Carbon monoxide and hydrocarbons are present in vehicle exhaust gases as a result of the incomplete combustion of fuel in the engine. Oxides of nitrogen are present in the air is at the temperature of the car engine.
The exhaust gases are passed through a catalytic converter a pipe containing a solid catalyst, plantinum and rhodium. The converter cannot remove oxides of nitrogen.
An engine can be fitted with a three-way catalytic system which can both oxidise fuel, carbon monoxide and also reduce oxides of nitrogen.
Lead poisons the catalyts in the converter.It destroys the catalytic action. Hence unleaded petrol must be used when using a catalytic converter.