CO 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 CO 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. CO 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, -70.8 kJ mol and -116.8 kJ mol on the (100), (110) and (111) facets, respectively. Compared to the bare Fe surfaces, we found weaker binding of the CO molecules on the Ni-deposited surfaces, where the adsorption energies are calculated at +47.2 kJ mol, -29.5 kJ mol and -65.0 kJ mol on the Ni-deposited (100), (110) and (111) facets respectively. We have also investigated the thermodynamics and activation energies for CO 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 CO into CO and O, although the instability of the activated CO on the Ni-covered surfaces will probably result in CO desorption from the nickel-doped iron surfaces, as is also seen on the Fe(110) surface.