Tungsten oxide, catalyst, and method for producing tungsten oxide

Abstract

The present invention addresses the problem of providing a compound that is usable as an ammonia oxidation catalyst and has high catalytic activity. A tungsten oxide represented by NixCu1-xWO4 (where 0<x<1). A catalyst containing the tungsten oxide.

Description

Tungsten Oxide, Catalyst, and Method for Producing Tungsten Oxide

[0001] The present invention relates to tungsten oxide represented by Ni ｘ Cu １－ｘ WO ４ (where 0 < x < 1), a catalyst containing the same, a method for producing tungsten oxide, an electrolytic cell in which tungsten oxide is supported as a catalyst, and a method for producing hydrogen.

[0002] In recent years, in order to solve problems such as global warming caused by the greenhouse effect of carbon dioxide, methods for producing hydrogen using renewable energy have attracted attention. As such methods, electrolysis of water (electrolysis), electrolysis of ammonia (electrolysis), etc. are cited and development is underway. Ammonia (NH ３ ) has been used as a chemical raw material including fertilizers, but since it stores 17.8% by mass of hydrogen in its molecule and serves as a hydrogen carrier, recently, its use in the energy field has begun to be studied. Ammonia is already widely used as liquefied ammonia, and since storage / transport technology and safety measures have been established, technology for storing and transporting hydrogen in the form of ammonia once and converting it to hydrogen at the place of use has attracted attention. If ammonia can be decomposed into hydrogen and nitrogen using electricity from renewable energy, zero emissions can be achieved. The decomposition reaction of ammonia by electrolysis is represented by the following equation, and only harmless nitrogen and hydrogen are generated. Also, thermodynamically, less energy (ΔE ０ = 0.06 V) is required compared to electrolysis of water (ΔE ０ = 1.23 V). (Anode) NH ３ (aq) + 3OH － → 1 / 2N ２ + 3H ２ O + 3e － E ０ = -0.77 V vs SHE (Cathode) 3H ２ O + 3e － → 3 / 2H ２ + 3OH － E ０ = -0.83 V vs SHE (Overall) NH ３ (aq) → 1 / 2N ２+3 / 2H ２ ΔE ０ = 0.06V

[0003] In the electrolysis of ammonia, hydrogen is generated at the cathode, while the ammonia oxidation reaction (AOR) takes place at the anode. The overall energy efficiency is determined by the ammonia oxidation, and a catalyst is needed to carry out ammonia oxidation quickly. Platinum-based materials are catalysts that show high activity for ammonia oxidation, but they are expensive and have problems such as being easily poisoned by adsorbed species or producing oxygen-containing nitrogen species as by-products. Therefore, there is a strong demand for the development of a catalyst that is abundant on Earth, highly active, and robust. As an ammonia oxidation catalyst to replace platinum-based materials, a material in which nickel ions and copper ions coexist between layers of multilayer manganese dioxide has been proposed (see Non-Patent Literature 1), but the development of further ammonia oxidation catalysts was desired.

[0004] Tungsten is a metal that is less expensive and more abundant than precious metals, and the use of tungsten compounds as catalysts for oxygen evolution reactions (OER) is being investigated. １－ｘ Fe ｘ WO ４ A composite of Ni and carbon nanotubes (CNTs) (Non-Patent Literature 2), a composite of Ni-Fe-W hydroxide and carbon fiber (Non-Patent Literature 3), Ni ｘ Fe １－ｘ WO ４ (Patent Document 1) and others have been proposed, but all of them relate to catalysts for oxygen evolution reactions in the electrolysis of water, etc. Moreover, the ones proposed in Non-Patent Documents 2 and 3 were composites with carbon nanotubes or carbon fibers.

[0005] Patent No. 7519642

[0006] K. Nagita, Y. Yuhara, K. Fujii, Y. Katayama, M. Nakayama, “Ni- and Cu-co-Intercalated Layered Manganese Oxide for Highly Efficient Electro-Oxidation of Ammonia Selective to Nitrogen”, ACS Appl. Mater. Interfaces, 13, 28098-28107 (2021)Composite Metal Oxide-Carbon Nanotube Electrocatalysts for the Oxygen Evolution and Oxygen Reduction Reactions,ChemElectroChem, 5, 2850-2856(2018)Jie Xu, Mingshuo Wang, FeiYang, Xiaoqian Ju,Xilai Jia, “Self-Supported Porous Ni-Fe-W HydroxideNanosheets on Carbon Fiber: A Highly Efficient Electrode for Oxygen EvolutionReaction” , Inorg. Chem. 58, 13037-13048 (2019).

