MXPA00004514A - Support composition based on a cerium oxide, a zirconium oxide and a scandium or rare earth oxide and use for treating exhaust gas - Google Patents

Abstract

The invention concerns a composition characterised in that it comprises a support based on cerium oxide, zirconium oxide and scandium or rare earth oxide other than cerium and a supported phase based on manganese and at least another element selected among alkaline, alkaline-earth and rare earth elements. Said composition can used to trap NOx and can therefore be used in a gas treatment process for reducing nitrogen oxides, in particular exhaust gas with excess of oxygen.

Description

COMPOSITION CONTAINING A SUPPORT BASED ON AN OXIDE DE CERIO, A ZIRCONIUM OXIDE AND A SCANDIO OXIDE OR RARE EARTHS AND THEIR USE FOR TREATMENT OF EXHAUST GASES FIELD OF THE INVENTION The present invention relates to a composition containing a support based on a cerium oxide, a zirconium oxide and a scandium or rare earth oxide and its use for the treatment of exhaust gases. BACKGROUND OF THE INVENTION It is known that the emissions of nitrogen oxide (NOx) in exhaust gases from motor vehicles are reduced, in particular, with the aid of three-way catalysts, which use stoichiometrically the reduction of gases present in the vehicle. mix. Any excess of oxygen leads to a pronounced deterioration in the performance of the catalyst. However, certain engines, for example, diesel engines or engines that depend on the burning of gasoline, save fuel but emit exhaust gases that permanently contain a large excess of oxygen, for example at least 5%. "This case is not effective as a standard three-way catalyst for NOx emissions. In addition, it has become imperative to limit NOx emissions due to the rigidity of the emission regulations of motor vehicles that have now been extended to these engines. There is therefore a genuine need for an efficient catalyst to reduce NOx emissions for this type of engines and, in general, to treat gases containing NOx. As a type of catalyst that can meet this need, systems referred to as NOx collectors have been proposed, which are capable of oxidizing NO into N02 and then absorbing eT N02 thus formed. Under certain conditions, the N02 is re-released after being reduced to N2 by reduction species contained in the exhaust gases. These NOx collectors are generally based on platinum. However, platinum is an "expensive" element Therefore, it would be beneficial to provide a platinum-free system in order to reduce the costs of the catalysts SUMMARY OF THE INVENTION The object of the invention is therefore to "develop a catalyst". that can be used as a NOx collector without the need to use plat. For this purpose, the composition of the invention is characterized in that it comprises a support based on a cerium oxide, a zirconium oxide and a scandium oxide or rare earths other than cerium and a supported phase based on manganese and minus another element selected from alkali metals, alkaline earth metals and rare earth metals. The invention also relates to a process for treating gases with a perspective to reduce nitrogen oxide emissions which are characterized in that a composition as defined above is used. Other features, details and advantages of the invention will be more fully apparent upon reading the following description, as well as the various concrete but non-limiting examples proposed to illustrate it DETAILED DESCRIPTION OF THE INVENTION The composition of the invention comprises a phase supported on a support The supported phase may correspond more "particularly to two variants. According to a first variant, this phase is based, in addition to manganese, on an alkaline metal and / or an alkaline earth metal. The alkali metal, can more particularly be sodium or potassium. The metaL to the inoterreo lime can more particularly be barium or strontium. - According to a second variant, the supported phase is based on manganese and at least one element-selected from the rare earths. Here and for the entire description, the term rare earths is proposed to mean the elements in the group consisting of yttrium and the elements in the Periodic Table that have an atomic number between 57 and 71 inclusive. The rare earths can more particularly be selected from lanthanum, cerium, praseodymium, neodymium, europium, samarium, gadolinium or terbium. As an advantageous embodiment in the purpose of this second variant, mention may be made of a supported phase based on manganese and praseodymium. Finally, it is entirely possible wn the scope of the present invention to have a "supported phase" based on manganese and at least two other elements, one being a rare earth and the other being selected from alkaline metals and alkaline alkaline metals. to a particular embodiment, the composition of the invention can be obtained by a process in which at least one of the two elements manganese and potassium is at least partially replaced by potassium permanganate.It should be noted that a single