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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