Automobile exhaust catalyst development

Automobile exhaust catalysts have been developed that maximize the catalyst surface area available to the flowing exhaust gas without incurring excessive pressure drop. Two types have been extensively studied the monolithic honeycomb type and the pellet type. 

One important catalyst design variable is the macroscopic, spatial profile of activity along the characteristic dimension of the catalyst particle. As with many new phenomena, this was first recognized in the patent literature [20, 21]. The first theoretical analysis was developed by Shadman-Yazdi and Petersen [22], Specific applications for automobile exhaust catalysts were proposed, e.g., by the influential papers of Becker and Wei [23, 24] these concepts were subsequently proved by experiment and used for the optimum design of automobile exhaust catalysts [25]. Figure 7 is one example of the effects that can be achieved. As Vayenas and Pavlou [26] (1988) pointed out, the theoretical analyses of optimum catalyst distributions became so popular that they are now way ahead of experimental verifica-... 

The introduction of automobile exhaust catalysts in the United States and elsewhere has produced a major market for platinum-type oxidation and reduction systems. An innovative consequence of this industry has been the development of ceramic honeycombed monoliths as catalyst supports. These structures contain long, parallel channels of less than 0.1 mm in diameter, with about SO channels per square centimeter. The monolith is composed of cordierite (2MgO - 2AI2O) SSiOj) and is manufactured by extrusion. A wash coat of stabilized alumina is administered prior to deposition of the active metal, either by adsorption or impregnation methods. 

The chemistry of palladium-carbonyl complexes has experienced extensive recent developments due to an increased interest in the role of palladium in surface catalysis especially in automobile exhaust catalysts. Palladium-carbonyl complexes are nevertheless stiU relatively rare, which is probably due to their relative instability in comparision with that of Ni complexes. The homoleptic Pd(CO)4 only exists at low temperatures (<80 K) in noble gas or CO matrices, in sharp contrast with isoleptic Ni(CO)4, which is stable at ambient temperature. Table 1 compiles some of the known and representative carbonyl complexes of palladium. 

Serious research in catalytic reduction of automotive exhaust was begun in 1949 by Eugene Houdry, who developed mufflers for fork lift trucks used in confined spaces such as mines and warehouses (18). One of the supports used was the monolith—porcelain rods covered with films of alumina, on which platinum was deposited. California enacted laws in 1959 and 1960 on air quality and motor vehicle emission standards, which would be operative when at least two devices were developed that could meet the requirements. This gave the impetus for a greater effort in automotive catalysis research (19). Catalyst developments and fleet tests involved the partnership of catalyst manufacturers and muffler manufacturers. Three of these teams were certified by the California Motor Vehicle Pollution Control Board in 1964-65 American Cyanamid and Walker, W. R. Grace and Norris-Thermador, and Universal Oil Products and Arvin. At the same time, Detroit announced that engine modifications by lean carburation and secondary air injection enabled them to meet the California standard without the use of catalysts. This then delayed the use of catalysts in automobiles. 

The relatively high cost and lack of domestic supply of noble metals has spurred considerable efforts toward the development of nonnoble metal catalysts for automobile exhaust control. A very large number of base metal oxides and mixtures of oxides have been considered, especially the transition metals, such as copper, chromium, nickel, manganese, cobalt vanadium, and iron. Particularly prominent are the copper chromites, which are mixtures of the oxides of copper and chromium, with various promoters added. These materials are active in the oxidation of CO and hydrocarbons, as well as in the reduction of NO in the presence of CO (55-59). Rare earth oxides, such as lanthanum cobaltate and lanthanum lead manganite with Perovskite structure, have been investigated for CO oxidation, but have not been tested and shown to be sufficiently active under realistic and demanding conditions (60-63). Hopcalities are out-... 

We have included in this volume two chapters specifically related to society s kinetic system. We have asked James Wei of the University of Delaware, recent Chairman of the consultant panel on Catalyst Systems for the National Academy of Sciences Committee on Motor Vehicle Emissions, to illustrate key problems and bridges between the catalytic science and the practical objectives of minimizing automobile exhaust emissions. We have also asked for a portrayal of the hard economic facts that constrain and guide what properties in a catalyst are useful to the catalytic practitioner. For this we have turned to Duncan S. Davies, General Manager of Research and Development, and John Dewing, Research Specialist in Heterogeneous Catalysts, both from Imperial Chemical Industries Limited. 

