Carbon dioxide reduction catalyst composed of metal nanocluster aggregate, electrode for carbon dioxide reduction, and carbon dioxide reduction apparatus

Abstract

To provide a carbon dioxide reduction catalyst employing a metal nanocluster aggregate, the catalyst having desirable catalytic activity and selectivity as well as durability.SOLUTION: The present invention provides a catalyst that comprises a metal nanocluster aggregate composed of two or more metal atoms, the aggregate being aggregated through a linker represented by the following formula. The metal atoms are at least one selected from Ag, Au, and Cu. The linker is an organic compound which is terminated at all ends with pyridine ligands and which includes one or more carbon-carbon triple bonds between the pyridine ligand and a group X. In the following formula, n is an integer of 2 or more and 8 or less. The group X is any of carbon, nitrogen, or phosphorus, or an aliphatic hydrocarbon group optionally having a substituent with 1 to 6 carbon atoms.SELECTED DRAWING: Figure 7

Description

Technical Field

[0001] The present invention relates to a carbon dioxide reduction catalyst for electrochemically reducing carbon dioxide. Specifically, the present invention relates to a carbon dioxide reduction catalyst composed of a metal nanocluster aggregate in which metal nanoclusters are regularly and periodically bonded and aggregated by a predetermined linker.

Background Art

[0002] As a technology for achieving carbon neutrality, which is one of the efforts to address environmental problems and greenhouse gas problems, artificial photosynthesis technology centered on carbon dioxide reduction has attracted attention. Carbon dioxide reduction is a technology that recovers carbon dioxide contained in exhaust gas and the like and electrochemically reduces it to convert it into hydrocarbons such as carbon monoxide, formic acid, methane, and ethane. Since the substances obtained by the reduction treatment of carbon dioxide can be used for the synthesis of fuels, alcohols, olefins, etc., they are all valuable substances. Therefore, artificial photosynthesis technology can contribute to addressing environmental problems as well as energy problems. In recent years, as such an artificial photosynthesis system, a carbon dioxide reduction device equipped with an electrode (cathode electrode) containing a carbon dioxide reduction catalyst and a solar cell for supplying electrical energy are being developed in combination.

[0003] The problems required of the carbon dioxide reduction catalyst applied in the above artificial photosynthesis technology include high activity for the carbon dioxide reduction reaction and good selectivity for preferentially proceeding the reduction reaction of the target product. One of the factors of such selectivity problems is that the reduction reactions to the above various valuable substances compete with the reaction of hydrogen generation (water decomposition). Taking carbon monoxide and formic acid as examples, the standard potential (RHE standard) of the reduction reaction from carbon dioxide to carbon monoxide is -0.10V, and the standard potential (RHE standard) of the reduction reaction to formic acid is -0.12V. Since the standard potentials of the reduction reactions to carbon monoxide and formic acid are lower than the standard potential of the hydrogen generation reaction (0V: RHE standard), when carbon dioxide is reduced in a moisture-containing atmosphere, it tends to be affected by the competition of the hydrogen generation reaction.

[0004] In addition, among the valuable substances produced by the above carbon dioxide reduction, some require a large number of charges (electrons) for their production. For example, since the number of electrons required for the reduction reaction from carbon dioxide to ethylene is 12, the reduction reaction to other substances with fewer charges may be prioritized over the reduction reaction to ethylene.

[0005] As a measure for the reduction activity and selectivity in the carbon dioxide reduction catalyst, optimization of the selection of the metal species serving as the catalyst can be mentioned. In the carbon dioxide reduction reaction, it is known that the adsorption force between the CO radical, which is a reaction intermediate, and the metal surface changes the reduction activity and the composition of the reaction product. Such an adsorption force is not preferable if it is too weak or too strong, and it is preferably moderately medium. Au, Ag, and Cu are considered to be suitable metals from this perspective, having a medium adsorption force for CO radicals, having good carbon dioxide reduction activity, and being able to cope with the problem of overvoltage when reducing to carbon monoxide or the like described above.

[0006] On the other hand, in order to improve the activity of the metal catalyst, it is known that increasing the metal surface area by ultrafine particle formation (nanoparticle formation) of the catalyst particles is effective. Also in the carbon dioxide reduction catalyst, the application of such metal nanoparticles is being attempted (Patent Document 1).

[0007] As a form of using metal nanoparticles as a catalyst, it is common to add a catalyst carrier such as carbon to an appropriate liquid phase solvent in which the metal nanoparticles are dispersed to fix the metal nanoparticles. Also in the above Patent Document 1, the carrier is immersed in a solution containing a metal salt, and the metal salt is adsorbed on the carrier while being reduced and particleized. However, at this time, the metal nanoparticles are irregularly and randomly dispersed in the dispersion liquid, so even after loading, they are irregularly distributed on the carrier and the particle intervals are also in a random state.

[0008] Metal nanoparticles irregularly dispersed on a carrier can exhibit a certain degree of catalytic activity even in such a state. However, when the catalyst particles are irregularly distributed, there are some aggregated particles in part, so initial activity is insufficient or activity decline over time is likely to occur. Also, in a catalyst in which catalyst particles are adsorbed and fixed to a catalyst carrier as in the past, when the binding force between the catalyst particles and the carrier is not sufficient, there is concern about activity decline due to aggregation by movement of the catalyst particles or desorption.

[0009] Here, the present inventors are focusing on the application of metal nanocluster aggregates as a new configuration for a catalyst obtained by supporting metal nanoparticles on a catalyst carrier. A metal nanocluster is an aggregate of a plurality of metal atoms, and in this regard, it is synonymous with metal nanoparticles, but in particular, those with a small number of constituent atoms are called metal nanoclusters. And a metal nanocluster aggregate is a structure formed by a plurality of metal nanoclusters being mutually bonded through a crosslinking material called a linker. Also, a linker is an organic compound containing a ligand capable of binding to a metal nanocluster, and is capable of binding to a metal nanocluster at its end or peripheral part.

[0010] In a metal nanocluster aggregate, since metal nanoclusters are regularly and periodically linked via a linker, it is considered to be more resistant to activity decline than a state where metal nanoparticles are random as in conventional catalysts. Also, an appropriate linker is considered to be able to suppress the movement and dropout of metal nanoclusters by strongly binding to the metal nanoclusters. That is, a metal nanocluster aggregate has the potential to become a catalyst with good periodic and regular arrangement and stability of metal nanoclusters having catalytic activity. And the present inventors are considering that the nanocluster aggregate of a metal (such as Ag) having the above-described carbon dioxide reduction activity is useful as a carbon dioxide reduction catalyst.

[0011] However, there are few specific examples of studies on metal nanocluster aggregates. As a prior example of a study on metal nanocluster aggregates, there is, for example, the Ag nanoparticle aggregate described in Patent Document 2. In this prior art, Ag nanoparticles (Ag nanoclusters) protected by a dispersant such as PVP are bound with a sulfur compound such as cysteine to describe an Ag nanoparticle aggregate. The Ag nanocluster aggregate of this Patent Document 2 can be said to be a structure that conforms to the definition in the sense of aggregating metal nanoclusters. However, the periodicity and regularity of the Ag nanoclusters by the linker, which are the greatest characteristics of the nanocluster aggregate, are not clear.

[0012] Furthermore, studies on metal nanocluster aggregates have so far mostly been limited to confirming the feasibility of aggregate formation. Although metal nanocluster aggregates are said to have several suitable properties, there are few examples of studies on their practical and industrial applicability.

