ELECTROCHEMICALLY TREATED SILVER NANOCLUSTER CATALYST, MANUFACTURING METHOD THEREOF, GAS DIFFUSION ELECTRODE INCLUDING SAME, ZERO-GAP CELL INCLUDING SAME, AND CARBON DIOXIDE CONVERSION METHOD OR SYNGAS PRODUCTION METHOD BY USING SAME

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application Nos. 10-2023-0132585 and 10-2024-0114786 filed on Oct. 5, 2023 and Aug. 27, 2024, respectively and all the benefits accruing therefrom under 35 U.S.C. § 119, the contents of which in its entirety are herein incorporated by reference.

BACKGROUND

1. Field

The present invention relates to a silver nanocluster catalyst for electrochemically converting carbon dioxide, reducing water, or producing syngas with controlled composition, its manufacturing method, a gas diffusion electrode containing it, a zero-gap cell (or zero-gap reactor) including it, and a method for converting carbon dioxide or producing syngas using the same.

2. Description of the Related Art

Nanoclusters are more stable compared to a single atom or nanoparticles and exhibit molecular properties more prominently than metallic ones, resulting in optical and electrochemical characteristics that are entirely different from those of nanoparticles. In particular, the optical, electrical, and catalytic properties of nanoclusters can vary sensitively depending on factors such as the number of metal atoms, the type of metal atoms, and the ligands. As a result, research on nanoclusters is being actively conducted across a wide range of fields.

Due to the recent global climate crisis, research on carbon dioxide conversion technologies, which address the greenhouse gas issue, is actively being pursued. Among these, the technology for converting carbon dioxide through electrochemical reduction is gaining attention. This technology involves applying electrical energy to generate a potential difference between electrodes, thereby facilitating the movement of electrons to reduce carbon dioxide into useful carbon compounds. It has the advantage of being able to perform the carbon dioxide reduction reaction under ambient temperature and pressure conditions. Additionally, since the only required raw materials are water and carbon dioxide, and the electrolyte can be recycled, it does not produce chemical waste. Moreover, the process is simple, which is a significant advantage.

In the case of such electrochemical carbon dioxide reduction technology, traditional methods have utilized flow electrolytic cells with a gap structure where the electrode and separator are separated by several millimeters. However, recent research has focused on reducing ionic resistance within the gap between the electrode and separator, as well as minimizing the increase in mass transfer resistance due to generated gas during the implementation of large-area electrodes. To address these issues, there is growing interest in using zero-gap reactors with a sandwich-like structure, where the anode and cathode are in contact with each other across a separator, eliminating the gap between the electrode and the separator.

In the case of such electrochemical reduction technology, the process is highly influenced by various reaction conditions, including the type of electrode catalyst, the nature/properties of the electrolyte, pH, temperature, and pressure. Therefore, to effectively reduce carbon dioxide and convert it into useful carbon compounds, it is especially important to conduct research on the types of electrode catalysts and the electrolytes used.

Currently, gold catalysts are primarily used as electrode catalysts, but their high cost and limited availability necessitate the development of alternative metal catalysts with excellent carbon dioxide conversion activity.

Meanwhile, among the electrochemical reduction technologies, the production of syngas, which can be integrated with the Fischer-Tropsch process (FT process), has recently gained attention. Syngas, a mixture of hydrogen and carbon monoxide, is an important gas used as an intermediate feedstock for synthesizing various types of compounds.

The Fischer-Tropsch (FT) process is a method of synthesizing liquid hydrocarbons from carbon monoxide and hydrogen through catalytic reactions. It is primarily used to produce synthetic petroleum or synthetic fuels as alternatives to conventional oil. This process is a valuable technology for the future post-petroleum era, as it allows for the active utilization of renewable energy sources like biomass.

In conventional syngas production technologies based on fossil fuels, the current density is very low, and a significant amount of unreacted carbon dioxide remains in the produced syngas, leading to a very low carbon conversion rate and reduced product purity. Therefore, there is a need to develop technologies that can address these issues.

SUMMARY

The purpose of the present invention is to provide a silver nanocluster catalyst with excellent performance in carbon dioxide conversion, water reduction, and syngas production, as well as a gas diffusion electrode containing this catalyst and a zero-gap reactor incorporating it.

Another objective of the present invention is to provide a method for converting carbon dioxide, a method for reducing water, and a method for producing syngas with controlled composition, all of which exhibit excellent conversion performance and selectivity using the aforementioned zero-gap reactor.

Furthermore, the present invention also aims to provide a method for manufacturing a silver nanocluster catalyst with significantly enhanced catalytic activity by including a step of electrochemical treatment.

The present invention relates to a silver nanocluster catalyst for carbon dioxide conversion, water reduction reactions, or syngas (syngas) synthesis composed of hydrogen and carbon monoxide, represented by the following Chemical Formula 1.

XAg₁₄(R¹)_n  [Chemical Formula 1]

wherein R¹ is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀ alkynyl, C₆-C₂₀ aryl, C₃-C₂₀ cycloalkyl, C₅-C₂₀ heteroaryl, C₃-C₂₀ heterocycloalkyl, C₆-C₂₀ arylalkyl, or S—R¹¹; R¹¹ is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀ alkynyl, C₆-C₂₀ aryl, C₃-C₂₀ cycloalkyl, C₅-C₂₀ heteroaryl, C₃-C₂₀ heterocycloalkyl, or C₆-C₂₀ arylalkyl; X is halogen; and n is an integer from 6 to 11.

In Chemical Formula 1, R¹ may be C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₃-C₁₀ alkynyl, C₆-C₁₀ aryl, C₃-C₁₀ cycloalkyl, C₅-C₁₀ heteroaryl, C₃-C₁₀ heterocycloalkyl, C₆-C₁₀ arylalkyl, or S—R¹¹; R¹¹ may be C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₃-C₁₀ alkynyl, C₆-C₁₀ aryl, C₃-C₁₀ cycloalkyl, C₅-C₁₀ heteroaryl, C₃-C₁₀ heterocycloalkyl, or C₆-C₁₀ arylalkyl; X may be halogen; and n may be an integer from 6 to 11.

In an embodiment of the present invention, R¹ of Chemical Formula 1 may be C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₃-C₁₀ alkynyl, C₆-C₁₀ aryl, or S—R¹¹; R¹¹ may be C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₃-C₁₀ alkynyl, C₆-C₁₀ aryl, or C₆-C₁₀ arylalkyl; X may be halogen; and n may be an integer from 6 to 11.

In an embodiment of the present invention, R¹ of Chemical Formula 1 may be C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, or C₃-C₁₀ alkynyl; X may be halogen; and n may be an integer from 6 to 11.

The present invention relates to a gas diffusion electrode for carbon dioxide conversion, water reduction reactions, or syngas (syngas) synthesis composed of hydrogen and carbon monoxide, which may include a porous support and a silver nanocluster catalyst according to an embodiment of the present invention fixed in the pores of the porous support, wherein the porous support may be a carbon body, and the average pore size of the porous support may range from 10 to 1000 nm.

In an embodiment of the present invention, the average particle size of the silver nanoclusters used in the gas diffusion electrode for carbon dioxide conversion, water reduction reactions, or syngas synthesis may range from 1 to 5 nm, and the silver nanocluster catalyst may be supported on the porous support at a density of 1 to 100 nmol/cm² per unit area.

The present invention relates to a zero-gap reactor for carbon dioxide conversion, water reduction reactions, or syngas synthesis, which may include an anode; a cathode comprising a silver nanoclusters according to an embodiment; and a separator positioned between the cathode and the anode.

The cathode may be disposed in contact with one surface of the separator, the anode may be composed of one or more selected from nickel, iron, and iridium, and the separator may be an ion-exchange membrane.

The present invention relates to a method for carbon dioxide conversion, which may include the steps of supplying carbon dioxide to one surface of the cathode in a zero-gap reactor for carbon dioxide conversion, water reduction reactions, or syngas synthesis according to an embodiment; and obtaining carbon monoxide converted from carbon dioxide from one surface of the cathode.

