The Success of the Automotive Catalytic Converter: BASF researchers improve cleanup of diesel emissions
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There are currently more than half a billion cars on the roads worldwide, plus around 200 million trucks. Although this intense automobile traffic raises many environmental issues, health hazardous exhaust gases do not necessarily have to be one of them. In theory, the hydrocarbons in the gasoline are combusted with atmospheric oxygen to give the nontoxic end products carbon dioxide and water. In the real world of the automobile this ideal combustion process is not that easy to accomplish. Incomplete combustion and the presence of minimal impurities in the fuel can result in the formation of toxic carbon monoxide, unburned hydrocarbons, nitrogen oxides and for diesels carbon particulates (soot). When optimally adjusted, modern engines can reduce emissions of these pollutants significantly. The key to clean exhaust gases, however, lies in the conversion of the harmful emissions into harmless end products by catalytic converters. The widespread introduction of such catalytic converter systems in North America (starting in 1976) and Europe (1986) resulted in a marked decrease in metropolitan air pollution from harmful tailpipe emissions despite a growing population of vehicles.
The development of the first three-way catalytic converter by the US American company Engelhard in 1979/80 marks a milestone in exhaust gas technology. This device was able to catalyze the conversion of the three main pollutants (unburned hydrocarbons, carbon monoxide and nitrogen oxides) simultaneously. “Ever since, Engelhard’s researchers have remained among the leading innovators in this field and are continuing their successful work following the integration into BASF", emphasizes Dr. Bob Farrauto, Research Fellow at BASF Catalysts LLC in Iselin, New Jersey.
As a general principle, a catalyst allows chemical conversions to desired products to occur more rapidly and at lower temperatures. For this reason, in addition to pollution control, industrial catalysts find use in a wide field, be it the processing of petroleum to produce transportation fuels or the production of chemicals including polymers and pharmaceuticals. Ideally, the catalyst itself is not chemically consumed during this process. Automotive emissions catalytic converters consist of special combinations of precious metals such as platinum, palladium and rhodium dispersed on high surface area carriers which in turn are coated onto the walls of ceramic or metallic monolithic structures. In the modern three-way catalytic converter, uncombusted fuel residues are oxidized with oxygen to produce carbon dioxide and water, nitrogen oxides are converted to ubiquitous nitrogen, and toxic carbon monoxide is oxidized with oxygen to carbon dioxide. Like this, a typical catalytic converter is capable of destroying around 98 percent of hydrocarbons, carbon monoxide and nitrogen oxides produced by the car’s engine.
“The principle of a catalyst may be simple, but it’s the details of the technological implementation that cause the headaches”, explains Bob Farrauto. “The catalyst needs the right operating temperature and an accurately adjusted residual oxygen content in the exhaust gas.” In today’s catalytic converters, the oxygen content is measured and regulated by lambda sensors. Through a computer controlled feedback system the mixture of air and fuel entering the engine is adjusted to meet the requirements for the three-way catalyst to function.
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Today three-way catalytic converters are well-established high technology products and a standard feature in more than 80 percent of new automobiles worldwide. Research continues in order to meet ever increasing emission standards and to improve the catalyst compositions to match changing engine control strategies established by auto makers for different models and for different parts of the world. In the United States a converter is required to meet tight emission standards over an individual lifetime of at least 150,000 miles.
Actually the biggest challenge for the exhaust gas experts are the tailpipe emissions of diesel powered vehicles, whose numbers are growing fast, especially in Europe. Their lower fuel consumption is not only good news for the wallet, but also helps preserving the earth’s crude oil resources. The main problem with diesel engines, which run at much lower temperatures, are the large amounts of carbon particulates and oxides of nitrogen in the exhaust gas. Three way converters are not totally effective due to excess oxygen present in the exhaust. “To solve this problem, we have developed special diesel oxidation catalysts combined with particulate filters which trap the soot and periodically oxidize it using a combination of catalysts and engine controls”, explains catalyst researcher Bob Farrauto. In public discussion, the simplified term “catalyzed soot filter” is often also used to describe this technology.
Moreover, diesel engines also require a “lean” air-fuel mixture that results in a high content of residual oxygen in the exhaust gas. This considerably impedes the conversion of nitrogen oxides (NOx) to nitrogen, which can only take place under oxygen-free conditions. But here too, the experts of BASF Catalysts are busy developing technical solutions: NOx storage devices or NOx traps incorporated into the catalyst first store the nitrogen oxides chemically while the engine is operated in the “lean” mode. When the storage capacity is exhausted, the engine automatically switches to a “rich” air-fuel mixture for a short time, allowing the catalyst to convert the stored nitrogen oxides into nitrogen. The storage catalyst is regenerated and the engine can switch back to the lean mixture, which both enhances engine performance and fuel economy. Alternatively an ammonia carrying liquid (i.e. urea) can be injected into the exhaust and passed over a highly selective catalyst which converts the NOx into N2.
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Other applications of catalytic converters
The environmental technologies portfolio of BASF Catalysts also includes catalysts for other applications:
* When the engine is switched off on hot summer days with high ozone levels, the vapors in the engine compartment contain particularly large amounts of hydrocarbons. To cope with this problem, BASF Catalysts’ experts have developed a hydrocarbon trap (made of zeolite, an aluminium-silicate mineral) deposited on a honeycomb monolith to adsorb evaporative emissions placed between a car’s air cleaner and engine. The adsorber traps the hydrocarbons and returns them for combustion when the engine is restarted.
* Millions of lawn-mowers, chain-saws, motorbikes or even model planes run on different kinds of combustion engines, but their exhaust gases are still rarely controlled by catalytic converters. BASF Catalysts’ experts develop tailored technical solutions for these smaller applications, which in their sum significantly add to the total of harmful emissions. On the other end of the scale, there is a broad field for catalytic technology in the control of emissions from power plants, mainly nitrogen oxides and carbon monoxides.
* Getting rid of hazardous emissions that are already in the air is the thought behind BASF Catalysts’ PremAir®, a catalytic coating for the car’s radiator, which converts ozone, the most dangerous compound of smog, into pure oxygen. Ozone can also build up in aircraft cabins during flight. Air filters based on catalytic conversion technology help to keep its levels down. At the same time, catalytic converters in the aircraft engines minimize the emission of residues like carbon monoxide, unburned fuel and nitrogen oxides.
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