The U.S. automobile industry introduced the catalytic converter, a device that uses chemical reactions to decrease air pollution from automobile exhaust gases. While the device helped to decrease carbon monoxide, unburned hydrocarbon, and nitrogen oxide emissions from automobiles, it did not reduce total emissions.
In 1974, the General Motors Corporation
The impetus for the development of the catalytic converter was the Clean Air Act of 1970.
The automobile industry fought the standards, which the industry claimed were technically impossible as well as economically disastrous. In 1973, the Environmental Protection Agency
Carbon dioxide and water are the theoretical by-products of the burning gasoline that provides power for motor vehicles. Carbon dioxide is a natural part of the ecological interdependence between green plants and animals: Animals inhale oxygen from the air, use the oxygen to release energy from their food, and exhale carbon dioxide; green plants absorb carbon dioxide and release oxygen into the atmosphere. Thus, the ideal by-products of combustion in an automobile engine are not in themselves harmful, but can contribute to the greenhouse effect. The problem with automobile engines is that the by-products of combustion are not entirely carbon dioxide and water, but also contain unburned or incompletely burned gasoline, carbon monoxide, and nitrogen oxides. The catalytic converter was designed to correct this problem.
Gasoline is essentially a mixture of hydrocarbons, molecules consisting of hydrogen and carbon atoms. Under ideal combustion conditions, with sufficient oxygen present, the hydrocarbons break down, the carbon atoms each combine with two oxygen atoms to form carbon dioxide, and each two hydrogen atoms combine with an oxygen atom to form water. Two basic problems arise that prevent ideal combustion to occur. One problem is the fact that the combustion takes place in air, which is approximately 78 percent nitrogen and only approximately 21 percent oxygen. At the operating temperatures of the automobile engine, some of the nitrogen and oxygen from the air combine to form nitrogen oxides. The other basic problem is that ideal combustion assumes that there is sufficient oxygen to completely burn the hydrocarbons and that the gasoline and oxygen—or gasoline and air, in the case of an automobile engine—are thoroughly mixed. The variables of engine speed, air and gasoline volume, and even the variation of the oxygen content of air with temperature all hinder the ability of the engine to burn the correct mixture of gasoline and oxygen. Engine technologies, such as hot-wire air mass sensors, microprocessor-based engine controls, fuel injection, turbochargers, and superchargers, exist to improve the efficiency of combustion within the engine. Despite these technologies, some of the hydrocarbons remain unburned, or partially burned, sometimes as volatile organic compounds (VOCs), and some of the carbon atoms are incompletely oxidized to form carbon monoxide instead of carbon dioxide.
The reactions that take place in an automobile catalytic converter convert carbon monoxide, unburned hydrocarbons, and nitrogen oxides, all pollutants, with oxygen to form carbon dioxide and water, the ideal combustion products, as well as nitrogen. The condition necessary for these reactions is a temperature of at least 700 degrees Fahrenheit, which is readily available in the exhaust system of an automobile. Because excess oxygen is needed to combine with carbon monoxide to form carbon dioxide, it is also necessary that the exhaust gases contain sufficient oxygen. A widely used catalyst is platinum, a precious metal. Although platinum is expensive, because it is the catalyst, it is not used up in the chemical reaction and is a onetime expense in the production of the automobile. A catalytic converter, therefore, is a canister containing a platinum surface that is placed in the exhaust system of an automobile, where the hot exhaust gases can pass over the platinum surface and react to provide the most desirable by-products of combustion.
When the catalytic converter became standard equipment in U.S. automobiles, it had a dramatic effect on carbon monoxide, hydrocarbon, and nitrogen oxide levels. When operating correctly, engines with catalytic converters have been shown to decrease carbon monoxide and hydrocarbon levels by 96 percent, and nitrogen oxides by 76 percent.
Motor vehicles represent the most significant source of carbon monoxide and nitrogen oxide emission. As older automobiles that are not equipped with catalytic converters have been taken out of service and replaced with newer automobiles that are equipped with catalytic converters, the emissions per mile-traveled of carbon monoxide, unburned hydrocarbons, and nitrogen oxides have decreased. Not all automobile emissions of unburned hydrocarbons occur in the exhaust gases, and other technologies such as charcoal canisters are used to reduce the other emissions of unburned hydrocarbons. The net result of the use of catalytic converters in automobiles, along with some changes in the emissions from stationary pollution sources, has been a reduction in the carbon monoxide levels in some cities.
The carbon monoxide, hydrocarbons, and nitrogen oxides that catalytic converters reduce are considered to be direct health threats, contributors to acid rain, and components of smog. Other sources also contribute to each of these categories, however, so the introduction of catalytic converters reduced, but did not eliminate, these dangers.
