JPH09248465A - Photocatalyst having catalytic activity in visible light region - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a photocatalyst showing catalytic activity against light within a visible light region and having a laminar structure by adding a transition metal element in the constitutional elements of a photocatalyst respresented by a specific structural formula. SOLUTION: A photocatalyst having catalytic activity within a visible light region composed of composite oxide decomposing water by light to generate hydrogen and/or oxygen is represented by general formula, AB2 C3-x Mx O10-x (wherein A is hydrogen or an alkali metal element, B is a divalent element containing an alkaline earth element, C is a pentavalent element, M is a trivalent element and (x) is an arbitrary number of 0-3). Concretely, M is Fe or Ni and A is H or Na and B is Ca or Cr and C is V or Nb. Since a transition metal element is added to constitutional elements, catalytic activity is shown even against light within a visible light region and, since the interlaminar parts of a laminar structure can be utilized as a place of hydrocracking reaction, a larger quantity of hydrogen can be generated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photocatalyst for water splitting used when water is split to produce at least one of hydrogen and oxygen.

[0002]

2. Description of the Related Art At present, various energy sources are used. Fossil fuels such as petroleum and coal have infinite reserves and emit carbon dioxide, nitrogen oxides and sulfur oxides when burned. As a result, carbon dioxide causes global warming, and nitrogen oxides and sulfur oxides cause acid rain and cause environmental destruction.
Further, nuclear power generation has been put into practical use as a new energy source, but has problems such as safety and waste disposal.

[0003] Under such circumstances, attention has been paid to hydrogen, which is one of the clean energy sources, as a method for solving the problems of energy resources and the global environment. Hydrogen only turns into water when burned, and does not cause environmental pollution. However, the use of fossil fuel or the like to generate hydrogen is meaningless. Sunlight is inexhaustible, and water is abundant on Earth. Hydrogen can be obtained by decomposing water using the energy of sunlight. One of the means is a photocatalyst for water decomposition. A photocatalyst is a type of semiconductor. When it absorbs energy above its band gap, it generates holes and electrons, and the holes react with water to produce oxygen and hydrogen ions, which react with the electrons to convert hydrogen. Occur.

[0004] Among photocatalysts, a substance having a layered structure is known as a useful substance because unlike a bulky substance, a high catalytic activity can be obtained by using a layer of the layered structure as a site for a water splitting reaction. . In other words, the layered material has more catalytically active sites than the bulk material. Furthermore, compared to a material without a layered structure,
The travel distance of holes and electrons is reduced, especially when the direction of travel is perpendicular to the layer plane, so that the rate of recombination of holes and electrons is reduced, and high catalytic activity is obtained.

Conventionally, KCa ₂ Nb ₃ O ₁₀ and KSr ₂ Nb ₃ O ₁₀ have been used as materials having a layered structure and high catalytic activity.
Although such substances were known, they showed high catalytic activity in the ultraviolet light, but did not show catalytic activity because they had no absorption in the visible light region.

[0006]

Conventional photocatalysts having a layered structure but exhibiting a catalytic activity only to ultraviolet light utilize only light which is contained in only 5% in the wavelength range of the solar spectrum. It was limited. Also, the ultraviolet light is 100
No photocatalyst that can be used for the% water splitting reaction is known. Therefore, when the conventional photocatalyst is made to act under sunlight, it is not possible to efficiently convert light energy into chemical energy.

Therefore, the object of the present invention is not only to utilize the light in the ultraviolet light region of sunlight, which is contained in only 5%, but also to use the light in the visible light region, which is approximately 9 times as much as the ultraviolet light region. It is also to provide a photocatalyst having a layered structure while exhibiting catalytic activity.

[0008]

The present invention relates to a photocatalyst having a catalytic activity in the visible light region, which is composed of a complex oxide that decomposes water by light to generate at least one of hydrogen and oxygen, Formula (I): represented by AB ₂ C _3-X M _X O _10-X , wherein in the general formula (I), A is one or more elements selected from the group consisting of alkali metal elements and hydrogen. , B is one or more elements selected from divalent elements including alkaline earth metal elements, C is one or more elements selected from pentavalent elements, and M is a trivalent element. Is a photocatalyst having a catalytic activity in the visible light region (claim 1), which is one or more elements selected from, and x is an arbitrary number of 0 <x ≦ 3. The photocatalyst according to the present invention contains, in its constituent elements, a transition metal element M having a function of absorbing visible light.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Since the photocatalyst of the present invention contains the transition metal M in the above general formula (I): AB ₂ C _3-X M _X O _10-X , it absorbs light in the visible light region. Further, in the constituent elements, the charge of the entire photocatalyst due to the substitution of the pentavalent element with the trivalent element is compensated by oxygen, so that there is a problem that the original layered structure cannot be maintained and collapses. Does not happen. Further, since it has a layered structure, the interlayer alkali metal can be ion-exchanged as in the conventional case, and the interlayer alkali metal can be exchanged with protons by performing ion exchange in an acid aqueous solution.