[0007] The object of the present invention is to provide a compound with high catalytic activity that can be used as an ammonia oxidation catalyst.

[0008] The inventors investigated new compounds with high catalytic activity that can be used as ammonia oxidation catalysts, and found that Ni ｘ Cu １－ｘ WO ４ (However, we found that tungsten oxides represented by 0 < x < 1 have very high catalytic activity. Conventionally, MWO ４ While there were documents describing various metal elements as M in the compound represented by , no compound in which M is a combination of Ni and Cu was known, and furthermore, it was not known that this compound could obtain very high catalytic activity. ｘ Cu１－ｘ WO ４ (However, tungsten oxides represented by 0 < x < 1) exhibit particularly excellent catalytic activity in ammonia oxidation reactions, but also excellent catalytic activity in oxygen evolution reactions such as the electrolysis of water.)

[0009] In other words, the present invention is defined by the following: (1) Ni ｘ Cu １－ｘ WO ４ (However, a tungsten oxide represented by 0 < x < 1.) (2) A catalyst containing the tungsten oxide of (1) above. (3) The catalyst of (2) above, which is a catalyst for ammonia oxidation reaction or an oxygen evolution reaction. (4) A Ni that synthesizes tungsten oxide by dissolving tungstate salts, nickel salts and copper salts in an organic solvent and heating the solution in which each of the salts is dissolved. ｘ Cu １－ｘ WO ４ (However, a method for producing tungsten oxide represented as 0 < x < 1) (5) The method for producing tungsten oxide according to (4) above, wherein the organic solvent is a polyol (6) An electrolytic cell comprising an anode chamber and a cathode chamber separated by an ion-permeable diaphragm, wherein an anode is placed in the anode chamber and a cathode is placed in the cathode chamber, wherein the anode is Ni ｘ Cu １－ｘ WO ４ (However, an electrolytic cell on which a tungsten oxide represented by 0 < x < 1 is supported as a catalyst.) (7) A method for producing hydrogen, wherein water containing ammonia is supplied to the anode chamber and water containing alkali metal hydroxide is supplied to the cathode chamber of the electrolytic cell of (6) above, and electrolysis is performed, thereby oxidative decomposition of ammonia in the anode chamber and hydrogen is generated in the cathode chamber.

[0010] The tungsten oxide of the present invention exhibits excellent catalytic activity, particularly in the ammonia oxidation reaction. Furthermore, it provides a catalyst that is less expensive than precious metal catalysts such as platinum.

[0011] Figure 1 shows the XRD pattern of the sample obtained in Example 3. Figure 2 shows the XRD patterns of the samples obtained in Examples 1 to 5 and Comparative Examples 1 and 2. Figure 3 shows the linear sweep voltammograms of the samples obtained in Examples 1 to 5 and Comparative Examples 1 and 2. Figure 4 shows the Tafel plots of the samples obtained in Examples 1 to 5 and Comparative Examples 1 and 2. Figure 5 shows the XRD patterns of the samples obtained in Examples 3, 6 and 7. Figure 6 shows the linear sweep voltammograms of the samples obtained in Example 3 and Comparative Examples 1 and 2. Figure 7 shows the linear sweep voltammograms of the samples obtained in Examples 1, 2, 4 and 5. Figure 8 shows one embodiment of the electrolytic cell of the present invention. Figure 9 shows one embodiment of the electrolytic cell of the present invention. Figure 10 shows one embodiment of the electrolytic cell of the present invention. Figure 11 shows one embodiment of the electrolytic cell of the present invention.

[0012] The tungsten oxide of the present invention is Ni ｘ Cu １－ｘ WO ４ (However, the compound is represented by the chemical formula 0 < x < 1.) x is preferably 0.05 to 0.95, preferably 0.10 to 0.90, preferably 0.15 to 0.85, preferably 0.2 to 0.8, and more preferably 0.4 to 0.6. Conventionally, platinum and the like have been used as ammonia oxidation catalysts, but the tungsten oxide of the present invention has a catalytic effect on the ammonia oxidation reaction without using precious metals such as platinum, making it cost-effective. Furthermore, these metals have toxicity issues, but the tungsten oxide of the present invention does not have toxicity issues, making it highly safe. Conventionally, tungsten oxides with catalytic activity have included Wolframite-type tungsten oxides such as cobalt and iron-containing Co １－ｘ Fe ｘ WO ４ (However, 0 < x < 1 and Ni ｘ Fe １－ｘ WO ４ (However, it is known that 0 < x < 1, Ni ｘ Cu １－ｘ WO ４(However, tungsten oxides represented by 0 < x < 1 were not known. Moreover, conventionally known tungsten oxides could be used as catalysts for oxygen evolution reactions, and none were known to exhibit catalytic activity as an ammonia oxidation catalyst. The present invention relates to Ni ｘ Cu １－ｘ WO ４ (However, a tungsten oxide represented by 0 < x < 1 was synthesized, and it was found that this compound is excellent as a catalyst for ammonia oxidation reactions. Furthermore, it was found that the tungsten oxide of the present invention can also be used as a catalyst for oxygen evolution reactions. In the present invention, wolframite includes low-crystallinity wolframite and wolframite precursors that become wolframite upon heat treatment. The tungsten oxide of the present invention is preferably low-crystallinity wolframite or a wolframite precursor that becomes wolframite upon heat treatment. Suitable crystallite sizes for the tungsten oxide of the present invention include 15.0 nm to 40.0 nm and 20.0 nm to 35.0 nm.

