Catalytic converter

The vehicle catalyst also briefly catalyst ( colloquially Kat ), the exhaust aftertreatment in vehicles with internal combustion engines used. Through the catalyst of pollutant emissions in the exhaust gas can be reduced drastically. In general, the entire exhaust gas aftertreatment system will be referred to as a vehicle catalytic converter.

History

Inventor of the first automotive exhaust catalyst was the French engineer Eugene Houdry. Around 1950, as the results of a preliminary investigation into smog in Los Angeles were published, he was worried about the impact of car exhaust gases to the air pollution and founded a special company, Oxy - Catalyst Company, which was to develop catalytic converters for gasoline engines - an idea of their time was far ahead. He developed the first automotive catalytic converters and received a patent for it in 1956 ( US2742437 ). However, this first car exhaust catalysts were not used because they were poisoned by the lead in the anti-knock agent tetraethyl lead.

Later John J. Mooney and Carl D. Keith developed the three-way catalyst in the Engelhard Corporation, which introduced him in 1973 in the market.

Construction

The catalyst generally consists of a plurality of vehicle components. Carrier serves as a temperature-stable ceramic honeycomb, usually cordierite, or metal foils (eg Metalit ) having a plurality of thin-walled channels. On the carrier is the so -called washcoat. It is made of porous alumina (Al2O3 ) as well as oxygen storage components, such as cerium (IV ) oxide, and serves to increase the surface area. Due to the high roughness of a large surface area is achieved of up to hundreds of square meters per gram. In the washcoat, the catalytically active noble metals are incorporated. In modern exhaust gas catalysts, these are the noble metals platinum, rhodium and / or palladium. The ceramic carrier is using special mounting mats, such as from high temperature wool, rare in combination with wire mesh, in a metal housing, the so-called Canning, stored.

Special mats or an additional metal housing are not required for the metal catalysts. The Canning is permanently installed in the exhaust system of the vehicle and has partly further connection possibilities, for example for lambda probes or thermocouples. There are also metal catalysts with integrated lambda probes.

Operation

The task of the vehicle catalyst is the chemical conversion of combustion pollutants hydrocarbons ( C m H n ), carbon monoxide (CO ) and nitrogen oxides (NOx ) in the non-toxic substances carbon dioxide ( CO2), water (H2O ) and nitrogen ( N2) by oxidation or reduction. Depending on the operating point of the motor and at optimal operating conditions conversion rates can be achieved close to 100%. The required operating temperature ( 500 ° C) is usually Reaches 3 to 5 minutes after starting the engine. This has the consequence that, for a 20 -minute drive away 80 to 90 percent of the pollutants are emitted within the starting phase.

Species

A controlled three-way catalyst

(Also called G- Kat ) oxidation of CO and C m H n and the reduction of NOx in parallel is the case of a closed-loop three -way catalytic converter instead: There are CmHn oxidized by O2 to CO2 and H2O, O2 oxidizes CO to CO2 and NOx reduced with CO to N2, O2 and CO2.

This requires a constant stoichiometric fuel ratio ( λ = 1) of 14.7 grams of air per gram of premium gasoline (octane 95) and 14.8 grams of air per gram of regular gasoline (octane 91). For fuel ethanol, for example, applies the ratio 9:1. Even a slight deviation in the lean region ( λ > 1) causes a sharp increase in nitrogen oxide emissions after the catalyst, as too little CO is present for the reduction. Therefore, the mixture between stoichiometric and slightly rich ratio is controlled. The three-way catalyst can be used only on vehicles with a petrol engine and lambda control. For diesel and lean-burn gasoline engines of excess oxygen in the exhaust gas prevents the reduction of NOx and makes specific catalysts required (see NOx catalytic converter).

Unregulated catalyst

In the early days of the catalyst technology found in particular in low-cost vehicles with a petrol engine also uncontrolled use of catalysts. Here, the composition of the air -fuel mixture is not monitored by an oxygen sensor, but only the exhaust gas flow guided through the catalytic block. Accordingly worse this was especially the nitrogen oxide reduction in the frequently used partial load range of the engine, in which a lean mixture ( excess air ) brings sufficient performance and a corresponding mixture composition for the sake of fuel economy was preferred in vehicle design. In particular, at the end of the 1980s mainly to existing engines with carburetors the control of air -fuel mixture was not by design with the same precision as possible for engines with fuel injection systems, which have therefore become established in the automotive industry since that time.

