REACTIONS: THERMODYNAMIC ASPECTS, KINETICS, AND CATALYSIS - Biomass as a Sustainable Energy Source for the Future - page 150

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FIGURE 5.4 Activation energies for catalyzed and noncatalyzed reactions and number
of molecules reacted (Source: Reproduced with permission from Pasquetto and Patrone,
1999, vol.3. © Zanichelli editore S.p.A).

mol −1 , the time to
halve the reactant concentration decreases by more than a million times!
The fundamental properties of a catalyst are:

This means that decreasing the activation energy with 80,000 J

a. At the end of a reaction, in principle, the catalyst has to be chemically
unchanged. Nothing can be said a priori about the deactivation.
b. The products of the catalyzed reaction can, at least in principle, be obtained
from an uncatalyzed reaction under the same conditions. There is, therefore,
no way of using catalysis to equilibrium. In practice, however, the unca-

talyzed reaction may be immeasurably slow or may yield a product distribution
different from the one obtained in a catalyzed reaction, in which the catalyst
may promote only some reactions.
c. Catalysts can be either homogeneous or heterogeneous, depending on whether
the catalyst exists in the same phase as the reactants or not.

“ cheat ”

5.3.1 Homogeneous Catalysis

Homogeneous catalysts function in the same phase (gas or liquid) as the reactants.
In this case, the reaction rate depends on the catalyst concentration. Generally,
a small quantity is enough to increase the quantity of the products in a short time.
The original reaction in the presence of a homogeneous catalyst takes place
through a new sequence of elementary reactions where the activation energy for
the formation of the unstable component is lower. Of particular

interest is the transition metal catalysis. The transition metal ion catalyzes the
original reaction by providing an alternative route between reactants and products
that has a lower activation energy. It can do this because transition metals can form
stable compounds in more than one oxidation state and the transition metal ions

“ catalyst - reactant ”