JPH06239613A - Production of barium hexaluminate - Google Patents

Abstract

PURPOSE:To obtain barium hexaluminate several tens nanometer size in particle diameter, causing no decrease in the surface area of the particles when held at elevated temperatures, by introducing a mixed solution of barium and aluminum into a plasma at a specified ratio. CONSTITUTION:This barium hexaluminate can be obtained by introducing a solution containing barium and aluminum so adjusted as to be (1:12) in the atom number ratio of Ba/Al into a plasma. The concentration of this solution is about 0.01-10mol in terms of a total of barium and aluminum ions. Barium and aluminum sources are pref. their chlorides, carbonates or nitrates. Plasma gas is pref. argon.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a method for producing barium hexaaluminate.

[0002]

2. Description of the Related Art Barium hexaaluminate (BaO.
6Al ₂ O ₃ ) has a relatively small decrease in specific surface area even when kept at high temperature, so the catalyst-supported one was developed as a catalyst for low-temperature combustion of high-temperature combustion boilers and removal of nitrogen oxides in exhaust gas. Is being promoted. In addition, studies have begun on its use as a sensor for high-temperature gas.
When used as the above catalyst carrier or high temperature gas sensor,
Barium hexaaluminate is an ultrafine particle having a particle size of several tens of nanometers, and it is necessary that the surface area is not reduced.

Conventionally, barium hexaaluminate is produced by mixing a carbonate or hydroxide of barium with an oxide or hydroxide of aluminum, followed by firing at 1300 to 1600 ° C. and pulverization. However, the particles manufactured by this method have a large particle diameter of several micrometers, the fluctuation of the atomic number ratio in the particles is large, and the barium concentration is extremely low in the central part of the particles.

There is also a method of producing barium hexaaluminate by a sol-gel method using alkoxide as a raw material. In this method, barium and aluminum alcoholate are dissolved in a large amount of alcohol, and a very homogeneous barium and aluminum hydroxide mixture is obtained by hydrolysis, and the mixture is baked at 1000 ° C to 1300 ° C to obtain a desired product. Produces a barium hexaaluminate. The barium hexaaluminate obtained by this sol-gel method has a particle size of 2 to 3 μm, which is smaller than that obtained by the above-mentioned firing method, but still has a large particle size, and the surface area decreases when kept at a high temperature. When applied to a catalyst, a step of supporting on a suitable carrier is required. In addition to this, the following is also a problem. 1. Since the alkoxide is expensive, the manufacturing cost is high. 2. Since alkoxides are easily hydrolyzed by the water in the air, when dissolved in alcohol solution, a large glove box that blocks the air and allows the flow of dry inert gas or air is required. become. 3. Since a large amount of vaporized alcohol is contained in the circulating gas, it has an explosive property, and a device for removing it is required, which causes the device to be complicated and the cost to increase.

[0005]

DISCLOSURE OF THE INVENTION The first object of the present invention is barium having a particle size of several tens of nanometers, which is smaller than that obtained by the sol-gel method, and has no decrease in surface area when kept at high temperature. The second purpose is to propose a method of directly supporting barium hexaaluminate on a substrate.

[0006]

As a result of various studies to find out a novel method for producing barium hexaaluminate, the present inventor found that barium hexaaluminate has a barium-aluminum atom number of 1:12. Therefore, the atomic ratio of barium to aluminum is 1:12.
Introducing a solution prepared so as to obtain plasma into barium hexaaluminate in the form of ultrafine particles, and the atomic ratio of barium to aluminum is 1: 1.
When the solution prepared so as to be 2 was introduced into plasma and barium hexaaluminate was deposited on the substrate placed downstream of the plasma, it was found that the deposited barium hexaaluminate was adhered to the substrate, and the present invention was completed. did.

That is, the present invention is a method for producing barium hexaaluminate, which comprises introducing a solution prepared so that the atomic ratio of barium and aluminum is 1:12, and barium hexaaluminate. Of barium hexaaluminate, characterized in that a solution prepared so that the atomic ratio of aluminum to aluminum is 1:12 is introduced into plasma and barium hexaaluminate is deposited on a substrate placed downstream of the plasma. Is the way.

The present invention will be further described below. It is preferable to use a barium salt as the barium source and an aluminum salt as the aluminum source, and it is preferable to use a water-soluble carbonate, chloride or nitrate as these salts. Since the purity of the salt affects the purity of barium hexaaluminate, which is the final target product, it is preferable to use a high purity salt, that is, 99.9% or more. Further, it is preferable to use an anhydrous salt, but if there is a salt having water of crystallization, it is better to use this because it is cheaper than the anhydrous salt. It is necessary to know in advance.

