Elementary reactions of CO and H2 on C-terminated χ-Fe5C2(0 0 1) surfaces

CO Dissociation at Vacancy Sites on Hägg Iron Carbide: Direct Versus Hydrogen-Assisted Routes Investigated with DFT

“…was calculated in the presence of vacancies relative to the defect-free surface, although once again adsorption in the vacancy site (a fourfold site) was degenerate with atop adsorption. Interestingly, with an increase in defect concentration on Fe 5 C 2 (100) -0.098 , further strengthening of CO adsorption was not obtained [54]. These results demonstrate that the degree to which vacancy sites can be considered to preferentially adsorb CO is a function of both vacancy concentration and surface termination.…”

“…3b) which is only 0.07 eV higher in energy also exists at this site. On Fe 5 C 2 (010) 0.25 [55] and Fe 5 C 2 (100) -0.098 [54], carbon vacancy sites have been reported to lower the CO dissociation barrier. However on Fe-terminated Fe 5 C 2 (010) 0.25 , dissociation at a site not associated with a carbon vacancy yields a TS (TS3, Fig.…”

“…Thus, as similarly concluded on Fe [41,42], Co [28,30] and Ru [27] surfaces, H-assisted CO dissociation through formation of an HCO intermediate is kinetically favoured over the COH mechanism. Comparison of direct and H-assisted CO dissociation pathways based on DFT calculations has been made on pure iron surfaces [20,[41][42][43][44][45] and to a lesser extent on surfaces of Hägg iron carbide [53][54][55][56]. On Fe(100) [41] and Fe(110) [20] it was argued that both direct and H-assisted CO dissociation paths can contribute under FT reaction conditions, although on Fe(110) sequential hydrogenation of CO to form a hydroxymethylene (HCOH) species was postulated at higher CO surface coverages [20].…”

“…In contrast, on Fe(310) only direct CO dissociation was concluded to be relevant on thermodynamic grounds [45]. On Hägg iron carbide surfaces, comparison of the energy profiles for direct and H-assisted CO dissociation has been reported on Fe 5 C 2 (510) [53], stoichiometric Fe 5 C 2 (010) 0.25 [55], Fe 5 C 2 (001) [56] and terminations of the Fe 5 C 2 (100) surface [54]. On Fe 5 C 2 (510), direct dissociation was concluded to be the preferred mechanism for CO activation [53], at sites that bear a structural similarity to the primary vacancy site on Fe-terminated Fe 5 C 2 (010) 0.25 investigated in the current work, at which direct dissociation is also predicted to be more favourable energetically.…”

“…In this work we therefore use DFT to investigate the role of direct and H-assisted paths for CO activation on Hägg iron carbide, extending our previous investigation of CO adsorption on v-Fe 5 C 2 [50]. We draw comparisons with previous studies examining direct [51,52] and H-assisted CO dissociation on v-Fe 5 C 2 surfaces [53][54][55][56] and place particular emphasis on the importance of considering a range of surface environments in arriving at a general picture of CO activation in Fe-based FT synthesis.…”