Oxygen Deficiency and Reactivity of Spinel NiCo2O4 (001) Surfaces

We carried out density functional theory (DFT) calculations with on-site Hubbard U corrections to investigate the structure, defects, and reactivity of (001) surfaces of spinel NiCo2O4 (NCO), a promising catalyst for CO and methane oxidation. By examining surfaces with different Co/Ni compositions, we find that the formation of surface oxygen vacancies (Vos) on NCO(001) is strongly affected by the neighboring cation in the third layer, the computed formation energy being largest (similar to 1.2 eV) for 0 vacancies coordinated to third layer Co and smallest (similar to 0.5 eV) for Vos coordinated to a Ni neighboring another Ni ion. As a result, Vo formation is generally much easier on NCO (001) than on Co3O4 (001) surfaces, suggesting that NCO may be a better catalyst than Co3O4 for oxidation reactions based on the Mars Van Krevelen mechanism. Surface oxygen vacancies on reduced NCO surfaces can be healed through dissociative water adsorption at room temperature. In contrast, adsorption of molecular oxygen at Vos is energetically unfavorable under ambient conditions, suggesting that O-2 adsorption may represent the thermodynamic limiting step for oxidation reactions on NCO(001) surfaces.