The TM surfaces were represented by six-layered supercell slab models, previously optimized so as to deliver slab-width converged results . During the DFT optimizations, the three bottommost layers were kept frozen to represent the materials bulk, while the topmost three layer were allowed to fully relax, along with the adsorbed CO molecule, or the adsorbed C or O adatom moieties, i.e. 3+3 approach. The slab models had a vacuum of 10 Å perpendicular to the modelled surface to prevent any spurious interactions between the periodically repeated layers. The supercells size was adjusted to maintain similar CO coverage between different surfaces. Thus, (3×3) supercells were used for fcc (111) surfaces, bcc (001) and (111) surfaces, and hcp (101 ̅ 0) surface, having nine atoms per layer, see Fig. .