Selective reduction of nitrogen oxide with hydrocarbons and hydrothermal aging of Cu-ZSM-5 catalysts

摘要

This paper deals with the hydrothermal deactivation, under an air + 10 vol. percent H_2O mixture between 923 and 1173 K, of Cu-MFI solids, catalysts for the selective reduction of NO by propane. Fresh and aged solids were characterized by various techniques and compared with a parent H-ZSM-5 solid. The catalytic activities were measured in the absence and in the presence of water. The differences between fresh and aged Cu-ZSM-5 catalysts (destruction of the framework, extent of dealumination-) were shown to be small in spite of the strong decreases in activity. Cu-ZSM-5 is more resistant to dealumination than the parent H-ZSM-5 zeolite. The rate of NO reduction into N_2 increases with the number of isolated Cu~(2+)/Cu~+ ions. These isolated ions partially migrate to inaccessible sites upon hydrothermal treatments. At very high aging temperatures a part of the copper ions agglomerates into CuO particles accessible to CO, but these bulk oxides are inactive. Under catalytic conditions and in the presence of water, dealumination is observed at a lower temperature (873 K) than under the (air + 10 percent H_2O) mixture, because of nitric acid formation linked to NO_2 which is either formed in the pipes of the apparatus or on the catalyst itself.