Ag cluster as active species for SCR of NO by propane in the presence of hydrogen over Ag-MFI

The effect of the addition of H-2 on SCR of NO by C3H8 over Ag-MFI was studied. SCR activity over Ag-MFI was significantly enhanced by the addition of H-2 below 673 K. Upon the removal of H-2 from reaction gases, NO reduction activity decreased to the same conversion before the addition of H-2, indicating that the promotion effect of H-2 on NO reduction activity was reversible. UV-vis spectroscopy was used to identify the active structure of Ag species during the C3H8-SCR. Ag+ ion was mainly in existence during the C3H8-SCR in the absence of H-2, while Ag-n(delta+) clusters (2 less than or equal to n less than or equal to 4), together with Ag+ ion, were formed in the presence of H-2. NO reduction rate and band intensity due to the Ag-n(delta+) cluster increased with an increase in H-2 concentration. On the other hand, formation of a metallic Ag-m cluster (3 less than or equal to m less than or equal to 5) and Ag metal increased the contribution of nonselective hydrocarbon combustion to the overall reaction. From the above results, it is indicated that a moderately agglomerated Ag-n(delta+) cluster is a highly active species for the SCR by C3H8 in the presence of H-2, and that the role of H-2 is reduction of Ag+ ion to Ag-n(delta+) clusters. Ag species is markedly influenced by concentrations of gaseous components not only H-2 but also NO and C3H8. In addition, the type of Ag species was reversibly changed among Ag+ ion, Ag-n(delta+) clusters, and metallic Ag-m clusters together with Ag metal, depending on the reaction atmosphere, which suggests that gaseous components control the balance among Ag species on the catalyst. (C) 2003 Elsevier Inc. All rights reserved.