JP2006027933A - Method for producing multiple oxide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a porous multiple oxide, which is excellent in the controllability of pore structure and high in the yield. <P>SOLUTION: The method for producing the porous multiple oxide comprises mixing a solution, obtained by dissolving a compound of a first metal element which forms a hydroxide or an oxide when it is hydrolyzed in an organic solvent and an emulsion containing ions of metal elements of second and further metal elements in a water phase inside a reverse micelle which is formed by a surfactant in an organic solvent, then hydrolyzing the compound of the first metal element at the interface of the reverse micelle, at the same time, incorporating the second and further metal elements, forming primary particles of a precursor of the multiple oxide by polycondensing them, further forming secondary particles by aggregating the primary particles in the system containing them, and aggregating the secondary particles. The concentration of cations except hydrogen ion in the water phase inside the reverse micelle is set to be ≥2 mol/L and the volume ratio (O/W) of an oil phase to a water phase is set to be ≤40 during hydrolysis. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for producing a composite oxide that can be used as a catalyst for purifying exhaust gas from an internal combustion engine.

  A composite oxide is an oxide in which two or more kinds of metal oxides form a compound, and an oxide of an oxoacid does not exist as a structural unit. One important application of this composite oxide is a catalyst and a catalyst carrier, and in particular, an exhaust gas purifying catalyst for an internal combustion engine is known. An example of the manufacturing method is described in Patent Document 1.

The method described in Patent Document 1 is a method that can be referred to as a so-called microemulsion method, in a water droplet (reverse micelle) forming a water-in-oil type (W / O type) microemulsion. Mixing and stirring the microemulsion in a solution in which a metal ion having catalytic activity is contained in the aqueous phase, and the supporting metal compound that is hydrolyzed to produce a hydroxide or oxide is dissolved. To cause hydrolysis of the metal compound for support at the interface of the reverse micelles and to produce primary particles in the reverse micelles. The primary particles agglomerate with each other in the aqueous phase in the reverse micelles to form secondary particles, and the reverse micelles repeatedly collide, fuse and separate, so that the aggregation of the secondary particles gradually proceeds, and thus to some extent Aggregates (so-called tertiary particles) of secondary particles grown to a size of 2 are washed, dried and then fired to obtain a metal composite oxide powder.
Japanese Patent No. 3466856

  The composite oxide used as the exhaust gas purification catalyst preferably has a porous structure in order to increase the surface area and improve the catalytic activity. In the method of Patent Document 1 described above, primary particles are aggregated. As a result, voids are formed between the primary particles, and the secondary particles having the voids aggregate to form voids between the secondary particles, resulting in a porous structure.

  However, depending on the form of agglomeration, the pore diameter and pore volume may not always satisfy the requirements. In other words, if reverse micelles collide and fuse while secondary particles do not grow sufficiently, the secondary particles in each reverse micelle will aggregate at that time, rather than primary particles agglomeration. May result in large aggregates of particles. The porous body produced in this way contains many small-diameter pores, so although it has a large surface area, it is heated to a high temperature while it is used as a catalyst, and sintering progresses early. In other words, a so-called clogging occurs and the flow of exhaust gas is hindered.

  The present invention has been made paying attention to the above technical problem, and can produce desired secondary particles stably, can easily control the pore diameter, and can produce a high yield composite oxide. It is intended to provide a method.

  In order to achieve the above object, the invention of claim 1 further aggregates secondary particles, which are aggregates of primary particles of a composite oxide containing two or more metal elements, and aggregates the secondary particles. In a method for producing a composite oxide having a porous structure in which an aggregate is fired, a solution in which a hydroxide or a compound of a first metal element that generates a hydroxide by hydrolysis is dissolved in an organic solvent, and in the organic solvent The aqueous phase inside the reverse micelle formed by the surfactant is mixed with an emulsion containing ions of the second and subsequent metal elements to hydrolyze the compound of the first metal element at the interface of the reverse micelle and the second Subsequent metal elements are taken in and polycondensed to form primary particles of a composite oxide precursor, and in the system including the primary particles, primary particles are aggregated to form secondary particles, and the secondary particles are further formed. Agglomerate The cation concentration excluding hydrogen ions in the aqueous phase in the reverse micelle during the hydrolysis is 2 mol / L or more, and the volume ratio of the oil phase to the aqueous phase is 40 or less. is there.

  In the present invention, as described in claim 2, it is preferable that the diameter of the reverse micelle during the hydrolysis is 20 nm or more.