Oxidation catalyst

Diesel engine

Diesel engines do not burn any fuel -air mixture is prepared. The fuel is inside the engine added to the compressed air. The combustion itself runs only locally stoichiometric or even a lack of oxygen. Since the fuel is not distributed evenly, the combustion results in their entirety to a high excess air and thus λ > 1 high oxygen concentrations, therefore, are present in the exhaust gas. Thus, the reduction of NOx as in the three-way catalyst is not possible. However CmHn and CO emissions can be reduced by the use of an oxidation catalyst. The oxidation reactions are the same terms as from the three-way catalyst. Because of the much lower exhaust gas temperatures as compared to the gasoline engine diesel oxidation catalysts are often installed close to the exhaust manifold, the washcoat contains only platinum and / or palladium.

The NOx reduction from diesel engines can be done first by in-engine measures, ie the targeted influence on the combustion, for example, by partial exhaust gas recirculation. However, this is only possible within narrow limits, otherwise increasing the soot emissions and engine performance drops. In the future, the increased use of NOx storage catalysts or SCR catalysts to reduce NOx emissions from diesel vehicles.

Recent work dealing with the use of perovskite catalysts for diesel engines in vehicles that are operated with excess oxygen to improve their efficiency. The oxygen contained in the exhaust gas prevents the use of conventional catalytic converters. The doping perovskite catalysts with palladium increases resistance to poisoning by sulfur.

Two-stroke gasoline engine

Also, two-stroke gasoline engines, as they are now fitted as yet in motorcycles with small displacement can be equipped with an oxidation catalyst. An oxidation catalyst may reduce the CO as well as the considerable when two-stroke gasoline engine emissions CmHn here. For older vehicles with two- stroke gasoline engine as the Trabant, there are retrofit oxidation catalysts. Generally speaking, the pollutant emissions from two-stroke gasoline engines compared to diesel and four-stroke gasoline engines due to the principle-related flushing with fresh mixture and the combustion of oil can be reduced but not so strong.

NOx storage catalyst

Modern lean-burn gasoline engines operate with excess oxygen to increase engine efficiency. Conventional catalysts can therefore not be used. The oxidation of CO and C m H n is in excess oxygen ( λ > 1), analogous to the conventional three-way catalyst further possible but must nitrogen oxides ( NOx) are cached. Their catalytic reduction can only be achieved in a stoichiometric to rich exhaust mixture. Therefore, these new engines require an advanced type of catalyst with additional chemical elements that enable storage of nitrogen oxides. To comply with future emission standards, even diesel cars be equipped in the future with NOx storage catalysts.

In order to achieve this temporary storage of the nitrogen oxides, are suitable carriers for the noble metal catalyst such as platinum and a NOx storage component, which is usually an alkaline earth metal such as barium applied. In the lean, that is, oxygen-rich atmosphere, the nitrogen oxides are oxidized by the action of the noble metal catalyst is absorbed to form nitrates such as barium nitrate in the catalyst and thus removed from the exhaust gas stream. Through regular short-term " enrichment " run these reactions in the opposite direction from, which issued the NOx molecules back into the exhaust stream and the present in the rich atmosphere reducing components such as CmHn - are reduced and / or CO on - incompletely burned hydrocarbons. Speicherkat can only store the NOx in a temperature range of 250 to 500 degrees Celsius. The temperature window is achieved by dreiflutige exhaust pipes or Auspuffbypässe.

If the recording capacity of the catalyst is exhausted, a fat, reducing exhaust gas mixture is by the engine electronics set briefly (about two seconds). In this short, fat cycle cached in the catalytic converter reduces nitrogen oxides to nitrogen and thus prepared the catalyst for the next memory cycle. By this procedure it is possible to minimize the emissions and fuel-efficient lean-burn engines comply with applicable limits of the Euro standards. The absorption capacity (about 60 to 90 seconds ) is monitored by a NOx sensor.

