Surface studies of cobalt-manganese oxide and industrial iron catalysts used in the Fischer-Tropsch synthesis
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Date
2015-01-08
Authors
Betts, Mark Justin
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Abstract
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.