JPH09285730A - Combustion catalyst for high temperature and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a combustion catalyst for high temp. having high activity and small in the deterioration or the fluctuation of oxidation activity at 700-1000 deg.C. SOLUTION: This combustion catalyst for high temp. is a catalyst having palladium oxide carried on a heat resistant inorganic carrier, the palladium oxide consists essentially of a hardly reducing palladium oxide having 880 deg.C dissociation temp. in an inert gas and the hardly reducing palladium oxide is present on the surface of palladium oxide particle. The palladium oxide is carried on the heat resistant inorganic carrier, fired in an oxygen-containing pressurized atmosphere and, after that, an easily reducing palladium oxide is reduced by reduction treating and the reduced palladium is removed by acid- treating. The catalyst having the high activity hardly reducing palladium oxide present on the particle surface is obtained by reducing the easily reducing palladium oxide present on the particle surface in the catalyst obtained by firing under the oxygen-containing pressurized atmosphere, and removing the reduced product by dissolving by the acid treatment.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high temperature combustion catalyst and a method for producing the same, and more particularly to a high temperature combustion catalyst which can be used in a high temperature combustor such as a gas turbine or a boiler, TECHNICAL FIELD The present invention relates to a high temperature combustion catalyst having a small decrease and fluctuation and a high oxidation activity and a method for producing the same.

[0002]

2. Description of the Related Art The catalytic combustion method is a method of premixing a fuel such as methane or propane with air and then performing flameless combustion in a catalyst layer, and has an advantage that the amount of nitrogen oxides generated is extremely small. As a combustion catalyst used in this catalytic combustion method, a catalyst in which palladium oxide is supported on a heat-resistant carrier such as alumina, zirconia, magnesia, or silica has been widely studied because of its high activity.

However, the supported palladium oxide catalyst has a low activity due to the particle growth of palladium oxide at high temperature.
Palladium oxide dissociates into palladium at temperatures above ℃ and the oxidation activity decreases significantly (Appl. Catal., A81,227 (1992)).
There is such a problem.

Conventionally, as a method for suppressing the grain growth of palladium oxide, an intermediate layer is provided between the palladium oxide and the carrier (JP-A-63-72345), and the oxide is dispersed on the palladium oxide ( JP-A-3-186347
Method) has been proposed. On the other hand, as a method of suppressing the dissociation of palladium oxide, a method of adding a lanthanum component (JP-A-4-27432) is proposed.

[0005]

However, even when the above-mentioned conventional improved supported palladium oxide catalyst is used,
There is a problem that it is not possible to sufficiently prevent the decrease and fluctuation of the oxidation activity in the temperature range of 1000 ° C.

The present invention solves the above-mentioned conventional problems, and provides a high temperature combustion catalyst and a method for producing the same, which is highly active and has a small decrease and fluctuation in oxidation activity even in the temperature range of 700 to 1000 ° C. To aim.

[0007]

A high temperature combustion catalyst according to claim 1 is a catalyst in which palladium oxide is supported on a heat-resistant inorganic carrier, the palladium oxide having a peak dissociation temperature mainly in an inert gas. It is characterized in that it is composed of a non-reducing palladium oxide having a temperature of 880 ° C. or higher, and that the non-reducing palladium oxide is present on the surface of the palladium oxide particles.

A method for producing a high temperature combustion catalyst according to claim 2 is
The method for producing a combustion catalyst for high temperature according to claim 1, wherein palladium oxide or a palladium compound that produces palladium oxide by firing is supported on a heat-resistant inorganic carrier, and after firing in an oxygen-containing pressurized atmosphere. A reduction treatment is performed to reduce easily reduced palladium oxide having a peak dissociation temperature of 850 ° C. or less in an inert gas, and then acid treatment is performed to remove the reduced palladium. .

That is, the inventors of the present invention have conducted extensive studies on a high temperature combustion catalyst in order to solve the above-mentioned problems, and as a result,
Of the palladium oxide supported on the heat-resistant inorganic carrier, palladium oxide with poor crystallinity is easily reduced, which causes activity fluctuations at high temperatures. Also, the difference in the ease of reduction of palladium oxide and reduction By utilizing the phenomenon that the formed palladium is easily dissolved in an acid, it has been found that the easily reducing palladium oxide, whose activity fluctuates at high temperature, can be selectively removed, and the present invention has been completed.

The refractory palladium oxide in the present invention means palladium oxide having a peak dissociation temperature of 880 ° C. or higher at a temperature rising rate of 20 ° C./min in an inert gas. Also, with easily reducing palladium oxide,
It means palladium oxide having a peak dissociation temperature of 850 ° C. or less at a temperature rising rate of 20 ° C./min in an inert gas.

