DE2811788A1 - Catalysts contg. rhodium and platinum - for use in gases contg. oxygen and sulphur di:oxide, esp. for exhaust gas purificn. - Google Patents

Abstract

Catalyst for use in gas streams contg. O2 and SO2 (esp. for oxidn. of CO and hydrocarbons in IC-engine exhaust gases) have a catalytically active phase comprising a mixt. of alloy of Rh and Pt with an Rh content of =25 (pref. 5-20, esp. 7.5) wt. %. The active phase can be supported on a pelleted or monolithic support, esp. wash-coated with an alkaline suspension (pH >8) of a refractory oxide. Suitable monolithic supports comprise zircon-mullite, mullite, gamma-alumina, sillimanite, Mg silicate, Zr petalite, spodumene, cordierite and aluminosilicates. Suitable pelleted supports comprise charcoal, carbon black, alumina, aluminosilicates, MgO, diatomaceous earth, bauxite, TiO2, ZrO2, limestone, Mg silicate and SIc. Suitable wash coats comprise oxides of B, Al, Si, Be, Mg, Ca, Sr, Ba, Sc, Y, ti, Zr, Hf, Th and/or lanthamides and/or actinides. The catalysts catalyse the oxidn. of CO and hydrocarbons more readily than the oxidn. of SO2, so that SO3 emission is minimised.

Description

Catalyst for use in an oxygen and sulfur

gas stream containing dioxide.

The invention relates to the monitoring of components, in particular unwanted components, in gas streams. More specifically, the invention relates to Monitoring of exhaust gases from internal combustion engines, explosion engines or so-called Internal combustion engines.

Under "internal combustion engines, explosion engines and internal combustion engines" are to be understood here as motors or prime movers in which energy is generated through combustion of fuels or fuels is produced, the combustion by a Spark or otherwise initiated.

Under a "monitoring or control of the exhaust gases" or an "exhaust gas control" in connection with internal combustion engines, explosion engines, internal combustion engines and the like is monitoring or control and in practice at least a reduction in the levels of harmful and other undesirable components to understand that occur in the exhaust gases produced by internal combustion engines, Explosion engines and internal combustion engines are ejected as Result of complete or partial combustion of fuels in the engine or motors.

To this day, the experts working in the field of exhaust gas monitoring have in general, their primary focus is on monitoring the exhaust gas content directed to nitrogen oxides, hydrocarbons and carbon monoxide, which in more or larger amounts in the exhaust gases of internal combustion engines and the like, in particular oil-powered engines and these engines occur.

It is well known in the context of such monitoring or control procedures the oxides of nitrogen (often referred to as NOx) by catalytic means Reduce nitrogen (where at the same time the oxidation products a reducing agent, i.e. a reducing gas used for reduction was used) and the hydrocarbons catalytically to water and carbon dioxide and to oxidize the carbon monoxide to carbon dioxide) by means of an oxidizing agent, such as oxygen or air).

This catalytic reduction and oxidation generally takes place by means of catalyst units with one or more metals of the platinum group, which are arranged within the exhaust system of the engine. Occasionally two such units are arranged in tandem around the desired To effect reduction and oxidation sequentially. Occasionally there will be a single Unit used with a catalyst that causes the reduction of NO or the Oxidation of hydrocarbons and carbon monoxide or both, depending on the conditions inside the catalyst unit. A catalyst of this latter type is occasionally used also referred to as a so-called "three-way catalyst" (in English t'TWC ").

A catalyst unit advantageously has one layer of one or more platinum group metals on a layer of a Material are deposited (generally a layer of a heat-resistant or refractory oxide, such as aluminum oxide), the layer being a has extensive surface and which in turn is deposited on a carrier is, which can for example consist of a ceramic material or metal. Such a carrier can advantageously have a cylindrical shape, whereby its arrangement within a housing in the exhaust system of a means of transport, for example a motor vehicle, wherein the carrier has a plurality of channels which extends longitudinally through the body of the wearer parallel to its axis of symmetry extend so that the gas enters at one end of the carrier and the carrier at the leaves the other end again. When viewed from one end, the walls of the canals give such a carrier a honeycomb-like appearance, which is why one such Carrier or body in the Experts also as a so-called honeycomb carrier ("honeycomb support").

The walls of the channels are generally covered with a heat-resistant one or refractory oxide or a precursor thereof coated, for example by a so-called "wash coating process" in which, for example, the honeycomb-like Layer support in an aqueous suspension of the oxide or the oxide precursor compound is immersed, whereupon after removal of superfluous liquid the honeycomb-shaped The support is subjected to heating or firing in order to form the oxide layer with the To connect carrier or to convert the precursor into the oxide, which then is connected to the carrier. The carrier does not need to be in an aqueous suspension of the oxide or the precursor to be immersed. Such a suspension can also for example also be sprayed on. One generated in the manner described Oxide layer is often referred to as a so-called "wash layer" and is honeycomb-like Carrier with an oxide layer covering at least the inner surfaces of the channels covered, is referred to as a so-called "wash-coated honeycomb carrier".

It has been found that the exhaust gases from internal combustion engines, Explosion engines and the like and in particular of liquid fuels processing machines, in addition to the components already mentioned, also sulfur trioxide (generally in the form of a sulfuric acid aerosol, at least in the lower end of the exhaust system) if the exhaust system has a catalytic converter unit for the control or monitoring of the content of hydrocarbons and carbon dioxide having.

This sulfur trioxide is created by the catalytic oxidation of sulfur dioxide, which in turn is presumably formed from the sulfur that is contained in the fuel as a result of the combustion of the fuel in the machine, although the sulfur dioxide itself is at least partly up catalytic Paths can be generated.

