Copper cluster, electrode for use in reduction of carbon dioxide, carbon dioxide reduction device, and method for producing methanol

Abstract

Provided are: a copper cluster which can be used as a catalyst capable of increasing the efficiency of a reaction for producing methanol by reducing carbon dioxide; an electrode for use in the reduction of carbon dioxide, which is prepared using the copper cluster; a carbon dioxide reduction device using the electrode for use in the reduction of carbon dioxide; and a method for producing methanol.　A copper cluster according to the present invention includes copper atoms, hydrogen atoms, and organic ligands, wherein the number of the copper atoms is 58, the number of the hydrogen atoms is 20, and the number of the organic ligands is 36 to 43 inclusive. The organic ligands preferably include at least one type of organic ligands selected from organic ligands each containing a phosphorus atom and organic ligands each containing a sulfur atom. In the copper cluster, the number of the organic ligands each containing a phosphorus atom is preferably 0 to 7 inclusive, and the number of the organic ligands each containing a sulfur atom is preferably 36.

Description

Copper cluster, carbon dioxide reduction electrode, carbon dioxide reduction device, and method for producing methanol

[0001] The present invention relates to a copper cluster, an electrode for carbon dioxide reduction, a carbon dioxide reduction device, and a method for producing methanol.

[0002] While renewable energy is being increasingly installed in the power generation sector, industries, transportation and forestry are facing serious CO2 emissions due to population growth and the modernization of developing countries. ２ Therefore, carbon sources such as raw materials for chemical products, raw materials for industrial products, and agricultural raw materials are being converted into CO ２ Specifically, for example, CO ２ A technique for producing methane, ethylene, ethane, etc. by reducing the above has been disclosed (Patent Document 1, etc.).

[0003] JP 2018-168410 A

[0004] where CO ２ The cost of recovering CO ２ Compounds that can be sold at prices exceeding the production costs from CO ２ Therefore, methanol, which has high added value and is in high demand, is produced from CO ２ Methanol can be used as a raw material for, for example, acetic acid, methyl methacrylate, and dimethyl ether, and is also being considered as an energy source for fuel cells, so demand for it is expected to grow further in the future.

[0005] However, in the prior art such as Patent Document 1, CO ２ In the reduction of CO, many compounds other than methanol are produced, resulting in low efficiency in the production of methanol. ２ Therefore, a catalyst capable of efficiently reducing methyl methyl ether to produce methanol is desired.

[0006] Therefore, an object of the present invention is to provide a copper cluster that can be used as a catalyst that can increase the efficiency of a reaction for producing methanol by reducing carbon dioxide, a carbon dioxide reduction electrode that uses the copper cluster, a carbon dioxide reduction device that uses the carbon dioxide reduction electrode, and a method for producing methanol.

[0007] The present inventors have discovered that the above-mentioned problems can be solved by a copper cluster containing copper atoms, hydrogen atoms, and organic ligands, the copper cluster having 58 copper atoms, 20 hydrogen atoms, and 36 to 43 organic ligands, and have thus completed the present invention. More specifically, the present invention is as follows.

[0008] (1) A copper cluster comprising copper atoms, hydrogen atoms, and organic ligands, wherein the number of copper atoms is 58, the number of hydrogen atoms is 20, and the number of organic ligands is 36 to 43.

[0009] (2) The copper cluster according to (1) above, wherein the organic ligand includes at least one of an organic ligand containing a phosphorus atom and an organic ligand containing a sulfur atom.

[0010] (3) The copper cluster according to (2) above, wherein the number of organic ligands containing phosphorus atoms is 0 to 7, and the number of organic ligands containing sulfur atoms is 36.

[0011] (4) A carbon dioxide reduction electrode for producing methanol by reducing carbon dioxide, the carbon dioxide reduction electrode comprising: an electrode substrate; and a copper cluster provided on the electrode substrate, the copper cluster being the copper cluster according to any one of (1) to (3) above.

