Thermal behaviour of Zn–Cr layered double hydroxides with hydrotalcite-like structures containing carbonate or decavanadate

Abstract

Hydrotalcite-like compounds containing Zn^II and Cr^III in the brucite-like layers have been prepared, containing carbonate or decavanadate in the interlayer, with the formulae [Zn_0.67Cr_0.33(OH)₂](CO₃)_0.165· 1.34H₂O and [Zn_0.67Cr_0.33(OH)₂](CO₃)_0.165· 1.34H₂O, respectively. The samples have been characterized by XRD, XAS, UV-VIS, FTIR and Raman spectrocopies, while the specific textuies of the samples were assessed by nitrogen adsorption, and reducibilities were studied by temperature-programmed reduction. A similar characterization study has been carried out on samples obtained after calcination of the parent samples in air at increasing temperatures. Thermal decomposition of the ZnCr hydrotalcite containing carbonate leads to oxidation of Cr^III to CrO₄^2– species at intermediate calcination temperatures, Zn^II cations then retaining their original octahedral coordination; at higher calcination temperatures ZnO and ZnCr₂O₄ are formed. However, the presence of interlayer decavanadate species inhibits the Cr^III→ CrO₄^2– oxidation during thermal decomposition of the vanadate-containing hydrotalcite, owing to preferential formation of vanadates. In this case the final product is a mixture of Zn pyrovanadate, Zn₂V₂O₇, and spinel ZnCr₂O₄, together with a phase consisting of a mixture of partially depolymerized decavanadate to β-VO₃^– chains, together with monomeric VO₄^3– species.