L08 Catalytic converters in cars

The large numbers of cars in the world are causing major environmental problems. The exhaust gases contain carbon monoxide, nitrogen oxides and unburned hydrocarbons. Leaded petrol also contains toxic lead compounds (lead tetra-ethyl). Carbon monoxide is very poisonous, blocking the oxygen-binding sites of haemoglobin, so that the blood can no longer transport oxygen. Nitrogen oxides are both harmful and a source of acid rain due to nitric acid formation. Volatile organic compounds react with nitrogen oxides under the influence of sunlight and contribute to the formation of photochemical smog and ozone pollution, see illustration L05.
The release of these substances has to be drastically reduced to keep the environment clean. One method is the use of a catalytic converter, as shown in illustration L08. A catalytic converter consists of a stainless steel chamber containing a ceramic structure with narrow channels lined with the three metals platinum, palladium and rhodium.

These metals act as a heterogeneous catalyst in:

1. The oxidation of poisonous carbon monoxide to carbon dioxide
   
   
2. The reduction of oxides of nitrogen to nitrogen.
   
   
3. The oxidation of unburned hydrocarbons to carbon
   dioxide and water.

Lead poisons the heterogeneous catalyst and therefore only lead-free petrol can be used in cars fitted with a catalytic converter. The exhaust gasses in the catalytic converter have a temperature of 300°C and stay for only about a tenth of a second. Under these conditions the nitrogen oxides are reduced to nitrogen and the liberated oxygen oxidises the carbon monoxide and the unburned hydrocarbons. To ensure that adequate oxygen is present, the converter also contains a lambda-probe which controls the fuel/air balance so that sufficient air is present to oxidise fully the chemical species present.