MXPA00005076A - COMPOSITION BASED ON MANGANESE AND USE FOR TRAPPING NOx FOR TREATING EXHAUST GASES - Google Patents

Abstract

The invention concerns compositions for trapping NOx. In a first embodiment, the composition comprises a supported phase containing manganese and at least another element selected among terbium, gadolinium, europium, samarium, neodymium and praseodymium and a support based on cerium oxide or a mixture of cerium oxide and zirconium oxide. In a second embodiment, the composition comprises a supported phase and a support of the same type as for the first embodiment. The invention is characterised in that manganese and potassium are brought by potassium permanganate. In another embodiment, the composition consists essentially of manganese and cerium oxide. Said compositions can be used in a process for treating gases for reducing nitrogen oxide emission.

Description

MANGANESE-BASED COMPOSITION AND ITS USE AS A NOx COLLECTOR FOR THE TREATMENT OF EXHAUST GASES TECHNICAL FIELD The present invention relates to a composition based on manganese and its use for the treatment of exhaust gases. BACKGROUND It is known that the emissions of nitrogen oxide (NOx) in exhaust gases from motor vehicles are reduced, in particular, with the help of three-way catalysts, which stoichiometrically use the reduction gases present in the mixture. 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%. Therefore, in this case, a standard three-way catalyst for NOx emissions is not effective. 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 in N02 and then absorbing the 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 platinum. For this purpose, and according to a first embodiment of the invention, the composition according to the invention comprises manganese, cerium oxide, or a mixture of cerium oxide and zirconium oxide and is characterized in that it also contains at least one other Selected element of terbium, gadolinium, europium, samarium, neodymium and praseodymium. According to a second embodiment, the composition according to the invention comprises manganese, and potassium, and cerium oxide, or a mixture of cerium oxide and zirconium oxide and is characterized in that it can be obtained by a process in which the minus one of the two elements of manganese and potassium is at least partially supplemented by permanganate of potassium io. The invention also relates to a process for treating gases with a perspective to reduce nitrogen oxide emissions when collecting said oxides, characterized in that a composition consisting essentially of manganese and cerium oxide is used. Finally, the invention relates to a process of the same type used by the compositions according to the two modalities mentioned above. 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 is characterized by the nature of the elements that form it and which were mentioned above. It will be noted here that in this composition the cerium oxide or the mixture of cerium oxide and zirconium oxide can form a support, the other elements forming a supported phase. This means that the cerium oxide or the mixture of cerium oxide and zirconium oxide can form the majority of the element or elements of the composition, on which element or elements the other elements are deposited. For simplicity, in the rest of the description, reference will be made to the support and the supported phase, but it will be understood that, if an element described as belonging to the supported phase was present in the support, for example, when entering it during the current preparation of the support, this will not depart from the scope of the present invention. In the scope of the first embodiment mentioned above, the composition may comprise a supported phase which is based on manganese in combination with terbium, gadolinium, samarium, neodymium or prasedimium, or alternatively a mixture of manganese and at least two of these elements. According to a variant of this first embodiment, the composition may further comprise an alkali metal which may more particularly be sodium or potassium. This alkali metal element can belong to the supported phase. In the second embodiment, the composition may comprise a supported phase which is based on manganese in combination with potassium. In addition, at least one of the two elements manganese and potassium is at least partially supplied by potassium permanganate during the process of preparing the composition. It should be noted that a single element can be replaced by the permanganate and only partially. Conversely and preferably, it is also possible to replace the two elements completely by the permanganate route. All the variants between these two possibilities can be contemplated. This embodiment makes it possible to obtain compositions having high NOx absorption capacities. The amounts of elements in the supported phase of the composition can vary in wide proportions. The minimum proportion is that below which NOx adsorption activity is no longer observed. The proportions of manganese can thus vary between 2 and 50%, more particularly between 5 and 30%. The proportions of terbium, gadolinium, samarium, neodymium, praseodymium and / or potassium can vary between 1 and 50%, more particularly between 5 and 30%. These proportions are expressed as% atomic with respect to the sum of the support and the relevant elements for the supported phase. It is pointed out here and throughout the description that manganese and other elements are present in the form of oxide in the compositions described. The supports based on cerium oxide or a mixture of cerium oxide and zirconium oxide are well known. More particularly, with respect to mixtures of cerium oxide and zirconium oxide, of those described in patent applications EP-A-605274 and EP-A-735984, the teachings of which are incorporated herein can be mentioned. . More particularly use can be made of supports based on cerium and zirconium oxides in which these oxides are present in the cerium / zirconium atomic ratio of at least 1. With respect to these same supports, those found 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 regards the supports that are rich in cerium, this phase really corresponds to that of a cubic 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 in the crystalline lattice of cerium oxide and hence the fact that a genuine solid solution is obtained. According to a variant of the invention, supports are used that are characterized by their specific surface at certain temperatures, as well as their capacity to store oxygen. The term specific surface is proposed to mean the BET specific surface area determined by nitrogen adsorption according to ASTM D 3663-78 standard established on the basis of the Brunauer-Emmett-Teller method described in the periodical publication "The Journal of the American Society, 60_, 309 (1938). " It is thus possible to use supports based on cerium oxide and zirconium oxide in a cerium / zirconium atomic ratio of at least 1 and having a specific surface area after calcination for 6 hours at 900 ° C of at least 35 hours. m2 / g. Another characteristic of the supports of this variant is its oxygen storage capacity. This ability, measured at 400 ° C is at least 1.5 ml 02 / g.

