COPPER-ZINC-COBALT-CHROMIUM HYDROXYCARBONATES AND OXIDES

Hydroxycarbonate precursors with different Cu/Zn/Co/Cr atomic ratios a nd Cu-Zn-Co-Cr oxides obtained by thermal treatment of the precursors at different temperatures (623, 723, and 973 K in air) have been exami ned, Characterization has been performed by X-ray powder diffraction, diffuse reflectance spectroscopy, thermal analysis, surface area deter mination, and measurement of magnetic susceptibility, X-ray diffractio n patterns show that the precursors are essentially hydrotalcite-like materials with the general formula (M(2+))(6)(M(3+))(2)(OH)(16)CO3 . 4 H(2)O (M(2+) = Cu, Zn, Co; M(3+) = Cr). Thermal decomposition of the p recursors occurs in four steps; the first three (up to T = 623 K) cons ist of complete dehydration of the sample, and the fourth (623 less th an or equal to T less than or equal to 773 K) is due to the release of CO2. The precursor structure collapses at T less than or equal to 623 K, giving rise to nearly amorphous materials, Crystalline oxide mixtu res are formed only after complete release of CO2. X-ray diffraction p atterns show that the oxides obtained by calcination of 973 K consist of a mixture of CuO, ZnO, and ZnCr2O4-ZnCo2O4-Co3O4 spinel solid solut ions. According to reflectance spectroscopy and magnetic susceptibilit y measurements, the inclusion of Zn2+ and Co3+ ions in the spinel-like solid solution seems to be privileged with respect to the formation o f separate Co3O4 and ZnO phases. (C) 1995 Academic Press, Inc.