  According to the invention of claim 1, by mixing and stirring the solution and the emulsion, hydrolysis of the first metal compound occurs at the interface of the reverse micelles, and the hydroxylation takes in the second metal element. Primary particles of products or oxides are generated, and aggregation of the primary particles occurs in the aqueous phase in the reverse micelle. In that case, since the concentration of the cation excluding hydrogen ions in the aqueous phase in the reverse micelle is adjusted to 2 mol / L or more, the volume ratio of the oil phase to the aqueous phase is as low as 40 or less. Even if the micelle concentration is high, electrical repulsion occurs between the reverse micelles, and fusion (or coalescence) is prevented or suppressed even when the reverse micelles collide. As a result, until the secondary particles grow to some extent in the reverse micelle, the secondary particles do not suppress or cause agglomeration, so the controllability of the diameter of the pores formed by the aggregation of the secondary particles is good. become.

  Further, the invention according to claim 1 separates the reverse micelles including the secondary particles by the electric repulsive force based on the electric potential of each, so that even if the reverse micelles collide, the reverse micelles are fused (or united). ) And the accompanying aggregation of secondary particles in each reverse micelle is prevented or suppressed. As a result, the relative concentration of the aqueous phase is increased while preventing or suppressing the aggregation of secondary particles between each reverse micelle. can do. Therefore, the yield is improved compared to the method of suppressing the reverse micelle fusion (or coalescence) by reducing the concentration of reverse micelles (or aqueous phase) containing secondary particles to the solvent (or oil phase). As a result, the productivity of the composite oxide can be improved and the cost can be reduced.

  In particular, according to the second aspect of the present invention, when the reverse micelle diameter is 20 nm or more, the growth field (or reaction field) of the secondary particles is large, or the amount of primary particles contained in the reverse micelles is large. Since it is sufficiently secured, it is possible to promote the growth of secondary particles and enlarge the secondary particles including pores.

  As an example, the composite oxide having a porous structure produced by the method of the present invention is an aggregate of primary particles 1 of a composite oxide having a particle size of about 5 to 15 nm, as shown in FIG. The secondary oxide 2 is agglomerated so that the composite oxide has not only pores between the primary particles 1 but also mesopores 3 having a diameter of 5 to 30 nm between the secondary particles 2. It is. In addition, the porous composite oxide of the present invention has almost no change in pore volume after firing at a high temperature of 600 ° C. compared to before firing, and pores having a diameter of 10 nm or more have a total pore volume. It accounts for more than 10%. In this specification, the pore diameter is a value measured by a nitrogen adsorption method at a liquid nitrogen temperature. The pore volume distribution is a value obtained by the BJH method, which is a general calculation method, from the nitrogen adsorption amount with respect to the nitrogen introduction pressure.

  The kind of the porous complex oxide in the present invention is not particularly limited as long as it is a complex oxide containing at least the first metal element and the second metal element. Complex oxide systems are known in many textbooks, handbooks, etc., and oxidation of many metal elements that form metal oxides such as alumina, zirconia, ceria, silica, iron oxide, manganese oxide, chromium oxide, yttrium oxide, etc. Most of the products can add a second and subsequent metal elements to form a composite oxide. It is known itself what elements form a complex oxide. The present invention can be applied to all the composite oxides as long as there are hydrolyzable raw materials or inorganic metal salt raw materials.

An example of such a composite oxide is a cerium-zirconium composite oxide. This composite oxide has a crystal structure of zirconium oxide Zr ₂ O, and a part of zirconium in this crystal structure is substituted with cerium. Conventionally, cerium oxide was supported on a carrier together with a catalyst metal. When a catalyst was used at a high temperature, the oxygen storage / release capacity (OSC) of cerium oxide decreased due to crystal growth of cerium oxide. When used as an oxide, the OSC can be prevented from lowering even when used at high temperatures. Furthermore, the composite oxide of the present invention has sufficiently large pores even after calcination at a high temperature, and can diffuse HC having a large molecular weight such as HC in diesel exhaust gas into the carrier. And can be purified by exhibiting OSC.

