Silver-functionalized silica aerogel: towards an understanding of aging on iodine sorption performance

The silver-functionalized silica aerogel (Ag⁰-aerogel) is being developed for the removal and sequestration of iodine compounds from the off-gas of a nuclear fuel reprocessing plant. This material shows high selectivity and sorption capacity for iodine. However, its sorption performance decreases when exposed to air containing H₂O and NO at 150 °C for extended periods of time. This phenomenon is referred to as "aging" and simulates exposure of the sorbent during plant idling. This extensive study of unaged and aged samples of Ag⁰-aerogel with and without iodine revealed that decreased efficiency of I capture after NO-aging can be attributed to an increase in size of silver nanoparticles and by the formation of free sulfate on their surfaces from oxidized thiol groups. The smaller reactive surface areas of bigger particles and thin sulfate layer on particle surfaces prevented a complete utilization of the silver. By contrast, formation of silver sulfate appears to be the main factor in decreasing the sorption capacity for samples aged in dry or humid air. It is hypothesized that a short period exposure of the Ag⁰-aerogel to a reducing gas stream would reduce oxidized silver back to metal and sulfate to sulfide. This may recover the sorption performance of Ag⁰-aerogel close to original levels.