CN1354689A

CN1354689A - Composition for use as NOX trap, combining two compositions based on manganese and another element selected among alkalines, alkaline-earths and rare earths and use thereof for treating exhaust gases

Info

Publication number: CN1354689A
Application number: CN00808416A
Authority: CN
Inventors: C·伯特; T·伯切姆; C·布里; M·吉昂; C·赫杜因; N·莫拉尔; T·庞蒂尔; C·萨达; T·希吉隆
Original assignee: ECIA Equipements et Composants pour lIndustrie Automobile SA; Renault SAS; Rhone Poulenc Chimie SA
Current assignee: ECIA Equipements et Composants pour lIndustrie Automobile SA; Rhodia Chimie SAS; Renault SAS
Priority date: 1999-05-07
Filing date: 2000-05-05
Publication date: 2002-06-19
Anticipated expiration: 2020-05-05
Also published as: BR0010365A; KR20020042528A; CN1131726C; WO2000067904A1; JP2002543964A; AU4413800A; MXPA01011243A; EP1187675A1; CA2373080A1

Abstract

The invention relates to a compound and its use as NOx trap in the treatment of exhaust gases. Said compound is characterized in that it comprises a combination of: a first composition comprising a support and an active phase, the active phase being based on manganese and at least another element A selected among the alkalines and alkaline earths, the manganese and the element A being chemically bound; a second composition comprising a support and an active phase based on manganese and at least another element B selected among the alkalines, the alkaline-earths and rare earths, said second composition having or capable of having a specific surface area of at least 10 m<2>/g after being calcined for eight hours at 800 DEG C.

Description

Composition combining two compositions based on manganese and another element selected from the group consisting of alkali metals, alkaline earth metals and rare earth elements and useful as NOx trap and use of the composition in exhaust gas treatment

The invention relates to a composition useful as a NOx trap and to the use thereof in the treatment of exhaust gases, said composition combining two compositions, the active phase of each composition being based on manganese and another element selected from the group consisting of alkali metals, alkaline earth metals and rare earth elements.

It is known to reduce nitrogen oxides (NOx) from the exhaust gases of automotive electric motors, in particular with a "three-way" catalyst which makes use of a reducing gas present in stoichiometric amounts in the mixture. Any excess oxygen will result in a significant deterioration of the catalyst performance.

Some electric motors, such as diesel or gasoline motors operated lean, are now economical in terms of fuel consumption, but release exhaust gases that often contain large amounts of excess oxygen, e.g., at least 5%. A standard three-way catalyst is therefore ineffective for releasing NOx in this case. It is also necessary to limit NOx emissions by the strict post-combustion standards used today for electric motor vehicles.

To solve this problem, systems known as NOx traps have been specifically proposed, which are capable of oxidizing NO to NO₂And then absorbs off the NO thus formed₂. Under certain conditions, NO₂Salts are formed and then reduced by the reducing agent contained in the exhaust gas. These NOx traps still have some drawbacks. It has therefore been observed that when old, they tend to operate only at very high temperatures and therefore their operating range is reduced. It would therefore be beneficial to have a system that is capable of operating over a wider temperature range. In addition, they are usually based on noble metals. These metals are now expensive and their feasibility may be problematic. To reduce costs, it would also be beneficial to have a catalyst that could be viable without precious metals.

It was therefore a subject of the present invention to develop a composition which can be used over a wide temperature range and which optionally can be used without any noble metal.

To this end, the composition of the invention useful as NOx trap is characterized in that it comprises a combination of the following compositions:

-a first composition comprising a support and an active phase based on manganese and at least one further element a selected from the group consisting of alkali metals and alkaline earth metals, the manganese and the element a being chemically bonded;

-a second composition comprising a support and an active phase based on manganese and at least one other element B chosen from alkali metals, alkaline earth metals and rare earths, the second composition having or being capable of having at least 10m after calcination at 800 ℃ for 8 hours²Specific surface area in g.

