Characterization of pumice-supported Ag-Pd and Cu-Pd bimetallic catalysts by X-ray photoelectron spectroscopy and X-ray diffraction

Bimetallic Ag-Pd and Cu-Pd catalysts supported on pumice have been prepared in order to be used in the selective hydrogenation of dienes. The catalysts were obtained by the classical impregnation method and in the case of the Cu-Pd system also by organometallic precursors. They were analysed by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). XPS allowed us to determine the surface distribution and chemical state of the two elements; XRD yielded the lattice parameters and allowed us to establish the possible formation of alloys. The two bimetallic systems behave differently. In the case of the Ag-Pd catalysts, Pd particles covered by silver atoms along with highly dispersed monometallic Ag particles interacting with the support were formed, without evidence of alloy formation. The possibility of a layer of silver segregated over the palladium particles would explain the Ag 3d binding energy shifts and also the differences between the overall and the surface Ag/Pd atomic ratios. Evidence of alloy formation was found for the Cu-Pd system obtained by impregnation. Analogously, alloy formation was detected in the organometallic samples reduced at higher temperature. An investigation of the oxidation behaviour of the Cu-Pd catalysts revealed a superior oxidation resistance of the alloyed particles. The differences between the Ag-Pd and Cu-Pd systems are discussed in terms of interaction with the support. (C) 1999 Academic Press.