JP5707773B2 - Composite photoelectrode and photoelectrochemical reaction system - Google Patents

Description

  The present invention relates to a composite photoelectrode and a photoelectrochemical reaction system in which a semiconductor electrode that generates photoexcited electrons and a catalyst that exhibits a reducing action of carbon dioxide coexist.

In Non-Patent Document 1, when a semiconductor catalyst powder such as TiO ₂ is suspended in water and irradiated with light from an artificial light source such as a xenon lamp or a high-pressure mercury lamp through carbon dioxide, formaldehyde, formic acid, methane A technique for producing methanol and the like is disclosed.

  Patent Document 1 discloses a method for efficiently producing hydrogen and oxygen from water using light energy by irradiating water in the presence of a zirconium oxide semiconductor, and a photolysis catalyst for water from water and carbon dioxide. A technique relating to a method capable of producing carbon monoxide simultaneously with hydrogen and oxygen using light energy in the presence of a catalyst is disclosed.

In Patent Document 2, a semiconductor electrode such as TiO ₂ in a hydroxide solution and a gas diffusion electrode carrying a Pd—Ru alloy catalyst in a hydrogen carbonate solution are short-circuited, and light is irradiated to the semiconductor electrode side to form a gas. A technique for reducing and producing carbon dioxide to formic acid on the diffusion electrode side is disclosed.

  In Patent Document 3, by irradiating light in the presence of cuprous oxide that is a visible light responsive photocatalyst and triethanolamine that is an electron donor, methanol is reacted in water and formic acid is reacted in acetonitrile. A technique for selectively generating the data is disclosed.

  Non-Patent Document 2 discloses that carbon monoxide is 89% by applying a constant current of 50 mA by irradiating light to a p-InP photoelectrode in a methanol solvent in which carbon dioxide is dissolved at a high pressure (40 atm). A technique for generating with a current efficiency of is disclosed.

  In Non-Patent Document 3, formic acid is silver-modified p-InP light with a Faraday efficiency of 29.9% by irradiating light to a lead-modified p-InP photoelectrode in a methanol solvent in which carbon dioxide is dissolved. A technique is disclosed in which carbon monoxide is generated with an Faraday efficiency of 80.4% by irradiating an electrode with light.

  Patent Document 4 discloses 0 in an organic solvent in which a photocatalyst selected from a metal complex having a metal-ligand charge absorption band from the ultraviolet part to the visible part and an electron donor selected from organic amines are dissolved. Disclosed is a carbon dioxide photoreduction method characterized by introducing carbon dioxide at a high pressure of 2-7.5 MPa and selectively reducing carbon dioxide to carbon monoxide by irradiation with light under the pressure. Yes.

Japanese Patent No. 2526396 JP-A-6-158374 Japanese Patent Laid-Open No. 7-112945 Japanese Patent No. 3590837

“Nature” 277 (1979) 637 "The Journal of Physical Chemistry" 102 (1998) 9834 "Applied Catalysis B: Environmental" 64 (2006) 139

  Non-Patent Document 1 discloses an example in which formaldehyde, formic acid, methane, methanol, and the like are simultaneously generated, and Patent Document 1 discloses an example in which carbon monoxide is generated only with hydrogen or simultaneously with hydrogen. It is a feature of inorganic semiconductor photocatalyst to generate hydrogen by photoirradiation or to simultaneously generate two or more carbon dioxide reduction products, but the product can be obtained with high selectivity when considering industrial use. That is important. In Patent Document 2, 32% of the photocurrent generated by light irradiation to titanium oxide is converted to formic acid, indicating high conversion efficiency. However, titanium oxide is an ultraviolet light-responsive semiconductor, and utilization efficiency of sunlight. Is a problem. Patent Document 3 uses a visible light responsive photocatalyst, but requires an electron donor in order to promote an oxidation reaction paired with a carbon dioxide reduction reaction. In Non-Patent Document 2, carbon dioxide is reduced using a visible light responsive photoelectrode, but it is necessary to dissolve carbon dioxide at a high pressure in order to increase the reactivity.

  Also in Non-Patent Document 3, carbon dioxide is reduced using a visible light responsive photoelectrode to produce formic acid and carbon monoxide with high Faraday efficiency, but -2.5 V (vs Ag / AgCl) and A high bias voltage is applied, and the merit of using the photoelectrode material is not utilized.

  In Patent Document 4, only an example using a rhenium complex is shown. In the case of using a rhenium complex, the feature that carbon monoxide is likely to be selectively produced is reported in an academic paper before the filing of Patent Document 4 and is well known. In the case of a rhenium complex, it is also known that when a photocatalytic carbon dioxide reduction reaction is realized, the absorption range of visible light is limited to a relatively short wavelength side of 450 nm or less in visible light. In Patent Document 4, although complexes using other metals are also described, there is no report that they have been realized only in a list of possible metal elements. In addition, it is well known in dye-sensitized solar cells that ruthenium complexes can absorb light of longer wavelengths depending on the configuration, but photocatalytic chemical reactions have not occurred with ruthenium complexes alone. Only electrochemical catalytic reactions when energized are realized. Thus, in the ruthenium complex, no photochemical reaction occurs, and only an electrochemical reaction is realized. However, this system is characterized in that formic acid is produced from carbon dioxide with high product selectivity.

