Method for producing carbon dioxide reduction catalyst

Abstract

To provide a carbon dioxide reduction catalyst with high current efficiency that has a wide potential window, is not easily inhibited by the hydrogen generation reaction in the electrolysis of an aqueous solution, and has excellent durability.SOLUTION: Provided is a manufacturing method for a carbon dioxide reduction catalyst which obtains the carbon dioxide reduction catalyst by ammonia plasma treatment of conductive amorphous carbon or water plasma treatment of conductive amorphous carbon containing nitrogen. Also provided is a method for imparting carbon dioxide reduction activity to conductive amorphous carbon by introducing nitrogen and oxygen to a surface of the conductive amorphous carbon and increasing the nitrogen, amino groups and C=O bonds on the surface of the conductive amorphous carbon.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing a carbon dioxide reduction catalyst using amorphous carbon, a carbon dioxide reduction catalyst electrode using the carbon dioxide reduction catalyst, and a method for imparting carbon dioxide reduction activity to amorphous carbon or improving the carbon dioxide reduction activity of amorphous carbon. [Background technology]

[0002] The world faces two major challenges: the need to reduce CO2 emissions to curb global warming, rising energy consumption, fossil fuel shortages, and energy shortages due to the decline of nuclear energy. Currently, research into the synthesis of fuel (alcohol) from CO2 through artificial photosynthesis is intensifying. Carbon monoxide (CO) and formic acid (HCOOH) are desirable reduction products, as their conversion technologies to olefins and fuels have been established. Research and development into the synthesis of fuel (alcohol) from CO2 (CO2 fixation) is a technology that can simultaneously solve environmental, resource, and energy issues. However, the standard potentials of the reduction reactions of CO2 to carbon monoxide (CO) and formic acid (HCOOH) are −0.53 V and −0.61 V (vs. NHE), respectively, which are more negative than the standard potential of the H2 production reaction (0 V vs. NHE). Therefore, electrochemical reduction of CO2 in aqueous solution without inhibiting the H2 production reaction is difficult.

[0003] The development of electrochemical catalytic electrodes for CO2 reduction has focused primarily on increasing catalytic activity for CO2 reduction and lowering the CO2 reduction overpotential relative to H2 production. Metals, alloys, and carbon nanostructures have been reported as electrocatalysts capable of selectively electrochemically reducing CO2. It has been reported that the products of the CO2 reduction reaction at metal electrodes vary depending on the adsorption strength of the CO radical (reaction intermediate) and the electrode surface. Hg, In, Sn, Cd, and Pb, which have weak CO2 radical adsorption strength, produce HCOOH, while Pt and Ni, which have strong adsorption strength, do not desorb the product and only the H2 production reaction occurs. Au, Ag, Cu, and Zn, which have moderate adsorption strength, have high CO2 reduction activity. Cu, in particular, can lower the reduction potential below the H2 production potential, allowing for highly efficient production of CO and CH4 without inhibiting H2 production. However, the activity of Cu electrodes decreases due to corrosion of the electrode surface and product adhesion. Therefore, a highly durable electrode material with long-term use has not been developed. Furthermore, by controlling the surface structure, it has not been possible to create an interface that exhibits the same adsorption power for CO radicals as Au, Ag, Cu, and Zn. In other words, a CO2 reduction electrode that combines high activity (separation of H2 generation and CO2 reduction) and high durability has yet to be realized.

[0004] As a carbon material, amorphous carbon with electrical conductivity has been developed (see Patent Document 1), and its use in gas permeable membranes for sensors (see Patent Document 2), electrodes for electrochemical sensors for cesium oxidation-reduction reactions, etc., and current collectors for lithium batteries (see Patent Document 3) has been proposed; however, it was not applicable to the reduction of CO2. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-189997 [Patent Document 2] Japanese Patent Application Laid-Open No. 2011-185910 [Patent Document 3] Japanese Patent Application Laid-Open No. 2012-188688 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a carbon dioxide reduction catalyst that has a wide potential window, is less susceptible to inhibition by the hydrogen production reaction during electrolysis of an aqueous solution, and has excellent durability and high current efficiency. [Means for solving the problem]

[0007] The present inventors investigated the use of amorphous carbon to solve the above-mentioned problems. While amorphous carbon is typically not conductive, conductive amorphous carbon has been developed by introducing impurity atoms to impart conductivity. While conductive amorphous carbon does not have carbon dioxide reduction activity, if it could be made carbon dioxide reduction active, its use as a catalyst or catalytic electrode for electrolytic carbon dioxide reduction would be expanded. Through further investigation, the inventors discovered that nitrogen, particularly amino groups, on the amorphous carbon surface are involved in the formation of carbon dioxide reduction active sites, and that oxygen-containing functional groups on the amorphous carbon surface increase the amount of carbon dioxide adsorbed onto the catalyst surface, thereby promoting carbon dioxide reduction activity. Conductive amorphous carbon, which increases the surface nitrogen, particularly amino groups and oxygen-containing functional groups, and increases their density on the surface, can induce a carbon dioxide reduction reaction with high current efficiency, separate from the hydrogen production reaction. This reaction separation is not possible with carbon materials such as graphite or diamond. Furthermore, it exhibits significantly higher stability than Cu electrodes, which are known as carbon dioxide reduction electrodes. This is how the present invention was completed.

