GOLD NANOCLUSTER CATALYST FOR CARBON DIOXIDE CONVERSION AND PREPARING THE SAME

Abstract

The present disclosure relates to a gold nanocluster catalyst for carbon dioxide conversion represented by [Chemical Formula 1] and a method for preparing the same:

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application Nos. 10-2023-0043024 and 10-2023-0192246 filed on Mar. 31, 2023 and Dec. 27, 2023, 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 disclosure relates to a gold nanocluster catalyst for carbon dioxide conversion and a method for preparing the same, more particularly to preparation of a ligand-controlled gold nanocluster catalyst for carbon dioxide conversion and application of the same to a gas diffusion electrode.

2. Description of the Related Art

In the superatomic orbital theory, a nanocluster is regarded as one gigantic atom defined by a specific number of metal atoms and ligands and valence electrons.

The nanocluster is more stable than single atoms or nanoparticles and has completely different optical and electrochemical properties from nanoparticles because of stronger molecular properties than metallic properties. In particular, the optical, electrical and catalytic properties of the nanocluster can vary sensitively depending on the number of the metal atoms, the type of the metal atoms and ligands, etc.

Meanwhile, although researches are being conducted for carbon dioxide reduction including storage and capture of carbon dioxide to cope with climate change, they are disadvantageous in that carbon dioxide is not consumed actually. Therefore, researches are being conducted on chemical conversion of carbon dioxide recently. Among them, electrochemical carbon dioxide conversion that can be linked to renewable energy is receiving attentions.

In particular, researches on conversion of carbon dioxide to carbon monoxide are drawing attentions because carbon monoxide is used as a precursor to synthesis gas and a key reactant for polymerization of various polymers and the market price and market size are established well. In the electrochemical conversion of carbon dioxide, electrolytes containing alkali metal cations such as potassium hydroxide (KOH), potassium chloride (KCl), etc. were used. Although a high-concentration electrolyte can be used to increase the production efficiency of carbon monoxide, there is limitation in using the high-concentration electrolyte in terms of the solubility of the electrolyte and economic feasibility.

Accordingly, there has been an attempt to increase concentration through electrostatic attraction of alkali metal cations near the catalyst by coating an ionomer containing anionic functional groups on the surface of an electrode. However, since process optimization is necessary for appropriate combinations of existing catalysts and ionomers and the introduction of an ionomer with low electrical conductivity and material permeability decreases the activity of the electrochemical catalyst, it is necessary to develop a catalyst or electrode that allows enrichment of cations on the surface without lowering the activity of the catalyst itself.

Therefore, based on the fact that the carbon monoxide production efficiency of the nanocluster catalyst is affected by the concentration of cations confirmed in the previous researches, the inventors of the present disclosure have found out that a gold nanocluster protected with a ligand having an anionic terminal can enrich alkali metal cations on the surface of a catalyst and exhibits high carbon dioxide conversion activity, and have completed the present disclosure.

REFERENCES OF THE RELATED ART

Patent Documents

• Patent document 1. Korean Patent Publication No. 10-2020-0125169.

Non-Patent Documents

• Non-patent document 1. J. Mater. Chem. A 2020, 8, 19493-19501.

SUMMARY

The present disclosure is directed to providing a gold nanocluster catalyst with superior carbon dioxide conversion activity and a method for preparing the same.

The present disclosure is also directed to providing a gas diffusion electrode including the gold nanocluster catalyst with superior carbon dioxide conversion activity.

The present disclosure provides a gold nanocluster catalyst for carbon dioxide conversion represented by [Chemical Formula 1]:

Au_n(SR)_m  [Chemical Formula 1]

• • wherein SR is an organic thiol-based ligand comprising an anionic terminal group, n is an integer between 25 and 102 and m is an integer between 18 and 44.

In Chemical Formula 1, R may be any one selected from a group consisting of a C₃-C₃₀ aryl group, a C₃-C₄₀ arylalkyl group, a C₃-C₃₀ heteroaryl group and a C₃-C₄₀ heteroarylalkyl group.

In Chemical Formula 1, the anionic terminal group may be COOH or SO₃H.

The Au_n(SR)_m of Chemical Formula 1 may be Au₂₅(SC₅H₁₀COOH)₁₈, Au₆₇(SC₅H₁₀COOH)₃₅, Au₁₀₂(SC₅H₁₀COOH)₄₄ or Au₂₅(SC₃H₆SO₃H)₁₈.

