Low-loaded Pd-Pb/alpha-Al2O3 catalysts: Indication of alloy formation from FTIR and x-ray photoelectron spectroscopy

Bimetallic Pd-Pb/alpha-Al2O3 catalysts with low metal concentrations have been characterized by Fourier transform infrared spectroscopy, H-2 chemisorption, and X-ray photoelectron spectroscopy (XPS), The samples have been prepared by reaction of tetra-n-butyl lead with prereduced monometallic Pd catalysts. The characterization of the base Pd/alpha-Al2O3 has been described in a previous paper (J. Goetz et at, J. Catal. 153, 86 (1995)); it was found that particles having a large interaction with the support are formed when the Pd content is low. The addition of lead to monometallic Pd catalysts results in a decrease of hydrogen adsorption and important differences in IR spectra, Modification of XPS core level and Auger transitions of Pd were also observed, all results proving the formation of a Pd-Pb alloy. Analysis of Pb XPS spectra indicates that lead is present in two forms, namely Pb forming an alloy with Pd and Pb oxide located in the vicinity of Pd particles, Interaction between the metal particles and the support was not modified when adding Pb to the Pd particles. A model for the bimetallic particles is proposed where the penetration of Pb into the bulk of Pd is limited for particles having a large interaction with the support. (C) 1997 Academic Press.