Control of Dispersion of Ni²⁺ Ions via Chelate Ligands in the Preparation of Ni/SiO₂ Materials. A XAFS Study

Abstract

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 °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 °C, 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 ≈ 2.04 Å) and short (d ≈ 1.75 Å) Ni−O distances was found, corresponding to hexacoordinated (Ni2+6c) and isolated tricoordinated (Ni2+3c) ions, respectively. Modeling of the structure of the Ni2+3c site indicated a distorted site with two short and one long Ni−O distance. The exchange mode with ethanediamine ligand led to isolated Ni2+3c 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 °C to NiO particles smaller than those produced from impregnation with Ni nitrate and therefore to smaller Ni particles after the subsequent reduction step.