Catalytic converters are devices which employ a catalyst to facilitate a chemical reaction. (A catalyst is a substance that changes the rate of a chemical reaction, but whose own composition is unchanged by that reaction.) For air pollution control purposes, such reactions involve the reduction of nitric oxide to molecular oxygen and nitrogen or oxidation of hydrocarbons and carbon monoxide to carbon dioxide and water. Using the catalyst, the activation energy of the desired chemical reaction is lowered. Therefore, exothermic chemical conversion will be favored at a lower temperature.

Traditional catalysts have normally been metallic, although nonmetallic materials, such as ceramics, have been coming into use in recent years. Metals used as catalysts may include noble metals, such as platinum, or base metals, including nickel and copper. Some catalysts are more effective in oxidation, others are more effective in reduction. Some metals are effective in both...