Page 72 - Basic physical chemistry for the atmospheric sciences

58 Basic physical chemistry

Since bromine does not appear in the net reaction, it is a catalyst.
Moreover, since it acts in the aqueous phase on aqueous-phase re­
actants, it is a homogeneous catalyst.
Heterogeneous catalysts often consist of finely divided metals or
metal oxides. Molecules from the gaseous or liquid phase can adsorb 6
onto the solid smfaces of the metals where their reactions with each
other may be enhanced. For example, the catalytic converter, which
is required on all automobiles in the United States, consists of beads
impregnated with cupric oxide [CuO(s)] and chromium oxide
[Cr203(s)]. Their purpose is as follow . Nitrogen oxides and various
s
n
unburned hydrocarbo s , CxHy(g), are emitted frorn cars and are in­
n
volved in the formation of photochemical smog. In additio , the air
pollutant carbon monoxide, CO(g), is present in the emission from
s
automobile . The catalytic converter is designed to reduce the nitro­
gen oxides to nitrogen gas and to oxidize the CO(g) and unburned
hydrocarbons to C0 2 (g) and water. Reduction of the nitrogen oxides
is favored thermodynamically, but the reaction is very slow; the most
effective catalysts are transition-metal oxides and noble meta s . Al­
l
though the same types of materials also catalyze the combustion of
CO(g) and hydrocarbons, two different catalysts are required for opti­
mum performance. For example, CuO(s) and Cr203(s) may be used to
catalyze the combustion of CO(g) and hydrocarbons. These catalysts
first adsorb the oxygen gas in the exhaust; this weakens the 0--0 bond
in O (g) and provides oxygen atoms to react with the adsorbed CO(g)
i
to form C02(g). Oxidation of the hydrocarbons probably occurs in a
similar manner, with the hydrocarbons first being adsorbed and then
by weakening a C-H bond. There are problems with the catalytic
converter. For example, the exhaust from automobiles contains traces
of SOi(g), and the conversion of S02 (g) to SOig) is catalyzed by the
converter. The S03(g) may then dissolve in water to form sulfuric
acid, which is a dangerous pollutant.

3. 7 Half-life, residence time, and renewal time
The half-Life (t 112) of a reactant is defined as the time required for the
concentration of the reactant to decrease to halfway between its initial
s
and final value . We can derive an expression for 1112 for a reactant A
for a reaction that is first order in A by substituting [A]1 = UA] and
0
t = 1112 in Eq. (3 .5), which yields