BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 54

of adsorbed probe molecules, which have different dimensions. However, there
is uncertainty about this method because of networking effects of some adsor-
bents including activated carbons and carbon nanostructures. Other experimental
techniques that usually implement for characterizing the pore structure of porous
materials are mercury porosimetry, XRD or SAXS, and immersion calorimetry. A
large number of simple and sophisticated models have been presented to obtain a
realistic estimation of PSD of porous adsorbents. Relatively simple but restricted
applicable methods such as Barret, Joyner and Halenda (BJH), Dollimore and
Heal (DH), Mikhail et al. (MP), Horvath and Kawazoe (HK), Jaroniec and Choma
(JC), Wojsz and Rozwadowski (WR), Kruk-Jaroniec-Sayari (KJS), and Nguyen
and Do (ND) were presented from 1951 to 1999 by various researchers for the
prediction of PSD from the adsorption isotherms [133, 139].
For example, the BJH method, which is usually recommended, for mesopo-
rous materials is in error even in large pores with dimension of 20 nm. The main
criticism of the MP method, in addition to the uncertainty regarding the multilayer
adsorption mechanism in micropores, is that we should have a judicious choice
of the reference isotherm. HK model was developed for calculating micropore
size distribution of slit-shaped pore; however, the HK method suffers from the
idealization of the micropore filling process. Extension of this theory for cylindri-
cal and spherical pores was made by Saito and Foley and Cheng and Ralph. By
applying some modifications on the HK theory, some improved models for calcu-
lating PSD of porous adsorbents have been presented. Gauden et al. extended the
Nguyen and Do method for the determination of the bimodal PSD of various car-
bonaceous materials from a variety of synthetic and experimental data. The pore
range of applicability of this model besides other limitations of ND method is its
main constraint. In 1985, Bunke and Gelbin determined the PSD of activated car-
bons based on liquid chromatography (LC). Choices of suitable solvent and pore
range of applicability of this method are two main problems that restrict its gen-
eral applicability. More sophisticated methods such as molecular dynamics (MD),
Monte Carlo simulation, Grand Canonical Monte Carlo simulations (GCMS), and
density functional theory (DFT) are theoretically capable of describing adsorption
in the pore system. The advantages of these methods are that they can apply on
wide range of pores width. But, they are relatively complicated and provide ac-
curate PSD estimation based on just some adsorbates with specified shapes [133,
140, 145].
1.1.10 NOVEL METHODS FOR CONTROLLING OF PORE STRUCTURE
IN CARBON MATERIALS
1.1.10.1 ACTIVATION
Activation processes are often classified into two, gas activation and chemical
activation. Different oxidizing gases, such as air, CO 2 , and water vapor, were used
for gas activation. For chemical activation, ZnC1 2 and KOH were used as an