Oxidative automotive emission control catalysts

Oxidative Automotive Emission Control Catalysts—Selected Factors Affecting Catalyst Activity... 

Tn the development of oxidative automotive emission control catalysts for use in the 1975 model year, certain requirements were recognized from the beginning. The catalyst had to be physically rugged and capable of withstanding both the mechanical and thermal abuse to which it would be subjected in an automobile driven by average drivers on real roads. The catalyst had to exhibit high levels of activity so that the catalytic units would be of reasonable size. The catalyst had to be stable for at least 50,000 miles and capable of withstanding chemical abuse from the exhausts of the various fuels to which it would be subjected. 

The main areas of commercial apphcation are automotive emission control catalysts (autocatalysts), oil refining, ammonia oxidation, hquid-phase ... 

Emission Control Catalysts. An appHcation of growing importance for cerium is as one of the catalyticaHy active components used to remove pollutants from vehicle (autoexhaust) emissions (36). The active form of cerium is the oxide that can be formed in situ by calciaation of a soluble salt such as nitrate or by deposition of slurried oxide (see Exhaust control, automotive). 

A catalytic oxidation system may cost 150 per car, but the catalyst cost is estimated to be 30, less than 1% of the cost of an automobile (2). In a few years, the gross sale of automotive catalysts in dollars may exceed the combined sale of catalysts to the chemical and petroleum industries (3). On the other hand, if the emission laws are relaxed or if the automotive engineers succeed in developing a more economical and reliable non-catalytic solution to emission control, automotive catalysis may turn out to be a short boom. Automotive catalysis is still in its infancy, with tremendous potential for improvement. The innovations of catalytic scientists and engineers in the future will determine whether catalysis is the long term solution to automotive emissions. 

Automotive emission control is a major catalyst market segment. These catalysts perform three functions (1) oxidize carbon monoxide to carbon dioxide (2) oxidize hydrocarbons to carbon dioxide and water and (3) reduce nitrogen oxides to nitrogen. The oxidation reactions use platinum and palladium as the active metal. Rhodium is the metal of choice for the reduction reaction. These three-way catalysts meet the current standards of 0.41 g hydrocarbon per mile, 3.4 g carbon monoxide per mile, and 0.4 g nitrogen oxides per mile. 

Air oxidation of CO to CO2 has been much studied in the context of automotive emission control. Studies of this reaction, equation (j), using heterogeneous catalysts have been reviewed . ... 

Palladium catalysts have long been recognized as having desirable performance properties in automotive emission control. In addition to its ready availability and low cost relative to platinum and rhodium, palladium is superior to platinum for CO oxidation and oxidation of unsaturated hydrocarbons [1]. 

Cerium-zirconium mixed metal oxides are used in conjunction with platinum group metals to reduce and eliminate pollutants in automotive emissions control catalyst systems. The ceria-zirconia promoter materials regulate the partial pressure of oxygen near the catalyst surface, thereby facilitating catalytic oxidation and reduction of gas phase pollutants. However, ceria-zirconia is particularly susceptible to chemical and physical deactivation through sulfur dioxide adsorption. The interaction of sulfur dioxide with ceria-zirconia model catalysts has been studied with Auger spectroscopy to develop fundamental information regarding the sulfur dioxide deactivation mechanism. 

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. 

Hysell, D. K. Moore, W. Jr. Hinners, R. Malanchuk, M. Miller, R. Stara, J. F. "The Inhalation Toxicology of Automotive Emissions as Affected by an Oxidation Exhaust Catalyst", Simiposium on Health Consequences of Environmental Controls Impact of Mobile Emission Control, NIEHS, NIH, PHS, HEW and NERC, ORD, EPA, Durham, NC, April 17-19,... 

The following reactions setup (reactions 25.1-25.6) has to be considered as a minimum reactions scheme in order to control the automotive emissions (a more complex reaction scheme is expected to be at work into the car catalysts, as for example that in Figure 25.1). Although "Oxidation catalysts were efficient to control HCs and CO emissions, via reactions (25.1) and (25.2), they were unable to reduce NO via reactions (25.3) and (25.4) the latter are often followed by the undesirable reactions (25.5) and (25.6), which correspond to the partial reduction of NO. 

John M. Vohs is the Carl V.S. Patterson Professor and chair of the Department of Chemical and Biomolecular Engineering at the University of Pennsylvania. He joined the faculty there after receiving a B.S. degree from the University of Illinois and a Ph.D. from the University of Delaware. Dr. Vohs research interest is in the field of surface and interfacial science, particularly the relationships between the local atomic structure of surfaces and their chemical reactivity. His work on structure-activity relationships for metal-oxide catalysts, especially those used for selective oxidation reactions and automotive emissions control systems, is widely known. In recent years, he has collaborated in the development of solid-oxide fuel cells that run on readily available hydrocarbon fuels, such as natural gas and diesel. Dr. Vohs has received numerous honors, including an NSF Presidential Young Investigator Award and two Union Carbide Research Innovation Awards, vohs seas.upenn.edu)... 

Pt-Rh/AROs catalysts are widely used in automotive-exhaust emission control. In these systems, Pt is generally used for the oxidation of CO and hydrocarbons and Rh is active for the reduction of nitric oxide to N2. HRTEM and AEM show two discrete particle morphologies and Pt-Rh alloy particles (Lakis et al 1995). EM studies aimed at understanding the factors leading to deactivation, surface segregation of one metal over the other and SMSI are limited. There are great opportunities for EM studies, in particular, of surface enrichment, and defects and dislocations in the complex alloy catalysts as sites for SMSI. 

Ohmic heating of catalyst is often used as a simple method of igniting the chemical reaction during reactor startup, for instance, in the oxidation of ammonia on platinum-rhodium gauze catalysts. Another application is the prevention of cold-start emissions from automotive catalysts responsible for much of the residual pollution still produced from this source (21). The startup times needed for the catalyst to attain its operating temperature can be cut by a factor of 5 or more by installing an electrically heated catalyst element with a metallic support upstream of the main catalyst unit. Direct electrical catalyst heating permits facile temperature control but requires a well-defined catalyst structure to function effectively. 

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