Water splitting method using light

Abstract

To provide a method for efficiently decomposing water into hydrogen and oxygen even under visible light irradiation by using a hydrogen-generating photocatalyst. The method for splitting water by light of the present invention is a method in which light is irradiated onto an aqueous solution containing one type of photocatalyst and a compound having oxidation-reduction ability dissolved therein.

Description

[Technical Field]

[0001] The present invention relates to a photocatalyst and a method for splitting water using light. [Background technology]

[0002] Building an inexpensive, high-performance photoenergy conversion system that utilizes renewable solar energy and abundant water is a particularly important challenge from the perspectives of carbon neutrality and environmental conservation. Hydrogen obtained by splitting water using solar energy (hydrogen obtained through solar hydrogen production) is a carbon-free energy carrier that can be stored and transported. Furthermore, hydrogen itself can be used as fuel for fuel cells, and it can also be converted into liquid fuels such as ammonia and methane, as well as various carbon resources. For this reason, active development of semiconductor photocatalysts suitable for this technology is underway around the world.

[0003] For this "solar-powered hydrogen production by water splitting using photocatalysts" to be put into practical use, further improvements in the solar energy conversion efficiency are essential, with the key being the efficient use of visible light, which is abundant in sunlight. For example, even if a photocatalyst absorbs all photons in the ultraviolet light range (200-400 nm) and splits water with a 100% quantum yield (the proportion of absorbed photons that contributed to the reaction), the conversion efficiency from solar energy to hydrogen energy (solar energy conversion efficiency) is only about 2% at most. In contrast, if the range of use is expanded to 600 nm in the visible light range, the number of absorbable photons increases significantly, raising the maximum conversion efficiency to 16%. Therefore, even if the average quantum yield is 30%, a solar energy conversion efficiency of about 5% can be expected.

[0004] For these reasons, the development of a photocatalyst that can efficiently absorb visible light and split water into hydrogen and oxygen has become an extremely important challenge. However, there are not many reported examples of materials with this potential, and there are still very limited examples of actually using these materials to steadily split water into hydrogen and oxygen under visible light irradiation.

[0005] Here, we will explain the relationship between the band levels of various photocatalysts and the potential for water splitting. As shown in Figure 1, when a semiconductor absorbs a photon with energy equal to or greater than the band gap, an excited electron is generated in the conduction band and a hole is generated in the valence band (Figure 1(a)). In order to proceed with water splitting, a semiconductor photocatalyst must satisfy three conditions: the potential at the bottom of the conduction band is more negative than the reduction potential of water, the potential at the top of the valence band is more positive than the oxidation potential of water, and the semiconductor is stable under light irradiation.

[0006] Metal oxide semiconductors, which have been widely used as photocatalysts to date, are highly stable, but the potential at the top of the valence band formed by the 2p orbital of oxygen (O) is approximately 3 V more positive than the reduction potential of water. As a result, most metal oxide semiconductors with a band gap smaller than 3.0 eV and capable of absorbing visible light do not have the ability to generate hydrogen, because the potential at the bottom of their conduction band is more positive than the reduction potential of water (Figure 1(b)).

[0007] On the other hand, it is known that doping a metal oxide semiconductor with an appropriate transition metal ion, which has a band edge potential capable of decomposing water into hydrogen and oxygen by ultraviolet light, creates an impurity level in the forbidden band, making it possible to absorb visible light (Non-Patent Document 1) (Figure 1(c)). However, although there are many metal oxide semiconductors that respond to ultraviolet light, the metal oxide semiconductors that function as visible light-responsive photocatalysts when doped with transition metal ions are limited to SrTiO3 and TiO2. It is also known that transition metal ion-doped semiconductor photocatalysts sometimes have extremely low oxygen generation capacity compared to their high hydrogen generation capacity.

[0008] In addition, (oxy)nitride, (oxy)sulfide, and (oxy)halide semiconductors, which contain nitrogen (N), sulfur (S), and halogens (Cl, Br, I) as constituent elements in addition to oxygen, have the following properties: 3- ,S 2- ,X - The electronegativity of the anion is O 2-The electronegativity of the anion is lower than that of the anion, and the potential at the top of the valence band is more negative than that of the corresponding metal oxide (Figure 1 (d)). Therefore, even if the band gap is smaller than 3.0 eV and the material can absorb visible light, the potential at the bottom of the conduction band is more negative than the reduction potential of water, and many of them have the ability to generate hydrogen. However, when a semiconductor with an anion other than oxygen is used as a photocatalyst for water splitting, in most cases the holes generated by light absorption are converted into N, not water. 3- ,S 2- ,X - This oxidizes the anions and easily causes them to lose their photocatalytic activity. For this reason, it is known that metal oxide semiconductors incorporating anions other than oxygen are unable to generate O2 (oxygen).

[0009] As mentioned above, in metal oxide semiconductors, O 2- Because the potential at the top of the valence band formed by the anion is too positive, it is difficult to achieve both visible light absorption and water reduction. On the other hand, introducing transition metal ions or anions other than oxygen provides visible light absorption, but the problem is that the water oxidation ability is lacking. As such, demonstrating water splitting under visible light using a single semiconductor as a photocatalyst is difficult, and until very recently only a few examples have been demonstrated (Non-Patent Document 2). Therefore, developing a method to impart water splitting functionality to candidate photocatalyst materials that have the band gap and band energy necessary to promote water splitting under visible light but have not yet been demonstrated to split water has become an extremely important challenge. Furthermore, because hydrogen is a flammable gas, from a safety standpoint, development is underway to develop methods for separating hydrogen gas from the mixture of hydrogen and oxygen produced by decomposing water, as well as methods for separating and producing hydrogen and oxygen by using different photocatalysts for hydrogen and oxygen production. [Prior art documents] [Non-patent literature]

[0010] [Non-Patent Document 1] J. Phys. Chem. B 2004, 108,8992-8995. [Non-patent document 2] Chem. Commun. 2014, 50, 2543-2546, J. Am. Chem. Soc. 2005, 127, 8286-8287. Summary of the Invention [Problem to be solved by the invention]

[0011] As mentioned above, although it is believed that there are many metal oxides, metal sulfides, metal nitrides, and other compounds with band gaps and band energies that can be used as photocatalytic materials to promote water splitting using visible light, a general-purpose method for utilizing this potential has not yet been found. As a result, although either hydrogen or oxygen can be produced, simultaneous production of hydrogen and oxygen through water splitting is difficult or the production efficiency is low. Therefore, the number of materials that can be used as photocatalysts for water splitting is extremely limited, hindering the demonstration of a highly efficient water splitting method using visible light.

