Role of Step Sites and Surface Vacancies in the Adsorption and Activation of CO on χ-Fe₅C₂ Surfaces

Abstract

Density functional theory calculations have been used to investigate CO adsorption on three surface terminations of -Fe 5 C 2 in the presence of carbon vacancy sites. CO did not show a strong energetic preference for a particular adsorption site on each surface, since similar adsorption energies were obtained for structurally distinct adsorption configurations. In addition, it was found that the adsorption of CO in a vacancy site is not necessarily more favorable energetically, compared with adsorption in alternative Fe sites. The presence of a subsurface carbon atom directly below a 4-fold site was found to inhibit or significantly destabilize adsorption of CO in that site. The role of step sites in activating CO has been investigated by comparing the calculated adsorption energies, structural properties, and vibrational stretching frequencies of CO adsorbed in equivalent sites in the presence or absence of steps. Coordination of CO to the surface through both ends of the molecule was associated with a lengthening of the C-O bond and a red-shift of the C-O stretching frequency, and such geometries were readily obtained for adsorption at the bottom of a step. Activation energies were calculated for the dissociation of CO initially adsorbed in a vacancy site in the presence and absence of steps. The step sites were found to lower the activation energy by at least 0.3 to 0.6 eV, without destabilizing the initial state.

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

“…Adsorption and reaction was considered on only one side of the slab, and a dipole correction was applied in the direction normal to the slab. Test calculations indicate that adsorption energies are converged to within 0.03 eV using these parameters [50].…”

“…Direct and H-assisted CO dissociation was investigated on the Fe 5 C 2 (010) 0.25 surface of Hägg iron carbide following the nomenclature adopted previously [50,68], where the subscript indicates the fractional displacement of the Miller plane in the surface normal direction from the origin of the conventional monoclinic bulk unit cell. The stoichiometric termination of Fe 5 C 2 (010) 0.25 exposes two surface C atoms per p(1 9 1) unit cell that lie above the surface plane of Fe atoms, and two C atoms that lie below, as shown in Fig.…”

“…1. Adsorption of CO on the Fe-terminated Fe 5 C 2 (010) 0.25 surface was studied in detail previously using DFT calculations [50]. No energetic preference for adsorption in vacancy sites was found in that adsorption in primary or secondary vacancy sites was calculated to be degenerate with adsorption in certain atop and non-vacancy sites on the surface, to within the accuracy of the DFT method employed [50].…”

“…Adsorption of CO on the Fe-terminated Fe 5 C 2 (010) 0.25 surface was studied in detail previously using DFT calculations [50]. No energetic preference for adsorption in vacancy sites was found in that adsorption in primary or secondary vacancy sites was calculated to be degenerate with adsorption in certain atop and non-vacancy sites on the surface, to within the accuracy of the DFT method employed [50]. This is in contrast to results for the stoichiometric Fe 5 C 2 (010) 0.25 termination where CO adsorption was reported to have a larger adsorption energy at a primary vacancy site introduced on the surface [55].…”

“…In some instances, FT activity has been related to the presence of Hägg iron carbide (v-Fe 5 C 2 ) in the catalyst [46][47][48][49] suggesting that this phase may play an important role in FT activity. 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.…”