STRUCTURE AND ACTIVITY OF SUPPORTED METAL-CATALYSTS .8. CHEMISORPTION AND BENZENE HYDROGENATION ON PALLADIUM-SILICA CATALYSTS

The chemisorption of CO and H2 on two series of Pd/silica catalysts, reduced at 300 or 450 °C and containing 0.5 to 25.0% Pd, was measured. Palladium areas were calculated on the basis of different assumptions about the crystal planes exposed and the stoichiometry of CO and H2 adsorption on Pd, and compared. X-ray line-broadening measurements of crystallite size provided some confirmation of the assumptions made in determining Pd areas by chemisorption. Electron micrographs of thin catalyst sections provided direct evidence of crystallite sintering or aggregation in catalysts with the higher Pd loadings. It was concluded that increasing Pd content only increased the crystallite size, but not apparently the distribution of exposed planes. Specific activity for benzene hydrogenation at 100 °C was independent of crystallite size for both series of catalysts within the size ranges 13 to 114 and 16 to 134 Å. The possibilities for observing crystallite size effects are discussed. Specific activity decreased with increasing reduction temperature; the effect was observed using different silica supports, Pd salts, and Pd contents. Specific activity also varied for catalysts prepared on different silicas but with a fixed reduction temperature and Pd content. Various explanations of the effect of reduction on specific activity are considered including the interaction of palladium and silica which certainly occurred when the reduction temperature was raised to 600 °C. © 1979.