JPH08175808A - Production of oxide powder - Google Patents

Abstract

PURPOSE: To grow the particles in the secondary dimensional direction on firing and prevent the particles from reducing their specific surface area by precipitating a lamella structure in which an oxide sol is oriented between platelet crystals in the secondary dimension from a mixed solution of a specific oxide sol and a solution of an organic substance forming platelet crystals and firing the lamella structure. CONSTITUTION: An oxide sol at an aspect ratio of 5 or more and a solution of an organic substance forming platelet crystals are mixed at a weight ratio of 100/15-100/5 the sol/the substance to prepare a mixed solution. This solution is cooled down under stirring to precipitate out the organic platelet crystals and give a lamella structure in which the oxide sol is secondarily oriented between the platelet crystals. The lamella structure is fired in an oxidative atmosphere to combust off the organic substance and grow particles of the oxide sol.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an oxide powder, and more particularly to a method for producing an oxide powder having a laminated structure in which particles are two-dimensionally grown and having a large specific surface area. The oxide powder obtained by the present invention is useful as a carrier for an exhaust gas purifying catalyst.

[0002]

2. Description of the Related Art For example, as a raw material for a carrier for an exhaust gas purifying catalyst, alumina powder having a large specific surface area and excellent adsorbability is generally used. This alumina powder is usually produced by heat treating alumina hydrate. Alumina has not only completely dehydrated α-type but also seven metastable crystal forms generated in the heat treatment step. The specific surface area of alumina depends on the kind of hydrate, but has a maximum specific surface area of about several hundred m ² / g when calcined at 300 to 800 ° C.

When the firing temperature is 800 ° C. or higher, the specific surface area gradually decreases, and in the state where the α phase alumina is completely transformed, the specific surface area decreases to about several m ² / g. 10 more
The small pores are completely extinguished by firing at a temperature of 00 ° C or higher. As described above, alumina has a tendency that the specific surface area thereof rapidly decreases in the vicinity of 1000 ° C. where the crystal form changes from the γ phase to the α phase.

Therefore, in order to suppress the decrease in specific surface area,
It is known to add impurities such as BaO and SrO to alumina. When alumina containing such impurities is fired, a hexaaluminate phase is formed, thereby suppressing a decrease in specific surface area. In addition, Japanese Patent Laid-Open No. 5-
No. 238728 discloses that an alumina thin film having a large specific surface area is produced by mixing an amphipathic compound having a polar group and a hydrophobic group at both ends of a molecule and an alumina sol, developing the mixture on a substrate and removing the solvent. A method of doing so is disclosed.

[0005]

In the conventional method of heat-treating a hydrate, the alumina particles are sintered in a three-dimensional manner, and the crystal growth also proceeds in a three-dimensional manner. Therefore, the primary particles of the obtained alumina powder are substantially spherical, and the particles are three-dimensionally bonded to each other during firing, which causes a decrease in the specific surface area.

Therefore, a method has been carried out in which impurities such as BaO are added to form a hexaaluminate phase to suppress grain growth. However, even with alumina powder having improved heat resistance by this method, some grain growth is inevitable during firing, and when fired at 1000 ° C., the specific surface area decreases to about 100 to 150 m ² / g. I will end up. In addition, impurities contained therein may cause defects (such as a decrease in heat resistance due to a solid-phase reaction at the interface), which is not suitable for applications such as high-temperature catalysts.

Further, the alumina thin film formed by the so-called molecular template method disclosed in the above publication has anisotropy in the orientation of particles and has a specific surface area of 20 even when fired at 1000 ° C.
A high value of 0 m ² / g can be maintained. However, the molecular template used is a special polymer material and is extremely expensive. Further, it is a method for forming a thin film and is not suitable for producing alumina powder.

The present invention has been made in view of such circumstances, and it is an object of the present invention to prevent a decrease in specific surface area by growing grains in a two-dimensional direction during firing.

[0009]

The oxide powder manufacturing method of the present invention for solving the above-mentioned problems has an aspect ratio (vertical / horizontal).
A first step of preparing a mixed solution by mixing an oxide sol having a ratio of 5 or more with a solution of an organic substance forming a plate crystal, and precipitating a plate crystal of an organic compound from the mixed solution and between plate crystals. A second step of forming a laminated body in which the oxide sol is two-dimensionally oriented, and a third step of firing the laminated body in an oxidizing atmosphere to burn and remove organic substances and grain-growing the oxide sol. It is characterized by consisting of.

