Effect of water on the space-time yield of different supported cobalt catalysts during Fischer–Tropsch synthesis Original Research Article

The effect of partial pressure of water on the Fischer–Tropsch (FT) rate of six cobalt-based catalysts supported on three different carrier materials (γ-Al2O3, α-Al2O3, TiO2) with varying Co particle sizes was investigated in a fixed-bed reactor by changing space velocity and by external water vapour addition. A typical catalyst pellet size (<100 μm) for industrial slurry-bed FT reactors was used. Water was found to have a positive kinetic effect, at least up to moderate amounts, on the FT rate of all catalysts in the present study, including the γ-Al2O3-supported catalyst with pores smaller than ∼10 nm. The reason for the apparent negative effect on the space-time yield at a direct exposure of Co supported on narrow-pore γ-Al2O3 to high partial pressures of water is due to a rapid and extensive deactivation. This could be ascribed to formation of hard-to-reduce oxidized cobalt species. The choice of support material was found to have a major effect on the response to changes in partial pressure of water, both with respect to deactivation behaviour and kinetics. However, there is a minor Co-particle size effect on the magnitude of the kinetic effect of water, larger Co particles showing a more positive response. Different extents of mass transfer limitations and/or differences in fugacities of H2, CO and water among the six catalysts could be ruled out as causes for the observed differences.