On the selectivity of cobalt-based Fischer–Tropsch catalysts: Evidence for a common precursor for methane and long-chain hydrocarbons

A total of 36 cobalt-based supported catalysts were investigated in the Fischer–Tropsch reaction at industrially relevant process conditions: 483 K, 20 bar, molar H2/CO ratio = 2.1, pellet size: 53–90 μm. The effect of adding water vapour to the feed was investigated for 20 of the catalysts, and a H2/CO ratio of 1.0 was used for a few catalysts. The catalysts differed in support material, Co loading, promoters, Cl content, Co particle size (larger than ∼6 nm), morphology, degree of reduction and preparation technique and showed a large variation in selectivity. For each set of process conditions, a linear relationship seems to exist between the selectivity to methane (and other light products) and C5+ indicating a common precursor, i.e. a common monomer pool, for all hydrocarbon products. A high selectivity to C5+ is mainly an effect of a high intrinsic chain-growth probability and unlikely to be a result of an enhanced α-olefin readsorption. The universal effect of external water addition on the hydrocarbon selectivities is limited to a decrease in the methane selectivity. A small proportion of the catalysts developed “pure methanation” sites upon exposure to high partial pressures of water.