Distinct NO₂ Effects on Cu-SSZ-13 and Cu-SSZ-39 in the Selective Catalytic Reduction of NO 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 NO 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 NO 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 NO 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, NO conversion was promoted by NO₂ on Cu-SSZ-39. This work provides an improved understanding of fast SCR on Cu-based small-pore zeolites.