A quantum-chemical study of the CO dissociation mechanism on low-index Miller planes of ⊖-Fe3C

Spin-polarized density functional theory was employed to determine the preferred CO bond dissociation mechanism on low-index Miller surfaces of circle minus-Fe3C in the context of Fischer-Tropsch synthesis. Compared to the most reactive (111) surface of bcc-Fe on which CO binds in a 7-fold coordination, CO prefers to locate in 3-fold or 4-fold sites on the carburized surfaces due to the presence of interstitial C atoms at or below the surface. An important finding is that the lowest activation energies for direct CO bond dissociation are associated with the presence of step-like sites, similar to the case of metallic surfaces. We could identify such sites for 3 out of the 9 investigated surfaces, namely the (111), (1 (1) over bar1), and (010) terminations of circle minus-Fe3C. On the other hand, H-assisted CO dissociation is preferred on the (0 (1) over bar1), (001), and (100) surfaces. The other (011), (110), and (101) surfaces are inert with CO dissociation barriers close to or exceeding the CO adsorption energy. A kinetic analysis shows that the (111) surface (direct CO dissociation) and the (0 (1) over bar1) surface (H-assisted CO dissociation via HCO) display comparable CO bond dissociation rates, much higher than the rates computed for the other surfaces. Together these two surfaces make up ca. 28% of the surface enclosing a Wulff nanoparticle of circle minus-Fe3C. Using an atomic population analysis, we show that the activation barrier for C-O bond dissociation correlates well with the bond order of adsorbed CO. This implies that pre-activation of CO is important for lowering the overall activation barrier. The present work demonstrates that the high-temperature circle minus-Fe3C phase is highly active towards CO bond dissociation, which is the essential first step in the Fischer-Tropsch reaction. Several of the exposed surfaces present lower overall CO dissociation barriers than a-Fe (known to be unstable under Fischer-Tropsch conditions) and the chi-carbide of Fe (usually assumed to be the most stable phase of Fe-carbide under Fischer-Tropsch conditions). Notably, the activity of the (111) surface is higher than that of a stepped cobalt surface.