JPH11278828A - Fine particles of gamma-alumina-magnesia multiple oxide and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To industrially produce fine particles of γ-alumina-magnesia multiple oxide exhibiting heat resistance and durability, less liable to a change in the crystal form, specific surface area and particle diameter even at high temp. and capable of maintaining catalyst carrier function and catalytic activity over a long period of time. SOLUTION: A starting material contg. Al and Mg is evaporated with thermal plasma in an oxygen atmosphere. Preferably an arc discharge is caused between an electrode comprising an Al-Mg alloy or based on the alloy and a counter electrode in a state that protects the latter electrode with an inert gaseous stream and allows the stream to orient toward the former electrode to generate a plasma flame. The Al-Mg alloy is evaporated with the plasma flame and an oxygen-contg. gas is introduced and brought into contact with a vapor stream generated in the plasma flame and/or on the downstream side of the plasma flame.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to gamma-type alumina-magnesia composite oxide fine particles and a method for producing the same. More specifically, the present invention provides a gamma-alumina-magnesia composite oxide fine particle having a crystal form, a specific surface area, and a particle diameter that is hard to change even at high temperature, and has excellent heat resistance and durability, and a gamma-alumina-magnesia composite including the fine particle. The present invention relates to a method for easily and efficiently producing oxide fine particles. The gamma-type alumina-magnesia composite oxide fine particles having the above characteristics of the present invention are used as catalysts requiring heat resistance, such as automobile exhaust gas purification catalysts, gasification catalysts, and catalyst carriers, and ceramics and ceramic additives. Useful as

[0002]

2. Description of the Related Art Heretofore, γ-alumina having a high specific surface area has been frequently used as a catalyst carrier for automobiles, but ordinary γ-alumina is δ- or θ-at 800 to 1000 ° C.
Phase transition occurs in α-alumina via alumina, and at the same time, the particle size is enlarged and the specific surface area is reduced, which causes a problem that the heat resistance and surface characteristics required for the catalyst carrier are not sufficient. Therefore, γ-alumina should be
Attempts to stabilize by firing at a high temperature of about 00 ° C. have been attempted, but heat resistance was still insufficient. In addition, in a field requiring higher heat resistance such as a ceramic turbine engine, there is almost no carrier having the required heat resistance.

[0003]

Recently, a heat-resistant alumina containing a small amount of magnesium in the crystal of γ-alumina has been proposed. However, this was only obtained by experiments using a gas phase method. Not reported (1996 1
The Nikkan Kogyo Shimbun, dated 029/29), an industrial manufacturing method has not yet been established. Under these circumstances, the present invention exhibits heat resistance and durability at a heat-resistant temperature of 1300 ° C. or higher, hardly changes crystal form, specific surface area, and particle size even at high temperatures, and maintains the catalyst support function and catalytic activity for a long period of time. It is an object of the present invention to provide a gamma-type alumina-magnesia composite oxide fine particle and a method for easily and efficiently producing the gamma-type alumina-magnesia composite oxide fine particle including the fine particle.

[0004]

Means for Solving the Problems The present inventors have developed various methods for industrially producing gamma-type alumina-magnesia composite oxide fine particles and for developing gamma-type alumina-magnesia composite oxide fine particles having the preferable characteristics described above. As a result of repeated research, among the fine particles obtained by a method of evaporating a raw material containing a predetermined element by thermal plasma in an oxygen atmosphere and fine particles obtained by the method, the specific surface area before heating is a predetermined value or more, and the heat resistance index is a predetermined value or more. The inventor found that some could be suitable for the purpose, and based on this finding, completed the present invention.

That is, the present invention provides a method for producing γ-type alumina-magnesia composite oxide fine particles, wherein a raw material containing aluminum and magnesium is evaporated by thermal plasma in an oxygen atmosphere, and the specific surface area before heating is reduced. 15 m ² / g or more and heat resistance index (heat treatment condition 100
(0 ° C.) is 65% or more. In the present invention, γ
The type-alumina-magnesia composite oxide is a general term for alumina-magnesia composite oxides including γ-alumina or spinel in which magnesium ions are coordinated in a spinel type crystal of γ-alumina.

