Stability of Cu–Mn bimetal catalysts based on different zeolites for NOxremoval from diesel engine exhaust

Cu–Mn bimetal catalysts were prepared to remove nitrogen oxides (NOx) from diesel engine exhaust at low temperatures. At a Cu/Mn ratio of 3:2, the NOxconversions at 200 °C reached 65% and 90% on Cu–Mn/ZSM-5 and Cu–Mn/SAPO-34, respectively. After a hydrothermal treatment and reaction in the presence of C3H6, the activity of Cu–Mn/SAPO-34 was more stable than that of Cu–Mn/ZSM-5. No obvious variations in the crystal structure or dealumination were observed, whereas the physical structure was best maintained in Cu–Mn/SAPO-34. The atomic concentration of Cu on the surface of Cu–Mn/SAPO-34 was quite stable, and the consumption of octahedrally coordinated Cu2+could be recovered. Conversely, the proportion of octahedrally coordinated Cu2+on the surface of Cu–Mn/ZSM-5 significantly decreased. Therefore, besides the structure, the redox cycle between Cu+and octahedrally coordinated Cu2+played an important role in the stability of the catalysts.