Table 2. Parameters and results of
the SCM.
It can be visualized that, at 200 ℃, certain abnormal behaviors exist
for both 104 and 1044 ppbv CH3I adsorption. To explain
such observation, the nitrogen titration, XPS and SEM-EDX analyses were
performed and a potential reaction pathway was proposed.
Water Equilibrium in Drying
Process
Because of multiple factors including the pellet size of
Ag0-Aerogel, the internal structure of
Ag0-Aerogel, etc., the water concentration in the
pellets after 350 ℃ vacuum drying may not be constant. Therefore, a
final drying is still required before the CH3I
adsorption and the pellets may adsorb or desorb water in different
trials. The water adsorbed was from the trace amount of moisture
contained in the carrier gas (Airgas Inc. Air, Ultra Zero). According to
the manufacturer, the air contains approximately 2 ppmv of water and the
value was also validated by experimental measurement using a dewpoint
meter. By selecting a water adsorption curve with the highest uptake
(therefore the initial water concentration in the pellet can be assumed
to be zero) , the water adsorption process can be modeled using multiple
models including the 1-D spherical Fick’s Law26 (Eq.
10), differential form of the diffusion term of the shrinking core model
(SCM)26 (Eq. 11), and the micropore term of the pore
diffusion model (Micro PD)13 (Eq. 12). For Fick’s law
and the shrinking core model, it is assumed that an equilibrium is
established on the surface immediately.
q(r,t) is the sorbate concentration (mol/cm3)
at radius r (cm) at time t (s), and the average sorbate
concentration q(t) can be calculated using . D is the pore
diffusivity (cm2/s), r is the radius at timet , V is the volume of the pellet (cm),Di is the micropore diffusivity
(cm2/s) and Ri is the radius of
the micropore (cm). Ri was measured by the
nitrogen adsorption method using ASAP 2020 and the value is
approximately 6.6 Å.