Table 1. Chemical composition of the Ag0-Aerogel determined by SEM-EDX and XPS.
The results in the first row were measured by Backscattered electron composition (BEC) mode of scanning electron microscopy (SEM, JEOL JSM IT100LA) with energy dispersive x-ray analysis (EDX). During the analysis, unlike reduced silver exchanged mordenite13, no obvious silver cluster in micrometer scale was observed on the pellet surface. The second row provides the composition measured by x-ray photoelectron spectroscopy (XPS, Scienta Omicron ESCA-2SR).
The reactions between Ag and organic iodides were proposed by Scheele et al.21; using CH3I as an example, the reaction is,\(\mathrm{2Ag\ +\ 2C}\mathrm{H}_{\mathrm{3}}\mathrm{I(g)\ \leftrightarrow\ 2AgI\ +\ }\mathrm{C}_{\mathrm{2}}\mathrm{H}_{\mathrm{6}}\mathrm{(g)}\). In this reaction, iodine binds with silver and the organic groups are released to the off-stream in gas form, which was also suggested by Zhou et al.22

Procedure Description

Using the continuous flow adsorption system, the mass changes of Ag0-Aerogel were recorded and the kinetic adsorption curves can be generated. In this presented work, the adsorption experiments of 104 ppbv and 1044 ppbv of CH3I at 100, 150 and 200 ℃ were performed. Since reaching equilibrium is not realistic at ppbv level concentrations, the experiments were stopped at approximately 300 hours.23 To satisfy the superficial velocity of 1.1 m/min,24 the flow rate of the gas through the adsorption column was set at 500 sccm (standard cubic centimeter per minute) for all experiments. Additionally, to prevent the concentration difference between the pellets, a single layer of Ag0-Aerogel (0.1-0.2 g) was carefully placed in the tray.
Before the start of the organic iodide adsorption, the pellets were air-dried at the same temperature as that of the adsorption experiment until the mass change in the past 24 hours is lower than 0.005 wt% (approximately 1/5 of the adsorption rate of 104 ppbv CH3I at 150 ℃). Especially, at 200℃, the drying process induced the loss of organic moiety of up to 9 -10 wt%, which a similar observation was reported in Matyáš and Engler’s thermogravimetric analysis (TGA).25
Therefore, to determine the effect of organic moiety loss, the comparison experiments were performed at 104 ppbv-CH3I-150℃ and 1044 ppbv-CH3I-150 ℃ using the Ag0-Aerogel with and without the organic moiety loss. The results showed no significant difference in the uptake curves (in wt%). To further accelerate the drying process (which usually takes weeks) and removing the organic moiety for all experiments, the pellets were vacuum dried overnight at 350 ℃ using the degas function of Surface Area and Porosity Analyzer (Micromeritics, ASAP 2020) and stored in N2 after the treatment. Similarly, after degassing, the adsorption curves (in wt%) and the maximum iodine capacities (approximately 37 – 38 wt%, measured by 50 ppmv I2 adsorption at 150 ℃) are similar to the untreated adsorbent. It is important to notice that during the 350 ℃ treatment, the organic moiety loss results in approximately 10 % loss of pellet mass. Therefore, the similar uptake rate by pellet mass indicates an approximately 10% loss of uptake rate by silver mass. However, this loss could be considered as a ‘worthy cost’ due to the successive significant increase of the organic iodide uptake rate at 200 ℃, which will be discussed in the following sections.

Results and Discussion

Adsorption Kinetics

The 104 ppbv and 1044 ppbv CH3I adsorptions on Ag0-Aerogel at 100, 150 and 200 ℃ were performed using the continuous flow adsorption system. For consistency purposes, pellets used for all trials were vacuum dried at 350 ℃ before the adsorption. After the pre-dry process, the pellets were moved to the adsorption system and the final water equilibrium process was performed before starting the adsorption. Once the mass change of the pellets in the past 24h was less than 0.005 wt%, the CH3I adsorption was started. During the CH3I adsorption, no significant mass gain/loss was observed neither in the initial part (first 1-2 days) nor after the adsorption ended and the desorption started, which indicating the physisorption of CH3I on Ag0-Aerogel is relatively minor.
The kinetic curves are shown in Figure 2, at 100 and 150 ℃, the tendency agrees well with the previous studies of I2 adsorption on Ag0Z24, that the adsorption rates increase slightly as the temperature increases. However, at 200 ℃, the uptake rate increases significantly (3 – 4 times higher) at 104 ppbv, and the curvature of the curve decreases at 1044 ppbv. To analyze such abnormal behaviors, the shrinking core model (SCM) was applied.