MXPA98002753A - Oxygen accumulator material, provided of high stability against temperatures, also a procedure for your manufacturer - Google Patents

Abstract

The invention relates to an oxygen storage material, provided with a high temperature stability, containing cerium oxide and at least one stabilizer taken from the group of praseodymium oxide, lanthanum oxide, yttrium oxide and neodymium oxide, the or the stabilizers and optionally cerium oxide present in an alternating dispersed form on the specific surface of a high-surface carrier material taken from the group of aluminum oxide, zirconium oxide, titanium oxide, silicon oxide, cerium oxide and mixtures thereof. same, and presenting the oxygen accumulator material, after its calcination in the air at a temperature of 900 degrees centigrade for 10 hours, still a specific surface of more than 20, preferably more than 30 square meters per gram.

Description

Material Oxygen Accumulator, Equipped with a High Stability Facing Temperatures, as well as a Procedure for its Manufacture Description of the invention The present invention relates to a material that accumulates or stores oxygen, and also refers to a process for its manufacture with a view to its use in the catalytic conversion of substances, in particular in the purification of the exhaust gases of the engines of internal combustion. The internal combustion engines emit together with the exhaust gases, substances that are essentially polluting or pe judicial such as carbon monoxide, CO, unburned HC hydrocarbons, and oxides of nitrogen, Nox that through modern catalysts for the purification of the exhaust gases are converted into a high percentage in the following harmless components: water, carbon dioxide and nitrogen. The conversion essentially takes place under stoichiometric conditions, ie, that the oxygen contained in the exhaust gases is regulated by a so-called "lambda probe" in such a way that the oxidation of carbon monoxide and hydrocarbons, as well as the the reduction of the nitrogen oxides in nitrogen can take place in an approximately quantitative form. The catalysts developed for this purpose receive the designation of "three-way catalysts". It usually contains, as catalytically active components, one or more metals of the Platinum Group of the Periodic Table of the Elements, on high-surface bearing materials such as gamma-aluminum oxide with specific surfaces of more than 100 square meters per gram. The stoichiometric conditions are presented with "lambda" air numbers (carburization indices) equal to l. The air number, lambda, is the air / fuel ratio, referred to stoichiometric conditions. The ratio: air / fuel, indicates how many kilograms of air are necessary for the complete combustion of one kilogram of fuel. In the case of the usual fuels for Otto-type engines, the stoichiometric ratio; air / fuel has a value of approximately 14.6. The exhaust gases of the motors indicate, depending on their load and rpm, periodic deviations more or less important of the number of air around the value "one". For a better conversion of oxidizable contaminants under these conditions, oxygen-accumulating components are applied, such as for example cerium oxide, which bind oxygen by modifying the oxidation state of Ce 3+ bringing it to Ce 4+, when the Oxygen is present in excess, and they return it for oxidant conversion, by means of the transition from Ce 4+ to Ce 3+, when oxygen is present in the exhaust gases. The catalytic converters for the exhaust gases of the vehicles are exposed to the temperatures of the exhaust gases, of up to 1100 degrees Celsius during the prolonged service of the engine and during the complete load. Therefore, these high temperatures require the use of materials resistant to high temperatures and stable in the long term, for catalysts. EP 0 44 470 Bl discloses a high surface cerium oxide, which consists of an intimate mixture of cerium oxide with from 5 to 25 mol percent, based