JP2022129732A - Carbon dioxide reduction catalyst, carbon dioxide reduction device, and artificial photosynthesis device - Google Patents

Abstract

To provide a carbon dioxide reduction catalyst that allows an electrochemical or photocatalytic carbon dioxide reduction reaction to proceed at low bias in an organic solvent and an aqueous solvent, and a carbon dioxide reduction device and an artificial photosynthesis device including the same.SOLUTION: A carbon dioxide reduction catalyst is a metal complex in which at least one molecule of polypyridine having an electron-donating substituent is coordinated to one of metal ions of Groups 6-12. The carbon dioxide reduction catalyst catalyzes an electrochemical or photocatalytic carbon dioxide reduction reaction.SELECTED DRAWING: None

Description

TECHNICAL FIELD The present invention relates to a carbon dioxide reduction catalyst, a carbon dioxide reduction device using the same, and an artificial photosynthesis device.

As one of the solutions to global environmental problems and the depletion of fossil fuels, catalyst technology that reduces carbon dioxide (CO ₂ ) at normal temperature and pressure to synthesize industrial raw materials such as CO and fuel (solar fuel) is extremely important. is. Among them, many reports show that metal complexes exhibit excellent _CO2 reaction selectivity and are useful as _CO2 reduction catalysts.

However, until now, highly active metal complex catalysts have used rare elements such as rhenium (Re), ruthenium (Ru), and iridium (Ir), or have complex chemical structures of ligands (e.g., patent documents 1). Since a large amount of catalyst is required for the practical application of carbon dioxide reduction technology, the development of metal complexes consisting of a central metal, which is abundant in resources, and a ligand with a simple structure, which is easy to synthesize. desired.

As ligands with simple structures, polypyridines such as bipyridine are well known. As a conventional technique, a carbon dioxide reduction catalyst using a bipyridine complex of cobalt (Co) has been reported.

For example, Patent Document 2 discloses a cobalt-tris-bipyridine complex ([Co(bpy) ₃ ] ²⁺ ) in which three molecules of unsubstituted bipyridine are coordinated to a Co ion and a photosensitizer (ruthenium tris-bipyridine ([ Ru(bpy) ₃ ] ²⁺ )) achieved CO ₂ reduction under light irradiation.

Patent Document 3 describes a carbon dioxide reduction catalyst in which bipyridine or the like is coordinated to rhenium, manganese, ruthenium, or iron as the central metal (M), as shown below.

In Non-Patent Document 1, a combination of a cobalt bis-bipyridine complex [Co(bpy) ₂ Cl ₂ ] and a photosensitizer (ruthenium tris-bipyridine ([Ru(bpy) ₃ ] ²⁺ )) shown below shows that light Realization of _CO2 reduction under irradiation is described.

However, the carbon dioxide reduction catalysts described in Patent Documents 2 and 3 and Non-Patent Document 1 have a high overvoltage required for carbon dioxide reduction, and further improvement in performance is required.

JP 2018-197225 A JP-A-2003-117402 International Patent Application Publication No. 2016/136433 pamphlet

Y.Yao, et al., J.Energy Chem., 2018, 27, pp.502-506

An object of the present invention is to provide a carbon dioxide reduction catalyst, a carbon dioxide reduction apparatus using the same, and an artificial carbon dioxide reduction catalyst capable of promoting an electrochemical or photocatalytic carbon dioxide reduction reaction in an organic solvent and an aqueous solvent at a low bias. An object of the present invention is to provide a photosynthesis device.

The present invention provides a metal complex in which one molecule or more of polypyridine having an electron-donating substituent is coordinated to one of group 6 to group 12 metal ions (provided that one molecule of bipyridine is coordinated to the metal ion If so, the metal ions are excluding rhenium ions, manganese ions, ruthenium ions, and iron ions), and are carbon dioxide reduction catalysts that catalyze electrochemical or photocatalytic carbon dioxide reduction reactions.

（１）
（式（１）中、Ｒ_１～Ｒ_８のうちの少なくとも１つは電子供与性の置換基であり、残りは水素原子またはビピリジンに置換可能な置換基を表す。） In the carbon dioxide reduction catalyst, it is preferable that the polypyridine is bipyridine represented by formula (1), and that the bipyridine is a metal complex in which one molecule or more and three molecules or less are coordinated to one of the metal ions.

