Key Takeaways & Executive Findings
- •• • 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.
Abstract
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.
1. Introduction
Iron-based catalysts are pivotal in converting carbonaceous resources via Fischer-Tropsch synthesis, yet their dynamic phase evolution under CO2/H2 atmospheres—carburization, oxidation, and reconstruction—creates a complex mixture of iron oxides, metallic iron, and carbides. This complexity hampers the identification of the true active phase and the control of catalyst stability, which are critical for industrial deployment. While phases like Fe5C2 and Fe3C are often cited as active, their intrinsic activities and transformation pathways under realistic reaction conditions remain poorly understood, leading to suboptimal catalyst design and operational challenges.
This study addresses this bottleneck by isolating high-purity single-phase χ-Fe5C2 and θ-Fe3C and systematically comparing their RWGS performance and structural evolution under controlled atmospheres. By employing fixed-bed reactions, pulse experiments, and in situ XRD, the research delineates the activity-stability trade-off and reveals atmosphere-dependent transformation mechanisms. These insights provide a rational basis for tailoring iron carbide phases to enhance both catalytic efficiency and longevity, directly tackling the industrial need for stable and selective RWGS catalysts.
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SUN Zeping, LIANG Ruikang, WEI Zidu, ZHANG Chao, XIANG Hongwei, WU Jianbing, LIU Xingwu (2026). Structural Evolution of χ-Fe5C2 and θ-Fe3C in the Reverse Water-Gas Shift Reaction. Journal of Fuel Chemistry and Technology. https://doi.org/10.3724/2097-213X.2025.JFCT.0035
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Frequently Asked Questions
What are the specific oxidation kinetics of χ-Fe5C2 and θ-Fe3C under CO2-rich conditions, and how do they impact catalyst lifetime?
Under 10% CO2/He at 340 °C and 0.1 MPa, in situ XRD shows χ-Fe5C2 fully oxidizes to Fe3O4 within 11 hours, while θ-Fe3C retains residual phase after 18.3 hours. This indicates θ-Fe3C has significantly higher oxidation resistance, which could translate to longer operational lifetime in CO2-rich feeds, albeit with lower activity.
Can θ-Fe3C be converted to the more active χ-Fe5C2 under RWGS conditions, and what are the implications for catalyst pretreatment?
Yes, under RWGS conditions with H2 (H2/CO2 = 1), θ-Fe3C partially transforms into χ-Fe5C2. This suggests that a controlled pretreatment in H2/CO2 can generate the more active phase in situ, potentially enhancing activity without sacrificing initial stability. However, the transformation is incomplete, and the resulting mixed-phase catalyst may exhibit intermediate performance.
What is the trade-off between activity and stability for these iron carbides, and how should one select the phase for a specific RWGS application?
χ-Fe5C2 offers higher CO2 conversion and lower onset temperature, but it is more prone to oxidation, leading to faster deactivation. θ-Fe3C is less active but more oxidation-resistant, maintaining structure longer. For applications requiring high conversion at lower temperatures, χ-Fe5C2 may be preferred if oxidation can be mitigated; for long-term stability under oxidizing conditions, θ-Fe3C is more suitable. The choice depends on the specific feed composition and operational targets.
How does the presence of H2 influence the phase transformation between χ-Fe5C2 and θ-Fe3C?
In the presence of H2 (RWGS conditions), θ-Fe3C can partially convert to χ-Fe5C2, indicating that H2 provides a carburizing driving force that stabilizes the higher-carbon χ phase. In contrast, in CO2-only atmospheres (no H2), both carbides directly oxidize to Fe3O4 without inter-carbide transformation, showing that H2 is essential for maintaining or transforming carbide phases.
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