Reduction reaction catalyst, reduction reaction catalyst electrode, and compound synthesis system

Abstract

To provide a reduction reaction catalyst, a reduction reaction catalyst electrode, and a compound synthesis system using the reduction reaction catalyst electrode, which can promote electrochemical reduction reactions with high efficiency.SOLUTION: A reduction reaction catalyst has a β-FeOOH structure and comprises a metal element compound configured to contain metal elements other than Fe within the structure or around the structure. A compound synthesis system uses a reduction reaction catalyst electrode prepared from the reduction reaction catalyst.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a catalyst for a reduction reaction, a catalytic electrode for a reduction reaction, and a compound synthesis system using the catalytic electrode for a reduction reaction. [Background technology]

[0002] Ammonia is widely used in nitrogen-based fertilizers, as a raw material for food and pharmaceuticals, and in the production of synthetic resins and fibers, with global demand exceeding 170 million tons per year. Furthermore, because ammonia molecules act as hydrogen carriers and can be transported as a liquid, they are attracting much attention as an energy source for fuel cells and as a carbon-free fuel.

[0003] Current industrial ammonia synthesis is carried out using the Haber-Bosch process, which involves reacting nitrogen from the air with hydrogen in the presence of a catalyst at high temperatures and pressures (e.g., 400-500°C, 100-300 atmospheres), and the hydrogen used is mainly derived from natural gas. Considering the creation of a carbon-neutral society, it is extremely important to develop an ammonia synthesis method that operates under milder conditions and reduces carbon dioxide emissions.

[0004] On the other hand, in recent years, the outflow and accumulation of nitrogen into the environment has become a problem, raising concerns about its adverse effects on ecosystems. - ) and nitrite ions (NO2 - The Water Pollution Control Act sets emission standards for nitrogen oxide ions such as nitrous oxide.

[0005] In order to solve the above problems, in recent years, attempts have been made to convert nitrate ions and nitrite ions in aqueous solutions into ammonia, an essential nitrogen source in the chemical industry, through a reduction reaction using electrochemical methods. The catalyst components used in such reduction reactions are preferably based on abundant elements such as iron, but due to low electrical conductivity and the difficulty of activating Fe on the catalytic site, high overvoltage is high and high energy is required for the reaction. In addition, when nitrate ions are used as the raw material, the nitrate ions (NO3- ) to nitrite ions (NO2 - Since this is a two-step sequential reduction reaction in which nitrite is converted to ammonia (NH3) via nitrite, it is necessary to improve the efficiency of each step. In addition, platinum, a precious metal with low overvoltage, is often used as the electrode (counter electrode) for the oxidation reaction on the oxidation side, so an ammonia synthesis system using a counter electrode without precious metals is desirable.

[0006] Patent Document 1 describes iron compound particles that contain a β-FeOOH crystalline phase and a metal element other than Fe doped into the β-FeOOH crystalline phase, the particles being rod-shaped, with the average major axis length of the primary particles being 1 to 50 nm and the average ratio of the major axis length to the minor axis length being 3 to 10, a method for producing the same, and an oxidation catalyst using the same.The iron compound particles are used as a catalyst for the oxidation reaction of water.

[0007] Patent Document 2 describes Fe-containing composite compound particles containing a β-FeOOH crystalline phase and a trivalent Ni-containing compound surrounding the β-FeOOH crystalline phase, a method for producing the same, and an Fe-containing composite compound electrode. The Fe-containing composite compound particles are based on the iron compound particles of Patent Document 1 and are an oxidation reaction catalyst that exhibits excellent oxidation catalytic activity in a solution in the neutral range.

[0008] Patent Document 3 describes a method for electrochemically synthesizing ammonia by nitrate ion reduction. In the ammonia synthesis method of Patent Document 3, ammonia is synthesized by electrochemically reducing a solution containing nitrate nitrogen (NO3 ions) in a reaction vessel using a diamond electrode as the cathode and an insoluble electrode as the anode.

[0009] Non-Patent Documents 1 and 2 describe the water oxidation activity of Ni-added β-FeOOH.

[0010] Non-Patent Document 3 describes the reduction of nitrite ions to ammonia using a NiFe layered double hydroxide based on a carbon material. However, the reduction of nitrite ions to ammonia using this NiFe layered double hydroxide requires a high bias (-0.6 V (vs. RHE)) and the current efficiency of ammonia production is low.

[0011] Non-Patent Document 4 describes the reduction of nitrate ions to ammonia using β-FeOOH. However, the reduction of nitrate ions to ammonia using β-FeOOH requires a high bias (−0.4 V (vs. RHE)). [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-119615 [Patent Document 2] Patent Publication No. 2021-107298 [Patent Document 3] Japanese Patent Application Laid-Open No. 2010-270385 [Non-patent literature]

[0013] [Non-Patent Document 1] Sustainable Energy Fuels, 2017, 1, 636-643 [Non-patent document 2] Bull. Chem. Soc. Jpn., 2018, 91, 778-796 [Non-patent document 3] Eur. J. Inorg. Chem., 2022, e202200291 [Non-patent document 4] ACS Appl. Mater. Interfaces, 2022, 14, 17312-17318 Summary of the Invention [Problem to be solved by the invention]

[0014] An object of the present invention is to provide a reduction reaction catalyst, a reduction reaction catalytic electrode, and a compound synthesis system using the reduction reaction catalytic electrode, which are capable of promoting an electrochemical reduction reaction with high efficiency. [Means for solving the problem]

[0015] The present invention is a reduction reaction catalyst that has a β-FeOOH structure and contains a metal element compound that contains a metal element other than Fe within the structure or around the structure.

[0016] In the reduction reaction catalyst, the metal element other than Fe is at least one selected from 3d and 4d transition metals other than Fe belonging to Groups 6 to 11 of the periodic table, and the atomic number ratio of the total number of atoms of the metal elements other than Fe to the number of atoms of Fe element (metal elements other than Fe / Fe element) in the reduction reaction catalyst is preferably in the range of 0.005 to 0.5.

[0017] In the reduction reaction catalyst, the metal element compound preferably contains a plurality of types of metal elements as the metal element other than Fe.

[0018] In the reduction reaction catalyst, the metal element compound preferably contains at least Cu as the metal element other than Fe.

[0019] The reduction reaction catalyst is preferably used for a reduction reaction that synthesizes at least one of ammonia and ammonium ions from nitrogen oxide ions.

[0020] The present invention is a catalytic electrode for a reduction reaction, which comprises a conductive substrate and the catalyst for a reduction reaction supported on the substrate.

[0021] The present invention is a compound synthesis system that includes a catalytic electrode for a reduction reaction on a cathode side and a catalytic electrode for an oxidation reaction on an anode side, and synthesizes a compound by electrochemically reducing a reaction substrate on the cathode side.

[0022] In the compound synthesis system, the anode-side oxidation reaction catalyst electrode is preferably an oxidation reaction catalyst electrode including a conductive base material and an oxidation reaction catalyst having a β-FeOOH structure supported on the base material.

