Method for producing carbon dioxide reduction catalyst

Abstract

To provide a carbon dioxide reduction catalyst with photocatalytic activity for carbon dioxide reduction, and furthermore, to provide a carbon dioxide reduction catalyst capable of producing C2 compounds through carbon dioxide reduction.SOLUTION: A method for producing a carbon dioxide reduction catalyst includes subjecting nitrogen-doped amorphous silicon carbide to water plasma treatment or ammonia plasma treatment to obtain the carbon dioxide reduction catalyst.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] The present invention relates to a method for producing a carbon dioxide reduction catalyst using nitrogen-doped amorphous silicon carbide, a carbon dioxide reduction catalyst electrode using the carbon dioxide reduction catalyst, and a method for imparting carbon dioxide reduction activity to nitrogen-doped amorphous silicon carbide or improving the carbon dioxide reduction activity of nitrogen-doped amorphous silicon carbide. [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 gaining momentum. Preferred reduction products of CO2 include carbon monoxide (CO), for which conversion technology has been established to produce olefins and fuel; oxalic acid ((COOH)2), a raw material for polyester that can realize carbon recycling; and formic acid (HCOOH), a hydrogen energy carrier. Research and development into the synthesis of fuel (alcohol) from CO2 (CO2 fixation) is a technology that can simultaneously solve environmental, resource, and energy problems. However, the standard potentials of the reduction reaction of CO2 to carbon monoxide (CO), formic acid (HCOOH), and oxalic acid ((COOH)2) are -0.52 V, -0.61 V, and -0.89 V (vs. NHE), respectively, which are lower potentials than the standard potential of the H2 production reaction, which is 0 V (vs. NHE). Therefore, it is difficult to reduce CO2 using a photocatalyst in an aqueous solution without inhibiting the H2 production reaction.

[0003] In the research and development of CO2-reducing photocatalysts, since there are few materials that exhibit CO2-reducing activity, a method of supporting a co-catalyst on a semiconductor with a wide band gap is commonly used. For example, BaLa4Ti4O, which has a band gap of 3.9 eV, 15These include systems in which Ag cocatalysts are supported on a catalyst, and systems in which Ag cocatalysts are supported on alkali metal-doped NaTaO3:M (M = Ca, Ba, Sr) with a band gap of 4.1 eV. Using Cu as a cocatalyst selectively produces alkanes, alkenes, and alcohols, while Au and Ag produce CO, and Pb and Hg produce HCOOH. These systems operate only under UV light irradiation, and cocatalysts such as Ag and Cu suffer from poor durability and reduced activity due to surface corrosion and product adhesion. Furthermore, most of the electrons and holes generated by photoexcitation disappear through recombination, with less than 1% consumed in the CO2 reduction reaction. This results in very poor light energy conversion efficiency, far below the 15% energy efficiency required for practical application. Furthermore, their quantum efficiency is significantly lower than that of solar cell-electrochemical CO2 reduction systems. Therefore, a highly efficient CO2-reducing photocatalyst that combines high activity (separation of H2 generation and CO2 reduction) and high durability has not yet been realized. One problem with co-catalysts used in photocatalysts is that the activity of Cu electrodes decreases due to corrosion of the electrode surface and the adhesion of products. Therefore, a photoelectrode material that is highly durable and can be used for a long period of time has not been realized. Patent Document 1 discloses a method for producing a carbon dioxide-reducing catalyst by treating nitrogen- or boron-doped amorphous carbon with ammonia plasma or nitrogen-doped amorphous carbon with water plasma, but the resulting catalyst does not have photocatalytic activity. Furthermore, no CO2-reducing photocatalyst that produces oxalic acid, a C2 compound, has yet been developed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2023-53820 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a carbon dioxide reduction catalyst that has photocatalytic activity for carbon dioxide reduction and is capable of producing a C2 compound by carbon dioxide reduction. [Means for solving the problem]

[0006] The present inventors investigated the use of amorphous silicon carbide to solve the above-mentioned problems. Through further investigation, the inventors discovered that by preparing nitrogen-doped amorphous silicon carbide and then treating it with water plasma or ammonia plasma, a catalyst with a high H2 generation overpotential during CO2 reduction and capable of efficiently reducing CO2 could be obtained. The nitrogen-doped amorphous silicon carbide catalyst thus obtained functions not only as a CO2 reduction catalyst by electrolysis but also as a CO2 reduction catalyst by light, making it possible to use it as a novel CO2 reduction photocatalyst. Furthermore, this catalyst was capable of producing C2 compounds (compounds with two carbon atoms), such as oxalic acid ((COOH)2), glycolic acid (HOCH2COOH), and acetic acid (CH3COOH). Furthermore, this catalyst was found to be more stable and less prone to activity degradation than catalysts using cocatalysts such as Cu, enabling long-term operation. This is how the present invention was completed.

