In-situ DRIFTS for the mechanistic studies of NO oxidation over α-MnO2, β-MnO2 and γ-MnO2 catalysts

- α-, β-, and γ-MnO2 catalysts with different tunnel structures were synthesized for NO oxidation.
- γ-MnO2 has outstanding catalytic activities due to the stacking faults for active oxygen species.
- The surface area was not the limiting factor for NO oxidation over MnO2 catalyst.
- Bridged nitrate as intermediate profited from active oxygens was decomposed to NO2 and new Mn-O-Mn.
- The reaction pathways of α-, β-, and γ-MnO2 catalysts for NO oxidation were proposed.

In this article, α-MnO2 and β-MnO2 nanorods, and urchin-like γ-MnO2 catalysts with different tunnel structures were synthesized by a hydrothermal synthesis method and evaluated for the catalytic oxidation of nitric oxide (NO). The experimental results showed the γ-MnO2 catalyst has the best catalytic activity among the three catalysts, with more than 80% NO conversion at 250 °C. The catalytic oxidation activities decreased in the order: γ- > β- ≈ α-MnO2. The XPS results implied that main manganese in all the catalysts was Mn4+ and the activity was in close correlation with the surface concentration of Oα species. The BET results showed that the surface area was not the suppression factor for NO oxidation. O2-TPO/TPD and In-situ DRIFTS experiments showed the catalytic activity of α-MnO2 with [2 × 2] tunnels was benefit from the chemisorbed oxygen species while not the lattice oxygens or Mn cations. For β-MnO2 with [1 × 1] tunnels and γ-MnO2 with [2 × 1] tunnels, both chemisorbed oxygen and lattice oxygen or Mn cations were the influencing factors on the catalytic oxidation activity, and the chemisorbed oxygens were the major. The main intermediate active species were monodentate nitrites at low temperature, while were bridged nitrates mainly profited from chemisorbed oxygen over three catalysts at high temperature, and further decomposed to NO2 and produced new Mn-O-Mn. The stacking faults of γ-MnO2 with the random intergrowth of ramsdellite and pyrolusite structures resulted in the main sources of active oxygen species, which were beneficial to the catalytic activity. The reaction pathways over α-, β-, and γ-MnO2 catalysts for NO oxidation were proposed.