Dr David Willock
Our interest in periodic DFT calculations for surfaces important in catalysis is exemplified by the images below on MoO3. This important oxidation catalyst is known to form as a defective (sub-stoichiometric) material with surface oxygen defects. Some oxidation processes also involve the use of lattice oxygen following the Mars-van Krevelen mechanism. The exchange of oxygen with the surface then becomes key to understand the oxygen supply.
Formally, the removal of oxygen from the MoO3 surface shown below involves a reduction of a single Mo centre from Mo6+ to Mo4+. GGA-DFT fails to reproduce this effect since the electrons left behind after O is removed delocalise through the conduction band of the material. We have used the Hubbard U parameter extension of DFT to consider this system, which increases the band gap (closer to the experimental) and gives localised states at the Mo centre. The spin density shows a triplet ground state for this defect.
Molecular oxygen will adsorb at this defect site, donate an electron to the Mo centre and produce a surface superoxo (O2-) species. The superoxo itself may play a role in oxidation catalysis, or go on to heal the defect.