The gold nanocluster catalyst for carbon dioxide conversion may have a size of 1.1-1.6 nm.

The present disclosure also provides a gas diffusion electrode including the gold nanocluster catalyst for carbon dioxide conversion represented by Chemical Formula 1.

The present disclosure also provides a method for preparing a gold nanocluster catalyst for carbon dioxide conversion represented by [Chemical Formula 1], which includes: (I) a step of preparing a precursor solution containing a gold precursor; (II) a step of preparing a solution of an organic thiol-based ligand having an anionic functional group; (III) a step of obtaining a reaction solution by mixing and stirring the precursor solution and the ligand solution and optionally adding a reaction catalyst; and (IV) a step of obtaining a nanocluster by adding a reducing agent to the reaction solution and stirring, filtering and drying the same:

Au_n(SR)_m  [Chemical Formula 1]

• • wherein SR is an organic thiol-based ligand containing an anionic terminal group, n is an integer between 25 and 102 and m is an integer between 18 and 44.

The gold precursor may be HAuCl₄·3H₂O.

The organic thiol-based ligand having an anionic functional group may be SC₅H₁₀COOH or SC₃H₆SO₃H.

The reaction catalyst may be NaOH or KOH.

The reducing agent may be any one selected from a group consisting of NaBH₄, LiBH₄, KBH₄ and LiAlH₄.

According to the present disclosure, a gold nanocluster catalyst with superior carbon dioxide conversion activity and a method for preparing the same may be provided.

In addition, a gas diffusion electrode including the gold nanocluster catalyst with superior carbon dioxide conversion activity may be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an electrospray ionization mass spectrometry (ESI-MS) result of an Au₂₅(SC₅H₁₀COOH)₁₈ nanocluster prepared in Example 1.

FIG. 2 shows an electrospray ionization mass spectrometry (ESI-MS) result of an Au₂₅(SC₃H₆SO₃H)₁₈ nanocluster prepared in Example 2.

FIG. 3 shows ultraviolet-visible (UV-Vis) absorption spectroscopy results of an Au₂₅(SC₅H₁₀COOH)₁₈ nanocluster prepared in Example 1, an Au₂₅(SC₃H₆SO₃H)₁₈ nanocluster prepared in Example 2 and an Au₂₅(SC₆H₁₃)₁₈ nanocluster prepared in Comparative Example 1.

FIG. 4 shows is an image showing the framework of a gold nanocluster according to the present disclosure excluding carbons in the ligand.

FIG. 5 shows an ultraviolet-visible (UV-Vis) absorption spectroscopy result of an Au₆₇(SC₅H₁₀COOH)₃₅ nanocluster prepared in Example 3, its transmission electron microscopic (TEM) image and a result of analyzing the size of the nanocluster.

FIG. 6 shows an ultraviolet-visible (UV-Vis) absorption spectroscopy result of an Au₁₀₂(SC₅H₁₀COOH)₄₄ nanocluster prepared in Example 4, its transmission electron microscopic (TEM) image and a result of analyzing the size of the nanocluster.

FIG. 7 shows the activity of an Au₂₅(SC₅H₁₀COOH)₁₈ nanocluster prepared in Example 1 and an Au₂₅(SC₆H₁₃)₁₈ nanocluster prepared in Comparative Example 1 for electrochemical carbon dioxide conversion reaction.

FIG. 8 shows the cross-sectional images and a result of relative elemental analysis for an Au₂₅(SC₅H₁₀COOH)₁₈ nanocluster prepared in Example 1 and an Au₂₅(SC₆H₁₃)₁₈ nanocluster prepared in Comparative Example 1 after carbon dioxide conversion reaction.

FIG. 9 shows the activity of an Au₂₅(SC₅H₁₀COOH)₁₈ nanocluster prepared in Example 1 and an Au₂₅(SC₆H₁₃)₁₈ nanocluster prepared in Comparative Example 1 depending on pH.

FIG. 10 shows the activity of an Au₂₅(SC₃H₆SO₃H)₁₈ nanocluster prepared in Example 2 and an Au₂₅(SC₆H₁₃)₁₈ nanocluster prepared in Comparative Example 1 for electrochemical carbon dioxide conversion reaction (FIG. 10-1) and the activity depending on pH (FIG. 10-2).

