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

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 NOx. 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.