Tungsten oxide, catalyst, and method for producing tungsten oxide

Abstract

The objective is to provide a compound with high catalytic activity that can be used as an ammonia oxidation catalyst. x Cu 1-x WO4 (however, 0

Description

Technical Field

[0001] The present invention relates to tungsten oxides represented by Ni x Cu 1-x WO4 (where 0 < x < 1), catalysts containing the same, a method for producing tungsten oxides, an electrolytic cell in which tungsten oxides are supported as a catalyst, and a method for producing hydrogen.

Background Art

[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 (NH3) has been used as a chemical raw material including fertilizers, but since it stores 17.8 mass% of hydrogen in its molecule and serves as a hydrogen carrier, its use in the energy field has recently begun to be studied. Ammonia is already widely used as liquefied ammonia, and since storage / transportation 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 derived from renewable energy, zero emissions can be achieved. The decomposition reaction of ammonia by electrolysis is represented by the following formula, and only harmless nitrogen and hydrogen are produced. Also, thermodynamically, less energy (ΔE 0 = 1.23V) is required than for electrolysis of water (ΔE 0 = 0.06V). (Anode) NH3(aq) + 3OH - → 1 / 2N2 + 3H2O + 3e - E 0 = -0.77V vs SHE (Cathode) 3H2O + 3e - → 3 / 2H2 + 3OH - E 0 = -0.83V vs SHE (Overall) NH3(aq) → 1 / 2N2 + 3 / 2H2 ΔE 0 = 0.06 V

[0003] In the electrolysis of ammonia, hydrogen is generated at the cathode, and during this process, the ammonia oxidation reaction (AOR) occurs at the counter electrode (anode). It is the ammonia oxidation that determines the overall energy efficiency, and a catalyst is required to perform ammonia oxidation rapidly. Platinum-based materials are catalysts that exhibit high activity towards ammonia oxidation, but they are expensive, prone to poisoning by adsorbed species, or produce oxygen-containing nitrogen species as by-products. Therefore, there is a strong demand for the development of catalysts that are 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 in the interlayer of manganese dioxide has been proposed (see Non-Patent Document 1), but further development of ammonia oxidation catalysts has been desired.

[0004] Tungsten is a metal with a lower cost and a larger resource amount than noble metals, and the use of tungsten compounds as catalysts for the oxygen evolution reaction (OER) has been investigated. Co 1-x Fe x Composites of WO4 and carbon nanotubes (CNT) (Non-Patent Document 2), composites of Ni-Fe-W hydroxide and carbon fiber (Non-Patent Document 3), Ni x Fe 1-x WO4 (Patent Document 1), etc. have been proposed, but all of them are related to catalysts for the oxygen evolution reaction in the electrolysis of water, etc. Moreover, those proposed in Non-Patent Documents 2 and 3 are composites with carbon nanotubes or carbon fibers.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Non-Patent Documents

[0006] [Non-Patent Document 1] 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) [Non-Patent Document 2] Composite Metal Oxide-Carbon Nanotube Electrocatalysts for the Oxygen Evolution and Oxygen Reduction Reactions,ChemElectroChem, 5, 2850-2856(2018) [Non-Patent Document 3] 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). [Overview of the project] [Problems that the invention aims to solve]

[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. [Means for solving the problem]

[0008] The inventors of the present invention studied a new compound with high catalytic activity that can be used as an ammonia oxidation catalyst, and found that a tungsten oxide represented by Ni x Cu 1-x WO4 (where 0 < x < 1) has very high catalytic activity. Conventionally, in the compound represented by MWO4, there are documents describing various metal elements as M, but a compound in which M is a combination of Ni and Cu has not been known, and furthermore, it has not been known that this compound can provide very high catalytic activity. Also, Ni x Cu 1-x WO4 (where 0 < x < 1) is particularly excellent in catalytic activity in the ammonia oxidation reaction, but is also excellent in catalytic activity in the oxygen generation reaction in electrolysis of water and the like.

