Preparation and characterization of uniform spherical silica particles coated with Ni and Co compounds

Uniform layers of Ni or Co compounds have been deposited over spherical silica particles of narrow size distribution (0.35 mu m) by aging at elevated temperatures (85-90 degrees C) aqueous dispersions of the preformed cores in NiSO4/urea or cobalt(II) acetate/sodium dodecyl sulfate (SDS) solutions. A detailed study of the effects of the reagent concentrations on the characteristics of the so prepared particles indicated that uniform coatings could only be obtained under very restrictive conditions, yielding thin shells which were amorphous to X-ray diffraction (XRD). The analysis of the X-ray photoelectron spectroscopy (XPS) spectra of the coated particles suggested that the deposited layers were constituted by mixed SiO2NiO or SiO2-CoO oxides, which involves the dissolution of a small amount of silica during aging and its further coprecipitation with the Ni2+ or Co2+ cations. On heating at 800 degrees C, segregation of NiO was observed in the Ni(II)/SiO2 particles while crystallization of CO2SiO4 was detected in the Co(II)/SiO2 system at the same temperature. In both cases, the spherical particle morphology was retained after calcination up to 1000 degrees C. (C) 1999 Elsevier Science B.V. All rights reserved.