on the moles of cerium oxide, of a stabilizer of the cerium oxide. Lanthanum, neodymium and yttrium can be used as stabilizers. The material is obtained by co-precipitating from the common solution of a cerium oxide precursor and a cerium oxide stabilizer precursor. According to EP 0 715 879 A1, it is possible to optimally use the ability of cerium oxide to store or accumulate oxygen, if it is deposited in the form of particles with diameters of 5 to 100 nm on a porous carrier material such such as for example aluminum oxide. For this purpose, a dispersion of the carrier material is prepared, in powder form, and a cerium oxide sol, the size of whose particles is in the indicated range. By means of this dispersion, an alveolar body is coated, which is then dried and subjected to a calcination of 650 degrees centigrade for one hour. It is also possible to use a cerium oxide sol together with a cerium oxide sol. Calcination has the effect of enlarging the size of the particles of cerium oxide on the carrier material, by more than 35 nm. If the cerium oxide sol and the zirconium oxide sol are used together, the calcination of the coating (at 750 degrees centigrade, for one hour) will result in a solid solution of cerium oxide and zirconium oxide, the particles of which have a size approximately 60 nm. In EP 0 337 809 Bl a catalytic composition is described which, among others, contains zirconium oxide particles stabilized with cerium oxide. To stabilize the zirconium oxide particles with the cerium oxide, the zirconium oxide is imbibed with a cerium salt solution. The embedded particles thus obtained are dried and calcined until the graphical representation of the X-ray diffraction spectrum (Roentgen) no longer shows any peak of the crystalline form of the cerium oxide. The cerium oxide is present in the mixture of cerium oxide / zirconium oxide, in an amount of 10 to 50 weight percent, based on the zirconium oxide. In addition to the cerium salt, it is also possible to apply an yttrium salt and / or a calcium salt. The material shows in the X-ray diffraction spectrum after a 10-hour calcination in the air at a temperature of 900 degrees centigrade, only a peak of the tetragonal zirconium oxide, and no peak of the cerium oxide. Therefore, in this material, the cerium oxide is present essentially in the form of a solid solution together with the zirconium oxide. The processes known in the state of the art for the preparation of a material that accumulates oxygen, also resort to co-precipitation processes and impregnation processes, in order to stabilize the cerium oxide by the addition of other stabilizing components, or effects of depositing cerium oxide on the carrier materials. As an alternative, the cerium oxide is directly deposited in the form of particles on the carrier materials, by using a cerium oxide sol. The disadvantages of the coprecipitation process is the fact that the material obtained consists of a high percentage of cerium oxide, which can not be used completely for the task of oxygen accumulation, since the accumulation of oxygen has essentially on the surface and that is why the areas located deeper in the material, are not available for accumulation. In the known case, of the impregnation process, that is, in the case of the separation of sols, the removal of the water from the treated material leads to chromatographic effects that lead to a non-uniform distribution of the cerium oxide on the carrier material . As it has been proved, the impregnation of the pore volumes does not lead to satisfactory results, in which the chromatographic effects are avoided by the fact that only a volume of solvent agent corresponding to the capacity of the carrier material to absorb water is used. . On the other hand, in this process the volume of the solvent is limited by the ability to absorb water, so it is not possible to dissolve in it arbitrary amounts of cerium salts. Van Dillen et al. (Proc. 6th Int.