[0013] The catalyst of the present invention is Ni ｘ Cu １－ｘ WO ４ (However, the catalyst of the present invention is represented by 0 < x < 1) and contains a tungsten oxide. The catalyst of the present invention may consist only of the tungsten oxide of the present invention, or it may contain other compounds in a range that has catalytic activity. It may also be supported on a carrier such as nickel foam, carbon material, or metal plate. The catalyst of the present invention can be used as a catalyst for ammonia oxidation reactions, a catalyst for oxygen evolution reactions, etc. For example, it can be used as a catalyst for ammonia oxidation reactions in ammonia electrolysis, a catalyst for oxygen evolution reactions in electrolysis (electrolysis), batteries, etc., and can be used as a catalyst for anodes or positive electrodes such as the anode in ammonia electrolysis, the anode in water electrolysis, the air electrode (positive electrode) in metal-air batteries, and the counter electrode in reduction reactions in carbon dioxide electrolysis.

[0014] The method for producing tungsten oxide according to the present invention is not particularly limited, but for example, one method is to dissolve a tungstate, nickel salt, and copper salt in an organic solvent and heat the solution in which each salt is dissolved to synthesize tungsten oxide. The organic solvent is not particularly limited as long as it can dissolve the tungstate, nickel salt, and copper salt used, but examples include alcohols, ethers, and polyols. Among these, the polyol method, which uses a polyhydric alcohol called a polyol as the organic solvent, is preferred. The polyol method is a method of obtaining the desired product by dissolving the raw material salt in a polyol and heating it. The polyol method includes the steps of dissolving various raw materials in a polyol and heating the polyol solution obtained in the above step, but when producing tungsten oxide according to the present invention by the polyol method, the polyol used is not particularly limited, and examples include ethylene glycol, propylene glycol, tetraethylene glycol, trimethylene glycol, tetramethylene glycol, diethylene glycol, dipropylene glycol, polyethylene glycol, etc. The salt to be dissolved in the polyol is a salt containing at least one of nickel, copper, and tungsten, which are constituent components of the tungsten oxide of the present invention. It is not particularly limited as long as it dissolves in the polyol used, and a combination of these salts is used to dissolve them so that the three components are contained in the polyol. Examples of tungsten sources include tungstate salts. Examples of tungstate salts include sodium tungstate, ammonium tungstate, and calcium tungstate. Examples of nickel and copper sources include acetates, sulfates, nitrates, and chlorides, respectively.

[0015] While there are no particular restrictions on the heating temperature in the polyol method, a temperature near or below the boiling point of the polyol used as the solvent is preferred. Furthermore, while there are no particular restrictions on the heating method, refluxing at a temperature near the boiling point of the polyol is preferred because it allows for the greatest amount of heat to be applied at atmospheric pressure during the synthesis reaction. The heating time can be appropriately selected to allow sufficient time for the synthesis reaction to proceed. For example, a nickel-containing salt, a copper-containing salt, and a tungsten-containing salt are dissolved in the polyol. Water may be added as appropriate, and the pH may be adjusted as needed. This solution is then heated under reflux. The heating temperature at this time varies depending on the type of polyol used, the amount of water added to the polyol, etc., but any temperature at which the solution can reflux is acceptable. The heating time is not particularly limited as long as sufficient time for the synthesis reaction to proceed, but examples include 30 minutes to 3 hours, 30 minutes to 2 hours, etc. After heating, the temperature of the solution is lowered to room temperature, and the solid components are recovered by separation operations such as centrifugation to obtain the synthesized tungsten oxide of the present invention. By using a polyol as a solvent, the polyol is thought to act as a protective agent on the surface of the generated tungsten oxide particles, preventing the growth of catalyst particles due to aggregation, thus enabling highly catalytically active Ni ｘ Cu １－ｘ WO ４ (However, a tungsten oxide represented by 0 < x < 1 can be obtained.)

