MXPA01000542A

MXPA01000542A - Process for synthesizing metal oxides and metal oxides having a perovskite or perovskite-like crystal structure

Info

Publication number: MXPA01000542A
Application number: MXPA/A/2001/000542A
Authority: MX
Inventors: Neste Andre Van; Serge Kaliaguine
Original assignee: UniversitéLaval
Priority date: 1998-07-16
Filing date: 2001-01-16
Publication date: 2002-06-05

Abstract

Metal oxides having a perovskite or perovskite-like crystal structure are prepared by a process comprising subjecting a mixture of starting powders to a high energy milling sufficient to induce chemical reaction of the components and thereby directly mechanosynthesize said metal oxide in the form of a perovskite or a perovskite-like material having a nanocrystalline structure as determined by X-ray diffractometry. The process according to the present invention is simple, efficient, not expensive and does not require any heating step for producing a perovskite that may easily show a very high specific surface area. Another advantage is that the perovskite obtained according to the present invention also has a high density of lattice defects thereby showing a higher catalytic activity, a characteristic which is highly desirable in their eventual application as catalysts and electronic conductors.

Description

PROCESS FOR SINTETIZING METALLIC AND OXIDE OXIDES METALLIC WHICH HAVE A PEROVSKITA OR STRUCTURE OF CRYSTAL SIMILAR TO PEROVSKITA Field of the Invention The present invention relates to a process for synthesizing a metal oxide having a perovskite or perovskite-like crystal structure by high energy milling. More particularly, a mixture of initial powders is subjected to a milling of high energy sufficient to induce the chemical reaction of the components and mechanics, thus directly synthesizing a metal oxide in the form of a perovskite or nanocrystalline structure similar to perovskite, as determined by diffractometry. X-ray.

BACKGROUND OF THE INVENTION In general, mixed metal oxides are crystalline compounds and are classified by general formulas and certain structural characteristics of naturally occurring minerals. Perovskite is a well-known type of mixed metal oxide. The perovskites have the general formula ABO3 where A and B remain as cations. More than one cation can be presented for each A and B. Another type of metal oxide includes "perovskite-like" materials, which comprise basic perovskite cells separated by the intervention of oxide layers. Perovskite-like materials have the general formula [(ABO3) n + CyOz] where A, β, and C remain as cations. More than one cation may be present for each A, B and C. Compounds derived from perovskite or perovskite-like materials are also known by substitution and deviations towards stoichiometry but maintaining their perovskite or perovskite-like crystal structure. Non-stoichiometric compounds derived from perovskites have the general formula (ABO3.x) and non-stoichiometric compounds derived from perovskite-like materials have the general formula [(ABO3-x) n + CyOz]. In all these non-stoichiometric compounds, metal ions with a different valence can replace both A and B ions, thus generating non-integral numbers of oxygen atoms in the formula. Lao.8Sr0.2CoO3-? and La0.8Sro.2MnO3- are examples of non-stoichiometric compounds derived from perovskites and Sr2FeO.x and Sr3Fe2O7.x are examples of non-stoichiometric compounds derived from perovskite-like materials. Other examples of such deviation towards stoichiometry are obtained by the elaboration of a perovskite or perovskite-like material deficient in oxygen. For example, the structure of brown millile (ABO2.5) is formed from perovskites (ABO3). It is therefore apparent that there is a fairly large number of compounds that fall within the scope of the term perovskite and perovskite-like materials. The compounds and their structures can be identified by X-ray diffraction. In the prior art, perovskite and perovskite-like compounds had been commonly used in the following fields: electrocatalysis, hydrogenation, dehydrogenation and exhaust gas self-purification. A disadvantage with the metal oxides having the perovskite and perovskite-like structure produced in the prior art is that, in general, they show a very low specific BET surface area (SS) of the order of 1 m2 / g. Accordingly, despite the fact that perovskite-like and perovskite-like metal oxides are not expensive to produce, that they normally exhibit good catalytic oxidation activities, that they are thermally stable and that they exhibit good resistance to poisoning, To date, a very limited application has been found instead of the catalysts based on noble metals used in the field of combating industrial pollution or the control of automobile emissions. The perovskite and perovskite-like compounds of higher surface area could thus have great potential as catalysts, particularly in the selective reduction of nitrogen oxides (Nox) and as electrocatalysts in the cathodic reduction of oxygen. Known methods for preparing perovskites and perovskite-like materials include sol-gel processes, co-precipitation, citrate complexing, pyrolysis, spray dehydration and lyophilization. In these, the precursors are prepared by a wet manner such as in a mixed gel or in the co-precipitation of metal ions in the form of hydroxides, cyanides, oxalates, carbonates or citrates. These precursors can be subjected to various treatments such as evaporation or combustion (SS ~ 1 -4 m2 / g), to the explosion method (SS <; 30 m2 / g), dehydration by plasma spray (SS ~ -20 m2 / g) and lyophilization (SS ~ 10-20 m2 / g). However, the disadvantages with all these methods are that either low specific surface area values are achieved or it is complicated and expensive to implement them. The most common method to prepare perovskite and perovskite-like catalysts is the traditional method called "ceramic". This method consists simply in the mixture of constituent powders (oxides, hydroxides or carbonates) and the sintering of the powder mixture thus formed at an elevated temperature. The problem with this method is that calcination at high temperature (usually above 100 ° C) is necessary to obtain crystalline perovskite or crystalline structure similar to perovskite. Another disadvantage is that a low specific surface area value (SS of about 1 m2 / g) is obtained. An example of such a method of heating at elevated temperature is described in the U.S. Patent. No. 5,093,301, where a perovskite structure to be used in a catalyst is formed after heating a dry powder mixture at 1300 ° C. The U.S. Patent No. 4, 134,852 (Volin ef al.) Issued in 1979 exposed a variant to the ceramic method by "making a mechanical alloy", in the strict sense of that expression, of the constituent powders necessary for the preparation of perovskite catalysts. However, it refers to conventional grinding in order to obtain a more or less homogeneous mixture of particles but without interfering with any chemical reaction between the components. It can be read in column 7, lines 5-8 of this patent that "[a] mechanically alloyed powder is one in which the precursor components have been intimately dispersed through each particle ...". Accordingly, a necessary step of the process set forth herein to obtain the desired perovskite structure is the heating of the "mechanically alloyed" powder composition to an elevated temperature greater than 800 ° C (column 7, lines 61-62). At present, the use of the term "mechanical alloy" or "mechanosynthesis" refers, inter alia, to a high energy grinding process wherein the nanostructural particles of the ground compounds are induced. Accordingly, it also relates to the production of metastable phases, for example, high temperature, high pressure or amorphous phases, from stable crystalline phases under ambient temperature and pressure. For example, the structural transformation of alumina (AI2O3), the preparation of ceramic oxides and the preparation of zirconias stabilized by high energy grinding or mechanical alloying, have already been described respectively in the following references: P.A. Zielinski er al. in J. Mater. Res., 1993, Vol. 8 p2985-2992; D. Michel ef al. , La revue de métallurgie-CIT / Sciences et Génies des matériaux, Feb. 1993; and D. Michel et al. , J. Am. Ceram. Soc, 1993, Voi. 76, p 2884-2888. The publication of E. Gaffet et al. in Mat. Trans., JIM, 1995, Vol. 36, (1995) p 198-209, gives a general view of the subject. However, even if these documents described the use of high energy grinding, their authors have only been able to transform their product from the beginning of one phase to another phase. The product resulting from the grinding has, therefore, still the same structure. In addition, none of them exposes the preparation of perovskite or perovskite-like materials. There is still still a need for a simple, low-cost process for the production of a metal oxide having perovskite or the cistral structure similar to perovskite. In addition, the perovskite and perovskite-like metal oxides produced according to all the above mentioned methods, known in the art, do not have a nanocrystalline structure. Accordingly, there is still a need for a metal oxide having a perovskite or perovskite-like nanocrystalline structure with a high specific surface area and the need for a process for synthesizing such compounds.

