Page 34 - Gas Adsorption Equilibria
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20 Chapter 1
Physico-chemical adsorption phenomena are characterized by weak
interactions of admolecules and sorbent atoms or molecules. However, due to
catalytic properties of the sorbent surface either dissociations or fairly strong
associations between admolecules may occur.
In chemisorption systems admolecules are normally strongly bound to the
surface atoms or molecules of the sorbent material and are subject to
chemical reactions. They also cannot reversibly be desorbed from the sorbent,
but only irreversibly by which the sorbent material is changed.
Though a clear decision between physisorption and chemisorption states is
not always possible – examples for this are ammonia or water sorption on
hydrophilic zeolites – it seems to be worthwhile to illustrate their basic
differences qualitatively in Table 1.1 as follows [1.2, 1.3, 1.11]:
We here restrict in what follows to physisorption phenomena. However,
some of the examples presented in the subsequent Chapters refer to physico-
chemical adsorption systems, cp. Fig. 6.29, and chemisorption systems, cp.
Fig. 3.24.
Physisorbates, i. e. adsorbed phases caused by physisorption phenomena
can exhibit many different structures reflecting the underlying molecular
mechanism. The most often types of these can be described as follows:
1. Monolayer adsorbates
Sorbent offers many energetically nearly homogenous adsorption sites.
Sorptive gas pressure (p) is well below saturation pressure at system
temperature:
