Chemisorption of NO2 on V‑Based SCR Catalysts: A Fundamental Study toward the Mechanism of “Fast SCR” Reaction

The “fast selective catalytic reduction (SCR)” process, with its superior efficiency, is promising for removing NO x from flue gas with low temperature and high gas velocity. Chemisorption of NO2 is a crucial step in this heterogeneous catalysis process. In this study, density functional theory calculations and experimental methods were implemented to study the chemisorption behaviors of NO2 on the industrial V-based SCR catalyst. Both the (001) and (101) surfaces of the TiO2 support were applied in the modeling and calculation process. Theoretical results show varied profiles of NO2 adsorption on the fresh and reduced V2O5/TiO2 catalysts. For the fresh catalyst, NO2 tends to adsorb on the TiO2 support rather than on the vanadium oxide. On the contrary, NO2 exhibits a strong interaction with the −OH site of the reduced vanadium oxide. NO2 temperature-programmed desorption, X-ray photoelectron spectroscopy, and a designed redox experiment were implemented to verify the calculation results. On the basis of the NO2 adsorption behaviors, we concluded two different reaction mechanisms of the “fast SCR” reaction. One is that NO2 significantly accelerates the reoxidation process of the reduced vanadia sites and the other is that the adsorbed NO2 on TiO2 reacts with NH3 and NO in the path of the “Nitrate route” with the assistance of an adjacent vanadia site.