A method for preparing a silver nanocluster catalyst represented by the following Chemical Formula 1 according to an embodiment of the present invention may include the steps of mixing a silver precursor, a ligand compound, an alkylammonium halide, and a reducing agent; and electrochemically treating the mixture.

XAg₁₄(R¹)_n  [Chemical Formula 1]

wherein R¹ is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀ alkynyl, C₆-C₂₀ aryl, C₃-C₂₀ cycloalkyl, C₅-C₂₀ heteroaryl, C₃-C₂₀ heterocycloalkyl, C₆-C₂₀ arylalkyl, or S—R¹¹; R¹¹ is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀ alkynyl, C₆-C₂₀ aryl, C₃-C₂₀ cycloalkyl, C₅-C₂₀ heteroaryl, C₃-C₂₀ heterocycloalkyl, or C₆-C₂₀ arylalkyl; X is halogen; and n is an integer from 6 to 11.

The ligand compound may be an alkyne compound with C₃-C₂₀, and the molar ratio of the silver precursor to the alkylammonium halide may range from 1:0.01 to 0.5. The silver precursor may be selected from one or more of AgNO₃, AgBF₄, AgCF₃SO₃, AgClO₄, AgO₂CCH₃, and AgPF₆.

The method for preparing a silver nanocluster catalyst represented by Chemical Formula 1 according to an embodiment of the present invention may include the steps of mixing a silver precursor, a ligand compound, and a halide compound in aqueous phase; and electrochemically treating the mixture.

XAg₁₄(R¹)_n  [Chemical Formula 1]

wherein R¹ is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀ alkynyl, C₆-C₂₀ aryl, C₃-C₂₀ cycloalkyl, C₅-C₂₀ heteroaryl, C₃-C₂₀ heterocycloalkyl, C₆-C₂₀ arylalkyl, or S—R¹¹; R¹¹ is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀ alkynyl, C₆-C₂₀ aryl, C₃-C₂₀ cycloalkyl, C₅-C₂₀ heteroaryl, C₃-C₂₀ heterocycloalkyl, or C₆-C₂₀ arylalkyl; X is halogen; and n is an integer from 6 to 11.

The ligand compound may be an alkyne compound with C₃-C₂₀, and the halide compound may be an alkali metal salt.

The silver nanoclusters of the present invention possesses structural stability within a specific range of atoms and ligands, and exhibits superior activity for the conversion reaction of carbon dioxide compared to conventional silver-based catalysts.

The silver nanocluster catalyst of the present invention is not only cost-effective but also highly uniform, making it very useful as a catalyst for the conversion of carbon dioxide.

The gas diffusion electrode comprising the silver nanoclusters of the present invention, and the zero-gap reactor including it, demonstrate superior performance compared to conventional methods using flow electrolysis cells and fossil fuel-based systems for carbon dioxide conversion. This system also exhibits greatly improved selectivity for the carbon dioxide conversion reaction, with minimal residual impurities, thus eliminating the need for additional purification processes.

Therefore, using the gas diffusion electrode for carbon dioxide conversion and the zero-gap reactor that includes it, as provided by the present invention, allows for the effective conversion of carbon dioxide with high selectivity and conversion rates, and also enables the production of syngas with controlled composition.

Furthermore, the method for preparing the silver nanocluster catalyst of the present invention, by exposing the active sites of the catalyst through electrochemical treatment, significantly enhances catalytic activity. Additionally, this method allows for the high-yield mass production of the catalyst through a simple process, making it highly valuable for industrial applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating a zero-gap reactor comprising a gas diffusion electrode for carbon dioxide conversion, water reduction reactions, or a synthesis of syngas composed of hydrogen and carbon monoxide.

FIG. 2 is a schematic diagram depicting the electrochemical treatment step in the method for preparing silver nanoclusters according to the present invention.

FIG. 3 is a graph showing the results of ESI-MS for Example 1.

FIG. 4 is a graph showing the UV-vis results for Example 1 and Example 2.

FIG. 5 is a graph showing the EXAFS results for Example 1 and Example 2.

FIG. 6 is a graph showing the UV-vis results for Examples 5 to 7.

FIG. 7 is a graph showing the carbon dioxide conversion activity results for Example 1 and Comparative Example 1.

FIG. 8 is a graph showing the carbon dioxide conversion activity results for Example 1 and Comparative Example 1.

FIG. 9 is a graph showing the electrochemical activity results for Example 1 and Comparative Example 1.

FIG. 10 is a graph showing the electrochemical activity results for Example 1 and Comparative Example 1.

FIG. 11 is a graph showing the electrochemical activity results for Examples 2 to 4.

FIG. 12 is a schematic diagram showing the electrochemical treatment process and the resulting nanoclusters for Example 2 and Example 8.

FIG. 13 is a graph showing the carbon dioxide conversion activity of the nanoclusters produced in Example 2 and Example 8 (−1.4 V vs. Ag/AgCl).

FIG. 14 is a schematic diagram and graph showing the single-crystal structure and mass spectrometry results of the silver nanoclusters synthesized according to Example 1 of the present invention.

FIG. 15 is a schematic diagram and graph showing the single-crystal structure and mass spectrometry results of the gold nanoclusters synthesized according to Comparative Example 1 of the present invention.

FIG. 16 is a graph showing the syngas production results in a zero-gap reactor using the [ClAg₁₄ (C₆H₉)₈]⁺ catalyst of Example 1 of the present invention.

FIG. 17 is a graph showing the syngas production results in a zero-gap reactor using the catalyst of Comparative Example 2 of the present invention.

FIG. 18 is a graph showing the syngas production results in a zero-gap reactor using the catalyst of Comparative Example 3 of the present invention.

FIG. 19 is a graph showing the gas selectivity as a function of current density when the target syngas composition is CO:H₂=1:3.

FIG. 20 is a graph showing the reaction selectivity of CO as a function of CO₂ partial pressure (atm) under an operating condition of 200 mA/cm².

FIGS. 21A to 21D are a graph showing FIG. 21A syngas production, FIG. 21B selectivity of each gas, FIG. 21C residual CO₂ percentage (%), and FIG. 21D carbon conversion efficiency (%) as a function of current density when the target syngas composition is CO:H₂=1:3.

FIG. 22 is a graph showing the gas selectivity in a 24-hour reaction under an operating condition of 400 mA/cm² when the target syngas composition is CO:H₂=1:3.

DETAILED DESCRIPTION

The following provides a detailed description of the electrochemically treated silver nanocluster catalyst for carbon dioxide conversion or syngas conversion, its preparation method, a gas diffusion electrode containing the catalyst, a zero-gap reactor containing the electrode, a method for converting carbon dioxide using this system, and a method for producing syngas, according to the present invention.

The singular forms used in this invention may include the plural forms as well, unless the context specifically indicates otherwise.

Furthermore, the numerical ranges used in this invention are intended to include the lower and upper limits, all values within the range, increments logically derived from the defined range, all values of dual-limited ranges, and all possible combinations of upper and lower limits of different numerical ranges. Unless otherwise defined in this specification, values outside the defined numerical range that could arise due to experimental error or rounding are also intended to be included within the defined numerical range.

The term “comprising,” as used in this invention, is an open-ended term equivalent to “including,” “containing,” “having,” or “characterized by,” and does not exclude elements, materials, or processes not explicitly listed.

The term “alkyl” as used in this invention refers to a straight-chain or branched acyclic hydrocarbon containing 1 to 10 carbon atoms, preferably 1 to 6 carbon atoms. In another embodiment, the alkyl group may have 1 to 3 carbon atoms.

The term “alkenyl” as used in this invention refers to a saturated straight-chain or branched non-cyclic hydrocarbon containing at least one carbon-carbon double bond. Examples include, but are not limited to, -vinyl, -allyl, -1-butenyl, -2-butenyl, -isobutyl, -1-pentenyl, -2-pentenyl, -3-methyl-1-butenyl, -2-methyl-2-butenic, -2,3-dimethyl-2-butenyl, -1-hexenyl, -2-hexenyl, -3-hexenyl, -1-heptenyl, -2-heptenyl, -3-heptenyl, -1-octenyl, -2-octenyl, -3-octenyl, -1-nonenyl, -2-nonenyl, -3-nonenyl, -1-decyl, -2-decyl, and -3-decyl. These alkenyl groups may be optionally substituted and may include radicals with cis and trans orientations, or alternatively, E and Z orientations.