[0008] That is, the present invention is specified by the following items. (1) A method for producing a carbon dioxide reduction catalyst, in which a conductive amorphous carbon is subjected to an ammonia plasma treatment or a nitrogen-containing conductive amorphous carbon is subjected to a water plasma treatment to obtain a carbon dioxide reduction catalyst. (2) A method for producing a carbon dioxide reduction catalyst according to (1) above, wherein the conductive amorphous carbon is nitrogen- or boron-doped amorphous carbon, and the nitrogen- or boron-doped amorphous carbon is subjected to an ammonia plasma treatment, or the nitrogen-doped amorphous carbon is subjected to a water plasma treatment. (3) A method for producing a carbon dioxide reduction catalyst electrode, comprising forming a conductive amorphous carbon layer on a substrate and subjecting the formed conductive amorphous carbon layer to an ammonia plasma treatment, or forming a nitrogen-containing conductive amorphous carbon layer on a substrate and subjecting the formed nitrogen-containing conductive amorphous carbon layer to a water plasma treatment, thereby obtaining a carbon dioxide reduction catalyst electrode. (4) A method for imparting carbon dioxide reduction activity to conductive amorphous carbon, comprising introducing nitrogen and oxygen onto the surface of the conductive amorphous carbon to increase the nitrogen and amino groups and C=O bonds on the surface of the conductive amorphous carbon, thereby imparting carbon dioxide reduction activity to the conductive amorphous carbon. (5) A method for imparting carbon dioxide reduction activity to nitrogen-doped amorphous carbon, comprising introducing oxygen onto the surface of the nitrogen-doped amorphous carbon to increase the number of C=O bonds on the surface of the nitrogen-doped amorphous carbon, thereby imparting carbon dioxide reduction activity to the nitrogen-doped amorphous carbon. (6) An amorphous carbon carbon dioxide reduction catalyst that is electrically conductive and further has nitrogen atoms and amino groups as well as oxygen atoms and C=O bonds on its surface, wherein the amount of the nitrogen atoms on the surface is 7 atom% or more and the amount of nitrogen atoms in the amino groups is 1.5 atom% or more, the amount of the oxygen atoms is 7 atom% or more and the amount of oxygen atoms in the C=O bonds is 7 atom% or more. [Effects of the Invention]

[0009] The production method of the present invention makes it possible to produce a carbon dioxide reduction catalyst and a carbon dioxide reduction catalyst electrode that have a wide potential window, are less susceptible to inhibition by the hydrogen evolution reaction during electrolysis of an aqueous solution, and have excellent durability and high current efficiency. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 shows cyclic voltammograms of the electrodes obtained in Example 1 and Comparative Example 1. [Figure 2] FIG. 2 shows the linear sweep voltammetry of the electrodes obtained in Examples 1 and 2. [Figure 3] FIG. 3 is a graph showing the linear sweep voltammetry of the electrode obtained in Comparative Example 1. [Figure 4] 4(a) to 4(c) are diagrams showing XPS spectra of nitrogen-doped amorphous carbon (before ammonia plasma irradiation) in Example 1. FIG. [Figure 5] 5(a) to 5(c) are diagrams showing XPS spectra of nitrogen-doped amorphous carbon after ammonia plasma irradiation in Example 1. FIG. [Figure 6] 6(a) to 6(c) are diagrams showing XPS spectra of nitrogen-doped amorphous carbon after water plasma irradiation in Example 2. FIG. [Figure 7] FIG. 7 shows the linear sweep voltammetry of the electrodes obtained in Example 3 and Comparative Example 2. [Figure 8] FIG. 8 is a diagram showing the mechanism of CO2 reduction in the present invention. [Figure 9] FIG. 9 is a diagram showing the reduction peak current value when the scanning speed of the amorphous carbon treated with ammonia plasma obtained in Example 1 is changed. [Figure 10] FIG. 10 is a diagram showing the UV absorption spectrum of the electrolyte solution obtained by constant-potential electrolysis. [Figure 11] FIG. 11 is a graph showing the rate of decrease in current value versus electrolysis time for the amorphous carbon treated with ammonia plasma obtained in Example 1 and the Cu electrode. [Figure 12] FIG. 12 is a photograph showing the state of the Cu electrode after constant-potential electrolysis. DETAILED DESCRIPTION OF THE INVENTION

[0011] The method for producing a carbon dioxide reduction catalyst of the present invention is a method for producing a carbon dioxide reduction catalyst by treating conductive amorphous carbon with ammonia plasma or by treating conductive amorphous carbon containing nitrogen with water plasma. 3 Carbon and graphite bond sp 2 It is defined as a carbon material with an amorphous structure in which both carbon and sp 3The carbon ratio varies from nearly 90% to about 30%, and the material is also called diamond-like carbon. The conductivity of the conductive amorphous carbon of the present invention is not particularly limited as long as it is within the range that allows it to be used as an electrode material for carbon dioxide reduction, but a range of 5 Ωcm to 0.05 Ωcm is preferred. Conductive amorphous carbon can be obtained by adding nitrogen, boron, phosphorus, sulfur, or the like to amorphous carbon, and two or more of these may be added. Preferred conductive amorphous carbons include nitrogen-doped amorphous carbon, which is amorphous carbon containing nitrogen added to amorphous carbon, and boron-doped amorphous carbon, which is amorphous carbon containing boron added to amorphous carbon. The nitrogen atom content in the nitrogen-doped amorphous carbon is preferably 0.5 atom% to 10 atom%, and the boron atom content in the boron-doped amorphous carbon is preferably 0.2 atom% to 5 atom%. Nitrogen-doped amorphous carbon and boron-doped amorphous carbon may contain elements other than nitrogen, boron, and carbon. The method for producing the conductive amorphous carbon used in the present invention is not particularly limited. For example, nitrogen-doped amorphous carbon can be produced by forming a film by plasmatizing a source gas, which is a compound containing carbon and nitrogen atoms. The source gas can be one or more of hydrogen cyanide, acetonitrile, ethane cyanide, propane cyanide, formamide, acetamide, methylamine, ethylamine, etc., and the production methods described in Patent Documents 1 and 2 can be used. Boron-doped amorphous carbon can be produced by using a hydrocarbon as the carbon source, an organoboron compound as the boron source, and argon gas as a reaction control gas, and then forming a film by plasmatizing the source gas. Examples of the hydrocarbon include saturated hydrocarbons such as n-hexane, and examples of the organoboron compound include trialkylboron and trialkoxyboron. The production method described in Patent Document 3 can be used.

