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

In situ XAFS spectroscopic studies have been carried out at 450 degree s C on the hydrogen reduction Of a rhenium-promoted Co3O4/Al2O3 cataly st. Reductions carried out using 100% hydrogen and 5% hydrogen in heli um gave different results. Whereas the reduction using dilute hydrogen yielded bulk-like metallic cobalt particles (hcp or fcc), the reactio n with pure hydrogen led to a more dispersed system with smaller cobal t metal particles (< 40 Angstrom) the crystal form of which could not be established so that the recently reported metastable nonclose-packe d body-centred cubic form cannot be excluded. Reoxidation of a similar catalyst in water-containing gas mixtures has been reported in the li terature; it is suggested that the different outcome in the case of th e 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 mixtu re. 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 vac ancies of the alumina support with Co-O bond lengths of 1.96 +/- 0.01 Angstrom. This dispersion: occurs during reduction and not calcination . The cobalt in these sites cannot be reduced at 450 degrees C, a temp erature that is too low to permit formation of the spinel CoAl2O4.