WO2000067904A1

WO2000067904A1 - Composition for use as nox trap, combining two compositions based on manganese and another element selected among the alkalines, alkaline-earths and rare earths and use thereof for treating exhaust gases

Info

Publication number: WO2000067904A1
Application number: PCT/FR2000/001227
Authority: WO
Inventors: Christian Bert; Thierry Birchem; Christophe Bouly; Marc Guyon; Catherine Hedouin; Najat Moral; Thierry Pontier; Christian Sarda; Thierry Seguelong
Original assignee: Rhodia Chimie; Ecia; Renault
Priority date: 1999-05-07
Filing date: 2000-05-05
Publication date: 2000-11-16
Also published as: KR20020042528A; CN1131726C; AU4413800A; BR0010365A; MXPA01011243A; JP2002543964A; CA2373080A1; EP1187675A1; CN1354689A

Abstract

The invention concerns a compound and its use as NOx trap in the treatment of exhaust gases. Said compound is characterised 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 m2/g after being calcined for 8 hours at 800 °C.

Description

COMPOUND USEFUL AS A NOx TRAP. COMBINING TWO

COMPOSITIONS BASED ON MANGANESE AND ANOTHER SELECTED ELEMENT.

AMONG ALKALIANS. ALKALINO-EARTHY AND RARE EARTHS AND

ITS USE IN THE TREATMENT OF EXHAUST GASES

The present invention relates to a compound which can be used as a Nox trap associating two compositions, the active phases of which are based on manganese and on another element chosen from alkalis, alkaline-earths and rare earths, and its use in the treatment of gases. exhaust

It is known that the reduction of emissions of nitrogen oxides (NOx) from the exhaust gases of automobile engines in particular, is carried out using “three-way” catalysts which use stoichiometrically the reducing gases present in the mixture All excess oxygen results in sudden deterioration in catalyst performance

However, certain engines such as diesel engines or gasoline engines operating in lean burn are fuel efficient but emit exhaust gases which permanently contain a large excess of oxygen of at least 5% for example A standard three-way catalytic converter therefore has no effect on NOx emissions in this case In addition, limiting NOx emissions is made imperative by the tightening of standards in automotive post combustion which now extend to these engines

To solve this problem, systems called NOx traps have been proposed, which are capable of oxidizing NO to NO2 and then adsorbing the O2 thus formed. Under certain conditions, the NO2 is released and then reduced to 2 by reducing species contained in the exhaust gases These NOx traps still have certain drawbacks. As a result of their aging, it has been observed that they tend to work only in a high temperature range and therefore have a reduced operating window It would therefore be advantageous to have a system capable of operating over a wider temperature range Furthermore, they are generally based on precious metals Gold, these metals are expensive and their availability can be problematic II would be also interesting to be able to have catalysts without precious metals to reduce costs

The object of the invention is therefore the development of a compound which can be used in a wider range of temperatures and, optionally, in the absence of precious metal For this purpose, the compound of the invention, which can be used as a NOx trap, is characterized in that it comprises a combination

- a first composition comprising a support and an active phase, the active phase being based on manganese and at least one other element A chosen from alkali and alkaline earth metals, manganese and element A being chemically related,

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, this second composition having or being capable of having a specific surface of at least 10m / g after calcination 8 hours at 800 ° C

Other characteristics, details and advantages of the invention will appear even more completely on reading the description which follows, as well as a concrete but nonlimiting example intended to illustrate it.

For the whole of the description, the term “rare earths” is understood to mean the elements of the group constituted by yttπum and the elements of the periodic classification with atomic number included inclusively between 57 and 71

Furthermore, the term “specific surface” is understood to mean the BET specific surface area determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 established on the basis of the BRUNAUER - EMMETT-TELLER method determined in the periodical "The Journal of the Amencan Chemical Society, 60, 309 (1938) "

The compound of the invention is characterized by the association of two specific compositions which will now be more particularly determined

These compositions comprise a support and an active phase The term support must be pπs in a broad sense to designate, in the composition, the majority of the element (s) and / or either without catalytic activity or proper trapping activity, or having a catalytic activity or trapping not equivalent to that of the active phase, and on which or on which the other elements are deposited To simplify, we will speak in the following description of support and active or supported phase but it will be understood that we would not leave not within the scope of the present invention in the case where an element described as belonging to the active or supported phase was present in the support, for example by having been introduced into it during the preparation of the support itself

The first composition is described below

This composition comprises an active phase which is based on manganese and at least one other element A chosen from alkali metals and alkaline earth metals. As alkaline element, mention may be made more particularly of sodium and potassium As alkaline earth element, barium can in particular be mentioned. As the composition may comprise one or more elements A, any reference below of the description in element A must therefore be understood as being able to also apply in the case where there are, several elements A

Furthermore, the elements manganese and A are present in this first composition in a chemically bonded form. By this is meant that there are chemical bonds between manganese and element A resulting from a reaction between them, these two elements not being simply juxtaposed as in a simple mixture Thus, the elements manganese and A can be present in the form of a compound or of a phase of mixed oxide type This compound or this phase can in particular be represented by the formula A _x Mn _y θ2 ± ^ (1) in which 0.5 <y / x <6, the value of δ depending on the nature of the element A and the oxidation state of manganese As phase or compound of formula (1) there may be mentioned, by way of example, those of the vernadite, hollandite, romanechite or psilomelane, birnessite, todorokite, nozzle or lithiophonte type. The compound may optionally be hydrated. (1) is given here by way of illustration, it would not be departing from the scope of the present invention if the compound had a different formula insofar as of course the manganese and the element A are well chemically linked

Analysis by X-ray or by electron microscopy makes it possible to demonstrate the presence of such a compound The degree of oxidation of manganese can vary between 2 and 7 and, more particularly between 3 and 7

In the case of potassium, this element and manganese can be present in the form of a compound of type K2Mn-4θ8 In the case of barium, it can be a compound of type BaMnθ3 The invention covers the case where the active phase of the first composition consists essentially of manganese and one or more other elements A chosen from alkali and alkaline earth metals, manganese and element A being chemically linked By "consists essentially" is meant that the composition of the invention may have a NOx trap activity in the absence in the active phase of any element other than manganese and the element (s) A, such as for example an element of the precious metal or other metal type usually used in catalysis

The first compositions further comprises a support. As a support, any porous support which can be used in the field of catalysis can be used. It is preferable that this support has chemical inertness with respect to the manganese elements and A sufficient to avoid a substantial reaction. of one or more of these elements with the support which would be likely to hinder the creation of a chemical bond between the manganese and the element A However, in the case of a reaction between the support and these elements, it is possible to implement more quantities important manganese and element A to obtain the desired chemical bond between these elements

This support can be based on alumina Any type of alumina can be used HERE capable of having a specific surface sufficient for an application in catalysis Mention may be made of aluminas resulting from the rapid dehydration of at least one aluminum hydroxide, such as bayeπte, hydrargillite or gibbsite, nordstrandite, and / or at least one aluminum oxyhydroxide such as boehmite, pseudoboehmite and diaspore

It is also possible to use a stabilized alumina. As a stabilizing element, mention may be made of rare earths, barium, silicon, titanium and zirconium. As rare earth, mention may be made particularly of cerium, lanthanum or the lanthanum-neodymium mixture.

The preparation of the stabilized alumina is carried out in a manner known per se, in particular by impregnating the alumina with solutions of salts, such as the nitrates of the abovementioned stabilizing elements or else by co-drying of an alumina precursor and of salts. of these elements then calcination

The support can also be based on an oxide chosen from cerium oxide, zirconium oxide or their mixtures

Mention may be made in particular for mixtures of cerium oxide and zirconium oxide those described in patent applications EP-A-605,274 and EP-A-735,984, the teaching of which is incorporated HERE The supports can more particularly be used based on cerium and zirconium oxide in which these oxides are present in a céπum / zirconium atomic proportion of at least 1 For these same supports, one can also use those which are in the form of a solid solution In this case, the X-ray diffraction spectra of the support reveal within the latter the existence of a single homogeneous phase For the supports richest in cerium, this phase corresponds in fact to that of a cenic oxide Ceθ2 cubic crystallized and whose the mesh parameters are more or less offset with respect to a pure cene oxide, thus reflecting the incorporation of zirconium in the crystal lattice of cerium oxide, and therefore obtaining a solid solution e true

Mention may also be made for mixtures of cerium oxide and zirconium oxide based on these two oxides and in addition of scandium oxide or of a rare earth other than cerium, and in particular those decπts in the application. WO 97/43214 whose teaching is incorporated HERE This application describes in particular compositions based on a cerium oxide, a zirconium oxide and a yttπum oxide, or even, in addition to the cerium oxide and zirconium oxide based on at least one other oxide chosen from scandium oxide and rare earth oxides with the exception of cerium, in a cerium / zirconium atomic ratio of at least 1 These compositions have a specific surface after calcination for 6 hours at 900 ° C. of at least 35 m2 / g and an oxygen storage capacity at 400 ° C. of at least 1.5 ml of 02 g.

