MXPA00003872A - High performance catalyst - Google Patents

Abstract

The invention relates to a high performance catalyst containing an inner and an outer layer on an inert carrier body comprising noble metals from the platinum group deposited on support materials. The catalyst is characterised in that, the inner layer comprises platinum deposited on a first support and on a first oxygen storage component and the outer layer comprises platinum and rhodium deposited on a second support only and the second layer further comprises a second oxygen storage component.

Description

High Performance Catalyst Description of the Invention: The present invention relates to a high performance three-way catalyst (TWC) containing an inner layer and an outer layer on an inert carrier body. The layers comprise noble metals of the platinum group deposited on support mats. The three-way catalysts are mainly used to convert the carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxides (NOx) contaminants contained in the exhaust gases of internal combustion engines into harmless substances. The known three-way catalysts with good activity and durability employ one or more catalytic components from the group of platinum metals such as platinum, palladium, rhodium and iridium deposited on a refractory oxide support of large surface area, for example, a alumina with a large surface area. The support is usually carried in the form of a thin layer or coating on a suitable substrate or carrier such as a monolithic carrier comprising a ceramic or metallic refractory honeycomb structure. The ever-increasing demand for improved activity and life in the catalysts has led to complex catalyst designs comprising multiple catalyst layers on the support structures, each of the layers containing support materials and selected catalyst components as well as those so-called promoters, stabilizers and oxygen storage compounds. US Patent No. 5,063,192 describes a three-way catalyst with improved thermal fatigue strength consisting of a first and a second catalyst layer. The first layer is directly applied on the surface of a monolithic honeycomb carrier and comprises alumina REF. : 119055 active and catalytic components fed on the tt? Isma comprising platinum and / or rhodium and at least one compound of zirconium, lanthanum or barium oxide. The second layer is deposited on top of the first layer and comprises active alumina, cerium oxide and a catalyst component comprising palladium. The zirconium, lanthanum or barium oxides prevent the active alumina particles from sintepting due to the high temperatures of the exhaust gas, thus improving the thermal resistance of the three-way catalyst. US Patent No. 5,677,258 describes a three-way catalyst containing barium oxide with improved resistance against poisoning with sulfur and water. The catalyst consists of two layers on a honeycomb carrier. The lower catalyst layer is located directly on the carrier and comprises at least barium or lanthanum. The upper layer comprises a water absorption component. The catalyst further comprises a catalytically active metal which is located at least in the lower or supepor layer. In a special embodiment the lower layer further comprises palladium and active aluminum oxide and the upper layer further comprises platinum and radium. US Patent No. 5,057,483 discloses a three-way catalyst comprising two individual layers on a monolithic carrier. The first or lower layer comprises a first support of activated alumina, a catalytically effective amount of a first catalytic component of platinum dispersed on the first support of alumina and a catalytically effective amount of cerium oxide without additives. The second or outer layer comprises a co-formed support of zirconium oxide-rare earth oxide, a catalytically effective amount of a first catalytic component of radium dispersed on the co-formed support of zirconium oxide-rare earth oxide, a second activated alumina support and a catalytically effective amount of a second catalytic component of platinum dispersed on the second alumina support.

PCT publication WO 95/35152 discloses another three-way catalyst consisting of two layers, which is thermally stable up to 900 ° C or more. The first layer comprises a first support, at least a first palladium component, optionally a first component of the platinum group, optionally a first stabilizer, optionally at least a first rare earth metal component and optionally a zirconium compound. The second layer comprises a second support, a second platinum component, a radio component, a second oxygen storage composition comprising a second diluted oxygen storage component and optionally a zirconium component.

German publication DE 197 26 322 A1 describes a three-way catalyst exhibiting improved thermal stability and activity and consisting of two layers of an inert carrier. The first or lower layer comprises various particulate matepales and one or more highly dispersed alkaline earth metal oxides and at least one metal of the platinum group which exhibits intimate contact with all the components of the first layer. The particulate materials of the first layer comprise at least one oxygen storage material in the form of particles and at least one other component in the form of particles. The second layer again comprises various materials in the form of particles and at least one metal of the platinum group. The particulate materials of the second layer comprise at least one oxygen storage material in the form of particles and another component in the form of particles. The platinum group metals of the second layer are selectively deposited on the particulate materials of the second layer. Preferably the metal of the platinum group in the first layer is palladium and the metals of the platinum group of the second layer are platinum and radium.