[0016] The electrolytic cell of the present invention comprises an anode chamber and a cathode chamber separated by an ion-permeable diaphragm, wherein an anode is disposed in the anode chamber and a cathode is disposed in the cathode chamber, and the anode is Ni ｘ Cu １－ｘ WO ４(However, the tungsten oxide of the present invention, represented by 0 < x < 1, is supported as a catalyst.) The ion-permeable diaphragm in the present invention is not particularly limited as long as it is an ion-permeable diaphragm that can be used in an electrolytic cell for electrolysis of aqueous solutions, etc. Examples include porous membranes made of asbestos or modified asbestos, porous membranes using polysulfone polymers, cloths using polyphenylene sulfide fibers, fluorine-based porous membranes, porous membranes using hybrid materials containing both inorganic and organic materials, and ion exchange membranes such as fluorine-based ion exchange membranes. The ion-permeable diaphragm in the present invention preferably has low gas permeability, low electrical conductivity, and high strength.

[0017] In the present invention, the anode is provided with the tungsten oxide of the present invention supported as a catalyst on a conductive substrate. The conductive substrate is not particularly limited as long as it is a substrate that can be used as an electrode in electrolysis, and examples include nickel, nickel alloys, titanium, titanium alloys, nickel iron, vanadium, molybdenum, copper, silver, manganese, platinum group elements, graphite, or chromium, or combinations thereof. The conductive substrate may be rigid or flexible. Examples of rigid conductive substrates include expanded metal and punched metal, and examples of flexible conductive substrates include wire mesh woven (or braided) with metal wire. The method and amount of the tungsten oxide of the present invention supported on the conductive substrate are not particularly limited as long as the tungsten oxide of the present invention can come into contact with the electrolyte and function as a catalyst, and examples of the supporting method include coating all or part of the surface of the conductive substrate, or adhering all or part of the surface of the conductive substrate. The cathode in this invention is not particularly limited as long as it is a substrate usable as an electrode in electrolysis, but it usually comprises a conductive substrate and a catalyst layer supported on the surface of the substrate. The conductive substrate is not particularly limited as long as it is a substrate usable as an electrode in electrolysis, and examples include nickel, nickel alloy, stainless steel, mild steel, or stainless steel or mild steel with nickel plating on the surface. The conductive substrate may be a rigid substrate or a flexible substrate. Examples of rigid conductive substrates include expanded metal and punched metal, and examples of flexible conductive substrates include wire mesh woven (or braided) with metal wire. Examples of the catalyst layer of the cathode include a catalyst layer made of a noble metal or noble metal oxide, nickel, cobalt, molybdenum or manganese, or oxides thereof. The anode and cathode are arranged in the anode chamber and cathode chamber of this invention, respectively.

[0018] In the electrolytic cell of the present invention, when electrolyzing ammonia, for example, water containing ammonia is supplied to the anode chamber and water containing alkali metal hydroxide is supplied to the cathode chamber, generating nitrogen in the anode chamber and hydrogen in the cathode chamber. In addition, in the electrolytic cell of the present invention, when electrolyzing water, water containing an electrolyte is supplied to both the anode and cathode chambers and electrolysis is performed, generating oxygen in the anode chamber and hydrogen in the cathode chamber. Figure 8 is a schematic diagram showing the configuration of the electrolytic cell of the present invention. The tungsten oxide of the present invention is supported on the anode. The left side of the diaphragm is the anode chamber, and the right side is the cathode chamber. The anode is located in the anode chamber and the cathode is located in the cathode chamber. In Figure 8, the anode and cathode are located at the ends of the anode and cathode chambers, respectively, but they may be located at positions other than the ends, for example, near the center. When electrolyzing ammonia in the electrolytic cell of the present invention, for example, water containing ammonium chloride is supplied to the anode chamber and water containing sodium hydroxide is supplied to the cathode chamber. The diaphragm is an anion-permeable membrane, allowing hydroxide ions to move from the cathode chamber to the anode chamber. At the anode, ammonia is oxidized and decomposed to produce nitrogen, while at the cathode, hydrogen is produced.