[0012] (5) The carbon dioxide reduction electrode according to (4) above, wherein the copper clusters are supported on a porous body.

[0013] (6) A carbon dioxide reduction device comprising the carbon dioxide reduction electrode according to (4) or (5) above.

[0014] (7) A method for producing methanol, comprising the step of producing methanol by reducing carbon dioxide using the carbon dioxide reduction electrode according to (4) or (5) above.

[0015] The present invention can provide a copper cluster that can be used as a catalyst that can increase the efficiency of a reaction for producing methanol by reducing carbon dioxide, a carbon dioxide reduction electrode that uses the copper cluster, a carbon dioxide reduction device that uses the carbon dioxide reduction electrode, and a method for producing methanol.

[0016] 1 is a schematic side view showing an example of a copper cluster according to the present embodiment; FIG. 2 is a diagram showing the results of gas chromatography (TCD and FID) in Example 1; １ Fig. 1 is a diagram showing the results of H NMR. Fig. 2 is a diagram showing the Faraday efficiency of Example 1. Fig. 3 is a diagram showing the Faraday efficiency of Example 1 when Ar gas is used. Fig. 4 is a diagram showing the Faraday efficiencies of Example 1 and Comparative Example 1.

[0017] <Copper Cluster> The copper cluster contains copper atoms, hydrogen atoms, and organic ligands. The copper cluster contains 58 copper atoms, 20 hydrogen atoms, and 36 to 43 organic ligands. That is, the copper cluster is a metal cluster in which 58 copper atoms are bonded, and contains 20 hydrogen atoms and 36 to 43 organic ligands.

[0018] Examples of organic ligands contained in copper clusters include organic ligands containing phosphorus atoms and organic ligands containing sulfur atoms. The copper cluster preferably has an organic ligand containing phosphorus atoms and an organic ligand containing sulfur atoms. Examples of organic ligands containing phosphorus atoms include triphenylphosphine, diphenyl(p-tolyl)phosphine, and tri(p-tolyl)phosphine. Examples of organic ligands containing sulfur atoms include alkanethiols having 1 to 4 carbon atoms. Specific examples of organic ligands containing sulfur atoms include methanethiol, ethanethiol, propanethiol, and butanethiol.

[0019] The copper cluster preferably has 0 to 7 organic ligands containing phosphorus atoms, more preferably 4 to 7 organic ligands containing phosphorus atoms, and even more preferably 6 to 7 organic ligands containing phosphorus atoms. The copper cluster preferably has 36 organic ligands containing sulfur atoms. The numbers and compositions of copper atoms, hydrogen atoms, and organic ligands (e.g., organic ligands containing phosphorus atoms, organic ligands containing sulfur atoms) of the copper cluster can be determined by X-ray diffraction and electrospray ionization mass spectrometry (ESI-MS).

[0020] As shown in the examples described below, the copper cluster described above can be used as a catalyst capable of increasing the efficiency of the reaction for producing methanol by reducing carbon dioxide. That is, by using the copper cluster described above, the reaction for producing methanol by reducing carbon dioxide can be increased in efficiency. In other words, by using the copper cluster described above, the selectivity of methanol in the reaction for reducing carbon dioxide can be increased. By using the copper cluster described above, the faradaic efficiency of methanol in the electrochemical reaction for producing methanol by reducing carbon dioxide can be increased to, for example, 50% or more, or even 60% or more. Furthermore, by using the copper cluster described above, the production of by-products other than methanol (such as formic acid and methane) can be significantly suppressed in the electrochemical reaction for producing methanol by reducing carbon dioxide. For example, the faradaic efficiency of by-products other than methanol can be reduced to, for example, 10% or less, 5% or less, or even 0%. The reaction for producing methanol by reducing carbon dioxide using the copper cluster described above can be carried out at room temperature and atmospheric pressure.

