XPS Studies of characterized Cu/Al2O3, Zn/Al2O3 and Cu-Zn/Al2O3, catalysts

By XPS it is confirmed that copper-containing catalysts supported on alumina and calcined at high temperature (673-873 K for 24 h), with or without zinc addition, have a 'surface spinel' structure. Combining the XPS results with previous results obtained by Vis-UV reflectance spectroscopy and XRD we show that the limit of solubility of Zn and Cu ions in γ-alumina, at a calcination temperature of 873 K, is about 20 and 10 wt.% (by weight), respectively. In the Cu-Zn/Al2O3 catalysts prepared from Zn/Al2O3, a surface enrichment of copper occurs if the zinc ions are already present in an amount larger than the surface solubility. The presence of zinc on the surface increases the reducibility under H2 of the Cu(II) ions compared to the same ions in the pure copper surface spinel. This may be due to the role of Zn(II)-O(II) aggregates in the activation of the hydrogen molecule and/or to the segregation of CuO species on the zinc surface spinel.