Conf. On Cat., London, Ed. G.C. Bold, P.B. Wells, F.C. Tomkins, 2667 (1976)), describe a process for the preparation of copper and nickel catalysts on high surface carrier material. In this process, the carrier material is dispersed together with a precursor of the active component, in water. By injecting an acidic or basic solution into the dispersion with the help of a capillary (capillary injection), the active component is deposited on the surface of the carrier material. In order to avoid rapid precipitation of the active component in the solution itself, precipitation must be undertaken only with low over-saturations of the dispersion as a whole. In order to guarantee a homogeneous precipitation and simultaneously in the entire solution, it is necessary that the basic or acid solution is introduced in small quantities therein per unit of time and is distributed evenly by agitation. The object of the present invention is to provide an oxygen accumulator material, which is distinguished by a high resistance to temperature and a long-term stability, and which can be manufactured economically. Said objective is achieved by an oxygen accumulator material, provided with a high stability to temperatures, containing cerium oxide and at least one stabilizer taken from the group of praseodymium oxide, lanthanum oxide, yttrium oxide and neodymium oxide. , the stabilizers being present in highly dispersed form on the surface of a high surface bearing material, and the oxygen accumulating material after being calcined in the air at a temperature of 900 degrees centigrade for 10 hours, still a specific surface area of more of 20, preferably more than 30 square meters per gram. By the designation "material that accumulates oxygen", it is designated, within the scope of the present invention, the combination of the carrier material with the stabilizers. In the material according to the present invention, the cerium oxide essentially has an oxygen accumulation function. But also praseodymium oxide can accumulate or store oxygen. However, its capacity to accumulate oxygen is lower than that of cerium oxide. Therefore, in the context of the present invention, the cerium oxide receives the designation "oxygen accumulator compound", while the praseodymium oxide, in spite of its capacity to accumulate oxygen, is included in the group of stabilizers . The oxygen storage compound of the material according to the invention can be completely contained in the carrier material. In this case, the carrier material is a mixture of oxides, one of the components of the oxide mixture being formed by cerium oxide. If the capacity to accumulate oxygen of the carrier material is already sufficient for the intended application, an additional coating of carrier material with the cerium oxide can be dispensed with. In the case of bearing materials that do not contain cerium oxide, it is necessary to provide the necessary capacity to accumulate oxygen by coating with cerium oxide, in which case the cerium oxide is present together with the stabilizers, in the form of highly dispersed Suitable carrier materials, which do not contain cerium oxide, are; aluminum oxide, zirconium oxide, titanium oxide, silicon oxide or mixtures thereof. Within the scope of the present invention, as materials with a high surface area, those materials whose specific surface, also referred to as the BET surface (measured according to DIN 66132), are considered to be at least 10 square meters per gram. The so-called active aluminum oxides comply with these conditions. These are aluminum oxides in the form of fine particles, which present the crystalline structure of the so-called transition phases of aluminum oxide. Among them are aluminum oxide chi-, delta-, gamma-, kappa-, teta-, and eta-. These materials have specific surfaces of between approximately 50 and 400 square meters per gram. Zirconium oxides and mixtures of cerium and zirconium oxides can also be obtained with similar high surfaces. For the purposes of the present invention, it is possible to obtain mixtures of cerium and zirconium oxides, according to EP 0 605 274 Al, by co-precipitation. They have a cerium oxide content of 60 to 90 percent by weight, based on the total weight of the oxide mixture. As an alternative, mixtures of cerium and zirconium oxide, rich in zirconium, with a cerium oxide content of only 10 to 40 weight percent, based on the total weight of the oxide mixture, can also be applied. The stabilizers and, eventually, the cerium oxide in highly dispersed form are applied to these bearing materials, for which appropriate techniques are used. It has been found that so-called carrier materials, if at least one stabilizer in the form of a high dispersion is present on their specific surface, present an extraordinary stability at temperatures and long-term stability. Within the scope of the present invention, it is considered as "stable at temperatures and over time", those materials that after a calcination in the air at a temperature of 900 degrees centigrade for 10 hours still have a surface of minus 20, preferably at least 30 square meters per gram. For this stabilization it is sufficient to apply stabilizers in an amount of 0.1 to 10 percent by weight based on the total weight of the oxygen accumulator material, with a content of less than 0.1 percent by weight, stabilization will no longer be sufficient. With an application superior to percent by weight, no appreciable increase in temperature stability is observed any more. The oxygen storage material has a particularly high stability against temperatures, if the stabilizers are present on the carrier material in crystallite forms of a size less than 7 nm. In this case, the stabilizers also receive the designation "amorphous from the point of view of the X-rays", since their crystallite substances have such small sizes, they no longer present a discrete spectrum of X-ray diffraction.