This last three-way catalyst exhibits excellent catalytic activity especially during the cold start-up phase of modern internal combustion engines that are operated with poor air / fuel mixtures during cold start to increase the exhaust gas temperature as fast as may be possible. The excellent behavior of the catalyst is essentially due to the use of palladium which, under the conditions of poor exhaust gas, allows obtaining "light off" temperatures lower than platinum, despite this excellent behavior, this catalyst faces the problem that it has developed a shortage of palladium in recent years resulting in an increase in prices and an uncertain supply situation.Another problem with existing three-way catalysts is the fact that they suffer from aging due to fuel cutting. Fuel cut describes the degradation in the performance of the catalyst due to the fuel cut after high load operation of the internal combustion engine.This situation frequently occurs during the fast running phase when an abrupt deceleration is required. fast gear the engine is operated at air / combus ratio Slightly below the stoichiometric value. The exhaust gases can reach temperatures well above 800 ° C which result in even higher catalyst temperatures due to the exothermic conversion reactions in the catalyst. In the event of abrupt deceleration, the electronic equipment of modern engines completely interrupts the supply of fuel to the engine, with the result that the standard air / fuel ratio (also called the lambda? Value) of the exhaust gas passes sharply from rich values to values poor These wide displacements of the normalized air / fuel ratio from rich values to poor values at high catalyst temperatures degrade the catalytic activity. The catalytic activity can be recovered at least partially by prolonged operation under stoichiometric exhaust gas conditions. The faster the catalytic activity recovers after aging by fuel cutting, the better the overall performance of the catalyst. Accelerate the recovery of catalytic activity after aging by cutting fuel is mandatory for modern three-way catalysts. It is an object of the present invention to develop a three-way catalyst based on platinum and rhodium which exhibits a catalytic performance similar to the known palladium / rhodium catalysts and which is commercially competitive with the latter. further, after aging by fuel cutting in poor exhaust conditions, the catalyst must recover its full efficiency of three routes quickly. The catalyst should also exhibit improved nitrogen oxide conversion to reduce the ozone formation potential of the purified exhaust gas. This and other objects are achieved with a catalyst containing an inner layer and an outer layer on an inert carrier body comprising noble metals of the platinum group deposited on support mats. The catalyst is characterized in that the inner layer comprises platinum deposited on a first support and on a first oxygen storage component and the extepor layer comprises platinum and rhodium deposited on a second support, the outer layer also comprising a second oxygen storage component . The catalyst of the present invention consists of a catalytic coating comprising an inner layer and an outer layer on an inert catalyst carrier, thereby forming a so-called double layer catalyst. With "inner layer" it is intended to designate the first layer of the catalytic coating on the catalyst carrier. The inner layer is covered with the "outer layer" or second layer. The exhaust gas to be treated with the catalyst comes directly into contact with the outer layer. The term "support material" or "support" is used in the present invention to designate a material in the form of particles on which catalytically active components such as the noble metals of the platinum group or other promoter components can be deposited in the form highly dispersed, that is, with crystallite sizes between 1 and 10 nm. For this purpose, the support materials must have a specific surface area (also called BET surface, measured in accordance with DIN 66132) of more than 5 m2 / g. The first and second oxygen storage components of the catalyst are also used in particulate form. Without intending to restrict the present invention to a particular theory, it is assumed that the contribution of the lower layer to the total catalytic performance of the catalyst consists primarily in the oxidation of hydrocarbons to carbon monoxide while the main function of the outer layer is the reduction of nitrogen oxides But the outer layer also contributes, particularly in the cold start phase, to the conversion of hydrocarbons and carbon monoxide. The superior properties of the catalyst according to the present invention with respect to aging by fuel cutting and conversion of nitrogen oxides is mainly attributed to the fact that in the outer layer the platinum and the radius are deposited only on the second material of support. It has been observed that depositing platinum and rhodium on the same support material shortens the recovery time of the catalytic activity after exposure to poor exhaust gas conditions at high temperatures. This, in turn, allows obtaining higher conversion efficiencies of nitrogen oxides over a complete running cycle. Depositing platinum and rhodium on the same support material means within the context of the present invention that platinum and rhodium are dispersed on the same particles of the second support mat, that is, platinum and rhodium are present at least in close neighborhood on the same particles. Other advantages can be obtained by ensuring an intimate contact between both noble metals. How this can be achieved will be discussed later. According to the present understanding of the invention, the reasons for the aging by fuel cutting of the three-way catalysts may be that the wide displacements of the normalized air / fuel ratio from the rich values to the poor values at high catalyst temperatures. degrades the catalytic activity especially of rhodium. Under stoichiometric or rich exhaust conditions the radius is reduced approximately to the zero oxidation state which is the most effective state for three-way catalysis. With poor exhaust gas and high catalyst temperatures, the radius is oxidized to the +3 oxidation level. This oxidation state of the radius is less active for the conversion of three pollutant pathways. On the other hand, because Rh2O3 is isomorphic in the crystallographic structure with respect to AI2O3, it can migrate at temperatures above 600 ° C within the lattice of aluminum oxide or other isomorphous support oxides of the general composition M2? 3 (M represents a metal atom), resulting in a permanent degradation of the catalytic activity. To recover its catalytic activity and to prevent rhodium losses in the aluminum oxide lattice, the rhodium must therefore be reduced as quickly as possible when the composition of the exhaust gas changes back to stoichiometric. According to the present understanding of the invention, the reduction of rhodium to the zero oxidation state is catalyzed by platinum.The more intimate the contact between platinum and rhodium, the better this reduction effect is. Rh2O3 to migrate within the isomorphic support oxides can be limited by an appropriate additive incorporation treatment (doping) of these oxides Beneficial are the "doping" components that are capable of generating activated hydrogen under reducing conditions. It helps to convert the radio oxide more quickly to the metallic form under reducing conditions and therefore the risk that the Rh2O3 migrates within the supporting oxide is further minimized.A suitable doping component for this purpose is oxide. But because the cerium oxide also exhibits an oxygen storage 'and release capacity the proportion of the "doping" with cenoxide should be kept low so as not to promote the oxidation of the radius by a very high level of cenoxide in the support oxide. The present invention will be understood more in depth with reference to the accompanying drawings, in which Figure 1 shows a schematic representation of the measurement principle for determining the CO / NOx crossing points. Now specific embodiments of the catalyst according to the present invention will be explained more in detail. The first and second supports of the catalyst can be the same or Different preferably the ppomer and the second support are selected from the group consisting of silica, alumina, titanium oxide, zirconium oxide, mixed oxides or mixtures thereof The term "mixed oxide" designates an intimate mixture of two or more oxides at an atomic level that can be considered as a new chemical compound, while the term mixture means the mechanical mixing of two or more oxide materials in the form of particles.Most advantageously, the supports are selected from activated aluminas, optionally complemented by zirconium oxide or a mixed oxide of zirconium oxide rich in zirconium oxide Activated aluminas exhibit specific surface areas of up to 400 m2 / g, the various phases comprising transitional aluminas that are formed by heating aluminum hydroxides in air (see Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, 1985, volume A1, pages 561 and 56) 2) For stability at the improved temperature, the activated aluminas can be stabilized with 0.5 to 20% by weight of lanthanum oxide. Such materials are commercially available. The frequently used stabilization of alumina with barium oxide is less preferred if the alumina is used as a support material for platinum because this involves the risk of formation of barium platinate. The term "rich in zirconium oxide" means that the matepal contains at least more than 50% by weight of zirconium oxide, preferably more than 60% and more preferably even more than 80% by weight, the remainder being formed by oxide of yttrium, neodymium oxide, calcium oxide, silica, lanthanum oxide or cenoxide which serve to stabilize zirconium oxide against thermal fatigue. What is most preferably used is a zirconium oxide / cerium oxide mixed oxide with zirconium oxide. The pure zirconium oxide and the stabilized zirconium oxide compounds will be summarized in the term "zirconium oxide component" in the following. The inner layer or first layer of the catalyst contains, in addition to the stabilized alumina and the optional zirconium oxide component, an oxygen storage material for an improved three-way conversion of the contaminants. It is well known that cerium oxide exhibits an oxygen storage capacity. Low leakage, the cerium is completely oxidized to the oxidation state Ce +, and under rich exhaust gas conditions, the cerium oxide releases oxygen and acquires the Ce3 + oxidation state. Instead of using pure cerium oxide as an oxygen storage compound, it is preferred to use cerium oxide / zirconium oxide mixed oxides rich in cerium oxide with an oxide concentration of from 60 to 90% by weight. relative to the total weight of the mixed oxide, such materials are obtainable with specific surface areas of 20 to 200 m2 / g and exhibit good stability at surface area temperature.Other improvements can be obtained by stabilizing this material with praseodymium oxide, of yttrium, neodymium oxide, lanthanum oxide or mixtures thereof For stabilization are sufficient concentrations of the stabilizing compounds from 0.5 to 10% by weight, relative to the weight tot to the stabilized material The stabilization of oxygen storage materials based on cenoxide using praseodymium oxide, neodymium oxide, lanthanum oxide or mixtures thereof is described in the German patent application DE 197 14 707 A1. According to the present invention both the support materials and the oxygen storage compound serve as supports for the platinum in the first layer. Platinum deposition on only one of these materials has been shown to provide inferior catalytic activities. The oxygen storage material of the outer layer may be the same as, or different from, the storage material of the inner layer. It is preferred to use the same storage material for the inner layer and the outer layer, especially mixed oxides of zirconium oxide / oxide stabilized with praseodymium oxide.The second oxygen storage material of the outer layer should be kept free The deposition of rhodium on the second oxygen storage material would lead to the deactivation of the reductive activity of the rhodium by oxidation of the rhodium.The extepor layer or second layer may additionally comprise a certain amount of active alumina in the form of This material may or may not be stabilized with lanthanum oxide or barium oxide Other improvements in catalytic activity and temperature stability may be achieved if the second layer is supplemented with a component highly dispersed selected from the group consisting of yttrium oxide, neodymium oxide, oxime of lanthanum or praseodymium oxide, with praseodymium oxide being preferred. These compounds can be introduced into the layer by adding a soluble precursor compound of these compounds to the coating composition of the second layer. The term "dispersed component" means that, contrary to a "particulate compound", this material is added to the coating composition in the form of a soluble precursor compound that acquires its final dispersed form after calcination of the catalytic coating. . The average particle size of the dispersed components can vary between 0.001 and 0.1 μm, while the particulate components exhibit average particle diameters between 1 and 15 μm. The dispersed compound of the second layer fulfills multiple functions. First, it stabilizes the particulate components (alumina support and cerium oxide / zirconium oxide oxygen storage component) of the second layer against thermal degradation. Therefore, when for example praseodymium oxide in dispersed form is added to the second layer, the cerium oxide / zirconium oxide must not be previously stabilized but must be stabilized in situ during the preparation of the coating. Secondly, praseodymium oxide also exhibits a storage and oxygen release function that helps improve the dynamic behavior of the final catalyst, although the oxygen storage capacity of the praseodymium oxide is not as pronounced as that of the oxide. cerium. The catalyst carrier body used in the present invention is in the form of a honeycomb monolith with a plurality of substantially parallel passageways extending therethrough. The passageways are defined by walls on which the catalytic coating comprising the inner layer and the outer layer is applied. The passageways of the carrier body serve as flow conduits for the exhaust gas of the internal combustion engine. By flowing through these passages, the exhaust gas comes into intimate contact with the catalytic coating, with which the contaminants contained in the exhaust gas are converted into benign products. The carrier bodies can be manufactured from any suitable material, such as from metallic or ceramic materials, as is well known in the art. The passageways are arranged in a regular pattern on the cross section of the carrier bodies The so-called cell density (passageways per cross-sectional area) can vary between 10 and 100 cm "2. Other suitable carrier bodies they can have an open cell foam structure.The metallic or ceramic foams can be used.The inner layer of the catalytic coating is applied to the carrier body in amounts of from about 50 to 250 g / l, and the outer layer is applied • in quantities of from 10 to 150 g / l of the carrier body. Advantageously, the inner layer comprises from 20 to 150 g / l of said first support component and from 10 to 100 g / l of said first oxygen storage component. The inner layer may further comprise from 5 to 60 g / l of zirconium oxide or of a zirconium oxide component. Platinum is present in the first layer in concentrations of from 0.01 to 5, preferably from 0.05 to 1% by weight, based on the total weight of the first layer. The concentration of platinum relative to the volume of the catalyst carrier varies from 0.01 to 12.5 g / l, with the most suitable concentrations being between 0.025 and 2 g / l. In the most preferred embodiment, the first support comprises an active alumina with a specific surface area between 50 and 200 m2 / g stabilized with lanthanum oxide, while the first oxygen storage component is advantageously selected from mixed oxide oxides of cerium / zirconium oxide rich in cerium oxide containing 60 to 90% by weight of cerium oxide and additionally stabilized with 0.5 to 10% praseodymium oxide (PrßO-n). It is believed that this composition of the first layer improves its catalytic function with respect to the oxidation of hydrocarbons (HC) and carbon monoxide (CO). The outer layer of the catalytic coating comprises from 5 to 100, preferably from 5 to 20 g / l of said second support, and from 5 to 100, preferably from 5 to 50 g / l of said second oxygen storage component. The outer layer may additionally comprise from 5 to 60 g / l of activated alumina. In the outer layer, platinum and rhodium are deposited on the second support. Compared with the inner layer, the concentration of the noble metals relative to the weight of the support material is preferably greater in the outer layer. In such a way, platinum concentrations plus radius between 0.5 and 20% by weight relative to the weight * • of the second support material can be selected, with concentrations between 1 and 15 being preferred. 0 / / or by weight. These concentrations correspond to concentrations relative to the volume of the catabolic carrier 0.025 and "2D-g / l, preferably between 0.05 and 15 g / L. As already explained, the intimate platinum with the radium in the The outer layer helps to reduce the radius oxide formed during the fuel cutting phases back to the metallic state.To fulfill this function, the mass ratio between the platinum and the radius should be selected between 5 1 to 1: 3, the relations being of mass between 3: 1 and 1: 1 the most effective As in the case of the inner layer, the second support is preferably selected from an active alumina with a specific surface area between 50 and 200 m2 / g stabilized with lanthanum oxide , while the second oxygen storage component is selected from mixed oxides of oxide of ceno / zirconium oxide rich in oxide of ceno containing from 60 to 90% by weight of oxide of ceno additionally stabilized with 0.5 to 10% by weight of praseodymium oxide (Pr.sub.O-n) As stated above, stabilization with praseodymium oxide or alternatively with yttrium oxide, neodymium oxide or lanthanum oxide, can also be achieved by addition of these compounds as highly dispersed components to the second layer In order to suppress the emission of hydrogen sulfide the first and second layers of the catalytic coating may additionally comprise from about 1 to 40 g / l of a nickel, iron or manganese component The catalyst of the present invention can be manufactured in different ways. Some of them will be discounted below- To obtain the inner layer, the passageways of the catalyst carrier can be coated with an aqueous coating composition comprising the support mat in the form of particles of the inner layer (including the first deoxygen storage material). The coating composition will also be called coating dispersion within the context of this invention. The techniques for coating catalyst carriers with said coating composition are well known to the skilled person. The coating is then dried and calcined in air. The drying is preferably carried out at an elevated temperature of up to 150 ° C. To heat the coating temperatures of 200 to 500 ° C must be applied for a period of 0, 1 to 5 hours. After calcination, the platinum can be dispersed on the coated carrier body by dipping the monolith into a solution containing a platinum precursor compound. The solution can be an aqueous or non-aqueous solution (organic solvent). Any platinum precursor compound can be employed, provided that the compound is soluble in the chosen solvent and decomposes after heating in air at elevated temperature. Illustrative of these platinum compounds are chloroplatinic acid, ammonium chloroplatinate, platinum tetrachloride hydrate, dichlorocarbonyl platinum dichloride, dinitrodiamino platinum, platinum nitrate, tetraamino platinum nitrate and tetraamino platinum hydroxide. After impregnation, the coating is again calcined at elevated temperatures between 200 and 500 ° C in air. Alternatively, the inner layer may be prepared by first impregnating the particulate materials of the inner layer with an aqueous solution of a soluble precursor compound of platinum, drying and calcining the materials in the form of impregnated particles to thermally fix the platinum thereon. This catalysed material is then used to prepare the aqueous coating composition to coat the -f walls of the passageways of the body portadoir? I coating is then dried and calcined as described above. In a preferred method for obtaining the inner layer, an aqueous dispersion is prepared from the particulate materials of the inner layer. To deposit and fix the platinum on the particulate materials of the dispersion, a solution of the platinum precursor compounds is slowly injected into the dispersion, the platinum compound then being precipitated on the particulate materials by adjustment. appropriate of the pH value of the dispersion to achieve the final coating composition. During injection and precipitation, the dispersion is continuously stirred to homogeneously distribute the injected solution throughout the volume of the dispersion. The precipitated compounds adhere firmly to the support materials. The method of precipitation by injection is described in German patent applications DE 197 14 732 A1 and DE 197 14 707 A1. In what follows it is also called injection-precipitation. The platinum precursor compounds suitable for this method of deposition are those already described above. In addition, solubilized platinum amino compounds such as methylethanolamine platinum (IV) hexahydroxide ((MEA) 2Pt (OH) 6 = ((OH-C2H4-NH2-CH3) 2 + Ptl (OH) 6) and ethanolamino hexahydroxide can be used. platinum (IV) ((EA) 2Pt (OH) 6 = (OH-C2H4-NH2-) 2 * Ptlv (OH) 6) or other organic derivatives of quaternary ammonium salts These complex platinum anionic compounds are known in the art. obtaining high dispersion deposits of platinum metal The solubilized ammo precursor compounds give highly basic aqueous solutions When alumina is used as the support material, the solubilized amino precursor compounds are easily fixed on the surface of the alumina by adsorption. the dispersion the adsorbed components can be chemically fixed.The coating dispersion thus prepared is then used to coat the walls of the passageways of the carrier body, after which the coating is dry or and calcined in air. The method described above for injection-precipitation is preferred because it involves only one drying and calcining step while the first two methods require in each case two stages of drying and calcination. Preferably, the first oxygen storage component for the lower layer is selected from a mixed oxide of cerium oxide / zirconium oxide rich in cerium oxide stabilized with praseodymium oxide. An already stabilized material can be used or the stabilization can be carried out in a separate manufacturing step. The cerium oxide / zirconium oxide can also be stabilized with praseodymium oxide in situ during the preparation of the first layer. For this purpose, a solution of a praseodymium oxide precursor compound can be prepared by dispersing the cerium oxide / zirconium oxide therein. Ammonia is then injected into the dispersion to precipitate the precursor compound on the cerium oxide / zirconium oxide. Suitable precursor compounds of praseodymium are acetate or praseodymium nitrate.

