Identification of active sites and reaction mechanism on low-temperature SCR activity over Cu-SSZ-13 catalysts prepared by different methods

Cu-SSZ-13 catalysts with similar Si/Al and Cu/Al ratios were prepared by aqueous solution ion-exchange (Cu-SSZ-13-I) and one-pot synthesis (Cu-SSZ-13-O) methods. NH₃-SCR tests, XRD, BET, SEM, HRTEM, EPR, H₂-TPR, NH₃-TPD, NO + O₂-TPD, in situ DRIFTS, and kinetic tests were performed for the catalytic measurement, bulk characterization and mechanism estimation. The NH₃-SCR results indicated that Cu-SSZ-13-O showed higher DeNO_x activity than Cu-SSZ-13-I in the absence or presence of H₂O across the entire temperature range. The results of EPR, H₂-TPR and NO + O₂-TPD showed that more Cu²⁺ ions existed in Cu-SSZ-13-O, which mainly accounts for the Lewis acid sites and the majority of the NO_x adsorption or activation. The DRIFTS results showed that NH₃ on Lewis acid sites was more active than that on Brønsted acid sites in the NH₃-SCR reaction. Furthermore, the DRIFTS results also indicated that monodentate nitrates are the most active nitrate species. Compared with Cu-SSZ-13-I, Cu-SSZ-13-O showed stronger Lewis acid site strengths and had more abundant monodentate nitrate species. Therefore, the NH₃-SCR reaction proceeded more easily over Cu-SSZ-13-O in comparison with Cu-SSZ-13-I. This could be the key factor that explains the more excellent low-temperature SCR activity of Cu-SSZ-13-O. In addition, at low reaction temperatures, Cu-SSZ-13-O was less affected by pore (i.e., intracrystalline) diffusion limitations.