JP2011078890A - Method for manufacturing catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a catalyst which enables a flocculating of a catalyst obtained by supporting a metallic microparticle with a catalyst carrier material. <P>SOLUTION: This method for manufacturing a catalyst involves the following procedures: first, a catalyst precursor is prepared by allowing a metallic raw material microparticle to be supported by the catalyst carrier material dispersed in a liquid medium, and then, a sample composed of a mixture of the precursor and the liquid medium is freeze-dried to remove the liquid medium. Thus, the flocculation of the precursor during drying can be prevented from occurring. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for producing a catalyst, and more particularly to an improvement in a method for producing a catalyst for a fuel cell.

A catalyst for a fuel cell is generally formed by supporting metal fine particles such as platinum on a catalyst carrier material such as carbon. Such a catalyst is produced, for example, by a colloid method. In this colloid method, a carbon carrier is gradually mixed with PtO ₂ colloid in the presence of water. Thereby, platinum fine particles are supported as metal fine particles on a carbon carrier as a catalyst carrier material, and a fuel cell catalyst is obtained.
In the state where this fuel cell catalyst is obtained, water is present as a medium, which is removed by heating and drying, and further reduced to activate the platinum fine particles.

United States Patent 4,044,193

Although the fuel cell catalyst obtained by the above conventional method is dried, the carbon particles constituting the carbon support are strongly bonded to each other to form secondary particles, and further, multi-particles exceeding the secondary particles. Sometimes. This is considered to be because the catalysts strongly aggregate due to capillary contraction or the like when water moves and evaporates during drying.
As a result, there is a possibility that the platinum fine particles inside cannot be sufficiently reduced in the reduction step. Also, when a polymer electrolyte and a catalyst are mixed to obtain a paste, the polymer electrolyte material may not penetrate into the catalyst. Therefore, all platinum fine particles do not function effectively.

The present invention has been made to solve the above problems, and the first aspect thereof is defined as follows. That is,
A catalyst preparation step of preparing a catalyst precursor by supporting metal raw material fine particles on a catalyst support material dispersed in a liquid medium; and
A drying step of removing the liquid medium by sublimation;
Is a method for producing a powdery catalyst.

According to the method for producing a catalyst of the first aspect thus defined, a method of removing the liquid medium by sublimation is employed as the drying step. For this reason, the catalyst precursor obtained in the catalyst adjustment step can be dried without agglomeration. If it is a catalyst obtained by the said manufacturing method, the dispersibility with respect to another substance will improve, for example.
For example, the fuel cell catalyst obtained by this method can ensure good dispersibility in the fuel cell electrode paste.
As a method for sublimation of the liquid medium in the drying step, a vacuum freezing method can be employed (second aspect). It is possible to obtain a catalyst with less aggregation by drying by a vacuum freezing method.
In the production method of the present invention, water can be used as the liquid medium, and carbon particles can be used as the catalyst support material.
In addition, since the catalyst has a property of being more easily aggregated as the particles of the catalyst carrier material used are smaller, the effect of the production method of the present invention is more exhibited.

  In the production method of the present invention, an impurity removal step can be interposed between the catalyst preparation step and the drying step (third aspect).

FIG. 1 is a conceptual diagram of a vacuum freeze-drying apparatus according to an embodiment of the present invention.

Hereinafter, a specific embodiment of the present invention will be described in detail by taking a fuel cell catalyst as an example.
<Catalyst preparation step>
Metal raw material fine particles are supported on a catalyst carrier material to form a fuel cell catalyst precursor.
Here, the catalyst support material is preferably electrochemically stable, has a wide area per unit volume, and has conductivity. Examples of the catalyst support material include carbon particles, conductive ceramics, conductive oxides, conductive resins, and the like.
The metal fine particles of the fuel cell catalyst provide an active site for the fuel cell reaction, and fine particles of platinum, ruthenium, and alloys thereof can be used.
The method for supporting the metal fine particles on the catalyst carrier material is appropriately selected from well-known methods such as impregnation method, colloid method and precipitation-precipitation method according to the material of both materials and the application of the catalyst.

