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.