A gold-based material has been formulated for use as a three-way catalyst in gasoline and diesel applications.28 This catalyst, developed at Anglo American Research Laboratories in South Africa, consisted of 1% Au supported on zirconia-stabilized-Ce02, ZrC>2 and TiC>2, and contained 1% CoOx, 0.1% Rh, 2% ZnO, and 2% BaO as promoters. The catalytically active gold-cobalt oxide clusters were 40-140 nm in size. This catalyst was tested under conditions that simulated the exhaust gases of gasoline and diesel automobiles and survived 773 K for 157 h, with some deactivation (see Section 11.2.7). 

Nieuwenhuys summarizes the understanding of supported metal catalysts for automobile exhaust abatement as it has developed from ultrahigh-vacuum surface science. The account is an impressive validation of the success of the surface science methods for elucidation of complex multicomponent catalysts. 

One of the early problems with catalytic control of automobile exhaust emissions was during the few minutes immediately after starting the engine when the cold catalytic systems did not function. This was solved by developing a porous zeolite which traps the unburned hydrocarbons while the catalysts are still cold [15]. Once the catalysts have warmed up, the zeolite canister also warms, releasing the trapped hydrocarbons to the catalytic systems to perform their important control reactions. 

Catalytic converters in automobile exhaust systems were developed to remove some of the carbon monoxide and unburned hydrocarbons from automobile exhaust. A catalyst is any substance that speeds a chemical reaction without being permanendy altered itself Some of the transition metals, such as platinum, palladium, iridium, and rhodium,... 

Tlie removal of NO from exliaust gases is an urgent issue, and environmental regulations are becoming more stringent tlian ever. The selective catalytic reduction (SCR) of NOx vvdth ammonia is the technique, which was extensively applied in stationary NOx sources from tlie 1970s, while the successful development of a three-way catalyst made it possible to reduce NOx in gasoline automobile exhaust. 

One notable exception has been the development of the catalytic exhaust system for automobiles, one of the most intense catalyst development efforts ever undertaken. An automotive catalyst normally consists of Pt/Pd and some Rh on a ceramic support. Catalytic exhaust control systems function under severe and rapidly changing conditions and must be active for several reactions that reduce automotive emissions—CO oxidation, hydrocarbon oxidation, and reduction (this is the so-called three-way catalyst). Typical operating conditions are temperatures of 400 to 600 C (or much greater under certain conditions) and 150,000 hr space velocity. Numerous reviews of the development and performance of these catalysts are available, and these catalysts are of interest because they are frequently used for control of VOC-emissions, particularly in conjunction with open flame preheaters. Unfortunately, these catalysts are not designed to resist poisoning by many VOC-type compounds, particularly those containing chlorine and sulfur. 

Legal limits for the emissions of the main pollutants in the automobile exhaust gases are becoming more and more strict The development of new and advanced catalytic converters demands not only experimental work, but also extensive and detailed modelling and simulation studies. The models become more complex, when all the important physical and chemical phenomena arc considered. Particularly the use of non-stationary kinetic models (microkinetics) with surface deposition of reaction components (Jirtit et al., 1999, e.g.) and the incorporation of diffusion effects in porous catalyst structure lead to a large system of partial differential equations. 

Furthermore, as clearly demonstrated by the three-way catalytic system for gasoline vehicles and DPNR explained in this chapter, the automobile exhaust gas treatment catalysts do not function alone, but rather require a highly controlled engine system. It is therefore necessary to further develop technologies to functionally integrate engine and after-treatment devices in which the soot oxidation catalyst is a component. 

To date there has been limited commercialization of devices that use the ER effect. However, ER fluids worthy of commercialization have been developed and this will accelerate further development of devices. For example, an ER cutting machine has been developed [75, 76]. This machine incorporates a variable rodless cylinder that functions by using an ER fluid with a sulfonated polymer. This machine cuts brittle ceramics, using ER fluid to control the cutting speed very accurately. It is used to manufacture a catalyst for automobile exhaust gas. 

The US Clean Air Act (CAA) of 1970 required that automobile exhaust emissions be regulated to meet new environmental standards. From 1975 all new models were to be fitted with catalytic combustion converters to reduce levels of carbon monoxide and unbumt hydrocarbons in the exhaust. This led to the phase-out of lead additives in gasoline between 1973 and 1996. To compensate for the loss of octane rating of the gasoline more reformate and alkylate needed to be added. Octane catalysts were also developed for FCC units and the aromat-... 

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