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

The CO activation mechanism on the chi-Fe5C2 Fischer-Tropsch synthesis catalysts is studied by spin-polarized density functional theory calculations. A series of low ((100), (010), (001), (110), (111), and (11 (1) over bar)) and high ((221), ((4) over bar 11), and (510)) Miller index surfaces are intensively investigated. Both the direct and H-assisted pathways through HCO, COH, HCOH, and CH2O 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 Fe5C2(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 Fe5C2(010), (110), ((4) over bar 11), (11 (1) over bar), 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 Fe5C2 facets might guide the rational design of catalysts with desired activities.