The term “alkynyl” as used in this invention refers to a saturated straight-chain or branched non-cyclic hydrocarbon containing at least one carbon-carbon triple bond. Examples include, but are not limited to, ethynyl, propynyl, butynyl, butadiynyl, pentynyl, pentadiynyl, hexynyl, hexadiynyl, and their isomers.

The term “cycloalkyl” as used in this invention refers to a monocyclic or polycyclic saturated ring containing carbon and hydrogen atoms, with no carbon-carbon multiple bonds. Examples include cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, and cycloheptyl. These cycloalkyl groups may be optionally substituted. Additionally, cycloalkyl may include structures where the ring is connected to one or more heteroatoms selected from B, O, N, C(═O), P, P(═O), S, S(═O)2, and Si.

The term “halogen” as used in this invention refers to fluorine, chlorine, bromine, or iodine.

The term “aryl” as used in this invention refers to an aromatic carbon ring group containing 5 to 20 ring atoms. Representative examples include phenyl, tolyl, xylyl, naphthyl, tetrahydronaphthyl, anthracenyl, fluorenyl, indenyl, azulenyl, and the like. Furthermore, aryl may include structures where the aromatic carbon ring group is connected via an alkylene or alkenylene group or via one or more heteroatoms selected from B, O, N, C(═O), P, P(═O), S, S(═O)2, and Si.

The carbon number in this invention refers to the number of carbon atoms excluding those in substituents. For example, C₁-C₁₀ alkyl refers to an alkyl group with 1 to 10 carbon atoms, excluding any carbon atoms in the substituents.

The following is a detailed description of the present invention. Unless otherwise defined, technical and scientific terms used herein have the meanings that are commonly understood by those skilled in the art. Descriptions of known functions and structures are omitted where they would unnecessarily obscure the essence of the present invention.

The technology for carbon dioxide conversion through electrochemical reduction proceeds as shown in Reaction Formula 1, where the reduction of carbon dioxide occurs in an aqueous electrolyte. Given that the reduction potential regions for hydrogen generation and carbon dioxide reduction are similar, these reactions may occur simultaneously, making selectivity a critical issue.

CO₂+e⁻+H₂O→COOH*+OH⁻120 m Vdec⁻¹

COOH*+e⁻→CO*+OH⁻40 m Vdec⁻¹

CO*→CO30 m Vdec⁻¹  [Reaction Formula 1]

The inventors of the present invention intensified their research efforts and discovered that the silver nanoclusters of the present invention is not only cost-effective and highly uniform but also exhibits excellent selectivity for carbon dioxide reduction. Furthermore, they found that the catalytic activity is significantly enhanced due to the exposure of active sites as ligands are removed through the electrochemical treatment process. This discovery led to the completion of the present invention.

Moreover, it was found that the catalyst of the present invention, due to its high selectivity for carbon dioxide reduction, produces high-purity syngas (CO+H₂) leaving behind almost no unreacted gases. Additionally, by adjusting the supply of water and carbon dioxide, it is possible to produce syngas with a desired ratio of CO to H₂, leading to the completion of the invention.

Syngas is a precursor used in various chemical production processes, such as Fischer-Tropsch synthesis. If syngas with a controlled CO to H₂ ratio, matching the target stoichiometry, can be obtained, it can be directly used in subsequent reactions without the need for additional purification steps. This reduces costs and offers additional economic benefits in related industrial fields.

The production of syngas (CO+H₂) using the silver nanocluster catalyst of the present invention proceeds according to Reaction Formula 2 below. Additionally, FIG. 5 illustrates the process in which this reaction unfolds, represented schematically as the reactions occurring in an electrolytic cell containing the silver nanocluster catalyst of the present invention.

[Reaction Formula 2]

CO₂+2e⁻+H₂O→CO+2OH⁻  (i)

CO₂+2OH⁻+6e⁻+5H₂O→CO₃²⁻+3H₂+6OH⁻  (ii)

2CO₂+8e⁻+6H₂O→CO+CO₃²⁻+3H₂+6OH⁻  (iii)

The silver nanocluster catalyst of the present invention exhibits extremely high selectivity for carbon dioxide reduction. When water and carbon dioxide are introduced as reactants into the reactor, the reduction of carbon dioxide proceeds before the reduction of water. If the catalyst achieves the theoretical maximum carbon conversion rate of 50%, then, according to Reaction Formula 2 (i), half of the supplied CO₂ is converted into CO, with each CO₂ molecule being reduced to CO by consuming two electrons.

Following the reduction of carbon dioxide, the remaining electrons, CO₂, and OH⁻ are utilized in the reduction reaction of water to produce hydrogen. According to Reaction Formula 2 (ii), the remaining carbon dioxide is converted into CO₃²⁻ ions, and water is reduced to produce hydrogen, where 2H₂O consumes two electrons to be reduced to 1H₂.

Therefore, the silver nanocluster catalyst of the present invention, with its high selectivity for carbon dioxide reduction, can theoretically achieve the maximum carbon conversion rate of 50%. When this 50% carbon conversion rate is achieved, supplying 2CO₂ and 6H₂O to the reactor, as described in Reaction Formula 2 (iii), will consume a total of 8 electrons, resulting in the production of 1CO and 3H₂. This means that by adjusting the amount and ratio of the reactants supplied, it is possible to control the composition of the produced syngas.

The present invention relates to a silver nanocluster catalyst for carbon dioxide conversion, water reduction reactions, or syngas (comprising hydrogen and carbon monoxide) synthesis, represented by Chemical Formula 1.

XAg₁₄(R¹)_n  [Chemical Formula 1]

wherein R¹ is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀ alkynyl, C₆-C₂₀ aryl, C₃-C₂₀ cycloalkyl, C₅-C₂₀ heteroaryl, C₃-C₂₀ heterocycloalkyl, C₆-C₂₀ arylalkyl, or S—R¹¹; R¹¹ is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀ alkynyl, C₆-C₂₀ aryl, C₃-C₂₀ cycloalkyl, C₅-C₂₀ heteroaryl, C₃-C₂₀ heterocycloalkyl, or C₆-C₂₀ arylalkyl; X is halogen; and n is an integer from 6 to 11.

The silver nanocluster catalyst of the present invention is not only highly active for the conversion reaction of carbon dioxide but also offers superior stability compared to conventional gold (Au) catalysts. It is cost-effective, highly uniform, and very useful as a catalyst for carbon dioxide conversion.

In Chemical Formula 1, R¹ may be C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₃-C₁₀ alkynyl, C₆-C₁₀ aryl, C₃-C₁₀ cycloalkyl, C₅-C₁₀ heteroaryl, C₃-C₁₀ heterocycloalkyl, C₆-C₁₀ arylalkyl, or S—R¹¹; R¹¹ may be C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₃-C₁₀ alkynyl, C₆-C₁₀ aryl, C₃-C₁₀ cycloalkyl, C₅-C₁₀ heteroaryl, C₃-C₁₀ heterocycloalkyl, or C₆-C₁₀ arylalkyl; X may be a halogen; and n may be an integer from 6 to 11.

In an embodiment of the present invention, R¹ of Chemical Formula 1 may be C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₃-C₁₀ alkynyl, C₆-C₁₀ aryl, or S—R¹¹; R¹¹ may be C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, C₃-C₁₀ alkynyl, C₆-C₁₀ aryl, or C₆-C₁₀ arylalkyl; X may be a halogen; and n may be an integer from 6 to 11.

Specifically, R¹ may be C₁-C₁₀ alkyl, C₂-C₁₀ alkenyl, or C₃-C₁₀ alkynyl; X may be a halogen; and n may be an integer from 6 to 11.

