Catalytic Converters
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Uncontrolled automobile emissions contain a variety of potentially harmful chemicals. Exhaust gases typically include hydrocarbons (HCs), carbon monoxide (CO), and nitrogen oxides (NOX). In urban areas, the environmental accumulation of such pollutants can threaten public health. Consequently, over the past several decades, the world's industrialized nations have formulated motor vehicle emission regulations. In response to these government mandates, automakers developed the catalytic converter. These devices facilitate the degradation of harmful compounds. Of all the different converters, the three-way catalytic converter has generally proven to be both durable and effective. Nonetheless, increasingly stringent environmental regulations have spurred continuing research into converter technologies. New devices currently being considered for the control of automotive emissions include such things as heated catalytic converters and hydrocarbon adsorbers. About the mid-1960s, the state of California enacted automotive emission controls which eventually led to the development of the catalytic converter ("Catalysts," 1989, pp. 69-75). As the United States' emissions standards became more stringent, it became necessary for new cars to have the devices installed as part of their exhaust system. Catalytic converters have been widely used in the United States since 1974 (1975 model year vehicles). The early converters typically consisted of an
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on monoxide are oxidized to CO2 and H2O, and nitric oxides are reduced to nitrogen. Under normal conditions, the gas-phase reactions of engine exhaust proceed very slowly. However, when the pollutants are chemisorbed onto catalyst, the reactions occur at rates rapid enough to be of "practical value for controlling emissions" ("Catalysts", 1989, pp. 69-75).
At present, catalysis is generally considered to occur around an "active site" ("Catalysts", 1989, pp. 69-75). At this particular location, ambient electronic forces act to enhance chemical reactivity. For example, once CO and O2 are chemisorbed onto a catalyst, their proximity and orientation results in rapid oxidation. This is because, during the process of adsorption, the catalytic surface sequesters some of the CO molecule's energy, and consequently weakens the carbon-oxygen bond. The atoms of the compromised CO molecule are then more easily attracted to other atoms such as oxygen. Thus, by facilitating the interaction between CO and O2, the catalyst facilitates CO degradation ("Catalysts", 1989, p. 6).
Platinum and palladium are both effective oxidation catalysts for carbon monoxide and hydrocarbons. In addition, both metals also promote the reduction of nitri
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Approximate Word count = 1750
Approximate Pages = 7 (250 words per page)
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