[0012] In the manufacturing method of the present invention, carbon dioxide reduction activity can be imparted to conductive amorphous carbon by treating the carbon dioxide reduction activity with ammonia plasma. Furthermore, in the case of nitrogen-doped amorphous carbon, carbon dioxide reduction activity can also be imparted by water plasma treatment. The ammonia plasma treatment in the present invention refers to a treatment in which conductive amorphous carbon is brought into contact with ammonia plasma, such as exposing the conductive amorphous carbon to ammonia plasma or irradiating the conductive amorphous carbon with ammonia plasma. The ammonia plasma in the present invention is a plasma formed by converting a compound containing an amino group and a compound containing a hydroxyl group into ammonia water, urea water, hydrazine aqueous solution, or the like into a plasma. Alternatively, a mixture of these raw materials may be used for plasma generation. To increase the efficiency of generating functional groups on the amorphous carbon surface, a raw material containing ammonia water is preferred, and using ammonia water alone is more preferred. When ammonia water is used as the raw material, the ammonia plasma contains NH3 + It is known that radicals, nitrogen radicals, hydrogen radicals, oxygen radicals, OH radicals, etc. are generated. The method for forming ammonia plasma is not particularly limited, but examples thereof include a method in which the above-mentioned raw materials are gasified and converted into plasma. The method for exciting the plasma is not particularly limited, but examples thereof include high frequency waves, microwaves, and hot filaments.

[0013] In the present invention, water plasma treatment refers to a treatment in which nitrogen-doped amorphous carbon is brought into contact with water plasma, such as exposing nitrogen-doped amorphous carbon to water plasma or irradiating nitrogen-doped amorphous carbon with water plasma. The water plasma in the present invention refers to a plasma containing hydrogen radicals, oxygen radicals, and OH radicals, and can be formed by converting a compound containing a hydroxyl group into plasma. An example of a compound containing a hydroxyl group is water. The method for generating water plasma is not particularly limited, but examples include a method in which water is gasified and then converted into plasma. The method for exciting the plasma is not particularly limited, but examples include high frequency, microwave, and hot filament.

[0014] The ammonia plasma treatment in the present invention can be performed by, for example, placing conductive amorphous carbon, such as nitrogen-doped amorphous carbon or boron-doped amorphous carbon, in a plasma generation vessel (chamber), generating ammonia plasma in the chamber, and contacting the conductive amorphous carbon with the ammonia plasma. The ammonia plasma treatment increases the amount of nitrogen (N), amino groups, and C═O bonds present on the surface of the conductive amorphous carbon. This allows carbon dioxide reduction activity to be imparted to conductive amorphous carbon that does not have carbon dioxide reduction activity. The water plasma treatment in the present invention can be performed by placing the nitrogen-doped amorphous carbon in a plasma generation vessel (chamber), generating water plasma in the chamber, and contacting the nitrogen-doped amorphous carbon with the water plasma. The water plasma treatment increases the amount of C═O bonds present on the surface of the nitrogen-doped amorphous carbon. The expression of carbon dioxide reduction activity in amorphous carbon is influenced by the amount of nitrogen, amino groups, and C=O bonds present on the surface, and nitrogen is required. However, in the case of nitrogen-doped amorphous carbon, a certain amount of nitrogen and amino groups already exists on the surface, so carbon dioxide reduction activity can be imparted by water plasma treatment alone. In the present invention, the nitrogen in "nitrogen and amino groups" refers to the total amount of nitrogen on the surface, including nitrogen derived from amino groups and nitrogen not derived from amino groups. Furthermore, when nitrogen-doped amorphous carbon is used as an electrochemical electrode, the generation of H2 and O2 by water electrolysis only occurs at very high potentials (the overpotential for the H2 and O2 generation reaction is high), exhibiting a wide potential window of up to 3 V. However, since the nitrogen in nitrogen-doped amorphous carbon does not function as an H2 generation catalyst, it is preferable to use nitrogen-doped amorphous carbon. According to the manufacturing method of the present invention, amorphous carbon having nitrogen atoms, amino groups, oxygen atoms, and C=O bonds on the surface and exhibiting carbon dioxide reduction activity can be obtained, and diamond-bonded sp 3 Carbon and graphite bond sp 2In addition to having an amorphous structure in which both carbon and nitrogen are present, the catalyst has a conductivity of approximately 5 Ωcm to 0.05 Ωcm, and further has a surface with a nitrogen atom content of 7 atom% or more, a nitrogen atom content of 1.5 atom% or more in amino groups, a oxygen atom content of 7 atom% or more, and a C=O bond content of 7 atom% or more. Furthermore, the catalyst can be suitably obtained with a nitrogen atom content of 15 atom% or more and a nitrogen atom content of 4 atom% or more in amino groups. Furthermore, nitrogen atoms and amino groups, as well as oxygen atoms and C=O bonds, can be introduced to the surface to the extent that they do not render the surface nonconductive. This allows for the suitable carbon dioxide reduction catalyst to be obtained with a nitrogen atom derived from amino groups and an oxygen atom derived from C=O bonds (C=O groups) of 33 atom% or less, respectively. In the present invention, the C=O bond refers to a C=O group contained in a functional group such as carbonyl, ester, or carboxylic acid. 3 Carbon and sp 2 The carbon composition ratio of carbon is sp 3 Carbon:sp 2 The carbon ratio is preferably 10:90 to 90:10, more preferably 10:90 to 40:60, and preferably, for example, about 25:75. In the present invention, the amounts of nitrogen atoms and amino groups, as well as oxygen atoms and C=O bonds at the surface, refer to the amounts at the surface measured by X-ray photoelectron spectroscopy. X-ray photoelectron spectroscopy can measure atomic information within a depth of about 2 to 8 nm from the surface, and the amounts of nitrogen atoms and amino groups, oxygen atoms and C=O bonds within the depth obtained by X-ray photoelectron spectroscopy measurement. In the present invention, the amount of C=O bonds is determined from the carbon peak measured by X-ray photoelectron spectroscopy. However, since the amount of carbon and the amount of oxygen in a C=O bond are the same, the amount of carbon atoms in the C=O bond is taken as the amount of oxygen atoms in the C=O bond. The conditions for the ammonia plasma treatment and water plasma treatment in the present invention, such as plasma output, treatment time, and concentration of each component in the plasma, can be appropriately selected by adjusting each of these conditions so as to impart carbon dioxide reduction activity.

