WO2023182282A1

WO2023182282A1 - Water splitting apparatus using photocatalyst, and water splitting system provided with same

Info

Publication number: WO2023182282A1
Application number: PCT/JP2023/010934
Authority: WO
Inventors: 竜阿部; 肇鈴木; 輝松岡; 朝葉城内; 冬樹八木; 浩平浦崎; 宏八木
Original assignee: 千代田化工建設株式会社; 国立大学法人京都大学
Priority date: 2022-03-22
Filing date: 2023-03-20
Publication date: 2023-09-28
Also published as: JP2023139749A

Abstract

[Problem] To realize Z scheme-type water splitting by means of a photocatalyst with a simple configuration. [Solution] This water splitting apparatus 3 comprises a photocatalyst panel 7 and a reaction container 9 for performing a water splitting reaction, wherein: the photocatalyst panel 7 includes a light transmitting substrate 11, a first photocatalyst layer 13 formed to overlap a first surface 11A of the substrate 11, and a second photocatalyst layer 13 formed to overlap a second surface 11B of the substrate 11; one among the first photocatalyst layer 13 and the second photocatalyst layer 13 is a photocatalyst layer which is for oxygen generation and includes a first semiconductor, and the other is a photocatalyst layer which is for hydrogen generation and includes a second semiconductor; and the reaction container 9 has an internal space divided by the photocatalyst panel 7, an oxygen generation tank 21 formed on the side of the photocatalyst layer for oxygen generation, a hydrogen generation tank 22 formed on the side of the photocatalyst layer for hydrogen generation, and a light incidence wall 9A through which light incident toward the first photocatalyst layer 13 of the photocatalyst panel 7 is transmitted.

Description

Water splitting device using photocatalyst and water splitting system equipped with the same

The present invention relates to a water splitting device using a photocatalyst that utilizes light energy, and a water splitting system equipped with the same.

In recent years, hydrogen has attracted attention as an energy medium that can efficiently store and transport energy. Hydrogen has superior storability compared to electricity, and unlike fossil fuels, it does not produce carbon dioxide when burned. Up until now, fuel cell vehicles and fuel cell cogeneration systems that use hydrogen have been put into practical use, but further expansion of hydrogen use is desired in order to realize a hydrogen society. The use of hydrogen to replace fossil fuels also contributes to reducing carbon dioxide emissions, which is one of the initiatives related to the Sustainable Development Goals adopted by the United Nations.

On the other hand, since hydrogen hardly exists alone in nature, it must be extracted from other compounds and used. Traditionally, much of industrial hydrogen is produced by steam reforming of fossil fuels (eg, methane, propane, etc.). However, when hydrogen is produced from fossil fuels, a large amount of carbon dioxide is emitted during the production process.

In response, technologies have been developed to decompose water (that is, to produce hydrogen) using photocatalysts and photoelectrodes that utilize solar energy (Patent Documents 1 to 6). If hydrogen production using renewable energy such as solar energy is put into practical use, it will be possible to obtain energy from hydrogen when needed while suppressing the generation of carbon dioxide during hydrogen production.

International Publication No. 2017/221866 JP 2017-124393 Publication International Publication No. 2012/008838 JP 2018-89589 Publication Japanese Patent Application Publication No. 2006-89336 Japanese Patent Application Publication No. 2007-117811

However, hydrogen production using conventional photocatalysts has a problem in that the conversion efficiency of solar energy (that is, the conversion efficiency into hydrogen energy) is low (for example, 1% or less). Therefore, the conventional technology is not at a level that can withstand practical use, at least from the viewpoint of solar energy conversion efficiency and hydrogen production cost. For example, in the hydrogen strategy basic scenario by Japan's Ministry of Economy, Trade and Industry, the cost of hydrogen production is currently around 100 yen per ^1Nm3 , and it is said that it will need to be significantly reduced to around 30 yen per ^1Nm3 by around 2030. ing. Similar goals have also been set by the US Department of Energy. Therefore, realizing a hydrogen society requires further technological innovation to improve the conversion efficiency of solar energy.

The key to improving the conversion efficiency of solar energy is to expand the usage efficiency of the sunlight spectrum (i.e., lengthen the usable wavelength) and improve the usage efficiency of the light (photons) absorbed by the photocatalyst (i.e., (improvement of quantum yield). For hydrogen production using photocatalysts, a solar energy conversion efficiency of about 5-10% is required for its practical use.

When a photocatalyst mainly uses light in the ultraviolet region (for example, 200 nm to 400 nm), the conversion efficiency of solar energy remains at about 2% at maximum. In other words, depending on the use of light in the ultraviolet region, even if the photocatalyst absorbs all of the photons and water is decomposed with a quantum yield of 100% (the proportion of photons that contributed to the reaction among the absorbed photons). , high conversion efficiency cannot be expected. On the other hand, if the wavelength range used by photocatalysts could be expanded to about 600 nm in the visible light range, the maximum conversion efficiency would increase to 16% due to a significant increase in the number of photons in the sunlight spectrum. In that case, even if the average quantum yield is 30%, a solar energy conversion efficiency of about 5% can be expected.

As a result of intensive research, the present inventors discovered that Z-scheme type (two-step excitation type) water splitting is effective as a hydrogen production technology to expand the wavelength range used in photocatalysts. Ta. In Z-scheme water splitting, two types of photocatalysts (hydrogen generation photocatalyst and oxygen generation photocatalyst) are used for hydrogen generation and oxygen generation, respectively, and it is also easy to separately recover the generated hydrogen and oxygen. On the other hand, in order to suppress hydrogen production costs, it is desirable to implement Z-scheme water splitting with a simpler configuration.

In view of the above background, it is an object of the present invention to provide a water splitting device using a photocatalyst that realizes Z-scheme type water splitting using a photocatalyst with a simple configuration, and a water splitting system equipped with the same.

In order to solve the above problems, an aspect of the present invention is a water decomposition device (3) using a photocatalyst that decomposes water into hydrogen and oxygen using light energy, the photocatalyst panel (7, 207, 307 ) and a reaction vessel (9, 209) that accommodates the photocatalyst panel and a reaction solution and performs a water decomposition reaction in the reaction solution, and the photocatalyst panel includes a light-transmitting substrate (11, 311). ), a first photocatalyst layer (13, 313) formed to overlap the first surface (11A) of the substrate, and a second surface (11B) that is paired with the first surface of the substrate. ), one of the first photocatalyst layer and the second photocatalyst layer containing a first semiconductor. and the other is a hydrogen generation photocatalyst layer containing a second semiconductor, and the reaction vessel has an internal space partitioned by the photocatalyst panel, and an oxygen generation tank (21) formed on the side of the oxygen generation photocatalyst layer. ), a hydrogen generation tank (22) formed on the side of the hydrogen generation photocatalyst layer, and a light incidence wall (9A) that transmits light incident on the first photocatalyst layer in the photocatalyst panel. The configuration is as follows.

According to this aspect, since the incident light that has passed through the first photocatalyst layer and the substrate can be used in the second photocatalyst layer, light different from the light incident on the first photocatalyst layer is transmitted to the second photocatalyst layer. Z-scheme water splitting using a photocatalyst can be realized with a simple configuration.

In the above embodiment, the substrate can be either electrically conductive or non-conductive (non-conductive), but is more preferably non-conductive.

According to this aspect, it is possible to significantly reduce costs compared to using a generally expensive conductive substrate, and since the transmittance of visible light is higher than that of a conductive substrate, the back side The light utilization efficiency in the second photocatalyst layer arranged in the second photocatalyst layer is improved.

In the above aspect, the light absorption spectrum of the second semiconductor is preferably located on the longer wavelength side compared to the light absorption spectrum of the first semiconductor.

According to this aspect, the incident light (particularly light in the visible light region) that has passed through the first photocatalyst layer and the substrate can be used more effectively in the second photocatalyst layer.

In the above aspect, the first semiconductor particles are one or more selected from metal oxides, metal oxynitrides, nitrides, metal oxysulfides, metal oxyhalides, and metal-organic structures. The second semiconductor includes a combination of metal oxynitride, metal nitride, metal oxysulfide, metal sulfide, metal oxyhalide, metal halide, organometallic halide, metal oxide, metal phosphorus. It may contain one or a combination of two or more selected from compounds, silicon, organic semiconductors, metal-organic structures, and covalent organic structures.

According to this aspect, the photocatalyst panel can be configured by the first and second semiconductors that can effectively utilize light energy.

In the above aspect, the first photocatalyst layer and the substrate portion of the photocatalyst panel have a transmission of 40% or more of the light incident on the first photocatalyst layer in a visible light region of at least 500 nm or more. It is good to have a rate.

According to this aspect, it becomes possible to more effectively utilize the optical energy in the visible light range that has passed through the first photocatalyst layer and the substrate in the second photocatalyst layer.

In the above aspect, the reaction solution includes a redox medium, and the redox medium is an ion pair that stably repeats redox in an aqueous solution, such as an iron ion pair, an ion pair containing iodine, and an ion containing vanadium. It may contain one type or a combination of two or more types selected from ion pairs, complex ion pairs, and polyoxometalate ion pairs.

According to this aspect, photocatalytic Z-scheme water splitting can be effectively performed using an appropriate redox medium.

In the above embodiment, a communication portion (G) may be formed between the reaction container and the photocatalyst panel to allow the reaction solution in the oxygen generation tank and the hydrogen generation tank to flow.

According to this aspect, there is no need to provide a structure for circulating the reaction solution in the oxygen generating tank and the hydrogen generating tank outside the reaction container, and Z-scheme type water splitting using a photocatalyst can be realized with a simpler structure. .

In the above embodiment, light may be incident on the reaction vessel only from the light entrance wall.

According to this aspect, the amount of light incident on the reaction vessel can be easily controlled, and as a result, the water splitting reaction (that is, the production of oxygen and hydrogen) can be easily controlled.

Another aspect of the present invention is a water splitting system including the water splitting device and a water storage tank (36, 236) that stores water supplied to the water splitting device.

According to this aspect, Z-scheme type water splitting using a photocatalyst can be realized with a simple configuration.

In the above aspect, a first circulation line (5, 105) that circulates a part of the reaction solution discharged together with oxygen from the oxygen generation tank to the hydrogen generation tank; A second circulation line (6, 106) for circulating a part of the reaction solution to the oxygen generation tank may be provided.

