Automotive exhaust converter

Ask the average person in the street what a catalyst is, and he or she will probably tell you that a catalyst is what one has under the car to clean up the exhaust. Indeed, the automotive exhaust converter represents a very successful application of catalysis it does a great job in removing most of the pollutants from the exhaust leaving the engines of cars. However, catalysis has a much wider scope of application than abating pollution. 

Sulfur in cmde oil is mainly present in organic compounds such as mercaptans (R-SH), sulfides (R-S-R ) and disulfides (R-S-S-R ), which are all relatively easy to desulfurize, and thiophene and its derivatives (Fig. 9.2). The latter require more severe conditions for desulfurization, particularly the substituted dibenzothiophenes, such as that shown in Fig. 9.2. Sulfur cannot be tolerated because it produces sulfuric add upon combustion, and it also poisons reforming catalysts in the refinery and automotive exhaust converters (particularly those for diesel-fueled cars). Moreover, sulfur compounds in fuels cause corrosion and have an unpleasant smell. 

Figure 10.4. Typical automotive exhaust converters. The one on the left has been cut open to reveal the monolith. The insert shows a blow up of the upper part of the monolith where a part has been chipped off.

As with the automotive exhaust converter, the SCR catalyst is designed to handle large flows of gas (e.g. 300 N s for a 300 MW power plant) without causing a significant pressure drop. Figure 10.12 shows a reactor arrangement with about 250 m of catalyst in monolithic form, sufficient for a 300 MW power plant. 

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%. 

The effect of alkali presence on the adsorption of oxygen on metal surfaces has been extensively studied in the literature, as alkali promoters are used in catalytic reactions of technological interest where oxygen participates either directly as a reactant (e.g. ethylene epoxidation on silver) or as an intermediate (e.g. NO+CO reaction in automotive exhaust catalytic converters). A large number of model studies has addressed the oxygen interaction with alkali modified single crystal surfaces of Ag, Cu, Pt, Pd, Ni, Ru, Fe, Mo, W and Au.6... 

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. 

Cant et al [21] focused their attention on the concentration of N20 in the automotive exhaust gas, which are rather low (14 ppm) but quite dependent on the air-to-fuel ratio. Typically 60-80% of NO is converted into N20 below the light-off temperature on Rh and then the selectivity drops at relatively high temperature 370°C [21,22] when the partial pressures of NO tends below lOTorr [22-25]. 

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... 

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. 

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... 

As discussed, the low temperature deNOx efficiency of SCR converters for automotive exhaust aftertreatment can be significantly enhanced by converting part of the nitric oxide to N02, e.g. by means of a DOC located upstream of the SCR. In fact, the so-called fast SCR reaction, involving the reaction between NH3 and equimolar amounts of NO and N02, can be faster by one order of magnitude than the standard SCR in the low-T region (Ciardelli et al., 2007a Koebel et al., 2001). Effective exploitation of fast SCR reactivity is certainly important... 

Exhaust emission legislation has become more and more stringent over the last years, demanding for lower engine raw emissions and more efficient exhaust converters. Simultaneous low emission limits for different species, e.g. PM and NOx, lead to the development of combined aftertreatment systems, consisting of different catalyst technologies and particulate filter. Simulation can make a considerable contribution to shorten the time and lower the cost of the system development. In this publication, the current status of exhaust aftertreatment simulation tools used in automotive industry is reviewed. The developed models for DOC with HC adsorption, NSRC and catalyst for SCR of NOx by NH3 (urea) were included into the common simulation environment ExACT, which enables simulation of complete combined exhaust aftertreatment systems. 

Another important application of heterogeneous catalysts is in automobile catalytic converters. Despite much work on engine design and fuel composition, automotive exhaust emissions contain air pollutants such as unburned hydrocarbons (CxHy), carbon monoxide, and nitric oxide. Carbon monoxide results from incomplete combustion of hydrocarbon fuels, and nitric oxide is produced when atmospheric nitrogen and oxygen combine at the high temperatures present in an... 

Some prominent industrial examples of packed-bed reactors are in ammonia, methanol or vinyl acetate synthesis, and in ethylene, methanol, naphthalene, xylene or SO2 oxidation. In recent years (since the 1975 model year), an important application of packed-bed reactors has been as catalytic converters for pollution control from automotive exhausts. 

A cutaway model of a catalytic converter used in automotive exhaust systems,... 

However, for the relatively low sulfur concentrations to which three-way catalyst are typically exposed, the precious metals are usually unaffected. Poisoning of ceria is much more serious than poisoning of the precious metal at the levels of 5 to 20 ppm SO2 currently present in the typical automotive exhaust. At these concentrations, SO2 interacis primarily with the ceria-containing component of in the catalytic converter and it is this poisoning of ceria that appears to be the primary problem associated with sulfur poisoning [6-11]. The evidence for this is strong. For... 

Many modeling studies and experimental investigations have demonstrated that the intentional unsteady operation of reactors can profoundly influence conversion and/or selectivity. Several reviews were published [28.29,31,44—46]. The effects for automotive exhaust gas converters were recently discussed [30]. 

The influence of the modulation frequency, see Fig. 19, shows beneficial effects in the frequency range 0-10 Hz, which is largest for NO reduction, with a maximum around the typical frequency of 1 Hz as met in the practice of automotive exhaust gas converters. 

Modeling of monolith reactors from first principles presents a valuable tool in the design of such reactors and in the analysis of the underlying phenomena. The results presented show that the reactor behavior can be adequately described and understood by a combination of the reactor s transport characteristics and the intrinsic kinetics obtained with a laboratory reactor of another type. As such we can generalize monolith models to other reaction networks, e.g., extend the given description of the dynamic operation for combined CO oxidation and NO reduction in the automotive exhaust gas converter to include other reactions, like the oxidation of various hydrocarbons and of hydrogen. The availability, however, of a proper kinetic model is a definite prerequisite. 

Not all catalysts need the extended smface provided by a porous structure, however. Some are sufficiently active so that the effort required to create a porous catalyst would be wasted. For such situations one type of catalyst is the monolithic catalyst. Monolithic catalysts are normally encountered in processes where pressure drop and heat removal are major considerations. Typical examples include the platinum gauze reactor used in the ammonia oxidation portion of nitric acid manufacture and catalytic converters used to oxidize pollutants in automobile exhaust. They can be porous (honeycomb) or non-porous (wire gauze). A photograph of a automotive catalytic converter is shown in Figure CD 11-2. Platinum is a primary catalytic material in the monolith. 

In the late 1960s, new antipollution initiatives were enacted to reduce nitrogen oxides, carbon monoxide, and lead pollutants from automotive exhaust. Nitrogen oxides were responsible for the brown haze that hung over cities that can still be seen today. The advent of the catalytic converter, a small canister that contained heavy metal catalysts embedded on a ceramic support, helped oxidize carbon monoxide and reverse the reaction that produced nitrogen oxides. However, lead in the exhaust stream deactivated the catalysts in the catalytic converter. The only solution was to remove tetraethyllead from the gasoline. 

Because of its ability to chemisorb NO dissociatively, Rh is the key catalyst in TWC converters for the reduction of NOx. Due to its rarity in nature in comparison with the other noble metals Pt and Pd ( 1 15) and its consequent significantly higher cost, a reduction in the amount of Rh present in automotive exhaust catalytic converters, via appropriate enhancement (promotion) of the catalytic activity of the other noble metals components (Pt or Pd) would be highly desirable. 

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