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

Abstract

To provide 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.SOLUTION: A copper cluster according to the present invention is a copper cluster comprising 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 of organic ligands containing a phosphorus atom and organic ligands containing a sulfur atom, and in the copper cluster, the number of the organic ligands containing a phosphorus atom is preferably 0 to 7 inclusive, and the number of the organic ligands containing a sulfur atom is preferably 36.SELECTED DRAWING: None

Description

Technical Field

[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.

Background Art

[0002] Compared with the field of power generation where the installation of renewable energy is progressing, in fields such as industry, transportation, and forest development, due to population growth and the modernization of developing countries, a significant increase in CO2 emissions is expected. Therefore, attempts are being made to use CO2 as a carbon source for chemical products, industrial products, agricultural raw materials, etc. Specifically, for example, technologies for reducing CO2 to produce methane, ethylene, ethane, etc. have been disclosed (Patent Document 1, etc.).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Here, if a compound that can be sold at a price higher than the cost of recovering CO2 and the production cost from CO2 is not produced from CO2, it does not match the production cost. For this reason, it is desired to be able to produce methanol, which has high added value and high demand, from CO2. Methanol can be used, for example, as a raw material for acetic acid, methyl methacrylate, and dimethyl ether, and is also being considered as an energy source for fuel cells, and is expected to have an increasing demand in the future.

[0005] However, in conventional technologies such as Patent Document 1, in the reduction of CO2, there is a problem that many compounds other than methanol are produced and the production efficiency of methanol is low. Therefore, a catalyst that can enhance the efficiency of the reaction for reducing CO2 to produce methanol is desired.

[0006] Accordingly, an object of the present invention is to provide a copper cluster that can be used as a catalyst capable of enhancing the efficiency of the reaction for reducing carbon dioxide to produce methanol, a carbon dioxide reduction electrode using the copper cluster, a carbon dioxide reduction apparatus using the carbon dioxide reduction electrode, and a method for producing methanol.

Means for Solving the Problems

[0007] The present inventors have found that the above problems can be solved by a 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, and have thus completed the present invention. More specifically, the present invention is as follows.

[0008] (1) A 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.

[0009] (2) The copper cluster according to (1) above, wherein the organic ligand contains 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 a phosphorus atom is 0 or more and 7 or less, and the number of organic ligands containing a sulfur atom is 36.

[0011] (4) A carbon dioxide reduction electrode for reducing carbon dioxide to produce methanol, 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 (1) to (3) above.

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

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

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

Advantages of the Invention

[0015] According to the present invention, it is possible to provide a copper cluster that can be used as a catalyst capable of enhancing the efficiency of the reaction for reducing carbon dioxide to produce methanol, a carbon dioxide reduction electrode using the copper cluster, a carbon dioxide reduction apparatus using the carbon dioxide reduction electrode, and a method for producing methanol.

Brief Description of the Drawings

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Embodiments for Carrying Out the Invention

[0017] ≪Copper Cluster≫ The copper cluster contains copper atoms, hydrogen atoms, and organic ligands. In the copper cluster, 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. That is, the copper cluster is a metal cluster in which 58 copper atoms are bonded, and contains 20 hydrogen atoms and 36 or more and 43 or less organic ligands.

[0018] Examples of the organic ligands contained in the copper cluster include organic ligands containing phosphorus atoms and organic ligands containing sulfur atoms. The copper cluster preferably has an organic ligand containing a phosphorus atom and an organic ligand containing a sulfur atom. Examples of the organic ligand containing a phosphorus atom include triphenylphosphine, diphenyl(p-tolyl)phosphine, and tri(p-tolyl)phosphine. Examples of the organic ligand containing a sulfur atom include alkanethiols having 1 to 4 carbon atoms. Specific examples of the organic ligand containing a sulfur atom include methanethiol, ethanethiol, 1-propanethiol, and 1-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. Also, the copper cluster preferably has 36 organic ligands containing sulfur atoms. Regarding the copper cluster, the number and composition of copper atoms, hydrogen atoms, and organic ligands (organic ligands containing phosphorus atoms, organic ligands containing sulfur atoms, etc.) can be determined by X-ray diffraction method and electrospray ionization mass spectrometry (ESI-MS).

[0020] As shown in the examples described below, the above copper cluster can be used as a catalyst that can enhance the efficiency of the reaction for reducing carbon dioxide to produce methanol. That is, by using the above copper cluster, the efficiency of the reaction for reducing carbon dioxide to produce methanol can be enhanced. In other words, by using the above copper cluster, the selectivity of methanol in the reaction for reducing carbon dioxide can be increased. By using the above copper cluster, in the electrochemical reaction for reducing carbon dioxide to produce methanol, the Faradaic efficiency of methanol can be, for example, 50% or more, and can further be 60% or more. Also, by using the above copper cluster, in the electrochemical reaction for reducing carbon dioxide to produce methanol, the generation of by-products (such as formic acid and methane) other than methanol can be extremely suppressed. For example, the Faradaic efficiency of by-products other than methanol can be, for example, 10% or less, 5% or less, and can further be 0%. Note that the reaction for reducing carbon dioxide to produce methanol using the above copper cluster can be carried out at normal temperature and pressure.

