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 H2 on SCR of NO by C3H8 over Ag-MFI was studied. SCR activity over Ag-MFI was significantly enhanced by the addition of H2 below 673 K. Upon the removal of H2 from reaction gases, NO reduction activity decreased to the same conversion before the addition of H2, indicating that the promotion effect of H2 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 H2, while Agnδ+ clusters (2⩽n⩽4), together with Ag+ ion, were formed in the presence of H2. NO reduction rate and band intensity due to the Agnδ+ cluster increased with an increase in H2 concentration. On the other hand, formation of a metallic Agm cluster (3⩽m⩽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 Agnδ+ cluster is a highly active species for the SCR by C3H8 in the presence of H2, and that the role of H2 is reduction of Ag+ ion to Agnδ+ clusters. Ag species is markedly influenced by concentrations of gaseous components not only H2 but also NO and C3H8. In addition, the type of Ag species was reversibly changed among Ag+ ion, Agnδ+ clusters, and metallic Agm clusters together with Ag metal, depending on the reaction atmosphere, which suggests that gaseous components control the balance among Ag species on the catalyst.