According to a particular embodiment of the invention, the support is based on cerium oxide and it also comprises silica. Supports of this type are described in patent applications EP-A-207857 and EP-A -547924 whose teaching is incorporated HERE

The total content of manganese, alkaline, and alkaline earth can vary within wide limits. The minimum content is that below which NOx adsorption activity is no longer observed. This content can be compπse in particular between 2 and 50%, more particularly between 5 and 30%, this content being expressed in atomic% relative to the sum of the moles of oxide (s) of the support and the elements concerned of the phase active. The respective contents of manganese, alkaline, and alkaline-earth can also vary within wide proportions, the manganese content can be in particular equal to, or close to that of alkaline or alkaline-earth.

According to an interesting variant of the invention, the alkali is potassium in a content (as expressed above) which can be between 10 and 50% and more particularly between 30 and 50%. The first composition of the compound of the invention can be prepared by a process in which the support is brought into contact with the manganese and at least one other element A or with precursors of manganese and at least one other element A and where the whole is calcined at a temperature sufficient to create a chemical bond between the manganese and the element A. A method which can be used for the abovementioned contact is impregnation. Thus, first of all, a solution or a slip of salts is formed. or of compounds of the elements of the supported phase.

As salts, one can choose the salts of inorganic acids such as nitrates, sulfates or chlorides. It is also possible to use the salts of organic acids and in particular the salts of saturated aliphatic carboxylic acids or the salts of hydroxycarboxylic acids. As examples, mention may be made of formates, acetates, propionates, oxalates or citrates. the support with the solution or the slip. More particularly dry impregnation is used. Dry impregnation consists in adding to the product to be impregnated a volume of an aqueous solution of the element which is equal to the pore volume of the solid to be impregnated. It may be advantageous to deposit the elements of the active phase in two stages. Thus, it is advantageously possible to deposit the manganese in a first step then the element A in a second.

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

The deposition of the active phase can also be done by atomization of a suspension based on salts or compounds of the elements of the active phase and of the support. The atomized product thus obtained is then calcined. The calcination is carried out, as indicated above, at a temperature sufficient to create a chemical bond between the manganese and the element A This temperature varies according to the nature of the element A but, in the case of calcination in air, it is generally at least 600 ° C. more particularly at least 700 ° C., it can in particular be between 800 ° C. and 850 ° C. Higher temperatures are generally not necessary insofar as the chemical bond between the manganese and the element A is already formed but on the other hand, they can lead to a reduction in the specific surface area of the support capable of reducing the catalytic properties of the composition. temperature and it is also fixed so as to be sufficient to create a chemical bond of the elements

The second composition of the compound of the invention will now be described. This composition also comprises a support and an active phase. What has been said above with regard to the active phase of the first composition also applies HERE, in particular as regards concerns the nature of the elements of this phase and their quantity Thus, element B can be more particularly sodium, potassium or barium

The active phase of the second composition can also be based on manganese and at least one rare earth This rare earth can be more particularly chosen from cerium, terbium, gadolinium, samaπum, neodymium and praseodymium The content total manganese, alkaline, alkaline earth or rare earth may vary between 1 and 50%, more particularly between 5 and 30% This content is expressed in atomic% relative to the sum of the moles of oxide (s) of the support and of the elements concerned in the supported phase The respective manganese, alkaline, alkaline-earth or rare earth contents may also vary in large proportions, the manganese content may in particular be equal to, or close to that of element B

The invention covers the case where the active phase consists essentially of manganese and one or more other elements B chosen from alkaline earths and rare earths. By "consists essentially" is meant that the composition of the invention can have a NOx trap activity in the absence in the active phase of any element other than manganese and the element (s) B, such as for example an element of the precious metal type or other metal usually used in catalysis

As indicated above, a characteristic of the second composition is that it has or is capable of having a specific surface of at least 10 m / g after calcination for 8 hours at 800 ° C. This specific surface can be

2 in particular at least 20 m / g after calcination at the same temperature and on the

2 same duration More particularly, this specific surface is at least 80 m / g and

2 still more particularly at least 100 m / g after calcination for 8 hours at 800 ° ^C. The specific surface is understood to mean the BET specific surface determined by nitrogen adsorption in accordance with standard ASTM D 3663-78 established on the basis of the method BRUNAUER - EMMETT- TELLER described in the periodical "The Journal of the American Chemical Society, 60, 309 (1938)"

This surface characteristic is obtained by the choice of a suitable support having in particular a sufficiently high specific surface.