Other characteristics, details and advantages of the invention will become more apparent on reading the description that follows and on reading the examples that illustrate it in particular, but not exclusively.

Throughout the specification, rare earth is understood to be the group of elements consisting of yttrium and the elements of the periodic table having atomic numbers from 57 to 71.

In addition, The specific surface area is understood to be The BET specific surface area, determined according to The nitrogen adsorption determined according to ASTM D3663-78 standard based on The BRUNAUER-EMMETT-TELLER method described in The journal "The journal of The American Chemical Society, 60, 309 (1938)",

the compositions of the present invention are characterized by the combination of two specific compositions, which are described in more detail below.

These compositions each comprise a carrier and an active phase. In the composition, the term support shall be used in its broadest sense to define a primary element and/or having no catalytic or trapping capacity per se, or having catalytic or trapping capacity not equivalent to activity, and having deposited thereon other elements. For simplicity, reference will be made in the following description to a support and an active or supported phase, but it is understood that it is still within the scope of the invention if the elements described as belonging to the active or supported phase are present in the support, as they are introduced during the preparation of the support.

The first composition is described below.

The composition contains an active phase based on manganese and at least one other element A chosen from the group consisting of alkali metals and alkaline earth metals. As the alkali metal element, sodium and potassium can be more specifically mentioned. As the alkaline earth metal element, barium may be more specifically mentioned. For the composition, one or more elements a may be present, so that references to elements a in the following description are understood to also apply to the case of several elements a.

In addition, manganese and element a are present in the first composition in a chemically bonded form. This means that there is a chemical bond between manganese and element a formed by the reaction between them, these elements not simply being juxtaposed in a simple mixture. The manganese and the element a may thus be present in a compound or in a phase of mixed oxide type. This compound or this phase may be represented in particular by the general formula A_xMn_yO₂+ -. delta (1), where 0.5. ltoreq. y/x. ltoreq.6, the value of delta depending on the nature of the element A and on the oxidation state of the manganese. As the phase or compound of the general formula (1), there may be mentioned, for example, hydrated pyrolusite, alkaline pyrolusite, heteropsilomete, or psilomete, birnessite, barium-magnesiate, buckerite or lithium-scleronite. The compound optionally may be a hydrate. In addition, the compound may have Cdl₂A layered structure of the type. The general formula (1) is given here by way of illustration, and it is not intended to be outside the scope of the invention if manganese and the element a are still chemically bonded, but have different general formulae.

XR or electron microscopy analysis can confirm the presence of such compounds.

The oxidation number of manganese may vary from 2 to 7, more particularly from 3 to 7.

In the case of potassium, the element and manganese may be present as compound K₂Mn₄O₈Exist in the form of (1). In the case of barium, this may then be BaMnO₃A compound of the type (I).

The invention covers the case where the active phase of the first composition consists essentially of manganese and one or more further elements a selected from the group consisting of alkali metals and alkaline earth metals, the manganese and the elements a being chemically bonded. "Main component" can be treatedIt is understood that the composition of the invention may have NO in the active phase without other elements than manganese and element a_xTrapping capabilities, e.g., no noble metal-based elements or other metals commonly used for catalysis.

The first composition further comprises a carrier. Any porous support that can be used in the field of catalysis can be used for the support. It is preferred for the support to be sufficiently chemically inert with respect to manganese and element a to avoid major reactions of one or both of these elements with the support, which may prevent the formation of chemical bonds between manganese and element a. However, in the case of a reaction of the support with these elements, it is possible to use larger amounts of manganese and element a in order to obtain the desired chemical bond between these elements.

The support may be alumina based. Any type of alumina that can have a specific surface area sufficient for catalysis can be used herein. Mention may be made of aluminas obtained by the rapid dehydration of at least one aluminium hydroxide, such as bayerite, gibbsite or gibbsite, nordstrandite and/or at least one aluminium hydroxide, such as boehmite, pseudoboehmite and diaspore.

Stabilized alumina may also be used. As stabilizing elements there may be mentioned rare earths, barium, silicon, titanium and zirconium. Cerium, lanthanum or lanthanum/neodymium mixtures may be mentioned in particular for the rare earths.

The preparation of the stabilized alumina is carried out in a manner known per se. In particular by impregnation of the alumina with a solution of the salts of the stabilizing elements mentioned previously, such as nitrates, or by simultaneous drying of the precursors of the alumina and of the salts of these elements, followed by calcination.

The support may also be based on oxides selected from cerium oxide, zirconium oxide or mixtures thereof.

It may be mentioned in particular that the present application incorporates the disclosure therein with respect to the cerium oxide and zirconium oxide mixtures described in patent applications EP-A-605274 and EP-A-735984. More particularly, cerium and zirconium oxide-based supports can be used, wherein these oxidesThe cerium/zirconium atomic ratio present is at least 1. For the same carrier, one in the form of a solid solution can also be used. In these cases, the X-ray diffraction pattern of the support revealed the presence of a single homogeneous phase inside the support. For the most cerium-containing support, this phase corresponds in fact to the crystallized cubic cerium oxide CeO₂The unit cell parameters are more or less altered with respect to pure ceria, thus converting the bound zirconium into a crystalline network of ceria, thus obtaining a true solid solution.

As mixtures of cerium oxide and zirconium oxide, mention may furthermore be made of mixtures based on two oxides, also scandium oxide or rare earths other than cerium, in particular those described in patent application WO97/43214, the disclosure of which is incorporated herein. This application describes in particular compositions based on cerium oxide, zirconium oxide and yttrium oxide, and also compositions based, in addition to cerium oxide and zirconium oxide, on at least one other oxide chosen from cerium oxide and rare earths other than cerium, in which the cerium/zirconium atomic ratio is at least 1. These compositions have a specific surface area of at least 35m after calcination at 900 ℃ for 6 hours²Per g, having an oxygen storage capacity of at least 1.5ml O at 400 DEG C₂/g。

According to a particular embodiment of the invention, the support is based on cerium oxide and further contains silicon dioxide. Such vectors are described in patent applications EP-A-207857 and EP-A-547924, the present application incorporating the disclosure therein.

The total content of manganese, alkali metal and alkaline earth metal can vary within a large range of proportions. Most preferablyLow levels are those below which NO NO is observed_xAnd (4) absorption behavior. In particular this content may be between 2 and 50%, more particularly between 5 and 30%. This content is expressed as atomic percent relative to the total number of moles of oxide and related elements in the active phase in the support. The respective contents of manganese, alkali metal and alkaline earth metal may also vary within a large range of proportions, in particular the content of manganese may in particular be equal to or close to the content of alkali metal or alkaline earth metal.

In a useful variant of the invention, the alkali metal is potassium, the content of which (as indicated above) may be between 10 and 50%, more particularly between 30 and 50%.

The first composition of the invention may be prepared by a process wherein a support is contacted with manganese and at least one element a or at least one precursor of manganese and at least one further element a, and wherein the entire composition is calcined at a temperature sufficient to form a chemical bond between the manganese and the element a.

One method that may be used for the above contacting process is impregnation. Thus, a solution or slurry of the salt or compound of the element in the carried phase is formed first.

For the salt, inorganic acid salts such as nitrate, sulfate or chloride may be selected.

Organic acid salts may also be used, in particular saturated fatty carboxylates or carboxylates. Mention may be made, by way of example, of formates, acetates, propionates, oxalates or citrates.

The support is then impregnated with the solution or slurry.

More specifically dry impregnation is used. Dry impregnation involves adding to the impregnated product a volume of aqueous elemental solution equal to the pore volume of the impregnated solid.

It is advantageous to carry out the deposition of the active phase element in two stages. It is therefore preferred that manganese is deposited in the first stage and element a is deposited in the second stage.

After impregnation, the support is optionally dried and then calcined. It should be noted that it is also possible to use a support which has not been calcined before impregnation.