BRIEF DESCRIPTION OF THE INVENTION An object of the present invention is to provide a process for producing a metal oxide that meets the above mentioned needs. According to the present invention, that object is achieved with a process for mechanosynthesizing a metal oxide having a perovskite or perovskite-like crystal structure and a predetermined stoichiometric content of oxygen, said metal oxide being selected from the group consisting of perovskites. of the general formula ABO3; perovskite-like materials of the general formula [(ABO3) n + CyOz]; non-stoichiometric compounds derived from perovskites and having the general formula (ABO3.x); and non-stoichiometric compounds derived from perovskite-like materials and having the general formula [(ABO3-x) n + CyOz], wherein: Á comprises at least one element selected from the group consisting of Al, Y, Na, K, Rb, Cs, Pb, La, Sr, Ba, Cr, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table; B comprises at least one element selected from the group consisting of Al, Ga, In, Zr, Nb, Sn, Ru, Rh, Pd, Re, Os, Ir, Pt, U, Co, Fe, Ni, Mn, Cr, Ti, Cu, Mg, V, Nb, Ta, Mo and W; C represents at least one element selected from the group consisting of Ga, In, Zr, Nb, Sn, Ru, Rh, Pd, Re, Os, Ir, Pt, U, Co, Fe, Ni, Mn, Cr, Ti, Cu, Mg, V, Nb, Ta, Mo, W, Al, Y, Na, K, Rb, Cs, Pb, The, Sr, Ba, Cr, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table; n represents an integer between 1 and 10; 0 < x < 3 and represents an integer between 1 and 5; z represents an integer between 1 and 5; the process comprising the step of attaching a mixture of initial powders formulated to contain the components represented by A, B and C in the formulas to a milling of high energy sufficient to induce the chemical reaction of the components and mechanes and thus directly synthesize said metal oxide in the process. the form of a perovskite or perovskite-like material having a nanocrystalline structure, as determined by X-ray diffractometry. According to a preferred variant of the invention, the high-energy grinding is carried out under a controlled atmosphere in order to of controlling the nanocrystalline structure and the stoichiometric oxygen content of the mechanosynthesized metal oxide. The controlled atmosphere preferably comprises a gas selected from the group consisting of He, Ar, N2, O2, H2, CO, CO2, NO2, NH3, H2S and mixtures thereof. In another preferred variant of the invention, the process is characterized in that it further comprises the step of selecting and grinding the initial powders in relative portions to control the nanocrystalline structure of the mechanically synthesized metal oxide. The present invention also provides a process for mechanically synthesizing a metal oxide having a perovskite or perovskite-like crystal structure, a predetermined stoichiometric oxygen content and a high specific BET surface area, said metal oxide being selected from the group consisting of perovskites of the general formula ABO3; perovskite-like materials of the general formula [(ABO3) n + CyOz]; non-stoichiometric compounds derived from perovskites and having the general formula (ABO3.x); and non-stoichiometric compounds derived from perovskite-like materials and having the general formula [(ABO3.x) n + CyOz], wherein: A comprises at least one element selected from the group consisting of Al, Y, Na, K, Rb, Cs, Pb, La, Sr, Ba, Cr, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table; • B comprises at least one element selected from the group consisting of Al, Ga, ln, Zr, Nb, Sn, Ru, Rh, Pd, Re, Os, Ir, Pt, U, Co, Fe, Ni, Mn , Cr, Ti, Cu, Mg, V, Nb, Ta, Mo and W; C represents at least one element selected from the group consisting of Ga, ln, Zr, Nb, Sn, Ru, Rh, Pd, Re, Os, Ir, Pt, U, Co, Fe, Ni, Mn, Cr, Ti, Cu, Mg, V, Nb, Ta, Mo, W, Al, Y, Na, K, Cs, Pb, La, Sr, Ba, Cr, Rb, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table; n represents an integer between 1 and 10; 0 < x < 3 and represents an integer between 1 and 5; z represents an integer between 1 and 5; the process comprising the steps of: a) fastening a mixture of initial powders formulated to contain the components represented by A, B and C in the formulas a high energy grind, sufficient to induce the chemical reaction of the components and mechanosinitiate thus directly said metal oxide in the form of a perovskite or perovskite-like material that has a nanocrystalline structure, as determined by light diffractometry X; b) increasing the BET specific surface area of the metal oxide obtained in step a) by subsequently attaching said metal oxide to high energy mill to obtain a metal oxide having a high specific BET surface area. Step a) is preferably carried out under a controlled atmosphere in order to control the nanocrystalline structure and the stoichiometric oxygen content of the mechanosynthesized metal oxide. Step b) is preferably carried out under a controlled atmosphere in order to control the BET specific surface area of the mechanosynthesized metal oxide. The controlled atmospheres preferably comprise a gas selected from the group consisting of H 2 O, He, Ar, N 2, O 2 > H2, CO, CO2, NO2, NH3, H2S and mixtures thereof. The process for mechano-synthesizing a metal oxide having a perovskite or perovskite-like crystal structure, a predetermined stoichiometric oxygen content and a high specific BET surface area according to the invention, may further comprise one or more additional steps. In another preferred embodiment, the process further comprises the step of adding a small amount of an aqueous solution to the metal oxide during grinding of step b) in order to obtain a wet metal oxide. In another preferred embodiment, the process further comprises the step of selecting and grinding the initial powders in relative portions to control the final nanocrystalline structure of the mechanically synthesized metal oxide. In a further preferred embodiment, the process further comprises the steps of c): adding a soluble non-reactive additive during grinding of step b); and d): subsequently rinsing said soluble additive. Preferably, the non-reactive soluble additive is selected from the group consisting of LiCl, NaCl, RbCI, CsCl, NH4Cl, ZnO and NaNO3. It is also an object of the invention to provide a metal oxide having a perovskite or nanocrystalline structure similar to perovskite and having a BET specific surface area of between 3.1 and 82.5 m2 / g, this metal oxide being obtained by using any of the processes mentioned above. Preferably, the metal oxide is characterized in that it consists of a millenium coffee having the formula AB02.5 or [(ABO2.s) n + CyOz] and more particularly a millenium coffee selected from the group consisting of Sr7Fe10O22, SrFeO2 5 and SrFeo.5Coo.5O2.5- As can be seen, the processes according to the present invention are simple, efficient, inexpensive and do not require any heating step to produce a metal oxide having a similar perovskite or nanocrystalline structure a perovskite that can easily show a very high specific surface area. Another advantage is that the perovskite or perovskite-like obtained according to the present invention also has a nanocrystalline structure and a high density of lattice defects thus showing a greater catalytic activity, a characteristic that is highly desirable in its eventual application as catalysts and conductors. electronic The fact that it is possible to synthesize brown yarns by using the processes of the invention is also a main advantage of the present invention. A non-restrictive description of the preferred embodiments of the present invention will now be given in relation to the accompanying drawings and tables.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an X-ray diffraction pattern (CuKa wavelength), shown at a diffraction angle scale (2T) of 10 ° to 80 °, of a sample taken after one hour of Ground at high energy. The peaks corresponding to the initial powders (La2O3 and Co3O4) and the obtained perovskite (LaCoO3) can be identified by using the corresponding bars in the lower part of the figure. Figure 2 is an X-ray diffraction pattern of a sample taken after four hours of ground at high energy. Figure 3 is an X-ray diffraction pattern of a sample taken after eight hours of grinding at high energy. Figure 4 is an X-ray diffraction pattern of a sample taken after sixteen hours of grinding at high energy.

