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 ).