Study on the NO2 production pathways and the role of NO2 in fast selective catalytic reduction DeNOx at low-temperature over MnOx/TiO2 catalyst

Selective catalytic reduction of NO by NH3 (SCR) at low temperature usually considered to be the combination of NO oxidation and "Fast SCR" process. However, the NO oxidation mechanism and the role of NO2 over "Fast SCR" process are controversial. Focusing on these problems, the in-deep studies were performed over the typical MnOx /TiO2 catalysts. Combining in-situ DRIFT, TPD-MS and DFT calculation results, an integrated NO oxidation mechanism was proposed based on an "Nitro-assisted oxygen activation" concept where the activation and transformation of O-2 is linked with the transformation of nitro species. The O=O bond of O-2 can be weakened by generating a variation of bidentate nitrates, which can promote the break of O=O bond and the formation of NO2. The formation of the variation of bidentate nitrates was the rate-determining step over NO oxidation. Additionally, we further reveal that the crucial role of NO2 over "Fast SCR" process lies in participating the hydrogen abstraction from coordinated NH3 and promoting the production of -NH2 active species which can direct react with NO to generate N-2 and H2O. The removability of NO2 can coupling the MnO2 (NO oxidation active component) and TiO2 support ("Fast SCR" active component) together. Our findings provide insight into the design and improvement of catalysts or reaction pathways over the low-temperature SCR process.