There are
t-plot, HK, SF, DR-plot, NLDFT and GCMC method for the evaluation of micropore.
t-plot and DR-plot are used to determine the pore volume and separation of internal
and external surface area of the particle. HK, SF, NLDFT and GCMC method are
used to determine the pore size distribution.
Since the
micropore analysis theories must describe the short-range interaction of
adsorbate and pore wall, it is not as easy as describing the flat surface
adsorption or mesopore adsorption. The typical assumption of these theories is
that the pore shape is a slit or cylinder. As the parameters, the surface atoms
of pore wall and adsorbate molecules must be selected (e.g. oxygen/carbon, N2/Ar). If the sample has uniform and
homogeneous pores, the calculated pore size will be accurate. However, most of
real materials have nonuniform and heterogeneous pores which are not fit to the
assumption of the theories. This disagreement is true not only for the pore
size distribution obtained from the gas adsorption but also for other
porosimetry and particle size measurement. The gas adsorption so far is the
best method for the evaluation of micropores compared to other methods because
the probe gas molecule size is below nm to detect micropores.
Our recommend
method of micropore analysis is as follows: For zeolitic materials, measure
them with the Ar adsorption isotherm at 87K and analyze by the cylindrical pore
model theory (SF, NLDFT, and GCMC). N2 molecules, which have strong quadrupole
moment, strongly attract to the cation sites and OH group on the surface. For
activated carbon materials, they are often measured with N2 adsorption isotherm at 77K and analyzed
by the slit pore model theory (HK, NLDFT, and GCMC).
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