element can be replaced by the permanganate and only partially, it is also possible to substitute the two elements completely by the permanganate route, all variants between these two possibilities can be contemplated.This modality makes it possible to obtain compositions having high NOx absorption capacities. Another important feature of the composition of the invention is the nature of the support the supported phase. As indicated above, the support is based on a cerium oxide, a zirconium oxide and a scandium or rare earth oxide other than cerium. This is emphasized herein and throughout the description, that the invention also applies to any support based on cerium oxide, zirconium oxide and as a third element, a combination of two or more selected oxides of scandium oxide and oxides of rare earths. The supports used are preferably those for which the atomic ratio cerro / zirconium is at least 1. As a rare earth included in the composition of the support, mention may be made more particularly of lanthanum, neodymium and paraseodymium. More particularly use can also be made of the support satisfying the complete formula CexZryMz02 where M represents at least one element selected from the group comprising eJTcandio ^ rare earths other than cerium and where x, y, and z satisfy the ratio 0 <; < 0.3, 1 < xZ / ~ y < 19 and x + y + z = 1. More particularly, x, y, and z can satisfy the following relations 0.02 < < 0.2, 1 < _x / y < 9, being possible, even more "" part icully "for the last proportion to be between 1.5 and 4, including these limits.According to a particular modality, the support is in the form of a" solid "solution. In this case, the X-ray diffraction spectrum of the support reveals the existence of a single homogeneous phase within it. As for the supports that are rich in cerium, this phase corresponds to that of a cerium oxide crystallized Ce02 whose reticular parameters are displaced to a greater or lesser degree in relation to a pure cerium oxide, thus reflecting the "incorporation of "zirconium and another element (scandium and rare earths other than pig) in the crystalline reticle of cerium oxide and hence the fact that a genuine solid solution is obtained. According to a preferred variant of the invention, supports are used that are characterized by their surface -specific at certain temperatures, as well as their capacity to store oxygen.The term "specific surface" is proposed to mean the specific surface BET determined by the absorption of nitrogen according to ASTM D 3663-78 standard established on the basis of the Braunauer ~ Emmett-Teller method described in the periodical publication "The Journal of the American Society, 6_0_, 309 (1938)." It is thus possible to use supports having a specific surface after calcination for 6 hours at 900 ° C of at least 35 m2 / g.This surface can more particularly be at least 40 m2 / g.It can still be more particularly at "less than 45 m2 / g. These supports can also have surfaces that are still considerable even after being calcined for 6 hours at 1000 ° C. This surface can be at least 14 m2 / g, more particularly at least 20 m2 / g and still more particularly at least 30 m2 / g. Another characteristic of the support of this variant is its ability to store oxygen. This capacity, measured at 400 ° C, is at least 1.5 ml 02 / g. It can be more particularly at least 178 ml 02 / g and still more particularly at least 2 ml 02 / g. In the best cases, this capacity can be at least 2.5 ml 02 / g. This capacity is determined by a test that evaluates the capacity of the support or the product, to successfully oxidize quantities of carbon monoxide injected with oxygen and consume the injected amounts of oxygen to reoxidize the product. The method used is referred to as an alternative method. The carrier gas is pure helium at a flow rate of 10"1 / hr The injections are carried out by means of a cycle containing 16 ml of gas.The amounts of CO are injected using a gas mixture containing 5 ml of gas. % of CO diluted in helium, while the quantities of 02 are injected using a gas mixture containing 2.5% of 02 diluted in helium .. Gases are analyzed by chromatography using a thermal conductivity detector.The amount of oxygen consumed makes It is possible to determine the oxygen storage capacity The characteristic value of the energy stored in the oxygen is expressed in ml of oxygen (under standard temperature and pressure conditions) per gram of product introduced and measured at 400 ° C. Oxygen storage capacity measurements are given here and in the rest of the description, it is taken from the pre-treated products at 900 ° C under air for 6 hours in a muffle furnace. of the invention can be prepared in the known manner. In this way they can be obtained by using a solid / solid reaction of the oxides or any other precursor such as carbonates. They can also be prepared by a wet route, which is to say by precipitation with a base of cerium salts, zirconium and the third element or elements, then calcining them.

