Impact of palladium silicide formation on the catalytic properties of Pd/SiO2 catalysts in liquid-phase semihydrogenation of phenylacetylene

In this study, palladium silicide was formed on the sol-gel derived SiO2 supported Pd catalysts when they were prepared by ion-exchange method using Pd(NH3)(4)Cl-2 as a palladium precursor. No other palladium phases (PdO or Pd-0) were evident after calcinations at 450 degrees C for 3d. The Pd/SiO2 catalysts with Pd silicide formation were found to exhibit superior performance than commercial SiO2 supported ones in liquid-phase semihydrogenation of phenyl acetylene. From XPS results, the binding energy of Pd 3d of palladium silicide on the Pd/SiO2 catalyst shifted toward larger binging energy, indicating that Pd is electron deficient. This could probably result in an inhibition of a product styrene on the Pd surface and hence high styrene selectivities were obtained at high phenylacetylene conversions. The formation of Pd silicide, however, did not have much impact on specific activity of the Pd catalysts since the TOFs were quite similar among the various catalysts with or without palladium silicides if their average particle sizes were large enough. The TOFs decreased by an order of magnitude when palladium dispersion was very high and their average particle sizes were smaller than 3-5 nm. (c) 2006 Elsevier B.V. All rights reserved.