A. M. Venezia

Particle size and metal-support interaction effects in pumice supported palladium catalysts

Abstract 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 Auger 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.

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

Abstract 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 3d 5/2 binding energy shifts of the different catalysts with respect to pure metal Pd 3d 5/2 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 1s 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.

Natural Pumice by XPS

X‐ray photoelectron spectroscopy was used for characterizing pumice: it is a natural aluminosilicate with a high percentage of silica (∼70%), a low surface area (∼5 m2u2009g−1 as determined by the BET method) and a density of 2.3 g cm−3. It has a fine, porous physical structure which makes the material easy to machine and also determines its abrasive property. In contrast to the zeolites, pumice has an amorphous structure. Apart from its main use in the construction industry, pumice has begun to be of scientific interest when used as a support for metallic catalysts. Core and valence spectra were compared with standard Al2O3 and SiO2. The aim of this study is to examine the surface of this material.