CONTROL OF DISPERSION OF NI2+ IONS VIA CHELATE LIGANDS IN THE PREPARATION OF NI SIO2 MATERIALS - A XAFS STUDY/

The influence of the Ni deposition mode on the dispersion of Ni2+ prec ursors was investigated in the preparation of Ni/SiO2 catalysts. The c oordination sphere of Ni complexes was mainly studied by XAFS spectros copy in the initial step (dried samples) and after a vacuum activating treatment at 700 degrees C. Four modes of deposition were compared, t wo leading to supported silicate phases (exchange with ammine ligands and deposition-precipitation) and the other ones using the ethanediami ne ligand (exchange and impregnation modes) which produced isolated Ni 2+ precursors in electrostatic interaction with the silica support. In this work, EXAFS spectroscopy has been found to be a suitable techniq ue to probe metal-support interactions in the first step of the prepar ation (dried samples). For samples activated at 700 OC, this spectrosc opy showed the presence of several categories of atoms in the first (o xygen backscatterers) and second (Ni and. Si backscatterers) shells. A distribution of long (d approximate to 2.04 Angstrom) and short (d ap proximate to 1.75 Angstrom) Ni-O distances was found, corresponding to hexacoordinated (Ni-6c(2+)) and isolated tricoordinated (Ni-3c(2+)) i ons, respectively. Modeling of the structure of the Ni-3c(2+). Site in dicated a distorted site with two short and one long Ni-O distance. Th e exchange mode with ethanediamine ligand led to isolated Ni-3c(2+) io ns which could then be photoreduced into Ni-3c(+) ions with the highes t yield. Impregnation with ethanediamine Ni nitrate was also found adv antageous as it led after thermal activation at 700 degrees C to NiO p articles smaller than those produced from impregnation with Ni nitrate and therefore to smaller Ni particles after the subsequent reduction step.