CO Direct versus H-Assisted Dissociation on Hydrogen Coadsorbed χ-Fe₅C₂ Fischer–Tropsch Catalysts

The CO activation mechanism on the χ-Fe 5 C 2 Fischer–Tropsch synthesis catalysts is studied by spin-polarized density functional theory calculations. A series of low ((100), (010), (001), (110), (111), and (111̅)) and high ((221), (4̅11), and (510)) Miller index surfaces are intensively investigated. Both the direct and H-assisted pathways through HCO, COH, HCOH, and CH 2 O intermediates are systematically examined on the hydrogen coadsorbed surfaces. Different activity and activation mechanisms are determined on the various surfaces as well as the varying sites. It is found that the high-index Fe 5 C 2 (510) and the relatively unstable (010) and (221) surfaces are active for direct CO dissociation due to the existence of the highly activated sites. H-assisted dissociation pathways are preferred on the Fe 5 C 2 (010), (110), (4̅11), (111̅), and (111) surfaces with the moderate stability. On the relatively stable (100) surface, CO dissociation can hardly take place. Such discrimination for the activity and the CO activation mechanisms on various Fe 5 C 2 facets might guide the rational design of catalysts with desired activities.