CZ20001451A3 - Extremely effective catalyst and process of its preparation - Google Patents

Abstract

The catalyst comprises an inner and outer layer on an inert support skeleton and platinum metals deposited on the underlying materials. The inner layer comprises platinum deposited on the first substrate and the first, the oxygen scavenging component and the outer the layer includes platinum and rhodium deposited on the other the substrate, wherein the outer layer further comprises a second oxygen capturing the component. The method of manufacture is that the walls the channels of the carrier carcass are coated with a coating composition containing the same a first substrate material and a first oxygen scavenger component. The formed coating is dried and calcined. Coated skeleton with dipped into a solution of the platinum precursor compound, calcined the outer layer is applied.

Description

Field of the invention

The present invention relates to a highly effective three-way catalyst (TWC) comprising an inner and an outer layer on an inert support frame. The layers include noble platinum group metals deposited on the underlying materials.

BACKGROUND OF THE INVENTION

The three-way catalysts are primarily used to convert the pollutants carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (N0 _X) contained in the exhaust gas of internal combustion engines into harmless substances. The known trivalent honor catalysts with good activity and durability use one or more platinum group metal catalyst components such as platinum, palladium, rhodium and iridium deposited on a large surface area, a refractory oxide substrate, such as a large surface area alumina. The substrate is typically a thin layer or coating on a suitable support or substrate, such as a monolithic support comprising a refractory ceramic or metal honeycomb structure.

The increasing demand for catalyst activity and lifetime has led to complex catalyst designs involving multiple supported catalyst layers.

E.

Each of the layers comprises selected support materials and catalyst components, as well as promoters, stabilizers and oxygen scavenging components.

US Patent Application No. 5,063,192 discloses a three-way catalyst with improved thermal resistance, which consists of a first and a second catalyst layer. The first layer is coated directly with the surface of the monolithic honeycomb support and comprises active alumina and catalyst components deposited thereon comprising platinum and / or rhodium and at least one zirconia, lanthanum or barium oxide compound. The second layer is coated on the surface of the first layer and comprises an active alumina, cerium oxide, and a catalyst component that includes palladium. The zirconium, lanthanum and / or barium oxides prevent sintering of the active alumina particles due to the high exhaust gas temperatures, thereby improving the heat resistance of the three-way catalyst.

U.S. Patent No. 5,677,258 discloses a three-way catalyst comprising barium oxide with improved sulfur and water poisoning resistance. The catalyst consists of two layers on a honeycomb support. The lower catalyst layer is located directly on the support and comprises at least barium or lanthanum. The topsheet comprises a water absorbing component. The catalyst further comprises a catalytically active metal which is disposed in at least one, bottom or top layer. In a particular embodiment, the backsheet further comprises palladium and active alumina and the topsheet further comprises platinum and rhodium.

• · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · ·

U.S. Patent No. 5,057,483 discloses a three-way catalyst comprising two discrete layers on a monolithic support. The first, backsheet comprises a first alumina, a catalytically active catalyst component dispersed on a first alumina substrate, and a catalytically effective amount of particulate oxide activated by an oxide substrate amount of a first platinum cerium formed

A second or external zirconia metal substrate, a catalytically effective amount of a first rhodium catalyst component dispersed on a co-formed rare earth metal zirconia substrate, a second activated alumina substrate, and a catalytically effective amount of a second platinum catalyst component dispersed on a second alumina substrate.

The layer includes simultaneously rare earth oxide

PCT application 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 substrate; at least one first palladium component, optionally a first platinum group component; optionally at least one first stabilizer; optionally at least one first rare-earth metal component and optionally a zirconium compound. The second layer comprises a second substrate; a second component of platinum; a rhodium component; a second oxygen scavenging composition comprising a dissolved second oxygen scavenging component; and optionally a zirconium component.

DE 197 26 322 A1 describes a three-way catalyst which exhibits improved activity and thermal stability and which consists of two layers on an inert support. The first or backsheet comprises a plurality of particulate particles.

- 4 - ·······. · «.. ·· ·».

and one or more highly dispersed alkaline earth metal oxides and at least one platinum group metal having close contact with all components of the first layer. The particulate materials of the first layer include at least one particulate oxygen scavenging material and at least one other particulate component. The second layer again comprises several particulate materials and at least one platinum group metal. The particulate materials of the second layer include at least a particulate oxygen scavenging material and a further particulate component. The platinum group metals of the second layer are selectively deposited on the particulate materials of the second layer. The platinum group metal in the first layer is preferably palladium and the platinum group metals of the second layer are platinum and rhodium.

The latter three-way catalyst exhibits excellent catalytic activity, especially during the cold start phase of modern internal combustion engines, which are driven by lean air / fuel mixtures during cold start to raise the exhaust gas temperature as quickly as possible. The excellent behavior of the catalyst is mainly due to the use of palladium, which provides a lower temperature drop than platinum under low exhaust conditions. Despite its excellent efficiency, this catalyst faces the problem that in recent years there has been a shortage of supply of palladium resulting from rising prices and an uncertain supply situation.

Another problem with the existing three-way catalysts is the fact that they suffer from aging due to fuel cut-off. Termination of aging due to fuel cut-off · · ················ 9 9 9 9 · 9 £ J “······ · 9 99 9 9 9-9 describes the degradation of the catalyst efficiency caused by a cut in the fuel supply after a high-power internal combustion engine operation. Such a situation often occurs during the rapid control phase when a sudden deceleration is required. During the quick steering phases, the engine operates at slightly below the stoichiometric value at air / fuel ratios. Exhaust gases can reach values well above 800 ° C, resulting in even higher catalyst temperatures due to exothermic conversion reactions in the catalyst. In the event of a sudden deceleration, a modern engine electronically shuts off the fuel supply completely to the engine, with the result that the normalized air / fuel ratio (also called lambda λ) of the exhaust gas jumps from rich to lean values.

These large fluctuations from the normalized air / fuel ratio from rich to lean values at high catalyst temperatures degrade catalytic activity. The catalytic activity can be at least partially restored by prolonged operation under stoichiometric exhaust gas conditions. The catalytic activity is again achieved after aging due to the interruption of the fuel supply, the faster the better the overall efficiency of the catalyst. Therefore, accelerating the recovery of catalytic activity after aging caused by a cut off of fuel supply for modern triple honor catalysts is mandatory.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a three-way catalyst based on platinum and rhodium, which exhibits:

A catalyst activity similar to that of the known palladium / rhodium catalysts and which competes commercially with the latter catalyst. In addition, the catalyst should rapidly regain its full three-honor efficiency after high temperature aging under low exhaust conditions. The catalyst should also exhibit improved conversion to reduce the ozone formation potential of the purified exhaust gas.

This and other objects are achieved by a catalyst comprising an inner and an outer layer on an inert support framework comprising noble platinum group metals deposited on the support materials. The catalyst is characterized in that the inner layer comprises platinum deposited on the first substrate and on the first oxygen scavenging component and the outer layer comprises platinum and rhodium deposited on the second support, and the outer layer further comprises a second oxygen scavenging component.