(1)
(In Formula (1), at least one of R ₁ to R ₈ is an electron-donating substituent, and the rest represent hydrogen atoms or substituents capable of substituting bipyridine.)

In the carbon dioxide reduction catalyst, the electron-donating substituent is preferably an alkyl group or an alkoxy group.

In the carbon dioxide reduction catalyst, when one molecule of the polypyridine is coordinated to the metal ion, the metal ion is preferably a cobalt ion, a nickel ion, or a molybdenum ion.

In the carbon dioxide reduction catalyst, when two or three molecules of the polypyridine are coordinated to the metal ion, the metal ion is a cobalt ion, an iron ion, a manganese ion, a nickel ion, a copper ion, or a molybdenum ion. Preferably.

The carbon dioxide reduction catalyst is preferably driven in a homogeneous system in which the carbon dioxide reduction catalyst is dissolved in an organic solvent or an aqueous solvent.

The present invention comprises an electrolytic solution obtained by dissolving the carbon dioxide reduction catalyst and an electrolyte in a solvent, a cathode electrode for promoting a reduction reaction of carbon dioxide, and an anode electrically connected to the cathode electrode and causing an oxidation reaction. and an electrode, wherein the cathode electrode and the anode electrode are immersed in the electrolyte solution.

The present invention is an artificial photosynthesis device comprising the carbon dioxide reduction device, and a solar cell that generates power to be supplied to the anode electrode and the cathode electrode.

INDUSTRIAL APPLICABILITY According to the present invention, a carbon dioxide reduction catalyst capable of promoting an electrochemical or photocatalytic carbon dioxide reduction reaction in an organic solvent and an aqueous solvent at a low bias, a carbon dioxide reduction device and an artificial photosynthesis device using the same can be provided.

1 is a schematic configuration diagram showing an example of a carbon dioxide reduction device according to an embodiment; FIG. 1 is a schematic configuration diagram showing an electrochemical cell used for electrochemical measurements in Examples. FIG. 4 is a graph showing a cyclic voltammogram (in NEt ₄ BF ₄ /DMA) of the complex catalyst of Example 1. FIG. 4 is a graph showing a cyclic voltammogram (in NEt ₄ BF ₄ /DMA) of the complex catalyst of Example 2. FIG. 4 is a graph showing a cyclic voltammogram (in NEt ₄ BF ₄ /DMA) of the complex catalyst of Example 3. FIG. 10 is a graph showing a cyclic voltammogram (in NEt ₄ BF ₄ /DMA) of the complex catalyst of Example 4. FIG. 4 is a graph showing a cyclic voltammogram (in NEt ₄ BF ₄ /DMA) of the complex catalyst of Comparative Example 1. FIG. 1 is a graph showing a cyclic voltammogram (in 0.01 M NaHCO ₃ aqueous solution) of the complex catalyst of Example 1;

An embodiment of the present invention will be described below. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

[Carbon dioxide reduction catalyst]
The carbon dioxide reduction catalyst according to the present embodiment is a metal complex in which one or more molecules of polypyridine having an electron-donating substituent is coordinated to one of group 6 to group 12 metal ions. Alternatively, it is a catalyst that catalyzes a photocatalytic carbon dioxide reduction reaction. However, in the carbon dioxide reduction catalyst according to the present embodiment, when one molecule of bipyridine is coordinated to a metal ion, rhenium ions, manganese ions, ruthenium ions, and iron ions are excluded as metal ions.

The present inventors have found that the introduction of electron-donating substituents into the ligand polypyridine enables the electrochemical or photocatalytic carbon dioxide reduction reaction to proceed at low bias. The carbon dioxide reduction catalyst according to the present embodiment is a complex catalyst in which one or more molecules of polypyridine into which an electron-donating substituent is introduced is coordinated to one of group 6 to group 12 metal ions. The carbon dioxide reduction reaction proceeds at a lower applied voltage than when polypyridine is coordinated.

This is because the introduction of electron-donating substituents into the ligand increases the electron density of the central metal, and as a result, facilitates the coordination of _CO2 to the central metal, resulting in the carbon dioxide reduction reaction. It is considered to be advantageous. In addition, by introducing a substituent containing a carbon compound as an electron-donating substituent, the surroundings of the complex catalyst become a hydrophobic environment, which makes it easier for CO ₂ molecules to approach.

