INFLUENCE OF ORDER-DISORDER PARAMETERS ON THE REDUCIBILITY OF NI-CONTAINING AND CU-CONTAINING SILICATES - APPLICATION TO TALC AND CHRYSOCOLLA

Reference phyllosilicates (Ni-talcs and chrysocolla) have been synthes ized with the aim of identifying supported layered silicates in the pr eparation of Ni/SiO2 and Cu/SiO2 materials. The analysis of temperatur e programmed reduction (TPR) profiles and X-ray diffraction (XRD) patt erns of talc synthesized at different temperatures shows a strong infl uence of the crystallinity of the Ni silicate on its reducibility. Chr ysocolla remains ill-crystallized whatever the temperature of synthesi s. The reducibility of the two silicates is discussed in terms of the structure of the silicate and the reducibility of the transition metal -ion. The asymmetric shape of the TPR curves of Ni-talcs is interprete d on the basis of the phase-boundary controlled model and the presence of a heterogeneous particle size distribution. The results emphasize the role of the crystallinity and show that the identification of supp orted silicate phases from TPR only may not be reliable.