In the case of the preferred variant described above using supports defined by their specific surface area and their oxygen storage capacity, the support can be obtained by a process in which a mixture is prepared in a liquid medium containing a cerium compound. , a scandium or rare earth compound and a zirconium solution, which is such that the amount of base necessary to reach the equivalence point during an acid-base titration of this solution satisfies the condition of molar ratio OH / Zr < 1.65; the aforementioned mixture is heated; the precipitate obtained is recovered and this precipitate is calcined. This process will now be described more specifically. The first stage of this process consists in preparing a mixture in a liquid medium, generally in the aqueous phase, containing at least one cerium compound, at least one zirconium compound and a scandium or rare earth compound. This mixture is prepared using a zirconium solution. The zirconium solution can be produced - by acid attack on a reagent containing zirconium. Examples of suitable reagents include carbonate hydroxide or zirconium oxide. The attack can be carried out using an inorganic acid such as nitric acid, hydrochloric acid or sulfuric acid. Nitric acid is the preferred acid and the use of a zirconyl nitrate produced by the nitric attack in a zirconium carbonate can thus be more particularly mentioned. The acid can also be an organic acid such as acetic acid or citric acid. This zircon solution must have the following characteristics. The amount of base necessary to reach the equivalence point during an acid-base titration of this solution must satisfy the condition of molar ratio OH / Zr < 1.65. More particularly this proportion can be at most 1.5 or even more particularly, at most 1.3. In general, the specific surface area of the product obtained tends to increase as this proportion decreases. The acid-base titration is carried out in a manner that is known. In order to perform it under optimum conditions, a solution which has been adjusted to a concentration of about 3.10 2 mol per liter, expressed in terms of the zirconium element, can be titrated.While stirring, a solution of sodium hydroxide is added to it. Under these conditions, the determination of the equivalence point (change of pH of the solution) takes place cleanly This equivalence point is expressed by the OH / Zr ratio The particular examples of cerium compounds that can be mentioned include salts of cerium salts such as cerium (IV) salts, such as ceric nitrates or ammonium nitrates, for example, which are here particularly suitable, Cericium nitrate is preferably used.The solution of cerium (IV) salts may contain cerium in the cerus but it is preferable that it contains at least 85% cerium (IV). For example, an aqueous solution of ceric nitrate can be obtained by reacting nitric acid with a ceric hydrated oxide, prepared conventionally by reacting a cerus salt solution for example nitrate cerus with a solution of ammonia in the presence of hydrogen peroxide. It is also possible to use a ceric nitrate solution which is obtained according to the process including electrolytic oxidation of a cerus nitrate solution, as described in FR-A-2 570 087 and which can be an advantageous raw material. Here it will be noted that the aqueous solution of cerium (IV) salts can have some degree of initial free acidity, for example a normality varies between 0.1 and 4 N. According to the present invention, it is equally possible to employ an initial solution of cerium (IV) salts that currently have some degree of free acidity, as mentioned above, as well as a solution that has previously been neutralized, more or less effectively, by adding a base such as, for example, a solution of ammonia or alkali metal hydroxides (sodium, potassium, etc.) but preferably a solution of ammonia, to limit this acidity. It is then possible, in the previous case, to define practically a degree of neutralization (r) for the initial cerium solution by means of the following equation in which neither represents the total number of moles of Ce (IV) present in the solution after neutralization; n2 represents the number of moles of OH ions currently needed to neutralize the initial free acidity contributed by the aqueous solution of the cerium (IV) salt; and n3 represents the total number of moles of OH ions contributed by the addition of the base. When the "neutralization" variant is used, the amount of base used in all cases will necessarily be less than the amount of base necessary to obtain the total precipitation of the hydroxide species Ce (OH) 4 (r = 4). In practice this will include a limitation of degree of neutralization of not more than 1, and more preferably not more than 0.5. Scandium or rare earth compounds are preferably compounds that are particularly soluble in water. As examples of scandium or rare earth compounds which can be used in the process in question, for example, mention may be made of salts of inorganic or organic acids, for example of the type of sulfate, nitrate, chloride or acetate. nitrate is particularly well suited.These compounds can also be supplied in the form of sols.These sols can be obtained, for example, by neutralizing a salt of these compounds using a base.The amount of cerium, zirconium and rare earth or scandium present in The mixture must correspond to the required stoichiometric proportions to obtain a support with the desired final composition - Once the initial mixture has been obtained in this way, it is then heated, according to the second stage of the process in question. which is carried out this thermotreatment, also referred to as thermohydrolysis, can be between 80 ° C and the critical temperature of the reaction medium, in particular between 80 and 350 ° C, preferably between 90 and 200 ° C. I Depending on the temperature conditions adopted, this treatment can be carried out either under normal atmospheric pressure or under a pressure such as, for example, the saturated vapor pressure corresponding to the temperature of the heat treatment. When the temperature of the treatment is selected above the reflux temperature of the reaction medium (ie in general above 100 ° C), for example, selected between 150 and 350 ° C, the "operation" is carried out when introducing the aqueous mixture containing the aforementioned species in a sealed vessel (closed reactor, more commonly referred to as an autoclave) in which case the pressure required results only from the heating of the reaction medium (autogenous pressure). Under the temperature conditions given above and in an aqueous medium, it may be indicated by way of illustration, that the pressure in the closed reactor varies between a value in excess of 1 bar (105 Pa) and 165 bar (165.105 Pa), preferably between 5 bar (5,105 Pa) and 165 bar (165,105 Pa). Of course it is also possible to exert an external pressure that is then added to that due to heating.

