Characterization by X-ray absorption, X-ray powder diffraction, and magnetic susceptibility of Cu-Zn-Co-Al-containing hydroxycarbonates, oxycarbonates, oxides, and their products of reduction

Copper-zinc-cobalt-aluminium-containing crystalline hydroxycarbonates having hydrotalcite structure have been prepared by coprecipitation. X-ray powder diffraction (XRPD), magnetic susceptibility, and extended X-ray absorption fine structure (EXAFS) indicate that Cu2+, Zn2+, and Co2+ are present in an octahedral environment. Calcination of the hydroxycarbonates at 723 K produces quasi-amorphous oxycarbonates where Cu2+ and Co2+ Still retain octahedral coordination and cobalt is almost completely oxidized to Co3+. The coordination of Zn2+, at this stage, is intermediate between the octahedral one of the precursors and the tetrahedral one of ZnO and Zn-based spinels. Further calcination at 973 K produces a mixture of crystalline oxides such as CuO, ZnO, CuAl2O4, ZnAl2O4, and ZnCo2O4. EXAFS analysis of these samples indicates that copper is mainly in a fourfold coordination (although two longer Cu-O distances are also detected), zinc is tetrahedral, and cobalt (as Co3+) is essentially octahedral. EXAFS and XANES investigations performed after in situ reduction (10% H-2/N-2, at 523 and 623 K) on the oxycarbonates and oxides reveal that the total Cu2+ --> Cu-0 reduction occurs only at 623 K in both series of samples, Co3+ is reduced to Co2+ only at 623 K in the oxycarbonates, and Zn2+ is never reduced. (C) 1996 Academic Press, Inc.