Effects of Ethanol Impregnation on the Properties of Silica-Supported Cobalt Catalysts

Silica-supported cobalt (6 wt%) catalysts were prepared by pore volume impregnation of either ethanol or aqueous cobalt nitrate solutions on calcined silica (500 and 1000°C), and characterized by infrared, hydrogen chemisorption, thermogravimetry–mass spectroscopy (TG-MS), and X-ray diffraction (XRD). XRD and TG show that the main cobalt phase is Co3O4(above 90%) after calcinationin vacuoto 300°C. The influence of the calcination temperature of silica on the particle size of Co3O4is not significant. Smaller Co3O4crystallites and more Co–SiO2interaction species were obtained when catalysts were prepared with ethanol instead of aqueous solution. In addition, ethoxyl groups were formed on the silica surface with a surface density of 0.17 to 0.09 ethoxyls/nm2depending on the calcination temperature of the pretreated silica. The decrease in Co3O4crystallite size on ethanol impregnation is attributed to the presence of surface ethoxyls which might hinder the aggregation of Co3O4during its formation from the thermal decomposition of cobalt nitrate. As a consequence, a larger percentage dispersion of cobalt metal was obtained after reduction at 400°C. TG-MS results showed that ethoxyl groups were decomposed into ethylene in the temperature range 500–600°C in He or H2. Hence, ethoxyl groups might affect the sintering process of the cobalt metal during reduction as well.