[0007] That is, the present invention is specified by the following items. (1) A method for producing a carbon dioxide reduction catalyst, in which nitrogen-doped amorphous silicon carbide is subjected to a water plasma treatment or an ammonia plasma treatment to obtain the carbon dioxide reduction catalyst. (2) The method for producing a carbon dioxide reduction catalyst according to (1) above, wherein the carbon dioxide reduction catalyst is a photocatalyst. (3) A method for producing a carbon dioxide reduction catalyst electrode, comprising forming a nitrogen-doped amorphous silicon carbide layer on a substrate and subjecting the formed nitrogen-doped amorphous silicon carbide layer to a water plasma treatment or an ammonia plasma treatment to obtain a carbon dioxide reduction catalyst electrode. (4) A method for imparting carbon dioxide reduction activity to nitrogen-doped amorphous silicon carbide, comprising increasing the proportion of oxygen atoms and nitrogen atoms on the surface of the nitrogen-doped amorphous silicon carbide, thereby increasing the number of C=O bonds, C=N bonds of amino groups, and C=N bonds and / or O=CN bonds on the surface of the nitrogen-doped amorphous silicon carbide, thereby imparting carbon dioxide reduction activity to the nitrogen-doped amorphous carbon. (5) A nitrogen-doped amorphous carbon carbon dioxide reduction catalyst having, on its surface, nitrogen atoms and oxygen atoms, as well as C-N bonds, C=O bonds and Si-O bonds of amino groups, wherein the amount of the nitrogen atoms on the surface is 0.7 atom % or more and the amount of nitrogen atoms in the C-N bonds of the amino groups is 0.2 atom % or more, the amount of the oxygen atoms on the surface is 40 atom % or more and the amount of oxygen atoms in the C=O bonds and Si-O bonds is 15 atom % or more. [Effects of the Invention]

[0008] The production method of the present invention makes it possible to produce a carbon dioxide reduction catalyst having photocatalytic activity for carbon dioxide reduction. The carbon dioxide reduction catalyst of the present invention has photocatalytic activity for carbon dioxide reduction and is also capable of producing C2 compounds. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing a cyclic voltammogram of Comparative Example 1. [Figure 2] FIG. 2 is a diagram showing a cyclic voltammogram of Example 1. [Figure 3] FIG. 3 is a diagram showing a cyclic voltammogram of Example 2. [Figure 4] FIG. 4 is a diagram showing the results of evaluation of the electrocatalytic activity by constant-potential electrolysis in Comparative Example 1. [Figure 5] FIG. 5 is a diagram showing the results of evaluating the electrocatalytic activity by constant-potential electrolysis in Example 1. [Figure 6]FIG. 6 is a diagram showing the results of evaluation of the electrocatalytic activity by constant-potential electrolysis in Example 2. [Figure 7] FIG. 7 is a diagram showing the configuration of a photoreaction cell used for evaluating photocatalytic activity. [Figure 8] FIG. 8 is a diagram showing the results of evaluating the photocatalytic activity of Comparative Example 1 (untreated Example 1). [Figure 9] FIG. 9 is a diagram showing the results of evaluating the photocatalytic activity of Example 1 (water plasma treatment). [Figure 10] FIG. 10 is a diagram showing the results of evaluating the photocatalytic activity of Example 2 (ammonia plasma treatment). [Figure 11] FIG. 11 shows the results of evaluating the photocatalytic activity of Examples 2, 3, and 4 (all treated with ammonia plasma), with the upper row showing the results of Example 3, the middle row showing the results of Example 2, and the lower row showing the results of Example 4. [Figure 12] 12(a) to (c) are diagrams showing XPS spectra of Comparative Example 1 (untreated Example 1), where (a) shows the result for C1s, (b) shows the result for N1s, and (c) shows the result for O1s. [Figure 13] 13(a) to (c) are diagrams showing XPS spectra of Example 1 (water plasma treatment), where (a) shows the results for C1s, (b) shows the results for N1s, and (c) shows the results for O1s. [Figure 14] 14(a) to (c) are diagrams showing XPS spectra of Example 2 (ammonia plasma treatment), where (a) shows the results for C1s, (b) shows the results for N1s, and (c) shows the results for O1s. DETAILED DESCRIPTION OF THE INVENTION