The active phase can also be deposited by atomizing a suspension based on the active phase and a salt or compound of the carrier element. The atomized product obtained is then calcined.

As described above, the calcination is carried out at a temperature sufficient to form a chemical bond between the manganese and the element a. This temperature varies according to the nature of element a, but when calcined in air, the temperature is generally at least 600 ℃, more particularly at least 700 ℃, and may in particular be between 800 ℃ and 850 ℃. With the chemical bonds between manganese and element a having been formed, higher temperatures are generally not required and, on the other hand, they may lead to a reduction in the specific surface area of the support, which may reduce the catalytic performance of the composition. The duration of the calcination depends in particular on the temperature, which is determined to be sufficient for the formation of chemical bonds between the elements.

The second composition of the present invention will be described below.

The composition also includes a carrier and an active phase.

The above remarks regarding the active phase of the first composition apply here as well, in particular regarding the nature of the elements of this phase and their amounts. More particularly, therefore, element B may be sodium, potassium or barium.

The active phase of the second composition may also be based on manganese and at least one rare earth element. More specifically, the rare earth element may be selected from cerium, terbium, gadolinium, samarium, neodymium and praseodymium. The total content of manganese, alkali metal, alkaline earth metal or rare earth element may vary between 1 and 50%, more particularly between 5 and 30%. This content is expressed as atomic percent relative to the total number of moles of support oxide and elements associated with the supported phase. The respective contents of manganese, alkali metal, alkaline earth metal and rare earth element may also vary within a large proportion range, in particular the content of manganese may be equal to or close to the content of element B.

The invention covers the case where the active phase consists essentially of manganese and one or more further elements B selected from the group consisting of alkaline earth metals and rare earth elements. By "consisting essentially of" is meant that the composition of the invention can have NO in the active phase without other elements than manganese and element B_xTrapping capabilities, e.g., no noble metal-based elements or other metals commonly used for catalysis.

As mentioned above, the second composition is characterized in that it has or can have at least 10m after calcination at 800 ℃ for 8 hours²Specific surface area in g. This specific surface area may in particular be at least 20m after calcination at the same temperature for the same time²(ii) in terms of/g. More particularly, the specific surface area is at least 80m after calcination at 800 ℃ for 8 hours²In terms of/g, evenTo more specifically at least 100m²/g。

The specific surface area is understood to be The BET specific surface area, determined according to The nitrogen adsorption determined according to ASTM D3663-78 standard based on The BRUNAUER-EMMETT-TELLER method described in The Journal of The American Chemical Society, 60, 309(1938) ",

this surface area characteristic is achieved by selecting a suitable support, in particular a support having a sufficiently high specific surface area.

Such supports may be based on alumina. Any type of alumina that can have a specific surface area sufficient for catalysis can be used herein. Mention may be made of aluminas obtained by the rapid dehydration of at least one aluminium hydroxide, such as bayerite, gibbsite or gibbsite, nordstrandite and/or at least one aluminium hydroxide, such as boehmite, pseudoboehmite and diaspore.

According to a particular embodiment of the invention, stabilised alumina is used. As stabilizing elements there may be mentioned rare earths, barium, silicon and zirconium. As rare earths, there may be mentioned in particular mixtures of cerium, lanthanum and neodymium.

Another method which may be mentioned for preparing stabilized alumina is to subject the alumina powder formed by the rapid dehydration of aluminum hydroxide or aluminum hydroxide to a slaking operation in the presence of a stabilizer consisting of a lanthanum compound and optionally a neodymium compound, more particularly wherein the stabilizer compound may be a salt. The slaking may be carried out by suspending the alumina in water and then heating to a temperature between 70-110 c. After aging, the alumina is heat treated.

Another preparation method consisted of a similar treatment method except that barium was used.

The stabilizer content is expressed as weight relative to the stabilizer oxide content of the stabilized alumina, and is typically 1.5 to 15%, more specifically 2.5 to 11%.

The support may also be silica-based.