Prior Art Documents

Patent Documents

[0013]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0014] The present invention has been made under the above background, and an object thereof is to clarify the structure of a metal nanocluster aggregate having applicability as a carbon dioxide reduction catalyst. In this problem, the present invention clarifies a suitable linker capable of forming nanoclusters of a predetermined metal having carbon dioxide reduction activity and further distributing them regularly and periodically. Then, a carbon dioxide reduction catalyst to which such a metal nanocluster aggregate is applied, which has suitable catalytic activity, selectivity, and durability, is clarified.

Means for Solving the Problems

[0015] In the process of solving the above problems, the present inventors examined an organic compound containing a pyridine ligand as a linker for forming a metal nanocluster aggregate. According to the examination by the present inventors, the pyridine ligand has good binding properties with metal nanoclusters such as Ag. Therefore, it is considered that an aggregate in which metal nanoclusters are regularly arranged can be obtained by using an organic compound having pyridine ligands at all terminals as a linker.

[0016] Among the organic compounds having pyridine ligands at all terminals that can act as a linker, the simplest organic compound is the following 4,4'-bipyridine-based compound. In the following formula, the pyridine ligands may each independently have a substituent.

[0017]

Chemical formula

[0018] The present inventors first examined the possibility of forming a metal nanocluster aggregate using the above 4,4'-bipyridine-based compound as a linker. As a result, it was confirmed that an organic compound having a pyridine ligand has an effect of autonomously forming a regular ordered structure during the binding process with a metal nanocluster. This metal nanocluster aggregate can behave as a pseudo-crystal in which metal nanoclusters are periodically and regularly arranged by a linker (organic compound containing a pyridine ligand) which is a crosslinking material. The present inventors refer to such behavior of the pyridine ligand as self-assembly.

[0019] Therefore, the present inventors examined the catalytic activity of a metal nanocluster aggregate using a 4,4'-bipyridine-based compound as a linker, and found that this metal nanocluster aggregate has an activity to be evaluated as a carbon dioxide reduction catalyst.

[0020] However, the study by the present inventors does not stop here. The present inventors examined the improvement of durability, which is a property required for a catalyst, for a metal nanocluster aggregate using an organic compound containing a pyridine ligand as a linker. As a result, the present inventors found that among organic compounds containing pyridine ligands at all terminals, those having a carbon-carbon triple bond in the structure have suitable thermal stability, and thus arrived at the present invention.

[0021] That is, the present invention is a carbon dioxide reduction catalyst comprising a metal nanocluster aggregate in which metal nanoclusters composed of two or more metal atoms are aggregated via a linker composed of an organic compound, wherein the metal atoms are at least any one of Ag, Au, and Cu, and the linker is an organic compound represented by the following Chemical Formula 2 and having pyridine ligands at all terminals.

[0022] [Chemical Formula] In the above formula, n is the number of pyridine ligands and is an integer of 2 or more and 8 or less. The group X is any one of carbon, nitrogen, and phosphorus, or an aliphatic hydrocarbon group which may have a substituent having 1 or more and 6 or less carbon atoms, or a monocyclic, linked, or condensed aromatic hydrocarbon group having 6 or more and 42 or less carbon atoms (these groups may each independently have a substituent), or a monocyclic, linked, or condensed heteroaromatic group having 3 or more and 68 or less carbon atoms (these groups may each independently have a substituent). Further, the pyridine ligands may each independently have a substituent. Furthermore, the linker contains one or more carbon-carbon triple bonds between the pyridine ligand and the group X.

[0023] Hereinafter, the structure and manufacturing method of the carbon dioxide reduction catalyst composed of the metal nanocluster aggregate according to the present invention will be described. And the usage aspect of the carbon dioxide reduction catalyst according to the present invention will be described.

[0024] A. Structure of the carbon dioxide reduction catalyst according to the present invention The carbon dioxide reduction catalyst of the present invention is composed of a metal nanocluster aggregate, and the metal nanocluster aggregate is composed of a metal nanocluster and an organic compound having a pyridine ligand as a linker. Hereinafter, the metal nanocluster and the linker that constitute the metal nanocluster aggregate will be described.

[0025] (i) Metal nanocluster As described above, the metal nanocluster is a fine particle containing two or more metal atoms. In the present invention, the metal nanocluster contains at least one of Ag, Au, and Cu metal atoms. These metals are applied because Ag, Au, and Cu have carbon dioxide reduction activity and good selectivity for the carbon dioxide reduction reaction. The metal atoms contained in the metal nanocluster may be only Ag atoms, Au atoms, or Cu atoms, or may contain two or more of these metal atoms. For example, when the metal nanocluster contains Ag atoms and Cu atoms, this metal nanocluster has catalytic activity as AgCu alloy particles.

[0026] In addition, the metal nanocluster is composed of two or more metal atoms. Preferably, it is composed of 4 or more and 78 or less metal atoms, more preferably 4 or more and 30 or less metal atoms.

[0027] The metal nanocluster may be composed of only metal atoms, or may contain atoms of other elements. For example, S (sulfur), O (oxygen), N (nitrogen), C (carbon), etc., which are atoms of elements with high bonding properties with Ag and Cu, can contribute to improving the stability and durability of the metal nanocluster, optimizing and activating the number of metal atoms constituting the metal nanocluster, etc. In the metal nanocluster composed of 2 or more and 78 or less metal elements described above, it may contain 1 or more and 43 or less atoms of the other elements.

[0028] (ii) Linker (ii-1) Basic composition of the organic compound serving as the linker As described above, the linker functions as a cross-linking material for the metal nanocluster and is a main component in the present invention that periodically and regularly accumulates the metal nanoclusters by self-assembly. The organic compound serving as this linker contains two or more pyridine ligands, and while the pyridine ligands are bonded to the group X, the pyridine ligands are located at all terminals. The linker binds to the metal nanocluster by the terminal pyridine ligands and acts as a cross-linking material that binds the metal nanoclusters to each other, ensuring periodicity and regularity.

[0029] The number n of pyridine ligands in the linker is 2 or more and 8 or less. When an organic compound having more than 8 pyridine ligands is used as the linker, crystallization (accumulation) is difficult due to steric hindrance, so the number of pyridine ligands is set to 8 or less. The number n of pyridine ligands is more preferably 2 or more and 4 or less.

[0030] In the linker applied in the present invention, as the bonding species between the pyridine ligand and the group X, one or more carbon-carbon triple bonds are included. Generally, the bonding force of the carbon-carbon bonding species is greater for a double bond than for a single bond, and a triple bond is greater than these. An increase in the bonding force between the pyridine ligand and the group X leads to an improvement in the stability of the linker. Further, since a triple bond has a high electron-withdrawing property, a linker containing a triple bond can be expected to improve the bonding strength with a metal nanocluster. Furthermore, a triple bond has a rigid structure with a short bonding distance and is difficult to rotate or bend compared to a single bond or the like. Due to these effects, a linker containing a triple bond has high stability itself and also has good bonding force with a metal nanocluster, so that a highly stable metal nanocluster aggregate can be formed.