[0021] Theoretical calculations suggest that the reason why the use of the above-described copper cluster can improve the efficiency of the reaction for producing methanol by reducing carbon dioxide is due to the following mechanism. A copper cluster containing copper atoms, hydrogen atoms, and organic ligands, where the number of copper atoms is 58, the number of hydrogen atoms is 20, and the number of organic ligands is 36 to 43, has a regular hexahedral structure as shown in FIG. 1 , in which the organic ligands are coordinated to the copper atoms. FIG. 1 shows an example of a copper cluster according to this embodiment. FIG. 1 is a schematic side view showing an example of a copper cluster according to this embodiment. In FIG. 1 , the organic ligands are triphenylphosphine (an organic ligand containing a phosphorus atom) and propanethiol (an organic ligand containing a sulfur atom), and the copper cluster shown has 7 triphenylphosphines and 36 propanethiol units. For simplicity of illustration, the carbon chain and benzene ring are represented by hydrogen atoms in FIG. 1 .

[0022] When the number of copper atoms is 58, the number of hydrogen atoms is 20, and the number of organic ligands is 36 to 43, at least one of the eight copper atoms located on each of the eight edges (corners) of the regular hexahedron has a structure in which an organic ligand can be coordinated but is not coordinated. More specifically, when the number of copper atoms is 58 and the number of hydrogen atoms is 20, a maximum of 44 organic ligands can be coordinated, but since the number of organic ligands in the above-mentioned copper cluster is 36 to 43, at least one of the eight copper atoms located on each of the eight edges (corners) of the regular hexahedron has a position in which an organic ligand is not coordinated. In FIG. 1, of eight copper atoms (Cu1 to Cu4 on the near side of the paper in FIG. 1 and four copper atoms on the far side of the paper in FIG. 1) located on each of the eight edges (corners) of a regular hexahedron, seven copper atoms are coordinated with triphenylphosphine and propanethiol, but one copper atom (Cu1) is not coordinated with triphenylphosphine.

[0023] In this way, since there are portions where no organic ligands are coordinated to the copper atoms at the edges of the regular hexahedron, the copper atom (Cu11) adjacent to the copper atom (Cu1) at the edge (corner) and the central portion between the two copper atoms adjacent to the copper atom (Cu11) (the gap (hollow sites) surrounded by three copper atoms shown by dotted lines in Figure 1)) contain protons H ＋ In addition, a proton H is present at the center of the adjacent copper atom (Cu11) and two copper atoms adjacent to the copper atom (Cu11). ＋ The reason why the copper atom (Cu1) at the edge is easily adsorbed is presumably because the presence of the organic ligand (steric hindrance) prevents the copper atom at the edge from moving outward from the copper cluster, and the copper atom (Cu1) at the edge is pushed into the inside of the copper cluster. As shown in FIG. 1, a proton H ＋ When adsorbed, the adsorbed protons react with carbon dioxide to easily form intermediates in the reaction that reduces carbon dioxide to produce methanol. This lowers the energy barrier for the reaction that reduces carbon dioxide to produce methanol, enabling highly efficient production of methanol.

[0024] On the other hand, a copper cluster containing copper atoms, hydrogen atoms, and organic ligands, in which the number of copper atoms is 58 and the number of hydrogen atoms is 20, but the number of organic ligands is 44, has an extremely low methanol production efficiency or is unable to produce methanol.

[0025] <<Method for Producing Copper Clusters>> The above-mentioned copper clusters containing copper atoms, hydrogen atoms, and organic ligands, where the number of copper atoms is 58, the number of hydrogen atoms is 20, and the number of organic ligands is 36 to 43, can be produced, for example, by mixing a solution containing a copper compound with the organic ligands, and then adding a reducing agent to reduce the copper compound. After mixing the solution containing the copper compound with the organic ligands and then adding a reducing agent to reduce the copper compound, the mixture may be washed with a solvent such as an organic solvent, as necessary.