[0015] The carbon dioxide reduction catalyst obtained by the production method of the present invention has electrical conductivity and carbon dioxide reduction activity, and therefore can be used as a carbon dioxide reduction catalyst electrode by itself after film formation, molding, etc.; however, to maintain the electrode strength, a carbon dioxide reduction catalyst layer may be formed on a substrate and used as a carbon dioxide reduction catalyst electrode. Specifically, a carbon dioxide reduction catalyst electrode can be produced by forming a conductive amorphous carbon layer on a substrate and subjecting the formed conductive amorphous carbon layer to ammonia plasma treatment, or by forming a nitrogen-doped amorphous carbon layer as the conductive amorphous carbon layer and subjecting the formed nitrogen-doped amorphous carbon layer to water plasma treatment. The substrate is not particularly limited, but examples include substrates that are conductive and corrosion-resistant, such as silicon wafers, stainless steel, nickel, and precious metals. The methods for forming a conductive amorphous carbon layer on a substrate, the ammonia plasma treatment, and the water plasma treatment can be the same as those described above for the carbon dioxide reduction catalyst.

[0016] The present invention is a method for imparting carbon dioxide reduction activity to conductive amorphous carbon, which involves introducing nitrogen and oxygen to the surface of the conductive amorphous carbon and increasing the number of nitrogen, amino groups, and C═O bonds on the surface of the conductive amorphous carbon, thereby imparting carbon dioxide reduction activity to the conductive amorphous carbon. The conductive amorphous carbon is amorphous carbon to which nitrogen, boron, phosphorus, sulfur, or the like has been added to impart conductivity to the amorphous carbon, and has a conductivity of 5 Ωcm to 0.05 Ωcm. The conductivity of the amorphous carbon in the present invention can be measured by performing Hall effect measurement. In the present invention, carbon dioxide reduction activity can be imparted to conductive amorphous carbon that does not have carbon dioxide reduction activity by introducing nitrogen and oxygen to the surface of the conductive amorphous carbon and increasing the number of nitrogen, amino groups, and C═O bonds on the surface of the conductive amorphous carbon. In the present invention, "imparting" includes imparting carbon dioxide reduction activity to conductive amorphous carbon that has no carbon dioxide reduction activity, and increasing the carbon dioxide reduction activity of conductive amorphous carbon that already has carbon dioxide reduction activity. Conductive amorphous carbon treated by the method of the present invention possesses both electrical conductivity and carbon dioxide reduction activity, making it suitable for use as a carbon dioxide reduction catalyst and a carbon dioxide reduction catalyst electrode. Methods for introducing nitrogen and oxygen to the surface of conductive amorphous carbon include, but are not limited to, ammonia plasma treatment. The present invention also provides a method for imparting carbon dioxide reduction activity to nitrogen-doped amorphous carbon by introducing oxygen to the surface of the nitrogen-doped amorphous carbon and increasing the number of C=O bonds on the surface of the nitrogen-doped amorphous carbon, thereby imparting carbon dioxide reduction activity to the nitrogen-doped amorphous carbon. Because nitrogen is already present in the amorphous carbon, a certain amount of nitrogen and amino groups are present on the surface. Therefore, increasing the number of C=O bonds on the surface can impart carbon dioxide reduction activity.The method for introducing oxygen into the surface of nitrogen-doped amorphous carbon is not particularly limited, but examples include water plasma treatment. Nitrogen-doped amorphous carbon treated by the method of the present invention possesses both electrical conductivity and carbon dioxide reduction activity, making it suitable for use as a carbon dioxide reduction catalyst and a carbon dioxide reduction catalyst electrode. Even for nitrogen-doped amorphous carbon, it is preferable to introduce nitrogen and oxygen onto the surface by ammonia plasma treatment or the like to further increase the amount of nitrogen on the surface. The amorphous carbon carbon dioxide reduction catalyst of the present invention is a carbon dioxide reduction catalyst produced by the carbon dioxide reduction catalyst manufacturing method of the present invention