The resulting ozone pollution or “summertime smog” may persist for several days and be transported over long distances.
Catalytic Converters
There is widespread concern about the emissions from motor vehicles that can cause smog and produce O3 forming gases. One attempt to stop this problem has been the introduction of catalytic converters in car exhaust systems.
A catalytic converter sits inside a car and uses a catalyst to convert three harmful compounds in car exhaust into harmless compounds.
The three harmful compounds are:
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Hydrocarbons (in the form of unburned gasoline)
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Carbon monoxide (formed by the combustion of gasoline)
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Nitrogen oxides (made when the heat in the engine forces nitrogen in the air to combine with oxygen)
In a catalytic converter, the catalyst (platinum and rhodium) is coated onto a ceramic honeycomb that is attached to the exhaust pipe. The catalyst helps to convert carbon monoxide into carbon dioxide. It converts the hydrocarbons into carbon dioxide and water. It also converts the nitrogen oxides back into nitrogen and oxygen. It does this in three stages:
Reduction
It uses platinum and rhodium to help reduce the NO emissions. When an NO or NO2 molecule touches the catalyst, the catalyst rips the nitrogen atom out of the molecule and holds on to it, freeing the oxygen in the form of O2. The nitrogen atoms bond with other nitrogen atoms that are also stuck to the catalyst, forming N2. For example:
2NO => N2 + O2 or 2NO2 => N2 + 2O2
Oxidisation
It reduces the unburned hydrocarbons and carbon monoxide by burning (oxidising) them over a platinum catalyst. This catalyst aids the reaction of the CO and hydrocarbons with the remaining oxygen in the exhaust gas. For example:
2CO + O2 => 2CO2
Control
A control system that monitors the exhaust gases, and uses this information to control the oxygen sensor on the catalytic converter. This sensor tells the engine computer how much oxygen is in the exhaust. The engine computer can increase or decrease the amount of oxygen in the exhaust to make sure that there is enough oxygen in the exhaust to allow the oxidisation catalyst to work properly.
Summary of catalytic converters
Temperature =424K
Catalyst =platinum + rhodium spread over a ceramic honeycomb support
The catalytic converter removes the harmful gases that in turn make O3 and smog.
Impurities
It is very important that the initial mixture of gases is pure, as impurities may poison the catalyst. A catalytic poison is a substance that adsorbs on to the catalyst but does not desorb, making the area of the catalyst on which it adsorbs ineffective, E.g. lead, which is why only unleaded petrol may be used with a catalytic converter.
Jennifer Ginley 12E