Mechanistic Aspects of CO Activation and C–C Bond Formation on the Fe/C- and Fe-Terminated Fe3C(010) Surfaces

To understand the reaction mechanisms of iron-based Fischer–Tropsch synthesis in converting synthesis gas into chemicals and hydrocarbons, we computed CO activation and C–C bond formation on the Fe3C(010) phase as an active catalyst systematically and comparatively on the basis of spin-polarized density functional theory. On the Fe/C-terminated Fe3C(010) surface, C–O dissociation of CO and CHxO (x = 1, 2, and 3) with higher barriers is not accessible, while consecutive hydrogenation of adsorbed CO to methanol and surface carbon atom to methane is accessible with low overall barriers and exothermic reaction energies. However, the formed CH3 from surface carbon hydrogenation can couple with adsorbed CO to form surface acetyl (CH3CO), resulting in the formation of the initial C–C bond with comparably low overall barrier and exothermic reaction energy. The formed acetyl can be deoxygenated to surface CH3C, which can couple with surface carbon to form CH3CC. The formation of CH3CO and CH3CC reveals that chain growth on the Fe/C-terminated Fe3C(010)-0.25 surface progresses alternatingly via both carbide and CO insertion mechanisms. The formed surface CH3CC can be hydrogenated into CH3CH2CH2 for next C–C coupling with CO, and this closes the initial chain growth cycle. The larger difference in the adsorption energy of CO and H2 or the H2/CO ratio will determine hydrogenation and C–C coupling. On the Fe-terminated Fe3C(010)-0.00 surface, however, CO prefers direct and H-assisted dissociation to C and CH at the B5 site; these two paths have close high overall barriers, and the formed carbon prefers to be hydrogenated into CH2, which can couple with CH to form CH2CH with low overall barrier and exothermic reaction energy, and only the carbide mechanism is operating for chain propagation. Comparison with the Fe/C- and Fe-terminated Fe5C2(100) surfaces as well as metallic Fe surfaces has been made.