Surface studies of cobalt-manganese oxide and industrial iron catalysts used in the Fischer-Tropsch synthesis

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2015-01-08

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Betts, Mark Justin

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The surfaces of pure and potassium promoted cobalt-manganese oxide as well as iron-based catalysis have been characterised before and after various treatments. The effects of reduction and carburisation of cobalt-manganese oxide catalyst surfaces is reported. CO-hydrogenation over pure and potassium-promoted cobalt-manganese oxide was also undertaken. Finally, various aspect; of hydrogen sulphide poisoning of an industrial iron-based catalyst during CO-hydrogenation were investigated. A miniature high pressure reactor (15 bar) interfaced to the UHV chamber and a gas chromatogram enabled ”m situ” surface analysis by XFS and AES to be carried out between successive catalyst treatments, as well as monitoring of CO-hydrogenation product distributions. The surface composition of the cobalt-manganese oxide catalysts was shown to differ considerably with respect to the bulk. There was significant depletion of surface cobalt, especially after reduction, and there was evidence to indicate that reduction of the cobalt was bulk initiated. Potassium segregated to the surface and inhibited efficient reduction of the surface cobalt in the Co/MnO catalyst. Pure CO re -oxidised the surface cobalt, reduced all the manganese to MnO, gave rise to graphite deposition, CO2 1 at the surface and bulk carbide formation. Large quantities of light hydrogenated carbon or nC2Hy” species were observed at the surface after CO-hydrogenation treatments, and a small amount of bulk-carbon was only detected with the potassium promoted Co/MnO. Both these factors serve as testimony to the higher hydrogenation activity and can also contribute to explaining the longer catalyst lifetimes shown by Co/MnO compared to Fe/Mn and industrial iron-based CO-hydrogenation catalysts. Small amounts of oxygenated carbon were also observed, and this was more pronounced over the potassium promoted catalyst. H)3 selectively attacked the potassium when sulphur poisoning treatments were administered to an industrial iron-based catalyst between CO-hydrogenation treatments. Correlations between hydrocarbon selectivities, sulphur dosages and treatment conditions, as well as surface studies, enabled a theory on the possible mechanism of H=S interaction with the catalyst to be proposed.

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