It can be more particularly at least 1.8 ml 02 / g. This capacity is determined by a test that evaluates the ability of the support or product to oxidize successfully amounts 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 / h. 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% CO diluted in helium, while the amounts of 02 are injected using a gas mixture containing 2.5% 02 diluted in helium. The gases are analyzed by chromatography using a thermal conductivity detector. The amount of oxygen consumed makes it possible to determine the storage capacity of oxygen. The characteristic value of the energy stored in oxygen is expressed in ml of oxygen (under standard temperature and pressure conditions) per gram of product introduced and measured at 400 ° C. The measurements of oxygen storage capacity given here and in the remainder of the description were taken from pre-treated products at 900 ° C under air for 6 hours in a muffle furnace. In the case of the variant described above, which uses 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 and a solution of zirconium, 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 and a zirconium compound. This mixture is prepared using a zirconium solution. This zirconium solution can be produced by acid attack in 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 nitric attack in a zirconium carbonate can 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 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 approximately 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) clearly takes place This equivalence point is expressed by the molar ratio OH / Zr The particular examples of cerium compounds that can be mentioned include salts cerium salts such as cerium (IV) salts, such as ceric nitrates or nitrates of ammonium, for example, which are particularly suitable here.Cornium nitrate is preferably used.The solution of cerium (IV) salts may contain cerium in the state 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 hydrous ceric oxide, prepared conventionally by reacting a solution of a cerus salt, for example cerus nitrate, 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 also possible to use an initial solution of salts of cerium (IV) that actually 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 hydroxides of alkali metal (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: n3-n2 neither in which nor represents the total number of moles of Ce (IV) present in the solution after neutralization; n2 represents the number of moles of OH ions actually necessary 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. The amount of cerium and zirconium present in the mixture must correspond to the tequistometric ratios required 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. The temperature at which this heat treatment is carried out, 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. 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 treatment temperature 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 mixture aqueous 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 step, a solid precipitate which can be separated from its medium is recovered by any conventional solid / liquid separation technique, for example filtration, decantation, desiccation or centrifugation. It may be advantageous to introduce a base, for example an ammonia solution, into the precipitation medium after the heating step.

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 is recovered, 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 heat treatment cycles are carried out. 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 thermohydride treatment and optionally after returning the product to a liquid medium and an additional treatment, it is possible to dry the directly obtained reaction medium by aspersion. The calcination is carried out at a temperature in general between 200 and 1200 ° C and preferably between 300 and 900 ° C. This 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 the Calcination employed increases. 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 that is carried out, for example, under an inert gas clearly is not discarded. It should be noted that in the case of a support based on a cerium oxide, it is possible to use a support which in addition to the cerium oxide comprises a specific surface stabilizer which is chosen from the group of rare earths. 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 praseodymium, terbium and lanthanum. It should be noted that it is thus possible to have a composition according to the invention comprising rare earths both in the supported phase and in the support. In the process for preparing the compositions of the invention comprising a support and a supported phase, the supported phase can be deposited on the support in a known manner. The method used may comprise a method of impregnation. An aqueous solution or suspension of salts or compounds of the elements in the supported phase will thus be formed first. Examples of salts that can be selected include salts of inorganic acids, such as nitrates, sulfates or chlorides. It is also possible to use salts of organic acids and in particular salts of saturated aliphatic carboxylic acids or salts of hydrocarboxylic acids. Examples that may be mentioned include formats, acetates, propionates, oxalates or cyclasts. "The support is then impregnated with the aqueous solution or suspension After the impregnation, the support is optionally dried and then calcined. support that has not yet been calcined before impregnation.