  The reason why the selectivity of the reaction product on the semiconductor photocatalyst is low is assumed as follows. The surface of the semiconductor film or powder is not uniform, and there are many defects and structural steps at the atomic level. Accordingly, the local surface energy differs depending on the site on the surface, and as a result, the adsorption performance of carbon dioxide, protons, solvent, gas, and reaction intermediates that are reactants differs. Accordingly, it is considered that various reaction products are generated because processes such as a probability and a speed of passing electrons to these substances are not constant.

  In addition, the use of visible light with a longer wavelength in complex photocatalysts and the reason for many photocatalytic limitations are not definitive, but the lifetime of photoexcited electrons is short, so the probability of moving to a reaction field on the complex is low. The reason is that it is low and does not lead to reaction even if it absorbs light.

In the composite photoelectrode according to the present invention, a semiconductor electrode and a metal complex catalyst exhibiting a carbon dioxide reducing action are bonded to the semiconductor electrode, or in the electrolyte solution of the metal complex catalyst. By contacting the metal complex catalyst, it coexists in a state where electrons can be exchanged, and the value of the lowest empty orbital energy level of the metal complex catalyst is subtracted from the value of the lowest energy level of the conduction band of the semiconductor. value is less than plus 0.2 eV, while the light irradiated on the semiconductor electrode, by excited electrons generated by applying a bias voltage moves the metal complex catalyst, by using a metal complex catalyst carbon dioxide Exhibits a reducing action.

The metal of the metal complex catalyst is preferably at least one metal selected from Group VII metal and Group VIII metal of the Periodic Table .

In addition, it is preferable that the semiconductor electrode can absorb light having a longer wavelength than a metal complex catalyst that exhibits a carbon dioxide reducing action.

In addition, the photoelectrochemical reaction system according to the present invention has a structure in which a semiconductor electrode and a metal complex catalyst exhibiting a carbon dioxide reducing action are joined, and from the value of the lowest energy level of the conduction band of the semiconductor, The value obtained by subtracting the energy level of the lowest empty orbit of the metal complex catalyst is plus 0.2 eV or less, and the excited electrons generated by irradiating the semiconductor electrode with light move to the metal complex catalyst. A composite photoelectrode that exhibits a carbon dioxide reduction action using a metal complex catalyst; and a bias voltage source that applies a bias voltage to the semiconductor electrode. The metal of the metal complex catalyst is preferably at least one metal selected from Group VII metal and Group VIII metal of the Periodic Table.

  Thus, according to the present invention, carbon dioxide can be reduced using a catalyst using light energy using a semiconductor electrode. Therefore, the reduction reaction can be performed at a relatively low voltage. In addition, since the catalyst can receive excited electrons from the semiconductor electrode, the complex itself does not need to be a photocatalyst, and the catalyst can be selected from a wide range including an electric complex catalyst, resulting in high reaction generation. It becomes possible to produce a reduced product from carbon dioxide with substance selectivity. Moreover, a reducing agent becomes unnecessary by supplying electrons from the power source to the semiconductor electrode.

It is a figure which shows the system configuration | structure of embodiment. It is a figure which shows the structure of a ruthenium complex. It is a figure which shows the TOF-SIMS positive ion spectrum of the sample which made the ligand adsorb | suck to GaP. It is a figure which shows the TOF-SIMS positive ion spectrum of the sample which made the InP adsorb | suck a ligand. It is a figure which shows the system configuration | structure at the time of omitting a reference electrode. It is a figure which shows the structure of a ruthenium complex. It is a figure which shows the relationship between a pyrrole addition rate and formic acid production amount. It is a figure which shows the relationship between an iron chloride (III) addition rate and the formic acid production amount.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

  In FIG. 1, the structure of the photoelectrochemical reaction system which concerns on embodiment is shown. The bias power source 10 is connected to the semiconductor electrode 12 and a counter electrode 14 which is a counter electrode thereof, and the bias power source 10 applies a predetermined bias voltage therebetween. In the example of FIG. 1, a potentiostat is used as the bias power source 10, and the reference electrode 16 is also connected to the bias power source 10, and a voltage can be applied to the semiconductor electrode 12 with the reference electrode 16 as a reference. Thus, in this system, the photoelectrochemical measurement can be performed using the semiconductor electrode 12 as a working electrode. In addition, since an intended reaction can occur without providing the reference electrode 16, it is not necessary to provide the reference electrode 16 in an actual apparatus, and an applied voltage between the counter electrode 14 and the semiconductor electrode 12 can be set. Control is sufficient.

  Then, the substrate 18 contacts the semiconductor electrode 12 in a state where electrons can be exchanged. In the illustrated example, a metal complex (ruthenium complex) is used as the substrate 18.

In such a system, when light is irradiated to the semiconductor electrode 12, wherein the photo-excited electrons e ^- are generated, the photo-excited electrons e ^- it is utilized in the reduction catalytic reaction of the substrate 18. In this example, carbon dioxide CO ₂ is reduced to formic acid (HCOOH). On the other hand, at the counter electrode 14, a reaction that oxidizes water (H ₂ O) to oxygen ((1/2) O ₂ ) occurs, and the electron e ⁻ generated here moves to the semiconductor electrode 12 by the bias voltage, and photoexcitation occurs. It combines with holes generated as electron pairs.

As described above, in this embodiment, the photoexcited electrons e ⁻ generated inside the semiconductor electrode 12 by the light irradiation move to the reaction site of the base material 18 that exhibits the carbon dioxide reducing action, whereby the carbon dioxide reduction reaction occurs. Is called. In particular, in order to utilize photoexcited electrons, carbon dioxide is reduced at a lower voltage than non-semiconductor electrode materials, and useful organic compounds can be synthesized with high efficiency and high reaction product selectivity.

