A theoretical study of the role of K on the reverse water-gas shift reaction on Hägg carbide

Potassium (K) is known to enhance the catalytic performance of Fe-based catalysts in the reverse water-gas shift (rWGS) reaction, which is highly relevant during Fischer-Tropsch (FT) synthesis of CO₂-H₂ mixtures. To elucidate the mechanistic role of K promoter, we employed density functional theory (DFT) calculations in conjunction with microkinetic modelling for two representative surface terminations of Hägg carbide (χ-Fe₅C₂), i.e., (010) and (510). K₂O results in stronger adsorption of CO₂ and H₂ on Hägg carbide and promotes C–O bond dissociation of adsorbed CO₂ by increasing the electron density on Fe atoms close to the promoter oxide. The increased electron density of the surface Fe atoms results in an increased electron-electron repulsion with bonding orbitals of adsorbed CO₂. Microkinetics simulations predict that K₂O increases the CO₂ conversion during CO₂-FT synthesis. K₂O also enhances CO adsorption and dissociation, facilitating the formation of methane, used here as a proxy for hydrocarbons formation during CO₂-FT synthesis. CO dissociation and O removal via H₂O compete as the rate-controlling steps in CO₂-FT.