Usually, the palladium oxide catalyst is 1
It is prepared by firing at a temperature of 000 ° C. or higher. As described above, the palladium oxide in the catalyst produced by baking at 1000 ° C. or higher in the air has many particles with a particle size of 50 nm or more, and the crystallinity is not good, and the dissociation temperature (in an inert gas, the temperature rising rate 2
The peak of the dissociation temperature at 0 ° C / min) is about 700 ° C.

On the other hand, the catalyst calcined under pressure in an oxygen-containing atmosphere has a better crystallinity of palladium oxide and a smaller particle size of palladium oxide than in the atmosphere. Then, when the dissociation temperature is examined, in an inert gas,
The peak of dissociation temperature at a temperature rising rate of 20 ° C / min is 880.
It can be seen that non-reducing palladium oxide having a temperature of ℃ or higher is produced.

However, in addition to the hardly reducible palladium oxide, easily reducible palladium oxide having a peak dissociation temperature of 850 ° C. or less is also produced, and its existence form is as follows: Of the palladium particles,
It is considered that there is hardly reducible palladium oxide on the carrier side of the particles, and easily reducible palladium oxide exists on the surface of the particles. It is speculated that this is because palladium oxide is stabilized by some interaction with the carrier.

The easily reducing palladium oxide present on the surface of the particles does not dissociate up to the reaction temperature of 900 ° C. and retains the catalytic activity, but dissociates at the reaction temperature of 1000 ° C. to form metallic palladium. It is considered that this is the cause of the decrease in catalytic activity. Although active and hardly reducing palladium oxide exists inside the particles, since the surface of the particles is covered with metallic palladium, this active and hardly reducing palladium oxide cannot contribute to the reaction.

Therefore, when combustion is carried out at a reaction temperature of 1000 ° C. using such a catalyst, the easily reducing palladium oxide existing on the surface of the particles is dissociated, and as a result, the surface of the particle is made of metallic palladium having a small activity. Because it will be covered
The catalytic activity is significantly reduced.

On the other hand, in the present invention, among the catalysts obtained by calcination in an oxygen-containing pressurized atmosphere, the easily reducing palladium oxide present on the particle surface is reduced by a reduction treatment, and The reduced palladium is dissolved and removed by an acid treatment to obtain a catalyst in which the non-reducing palladium oxide is present on the particle surface.

Therefore, the high temperature combustion catalyst of the present invention has a high dissociation temperature of palladium oxide, and the decrease in the oxidation activity due to the dissociation of palladium oxide is extremely small even at a high temperature of 700 to 1000 ° C.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below in accordance with the procedure for producing a high temperature combustion catalyst according to the present invention.

In the present invention, the heat resistant inorganic carrier for supporting palladium oxide is not particularly limited, but 10
It is preferable that the specific surface area is less reduced when heated to 00 ° C., and ceramic powders such as alumina, zirconia, and magnesia, or powders obtained by stabilizing these ceramic powders with various additives can be used.

In the present invention, it is preferable to load palladium oxide on such a heat-resistant inorganic carrier in a proportion of 0.01 to 5.0% by weight.

The method for supporting palladium oxide on the heat-resistant inorganic carrier is not particularly limited, and the method used for usual catalyst preparation can be adopted. For example, an impregnation method is used in which a heat-resistant inorganic carrier is impregnated with a solution containing a palladium salt such as palladium acetylacetonate, palladium nitrate or palladium acetate. Since the palladium salt decomposes during firing to generate a gas, the mixture obtained by the impregnation method is dried in advance and then dried in air at 300
It is preferable to pyrolyze at a temperature of up to 800 ° C. and then bake in an oxidizing atmosphere.

In the present invention, the heat-resistant inorganic carrier carrying palladium oxide is fired in an oxygen-containing pressurized atmosphere in this manner. The pressure of this firing atmosphere is 2 k.
gf / cm ² or more, preferably 10 kgf / cm ² or more, particularly preferably 50 kgf / cm ² or more.
By firing in an oxygen-containing atmosphere at a pressure lower than 2 kgf / cm ^2, the effect of forming the non-reducing palladium oxide cannot be fully expected.

The oxygen partial pressure in the firing atmosphere is 0.
5 kgf / cm ² or more, preferably 1 kgf / cm ² or more, particularly preferably at 25 kgf / cm ² or more.
When firing at an oxygen partial pressure lower than 0.5 kgf / cm ² ,
The high temperature stability of palladium oxide is the same as when it is fired in the air, and it is difficult to obtain a non-reducing palladium oxide.

As a gas component other than oxygen in the oxidizing atmosphere, argon, nitrogen or the like is used.
Firing is performed in an oxidizing atmosphere of 2 to 50% oxygen and 50 to 98% other gas.

