JP2008150242A - Method of producing oxide - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst support material having excellent heat resistance and keeping high specific surface area even when being used under a high temperature atmosphere for a long period of time by controlling the size of particles to have a desired optimum size. <P>SOLUTION: After an aqueous precipitate solution formed by a chemical reaction is aged, a step for making a raw material to become the source of particle growth, which is a particle having the same composition and smaller particle diameter, coexist and be subjected to aging again, and the step is repeated to grow the particle constituting the precipitate to have the desired particle size. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing an oxide, and more particularly to a method for producing an oxide having a desired optimum particle size by controlling the particle size of oxide particles used as a catalyst support.

  Various oxides are widely used as catalyst support materials. In order to improve the catalyst performance, it is preferable to use a material having a large specific surface area as the catalyst carrier in order to suppress grain growth of the noble metal exhibiting the catalytic function. In order to increase the specific surface area, generally, the support material should be made porous or the particle diameter of the support material should be made fine.

  However, when used in a high temperature atmosphere such as an automobile exhaust gas, the surface energy of the particles increases as the material particles become finer. It is not possible to maintain a high specific surface area. In addition, particles having a large particle diameter are excellent in thermal durability, but a specific surface area is insufficient and a sufficient amount of noble metal responsible for catalytic activity cannot be stably maintained. This means that in order to realize heat resistance at high temperatures, the material particles constituting the catalyst carrier have an optimum particle diameter.

  Usually, the catalyst carrier is produced by a liquid phase method such as a coprecipitation method that can easily synthesize a powder having a high specific surface area. However, the crystal size of material particles obtained by a conventional method such as a coprecipitation method is a minute size less than 10 nm. For this reason, such a material is insufficient in heat resistance to be used as, for example, a catalyst carrier for purifying high-temperature exhaust gas, and is not suitable for long-term use.

  Conventionally, for example, a method is proposed in which a cerium salt aqueous solution and an ammonium carbonate aqueous solution are mixed at a constant rate, thermally formed in a temperature range of 50 to 85 ° C., and the obtained cerium carbonate is dried and calcined to obtain nano-acicular ceria powder. (See, for example, Patent Document 1). This method is a technique for synthesizing needle-like crystals by devising the ripening process, but the crystallite size obtained is a fine one having a uniform particle diameter and less than the growth up to around 10 nm. In addition, a method for producing an oxide by aging an aqueous solution containing a zirconium hydroxide precipitate at a temperature of 50 ° C. or higher has been proposed (for example, see Patent Document 2). However, the oxide obtained by this method is also composed of ultrafine particles having a specific surface area of 250 m 2 / g or more. Since the oxide produced by any of these methods is composed of fine oxide particles, it cannot be said that it is suitable as a catalyst carrier for use in a high temperature atmosphere near 1000 ° C.

JP 2003-252622 A JP 2004-323257 A

  Accordingly, the present invention is intended to provide a method that solves the problems of the conventional methods described above, and specifically, an oxide that can control the size of the particle diameter to a desired optimum size. It aims at providing the manufacturing method of. Accordingly, an object of the present invention is to provide a catalyst carrier material excellent in thermal stability that can maintain a high specific surface area even when used in a high temperature atmosphere for a long period of time.

  In order to solve the above-described problems, the present invention is a precursor of oxides, and after aging an aqueous precipitate of a precipitate generated by a chemical reaction, a raw material that is a source of particle growth, that is, a composition having the same composition. Moreover, it was made based on the idea of growing the particles constituting the precipitate to a desired particle size by repeating the ripening in the presence of particles having a smaller particle size. Specific means for solving the problems are as follows.

The method for producing an oxide according to claim 1 is a method in which the precipitate is separated from an aqueous precipitate containing a precipitate generated by a chemical reaction, dried, and then fired to produce the oxide. The precipitation solution is divided into the following two steps
(A) a first step of heating the aqueous precipitation liquid to age the constituent particles of the precipitate;
(B) a second step in which particles having the same composition having a particle size smaller than the average particle size of the particles constituting the precipitate are coexisted in the aqueous precipitation liquid containing the aged precipitate particles, and the heat aging is performed again. When,
It is characterized by processing including.

