The Foundations of Chemistry

Nitrogen oxide is a serious air pollutant because it is oxidized to nitrogen dioxide, NO 2 ,

which reacts with water to form nitric acid and with other products of the incomplete

combustion of hydrocarbons to form nitrates. The latter are eye irritants in photochem-

ical smog.

These three reactions, catalyzed in catalytic converters, are all exothermic and ther-

modynamically favored. Unfortunately, other energetically favored reactions are also

accelerated by the mixed catalysts. All fossil fuels contain sulfur compounds, which are

oxidized to sulfur dioxide during combustion. Sulfur dioxide, itself an air pollutant, under-

goes further oxidation to form sulfur trioxide as it passes through the catalytic bed.

See the Chemistry in Use essay
“Nitrogen Oxides and Photochemical
Smog” in Chapter 24.

692 CHAPTER 16: Chemical Kinetics

Catalyst surface

Adsorption: CO and O 2 reactant molecules become

bound to the surface:

CO(g) CO(surface) and O 2 (g) O 2 (surface)

The CO molecules are linked through their C atoms

to one or more metal atoms on the surface. The O 2

molecules are more weakly bound.

(a) Activation: The O 2 molecules dissociate into O atoms,

which are held in place more tightly:

O 2 (surface) 2O(surface)

The CO molecules stick to the surface, but they migrate

easily across the surface.

(b)

Desorption: The weakly bound CO 2 product molecules

leave the surface:

CO 2 (surface) CO 2 (g)

Fresh reactant molecules can then replace them to

start the cycle again [back to step (a)].

(d) Reaction: O atoms react with bound CO molecules,

to form CO 2 molecules:

CO(surface) + O(surface) CO 2 (surface)

The resulting CO 2 molecules bind to the surface

very poorly.

(c)

Figure 16-18 A schematic representation of the catalysis of the reaction

2CO(g)
O 2 (g)88n2CO 2 (g)

on a metallic surface.