[0026] The copper compound is tetrakis(acetonitrile)copper(I) tetrafluoroborate (Cu(CH ３ CN) ４ BF ４ ), tetrakis(acetonitrile)copper(I) hexafluorophosphate (Cu(CH ３ CN) ４ ) PF ６ ), copper(II) acetate (Cu(CH ３ COO) ２ ), copper(II) trifluoroacetate (Cu(CF ３ COO) ２ ), copper(II) nitrate (Cu(NO ３ ) ２ ), copper(I) thiocyanate (CuSCN), copper(I) chloride (CuCl), copper(I) bromide (CuBr), etc. The organic ligands are as described above in the section on copper clusters. The reducing agent is sodium borohydride (NaBH ４ ), lithium borohydride (LiBH ４ ), diphenylsilane (Ph ２ SiH ２ ), triethylamine ((CH ３ CH ２ ) ３ N), borane tert-butylamine (tert-BuNH ２ ・BH ３ The temperature at which the copper compound is reduced by adding the reducing agent is preferably −10° C. or higher and 25° C. or lower, and more preferably 0° C. or higher and 10° C. or lower.

[0027] When two or more kinds of organic ligands are used as the organic ligand, the solution containing the copper compound and the two or more kinds of organic ligands may be mixed simultaneously, but it is preferable to mix the organic ligands one by one into the solution containing the copper compound. For example, when the organic ligands include an organic ligand containing a phosphorus atom and an organic ligand containing a sulfur atom, it is preferable to mix the organic ligand containing a phosphorus atom into the solution containing the copper compound, and then mix the organic ligand containing a sulfur atom into the solution containing the copper compound.

[0028] Here, the copper cluster described above contains copper atoms, hydrogen atoms, and organic ligands, and the number of copper atoms is 58, the number of hydrogen atoms is 20, and the number of organic ligands is 36 to 43. Thus, the number of atoms in a copper cluster having a specific number of atoms can be adjusted by adjusting the amount of raw materials. For example, the number of organic ligands in the produced copper cluster can be reduced by reducing the number of moles of the raw material for the organic ligand. For example, the ratio of the number of moles of organic ligand to the number of moles of copper compound used in the production of the copper cluster (number of moles of organic ligand / number of moles of copper compound) is preferably 0.0 to 100, more preferably 0.7 to 2.0. Furthermore, when an organic ligand containing a phosphorus atom and an organic ligand containing a sulfur atom are used as the organic ligand, the ratio of the number of moles of the organic ligand containing a phosphorus atom to the number of moles of the copper compound used in the production of the copper cluster (number of moles of organic ligand containing a phosphorus atom / number of moles of copper compound) is preferably 0.0 or more and 100 or less, more preferably 0.1 or more and 1.2 or less, and the ratio of the number of moles of the organic ligand containing a sulfur atom to the number of moles of the copper compound (number of moles of organic ligand containing a sulfur atom / number of moles of copper compound) is preferably 0.0 or more and 100 or less, more preferably 0.5 or more and 0.8 or less.

[0029] The copper clusters described above may also be produced by mixing a solution containing a copper compound with an organic ligand, reducing the copper compound with a reducing agent to obtain copper clusters containing the organic ligands, and then partially removing the organic ligands. For example, the copper clusters described above may also be produced by mixing a solution containing a copper compound with an organic ligand containing a phosphorus atom and an organic ligand containing a sulfur atom, adding a reducing agent to reduce the copper compound, obtaining organic ligands containing a phosphorus atom and organic ligands containing a sulfur atom, and then partially removing the organic ligands containing a phosphorus atom.