This support can be based on alumina. Any type of alumina capable of having a specific surface sufficient for a catalysis application can be used here. The aluminas resulting from the rapid dehydration of at least one aluminum hydroxide may be mentioned, such as bayeπte, hydrargillite or gibbsite, nordstrandite, and / or at least one aluminum oxyhydroxide such as boehmite, pseudoboehmite and diaspore

According to a particular embodiment of the invention, a stabilized alumina is used. As stabilizing element, mention may be made of rare earths, barium, silicon and zirconium. As rare earth, mention may be made very particularly of cerium, lanthanum or the mixture lanthanum-neodymium

The preparation of the stabilized alumina is carried out in a manner known per se, in particular by impregnating the alumina with solutions of salts, such as nitrates, of the abovementioned stabilizing elements or else by co-drying of an alumina precursor and of salts of these elements followed by calcination One can also cite another preparation of stabilized alumina in which the alumina powder resulting from the rapid dehydration of an aluminum hydroxide or oxyhydroxide is subjected to an operation of ripening in the presence of a stabilizing agent consisting of a lanthanum compound and, optionally, a neodymium compound, this compound possibly being more particularly a salt The ripening can be carried out by suspending the alumina in water then heating at a temperature compπse for example between 70 and 110 ° C. After curing, the alumina is subjected to a heat treatment Another preparation consists of a similar type of treatment but with barium

The stabilizer content expressed by weight of stabilizer oxide relative to the stabilized alumina is generally between 1.5 and 15%, more particularly between 2.5 and 11%.

The support can also be based on silica

It can also be based on silica and titanium oxide in an atomic proportion TI / TI + SI compπse between 0.1 and 15% This proportion can be more particularly compπse between 0.1 and 10% Such a support is described in particular in patent application WO 99/01216, the teaching of which is incorporated HERE

As other suitable support, those based on cerium oxide and zirconium oxide can be used, these oxides possibly being in the form of a mixed oxide or of a solid solution of zirconium oxide in l 'ceπum oxide or vice versa These supports are obtained by a first type of process which comprises a step in which a mixture is formed comprising zirconium oxide and cerium oxide and the mixture thus formed is washed or impregnated by an alkoxylated compound having a number of carbon atoms greater than 2 The impregnated mixture is then calcined

The alkoxylated compound can be chosen in particular from the products of formula (2) Rl - ((CH2) χ-0) _n -R2 in which Ri and R2 represent linear or non-linear alkyl groups, or H or OH or Cl or Br or I, n is a number between 1 and 100 and x is a number between 1 and 4, or those of formula (3) (R3, R-4) - ^ - ((CH2) χ-0) _n -

OH in which φ denotes a benzene ring, R3 and R4 are identical or different substituents on this ring and represent hydrogen or linear or non-alkyl groups having from 1 to 20 carbon atoms, x and n being defined as above, or also those of formula (4) R4θ - ((CH2) χ-O) nH where R4 represents a linear or non-linear alcohol group, having from 1 to 20 carbon atoms, x and n being defined as above, and those of formula (5) R5-S - ((CH2) χ-O) _n -H where R5 represents a linear or non-linear alkyl group having from 1 to 20 carbon atoms, x and n being defined as above We can refer to these products to patent application WO 98/16472, the teaching of which is incorporated HERE

These supports can also be obtained by a second type of process which comprises a stage in which a solution of a cerium salt, a solution of a zirconium salt and an additive chosen from anionic surfactants, non-surfactants, are reacted. ionic, polyethylene glycols, carboxylic acids and their salts, the reaction possibly being able to take place in the presence of a base and / or an oxidizing agent As anionic surfactants, carboxylates, phosphates, sulfates and sulfonates can be used more particularly. Among the nonionic surfactants, it is possible to use ethoxylated alkyl phenols and ethoxylated amines. The reaction between the zirconium and cerium salts can be carried out. by heating the solution containing the salts, it is then a thermohydrolysis reaction It can also be done by precipitation by introducing a base into the solution containing the salts

We can refer for these products to patent application WO 98/45212, the teaching of which is incorporated HERE

The preparation of the second composition can be done with the same methods as those which were given above for the first. It will be noted that after bringing the support into contact with the elements of the active phase, the whole is calcined at a sufficient temperature for these elements to be present in the form of oxides Generally, this temperature is at least 500 ° C., more particularly at least 600 ° C.