or a carbon dioxide reduction catalyst to which carbon dioxide reduction activity has been imparted by the method of imparting carbon dioxide reduction activity of the present invention. The amorphous carbon carbon dioxide reduction catalyst of the present invention also includes those obtained by other methods, as long as they have the same structure and properties as the amorphous carbon carbon dioxide reduction catalyst obtained by the manufacturing method or imparting method of the present invention. The carbon dioxide reduction catalyst of the present invention is a diamond-bonded sp. 3 Carbon and graphite bond sp 2The carbon dioxide reduction catalyst has an amorphous structure containing both carbon and nitrogen atoms, a conductivity of about 5 Ωcm to 0.05 Ωcm, and a surface containing nitrogen atoms, amino groups, oxygen atoms, and C═O bonds, and has carbon dioxide reduction activity. The surface preferably contains 7 atom% or more of nitrogen atoms and 1.5 atom% or more of nitrogen atoms in the amino groups, and 7 atom% or more of oxygen atoms and 7 atom% or more of oxygen atoms in the C═O bonds. The nitrogen atom content is preferably 15 atom% or more and 4 atom% or more of nitrogen atoms in the amino groups. If the surface of amorphous carbon is completely covered with an oxide film of C═O bonds, the surface becomes a passivation material, and the hydrogen atoms in the amino groups become sterically hindered. Therefore, the upper limits of the amounts of nitrogen atoms and amino groups, oxygen atoms, and C═O bonds are not particularly limited as long as they can be introduced onto the surface without passivating the surface. However, it is preferred that the nitrogen atoms derived from amino groups and the oxygen atoms derived from C═O bonds (C═O groups) each be 33 atom% or less. In the present invention, the C=O bond refers to a C=O group contained in a functional group such as a carbonyl, ester, or carboxylic acid. 3 Carbon and sp 2 The carbon composition ratio of carbon is sp 3 Carbon:sp 2 The carbon ratio is preferably 10:90 to 90:10, more preferably 10:90 to 40:60, and for example, approximately 25:75. In the present invention, the amounts of nitrogen atoms and amino groups, as well as oxygen atoms and C=O bonds at the surface, refer to the amounts at the surface measured by X-ray photoelectron spectroscopy. X-ray photoelectron spectroscopy can measure atomic information within a depth of approximately 2 to 8 nm from the surface, and the amounts of nitrogen atoms and amino groups, oxygen atoms and C=O bonds within the depth obtained by measurement by X-ray photoelectron spectroscopy. In the present invention, the amount of C=O bonds is determined from the carbon peak measured by X-ray photoelectron spectroscopy. However, since the amount of carbon and the amount of oxygen in a C=O bond are the same, the amount of carbon atoms in the C=O bond is taken to be the amount of oxygen atoms in the C=O bond. The conductive amorphous carbon in the carbon dioxide reduction catalyst of the present invention is preferably nitrogen-doped amorphous carbon or boron-doped amorphous carbon. [Example]

[0017] The present invention will be described below with reference to examples, but the present invention is not limited to these specific embodiments.

[0018] [Example 1] (Preparation of nitrogen-doped amorphous carbon) A cathode-coupled radio-frequency plasma-enhanced chemical vapor deposition (RF-PeCVD) system (13.56 MHz, SAMCO Co., Ltd. Model BP-1) was used as the plasma CVD system. Nitrogen-doped amorphous carbon (N-doped aC) films were deposited on silicon wafers placed on electrodes using acetonitrile vaporized at 50°C as the source gas. The acetonitrile flow rate was 1 sccm, the chamber pressure was 10 Pa, and the plasma power was 175 W. The deposition time was 40 minutes, the substrate temperature during the reaction was 260°C, and the resulting film thickness was 1600 nm at a deposition rate of 40 nm / min. (ammonia plasma treatment) After the nitrogen-doped amorphous carbon film was formed, the chamber was evacuated to reduce the internal pressure to 0.4 Pa or less, and then ammonia water (concentration 28 mass%) that had been heated to 30°C and vaporized was introduced into the chamber. The exhaust pressure was adjusted to set the pressure inside the chamber to 30 Pa, and plasma was generated. The plasma output was 30 W, and the treatment time was 2 minutes. In this way, the nitrogen-doped amorphous carbon inside the chamber was subjected to ammonia plasma treatment, yielding ammonia plasma-treated nitrogen-doped amorphous carbon.