The resulting dispersion is then used to prepare the final coating composition by the subsequent addition of active alumina and optionally a zirconium oxide component in the form of particles. The particulate materials of this dispersion are then catalyzed with platinum by the injection-precipitation already discussed. After having deposited the inner layer on the catalyst carrier, the outer layer is prepared as detailed below: First, the second support that HeviPplatinum and rhodium is prepared by impregnating this support with an aqueous solution of soluble precursors of platinum and rhodium, and drying and calcining the impregnated support, after which the catalyzed support, the second oxygen storage compound and additional active alumina, are dispersed in water to obtain a coating composition. This coating composition is used to apply the outer layer on top of said inner layer. Finally, the coated carrbody is again dried and calcined as described herein. Suitable precursor compounds for platinum are those already mentioned above. As radio precursors, hexaminorodium chloride, rhodium trichloride, rhodium carbonyl chloride, rhodium trichloride hydrate, rhodium nitrate and rhodium acetate may advantageously be used, but rhodium nitrate is preferred. The second support can be impregnated with platinum and radio precursors sequentially in any order or simultaneously from a common solution. However, as noted above, it is highly desirable to obtain a contact between platinum and rhodium as intimately as possible. It has been found that this is best achieved by first depositing the platinum and then the rhodium on the support material by the above described method of injection-precipitation. For this purpose, the precursor compound for platinum is selected from solubilized platinum amino as ethanolamino platinum (IV) hexahydroxide, the precipitation of the platinum being effected by appropriate adjustment of the pH value of the dispersion. After precipitation of the platinum, the support is not dried and calcined but the rhodium is then precipitated directly from a solution of a rhodium acid precursor compound such as rhodium nitrate.