[0031] The present invention is a carbon dioxide reduction catalyst composed of a metal nanocluster aggregate. The carbon dioxide reduction catalyst is a material that continuously receives an electrical load by electrolysis. The electrical load is a factor in the alteration of the metal nanocluster aggregate due to decomposition of the linker or the like. In the present invention, by applying a linker having good stability containing a triple bond, a metal nanocluster aggregate having good stability against an electrical load is applied. Specific examples of such a linker containing a triple bond will be described in detail later.

[0032] The group X that binds to the pyridine ligand may be any of carbon, nitrogen, phosphorus, or an aliphatic hydrocarbon group that may have a substituent having 1 or more and 6 or less carbon atoms, or a monocyclic, linked, or condensed aromatic hydrocarbon group having 6 or more and 42 or less carbon atoms (each of these groups may independently have a substituent), or a monocyclic, linked, or condensed heteroaromatic group having 3 or more and 68 or less carbon atoms (each of these groups may independently have a substituent).

[0033] When the group X is an aliphatic hydrocarbon group having 1 to 6 carbon atoms, the group X is preferably an aliphatic hydrocarbon group composed of carbon, oxygen, nitrogen, and sulfur. Examples of the aliphatic hydrocarbon group include saturated aliphatic hydrocarbon groups and unsaturated aliphatic hydrocarbon groups. Further, the aliphatic hydrocarbon group may be linear, branched, or cyclic. Examples of the saturated aliphatic hydrocarbon group include an alkyl group, an alkoxy group, and a cycloalkyl group. Examples of the unsaturated aliphatic hydrocarbon group include an alkenyl group, an alkynyl group, and a cycloalkenyl group.

[0034] When the group X is a monocyclic, linked, or fused aromatic hydrocarbon group having 6 to 42 carbon atoms, the group X includes, for example, a phenyl group, a biphenylyl group, a terphenyl group, a naphthalenyl group, a phenylnaphthyl group, a naphthylphenyl group, an anthracenyl group, a pyrenyl group, a phenanthrenyl group, a perylenyl group, a triphenylenyl group, a tetracenyl group, a benzanthracenyl group, a chrysenyl group, a benzophenanthrenyl group, and the like.

[0035] When the group X is a monocyclic, linked, or fused heteroaromatic group having 3 to 68 carbon atoms, the group X includes, for example, a pyridyl group, a bipyridyl group, a triazinyl group, a pyrrolyl group, an imidazolyl group, a thiazolyl group, a benzothiadiazole group, a furanyl group, a benzofuranyl group, a dibenzofuranyl group, a thienyl group, a benzothienyl group, a dibenzothienyl group, a pyridinyl group, a pyrimidinyl group, a pyrazinyl group, a quinolinyl group, an isoquinolinyl group, a carbazolyl group, a pyridyl-phenyl group, a phenyl-pyridyl group, a pyridyl-biphenylyl group, a pyrimidyl-phenyl group, a bicyclo[1.1.1]pentenyl group, a porphyrinyl group, a phthalocyanyl group, or a phenyl-pyrimidyl group, and the like.

[0036] Furthermore, Group X can each independently have a substituent. Examples of the substituent include linear or branched alkyl groups having 1 to 20 carbon atoms (such as methyl group, ethyl group, n-propyl group, i-propyl group, n-butyl group, sec-butyl group, tert-butyl group, i-butyl group, etc.), alkene groups having 2 to 20 carbon atoms, alkenyl groups, alkoxy groups, hydroxy groups, cyano groups, carboxy groups, amide groups, nitro groups, carbonyl groups, aldehyde groups, sulfonyl groups, sulfinyl groups, ester groups, silyl groups, siloxanyl groups, and halogen groups such as fluorine atom, chlorine atom, and bromine atom.

[0037] Furthermore, each of the plurality of pyridine ligands included in the linker may independently have a substituent. Examples of the substituent in this case include linear or branched alkyl groups having 1 to 20 carbon atoms (such as methyl group, ethyl group, n-propyl group, i-propyl group, n-butyl group, sec-butyl group, tert-butyl group, i-butyl group, etc.), alkene groups having 2 to 20 carbon atoms, alkenyl groups, alkoxy groups, hydroxy groups, cyano groups, carboxy groups, amide groups, nitro groups, carbonyl groups, aldehyde groups, sulfonyl groups, sulfinyl groups, ester groups, silyl groups, siloxanyl groups, and halogen groups such as fluorine atom, chlorine atom, and bromine atom.

[0038] (ii-2) Detailed structural examples of the organic compound serving as the linker Regarding the specific structure of a suitable organic compound serving as the linker of the metal nanocluster aggregate constituting the carbon dioxide reduction catalyst according to the present invention, when classified by the number of pyridine ligands, it is as follows.

[0039] (ii-2-1) Linker composed of two pyridine ligands Examples of the linker having two pyridine ligands (n = 2) include an organic compound (4-(2-pyridin-4-ylethynyl)pyridine) represented by the following Chemical Formula 3, in which the pyridine ligands are bonded via an ethynyl group and contain one triple bond. Further, examples of the linker having two pyridine ligands include organic compounds containing two or more triple bonds represented by the following Chemical Formula 4.

[0040] [Chemical formula]

[0041] [Chemical formula]

[0042] In the organic compound represented by Chemical formula 4 above, the group Y1 is preferably carbon, an aliphatic hydrocarbon group which may have a substituent having 1 or more and 6 or less carbon atoms, a monocyclic, linked, or condensed aromatic hydrocarbon group having 6 or more and 22 or less carbon atoms (each of these groups may independently have a substituent), or a monocyclic, linked, or condensed heteroaromatic group having 3 or more and 44 or less carbon atoms (each of these groups may independently have a substituent).

[0043] (ii-2-2) A linker composed of three pyridine ligands Preferred organic compounds as linkers having three pyridine ligands (n = 3) include organic compounds containing three or more triple bonds between the pyridine ligand represented by Chemical formula 5 below and the group X.

[0044] [Chemical formula]

[0045] In the organic compound represented by the above formula, the group Y2 is preferably carbon, nitrogen, phosphorus, an aliphatic hydrocarbon group which may have a substituent having 1 or more and 6 or less carbon atoms, or a monocyclic, linked, or condensed aromatic hydrocarbon group having 6 or more and 30 or less carbon atoms (each of these groups may independently have a substituent), or a monocyclic, linked, or condensed heteroaromatic group having 3 or more and 27 or less carbon atoms (each of these groups may independently have a substituent).

[0046] (ii-2-3) A linker composed of four pyridine ligands Preferred organic compounds as linkers having four pyridine ligands (n = 4) include organic compounds containing four or more triple bonds between the pyridine ligand represented by the following Chemical Formula 6 and group X.

[0047]

Chemical Formula

[0048] In the organic compound represented by the above formula, group Y3 is preferably a carbon, an aliphatic hydrocarbon group which may have a substituent having 1 to 6 carbon atoms, or a monocyclic, linked, or condensed aromatic hydrocarbon group having 6 to 25 carbon atoms (each of these groups may independently have a substituent), or a monocyclic, linked, or condensed heteroaromatic group having 3 to 44 carbon atoms (each of these groups may independently have a substituent).

