The catalytic converter for automobile exhaust

The metallic catalysts for exhaust pollution control are designed to perform three functions. The air/fuel ratio employed in combustion engines creates exhaust products which are a mixture of hydrocarbons, carbon oxides, and nitrogen oxides. These must be rendered environmentally innocuous by reactions on the catalyst such as 

There is a significant difference between rhodium and the other metals in that rhodium forms a relatively stable oxide, Rh203. The Gibbs energy of formation of this oxide is given by the equation 

The kinetics of the oxidation of CO on a platinum surface indicate that CO and oxygen are adsorbed to about the same extent. The rate of oxidation depends on the oxygen partial pressure when CO is in excess, and on the CO partial pressure when oxygen is in excess. 

One feature of oxides is that, like all substances, they contain point defects which are most usually found on the cation lattice as interstitial ions, vacancies or ions with a higher charge than the bulk of the cations, referred to as positive holes because their effect of oxygen partial pressure on the electrical conductivity is the opposite of that on free electron conductivity. The interstitial ions are usually considered to have a lower valency than the normal lattice ions, e.g. Zn+ interstitial ions in the zinc oxide ZnO structure. 

An effect which is frequently encountered in oxide catalysts is that of promoters on the activity. An example of this is the small addition of lithium oxide, Li20 which promotes, or increases, the catalytic activity of the alkaline earth oxide BaO. Although little is known about the exact role of lithium on the surface structure of BaO, it would seem plausible that this effect is due to the introduction of more oxygen vacancies on the surface. This effect is well known in the chemistry of solid oxides. For example, the addition of lithium oxide to nickel oxide, in which a solid solution is formed, causes an increase in the concentration of the major point defect which is the Ni3+ ion. Since the valency of the cation in the alkaline earth oxides can only take the value two the incorporation of lithium oxide in solid solution can only lead to oxygen vacancy formation. Schematic equations for the two processes are 

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In this chapter we consider systems in which a reaction between two gaseous species is carried out in the adsorbed state on the surface of a solid. The products of the reaction will be gaseous, and the solid acts to increase the rate of a reaction which, in the gaseous state only, would be considerably slower, but would normally yield the same products. This effect is known as catalysis and is typified in industry by the role of adsorption in increasing the rate of synthesis of many organic products, and in the reduction of pollution by the catalytic converter for automobile exhaust. 

The standard, when one considers multiphase reactors, has become more complex over the years. Most can be classified as reactions over heterogenous catalysts. The catalytic activity occurs in one phase, the solid phase, while transport of the reactants occurs in a gas or liquid phase, or both. A common example is the catalytic converter for automobile exhaust gas. The key steps for a packed bed reactor are ... 

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.)... 

The structured packing for column and ceramic monoliths has been in use for several decades. The latter is used in catalytic converters for automobile exhausts for the in-line conversion of CO and oxides of nitrogen into harmless oxides. It is also used for catalyst and adsorbent supports in process industries. They offer higher specific surface area and low pressure drop. A comprehensive review of monolith reactors as alternatives to trickle-bed, slurry, and slurry bubble column reactors is available in Roy et al. [65]. The monolith reactor... 

Catalysis. Platinum and Pt alloys are preferably applied in heterogeneous catalysis as wire nets or powders with a high specific surface area ranging from 20 to 1000 m /g ( platinum black, palladium black ) on carbon or AI2O3 supports. The catalytic effec-tivity is structure-sensitive. Figure 3.1-300 show an example of the catalytic action of Pt for the reaction rate and the product selectivity on different crystal planes [1.218]. Pt—Pd—Rh alloys are the main active constituents of catalytic converters for automobile exhaust gas cleaning. 

Platinum s use in catalytic converters for automobile exhaust emissions is mentioned in the palladium section and its use in thermocouples in the rhodium section. 

The solver is implemented in Fortran, using optimized treatment of diagonal-band matrices and analytical derivatives of reaction rates to minimize computation time. The software structure is modular, so that different reaction-kinetic modules for individual types of catalysts can be easily employed in the monolith channel model. The compiled converter models are then linked in the form of dynamic libraries into the common environment (ExACT) under Matlab/Simulink. Such combination enables fast and effective simulation of combined systems of catalytic monolith converters for automobile exhaust treatment. 

