Page 195 - Introduction to chemical reaction engineering and kinetics

8.1 Catalysis and Catalysts 177

(1) The primary characteristic is that a catalyst increases the rate of a reaction, rela-
tive to that of the uncatalyzed reaction.
(2) A catalyst does not appear in the stoichiometric description of the reaction, al-
though it appears directly or indirectly in the rate law and in the mechanism. It
is not a reactant or a product of the reaction in the stoichiometric sense.
(3) The amount of catalyst is unchanged by the reaction occurring, although it may
undergo changes in some of its properties.
(4) The catalyst does not affect the chemical nature of the products. This must be
qualified if more than one reaction (set of products) is possible, because the cat-
alyst usually affects the selectivity of reaction.
(5) Corresponding to (4), the catalyst does not affect the thermodynamic affinity of
a given reaction. That is, it affects the rate but not the tendency for reaction to
occur. It does not affect the free energy change (AG) or equilibrium constant
(K,,) of a given reaction. If a catalyst did alter the position of equilibrium in a
reaction, this would be contrary to the first law of thermodynamics, as pointed out
by Ostwald many years ago, since we would then be able to create a perpetual-
motion machine by fitting a piston and cylinder to a gas-phase reaction in which
a change in moles occurred, and by periodically exposing the reacting system to
the catalyst.
(6) Since a catalyst hastens the attainment of equilibrium, it must act to accelerate
both forward and reverse reactions. For example, metals are good hydrogenation
and dehydrogenation catalysts.
(7) Although it may be correct to say that a catalyst is not involved in the stoichiom-
etry or thermodynamics of a reaction, it is involved in the mechanism of the re-
action. In increasing the rate of a reaction, a catalyst acts by providing an easier
path, which can generally be represented by the formation of an intermediate
between catalyst and reactant, followed by the appearance of product(s) and
regeneration of the catalyst. The easier path is usually associated with a lower
energy barrier, that is, a lower EA.
Catalysis is a special type of closed-sequence reaction mechanism (Chapter 7). In this
sense, a catalyst is a species which is involved in steps in the reaction mechanism, but
which is regenerated after product formation to participate in another catalytic cycle.
The nature of the catalytic cycle is illustrated in Figure 8.1 for the catalytic reaction used
commercially to make propene oxide (with MO as the catalyst), cited above.
This proposed catalytic mechanism (Chong and Sharpless, 1977) requires four reac-
tion steps (3 bimolecular and 1 unimolecular), which take place on a molybdenum metal
center (titanium and vanadium centers are also effective), to which various nonreactive
ligands (L) and reactive ligands (e.g., O-R) are bonded. Each step around the catalytic
cycle is an elementary reaction and one complete cycle is called a turnover.

Figure 8.1 Representation of pro-
posed catalytic cycle for reaction to
produce C3H60 (Chong and Sharp-
less, 1977)