Page 59 - Gas Adsorption Equilibria

1. Basic Concepts 45

use to calculate the pore spectrum of the sorbent from a measured gas
adsorption isotherm. Naturally, they often will lead to different results.
Examples for these methods are the t-plot method, the the
Horvath-Kavazoe procedure, the density functional theory (DFT) procedure
and also its non-local generalization (NLDFT). All of these are presented and
discussed in today’s literature [1.2, 1.42]. Actually, the above mentioned
difficulties with gas adsorption measurements are condensed in two important
draft documents of the International Standardization Organization (ISO),
Geneva [1.55, 1.56] providing recommendations for both the experimental
procedure of measuring gas adsorption isotherms and to calculate from these
data the macro-, meso-, and micropore spectrum of the solid material
considered, However, it must be emphasized that these documents today
(2004) are still drafts and do not provide international standards. Even though,
it can be expected that they sometime will be promoted, probably in an only
slightly different form.

A characteristic parameter of a porous material is its so-called BET-
surface. This is the surface of a monolayer adsorbate of at
boiling temperature of at namely 77.3 K [1.3]. It is determined
by nitrogen adsorption experiments at this temperature for pressures
The mass of the monolayer load is determined by fitting data
to the adsorption isotherm equation developed by Brunauer, Emmett and
Teller in 1938, originally designed for multilayer adsorbates [1.1-1.3, 1.57].

Here (C, ) are parameters to be determined by a data fitting procedure,
preferably restricted to the region A theoretical analysis of
Eq. (1.9) is presented in brief in Sect. 3.4 of Chap. 7.

Assuming a cross section of per [1.3], the
BET-surface of the monolayer simply can be calculated as

being the molar mass of nitrogen. An example for such
measurements is given in Figure 1.12, [1.36]. It presents Gibbs excess masses
of nitrogen adsorbed on reference material CRM BAM-PM-104 at 77 K
determined by the volumetric/manometric method in a closed
system and without carrier gas. Data show hysteresis between the adsorption-