In a special embodiment of the present invention, a mixture of cerium and zirconium oxides is used as a high-surface carrier material with a content of 60 to 90 weight percent cerium oxide, based on the total weight of the oxide mixture. For the stabilization of this material it is sufficient to apply only the stabilizer (s) in highly dispersed form, on the surface. In another embodiment of the present invention, zirconium oxide or aluminum oxide function as high-surface carrier materials. In order to give the carrier material a sufficient capacity to accumulate oxygen and at the same time to ensure outstanding resistance against temperatures and outstanding long-term stability, the cerium oxide and one or more stabilizers are applied in a highly dispersed form. the zirconium oxide or aluminum oxide, in which case the content of the cerium oxide in the finished material is from 5 to 60 weight percent, based on the total weight of the oxygen accumulator material. For the stabilization of the oxygen storage material, the praseodymium oxide is preferably used. The stabilizers and possibly the cerium oxide can be applied in various ways on the chosen carrier material. The important thing is that the separation process guarantees the separation of the stabilizers from the cerium, in a highly dispersed form. The procedure of homogeneous precipitation has proved particularly suitable. In said process, the stabilizers and possibly the cerium are used in the form of soluble precursor compositions. Suitable are, for example, nitrates, acetates, chlorides and other soluble compositions. In the case of impregnation of the pore volume, frequently used in the current state of the art, the precursor compounds are dissolved in an amount of water that preferably corresponds to 70 to 100, preferably 80 to 90 percent, of the previously established capacity of the carrier material present to absorb water. Said solution is completely divided, for example, by means of spray or spray nozzles in a uniform manner over the carrier material which is rotated in a drum. The resulting powder is still capable of being drained, despite the water content. Finally, for fixing the stabilizers, or the cerium on the specific surface of the carrier material, it is calcined in the air or under an inert atmosphere. For this purpose, a calcination temperature of between 300 and 800 is used, preferably between 500 to 700 degrees centigrade for 0.5 to three hours. The disadvantage of this method is, among others, the fact that only a limited volume of solvent is available for the precursor compounds. Therefore, depending on the solubility of said compounds, by means of this method only limited quantities can be separated on the carrier material. If larger loads are required, the impregnation of the pore volumes should be repeated several times, for which, prior to each new impregnation, thermal decomposition of the precursor compounds of the stabilizers and / or cerium should take place. However, with this there is the danger of an unwanted development of crystallites, so that after the repeated impregnations are finished, the stabilizers and eventually the cerium are no longer present in the amorphous form, preferred, to the X-rays. This limitation of the quantities does not occur in the homogeneous precipitation procedure described. by von Dillen et al. for the separation of nickel and copper. For the implementation of this procedure, the carrier material, in the form of fine particles, can be dissolved, or dispersed in water, together with the cerium precursor compounds and the stabilizers. However, it is preferable to prepare a separate solution of the precursor compounds, which is slowly introduced under stirring, into the dispersion of the carrier material. For this purpose, for example, the so-called capillary injection can be applied; the solution of the precursor compounds is injected by means of one or several capillaries below the surface of the aqueous dispersion of the carrier material, and distributed by means of a stirrer rapidly over the entire volume of the dispersion. In the preparation of the mixture of the dispersion and solution of the precursor compounds, a given adsorption of the precursor compounds on the surface of the carrier material and thus the formation of "parent crystals" (crystallization germs) can easily occur. The degree of this adsorption depends on the combination of the materials of the carrier material and the stabilizers. However, the actual separation of the stabilizers and eventually the cerium is carried out by chemical precipitation. To this end, a dilute aqueous base is added to the dispersion, preferably a dilute ammoniacal solution (from 1 to 5 weight percent of NH3 in water), also by capillary injection. The speed of the injection is chosen in such a way as to ensure a distribution as quickly and homogeneously as possible of the ammoniacal solution in the volume of the dispersion. The speeds (flow rates) of injection, R, of 1 to 4.10E-5 ml of ammoniacal solution per minute and ml of water as well as g of carrier material have been shown to be adequate: R = 1 ... 4 x 10E-5 [ ml of NH3 solution] / [min x ml H20 xg of carrier material. The preceding formula represents a guideline value for the appropriate injection speed. To establish the speed of the injection, it is important that the precipitation of the stabilizers does not take place in the aqueous phase of the dispersion, but in the condensation germs formed on the surface of the carrier. It is easy to determine the proper injection speed through some preliminary tests. By injecting the basic solution, or ammoniacal, the pH value, which in the first instance is found in the acidic environment, of the mixture consisting of the dispersion and in the dissolved precursor compounds (pH value: between 1 and 7, in function of the acidity of the carrier material), is slowly raised to a value of 8 to 10. Thus the precursor compounds are separated in the form of their hydroxides, and deposited on the surface of the carrier material. The procedure of homogeneous precipitation provides a chemical fixation of the precursor compounds on the carrier material. No subsequent thermal fixation is necessary by calcination, as in the case of impregnation of the pore volumes. For the preparation of a coating dispersion for the inert carrier body of the catalyst, and once the precipitation is complete, the water can be removed from the dispersion, after which the dispersion is dried and eventually calcined, before the accumulator material Oxygen, thus obtained, is again dispersed together with other components of the coating material and said oxygen accumulator material is applied to the carrier body by immersion. However, due to the chemical fixation of the precursor compounds on the carrier material, there is also the possibility of further elaboration of the dispersion, once the chemical precipitation is finished, obtaining directly a dispersion of coating material, by adding other components of the material Coating. Below are some examples intended for a better illustration of the present invention. In the drawings: Figure 1 is a sketch of the apparatus (set of apparatuses) necessary to carry out the homogeneous precipitation; Figure 2 is a diagram of X-ray diffraction (Roentgen) of powders H and I in fresh state after calcination at a temperature of 600 degrees centigrade for 2 hours; Figure 3 is a diagram of X-ray diffraction (Roentgen) of powders H and I after aging, (calcination at a temperature of 600 degrees centigrade for 10 hours; Figure 4 is a dependence as a function of time, the conversion coefficients of carbon monoxide and nitrogen oxides by varying the number of air "lambda", to determine the crossing point.