[0010] The method for producing a carbon dioxide reduction catalyst of the present invention involves subjecting nitrogen-doped amorphous silicon carbide to water plasma or ammonia plasma treatment to obtain a carbon dioxide reduction catalyst. Nitrogen-doped amorphous silicon carbide is a material obtained by adding a trace amount of nitrogen atoms as an n-type impurity to amorphous silicon carbide, which is an amorphous material composed of silicon atoms and carbon atoms in a given composition ratio. In the present invention, the nitrogen atom content in the nitrogen-doped amorphous silicon carbide before treatment with water plasma or ammonia plasma is preferably 1 to 5 atom%, more preferably 1 to 2 atom%, based on the total carbon and silicon atoms. Furthermore, in the present invention, the silicon atom content in the nitrogen-doped amorphous silicon carbide before treatment with water plasma or ammonia plasma is preferably 25 to 50 atom%, more preferably 40 to 50 atom%, based on the total carbon and silicon atoms. In the present invention, the optical gap of the nitrogen-doped amorphous silicon carbide before treatment with water plasma or ammonia plasma is preferably 2.8 eV or more. From the viewpoint of not only producing oxalic acid by water plasma or ammonia plasma treatment but also re-reducing (and further sequentially reducing) the oxalic acid to obtain glycolic acid or acetic acid, an optical gap of 3.0 eV is preferred. The optical gap of the nitrogen-doped amorphous silicon carbide remains almost unchanged even after water plasma or ammonia plasma treatment. The method for producing the nitrogen-doped amorphous silicon carbide used in the present invention before treatment with water plasma or ammonia plasma is not particularly limited. For example, it can be produced by converting a source gas containing carbon atoms, silicon atoms, and nitrogen atoms into plasma and forming a film. The source gas containing carbon atoms, silicon atoms, and nitrogen atoms can be produced by vaporizing a compound containing at least one of carbon atoms, silicon atoms, and nitrogen atoms, either alone or in combination.Examples of compounds used in the source gas include compounds containing carbon and silicon atoms such as silane compounds such as tetramethylsilane, triethylsilane, and tetraphenylsilane, and hexamethyldisiloxane; compounds containing carbon and nitrogen atoms such as hydrogen cyanide, acetonitrile, ethane cyanide, propane cyanide, formamide, acetamide, methylamine, and ethylamine; and compounds containing carbon, silicon, and nitrogen atoms such as hexamethyldisilazane, triethylsilazane, hexaethyldisilazane, and bis(trimethylsilyl)amine. The nitrogen-doped amorphous silicon carbide of the present invention before treatment with water plasma or ammonia plasma can be produced by, for example, mixing a source gas obtained by vaporizing one or more of these compounds with argon gas as a reaction control gas, generating plasma, and then forming a film.

[0011] In the manufacturing method of the present invention, nitrogen-doped amorphous silicon carbide is subjected to water plasma treatment or ammonia plasma treatment, thereby imparting carbon dioxide reduction activity to the nitrogen-doped amorphous silicon carbide. In particular, photocatalytic activity for carbon dioxide reduction can be imparted to the nitrogen-doped amorphous silicon carbide. The water plasma treatment in this invention refers to a treatment in which the nitrogen-doped amorphous silicon carbide is brought into contact with water plasma, such as exposing the nitrogen-doped amorphous silicon carbide to water plasma or irradiating the nitrogen-doped amorphous silicon carbide with water plasma. The water plasma in this 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. Examples of compounds containing a hydroxyl group include 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 waves, microwaves, and hot filaments. The water plasma treatment in the present invention can be carried out by placing nitrogen-doped amorphous silicon carbide in a plasma generation vessel (chamber), generating water plasma in the chamber, and bringing the nitrogen-doped amorphous silicon carbide into contact with the water plasma.

[0012] In the present invention, the ammonia plasma treatment refers to a treatment in which nitrogen-doped amorphous silicon carbide is brought into contact with ammonia plasma, such as exposing nitrogen-doped amorphous silicon carbide to ammonia plasma or irradiating nitrogen-doped amorphous silicon carbide 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, and can be formed by converting ammonia water, urea water, hydrazine aqueous solution, or the like into a plasma as a raw material. Alternatively, a mixture of these raw materials may be converted into a plasma. In order to increase the efficiency of generating functional groups on the surface of the nitrogen-doped amorphous silicon carbide, a raw material containing ammonia water is preferred, and it is more preferred to use ammonia water alone as the raw material. 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, and 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, and examples thereof include high frequency, microwave, and hot filament. The ammonia plasma treatment in the present invention can be performed, for example, by placing nitrogen-doped amorphous silicon carbide in a plasma generation vessel (chamber), generating ammonia plasma in the chamber, and bringing the ammonia plasma into contact with the nitrogen-doped amorphous silicon carbide.