According to this aspect, the products (redox medium, etc.) generated in one of the oxygen generation tank and the hydrogen generation tank are sent to the other and used for the water splitting reaction, so that the Z-scheme type photocatalyst is used in the reaction vessel. The water splitting reaction can be carried out continuously.

In the above aspect, the first circulation line is connected to an oxygen separation line (5D) that transports the oxygen separated from the reaction solution, and the second circulation line is connected to the oxygen separation line (5D) that transports the oxygen separated from the reaction solution. A hydrogen separation line (6D) for transporting the hydrogen may be connected.

According to this aspect, each of oxygen and hydrogen produced in the reaction vessel can be easily obtained.

In the above aspect, the water storage tank may be provided in at least one of the first circulation line and the second circulation line.

According to this aspect, water can be supplied to the water splitting device with a simple configuration.

In the above embodiment, a gas-liquid separator (35) may be provided upstream of the water storage tank.

According to this aspect, a part of the reaction solution discharged together with oxygen or hydrogen can be circulated to the water splitting apparatus with a simple configuration.

According to the above embodiments, it becomes possible to realize Z-scheme type water splitting using a photocatalyst with a simple configuration.

A configuration diagram showing a water splitting system according to the first embodiment An explanatory diagram of the water splitting reaction by the water splitting device in Figure 1 Diagram showing a modification of the water splitting device in Figure 1 A configuration diagram showing a water splitting system according to the second embodiment A configuration diagram showing a water splitting system according to the third embodiment Configuration diagram of the evaluation device (water splitting device) used in each example, each comparative example, and each reference example Graph showing the UV-visible diffuse reflectance spectra of various semiconductors Graph showing the transmittance to the hydrogen generation catalyst layer side in the photocatalyst panel of Example 1 Graph showing the transmittance of the photocatalyst panel of Comparative Example 1 Graph showing the transmittance of the photocatalyst panel of Comparative Example 2 Graph showing the transmittance of the photocatalyst panel of Comparative Example 3 Graph showing the transmittance to the hydrogen generation catalyst layer side in the photocatalyst panel of Comparative Example 8 Graph showing temporal changes in water splitting reaction in Example 1 Graph showing changes over time in water splitting reaction in Comparative Example 7 Graph showing changes over time in water splitting reaction in Comparative Example 8 Graph showing time-dependent changes in water splitting reaction in Example 4 Graph showing changes over time in water splitting reaction in Example 5 Graph showing changes over time in water splitting reaction in Example 3 Graph showing time-dependent changes in water splitting reaction in Reference Example 1 Graph showing changes over time in water splitting reaction in Reference Example 2

Hereinafter, a water splitting device using a photocatalyst according to an embodiment and a water splitting system equipped with the same will be described with reference to the drawings.

(First embodiment)
FIG. 1 is a configuration diagram showing a water splitting system 1 according to the first embodiment. FIG. 2 is an explanatory diagram of a water splitting reaction by the water splitting apparatus 3 in FIG.

As shown in FIG. 1, the water splitting system 1 includes a water splitting device 3 that splits water into hydrogen and oxygen using solar energy, and a first circulation line 5 and a second circulation line connected to the water splitting device 3, respectively. 2 circulation lines 6.

The water decomposition device 3 includes a photocatalyst panel 7 that uses a photocatalyst, and a reaction container 9 that accommodates the photocatalyst panel 7 and a reaction solution (aqueous solution) and performs a decomposition reaction of water in the reaction solution.

The photocatalyst panel 7 includes a substrate 11 having optical transparency, a photocatalytic layer 13 for oxygen generation (first photocatalyst layer) formed so as to overlap the front surface 11A (first surface) of the substrate 11, and It includes a hydrogen generation photocatalyst layer 14 (second photocatalyst layer) formed so as to overlap the rear surface 11B (second surface) that is paired with the front surface 11A.

The substrate 11 has a rectangular flat plate shape and is made of a transparent material with high light transmittance. As the transparent material, for example, glass, acrylic resin, polycarbonate, etc. can be used. Further, the substrate 11 is preferably insulating or non-conductive. Regarding the insulation or non-conductivity of the substrate 11, it is preferable that the volume resistivity (see JIS C 2139-3-1 Part 3-1) is 10 ¹⁰ Ω·m or more. Note that the shape and size of the substrate 11 can be changed as appropriate.

The oxygen generation photocatalyst layer 13 is formed so as to overlap the front surface 11A of the substrate 11. As will be described in detail later, the oxygen generation photocatalyst layer 13 includes an oxygen generation semiconductor (first semiconductor) that constitutes the oxygen generation photocatalyst.

The hydrogen generation photocatalyst layer 14 is formed so as to overlap the rear surface 11B of the substrate 11. As will be described in detail later, the hydrogen generation photocatalyst layer 14 includes a hydrogen generation semiconductor (second semiconductor) that constitutes the hydrogen generation photocatalyst.

These oxygen-generating semiconductors and hydrogen-generating semiconductors are fixed on the substrate 11 as particles (groups) or thin films. The shape of such particles is not particularly limited, and examples include spherical, ellipsoidal, plate-like, fibrous, scale-like, and porous shapes. Further, such thin films include, but are not particularly limited to, dense films of semiconductors, porous films formed by depositing semiconductor particles, porous films with regular structures, and the like.

The oxygen generating semiconductor absorbs visible light by having a band gap of 3.0 eV or less or an energy gap derived from an impurity level. The upper end of the valence band of the oxygen generating semiconductor is on the positive side of the oxidation potential of water (O ₂ /H ₂ O: +1.23V). The reference potential is the potential of a standard hydrogen electrode at pH=0.

The oxygen generating semiconductor is not particularly limited as long as it can decompose water and generate oxygen. Examples of the semiconductor for oxygen generation include metal oxides, metal oxynitrides, metal nitrides, metal oxysulfides, metal oxyhalides, metal organic frameworks (MOF), and the like. In the water splitting device 3, one kind or a combination of two or more kinds selected from these materials can be used as the oxygen generating semiconductor.

Examples of the metal oxide include WO ₃ , BiVO ₄ , H ₂ WO ₄ and the like.

Examples of the metal oxynitride include TaON and BaTaO ₂ N. Examples of the metal nitride include Ta ₃ N ₅ and the like.

Examples of the metal oxysulfide include Sm ₂ Ti ₂ S ₂ O ₅ and the like.

Examples of the acid halides include Bi ₄ NbO ₈ Cl and Ba ₂ Bi ₃ Nb ₂ O ₁₁ Br _1- x I _x (0≦x≦1).

Examples of the above-mentioned metal-organic framework include MOF containing iron (III) oxo cluster and terephthalic acid as constituent elements.

The oxygen generating semiconductor may be a material doped with impurities or may be a material not doped with impurities. By doping the oxygen-generating semiconductor with an impurity to form an impurity level, electrons can be excited from the impurity level to the conduction band or from the valence band to the impurity level.

A semiconductor for hydrogen generation absorbs visible light by having a band gap of 3.0 eV or less or an energy gap derived from an impurity level. The lower end of the conduction band of the hydrogen generating semiconductor is on the negative side of the proton reduction potential (H ⁺ /H ₂ : 0V).

The hydrogen generating semiconductor is not particularly limited as long as it can decompose water and generate hydrogen. Semiconductors for hydrogen generation include metal oxynitrides, metal nitrides, metal oxysulfides, metal sulfides, metal oxyhalides, metal halides, organometallic halides, metal oxides, metal phosphides, silicon, and organic Examples include semiconductors, MOFs, and covalent organic frameworks (COFs). In the water splitting device 3, one kind or a combination of two or more kinds selected from these materials can be used as the hydrogen generating semiconductor.

Examples of the metal oxynitride include TaON, BaTaO ₂ N, CaTaO ₂ N, and SrTaO ₂ N. Examples of the metal nitride include Ta ₃ N ₅ and the like.

Examples of the metal oxysulfide include Sm ₂ Ti ₂ S ₂ O ₅ and the like. Examples of the metal sulfides include CdS and Cu _x Ag _x In _2x Zn _2-2x S ₂ (0≦x≦1).

Examples of the metal acid halides include Bi ₄ NbO ₈ Cl and Ba ₂ Bi ₃ Nb ₂ O ₁₁ Br _1-x I _x (0≦x≦1). Examples of the metal halide include Cs ₂ AgBiBr ₆ and the like. Examples of the organometallic halide include CH ₃ NH ₃ PbI ₃ and the like.

Examples of the metal oxide include Cu ₂ O and SnNb ₂ O ₆ .

Examples of the metal phosphide include GaP.

Examples of the organic semiconductor include carbon nitride (g-C ₃ N ₄ ) and dibenzothiophene polymer.

Examples of the metal-organic framework include aminobenzenedicarboxylic acid titanium MOF.

Examples of the covalent organic structure include COF, which has 1,3,5-triformylphloroglucinol and 4,4''-diamino-p-terphenyl as constituent elements.

The hydrogen generating semiconductor may be a material doped with impurities or may be a material not doped with impurities. When a hydrogen generating semiconductor is doped with an impurity to form an impurity level, electrons can be excited from the impurity level to the conduction band or from the valence band to the impurity level.

Note that the oxygen-generating photocatalyst may further include an oxygen-generating co-catalyst. The oxygen generation co-catalyst is supported on the oxygen generation semiconductor and promotes a reaction that generates oxygen from water and holes generated in the oxygen generation semiconductor when the oxygen generation semiconductor is irradiated with visible light. When the oxygen generation co-catalyst is included in the oxygen generation photocatalyst, the oxygen generation activity of the oxygen generation photocatalyst is significantly improved.

Examples of the oxygen generation promoter include metal oxides, metal hydroxides, metal phosphates, and the like. The cocatalyst for oxygen generation may include at least one transition metal or noble metal. Transition metals include Mn, Fe, Ni, and Co. Examples of noble metals include Ru and Ir.

Moreover, the photocatalyst for hydrogen generation may further contain a co-catalyst for hydrogen generation. A hydrogen generation co-catalyst is supported on a hydrogen generation semiconductor, and is a reaction that generates hydrogen from excited electrons and protons (or water) generated in the hydrogen generation semiconductor when the hydrogen generation semiconductor is irradiated with visible light. promote. When the hydrogen generation co-catalyst is included in the hydrogen generation photocatalyst, the hydrogen generation activity of the hydrogen generation photocatalyst is significantly improved.

