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CATALYTIC CONVERTERS.
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Use in controlling auto emissions, chemical process, materials, effectiveness, future developments.... More...
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Paper Abstract:
Use in controlling auto emissions, chemical process, materials, effectiveness, future developments.
Paper Introduction:
Catalytic Converters
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
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The calculation of a catalyticconverter's efficiency generally involves a comparison of the vehicle'sexhaust emissions with some standard. These may include, for example,cerium, lanthanum, nickel, and iron. This device is typicallyinstalled in front of a standard catalytic converter. Automotive Engineering, 97, 69-75."Catalytic converters: The next generation." (1991, October 7). Other ideas being considered for the reduction of cold-startemissions include heat-retention schemes, burner systems, close-coupledcatalysts, and HC adsorber traps. 69-75). For instance, leadedgasoline can poison a catalyst by covering up its entire surface. For this to occur though, the rhodium-containing catalyticconverter must be operated at an air/fuel ratio close to thestoichiometrically balanced composition of 14.6. Of all thedifferent converters, the three-way catalytic converter has generallyproven to be both durable and effective. At this particular location,ambient electronic forces act to enhance chemical reactivity. Over the years, however, the implementation of stricter emissionrequirements has resulted in the introduction of more complex emissioncontrol systems. In A. Otherpotential catalyst poisons include phosphorous and sulfur. Automotive catalysts operate over a temperature range from 2 ?C to11 ?C. When substances fail to desorb, they essentially"poison" the catalyst ("Catalysts", 1989, pp. Various strategies have been considered for the control of cold-startemissions. Therefore, the early converters used eitherplatinum or platinum/palladium mixtures ("Catalysts," 1989, pp. However, whenthe pollutants are chemisorbed onto catalyst, the reactions occur at ratesrapid enough to be of "practical value for controlling emissions"("Catalysts", 1989, pp. Mechanical Engineering-CIME, 116, 81-82."Catalysts in automobiles: A history." (1989, June). Theythen passed through the catalyst bed and exited through a lower increasingoutlet plenum. (1987). References"Alternator-powered, electrically heated catalyst." (1994, April). Automobile catalytic converters. These devices heat up quickly and are then able toremove HCs. 97- 116). The catalyst's primeactive ingredients are platinum, rhodium, and palladium. Nonetheless, increasinglystringent environmental regulations have spurred continuing research intoconverter technologies. 6/28/95Michael or James, There is one illustration that goes along with this report(i.e., 1 photocopy). Exhaust gases flowed in at the top of theconverter through the decreasing inlet plenum (see illustration). The systems, however, are generally too complex. These preciousmetals are applied to a ceramic honeycomb with a large internal surfacearea, thus enabling them to trap and degrade exhaust pollutants (Ashley,1994, pp. Platinum and palladium are both effective oxidation catalysts forcarbon monoxide and hydrocarbons. In addition to platinum, palladium, and rhodium, three-way catalystsalso contain a number of other metals. Likewise, conditions more oxidizing than 14.6compromise a rhodium-containing catalytic converter's ability to reducenitrogen oxides. Theatoms of the compromised CO molecule are then more easily attracted toother atoms such as oxygen. 51-54). 97-115). 29). Under normal conditions, thegas-phase reactions of engine exhaust proceed very slowly. Design News, 47, 29.Heimrich, M. They therefore don't function properly until hot exhaust warmsthem to their effective operating, or "light-off", temperature (Ashley,1994, pp. 81-82). The early converters typically consisted of an"inlet plenum, a narrow louvered catalyst bed, and an exhaust plenum"(Taylor, 1987, pp. New devices currently being considered for thecontrol of automotive emissions include such things as heated catalyticconverters and hydrocarbon adsorbers. With three-way catalyst, the followingreactions occur: HC + O2 ---> CO2 + H2O CO + O2 ---> CO2 NO + CO & H2 ---> N2 + CO2 & H2O Hence, hydrocarbons and carbon monoxide are oxidized to CO2 and H2O,and nitric oxides are reduced to nitrogen. 81-82). Catalysts have only been used for automobile exhaust emission controlsince the 197 s. Thus, by facilitating the interaction betweenCO and O2, the catalyst facilitates CO degradation ("Catalysts", 1989, p.6). In 1994, the acceptableHC emission level as determined by the Federal test cycle was set at .25gram of HCs per mile. Finally, yet another promising technology which may be used tocontrol automotive emissions is the HC adsorber. 81-82). As the United States' emissionsstandards became more stringent, it became necessary for new cars to havethe devices installed as part of their exhaust system. Automotive Engineering, 1 2, 51-54.Ashley, S. The devices--which simultaneously convert CO, HCs, and NOXs overa single catalyst--employ a "three-way catalyst" (Taylor, 1987, pp. By reducing motor vehicle emissions tominuscule levels, catalytic converters are of inestimable benefit to boththe environment and public health. As recently as 1991, three-way oxidation/reduction catalysts wereinstalled on as many as 2 million automobiles in North America, Europe,and Asia ("Catalytic converters", 1991, p. 69-75). However, they are not as effective asrhodium for the catalysis of nitric oxide reactions. Since 1981, for example, emission standards regulatingnitrogen oxides have required additional emission controls. At low temperatures,the adsorber electrically attracts and retains HCs. In urban areas, theenvironmental accumulation of such pollutants can threaten public health.Consequently, over the past several decades, the world's industrializednations have formulated motor vehicle emission regulations. For example,once CO and O2 are chemisorbed onto a catalyst, their proximity andorientation results in rapid oxidation. TheHCs are then released, and flow into the operational catalytic converter(Ashley, 