  Another example is a complex oxide of a rare earth metal such as lanthanum and zirconium. When a part of zirconium in the crystal structure of zirconium oxide is substituted with lanthanum, since zirconium is tetravalent and lanthanum is trivalent, oxygen defects in which no oxygen is present are formed in the crystal lattice. When an alkali metal is added to this composite oxide, electrons are donated to oxygen defects. Electron-donated oxygen vacancies have a very strong basicity, and thus electron-donated oxygen vacancies constitute strong base points. At such a strong base point, nitric oxide NO in the exhaust gas is captured, and as a result, a large amount of nitric oxide is adsorbed in the composite oxide. That is, this composite oxide has a NOx occlusion action and can be used as a NOx occlusion reduction catalyst. The lanthanum-zirconium composite oxide has sufficiently large pores even after firing at a high temperature, can diffuse the exhaust gas quickly, and can make exhaust gas purification more efficient.

  Next, the manufacturing method of the composite oxide of this invention is demonstrated. In the method of the present invention, primary particles of the composite oxide are generated, the primary particles are aggregated to generate secondary particles, and the secondary particles are aggregated together. Suppress until the next particle grows to a certain size. In particular, in the present invention, reverse micelles (preferably reverse micelles having a diameter of 20 nm or more) dispersed in an organic solvent are formed as a reaction field for primary particle generation and secondary particle generation / growth by aggregation thereof, Adjust so as to suppress the fusion (or coalescence) of the reverse micelles. Specifically, this is done by adjusting the concentration of cations (excluding hydrogen ions) inside the reverse micelle to 2 mol / L or more, and the volume ratio of the oil phase to the water phase of the microemulsion is It is adjusted to 40 or less. The state is schematically shown in FIG. 1, and the reverse micelle 6 dispersed by the surfactant 5 in the organic solvent 4 is charged by the adsorption of the cation of the aqueous phase 7 therein. As a result, electrical repulsion between the reverse micelles 6 occurs, and even if the reverse micelles 6 collide with each other, fusion is suppressed, and aggregation of secondary particles in the aqueous phase 7 inside thereof is avoided.

  In the method for producing a porous composite oxide of the present invention, a solution in which a compound of a first metal element that is hydrolyzed to form a hydroxide or an oxide is dissolved in an organic solvent, and a surfactant is added in the organic solvent. The aqueous phase inside the reverse micelle to be formed is mixed with an emulsion containing ions of the second and subsequent metal elements to hydrolyze the first metal element compound at the interface of the reverse micelle and the second and subsequent metal elements. And polycondensate to form primary particles of a composite oxide precursor.

  If the compound of the first metal element that forms a hydroxide by hydrolysis is referred to as a first metal compound, the metal constituting the first metal compound is not a metal in a narrow sense, but M-OM It means element M in general that can form a bond.

  As the first metal compound, a metal compound generally used in a so-called sol-gel method can be used. For example, metal alkoxide, acetylacetone metal complex, metal carboxylate, metal inorganic compound (for example, nitrate, oxychloride, chloride, etc.) can be used.

  The metal element M that forms the metal alkoxide includes elements from Group 1 to Group 14, group 16 includes sulfur, selenium, tellurium, group 15 includes phosphorus, arsenic, antimony, and bismuth. And some lanthanoid elements are said not to form alkoxides. For example, silicon alkoxide and germanium alkoxide are also referred to as metal alkoxide. As metal alkoxides, various metal alkoxides are commercially available, and since the production methods are known, they are easily available.

Hydrolysis reactions of metal alkoxides M (OR) n (where M is a metal and R is an alkyl group such as methyl, ethyl, propyl, butyl, etc.) are also known, and formally, M (OR) _n + nH ₂ O → M (OH) _n + nROH and then M (OH) _n → MO _{n / 2} + n / 2H ₂ O.

Hydrolysis reaction of acetylacetone metal complex (CH ₃ COCH ₂ COCH ₃ ) _n M (where M is a metal) is also known, (CH ₃ COCH ₂ COCH ₃ ) _n M + nROH → nCH ₃ COCH ₂ C (OH) CH ₃ + M (OH) _n , then M (OH) _n → MO _{n / 2} + n / 2H ₂ O

  The acetylacetone metal complex is easily available because various metal complexes are commercially available and the production methods are also known. Typically, there are aluminum acetonate, barium acetonate, lanthanum acetonate, platinum acetonate, and the like, and there are many more than alkoxides.

  Organometallic compounds such as metal alkoxides and acetylacetone metal complexes are easily dissolved if an appropriate solvent is selected from alcohols, polar organic solvents, hydrocarbon solvents and the like. As the solvent of the present invention, it is preferable to use a hydrophobic (oil-based) organic solvent that can be separated into two phases from the aqueous phase.

