ESR STUDY OF THE DEACTIVATION OF CU-ZSM-5 IN A NET OXIDIZING ATMOSPHERE

A microscopic study of the deactivation of Cu-ZSM-5 was carried out. T he activity of Cu-ZSM-5 for NOx reduction with hydrocarbons decreased when heated above 600 degrees C in a simulated net oxidixing exhaust g as. Al-27 and Si-29 nuclear magnetic resonance results revealed that a dealumination of the zeolite occurs in the deactivated Cu-ZSM-5. Elec tron spin reonance (ESR) results indicated that aggregation of copper ions did not occur and copper remained as isolated ions in Cu-ZSM-5 ev en after the deactivation. The ESR spectra of dehydrated Cu-ZSM-5 sugg ested the migration of copper ions in the zeolite after the deactivati on. To investigate the interactions between copper ions and NO/C3H6, t he ESR spectra were measured after exposure to nitric oxide and propen e. The ESR results indicated that the oxidation state and coordination structure of the copper ions were changed by the adsorption of nitric oxide or propene in active Cu-ZSM-5. On the other hand, no such react ion was observed in the deactivated Cu-ZSM-5. In the deactivated Cu-ZS M-5, copper ions remain atomically dispersed but cannot interact with nitric oxide or propene. In conclusion, the deactivation of Cu-ZSM-5 i s suggested to occur through the migration of copper ions to sites whe re gas molecules like nitric oxide and propene cannot reach them. This migration is triggered by the dealumination of the zeolite.