[0023] In the compound synthesis system, it is preferable that at least one of ammonia and ammonium ions is synthesized as the compound by electrochemically reducing nitrogen oxide ions as the reaction substrate. [Effects of the Invention]

[0024] The present invention can provide a reduction reaction catalyst, a reduction reaction catalytic electrode, and a compound synthesis system using the reduction reaction catalytic electrode, which can promote an electrochemical reduction reaction with high efficiency. [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a graph showing XRD patterns of various reduction reaction electrode catalysts synthesized using FeCl 3 as the Fe source in the metal ion-containing raw material aqueous solution (raw material solution A). [Figure 2] 1 is a graph showing XRD patterns of various reduction reaction electrode catalysts synthesized using different ratios of FeCl 3 as the Fe source in the metal ion-containing raw material aqueous solution (raw material solution A). [Figure 3] 1 is a graph showing the 57Fe Mössbauer spectra and peak fitting (component 1, component 2) results of reduction reaction electrode catalysts 1, 4, 6, and 7 at room temperature. [Figure 4] 1A is an SEM backscattered electron image of the reduction reaction electrode catalyst 1, and FIG. 1B is an SEM-EDX spectrum graph of the portion marked with a black circle in FIG. 1A. [Figure 5]FIG. 1 is a schematic diagram showing the configuration of an electrochemical cell used in the electrochemical measurements of Examples 1 to 12 and Comparative Examples 1 to 4. [Figure 6] This is a reduction scheme of nitrate ions. [Figure 7] 1 is a graph showing the results of measuring the change over time (6 hours) in Example 1. [Figure 8] 1 is a graph showing the results of measuring the change over time (6 hours) in Comparative Example 1. [Figure 9] 10 is a graph showing the results of measuring the change over time (3 hours) in Example 9. [Figure 10] FIG. 1 is a schematic diagram showing the configuration of an electrochemical cell used in the electrochemical measurements of Examples 13 and 14. [Figure 11] 10 is a graph showing the results of measuring the change over time (6 hours) in Example 13. [Figure 12] 10 is a graph showing the results of measuring the change over time (6 hours) in Example 14. DETAILED DESCRIPTION OF THE INVENTION

[0026] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes an embodiment of the present invention. The embodiment is an example of implementing the present invention, and the present invention is not limited to the embodiment.

[0027] <Catalyst for reduction reaction> The reduction reaction catalyst according to this embodiment has a β-FeOOH structure and contains a metal element compound containing a metal element other than Fe (sometimes referred to as a "different metal element" in this specification) within the structure or around the structure.

[0028] The present inventors conducted research to develop a reduction catalyst that can promote electrochemical reduction reactions with high efficiency using a catalyst based on β-FeOOH, a type of iron rust that is abundant and low cost. As a result, by adding a metal element compound containing metal elements other than Fe that are useful for reduction reactions to β-FeOOH, which has low activity by itself, they discovered a reduction catalyst that can promote electrochemical reduction reactions with high efficiency, which was difficult to achieve with conventional Fe-based catalysts (including oxidation catalysts). The reduction catalyst discovered by the present inventors is particularly capable of achieving a highly efficient (high reaction rate and current efficiency) sequential reduction reaction of nitrate ions to ammonia, driven at low potential.

[0029] (Composition of catalyst components) The reduction catalyst according to this embodiment has a β-FeOOH structure. The reduction catalyst according to this embodiment may also contain iron compounds other than those having a β-FeOOH structure. Examples of such iron compounds other than those having a β-FeOOH structure include other iron oxyhydroxides such as α-FeOOH, γ-FeOOH, and δ-FeOOH, iron oxides such as ferrihydrite, FeO, Fe2O3, and Fe3O4, hydroxides such as Fe(OH)2 and Fe(OH)3, components contained in iron rust, and amorphous components thereof.

[0030] The β-FeOOH structure is 57 This can be determined by Fe Mössbauer measurement. In Patent Documents 1 and 2, the presence of the β-FeOOH structure is determined by X-ray diffraction (XRD). However, 57 Fe Mössbauer measurement can distinguish the β-FeOOH structure with higher sensitivity than XRD, so 57 It is preferable to determine whether a sample has a β-FeOOH structure by Fe Mössbauer measurement. The Mössbauer spectrum of the β-FeOOH structure is characteristic and shows a paramagnetic doublet. As long as the doublet structure resulting from this β-FeOOH structure can be identified, it does not matter if the sample contains other iron compounds other than the β-FeOOH structure.

[0031] The content of β-FeOOH structure is 57 It is preferable that the content of iron compounds detected by Fe Mössbauer analysis be 50 mol% or more. If the content of iron compounds other than the β-FeOOH structure is high, the superiority of the properties derived from β-FeOOH may be reduced. The content can be roughly calculated from the intensity ratio of the doublet peak due to the β-FeOOH structure to the peak of all Fe compounds.

[0032] The reduction catalyst according to this embodiment includes a metal element compound containing a metal element other than Fe (heterogeneous metal element) within the β-FeOOH structure or around the β-FeOOH structure. The metal element compound containing the heterogeneous metal element may be the heterogeneous metal element itself. For example, in the reduction catalyst according to this embodiment, the heterogeneous metal element may be doped into the β-FeOOH structure, or the β-FeOOH structure may be surrounded by a metal element compound containing the heterogeneous metal element. Alternatively, the β-FeOOH structure may be doped with a heterogeneous metal element, and the β-FeOOH structure may be surrounded by a metal element compound containing the heterogeneous metal element. Here, examples of the metal element compound containing the heterogeneous metal element include oxides, hydroxides, and iron oxyhydroxides of the heterogeneous metal element, and these may be crystalline or amorphous.

[0033] As will be described later, the metal element compound containing a different metal element may be added during the synthesis of β-FeOOH, or may be added later to the β-FeOOH electrode.

[0034] The particle shape and particle size of β-FeOOH are not particularly limited, but a smaller particle size is preferable from the viewpoint of denser deposition on a substrate when used as an electrode. The particle size of β-FeOOH is, for example, in the range of 1 to 500 nm, preferably 1 to 100 nm, in terms of the average length of the major axis. If the particle size of β-FeOOH exceeds 500 nm, the catalyst particles may be sparsely deposited on the electrode, which may reduce the efficiency of electron transfer between catalyst particles.

[0035] The particle size of β-FeOOH can be measured by measuring the length of particles using a scanning electron microscope (SEM) or a transmission electron microscope (TEM).

[0036] The metal element other than Fe (different metal element) is not particularly limited, but is preferably a metal element selected from 3d transition metals and 4d transition metals other than Fe that belong to Groups 6 to 11 of the periodic table. The 3d transition metals and 4d transition metals other than Fe that belong to Groups 6 to 11 of the periodic table are Cr, Mn, Co, Ni, Cu, Mo, Tc, Ru, Rh, Pd, and Ag.