  Examples of the organic solvent include hydrocarbons such as cyclohexane and benzene, linear alcohols such as hexanol, and ketones such as acetone. The selection criteria for the organic solvent include the solubility of the surfactant, the area of the microemulsion to be formed (the molar ratio of water / surfactant is large), and the like.

  It is known that when water is added to the organic phase in which the first metal element compound that is hydrolyzed to form a hydroxide or oxide is dissolved in this way, the hydrolysis reaction of the organometallic compound starts and proceeds. It has been. Generally, water can be added to the organic phase in which the first metal compound is dissolved and stirred to obtain a metal hydroxide or metal oxide.

  In this invention, a water-in-oil emulsion containing ions of the second and subsequent metal elements is formed in the aqueous phase inside the reverse micelle in which the aqueous phase is finely dispersed with a surfactant in the organic phase, By adding the solution of the first metal compound to the emulsion, stirring and mixing, in the aqueous phase surrounded by the surfactant in the reverse micelle, it is allowed to react with ions of the second and subsequent metal elements to add water. Disassemble. In this method, it is considered that a large number of reverse micelles become reaction nuclei, or fine particles of the product can be obtained by stabilizing the generated hydroxide fine particles by the surfactant.

  In the hydrolysis reaction as described above, by dissolving a plurality of hydrolyzable metal compounds in the organic phase, when brought into contact with water, the plurality of metal compounds are hydrolyzed to form a plurality of metal compounds. It is also known that hydroxide is formed at the same time.

  In the present invention, one kind of the hydrolyzable metal compound (compound containing the first element) is present in the organic phase, and when the organic phase and the aqueous phase are brought into contact, the second metal element, Further, the third and subsequent metal elements are allowed to exist as ions in the aqueous phase in the reverse micelle.

  For the presence of ions in the aqueous phase, water-soluble metal salts, particularly inorganic acid salts such as nitrates and chlorides, and organic acid salts such as acetates, lactates and oxalates can be used. The ions of the second element present in the aqueous phase may be complex ions containing the second element in addition to the simple metal ions. The same applies to ions of the third and subsequent elements.

  When the organic phase and the aqueous phase are brought into contact with each other, the organometallic compound in the organic phase is brought into contact with water to cause a hydrolysis reaction to produce a first metal hydroxide or oxide. According to the invention, it has been found that metal ions present in the aqueous phase are incorporated into the first metal hydroxide (or oxide) which is the hydrolysis product. This phenomenon has not been known so far. The reason why the ions in the aqueous phase are taken into the hydroxide without performing a special sedimentation operation is not fully understood. However, when the first metal compound is an alkoxide, the alkoxide is hydrolyzed. When the second metal ion in the aqueous phase induces alkoxide and hydrolysis proceeds, or the alkoxide hydrolyzed fine hydroxide captures and aggregates a predetermined amount of metal ion in the aqueous phase. It is thought that it will go.

  According to the present invention, in particular, in this novel production method, the second metal element present in the aqueous phase is contained in the hydroxide obtained by hydrolysis of the compound of the first metal element in the organic phase. Although ions are taken in, a hydroxide in which the first metal element and the second and subsequent metal elements in the obtained hydroxide are dispersed very uniformly can be obtained, and the uniformity is obtained by the conventional alkoxide method, That is, it has been found that it can be remarkably superior to the case where a plurality of metal alkoxides are present in the organic phase. A composite oxide (solid solution) in which the first metal element and the second metal element of the composite oxide after firing were ideally mixed at the atomic level was obtained even at a relatively low firing temperature. Such a thing has not been achieved by the conventional metal alkoxide method. In the conventional metal alkoxide method, since the stability varies depending on the type of metal alkoxide, only a non-uniform product can be obtained between the first metal element and the second metal element.

  The relative ratio of the first metal element and the second metal element in the composite oxide obtained by the present invention is the ratio of the amount of the first metal element in the organic phase and the amount of the second metal element in the aqueous phase. Can be adjusted.

In the present invention, the reaction system is preferably a water-in-oil emulsion system or a microemulsion system. In this case, first, the reverse micelle diameter is extremely small, from several nanometers to several tens of nanometers, and the interface between the organic phase and the aqueous phase is extremely wide (approximately 8000 m ² / liter when the diameter is 10 nm), thereby increasing the hydrolysis rate. Secondly, the aqueous phase is divided into shells, and it is considered that this is due to the effect of homogenization by containing only a small amount of metal ions (about 100) per one.

