The catalytic converter in an automobile is an expanded section of exhaust pipe occurring upstream of the muffler in which pollutants generated in the engine are converted to normal atmospheric gases. It is an essential element in the emissions control system of modern automobiles. This technology was introduced in the United States in the late 1970s and became legally required by the early 1980s because of more stringent exhaust emission control standards. Early catalyst systems, as applied to vehicles with carburetors, attempted to oxidize carbon monoxide (CO) and unburned hydrocarbons (HC) to carbon dioxide (CO2) and water vapor, using air added by means of an air pump or rapidly actuating valve system. Although constructed from a high-surface-area alumina substrate with a noble metal (usually platinum) on the surface, their effectiveness was limited by extreme conditions of service. These problems include high temperatures (greater than 1,000°C) exacerbated by a large and variable "engine-out" pollutant load and constant vibration from roadway and engine sources.
The replacement of carburetors with computer-controlled, port fuel injection and precise air/fuel ratio control based on exhaust oxygen sensing has allowed catalytic converters to operate with close to 100 percent efficiency and better longevity, often exceeding 100,000 miles. The addition of a ceria wash coat in the form of a thin layer of porous cerium oxide and rhodium metal in conjunction with the platinum now allow for both good longevity and "three-way" operation. Not only are the small amounts of residual CO and HC oxidized, but the nitric oxide pollution emissions are simultaneously reduced to nitrogen (and some nitrous oxide, a potent atmospheric greenhouse gas, but otherwise nonpoisonous). The tetraethyl lead present in gasoline as an octane booster in the 1970s was removed not because of its effects on human health, but because it rapidly poisoned catalysts. Its removal, although coincidental, has had enormous benefits for human and environmental health. Current pressure to reduce the sulfur content of fuel arises, in part, from evidence that sulfur has a similar, but much smaller effect on catalyst longevity and effectiveness.
see also Greenhouse Gases; Lead; Ozone; Vehicular Pollution.
Kovark, William, and Hermes, Matthew E. "The Role of the Catalytic Converter in Smog Reduction." Available from http://chemcases.com/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. ]
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.
Amato, I. "Catalytic Conversion Could Be a Gas." Science 259 (15 January 1993): 311.