Role of Surface Functional Groups in the Adsorption Kinetics of Water Vapor on Microporous Activated Carbons

Abstract

Activated carbons have both hydrophilic surface oxygen functional groups, which act as primary adsorption centers for water vapor and hydrophobic graphene layers on which non-polar species are primarily adsorbed. The aim of this research was to investigate the effects of oxygen surface functional groups, in activated carbons, on the adsorption characteristics of water vapor. Activated carbon G, was oxidized using nitric acid and then heat treated in the range 387-894 K to produce a suite of adsorbents with varying oxygen contents in the range 0•4-21•5 %, but very similar porous structure characteristics, thereby minimizing effects due to changes in porous structure. The type and concentration of surface oxygen groups present on each sample was assessed using TPD, FTIR and Boehm titration methods. Water vapor adsorption at low relative pressure was dramatically enhanced by the presence of functional groups, in particular, carboxylic groups. Kinetic profiles for each pressure increment were modeled using a set of nested kinetic models, which allow the adsorption kinetics to be interpreted in relation to the adsorption mechanism. The results establish a clear relationship between water adsorption kinetics and the type and concentration of oxygen surface functional groups. A linear relationship was observed between the rate constants in the low pressure region and the inverse of the Henry's Law constant. This indicates the importance of adsorbate-adsorbent interactions in water adsorption kinetics and is consistent with a site-to-site hopping mechanism between functional groups.

Effects of active carbon pore size distributions on adsorption of toxic organic compounds

“…The rate limiting step in highly microporous carbons is pore entry. A number of dynamic models based on Fickian, barrier resistance/Fickian and linear driving force (LDF) model (Reid and Thomas 2001;Fletcher et al 2007;Rutherford and Coons 2004) have been used to describe adsorption by granular active carbons. For microporous carbons, where pore entry is rate determining, the kinetics follow the LDF model which takes the following form:…”

“…The DFT method has been used to extract additional pore size distributions from the nitrogen data. Kinetic profiles have been analysed using the Linear Driving Force method (Reid and Thomas 2001;Fletcher et al 2007;Rutherford and Coons 2004) where adsorption by an active carbon follows a pseudo first-order mass transfer process from the free adsorptive phase to the carbon surface/porosity. This allows the determination of an adsorption rate constant, k, which allows comparisons of adsorption kinetics for different adsorbate/carbon systems which can then be correlated with pore characteristics.…”

Effect of pH conditions on the depolymerization of Wucaiwan coal by mixed acids/ultrasound method and the product structures and performance

“…Second, sample 2 and 3 showed a major weight loss peak at 284 and 293°C, respectively, corresponding to the volatilization of small molecular compounds and the decomposition or dehydration condensation of oxygen-containing groups such as carboxyl and hydroxyl groups, this is distinctly different from the weight loss behavior of raw coal sample, it indicates that by treatment with oxidative HNO 3 , a large number of active oxygen-containing functional groups appeared in the coal structure. Noted that within this range sample 3 has lower temperatures for water loss peak and decomposition peak of oxygen-containing groups than that of sample 2, demonstrating that the content of oxygen-containing groups in sample 3 is higher than that in sample 2, because the existent of oxygen-containing groups can not only facilitate the decomposition and dehydration condensation of coal but also induce the pyrolysis of organic matter and decrease the pyrolysis temperature (Fletcher et al 2007;Ruiz et al 2006). Moreover, the oxygen-containing groups in sample 2 and 3 were basically removed by the first pyrolysis, resulting in higher dehydrogenation condensation coking temperature than that of raw coal sample during the second pyrolysis; therefore, both sample 2 and 3 exhibited weight loss peaks between 500 and 600°C.…”

Kinetic Analysis of the Adsorption of Polar and Nonpolar Molecules onto Ordered Mesoporous Silica Using the Pressure-feedback Method

“…Furthermore, knowledge about the rate of adsorption of water is important because the adsorption of the target gas is frequently obstructed by the adsorption of water. 10 Recently, the discovery of new porous materials, such as metalorganic frameworks (MOFs), porous coordination polymers (PCPs), and periodic mesoporous organosilicates (PMOs), 11 has enabled the design of pore structure and pore functionality. Most of the investigations on these materials have been focused on the equilibrium state of the adsorption process; the kinetics have not yet been investigated enough because they are difficult to measure.…”