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To hunt the correlation between the surfaces of Fe5C2and their corresponding activities, the CO adsorption and dissociation on a series of both low ((010), (001), (110), (111), (111̅)) and high Miller index surfaces of Fe5C2((221), (4̅11), and (510)) surfaces are systemically investigated. For the CO adsorption, configurations with bonding to surface Fe sites are much stronger than that on C sites. For the CO dissociation, direct C–O cleavage can take place on the (221), (510), (010), and (111̅) surfaces due to the low activation energy. More importantly, to correlate the surface character and the activity of CO dissociation we proposed a concept of sum bond valence. It is found that the adsorbed CO with more bond valence can dissociate easier, and a linear relationship between the activation energies and the CO bond valence can be established. It can be inferred that the activity of Fe5C2surfaces for CO dissociation strongly relies on the binding characteristics. The relatively stable (100) and (111) surfaces are not active for direct CO dissociation. In this work, the CO bond valence is suggested to be an important descriptor to correlate complicated surfaces and their activities. Furthermore, such a finding can guide the rational design of catalysts with desired activities.

As active phases in low-temperature Fischer–Tropsch synthesis for liquid fuel production, epsilon iron carbides are critically important industrial materials. However, the precise atomic structure of epsilon iron carbides remains unclear, leading to a half-century of debate on the phase assignment of the ε-Fe2C and ε′-Fe2.2C. Here, we resolve this decades-long question by a combined theoretical and experimental investigation to assign the phases unambiguously. First, we have investigated the equilibrium structures and thermal stabilities of ε-FexC (x= 1, 2, 2.2, 3, 4, 6, 8) by first-principles calculations. We have also acquired X-ray diffraction patterns and Mössbauer spectra for these epsilon iron carbides and compared them with the simulated results. These analyses indicate that the unit cell of ε-Fe2C contains only one type of chemical environment for Fe atoms, while ε′-Fe2.2C has six sets of chemically distinct Fe atoms.