1994, pp. Consequently, 7 percent to 8 percent ofFederal Test Procedure emissions are emitted during the first minute ofcold engine operation. This is because, during theprocess of adsorption, the catalytic surface sequesters some of the COmolecule's energy, and consequently weakens the carbon-oxygen bond. The microprocessor receives signals from the exhaustsystem's oxygen sensor and subsequently adjusts such things as "engine air-fuel, spark timing, transmission converter clutch, and a variety of otherengine and drivetrain functions" (Taylor, 1987, pp. (1994, November). Typically such converters are prewarmed with resistance-heating elements. In order for this to be doneaccurately, however,an established driving cycle and sampling method must be employed. Most of these approaches involve heating the catalyst prior tostarting the engine. However, this level is being reduced in stages until1998. The following two basic supportconfigurations were used: (1) a catalyst support consisting of aluminapellets; and (2) an alumina-coated ceramic monolith. Such asystem might resemble a thermos bottle that would fit over the converter.Heat-storage systems may be prone to overheating during engine operation.Second, fuel-burner systems might consist of a small fuel-injector andigniter. At present, airinjection systems are being developed to preclude this problem (Heimrich etal., 1992, pp. Crucq & A. When combined withplatinum, rhodium can potentially catalyze both oxidation and reductionreactions. For one, heat-storage systems aredesigned to keep catalytic converters hot when they are not in use. 69-75). 69-75). Once any given reaction has been catalyzed, its products must desorbfrom the catalyst. Journal of Engineering for Gas Turbines and Power, 114, 496-5 .Taylor, K. New York, NY: Elsevier. Frennet (Eds.), Catalysis and Automotive Pollution Control (pp. These metal additives are thought toboth improve catalyst performance, and protect the catalyst supportstructures. 29). In response tothese government mandates, automakers developed the catalytic converter.These devices facilitate the degradation of harmful compounds. About the mid-196 s, the state of California enacted automotiveemission controls which eventually led to the development of the catalyticconverter ("Catalysts," 1989, pp. In addition, the first catalytic converters alsoemployed ceramic catalyst supports. 81-82). If conditions are morereducing than the 14.6 air-fuel ratio, the catalyst's ability to oxidizeHCs and CO declines. The company's 47 XT substrate has14 percent more surface area than conventional catalyst supports. It should be noted that even heated catalysts are sometimes impairedby a lack of oxygen in the exhaust. Exhaust gases typically include hydrocarbons (HCs),carbon monoxide (CO), and nitrogen oxides (NOX). In order to provide vehicles with stoichiometricallybalanced exhaust gas composition, air/fuel ratio control systems have beendeveloped. The resultant low exhaustoxygen levels can limit the oxidation of HCs and CO. Desorption frees the catalyst's active site forsubsequent reactions. More recently, a new generation of catalytic converters has beendeveloped for 1994 automobiles. C. In general, a "catalyst" may be definedas "an agent which changes the rate or speed of a chemical reaction"("Catalysts", 1989, pp. Using a dense cordierite ceramiccomposition and thin-wall extrusion technology, Corning, Inc., has produceda thin-wall ceramic catalyst support. 97-115). 69-75). Motor vehicle exhaust emissions vary as a function of such things asdriving mode and ambient conditions. Sincerely, Bill----------------------- 1 In addition tooxidation catalysts, current converters were also equipped with a reducingcatalyst. Third, close-coupled catalysts may actually be used by the automakers one day. As exhausttemperatures increase, however, the heat breaks the electric bonds. At that time, catalytic converters represented acompletely new catalyst technology. Reducing tailpipe emissions with catalytic converters. At present, catalysis is generally considered to occur around an"active site" ("Catalysts", 1989, pp. 69-75). 496-5 ). Several electrically heated catalysts have beeninvestigated. In 1975, United States Environmental ProtectionAgency regulations addressed only CO and HC emissions ("Catalyticconverters", 1991, p. 45 gramsof HCs per mile (Ashley, 1994, pp. These supports werethen coated with catalyst. The general purpose of this design was to create exhaustflow uniformity, reduce flow restriction, and minimize the potential forcatalyst movement. In addition, both metals also promotethe reduction of nitric oxide. By minimizing the heater's impacton vehicle batteries, such systems might be less expensive and morereliable ("Alternator-powered", 1994, pp. However, in only a few decades, thedevices have advanced considerably. I'm going to post it by regular mail. Unfortunately, the technology requires extra batteriesand complex electronic controls. By then, ultra-low emission vehicles should produce only . (1992, July). It couldarrive on Friday. Thisincreases its ability to degrade noxious gases while minimizing any adverseeffects on engine performance ("Catalytic converters", 1991, p. The chemical mechanisms responsible for catalytic convertereffectiveness are rather simple. Electrically heated catalysts for cold-start emission control on gasoline- and methanol- fueled vehicles. Yet another approach involves heating thecatalyst using the vehicle's alternator. Catalytic Converters Uncontrolled automobile emissions contain a variety of potentiallyharmful chemicals. Depending on the distance between the exhaust manifoldand the catalyst, this can take as long as 1 to 2 minutes ("Alternator-powered", 1994, pp. These generally involve an exhaust oxygen sensor and an on-board microprocessor. 29). Theytypically involve an all-palladium, pre-catalyst system placed close to theexhaust manifold. As clean-air regulations have become increasinglymore stringent, automakers have been faced with the problem of cold-startemissions reduction. Catalyticconverters have been widely used in the United States since 1974 (1975model year vehicles). 97-115). Many vehicle fuel systems use fuel-rich mixtures during cold engine operations. In theUnited States, a Federal Test Procedure is used. J.; Albu, S., & Ahuja, M. 51-54).
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