For this reason, the coating dispersion -icuous for the outer layer is prepared by making a first aqueous dispersion of the second support material, preferably active alumina and then injecting an aqueous solution of a platinum amino precursor compound solubilized in the dispersion. The solubilized platinum amino precursor compound is rapidly adsorbed onto the active alumina. An aqueous solution of a rhodium acid precursor compound is then injected into the dispersion and the pH value of the dispersion is adjusted appropriately to fix the platinum and radio compounds on the second support. After this the second catalyzed support material can be separated from the liquid phase of the first dispersion and dried and calcined before its redispersion together with the second oxygen storage component and optionally additional active alumina, to form the coating dispersion for the outer layer. The most suitable is the use of spray or instant calcination to calcify the catalyzed support material. In the case of spray or flash calcination, the wet material is injected into a hot stream of a gas with a temperature between 700 and 1000 ° C, resulting in drying and decomposition of the precursor compounds within a few days. seconds or even in less than a second. This results in a high dispersion of the noble metal cpstalites that are formed. However, it is preferred to avoid the intermediate step of drying and calcining the second catalyzed support material and directly adding the second oxygen storage component and optionally additional active alumina, to the first dispersion containing the second catalyzed support. This is possible because platinum and rhodium are firmly fixed to the second support material by the described injection-precipitation.