- - The heating can be carried out either under an air atmosphere or under an atmosphere of inert gas, preferably nitrogen. The treatment time is not critical and can thus vary within wide limits, for example between 1 and 48 hours, preferably between 2 and 24 hours.At the end of the heating stage, a solid precipitate is recovered which can be separated from its It can be advantageous to introduce a base, for example a solution of ammonia, into the precipitation medium after the heating step by means of any conventional solid / liquid separation technique, for example filtration, decanting, drying or centrifugation.

This makes it possible to increase the yields with which precipitated species are recovered. It is also possible to add hydrogen peroxide in the same way, after the heating stage. The product, as it recovers, it can then be washed with water and / or aqueous ammonia, at a temperature between room temperature and boiling point. In order to remove the waste water, the washed product can finally, if appropriate, be dried for example to air, doing this at a temperature that can vary between 80 and 300 ° C, preferably between 100 and 150 ° C, continuing the drying to obtain a constant weight. It will be noted that, of course, it is possible for a heating step as described above, to repeat it one or more times, in an identical or different manner after the recovery of the product and the optional addition of the base or hydrogen peroxide, in which case the product is returned to a liquid medium, in particular in water and for example to carry out ermotreatment cycles. In a final stage of the process, the recovered precipitate is calcined, after washing and / or optional drying. According to a particular embodiment, after the thermohydrolysis treatment and optionally after returning the product to a liquid medium and an additional treatment, it is possible to dry the obtained reaction medium directly by spraying. ~ The calcination is carried out at a temperature in general between L200 and -1200 ° C and preferably between 300 and 900 ° C. JThis calcination temperature must be sufficient to convert the precursors- into oxides and is also selected in the bases of future operating temperature of the support and taking into account the fact that the specific surface of the product becomes commensurably lower as the temperature of calcining used is increased. For its part, the calcination time can vary within wide limits, for example between 1 and 24 hours, preferably between 4 and 10 hours. The calcination is generally carried out under air, but the calcination which is carried out, for example, under an inert gas is clearly not discarded The supported phase can be deposited on the support in a known manner The method used can comprise An impregnation method An aqueous solution or suspension of salts or compounds of the elements in the supported phase will first be formed.Examples of salts that can be selected include salts of inorganic acids, such as nitrates, sulfates or chlorides. salts of organic acids and in particular salts of saturated aliphatic carboxylic acids or salts of hydrocarboxylic acids, examples which may be mentioned include formats, acetates, propionates, oxalates or citrates.The support is then impregnated with the aqueous solution or suspension. the impregnation, the support is optionally dried and then calcined. It is possible to use a support that has not yet been calcined before impregnation. The supported phase can also be deposited by atomizing a suspension based on salts or compounds of the elements of the supported phase and of the support. It may be advantageous to deposit the elements of the supported phase in two stages. Thus in the case of supported phases based on manganese and potassium and manganese and praseodymium reciprocally, it can advantageously be deposited in the first case the manganese then the potassium and in the second case the praseodymium then the manganese. As indicated above, for the particular embodiment that applies to the case in which the supported phase comprises manganese and potassium, at least one of the elements manganese and potassium can be at least partially replaced by potassium permanganate. Finally, it should be noted that it is possible within the scope of the present invention for at least one of the elements of the supported phase to enter the support during the actual preparation of the latter. _ The levels of manganese, alkali metals, alkaline earth metals and rare earths can vary in a large proportion. The minimum proportion is that below which NOx adsorption activity no longer occurs. They can be in particular between 2 and. 50% and more particularly between 5 and 30%, these levels being expressed as atomic% with respect to the sum of the elements of the support and of the relevant elements for the supported phase. The compositions of the invention, as described above, are in the form of powders but can be optionally shaped to be in the form of granules, spheres, cylinders, honeycombs of varying sizes.The compositions can thus be used in catalyst systems comprising a thin coating having catalytic properties and based on these compositions, on a substrate of, for example, the metallic type or ceramic monolith.The invention also relates to a process for treating gases with a focus to "reduce emissions". of nitrogen oxide using the compositions of the invention. The gases that can be treated by the present invention are, for example, those gas turbine outputs, thermal power combustion chambers or alternatively internal combustion engines. In the latter case, these may be in particular diesel engines or engines that depend on combustion.

