Silver Ion-Exchanged Anionic Metal-Organic Frameworks for Iodine Adsorption: Silver Species Evolution from Ions to Nanoparticles

Ag clusters and nanoparticles have attracted much attention in the fields of environmental catalysis and pollutant removal. It is a common strategy to fabricate highly dispersed Ag nanoparticles in porous scaffolds via in situ growth. Here, anionic metal-organic frameworks (MOFs) have been employed to provide robust supports for Ag species. Ag nanoparticles with well-defined sizes are observed in MOFs when internal NH2Me2+ is exchanged with extrinsic Ag+ ions. The extended X-ray absorption fine structure and X-ray photoelectron spectroscopy spectra confirm that about 20% of Ag+ are reduced into Ag-0, while the rest 80% are still Ag+ species. The comparison of Ag loading into anionic and neutral Cu-BTC MOFs suggests that the exchangeable cations facilitate Ag+ migration into the interior of MOFs and thus alleviate its aggregation and spontaneous reduction. Besides, the adsorption tests verify that all the Ag species in MOFs are accessible to iodine vapor to form AgI. The charge transfer from frameworks to trapped iodine and the water in the air promote the formation of iodide ions, which react with Ag+ to produce AgI. The iodine adsorption performance of MOF beads was also evaluated using the pore diffusion model. This work provides insights into the evolution of Ag species in MOFs and potential Ag-based adsorbents for radioiodine capture.