The catalyst of the present invention consists of a catalytic coating comprising an inner and an outer layer on an inert catalyst support and therefore forms a so-called two-layer catalyst. The inner layer is to be the first layer of the catalytic coating deposited directly on the catalyst support. The inner layer is covered by an outer layer or a second layer. The catalyst treated exhaust gas comes into contact directly with the outer layer.

The term "backing material" or "backing material" is used in the present invention to refer to a particulate material onto which catalytically active components such as precious metals of the noble metals can be deposited in highly dispersed form. Platinum groups of elements or other promoter components, i.e. with crystallite sizes between 1 and 10 nm. For this purpose, the substrate materials should have a specific surface area (also called BET surface, measured according to DIN 66132) of greater than 5 m ² / g. The first and second oxygen scavenging catalyst components are also used in particulate form.

Without limiting the present invention to a specific theory, it is believed that the contribution of the lower layer to the overall catalytic efficiency of the catalyst lies primarily in the oxidation of hydrocarbons and carbon monoxide, while the main purpose of the outer layer is to reduce nitrogen oxides. However, the outer layer also contributes, in particular in the cold-start phase, to the conversion of hydrocarbons and carbon monoxide.

The improved aging properties of the catalyst of the present invention with respect to fuel cut off and carbon monoxide conversion are mainly attributed to the fact that platinum and rhodium are deposited in the outer layer only on the second substrate.

It has been found that deposition of platinum and rhodium on the same support material shortens the time required to recover catalytic activity after exposure to lean exhaust conditions at high temperatures. This in turn provides greater conversion efficiency for nitrogen oxides in the full cycle. Placing platinum and rhodium on the same substrate in the context of the present invention means that platinum and rhodium are dispersed on the same particles of the second substrate, which means that platinum and rhodium at least immediately

-8 '· · · φ φ φ φ φ φ φ φφφφ; .φφ adjacent to the same particles. Further improvements can be obtained by ensuring close contact between the two noble metals. How this can be achieved will be discussed below.

According to the present invention, the reason for the aging caused by the interruption of the fuel supply of the three-way catalysts can be large deviations from the normalized air / fuel ratio from rich to lean values at high catalyst temperatures degrading the catalytic activity of rhodium in particular. Rhodium is reduced to almost zero oxidation state under stoichiometric or rich exhaust conditions, which is the most efficient state for three-way catalysis. For lean exhaust gases and high catalyst temperatures, rhodium oxidizes to +3. This rhodium oxidation state is less active for the trivalent conversion of pollutants. In addition, since Rh ₂ O ₃ is isomorphic in a crystallographic structure with Al ₂ O ₃ , it can migrate at temperatures above 600 ° C to the alumina grid or other isomorphic underlying oxides of the general composition M ₂ O ₃ (M represents a metal atom) causing permanent degradation catalytic activities.

In order to regain its catalytic activity and prevent the loss of rhodium into the alumina grid, the rhodium must be reduced as quickly as possible when the exhaust gas mixture changes back to stoichiometric. According to the present invention, the reduction of the rhodium to the oxidation state of zero is catalyzed by platinum. The closer the contact between platinum and rhodium, the better the reduction effect.

4

-9 • · 4 4 • · ·· φ 4 · 4 φ φφ φ

Φ Φ ΦΦΦ Φ · · · · φ Φ • · Φ. 4 · · Φ 4 · φ φ • ΦΦΦ ΦΦ 4 · 4 ΦΦ

In addition, the tendency of Rh ₂ O ₃ to migrate to isomorphic backing oxides may be limited by appropriate alloying of these oxides. Useful alloying components are capable of generating activated hydrogen under reducing conditions. Activated hydrogen helps to convert rhodium oxide to metal form more rapidly under reducing conditions, and therefore the risk of Rh ₂ O ₃ migration to the underlying oxide is further minimized. A suitable alloying component for this purpose is cerium oxide. However, since cerium oxide also exhibits oxygen scavenging and release properties, the cerium oxide alloying rate must be kept low so that too high a level of cerium oxide in the underlying oxide does not cause rhodium oxidation.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described with reference to the accompanying drawings, in which: FIG. 1 is a schematic representation of the measuring principle for determining CO / _NOx intersections.

Examples

The specific embodiment of the catalyst according to the invention will now be explained in more detail.

The first and second catalyst supports may be the same or different. Preferably, the first and second substrates are selected from the group consisting of silica, alumina, titanium dioxide, zirconia, mixed oxides, or their oxides.

• 4 • · * 4

4 4 4

-10'4.4 4 ·. 4 4 mixtures. The term "mixed oxide" refers to a well mixed mixture of two or more oxides at the atomic level, which can be considered a new chemical compound, while the term mixture refers to a mechanical mixture of two or more particulate oxide materials.

The substrates are most preferably selected from activated alumina, optionally supplemented with zirconia or mixed zirconia rich in zirconia. The activated alumina has a specific surface area of up to 400 m ² / g. They include different phases from the transient stage of alumina, which are formed by heating the aluminum hydroxides in the air (see Ullman's Encyclopedia of Industrial Chemistry, 5th edition, 1985, Volume A1, pages 561 and 562). For improved thermal stability, the active alumina can be stabilized with 0.5 to 20 wt. % lanthanum oxide. Such materials are commercially available. The frequently used stabilization of alumina with barium oxide is less advantageous when alumina is used as a platinum support material, as this carries the risk of forming barium platinate.

The term "rich in zirconium oxide means that the material contains more than 50% by weight of zirconia, preferably more than 60 and most preferably by more than 80% by weight, and _is formed by a balance with yttrium oxide, neodymium oxide, calcium silica , lanthanum oxide or cerium oxide, which serve to stabilize the zirconium oxide against thermal changes. Most preferably, a mixed zirconia / cerium oxide rich in zirconia is used. Pure zirconia and stabilized zirconia compounds will be further summarized under the term "zirconia component".

The inner or first catalyst layer contains, in addition to the stabilized alumina and optionally a zirconia compound, an oxygen scavenger material for improved three-way pollutant conversion. Cerium oxide is known to exhibit oxygen scavenging capacity. Under lean exhaust conditions, the cerium is completely oxidized to the Ce ⁴⁺ oxidation state. Under conditions of a rich exhaust gas, cerium dioxide releases oxygen and reaches a Ce ³⁺ oxidation state. Instead of using pure cerium oxide as the oxygen scavenging compound, it is preferable to use a mixed cerium / zirconia rich cerium oxide compound with a cerium oxide concentration of from 60 to 90% by weight based on the total weight of the mixed oxide. Such materials are achievable with specific surface areas of 20 to 200 m ² / g and exhibit good thermal stability of the surface area. Further improvements can be achieved by stabilizing this material with praseodymium oxide, yttrium oxide, neodymium oxide, lanthanum oxide or mixtures thereof. They are suitable for stabilizing the stabilizing compound concentrations from 0.5 to 10% by weight, based on the total weight of the stabilized material. The stabilization of oxygen scavenging materials based on cerium oxide using praseodymium oxide, neodymium oxide, lanthanum oxide or mixtures thereof is described in German patent application DE 197 14 707 A1.