[0006]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The raw materials used in the method of the present invention may contain aluminum and magnesium, but preferably those containing only aluminum and magnesium as metal elements are selected. The atomic ratio of aluminum to magnesium in the raw material is not particularly limited, but is preferably 99: 1 to 2: 1, more preferably 23: 2 to 2.
It may be in the range of 3: 1. As such a raw material, for example, an aluminum-magnesium alloy or a material mainly containing the alloy, or containing only aluminum and magnesium as metal elements, at least two kinds selected from metals and other conductive substances A mixture or a mixture of at least one selected from metals and other conductive substances with ceramics may be used.

The oxygen atmosphere used in the method of the present invention includes, for example, air and oxygen as well as those further containing argon.

The thermal plasma used in the method of the present invention can be generated by, for example, an arc discharge method, an induction heating method using a high-frequency magnetic field, or the like, particularly preferably an arc discharge method. The arc discharge method causes an arc discharge between electrodes in an electric field. The arc discharge is preferably performed while the inert gas flow is directed to the other electrode while protecting one of the pair of electrodes.

Advantageously, such a method using an arc discharge is a method using an aluminum-magnesium alloy or a material mainly containing the alloy as one of a pair of electrodes (hereinafter referred to as an alloy-based electrode method). A method using the above mixture (hereinafter, referred to as a mixed electrode method) is exemplified.

The alloy-based electrode method is advantageous in that an electrode made of or mainly containing an aluminum-magnesium alloy (hereinafter referred to as an alloy-based electrode) and an electrode paired with the electrode (hereinafter referred to as a counter electrode) are used. While protecting the latter electrode with the inert gas flow, orienting it to the former electrode, arc discharge is performed to generate a plasma flame,
A method of evaporating an aluminum-magnesium alloy and introducing and contacting an oxygen-containing gas with the resulting vapor flow in and / or downstream of the plasma flame, wherein the alloy-based electrode further comprises aluminum and magnesium. The atomic ratio of magnesium is 99: 1
~ 2: 1, preferably 23: 2 to 3: 1.

[0011] The mixed electrode method is advantageous in that it contains only aluminum and magnesium as metal elements, and is a mixture of at least two kinds selected from metals and other conductive substances, or a mixture of metals and other conductive substances. An inert gas flow is protected between an electrode made of a mixture of at least one selected from the above and a ceramic (hereinafter referred to as a mixed electrode) and an electrode (hereinafter referred to as a counter electrode) forming a pair with the electrode to protect the latter. While orienting to the former electrode,
Arc discharge to generate a plasma flame, evaporating the former electrode, and introducing and contacting an oxygen-containing gas with the resulting vapor flow in the plasma flame and / or downstream of the plasma flame. In the mixed electrode, the atomic ratio of aluminum to magnesium is 99: 1 to 2: 1, preferably 23: 2.
It is preferable to use one in the range of 2-3: 1.

When ceramics are used as a constituent component of the mixed electrode, examples of the ceramics include oxides and metal carbonates, and preferably oxides, especially magnesium oxide and aluminum oxide. These ceramics may be used alone or in combination of two or more. Also,
When a metal is used as a component in the mixed electrode, for example, aluminum or magnesium is used as the metal. For metals and other conductive materials,
At least one type is used when ceramics are used in combination, and at least two types are used when ceramics are not used in combination.

The inert gas flow used in the alloy-based electrode method and the mixed-electrode method mainly shields one or the other of the pair of electrodes from the oxidizing gas from the oxidizing gas, and causes the contact oxidation of the electrode. Helps to prevent wear by. Examples of the inert gas used at that time include nitrogen and argon.