Examples of the hydrogen generation promoter include metals, metal oxides, and metal sulfides. The hydrogen production promoter may contain at least one noble metal. The hydrogen generation co-catalyst may contain Cr in addition to the noble metal. Noble metals include Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au. Among them, Pt and/or Rh are preferred.

The hydrogen generation promoter may include at least one member selected from the group consisting of a core-shell type material having a noble metal core and a chromium oxide (CrO _x ) shell, and a mixed oxide of a noble metal and Cr. . The mixed oxide of noble metal and Cr is, for example, an oxide containing rhodium (III) and chromium (III) (Rh _x Cr _2-x O ₃ ). These cocatalysts for hydrogen production can promote the reaction of producing hydrogen from excited electrons and protons (or water) even in the presence of an oxidized form of the redox medium, which will be described later. By reducing the oxidized form of hydrogen, the excited electrons are prevented from disappearing without being used for hydrogen production. As a result, in the presence of a redox medium, the reduction of protons (or water) proceeds selectively and smoothly, making it possible to achieve high solar energy conversion efficiency.

These co-catalysts can be supported on semiconductor particles in the form of particles. The particle size of the promoter is not particularly limited. The average particle diameter of the cocatalyst particles is usually 0.1 nm or more and 100 nm or less.

In the water splitting system 1 according to the present embodiment, the oxygen generating semiconductor (oxygen generating photocatalyst) uses sunlight incident on the water splitting device 3, and the hydrogen generating semiconductor (hydrogen generating photocatalyst) uses oxygen generating semiconductor (hydrogen generating photocatalyst) to generate oxygen. The light that has passed through the photocatalyst layer 13 and the substrate 11 is used. Therefore, it is preferable that the oxygen generation semiconductor and the hydrogen generation semiconductor used in the water splitting device 3 have different absorption wavelength regions with respect to sunlight (particularly light in the visible light region). For example, the light absorption spectrum of the semiconductor for hydrogen generation is preferably located on the longer wavelength side compared to the light absorption spectrum of the semiconductor for oxygen generation used in combination. As a result, the oxygen-generating semiconductor mainly absorbs light in a wavelength range with a shorter upper limit (e.g., 200-500 nm), and the hydrogen-generating semiconductor mainly absorbs light in a wavelength range with a longer upper limit (e.g., 200-650 nm). can be mainly absorbed.

Table 1 shows examples of suitable combinations of oxygen generating semiconductors and hydrogen generating semiconductors in the water splitting apparatus 3 according to the present embodiment.

As shown in Table 1, oxygen generating semiconductors have a lower upper limit on the usable wavelength (i.e., they mainly absorb light in the shorter wavelength range) and semiconductors have a higher upper limit on the usable wavelength (i.e., they mainly absorb light in the shorter wavelength range). A combination with a hydrogen generating semiconductor (which mainly absorbs light in a wavelength region with a longer upper limit) is suitable. For example, the usable wavelength of WO ₃ as a semiconductor for oxygen generation is approximately 450 nm or less, and TaON, Ta ₃ N ₅ , Sm ₂ Ti ₂ S ₂ O ₅ , The available wavelengths of BaTaO ₂ N and BaTaO 2 N are about 500 nm or less, about 600 nm or less, about 600 nm or less, and about 650 nm or less, respectively. Such a combination of the oxygen generation semiconductor and the hydrogen generation semiconductor allows the energy of the incident light to be effectively used in both the oxygen generation photocatalyst layer 13 and the hydrogen generation photocatalyst layer 14.

The reaction container 9 has a box shape and holds a reaction solution containing water as a main component in its internal space. The reaction solution includes a redox medium (combination of oxidant and reductant). In this embodiment, an example is shown in which a Fe ³⁺ /Fe ²⁺ ion pair is used as the redox medium. Fe ²⁺ may be present in both the reaction solutions of the oxygen generation tank 21 and the hydrogen generation tank 22 at the time of start-up (at the start of the water splitting reaction). Alternatively, Fe ³⁺ may be present in both the reaction solutions of the oxygen generation tank 21 and the hydrogen generation tank 22 at startup. Alternatively, Fe ²⁺ and Fe ³⁺ may be present in the reaction solutions of the oxygen generation tank 21 and the hydrogen generation tank 22, respectively, at startup. The following explanation will be given under the condition that water in which FeCl ₂ is dissolved as a redox source is injected into the reaction vessel 9 at startup, and Fe ²⁺ is present in the reaction solutions in both the oxygen generation tank 21 and the hydrogen generation tank 22. . In addition, in FIG. 1, regarding the arrangement of the reaction vessels 9, the vertical direction of the drawing generally corresponds to the vertical direction.

In the reaction vessel 9, so-called Z scheme type (two-step excitation type) water splitting is performed. In Z-scheme water splitting, two different types of photocatalysts (i.e., oxygen generation photocatalyst layer 13 and hydrogen generation photocatalyst layer 14) are used, and electron transfer between both photocatalysts is performed via a redox medium in the reaction solution. It will be done.

The redox reaction of the Fe ³⁺ /Fe ²⁺ ion pair is a one-electron process involving one electron. Therefore, when the Fe ³⁺ /Fe ²⁺ ion pair is used, electron transfer between the oxygen generation photocatalyst and the hydrogen generation photocatalyst proceeds smoothly. Furthermore, light absorption by Fe ³⁺ /Fe ²⁺ ion pairs is also small. Furthermore, since the redox potential of Fe ³⁺ /Fe ²⁺ ion pair is 0.77 V (vs. SHE), if Fe ³⁺ /Fe ²⁺ ion pair is used as a redox medium, many semiconductors for hydrogen generation and oxygen generation Transfer of electrons between the semiconductor and the redox medium can proceed smoothly.

In the water splitting device 3, iron ions, iodine-containing ions, vanadium-containing ions and complex ions, polyoxometalate ions, and the like may be used as redox media. Examples of iron ions include the above-mentioned Fe ³⁺ /Fe ²⁺ ion pair. Examples of iodine-containing ions include the IO ₃ ⁻ /I ⁻ ion pair and the I ₃ ⁻ /I ⁻ ion pair. Examples of ions containing vanadium include VO ₂ ⁺ /VO ²⁺ and the like. Complex ions include [Fe(CN) ₆ ] ^3- / [Fe(CN) ₆ ] ^4- ion pair, [Co(bpy) ₃ ] ³⁺ /[Co(bpy) ₃ ] ²⁺ ion pair, and [Co Examples include (phen) ₃ ] ³⁺ /[Co(phen) ₃ ] ²⁺ ion pair. Examples of polyoxometalate ions include the [SiW ₁₁ O ₃₉ Mn(H ₂ O)] ^5- /[SiW ₁₁ O ₃₉ Mn(H ₂ O)] ^6- ion pair. In the water splitting device 3, one or more ion pairs selected from these can be used as a redox medium.

Note that the above-mentioned photocatalyst for oxygen production may further include a reduction co-catalyst for redox media. The reduction co-catalyst for the redox medium is supported on the semiconductor for oxygen generation, and promotes the reduction reaction of the redox medium by excited electrons generated in the semiconductor for oxygen generation when the semiconductor for oxygen generation is irradiated with visible light. When the reduction promoter for redox media is included in the oxygen generation photocatalyst, recombination by excited electrons and holes generated in the oxygen generation semiconductor is suppressed, and the oxygen generation activity of the oxygen generation photocatalyst is significantly improved. .

Examples of reduction promoters for redox media include metals and metal oxides. The reduction cocatalyst for the redox medium may include at least one transition metal or noble metal. Examples of transition metals include Fe. Noble metals include Ru, Ir, and Pt.

Moreover, the above-mentioned photocatalyst for hydrogen production may further contain an oxidation promoter for redox media. The oxidation promoter for a redox medium is supported on a semiconductor for hydrogen generation, and promotes the oxidation reaction of the redox medium by holes generated in the semiconductor for hydrogen generation when the semiconductor for hydrogen generation is irradiated with visible light. When the oxidation cocatalyst for redox media is included in the photocatalyst for hydrogen generation, recombination by holes generated in the semiconductor for hydrogen generation and excited electrons is suppressed, and the hydrogen generation activity of the photocatalyst for hydrogen generation is significantly improved. .

Examples of the oxidation promoter for redox media include oxides containing Ir or metal cyanometalates. Examples of metal cyanometalates include indium (III) hexacyanoferrate.

The reaction container 9 has a front wall 9A (light incidence wall) arranged to face the oxygen-generating photocatalyst layer 13. The front wall 9A is arranged diagonally upward so that its outer surface is generally orthogonal to the direction of incidence of sunlight. The internal space of the reaction vessel 9 is partitioned by a photocatalyst panel 7 arranged substantially parallel to the front wall 9A. More specifically, the upper edge of the photocatalyst panel 7 is in close contact with the upper wall 9C of the reaction vessel 9, and the lower edge of the photocatalyst panel 7 is in close contact with the lower wall 9D of the reaction vessel 9.

Thereby, in the reaction vessel 9, the oxygen generation tank 21 is formed on the front side of the photocatalyst panel 7 (on the side of the oxygen generation photocatalyst layer 13). The oxygen generation photocatalyst layer 13 forms a part of the inner surface of the oxygen generation tank 21 . Further, in the reaction vessel 9, a hydrogen generation tank 22 is formed on the rear side of the photocatalyst panel 7 (on the side of the hydrogen generation photocatalyst layer 14). The hydrogen generation photocatalyst layer 14 forms part of the inner surface of the hydrogen generation tank 22 .

The outer wall of the reaction vessel 9 is made of a transparent material with high light transmittance, similar to the substrate 11 described above. However, in the outer wall of the reaction container 9, it is sufficient that at least the light incidence part (here, the front wall 9A serving as the light incidence wall) through which sunlight enters is made of a transparent material. For example, only the light incident area is made of a transparent material (for example, the outer wall other than the light incident area has no transparency, or the transmittance of the area other than the light incident area is higher than the transmittance of the light incident area). In this configuration, the amount of light incident on the reaction vessel 9 (and thus the water splitting reaction) can be easily controlled.

