Environmental Engineering Reference
In-Depth Information
leads to valuable information on the pore structures. A non-porous solid has a
single line passing the origin, while the line from the origin for the alpha plot of
the mesoporous system bends. The slopes of the straight line through the origin
and the line at the high as region give the total and mesoporous surface areas,
respectively. The type of the alpha plot suggests the presence of ultra micropores
and/or super micropores. Detailed analysis results will be shown for the micro-
pore analysis [43, 46].
In the discussion of the mesopore shape, the contact angle is assumed to be
zero (uniform adsorbed film formation). The lower hysteresis loop of the same
adsorbate encloses at a common relative pressure depending to the stability of
the adsorbed layer regardless of the different adsorbents due to the so called ten-
sile strength effect. This tensile strength effect is not sufficiently considered for
analysis of mesopore structures. The Kelvin equation provides the relationship
between the pore radius and the amount of adsorption at a relative pressure. Many
researchers developed a method for the calculation of the pore size distribution
on the basis of the Kelvin equation with a correction term for the thickness of
the multilayer adsorbed film. They so-called BJH (Barret-Joyer-Halenda) and DH
(Dollimore-Heal) methods have been widely used for such calculations. As math-
ematical details are shown in other articles, only the simple Fortran program for
the DH method. (This program can be easily used for the analysis of the meso-
pore size distribution).The thickness correction is done by the Dollimore-Heal
equation. One can calculate the mesopore size distribution for cylindrical or slit
shaped mesopores with this program. Therefore, the adsorption branch provides
more reliable results. However, the adsorption branch gives a wide distribution
compared to the desorption branch due to gradual uptake. Theoretical studies on
these points are still done [133].
The pore size distribution from the Kelvin equation should be limited to meso-
pores due to the ambiguity of the meniscus in the microporous region. It is well
known that the presence of micropores is essential for the adsorption of small
gas molecules on activated carbons. However, when the adsorbate is polymer,
dye or vitamin, only mesopores allow the adsorption of such giant molecules and
can keep even bacteria. The importance of mesopores has been pointed out not
only for the giant molecule adsorption, but also for the performance of new ap-
plications such as electric double layer capacitors. Thus, the design and control
of mesoporosity is very desirable both for the improvement of performance of
activated carbon and for the development of its new application fields [1-3].
Important parameters that greatly affect the adsorption performance of a po-
rous carbonaceous adsorbent are porosity and pore structure. Consequently, the
determination of pore size distribution (PSD) of carbon nano structures adsorbents
is of particular interest. For this purpose, various methods have been proposed to
study the structure of porous adsorbents. A direct but cumbersome experimen-
tal technique for the determination of PSD is to measure the saturated amount
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