JP3592727B2 - Photocatalyst - Google Patents

Description

[0001]
[Industrial applications]
The present invention relates to a photocatalyst used for water purification, deodorization, sterilization, wastewater treatment, water decomposition, growth suppression of algae, various organic chemical reactions, and the like.
[0002]
[Prior art]
When a semiconductor is irradiated with light of an appropriate wavelength having energy equal to or greater than its band gap, photoexcitation causes transition of electrons from the valence band to the conductor, and at the same time, generation of holes in the valence band, so-called charge separation occurs. . In addition, when water or a solution is brought into contact with a semiconductor while being irradiated with light, a junction similar to a Schottky barrier is formed. When the semiconductor is an n-type, holes are produced, and when the semiconductor is a p-type, electrons are produced. It is well known that they gather on the surface on the solid-liquid interface side. In the case of an n-type semiconductor, holes extract electrons from water or a solution species, and as a result, water is decomposed or a solute in the solution is oxidized. In the case of a p-type semiconductor, electrons are given to adjacent water or solution species, and a reduction reaction of the water or solution species occurs. Such a semiconductor that promotes the photooxidation-reduction reaction is particularly called a semiconductor photocatalyst or simply a photocatalyst.
[0003]
Conventionally, as a redox reaction or a redox reaction operation using a photocatalyst, a decomposition reaction of water, a reaction for killing microorganisms, a deodorization reaction, a sterilization reaction, a water purification wastewater treatment, and various other organic chemical reactions have been proposed. As a photocatalyst, specifically, titanium oxide as an n-type semiconductor is most widely used because of its chemical stability. Titanium oxide may be used in the form of a powder suspended in a solution or in the form of being supported on some substrate. From the viewpoint of photocatalytic activity, the former is generally more active due to its large surface area, but from a practical standpoint, the latter must be adopted from the viewpoint of ease of handling. Often not.
[0004]
Various methods have been proposed for supporting a photocatalyst on a substrate. For example, (A) a titanium oxide fine powder is applied to a substrate made of a light-transmitting substance such as nitrocellulose, glass, polyvinyl chloride, nylon, methacrylic resin, polypropylene, etc. in the form of a film, beads, board, fiber, etc. (B) A porous glass support is impregnated with an alcohol solution of titanium (IV) tetrabutoxy oxide and heated to form an anatase type titanium oxide. (Japanese Patent Laid-Open No. 50154/1990), (C) In a porous polymer membrane (for example, polyfluoroethylene resin) having a photosensitizer such as a dye or a metal complex as a side chain. For holding and fixing the semiconductor catalyst powder by press-fitting, impregnation, deposition, etc. (Japanese Patent Application Laid-Open No. 58-125602), (D) polypropylene A method of supporting titanium oxide on a filter made of fiber or ceramic (Japanese Patent Application Laid-Open No. 2-68190). (E) Titanium oxide powder is held and fixed in the entanglement of quartz, glass, and plastic fibers, and light is transmitted through both surfaces. (U.S. Pat. No. 4,888,101). (F) Platinum is fixed to an alumina substrate by a sputtering method, and anatase-type titanium oxide powder and an organic solvent solution of methyl methacrylate are further placed thereon. A method in which the mixed dispersion is applied by a spin coating method, and thereafter, methyl methacrylate as a binder is thermally decomposed and anatase-type titanium oxide is converted to rutile-type titanium oxide << Robert, E, Hetric-Robert E . Hetric, Applied Physics Communications, 5 , (3), 177-187 (1985) >>, (G) A method of spray-supporting titanium oxide on the surface of a polyester cloth by a low-temperature spraying method (Takashi Sakurada, Surface Technology Vol. 41, No. 10) , P60 (1990), and the like.
[0005]
[Problems to be solved by the invention]
Considering the conventional method of holding and fixing the photocatalyst to the substrate as described above, first, in the method (B) or (G), the titanium oxide itself as the photocatalyst is a bulk semiconductor that is a large lump rather than a fine powder. Therefore, its activity as a photocatalyst is relatively lower than that of a photocatalyst in which a powdered semiconductor photocatalyst is immobilized.
