JP2000015097A - Production of catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a catalyst producing method capable of uniformly dispersing noble metal particles in a carrier and easily controlling the particle size of the noble metal particles. SOLUTION: A catalyst producing method consists of a process for preparing a noble metal colloid soln. by mixing a noble metal salt with an org. polymer, a metal alkoxie decomposing process mixing metal alkoxide with the noble metal colloid soln. and subsequently hydrolyzing metal alkoxide and a baking process drying and baking the mixture obtained by the metal alkoxide decomposing process to form a catalyst powder.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for producing a catalyst such as an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine such as an automobile engine.

[0002]

2. Description of the Related Art As one type of catalyst, there is an exhaust gas purifying catalyst for purifying exhaust gas generated in an internal combustion engine. Above all, a three-way catalyst capable of purifying these air pollutants by simultaneously oxidizing carbon monoxide (CO) and hydrocarbons (HC) and reducing nitrogen oxides (NO _x ) is used in automobiles. It is widely used as an exhaust gas purifying catalyst for purifying exhaust gas generated by an engine.

As such an exhaust gas purifying catalyst, for example, a porous carrier layer made of alumina is formed on the surface of a heat-resistant carrier substrate made of cordierite or the like, and platinum (Pt), Those supporting noble metals such as rhodium (Rh) and palladium (Pd) are widely known. In the production, conventionally, various noble metal salts have been adsorbed on the support layer and then subjected to an appropriate heat treatment, whereby the noble metal is supported on the support layer.

However, in this manufacturing method, the noble metal is locally supported on the surface of the supporting layer. Therefore, the exhaust gas purifying catalyst obtained by this production method is liable to cause sintering of a noble metal when heated to a high temperature in an oxidizing atmosphere. In order to prevent such sintering of the noble metal, a technique for uniformly dispersing the noble metal particles throughout the supporting layer is required.

On the other hand, JP-A-6-31170 discloses that
A carrier comprising a composite obtained by co-hydrolytic polymerization of an organic polymer and at least two kinds of metal alkoxides in the presence or absence of metal salts (noble metal salts such as Pt and Pd) and a porous inorganic oxide A hydrotreating catalyst supporting at least one metal selected from Group 6A and Group 8 of the periodic table is disclosed. If this production method is used, it is possible to obtain one in which noble metal particles are dispersed throughout the support layer.

However, when the organic polymer, the metal alkoxide, and the noble metal salt are polymerized in the coexisting state, each of them becomes a highly mixed composite, so that the size of the noble metal particles formed at the time of firing can be easily reduced. Can't control. Therefore, the sintering of the noble metal cannot be sufficiently suppressed in the catalyst produced by using this production method.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned circumstances, and it is an object of the present invention to disperse noble metal particles uniformly in a carrier and to easily control the particle size of the noble metal particles. It is an object of the present invention to provide a method for producing a catalyst that can be used.

[0008]

According to a first aspect of the present invention, there is provided a method for producing a catalyst, which comprises mixing a noble metal salt and an organic polymer to prepare a noble metal colloid solution. Mixing a metal alkoxide with the noble metal colloid solution and then hydrolyzing the metal alkoxide; and a firing step of drying and firing the mixture obtained in the metal alkoxide decomposition step to form a catalyst powder. And characterized by the following.

According to a second aspect of the present invention, there is provided a method for producing a catalyst according to the first aspect of the present invention, wherein the organic polymer has at least a hydroxyl group.

[0010]

DETAILED DESCRIPTION OF THE INVENTION The method for producing a catalyst according to claim 1 comprises a series of steps of a noble metal colloid preparation step, a metal alkoxide decomposition step and a calcination step. In the noble metal colloid solution preparation step, a noble metal salt and an organic polymer are mixed to prepare a noble metal colloid solution.

As the kind of the noble metal element, Pt, R
In addition to h and Pd, iridium (Ir), silver (Ag), and the like can be given. For example, if the noble metal element is Pt, on its noble metal _{salt, PtCl 4 · 5H 2 O,} H 2 P
Examples include tCl ₄ and platinum nitrate. Also,
Examples of the organic polymer include polyvinyl alcohol, polyacrylic acid, and polypropyl alcohol.

