Pillared hydroxycarbonates and mixed oxides. Part 1.—Copper–zinc–cobalt–aluminium system

Abstract

Cu–Zn–Co–Al mixed oxides with different Cu : Zn : Co : Al atomic ratios have been obtained by thermal treatment of hydroxycarbonate precursors at different temperatures (623, 723, 823 and 973 K in air). The characterization has been performed by X-ray powder diffraction (XRPD), diffuse reflectance spectroscopy (DRS), thermal analysis (TG/DTA), BET surface-area determination and measurements of magnetic susceptibility. The XRPD patterns showed that the precursors were almost pure pillared-layered (PLS) hydrotalcite-like materials with general stoichiometric formula M₆^II M₂^III(OH)₁₆CO₃.4H₂O (M^II= Cu, Zn, Co, M^III= Al). The thermal decomposition of the precursors occurred in four steps, the first three (up to T= 673 K) consisting of complete dehydration of the sample, the fourth (843 ⩽T/K ⩽ 923) due to the release of CO₂. The precursor structure collapsed at T⩽ 623 K, giving rise to poorly crystalline materials, in which the Cu²⁺ ions retained a distorted octahedral symmetry. Crystalline oxide mixtures were formed only after complete release of CO₂. XRPD showed that the oxides obtained by calcination at 973 K consisted of a mixture of CuO, ZnO and spinels such as CuAl₂O₄, ZnAl₂O₄, ZnCo₂O₄, and/or spinel solid solutions. According to DRS and magnetic susceptibility, the formation of ZnCo₂O₄ seems to be privileged with respect to that of Co₃O₄ and ZnO.