Dry impregnation is more particularly used. Dry impregnation consists in adding the product to be impregnated, a volume of an aqueous solution of the element that is equal to the pore volume of the solid to be impregnated. In the case of the second embodiment described above, potassium permanganate is very clearly used as the manganese and potassium salt. If appropriate, the complementary portion of manganese and / or potassium can be supplied by salts of the type described above. In the case of compositions whose supported phase contains a rare earth, it may be advantageous to first deposit the rare earth, followed by the manganese. The compositions of the invention, as described above, are in the form of powders but can optionally be 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 or ceramic monolith type. The invention also relates to a process for treating gases with an approach to reduce nitrogen oxide emissions employing the compositions of the invention. As indicated above, the invention further relates to the use in this treatment of a composition consisting essentially of manganese oxide and cerium. The term "consists essentially" is proposed to mean that this composition may have catalytic activity (ie, in this case, NOx collecting activity) in the absence of any element other than manganese oxide and cerium. It will be noted that, in the case of using a composition consisting essentially of manganese oxide and cerium, the cerium oxide can form a support with the manganese forming a supported phase. The gases that can be treated by the present invention are, for example, those emitted by gas turbines, thermal plant 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 contacted with gases having a high oxygen level, the composition of the invention functioned as NOx collectors. The term gases having a high level of oxygen 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 a 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 higher level of oxygen, for example gases coming from engines of the diesel type, 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? it 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. The invention also relates to a catalytic system for treating gases with an approach to reduce 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. This system comprises a composition as described above. Now we will give examples. In these examples the catalytic test is carried out as follows: 0.15 g of the NOx collector is introduced in powder form in a quartz reactor. The powder used has been previously compacted and ground and holed in order to isolate the fraction of the particle size between 0.125 and 0.250 mm. The reaction medium at the reactor inlet has the following composition (by volume): - NO: 3 0 0 vpm - 02: 1 0% - C02: 1 0% - H20: 1 0% - N2: sufficient quantity for 100 % The total flow rate is 30 l (stp) / h. The space velocity per hour is of the order of 150, 000 h "1. The NO and NOx signals (NOx = NO + N05 are recorded continuously, depending on the temperature in the reactor.) The NO and NOx signals are given by a NOx Ecophysics analyzer based on the principles of chemiluminence The evaluation of the NOx collector is in two parts: First, the maximum absorption temperature is determined, by absorbing the NOx at 125 ° C for 15 minutes and subsequently heating it under the same mixture at 600 ° C. The NOx profile shows a maximum absorption at a certain temperature. Second, the amount adsorbed isothermally at the maximum adsorption temperature is determined. The amount is calculated by integration.

EXAMPLE -1"This example refers to the preparation and use of a catalyst whose supported phase is based on manganese and potassium, using potassium permanganate (99.5% KMn04) and supporting the HSA5® cerium oxide from Rhone-Poulenc , with a volume of pore determined with water of 0.4 c 3 / g that is not calcined before the deposition of the active element The levels of Mn and K deposited are equal to 16% respectively ([Mn] / ([Mn] + [CeO) = 0.16.) Potassium and manganese are deposited by dry impregnation, impregnating the support with KMn04 solution whose volume is equal to the pore volume of the support and whose concentration makes it possible to obtain the desired levels of Mn and K. The impregnated support is then dried in an oven at 110 ° C then calcined at 750 ° C for 2 hours with a temperature rise of 5 ° C / min.The BET specific surface area (BET SS) is equal to 9 2 / g.