The object of the invention is to provide a selective oxidation catalyst indicate, for example, the oxidation of hydrocarbons to water and Carbon dioxide and the oxidation of carbon monoxide to carbon dioxide easier and faster catalyzes as the oxidation of sulfur dioxide to sulfur trioxide.

The invention was based on the knowledge that the asked Task can be solved with a catalyst, which is characterized by a catalytic active phase of a mixture or alloy of rhodium and platinum, in the rhodium in a concentration of traces up to 25 wt. t, based on the weight of the catalytically active phase.

After extensive investigations it has been found that, if a gas stream, containing at least sulfur dioxide and oxygen, via an oxidation catalyst among those allow the catalyst to oxidize sulfur dioxide To promote sulfur trioxide, the proportion of sulfur dioxide that is oxidized, in the gas stream is decreased when one or more of the following Conditions are met.

The invention was based on the knowledge that in particularly advantageous Way the relative amounts of sulfur trioxide or sulfuric acid or both that through the exhaust pipe of an internal combustion engine or an explosion engine or an internal combustion engine, if in the exhaust system a platinum group metal catalyst is significantly reduced leave if: 1.) the catalytically active phase of the catalyst from a mixture or alloy of rhodium and platinum, the proportion of rhodium in the Case of a catalytic converter of the exhaust system of an internal combustion engine or an explosion engine or an internal combustion engine is preferably so high, that sufficient or satisfactory oxidation of the hydrocarbons and of carbon monoxide takes place and the Total metal loading of the catalyst is preferably no greater than 25 grams per 0.02832 m3 of the catalyst; 2.) that Washcoat material (or precursor thereof) that is the catalyst metal carries, is applied to the final support from an alkaline suspension before there is a fire; A ceramic carrier is preferably used as the carrier used with honeycomb structure as mentioned above; preferably lies further the pH of the suspension of the wash coating material or the precursor over 8; 3.) a carrier with a honeycomb structure is used, with the number the channels per unit area on the end side is advantageously so small that how it allows effective hydrocarbon and carbon monoxide oxidation; 4.) the catalyst unit is used at a temperature as high as possible is; in the case of a catalytic converter of an exhaust system of an internal combustion engine and the like, for example, this is made possible by making the catalyst so tight or as close as possible to the exhaust stub or exhaust manifold or exhaust manifold is arranged; alternatively, however, the catalyst can also be as low as this how possible temperature to be used; in this case the catalyst becomes used in the case of an internal combustion engine and the like at a temperature which is as low as there is an effective monitoring of the hydrocarbon content and carbon monoxide in the exhaust system, in the latter case the catalyst unit advantageously at the end of the exhaust pipe or closer is housed at the end of the exhaust pipe; ~ 5.) The percentage of oxygen in the Gas flow at least shortly before it is passed over the catalyst, so low as is possible; in the case of using a catalytic converter in the exhaust system one Internal combustion engine and the like should, if possible, the oxygen content be as low as there is effective monitoring or control of the content Allows hydrocarbon and carbon monoxide in the exhaust gas stream; 6.) two or several catalyst units are arranged in parallel in the gas stream, with measures be taken to prevent the gas flow through one or more of the Units can get; in the case of a catalytic converter unit in the exhaust system of a Motor vehicle such measures can be taken in an advantageous manner, that the driver can divert the flow of exhaust gas from one unit to the other unit; alternatively, it is also possible to take such measures that an automatic Switching as a result of certain changes in one or more measured Parameters of the exhaust system takes place; the two units are preferably located in poor thermal contact with each other, so that the gas flow from one first catalyst unit can be passed into another catalyst unit, so that the first catalyst unit can cool down before it is reloaded will; this is especially- <. which is advantageous if the catalytically active Phase consists of a rhodium-platinum alloy, for reasons that will be explained later will be explained in more detail.

According to an advantageous embodiment of the invention thus has a catalyst unit for use in a gas stream containing Oxygen and sulfur dioxide (and especially a unit in the exhaust gas stream of a Internal combustion engine and the like for controlling and checking the content of hydrocarbons and carbon monoxide and, if applicable, oxides of nitrogen, which may be present in the exhaust gas flow) one or more of the listed features 1, 2, 3 or 6 on and / or is in accordance with the features 4 or 5 or both operated to avoid the emission of sulfur trioxide and / or sulfuric acid the exhaust system of the engine or the engine.

As already stated, sulfur dioxide, which occurs in the exhaust gases of internal combustion engines and the like, and which is formed by the combustion of sulfur, which is found in practically all commercially available fuels, for example liquid fuels, can be in the presence of excess oxygen and a catalyst that is used to oxidize the existing hydrocarbons and carbon monoxide is intended to be converted into sulfur trioxide. I) The oxidation proceeds according to the following reversible reaction equation: a subsequent reaction of the SO3 with water droplets forms an aerosol of sulfuric acid, which is ejected from the exhaust pipe.

For the sake of simplicity, an emission or a discharge will hereinafter be used of sulfuric acid with or without sulfur trioxide or of sulfur trioxide alone referred to as "sulfate emission".

The rate of oxidation of sulfur dioxide to sulfur trioxide depends on half the force (0.5 power) of the oxygen partial pressure and it follows that the limitation of the oxygen partial pressure in the exhaust gas leads to the Sulfate emission is limited.

The values of the equilibrium constants K for Equation 1 within of the temperature range of the emission control catalytic converters in the exhaust system of Motor vehicles reached, namely 350 to 8000C, are such that the equilibrium conversions from SO2 to SO, reach 100% until the catalyst temperature exceeds 5000C.