[0019] [Example 2] (Water plasma treatment) Nitrogen-doped amorphous carbon was deposited in the same manner as in Example 1, and the chamber was evacuated to reduce the internal pressure to 0.4 Pa or less. Then, pure water (ion-exchanged water: electrical resistivity 0.1 MΩcm or more) that had been heated to 30°C and vaporized was introduced into the chamber, and the exhaust pressure was adjusted to set the pressure inside the chamber to 30 Pa, and plasma was generated. The plasma output was 30 W, and the treatment time was 1 minute. In this way, the nitrogen-doped amorphous carbon inside the chamber was subjected to water plasma treatment, and water plasma-treated nitrogen-doped amorphous carbon was obtained.

[0020] [Example 3] (Preparation of boron-doped amorphous carbon) The same plasma CVD apparatus as in Example 1 was used. A liquid mixture of trimethylborane (TMOB) and n-hexane at a volume ratio of 6.3:1 was used as the source gas. Vacuuming and vaporizing the mixture at room temperature, a boron-doped amorphous carbon (boron-doped aC) film was formed on a silicon wafer placed on an electrode. The source gas and argon were introduced into the chamber at a source gas flow rate of 5 sccm and an argon flow rate of 100 sccm. The source gas partial pressure in the chamber was 10 Pa (total pressure 40 Pa), the plasma output was 175 W, the film formation time was 40 minutes, the substrate temperature during the reaction was 260°C, and the resulting film thickness was 700 nm. (ammonia plasma treatment) The boron-doped amorphous carbon in the obtained chamber was subjected to an ammonia plasma treatment in the same manner as in Example 1, to obtain ammonia-plasma-treated boron-doped amorphous carbon.

[0021] [Comparative Example 1] (Fabrication of boron-doped diamond) The plasma CVD apparatus used was an ASTeX Model AX-5400. The source gas was a mixture of 10 parts by volume of acetone and methanol, with B2O3 dissolved in the mixture at a boron atom (B) to carbon atom (C) ratio of 10,000 ppm (by mass). Hydrogen gas was bubbled through the solution, vaporizing the source gas and introducing it into the chamber. Boron-doped diamond films were deposited on silicon wafers placed on electrodes. The hydrogen gas flow rate was 532 sccm, the source gas flow rate was 30 sccm, the chamber pressure was 113 Pa, and the plasma power was 5 kW. The deposition time was 8 hours, the substrate temperature during the reaction was 500°C, and the resulting film thickness was 10 μm.

[0022] Comparative Example 2 The boron-doped amorphous carbon obtained in Example 3 was subjected to a water plasma treatment in the same manner as in Example 2, to obtain water-plasma-treated boron-doped amorphous carbon.

[0023] (Measurement of potential window) The potential windows of the ammonia plasma-treated nitrogen-doped aC obtained in Example 1 and the boron-doped diamond obtained in Comparative Example 1 were measured. Measurements were performed using a three-electrode cell, with a graphite electrode as the counter electrode, an Ag / AgCl electrode as the reference electrode, and a 0.1M H2SO4 aqueous solution with carbon dioxide dissolved in it as the electrolyte. The ammonia plasma-treated nitrogen-doped aC and the boron-doped diamond, each deposited on a silicon wafer, were used as the working electrode, and potential sweep measurements were performed at a rate of 100mV / s. Figure 1 shows the obtained cyclic voltammogram. Figure 1 also shows the cyclic voltammogram of polycrystalline platinum for reference. The ±0.2mA / cm of each electrode was used. 2 The oxygen evolution potential and hydrogen evolution potential are defined as the potentials reached by the ammonia plasma treatment. However, the hydrogen evolution potential of the ammonia plasma-treated nitrogen-doped aC is 0.46 V lower than that of the boron-doped diamond, indicating that the ammonia plasma-treated nitrogen-doped aC can suppress hydrogen evolution. Table 1 shows the hydrogen evolution potential and oxygen evolution potential of each electrode.

[0024] [Table 1]

[0025] (Linear Sweep Voltammetry) Using the same three-electrode cell as used in the potential window measurement, linear sweep voltammetry was performed by sweeping the potential at a rate of 100 mV / s. The results are shown in Figure 2. In Figure 2, (1) CO2 2.5mV / s indicates the case where untreated nitrogen-doped aC was used and measurement was started after bubbling CO2 into the electrolyte for 30 minutes. (2) Water plasma Ar indicates the case where water plasma-treated nitrogen-doped aC obtained in Example 2 was used as the working electrode and measurement was started after bubbling Ar into the electrolyte for 30 minutes. (3) Water plasma CO2 indicates the case where water plasma-treated nitrogen-doped aC obtained in Example 2 was used as the working electrode and measurement was started after bubbling CO2 into the electrolyte for 30 minutes. (4) Ammonia plasma Ar indicates the case where ammonia plasma-treated nitrogen-doped aC obtained in Example 1 was used as the working electrode and measurement was started after bubbling Ar into the electrolyte for 30 minutes. (5) Ammonia plasma CO2 indicates the case where ammonia plasma-treated nitrogen-doped aC obtained in Example 1 was used as the working electrode and measurement was started after bubbling CO2 into the electrolyte for 30 minutes. Figure 2 shows that nitrogen-doped aC after both ammonia plasma and water plasma treatments exhibited CO2 reduction activity. The CO2 reduction peak was observed near -1.5 V vs Ag|AgCl for water plasma-treated nitrogen-doped aC, and the current efficiency for CO2 reduction at this potential ((current value of (3) - current value of (2)) ÷ current value of (3) × 100) was 81%. The CO2 reduction peak was observed near -1.2 V vs Ag|AgCl for ammonia plasma-treated nitrogen-doped aC, and the current efficiency for CO2 reduction at this potential ((current value of (5) - current value of (4)) ÷ current value of (5) × 100) was 95%. The numbers in parentheses ( ) for (1), (2), etc., correspond to the numbers in circles in Figure 2.