[0019] When electrolyzing water in the electrolytic cell of the present invention, for example, water containing NaCl and KOH is supplied to the anode chamber, and water containing NaCl is supplied to the cathode chamber. The diaphragm is an anion-permeable membrane, and OH －The hydrogen moves from the cathode chamber to the anode chamber. Oxygen is generated near the anode, and hydrogen is generated near the cathode. Using the electrolytic cell of the present invention, brine containing alkali can be supplied to the anode chamber and brine can be supplied to the cathode chamber to electrolyze the brine. Here, alkali refers to a compound that dissolves in water and exhibits basicity, and examples include alkali metal hydroxides and alkaline earth metal hydroxides. Figure 9 shows an example in which KOH is used as the alkali, but other than KOH, for example, NaOH, LiOH, CsOH, etc. can be used. Brine refers to an aqueous solution containing NaCl. The electrolytic cell of the present invention can also perform alkaline water electrolysis by supplying alkali-containing water without NaCl to the anode chamber and cathode chamber and electrolyzing the supplied water, or it can perform alkaline brine electrolysis by supplying water containing NaCl and alkali as described above and electrolyzing the supplied water.

[0020] Furthermore, in other embodiments of the electrolytic cell of the present invention, in addition to the anode chamber, diaphragm, and cathode chamber, a gas diffusion layer for supplying carbon dioxide to the cathode may be provided to reduce carbon dioxide. Figure 10 is a schematic diagram showing the configuration of such an electrolytic cell. The anode is supported with the tungsten oxide of the present invention. In Figure 10, a composite cathode is formed by the cathode, a carbon dioxide reduction catalyst on the cathode surface, and an anion exchange membrane. A gas diffusion layer is provided on the side of the cathode opposite to the anode, and carbon dioxide reaches the cathode and the catalyst on the cathode through this gas diffusion layer, reducing the carbon dioxide to carbon monoxide. An aqueous KOH solution is supplied to the anode chamber, and oxygen is generated near the anode. In this embodiment, the composite cathode also serves as the cathode chamber. The electrolytic cell of the present invention performs reduction of alkaline water CO by electrolysis while electrolyzing alkaline water. ２Electrolysis can be performed. In addition, in other forms of the electrolytic cell of the present invention, in addition to the anode chamber, diaphragm, and cathode chamber, a carbon dioxide introduction section may be provided on the opposite side of the cathode chamber from the anode chamber to introduce carbon dioxide so that it comes into contact with the cathode, and the reduction of carbon dioxide may be performed in the carbon dioxide introduction section. Figure 11 is a schematic diagram showing the configuration of such an electrolytic cell. In Figure 11, the anode chamber is provided on the left side of the diaphragm and the cathode chamber is provided on the right side, but a carbon dioxide introduction section is provided on the right side of the cathode chamber, that is, on the opposite side of the cathode chamber from the anode chamber. The carbon dioxide introduced here comes into contact with the cathode and is reduced to carbon monoxide. The carbon dioxide introduction section is not particularly limited as long as carbon dioxide can be introduced so that it comes into contact with the cathode, for example, a structure that provides a passage for carbon dioxide to flow, a structure that provides a gas diffusion layer, etc. Water containing NaCl and NaOH is supplied to the anode chamber, and water containing NaCl is supplied to the cathode chamber, so that oxygen is generated near the anode supporting the tungsten oxide of the present invention and hydrogen is generated near the cathode. The electrolytic cell of the present invention performs reduction of alkaline salt water by electrolysis of carbon dioxide. ２ Electrolysis can be performed. The electrolytic cells and electrolytic methods using them shown in Figures 10 and 11 utilize the excellent oxygen evolution reaction activity of the tungsten oxide catalyst of the present invention, and activate the carbon dioxide reduction reaction by improving the activity of the entire reaction system with the driving force from the oxygen evolution reaction. The above is just one example of reducing carbon dioxide to carbon monoxide, but the present invention is not limited to this. For example, other substances produced by reducing carbon dioxide include formic acid (HCOOH) and methane (CH₂). ４ ), methanol (CH ３ OH), ethane (C ２ H ６ ), ethylene (C ２ H ４ ), ethanol (C ２ H ５ OH), formaldehyde (HCH), acetaldehyde (CH) ３ CHO), acetic acid (CH ３ COOH) Ethylene Glycol (HOCH) ２ CH ２OH), 1-propanol (CH ３ CH ２ CH ２ OH), and other carbon compounds can be mentioned.

[0021] Hereinafter, the present invention will be specifically described with reference to examples of the present invention, but the technical scope of the present invention is not limited to these examples.