It follows that a motor vehicle equipped with a catalytic converter which is active enough to catalyze reaction (1), up to one Equilibrium under the conditions the temperature of the flow velocities and at the relative concentrations of gases found in the case of exhaust systems and using a fuel with 0.03 wt% sulfur at 20 mpg one Can cause sulfate emissions of over 150 mg / 1.6094 km. The very great importance the activity of the catalyst in relation to the reaction given by the equation 1 is shown, for the determination of sulfate emissions is therefore evident.

It is of course possible to have generated sulfur trioxide over the catalyst to reduce to sulfur dioxide in the presence of a reducing agent, where hydrogen, propylene, propane and carbon monoxide are already used for this reaction indicated as active. The presence of such substances in the exhaust gases would consequently lead to a weakening of the sulphate emission, whereby it should be mentioned that that carbon monoxide is less active than the other three compounds listed.

The phenomenon of sulfate emission is complicated by a sulfate storage effect, which is presumably based on the action of sulfur trioxide with the wash coating, for example an aluminum oxide wash coating of the catalyst unit according to the following reaction equation: As a result, the following reactions can occur in the case of emissions control catalysts under appropriate conditions: and then on the catalyst and / or subsequently: The reaction (1) is exothermic and is consequently shifted to the left at high temperatures. Reaction (2), which is the "storage" reaction, is favored at a comparatively low temperature, while the reverse reaction occurs at the decomposition temperature of Al2 (SO4) 3, which is 7700C. The reaction (2) therefore does not need to be a strictly reversible reaction, since this assumes the existence of equilibrium conditions at a given temperature. In the present case, however, the reaction is shown as a reversible reaction.

The sulfate emission is a complex function of, among other things, the Activity of the catalyst for sulfur dioxide oxidation and the ability of the Catalyst wash coating to store and release sulfate.

The activity of a catalyst is subject to its use Changes, for example when used in an exhaust system of a motor vehicle. These changes are due to changes in the degree of dispersion of the catalytic active metallic phase on the wash layer, for example due to sintering of the metallic phase. The changes may be also to changes in the surface structure of the catalytically active metallic Phase based.

At comparatively low temperatures under thermodynamically favorable conditions Conditions for sulphate production are also conditions for sulphate storage cheap. Conversely are at high temperatures, if due to thermodynamic Considerations of low sulfate emissions would be expected, the condition / as well beneficial for the release of stored sulfate for the wash layer, so that an unexpectedly high sulfate emission can occur.

To further complicate matters is the sulfate emission all the exhaust pipe of a motor vehicle is also a function of history or Pre-treatment of the catalyst. It has been found that a catalyst is used in the exhaust system of a motor vehicle that is a platinum group metal contains or a metal alloy from the platinum group on an Alumijiiumoxid-i't'asci # sciiiciit, which was and is applied to a ceramic honeycomb pattern light carrier the catalytic converter first the ases of an engine with an operating time of 804.67 km hot exposed to a constant speed of about 80.467 km per hour and then the catalyst is allowed to cool to normal temperature and finally again the action of exhaust gases from an engine for another 804 km at one When exposed to speeds of around 80 km per hour, this is the sulfate emission from the exhaust pipe at the beginning of the second 804 km and lower in some cases much lower than the sulphate emission from the exhaust pipe at the end of the first 804 km. In other words, the rate at which sulfur dioxide is catalytic is oxidized to sulfur trioxide, from which sulfuric acid is subsequently formed is considerably higher shortly before the catalyst has cooled down than after cooling and with the new use of the catalyst in the exhaust gas flow. This means that Obviously some beneficial changes in the catalyst as a result of the cooling occur.

The information that the catalytic converter unit was "exposed" to the exhaust gases, means that the exhaust gases were passed over the catalytic converter or, if applicable through the channels of the honeycomb support so that they are in contact with the catalytically active metal phase on the wash layer.

The catalyst substrate can be made of a metallic or a non-metallic Material, for example a ceramic material exist.

Advantageously, metallic substrates in the form of metallic Monoliths are used as a carrier, as these are generally less Show pressure drop and a relationship from surface to volume have, which can be 1 1/2 to 3 times as large as the ratio of a ceramic honeycomb substrate.

For example, a normal ceramic substrate may have a surface area to volume ratio on the order of 55.74 to 65.03 m2 (000 to 700 square feet) per 0.02832 mS (per cubicfoot) substrate.

Examples of typical ceramic substrates are commercially available Products, such as the product EK20 (Corning) with a surface of 52.67 mZZ0.02832 m3 (576 square feet / ft3) and the single surface area product Grace 400 of 72.46 m2 / 0.02832 jn3 (780 square foot / Et3). Ilierhei is about the highest preserved surface, which is comparable with 102.1 m2 / 0.02832 m3 (1100 square foot / ft3) for a 0.0076 cm thick Kanthal D sheet and 185.8 m2 / 0.02832 m3 (2000 square foot / £ t3) for a 0.00508 cm thick Kanthal D sheet.

A suitable metal substrate can advantageously be omitted a corrugated, corrugated or corrugated foil with a thickness of for example Make 0.0038 cm to 0.0114 cm (and preferably a thickness of 0.00508 cm), which is put together in such a way that a structure with, for example, 400 cells, seen in cross-section, is obtained on a cross-sectional area of 6.4516 cm2.

The cell size is advantageously around 200 to 800 Cells per 6.4516 cm. An advantageous surface-to-volume ratio is around 111.4 m2 / 0.02832 m3 (1200 sq ft per cubic foot) with e.g. 400 cells 2 2 len per 6.4516 cm and 185.8 m2 / 0.02832 m3 (2000 sq. ft. per cubic foot) with about 800 cells per 6.4516 cm2.

