Spontaneous activation of CO ₂ and possible corrosion pathways on the low-index iron surface Fe(100)

Spin-polarized density functional theory (DFT) calculations and periodic slab models were used to study the reactive pathways leading to corrosion products on the low-index (100) surface of iron. We determined the binding energies of CO ₂ and the barrier to decomposition of adsorbed CO ₂ to O + CO, as well as to the formation of adsorbed CO ₃ ^2- and H ₂CO ₃ on the Fe(100) surface. The barriers of these pathways were determined with nudged elastic band (NEB) calculations. Short trajectories with DFT-based dynamics were employed to identify the most important species. These simulations (up to 0.5 ML coverage) show that CO ₂ is spontaneously activated and can bind with two or all three atoms assuming bent configurations strongly reminiscent of the radical CO ₂-. This spontaneous activation of CO ₂ is possible through charge rearrangement of the slab density. The CO ₂ decomposition to O + CO has a barrier of 5.0 kcal/mol. The subsequent formation of CO ₃ ^2- by reaction with an incoming CO ₂ is strongly favored thermodynamically. Interaction of H ₂O with the adsorbed CO ₂ forms a loosely bound complex that leads to the formation of surface-bound carbonic acid, with a barrier of approximately 35.0 kcal/mol.