The method of mixing the noble metal salt and the organic polymer is not particularly limited, but it is preferable to mix the noble metal salt and the organic polymer in a dispersion medium. By this mixing, noble metal colloid particles composed of a noble metal salt and an organic polymer are formed, and a noble metal colloid solution in which the noble metal colloid particles are dispersed in a dispersion medium can be obtained. Further, by selecting a combination of the kind (compound type) of the noble metal salt and the organic polymer, a predetermined amount of the noble metal salt can be incorporated into the organic polymer. That is,
Noble metal colloid particles into which a desired amount of a noble metal element has been incorporated can be formed.

In this step, it is desirable to appropriately select and mix the temperature of the dispersion medium and the concentrations of the noble metal salt and the organic polymer so that the noble metal colloid particles are formed efficiently and in large quantities. In the metal alkoxide decomposition step, a metal alkoxide is mixed with the noble metal colloid solution, and then the metal alkoxide is hydrolyzed.

As the metal alkoxide, an alkoxide composed of a metal element serving as a main component of the carrier is used. The carrier includes alumina (Al ₂ O ₃ ), silica (SiO ₂ ), zirconia (ZrO ₂ ), titania (TiO ₂ ), silica
Alumina, zeolite and the like are known. For example, if the carrier is composed of alumina, the metal element which is the main component of the carrier is aluminum, and the metal alkoxide includes Al (OC ₂ H ₅ ) ₃ and Al (OC ₃ H ₇ ) _{3 and the} like can be used.

In this step, the method of mixing the metal alkoxide is not particularly limited, and the metal alkoxide may be directly mixed with the noble metal colloid solution, or a metal alkoxide solution in which the metal alkoxide is dissolved is prepared. This metal alkoxide solution may be mixed with a noble metal colloid solution. When hydrolyzing the metal alkoxide, water may be added as necessary. Further, it is preferable that the metal alkoxide is mixed with the noble metal colloid solution, refluxed, and then the metal alkoxide is hydrolyzed. By carrying out the reflux, the reaction is promoted and a uniform colloid / alkoxide solution can be obtained.

Further, it is desirable that the temperature of the solution, the concentration of the metal alkoxide and the like be appropriately selected and mixed so that the metal alkoxide is efficiently and largely hydrolyzed. In the firing step, the mixture obtained in the metal alkoxide decomposition step is dried and fired to form a catalyst powder. The method for drying the mixture is not particularly limited as long as the noble metal colloid particles and the decomposition product containing the metal element constituting the metal alkoxide can be obtained together. If the amount of the solvent in the mixture is large, the mixture may be filtered and then dried.

Next, when the dried product is fired, the metal element of the decomposition product of the metal alkoxide is oxidized to an oxide, and oxide particles containing the metal element as a main component are formed. At the same time, the noble metal element in the noble metal colloid particles is singulated to form noble metal particles. At this time, since the noble metal element is surrounded by the metal element of the metal alkoxide via the organic polymer, the noble metal element can be aggregated only between the same noble metal colloid particles. Therefore, the noble metal particles formed are controlled to a small particle size. As a result, a catalyst powder composed of oxide particles mainly composed of the metal element constituting the metal alkoxide and noble metal particles finely dispersed in the oxide particles is synthesized.

The firing conditions such as firing atmosphere, firing temperature and firing time in firing are appropriately selected. The use of the catalyst powder thus obtained can be variously selected depending on the kind of the noble metal particles (noble metal element) contained therein, and is not particularly limited. For example, Pt, R
The catalyst powder containing noble metal particles such as h and Pd can be used as an exhaust gas purifying catalyst for purifying exhaust gas discharged from an internal combustion engine such as an automobile engine.

As described above, in the method for producing a catalyst according to the first aspect, a precious metal colloid solution preparing step of preparing a precious metal colloid solution using an organic polymer is provided before mixing the precious metal salt and the metal alkoxide. This is a significant feature, and the particle size of the noble metal particles supported on the support layer can be easily reduced. Therefore, a catalyst produced by controlling the particle size of the noble metal particles to a small value by this production method does not easily cause sintering of the noble metal particles even when heated at a high temperature in an oxidizing atmosphere.

Further, in the firing step, by utilizing the fact that the noble metal element can be aggregated only between the same noble metal colloid particles, the noble metal colloid particles having a desired amount of noble metal element incorporated therein are formed.
Noble metal particles having a desired particle size can be easily formed. In the method for producing a catalyst according to claim 2, except that an organic polymer having at least a hydroxyl group is used,
It can be carried out in the same manner as in the method for producing a catalyst according to claim 1.