Metals that are used to manufacture monolithic beams are those that allow high temperatures and strong oxidation conditions be able to resist. Examples of suitable base metal alloys are nickel and chrome alloys Init an aggregated Ni + Cr content of greater than 2 £) wt .-% as well as alloys of iron and at least one of the elements: Chrome (7 # I) is 40 Cew. -%), aluminum (1 to 10 (% by weight), cobalt (traces up to 5 (% by weight), Nickel (traces up to 72% by weight) and carbon (traces up to 0.5% by weight). Other trace elements in such alloys for the purpose of improving their strength and oxidation and heat resistance are:% W / W Si 0.2 0.9 0.2 0.7 Zr 0, () 1 0.20 Cu 0.01 0.15 N, Ot 0.3 l'a 0.8 1.2 Ti 0> 8 1.2 Ce 6.01 T, .O Ca 0.01 0.5 More examples of base metal 1 alloys that have rigorous conditions too are able to withstand and can be used according to the invention are iron-aluminum-chromium alloys, which also contain yttrium. Such alloys include, for example 0.2 to 12 wt% Al, 0.1 to 3.0 wt% Y and 0 to 20 wt% chromium, the remainder consists of Fa. Such alloys are disclosed, for example, in US Pat. No. 3,298,826 known. Other advantageous alloys that can be used according to the invention are Fe-Cu-Al-Y alloys with 0.5 to 4 wt% Al, 0.5 to 3.0 wt% Y, 20.0 to 95.0 wt% Cr and Fe 100 t. Such alloys are known, for example, from US Pat. No. 3,027,252.

A metal substrate that has been mechanically deformed to to achieve an enlarged surface has in comparison with a flat, undeformed substrate has a considerably increased surface area at the same Volume. In a typical way can be such an enlarged Outer surfaces obtained by corrugation or folding by means of an appropriate device cloning and co-winding of a flat film and a corrugated or folded foil into a tube with a spiral cross-section.

According to an advantageous embodiment of the invention, the metal substrate, which is used as a monolithic support for the catalyst, initially bent, curled or folded, stranded with grooves or corrugations, folded, serrated or notched and / or perforated in such a way that a considerably larger surface area is produced. The surface created by the pretreatment is usually considerably larger than that obtained when using a ceramic roll carrier can be obtained or with a particulate catalyst support with the same volume.

An example of a metal substrate used in the present invention is a roll of a sheet of paper with handles or grooves a heat-resistant alloy that is shared with a non-corrugated or folded sheet of such an alloy is used, e.g. Roll of different layers of the two sheets. On the other hand, for example also use two corrugated or ribbed blades, with the corrugations or ribs in the case of each sheet parallel or transverse to one another are located. Starting from such sheets or foils, coil-like can be created Prepare substrates with a firmly adhering oxide coating that is porous and has an absorbent effect and has a large surface area and acts as a carrier for the second catalytically active layer acts, the one or more of the invention usable, catalytically active mixtures or alloys, e.g. in the form of intermetallic compounds.

Heat or heat-resistant alloys with the expanded Metal substrates are alternatively alloys with a minimum nickel and chromium content of 20 wt. "o. Typical alloys used for the expanded metal substrate are rust-free steels with a high nickel and chromium content. Such Steels are available in ilandel, for example under the name "IOL" 600 as well as TINCONLL t01.

The metallic honeycomb structure with a first coating A heat-resistant metal oxide can then be further impregnated or coated with one or more of the intermetallic compounds given above Links. Suitable heat or refractory bfetal oxides that are the first coating or make up or are present in the first layer are one or more of the Oxides of ß, Al, Si, Be, Mg, Ca, Sr, Ba, Sc, Y, Ti, Lr, Hf, Th, of the lanthanides and the actinides.

These are particularly advantageous heat-resistant metal oxide layers from or with from the family of γ-aluminum oxides and activated aluminum oxides. These can be created, for example, by precipitating a water-containing aluminum oxide gel and drying and calcination of the same for the purpose of splitting off Ilydrat water and recovery of active γ-alumina. A particularly beneficial, active, Refractory metal oxide can be obtained by drying and calcining a so-called precursor mixture of hydrous aluminum oxide phases, the consist predominantly of crystalline trihydrate, i.e. over 50% by weight of the total aluminum hydrate mass, preferably 65 to 95% by weight of one or more the trihydrate forms of gibbsite, bayerite or norstrandite according to X-ray diffraction exists, obtained at temperatures of 300 to 800 0C.

According to the invention, an aluminum oxide hydrate can advantageously be used of the type British Aluminum Co., grade Mit170 alumina hydrate that use through Drying and calcining or firing in the manner described in activated Alumina is transferred. In a particularly advantageous manner, the metallic Substrate with a first firmly adhering oxide layer in an essentially two-stage process Process made. In the first stage, the metallic substrate becomes thermal oxidized, producing a thin first oxide layer that acts as a "key". In an advantageous manner, thermal oxidation is brought about by the fact that the formed or shaped metallic substrate at a temperature is heated from 1000 to 12000C in air or in damp cracked ammonia vapors, for example for 1 hour. The higher temperature is in the case of very Oxidation-resistant alloys required, e.g. the Kanthal area, while a moist hydrogen atmosphere is preferred in the case of alloys with a high Ni content is used.

The adhesive oxygen-containing film or oxide film in the case of Both metallic and non-metallic substrates can be processed according to customary known methods, including chemical processes. The film is supposed to be of sufficient thickness to provide adequate absorption capacity for the Retention of the catalytically active alloy with one or more of the metals to have the platinum group. The film preferably has a thickness of 0.0010 up to 0.00254 cm.

