Promoting effect of Ce and Mn addition on Cu-SSZ-39 zeolites for NH3-SCR reaction: Activity, hydrothermal stability, and mechanism study

In this work, Ce and Mn were added to Cu-SSZ-39 zeolites by ion-exchange process, and their activity performance for selective catalytic reduction of NOx with ammonia (NH3-SCR), especially hydrothermal stability (hydrothermal aging at 750, 800, 850 °C) was investigated systematically. It is found that the addition of Ce and Mn, Mn more evidently, both can greatly improve the activity of Cu-SSZ-39 catalyst. Moreover, MnCu-SSZ-39 catalyst have shown superior hydrothermal stability compared with Cu-SSZ-39 catalyst, and outstandingly its activity is not only not decreased significantly like Cu-SSZ-39 but improved obviously after hydrothermal aging at as high as 850 °C. The high and improved activity of Ce and Mn added Cu-SSZ-39 is ascribed to more active CuO crystallites formation in the inner channel and higher acidity strength. The superior hydrothermal stability of MnCu-SSZ-39 may be due to its well-preserved structure and acid density even after hydrothermal aging at as high as 850 °C. Moreover, during the hydrothermal aging process, not only Cu oxides aggregation is inhibited, but also more Cu2+ ion species are induced to ion exchange sites in MnCu-SSZ-39, which may account for its excellent hydrothermal stability (even higher than the activity of fresh samples). The mechanism analysis has shown NH3-SCR reaction over Cu-SSZ-39 and CeCu-SSZ-39 samples follow the “Langmuir–Hinshelwood” (L-H) mechanism, while on MnCu-SSZ-39, it follows both “L–H” and “Eley–Rideal” (E–R) mechanisms, which may also be another reason for its excellent activity and hydrothermal stability. Moreover, “Fast SCR” mechanisms also occur and contribute over hydrothermal aged A-MnCu-SSZ-39 sample, in addition to “L-H” and “E-R” mechanisms. Thus, the easily prepared MnCu-SSZ-39 catalyst has shown the excellent activity and super hydrothermal stability, with 90% NO conversion at 245 °C and broader temperature window of 245 °C to 550 °C, even after hydrothermal aging at 850 °C, therefore indicate great potential for future application in NOx purifying from diesel engine exhaust.