The aluminum-magnesium alloy, alloy-based electrode or mixed-based electrode used in the method of the present invention is preferably continuously supplied by a feeder or the like in accordance with the amount consumed by the evaporation. The supply direction is not particularly limited, but is preferably horizontal or vertical.

According to the method of the present invention, an oxidation reaction or complexation occurs, and γ-type alumina-magnesia composite oxide fine particles can be easily obtained by an industrial method.

Γ-type alumina obtained by the method of the present invention
Among the magnesia composite oxide fine particles, the specific surface area (referred to as the specific surface area before heating) of the fine particles immediately after being obtained (hereinafter referred to as “before heating”) is 15 m ² / g or more, and particularly 20 m ² / g or more. And heat resistance index (heat treatment conditions 100
0 ° C.) is 65% or more, especially 70% or more, and further, the heat resistance index (heat treatment condition 1200 ° C.) is 20% or more, especially 30% or more, especially the specific surface area before heating is 20 m ² / g or more and heat resistance index (heat treatment condition 1
(000 ° C.) is preferably 70% or more, because the crystal form does not easily change even at a high temperature, the heat resistance and the durability are excellent, and it is effective as a catalyst or a catalyst carrier used at a high temperature such as an exhaust gas purifying catalyst for automobiles. Here, the heat resistance index is a percentage of the specific surface area after the heat treatment at a predetermined temperature for 6 hours with respect to the specific surface area before the heat treatment. The predetermined temperature condition of the heat treatment is usually 1000 ° C. Or 1
Although 200 ° C. is used, it is a rough guide, and other temperatures that can be considered in view of heat resistance, for example, 1100 ° C., 13 ° C.
00 ° C., 1400 ° C., etc. are also used.

The fine particles having such a specific surface area preferably have an average particle diameter of 100 nm or less before heating and 150 nm or less after heat treatment at 1200 ° C. for 6 hours because the specific surface area can be increased. Average particle size before heating is 1
The thickness of 0 to 80 nm is preferable because the specific surface area can be increased not only before heating but also after heating, and the crystal form mainly composed of γ-alumina can be maintained even when heated to 1000 ° C. or more.

Next, an example of a manufacturing apparatus suitable for carrying out the method of the present invention will be described. In this manufacturing apparatus, an alloy-based electrode or a mixed-system electrode is provided in a chamber as an upper cathode, and as a lower anode. The cathode is located at the center of the torch with a nozzle at the tip oblique to the anode and is protected by a flow of inert gas supplied into the torch and is primarily shielded from the oxidizing environment.

The inside of the chamber has an absolute pressure of 250 to 10
00 torr (about 3.3 × 10 ⁴ Pa to about 1.3 × 10 ⁵
Pa) (about 20 inches of mercury in relative pressure (about 6.8 × 1
From 0 ⁴ Pa) to +3 psi (about 2.1 × 10 ⁴ P)
It is preferred to maintain the pressure in the range of a) positive pressure).

In this manufacturing apparatus, when a current is caused to flow between the electrodes to cause arc discharge while protecting the cathode with the inert gas flow as described above, the nozzle is inclined with respect to the anode. A plasma flame with an elongated tail that is oriented and extended obliquely is generated, and the anode can be evaporated at its upper end. The length of the plasma frame tail can be adjusted according to the magnitude of the electric field, and acts as a high-temperature gradient furnace. An oxygen-containing gas is introduced from a nozzle into contact with the vapor flow of the anode material in or downstream of the plasma frame generated in this manner.

In order to increase the temperature gradient in the plasma flame and to increase its length, it is necessary to use a dissociable inert gas such as nitrogen, hydrogen or a mixture of both as an inert gas for protecting the cathode. In that case, the mixing amount of the dissociable inert gas is usually 1 to 70.
It is selected in the range of weight%.

The anode is preferably mounted on a feeder provided outside the chamber, and the amount of consumption by evaporation is preferably supplied continuously. By doing so, continuous production becomes possible, and production conditions are easily controlled. Such continuous production is preferred over batch operation because it can operate on a more consistent basis.