[0009] That is, the present invention is specified by the following matters. (1) A tungsten oxide represented by Ni x Cu 1-x WO4 (where 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 oxygen generation reaction. (4) A method for producing a tungsten oxide represented by Ni x Cu 1-x WO4 (where 0 < x < 1), which comprises dissolving a tungstate, a nickel salt and a copper salt in an organic solvent and heating the solution in which each salt is dissolved to synthesize the tungsten oxide. (5) The method for producing a tungsten oxide of (4) above, wherein the organic solvent is a polyol. (6) An electrolytic cell provided with an anode chamber and a cathode chamber partitioned by an ion-permeable diaphragm, in which an anode is disposed in the anode chamber and a cathode is disposed in the cathode chamber, and a tungsten oxide represented by Ni x Cu 1-x WO4 (where 0 < x < 1) is supported as a catalyst on the anode. (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 described in (6) above, and electrolysis is performed, thereby oxidative decomposition of ammonia in the anode chamber and hydrogen is generated in the cathode chamber. [Effects of the Invention]

[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. [Brief explanation of the drawing]

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

Embodiments for Carrying Out the Invention

[0012] The tungsten oxide of the present invention is a compound represented by the chemical formula of Ni x Cu 1-x WO4 (where 0 < x < 1). x is preferably 0.05 or more and 0.95 or less, preferably 0.10 or more and 0.90 or less, preferably 0.15 or more and 0.85 or less, preferably 0.2 or more and 0.8 or less, and more preferably 0.4 or more and 0.6 or less. Conventionally, platinum or the like has been used as an ammonia oxidation catalyst. However, since the tungsten oxide of the present invention has a catalytic effect on the ammonia oxidation reaction without using a noble metal such as platinum, it is excellent in terms of cost. In addition, although these metals have toxicity problems, since there are no toxicity problems, the tungsten oxide of the present invention is excellent in safety. Conventionally, as a tungsten oxide having catalytic activity, Co 1-x Fe x WO4 (where 0 < x < 1) and Ni x Fe 1-x WO4 (where 0 < x < 1) are known. However, Ni x Cu 1-x WO4 (where 0 < x < 1) has not been known. Moreover, the conventionally known tungsten oxides can be used as catalysts for the oxygen generation reaction, and those showing catalytic activity as ammonia oxidation catalysts have not been known. The present invention is Ni x Cu 1-xTungsten oxide represented by WO4 (where 0 < x < 1) is synthesized, and it has been found that it is excellent as a catalyst for ammonia oxidation reaction as one of the uses of this compound. Also, it has been found that the tungsten oxide of the present invention can also be used as a catalyst for oxygen generation reaction. In the present invention, wolframite includes low-crystalline wolframite and a wolframite precursor that becomes wolframite when heat-treated. The tungsten oxide of the present invention is preferably low-crystalline wolframite or a wolframite precursor that becomes wolframite when heat-treated. Suitable crystallite sizes of the tungsten oxide of the present invention include 15.0 nm to 40.0 nm, 20.0 nm to 35.0 nm, etc.

[0013] The catalyst of the present invention is Ni x Cu 1-x A catalyst containing the tungsten oxide of the present invention represented by WO4 (where 0 < x < 1). The catalyst of the present invention may consist only of the tungsten oxide of the present invention, or may contain other compounds within the range having catalytic activity. Also, it may be supported on a carrier such as nickel foam, carbon material, metal plate, etc. The catalyst of the present invention can be used as a catalyst for ammonia oxidation reaction, oxygen generation reaction, etc. For example, it can be used as a catalyst for ammonia oxidation reaction in electrolysis of ammonia, a catalyst for oxygen generation reaction in electrolysis (electrolytic decomposition), battery, etc., and can be used as a catalyst for use in an anode in electrolysis of ammonia, an anode in electrolysis of water, an air electrode (positive electrode) in a metal-air battery, a counter electrode for reduction reaction in electrolysis of carbon dioxide, etc., i.e., an anode or a positive electrode.

[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 polyol contains the three components. 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] The heating temperature in the polyol method is not particularly limited, but a temperature near or below the boiling point of the polyol used as the solvent is preferred. Also, the heating method is not particularly limited, but since the most heat can be added at normal pressure during the synthesis reaction, it is preferable to reflux at a temperature near the boiling point of the polyol used. The heating time can be appropriately selected as the time when the synthesis reaction is sufficiently carried out. For example, a salt containing nickel, a salt containing copper, and a salt containing tungsten are dissolved in a polyol. At this time, water may be added as appropriate, and the pH may be adjusted as necessary. This solution is heated to 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 the synthesis reaction is sufficiently carried out, and 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 synthesized tungsten oxide of the present invention can be obtained by recovering the solid content by a separation operation such as centrifugation. By using a polyol as the solvent, the polyol is considered to act as a protective agent on the surface of the generated tungsten oxide particles and prevent growth due to aggregation of the catalyst particles, and Ni with high catalytic activity x Cu 1-x a tungsten oxide represented by WO4 (where 0 < x < 1) can be obtained.