The compound of the invention as described above is in the form of a powder but it can optionally be shaped to be in the form of granules, beads, cylinders or honeycombs of variable dimensions The invention relates also a system, usable for trapping NOx, which comprises a coating (wash coat) with catalytic properties and incorporating the compound according to the invention, this coating being deposited on a substrate of the type, for example metallic or ceramic monolith.

This system can be presented in different embodiments. According to a first mode, the system comprises a substrate and a coating consisting of two superposed layers. In this case, the first layer contains the first composition of the compound and the second layer contains the second composition L the order of the layers can be arbitrary, that is to say that the internal layer in contact with the substrate can contain either the first or the second composition, the external layer deposited on the first then containing the other composition of the compound

According to a second embodiment, the compound is present in the coating in the form of a single layer which in this case comprises the two aforementioned compositions, in the form of a mixture, obtained for example by mechanical mixing. realization is possible In this case, the system comprises two juxtaposed substrates, each substrate comprising a coating The coating of the first substrate contains the first composition and the coating of the second substrate contains the second composition The invention further relates to the use of a compound as described above in the manufacture of such a system

The invention also relates to a gas treatment process with a view to reducing the emissions of nitrogen oxides using the compound of the invention.

The gases capable of being treated in the context of the present invention are, for example, those from gas turbines, boilers of thermal power plants or even internal combustion engines. In the latter case, it may especially be diesel engines or engines operating in lean mixture The compound of the invention functions as NOx traps when it is brought into contact with gases which have a high oxygen content. By gases having a high oxygen content, we mean gases. having an excess of oxygen compared to the quantity necessary for the stoichiometric combustion of fuels and, more precisely, of gases having an excess of oxygen compared to the stoichiometric value λ = 1, that is to say the gases for which the value of λ is greater than 1. The value λ is correlated with the air / fuel ratio in a manner known per se, in particular in the field of internal combustion engines. Such gases may be those of an engine operating in a lean burn mixture and which have an oxygen content (expressed by volume) for example of at least 2%, as well as those which have an even higher oxygen content, for example. example of diesel engine gases, that is to say at least 5% or more than 5%, more particularly at least 10%, this content being, for example, between 5% and 20%.

The invention also applies to gases of the above type which may also contain water in an amount of the order of 10% for example. An example will now be given

In this example, the NOx trap evaluation test is carried out as follows:

0.15 g of each of the NOx traps (first composition and second composition) in powder is loaded into a quartz reactor. The powder used has previously been compacted, then ground and sieved so as to isolate the particle size range between 0.125 and 0.250 mm.

The reaction mixture at the inlet of the reactor has the following composition (by volume) - - NO: 300 vpm

- O2: 10% - CO2. 10% - N2 qs 100% The overall flow rate is 30 Nl / h The WH is around 150,000 h ^"1

The NO and NOx signals (NOx = NO + NO2) are permanently recorded as well as the temperature in the reactor

NO and NOx signals are given by an ECOPHYSICS NOx analyzer, based on the principle of chemiluminescence

The evaluation of NOx traps is carried out by determining the total amount of NOx adsorbed (expressed in mgNO / g of trap or active phase) until the trap phase is saturated The experiment is repeated at different temperatures between 250 ° C. and 500 ° C It is thus possible to determine the optimal temperature zone for the operation of the NOx traps

A) First composition

Raw materials

Manganese nitrate Mn (N03) 2.4H ₂ 0 and potassium nitrate KNO3 99.5% are used

The support used is of the type described in Example 4 of patent application WO 97/43214, that is to say a support based on cerium oxide, zirconium oxide and lanthanum oxide but in the respective proportions by weight relative to the oxides of 72/24/4

Preparation of the composition We proceed in two stages for the deposition

1st step . Deposit of the first active element

This step consists in depositing the active element Mn in an amount equal to 10 atomic% and calculated as follows

[Mn] / ([Mn] + [CeO 2] + [Zr0 _2] + [La ₂ θ3]) = 0.10 2'è ^me step Tabled second active element

This step consists in depositing the second active element K in an amount of 10 atomic% and calculated as follows

[K] / ([Mn] + [K] + [Ce0 ₂ ] + [Zrθ2] + [La ₂ θ3]) = 0.10 The deposition is done by the dry impregnation method This method consists in impregnating the support considered with the element of the active phase dissolved in a solution of volume equal to the pore volume of the support (determined with water. 0 5 cm ^ / g) and of concentration making it possible to reach the desired concentration In the present case, the elements are impregnated on the support one after the other.