[0012] The present invention has been made in view of the above points, and aims to provide a method for efficiently decomposing water into hydrogen and oxygen even under visible light irradiation, preferably using a hydrogen-generating photocatalyst. [Means for solving the problem]

[0013] As a result of extensive research aimed at achieving the above-mentioned object, the present inventors have devised a method in which the ability to oxidize water can be increased by using a compound with photocatalyst-induced oxidation-reduction ability, such as a compound with oxidation-reduction ability that is thought to capture holes generated by light irradiation (hereinafter also referred to as a "hole reservoir"), and the oxidation reaction of water can be accelerated, and this can be used as a starting point to efficiently decompose water into hydrogen and oxygen. The inventors then discovered that the above-mentioned problems can be solved by using a compound with oxidation-reduction ability, preferably a polyoxometalate represented by a specific composition formula, in combination with a photocatalyst, and have thus completed the present invention.

[0014] That is, the present invention relates to a method for splitting water using a photocatalyst in which the following compound having oxidation-reduction ability (for example, polyoxometalate) is used in combination as a hole reservoir.

[0015] (1) A method for splitting water by light, in which light is irradiated onto an aqueous solution containing one type of photocatalyst and a compound having oxidation-reduction ability dissolved therein. (2) The method for splitting water by light according to (1), wherein the compound having oxidation-reduction ability is a polyoxometalate. (3) The method for splitting water by light according to (2), wherein the polyoxometalate is a compound that oxidizes water, and the aqueous solution contains the polyoxometalate and a compound that oxidizes water other than the polyoxometalate. (4) The method for splitting water by light according to (2), wherein the polyoxometalate contains any one of Si, W, and Co. (5) The method for splitting water by light according to (2), which comprises a step of oxidizing the polyoxometalate with visible light. (6) The method for splitting water by light according to (2), wherein the photocatalyst generates hydrogen and the polyoxometalate or a water oxidation catalyst other than the polyoxometalate generates oxygen in separate steps. [Effects of the Invention]

[0016] According to the present invention, a method can be provided in which water can be efficiently decomposed into hydrogen and oxygen even under visible light irradiation using a hydrogen-generating photocatalyst.

[0017] Furthermore, a water splitting method using a compound with redox activity, such as the polyoxometalate of the present invention, can completely split water, preferably using visible light, even with photocatalysts that have visible light absorption ability and a band-edge potential capable of splitting water into hydrogen and oxygen, but have not previously been demonstrated to completely split water using the photocatalyst alone. In other words, the present invention significantly expands the range of photocatalysts that can be used for water splitting, and therefore provides a water splitting method using a wide variety of photocatalysts, preferably using visible light. Despite having physical properties that satisfy the above conditions, the number of photocatalysts that have not yet been able to completely split water using conventional technology is far greater than the number of photocatalysts that have already been demonstrated to split water. Therefore, the method provided by the present invention is expected to significantly expand the range of semiconductor photocatalysts that can be used for water splitting. [Brief explanation of the drawings]

[0018] [Figure 1] FIG. 1 is a diagram showing the relationship between the band level and the potential for water splitting for various semiconductor photocatalysts. [Figure 2] 2A and 2B are configuration diagrams of a water-splitting system according to a modified example and another modified example, respectively. [Figure 3] Fig. 3A is a graph showing the amount of gas produced in the water-splitting reaction of Example 2 and Comparative Example 2. Fig. 3B is a graph showing the absorption spectra of the reaction solutions after the water-splitting reaction of Example 2 and Comparative Example 2. [Figure 4] 4A is a graph showing the amount of oxygen produced in the thermal oxygen production reaction using the hole reservoirs of Reference Examples 5 and 6 and Examples 7 and 8. FIG. 4B and FIG. 4C are graphs showing the absorption spectra of the reaction solutions after the thermal oxygen production reaction using the hole reservoirs of Reference Examples 5 and 6 and Examples 7 and 8. [Figure 5] 5A is a graph showing the amount of oxygen produced in a thermal oxygen production reaction using the hole reservoirs and oxygen production catalysts of Examples 9 and 10 and Comparative Example 7. FIG. 5B is a graph showing the absorption spectrum of the solution after the thermal oxygen production reaction using the hole reservoirs and oxygen production catalysts of Examples 9 and 10 and Comparative Example 7. [Figure 6]Fig. 6A is a graph showing the results of the photocatalytic reactions in Example 11 and Reference Example 8, and a graph showing the absorption spectrum of the reaction solution. Fig. 6B is a graph showing the amount of oxygen generated in Example 11. Fig. 6C is an absorption spectrum of the reaction solution after the photocatalytic reaction in Example 11 and after thermal oxygen generation. DETAILED DESCRIPTION OF THE INVENTION

[0019] The present inventors have concluded that the reason why complete water decomposition does not proceed despite the photocatalyst having visible light absorption ability and a band edge potential sufficient to decompose water into hydrogen and oxygen is that the water oxidation reaction by holes, which involves a four-electron, four-proton transfer and the formation of an oxygen-oxygen bond, does not proceed easily on the photocatalyst surface. As a result of extensive research, they have developed a new method using a compound with redox ability, preferably a polyoxometalate. This method temporarily reserves the oxidizing power of holes in a redox medium with an appropriate redox potential (described below), and then releases the high oxidizing power of the reserved holes by combining it with a thermal catalytic reaction or the like to oxidize water, thereby developing a new method that can efficiently achieve water decomposition even in an embodiment using visible light, for example.

[0020] The water splitting method of the present invention, which involves irradiating light onto an aqueous solution containing one type of photocatalyst and a compound with redox ability dissolved therein (hereinafter also referred to as the "water splitting method of the present invention"), is characterized by the fact that, for example, holes generated by the hydrogen-generating photocatalyst are temporarily accumulated in a compound with redox ability such as a polyoxometalate, and the resulting positive charges have the potential to oxidize water. The water splitting method of the present invention will be described in detail below.