When a water-soluble salt is used as a raw material, the water used is, for example, ion-exchanged water containing no impurities,
It is preferably dissolved in this. The solution concentration is 0.01 to 10 M as the total amount of barium and aluminum ions.
ol, preferably 0.1 to 3 mol.
If it is less than 0.01 mol, the yield of barium hexaaluminate is small, which is not preferable, and particles of barium hexaaluminate exceeding 10 mol are coarsened and an impurity compound phase is generated, which is not suitable. Further, when the salt is dissolved in water in a relatively high concentration within the above solution concentration range, even if the salt solution is used by stabilizing the salt solution by making the liquid solution acidic or alkaline with hydrochloric acid, ammonia, or the like, This method can also be used because it does not affect the physical properties of the obtained barium hexaaluminate.

The plasma used has a frequency of about 4 to 6 MHz using an ordinary high frequency oscillator, and the output is 10 to 10.
It is preferable to use the one of about 50 kW. Although other frequencies can be used, it is preferable to use the above frequency because the device becomes large. If the output is less than 10 KW, the decomposition of the salt in the solution may be insufficient and the characteristics of the target product may deteriorate, which is not preferable.
If it is larger than 50 kW, it does not affect the quality of the target product, but the device becomes large in size and consumes a large amount of power, which is uneconomical and is not preferable.

As the plasma gas, it is preferable to use the commonly used argon, which has a purity of 99.9.
% Or more is preferable. When introducing the prepared solution into plasma, for example, a solution is made into a mist by a nebulizer using ordinary ultrasonic waves, and this is transported by an inert gas such as nitrogen or argon gas and introduced into plasma. preferable.

By providing a substrate downstream of plasma, barium hexaaluminate can be deposited on the substrate, and the substrate thus deposited can be used as a catalyst carrier itself. As the substrate, it is preferable to use oxide ceramics such as zirconium oxide, aluminum oxide, and mullite, which have little deterioration at high temperatures, and high melting point ceramics such as non-oxides such as silicon nitride, silicon carbide, and titanium nitride. The thickness of the substrate is 0.1
Approximately 5 mm is preferable. Among them, when used as a catalyst carrier, a relatively thin one within this thickness range is used, and when mechanical strength such as a sensor is required, a relatively thick one is used. Is preferred. It is preferable to keep the substrate at a high temperature, for example, a high-temperature flame such as an oxygen-hydrogen flame or an acetylene-oxygen flame, laser light, or a method of applying the substrate from the backside by infrared concentrated heating or a substrate on a heating element such as silicon carbide. It is effective to keep it at a high temperature by placing it. The plasma gas and the gas used for transporting the raw material are economical because they can be recycled by a compressor or the like by passing through a pipe provided with a simple moisture absorption layer filled with silica gel or the like.

[0013]

[Function] The atomic ratio of barium to aluminum is 1: 1.
When the solution prepared so as to be 2 is introduced into the plasma, the solvent in the solution is instantly evaporated, and the barium salt of the barium source and the aluminum salt of the aluminum source are dissociated to form barium atom, aluminum atom and anion. It becomes a constituent atom, and a barium atom and an aluminum atom are bonded to each other to form an oxide and barium hexaaluminate. In addition,
When a substrate is provided downstream of plasma, barium hexaaluminate is deposited on the substrate. The barium hexaaluminate obtained as described above has a high specific surface area and has no decrease in surface area even when kept at about 1000 ° C. Also, when barium hexaaluminate is deposited on the substrate, the adhesion strength between the substrate and barium hexaaluminate depends on the substrate, but when zirconium oxide is used as the substrate, it does not peel off even with an iron tool and has good adhesion. Have strength. Therefore, on the deposited barium hexaaluminate, by introducing a solution of a catalytically active element such as a noble metal such as cobalt or platinum into the plasma from the latter half of the barium hexaaluminate production,
It can be uniformly supported on the deposited barium hexaaluminate surface layer, and can serve as an excellent catalyst in a high temperature range. Further, when a substance exhibiting good characteristics as a gas sensor such as titanium oxide is similarly carried, a gas sensor in a high temperature range can be produced. Furthermore, when barium hexaaluminate is deposited on a substrate, different so-called anisotropy may occur depending on the crystal orientation of the deposited barium hexaaluminate, and such special characteristics can be utilized.