The present invention also relates to a gas diffusion electrode for carbon dioxide conversion, water reduction reactions, or syngas synthesis composed of hydrogen and carbon monoxide, which may include a porous support and a silver nanocluster catalyst according to an embodiment of the present invention fixed in the pores of the porous support.

The porous support may be a microporous layer (MPL) with conductivity, and it may specifically be a carbon-based material. Examples of carbon-based materials include, but are not limited to, carbon black, carbon nanotubes, graphene, carbon nanofibers, and graphitized carbon black. More specifically, it may be carbon black.

The average pore size of the porous support may range from 10 to 1000 nm, more specifically from 10 to 500 nm, and even more specifically from 10 to 100 nm, but it is not limited to these ranges.

In an embodiment of the present invention, the average particle size of the silver nanoclusters used in the gas diffusion electrode for carbon dioxide conversion, water reduction reactions, or syngas synthesis may range from 1 to 5 nm. The silver nanocluster catalyst may be supported on the porous support at a density of 1 to 100 nmol/cm² per unit area, preferably at a density of 1 to 50 nmol/cm², and more preferably at a density of 1 to 30 nmol/cm².

A method of supporting metal nanoclusters on a porous support, according to an embodiment, may involve preparing a solution in which metal nanoclusters are dispersed, dropping this solution onto the porous support, drying it, and then heat-treating it. This method allows the metal nanoclusters to be easily applied across the entire surface of the support via capillary action, resulting in a uniformly coated and stably supported material. The solvent used in this method can be any organic solvent commonly recognized in the field, including, but not limited to, ethanol, methanol, isopropanol, butanol, pentanol, hexanol, dichloromethane, hexane, acetone, ethylene glycol, diethylene glycol, glycerol, and propylene glycol.

Due to the inclusion of metal nanoclusters as a catalyst compound in the cathode, the metal nanoclusters can be dispersed and adsorbed at the molecular level within the pores of the porous support, in contrast to conventional metal nanoparticle catalysts. As a result, cathodes containing metal nanoclusters can exhibit significantly enhanced electrochemical carbon dioxide conversion properties compared to those containing metal nanoparticle catalysts.

In an embodiment of the present invention, the cathode may reduce carbon dioxide to carbon monoxide. More specifically, the zero-gap reactor of the present invention, as illustrated in FIG. 1, allows the carbon dioxide supplied to the cathode to be converted to carbon monoxide, which is then released. The electrolyte in the zero-gap reactor may be an aqueous solution of KCl, NaOH, or KOH. Specifically, the electrolyte may have a pH of 7 to 14, and more specifically, a pH of 8 to 14. The concentration of the aqueous solution may range from 0.1 to 10 M, more specifically from 0.5 to 5 M, and even more specifically from 0.5 to 3 M, but it is not limited to these ranges.

The present invention relates to a zero-gap reactor for carbon dioxide conversion, water reduction reactions, or syngas (a mixture of hydrogen and carbon monoxide) synthesis. The zero-gap reactor may include an anode, a cathode comprising silver nanoclusters according to an embodiment of the invention, and a separator positioned between the cathode and the anode.

The gas diffusion electrode containing the silver nanoclusters and the zero-gap reactor incorporating it demonstrate superior performance compared to conventional flow electrolysis cells for carbon dioxide conversion, with significantly enhanced selectivity for the carbon dioxide conversion reaction.

The separator may be positioned in a sandwich configuration between the cathode and the anode. Specifically, the cathode can be placed in contact with one surface of the separator, and the separator may be in contact with the microporous layer (MPL) of the gas diffusion electrode, which includes the silver nanoclusters as the cathode.

The anode may be made of a conductive metal, specifically selected from one or more of nickel, iron, and iridium. More specifically, the anode may be in the form of a conductive metal foam, such as a porous nickel foam, though it is not limited to this material.

The separator should be durable in strong alkaline environments, have low gas permeability, and possess high ionic conductivity. In an embodiment, the separator may be an ion-exchange membrane. These ion-exchange membranes can be manufactured from ion-exchange resins and ionomers known in the industry or purchased as films. For example, Nafion™ membranes, which are perfluorinated sulfonic acid-containing polymers from DuPont, may be used. Other similar commercial membranes include the Aquivion PFSA membranes from Solvay, Fumapem ion-exchange membranes from Fumatech (including both cation and anion exchange membranes), anion exchange membranes from Dioxide Materials, Orion Polymer, Aciplex-S membranes from Asahi Chemicals, Dow membranes from Dow Chemicals, Flemion membranes from Asahi Glass, and GoreSelect membranes from Gore & Associates, though the invention is not limited to these examples.

The present invention also relates to a method for carbon dioxide conversion. This method may include the steps of supplying carbon dioxide to one surface of the cathode in the zero-gap reactor for carbon dioxide conversion, water reduction reactions, or syngas synthesis according to an embodiment of the invention, and obtaining carbon monoxide converted from carbon dioxide from one surface of the cathode.

Using the gas diffusion electrode for carbon dioxide conversion and the zero-gap reactor containing it, according to the present invention, allows for the effective conversion of carbon dioxide with high selectivity and conversion efficiency.

The method for preparing the silver nanocluster catalyst represented by Chemical Formula 1 according to an embodiment of the present invention may include the steps of mixing a silver precursor, a ligand compound, an alkylammonium halide, and a reducing agent, and electrochemically treating the mixture.

XAg₁₄(R¹)_n  [Chemical Formula 1]

wherein R¹ is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀ alkynyl, C₆-C₂₀ aryl, C₃-C₂₀ cycloalkyl, C₅-C₂₀ heteroaryl, C₃-C₂₀ heterocycloalkyl, C₆-C₂₀ arylalkyl, or S—R¹¹; R¹¹ is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀ alkynyl, C₆-C₂₀ aryl, C₃-C₂₀ cycloalkyl, C₅-C₂₀ heteroaryl, C₃-C₂₀ heterocycloalkyl, or C₆-C₂₀ arylalkyl; X is halogen; and n is an integer from 6 to 11.

In more detail, the ligand compound used in the present invention may be an alkyne compound with C₃-C₂₀, more specifically, C₃-C₁₀, or even C₃-C₇. The molar ratio of the silver precursor to the ligand compound may range from 1:0.1 to 10, preferably from 1:0.2 to 7, and more preferably from 1:0.5 to 5. This range is preferred because it ensures high synthesis efficiency while reducing reaction impurities.

In the catalyst preparation method according to an embodiment, the molar ratio of the silver precursor to the reducing agent may be from 1:0.1 to 10, preferably from 1:0.2 to 7, and more preferably from 1:0.5 to 5, although it is not limited to these ranges.

Additionally, the molar ratio of the silver precursor to the alkylammonium halide may be from 1:0.01 to 0.5, preferably from 1:0.02 to 0.3, and more preferably from 1:0.03 to 0.2.

In an embodiment, the silver precursor may be selected from one or more of AgNO₃, AgBF₄, AgCF₃SO₃, AgClO₄, AgO₂CCH₃, and AgPF₆, more specifically from AgNO₃, AgBF₄, and AgPF₆, and even more specifically from AgNO₃ or AgBF₄, though it is not limited to these examples.

In an embodiment, the preparation method may further include a solvent, which can be any solvent commonly used in the industry without particular limitation. Specific examples include C₁-C₅ alcohols, acetonitrile, dimethyl sulfoxide, dimethylformamide, acetone, tetrahydrofuran, and 1,4-dioxane, alone or in combination. Preferably, tetrahydrofuran may be used, but it is not limited to this.

The preparation method of the silver nanoclusters may also include a step of precipitation and separation using a non-polar solvent. The non-polar solvent may be selected from one or more of n-pentane, n-hexane, n-heptane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, cis-cyclooctene, toluene, m-, o-, p-xylene, t-butyl methyl ether, and di-n-butyl ether. More specifically, it may be selected from one or more of n-pentane, n-hexane, n-heptane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, and cis-cyclooctene. Even more specifically, it may be selected from one or more of n-pentane, n-hexane, and n-heptane, though it is not limited to these.