Is aluminum present in the alloy forming the extended metal substrate the oxide film can be formed by treating the aluminum-containing surface with a solution of an alkali carbonate, for example a sodium carbonate chromate solution be generated. The film can also by anodic oxidation of the Generate metal surface, in which case the metal acts as an anode in an electrolytic Solution is switched. For anodizing surfaces containing aluminum For example, a 15 strength sulfuric acid solution can be used as the electrolyte. However, other acidic electrolytes can also be used, such as Chromic acid, oxalic acid, phosphoric acid and optionally boric acid. The oxide film becomes intentionally applied and does not consist of the comparatively thin ones natural oxide layers that occasionally appear on metal surfaces that have been exposed to the action of the atmosphere.

A method of producing an aluminum oxide layer on the alloys, which do not contain a sufficient amount of aluminum necessary for the formation of a own aluminum oxide layer is required by oxidation consists in the application a so-called Calorization process. In this procedure a vapor deposition takes place on an aluminum layer with subsequent anodization or heating in a gas containing oxygen. Alternatively, layers, such as chromate, phosphate, silica or silicate or zirconium oxide layers be deposited by known methods.

The ceramic layer supports which can be used according to the invention can consist either of particulate material, e.g. so-called pellets or but the beams may have a monolithic structure. Monolithic ceramic Carriers are preferably of the "honeycomb" type having a regular arrangement of channels through which gas can flow. Suitable substances for the production can be used by ceramic carriers are, for example, zirconium mullite, Mullite, a-aluminum oxide, sillimanite, magnesium silicates, kaolin clays, zircon, petalite, Spodumene, cordierite and a wide variety of aluminosilicates.

Products which can be used according to the invention are, for example, in the U.S. Patents 3,397,154 and 3,498,927 and British Patent 882,484.

Examples of suitable carriers available in Ilandel are ~ Torvex " which is a honeycomb mullite product with 8 corrugations or Grooves on 2.54 cm and an alumina wash coat; "Thermacomb", a honeycomb-like one Cordierite product made by American Lava Corporation and EX 20, a honeycomb Cordierite product, manufacturer Corning Glass.

Advantageous carriers of the pellet type consist, for example, of porous Silicic acid and are, for example, in Flandel under the llandels name "Silocel" available. Also to be mentioned are carriers made of charcoal granulate, carriers made of a- or γ-aluminum oxide granules or corresponding pellets, carriers made from pellets naturally occurring or synthetic aluminosilicates, magnesium oxide, diatomaceous earth, Bauxite, titanium dioxide, zirconium dioxide, limestone, magnesium silicate, silicon carbide as well as activated or non-activated carbon. These substances can be in the form of Particles of uniform or non-uniform shape can be used and for example the shape of tubes, e.g. capillary tubes, extrudates, rods, spheres, broken ones Have particles or a wide variety of fittings.

It preferably comprises a particulate ceramic material existing carrier or the honeycomb-like structure a first deposit or a first layer of a refractory metal oxide that continues with a or more of the described or mentioned intermetallic compounds or Alloys coated or impregnated. Suitable advantageous heat-resistant or refractory metal oxides used to form the first layer are one or more of the oxides of B, Al, Si, Be, Mg, Ca, Sr, Ba, Sc, Y, Ti, Zr, Hf, Th, the lanthanides and actinides. Particularly advantageous heat-resistant Metal oxide layers are those with oxides of the y-oxide family and activated Aluminum oxides. I) such oxides can be produced, for example by precipitation of an aqueous aluminum oxide gel and subsequent drying and calcining to drive off the hydrated water to produce active y-alumina.

A particularly advantageous active refractory metal oxide leaves obtained by drying and calcining a precursor mixture of hydrous Aluminum oxide phases, which predominantly consist of crystalline trihydrate, i.e. more than 50% by weight, based on the total mass of alumina hydrate, preferably 65 to 95% by weight of one or more of the trihydrate forms of gibbsite, bayerite and / or norstrandite as determined by X-ray diffraction Temperatures from 400 to 8000C.

A commercially available aluminum oxide hydrate can be used in a particularly advantageous manner of the type British Aluminum Co. grade MH 170 alumina hydrate, made by Drying and calcining or firing as described in activated alumina is convicted.

There are several different methods for making a catalytic active wash layer or wash coating made up of a heat-resistant metal oxide with a high surface area with one or more of the heat-resistant metal oxides possible, the advantageous properties in terms of deposition and inertness on the deposited intermetallic compounds at high temperatures under oxidizing and reducing Transfer conditions.

An advantageous adhesive oxide layer on a metallic or non-metallic substrate can be deposited, consists of aluminum oxide, its surface area ranges from about 50 to 500 square meters per gram of alumina having.

One method for the deposition of aqueous alumina is, for example known from US-PS 2,406,420. The most varied of starting compounds can be used Aluminum compounds such as alkali metal aluminates and aluminum salts be used. Either acidic or basic precipitants can be used for the precipitation can be used depending on the character of the starting material. Suitable acidic precipitants are for example ammonium chloride, ammonium sulfate, ammonium nitrate, hydrochloric acid and nitric acid. Suitable basic precipitants are, for example, ammonium hydroxide, Sodium hydroxide, hexamethylenetetramine and the like.

According to the invention it is also possible to use the aqueous aluminum oxide from an alkali metal aluminate solution with excess aluminum and alkali metal hydroxide to be applied directly to the metal substrate or ceramic substrate. Becomes an aluminate solution a temperature of 600 to 850C is used, a film or a layer precipitated from α-aluminum oxide trihydrate (gibbsite).

Subsequent heating to a temperature of 250 to 1800C the trihydrate is converted into the monohydrate and by further heating to, for example 5400C the monohydrate is converted into γ-aluminum oxide without any impairment the very large surface layer that is created by this method occurs.