Outlandish as it may seem, a new scheme has been proposed to turn automobiles into air purifiers, devouring the pollutants ozone and carbon monoxide. BASF, an Iselin, New Jersey, company that specializes in the manufacture of catalytic converters for automotive exhaust systems, has developed a catalyst that decomposes ozone to oxygen and converts carbon monoxide to carbon dioxide. BASF proposes to paint the catalyst on automobile... 

Typical ceramic materials produced on a co-rotating twin screw extruder are for example catalyst carriers. They are commonly shaped into granules for use as bulk material in reactors in the chemical industry or into honeycombs for catalytic converters in automobiles exhaust systems (Fig. 12). After extrusion, the catalyst carriers are cut oversized in the lineal direction, dried and then cut to the proper length. Afterwards the binder is removed and the carriers are calcinated or sintered. Finally, to provide them with catalytic properties, they are impregnated with an active film in a bath [Fri76]. 

Oxidation. Carbon monoxide can be oxidized without a catalyst or at a controlled rate with a catalyst (eq. 4) (26). Carbon monoxide oxidation proceeds explosively if the gases are mixed stoichiometticaHy and then ignited. Surface burning will continue at temperatures above 1173 K, but the reaction is slow below 923 K without a catalyst. HopcaUte, a mixture of manganese and copper oxides, catalyzes carbon monoxide oxidation at room temperature it was used in gas masks during World War I to destroy low levels of carbon monoxide. Catalysts prepared from platinum and palladium are particularly effective for carbon monoxide oxidation at 323 K and at space velocities of 50 to 10, 000 h . Such catalysts are used in catalytic converters on automobiles (27) (see Exhaust CONTHOL, automotive). 

Emission Control Technologies. The California low emission vehicle (LEV) standards has spawned iavestigations iato new technologies and methods for further reducing automobile exhaust emissions. The target is to reduce emissions, especially HC emissions, which occur during the two minutes after a vehicle has been started (53). It is estimated that 70 to 80% of nonmethane HCs that escape conversion by the catalytic converter do so during this time before the catalyst is fully functional. 

In this section the models employed for simulation of catalytic monolith reactor are discussed, focusing on effective description of heat and mass transfer phenomena in monolith channel. The number of different mathematical models developed for converters of automobile exhaust gases over the last decades is huge—cf., e.g. Heck et al. (1976), Young and Finlayson (1976), Oh and Cavendish (1982), Zygourakis and Aris (1983), Chen et al. (1988),... 

The following, well-acceptable assumptions are applied in the presented models of automobile exhaust gas converters Ideal gas behavior and constant pressure are considered (system open to ambient atmosphere, very low pressure drop). Relatively low concentration of key reactants enables to approximate diffusion processes by the Fick s law and to assume negligible change in the number of moles caused by the reactions. Axial dispersion and heat conduction effects in the flowing gas can be neglected due to short residence times ( 0.1 s). The description of heat and mass transfer between bulk of flowing gas and catalytic washcoat is approximated by distributed transfer coefficients, calculated from suitable correlations (cf. Section III.C). All physical properties of gas (cp, p, p, X, Z>k) and solid phase heat capacity are evaluated in dependence on temperature. Effective heat conductivity, density and heat capacity are used for the entire solid phase, which consists of catalytic washcoat layer and monolith substrate (wall). 

The primary use of platinum and other platinum metals is as catalysts. A catalyst is a substance used to speed up a chemical reaction without undergoing any change itself. For example, the catalytic converter in an automobile s exhaust system may contain a platinum metal. 

Catalytic combustion applications can be classified as either primary or secondary pollution control, that is, emissions prevention or emissions clean-up. The most common example of catalytic combustion for emissions clean-up is the catalytic converter in the exhaust system of automobiles. Catalytic combustion is also increasingly used for the removal of volatile organic compounds (VOCs) from industrial exhaust streams. The use of catalytic combustion in exhaust gas clean-up is discussed in other sections of this Handbook this section deals only with primary control applications. 

Teti amethyllead, which once represented a significant demand for methanol, has declined drastically as the U.S. automobile has been equipped with catalytic converters to eliminate exhaust pollution, with the result that tetramethyllead has been eliminated from gasoline except for some commercial vehicles. 

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