REDUCIBILITY OF COBALT SPECIES IN SILICA-SUPPORTED FISCHER-TROPSCH CATALYSTS

Reducibility of Co species in silica supported Fischer-Tropsch catalys ts was studied using in situ XRD, in situ EXAFS, and FTIR spectroscopy with carbon monoxide as a molecular probe. Crystalline Co phases in t he oxidised catalysts were characterised using XRD. In the oxidised sa mples with a large concentration of amorphous phase, EXAFS showed the presence of small oxide clusters including several Co atoms. It was fo und (in situ XRD, EXAFS) that calcination of oxidised Co catalysts und er inert atmosphere resulted in a selective transformation of Co3O4 to CoO at 623-673 K. FTIR spectroscopy with CO as a molecular probe reve aled the presence of different sites associated with Co after the redu ction of the catalysts with hydrogen at 723 K: Co metal sites (v(CO) = 2025 cm(-1)), Co2+ ions in the crystalline phase of CoO (v(CO) = 2143 cm(-1)), and Con+ species in the amorphous phase (v(CO) = 2181 cm(-1) ). The results (XRD, EXAFS, FTIR) showed that the hydrogen reduction p roperties of particles of cobalt oxide on silica depended on the size of the Co3O4 crystallites. The ease of reduction to metal species decr eased from larger (200-700 Angstrom) to smaller (60 Angstrom) particle s. (C) 1997 Academic Press.