Particle size effects in vinyl acetate synthesis over Pd/SiO 2

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Pd catalysts with varying particle size supported on a high-surface-area (600 m 2/g) SiO 2 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 ± 0.1 nm over the reduced Pd(5.0 wt%)/SiO 2 and Pd(1.0 wt%)/SiO 2 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 (PdC x ) 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%)/SiO 2 to 39.0 kJ/mol over Pd(1.0 wt%)/SiO 2, 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.