[0049] In the above organic compounds of Chemical Formula 4 to Chemical Formula 6, when groups Y1 to Y3 are aliphatic hydrocarbon groups having 1 to 6 carbon atoms, groups Y1 to Y3 are preferably aliphatic hydrocarbon groups composed of carbon, oxygen, nitrogen, and sulfur. The aliphatic hydrocarbon groups include saturated aliphatic hydrocarbon groups and unsaturated aliphatic hydrocarbon groups. Further, the aliphatic hydrocarbon groups may be linear, branched, or cyclic. Examples of the saturated aliphatic hydrocarbon groups include alkyl groups, alkoxy groups, cycloalkyl groups, etc. Examples of the unsaturated aliphatic hydrocarbon groups include alkenyl groups, alkynyl groups, cycloalkenyl groups, etc.

[0050] Also, when groups Y1 to Y3 are monocyclic, linked, or condensed aromatic hydrocarbon groups, groups Y1 to Y3 include, for example, phenyl group, biphenylyl group, terphenyl group, naphthalenyl group, phenylnaphthyl group, naphthylphenyl group, anthracenyl group, pyrenyl group, phenanthrenyl group, perylenyl group, triphenylenyl group, tetracenyl group, benzanthracenyl group, chrysenyl group, benzophenanthrenyl group, etc.

[0051] When the groups Y1 to Y3 are monocyclic, linked, or condensed heterocyclic aromatic groups, the groups Y1 to Y3 are, for example, a pyridyl group, a bipyridyl group, a triazinyl group, a pyrrolyl group, an imidazolyl group, a thiazolyl group, a benzothiadiazole group, a furanyl group, a benzofuranyl group, a dibenzofuranyl group, a thienyl group, a benzothienyl group, a dibenzothienyl group, a pyridinyl group, a pyrimidinyl group, a pyrazinyl group, a quinolinyl group, an isoquinolinyl group, a carbazolyl group, a pyridyl-phenyl group, a phenyl-pyridyl group, a pyridyl-biphenylyl group, a pyrimidinyl-phenyl group, a bicyclo[1.1.1]pentenyl group, a porphyrinyl group, a phthalocyanyl group, or a phenyl-pyrimidinyl group, etc.

[0052] And the groups Y1 to Y3 can each independently have a substituent. Examples of the substituent include a linear or branched alkyl group having 1 to 20 carbon atoms (such as a methyl group, an ethyl group, an n-propyl group, an i-propyl group, an n-butyl group, a sec-butyl group, a tert-butyl group, an i-butyl group, etc.), an alkene group having 2 to 20 carbon atoms, an alkenyl group, an alkoxy group, a hydroxy group, a cyano group, a carboxy group, an amide group, an amino group, a nitro group, a carbonyl group, an aldehyde group, a sulfonyl group, a sulfinyl group, an ester group, a silyl group, a siloxanyl group, a halogen group such as a fluorine atom, a chlorine atom, a bromine atom, etc.

[0053] (ii-2-4) Specific examples of the organic compound serving as a linker More specific examples of the organic compound serving as a linker in the carbon dioxide reduction catalyst of the present invention include the following organic compounds.

[0054]

Chemical formula

[0055]

Chemical formula

[0056]

Chemical formula

[0057] [Chemical]

[0058] [Chemical]

[0059] [Chemical]

[0060] (iii) Other components that may be included in the metal nanocluster aggregate In the metal nanocluster aggregate constituting the carbon dioxide reduction catalyst according to the present invention, organic ligands other than the pyridine ligand which is the main component of the linker described above may be bonded. This organic ligand is an organic ligand derived from a complexing agent used when synthesizing an Ag complex which is a precursor of the metal nanocluster before forming the aggregate, or a regulator / protective agent used for shape adjustment and aggregation suppression of the manufactured metal nanocluster. These organic ligands may be bonded to the metal nanocluster even after the metal nanocluster is formed into an aggregate.

[0061] Examples of the organic ligands other than the pyridine ligand that can be bonded to the metal nanocluster of the present invention include thiol group-containing ligands, ethynyl group-containing ligands, carboxylate group-containing ligands, and the like. Specific examples of these organic ligands will be described in the explanation of the manufacturing method described later.

[0062] In the carbon dioxide reduction catalyst composed of the metal nanocluster aggregate described above, it is preferable that the average particle diameter of the metal nanocluster aggregate is 0.1 μm or more and 10 μm or less. Considering the use of the catalyst, the metal nanocluster aggregate within the above particle diameter range is advantageous from the viewpoint of specific surface area.

[0063] B. Method for Producing Carbon Dioxide Reduction Catalyst (Metal Nanocluster Aggregate) According to the Present Invention As described above, the metal nanocluster aggregate constituting the carbon dioxide reduction catalyst of the present invention is a structure in which metal nanoclusters containing a plurality of metal atoms are aggregated via linkers. Therefore, its production can be carried out by forming an aggregate of a plurality of metal atoms and adding a linker to the state in which this aggregate is dispersed.

[0064] Here, examples of the aggregate of a plurality of metal atoms include metal complexes in addition to the metal nanoclusters described so far. A metal complex is a compound that can be a precursor of a metal nanocluster. A metal complex is formed by adding and reacting an organic ligand to a solution of a metal salt (metal ion), and is a complex containing a plurality of metal atoms and having an organic ligand coordinated to the metal atoms. Incidentally, as the above-mentioned metal salt, nitrates, chlorides, sulfates, oxalates, oxides, carbonates, etc. can be used. And, as suitable organic ligands for forming metal complexes in the present invention, thiol group-containing ligands and ethynyl group-containing ligands can be mentioned. Specific compounds as organic ligands include 1-adamantanethiol and tert-butylthiol (tert-butyl mercaptan), which are tertiary thiol group-containing ligands, 3-mercapto-3-methyl-1-butanol, bicyclo[1.1.1]pentanethiol, 8-mercaptomentone, O-carborane-1,2-dithiol, etc., and ethynyl group-containing ligands such as tert-butylacetylene and 1-ethynyladamantane, 3,3-dimethyl-1-pentene, 1-ethynylbicyclo[1.1.1]pentane, 1-ethynyl-3,5-dimethyladamantane, etc.

[0065] Also, metal nanoclusters can be formed by reduction treatment of the above-mentioned metal salt or metal complex, or by a ligand exchange method in which another organic ligand is added to the above metal complex.

[0066] In the production of the metal nanocluster aggregate of the present invention, it is preferable to form metal nanoclusters by a ligand exchange method in which another organic ligand is added to the metal complex formed by the above-described organic ligand, and then a linker is reacted to form an aggregate. Although the metal complex is composed of a plurality of atoms, the number of its components is often not defined in a fixed manner. For the metal complex formed by the thiol group-containing ligand or the like described above, the number of metal atoms can be adjusted by adding another organic ligand, and metal nanoclusters can be synthesized from the metal complex by ligand exchange. As this other organic ligand, it is preferable to add a carboxylate group-containing ligand such as trifluoroacetic acid or trifluoroacetate. The carboxylate group-containing ligand can contribute to maintaining the dispersibility while generating metal nanoclusters from the metal complex.

[0067] In a state where the metal complex or metal nanoclusters produced as described above are dispersed in a dispersion medium, an organic compound serving as a linker is added and reacted to form a metal nanocluster aggregate. When obtaining a state where the metal nanoclusters are dispersed in a dispersion medium, the metal complex formed above may be recovered, dispersed in the dispersion medium, and then made into metal clusters. Metal nanoclusters may be produced and dispersed in a dispersion medium.

