Pt
2SO 2 (g)O 2 (g)8888n2SO 3 (g)
NiOSulfur trioxide is probably a worse pollutant than sulfur dioxide, because SO 3 is the acid
anhydride of strong, corrosive sulfuric acid. Sulfur trioxide reacts with water vapor in the
air, as well as in auto exhausts, to form sulfuric acid droplets. This problem must be over-
come if the current type of catalytic converter is to see continued use. These same catalysts
also suffer from the problem of being “poisoned”—that is, made inactive—by lead.
Leaded fuels contain tetraethyl lead, Pb(C 2 H 5 ) 4 , and tetramethyl lead, Pb(CH 3 ) 4. Such
fuels are not suitable for automobiles equipped with catalytic converters and are excluded
by U. S. law from use in such cars.
Reactions that occur in the presence of a solid catalyst, as on a metal surface (hetero-
geneous catalysis) often follow zero-order kinetics. For instance, the rate of decomposition
of NO 2 (g) at high pressures on a platinum metal surface does not change if we add more
NO 2. This is because only the NO 2 molecules on the surface can react. If the metal surface
is completely covered with NO 2 molecules, no additional molecules can be adsorbeduntil
the ones already there have reacted and the products have desorbed.Thus, the rate of the
Maintaining the continued efficiency
of all three reactions in a “three-way”
catalytic converter is a delicate matter.
It requires control of such factors as
the O 2 supply pressure and the order
in which the reactants reach the
catalyst. Modern automobile engines
use microcomputer chips, based on an
O 2 sensor in the exhaust stream, to
control air valves.16-9 Catalysts 693A 30% hydrogen peroxide, H 2 O 2 , solution at room temperature decomposes very slowly
to form O 2 and H 2 O. (a) This reaction takes place more rapidly if the solution is heated.
(b) A very small amount of a transition metal oxide is added. (c) This oxide catalyzes the
decomposition reaction. The catalyzed reaction is rapid, so the exothermic reaction
quickly heats the solution to the boiling point of water, forming steam. The temperature
increase further accelerates the decomposition. Never use a syringe with a metal tip to
withdraw a sample from a 30% hydrogen peroxide solution. (d) The bombardier beetle
uses a catalyzed decomposition of hydrogen peroxide as a means of defense. An enzyme
produced by the beetle catalyzes the rapid exothermic reaction. The resulting steam,
along with other irritating chemicals, is ejected.
(a) (b) (c)
(d)(text continues on page 696)