Figure 3. The adsorption curve and
the modeling results of the water adsorption process. (Fick’s Law and
Micro PD are overlapped)
The adsorption curve and the modeling results are shown in Figure 3. The
diffusivities determine using Fick’s Law, SCM and Micro PD are
9.6×10-9 cm2/s,
2.6×10-8 cm2/s and
4.4×10-21 cm2/s respectively. By
comparing with multiple literature works28,29, the
values of the diffusivities clearly fit in the range of micropore
diffusion, indicating that the water adsorption/desorption in
Ag0-Aerogel is a micropore diffusion process.
Nitrogen Adsorption
Analyses
Since the carrier gas contains approximately 2 ppmv of water, the
Ag0-Aerogel tends to reach moisture equilibrium with
the ambient air during the adsorption of CH3I. The
moisture levels in the pellet at 100, 150, 200℃ were measured using the
pre-dried pellet and the continuous flow adsorption system. The
pre-dried (350 ℃ degassed) pellets were placed into the adsorption
system with identical settings except flowing pure air instead of
air/CH3I mixture. The temperature of the adsorption
column was firstly set at 200 ℃, and the pellets started to adsorb water
from the ambient air, with the mass change collected continuously. Once
the mass change in the past 24h was less than 0.005 wt%, the
temperature was decreased to 150 ℃ and further down to 100 ℃ when the
same criterion was met. The time to settle a new equilibrium was 100 –
150 hours.
The results indicated that the differences in water concentration
between each temperature were approximately 0.15 – 0.2 wt%. In other
words, if the water concentration in the pellets at 200 ℃ was set as the
zero point, the water adsorbed at 150 and 100 ℃ were approximately 0.2
and 0.4 wt%. To examine such observation, the water desorption
experiment in the reverse temperature order was also performed and the
differences between each level were comparable.
In order to determine the effect of different concentrations of moisture
in the pellets at various temperatures, the nitrogen adsorption analyses
were performed using the Surface Area and Porosity Analyzer
(Micromeritics, ASAP 2020). Traditionally, the samples used for nitrogen
adsorption analysis were fully dried to prevent any residual water to
influence the results. Since the standard drying conditions for
Ag0-Aerogel were not reported, the fully dried result
was the one with the highest pore volume and surface area, selected from
multiple 350 ℃ degassed trials.
Instead, to determine the surface area and pore volume at real
adsorption temperatures, the Ag0-Aerogel was air dried
using the adsorption column at the target temperatures (100, 150 and 200
℃) with the same criterion described above. Once the drying process was
completed, the pellets were transferred to ASAP 2020 as quickly as
possible (2-3 min). To minimize the potential water gain/loss during the
analyses, multiple methods were applied including, minimize the time of
transferring sample, measure the free volume of the test tube in advance
(therefore, exposing the dried sample under vacuum at room temperature
could be avoided), submerge the sample in liquid nitrogen immediately,
only start the vacuum pump after the sample was submerged, and minimize
the analysis time by selecting the minimal amount of sample points. Once
the analysis was completed, the sample was transferred back to the
adsorption column to measure the mass change during the porosity
analysis. Since no significant mass changes were observed, the water
gain/loss during the analysis appears to be minor.
As a reference of the highest level of water that could be contained in
the pellet during actual industrial usage, the nitrogen adsorption
experiment without any sample pretreatment was also performed using
normally stored Ag0-Aerogel (named as Room Condition
Equilibrium in Figure 4, short as RC.EQ). The storage conditions
measured by laboratory thermometer and hydrometer are, 20 – 23 ℃ and 20
– 30% humidity.