REACTIONS: THERMODYNAMIC ASPECTS, KINETICS, AND CATALYSIS - Biomass as a Sustainable Energy Source for the Future - page 152

H H

HIS HIS

O O

O - O

C C

HIS

Zn ++

HIS

Zn ++

O

O-

HIS HIS

FIGURE 5.5 Carbonic anhydrase as a catalyst. It brings its two substrates into close
proximity and optimizes their orientation for reaction (see also Raven et al., 2005).

5.3.2 Heterogeneous Catalysis

Heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst
differs from that of the reactants. The great majority of practical heterogeneous
catalysts are solids (metal, oxide, or salt), and most reactants are liquids or gases.
In solid-catalyzed reactions, the catalytic activity is proportional to the catalyst -

reactant contact surface; this is why a pulverized or spongy catalyst is normally used.
The catalyst surface has active sites , regions with an irregular atom distribution where
the valences are not completely saturated. This is why the catalyst atoms can combine
with the reactant molecules, producing transition states with lower activation energy.
The atoms of the solid are in this state chemically bound to the atoms of the crystal
lattice, and in this way, it is possible to win the reactant(s) chemical inertia, and the
final stadium of the reaction is easily reached. A solid catalyst makes the reactants

'

bonds less strong by combining with the reactants molecules, weakening the original

bonds and, in this way, speeding up the products formation.
The steps of the overall process are:

'

￿ Diffusion of the reactants to the solid catalyst surface

￿ Adsorption of the reactants onto this surface (physically, chemically, or chem-

ically - physically)

￿ Formation of chemical bonds

￿ Formation of products and restoration of the catalyst

￿ Desorption of the products desorb from the catalyst surface

￿ Diffusion of the products away from the catalyst surface

Some examples of heterogeneous catalysis are:

1. Synthesis of sulfuric acid on platinum:

SO 2 + 1

2 O 2 !

SO 3

Opening of the oxygen double bond is possible as a result of the combination
with platinum (Pt 0 ).