[0021] (Water splitting method using light) A method for splitting water by light according to one embodiment of the present invention is a method in which light is irradiated onto an aqueous solution containing one type of photocatalytic material and a compound having oxidation-reduction ability dissolved therein.

[0022] The water splitting method of the present invention includes a photocatalyst and a compound having redox ability (hole reservoir). The photocatalyst is preferably in contact with water, and the compound having redox ability (hole reservoir) is preferably a compound that dissolves in the water. That is, the photocatalyst and the hole reservoir are positioned to be in direct contact or in contact via water. The water splitting method of the present invention may also enable water splitting using visible light.

[0023] (Compounds with oxidation-reduction ability) The compound having redox ability (hole reservoir) in the present invention is a compound that exhibits redox ability by changing its oxidation number electrochemically, photochemically, photocatalytically, or catalytically, for example. Preferably, it is a compound that exhibits redox ability when irradiated with light including visible light in the presence of a photocatalyst or when heated.

[0024] The hole reservoir is preferably a compound that can be oxidized by holes generated by the photocatalyst in the presence of light. It is even more preferable that the hole reservoir is a redox medium that can be oxidized by holes generated by the photocatalyst in the presence of visible light, more preferably sunlight on Earth, and accumulate oxidizing power. Furthermore, it is preferable that the accumulated oxidizing power can catalytically oxidize water. Examples of such compounds include polyoxometalates. In the examples of the present invention, an example is disclosed in which the polyoxometalate is a compound that functions as a hole reservoir.

[0025] Examples of polyoxometalates include compounds containing silicon, tungsten, and cobalt, and compounds containing silicon, molybdenum, and cobalt. Examples of the structure of such compounds include cluster molecules formed by the condensation of multiple tungstic acid or molybdic acid molecules. Polyoxometalates have structures represented by the following composition formulas (1) to (3), for example:

[0026] [ka]

[0027] In formulas (1) to (3), X is Be 2+ , B 3+ , Al 3+ , Si 4+ , P 5+ , S 6+ , V 5+ , Co 2+ , Cu 2+ , Zn 2+ , Ga 3+ , Ge 4+ or As 5+ and M' is W 6+ or Mo 6+ and M” is V 4+ , Cr 3+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , W 6+ , Mo 6+ or Ru 2+ where n=0,1,2,3.

[0028] When at least one of the polyoxometalates represented by the above composition formula is dissolved as a hole reservoir in an aqueous solution, and preferably one type of hydrogen-generating photocatalyst is added to the solution, and the solution is irradiated with visible light, water decomposition tends to proceed efficiently.

[0029] The redox potential of one of the metals contained in the polyoxometalate is preferably more negative than the potential at the top of the valence band of the hydrogen-generating photocatalyst and more positive than the oxidation potential of water. When these requirements are met, the polyoxometalate can be considered to function as a hole reservoir, smoothly accepting holes and retaining the oxidizing power to oxidize water.

[0030] The redox potential of polyoxometalates can be controlled by the constituent elements. For example, when M' in the composition formula (1) is W, 6+ , M” is Co 2+ , polyoxometalates [XM' 11 Co(HO)O39 ] m- Heterocation X to B 3+ , Si 4+ , P 5+ Using Co 3+ / Co 2+ The redox potentials of these are approximately +0.70 V, approximately +1.20 V, and approximately +1.40 V (vs. SHE (standard hydrogen electrode)), respectively.

[0031] Among the above polyoxometalates, for example, X is Si 4+ , M' is W 6+ , M” is Co 2+ A compound having the structure of 11 In an aqueous solution containing this compound, the Co in the polyoxometalate 3+ / Co 2+ The redox potential of Co is approximately +1.20V (vs. SHE). 2+ The species captures the photocatalytic holes and generates Co 3+ It is believed that the species has the potential to oxidize water and thus functions as a hole reservoir. Of course, the composition of the polyoxometalate that serves as the hole reservoir is not limited to this.

[0032] The method for producing polyoxometalates is not particularly limited. Each polyoxometalate can be synthesized by a known method. Compounds having redox ability, including polyoxometalates, can be used singly or in combination. The compounds having redox ability of the present invention are thought to accumulate the above-mentioned holes and have high oxidizing ability, as is suggested by the structural change (increase in oxidation number) of the compounds having redox ability during the water decomposition reaction, as explained in the Examples. The structural change can be identified by methods such as ultraviolet-visible absorption spectroscopy.

[0033] (Photocatalyst) The photocatalyst in one embodiment of the present invention can be a known photocatalyst. The photocatalyst is preferably a semiconductor photocatalyst known as a hydrogen-generating photocatalyst. Examples of the photocatalyst include metal oxides, metal sulfides, and metal nitrides. From the viewpoint of compound stability, metal oxides are preferred, and compounds such as SrTiO3, TiO2, and SnNbO6, which have been reported to completely decompose water, are more preferred. Of these, SrTiO3 is particularly preferred. Specific examples of hydrogen-generating catalysts will be described later. Among these, hydrogen-generating photocatalysts are preferred. Furthermore, the hydrogen-generating photocatalyst is not limited to the one used in the examples, as long as it has a band gap of 3.0 eV or less and a band-edge potential capable of generating hydrogen and oxygen. The inventors believe the reason why the method of the present invention can efficiently perform water splitting is as follows. Below, a preferred hydrogen-generating photocatalyst will be described as a representative example.

[0034] The hydrogen-generating photocatalyst oxidizes the hole reservoir with holes and reduces protons with excited electrons to generate hydrogen. The hole reservoir stores the holes from the hydrogen-generating photocatalyst as positive charges and uses the positive charges to oxidize water. However, the photocatalyst may also be equipped with a water oxidation catalyst separate from the hole reservoir.

[0035] The hydrogen generating photocatalyst is usually in the form of particles. The shape of the particles is not particularly limited. Examples of the particle shape include spherical, oval, plate, fibrous, and scale shapes.

[0036] The hydrogen-generating photocatalyst is not particularly limited as long as it can oxidize the hole reservoir and generate hydrogen. Examples of hydrogen-generating photocatalysts include SrTiO3:Rh, TaON, BaTaO2N, Ta3N5, Sm2Ti2S2O5, Bi4NbO8Cl, and Ba2Bi3Nb2O. 11 Br 1-x I x (0≦x≦1), g-C3N4, CaTaO2N, SrTaO2N, CdS, etc. However, the hydrogen generation photocatalyst is not limited to the above compounds.