In the present invention, the firing temperature is preferably lower than the temperature at which palladium oxide dissociates under the firing atmosphere pressure. When calcined at a temperature at which the palladium oxide is dissociated or higher, hardly-reducible palladium oxide is less likely to be produced, and the oxidation activity of the combustion catalyst is significantly reduced and fluctuated.
The firing is preferably performed at a temperature of 500 to 1200 ° C. for about 1 to 5 hours.

According to such a method, it is possible to obtain a catalyst carrying a hardly reducing palladium oxide. However, even if prepared in this way, palladium oxide which is easily reduced is simultaneously produced together with the hardly reducing palladium oxide. As described above, this easily reducing palladium oxide causes fluctuations in activity at high temperatures, so in the present invention, it is removed by a reduction treatment after firing and a subsequent acid treatment.

The reduction treatment of the easily reducing palladium oxide is
A wet method using a reducing agent such as an aqueous solution of hydrazine may be used, or heating may be performed in a reducing gas such as hydrogen gas or methane gas or an inert gas such as helium gas, nitrogen gas, and argon gas. A dry method may be used. Usually, a dry method is used because it is easy to process.

In this case, the concentration of hydrogen is not particularly limited, but the reduction temperature is preferably 400 ° C. or lower, because if the temperature is too high, it may be reduced to the hardly reducible palladium oxide. . Treatment in an inert gas has a weaker reducing power than when a reducing gas is used.
It is necessary to raise the temperature, but even in this case, if the temperature is too high, it is possible to reduce even the hardly-reducing palladium oxide. The reduction time depends on the temperature and the atmosphere, but is 1
It is enough to go for about an hour.

For the acid treatment after the reduction treatment, nitric acid, sulfuric acid, hydrochloric acid, aqua regia, etc. can be used. However, since it is difficult to dissolve in hydrochloric acid and sulfuric acid and even the carrier may be dissolved in aqua regia, the acid treatment The treatment is usually carried out by immersion in nitric acid at room temperature. In this case, the immersion time varies depending on the nitric acid concentration and time, but if it is concentrated nitric acid, the immersion treatment is performed for about 5 minutes to 1 hour.

After this acid treatment, it is washed with water and dried by heating to obtain a high temperature combustion catalyst.

According to such a method, it is possible to obtain a supported palladium oxide catalyst in which 60% or more of the palladium oxide which is hardly reducible among the palladium oxides in contact with the gas phase is used.

[0032]

EXAMPLES The present invention will be described in more detail below with reference to Examples and Comparative Examples, but the present invention is not limited to the following Examples unless it exceeds the gist.

Example 1 δ-alumina powder was impregnated with an aqueous ethanol solution containing palladium acetylacetonate and dried at 100 ° C. The obtained dry mixture was pyrolyzed in air at 500 ° C. for 4 hours to prepare a supported palladium oxide catalyst carrying 0.5 g of palladium oxide per 100 g of δ-alumina.

Next, 10 times this supported palladium oxide catalyst was used.
Under 5% oxygen 95% argon mixed gas atmosphere at a pressure of 00kgf / cm ² (partial pressure of oxygen 50kgf / cm ^2),
It was baked at 1100 ° C. for 4 hours.

This catalyst was passed under hydrogen flow (100% hydrogen, 3%
(0 cc / min) at 300 ° C. for 30 minutes. After that, the catalyst was immersed in concentrated nitric acid at room temperature for 30 minutes, washed, and then dried by heating to prepare a high-temperature catalyst from which easily reducible palladium oxide was removed.

Example 2 A supported palladium oxide catalyst was prepared in the same manner as in Example 1, and 2000 kgf / c of this supported palladium oxide catalyst was used.
Firing was performed at 1100 ° C. for 4 hours in a mixed gas atmosphere of 5% oxygen-95% argon with a pressure of m ² .

This catalyst was passed through helium (helium 10
0%, 30 cc / min), and reduction treatment was performed at 830 ° C. for 10 minutes. After that, the catalyst was immersed in concentrated nitric acid at room temperature for 45 minutes, then washed and dried by heating to prepare a catalyst for high temperature from which easily reducible palladium oxide was removed.

Example 3 A high temperature combustion catalyst was prepared in the same manner as in Example 1 except that the reduction treatment was performed at 250 ° C. for 60 minutes.

Comparative Example 1 A combustion catalyst was prepared in the same manner as in Example 1, except that the firing was performed in the atmosphere.

Comparative Example 2 A combustion catalyst was prepared in the same manner as in Example 1, except that the reduction treatment and the subsequent acid treatment were not performed.

Comparative Example 3 A combustion catalyst was prepared in the same manner as in Comparative Example 1, except that the reduction treatment and the subsequent acid treatment were not performed.