"semiconductor"
Here, the semiconductor used for the semiconductor electrode 12 has the lowest energy level value among the molecular orbitals not occupied by the electrons of the base material, which will be described later, from the lowest energy level value of the conduction band. The material with a value obtained by subtracting 0.2 eV or less. For example, tantalum oxide, tantalum nitride, tantalum oxynitride, nickel-containing zinc sulfide, copper-containing zinc sulfide, nitrogen-doped tantalum oxide, zinc sulfide, indium phosphide, gallium phosphide, iron oxide, silicon carbide, copper oxide be able to.

  Tantalum nitride and tantalum oxynitride can be generated by heat-treating tantalum oxide in an atmosphere containing ammonia gas. Ammonia is preferably diluted with a non-oxidizing gas (argon, nitrogen, etc.). For example, it is preferable to arrange and heat tantalum oxide in a gas stream in which ammonia and argon are mixed at the same flow rate. It is. The heating temperature is preferably 500 ° C. or higher and 900 ° C. or lower, and more preferably 650 ° C. or higher and 850 ° C. or lower. The treatment time is preferably 6 hours or more and 15 hours or less. As the tantalum oxide before ammonia treatment, commercially available tantalum oxide or amorphous one obtained by subjecting a tantalum-containing compound solution such as tantalum chloride to a hydrolysis treatment or the like can be used.

  In addition, nickel-containing zinc sulfide dissolves nickel-containing hydrate and zinc-containing hydrate, and an aqueous solution in which sodium sulfide hydrate is dissolved is added and stirred, and then centrifuged and redispersed. It can be obtained by drying after removing the supernatant. The nickel-containing hydrate can be, for example, nickel (II) nitrate hexahydrate. The zinc-containing hydrate can be, for example, zinc (II) nitrate hexahydrate. Here, nickel chloride, nickel acetate, nickel perchlorate, nickel sulfate, etc. can be used as the nickel source. Further, zinc chloride, zinc acetate, zinc perchlorate, zinc sulfate, etc. can be used as the zinc source.

  Similarly, copper-containing zinc sulfide dissolves copper-containing hydrate and zinc nitrate hydrate, and then stirs sodium hydrate into it, which is then centrifuged and redispersed to remove the supernatant. And can be obtained by drying. The copper-containing hydrate can be, for example, copper (II) nitrate di · 5- (2.5) hydrate. The zinc-containing hydrate can be, for example, zinc (II) nitrate hexahydrate. Here, copper chloride, copper acetate, copper perchlorate, copper sulfate, etc. can be used as the copper source. Further, zinc chloride, zinc acetate, zinc perchlorate, zinc sulfate, etc. can be used as the zinc source.

"Base material"
The base material 18 is a substance whose energy level value of the vacant orbit is lower than that of the lowest energy level of the semiconductor conduction band of the semiconductor electrode 12 described above or higher than 0.2V. The substrate 18 can be a metal complex, for example, a rhenium complex having a carboxybipyridine ligand ((Re (dcbpy) (CO) ₃ P (OEt) ₃ )), ((Re (dcbpy) (CO ) ₃ Cl)), Re (dcbpy) (CO) ₃ MeCN, Re (dcbqi) (CO) ₃ MeCN, Ru complex [Ru (dcbpy) (bpy) (CO) ₂ ] ²⁺ (bpy = 2, 2 ′ -Bipyridine, dcbpy = 4,4'-dicboxy-2,2'-bipyridine) is used.

  Further, the semiconductor electrode 12 and the base material 18 are made to coexist so that electrons can be exchanged. For example, the base material 18 may be suspended in the electrolytic solution, or the base material 18 is mixed with a solvent, and the semiconductor electrode 12 is inserted therein to adhere the base material 18 to the surface. And the photoelectrode which joined the base material 18 on the surface of the semiconductor electrode 12 is obtained by drying this. The solvent can be an organic solvent, and for example, acetonitrile, methanol, ethanol, acetone and the like can be applied.

  The base material 18 is not particularly limited as long as it is a compound that exhibits carbon dioxide reduction activity by utilizing electrons. In the case of a metal complex, a complex of at least one metal selected from Group VII metal and Group VIII metal of the Periodic Table can be mentioned. For example, ruthenium, rhenium, manganese, iron, osmium, cobalt, rhodium, iridium, nickel And complexes of metals such as palladium, platinum and ligands.

The ligand is not particularly limited, but typical main ligands include nitrogen-containing heterocyclic compounds, oxygen-containing heterocyclic compounds, sulfur-containing heterocyclic compounds and the like. Examples of auxiliary ligands include CO, halogen, and phosphines. In the course of the reaction, the auxiliary ligand may be partly dissociated by contact with CO ₂ , base, or water and converted into the following subligand. By converting to a secondary ligand, CO ₂ reduction activity is expressed. Sub-ligands include various solvent molecules such as acetonitrile, DMF, water and the like. These ligands can be used alone or in combination of two or more.

  Examples of nitrogen-containing heterocyclic compounds include pyridine, bipyridine, phenanthroline, terpyridine, pyrrole, indole, carbazole, imidazole, pyrazole, quinoline, isoquinoline, acridine, pyridazine, pyrimidine, pyrazine, phthalazine, quinazoline, quinoxaline, and the like. Examples of the heterocyclic compound include furan, benzofuran, oxazole, pyran, pyrone, coumarin, and benzopyrone. Examples of the sulfur-containing heterocyclic compound include thiophene, thionaphthene, and thiazole. Such ligands can be used alone or in combination of two or more.