Carbon monoxide is a well-known human health risk. Carbon monoxide reduces the blood’s ability to transport oxygen, and there is evidence that it may have other, unknown, physiological effects. Exposure to carbon monoxide can cause headaches, coma, and even death, depending on the level of carbon monoxide exposure and the physical condition of the exposed person. Carbon monoxide is also a component of smog. Depending on atmospheric conditions, carbon monoxide can be converted to carbon dioxide, which contributes to the greenhouse effect and can be a minor factor in the formation of acid rain.
Nitrogen oxides can cause respiratory system damage in humans. The chemical compounds also contribute to photochemical smog and can react in the atmosphere with water to form nitric acid, a component of acid rain. The prevalent reactions that form acid rain, however, are sulfur-based.
Hydrocarbons from automobile emissions actually make up a larger group of compounds with varying properties and danger levels. Air samples have found as many as six hundred different hydrocarbons, some of which have been found to be carcinogenic. The main environmental problem with hydrocarbons in the atmosphere, however, is the formation of photochemical smog. When hydrocarbons and nitrogen oxides are exposed to strong sunlight in the correct atmospheric conditions, a string of chemical reactions takes place in twenty-four-hour cycles. The reactions produce toxic aldehydes, then ozone, then the toxic eye-irritating peroxyacyl nitrates in succession.
There is a secondary effect of the introduction of catalytic converters to automobile exhaust systems. The platinum catalyst is not consumed in the catalytic reactions that take place and, therefore, does not require routine replacement. Lead additives, however, can destroy the platinum catalyst. This fact led to the development of unleaded gasolines, which replaced the leaded gasolines that had been in use before the introduction of catalytic converters. The tetra-ethyl lead controlled the combustion to keep automobile engines from knocking, but the lead itself was discharged into the atmosphere in the exhaust gases. The lead thus emanated became a source for low-level lead poisoning. Lead emissions decreased by 75 percent between the mid-1970’s and the mid-1980’s. Thus, an additional benefit of the introduction of catalytic converters was the mandatory change in the composition of gasolines that removed another pollutant from automobile emissions.
While the introduction of the catalytic converter decreased the emission levels per mile-traveled of carbon monoxide, unburned hydrocarbons, and nitrogen oxides, it did nothing to decrease the emissions per mile of carbon dioxide. In fact, the catalytic converter increased carbon dioxide emissions slightly, as unburned hydrocarbons and carbon monoxide were both changed to carbon dioxide. Atmospheric carbon dioxide levels have increased since the introduction of the catalytic converter, but this is a result of the increase in number of vehicle miles traveled and because of the increased general use of fossil fuels. Carbon dioxide is one of the prime gases that contribute to the greenhouse effect, the cause of global warming. Water vapor also contributes to the greenhouse effect.
Catalytic converters only work when they reach approximately 700 degrees Fahrenheit, thus most air pollution occurs during the first few minutes of running a cold engine. Current research is aimed at developing a catalytic converter that will work effectively from the first few seconds of engine operation.
Catalytic converters have had a beneficial effect on automobile emissions, with the exception of carbon dioxide emissions, but even in the late 1980’s it was evident that nearly one-half of all Americans lived in areas where air-quality standards were not always met. This led to a number of laws and regulations designed, in part, to reduce motor vehicle emissions further. These laws included the Electric and Hybrid Vehicle Research, Development, and Demonstration Act of 1976;
Because catalytic converters have no effect on the carbon dioxide emissions that contribute to the greenhouse effect, a number of laws and regulations were also passed to improve the fuel efficiency of automobiles, which would in turn lower the carbon dioxide emissions. These laws included setting a national highway speed limit of 55 miles per hour in 1973; the Energy Policy and Conservation Act of 1975,
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Gay, Kathyln. Air Pollution. New York: Franklin Watts, 1991. A high school-level book that clearly presents the various facets of air pollution in a nontechnical style. The chapters on smog, acid rain, and global warming are particularly appropriate to this subject. -
Gore, Al. Earth in the Balance: Ecology and the Human Spirit. 1992. Reprint. Emmaus, Pa.: Rodale Press, 2006. An important book that places air pollution, and pollution in general, into a global perspective. -
Nadis, Steve, and James J. MacKenzie with Laura Ost. Car Trouble. Boston: Beacon Press, 1993. A good description of the pollution problems associated with automobiles and the growth of suburbs. Includes a wide range of alternatives, including alternative automobile technologies as well as other transportation options. -
Rosenbaum, Walter A. Environmental Politics and Policy. 6th ed. Washington, D.C.: CQ Press, 2004. An acclaimed review of environmental law and regulation and the politics involved in environmental law enforcement. -
Wellburn, Alan. Air Pollution and Acid Rain: The Biological Impact. New York: John Wiley & Sons, 1988. A complete treatment of the technical aspects of air pollution, including its sources, control, and effects. The author, a biochemist, provides information about the chemical reactions that produce air pollutants, the chemical reactions that occur in the atmosphere, the reactions that break down pollution products, and the reactions that pollutants have with plants and animals, including humans. Where multiple theories exist, the author provides each one. Excellent charts and graphs.
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