The alkylammonium halide may be a C₁₀-C₃₀ alkylammonium halide, preferably a C₁₂-C₂₀ alkylammonium halide, and more preferably a C₁₄-C₁₈ alkylammonium halide.

The reducing agent may be selected from one or more of triethylamine, oleylamine, carbon monoxide, and sodium borohydride, more specifically triethylamine, but it is not limited to these.

In the catalyst preparation method according to an embodiment, the electrochemical treatment step may include dispersing the mixture of silver precursor, ligand compound, alkylammonium halide, and reducing agent onto a working electrode; preparing another counter electrode and a reference electrode; and applying a potential of −0.01 to −3.0 V to the working electrode. Specifically, the reference electrode may be Ag/AgCl, and the counter electrode may include nickel, though these are not limitations.

In an embodiment, the working electrode may be a porous electrode, specifically based on a metal selected from the group consisting of carbon, nickel, and silver, though it is not limited to these.

The step of applying the potential may be performed for 1 minute to 10 hours, preferably for 30 minutes to 3 hours.

The method for preparing the silver nanocluster catalyst represented by Chemical Formula 1 according to an embodiment of the present invention may include mixing a silver precursor, a ligand compound, and a halide compound in an aqueous solution; and electrochemically treating the mixture.

XAg₁₄(R¹)_n  [Chemical Formula 1]

wherein R¹ is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀ alkynyl, C₆-C₂₀ aryl, C₃-C₂₀ cycloalkyl, C₅-C₂₀ heteroaryl, C₃-C₂₀ heterocycloalkyl, C₆-C₂₀ arylalkyl, or S—R¹¹; R¹¹ is C₁-C₂₀ alkyl, C₂-C₂₀ alkenyl, C₃-C₂₀ alkynyl, C₆-C₂₀ aryl, C₃-C₂₀ cycloalkyl, C₅-C₂₀ heteroaryl, C₃-C₂₀ heterocycloalkyl, or C₆-C₂₀ arylalkyl; X is halogen; and n is an integer from 6 to 11.

Specifically, the ligand compound in the present invention may be an alkyne compound with a carbon chain length of C₃-C₂₀, more specifically C₃-C₁₀, and even more specifically C₃-C₇. The molar ratio of the silver precursor to the ligand compound can range from 1:0.1 to 10, preferably from 1:0.2 to 7, and more preferably from 1:0.5 to 5. This range is preferred as it provides excellent synthesis efficiency while minimizing reaction impurities.

Additionally, the mole ratio of the silver precursor to the halide compound can range from 1:0.01 to 0.5, preferably from 1:0.02 to 0.3, and more preferably from 1:0.03 to 0.2.

In an embodiment, the silver precursor may be selected from one or more of AgNO₃, AgBF₄, AgCF₃SO₃, AgClO₄, AgO₂CCH₃, and AgPF₆, more specifically from AgNO₃, AgBF₄, and AgPF₆, and even more specifically from AgNO₃ or AgBF₄, though it is not limited to these.

The method for preparing the silver nanoclusters may further include a step of precipitating and separating the product using a non-polar solvent. The non-polar solvent may be selected from one or more of n-pentane, n-hexane, n-heptane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, cis-cyclooctene, toluene, m-, o-, p-xylene, t-butyl methyl ether, and di-n-butyl ether. More specifically, it may be selected from one or more of n-pentane, n-hexane, n-heptane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, and cis-cyclooctene. Even more specifically, it may be selected from one or more of n-pentane, n-hexane, and n-heptane, though it is not limited to these.

In an embodiment, the halide compound may be an alkali metal salt, specifically selected from LiCl, NaCl, KCl, LiF, NaF, LiBr, NaBr, KBr, LiI, and KI, with NaCl being more preferred.

In the catalyst preparation method according to an embodiment, the electrochemical treatment step may include dispersing the mixture of the silver precursor, ligand compound, and halide compound onto a working electrode; preparing a counter electrode and a reference electrode; and applying a potential of −0.01 to −3.0 V to the working electrode. Specifically, the reference electrode may be Ag/AgCl, and the counter electrode may include nickel, though these are not limitations.

The working electrode may be a porous electrode, specifically based on a metal selected from the group consisting of carbon, nickel, and silver, though it is not limited to these.

The step of applying the potential may be performed for 1 minute to 10 hours, preferably for 30 minutes to 3 hours.

A schematic diagram of the electrochemical treatment step of the present invention is illustrated in FIG. 2. Through this step, the ligands on the silver nanocluster catalyst are removed, exposing the active sites of silver, thereby significantly enhancing the carbon dioxide conversion activity.

Moreover, the catalyst preparation method described above allows for high-yield mass production, leveraging the aggregation properties of silver ions around anions, making it highly valuable for industrial applications.

The following detailed examples provide further explanation of the silver nanocluster catalyst for carbon dioxide conversion, water reduction reactions, or syngas (a mixture of hydrogen and carbon monoxide) synthesis, the gas diffusion electrode containing it, the zero-gap reactor incorporating it, and the method for converting carbon dioxide using these components according to the present invention.

However, these examples are merely illustrative and are not intended to limit the scope of the invention, as the invention can be implemented in various forms. Additionally, the terminology used in describing the invention is intended to effectively describe specific examples and is not intended to limit the scope of the invention.

Preparation Example 1: Synthesis of [Ag₁₄ (C≡CtBu)₁₂Cl]⁺ Nanocluster

In a flask, 0.097 g (0.5 mmol) of AgBF₄, 0.061 mL (0.5 mmol) of t-butylacetylene, 0.070 mL (0.5 mmol) of triethylamine, and 0.011 g (0.04 mmol) of tetrabutylammonium chloride were added, followed by the addition of 1 mL of tetrahydrofuran. The mixture was stirred for 4 hours. The solvent was removed by vacuum distillation, and the residue was washed with pure water and n-pentane to remove impurities, yielding the [Ag₁₄ (C≡CtBu)₁₂Cl]⁺ nanocluster.

The ESI-MS analysis of the nanoclusters synthesized in Example 1, as shown in FIG. 3, confirmed that the results matched the theoretical values.

Preparation Example 2: Synthesis of [Ag₁₄ (C≡CtBu)₈Cl]⁺ Nanoclusters Through Electrochemical Treatment

The [Ag₁₄ (C≡CtBu)₁₂Cl]⁺ nanoclusters obtained in Example 1 was fixed onto a 100 cm² porous electrode (0.5 mg). This electrode was used as the working electrode, another porous nickel electrode was used as the counter electrode, and Ag/AgCl was used as the reference electrode. A constant potential of −2.0 V (vs. Ag/AgCl) was applied to the working electrode in a 1.0 M KOH solution for 1 hour. After washing the working electrode with pure water and diethyl ether, the [Ag₁₄ (C≡CtBu)₈Cl]⁺ nanoclusters were obtained using dichloromethane as the solvent.

The UV-vis and EXAFS (Extended X-ray Absorption Fine Structure) spectra of the nanoclusters synthesized in Examples 1 and 2 are shown in FIGS. 4 and 5, respectively. The UV-vis results confirmed that the overall structure of the cluster was maintained after electrochemical treatment.

Moreover, the EXAFS spectra revealed that the Ag—C peak of the nanoclusters in Example 2 decreased compared to that in Example 1, indicating that the ligand was removed during the electrochemical treatment, thereby exposing the active sites of the catalyst.

Preparation Example 3: Synthesis of [Ag₁₄ (C≡CtBu)₈F]⁺ Nanocluster

The same procedure as in Example 1 was followed, except that tetrabutylammonium fluoride (0.04 mmol) was used instead of tetrabutylammonium chloride. The resulting [Ag₁₄ (C≡CtBu)₁₂F]⁺ nanoclusters were then subjected to the same electrochemical treatment as in Example 2 to yield the [Ag₁₄ (C≡CtBu)₈F]⁺ nanocluster.

Preparation Example 4: Synthesis of [Ag₁₄ (C≡CtBu)₈Br]⁺ Nanocluster

The same procedure as in Example 1 was followed, except that tetrabutylammonium bromide (0.04 mmol) was used instead of tetrabutylammonium chloride.

