Investigation of Surface Carburization on Fe5C2(111) Based on the GRL Framework

Abstract In iron-based Fischer–Tropsch synthesis (FTS) catalysts, χ-Fe5C2 (Hägg carbide) is regarded as the key active phase; however, its dynamic structural evolution and stabilization mechanism under realistic reaction conditions remain unclear. To elucidate the carburization mechanism of the Fe5C2(111) surface under realistic reaction conditions at the atomic scale, this study develops a simulation-analysis framework named GRL (GCMC-RF-LAS). This framework integrates Grand Canonical Monte Carlo (GCMC) simulations based on the Reactive Force Field (ReaxFF), the Random Forest (RF) algorithm, and a self-developed local coordination environment-based structural identification method (FeC-LAS, Fe–C Local Atomic Structure Classifier). The results indicate that the carburization process on the Fe5C2(111) surface can be divided into three stages, with both the rate and extent of carburization strongly influenced by temperature. Notably, temperature exerts a significantly stronger regulatory effect on kinetics than on thermodynamics. Importance analysis using Random Forest reveals that the number of carbon atoms surrounding an iron atom (cnFeC) is the most significant descriptor contributing to surface energy. Thermodynamic phase diagrams further confirm that structures with high carbon coverage are more stable. Dynamic analysis of the subsurface structure using the FeC-LAS method reveals that the proportion of the Fe5C2 phase continuously decreases, while the content of the metastable intermediate phase Fe2C initially increases and subsequently enters a regime of wide-range dynamic oscillation. Moreover, the steady-state proportions of these two phases show no clear linear correlation with the carbon chemical potential, and a more detailed statistical analysis of the Fe2C proportion further confirms this lack of monotonic correlation with temperature and pressure, underscoring that kinetics dominate the subsurface evolution. Collectively, these observations indicate that kinetics dominate the carburization process on the Fe5C2(111) surface. The GRL framework established in this study provides a systematic approach for resolving catalyst surface phase evolution under realistic reaction conditions at the atomic scale, deepening the understanding of the dynamic structural evolution and deactivation micromechanisms of iron-based Fischer–Tropsch catalysts.