An XPS study of the reduction process of CuO-ZnO-Al2O3 catalysts obtained from hydroxycarbonate precursors

CuO-ZnO-Al2O3 catalysts with different Cu/Zn/Al atomic ratios were obtained by thermal decomposition of hydroxycarbonate precursors. The hydroxycarbonate precursors, prepared by coprecipitation at pH = 8, depending on the Cu/Zn/Al atomic ratios contain malachite Cu2CO3(OH)(2), zincian-malachite Cu2-xZnxCO3(OH)(2), aurichalcite Zn5-xCux(CO3)(2)(OH)(6), hydrozincite Zn-5(CO3)(2)(OH)(6), and an hydrotalcite-like: Cu6-xZnxAl2(CO3)(2)(OH)(16). Upon calcination, well interdispersed oxidic phases are obtained. In the ternary Cu/Zn/Al catalysts, the aluminium-containing phase was found to be amorphous and, regardless of the composition, only CuO and ZnO oxides are identified by X-ray diffraction analysis. The reduction process in H-2 was studied by XPS and XAES. The reduction of the Cu(II) species leads to the formation of both Cu(I) (Cu2O) and Cu(0) species. The reduction process is much slower for the alumina-containing samples compared to the binary CuO/ZnO specimen. Alumina and ZnO play an important role in the stabilization of Cu(II) and Cu(I) chemical states at low temperature. Upon reduction at high temperature, alumina plays a key role by preventing the sintering process of the Cu(0) particles.