JP6990815B2 - Hydrogen recombination catalyst - Google Patents

Description

The present invention relates to a hydrogen recombination catalyst used to reduce the concentration of hydrogen generated during storage of hydrous radioactive waste.

Currently, at the Fukushima Daiichi Nuclear Power Station, treatment is being carried out to remove radioactive cesium from contaminated water in the reactor building, etc. toward decommissioning. In this treatment, radioactive cesium contained in the contaminated water is removed by the adsorption tower, but the used adsorption tower will be stored for a long time until the radioactive concentration decreases. During this long-term storage, the water remaining in the adsorption tower is decomposed into hydrogen and oxygen by radiation, and the hydrogen accumulated in the storage container may ignite and explode due to the temperature rise due to the decay heat of radioactive cesium. Therefore, at present, the valve of the storage container is opened so that the hydrogen gas generated in the storage container does not stay in the storage container. However, with this method, there was a risk that radioactive substances would be released into the environment.

Therefore, it has been studied to recombinate generated hydrogen and oxygen by using a static catalytic hydrogen recombination vessel equipped with a hydrogen recombination catalyst. In the United States, a hydrogen recombination catalyst made of platinum is partially used for storage of fuel debris generated in the accident at Three Mile Island Nuclear Generating Unit 2, but it has not been used in Japan yet. Since this hydrogen rebonding catalyst requires a large amount of hydrogen bonding catalyst to treat a large amount of contaminated water, it can be mass-produced, is inexpensive, and has excellent long-term stability. Is required.

By the way, geopolymer is an amorphous inorganic polymer material based on aluminosilicate, and has excellent properties such as composition flexibility, heat resistance, chemical stability, and radiation resistance. .. Since it is dense and strong, it is attracting attention as a substitute material for cement in the field of structural materials (for example, Patent Documents 1 and 2). In addition, the geopolymer is produced from alumina silica powder as an active filler and an alkali silica solution. When the alumina silica powder and the alkali silica solution are mixed and cured, the polymerization reaction proceeds and the solidification produces a geopolymer. Fly ash or blast furnace slag can be used as the raw material alumina silica powder.

Therefore, if a geopolymer can be used as a supporting material for a hydrogen rebinding catalyst, it is considered that a hydrogen bonding catalyst that can be mass-produced, is inexpensive, and has excellent long-term stability can be obtained (non-patent statement). 1).

When the geopolymer is used as a supporting material for the hydrogen recombination catalyst, it is desirable to increase the specific surface area of the geopolymer in order to efficiently use the expensive noble metal used as the catalyst and increase the hydrogen recombination ability. Therefore, in order to increase the specific surface area, it was considered to make the geopolymer into a honeycomb structure or a porous body (Non-Patent Document 2). However, even if the geopolymer has a high strength enough to attract attention as a structural material, a decrease in strength is unavoidable when a honeycomb structure or a porous body is used.

On the other hand, in the work of handling the hydrogen recombination catalyst, in order to protect the worker from radiation, it is necessary to use a remote-controlled robot or, in the case of manual intervention, to complete the work in a short time, and the hydrogen recombination catalyst. Handling tends to be crude. Therefore, the hydrogen bond catalyst is required to have a certain strength or higher that can withstand rough handling.

Japanese Unexamined Patent Publication No. 2012-116677 Japanese Unexamined Patent Publication No. 2008-239446

Nihon Keizai Shimbun, "Radioactive waste, long-term safe storage in containers, Nagaoka University of Technology, etc.", [online], February 11, 2017, Nihon Keizai Shimbun, [Search on August 9, 2017], Internet <URL: http://www.nikkei.com/article/DGXLASFB10H10_Q7A210C1L21000/> Nagaoka University of Technology, "Research and Development on Technology for Reducing the Concentration of Combustible Gas Generated in Long-Term Waste Storage Containers", [online], Japan Science and Technology Agency, [Search on August 9, 2017], Internet <URL : https://www.jst.go.jp/nuclear/field/h28/hairo01_04.pdf>

Therefore, an object of the present invention is to provide a novel hydrogen recombination catalyst having sufficient strength and high hydrogen recombination ability by using an inorganic material containing a geopolymer as a supporting material.

The hydrogen recombination catalyst of the present invention is characterized by comprising a carrier made of an inorganic material containing a geopolymer, an inorganic powder supported on the surface of the carrier, and a catalyst supported on the surface of the inorganic powder. ..

