Catalysts - how do they work and what do they do?

"Cats that clean up after themselves."

What is a cat and what does it do?

In an ideal world a car engine would efficiently burn all the petrol supplied to the engine and emit nothing but carbon dioxide and water vapour.

Sadly, the internal combustion engine is not a very efficient beast, as a result several different pollutants are emitted in the exhaust gases.

Catalytic converters were made mandatory in the UK for all new cars from 1^st January 1993 in order to reduce pollution. They are frequently blamed for a loss of power which is something we address in our sports catalyst article. 

The incomplete combustion of the petrol gives rise to carbon monoxide and various Volatile Organic Compounds (VOCs).  This problem is worst when idling or decelerating. Carbon monoxide is poisonous and a greenhouse gas.  Volatile Organic Compounds are harmful to health and some are linked to cancer.

The very high temperatures in the engine (over 1500°C) causes nitrogen from the air to react with the  oxygen from the air to form nitrous oxides. These cause various problems, nitrogen dioxide for example reacts with the water in the atmosphere to form dilute nitric acid which gives rise to acid rain.

The action of sunlight on this mix of pollutants gives rise to photochemical smogs and the formation of other pollutants such as ozone.

You can think of your catalytic converter as a dating agency for exhaust pollutants, helping them meet their ideal partner and converting them to carbon dioxide and water vapour. Some of the chemical reactions that take place in the catalytic converter are shown below.

carbon monoxide + oxygen → carbon dioxide

Volatile Organic Compounds (partly burnt petrol) + oxygen → carbon dioxide + water

nitrogen monoxide + carbon monoxide → carbon dioxide + nitrogen

The catalytic converter is a marvel of engineering design. When you turn on your engine it needs to cope with a rapid temperature rise of several hundred °C in less than a minute. Operating temperatures are up to 1000°C, on a highly tuned TorqueCars members engine this becomes more frequently at the upper end of this range.

The catalyst needs to make good contact with the exhaust gases without impeding its flow. The speed at which the exhaust gases flow means the catalyst has a matter of milliseconds to work its magic.

The answer is a ceramic lattice with around 400 channels per square inch coated with a mixture containing the precious metals platinum, rhodium and palladium as well as other metal oxides.

One of the main causes of damage to catalytic converters is high temperatures. The usual operating temperature is 150-600°C, but misfiring and high driving speeds can lead to temperatures of up to 1000°C. This damages the surface of the catalyst reducing its efficiency and in extreme circumstances can melt the ceramic.

Another problem is poisoning due to contaminants in the exhaust gases. Sulphur in the petrol or phosphorous from engine oil can both permanently damage the effectiveness of the catalyst.

Please join us in our friendly forum to catch up with the latest trends in catalysts and exhaust systems for your car.