Examples of polypyridine ligands include bipyridine, terpyridine, quaterpyridine, phenanthroline and derivatives thereof. Among them, a ligand having a structure represented by the following formula (1) is particularly preferable. A bipyridine ligand having a structure represented by the following formula (1) coordinates 1 to 3 molecules with a metal ion as a main ligand to form a complex. All the ligands in the polybipyridine complex molecule may have the same structure or different structures.

（１）
（式（１）中、Ｒ_１～Ｒ_８のうちの少なくとも１つは電子供与性の置換基であり、残りは水素原子またはビピリジンに置換可能な置換基を表す。）

(1)
(In Formula (1), at least one of R ₁ to R ₈ is an electron-donating substituent, and the rest represent hydrogen atoms or substituents capable of substituting bipyridine.)

At least one of the substituents R ₁ to R ₈ of bipyridine in formula (1) is an electron-donating substituent, and the rest are hydrogen atoms or substituents capable of substituting bipyridine. The electron-donating substituent is not particularly limited as long as it has electron-donating properties. Examples of electron-donating substituents include straight-chain alkyl groups such as methyl, ethyl, propyl, and butyl groups, cyclic alkyl groups, and branched alkyl groups. Although the length of the alkyl group is not particularly limited, it is preferably in the range of C ₁ to C ₄ in consideration of the solubility of the metal complex in solvents. Examples of electron-donating substituents include linear alkoxy groups such as methoxy, ethoxy, propyloxy, and butyloxy groups, cyclic alkoxy groups, and branched alkoxy groups. Although the length of the alkoxy group is not particularly limited, it is preferably in the range of C ₁ to C ₄ in consideration of the solubility of the metal complex in solvents. Other electron-donating substituents include a hydroxyl group, an acetoxy group, an amino group, an acetamide group, a dimethylamino group, and the like. In addition, an alkylphosphonic acid group in which a phosphonic acid group is linked to the end of an alkyl group, an alkylcarboxyl group in which a carboxyl group is linked to the end of an alkyl group, an alkylsulfonic acid group in which a sulfonic acid group is linked to the end of an alkyl group, an alkyl group An alkylsilanol group having a silanol group linked to the end, an alkylmercapto group having a mercapto group linked to the end of an alkyl group, and derivatives thereof may be used. Among these, the electron-donating substituent is preferably an alkyl group or an alkoxy group in terms of solubility in a solvent, ease of synthesis, and the like. Other substituents may be hydrogen atoms or any substituents that can be substituted on bipyridine, but hydrogen atoms are preferred. Substituents that can be substituted on bipyridine include, for example, a carboxyl group, a phosphonic acid group and derivatives thereof.

The position of the electron-donating substituent may be any of R ₁ to R ₈ , but from the viewpoint of ease of complex formation, etc., the 4,4′ positions (R ₃ , R ₆ ) of bipyridine or The 5,5′ positions (R ₂ , R ₇ ) are preferred.

Besides the polybipyridine ligand, the ligand to which the metal ion is coordinated is not particularly limited, either as the main ligand or the auxiliary ligand. Examples of main ligands include nitrogen-containing heterocyclic compounds, oxygen-containing heterocyclic compounds, oxygen-containing compounds, sulfur-containing heterocyclic compounds, and the like. Examples of ancillary ligands include halogen atoms such as Cl and Br, carbonyl, aqua, ammine, oxo, cyano, nitro, nitrite, thiocyanato, pyridine, triphenylphosphine, triethylphosphine, and triethylphosphite. During the course of the reaction, the primary ligand and the ancillary ligand may come into contact with CO ₂ , a base, or water and partly dissociate and be converted into an ancillary ligand. Examples of auxiliary ligands include -CO, -CO ₂ , -COOH, -COH, -(CO ₂ )-, -OH, -OH ₂ and the like. These ligands may be used alone or in combination of two or more. In these ligands, the element that coordinates to the metal is not particularly limited, and examples thereof include O, N, C, P, S, Si, and halogen. One of such elements may be coordinated, or two or more may be coordinated.

Examples of nitrogen-containing heterocyclic compounds include pyridine, bipyridine, diphosphonatepyridine, phenanthroline, terpyridine, quaterpyridine, pyrrole, indolecarbazole, imidazole, pyrazole, quinoline, isoquinoline, acridine, pyridazine, pyrimidine, pyrazine, phthalazine, and quinazoline. , quinoxaline and its derivatives. Oxygenates include polyoxometalates and their derivatives. Sulfur-containing heterocyclic compounds include thiophene, thionaphthene, thiazole and derivatives thereof.