Preparation Example 5: Synthesis of Aqueous-Based [Ag₁₄ (C≡CtBu)₈Cl]⁺ Nanocluster

In a flask, 0.100 g (0.51 mmol) of AgBF₄ was dissolved in 5 mL of pure water. Then, 0.054 mL (0.44 mmol) of t-butylacetylene and 0.004 g (0.07 mmol) of sodium chloride were added to the solution, and the mixture was stirred for 12 hours. As the cluster catalyst formed, it precipitated due to the difference in solubility. The precipitate was washed with pure water and n-pentane to remove impurities, and the [Ag₁₄ (C≡CtBu)₁₂Cl]⁺ nanoclusters were obtained. This nanoclusters were then subjected to the same electrochemical treatment as in Example 2 to yield the [Ag₁₄ (C≡CtBu)₈Cl]⁺ nanocluster.

Preparation Example 6: Synthesis of Aqueous-Based [Ag₁₄ (C≡CtBu)₈F]⁺ Nanocluster

The same procedure as in Example 5 was followed, except that sodium fluoride (0.07 mmol) was used instead of sodium chloride.

Preparation Example 7: Synthesis of Aqueous-Based [Ag₁₄ (C≡CtBu)₈Br]⁺ Nanocluster

The same procedure as in Example 5 was followed, except that sodium bromide (0.07 mmol) was used instead of sodium chloride.

The UV-vis spectra of the nanoclusters synthesized in Examples 5 to 7 are shown in FIG. 6.

Preparation Example 8: Synthesis of [Ag₁₄ (C≡CtBu)₈Cl]⁺ [Ag₁₄ (C≡CtBu)₁₀Cl]⁺, and [Ag₁₄ (C≡CtBu)₁₁Cl]⁺ Nanoclusters

In Example 2, the electrochemical treatment time was varied to 45 minutes, 30 minutes, and 15 minutes instead of 1 hour, resulting in the [Ag₁₄ (C≡CtBu)₉Cl], [Ag₁₄ (C≡CtBu)₁₀Cl]⁺, and [Ag₁₄ (C≡CtBu)₁₁Cl]⁺ nanoclusters, respectively. The schematic diagram illustrating the electrochemical treatment process and the resulting nanoclusters for Preparation Examples 2 and 8, along with the graph showcasing the CO₂ conversion activity of the nanoclusters synthesized in these Examples (−1.4 V vs. Ag/AgCl), are presented in FIGS. 12 and 13, respectively.

Preparation Example 9: Synthesis of [Ag₁₄ (C≡CC₂H₄NH₂)₈Cl]⁺, [Ag₁₄ (C≡CC₂H₄NH₂)₉Cl]⁺, [Ag₁₄ (C≡CC₂H₄NH₂)₁₀Cl]⁺, and [Ag₁₄ (C≡CC₂H₄NH₂)₁₁Cl]⁺ Nanoclusters

In a flask, 0.097 g (0.5 mmol) of AgBF₄, 0.035 g (0.5 mmol) of 1-amino-3-butyne (with the structure shown below), 0.070 mL (0.5 mmol) of triethylamine, and 0.011 g (0.04 mmol) of tetrabutylammonium chloride were added, followed by the addition of 1 mL of tetrahydrofuran. The mixture was stirred for 4 hours. The solvent was removed by vacuum distillation, and the residue was washed with pure water and n-pentane to remove impurities, yielding the [Ag₁₄ (C≡CC₂H₄NH₂)₁₂Cl]⁺ nanocluster.

Then, following the procedure of Examples 2 and 8, electrochemical treatment was carried out for 1 hour, 45 minutes, 30 minutes, and 15 minutes to obtain the [Ag₁₄ (C≡CC₂H₄NH₂)₈Cl]⁺, [Ag₁₄ (C≡CC₂H₄NH₂)₉Cl]⁺, [Ag₁₄ (C≡CC₂H₄NH₂)₁₀Cl]⁺, and [Ag₁₄ (C≡CC₂H₄NH₂)₁₁Cl]⁺ nanoclusters, respectively.

Preparation Example 10: Synthesis of [Ag₁₄ (C≡CC₄H₃NH)₈Cl]⁺ [Ag₁₄ (C≡CC₄H₃NH)₁₁Cl]⁺, [Ag₁₄ (C≡CC₄H₃NH)₁₀Cl]⁺, and [Ag₁₄ (C≡CC₄H₃NH)₁₁Cl]⁺ Nanoclusters

In a flask, 0.097 g (0.5 mmol) of AgBF₄, 0.045 g (0.5 mmol) of 2-ethynylpyrrole (with the structure shown below), 0.070 mL (0.5 mmol) of triethylamine, and 0.011 g (0.04 mmol) of tetrabutylammonium chloride were added, followed by the addition of 1 mL of tetrahydrofuran. The mixture was stirred for 4 hours. The solvent was removed by vacuum distillation, and the residue was washed with pure water and n-pentane to remove impurities, yielding the [Ag₁₄ (C≡CC₄H₃NH)₁₂Cl]⁺ nanocluster.

Then, following the procedure of Examples 2 and 8, electrochemical treatment was carried out for 1 hour, 45 minutes, 30 minutes, and 15 minutes to obtain the [Ag₁₄ (C≡CC₄H₃NH)₈Cl]⁺, [Ag₁₄ (C≡CC₄H₃NH)₉Cl]⁺, [Ag₁₄ (C≡CC₄H₃NH)₁₀Cl]⁺, and [Ag₁₄ (C≡CC₄H₃NH)₁₁Cl]⁺ nanoclusters, respectively.

Preparation Example 11: Synthesis of [Ag₁₄ (C≡CC₆H₁₁)₈Cl]⁺ [Ag₁₄ (C≡CC₆H₁₁)₁₁Cl]⁺ [Ag₁₄ (C≡CC₆H₁₁)₁₀Cl]⁺, and [Ag₁₄ (C≡CC₆H₁₁)₁₁Cl]⁺ Nanoclusters

In a flask, 0.097 g (0.5 mmol) of AgBF₄, 0.054 g (0.5 mmol) of ethynylcyclohexane (with the structure shown below), 0.070 mL (0.5 mmol) of triethylamine, and 0.011 g (0.04 mmol) of tetrabutylammonium chloride were added, followed by the addition of 1 mL of tetrahydrofuran. The mixture was stirred for 4 hours. The solvent was removed by vacuum distillation, and the residue was washed with pure water and n-pentane to remove impurities, yielding the [Ag₁₄ (C≡CC₆H₁₁)₁₂Cl]⁺ nanocluster.

Then, following the procedure of Examples 2 and 8, electrochemical treatment was carried out for 1 hour, 45 minutes, 30 minutes, and 15 minutes to obtain the [Ag₁₄ (C≡CC₆H₁₁)₈Cl]⁺, [Ag₁₄ (C≡CC₆H₁₁)₉Cl]⁺, [Ag₁₄ (C≡CC₆H₁₁)₁₀Cl]⁺, and [Ag₁₄ (C≡CC₆H₁₁)₁₁Cl]⁺ nanoclusters, respectively.

Preparation Example 12: Synthesis of [Ag₁₄ (C≡CC₆H₅CF₃)₈Cl]⁺ [Ag₁₄ (C≡CC₆H₅CF₃)₉Cl]⁺ [Ag₁₄ (C≡CC₆H₅CF₃)₁₀Cl]⁺, and [Ag₁₄ (C≡CC₆H₅CF₃)₁₁Cl]⁺ Nanoclusters

In a flask, 0.097 g (0.5 mmol) of AgBF₄, 0.085 g (0.5 mmol) of 4-(trifluoromethyl)phenylacetylene (with the structure shown below), 0.070 mL (0.5 mmol) of triethylamine, and 0.011 g (0.04 mmol) of tetrabutylammonium chloride were added, followed by the addition of 1 mL of tetrahydrofuran. The mixture was stirred for 4 hours. The solvent was removed by vacuum distillation, and the residue was washed with pure water and n-pentane to remove impurities, yielding the [Ag₁₄ (C≡CC₆H₅CF₃)₁₂Cl]⁺ nanocluster.