[0006]
In the methods (A), (D), (E) and (F), the photocatalyst powder is held and fixed on the substrate because the binder is not contained or the binder is not finally deposited. The required strength is not practically sufficient. Further, when an organic polymer emulsion such as an emulsion polymerization suspension of polytetrafluoroethylene is added as a binder to these systems, the active sites on the catalyst surface can be covered with the binder, or the photocatalyst can be coated. However, there is a problem that these binders themselves are decomposed due to the strong oxidizing power of the photocatalyst, or the photocatalyst powder is poisoned by the decomposition products and the catalytic activity is remarkably reduced.
[0007]
In addition, when compared to the case where the photocatalyst is used in powder form, the photocatalyst powder has a weaker binding strength than the case where the photocatalyst is used as powder in any of the above-described methods. In addition, there is a problem that the catalyst activity is relatively reduced.
[0008]
[Means for Solving the Problems]
The present invention relates to a photocatalyst obtained by supporting and fixing a photocatalyst powder on a substrate, wherein a porous metal oxide produced by heat-treating a sol of a metal oxide at a temperature of 400 ° C. or lower is used as a photocatalyst powder support and fixing agent. It is intended to solve the above-mentioned problem by using the same.
[0009]
[Action]
A feature of the photocatalyst according to the present invention is that the photocatalyst powder is supported and fixed on a substrate, has a certain mechanical strength, and maintains a high photocatalytic activity as a powder. Hereinafter, the production process, configuration, and significance of the photocatalyst according to the present invention will be described in detail.
[0010]
There are various methods for producing the photocatalyst according to the present invention.
First, as the material used as the base, any material can be applied as long as it can support the photocatalyst powder by coating, impregnation, or the like, or can form a film by dip coating, spin coating, or the like. For example, a three-dimensional space such as a ceramic mat, porous glass, or a fiber product such as nylon, acrylic, or polyester obtained by dispersing glass or ceramic fibers in water and forming the paper in a papermaking manner. Examples thereof include those having a hole, glass, and translucent plate-like, linear, and tubular materials such as PMMA, and molded products of ceramic, metal, and plastic.
[0011]
Next, as the semiconductor photocatalyst powder, TiO ₂ , ZnO, SrTiO ₃ , CdS, GaP, InP, GaAs, BaTiO ₃ , K ₂ NbO ₃ , Fe ₂ O ₃ , Ta ₂ O ₅ , WO ₃ , SnO ₂ , Bi ₂ O ₃ , NiO, Cu ₂ O, SiC, SiO ₂ , MoS ₂ , InPb, RuO ₂ , CeO ₂ , etc., and Pt, Rh, RuO ₂ , Nb, Cu, Sn, NiO, etc. All conventionally known materials supporting metals and metal oxides can be applied.
[0012]
One typical production method is a method in which a semiconductor photocatalyst powder is first supported on such a substrate, and then a metal oxide sol is applied by a method such as impregnation. That is, first, if the substrate has three-dimensional pores such as a ceramic mat or a fiber product, the semiconductor photocatalyst powder is applied to the pores of the substrate in advance by a method such as coating and impregnation. In addition, if the surface has a relatively smooth surface, such as glass, PMMA, ceramic, metal, or plastic moldings, the surface is sprayed in advance by a method such as spraying, dip coating, or spin coating. A photocatalyst powder layer is formed in advance. A metal organic compound such as a metal alkoxide, acetylacetonate, carboxylate or the like employed in a sol-gel method, or a chloride alcohol solution such as titanium tetrachloride is applied to the photocatalyst powder carrier to which the photocatalyst powder is merely attached. A sol of a metal oxide obtained by hydrolysis in the presence of an acid or an alkali catalyst is impregnated, and a solvent component is skipped to gel to form a metal oxide gel, thereby obtaining a desired photocatalyst.
