Environmental Engineering Reference
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eling the system for PSD calculating, first BET model to determine the surface
area of each sample and the quantities evaluated from adsorption. The sample
cell was then placed in a liquid nitrogen bath, which created an analysis tem-
perature of approximately 77 K with nitrogen gas used as the adsorbate. The
density functional theory (DFT) was used to develop the pore size distribution
data in the micropores (<2 nm) from the nitrogen isotherm with a relative pres-
sure range from 10
-6
to 1, while the Barrett, Joyner, Halenda (BJH) model was
used for all pores greater than 2 nm. In recent works the complexity of crystallo-
graphical and geometrical structure of many solids and their complex chemical
composition are the main sources of adsorbent heterogeneity. As it was pointed
out by Jaroniec and Brauer the main sources of surface heterogeneity are the
following: different types of crystal planes, growth steps, crystal edges and cor-
ners, various atoms and functional groups exposed at the surface and available
for adsorption, irregularities in crystallographical structure of surface and im-
purities strongly bounded with the surface. Clearly, surface heterogeneity plays
generally very important role in adsorption on solids. The process of adsorption
in micropores is strongly affected by the presence of small pores of different
dimensions and geometrical forms. Furthermore it is well known that uniform
micropores are the source of the energetic heterogeneity in the sense that mol-
ecules adsorb in these micropores with different adsorption energies. This is
caused by the change of adsorption potential field in the micropore volume. So,
for microporous solids absolute heterogeneity should be identified as a super-
position of non-uniformity of microporous structure and sources of, mentioned
above, surface heterogeneity. At the first sight the complete description of such
three-dimensional complicated structure is very difficult. Up to the present only
simplified models have been developed. Clearly, different models rely on dif-
ferent assumptions in order to obtain relationships allowing the calculation of
the main characteristics of adsorbent structural heterogeneity. Let us consider
the assumptions introduced by Astinov and Do while developed the improved
HK method [126, 133]:
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