BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 7

sized from polymeric carbon precursors, ACFs contain narrow and uniform pore
size distributions with widths on the order of 1 nm. Images of ACFs from scan-
ning tunneling microscopy have revealed networks of elongated slit-shaped and
ellipsoid shaped pores. Edge terminations in graphitic layers are thought to be the
most reactive sites during the steam/carbon dioxide activation process, resulting
in a gradual lengthening of slit-shaped pores as a function of burn-off. ACFs sub-
jected to less burn-off will have smaller pore volumes and a greater abundance of
narrow pores widths. With longer activation times, the pore volume increases and
the pores grow wider. This offers a convenient control for an experimental study
of the correlation between pore structure and hydrogen adsorption. In the current
study, the pore size distribution (PSD) of activated carbon fibers is used to inter-
pret the enthalpy and the capacity of supercritical H 2 adsorption [2].
1.1.3 MOLECULAR SIEVING CARBONS (MSCS): PROPERTIES AND
APPLICATION
Molecular sieving carbons (MSCs) have a smaller pore size with a sharper
distribution in the range of micropores in comparison with other activated carbons
for gas and liquid phase adsorbates. They have been used for adsorbing and elimi-
nating pollutant samples with a very low concentration (ethylene gas adsorption
to keep fruits and vegetables fresh, filtering of hazardous gases in power plants,
etc.) An important application of these MSCs was developed in gas separation
systems [1, 2]. The adsorption rate of gas molecules, such as nitrogen, oxygen,
hydrogen and ethylene, depends strongly on the pore size of the MSC; the ad-
sorption rate of a gas becomes slower for the MSC with the smaller pore size.
The temperature also governs the rate of adsorption of a gas because of activated
diffusion of adsorbate molecules in micropores: the higher the temperature, the
faster the adsorption [47-49].
By controlling (swinging) these parameters, temperature and pressure of ad-
sorbate gas, gas separation can be performed. Depending on which parameter is
controlled, swing adsorption method is classified into two modes; temperature
swing adsorption (TSA) and pressure swing adsorption (PSA). Adsorption of ox-
ygen into the MSC completes within 5 min, but nitrogen is adsorbed very slowly,
less than 10% of equilibrium adsorption even after 15 min. From the column
of MSC, therefore, nitrogen rich gas comes out on the adsorption process, and
oxygen-rich gas is obtained on the desorption process. By using more than two
columns of MSC and repeating these adsorption/desorption processes, nitrogen
gas is isolated from oxygen. This swing adsorption method for gas separation has
advantages such as low energy cost, room temperature operation, and compact
equipment [50-53].