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The use of catalytic converters for the purification of automotive exhaust gases is a relatively new technology which was brought into existence by social pressures for the preservation of acceptable environmental conditions. The majority of catalytic practitioners have been able to watch the growth of this technology from its inception to its current state of sophistication. Automotive catalytic converter technology is now in a mature state, and this chapter from Vol. 5 Catalysis: Science and Technology by Dr. K. C. Taylor provides a review which covers both the process chemistry and the most important converter design factors. Contents 1. Introduction. . . . . . . . . . . . . . 2. Emission Regulations in the United States. 3. Exhaust Emission Characteristics. . 3 4. 1981 Emission Control Technology. 5 A. Converters. . . 5 B. Control System. 7 8 ~. Catalyst Screening . 6. Laboratory Testing. .10 7. The Chemical Reactions 13 8. Composition of Three-Way Catalysts. 16 A. Rhodium 17 21 B. Platinum. C. Palladium 22 D. Iridium . 22 23 E. Ruthenium and Nickel. F. Cerium Oxide ..... 23 G. Search for Alternatives to Nohle Metals 24 9. Catalyst Supports . 25 A. Pellets .... 26 B. Monoliths . . 26 10. The Transient Behavior of Three-Way Catalysts 27 II. Deterioration of Three-Way Catalysts. 35 A. Thermal Effects. . . . 35 B. Phosphorus Poisoning. . . 37 C. Lead Poisoning. . . . . . · 38 D. Catalyst Poisoning by Sulfur · 40 12. The 0.4 NO,; Research Objective. · 41 13. Control of Diesel Particulate Emissions.

Inhaltsverzeichnis

Frontmatter

1. Introduction

Abstract
Catalysts have been widely used to lower the emissions of carbon monoxide (CO) and hydrocarbons (HC) in the exhaust of automobiles in the United States since the introduction of 1975 models in the fall of 1974. These catalysts, contained in so-called catalytic converters in the exhaust system of automobiles, promote the oxidation of CO and HC to CO2 and H2O under net oxidizing conditions (e.g. A/F > 14.6). Until 1978, emission control requirements for nitrogen oxide (NOx) emissions were met through non- catalyst technology, primarily exhaust gas recirculation (EGR) [1, 2]. Starting with some vehicles sold in California in 1977, NOx emissions from gasoline engines have been subject to catalytic control. The catalyst here has the additional function to promote the reduction of NO to N2 via reaction of NO with hydrogen or CO. Catalyst systems designed to reduce NOx are considerably more complex than the earlier control systems. For example, the control system introduced by General Motors on some 1978 model year cars has closed-loop air-fuel ratio control (closed-loop fuel metering system, exhaust gas oxygen sensor, and an electronic control unit) as well as a three-way catalyst which simultaneously promotes the conversion of HC, CO, and NOx [3]. Stringent federally mandated emission control requirements of 1 gram per mile (g mi-1) for NOx have led to the further application of three-way catalysts. This review will emphasize the state-of- the-art of catalytic control of automobile exhaust emissions since 1978, specifically three-way catalysts. A recent review by J. Kummer covers part of this period and earlier years [4]. Other reviews of this subject are listed in the reference section [5–14].
Kathleen C. Taylor

2. Emission Regulations in the United States

Abstract
Table 1 lists the passenger car emission control requirements (current as of September, 1982) for all passenger cars sold in the United States except where waivers have been granted by the Environmental Protection Agency for specific vehicles. The law is currently under review (1982) so these requirements may be changed for future years. The 1981 exhaust standards represent a reduction from uncontrolled 1960 levels of 96% for HC, 96% for CO, and 76% for NOx.
Kathleen C. Taylor