FIG. 11 shows the selectivity of an Au₂₅(SC₃H₆SO₃H)₁₈ nanocluster prepared in Example 2 for carbon monoxide production under the condition of pH 1 for 24 hours.

FIG. 12 shows the carbon dioxide conversion activity of anionic ligand-protected Au₆₇(SC₅H₁₀COOH)₃₅ and Au₁₀₂(SCH₁₀COOH)₄₄ nanoclusters prepared in Examples 3 and 4 and an Au₂₅(SC₆H₁₃)₁₈ nanocluster having a neutral ligand prepared in Comparative Example 1.

DETAILED DESCRIPTION

Hereinafter, a novel gold nanocluster catalyst for carbon dioxide conversion according to the present disclosure, a method for preparing the same and a gas diffusion electrode including the gold nanocluster catalyst for carbon dioxide conversion will be described in detail referring to the attached drawings.

The present disclosure discloses a gold nanocluster catalyst protected by a ligand having an anionic terminal the structure and composition of which are identified clearly at the atomic level. Because the gold nanocluster catalyst according to the present disclosure maintains the catalyst structure during carbon dioxide conversion reaction and has an anionic functional group at the terminal of the ligand, it can be used as an excellent electrochemical catalyst for carbon dioxide conversion with enriched alkali metal cations on the catalyst surface. In addition, since various anionic functional groups can be utilized, it can be universally applied to nanocluster materials, including the gas diffusion electrode.

First, the present disclosure provides a gold nanocluster catalyst for carbon dioxide conversion represented by [Chemical Formula 1]:

Au_n(SR)_m  [Chemical Formula 1]

• • wherein SR is an organic thiol-based ligand containing an anionic terminal group, n is an integer between 25 and 102 and m is an integer between 18 and 44.

The nanocluster catalyst for carbon dioxide conversion represented by Chemical Formula 1 wherein 25-102 gold atoms and the organic thiol-based ligand having an anionic terminal group are bound with a specific structure exhibits superior carbon dioxide conversion rate, i.e., the ability of converting carbon dioxide to carbon monoxide, as compared to the conventional nanocluster catalyst protected by a neutral ligand.

In Chemical Formula 1, R may be any one selected from a group consisting of a C₃-C₃₀ aryl group, a C₃-C₄₀ arylalkyl group, a C₃-C₃₀ heteroaryl group and a C₃-C₄₀ heteroarylalkyl group.

In Chemical Formula 1, the anionic terminal group may be COOH or SO₃H.

Specifically, the Au_n(SR)_m of Chemical Formula 1 may be Au₂₅(SC₅H₁₀COOH)₁₈, Au₆₇(SC₅H₁₀COOH)₃₅, Au₁₀₂(SC₅H₁₀COOH)₄₄ or Au₂₅(SC₃H₆SO₃H)₁₈.

Specifically, the gold nanocluster catalyst for carbon dioxide conversion represented by Chemical Formula 1 according to the present disclosure may have a size of 1.1-1.6 nm. If the size of the catalyst is smaller than 1.1 nm, it may be difficult to prepare a homogeneous nanocluster. And, if the size exceeds 1.6 nm, carbon dioxide conversion efficiency may decrease due to decreased surface area.

The present disclosure also provides a gas diffusion electrode including the gold nanocluster catalyst for carbon dioxide conversion represented by Chemical Formula 1. Other components of the gas diffusion electrode except for the use of the gold nanocluster catalyst for carbon dioxide conversion represented by Chemical Formula 1 may be the same as those commonly used in a gas diffusion electrode by those skilled in the art.

The present disclosure also provides a method for preparing a gold nanocluster catalyst for carbon dioxide conversion represented by [Chemical Formula 1], which includes: (I) a step of preparing a precursor solution containing a gold precursor; (II) a step of preparing a solution of an organic thiol-based ligand having an anionic functional group; (III) a step of obtaining a reaction solution by mixing and stirring the precursor solution and the ligand solution and optionally adding a reaction catalyst; and (IV) a step of obtaining a nanocluster by adding a reducing agent to the reaction solution and stirring, filtering and drying the same:

Au_n(SR)_m  [Chemical Formula 1]

• • wherein SR is an organic thiol-based ligand containing an anionic terminal group, n is an integer between 25 and 102 and m is an integer between 18 and 44.

In Chemical Formula 1, R may be any one selected from a group consisting of a C₃-C₃₀ aryl group, a C₃-C₄₀ arylalkyl group, a C₃-C₃₀ heteroaryl group and a C₃-C₄₀ heteroarylalkyl group.

