JP7392482B2 - carbon dioxide reduction catalyst - Google Patents

Description

The present invention relates to a carbon dioxide reduction catalyst.

Research into catalysts for reducing carbon compounds such as carbon dioxide is being carried out around the world in order to solve problems such as global warming caused by rising concentrations of carbon dioxide (CO ₂ ) in the atmosphere and the depletion of energy and carbon resources. It is being actively carried out. Among these, many metal complexes have been reported to have excellent properties as catalysts for reducing carbon compounds.

For example, Patent Document 1 discloses a technique for producing CO by electrically reducing an aqueous solution containing CO ₂ using a porphyrin complex that is a carbon compound reduction catalyst.

Further, Patent Document 2 discloses a carbon compound reduction catalyst including a metal complex in which a central metal, a ligand capable of accumulating electrons, and a monodentate ligand are bonded.

Furthermore, Non-Patent Document 1 discloses a technique for reducing CO ₂ and producing CO using a Cu complex as a carbon compound reduction catalyst.

Furthermore, Non-Patent Document 2 describes the use of Cu(OH) ₂ as a carbon compound reduction catalyst.

Japanese Patent Application Publication No. 2018-034136 Japanese Patent Application Publication No. 2013-193056

J. Am. Chem. Soc. , 2018, 140(49), p17241-17254 J. Am. Chem. Soc. , 2018, 140(28), p8681-8689 ACS Catal. 2017, 7, p8382-8385

By the way, in the reduction reaction of carbon dioxide, a catalyst is desired that can efficiently generate a multi-electron reduction product having more than two electrons, which is more useful than a two-electron reduction product in terms of energy resources. Note that two-electron reduction products produced by two-electron reduction of carbon dioxide include, for example, CO, HCOOH, and the like. Further, multi-electron reduction products generated by multi-electron reduction of carbon dioxide exceeding 2 electrons include, for example, CH ₄ (8-electron reduction product), C ₂ H ₄ (12-electron reduction product), C ₂ H ₅ OH (12-electron reduction product), etc.

However, with conventional catalysts using metal complexes, two-electron reduction products are preferentially produced, and it is difficult to produce multi-electron reduction products.

Therefore, an object of the present invention is to provide a carbon compound reduction catalyst that can promote the production of multi-electron reduction products such as CH ₄ , C ₂ H ₄ , C ₂ H ₅ OH, etc. in the reduction reaction of carbon dioxide. There is a particular thing.

The present invention provides a carbon dioxide comprising a metal complex having a ligand having a nitrogen atom, a ligand having a phosphorus atom, and a metal halide salt having two metal atoms, at least one of which is Cu. It is a carbon reduction catalyst.

Further, in the carbon dioxide reduction catalyst, it is preferable that the metal complex has a crystal structure.

Further, in the carbon dioxide reduction catalyst, the metal complex is represented by the general formula A: [{CuMX ₂ (Y) ₂ } (L)] _n , and in the general formula A, M is Cu, Ag or Ni. , X is a halogen atom, Y is a ligand having a phosphorus atom, and L is preferably a ligand having a nitrogen atom.

Furthermore, in the carbon dioxide reduction catalyst, X in the general formula A is preferably selected from Cl, Br, and I.

Further, in the carbon dioxide reduction catalyst, Y in the general formula A is preferably triphenylphosphine.

Further, in the carbon dioxide reduction catalyst, L in the general formula A is preferably a bipyridine derivative.

Further, in the carbon dioxide reduction catalyst, the metal complex is represented by the general formula B: CuMX ₂ (Y) ₂ L ₂ , where M is Cu, Ag or Ni, and X is a halogen atom. It is preferable that Y is a ligand having a phosphorus atom, and L is a ligand having a pyridine ring.

Further, in the carbon dioxide reduction catalyst, X in the general formula B is preferably selected from Cl, Br, and I.

Further, in the carbon dioxide reduction catalyst, Y in the general formula B is preferably triphenylphosphine.

Further, in the carbon dioxide reduction catalyst, L in the general formula B is preferably 4-phenylpyridine.

According to the present invention, the production of multi-electron reduction products such as CH ₄ , C ₂ H ₄ and C ₂ H ₅ OH can be promoted in the reduction reaction of carbon dioxide.