It may also be based on silica and titanium oxide with an atomic ratio Ti/Ti + Si of between 0.1 and 15%. More specifically, the ratio is 0.1 to 10%. Such vectors are described in particular in patent application WO99/01216, the disclosure of which is incorporated herein by reference.

For other suitable supports, those based on ceria and zirconia may be used, these oxides being present in the form of mixed oxides or a solid solution of zirconia in ceria or a solid solution of ceria in zirconia. These supports are obtained by a first type of process comprising a stage of forming a mixture comprising zirconium oxide and cerium oxide, and washing or impregnating the mixture thus formed with an alkoxide having a number of carbon atoms greater than 2. The impregnated mixture is then calcined.

The alkoxylated compounds may in particular be chosen from those of formula (2) R₁-((CH₂)_x-O)_n-R₂The product of (1), wherein R₁And R₂Represents linear or non-linear alkyl, or H or OH or Cl or Br or I; n is a number between 1 and 100; x is a number between 1 and 4, or may be of formula (3) (R)₃，R₄)-φ-((CH₂)_x-O)_nProducts of-OH, where φ denotes a benzene ring, R₃And R₄Are identical or different substituents in the ring, and represent hydrogen or a linear or nonlinear alkyl group having 1 to 20 carbon atoms, x and n being as defined above; or may be of the formula (4) R₄O-((CH₂)_x-O)_nProducts of formula (I) -H, wherein R₄Is a linear or non-linear alcohol radical having from 1 to 20 carbon atoms, x and n being as defined hereinbefore; and general formula (5) R₅-S-((CH₂)_x-O)_nProducts of formula (I) -H, wherein R₅Is a linear or non-linear alkyl group having 1 to 20 carbon atoms, and x and n are as defined above. For these products, reference may be made to patent application WO98/16472, which is incorporated herein by referenceThe disclosure of which is herein incorporated.

These supports can also be obtained by a second type of process comprising a stage in which a solution is reacted with a cerium salt solution, a zirconium salt solution and a compound chosen from anionic surfactants, nonionic surfactants, polyvinyl glycols, carboxylic acids and their salts, optionally in the presence of a base and/or an oxidizing agent.

More specifically, carboxylates, phosphates, sulfates, and sulfonates can be used as the anionic surfactant. Among the nonionic surfactants, ethoxylated alkylphenols and ethoxylated amines can be preferably used.

The reaction between the zirconium and cerium salts can be carried out by heating a solution containing these salts, thus involving a thermal hydrolysis reaction. It can also be carried out by introducing a base into a solution containing these salts to form a precipitate.

For these products, reference is made to patent application WO98/45212, the disclosure of which is incorporated herein.

The second composition may be prepared using the same methods given above for preparing the first composition. It should be noted that after the support has been brought into contact with the elements of the active phase, the whole should be calcined at a temperature sufficient to allow the elements to be present in the form of oxides. Typically, this temperature is at least 500 deg.C, more particularly at least 600 deg.C.

The compositions of the invention described above are in powder form, but optionally they may be shaped to exist in the form of granules, pellets, cylinders or honeycombs of various sizes.

The invention also relates to a system that can be used for trapping NOx, comprising a wash coat (wash coat) having catalytic properties and incorporating the composition of the invention, the wash coat being deposited on a substrate, such as a monolithic substrate of metal or ceramic.

The system may exist in different embodiments.

In a first embodiment, the system includes a substrate and a washcoat layer comprised of two stacked layers. In this case, the first layer contains a first composition of the composition and the second layer contains a second composition. The layers may be in any order, i.e. the inner layer in contact with the substrate may contain the first or second composition, while the outer layer deposited on the first layer contains the other composition of the compositions.

According to a second embodiment, the composition is present in the washcoat as a single layer, in which case the single layer comprises both compositions as a mixture, for example by mechanical mixing.

A third embodiment is conceivable. In this case, the system comprises two substrates arranged in parallel, each substrate comprising a washcoat. The first substrate washcoat layer comprises a first composition and the second substrate washcoat layer comprises a second composition.

