Relationship between Iron Carbide Phases (ε-Fe₂C, Fe₇C₃, and χ-Fe₅C₂) and Catalytic Performances of Fe/SiO₂ Fischer-Tropsch Catalysts

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 Fe₇C₃ has the highest intrinsic activity (TOF = 4.59 × 10^-2 s^-1) among the three candidate carbides (ε-Fe₂C, Fe₇C₃, and χ-Fe₅C₂) in typical mediumerature Fischer-Tropsch (MTFT) conditions (260-300 °C, 2-3 MPa, and H₂/CO = 2). Moreover, FTS over ε-Fe₂C leads to the lowest methane selectivity.