The adsorption and dissociation of CO on Fe(111)

We present DFT calculations relating to fundamental aspects of the adsorption, molecular diffusion and dissociation pathways available for CO on the Fe(111) surface. On the clean surface CO dissociates most easily from di-bridge (DB) sites with both carbon and oxygen atoms interacting with the surface via a tilted configuration. This adsorption site is 0.52 eV higher in energy than the lowest energy at the shallow hollow site and so CO bond cleavage takes place following molecular migration. The lowest calculated barriers are also found when the molecule re-orientates during the dissociation process to maintain a surface co-ordination for the O atom of at least two. arbon is pre-adsorbed on the surface we find a small stabilisation of the molecularly adsorbed state when the CO⋯C separation is ~ 3 Å, but repulsive interactions reduce the binding energy at shorter distances. The molecularly adsorbed states are affected by the presence of surface carbon with some structures that are transition states for molecular diffusion becoming minima with co-adsorbed carbon. This also leads to lower energy pathways for CO bond cleavage so that our results indicate that surface carbide formation is auto-catalytic at low C coverage.