Ag clusters as active species for HC-SCR over Ag-zeolites

The addition of hydrogen in the reaction atmosphere is effective in promoting the activity of Ag/alumina and Ag-zeolites on the selective reduction of NO by hydrocarbons (HC-SCR) at low temperatures. The increment of NO conversion over Ag-MFI corresponds to the periodic addition of hydrogen into C3H9-SCR conditions. The UV-VIS spectra of Ag-MFI have revealed that the addition of hydrogen results in the formation of Ag-n(delta+) clusters due to partial reduction and agglomeration of Ag species. The coincidence of the formation of the Ag-n(delta+) clusters and the increment of NO conversion suggests that Ag-n(delta+) clusters are the highly active species for HC-SCR. From analysis by H-2-TPR, UV-VIS, and EXAFS, the structure of Ag-n(delta+) clusters on Ag-MFI is identified as being Ag-4(2+) on average. The formation of Ag clusters was strongly affected by the type of zeolites: The major Ag species are Ag+ ions for MOR, Ag-n(delta+) clusters for MFI and BEA, and relatively large metallic Ag-m particles for Y. The sequence of Ag agglomeration (MOR < MFI < BEA < Y) is in accordance with the strength of the acid sites of zeolites. It can be expected that the interaction between the positive charge of Ag-n(delta+) clusters and acid sites, i.e., the ion-exchange site of zeolites, stabilizes Ag-n(delta+) clusters. The type of Ag Species under HC-SCR conditions depends on the concentration of gas-phase oxidants (NO, O-2) and reductants (H-2, HC), and also on the number and strength of the zeolite acid sites.