CO Activation Pathways of Fischer–Tropsch Synthesis on χ-Fe₅C₂ (510): Direct versus Hydrogen-Assisted CO Dissociation

Abstract

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.

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

“…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.…”

“…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. On stoichiometric Fe 5 C 2 (010) 0.25 , Huo et al [55] argued in favour of direct dissociation at a primary vacancy introduced on the surface, although comparison with a H-assisted path at this site was not reported.…”

“…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.…”

Recent Progresses in Understanding of Co-Based Fischer–Tropsch Catalysis by Means of Transient Kinetic Studies and Theoretical Analysis

“…More importantly, it can distinguish between those possibilities that were left open, such as various reaction pathways. Since the mechanism of FTS is inconclusive and controversial, more and more DFT calculations have been devoted to tackle these issues [18,19], especially for the mechanism of CO activation [27][28][29][30][31], methane formation [32][33][34][35], and chain growth [36][37][38]. Despite the successful implementations of DFT in catalysis process, it still holds some drawbacks such as failure in the calculation of weak interactions (such as Van der Waals interaction) [25].…”