Characterization of well dispersed copper species on the surface of ZnO by x-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy was employed to characterize Cu(II) species on the surface of calcined CuO-ZnO systems with Cu/Zn atomic ratios {precedes above single-line equals sign} 0.10. Such solids are of interest as precursors of Cu-ZnO catalysts. It is shown that well dispersed Cu(II) species on the surface of ZnO have unique spectroscopic properties. For the Cu(2p3/2) transition the intensity of the satellite structure, typical of bulk Cu(II) compounds, is strongly reduced or even absent in the more dilute samples. To explain this result, it is suggested that a stronger Cu-O covalent bond than in bulk CuO is formed at the interface between the surface oxygen ions of ZnO and the supported copper species. The strong interaction between Cu(II) species and the support leads on reduction to the stabilization of Cu(I) species at the periphery of metallic copper particles which could play a role in the reaction mechanisms of CO hydrogenation at the surface of Cu-ZnO catalysts.