• · · · · · · · · «φ φ φφφ φφφ φ φ φ φ φ * φ φ φ φ φ φφφ φ φ φφφ 1Ο φφφφφ φ φφφφ £ -12- φφφφφφ φφφφ φφφφ

According to the present invention, both the support materials and the oxygen scavenging compound serve as substrates for the platinum in the first layer. Placing platinum on only one of these materials was able to provide lower catalytic activities.

The oxygen scavenger material of the outer layer may be the same as the scavenger material of the inner layer, or it may be different. Preferably, the same scavenger material is used for the inner and outer layers, especially the mixed cerium oxide / zirconia oxide stabilized with praseodymium oxide. The second oxygen scavenging material of the outer layer must be rhodium free. The deposition of rhodium on the second oxygen scavenging material may lead to deactivation of the rhodium reducing activity by oxidation of rhodium.

The outer or second layer may further comprise a certain amount of active alumina in particulate form that serves as a diluent material. This material may or may not be stabilized by lanthanum oxide or barium oxide.

A further improvement in catalytic activity and thermal stability can be obtained if the second layer is supplemented with a highly dispersed component selected from the group consisting of yttrium oxide, neodymium oxide, lanthanum oxide 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.

Tt ttt tttt tttt ttt ttttt tttttt tttttt tttttt tttttt tttttttt ttt tttttt £ -13 - ···· · · ·· ·· · * · »

The term "dispersed component" means that, as opposed to the "particulate component," this material is added to the coating composition in the form of a soluble precursor compound that reaches its final dispersed form when calcining the catalytic coating. The average particle size of the dispersed components may be between 0.001 and 0.1 μιη, while the particulate components usually have average particle diameters between 1 and 15 μιη.

The dispersed component of the second layer has several functions. First, it stabilizes the particulate components (alumina substrate and oxygen scavenging component cerium oxide / zirconia) of the second layer against thermal degradation. Therefore, if, for example, praseodymium oxide in dispersed form is added to the second layer, the cerium oxide / zirconia may not be stabilized in advance, but will be stabilized in situ during the manufacture of the coating. Second, praseodymium oxide also exhibits an oxygen scavenging and releasing function that helps to improve the dynamic behavior of the final catalyst, although the ability to scaveng praseodymium oxide is not as distinct as cerium oxide.

The catalyst support frame used in the present invention has the shape of a honeycomb monolith with a plurality of substantially parallel channels extending therethrough. The channels are delimited by walls to which a catalytic coating comprising an inner and an outer layer is applied.

The carcass channels serve as a flow guide for the exhaust gas of an internal combustion engine. When the exhaust gas flows through these channels, it comes in close contact with the catalytic • φ φφ φφ ** φφ φφ • * φ. * Φ • • • • • • φ φ φ φ φ φ φ φ

-. 14 φ povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla povla kem kem povla kem povla povla povla povla kem povla povla kem The carcasses may be made of any suitable material, such as metal or ceramic, as is known in the art. The channels are arranged in the cross-section of the carrier frameworks in a regular pattern. The so-called cell density (channels in the cross-sectional area) can vary between 10 and 200 cm ² . Other suitable support frameworks may have an open-cell foam structure. Metal or ceramic foams may be used.

The inner layer of the catalytic coating is applied to the support frame in amounts of 50 to 250 g / l, and the outer layer is applied in amounts of 10 to 150 g / l of support frame. The inner layer preferably comprises from 20 to 150 g / l of said first substrate component and from 10 to 100 g / l of said first oxygen scavenging component. The inner layer may further comprise 5 to 60 g / l of zirconia or zirconia component. The platinum is present in the first layer at a concentration of from 0.01 to 5, preferably from 0.05 to 1 wt. %, based on the total weight of the first layer. The concentration of platinum relative to the volume of the catalyst support is in the range of 0.01 to 12.5 g / l, with concentrations between 0.25 and 2 g / l being most suitable.

In a most preferred embodiment, the first substrate comprises active alumina with a specific surface area between 50 and 200 m ² / g stabilized by lanthanum oxide, while the first oxygen scavenging component is preferably selected from mixed cerium oxide / zirconia rich cerium oxide containing 60 to 90 % by weight of cerium oxide and, in addition,

- 15stabilizovaných 0.5 to 10 wt.% Praseodymium oxide (Pr _E O _L). This first layer mixture is believed to improve 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 substrate, and from 5 to 100, preferably from 5 to 50 g / l of said second oxygen scavenging component. The outer layer may further comprise from 5 to 60 g / l of activated alumina. In the outer layer, platinum and rhodium are deposited on a second substrate. Compared to the inner layer, the outer layer preferably has a higher concentration of noble metals relative to the weight of the underlying materials. Consequently, a concentration of platinum plus rhodium of between 0.5 and 20% by weight relative to the weight of the second support material can be selected, with a concentration of between 1 and 15% by weight being preferred. These concentrations correspond to concentrations with respect to the volume of the catalyst support of between 0.025 and 20 g / l, preferably between 0.05 and 15 g / l.

As already explained, platinum in close contact with the rhodium in the outer layer helps to reduce the rhodium oxide formed during the interruption phases of the fuel supply back to the metal state. To accomplish this task, the weight ratio between platinum and rhodium should be selected between 5: 1 and 1: 3. The most effective are weight ratios between 3: 1 and 1: 1.

As in the case of the inner layer, preferably an active alumina with a specific surface area of between 50 and 200 m / g stabilized by lanthanum oxide is chosen as the second substrate. · · · · ·; ······ '····· · · ·

-17- «·« «· ·. whereas the second oxygen scavenging component is selected from mixed cerium oxide / cerium oxide rich cerium oxides containing 60 to 90 wt% cerium oxide additionally stabilized with 0.5 to 10 wt% praseodymium oxide (Pr ₆ 0 _n ).

As mentioned above, stabilization with praseodymium oxide, or alternatively yttrium oxide, neodymium oxide or lanthanum oxide, can also be achieved by adding these compounds as highly dispersed components to the second layer.

In order to suppress the emission of hydrosulfide, the first and second layers of the catalytic coating may further comprise from about 1 to 40 g / l of nickel, iron or manganese component.

The catalyst of the present invention can be produced in various ways. Some of them will be described below:

To provide the inner layer, the catalyst channels may be coated with an aqueous coating composition comprising particulate backing materials of the outer layer (including the first oxygen scavenging material). In the context of the present invention, the coating composition will also be referred to as a coating dispersion. Methods for coating catalyst supports with such a coating composition are well known to those skilled in the art. The coating is then dried and calcined in air. The drying is preferably carried out at elevated temperatures up to 150 ° C. Temperatures from 200 to 500 ° C for 0.1 to 5 hours should be used for calcination.

After calcination, the platinum can be dispersed on the coated backbone by dipping the monolith into a solution containing the platinum precursor compound. The solution may be aqueous or 9 9 9 9 9 9 9 9 9.

