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 (V_Os) on NCO(001) is strongly affected by the neighboring cation in the third layer, the computed formation energy being largest (∼1.2 eV) for O vacancies coordinated to third layer Co and smallest (∼0.5 eV) for V_Os coordinated to a Ni neighboring another Ni ion. As a result, V_O 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 V_Os is energetically unfavorable under ambient conditions, suggesting that O₂ adsorption may represent the thermodynamic limiting step for oxidation reactions on NCO(001) surfaces.