[Evaluation of Catalyst] Measurement of Change in Methane Oxidation Activity with Change in Reaction Temperature The methane oxidation activity of the catalysts produced in Examples 1 to 3 and Comparative Examples 1 to 3 was examined. First, 0.4 g of each prepared combustion catalyst was filled in a fixed bed flow reactor, and 1.7 liters of a mixed gas of 0.5% methane-99.5% air was flowed per minute.

Next, the temperature of the electric furnace is raised to 1000 ° C. to heat the catalyst layer, the methane conversion rate is measured, and subsequently, the methane conversion rate in the temperature lowering process is measured while sequentially lowering the reaction temperature to a predetermined temperature. did.

The methane conversion rates of the high temperature combustion catalysts of Examples 1 to 3 are shown in FIGS. 1 to 3, respectively. From FIGS. 1 to 3, it can be seen that the high temperature combustion catalysts of Examples 1 to 3 have little drop in the methane conversion rate at 700 to 800 ° C.

The methane conversion of the combustion catalyst of Comparative Example 1 is shown in FIG.
Shown in As is clear from FIG. 4, the methane conversion rate rapidly decreased at the reaction temperature of 800 ° C. or lower, and did not increase even when the temperature was further lowered. This is because the non-reducing palladium oxide did not exist because it was not fired under pressure, and the palladium oxide was almost removed by the reduction treatment and the subsequent acid treatment.

FIG. 5 shows the methane conversion rate of the combustion catalyst of Comparative Example 2.
Shown in As is clear from FIG. 5, the decrease in the methane conversion rate at 750 ° C. is large as compared with Examples 1 to 3.

FIG. 6 shows the methane conversion rate of the combustion catalyst of Comparative Example 3.
Shown in As is clear from FIG. 6, the decrease in the methane conversion rate at 750 ° C. is large as compared with Examples 1 to 3.

Measurement of Particle Size of Each Catalyst The proportion of particles having a particle size of 50 nm or more in each combustion catalyst was examined by observation with a transmission electron microscope (TEM) (20000 times), and the results are shown in Table 1.

[0049]

[Table 1]

From Table 1, it can be seen that the high temperature combustion catalysts of Examples 1 to 3 have few coarse particles and have high activity.

Measurement of Dissociation Temperature of Combustion Catalyst For the combustion catalysts of Example 1 and Comparative Example 2 as a representative example, about 0.3 g of a sample was passed under helium gas flow (helium 1
00%, 30 cc / min), heating rate 20 ° C / min
The dissociation temperature when the temperature was raised at was measured and is shown in FIG.

FIG. 7 shows that the high temperature combustion catalyst of the present invention is excellent in high temperature stability.

[0053]

As described above in detail, according to the combustion catalyst for high temperature and the method for producing the same of the present invention, the oxidation activity is high, and even after exposure to a high temperature of 1000 ° C. or higher, 70
Provided is a high-performance high temperature combustion catalyst in which the oxidation activity does not decrease or fluctuate even in a high temperature range of 0 to 1000 ° C.

[Brief description of drawings]

1 is a diagram showing a methane conversion rate of a high temperature combustion catalyst prepared in Example 1. FIG.

2 is a diagram showing a methane conversion rate of a high temperature combustion catalyst prepared in Example 2. FIG.

3 is a diagram showing a methane conversion rate of a high temperature combustion catalyst prepared in Example 3. FIG.

FIG. 4 is a diagram showing a methane conversion rate of a combustion catalyst prepared in Comparative Example 1.

5 is a diagram showing a methane conversion rate of a combustion catalyst prepared in Comparative Example 2. FIG.

FIG. 6 is a diagram showing a methane conversion rate of a combustion catalyst prepared in Comparative Example 3.

7 is a diagram showing measurement results of dissociation temperatures of a high temperature combustion catalyst prepared in Example 1 and a combustion catalyst prepared in Comparative Example 2. FIG.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akio Nishida 5 1978, Kozugushi, Ube City, Yamaguchi Prefecture Ube Kosan Co., Ltd. Ube Laboratory

Claims (2)

[Claims] 1. A catalyst in which palladium oxide is supported on a heat-resistant inorganic carrier, wherein the palladium oxide is mainly composed of non-reducing palladium oxide having a peak dissociation temperature of 880 ° C. or higher in an inert gas, A combustion catalyst for high temperature, characterized in that the hardly reducing palladium oxide is present on the surface of the palladium oxide particles. 2. A heat-resistant inorganic carrier is loaded with palladium oxide or a palladium compound which produces palladium oxide by calcination, and calcinated in an oxygen-containing pressurized atmosphere, and then subjected to reduction treatment to reduce the dissociation temperature in an inert gas. 850 peak
The method for producing a high temperature combustion catalyst according to claim 1, wherein the easily reducing palladium oxide having a temperature of not higher than 0 ° C. is reduced, and then the reduced palladium is removed by acid treatment.