Depending on the loading method, there are a case where the metal fine particles are supported on the carrier material in a metal state as it is, and a case where the metal fine particles are supported on the carrier material in the form of a metal compound such as a metal oxide. In the latter case, after removing the liquid medium, atoms bonded to the metal are removed by reduction or the like to form fine particles of a single metal.
In this specification, a material in which metal fine particles or metal compound fine particles are supported on a carrier material under a liquid medium is called a “precursor of a fuel cell catalyst”. In this precursor, metal fine particles and metal compound fine particles are referred to as “metal raw material fine particles”.

Since the precursor of the fuel cell catalyst exists in the liquid medium, it can be relatively easily dispersed by adjusting the amount of the medium, adding a dispersant, supplying external energy such as ultrasonic waves, and the like. . By obtaining a highly dispersed state in the liquid medium, a finer particle fuel cell catalyst can be obtained after sublimation of the liquid medium.
In order to obtain high dispersibility of the fuel cell catalyst, it is preferable to break the agglomeration of the support material itself so that it is in a highly dispersed state in the liquid medium and to support the metal raw material fine particles thereon. Further, after the metal raw material fine particles are supported on the support material, the support material can be agglomerated to be in a highly dispersed state in the liquid medium.
In either case, wet pulverization can be used because a liquid medium is present. A ball mill method or a bead mill method is used as the wet pulverization.

  In the above ball mill method and bead mill method, the energy for finely pulverizing the catalyst carrier material can vary widely, such as the number of grinding revolutions, the grinding time, the diameter and number of media, and the ratio between the catalyst carrier material and the liquid to media Can be controlled.

In preparing the fuel cell catalyst precursor, various impurities may be mixed and impurities adsorbed on the carrier material itself may be discharged. In particular, when wet pulverization is performed on the support material or precursor, impurities adsorbed on the support material may be discharged.
Therefore, it is necessary to clean the obtained fuel cell catalyst precursor to remove impurities.
It is preferable to use the same or the same type of liquid as that used for the cleaning, but the medium can be exchanged depending on the type of impurities. For example, water is used as a medium during catalyst preparation, and an organic medium is used during washing to dissolve and remove oily impurities.

<Drying step>
The liquid medium is removed by sublimation from the sample (mixture of the precursor of the fuel cell catalyst and the liquid medium) prepared by the catalyst preparation step and obtained by removing impurities as necessary.
When performing normal drying, for example, heat drying at 60 ° C. in the atmosphere, when the liquid moves in the sample during drying or when the liquid evaporates from the sample, the precursor of the fuel cell catalyst is caused by a capillary contraction phenomenon or the like. Since the body particles aggregate with a strong binding force, it is difficult to obtain a fuel cell catalyst while maintaining a dispersed state.
If the liquid medium is removed by sublimation, the excess liquid medium in the sample is removed from the solid state as a gas by sublimation, so that the liquid does not move. Therefore, the fuel cell catalyst precursor can be dried without agglomeration to obtain a powdered fuel cell catalyst.

The drying step is not limited as long as the liquid medium can be removed by sublimation without substantial movement of the medium.
An example of such a sublimation method is lyophilization under reduced pressure. Freezing and decompression conditions can be arbitrarily selected depending on the material of the medium and the catalyst.
For example, when carbon particles are selected as a carrier and water is selected as a medium, the vacuum condition varies depending on the temperature, but at a pressure at which water can be frozen and sublimated from a solid to a gas, that is, a pressure lower than the water sublimation curve. It is necessary to be.
The freezing condition is not particularly limited as long as it is 0 ° C. or lower, but it is preferable to perform quick freezing in order to prevent aggregation of the catalyst.
Moreover, before freezing, it is preferable to remove the part of the medium that exceeds the required amount by a method such as filtration or heating.

When freeze-drying, a cold trap such as a liquid nitrogen trap may be installed between the sample and the vacuum pump. This is because the vacuum rate can be further increased by installing a cold trap, so that the drying rate can be improved. As a means for improving the drying rate, the sample temperature may be appropriately increased by heating the container containing the sample with a heater or the like. Any one of these means for improving the drying speed may be employed or may be used in combination.
Since it is desired to strictly control the mixing ratio with a liquid such as water when preparing the catalyst after lyophilization and the electrode paste described later, it is necessary to dry it reliably.

