Improvement of catalyst for selective reduction of NO by controlling reaction atmospheres

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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 C_3H_8-SCR conditions. The UV-Vis spectra of Ag-MFI have revealed that the addition of hydrogen results in the formation of Ag_n^<□+> clusters due to partial reduction and agglomeration of Ag species The coincidence of the formation of the Ag_n^<□+> clusters and the increment of NO conversion suggests that Ag_n^<□+> clusters are the highly active species for HC-SCR. From analysis by H_2-TPR, UV-Vis, and EXAFS, the structure of Ag_n^<□+> 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^<□+> 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^<□+> clusters and acid sites, i.e., the ion-exchange site of zeolites, stabilizes Ag_n^<□+> 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.