In the carbon dioxide reduction catalyst according to the present embodiment, a complex in which three molecules are coordinated desorbs and decomposes a part of the ligand in the reaction system, resulting in a monomolecular coordination structure or a bimolecular coordination structure. may change to It may also exist in the system as a mixture of two or more structures selected from the monomolecular coordination structure, the bimolecular coordination structure, and the trimolecular coordination structure.

Examples of metal complexes include complexes of ligands and at least one metal selected from metals belonging to any one of Groups 6 to 12 of the periodic table. However, when one molecule of bipyridine is coordinated to a metal ion, rhenium ion, manganese ion, ruthenium ion and iron ion are excluded as metal ions. The metal ions are selected from the group consisting of ions of cobalt (Co), iron (Fe), copper (Cu), nickel (Ni), manganese (Mn) and molybdenum (Mo) from the viewpoint of catalytic activity. Co and Fe are more preferable because they are abundant in resources. Such complexes may be mononuclear or polynuclear with two or more nuclei. Moreover, two or more kinds of metals may be contained.

In the carbon dioxide reduction catalyst, when one molecule of polypyridine is coordinated to a metal ion, the metal ion is preferably cobalt ion, nickel ion, or molybdenum ion. When polypyridine is bimolecularly or trimolecularly coordinated to a metal ion, the metal ion is preferably cobalt ion, iron ion, manganese ion, nickel ion, copper ion, or molybdenum ion.

Specific examples of metal complex catalysts include 4,4′-dimethyl-2,2′-bipyridine (4,4′-dmbpy), 4-methyl-2,2′-bipyridine (4-mbpy), 4 ,4′-diethyl-2,2′-bipyridine (4,4′-debpy), 4,4′-dibutyl-2,2′-bipyridine (4,4′-dbbpy), 4,4′-ditert -butyl-2,2'-bipyridine (4,4'-dtbbpy), 4,4'-dimethoxy-2,2'-bipyridine (4,4'-dmobpy), 4,4'-diethoxy-2,2 '-bipyridine (4,4'-deobpy), 4,4'-dimethylphosphonate-2,2'-bipyridine (4,4'-dmpbpy), 4,4'-dimethylphosphonate-ethyl-2,2'-bipyridine (4,4′-dmpebpy), 4,4′-dimethylcarboxy-2,2′-bipyridine (4,4′-dmcbpy), 5,5′-dimethyl-2,2′-bipyridine (5,5′ -dmbpy) having at least one selected from the group consisting of Co, Fe, Cu, Ni, Mn, and Mo complexes as main ligands.

More specifically, in the case of a Co complex, [Co(4,4′-dmbpy) ₃ ] ²⁺ , [Co(4,4′-dmbpy) ₃ ] ³⁺ , [Co(4,4′-dmbpy) ₂ (CO ₃ )] ²⁺ , [Co(4,4′-dmbpy) ₂ (CO ₃ )], [Co(4,4′-dmbpy) ₂ (CO ₃ )] ⁺ , [Co(4,4′ -dmbpy) ₂ (bpy)] ²⁺ , [Co(4,4′-dmbpy) ₂ Cl ₂ ], [Co(4,4′-dmbpy) ₂ Cl ₂ ] ⁺ , [Co(4-mbpy) ₃ ] ²⁺ , [Co(4,4′-debpy) ₃ ] ²⁺ , [Co(4,4′-dbbpy) ₃ ] ²⁺ , [Co(4,4′-dtbbpy) ₃ ] ²⁺ , [Co(4,4 '-dmobpy) ₃ ] ²⁺ , [Co(4,4'-deobpy) ₃ ] ²⁺ , [Co(4,4'-dmpbpy)(bpy) ₂ ] ²⁺ , [Co(4,4'-dpbpy) ₃ ] ²⁺ , [Co(4,4′-dmpebpy) ₃ ] ²⁺ , [Co(4,4′-dmcbpy) ₃ ] ²⁺ , [Co(5,5′-dmbpy) ₃ ] ²⁺ and the like. Moreover, the metal complex polymer which polymerized these may be used.