[0068] In addition, when dispersing the metal nanoclusters in a dispersion medium to form a metal nanocluster aggregate as described above, the dispersion medium is preferably an organic solvent such as alcohol (ethanol, methanol, etc.), acetonitrile, acetone, cyclohexane, normal hexane, chloroform, toluene, triethylamine, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, methyl ethyl ketone, methyl isobutyl ketone, methyl hexyl ketone, diisobutyl ketone, ethyl acetate, diethyl ether, methyl tert-butyl ether, tetrahydrofuran, dichloromethane, dimethyl sulfoxide, etc. These organic solvents can be used alone or in combination as the dispersion medium.

[0069] Also, when adding the linker, it is preferable to dissolve the linker in the same organic solvent as described above to form a solution (linker solution) and then add it. The amount of the linker added is preferably an excessive amount with a guideline of 10 times the number of moles of the metal element of the metal nanoparticles.

[0070] The metal nanocluster aggregate is formed by reacting the linker with the metal nanoclusters after adding the linker solution. As the reaction conditions at this time, it is preferable that the reaction temperature is -40°C or higher and 80°C or lower in the air or in an inert gas, and the reaction time is 3 hours or longer. The bonding between the linker and the metal nanoparticles proceeds by ligand exchange between the organic ligand bonded to the metal nanoclusters and the pyridine ligand at the end of the linker. Then, the metal nanocluster aggregate can be obtained by self-organization due to the reaction between the linker and the metal nanoclusters.

[0071] After the reaction is completed, the metal nanocluster aggregate can be separated and recovered by centrifugation followed by vacuum drying. The metal nanocluster aggregate can be appropriately washed with alcohol (such as methanol). The metal nanocluster aggregate obtained through the above steps serves as a carbon dioxide reduction catalyst.

[0072] C. Electrode for Carbon Dioxide Reduction and Carbon Dioxide Reduction Device According to the Present Invention The carbon dioxide reduction catalyst according to the present invention composed of the above-described metal nanocluster aggregate can be used as an electrode for carbon dioxide reduction by combining it with a support. The material of the support is not particularly limited as long as it is a conductive material that can supply electrons to the carbon dioxide reduction catalyst. For example, metals, carbon, solid electrolytes, etc. can be applied. Also, there are no restrictions on the shape and structure of the support, which can be plate-shaped, net-shaped, cylindrical, etc., and can be any of bulk bodies, sintered bodies, fibers, and powders. There are no restrictions on the dimensions of the support.

[0073] There are no particular restrictions on the method of using the carbon dioxide reduction catalyst and the support as electrodes. For example, an electrode can be formed by applying a dispersion liquid in which a carbon dioxide reduction catalyst is dispersed in an appropriate dispersion medium to a support. As the dispersion medium at this time, alcohols (such as methanol, ethanol, and propanol) can be used. Further, for example, when a solid polymer electrolyte or the like is used as the support, the support and the carbon dioxide reduction catalyst can be mixed to form an electrode.

[0074] And by using the electrode for carbon dioxide reduction according to the present invention as a cathode, a carbon dioxide reduction device can be obtained. The basic structure of the carbon dioxide reduction device is an electrolytic cell, and the cathode, anode, and electrolyte are the main components. At this time, as the anode, metals such as Ni, Ti, and Fe, alloys thereof, or noble metals or noble metal alloys such as Pt are often used, but there is no particular limitation. The shape of the anode can also be appropriately selected, such as a plate shape or a mesh shape.

[0075] As a treatment form of the carbon dioxide reduction device, there is a device that reduces an aqueous solution in which carbon dioxide is dissolved in a solvent such as water. However, since the solubility of carbon dioxide in water is low, and further, the hydrogen generation reaction due to the decomposition of water is likely to compete, this form has low efficiency in the efficient reduction of carbon dioxide and the production of carbon monoxide and the like. Therefore, in recent years, the development of a carbon dioxide reduction device of a gas diffusion type electrolytic flow cell that treats a gas containing carbon dioxide has been progressing. The carbon dioxide reduction catalyst according to the present invention can be applied to any form of carbon dioxide reduction device.

[0076] In a carbon dioxide reduction device of a gas diffusion type electrolytic flow cell, in order to efficiently reach the gas to be treated to the carbon dioxide reduction catalyst, it is preferable to form a catalyst layer on a gas diffusion layer (GDL). At this time, the gas diffusion layer serves as a support for the carbon dioxide reduction catalyst, and a combination of these forms a cathode electrode.

Advantages of the Invention

[0077] As described above, the carbon dioxide reduction catalyst according to the present invention is composed of a metal nanocluster aggregate in which metal nanoclusters are mutually bonded and aggregated by a linker. In the present invention, as the linker for forming the metal nanocluster aggregate, an organometallic compound having a pyridine ligand at the terminal and containing one or more triple bonds in the structure is applied.

[0078] In the present invention, due to the self-assembly action of the linker having a pyridine ligand, the metal nanoclusters are bonded periodically and regularly. And the triple bond in the structure of the linker imparts stability and durability to the metal nanocluster aggregate. The carbon dioxide reduction catalyst according to the present invention can sufficiently exhibit the carbon dioxide reduction activity and selectivity of Ag, Au, and Cu serving as the catalyst metal.

Brief Description of the Drawings

[0079]

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Mode for Carrying Out the Invention

[0080] First Embodiment (Carbon Dioxide Reduction Catalyst Composed of Ag Nanocluster Aggregates) : Hereinafter, embodiments of the present invention will be described. In this embodiment, an Ag metal nanocluster assembly (Ag nanocluster assembly) was produced and its activity was evaluated as a carbon dioxide reduction catalyst.

[0081] In this embodiment, as a linker for forming the Ag nanocluster assembly, an Ag nanocluster assembly to which 1,4-bis(pyridin-4-ylethynyl)benzene (hereinafter sometimes referred to as 2EBbpy) listed in Chemical Formula 7 above was applied was produced (Example 1). Further, as a reference example, an Ag nanocluster assembly to which 4,4'-bipyridine (Chemical Formula 1: hereinafter sometimes referred to as bpy), which is a linker of the metal nanocluster assembly on which the present invention is based, was applied was produced (Reference Example 1). The production process of each Ag nanocluster assembly is as follows.

[0082] Example 1 (Ag / 2EBbpy) 0.55 g (Ag 3.24 mmol) of silver nitrate (AgNO3) was dissolved in 7.5 mL of acetonitrile, and 1.25 mL of tert-butylthiol (HS-tBu) was added to this solution to form a precursor Ag complex (referred to as AgStBu). The formed Ag complex was washed with alcohol or the like and then recovered by centrifugation and vacuum drying.

[0083] Next, as a solvent, 5 mL of a mixed solution in which dimethylacetamide (DMAc) was dissolved in acetonitrile at a volume ratio of 1:1 was prepared. Then, 30 mg of the Ag complex (AgStBu) produced above and 22.5 mg of silver trifluoroacetate (CF3COOAg) were added to this solvent, and the mixture was stirred well to obtain an Ag nanocluster dispersion.

[0084] While synthesizing the Ag nanocluster dispersion as described above, 45 mg of 1,4-bis(pyridin-4-ylethynyl)benzene as a linker was weighed and then added to 5 mL of the same mixed solution of dimethylacetamide and toluene as above, and the mixture was stirred well to prepare a linker solution.

[0085] Then, the Ag nanoparticle dispersion and the linker solution were mixed and left at 5°C for 24 hours. After recovery by centrifugation, it was washed with methanol to obtain an Ag nanocluster aggregate (Ag / 2EBbpy). This Ag nanocluster aggregate was used as a carbon dioxide reduction catalyst (Example 1).

