Resolving CO2 activation and hydrogenation pathways over iron carbides from DFT investigation

In this work, periodic density functional theory (DFT) calculations were performed to investigate the CO2 activation mechanism over thermodynamically stable χ-Fe5C2 (510) and θ-Fe3C (031) facets. Four major pathways of CO2 activation were examined, including the direct dissociation of CO2 and the H-assisted intermediates of *COOH, *HCOO, and *CO + *OH. Both χ-Fe5C2 and θ-Fe3C have proven to be active for CO2 direct dissociation (Ea =0.17 eV). As for H assisted CO2 activation, the one-step formation of *CO + *OH is feasible on χ-Fe5C2 (Ea =0.24 eV). Furthermore, θ-Fe3C favors the *HCOO pathway (Ea =0.20 eV) and *CO + *OH formation (Ea = 0.11 eV), while neither phase favors the formation of *COOH. Both CO2 direct activation and H-assisted CO2 activation pathways are of vital importance under high ratio of H/C in CO2 hydrogenation reaction. This work sheds light on CO2 activation mechanism over iron carbides, improving the rational design of CO2 hydrogenation catalysts.