[0030] <Carbon Dioxide Reduction Electrode> The above-mentioned copper cluster containing copper atoms, hydrogen atoms, and organic ligands, wherein the number of copper atoms is 58, the number of hydrogen atoms is 20, and the number of organic ligands is 36 or more and 43 or less, can be used as a catalyst for the carbon dioxide reduction reaction in a carbon dioxide reduction electrode for producing methanol by reducing carbon dioxide. Such a carbon dioxide reduction electrode for producing methanol by reducing carbon dioxide includes an electrode substrate and a copper cluster provided on the electrode substrate, and the copper cluster is the above-mentioned copper cluster containing copper atoms, hydrogen atoms, and organic ligands, wherein the number of copper atoms is 58, the number of hydrogen atoms is 20, and the number of organic ligands is 36 or more and 43 or less.

[0031] By using the above-described carbon dioxide reduction electrode, the efficiency of the reaction for producing methanol by reducing carbon dioxide can be increased.

[0032] The electrode substrate of the carbon dioxide reduction electrode is not particularly limited, and any known conductive substrate can be used. Examples of the electrode substrate for the carbon dioxide reduction electrode include known substrates used as electrodes for carbon dioxide reduction, and specific examples include metal substrates, carbon substrates, and glass substrates.

[0033] Examples of metal substrates include substrates of simple metals such as nickel, titanium, iron, and copper, and substrates of alloys. Examples of carbon substrates include glassy carbon (GC), carbon paper, carbon fiber paper, and carbon rods. Examples of glass substrates include conductive glass. The electrode substrate may be a porous material.

[0034] In the carbon dioxide reduction electrode, the copper clusters are preferably supported on a porous body. When the copper clusters are supported on a porous body in the carbon dioxide reduction electrode, the copper clusters may be supported on a porous body, and the porous body may be provided on the surface of an electrode substrate, or the copper clusters may be supported on an electrode substrate made of a porous body. Examples of porous bodies that support copper clusters include carbon black and metal oxides.

[0035] The method for producing the carbon dioxide reduction electrode is not particularly limited. For example, a catalyst can be produced by impregnating a porous body such as carbon black with a solution of the copper clusters described above, and then applying a liquid containing the catalyst (e.g., catalyst slurry) to an electrode substrate, thereby producing the carbon dioxide reduction electrode. Note that the catalyst is preferably produced without calcining the copper clusters.

[0036] <Carbon dioxide reduction device> The carbon dioxide reduction electrode described above can be used as an electrode in a carbon dioxide reduction device. Such a carbon dioxide reduction device includes the carbon dioxide reduction electrode described above. More specifically, the carbon dioxide reduction device includes the carbon dioxide reduction electrode described above as a cathode electrode, and an anode electrode. By including the carbon dioxide reduction electrode described above, the carbon dioxide reduction device can reduce carbon dioxide and produce methanol with high efficiency.

[0037] The carbon dioxide reduction electrode as the cathode electrode provided in the carbon dioxide reduction device is as described above in <<Carbon Dioxide Reduction Electrode>>. As the anode electrode provided in the carbon dioxide reduction device, a known anode electrode can be used, for example, a platinum electrode.

[0038] The carbon dioxide reduction device includes, for example, the above-described carbon dioxide reduction electrode as a cathode electrode, an anode electrode, and an electrolytic cell containing an electrolytic solution. By applying a voltage to the electrodes, an electrochemical reaction that reduces carbon dioxide occurs at the cathode electrode, producing methanol. The voltage applied to the electrodes is preferably −0.9 V to −0.7 V.

[0039] <Method for Producing Methanol> The carbon dioxide reduction electrode described above can be used in a method for producing methanol by reducing carbon dioxide. Such a method for producing methanol includes a step of producing methanol by reducing carbon dioxide using the carbon dioxide reduction electrode described above. By using the carbon dioxide reduction electrode described above, carbon dioxide can be reduced to produce methanol with high efficiency. Note that the voltage applied when reducing carbon dioxide is preferably −0.9 V to −0.7 V.