SCR ( Selective Catalytic Reduction )

Another method for the reduction of nitrogen oxides, the selective catalytic reduction. This is continuously an aqueous urea solution (trade name AdBlue) is injected, for example, by means of metering pump into the exhaust stream, from which arise by hydrolysis of water and ammonia. The resulting ammonia reduces nitrogen oxides in the exhaust gas to normal nitrogen (N2). The SCR process is now being used in many commercial vehicles to fall below especially the emission limits according to Euro V and Euro VI.

Meeting current exhaust emission limits

By dramatically reducing the cold phase vehicle emissions could be significantly reduced. The cold-start phase can be reduced by the following measures:

• Close to the engine as possible catalyst assembly, for example, directly behind the exhaust manifold. This measure requires very heat resistant materials as well as a very good flow to the catalyst
• Air injection upstream of the catalyst to heat up more quickly by an exothermic reaction ( post-combustion of the remaining fuel components ) to the catalyst.
• Double walled exhaust pipes so that the hot exhaust gases can not cool down as quickly
• Electrically heated catalyst,
• Retarded ignition.

For most standard vehicles ( about 60%) the close-coupled catalyst arrangement has prevailed, as this is the most cost-effective and fuel-efficient deste method.

As a retrofit solution ( primarily for older cars with standard Euro 1 classification) to offer so-called Aufrüstkats. In addition to the effect of a reduced environmental impact, both in the cold phase as well as in warm operating state (as opposed to air systems ) include a classification in a better Emission class is thus generally associated, which is a partly significant savings in vehicle tax may result.

Development

In order to meet future, more stringent emission limits, different steps of development in the catalysts are currently necessary:

• Improved coatings, depending on the application for nitrogen oxides ( NOx) to an intermediately during the cold start phase and then converted in hot catalyst into harmless exhaust gasses.
• It is by producing extremely thin-walled catalyst monoliths to achieve a rapid catalyst light-off, while reducing the exhaust back pressure (so connected to a lower fuel consumption).
• By the production of transverse grooves and / or openings into the catalyst monolith is produced turbulent flow profiles can thus improved mass transfer of the harmful exhaust gas molecules with the noble metals to achieve (the small and the long channels conventional catalyst monoliths generate namely a laminar flow profile). Lateral grooves and / or breakthroughs, there are already at the metal catalysts, which are already used in large series.

Emissions legislation

In parts of the United States catalysts were since 1974 (then still unregulated) prescribed. In Europe, first wrote to Switzerland alone from 1986 for all new vehicles catalysts before; other countries such as Austria and Sweden moved to soon. End of 1984, Germany decided to make the installation of catalytic converters in new vehicles from 1989 edition. Through tax incentives, the use of catalysts has been significantly accelerated, as of 1993, only new vehicles with three -way catalytic converter were really admitted. The introduction of catalyst technology was delayed due to the fact that until the mid- 1980s only leaded fuel was sold and the users of the introduction of unleaded fuels initially critical stood against.

With the proliferation of mobile emissions catalysts, a low concentration noble metal was found at the edge of motorways. This is consistent with previous studies in the United States. Causes are mainly the loss of catalyst material in the destruction of the catalyst by engine malfunction, and low losses during normal operation. A biogenic effects on the human organism has not been established. Today's automotive catalysts are monitored by a diagnostic system. In case of malfunction of the catalyst, the driver is prompted to visit a workshop.

Spent catalysts are collected. The noble metal is recovered and recycled.

Criticism

In the Critique are caused by vehicle catalysts emission of platinum aerosols (supposedly 50 trillion platinum atoms per kilometer ), releases of sulfur trioxide, hydrogen sulfide and hydrogen cyanide (prussic acid ) and the admixed in a necessary for catalysts unleaded gasoline antiknock additive methyl tert-butyl ether (MTBE ), the can produce highly toxic compounds result from the engine oil with zinc dithio phosphate, and benzene.