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

• CO 2 activation is more convenient on χ-Fe 5 C 2 and θ-Fe 3 C than Fe 3 O 4. • *CO is the dominating intermediate species on χ-Fe 5 C 2. • CO 2 activation undergoes both direct dissociation and *HCOO pathways on θ-Fe 3 C. In this work, periodic density functional theory (DFT) calculations were performed to investigate the CO 2 activation mechanism over thermodynamically stable χ-Fe 5 C 2 (510) and θ-Fe 3 C (031) facets. Four major pathways of CO 2 activation were examined, including the direct dissociation of CO 2 and the H-assisted intermediates of *COOH, *HCOO, and *CO + *OH. Both χ-Fe 5 C 2 and θ-Fe 3 C have proven to be active for CO 2 direct dissociation (E a =0.17 eV). As for H assisted CO 2 activation, the one-step formation of *CO + *OH is feasible on χ-Fe 5 C 2 (E a =0.24 eV). Furthermore, θ-Fe 3 C favors the *HCOO pathway (E a =0.20 eV) and *CO + *OH formation (E a = 0.11 eV), while neither phase favors the formation of *COOH. Both CO 2 direct activation and H-assisted CO 2 activation pathways are of vital importance under high ratio of H/C in CO 2 hydrogenation reaction. This work sheds light on CO 2 activation mechanism over iron carbides, improving the rational design of CO 2 hydrogenation catalysts. [ABSTRACT FROM AUTHOR]