Due to its high iodine selectivity and sorption capacity, silver-functionalized silica aerogel (Ag0-Aerogel) has considerable potential as a material for the capture of iodine radioisotopes released as byproducts of spent nuclear fuel reprocessing. Ag0-Aerogel, nonetheless, experiences a steady capacity loss when exposed to other off-gas components. This sorption capacity reduction is known as aging though its governing processes remain unclear. To investigate aging, we exposed Ag0-Aerogel samples to 2% NO2 in dry air (i.e. a relevant gas stream with a strong aging effect) at 150 C for up to one month. The pristine Ag0-Aerogel and aged samples were then characterized using Scanning Electron Microscopy, Tunnel Electron Microscopy, X-ray Diffraction, Fourier-transform infrared spectroscopy, X-ray Photoelectron Spectroscopy and X-ray Absorption Spectroscopy. Our results coherently indicate that a pristine Ag0-Aerogel contains three Ag species: (i) Ag0 in the form of metallic nanoparticles that represent approximately 20 mol % of the system’s total Ag, (ii) Ag propanethiolates bound to the metallic nanoparticles as a covering layer or distributed on the silica aerogel backbone as a consequence of incomplete reduction, and (iii) Ag-S complexes (e.g., AgmSn) with some similarities to, but that are nevertheless distinct from, crystalline Ag2S. Aging of Ag0-Aerogel in NO2 involves two predominant processes: (i) propanethiolate is oxidized first to propanesulfonate and then to sulfate ultimately forming silver sulfate molecules; (ii) silver sulfate molecules then aggregate together yielding silver sulfate crystals. Iodine adsorption on these silver sulfate crystals is unfavorable which directly reduces the iodine capacity. Plausible reaction pathways and aggregation mechanisms are explored. These findings elucidate the physical chemical processes that lead to aging in Ag0-Aerogel, and may be used to guide the development of accurate predictive models and more advanced materials for iodine capture.

Iodine capture using silver-functionalized silica aerogel: How aging affects the adsorption performance?

ACS Spring 2022

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