The gold precursor is not specially limited as long as it is one commonly used in the art. Specifically, one selected from HAuCl₄·3H₂O, HAuCl₄·xH₂O, Au(OH)₃, etc. may be used. More specifically, HAuCl₄·3H₂O may be used because the synthesis efficiency can be improved.

The organic thiol-based ligand having an anionic functional group may be SC₅H₁₀COOH or SC₃H₆SO₃H.

In addition, in the step (III), a reaction catalyst may be added optionally, if necessary, when obtaining the reaction solution by mixing and stirring the precursor solution and the ligand solution, to improve reaction rate. The reaction catalyst may be NaOH or KOH.

In addition, the reducing agent in the step (IV) may be any one selected from a group consisting of NaBH₄, LiBH₄, KBH₄ and LiAlH₄.

Hereinafter, the present disclosure will be described in detail with specific examples and a comparative example.

(Example 1) Preparation of Au₂₅(SC₅H₁₀COOH)₁₈

A precursor solution containing a gold precursor was prepared by dissolving 59 mg of chloroauric(III) acid trihydrate (HAuCl₄·3H₂O) in 0.75 mL of ultrapure water. A solution of an organic thiol-based ligand having an anionic functional group was prepared by dissolving 45 μL of 6-mercaptohexanoic acid (SC₅H₁₀COOH) in 13 mL of ultrapure water. After mixing the precursor solution and the ligand solution at once and stirring the mixture uniformly using an ultrasonicator, a reaction solution was obtained by adding 1.3 mL of 1 M sodium hydroxide (NaOH) as a reaction catalyst and stirring the mixture. After adding a reducing agent solution in which 43 mg of sodium borohydride (NaBH₄) was dissolved in 1 mL of a 0.2 M sodium hydroxide solution to the reaction solution, the mixture was stirred, filtered and then dried. Then, impurities were removed from the dried product by precipitating with a mixture of ultrapure water and acetonitrile (CH₃CN) at a volume ratio of 3:4 or 3:5 (mL/mL) and conducting centrifugation. Finally, the gold nanocluster catalyst Au₂₅(SC₅H₁₀COOH)₁₈ was prepared by precipitating the pure product in a 3:6 mixture solution of ultrapure water and acetonitrile.

(Example 2) Preparation of Au₂₅(SC₃H₆SO₃H)₁₈

A precursor solution containing a gold precursor was prepared by dissolving 78.8 mg of chloroauric(III) acid trihydrate (HAuCl₄·3H₂O) in 4.5 mL of methanol. A solution of an organic thiol-based ligand having an anionic functional group was prepared by dissolving 155.3 mg of 3-mercapto-1-propanesulfonic acid (SC₃H₆SO₃H) in 13.5 mL of ultrapure water. After mixing the ligand solution with the precursor solution by adding dropwise, a reaction solution was obtained by stirring the same. After adding a reducing agent solution in which 22.4 mg of sodium borohydride (NaBH₄) was dissolved in 1.5 mL of ultrapure water at once, the mixture was stirred. Sequentially, after adding a reducing agent solution in which 15 mg of sodium borohydride (NaBH₄) was dissolved in 0.3 mL of ultrapure water at once, the mixture was stirred, filtered and then dried. Then, impurities were removed from the dried product by precipitating with a mixture of ultrapure water and methanol at a volume ratio of 4:4, 4:8 or 4:12 (mL/mL) and conducting centrifugation. Finally, the gold nanocluster catalyst Au₂₅(SC₃H₆SO₃H)₁₈ was prepared by precipitating the pure product in a 3:16 mixture solution of ultrapure water and methanol.

