Mechanism and activity of CO oxidation on (001) and (110) surfaces of spinel Co3O4, NiCo2O4 and NiFe2O4: A DFT + U study

We carried out density functional theory (DFT) calculations with on-site Hubbard U repulsion (DFT + U) to investigate the mechanism and activity of the CO oxidation reaction (COOR) on pristine and oxygen-deficient (001) and (110) surfaces of spine] Co3O4, NiCo2O4 and NiFe2O4. On all investigated (001) surfaces, reaction of adsorbed CO with a surface oxygen atom leads to the spontaneous formation of a CO2 molecule and a surface oxygen vacancy (V-O). On these surfaces, the COOR is controlled by the thermodynamics of surface re-oxidation through O-2 adsorption at Vo sites. On the Co3O4 (110) surface, instead, the kinetic barrier for CO2 formation is rate determining, while re-oxidation is thermodynamically favorable also at relatively high temperature (T - 500 K). In humid environment, water adsorption at surface VOS inhibits O-2 adsorption thus blocking the reaction. This effect is particularly important on NiFe2O4(001), which may contribute to the limited COOR activity of this material.