A vehicle fitted with a catalytic converter must use unleaded petrol because lead:

The correct answer is B, inactivates the catalyst in the converter. A catalytic converter holds platinum, palladium and rhodium on a honeycomb support, which oxidise carbon monoxide and unburnt hydrocarbons to carbon dioxide and water and reduce the oxides of nitrogen to nitrogen. Lead compounds from leaded petrol coat the surface of these metals and destroy their catalytic action, a process called catalyst poisoning, after which the converter no longer cleans the exhaust. Option A is wrong because the problem is chemical poisoning, not a rise in temperature. Option C is wrong because lead does not form a solid with carbon monoxide in the exhaust. Option D is wrong because tetraethyl lead was in fact added to petrol to raise its octane number, not to lower it. India therefore phased out leaded petrol before the tighter Bharat Stage norms, since the converter is the main device by which a modern engine meets them.