PARTICLE-SIZE AND METAL-SUPPORT INTERACTION EFFECTS IN PUMICE SUPPORTED PALLADIUM CATALYSTS

Pumice supported palladium catalysts with different metal loading and different metal particle sizes have been analyzed with X-ray photoelectron spectroscopy (XPS). The observed negative shift of the Pd 3d doublet binding energies of all the catalysts with respect to the corresponding binding energies of palladium metal powder suggested an increased electron density at the metal centers. Through the combination of the photoelectron and Anger shifts, yielding the Auger parameter shift, relative to the palladium metal, the charge distribution on the palladium atoms was estimated. The variation of that parameter with particle size is attributed to the interaction between the metal and the support rather than to an intrinsic size effect. The XPS intensity ratios of Pd 3d and Si 2p peaks were well reproduced by the Kerkhof and Moulijn model, especially at lower metal loading. Depletion of sodium, naturally present in pumice, has been observed in the catalysts with lower palladium content. An explanation of the catalytic behavior of these samples, in terms of electronic effects and surface elemental composition, is given.