FIG. 1 is a schematic configuration diagram showing an example of a carbon compound reduction device according to the present embodiment. It is a schematic block diagram which shows another example of the carbon compound reduction apparatus based on this embodiment. 1 is an FE-SEM image of the reduction reaction electrodes of Examples 1 to 7. FIG. 1 is a schematic configuration diagram showing an example of a carbon compound reduction device according to the present embodiment.

The carbon compound reduction catalyst according to the present embodiment includes a metal halide salt having a ligand having a nitrogen atom, a ligand having a phosphorus atom, and two metal atoms, at least one of which is Cu. including metal complexes having

The reduction reaction of carbon dioxide proceeds as a metal complex that receives electrons from an electrode or the like reacts with carbon dioxide coordinated to a catalytic site. Here, in the reduction reaction of carbon dioxide, it is more efficient to generate multi-electron reduction products such as CH ₄ , C ₂ H ₄ , C ₂ H ₅ OH than to generate 2-electron reduction products such as CO and HCOOH. of electrons are required. Therefore, in order to promote the production of multi-electron reduction products, it is preferable to use a metal complex having a structure that accumulates many electrons around the catalytic site. A metal complex having a ligand having a nitrogen atom, a ligand having a phosphorus atom, and a metal halide salt having two metal atoms, at least one of which is Cu, has a catalytic site ( Since many electrons can be accumulated around the metal element (substantially Cu) constituting the metal halide salt, the multi-electron reduction reaction is promoted (that is, the generation of multi-electron reduction products is facilitated).

Ｍ：金属原子
Ｘ：ハロゲン
Ｙ：リン原子を含む配位子
Ｌ：窒素原子を含む配位子 The metal complex of this embodiment preferably has a crystal structure. A crystalline metal complex has, for example, a structure in which the following metal complex units extend on a one-dimensional main chain and overlap. The crystal structure can be analyzed by powder X-ray diffraction (XRD) measurement (Rigaku Co., Ltd., Ultima IV).

M: Metal atom X: Halogen Y: Ligand containing phosphorus atom L: Ligand containing nitrogen atom

When the metal complex of this embodiment forms a crystal structure, multiple halogenated metal salts come together and multiple metal elements (catalyst sites) are in close proximity, so many electrons are transferred from the catalytic sites to carbon dioxide reduction. Can be used for reactions. As a result, the production of multi-electron reduction products is promoted.

The crystalline metal complex is represented by, for example, the general formula A: [{CuMX ₂ (Y) ₂ }(L)] _n (n is, for example, 2 or more). In general formula A, M may be any metal element that constitutes a metal halide salt together with Cu, but it should be Cu, Ag, or Ni because it has high catalytic activity and promotes the production of multi-electron reduction products. is preferable, and Cu is more preferable. In general formula A, X is not particularly limited as long as it is a halogen atom, but is preferably selected from Br, Cl and I in terms of stability of crystal structure and amount of multi-electron reduced products, with Br being particularly preferred. preferable.

In general formula A, Y is not particularly limited as long as it is a ligand having a phosphorus atom, but examples include trimethylphosphine, triethylphosphine, tri-n-propylphosphine, triisopropylphosphine, tri-n-butylphosphine, tri- tert. -Butylphosphine, tri-n-pentylphosphine, tricyclopentylphosphine, tri-n-hexylphosphine, tricyclohexylphosphine, tri-n-heptylphosphine, tri-n-octylphosphine, triphenylphosphine, tris(2-methoxyphenyl) ) phosphine, tris(4-methoxyphenyl)phosphine, tris(2,6-dimethoxyphenyl)phosphine, tris(2-furyl)phosphine, tris(4-dimethylaminophenyl)phosphine, tri-p-tolylphosphine, tris- Examples include (4-fluorophenyl)phosphine, tris[3,5-bis(trifluoromethyl)phenyl]phosphine, and tris(pentafluorophenyl)phosphine. Among these, triphenylphosphine (PPh ₃ ) is preferred because of its low steric hindrance, hydrophobicity, and high stability.

In general formula A, L is not particularly limited as long as it is a ligand having a nitrogen atom, but it is preferably a ligand that carries the lowest unoccupied molecular orbital (LUMO) that receives electrons from an electrode, such as a bipyridine derivative. etc. Examples of bipyridine derivatives include 4,4'-di(4-pyridyl)biphenyl, 4,4'-dipyridyl, 1,2-di(4-pyridyl)ethane, 1,2-di(4-pyridyl)ethylene , N,N'-di(4-pyridyl)-1,4,5,8-naphthalenetetracarboxydiimide, 4-(4-piperidyl)pyridine, and the like. Among these, for example, 4,4'-di( 4-pyridyl)biphenyl is preferred. Note that the ligand L in the general formula A acts as a linking group in which the basic unit of the general formula A is repeated via the ligand L.

