Distinct NO2 Effects on Cu-SSZ-13 and Cu-SSZ-39 in the Selective Catalytic Reduction of NOx with NH3

Cu-SSZ-13 and Cu-SSZ-39, with similar structures, are both highly active and hydrothermally stable in the selective catalytic reduction of NOx with NH3 (NH3-SCR), attracting great attention for applications on diesel vehicles. In this study, it was interestingly found that NO2 has distinct effects on the NOx conversion over Cu-SSZ-13 and Cu-SSZ-39, with an inhibiting effect for Cu-SSZ-13 but a promoting effect for Cu-SSZ-39. The distinct NO2 effects were found to be associated with the differences in the reactivity of surface NH4NO3, a key intermediate for NH3-SCR, on these two Cu-based small-pore zeolites. Cu-SSZ-13 has excellent standard SCR activity, but the reactivity of surface NH4NO3 with NO is relatively low, which would induce the accumulation of NH4NO3 on the surface and thus inhibit NOx conversion. Surface Brønsted acid sites play key roles in the reduction of surface NH4NO3 by NO, and Cu-SSZ-39 showed much higher surface acidity than Cu-SSZ-13. Compared with Cu-SSZ-13, the intrinsic standard SCR activity of Cu-SSZ-39 was lower but NH4NO3 could be reduced by NO rapidly on Cu-SSZ-39, even faster than the reduction of NO by the adsorbed NH3 on Cu active sites; thus, NOx conversion was promoted by NO2 on Cu-SSZ-39. This work provides an improved understanding of fast SCR on Cu-based small-pore zeolites.