[0010]

In the first step, the oxide sol and the organic material solution are mixed to prepare a mixed solution. In the mixed solution, the organic matter is dissolved in the solvent, and the fine particles of the oxide sol are uniformly dispersed in the solution. In the second step, plate crystals of an organic substance are precipitated from the mixed solution, and the plate crystals tend to be stacked. At this time, since the oxide sol has an aspect ratio (longitudinal / horizontal) of 5 or more, it is sandwiched between the plate-like crystals in a two-dimensional orientation parallel to the plate-like crystals, and the oxide sol is separated from the plate-like crystals. A laminate comprising a three-dimensionally oriented oxide sol layer is formed.

In the third step, plate crystals of an organic material are burned and removed by firing in an oxidizing atmosphere. On the other hand, in each oxide sol layer, since the particles are two-dimensionally oriented, the grain growth also two-dimensionally grows in a layered form. Therefore, a layered oxide powder in which a plurality of layers are laminated is obtained,
The specific surface area is higher than that of the conventional oxide powder that is three-dimensionally grain-grown.

[0012]

【Example】

[Specific Examples of the Invention] Examples of the oxide according to the present invention include alumina, silica, titania, zirconia, silica-alumina, silica-titania, alumina-zirconia, and the like. ) Is 5
The above is used. If the aspect ratio of the sol is less than 5, it becomes difficult to two-dimensionally orientate between the plate-like crystals in the second step, and a component that three-dimensionally orients appears, leading to a decrease in the specific surface area.

Paraffin wax, anthracene, naphthalene and the like are exemplified as the organic substance which forms the plate crystal. As the solvent, those capable of easily precipitating plate-like crystals from the dissolved organic substance are desirable, and various organic solvents or water can be selected and used according to the type of organic substance. A poor solvent that dissolves organic matter when heated and precipitates organic matter when cooled is particularly desirable.

The mixing ratio of the oxide sol and the organic substance is, by weight ratio, oxide sol: organic substance = 100: 15 to 10
The range of 0: 5 is desirable. If the oxide sol is more than this range, two-dimensional orientation becomes difficult and the specific surface area decreases.
On the other hand, if the amount of the organic substances is more than this range, the orientation of the oxide sol may be disturbed due to the aggregation of the organic substances and the specific surface area may decrease. [Examples] Hereinafter, specific examples will be described. (Example 1) (1) First step Alumina sol aqueous solution ("A
S200 "Nissan Chemical Co., Ltd., aspect ratio 10) 2k
While heating g to 80 ° C., it was dried under reduced pressure with an aspirator and concentrated to 500 g, and 1.5 liter of ethanol was added and sufficiently stirred.

There, paraffin wax (melting point 56-5
(8 ° C.) 20 g was added, the mixture was heated to 70 ° C. and stirred for 1 hour to completely dissolve the paraffin wax. (2) Second step While stirring the obtained mixed solution, the temperature decreasing rate is 10 ° C./h
It was cooled to 15.degree. As a result, plate-like crystals of paraffin wax were deposited. 15 in a constant temperature / humidity tank
The temperature was maintained at 60 ° C. and humidity of 60 RH% for 6 days to grow plate crystals and dry ethanol. (3) Third step The obtained powder was put into a furnace at 1000 ° C.
Alumina powder was obtained by heating and firing for 0 hours. The obtained alumina powder had a very high specific surface area of 190 m ² / g.

A micrograph showing the crystal structure of the obtained alumina powder is shown in FIG. 1 (about 2 cm corresponds to 6.0 μm).
2 and FIG. 2 (about 2 cm corresponds to 600 nm). FIG.
Also, from FIG. 2, it is found that the alumina powder obtained in Example 1 has a layered crystal structure, and the primary particles have a fine plate-like shape. Example 2 Example 1 except that 20 g of naphthalene (melting point 80.5 ° C.) was used instead of the paraffin wax.
Alumina powder was produced in the same manner as in. The obtained alumina powder had an extremely high specific surface area of 180 m ² / g. (Example 3) The addition amount of paraffin wax or naphthalene was selected in several levels from 1% by weight to 20% by weight based on the alumina sol, and alumina powder was produced in the same manner as in Example 1. The specific surface areas of the obtained alumina powders were measured, and the results are shown in FIG.

From FIG. 5, it can be seen that the alumina powder produced has a high specific surface area when the amount of the organic substance added to the alumina sol is in the range of 5 to 15% by weight. The specific surface area is drastically reduced when the amount of the organic substance added is less than 5% by weight or more than 15% by weight. This is presumably because when the amount is less than 5% by weight, the number of plate-like crystals is small and the orientation of the alumina sol that cannot fit between the plate-like crystals is disturbed. If the amount is more than 15% by weight, it is considered that the organic substances aggregate to disturb the orientation of the alumina sol. (Comparative Example 1) The alumina sol used in Example 1 was dried,
It was placed in a furnace at 1000 ° C. in the same manner as in Example 1 and heated in the air for 10 hours for firing to obtain an alumina powder. The obtained alumina powder had a low specific surface area of 110 m ² / g.