In addition, the present invention also relates to the use of the composition described above in the manufacture of such a system.

The invention also relates to a gas treatment process with the aim of reducing nitrogen oxide emissions using the composition according to the invention.

Gases which can be treated within the scope of the invention are gases emitted by gas turbines, power station boilers or internal combustion engines. In the last case, diesel engines and lean burn engines may be included.

The composition of the present invention can be used as a NOx trap when it is contacted with a gas having a high oxygen content. A gas with a high oxygen content means that it contains an excess of oxygen with respect to the amount of oxygen necessary for the stoichiometric combustion of the motor fuel, more precisely with respect to the stoichiometric ratio λ 1, i.e. it has a λ value greater than 1. In particular, the lambda value is correlated with the air/fuel ratio in a manner known per se in the field of internal combustion engines. Such gases may be those of lean burn electric motors which contain at least 2% oxygen (by volume) and gases containing higher amounts of oxygen, such as diesel engine gases, i.e. at least 5% or more than 5%, more particularly at least 10%, for example this content may be between 5% and 20%.

The invention is also applicable to the case where the gas also contains 10% water.

An example will be given below.

In this example, NO_xThe evaluation test of the collector was carried out as follows:

each NO is added_x0.15g each of the collectors (first composition and second composition) was charged in the form of powder into a quartz reactor. The powders used are preferably compacted and then ground and sieved in order to separate the fraction of particles with a size of 0.125-0.25 mm.

The reaction mixture at the reactor inlet had the following composition (by volume):

-NO： 300vpm

-O₂：10％

-CO₂：10％

-H₂O：10％

-N₂: balance of

The total flow rate was 30 Nl/h.

VVH of the order of 150,000h^-1。

Recording of NO and NO in the reactor at all times_x( ) And a temperature signal.

NO and NO_xThe signal is ECOPHYSICS NO_xThe analyzer is given in the principle of chemiluminescence.

For NO_xThe collector was evaluated by determining the NO absorbed when the collector phase was saturated_xThe total amount (expressed as mgNO/g trapping phase or active phase). The experiment was repeated at different temperatures between 250 ℃ and 500 ℃. Thus, NO can be determined_xThe optimal temperature range for the trapping agent to function.

A) First composition

Raw materials:

using manganese nitrate Mn (NO)₃)₂·4H₂O and potassium nitrate KNO₃99.5％。

The support used is of the type described in example 4 of patent application WO97/43214, i.e. a support based on cerium oxide, zirconium oxide and lanthanum oxide, in a respective weight ratio of 72/24/4 with respect to the oxide.

Preparation of the composition:

two deposition phases are performed.

Stage 1: depositing a first active element

This stage consists in depositing the active element Mn by an atomic weight equal to 10%, calculated as follows:

[Mn]/([Mn]+[CeO₂]+[ZrO₂]+[La₂O₃])＝0.10

and (2) stage: depositing a second active element;

this phase consists in depositing the second active element K in an atomic ratio equal to 10%, calculated as follows:

[K]/([Mn]+[K]+[CeO₂]+[ZrO₂]+[La₂O₃])＝0.10

the deposition is performed using dry impregnation. This method consists in impregnating the support concerned with the active phase element dissolved in a solution having a volume equal to the pore volume of the support (determined with water: 0.5 cm)³In g) in such a concentration that the desired concentration is achieved.

In this case, the elements are impregnated one by one on the support.

The operation procedure is as follows:

dry impregnation of the first element

Drying in an oven (110 ℃, 2 hours)

Calcination at 600 ℃ for 2 hours

Dry impregnation of the second element

Drying in an oven (110 ℃, 2 hours)

Calcination at 800 ℃ for 2 hours

B) Second composition

Raw materials:

The support used was Condea SB3 alumina, which had been stabilized by impregnation with 10% atomic weight Ce and then calcined at 500 ℃ for 2 hours.

