Structural Evolution of χ-Fe5C2 and θ-Fe3C in the Reverse Water-Gas Shift Reaction
Iron-based catalysts in CO2/H2 atmospheres undergo dynamic carburization and oxidation phase transitions, complicating active-phase identification and stability control. This study prepared high-purity single-phase χ-Fe5C2 (Hägg carbide) and θ-Fe3C (cementite) via gas-solid carburization, with purity confirmed by XRD and Mössbauer spectroscopy. Fixed-bed reactor tests (H2/CO2 = 1, 0.1 MPa, 270–420 °C), pulse experiments (270 °C), and in situ XRD (10% CO2/He, 340 °C) were employed to investigate catalytic performance and structural evolution in the reverse water-gas shift (RWGS) reaction. Results show that χ-Fe5C2 exhibits higher RWGS activity but is more susceptible to oxidation, whereas θ-Fe3C demonstrates superior oxidation resistance but lower activity. Under RWGS conditions with H2, θ-Fe3C partially transforms into χ-Fe5C2; however, in 10% CO2 atmosphere, both carbides directly oxidize to Fe3O4 without inter-carbide transformation. In situ XRD at 340 °C and 0.1 MPa revealed that χ-Fe5C2 fully oxidizes within 11 h, while θ-Fe3C retains residual phase after 18.3 h, confirming its higher oxidation stability. These findings elucidate the atmosphere-dependent evolution mechanisms of χ-Fe5C2 and θ-Fe3C, providing experimental basis for phase-structure regulation and operational stability optimization in iron-based Fischer-Tropsch and RWGS catalysts.