XAPS STUDY OF STRUCTURAL EVOLUTION DURING CALCINATION OF 2 ZINC-CHROMIUM BASED LAMELLAR DOUBLE HYDROXIDES

Two [Zn-Cr-III] Lamellar Double Hydroxides respectively intercalated b y chloride and sulphate anions were prepared and two series of samples were obtained by heating these two compounds at increasing temperatur es. At temperatures higher than 200 degrees C these compounds become i ll-organized materials. The lamellar structure collapses at about 300 degrees C leading to highly divided amorphous oxides. A spinel-like ph ase and zinc oxide crystallise at high temperatures. The EXAFS study a t Zn and Cr K-edges shows clearly the presence of a pre-spinel organis ation 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 ar e observed at slightly different temperatures for both sulphate and ch loride intercalated precursors.