SmMn2O5 mullite has recently been reported to be a promising alternative to traditional Pt-based catalysts for environmental and energy applications. By performing density functional theory calculations, we have systematically investigated lattice oxygen reactivity and oxygen adsorption/dissociation/migration behaviors on low-index surfaces of SmMn2O5 mullite with different terminations. On the basis of the oxygen chemistry and thermodynamic stability of different facets, we conclude that (100)3+, (010)4+, and (001)4+ are reactive toward NO oxidation via either the Mars van Krevelen (MvK) or Eley–Rideal (ER) mechanism. Concrete NO → NO2 reaction paths on these candidate mechanisms have also been calculated. Both the (010)4+ and (001)4+ surfaces presented desirable activities. Bridge MnO sites on (010)4+ surface are identified to be the most active for NO oxidation through the ER mechanism with the lowest barrier of ∼0.38 eV. We have also identified that on all active sites considered in the current study,...