Catalytic converters can be affected by catalyst poisoning. This happens when something which isn't a part of the reaction gets very strongly adsorbed onto the surface of the catalyst, preventing the normal reactants from reaching it.
Lead is a familiar catalyst poison for catalytic converters. It coats the honeycomb of expensive metals and stops it working.
In the past, lead compounds were added to petrol (gasoline) to make it burn more smoothly in the engine. But you can't use a catalytic converter if you are using leaded fuel. So catalytic converters have not only helped remove poisonous gases like carbon monoxide and nitrogen oxides, but have also forced the removal of poisonous lead compounds from petrol.