Stability and Structures of Silver Subnanometer Clusters in EMT Zeolite with Maximum Aluminum Content

The stability of small silver clusters, Ag-n with n = 3, 4, 6, and 8, supported in a sodalite of EMT zeolite with a maximum possible aluminum content (Si/Al = 1) was studied with the density-functional-theory-based cluster approach. The results showed that various silver clusters, partially or completely reduced, can be stabilized in this zeolite with the maximum negative defects. Computed formation energies for two reduction reactions reveal that completely reduced Ag clusters can be stabilized through hydrogenation, giving some preference to the formation of hydrogenated Ag-6 species. The hydrogenated neutral AgnHm/EMT clusters remain stable at T = 300 K, as obtained from the analysis of the Born - Oppenheimer molecular dynamics trajectories. Ion exchange of Na+ by Ag+ cations is obtained to be exothermic with energy gain of 0.5 eV per Ag cation. The formation of Ag-O ionic bonds, known to stabilize metal clusters in zeolitic frameworks, is found to be preponderant in the structures with fewer than six silver atoms, which leads to strong geometrical deformations especially for Ag-3 and Ag-4 species. The largest possibly stabilized in the sodalite completely reduced silver cluster is Ag-8. Partially reduced intact structures are obtained stable for silver clusters with n >= 4. Formation energies predict a coexistence of silver exchanged cations and partially reduced silver clusters with several oxidation states in the sodalite. The density of states spectra (DOSS) of the supported completely and partially reduced Ag-4 and Ag-8 clusters are very similar despite the different cluster size, topology, and reduction degrees. DOSS of the host zeolite did not undergo any changes due to the interaction with silver clusters.