[0086] Reference Example 1 (Ag / bpy) An Ag complex (AgStBu) was synthesized in the same procedure as in Example 1, and 5 mL of the same mixed solution of ethanol and acetonitrile as in Example 1 was prepared as a solvent. After adding 30 mg of the Ag complex and 22.5 mg of silver trifluoroacetate (CF3COOAg) to this solvent, the mixture was stirred well to obtain a dispersion of Ag nanoclusters.

[0087] On the other hand, 20 mg of 4,4'-bipyridine as a linker was weighed and then added to 5 mL of the same mixed solution of ethanol and acetonitrile as above as a solvent, and the mixture was stirred well to prepare a linker solution.

[0088] Then, the Ag nanocluster dispersion and the linker solution were mixed in an ice bath and left standing in a cool, dark place for 24 hours. After that, they were recovered by centrifugation and washed with methanol to obtain an Ag nanocluster aggregate (Ag / bpy). This Ag nanocluster aggregate was used as a carbon dioxide reduction catalyst (Reference Example 1).

[0089] [Observation of the Appearance of the Ag Nanocluster Aggregate (Carbon Dioxide Reduction Catalyst)] Regarding the Ag nanocluster aggregates (carbon dioxide reduction catalysts) of Example 1 and Reference Example 1 produced above, the appearance was observed with a scanning electron microscope (SEM). Figure 1 shows the SEM images of these examples. The Ag nanocluster aggregates were in a particulate form, and when the average particle size was evaluated based on the SEM images, it was about 1 μm for both Example 1 and Reference Example 1.

[0090] [Powder X-ray Diffraction Analysis of the Ag Nanocluster Aggregate] Next, powder X-ray diffraction analysis (P-XRD) was performed on the Ag nanocluster aggregates of Example 1 and Reference Example 1. In the P-XRD analysis, after adjusting the sample by appropriately pulverizing the Ag nanocluster aggregate with a glass rod, it was analyzed with a multi-purpose X-ray diffractometer (manufactured by Rigaku Corporation: Ultima IV) using a Cu X-ray source with an analysis range of 2θ = 5° to 50°.

[0091] The diffraction patterns of each Ag nanocluster aggregate by P-XRD are shown in Figure 2. Crystalline peaks were observed in the diffraction patterns of both the Ag nanocluster aggregates of Example 1 and Reference Example 1. However, in the Ag nanocluster aggregate of Example 1, clearer peaks appeared, and it is considered that the periodicity of the Ag nanoclusters is particularly good.

[0092] [Structure Analysis of the Ag Nanocluster Aggregate] Therefore, for the Ag nanocluster aggregate (Ag / 2EBbpy) of Example 1, single-crystal X-ray diffraction analysis (SC-XRD) was performed, and structural analysis based on the diffraction data was carried out. The SC-XRD analysis was performed using a single-crystal X-ray diffractometer (manufactured by Rigaku Corporation: XtaLAB Synergy-R / DW) with the sample surface-coated with a paraffin dispersion. The analysis was carried out by irradiating monochromatic X-rays from a multilayer mirror monochromatic Cu source at -173 °C to collect diffraction data. Then, for the obtained diffraction data, analysis was performed using crystal structure analysis software (Olex2) to simulate the structure of the Ag nanocluster aggregate.

[0093] In the structural analysis by SC-XRD, for the Ag nanocluster aggregate of each example, the structure of the unit unit of the Ag nanocluster and the periodic structure of the Ag nanocluster aggregate were modeled. Fig. 3 shows the analysis results of the Ag nanocluster aggregate of Example 1.

[0094] Referring to Fig. 3, from Fig. 3(a), it can be seen that the Ag nanocluster, which is the unit unit of the aggregate, is composed of 12 Ag atoms and 6 S atoms, and the Ag atoms are bonded with a cuboctahedral-shaped skeleton. In addition, thiol and trifluoroacetic acid, which are organic ligands, are coordinated to the Ag nanocluster. Then, from Fig. 3(b), in this Ag nanocluster aggregate, the Ag nanoclusters are bonded to 6 linkers to form a regular hexagon and are two-dimensionally arranged periodically and at equal intervals. Periodicity and regularity are observed in both the two-dimensional structure (plane structure) and the three-dimensional structure (layer structure).

[0095] [Characteristics Evaluation of Carbon Dioxide Reduction Catalyst (CO2 Electrolysis Test)] The carbon dioxide reduction activities of the carbon dioxide reduction catalysts (Ag nanocluster aggregates) produced in Example 1 and Reference Example 1 were evaluated. In this evaluation test, the catalyst was applied to a substrate to prepare an electrode (cathode electrode), which was incorporated into a carbon dioxide reduction device to electrolyze a carbon dioxide-containing gas and analyze the reaction products (gas components and liquid components), and the presence or absence and selectivity of the carbon dioxide reduction activity were examined.

[0096] Figure 4 is a diagram schematically showing the configuration of the carbon dioxide reduction device used in the CO2 electrolysis test of this embodiment. This carbon dioxide reduction device is a gas diffusion type electrolytic CO2 flow cell (GDE type CO2 flow cell). This GDE type CO2 flow cell has a configuration in which a cathode electrode composed of a gas diffusion layer (GDL) and the carbon dioxide reduction catalyst layer of this embodiment and an anode electrode sandwich an ion exchange membrane (polymer solid electrolyte). In the CO2 electrolysis test, a carbon dioxide-containing gas is passed through the flow path on the back side of the cathode electrode, and an electrolytic solution (0.1 M aqueous KHCO3 solution) is circulated on both sides of the ion exchange membrane for electrolysis treatment.

[0097] In this embodiment, carbon paper (MFK-A manufactured by Mitsubishi Chemical Corporation, dimensions: 2 cm × 2 cm) was used as the gas diffusion layer, and a dispersion of a carbon dioxide reduction catalyst (Ag nanocluster aggregate) was applied to and dried on the surface to fabricate a cathode electrode. The dispersion medium of the dispersion at this time was 2-propanol (IPA), the dispersion was spray-coated with a superfine airbrush, and dried at a temperature of 50 °C or higher in the atmosphere. The coating amount of the carbon dioxide reduction catalyst on the carbon paper was set to about 3 mg.

[0098] The GDE type CO2 flow cell used in this embodiment used a Pt mesh as the anode electrode and Nafion (registered trademark) NR212 as the ion exchange membrane in addition to the above cathode electrode. In the carbon dioxide reduction test, 100% CO2 gas (20 sccm) was passed through the cathode side and 0.5 M potassium hydrogen carbonate solution (400 mL / h) was passed through the cathode side for electrolysis. In this test, conditioning was performed by holding at -0.2 V for 1 hour and -0.25 V for 1 hour before the test. Then, a carbon dioxide-containing gas was passed through for electrolysis treatment. The applied potential on the cathode side at this time was set to -1.0 V, -1.5 V, -2.1 V, -2.5 V, -3.0 V, and the current density after holding at each potential for 30 minutes was measured and the reaction products were collected and analyzed. Note that the above potentials are based on RHE. Also, the potential operation during electrolysis was performed using a potentiostat.