[0037] The type of heterometal element is not particularly limited, but it is preferable that the heterometal element contains multiple types of metal elements. "Containing multiple types of metal elements" means that multiple types of heterometal elements, i.e., a first heterometal element to an nth heterometal (n is an integer of 2 or more), may be contained within or around the β-FeOOH structure, or β-FeOOH doped with the first heterometal element to β-FeOOH doped with the nth heterometal element (n is an integer of 2 or more) may be mixed. The β-FeOOH structure may be surrounded by a metal element compound containing the first heterometal element to a metal element compound containing the nth heterometal element (n is an integer of 2 or more).

[0038] The type of heterometal element to be combined with β-FeOOH can be selected depending on the target of the reduction reaction. For example, Cu is superior in reducing nitrate ions to nitrite ions, while Ni or Ru (in the presence of Cu) are superior in reducing nitrite ions to ammonia. The coexistence of metal element compounds containing these elements within or around the β-FeOOH structure produces a synergistic effect that cannot be predicted from the effects of reduction catalysts containing individual metal element compounds. Furthermore, because Cu has a high electrochemical ability to reduce carbon dioxide, reduction catalysts containing at least Cu as a heterometal element are expected to exhibit excellent effects in carbon dioxide reduction reactions.

[0039] From these points of view, it is preferable that the different metal elements contain at least Cu. When the target is the sequential reduction of nitrate ions to ammonia via nitrite ions, it is preferable that the first different metal element contains Cu and the second different metal element contains at least one of Ni and Ru.

[0040] The atomic ratio of the total number of atoms of metal elements other than Fe to the number of atoms of Fe in the reduction catalyst (metal elements other than Fe / Fe element) is, for example, in the range of 0.005 to 0.5, and preferably in the range of 0.01 to 0.4, throughout the bulk (particles). If this atomic ratio (different metal elements / Fe element) is less than 0.005, the reduction catalytic activity may not be improved, whereas if it exceeds 0.5, the growth of the β-FeOOH crystal phase may be hindered, resulting in non-uniform doping and modification with different metal elements, and reduced catalytic activity.

[0041] This atomic ratio (different metal elements / Fe element) can be quantified by scanning electron microscope / energy dispersive X-ray spectroscopy (SEM-EDX).

[0042] (Use of catalyst for reduction reaction) The reduction catalyst according to this embodiment can be used as a catalyst for an electrochemical reduction reaction. The electrochemical reduction reaction that can be used is not particularly limited, but includes the reduction of nitrogen oxide ions (NO3 - , NO2 - ) into ammonia (NH3) and ammonium ion (NH4 + ), i.e., nitrate ion (NO3 - ) to nitrite ions (NO2 - ) to ammonia (NH3) and ammonium ion (NH4 + ) and nitrite ion (NO2 - ) into ammonia (NH3) and ammonium ion (NH4 + ) Other electrochemical reduction reactions that can be used include the reduction of nitrate ions (NO3 - ) to nitrite ions (NO2 - ), a reaction of reducing carbon dioxide (CO2) to at least one of carbon monoxide (CO), methane (CH4), formic acid (HCOOH), ethylene (C2H4), acetic acid (CH3COOH), methanol (CH3OH), and ethanol (C2H5OH), a reaction of reducing carbon monoxide (CO) to at least one of methane (CH4), formic acid (HCOOH), ethylene (C2H4), acetic acid (CH3COOH), methanol (CH3OH), and ethanol (C2H5OH), and a reaction of reducing water to hydrogen (H2). Of these, the reduction reaction catalyst according to this embodiment is - , NO2 - ) into ammonia (NH3) and ammonium ion (NH4 + ) can be suitably used in a reaction to reduce the

[0043] <Catalyst electrode for reduction reaction> The catalytic electrode for reduction reaction according to this embodiment is an electrode having a conductive substrate and the above-mentioned catalyst for reduction reaction carried on this substrate.

[0044] The substrate is a member that supports a reduction catalyst. The conductive substrate may be any member that is conductive, conducts electricity, and can support a reduction catalyst. Examples of the conductive substrate include, but are not limited to, a composite substrate of carbon and carbon that has been heat-treated at high temperatures, such as carbon paper and carbon cloth. Other examples of the conductive substrate include porous materials and metal plates, such as metal foams and meshes made of nickel, stainless steel, titanium, or tantalum. Carbon paper is a sheet-like material obtained by impregnating organic fibers, such as polyacrylonitrile (PAN) fibers, with a dispersion of polyvinyl alcohol and an aqueous medium, carbonizing the fibers at approximately 2000°C, and bonding the fibers together. Carbon cloth is a woven carbon fiber obtained by baking and carbonizing organic fibers. To support a larger amount of catalyst, a substrate with a larger surface area is preferred, and a porous material, such as carbon paper or carbon cloth, is preferred. Carbon paper and carbon cloth have, for example, numerous pores of several tens of μm (approximately 10 μm to 100 μm). The thickness of the substrate is not particularly limited as long as it functions as a carrier, but is generally in the range of 0.1 mm to 5 mm per sheet.

[0045] <Method of manufacturing a reduction reaction catalyst and a method of manufacturing a reduction reaction catalyst electrode> The reduction catalyst according to this embodiment can be obtained, for example, by preparing a metal ion-containing solution containing Fe ions, ions of a metal other than Fe, and a solvent such as water, and then mixing and stirring the solution at a predetermined temperature for a predetermined time at a predetermined pH. For example, a metal ion-containing solution is prepared by dissolving an Fe salt and a salt of a metal other than Fe in a solvent such as water in a container, and a neutralizing agent such as sodium hydroxide is dissolved in a solvent such as water in a container to prepare a neutralizing agent-containing solution. The metal ion-containing solution and the neutralizing agent-containing solution are then mixed in a reaction vessel to obtain a mixed solution of a predetermined pH, and the mixed solution is stirred at a predetermined temperature for a predetermined time to obtain a colloidal solution containing the reduction catalyst.

[0046] The catalytic electrode for reduction reaction according to this embodiment can be obtained by, for example, applying or immersing this colloidal solution on a substrate, drying it at a predetermined temperature for a predetermined time, and washing it with a washing liquid such as water as necessary, to obtain a catalytic electrode for reduction reaction having a conductive substrate and a catalyst for reduction reaction supported on the substrate.

[0047] The metal element compound containing different metal elements may be added during the synthesis of β-FeOOH as described above, or may be added later to the β-FeOOH electrode as follows.

[0048] The catalytic electrode for a reduction reaction according to this embodiment may be prepared by, for example, dissolving an Fe salt in a solvent such as water in a container to prepare an Fe ion-containing solution, dissolving a neutralizer such as sodium hydroxide in a solvent such as water in a container to prepare a neutralizer-containing solution, mixing the Fe ion-containing solution and the neutralizer-containing solution in a reaction container to obtain a mixed solution of a predetermined pH, and stirring the resulting solution at a predetermined temperature for a predetermined time to obtain a colloidal solution. This colloidal solution is then applied to a substrate, and a different metal ion-containing solution prepared by dissolving a metal salt other than Fe in a solvent such as water is then applied thereon, followed by drying at a predetermined temperature for a predetermined time and washing with a washing liquid such as water, as necessary, to obtain a catalytic electrode for a reduction reaction having a conductive substrate and a catalyst for a reduction reaction supported on the substrate.