  The method for producing an oxide described in claim 2 is characterized in that the heat aging in the first step and the second step is performed at a temperature of 80 to 250 ° C. in the method described in claim 1.

  The method for producing an oxide according to claim 3 is the method according to claim 1 or 2, wherein the second step is repeated twice or more times.

The oxide production method according to claim 4 is the method according to any one of claims 1 to 3, wherein the average particle size of the constituent particles of the precipitate aged in the second step is set. B, where A is the particle size of the particles having the same composition to be coexisted, the relationship between them is
1.5 <B / A <1000
It is characterized in that is established.

  The method for producing an oxide according to claim 5 is the method according to any one of claims 1 to 4, wherein the precipitate is a coprecipitate composed of a plurality of types of compounds. .

  The method for producing an oxide according to claim 6 is the method according to any one of claims 1 to 5, wherein the precipitate is made of a metal hydroxide.

  The method for producing an oxide according to claim 7 is the method according to any one of claims 1 to 6, wherein the precipitate contains at least one hydroxide of a rare earth element. And

  The method for producing an oxide according to claim 8 is the method according to any one of claims 1 to 7, wherein the precipitate contains cerium hydroxide and zirconium hydroxide. .

  According to the method of the present invention, the particle size of the oxide can be controlled to an arbitrary size while maintaining a high specific surface area, so that the particle size having the optimum thermal stability according to the purpose of use, etc. It is possible to produce an oxide having

  In the present invention, a precipitate generated by a chemical reaction means a precipitated substance that forms an oxide when the precipitate is baked. Typically, there are metal hydroxides such as aluminum, silicon, titanium, iron, zirconium, and cerium and other rare earth elements. Such a metal hydroxide can be prepared by neutralizing an aqueous strong salt solution of the metal with an alkaline substance such as aqueous ammonia.

  Metal strong acid salts include nitrates, chlorides, sulfates and oxynitrates, oxysulfates, oxychlorides, but of course are not limited to these compounds. Metal salt compounds that can be particularly preferably used for the production of automobile exhaust gas catalyst carriers include cerium (III) nitrate, cerium (IV) ammonium nitrate, cerium sulfate (III), cerium sulfate (IV), and cerium chloride. (III), zirconium oxynitrate, zirconium oxychloride and the like.

  As the alkaline substance, it is preferable to use a volatile alkaline substance such as ammonia water. This is because it does not remain as an impurity in the fired oxide. In the case of using an alkali metal hydroxide such as sodium hydroxide as well as other alkaline substances, it is necessary to perform sufficient washing so that the alkaline substance does not remain in the resulting metal hydroxide structure.

  The aqueous precipitate containing the precipitate generated by the chemical reaction as described above is heated and aged at a temperature of 80 ° C. to 250 ° C., more preferably at a temperature of 100 ° C. or more, so that the particle size of the particles constituting the precipitate is uniform. And particle growth. If the ripening temperature is lower than 80 ° C, the equilibrium of dissolution of fine precipitate particles and reprecipitation into large particles for Ostwald growth cannot be activated sufficiently, so that the ripening rate is too slow and the particles are sufficiently grown. Absent. On the other hand, when the temperature is higher than 250 ° C., the crystal structure of the precipitate-constituting particles starts to collapse, and the intended effect of the present invention cannot be obtained.

  In order to heat and age at a temperature of 100 ° C. or higher, a hydrothermal aging method is used. The hydrothermal aging method refers to a method in which an object sample is aged in a hydrothermal state under a high temperature and pressure of 100 ° C. or higher in a pressure-resistant sealed container such as an autoclave. As a matter of course, the aqueous precipitate cannot be heated to a temperature of 100 ° C. or higher under atmospheric pressure, and in order to mature at a temperature of 100 ° C. or higher, heating under sealed pressure is required. Among them, the reaction is carried out under hydrothermal pressure conditions. In this specification, the term “heat aging” is used in the meaning including hydrothermal aging. However, when treated by hydrothermal aging, or when treated, especially when it is preferable to use hydrothermal aging and a limited terminology for understanding the invention, the term “hydrothermal aging” is used. I decided to use it.

