Evolution of nickel speciation during preparation of Ni–SiO2catalysts: effect of the number of chelating ligands in [Ni(en)x(H2O)6−2x]2+ precursor complexes

Abstract

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.