JPH0442809A - Heat-resistant fibrous alumina having high surface area and its production - Google Patents

Abstract

PURPOSE:To obtain thermally stable alumina having high specific surface area by atomizing a solution of alumina sol comprising fibrous aluminum hydroxide and burning. CONSTITUTION:Alumina sol comprising fibrous aluminum hydroxide is prepared. The alumina sol is atomized, instantly dried with air and burnt to give the title alumina having >=30m<2>/g specific surface area after burning at <=1,200 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a thermally stable alumina and a method for producing the same.

[Prior Art] Catalysts used for generating high-temperature gas in gas turbines and for treating automobile exhaust gas are required to have heat resistance. These catalysts are typically coated with a refractory oxide such as alumina or silica on a support or monolithic structure, which is then coated with platinum (Pi).
), palladium (Pd), rhodium (Rh)
However, the dispersibility depends on the surface area of the carrier.

Even heat-resistant oxides such as alumina and silica are 1.0
At high temperatures exceeding 00°C, the surface area decreases rapidly with the passage of reaction time, resulting in a decrease in the dispersibility of catalyst components and deterioration of catalyst activity. The decrease in the surface area of alumina at high temperatures is thought to be due to the pores on the alumina surface being clogged during crystallization into α-alumina. Therefore,
It is believed that by improving the heat resistance of alumina, it is possible to prevent the surface area of alumina from decreasing to some extent under such high temperatures. As a method for this purpose, it was proposed in JP-A-54-1 to increase the crystallization temperature of alumina by adding alkaline earth metals or rare earth metals to make alumina stable at high temperatures.
It is described in Publication No. 17387 and the like.

Research is also being conducted to react an added alkaline earth metal oxide with alumina to produce an alkaline earth metal aluminate with better heat resistance.

[Problems to be Solved by the Invention] However, there has been a demand for a method of improving the heat resistance of alumina using only alumina without adding any of the above additives.

[Means for Solving the Problems] The surface area of alumina is considered to be the total surface area of the fine particles constituting the alumina powder. Therefore, in order to create alumina with a high surface area, the constituent alumina particles may be made fine. However, in aggregates of fine particles, the contact area between particles is large, and reactions between particles occur at high temperatures.
Growth into larger particles results in a decrease in surface area. Therefore, it is thought that if alumina particles, which are aggregates of fine particles but have a small contact area with each other, are made, it will be possible to prepare alumina powder with excellent heat resistance.
The present invention has been developed.

That is, the present invention provides a heat-resistant, high-surface-area fibrous alumina and a method for producing the same. It is characterized by being atomized and instantly dried and fired with air or oxygen.

The fibrous alumina has a specific surface area of at least 30 m2/g after being fired at 1,200°C or less.

Note that if a sol solution of fibrous aluminum hydroxide is dried and fired as it is, it becomes an aggregate of fibrous alumina particles, but the contact area between the fibers is large and particle growth progresses at high temperatures. Undesirable. Furthermore, as described below, at high temperatures of 1.300° C. or higher, the surface area decreases significantly.

[Function] The heat-resistant high surface area fibrous alumina produced by the present invention is
As alumina particles with a small contact area, fibrous alumina becomes particles with a three-dimensionally intricate structure and a high porosity, making it possible to prepare alumina powder with excellent heat resistance.

[Example 1 20g of aluminum isopropoxide (AIF)
A solution in which 400 mJll was dissolved in hot water and 1 rrl of nitric acid was added thereto was stirred at 85° C. for 1 hour and then cooled to room temperature (solution A).

Next, approximately 100 mN of water is added to this solution A to make it a sol, and then this is poured into the solution reservoir 1 from the solution inlet 9 of the solution reservoir 1 of the spray combustion apparatus shown in FIG. 1, and the ultrasonic generator 2 It is atomized by 165MHz ultrasonic waves generated by the air or oxygen inlet 10 and passed through the flowmeter 3 at 1541
With oxygen introduced into the solution reservoir 1 at a flow rate of /win,
The alumina particles are introduced into a reaction tube consisting of a glass tube 7 in an electric furnace 4 maintained at 600° C., and are instantly dried and sintered to form approximately spherical alumina particles. It was collected by. After drying this at 110°C, it was fired at 500°C for 5 hours to obtain a small fibrous alumina powder. Note that the electric furnace 4 was provided with a thermocouple 5 and a thermostat 6 for temperature adjustment.

Example 2 A small fibrous alumina powder was obtained in the same manner as in Example 1 except that air was used instead of oxygen in Example 1.

Comparative Example 1 20g aluminum isopropoxide (AIP)
A solution obtained by dissolving 400 mg of hot water and adding 1 r+dl of nitric acid was stirred at 85° C. for 1 hour and then cooled to room temperature (solution A).

Next, about 100 mN of water was added to this solution A to form a sol, and the solution was dried at 110°C, and then calcined at 500°C for 5 hours to obtain fibrillar alumina powder.