Π · 9 9 9 9. · Non-aqueous solution (organic solvent). Any platinum precursor compound that is soluble in the selected solvent and decomposes when heated in air at elevated temperatures may be used. Among these platinum compounds, illustrative is chloroplatinic acid, ammonium chloroplatinate, platinum (II) chloride hydrate, dichlorocarbonyl (II) dichloride, diammonium (II) nitrate, platinum (II) nitrate, tetram (II) nitrate, tetram (II) hydroxide. After impregnation, the coating is again calcined at 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 platinum precursor compound, drying and calcining the impregnated particulate materials to heat mount the platinum thereto. The catalyzed material is then used to prepare an aqueous coating composition for coating the walls of the carcass channels. The coating is then dried and calcined as described above.

A preferred method of providing the inner layer is to prepare an aqueous dispersion from the particulate materials of the inner layer. To deposit and mount platinum on the dispersion particulate materials, a solution of the platinum precursor compound compounds is slowly injected into the dispersion, and then the platinum compound is precipitated onto the particulate materials by appropriately adjusting the pH of the dispersion to obtain a final coating composition. During injection and precipitation, the dispersion is continuously stirred to rapidly disperse the injection solution homogeneously throughout the dispersion volume. The precipitated compounds adhere firmly to the underlying materials.

to ·

-18 • to · to · to · to · this ·> <i>

A method of injection molding is described in German patent applications DE 197 14 732 A1 and DE 197 14 707 A1. This method is also called injection-molding.

Suitable platinum precursor compounds for this deposition method are those already described above. In addition, amine-solubilized platinum compounds such as platinum (II) methylethanolamine hexahydroxide ((MEA) ₂ Pt (OH) ₆ = (OHC ₂ H ₄ -NH ₂ -CH ₃ ) ₂ ⁺ Pt ^IV (OH) ₆ ) and ethanolamine platinum hexahydroxide may be used or other organic ((EA) ₂ Pt (OH) ₆ = (OH-C ₂ H ₄ -NH ₃ ) ₂ ⁺ Pt ^IV (OH) ₆ ) quaternary ammonium salt derivatives. It is known that highly dispersed platinum metal coatings can be obtained with these anionic platinum complex compounds.

The amine-solubilized precursor compounds provide highly basic aqueous solutions. When alumina is used as the backing material, the amine-solubilized precursor components are readily attached to the alumina surface by adsorption. The adsorbed substance can be fixed chemically by neutralizing the dispersion.

The coating dispersion thus prepared is then used to coat the walls of the carcass channels. Then, the coating is dried and calcined in air.

The above-described injection shrinkage method is preferred because it involves only one drying and calcining step, the first two methods both requiring two drying and calcining steps.

Φ · φ · · · · ·

-19 ..'- φ · · · • • • • • • »prek» »»

The first oxygen scavenging component for the backsheet is preferably a mixed cerium oxide / zirconium oxide rich cerium oxide stabilized with praseodymium oxide. An already stabilized material may be used, or the stabilization may be carried out in a separate manufacturing step. Cerium (IV) / zirconia may also be stabilized by praseodymium oxide in situ during the preparation of the first layer. For this purpose, a solution of the praseodymium oxide compound and zirconium oxide dispersed therein can be prepared. Ammonia is then injected into the dispersion to precipitate the precursor compound onto cerium oxide / zirconia. Suitable praseodymium precursor compounds are praseodymium acetate or praseodymium nitrate.

The resulting dispersion is then used to prepare the final coating composition by further adding the active alumina and optionally the particulate zirconia component. The particulate materials of this dispersion are then catalyzed by the platinum injection precipitation described above.

After the inner layer has been deposited on the catalyst support, the outer layer can be prepared as follows:

First, a second platinum-and-rhodium-bearing substrate is prepared by impregnating the substrate with an aqueous solution of soluble platinum-rhodium precursor compounds, and drying and calcining the impregnated substrate. Thereafter, the catalyst, the second oxygen scavenging compound, and the additional catalyst are catalyzed. . · · · · ······ 444 4 4 4 4 · £ - 20 “: ···· ·· · · ·? The active alumina dispersed in water to obtain a coating composition. This coating composition is used to apply an outer layer to the surface of said inner layer. Finally, the coated backing is dried and calcined again as described herein.

Suitable platinum precursor compounds are those already mentioned above. As rhodium precursors, hexaaminorhodium chloride, rhodium chloride, rhodium carbonyl chloride, rhodium chloride hydrate, rhodium nitrate and rhodium acetate may be used, but preferably rhodium nitrate.

The second substrate may be impregnated with platinum and rhodium precursors in any order or simultaneously from one common solution. However, as pointed out above, it is highly desirable to obtain the closest contact between platinum and rhodium. It has been found that this is best achieved by depositing first platinum and then rhodium on the substrate material by the above-described injection precipitation. For this purpose, the amine-solubilized ethanolamine platinum hydroxide is selected as the platinum precursor compound, and platinum precipitation is affected by appropriate adjustment of the pH of the dispersion. After platinum precipitation, the substrate is not dried and calcined, but rhodium is then directly precipitated from a solution of an acidic rhodium precursor compound such as rhodium nitrate.

For this reason, an aqueous coating dispersion for the outer layer is prepared by preparing a first aqueous dispersion from a second carrier material, preferably active alumina, and then injecting the aqueous solution with an amine solubilized. »·» «« »» »» »» »» »» »»

-21- «·». ·. Platinum precursor compounds into a dispersion. The amine solubilized platinum precursor compound is readily adsorbed on active alumina. Then, an aqueous solution of the rhodium acid precursor compound is injected into the dispersion and the pH of the dispersion is suitably adjusted to attach the platinum and rhodium compounds to the second substrate.

Thereafter, the catalysed second support material may be separated from the liquid phase of the first dispersion and dried and calcined prior to re-dispersing it with the second oxygen scavenger, and optionally with another active alumina, to form a coating dispersion for the outer layer. Most preferably, spray calcination or flash calcination can be used to calculate the catalyzed support material. In the case of spray or flash calcination, the wet material is injected into a hot gas stream at a temperature between 700 and 1000 ° C, causing drying and decomposition of the precursor compounds in a few seconds or even sooner than one second. This causes a large dispersion of precious metal crystallites.

However, it is preferable to avoid the intermediate drying and calcining step of the catalyzed second support material and directly add the second oxygen scavenging component and optionally additional active alumina to the first dispersion containing the catalyzed (second support. This is possible because platinum and rhodium are fixed to the the substrate material as described by injection precipitation.

• 9 9 9

-22 · ♦ 98 ·· «·

The coating dispersion thus obtained is then used to apply the outer layer to the surface of the inner layer followed by drying and calcining the coated catalyst support. The latter method of preparing the outer layer is more advantageous than the previously described method because it avoids separate thermal treatment of the catalyzed second substrate.

The second oxygen scavenging component is preferably a mixed cerium oxide / zirconium oxide stabilized with praseodymium oxide. The stabilization of the cerium oxide / zirconia can be achieved in situ as described above. To this end, a solution of the praseodymium precursor compound, cerium oxide / zirconium oxide and, optionally, alumina is added to a platinum-rhodium-catalyzed aluminum oxide dispersion. The resulting dispersion is then used to apply a second coating layer. In calcining this layer, the praseodymium precursor forms a highly dispersed praseodymium oxide on the surface of the particulate materials of the second layer. This stabilizes the cerium oxide / zirconium oxide against thermal changes, and in addition, the oxygen scavenging capacity of the catalyst is increased by the oxygen scavenging capacity of praseodymium oxide.