  Here, it is preferable that the semiconductor electrode 12 and the base material 18 are chemically bonded by a linking group. The linking group is not particularly limited as long as it is chemically bonded, and examples thereof include a carboxyl group, a phosphoric acid group, a sulfonic acid group, a silanol group, and derivatives thereof. Here, the linking group may have a structure in which protons are eliminated or a structure in which a metal and an oxygen atom are coordinated in a state where the linking group is connected to the semiconductor electrode 12. These linking groups may be used alone or in combination of two or more. A plurality of uses may be used.

  Moreover, the connection method of the semiconductor electrode 12 and the base material 18 should just be chemically couple | bonded with the semiconductor and the base material, and is not specifically limited. For example, (1) a metal complex in which a linking group is introduced into a ligand is adsorbed on a semiconductor, (2) a ligand into which a linking group is introduced is adsorbed on a semiconductor, and then a complex is formed directly. For example, a metal complex is bonded to a semiconductor into which a group is introduced.

  The coverage of the substrate 18 is preferably 1% or more and 90% or less with respect to the surface area of the semiconductor electrode 12. When the coverage of the base material 18 is less than 1%, the amount of the base material is too small and sufficient carbon dioxide reduction activity is not exhibited. On the other hand, when it is more than 90%, the surface of the semiconductor is coated with a large amount of metal complex, so that the visible light absorption of the semiconductor is inhibited, or the oxidation site of the semiconductor is coated, so that the photoelectrode performance is deteriorated. To do.

  Particularly preferred combinations include a linking group such as a phosphate group when the semiconductor is a metal oxide, and a linking group such as a phosphate ester when the semiconductor is a compound semiconductor such as GaP or InP.

Alternatively, the Ru complex may be electrochemically deposited on the semiconductor electrode 12. For example, the semiconductor electrode 12 and the counter electrode in an electrolytic solution in which [Ru {4,4′-di (1-H-1-pyrrolopropyl carbonate) -2,2′-bipyridine] (CO) ₂ Cl ₂ ] is dissolved. 14 is immersed, and the Ru complex can be bonded onto the semiconductor electrode 12 by electrodeposition. Thus, by immobilizing the ruthenium complex (Ru complex) on the semiconductor electrode 12 by electrolytic polymerization, electron transfer between the semiconductor electrode and the complex is promoted, and the reactivity is improved. This is shown in Example 5.

  Further, the Ru complex can be polymerized and immobilized on the semiconductor material also by a chemical polymerization method. The advantage of the chemical polymerization method is that the Ru complex can be immobilized even on an unstable semiconductor electrode that is dissolved and altered during the electropolymerization process. This is shown in Examples 8-10. By adding pyrrole and iron (III) chloride to the Ru complex, polymerization of the Ru complex on the semiconductor electrode proceeds, and the Ru complex can be polymerized and fixed on the semiconductor electrode.

"Counter electrode, reference electrode"
The counter electrode 14 must be an anode that does not dissolve, and platinum or the like is used in addition to carbon. Further, as the reference electrode 16, a silver / silver chloride (Ag / AgCl) electrode, an iodine electrode, or the like is used so that a reference potential can be maintained.

  In such a system, for example, the electrodes 12, 14, and 16 are immersed in water in which carbon dioxide is dissolved, a predetermined voltage is applied, and light is irradiated. Thus, as described above, formic acid is generated from carbon dioxide in the water by the reduction catalytic reaction in the substrate 18, and water is oxidized to oxygen gas at the counter electrode 14. In addition, it becomes possible to synthesize not only formic acid but also useful organic substances such as alcohol from carbon dioxide by selecting the substrate 18 and causing a catalytic reaction in an appropriate environment.

[Example"
In the system of FIG. 1, an electrochemical analyzer (BAS) was used, and photoelectrochemical measurement was performed by a three-electrode system using the semiconductor electrode 12, the counter electrode 14, and the reference electrode 16. A cylindrical Pyrex (registered trademark) glass cell was used as the cell. As a light source, a 300 W xenon lamp (Asahi Spectroscopy, MAX-302) was used, and only visible light was irradiated using a cutoff filter with λ> 422 nm. Here, only the visible light was irradiated to confirm the operation with visible light. In actual operation, ultraviolet light may be irradiated, or sunlight, light from a fluorescent lamp, or the like is irradiated. The An ion chromatograph (DIONEX, with ICS-2000 autosampler AS) was used for evaluation of the product accompanying the photoelectrochemical measurement. IonPac AS15 was used for the column, KOH eluent was used for the eluent, and a conductivity detector was used for the detector.

  Specific examples will be described below.

<Example 1>
The semiconductor electrode 12 is a p-type semiconductor zinc-doped indium phosphide (p-InP-Zn) wafer (8 mm × 20 mm), the counter electrode 14 is a glassy carbon electrode, and the reference electrode 16 is a silver / chloride. A silver electrode was used. As the electrolytic solution, a solution obtained by dissolving 3 mg of ruthenium complex (Ru (bpy) ₂ (CO) ₂ ) in 250 μl of acetonitrile and then adding 5 ml of distilled water was used. After argon gas was bubbled into the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled into the solution for about 10 minutes, and then measurement was performed in a carbon dioxide gas atmosphere. A potential of −0.6 V was applied to the reference electrode.

