Converters, catalytic

In automotive catalytic converters, Ce(III)oxide is used as a catalyst for the oxidation of CO and NOx. See Fig. 5.5 for a schematic image of a catalytic converter. 

Numerous permutations in composition exist, but usually the precise composition, particularly that of the washcoat, is a commercial secret. Detailed accounts of the three-way catalyst have been given by Heck and Farrauto [R.M. Heck and R.J. Farrauto, Catalytic Air Pollution Control, (2002, 2 Edition), WUey, New York.]. Here we briefly describe the functions of the catalyst ingredients. 

The ideal operating temperatures for the three-way catalyst lie between 350 and 650 °C. After a cold start it takes at least a minute to reach this temperature, implying that most CO and hydrocarbons emission takes place directly after the start. Temperatures above 800 °C should be avoided to prevent sintering of the noble metals and dissolution of rhodium in the support. 

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Several parameters come into the relation between density and equivalence ratio. Generally, the variations act in the following sense a too-dense motor fuel results in too lean a mixture causing a potential unstable operation a motor fuel that is too light causes a rich mixture that generates greater pollution from unburned material. These problems are usually minimized by the widespread use of closed loop fuel-air ratio control systems installed on new vehicles with catalytic converters. 

These compounds have been incorporated in the gasolines of the entire world since 1922. Their use will disappear with the general acceptance of catalytic converters with which they are totally incompatible. 

The lead alkyls and scavengers contained in fuels cause rapid poisoning ol exhaust gas catalytic converters. They are tolerated only in trace quantities in fuels for vehicles having that equipment. The officially allowed content is 0.013 g Pb/1, but the contents observed in actual practice are less than 0.005 g Pb/1. 

As of 1 July 1991 all new vehicles registered in Europe should have been adapted for using Eurosuper. However, another quality of unleaded fuel had already appeared in 1988 and has since been well established in some countries, notably France. It is called Superplus, the highest octane level, RON 98, MON 88. Superplus is suitable for vehicles having catalytic converters as well as for a large part of the former automotive fleet requiring an RON of 97 and higher. 

However, such a level can still be considered too high for vehicles having 3-way catalytic converters. In fact, results observed in the United States (Benson et al., 1991) and given in Figure 5.20 show that exhaust pollutant emissions, carbon monoxide, hydrocarbons and nitrogen oxides, increase from 10 to 15% when the sulfur level passes from 50 ppm to about 450 ppm. This is explained by an inhibiting action of sulfur on the catalyst though... 

Influence of the sulfur content in diesel fuel on particulate emissions as a, function of the catalytic converter inlet temperature. 

The implementation of very effective devices on vehicles such as catalytic converters makes extremely low exhaust emissions possible as long as the temperatures are sufficient to initiate and carry out the catalytic reactions however, there are numerous operating conditions such as cold starting and... 

Eurosuper gasoline Engine speed 1500 rpm Equivalence ratio 1,00 Sample point upstream of catalytic converter Relative reactivity factor Total teactivity 2.66 g O lg HC... 

The development of catalytic converters for combustion of unburned hydrocarbons prohibits a return to lead compounds and henceforth refiners are turning to oxygenated compounds that must be used as a gasoline component therefore, in amounts much greater than those of lead compounds. 

The protection of the environment implies the elimination of lead compounds, first of all because of their individual toxicities and second because these derivatives or their products of decomposition poison catalytic converter catalysts. 

The increase in the oil-change interval has already been a strong incentive for improving lubricant formulations. The increase in engine operating temperatures and the development of catalytic converters are without doubt two orientations that will have consequences on lubricant additives. 

We consider next perhaps the bet understood catalyzed reaction the oxidation of CO over group VIII metal catalysts. The reaction is an important environmental one since it involves the conversion of CO to CO2 in automobile catalytic converters. The mechanism is straightforward ... 

Another even more significant use of methyl alcohol can be as a fuel in its own right in fuel cells. In recent years, in cooperation with Caltech s Jet Propulsion Laboratory (JPL), we have developed an efficient new type of fuel cell that uses methyl alcohol directly to produce electricity without the need to first catalytically convert it to produce hydrogen. 

Although the naturally occurring concentration of ozone at the earth s surface is low, the distribution has been altered by the emission of pollutants, primarily by automobiles but also from industrial sources which lead to the formation of ozone. The strategy for controlling ambient ozone concentrations arising from automobile exhaust emissions is based on the control of hydrocarbons, CO, and NO via catalytic converters. As a result, peak ozone levels in Los Angeles, for instance, have decreased from 0.58 ppm in 1970 to 0.33 ppm in 1990, despite a 66% increase in the number of vehicles. 

RCF is sold in a variety of forms, such as loose fiber, blanket, boards, modules, cloth, cements, putties, paper, coatings, felt, vacuum-formed shapes, rope, braid, tape, and textiles. The products are principally used for industrial appHcations as insulation in furnaces, heaters, kiln linings, furnace doors, metal launders, tank car insulation, and other uses up to 1400°C. RCF-consuming industries include ferrous and nonferrous metals, petrochemical, ceramic, glass, chemical, fertiH2er, transportation, constmction, and power generation/incineration. Some newer uses include commercial fire protection and appHcations in aerospace, eg, heat shields and automotive, eg, catalytic converters, metal reinforcement, heat shields, brake pads, and airbags. 

Type 409, developed as a less expensive replacement for Type 430 in automotive appHcations such as trim and catalytic converters, has become the principal alloy in this area. 

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). 

Some catalyst supports rely on a relatively low surface area stmctural member coated with a layer of a higher surface area support material. The automotive catalytic converter monolith support is an example of this technology. In this appHcation, a central core of multichanneled, low surface area, extmded ceramic about 10 cm in diameter is coated with high surface area partially hydrated alumina onto which are deposited small amounts of precious metals as the active catalytic species. 

Automotive Catalytic Converter Catalysts. California environmental legislation in the early 1960s stimulated the development of automobile engines with reduced emissions by the mid-1960s, led to enactment of the Federal Clean Air Act of 1970, and resulted in a new industry, the design and manufacture of the automotive catalytic converter (50). Between 1974 and 1989, exhaust hydrocarbons were reduced by 87% and nitrogen oxides by 24%. 

In principle, the catalytic converter is a fixed-bed reactor operating at 500—620°C to which is fed 200—3500 Hters per minute of auto engine exhaust containing relatively low concentrations of hydrocarbons, carbon monoxide, and nitrogen oxides that must be reduced significantly. Because the auto emission catalyst must operate in an environment with profound diffusion or mass-transfer limitations (51), it is apparent that only a small fraction of the catalyst s surface area can be used and that a system with the highest possible surface area is required. 

The most widely used exhaust control device consists of a ceramic monolith with a thin-waHed open honeycomb stmcture. The accessible surface of this monolith system is iacreased by applyiag a separate coatiag, a wash coat, of a high surface area material such as gamma-alumiaa with the catalyticaHy active species impregaated iato this washcoat. The catalyst aeeds to oxidize hydrocarboas, coavert CO to CO2, and reduce NO. The whole system forms a catalytic converter that, suitably encased, is placed between the engine and the muffler/silencer unit. 

K. C. Taglot, Automobile Catalytic Converters, Sptinger-Verlag, New York, 1984. 

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