Chemical Reactions on Solid Surfaces - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 415

n AS

0

0

C A

FIGURE 9.10 A schematic of nonideal isotherm with
adsorbate e adsorbate interactions.

FIGURE 9.11 Uniform hard spheres packed
closely on a flat surface.

flat “hard surface”. Therefore, there can be up to seven pseudo-layers of adsorption for hard
spheres on flat surfaces as there are differences on the number of interacting hard spheres.
For real adsorbate and adsorbent surface, the situation can be more completed. Therefore,
the apparent layer of adsorption can be more than two. While the adsorbate e adsorbent
interaction can be chemical (i.e. chemical bonds formed), the interactions between adsorbate
and adsorbate near or at contact is physical. The orderly state of adsorbate molecules on
the adsorbent surface is at a lower energy well than that in the bulk fluid phase. While the
actual physical orientation of the adsorbate molecules on the adsorbent surface is monolayer,
the adsorption mechanism needs not be. Thus, modeling the adsorbate e adsorbent and
adsorbate e adsorbent e adsorbate interactions is best to be accomplished with multiple layers.
We will deal with this idealization in the proceeding sections.

9.1.4.1. Chemisorption, Physisorption and the BET Theory
We have so far been focused on the interactions between the adsorbent surface and the
adsorbate molecules. The difference on the adsorbent surface has led us to a pseudo-two
layer adsorption model. The adsorption models so far are applicable to both chemisorptions
and physisorption. What happens if more than one layers of adsorbate can truly be adsorbed
on the surface? This would lead to pure physical adsorptions, since the interaction between
the adsorbate molecules and the adsorbent surfaces become negligible on layers above the
adsorbed molecules. If we summarize to this point, chemisorption is a chemical interaction
between the adsorbate and the surface. The heats and activation energies of chemisorption
are typical of those of a chemical reaction, and that is exactly what it is: a chemical reaction,
albeit three-dimensional (in the bulk fluid phase) on one side of the arrow and two-dimen-
sional (on the adsorbent surface) on the other side. The activation energies are such that
the species involved have sufficient energy to cross the activation energy barrier at temper-
ature levels that are experimentally accessible and of practical importance.
Now we need to talk about physical adsorption, which, though it is not generally consid-
ered to be a crucial factor in surface reactions, is of importance in relation to the topic as