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Ab initio atomistic thermodynamics is utilized to achieve the understanding of the surface structure and stability of χ-Fe₅C₂ under CO and synthesis gas (H₂/CO) pretreatment conditions in catalyst activation for Fischer–Tropsch synthesis (FTS). On the basis of the computed surface free energy (γ) as a function of the carbon chemical potential (μ_C) by considering temperature, pressure, and H₂/CO ratios, it is found that CO pretreatment favors stable carbon-rich facets, while small amount of H₂ added into CO leads to a large decrease in μ_C and thus stabilizing carbon-poor facets. The high activity of surface carbon toward hydrogenation might explain the enhanced initial activity of the FTS catalysts activated from CO pretreatment. Moreover, under both CO and H₂/CO pretreatments, either low temperature or low pressure can lead to stable carbon-rich facets.