Interactive video lesson plan for: What Are Catalytic Converters | Chemistry for All | FuseSchool

Learn the basics about catalytic converters, as a part of the overall environmental chemistry topic.

A catalyst is a substance that causes a reaction to proceed more quickly, without being used up.

A catalytic converter accelerates a reaction whereby more favourable products are formed.

Carbon monoxide, a highly toxic gas, is released from the incomplete combustion of petrol. Both of these products have the potential to leave the car as exhaust gases. Instead of releasing these products, the catalytic converter would release less harmless gases into the atmosphere.

In a catalytic converter, there are two different types of catalyst at work, a reduction catalyst and an oxidation catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. These metals are usually arranged in a honeycomb arrangement.

The honeycomb structure exposes the maximum surface area of catalyst to the exhaust stream, while also minimizing the amount of catalyst required, as these metal catalysts are extremely expensive. Typically cars, nowadays, are fitted with three-way catalytic converters. The reduction catalyst is the first stage of the catalytic converter. It uses platinum and rhodium to help minimise the NOx emissions. When an NO or NO2 molecule contacts the catalyst the catalyst causes the nitrogen-oxygen bond to break, so that an O2 molecule (a harmless product is formed), along with a nitrogen molecule. All of this happens on the surface of the catalyst.

The oxidation catalyst is the second stage of the catalytic converter. It oxidises the unburned hydrocarbons and carbon monoxide by burning them over a platinum and palladium catalyst.

Finally, the catalytic converter is able to monitor the level of oxygen in the exhaust stream with an oxygen sensor.

This sensor is linked to an automatic control system which adjusts the carburetor in order to change the air:fuel ratio. This allows for more or less air to mix with the car's fuel before it enters the cylinders.

'Changing the air-fuel ratio as required ensures that there is just enough oxygen in the exhaust to allow for the oxidation of unburned hydrocarbons and CO.

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