  In Example 1, electrons are supplied to the substrate 18 when the substrate 18 in the liquid contacts the surface of the semiconductor electrode 12 without fixing the substrate 18 to the semiconductor electrode 12. Zinc-doped indium phosphide (p-InP-Zn) is used for the semiconductor electrode 12.

<Example 2>
The semiconductor electrode 12 is a p-type semiconductor zinc-doped gallium phosphide (p-GaP-Zn) wafer (8 mm × 20 mm), the counter electrode 14 is a glassy carbon electrode, and the reference electrode 16 is silver / silver chloride. An electrode was used. As the electrolytic solution, a solution obtained by dissolving 3 mg of ruthenium complex (Ru (bpy) ₂ (CO) ₂ ) in 250 μl of acetonitrile and then adding 5 ml of distilled water was used. After argon gas was bubbled into the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled into the solution for about 10 minutes, and then measurement was performed in a carbon dioxide gas atmosphere. A potential of −0.6 V was applied to the reference electrode.

  The second embodiment is different from the first embodiment in that zinc-doped gallium phosphide (p-GaP-Zn) is used for the semiconductor electrode 12.

<Example 3>
The semiconductor electrode 12 is a p-type semiconductor zinc-doped indium phosphide (p-InP-Zn) wafer (8 mm × 20 mm), a glassy carbon electrode for the counter electrode, and a silver / silver chloride electrode for the reference electrode. used. As an electrolytic solution, a solution obtained by dissolving 3 mg of ruthenium complex (Ru (bpy) ₂ (CO) ₂ ) in 250 μl of acetonitrile and adding 5 ml of an aqueous sodium hydroxide solution (pH 9) was used. After argon gas was bubbled into the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled into the solution for about 10 minutes, and then measurement was performed in a carbon dioxide gas atmosphere. A potential of −0.6 V was applied to the reference electrode.

  Example 3 differs from Example 1 by using an aqueous sodium hydroxide solution.

<Example 4>
The semiconductor electrode 12 was a zinc-doped gallium phosphide (p-GaP-Zn) wafer (8 mm × 20 mm), the counter electrode 14 was a glassy carbon electrode, and the reference electrode 16 was an iodine electrode. As an electrolytic solution, a solution obtained by dissolving about 2 mg of a ruthenium complex (Ru (bpy) ₂ (CO) ₂ ) in 5 ml of acetonitrile and then adding 250 μl of distilled water was used. After argon gas was bubbled into the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled into the solution for about 10 minutes, and then measurement was performed in a carbon dioxide gas atmosphere. A potential of −0.8 V was applied to the reference electrode 16.

  In Example 4, the voltage of the semiconductor electrode 12 with respect to the reference electrode 16 is changed by using acetonitrile as a main solvent and using an iodine electrode as a reference electrode, as compared with Example 2.

<Example 5>
A ruthenium complex [Ru {4,4′-di (1-H-1-pyrrolopropyl carbonate) -2,2′-] using a zinc-doped indium phosphide (p-InP—Zn) wafer (8 mm × 20 mm). Electrodeposition was performed in a solvent containing bipyridine} (CO) ₂ Cl ₂ ], and a ruthenium complex polymer was deposited on the surface of the zinc-doped indium phosphide. The semiconductor electrode 12 used was the zinc-doped indium phosphide semiconductor electrode 12 on which the ruthenium complex polymer was deposited, the counter electrode 14 was a glassy carbon electrode, and the reference electrode 16 was a silver / silver chloride electrode. 5 ml of distilled water was used as the electrolytic solution. After argon gas was bubbled into the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled into the solution for about 10 minutes, and then measurement was performed in a carbon dioxide gas atmosphere. A potential of −0.6 V was applied to the reference electrode.

The structure of the above-described ruthenium complex [Ru {4,4′-di (1-H-1-pyrrolopropyl carbonate) -2,2′-bipyridine] (CO) ₂ Cl ₂ ] is shown in FIG.

  Example 5 is different from Example 1 in that a ruthenium complex is electrodeposited and deposited on a zinc-doped indium phosphide semiconductor electrode 12.

<Example 6>
Zinc-doped gallium phosphide (p-GaP-Zn) diphosphonate ethylbipyridine ligand (dpebpy) having a ruthenium complex [Ru (dpebpy) (bpy) (CO) ₂ ] ²⁺ (FIG. 2) It was made to adsorb | suck by the method of. In a Teflon (registered trademark) container, 1 ml of a 2 mM [Ru (dpebpy) (bpy) (CO) ₂ ] ²⁺ dichloromethane / methanol mixed solution and a p-GaP-Zn wafer (5 mm × 5 mm) are placed overnight at room temperature. I left it alone. The next morning, the membrane was taken out, washed twice with a solvent (dichloromethane / methanol), and dried in vacuum at 40 ° C. The presence or absence of ligand adsorption on the semiconductor substrate was analyzed by time-of-flight secondary ion mass spectrometry (TOF-SIMS).

  As the semiconductor electrode 12, a zinc-doped gallium phosphide (p-GaP · Zn) wafer (8 mm × 20 mm) connected with the above-described ruthenium complex is used, a glassy carbon electrode is used as a counter electrode, and an iodine electrode is used as a reference electrode. used. As the electrolytic solution, a solution obtained by adding 250 μl of distilled water to 5 ml of acetonitrile was used. After argon gas was bubbled into the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled into the solution for about 10 minutes, and then measurement was performed in a carbon dioxide gas atmosphere. A potential of −0.8 V was applied to the reference electrode.

