Selective catalytic reduction of NO with hydrocarbon on Cu²⁺-exchanged pillared clay: An IR study of the NO decomposition mechanism

In an earlier study, it has been found that Cu²⁺ ion-exchanged pillared clay (Cu-PILC) has a substantially higher activity for the selective catalytic reduction of NO by ethylene over Cu-ZSM-5. Moreover, it is not significantly deactivated by water vapor and SO₂. In this study, the activity for direct NO decomposition in the presence of O₂ on Cu-PILC was studied and an in situ IR study for the key intermediates and the reaction mechanism was made. The direct NO decomposition activities for Cu-PILC and Cu-ZSM-5 were similar. Under in situ NO and O₂ reaction conditions at temperatures up to 300°C, IR absorption bands at well-defined peak positions are identified. The band at 1699 cm^-1 is assigned to a dinitrosyl species on Cu⁺. The bands with peaks at 1609, 1530-1480 and in the region of 1440- 1335 cm^-1 are assigned to bidentate nitrate, monodentate nitrate and nitro species bonded to Cu²⁺. A redox mechanism is proposed for NO decomposition. The limiting step is thought to be the N-N coupling between surface nitrate and gaseous nitric oxide to form nitrogen. The existence of substantial amounts of nitrate formed from NO alone indicates the important role of the large amount of lattice oxygen that is available on Cu-PILC. As a result, the role of external oxygen supply is only to replenish the consumed lattice oxygen. The proposed NO decomposition mechanism suggests that the redox property of Cu-PILC is crucial for this reaction.