The crystal-structured metal complex is preferably fine particles having a particle size of, for example, 10 nm to 1000 nm in terms of catalytic activity for carbon dioxide reduction.

Moreover, the metal complex of this embodiment may be, for example, a metal complex represented by the following structural formula. That is, a metal complex represented by the general formula B: CuMX ₂ (Y) ₂ L ₂ may be used.

In general formula B, M may be any metal element that constitutes a metal halide salt together with Cu, but it should be Cu, Ag, or Ni because it has high catalytic activity and promotes the production of multi-electron reduction products. is preferable, and Cu is more preferable. In general formula B, X is not particularly limited as long as it is a halogen atom, but is preferably selected from Br, Cl and I in terms of stability of the crystal structure and amount of multi-electron reduced products, with Br being particularly preferred. preferable.

In general formula B, Y is not particularly limited as long as it is a ligand having a phosphorus atom, but examples include trimethylphosphine, triethylphosphine, tri-n-propylphosphine, triisopropylphosphine, tri-n-butylphosphine, tri- tert. -Butylphosphine, tri-n-pentylphosphine, tricyclopentylphosphine, tri-n-hexylphosphine, tricyclohexylphosphine, tri-n-heptylphosphine, tri-n-octylphosphine, triphenylphosphine, tris(2-methoxyphenyl) ) phosphine, tris(4-methoxyphenyl)phosphine, tris(2,6-dimethoxyphenyl)phosphine, tris(2-furyl)phosphine, tris(4-dimethylaminophenyl)phosphine, tri-p-tolylphosphine, tris- Examples include (4-fluorophenyl)phosphine, tris[3,5-bis(trifluoromethyl)phenyl]phosphine, and tris(pentafluorophenyl)phosphine. Among these, triphenylphosphine (PPh ₃ ) is preferred because of its hydrophobicity and high stability.

In general formula B, L is not particularly limited as long as it is a ligand having a pyridine ring. However, the ligand L does not act as a linking group in which the basic unit of general formula B is repeated via the ligand L. Ligands having such a pyridine ring include, for example, 2-methylpyridine, 2-ethylpyridine, 4-propylpyridine, 2-vinylpyridine, N,N-dimethyl-4-aminopyridine, 4-phenylpyridine, 2-hydroxypyridine, 1,5-naphthyridine, 2,2'-bipyridyl, 1,3-di(4-pyridyl)propane, 4-pyridyl-4'-methylpyridylbiphenyl, 4-methylpyridine, 3-benzylpyridine etc. Among these, 4-phenylpyridyl is preferred, for example, in that it can promote the production of multi-electron reduction products and in that it suppresses the hydrogen production reaction, which is a side reaction during the reduction reaction of carbon dioxide.

Although the general formula B (CuMX ₂ (Y) ₂ L ₂ ) does not have a repeating structure in which many basic units are connected like the general formula A, as a result of intensive study by the present inventors, the general formula B ( It was found that even in the metal complex represented by CuMX ₂ (Y) ₂ L ₂ ), many electrons are utilized from the catalytic site for the carbon dioxide reduction reaction, promoting the production of multi-electron reduction products. .

In addition to the above-mentioned metal complex, the carbon compound reduction catalyst according to the present embodiment may contain a conventionally known carbon compound reduction catalyst to the extent that the effects of the present embodiment are not impaired.

Below, an example of a carbon compound reduction device and an artificial photosynthesis device using the carbon compound reduction catalyst according to the present embodiment will be described.

1 and 4 are schematic configuration diagrams showing an example of a carbon compound reduction apparatus according to this embodiment. The carbon compound reduction apparatus 1 shown in FIGS. 1 and 4 is electrically connected to a reduction reaction electrode 10 (cathode) used for a reduction reaction of carbon compounds such as carbon dioxide, and is electrically connected to the reduction reaction electrode 10 to perform an oxidation reaction. It is configured to include an oxidation reaction electrode 12 (anode) that causes oxidation reaction, a diaphragm 11 disposed between the reduction reaction electrode 10 and the oxidation reaction electrode 12, and an electrolyte 14.