  When heat aging, small particles out of the large and small particles mixed in the precipitate in the aqueous precipitation liquid dissolve, precipitate into large particles in the cooling process, and as the particles grow, the particle size distribution becomes uniform and smaller. Become. Once ripening is performed, there is no variation in the particle size in the aqueous precipitation liquid, that is, the amount of small particles that dissolve even when heated is reduced. Little growth occurs.

  The present invention is characterized in that the particles grow by re-heating and aging in the presence of smaller particles in the aqueous precipitation liquid in which the remaining amount of small particles is extremely small after the heat aging. There is. In this way, by repeating the process of heating and aging in the presence of particles having a small particle size, in other words, by constantly performing heating and aging in multiple stages in the presence of small particles, the size of the particles constituting the precipitate is increased. The size can be controlled to grow to the desired size. Thereby, for example, an oxide having a particle diameter excellent in heat resistance can be produced.

FIG. 1 shows that by repeating hydrothermal ripening at a temperature of 250 ° C., that is, by repeating the hydrothermal ripening process in multiple stages while coexisting particles having a smaller particle size in the aqueous precipitate after hydrothermal ripening. It is the graph which showed how the particle size distribution and volume fraction of the precipitate which are obtained change. Since such a measurement cannot be performed on the precipitate itself, the actual measurement was performed on the oxide after the precipitate was separated and dried and then calcined to form an oxide. In this figure, the volume fraction means the ratio of particles having a specific particle size to the total volume of the sample (oxide) .The test method is cerium nitrate (iv), zirconium oxynitrate, lanthanum nitrate and A mixed aqueous solution of praseodymium nitrate (the weight ratio in terms of oxide is CeO ₂ : ZrO ₂ : La ₂ O ₃ : Pr ₂ O ₃ = 60: 30: 3: 7) is mixed with ammonia water equivalent to 1.2 times the equivalent amount. These hydroxide coprecipitates were prepared and hydrothermally aged for 4 hours using an autoclave at a temperature of 250 ° C. The above-mentioned metal salt was dissolved in the same mixture ratio in the precipitate liquid after hydrothermal aging. (However, the addition amount was 1/2 of the initial amount) Ammonia water was added to form a virgin hydroxide coprecipitate, and the hydrothermally aged precipitate and the virgin precipitate were mixed. Same conditions as above for aqueous precipitate again And hydrothermal aging.

  From the graph of FIG. 1, the precipitate produced by the chemical reaction has a wide particle size distribution, and the average particle size is also small (curve A). When hydrothermal aging is performed in this state, the small particles in the aqueous precipitation liquid dissolve and precipitate into large particles, and the particles grow. As a result, the particle size distribution curve becomes sharper and the spread of the particle size distribution becomes smaller (curve B, first aging step). In this state, when particles having a small particle size coexist in this state and hydrothermal aging is performed again, the small particles coexisting are dissolved and precipitated into larger particles, and the particles further grow. Thus, as the number of hydrothermal aging increases, the particle size distribution changes to the right, that is, in the direction in which the particle size increases (curves C, D, E. Each time in the second aging step, the first time). , Corresponding to the 9th and 19th maturation).

With respect to the oxide sample used for the measurement in FIG. 1 and other oxides prepared under the same conditions, tests were performed to see what changes in specific surface area after 1000 ° C. endurance occurred. This is a test that simulates the actual usage condition of an automobile. In order to test the durability of the oxide that is the catalyst support material that is exposed to high-temperature exhaust gas, the oxide is put into a firing furnace and introduced into the furnace. By adjusting the amount of combustible gas to be produced, an atmosphere of reduction (Rich) and oxidation (Lean) is alternately and alternately formed, and then exposed at a temperature of 1000 ° C. for 5 hours. The specific surface area of the oxide after such heat treatment is referred to as “1000 ° C. R / L specific surface area after endurance” or simply “specific surface area after 1000 ° C. endurance”. The degree of deterioration of the oxide can be known by comparing with the specific surface area before performing such a durability test. A gas adsorption method is generally used for measuring the specific surface area.
FIG. 2 is a graph showing the results of this test. As the number of aging steps increases, the particle diameter increases, and the specific surface area after 1000 ° C. endurance increases accordingly. However, after the peak of 7 to 10 times, the specific surface area after 1000 ° C. endurance decreases rapidly thereafter. When the second aging step is repeated about 20 times, the specific surface area after the endurance of the oxide to be produced is almost the same as that in the first aging step. The reason is considered as follows. As a result of repeated aging, the precipitate is left in a high temperature aging state for a long time, resulting in a phenomenon similar to sintering, the particle size increases while the particle size distribution expands, and the specific surface area is already small at the initial time before heat resistance. This is because it becomes.