[0037] The method for producing the hydrogen generating photocatalyst is not particularly limited.

[0038] The hydrogen-generating photocatalyst may contain a reduction promoter, which is preferably supported on the surface of the hydrogen-generating photocatalyst, and which generates hydrogen in water when the hydrogen-generating photocatalyst is irradiated with visible light.

[0039] The reduction promoter may contain at least one noble metal. The reduction promoter may contain Cr in addition to the noble metal. The noble metal used as the reduction promoter is preferably Pt, Ru, or Rh. It is preferable that the reduction promoter selectively and smoothly reduces protons or water and does not re-reduce the polyoxometalate that has reserved holes.

[0040] The hydrogen generating photocatalyst may contain an oxidation promoter supported on the surface of the hydrogen generating photocatalyst, and smoothly oxidizes the polyoxometalate using holes generated in the hydrogen generating photocatalyst when the hydrogen generating photocatalyst is irradiated with visible light.

[0041] The method for supporting the reduction promoter is not particularly limited.

[0042] The amount (total amount) of the reduction promoter supported is not particularly limited. The supported amount is not particularly limited, but is preferably 0.05 mass % to 30 mass % based on the hydrogen generating photocatalyst. A more preferable lower limit is 0.1 mass %, and even more preferably 0.3 mass %. A more preferable upper limit is 20 mass %, and even more preferably 15 mass %. Note that Example 2 and the like of the present application disclose an example in which the promoter supported amount is 0.7 mass % as a preferred embodiment when a Ru promoter is used. Note that the above "supported amount" refers to the mass occupied by the metal element in the promoter relative to the hydrogen generating photocatalyst.

[0043] In the water splitting method of the present invention, the water in which the hydrogen-generating photocatalyst is suspended and the polyoxometalate is dissolved may or may not contain an electrolyte.

[0044] The pH range of the water is not particularly limited as long as the compound having oxidation-reduction ability is stable. Preferably, the pH range is 1.0 to 10. The water may or may not contain an electrolyte, but preferably contains an electrolyte. When an electrolyte is contained, the pH of the water is more preferably in the acidic range. In this case, the preferred pH range of the water is 1.0 to 6.8. The more preferred lower limit of the pH of the water is 1.5, even more preferably 2.0, and particularly preferably 2.5. The more preferred upper limit of the pH of the water is 6.5, even more preferably 6.3, and particularly preferably 6.2. In the present invention, in order for the polyoxometalate to stably exert its effect, there may be a suitable pH range for the water depending on the type of polyoxometalate used. For such pH adjustment, a method using a buffer solution such as a phosphate buffer is a preferred example. For example, in the case of SiW 11 When Co is used, the pH of water tends to be stable in the pH range of 3 to 6, resulting in a stable oxidation-reduction ability.

[0045] The concentration of the polyoxometalate is not particularly limited. For example, the SiW 11 Co II is operated as a hole reservoir at 0.5 mmol / L to 2 mmol / L.

[0046] A polyoxometalate with accumulated holes may oxidize water if its positive charges have the potential to oxidize water. The polyoxometalate itself may or may not have the ability to oxidize water. For example, if the polyoxometalate itself does not (apparently) have the ability to oxidize water, it may be used in combination with a water oxidation catalyst other than the polyoxometalate for the purpose of oxidizing water. Furthermore, changing the pH of the water in the system may sometimes promote water oxidation. As a water oxidation catalyst other than the polyoxometalate, oxides of elements in Groups 7 to 9 of the periodic table are preferred, and oxides of elements in Group 8 of the periodic table are more preferred. Among oxides of elements in Groups 7 to 9 of the periodic table, iridium oxide is preferred. In the present invention, even if a polyoxometalate does not apparently oxidize water, a compound that oxidizes water when appropriate conditions are selected is considered a "compound with redox ability" in the present invention.

[0047] Polyoxometalates with accumulated holes can thermally oxidize water, whether or not a water oxidation catalyst different from the polyoxometalate is used in combination with the polyoxometalate, and can also oxidize water through further oxidation by the holes in the photocatalyst. A method including a step of promoting oxidation by changing the reaction temperature can also be employed. Furthermore, in the water splitting method of the present invention using a single photocatalyst, for example, by using a compound such as the iridium oxide in combination with the polyoxometalate or by changing the reaction temperature, it is possible to generate oxygen by creating a separate oxygen generating field separate from the hydrogen generating step.

[0048] The temperature at which polyoxometalates oxidize water is not particularly limited. Water oxidation by polyoxometalates with accumulated holes may be effective at room temperature (25°C), but may be accelerated at higher temperatures such as 40°C.

[0049] As described above, the water splitting method of the present invention is characterized in that it can split water even with visible light. Of course, the light that can be used in the water splitting method of the present invention is not limited to visible light. For example, ultraviolet light, infrared light, etc. can also be used.

[0050] The water splitting method of the present invention is a method in which an aqueous solution, a hydrogen-generating photocatalyst, and a hole reservoir are mixed in a single tank and then irradiated with visible light, preferably. However, as will be explained below, multiple tanks may be used instead of a single tank.

[0051] FIG. 2A is a schematic diagram of a water splitting system according to a modified example (hereinafter, also referred to as "water splitting system A"). Water splitting system A includes a bifacial cell. The bifacial cell is separated into two compartments by a partition wall, one side of which is coated with a hydrogen-generating photocatalyst sheet and the other side with an oxygen-generating sheet. An aqueous solution containing dissolved hole reservoirs flows through the compartment. When sunlight or other light is irradiated onto water splitting system A, the reduced-state hole reservoir (Red in FIG. 2A) is oxidized in the compartment facing the hydrogen-generating photocatalyst sheet of the bifacial cell, generating hydrogen and creating an oxidized-state hole reservoir (Ox in FIG. 2A). When this reaction solution flows into the compartment facing the oxygen-generating sheet, oxygen is generated by the positive charge stored in the oxidized-state hole reservoir due to the heat generated by the water oxidation catalyst and / or sunlight, and the reduced-state hole reservoir is regenerated. Water splitting system A allows hydrogen and oxygen to be collected separately.

