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+ precursors was investigated in the preparation of Ni/SiO2 catalysts. The coordination sphere of Ni complexes was mainly studied by XAFS spectroscopy in the initial step (dried samples) and after a vacuum activating treatment at 700 degrees C. Four modes of deposition were compared, two leading to supported silicate phases (exchange with ammine ligands and deposition-precipitation) and the other ones using the ethanediamine ligand (exchange and impregnation modes) which produced isolated Ni2+ precursors in electrostatic interaction with the silica support. In this work, EXAFS spectroscopy has been found to be a suitable technique to probe metal-support interactions in the first step of the preparation (dried samples). For samples activated at 700 OC, this spectroscopy showed the presence of several categories of atoms in the first (oxygen backscatterers) and second (Ni and. Si backscatterers) shells. A distribution of long (d approximate to 2.04 Angstrom) and short (d approximate to 1.75 Angstrom) Ni-O distances was found, corresponding to hexacoordinated (Ni-6c(2+)) and isolated tricoordinated (Ni-3c(2+)) ions, respectively. Modeling of the structure of the Ni-3c(2+). Site indicated a distorted site with two short and one long Ni-O distance. The exchange mode with ethanediamine ligand led to isolated Ni-3c(2+) ions which could then be photoreduced into Ni-3c(+) ions with the highest yield. Impregnation with ethanediamine Ni nitrate was also found advantageous as it led after thermal activation at 700 degrees C to NiO particles smaller than those produced from impregnation with Ni nitrate and therefore to smaller Ni particles after the subsequent reduction step.