  In order to allow smaller particles to coexist in the heat-ripened aqueous precipitate, the aqueous precipitate before ripening may be mixed with the heat-ripened aqueous precipitate. On the contrary, an aqueous precipitate that has been heat-aged and aged may be mixed into an aqueous precipitate before aging. In addition to these methods, in the heat-ripened aqueous precipitate, the same method as that used to produce the first virgin precipitate, that is, dissolve the strong metal salt and attach the alkaline substance to neutralize the precipitate. It may be generated.

Regarding the particle size of the constituent particles between the coexisting precipitate and the aged precipitate, when the average particle size of the component particles of the aged precipitate is B and the particle size of the coexisting average particles is A And the following relation:
1.5 <B / A <1000
Is preferably satisfied. The reason is that if this ratio is smaller than 1.5, the particle A is stable and difficult to dissolve with respect to the particle B, and Ostwald growth cannot be used. If this ratio is larger than 1000, the nucleus size of the particle is reduced. On the other hand, it is not realistic that the maximum particle size (crystallite) that can be synthesized by the solution method is 1000 times or more.

  The upper limit of the number of repetitions of hydrothermal aging in which a precipitate with a small particle size coexists in a hydrothermally aged aqueous precipitate, or the optimum number of times varies depending on the amount of small particles to be coexisted, and should be uniformly specified. I can't. Generally, the larger the amount of coexistence, the smaller the upper limit number and the optimum number. However, if the amount of coexistence is large at once, the solubility at the hydrothermal aging temperature is exceeded, and there is a possibility that an inconvenient problem that the particles A are not completely dissolved and remains. As the particle size of hydrothermally ripened particles grows larger, the growth rate gradually decreases. However, it may be appropriately selected in consideration of the desired particle size and the amount of precipitate that coexists in one hydrothermal ripening. Examples of the present invention will be described below.

Various hybrid oxides composed of cerium-zirconium-lanthanum-praseodymium were prepared by the following method and treated at high temperature to compare the deterioration of their specific surface areas.
(I) Preparation of Coprecipitated Hydroxide Mixing aqueous solution containing cerium nitrate (iv), zirconium oxynitrate, lanthanum nitrate and praseodymium nitrate in the proportions shown in Table 1 was mixed with ammonia water equivalent to 1.2 times the equivalent amount. Coprecipitates of these hydroxides were prepared.