[0052] FIG. 2B is a schematic diagram of a water splitting system according to another modified example (hereinafter, also referred to as "water splitting system B"). Water splitting system B includes a photocatalyst tank and an oxygen generation tank. An aqueous solution containing a hole reservoir is circulated through the photocatalyst tank and the oxygen generation tank. The photocatalyst tank includes a panel coated with a hydrogen-generating photocatalyst, and the oxygen generation tank includes a water oxidation catalyst and / or a heating device. When water splitting system B is irradiated with light such as sunlight, the reduced-state hole reservoir (Red in FIG. 2B) is oxidized in the photocatalyst tank as hydrogen is generated, generating the oxidized-state hole reservoir (Ox in FIG. 2B). When this reaction solution flows through the oxygen generation tank, oxygen is generated by the positive charge stored in the oxidized-state hole reservoir via the water oxidation catalyst or induced by heat, and the reduced-state hole reservoir is regenerated. Water splitting system B allows hydrogen and oxygen to be collected separately.

[0053] In the present invention, water can be decomposed into hydrogen and oxygen whether a single photocatalyst or a plurality of photocatalysts are used in combination. In this case, it is also possible to use a known hydrogen generating co-catalyst or oxygen generating co-catalyst in combination. Furthermore, it is of course possible to apply the present invention to a method of decomposing water by using a hydrogen generating catalyst and an oxygen generating catalyst in combination. [Example]

[0054] The present invention will be specifically described below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0055] The methods used to evaluate the physical properties of the samples (polyoxometalates, hydrogen-generating photocatalysts) synthesized in the examples and comparative examples (infrared absorption characteristics, visible light absorption characteristics of solution samples, crystal structure analysis, visible light absorption characteristics of solid samples), electrochemical measurements using polyoxometalates, thermochemical oxygen-generating activity, and photocatalytic water-splitting activity using hydrogen-generating photocatalysts and polyoxometalates are as follows.

[0056] <Method for synthesizing polyoxometalates> K6[SiCo II (H2O)W 11O 39 ] was synthesized according to the following procedure as previously reported. 12 O 40 ]·nH2O, 4.0 mmol) was dissolved in 26 mL of 12% by volume aqueous acetic acid, and potassium bicarbonate was added to adjust the pH to 6.0. The solution was heated to 70°C, and Co(OCOCH3)2·4H2O (1.6 mmol) dissolved in Milli-Q water (8 mL) and a CH3COOH aqueous solution (6% by volume, 8 mL) containing CH3COOK (15 g) were slowly added. After heating and stirring for 4 minutes, the precipitate was removed by suction filtration, and the filtrate was allowed to stand at 5°C for 15 hours. The resulting dark red precipitate was recrystallized using Milli-Q water to give K6[SiCo II (H2O)W 11 O 39 ] was obtained (hereafter, SiW 11 Co II (Hereafter referred to as "(

[0057] <Method for synthesizing semiconductor photocatalyst> Rhodium-doped strontium titanate was prepared by the solid-state method according to a previously reported method. The raw materials, SrCO3, TiO2, and Rh2O3, were weighed to a molar ratio of Sr:Ti:Rh = 1.07:0.99:0.01. These materials were mixed in an agate mortar for 40 minutes with the addition of an appropriate amount of methanol to obtain a mixed powder. The mixed powder was then loaded into an alumina crucible and sintered in air. The temperature was increased at 10°C / min, and the mixture was fired at 800°C for 2 hours and then allowed to cool naturally. The resulting powder was mixed in the agate mortar, then loaded into an alumina crucible and sintered in air. The temperature was increased at 10°C / min, and the mixture was fired at 1000°C for 5 hours and then allowed to cool naturally. This procedure was repeated twice to synthesize the sample (hereafter referred to as SrTiO3:Rh).

[0058] <Synthesis of IrO2 nanocolloids as a water oxidation catalyst> An IrO2 nanocolloidal solution was prepared according to a previous report. Na2IrCl6·6H2O (approximately 20 mg) was dissolved in Milli-Q water (50 mL) and the pH was adjusted to 12 using aqueous NaOH. The solution was heated and stirred at 80°C for 30 minutes, then allowed to cool to room temperature in ice water. The pH was then adjusted to 9 using aqueous HNO3, and the solution was again heated and stirred at 80°C for 30 minutes. After cooling again in ice water, a deep blue IrO2 nanocolloidal solution was obtained (approximately 7 mg of Ir / 50 mL).

[0059] <Infrared absorption characteristics> The synthesized polyoxometalates were identified by comparing their FT-IR transmission spectra with those previously reported. FT-IR transmission spectra were measured using a Fourier transform infrared spectrophotometer (JASCO Corporation, FT-4200). Each sample was mixed with dried KBr to a concentration of 1% by mass, and the mixture was pressed in a tablet press to form tablets, which were then used for spectral measurement. The resolution was 2 cm. -1 The total number of times was 64.

[0060] <Light absorption characteristics of solution samples> The absorption spectrum of the solution containing the synthesized polyoxometalate was measured using a UV-visible spectrophotometer (Shimadzu UV-1800). The solution was filled into a quartz cell with an optical path length of 1 cm, and the solution sample was measured under the conditions of a scanning range of 200 to 800 nm, a scanning speed of 200 nm / min, and a sampling interval of 0.5 nm.

[0061] <Crystal structure analysis> The synthesized photocatalyst powder was identified by comparing the XRD pattern obtained by crystal structure analysis with that previously reported. Using an X-ray diffractometer (Rigaku, MiniFlex II), CuKα radiation generated at 30 kV and 15 mA was monochromated with a carbon monochromator, and measurements were taken in the range of 2θ = 3 to 70 deg at a scanning rate of 10 deg / min.

[0062] <Visible light absorption characteristics of solid samples> The optical absorption characteristics of the photocatalyst powder were evaluated using a UV-visible spectrophotometer (JASCO, V-670). Measurements were performed using an integrating sphere with a standard reflector (BaSO4) as a reference. The UV-visible diffuse reflectance spectrum of the sample powder was measured under the following conditions: scanning range 200 to 800 nm, scanning speed 1000 nm / min, and sampling interval 0.5 nm.

[0063] <Electrochemical measurements> SiW 11 Co II Cyclic voltammetry was evaluated using a three-electrode cell with a glassy carbon working electrode, a platinum wire counter electrode, and a silver-silver chloride (Ag / AgCl) reference electrode. Various polyoxometalates were dissolved in a 0.5 mol / L phosphate buffer solution adjusted to pH 3.2 or 6.0. Dissolved oxygen was then removed by bubbling with argon gas. The potential applied to the working electrode was scanned at 50 mV / s using a potentiostat (Princeton Applied Research, PARSTAT 2263).