[0049] The application method is not particularly limited, but examples thereof include a method in which the substrate is immersed in the solution and then dried, dip coating, spray coating, nozzle coating, and the like.

[0050] The Fe salt may be a divalent or trivalent salt of iron. Examples of Fe salts include inorganic salts such as iron chloride, iron nitrate, and iron sulfate, and organic salts such as citrate. Examples of salts of metals other than Fe include inorganic salts such as chloride salts, nitrate salts, and iron sulfate of metals other than Fe, and organic salts such as citrate of metals other than Fe.

[0051] The concentration of Fe ions in the metal ion-containing solution or the Fe ion-containing solution is not particularly limited, but may be, for example, in the range of 0.01 to 1 mol / L. The concentration of metal ions other than Fe ions in the metal ion-containing solution or the different metal ion-containing solution is not particularly limited, but may be, for example, in the range of 0.0001 to 0.8 mol / L.

[0052] The neutralizing agent is not particularly limited as long as it is a basic compound having a neutralizing effect, and examples thereof include inorganic basic compounds such as sodium hydroxide, potassium hydroxide, and ammonia, and organic basic compounds such as ethylenediamine, hydrazine, monoethanolamine, diethanolamine, and triethanolamine. However, when the product of the reduction reaction is ammonia, it is desirable to avoid using nitrogen-containing compounds, as there is a risk that the product will be contaminated with the remaining neutralizing agent, making accurate quantification impossible.

[0053] The concentration of the neutralizing agent in the neutralizing agent-containing solution is not particularly limited, but may be in the range of 0.01 to 1 mol / L, for example.

[0054] The solvent is not particularly limited as long as it can dissolve Fe salts and salts of metals other than Fe. Examples of the solvent include water such as ion-exchanged water and pure water, water-soluble organic solvents such as methanol, ethanol, propanol, isopropanol, butanol, acetone, acetonitrile, and dimethylformamide, and mixed solvents of water and water-soluble organic solvents.

[0055] In the method for producing a reduction reaction catalyst according to this embodiment, a dispersant such as aminocaproic acid or ε-caprolactam may be used as needed. Such a dispersant may be added to either the metal ion-containing solution, the Fe ion-containing solution, or the different metal ion-containing solution, or the neutralizing agent-containing solution, but is preferably added to the neutralizing agent-containing solution.

[0056] The metal ion-containing solution or the Fe ion-containing solution may be mixed with the neutralizer-containing solution so that the pH of the colloidal solution falls within the range of 1.8 to 5.0. When the pH of the colloidal solution is less than 1.8, the Fe 3+ If the pH exceeds 5.0, the average particle size of the reduction catalyst particles becomes significantly large, which may result in a decrease in reduction catalytic activity. From the viewpoint of facilitating the production of reduction catalyst particles with a small average particle size, the pH of the colloidal solution is preferably in the range of 1.9 to 4.0, and more preferably in the range of 2.0 to 3.0.

[0057] The temperature at which the metal ion-containing solution or the Fe ion-containing solution and the neutralizing agent-containing solution are mixed is not particularly limited, but may be, for example, 50° C. or lower, and preferably in the range of 10 to 30° C. If the mixing temperature exceeds 50° C., the average particle size of the particles of the reduction reaction catalyst increases, which may result in a decrease in reduction catalytic activity.

[0058] The method and stirring time for mixing the metal ion-containing solution or Fe ion-containing solution with the neutralizing agent-containing solution are not particularly limited as long as they are a method and stirring time that allow sufficient stirring.

[0059] The method, drying temperature, and drying time for drying the colloidal solution after it has been applied to the substrate are not particularly limited, as long as they are sufficient to dry the substrate sufficiently.

[0060] <Compound synthesis system> The compound synthesis system according to this embodiment is a compound synthesis system that includes the above-described catalytic electrode for reduction reaction on the cathode side and a catalytic electrode for oxidation reaction on the anode side, and synthesizes a compound by electrochemically reducing a reaction substrate on the cathode side.

[0061] The anode-side oxidation catalyst electrode can be a conventional metal electrode such as platinum, but from the perspective of no precious metal-free operation, it can also be an oxidation catalyst electrode having a conductive substrate and an oxidation catalyst having a β-FeOOH structure supported on this substrate. An oxidation catalyst electrode containing this oxidation catalyst having a β-FeOOH structure can also exhibit performance equivalent to that of platinum. By replacing the anode-side platinum electrode with an Fe-based catalyst, a noble metal-free compound synthesis system such as an ammonia synthesis system can be realized.

[0062] The anode-side oxidation catalyst having a β-FeOOH structure may be the oxidation catalyst described in Patent Document 1, i.e., iron compound particles containing a β-FeOOH crystalline phase and a metal element other than Fe doped into the β-FeOOH crystalline phase, wherein the metal element other than Fe is at least one metal element selected from the group consisting of 3d and 4d transition metal elements other than Fe belonging to Groups 4 to 12 of the periodic table and Al, the atomic ratio of the metal element other than Fe to Fe (metal element other than Fe / Fe) being 0.001 to 0.5, and satisfying at least one of the following conditions (A) and (B): (A) The crystallite diameter measured by X-ray diffraction is 1 to 60 nm. (B) The average particle size measured by dynamic light scattering in a solvent is 1 to 600 nm.

[0063] The atomic ratio (metal elements other than Fe / Fe element) and atomic number ratio (Ni element / Fe element) can be quantified by scanning electron microscope / energy dispersive X-ray spectroscopy (SEM-EDX), similarly to the reduction reaction catalyst described above.

[0064] The conductive substrate may be the same as that of the catalytic electrode for reduction reaction.

[0065] The compound synthesis system according to this embodiment can perform an electrochemical reduction reaction using the reduction reaction catalyst electrode and the oxidation reaction catalyst. The electrochemical reduction reaction is not particularly limited, but may be performed using nitrogen oxide ions (NO3 - , NO2 - ) into ammonia (NH3) and ammonium ion (NH4 + ), i.e., nitrate ion (NO3 - ) to nitrite ions (NO2 - ) to ammonia (NH3) and ammonium ion (NH4 + ) and nitrite ion (NO2 - ) into ammonia (NH3) and ammonium ion (NH4 + ) as a reaction substrate. - ) to nitrite ions (NO2 - ), a reaction in which carbon dioxide (CO2) is used as a reaction substrate to at least one of carbon monoxide (CO), methane (CH4), formic acid (HCOOH), ethylene (C2H4), acetic acid (CH3COOH), methanol (CH3OH), and ethanol (C2H5OH); a reaction in which carbon monoxide (CO) is reduced to at least one of methane (CH4), formic acid (HCOOH), ethylene (C2H4), acetic acid (CH3COOH), methanol (CH3OH), and ethanol (C2H5OH); and a reaction in which water is reduced to hydrogen (H2).

