title: Evolution of nickel speciation during preparation of Ni-SiO(2) catalysts: 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-SiO(2) 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)(H(2)O)((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)(H(2)O)(4)](2+) (64% of all Ni complexes), [Ni(en)(2)(H(2)O)(2)](2+) (81%) and [Ni(en)(3)](2+) (61%), respectively. Equilibrium adsorption of [Ni(en)(x)(H(2)O)((6-2x))](2+) on SiO(2) results in the selective grafting of [Ni(en)(H(2)O)(4)](2+) and [Ni(en)(2)(H(2)O)(2)](2+), through the substitution of two labile H(2)O ligands by two surface SiO(-) groups. The surface [Ni(en)(H(2)O)(2)(SiO)(2)] complex formed by the grafting of [Ni(en)(H(2)O)(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)(H(2)O)(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 Ni(2+) ions, which are reduced into small metallic Ni particles with a more homogeneous size distribution, even at higher Ni loading.
authors.affiliation: Laboratoire de Réactivité de Surface (UMR 7609-CNRS), Université Pierre et Marie Curie, 4 place Jussieu, 75252 Paris Cedex 05, France.
authors.forename: Ke-Qiang
authors.initials: KQ
authors.lastname: Sun
authors.forename: Eric
authors.initials: E
authors.lastname: Marceau
authors.forename: Michel
authors.initials: M
authors.lastname: Che
mesh_terms: D002384:Catalysis; D002614:Chelating Agents; D008024:Ligands; D008697:Methane / Q000737:chemistry; D009532:Nickel; D010084:Oxidation-Reduction; D013053:Spectrophotometry; D013458:Sulfur Dioxide; D014867:Water
chemical_list: D002614:Chelating Agents; D008024:Ligands; D014867:Water; D013458:Sulfur Dioxide; D009532:Nickel; D008697:Methane