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

Abstract

Hydrotalcite-like compounds containing ZnII and CrIII in the brucite-like layers have been prepared, containing carbonate or decavanadate in the interlayer, with the formulae [Zn0.67Cr0.33(OH)2](CO3)0.165· 1.34H2O and [Zn0.67Cr0.33(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 CrIII to CrO42– species at intermediate calcination temperatures, ZnII 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 CrIII→ CrO42– 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 VO43– species.