BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 104

constrictions in the microporous network may cause molecular sieve effects and
molecular shape selectivity [1, 2, 133].
Diffusion effects may also occur when using N 2 at 77 K as the adsorbate since
at such low temperatures the kinetic energy may be insufficient to penetrate all
the micropores. For this reason adsorption of CO 2 at higher temperatures (273 K)
is also used. CO 2 and N 2 isotherms are complementary. Thus, whereas from the
CO 2 isotherm micropores of up to approximately 10 −9 m width can be measured,
the N 2 can be used to test larger pores. Despite these limitations the BET surface
area is the parameter most commonly used to characterize the specific surface
area of carbon adsorbents. On the basis of volume-filling mechanism and thermo-
dynamic considerations, Dubinin and Radushkevich found empirically that the
characteristic curves obtained using the Potential Theory for adsorption on many
microporous carbons could be linearized using the Dubinin-Radushkevich (DR)
equation. For some microporous carbons the DR equation is linear over many or-
ders of magnitude of pressure. For others, however, deviations from the DR equa-
tion are found. For such cases the Dubinin-Astakhov equation has been proposed
in which the exponent of the DR equation is replaced by a third adjustable param-
eter, n, where 1 <n < 3.Both the BET and the Dubinin models are widely thought
to adequately describe the physical adsorption of gases on solid carbons. BET
surface areas from many microporous carbons range from 500 to 1500 m 2 g -1 .
However, values of up to 4000 m 2 g -1 are found for some super activated carbons
and these are unrealistically high. The relatively high values of the surface areas
of activated carbons are mainly due to the contribution of the micropores and
most of the adsorption takes place in these pores. At least90-95% of the total sur-
face area of an activated carbon may correspond to micropores. However, meso
and macropores also play a very important role in any adsorption process since
they serve as the passage through which the adsorbate reaches the micropores.
Thus, the mesopores, which branch off from the macropores, serve as passages
for the adsorptive to reach the micropores. In such mesopores capillary condensa-
tion may occur with the formation of a meniscus in the adsorbate. Although the
surface area of the mesopores is relatively low inmost activated carbons, some
may have a well-developed mesoporosity (200 m 2 g -1 or even more). In addition,
depending on the size of the adsorbate molecules, especially in the case of some
organic molecules of a large size, molecular sieve effects may occur either be-
cause the pore width is narrower than the molecules of the adsorbate or because
the shape of the pores does not allow the molecules of the adsorbate to penetrate
into the micropores. Thus, slit-shaped micropores formed by the spaces between
the carbon layer planes are not accessible to molecules of a spherical geometry,
which have a diameter larger than the pore width. This means that the specific
surface area of a carbon is not necessarily proportional to the adsorption capacity
of the activated carbon. Pore size distribution, therefore, is a factor that cannot
be ignored. The suitability of a given activated carbon for a given application