JPH0889800A - photocatalyst - Google Patents

Abstract

(57) [Summary] (Modified) [Purpose] Light can be used efficiently and has high catalytic activity in the visible light range. Used for photolysis of water to produce at least one of hydrogen and oxygen. A photocatalyst is provided. [Structure] General formula (I): AB ₂ Nb _3-x M _x O ₁₀ (wherein A is one or more elements selected from an alkali metal element or hydrogen, B is an alkaline earth metal element or One or more elements selected from lead, M is vanadium,
It is one or more elements selected from tungsten or molybdenum, and x is any numerical value of 0 <x ≦ 3. As a photocatalyst composed of an oxide represented by), a photocatalyst having a catalytic activity in the visible light region can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photocatalyst used in the photolysis of water to produce hydrogen and / or oxygen.

[0002]

2. Description of the Related Art At present, various human energy sources are used. However, the oil depletion problem for oil resources has begun to be highlighted since the 1980s. Also, since the 90's, global-scale environmental pollution has become a hot topic, and one of the causes thereof is carbon dioxide, nitrogen oxides, etc. emitted when fossil resources are used as fuel to obtain energy. Sulfur oxides are mentioned.

An increase in carbon dioxide causes global warming, and nitrogen oxides and sulfur oxides react with moisture in the air to generate nitric acid and sulfuric acid, which cause acid rain. On the other hand, despite the fact that nuclear energy has been a source of turmoil as a dream energy, social consensus on safety has not yet been obtained. for that reason,
Recently, it is difficult to construct new nuclear power plants in each country. The newspaper is also busy with the reprocessing of spent nuclear fuel. In this way, nuclear power is still a socially problematic energy source because of the fear of radiation.

Under these circumstances, hydrogen has recently attracted attention as a clean energy source that does not emit carbon dioxide, nitrogen oxides, sulfur oxides, etc. and emits no radioactivity. The reaction that decomposes water to generate hydrogen requires some energy, but if fossil resources or the like are used as this energy source, clean energy that solves the above problems cannot be obtained. Therefore, it is useful to use light such as sunlight.

A photocatalyst is one of means for obtaining hydrogen by utilizing light. When the photocatalyst absorbs light energy larger than the band gap of the semiconductor used, it forms holes in the valence band and electrons in the conduction band. The holes can oxidize water to produce oxygen, and the electrons can reduce water to produce hydrogen.

[0006]

The sunlight obtained on the surface of the earth has a spectral distribution having a peak near 500 nm. The distribution is about 5% in the ultraviolet region (400 nm or less) and about 43 in the visible light region (400 to 750 nm).
%, And the infrared region (750 nm or more) is about 52%.
Therefore, in order to use light efficiently, a photocatalyst whose bandgap energy corresponds to low light energy in the visible light region or higher is desired.

On the other hand, the inventors of the present invention previously disclosed in Japanese Patent Laid-Open No. 2-172.
As a “new water photolysis catalyst” of 535, a compound represented by the general formula I: [A _n-1 B _n O _{3n + 1} ] ^-u (wherein A is an alkali metal element, an alkaline earth metal element, a rare earth element and N is at least one atom selected from the group consisting of transition metal elements, B is at least one atom selected from Ti, Nb, or Ta, and n
Is an integer of 1 to 7, and u is the valence of the atomic group), and a group consisting of a layered anion atomic group represented by the following formula, and hydrogen, an alkali metal element, an alkaline earth element, and a Group 3B element present between the layers. And one or more cations M selected from the group consisting of the chemical formula II: M _m [A _n-1 B _n O
_{[3n + 1} ]], a photocatalyst composed of a compound having a laminated structure is shown.

The photocatalyst represented by the chemical formula II does not require a process of supporting a noble metal and other supporting components,
And because no precious metal is used, the manufacturing cost is low,
The catalytic activity was high. However, the study on the wavelength region of light showing its catalytic activity was insufficient. Therefore,
It is an object of the present invention to provide a “photocatalyst used for photolyzing water to generate at least one of hydrogen and oxygen” which has high catalytic activity in the visible light region and which can efficiently utilize light. And

[0009]

Means for Solving the Problems As a result of intensive studies, the inventors have found that a photocatalyst that has a high catalytic activity with respect to light in the visible region and photolyzes water to produce at least one of hydrogen and oxygen. I found it. Therefore, in the general formula (I): AB ₂ Nb _3-x M _x O ₁₀ (wherein, A is one or more elements selected from alkali metal elements or hydrogen, and B is an alkaline earth metal element or lead). Is one or more elements selected from, M is one or more elements selected from vanadium, tungsten, or molybdenum, and x is any numerical value of 0 <x ≦ 3.). There is provided a photocatalyst (claim 1), characterized in that it is composed of an oxide, and decomposes water by light to generate at least one of hydrogen and oxygen.

[0010]

In the general formula (I), an oxide in which x is 0 has already been disclosed in JP-A-2-172535.
This oxide shows no catalytic activity in the visible light region. However, the present inventors have found that a photocatalyst represented by the general formula (I) can be obtained, in which the oxide exhibits high catalytic activity in the visible light region.

This substitution amount x may be any amount as long as it is greater than 0 and 3 or less. However, since the photocatalyst of the present invention has a layered structure capable of ion exchange, it exhibits a high catalytic activity. The layered structure must not be disrupted by. B is one or more elements selected from alkaline earth metal elements or lead. Among them, in the case of calcium, strontium or lead, the obtained oxide is stable and is particularly useful as a photocatalyst.