  Example 6 differs from Example 2 in that a ruthenium complex is connected to the surface of the semiconductor electrode 12 using a connecting group.

<Example 7>
Ruthenium complex [Ru (dpebpy) (bpy) (CO) ₂ ] ²⁺ (FIG. 2) having diphosphonate ethylbipyridine ligand (dpebpy) to zinc-doped indium phosphide (p-InP-Zn) It was made to adsorb | suck by the method of. In a Teflon (registered trademark) container, 1 ml of a 2 mM [Ru (dpebpy) (bpy) (CO) ₂ ] ²⁺ dichloromethane / methanol mixed solution and a p-InP-Zn wafer (5 mm × 5 mm) were placed overnight at room temperature. I left it alone. The next morning, the membrane was taken out, washed twice with a solvent (dichloromethane / methanol), and dried in vacuum at 40 ° C. The presence or absence of ligand adsorption on the semiconductor substrate was analyzed by time-of-flight secondary ion mass spectrometry (TOF-SIMS).

  The semiconductor electrode 12 is a zinc-doped indium phosphide (p-InP.Zn) wafer (8 mm × 20 mm) connected with the ruthenium complex described above, a glassy carbon electrode as a counter electrode, and an iodine electrode as a reference electrode. used. As the electrolytic solution, a solution obtained by adding 250 μl of distilled water to 5 ml of acetonitrile was used. After argon gas was bubbled into the solution for about 20 minutes to remove the dissolved gas, carbon dioxide gas was bubbled into the solution for about 10 minutes, and then measurement was performed in a carbon dioxide gas atmosphere. A potential of −0.8 V was applied to the reference electrode.

  Example 7 differs from Example 1 in that a ruthenium complex is connected to the surface of the semiconductor electrode 12 using a connecting group.

<Example 8>
A zinc-doped indium phosphide (p-InP-Zn) photoelectrode is obtained by cutting a commercially available p-InP-Zn wafer (manufactured by Sumitomo Electric Industries, Ltd.) into an 8 mm × 20 mm square and connecting it to the upper end of the electrode as a terminal connection. In solder was deposited. Glass was stacked on the back side of the light irradiation surface of p-InP-Zn, and the periphery was sealed with silicon rubber.

The ruthenium complex polymer modification of the p-InP-Zn photoelectrode was performed by the following procedure. 0.25 mg of Ru complex [Ru {bipyridyl-4,4′-dicarboxic acid di (3-pyrol-1-yl) ester} (CO) ₂ (CH ₂ CN) ₂ ] is dissolved in 0.25 ml of acetonitrile, After mixing 5 μl of pyrrole solution (molar ratio of pyrrole to Ru complex was 1.1%), 5 μl of 0.2 M iron (III) chloride solution (molar ratio of iron chloride to Ru complex was 3.1 times) was added. . The pyrrole solution was prepared by diluting 50 μl of pyrrole with 1 ml of acetonitrile, and the iron (III) chloride solution was prepared by dissolving 1.08 g of iron (III) chloride hexahydrate in 20 ml of ethanol. As the iron (III) chloride solution was added, the solution gradually turned black. 50 μl of this solution was applied onto a p-InP—Zn photoelectrode and dried in an oven at 45 ° C. The application and drying of the solution were repeated 5 times to produce a Ru-polymer (CP) / p-InP-Zn photoelectrode.

Using a Ru-polymer (CP) / p-InP-Zn photoelectrode, CO ₂ reduction by a photoelectrochemical reaction was performed. After 3 hours of light irradiation, a charge of 0.565C was observed and 0.34 mM formic acid was produced. The Faraday efficiency was 58.3%. As in Example 5, the electrolytic solution was 100% water, the gas was CO ₂ , and the applied voltage was -0.6V.

<Example 9>
In the preparation method shown in Example 8, a polymer film was synthesized on the p-InP-Zn photoelectrode by changing the ratio of the pyrrole addition amount to the Ru complex amount in the range of 0 to 5.6%.

The results of CO ₂ reduction by photoelectrochemical reaction are shown in FIG. Thus, when the amount of pyrrole added is in the range of 0.22 to 0.56%, the amount of formic acid increased as compared to the sample without pyrrole, and when added over 1.1%, the reactivity is higher than that of the sample without pyrrole added. Also declined.

  When only pyrrole was applied without using a Ru complex or an iron (III) chloride solution, formic acid was hardly generated.

  Since the Ru complex itself also has a pyrrole group, the chemical polymerization reaction proceeds by adding iron (III) chloride without adding pyrrole, and a polymer film of the complex is formed on the photoelectrode. And exhibits high activity in the formic acid production reaction. On the other hand, the addition of pyrrole is presumed to increase the degree of polymerization of the complex, improve the durability of the complex to oxygen, and improve the contact between the photoelectrode and the complex. Moreover, when there is too much addition amount of pyrrole, it is guessed that the activity will fall, for example, the excessively produced | generated pyrrole film will inhibit the incident light to a photoelectrode.

<Example 10>
In the preparation method shown in Example 8, the ratio of the addition amount of iron (III) to the amount of Ru complex was changed in the range of 0 to 15.4 times to synthesize a polymer film on the p-InP-Zn photoelectrode.