Figure 1 shows the realization of an apparatus (set of apparatuses) to carry out the procedure of homogeneous precipitation, which is preferred. In the vessel 1, which has a volume of two liters, an aqueous dispersion, 2, of the carrier material is introduced. From the container-tank, 4, a common solution, prepared in advance, of the precursor compounds 5 of the stabilizers and, optionally, the cerium, is injected under the surface in the dispersion 2, by means of one or more capillary hoses 6 (internal diameter: about 1 mm). Together with this, the dispersion is subjected to a good constant agitation by means of a stirrer 3 (1200 rpm). The baffle plates 8 ensure intensive mixing of the dispersion. A hose type pump is indicated by reference number 7. After injection of the solution, an ammoniacal solution diluted, in a completely analogous manner, in the solution, now present, consisting of the dispersion and solution of the compounds, is injected from another container-tank, not represented here. precursors, and the precursor compounds are precipitated on the carrier material because the pH of the mixture acquires a value higher than 8 to 10. The set of apparatuses shown here, is suitable for operating with materials in small scale, and will also be used in this way in the following examples. However, the process of homogeneous precipitation can also be carried out in a continuous manner, for which the dispersion of the carrier material is carried in the form of a direct current through an aggregate agitator or milling device. For the combined homogeneous mixing of the precursor compounds and the ammoniacal solution, it is possible to arrange two aggregate devices of this type, one after the other. It is convenient that the solution of the precursor compounds and the ammonia solution are introduced directly into the aggregate agitator device or into the aggregate grinding device. For the following examples a mixture of cerium and zirconium oxides is used, as well as a pure zirconium oxide, as carrier materials for the praseodymium oxide and the cerium oxide. The properties of said carrier materials have been reported in Table 1. By means of the oxygen-accumulating materials, in powder form, prepared in the examples, and under the addition of aluminum oxide and by activation with palladium, model catalysts were prepared, which were studied in terms of their activation behavior and percentage conversion coefficient at the crossing point of the CO and NOx conversion curves (crossing point). In addition to the powders modified by praseodymium oxide, or by cerium oxide, the pure powders themselves were also used for the preparation of catalysts. In the following examples, the various powders have been designated by the letters A to I. "A" represents the pure mixture of cerium oxide / zirconium oxide, and "E" represents the pure zirconium oxide.

