Catalytic converter 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%. 

D. W. Wendland and W. R. Matthes, Visualicyation of Automotive Catalytic Converter Internal Flom, SAE 861554,1986 D. W. Wendland, P. L. Sorrell, and J. E. Kreucher, Sources of Monolithic Catalytic Converter Pressure Eoss, SAE 912372, Society of Automotive Engineers, Warrendale, Pa., 1991. 

Catalytic reactors have worked to the benefit of the chemical and petroleum industries for many decades, under the watchful eyes of plant engineers and batteries of monitor and control instruments. The automotive catalytic converter will be the first mass produced catalytic reactor placed directly in the hands of the public, who can provide little more than benign neglect. 

Many elements of a mathematical model of the catalytic converter are available in the classical chemical reactor engineering literature. There are also many novel features in the automotive catalytic converter that need further analysis or even new formulations the transient analysis of catalytic beds, the shallow pellet bed, the monolith and the stacked and rolled screens, the negative order kinetics of CO oxidation over platinum,... 

There is a general trend toward structured packings and monoliths, particularly in demanding applications such as automotive catalytic converters. In principle, the steady-state performance of such reactors can be modeled using Equations (9.1) and (9.3). However, the parameter estimates in Figures 9.1 and 9.2 and Equations (9.6)-(9.7) were developed for random packings, and even the boundary condition of Equation (9.4) may be inappropriate for monoliths or structured packings. Also, at least for automotive catalytic converters. 

At the heart of an automotive catalytic converter is a catalyzed monolith which consists of a large number of parallel channels in the flow direction whose walls are coated with a thin layer of catalyzed washcoat. The monolith catalyst brick is wrapped with mat, steel shell and insulation to minimize exhaust gas bypassing and heat loss to the surroundings. 

The widespread use of platinum, palladium and other metals in automotive catalytic converters has been driven by environmental considerations and the increasing costs of the metals. This has not been matched by the development of clean reprocessing technologies for the catalysts themselves. The spent catalyst metals are oxidized to their cations via leaching into concentrated acids. 

Case study Man-made mineral fibres in automotive catalytic converters ... 

Technical advantage/fimction Ceramic fibres are used in automotive catalytic converters as bearing and adjustment materials for the catalytic converter (monolith), where the chemical reactions for exhaust cleaning take place. They are also used for thermal and acoustic insulation. Series-tested ceramic fibre substitutes for converter-specific usage conditions are not yet available... 

British scientist William Hyde Wollaston Hard, anticorrosive metal whose salts have a red color, its name is derived from the Greek word for red used in automotive catalytic converters. 

This is an extremely important reaction to which we wiU refer throughout this book. It is responsible for all NO, formation in the atmosphere (the brown color of the air over large cities) as well as nitric acid and acid rain. This reaction only occurs in high-temperature combustion processes and in lightning bolts, and it occurs in automobile engines by free-radical chain reaction steps, which will be the subject of Chapter 10. It is removed from the automobile exhaust in the automotive catalytic converter, which wiU be considered in Chapter 7. 

The largest market for chemical reactors by far is the automotive catalytic converter (ACC), both in number of reactors in existence (many million sold per year) and in amount of reactants processed (mUhons of tons per year). There are >50 mtUion automotive catalytic converters operating (or at least existing) throughout the world, and everyone owns one if he or she has a car less than 10 years old. 

Figure 7-16 A highly simplified sketch of an automohile engine and catalytic converter with typical gas compositions indicated before and after the automotive catalytic converter. The catalytic converter is a tube wall reactor in which a noble-metal-impregnated wash coat on an extruded ceramic monolith creates surface on which reactions occur.

There are a number of examples of tube waU reactors, the most important being the automotive catalytic converter (ACC), which was described in the previous section. These reactors are made by coating an extruded ceramic monolith with noble metals supported on a thin wash coat of y-alumina. This reactor is used to oxidize hydrocarbons and CO to CO2 and H2O and also reduce NO to N2. The rates of these reactions are very fast after warmup, and the effectiveness factor within the porous wash coat is therefore very smaU. The reactions are also eternal mass transfer limited within the monohth after warmup. We wUl consider three limiting cases of this reactor, surface reaction limiting, external mass transfer limiting, and wash coat diffusion limiting. In each case we wiU assume a first-order irreversible reaction. 