The hydroxyl group of the organic polymer has an affinity for a water-soluble liquid. Therefore, if a water-soluble solution is prepared in the step of preparing a noble metal colloid solution, the organic polymer takes in the noble metal salt by directing its hydroxyl group to the outside (solution). At this time, it is preferable to perform reflux.
By this reflux, the noble metal element of the noble metal salt can be reduced. The noble metal element thus reduced can be easily bonded to the hydroxyl group of the organic polymer.

In the subsequent metal alkoxide decomposition step,
If the metal alkoxide is mixed with the noble metal colloid solution under appropriate mixing conditions, hydrolysis can be caused by one metal alkoxide and reacted with a hydroxyl group by the other metal alkoxide. As a result, precious metal colloid particles having a metal element of a metal alkoxide bonded to the surface are generated.

Next, in the firing step, when the mixture obtained in the metal alkoxide decomposition step is dried, the noble metal colloid particles having the metal element bonded to the surface further decomposes the metal alkoxide-containing decomposition product containing the metal element. Surrounded by Therefore, the noble metal element is further surrounded by the metal element of the metal alkoxide via the organic polymer. Therefore, in the subsequent firing, the aggregation of the noble metal element is less likely to occur, and the metal particles containing the metal element constituting the metal alkoxide as a main component and the noble metal particles having a smaller particle size dispersed in the metal particles are used. Catalyst powder is synthesized.

In this catalyst powder, the particle size of the noble metal particles is controlled to be extremely small. Therefore, even when the catalyst powder is heated to a high temperature in an oxidizing atmosphere, the sintering of the noble metal particles hardly occurs.

[0025]

EXAMPLE If a catalyst powder containing noble metal particles made of Pt, Rh, Pd, or the like is produced by the method for producing a catalyst according to claim 1 or 2, a pellet is formed using the catalyst powder. By molding the body, an exhaust gas purifying catalyst can be obtained.

Further, by preparing a carrier substrate and forming a layer made of a catalyst powder on the surface of the carrier substrate, a carrier substrate and a porous carrier layer covering the surface of the carrier substrate are formed. It is also possible to obtain an exhaust gas purifying catalyst comprising the noble metal supported on the support layer. In this case, a monolithic carrier substrate made of a heat-resistant ceramic such as cordierite or a metal carrier made of a metal foil can be used as the carrier substrate.

In the case of producing such an exhaust gas purifying catalyst, the noble metal supporting layer can be formed, for example, by the following procedure. First, a slurry is prepared by mixing the catalyst powder with an appropriate dispersion medium, and the slurry is coated on the surface of the carrier substrate. Next, firing is performed under appropriate firing conditions. At this time, the method of coating the slurry is not particularly limited. For example, the slurry can be easily coated by immersing the carrier substrate in the slurry. The firing conditions such as firing atmosphere, firing temperature and firing time in firing are appropriately selected according to the type of the catalyst powder.

It should be noted, supported layer may be one NO _X absorbent element or cerium oxide is supported. As the NO _x absorbing element, at least one selected from alkali metals, alkaline earth metals, and rare earth elements can be used. In addition, since cerium oxide has an oxygen storage / release function, more excellent catalytic performance can be obtained. This Ce is preferably supported in a stabilized state as a composite oxide with Zr (Ce-Zr oxide). Note that C
The e / Zr ratio is preferably in the range of 0.1 to 10.

As a method for supporting the NO _x absorbing element and the Ce—Zr oxide on the carrier layer, when the carrier layer is formed,
NO _X absorbent element or Ce-Zr oxide to a slurry of the catalyst powder, or added their raw materials, and a method of firing the coated slurry to the surface of the support substrate. During this firing, NO _X absorbent element or Ce-Zr
The oxide is supported on the support layer.

Alternatively, after forming the carrier layer, NO
There is also a method in which an _X- absorbing element or Ce-Zr oxide is supported on the supporting layer. For example, a solution containing NO _X absorbent element or Ce-Zr oxide was prepared by impregnating the solution onto the surface of the carrier layer, and then by calcining under appropriate firing conditions, NO
_{An X-} absorbing element or Ce-Zr oxide can be supported on the supporting layer.