Table I: d5o diameter of the granules, greater than or equal to, the diameter of 50 percent by weight of the material.

Example 1: Powder A was impregnated by impregnation of the pore volume with an aqueous solution of praseodymium nitrate (Pr (N03) 3.5H20). In the impregnation of the pore volume, the component to be applied by impregnation was dissolved in a volume of solvent corresponding to 80 to 100 weight percent of the capacity of the aforementioned solvent to absorb the powder. After drying the impregnated powder at a temperature of 120 degrees centigrade, it was proceeded to calcined for 2 hours at a temperature of 600 degrees Celsius, in air, in order to transform the praseodymium nitrate into the corresponding oxide. In order to guarantee a uniform heating through the present powder, the temperature was raised to the desired 600 degrees centigrade, in a period of 4 hours. The finished powder contained 5 weight percent praseodymium oxide (Pr60u), based on the total weight of the powder. In the following, this material is called "Powder B".

Example 2 To be able to make a comparison of Dust B with a material of the same composition, but prepared by co-precipitation, an aqueous solution of cerium nitrate, zirconium nitrate and praseodymium nitrate was prepared. The pH of the solution had a value of 2. Through the slow addition of a diluted ammoniacal solution, the cerium, zirconium and praseodymium were precipitated simultaneously, in the form of their respective hydroxides. The precipitated material was filtered, washed, dried at 120 degrees centigrade and then subjected to calcination for two hours at a temperature of 600 degrees Celsius to the air, to transform the hydroxides into the corresponding oxides. The gradient of the temperatures, for a final temperature of 600 degrees Celsius, was chosen equal to that of Example 1. The finished material (Powder C) contained, as well as Powder B, 66.55 weight percent cerium oxide, 28.5 percent by weight of zirconium oxide, and 5 percent by weight of praseodymium oxide.

Example 3: Powder A was dispersed in water and coated by the procedure of homogeneous precipitation with praseodymium, using praseodymium acetate and a 5 weight percent ammonia solution. As in the previous examples, the coated powder was subjected to filtration, drying at 120 degrees centigrade and then subjected to calcination at a temperature of 600 degrees centigrade for 2 hours in air. In the following, this powder receives the designation "Powder D". It had the same composition as powders B and C.

Example 4: Powder E was coated by impregnation of the pore volume, using cerium nitrate with a total of 20 weight percent cerium oxide, based on the total weight of the finished material. The drying and calcination were carried out in the same manner as in the preceding examples. In the following, this powder receives the designation "Powder F".

Example 5: Powder E was coated by homogeneous precipitation, using cerium nitrate with a total of 20 weight percent cerium oxide, based on the total weight of the finished material. The drying and calcination were carried out in the same manner as in the previous examples. In the following, this powder receives the designation "Powder G".

Example 6: The powder E was coated by impregnation of the pore volume, using praseodymium nitrate and cerium nitrate (Ce (N03) 3.6 H; 0), in a molar ratio Ce / Pr = 10/1. The drying and calcination of the material were carried out in the same manner as in the previous examples. The finished powder contained 79 percent zirconium oxide, 19 weight percent cerium oxide and 2 percent praseodymium oxide, in each case, based on the total weight of the finished powder. In the following, this powder receives the designation "Dust H".

Example 7: In a manner analogous to Example 6, the powder E is coated with the same amounts of cerium oxide and praseodymium oxide. Unlike example 6, the coating is effected by the homogeneous precipitation process. In the following, the powder receives the designation "Powder I". Table 2 gives a summary of the powders used for the following investigations: Table 2: Powdered Materials Table 2: Powdered Materials (continued) Table 2: Powdered Materials (continued) Example 8 The powdery materials of Table 2 were aged by calcination at a temperature of 900 degrees centigrade for 10 hours. The gradient of elevation of the temperature until reaching the final temperature of 900 degrees centigrade, was 225 degrees Celsius per hour. Then the specific surfaces according to DIN 66132 were determined for all materials. The results of these measurements have been recorded in table 3.

Table 3: The magnitudes of the crystallites were determined from the powder materials F to I by X-rays (Roentgen), of the zirconium oxide in the powder granules in the fresh state, that is, after the first calcination at 600 degrees centigrade and after of aging, that is to say after a calcination at 900 degrees centigrade for 10 hours. The results are reported in the following table 4.

Table 4: Figure 2 shows the X-ray diffraction diagram of Powder H (curve a) and I (curve b) in fresh state after the final calcination at 600 degrees centigrade for 2 hours.

