Silver clusters in the cages of zeolites: A quantum chemical study

The electronic structure of zeolite A is developed in a step by step procedure from the simple O_hH₈Si₈O₁₂ molecule, to the _∞ ¹ [(-O)₂H₄[Si₈O₁₂)] chain, to the _∞ ² [(-O₄)(Si₈O₁₂)] layer, and finally to the silica zeolite A framework _∞ ³ [Si₂₄O₄₈]. It is remarkable how well the calculated band structures of both, _∞ ² [(-O)₄(Si₈O₁₂)] and _∞ ³ [Si₂₄O₄₈] correspond to the experimentally determined band structure of α-quartz with a Fermi level of -10.55 eV. The HOMO region consists in each case of nonbonding 2p-oxygen bands which in a localized language can be denoted as oxygen lone pairs ( | O<). We observe in each case the typical behaviour of an insulator with saturated valencies whose electronic structure can be described as being localized and is already present in the starting O_h-H₈Si₈O₁₂ molecule. The double-8-rings D8R of the _∞ ² [(-O)₄(Si₈O₁₂)] layer have a pore diameter of 4.1 Å, the same as the pore opening of zeolite A. It is large enough to accept up to four Ag, forming _∞ ² [(-O)₄(Si₈O₁₂)Ag_n], n = 1, 2, 3, 4, layers, suitable for modelling the electronic interactions between the zeolite cavity embedded silver clusters and between the clusters and the zeolite framework. With one Ag per D8R the band structure is simply a superposition of the 4d, 5s and 5p levels of a layer of nearly noninteracting Ag and the silicon dioxide layer. The Ag-d band lies below the oxygen lone pairs, the Ag-s band lies about 3 eV above the oxygen lone pairs, and the Ag-5p bands are in the antibonding silicon dioxide region. The first electronic transition is of oxygen lone pair to Ag-5s LMCT type. Increasing silver content results in progressive splitting of the 5σ Ag bands and shifts the first (Ag^m+ _n)? ← (| O<) charge transfer transition to lower energies. The filled Ag 4d-bands lie always significantly below the (| O<) HOCOs (highest occupied crystal orbitals) but their band width increases with increasing silver content. In all cases the zeolite environment separates the Ag clusters through antibonding Ag-(← O<) interactions so that the coupling remains weak and it makes sense to describe the Ag clusters in the D8R as quantum dots weakly interacting with each other.