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

Abstract

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 NO 2 in the off-gas. This phenomenon is known as aging and its governing processes remain unclear. We exposed Ag 0 -Aerogel samples to 2% NO 2 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, X-ray 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 Ag 2 S. We unraveled the two predominant underlying processes of aging in NO 2 : (i) Ag–S-r is oxidized by NO 2 forming silver sulfonate (Ag–SO 3 -r) which is oxidized further to silver sulfate (Ag 2 SO 4 ) molecules, (ii) Ag 2 SO 4 molecules then migrate from the pores to the aerogel surface and aggregate yielding Ag 2 SO 4 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. • The iodine capacity of Ag-aerogel is reduced by NO 2 at elevated temperatures. • Ag nanoparticles and Ag–S compounds coherently adsorb iodine. • NO 2 oxidizes Ag-thiolates forming Ag 2 SO 4 molecules that favor iodine adsorption. • Ag 2 SO 4 molecules migrate and accumulate on particles preventing iodine adsorption. • Thiolation step directly impacts Ag-aerogel aging in NO 2 .