Environmental Engineering Reference
In-Depth Information
nanotechnology attracted the attention of many scientists recently, the pore struc-
ture has been required to be controlled closely. When scientists wanted to express
that they are controlling pores in the nanometer scale, some of them preferred to
call the smallest pores nano-sized pores, instead of micro/mesopores [1, 2].
Pores can also be classified on the basis of their state, either open or closed. In
order to identify the pores by gas adsorption (a method which has frequently been
used for activated carbons), they must be exposed to the adsorbate gas. If some
pores are too small to accept gas molecules they cannot be recognized as pores
by the adsorbate gas molecules. These pores are called latent pores and include
closed pores. Closed pores are not necessarily in small size. Pores in carbon mate-
rials have been identified by different techniques depending mostly on their sizes.
Pores with nano-meter sizes, that is, micropores and mesopores, are identified by
the analysis of gas adsorption isotherms, mostly of nitrogen gas at 77 K [41, 46].
The basical theories, equipments, measurement practices, analysis procedures
and many results obtained by gas adsorption have been reviewed in different
publications. For macropores, mercury porosimetry has been frequently applied.
Identification of intrinsic pores, the interlayer space between hexagonal carbon
layers in the case of carbon materials, can be carried out by X-ray diffraction
(XRD). Recently, direct observation of extrinsic pores on the surface of carbon
materials has been reported using microscopy techniques coupled with image pro-
cessing techniques, namely scanning tunneling microscopy (STM) and atomic
force microscopy (AFM) and transmission electron microscopy (TEM) for mi-
cropores and mesopores, and scanning electron microscopy (SEM) and optical
microscopy for macropores [1-3].
The most important Pore Characterization methods are include: STM, AFM,
TEM, Gas adsorption, Calorimetric methods, Small-angle X-ray scattering
(SAXS), Small-angle Neutron Scattering (SANS), Positron Annihilation Lifetime
Spectroscopy (PALS), SEM, Optical Microscopy, Mercury Porosimetry and Mo-
lecular Resolution Porosimetry. Adsorption from solution using macromolecules
has been applied to macropore analysis, but we still need more examinations. It
is difficult to compare one adsorption isotherm with another, but determination of
the deviation from the linearity using a standard adsorption isotherm is accurate.
The plot constructed with the aid of standard data is called a comparison plot. The
representative comparison plots are the t and alpha plots [41, 42].
The molecular adsorption isotherm on non-porous solids, which can be well
described by the BET theory. The deviation from the linearity of the t-plot gives
information on the sort of pores, the average pore size, the surface area, and the
pore volume. However, the t plot analysis has the limited applicability to the
microporous system due to the absence of explicit monolayer adsorption. The
construction of the alpha plot does not need the monolayer capacity, so that it is
applicable to microporous solids. The straight line passing the origin guarantees
multilayer adsorption, that is, absence of meso- and/or micropores; the deviation
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