EFFECT OF SODIUM ON THE ELECTRONIC-PROPERTIES OF PD SILICA-ALUMINA CATALYSTS/

The effect of the alkali ion content on the electronic properties of p alladium, supported on natural pumice and on synthetic aluminosilicate s, was studied by X-ray photoelectron spectroscopy (XPS) measurements. To this aim a series of catalysts of palladium supported on aluminosi licates with various amounts of sodium and potassium in the bulk struc ture were prepared and analysed. The Pd 3d(5/2) binding energy shifts of the different catalysts with respect to pure metal Pd 3d(5/2) bindi ng energy were followed as a function of the Na/Pd atomic ratio, For r easons of comparison, a silica supported Pd catalyst whose surface had been doped with sodium was also studied. A correlation of the Pd 3d b inding energy with the sodium content was found, regardless on whether sodium was present in the bulk structure or just at the surface of th e catalyst. The effect was independent of the catalyst preparation pro cedure. The analysis of the intensity of the photoelectron peaks of Pd 3d and Si 2p of the lower surface area catalysts, using the Kerkhof-M oulijn 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 distributi on was found. The intensity ratios of the Na Is and Si 2p photoelectro n peaks are in accordance with those estimated from the bulk Na/Si ato mic ratios of the synthetic and natural pumice support. Therefore any segregation of the alkali ions on the palladium particle of the corres ponding catalysts are ruled out, The electronic effect is thus discuss ed in terms of the formation of an electron donor composite formed by the metal, the oxygen and the alkali ion.