[0021] The reason why the efficiency of the reaction for reducing carbon dioxide to produce methanol can be enhanced by using the above copper cluster is presumed to be the following mechanism by theoretical calculation. A copper cluster containing copper atoms, hydrogen atoms, and organic ligands, with 58 copper atoms, 20 hydrogen atoms, and the number of organic ligands being 36 or more and 43 or less, has a regular hexahedron structure as shown in FIG. 1, and the organic ligands are coordinated to the copper atoms. FIG. 1 shows an example of the copper cluster of the present embodiment. FIG. 1 is a schematic side view showing an example of the copper cluster of the present embodiment. In FIG. 1, the organic ligands are triphenylphosphine (an organic ligand containing a phosphorus atom) and 1-propanethiol (an organic ligand containing a sulfur atom), and a copper cluster is exemplified in which the number of triphenylphosphine is 7 and the number of 1-propanethiol is 36. In FIG. 1, for the sake of simplicity of the figure, the carbon chains and benzene rings are represented by hydrogen atoms.

[0022] When 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, among the eight copper atoms located at each of the eight edges (corners) of the regular hexahedron, at least one copper atom 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. However, in the above-mentioned copper cluster, the number of organic ligands is 36 or more and 43 or less. Therefore, among the eight copper atoms located at each of the eight edges (corners) of the regular hexahedron, at least one copper atom has a portion where an organic ligand is not coordinated. In FIG. 1, among the eight copper atoms (Cu1 to Cu4 existing on the front side of the paper surface in FIG. 1 and the four copper atoms located on the back side of FIG. 1) located at each of the eight edges (corners) of the regular hexahedron, triphenylphosphine and 1-propanethiol are coordinated to seven copper atoms, but triphenylphosphine is not coordinated to one copper atom (Cu1).

[0023] Thus, since there is a portion where the organic ligand is not coordinated to the copper atom at the edge of the regular hexahedron, the central portion (the gap (Hollow sites) surrounded by the three copper atoms indicated by the dotted line in FIG. 1) of the copper atom (Cu11) adjacent to the copper atom (Cu1) at the edge (corner) and the two copper atoms adjacent to the copper atom (Cu11) is prone to adsorb the proton H + Moreover, the proton H +The reason for the easier adsorption is presumably that due to the presence of the organic ligand (steric hindrance), the separation of the copper atoms at the edge to the outside of the copper cluster is suppressed, and the copper atoms (Cu1) at the edge are pushed into the inside of the copper cluster. At the central part of the copper atom (Cu11) adjacent to the copper atom (Cu1) at the edge (corner) and two copper atoms adjacent to the copper atom (Cu11), as shown in Fig. 1, a proton H + When adsorbed, the adsorbed proton reacts with carbon dioxide, making it easier to form an intermediate in the reaction of reducing carbon dioxide to produce methanol. Therefore, the energy barrier for the reaction of reducing carbon dioxide to produce methanol becomes lower, and methanol can be produced with high efficiency.

[0024] On the other hand, a copper cluster containing copper atoms, hydrogen atoms, and organic ligands, with 58 copper atoms, 20 hydrogen atoms, and 44 organic ligands, has extremely low methanol production efficiency or cannot produce methanol.

[0025] ≪Method for Producing Copper Cluster≫ The above-mentioned copper cluster containing copper atoms, hydrogen atoms, and organic ligands, with 58 copper atoms, 20 hydrogen atoms, and 36 to 43 organic ligands, can be produced, for example, by mixing a solution containing a copper compound and an organic ligand, and then adding a reducing agent to reduce the copper compound. After mixing the solution containing the copper compound and the organic ligand, and then adding a reducing agent to reduce the copper compound, it may be washed with a solvent such as an organic solvent if necessary.

[0026] Examples of the copper compound include tetrakis(acetonitrile)copper(I) tetrafluoroborate (Cu(CH3CN)4BF4), tetrakis(acetonitrile)copper(I) hexafluorophosphate (Cu(CH3CN)4)PF6), copper(II) acetate (Cu(CH3COO)2), copper(II) trifluoroacetate (Cu(CF3COO)2), copper(II) nitrate (Cu(NO3)2), copper(I) thiocyanate (CuSCN), copper(I) chloride (CuCl), copper(I) bromide (CuBr), and the like. The organic ligand is as described in the above-mentioned <<Copper Cluster>>. Examples of the reducing agent include sodium borohydride (NaBH4), lithium borohydride (LiBH4), diphenylsilane (Ph2SiH2), triethylamine ((CH3CH2)3N), borane tert-butylamine (tert-BuNH2·BH3), and the like. The temperature for reducing the copper compound 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 types of organic ligands are used as the organic ligand, the solution containing the copper compound and the two or more types 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 ligand includes 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.

