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

The effects of various alkali metal promoters on the structure-performance of cobalt-manganese (CoMn) catalysts for Fischer-Tropsch synthesis (FTS) reaction have been investigated. The electronic promotion effects are correlated with the ionic potential (IP) of alkali metals which stands for the electronic polarizability. The presence of alkali metal promoters in the CoMn catalysts benefited the formation and stabilization of Co2C phase. With the increase of IP of alkali metal promoters, the active phase was found from Co2C nanospheres to Co2C nanoprisms, then to a mixture of fcc Co0 and Co2C nanospheres. In addition, the catalytic performance also changed with the transition of active phase. For Co2C nanospheres, high CH4 selectivity and low activity were observed. Promising catalytic performance for Fischer-Tropsch to olefins (FTO) with low CH4 selectivity and high C2-4= selectivity was achieved for Co2C nanoprisms. However, low C2-4= selectivity with high activity was found for the mixture of fcc Co0 and Co2C nanospheres. It is suggested that the alkali metal promoters in the CoMn catalysts had two different promotion effects enhanced with IP: (1) the electronic effect to enhance CO dissociation probability and benefit the Co2C formation; (2) the facets stabilization effect to stabilize Co2C nanoprisms with specific exposed facets of (101) and (020) under the FTO reaction conditions.