The carbon compound reduction apparatus 1 shown in FIGS. 1 and 4 includes a storage container 16, and the inside of the storage container 16 is divided into a gas supply chamber 18 and an electrolyte storage chamber 20 by a reduction reaction electrode 10. There is. One surface of the reduction reaction electrode 10 faces the gas supply chamber 18 , and the other surface of the reduction reaction electrode 10 faces the electrolyte storage chamber 20 . Note that the reduction reaction electrode 10 may be structurally supported by, for example, a frame material (not shown).

In the carbon compound reduction apparatus 1 shown in FIG. 1, a gas supply port 22 and a gas discharge port 24 are provided in the side wall of the storage container 16 on the gas supply chamber 18 side. Then, gas containing carbon compounds such as carbon dioxide is supplied into the gas supply chamber 18 from the gas supply port 22, and the gas within the gas supply chamber 18 is discharged from the gas discharge port 24. It is desirable that the gas containing the carbon compound contains moisture such as water vapor. The electrolytic solution 14 is accommodated in the electrolytic solution storage chamber 20 . Further, a diaphragm 11 and an oxidation reaction electrode 12 are installed in the electrolyte storage chamber 20 in which the electrolyte 14 is stored. In the carbon compound reduction apparatus 1 shown in FIG. 4, a gas supply port 22 is provided in the side wall of the container 16 on the gas supply chamber 18 side, and a gas exhaust port 22 is provided in the upper wall of the container 16 on the electrolyte storage chamber 20 side. An outlet 24 is provided. Gas containing carbon compounds such as carbon dioxide is supplied into the gas supply chamber 18 from the gas supply port 22, passes through the reduction reaction electrode 10 and the diaphragm 11, passes through the electrolyte storage chamber 20, and passes through the gas discharge port. It is discharged from 24.

Electrolyte solution 14 includes an electrolyte and a solvent. Examples of electrolytes include potassium chloride (KCl), sodium hydrogen carbonate (NaHCO ₃ ), potassium hydrogen carbonate (KHCO ₃ ), potassium sulfate (K ₂ SO ₄ ), potassium carbonate (K ₂ CO ₃ ), and potassium tetraborate. (K ₂ B ₄ O ₇ ), dipotassium hydrogen phosphate (K ₂ HPO ₄ ), potassium dihydrogen phosphate (KH ₂ PO ₄ ), potassium hydroxide (KOH), and the like. Examples of the solvent include water.

The concentration of the electrolyte is, for example, in the range of 0.01 mol/L to 10 mol/L, preferably in the range of 0.1 mol/L to 1.0 mol/L.

The reduction reaction electrode 10 is an electrode used to reduce carbon dioxide. The reduction reaction electrode 10 is formed, for example, by supporting the carbon compound reduction catalyst described above on a base material having a porous structure. The base material preferably has electrical conductivity, and examples thereof include carbon nanotubes, graphene, carbon black, carbon cloth, carbon paper, glassy carbon, graphite, tantalum (Ta), and the like.

The oxidation reaction electrode 12 is an electrode used to oxidize a substance by an oxidation reaction. The oxidation reaction electrode 12 includes, for example, a substrate and an oxidation catalyst supported on the substrate. Examples of the oxidation catalyst include nickel compounds such as hydroxides, iron compounds such as hydroxides, cobalt compounds such as phosphoric acid or boric acid, iridium oxide (IrO _x , x = 1 to 2), semiconductors, etc. . Examples of the substrate include stainless steel, carbon, fluorine-containing tin oxide (FTO), tin-doped indium oxide (ITO), and semiconductors. Examples of the semiconductor include tungsten oxide (WO ₃ ), bismuth vanadate (BiVO ₄ ), iron oxide (Fe ₂ O ₃ ), silicon (Si), tantalum oxynitride (TaON), and the like.

The diaphragm 11 used in the carbon compound reduction apparatus shown in FIGS. 1 and 4 is an anion exchange membrane, a cation exchange membrane, or a combination of both. In addition, in that it can promote the production of multi-electron reduction products, it has an anion exchange membrane disposed on the reduction reaction electrode side and a cation exchange membrane disposed on the oxidation reaction electrode side. Preference is given to using composite membranes.

The diaphragm 11 may be structurally supported, for example, together with the reduction reaction electrode 10 by a frame material (not shown).

