Manganese promotion of a cobalt Fischer-Tropsch catalyst to improve operation at high conversion

Abstract

The addition of manganese to cobalt-based Fischer-Tropsch catalysts increases activity and significantly improves liquid fuel yield by decreasing CO2 and CH4 selectivity. These effects are particularly noticeable at high CO conversions where the catalyst is exposed to high H2O, and low CO and H2, partial pressures. Under these conditions, manganese-promotion causes a shift in the maximum liquid fuel yield from 61 C-% to 79 C-% (at CO-conversion levels of 78% and 91%, respectively), thus enabling once-through operation. Our CO-TPD and DFT studies show that manganese promotion cannot to be linked to enhanced CO adsorption nor promotion of the C-O bond cleavage (both typically postulated mechanisms). Rather, our DFT-calculations indicate that manganese promotion modelled as a manganese oxide cluster on Co(111) is associated with improved oxygen removal from the catalytically active surface, explaining the reduced selectivity towards CO2 at high conversion and, thus, reduced methane selectivity and improved liquid fuel yield.