The coating dispersion thus obtained is then used to apply the outer layer on top of the inner layer followed by drying and calcination of the coated catalyst carrier. The last preparation method for the outer layer is preferred with respect to the first described method because it avoids the thermal treatment separated from the second catalyzed support. The second oxygen storage component is preferably selected from a mixed oxide of cerium oxide / zirconium oxide stabilized with praseodymium oxide. The stabilization of the cerium oxide / zirconium oxide can be advantageously achieved by the in situ method already described above. For this purpose, the solution of the praseodymium precursor compound, the cerium oxide / zirconium oxide and optionally alumina are added to the alumina-containing dispersion catalyzed by platinum and rhodium. The resulting dispersion is then used to apply the second coating layer. After calcination of this layer, the praseodymium precursor forms highly dispersed praseodymium oxide on the surface of the particulate materials of the second layer. In this way the cerium oxide / zirconium oxide is stabilized against thermal fatigue and furthermore the oxygen storage capacity of the catalyst is enhanced by the oxygen storage capacity of the praseodymium oxide. In summary, in one of the most preferred embodiments of the invention, the inner layer of the catalyst comprises platinum deposited on active alumina and on a mixed oxide of zirconium oxide / cerium oxide rich in cerium oxide and the outer layer of the catalyst comprises platinum and rhodium deposited on active alumina, the outer layer also containing mixed oxide of cerium oxide / zirconium oxide rich in cenoxide. This catalyst is obtainable by the following process steps: a) Preparation of a precursor praseodymium solution, adding mixed oxide of cerium oxide / zirconium oxide and adjusting the pH value of the dispersion to thereby precipitate the precursor of praseodymium oxide on cerium oxide / zirconium oxide; b) additional addition of alumina and optionally a zirconium oxide component to the dispersion of step a); c) injection of a solution of a platinum precursor into the dispersion of step b) and precipitation thereof on the alumina, cerium oxide / zirconium oxide and optionally the zirconium oxide component to obtain a first coating composition for the inner layer of the catalyst; d) coating a monolithic carrier with said first coating composition and drying and calcining the coating to thereby obtain a carrier coated with said inner layer; e) preparing an active alumina dispersion and injecting a solution of a platinum compound into said dispersion; f) injection followed by an aqueous solution of a soluble rhodium precursor in the dispersion of step e) and adjustment of the pH value of the dispersion to thereby obtain an active alumina dispersion catalyzed by platinum and rhodium; g) addition of active alumina, mixed oxide of cerium oxide / zirconium oxide rich in cerium oxide and optionally a solution of a praseodymium precursor to the dispersion of step f) to obtain a second coating composition for the outer layer of the catalyst; h) use of said second coating composition for applying said outer layer on top of said inner layer and i) drying and calcination of said coated monolithic carrier.

It is most preferred to be used in steps a) and d) for the inner and outer layers to be buffered with 0.5 to 20 wt% of lanthanum oxide. In the method described above, the support materials and the cerium oxide / zirconium oxide are stabilized in situ with praseodymium oxide. Alternatively stabilization of the cerium oxide / zirconium oxide with praseodymium oxide, yttrium oxide, neodymium oxide, lanthanum oxide or mixtures thereof can be achieved in a separate step with the previous "doping" compounds by impregnation, injection , precipitation, co-precipitation or co-thermohydrolysis. For the stabilization of cerium oxide / zirconium oxide by impregnation, the zirconium oxide / oxide in the form of particles is wetted with an aqueous solution of precursor compounds of the desired doping element and then dried and calcined. Frequently impregnation by pore volume is used for this purpose. In that case the precursor compounds are dissolved in an amount of water corresponding to the water absorption capacity of the zirconium oxide / ceno oxide. The injection-precipitation for the deposition of the noble metal compounds on the support materials has already been explained above. For the stabilization of zirconium oxide / oxide by co-precipitation, a solution of precursor compounds of zirconium oxide and of zirconium oxide and of a precursor compound of the stabilizing element is prepared. The three compounds are then precipitated simultaneously by the addition of a suitable precipitating agent. Thus, cerium oxide / zirconium oxide stabilized with praseodymium can be manufactured by preparing a common solution of nitrate nitrate, zirconium nitrate and praseodymium nitrate, adding ammonium carbonate or ammonium oxalate so that cerium, zirconium and the praseodymium precipitate simultaneously as carbonates or oxalates.

After filtration and drying, the desired cerium oxide / zirconium oxide is obtained by calcination. Alternatively, the co-precipitation can also be carried out in a basic medium. For the stabilization of cerium oxide / zirconium oxide by co-thermohydrolysis, a sol of hydroxy nitrate nitrate, zirconium hydroxy nitrate and the hydroxynitrate of the doping element is prepared. Then the sun is dehydrated by increasing the temperature. In this way the hydroxynitrates decompose to form the corresponding oxides. Co-thermohydrolysis is described, for example, in WO 98/16472. The beneficial properties of the catalyst according to the invention will now be explained in more detail with the help of the following examples. Figure 1 shows a schematic representation of the measurement principle to determine the CO / NOx crossing points.

Comparison Example 1: A conventional single-layer platinum / rhodium catalyst CC1 (comparison catalyst 1) is prepared as follows: Centrium carbonate and zirconium carbonate are treated with water and acetic acid overnight at room temperature to partially form the corresponding acetates. To the resulting dispersion stabilized alumina and bulk cured oxide of low surface area are added. After wet grinding, a monolithic carrier is coated with the slurry by a conventional immersion technique. The coated substrate is dried in air and calcined for 2 hours at 500 ° C in air. The total absorption by embedding of the carrier was 205 g / l consisting of 112 g / l of stabilized alumina, 34 g / l of bulk cenoxide, 34 g / l of cerium oxide and 25 g / l of zirconium oxide , these last two compounds originated from acetate precursors. The embedding layer was impregnated with chloride-free platinum and radio salts (tetramine platinum nitrate and rhodium nitrate). The mass ratio between platinum and rhodium was 5Pt / 1Rh at a concentration of 1.41 g / l (40 g / p3). The final catalyst had the composition indicated in Table 1: Table 1: Composition of the comparison catalyst (CC1) Example 2: A double-layer catalyst C1 according to the invention was prepared in the following manner: Preparation of the first layer (interior): To a solution of praseodymium acetate was added a cerium-rich oxygen storage component ( 70% by weight of cerium oxide, 30% by weight of zirconium oxide). By the controlled injection of ammonium hydroxide and stirring for about 30 minutes, the praseodymium acetate was precipitated on the cerium oxide / zirconium oxide.

