Page 28 - Adsorption by Powders and Porous Solids

CHAPTER I. IIVRODUCTION 13

barriers between adsorption sites are small enough to be overcome easily at the oper-
ational temperature: the adsorbed molecules therefore retain two translational
degrees of freedom and can be regarded as mobile. On the other hand, if the energy
bhers are much larger than kT, the adsorbed molecules are said to be localized since
they spend most of their time on particular surface sites.
In the hypothetical case of a perfectly homogeneous surface, there is no variation
of #i (z) in the xy plane - see Figure 1.4a. It is more realistic to picture a uniform
which gives rise to energy wells of the same depth. Now, the potential
energy profdes corresponding to mobile and localized adsorption are shown respec-
tively in Figures 1.4b and 1.4~. In the former case, there is a random distribution of

I
Moblle adsorption on an ideal, homogeneous sulfate

J
Mobile adsorption in a uniform surface

I Localized adsorption bn a uniform surface J

a heterogeneous surface A
I
Mixed mobile and localized adsorption on
Figure 1.4. Distribution of adsorbed molecules on hypothetical surfaces (left) and corresponding vari-
ations of potential energy (right). M, mobile; L, localized. Adsorbate-adsorbate interactions are not taken
into account.