CHARACTERIZATION OF CuO-ZnO CATALYSTS BY PHOTOELECTRON SPECTROSCOPY: PRECURSORS, CALCINED AND REDUCED SAMPLES

X-ray photoelechon spectroscopy was applied to characterize CuO-ZnO catalysts (Cu/Zn atomic ratios > 1) prepared by thermd decomposition at 350°C in air of hydroxycarbonrte precunnrs obtained by the coprecipitation method at constant pH. For the precursors, the surface Cu/Zn atomic ratios have, within experimental error, the same valucs as that obtaind by chemical analysis. This shows both a homogeneorn rilispersion of zinc in the zincian-malachite solid solutior for the samples Ca/Zn - 92/8, 85/15 and 77/23, and e good interdispersion of the zincian-malachite and arnichalcite phases in 67/33 and 50/50 semples. In alr calcined samples we find a strong interaction between the small oxide particles but no eúdence of Cu2+/ZnO and Zn2+/CuO solid solutions. In the catatysts with low zinc loading the ZnO phase (amorphous or with particles of a size <40 A) tends to cover the CuO particles. The catalysb with high zinc loading present the differential charging phenomenon due to the peculiar morphologicd situation of CuO and ZnO particles, as shown by the XPS measuremens of samples supported on a biased sample holder. The reduction in situ under H2 at 135 and 200°C of these catalysts shows that Cu and Cu+ (Cu2O-like) species are formed. Cu+ species are present even at high extents of reduction but the kinetics of the reduction and the relative amounts do not seem to be influenced by the prescnce of the ZnO phase. These ffndings are in agreement with our previons characterization with bulk techniques. A critical comparison with previous published papers dealing with XPS characterization of real and model catalysts is presented.