[0016] The electrolytic cell of the present invention is an electrolytic cell comprising an anode chamber and a cathode chamber partitioned by an ion-permeable diaphragm, with an anode disposed in the anode chamber and a cathode disposed in the cathode chamber, wherein the anode is Ni x Cu 1-xThe tungsten oxide of the present invention represented by WO4 (where 0 < x < 1) is supported as a catalyst. The ion-permeable separator in the present invention is not particularly limited as long as it is an ion-permeable separator that can be used in an electrolytic cell for electrolysis such as an aqueous solution. For example, a porous membrane made of asbestos or modified asbestos, a porous separator using a polysulfone-based polymer, a cloth using polyphenylene sulfide fibers, a fluorine-based porous membrane, a porous membrane such as a porous membrane using a hybrid material containing both an inorganic material and an organic material, and an ion exchange membrane such as a fluorine-based ion exchange membrane can be mentioned. As the ion-permeable separator in the present invention, it is preferable that the gas permeability is low, the electrical conductivity is small, and the strength is high.

[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, such as 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 tungsten oxide of the present invention is supported on the anode. 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 can perform alkaline water CO2 electrolysis, which reduces carbon dioxide by electrolysis while electrolyzing alkaline water. Furthermore, in other embodiments 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 is 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 enables alkaline salt water CO2 electrolysis, which involves reducing alkaline salt water by electrolysis of carbon dioxide while simultaneously electrolyzing the salt water.The electrolytic cells and electrolytic methods using them shown in Figures 10 and 11 utilize the excellent oxygen evolution activity of the tungsten oxide catalyst of the present invention to enhance the activity of the entire reaction system through the driving force of the oxygen evolution reaction, thereby activating the carbon dioxide reduction 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 carbon compounds such as formic acid (HCOOH), methane (CH4), methanol (CH3OH), ethane (C2H6), ethylene (C2H4), ethanol (C2H5OH), formaldehyde (HCH), acetaldehyde (CH3CHO), acetic acid (CH3COOH), ethylene glycol (HOCH2CH2OH), and 1-propanol (CH3CH2CH2OH). [Examples]

[0021] The present invention will be described in detail below 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 the pH was adjusted to 5.5 with hydrochloric acid diluted with distilled water. After raising the temperature of 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 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 with vigorous stirring. 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 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 (Ni) containing nickel and copper in a 2:8 ratio. 0.2 Cu 0.8 WO4) was obtained.

[0023] [Example 2] Except for changing the amount of nickel(II) acetate tetrahydrate added to 0.50 g and the amount of copper(II) nitrate trihydrate added to 0.72 g, the same procedure as in Example 1 was carried out to obtain a Wolframite-type tungsten oxide (Ni) incorporating nickel and copper in a 4:6 ratio. 0.4 Cu 0.6 WO4) was obtained.

[0024] [Example 3] Except for changing the amount of nickel(II) acetate tetrahydrate added to 0.62 g and the amount of copper(II) nitrate trihydrate added to 0.60 g, the same procedure as in Example 1 was carried out to obtain a Wolframite-type tungsten oxide (Ni) incorporating nickel and copper in a 1:1 ratio. 0.5 Cu 0.5 WO4) was obtained.

[0025] [Example 4] Except for changing the amount of nickel(II) acetate tetrahydrate added to 0.75 g and the amount of copper(II) nitrate trihydrate added to 0.48 g, the same procedure as in Example 1 was carried out to obtain a Wolframite-type tungsten oxide (Ni) incorporating nickel and copper in a 6:4 ratio. 0.6 Cu 0.4 WO4) was obtained.

[0026] [Example 5] Except for changing the amount of nickel(II) acetate tetrahydrate added to 1.0 g and the amount of copper(II) nitrate trihydrate added to 0.24 g, the same procedure as in Example 1 was carried out to obtain a Wolframite-type tungsten oxide (Ni) incorporating nickel and copper in an 8:2 ratio. 0.8 Cu 0.2 WO4) was obtained.

[0027] [Example 6] Except for changing the heating temperature from 150°C to 130°C, the process was carried out in the same manner as in Example 3, and a wolframite-type tungsten oxide (Ni) incorporating nickel and copper in a 1:1 ratio was obtained. 0.5 Cu 0.5 WO4) was obtained.

[0028] [Example 7] Except for raising the temperature to 150°C and changing the reflux temperature to 180°C, the process was carried out in the same manner as in Example 3, and a wolframite-type tungsten oxide (Ni) incorporating nickel and copper in a 1:1 ratio was obtained. 0.5 Cu 0.5 WO4) 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. 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 distilled water was added and centrifuged several times. The residue was vacuum-dried at room temperature for 5 hours to obtain copper-containing Wolframite-type tungsten oxide (CuWO4).

