EXAFS characterization of the adsorption sites of nickel ammine and ethylenediamine complexes on a silica surface

Abstract

The evolution of the first and the second coordination shells of Ni 2+ ions in a nickel ion exchanged silica is monitored by EXAFS at the nickel K edge in the early stages of a Ni/SiO 2 catalyst preparation. There is a strong pH and ligand effect on the nature of the adsorbed state of Ni 2+ ions. At pH 9 in the wet gel, the coordination sphere of Ni 2+ ions completed by ammonia or ethylenediamine is not modified by electrostatic adsorption on the support. Below pH 9, ammonia can be substituted by water molecules to form penta- or tetraaammine complexes which can be thus grafted to the surface via a ligand substitution involving surface oxygens. Under theseconditions, the majority of nickel ions are engaged in the formation of a nickel surface layer which has a silicate structure. After drying at 353 K and whatever the pH of the exchange solution, the silicate is always formed. Below pH 7, no substantial cation exchange occurs on silica. On the contrary, the tris ethylenediamine nickel complex which is very stable above pH 7, does not lead to the silicate layer formation. The shift from electrostatic adsorption toward grafting only occurs under drying at 353 K leading to isolated grafted complexes. In all cases, a distance of 3.25–3.31 Å between nickel and 2 silicon ions is consistent with a grafting to a pair of vicinal silanol groups.