[0013] In the present invention, water plasma treatment or ammonia plasma treatment increases the proportion of oxygen and nitrogen atoms present on the surface of nitrogen-doped amorphous silicon carbide, thereby increasing the number of C=O bonds, C-N bonds in amino groups, and C=N and / or O=CN bonds on the surface of the nitrogen-doped amorphous silicon carbide. By increasing the amount of oxygen and nitrogen atoms on the surface of nitrogen-doped amorphous silicon carbide and increasing the number of C=O bonds, C-N bonds in amino groups, and C=N and / or O=CN bonds on the surface, carbon dioxide reduction activity can be imparted to nitrogen-doped amorphous silicon carbide that does not have carbon dioxide reduction activity. In particular, photocatalytic activity for carbon dioxide reduction can be imparted. In the present invention, the amounts of various atoms, groups, and bonds on the surface of nitrogen-doped amorphous silicon carbide refer to the amounts on 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 various atoms, groups, and 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 as the amount of oxygen atoms in the C=O bond. The plasma output, treatment time, and concentration of each component in the plasma in the ammonia plasma treatment and water plasma treatment in the present invention can be appropriately adjusted and selected to impart carbon dioxide reduction activity. In the present invention, the amount of oxygen atoms refers to the total amount of oxygen on the surface, including the amount of oxygen atoms derived from C=O bonds and the amount of oxygen atoms other than those derived from C=O bonds. Furthermore, the amount of nitrogen atoms refers to the total amount of nitrogen on the surface, including the amount of nitrogen atoms derived from C=N bonds, C=N bonds, and O=CN bonds of amino groups, as well as the amount of nitrogen atoms other than those derived from these bonds.In the present invention, when producing a carbon dioxide reduction catalyst or imparting carbon dioxide reduction activity, it is preferable to set the nitrogen atom content on the surface of the nitrogen-doped amorphous silicon carbide to 0.7 atom% or more, the nitrogen atom content in the C-N bonds of the amino groups to 0.2 atom% or more, the oxygen atom content on the surface to 40 atom% or more, and the oxygen atom content in the C=O bonds and Si-O bonds to 15 atom% or more. It is also preferable to set the nitrogen atom content in the C=N bonds and O=CN bonds on the surface to 0.2 atom% or more. In the present invention, it is believed that the production of oxalic acid by carbon dioxide reduction can be promoted by increasing the C-N bonds of the amino groups on the surface of the nitrogen-doped amorphous silicon carbide, more preferably by increasing the C=N bonds and O=CN bonds in addition to these, and even more preferably by increasing the N=O bonds in addition to these. It is also believed that the production of glycolic acid by oxalic acid reduction can be promoted by increasing the C=O bonds (carbonyl groups and carboxy groups) on the surface of the nitrogen-doped amorphous silicon carbide. Furthermore, it is believed that an increase in Si-O bonds can promote the production of acetic acid by the reduction of glycolic acid. Furthermore, unlike amorphous carbon, nitrogen-doped amorphous silicon carbide possesses Si-N bonds, which are believed to contribute to the reduction of carbon dioxide, particularly the production of acetic acid. In the present invention, the types and amounts of C2 compounds produced can be controlled by adjusting the amounts of nitrogen and oxygen atoms and each bond on the nitrogen-doped amorphous silicon carbide surface. Furthermore, there are no particular upper limits on the amounts of nitrogen and oxygen atoms and each bond, as long as they can coexist on the surface. In the present invention, it is preferable that the amount of nitrogen atoms on the nitrogen-doped amorphous silicon carbide surface be 2 atom% or more, the amount of nitrogen atoms in C-N bonds of amino groups be 0.9 atom% or more, the amount of oxygen atoms on the surface be 40 atom% or more, and the amount of oxygen atoms in C=O bonds and Si-O bonds be 20 atom% or more. It is also preferable that the amount of nitrogen atoms in C=N bonds and O=CN bonds on the surface be 0.7 atom% or more.Furthermore, the amount of nitrogen atoms in N═O bonds on the surface is preferably 0.2 atom % or more, and more preferably 0.3 atom % or more.

[0014] The carbon dioxide reduction catalyst obtained by the production method of the present invention has both carbon dioxide reduction activity and electrical conductivity, and can be used as a carbon dioxide reduction catalyst electrode, such as a photocatalytic electrode or an electrode for electrolysis, after being formed into a film or molded. The carbon dioxide reduction catalyst electrode of the present invention may be used by forming a carbon dioxide reduction catalyst layer on a substrate to maintain the electrode's strength. Specifically, a carbon dioxide reduction catalyst electrode can be produced by forming a nitrogen-doped amorphous silicon carbide layer on a substrate and then subjecting the formed nitrogen-doped amorphous silicon carbide layer to water plasma treatment or ammonia plasma treatment. The substrate is not particularly limited, but examples include substrates that are electrically conductive and corrosion-resistant, such as silicon wafers, stainless steel, nickel, and noble metals. Methods for forming a nitrogen-doped amorphous silicon carbide layer on a substrate, water plasma treatment, and ammonia plasma treatment can be the same as those described above for the carbon dioxide reduction catalyst. [Example]

[0015] The present invention will be described below with reference to examples, but the present invention is not limited to these specific embodiments.

