9th November 2003
The aim of the catalytic converter is to dramatically reduce the three most harmful exhaust emissions — carbon monoxide, hydrocarbons and nitrogen oxides — hence the term “three-way catalytic converter”. The device is required on unleaded petrol-engine passenger cars first used on and after 1 August 1992.
It’s called a converter because the emissions that go into the catalyst are converted to more benign substances such as carbon dioxide, nitrogen and water vapour. The device is fitted in the exhaust system just under the driver or passenger footwell and looks like a conventional silencer box, albeit flatter and squarer.
The converter has two sections: a reduction catalyst and an oxidisation catalyst. Each is made of a ceramic honeycomb with a metal coating. The honeycombs create the maximum surface area for exhaust gas to pass over.
Inside the reduction catalyst, the honeycomb is coated with platinum and rhodium, which are used to reduce nitrogen oxides in emissions. As the gas passes over the platinum and rhodium, a chemical reaction takes place that splits the nitrogen atoms from the oxygen. Both are harmless in their separate forms.
In the second stage, hydrocarbons (from fuel) that have passed unburnt through the engine, together with carbon monoxide, are oxidised (burnt, basically) on a catalyst coated with platinum and palladium.
It is because so many rare and precious metals are used that a replacement converter costs so much (between £100 and £500) but without it, your car will not pass the MoT test. Eventually, a converter will wear out (three to four years would be typical), though it can be physically damaged by knocking it on a kerb, or by an engine backfire. Mistaken use of leaded petrol will also ruin a converter.
The Sunday Times, 9 November 2003