Effect of surface carbon of iron carbide on Fischer-Tropsch synthesis: A density functional theory study

The surface iron of iron carbide has been identified as active in Fischer-Tropsch synthesis (FTS), but the effect of surface carbon remains a topic of debate. Herein, to clarify the effect of surface carbon of iron carbide on FTS, we conducted a density functional theory (DFT) study on CO adsorption, H-2 activation and CO hydrogenation over epsilon-Fe2C and theta-Fe3C. Compared to Fe3C(111), the Fe2C(111) facet exhibits more low-coordinated carbon, benefiting for CO adsorption and H-2 activation. Because of the low-coordinated carbon, the Fe2C(111) is more active for FTS than Fe3C(111), not only through the formation of CH2 species with H atoms, but also through the direct C-C coupling for the carbon chain growth. The consumed carbon on Fe2C(111) can be recovered timely by absorption and direct dissociation of CO, ensuring the sustainability of the carbon chain growth. This work contributes to a comprehensive understanding of the FTS mechanism over iron carbide surface.