The counter ion of the complex is not particularly limited as long as it is anionic. The carbon dioxide reduction catalyst according to the present embodiment can be made water-soluble or organic solvent-soluble by selecting the counter ion of the metal complex of polypyridine having an electron-donating substituent. If it is a water-soluble metal complex, it can be a homogeneous system-driven carbon dioxide reduction catalyst dissolved in an aqueous solvent, and if it is organic solvent-soluble, it can be a homogeneous system-driven carbon dioxide dissolved in an organic solvent. It can be a reduction catalyst. Counter ions that dissolve in water include nitrate ions, sulfate ions, chloride ions, carbonate ions, and halogens such as Cl and Br. Counter ions that are insoluble in water but soluble in organic solvents include PF ₆ ion, BF ₄ ion, and the like.

[Carbon dioxide reduction device and artificial photosynthesis device]
The carbon dioxide reduction device according to the present embodiment includes an electrolyte solution obtained by dissolving the carbon dioxide reduction catalyst and the electrolyte in a solvent, a cathode electrode for advancing the carbon dioxide reduction reaction, and electrically connected to the cathode electrode, and an anode electrode that causes an oxidation reaction, wherein the cathode electrode and the anode electrode are immersed in an electrolyte solution.

FIG. 1 shows a schematic configuration of an example of a carbon dioxide reduction device according to this embodiment. The carbon dioxide reduction apparatus 1 shown in FIG. 1 has, for example, an electrolyte solution 14 in which the carbon dioxide reduction catalyst and the electrolyte are dissolved in a solvent is contained in a container 16, and a cathode electrode (action) that advances the reduction reaction of carbon dioxide. 12 and an anode electrode (counter electrode) 10 that causes an oxidation reaction are immersed in an electrolytic solution 14, and the cathode electrode 12 and the anode electrode 10 are electrically connected. The electrolyte solution 14 contains carbon dioxide. Carbon dioxide may be saturated in advance in the electrolyte solution, or may be in a state in which carbon dioxide gas is allowed to flow in the system.

At the anode electrode 10, an oxidation reaction occurs and a potential is obtained. At the cathode electrode 12 , carbon monoxide (CO), formic acid (HCOOH), and the like are generated by reducing carbon dioxide (CO ₂ ), for example, by obtaining a potential from an electrode that causes an oxidation reaction.

The cathode electrode 12 and the anode electrode 10 are electrically connected, and an appropriate bias voltage is applied. Means for applying a bias voltage are not particularly limited, and include chemical batteries (including primary batteries, secondary batteries, etc.), constant voltage sources, solar batteries, and the like. At this time, the positive electrode is connected to the anode electrode 10 and the negative electrode is connected to the cathode electrode 12 .

By using a solar cell as a means for applying a bias voltage, an artificial photosynthesis device can be provided that includes the carbon dioxide reduction device and a solar cell that generates power to be supplied to the anode electrode and the cathode electrode. . When a solar cell is used as means for applying a bias voltage, the positive electrode of the solar cell should be connected to the anode electrode 10 and the negative electrode should be connected to the cathode electrode 12 . In the artificial photosynthesis apparatus according to this embodiment, the anode electrode 10 and the cathode electrode 12 of the carbon dioxide reduction apparatus 1 are connected via a solar cell, and driven using sunlight as an energy source.

As the solvent, any medium can be appropriately used as long as it dissolves the complex catalyst. The solvent is preferably an electrolyte solution in order to facilitate the progress of the electrochemical reaction. In the case of an aqueous medium, in addition to containing water and an electrolyte, it contains a water-soluble organic solvent such as methanol, methanol, acetonitrile, N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA). good too. Alternatively, an organic solvent containing no water may be used alone.

Electrolytes in the case of organic solvents alone include tetraethylammonium tetrafluoroborate (NEt ₄ BF ₄ ), sodium perchlorate, tetraethylammonium hexafluorophosphate, and the like. Electrolytes for aqueous solvents include, for example, oxoacids and oxoacid salts. Examples of oxoacids include carbonic acid, phosphoric acid, nitric acid, sulfuric acid, boric acid, halogen oxoacids (hypochlorite, chloric acid and perchlorate) and the like. Examples of cations constituting oxoacid salts include alkali metals such as sodium, potassium, and cesium. Examples of specific electrolytes include carbonates such as sodium carbonate and potassium carbonate, bicarbonates such as sodium hydrogen carbonate and potassium hydrogen carbonate, phosphates such as potassium phosphate and sodium phosphate (dihydrogen phosphate, including hydrogen phosphate), sulfates such as sodium sulfate and potassium sulfate, borates such as potassium borate, and perchlorates such as sodium perchlorate and potassium perchlorate. Carbonates, bicarbonates, phosphates and borates are preferred as electrolytes used in aqueous media. Also, a combination of two or more kinds may be used.

