Automobile emission catalysts poisons

Cooper, B. J., Renny, L. V., and White, R. J., Lead Poisoning of Automobile Emission Control Catalysts—Influence of Emission System and Catalyst Design Characteristics on the Poisoning Mechanism, Am. Chem. Soc.. Symp. Automot. Catal., Chicago Meet., 1975. 

Automobile and Hydrocarbon Emissions. The oxidation of carbon monoxide and hydrocarbons is catalyzed by platinum/palladium/rhodium on alumina. If catalyst poisons such as lead and phosphorus are not present, the major problems become initiation of oxidation at low temperature, thermal stability at high temperature, resistance to thermal schock, and a high external surface area catalyst configuration. 

If a process feed contains known catalyst poisons, such as lead compounds in the exhaust of automobile emissions, it may be wise to locate the active species more... 

Noble metal catalysts are highly active for the oxidation of carbon monoxide and therefore widely used in the control of automobile emissions. Numerous recent studies on noble metal-based three-way catalysts have revealed characteristics of good thermal stability and poison resistance(l). Incorporation of rare earth oxides as an additive in automotive catalysts has improved the dispersion and stability of precious metals present in the catalyst as active components(2). Monolith-supported noble-metal catalysts have also been developed(3). However, the disadvantages of noble metal catalysts such as relative scarcity, high cost and requirement of strict air/fuel ratio in three-way function have prompted attention to be focused on the development of non-noble metal alternatives. 

When automobile emission regulations were first introduced, only platinum and palladium catalysts were used for the oxidation of carbon monoxide and residual hydrocarbons. Both metals were active oxidation catalysts although palladium was more temperature resistant than platinum but was more readily poisoned by sulfur. Rhodium became an important part of the three-way autocatalysts because it had high activity for the reduction of nitric oxide. Three-way catalysts were used in all cars from 1981 because of US Federal regulations although they had been introduced by California for 1978 model cars. 

Beginning with the 1975 U.S. automobiles, catalytic converters were added to nearly all models to meet the more restrictive emission standards. Since the lead used in gasoline is a poison to the catalyst used in the converter, a scheduled introduction of unleaded gasoline was also required. The U.S. petroleum industry simultaneously introduced unleaded gasoline into the marketplace. 

One notable exception has been the development of the catalytic exhaust system for automobiles, one of the most intense catalyst development efforts ever undertaken. An automotive catalyst normally consists of Pt/Pd and some Rh on a ceramic support. Catalytic exhaust control systems function under severe and rapidly changing conditions and must be active for several reactions that reduce automotive emissions—CO oxidation, hydrocarbon oxidation, and reduction (this is the so-called three-way catalyst). Typical operating conditions are temperatures of 400 to 600 C (or much greater under certain conditions) and 150,000 hr space velocity. Numerous reviews of the development and performance of these catalysts are available, and these catalysts are of interest because they are frequently used for control of VOC-emissions, particularly in conjunction with open flame preheaters. Unfortunately, these catalysts are not designed to resist poisoning by many VOC-type compounds, particularly those containing chlorine and sulfur. 

Auto emissions have been closely monitored, and strict controls have been put Into place to minimize the amount of unbumed hydrocarbons released into the atmosphere. The Clean Air Act of 1990 was passed to help reduce hydrocarbon emissions from automobiles. The catalytic converter was developed to help react the unburned hydrocarbon and produce a less dangerous emission of carbon dioxide and water. (As a side benefit, lead had to be eliminated from gasoline because it poisoned the catalyst and made the catal3dic converter useless. The big campaign to get the lead out removed a major source of the deadly heavy metal from the environment.)... 

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