Hereinafter, the present invention will be described specifically with reference to examples. In the following description, “parts” means parts by weight. (Example 1) A catalyst powder comprising alumina oxide particles and Pt noble metal particles dispersed in the oxide particles was produced by the catalyst production method of the present invention as follows. FIG. 1 shows the flow of the process. [Preparation process of precious metal colloid solution] PtC which is a salt of Pt
l ₄ · 5H ₂ O was 0.438g weighed, which was dissolved in ethanol 500 g. Subsequently, an organic polymer, polyvinyl alcohol (PVA) was mixed with the solution.
Here, PtCl ₄ .5H ₂ O is about 50 in terms of Pt mole.
Twice the amount of PVA was mixed. Subsequently, the solution in which the PVA was mixed was refluxed at about 80 ° C. As a result, the noble metal salt was reduced, and the reduced noble metal salt was wrapped in PVA to form noble metal colloid particles. Thus, a noble metal colloid solution in which noble metal colloid particles were uniformly dispersed was prepared. [Metal alkoxide decomposition step] Aluminum ethoxide (Al (OC ₂ H ₅ ) ₃ )
6.7 g was weighed and dissolved in about 500 g of ethanol. Further, an appropriate amount of ethanol was added to reduce the concentration of the solution to 0.1.
It was adjusted to 5 mol / l. This solution was mixed with the noble metal colloid solution obtained in the noble metal colloid solution preparation step. After the mixed solution was refluxed at about 80 ° C., pure water diluted with alcohol was slowly added to hydrolyze aluminum ethoxide. As a result, aluminum hydroxide containing a noble metal component was precipitated. [Firing Step] The solution obtained in the metal alkoxide decomposition step was filtered under reduced pressure to separate the noble metal colloid particles and aluminum hydroxide from the solution together. The thus separated noble metal colloid particles and aluminum hydroxide were washed with an appropriate washing solution and then dried. Then
The calcination was performed slowly at a calcination temperature of 600 ° C. for 5 hours. As a result of this firing, a catalyst powder (Pt-dispersed alumina powder) composed of metal particles made of alumina and noble metal particles made of Pt (Pt particles) finely dispersed in the metal particles was synthesized. [Formation of Exhaust Gas Purifying Catalyst] Using the catalyst powder thus produced, a honeycomb carrier made of cordierite and alumina, a porous carrier layer covering the surface of the honeycomb carrier, and a porous carrier layer supported on the carrier layer An exhaust gas purifying catalyst composed of Pt particles and a Ce-Zr oxide also supported on a support layer was formed as follows.

In addition to 100 parts of Pt-dispersed alumina powder, 70 parts of alumina sol (the content of alumina is 10 parts with respect to 100 parts of Pt-dispersed alumina powder. The same applies to the following.) 15 parts of an aluminum nitrate aqueous solution (40% by weight of aluminum nitrate is dissolved in the whole aluminum nitrate aqueous solution. The same applies to those used below), 40 parts of Ce-Zr oxide (Ce / Zr ratio) And 4 parts of water and 30 parts of water were prepared, and these were stirred and mixed to prepare a slurry for coating. The Ce-Zr oxide was prepared in advance as follows.

First, a CeO ₂ powder having a particle size distribution of 20 μm or less and a specific surface area of 50 m ² / g or more was prepared. The CeO ₂ powder was immersed in an aqueous solution of zirconium oxynitrate, pulled up and dried at 200 ° C. for 3 hours.
The sintering was performed in air at a sintering temperature of 600 ° C. for 5 hours. Thus, a Ce—Zr oxide that was a composite of cerium oxide and zirconium oxide was obtained. The Ce / Zr ratio was set to 4.

On the other hand, the cordierite honeycomb carrier was immersed in water to make it wet well, pulled up and blown off excess water. This honeycomb carrier was immersed in the above-mentioned slurry for coating, pulled up to blow off excess slurry, and then dried at a temperature of 80 ° C. for 20 minutes. Then, it was fired in air at a firing temperature of 600 ° C. for 1 hour. The fired body was immersed again in the coating slurry, pulled up and blown off excess slurry, and then dried and fired in the same manner as above.