[0030] [Comparative Example 2] Except for adding 1.25 g of nickel(II) acetate tetrahydrate instead of copper(II) nitrate trihydrate, the procedure was carried out in the same manner as in Comparative Example 1 to obtain nickel-containing wolframite-type tungsten oxide (NiWO4).

[0031] The samples obtained in the examples and comparative examples were evaluated by the following method. (X-ray diffraction (XRD)) The XRD patterns were measured using an X-ray diffractometer (RigakuUltima4) equipped with CuKα radiation (40kV, 30mA). (Linear sweep voltammetry (LSV)) A mixed solution containing 350 μL of ethanol, 350 μL of water, and 95 μL of Nafion was mixed with 5 mg of each sample and 5 mg of acetylene carbon black (conductive carbon), and ultrasonic dispersion 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 content: 0.32 mg / cm³). 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 (1MNaOH) 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 formula was used. The pH was 14. To confirm the catalytic activity of the ammonia oxidation reaction, 0.5 M NaOH containing 50 mM NH4Cl, purged with O2 for 30 minutes, was used as the electrolyte. In addition, to confirm the catalytic activity of the oxygen evolution reaction, 0.5 M NaOH, purged with O2 for 30 minutes, was used as the electrolyte.

[0032] Figure 1 shows the XRD pattern of the sample obtained in Example 3. Ni in Figure 1 0.5 Cu 0.5 WO4_150℃ is the XRD pattern of the sample obtained in Example 3, Ni 0.5 Cu 0.5WO4_150℃_cal is the XRD pattern of the sample obtained in Example 1 after heating at 600℃ in air for 3 hours. NiWO4 and CuWO4 are X-ray diffraction patterns from the ICDD database. The sample obtained in Example 1 has a typical crystalline structure of wolframite after heating at 600℃, so the obtained sample can be said to be 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 show a crystalline structure similar to that of Example 3, and can be said to be 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 1-5 and Comparative Examples 1 and 2. The linear sweep voltammograms shown in Figure 3 were obtained using 0.5 M NaOH containing 50 mM NH4Cl purged with N2 for 30 minutes as the electrolyte. As is clear from Figure 3, the samples obtained in Examples 1-5 showed a low starting potential and a sharp rise in current, with a current of 10 mA·cm. -2 The potential reached was also low, and it showed much higher catalytic activity than CuWO4 in Comparative Example 1 and NiWO4 in Comparative Example 2.

[0034] A Tafel plot was created to analyze the rising portion of Figure 3. Figure 4 shows a plot of the current density from Figure 3 with the common logarithm on the x-axis and the reversible hydrogen electrode potential on the y-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 1-5 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 1-5 have significantly faster reaction rates than the samples in Comparative Examples 1 and 2.

[0035] [Table 1]

[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 formula. Crystallite size = Kλ / (βcosθ) (In the above formula, 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 (half 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 1-5. -2 The potential reached at this point was shown, indicating a remarkably fast reaction rate and high catalytic activity.

[0038] [Table 2]

[0039] The linear sweep voltammograms of the samples obtained in Example 3 and Comparative Examples 1 and 2 are shown in Figure 6, along with the case where 0.5 M NaOH (without NH4Cl) obtained by purging N2 for 30 minutes was used as the electrolyte, and the case where 0.5 M NaOH containing 50 mM NH4Cl obtained by purging N2 for 30 minutes was used as the electrolyte. In Figure 6, "with NH3" indicates the case where NH4Cl is included in the electrolyte, and "without NH3" indicates the case where NH4Cl is not included in the electrolyte. 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 catalytic activity in the ammonia oxidation reaction. Figure 7 shows the linear sweep voltammograms of the samples obtained in Examples 1, 2, 4, and 5, with and without NH4Cl in the electrolyte, similar to Example 3. Figure 7(a) shows the linear sweep voltammogram of the sample obtained in Example 1, Figure 7(b) shows the sample obtained in Example 2, Figure 7(c) shows the sample obtained in Example 4, and Figure 7(d) shows 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. [Industrial applicability]

[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. A catalyst for ammonia oxidation reaction or an oxygen evolution reaction comprising a tungsten oxide represented as Ni x Cu 1-x WO 4 (where 0 < x < 1).

2. 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).

3. A method for producing tungsten oxide according to claim 2, wherein the organic solvent is a polyol.

4. 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.)

5. 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 an electrolytic cell according to claim 4, thereby performing electrolysis to oxidatively decompose ammonia in the anode chamber and generate hydrogen in the cathode chamber.

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