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 spinel 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 (VO). On these surfaces, the COOR is controlled by the thermodynamics of surface re-oxidation through O2 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 O2 adsorption thus blocking the reaction. This effect is particularly important on NiFe2O4(001), which may contribute to the limited COOR activity of this material.
All Science Journal Classification (ASJC) codes
- Condensed Matter Physics
- Surfaces and Interfaces
- Surfaces, Coatings and Films
- Materials Chemistry