Note that the shape and size of the reaction container 9 can be changed as appropriate. In the reaction vessel 9 shown in FIG. 1, the oxygen generation photocatalyst layer 13 is arranged on the sunlight incident side (that is, facing the front wall 9A). The photocatalyst layer 14 for hydrogen generation may be arranged on the light incident side.

As shown in FIG. 2, in the oxygen generation tank 21, the oxygen generation semiconductor in the oxygen generation photocatalyst layer 13 absorbs light, so that electrons (e ^- ) are generated in the conduction band and holes (e - ) are generated in the valence band. h ⁺ ) occurs. As a result, in the oxygen generation photocatalyst layer 13, O ₂ is generated from H ₂ O (or OH ^- ) by holes, and a reductant (Red) is generated from an oxidant (Ox) by electrons. Similarly, in the hydrogen generation tank 22, the hydrogen generation semiconductor (semiconductor photocatalyst) in the hydrogen generation photocatalyst layer 14 absorbs light, thereby generating electrons (e ^- ) and holes (h ⁺ ). As a result, in the hydrogen generation photocatalyst layer 14, H ₂ is generated from H ⁺ (or H ₂ O) by electrons, and oxidant (Ox) is generated from reductant (Red) by holes.

Referring to FIG. 1 again, the first circulation line 5 is provided with a first gas-liquid separator 25, a first water storage tank 26, and a first circulation pump 27.

The first circulation line 5 includes an oxygen discharge line 5A made up of piping that connects an upper opening 29 located at the upper part of the oxygen generation tank 21 and the first gas-liquid separator 25. Oxygen discharged from the upper opening 29 is transported to the first gas-liquid separator 25 through the oxygen discharge line 5A. The fluid transported by the oxygen discharge line 5A contains a portion of the reaction solution along with oxygen. In the first gas-liquid separator 25, the reaction solution (containing Fe ²⁺ ) is separated from the oxygen flowing through the oxygen discharge line 5A. The oxygen separated in the first gas-liquid separator 25 is supplied to an oxygen storage tank (not shown) through an oxygen separation line 5D connected to its upper part (for example, a portion corresponding to the gas phase in the drum body of the knockout drum). Ru. However, the oxygen separated by the first gas-liquid separator 25 may be released to the atmosphere.

Further, the first circulation line 5 is composed of piping that connects the bottom of the first gas-liquid separator 25 (for example, the portion corresponding to the liquid phase in the drum body of the knockout drum) and the first water storage tank 26. It includes a water discharge line 5B. The reaction solution discharged from the first gas-liquid separator 25 is transported to the first water storage tank 26 through the water discharge line 5B. In the first water storage tank 26, the reaction solution discharged from the first gas-liquid separator 25 is stored. The first water storage tank 26 can be replenished with water, a redox medium (or a redox source as its raw material), etc. from the outside as necessary.

Furthermore, the first circulation line 5 includes a first water supply line 5C that is constituted by piping that connects the first water storage tank 26 and the lower opening 31 located at the lower part of the hydrogen generation tank 22. A first circulation pump 27 is provided in the first water supply line 5C. The reaction solution stored in the first water storage tank 26 is transported to the reaction container 9 through the first water supply line 5C by the first circulation pump 27.

The second circulation line 6 is provided with a second gas-liquid separator 35, a second water storage tank 36, and a second circulation pump 37.

The second circulation line 6 includes a hydrogen discharge line 6A formed of a pipe connecting between an upper opening 39 located at the upper part of the hydrogen generation tank 22 and the second gas-liquid separator 35. Hydrogen discharged from the upper opening 39 is transported to the second gas-liquid separator 35 through the hydrogen discharge line 6A. The fluid transported by the hydrogen discharge line 6A contains hydrogen and a portion of the reaction solution. In the second gas-liquid separator 35, the reaction solution (containing Fe ³⁺ ) is separated from the hydrogen flowing through the hydrogen discharge line 6A. The hydrogen separated by the second gas-liquid separator 35 is supplied to a hydrogen storage tank (not shown) through a hydrogen separation line 6D connected to the upper part of the hydrogen separation line 6D.

Further, the second circulation line 6 includes a second water discharge line 6B that is configured from a pipe that connects the lower part of the second gas-liquid separator 35 and the second water storage tank 36. The reaction solution discharged from the second gas-liquid separator 35 is transported to the second water storage tank 36 through the second water discharge line 6B. In the second water storage tank 36, the reaction solution discharged from the second gas-liquid separator 35 is stored. The second water storage tank 36 can be replenished with water, a redox medium (or a redox source as a raw material thereof), etc. from the outside as necessary.

Further, the second circulation line 6 includes a second water supply line 6C that is constituted by piping that connects the second water storage tank 36 and the lower opening 41 located at the lower part of the oxygen generation tank 21. A second circulation pump 37 is provided in the second water supply line 6C. The water stored in the second water storage tank 36 is transported to the reaction vessel 9 through the second water supply line 6C by the second circulation pump 37.

Thus, in the water splitting system 1, the upstream end of the first circulation line 5 is connected to the oxygen generation tank 21, and the downstream end of the first circulation line 5 is connected to the hydrogen generation tank 22. Thereby, the reductant (here, Fe ²⁺ ) generated in the oxygen generation tank 21 is supplied to the hydrogen generation tank 22 . Further, the upstream end of the second circulation line 6 is connected to the hydrogen generation tank 22, and the downstream end of the second circulation line 6 is connected to the oxygen generation tank 21. As a result, the oxidant (here, Fe ³⁺ ) generated in the hydrogen generation tank 22 is supplied to the oxygen generation tank 21 . Therefore, in the water splitting system 1, even if the oxygen generation tank 21 and the hydrogen generation tank 22 are completely separated by the photocatalyst panel 7, it is possible to sequentially supply the redox medium required by one tank from the other tank. can.

In the water splitting system 1, when sunlight is incident on the front wall 9A of the reaction container 9, the water in the container is decomposed into oxygen and hydrogen by the photocatalyst panel 7 that utilizes the light energy. The sunlight that has entered the oxygen generation photocatalyst layer 13 through the front wall 9A further passes through the substrate 11 and reaches the hydrogen generation photocatalyst layer 14. Thereby, in the photocatalyst layer for hydrogen generation 14, it is possible to generate hydrogen using the light not absorbed by the photocatalyst layer for oxygen generation 13. Therefore, in the reaction vessel 9, there is no need to input sunlight for water splitting from the side of the photocatalyst layer for hydrogen generation 14 (that is, from the rear wall 9B side of the reaction vessel 9), and a simple configuration is realized. The water splitting system 1 can function as a hydrogen production system whose main purpose is to produce hydrogen.

Note that although only Fe ²⁺ exists as a redox medium in the initial reaction vessel 9, Fe ³⁺ generated together with hydrogen in the hydrogen generation tank 22 is supplied to the oxygen generation tank 21 through the second circulation line 6. Thereby, in the oxygen generation tank 21, oxygen is generated, and Fe ²⁺ is generated again from Fe ³⁺ by the electrons of the oxygen generation photocatalyst layer 13.

(Modified example of the first embodiment)
FIG. 3 is a diagram showing a modification of the water splitting apparatus 3 shown in FIG. 1. In FIG. 3, the same components as those in the water splitting apparatus 3 shown in FIG. 1 are designated by the same reference numerals. Further, regarding the modification, matters that are not particularly mentioned below are the same as those in the first embodiment, so detailed explanations will be omitted.

In the first embodiment, sunlight is directly incident on the reaction container 9, but the invention is not limited to this, and as shown in FIG. You may let them. The reaction container 9 shown in FIG. 3 has a cylindrical shape with a bottom, and light enters from one side of the peripheral wall (here, the side of the oxygen-generating photocatalyst layer 13). In this way, by using the light condensing plate 45, it is possible to use sunlight more effectively without increasing the size of the water splitting device 3.

(Second embodiment)
FIG. 4 is a configuration diagram showing a water splitting system 1 according to the second embodiment. In FIG. 4, the same components as those of the water splitting apparatus 3 shown in FIG. 1 are designated by the same reference numerals. Furthermore, regarding the second embodiment, matters not specifically mentioned below are the same as those in the first embodiment, so detailed explanations will be omitted.

The water splitting system 1 according to the second embodiment has the same reaction vessel 9 as the first embodiment. On the other hand, in the water splitting system 1 according to the second embodiment, the configurations of the first circulation line 105 and the second circulation line 106 are different from the first circulation line 5 and the second circulation line 6 in the first embodiment, respectively.

The first circulation line 105 includes an oxygen discharge line 5A and a water discharge line 5B. On the other hand, in the first circulation line 105, the first water supply line 5C, the first water storage tank 26, and the first circulation pump 27 in the first embodiment (see FIG. 1) are omitted.

In the first circulation line 105, the first gas-liquid separator 25 is arranged at a higher position than the reaction vessel 9. The downstream end of the oxygen discharge line 5A is connected to the first gas-liquid separator 25. The upstream end of the water discharge line 5B is connected to the bottom of the first gas-liquid separator 25. On the other hand, the downstream end of the water discharge line 5B is connected to the upper opening 39 of the hydrogen generation tank 22. The fluid flowing through the oxygen discharge line 5A is pushed out from the oxygen generation tank 21 side by the second circulation pump 37 provided in the second circulation line 106 and sent to the first gas-liquid separator 25. Further, in the water discharge line 5B, the reaction solution (containing Fe ²⁺ ) separated by the first gas-liquid separator 25 flows under its own weight from its upstream end to its downstream end.

With this configuration, the first circulation line 105 can supply the reaction solution (containing Fe ²⁺ ) separated by the first gas-liquid separator 25 to the hydrogen generation tank 22 without the need for a pump. can.

Note that in the first circulation line 105, a degassing device (for example, a degassing valve) having a known configuration can be provided at least one of the upper part of the reaction vessel 9 and the oxygen exhaust line 5A. Thereby, in the first circulation line 5, the first gas-liquid separator 25 can be omitted. In that case, the water discharge line 5B is omitted, and the downstream end of the oxygen discharge line 5A is connected to the upper opening 39 of the hydrogen generation tank 22.