  On the other hand, the aqueous phase in the reverse micelle is a so-called reaction field that generates primary particles and secondary particles by primary particle aggregation and secondary particle aggregation. The size of the reverse micelle affects the vacancies generated in the film and the vacancy structure of the complex oxide resulting therefrom. In the present invention, in consideration of this point, the diameter of the water phase of the reverse micelle is preferably 20 nm or more.

  Methods for forming water-in-oil emulsion systems or microemulsion systems are known. As the organic phase medium, hydrocarbons such as cyclohexane and benzene, linear alcohols such as hexanol, ketones such as acetone and the like can be used. Surfactants that can be used in this invention are various, such as nonionic surfactants, anionic surfactants, and cationic surfactants, and can be used in combination with organic phase components according to the application. it can.

  Nonionic surfactants include polyoxyethylene (n = 5) nonylphenyl ether represented by polyoxyethylene nonylphenyl ether and polyoxyethylene (n = 10) octylphenyl ether. Polyoxyethylene alkyl ether surfactants typified by ethylene octyl phenyl ether, polyoxyethylene (n = 7) cetyl ether, polyoxyethylene sorbitan surfactants typified by polyoxyethylene sorbitan trioleate, etc. Can be used.

  As the anionic surfactant, sodium di-2-ethylenehexyl sulfonate can be used, and as the cationic surfactant, cetyltrimethylammonium chloride, cetyltrimethylammonium bromide, or the like can be used.

  In the present invention, when producing a composite oxide of three or more elements, the third and subsequent elements are present in the aqueous phase in the reverse micelle. This is because when a plurality of hydrolyzable metal compounds are present in the organic phase, the organic phase becomes a heterogeneous product due to the difference in stability between the hydrolyzable metal compounds. However, it is necessary to be uniform between the first metal element and the second metal element, but if uniformity is not important between the first metal element and the third metal element, the third element These metal compounds may be present in the organic phase.

  The reverse micelle containing the second metal element ion is prepared by an injection method in which the surfactant is dissolved in the organic phase medium, an aqueous solution containing the second metal element ion is added thereto, and the mixture is stirred. Can be formed.

  Thus, the precursor of the complex oxide containing the first metal element and the second metal element by contacting the solution of the first metal compound and the reverse micelle containing the ion of the second metal element in the aqueous phase, and hydrolysis. After the primary particles are formed, the system containing the primary particles is aged at a predetermined temperature (30 ° C. to 80 ° C.) for a predetermined time (2 hours). In this ripening step, the primary particles aggregate to form secondary particles. At this time, rather than agglomerating all the primary particles to form large secondary particles, a relatively small secondary particle is once formed, and then sufficiently large pores are formed between the secondary particles. In order to aggregate the secondary particles, hydrolysis (progression) between reverse micelles is prevented or suppressed, and hydrolysis proceeds, and primary particles and secondary particles are aged.

Specifically, as shown in FIG. 1, the reverse micelle 6 formed by the surfactant 5 having the lipophilic group on the outside and the hydrophilic group on the inside is formed, and the second water phase 7 is formed in the internal aqueous phase 7. It contains metal element ions. Here, the concentration of cations in the aqueous phase 7 excluding hydrogen ions (H ⁺ ) is adjusted to 2 mol / L or more. This can be done by increasing the ion concentration of the second metal element, or by adding alkaline water such as ammonia water and an appropriate buffer solution. As a result, the reverse micelles 6 are charged by internal cations, and electrical repulsion between the reverse micelles 6 occurs. The volume ratio (O / W) between the oil phase (organic solvent) and the water phase (reverse micelle) is set to 40 or less within the range in which the microemulsion maintains the water-in-oil type (W / O type). .

  The concentration of cations (excluding hydrogen ions) in the aqueous phase was 2 mol / L or more. If the concentration of cations (excluding hydrogen ions) was lower than this, the reverse micelles would not be sufficiently charged, and brown This is because the reverse micelle fusion (unification) and the accompanying aggregation of secondary particles are increased due to the collision caused by the motion. In addition, the volume ratio (O / W) of the oil phase (organic solvent) to the water phase (reverse micelle) was set to 40 or less within the range in which the microemulsion maintained the water-in-oil type (W / O type). This is because it is necessary to secure the amount of production per day when industrializing.

