Defects on the Surfaces of Transition Metal Oxides

Anion defects on oxide surfaces are important centers for adsorption and acbva-bon of molecules in catalysis. For example, oxidabon reacbons following a Mars-van Krevelen mechanism require oxygen bansfer from a surface to an 

In a calculation, m and n define the stoichiometry of the simulation ceU, and the two electrons that remain after a surface ion has formed part of the neutral 02(g) molecule have been written explicitly. Using a periodic approach it should be remembered that the defect will be repeated in neighboring cells. To obtain results relevant to isolated defect sites requires ceU sizes large enough to aUow interactions between the defect and its own images to be neglected. 

The map extends beyond the central octahedron and includes part of the interlayer space, denoted by A. Taken from ref [128]. 

For transition metal oxides with partially filled d states, the balance between Coulomb and exchange interactions is even more critical. DFT strongly underestimates the exchange gap present in materials with magnetic order such as NiO, while it is again overestimated in HF. This can be corrected either through hybrid functionals or with the DFT -i- U approach. 

In defects on transition metal oxides DFT again fails, giving structures that show unlikely relaxations and tending to delocalize electrons associated with the defect into conduction band states. Hybrid functionals and DFT + U have also been used to correct the models in these cases, giving a localized picture of surface reduction. These methods are now able to give useful descriptions of reactions at these defect sites, including the transfer of electrons between surface and adsorbate required in redox chemistry. 

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