Silver-functionalized silica aerogel for iodine capture: Adsorbent aging by NO2 in spent nuclear fuel reprocessing off-gas

Silver-functionalized silica aerogel (Ag-0-Aerogel) is considered a promising porous adsorbent for the capture of iodine from nuclear fuel reprocessing off-gas. The Ag-0-Aerogel, nevertheless, experiences a steady loss of capacity when exposed to NO2 in the off-gas. This phenomenon is known as aging and its governing processes remain unclear. We exposed Ag-0-Aerogel samples to 2% NO2 in dry air at 150 C for up to 1 month, followed by I-2 loading. Samples were characterized using scanning electron microscopy, transmission electron microscopy, Xray diffraction, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. Our results elucidated the precise Ag species in Ag-0-Aerogel that adsorb I-2: (i) Ag-0 nanoparticles, (ii) Ag thiolates (Ag-S-r) covering the nanoparticles and the aerogel backbone, and (iii) Ag-S complexes including amorphous Ag2S. We unraveled the two predominant underlying processes of aging in NO2: (i) Ag-S-r is oxidized by NO2 forming silver sulfonate (Ag-SO3-r) which is oxidized further to silver sulfate (Ag2SO4) molecules, (ii) Ag2SO4 molecules then migrate from the pores to the aerogel surface and aggregate yielding Ag2SO4 particles that do not adsorb I-2. Plausible reaction pathways and aggregation mechanisms are explored. These findings may be used to guide the discovery of more advanced materials for iodine capture and the development of accurate predictive models.