Examples of the present invention will be described below.
(Example)
A colloidal method is executed to obtain colloidal PtO ₂ as metal raw material fine particles. That is, NaHSO ₃ is added using chloroplatinic acid as a starting raw material and water as a medium, and stirred. To the obtained aqueous solution of H ₃ Pt (SO ₃ ) ₂ OH, Na ₂ CO ₃ is mixed, and hydrogen peroxide solution and aqueous sodium hydroxide solution are further added to obtain colloidal PtO ₂ .
When carbon particles are mixed with PtO ₂ using water as a medium, PtO ₂ is adsorbed and supported on the carbon particles (a precursor of a catalyst for a fuel cell).
At this time, since impurities may be adsorbed to the carbon particles, the precursor is washed with pure water.

If the carbon particles themselves are aggregated, the aggregation is not solved even when freeze drying is performed. Therefore, it is preferable to pulverize the carbon particles themselves. This grinding may be before or after contact with the PtO ₂ colloid.
The carbon particles are preferably pulverized together with water as a medium by wet pulverization such as a ball mill method or a bead mill method. This is because fine powdering is thoroughly performed and impurities attached to the carbon particles can be removed.

Next, the precursor produced in the catalyst preparation step is dried by a vacuum freeze-drying method. In FIG. 1, the outline of the vacuum freeze-drying apparatus 1 performed in the present Example is shown.
A sample containing the precursor and pure water produced in the catalyst preparation step is put in a stainless steel container and placed in a constant temperature / vacuum chamber 2. In this example, an appropriate amount of pure water (for example, about 100% by weight with respect to the weight of the support) can be added after the preparation of the precursor so that the sample is not naturally dried and the precursor is aggregated. .

First, in a normal pressure state, the inside of the chamber 2 is set to a temperature of 0 ° C. or lower, and the sample is completely frozen. Subsequently, using the vacuum pump 3 connected to the chamber 2, the inside of the chamber 2 is depressurized and drying by sublimation of ice is performed. In this embodiment, a liquid nitrogen trap is installed as a cold trap 4 between the chamber 2 and the vacuum pump 3 in order to increase the degree of vacuum and improve the drying speed. Further, in order to further improve the drying rate, the sample 5 is heated appropriately by the heater 5 installed at the lower part of the chamber 2 to raise the sample temperature appropriately.
By this freeze-drying method, all the pure water in the sample is removed by sublimation, and the precursor of the fuel cell catalyst produced by the catalyst preparation step (carbon particles carrying metal raw material fine particles) is not agglomerated. Collect in powder form.
Finally, the obtained precursor is heat-treated at 170 ° C. for 3 hours in a hydrogen stream, thereby reducing the platinum oxide fine particles supported on the carbon surface to metal platinum fine particles. By performing this reduction step, the function of the platinum catalyst can be expressed.
When the metal raw material particles supported on the support material in the precursor of the fuel cell catalyst are made of the metal itself, the reduction step is not necessary.

The fuel cell catalyst produced by the above method maintains a fine particle state without being agglomerated. Such a fuel cell catalyst can be quickly dispersed in a liquid and aggregated by simply mixing a solvent, an electrolyte solution, etc., or further applying energy such as ultrasonic waves when producing an electrode paste. This makes it possible to produce a highly dispersed electrode paste.
Moreover, by using such an electrode paste, a fuel cell electrode having a more uniform structure can be produced, and the electrode performance can be improved.

  The present invention is not limited to the description of the embodiments and examples of the invention described above. Various modifications may be included in the present invention as long as those skilled in the art can easily conceive without departing from the description of the scope of claims.

1 Vacuum freeze-drying device 2 Constant temperature / vacuum chamber 3 Vacuum pump 4 Cold trap 5 Heater

Claims (3)

A catalyst preparation step of preparing a catalyst precursor by supporting metal raw material fine particles on a catalyst support material dispersed in a liquid medium; and
A drying step of removing the liquid medium by sublimation;
A process for producing a powdery catalyst, comprising:   The method for producing a catalyst according to claim 1, wherein sublimation of the liquid medium in the drying step is performed by a vacuum freezing method.   The method for producing a catalyst according to claim 1, wherein an impurity removal step is interposed between the catalyst preparation step and the drying step.

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