[0013]
However, as another method, a metal organic compound such as metal alkoxide, acetylacetonate, carboxylate or the like employed in the sol-gel method, or a chloride alcohol solution such as titanium tetrachloride is hydrolyzed in the presence of an acid or alkali catalyst. A mixed solution of the metal oxide sol and the photocatalyst powder obtained by the above is prepared, and instead of supporting only the semiconductor photocatalyst powder on the substrate, the mixed solution is impregnated, coated, or supported by various coating methods. Then, a photocatalyst having the same effect is obtained in the method of gelation.
[0014]
Alternatively, before obtaining a metal oxide sol, a photocatalyst powder obtained by first mixing a photocatalyst powder with an alcohol solution of the above-mentioned metal organic compound or chloride and hydrolyzing in the presence of an acid or alkali catalyst. A similar effect may be obtained by a method in which the mixed sol is impregnated on a substrate, coated or supported by various coating methods, and gelled.
[0015]
The metal oxide sol used in any of these methods may be effective when two or more metal oxide sols are mixed and used as a metal composite oxide sol.
[0016]
As described above in detail, in the production process of the photocatalyst according to the present invention, it is most important that the porous metal oxide formed from the photocatalyst powder and the metal oxide sol has a good structure. However, there is no limitation on the raw materials, handling methods, and the like of the two.
[0017]
Here, it is considered that the favorable configuration of the photocatalyst powder and the supported fixing agent is as follows. In other words, the photocatalyst powder supported on the substrate by a method such as coating or impregnation has a very low adhesion to the substrate, but has a large porosity and surface area and maintains a catalytic activity relatively close to a powder state. I have. Next, when the metal oxide sol was impregnated into the powder photocatalyst carrier and gelled, it became a network of metal oxide gel having a large number of pores or penetrating between the photocatalyst powder particles and the substrate, and was carried on the substrate. The powder photocatalyst is held so as to be taken into the so-called mesh, so that it becomes a strong binder. The concern here is that the metal oxide gel derived from the metal oxide sol or the metal oxide itself obtained by heat treatment acts as a photocatalyst or coats the surface of the photocatalyst powder to photoexcit the photocatalyst powder. UV light scattered or absorbed, and may hinder photoexcitation of the photocatalyst powder. However, in the photocatalyst according to the present invention, as shown in Examples, for example, when titanium oxide is used for the metal oxide sol, the titanic acid polymer or titanium oxide derived from the titanium oxide sol is immobilized on the photocatalyst powder. While acting as an agent, it did not exhibit such a so-called obstructive effect at all. That is, as a result, the surface of the photocatalyst powder is sufficiently in contact with the solution interface, and the metal oxide gel is configured not to cover the active sites of the photocatalytic reaction.
[0018]
Unfortunately, the above configuration is only one guess, and the present inventors have not been able to elucidate the mechanism of action of the product of the present invention. However, a photocatalyst having excellent catalytic activity and a certain mechanical strength, which cannot be obtained only with a conventionally known photocatalyst powder and a metal oxide gel alone, can be obtained only by combining a photocatalyst powder and a metal oxide sol. It became possible.
[0019]
【Example】
<Example 1>
1.5 g of glass fiber having a fiber diameter of 19 μm was dispersed in 1 liter of water, and the resultant was made into a paper to obtain a glass mat having an area of 100 cm 2 (100 mm × 100 mm) and a thickness of 1.1 mm as a substrate. 10 g of anatase-type titanium oxide (specific surface area: 50 m3 / g, particle size: 0.03 μm) as a photocatalyst powder was suspended in 100 ml of water to form a water slurry, which was impregnated into a substrate prepared in advance, and dried to obtain a photocatalyst powder. Was carried. The supported amount of titanium oxide was 2.0 g. Next, 10 g of titanium isopropoxide was dissolved in 100 ml of ethanol, and a mixed solution of 50 ml of ethanol, 5 ml of water, and 0.7 ml of concentrated hydrochloric acid was added thereto and hydrolyzed to obtain a titanium oxide sol. . A glass mat pre-impregnated with titanium oxide powder was immersed in the titanium oxide sol, quickly pulled up, dried at 50 ° C. for 1 hour to remove solvent alcohol, and then heat-treated at 350 ° C. for 2 hours to form a photocatalyst. . The supported amount of titanium oxide was 2.06 g.
