Automobile exhaust metals

Fossil fuel combustion—S N oxides and acid radicals Pb, PAHs from automobile exhausts Metal smelting—As, Cd, Cu, Cr, Ni, Pb, Sb, Tl, Zn Chemical industries—organic micropollutants, Hg Waste incineration—TCDDs, TCDFs... 

Occurrence. Carbon monoxide is a product of incomplete combustion and is not likely to result where a flame bums in an abundant air supply, yet may result when a flame touches a cooler surface than the ignition temperature of the gas. Gas or coal heaters in the home and gas space heaters in industry have been frequent sources of carbon monoxide poisoning when not provided with effective vents. Gas heaters, though properly adjusted when installed, may become hazardous sources of carbon monoxide if maintained improperly. Automobile exhaust gas is perhaps the most familiar source of carbon monoxide exposure. The manufacture and use of synthesis gas, calcium carbide manufacture, distillation of coal or wood, combustion operations, heat treatment of metals, fire fighting, mining, and cigarette smoking represent additional sources of carbon monoxide exposure (105—107). 

Reclamation, Disposal, and Toxicity. Removal of poisons and inorganic deposits from used catalysts is typically difficult and usually uneconomical. Thus some catalysts are used without regeneration, although they may be processed to reclaim expensive metal components. Used precious metal catalysts, including automobile exhaust conversion catalysts, are treated (often by the suppHers) to extract the metals, and recovery efficiencies are high. Some spent hydroprocessing catalysts may be used as sources of molybdenum and other valuable metals. 

This reaction takes place similarly in automobile exhaust converters, although the metals in these catalysts are platinum and rhodium, among other components, and other reactions occur. 

Two classes of metals have been examined for potential use as catalytic materials for automobile exhaust control. These consist of some of the transitional base metal series, for instance, cobalt, copper, chromium, nickel, manganese, and vanadium and the precious metal series consisting of platinum [7440-06-4], Pt palladium [7440-05-3], Pd rhodium [7440-16-6], Rh iridium, [7439-88-5], Ir and mthenium [7440-18-8], Ru. Specific catalyst activities are shown in Table 3. 

The precious metals possess much higher specific catalytic activity than do the base metals. In addition, base metal catalysts sinter upon exposure to the exhaust gas temperatures found in engine exhaust, thereby losing the catalytic performance needed for low temperature operation. Also, the base metals deactivate because of reactions with sulfur compounds at the low temperature end of auto exhaust. As a result, a base metal automobile exhaust... 

The mechanism of poisoning automobile exhaust catalysts has been identified (71). Upon combustion in the cylinder tetraethyllead (TEL) produces lead oxide which would accumulate in the combustion chamber except that ethylene dibromide [106-93-4] or other similar haUde compounds were added to the gasoline along with TEL to form volatile lead haUde compounds. Thus lead deposits in the cylinder and on the spark plugs are minimized. Volatile lead hahdes (bromides or chlorides) would then exit the combustion chamber, and such volatile compounds would diffuse to catalyst surfaces by the same mechanisms as do carbon monoxide compounds. When adsorbed on the precious metal catalyst site, lead haUde renders the catalytic site inactive. 

J. T. Kummer, "The Use of Noble Metals in Automobile Exhaust Catalyst," Chicago Symposium, 1985. 

The relatively high cost and lack of domestic supply of noble metals has spurred considerable efforts toward the development of nonnoble metal catalysts for automobile exhaust control. A very large number of base metal oxides and mixtures of oxides have been considered, especially the transition metals, such as copper, chromium, nickel, manganese, cobalt vanadium, and iron. Particularly prominent are the copper chromites, which are mixtures of the oxides of copper and chromium, with various promoters added. These materials are active in the oxidation of CO and hydrocarbons, as well as in the reduction of NO in the presence of CO (55-59). Rare earth oxides, such as lanthanum cobaltate and lanthanum lead manganite with Perovskite structure, have been investigated for CO oxidation, but have not been tested and shown to be sufficiently active under realistic and demanding conditions (60-63). Hopcalities are out-... 

Almost all the materials which are being considered as components in automobile exhaust catalyst are somewhat toxic (74)- Most of the compounds considered are low vapor pressure solids which can only escape from the exhaust system as very fine airbone dust particles formed by catalyst attrition. A few compounds, such as the highly toxic metal carbonyls and ruthenium tetroxides, are liquid under ambient conditions and have boiling points less than 100 °C. These compounds are not present in... 

Organometallic compounds are included as it is becoming increasingly apparent that these compounds occur extensively throughout the ecosystem either as direct pollutants eg alkylead compounds from automobile exhausts or by biomethylation of inorganic metals occurring in sediments. 

The oxidation of CO by Oj over group VIII metal catalysts has been the subject of a large body of ultrahigh vacuum surface science and high pressure catalysis work due to its importance in pollution control. Currently, the removal of CO as CO2 from automobile exhaust is accomplished by catalytic converters which employ a supported Pt, Pd, and Rh catalyst. The importance of CO oxidation has led to numerous recent studies of the kinetics of this reaction on supported metal catalysts and transient kinetic studies on polycrystalline foils , which have sought to identify and quantify the parameters of the elementary mechanistic steps in CO oxidation. 

Japanese chemists succeeded in obtaining good yields of methane by reaction of H2 with a mixture of carbon monoxide and carbon dioxide, at temperatures as low as 270 °C, by use of a special mixed catalyst containing nickel as the most important metallic constituent. Why is nickel used In the same vein, why is platinum or platinum-rhodium alloy (but not nickel) used in catalytic converters for automobile exhausts (See also Section 17.4.)... 

Olefins - [FEEDSTOCKS - COALCHEMICALS] (Vol 10) - [FEEDSTOCKS-PETROCHEMICALS] (VollO) - [HYDROCARBONS - SURVEY] (Vol 13) -m automobile exhaust [EXHAUSTCONTROL, AUTOMOTIVE] (Vol 9) -catalyst for stereospeafic polymerization [TITANIUMCOMPOUNDS - INORGANIC] (Vol 24) -esters from [ESTERIFICATION] (Vol 9) -hydroxylation using H202 [HYDROGEN PEROXIDE] (Vol 13) -luminometer ratings [AVIATION AND OTHER GAS TURBINE FUELS] (Vol 3) -osmium oxidations of [PLATINUM-GROUP METALS, COMPOUNDS] (Vol 19) -polymerization [SULFONIC ACIDS] (Vol 23) -reaction with EDA [DIAMINES AND HIGHER AMINES ALIPHATIC] (Vol 8) -silver complexes of [SILVER COMPOUNDS] (Vol 22)... 

Recent evaluations of S02 oxidation over noble metal catalysts (Pt, Pd, and Rh) have given some information on one particular secondary reaction. It was observed in car tests that S03 formation under the conditions of automobile exhaust is highly vulnerable to catalyst deactivation either by thermal sintering or by poisoning (78, 79). At the same time, the data indicated a lesser sensitivity of CO and hydrocarbon oxidation to catalyst aging. The results were confirmed in laboratory experiments (80). This is one example of preferential suppression of an undesirable side reaction. Obviously, the importance of a given poison on the different secondary reactions will vary widely with catalyst formulation and operating conditions. 

In the field of toxic substances which pollute our environment, special importance must be attached to the heavy metal lead, which, until the late eighties arose primarily from automobile exhaust [EWERS and SCHLIPKOTER, 1984], For quantitative measurement of the impact of this lead, it is essential to establish the correlation between the lead content of the plants and soils, the traffic density, and the distance from the road. 

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