By electrically connecting the reduction reaction electrode 10 and the oxidation reaction electrode 12 and applying an appropriate bias voltage, an oxidation reaction occurs in the oxidation reaction electrode 12, and the reduction reaction In the electrode 10, a reduction reaction of carbon compounds such as carbon dioxide supplied from the gas supply port 22 into the gas supply chamber 18 proceeds. At the reduction reaction electrode, for example, two-electron reduction products such as CO and HCOOH and multi-electron reduction products such as CH ₄ , C ₂ H ₄ , and C ₂ H ₅ OH are generated by the reduction reaction of carbon dioxide. However, the catalytic action of the carbon compound reduction catalyst described above promotes the production of multi-electron reduction products such as CH ₄ , C ₂ H ₄ and C ₂ H ₅ OH.

The means for applying a bias voltage to the reduction reaction electrode 10 and the oxidation reaction electrode 12 is not particularly limited, and may include chemical batteries (including primary batteries, secondary batteries, etc.), constant voltage sources, solar cells, etc. can be mentioned.

By using a solar cell as a means for applying a bias voltage, the carbon compound reducing apparatus 1 shown in FIGS. An artificial photosynthesis device can be provided. In the artificial photosynthesis device according to this embodiment, the reduction reaction electrode 10 and the oxidation reaction electrode 12 of the carbon compound reduction device are connected via a solar cell, and the artificial photosynthesis device is driven using sunlight as an energy source.

FIG. 2 is a schematic configuration diagram showing another example of the carbon compound reduction apparatus according to the present embodiment. In the carbon compound reduction apparatus 2 shown in FIG. 2, the inside of the storage container 16 is divided into a cathode chamber 28 and an anode chamber 30 by a diaphragm 11, the reduction reaction electrode 10 is arranged in the cathode chamber 28, and the anode chamber 30 is divided into a cathode chamber 28 and an anode chamber 30. An oxidation reaction electrode 12 is placed inside. Further, the cathode chamber 28 and the anode chamber 30 contain an electrolytic solution 14. Note that the electrolytes 14 contained in the cathode chamber 28 and the anode chamber 30 may be the same or different. In the carbon compound reducing apparatus 2 shown in FIG. 2, carbon dioxide-containing gas is directly supplied to the cathode chamber 28 partitioned by the diaphragm 11.

The carbon compound reduction apparatus (1-2) shown in FIGS. 1-2 and 4 is a two-electrode type using an electrode 10 for reduction reaction and an electrode 12 for oxidation reaction, but is not limited thereto. A combined three-electrode type may also be used. In the case of a two-chamber type device including a cathode chamber 28 and an anode chamber 30, such as the carbon compound reduction device 2 in FIG. 2, the reference electrode is installed on the cathode chamber 28 side, for example.

EXAMPLES Hereinafter, the present invention will be further explained with reference to Examples, but the present invention is not limited to these Examples.

<Metal complex preparation method 1>
PPh ₃ nanocrystals/bipyridine derivative nanocrystals were mixed by injecting triphenylphosphine (PPh ₃ ) acetone solution (400 μL) and bipyridine derivative dimethylformamide solution (400 μL) into stirred ultrapure water (20 mL) using a syringe. A dispersion liquid was prepared. Finally, copper bromide (CuBr) acetonitrile solution (400 μL) was added dropwise to the PPh ₃ nanocrystal dispersion using a syringe, and the mixture was left to stand for _{10 minutes} . ₃ ) ₂ }(L)] _n (wherein L is a bipyridine derivative) was obtained.

<Metal complex preparation method 2>
A mixed dispersion of PPh ₃ nanocrystals/bipyridine derivative nanocrystals was obtained by injecting a triphenylphosphine (PPh ₃ ) acetone solution (400 μL) into the stirred bipyridine derivative-containing aqueous solution (20 mL) using a syringe. Finally, copper bromide (CuBr) acetonitrile solution (400 μL) was added dropwise to the PPh ₃ nanocrystal dispersion using a syringe, and the mixture was left to stand for _{10 minutes} . ₃ ) ₂ }(L)] _n (wherein L is a bipyridine derivative) was obtained.