The photocatalyst of the present invention is synthesized by a usual solid phase method, that is, by mixing salts such as oxides or carbonates or nitrates of respective metal components as raw materials in a desired composition ratio and firing the mixture. It may be synthesized by any other wet method or gas phase method. When A is hydrogen, first, an oxide of the general formula (I) in which A is an alkali metal is synthesized,
Thereafter, this oxide is ion-exchanged in an aqueous acid solution such as nitric acid to synthesize the alkali metal A by ion-exchange with hydrogen. Of course, even when A is an alkali metal other than hydrogen, the target oxide can be similarly synthesized by an ion exchange reaction.

Further, as long as the production cost is not increased, the photocatalyst of the present invention can be subjected to the usual modification such as carrying of Pt or NiO which is a promoter. In addition, in order to effectively use the interlayers that become the reaction fields of the photolysis reaction, pillars of inorganic substances are laid, or the interlayer distance is expanded by ion exchange of alkylammonium,
The catalytic activity can be increased.

Water used in the photocatalytic reaction of water with the photocatalyst of the present invention does not need to be limited to pure water. A reagent may be used without any problem, and a photolysis reaction of water mixed with a salt such as a carbonate or a hydrogen carbonate may be performed. Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited thereto.

[0015]

【Example】

[0016]

Example 1 (Preparation of Photocatalyst) KSr ₂ Nb ₃ O ₁₀ was partially replaced with 5% vanadium in a molar ratio to niobium, KS
r ₂ Nb _2.85 V _0.15 O ₁₀ was synthesized. K ₂ CO ₃ 0.9
0g, SrCO ₃ 3.90g, Nb ₂ O ₅ 5.00g
In addition to Niobium as an oxide, V ₂ O ₅ 0.1
8 g of each was weighed, pulverized and mixed, put into a platinum crucible, and baked in air at 1200 ° C. for 10 hours.

The calcined product was crushed in a mortar to a particle size of 10 μm or less, and the resulting particles were identified by powder X-ray diffraction. Moreover, it was confirmed from the measurement result of powder X-ray diffraction that the collapse of the layered structure and the appearance of another crystal phase due to the substitution of vanadium for niobium did not occur. Next, 5 g of this oxide was stirred in 100 ml of 5N nitric acid for 3 days to obtain a proton exchanger. This proton exchanger was replaced with HSr ₂ Nb
_It is expressed as _2.85 V _0.15 O ₁₀ .

(Evaluation of Catalytic Activity) The catalytic activity of the photocatalyst was evaluated by using a closed-circulation system catalytic reactor to generate hydrogen from an aqueous methanol solution containing methanol as a sacrificial reagent. 1 g of the above-prepared catalyst and chloroplatinic acid aqueous solution corresponding to 0.1 wt% of platinum as a supporting reagent were put in 300 ml of an aqueous methanol solution (250 ml of water, 50 ml of methanol) and suspended by a magnetic stirrer, and a xenon lamp The wavelength was cut below 420 nm). The formed hydrogen was detected and quantified by gas chromatography. Table 1 shows the measurement results.

[0019]

Example 2 (Preparation of Photocatalyst) A part of niobium of KSr ₂ Nb ₃ O ₁₀ was replaced with 10% vanadium in molar ratio to niobium, K
Sr ₂ Nb _2.7 V _0.3 O ₁₀ was synthesized. K ₂ CO ₃ 0.9
0g, SrCO ₃ 3.69g, Nb ₂ O ₅ 5.00g
In addition to the above, as an oxide substituting niobium, V ₂ O ₅ is 0.3
6 g of each was taken, pulverized and mixed, put in a platinum crucible, and fired in air at 1200 ° C. for 10 hours.

The calcined product was crushed in a mortar to a particle size of 10 μm or less, and the resulting particles were identified by powder X-ray diffraction. Moreover, it was confirmed from the measurement result of powder X-ray diffraction that the collapse of the layered structure and the appearance of another crystal phase due to the substitution of vanadium for niobium did not occur. Next, 5 g of this oxide was stirred in 100 ml of 5N nitric acid for 3 days to obtain a proton exchanger. This proton exchanger was replaced with HSr ₂ Nb
_It is expressed as _2.7 V _0.3 O ₁₀ .

(Evaluation of catalytic activity) The same evaluation as in Example 1 was carried out, and the measurement results are shown in Table 1.

[0022]

[Table 1]

[0023]

As described above, the photocatalyst represented by the general formula (I) of the present invention has a high catalytic activity in the visible light region and photolyzes water to generate at least one of hydrogen and oxygen. be able to. Since this oxide has a layered structure, it is easy to modify the layers by a method other than ion exchange. Of course, even if a noble metal such as Pt is supported on these oxides, the catalytic activity is maintained, and therefore, there is no problem even if these noble metals are supported.

Further, it does not matter if these photocatalysts are used for other chemical reactions than water decomposition reaction. For example, it can be applied to decomposition reactions of organic substances and reduction reactions of metal ions.

Claims (4)

[Claims] 1. General formula (I): AB ₂ Nb _3-x M _x O ₁₀ (wherein A is an alkali metal element or one or more elements selected from hydrogen, and B is an alkaline earth metal). One or more elements selected from elements or lead, M is one or more elements selected from vanadium, tungsten or molybdenum, and x is an arbitrary numerical value of 0 <x ≦ 3.). A photocatalyst that decomposes water with light to generate at least one of hydrogen and oxygen, characterized by comprising an oxide represented. 2. The photocatalyst according to claim 1, wherein B in the general formula (I) is calcium, and water is decomposed by light to produce at least one of hydrogen and oxygen. Photocatalyst. 3. The photocatalyst according to claim 1, wherein B in the general formula (I) is strontium, and water is decomposed by light to produce at least one of hydrogen and oxygen. Photocatalyst. 4. The photocatalyst according to claim 1, wherein B in the general formula (I) is lead, and water is used to decompose water to generate at least one of hydrogen and oxygen. Photocatalyst.