The large surface area is based on the formation of hexagonal crystal aggregates an approximate size of 8 ~ 8 ~ 20 microns.

In the hexagonal crystal aggregates there are micropores of one size of 40 i - diameter, but these play quite obviously for the catalytic Activity of the structure no or at most a subordinate role.

A washcoat loading has proven to be particularly advantageous from 5 to 30% by weight, based on the metallic onolyte substrate. One suitable loading of Al203 on Kanthal D (registered trademark) with, for example 400 cells per 2.54 cm2 is, for example, 10% by weight. The surface of the aluminum oxide is 50 to 500 m2 per gram of aluminum oxide. The Alwninat method described above the deposition of 2 aluminum oxide leads to surfaces of 120 to 160 m per gram Alumina.

Another advantageous method for the deposition of an adhesive Alumina wash coating on a metallic or ceramic substrate consists in the production of a slurry of a pre-activated gibbsite (aluminum oxide trihydrate) and an alumina monohydrate having a solid to liquid ratio from 25 to 50 t and a pH of less than 7 and using this slurry for the impregnation of the molded substrate by complete immersion. The strenght or concentrations of the slurry that will be used (and that will result from an attempt to determine) should be sufficient to apply an alumina wash coat of the required thickness. The substrate will then dry in warm air left and finally heated or fired at 4500C for example for 2 hours, forming Xc and y-alumina in an adhesive layer of one thickness of up to, for example, 0.005 cm on the metallic substrate. There are crystal aggregates 3 to 7 microns in diameter created with micropores of approximately the same size Size, i.e. 40 in diameter. Another advantageous method of deposition an adherent Alumina wash coating on the substrate consists in using a slurry of alpha alumina monohydrate.

After heating or firing to 4500C, y-aluminum oxide is also formed a surface area of 180 to 300 m2 per gram. The γ-aluminum oxide monohydrate is α-alumina added at the time of slurry and before heating, to create a thixotropic mixture. Crystallite or crystal aggregates are formed a diameter of 20 to 100 i.d. The micropore diameter remains the same 40 R.

Suitable aluminum oxide trihydrates (gibbsite) are commercially available, for example under the Flanders name "FRF 80", manufacturer British Aluminum Chemicals Ltd. and "C 333", manufacturer Conoco. Suitable aluminum oxide monohydrates (Boehmite) are also available in stores, for example under the llandels name "Sol-Gel Alumina", manufacturer United Kingdom Atomic Energy Authority. To mention is also, for example, the product "Dispal M", manufacturer Conoco and the product "Condea F", which is supplied by the Condea Group. For example Gibbsite can be converted into the "Sol-Gel Alumina" (which is a microcrystalline Boehmite trades) during the slurry stage with the aim of formation a thixotropic mixture.

Optionally, one or more of the aluminum oxide used can be used Oxides such as titanium dioxide and corresponding oxides of zirconium, flafnium and / or thorium are present in order to provide additional stabilization of the intermediate oxide layer (Wash layer) to bring about. Instead of the listed oxides or with these together other rare earth oxides, alkaline earth oxides and alkali metal oxides be used.

Many of the aluminum containing metallic substrates of the invention have the property of oxidizing inwards. It is assumed that a factor which contributes to the success of the present invention is that the extensive or enlarged metal substrate itself, which is part of the invention catalytic structure forms, tends to oxidize under has very strong oxidizing conditions in such a way that the first layer of the adherent oxide film does not tend to over the outermost layer of intermetallic Connection to grow or cover them.

As already stated, the layer or coating of the intermetallic Connection be continuous or discontinuous. If desired, the Layer also be in the form of a fine microcrystalline dispersion.

The experiments described in the following are intended to provide a better understanding the invention. The experiments relate to the phenomenon of sulfate emissions the exhaust gases from automobiles. The effects that can be achieved according to the invention are thereby achieved explained with reference to the accompanying drawings. In detail are shown in: Fig. 1 a diagram from which the sulfate emission of the exhaust of a 1.8 1 British Leyland Marine motor car depending on the distance traveled The number of kilometers at four continuous speeds results; Fig. 2 is a graphical representation, from which the sulfate conversion efficiency depends on the catalyst temperature, seen from the theoretical point of view, results; 3 is a graphical representation, from which the sulfate conversion efficiency depends on the amount covered Kilometers per hour; 4 is a diagram showing the degree of sulfate emission depending on the distance covered at a steady speed of 80.46 km per hour for a total of 1609 km, with a test interruption after Gives 804.6 km; 5 shows a graph of the sulfate emission as a function on the number of kilometers traveled in the case of catalytic converters with Wash coatings of four different precursors and FIG. 6 a series of Ilistograms showing sulfate emissions for a number of test runs using Illustrate catalysts with washcoats of three different precursors.

The various sulphate emissions of the engine are shown in FIG at sustained speeds of 48.28; 64.37; 80.46 and 96.56 km / hour with one Mileage covered of 804.6 km. The motor used was with an air pump and an exhaust gas recycling facility. The following were also used: a continuously working pre-distributor, a low-density cylinder head (7.5: 1) and a platinum catalyst with 7.5% by weight in the exhaust line.

The catalyst metal was on an alumina washcoat applied, which in turn is deposited on a ceramic carrier with a honeycomb structure was. The air pump was set so that there was a large excess of oxygen in the exhaust gas was present in order to achieve maximum sulphate formation and sulphate emission.