(Example 1)
<Electrode for reduction reaction>
Using 4,4'-di(4-pyridyl)biphenyl (hereinafter referred to as DPB1) as a bipyridine derivative, [{Cu ₂ Br ₂ (PPh ₃ ) ₂ } (DBP1) _] was prepared by the above production method 1. We prepared the metal complex nanocrystals shown below.

20 mL of the metal complex nanocrystals prepared above were filtered on each side of a 50 mm x 50 mm water-repellent carbon paper, and the metal complex nanocrystals were transferred to the carbon paper.

< _CO2 reduction reaction test>
For electrochemical measurements, a three-electrode device shown in FIG. 1 was used. The above electrode for reduction reaction was used as a working electrode, platinum foil (manufactured by Nilaco, PT-353212, φ20 mm x 0.02 mm, 99.98%) was used as a counter electrode (electrode for oxidation reaction), and silver/silver chloride electrode (manufactured by Nilaco Co., Ltd.) was used as a reference electrode. EC Frontier, RE-14) was used. Further, as a diaphragm, a composite membrane was used that had an anion exchange membrane (manufactured by Astom, ASE) placed on the working electrode side and a cation exchange membrane (manufactured by DuPont, Nafion 117) placed on the counter electrode side. Furthermore, a 0.5M potassium hydrogen carbonate aqueous solution was used as the electrolyte. The working electrode, counter electrode, and reference electrode were connected to an electrochemical measurement system (Bio-Logic Science Instruments, SP-150), and while carbon dioxide gas (99.995%) was flowing through the gas supply chamber at 10 ml/min, the working electrode was Electrochemical measurements were performed by applying a voltage of -1.6 V (vs.Ag/AgCl) to the poles for 2 hours. An online gas chromatograph (SRI Instruments, Multiple Gas Analyzer #5) was used for product identification and quantification associated with electrochemical measurements. MOLECULAR SIEVE SA and HAYESEP-D were used as columns, and a thermal conductivity type detector (TCD) and a flame ionization detector (FID) were used as detectors.

(Example 2)
Using 4,4'-dipyridyl (hereinafter referred to as BPY) as a bipyridine derivative, the metal complex nano represented by [{Cu ₂ Br ₂ (PPh ₃ ) ₂ } (BPY)] _n was prepared by the above production method 2. A reduction reaction electrode was prepared in the same manner as in Example 1, except that crystals were prepared, and a CO ₂ reduction reaction test was conducted in the same manner as in Example 1 using the reduction reaction electrode.

(Example 3)
Using 1,2-di(4-pyridyl)ethane (hereinafter referred to as DPE1) as a bipyridine derivative, by the above production method 2, a compound represented by [{Cu ₂ Br ₂ (PPh ₃ ) ₂ } (DPE1)] _n A reduction reaction electrode was prepared in the same manner as in Example 1, except that metal complex nanocrystals were prepared, and a CO ₂ reduction reaction test was conducted in the same manner as in Example 1 using the reduction reaction electrode. .

(Example 4)
Using 1,2-di(4-pyridyl)ethylene (hereinafter referred to as DPE2) as a bipyridine derivative, by the above production method 2, a compound expressed by [{Cu ₂ Br ₂ (PPh ₃ ) ₂ } (DPE2)] _n A reduction reaction electrode was prepared in the same manner as in Example 1, except that metal complex nanocrystals were prepared, and a CO ₂ reduction reaction test was conducted in the same manner as in Example 1 using the reduction reaction electrode.

(Example 5)
[{Cu ₂ Br ₂ (PPh ₃ ) ₂ } (NDI)] A reduction reaction electrode was prepared in the same manner as in Example 1, except that metal complex nanocrystals represented by _n were prepared, and the reduction reaction electrode was used to carry out the experiment. A CO ₂ reduction reaction test was conducted in the same manner as in Example 1.

(Example 6)
Using 4-(4-piperidyl)pyridine (hereinafter referred to as PPP) as a bipyridine derivative, the metal represented by [{Cu ₂ Br ₂ (PPh ₃ ) ₂ } (PPP)] _n A reduction reaction electrode was prepared in the same manner as in Example 1, except that complex nanocrystals were prepared, and a CO ₂ reduction reaction test was conducted in the same manner as in Example 1 using the reduction reaction electrode.

(Example 7)
Using 4-(4-piperidyl)pyridine (hereinafter referred to as PPP) as a bipyridine derivative, the metal represented by [{Cu ₂ Cl ₂ (PPh ₃ ) ₂ } (PPP)] _n A reduction reaction electrode was prepared in the same manner as in Example 1, except that complex nanocrystals were prepared, and a CO ₂ reduction reaction test was conducted in the same manner as in Example 1 using the reduction reaction electrode.

