Selective synthesis of the core–shell structured catalyst χ-Fe5C2 surrounded by nanosized Fe3O4 for the conversion of syngas to liquid fuels

Abstract

Enhancing liquid hydrocarbon selectivity and simultaneously suppressing CO₂ formation are highly desirable yet challenging in iron-based Fischer–Tropsch synthesis. Herein, we report an in situ oxidation method for the fabrication of a functional configuration catalyst with a core–shell structure, namely, a χ-Fe₅C₂ core surrounded by a nanosized Fe₃O₄ shell, which promotes the formation of long-chain hydrocarbons on the inner χ-Fe₅C₂ as well as the depletion of primarily formed CO₂ on the outer Fe₃O₄via the reverse water-gas shift reaction. The key to its success is the pretreatment of Fe₃O₄ under a syngas atmosphere with a H₂/CO ratio of 2 at a high pressure of 20 bar and a high temperature of 400 °C. The pretreatment leads to an in situ formed high oxidation chemical potential atmosphere, which promotes the formation of the χ-Fe₅C₂@Fe₃O₄ core–shell structure. Moreover, the high-pressure pretreatment facilitates the formation of surface-graphitized carbon layers, leading to enhanced selectivity for long-chain hydrocarbons due to the electronic promotion effect of the graphitic structures. Such a functional configuration catalyst achieves a high C₅₊ hydrocarbon selectivity of 45% and a simultaneously low CO₂ selectivity of only 7% with a CO conversion of 19% at 230 °C. These results offer a valuable reference for the rational fabrication of catalysts to suppress the usually high CO₂ selectivity.