Characterization by UV-vis-NIR reflectance spectroscopy of the exchange sites of nickel on silica

Abstract

The coordination of Ni 2+ ions in Ni/SiO 2 catalysts has been studied by UV-vis-NIR reflectance spectroscopy at various stages of the preparation, drying, and calcination followed by rehydration. The catalysts were prepared by competitive cation exchange from an ammoniacal nickel solution. The nickel is first exchanged as a hexammine complex on the silica surface via a purely electrostatic mechanism at pH 9.8 or 10.4. Upon washing with water, substitution reactions occur in the coordination sphere of Ni 2+ ions, which remain in octahedral symmetry and become grafted to the silica surface via ligand displacement with surface oxygen. The so formed [ Ni(≡ SiO) 2 L 4] surface complexes contain ligans L from the solution (H 2O or NH 3) and ≡SiO mononegative ligands from the support forming ionic bonds with Ni 2+. Distortion of the grafted nickel from octahedral symmetry increases when H 2O substitutes for NH 3 ligands. This is attributed to hydrogen bonds between the ligands L and the unexchanged ≡SiO − surface groups. From these results, a model of the exchange sites and the exchange mechanism are proposed.