X-ray photoelectron spectroscopy was applied to characterize CuO–ZnO catalysts (Cu/Zn atomic ratio ≥ 1) prepared by thermal decomposition at 350°C in air of hydroxycarbonate precursors obtained by the coprecipitation method at constant pH.
For the precursors, the surface Cu/Zn atomic ratios have, within experimental error, the same values as that obtained by chemical analysis. This shows both a homogeneous dispersion of zinc in the zincian-malachite solid solutions for the samples Cu/Zn = 92/8, 85/15 and 77/23, and a good interdispersion of the zincian-malachite and aurichalcite phases in the 67/33 and 50/50 samples.
In our calcined samples we find a strong interaction between the small oxide particles but no evidence of Cu2+/ZnO and Zn2+/CuO solid solutions. In the catalysts with low zinc loading, the ZnO phase (amorphous or with particles of a size < 40 Å) tends to cover the CuO particles. The catalysts with high zinc loading present the differential charging phenomenon due to the peculiar morphological situation of CuO and ZnO particles, as shown by the XPS measurements 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 presence of the ZnO phase. These findings are in agreement with our previous characterization with bulk techniques.
A critical comparison with previous published papers dealing with XPS characterization of real and model catalysts is presented.