XAFS Study of Structural Evolution During Calcination of Two Zinc-Chromium Based Lamellar Double Hydroxides

Abstract
Two [Zn-Cr^III] Lamellar Double Hydroxides respectively intercalated by chloride and sulphate anions were prepared and two series of samples were obtained by heating these two compounds at increasing temperatures. At temperatures higher than 200°C these compounds become ill-organized materials. The lamellar structure collapses at about 300°C leading to highly divided amorphous oxides. A spinel-like phase and zinc oxide crystallise at high temperatures. The EXAFS study at Zn and Cr K-edges shows clearly the presence of a pre-spinel organisation in the ill-organized samples at intermediate temperature range. The XANES spectra at Cr K-edge evidence the formation of small amounts of Cr^VI which disappears at higher temperatures. These behaviours are observed at slightly different temperatures for both sulphate and chloride intercalated precursors.