Distinct NO Effects on Cu-SSZ-13 and Cu-SSZ-39 in the Selective Catalytic Reduction of NO with NH.

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 NH (NH-SCR), attracting great attention for applications on diesel vehicles. In this study, it was interestingly found that NO 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 NO effects were found to be associated with the differences in the reactivity of surface NHNO, a key intermediate for NH-SCR, on these two Cu-based small-pore zeolites. Cu-SSZ-13 has excellent standard SCR activity, but the reactivity of surface NHNO with NO is relatively low, which would induce the accumulation of NHNO on the surface and thus inhibit NO conversion. Surface Brønsted acid sites play key roles in the reduction of surface NHNO 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 NHNO could be reduced by NO rapidly on Cu-SSZ-39, even faster than the reduction of NO by the adsorbed NH on Cu active sites; thus, NO conversion was promoted by NO on Cu-SSZ-39. This work provides an improved understanding of fast SCR on Cu-based small-pore zeolites.