When these were brought into contact with gases having a high oxygen level, the composition of the invention functioned as a Nox collector. The term gases having a high oxygen level is proposed to mean gases having an excess of oxygen with respect to the amount necessary for the stoichiometric combustion of fuels and, more precisely, gases having an excess of oxygen with respect to the value estequimétrico? = 1. The value? it correlates with the air / fuel ratio in the manner that is known per se, particularly in the field of internal combustion engines. Such gases are those coming from an engine that depends on the combustion that has an oxygen level (expressed in volume) of at least 2%, as well as those that still have a high level of oxygen, for example gases coming from engines of the type diesel, ie at least 5% or more than 5%, more particularly 10%, it being possible for this level to be, for example, between 5 and 20%. The compositions of the invention can be associated with complementary emission control systems, such as three-way catalysts, which are efficient when the value of? is less than or equal to 1 in the gases, or alternatively in systems that include fuel injection or exhaust gas recirculation (EGR) for diesel engines. It can also be associated with Nox catalysts for diesel engines. The invention also relates to a catalytic system for treating gases with an approach for reducing nitrogen oxide emissions whose gases can be of the aforementioned type and, more particularly, those having an excess of oxygen relative to the stoichiometric value. The system is characterized in that it comprises a composition as described above. Examples will now be given EXAMPLES Preparation of the support _ "A support based on cerium oxide, zirconium oxide and lanthanum oxide is used in respective weight proportions of 72/24/4 with respect to the oxides. as follows: In the proportions-stoichiometric required to obtain the above composition, a preneutralized ceric nitrate solution is mixed by adding NH40H so that r = -0.22 (where r is defined above), a nitrate solution of Lanthanum and a solution of zirconium nitrate which, in the sense defined above, satisfies the condition of molar ratio OH '/ Zr = 1.17, are mixed in. The concentration of this mixture (expressed as oxides of the various elements) is adjusted to 80g. This mixture is then heated to 150 ° C for 4 hours, then a solution of ammonia is added to the reaction medium so that the pH is greater than 8.5.The reaction medium thus obtained is heated. ta boiling for 2 hours. After the sedimentation is then extracted, the solid product is re-suspended and the medium thus obtained is treated for 1 hour at 100 ° C. Then the product is filtered. The product proposed to be used as a support, for the rest of the examples, is calcined at a temperature of 550 ° C for 2 hours. It should be noted that the product calcined 900DC for 6 hours has a specific surface area of 51 m2 / g. Its oxygen storage capacity measured under the conditions given above is 2.8 ml 02 / g. catalyst preparation Raw materials: Manganese nitrate Mn (N03) 2, praseodymium nitrate Pr (N03) 3, potassium nitrate KN03, potassium permanganate KMn04 and barium acetate Ba (C2H302) 2. Elements of the Supported phase, level and deposition: The procedure for the deposition includes two stages. 