CO₂ activation and dissociation on the low miller index surfaces of pure and Ni-coated iron metal: A DFT study

We have used spin polarized density functional theory calculations to perform extensive mechanistic studies of CO₂ dissociation into CO and O on the clean Fe(100), (110) and (111) surfaces and on the same surfaces coated by a monolayer of nickel. CO₂ chemisorbs on all three bare facets and binds more strongly to the stepped (111) surface than on the open flat (100) and close-packed (110) surfaces, with adsorption energies of -88.7 kJ mol^-1, -70.8 kJ mol^-1 and -116.8 kJ mol^-1 on the (100), (110) and (111) facets, respectively. Compared to the bare Fe surfaces, we found weaker binding of the CO₂ molecules on the Ni-deposited surfaces, where the adsorption energies are calculated at +47.2 kJ mol^-1, -29.5 kJ mol^-1 and -65.0 kJ mol^-1 on the Ni-deposited (100), (110) and (111) facets respectively. We have also investigated the thermodynamics and activation energies for CO₂ dissociation into CO and O on the bare and Ni-deposited surfaces. Generally, we found that the dissociative adsorption states are thermodynamically preferred over molecular adsorption, with the dissociation most favoured thermodynamically on the close-packed (110) facet. The trends in activation energy barriers were observed to follow that of the trends in surface work functions; consequently, the increased surface work functions observed on the Ni-deposited surfaces resulted in increased dissociation barriers and vice versa. These results suggest that measures to lower the surface work function will kinetically promote the dissociation of CO₂ into CO and O, although the instability of the activated CO₂ on the Ni-covered surfaces will probably result in CO₂ desorption from the nickel-doped iron surfaces, as is also seen on the Fe(110) surface.