EXAFS CHARACTERIZATION OF THE ADSORBED STATE OF NI(II) IONS IN NI/SIO2 MATERIALS PREPARED BY DEPOSITION PRECIPITATION

Ni/SiO2 materials have been prepared by the deposition-precipitation technique. After washing and drying, the unreduced and uncalcined materials have been investigated by EXAFS spectroscopy, temperature-programmed reduction (TPR), and electron microscopy. The formation of nickel silicates likely of nepouite structure, Ni3Si2O5(OH)4, is established by EXAFS and confirmed by TPR experiments in contrast to X-ray diffraction which does not provide any information. The obtainment of high nickel content involves a partial dissolution of silica followed by a reprecipitation leading to the silicate formation. While EXAFS indicates the formation of silicates and excludes the precipitation of Ni(OH)2 epitaxially "glued" onto silicate layers, TPR excludes the precipitation of Ni(OH)2 in the pores of the washed materials. © 1991.