The operating protocol is as follows

- Dry impregnation of the first element - Drying in the oven (110 ° C, 2H)

- Calcination 2h 600 ° C

- Dry impregnation of the second element

- Drying in the oven (110 ° C, 2H)

- Calcination 2h 800 ° C

B) Second composition

Raw materials

Manganese nitrate Mn (N03) 2.4H ₂ 0 and potassium nitrate KN03 99.5% are used

The support used is an SB3 Condea alumina which has been stabilized by impregnation with 10 atomic% of Ce and then calcination for 2 hours at 500 ° C.

Preparation of the composition

The procedure is carried out in two stages for the deposition in the same manner as for the first composition (dry impregnation, pore volume of the support determined with water of 0.8 cm ³ / g) with the elements Mn and K in the same quantities, the first calcination taking place at 500 ^β C and the second at 800 ° C. The composition thus obtained has a BET surface area of 117 m ² / g.

The results in catalysis are given below

NOx trap activity is therefore observed over the entire temperature range between 250 ° C. and 500 ° C. This activity takes place in the absence of precious metal.

Claims

1- Compound usable as a NOx trap, characterized in that it includes a combination

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, this second composition having or being capable of having a specific surface of at least 10m ² / g, more particularly at least 20m ² / g and even more particularly at least

80m ² / g after 8 hours calcination at 800 ° C

2- Compound according to claim 1, characterized in that the second composition pprréésseennttee ou eesstt ssuusscceeppttiibbllee ddee pprréé: feel a specific surface of at least 100m ² / g after calcination 8 hours at 800 ° C

3- Compound according to claim 1 or 2, characterized in that the elements A and B are chosen from potassium, sodium or barium

4- Compound according to one of the preceding claims, characterized in that the support of the second composition is based on alumina or alumina stabilized by silicon, zirconium, barium or a rare earth

5- Compound according to one of claims 1 to 3, characterized in that the support of the second composition is based on silica

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

7- Compound according to one of claims 1 to 3, characterized in that the support of the second composition is based on céπum oxide and zirconium oxide, this support having been obtained by a process in which one forms a mix comprising zirconium oxide and ceπum oxide and the mixture thus formed is washed or impregnated with an alkoxylated compound having a number of carbon atoms greater than 2

8- Compound according to one of claims 1 to 3, characterized in that the support of the second composition is based on cerium oxide and zirconium oxide, this support having been obtained by a process in which one makes reacting a solution of a cerium salt, a solution of a zirconium salt and an additive chosen from anionic surfactants, nonionic surfactants, polyethylene glycols, carboxylic acids and their salts, the reaction possibly being able to take place presence of a base and / or an oxidizing agent

9- Compound according to one of the preceding claims, characterized in that the support of the first composition is based on an oxide chosen from alumina, cerium oxide, zirconium oxide or their mixtures

10- Compound according to one of the preceding claims, characterized in that the support of the first composition is based on cerium oxide and it further comprises silica

11- System comprising a substrate and a coating incorporating the compound according to one of claims 1 to 10, on this substrate, the compound being present in the coating in the form of a first layer containing the aforementioned first composition and a second layer containing the aforementioned second composition

12- System comprising a substrate and a coating incorporating the compound according to one of claims 1 to 10, on this substrate, the compound being present in the coating in the form of a mixture of the two above-mentioned compositions

13- System comprising two juxtaposed substrates, a coating on each of said substrates and a compound according to one of claims 1 to 10, said compound being present in the coating on the first substrate in the form of the aforementioned first composition and in the coating on the second substrate in the form of the aforementioned second composition

14- A method of treating gas with a view to reducing the emissions of nitrogen oxides, characterized in that a compound according to one of claims 1 to 10 or a system according to one of claims 11 to 13 is used 15- Method according to claim 14, characterized in that one treats an exhaust gas of internal combustion engine.

16- A method according to claim 15, characterized in that a gas having an excess of oxygen relative to the stoichiometric value is treated.

17- Use of a compound according to one of claims 1 to 10 for the manufacture of a system for the treatment of an exhaust gas from an internal combustion engine.