Figure 4. The surface area and pore volume of micropores, macropores and total pores measured using nitrogen titration method at different drying conditions.
The micro, macro and total pore surface area and volume are plotted in Figure 4. The total pore surface area of the 350 ℃ degassed sample is 297.6 m2/g and the total pore volume is 0.34 cm3/g, which are comparable with the trends of surface area and pore volume to silver composition in silica aerogel reported by Balkis Ameen et al.30 Additionally, the plot shows that as the temperature decreases and the in-pellet water concentration increases, both pore surface area and pore volume decrease. As the temperature decreases to 100 ℃, the micropores are significantly blocked by water and become fully blocked at room condition equilibrium.
Moreover, the diffusion of CH3I and related gas form products in Ag0-Aerogel may also be impacted by the porosity change. The porosity (ε = total pore volume / density) of the pellet is larger at higher temperature, and an experimental relationship between pore diffusivity and porosity is given as,31
where m is usually between 2 – 4.5 depending on the material. This relationship indicates that the pore diffusivity of both CH3I and gas form products may increase with increasing temperature, and therefore decreasing the diffusion limitations and accelerating the adsorption process.

Pore Distribution and Silver Sites Availability

Shown in Figure 4, the drying temperature impacts the pore surface area and the pore volume of the pellets by changing the water concentration in the pellet. When temperature decreases to below 100 ℃, the micropores are significantly impacted. This can be further visualized by comparing the pore distribution plots of the Barrett-Joyner-Halenda (BJH) desorption method and the density function theory (DFT) of 350 ℃ degassed and room condition equilibrium samples.
The pore distribution plots (Figure 5) show that the Ag0-Aerogel mostly consists of mesopores (20-500 Å) and micropores (<20 Å) with a limited amount of macropores (> 500 Å), which agree with the SEM images that no significant structure was observed in the micrometer scale. Comparing the two curves in Figure 5. b, two sharp peaks at approximately 6 and 12 Å were observed only in the fully dried sample, whereas the room condition equilibrium one only contained a small peak at 15-17 Å. A similar trend is also revealed in Figure 5. a: the 350 ℃ degassed sample contains a ‘tail’ at 20-25 Å.
Such observation further supports that the micropores (< 2 nm) are mostly blocked as the in-pellet water concentration increases (Discussed in Nitrogen Adsorption Analyses ) and the water adsorption/desorption process in Ag0-Aerogel is a micropore diffusion process (Discussed in Water Equilibrium in Drying Process ).