Some of the important reactions in contemporary technology involve NO, which is a designation of N2O, NO, and NO2, and was one of the first examples in this book. The formation of these molecules in combustion processes is a major source of air pollution, and the catalytic oxidation of NH3 to NO on R surfaces is used to produce nitric acid, a major industrial chemical. The decomposition of NO, to N2 is a major process in the automotive catalytic converter. 

NO must be decomposed by bimolecular reactions to keep the product oxygen from blocking the surface sites on which NO must adsorb. In the automotive catalytic converter the NO in fact reacts with CO, which is another pollutant reduced in combustion. The reactions may be written as... 

This reaction is sufficiently fast that it proceeds at low enough temperature to be useful to eliminate NO in the automotive catalytic converter. We assumed that the surface reaction step is... 

Considerable research and engineering are being devoted to improving the performance of the automotive catalytic converter to reduce emissions that occur during startup. 

The most important reactor by far in twentieth century technolo is the fluidized catalytic cracker. It processes more chemicals than any other reactor (except the automotive catalytic converter), the products it creates are the raw materials for most of chemical technology, and this reactor is undoubtedly the largest and most complex piece of equipment in our business. Yet it is very possible that a student can receive a B.S. degree in chemical engineering without ever hearing of it... 

In fact, most of us benefit from the use of catalysis. Automotive catalytic converters have represented the most massive application of environmental catalysis and one of the most challenging and successful cases in catalysis, generally. Automobile catalysts deseive a few more comments. The engine exhaust emission is a complex mixture, whose composition and flow rate change continuously depending on a variety of factors such as driving conditions, acceleration, and speed. Despite the variability of the conditions, three-way catalysts have achieved the reduction of exhaust carbon monoxide, hydrocarbons, and... 

Monolithic catalysts have found a wide range of applications in the removal of pollutants from air, especially in the automotive industry. Specifically, the demand for large surface to small volume, high conversions achieved for low retention times, and low pressure drop led to the development of monolithic supports. More information on automotive catalytic converters has been given in Chapter 1. Usually, a thin layer of alumina is deposited onto a monolith for keeping the precious metal used for air pollutants abatement dispersed. The oxidations that take place are highly exothermic and the reaction rates achieved are in turn high. Hence, the reactants diffuse only a small distance... 

The catalyst of an automotive catalytic converter is typically a form of platinum, Pt palladium, Pd or rhodium, Rd. The production of catalytic converters is by far the largest market for these precious and semiprecious metals. 

ICP-MS is useful for analysis of catalysts from two perspectives The composition of the catalysts must be carefully controlled, particularly because the active elements are often expensive. The catalysts are often finely distributed in a substrate material so their concentration in the bulk material may be quite low. Second, catalysts, particularly those used in automotive catalytic converters, can be a significant source of platinum group elements in the environment. Re and Pt have been measured in catalysts by ICP-MS [193], Procedures for the analysis of used catalytic converter materials by ICP-MS have been reported [355]. Accurate measurements are essential for many of these applications so isotope dilution-based concentration calibration is commonly used. 

R. Merget and G. Rosner, Evaluation of the health risk of platinum group metals emitted from automotive catalytic converters, Sci. Total Environ., 270 (2001), 165D173. 

S. Caroli, F. Petrucci, B. Bocca, M. Krachler, F. Forastiere, A. Alimonti, Exposure to platinum-group metals released by automotive catalytic converters the case of urban youngsters, in A. M. Roussel, R. A. Anderson, A. E. Favier (eds), Trace Elements in Man and Animal, Kluwer Academic/Plenum Publishers, New York, vol. 10, 2000, pp. 667 D 670. 

Cerium 58 Ce Polishing powders, opacifier for porcelain coatings, glass decolouriser and melting accelerator, photographic materials, textiles, arc lamps, ferrous and non-ferrous alloys including high-temperature Mg alloys, automotive catalytic converters, misch metal... 

Complex oxides of the perovskite structure containing rare earths like lanthanum have proved effective for oxidation of CO and hydrocarbons and for the decomposition of nitrogen oxides. These catalysts are cheaper alternatives than noble metals like platinum and rhodium which are used in automotive catalytic converters. The most effective catalysts are systems of the type Lai vSrvM03, where M = cobalt, manganese, iron, chromium, copper. Further, perovskites used as active phases in catalytic converters have to be stabilized on the rare earth containing washcoat layers. This then leads to an increase in rare earth content of a catalytic converter unit by factors up to ten compared to the three way catalyst. 

---