Thus, a porous carrier layer composed of alumina carrying Pt particles and Ce-Zr oxide was formed on the surface of the honeycomb carrier. The amount of the support layer formed here was 120 g per liter of the honeycomb carrier. (Example 2) In this example, after obtaining a catalyst powder in the same manner as in Example 1, a catalyst for purifying exhaust gas was formed as follows.

100 parts of Pt-dispersed alumina powder, 70 parts of alumina sol, 15 parts of aluminum nitrate aqueous solution, and water 3
0 parts were prepared and mixed with stirring to prepare a slurry for coating. On the other hand, the cordierite honeycomb carrier was immersed in water to moisten it well, then pulled up to blow off excess water. The honeycomb carrier was immersed in the previously prepared coating slurry, pulled up to blow off excess slurry, and then dried at a temperature of 80 ° C. for 20 minutes. Next, it was fired at a firing temperature of 600 ° C. for 1 hour.
The fired body was immersed again in the coating slurry, pulled up and blown off excess slurry, and then dried and fired in the same manner as above.

Thus, a porous carrier layer made of alumina carrying Pt particles was formed on the surface of the honeycomb carrier. The amount of the support layer formed here was 120 g per liter of the honeycomb carrier. Then, cerium nitrate (C
e (NO ₃ ) ₃ ) and zirconium oxynitrate (ZrO
The honeycomb carrier having the carrier layer formed thereon was immersed in a mixed aqueous solution in which (NO ₃ ) ₂ ) was dissolved for 1 minute, pulled up to remove excess moisture, and then dried at a temperature of 200 ° C. for 3 hours. Next, the support was fired in air at a firing temperature of 600 ° C. for 5 hours to support the Ce—Zr oxide on the support layer.
Also in this case, as a mixed aqueous solution of cerium nitrate and zirconium oxynitrate, the concentration of cerium nitrate was 0.32 mol / l and the concentration of zirconium oxynitrate was 0.08 mol / l so that the Ce / Zr ratio was 4. Was used. (Comparative Example 1) Commercially available alumina powder (manufactured by Grace; MI
-386) 100 parts, alumina sol 70 parts, aluminum nitrate aqueous solution 15 parts, and water 30 parts were prepared, respectively.
These were stirred and mixed to prepare a coating slurry.

On the other hand, a cordierite honeycomb carrier was immersed in water to moisten it well, pulled up and blown off excess water. The honeycomb carrier was immersed in the slurry for coating, pulled up to blow off excess slurry, and then dried at a temperature of 80 ° C. for 20 minutes. Then 600 ° C
For 1 hour. The fired body was immersed again in the coating slurry, pulled up and blown off excess slurry, and then dried and fired in the same manner as above.

In this manner, a porous carrier layer made of alumina was formed on the surface of the honeycomb carrier. The amount of the support layer formed here was 120 g per liter of the honeycomb carrier. A Pt nitrate aqueous solution containing a predetermined concentration of Pt nitrate was prepared, and the honeycomb carrier was immersed in the Pt nitrate aqueous solution, pulled up, and dried at 250 ° C.
Thus, Pt was carried on the carrying layer. Note that P
The carried amount of t was set to be 2 g per liter of the honeycomb carrier. Comparative Example 2 A porous carrier layer made of alumina was formed on the surface of the honeycomb carrier in the same manner as in Comparative Example 1.

In the same manner as in Example 2, a mixed aqueous solution of cerium nitrate and zirconium oxynitrate was prepared. The honeycomb carrier having the support layer formed thereon was immersed in the mixed aqueous solution for 1 minute, pulled up to blow off excess water, and then dried at 200 ° C. for 3 hours. Next, the support was fired in air at a firing temperature of 600 ° C. for 5 hours to support the Ce—Zr oxide on the support layer.

Next, a Pt nitrate aqueous solution containing a predetermined concentration of Pt nitrate was prepared, and the honeycomb carrier was immersed in the Pt nitrate aqueous solution, pulled up, and dried at 250 ° C. Thus, Pt was carried on the carrying layer. The amount of Pt carried here was also set to be 2 g per liter of the honeycomb carrier.

[0042]

[Evaluation method] Each of the exhaust gas purifying catalysts produced in the above Examples and Comparative Examples was housed in a catalytic converter, and 2 L of an automobile was used.
It was mounted on the exhaust system of the engine and a purification test of exhaust gas generated by the engine was performed. In this test, the following three types of purification tests were performed. (Test 1) The lower limit temperature of the exhaust gas at which the exhaust gas purifying catalyst can purify the exhaust gas by 50% was tested.

