Utilizing super-atom orbital ideas to understand properties of silver clusters inside ZSM-5 zeolite
2017
Yumura, Takashi | Kumondai, Mitsuhiro | Kuroda, Yasushige | Wakasugi, Takashi | Kobayashi, Hisayoshi
The energetic properties of Agₙ clusters in ZSM-5 zeolite were investigated using density functional theory (DFT) calculations with the B3PW91 functional. Several optimized geometries (Agₙ–ZSM-5(Alₘ), 3 ≤ n ≤ 6 and 1 ≤ m ≤ 5) were obtained using this method, where m is the number of Al atoms substituted for Si atoms of a ten-membered ring of ZSM-5. DFT calculations found that an Agₙ cluster is well stabilized within a ZSM-5(Alₘ) cavity at n = m + 2. The stabilization conditions can be explained by frontier orbital theory because the HOMO of Agₘ₊₂–ZSM-5(Alₘ) zeolites is composed by totally symmetric 5s-based orbitals on silver atoms. The totally symmetric 5s-based orbital corresponds to a superatom S-orbital in cluster chemistry. Accordingly, the Agₘ₊₂–ZSM-5(Alₘ) zeolites have an S² electronic configuration, being similar to magic-number silver clusters in the gas phase. Time-dependent DFT calculations found significant oscillator strength at the electronic transition between 5s-based orbitals from the totally symmetric orbital (S-orbital) to that with one node (P-orbital). The S → P electronic transitions in Agₘ₊₂–ZSM-5(Alₘ) follow the selection rule of electronic transitions of bare clusters. Because the excitation energies (λₘₐₓ) change with an increase in the number of contained silver atoms, the S → P electronic transitions of Agₘ₊₂–ZSM-5(Alₘ) could be used to identify the state of the inner silver atoms.
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