[0064] <Thermal oxygen generation evaluation> Ten milliliters of phosphate buffer solution containing 5 mmol / L of polyoxometalate was placed in a Pyrex® glass cell, sealed, and then argon gas was bubbled through to remove oxygen from the cell. The temperature of the solution was maintained constant using a hot stirrer. The oxygen concentration in the gas phase of the cell was quantified using an optical oxygen sensor (FireSting Oxygen Monitor, manufactured by PyroScience GmbH).

[0065] <Photocatalytic water splitting activity evaluation> The photocatalytic reaction was carried out in a Pyrex glass cell connected to a closed circulation system. 100 mL of phosphate buffer solution containing the hole reservoir polyoxometalate was placed in a side-illuminated Pyrex glass cell along with 0.1 g of photocatalyst powder. The cell was then connected to the closed circulation system, after which the system was degassed and argon (Ar) gas was introduced. A xenon lamp (300 W, Cermax) equipped with a UV cutoff filter (L-42, HOYA) was used as the light source, and only visible light with a wavelength of 400 nm or greater was irradiated. The temperature of the reaction solution was controlled using a cooling water circulator to maintain it at 288 K. The amount of generated gas was analyzed and measured using a gas chromatograph (GC-8A, Molecular sieve 5A column, Shimadzu Corporation) with Ar gas as the carrier gas.

[0066] Each example and comparative example will be described below.

[0067] Below, we used SrTiO3:Rh as the hydrogen generation photocatalyst and SiW as the hole reservoir. 11 Co II The water splitting activity under visible light irradiation is shown.

[0068] Example 1 SiW as a hole reservoir 11 Co II The photocatalytic water splitting was evaluated by irradiating the aqueous suspension with visible light, which was prepared by suspending SrTiO3:Rh powder, the hydrogen generation photocatalyst of Example 1, in a reaction aqueous solution containing SiW. 11 Co II The concentration of was 2 mmol / L, the pH was 3.2, and the concentration of phosphate ions was 50 mmol / L.

[0069] Example 2 RuO was prepared by the photodeposition method shown below. x was supported on SrTiO3:Rh.

[0070] A dispersion was obtained by dispersing SrTiO3:Rh powder in 100 mL of a methanol solution containing RuCl3·nH2O. The methanol solution was adjusted to a methanol concentration of 20% by volume and a Ru content of 0.7% by mass relative to the SrTiO3:Rh.

[0071] Using the same procedure as in the evaluation of photocatalytic water splitting activity described above, the dispersion was irradiated with visible light for 5 hours under an Ar atmosphere. After the light irradiation, the powder was collected by suction filtration, washed with Milli-Q water, and then vacuum dried overnight. This resulted in the formation of RuO, the photocatalyst of Example 2. x / SrTiO3:Rh was obtained.

[0072] In Example 2, RuO was used as the hydrogen generation photocatalyst. x The photocatalytic water splitting activity was evaluated in the same manner as in Example 1, except that / SrTiO3:Rh was used.

[0073] Example 3 SiW as a hole reservoir 11 Co II The same photocatalyst (RuO) as in Example 2 was used, except that the concentration of x / SrTiO3:Rh) was used to evaluate the photocatalytic water splitting activity in the same manner as in Example 2.

[0074] Example 4 SiW 11 Co II A phosphate buffer solution (pH 3.2) containing SiW was electrolytically oxidized using an electrochemical cell. 11 Co III A solution was obtained. A porous carbon electrode was used as the working electrode, a Pt coil as the counter electrode, and an Ag / AgCl electrode as the reference electrode. A two-compartment cell was used in which the Pt coil was separated by porous glass. The concentration of this aqueous solution was measured using a SiW 11 Co III The concentration was adjusted to 0.5 mmol / L, the phosphate ion concentration was adjusted to 50 mmol / L, and the pH was adjusted to 3.2.

[0075] The reaction solution in Example 4 contained 0.5 mmol / L of SiW as a hole reservoir. 11 Co III The same photocatalyst (RuO) as in Example 2 was used, except that a phosphate buffer solution containing x / SrTiO3:Rh) was used to evaluate the photocatalytic water splitting activity in the same manner as in Example 2.

[0076] (Comparative Example 1) The photocatalytic water splitting activity was evaluated in the same manner as in Example 1 using the same photocatalyst (SrTiO3:Rh) as in Example 1, except that a reaction solution not containing a hole reservoir was used.

[0077] (Comparative Example 2) The same photocatalyst (RuO) as in Example 2 was used, except that a reaction solution containing no hole reservoir was used. x / SrTiO3:Rh) was used to evaluate the photocatalytic water splitting activity in the same manner as in Example 2.

[0078] (Comparative Example 3) The photocatalytic water splitting activity was evaluated in the same manner as in Example 2, except that a reaction solution to which no photocatalytic powder was added was used.

[0079] Comparative Example 4 In the same manner as in Example 4, 0.5 mmol / L of SiW 11 Co III A phosphate buffer solution containing the following was prepared.

[0080] The photocatalytic water splitting activity was evaluated in the same manner as in Example 4, except that a reaction solution to which no photocatalytic powder was added was used.

[0081] (Comparative Example 5) The water splitting activity was evaluated in the same manner as in Example 1, except that visible light was not irradiated.

[0082] (Comparative Example 6) The water splitting activity was evaluated in the same manner as in Example 2, except that visible light was not irradiated.

[0083] Table 1 shows the rates of hydrogen and oxygen production obtained in Examples 1 to 4 and Comparative Examples 1 to 6.

[0084] [Table 1]

[0085] (Discussion of Examples 1 to 4 and Comparative Examples 1 to 6) SiW 11 Co II SrTiO3:Rh (Comparative Example 1) and RuO x / SrTiO3:Rh (Comparative Example 2) did not generate oxygen even when irradiated with visible light, but SiW 11 Co II By adding SrTiO3:Rh (Example 1) and RuO x / SrTiO3:Rh (Example 2) generated oxygen along with hydrogen under visible light irradiation, and water decomposition proceeded (see Figure 3A). Therefore, in order to impart water decomposition to SrTiO3:Rh, SiW 11 Co II The effectiveness of adding SiW was demonstrated. At this time, the UV-visible absorption spectrum of the solution after the reaction showed that SiW 11 Co II With the decrease of SiW 11 Co III The formation of SrTiO3:Rh or RuO was confirmed (see Figure 3B). x / SrTiO3:Rh is a hole and SiW 11 Co II The excited electrons oxidize the silicon dioxide, reducing water and simultaneously generating SiW. 11 Co III This suggests that the water was oxidized.

