Sol-gel processing of alkoxysilyl-substituted nickel complexes for the preparation of highly dispersed nickel in silica

Abstract

Nanocomposites containing nickel nanoparticles in silica were obtained by sol-gel processing of Ni(NO₃)₂, Ni(OAc)₂ (OAc = acetate), NiCl₂, or Ni(acac)₂ (acac = acetylacetonate), a complexing silane [(EtO)₃Si(CH₂)₃NHCH₂CH₂NH₂, (EtO)₃Si(CH₂)₃NH₂ or (RO)₃Si(CH₂)₃NHCH₂CH₂NHCH₂CH₂NH₂] and Si(OEt)₄, followed by calcination of the metal-complex-containing xerogels in air and reduction in hydrogen. Two types of nanocomposite materials were prepared, both containing about 8–8.5 wt % of elemental nickel: (i) Ni/SiO₂ nanocomposite powders in which the silica matrix is mainly formed from Si(OR)₄, and (ii) Ni particles on pre-formed silica spheres. The latter were prepared by impregnating silica spheres with sols from the particular complexing silane–nickel salt combination followed by calcination and reduction. In both types of materials, the average nickel particle size is influenced by the composition of the precursor mixture, particularly the counter-ion of the employed nickel salt. The smallest particles (diameter below 3 nm) were obtained from Ni(NO₃)₂ [and Ni(OAc)₂ for the powders], while the use of Ni(acac)₂ and NiCl₂ resulted in larger nickel particles. The variation of the nickel particle diameters was much larger for the powders than for the impregnated spheres.