Structural Evolution Regulation of χ-Fe5C2 and θ-Fe3C in the Reverse Wa-ter-Gas Shift Reaction

Iron-based catalysts often undergo dynamic carburization and oxidation phase transitions in a CO2/H2 atmosphere. The identification of the active phase and the control of stability remain key challenges affecting their industrial application. This study employed a gas-solid carburization method to prepare high-purity single-phase χ‑Fe5C2 (Hägg carbide) and θ‑Fe3C (cementite), with their structural purity systematically confirmed by XRD and Mössbauer spectroscopy (MES). Using techniques including 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), the catalytic performance and structural evolution pathways of these carbides in the reverse water-gas shift (RWGS) reaction were thoroughly investigated. The results indicate that χ‑Fe5C2 exhibits higher RWGS activity but is more susceptible to oxidation during the reaction. In contrast, θ‑Fe3C demonstrates stronger anti-oxidation stability but lower RWGS activity. Under RWGS conditions with H2, θ‑Fe3C can partially transform into χ‑Fe5C2. However, in 10 % CO2 atmosphere, both carbides are directly oxidized to Fe3O4, with no observed phase transformation between the iron carbides. These findings reveal the differences in the evolution mechanisms of χ‑Fe5C2 and θ‑Fe3C under different atmospheres, providing experimental basis for the regulation of phase structure and optimization of operational stability in iron-based Fischer-Tropsch and RWGS catalysts.