Further, it is characterized by having a substantially spherical shape having an average diameter of 3 to 50 mm.

Further, the inorganic powder is characterized by having a specific surface area of 40 to 200 m ² / g.

Further, the inorganic powder is characterized by being a γ-alumina powder.

Further, the catalyst is characterized by being made of platinum or palladium.

Further, the catalyst contains platinum or palladium, and the total amount of platinum, palladium, or platinum and palladium is 30% by mass or more.

According to the present invention, an inorganic material containing a high-strength geopolymer is used as a carrier to have sufficient strength, and a catalyst is supported on the surface of an inorganic powder having a large specific surface area to obtain high hydrogen recombination ability. A hydrogen recombination catalyst having is provided.

It is a schematic diagram of the hydrogen recombination catalyst of this invention. FIG. 5 is a flow chart showing an embodiment of a step of supporting a catalyst made of platinum or palladium on alumina powder in the process of manufacturing a hydrogen recombination catalyst of the present invention. It is a flow diagram which shows an Example of the step of molding a geopolymer and supporting an alumina powder on a geopolymer in the process of manufacturing a hydrogen recombination catalyst of the present invention. It is an appearance photograph which shows one Example of the hydrogen bond catalyst of this invention. It is a transmission type electron microscope image of the alumina powder carrying platinum in one Example of the hydrogen bond catalyst of this invention. It is an electron beam diffraction image of the alumina powder carrying platinum in one Example of the hydrogen bond catalyst of this invention. It is a graph which shows the catalyst performance in one Example of the hydrogen bond catalyst of this invention.

As schematically shown in FIG. 1, the hydrogen recombination catalyst of the present invention uses an inorganic material containing a geopolymer as a supporting material. In addition, a has a support material made of only a geopolymer, b has an inorganic aggregate other than the geopolymer in the geopolymer, and c has a central core made of an inorganic sphere other than the geopolymer. The one in which the geopolymer layer was formed on the surface layer is shown.

The geopolymer of the supporting material is an inorganic polymer material having an amorphous structure similar to zeolite, and the SiO4 _tetrahedron and AlO4 _tetrahedron are three-dimensionally connected via oxygen atoms to maintain charge balance. Therefore, cations are incorporated into the structure. The curing reaction formula is said to be as shown below.

The carrier made of an inorganic material containing the geopolymer constituting the hydrogen recombination catalyst of the present invention is preferably solid in order to secure high strength and easy to manufacture, but if the strength can be ensured, the carrier made of the inorganic material is used. It does not preclude the inclusion of an aggregate, which may be hollow, and the use of spheres of other inorganic materials for the core. Further, the shape of the carrier is not limited, but it is preferably substantially spherical from the viewpoint of ease of handling. The size of the carrier is not limited, but practically, a diameter of 3 to 50 mm is desirable in consideration of loading into a storage container and holding of the catalyst.

Further, it has a structure in which an inorganic powder such as alumina powder is supported on the surface of the inorganic material containing the geopolymer, and a catalyst such as platinum or palladium is supported on the surface of the inorganic powder. This structure ensures high strength and a large specific surface area of the hydrogen recombination catalyst, resulting in high catalytic performance.

Examples of the inorganic powder constituting the hydrogen recombination catalyst of the present invention include metal oxide powders such as alumina powder, silica powder, zirconia powder, ceria powder and titanium oxide powder, and metal carbide powders such as silicon carbide and boron carbide. Examples thereof include metal nitride powders such as silicon nitride and boron nitride. Among these, alumina powder is preferable because it is inexpensive, chemically stable, has many types of particle sizes available, and is a kind of raw material for geopolymers and is easily compatible with the geopolymer of the carrier. Among the alumina powders, γ-alumina powder having a large specific surface area is more preferably used. The particle size of the inorganic powder is preferably several tens to several hundreds μm, which strengthens the bonding (adhesive) force with the geopolymer, and is preferably 40 to 200 μm within the available range. Further, as for the specific surface area of the inorganic powder, the larger the specific surface area, the more platinum is dispersed and the particle size of platinum itself becomes smaller, and the surface area of platinum also becomes larger. Therefore, the larger the specific surface area, the better. It is preferably about 200 m ² / g.

