Si/Al Ratio Determines the SCR Performance of Cu-SSZ-13 Catalysts in the Presence of NO2

A comparative study was performed to investigate the NH₃-selective catalytic reduction (SCR) reaction activity of Cu-SSZ-13 zeolites having Si/Al ratios (SARs) of 5, 18, and 30. Remarkably, the Cu-SSZ-13 zeolite catalysts exhibited completely opposite behaviors as a function of SAR under standard SCR (SSCR) and fast SCR (FSCR) reaction atmospheres. Under SSCR conditions, the NO_x conversion increased as expected with the decreasing SAR. Under FSCR conditions, however, the NO_x conversion decreased as the SAR decreased, contrary to expectations. In this study, based on characterization of the catalysts by X-ray diffraction, transmission electron microscopy, electron paramagnetic resonance, H₂-temperature-programmed reduction, temperature-programmed desorption, and diffuse reflectance infrared Fourier transform spectroscopy, together with theoretical calculations, the authors found that the amount of Brønsted acid sites goes up while the SAR goes down, leading to an increase in the accumulation of NH₄NO₃ under FSCR reaction conditions. Moreover, the accumulated NH₄NO₃ is of greater stability for those low SAR Cu-SSZ-13 catalysts. These two reasons cause the FSCR performance of Cu-SSZ-13 to decrease with a decrease in SAR. As a result, the NO₂ effect on SCR activity changes from promotion to inhibition as the SAR decreases.