Utilizing super-atom orbital ideas to understand properties of silver clusters inside ZSM-5 zeolite

The energetic properties of Ag-n clusters in ZSM-5 zeolite were investigated using density functional theory (DFT) calculations with the B3PW91 functional. Several optimized geometries (Ag-n-ZSM-5(Al-m), 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(Al-m) 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(Al-m) zeolites have an S-2 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(Al-m) follow the selection rule of electronic transitions of bare clusters. Because the excitation energies (lambda(max)) change with an increase in the number of contained silver atoms, the S -> P electronic transitions of Agm+2-ZSM-5(Al-m) could be used to identify the state of the inner silver atoms.