[0016] [Example 1] (Preparation of nitrogen-doped amorphous silicon carbide) A cathode-coupled radio-frequency plasma-enhanced chemical vapor deposition (RF-PeCVD) system (60.00 MHz, SAMCO Co., Ltd. Model BPD-1) was used as the plasma CVD system. The source gases were tetramethylsilane and hexamethyldisilazane, vaporized at 65°C, in a tetramethylsilane:hexamethyldisilazane molar ratio of 50:4. Nitrogen-doped amorphous silicon carbide (nitrogen-doped a-SiC) films were deposited on silicon wafers mounted on electrodes. The tetramethylsilane and hexamethyldisilazane mixture was supplied at a flow rate of 5 sccm, and hydrogen gas was added at a flow rate of 200 sccm. The chamber pressure was 110 Pa, the plasma power was 5 W, and the deposition time was 80 min. The substrate temperature during the reaction was 200°C. The resulting film thickness was 150 nm, and the deposition rate was 12 nm / min. (Water plasma treatment) After the nitrogen-doped amorphous silicon carbide film was formed, the chamber was evacuated to reduce the internal pressure to 2 Pa or less, and then pure water (ultrapure water: 18 MΩ cm or more) vaporized at room temperature (25°C) was introduced into the chamber, and the pressure inside the chamber was set to 30 Pa by adjusting the exhaust pressure, and plasma was generated. The plasma output was 5 W, and the treatment time was 1 minute. In this way, the nitrogen-doped amorphous silicon carbide inside the chamber was subjected to water plasma treatment, and the water plasma-treated nitrogen-doped amorphous silicon carbide of Example 1 was obtained.

[0017] [Example 2] (ammonia plasma treatment) Nitrogen-doped amorphous silicon carbide was deposited in the same manner as in Example 1, and the chamber was evacuated to reduce the internal pressure to 2 Pa or less. Then, ammonia water (concentration: 28.0 to 30.0 mass %) vaporized at room temperature (25°C) was introduced into the chamber, and the pressure inside the chamber was set to 30 Pa by adjusting the exhaust pressure, and plasma was generated. The plasma output was 5 W, and the treatment time was 1 minute. Thus, the nitrogen-doped amorphous silicon carbide in the chamber was subjected to an ammonia plasma treatment, and the ammonia plasma-treated nitrogen-doped amorphous silicon carbide of Example 2 was obtained.

[0018] [Example 3] Nitrogen-doped amorphous silicon carbide was deposited on a silicon wafer placed on an electrode in the same manner as in Example 1, except that the pressure in the chamber was 95 Pa and the substrate temperature during the reaction was 180°C. The resulting film had a thickness of 150 nm and a deposition rate of 12 nm / min. The deposited nitrogen-doped amorphous silicon carbide was subjected to an ammonia plasma treatment in the same manner as in Example 2, to obtain ammonia-plasma-treated nitrogen-doped amorphous silicon carbide of Example 3.

[0019] [Example 4] Nitrogen-doped amorphous silicon carbide was deposited on a silicon wafer placed on an electrode in the same manner as in Example 1, except that the molar ratio of tetramethylsilane to hexamethyldisilazane used as the raw material gases was 50:1 and the substrate temperature during the reaction was 220°C. The resulting film had a thickness of 150 nm and a deposition rate of 12 nm / min. The deposited nitrogen-doped amorphous silicon carbide was subjected to an ammonia plasma treatment in the same manner as in Example 2, to obtain ammonia-plasma-treated nitrogen-doped amorphous silicon carbide of Example 4.

[0020] [Comparative Example 1] Nitrogen-doped amorphous silicon carbide was formed as a film in the same manner as in Example 1, and nitrogen-doped amorphous silicon carbide of Comparative Example 1 was obtained without performing water plasma treatment or ammonia plasma treatment.

[0021] (Measurement of potential window) The potential window of the nitrogen-doped amorphous silicon carbide obtained in Example 1 before water plasma treatment was measured. The measurement was carried out using a three-electrode cell, with a graphite electrode as the counter electrode, an Ag / AgCl electrode as the reference electrode, and a 0.1 M H2SO4 aqueous solution with carbon dioxide dissolved therein as the electrolyte. The nitrogen-doped amorphous silicon carbide film formed on a silicon wafer was used as the working electrode, and the potential was swept at a rate of 10 mV / s. The potential window of each electrode was measured at -0.5 μA / cm 2The potentials reached at these points are the oxygen evolution potential and hydrogen evolution potential. Since this is an n-type semiconductor electrode, the oxygen evolution potential was not observed, and the hydrogen evolution potential was −1.32 V (vs. Ag|AgCl).