  FIG. 2 is a three-phase map showing the relationship between water, surfactant and oil (organic solvent) constituting the microemulsion, and a predetermined amount or more of surfactant is required to form a dispersed phase. Therefore, there is a two-phase separation region on the lower side of the three-phase map, and it is necessary to reduce the surfactant concentration to some extent in order to set the micelle or reverse micelle diameter to a certain extent. A region surrounded by a thick line is a region that becomes a dispersed phase of a microemulsion (water droplet) having a diameter of 20 nm or more. Also, when the volume ratio of water is large in this region, it becomes an oil-in-water type (O / W type), whereas when the volume ratio of oil is large, it becomes a water-in-oil type (W / O type). In the middle, there is an intermediate area (area surrounded by a broken line in FIG. 2). In this intermediate region, a two-phase continuous type in which each of the oil phase and the aqueous phase is continuous is obtained.

  As described above, in the method of the present invention, the volume ratio of the oil phase to the water phase is set to 40 or less in the range of the water-in-oil type (W / O type). This is shown by hatching in FIG. It becomes an area. That is, a surfactant is used to increase the reverse micelle diameter as much as possible, which is adjacent to the intermediate region and is a reaction field for primary particle generation and secondary particle generation / growth due to the high concentration of the aqueous phase. This is a region where the concentration of is as low as possible.

Example 1
A beaker having an internal volume of 5 liters was charged with 3.0 liters of cyclohexane and 224 g of polyoxyethylene (n = 5) nonylphenyl ether, and an aqueous solution composed of 19 g of cerium nitrate and 120 mL of distilled water was added and stirred. Reverse micelles (water-in-oil microemulsions, water droplets measured diameter of 30 nm) were prepared by stirring using a magnetic stirrer at room temperature. Separately, a zirconium alkoxide solution was prepared by dissolving 0.23 mol of zirconium butoxide in 0.8 liter of cyclohexane, and this was added to the microemulsion. At the same time, the pH was adjusted to 5. The volume ratio (O / W) of water (aqueous phase) to cyclohexane (organic phase) at this time is 18. The concentration of cations (excluding hydrogen ions) in the aqueous phase in the reverse micelle was 3.5 mol / L by adding ammonium nitrate for buffering. When this mixture was well stirred at room temperature, the inside of the beaker immediately became cloudy white and colloidal particles (secondary particles, particle size of about 20 nm) were formed.

Next, the pH was adjusted to 8 with aqueous ammonia to control colloid aggregation. Further, the mixture was aged for about 1 hour with stirring. The mother liquor was separated by filtration, and the resulting precipitate was washed with ethanol three times, dried overnight at 80 ° C., and then calcined in the atmosphere at 600 ° C. for 2 hours to obtain a composite oxide containing cerium and zirconium (ceria zirconia: Ce _0.44). Zr _0.6 O _x ) was obtained. The Ce / Zr molar ratio of the composite oxide was 4/6. Further, the yield of the composite oxide was about 1.6 kg / container 100 L.

Example 2
This is an example of producing La _0.2 Zr _0.8 O _1.9 . A beaker having a volume of 5 liters was charged with 3.0 liters of cyclohexane and 202 g of polyoxyethylene (n = 5) nonylphenyl ether, and an aqueous solution composed of 0.058 mol of lanthanum nitrate and 120 mL of distilled water was added and stirred. A reverse micelle (water-in-oil microemulsion, measured water droplet diameter of 30 nm) was prepared by stirring with a magnetic stirrer at room temperature. Separately, a zirconium butoxide / cyclohexane solution was prepared and added to the microemulsion solution. At the same time, the pH was adjusted to 6. The volume ratio (O / W) of water (aqueous phase) to cyclohexane (oil phase) at this time is 18. The concentration of cations (excluding hydrogen ions) in the aqueous phase in the reverse micelle was 4.3 mol / L by adding ammonium nitrate for buffering. When this mixture was well stirred at room temperature, the inside of the beaker immediately became cloudy and colloidal particles (secondary particles, particle diameter of about 20 nm) were formed.

Next, the pH was adjusted to 8.9 with aqueous ammonia to adjust the aggregation of the colloid. Further, the mixture was stirred for about 1 hour for aging. The mother liquor was separated by filtration, and the resulting precipitate was washed three times with ethanol, dried overnight at 80 ° C., and then calcined at 600 ° C. for 2 hours in the atmosphere to obtain a composite oxide containing lanthanum and zirconium (lanthanum zirconia: La _0.2 to obtain a Zr _0.8 O _1.9). The complex oxide had a La / Zr molar ratio of 2/8. Further, the yield of the composite oxide was about 1 kg / 100 L in a container.