In this way, an oxygen-containing gas is introduced into and brought into contact with the vapor stream generated by evaporation from the anode, and an air stream containing primary particles generated by the fan is sent out to the collection chamber side by a fan, and cooling is sent in the middle of the stream. The gas is cooled by contact with the gas for use, flows into the collection chamber, expands and cools, and the fine particles are loosely aggregated. By capturing these fine particles with a filter medium, then stopping the fan and sending compressed air to the filter medium,
The captured fine particles are released, dropped, and collected in a receiver provided at the bottom of the collection chamber. Examples of the cooling gas include, for example, air, nitrogen, argon, helium, or a combination of these gases, with air being particularly preferred. Examples of the filtering material include a cloth fiber having a heat-resistant coating, a porous sintered body, and a bag filter.

[0024]

Next, the present invention will be described in more detail by way of examples, which should not be construed as limiting the present invention.

Various physical properties of the sample particles were determined, tested, and observed by the following methods. (1) Specific surface area Measured by BET method using ASAP-2010 (manufactured by Shimadzu Corporation, specific surface area measuring device). (2) Average particle size Specific surface area [m ² / g] and density [g / cm ³ ] of sample particles
Was calculated by the following equation. Average particle size (nm) = 6 × 10 ³ / (density × specific surface area) (3) Heat resistance test Using an electric furnace, heating at 1000 ° C. or 1200 ° C.
After holding for a time, the heat resistance index was determined. (4) Electron micrograph A photograph was taken with a transmission electron microscope H-8100 (manufactured by Hitachi, Ltd.) at an acceleration voltage of 200 kV.

Example 1 An arc discharge type plasma generator was used, and the inside of the chamber was maintained at a reduced pressure of 0.5 atm. Aluminum-magnesium alloy (atomic ratio of aluminum to magnesium =
A round bar composed of 9: 1) was set as an anode so that it could be replenished as needed from a feeder arranged outside the chamber. An electric power of 250 A and 50 V was applied between the anode in the chamber and the cathode diagonally above to cause arc discharge to generate a plasma flame. Argon gas was introduced into the chamber at a flow rate of 0.7 m ³ / h around the cathode. This argon gas flow was oriented from the cathode side to the anode side. The anode is evaporated from the upper end of the anode by the plasma flame generated by the arc discharge, and oxygen gas is added to the generated steam flow by 5 times.
The particles were sprayed at a flow rate of m ³ / h to form fine particles. The anode is continuously supplied from the feeder in an amount corresponding to the anode consumed by the evaporation. The fine particles generated in this manner were carried in a floating state along the flow of air created by a blower in the reaction chamber, and collected on a cloth filter in the collection chamber. The collected fine particles were separated from the filter medium by blowing compressed air from the collection chamber, and collected in a lower bottle. FIGS. 1 and 2 show transmission electron micrographs of the fine particles and those obtained by heat-treating the fine particles at 1400 ° C. for 6 hours. It can be seen that the shape of the fine particles is spherical and the particle size does not change much. In addition, the crystal form of the fine particles retained the structure mainly composed of γ-alumina even after heating at 1200 ° C. for 6 hours, as before heating.

Examples 2 to 5, Comparative Examples Fine particles were obtained in the same manner as in Example 1 except that the contents of aluminum and magnesium were variously changed as shown in Table 1.
The crystal form of the fine particles of each example retained the structure mainly composed of γ-alumina even after heating at 1200 ° C. for 6 hours, as before heating. In contrast, the fine particles of the comparative example were γ-alumina before heating, but had undergone a phase transformation to α-alumina after heating as in the examples.

Table 1 also shows various physical properties obtained for the fine particles of each of the above examples.