In the second circulation line 106, the upstream portion of the hydrogen discharge line 6A is connected to the lower opening 31 located at the lower part of the hydrogen generation tank 22. However, in the hydrogen generation tank 22, there is a possibility that the generated hydrogen will stay in the upper part of the tank. Therefore, in order to smoothly discharge hydrogen from the hydrogen generation tank 22, the upstream end (hydrogen discharge port) of the hydrogen discharge line 6A is preferably located at the upper part of the hydrogen generation tank 22 through the lower opening 31.

In the water splitting system 1 according to the second embodiment, the upstream end of the first circulation line 105 is connected to the oxygen generation tank 21, and the downstream end of the first circulation line 105 is connected to the hydrogen generation tank 22. Thereby, the reductant (here, Fe ²⁺ ) generated in the oxygen generation tank 21 is supplied to the hydrogen generation tank 22 . Further, the upstream end of the second circulation line 106 is connected to the hydrogen generation tank 22, and the downstream end of the second circulation line 106 is connected to the oxygen generation tank 21. As a result, the oxidant (here, Fe ³⁺ ) generated in the hydrogen generation tank 22 is supplied to the oxygen generation tank 21 . Therefore, in the water splitting system 1, even if the oxygen generation tank 21 and the hydrogen generation tank 22 are completely separated by the photocatalyst panel 7, it is possible to sequentially supply the redox medium required by one tank from the other tank. can.

(Third embodiment)
FIG. 5 is a configuration diagram showing a water splitting system 1 according to the third embodiment. In FIG. 5, the same reference numerals are given to the same components as those of the water splitting apparatus 3 shown in FIG. Furthermore, regarding the third embodiment, matters not specifically mentioned below are the same as those in the first embodiment, so detailed explanations will be omitted.

In the water splitting system 1 according to the third embodiment, the configurations of a photocatalyst panel 207 and a reaction container 209 are different from those of the photocatalyst panel 7 and reaction container 9 in the first embodiment (see FIG. 1). Furthermore, in the water splitting system 1 according to the third embodiment, a circulation line for circulating the reaction solution between the oxygen generation tank 21 and the hydrogen generation tank 22 is omitted.

In the first and second embodiments described above, the internal space of the reaction vessel 9 is partitioned by the photocatalyst panel 7, so that the oxygen generation tank 21 and the hydrogen generation tank 22 that do not communicate with each other can be formed. That is, inside the reaction vessel 9, movement of fluid between the oxygen generation tank 21 and the hydrogen generation tank 22 is not essential.

On the other hand, the internal space of the reaction container 209 is not completely partitioned by the photocatalyst panel 207, and the lower edge 207A of the photocatalyst panel 207 is spaced apart from the lower wall 209D of the reaction container 209. Thereby, in the reaction vessel 209, the oxygen generation tank 21 and the hydrogen generation tank 22 are in a state of communication with each other via the space G (communication portion) between the photocatalyst panel 207 and the lower wall 209D.

The upper opening 29 of the oxygen generation tank 21 is connected to the upstream end of the oxygen separation line 205D. Oxygen discharged from the oxygen generation tank 21 through the oxygen separation line 205D is either supplied to an oxygen storage tank (not shown) or released to the atmosphere.

The upper opening 39 of the hydrogen generation tank 22 is connected to the upstream end of the hydrogen separation line 206D. Hydrogen discharged from the hydrogen generation tank 22 through the hydrogen separation line 206D is supplied to a hydrogen storage tank (not shown).

Additionally, the water splitting device 3 is provided with a water storage tank 236 and a water supply line 206C whose upstream end is connected to the water storage tank 236. A water supply pump 227 is provided in the water supply line 206C. The reaction solution stored in the water storage tank 236 is transported to the reaction container 209 by the water supply pump 227 through the water supply line 206C. The downstream end of the water supply line 206C is connected to a lower opening 231 provided in the lower wall 209D of the reaction vessel 209. The lower opening 231 is arranged at a position facing the lower end of the photocatalyst panel 207 (that is, near the communication section).

As described above, in the water splitting system 1 according to the third embodiment, since the oxygen generation tank 21 and the hydrogen generation tank 22 are configured to communicate with each other in the reaction container 9, the reaction solution (including the redox medium) in the reaction container 209 is can be moved between both tanks. As a result, the reductant (here, Fe ²⁺ ) generated in the oxygen generation tank 21 moves to the hydrogen generation tank 22, and the oxidant (here, Fe ³⁺ ) generated in the hydrogen generation tank 22 moves to the oxygen generation tank 22. Move to tank 21. Therefore, the circulation line for circulating the reaction solution in the reaction container 209 as in the first and second embodiments can be omitted.

Hereinafter, specific examples of photocatalyst panels used in the water splitting system 1 and the water splitting device 3 will be described based on some examples, comparative examples, and reference examples. However, the present invention is not limited to these examples, and various changes can be made without departing from the scope of the present invention.

In each Example, each Comparative Example, and each Reference Example, the photocatalyst and its photocatalytic activity were evaluated by the following method.

(Evaluation of photocatalyst)
<Powder X-ray diffraction measurement>
Crystal structure analysis of the sample was performed using an X-ray diffraction device (Rigaku Corporation, MiniFlex II). The CuKα rays generated under the conditions of 30 kV and 15 mA were made monochromatic using a carbon monochromator, and the range of 2θ=3 to 70 deg was measured at a scanning speed of 10 deg/min.

<UV-visible spectroscopy measurement>
The light absorption properties of the samples were evaluated using an ultraviolet-visible spectrophotometer (manufactured by JASCO Corporation, V-650). Measurement was performed using an integrating sphere using a standard reflector (BaSO ₄ ) as a reference, and the UV-visible diffuse reflectance spectrum of the sample powder was measured under the conditions of a scanning range of 200 to 800 nm, a scanning speed of 1000 nm/min, and a sampling interval of 0.5 nm. did.

<Shape observation>
The particle shape of the powder sample was observed using a transmission electron microscope (manufactured by JEOL Ltd., JEM-2100F). The sample was immobilized on a mesh attached to a microgrid and observed.

(Evaluation of photocatalytic activity)
<Evaluation device>
FIG. 6 is a configuration diagram of the evaluation device 303 (water splitting device) used in each Example, each Comparative Example, and each Reference Example. The evaluation device 303 includes a photocatalyst panel 307 that uses a photocatalyst, and a downward irradiation type reaction cell 51 (reaction container) that accommodates the photocatalyst panel 307 and a reaction solution and performs a water decomposition reaction.

The evaluation device 303 has the same configuration as the water splitting device 3 described above, except for matters specifically mentioned below. The photocatalyst panel 307 includes a light-transmitting substrate 311, an oxygen-generating photocatalytic layer 313 formed so as to overlap a lower surface 311A (first surface) of the substrate 311, and an upper surface of the substrate 311 that is paired with the lower surface 311A. A photocatalyst layer 314 for hydrogen generation formed so as to overlap 311B (second surface). Photocatalyst panel 307 is supported by spacer 53. In the evaluation device 303, light from a xenon lamp was used instead of sunlight.

The evaluation device 303 was prepared as follows. After adding 40 mL of various reaction solutions to a downward irradiation type reaction cell 51 with a Pyrex (registered trademark) window (circular with a diameter of 7.0 cm), photocatalyst panels 307 of each example, each comparative example, and each reference example were added. was placed horizontally against a Pyrex window. Spacers 53 made of carbon tape were attached to all sides of the photocatalyst-uncoated portion of the photocatalyst panel 307 to separate the bottom surface of the photocatalyst panel 307 from the bottom surface of the cell 51. The reaction cell 51 containing the reaction solution and the photocatalyst panel 307 was attached to a closed circulation system evaluation device, and after evacuation was performed, Ar gas was introduced.

<Water splitting reaction and its evaluation>
The water splitting reaction in the evaluation device 303 was carried out as follows. Visible light was directed vertically from the Pyrex window side of the reaction cell 51 (from the bottom to the top in Figure 6) using a 300W xenon lamp with a CM-1 cold mirror and a cutoff filter (L42, λ>400nm). ) irradiated. The distance between the xenon lamp (light irradiation port) and the irradiation window was 10 mm or less. The temperature of the reaction solution was controlled by a cooling water circulation device so that the temperature of the reaction solution was maintained at 288K. The amount of generated gas was analyzed and measured using a gas chromatograph analyzer (manufactured by Shimadzu Corporation, GC-8A, Molecular sieve 5A column) using Ar gas as a carrier gas.

<Preparation of photocatalyst powder for oxygen generation>
The oxygen-generating photocatalyst powder used for the oxygen-generating photocatalyst layer 313 was prepared as follows.

《Synthesis of Fe-H-Cs-WO ₃ 》
The WO ₃ powder was classified and coarse particles were collected. 1 g of the classified and collected WO ₃ was added to an evaporating dish, and 950 μL of a CsCl aqueous solution (0.1 M) was added dropwise, and the water was evaporated to dry the sample. The amount of CsCl aqueous solution added dropwise was adjusted so that Cs was 2.1 mol % relative to WO ₃ . The sample, still in the evaporating dish, was placed in an electric furnace and fired in air at 500°C for 30 minutes. 0.5 g of the collected sample was suspended in 50 mL of an aqueous H ₂ SO ₄ solution (concentration 1M) containing an aqueous FeSO ₄ solution (concentration 50 mM) and stirred for 30 minutes. Thereafter, the product was collected by suction filtration, washed with ultrapure water, and then dried at room temperature. As a result, a powder of Fe-H-Cs-WO ₃ was obtained.

<Preparation of photocatalyst powder for hydrogen generation>
The photocatalyst powder for hydrogen generation used in the photocatalyst layer 314 for hydrogen generation was prepared as follows.

《Synthesis of ZrO ₂ /TaON》
As raw materials, a powder reagent of Ta ₂ O ₅ and a powder reagent of ZrO(NO ₃ ) _2.2H ₂ O were used. _Ta2O5 and _ZrO ( _NO3 ) _2.2H2O were weighed so that the molar ratio of Zr/Ta ₌ 0.1. These powder samples and a small amount of methanol were placed in a mortar and mixed to obtain a mixed powder. The mixed powder was filled into an alumina crucible, dried at 70°C for 1 hour, and then fired in air at 800°C for 2 hours. Thereby, TaON modified with ZrO ₂ was obtained.