This precipitate was separated, washed twice with purified water, and then peptized with dilute nitric acid to prepare an aqueous liquid. This aqueous precipitate was divided equally into 8 containers.
(B) Hydrothermal aging of coprecipitated hydroxide (first step)
The aqueous precipitates described above were placed in an autoclave, and 5 pieces were aged at 150 ° C. and the other 3 pieces were aged at 250 ° C. for 4 hours under hydrothermal pressure. In this way, 8 types of two aqueous precipitation liquids having different aging temperatures were prepared.
(C) Coexistence of small-sized particles (first round of the second step)
To each of the eight aqueous precipitation liquids, the salt of each component corresponding to 1/2 of the initial blending amount was added and stirred to dissolve. Next, ammonia water was added to again form a hydroxide co-precipitate of each component, and eight aqueous precipitates mixed with aged hydroxide co-precipitate and virgin hydroxide co-precipitate were prepared. Prepared.
(D) Hydrothermal ripening (first round of the second step)
Each precipitate was separated from the aqueous precipitation solution described above, washed twice with purified water, then dispersed again in dilute nitric acid and peptized. These peptized aqueous precipitates were placed in an autoclave and hydrothermally aged for 4 hours at a temperature of 155 ° C to 250 ° C.
(E) Coexistence of small particles (second step of the second step)
To the aged aqueous precipitate, salts of each component corresponding to 1/2 of the initial blending amount were added and stirred to dissolve. Next, ammonia water was added to again form a hydroxide coprecipitate of each component, and an aqueous precipitation liquid in which an aged hydroxide coprecipitate and a virgin hydroxide coprecipitate were mixed was prepared.
(F) Hydrothermal aging (second time in the second step)
Each precipitate was separated from the aqueous precipitation solution, washed twice with purified water, then dispersed again in dilute nitric acid and peptized. These two kinds of aqueous precipitation liquids were put into an autoclave and hydrothermally aged at a temperature of 160 to 250 ° C. for 4 hours.
By repeating the treatment described above, eight types of aqueous precipitation liquids were prepared with different hydrothermal aging conditions such as the hydrothermal temperature and the number of aging treatments as shown in Table 2.
(G) Neutralization of aqueous liquid after hydrothermal treatment After completion of a predetermined number of aging treatments, ammonia water was added to each of the aqueous precipitation liquids to increase the pH, and the precipitates were aggregated at an isoelectric point.
(H) Dry calcination The precipitate was filtered and dried at a temperature of 120 ° C. After drying, this powder was calcined by holding at 700 ° C. (temperature rising rate 300 ° C./h) for 3 hours to prepare the above-mentioned four kinds of oxide samples.
[Preparation of Comparative Sample]
According to the preparation of the coprecipitation hydroxide and (b) hydrothermal aging of the coprecipitation hydroxide (first step) described in the above examples, a mixed aqueous solution consisting of the same components as the examples is prepared. Ammonia water was added to prepare a coprecipitate. The precipitate was separated and washed, then peptized in dilute nitric acid solution, and hydrothermally aged for 4 hours at various temperatures. After hydrothermal aging, an oxide composed of the same components as in Examples was prepared by the same method as described in the above (h) dry firing.

  The oxide obtained as described above was heat-treated and the specific surface area after endurance at 1000 ° C. (gas adsorption method) was measured and compared with the specific surface area before endurance. The results are shown in Table 2.

  In this table, the XRD crystallite diameter is the size of a crystallite obtained by calculating the spread of the (111) peak of Ce-Zr oxide detected by XRD (X-ray diffraction) by Cauchy function approximation of the Sherrer method. Say. The SAXS particle size distribution value is a dispersion value calculated by assuming that the particle size distribution has a γ distribution shape in the particle size distribution measurement by the SAXS (X-ray small angle scattering) method.

  As described above, according to the method of the present invention, the particle size of the oxide particles can be grown to an arbitrary size, and an oxide having excellent heat resistance can be prepared.

It is a graph showing the change of the particle size distribution and volume fraction of the oxide obtained by the heat aging frequency. It is a graph which shows the relationship between the heat aging frequency of a 2nd process after the heat aging of a 1st process, and the specific surface area after 1000 degreeC durability of the obtained oxide.

Claims (8)

In the method of separating the precipitate from the aqueous precipitate containing the precipitate generated by the chemical reaction, drying, and calcining to produce the oxide, the aqueous precipitate is divided into the following two steps:
(A) a first step of heating the aqueous precipitation liquid to ripen the constituent particles of the precipitate;
(B) a second step in which particles having the same composition having a particle size smaller than the average particle size of the particles constituting the precipitate are coexisted in the aqueous precipitation liquid containing the aged precipitate particles, and the heat aging is performed again. When,
A process for producing an oxide, comprising:   The method for producing an oxide according to claim 1, wherein the heat aging in the first step and the second step is performed at a temperature of 80 to 250 ° C.   The method for producing an oxide according to claim 1 or 2, wherein the second step is repeated twice or more times. In the second step, when the average particle size of the constituent particles of the aged precipitate is B and the particle size of the particles having the same composition to coexist is A, the following relational expression is established between the two:
1.5 <B / A <1000
The method for producing an oxide according to any one of claims 1 to 3, wherein: is established.   The method for producing an oxide according to any one of claims 1 to 4, wherein the precipitate is a coprecipitate composed of a plurality of kinds of compounds.   The method for producing an oxide according to any one of claims 1 to 5, wherein the precipitate comprises a metal hydroxide.   The method for producing an oxide according to any one of claims 1 to 6, wherein the precipitate contains at least one rare earth element hydroxide.   The method for producing an oxide according to any one of claims 1 to 7, wherein the precipitate contains cerium hydroxide and zirconium hydroxide.

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