1st stage: The deposition of the first active element. This step consists of depositing the active element in an amount that varies between 5 and 10 of atomic% in the case of Mn and 15 of atomic% in the case of Ba and Pr, and is calculated as follows: - [X] / ([X] + [Ce02] + [Zr02] + [La203]) = 0.05 or 0.1 with X = Mn [X] / ([X] + [Ce02] + [Zr02] + [La203]) = 0.15 with X = Pr, Ba 2 a Stage: Deposition of the second ^ element ^ active. This stage consists of the deposition of the second active element, in an amount that varies between 15 and 20 of atomic% in the case of K, and of 5 of atomic% in the case of Mn and is calculated as follows: [Y ] / ([X] + [Y] + [Ce02] + [Zr02] + [La203]) = 0.15 or 0.2 with Y = K [Y] / ([X] + [Y] + [Ce02] + [ZrO + [La203]) = 0.05 _ with Y = Mn In the case where potassium permanganate (KMn04) is used as a precursor, the deposition will be carried out in a single stage. The number of moles deposited is equal to 13.6% calculated as follows: [X] / ([X] + [Ce02] + [Z r02] + [La203]) = 0. 13 6 with X = KMnO.

The two methods of deposition are used: dry impregnation for Example 1 and atomization of Bucki® in the other case. Dry impregnation: ~~ This involves impregnating the support in question with the element of the supported phase dissolved in a solution whose volume is equal to the pore volume of the support (determined with water: 0.5 cm3 / g) and whose concentration makes it possible to achieve the desired concentration. In the present case, the elements are impregnated in the support one after the other. The operation protocol is as follows: - dry impregnation of the first element - drying in an oven (110 ° C, 2h ") -calcination for 2 h at 600 ° C (5 ° C / min.) - dry impregnation of the second element - drying in an oven (110 ° C, 2h) -calcination for 2h at 600 ° C. (5 ° C / min.) Atomization of Buchi < A suspension is prepared containing the element to be deposited and the support (C = 150 g / l) The suspension is then atomized by Buchi.The dried solid is calcined for 2 hours at 600 ° C (raise the proportion 5 ° C / min. .) The operation is repeated with the second element to be deposited Buchi® inlet and outlet temperatures are respectively equal from 210 ° C to 120 ° C. - Catalytic test The catalytic test is carried out in the following way: 0.15 g of the NOx collector is introduced in the form of powder in a quartz reactor The powder used has been previously compacted and milled and holed in order to isolate the fraction of particle size between 0.125 and 0.250 mm. at the entrance of the reactor has the following composition (by volume): - NO: 3 0 0 vpm - 02: 1 0% - C02: 1 0% - H20: 1 0% - N2: sufficient amount for 100% The total speed of flow-is 30 l (stp) / h.The speed of space per hour is of the order of 150.0 00 h "1. The NO and NOx signals (NOx = NO + N02) are continuously recorded, according to the temperature in the reactor. The NO and NOx signals are given by a NOx Ecophysics analyzer based on the principles of chemiluminicence. The evaluation of the NOx collector is in two parts: Firstly, the maximum absorption temperature Tmax is determined, by absorbing the NOx at 125 ° C for 15 minutes, and subsequently heating it with the same mixture at 600 ° C. The NOx profile shows a maximum absorption at a certain temperature, referred to as Tmax. • Secondly, the amount isothermally adsorbed (according to the number of moles of NO) at the maximum adsorption temperature The amount is calculated by integration.