In summary, in a most preferred embodiment of the invention, the inner layer of the catalyst comprises platinum deposited on the active alumina and mixed cerium oxide / zirconia rich in cerium oxide, and the outer layer of the catalyst comprises platinum and rhodium deposited on the active alumina, and the outer layer further comprises the mixed oxide oxide

-23 · β · cer cer φ cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer cer This catalyst can be obtained by a process comprising the following steps:

(a) preparing a solution of praseodymium precursor, mixed cerium dioxide / zirconia and adjusting the pH of the dispersion to precipitate praseodymium precursor to cerium dioxide / zirconia;

b) further adding the alumina and optionally the zirconia component to the dispersion according to step a),

c) injecting the platinum precursor solution into the dispersion according to step b) and precipitating it onto alumina, cerium oxide / zirconia and optionally onto the zirconia component to obtain a first coating composition for the inner catalyst layer,

d) coating said monolithic carrier with said first coating composition and drying and calcining the coating to obtain a carrier coated with said inner layer,

e) preparing an active alumina dispersion and injecting a platinum compound solution into the dispersion,

f) then injecting an aqueous solution of the soluble rhodium precursor into the dispersion according to step e) and adjusting the pH of the dispersion to obtain a platinum-rhodium-catalyzed active alumina dispersion,

-24 ♦ * * * «« «« 24 24 24 24 24 24

g) adding the active alumina, mixed cerium oxide / cerium oxide rich zirconia and optionally the praseodymium precursor solution to the dispersion according to step f) to obtain a second coating composition for the outer catalyst layer,

h) using said second coating composition to apply said outer layer to the surface of said inner layer; and

(i) drying and calcining the coated monolithic support.

Most preferably, the active alumina used in steps a) and d) for the inner and outer layers is stabilized with 0.5 to 20 wt% lanthanum oxide. In the method described above, the support materials and cerium oxide / zirconia in situ are stabilized with praseodymium oxide. Alternatively, stabilization of the cerium oxide / zirconia with praseodymium oxide, yttrium oxide, neodymium oxide, lanthanum oxide or mixtures thereof can be achieved in a separate step with the above alloying compounds by impregnation, injection precipitation, simultaneous precipitation or simultaneous thermal hydrolysis.

To stabilize the ceria / zirconia by impregnation, the particulate ceria / zirconia is wetted with an aqueous solution of the precursor compounds of the desired alloying elements and then dried and calcined. Pore volume impregnation is often used for this purpose. In this case, the precursor compounds are dissolved in an amount of water, φ · <··· ·· £

-25 • · · φ Φ

Odpovídá ΦΦ which corresponds to the absorption capacity of cerium oxide / zirconia.

Injection precipitation has already been explained above for depositing noble metal compounds on the underlying materials.

To stabilize the cerium oxide / zirconia by co-precipitation, a conventional solution is prepared from the cerium oxide and zirconia precursor compounds and the stabilizer element precursor compound. Then, the three compounds are co-precipitated by addition of a suitable coagulant. Thus, praseodymium-stabilized cerium dioxide / zirconia can be prepared by preparing a conventional solution of cerium nitrate, zirconium nitrate and praseodymium nitrate, and adding ammonium carbonate or ammonium oxalate such that cerium, zirconium and praseodymium are precipitated simultaneously as carbonates or propyodymium. After filtration and drying, the desired stabilized cerium oxide / zirconia is obtained by calcination. Alternatively, the co-precipitation can also be carried out in a basic environment.

To stabilize the cerium oxide / zirconia by simultaneous thermal hydrolysis, a salt of cerium hydroxide nitrate, zirconium hydroxide nitrate and an alloy of the alloying element is prepared. The sol is then dewatered by rising temperature. Thereby, the hydroxynitrates are decomposed into the corresponding oxides. Current thermal hydrolysis is described, for example, in WO 98/16472.

• 9

9 99 ·

► · »· ♦ 98

-26999-99 99 9

9 9 9 9 9 9 99 99 9

9 9 9 9 9

99 99 99

The useful properties of the catalyst according to the invention will now be explained with reference to the following examples.

Figure 1 shows a schematic representation of the measurement principle for determining CO / NO _X intersections.

Comparative Example 1

The usual single layer platinum / rhodium catalyst CC1 (comparison catalyst 1) was prepared as follows:

Cerium carbonate and zirconium carbonate were treated with water and acetic acid overnight at ambient temperature to partially form the corresponding acetates. Stabilized alumina and free-flowing cerium oxide were added to the resulting dispersion. After wet milling, the monolithic support was coated with a slurry in a conventional soaking process. The coated substrate was air dried and calcined for 2 hours at an air temperature of 500 ° C.

The total coating of the vertical channels of the carrier was 205 g / l consisting of 112 g / l stabilized alumina, 34 g / l bulk cerium oxide, 34 g / l cerium oxide and 25 g / l zirconia, both of which are derived from acetate precursors.

The coating layer was impregnated with chloride-free platinum and rhodium salts (aminoplatinate and nitrate

-27 • 9

9 9 ·

9 · * 9 • »·. · '«• 9 9 9 9 · · ·

999.9 99 99 99 99 rhodium). The platinum / rhodium weight ratio was 5Pt / lRh at a concentration of 1.41 g / l (40 g / ft ³ ).

The final catalyst had the composition shown in Table 1:

Table 1: Comparative catalyst composition CC1

component concentration (g / l) alumina (stabilized by 3 wt.%) 112 La ₂ O ₃ ) CeO ₂ (loose) 34 CeO ₂ (from acetate) 34 ZrO ₂ (from acetate) 25 total oxide content 205 platinum 1,175 rhodium 0.235 total precious metal content 1.41

Example 1

The bilayer catalyst C1 of the invention was prepared as follows:

Preparation of the first (inner) layer:

To the praseodymium acetate solution was added the oxygen scavenging component rich in cerium (70 wt% cerium oxide, 30 wt% zirconia). Controlled ammonia injection and stirring for 30 minutes made praseodymium acetate ζ

precipitated to cerium oxide / zirconia. Subsequently, stabilized alumina (3 wt% La ₂ O ₃ , 97 wt% Al ₂ O ₃ ) and bulk zirconia were added. Thereafter, the platinum solution ((EA) ₂ Pt (OH) ₆ ) was injected into the slurry and the platinum was precipitated onto alumina and cerium oxide / zirconia by appropriately adjusting the pH of the dispersion with acetic acid. After grinding the slurry, the monolithic carrier was dipped into the slurry to apply the first layer.

The finished coating of the vertical channels was 160 g / l. The first layer was finally dried and then calcined in air at 500 ° C.

Preparation of the second (outer) layer:

Stabilized alumina (4 wt% La ₂ O ₃ , 96 wt% Al ₂ O ₃ ) was dispersed in water. Thereafter, the chloride-free ((EA) ₂ Pt (OH) ₆ ) platinum salt was injected and was easily adsorbed on alumina. The rhodium nitrate was then injected. By adjusting the pH of both catalyst components, it was fixed to the alumina backing.