[0022] (cyclic voltammetry) The nitrogen-doped amorphous silicon carbide films of Examples 1 and 2 and Comparative Example 1 were formed on silicon wafers and used as test electrodes. Cyclic voltammetry was performed using a three-electrode beaker cell. A silver-silver chloride electrode (manufactured by DKK-TOA Corporation, HS-250C purity) was used as the reference electrode, and a platinum wire (manufactured by Nilaco Corporation, 99.95% purity) was used as the counter electrode. The electrolyte was a 0.1 M aqueous sodium sulfate solution. The scan range was 0 to −2.4 V vs. Ag|AgCl, and the scan rate was standardized to 25 mV / s, for one cycle. The results for the nitrogen-doped amorphous silicon carbide films of Examples 1 and 2 and Comparative Example 1 are shown in Figures 1 to 3, respectively. Figure 1 shows the results for Comparative Example 1, Figure 2 shows the results for Example 1, and Figure 3 shows the results for Example 2. The dotted circles in Figures 2 and 3 indicate the CO reduction peaks.

[0023] (Electrocatalytic activity evaluation by controlled potential electrolysis) The same three-electrode cell as used for potential window measurements was used. The nitrogen-doped amorphous silicon carbide films of Examples 1 and 2 and Comparative Example 1 were formed on silicon wafers and used as the test electrode (working electrode). A silver-silver chloride electrode (manufactured by DKK-TOA Corporation, HS-250C purity) was used as the reference electrode, and a platinum wire (manufactured by Nilaco Corporation, 99.95% purity) was used as the counter electrode. A 0.1 M aqueous sodium sulfate solution was used as the electrolyte. The electrocatalytic activity was evaluated by constant-potential electrolysis at a potential of −2.4 V vs. Ag|AgCl for 12 hours. The results for the nitrogen-doped amorphous silicon carbide films of Examples 1 and 2 and Comparative Example 1 are shown in Figures 4 to 6, respectively. Figure 4 shows the results for Comparative Example 1, Figure 5 shows the results for Example 1, and Figure 6 shows the results for Example 2.

[0024] (Photocatalytic activity evaluation) A photoreaction cell as shown in FIG. 7 was constructed, and a photocatalytic activity evaluation test was conducted on the nitrogen-doped amorphous silicon carbide samples of Examples 1 to 4 and Comparative Example 1. The test consisted of immersing each nitrogen-doped amorphous silicon carbide sample in ultrapure water (resistivity 18 MΩcm or higher) saturated with CO2. The nitrogen-doped amorphous silicon carbide sample was then irradiated with ultraviolet light from a mercury-xenon lamp (USHIO SP-2, center wavelength 365 nm) at an output of 1.0 W for 6 hours from the direction of the nitrogen-doped amorphous silicon carbide surface. The distance from the tip of the lamp to the nitrogen-doped amorphous silicon carbide surface was 10 mm. A carbon dioxide reduction reaction occurs on the front side of the nitrogen-doped amorphous silicon carbide, while a water oxidation reaction (oxygen generation reaction) occurs on the silicon on the back side. However, since the oxygen generation reaction is difficult to occur on silicon, gold was vapor-deposited on the surface of the silicon wafer opposite the surface on which the nitrogen-doped amorphous silicon carbide film was formed to facilitate this reaction. After 6 hours, a sample solution was collected from the solution, and products in the sample solution were detected by high-performance liquid chromatography (HPLC). The results are shown in Figures 8 to 11. Figure 8 shows the results for Comparative Example 1 (same as untreated Example 1), Figure 9 shows the results for Example 1 (water plasma treatment), Figure 10 shows the results for Example 2 (ammonia plasma treatment), and Figure 11 shows the results for Example 3 (ammonia plasma treatment) in the top row, Example 2 (ammonia plasma treatment) in the middle row, and Example 4 (ammonia plasma treatment) in the bottom row.

[0025] (X-ray photoelectron spectroscopy measurement) (1) The nitrogen-doped amorphous silicon carbide film formed in Comparative Example 1 (the same as the nitrogen-doped amorphous silicon carbide before the water plasma treatment in Example 1) and the nitrogen-doped amorphous silicon carbide films formed in Examples 3 and 4 before the ammonia plasma treatment were subjected to Ar + After etching with ions (3000 eV) for 5 seconds, X-ray photoelectron spectroscopy measurements were performed. (2) The nitrogen-doped amorphous silicon carbide after water plasma treatment obtained in Example 1, the nitrogen-doped amorphous silicon carbide after ammonia plasma treatment obtained in Example 2, and the nitrogen-doped amorphous silicon carbide of Comparative Example 1 were analyzed using Ar +X-ray photoelectron spectroscopy measurements were performed without ion etching. The X-ray photoelectron spectroscopy measurements (1) and (2) were performed using an X-ray photoelectron spectroscopy measurement device (Model K-Alpha TM + X-ray Photoelectron Spectrometer System (manufacturer: Thermo Scientific) was used, and Al Kα (1468.6 eV) was used as the X-ray source. The angle between the sample surface and the X-ray detector was set to 90°, and the scanning speed was 1 eV min -1 The binding energy resolution in the measurements was 0.5 eV.

