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The chemical–physical environment around iron based FTS catalysts under working conditions is used to estimate the influences of carbon containing gases on the surface structures and stability of ε-Fe₂C, χ-Fe₅C₂, θ-Fe₃C and Fe₄C from combined density functional theory and atomistic–thermodynamic studies. Higher carbon content gas has higher carburization ability; while higher temperature and lower pressure as well as higher H₂/CO ratio can suppress carburization ability. Under wide ranging gas environment, ε-Fe₂C, χ-Fe₅C₂ and θ-Fe₃C have different morphologies, and the most stable non-stoichiometric termination changes from carbon-poor to carbon-rich (varying surface Fe/C ratio) upon the increase in Δμ_C. The most stable surfaces of these carbides have similar surface bonding pattern, and their surface properties are related to some common phenomena of iron based catalysts. For these facets, χ …