Page 39 - Adsorption, Ion Exchange & Catalysis- 2007, Elsevier - Copy

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2.1 Def initions 35

Catalysis can be generally described as the action of a catalyst, which is a substance that
accelerates the rate of a chemical reaction, while itself remaining unchanged by the reac-
tion. There are two main categories of catalysis:

• homogeneous catalysis, where the catalyst is in the same phase with the reactants,
• heterogeneous catalysis, where the catalyst is present in a different phase from the reac-
tants in the reaction.

Heterogeneous catalysis is often called surface catalysis as it mainly occurs between a
solid surface and a gas. There are basically three stages in this process:

• adsorption (physisorption or chemisorption as described previously) of reactants on the
catalyst surf ace,
• chemical reaction on the surf ace,
• desorption of products from the catalyst surf ace.

The following points hae to be noted about the action of catalysts. First, catalysts do
v
not alter the thermodynamics of the reactions. No catalyst fors a thermodynamically v a
,
unfeasible reaction. Consequently the reaction would proceed een without the catalyst, v
though perhaps too slowly to be observed or be of use in a given context. Furthermore, the
use of a catalyst does not change the equilibrium composition because it increases the rates
of the forward and reerse reactions by the same extent. Here arises the question: since a
v
catalyst cannot change the position of equilibrium, why is it said that from the practical
,
w
point of vie the most important characteristic of a catalyst is its selecti W e should vity?
ork,
v
keep in mind that in the eent of a complex reaction netw which is often the case, the
catalyst might affect each reaction to a dif thus changing the o ferent e xtent, erall reaction v
selectivity. For example, the reaction between gaseous ethanol and oxygen in nitrogen, at
150–300 °C under atmospheric pressure, is not just a simple oxidation reaction but a net-
work consisting of the following reactions (Poulopoulos et al ., 2002):
C H OH 3O 5 2CO 3H O 2 2 2 (2.1)
2

C H OH 0.5O 5 CH CHO H O 2 3 2 (2.2)
2
C H O CH CH H 3 2 (2.3)
O
H
2
5
CHO CH H O 2 4 (2.4)
H
5
2
2
2C H O ( H ) O H O 2 5 2 (2.5)
H
C
2
2
5
C H OH 2CH 5 0.5O 4 2 (2.6)
2
Consequently, under these conditions, ethanol may be totally oxidized to carbon dioxide via
the first reaction, partially oxidized to acetaldehyde via the second reaction, dehydrogenized