Then, following the procedure of Examples 2 and 8, electrochemical treatment was carried out for 1 hour, 45 minutes, 30 minutes, and 15 minutes to obtain the [Ag₁₄ (C≡CC₆H₅CF₃)₈Cl]⁺, [Ag₁₄ (C≡CC₆H₅CF₃)₉Cl]⁺, [Ag₁₄ (C≡CC₆H₅CF₃)₁₀Cl]⁺, and [Ag₁₄ (C≡CC₆H₅CF₃)₁₁Cl]⁺ nanoclusters, respectively.

Preparation Example 13: Synthesis of [Ag₁₄ (C≡C(CH₂)₂C₆H₅)₈Cl]⁺ [Ag₁₄ (C≡C(CH₂)₂C₆H₅)₁₁Cl]⁺ [Ag₁₄ (C≡C(CH₂)₂C₆H₅)₁₀Cl]⁺, and [Ag₁₄ (C≡C(CH₂)₂C₆H₅)₁₁Cl]⁺ Nanoclusters

In a flask, 0.097 g (0.5 mmol) of AgBF₄, 0.065 g (0.5 mmol) of 3-butyn-1-benzene (with the structure shown below), 0.070 mL (0.5 mmol) of triethylamine, and 0.011 g (0.04 mmol) of tetrabutylammonium chloride were added, followed by the addition of 1 mL of tetrahydrofuran. The mixture was stirred for 4 hours. The solvent was removed by vacuum distillation, and the residue was washed with pure water and n-pentane to remove impurities, yielding the [Ag₁₄ (C≡C(CH₂)₂C₆H₅)₁₂Cl]⁺ nanocluster.

Then, following the procedure of Examples 2 and 8, electrochemical treatment was carried out for 1 hour, 45 minutes, 30 minutes, and 15 minutes to obtain the [Ag₁₄ (C≡C(CH₂)₂C₆H₅)₈Cl]⁺, [Ag₁₄ (C≡C(CH₂)₂C₆H₅)₉Cl]⁺, [Ag₁₄ (C≡C(CH₂)₂C₆H₅)₁₀Cl]⁺, and [Ag₁₄ (C≡C(CH₂)₂C₆H₅)₁₁Cl]⁺ nanoclusters, respectively.

Preparation Example 14: Synthesis of [Ag₁₄ (C≡C(CH₂)₆ (OH) CtBu)₈Cl]⁺, [Ag₁₄ (C≡C(CH₂)₆ (OH) CtBu)₉Cl]⁺, [Ag₁₄ (C≡C(CH₂)₆ (OH) CtBu)₁₀Cl]⁺ and [Ag₁₄ (C≡C(CH₂)₆ (OH) CtBu)₁₁Cl]⁺ Nanoclusters

In a flask, 0.097 g (0.5 mmol) of AgBF₄, 0.091 g (0.5 mmol) of 2,2-dimethyl-9-decin-3-ol (with the structure shown below), 0.070 mL (0.5 mmol) of triethylamine, and 0.011 g (0.04 mmol) of tetrabutylammonium chloride were added, followed by the addition of 1 mL of tetrahydrofuran. The mixture was stirred for 4 hours. The solvent was removed by vacuum distillation, and the residue was washed with pure water and n-pentane to remove impurities, yielding the [Ag₁₄ (C≡C(CH₂)₆ (OH) CtBu)₁₂Cl]⁺ nanocluster.

Then, following the procedure of Examples 2 and 8, electrochemical treatment was carried out for 1 hour, 45 minutes, 30 minutes, and 15 minutes to obtain the [Ag₁₄ (C≡C(CH₂)₆ (OH) CtBu)₈Cl]⁺, [Ag₁₄ (C≡C(CH₂)₆ (OH) CtBu)₉Cl]⁺, [Ag₁₄ (C≡C(CH₂)₆ (OH) CtBu)₁₀Cl]⁺, and [Ag₁₄ (C≡C(CH₂)₆ (OH) CtBu)₁₁Cl]⁺ nanoclusters, respectively.

Example 1

A nanoclusters composite dispersion was prepared by mixing the solution, where 170 μg of the nanoclusters obtained from Preparation Example 2 was dissolved in 160 μL of dichloromethane, with 160 μL of acetone, followed by ultrasonic dispersion for about 1 minute. Next, the prepared nanoclusters composite dispersion was deposited onto a gas diffusion electrode (GDE (W1S1011, Ce-Tech)) with a 2.5×2.5 cm² area to produce a nanoclusters composite gas diffusion electrode. The average loading amount of the nanoclusters were measured to be 10.6 nmol/cm².

The prepared gas diffusion electrode was used as the cathode, and a 3×3 cm² area Ni foam (NiF, 29-04275-01, Invisible Inc.) was used as the anode. An AEM (Sustainion X37-50, RT grade, Dioxide Materials) membrane, pretreated with 1.0 M KOH, was placed between the cathode and the anode. A zero-gap reactor was then fabricated by compressing the assembly using stainless steel.

Example 2

This was carried out in the same manner as in Example 1, except that Preparation Example 5 was used instead of Preparation Example 2.

Example 3

This was carried out in the same manner as in Example 1, except that Preparation Example 6 was used instead of Preparation Example 2.

Example 4

This was carried out in the same manner as in Example 1, except that Preparation Example 7 was used instead of Preparation Example 2.

Comparative Example 1

This was carried out in the same manner as in Example 1, except that Preparation Example 1 was used instead of Preparation Example 2.

Experimental Example 1

The conversion of carbon dioxide was carried out using the zero-gap reactors from Examples 1 to 4 and Comparative Example 1.

While supplying CO₂ gas containing water vapor to the cathode at 30 sccm, a 1.0 M KOH solution was circulated at a flow rate of 3 mL/min. A cold trap was installed at the outlet of the cathode to remove moisture generated from the water vapor, and finally, the final products were analyzed using gas chromatography. Example 2 had an identical structure and produced identical results as Example 1.

FIGS. 7 and 8 show the results of carbon dioxide conversion activity for Example 1 and Comparative Example 1, demonstrating that the nanoclusters in the Example exhibited significantly improved carbon dioxide conversion rates.

Additionally, FIGS. 9 and 10 show the electrochemical activity results for Example 1 and Comparative Example 1, indicating that the nanoclusters in the Example had higher electrochemical activity compared to the Comparative Example.

Furthermore, FIG. 11 shows the electrochemical activity results for Examples 2 to 4, confirming that the catalyst with excellent carbon dioxide conversion performance in aqueous solutions was successfully developed.

Comparative Preparation Example 1: Synthesis of Gold Nanoclusters

A reaction solution was obtained by adding and stirring 0.32 mL of 6-hexanethiol (HSC₆H₁₃) dropwise into a solution of 197 mg of chloroauric acid (III) and 317 mg of tetraoctylammonium bromide dissolved in 15 mL of tetrahydrofuran. Sodium borohydride, 190 mg, was then added dropwise to the reaction solution while stirring, where the sodium borohydride was dissolved in 5 mL of ultrapure water. The resulting precipitate was collected through washing and centrifugation using ultrapure water and methanol. The precipitate was then dissolved in acetonitrile to isolate and dry the Au₂₅-sized product, yielding the target compound Au₂₅ (SC₆H₁₃)₁₈.

Comparative Example 2

A zero-gap reactor was fabricated in the same manner as in Example 1, except that 170 μg of the gold nanoclusters obtained from Comparative Preparation Example 1 was used instead of 170 μg of silver nanocluster.

Comparative Example 3

A zero-gap reactor was fabricated in the same manner as in Example 1, except that a commercial silver nanoparticle-based electrode (Dioxide Materials™) was used instead of the gas diffusion electrode obtained in Example 1.

Experimental Example 2: Comparative Analysis of Nanoclusters

The mass spectrometry results of the single-crystal structures of the nanoclusters synthesized according to Preparation Example 1 of the present invention and the nanoclusters synthesized according to Comparative Preparation Example 1 are shown in FIGS. 14 and 15, respectively.

