Ab-initio calculations of the direct and hydrogen-assisted dissociation of CO on Fe(3 1 0)

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

“…Nevertheless, the overall energy barrier calculated for direct CO dissociation on Fe(111) exceeds 1.7 eV [42,43]. 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].…”

“…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].…”

“…Coadsorbed CO was found to significantly alter the stability of intermediates adsorbed on the surface. Finally, on the stepped Fe(310) surface direct CO dissociation was concluded to be the only feasible route given that the endothermic formation of both COH and HCO exceeded the energy barrier for direct dissociation at the step sites [45]. From these DFT studies it may be concluded that the extent to which CO dissociation is assisted by addition of hydrogen prior to C-O bond scission is sensitive to both the surface structure and reaction conditions, which control the availability of free sites and the coverage of coadsorbates on the catalyst surface.…”

Methane, formaldehyde and methanol formation pathways from carbon monoxide and hydrogen on the (0 0 1) surface of the iron carbide χ-Fe5C2

“…Among the mixture of carbide and oxide phases [5][6][7][8] present under process conditions, the Hägg carbide (v-Fe 5 C 2 ) is generally regarded as the active phase for FTS [2,5,6,[9][10][11][12][13][14][15][16][17][18], although other phases such as e-Fe 2.2 C or h-Fe 3 C may exhibit activity as well [9]. Mechanistic studies on CO hydrogenation and FTS on metallic iron surfaces are available in the literature [4,14,[19][20][21][22][23][24][25][26][27][28][29], but studies on the catalytic properties of iron carbides in FTS are scarce, we refer to de Smit and Weckhuysen [13] for a recent review and to other literature [3,5,[8][9][10][11]13,15,24,[30][31][32][33][34][35]…”

“…For CO dissociation, the activation barrier depends strongly on the catalyst surface [19][20][21][22]37,38] and particularly on its structure [6,8,[21][22][23][24]39]. Hydrogen addition to surface carbon forms CH 2 , which is the monomer for chain propagation in the carbene mechanism proposed by Fischer and Tropsch [25,26,40].…”