The concentration of the carbon dioxide reduction catalyst in the solvent may be determined according to the solubility of the carbon dioxide reduction catalyst in the solvent.

The concentration of the electrolyte with respect to the solvent is, for example, in the range of 0.001 mol/L to 0.5 mol/L, preferably in the range of 0.005 mol/L to 0.25 mol/L.

The cathode electrode 12 (electrode for reduction reaction) is an electrode used for reducing a substance by a reduction reaction. As the cathode electrode 12, a carbon material such as glassy carbon, a metal such as platinum or gold, or the like can be used.

The anode electrode 10 (electrode for oxidation reaction) is an electrode used for oxidizing a substance by an oxidation reaction. As the anode electrode 10, a carbon material, a metal such as platinum or gold, or the like can be used.

As the storage container 16, for example, a sealed container made of metal, plastic, glass, or the like, a reaction container having a mechanism for circulating gas, or the like can be used.

In the carbon dioxide reduction device, the rising potential of the reduction current (CO ₂ reduction wave) accompanying the reduction reaction of carbon dioxide is −1.6 V or less based on the Ag/AgNO ₃ electrode when the solvent is N,N-dimethylacetamide. , preferably -1.2 V or less, and when an aqueous solvent is used, -1.9 V or less, preferably -1.6 V or less based on the Ag/AgCl electrode.

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples, but the present invention is not limited to the following examples.

1. Synthesis of complex catalyst <Example 1>
Synthesis of [Co(4,4′-dimethyl _- 2,2′ _- bipyridine) ₃ ] ₍ NO ₃ ) ₂ , 4′-dimethyl-2,2′-bipyridine (Tokyo Kasei Co., Ltd.) (4.5 mmol) was dissolved in methanol, stirred at room temperature (20±5° C.) for 2 hours, and then concentrated under reduced pressure. The residue was recrystallized with methanol and diethyl ether to obtain 1.09 g of [Co(4,4′-dimethyl-2,2′-bipyridine) ₃ ](NO ₃ ) ₂ shown below.

<Example 2>
Synthesis of [Co(4,4′-dimethyl-2,2′-bipyridine) ₃ ](NO ₃ ) ₂ In the synthesis of Example 1, 4,4′-dimethyl-2,2′-bipyridine was used as a ligand. 0.96 g of the following [Co(4,4′-dimethoxy-2,2′- bipyridine) ₃ ](NO ₃ ) ₂ was obtained.

<Example 3>
Synthesis of [Co(5,5′-dimethyl-2,2′-bipyridine) ₃ ](NO ₃ ) ₂ In the synthesis of Example 1, 4,4′-dimethyl-2,2′-bipyridine as a ligand 0.95 g of the following [Co(5,5′-dimethyl-2,2′- bipyridine) ₃ ](NO ₃ ) ₂ was obtained.

<Example 4>
Synthesis of [Co(4,4′-dimethyl-2,2′-bipyridine) ₂ (CO ₃ )](NO ₃ ) ₁ mmol of Co(NO ₃ ) 2.6H ₂ O (Wako Pure Chemical Industries) as a ligand After dissolving 2 mmol of 4,4'-dimethyl-2,2'-bipyridine (Tokyo Kasei) in 25 mL of methanol, 25 mL of 0.16M NaHCO ₃ (Wako Pure Chemical) aqueous solution was added and stirred at 40° C. for 2 hours. After that, the mixture was oxidized by stirring overnight in the air, and then the solvent was distilled off under reduced pressure until the solvent became small, followed by filtration. The residue was washed with water and acetonitrile and air-dried to give 504.8 mg (0.919 mmol) of [Co(4,4′-dimethyl-2,2′-bipyridine) ₂ (CO ₃ )] ( NO ₃ ) was obtained.

