Reducibility of Cobalt Species in Silica-Supported Fischer-Tropsch Catalysts

Author： Khodakov A.Y. Lynch J. Bazin D. Rebours B. Zanier N. Moisson B. Chaumette P.

ISSN： 0021-9517

Source： Journal of Catalysis, Vol.168, Iss.1, 1997-01, pp. : 16-25

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Abstract

Reducibility of Co species in silica supported Fischer-Tropsch catalysts was studied using in situ XRD, in situ EXAFS, and FTIR spectroscopy with carbon monoxide as a molecular probe. Crystalline Co phases in the oxidised catalysts were characterised using XRD. In the oxidised samples with a large concentration of amorphous phase, EXAFS showed the presence of small oxide clusters including several Co atoms. It was found ( in situ XRD, EXAFS) that calcination of oxidised Co catalysts under inert atmosphere resulted in a selective transformation of Co 3 O 4 to CoO at 623-673 K. FTIR spectroscopy with CO as a molecular probe revealed the presence of different sites associated with Co after the reduction of the catalysts with hydrogen at 723 K: Co metal sites (nu CO =2025 cm -1 ), Co 2+ ions in the crystalline phase of CoO (nu CO =2143 cm -1 ), and Co n+ species in the amorphous phase (nu CO =2181 cm -1 ). The results (XRD, EXAFS, FTIR) showed that the hydrogen reduction properties of particles of cobalt oxide on silica depended on the size of the Co 3 O 4 crystallites. The ease of reduction to metal species decreased from larger (200-700 A) to smaller (60 A) particles.