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 catalyst s 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-ra y photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). XPS allowed us to determine the surface distribution and chemical state of the two ele ments; XRD yielded the lattice parameters and allowed us to establish the p ossible formation of alloys. The two bimetallic systems behave differently. In the case of the Ag-Pd catalysts, Pd particles covered by silver atoms a long with highly dispersed monometallic Ag particles interacting with the s upport were formed, without evidence of alloy formation. The possibility of a layer of silver segregated over the palladium particles would explain th e 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 w ith the support. (C) 1999 Academic Press.