Subsequently, stabilized alumina (3% by weight of La2O3l 97% by weight of AI2O3) and zirconium oxide in bulk were added. After this a solution of platinum ((EA) 2Pt (OH) 6) was injected into the slurry, precipitating the platinum on the alumina and the oxide of ceno / zirconium oxide by appropriate adjustment of the pH value of the dispersion with acid. acetic. After grinding the slurry, a catalytic carrier was immersed therein to apply the first layer. The complete absorption by embedding was 160 g / l. Finally the first layer was dried and then calcined in air at 500 ° C. Preparation of the second (outer) layer: Stabilized alumina (4% by weight of La2O3, 96% by weight of AI2O3) was dispersed in water. Next, a chlorine-free platinum salt ((EA) 2Pt (OH) 6) was injected and rapidly adsorbed onto the alumina. Next, rhodium nitrate was injected. Adjusting the pH value, both catalytic components were fixed on the supporting alumina. To finish the embedding alumina, praseodymium acetate and an oxygen storage component rich in cenoxide (70% by weight of cenoxide, 30% by weight of zirconium oxide) were introduced. Prior to coating a monolithic substrate, the pH of the slurry was adjusted to approximately 6 and ground. The total absorption by embedding of the second layer was 70 g / l. The catalyst was dried and calcined at 500 ° C in air. The final catalyst had the composition set forth in Tables 2 and 3: Table 2: Composition of the Wf or catalyst layer C1 Table 3: Composition of the outer layer of the C1 catalyst The mass ratio of platinum to radium was 1 Pt / 1 Rh in the upper layer. The total content of platinum and radium was 1.41 g / l (1.175 g of Pt / I and 0.235 g of Rh / I) with a mass ratio of 5Pt / 1 Rh (mass ratio; combined for both layers) Comparison Example 2: A double-layer catalyst CC2 was prepared in the same manner as the catalyst of Example 1. Contrary to Example 1, the platinum in the first layer was deposited only on the alumina in a step of separate preparation before the coating slurry was prepared for the first layer.

Comparison Example 3: A double-layer catalyst CC3 was prepared in the same manner as the catalyst of Example 1. Contrary to Example 1, the total amount of platinum was applied only to the first layer. Thus, in the resulting comparison catalyst the platinum and rhodium were completely separated from each other.

Evaluation of the catalysts: a) Motor tests: The catalysts according to the previous examples and comparison examples were previously aged in an internal combustion engine (engine displacement: 2.8 I) for 76 hours at a gas temperature of escape in front of the catalysts of 850 ° C. Then, the "light off" temperatures for the conversion of HC, CO and NOx and the CO / NOx crossing points were determined.The term "light off temperature" designates the exhaust gas temperature at which the 50% of the respective pollutant is converted by the catalyst. The "light off" temperature can be different for HC, CO and NOx Two aging operations were carried out In the first operation, a sample of catalyst C1 and comparison catalyst CC1 were aged together, while in the second operation, another sample of the catalyst C1 was aged in conjunction with the comparison catalysts CC2 and CC3. Because the aging operations can not be reproduced exactly, the catalysts of the two aging operations differ slightly, therefore, only catalysts aged within the same aging operation can be compared to each other. 'were carried out at a space velocity of 65000 h "1 with engine exhaust gas temperatures that were gradually increased (38 K / mm) The measurements of the CO / NOx crossing points are shown schematically in Figure 1 The lambda value of the air / fuel mixture supplied to the engine was periodically changed from 0.98 (rich air / fuel mixture) to 1.02 (poor air / fuel mixture) and vice versa. The dwell times a? = 0.98 and? = 1, 02 were set at 1 minute in each case The passage from rich to poor and vice versa was done in 3 minutes The corresponding lambda sweep is indicated in Figure 1 (lower curve) The associated conversion curves for CO and NOx they are also shown in Figure 1 During the lean period, the CO conversion is virtually 100% and decays to approximately 50 to 60% during the rich period. The conversion curve for NOx behaves in a reciprocal manner during the rich period, the conversion of NOx is close to 100%, while in the poor period the conversion of NOx decreases to values between 50 and 60%. At a lambda value of 1 both conversion curves cross each other. The corresponding conversion value is the highest conversion that can be achieved simultaneously for CO and NOx.

The higher the crossing point, the better is the dynamic behavior of the catalytic activity of the catalyst. The recently described determination of the crossing point uses a lambda sweep called static. A dynamic lambda sweep can also be used. In this case, the sweep curve for the lambda value is further modulated with a frequency of 1 Hz or 0.5 Hz. The amplitude can be A / C ± 1 or A / C ± 0.5 (air / fuel) ). This amplitude is usually greater than the amplitude of the sweep curve of ± 0.02, corresponding to an A / C amplitude of ± 0.3. The dynamic crossing points for the catalysts of the preceding Examples were measured at a space velocity of 65000 h "1 and at 4540 ° C and 400 ° C exhaust gas temperature.The air / fuel ratio was modulated with a frequency of 1. Hz and an amplitude of A / C 1 (1 Hz air / fuel ± 1) at 450 ° C exhaust gas temperature At 400 ° C exhaust gas temperature the amplitude of modulation was reduced to A / C 0 , 5 (1 Hz air / fuel ± 0.5) The measured results are summarized in Tables 4 and 5. Table 4 compares the catalysts that were aged during the first aging operation, while Table 5 compares the catalysts aged during the second aging operation.

