Density Functional Theory Study of the Oxygen Chemistry and NO Oxidation Mechanism on Low-Index Surfaces of SmMn₂O₅ Mullite

Abstract

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, the rate-determining step in NO → NO₂ oxidation reaction is the NO₂ desorption. Our study gives an insight into the mechanisms of NO oxidation on SmMn₂O₅ mullite at the atomic level and can be used to guide further improvement of its catalytic performance