[0040] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0041] Example 1 (Synthesis) 0.16 mmol of tetrakis(acetonitrile)copper(I) tetrafluoroborate (Cu(CH ３ CN) ４ BF ４ ) and 0.1 mmol of triphenylphosphine were dissolved in a mixture of 2 mL of acetonitrile and 0.5 mL of chloroform at room temperature to produce a colorless, transparent solution. After stirring for 5 minutes, 0.1 mmol of 1-propanethiol was added to the reaction mixture and stirring was continued. Then, 1 mmol of sodium borohydride was dissolved in 2.5 mL of methanol and immediately added to the mixture at room temperature. The solution changed color from clear to red. The reaction was continued with stirring for another hour. After the reaction was completed, the mixture was centrifuged to obtain a red precipitate. The red precipitate was then washed three times with methanol and dried overnight. The red precipitate was then dissolved in a chloroform / hexane mixed solvent (volume ratio 1:1). The final transparent liquid was crystallized at room temperature. After six days, red plate-like crystals (copper clusters) were obtained.

[0042] (Identification) The obtained red plate-like crystals (copper clusters) were identified by X-ray diffraction and ESI-MS to have the composition [Cu ５８ H ２０ (SCH ２ CH ２ CH ３ ) ３６ (PPh ３ ) ７It was confirmed that the compound was a phenyl group.

[0043] <Preparation of catalyst> Carbon black (manufactured by Fuel Cell Earth, product name: VULCAN XC-72) and copper clusters (red plate-like crystals) dissolved in chloroform were added to an agate mortar and impregnated. This was then evacuated overnight in a desiccator to prepare a Cu-supported catalyst in which copper clusters were supported on carbon black. The copper cluster concentration was determined by ICP-MS, and the copper clusters were added so that 10 mg of Cu was added per 100 mg of carbon black.

[0044] <Preparation of Carbon Dioxide Reduction Electrode> The prepared Cu-supported catalyst was added to a mixed solution consisting of ultrapure water (2 mL), 2-propanol (0.5 mL), and polymer electrolyte (Nafion (registered trademark) solution, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) (10 μL). The resulting mixed solution was subjected to ultrasonic treatment in an ice bath for 30 minutes to disperse the Cu-supported catalyst in the mixed solution, thereby preparing a catalyst slurry. The catalyst slurry (1.1 mL) was sprayed onto carbon paper (manufactured by SGL Carbon, 22BB) using a sprayer, and then a vacuum was drawn to prepare a carbon dioxide reduction electrode.

[0045] <Measurement> Electrochemical measurements were carried out using an H-type cell. The electrodes used were the carbon dioxide reduction electrode (working electrode), a platinum mesh electrode (counter electrode), and a silver-silver chloride electrode (reference electrode), and the electrolyte was a 0.1 M potassium bicarbonate aqueous solution. ２ After bubbling the gas for 15 minutes, the gas was allowed to flow at a rate of 15 mL / min. First, cyclic voltammetry was performed at 200 mVs in the range of 0 to 1.2 V (vs. RHE). －１ The electrode was cleaned by scanning 100 times at a scanning speed of 0.5 V, 0.6 V, 0.7 V, 0.8 V, and 0.9 V (vs. RHE). After cleaning, amperometry was performed for 30 minutes at potentials of -0.5 V, -0.6 V, -0.7 V, -0.8 V, and -0.9 V (vs. RHE) to measure the activity. The generated gas components were analyzed by gas chromatography, and the generated liquid components were analyzed by HPLC. １The content was quantified by H NMR. The results of gas chromatography (TCD (Thermal Conductivity Detector) and FID (Flame Ionization Detector)) when amperometry was performed at a potential of −0.9 V for 30 minutes are shown in FIG. １ The results of H NMR are shown in Figure 3. From the above measurements, MeOH (methanol), HCOOH, H ２ , CO, CH ４ The results of determining the Faraday efficiency for each of the above are shown in FIG.

[0046] <Measurement under Ar> CO ２ The same procedure as in the above <Measurement> was carried out except that Ar gas was used instead of CO gas and the potential was set to −0.9 V. The results are shown in FIG. ２ The results when the potential was set to −0.9 V using gas are also shown.