TaON modified with ZrO ₂ was filled into an alumina boat and placed inside a quartz tube. The quartz tube was connected to a tube furnace, and the ZrO ₂ modified TaON was heated in a 12 mL/min ammonia flow. The alumina boat was heated to 850°C at a rate of 10°C/min and held at that temperature for 15 hours. After holding for 15 hours, the product was naturally cooled to room temperature. The alumina boat was taken out of the furnace and the ZrO ₂ modified TaON powder was collected. The average particle size of the particles of ZrO ₂ modified TaON was in the range of 200-500 nm.

In addition, in this specification, the term " _ZrO2 /TaON" means TaON modified with _ZrO2 . ZrO ₂ suppresses the generation of reduced species of Ta (Ta ^IV ) during the nitriding process of Ta ₂ O ₅ . Ta ^IV serves as a recombination center for electrons and holes and reduces the activity of the photocatalyst. By modifying the surface of TaON particles with ZrO ₂ , the generation of Ta ^IV is suppressed, thereby obtaining TaON particles with higher photocatalytic activity.

《Synthesis of InHCF nanoparticles》
InCl _3.4H ₂ O (30 mmol) was dissolved in ultrapure water to obtain a 30 mL solution. K ₄ [Fe(CN) ₆ ].3H ₂ O (30 mmol) was dissolved in ultrapure water to obtain a 60 mL solution. These solutions were mixed and stirred vigorously for 5 minutes. The obtained precipitate was washed three times with ultrapure water, once with methanol, and centrifuged. Thereafter, the precipitate was dried in a vacuum dryer at 35°C. As a result, a powder of indium (III) hexacyanoferrate was obtained.

Powder X-ray diffraction and infrared spectroscopy measurements of the precipitate revealed that indium (III) hexacyanoferrate (K _x In[Fe(CN) ₆ ] _y ·nH ₂ O, 0≦x≦1, 0.75≦y ≦1, 0≦n). In addition, in this specification, the term "InHCF" means indium (III) hexacyanoferrate.

From observation using a transmission electron microscope, it was confirmed that InHCF was a nanoparticle having an average particle size of 80 nm or less.

《Supporting co-catalyst on ZrO ₂ /TaON》
Rh _x Cr _2-x O ₃ (reduction co-catalyst) and InHCF (oxidation co-catalyst) were sequentially supported on ZrO ₂ /TaON by the following method.

ZrO ₂ /TaON powder was dispersed in 150 mL of an aqueous methanol solution in which Na ₃ RhCl ₆ .nH ₂ O and K ₂ CrO ₄ were dissolved to obtain a dispersion. The methanol aqueous solution was prepared such that the concentration of methanol was 20 vol%, the amount of Rh was 1% by mass relative to TaON, and the amount of Cr was 1.5% by mass relative to TaON.

Before light irradiation, dissolved oxygen in the dispersion was removed by Ar bubbling at a flow rate of 200 mL/min. Ar bubbling was continued at a flow rate of 50 mL/min during light irradiation. The dispersion was irradiated with visible light (λ>400 nm) for 6 hours using a 300 W xenon lamp equipped with an L-42 cutoff filter and a CM-1 cold mirror. After light irradiation, the powder was collected by suction filtration, washed with ultrapure water, and vacuum dried at 35° C. overnight. Hereinafter, this sample will be referred to as "Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON".

Next, Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON was suspended in a small amount of water in which InHCF was dispersed to obtain a suspension. The amount of InHCF was adjusted so that the amount of Fe in InHCF was 10 mol% relative to TaON. The suspension was uniformly mixed by ultrasonication for 30 seconds. Thereafter, water was evaporated from the suspension to dry the sample. The sample was fired at 100° C. for 1 hour in an Ar flow at a flow rate of 20 mL/min. As a result, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON, which is a photocatalyst for hydrogen generation in Example 1 to be described later, was obtained.

《Synthesis of SrTiO ₃ :Rh》
A SrCO ₃ powder reagent, a TiO ₂ powder reagent, and a Rh ₂ O ₃ powder reagent were used as raw materials. The powder reagent was weighed so that the molar ratio of Sr:Ti:Rh=1.07:1.00:0.01. These powder reagents and a small amount of methanol were placed in a mortar and mixed to obtain a mixed powder. The mixed powder was filled into an alumina crucible and fired in air at 600°C for 2 hours, and then further fired in air at 1000°C for 10 hours. As a result, Rh-doped SrTiO ₃ was obtained. Note that in this specification, the term "SrTiO ₃ :Rh" means Rh-doped SrTiO ₃ .

《SrTiO ₃ :Supporting Ru co-catalyst on Rh》
The Ru promoter was sequentially supported on ZrO ₂ /TaON by the following method. A dispersion liquid was obtained by dispersing SrTiO ₃ :Rh powder in 150 mL of an aqueous methanol solution in which RuCl ₃ .nH ₂ O was dissolved. The methanol aqueous solution was prepared such that the methanol concentration was 20 vol % and the Ru amount was 0.7 mass % based on TaON.

Before light irradiation, dissolved oxygen in the dispersion was removed by Ar bubbling at a flow rate of 200 mL/min. Ar bubbling was continued at a flow rate of 50 mL/min during light irradiation. The dispersion was irradiated with visible light (λ>400 nm) for 6 hours using a 300 W xenon lamp equipped with an L-42 cutoff filter and a CM-1 cold mirror. After light irradiation, the powder was collected by suction filtration, washed with ultrapure water, and vacuum dried at 35° C. overnight. Hereinafter, this sample will be referred to as "Ru/SrTiO ₃ :Rh."

FIG. 7 is a diagram showing the ultraviolet-visible diffuse reflection spectra of various synthesized semiconductors (ZrO ₂ /TaON, SrTiO ₃ :Rh, WO ₃ ). In FIG. 7, the vertical axis indicates the value of the diffuse reflectance obtained by Kubelka-Munk transformation. In particular, in ZrO ₂ /TaON, the absorption edge wavelength is located on the longer wavelength side compared to other semiconductors.

Next, details of Examples 1-5 will be explained.

[Example 1]
First, BiVO ₄ was applied as a photocatalyst for oxygen generation onto one side of a quartz glass substrate (3.8 cm x 5 cm x thickness 1.0 mm). The coating area was 11.4 cm ² (3.8 cm x 3 cm). As precursor solutions, an acetic acid solution (1M) in which Bi(NO ₃ ) _3.5H ₂ O was dissolved and a methanol solution (0.15M) in which VO(acac) ₂ was dissolved were prepared. These two types of solutions were mixed at a molar ratio of Bi/V=1 to obtain a precursor solution. After dropping 100 μL of this precursor solution onto a quartz glass substrate, spin coating was performed at 750 rpm for 2 minutes. Thereafter, it was baked for 30 minutes on a hot plate at 500°C. The amount of BiVO ₄ deposited can be controlled by repeating the cycle of spin coating the precursor solution and baking on a hot plate. In Example 1, this cycle was repeated four times to deposit 2.5 mg of BiVO ₄ on the quartz glass substrate.

FIG. 8 shows the transmittance of the glass substrate on which BiVO ₄ of Example 1 was deposited (that is, the transmittance toward the hydrogen generation catalyst layer side in the photocatalyst panel). As shown in FIG. 8, the transmittance of the substrate 311 on which the oxygen generation photocatalyst layer 313 is formed is in the wavelength region of 500 nm or more (at least 500 to 650 nm) of the light incident on the oxygen generation photocatalyst layer 313. , preferably at least 40% or more. Thereby, the optical energy in the visible light range that has passed through the oxygen generation photocatalyst layer 313 and the substrate 311 can be used more effectively in the hydrogen generation photocatalyst layer 314.

Next, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON was applied as a photocatalyst for hydrogen generation on the surface opposite to the surface coated with BiVO ₄ , with a coating area of 11.4 cm ² (3.8 cm x 3 cm), and the amount of deposition was 6.6 mg, according to the following procedure. Using an agate mortar, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON was well dispersed in a small amount of ultrapure water. This dispersion liquid was dropped onto the surface opposite to the BiVO ₄ -coated surface of the glass substrate coated with BiVO ₄ and spread uniformly with a glass rod. After coating, it was dried at room temperature. Hereinafter, this photocatalyst panel will be referred to as "InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON|Glass|BiVO ₄ ".

The water-splitting activity of the InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON|Glass|BiVO ₄ photocatalyst panel of Example 1 was evaluated in an aqueous FeCl ₂ solution. The evaluation method was as described above. The Fe ²⁺ cation concentration in the FeCl ₂ aqueous solution was 2 mM, and the pH before reaction (before light irradiation) was 2.3.

[Example 2]
In Example 2, a photocatalyst panel was created using the same method as in Example 1. The water-splitting activity of the photocatalyst panel of Example 2 was evaluated by the same method as in Example 1, except that the pH before the reaction in the FeCl ₂ aqueous solution was changed to 2.5.

[Example 3]
In Example 3, a photocatalyst panel was created by the same method as in Example 2. The water-splitting activity of the photocatalyst panel of Example 3 was evaluated by the same method as in Example 1, except that the FeCl ₂ aqueous solution in the reaction solution was changed to an Fe(NO ₃ ) ₃ aqueous solution.

[Example 4]
First, BiVO ₄ was applied as a photocatalyst for oxygen generation onto a quartz glass substrate by the same method as in Example 1. The coating area was 11.4 cm ² (3.8 cm x 3 cm). The amount applied was 2.7 mg.

Next, Ru/SrTiO ₃ :Rh was applied as a photocatalyst for hydrogen generation to the surface opposite to the surface coated with BiVO ₄ , with a coating area of 11.4 cm ² (3.8 cm x 3 cm) and a deposited amount of 6.0 mg. It was applied according to the following procedure. Ru/SrTiO ₃ :Rh was well dispersed in a small amount of ultrapure water using an agate mortar. This dispersion liquid was dropped onto the surface opposite to the BiVO ₄ -coated surface of the glass substrate coated with BiVO ₄ and spread uniformly with a glass rod. After coating, it was dried at room temperature. Hereinafter, this photocatalyst panel will be referred to as "SrTiO ₃ |Glass|BiVO ₄ ".