[0066] In the compound synthesis system according to this embodiment, the reduction reaction catalyst electrode and the oxidation reaction catalyst are used to generate nitrogen oxide ions (NO3 - , NO2 - ) is electrochemically reduced to produce the compounds ammonia (NH3) and ammonium ion (NH4 + It is preferable to synthesize at least one of

[0067] The method of ammonia synthesis is to use an electrochemical method, in which the reaction solution contains nitrogen oxide ions (NO3 - , NO2 - ) and water as a proton source. Since a sequential reaction from nitrate ions to nitrite ions to produce ammonia is possible, it is more effective for the reaction solution to contain nitrate ions.

[0068] Similarly, in the case of other reduction reactions, a reaction solution containing a reaction substrate may be used.

[0069] The present specification includes the following embodiments. (1) A catalyst for reduction reaction, which has a β-FeOOH structure and contains a metal element compound containing a metal element other than Fe within or around the structure.

[0070] (2) The catalyst for reduction reaction according to (1), the metal element other than Fe is at least one selected from 3d and 4d transition metals other than Fe belonging to Groups 6 to 11 of the periodic table, The reduction reaction catalyst has an atomic number ratio (metal elements other than Fe / Fe element) of the total number of atoms of the metal elements other than Fe to the number of atoms of Fe element in the reduction reaction catalyst, in the range of 0.005 to 0.5.

[0071] (3) The catalyst for reduction reaction according to (1) or (2), The reduction reaction catalyst, wherein the metal element compound contains a plurality of metal elements as the metal element other than Fe.

[0072] (4) The catalyst for reduction reaction according to any one of (1) to (3), The reduction reaction catalyst, wherein the metal element compound contains at least Cu as the metal element other than Fe.

[0073] (5) The catalyst for reduction reaction according to any one of (1) to (4), A catalyst for reduction reaction, which is used in a reduction reaction to synthesize at least one of ammonia and ammonium ions from nitrogen oxide ions.

[0074] (6) A catalytic electrode for reduction reactions, comprising a conductive substrate and the catalyst for reduction reactions according to any one of (1) to (5) supported on the substrate.

[0075] (7) a catalytic electrode for reduction reaction according to (6) on the cathode side; an anode-side catalytic electrode for oxidation reaction; wherein a compound is synthesized by electrochemically reducing a reaction substrate on the cathode side.

[0076] (8) The compound synthesis system according to (7), the anode-side catalytic electrode for oxidation reaction is an oxidation reaction catalytic electrode including a conductive substrate and an oxidation reaction catalyst supported on the substrate and having a β-FeOOH structure.

[0077] (9) The compound synthesis system according to (7) or (8), A compound synthesis system that synthesizes at least one of ammonia and ammonium ions as the compound by electrochemically reducing nitrogen oxide ions as the reaction substrate. [Example]

[0078] EXAMPLES The present invention will be explained in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0079] [Preparation of electrode catalyst for reduction reaction and catalytic electrode for reduction reaction] 1. Preparation of electrode catalysts and electrodes used in the examples <Electrode catalyst 1> In a beaker, FeCl3·H2O (Fujifilm Wako Co., Ltd., 13.65 g), Cu(NO3)2·3H2O (Fujifilm Wako Co., Ltd., 0.64 g), and NiCl2·6H2O (Fujifilm Wako Co., Ltd., 0.63 g) were dissolved in ion-exchanged water (250 mL) to prepare a metal ion-containing solution (raw material solution A) with an Fe ion concentration of 0.2 mol / L. Also, in a beaker, 250 mL of a neutralizing agent-containing solution (raw material solution B) was prepared by diluting aqueous sodium hydroxide solution (Fujifilm Wako Co., Ltd.) with ion-exchanged water. Raw material solutions A and B were mixed in a beaker at room temperature (25 °C) using a magnetic stirrer (rotation speed: 400 rpm) for 30 minutes to produce a colloidal solution of iron compounds. The pH of the resulting colloidal solution was measured using a pH meter and found to be 2.2. A small amount of ethanol was mixed with 1 mL of this colloidal solution and dropped onto carbon paper (Toray, TGP-H-060, 1.8 x 2.2 cm). After drying at room temperature (25°C) for 6 hours, the paper was vacuum dried at 40°C overnight. The electrode was then washed with a 0.1 M KOH solution and water, and dried at room temperature (25°C) to obtain an electrode carrying a β-FeOOH catalyst containing Cu and Ni.

[0080] <Electrode catalyst 2> A colloidal solution of iron compounds was prepared in the same manner as for electrode catalyst 1, except that the amount of Cu(NO3)2·3H2O was increased to 1.34 g and the amount of NiCl2·6H2O was increased to 1.32 g. The solution was then supported on carbon paper, and an electrode supported with a β-FeOOH catalyst containing Cu and Ni was obtained.

[0081] <Electrode catalyst 3> A colloidal solution of iron compounds was prepared in the same manner as for electrode catalyst 1, except that the amount of Cu(NO3)2·3H2O was increased to 3.04 g and the amount of NiCl2·6H2O was increased to 2.97 g. The solution was then supported on carbon paper, and an electrode supported with a β-FeOOH catalyst containing Cu and Ni was obtained.

[0082] <Electrode catalyst 4> A colloidal solution of iron compounds was prepared in the same manner as for electrode catalyst 1, except that RuCl3·nH2O (0.55 g) was used instead of NiCl2·6H2O, and the solution was supported on carbon paper to obtain an electrode supported with a β-FeOOH catalyst containing Cu and Ru.

[0083] <Electrode catalyst 5> A colloidal solution of iron compounds was prepared in the same manner as for electrode catalyst 2, except that RuCl3·nH2O (1.15 g) was used instead of NiCl2·6H2O, and the solution was supported on carbon paper to obtain an electrode supported with a β-FeOOH catalyst containing Cu and Ru.

[0084] <Electrode catalyst 6> A colloidal solution of iron compounds was prepared in the same manner as for electrode catalyst 1, except that FeCl3·H2O (6.83 g) and Fe(NO3)3·9H2O (10.11 g) were used instead of FeCl3·H2O (13.65 g). The solution was then supported on carbon paper to obtain an electrode supported with a β-FeOOH catalyst containing Cu and Ni.

[0085] <Electrode catalyst 7> A colloidal solution of iron compounds was prepared in the same manner as for electrode catalyst 1, except that Fe(NO3)3·9H2O (20.22 g) was used instead of FeCl3·H2O (13.65 g), and the solution was supported on carbon paper to obtain an electrode supported with a β-FeOOH catalyst containing Cu and Ni.

[0086] <Electrode catalyst 8> A colloidal solution of iron compounds was prepared in the same manner as for electrode catalyst 1, except that Cu(NO3)2·3H2O and NiCl2·6H2O were not used, and then supported on carbon paper. A small amount of ethanol was added to 1 mL of a mixed aqueous solution of Cu(NO3)2·3H2O and NiCl2·6H2O, equivalent to the amount of electrode catalyst 1, and the mixture was dropped onto the prepared electrode. After drying for 6 hours at room temperature (25°C), the mixture was vacuum dried overnight at 40°C. The electrode was then washed with a 0.1M KOH aqueous solution and water, and dried at room temperature (25°C) to obtain a β-FeOOH catalyst electrode with post-supported Cu and Ni components.