The catalyst constituting the hydrogen recombination catalyst of the present invention includes, for example, precious metals such as platinum, palladium, rhodium and ruthenium, and metal oxides such as titanium oxide, copper oxide, zinc oxide, nickel oxide and vanadium oxide. Can be. Among these, a catalyst made of platinum or palladium is preferably used because of its high hydrogen recombination ability. Further, it is desirable that the catalyst constituting the hydrogen recombination catalyst of the present invention has platinum, palladium, or a total of platinum and palladium of 30% by mass or more.

As described above, the hydrogen recombination catalyst of the present invention has sufficient strength by using an inorganic material containing a high-strength geopolymer as a supporting material, and supports the catalyst on the surface of an inorganic powder having a large specific surface area. It has a high hydrogen recombination ability by making it. By installing the hydrogen recombination catalyst of the present invention in a storage container for storing water-containing radioactive waste, hydrogen and oxygen generated by radiolysis of water are recombined and returned to water to concentrate the hydrogen in the container. Can be suppressed from rising.

The hydrogen recombination catalyst of the present invention can be obtained, for example, by the following procedure.

First, an inorganic powder on which a catalyst is supported is prepared. The inorganic powder on which the catalyst is supported can be produced by a known method depending on the type of the inorganic powder or the catalyst.

Next, the alumina silica powder as an active filler, which is a raw material of the geopolymer, and the alkaline silica solution are mixed to prepare a geopolymer solution. As the alumina silica powder, a mixture of metakaolin and microsilica (silica fume) is preferably used. Instead of microsilica, fly ash containing silicon dioxide, blast furnace slag, sewage sludge, or the like may be used. Further, as the alkali silica solution, a mixture of potassium silicate solution, potassium hydroxide and water is preferably used. A sodium silicate solution (water glass) may be used instead of the potassium silicate solution, and sodium hydroxide may be used instead of potassium hydroxide.

Then, this geopolymer solution is poured into a mold and cured, and the geopolymer whose surface is not completely solidified is taken out from the mold. The curing conditions are preferably, for example, 70 to 90 ° C. for 20 to 50 minutes.

Then, the catalyst-supported inorganic powder is coated on the surface of the geopolymer by adhering the catalyst-supported inorganic powder to the surface of the geopolymer.

It is then cured to completely solidify the geopolymer. The curing conditions are preferably, for example, 70 to 90 ° C. for 1 to 3 days.

Alternatively, the catalyst may be supported on the surface of the geopolymer in the same procedure after the inorganic powder on which the catalyst is not supported is attached and coated and cured.

By the above procedure, the hydrogen recombination catalyst of the present invention can be obtained.

Hereinafter, the hydrogen recombination catalyst of the present invention will be specifically described. The present invention is not limited to the following examples, and various modifications can be carried out.

[Supporting platinum on alumina powder]
Alumina powder was immersed in a platinum nitrate solution, mixed with the platinum nitrate solution, then dried and calcined in the air to prepare an alumina powder having platinum supported on its surface. As a platinum material, a dinitrodiammine platinum (II) nitric acid solution (Pt (NH ₃ ) ₂ (NO ₂ ) ₂ / HNO ₃ ) was used. As the alumina powder supporting platinum, γ-alumina powder having a large specific surface area was used. For comparison, three types of alumina powder supporting platinum were prepared. The physical characteristics of each alumina powder are as shown in Table 1.

As shown in FIG. 2, a dinitrodiammine platinum (II) nitric acid solution diluted with water so that platinum is 0.1 to 1% by mass with respect to the alumina powder to be used is prepared, and the alumina powder is used as this platinum nitrate solution. Soaked in. Then, ethanol was used as the dispersion medium, and the alumina powder and the platinum nitrate aqueous solution were mixed for 10 minutes by a ball mill at a rotation speed of 520 times / min. The mixed sample was dried at 90 ° C. for 4 hours by a dryer, pulverized, and calcined at 500 ° C. for 3 hours in an air atmosphere to obtain an alumina powder carrying platinum.

[Supporting platinum-supported alumina powder on a geopolymer carrier]
A substantially spherical geopolymer carrier was prepared, and the surface thereof was coated with an alumina powder carrying platinum. Table 2 shows the mixing ratio of the raw materials of the geopolymer carrier. The coating was performed by adhering alumina powder to the surface of the geopolymer carrier during curing of the geopolymer carrier and before the geopolymer was completely solidified.

