Cuo–ZnO–Al2O3 mixed oxides: preparation, bulk and surface characterization

Abstract

CuO–ZnO–Al₂O₃ mixed oxides with different Cu : Zn : Al atomic ratios were obtained by thermal decomposition of hydroxycarbonate) 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 Cu²⁺ species in C_3v and in D_4h 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 spinet’ 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.