Comparative Example 1
A beaker with an internal volume of 30 liters was charged with 15 liters of cyclohexane and 625 g of polyoxyethylene (n = 5) nonylphenyl ether, and an aqueous solution consisting of 19 g of cerium nitrate and 120 mL of distilled water was added and stirred. Reverse micelles (water-in-oil microemulsions, water droplets measured diameter of 30 nm) were prepared by stirring using a magnetic stirrer at room temperature. Separately, a zirconium alkoxide solution was prepared by dissolving 0.23 mol of zirconium butoxide in 0.8 liter of cyclohexane, and this was added to the microemulsion. At the same time, the pH was adjusted to 5. The volume ratio (O / W) of water (aqueous phase) to cyclohexane (organic phase) at this time is 80. The concentration of cations (excluding hydrogen ions) in the aqueous phase in the reverse micelle is 0.25 mol / L. When this mixture was well stirred at room temperature, the inside of the beaker immediately became cloudy white and colloidal particles (secondary particles, particle size of about 20 nm) were formed.

The composite oxide containing cerium and zirconium (ceria zirconia: Ce _0.4 Zr _0.6 O _X ) obtained in the same manner as in Example 1 was obtained by adjusting colloid aggregation, washing the precipitate, drying, and firing. The Ce / Zr molar ratio of the composite oxide was 4/6. The yield of the composite oxide was 0.4 kg / 100 L container.

Comparative Example 2
A beaker having a volume of 5 liters was charged with 3.0 liters of cyclohexane and 224 g of polyoxyethylene (n = 5) nonylphenyl ether, and an aqueous solution composed of 19 g of cerium nitrate and 120 mL of distilled water was added and stirred. A reverse micelle (water-in-oil microemulsion, measured water droplet diameter of 30 nm) was prepared by stirring with a magnetic stirrer at room temperature. Separately, a zirconium butoxide / cyclohexane solution was prepared and added to the microemulsion solution. At the same time, the pH was adjusted to 5. The volume ratio (O / W) of water (aqueous phase) to cyclohexane (oil phase) at this time is 18. The concentration of cations (excluding hydrogen ions) in the aqueous phase in the reverse micelle was set to 1.5 mol / L by lowering the buffer ammonium nitrate concentration from that in Example 1. When this mixture was well stirred at room temperature, the inside of the beaker immediately became cloudy white and colloidal particles (secondary particles, particle diameter of about 20 nm) were generated.

Next, the pH was adjusted to 8 with aqueous ammonia in order to adjust the aggregation of the colloid. Further, the mixture was stirred for about 1 hour for aging. The mother liquor was separated by filtration, and the resulting precipitate was washed with ethanol three times, dried overnight at 80 ° C., and then calcined in the atmosphere at 600 ° C. for 2 hours to obtain a composite oxide containing cerium and zirconium (ceria zirconia: Ce _0.44). Zr _0.6 O _x ) was obtained. The Ce / Zr molar ratio of the composite oxide was 4/6. Further, the yield of the composite oxide was about 1 kg / 100 L in a container.

Comparative Example 3
A composite oxide (lanthanum zirconia: La _0.2 Zr _0.8 O _1.9 ) containing lanthanum and zirconium was obtained in the same manner as in Comparative Example 1 except that “cerium nitrate” in Comparative Example 1 was replaced with lanthanum nitrate. It was.

Evaluation About the porous complex oxide obtained in Example 1 and Comparative Examples 1 and 2, the pore volume after firing at 900 ° C. was measured by nitrogen adsorption at a liquid nitrogen temperature. The result is shown in FIG. As is clear from the results shown in FIG. 3, the distribution of pores in the composite oxide obtained by the method according to the present invention (Example 1) increases the proportion of the oil phase and suppresses the aggregation of secondary particles. The distribution of pores in the composite oxide according to Comparative Example 1 is almost the same, and a composite oxide having an intended porous structure can be obtained. Therefore, the concentration of cations (excluding hydrogen ions) in the aqueous phase in the reverse micelle is reduced. It was recognized that the pore diameter and pore volume can be controlled by increasing the relative height. Further, the yield of the composite oxide by the method according to the present invention is 1.6 kg / container 100 L, and it is recognized that the yield of about 4 times can be achieved with respect to the yield 0.4 kg / container 100 L of the method according to Comparative Example 1. It was. Furthermore, according to the method of the present invention, the amount of the surfactant used for the synthesis is reduced to about 1/3, which is excellent in terms of cost and environment.

