Metallic clusters and color changes in silver-exchanged zeolites: ¹⁰⁹Ag solid state NMR and optical studies

We have applied ¹⁰⁹Ag solid-state NMR and optical methods to probe dynamics and local environment of silver cations in a series of silver-exchanged zeolites as a function of hydration. In dehydrated samples of Ag-Rho, a Knight shift produced by the conduction electrons of metallic silver particles with cluster size above 400 Å is observed. Temperature dependent color change upon dehydration in zeolites Ag-X and Ag-A is also present. Although the color change in both zeolites is optically similar, NMR spectra demonstrate that fundamentally different effects lead to the color change. The color change in Ag-X zeolite is accompanied by stronger interaction of silver cations with the zeolite substrate that causes a paramagnetic contribution to the Ag shift as dehydration occurs. In Ag-A zeolite the loss of coordinated water dominates the dynamics of silver cations and leads to a more shielded Ag NMR signal upon dehydration.