Formation and redispersion of silver clusters in Ag-MFI zeolite as investigated by time-resolved QXAFS and UV-vis

Dynamic structural changes of Ag in Ag-MFI upon H-2 reduction and subsequent reoxidation treatments were investigated by various characterization methods. Results of in situ time-resolved spectroscopies, UV-vis and quick XAFS, at 573 K show that Ag+ ions at a cation-exchange site of preoxidized Ag-MFI are reduced by H-2 to a Ag-4(delta+) cluster. Protons on the zeolite (Bronsted acid sites) are also produced by the H-2 reduction of Ag-MFI as evidenced by IR measurements after D-2 adsorption. At higher reduction temperatures, the aggregation of Ag-4(delta+) clusters results in the formation of larger Ag-n (n > 8) clusters, and Ag metal particles appear on the external surface of the zeolite. Upon reoxidation at 573 K, the silver clusters are redispersed to Ag+ ions. The rate constant of cluster redispersion at 573 K depends on the oxidants used and increases in the order of NO(0.1%) + O-2(10%) > O-2(10%) > NO(0.1%). The activation energy for cluster formation in H2 was lower than that for cluster redispersion during reoxidation with O-2. The effect of Ag exchange level on the rate constants and apparent activation energies for Ag cluster formation in H-2 and Ag cluster redispersion in O-2 are also examined.