Evolution of nickel speciation during preparation of Ni-SiO_2 catalysts:effect of the number of chelating ligands in [Ni(en)_x(H_2O)_(6-2x)]~(2+) precursor complexes

摘要

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_2O)_(6-2x)]~(2+) (en=ethylene-diamine,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_2O)_4]~(2+) (64% of all Ni complexes),[Ni(en)_2(H_2O)_2]~(2+) (81%) and [Ni(en)_3]~(2+) (61%),respectively.Equilibrium adsorption of [Ni(en)_x(H_2O)_(6-2x)]~(2+) on SiO_2 results in the selective grafting of [Ni(en)(H_2O)_4]~(2+) and [Ni(en)_2(H_2O)_2]~(2+),through the substitution of two labile H_2O ligands by two surface SiO~- groups.The surface [Ni(en)(H_2O)_2(SiO)_2] complex formed by the grafting of [Ni(en)(H_2O)_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_2O)_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.