[0022] [Example 1] 25 mL of diethylene glycol was placed in a beaker and adjusted to pH 5.5 with hydrochloric acid diluted with distilled water. After heating this solution to 80 °C, 0.25 g of nickel(II) acetate tetrahydrate and 0.97 g of copper(II) nitrate trihydrate were added, and it was strongly stirred using a magnetic stirrer until it became uniform. The solution in the beaker was transferred to a four-neck flask, and a solution prepared by dissolving 1.67 g of sodium tungstate dihydrate in 2.5 mL of distilled water was added, and the temperature was raised to 150 °C within 15 to 20 minutes. This solution was refluxed at 150 °C for 1 hour while being strongly stirred. After reflux, it was naturally cooled to room temperature. Acetic acid and ethanol were added to the obtained mixed solution, and centrifugation was performed several times. Then, only distilled water was added, and centrifugation was performed several times. The residue was vacuum dried at room temperature for 5 hours to obtain a wolframite-type tungstate (Ni ０．２ Cu ０．８ WO ４ ) incorporating nickel and copper in a ratio of 2:8.

[0023] [Example 2] The treatment was carried out in the same manner as in Example 1, except that the addition amount of nickel(II) acetate tetrahydrate was changed to 0.50 g and the addition amount of copper(II) nitrate trihydrate was changed to 0.72 g, to obtain a wolframite-type tungstate (Ni ０．４ Cu ０．６ WO ４ ) incorporating nickel and copper in a ratio of 4:6.

[0024] [Example 3] The treatment was carried out in the same manner as in Example 1, except that the addition amount of nickel(II) acetate tetrahydrate was changed to 0.62 g and the addition amount of copper(II) nitrate trihydrate was changed to 0.60 g, to obtain a wolframite-type tungstate (Ni ０．５ Cu ０．５ WO ４ ) incorporating nickel and copper in a ratio of 1:1.

[0025] [Example 4] The treatment was carried out in the same manner as in Example 1, except that the addition amount of nickel (II) acetate tetrahydrate was changed to 0.75 g and the addition amount of copper (II) nitrate trihydrate was changed to 0.48 g, and a wolframite-type tungstate (Ni ０．６ Cu ０．４ WO ４ ) incorporating nickel and copper in a ratio of 6:4 was obtained.

[0026] [Example 5] The treatment was carried out in the same manner as in Example 1, except that the addition amount of nickel (II) acetate tetrahydrate was changed to 1.0 g and the addition amount of copper (II) nitrate trihydrate was changed to 0.24 g, and a wolframite-type tungstate (Ni ０．８ Cu ０．２ WO ４ ) incorporating nickel and copper in a ratio of 8:2 was obtained.

[0027] [Example 6] The treatment was carried out in the same manner as in Example 3, except that the temperature increase to 150 °C and the reflux temperature were changed to 130 °C, and a wolframite-type tungstate (Ni ０．５ Cu ０．５ WO ４ ) incorporating nickel and copper in a ratio of 1:1 was obtained.

[0028] [Example 7] The treatment was carried out in the same manner as in Example 3, except that the temperature increase to 150 °C and the reflux temperature were changed to 180 °C, and a wolframite-type tungstate (Ni ０．５ Cu ０．５ WO ４ ) incorporating nickel and copper in a ratio of 1:1 was obtained.

[0029] [Comparative Example 1] 25 mL of diethylene glycol was placed in a beaker and the pH was adjusted to 5.5 with hydrochloric acid diluted with distilled water. After raising the temperature of this solution to 80°C, 1.21 g of copper(II) nitrate trihydrate was added and the mixture was vigorously stirred with a stirring bar until homogeneous. The solution in the beaker was transferred to a four-necked flask, and a solution of 1.67 g of sodium tungstate dihydrate dissolved in 2.5 mL of distilled water was added and the temperature was raised to 150°C within 15-20 minutes. This solution was refluxed at 150°C for 1 hour while being vigorously stirred. After refluxing, it was allowed to cool naturally to room temperature. Acetic acid and ethanol were added to the resulting mixed solution and centrifuged several times, then only distilled water was added and centrifuged several times. The residue was vacuum-dried at room temperature for 5 hours to obtain a wolframite-type tungsten oxide (CuWO) incorporating copper. ４ ) was obtained.

[0030] [Comparative Example 2] The same procedure as in Comparative Example 1 was carried out, except that 1.25 g of nickel(II) acetate tetrahydrate was added instead of copper(II) nitrate trihydrate, to obtain a nickel-incorporated wolframite-type tungsten oxide (NiWO ４ ) was obtained.

