Sol–gel synthesis of silica–cobalt composites by employing Co3O4 colloidal dispersions

Silica–cobalt composites with cobalt contents (w/w) around 10% were synthesized by employing the sol–gel method, tetraethylorthosilicate as the silica precursor and either a solution of cobalt nitrate or a colloidal dispersion of Co3O4 or Co(OH)2 nanoparticles as the cobalt precursors. Cobalt oxide was obtained by the precipitation of Co2+ ions in alkaline medium and by subsequent thermal treatment. Stable colloidal dispersions of cobalt oxide were prepared by using high power ultrasound and by covering nanoparticles with lauric acid bilayers. All materials were characterized by X-ray diffraction, infrared and Raman spectroscopy, transmission electron microscopy, and adsorption/desorption of nitrogen. Further, the reducibility of cobalt species was studied using the thermal-programmed reduction technique. All the Co3O4/silica composites were mesoporous (3.9–5.4 nm) with considerably high porosity (321–567 m2 g−1). The TEM mean size of Co3O4 nanoparticles within the calcined composites varied from 10 to 18 nm, according to the cobalt precursor employed in the synthesis. The reducibility of cobalt species also depended on the employed cobalt precursor, as shown by the thermal programmed reduction experiments. The composites which were prepared from the Co3O4 colloidal dispersions were reduced at lower temperatures; 80% of the total Co amount was reduced in the range of 300–500 °C. On the other hand, only 53% of total Co in the sample prepared from cobalt nitrate solution was reduced from 200 up to 900 °C. The difference on the reducibility was attributed to morphological characteristics of the composite samples.