Mechanism of NH3 Selective Catalytic Reduction Reaction for NOx Removal from Diesel Engine Exhaust and Hydrothermal Stability of Cu-Mn/Zeolite Catalysts

Zeolite-supported catalysts are effective in selective catalytic reduction (SCR) of NOx from the diesel engine exhaust, whereas the reaction mechanism remains unclear. In this work, in situ diffuse reflectance infrared Fourier transform (in situ DRIFT) spectroscopy was used to investigate the SCR reaction mechanism on Cu-Mn/ZSM-5 and Cu-Mn/SAPO-34. Langmuir-Hinshelwood (L-H) and Eley-Rideal (E-R) reaction mechanisms were both involved in the SCR reaction. In the L-H reaction mechanism, NO2 and NH4+ Were two important intermediates. The transformation of bidentate nitrates to non nitrates, and further to NO2, played an important role. The amount of bidentate nitrates and monodentate nitrates on Cu-Mn/ZSM-5 was much less than that on Cu-Mn/SAPO-34. It led to less NO removal-through the L-H reaction mechanism on Cu-Mn/ZSM-5. The formation and consumption of coordinated NH3, as the intermediate in the E-R reaction mechanism, was similar on both catalysts. Therefore, the different catalytic activities between the two-catalysts should be mainly due to the L-H reaction, mechanism, the main mechanism at low temperature. The hydrothermal aging treatment significantly reduced the amount of NH4+ and NOx complexes on Cu-Mn/ZSM-5, and the variation of the SCR reaction through the L-H mechanism was also the main reason for different hydrothermal stabilities of the two catalysts.