Table 4: ['Catalyst' T5o [° C] CO / NOx [%]. ! HC CO NOx 1 HzAC ± 1 | 1 HzAC ± 0.5 ! C1! 360 I 363 I 354 84 88 I CC1 | 387, '407 and 382 76 62 CC: comparison catalyst; C: T50 catalyst: "light off" temperature for 50% conversion. and * and * Table 5: CC3 389 391 379 72 79 b) model gas tests After aging the catalysts of Example 1 and the comparison of Example 3 for 16 hours at 985 ° C in a mixture of synthetic lean gas containing 6% by volume of O2, 10% by volume of H2O, 20 ppm of SO2, the rest nitrogen, the crossing points CO / NOx were determined at a temperature of 400 ° C and at a space velocity of 100000 h "The crossing points were determined for 3 different concentrations of SO2 of the mixture of gases (0, 5 and 20 ppm) The results are shown in Table 6 Table 6: Example 2: An additional sene of 4 different catalysts was prepared, C2, C3, C4 and C5 according to Example 1. In contrast to Example 1 all the catalysts were manufactured with a total noble metal charge of 2.12 g / l. (60 g / ft) The weight ratio of the platinum to the radius in the upper layer was "vanished" to determine its influence on the catalytic properties of the catalysts. The noble metal distribution of these catalysts is set forth in Table 7 Table 7: Distribution of the noble metal Before determining the CO / NOx crossing points, the four catalysts in their entirety were aged for 12 hours at an exhaust gas temperature versus the catalysts at 1100 ° C in a 6% by volume oxygen gas mixture. , 10% by volume of water vapor, 20 ppm of sulfur dioxide, with the rest being nitrogen The static crossing points of these catalysts were determined at an exhaust gas temperature of 400 ° C and at a space velocity of 100000 h " 1 During the test the lambda value of the exhaust gas was increased from 0.98 to 1.02 in 5 minutes The lambda value at 1.02 was kept constant for 1 minute. Then the lambda value was again lowered to 0.98. in 5 nm After a dwell time of 1 minute, the descppto cycle was repeated again for 2 times The CO / NOx crossing values indicated in Table 8 are average values taken from the last two test cycles Table 8: Points of crossing CO / NOx * 'there is no crossing point For these measurements the model gas had the following composition CO 1, 40% by volume H2 0.47% by volume NO 0.1% by volume CO2 14.0% by volume SO2 20 ppm H2O 10% by volume O2 0.93-1, 77% by volume N2 remainder carry out the lambda sweep ef oxygen content of the model gas was vain between 0.93 and 1.77% by volume Example 3: Two other catalysts, C6 and C7, were prepared with a total noble metal charge of 1.41 g / l (40 g / p? E3) according to Example 1. For the preparation of the C6 catalyst, exactly duplicated Example 1 while for the preparation of the C7 catalyst the platinum and radio impregnation sequence was inverted for the second support. Radio was first deposited on the activated alumina support and then only the platinum. Both catalysts were tested to determine their behaviors regarding the CO / NOx crossing and its "light off" temperatures The results are shown in Table 9 Table 9 Catalyst T5o [° C] CO / NOx [%] HC CO NOx 1 Hz AC ± 025 1 1 Hz A / C ± 0.5 ¡1 HzA ± 1; C6 360 362 354 99 I 95 ¡90 f C7 359 355 353 95! 90 I 82 i From the results of Table 9 it can be seen that the dynamic behavior of the C6 catalyst is much better than that of the C7. Without pretending to be limited by any theory, this effect can be explained by a more intimate contact between platinum and rhodium if platinum is deposited first and then rhodium.

Example 4: Four other catalysts, C8, C9, C10 and C11 were prepared according to Example 1 with the following alterations: The total charge of noble metal was set at 1.77 g / l (50 g / pie3). The platinum / radius ratio was modified to 3: 2. In addition, different amounts of MnO and NiO in the form of particles were added to the coating dispersions for the inner layers of catalysts C9 and C11. These hydrogen sulfide suppressant components were added to the coating compositions after the injection of the platinum compound. To measure the emission of hydrogen sulphide from these catalysts, they were first charged under poor conditions with sulfur (space velocity 65000 h "1, temperature 550 ° C, lambda 1, 01, approximate sulfur content of the fuel: 200 ppm; approximate load> 0.5 h) Then the lambda value was lowered to 0.88 and the emission of hydrogen sulphide was measured with a mass spectrometer on the line.The maximum emission peak of hydrogen sulphide indicated in Table 10 for catalysts C8 to C11. and Table 10: Suppression of H2S emission by Mn02 and NiO C9 3 2 1, 77 I 20 MnO2 380 'C10 3 2 177! 40 MnO2 330 C1 1 3 2 1, 77 i 5 N? O j 100 l Example 5: The catalyst according to the present invention does not contain palladium However, it has been shown that comparatively low emissions of hydrocarbons, carbon monoxide and nitrogen oxides are obtained as are obtained with catalysts using palladium and radium. Another objective of The present invention was to reduce the costs of the platinum group metals (PGM) of the novel platinum / rhodium catalysts compared to the conventional palladium / rhodium double layer catalysts given the PGM prices of April 1999. Accordingly, the catalysts according to Example 1 with charges of different noble metals and variable platinum / rhodium ratios were compared with respect to the activity of exhaust gas purification and PGM costs. The catalysts were tested in a vehicle with EU-II certificate. as main catalysts located under the floor with a ratio of catalyst volume / engine capacity of 0.67 All the catalysts were measured after an aging of 16 hours at 985 ° C with 10% by volume of water in nitrogen. The tests were carried out with cold start stoichiomepco according to the new European test cycle MVEG-EU III. Relative emissions are summarized in the Tablar li with the values for the palladium / rhodium comparison catalysts (14Pd / 1Rh) set at 100: Table 11: Relative emissions with respect to PGM costs As shown in table 11, the conversion of HC, CO and NOx is strongly influenced by the radio load, being for a given emission target, beneficial reduction of platinum content in favor of radio content. For example, a radio enriched charge of 1.77 g / l (3 Pt / 2 Rh) in the EU-II certified vehicle shows lower emission for the three pollutant components as a whole compared to the Pd / Rh reference ( 3.53 g / l, 14 Pd / 1 Rh), the vanant with high platinum content of 3.32 g / l (45 Pt / 2 Rh), remains behind the results of the loading of 1.77 g / l (3 Pt / 2 Rh), despite a higher total charge and sharply higher precious metal costs. Other variations and modifications of the foregoing will be apparent to those skilled in the art and it is intended that they be understood by the claims appended hereto. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (13)

