Iron carbides, especially χ-Fe5C2, among the active iron species in Fischer–Tropsch synthesis (FTS), are considered to be responsible for high FTS activity. CO activation pathways as the initial steps of FTS over χ-Fe5C2 were explored by spin-polarized density functional theory calculations. Surface energies of χ-Fe5C2 facets observed from the XRD patterns were first calculated, and then the corresponding equilibrium χ-Fe5C2 shape was obtained by Wulff construction. The thermodynamically stable (510) surface was predicted to have the largest percentage among the exposed crystal facets. Subsequently, the adsorption properties of CO on χ-Fe5C2 (510) were studied. Despite exhibiting lower binding energy than that at the 3F-4 site as the most stable configuration, CO adsorption at the 4F-1 site led to significant weakening of the C–O bond from both the structural and electronic properties’ points of view. Furthermore, two kinds of CO activation mechanisms (i.e., the direct and H-assisted CO dissociation) and the corresponding six kinds of CO activation pathways on χ-Fe5C2 (510) were comparatively investigated on the basis of the evolution of carbon species, in which the C–O bond cleavage and further hydrogenation of surface species were concerned. The systematic analysis of the activation properties of CO suggests the direct CO dissociation as the preferred activation pathway.

Probing the product selectivity of Fischer–Tropsch catalysts is of prime scientific and industrial importancewith the aim to upgrade products and meet various end-use applications. In this work, the mechanisms for CH4 formation and C1–C1 coupling on a thermodynamically stable, terraced-like χ-Fe5C2 (510) surface were studied by DFT calculations. It was found that this surface exhibits high effective barriers of CH4 formation for the three cases (i.e., 3.66, 2.81, and 2.39 eV), indicating the unfavorable occurrence of CH4 formation under FTS conditions. The C + CH and CH + CH are the most likely coupling pathways, which follow the carbide mechanism. Subsequently, the effective barrier difference between CH4 formation and C1–C1 coupling was used as a descriptor to quantify FTS selectivity. A comparison of the selectivity between this surface and the reported FTS catalysts’ surfaces was discussed in detail. More interestingly, this surface shows unexpectedly high C2+ selectivity. This indicates that manipulating the crystal facet of χ-Fe5C2 catalyst can effectively tune the FTS selectivity, which will open a new avenue for highly selective Fe-based FTS catalysts.

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