[0031] The samples obtained in the examples and comparative examples were evaluated by the following methods: (X-ray diffraction (XRD)) The XRD patterns were measured using an X-ray diffractometer (Rigaku Ultima 4) equipped with CuKα radiation (40 kV, 30 mA). (Linear sweep voltammetry (LSV)) 5 mg of each sample and 5 mg of acetylene carbon black (conductive carbon) were added to a mixed solution containing 350 μL of ethanol, 350 μL of water, and 95 μL of Nafion, and ultrasonic dispersion treatment was performed for 60 minutes. 10 μL of the resulting dispersion was added dropwise to a disk electrode (5 mm in diameter) polished with alumina (active material amount: 0.32 mg / cm²). ２). Subsequently, the disk electrode was dried at room temperature in air and used as the working electrode. A three-electrode cell was used, with a carbon rod as the counter electrode (control electrode) and Hg / HgO (1M NaOH) as the reference electrode. The sweep speed was set to 10 mV / s, and the rotation speed was set to 1600 rpm to remove oxygen bubbles on the working electrode. The resistance of the solution between the working electrode and the reference electrode was compensated with a feedback rate of 60%. In the ammonia oxidation reaction and the oxygen evolution reaction, protons are produced, so the pH of the electrolyte decreases and the hydroxide potential changes. By converting to a reversible hydrogen electrode (RHE), the effect of pH can be canceled. For the conversion, E ＲＨＥ =0.059×14+0.114+E Ｈｇ／ＨｇＯ The following formula was used. The pH was 14. To confirm the catalytic activity of the ammonia oxidation reaction, O was added to the electrolyte. ２ 50 mM NH after purging for 30 minutes ４ 0.5 M NaOH containing Cl was used. In addition, to confirm the catalytic activity of the oxygen evolution reaction, O was added to the electrolyte. ２ 0.5 M NaOH was used, which had been purged for 30 minutes.

[0032] Figure 1 shows the XRD pattern of the sample obtained in Example 3. Ni in Figure 1 ０．５ Cu ０．５ WO ４ _150℃ is the XRD pattern of the sample obtained in Example 3, Ni ０．５ Cu ０．５ WO ４ The value _150℃_cal is the XRD pattern obtained after heating the sample from Example 1 in air at 600℃ for 3 hours. NiWO ４ and CuWO ４This is the X-ray diffraction pattern from the ICDD database. The sample obtained in Example 1, after heating at 600°C, exhibits a crystal structure typical of wolframite. Therefore, the obtained sample can be considered a wolframite precursor, or wolframite with the same composition but lower crystallinity, meaning it has the same composition as wolframite but is not yet crystallized. Furthermore, the X-ray diffraction patterns of the samples obtained in Examples 1, 2, 4, and 5, and Comparative Examples 1 and 2, are shown in Figure 2 along with the X-ray diffraction pattern of Example 3. The samples obtained in Examples 1, 2, 4, and 5 exhibit a crystal structure similar to that of Example 3, and can be considered a wolframite precursor, or wolframite with the same composition but lower crystallinity, meaning it has the same composition as wolframite but is not yet crystallized.

[0033] Figure 3 shows the linear sweep voltammograms of the samples obtained in Examples 1 to 5 and Comparative Examples 1 and 2. The linear sweep voltammograms shown in Figure 3 use N as the electrolyte. ２ 50 mM NH after purging for 30 minutes ４ This method uses 0.5 M NaOH containing Cl. As is clear from Figure 3, the samples obtained in Examples 1 to 5 show a low starting potential and a sharp rise in current, reaching 10 mA·cm. －２ The potential when it reaches is also low, and in Comparative Example 1, CuWO ４ and NiWO of Comparative Example 2 ４ It showed much higher catalytic activity.

[0034] A Tafel plot was created to analyze the rising portion of Figure 3. Figure 4 shows a plot of the current density in Figure 3 with the common logarithm on the horizontal axis and the reversible hydrogen electrode potential on the vertical axis. The parameters calculated from Figures 3 and 4 (the starting potential is defined as the endpoint on the low-potential side of the linear region in Figure 4) are shown in Table 1. The Tafel gradient was calculated from the overlapping portion of the plot and the line in Figure 4 and approximated by the Tafel equation [η = a + b・log(j)]. Here, a is the Tafel constant, b is the Tafel gradient, and j is the current density. The samples obtained in Examples 1 to 5 have lower Tafel gradients than the samples in Comparative Examples 1 and 2. The Tafel gradient is the potential change required for the current value to increase tenfold; a smaller value indicates a faster reaction rate, higher activity, and lower overpotential, representing the speed of electron transfer at the electrode / electrolyte interface. Therefore, the results in Table 1 show that the samples obtained in Examples 1 to 5 have significantly faster reaction rates than the samples in Comparative Examples 1 and 2.