The sulfate emissions were determined by mixing the gas out the exhaust pipe with a countercurrent flow of filtered laboratory air in a dilution tunnel development section measuring 3.81 m ~ 11 cm in Diameter at a flow rate of 4890 l / minute. From the development section samples were isokinetically using a probe with a diameter of 0.38 cm, a fiberglass filter pad, a gas meter and a pump. The samples were passed through the filter pad at a rate of 12 1 per Minute subtracted, creating a sample flow to development flow ratio (development flor) of 0.245 t. A second Filter pad was used for samples of the ambient air and served as a reference standard.

The filter pads consisted of fiberglass pads of one mesh size from 0.2 micron manufacturer Whatman GFA glass fiber paths There were circular cutouts a diameter of 5.8 cm exposed to the action of the streams of robes.

To get the "sulfate" (mainly sulfuric acid droplets) present in the Filters remained to determine1 these were extracted three times with water and centrifuged each time. The extract was then treated with an ion exchange resin for one hour (I) owex 5OWXA) shaken to remove interfering ions, followed by centrifugation became. The resulting liquid was then poured into an 80 percent by volume Isopropanol-water solution transferred, whereupon 25 mg barium chloranilate were added, whereupon the mixture was shaken and then centrifuged. Finally it was the absorption liquid determined at 310 mji using K2504 standards.

As already stated, tests were carried out with an engine running time of 804> 6 km (500 miles) with sustained speeds of 48.28; 64.37; 80.46 or 96.56 km per hour. A new catalyst sample was used before each experiment introduced, and during the implementation of the tests, the filter pads described were replaced by new ones every 80.46 km. Sulphate emissions were also determined without a catalyst in the exhaust line under the four specified test conditions, the results obtained have been subtracted from the measurement data obtained when using of the engine equipped with the catalyst were obtained in order to increase the To be able to determine sulphate emissions on the basis of the existing catalyst.

After all, the same fuel was used for all measurements. This contained: 0.033% by weight sulfur, 0.02 g lei to 3.785 l, 0.001 g of phosphorus to 3.7 # 5 l, 100 ppm (; Iilor and 15 ppnI ßrom.

As can be seen from Fig. 1, there was an increase in sulfate emission in the first 321 attempts. This is evidence of the occurrence of the mentioned storage effect, whereby the progressive increase in sulphate emissions is likely is based on the fact that the wash layers are in a "state of saturation" with sulfate approach and after approximately 200 miles (321 km) are effectively saturated. The later diminution the sulfate emission is probably due to a reduction in the catalyst activity, Can be used as a result of catalyst degradation, e.g. through sintering and fermentation the active sites of the catalyst.

From Fig. 2 the effect of the catalyst temperature (and consequently the position of the catalytic converter in the exhaust pipe) on sulphate emissions. Curve A relates to calculated conversion efficiencies under equilibrium conditions in accordance with the reversible response represented by the equation (1) is, in the case of 7 vol .-% oxygen. s) he curve B refers to calculated Conversion efficiencies under kinetic conditions when equilibrium fails was achieved.

I) ic curve C shown in dashed lines results from curves A and ß and illustrates the actual change in sulfate conversion effectiveness with temperature. It follows that the sulfate conversion efficiency and consequently the sulfate emiss ions are very temperature sensitive and that a peak at about 600 ° C occurs at oxygen concentrations of about 7% by volume. Therefore should the catalyst, as already stated, reduces sulfate emissions at high oxygen concentrations at temperatures as low as possible or as high as possible be used.

The temperature at which the peak appears in the resulting curve and the actual peak of sulfate conversion potency falls in the malae, in which the oxygen concentration falls. This means that with falling oxygen concentrations the equilibrium curve A is shifted to the left and downwards until at one Value of 0.1 Vol.-t 02 the position of curve 0 is reached. At the same time will the kinetic curve B shifted to the right because at any given temperature the rate of oxidation increases and, consequently, the conversion is effective on half (0.5 power) of the oxygen partial pressure is dependent. it follows from this, that the resulting curve for 0.1 vol% oxygen peaks at a lower one Value of sulfate conversion efficiency than that of the intermediate section I of curves B and D. This means that at low oxygen concentrations Sulphate emissions are less dependent on the catalyst temperature than at high concentrations.

In Fig. 3, curve A represents the theoretical equilibrium curve as in Fig. 2, whereas curve C represents the sulfate conversion efficiency, which actually achieved at different speeds of the motor vehicle and, consequently, the sulfate conversion efficiencies at various catalyst operating temperatures. The four points on curve C were from the sulfate emission values of cruising speed (plateau) sections of the four curves of Fig. 1 obtained. The curve C has the same Waveform like curve C of Figure 2 except that the maximum conversion efficiency at only about 30: is. This means that the degree of oxidation in use of the 7 1/2 wt.% Rh / Pt catalyst of FIG. 1 at a sustained speed of 48.28 to 96.56 km per hour was so low that the closest approach to the Equilibrium conversion efficiencies were not greater than about 30 tons.

In Fig. 4 are the sulfate emissions from a 1609 km trial at a Continuous speed of 80.46 km per hour with the described car with the 1.8 l marine engine shown, which was used to obtain the test results for Fig. 1 was used.

The exhaust system of the motor vehicle used was with a Rh / Pt catalyst equipped. 1) The test conditions were practically the same as those that did to achieve the results of Fig. 1 were used with the exception, however, that in the present case the experiment was interrupted after 500 haste for so long, that the catalyst could cool to ambient temperature.

The experiment was then continued, i.e. there were another 804.6 km driven so that the total distance covered was 1609 km.

As can be seen from Fig. 4, the first part of the curve of Fig. 4 in accordance with the 80.46 km curve of FIG. 1, since the emissions have a maximum of Reach about 20 mg per 1.609 km at about 321.9 km and then by one im substantial constant value fluctuate.