(Example 3) Preparation of Au₂₅(SC₅H₁₀COOH)₃₅

A precursor solution containing a gold precursor was prepared by dissolving 59 mg of chloroauric(III) acid trihydrate (HAuCl₄·3H₂O) in 0.75 ml of ultrapure water. A solution of an organic thiol-based ligand having an anionic functional group was prepared by dissolving 45 μL of 6-mercaptohexanoic acid (SC₅H₁₀COOH) in 13 mL of ultrapure water. After mixing the precursor solution and the ligand solution at once and stirring the mixture uniformly using an ultrasonicator, a reaction solution was obtained by adding 1.3 mL of 1 M sodium hydroxide (NaOH) as a reaction catalyst and stirring the mixture. After adding a reducing agent solution in which 43 mg of sodium borohydride (NaBH₄) was dissolved in 1 mL of a 0.2 M sodium hydroxide solution to the reaction solution, the mixture was stirred, filtered and then dried. Then, impurities were removed from the dried product by precipitating with a mixture of ultrapure water and acetonitrile (CH₃CN) at a volume ratio of 3:4 (mL/mL) and conducting centrifugation. Finally, the gold nanocluster catalyst Au₆₇(SC₅H₁₀COOH)₃₅ was prepared by precipitating the pure product in a 3:5 mixture solution of ultrapure water and acetonitrile.

(Example 4) Preparation of Au₁₀₂(SC₅H₁₀COOH)₄₄

A precursor solution containing a gold precursor was prepared by dissolving 59 mg of chloroauric(III) acid trihydrate (HAuCl₄·3H₂O) in 0.75 mL of ultrapure water. A solution of an organic thiol-based ligand having an anionic functional group was prepared by dissolving 45 μL of 6-mercaptohexanoic acid (SC₅H₁₀COOH) in 13 mL of ultrapure water. After mixing the precursor solution and the ligand solution at once and stirring the mixture uniformly using an ultrasonicator, a reaction solution was obtained by adding 1.3 mL of 1 M sodium hydroxide (NaOH) as a reaction catalyst and stirring the mixture. After adding a reducing agent solution in which 43 mg of sodium borohydride (NaBH₄) was dissolved in 1 mL of a 0.2 M sodium hydroxide solution to the reaction solution, the mixture was stirred, filtered and then dried. Then, the gold nanocluster catalyst Au₁₀₂(SC₅H₁₀COOH)₄₄ was prepared by precipitating the dried pure product with a mixture solution of ultrapure water and acetonitrile (CH₃CN) solution at a volume ratio 3:3 (mL/mL).

(Comparative Example 1) Preparation of Au₂₅(SC₆H₁₃)₁₈

A reaction solution was obtained by adding 0.32 mL of 2-phenylethanethiol (HSEtPh) dropwise to a solution in which 197 mg of chloroauric(III) acid and 317 mg of tetraoctylammonium bromide were dissolved in 15 mL of tetrahydrofuran and stirring the mixture. The reaction solution was stirred while adding a solution in which 190 mg of sodium borohydride was dissolved in 5 mL of ultrapure water dropwise. Then, a precipitate was obtained by washing with ultrapure water and methanol conducting centrifugation. The target product Au₂₅(SC₆H₁₃)₁₈ was prepared by dissolving the precipitate in acetonitrile, obtaining only the product with a size of Au₂₅ and drying the same.

[Confirmation of Preparation of Au₂₅(SC₅H₁₀COOH)₁₈ Catalyst and Au₂₅(SC₃H₆SO₃H)₁₈ Catalyst]

FIGS. 1 and 2 show the electrospray ionization mass spectrometry (ESI-MS) result for the Au₂₅(SC₅H₁₀COOH)₁₈ nanocluster prepared in Example 1 and the Au₂₅(SC₃H₆SO₃H)₁₈ nanocluster prepared in Example 2.

As seen from FIG. 1, the nanocluster prepared in Example 1 showed two large peak signals, indicating that Au₂₅(SC₅H₁₀COOH)₁₈ was prepared as the pure target product with charges of 2⁻ and 3⁻, respectively.

In addition, it was also confirmed from FIG. 2 that the Au₂₅(SC₃H₆SO₃H)₁₈ nanocluster was prepared as a pure product.

FIG. 3 shows ultraviolet-visible (UV-Vis) absorption spectroscopy results of the Au₂₅(SC₅H₁₀COOH)₁₈ nanocluster prepared in Example 1, the Au₂₅(SC₃H₆SO₃H)₁₈ nanocluster prepared in Example 2 and the Au₂₅(SC₆H₁₃)₁₈ nanocluster prepared in Comparative Example 1.

As seen from FIG. 3, the gold nanocluster catalysts prepared in Examples 1 and 2 showed the same electronic structure. Since they showed the same absorption spectrum as that of Comparative Example 1, it can be seen that they have the same structure regardless of the ligand type.

FIG. 4 shows is an image showing the framework of the gold nanocluster according to the present disclosure excluding carbons in the ligand. It can be seen that the gold nanocluster exhibits the same structure regardless of the terminal ligand type.

