Effect of sodium on the electronic properties of Pd/silica-alumina catalysts

The effect of the alkali ion content on the electronic properties of palladium, supported on natural pumice and on synthetic aluminosilicates, was studied by X-ray photoelectron spectroscopy (XPS) measurements. To this aim a series of catalysts of palladium supported on aluminosilicates with various amounts of sodium and potassium in the bulk structure were prepared and analysed. The Pd 3d5/2 binding energy shifts of the different catalysts with respect to pure metal Pd 3d5/2 binding energy were followed as a function of the Na/Pd atomic ratio. For reasons of comparison, a silica supported Pd catalyst whose surface had been doped with sodium was also studied. A correlation of the Pd 3d binding energy with the sodium content was found, regardless on whether sodium was present in the bulk structure or just at the surface of the catalyst. The effect was independent of the catalyst preparation procedure. The analysis of the intensity of the photoelectron peaks of Pd 3d and Si 2p of the lower surface area catalysts, using the Kerkhof-Moulijn model, yielded particle sizes in agreement with values obtained from small angle X-ray diffraction measurements (SAXS). For the large surface area supported catalysts a monolayer type of metal distribution was found. The intensity ratios of the Na Is and Si 2p photoelectron peaks are in accordance with those estimated from the bulk Na/Si atomic ratios of the synthetic and natural pumice support. Therefore any segregation of the alkali ions on the palladium particle of the corresponding catalysts are ruled out. The electronic effect is thus discussed in terms of the formation of an electron donor composite formed by the metal, the oxygen and the alkali ion.