Characterization of cobalt Fischer-Tropsch catalysts I. Unpromoted cobalt-silica gel catalysts

Abstract

The effect of preparation conditions on cobalt-silica gel catalysts has been investigated by temperature-programmed reduction, differential thermal analysis, X-ray photoelectron spectroscopy, and magnetic susceptibility measurements. The results have been correlated with the performance of the catalysts for the conversion of syngas. It has been found that active Fischer-Tropsch (F-T) cobalt catalysts can be partially or totally reduced to the metal phase during activation. However, reducibility of the catalysts is a necessary but not sufficient characteristic in ensuring good activity. The pH of the impregnating solution influences surface charge and solubility of silica gel. Consequently, the physical and chemical properties of the cobalt species on the surface of silica gel are strongly affected by this parameter. At pH<5 cobalt deposits as a cation. At pH<2 the silica surface is positively charged so that the dispersion of cobalt and consequently catalytic activity are poor. At pH≥5, cobalt reacts to form silicates or hydrosilicates which are irreducible. Active catalysts are shown to adsorb hydrogen by spillover on to the silica support.