Surface-dependent CO2 and CO activation on iron and iron carbides: Insights from density functional theory

Conventionally, iron catalysts produce a variety of in-situ carbides, which are sometimes identified as the active phase in CO₂ and CO hydrogenation, Reverse water gas shift (RWGS) and Fischer–Tropsch synthesis (FTS) reactions. However, the key questions arise towards the role of surface Fe and C atoms of iron carbides towards CO₂ and CO activation. Spin-polarised Density Functional Theory (DFT) calculations performed on Fe metal and Fe-C-terminated surfaces of η-Fe₂C, θ-Fe₃C, and χ-Fe₅C₂ iron carbides reveal that, (i) surface carbon brings more stability to Fe-C-terminated surfaces compared to Fe-terminated iron carbides and metallic iron, (ii) Fe-terminated iron carbides and metallic iron are more conducive towards the exothermic CO₂, CO adsorption than the Fe-C-terminated iron carbides. Moreover, electronic structure analysis unveils that Fe-C-terminated surfaces are comparatively less reactive due to the occurrence of the d-band center very far from the Fermi Level. Furthermore, moderately stable Fe(110) is found as the most preferred surface, favouring direct dissociation of both CO₂ and CO kinetically and thermodynamically compared to all carbides considered. Overall, this study systematically analysed the role of surface energy, termination, surface Fe/C ratio, and surface C atom in iron carbides on the activation of CO₂ and CO.