(Comparative example 1)
A reduction reaction electrode was prepared by adding an acetonitrile solution (400 μL) containing copper bromide (CuBr) to water-repellent carbon paper using a drop-casting method, and using the reduction reaction electrode, Example 1 A CO ₂ reduction reaction test was conducted in the same manner.

(Comparative example 2)
A copper film was formed on a 50 mm x 50 mm water-repellent carbon paper by a sputtering method using an RF magnetron sputtering device (manufactured by Canon Tokki, SPK-404L). A copper disk (manufactured by Kojundo Kagaku Kenkyusho, φ101.6×t1, 99.99%) was used as the target. The applied power was 100 W, the sputtering gas was Ar, the back pressure was 0.5 Pa, and a copper film was formed to a thickness of about 100 nm. Using this as an electrode for reduction reaction, a CO ₂ reduction reaction test was conducted in the same manner as in Example 1.

FIG. 3 shows FE-SEM images of the reduction reaction electrodes of Examples 1 to 7. FE-SEM images were obtained using a scanning electron microscope S-5500 (Hitachi High Technologies). As shown in FIG. 3, Examples 1 to 7 had different crystal structure forms of the metal complexes. This is thought to be due to differences in the ligands of the metal complexes, and more specifically, to differences in rigidity and rotational freedom of the ligands.

Tables 1 and 2 show the current efficiency and amount of products produced in the CO ₂ reduction reaction tests of Examples 1 to 7 and Comparative Examples 1 and 2.

From the results in Tables 1 and 2, it can be seen that in Examples 1 to 7, the production ratio of multi-electron reduction products such as CH ₄ , C ₂ H ₄ , C ₂ H ₅ OH, etc. was higher than in Comparative Examples 1 and 2. increased. That is, the metal complexes used in Examples 1 to 7 were able to promote the production of multi-electron reduction products in the reduction reaction of carbon dioxide. Among Examples 1 to 7, Example 1, which used a metal complex coordinated with 4,4'-di(4-pyridyl)biphenyl (DPB), had a high production rate of multi-electron reduction products and also Hydrogen production rate was low. This is considered to be because the hydrophobicity and electron-withdrawing properties of the DPB ligand are higher than those of other ligands.

(Example 8)
Triphenylphosphine (PPh ₃ ) acetone solution (400 μL) and 4-phenylpyridine (4PP) acetone solution (400 μL) were mixed, and copper bromide (CuBr) acetonitrile solution (400 μL) was added to the resulting mixture using a syringe. A metal complex molecule (CuBr(PPh ₃ ) 4PP) was obtained by dropping the mixture and leaving it for 10 minutes.

The metal complex molecules prepared above in an amount corresponding to 2 μM were filtered under reduced pressure on both sides of a 20 mm x 20 mm water-repellent carbon paper, and the metal complex molecules prepared above were transferred to the carbon paper. This was used as an electrode for reduction reaction.

< _CO2 reduction reaction test>
For the electrochemical measurements, the apparatus shown in FIG. 2 was used. The above electrode for reduction reaction was used as a working electrode, platinum foil (manufactured by Nilaco, PT-353212, φ20 mm x 0.02 mm, 99.98%) was used as a counter electrode (electrode for oxidation reaction), and silver/silver chloride electrode (manufactured by Nilaco Co., Ltd.) was used as a reference electrode. EC Frontier, RE-14) was used. A reduction reaction electrode and a reference electrode were installed in the cathode chamber, and a counter electrode was installed in the anode chamber. An anion exchange membrane (manufactured by Astom, ASE) was used as a diaphragm that partitioned the cathode chamber and the anode chamber. Furthermore, a 0.5M potassium hydrogen carbonate aqueous solution was used as the electrolyte.

The working electrode, counter electrode, and reference electrode were connected to an electrochemical measurement system (Bio-Logic Science Instruments, SP-150), and carbon dioxide gas (99.995%) was supplied to the electrolyte in the cathode chamber at a rate of 10 ml/min. Electrochemical measurements were performed by applying a voltage of -2.0 V (vs.Ag/AgCl) to the working electrode for 1 hour. An online gas chromatograph (SRI Instruments, Multiple Gas Analyzer #5) was used for product identification and quantification associated with electrochemical measurements. MOLECULAR SIEVE SA and HAYESEP-D were used as columns, and a thermal conductivity type detector (TCD) and a flame ionization detector (FID) were used as detectors.

