Metal catalyst
Metal catalysts consist of a steel support (monolith) and the catalytic coating. In addition to ceramic catalytic converters, metal catalytic converters are an alternative in many branches of industry. In addition to being used as vehicle catalytic converters, metal catalytic converters are used in small engines such as chain saws and lawnmowers, as well as in industrial plants (combined heat and power plants, diesel generators, etc.).
The first metal catalysts were invented decades before the development and use of ceramic catalysts.
construction
Metal catalytic converters usually consist of thin, spiral-wound metal foils (usually made of alternating smooth and corrugated layers) or of layered plates. Nowadays, extremely thin sheets (with a thickness usually less than 50 µm) are made of high-alloy special stainless steels with a relatively high aluminum content (up to 10%). Often the term "aluminum steel" is used. Due to the high proportion of aluminum, which is essential for applying the washcoat and the precious metal layer, the metal foils cannot be welded together. In a special process, the contact points (the foil layers below each other and these in turn with the metal jacket) are therefore brazed in a high vacuum furnace so that high mechanical strength and a precise cell structure are achieved. Brazing also prevents the dreaded "spiraling out", especially under load, i.e. in daily vehicle use.
After vacuum brazing, the so-called coaters coat the metal catalysts with the washcoat and the precious metals. Then they are sent to the vehicle manufacturers or other OEMs.
advantages
One of the main advantages of a metal catalyst is its low cell wall thickness. Metal monoliths with a film thickness of only 20 µm are currently in use. For comparison, the thinnest ceramic monoliths have wall thicknesses of 40 µm. Small cell wall thicknesses offer a particularly favorable ratio of the open flow cross-section to the geometric surface. In addition, they reduce the exhaust gas back pressure and thus reduce a possible loss of power. After all, this was still 10% for the first (ceramic) catalytic converters.
Another great advantage is the lower heat capacity. This means that metal catalysts start up much faster and can therefore convert the exhaust gas pollutants into non-toxic gases more quickly. A high thermal and mechanical stability even with strong temperature and load changes (temperature shocks) are further advantages of metal catalysts. In addition, there is a particularly flexible design of the catalyst cross-section. Round, oval (also racetrack shapes), square, hexagonal (etc.) are used.
Since the metal supports can be welded directly into the exhaust system, the installation effort is considerably reduced compared to ceramic substrates. Last but not least, fiber mats, which are indispensable for storing the ceramic monoliths, are not necessary. After all, these fiber mats made of aluminum silicate (still over 90% in use) are considered carcinogenic according to EU classification.
The all-metal construction of the metal catalysts also improves their recycling properties.
The manufacturers of metal carrier / catalysts currently still active on the market are the companies Ecocat, Vihtavuori / Finland, Emitec, Lohmar near Cologne, Oberland Mangold, Eschenlohe / Bavaria, Emission Partner / Lower Saxony and the company Lindo Gobex, Gorzów / Poland. Only Emitec is active as the first supplier for the automobile and commercial vehicle manufacturers.
As one of the largest manufacturers of metal supports (the actual catalytic coating is carried out by other suppliers), Emitec has recently specialized in metal monoliths with profiles that create turbulence. These metal catalysts with longitudinal and / or transversal profiles / openings or with perforated (perforated) metal foils are up to 30% more efficient than comparable ceramic catalysts. The structured channels break up the laminar flow and create turbulence. These in turn increase the exchange of substances between the toxic exhaust gas molecules and the catalytically active components on the catalyst wall and ultimately increase the catalytic activity. Due to the structured metal foils that generate turbulence, catalysts that are 30% smaller can be produced with the same cleaning performance. This can save 30% of the expensive precious metals.
The many advantages of metal catalytic converters ensured that they were widely used, especially in vehicles with catalytic converters close to the engine. For the first time, all BMW 3 Series E46 series with gasoline engines were fitted with close-coupled metal catalytic converters.
S and SM shape
When driving, the coated metal carrier is exposed to extreme temperature fluctuations, vibrations and impacts. It expands when heated and shrinks when cooled. From a certain diameter of the package on, the second winding layer on the circumference of the jacket came off the first and tore it. The very stable soldered connection made thereupon, in turn, was not able to cope with the foils themselves. A multitude of new ideas for the production of stretchable structures also regularly failed. It was therefore a matter of distributing the loads over the entire cross-section of the beam.
In parallel to the development of suitable steel foil formulations, numerous hard soldering tests and the development of a quality test method for the soldered supports in cold condition were carried out. The soldering process was also developed in small steps until the entire hard soldering process could be mastered using vacuum technology. In doing so, they realized that the way the foils were wound also had a decisive influence on the durability of the soldered joint.
