Reaction Pathway Investigation on the Selective Catalytic Reduction of NO with NH₃ over Cu/SSZ-13 at Low Temperatures

Abstract

The mechanism of the selective catalytic reduction of NO with NH₃ was studied using Cu/SSZ-13. The adspecies of NO and NH₃ as well as the active intermediates were investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy and temperature-programmed surface reaction. The results revealed that three reactions were possible between adsorbed NH₃ and NO_<i>x</i>. NO₂^– could be generated by direct formation or NO₃^– reduction via NO. In a standard selective catalytic reduction (SCR) reaction, NO₃^– was hard to form, because NO₂^– was consumed by ammonia before it could be further oxidized to nitrates. Additionally, adsorbed NH₃ on the Lewis acid site was more active than NH₄⁺. Thus, SCR mainly followed the reaction between Lewis acid site-adsorbed NH₃ and directly formed NO₂^–. Higher Cu loading could favor the formation of active Cu-NH₃, Cu-NO₂^–, and Cu-NO₃^–, improving the SCR activity at low temperature