[0047] Comparative Example 1 (Synthesis) 0.16 mmol of tetrakis(acetonitrile)copper(I) tetrafluoroborate (Cu(CH ３ CN) ４ BF ４ ) and 0.19 mmol of triphenylphosphine were dissolved in a mixture of 2 mL of acetonitrile and 0.5 mL of chloroform at room temperature to produce a colorless, transparent solution. After stirring for 5 minutes, 0.12 mmol of 1-propanethiol was added to the reaction mixture and stirring was continued. Subsequently, 1.32 mmol of sodium borohydride was dissolved in 2.5 mL of methanol and added dropwise to the reaction system, maintaining the temperature at 5-10°C. The solution changed color from clear to red. The reaction was continued with stirring for another hour. After the reaction was completed, the mixture was centrifuged and a red precipitate was collected. The collected red precipitate was dried and dissolved in a chloroform / hexane mixed solvent (volume ratio 1:1). The final transparent liquid was crystallized at room temperature. After 10 days, red box-shaped crystals (copper clusters) were obtained.

[0048] (Identification) The obtained red box-shaped crystals (copper clusters) were identified by X-ray diffraction and ESI-MS to have the composition [Cu ５８ H ２０ (SCH ２ CH２ CH ３ ) ３６ (PPh ３ ) ８ It was confirmed that the compound was a phenyl group.

[0049] The same operations as in the <Preparation of catalyst>, <Preparation of electrode for carbon dioxide reduction> and <Measurement> of Example 1 were carried out, except that in the <Preparation of catalyst>, the red box-shaped crystals (copper clusters) obtained in Comparative Example 1 were used instead of the copper clusters (red plate-like crystals), and in the <Measurement>, the potential was set to -0.9 V. The results are shown in Figure 6. ２ The results when the potential was set to −0.9 V using gas are also shown.

[0050] As shown in FIG. 4, in Example 1 using a copper cluster containing copper atoms, hydrogen atoms, and organic ligands, the number of copper atoms is 58, the number of hydrogen atoms is 20, and the number of organic ligands is 36 to 43, the CO ２ It can be seen that methanol can be produced with extremely high efficiency (Faraday efficiency). In Example 1, HCOOH and CH ４ Furthermore, as shown in FIG. 5, methanol was not produced under Ar, which explains the CO ２ Methanol produced under ２ It was confirmed that the methanol was produced by the reduction of

[0051] On the other hand, as shown in FIG. 6 , in Comparative Example 1, which used a copper cluster containing copper atoms, hydrogen atoms, and organic ligands, with 58 copper atoms, 20 hydrogen atoms, and 44 organic ligands, no methanol was detected, indicating that no methanol was produced.

Claims

1. A copper cluster comprising copper atoms, hydrogen atoms, and organic ligands, wherein the number of copper atoms is 58, the number of hydrogen atoms is 20, and the number of organic ligands is 36 or more and 43 or less.

2. The copper cluster according to claim 1, wherein the organic ligand contains at least one of an organic ligand containing a phosphorus atom and an organic ligand containing a sulfur atom.

3. The copper cluster according to claim 2, wherein the number of organic ligands containing a phosphorus atom is 0 or more and 7 or less, and the number of organic ligands containing a sulfur atom is 36.

4. A carbon dioxide reduction electrode for producing methanol by reducing carbon dioxide, comprising an electrode substrate and a copper cluster provided on the electrode substrate, wherein the copper cluster is the copper cluster according to any one of claims 1 to 3.

5. The carbon dioxide reduction electrode according to claim 4, wherein the copper cluster is supported on a porous body.

6. A carbon dioxide reduction device comprising the carbon dioxide reduction electrode according to claim 4.

7. A method for producing methanol, comprising a step of reducing carbon dioxide to produce methanol using the carbon dioxide reduction electrode according to claim 4.

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