Preparation of the composition:

the same way as when preparing the first composition (dry impregnation, pore volume of the support using 0.8 cm)³Water/g) with the same amounts of Mn and K elements, the first calcination being carried out at 500 c and the second at 800 c. The BET surface area of the composition thus obtained was 117m²/g。

The catalytic results are given below.

T℃	NO_xStorage capacity (mg NO)_xPer g active phase)
T℃	NO_xStorage capacity (mg NO)_xPer g active phase)	250	10
300	12	250	10
300	12	350	22
400	19	350	22
400	19	450	16
500	9	450	16

Thus, the entire temperature range between 250-500 ℃ was tested for NO_xThe trapping activity of (1). This activity is produced in the absence of noble metals.

Claims

1. A composition useful as a NOx trap, characterized in that it comprises a combination of:

-a second composition comprising a support and an active phase based on manganese and at least one other element B chosen from alkali metals, alkaline earth metals and rare earths, the second composition having or being capable of having at least 10m after calcination at 800 ℃ for 8 hours²A specific surface area per gram, more particularly at least 20m²A/g, even more particularly 80m²/g。

2. The composition of claim 1, characterized in that the second composition has or is capable of having at least 100m after calcination at 800 ℃ for 8 hours²Specific surface area in g.

3. Composition according to claim 1 or 2, characterized in that the elements a and B are selected from potassium, sodium or barium.

4. Composition according to one of the preceding claims, characterized in that the support of the second composition is based on alumina or alumina stabilized with silicon, zirconium, barium or a rare earth element.

5. A composition according to any of claims 1 to 3, characterized in that the carrier of the second composition is silica-based.

6. A composition according to any one of claims 1 to 3, characterized in that the support of the second composition is based on silica and titanium oxide having an atomic ratio Ti/Ti + Si of between 0.1 and 15%.

7. A composition according to claims 1-3, characterized in that the carrier of the second composition is based on cerium oxide and zirconium oxide, which carrier is obtained by a method in which a mixture containing zirconium oxide and cerium oxide is formed and the mixture thus formed is washed or impregnated with an alkoxide having a number of carbon atoms greater than 2.

8. A composition according to any of claims 1-3, characterized in that the carrier of the second composition is based on cerium oxide and zirconium oxide, which carrier is obtained by a process in which a cerium salt solution and a zirconium salt solution are reacted with an additive selected from the group consisting of anionic surfactants, non-ionic surfactants, polyvinyl glycols, carboxylic acids and their salts, optionally in the presence of a base and/or an oxidizing agent.

9. Composition according to one of the preceding claims, characterized in that the support of the first composition is based on an oxide selected from the group consisting of alumina, ceria, zirconia or mixtures thereof.

10. Composition according to one of the preceding claims, characterized in that the support of the first composition is based on cerium oxide and further comprises silicon dioxide.

11. A system comprising a substrate and a washcoat, wherein the washcoat incorporates the composition of any of claims 1-10, the composition being present in the washcoat on the substrate in such a manner that a first layer comprises the first composition and a second layer comprises the second composition.

12. A system comprising a substrate and a washcoat, wherein the washcoat incorporates a composition according to any one of claims 1 to 10 in the form of a mixture of the two aforementioned compositions in the washcoat on the substrate.

13. A system comprising two substrates disposed side-by-side, each of said substrates having a washcoat layer and a composition according to any one of claims 1 to 10 in the form of said first composition in a washcoat layer on a first substrate and in the form of said second composition in a washcoat layer on a second substrate.

14. A method for treating gases with the aim of reducing nitrogen oxide emissions, characterized in that a composition according to one of claims 1 to 10 or a system according to one of claims 11 to 13 is used.

15. The method of claim 14, wherein exhaust gas from an internal combustion engine is treated.

16. The method of claim 15, characterized in that the treated gas contains an excess of oxygen with respect to the stoichiometric value.

17. Use of a composition according to any one of claims 1 to 10 in the manufacture of a system for treating exhaust gases of an internal combustion engine.