[0026] (Optical gap measurement) The optical gaps of the nitrogen-doped amorphous silicon carbide films formed in Comparative Example 1 (same as the nitrogen-doped amorphous silicon carbide before water plasma treatment in Example 1) and the nitrogen-doped amorphous silicon carbide films formed in Examples 3 and 4 before ammonia plasma treatment were measured by UV absorption spectroscopy (UV-visible absorptiometer, JASCO Corporation, V-670, automatic absolute reflectance measurement unit, ARMN-735). A Taucplot was created from the obtained transmission and reflection spectra, and the optical gap value was calculated from the intersection with the x-axis. For the optical gap measurement, a glass substrate was used instead, and nitrogen-doped amorphous silicon carbide films before plasma treatment were formed in the same manner as in Comparative Example 1 and Examples 3 and 4. Transmission spectra (in the light energy range of 0.6 to 4 eV) were measured to measure the extinction coefficient of the thin film. Subsequently, reflection spectra were measured to calculate the reflection rate at the thin film surface and subtract the reflection rate contained in the absorption spectrum. The transmittance T and reflectance R of the two spectra obtained are substituted into the Tauc formula below (where d is the thickness of the thin film) to calculate α, (αhν) 1 / 2 A Taucplot was created by converting wavelength into light energy on the horizontal axis and the vertical axis. α=-ln(T / (1-R))×10 8 / d The energy value at the intersection of the extrapolated line of the obtained plot and the horizontal axis was taken as the optical gap value.

[0027] The optical gap changes depending on the composition of the material itself (inside the material). + After etching with ions (3000 eV) for 5 seconds, X-ray photoelectron spectroscopy was performed. The results showed that the optical gap of Comparative Example 1 (untreated Example 1) was 3.0 eV, the optical gap of Example 3 before ammonia plasma treatment (untreated Example 3) was 3.2 eV, and the optical gap of Example 4 before ammonia plasma treatment (untreated Example 4) was 2.8 eV. Table 1 shows the abundance ratio of elements in each bonding state. The composition ratio Si / (Si+C) was 48.4 atm% for Comparative Example 1 (untreated Example 1), 49.5 atm% for untreated Example 3, and 44.2 atm% for untreated Example 4.

[0028] [Table 1]

[0029] In order to observe the surface activity, in (2) above, the film surface was exposed to Ar + X-ray photoelectron spectroscopy (XPS) measurements were performed without etching with ions. Tables 2 to 4 show the abundance ratios of elements in each bonding state measured. The obtained XPS spectra are shown in Figures 12 to 14, respectively. Figure 12 shows the results for Comparative Example 1 (untreated Example 1), Figure 13 shows the results for the nitrogen-doped amorphous silicon carbide obtained in Example 1 after water plasma treatment (Example 1 (water plasma treatment)), and Figure 14 shows the results for the nitrogen-doped amorphous silicon carbide obtained in Example 2 after ammonia plasma treatment (Example 2 (ammonia plasma treatment)). In Figures 12 to 14, (a) shows the results for C1s, (b) shows the results for N1s, and (c) shows the results for O1s, respectively.

[0030] [Table 2]

[0031] [Table 3]

[0032] [Table 4]

[0033] [Table 5]