3. Exhaust Emission Characteristics

Abstract
The engine-out exhaust emissions of CO, HC, and NOx vary as a function of air-fuel ratio as well as several other parameters such as ignition timing and EGR. Here we shall consider only the relationship to air-to-fuel ratio (A/F). A general relationship between engine out CO, HC, and NOx emissions and A/F is shown in Figure 1. Engine operation at lean A/F (net oxidizing condition) results in lower HC and CO emissions and more O2. These conditions favor subsequent catalytic oxidation reactions. At lean A/F the exhaust contains insufficient reducing agents to react with all the O2 and all the NO. By operating closer to the stoichiometric A/F more NO can react but the volume of CO which must be oxidized increases. The stoichiometric A/F occurs at about 14.6. Here, the concentrations of oxidizing gases and reducing gases are matched, and equilibration of the exhaust mixture would yield only CO2, H2O, and N2. The application of three-way catalytic converters has this objective. Three-way catalysts operate in a narrow A/F band between 14 and 15.
Kathleen C. Taylor

4. 1981 Emission Control Technology

Abstract
The catalytic converters used on most 1981 model year vehicles fall into two general categories: three-way converters and dual-bed converters. (Some manufacturers employ dual converters. The distinction between dual-bed and dual converters is whether the two catalysts are housed in the same container or in separate containers.) Both contain a three-way catalyst, but with the dual-bed converter (and dual converter) the three-way catalyst is followed by an oxidation catalyst to provide increased oxidation capability. Supplemental air is added to the exhaust ahead of the oxidation catalyst. The pellet type dual-bed catalytic converter used by General Motors is shown in Figure 2. Both the three-way catalyst and oxidation catalyst are enclosed in the same converter, separated by the air plenum. Operation of the three-way catalyst requires that the exhaust A/F be controlled close to the stoichiometric composition. A diagram which illustrates the relationship between conversion efficiency and A/F for the two converter types is presented in Figure 3. A closed-loop feed-back control system holds the A/F in a narrow region near stoichiometry.
Kathleen C. Taylor

5. Catalyst Screening

Abstract
The automobile manufacturers have developed dynamometer-controlled engine facilities which they use to screen catalysts before certification [30, 32–35]. Aging of catalysts by tests on a dynamometer allows catalyst durability to be measured under precisely controlled conditions and possibly completed in fewer days (by running 24 h days) than for vehicle aging. Dynamometer-controlled engine facilities are used for accelerated aging in which the catalysts are exposed to poisons and high temperatures as well as for short emissions tests. A short description of catalyst screening tests as described in published reports follows.
Kathleen C. Taylor

6. Laboratory Testing

Abstract
Laboratory testing facilities used for the characterization and preliminary evaluation of three-way catalysts have been described in several reports. Descriptions of several of these reactor systems are given here.
Kathleen C. Taylor

7. The Chemical Reactions

Abstract
The essential requirement for an effective three-way catalyst is high conversions of NOx, CO, and hydrocarbons at and near the stoichiometrically balanced exhaust composition. In general, performance is limited by low conversions of CO and hydrocarbons as the A/F is changed in the reducing (rich) direction and decreased conversion of NO(NOx) as the A/F is changed in the oxidizing (lean) direction. The narrow range of A/F around the stoichiometric point where conversions are high is commonly referred to as the operating “window”. A wide A/F range or window of high simultaneous NOx, CO, and HC conversions is a desirable catalyst characteristic because it lessens the need for tight A/F control.
Kathleen C. Taylor

8. Composition of Three-Way Catalysts

Abstract
Three-way catalysts being used to meet 1981 and 1982 emission control requirements in the United States contain the noble metals rhodium, platinum, and often palladium. Many catalyst compositions are in use. Properties which distinguish these catalysts are the noble metal loadings, the identity of base metal additives, and the supports.
Kathleen C. Taylor

9. Catalyst Supports

Abstract
Supports for automobile exhaust catalysts are of two general types: alumina pellets (spheres and extrudates) and ceramic monoliths coated with a thin alumina washcoat (Figure 14). The reasons for choosing alumina supports for the noble metals include high surface area, attrition resistance, stable structure under typical exhaust conditions, favorable pore structure, and adequate supply [109]. Both support types have perceived advantages and disadvantages which have influenced their selection for use by automobile manufacturers [110]. Factors considered include converter size and cost, catalyst performance and durability on a particular vehicle, availability of support material from suppliers, and ease of replacement. In this section the recent literature on both support types will be reviewed.
Kathleen C. Taylor