EXAMPLES 2 TO 7 Raw materials: Manganese nitrate Mn (NOa) 2, praseodymium nitrate Pr (N03) 3, gadolinium nitrate Gd (N03) 3, samarium nitrate Sm (N03) 3, neodymium nitrate are used Nd (N03) 3 and terbium nitrate Tb (N03) The support used is cerium oxide calcined for 2 hours at 500 ° C. Levels of the supported elements: 1 stage: Deposition of the first supported element. This consists of depositing the supported element, specifically at 10% atomic in the case of - 2: Gd, Sm, Nd, Pr, Tb with respect to the sum of the number of moles of the element and moles of the cerium oxide, that is: [X] / ([X] + [Ce02]) = 0.1 with X = Gd , Sm, Nd, Pr, Tb 2 a Stage: Deposition of the second supported element. This consists in the deposition of the second supported element, specifically at 10 atomic% of Mn with respect to the sum of the number of moles of the elements and moles of cerium oxide, that is to say: [Y] / ([X] + [ Y] + [Ce02]) = 0.1 with Y = Mn Synthesis routes: Dry impregnation is used in all cases. Dry impregnation: This involves impregnating the support in question with the supported element dissolved in a solution whose volume is equal to the pore volume of the support (determined with water: 0.4 cm3 / g) and whose concentration makes it possible to achieve the desired impurification. 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 2h at 500 ° C. (5 ° C / min.) - dry impregnation of the second element - drying in an oven (110 ° C, 2h) - calcination for 2h at 500 ° C. (5 ° C / min.) Products obtained: Example 2: [Gd] = 10 'atomic, iMn] 10% atomic, BET SS = 107 m2 / g Example 3: [Sm] = 10% atomic, [Mn] = 10 atomic%, BET SS = 107 m2 / g Example 4: [Nd] = 10 atomic%, [Mn] = 10 atomic%, BET SS = 106 m2 / g Example 5: [Pr] = 10 atomic%, [ Mn] = 10 atomic%, BET SS = 100 m2 / g Example 6: [Tb] = 10 atomic%, [Mn] = 10 atomic%, BET SS = 125 mz / g Example 7: [Mn] = 10% atomic , BET SS = 128 m2 / g For Example 7, a single element (Mn) was deposited. EXAMPLES 8 TO 10 The same raw materials and the same support as in examples 2 to 7 are used in addition with sodium nitrate and potassium nitrate. The procedure for the deposition comprises three stages. The first consists of depositing manganese in an amount of 10 atomic% with respect to the sum of the number of moles of manganese and moles of cerium oxide, that is to say: [Mn] / ([Mn] + [Ce02]) = 0.1 In the second stage, the second supported element is deposited in an amount of 10 atomic% with respect to the sum of the number of moles of the elements and moles of the cerium oxide, ie: [Y] / ([Mn] + [Y] + [Ce02]) = 0.1 with Y = Pr, Nd In the third stage, the third supported element is deposited in an amount of 5 atomic% with respect to the sum of the number of moles of the elements and moles of the cerium oxide, that is to say: [Z] / ([Mn] + [Y] + [Z] + [Ce02]) = 0.05 with Z = Na, K La dry impregnation is used as in examples 2 to 7. Products obtained: Example 8: [Mn] = 10 atomic%, [Nd] = 10 atomic%, [K] = 5 atomic%; BET SS = 91 m2 / g Example 9: [Mn] = 10 atomic%, [Pr] = 10 atomic%, [K] = 5 atomic%; BET SS = 92 m / g Example 10: [Mn] = 10 atomic%, [Pr] = 10 atomic%, [Na] = 5 atomic%; BET SS = 90 m2 / g The results of the catalytic test are given in the following table.

Claims (15)

1. A process is used to treat gases with a view to reducing nitrogen oxide emissions by the collection of the oxides, characterized in that the composition consists essentially of manganese and cerium oxide

2. A process is used to treat gases in view of the reduction of nitrogen oxide emissions by the collection of the oxides, characterized in that the composition comprises manganese, cerium oxide or a mixture of cerium oxide and zirconium oxide and also at least one other element selected from terbium, gadolinium, europium, samarium, neodymium, and prasedomio.

3. The process according to claim 2 is used, characterized in that the composition further comprises an alkali metal.

4. A process for treating gases with a view to reducing nitrogen oxide emissions is used by the collection of the oxides, characterized in that the composition comprises manganese and potassium, cerium oxide or a mixture of cerium oxide and zirconium oxide and which has been obtained by a route in which at least one of the two manganese and potassium elements is at least partially supplied by potassium permanganate.

5. The process according to one of the preceding claims is used, characterized in that the composition in which the cerium oxide or the mixture of cerium oxide and zirconium oxide forms a support, the manganese and the other elements form a phase supported.

The process according to one of the preceding claims, characterized in that an exhaust gas from internal combustion engines is treated.

The process according to claim 6, characterized in that a gas having a relative excess of oxygen is treated for the stoichiometric value.

The process according to one of claims 6 and 7, characterized in that the oxygen level in the gases is at least 5% by volume.

9. A composition comprising manganese, cerium oxide or a mixture of cerium oxide and zirconium oxide, characterized in that it also contains at least one other element selected from terbium, gadolinium, europium, samarium, neodymium and para-somium.

10. The composition according to claim 9, characterized in that it also comprises at least one alkali metal.

11. A composition comprising manganese and potassium and cerium oxide or a mixture of cerium oxide and zirconium oxide, characterized in 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.

The composition according to one of claims 9, 10 or 11, characterized in that the cerium oxide or the mixture of cerium oxide and zirconium oxide form a support, the other elements form a supported phase.

The composition according to claim 12, characterized in that the support is based on a cerium oxide and a zirconium oxide in a cerium / zirconium atomic ratio of at least 1, and in that it has a specific surface area after the calcination for 6 hours at 900 ° C of at least 35 mg AND an oxygen storage capacity at 400 ° C of at least 1.5 ml 02 / g.

The composition according to claim 12 or 13 characterized in that the support is based on a cerium oxide and a zirconium oxide in a cerium / zirconium atomic ratio of at least 1 and that is obtained by a process in the which is prepared a mixture in a liquid medium containing a cerium 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 0H "/ Zr < 1.65; the mixture is heated, the precipitate obtained is recovered and this precipitate is calcined

15. A catalytic system for carrying out the process according to one of claims 1 to 8.