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The influence of different iron carbides on the activity and selectivity of iron-based Fischer–Tropsch catalysts has been studied. Different iron carbide phases are obtained by the pretreatment of a binary Fe/SiO₂ model catalyst (prepared by coprecipitation method) to different gas atmospheres (syngas, CO, or H₂). The phase structures, compositions, and particle sizes of the catalysts are characterized systematically by XRD, XAFS, MES, and TEM. It is found that in the syngas-treated catalyst only χ-Fe₅C₂ carbide is formed. In the CO-treated catalyst, Fe₇C₃ and χ-Fe₅C₂ with a bimodal particle size distribution are formed, while the H₂-treated catalyst exhibits the bimodal size distributed ε-Fe₂C and χ-Fe₅C₂ after a Fischer–Tropsch synthesis (FTS) reaction. The intrinsic FTS activity is calculated and assigned to each corresponding iron carbide based on the phase composition and the particle size. It is identified that …