[0028] Here, the above-mentioned copper cluster includes a copper atom, a hydrogen atom, and an organic ligand. 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. Thus, the number of atoms of the copper cluster having a specific number of atoms can be adjusted by adjusting the amount of the raw materials. For example, by reducing the number of moles of the raw material of the organic ligand, the number of organic ligands in the produced copper cluster can be reduced. For example, the ratio of the number of moles of the organic ligand to the number of moles of the copper compound (number of moles of organic ligand / number of moles of copper compound) used in the production of copper clusters is preferably 0.0 or more and 100 or less, more preferably 0.7 or more and 2.0 or less. 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 (number of moles of organic ligand containing a phosphorus atom / number of moles of copper compound) used in the production of copper clusters is preferably 0.0 or more and 100 or less, more preferably 0.1 or more and 1.2 or less. Also, 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] After mixing a solution containing a copper compound and an organic ligand, the copper compound is reduced with a reducing agent to obtain a copper cluster containing the organic ligand, and then a part of the organic ligand is removed to produce the above-mentioned copper cluster. For example, a solution containing a copper compound is mixed with an organic ligand containing a phosphorus atom and an organic ligand containing a sulfur atom, and then a reducing agent is added to reduce the copper compound to obtain an organic ligand containing a phosphorus atom and an organic ligand containing a sulfur atom, and then a part of the organic ligand containing a phosphorus atom is removed to produce the above-mentioned copper cluster.

[0030] ≪Carbon dioxide reduction electrode≫ The copper cluster containing the above-mentioned copper atom, hydrogen atom, and organic ligand, having 58 copper atoms, 20 hydrogen atoms, and the number of organic ligands being 36 or more and 43 or less, can be used as a catalyst for the carbon dioxide reduction reaction of a carbon dioxide reduction electrode for producing methanol by reducing carbon dioxide. The carbon dioxide reduction electrode for producing methanol by reducing carbon dioxide as described above includes an electrode substrate and a copper cluster provided on the electrode substrate. The copper cluster includes the above-mentioned 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 a copper cluster of 36 or more and 43 or less.

[0031] By using the above-mentioned carbon dioxide reduction electrode, the reaction of reducing carbon dioxide to produce methanol can be made more efficient.

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

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

[0034] In the carbon dioxide reduction electrode, the copper cluster is preferably supported on a porous body. In the carbon dioxide reduction electrode, when the copper cluster is supported on a porous body, the copper cluster may be supported on the porous body, and the one in which this copper cluster is supported on the porous body may be provided on the surface of the electrode substrate, or the copper cluster may be supported on the electrode substrate made of a porous body. Examples of the porous body for supporting the copper cluster include carbon black and metal oxides.

[0035] The method for manufacturing the electrode for carbon dioxide reduction is not particularly limited. For example, after manufacturing a catalyst by impregnating a porous material such as carbon black with the above-described solution of copper clusters, a liquid containing the catalyst (for example, a catalyst slurry) is applied to an electrode substrate to manufacture an electrode for carbon dioxide reduction. Note that it is preferable to manufacture the catalyst without firing the copper clusters.

[0036] ≪Carbon Dioxide Reduction Device≫ The above-described electrode for carbon dioxide reduction can be used as an electrode of a carbon dioxide reduction device. Such a carbon dioxide reduction device includes the above-described electrode for carbon dioxide reduction. More specifically, the carbon dioxide reduction device includes the above-described electrode for carbon dioxide reduction as a cathode electrode and an anode electrode. By including the above-described electrode for carbon dioxide reduction, the carbon dioxide reduction device can reduce carbon dioxide and produce methanol with high efficiency.

[0037] The electrode for carbon dioxide reduction as the cathode electrode included in the carbon dioxide reduction device is as described in the above ≪Electrode for Carbon Dioxide Reduction≫. As the anode electrode included in the carbon dioxide reduction device, a known anode electrode can be used, and examples thereof include a platinum electrode.