1. CLAIMS Having described the invention as above, the claim contained in the claims is claimed as property: 1. High performance catalyst containing an inner layer and an outer layer on an inert carrier body comprising noble metals of the platinum group deposited on support materials, characterized in that the inner layer comprises platinum deposited on a first support and on a first oxygen storage component and the outer layer comprises platinum and rhodium deposited on a second support, the outer layer also comprising a second storage component of oxygen.
2. 2. The catalyst according to claim 1, characterized in that said first and second supports are the same or different and are compounds selected from the group consisting of silica, alumina, titanium oxide, zirconium oxide, mixed oxides or mixtures thereof and oxide mixed zirconium oxide / cerium oxide rich in zirconium oxide.
3. 3. The catalyst according to claim 2, characterized in that said first and second supports are activated alumina stabilized with 0.5 to 20% by weight of lanthanum oxide.
4. 4. The catalyst according to claim 3, characterized in that said first support further comprises a zirconium oxide component.
5. The catalyst according to claim 1, characterized in that said first and second oxygen storage components are the same or different and comprise cerium oxide / zirconium oxide mixed oxide-cente compounds.
6. 6. The catalyst according to claim 5, characterized in that said mixed oxide cerium oxide / zirconium oxide compounds are stabilized with praseodymium oxide, yttrium oxide, neodymium oxide, lanthanum oxide or mixtures thereof.
7. 7. The catalyst according to claim 6, characterized in that said outer layer also comprises activated alumina.
8. 8. The catalyst according to the laugh? 1, characterized in that said outer layer further comprises highly dispersed yttrium oxide, neodymium oxide, lanthanum oxide or praseodymium oxide.
9. 9. The catalyst according to claim 1, characterized in that the platinum and the radius are present on said second support in intimate contact with each other. The catalyst according to one of claims 1 to 9, characterized in that said carrier body is in the shape of a beehive with a plurality of substantially parallel passageways extending therethrough, the tracks being passage defined by walls 10 on which the inner layer is applied in amounts of from about 50 to 250 g / l and the outer layer is applied in amounts of from about 10 to 150 g / l of the carrier body. The catalyst according to claim 10, characterized in that said first support is present in amounts of from 20 to 150 g / l, said The first oxygen storage component is present in amounts of from 10 to 100 g / l and the zirconium oxide support is present in amounts of from 5 to 60 g / l. 12. The catalyst according to claim 11, characterized in that The platinum is present in said inner layer in concentrations of from 0.01 to 5% by weight with respect to the total weight of said inner layer. The catalyst according to claim 12, characterized in that said second support is present in amounts of from 5 to 100 g / l, said second oxygen storage component is present in _-: > amounts of from 5 to 100 g / l and activated alumina is present in amounts of from 5 to 60 g / l. The catalyst according to claim 13, characterized in that platinum and radium are present in said outer layer in concentrations from 0, 5 to 20% by weight with respect to the total weight of said outer layer and the platinum / radius mass ratio is selected from the range of from 1 to 15. The catalyst according to claim 14, characterized in that at least one of said inner and outer layers further comprises from about 1 to 40 g / l of nickel, iron or manganese components. A method for manufacturing a catalyst according to claim 1 characterized in that the method comprises the steps of a) coating the walls of the passageways of the carrier body with a coating composition containing particulate materials comprising said first support material and said first oxygen storage component b) drying and calcining said coating c) immersing the carrier body coated in a solution of a soluble platinum precursor compound and calcining the coating, and d) applying the outer layer on top of the inner layer 1 A method for manufacturing a catalyst according to claim 1, characterized in that the method comprises the steps of a) catalyzing materials in the form of particles comprising said first support material and said first oxygen storage component by impregnating them with a solution of a soluble platinum precursor compound by drying and calcining the materials to thermally fix the platinum thereon; b) preparing an aqueous re-builder composition with the catalyzed materials of step a) and coating the walls of the passageways of the carrier body with this coating composition, c) drying and calcining said coating, d) applying the outer layer on top of the inner layer 18 A method for manufacturing a catalyst according to claim 1, characterized in that the method comprises the steps of a) preparing a dispersion of materials in the form of particles comprising said first mater of the support and said first oxygen storage component and injecting a solution of a soluble platinum precursor compound, b) fixing the platinum compound on all the particulate materials by adjusting the pH of the dispersion to thereby obtain a coating composition, c) coating the walls of the passageways of the carrier body with the aqueous coating composition of step a), d) drying and calcining said coating, and e) applying the outer layer on top of the inner layer 19 A method for manufacturing a catalyst according to any of claims 16 to 18, characterized in that the method comprises the steps of a) impregnating said second support with a solution of a soluble platinum precursor compound and a radio compound, drying and calcining the impregnated support to thereby obtain a catalyzed support, • • b) prepare an aqueous coating composition from said catalyst support, said second storage component of oxygen and additional active alumina, and c) using said coating composition to apply said outer layer on top of said inner layer. A method for manufacturing a catalyst according to any of claims 16 to 18, characterized in that the method further comprises the steps of a) preparing a dispersion of said second support material and injecting a solution of a soluble platinum precursor compound, b) then injecting a solution of a soluble radium precursor compound into the dispersion of step a) and adjusting the pH value of the dispersion to thereby obtain a platinum and radio catalyzed support, c) preparing a coating composition of the dispersion of step b) by adding said second storage compound of oxygen and additional active alumina, d) using said coating composition to apply said outer layer on top of said inner layer and e) drying and calcining the coated monolithic carrier. high performance according to claim 1, characterized in that the inner layer comprises platinum deposited on active alumina and on mixed oxide of oxide of ceno / zirconium oxide rich in oxide of ceno and the outer layer comprises platinum and radium deposited on active alumina, the outer layer also comprising mixed oxide of Ceno oxide / zirconium oxide rich in cenoxide, the catalyst being obtainable by a) preparing a solution of a praseodymium precursor, adding mixed oxide of cenoxide / zirconium oxide and adjusting the pH value of the dispersion to thereby precipitate the praseodymium oxide precursor on the zirconium oxide / ceno oxide, b) additional alumina aggregate to the dispersion of step a), c) injection of a solution of a platinum precursor into the dispersion from stage b) by precipitating it on alumina and the oxide of ceno / zirconium oxide to obtain a first coating composition for the inner layer of the catalyst; d) coating a monolithic carrier with said first coating composition and drying and calcining the coating to thereby obtain a carrier coated with said inner layer; e) preparing an active alumina dispersion and injecting a solution of a platinum compound into said dispersion; f) injecting followed by an aqueous solution of a soluble precursor of radium in the dispersion of step e) and adjusting the pH value of the dispersion to thereby obtain a dispersion of active alumina catalyzed by platinum and radium; g) addition of active alumina, mixed cerium oxide / zirconium oxide oxide rich in cerium to the dispersion of step f), h) use of said second coating composition to apply said outer layer on top of said inner layer and i) drying and calcination of said coated monolithic carrier. 22. Catalyst according to claim 21, characterized in that the active aluminas of steps b) and e) are stabilized with 0.5 to 20% by weight of lantapo oxide. The catalyst according to claim 21, characterized in that in step b) an additional component of zirconium oxide is added. 24. Catalyst according to claim 21, characterized in that a praseodymium precursor compound is added to the dispersion of step e).