To finish the alumina coating, praseodymium acetate and an oxygen scavenging component rich in cerium oxide (70 wt% cerium oxide, 30 wt% zirconia) were introduced.

Before coating the monolithic substrate, the pH of the slurry was adjusted to about 6 and the slurry was ground. The complete coating of the vertical channels of the second layer was 70 g / l. The catalyst was dried and calcined at 500 ° C in air.

• φ φ · ί

-29φ φ φ φ φ φ · Φ · * ·· φφ

The final catalyst had the composition shown in Tables 2 and 3:

Table 2: composition of the inner layer of catalyst C1

component concentration (g / l) alumina (stabilized by 3 wt.%) LČ12O3) 80 CeO ₂ / ZrO ₃ (70 wt% CeO ₂ ; 30 wt% ZrO ₂ ) Pr ₆ O _xl 51.7 ZrO ₂ 4.3 24 total oxide content 160 platinum 0, 94

Table 3: composition of the outer layer of catalyst C1

component concentration (g / l) alumina (stabilized by 3 wt.%) LCÍ2O3) 10 CeO ₂ / ZrO ₃ (70 wt% CeO ₂ ; 30 wt% ZrO ₂ ) Pr ₆ 0n 18.5 alumina (unstabilized) 1,5 ⁴⁰ total oxide content 70 platinum 0.235 rhodium 0.235 total precious metal content 0.47

The platinum-rhodium weight ratio was lPt / lRh in the upper layer. The total platinum and rhodium contents were 1.41 g / l (1.175 g Pt / l and 0.235 g Rh / l) at a weight ratio of 5Pt / lRh (combined weight ratios of the two layers).

Comparative Example 2

The dual-layer catalyst CC2 was prepared in the same manner as the catalyst of Example 1. In contrast to Example 1, platinum in the first layer was deposited in a separate alumina preparation step only before the coating slurry was prepared for the first layer.

Comparative example

The dual-layer catalyst CC3 was prepared in the same manner as the catalyst of Example 1. In contrast to Example 1, the entire amount of platinum was used only in the first layer. Thus, in the resulting comparative catalyst, platinum and rhodium were completely separated from each other.

Evaluation of catalysts

(a) engine tests:

The catalysts of the above Examples and Comparative Examples first aged in an internal combustion engine (engine displacement: 2.8 L) for 76 hours at an exhaust gas temperature upstream of the catalyst of 850 ° C. Then the quenching temperatures of the conversion of HC, CO and NO _x and CO / NO _X intersections were determined.

• 9 9 9 £

-31 ·· ·· ·· ··

The term "quenching temperatures" refers to the exhaust gas temperature at which 50% of the respective pollutants are converted by the catalyst. The extinguishing temperature may be different for HC, CO and NO _x . .

Two separate aging procedures were performed. In the first run, the catalyst sample C1 and the comparative catalyst CC1 were aged together, while in the second run, another catalyst sample C1 was aged together with the comparative catalysts CC2 and CC3. Since aging runs cannot be accurately reproduced, the catalysts from these two aging processes differ slightly. Therefore, only catalysts that have aged in the same aging run can be compared.

The quenching tests were performed at a flow rate of 65,000 h ^-1 with a gradually increasing engine exhaust temperature (38 K / min).

The measurement of CO / NO _X intersections is shown schematically in Figure 1. The Lambda value of the air / fuel mixture supplied to the engine is periodically changed from 0.98 (rich air / fuel mixture) to 1.02 (lean air / fuel mixture) and vice versa. Both residence times at λ = 0.98 and λ = 1.02 were determined for 1 minute. The transition from rich to poor and back occurred in 3 minutes. The corresponding λ curve is shown in Figure 1 (lower curve). The associated conversion curves for CO and NO _X are also shown in Figure 1. During the lean phase, CO conversion is actually 100% and falls to about 50 to 60% during the rich phase. The NO _X conversion curve behaves in an inverse way. During the rich phase, the NO _x conversion is approximately 100%, while during the lean phase

32 · 44 * 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 44 4444 9 444 9 4 4 4 9 f 9 4 4

4444 44 44 ·· ·· ·· The NO _X conversion phase falls to between 50 and 60%. At lambda 1, the two conversion curves intersect. The corresponding conversion value is the highest conversion which can be achieved simultaneously for CO and N0 _x. The higher this intersection, the better the dynamic functioning of the catalytic activity of the catalyst.

The intersection just described uses the so-called constant lambda curve. A dynamic lambda curve can also be used. In this case, the curve curved to the lambda value is additionally modulated at 1 Hz or 0.5 Hz. The amplitude can be + 1 V / P or ± 0.5 V / P (air / fuel). This amplitude is usually greater than the amplitude of the curve curved to +0.02 λ, corresponding to a V / P amplitude of ± 0.3.

The dynamic intersections for the catalysts of the previous examples were measured at a volumetric velocity of 65,000 ^-1 and at 450 ° C and at an exhaust gas temperature of 400 ° C. At an exhaust gas temperature of 450 ° C, the air / fuel ratio was modulated at a frequency of 1 Hz and a amplitude of 1 V / P (1 Hz ± 1 air / fuel). At an exhaust gas temperature of 400 ° C, the modulation amplitude was reduced to 0.5 V / P (1 Hz ± 0.5 air / fuel).

The measured results are summarized in Tables 4 and 5. Table 4 compares the catalysts that aged during the first aging run, while Table 5 compares the catalysts that aged during the second aging run:

Table 4 kataly50 (° C)

CO / NO _X (%)

44 44 • 44 ί

44 · 44

4 4 43 4

4 4 4 4 4 5

443 44 4 4 4

4 4 4 4 4 5

44 44 44

zátor HC WHAT NO _X 1 Hz ± 1 IN P 1 Hz ± 0.5 IN P Cl 360 363 354 84 88 CC1 387 407 382 76 62

CC: comparison catalyst; C: catalyst; T ₅₀ : switch-off temperature for 50% conversion

Table 5

kataly- T ₅₀ (° C) CO / NO _X zátor HC WHAT N0 _x 1 Hz ± 1 1 Hz ± 0.5 IN P IN P Cl 366 355 354 90 87 CC1 373 374 359 85 CC3 389 391 379 72 79

(a) model gas tests:

After aging the catalyst of Example 1 and Comparative Example 3 for 16 hours at 985 ° C in a lean synthetic gas mixture containing 6 vol% O ₂ , 10 vol% H ₂ O, 20 ppm

SO ₂ and the rest of N ₂ were determined by CO / NO _X intersections at a gas temperature of 400 ° C and a volume velocity of 100,000 h ^-1 . Intersections were determined for three different concentrations of SO ₂ gas mixture (0.5 and 20 ppm). The results are shown in Table 6:

Table 6

CO / NO _X catalyst 0 ppm SO ₂ 5 ppm SO ₂ 20 ppm SO ₂ Cl 65 61 52

tt · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · t · «· -34- ···· ·· ·· ·· ·· ··

CC3 42 35 30

Example 2

According to Example 1, another set of four different catalysts C2, C3, C4 and C5 was prepared. Unlike Example 1, all catalysts were made with a total noble metal load of 2.12 g / l (60 g / ft ³ ). The weight ratio of platinum to rhodium in the upper layer was varied to determine its effect on the catalytic properties of the catalysts. The distribution of noble metals on these catalysts is shown in Table 7:

Table 7: Distribution of precious metals

kataly- internal outer layer external both zátor layer layer layers Pt (g / l) Pt (g / l) Rh (g / l) Pt / Rh Pt / Rh C2 1.41 0.35 0.35 1 - 1 It was 5 - 1 at the break C3 1.73 0,035 0.35 1:10 It was 5 - 1 at the break C4 0.71 1.01 0.35 3 - 1 It was 5 - 1 at the break C5 1.77 0.0 0.35 0 - 1 It was 5 - 1 at the break

Before determining the CO / NO _X intersections, all four catalysts were aged for 12 hours at an exhaust gas temperature in front of the catalysts of 1100 ° C in a 6 vol. % oxygen, 10% water vapor, 20 ppm sulfur dioxide, and the remainder nitrogen.

