Aging kinetics on silver-functionalized silica aerogel in off-gas streams including dry air, humid air, NO and NO2

Silver Sulfide and Silver Sulfate as Aging Byproducts and Adsorbents for Gaseous Iodine Capture in Spent Nuclear Fuel Reprocessing

“…The primary motivation of this study is to provide experimental evidence for our previous studies on the aging of Ag adsorbents, i.e., how the major aging byproducts of Ag 0 -aerogel, Ag 2 S and Ag 2 SO 4 , react with I 2 . , Although the Ag 2 S and Ag 2 SO 4 used in this study were commercial products, the similarities between their characterization patterns and those of the aging byproducts that we have previously observed suggest they are appropriate representatives. , In the NO 2 aged Ag 0 -aerogel, Ag 2 SO 4 was generated via the oxidation of Ag thiolates and eventually aggregated into the form of crystalline particles . The adsorption experiments in this study validate our previous hypothesis that Ag 2 SO 4 particles are generally unable to capture I 2 .…”

“…Adsorption experiments were performed using the continuous-flow system that was described in our previous works (Figure ). − , Specifically, the Ag 2 S and Ag 2 SO 4 powders (∼200 mg) were packed one at a time as a thin layer onto a sample holding plate. Before I 2 loading, the Ag 2 S and Ag 2 SO 4 samples were first dried for 4 h at 150 °C in a stream of compressed dry air (Ultra Zero grade, dew point < −70 °C) with a flow rate of 500 mL/min to remove excess physisorbed water.…”

“…Thus, while the formation of AgNO 3 is an important component of the aging process, it does not directly diminish the capacity of the mordenite. In our most recent investigations of Ag 0 -aerogel, crystallized Ag 2 SO 4 particles at micrometer sizes were observed in NO 2 aged samples, while Ag 2 S crystals were observed after aging in dry air, humid air, and 1% NO in N 2 . The formation of these byproducts was attributed to the Ag–S species that are produced when the Ag 0 nanoparticles react with the thiol (−SH) functional group of Ag 0 -aerogel .…”

“…One major obstacle to the use of Ag adsorbents is, however, their tendency to steadily lose capacity (i.e., age) when they are exposed to other SNF reprocessing off-gas components (e.g., O 2 , H 2 O, NO x ) at elevated temperatures for prolonged periods of time. Our previous studies have focused on the processes governing aging in two prototypical porous Ag adsorbents, Ag 0 Z and reduced Ag functionalized silica aerogel (Ag 0 -aerogel), in gas streams containing the aforementioned off-gas components. − Ag 0 -aerogel is a relatively new material developed at Pacific Northwest National Laboratory (PNNL), and its high Ag content and adequate specific surface area aid in iodine capture, making it an appealing alternative to zeolites that have traditionally been the primary focus in this area. , By exposing Ag 0 Z and Ag 0 -aerogel to gas streams containing the aforementioned off-gas components at 150 °C, we found that NO 2 significantly reduced the I 2 capacity for both materials (∼90% reduction after 1 month), , while O 2 , H 2 O, and NO were less impactful (30%–80% reduction for Ag 0 Z and ∼20% reduction for Ag 0 -aerogel after 1 month). , Though the existence of the aging phenomenon is well established, there are still, nevertheless, some discrepancies that arise when attempting to understand its underlying processes.…”

“…As such, this manuscript is intended to facilitate our understanding of the aging process in Ag 0 -aerogel by examining the adsorption of I 2 on Ag 2 S and Ag 2 SO 4 . Commercial Ag 2 S and Ag 2 SO 4 powders underwent adsorption in a dry air steam with 50 ppmv I 2 at 150 °C (a prototypical I 2 -loading conditions applied to other Ag adsorbents). − , The mass change was recorded to help us calculate the I 2 capacity. To examine the evolution of the adsorbents’ physical and chemical properties, we characterized pristine and I 2 -loaded samples through scanning electron microscopy with energy dispersive X-ray spectroscopy analysis (SEM-EDX), powder X-ray diffraction (PXRD), diffuse reflectance UV–visible spectroscopy (UV–vis DRS), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS).…”