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.