(Example 9)
In preparing the metal complex molecule, the 4-phenylpyridine (4PP) acetone solution (400 μL) was changed to the 4-pyridyl-4'-methylpyridylbiphenyl (mDPB) acetone solution (400 μL), and the metal complex molecule (CuBr(PPh) ₃ ) A reduction reaction electrode was produced in the same manner as in Example 8, except that mDPB) was obtained. Then, using the reduction reaction electrode, a CO ₂ reduction reaction test was conducted in the same manner as in Example 8.

(Comparative example 3)
Using the reduction reaction electrode of Comparative Example 2, a CO ₂ reduction reaction test was conducted in the same manner as in Example 8.

Tables 3 and 4 show the current efficiency and amount of products produced in the CO ₂ reduction reaction tests of Examples 8 to 9 and Comparative Example 3.

The results of the current efficiency of the product produced by the CO ₂ reduction reaction test (the amount produced in parentheses) were as follows: In Example 8, CH ₄ : 50.7% (47.1 μmol), H ₂ : 14.1% ( 52.6 μmol), C ₂ H ₄ : 15.9% (9.1 μmol), C ₂ H ₅ OH: 9.1% (5.2 μmol), and in Example 9, CH ₄ : 31.7%. (18.5 μmol), H ₂ : 30.0% (70.3 μmol), C ₂ H ₄ : 2.3% (1.2 μmol), C ₂ H ₅ OH: 1.8% (0.9 μmol). Yes, in Comparative Example 3, CH ₄ : 13.3% (11.3 μmol), H ₂ : 43.0% (146.5 μmol), C ₂ H ₄ : 11.1% (6.3 μmol), C ₂ H ₅ OH: 4.4% (2.5 μmol). That is, in both Examples 8 and 9, compared to Comparative Example 3, the production rate of CH ₄ was increased and the production rate of H ₂ as a byproduct was able to be decreased. In particular, in Example 8 using a metal complex coordinated with 4-phenylpyridine, not only the production rate of CH ₄ but also the production rate of C ₂ H ₄ and C ₂ H ₅ OH increased.

Reference Signs List 1, 2 Carbon compound reduction device, 10 Electrode for reduction reaction, 11 Diaphragm, 12 Electrode for oxidation reaction, 14 Electrolyte, 16 Storage container, 18 Gas supply chamber, 20 Electrolyte storage chamber, 22 Gas supply port, 24 Gas exhaust Exit, 28 cathode chamber, 30 anode chamber.

Claims (10)

a ligand having a nitrogen atom;
a ligand having a phosphorus atom;
A carbon dioxide reduction catalyst comprising a metal complex having a metal halide salt having two metal atoms, at least one of which is Cu. The carbon dioxide reduction catalyst according to claim 1, wherein the metal complex has a crystal structure. The metal complex is represented by the general formula [{CuMX ₂ (Y) ₂ } (L)] _n , where M is Cu, Ag or Ni, X is a halogen atom, and Y is a phosphorus atom. 3. The carbon dioxide reduction catalyst according to claim 1, wherein L is a ligand having a nitrogen atom. 4. The carbon dioxide reduction catalyst according to claim 3, wherein X in the general formula is selected from Cl, Br, and I. The carbon dioxide reduction catalyst according to claim 3 or 4, wherein Y in the general formula is triphenylphosphine. The carbon dioxide reduction catalyst according to any one of claims 3 to 5, wherein L in the general formula is a bipyridine derivative. The metal complex is represented by the general formula CuMX ₂ (Y) ₂ L ₂ , where M is Cu, Ag or Ni, X is a halogen atom, and Y is a ligand having a phosphorus atom. 2. The carbon dioxide reduction catalyst according to claim 1, wherein L is a ligand having a pyridine ring. The carbon dioxide reduction catalyst according to claim 7, wherein X in the general formula is selected from Cl, Br, and I. The carbon dioxide reduction catalyst according to claim 7 or 8, wherein Y in the general formula is triphenylphosphine. The carbon dioxide reduction catalyst according to any one of claims 7 to 9, wherein L in the general formula is 4-phenylpyridine.