CuO-ZnO-Al2O3 mixed oxides: preparation, bulk and surface characterization

CuO-ZnO-Al2O3 mixed oxides with different Cu:Zn:Al atomic ratios were obtained by thermal decomposition of hydroxy(carbonate) precursors at 623 K in air for 24 h. Both precursors and mixed oxides were characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS) and X-ray photoelectron spectroscopy (XPS); surface area determination was also performed. The XRD patterns of the precursors show that, as a function of the Cu:Zn:Al atomic ratio, parent phases to hydrozincite, aurichalcite, malachite, gerhardtite, hydrotalcite and boehmite are present. In the calcined samples zincite, tenorite and spinel-like phases are present. The occurrence of Cu 2+ species in C3v and in D4n symmetries has been evidenced by DRS in the most zinc-rich CuO-ZnO and in the most aluminum-rich CuO-ZnO-Al2O3 samples, respectively. A comparative study of the photoelectron and X-ray excited Auger transition of Cu and Zn species shows an increasing covalency of the Cu-O chemical bond in the CuO-ZnO samples with low copper loading and the formation of a copper-zinc 'surface spinel' in the oxide samples with high aluminum content. The quantitative XPS investigation has shown that the surface Cu:Zn atomic ratios are lower than those found by chemical analysis, thus pointing to a surface segregation of ZnO and/or of zinc 'surface spinel' phases. The surface segregation tends to be higher when the catalyst precursors are polyphasic.