As shown in FIG. 3, potassium silicate, pure water, potassium hydroxide, microsilica, and metacaolin are blended and mixed at the blending ratios shown in Table 2 to prepare a geopolymer solution, and the geopolymer solution is substantially spherical. I poured it into a mold. After curing at 70 ° C. for 20 to 50 minutes, the geopolymer whose surface is not completely solidified is removed from the mold, and platinum-supported alumina powder is adhered to the surface of the geopolymer, whereby platinum-supported alumina is attached to the surface of the geopolymer. The powder was coated. Then, it was cured at 70 ° C. for 1 to 3 days to completely solidify the geopolymer, and a geopolymer carrier carrying platinum-supported alumina powder, that is, the hydrogen recombination catalyst of the present invention was obtained. The appearance of the hydrogen recombination catalyst of the present invention actually obtained is shown in FIG.

[Electron microscope observation]
Using a transmission electron microscope (TEM), the alumina powder supporting platinum on the surface was observed. The obtained TEM image is shown in FIG. In addition, electron diffraction of alumina powder supporting platinum on the surface was performed. Then, the radius was measured from the obtained electron diffraction image, the d value, which is the surface spacing, was obtained from the reciprocal of the radius, and the chemical state of platinum was identified. The electron diffraction image is shown in FIG. 6, and the obtained d value, the alumina card data, and the platinum card data are shown in Tables 3, 4, and 5, respectively. As a result, the d value corresponding to the (200) plane of platinum was obtained from the electron beam diffraction image, and it was confirmed that the metallic platinum was present.

[Evaluation of catalyst performance]
The catalytic performance of the alumina powder obtained in Example 1 in which platinum was supported on the surface was evaluated. The evaluation conditions were as follows.
Sample amount: 250 mg
Gas composition: 1.0% -H ₂ , 1.0% -O ₂ , remaining -Ar
Gas flow rate: 50cc / min (total amount)
Temperature rise rate: 10 ° C./min Figure 7 shows the evaluation results of the catalytic performance of the alumina powder carrying platinum on the surface (◆, ■, ×). The hydrogen conversion rate, which indicates the ratio of hydrogen bonded to oxygen, was high, indicating that it has good catalytic performance as a hydrogen recombination catalyst.

[Supporting palladium on alumina powder]
Dinitrodiammine Palladium (II) Nitric Acid Solution (Pd (NH ₃ ) ₂ (NO ₂ ) ₂ / HNO ₃ ) Instead of Dinitrodiammine Platinum (II) Nitric Acid Solution (Pt (NH ₃ ) ₂ (NO ₂ ) ₂ / HNO ₃ ) ) Was used to make the amount of palladium 0.1% by mass with respect to the alumina powder used, and the firing temperature was set to 550 ° C. Made.

[Evaluation of catalyst performance]
The catalytic performance of the alumina powder obtained in Example 3 on which palladium was supported on the surface was evaluated. The evaluation conditions were the same as in Example 2.

FIG. 7 shows the evaluation results of the catalytic performance of the alumina powder supporting palladium on the surface (▲, +). The hydrogen conversion rate, which indicates the ratio of hydrogen bonded to oxygen, was high, indicating that it has good catalytic performance as a hydrogen recombination catalyst.

[Support of platinum and palladium on alumina powder]
Dinitrodiammine platinum (II) nitric acid solution (Pt (NH ₃ ) ₂ (NO ₂ ) ₂ / HNO ₃ ) and dinitrodiammine palladium (II) nitric acid solution (Pd (NH ₃ ) ₂ (NO ₂ ) ₂ / HNO ₃ ) Using both, platinum and palladium were applied to the surface in the same manner as in Example 1 except that the total of platinum and palladium was 0.1% by mass with respect to the alumina powder used and the firing temperature was 550 ° C. A supported alumina powder was prepared.

[Evaluation of catalyst performance]
The catalytic performance of the alumina powder obtained in Example 5 on which platinum and palladium were supported was evaluated. The evaluation conditions were the same as in Example 2.

FIG. 7 shows the evaluation results of the catalytic performance of the alumina powder supporting platinum and palladium on the surface (●). The hydrogen conversion rate, which indicates the ratio of hydrogen bonded to oxygen, was high, indicating that it has good catalytic performance as a hydrogen recombination catalyst.

Claims (1)

It consists of a carrier made of an inorganic material containing a geopolymer, an inorganic powder supported on the surface of the carrier, and a platinum or palladium catalyst supported on the surface of the inorganic powder, and has a substantially spherical shape with an average diameter of 3 to 50 mm. Hydrogen recombination catalyst characterized by being present

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