The silver nanoclusters synthesized according to Preparation Example 1 has the single-crystal structure shown in FIG. 14, and the ESI-MS analysis confirmed that a single signal was observed. Isotope analysis further confirmed that the [ClAg₁₄ (C₆H₉)₁₂]⁺ catalyst was obtained with high purity.

The gold nanoclusters synthesized according to Comparative Preparation Example 1 has the single-crystal structure shown in FIG. 15, and the ESI-MS analysis confirmed that a single signal was observed. Isotope analysis further confirmed that the [Au₂₅ (SC₆H₁₃)₁₈]⁻ catalyst was obtained with high purity.

Experimental Example 3: Analysis of Syngas Composition Control

Syngas was produced under operational conditions of 400 mA/cm² using the zero-gap reactors from Example 1 and Comparative Examples 2 and 3. A gas mixture of CO₂ and H₂O was supplied to the cathode at 30 sccm, while a 1.0 M KOH solution was circulated at a flow rate of 3 mL/min. A cold trap was installed at the outlet of the cathode to remove moisture generated from the water vapor, and the final products were analyzed using gas chromatography. The results are shown in FIGS. 16 to 18. FIG. 16 graphically shows the syngas production results from the zero-gap reactor containing the [ClAg₁₄ (C₆H₉)₈]⁺ catalyst of Example 1.

FIG. 16(a) shows the composition of the target syngas and the selectivity (%) of each gas produced in the reactor, indicating that the composition of the produced syngas matches or nearly matches the target composition.

FIG. 16(b) shows the composition of the target syngas and the proportion (%) of residual CO₂ after the reaction, confirming that the syngas produced through Example 1 contains almost no residual CO₂, with less than 10% remaining, resulting in a high-purity product.

FIG. 16(c) shows the composition of the target syngas and the carbon conversion efficiency (%), demonstrating that the carbon conversion efficiency reaches or nearly reaches the theoretical maximum of 50% in all compositions.

FIG. 17 graphically shows the syngas production results from the zero-gap reactor containing the [Au₂₅ (SC₆H₁₃)₁₈]⁻ catalyst of Comparative Example 2.

FIG. 17(a) shows the composition of the target syngas and the selectivity (%) of each gas produced in the reactor, indicating that there is a slight difference between the target composition and the produced syngas composition compared to Example 1.

FIG. 17(b) shows the composition of the target syngas and the proportion (%) of residual CO₂ after the reaction, indicating that some unreacted CO₂ remains in the produced syngas.

FIG. 17(c) shows the composition of the target syngas and the carbon conversion efficiency (%), showing that the carbon conversion efficiency does not reach the theoretical maximum of 50% and is somewhat lower compared to Example 1.

FIG. 18 graphically shows the syngas production results from the zero-gap reactor containing the commercial silver nanoparticle-based electrode of Comparative Example 3.

FIG. 18(a) shows the composition of the target syngas and the selectivity (%) of each gas produced in the reactor, revealing a significant difference between the target and produced syngas compositions, confirming that it is not possible to produce syngas with the desired composition.

FIG. 18(b) shows the composition of the target syngas and the proportion (%) of residual CO₂ after the reaction, indicating that an excessive amount of residual CO₂ remains in the produced syngas.

FIG. 18(c) shows the composition of the target syngas and the carbon conversion efficiency (%), demonstrating that the carbon conversion efficiency falls significantly short of the theoretical maximum of 50% and is much lower compared to both Example 1 and Comparative Example 2. Therefore, it can be seen that when using a reactor containing the silver nanocluster catalyst according to the present invention, it is possible to obtain syngas controlled to the desired composition with high purity.

Experimental Example 4: Gas Selectivity Analysis

The results of analyzing the gas selectivity (%) during the production of syngas using the zero-gap reactors according to Example 1 of the present invention and Comparative Examples 2 and 3 are shown in FIGS. 19 and 20.

FIG. 19 is a graph showing the gas selectivity at different current densities when the target composition of the syngas is CO:H₂=1:3 (CO=25%).

As shown in FIG. 19, Example 1 consistently maintains the gas selectivity of CO at 25%, the target syngas composition, even as the current density increases. This confirms that it is an excellent catalyst capable of producing syngas with the desired composition under high current density conditions.

In contrast, Comparative Example 2 shows a decrease in gas selectivity as the current density increases, indicating a limitation in maintaining the desired syngas composition. Comparative Example 3 shows low gas selectivity even at low current densities, making it impossible to control the syngas composition, and the selectivity further decreases as the current density increases.

FIG. 20 is a graph showing the reaction selectivity of CO according to the partial pressure of CO₂ (atm) under operating conditions of 200 mA/cm².

To achieve the target syngas composition, all injected CO₂ must be fully converted with high reactivity. Example 1 shows high CO selectivity even at low CO₂ partial pressures, whereas the gold nanocluster catalyst of Comparative Example 2 and the commercial silver nanoparticle catalyst of Comparative Example 3 show a decrease in CO selectivity as the CO₂ partial pressure decreases.

Therefore, it can be concluded that the silver nanocluster catalyst of Example 1 is an excellent catalyst most suitable for composition control and the production of high-purity syngas, demonstrating superior reactivity at high current densities and low CO₂ partial pressures.

Experimental Example 5: Syngas Production According to Current Density

FIGS. 21A to 21D show the results of syngas production with varying current densities using a zero-gap reactor according to an embodiment of the present invention, where the syngas produced is a mixture of carbon monoxide and hydrogen. The target syngas composition is CO:H₂=1:3 (CO=25%).

FIG. 21A shows the amount of syngas produced as the current density increases, confirming that the production of syngas increases with increasing current density, and stable syngas production is possible even at high current densities.

FIG. 21B shows the selectivity of each gas component according to current density, confirming that gas selectivity remains constant at 25% CO even as the current density increases.

FIG. 21C shows the proportion (%) of residual CO₂ according to current density, confirming that residual CO₂ is almost nonexistent, with less than 2%, indicating that the produced syngas is a high-purity product with almost no residual CO₂.

FIG. 21D shows the carbon conversion efficiency (%) according to current density, confirming that it achieves or closely approaches the theoretical maximum of 50% across all current densities.

Experimental Example 6: Stability Evaluation

FIG. 22 shows the stability of syngas production using a catalyst when producing syngas composed of carbon monoxide and hydrogen, using a zero-gap reactor according to an embodiment of the present invention. FIG. 22 graphically represents the selectivity of each gas component over a 24-hour reaction period under high current density conditions of 400 mA/cm², with the target composition of CO:H₂=1:3 (CO=25%).

As shown in FIG. 22, the selectivity of CO and H₂ remains consistent during the 24-hour operation of the reactor.

Therefore, it is evident that the silver nanocluster catalyst of the present invention can reliably produce syngas with the desired composition under high-current operating conditions for extended periods.

Furthermore, the manufacturing examples and embodiments that presented the above experimental results, as well as other manufacturing examples and embodiments not explicitly detailed in the experimental results, were observed to yield nearly comparable effects to those of the aforementioned Preparation Examples and Embodiments that presented experimental results.

Thus, the silver nanoclusters of the present invention is not only economically superior to gold (Au) catalysts but also exhibits better carbon dioxide conversion activity than conventional silver (Ag) catalysts. Therefore, gas diffusion electrodes or zero-gap reactors using this nanoclusters are highly useful as high-performance electrochemical carbon dioxide conversion systems for industrial applications.

Additionally, the silver nanoclusters of the present invention offers a simple and environmentally friendly manufacturing process, enabling mass production with high yield, making it highly valuable for industrial use.

While the present invention has been described in detail with specific elements, limited embodiments, and comparative examples, these are provided to enhance the overall understanding of the invention. The invention is not limited to these embodiments. Those skilled in the art may devise various modifications and variations from these descriptions.

Thus, the spirit of the present invention should not be restricted to the disclosed embodiments but should encompass all equivalents or modifications within the scope of the following claims and their equivalents.