A micrograph showing the crystal structure of the obtained alumina powder is shown in FIG. 3 (about 2 cm corresponds to 3.0 μm).
And FIG. 4 (about 2 cm corresponds to 600 nm). FIG.
4 and FIG. 4, it is clear that the alumina powder obtained in Comparative Example 1 has a substantially spherical granular crystal structure, which is clearly different from FIGS. 1 and 2. (Comparative Example 2) An alumina powder was produced in the same manner as in Example 1 except that an alumina sol having an aspect ratio of 2 to 4 was used. The obtained alumina powder had a specific surface area of 110 m ² / g, which was lower than that in Example 1. Therefore, it is necessary to set the aspect ratio to 5 or more. (Practical Example) Using the alumina powders obtained in Example 1, Example 2 and Comparative Example 1, the purification performance was examined when each was used as a carrier for an exhaust gas purification catalyst.

First, the alumina powder 10 obtained in Example 1
A slurry was prepared by mixing 0 part by weight of 45 parts by weight of an aqueous solution of aluminum nitrate having a concentration of 40% by weight, 3 parts by weight of an alumina sol having an alumina content of 10% by weight, and 100 parts by weight of water. A cordierite honeycomb carrier substrate having a volume of 1.5 L was immersed in this slurry, pulled up to blow off excess slurry, dried at 120 ° C., and baked at 500 ° C. for 2 hours to form a coat layer. Then, it was immersed in a dinitrodiammine platinum aqueous solution of a predetermined concentration, pulled up to blow off excess water, and then dried at 250 ° C. for 1 hour to support Pt. Further, Rh was similarly loaded using a rhodium nitrate solution having a predetermined concentration. The supported amount of each catalytic metal is
Pt is 2 g and Rh is 0.2 g per 1 L of the carrier substrate.

The alumina powders obtained in Example 2 and Comparative Example 1 were similarly slurried to form a coat layer, and each exhaust gas-purifying catalyst was prepared in the same manner. Table 1 shows the obtained three types of exhaust gas-purifying catalysts.
The A / F = 16 equivalent durability model gas shown in went each durability test flow 10 hours under the conditions of a space velocity 50000h ^-1. Inlet gas temperature is 600 ° C, 800 ° C and 10
Three levels of 00 ° C.

Next, for each exhaust gas purifying catalyst after the durability test, the evaluation model gas corresponding to A / F = 14.5 shown in Table 1 was used at A / F = 14.5 ± 1 and a cycle of 0.1 Hz. Flowing while changing the inlet gas temperature from low temperature to high temperature while changing the conditions, the hydrocarbon (HC) purification rate is 50%
The temperature (T ₅₀ ) at which FIG. 6 shows the results.

[0022]

[Table 1] It can be seen from FIG. 6 that the exhaust gas-purifying catalyst using the alumina powder of the example has a lower T _{50 and} is excellent in the initial activity as compared with the comparative example, and the difference is larger as the temperature of the durability test is higher. That is, it is clear that the alumina powders of the examples have extremely excellent durability even when used as a carrier for an exhaust gas purifying catalyst.

[0023]

That is, according to the method for producing an oxide powder of the present invention, a layered oxide powder having a large specific surface area can be produced easily and stably at low cost.

[Brief description of drawings]

FIG. 1 is a micrograph showing a crystal structure of alumina powder obtained in one example of the present invention.

FIG. 2 is a micrograph showing a crystal structure of an alumina powder obtained in one example of the present invention.

FIG. 3 is a micrograph showing a crystal structure of an alumina powder obtained in a comparative example of the present invention.

FIG. 4 is a micrograph showing a crystal structure of an alumina powder obtained in a comparative example of the present invention.

FIG. 5 is a graph showing the relationship between the added amount of organic substances and the specific surface area of the obtained alumina powder in one example of the present invention.

FIG. 6 is a durability processing temperature and HC5 in a practical example of the present invention.
It is a graph which shows the relationship with 0% purification temperature.

Claims (1)

[Claims] 1. A first step of preparing a mixed solution by mixing an oxide sol having an aspect ratio (longitudinal / horizontal) of 5 or more and a solution of an organic substance that forms a plate crystal, and a step of preparing the mixed solution from the mixed solution. A second step of precipitating the plate crystals of an organic substance and forming a laminate in which the oxide sol is two-dimensionally oriented between the plate crystals; and firing the laminate in an oxidizing atmosphere. And a third step of burning and removing the organic matter and growing the particles of the oxide sol.