Electrochemical system and method for producing carbonyl compound

Abstract

An electrochemical system 20 comprises: an electrochemical cell 10 provided with an electrolytic solution 13, a second electrode 12, and a first electrode 11 for reducing carbon dioxide to carbon monoxide; and a catalyst for synthesizing, from carbon monoxide, at least one carbonyl compound selected from the group consisting of organic carbonates and organic oxalates, the electrolytic solution 13 containing an organic redox species selected from the group consisting of quinone derivatives and anthraquinone derivatives.

Description

Electrochemical system and method for producing carbonyl compounds

[0001] The present invention relates to an electrochemical system for producing a carbonyl compound using carbon dioxide as a raw material, and a method for producing a carbonyl compound.

[0002] Carbon dioxide reduction devices, which generate valuable resources by electrically reducing carbon dioxide, have attracted attention as a method for reducing carbon dioxide emissions and storing natural energy, and research and development is being conducted on them. Carbon dioxide reduction is known to be performed using an electrochemical cell, in which case carbon dioxide is generally reduced on the cathode side. There are also many known methods for oxidizing organic compounds using the anode of an electrochemical cell to obtain new valuable resources.

[0003] In recent years, methods for producing valuable materials from carbon dioxide using both a cathode and an anode have been studied in order to produce valuable materials from carbon dioxide more efficiently. For example, Patent Document 1 discloses an electrochemical cell including a cathode chamber provided with a cathode, an anode chamber provided with an anode and filled with an electrolyte solution containing a reaction substrate such as an alcohol-based compound, an ion transport membrane separating the cathode chamber from the anode chamber, and a connecting path connecting the cathode chamber to the anode chamber, the electrolyte solution further containing a catalyst and a redox species. In the electrochemical cell of Patent Document 1, carbon dioxide is reduced to carbon monoxide at a first electrode, and the generated carbon monoxide is discharged into the anode chamber via the connecting path. In the anode chamber, valuable materials such as carbonate compounds are produced from the carbon monoxide and the reaction substrate in the presence of the catalyst and the redox species.

[0004] International Publication No. 2023 / 038091

[0005] In carbon dioxide reduction devices, when redox species are used to synthesize carbonate compounds, inorganic metal salts such as alkali metal salts are typically used as the redox species. However, when redox species consisting of inorganic metal salts such as alkali metal salts are added to the electrolyte, the inorganic metal salts precipitate as the reaction proceeds, reducing catalytic activity and durability and causing side reactions, making it difficult to synthesize the target carbonate compound with high selectivity over a long period of time. Furthermore, the addition of inorganic metal salts to the electrolyte increases the number of steps required to separate and purify the target product, resulting in reduced production efficiency.

[0006] Therefore, an object of the present invention is to provide an electrochemical system for producing carbonyl compounds such as organic carbonates from carbon monoxide using an electrochemical reaction, which is capable of synthesizing the target carbonyl compounds with high selectivity over a long period of time and also has good production efficiency.

[0007] The present invention provides the following items [1] to

[16] . [1] An electrochemical system comprising an electrochemical cell including a first electrode that reduces carbon dioxide to carbon monoxide, a second electrode, and an electrolyte; and a catalyst that synthesizes at least one carbonyl compound selected from the group consisting of organic carbonates and organic oxalates from carbon monoxide, wherein the electrolyte contains an organic redox species selected from the group consisting of quinone derivatives and anthraquinone derivatives. [2] The electrochemical system according to item [1] above, wherein the electrolyte is substantially free of alkali metals and alkaline earth metals. [3] The electrochemical system according to item [1] or [2] above, wherein the organic redox species contains a quinone derivative. [4] The electrochemical system according to any one of items [1] to [3] above, wherein the organic redox species contains an anthraquinone derivative. [5] The electrochemical system according to any one of items [1] to [4] above, wherein the catalyst is contained in the electrolyte. [6] The electrochemical system according to any one of items [1] to [5] above, wherein the catalyst is not contained in the second electrode. [7] The electrochemical system according to any one of [1] to [6] above, wherein the electrochemical cell comprises an ion exchange membrane separating a region on the first electrode side from a region on the second electrode side. [8] The electrochemical system according to [7] above, wherein the first electrode is a diffusion electrode, and the first electrode and the ion exchange membrane are integrated. [9] The electrochemical system according to [7] or [8] above, wherein the second electrode is a diffusion electrode, and the second electrode and the ion exchange membrane are integrated.

[10] The electrochemical system according to any one of [7] to [9] above, wherein carbon monoxide produced in the region on the first electrode side is supplied to the region on the second electrode side.

[11] The electrochemical system according to any one of [7] to

[10] above, wherein convection currents are present in which the electrolytic solution is circulated from the region on the second electrode side to the outside of the electrochemical cell and then returns to the region on the second electrode side.

[12] The electrochemical system according to any one of the above [1] to

[11] , wherein the catalyst is at least one selected from the group consisting of a salt of a metal selected from the group consisting of palladium or copper, and a catalyst containing active particles having a metal element and a carbon compound supporting the active particles.

[13] The electrochemical system according to any one of [1] to

[12] above, wherein the first electrode contains a reduction catalyst that reduces carbon dioxide to carbon monoxide, and the reduction catalyst is at least one selected from the group consisting of a metal or a salt of a metal selected from the group consisting of cobalt and silver, and a catalyst containing active particles having a metal element and a carbon compound supporting the active particles.

[14] The electrochemical system according to any one of [1] to

[13] above, wherein the electrolyte contains a reaction substrate.