Both powders contain equal amounts of cerium oxide and praseodymium oxide on the zirconium oxide carrier. Both diffraction diagrams clearly show the structure of the carrier. It is a mixture of onoclinal and tetragonal phases of zirconium oxide, recognizable in the third diffraction reflection with a diffraction angle of 2 theta = 30 degrees. In curve a) there are additional superimposed diffraction reflections, which can be unambiguously attributed to cerium oxide. The vertical lines of figure 2 reproduce the position of the diffraction reflections of the pure cerium oxide. In contrast, the dust diffraction diagram I (curve b) does not show the superimposed reflections, despite the same cerium oxide content. Based on these facts it can be concluded that the cerium oxide is present in the powder H, which was prepared by impregnating the volume of the pores, in the form of relatively large crystallites on the sodium oxide carrier. On the other hand, the separation of the cerium oxide on the zirconium oxide took place by homogeneous precipitation in an "amorphous according to the X-rays" form. The formation of a mixture of oxides, cerium oxide and zirconium oxide, given the chosen temperature for calcination, of 600 degrees centigrade can be excluded. On the other hand, the formation of a mixture of oxides should be manifested by a shifting or displacement of the diffraction reflections of the zirconium oxide. However, this is not the case. Figure 3 shows the diffraction diagram of powders H and I after aging at 900 degrees centigrade for 10 hours. The curve a) of the powder H shows, on the other hand, the additional diffraction reflections of the cerium oxide. The amplitude of these diffraction reflections has decreased due to aging, which allows to conclude in a growth or development of cerium oxide crystallites. However, on the other hand curve b) of powder I shows only the diffraction reflections of zirconium oxide, that is to say that the content of cerium oxide of this powder is present in amorphous form to X-rays, on the oxide of zirconium. No particle growth has taken place; nor can the form of a mixture of oxides be observed. Therefore, the method of homogeneous precipitation according to the present invention leads to a separation of cerium oxide and stabilizers on the carrier material of the case, in a highly dispersed, amorphous way according to X-rays. dispersion of separate substances, is stable up to temperatures of 900 degrees Celsius. Said positive properties of the oxygen accumulator materials, according to the present invention, are also highlighted by the stability of the specific surfaces (see table 3), as well as in their application in the catalysts for the purification of the exhaust gases.

Application Example: Catalysts are prepared by means of Powders A to D, and their activation temperatures, T50, are determined for the conversion of the CO carbon monoxide contaminants, the HC hydrocarbons and the NOx nitrogen oxides. The term "activation temperature" is understood to mean those exhaust gas temperatures at which the pollutant considered in each case is converted to 50% by the catalyst. For the preparation of the catalysts, the mentioned powdery materials were, each of them, dispersed in water together with an active aluminum oxide, of high surface area, (specific surface, 140 square meters / grams), with a weight ratio 1: 5. By means of this dispersion of coating material, alveolate cordierite bodies with a density of cells (number of circulation channels per square centimeter of cross section) of 62 per square centimeter having the walls of the circulation channels were coated by immersion. a thickness of 0.2 mm. The coating was then dried and subjected to calcination at a temperature of 500 degrees centigrade for 3 hours. Next, the coating was impregnated by immersing the honeycomb body in a palladium nitrate solution, again drying at a temperature of 300 degrees centigrade, and subjected to calcination. The finished catalysts contained 100 g of aluminum oxide, 20 g of the corresponding powdery material as well as 2.1 g of palladium, in each case, based on one liter of the volume of the honeycomb body. The manufactured catalysts were designated with the letters A to D, according to the oxygen accumulator material used for their manufacture. Prior to the measurement of the activation temperature for the three pollutants CO, HC and Nox and the so-called CO / Nox crossing point, the catalysts were subjected to hydro thermal aging at a temperature of 690 degrees centigrade for 16 hours. For hydrothermal aging, the catalysts were stored in a gaseous stream containing 10 volume percent of water vapor, the rest being nitrogen at the aforementioned temperature. Table 5 shows the activation temperatures measured in the aged catalysts, for the three pollutants, as well as for the CO / Nox crossing points, for the conversion of carbon monoxide and nitrogen oxides, under the conditions of dynamic service.