Permanent intersections of these catalysts were determined at an exhaust gas temperature of 400 ° C and a volume velocity of 100,000 h -1.

-35 ¹ . During the test, the exhaust lambda value increased from 0.98 to 1.02 in 5 minutes. At 1.02, the lambda value was kept constant for 1 minute. Then the lambda value was again reduced to 0.98 in 5 minutes. After a delay of 1 minute, the described cycle was repeated twice. The CO / NO _X intersection values shown in Table 8 are the average of the last two test cycles.

Table 8: CO / NO _X intersections

catalyst CO / NO _X (%) constant C2 90 C3 55 C4 94 C5 <30 * ’

*): no intersection

For these measurements, the model gas had the following composition:

WHAT 1.40 ob j. % h ₂ 0.47 Vol. % NO 0.1 ob j. % CO ₂ 14.0 Vol. % SO ₂ 20 May ppm h ₂ o 10 Vol. % c ₃ h ₆ 666 ppm c ₃ h ₈ 333 ppm o ₂ 0.93 - 1,77% vol N2 residue

To generate the lambda curve, the oxygen content of the model gas varied between 0.93 and 1.77 vol%.

Example 3 • »φ φ φ ί

-36 • · φ ··

According to Example 1, two additional catalysts, C6 and C7, were prepared with a total noble metal loading of 1.41 g / l (40 g / ft ³ ). Example 1 was precisely copied to prepare catalyst C6, while the order of impregnation of the second substrate with platinum and rhodium was reversed for catalyst C7. First, rhodium was deposited on the activated alumina substrate, and then platinum.

Both catalysts were tested for their behavior in CO / NO _x, and their intersections snuffer temperature. The results are shown in Table 9.

Table 9

kataly- zátor HC T ₅₀ (° C) 1 Hz 0.25 + IN P CO / ΝΟχ% WHAT NO _X 1 Hz ± 0.5 V / P 1 Hz + 1 V / P C6 360 362 354 99 95 90 C7 359 355 353 97 90 82

The results in Table 9 show that the dynamic behavior of the catalyst C6 is much better than that of the catalyst C7. Without being limited by any theory, this result can be explained by a closer contact between platinum and rhodium if the first platinum and then rhodium is deposited.

Example 4

Four additional catalysts C8, C9, C10 and C11 were prepared according to Example 1 with the following changes:

9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9

-379999 99

The total noble metal load was determined to be 1.77 g / l (50 g / ft ³ ). The platinum / rhodium ratio was changed to 3: 2. In addition, different amounts of MnO ₂ and NiO in particulate form were added to the coating dispersion for the inner layers of C9-C11 catalysts. These hydrosulfide suppressants were added to the coating dispersions after injection of the platinum compound.

To measure the hydrosulfide emissions of these catalysts, they were initially loaded with sulfur under low conditions (volumetric rate of 65,000 h ^-1 ; temperature 550 ° C; lambda 1.01; approximate sulfur content in the fuel 200 ppm; approximate loading time> 0.5 h). Thereafter, emission was and was measured by a spectrometer. The peak reduced lambda value to 0.88 hydrosulfide by the attached mass maximum emission of hydrosulfide is shown in Table 10 for C8-C11 catalysts.

Table 10: Suppression of H ₂ S emission by MnO ₂ and NiO

kataly- both celekové inner layer maximum zátor layers amount suppressing component H ₂ S Pt / Rh Pt + Rh (g / l) H ₂ S (g / l) (ppm) C8 3 - 2 1.77 750 C9 3 - 2 1.77 20 MnO ₂ 380 CIO 3 - 2 1.77 40 MnO ₂ 330 Target 3 - 2 1.77 5 NiO 100 ALIGN!

Example 5

I · · · · · · · · · · · · · · · ·

The catalysts of the invention do not contain palladium. However, they were able to provide comparatively low emissions of hydrocarbons, carbon monoxide and nitrogen oxides as palladium and rhodium catalysts.

Another object of the invention was to reduce the cost of platinum group metals (PGMs) new compared to conventional catalysts at the platinum / rhodium catalyst prices of double-layered palladium / rhodium catalysts of April 1999. Therefore, the catalysts of Example 1 were produced with different total noble metal loads. and varying the platinum / rhodium ratio and compared with respect to exhaust purification activity and PGM cost.

Catalysts were tested on an EU-II-tested vehicle as the main catalytic converter catalysts of 0.67 ratio ratios. All underfloor volume / capacity catalysts were measured following aging for 16 hours at 985 ° C in nitrogen with 10% v / v. water. The tests were performed with a stoichiometric cold start according to the new European test cycle MVEG-EU III.

The relative emissions are shown in Table 11, where the values of the palladium / rhodium comparison catalyst (14Pd / 1Rh) were set to 100.

£.

-39 · • * 39 39

Table 11: relative emissions versus PGM costs

overall PGM relative emissions PGM PGM load (g / 1) ratio HC WHAT NO _X costs 3.53 14Pd / lRh 100 ALIGN! 100 ALIGN! 100 ALIGN! 100 ALIGN! 1.41 5Pt / lRh 119 110 112 51 1.77 3Pt / 2Rh 92 91 83 75 3.32 45Pt / 2Rh 128 108 110 108 3.89 9Pt / 2Rh 80 75 56 142

As shown in Table 11, the conversion of HC, CO and N0 _x is strongly influenced by the rhodium loading, which is beneficial for a given emission target, which is reducing the platinum content in favor of the rhodium content. For example, a rhodium-enriched load of 1.77 g / l (3Pt / 2Rh) in an EU-II tested vehicle shows lower emissions for all three pollutants compared to Pd / Rh (3.53 g / 1, 14Pd / lRh), a variant 3.32 g / l (45Pt / 2Rh) with a high platinum content falls below the results of 1.77 g / l load (3Pt / 2Rh), despite higher total load and considerably higher precious metal costs.

Other variations and modifications of the foregoing will be apparent to those skilled in the art and should be included in the appended claims.

