Why we use it?
Car exhausts are major air pollutants in the world. The most significant gases in it are CO, NOx and unburnt hydrocarbons CxHy. Some developed countries (particularly the US) has set emission standards on various common pollutant gases in new cars that aims at reducing these gases progressively. They are shown in the table below.
|
Model Year
|
emission / mile-1
|
||
|
CO
|
CxHy
|
NOx
|
|
|
1970
|
34
|
4.1
|
4.0
|
|
1980
|
7.0
|
0.41
|
2.0
|
|
1983
|
3.4
|
0.41
|
1.0
|
|
1992
|
3.4
|
0.25
|
0.4
|
The mechanism
A
catalytic convertor removes the pollutant gases from the exhaust by reducing
or oxidising them. Precious metals are used as catalysts in the process,
like an alloy of platinum and rhodium (35% and 73% respectively in US).
Palladium metal may also be the choice.
Reactions (heterogenous catalysis) take place in the
convertor. Both NOx and CO are eliminated together by a redox reaction
on rhodium catalyst. NOx oxidises CO to CO2, and is reduced
to harmless nitrogen gas (N2).
CO and CxHy are oxidised by air on platinum
catalyst. (C7H16 is used to represent the unburnt hydrocarbon)
For all these reactions to happen, the convertor is designed as a 'three-way convertor' and has an oxygen monitor fitted to the engine. The monitor checks the quantity of oxygen into the engine to make sure there is enough oxygen in carrying out oxidation reactions.
Significants
The overall result of passing exhaust gases through the chamber is to convert the harmful CO, NOx and CxHy to relatively harmless N2, CO2 and H2O. Though the exhaust gases are in contact with the catalyst for only 100 to 400 ms, 96% of hydrocarbons and CO are converted to CO2 and H2O. The emission of nitrogen oxides is reduced by 76%.
|
Gas
|
% by volume
|
|
|
Non-catalysed car
|
Catalysed car
|
|
|
carbon dioxide
|
14
|
15
|
|
oxygen
|
0.7
|
0
|
|
hydrogen
|
0.25
|
0
|
|
carbon monoxide
|
1.0
|
0.2
|
|
hydrocarbons
|
0.06
|
0.01
|
|
aldehydes
|
0.004
|
0.002
|
|
nitrogen oxides
|
0.2
|
0.02
|
|
sulphur dioxide
|
0.005
|
0.005
|
|
lead (as solids if leaded petrol is used)
|
4 mg m-3
|
0
|
Drawbacks
Inefficient at low temperatures
However, the catalytic reactions do not start until a temperature of 200°C
is reached. The convertor is inefficient when the motor initially starts.
Expansive
Catalytic convertors are very expansive. The one used in the US comprises
73% rhodium and 35% platinum, and the cost of the metals are far higher than
gold.
Catalytic poison
The catalysts are imcompatible with lead-containing anti-knocking agents
that used to petrol to enhance engine performance. One of such, tetraethyl lead
[(C2H5)4Pb], blocks the catalytic sites and
decreases catalytic activity. The poisoning of catalysts results from
the use of unleaded petrol. Cars
built since 1975 have been engineered to use unleaded petrol.
Related topics
Carbon monoxide
Nitrogen oxides
Hydrocarbons
Unleaded petrol
I would love to hear your comments. Please e-mail to xavier_fung@sinaman.com