The water splitting activity of the photocatalyst panel of Example 4, SrTiO ₃ |Glass|BiVO ₄ , was evaluated by the same method as in Example 1.

[Example 5]
First, apply Fe-H-Cs-WO ₃ as a photocatalyst for oxygen generation to a quartz glass substrate using the following procedure so that the coating area is 11.4 cm ² (3.8 cm x 3 cm) and the deposit amount is 6.0 mg. It was coated with. Ru/SrTiO ₃ :Rh was well dispersed in a small amount of ultrapure water using an agate mortar. This dispersion was dropped onto a quartz glass substrate and spread uniformly with a glass rod. After coating, it was dried at room temperature.

Next, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON was applied as a photocatalyst for hydrogen generation on the surface opposite to the surface coated with Fe-H-Cs-WO ₃ using the same method as in Example 1. It was applied by. The coating area was 11.4 cm ² (3.8 cm x 3 cm), and the amount deposited was 6.0 mg. Hereinafter, this photocatalyst panel will be referred to as "InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON|Glass|Fe-H-Cs-WO _3. "

The water splitting activity of InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON|Glass|Fe-H-Cs-WO ₃ , which is the photocatalyst panel of Example 5, was evaluated in an aqueous FeCl ₂ solution. The Fe ²⁺ cation concentration in the FeCl ₂ aqueous solution was 2 mM, and the pH before reaction (before light irradiation) was 2.5.

Next, details of Comparative Examples 1-8 and Reference Examples 1 and 2 will be explained.

[Comparative example 1]
In the same manner as in Example 1, BiVO ₄ was applied as a photocatalyst for oxygen generation onto one side of a glass substrate. The amount applied was 1.2 mg. Nothing was applied to the back side. Hereinafter, this photocatalyst panel will be referred to as "No | Glass | BiVO ₄ (1)." FIG. 9 shows the transmittance of the obtained free|glass|BiVO ₄ (1) (ie, the transmittance of the photocatalyst panel).

The oxygen production activity of the photocatalytic panel of Comparative Example 1, ie, the non-glass |BiVO ₄ (1), was evaluated in an aqueous FeCl ₃ solution. The evaluation method was as described above. The Fe ²⁺ cation concentration in the FeCl ₃ aqueous solution was 2 mM, and the pH before reaction (before light irradiation) was 2.5.

[Comparative example 2]
In the same manner as in Example 1, BiVO ₄ was applied as a photocatalyst for oxygen generation onto one side of a glass substrate. The amount applied was 2.4 mg. Nothing was applied to the back side. Hereinafter, this photocatalyst panel will be referred to as "No | Glass | BiVO ₄ (2)." FIG. 10 shows the transmittance of the obtained free|glass|BiVO ₄ (2) (ie, the transmittance of the photocatalyst panel).

The oxygen production activity of the photocatalytic panel of Comparative Example 2, ie, non-glass |BiVO ₄ (2), was evaluated in an aqueous FeCl ₃ solution. The evaluation method was as described above. The Fe ²⁺ cation concentration in the FeCl ₃ aqueous solution was 2 mM, and the pH before reaction (before light irradiation) was 2.5.

[Comparative example 3]
In the same manner as in Example 1, BiVO ₄ was applied as a photocatalyst for oxygen generation onto one side of a glass substrate. The amount applied was 3.6 mg. Nothing was applied to the back side. Hereinafter, this photocatalyst panel will be referred to as "No | Glass | BiVO ₄ (3)." FIG. 11 shows the transmittance of the obtained free|glass|BiVO ₄ (3) (ie, the transmittance of the photocatalyst panel).

The oxygen production activity of the photocatalytic panel of Comparative Example 3, ie, glass-free |BiVO ₄ (3), was evaluated in an aqueous FeCl ₃ solution. The evaluation method was as described above. The Fe ²⁺ cation concentration in the FeCl ₃ aqueous solution was 2 mM, and the pH before reaction (before light irradiation) was 2.5.

[Comparative example 4]
InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON was coated on one side of a glass substrate as a photocatalyst for hydrogen generation using the same method as in Example 1. The amount applied was 6.5 mg. Nothing was applied to the opposite side. Hereinafter, this photocatalyst panel will be referred to as "InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON|Glass|None."

The hydrogen production activity of the InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON|Glass|None photocatalyst panel of Comparative Example 4 was evaluated in an aqueous FeCl ₂ solution. The evaluation method was as described above. The Fe ²⁺ cation concentration in the FeCl ₂ aqueous solution was 2 mM, and the pH before reaction (before light irradiation) was 2.5.

[Comparative example 5]
As a photocatalyst panel of Comparative Example 5, a photocatalyst panel was created by the same method as the photocatalyst panel of Comparative Example 4. The water-splitting activity of the photocatalyst panel of Comparative Example 5 was evaluated by the same method as Comparative Example 4, except that the pH before the reaction in the FeCl ₂ aqueous solution was changed to 2.3.

[Comparative example 6]
As a photocatalyst panel of Comparative Example 6, a photocatalyst panel was created by the same method as the photocatalyst panel of Comparative Example 4. The water-splitting activity of the photocatalyst panel of Comparative Example 6 was evaluated by the same method as Comparative Example 4, except that the pH of the FeCl ₂ aqueous solution before the reaction was changed to 2.1.

[Comparative Example 7]
As a photocatalyst panel of Comparative Example 7, a photocatalyst panel was created by the same method as the photocatalyst panel of Example 2. The water-splitting activity of the photocatalyst panel of Comparative Example 7 was evaluated in the same manner as in Example 2, except that FeCl ₂ as a redox medium source was not added to the reaction solution for evaluating the water-splitting activity.

[Comparative example 8]
BiVO ₄ was deposited as a photocatalyst for oxygen generation on one side of an alumina substrate (3.8 cm x 5 cm x thickness 1.0 mm) using the same procedure as in Example 1. The coating area was 11.4 cm ² (3.8 cm x 3 cm). The amount applied was 2.9 mg. FIG. 12 shows the transmittance of an alumina substrate and a glass substrate not coated with a photocatalyst. As shown in FIG. 12, the alumina substrate has lower light transmittance than the glass substrate.

Next, in the same manner as in Example 1, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON was applied as a photocatalyst for hydrogen generation on the opposite side to the side on which BiVO ₄ was applied. The coating area was 15 cm ² (3 cm x 5 cm), and the amount deposited was 5.7 mg. Hereinafter, this photocatalyst panel will be referred to as "InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON|Alumina|BiVO ₄ ".

The hydrogen production activity of the InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON|alumina|BiVO ₄ photocatalyst panel of Comparative Example 8 was evaluated in an aqueous FeCl ₂ solution. The evaluation method was as described above. The Fe ²⁺ cation concentration in the FeCl ₂ aqueous solution was 2 mM, and the pH before reaction (before light irradiation) was 2.3.

[Reference example 1]
A photocatalyst panel of Reference Example 1 was created by the same method as the photocatalyst panel of Example 2. The water-splitting activity of the photocatalyst panel of Reference Example 1 was evaluated by the same method as in Example 2, except that the reaction solution of FeCl ₂ aqueous solution was changed to FeSO ₄ aqueous solution.

[Reference example 2]
A photocatalyst panel of Reference Example 2 was created by the same method as the photocatalyst panel of Example 2. The water-splitting activity of the photocatalyst panel of Reference Example 2 was evaluated in the same manner as in Example 2, except that the FeCl ₂ aqueous solution in the reaction solution was changed to an Fe(ClO ₄ ) ₂ aqueous solution.

The configurations and redox sources of the photocatalyst panels used in Examples 1-5, Comparative Examples 1-8, and Reference Examples 1 and 2 described above are summarized in Table 2. Table 2 shows a simplified description of the oxygen-generating photocatalyst and hydrogen-generating photocatalyst in each Example, each Comparative Example, and each Reference Example, and the details thereof are as described above.

Table 3 summarizes the results of the water splitting reaction (hydrogen and oxygen production rate) and the initial pH of the reaction solution in Examples 1-5, Comparative Examples 1-8, and Reference Examples 1 and 2 described above. In Table 3, the gas production rates (hydrogen production rate, oxygen production rate) of Examples 1-5 show that the ratio of the hydrogen production rate to the oxygen production rate is approximately 2:1 (that is, the gas production rate corresponds to the complete decomposition of water). stoichiometric ratio). Further, the gas generation rate in Comparative Examples 1-8 and Reference Examples 1 and 2 is based on the amount of gas generated from the start of light irradiation to the time point when one hour has passed.

(1) Necessity of a photocatalyst for hydrogen production and a photocatalyst for oxygen production As shown in Table 3, simultaneous production of hydrogen and oxygen was confirmed in Example 1 (photocatalyst panel containing a photocatalyst for hydrogen production and a photocatalyst for oxygen production). It was done. FIG. 13 is a graph showing changes over time in the water splitting reaction in Example 1. FIG. 13 shows changes over time in the hydrogen production rate, oxygen production rate, and their ratio (oxygen production rate/hydrogen production rate).

In Example 1, the amount of hydrogen produced after 6.0 hr of light irradiation time exceeded the stoichiometric amount (40 μmol). Further, in Example 1, the ratio of the oxygen production rate to the hydrogen production rate finally approached 0.5 (stoichiometric ratio corresponding to complete decomposition of water). These results indicate that the hydrogen generation photocatalyst in the upper hydrogen generation photocatalyst layer 314 generates hydrogen while oxidizing Fe ²⁺ to generate Fe ³⁺ , and the oxygen generation photocatalyst in the lower oxygen generation photocatalyst layer 313 generates hydrogen. This shows that the photocatalyst generated oxygen while reducing Fe ³⁺ to generate Fe ²⁺ . That is, in Example 1, it was shown that photocatalytic water splitting progressed by the Z scheme mechanism.

In Comparative Example 1-3 (photocatalyst panel not containing a photocatalyst for hydrogen generation), no hydrogen generation could be confirmed. Furthermore, in Comparative Example 4-6 (a photocatalyst panel not containing an oxygen-generating photocatalyst), no oxygen generation could be confirmed. This shows that both photocatalysts are necessary for Z-scheme water splitting.

₍ ² ) _Need _for _a _redox medium FIG. The components of the graph in FIG. 14 are the same as those in the graph in FIG. 13.