[0087] <Electrode catalyst 9> A colloidal solution of iron compounds was prepared in the same manner as electrode catalyst 2, except that NiCl2·6H2O was not used, and the solution was supported on carbon paper to obtain an electrode supported with a β-FeOOH catalyst containing Cu.

[0088] <Electrode catalyst 10> A colloidal solution of iron compounds was prepared in the same manner as electrode catalyst 2, except that Cu(NO3)2·3H2O was not used, and the solution was supported on carbon paper to obtain an electrode supported with a Ni-containing β-FeOOH catalyst.

[0089] <Electrode catalyst 11> A colloidal solution of iron compounds was prepared in the same manner as electrode catalyst 5, except that Cu(NO3)2·3H2O was not used, and the solution was supported on carbon paper to obtain an electrode supported with a Ru-containing β-FeOOH catalyst.

[0090] 2. Preparation of electrode catalyst and electrode used in comparative example <Electrode catalyst 12> A colloidal solution of iron compounds was prepared in the same manner as electrode catalyst 1, except that Cu(NO3)2·3H2O and NiCl2·6H2O were not used, and the solution was supported on carbon paper. An electrode supported with a β-FeOOH catalyst containing no other elements than Fe was obtained.

[0091] <Electrode catalyst 13> α-FeOOH powder (Alfa Aesar, 3.6 mg) was mixed with 1 mL of a water / ethanol (volume ratio 1 / 1) mixed solution, dispersed ultrasonically, applied to carbon paper, and dried. Next, a small amount of ethanol was added to 1 mL of a mixed solution of Cu(NO3)2·3H2O and NiCl2·6H2O, equivalent to the amount of electrode catalyst 1, and the mixture was dropped onto the prepared electrode. After drying for 6 hours at room temperature (25°C), the mixture was vacuum dried at 40°C overnight. The electrode was then washed with a 0.1 M KOH solution and water, and dried at room temperature (25°C) to obtain an α-FeOOH catalyst electrode with post-supported Cu and Ni components.

[0092] <Electrode catalyst 14> A catalyst electrode was obtained in which only Cu and Ni components in amounts corresponding to those of Electrocatalyst 1 were supported directly on the CP in the same manner as Electrocatalyst 13, except that the Fe colloidal solution was not supported.

[0093] [Preparation of electrode catalysts for oxidation reactions and catalytic electrodes for oxidation reactions] 3. Preparation of the electrode catalyst and electrode used as the counter electrode catalyst (used in Examples 9 and 10) <Counter electrode catalyst 1> An electrode catalyst for oxidation reactions was obtained by supporting the iron on carbon paper in the same manner as in Electrocatalyst 1, except that a Ni-containing β-FeOOH solution (the same solution as described in Example 17 of Patent Document 1 (JP 2017-119615)) was used as the iron colloid solution. The iron colloid solution was prepared as follows.

[0094] In a beaker, FeCl3·6H2O (27.30 g, 101 mmol) and Ni(NO3)2·6H2O (14.55 g, 50.0 mmol) were dissolved in ion-exchanged water (500 mL) to prepare a metal ion-containing solution (raw solution A) with an Fe ion concentration of 0.2 mol / L. In a beaker, ethylenediamine solution (11 mL) diluted 1 / 2 with ion-exchanged water was dissolved in ion-exchanged water (500 mL) to prepare a neutralizing agent-containing solution (raw solution B). Raw solutions A and B were mixed in a beaker at room temperature (25 °C) using a magnetic stirrer (rotation speed: 400 rpm) for 30 minutes to prepare a colloidal solution of iron compounds. The pH of the resulting colloidal solution was measured using a pH meter and found to be 2.1.

[0095] The resulting colloidal solution was dropped onto carbon paper and allowed to dry naturally. The sample was then washed with water and 0.1 M KOH solution to prepare a measurement sample. X-ray diffraction (XRD) measurements were performed using a powder X-ray diffractometer (Rigaku Corporation, "Ultima IV") under the following conditions: tube voltage: 40 kV, tube current: 40 mA, and X-ray: CuKα radiation (wavelength λ = 1.5418 Å). The crystallite size was calculated from the half-width of the peak derived from the crystalline phase using the Scherrer equation, and was found to be 5 nm. The atomic ratio of Ni to Fe (Ni / Fe) was 0.29.

[0096] [Characteristics evaluation of electrode catalysts for reduction reactions] (1) X-ray diffraction (XRD) measurement The XRD measurement of the above-mentioned reduction reaction electrode catalyst was carried out using an Ultima IV powder X-ray diffractometer manufactured by Rigaku Corporation under conditions of a tube voltage of 40 kV and a tube current of 40 mA (using Cu-Kα radiation).

[0097] (2) Mössbauer measurement The microstructure of Fe in the reduction reaction electrode catalyst is as follows: 57 Fe diffusion in the Rh matrix was measured by Mössbauer spectroscopy. 57 Measurements were performed at room temperature (25°C) using Co as the radiation source and collimated with lead. The sample was made by wrapping more than 30 mg of crushed electrode catalyst in a paper bag. The speed range was ±14.0 mm / s. -1 The velocity calibration was performed using α-Fe obtained at room temperature (25°C).

[0098] (3) Calculation of atomic ratio by scanning electron microscope (SEM)-energy dispersive X-ray analysis (EDX) The atomic ratio of the reduction reaction electrode catalyst was calculated using SEM-EDX (SU3500, manufactured by Hitachi High-Technologies Corporation). For each electrode catalyst, the average atomic ratio relative to Fe was calculated from the quantitative values ​​of 20 spots.

[0099] [Characterization results of the prepared electrode catalyst for reduction reaction] 1. XRD measurement results of electrode catalyst for reduction reaction To investigate the structure of the electrode catalyst for reduction reactions, we performed XRD measurements on various electrodes in which the electrode catalyst for reduction reactions was supported on a substrate (carbon paper (CP)). Representative results are shown in Figures 1 and 2.

[0100] Figure 1 shows the XRD patterns of electrode catalysts 1, 2, 4, 5, and 9 and the substrate carbon paper (CP) as examples. In all samples in which FeCl3 was used as the Fe source in raw solution A, a peak due to crystalline β-FeOOH (corresponding to the * in Figure 1) was confirmed. Even when Cu, Ni, or Cu, Ru were added as foreign metals other than Fe (feed amounts of 5 atom% or 10 atom% relative to Fe, respectively), a clear β-FeOOH structure was confirmed (electrode catalysts 1, 2, 4, and 5). However, no peaks due to foreign elements other than the added Fe were confirmed.

