Formation and Redispersion of Silver Clusters in Ag-MFI Zeolite as Investigated by Time-Resolved QXAFS and UV−Vis

Abstract

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 δ+ cluster. Protons on the zeolite (Brønsted 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 δ+ 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 H 2 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.

Identification of the silver state in the framework of Ag-containing zeolite by XRD, FTIR, photoluminescence,109Ag NMR, EPR, DR UV-vis, TEM and XPS investigations

“…However, there is still lack of data in the literature concerning application of silver-containing zeolites in the SCR of NO x . According to previous reports, dispersed silver species seem to be active catalytic centers of SCR of NO on Ag-MFI zeolites [18,19]. However, silver is not well dispersed in such materials, which contain silver in the form of various species, i.e., ions, nanoclusters, and metallic nanoparticles.…”

Silver Cluster Catalysts for Green Organic Synthesis

“…Our group has paid attention to Ag clusters catalysts for automotive emission control 43)～45) and green organic syntheses 46)～52) . Based on the fact that the surface of metal particle with size below a few nanometer contains large number of low-coordinated metal atoms having high reactivity, we paid special emphasis on the catalysis of metallic Ag clusters with a size of subnanometer to a few nanometers.…”

Oxidant‐Free Dehydrogenation of Alcohols Heterogeneously Catalyzed by Cooperation of Silver Clusters and Acid–Base Sites on Alumina

“…[14][15][16][17][18][19][20][21][22][23][24] We have been paying attention to cluster-specific catalysis by supported silver catalysts for environmental applications and have reported a series of studies on size-and supportspecific catalysis by silver clusters for the selective catalytic reduction of NOx with hydrocarbons for the removal of NOx emissions from diesel engine exhausts. [20][21][22] It has been shown that, of various silver catalysts, silver clusters on alumina constitute the best catalyst, with activity much higher than that of Pt/Al 2 O 3 catalyst. Fundamental studies showed that both the silver clusters and the acid-base sites on the support surface are responsible for the high activity of silver clusters on alumina.…”

“…Fundamental studies showed that both the silver clusters and the acid-base sites on the support surface are responsible for the high activity of silver clusters on alumina. [20][21][22] For this study we attempted an extension of the acid-base-assisted silver cluster catalysis to selective oxidation.…”