Catalytic converter

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 kinds of reactions. The catalyst material is normally coated on a porous, inert support structure of varying design. Examples include honeycomb ceramic structures with long channels and pellet beds. The goal is to channel exhaust over a large surface area of catalyst without an unacceptable pressure drop.

In some cases, reduction and oxidation catalysts are combined to control oxides of nitric oxide, carbon monoxide, and hydrocarbon emissions in exhaust from internal combustion engines. The reduction and oxidation processes can be conducted sequentially or simultaneously. Dual catalysts are used in sequential reduction-oxidation. In this case, the exhaust gas from a rich-burn engine initially enters the reducing catalyst to reduce nitric oxide. Subsequently, as the exhaust enters an oxidation catalyst, it is diluted with air to provide oxygen for oxidation. Alternatively, three-way catalysts can be used for simultaneous reduction and oxidation. Engines exhausting to such catalysts run slightly rich and require tight regulation of air-fuel ratio.

Reducing catalysts can be made more efficient using a reducing agent, such as ammonia. This method of control, referred to as selective catalytic reduction, has been employed successfully on large turbines. In this case, a reducing agent is introduced upstream of a reducing catalyst, allowing for greater rates of nitric oxide reduction.

[Robert B. Giorgis Jr. ]

RESOURCES

BOOKS

Silver, R. G., ed. Catalytic Control of Air Pollution: Mobile and Stationary Sources. Washington, DC: American Chemical Society, 1992.

Yaverbaum, L. H. Nitrogen Oxides Control and Removal: Recent Developments. Park Ridge, NJ: Noyes Data Corp., 1979.

PERIODICALS

Amato, I. "Catalytic Conversion Could Be a Gas." Science 259 (15 January 1993): 311.