By using the bars in the lower part of the figure, it can be seen that all the main peaks correspond to the perovskite produced LaCoO3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to a new process called "mechanical alloying" or "mechanosynthesis" for producing metal oxides having a perovskite or a similar perovskite nanocrystalline structure by means of the simple attachment of a mixture of starting powders milled high energy, being sufficient the ground at high energy to induce chemical reaction of the components and thus mecanosintetizar directly a metal oxide in the form of a perovskite or structure similar to perovskite nanocrystalline, as determined by ray diffractometry X. as indicates throughout the description, the term "high energy grind" refers to the condition that develops in the container of a "high energy crusher" and where the nanostructural particles of the components in the crusher are induced. Examples of such high energy mill include: planetary crusher machine (called G5 and G7), planetary crusher machine PULVERISETTED (P5 and P7), ASI UNI horizontal grinder-BALL MILL II ™ and SPEX ™. During grinding, the spheres project violently away and closer into the container of the crusher. The spheres also shoot each other inside the container. When sufficient mechanical energy is applied to the total charge (waits and powders), it is believed that a substantial portion of the charge is maintained continuously and kinetically in a state of relative motion. To achieve "mechanosynthesis", the impact energy developed by these repetitive shocks must be sufficient to induce nanostructural particles of the components in the order of 1 0 to 1 00 nanometers in order to generate physical-chemical reactions only through mechanical forces . To illustrate the invention and to give those skilled in the art a better understanding of the invention, the results obtained for the preparation of various perovskites and perovskite-like materials are given below.

