Influence of the characteristics of γ-aluminas on the dispersion and the reducibility of supported cobalt catalysts

Four commercial aluminas and different cobalt loadings have been selected to better understand the interactions between Co oxides:rides and metallic cobalt with alumina surfaces. The solids have been characterized, after drying and calcination procedures or after reduction under Hz, using thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The information has been complemented by specific surface area (SSA), porosimetry, and magnetic measurements. Both the effect of the cobalt content and the texture of supports have been examined. It appears that the cobalt reducibility is influenced by the cobalt loading and the alumina porosity mainly because of the heterogeneity in the Co distribution. Two cobalt reduction steps were identified. The first one which transforms Co3O4 into CoO appears to be easy from 500 K whereas the second one which produces metallic Co occurs in a wide range of temperatures (623-873 K) and is highly dependent on the support characteristics. The use of high specific surface area supports leads to catalysts with a good dispersion when their macroporosity (pore diameter phi > 0.05 mu m) is large, However, in all the cases, the grain edges of the powdered sample or the periphery of pellets are richer in cobalt than the bulk, and the cobalt is less dispersed. The reduction of cobalt oxide localized at the periphery is facilitated by its poor dispersion. On the other hand, the well-dispersed cobalt species in the core of the pellets are more strongly interacting with the support and are not so easily reduced.