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 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.