In this test, the engine was operated at the air-fuel ratio A / F =
The gas was burned at 14.6 (stoichiometric) to generate exhaust gas. The exhaust gas is intended to include a large amount of CO, HC and NO _X. While increasing the temperature of the exhaust gas from a low temperature side, the exhaust gas was purified by the exhaust gas purifying catalyst. In this case the CO contained in the exhaust gas, the purification rate of HC and NO _X is 50% and is temperature were measured. Table 1 shows the test results of the exhaust gas purifying catalysts of the examples and comparative examples. (Test 2) The purification performance of the exhaust gas purifying catalyst when the temperature of the exhaust gas was 400 ° C. was tested.

In this test, the engine was operated at the air-fuel ratio A / F =
It was burned at 14.6 to generate exhaust gas. The temperature of the exhaust gas was set to 400 ° C., and the exhaust gas was purified by the exhaust gas purifying catalyst. CO at this time in the exhaust gas was measured HC and NO _X exhaust emission reduction rate. Table 1 also shows the test results of the exhaust gas purifying catalysts of the examples and comparative examples. (Test 3) If the exhaust gas purifying catalyst is used for a long time to purify the high-temperature exhaust gas, the noble metal particles carried on the carrier layer may cause sintering, and the purification performance may be reduced. Therefore, durability performance against high-temperature exhaust gas was tested using the particle size of Pt particles as an index.

In this durability test, the air-fuel ratio A / F was set to 14.
5 centered, amplitude: about 1A / F, period: about 0.5H
The engine was burned for 50 hours while varying in z to generate exhaust gas. During this time, the temperature of the exhaust gas was set to 900 ° C., and the exhaust gas was purified by the exhaust gas purifying catalyst. The average particle size of the Pt particles after the test was measured by an X-ray diffraction method. Table 1 also shows the test results of the exhaust gas purifying catalysts of the examples and comparative examples.

[0046]

[Table 1] From Table 1, each of CO, HC and NO contained in the exhaust gas
_It can be seen that the temperature of the exhaust gas purifying catalyst of the example is lower than that of the comparative example at the temperature where the purification rate of _X is 50%. From these results, it is clear that the exhaust gas purifying catalysts of the examples are superior in catalytic performance at low temperatures as compared with those of the comparative example.

When the temperature of the exhaust gas is at 400 ° C.
, HC and NO in flue gas _XAbout purification rate of
Therefore, the purification rate of any of the exhaust gas purifying catalysts
It turns out that it is higher than that of the comparative example. from this result,
Exhaust gas purifying catalyst of the example, compared with the comparative example,
For exhaust gas purification performance when the temperature of exhaust gas is at 400 ° C
It turns out that it is excellent.

Further, regarding the average particle size of the Pt particles supported on the support layer after the durability test, it can be seen that the average particle size of the exhaust gas purifying catalyst of the example is smaller than that of the comparative example. From these results, it is clear that in the exhaust gas purifying catalyst of the example, the frequency of sintering noble metal particles is smaller and the purification performance is less reduced than that of the comparative example.

The average particle size of the Pt particles after the durability test is slightly smaller in Example 2 than in the exhaust gas purifying catalyst of Example 1. The reason is considered as follows. In Example 2, the support of Ce—Zr oxide was Ce—Zr oxide.
Since the Pt particles are supported by impregnating and supporting the Zr oxide from the raw material solution in a separate step, the Ce-Zr oxide is more finely dispersed than that of the first embodiment.
It is considered that the Ce-Zr oxide further suppressed the sintering of the Pt particles.

[Brief description of the drawings]

FIG. 1 is a chart of a process in manufacturing an exhaust gas purifying catalyst of Example 1.

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Claims (2)

[Claims] A noble metal colloid solution preparing step of mixing a noble metal salt and an organic polymer to prepare a noble metal colloid solution; mixing a metal alkoxide with the noble metal colloid solution;
Then, a metal alkoxide decomposition step of hydrolyzing the metal alkoxide, and a firing step of drying and firing the mixture obtained in the metal alkoxide decomposition step to form a catalyst powder, comprising: . 2. The method according to claim 1, wherein the organic polymer has at least a hydroxyl group.