[0026] Figure 3 shows the results of linear sweep voltammetry performed on the boron-doped diamond obtained in Comparative Example 1, similar to the measurement in Figure 2. No significant current increase resulting from CO2 reduction was observed with the boron-doped diamond. When linear sweep voltammetry measurements were performed on boron-doped diamond in the presence of CO2, the reduction current obtained may have included a reduction current resulting from the CO2 reduction reaction. However, since the increase in current compared to the linear sweep voltammetry current in the absence of CO2 was extremely small, the current efficiency for the CO2 reduction reaction was extremely low (current efficiency of 9.9% at -1.5V).

[0027] The nitrogen-doped aC after film formation (before ammonia plasma treatment) obtained in Example 1, the nitrogen-doped aC after ammonia plasma treatment obtained in Example 1, and the nitrogen-doped aC after water plasma treatment obtained in Example 2 were subjected to X-ray photoelectron spectroscopy (measurement device name: Model. K-Alpha TM The X-ray source was Al Kα (1468.6 eV). The angle between the sample surface and the X-ray detector was 90°, and the scanning speed was 1 eV min -1The measurements were performed at 1000 K. The binding energy resolution in the measurements was 0.5 eV. The resulting XPS spectra are shown in Figures 4 to 6. Figures 4(a) to 4(c) show the results for nitrogen-doped aC before ammonia plasma treatment, Figures 5(a) to 5(c) show the results for nitrogen-doped aC after ammonia plasma treatment, and Figures 6(a) to 6(c) show the results for nitrogen-doped aC after water plasma treatment. After ammonia plasma treatment, the intensity of the XPS C1s peak near C=O increased by 1.4 times. Furthermore, after ammonia plasma treatment, the intensity of the entire XPS N1s peak (all three peaks) increased by 2.5 times, with the amine (amino group) in particular increasing. Tables 2 to 4 show the abundance ratios of elements in each bonding state. Tables 2 and 3 show that the ammonia plasma treatment approximately doubled the atom % of N compared to before treatment, with the atom % of amine (amino group) increasing significantly. Table 4 shows that the ammonia plasma treatment increased the C=O atom % to about twice that before treatment, and that the water plasma treatment also increased the C=O atom % to about twice that before treatment.

[0028] [Table 2]

[0029] [Table 3]

[0030] [Table 4]

[0031] Linear sweep voltammetry was performed on boron-doped aC in the same manner as for nitrogen-doped aC. The results are shown in Figure 7. In Figure 7, (1) CO2 2.5 mV / s indicates the case where untreated boron-doped aC was used and measurement was started after CO2 was bubbled into the electrolyte for 30 minutes; (2) Ar 2.5 mV / s indicates the case where untreated boron-doped aC was used and measurement was started after Ar was bubbled into the electrolyte for 30 minutes; (3) ammonia plasma CO2 indicates the case where ammonia plasma-treated boron-doped aC obtained in Example 3 was used on the working electrode and measurement was started after CO2 was bubbled into the electrolyte for 30 minutes; and (4) ammonia plasma Ar (a) indicates the case where ammonia plasma-treated boron-doped aC obtained in Example 3 was used as the working electrode, and Ar was bubbled into the electrolyte for 30 minutes before measurement was initiated. (5) Water plasma Ar indicates the case where water plasma-treated boron-doped aC obtained in Comparative Example 2 was used as the working electrode, and Ar was bubbled into the electrolyte for 30 minutes before measurement was initiated. (6) Water plasma CO indicates the case where water plasma-treated boron-doped aC obtained in Comparative Example 2 was used as the working electrode, and CO was bubbled into the electrolyte for 30 minutes before measurement was initiated. It can be seen that CO reduction activity was also achieved with boron-doped aC by ammonia plasma treatment. The CO reduction peak was observed near -1.3 V vs Ag|AgCl for ammonia plasma-treated boron-doped aC, and the current efficiency of carbon dioxide reduction at this potential ((current value of (3) - current value of (4)) ÷ current value of (3) × 100) was 91%. On the other hand, water plasma treatment did not result in CO reduction activity. The numbers in parentheses in (1), (2), etc. above correspond to the numbers in circles in FIG.

[0032] 8 is a diagram showing the mechanism of carbon dioxide reduction in the carbon dioxide reduction catalyst produced by the present invention. It is presumed that carbon dioxide is adsorbed to oxygen-containing functional groups such as C═O, and the adsorbed carbon dioxide is supplied to active sites formed by nitrogen and amino groups, where electrochemical reduction takes place.