[Confirmation of Preparation of Au₆₇(SC₅H₁₀COOH)₃₅ Catalyst and Au₁₀₂(SC₅H₁₀COOH)₄₄ Catalyst]

As seen from FIG. 5, the nanocluster prepared in Example 3 showed the same UV-visible absorption spectrum as the previously reported Au₆₇(SC₆H₁₃)₃₅ nanocluster protected with an organic ligand, indicating that the Au₆₇(SC₅H₁₀COOH)₃₅ was prepared as intended. In addition, the transmission electron microscopic (TEM) image shows that the nanocluster catalyst Au₆₇(SC₅H₁₀COOH)₃₅ was prepared homogeneously with a size of about 1.4 nm.

As seen from FIG. 6, the nanocluster prepared in Example 4 showed the same UV-visible absorption spectrum as the previously reported Au₁₀₂(SC₆H₁₃)₄₄ nanocluster protected with an organic ligand, indicating that the Au₁₀₂(SC₅H₁₀COOH)₄₄ was prepared as intended. In addition, the transmission electron microscopic (TEM) image shows that the nanocluster catalyst Au₁₀₂(SC₅H₁₀COOH)₄₄ was prepared homogeneously with a size of about 1.6 nm.

[Evaluation of Carbon Dioxide Conversion Activity of Gold Nanocluster Catalysts]

Electrodes for carbon dioxide conversion were prepared using the gold nanocluster catalysts prepared in Examples 1-4 and Comparative Example 1. Specifically, for Examples 1-4, a nanocluster solution was prepared by dissolving 66.25 nmol of each catalyst in a mixed solvent of 0.1 mL of ultrapure water and 0.3 mL of acetone. For Comparative Example 1, a nanocluster solution was prepared by dissolving 66.25 nmol of the catalyst in a mixed solvent of 0.2 mL of acetone and 0.2 mL of dichloromethane. Then, an electrode for carbon dioxide conversion was prepared by depositing the prepared nanocluster solution in a gas diffusion electrode including a microporous layer with an area of 2.5×2.5 cm².

FIG. 7 shows the activity of the Au₂₅(SC₅H₁₀COOH)₁₈ nanocluster prepared in Example 1 and the Au₂₅(SC₆H₁₃)₁₈ nanocluster prepared in Comparative Example 1 for electrochemical carbon dioxide conversion reaction.

As seen from FIG. 7, Example 1 showed 2 times or higher current density for carbon monoxide (CO) production as compared to Comparative Example 1 despite the same gold-ligand framework, suggesting that the anionic ligand of Example 1 provides improved efficiency.

FIG. 8 shows the cross-sectional images and the result of relative elemental analysis for the Au₂₅(SC₅H₁₀COOH)₁₈ nanocluster prepared in Example 1 and the Au₂₅(SC₆H₁₃)₁₈ nanocluster prepared in Comparative Example 1 after carbon dioxide conversion reaction.

As seen from FIG. 8, unlike Comparative Example 1, the presence of the alkali metal cation K⁺ at high density was confirmed in the porous catalytic layer (indicated as MPL) in Example 1. Due to the introduction of the anionic ligand, the alkali metal cation in the electrolyte was concentrated near the nanocluster catalyst, suggesting that the activity for carbon dioxide conversion was increased.

FIG. 9 shows the activity of the Au₂₅(SC₅H₁₀COOH)₁₈ nanocluster prepared in Example 1 and the Au₂₅(SC₆H₁₃)₁₈ nanocluster prepared in Comparative Example 1 depending on pH.

As seen from FIG. 9, for Example 1, the activity was low at the acidic electrolyte pH of 3.7 or lower. This implies that the concentration of the alkali metal cation near the catalyst was not achieved as the terminal of the anionic ligand was protected by hydrogen ions.

FIG. 10 shows the activity of the Au₂₅(SC₃H₆SO₃H)₁₈ nanocluster prepared in Example 2 and the Au₂₅(SC₆H₁₃)₁₈ nanocluster prepared in Comparative Example 1 for electrochemical carbon dioxide conversion reaction (FIG. 10-1) and the activity depending on pH (FIG. 10-2).

As seen from FIG. 10, Example 2 showed high current density and also high activity regardless of the electrolyte pH as compared to Comparative Example 1. It may be because the anionic terminal group is maintained even under the strongly acidic condition due to the high acidity of the sulfonate group.

