• • χ-Fe5C2 exhibits higher RWGS activity than θ-Fe3C, with lower onset temperature and higher CO2 conversion under H2/CO2 = 1, 0.1 MPa, 270–420 °C; however, it is more prone to oxidation to Fe3O4, limiting long-term stability.
• • θ-Fe3C demonstrates superior oxidation resistance: under 10% CO2/He at 340 °C and 0.1 MPa, it retains residual phase after 18.3 h, whereas χ-Fe5C2 fully oxidizes within 11 h, as shown by in situ XRD.
• • Under RWGS conditions with H2, θ-Fe3C partially transforms into χ-Fe5C2, indicating a reversible phase transformation driven by carburization; in contrast, in CO2-only atmosphere, both carbides directly oxidize to Fe3O4 without inter-carbide conversion.
• • The trade-off between activity and stability (χ-Fe5C2: high activity, low stability; θ-Fe3C: low activity, high stability) underscores the need for atmosphere-specific phase management to optimize iron-based catalysts for RWGS and Fischer-Tropsch applications.