Oxidative automotive emission

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 ... 

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. 

It is fair to state that by and large the most important application of structured reactors is in environmental catalysis. The major applications are in automotive emission reduction. For diesel exhaust gases a complication is that it is overall oxidizing and contains soot. The three-way catalyst does not work under the conditions of the diesel exhaust gas. The cleaning of exhaust gas from stationary sources is also done in structured catalytic reactors. Important areas are reduction of NOv from power plants and the oxidation of volatile organic compounds (VOCs). Structured reactors also suggest themselves in synthesis gas production, for instance, in catalytic partial oxidation (CPO) of methane. 

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. 

New regulations in the United States and Europe mandate that automotive emissions must decrease substantially from current levels. As a result, there is a strong incentive to develop improved TWC with better oxidation activity at low temperatures since most of the hydrocarbons and CO are emitted immediately following cold starts of engines. As previously mentioned, the addition of transition metal oxides can have a beneficial effect on the performance of Au catalysts in CO oxidation. Combinations of Pt or Pd with transition metal oxides are also active in CO oxidation at low temperatures 50). Figure 11 shows examples of the reaction over Pt/MO/ Si02 catalysts. 

The main role of rhodium in catedysts used for the control of automotive emissions is to promote the reduction of NOx (9). The high cost and the limited availability of this metal provide a strong incentive to develop methods for its more effective utilization. Indeed, it is well known that Rh supported on y-AhOs, when exposed to high temperatures in oxidizing atmosphere, interacts with the support leading to the diffusion of a part of rhodium into... 

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]. 

Selective non catalytic reduction (SNCR) with NH3 is limited to industrial boilers in consequence of the relatively narrow temperature range for the reaction. Selective catalytic reduction (SCR) by ammonia has high efficiency and it can be used for many stationary sources, especially for nitric acid plants [1], and it is based on the catalytic pairing of nitrogen atoms, one fi-om nitric oxide, one fi om ammonia. This method, however, is imsuitable for small sources and vehicles. As far as automotive emission is concerned nonselective catalytic reduction (NSCR) by hydrocarbons, CO and H2 from the exhaust stream has been reported over various catalysts recently [1,3,4]. 

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,... 

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