The results of CO ₂ reduction by photoelectrochemical reaction are shown in FIG. Thus, the amount of formic acid produced in the sample to which no iron (III) chloride was added was as extremely low as 0.04 mM, and the amount of formic acid produced was improved by adding iron (III) to the Ru complex. When iron (III) chloride was added 6.2 times, the amount of formic acid produced increased to 0.4 mM.

  Note that the reactivity decreased when iron (III) chloride was added 15.4 times. When only iron (III) chloride was applied without using a Ru complex or pyrrole, formic acid was hardly generated.

  Although the complex itself has a pyrrole group, the chemical polymerization reaction does not proceed unless iron (III) chloride is added. Therefore, the activity in the formic acid formation reaction is low when no iron (III) chloride is added. By adding iron (III) chloride, a chemical polymerization reaction proceeds, a polymer film of a complex is formed on the photoelectrode, and exhibits high activity in the formic acid production reaction. In addition, when the amount of iron (III) added is excessive, excess iron chloride (III) is deposited on the photoelectrode in the form of iron hydroxide, iron oxide, etc., and the incident light on the photoelectrode is inhibited. It is assumed that the activity decreases.

Here, in the chemical polymerization of the complex, it functions as an oxidizing agent in the same manner as iron (III) chloride.
(I) a compound containing iron ion (III) such as iron (III) nitrate, iron (III) sulfate, iron (III) perchlorate,
(Ii) a compound containing copper ion (II) such as copper (II) chloride, copper (II) nitrate, copper (II) sulfate, copper (II) perchlorate,
(Iii) oxidizing agents such as ammonium persulfate, hydrogen peroxide, ammonium peroxodisulfate, iodine,
Etc.

<Comparative Example 1>
In Example 1, the measurement was performed in an argon gas atmosphere without bubbling carbon dioxide gas.

<Comparative Example 2>
In Example 2, the measurement was performed in an argon gas atmosphere without bubbling carbon dioxide gas.

<Comparative Example 3>
In Example 1, measurement was performed by changing the working electrode to a glassy carbon electrode.

"Experimental result"
In Example 1, 0.19 mM formic acid was detected in 20 hours in a carbon dioxide gas atmosphere, whereas in the argon gas atmosphere in Comparative Example 1, only 0.01 mM formic acid was detected in 20 hours. It was suggested that carbon dioxide was reduced to formic acid in an aqueous solution (water 95%) coexisting with an indium phosphide electrode. In Example 3, 0.42 mM formic acid was detected in 20 hours in a carbon dioxide gas atmosphere, and the amount of production increased compared to Example 1 using distilled water. The increase in the concentration of dissolved carbon dioxide due to the use of an alkaline aqueous solution is considered to be a cause of the improvement in reactivity (Table 1).

  In Example 2, 0.07 mM formic acid was detected in 9.7 hours in a carbon dioxide gas atmosphere, whereas in the argon gas atmosphere in Comparative Example 2, formic acid was not detected even when irradiated with light for 19 hours. It was suggested that carbon dioxide was reduced to formic acid in an aqueous solution (95% water) in which the complex and the gallium phosphide electrode coexist (Table 1).

  In Comparative Example 3, formic acid was not detected even when a voltage of −0.6 V (vs Ag / AgCl) was applied to the glassy carbon electrode for 20 hours in a carbon dioxide gas atmosphere. It was suggested that the indium phosphide electrode and the gallium phosphide electrode can generate formic acid at a low voltage using light energy (Table 1).

  In Example 4, 0.06 mM formic acid was detected in a carbon dioxide gas atmosphere in 8.8 hours, and the production of formic acid was also confirmed in an acetonitrile solution (5% water) in which the complex and the gallium phosphide electrode coexist.

  In Example 5, 0.7 mM formic acid was detected in a carbon dioxide gas atmosphere in 20 hours, and the production of the largest amount of formic acid was confirmed. By immobilizing the complex exhibiting catalytic action on the surface of the indium phosphide electrode as a polymer, the electron transfer between the semiconductor electrode and the complex is promoted, and the reactivity is considered to be improved (Table 1).

  In Example 6, from the result of TOF-SIMS measurement, a spectrum corresponding to ruthenium ions was observed, and it was found that the ruthenium complex was adsorbed on the GaP substrate (see FIG. 3).

  In the photoelectrochemical measurement, 0.2 mM formic acid was detected in a carbon dioxide gas atmosphere in 20 hours, and the amount of formic acid produced was higher than that in Example 4. It is considered that the reactivity of the gallium phosphide electrode surface was enhanced by linking the complex exhibiting a catalytic action with a ligand, thereby promoting the electron transfer between the semiconductor electrode and the complex (Table 1).

  In Example 7, from the result of TOF-SIMS measurement, a spectrum corresponding to ruthenium ions was observed, and it was found that the ruthenium complex was adsorbed on the InP substrate (see FIG. 4).

  In the photoelectrochemical measurement, 0.3 mM formic acid was detected in 20 hours in a carbon dioxide gas atmosphere (Table 1).

"System configuration"
As an embodiment, it is considered that a two-electrode photoelectrochemical reaction cell shown in FIG. 5 is mainly used. A semiconductor electrode 12 is connected to the cathode of the bias power supply 10, and a counter electrode 14 is connected to the anode. Accordingly, the reference electrode 16 in FIG. 1 is omitted, and a simple DC power supply is adopted as the bias power supply 10.

