Three-way catalyst cleans auto exhaust
Radiation Effects on Solid Surfaces Advances in Chemistry Series No. 158 Manfred Kaminsky, Editor A symposium sponsored by the Division of Colloid and Surface Chemistry of the American Chemical Society. A new and fully informative volume containing the latest research from top government, academic, and industrial laboratories in three significant areas: • surface processes resulting from surface irradiation with atoms, ions, neutrons, electrons, positrons, and photons • recently developed analysis techniques based on particle and/or photon irradiation of surfaces • surface irradiation effects in solar and nuclear applications. The eighteen papers in this timely collection represent an important contribution to applied fields of accelerator and plasma technology, controlled thermonuclear fusion, and solar and nuclear (fission) energy. Surface chemists and physicists, material scientists, plasma physicists, metallurgists, and nuclear engineers will find this volume to be a key professional reference on radiation damage and surface science. 397 pages (1976) clothbound $32.50 LC 76-51209 ISBN 0-8412-0331-8
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To the process designer in the chemical industry, instrumenting an internal combustion engine for maximum efficiency and minimum pollution may appear a fairly mundane task. In principle it may be, but the experiences of auto makers suggest the contrary. A legislative muddle in the U.S. and abroad, a surplus of "expert" commentators, and the growing anger of potential auto purchasers all militate against an orderly solution of the vehicle pollution problem. One observer at the Society of Automotive Engineers' Congress & Exposition in Detroit early this month likened the situation to a basketball game with 52 levels of referees and the right of every spectator to halt the game at any time. But as involved as the situation may be, a kind of developmental gravity and the proximity of legislative time limits have all but guaranteed that the coming generation of vehicle exhaust control will center on the three-way catalyst (TWC) converter. The reason vehicle engines present such a problem is that they don't run at steady-state conditions, and probably never will. Ensuring a constant composition of the exhaust thus requires either an incredibly complex system that varies engine operation in response to many variables, or a final stage in the exhaust system that converts everything to acceptable emissions. In fact, both are under investigation and are used to some extent in even the simplest emission control systems. However, the TWC converter is the front-runner at the moment. The TWC catalytically converts exhaust hydrocarbons, carbon monoxide, and nitrogen oxides to carbon dioxide, water, and nitrogen. The conversion is not 100% effective but it is possible to reduce them to acceptable levels. Of the 56 hydrocarbons routinely monitored by the Environmental Protection Agency, all are converted in some degree, but the total reaction mechanism is very complex. From the thermodynamic viewpoint, everything is favorable for conversion in the general temperature range from 200° to 900° C, and an acceptable TWC would exhibit good activity at or near the stoichiometric conversion point in this range. Since the thermodynamics are favorable, it follows that reducing pollution of engine exhausts is more a matter of kinetics than anything else. Hence, the TWC. The active catalysts most often used in TWC's are platinum and platinum-rhodium combinations. They are supported on either pellets or a monolithic honeycomb. Some common metal oxides have been found that also convert exhaust gases, but they are handicapped by insufficient low-temperature activity and unfavorable aging characteristics. The TWC's require only one catalyst bed, although more than one has been proposed on occasion. They also require no secondary air, produce no oxidation of sulfur dioxide to sulfur trioxide, and do
not form secondary pollutants such as hydrogen cyanide and hydrogen sulfide. One disadvantage of TWC's is that they require more closely controlled air-to-fuel ratios in the engine, which can only be obtained from more expensive carburetors. There also is the problem of expense in working with precious metals, particularly rhodium. A major developer of precious metal TWC's is Engelhard Industries. Extensive tests at the bench and in service indicate that the converters are effective and will become more so as the amount of lead additives in gasoline is reduced over the next few years. According to Engelhard, the simultaneous removal of hydrocarbons, carbon monoxide, and nitrogen oxides is obtained by controlling the air/fuel ratio in a narrow range around the stoichiometric value. This control requires an air/fuel ratio meter, a sensor to determine stoichiometric conditions, a control logic system to continuously correct the air/fuel ratio, and the appropriate catalyst—a proprietary combination, Engelhard says, of rhodium and platinum. Increasing the rhodium content improves nitrogen oxides conversion and catalyst durability. Some problems with rhodium-platinum TWC's are cited by Johnson Matthey Research of the U.K. It notes that TWC's can show decreased hydrocarbon oxidation in some cases. They also may have decreased durability if there is an excess of oxygen. These effects are attributed in part to surface enrichment of the catalyst by rhodium, followed by oxidation to produce surface species that are both inactive to sulfate formation and to reduced oxidation activity. One possible solution suggested by Johnson Matthey may be to separate the rhodium and platinum into split beds. Tests have shown that this solution significantly increases converter performance. Catalyst supports for the rhodiumplatinum combinations are invariably silica, alumina, or combinations of the two. However, they exhibit different aging characteristics. Tests conducted by W. R. Grace & Co. show that all three supports have good catalytic activity when fresh. Upon aging, alumina-supported catalysts show the least susceptibility to poisoning by lead, phosphorus, and sulfur. A problem facing auto makers is selecting a catalytic conversion system for general production use, and they have many unanswered questions about the proprietary catalysts being offered. The auto makers thus have attacked the problem in many ways, including development of their own catalysts and development of accelerated testing procedures for them. But even if catalyst producers and auto makers come up with satisfactory control devices, all of them now under consideration are far from foolproof and will raise the price of autos. One topic discussed often at the meeting was penalties for system failure in service. The basketball analogy is still appropriate: Every level of officialdom controlling the game probably will devise penalties as the game proceeds. D