[0099] Analysis of the reaction products at each potential was performed for both the gas component and the liquid component. For the gas component analysis, after passing through a methanizer, it was analyzed by gas chromatography (GC-2014 manufactured by Shimadzu Corporation) equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD) using Shincarbon-ST 50 / 80 (2m) manufactured by Shinwa Chemical Industries Co., Ltd. at a constant temperature of 150 °C by the isothermal method. Also, for the analysis of the liquid phase component, it was analyzed by FT-NMR (JNM-ECS400 manufactured by JEOL Ltd.) in accordance with the absolute calibration curve method. Then, the production amounts of carbon monoxide, hydrogen, methane, ethylene, formic acid, and ethanol in the reaction products were evaluated. For carbon monoxide, hydrogen, methane, and ethylene, based on the chromatogram obtained by GC analysis, the production amount of each component was evaluated by the Faraday selectivity based on the peak intensity.

[0100] The I-V curves during CO2 electrolysis using the carbon dioxide reduction catalysts of Example 1 and Reference Example 1 are shown in Fig. 5. Also, the results of the compositional analysis of the reaction products when electrolyzed at each potential are shown in Fig. 6.

[0101] Referring to the I-V curve in Fig. 5, it can be seen that in both the catalysts of Example 1 and Reference Example 1, as the potential increases on the negative side, the reaction current density (cathode current density) increases, indicating that carbon dioxide reduction proceeds.

[0102] Then, referring to Fig. 6, it can be seen that in the carbon dioxide reduction using the Ag nanocluster aggregates of Example 1 and Reference Example 1 as the catalyst, the production amount of carbon monoxide tends to increase proportionally as the potential increases. Also, at a potential on the negative side of -2.0 V (RHE) or more, the production of formic acid is also observed. On the other hand, hydrogen generation is also observed, but compared with the production amount (peak intensity ratio) of carbon monoxide, it can be said that the hydrogen generation reaction is suppressed.

[0103] Therefore, by estimating the number of electrons consumed in each reaction based on the production amount, the Faraday selectivity for each compound produced by the catalysts of Example 1 and Reference Example 1 was determined. The results are shown in Fig. 7. Depending on the applied potential, in the carbon dioxide reduction reaction using the catalysts of Example 1 and Reference Example 1, most of the introduced charge contributes to the production of carbon monoxide and formic acid. From these results, it was confirmed that the carbon dioxide reduction catalyst (Ag nanocluster aggregate) of the present embodiment has carbon dioxide reduction activity and good selectivity for the production reaction of carbon monoxide and formic acid and the like.

[0104] Fig. 8 shows the SEM photographs of the cathode electrode surface (catalyst layer surface) before and after electrolysis and the results of elemental analysis (EDX) after electrolysis in the CO2 electrolysis test. Regarding the surface morphology of the catalyst layer before and after electrolysis, a relatively large change was observed in Reference Example 1 (Ag / bpy), while the change in Example 1 (Ag / 2EBbpy) was small. Also, referring to the results of elemental analysis, in the catalyst of Reference Example 1 after electrolysis, the elements S, N, and F derived from thiol and trifluoroacetic acid, which are organic ligands of the Ag nanocluster aggregate, disappeared (this is why the atomic ratio of Ag in Reference Example 1 after electrolysis is high). From this, it is considered that there is a possibility that the structure of the Ag nanocluster aggregate has changed due to electrolysis in Reference Example 1. On the other hand, in the catalyst of Example 1, the remaining of S, N, and F was confirmed. From this, it can be seen that by applying a compound in which a triple bond is introduced to the linker of the Ag nanocluster aggregate, a more durable carbon dioxide reduction catalyst can be obtained.

[0105] Second Embodiment (Carbon Dioxide Reduction Catalyst Composed of Cu Nanocluster Aggregates) : In the present embodiment, a metal nanocluster aggregate of Cu (Cu nanocluster aggregate) was produced and its activity was evaluated as a carbon dioxide reduction catalyst. In the present embodiment, a Cu nanocluster aggregate (Example 2) to which 1,4-bis(pyridin-4-ylethynyl)benzene (2EDbpy) was applied as a linker of the Cu nanocluster aggregate and a Cu nanocluster aggregate (Reference Example 2) to which 4,4'-bipyridine (bpy) was applied as a linker were produced.

[0106] Example 2 (Cu / 2EBbpy) Dissolve 0.36 g (1.5 mmol of Cu) of copper nitrate (Cu(NO3)2·3H2O) in 5 mL of acetonitrile. After adding 5 mL of triethylamine to this solution, add 0.5 mL of tert-butyl mercaptan (HS-tBu) to form a Cu complex (CuStBu) as a precursor. The formed Cu complex was washed, centrifuged, and vacuum dried in the same manner as in Example 1 and then recovered.

[0107] Prepare 5 mL of a mixed solution in which ethanol is dissolved in chloroform at a volume ratio of 1:1 as a solvent. Then, after adding 30 mg of the Cu complex (CuStBu) prepared above and 23 mg of copper trifluoroacetate (Cu(CO2CF3)2·xH2O) to this solvent, stir well to obtain a dispersion of Cu nanoclusters.

[0108] While synthesizing the dispersion of Cu nanoclusters, weigh 22.5 mg of 1,4-bis(pyridin-4-ylethynyl)benzene as a linker, then add it to 5 mL of the same mixed solution of chloroform and ethanol as above and stir well to prepare a linker solution.

[0109] After that, mix the Cu nanocluster dispersion and the linker solution at room temperature, then leave them in a cool and dark place for 24 hours. After recovery by centrifugation, wash with methanol to obtain a Cu nanocluster aggregate (Cu / 2EBbpy). This Cu nanocluster aggregate was used as a carbon dioxide reduction catalyst (Example 2).

[0110] Reference Example 2 (Cu / bpy) After generating a Cu complex (CuStBu) under the same conditions and procedure as in Example 2, a dispersion of Cu nanoclusters was obtained. On the other hand, weigh 20 mg of 4,4'-bipyridine as a linker, then add it to 5 mL of the same mixed solution of chloroform and ethanol as in Example 2 and stir well to prepare a linker solution.

[0111] Then, the Cu nanocluster dispersion prepared above and the linker solution were mixed at room temperature and left standing for 24 hours. After that, they were collected by centrifugation and washed with methanol to obtain a Cu nanocluster aggregate (Cu / bpy). This Cu nanocluster aggregate was used as a carbon dioxide reduction catalyst (Reference Example 2).

[0112] [Observation of the Appearance of Cu Nanocluster Aggregate (Carbon Dioxide Reduction Catalyst)] Regarding the Cu nanocluster aggregates (carbon dioxide reduction catalysts) of Example 2 and Reference Example 2 produced above, the appearance was observed by SEM. Figure 9 shows the SEM images of the Cu nanocluster aggregates of these Examples and Reference Examples. All of the Cu nanocluster aggregates were in a particulate form, and the average particle size was about 1 μm (Example 2) and about 2 μm (Reference Example 2).

[0113] [Powder X-ray Diffraction Analysis of Cu Nanocluster Aggregate] Next, P-XRD analysis was performed on the Cu nanocluster aggregates of Example 2 and Reference Example 2. The analysis method and conditions at this time were the same as those in the first embodiment.

[0114] The diffraction patterns of each Cu nanocluster aggregate by P-XRD are shown in Figure 10. Similar to the Ag nanocluster aggregate of the first embodiment, the diffraction patterns of the Cu nanocluster aggregates of Example 2 and Reference Example 2 show crystallinity. And the peaks of the Cu nanocluster aggregate of Example 2 are clearer and the periodicity is better.

