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 photoelec tron spectroscopy (XPS). The observed negative shift of the Pd 3d doub let binding energies of all the catalysts with respect to the correspo nding binding energies of palladium metal powder suggested an increase d electron density at the metal centers. Through the combination of th e photoelectron and Anger shifts, yielding the Auger parameter shift, relative to the palladium metal, the charge distribution on the pallad ium atoms was estimated. The variation of that parameter with particle size is attributed to the interaction between the metal and the suppo rt rather than to an intrinsic size effect. The XPS intensity ratios o f Pd 3d and Si 2p peaks were well reproduced by the Kerkhof and Moulij n model, especially at lower metal loading. Depletion of sodium, natur ally present in pumice, has been observed in the catalysts with lower palladium content. An explanation of the catalytic behavior of these s amples, in terms of electronic effects and surface elemental compositi on, is given.