The problem of durability in metal girders was solved by using an S-shaped winding instead of a simple spiral winding. Emitec applied for a patent in 1985. With this arrangement, there is no longer position on position parallel to the housing wall, but only the ends of the individual layers come into contact with it and are brazed to it. This construction results in a carrier of great stability with high elasticity at the same time, which is absolutely necessary for thermal expansion. Initially two mandrels (for an S-winding) became three mandrels (for an M-winding) over the years. In addition, four mandrels and more are also possible if carriers with large diameters are to be produced for commercial vehicles or stationary engines, for example.
Designs and areas of application
The efficiency of the catalysts is determined by the type of coating, the cell density cpsi (number of cells per square inch from 150 to 2,400 cpsi are possible) and the carrier volume made available. In terms of production, beam diameters from 28 to 1,200 mm can be realized. In the case of very large support diameters, however, individual, hexagonal catalysts that can be put together as a matrix have often become established.
Metal catalysts are used in many areas of application such as in vehicles (cars, trucks, motorcycles, non-road vehicles such as construction machinery, industrial trucks such as forklifts and agricultural machinery, among other things in tractors, railroad locomotives and ships). They are also used in small engines (e.g. chainsaws, lawn mowers) and in industrial plants (conventional power plants, combined heat and power plants (CHP) with diesel engines or gas turbines or stationary electricity generators, etc.).
species
Otto and diesel engines
Metal catalysts are available as three-way catalysts for gasoline engines and as oxidation catalysts for diesel engines.
In 2004, after many years of trials, the commercial vehicle manufacturer MAN used the newly developed PM (Particulate Matter) Metalit diesel particulate filter (DPF), the PM-Kat , as standard in commercial vehicle engines of the D20 series for the first time. In automobiles, BMW was the first to use the PM-Metalit in the 1-series diesel car. Later, it was used in series production of the Diesel Smart. In addition to reducing the particle mass by more than 30%, the number of ultra-fine particles (diameter less than 400 nm), which are particularly harmful to health, is reduced by around 80%.
Since the legislature does not make a distinction between coarse particles, which are responsible for the particle mass, and the finest particles, which are harmful to health, the metal diesel particle filter as a bypass filter has not yet established itself in the automotive sector as a bypass filter compared to the wall-flow ceramic filter.
The situation is completely different in the commercial vehicle sector and especially with non-road vehicles. Due to their high degree of robustness, the metal diesel particulate filters or SCR systems (SCR = Selective Catalytic Reduction) dominate there, which are essentially based on the design of the metal diesel particulate filter.
Two-wheel metal catalytic converters
As early as 1987, metal supports for two-wheel catalytic converters (four-stroke and two-stroke engines) were developed and used. Above all, the emerging economies of China and India wanted to get the emissions of their fast growing two-wheeler and three-wheeler market under control.
In India, for example, metal catalytic converters are mainly used for two-stroke engines with 125 cubic centimeters. These engines drive a multitude of vehicles from two-wheelers to bizarre three-wheeled transport vehicles with the name "Tuk Tuk". The built-in metal catalytic converters help the previously smelly engines to achieve exhaust gas values at the Euro 2 level.
Today, high-performance motorcycles of the type RSV4 (company Aprilia), R 1200 and S 1000 RR (company BMW) or the Super Duke models (company KTM) are exclusively equipped with metal catalysts. Fragile ceramic monoliths are ruled out from the start.
From Aprilia to Honda, Kawasaki to Suzuki and Triumph, all motorcycle manufacturers only use metal catalytic converters. Even the rugged Neander diesel motorcycle is equipped with a metal particle filter. By the way, this is the first turbo-diesel motorcycle in the world!
Industrial catalysts
According to their own statements, Oberland-Mangold produces metal beams up to 1,200 mm in diameter. Since these large and, above all, very heavy individual catalytic converters cause certain problems in handling and assembly, Emitec, for example, developed the HEXA catalytic converter modules. These are compact hexagonal, i.e. hexagonal, metal supports (width at the narrow point corresponds to the width across flats of 250 mm), which can be combined to form larger catalyst packages. This means that even the largest exhaust systems for machines and power plants in the MW range can be implemented relatively easily, flexibly and, above all, inexpensively.