A portion of solution A was collected with a pipette, dried on a Microgrilad, and observed under a transmission electron microscope.

A microscopic photograph thereof is shown in Fig. 2. This solution has a diameter of 10
It can be seen that it is composed of small fibrous aluminum hydroxide with a diameter of 0A and a length of 1.0OOA.

The fibrillar alumina powders obtained in Example 1 and Comparative Example 1 were calcined in air at various temperatures up to 1,400° C. for 5 hours, and the results of specific surface area measurements are shown in FIG. It can be seen that the fine fibrous alumina powder of the present invention has a high surface area of about 50 m2/g when fired at 1,200°C, although the surface area decreases significantly at high temperatures of 1,300°C or higher.

After drying the small fibrous alumina powder obtained in Example 1,
5 at temperatures of 500°C, 1,200°C and 1.400°C
FIG. 4 shows an electron micrograph of the time-sintered alumina powder observed with a transmission electron microscope. When fired at temperatures below 1,200°C, alumina particles exist as aggregates of small fibrous alumina, but at 1,400°C, no fibrous alumina is observed, and some alumina spheres melt and particle growth occurs. I can see that you are doing it.

The fine fibrous alumina powder obtained in Example 1 was
Figure 5 shows the X-ray diffraction spectra of alumina powder fired for 5 hours at temperatures of 11,300°C and 1,400°C. The formation of α-alumina was not confirmed, and the diffraction peaks were all co-alumina and θ-alumina.

1. Generation of α-alumina was confirmed by firing at 300°C or higher, and all diffraction peaks were δ-alumina and θ-alumina. At 1,400°C, it became a single phase of α-alumina.

1. The decrease in surface area during firing at temperatures above 300°C is α
- It can be seen that this is caused by the melting that occurs during crystallization into alumina and the accompanying particle growth.

The fine fibrous alumina powder obtained in Example 1 was
After firing at 0° C. for up to 25 hours, the change in alumina specific surface area over time was measured, and the results are shown in FIG.

The rate of decrease in specific surface area is quite large up to 10 hours of firing, but almost no decrease is seen after firing for 15 hours or more.
It can be seen that the value is approximately constant at 30 m2/g.

[Effects of the invention] The present invention has a specific surface area of 30 m2 after firing at 1,200°C without adding alkaline earth or rare earth metal to alumina.
It can be seen that it is a thermally stable alumina having a weight of 1.5% or more.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the outline of a spray combustion apparatus for producing heat-resistant, high-surface-area fibrous alumina of the present invention. FIG. Figure 3 is an electron micrograph observed with a microscope. The fibrillar alumina powder obtained in Example 1 and Comparative Example 1 was calcined in air at various temperatures up to 1.400°C for 5 hours. Figure 4 shows the relationship between temperature and specific surface area at 500°C, 1,200°C, and 1.400°C after drying the small fibrous alumina powder obtained in Example 1. An electron micrograph of time-calcined alumina powder observed with a transmission electron microscope. 1,300
The X-ray diffraction spectra of alumina powder calcined at temperatures of 1,400°C and 1,400°C for 5 hours, and FIG. FIG. 3 is a graph showing the change over time in the specific surface area of alumina after firing to a certain temperature. 1... Solution reservoir, 2... Ultrasonic generator, 3... Flow meter, 4... Electric furnace, 5... Thermocouple, 6... Thermostat, 7... Glass tube, 8... compensator, 9.
...Solution inlet, 10...Air or oxygen inlet. Applicant's representative Patent attorney Takehiko Suzue; A, L (@C) Figure 3 Figure 6 Procedural amendment (method) % formula % Indication of case Patent application No. 148593 Name of invention Heat resistance High surface area fibrous alumina and its manufacturing method 3, relationship with the case of the person making the amendment Patent applicant Cataler Engineering Co., Ltd. 7 Contents of the amendment (1) Specification box page 7, line 2, “Microscopic photograph thereof” was changed to “ "A microscopic photograph showing the particle structure of solution A." (2) On page 7, lines 14 and 15, "alumina powder" is corrected to "particle structure of alumina powder." (3) On page 9, lines 11 to 13, “electron micrograph of alumina sol observed with a transmission electron microscope” is changed to “
"An electron micrograph showing the particle structure of Solution A." (4) On page 10, lines 1 and 2, "electron micrograph of powder observed with a transmission electron microscope" is corrected to "electron micrograph showing the particle structure of powder." Target of correction

Claims (2)

[Claims] (1) Alumina made by atomizing alumina sol consisting of fibrous aluminum hydroxide, instantaneously drying and firing with air or oxygen, and having a specific surface area of at least 30m^2/g after firing at 1,200°C or less. A heat-resistant, high surface area fibrous alumina. (2) A method for producing heat-resistant, high-surface-area fibrous alumina, which comprises producing an alumina sol from fibrous aluminum hydroxide, atomizing the sol solution, and instantaneously drying and firing it with air or oxygen.