In a first mode, the mechanosynthesized perovskite is LaCoO3. Therefore, for this example, La remains for A and Co remains for B in the empirical formula ABO3. However, a person skilled in the art will understand that the range of application of the current process is much greater since A comprises at least one element selected from the group consisting of Al, Y, Na, K, Rb, Cs, Pb , La, Sr, Ba, Cr, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table; and B comprises at least one element selected from the group consisting of Al, Ga, In, Zr, Nb, Sn, Ru, Rh, Pd, Re, Os, Ir, Pt, U, Co, Fe, Ni, Mn , Cr, Ti, Cu, Mg, V, Nb, Ta, Mo and W. As will be explained in greater detail below, CeCuO3, LaAIO3, LaMnO3, LalnO3, YC0O3 and SrFeO3 have also mecanosintetizado using the process of the present invention. According to the process of the invention, perovskites of the formula Ai-aA'aBT-bB'bOs can also be prepared, where A and A 'are of equal or different valence and B has the same or another valence as B'. Multiple oxides (triple, quad, etc.) can also be produced such as LaaSp.aCoO-5, LasSr .aCo Fe? .bO3 and Laa? Sra2Ba? .ai-a2C? Bi Feb2Ni? - i-b2? 3, only by the selection and mixing of the initial powders according to the stoichiometric proportion that constitutes the desired perovskite. Among these potential products, those with the most important commercial values are LaCoO3, La0.8Sr0.2C? O.85Feo.i5? 3 and NdMnO3. In the present, examples of Lao mechanosynthesis are included.6Sro. C? O.8 Feo.2? 3, Lao.6Sr0. CoO3, La0.6Sro. MnO3 and LaMn0 8Mg0 2O3. In another preferred embodiment, the mechano-synthesized metal oxide is "perovskite-like" material comprising basic perovskite cells separated by intervening oxide layers. Perovskite-like materials have the general formula [(ABO3) n + CyOz] where A remains as a cation selected from the group consisting of Al, Y, Na, K, Rb, Cs, Pb, La, Sr, Ba , Cr, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table; B remains as a cation selected from the group consisting of Al, Ga, In, Zr, Nb, Sn, Ru, Rh, Pd, Re, Os, Ir, Pt, U, Co, Fe, Ni, Mn, Cr , Ti, Cu, Mg, V, Nb, Ta, Mo and W; and C remains as a cation selected from the group consisting of the cations of groups A and B combined. More than one cation may be presented for each A, B and C. An example of the mechanosynthesis of [SrFe0.5C? O.5? 3 + Feo.5? X «] or SrFeCo0.5? 3+ is included herein. x * where 0 < x * < 10. In a further preferred embodiment of the invention, the mechano-synthesized metal oxide are non-stoichiometric compounds derived from perovskite or perovskite-like materials. These non-stoichiometric compounds are characterized in that although they maintain the crystal structure of the perovskite or perovskite-like materials, their oxygen content in the perovskite part deviates from the regular stoichiometric content of ABO3. Non-stoichiometric compounds derived from perovskite have the general formula (ABO3.?) And non-stoichiometric compounds derived from perovskite-like materials have the general formula [(ABO3.x) n + CyOz]. In all these non-stoichiometric compounds, metal ions with a different valence can replace ions both A and B, thus generating the non-integral numbers of oxygen in the formula. La0.8Sr0.2CoO3.x and La0.8Sro.2Mn? 3-x are examples of non-stoichiometric compounds derived from perovskite-like materials. Other examples of such deviations to stoichiometry are obtained by making a perovskite or perovskite-like material deficient in oxygen. For example, the structure of brown millile (ABO2.5) is formed from perovskites (ABO3). Examples of the mecanosynthesis of SrFeO2.s are included herein. [SrFeO2.5 + Fe0.5? 0.5+?] Or SrFe? .5O3 + x and [(SrFeO2.5) 7 + Fe3O4.5] or S ^ Fe ^ Oz ?. To form the perovskite or perovskite-like crystal structure, the starting materials are selected on a basis of availability and cost, taking into account that the shape is adequate (ie, fine powder) and the unwanted additives are not introduced. in the product. By selecting and grinding the initial powders in specific relative portions, it is also possible to control the nanocrystalline structure of the mechanically synthesized metal oxide. In the preferred embodiments of the invention, lanthanum has been introduced as the elemental oxide La2O3, strontium as the elemental oxide SrO, cobalt as the elemental oxide Co3O4 and iron as the elemental oxide Fe2O3. Compounds such as hydroxides, carbonates, nitrates, oxalates, chlorides could also be used. The high energy grinding can be operated under controlled atmosphere, either neutral oxidant or reduction and under pressure or partial vacuum. Suitable atmospheres comprise a gas selected from the group consisting of H2O, He, Ar, N2, O2, H2, CO, CO2, NO, NH3, H2S and mixtures thereof. The temperature can also be controlled to a certain degree. Through the appropriate choice of starting materials and their quantities, oxides and suboxides and the crushing conditions, mainly the atmosphere and its partial pressure of oxygen, it is possible to obtain the same metallic elements, the oxygen-rich cubic phase or the ideal perovskite or the orthorhombic phase deficient in oxygen, the brown milerite phase. This is the case for the strontium-cobalt-iron system. As an example, it is possible to obtain SrCoO3 and SrFeO3, which are cubic perovskites or SrCoO2.5 and SrFeO2.5, which are orthorhombic, the structure of the brown Millerite. The orthohombic structure, with other structures such as the rhombohedral structure, are generally called "perovskite-type" or "perovskite-like" structures and can be interpreted to a certain degree as a deformation of the ideal cubic perovskite. The impurification by other metallic elements as well as the control of the stoichiometry and the concentration of resulting defects (vacancies) are easily controlled by the relative amounts of the starting materials and by the amount of oxygen introduced into the grinding container. Alternately or simultaneously with the introduction of a reactive gas into the atmosphere of high energy grinding, it is possible to add an additive during stage b) of the process. The role of this additive is to provide a layer of non-reactive material, which is sandwiched between the newly created surfaces of the perovskite after impact. This layer prevents the formation of chemical bonds between the two surfaces created when the particles are fractioned, thus maintaining a high specific surface area. The additive must not react with the perovskite in the sense that it should not diffuse into the perovskite grid. In addition, it must be soluble in water or any other solvent so that it can be rinsed from the perovskite or final product similar to perovskite. Preferably, the additive is added in a solid form although it could also be added in an aqueous form. Suitable additives include LiCl, NaCl, RbCI, CsCl, NH4Cl, ZnO and NaNO3. EXAMPLE I Under normal milling conditions, the initial powders are weighed and mixed in the desired proportion that leads to the composition of the final compound. In this specific example, 3.3 were introduced. g of lanthanum oxide (La2O3) and 1 .7 g of cobalt oxide (Co3O4) in a cylindrical hardened steel container that is 5 mm thick on its wall with three tempered steel spheres [two of 7/16 inches of diameter (1 1 mm) and one of 9/16 inches in diameter (14 mm)]. Preferably, the total weight of the powder inserted into the container is about 5 to 7 g. The container is closed with a thick cover and sealed with a VITOND circular ring. To vary the energy of the grinding impacts, different sets of spheres having different sizes and specific densities can be used. The container is inserted horizontally in a laboratory SPEX ™ stirrer and grinding usually proceeds at a stirring speed of 1000 cycles per minute for a period ranging from 1 to 20 hours. Although the process of milling at room temperature, the shocks of the numerous spheres inside the container increase its temperature. In this way, the container is cooled by ventilation and its temperature is thus kept below 40 ° C. Sampling was also carried out at 1, 4, 8, 16 and 20 hours of grinding and the crystalline structure of the product was determined by X-ray diffractometry using a PHILIPS ™ diffractometer or a SIEMENS D5000 ™. In both cases, CuKa (lambda = 1 .54 Angstrom) was used. The spectrum was recorded in a stage that explored from 10 to 80 ° at a 2T angle with 2.4 s for each 0.05 ° stage. The correct identification of the compounds was carried out by comparing the patterns obtained with the patterns found in a pattern library. The specific surface area of the product was determined by using the Brunauer, Emmet and Teller (BET) method using a computer controlled absorption analyzer (OMNISORB 100 ™) from Omicron, operating in a continuous mode. Samples of approximately 1 g were heated under a vacuum at various temperatures (see Table 1) until complete removal of moisture (20 to 24 hours) prior to the adsorption-desorption experiments. The measurement of nitrogen adsorption was carried out at liquid nitrogen temperature, with a scanning pressure of up to 75 Torr. Figures 1 to 4 illustrate patterns of X-rays that allow the deduction of the evolution of the crystalline structure from the product found in the container at different periods during grinding.

