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.