Insight into the Superior Catalytic Activity of MnO₂ for Low-Content NO Oxidation at Room Temperature

To achieve efficient low-content NO oxidation at room temperature is a hot but challenging topic in heterogeneous catalysis, and MnO₂-based oxide catalysts have recently drawn so much attention owing to the potential activity. However, the activity origin of MnO₂ and the critical rate-limiting factor are vague, impeding further catalyst optimization. Herein, by combining the first-principles calculations and microkinetic analyses, we systematically investigate the low-content NO oxidation process catalyzed by MnO₂. On MnO₂(110) exposed by two kinds of active sites (Mn₅c and the lattice Obᵣᵢ), the favorite pathway contributing to NO oxidation is figured out by examining the complicated reaction network. This reveals that the Mars–van Krevelen mechanism with the lattice Obᵣᵢ involved is preferred rather than the Langmuir–Hinshelwood one occurring at the Mn₅c sites alone. First, NO adsorbs at Mn₅c (as NO*) and is oxidized by the lattice Obᵣᵢ, forming NO₂# (# denotes the Obᵣᵢ vacancy, Oᵥₐc) that can desorb with an Oᵥₐc left. Second, O₂ can adsorb at Oᵥₐc (O₂#) and react with NO* into an intermediate ONOO, which can break its O–O bond and release NO₂. It is also found that Mn₅c can exclusively adsorb NO and guarantee the coverage of NO* even for the low-content NO(g) and the lattice Obᵣᵢ is very reactive and provides oxidative species, showing a synergetic catalytic role accounting for high activity of MnO₂ for low-content NO oxidation at room temperature. Quantitatively, the adsorption energies of NO at Mn₅c (Eₐdₛ(NO@Mn₅c)) and O₂ at Oᵥₐc (Eₐdₛ(O₂@Oᵥₐc)) can serve as the important activity descriptors and increasing either of them could improve the activity of MnO₂. In addition, our microkinetic results show that the NO# (NO adsorbed at the Oᵥₐc) would be a key poisoning species and deactivate MnO₂ owing to its stronger adsorption in comparison with O₂, whereas the nitrate/nitrite species could not cause the blockage of active sites as expected. This work could provide a significant insight into low-content NO oxidation at room temperature catalyzed by MnO₂.