Formation of Ag clusters in zeolite X studied by in situ EXAFS and infrared spectroscopy

The in situ investigation of local structures by means of Ag K-edge EXAFS and infrared spectra for fully and partially Ag+ exchanged zeolite X (Agx–X; x = 86, 66 and 12, where x is number of Ag atoms in unit cell) were carried out from room temperature to 300 °C under vacuum. By evacuation at room temperature the coordination number of oxygen around Ag, NAg–O, decreases due to the removal of water molecules. By heating samples under vacuum, Ag clusters grew up via step that Ag–Ag interaction was weakened for Ag86–X and Ag66–X. On the other hand, for Ag12–X, the process with weak Ag–Ag interaction did not exist but Ag cluster was formed just by heating in a vacuum. These differences may have depended on the amount of residual Na+ ions in the zeolite cavity. From the infrared and EXAFS results it was suggested that the mobile Na+ ions in the cavity accelerated to produce Ag clusters for Ag12–X and Ag66–X in low temperature. On the other hand, Ag+ ions were only thermally excited in Ag86–X because there was no Na+ ion in the zeolite cavity. EXAFS result indicated the two types of Ag clusters which were characterized by the Ag coordination number, NAg–Ag = 3.0 ± 0.4 (cluster I) and 4.7 ± 0.4 (cluster II). The Ag cluster formation proceeded step by step as these two types of clusters. It was contrasted to Ag zeolite 4A that only the middle size Ag clusters that NAg–Ag = 3.8 ± 0.4 exist.

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► By heating samples under vacuum Ag clusters grow up via step that Ag–Ag interaction is weakened for Ag86–X and Ag66–X.
► For Ag12–X, the process with weak Ag–Ag interaction does not exist but Ag cluster is formed just by heating in vacuum.
► EXAFS results indicates the two types of Ag clusters.
► Ag cluster formation proceeds step by step between these two types of cluster.