[0033] Figure 9 shows the reduction peak current value at -1.5 V when the sweep rate was changed using a three-electrode cell similar to that used in the potential window measurement, with the working electrode being a nitrogen-doped aC treated with ammonia plasma. The reduction peak current value was proportional to the 1 / 2 power of the sweep rate. The pH of 0.2 M Na2SO4 saturated with CO2 was 5.6, and the CO2 and HCO3 - The ratio of CO2 to HCO3 is approximately 8:2. - = 26.88mM:6.72mM. Assuming a reversible reaction, HCO3 - If the reaction is taking place, the number of reactive electrons, n, is 0.517, and HCO3 - It is highly likely that a reduction reaction involving n = 1 is occurring. (Randles-Sevcik formula) I P =2.69×10 5 n 1.5 AD 0.5 CV 0.5 I P : reduction peak current (A), n: number of reaction electrons, A: electrode area (cm 2 ), D: diffusion coefficient (cm 2 / s), C: Concentration (mol / cm 3 ), v: Sweep speed (V / s)

[0034] Figure 10 shows the results of constant potential electrolysis at −1.6 V (average current density −223.78 μA cm) while bubbling CO2, using the same three-electrode cell as in the potential window measurement, with the working electrode being nitrogen-doped aC treated with ammonia plasma and the working electrode being nitrogen-doped aC before ammonia plasma treatment. -210 shows the UV absorption spectra of the electrolyte solution after electrolysis. The UV absorption spectra were measured using a UV-visible-near-infrared spectrophotometer (instrument name: V-670, manufacturer: JASCO). In Figure 10, the spectrum with the highest absorption peak derived from formic acid is obtained when ammonia plasma-treated nitrogen-doped aC was electrolyzed for 6 hours, the second highest spectrum is obtained when ammonia plasma-treated nitrogen-doped aC was electrolyzed for 3 hours, and the lowest spectrum is obtained when nitrogen-doped aC before ammonia plasma treatment was electrolyzed for 3 hours.

[0035] Figure 11 shows the change in the rate of current decrease versus electrolysis time when CO2 was bubbled through the three-electrode cell used in the potential window measurements. The working electrodes were the ammonia plasma-treated nitrogen-doped aC prepared in Example 1 and a copper (Cu) plate. Potential electrolysis was performed for 6 hours at -1.6 V for the nitrogen-doped aC and -1.5 V for the copper (Cu) plate. While the CO2 reduction current significantly decreased over 6 hours of potential electrolysis, the ammonia plasma-treated nitrogen-doped aC electrode maintained a nearly constant potential electrolysis current, demonstrating its high durability. Figure 12 shows a photograph of the Cu electrode after potential electrolysis, demonstrating corrosion. The photograph on the right side of Figure 12 is an enlarged view of the area circled in the photograph on the left side. The area within the dotted circle is green, indicating corrosion. [Industrial Applicability]

[0036] The carbon dioxide reduction catalyst and carbon dioxide reduction catalyst electrode produced by the production method of the present invention are unlikely to be inhibited by the hydrogen production reaction during electrolysis of an aqueous solution and have excellent durability. Therefore, they can be suitably used as catalysts for carbon dioxide reduction by electrochemical treatment, and can be used, for example, in electrochemical treatment facilities for producing carbon monoxide or formic acid.

Claims

1. A method for producing a carbon dioxide reduction catalyst is provided by treating conductive amorphous carbon with ammonia plasma, which is a plasma of a compound containing an amino group and a compound containing a hydroxyl group, or by treating conductive amorphous carbon containing nitrogen with water plasma.

2. 2. The method for producing a carbon dioxide reduction catalyst according to claim 1, wherein the conductive amorphous carbon is nitrogen- or boron-doped amorphous carbon, and the nitrogen- or boron-doped amorphous carbon is treated with ammonia plasma obtained by converting a compound containing an amino group and a compound containing a hydroxyl group into a plasma, or the nitrogen-doped amorphous carbon is treated with water plasma.

3. A method for producing a carbon dioxide reduction catalyst electrode, comprising: forming a conductive amorphous carbon layer on a substrate; and treating the formed conductive amorphous carbon layer with ammonia plasma, which is obtained by converting a compound containing an amino group and a compound containing a hydroxyl group into a plasma; or forming a nitrogen-containing conductive amorphous carbon layer on a substrate; and treating the formed nitrogen-containing conductive amorphous carbon layer with water plasma, thereby obtaining a carbon dioxide reduction catalyst electrode.

4. A method for imparting carbon dioxide reduction activity to conductive amorphous carbon, comprising treating the conductive amorphous carbon with ammonia plasma, which is a plasma of a compound containing an amino group and a compound containing a hydroxyl group, thereby introducing nitrogen and oxygen onto the surface of the conductive amorphous carbon and increasing the nitrogen and amino groups and C═O bonds on the surface of the conductive amorphous carbon, thereby imparting carbon dioxide reduction activity to the conductive amorphous carbon.

5. A method for imparting carbon dioxide reduction activity to nitrogen-doped amorphous carbon, comprising treating the nitrogen-doped amorphous carbon with ammonia plasma, which is a plasma of a compound containing an amino group and a compound containing a hydroxyl group, or treating the nitrogen-doped amorphous carbon with water plasma, thereby introducing oxygen onto the surface of the nitrogen-doped amorphous carbon and increasing the number of C═O bonds on the surface of the nitrogen-doped amorphous carbon, thereby imparting carbon dioxide reduction activity to the nitrogen-doped amorphous carbon.

6. An amorphous carbon carbon dioxide reduction catalyst that is electrically conductive and further has nitrogen atoms, amino groups, oxygen atoms, and C=O bonds on its surface, wherein the amount of the nitrogen atoms on the surface is 7 atom % or more and the amount of nitrogen atoms in the amino groups is 1.5 atom % or more, the amount of the oxygen atoms is 7 atom % or more, and the amount of oxygen atoms in the C=O bonds is 7 atom % or more.

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