As seen in figure 1, after one hour of grinding, the typical patterns of the two starting oxides La2O3 and Co3O4 are clearly observed. The structure of perovskite type (LaCoO3) begins to become patent. As shown in Figures 2 and 3, after 4 and 8 hours of grinding, the intensities of the starting oxide peaks gradually decrease. It can also be seen that the peaks of the perovskite type structure grows according to the above. After sixteen hours of grinding (Figure 4), the contents of the container practically all turn into perovskite since the patterns of the two starting oxides have almost completely disappeared. The main peaks thus allow the presence of the perovskite structure compound to be shown exclusively. In fact, this X-ray diffraction pattern shows that approximately 95% of the dust content inside the container consists of perovskite after sixteen hours of grinding. Measurements of the specific surface area revealed that this final compound has a specific surface area of approximately 16 m2 / g, a value considerably greater than that of the conventional method which is only of the order of a few m2 / g. It has also been found that milling performance can be increased by replacing the normal milling atmosphere. For example, in a second variant of the process, the grinding atmosphere was replaced by injecting pure oxygen (O2) into the container. This caused the speed of the reaction to increase slightly. The complete conversion (as evaluated by X-ray diffraction) of the oxides of initiation to perovskite was obtained in 14 hours compared to 16 hours when the atmosphere was not changed. Similarly, it is believed that the use of other gases such as CO2, NO2, NH3 and H2S, instead of using ambient air as the reaction atmosphere, can have a positive effect on the grinding reaction. More particularly, it increases the speed of the reaction and / or increases the specific surface area of the resulting perovskite. Since grinding is normally carried out in a steel container, iron contamination in the final compound was measured. The analysis showed that after 20 hours of normal milling, this contamination is lower since it constitutes less than 1% of the final compound as detected by scanning electron microscopy (data not shown). In order to improve the specific surface area of the perovskite obtained under normal milling conditions, various grinding conditions were examined. These include: replacing the tempered steel spheres and the container with tungsten ones; the increase in the duration of the grind; and the subjection of the perovskite obtained after normal milling conditions to a subsequent grinding stage, called a post-treatment under a modified atmosphere. EXAMPLE II It is believed that during high-energy grinding by the use of a sealed container, the oxygen contained in trapped air is rapidly consumed by metal atoms exposed to the surface created by the glass breaking under the impacts -petitive inside. of the container. In this way, grinding is carried out very rapidly under an atmosphere of inert nitrogen. In such indications, the exposed surfaces "stick to the back" in snjunto, giving perovskita with a lower specific surface area. Therefore, as for the first stage, the Applicant modified the normal grinding atmosphere in order to increase the specific surface area. For example, in a third embodiment, the perovskite was first synthesized in a sealed container according to example 1. then, the newly synthesized perovskite was ground to energy "further lifted for a period of up to 72 hours under an atmosphere of constant oxygen. The oxygen level was maintained at a normal level (air) by replacing the sealed gasket of the container with a filter paper ring in order to allow normal air to enter the container. In doing so, the BET specific surface area of the milled perovskite was increased from about 16 m2 / g to about 23 m2 / g. Likewise, it is believed that the use of other reactive gases such as CO2, NO, NH3 and H2S, instead of using ambient air as the reaction atmosphere, can have a positive effect on the grinding reaction (increase in speed of the reaction and / or increase of the specific surface area of the resulting perovskite, etc.). EXAMPLE III In a fourth variant, the perovskite was obtained after normal grinding conditions in a tungsten carbide container. However, since the density of the tungsten carbide spheres is greater than that of the spheres of hardened steel, the speed of agitation must be reduced to avoid the destruction of the container or spheres. In a fifth variant, the perovskite obtained under normal milling conditions was post-treated. This post-treatment comprises the additional high-energy grinding of the perovskite under a humidified atmosphere. Preferably, upon obtaining said humidified atmosphere, a small amount of water (six drops) was simply added to said perovskite (~ 5 g), the container was sealed and completely subjected to a subsequent normal grind of one to six hours. Table 1 presents the measurements of the specific surface area by using the BET method after grinding lanthanum oxide (La2O3) and cobalt oxide (Co3O4) in order to obtain a perovskite structure of the LaCoO3 type according to the preferred embodiments. first, fourth or fifth of the invention. As can be appreciated, grinding in a tungsten carbide container (samples 1 and 2) does not improve the specific surface area of the resulting perovskite compared to the perovskite obtained inside a hardened steel container (example 1, sample 3) . However, the additional high-energy grinding of the perovskite under a humidified atmosphere provides a perovskite having a specific surface area of up to about 36 m2 / g (sample 4), one of the highest values achieved in the matter. The atmosphere of humidified grinding, created by the addition of water during the subsequent grinding of the perovskite, is therefore one of the factors that have a significant positive influence on the increase of the specific surface area of the perovskite obtained according to this process . The catalytic activity of post-treated perovskite (sample 4) was also evaluated and compared with the catalytic activity of sample 1. As seen in Table 2, the perovskite obtained after the post-treatment has a Minimum Total Conversion Temperature (MTTTC) lower than that of the untreated perovskite. It has been calculated that this difference of 70 ° in favor of the post-treated sample corresponds to a higher catalytic activity by a factor of approximately 600 to 2000 times over sample 1. Such an increase is vastly higher (from about 50 to 200 times) than it should have been for a perovskite that has a specific surface area of 36 m2 / g and the same activity per unit surface area as sample 1, since the proportion of the area Specific surface area of the post-treated perovskite (sample 4) over the untreated perovskite (sample 1) is only 1 1 .6 (36 / 3.1). These results show that, apart from having a high specific surface area, the post-treated perovskite obtained according to this variant of the process of the invention also has a high density of lattice defects, thus having a greater catalytic activity. A high density of lattice defects is a feature that is highly desirable for the eventual application of perovskite as a catalyst and in electronic conductive components.

