In situ X-ray absorption spectroscopic studies at the cobalt K-edge on an Al₂O₃-supported rhenium-promoted cobalt Fischer-Tropsch catalyst. Comparing reductions in high and low concentration hydrogen

Abstract

In situ XAFS spectroscopic studies have been carried out at 450°C on the hydrogen reduction of a rhenium-promoted Co₃O₄/Al₂O₃ catalyst. Reductions carried out using 100% hydrogen and 5% hydrogen in helium gave different results. Whereas the reduction using dilute hydrogen yielded bulk-like metallic cobalt particles (hcp or fcc), the reaction with pure hydrogen led to a more dispersed system with smaller cobalt metal particles (<40 Å) the crystal form of which could not be established so that the recently reported metastable nonclose-packed body-centred cubic form cannot be excluded. Reoxidation of a similar catalyst in water-containing gas mixtures has been reported in the literature; it is suggested that the different outcome in the case of the 100% hydrogen protocol may be due to a similar mechanism. This would involve the in situ water produced by the reduction with reoxidation-reduction of cobalt metal particles in the water vapour-hydrogen mixture. However, this mechanism cannot be established by the present study. Additionally, in both reduction protocols a small fraction (3-4 wt.%) of the cobalt content is randomly dispersed over the tetrahedral vacancies of the alumina support with Co-O bond lengths of 1.96±0.01 Å. This dispersion occurs during reduction and not calcination.The cobalt in these sites cannot be reduced at 450°C, a temperature that is too low to permit formation of the spinel CoAl₂O₄.

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