<Comparative Example 1>
Synthesis of [Co(2,2′-bipyridine) ₃ ](NO ₃ ) ₂ In the synthesis of Example 1, instead of 4,4′-dimethyl-2,2′-bipyridine as a ligand, 2,2′ -Bipyridine was synthesized in the same manner to obtain 0.934 g of [Co(2,2'-bipyridine) ₃ ](NO ₃ ) ₂ shown below.

2. Electrochemical _CO2 reduction (in organic solvent)
The electrochemical cell shown in FIG. 2 was used for the electrochemical measurement. Using a potentiostat, a three-electrode method was used with a glassy carbon electrode (Φ3 mm) as the working electrode, a platinum (Pt) wire as the counter electrode, and a 10 mmol/L Ag/AgNO ₃ electrode as the reference electrode. The electrolyte solution was prepared by dissolving 0.1 mol/L of tetraethylammonium tetrafluoroborate (NEt ₄ BF ₄ ) and 0.5 mmol/L of various complexes of the measurement sample as electrolytes in an N,N-dimethylacetamide (DMA) solution. I used what I made. As the storage container, a Pyrex (registered trademark) glass container was used. Measurements were taken after passing argon (Ar) or carbon dioxide (CO ₂ ) through the electrolyte solution. Cyclic voltammetry (CV) was measured in the potential range from 0 V to −2.0 V (or −1.5 V) vs Ag/AgNO ₃ at a sweep rate of 10 mV/s.

3. Electrochemical _CO2 reduction (in aqueous media)
The electrochemical cell shown in FIG. 2 was used for the electrochemical measurement. Using a potentiostat, a three-electrode system was used, using a glassy carbon electrode (Φ3 mm) as a working electrode, a platinum (Pt) wire as a counter electrode, and an Ag/AgCl electrode as a reference electrode. The electrolyte solution was a 0.01 mol/L _NaHCO3 solution (in the case of Ar atmosphere, the pH was adjusted to 5.9 with a dilute sulfuric acid solution in advance, and in the case of _CO2 atmosphere, the pH was adjusted to 5.9 by saturating with _CO2 in advance). ) in which 0.5 mmol/L of various complexes of the measurement sample were dissolved respectively. As a storage container, a Pyrex glass container was used. Measurements were taken after passing argon (Ar) or carbon dioxide (CO ₂ ) through the electrolyte solution. Cyclic voltammetry was measured in the potential range from 0 V to −1.5 V vs Ag/AgCl at a sweep rate of 10 mV/s.

4. Results [Electrochemical _CO2 reduction in organic solvents]
The cyclic voltammograms of the complex catalysts of Examples 1-4 and Comparative Example 1 in an organic solvent under an argon (Ar) atmosphere and a _CO2 atmosphere _are shown in FIGS. solid line).

In the cyclic voltammogram in Ar, peaks attributed to redox of the catalyst itself are observed. On the other hand, in _CO2 , when acting as a _CO2 reduction catalyst, a _CO2 reduction wave is generated due to donating electrons to _CO2 . That is, if there is a difference in the cyclic voltammograms in Ar and _CO2 , it means that it acts as a catalyst for _CO2 reduction.

As shown in FIG. 7, when the complex catalyst of Comparative Example 1 was used, there was almost a clear transition between the Ar atmosphere and the CO ₂ atmosphere in the region of −0.4 V to −1.9 V vs Ag/AgNO ₃ . No difference was observed in click voltammograms. Some CO ₂ reduction current occurred only at a negative bias of -1.9 V or higher. From this, it was found that the complex catalyst of Comparative Example 1 functions as an electrochemical CO ₂ reduction catalyst, but requires a high applied potential on the negative side, and its function is extremely low. Requiring a high applied voltage consumes more energy, and in addition, the hydrogen production reaction, which is a by-product on the working electrode, proceeds. Therefore, it is desirable to drive with a bias as low as possible.

On the other hand, when using the complex catalysts of Examples 1 to 3 in which the bipyridine having an electron-donating substituent introduced into the ligand is tricoordinated and the complex catalyst of Example 4 in which bipyridine is bicoordinated, FIG. 6, a larger current was observed in the CO ₂ atmosphere than in the Ar atmosphere at a negative bias from around -0.7V. That is, it can be seen that the complex catalysts of Examples 1 to 4 act as excellent CO ₂ reduction catalysts driven at a low bias.