EXAMPLE IV Table 3 presents the measurements of the specific surface area by the use of the BET method after the mechanization of various perovskite products. According to the sixth preferred embodiment of the invention, a grinding additive is introduced into the container during the post-treatment of a perovskite sample. The role of this additive is to provide a thin film that is sandwiched between the two faces of an invoice in the perovskite crystal lattice as it forms after impact with the spheres in the grinding process. This film prevents the two surfaces from recombining with each other and preserves a high specific surface area in the final product. The additive should not react with the perovskite and in particular, should not diffuse into the perovskite grid. In addition, it must be soluble in water or in another solvent that allows it to be separated by filtration of the mixture after the post-treatment. As shown in Table 3, in the case of LaCoO3 perovskite, several additives have been shown to be efficient, including Lithium chlorides (samples 6 and 7), sodium (samples 5 and 8) and ammonia (sample 1 8) , sodium nitrate (sample 9). The highest specific surface area of 82.5 m2 / g was achieved through the use of zinc oxide (sample 17). ZnO was filtered out of the sample by the use of a solution of ammonium chloride. In Table 3, different values of BET surface area are reported for some of the samples (samples 5 and 8). Different values are obtained for the same sample treated for two hours in pure oxygen at the reported temperature. This shows that mechanosynthesized perovskites can maintain rather high surface areas even after calcination at 300-500 ° C. The additive can be introduced into the container as a powder or as a saturated solution. The results in Table 3 indicate that the process involving an additive in the post-treatment stage also produced a high surface area of BET for such solids as CeCuO3 (samples 10 and 11), YCoO3 (sample 14) and perovskites more complexes such as Lao.6Sr0. C? O.8Fe0.2? 3 (sample 12), La0.ßSr0 4CoO3 (sample 1 3), La0.6Sr0. MnO3 (sample 1 5) and LaMn0.8Mg0.2? 3 (sample 16). These results demonstrate that the present invention provides a very simple process that avoids any heating at elevated temperature for the preparation of perovskites of unprecedented high surface area. Consequently, the resulting solids have potential applications as highly active catalysts for low temperature oxidations and as electrocatalysts for the Catholic reduction of oxygen. EXAMPLE V Table 4 presents the mechanosynthesis of various products similar to perovskite. In these experiments, the objective was not to improve the specific surface area but to demonstrate the ability of the technique to control the stoichiometry of perovskite-like materials and their deviation towards the stoichiometric content of oxygen. The comparison of samples 21 and 22 shows how easy it is to produce either the brown millile or the perovskite of the same cationic composition. Starting with one mole of SrO and one and a half mole of Fe2O3, the brown milliliter of SrFeO2.s is obtained (sample 21) as demonstrated by X-ray diffractometry, taking into account that additional oxygen is not admitted during grinding. In the other case, that is, when the oxygen gas is deliberately introduced during grinding simply by the frequent opening of the grinding, the perovskite of SrFeO3 is obtained (sample 22). Experiments 19 and 23 show that similar control of oxygen vacancies with perovskite-like materials is possible. The sample 19 obtained with 1 mole of SrO and% of one mole of Fe2O3 as the starting material and without readmitting oxygen in the grind after it was first closed in the air, is a compound similar to brown milerite of type [SrFeO2 .5 + Fe0.5? O.5 + x *] where 0 <; x * < 10. Sample 23 prepared from 1 mole of SrO, one mole and a half of Fe2O3 and 1/6 mole of Co3O4 in an oxygen-rich grinding atmosphere is a perovskite-like material of composition [SrFe2.5Co0.5 ? 3 + Fe0.5Ox «] where 0 < x * < 10. Normally, the two samples 19 and 23 have a very similar total composition, ie SrFe? .5O3 + x. (sample 19) and SrFeC? o.sO3 + x. (sample 23), but its basic crystal structure is a brown mill in the first case and a perovskite in the latter case, the difference being induced by the addition of oxygen gas in the grinding atmosphere. The sample 20 was obtained from a powder prepared in the same manner as the sample 19 but also subjected to an agglutination process in a press that exerts a pressure of 2 ton / cm2 for 1 hour at a temperature of 1 100 ° C. . sample 20 was found in the form of a crack-free ceramic, wafer 0.5 μm thick, uniform, with a brown millile structure. This experiment demonstrated that the perovskite and perovskite-like materials manufactured according to the present invention are especially well suited for the preparation of thin ceramic membranes of uniform composition. The basic reason is that the compounds of milled by sphere to high energy of the perovskite type, ensure the spatial uniformity in the composition of the material at atomic level after the process of molding the ceramics. There is a very important advantage of the present invention, the products of which will produce high quality ceramics to be produced on a commercial scale. According to another aspect of the invention, the metal oxides obtained according to the process of the present invention can be contaminated with a transition group metal or a precious group metal. In industry, the doping of a metal oxide used as a catalyst allows the reduction of sulfur poisoning (SO2). Preferably, the doped metal used to contaminate the metal oxides is selected from the group consisting of osmium, iridium, platinum, ruthenium, rhodium, palladium, iron, cobalt, nickel and copper. Doping is preferably carried out only once the metal oxide has been synthesized since early doping would reduce the specific surface area of the synthesized metal oxide. Advantageously, the doping can be carried out during the post-treatment step with the. help of a piece of the selected doping target, inserted in the container during subsequent high-power grinding. However, the metal oxide obtained after grinding could also be doped by treating this metal oxide with a deposit of the doping metal by using methods known to a person skilled in the art. Although preferred variants of the invention according to the present invention have been described in detail herein and are illustrated in the accompanying figures and tables, it should be understood that the invention is not limited to these precise modalities and that various changes can be made and modifications in it without departing from the spirit or scope of the invention. For example, a planetary grinding machine could be used by spheres instead of a horizontal crusher. The initial powders could also be ground before grinding at high power. Similarly, the perovskite obtained after grinding according to the process of the invention, could also be treated to increase its catalytic activity by removing its contamination by iron.

