The Foundations of Chemistry

tionoccurs more readily than would otherwise be possible. In a final step, desorption,the
product molecules leave the surface, freeing reaction sites to be used again. Most contact
catalysts are more effective as small particles, because they have relatively large surface
areas.
Transition metals and their compounds function as effective catalysts in many homo-
geneous and heterogeneous reactions. Vacant dorbitals in many transition metal ions can
accept electrons from reactants to form intermediates. These subsequently decompose to
form products. Three transition metals, Pt, Pd, and Ni, are often used as finely divided
solids to provide surfaces on which heterogeneous reactions can occur.
The catalytic converters (Figure 16-17) built into automobile exhaust systems contain
two types of heterogeneous catalysts, powdered noble metals and powdered transition
metal oxides. They catalyze the oxidation of unburned hydrocarbon fuel (reaction 1) and
of partial combustion products such as carbon monoxide (reaction 2, shown in Figure
16-18).

Pt

1. 2C 8 H 18 (g)
   25O 2 (g)8888n16CO 2 (g)
   18H 2 O(g)
   NiO
   h
   isooctane (a component of gasoline)
   Pt


2. 2CO(g)
   O 2 (g)8888n2CO 2 (g)
   NiO

It is desirable to carry out these reactions in automobile exhaust systems. Carbon
monoxide is very poisonous. The latter reaction is so slow that a mixture of CO and O 2
gas at the exhaust temperature would remain unreacted for thousands of years in the
absence of a catalyst! Yet the addition of only a small amount of a solid, finely divided
transition metal catalyst promotes the production of up to a mole of CO 2 per minute.
Because this reaction is a very simple but important one, it has been studied extensively
by surface chemists. It is one of the best understood heterogeneously catalyzed reactions.
The major features of the catalytic process are shown in Figure 16-18.
The same catalysts also catalyze another reaction, the decomposition of nitrogen oxide,
NO, into harmless N 2 and O 2.

Pt

2NO(g)8888nN 2 (g)
O 2 (g)

NiO

At the high temperatures of the

combustion of any fuel in air, nitrogen

and oxygen combine to form nitrogen

oxide.

16-9 Catalysts 691

Figure 16-17 (a) The arrangement of a catalytic converter in an automobile. (b) A cutaway
view of a catalytic converter, showing the pellets of catalyst.

Exhaust manifold

Exhaust

pipe

Catalytic

converter

Tailpipe

Air

compressor

in

O 2 , CO, NO,

hydrocarbons

out
O 2 , CO 2 ,
N 2 , H 2 O
(a) (b)