Revealing the Mechanism and Kinetics of Fe₅C₂ Formation From Ferrous Oxalate under CO₂ Fischer‐Tropsch Conditions Using Time‐Resolved In Situ X‐Ray Absorption Spectroscopy

Abstract The fundamentals of in situ formation of iron carbides are required for the tailored design of Fe‐based catalysts for the efficient conversion of CO 2 to higher hydrocarbons. Herein, time‐resolved in situ X‐ray absorption spectroscopy has been used to elucidate the mechanism of the formation of Fe 5 C 2 from ferrous oxalate (FeC 2 O 4 ) at 350 °C using a H 2 /CO=3 reaction feed. Regardless of the kind of alkali metal promoter and reaction pressure (1 or 7.5 bar), FeC 2 O 4 is first decomposed to FeO followed by the conversion of the latter to Fe 5 C 2 . Further insights into the above transformations were derived by kinetic analysis using a Johnson–Mehl–Avrami–Erofeev–Kolmogorov model and kinetics‐constrained neural ordinary differential equations method. Both approaches revealed that the formation of FeO at 1 bar follows a nucleation mechanism, while a diffusion mechanism has a higher contribution at 7.5 bar. The latter mechanism is valid for the conversion of FeO to Fe 5 C 2 at both pressures. Alkali metal promoters were found to accelerate the rate of Fe 5 C 2 formation. This rate decreases with increasing total pressure due to the stabilization of FeO.