[Table 1]

From the above results, the γ-alumina fine particles of the comparative example undergo a phase transformation to α-form by heat treatment, and the strength is reduced, and the particles are enlarged and the specific surface area is significantly reduced. While it is difficult to exhibit a sufficient function as a catalyst carrier or a catalyst, the fine particles of Examples
In the heat treatment at ℃, the particle size does not change much compared to before heating,
Even in the heat treatment at 1200 ° C., the change in the particle size is smaller than that in the comparative example, the growth of the particles is suppressed, and the specific surface area satisfies the level usually required as a catalyst carrier or a catalyst before heating. Even after the heat treatment, the heat treatment at 1000 ° C. does not decrease much compared to before the heat treatment, and the change at the heat treatment at 1200 ° C. is smaller than that of the comparative example. It turns out that it can be utilized as a catalyst.

[0030]

The gamma-alumina-magnesia composite oxide fine particles of the present invention exhibit heat resistance and durability at a heat resistance temperature of 1300 ° C. or higher, and their crystal form, specific surface area and particle size are hardly changed even at high temperatures, and the catalyst carrier function In addition to being used as a catalyst or a catalyst carrier requiring heat resistance, such as an exhaust gas purifying catalyst for automobiles and a gasification catalyst, it is also useful as ceramics and ceramic additives. Further, according to the method of the present invention, including the fine particles,
γ-type alumina-magnesia composite oxide fine particles can be easily and industrially produced efficiently. Further, when the anode is continuously supplied in the method of the present invention, the production amount is restricted in the batch system, and it is difficult to control the production conditions with a decrease in the volume of the raw material. In addition, control of manufacturing conditions is also facilitated.

[Brief description of the drawings]

FIG. 1 is a transmission electron micrograph showing the particle structure of the fine particles obtained in Example 1.

FIG. 2 is a transmission electron micrograph showing the particle structure of a heat-treated product of fine particles obtained in Example 1.

Claims (10)

[Claims] 1. A method for producing fine particles of γ-type alumina-magnesia composite oxide, wherein a raw material containing aluminum and magnesium is evaporated by thermal plasma in an oxygen atmosphere. 2. The method according to claim 1, wherein the atomic ratio of aluminum to magnesium in the raw material is in the range of 99: 1 to 2: 1. 3. The method according to claim 1, wherein the thermal plasma is generated by arc discharge between the electrodes. 4. An inert gas flow directed between an electrode made of or mainly containing an aluminum-magnesium alloy and a paired electrode while protecting the latter electrode and directing the inert gas flow to the former electrode. While discharging, a plasma flame is generated by an arc discharge, an aluminum-magnesium alloy is evaporated, and an oxygen-containing gas is introduced into and brought into contact with the resulting vapor flow in the plasma flame and / or downstream of the plasma flame. A method for producing gamma-type alumina-magnesia composite oxide fine particles. 5. A mixture containing only aluminum and magnesium as metal elements, and at least two kinds of mixtures selected from metals and other conductive substances or at least selected from metals and other conductive substances. An arc is discharged between an electrode made of a mixture of one type of ceramics and a pair of electrodes, while protecting the latter electrode and orienting the inert gas flow to the former electrode, thereby performing an arc discharge to form a plasma frame. Gamma-type alumina-magnesia composite oxide fine particles, wherein an oxygen-containing gas is introduced into and brought into contact with a vapor flow generated in the plasma flame and / or downstream of the plasma flame. Manufacturing method. 6. The method according to claim 5, wherein the ceramic is an oxide or metal carbonate. 7. A gamma-type alumina-magnesia composite oxide fine particle produced by the production method according to claim 1. 8. A specific surface area before heating is 15 m ² / g or more,
Γ-type alumina-magnesia composite oxide fine particles having a heat resistance index (heat treatment conditions of 1000 ° C.) of 65% or more. 9. The γ-type alumina-magnesia composite oxide fine particles according to claim 8, wherein the average particle diameter before heating is 100 nm or less. 10. The γ-type alumina-magnesia composite oxide fine particles according to claim 8 or 9, produced by the production method according to any one of claims 1 to 6.