Adsorption of Methyl Iodide on Reduced Silver-Functionalized Silica Aerogel: Kinetics and Modeling

Abstract

The low concentration methyl iodides (CH 3 I) adsorption process on reduced silver-functionalized silica aerogel (Ag 0 -Aerogel) was studied. The kinetic data were acquired using a continuous flow adsorption system. Because the corresponding physical process was observed, the shrinking core model (SCM) was modified and applied. An average CH 3 I pore diffusivity was calculated, the CH 3 I-Ag 0 -Aerogel reaction was identified as a 1.37 order reaction instead of first order reaction, and the n th order reaction rate constant was determined. This modified SCM significantly increases the accuracy of adsorption behavior prediction at low adsorbate concentration. Modeling results indicate that the overall adsorption process is controlled by the pore diffusion. However, at low adsorbate concentration (<100 ppbv), the CH 3 I adsorption is limited to the surface reaction due to the low uptake rate in a predictable time period.

Methyl Iodide Adsorption on Reduced Silver-Functionalized Silica Aerogel: How Temperature Impacts the Adsorption?

“…48,49 Therefore, the higher iodine compositions for partially loaded Ag 0 -Aerogel observed by XPS indicate the existence of a strong concentration gradient in the pellet during the CH 3 I adsorption. Additionally, it can also be visualized that the concentrations of I on the surface are much higher than the experimental measured iodine uptakes (same as CH 3 I uptake, assuming CH 3 group diffuse out in C 2 H 6 form), which further supports the existence of the surface reaction proposed by Tang et al 23…”

“…For the chemisorption with gas phase product generated, Zhou and White 32 and Robb and Harriott 33 suggested that the formation of gases and the successive diffusion limitations may result in a fractional order reaction, which has been observed in the CH 3 I adsorption on Ag 0 -Aerogel by Tang et al 23 To explain this observation, a potential reaction pathway is proposed. Figure 6.…”

“…Since reaching equilibrium is not realistic at ppbv level concentrations, the experiments were stopped at approximately 300 hours. 23 To satisfy the superficial velocity of 1.1 m/min, 24 the flow rate of the gas through the adsorption column was set at 500 sccm (standard cubic centimeter per minute) for all experiments. Additionally, to prevent the concentration difference between the pellets, a single layer of Ag 0 -Aerogel (0.1-0.2 g) was carefully placed in the tray.…”

“…Two CH 3 * radicals bind with each other and form C 2 H 6 (2); I * radical binds with Ag and forms AgI (3); and C 2 H 6 diffuses out through the reacted layer (4). In this process, to satisfy the observed 1.4 reaction order by Tang et al 23 , the reaction order of CH 3 I is assumed to be n ; the orders of other reactions are 1 to each component; and the reason will be discussed in the following contents.…”

“…The well-known Arrhenius relationship is commonly used in describing the temperature dependence of reaction rate constant and diffusivity, which is, 35 and the linear form is, Where k s 0 is the pre-exponential factor (unit same as k s ),ΔΕ is the activation energy (kJ/mol) and R is the gas constant (kJ/mol/K). Reported by Tang et al 23 , the CH 3 I adsorption on Ag 0 -Aerogel may be a 1.4 order shrinking core process, where the calibratedk s * ((cm/s)[? ](mol/cm 3 ) 1-n ) can be represented as .…”