Evolution of nickel speciation during preparation of Ni–SiO2 catalysts: effect of the number of chelating ligands in [Ni(en)x(H2O)6–2x]2+ precursor complexes

The evolution of nickel speciation during the successive preparation steps of Ni–SiO2 catalysts is studied by UV-Vis-NIR, FT-IR, DTG, TPR and TEM. The study focuses on the effect of the number of chelating ligands in the precursor complexes [Ni(en)x(H2O)(6−2x)]2+ (en = ethylenediamine, x = 1, 2, 3) on the adsorption on silica, and on nickel speciation after thermal treatment. When the en:Ni ratio in solution increases from 1 to 3, the most abundant complex is [Ni(en)(H2O)4]2+ (64% of all Ni complexes), [Ni(en)2(H2O)2]2+ (81%) and [Ni(en)3]2+ (61%), respectively. Equilibrium adsorption of [Ni(en)x(H2O)(6−2x)]2+ on SiO2 results in the selective grafting of [Ni(en)(H2O)4]2+ and [Ni(en)2(H2O)2]2+, through the substitution of two labile H2O ligands by two surface SiO− groups. The surface [Ni(en)(H2O)2(SiO)2] complex formed by the grafting of [Ni(en)(H2O)4]2+ onto silica tends to transform into NiO and nickel phyllosilicate after calcination, which consequently leads to large and heterogeneously distributed metallic Ni particles upon reduction. In contrast, [Ni(en)2(SiO)2], resulting from the grafting of [Ni(en)2(H2O)2]2+ onto silica, no longer has aqua ligands able to react with other nickel complexes or silicium-containing species. Calcination transforms these complexes into isolated Ni2+ ions, which are reduced into small metallic Ni particles with a more homogeneous size distribution, even at higher Ni loading.