[0101] Figure 2 shows the XRD patterns of electrocatalysts 6 and 7, which were synthesized by varying the ratio of FeCl3 in the Fe source of raw material solution A. For comparison, the results for electrocatalyst 1, which used only FeCl3, are also shown. In all cases, the amounts of Cu and Ni components charged were the same, at 5 atom% relative to Fe. In electrocatalyst 6, which was prepared by mixing FeCl3 and Fe(NO3)3 in equimolar amounts as the Fe source, the peak of crystalline β-FeOOH (located at the position marked with an asterisk in Figure 2) was barely detectable, but in the case of electrocatalyst 7, which used only Fe(NO3)3, the peak derived from β-FeOOH was barely detectable.

[0102] 2. Room temperature Mössbauer measurement results of electrode catalysts for reduction reactions Next, we performed room-temperature Mössbauer analysis, which is more sensitive than XRD and can clearly examine the microstructure of Fe. Figure 3 shows the Mössbauer spectrum results for a representative electrode catalyst.

[0103] Figure 3 shows the room-temperature Mössbauer measurement results for electrocatalysts 1, 4, 6, and 7. All electrocatalysts showed spectra typical of β-FeOOH, which is composed of two paramagnetic components, component 1 and component 2. Furthermore, all Fe was trivalent, and no divalent components were present. From these results, it was confirmed that electrocatalyst 7 (Figure 2), for which the β-FeOOH structure could not be confirmed by the sensitivity of XRD, has a clear β-FeOOH structure through Mössbauer microstructural analysis. Furthermore, it was found that Mössbauer spectroscopy is an effective means of confirming the β-FeOOH structure.

[0104] 3. SEM-EDX analysis results of electrode catalyst for reduction reaction To confirm the actual amounts of elements other than Fe added and calculate the number of atoms, SEM-EDX analysis was performed. Typical results are shown in Figure 4.

[0105] As an example, Figure 4 shows the EDX spectrum (b) of the black circle in the SEM image (a) of electrode catalyst 1. The added foreign elements Cu and Ni were detected along with the Fe and Cl components, making it clear that Cu and Ni are contained in the catalyst. The actual amount of added foreign elements was determined by calculating the ratio to Fe from the atomic number of each element in the EDX spectrum (b), and then calculating the average value of 20 spots.

[0106] 4. Summary of characterization results The results of elemental analysis by XRD, Mössbauer measurement, and SEM-EDX for the various reduction reaction electrode catalysts that were prepared are summarized in Table 1. These results confirmed that electrode catalysts containing each heterogeneous element were successfully prepared.

[0107] [Table 1]

[0108] [Catalytic activity evaluation method and results] <Examples 1 to 12 and Comparative Examples 1 to 4> (Electrochemical reduction of nitrate ions) 1. Electrochemical Cell Construction The electrochemical nitrate ion reduction in Examples 1 to 12 and Comparative Examples 1 to 4 was evaluated using the electrochemical cell 1 shown in FIG. 5 (see Table 2 for specific experimental methods). An electrochemical analyzer (ALS, Model 612E) equipped with a potentiostat was used as the electrochemical measurement device 16. The evaluation was performed in a three-electrode system using an H-shaped, two-compartment cell with a bipolar membrane (Astrom, BP-1E) as the diaphragm 24. The working electrode 18 was the reduction reaction catalyst electrode prepared above, the counter electrode 26 was a Pt wire, and the reference electrode 20 was Ag / AgCl. The cathode cell 10 and anode cell 12 were made of Pyrex (registered trademark) glass. The cathode electrolyte used was 15 mL of an aqueous solution containing 0.1 M KOH and 0.1 M KNO3, and the anode electrolyte used was 15 mL of a 0.1 M KOH aqueous solution. Ar gas, an inert gas, was flowed into the cathode cell 10 and the anode cell 12 via the gas supply pipe 22 and the gas supply pipe 28, respectively. + In the alkaline solution, a portion of the NH4 gas is vaporized to ammonia gas, making it impossible to accurately measure the amount. Therefore, the NH4 gas is again transferred to the trap cell 14 containing a 10 mM HCl aqueous solution via the gas component transfer pipe 30. + The compound was captured in a dissolved state in the solution.

[0109] 2. Evaluation of nitrate ion reduction properties Using the H-type electrochemical cell 1 shown in Figure 5, constant-potential electrolysis was carried out at -0.1 V or 0.0 V vs. RHE (standard hydrogen electrode potential) for 3 or 6 hours. At the specified times, the electrolyte was sampled and ammonium ions (NH4 + ) was determined by headspace gas chromatography mass spectrometry using a gas chromatograph mass spectrometer (Shimadzu Corporation, GCMS-QP2010), and nitrate ions (NO3 - ) and nitrite ion (NO2 - The quantitative determination of ammonium ions was carried out by ion chromatography using an ion chromatograph (ICS-2100, manufactured by Dionex). The amount of ammonium ions was the total value of the electrolyte solution in the cathode cell 10 and the solution in the trap cell 14. The amount of nitrate ions (NO3- The reduction scheme of nitrate ions (NO3 - ) to nitrite ions (NO2 - ) via ammonium ion (NH4 + ) is a total of eight-electron reaction. The current efficiency for ammonium ion synthesis was calculated according to this scheme.

[0110] (Results of electrochemical reduction of nitrate ions) 3. Summary of electrochemical measurement results Nitrate ion reduction experiments were performed using reduction reaction electrode catalysts 1 to 14. The maximum ammonium ion production rate and its current efficiency (after 3 hours of electrolysis), as well as the amounts of nitrogen compounds (nitrate ions, nitrite ions, and ammonium ions) (after 3 hours of electrolysis) are summarized in Table 2. For reference, the results of measuring the changes in these amounts over time in Example 1, Comparative Example 1, and Example 9 are shown in Figures 7 to 9.

[0111] [Table 2]

[0112] In the prior art of β-FeOOH catalysts (Non-Patent Document 4), a high bias (-0.4 V vs. RHE) was required for the reduction of nitrate ions. In this example, as shown in Examples 1 to 8 in Table 2, the combination of an Fe compound with a β-FeOOH structure with a Cu-Ni compound or a Cu-Ru compound allowed the sequential generation reaction from nitrate ions to ammonium ions to proceed at a low bias of -0.1 V, producing ammonium ions with a high current efficiency (over 90%). Ammonia synthesis was also confirmed at a lower bias of 0.0 V (Example 12). Furthermore, as shown in Example 8, ammonium ions were generated with a current efficiency of 100% even when Cu and Ni compounds were added to the β-FeOOH electrode.

[0113] As reference data, Figure 7 shows the nitrate ion (NO3 - ), nitrite ion (NO2 - ), ammonium ion (NH4 +) over time. The raw material nitrate ions decrease over time, indicating that they are used in the reaction as a nitrogen source. The reduction product nitrite ions reach their highest concentration one hour after electrolysis and then gradually decrease. On the other hand, the final reduction product ammonium ions increase almost linearly from the start of electrolysis until four hours later. These results indicate that this reaction is mainly driven by the nitrate ions (NO3 - ) → nitrite ion (NO2 - ) → ammonium ion (NH4 + ) reactions proceed sequentially.