TABLE 1 Measurements of the specific surface area by using the BET method TABLE 1 Measurements of the specific surface area by using the BET method Surface area measurements (BET) Sample Conditions and duration Temperature Surface grinding area Heating (m / g) ° C "Initial powders and their relative molar ratio TABLE 2 Comparison of the catalytic activity * of standard perovskite with the post-treated perovskite obtained according to the method of the invention Perovskite Surface area MTTC * specific (m2 / g) Not treated (sample 1) 3.1 295 ° C Post-treated (sample 36 225 ° C * Measured by the conversion of n-Hexane (Conditions: ycei = 1%, yo2 = 89.1%, Catalyst weight = 0.105 + 0.0015 g) ** MTTC = Minimum Total Conversion Temperature at a space velocity of 22,500 h "1 TABLE 3 Measurements of the specific surface area by using the BET method TABLE 3 Measurements of the specific surface area by using the BET method TABLE 3 Measurements of the specific surface area by using the BET method TABLE 3 Measurements of the specific surface area by using the BET method TABLE 3 Measurements of the specific surface area by using the BET method TABLE 3 Measurements of the specific surface area by using the BET method TABLE 3 Measurements of the specific surface area by using the BET method TABLE 3 Measurements of the specific surface area by using the BET method "Initial powders and their relative molar ratio TABLE 4 Preparation of brown mills and other perovskite-like materials * Initial powders and their relative molar ratio