Table 5 For the use of the oxygen accumulator material, in the catalytic converters for the exhaust gas cleaning of internal combustion engines, the lowest possible activation temperatures are possible for the catalysts prepared with it, as well as crossing points as much as possible. high possible. Overall, catalyst D offers a combination of more favorable properties than the check catalysts A, B and C. For the determination of the crossing point of table 5, the catalysts were tested in a synthetic gas installation with a synthetic exhaust gas having the composition indicated in figure 6, at a temperature of 400 degrees centigrade and with a special speed of 50,000 / h.

Table 6: Escape Gas Composition Table 6: Escape Gas Composition To determine the crossing point, the "lambda" air number of the exhaust gas was increased by the addition of oxygen within ten minutes, linearly from an air number 0.99, to an air number 1.03 and then again reduced the "lambda" with the same gradient. This slow variation of the air number was modulated by a periodic variation of the air number with a frequency of 0.5 Hz and an amplitude of delta lambda approximately equal to ± 0.059 (dynamic service conditions). During this solicitation of the catalysts, we proceeded to measure their conversion coefficients for carbon monoxide CO and oxides of nitrogen NOx and these data were transferred in a diagram according to figure 4, as a function of time. The diagram in figure 4 represents the measurement curves for catalyst D. In this diagram it is possible to read the percentage conversion for equal conversions for carbon monoxide and nitrogen oxides (crossing point).

It is noted that in relation to this date, the best method known by the applicant to carry out the present invention, is the conventional one for the manufacture of the objects to which it refers. Having described the invention as above, the content of the following is claimed as property:

Claims (11)

1. Oxygen storage material, provided with a high temperature stability, characterized in that it contains cerium oxide and at least one stabilizer taken from the group of praseodymium oxide, lanthanum oxide, lithium oxide and neodymium oxide, the stabilizers being present in a highly dispersed form on the surface of a high-surface carrier material, and the oxygen accumulator material after being calcined in the air at a temperature of 900 degrees centigrade for 10 hours, still a surface of more than 20, preferably more of 30 square meters per gram.

2. Oxygen storage material, according to claim 1, characterized in that the carrier material is a cerium oxide or a mixture of cerium oxide and zirconium oxide.

3. Oxygen accumulator material, according to claim 2, characterized in that the carrier material is a mixture of cerium oxide and zirconium oxide with a content of cerium oxide of 60 to 90 weight percent, based on the total weight of the mixture of oxides.

4. Oxygen accumulator material, according to claim 1, characterized in that the carrier material is an aluminum oxide, zirconium oxide, titanium oxide, silicon oxide, or a mixture of said oxides, on which the oxide is present of cerium together with the stabilizers in highly dispersed form.

5. Oxygen accumulator material, according to claim 4, characterized in that the carrier material is zirconium oxide or aluminum oxide, on which are present in cerium oxide and the stabilizer (s) in highly dispersed form, the content of the cerium oxide in the oxygen accumulator material, from 5 to 60 weight percent, preferred to the total weight of the oxygen accumulator material.

6. Oxygen accumulator material, according to any of the preceding claims, characterized in that it contains from 0.1 to 10 weight percent of stabilizers, based on the weight of stabilizers, based on the total weight of the oxygen accumulator material.

7. Oxygen accumulator material, according to claim 6, characterized in that the stabilizer (s) are present on the carrier material in an amorphous form according to X-rays, the crystallites having a size of less than 7 nm.

8. Oxygen accumulator material, according to claim 7, characterized in that praseodymium oxide is used as a stabilizer.

9. Oxygen accumulator material, characterized in that it contains a mixture of several oxygen accumulators according to claims 6 to 8.

10. The use of oxygen accumulator materials, according to any of the preceding claims, characterized in that the catalysts for the purification of exhaust gases in internal combustion engines.

11. Process for the manufacture of an oxygen accumulator material, according to any of the preceding claims, characterized in that the dissolved precursors of the stabilizer (s) and optionally the cerium oxide are introduced into an aqueous dispersion prepared in advance of the selected material, under constant agitation, after which, they are precipitated on the carrier material by slowly raising the pH value of the dispersion to a value of 8 to 10, by the addition of a base.