I believe that you are referring to the catalytic converter on a car. All automobiles have a device called a catalytic converter that the exhaust stream passes through. Because cars burn a hydrocarbon based fuel for energy, carbon dioxide and water vapor are the main gas byproducts of combustion. But some amounts of carbon monoxide and nitrogen oxides are also produced. These gasses are poisonous to humans and can contribute to smog and acid rain. So the catalytic converter contains different chemical catalysts that convert the carbon monoxide to carbon dioxide (not toxic to humans) and the nitrogen oxides to nitrogen gas (N2, a completely inert gas) and water vapor. These chemically altered gasses are then released through the muffler and the tailpipe into the atmosphere.