  The energy at the lowest end of the conduction band of GaP mentioned in the above embodiment is -2.4V with respect to the standard hydrogen electrode potential, that is, NHE, and that of InP is -1.7V. On the other hand, the energy of the lowest vacant orbit of the ruthenium complex of Example 1 is about -0.7V, and the energy of the lowest vacant orbit of the ruthenium complex of Example 5 is about -0.8V.

  The effect of the present embodiment is preferably such that the energy at the lowest end of the conduction band of the semiconductor is higher than the energy of the lowest empty orbit of the complex, that is, at a position closer to the vacuum level. More specifically, it is preferable that the value obtained by subtracting the energy of the lowest empty orbit of the complex from the energy at the lowest end of the conduction band is plus 0.2 eV or less.

  For example, another example is a p-type semiconductor. In the case of nitrogen-doped tantalum oxide, the energy at the lowest end of the conduction band is −1.5 V, and the effects of this embodiment can be exhibited using the complexes of Example 1 and Example 5 as catalysts.

  Even in such a configuration, the semiconductor electrode 12 that exchanges electrons with the substrate 18 and the counter electrode 14 are immersed in the electrolytic solution. The substrate 18 may be fixed on the semiconductor electrode 12 or may be suspended in the solution. The semiconductor electrode 12 is irradiated with light. As a result, photoexcited electrons are generated in the semiconductor electrode 12, which moves to the ruthenium complex and is used for a catalytic reaction that reduces carbon dioxide in the solution to formic acid. In this system, electrons are supplied from the bias power source 10, so that a reducing agent for supplying electrons is not necessary, and an efficient reduction treatment can be performed because a photoelectrochemical reaction is used.

"Effect of this embodiment"
According to this embodiment, by using light energy, a useful organic compound can be synthesized from carbon dioxide with high reaction product selectivity at a lower voltage than when a non-semiconductor electrode material is used. That is, the photoexcited electrons generated inside the semiconductor by the light irradiation move to the reaction site of the catalyst exhibiting the carbon dioxide reduction action, thereby driving the carbon dioxide reduction reaction. In the present invention, in order to use photoexcited electrons, carbon dioxide is reduced at a lower voltage than non-semiconductor electrode materials, and useful organic compounds can be synthesized with high efficiency and high reaction product selectivity.

  When the absorption wavelength range of the semiconductor is wider than that of the complex, the light utilization efficiency is improved as compared with the case of the complex alone. That is, the semiconductor in the above-described embodiment generates photoexcited electrons by visible light. Therefore, by utilizing the photoexcited electrons of the semiconductor in this way, a complex that does not exhibit a photocatalytic action or a complex that exhibits a photocatalytic action only with ultraviolet light can be used as a catalyst with high reaction product selectivity.

  Further, by using a semiconductor electrode, a bias voltage can be applied, and separation of electrons and holes generated by photoexcitation can be promoted without using an electron donor.

  By using a semiconductor electrode, a flow reaction system can be constructed, and the product can be easily separated from the reaction system. In particular, if the substrate 18 is fixed to the semiconductor electrode 12, an electrolytic solution in which carbon dioxide is dissolved can be circulated to obtain a solution containing the generated organic substance (for example, formic acid).

  As described above, according to the present embodiment, the photoexcited electrons generated inside the semiconductor are transferred to a catalyst such as a metal complex, thereby reducing carbon dioxide or the like characterized by having high efficiency and high reaction product selectivity. A composite photoelectrode having the function of Therefore, carbon dioxide can be converted into a useful organic compound using sunlight or artificial light.

  10 bias power source, 12 semiconductor electrode, 14 counter electrode, 16 reference electrode, 18 substrate.

Claims (5)

The semiconductor electrode and the metal complex catalyst exhibiting the reducing action of carbon dioxide are bonded to the semiconductor electrode, or the metal complex catalyst is in contact with the semiconductor electrode in the electrolyte solution of the metal complex catalyst. Coexisting in a state where electrons can be exchanged, the value obtained by subtracting the energy level of the lowest orbital of the metal complex catalyst from the value of the lowest energy level of the conduction band of the semiconductor is plus 0.2 eV or less And
With light irradiation of the semiconductor electrode, by excited electrons generated by applying a bias voltage moves the metal complex catalyst, a composite photoelectrode by using a metal complex catalyst exhibits a reducing action of carbon dioxide. The composite photoelectrode according to claim 1,
The composite photoelectrode, wherein the metal of the metal complex catalyst is at least one metal selected from Group VII metal and Group VIII metal of the Periodic Table . The composite photoelectrode according to claim 1 or 2 ,
Before SL semiconductors electrodes, as compared to the metal complex catalyst that exhibits a reducing action of carbon dioxide, the composite photoelectrode capable of absorbing light of longer wavelength. It has a structure in which a semiconductor electrode and a metal complex catalyst exhibiting an action of reducing carbon dioxide are joined, and from the value of the lowest energy level of the conduction band of the semiconductor, the value of the energy level of the lowest empty orbit of the metal complex catalyst minus not more than plus 0.2 eV, by excited electrons generated by light irradiation of the semiconductor electrode is moved to a metal complex catalyst, a reducing action of carbon dioxide by using a metal complex catalyst A composite photoelectrode present;
A bias voltage source for applying a bias voltage to the semiconductor electrode;
A photoelectrochemical reaction system.   The photoelectrochemical reaction system according to claim 4,
  The photoelectrochemical reaction system wherein the metal of the metal complex catalyst is at least one metal selected from Group VII metal and Group VIII metal of the Periodic Table.

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