Insights into the Hydrogen Coverage Effect and the Mechanism of Fischer–Tropsch to Olefins Process on Fe₅C₂ (510)

Abstract

Direct production of light olefins from synthesis gas through the Fischer–Tropsch to Olefins (FTO) process over iron carbide catalysts is promising. Several mechanisms have been proposed to explain the formation of different hydrocarbons on clean surfaces. However, when these mechanisms were discussed before, the influence of adsorbed surface species, especially surface H, on the mechanisms is less understood. Hence, we reported a density functional theory (DFT) study on the hydrogen coverage effect and the mechanism of the FTO process on Fe5C2 (510). It is found that the Fe5C2 (510) surface may be covered with a layer of H in the FTO process. The mechanism of the Fischer–Tropsch synthesis on the Fe5C2 (510) surface prefers to proceed via the CO insertion mechanism rather than the carbide mechanism in the presence of adsorbed H. The optimized C–C coupling pathways feature H-assisted CO coupling pathways in which the CHCHO pathways (Ea = 1.15 eV) and the CH2CHO pathways (Ea = 1.10 eV) are favorable. The methane formation pathway is an alternative major pathway on the χ-Fe5C2 (510) surface due to its relatively low barrier energies (Ea = 0.80 eV). The formation of oxygenates is unfavorable due to the high C–O dissociation reactivity. The olefin ratio for ethylene and ethane is influenced by two competitive processes in a stepwise hydrogenation. It is suggested from the mechanism that high C–C coupling activity and the matching hydrogenation activity with the former are the key directions to modify iron-based FTO catalysts.