Comparing the water splitting activities in the presence and absence of redox medium (Fe ²⁺ ), it was found that clear water was observed in the presence of Fe ²⁺ (Example 1) than in the absence of Fe ²⁺ (Comparative Example 7). Degradation activity was confirmed. From this, the redox medium (Fe ³⁺ /Fe ²⁺ ) present in the reaction solution is responsible for electron transfer between the upper (rear) photocatalyst for hydrogen production and the lower (front) photocatalyst for oxygen production, and the Z-scheme It can be seen that this is essential for the mechanism to proceed with water splitting. In addition, it was found that water splitting, which occurs when at least one of the hydrogen generation photocatalyst and the oxygen generation photocatalyst peels off from the substrate and transfers electrons when they come into direct contact with each other, hardly progresses here. .

(3) Influence of substrate material FIG. 15 is a graph showing changes over time in the water splitting reaction in Comparative Example 8 (photocatalyst panel using an alumina substrate). It was confirmed that the water-splitting activity when using the alumina substrate was very low compared to the water-splitting activity when using the glass substrate (Example 1). As shown in FIG. 12, the alumina substrate has lower light transmittance than the glass substrate (less than 40% in the visible light region). From this, when an alumina substrate is used, almost no light enters the upper photocatalytic layer 314 for hydrogen generation from the lower photocatalytic layer 313 for oxygen generation through the substrate 311 (alumina substrate), and the photocatalyst layer 314 for hydrogen generation It is thought that the light could not be excited sufficiently. Therefore, the substrate material is required to have sufficient light transmittance so that the photocatalyst can be photoexcited and exhibit its photocatalytic activity.

(4) Influence of the type of photocatalyst FIG. 16 shows the change over time in the water splitting reaction of Example 4 (a photocatalyst panel containing Ru/SrTiO ₃ :Rh as a photocatalyst for hydrogen production). In Example 4, the amount of hydrogen produced after 30 hours of light irradiation exceeded the stoichiometric amount (40 μmol). Further, in Example 4, the ratio of the oxygen production rate to the hydrogen production rate finally approached 0.5. These results showed that even when Ru/SrTiO ₃ :Rh was used as a photocatalyst for hydrogen generation, photocatalytic water splitting proceeded by the Z-scheme mechanism.

FIG. 17 shows the change over time in the water splitting reaction of Example 5 (a photocatalyst panel containing Fe-H-Cs-WO ₃ as a photocatalyst for oxygen generation). In Example 5, the amount of hydrogen produced after 6.5 hours of light irradiation exceeded the stoichiometric amount (40 μmol). Further, in Example 5, the ratio of the oxygen production rate to the hydrogen production rate finally approached 0.5. These results showed that even when Fe-H-Cs-WO ₃ was used as a photocatalyst for oxygen production, photocatalytic water splitting proceeded by the Z-scheme mechanism.

(5) Influence of the type of counter anion in the redox source FIG. 18 is a graph showing changes over time in the water splitting reaction of Example 3 (Fe(NO ₃ ) ₃ aqueous solution). FIG. 19 is a graph showing changes over time in the water decomposition reaction of Reference Example 1 (FeSO ₄ aqueous solution). FIG. 20 is a graph showing changes over time in the water decomposition reaction of Reference Example 2 (Fe(ClO ₄ ) ₂ aqueous solution).

As shown in FIG. 8 and FIGS ^. 18 to 20, in Examples 1 and ³ and Reference Examples 1 and 2, the types of anions ( ^Cl− , NO ₃₋ , SO ₄ ²⁻ , ClO ₄₋ ) in the redox source The water-splitting activity varied accordingly. Hydrogen and oxygen were generated simultaneously and continuously only when the counter anion was Cl- ^or ^NO _3- (Examples 1 and 3). From these results, it is considered that photocatalytic activity (water splitting activity) can be optimized by optimizing not only the photocatalytic layer but also the type of counter anion in the redox source.

(6) Influence of pH of reaction solution When the pH at the initial stage of the reaction in Example 2 was 2.5, brown precipitate by-products were observed on the photocatalyst panel 307 after the reaction. This is probably because the Fe source added as a redox medium precipitated as Fe(OH) ₃ etc. due to the local pH increase.

As in Comparative Examples 5 and 6, the pH at the initial stage of the reaction was set to 2.3 or 2.1, and the hydrogen production reaction was performed using InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON | glass | As a result, it showed hydrogen production activity without producing any brown precipitate. Furthermore, as in Example 1, when a water splitting reaction is carried out using InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON | glass | BiVO ₄ with the initial pH of the reaction set to 2.3, a brown precipitate is formed. The substance showed water-splitting activity without being confirmed. These results showed that from a practical standpoint, appropriate pH control is important in order to proceed with Z-scheme water decomposition without producing by-products.

Although the description of the embodiments including specific examples is completed above, the present invention is not limited to the above-described embodiments and modifications, and can be implemented in a wide range of modifications. All of the components of the water splitting apparatus using a photocatalyst and the water splitting system equipped with the same shown in the embodiments (including each example) described above are not necessarily essential, and at least those skilled in the art will understand the present invention. As long as it does not deviate from the above range, it is possible to make a selection as appropriate.

1: Water splitting system 3: Water splitting device 5: First circulation line 5A: Oxygen discharge line 5B: Water discharge line 5C: First water supply line 5D: Oxygen separation line 6: Second circulation line 6A: Hydrogen discharge line 6B : Second water discharge line 6C : Second water supply line 6D : Hydrogen separation line 7 : Photocatalyst panel 9 : Reaction vessel 9A : Front wall (light incidence wall)
9B: Rear wall 9C: Upper wall 9D: Lower wall 11: Substrate 11A: Front (first surface)
11B: Rear surface (second surface)
13: Oxygen generation photocatalyst layer (first photocatalyst layer)
14: Hydrogen generation photocatalyst layer (second photocatalyst layer)
21: Oxygen generation tank 22: Hydrogen generation tank 25: First gas-liquid separator 26: First water storage tank 27: First circulation pump 29: Upper opening 31: Lower opening 35: Second gas-liquid separator 36: Second Water storage tank 37: Second circulation pump 39: Upper opening 41: Lower opening 45: Light collecting plate 51: Downward illumination type reaction cell 53: Spacer 105: First circulation line 106: Second circulation line 205D: Oxygen separation line 206C: Water Supply line 206D: Hydrogen separation line 207: Photocatalyst panel 207A: Lower edge 209: Reaction container 209D: Lower wall 226: Water storage tank 227: Water supply pump 231: Lower opening 303: Evaluation device 307: Photocatalyst panel 311: Substrate 311A: Lower surface 311B: Top surface 313: Photocatalytic layer for oxygen generation 314: Photocatalytic layer for hydrogen generation

Claims

A water splitting device using a photocatalyst that uses light energy,
photocatalyst panel,
a reaction vessel that accommodates the photocatalyst panel and the reaction solution and performs a water decomposition reaction in the reaction solution;
Equipped with
The photocatalyst panel is
a substrate having optical transparency;
a first photocatalyst layer formed to overlap the first surface of the substrate;
a second photocatalyst layer formed to overlap a second surface of the substrate that is paired with the first surface;
including;
One of the first photocatalyst layer and the second photocatalyst layer is an oxygen generation photocatalyst layer containing a first semiconductor, and the other is a hydrogen generation photocatalyst layer containing a second semiconductor,
The reaction vessel is
The interior space is partitioned by the photocatalyst panel,
an oxygen generation tank formed on the side of the oxygen generation photocatalyst layer;
a hydrogen generation tank formed on the side of the hydrogen generation photocatalyst layer;
A water splitting device, comprising: a light entrance wall that transmits light incident toward the first photocatalyst layer in the photocatalyst panel.
The water splitting device according to claim 1, wherein the substrate is non-conductive.
The water splitting device according to claim 1 or 2, wherein the light absorption spectrum of the second semiconductor is located on a longer wavelength side compared to the light absorption spectrum of the first semiconductor.
The first semiconductor includes one or a combination of two or more selected from metal oxides, metal oxynitrides, nitrides, metal oxysulfides, metal oxyhalides, and metal organic structures,
The second semiconductor is a metal oxynitride, a metal nitride, a metal oxysulfide, a metal sulfide, a metal oxyhalide, a metal halide, an organometallic halide, a metal oxide, a metal phosphide, silicon, an organic The water splitting device according to any one of claims 1 to 3, comprising one or a combination of two or more selected from semiconductors, metal-organic structures, and covalent organic structures.
A portion of the first photocatalyst layer and the substrate in the photocatalyst panel has a transmittance of 40% or more in the visible light region of at least 500 nm or more of the light incident on the first photocatalyst layer. The water splitting apparatus according to any one of claims 1 to 4.
The reaction solution includes a redox medium,
Claim 1, wherein the redox medium contains one or a combination of two or more selected from iron ion pairs, iodine-containing ion pairs, vanadium-containing ion pairs, complex ion pairs, and polyoxometalate ion pairs. The water splitting apparatus according to claim 5.
Any one of claims 1 to 6, wherein a communication portion is formed between the reaction container and the photocatalyst panel to allow the reaction solution of the oxygen generation tank and the hydrogen generation tank to flow. The water splitting device according to item 1.
The water decomposition apparatus according to any one of claims 1 to 7, wherein light is incident on the reaction vessel only from the light entrance wall.
A water splitting system comprising the water splitting device according to any one of claims 1 to 8, and a water storage tank that stores water supplied to the water splitting device.
a first circulation line for circulating a part of the reaction solution discharged from the oxygen generation tank together with oxygen to the hydrogen generation tank; and a first circulation line for circulating a part of the reaction solution discharged together with hydrogen from the hydrogen generation tank; a second circulation line that circulates to the oxygen generation tank;
The water splitting system according to claim 9, comprising:
An oxygen separation line for transporting the oxygen separated from the reaction solution is connected to the first circulation line,
The water splitting system according to claim 10, wherein a hydrogen separation line that transports the hydrogen separated from the reaction solution is connected to the second circulation line.
The water splitting system according to claim 10 or 11, wherein the water storage tank is provided in at least one of the first circulation line and the second circulation line.
The water splitting system according to claim 12, wherein a gas-liquid separator is provided upstream of the water storage tank.