history
The metal carrier as the basis for the car catalytic converter has employed generations of engineers long before the ceramic carrier was presented as a solution. As early as 1929, Joseph Christie Whitney Frazer (* October 30, 1875, † July 28, 1944) from Baltimore / USA recognized the advantages of the metal carrier and had his developments in this regard patented in the same year. The patent describes a catalyst carrier made of corrugated steel foil, the corrugated foils being separated from one another by smooth foils. Other claims name screens or perforated sheets instead of the corrugated metal foil. This was by no means the first proposal for a catalytic converter, but it seems that Joseph Christie Whitney Frazer was the forefather of today's steel support for metal catalytic converters. Frazer also applied for a patent for his steel support for metal catalysts in Germany and was granted the German Imperial Patent No. 563757 in 1930
The description of both the steel support and the metal oxide catalyst seems extremely modern. Since the engines of the time also contained large amounts of unburned hydrocarbons (HC) in addition to carbon monoxide, Frazer also provided for secondary air injection, which was only introduced in automotive engineering decades later. The inventor Frazer wanted to regulate the amount of air that was to be blown in before the catalytic converter as a function of the engine speed. Frazer also recognized very early on that the catalytic converter is only able to oxidize carbon monoxide from an exhaust gas temperature of 450 ° C. That is why he recommended heating the exhaust gas after a cold start, for example with an electric heater. The heated catalyst only became a reality decades later.
With the heated EMICAT metal catalytic converter, the BMW Alpina B12 5.7 and shortly thereafter the 12-cylinder series model from BMW AG were able to significantly fall below the 1996 emission limits in Europe, Japan and the USA.
Joseph CW Frazer points out several times that the carrier must be made of a thermally conductive metal. It is therefore necessary for the carrier to touch the wall of the container surrounding it. Frazer also wanted to use the heat generated in the carrier to preheat the exhaust gases, which were too cold, for example when idling. The description is so detailed that tests were certainly carried out at the time. Sieve-like supports made of high-alloy steel existed before Frazer, and they obviously come from the petrochemical industry. Frazer is therefore considered to be the forefather of perforated metal supports.
After Frazer's work, the idea of using steel beams for car catalytic converters did not reappear until after 1950, after further demands to reduce pollutants in car exhaust became ever stronger. One of the first manufacturers to investigate the metal carrier was UOP (Universal Oil Products) in the USA, which was later (1988) taken over by Allied Signal and Union Carbide. On the part of the automobile manufacturers, General Motors, Mercedes-Benz, Audi, VW and others should be mentioned. All failed, however, because the spirally wound metal foils were sitting loosely in their tubular container and were not up to the dynamic operation. They spiraled quickly and then quickly broke.
These mechanically harsh conditions led to the end of a large number of experiments much later until modern times, most of which became known through numerous patents. They came from Ford, General Motors, Mitsui, the UK Nuclear Energy Agency, Degussa and other companies. All showed corrugated metal foils, some also the alternate use of smooth and corrugated foils. In addition to spirally wound foils, there have also been proposals to lay the foils on top of one another as plates and to accommodate them in rectangular housings. Mitsui even patented a variant in 1974 in which the plate pack was interchangeable. In addition, the plates were fixed with screws, but these could not avoid fatigue failure. The problem of loose foil packs was well known from painful experience, but a final solution was still to be found.
Although the first so-called three-way catalysts with ceramic monolith were successfully launched on the market in 1973, the automotive industry supported all attempts to use steel foils as carriers for decades. In addition to the many advantages of metal catalysts, they wanted to make themselves more independent of the then two large ceramic monolith manufacturers (the companies Corning, USA and NGK, Japan; the Japanese Denso was added much later).
At Audi, Ford, General Motors and VW, for example, numerous attempts were made from around 1960 to introduce metal beams into series production. In tests, excellent exhaust gas cleaning performance could be achieved with it, but the durability of these metal supported catalysts was not given. The spirally wound and then loosely inserted foils were first attempted to be fixed at the ends using cross-shaped holders. In practice, however, during dynamic operation, these cross-shaped holders chafed through, so that the monolith (carrier) nevertheless spiraled out. In 1985, the Finnish company Kemira (today EcoCat) presented a metal catalyst that consisted of foils already coated with precious metal, which were then wound in a spiral. In order to prevent it from spiraling out, the wrapped foil package was fixed to the jacket with nail-like metal pins. One also spoke of the “nailed metal catalyst”. In 1987 Kemira built a complete production facility in Vihtavuori, Laukaa, for the production of these coated nail-metal catalysts. Series production started in 1990 and in 1992, for example, the Opel Corsa model was equipped with this nail-metal catalytic converter. In addition to Opel, Fiat was also supplied with these metal catalysts.
These Kemira metal cats also showed a moderate durability. Above all, the trend towards catalytic converters arranged close to the engine ensured that the original equipment manufacturers (OEMs) decided not to use Kemira metal catalytic converters. The Kemira successor company EcoCat currently manufactures metal catalytic converters for gas-powered internal combustion engines as well as metal catalytic converters and metal and ceramic particle filters for diesel vehicles.
