A. Cimino

The characterization of CrOxSiO2 catalysts by photoelectron spectroscopy (XPS), X-ray and optical measurements

X-ray photoelectron spectroscopy (XPS or ESCA) has been applied, together with X-ray diffraction and diffuse reflectance spectroscopy, to the study of silica-supported CrO3 in the concentration range 1 to 9 wt% Cr. The progressive reduction of Cr(VI) to Cr(III) caused by heat treatment has been followed. The stabilizing influence of the support on Cr(VI) at lower Cr contents is apparent. The X-ray diffraction pattern of the samples of higher Cr concentration reveals the presence of intermediate phases (Cr3O8, Cr2O5, CrO2 for 9% Cr; CrO3 for 5% Cr) which are known to occur in the decomposition of pure CrO3. A drastic decrease of the intensity of the XPS 2p12 peak of Cr as a function of the temperature indicates an agglomeration of Cr2O3 into considerably large particles. The Cr(VI)Cr(III) ratio as determined by XPS is larger than that obtained by chemical analysis. This discrepancy is explained by a model, valid for samples calcined at high temperature, in which a small part of Cr2O3 is present in a dispersed state (particles of diameter about equal to the probing depth of XPS) while the remaining part forms large particles of several hundred A diameter.

Investigation of the surface composition of NiOMgO solid solutions by X-ray photoelectron spectroscopy

Abstract The surface composition of NiOMgO solid solutions has been investigated by XPS. It has been found that no appreciable deviation from bulk composition is present, or, if present, it goes in the direction of a slight depletion in Ni2+ ions at the surface. The approximations involved and the problems encountered in the determination of the surface composition of air-exposed oxide samples are discussed.

Investigation of the surface composition of MgOZnO solid solutions by X-ray photoelectron spectroscopy

Abstract The surface composition of well-characterized samples of solid solutions MgO ZnO, containing up to 25 mol% ZnO, has been studied by recording the intensity of XPS and AES peaks. The influence on the measured intensifies of factors such as surface carbonation and the energy of the measured peaks has been pointed out. The Zn concentration in the outer layer has been found to be considerably higher than in the bulk. The extent of surface enrichment in Zn is in reasonable agreement with the thermodynamic prediction based on the ideal solution approximation.