Identification of Iron Carbides in Fe(−Na−S)/α-Al₂O₃ Fischer-Tropsch Synthesis Catalysts with X-ray Powder Diffractometry and Mössbauer Absorption Spectroscopy

In Fe-based Fischer-Tropsch Synthesis (FTS), the Fe carbides form under the carburizing H₂ : CO reaction atmosphere providing the active phases for hydrocarbon synthesis. H₂ reduced Fe(−Na−S)/α-Al₂O₃ catalyst materials, with and without Na−S promotion, were carburized under CO at 240–440 °C to form Fe carbides. X-ray Powder Diffractometry (XRPD) with Rietveld Quantitative Phase Analysis (R-QPA) and Mössbauer Absorption Spectroscopy (MAS) were used to identity and quantify the formed Fe carbide phases. The Fe carbides formed in order of increasing temperature are ϵ-Fe₃C, η-Fe₂C, χ-Fe₅C₂ and θ-Fe₃C. θ-Fe₇C₃ and a distorted χ-Fe₅C₂ phase are formed at 25 bar CO (340 °C) from a Fe oxide precursor. Fe carbide formation was unaffected by Na−S addition, but it did increase Fe oxidation (≤290 °C) and preferred formation of χ-Fe₅C₂ over θ-Fe₃C phase (≥390 °C). The results unify the often ambiguous Fe carbide identification and nomenclature and specify the role of Na−S in the carburization process.