  On the other hand, when the concentration of cations (excluding hydrogen ions) in the aqueous phase in the reverse micelle is less than 2 mol / L (Comparative Example 2), the volume ratio (O / W) of the oil phase to the aqueous phase is 40. In combination with the high density of reverse micelles, the pore distribution was shifted to a smaller one, and it was found that it could not be controlled as intended. Specifically, as shown by black squares in FIG. 3, in Comparative Example 2, the pore distribution shifted to a smaller one, and there were almost no pores of 20 nm or more. This phenomenon shows that even if the reverse micelle diameter as a reaction field is the same, the growth of secondary particles in the reverse micelle is suppressed when the amount of cation contained in the aqueous phase is small.

  Further, when the volume ratio (O / W) of the oil phase to the water phase is adjusted to exceed 40 (Comparative Example 1), the structure / characteristic composite oxidation obtained in Example 1 according to the present invention is used. A composite oxide similar to the product could be obtained, but the yield was as low as 0.4 kg / 100 L or less, and it was not practically usable.

  On the other hand, with respect to the composite oxide obtained in Example 2 according to the present invention and the composite oxide obtained in Comparative Example 3, the pore volume after baking at 800 ° C. for 2 hours was measured under liquid nitrogen temperature. Measured by nitrogen adsorption. The result is shown in FIG. As is apparent from the results shown in FIG. 4, the distribution of pores in the composite oxide obtained by the method according to the present invention (Example 2) increases the proportion of the oil phase and suppresses the aggregation of secondary particles. The distribution of pores in the composite oxide according to Comparative Example 3 is almost the same, and a composite oxide having an intended porous structure can be obtained. Therefore, the cation (excluding hydrogen ions) concentration in the aqueous phase in the reverse micelle can be increased. It was recognized that the pore diameter and pore volume can be controlled by increasing the relative height. Further, the yield of the composite oxide by the method according to the present invention is 1.6 kg / container 100 L, and it is recognized that the yield of about 4 times can be achieved with respect to the yield 0.4 kg / container 100 L of the method according to Comparative Example 3. It was. Furthermore, according to the method of the present invention, the amount of the surfactant used for the synthesis is reduced to about 1/3, which is excellent in terms of cost and environment.

It is a figure which shows typically a mode that fusion (unification) of a reverse micelle is suppressed by the method of this invention. It is a three-phase map which shows the area | region of the microemulsion used by this invention. It is a figure which shows the measurement result of the pore distribution of the complex oxide obtained by Example 1 of this invention, Comparative Example 1, and Comparative Example 2. FIG. It is a figure which shows the measurement result of the pore distribution of the complex oxide obtained by Example 2 and Comparative Example 3 of this invention. It is a figure which shows typically the mode of the production | generation and aggregation of a secondary particle.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Primary particle, 2 ... Secondary particle, 3 ... Pore, 4 ... Organic solvent, 5 ... Surfactant, 6 ... Reverse micelle, 7 ... Aqueous phase.

Claims (2)

In the method for producing a composite oxide having a porous structure, further aggregating secondary particles, which are aggregates of primary particles of a composite oxide containing two or more metal elements, and firing the aggregates of the secondary particles,
A solution in which a hydroxide or a compound of a first metal element that generates an oxide by hydrolysis is dissolved in an organic solvent, and a second aqueous phase in a reverse micelle formed by a surfactant in the organic solvent. Mix with emulsion containing ions of the following metal elements,
Hydrolyzing the compound of the first metal element at the interface of the reverse micelle and taking in the second and subsequent metal elements,
Polycondensation to form primary particles of a composite oxide precursor,
In the system containing the primary particles, the primary particles are aggregated to form secondary particles,
Further comprising agglomerating the secondary particles,
A method for producing a composite oxide, wherein the cation concentration excluding hydrogen ions in the aqueous phase in the reverse micelle during the hydrolysis is 2 mol / L or more and the volume ratio of the oil phase to the aqueous phase is 40 or less. .   The method for producing a composite oxide according to claim 1, wherein the diameter of the reverse micelle during the hydrolysis is 20 nm or more.

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