Revealing the Mechanism and Kinetics of $Fe_5C_2$ Formation From Ferrous Oxalate under $CO_2$ Fischer‐Tropsch Conditions Using Time‐Resolved In Situ X‐Ray Absorption Spectroscopy

The fundamentals of in situ formation of iron carbides are required for the tailored design of Fe-based catalysts for the efficient conversion of CO2 to higher hydrocarbons. Herein, time-resolved in situ X-ray absorption spectroscopy has been used to elucidate the mechanism of the formation of Fe5C2 from ferrous oxalate (FeC2O4) at 350 °C using a H2/CO=3 reaction feed. Regardless of the kind of alkali metal promoter and reaction pressure (1 or 7.5 bar), FeC2O4 is first decomposed to FeO followed by the conversion of the latter to Fe5C2. 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 Fe5C2 at both pressures. Alkali metal promoters were found to accelerate the rate of Fe5C2 formation. This rate decreases with increasing total pressure due to the stabilization of FeO.