Effects of alkali metal promoters on the structure–performance relationship of CoMn catalysts for Fischer–Tropsch synthesis

Abstract

The effects of various alkali metal promoters on the structure–performance relationship of cobalt–manganese (CoMn) catalysts for the Fischer–Tropsch synthesis (FTS) reaction have been investigated. The electronic promotion effects are correlated with the ionic potential (IP) of the alkali metals, which represents their electronic polarizability. The presence of alkali metal promoters in the CoMn catalysts is beneficial for the formation and stabilization of the Co₂C phase. With an increase in the IP of the alkali metal promoters, the active phase was found to transition from Co₂C nanospheres to Co₂C nanoprisms and then to a mixture of fcc Co⁰ and Co₂C nanospheres. In addition, the catalytic performance also changed with the transition in the active phase. For the Co₂C nanospheres, high CH₄ selectivity and low activity were observed. A promising catalytic performance for Fischer–Tropsch to olefins (FTO) with low CH₄ selectivity and high C_2–4⁼ selectivity was achieved for the Co₂C nanoprisms. However, low C_2–4⁼ selectivity with high activity was found for a mixture of fcc Co⁰ and Co₂C nanospheres. It is suggested that the alkali metal promoters in the CoMn catalysts have two different promotion effects that are enhanced with IP: (1) the electronic effect to enhance the CO dissociation probability and benefit the formation of Co₂C and (2) the facet stabilization effect to stabilize the Co₂C nanoprisms with specific exposed (101) and (020) facets under the FTO reaction conditions.