[0086] SiW 11 Co II (Comparative Example 3) or SiW 11 Co IIIWhen visible light was irradiated onto the reaction solution containing the photocatalyst (Comparative Example 4), no hydrogen or oxygen was generated. 11 It was shown that both the Co hole reservoir and the ZnO hole reservoir are necessary.

[0087] SiW 11 Co II or SiW 11 Co III In the presence of SrTiO3:Rh (Comparative Example 5) and RuO x From the above results, it was found that the SrTiO3:Rh photocatalyst and SiW photocatalyst were irradiated with visible light, and hydrogen and oxygen were not generated from the aqueous suspension of SiW / SrTiO3:Rh (Comparative Examples 6 and 7). 11 It was shown that the Co hole reservoir is driven and water splitting proceeds.

[0088] SiW 11 Co II In the evaluation of water splitting activity in the presence of RuO supported on a reduction promoter, under visible light irradiation, the reduction promoter was more effective than SrTiO:Rh (Example 1). x / SrTiO3:Rh (Example 2) showed higher water splitting activity. Therefore, it was demonstrated that the reduction promoter contributes to improving water splitting activity.

[0089] RuO x In the evaluation of water splitting activity using SiW / SrTiO3:Rh, 11 Co II The rate of hydrogen and oxygen production hardly changed when the concentration was reduced from 2 mmol / L to 0.5 mmol / L. This indicates that the excited electrons generated in the semiconductor accumulated positive charges in the SiW 11 Co III This suggests that water splitting may be suppressed by back electron transfer, which reduces the

[0090] Below, SiW 11 Co II SiW prepared by electrochemical oxidation of 11 Co IIIThis shows the evaluation of the thermal water oxidation reaction by

[0091] (Reference example 5) In the same manner as in Example 4, SiW 11 Co III A reaction solution was prepared as a phosphate buffer solution containing SiW. 11 Co III The concentration of phosphate ions was 0.5 mol / L, and the pH was 3.2.

[0092] At room temperature (25°C), SiW 11 Co III The oxygen generated from 10 ml of the reaction solution containing SiW was measured. 11 Co III Thermal oxygen production by the oxidative stress was evaluated.

[0093] (Reference example 6) SiW was prepared in the same manner as in Reference Example 5, except that the solution temperature was maintained at 40°C. 11 Co III Thermal oxygen production by the oxidative stress was evaluated.

[0094] Example 7 SiW was prepared in the same manner as in Reference Example 5, except that the pH of the solution was adjusted to 6.0. 11 Co III Thermal oxygen production by the oxidative stress was evaluated.

[0095] Example 8 SiW was prepared in the same manner as in Reference Example 5, except that the pH of the solution was adjusted to 6.0 and the temperature of the solution was maintained at 40°C. 11 Co III Thermal oxygen production by the oxidative stress was evaluated.

[0096] The time course of the amount of oxygen produced in Reference Examples 5 and 6 and Examples 7 and 8 is shown in FIG. 4A, and the absorption spectrum of the solution after the reaction is shown in FIG. 4B.

[0097] (Discussion of Reference Examples 5 and 6, Examples 7 and 8) As shown in Figure 4A, at pH 3.2, SiW11 Co III Almost no oxygen was generated from SiW, regardless of the temperature (Reference Examples 5 and 6). At this time, the absorption spectrum of the solution shown in Figure 4B hardly changed, which also indicates that SiW 11 Co III Therefore, in Examples 1 and 2, oxygen generation proceeded under visible light irradiation despite the pH being 3.2. 11 Co III is SrTiO3:Rh or RuO x This suggests that the oxidation of water was promoted by further oxidation by the holes in / SrTiO3:Rh.

[0098] On the other hand, at pH 6.0, oxygen was generated at 25°C (Example 7), and when heated to 40°C, even more oxygen was generated (see Figure 4A). At this time, the change in the absorption spectrum of the solution indicated that SiW 11 Co II It was confirmed that SiW was generated (see Figure 4B). 11 Co III The potential for water oxidation (O2 / H2O) was +1.04 V (vs. SHE) at pH 3.2 and +0.88 V (vs. SHE) at pH 6.0, whereas SiW 11 Co contained in Co III / II The oxidation-reduction potential of SiW is +1.20 V (vs. SHE) regardless of pH. 11 Co III has a sufficient overpotential for water oxidation, and it is thought that water oxidation proceeded at room temperature and 40°C. On the other hand, at pH 3.2, SiW 11 Co III It is thought that SiW does not have sufficient overpotential for water oxidation and only a small amount of oxygen is produced. 11 Co II However, it was shown that it also functions as a water oxidation catalyst.

[0099] Below, SiW 11 Co III We present an evaluation of the thermal water oxidation reaction when a nanocolloidal solution of IrO2, a water oxidation catalyst different from polyoxometalates, is added to an aqueous solution containing the compound.

[0100] Example 9 In the same manner as in Example 4, SiW 11 Co III The reaction solution was prepared by dissolving SiW in a phosphate buffer solution. 11 Co III The concentration of phosphate ions was 0.5 mol / L, and the pH was 3.2.

[0101] At room temperature (25°C), 5 mmol / L of SiW 11 Co III The oxygen generated from 10 ml of the solution was measured and SiW 11 Co III At this time, 1 ml of IrO2 nanocolloid solution from which dissolved oxygen had been removed by Ar bubbling was added two hours after the start of measurement.

[0102] Example 10 SiW was prepared in the same manner as in Example 9, except that the solution was kept at 40°C. 11 Co III Thermal oxygen production by the oxidative stress was evaluated.

[0103] (Comparative Example 7) SiW 11 Co III Thermal oxygen production was evaluated in the same manner as in Example 9, except that a reaction solution containing no

[0104] The amounts of oxygen produced in Examples 9 and 10 and Comparative Example 7 are shown in FIG. 5A, and the absorption spectra of the solutions after the reaction are shown in FIG. 7B.

