XPS study on barium-promoted copper chromite catalysts

Copper chromite catalysts are used nowadays in catalytic reactions during the incineration of organic air pollutants, which is a subject of growing interest in environmental safety. The aim of this work was to investigate by XPS the surface species, on a sample prepared by ourselves and to study the modifications of these species after various heat treatments. This study confirmed the deep surface transformation on the sample. We observed a complete reduction of the copper atoms from oxidation state II to I. In contrast, this behaviour is not followed by the chromium atoms, which are reduced to oxidation state III only to a small degree. This fact could be explained by the presence of barium which was introduced as a promoter in the catalyst

Interfacial electrostatic behaviour of oxides : correlations with structural and surface parameters of the phase

The electrochemical behaviour of aqueous suspensions of ZrO2 powders, either pure phase (monoclinic and tetragonal) or mixtures of polymorphs, was investigated. Surface charge and total capacitance vs. pH curves, for the pure oxides, were regular showing different values of points of zero charge and low inner layer capacitance values. The mixtures of the polymorphs presented atypical interfacial electrochemical features which were interpreted as due to patch wise heterogeneity. This latter hypothesis was supported by XPS analyses of the powders which indicated, in the case of the mixtures of the polymorphs, the presence of heterogeneity both in composition and in the acid-base character of the surface sites. \ua9 1994

An XPS study on ruthenium compounds and catalysts

The binding energy (BE) of the relevant peaks of several ruthenium compounds have been measured with a monochromatic small spot XPS. The BE of the 3d,, level of ruthenium is in the range 279.91-282.88 eV. The variation of BE is due either to the variation of the oxidation state or to the different counter-ion. A series of catalysts with varying amounts of ruthenium supported on alumina and prepared using different precursors was also analyzed. The presence of more ruthenium species other than the metal was observed. On the basis of the values previously obtained on unsupported compounds, the species with higher BE were assigned to oxides. On all the samples prepared from RuCl,, an additional peak at a very high BE (283.79 eV) has been observed. This peak is related to the presence of chlorine on the surface: it is suggested that it is related to a charge transfer interaction. The influence of this species on the CO reactivity in the Fischer-Tropsch reaction is discussed

Hydrothermal "route" to pure phase ZrO2. Interfacial reactivity by XPS and electrochemical determinations

The precipitation of pure monoclinic ZrO2 from acid solutions of zirconium salts aged at 100\ub0C was investigated. The rate of nucleation depends not only on the initial pH and pZr but also on the nature of the precursor salt and the electrolyte concentration [Cl-]. All the different experimental conditions adopted yielded monoclinic ZrO2, without metastable polymorphs. Analyses of the samples by thermogravimetric analysis and differential thermal analysis revealed both physisorbed and chemisorbed water. X-ray photoelectron spectroscopy showed the coordination of the surface cations to be regular and gave the stoichiometry and density of surface sites. The reactivity of the electrical double layer of the products was assessed by obtaining charge versus pH curves of aqueous suspensions. The value of the point of zero charge, determined electrochemically, was compared with data from other sources. \ua9 1994

A procedure for achieving single crystal-like zirconia particles

The paper contains an improvement of a previously reported procedure aimed at obtaining pure phase monoclinic zirconia powders. The former method, implying the combination of calcination and grinding stages of hydrothermal precursors, determined the presence of surface roughness and chemical surface defects. By modulating duration and alternation of mechanical and firing treatments, the present procedure determines the formation of single crystal, smooth particles, with regularly coordinated surface cations. \ua9 1993

An experimental correlation between points of zero charge and X-ray photoelectron spectroscopy chemical shifts of oxides

Measurements are reported of X-ray photoelectron spectroscopy (XPS) investigations, performed under conditions of ultrahigh vacuum, of several metal oxides either produced commercially (high purity) or prepared in the laboratory. The same oxides were also subjected to electrochemical measurements in aqueous solutions for the determination of the point of zero charge (PZC) values and/or the isoelectric points (IEPs). The PZC (IEP) values appear to be linearly correlated with the sum of the metal and oxygen XPS chemical shifts (DM + DO). The higher (more alkaline) the PZC (IEP), the lower the XPS (DM + DO) term and vice versa. The weighting of the different components in the accuracy of the correlation is discussed. It is shown how a cross comparison between experimental XPS and electrochemical data obtained for the same samples can be used as a diagnostic criterion for the occurrence of secondary phenomena at the solid/liquid boundary