The reason for this is that by introducing an electron-donating substituent to the ligand, the electron density of the Co ion of the central metal is improved, electrons are easily accumulated, and _CO2 is easily coordinated to this site. It is presumed that the _CO2 reduction reaction is accelerated. In addition, the introduction of a substituent containing a carbon compound creates a hydrophobic environment around the complex catalyst, making it easier for _CO2 molecules to approach, suppressing the reduction reaction with protons (H ⁺ ), which are the raw materials for hydrogen generation. Presumed.

[Electrochemical _CO2 reduction in aqueous solvents]
Cyclic voltammograms of the complex catalyst of Example 1 in an aqueous solvent under an argon (Ar) atmosphere and a _CO2 atmosphere are shown in FIG. 8 (Ar: dotted line, _CO2 : solid line). Even in an aqueous solvent in which the hydrogen generation reaction proceeds easily, a larger current was observed in the CO ₂ atmosphere than in the Ar atmosphere at a negative bias near -0.4 V (vs Ag/AgCl). When a potential of −1.2 V was applied, the gas phase in the cell was sampled and analyzed by a gas chromatograph. As a result, CO and hydrogen were produced at a ratio of 1:1. Also, formic acid was detected by ion chromatography in solution. From this, it was confirmed that the CO ₂ reduction reaction proceeded even in the aqueous solvent.

In this way, metal complexes using abundantly abundant elements and simple ligands can be used to promote the electrochemical _CO2 reduction reaction at low bias not only in organic solvents but also in aqueous solvents. ₂ reduction catalyst was obtained.

As described above, according to the examples, carbon dioxide reduction catalysts were obtained that are capable of causing an electrochemical or photocatalytic carbon dioxide reduction reaction to proceed at a low bias in organic solvents and aqueous solvents. Also, a carbon dioxide reduction device using the carbon dioxide reduction catalyst was obtained. This carbon dioxide reduction device is applicable to an artificial photosynthesis device.

1 carbon dioxide reduction device, 10 anode electrode, 12 cathode electrode, 14 electrolyte solution, 16 container.

Claims (8)

A metal complex in which one molecule or more of polypyridine having an electron-donating substituent is coordinated to one of group 6 to group 12 metal ions (however, when one molecule of bipyridine is coordinated to the metal ion, A carbon dioxide reduction catalyst characterized in that the metal ions are excluding rhenium ions, manganese ions, ruthenium ions, and iron ions, and catalyzes an electrochemical or photocatalytic carbon dioxide reduction reaction. The carbon dioxide reduction catalyst according to claim 1,
The polypyridine is bipyridine represented by formula (1),
A carbon dioxide reduction catalyst, wherein the bipyridine is a metal complex in which one molecule or more and three molecules or less are coordinated to one of the metal ions.

(1)
(In Formula (1), at least one of R ₁ to R ₈ is an electron-donating substituent, and the rest represent hydrogen atoms or substituents capable of substituting bipyridine.) The carbon dioxide reduction catalyst according to claim 1 or 2,
The carbon dioxide reduction catalyst, wherein the electron-donating substituent is an alkyl group or an alkoxy group. The carbon dioxide reduction catalyst according to any one of claims 1 to 3,
The carbon dioxide reduction catalyst, wherein one molecule of the polypyridine is coordinated to the metal ion, and the metal ion is a cobalt ion, a nickel ion, or a molybdenum ion. The carbon dioxide reduction catalyst according to any one of claims 1 to 3,
When the polypyridine is coordinated with two or three molecules of the metal ion, the metal ion is a cobalt ion, an iron ion, a manganese ion, a nickel ion, a copper ion, or a molybdenum ion. Carbon reduction catalyst. The carbon dioxide reduction catalyst according to any one of claims 1 to 5,
A carbon dioxide reduction catalyst characterized by being driven by a homogeneous system in which the carbon dioxide reduction catalyst is dissolved in an organic solvent or an aqueous solvent. an electrolyte solution obtained by dissolving the carbon dioxide reduction catalyst according to any one of claims 1 to 6 and an electrolyte in a solvent;
a cathode electrode that advances the reduction reaction of carbon dioxide;
an anode electrode that is electrically connected to the cathode electrode and causes an oxidation reaction;
has
A carbon dioxide reduction apparatus, wherein the cathode electrode and the anode electrode are immersed in the electrolyte solution. The carbon dioxide reduction device according to claim 7;
a solar cell that generates power to be supplied to the anode electrode and the cathode electrode;
An artificial photosynthesis device comprising:

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