[0114] On the other hand, when an electrode catalyst 12 containing only β-FeOOH and no other elemental components was used (Comparative Example 1), the consumption of nitrate ions was slight, and the production of nitrite ions and ammonium ions was also extremely small, with the production rate of ammonium ions being 1 / 10 of that in the example, as shown in Figure 8. This indicates that other elemental components than Fe are essential for promoting the reduction reaction.

[0115] As shown in Examples 9 to 11, the addition of one kind of heteroelement other than Fe to β-FeOOH hardly promoted the subsequent reaction of nitrate ions to ammonium ions. However, in the case of β-FeOOH electrode catalyst 9, which contained only a Cu compound in Example 9, it was extremely effective in the initial reaction of reducing nitrate ions to nitrite ions, as shown in Figure 9. Furthermore, the activity of the subsequent reaction of reducing nitrite ions to ammonium ions was low, resulting in a large amount of nitrite ions being contained in the system. On the other hand, in the case of the addition of only a Ni compound or a Ru compound in Example 10 or Example 11, the content of nitrite ions was one order of magnitude lower than in the other Examples, and a small amount of ammonium ions was produced, indicating that the reduction reaction of nitrite ions to ammonium ions proceeded more rapidly than the reduction reaction of nitrite ions to nitrite ions. These results demonstrate that the subsequent reaction of nitrate ions to ammonium ions is difficult to achieve with the addition of a single heterometal component, and that the addition of a Cu compound, which excels in reducing nitrite ions to nitrite ions, and a heterometal (Ni or Ru), which excels in reducing nitrite ions to ammonium ions, is desirable.

[0116] Furthermore, as shown in Comparative Example 2, when commercially available α-FeOOH was used instead of β-FeOOH and Cu and Ni compounds were added, the amount and efficiency of ammonia production were significantly reduced compared to the experiment using β-FeOOH under the same conditions (Example 8). This suggests that the β-type Fe compound is suitable. Although the reason for this is unclear, it is speculated that only the β-type FeOOH contains Cl ions in its tunnels, and that these Cl ions play a role in promoting mass transfer and catalytic reactions with the anions of the reactants, nitrate ions and nitrite ions.

[0117] Furthermore, when electrocatalyst 14, which did not contain β-FeOOH but contained the same amounts of Cu and Ni compounds as electrocatalyst 1 in Example 1, was used (Comparative Example 3), the amount of ammonium ions produced and the current efficiency were significantly reduced compared to Example 1. As described in Non-Patent Document 4, β-FeOOH itself is an ammonia-producing catalyst that is driven at a high bias, but it is speculated that by adding a compound of a different metal element other than Fe, the electronic state of Fe at the catalytic active site is activated, enabling highly efficient ammonia conversion at a low bias, although the details of the mechanism are unknown.

[0118] As shown in Comparative Example 4, when the electrolyte did not contain nitrate ions as a nitrogen source, ammonium ions were not produced.

[0119] [Electrochemical reduction of nitrate ions using a β-FeOOH-based catalyst electrode as the counter electrode and an electrocatalyst for oxidation reactions] <Examples 13 and 14> 1. Electrochemical Cell Construction Instead of the Pt wire, which is a precious metal, used as the counter electrode 26 in the electrochemical cell 1 shown in Fig. 5, an electrochemical cell 2 having a catalyst electrode supporting a β-FeOOH-based catalyst with Ni added as an electrode catalyst for oxidation reaction as the counter electrode 32, as shown in Fig. 10, was used for evaluation. The configuration other than the counter electrode 32 is the same as that of the electrochemical cell 1 shown in Fig. 5. The cell configuration of the electrochemical cell 2 is shown in FIG.

[0120] 2. Evaluation of nitrate ion reduction properties Using the H-type electrochemical cell 2 shown in Figure 10, constant potential electrolysis was carried out for 6 hours at -0.1 V vs. RHE (standard hydrogen electrode potential). The quantification of the product was the same as in the previous section.

[0121] In Example 13, the electrode catalyst 2 was used for the working electrode 18 .

[0122] In Example 14, the electrode catalyst 5 was used for the working electrode 18 .

[0123] 3. Summary of electrochemical properties The results of measuring the change over time in Examples 13 and 14 are shown in FIGS.

[0124] It was found that ammonium ions were produced from nitrate ions by successive reactions with high efficiency, almost the same as in the case of electrolysis under the same conditions using a Pt wire as the counter electrode 26. In particular, in Example 12, it was possible to realize the construction of an ammonia synthesis system without precious metals.

[0125] Thus, it was found that the electrocatalyst for reduction reaction of the example can promote the electrochemical reduction reaction with high efficiency. [Explanation of symbols]

[0126] 1, 2 electrochemical cell, 10 cathode side cell, 12 anode side cell, 14 trap cell, 16 electrochemical measurement device, 18 working electrode, 20 reference electrode, 22, 28 gas supply piping, 24 diaphragm, 26, 32 counter electrode, 30 piping for transferring gas components.

Claims

1. A catalyst for reduction reaction, characterized by having a β-FeOOH structure and containing a metal element compound containing a metal element other than Fe within or around the structure.

2. The reduction reaction catalyst according to claim 1, the metal element other than Fe is at least one selected from the 3d and 4d transition metals other than Fe belonging to Groups 6 to 11 of the Periodic Table, The reduction reaction catalyst is characterized in that the atomic number ratio (metal elements other than Fe / Fe element) of the total number of atoms of the metal elements other than Fe to the number of atoms of Fe element in the reduction reaction catalyst is in the range of 0.005 to 0.

5.

3. The reduction reaction catalyst according to claim 1, The reduction reaction catalyst is characterized in that the metal element compound contains a plurality of types of metal elements as the metal element other than Fe.

4. The reduction reaction catalyst according to claim 1, The reduction catalyst is characterized in that the metal element compound contains at least Cu as the metal element other than Fe.

5. The reduction reaction catalyst according to claim 1, A catalyst for reduction reactions, characterized in that it is used in reduction reactions for synthesizing at least one of ammonia and ammonium ions from nitrogen oxide ions.

6. A catalytic electrode for a reduction reaction, comprising: a conductive substrate; and the catalyst for a reduction reaction according to any one of claims 1 to 5 supported on the substrate.

7. a reduction reaction catalyst electrode according to claim 6 on a cathode side; an anode-side catalytic electrode for oxidation reaction; and synthesizing a compound by electrochemically reducing a reaction substrate on the cathode side.

8. The compound synthesis system according to claim 7, a compound synthesis system, characterized in that the anode-side oxidation reaction catalytic electrode is an oxidation reaction catalytic electrode having a conductive substrate and an oxidation reaction catalyst supported on the substrate and having a β-FeOOH structure.

9. The compound synthesis system according to claim 7, A compound synthesis system characterized by electrochemically reducing nitrogen oxide ions as the reaction substrate to synthesize at least one of ammonia and ammonium ions as the compound.

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