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

Four commercial aluminas and different cobalt loadings have been selected t o better understand the interactions between Co oxides:rides and metallic c obalt with alumina surfaces. The solids have been characterized, after dryi ng and calcination procedures or after reduction under Hz, using thermograv imetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectr oscopy (XPS), and scanning electron microscopy (SEM). The information has b een complemented by specific surface area (SSA), porosimetry, and magnetic measurements. Both the effect of the cobalt content and the texture of supp orts have been examined. It appears that the cobalt reducibility is influen ced by the cobalt loading and the alumina porosity mainly because of the he terogeneity in the Co distribution. Two cobalt reduction steps were identif ied. 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 ra nge of temperatures (623-873 K) and is highly dependent on the support char acteristics. The use of high specific surface area supports leads to cataly sts 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 powder ed sample or the periphery of pellets are richer in cobalt than the bulk, a nd the cobalt is less dispersed. The reduction of cobalt oxide localized at the periphery is facilitated by its poor dispersion. On the other hand, th e well-dispersed cobalt species in the core of the pellets are more strongl y interacting with the support and are not so easily reduced.