 kJ/mol over Pd(1.0 wt%)/SiO2, and the rate of VA formation was significantly enhanced. On the other hand, the reaction orders (i.e., negative order with respect to ethylene and positive order with respect to oxygen) showed very little dependence on the Pd particle size.