The effect of O-2 toward selective catalytic reduction (SCR) of NO2 with CH4 on the surface of galliummodified ZnAl2O4 catalyst has been investigated by first-principles calculations based on density functional theory (DFT). The studies show that NO2 interacts with Ga via its N side with a high adsorption energy of -56.33 kcal mol(-1) stronger than other molecules, such as H2O, O-2, CH4, CO2 and N-2. Among them, the H2O prefers to adsorb on Zn rather than Ga as the active component. Additionally, O-2 and NO2 co-adsorption is strongly repulsive. O-2 has a negligible effect on stiffness of H-C (O-N) bond. Repulsion between the reactant and O-2 is responsible for the formation of much weaker metal-adsorbate bonds (Ga N and Al C bonds). Instead, negative repulsion between the product and O-2 results in formation of much stronger surface-adsorbate bonds (Zn O, Al C and Ga N bonds). In addition, enthalpies of reaction in the presence and absence of O-2 are -195.53 and -165.30 kcal mol(-1), respectively. Both reactant and product are raised to higher energy level after O-2 adsorption on the catalyst surface. O-2 decreases activation energy of CH4-NO2 SCR reaction and causes a lowering of SCR reaction temperature. (C) 2015 Elsevier B.V. All rights reserved.