[0020]
<Example 2>
5 g of silicon tetraethoxide was dissolved in 100 ml of ethanol, and a mixed solution of 50 ml of ethanol, 5 ml of water and 0.7 ml of concentrated hydrochloric acid was added thereto and hydrolyzed to obtain a silica sol. This silica sol was impregnated with a water slurry of titanium oxide in advance and immersed in a glass mat carrying titanium oxide in the same manner as in Example 1, and was quickly pulled up and dried at 50 ° C. for 1 hour to evaporate the solvent alcohol. Heat treatment was performed at 400 ° C. for 2 hours to obtain a photocatalyst. The supported amount of titanium oxide was 2.0 g.
[0021]
<Example 3>
3 g of titanium isopropoxide was dissolved in 50 ml of ethanol, and a mixed solution of 20 ml of ethanol, 2 ml of water and 0.3 ml of concentrated hydrochloric acid was added thereto to obtain a titanium oxide sol. 5 g of anatase-type titanium oxide (particle size: 0.2 μm), which had been heat-treated at 350 ° C. for 2 hours, was dispersed therein, and a photocatalyst layer was formed on a 30 mm × 30 mm glass plate by dip coating. At this time, the supported amount of titanium oxide was 1.1 mg.
[0022]
[Test Example on Effectiveness of Catalyst Body]
<Test Example 1>
The photocatalyst bodies obtained in Example 1 and Example 2 were cut into 30 mm × 30 mm and each was taken as (a) and (b) (amount of titanium oxide (a): 0.18 g, (b): 0.17 g), 200 ml Placed on the bottom of the beaker. 100 ml of a phenol aqueous solution having a concentration of 0.1 mmol / l was added thereto, and ultraviolet light of 400 nm to 300 nm from a 1 kW high-pressure mercury lamp was irradiated from the bottom of the beaker. . The ultraviolet intensity at this time was 8.79 μe Einstein / min.
[0023]
As a result, in both (a) and (b), the concentration of phenol was reduced by half about 20 minutes after the start of light irradiation, and decomposed below the detection limit in about 3 hours. For comparison, 0.18 g of titanium oxide powder supported on a photocatalyst was suspended as powder (c), and titanium oxide obtained by hydrolysis of titanium isopropoxide was supported on a glass mat (d). ) And 0.18 g of a photocatalyst powder mixed with a PTFE dispersion to form a solid and heat-treated at 200 ° C. (e). The same examination was performed. FIG. 1 shows the phenol decomposition characteristics of these photocatalysts. Other photocatalysts were not decomposed even after irradiation for 4 hours.
[0024]
<Test Example 2>
The photocatalyst obtained in Example 3 was placed inside a glass tube having a diameter of 50 mm to form a closed system. When the photocatalyst was irradiated with 400-300 nm ultraviolet light of a 300 W xenon lamp at 30 μE / min while refluxing air containing, acetaldehyde was almost decomposed in about 15 minutes after the start of light irradiation. For comparison, 1.1 mg of titanium oxide prepared by the sol-gel method was subjected to the same examination, and was not decomposed even after irradiation for 4 hours.
[0025]
【The invention's effect】
As described in detail above, the photocatalyst according to the present invention is a novel photocatalyst having a photocatalyst powder supported and fixed on a substrate, having a certain mechanical strength, and maintaining a high photocatalytic activity as a powder. Photocatalyst, and its industrial value is extremely large.
[Detailed description of drawings]
FIG. 1 is a graph showing the photocatalytic decomposition characteristics of Test Example 1 of a photocatalyst according to an example of the present invention.

Claims (1)

A photocatalyst obtained by supporting and fixing a photocatalyst powder on a substrate, wherein a porous metal oxide produced by heat-treating a metal oxide sol at a temperature of 400 ° C. or lower is used as a support and fixing material for the photocatalyst powder. A photocatalyst comprising:

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