Since 1985 the company Oberland Mangold in southern Bavaria has been producing metal supports for catalytic converters for exhaust gas purification. Since the beginning, however, almost exclusively in the area of replacement and retrofit catalyst systems. Soot particle filter systems (metal particle filters) and mini-cats made of metal were added later. Since it was founded, the company has successfully positioned itself in the catalytic converter market.
At the Siemens subsidiary Interatom in Bensberg, the focus of reactor technology was often on combining exotic materials. The basic patent for joining metals with ceramic surfaces, which was later to allow mastering the manufacture of metal carriers, was registered by Interatom in 1978. That was also the reason why Degussa turned to Interatom for help at that time.
The first application of the Interatom brazing process, however, was not carried out by Degussa or Interatom, but by the Stuttgart cooler specialist Behr. Due to the close proximity to Mercedes-Benz, Behr decided to manufacture spirally wound metal supports that Mercedes-Benz wanted to use as starter catalysts close to the engine.
The Mercedes engineers Dr. Jörg Abthoff and Dieter Schuster published in 1984. Initially, ceramic monoliths were used for the regulated starting catalytic converters. However, since these could not withstand the high thermal and mechanical loads directly on the exhaust manifold, they had to be installed away from the engine. Due to the long distance between the exhaust valves and the starting catalytic converter and the main catalytic converter connected to it, the exhaust gas cooled down too much and the catalytic converters started much too late. The consequence of this was that the exhaust gases contained too high a proportion of pollutants immediately after the cold start, because they passed the catalytic converter without conversion. With this first solution, the future, stricter exhaust emission limits, especially in Japan and the USA, could not be observed. For this reason, Mercedes-Benz used close-coupled starter catalysts based on metal substrates from Behr, which withstood the high thermal and mechanical loads.
For Behr it was an advantage that the starting catalysts only had to have a small diameter, for which the simple spiral winding of the foils did not yet prove to be a disadvantage. They carried the brand name "Metalit" and were introduced in Japan and the USA, with some very good results. The disadvantages of the simple spiral winding only became apparent later, when the Behr company manufactured metal carriers with a larger diameter at the customer's request.
When the political turn away from nuclear energy became apparent around 1980, Interatom managers began looking for other tasks with which their own scientists, engineers and highly specialized technicians could continue to occupy themselves. In addition to the employees, unique calculation and analysis methods, machining methods and metallurgical knowledge were available that stood out well above the usual industrial level.
A “New Technologies” department was founded at Interatom in 1984, headed by Dr. Rolf Kottmann was headed. The "Automotive Industry" department was created within this umbrella term. It should develop new products and take over the marketing of Interatom technologies. In the same year, Kottmann appointed Wolfgang Maus , who was about the same age, to head this “New Technologies” department. Wolfgang Maus had been involved in research and development work for components of the high-temperature reactor (HTR). In the newly founded department, Wolfgang Maus was to develop future products and market services for the automotive industry in the nuclear industry. In this way, Interatom hoped to survive the end in the field of nuclear energy technology and thus save the extensive knowledge of its employees.
In addition to the hollow camshaft, the second far-reaching idea that was considered was the metal carrier for car catalytic converters, which has already failed so many engineers. In addition, there was the idea of gas-dynamic bearings (air bearings) for exhaust gas turbochargers. For this purpose, the first simulation software for combustion calculations was created, which was derived from nuclear safety analysis programs.
In order to be able to continue the metal beam project, the Interatom engineers began looking for interested parties in the automotive industry. After a successful presentation in Japan in 1984, they met with great interest from some of the employees of Japanese automobile manufacturers present, especially from Toyota engineers. The new strategic orientation of the Interatom parent company turned out to be another stroke of luck. At that time, Siemens wanted to get more involved in the automotive industry and founded the "Automotive Technology" division, which was later incorporated into VDO and which then developed into a profitable business area. But a manufacturer of exhaust systems also showed interest in a joint venture with Siemens.
Then on August 11, 1986, Siemens founded the Emitec joint venture together with the automobile manufacturer Uni Cardan. The name originated from Emitec " Emi ssions tech nology" and is now a global company, which is by far the market leader in the production of metal supported catalysts. In 2011 Emitec celebrated its 25th company anniversary.
Web links
- 25 years of metal catalysts
- Metal Kat patent Joseph Christie Whitney Frazer (excerpt)
- Metal-Kat patent "start catalyst"
- Metal-Kat patent "start catalyst"
- Metal cat patent for close-coupled starter catalysts from Mercedes engineers Dr. Jörg Abthoff and Dieter Schuster - 1984
- SAE-Paper: New Developments in Catalytic Converter Technology as Demonstrated by Mercedes-Benz 300 SL and 300 CE
Individual evidence
- ↑ Biogas catalysts | Emission reduction bonus with Emission Partner. Accessed December 7, 2017 (German).