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

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

Example

[0040] Hereinafter, the present invention will be described in more detail 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(CH3CN)4BF4) and 0.1 mmol of triphenylphosphine were dissolved in a mixed solution of 2 mL of acetonitrile and 0.5 mL of chloroform at room temperature to form 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, and the color of the solution changed from transparent to red. Stirring was continued for another 1 hour to continue the reaction. After the reaction was completed, the mixture was centrifuged to obtain a red precipitate. The red precipitate was washed three times with methanol and dried overnight. Then, the red precipitate was dissolved in a chloroform / hexane mixed solvent (volume ratio 1:1). The finally obtained transparent liquid was crystallized at room temperature. After 6 days, red plate-like crystals (copper clusters) were obtained.

[0042] (Identification) By X-ray diffraction method and ESI-MS, it was confirmed that the obtained red plate-like crystals (copper clusters) had a composition of [Cu 58 H 20 (SCH2CH2CH3) 36 (PPh3)7]. Here, Ph represents 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. By subjecting this to vacuum pumping in a desiccator overnight, a Cu-supported catalyst in which copper clusters were supported on carbon black was prepared. The concentration of the copper clusters was determined by ICP-MS, and Cu was added so that the amount of Cu was 10 mg 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 a polyelectrolyte (Nafion (registered trademark) solution, manufactured by Fujifilm Wako Pure Chemical Corporation) (10 μL). The obtained mixed solution was sonicated 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 vacuum pumping was performed to prepare a carbon dioxide reduction electrode.

[0045] <Measurement> Electrochemical measurements were performed using an H-type cell. As the electrodes, the carbon dioxide reduction electrode (working electrode) obtained above, a platinum mesh electrode (counter electrode), and a silver / silver chloride electrode (reference electrode) were used, and as the electrolyte, a 0.1 M aqueous potassium hydrogen carbonate solution was used. The measurement was carried out by bubbling CO2 gas for 15 minutes and then flowing it at a rate of 15 mL / min. First, cyclic voltammetry was performed 100 times in the region of 0 to 1.2 V (vs. RHE) at a scanning rate of 200 mV / s to clean the electrode. After cleaning, amperometry was performed for 30 minutes at each potential 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 liquid components were -1 analyzed by 1Quantification was performed by \(^{1}\)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 Figure 2, 1 The results of \(^{1}\)H NMR are shown in Figure 3. The results of obtaining the Faraday efficiency for MeOH (methanol), HCOOH, H\(_2\), CO, and CH\(_4\) respectively from the above measurements are shown in Figure 4.

[0046] <Measurement under Ar> Using Ar gas instead of CO\(_2\) gas and setting the potential to -0.9 V, the same operations as in the above <Measurement> were performed. The results are shown in Figure 5. In Figure 5, the results when using CO\(_2\) gas and setting the potential to -0.9 V are also shown together.

[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 mixed solution of 2 mL of acetonitrile and 0.5 mL of chloroform at room temperature to form 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. Then, 1.32 mmol of sodium borohydride was dissolved in 2.5 mL of methanol and dropped into the reaction system maintained at a temperature of 5 - 10 °C, and the color of the solution changed from transparent to red. Stirring was continued for another 1 hour to continue the reaction. After the reaction was completed, the mixture was centrifuged and the red precipitate was collected. The collected red precipitate was dried and dissolved in a chloroform / hexane mixed solvent (volume ratio 1:1). The finally obtained transparent liquid was crystallized at room temperature. After 10 days, red box-shaped crystals (copper clusters) were obtained.

[0048] (Identification) By X-ray diffraction method and ESI-MS, the obtained red box-shaped crystals (copper clusters) had a composition of [Cu 58 H 20 (SCH\(_2\)CH\(_2\)CH\(_3\))36 It was confirmed that it is [(PPh3)8]. Here, Ph represents a phenyl group.

[0049] In <Preparation of Catalyst>, instead of the copper cluster (red plate-like crystal), the red box-shaped crystal (copper cluster) obtained in Comparative Example 1 was used. In <Measurement>, except that the potential was set to -0.9 V, the same operations as in <Preparation of Catalyst>, <Preparation of Electrode for Carbon Dioxide Reduction>, and <Measurement> of Example 1 above were performed. The results are shown in Fig. 6. In Fig. 6, the results of Example 1 using CO2 gas with a potential of -0.9 V are also shown together.

[0050] As shown in Fig. 4, in Example 1 using 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 or more and 43 or less, it can be seen that methanol can be produced from CO2 with extremely high efficiency (Faraday efficiency) when the voltage is in the range of -0.9 V to -0.7 V. In Example 1, HCOOH and CH4 were not produced at any voltage in the range of -0.9 V to -0.5 V. Also, as shown in Fig. 5, since methanol was not produced under Ar, it was confirmed that the methanol produced under CO2 shown in Fig. 4 was methanol produced by reducing CO2.

[0051] On the other hand, as shown in Fig. 6, in Comparative Example 1 using 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 44, methanol was not detected, indicating that methanol was not 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 comprises 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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