After the catalyst had cooled down after the first 804.6 km, the sulphate emissions decreased very quickly from about 20 to about 4 mg per 1.609 km if the attempt is restarted. This effect is essential as the changes that caused the loss of "sulfate activity" are evident occurred during the cool-down period. It is believed that these changes at least partly due to the migration of rhodium to the surface of the Rh / Pt crystallites based, so that the relative concentration of rhodium at and near the surface increases considerably. On the other hand, because rhodium is less active in terms of oxidation from sulfur dioxide to sulfur trioxide is considered a rhodium-platinum alloy and a lot less active than platinum, the result is a reduction in the sulphate emissivity.

Figures 5 and 6 illustrate the effect on sulfate emission by changing the precursor from which the catalyst wash layers are derived.

The sulfate emission tests to which the results shown in FIG underlying, were carried out with the motor vehicle already described. The four catalysts used were 7 1/2% by weight rhodium-platinum catalysts, the on wash layers had been deposited, which in turn were essentially made of aluminum oxide on monolithic supports with honeycomb structure (Corning FX20 honeycomb monolith supports) passed.

Each of the four wash layers were derived from a different precursor away. The emission data from 804.6 km tests at a cruising speed of 80.46 km per hour are shown in Fig. 4 in the form of curves A, B, C and T). I) ie p values of the wash layers are different, with the pII value of B over 1) and A to C increased. Wash layers 13 and 3 were the same except that the layer D additionally contained barium oxide (BaO). From Fig. 4 it follows that that the sulphate emission decreased to the extent that the pIS value of the wash layer system rise.

In the case of FIG. 6, the left group of 3 relates to ilistograms for the wash shifts A, 13 and C, marked with (i), over 804.6 km of tests at a continuous speed of 80.46 km per hour, which is the subject of Fig. 5 is.

After the 804.6 km test with each of the catalysts, subjected the motor vehicle to the following test conditions: (ii) cruising for 15 minutes 96.56 km per hour; (iii) Test method according to U.S. Federal test procedure (FTP); (iv) sustained speed of 96.56 km per hour for 15 minutes; (v) 15 minutes long cruising speed of 64.37 km per hour; (vi) Continuous speed for 15 minutes of 96.56 km per hour.

In the 96.56 km per hour test (ii) following the 804.6 km Endurance speed test (i) the sulfate concentrations sen were higher than from the initial test result / was to be expected, which are shown in FIG.

The test run (iii), that is to say the FTP test, shows considerable results Differences in emissivity during the second 96.56 km per hour test (Test (iv)) low sulphate emissions compared to test (ii) i.e. the first 96.56 km hourly test shows. From this there is a recovery the storage capability. This is confirmed by the low Values obtained in the 64.37 km-hour test (test (v)) and the sulfate concentration increase in the case of test (vi), i.e. the third 96, S6 km hour test.

From the data obtained it can initially be seen that during the 804.6 km test significant amounts of sulphate have been deposited by all of the catalysts, that at least a portion of this deposited sulfate is released during test (ii) and that sulfate was deposited or stored during subsequent testing and was released. From the results obtained, it can be seen that the release sulfate is a faster process than sulfate deposition or storage and that the decomposition of sulphate occurs at higher rates if the catalyst temperature is higher.

Although the experimental data described in experiments with an automobile using a catalyst in the exhaust pipe or in the exhaust, the invention is in no way based on this limited, i.e. the catalysts of the invention can also be used in others Use cas streams containing sulfur dioxide from any other supplier.

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Claims (11)

Catalyst for use in an oxygen and sulfur dioxide gas stream containing claims 1. Catalyst for use in a Gas stream containing oxygen and sulfur dioxide, characterized by a catalytic active phase of a mixture or alloy of rhodium and platinum, in the rhodium in a concentration of traces up to 25 wt. ~ based on the weight of the catalytically active phase is present. 2. Catalyst according to claim 1, characterized in that the rhodium is present in the catalytically active phase in a concentration of 5 to 20 wt. 3. Catalyst according to claim 2, characterized in that the rhodium is present in the catalytically active phase in a concentration of 5 to 10% by weight. 4. Catalyst according to claim 3, characterized in that the rhodium in the catalytically active phase at a concentration of 7, 5 wt. T is present z. Catalyst according to one of Claims 1 to 4, characterized in that the catalytically active phase is arranged on a carrier. 6. Catalyst according to claim 5, characterized in that the carrier between the carrier and the catalytically active phase a layer of a heat-resistant or refractory oxide auE-shows. 7. Catalyst according to claim 6, characterized in that the layer of refractory or refractory oxide on the support in the form of an alkaline Suspension has been applied. 8. Catalyst according to claim 7, characterized in that the alkaline Suspension has a pH greater than 8. 9. Catalyst according to one of claims 1 to 8, characterized in that that the substrate is monolithic and made of zirconium mullite, mullite, α-aluminum oxide, Siliamanite, blagneslum silicates, zircon petalite, spondumene, cordierite and / or aluminosilicates consists. 10. Catalyst according to one of claims 1 to 9> characterized in that that the substrate is in the form of pellets, made from a wood grain granulate, a- or y-aluminum oxide granules or corresponding pellets, naturally occurring or synthetic aluminosilicates, magnesium oxide, diatomaceous earth, bauxite, titanium dioxide, Zirconium dioxide, limestone, magnesium silicates, silicon carbide and / or activated or non-activated carbon or soot. 11. Catalyst according to claim 6 * characterized in that it is used as heat-resistant or refractory oxide at least one oxide of B, Al, Si, Be, Mg, Ca, Ca, Sir, Ba, Sc, Y, Ti, Zr, tsf, Th and / or the lanthanides and / or actinides having.