Automobile catalysts exhaust

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. 

Catalyst Function. Automobile exhaust catalysts are perfect examples of materials that accelerate a chemical reaction but are not consumed. Reactions are completed on the catalyst surface and the products leave. Thus the catalyst performs its function over and over again. The catalyst also permits reactions to occur at considerably lower temperatures. For instance, CO reacts with oxygen above 700°C at a substantial rate. An automobile exhaust catalyst enables the reaction to occur at a temperature of about 250°C and at a much faster rate and in a smaller reactor volume. This is also the case for the combustion of hydrocarbons. 

The mechanism of poisoning automobile exhaust catalysts has been identified (71). Upon combustion in the cylinder tetraethyllead (TEL) produces lead oxide which would accumulate in the combustion chamber except that ethylene dibromide [106-93-4] or other similar haUde compounds were added to the gasoline along with TEL to form volatile lead haUde compounds. Thus lead deposits in the cylinder and on the spark plugs are minimized. Volatile lead hahdes (bromides or chlorides) would then exit the combustion chamber, and such volatile compounds would diffuse to catalyst surfaces by the same mechanisms as do carbon monoxide compounds. When adsorbed on the precious metal catalyst site, lead haUde renders the catalytic site inactive. 

J. T. Kummer, "The Use of Noble Metals in Automobile Exhaust Catalyst," Chicago Symposium, 1985. 

Platinum compounds Hydrosilation cross-linking of silicone polymers Hydrogenation, isomerization and hydroformylation of alkenes Automobile exhaust catalyst Sensitization dermatitis... 

Almost all the materials which are being considered as components in automobile exhaust catalyst are somewhat toxic (74)- Most of the compounds considered are low vapor pressure solids which can only escape from the exhaust system as very fine airbone dust particles formed by catalyst attrition. A few compounds, such as the highly toxic metal carbonyls and ruthenium tetroxides, are liquid under ambient conditions and have boiling points less than 100 °C. These compounds are not present in... 

Ceria is another type of mixed conducting oxide which has been shown already to induce electrochemical promotion.71 Ceria is a catalyst support of increasing technological importance.73 Due to its nonstoichiometry and significant oxygen storage capacity it is also often used as a promoting additive on other supports (e.g. y-A Cb) in automobile exhaust catalysts.79 It is a fluorite type oxide with predominant n-type semiconductivity. The contribution of its ionic conductivity has been estimated to be 1-3% at 350°C.71... 

Metcalfe, I. Sundaressan, S. 1986 Oxygen storage in automobile exhaust catalyst. Chem. Engng Sci. 41(4), 1109-1115. 

Zereini, F., Skerstupp, B., Alt, F., Helmers, E., and Urban, H. (1994). Geochemical behaviour of platinum group elements (PGE) in particulate emissions by automobile exhaust catalysts Experimental results and environmental investigations. Sci. Total Environ. 206, 137—146. 

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-... 

Figure 7. Effects of Rh location on the performance of fresh automobile exhaust catalyst beads as a function of engine air-fuel ratio (A/F). Adapted from Hegedus et ai [33] and reprinted from Hegedus and McCabe [25], p. 94, by courtesy of Marcel Dekker, Inc.

Automobile exhaust catalysts typically contain noble metals such as Pt, Pd and Rh with a ceria promoter supported on alumina. Traditionally, the principal function of the Rh is to control emissions of nitrogen oxides (NO ) by reaction with carbon monoxide, although the increasing use of Pd has been proposed. For example, recent X-ray absorption spectroscopy studies of Holies and Davis show that the average oxidation state of Pd was affected by gaseous environment with an average oxidation slate between 0 and +2 for a stoichiometric mixture of NO and CO. Exposure of Pd particles to NO resulted in the formation of chemisorbed oxygen and/or a surface oxide layer. 

Ceria is effective in the removal of trace amounts of toxic metal species and radionuclides from aqueous solutions and contaminated soils [17], The behaviour of hydrous ceria as selective anion exchanger has also been described in the literature [18] For these applications the preparation of high surface area, thermally and chemically stable ceria phases as well as the study of the parameters which control structural and textural properties of the solid are of particular interest, as they are in the case of the automobile exhaust catalysts... 

Matsumoto, S. (2004) Recent advances in automobile exhaust catalysts. Catal. Today, 90, 183-190. 

Monolith reactors have been modeled extensively for combustion applications, with most of the models relating to the combustion of pollutants in exhaust gases. Details of the various models for combustion in monolith reactors can be found in the review by Cybulski and Moulijn [16]. The models have been important tools in the design of catalysts for the control of pollutants, especially automobile exhaust catalysts. Many of the insights obtained through modeling the combustion of pollutants are transferable to fuel-combustion catalysts. 

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 sodium and silica content of the aluminas, originally present in small quantities in the hydrates or added in processing, can be important for certain catalyst applications. Besides affecting the catalytic properties, they can influence thermal stability greatly. One procedure for imparting thermal stability to support materials is by doping. Thus, alumina can be stabilized to prevent conversion to the low area a form (when area is reduced from about 250 to 1 m /g). If alumina is doped with small amunts of oxides from group IIA such as CaO and subsequently calcined at 1200°C for 2 hs, a stable surface area of 20-100 m /g is obtained for use as a thermally stabilized support. Such materials have found applications iln automobile exhaust catalysts and other combustion catalysts. 

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