[0105] (Discussion of Examples 9 and 10 and Comparative Example 7) In the above-mentioned Reference Examples 5 and 6, SiW11 Co III However, by adding IrO2 nanocolloids, clear oxygen generation was confirmed at 25℃ and 40℃. At this time, the absorption spectrum of the solution after the reaction showed that the amount of oxygen generated corresponded to the amount of SiW 11 Co III Decrease in SiW 11 Co II Therefore, IrO2 acts as a water oxidation catalyst, and SiW 11 Co III These results demonstrate that the use of an appropriate water oxidation catalyst other than polyoxometalates is effective in promoting water oxidation using the positive charges stored in the hole reservoir.

[0106] Example 11 CrO was prepared by the photo-deposition method shown below. x / Rh was supported on SrTiO3:Rh.

[0107] A dispersion was obtained by dispersing SrTiO3:Rh powder in 80 mL of aqueous methanol solution containing dissolved Na3RhCl6·nH2O. The aqueous methanol solution was adjusted to a methanol concentration of 20% by volume and a Rh content of 1% by mass relative to SrTiO3:Rh. Following the same procedure as in the evaluation of photocatalytic water splitting activity described above, the dispersion was irradiated with visible light for 5 hours under an Ar atmosphere. After light irradiation, the powder was collected by suction filtration to obtain Rh / SrTiO3:Rh. The obtained powder was dispersed in 80 mL of aqueous methanol solution containing dissolved K2CrO4 to obtain a dispersion. The aqueous methanol solution was adjusted to a methanol concentration of 20% by volume and a Cr content of 1.5% by mass relative to SrTiO3:Rh. After light irradiation, the powder was collected by suction filtration. The powder was washed with Milli-Q water and then vacuum dried overnight. This resulted in the formation of the CrO, the photocatalyst of Example 11. x / Rh / SrTiO3:Rh was obtained.

[0108] In Example 11, CrO was used as a hydrogen generating photocatalyst. xThe photocatalytic water splitting activity was evaluated in the same manner as in Example 1, except that / Rh / SrTiO3:Rh was used.

[0109] After 70 hours of visible light irradiation, the aqueous suspension was suction filtered to obtain a post-reaction solution.

[0110] At 40°C, the amount of oxygen generated from a mixed solution of 10 ml of the reaction solution and 1 ml of the IrO2 nanocolloid solution was quantified. 11 Co III Thermal oxygen production by the oxidative stress was evaluated.

[0111] (Reference example 8) In Reference Example 8, the photocatalytic water splitting activity was evaluated in the same manner as in Example 1, except that Rh / SrTiO3:Rh was used as the hydrogen-generating photocatalyst.

[0112] The results of the photocatalytic reactions in Example 11 and Reference Example 8 are shown in FIG. 6A, the results of the thermal oxygen production in Example 11 are shown in FIG. 6B, and the absorption spectrum of the solution in Example 11 is shown in FIG. 6C.

[0113] (Discussion of Example 11 and Reference Example 9) SiW 11 Co II When irradiated with visible light in an aqueous solution containing CrO x Neither Rh / Rh / SrTiO3:Rh (Example 11) nor Rh / SrTiO3:Rh (Reference Example 8) produced oxygen, but hydrogen, and the hydrogen production eventually stopped (see Figure 6A). x Hydrogen was produced at a higher rate than with Cr / Rh / SrTiO:Rh, but then the rate significantly decreased, with the final hydrogen production being approximately 13 μmol. x The Rh / SrTiO3:Rh catalyst produced hydrogen for a long time, ultimately producing 33 μmol of hydrogen. This is because the Rh / SrTiO3:Rh catalyst was produced by heating SiW on the Rh reduction promoter. 11 Co III While the back electron transfer that reduces CrO x / Rh / SrTiO3: Rh is a reduction promoter and Rh field is CrOx This is thought to be because the back electron transfer was suppressed by the species covering the catalyst, resulting in the selective reduction of water.

[0114] Example 11 CrO x The absorption spectrum of the solution after the photocatalytic reaction using SiW / Rh / SrTiO3:Rh (Figure 6C) shows that 11 Co II and 47 μmol of SiW 11 Co III This confirmed the formation of CrO under visible light irradiation. x / Rh / SrTiO3:SiW with Rh 11 Co II It was shown that hydrogen production accompanied by oxidation of

[0115] In 10 ml of the solution after the photocatalytic reaction in Example 11, SiW 11 Co III The total amount of SiW 11 Co III When 1 mL of IrO2 nanocolloid solution was added to this solution and heated to 40°C, approximately 0.5 μmol of oxygen was generated (Figure 6B). 11 Co III Consumption and SiW 11 Co II The formation of SiW was confirmed (Fig. 6C). 11 Co III The amount of oxygen consumed was about 2.6 μmol, and the corresponding amount of oxygen produced was about 0.7 μmol, which roughly matches the amount of oxygen actually measured. 11 Co III It is thought that oxygen production by this process progressed.

[0116] From the above results, CrO x / Rh / SrTiO3: Rh photocatalytically reacts with hydrogen and SiW 11 Co III and SiW 11 Co IIIIt can be said that oxygen was thermally generated by reacting IrO2 with SiW. 11 The separate production of hydrogen and oxygen using Co as a hole reservoir was demonstrated.

Claims

1. A method for splitting water by light, in which light is irradiated onto an aqueous solution containing one type of photocatalyst and a compound having oxidation-reduction ability dissolved therein.

2. 2. The method for splitting water by light according to claim 1, wherein the compound having oxidation-reduction ability is a polyoxometalate.

3. the polyoxometalate is a compound that oxidizes water, 3. The method for splitting water by light according to claim 2, wherein the aqueous solution contains the polyoxometalate and a compound that oxidizes water and is different from the polyoxometalate.

4. The method for splitting water by light according to claim 2 , wherein the polyoxometalate contains any one of Si, W, and Co.

5. The method for splitting water by light according to claim 2, comprising the step of oxidizing the polyoxometalate with visible light.

6. 3. The method for splitting water by light according to claim 2, wherein a stage where the photocatalyst generates hydrogen is separated from a stage where the polyoxometalate or a water oxidation catalyst different from the polyoxometalate generates oxygen.

Images