[0115] [Characteristic Evaluation of Carbon Dioxide Reduction Catalyst] The carbon dioxide reduction activities of the carbon dioxide reduction catalysts (Cu nanocluster aggregates) produced in Example 2 and Reference Example 2 were evaluated. For this evaluation test, the same carbon dioxide reduction apparatus (GDE type CO2 flow cell) as in the first embodiment was used. Regarding the cathode electrode, a dispersion of the carbon dioxide reduction catalyst (Cu nanocluster aggregate) was also applied and dried on the same carbon paper (gas diffusion layer) as in the first embodiment. The other configurations of the GDE type CO2 flow cell were the same as those in the first embodiment, and the test conditions were also the same.

[0116] The I-V curves of the CO2 electrolysis tests using the carbon dioxide reduction catalysts of Example 2 and Reference Example 2 are shown in FIG. 11. Also, the results of the composition analysis of the reaction products when electrolyzed at each potential are shown in FIG. 12. From FIG. 10, it can be seen that in both Example 2 and Reference Example 2, which are Cu nanocluster aggregates, as the cathode potential increases, the reaction current density increases and carbon dioxide reduction proceeds.

[0117] Referring also to FIG. 12 showing the composition of the reaction products, it can be seen that in the carbon dioxide reduction by the Cu nanocluster aggregate, in addition to carbon monoxide and formic acid, methane and ethylene are being produced. It is characteristic that in Example 2, the production of ethanol is further observed. The Faraday selectivity for each product of these carbon dioxide reduction catalysts is shown in FIG. 13. Regarding the carbon dioxide reduction catalyst composed of a Cu nanocluster aggregate, when the cathode potential is low, charge tends to be utilized for hydrogen production, but by increasing the cathode potential, the utilization efficiency for carbon monoxide, formic acid, methane, and ethylene increases. Therefore, depending on the setting of the electrolysis conditions, the Cu nanocluster aggregate can also exhibit suitable selectivity as a carbon dioxide reduction catalyst. In particular, the Cu nanocluster aggregate can be said to be a useful carbon dioxide reduction catalyst in that it produces versatile valuable substances such as methane and ethylene.

[0118] FIG. 14 shows the SEM photographs of the surface of the catalyst layer before and after electrolysis of the carbon dioxide reduction catalysts of Example 2 and Reference Example 2 and the elemental analysis results after electrolysis. A large change is observed in the surface morphology of the catalyst layer in Reference Example 2 (Cu / bpy) before and after electrolysis. Referring to the results of the elemental analysis, in the catalyst of Reference Example 1 after electrolysis, among S, N, and F, which are elements derived from the organic ligand of the Cu nanocluster aggregate, S has disappeared and N has also decreased significantly. In the Cu nanocluster aggregate of the example, although a decrease in S, N, and F is observed, S remains and the ratios of N and F are also high. Therefore, similar to the first embodiment, it is considered that a highly durable carbon dioxide reduction catalyst can be obtained by using a compound with a triple bond introduced as a linker also in the Cu nanocluster aggregate.

Industrial Applicability

[0119] As described above, in the carbon dioxide reduction catalyst composed of the metal nanocluster aggregate according to the present invention, the metal nanoclusters are periodically and regularly bonded by a predetermined linker having a self-assembling action. Such a structure of the metal nanocluster aggregate can sufficiently exhibit the carbon dioxide reduction activity and selectivity of the catalytic metals Ag and Cu. The present invention can contribute to the achievement of carbon neutrality related to recent environmental problems, and can also contribute to addressing energy problems caused by the generation of hydrocarbons such as carbon monoxide and methane.

Claims

**Claim 1** A carbon dioxide reduction catalyst comprising a metal nanocluster aggregate formed by aggregating metal nanoclusters composed of two or more metal atoms via a linker composed of an organic compound, wherein the metal atoms are at least one of Ag, Au, and Cu, and the linker is an organic compound represented by the following Chemical Formula 1 and having all terminals as pyridine ligands. 【Chemical 1】 In the above formula, n is the number of pyridine ligands and is an integer of 2 or more and 8 or less. Group X is any one of carbon, nitrogen, and phosphorus, or an aliphatic hydrocarbon group which may have a substituent having 1 or more and 6 or less carbon atoms, or a monocyclic, linked, or condensed aromatic hydrocarbon group having 6 or more and 42 or less carbon atoms (these groups may each independently have a substituent), or a monocyclic, linked, or condensed heteroaromatic group having 3 or more and 68 or less carbon atoms (these groups may each independently have a substituent). Also, each pyridine ligand may independently have a substituent. Further, the linker contains one or more carbon-carbon triple bonds between the pyridine ligand and group X. **Claim 2** The carbon dioxide reduction catalyst according to Claim 1, wherein the linker is any one of the organic compounds represented by the following Chemical Formulas 2 to 5. [Chemical 2] 【Chemical Formula 3】 In the formula, the two pyridine ligands can each independently have a substituent. Between the pyridine ligand and Y １ one or more carbon-carbon triple bonds are included. Group Y １ is carbon, an aliphatic hydrocarbon group which may have a substituent having 1 to 6 carbon atoms, or a monocyclic, linked, or condensed aromatic hydrocarbon group having 6 to 22 carbon atoms (each of these groups may independently have a substituent), or a monocyclic, linked, or condensed heteroaromatic group having 3 to 44 carbon atoms (each of these groups may independently have a substituent). 【Chemical Formula 4】 In the formula, the three pyridine ligands can each independently have a substituent. Between the pyridine ligand and the group Y ２ one or more carbon-carbon triple bonds are included. The group Y ２ is any one of carbon, nitrogen, and phosphorus, or an aliphatic hydrocarbon group which may have a substituent having 1 or more and 6 or less carbon atoms, or a monocyclic, linked, or condensed aromatic hydrocarbon group having 6 or more and 30 or less carbon atoms (these groups may each independently have a substituent), or a monocyclic, linked, or condensed heteroaromatic group having 3 or more and 27 or less carbon atoms (these groups may each independently have a substituent). 【Chemical Formula 5】 In the formula, the four pyridine ligands can each independently have a substituent. Between the pyridine ligand and group Y ３ one or more carbon-carbon triple bonds are included. Group Y ３ is carbon, an aliphatic hydrocarbon group which may have a substituent having 1 to 6 carbon atoms, or a monocyclic, linked, or condensed aromatic hydrocarbon group having 6 to 25 carbon atoms (these groups may each independently have a substituent), or a monocyclic, linked, or condensed heteroaromatic group having 3 to 44 carbon atoms (these groups may each independently have a substituent). **Claim 3** The carbon dioxide reduction catalyst according to Claim 1 or Claim 2, wherein the metal nanocluster contains 4 or more and 78 or less metal atoms. **Claim 4** The carbon dioxide reduction catalyst according to Claim 1 or Claim 2, wherein the average particle diameter of the metal nanocluster aggregate is 0.1 μm or more and 10 μm or less. **Claim 5** A carbon dioxide reduction electrode formed by combining the carbon dioxide reduction catalyst according to Claim 1 or Claim 2 with a support. **Claim 6** A carbon dioxide reduction device comprising a cathode including the carbon dioxide reduction electrode according to Claim 5.

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