[0035]

[0036] X-ray diffraction measurements were performed on the samples obtained in Examples 6 and 7 in the same manner as in Example 3. The XRD patterns of Examples 6 and 7 are shown in Figure 5, along with the XRD pattern of Example 3. The samples obtained in Examples 6 and 7 showed the same crystal structure as the sample obtained in Example 3. The crystallite sizes of the samples obtained in Examples 3, 6, and 7 were calculated using the following Scherrer equation: Crystallite size = Kλ / (βcosθ) (In the above equation, K is the Bragg constant (=0.9), λ is the wavelength of the X-rays used (CuKα radiation: 1.54051 Å), β is the full width at half maximum of the 30° peak, and θ is the Bragg angle (1 / 2 of the diffraction angle 2θ) As a result, the crystallite sizes of the samples obtained in Examples 3, 6, and 7 were 23.4 Å, 24.4 Å, and 30.8 Å, respectively.

[0037] The samples obtained in Examples 6 and 7 were subjected to LSV in the same manner as in Example 3, and Tafel plots were created. The results are shown in Table 2. The samples obtained in Examples 6 and 7 also showed the same Tafel gradient, onset potential, and 10 mA·cm as the samples obtained in Examples 1 to 5. －２ The potential reached at this point was shown, indicating a remarkably fast reaction rate and high catalytic activity.

[0038]

[0039] For the samples obtained in Example 3 and Comparative Examples 1 and 2, N was used as the electrolyte. ２ 0.5M NaOH(NH₃) was purged for 30 minutes. ４ A linear sweep voltammogram using (Cl-free) with N as the electrolyte. ２ 50 mM NH after purging for 30 minutes ４ Figure 6 shows the case using 0.5 M NaOH containing Cl. In Figure 6, with NH ３ The electrolyte contains NH ４ This is the case when Cl is included, without NH ３ The electrolyte contains NH ４ This is the case where Cl is not included. Figure 6(a) is the linear sweep voltammogram of Example 3, Figure 6(b) is the linear sweep voltammogram of Comparative Example 1, and Figure 6(c) is the linear sweep voltammogram of Comparative Example 2. The sample obtained in Example 3 differed from the samples obtained in Comparative Examples 1 and 2 in that it showed catalytic activity in the oxygen evolution reaction as well as in the ammonia oxidation reaction. Figure 7 shows the samples obtained in Examples 1, 2, 4 and 5 with NH added to the electrolyte. ４ Linear sweep voltammograms with and without Cl are shown in the same manner as in Example 3. Figure 7(a) is the linear sweep voltammogram of the sample obtained in Example 1, Figure 7(b) is the linear sweep voltammogram of the sample obtained in Example 2, Figure 7(c) is the linear sweep voltammogram of the sample obtained in Example 4, and Figure 7(d) is the linear sweep voltammogram of the sample obtained in Example 5. The catalytic activity in the oxygen evolution reaction, as well as the catalytic activity in the ammonia oxidation reaction, was also shown for the samples obtained in Examples 1, 2, 4, and 5.

[0040] The tungsten oxide of the present invention can be suitably used as a catalyst for various reactions, such as ammonia oxidation reactions and oxygen evolution reactions, and is particularly suitable as a catalyst for use as an anode or cathode in these reactions by electrolysis. The manufacturing method of the present invention is suitable for manufacturing the tungsten oxide of the present invention. An electrolytic cell equipped with the tungsten oxide of the present invention can be suitably used to generate hydrogen by performing an ammonia oxidation reaction or an oxygen evolution reaction by electrolysis.

Claims

1. Ni ｘ Cu １－ｘ WO ４ (However, tungsten oxide represented as 0 < x < 1.) 2. A catalyst comprising the tungsten oxide described in claim 1.

3. The catalyst according to claim 2, which is a catalyst for an ammonia oxidation reaction or a catalyst for an oxygen evolution reaction.

4. Tungsten oxide is synthesized by dissolving tungstate, nickel salt, and copper salt in an organic solvent and heating the solution containing each of the salts. ｘ Cu １－ｘ WO ４ A method for producing tungsten oxide (where 0 < x < 1).

5. The method for producing tungsten oxide according to claim 4, wherein the organic solvent is a polyol.

6. An electrolytic cell comprising an anode chamber and a cathode chamber partitioned by an ion-permeable diaphragm, wherein an anode is disposed in the anode chamber and a cathode is disposed in the cathode chamber, and wherein a tungsten oxide represented by Ni ｘ Cu １－ｘ WO ４ (where 0 < x < 1) is supported as a catalyst.

7. A method for producing hydrogen, comprising supplying water containing ammonia to the anode chamber and water containing alkali metal hydroxide to the cathode chamber of the electrolytic cell according to claim 6, thereby performing electrolysis to oxidatively decompose ammonia in the anode chamber and generate hydrogen in the cathode chamber.

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