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 each metal component as a raw material at a target composition ratio and calcining, but otherwise. It may be synthesized by the wet method or the gas phase method. When A is hydrogen in the general formula (I), a composite oxide of the general formula (I) in which A is an alkali metal is first synthesized, and then the composite oxide is treated with an acid such as nitric acid. By performing ion exchange in an aqueous solution, the alkali metal ions are exchanged for hydrogen ions to synthesize. Of course, even when A is an alkali metal other than hydrogen, a composite oxide having a desired composition can be similarly synthesized by an ion exchange reaction.

The shape of the photocatalyst of the present invention is preferably a particle having a large surface area in order to effectively utilize light,
Generally, the particle size is from 0.1 to 10 μm, preferably 0.1.
~ 1 µm is appropriate. Conventional means for obtaining such a particle size include, for example, grinding with a ball mill. Further, the photocatalyst of the present invention also has a promoter such as Pt or N
Modifications such as iO loading that are commonly used in photocatalyst production can be made. Further, the catalytic activity can be enhanced by erecting a pillar of an inorganic substance in order to effectively use the interlayer that is a site of the water splitting reaction, or by widening the interlayer distance by ion exchange of alkylammonium.

When carrying out the water-splitting reaction of water with the photocatalyst of the present invention, water is not limited to pure water, but sacrificial reagents such as alcohol and silver ions are used so that water is often used in the water splitting reaction. However, water in which salts such as carbonate and hydrogen carbonate are mixed may be used. Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples.

[0013]

【Example】

(Method for producing photocatalyst) First, by the solid phase method, CsCa ₂
Nb ₂ NiO ₉ is synthesized. 9.16 g of Cs ₂ CO ₃ , C
aCO ₃ 11.25 g, Nb ₂ O ₅ 14.94 g, Ni ₂ O
4.65 g of ₃ was sampled in a platinum crucible and stored at 1200 ° C for 10
The firing was performed for a time. After firing, this fired product was crushed to 10 μm or less in a mortar, and the particles obtained were identified by powder X-ray diffraction, and as a result, the structure of the desired substance was obtained.

Further, 5 g of the composite oxide is mixed with 5 N nitric acid.
The mixture was stirred in 100 ml for 3 days to perform proton exchange to obtain a proton exchanger. (Evaluation of catalytic activity) The catalytic activity of the photocatalyst for water splitting was evaluated by using a closed-circulation catalytic reactor and generating hydrogen from a methanol aqueous solution to which methanol was added as a sacrificial reagent. 1 g of the proton exchanger produced by the above method was placed in 350 ml of an aqueous methanol solution (300 ml of water, 50 ml of methanol), and irradiated with light from outside while stirring with a magnetic stirrer. A 500 W xenon lamp was used as a light source, and light with a wavelength of 420 nm or less was cut by a filter. As the material of the reaction tube, one made by Pyrex was used. The detection and quantification of the generated hydrogen were performed by gas chromatography.

As a result of the measurement, generation of hydrogen was recognized, and the steady hydrogen activity was 0.6 μmol / hour. Thus, the catalytic activity was exhibited even when the light of short wavelength was cut.

[0016]

As described above, since the photocatalyst of the present invention contains a transition metal element in its constituent elements, it exhibits catalytic activity even in light in the visible light region. Moreover, since the layers of the layered structure can be used as a field for the water splitting reaction, more hydrogen can be generated. Therefore, according to the present invention, visible light contained in a large amount in the sunlight spectrum can also be utilized for the water-splitting reaction, so that the use of solar energy rather than the photocatalyst which shows a catalytic activity only for light in the ultraviolet region. The efficiency can be significantly improved.

Claims (5)

[Claims] 1. A photocatalyst having a catalytic activity in the visible light region, which comprises a composite oxide that decomposes water by light to generate at least one of hydrogen and oxygen, and has a general formula (I): AB ₂ C. _{Represented by 3-X} M _X O _10-X , in the general formula (I), A is one or more elements selected from the group consisting of alkali metal elements and hydrogen, and B is an alkaline earth metal element. Is one or more elements selected from divalent elements including, C is one or more elements selected from pentavalent elements, and M is one or more elements selected from trivalent elements. A photocatalyst having catalytic activity in the visible light region, which is an element and x is an arbitrary number of 0 <x ≦ 3. 2. The M is Fe, Ni, Cr, Co, Mn
The photocatalyst having a catalytic activity in the visible light region according to claim 1, wherein the photocatalyst is one or more elements selected from the following. 3. The photocatalyst having a catalytic activity in the visible light region according to claim 1, wherein the A is one or more elements selected from H, Na, K, Rb and Cs. 4. The photocatalyst having a catalytic activity in the visible light region according to claim 1, wherein the B is one or more elements selected from Ca, Sr, Ba, and Pb. 5. The photocatalyst having a catalytic activity in the visible light region according to claim 1, wherein the C is one or more elements selected from V, Nb and Ta.