[0034] As can be seen from Figures 1 to 3 and 4 to 6, the nitrogen-doped amorphous silicon carbide of Comparative Example 1, which was not treated with water plasma or ammonia plasma, did not exhibit catalytic activity for electrolytic carbon dioxide reduction, whereas the nitrogen-doped amorphous silicon carbide of Example 1, which was treated with water plasma, and Example 2, which was treated with ammonia plasma, exhibited catalytic activity for electrolytic carbon dioxide reduction. In particular, the generation of glycolic acid was observed in Example 1, which was treated with water plasma. As can be seen from Figures 8 to 11, the nitrogen-doped amorphous silicon carbide of Comparative Example 1, which was not treated with water plasma or ammonia plasma, exhibited such low photocatalytic activity for carbon dioxide reduction that no clear generation of oxalic acid was observed. However, the nitrogen-doped amorphous silicon carbide of Example 1, which was treated with water plasma, and Examples 2 to 4, which were treated with ammonia plasma, exhibited photocatalytic activity for carbon dioxide reduction. The generation of glycolic acid was significant in Example 1, which was treated with water plasma, and the generation of oxalic acid and acetic acid was significant in Example 2, which was treated with ammonia plasma, although glycolic acid was also produced. Both the nitrogen-doped amorphous silicon carbide photocatalysts treated with water plasma and the ammonia plasma have photocatalytic activity to reduce carbon dioxide and produce oxalic acid. -It is believed that the generation of radicals contributes to the generation of oxalic acid. Tables 2 to 5 and Figures 12 to 14 show that, on the surface of the nitrogen-doped amorphous silicon carbide of Example 1 that had been water plasma-treated, the intensities of the XPSC1s peaks near C=O increased by 2.7 times for C=O and 2.9 times for C(=O)OR, compared to before the treatment. For the XPSN1s peaks, the intensities of the C=N bond and the C=N bond and the O=CN bond increased by 3.3 times and 2.7 times, respectively (Table 4). Furthermore, for the XPSO1s peaks, the intensities of the aliphatic C=O and Si-O (aliphatic C=O / Si-O) increased by 1.4 times (Table 5). Tables 2 to 5 and Figures 12 to 14 show that, on the surface of the nitrogen-doped amorphous silicon carbide of Example 2 treated with ammonia plasma, the intensities of the XPSC1s peaks near C=O increased by 2.2 times for C=O and 2.7 times for C(=O)OR compared to before treatment. In the XPSN1s peak, the intensities of the C=O bond increased by 13.1 times, and the C=N bond and O=CN bond increased by 7.4 times (Table 4). In the XPSO1s peak, the intensities of the aliphatic C=O and Si-O (aliphatic C=O / Si-O) increased by 1.7 times (Table 5). The C=O bond in this invention refers to the combination of C=O and C(=O)OR in the XPSC1s peak and the aliphatic C=O in the XPS O1s peak. The amount of C=O bonds in the amino group in this invention is expressed by the amount of C=O bonds in the XPS N1s peak, which are C=O bonds in the amino group. In the present invention, the C=N bond and O=CN bond refer to the C=N and O=CN in the XPS N1s peak. The Si-O bond in the present invention refers to the Si-O of aliphatic C=O and Si-O in the XPS N1s peak. The N=O bond in the present invention refers to the N=O in the XPS N1s peak. From the results of cyclic voltammetry, product measurement after electrolysis, and product measurement after evaluation of photocatalytic activity, it is presumed that the C=O groups (carbonyl groups and carboxy groups) introduced onto the surface act to promote the reduction of oxalic acid to glycolic acid, and functional groups containing oxygen or nitrogen (the C=N bond of an amino group, the C=N bond, the O=CN bond, and the N=O bond) act to promote the reduction of CO2 -It is thought that this stabilizes the radicals and promotes the production of oxalic acid. Si-containing functional groups (Si-O groups or Si-N groups) are also thought to promote the reduction of glycolic acid to acetic acid. Figure 11 shows that oxalic acid was produced when the optical gap was 2.8 eV, and when the optical gap was 3.0 eV or higher, it was possible to produce glycolic acid and acetic acid in addition to oxalic acid. [Industrial Applicability]

[0035] The carbon dioxide reduction catalyst and carbon dioxide reduction catalyst electrode produced by the production method of the present invention are resistant to inhibition by the hydrogen production reaction and have excellent durability, and therefore can be suitably used as a catalyst for carbon dioxide reduction using light. For example, they can be used in the synthesis of C2 compounds such as oxalic acid, glycolic acid, and acetic acid by fixation of CO2 using a photocatalyst.

Claims

1. A method for producing a carbon dioxide reduction catalyst, comprising subjecting nitrogen-doped amorphous silicon carbide to a water plasma treatment or an ammonia plasma treatment to obtain the carbon dioxide reduction catalyst.

2. 2. The method for producing a carbon dioxide reduction catalyst according to claim 1, wherein the carbon dioxide reduction catalyst is a photocatalyst.

3. A method for producing a carbon dioxide reduction catalyst electrode, comprising forming a nitrogen-doped amorphous silicon carbide layer on a substrate and subjecting the formed nitrogen-doped amorphous silicon carbide layer to a water plasma treatment or an ammonia plasma treatment to obtain a carbon dioxide reduction catalyst electrode.

4. A method for imparting carbon dioxide reduction activity to nitrogen-doped amorphous silicon carbide, comprising increasing the proportion of oxygen atoms and nitrogen atoms on the surface of the nitrogen-doped amorphous silicon carbide, thereby increasing the number of C═O bonds, C—N bonds of amino groups, and C═N bonds and / or O═C—N bonds on the surface of the nitrogen-doped amorphous silicon carbide, thereby imparting carbon dioxide reduction activity to the nitrogen-doped amorphous carbon.

5. A nitrogen-doped amorphous carbon carbon dioxide reduction catalyst having, on its surface, nitrogen atoms and oxygen atoms, as well as C-N bonds, C=O bonds and Si-O bonds of amino groups, wherein the amount of the nitrogen atoms on the surface is 0.7 atom % or more and the amount of nitrogen atoms in the C-N bonds of the amino groups is 0.2 atom % or more, the amount of the oxygen atoms on the surface is 40 atom % or more and the amount of oxygen atoms in the C=O bonds and Si-O bonds is 15 atom % or more.

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