Automobile emission catalysts operation

Reduction of pollutant gases of CO, HCs and NO c from automobile emissions is currently achieved by the use of three-way catalysts (TWC), which can oxidise CO and HCs into CO2 and H2O, while simultaneously reducing NO into N2, provided the TWC is operated at stoichiometric air/fuel ratio, as shown in figs. 32 and 33. The air/fiiel ratio is normally defined in terms of lambda (A) ... 

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. 

While the discovery of the catalytic properties of zeolites was driven by the desire to improve industrial prcKessing, the development of emission control catalysts was necessitated by governmental fiat. The first requirement was for 90+% removal of CO and of hydrocarbons, a goal which could not be met by oxidation with base metal oxides. To achieve the required spedfications during automobile operations, it was necessary to develop supported platinum catalysts. Originally the support was alumina in pellet form. Later platinum on cordierite was used in honeycomb form, containing 200-400 square channels per square inch. 

There is currently a great interest in improving automobile fuel economy and to reduce emissions of carbon dioxide into the atmosphere. One step in this direction is to use a technique where a gasoline engine is operated at lean-bum conditions. Using this concept it is possible to improve fuel economy significantly [1,2] compared to the normal stoichiometric operation. A setback with lean-bum technology is that Ae common three-way catalyst is... 

Further progress is expected from new developments and combinations of processes. Thus, it would be possible to make the disposal of the gaseous (and highly pure) waste gas streams (residual propane content of the propylene feed) cost-effective and a source of electric power by connection to novel, compact, membrane fuel cells. Potential synergisms would also occur in the operating temperature of the cells (medium-temperature cells at 120 °C using the residual exothermic heat of reaction from the oxo reaction), the membrane costs by means of combined developments (e.g., for membrane separations of the catalysts [22]), and also in the development of the zero-emission automobile by the automotive industry. The combination of hydroformylation with fuel cells would further reduce the E-factor - thus approaching a zero-emission chemistry. ... 

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. 

Storage and reduction catalysts (NSR) offer the possibility of controlling NOx emissions from automobile sources while permitting operation under predominantly lean-burn conditions. °" The concept is based upon the storage of NO under lean conditions on an alkaline-earth oxide component, such as baria, which is then released during intermittent rich/stoichiometric periods where the stored NO is released and reduced by Hg, CO or HC over the noble metal component (Fig. 10.9). 

Up to 90% of CO, NO, and VOCs are typically eliminated from automobile exhaust by a catalytic converter. Although catalytic converters are beneficial to our environment, they could still be improved. Catalysts that work at lower temperatures would reduce an automobile s emission during the first few minutes of operation. Other gases that are emitted by cars may also pose problems for the environment. Nitrous oxide, N2O, can be formed from the incomplete reduction of NO in catalytic converters. Unlike the NO gases, N2O can travel to the upper atmosphere, where it can destroy ozone. As a greenhouse gas, N2O is more than 300 times more potent than CO2. 

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