Claims

CLAIMS 1. A process for mechanically synthesizing a metal oxide which has a perovskite or perovskite-like crystal structure and a predetermined stoichiometric oxygen content, with the metal oxide being selected from the group consisting of perovskites of the general formula ABO3; perovskite of the general formula [(ABO3) n + CyOz], non-stoichiometric compounds derived from perovskites and having the general formula (ABO3.x), and non-stoichiometric compounds derived from materials similar to perovskite and having the general formula [ (ABO3.x) n + CyOz], wherein: • A comprises at least one element selected from the group consisting of Al, Y, Na, K, Rb, Cs, Pb, La, Sr, Ba, Cr, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table - B comprises at least one element selected from the group consisting of Al, Ga, ln, Zr, Nb, Sn, Ru , Rh, Pd, Re, Os, Go, Pt, U, Co, Fe, Ni, Mn, Cr, Ti, Cu, Mg, V, Nb, Ta, Mo and W; • C represents at least one element selected from the group consisting of Al, Ga, In, Zr, Nb, Sn, Ru, Rh, Pd, Re, Os, Go, Pt, U, Co, Fe, Ni, Mn , Cr, Ti, Cu, Mg, V, Nb, Ta, Mo, W, Al, Y, Na, K, Rb, Cs, Pb, La, Sr, Ba, Cr, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table; • n represents an integer between 1 and 10; • 0 < x < 3 • y represents an integer between 1 and 5; • z represents an integer between 1 and 5; said process comprising the step of holding a mixture of initial powders formulated to contain the components represented by A, B and C in the formulas to a milling of high energy sufficient to induce the chemical reaction of the components and mechanes and thus directly synthesize said metal oxide in the form of a perovskite or perovskite-like material having a nanocrystalline structure, as determined by X-ray diffractometry. The process according to claim 1, characterized in that the high-energy grinding is carried out under a controlled atmosphere at In order to control the nanocrystalline structure and the stoichiometric oxygen content of the mechanosynthesized metal oxide. 3. The process according to claim 2, characterized in that the atmosphere comprises a gas selected from the group consisting of He, Ar, N2, O2, H2, CO, CO2, NO, NH3, H2S and mixtures thereof. The process according to any of claims 1 to 3, characterized in that it further comprises the step of selecting and grinding the initial powders in relative portions to control the nanocrystalline structure of the mechanically synthesized metal oxide. 5. A process for mechanically synthesizing a metal oxide having a perovskite or perovskite-like crystal structure and a predetermined stoichiometric content of oxygen and a high specific surface area of BET, said metal oxide being selected from the group consisting of perovskites of the general formula ABO3; materials similar to perovskite of the general formula [(ABO3) n + CyOz]; non-stoichiometric compounds derived from perovskites and having the general formula (ABO3_x); and non-stoichiometric compounds derived from perovskite-like materials and having the general formula [(ABOs-x) n + CyOz], wherein: - A comprises at least one element selected from the group consisting of Al, Y, Na , K, Rb, Cs, Pb, La, Sr, Ba, Cr, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table; • B comprises at least one element selected from the group consisting of Al, Ga, In, Zr, Nb, Sn, Ru, Rh, Pd, Re, Os, Ir, Pt, U, Co, Fe, Ni, Mn , Cr, Ti, Cu, Mg, V, Nb, Ta, Mo and W; • C represents at least one element selected from the group consisting of Al, Ga, In, Zr, Nb, Sn, Ru, Rh, Pd, Re, Os, Ir, Pt, U, Co, Fe, Ni, Mn , Cr, Ti, Cu, Mg, V, Nb, Ta, Mo, W, Al, Y, Na, K, Rb, Cs, Pb, La, Sr, Ba, Cr, Ag, Ca, Pr, Nd, Bi and the elements of the lanthanide series of the periodic table; • n represents an integer between 1 and 10; • 0 < x < 3; • and represents an integer between 1 and 5; • z represents an integer between 1 and 5; said process comprising the step of subjecting a mixture of initial powders formulated to contain the components represented by A, B and C in the formulas to a milling of high energy sufficient to induce the chemical reaction of the components and mechanics and thus directly synthesize said metal oxide in the form of a perovskite or perovskite-like material having a nanocrystalline structure, as determined by X-ray diffractometry; b) increasing the BET specific surface area of the metal oxide obtained in step a) by further clamping said metal oxide to high energy mill to obtain a metal oxide having a high specific BET surface area. The process according to claim 5, characterized in that the high-energy milling of step a) is carried out under a controlled atmosphere in order to control the nanocrystalline structure and the stoichiometric oxygen content of the mechanosynthesized metal oxide. The process according to claim 5 or 6, characterized in that it further comprises the step of adding a small amount of an aqueous solution to the metal oxide during grinding of step b) in order to obtain a humidified metal oxide. The process according to claims 5, 6 or 7, characterized in that the high energy milling of step b) is carried out under a controlled atmosphere in order to control the BET specific surface area of the mechanosynthesized metal oxide. 9. The process according to any of claims 5 to 8, characterized in that the atmosphere comprises a gas selected from the group consisting of H 2 O, He, Ar, N 2, O 2, H 2, CO, CO, NO 2, NH 3, H 2 S and mixtures thereof. 10. The process according to any of claims 5 to 9, characterized in that it further comprises the step of selecting and grinding the initial powders in relative portions to control the final nanocrystalline structure of the mechanosynthesized metal oxide. eleven . The process according to any of claims 5 to 10, characterized in that it further comprises the steps of: c) adding a non-reactive soluble additive during grinding of step b); and d) subsequently rinsing said soluble additive. The process according to claim 1, characterized in that the non-reactive soluble additive is selected from the group consisting of LiCl, NaCl, RbCI, C3CI, NH4Cl, ZnO, NaNO3 and mixtures thereof. 13. A metal oxide having a perovskite or nanocristalline structure similar to perovskite obtained according to any of the processes of claims 1 to 12. The metal oxide according to claim 13, characterized in that it consists of a brown millimeter having the formula ABO2 5 or [(ABO2.5) n + CyOz]. 15. The metal oxide according to claim 14, characterized in that the brown milliate is selected from the group consisting of Sr7Fe 0O22, SrFeO2.5 and SrFeo.5Coo.5O2.5-16. The metal oxide according to claim 13, characterized because it has a specific surface area of BET of between 3.1 and 8.2 m2 / g. 17. A perovskite having the formula LaCoO3, characterized in that it has a specific surface area of BET of at least 20 m2 / g. 18. The perovskite according to claim 1 7, characterized in that it has a specific surface area of BET of between 20 m2 / g and 82.5 m2 / g. 1 9. A perovskite having the formula CeCuO3, characterized in that it has a specific surface area of BET of at least 30.3 m2 / g. 20. The perovskite according to claim 1 9, characterized in that it has a specific surface area of BET of between 30.3 m2 / g and 39. 2 m2 / g. twenty-one . A perovskite having the formula YCoO3, characterized in that it has a specific surface area of BET of at least 9.6 m2 / g. 22. The perovskite according to claim 21, characterized in that it has a specific BET surface area of between 9.6 and 24.2 m2 / g. 23. A perovskite having the formula La0 ßSr0 CoO3, characterized in that it has a specific surface area of BET of at least 12.7 m2 / g. 24. The perovskite according to claim 23, characterized in that it has a specific BET surface area of between 12.7 and 30.2 m2 / g. 25. A perovskite having the formula La0.6Sro.4MnO3, characterized in that it has a specific surface area of BET of at least 45.4 m2 / g. 26. A perovskite that has the formula Lao.6Sro. C? O.8Fe0.2? 3, characterized in that it has a specific BET surface area of at least 20.2 m2 / g. 27. The perovskite according to claim 23, characterized in that it has a specific BET surface area of between 20.2 and 47.8 m2 / g.