Table 3. The Ag and I concentrations measured by SEM-EDX and XPS (I compositions have been calibrated as additional mass).
Both SEM-EDX and XPS are the surface analysis techniques whereas the penetration depth of XPS is approximately 5-10 nm and that of SEM-EDX is 1-10 μm.48,49 Therefore, the higher iodine compositions for partially loaded Ag0-Aerogel observed by XPS indicate the existence of a strong concentration gradient in the pellet during the CH3I adsorption. Additionally, it can also be visualized that the concentrations of I on the surface are much higher than the experimental measured iodine uptakes (same as CH3I uptake, assuming CH3 group diffuse out in C2H6 form), which further supports the existence of the surface reaction proposed by Tang et al.23

Conclusion and Recommendation

Traditionally, after the adsorption, Ag0-Aerogel will be consolidated by compressing at high temperature, and removing the organic moieties at 350 ℃ before compressing benefits the consolidation results (higher product density and lower porosity).25In this presented work, a novel pre-treating method was applied. The Ag0-Aerogel was vacuum dried at 350 ℃ before the CH3I adsorption to remove the organic moieties, and the uptake rate and maximum iodine adsorption capacity remain similar to the untreated one. Therefore, removing the organic moieties before the adsorption could be a practicable alternative, and the potential iodine contamination during the traditional organic moieties removing process could be avoided.
In the 104 and 1044 ppbv CH3I adsorptions on Ag0-Aerogel at 100, 150 and 200 ℃, an abnormal behavior was observed in 104 ppbv adsorption at 200 ℃; the uptake rate was approximately 3 – 4 times higher than those of 100 and 150 ℃ adsorptions at the same concentration. The most intuitive explanation is the well-known Arrhenius relationship, the increase of temperature results in the increase of reaction rate and diffusivity.
Additionally, more potential explanations are proposed based on successive experimental and theoretical analyses. The nitrogen adsorption analyses were performed using the pellets at different drying conditions, and the results indicated that the increasing temperature decreases the water concentration in the pellet and therefore may increase the silver sites availability and the pore diffusivities of CH3I and the gas form product. The gas form product, believed to be C2H6, is considered as a ‘diffusion limitation’ for the adsorption process in the proposed reaction pathway, and the increase of its diffusivity may vary the reaction rate in another perspective.
Moreover, the XPS and SEM-EDX analyses were also performed and the results indicated that at 104 ppbv/ 200℃ condition, some additional Ag-I compounds were generated. By comparing the binding energies of the peaks with previous studies, we presumed that the additional Ag-I compounds may contain a certain amount of Ag2+. However, identifying the peak group 3 in Figure 9 remains unsolved. To further determine the composition formed in CH3I adsorption process, performing additional physical analyses such as regional scans of other elements in XPS, x-ray absorption spectroscopy (XAS) and Raman spectroscopy is recommended.
In the presented work, we observed an unusually high uptake rate for 104 ppbv CH3I adsorption on Ag0-Aerogel at 200 ℃, and suggested multiple explanations for this behavior. Our discoveries offer a new perspective in determining the optimum temperature for CH3I adsorptions, whereas a commonly used temperature is 150 ℃. However, for the purpose of carefulness and accuracy, we may not recommend 200 ℃ as the optimum adsorption temperature until further evidence has been revealed.

Acknowledgement

This research was funded by the Nuclear Energy University Program of the U.S. Department of Energy, Office of Nuclear Energy (Grant No. DE-NE0008761). A portion of the presented work may be used in Siqi Tang’s Ph.D. thesis and the technical report submitted to DOE.

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