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 H 2 : CO reaction atmosphere providing the active phases for hydrocarbon synthesis. H 2 reduced Fe(−Na−S)/α‐Al 2 O 3 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‐Fe 3 C, η‐Fe 2 C, χ‐Fe 5 C 2 and θ‐Fe 3 C. θ‐Fe 7 C 3 and a distorted χ‐Fe 5 C 2 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‐Fe 5 C 2 over θ‐Fe 3 C 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.