Chemical Reactions on Solid Surfaces - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 394

Another characteristic of catalytic reactions is that it cannot in general be assumed, once
reaction is established at a certain rate under given conditions, that the rate will remain
constant with the passage of time. Catalysts normally lose some or all of their specific activity
for a desired chemical transformation with time of utilization. This effect, normally referred
to as deactivation , can come from a number of different sources and is often very important in
the analysis and/or design of catalytic processes and reactors.
It is most convenient to consider surface reactions as a series of several steps, in essence
a type of chain reaction, which (as we have seen in Chapter 3) employs the active centers
of reaction in a closed sequence. Let us again turn to the simple example of an isomerization
reaction, A

B, now taking place on a surface containing one type of active center, s . The
individual reaction steps are:

/

A þ s % A $s (9.1a)

A $s / B $s (9.1b)

(9.1c)
The first step in this sequence, which we have written as a chemical reaction, represents
the adsorption of A on the surface; the second is the reaction of the surface-adsorbed
A species, A $s , to the corresponding B species on the surface, B $s ; and third is the desorption
of product B from the surface. We will have to develop our ideas concerning surface reactions
by considering each of three steps individually, much as we did in examining the Lindemann
scheme. The development is based mostly on gas/solid systems, which comprise the most
common types of heterogeneous catalytic reactions, however the analysis is also generally
valid for liquid/solid systems. Normally for gas/solid systems the expressions for adsorp-
tion equilibrium and reaction rate are written in terms of partial pressures of reactant and
product species, whereas in liquid/solid systems concentrations are employed. This is not
the first time that we have considered kinetics in heterogeneous (i.e. more than one phase)
reaction systems, as enzyme can effectively be considered as “solid.” Still, it is not an easy
transition to make to “two-dimensional chemistry,” whereby concentrations are based on
area (surface area) rather than volume.

B $s % B þ s

9.1. ADSORPTION AND DESORPTION

According to the first step of reaction (9.1a) given above, an essential feature of catalysis is
the adsorption of the reacting species on the active surface prior to reaction. As indicated, this
type of adsorption is generally a very specific interaction between surface and adsorbate
which is a chemical reaction in itself and is called chemisorption . Desorption is just the reverse
of this, so it is logical to discuss the two together.
To beginwith, let us consider the rate at whichAmolecules in a homogeneous gas (or liquid)
will strike a solid surface. For once, this is a problem we have already solved (Chapter 6).

s

RT
2pM A

Z cT ð A ; surface Þ¼N AV C A (6.21)