Identifying Accessible Ag+ Sites as the Key Active Sites Governing Xenon Capture in Ag-Exchanged FAU Zeolite

To address the challenge of identifying the true structure–performance relationship governing xenon adsorption in silver-exchanged zeolites, a series of Ag/13X adsorbents with tunable silver loadings were synthesized via liquid-phase ion exchange. The unique FAU-type topology of 13X zeolite, composed of spacious supercages interconnected by large 12-membered-ring windows, enables high silver dispersion while minimizing diffusion limitations. This unique structural advantage makes Ag/13X an ideal model system for investigating the intrinsic role of silver active sites in Xe adsorption. We identify a pronounced volcano-shaped dependence of Xe adsorption on silver loading, establishing that total silver content is not the governing factor for Xe capture. Combined structural characterization and spectroscopic analyses reveal that increasing silver loading continuously alters silver speciation from highly dispersed Ag+ ions toward more aggregated silver species, which may reduce the fraction of accessible Ag+-related adsorption sites and simultaneously modify the local electronic environment of silver species. At an optimal AgNO3 exchange concentration of 0.5 M, the predominance of well-dispersed Ag+ species maximizes accessible active-site density and leads to the highest Xe uptake. Charge-corrected GCMC simulations provide molecular-level validation by showing that these accessible Ag+ sites reinforce Xe adsorption through stronger ion-induced dipole interactions. Consequently, this work identifies the density of accessible Ag+ sites as an important descriptor for Xe capture, offering a design guideline for engineering high-efficiency noble-gas adsorbents.