Combined Experimental and Computational Approaches To Elucidate the Structures of Silver Clusters inside the ZSM-5 Cavity

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• 作者：Takashi Yumura ; Akira Oda ; Hiroe Torigoe ; Atsushi Itadani ; Yasushige Kuroda ; Takashi Wakasugi ; Hisayoshi Kobayashi
• 刊名：Journal of Physical Chemistry C
• 出版年：2014
• 出版时间：October 16, 2014
• 年：2014
• 卷：118
• 期：41
• 页码：23874-23887
• 全文大小：620K
• ISSN：1932-7455

文摘

A combination of experimental and computational analyses suggested the presence of Ag₃ or Ag₄ clusters inside a nanometer-sized cavity in Ag鈥揨SM-5 zeolites, which were formed from H鈥揨SM-5 using the conventional ion-exchange method in an aqueous silver nitrate solution. During the experimental analyses, we investigated the structural and absorption properties of Ag鈥揨SM-5 through UV鈥搗is diffuse reflectance and X-ray absorption fine structure (XAFS) measurements. The results from the extended XAFS (EXAFS) analysis indicated that clusters contained in the ZSM-5 cavity have Ag鈥揂g separations of approximately 2.6 脜. The UV鈥搗is measurements indicated that the clusters located in the cavity present three prominent absorption bands centered at approximately 255, 287, and 331 nm for the sample treated at 473 K. The Ag鈥揨SM-5 treated at 573 or 673 K presents new UV鈥搗is bands at approximately 303 and 319 nm. The experimental results regarding the structural and absorption properties of Ag鈥揨SM-5 could be well-reproduced by DFT calculations when a model large enough to represent a 10-membered ring of ZSM-5 was used. DFT optimization indicated that the ZSM-5 cavity can accommodate a triangular Ag₃ cluster and a butterfly Ag₄ cluster whose Ag鈥揂g separations range from 2.7 to 2.9 脜. According to time-dependent DFT calculations, these clusters have electronic transitions from a completely symmetric 5s-based orbital to a 5s-based orbital with one node. The electronic excitations between the 5s-based orbitals are modulated by the ZSM-5 encapsulation through the resulting deformation of the cluster and interactions between the cluster and framework oxygen atoms. The electronic transitions between the 5s-based orbitals that appropriately explain the UV鈥搗is absorption properties would become fingerprints for identifying the shapes and sizes of clusters inside a zeolite cavity. Our multidisciplinary analyses conclusively determined the origin of the absorption peaks in Ag鈥揨SM-5 and successfully obtained atomistic information about the states of silver clusters inside a ZSM-5 cavity. The findings of this study will provide useful information for elucidating the structures of active sites in Ag-ZSM-5 and their important role in catalytic reactions such as C鈥揌 bond activation in hydrocarbons.