Submonolayers of carbon onα-Fefacets: Anab initiostudy

Abstract

Motivated by recent in situ studies of carbon nanotube growth from large transition-metal nanoparticles, we study various α−iron (ferrite) facets at different carbon concentrations using ab initio methods. The studied (110), (100) and (111) facets show qualitatively different behaviour when carbon concentration changes. In particular, adsorbed carbon atoms repel each other on the (110) facet, resulting in carbon dimer and graphitic material formation. Carbon on the (100) facet forms stable structures at concentrations of about 0.5 monolayer and at 1.0 monolayer this facet becomes unstable due to a frustration of the top layer iron atoms. The stability of the (111) facet is weakly affected by the amount of adsorbed carbon and its stability increases further with respect to the (100) facet with increasing carbon concentration. The exchange of carbon atoms between the surface and sub-surface regions on the (111) facet is easier than on the other facets and the formation of carbon dimers is exothermic. These findings are in accordance with a recent in situ experimental study where the existence of graphene decorated (111) facets is related to increased carbon concentration.

Structures and energies of Cu clusters on Fe and Fe3C surfaces from density functional theory computation

“…During FTS and metallurgy processes, carburization of α-Fe to Fe 3 C is prevailing, and the carburization process differs from surface to surface. For example, Fe(110) and Fe(100) have higher carbon diffusion barriers (1.47 and 1.18 (1.35) eV) 66,67 than Fe(111) (0.77 eV), 68 which is also lower than the bulk diffusion barrier from computation (0.86 eV) 69 or experiment (0.87 eV). 70 This result also indicates the easy carburization of Fe(111) suface.…”

α-iron facet with enhanced carbon mobility

“…The analysis of adsorption sites on various facets is simplified once it is realized that carbon prefers to adsorb into similar octahedral-like sites on the surface as in the bulk. 27 In Figs. 1-3 several of these octahedral sites on the surface and deeper in the bulk have been illustrated.…”

“…1. 27,35 From this site the carbon atom is migrated to the nearest sub-surface adsorption site marked as (B) in Fig. 1 (diffusion path A → B is identical to that considered in Ref.…”

“…The computational details are identical to Ref. 27, namely the projector-augmented waves were used with kinetic cutoff energy of 420 eV. We utilized the Perdew-Burke-Ernzerhof (PBE) generalized gradient (GGA) exchange correlation (XC) functional 28 and surface Brillouin zone was sampled with a 7 × 7 Monkhorst-Pack sampling.…”

Identifying the active sites for CO dissociation on Fe–BCC nanoclusters

“…Differences in nanoparticle and surface reactivity have been attributed to several phenomena, e.g., presence special and lowcoordinated sites, strain effects, differences in atomic mobility as well as electronic confinement effects [44][45][46]. Effect of special sites and strain effects are often explained using the d-band model [1,2].…”