10. The Transient Behavior of Three-Way Catalysts

Abstract
A typical closed-loop control system causes the A/F to cycle rapidly about the stoichiometrically balanced composition with a frequency of about 1 Hz. This operating condition has generated considerable interest in how the A/F perturbations influence catalyst performance compared with a non-cycled stoichiometric feedstream and how catalysts may be formulated to perform optimally in cycled feeds. The recent literature aimed at understanding the transient behavior of three-way catalysts will be reviewed here.
Kathleen C. Taylor

11. Deterioration of Three-Way Catalysts

Abstract
The deterioration of the activity of three-way catalysts during use is an important characteristic because the Federal emission standards require that performance be maintained for 80,000 km, as described earlier. Three-way catalysts lose activity both due to thermal effects and due to poisoning by contaminants in exhaust, namely phosphorus, lead, and sulfur. Combined poisoning and thermal effects and interaction among poison species complicate deterioration studies. A considerable incentive exists to minimize catalyst deterioration. If deterioration were eliminated, catalysts could have lower noble metal loadings and vehicles might be calibrated for greater fuel economy.
Kathleen C. Taylor

12. The 0.4 NO x Research Objective

Abstract
Two vehicle manufacturers have published papers in which they describe their research aimed at the development of emission control systems which meet the Federal research objective emission levels of 0.41 g mi-1 HC, 3.4 g mi-1 CO, and 0.4 g mi-1 NO x [18a, 31]. The development programs are aimed at achieving these emission levels at low mileage and at establishing the necessary durability of the emission control systems using the 80,000 km certification durability schedule [18a, 31]. Of course, vehicle emission levels far below the research objective are sought at low mileage in order to allow for vehicle-to-vehicle variation in emissions and for catalyst deterioration during use. The published reports of these programs indicate that further development is required before the experimental systems can be considered viable [18a, 31].
Kathleen C. Taylor

13. Control of Diesel Particulate Emissions

Abstract
Federal emission control requirements for diesel-fueled automobiles include a particulate standard of 0.6 g mi-1 in 1982 and 0.2 g mi-1 in 1985. Development programs aimed at meeting these requirements include both engine modifications and the use of aftertreatment devices. Only aftertreatment will be considered in this review.
Kathleen C. Taylor

14. Exhaust Gas Purification for Europe

Abstract
Exhaust emissions are currently controlled without the use of catalysts in Europe; however, exhaust emission standards have been proposed which might lead to catalyst systems which differ significantly from those used in the United States and Japan [171]. Exhaust emission standards being discussed are aimed at 90% reductions in emissions compared with 1969 levels [171]. Koberstein et al [171] state that the proposed CO emission standard varies with the inertial weight of the vehicle, while the HC and NOx emissions are added together to meet a single standard. No durability schedule has yet been adopted. The emissions test method (called the ECE-test after the Economic Commission for Europe) differs from the CVS-test used in the United States, and emissions measured with the two tests cannot be directly compared. These two driving schedules are shown in Figure 23. A main difference between the European and U.S. approach to exhaust emission control is that lead-free gasoline will not be available in Europe (except perhaps in Switzerland). Accordingly catalyst systems must be developed which are compatible with lead content in the range 0.15 to 0.4 g l-1 [171]. No catalyst schemes have been identified which can be used to meet U.S. exhaust emission standards at such high lead content.
Kathleen C. Taylor

15. Concluding Remarks

Abstract
The research challenges in emission control catalysis have not been exhausted with the introduction of the three-way catalyst.
Kathleen C. Taylor

16. References

Without Abstract
Kathleen C. Taylor

Backmatter

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