• Results The results are given in the following table It can be noted that Tmax can be equal to or greater than 300 ° C for certain compositions

Claims (13)

1. CLAIMS 1.
2. The composition characterized in that it comprises a support based on a cerium oxide, a zirconium oxide and a scandium or rare earth oxide other than cerium, and a supported phase based on manganese and at least one other element selected from metals alkaline, alkaline earth metals and rare earths. - - The composition according to claim 1, characterized in that the other elements mentioned above are selected from alkali metals and alkaline earth metals, being more particularly possible for the alkali metal to be potassium, and being more particularly "possible for the alkaline earth metal being barium
3. 3. The composition according to claim 1, characterized in that the other elements mentioned above are selected from the rare earths,
4. 4. The composition according to claim 1 or 3, characterized in that the supported is based on manganese and praseodymium.
5. 5. The composition according to one of the preceding claims, characterized in that the atomic ratio cerium / zirconium in the support is at least 1.
6. The composition according to one of the preceding claims, characterized in that the support has a specific surface after the I calcination for 6 hours at 900 ° C of at least 35 m2 / g, more particularly of at least 40 m2 / g.
7. The composition according to one of the preceding claims, characterized in that the support has an oxygen storage capacity at 400 ° C of at least 1.5 ml 02 / g, more particularly at least 2 ml 02 / g.
8. 8. The composition according to claim 1, characterized in that the support satisfies the formula CexZryMz02 wherein M represents at least one element selected from the group comprising scandium and the rare earths other than cerium, and wherein < z < 0.3, and more particularly.

   0. 02 < < 0.2 and 1 < x / y < 19 and more particularly 1 < x / y < 9, and x, y, and z are connected by the equation x + y + z = l.
9. 9. The composition according to one of the preceding claims, characterized in that the support is in the form of a solid solution. The composition according to one of the preceding claims, characterized in that the rare earths included in the composition of the support are lanthanum, neodymium or paraseodymium. The composition according to one of the preceding claims, characterized in that the support is obtained by a process in which a mixture is prepared in a liquid medium containing a cerium compound, a scandium or rare earth compound and a zirconium solution, which is such that the amount of base necessary to reach the equivalence point during an acid-base titration of this solution satisfies the condition of molar ratio OH / Zr < 1.65; the aforementioned mixture is heated; the precipitate obtained is recovered and this precipitate is calcined. 12. The composition according to one of the preceding claims, characterized in that the catalytic phase comprises manganese and potassium and that it can be obtained by a process in which at least one of the two elements manganese and potassium is supplied at least partially by potassium permanganate 13. The process for treating gases with a "perspective to reduce nitrogen oxide emissions, characterized in that a composition according to one of the preceding claims is used 14 The process according to the claim 13 characterized in that an exhaust gas from an internal combustion engine is treated. The process according to claim 14, characterized in that a gas having an excess of oxygen is treated in relation to the stoichiometric value. The process according to one of claims 14 and 15, characterized in that the oxygen level in the gases is at least 5% by volume 17. The catalytic system for the treatment of an exhaust gas from combustion engines internal, characterized in that it comprises a composition according to one of claims 1 to 12. 18. The catalytic system according to claim 17, for the treatment of a gas having an excess of oxygen in relation to the stoichiometric value.