Claims (23)

What is claimed is: 1. A high performance catalyst comprising an inner and an outer layer on an inert support skeleton comprising noble platinum group metals deposited on support materials, wherein the inner layer comprises platinum deposited on the first substrate and the first oxygen scavenging component; rhodium deposited on the second substrate, and the outer layer further comprises a second oxygen scavenging component. 2. The catalyst of claim 1 wherein said first and second substrates are the same or different and are compounds selected from the group consisting of silica, alumina, titanium dioxide, zirconia, mixed oxides or mixtures thereof, and mixed oxide. zirconium dioxide / zirconia rich cerium oxide. Catalyst according to claim 2, characterized in that said first and second substrates are activated alumina stabilized with 0.5 to 20% by weight of lanthanum oxide. 4. The catalyst of claim 3 wherein said support further comprises a zirconia component. ·· ·· · · · · · · · · · · -41 5. The catalyst of claim 1 wherein said first and second oxygen scavenging components are the same or different and include mixed cerium / zirconia rich cerium oxide compounds. Catalyst according to claim 5, characterized in that said mixed ceria / zirconia compounds are stabilized with praseodymium oxide, yttrium oxide, neodymium oxide, lanthanum oxide or mixtures thereof. The catalyst of claim 6, wherein said outer layer further comprises activated alumina. The catalyst of claim 1, wherein said outer layer further comprises highly dispersed yttrium oxide, neodymium oxide, lanthanum oxide or praseodymium oxide. 9. The catalyst of claim 1 wherein the platinum and rhodium are present on said second substrate in intimate contact with each other. Catalyst according to one of Claims 1 to 9, characterized in that said carcass is honeycomb-shaped with a plurality of substantially parallel channels extending therethrough, the channels being delimited by walls on which the inner layer is applied in an amount of from about 50 and - 42 - * W w w - • · · · · . * ·· '·' · · · • · · * ·· · ♦ ·· do 250 g / l and the outer layer is applied in an amount from 10 to 150 g / l supporting skeletons. 11 Catalyst according to claim 10, v y z n a č u .mu.Ci se t wherein said first substrate is present in amount from 20 to 150 g / l, said first oxygen scavenging component is present in an amount from 10 to 100 g / l, and the zirconia support is present in an amount from 5 to 60 g / l. Catalyst according to claim 11, characterized in that the platinum is present in said inner layer in a concentration of 0.01 to 5% by weight with respect to the total weight of said inner layer. Catalyst according to claim 12, characterized in that said second support is present in an amount of 5 to 100 g / l, said second oxygen scavenging component is present in an amount of 5 to 100 g / l and the activated alumina is present in an amount of 5 to 100 g / l up to 60 g / l. 14. The catalyst of claim 13 wherein the platinum and rhodium are present in said outer layer at a concentration of 0.5 to 20% by weight relative to the total weight of said outer layer and the platinum / rhodium weight ratio is selected from 5: 1 to 1: 3. The catalyst of claim 14, wherein at least one of said inner and outer layers further comprises from about 1 to 40 g / L of a nickel, iron or manganese component. • φ ··) »· · -43 φφφ φ 'φ φ 16. The process of claim 1 wherein the process comprises the steps of: a) coating the walls of the channels of the carrier skeleton with a coating composition comprising particulate materials comprising said first backing material and said first oxygen scavenging component; b) drying and calcining said coating, c) dipping the coated backbone into a solution of the soluble platinum precursor compound and calcining the coating; and d) applying an outer layer to the inner layer surface. 17. The process of claim 1 wherein the process comprises the steps of: (a) catalyzing particulate materials comprising said first carrier material and said first oxygen scavenging component by impregnating them with a solution of a soluble platinum precursor compound, drying and calcining the materials for thermally attaching platinum to said materials, b) preparing an aqueous coating composition from the catalysed materials of step a) and coating the walls of the carcass channels with the coating composition, c) drying and calcining said coating; and d) applying an outer layer to the surface of the inner layer. A process for the production of a catalyst according to claim 1, wherein the process comprises the steps of: • · · · · · · · · · · · · · · a) preparing a dispersion from particulate materials comprising said first support material and said first oxygen scavenging component and injecting a solution of a soluble platinum precursor compound, b) attaching the platinum compound to all particulate materials by adjusting the pH of the dispersion to obtain a coating composition, c) coating the walls of the channels of the carcass with the aqueous coating composition of step a), d) drying and calcining said coating; and e) applying an outer layer to the surface of the inner layer. The process for producing a catalyst according to any one of claims 16 to 18, wherein the process further comprises the steps of: a) impregnating said second substrate with a solution of a soluble platinum precursor compound and a rhodium compound, drying and calcining the impregnated substrate to obtain a catalyzed substrate, b) preparing an aqueous coating composition from said catalyzed support, said second oxygen scavenger component and additional active alumina, and c) using said coating composition to apply said outer layer to the surface of said inner layer. A process for producing a catalyst according to any one of claims 16 to 18, wherein the process further comprises the steps of: ·· 44 • 4 4 4 4 49 4 4 4 4 9 9 4949 44 Preparing a dispersion from said second undercoat 4 94 4 4 4 material and injection of the platinum precursor, b) then injecting the soluble compound solution of the soluble rhodium precursor compound solution into the dispersion of step a) and adjusting the pH of the dispersion to obtain a platinum and rhodium catalysed substrate, c) preparing a coating composition from the dispersion according to step b) by adding said second oxygen scavenging compound and additional active alumina, d) using said coating composition to apply said outer layer to the surface of said inner layer; and e) drying and calcining the coated monolithic support. 21. The high performance catalyst of claim 1 wherein the inner layer comprises platinum deposited on the active alumina and mixed cerium oxide / cerium oxide zirconia and the outer layer comprises platinum and rhodium deposited on the active alumina and the outer layer. it further comprises a mixed cerium oxide / zirconia rich in cerium oxide and the catalyst is obtainable (a) preparing a praseodymium precursor solution, adding mixed cerium dioxide / zirconia and adjusting the pH of the dispersion to precipitate the praseodymium precursor to cerium dioxide / zirconia; b) further adding alumina to the dispersion according to step a), c) injecting a solution of the platinum precursor compound into the dispersion according to step b) and precipitating it onto alumina and φ · φφ ·· ·· φ φ φ · · · · · · · · · · -46 cerium oxide / zirconium oxide to obtain a first coating composition for the inner catalyst layer, d) coating the monolithic carrier with said first coating composition and drying and calcining the coating to obtain a carrier coated with said inner layer, e) preparing an active alumina dispersion and injecting a platinum compound solution into the dispersion, f) then injecting a solution of the soluble rhodium precursor compound into the dispersion of step e) and adjusting the pH of the dispersion to obtain an aqueous dispersion of active platinum-rhodium-catalyzed active alumina, g) adding to the dispersion according to step f) an active alumina, a mixed cerium oxide / zirconia rich in cerium oxide; h) using said second coating composition for applying said outer layer to the surface of said inner layer; and i) drying and calcining the coated monolithic support. Catalyst according to claim 21, characterized in that the active alumina of steps b) and e) are stabilized with 0.5 to 20% by weight of lanthanum oxide. Catalyst according to claim 21, characterized in that in step b) an additional zirconia component is added 24. The catalyst of claim 21 wherein a solution of the praseodymium precursor compound is added to the dispersion of step e).