[0014]

EXAMPLES The present invention will be described below with reference to examples in which an aqueous solution prepared by using barium chloride as a barium source and aluminum chloride as an aluminum source was introduced into plasma to deposit an object on a substrate provided downstream of the plasma. Although further described, it goes without saying that the present invention is not limited to these examples. Example 1 Barium chloride (BaCl ₂ ·.
2H ₂ O, special grade reagent, purity 99.9%) and aluminum chloride _(AlCl 3 · _6H 2 O, special grade reagent, purity 99.9
%) And 0.380 g and 4.680 g, respectively, and were dissolved in 30 ml of ion-exchanged water. The metal ion concentration of this aqueous solution is 0.7 Mol solution. A 20 cm long plasma is generated under the conditions of 6 MHz and 15 kW using argon gas as the plasma gas, a zirconium oxide substrate (1 mm thick) is arranged downstream of the plasma near the tail of the plasma, and an oxygen-hydrogen flame is applied from the back side to maintain a high temperature. did. The aqueous solution was made into a mist by an ultrasonic nebulizer, and an argon gas was passed through to introduce the aqueous solution into plasma and deposit it on the substrate for about 10 minutes. The deposit was not easily peeled off even if scratched by an iron tool, and the adhesion strength was good. In addition, from the X-ray diffraction, only the barium hexaaluminate phase was found, and no other impurity crystal phase was observed.
When the particle size was measured, it was 20 to 40 nm. Moreover, the specific surface area was measured and found to be 20 m ² / g.
Even when kept at 0 ° C for 24 hours, there was no decrease in the specific surface area.

Example 2 Barium hexaaluminate was deposited in the same manner as in Example 1 except that titanium nitride was used as the substrate and a resistance heating element made of silicon carbide was used as the substrate heating source. The specific surface area was 18 m ² / g, and there was no decrease in the specific surface area even when kept at 1000 ° C. for 24 hours.

Example 3 The same procedure as in Example 1 was carried out except that the metal ion concentration of the aqueous solution was changed to 3 Mol, and the barium hexaaluminate deposited on the substrate showed C-axis orientation. Particle size is 4
The surface area was 0 to 60 nm and the specific surface area was 15 m ² / g. It should be noted that no decrease in specific surface area was observed even after holding at 1000 ° C. for 24 hours.

Comparative Example 1 Barium isopropoxide (purity: 99.99%) which is an alcoholate of barium, and aluminum isopropoxide (purity of 99.9) which is an alcoholate of aluminum.
9%) was dissolved in propanol and kept at 80 ° C. for 5 hours to uniformly mix both. Next, ion-exchanged water diluted with propanol was carefully added dropwise slowly with stirring to precipitate both hydroxides (gel-like substances), and at 80 to 90 ° C, only propanol was fractionated to concentrate the precipitate. All the operations up to this point were performed in a glove box in which dry nitrogen with completely removed water was passed. The propanol in the flowing gas was condensed and collected in the trap kept at -100 ° C, and circulated again in the glove box. Then, the concentrated gel-like material is applied to a zirconium oxide substrate to form 130
It was calcined at a temperature of 0 ° C. for 12 hours to form barium hexaaluminate. It took about 2 days from the dissolution of the raw material alcoholate in propanol to the completion of firing. Although barium hexaaluminate obtained by this alkoxide method adheres to the zirconium oxide substrate,
It easily peeled off with a wooden spatula, and the adhesive strength was weak. X
From the line diffraction, only barium hexaaluminate was present and no other impurity phase was present, but the particle size was large, 2-3 μm. The specific surface area was small and was 8 m ² / g. When kept at 1000 ° C. for 24 hours, the specific surface area decreased by 7%.

[0018]

According to the present invention, barium hexaaluminate can be produced only by introducing into the plasma a solution in which the atomic ratio of barium to aluminum is 1:12. Therefore, the production process is simple and the production cost is low. Is also cheap. Moreover, the obtained barium hexaaluminate has a larger specific surface area than that produced by the conventional alkoxide method, and there is no reduction in the specific surface area even when kept at a high temperature.
It is of good quality. Therefore, the barium hexaaluminate produced according to the present invention is a good carrier for various catalysts, and is suitable for supporting a substance having properties as a gas sensor when producing a high temperature gas sensor.

Claims (6)

[Claims] 1. A method for producing barium hexaaluminate, which comprises introducing a solution prepared so that the atomic ratio of barium and aluminum is 1:12 into plasma. 2. A solution prepared so that the atomic ratio of barium to aluminum is 1:12 is introduced into plasma, and barium hexaaluminate is deposited on a substrate placed downstream of the plasma. Method for producing barium hexaaluminate. 3. The method for producing barium hexaaluminate according to claim 1, wherein a barium salt is used as a barium source and an aluminum salt is used as an aluminum source. 4. The method for producing barium hexaaluminate according to claim 1 or 2, wherein the solution is an aqueous solution. 5. The method for producing barium hexaaluminate according to claim 1 or 2, wherein the concentration of the solution is 0.01 to 10 mol as the total amount of metal ions of barium and aluminum. 6. The barium salt is barium chloride, carbonate, nitrate, and the aluminum salt is aluminum chloride,
The method for producing barium hexaaluminate according to claim 3, which is a carbonate or a nitrate.