FIG. 11 shows the selectivity of the Au₂₅(SC₃H₆SO₃H)₁₈ nanocluster prepared in Example 2 for carbon monoxide production under the condition of pH 1 for 24 hours.

As seen from FIG. 11, it can be seen that Example 2 maintains the carbon monoxide production selectivity at 96% or higher without significant change in the applied voltage even under long-term operation for 24 hours at the acidic condition of pH 1. This suggests that the nanocluster catalyst can maintain high activity for a long time.

FIG. 12 shows the carbon dioxide conversion activity of the anionic ligand-protected Au₆₇(SC₅H₁₀COOH)₃₅ and Au₁₀₂(SC₅H₁₀COOH)₄₄ nanoclusters prepared in Examples 3 and 4 and the Au₂₅(SC₆H₁₃)₁₈ nanocluster having a neutral ligand prepared in Comparative Example 1.

As seen from FIG. 12, the Au₆₇(SC₅H₁₀COOH)₃₅ and Au₁₀₂(SC₅H₁₀COOH)₄₄ nanoclusters of different sizes protected with the anionic ligand SC₅H₁₀COOH according to Examples 3 and 4 showed higher current density than Comparative Example 1 having a neutral ligand and also showed similar performance to Example 1. Therefore, it is though that the anionic ligand increases catalyst activity through concentration of cations on the catalyst surface regardless of the size and type of the nanocluster.

Accordingly, it is expected that the activity of the electrochemical nanocluster catalyst for carbon dioxide conversion according to the present disclosure can be maximized by introducing various anionic terminal groups.

Claims

1. A gold nanocluster catalyst for carbon dioxide conversion represented by [Chemical Formula 1]: Aun(SR)m  [Chemical Formula 1]wherein SR is an organic thiol-based ligand comprising an anionic terminal group, n is an integer between 25 and 102 and m is an integer between 18 and 44.

2. The gold nanocluster catalyst for carbon dioxide conversion according to claim 1, wherein, in Chemical Formula 1, R is any one selected from a group consisting of a C3-C30 aryl group, a C3-C40 arylalkyl group, a C3-C30 heteroaryl group and a C3-C40 heteroarylalkyl group.

3. The gold nanocluster catalyst for carbon dioxide conversion according to claim 1, wherein, in Chemical Formula 1, the anionic terminal group is COOH or SO3H.

4. The gold nanocluster catalyst for carbon dioxide conversion according to claim 1, wherein the Aun(SR)m of Chemical Formula 1 is Au25(SC5H10COOH)18, Au67(SC5H10COOH)35, Au102(SC5H10COOH)44 or Au25(SC3H6SO3H)18.

5. The gold nanocluster catalyst for carbon dioxide conversion according to claim 1, wherein the catalyst has a size of 1.1-1.6 nm.

6. A gas diffusion electrode comprising the gold nanocluster catalyst for carbon dioxide conversion according to claim 1.

7. A method for preparing a gold nanocluster catalyst for carbon dioxide conversion represented by [Chemical Formula 1], comprising: (I) a step of preparing a precursor solution comprising a gold precursor;(II) a step of preparing a solution of an organic thiol-based ligand having an anionic functional group;(III) a step of obtaining a reaction solution by mixing and stirring the precursor solution and the ligand solution and optionally adding a reaction catalyst; and(IV) a step of obtaining a nanocluster by adding a reducing agent to the reaction solution and stirring, filtering and drying the same: Aun(SR)m  [Chemical Formula 1]wherein SR is an organic thiol-based ligand comprising an anionic terminal group, n is an integer between 25 and 102 and m is an integer between 18 and 44.

8. The method for preparing a gold nanocluster catalyst for carbon dioxide conversion according to claim 7, wherein the gold precursor is HAuCl4·3H2O.

9. The method for preparing a gold nanocluster catalyst for carbon dioxide conversion according to claim 7, wherein the organic thiol-based ligand having an anionic functional group is SCH10COOH or SC3H6SO3H.

10. The method for preparing a gold nanocluster catalyst for carbon dioxide conversion according to claim 7, wherein the reaction catalyst is NaOH or KOH.

11. The method for preparing a gold nanocluster catalyst for carbon dioxide conversion according to claim 7, wherein the reducing agent is any one selected from a group consisting of NaBH4, LiBH4, KBH4 and LiAlH4.