[15] The electrochemical system according to

[14] above, wherein the reaction substrate is an alcohol-based compound.

[16] The electrochemical system according to any one of [1] to

[15] above, wherein carbon monoxide is synthesized from carbon dioxide at the first electrode, and at least one carbonyl compound selected from the group consisting of organic carbonates and organic oxalates is electrochemically synthesized from the carbon monoxide using the catalyst.

[0008] In the present invention, in a system for producing carbonyl compounds such as organic carbonates from carbon monoxide using an electrochemical reaction, the target carbonyl compounds can be synthesized with high selectivity over a long period of time, and production efficiency can also be improved.

[0009] 1 is a schematic diagram showing an electrochemical system according to a first embodiment; and FIG. 2 is a schematic diagram showing an electrochemical system according to a second embodiment.

[0010] The electrochemical system and the method for producing a carbonyl compound using the electrochemical system of the present invention will be described below with reference to the drawings. In the following description, elements having the same configurations will be designated by the same reference numerals.

[0011] First Embodiment An electrochemical system 20 according to a first embodiment of the present invention is an electrochemical system that reduces carbon dioxide to carbon monoxide and electrochemically synthesizes at least one compound selected from the group consisting of organic carbonates and organic oxalates (hereinafter, sometimes referred to as a "carbonyl compound") from the carbon monoxide. The electrochemical system 20 according to this embodiment includes an electrochemical cell 10 including a cathode (also referred to as a "first electrode") 11 that reduces carbon dioxide to carbon monoxide, an anode (also referred to as a "second electrode") 12, and an electrolyte 13, and a catalyst (second catalyst) that synthesizes the carbonyl compound from carbon monoxide. The first electrode 11 typically has a reduction catalyst (first catalyst 14) that reduces carbon dioxide to carbon monoxide.

[0012] The electrochemical system 20 according to this embodiment includes an ion exchange membrane as a separator 25 between the first electrode 11 and the second electrode 12, and is partitioned by the separator 25 into a cathode chamber 10X, which is a region on the cathode (first electrode) side, and an anode chamber 10Y, which is a region on the anode (second electrode) side. The electrochemical system 20 also includes a reaction chamber 21. The reaction chamber 21 constitutes a reaction chamber separate from the electrolysis chamber constituted by the electrochemical cell 10B. The anode chamber 10Y and the reaction chamber 21 are filled with an electrolyte 13.

[0013] The electrochemical cell 10 is provided with first and second supply ports 16X, 16Y as supply ports for the electrochemical cell 10 and first and second exhaust ports 17X, 17Y as exhaust ports. The first supply port 16X and the first exhaust port 17X are provided in the cathode chamber 10X. The first supply port 16X is connected to a carbon dioxide supply source (not shown) via a supply path 24, and carbon dioxide is supplied to the cathode chamber 10X. Carbon dioxide may be supplied to the cathode chamber 10X alone or together with other gases such as an inert gas such as argon or nitrogen, oxygen gas, or water vapor. Water vapor is preferred as the other gas. The use of water vapor facilitates the carbon dioxide reduction reaction at the cathode 11. The carbon dioxide supply source is not particularly limited, but may be a gas cylinder or the like. Carbon dioxide may also be obtained from exhaust gas emitted from facilities such as power plants, steel mills, cement plants, and waste incinerators, and any of these facilities may serve as the carbon dioxide supply source. These facilities generate large amounts of exhaust gas, which generally contains a large amount of carbon dioxide. Therefore, the exhaust gas generated by each of these facilities may be supplied to the electrochemical cell 10 .

[0014] The first outlet 17X allows carbon monoxide, which has been reduced from carbon dioxide in the cathode chamber 10X, to be discharged from the cathode chamber 10X and supplied to the reaction chamber 21 through the supply port 21A via the connecting path 18A along the flow of convection F. This connects the cathode chamber 10X and the reaction chamber 21 so that carbon monoxide is supplied from the cathode chamber 10X to the reaction chamber 21. In this embodiment, carbon dioxide is reduced to carbon monoxide by a gas-phase reaction in the cathode chamber 10X. That is, gaseous carbon dioxide comes into contact with the first catalyst 14 on the cathode 11, generating carbon monoxide. Therefore, carbon monoxide is supplied as a gas from the cathode chamber 10X to the reaction chamber 21. Note that the gas supplied from the cathode chamber 10X to the reaction chamber 21 usually also contains unreacted carbon dioxide.

[0015] The second exhaust port 17Y is connected to the supply port 21A of the reaction chamber 21 via a connection path 18A, and the second supply port 16Y is connected to the exhaust port 21B of the reaction chamber 21 via a connection path 18B. The connection paths and the supply path 24 described below may be configured using, but are not limited to, piping or the like. While FIG. 1 shows the first and second exhaust ports 17X and 17Y each connected to the supply port 21A of the reaction chamber 21 via the same connection path 18A, they may also be connected via separate connection paths (not shown). Alternatively, the reaction chamber 21 may be provided with two supply ports, and the two supply ports may be connected to the respective exhaust ports 17X and 17Y via separate connection paths.

[0016] The electrochemical system 20 includes an electrolytic solution 13. In this embodiment, the electrolytic solution 13 contains a reaction substrate, a redox species, and a second catalyst, and is filled inside the anode chamber 10Y and the reaction chamber 21. The redox species typically serves as an electrolyte. The redox species undergo an oxidation reaction in which they are oxidized at the anode 12. In this embodiment, the redox species is an organic redox species, and specifically, is at least one of a quinone derivative and an anthraquinone derivative. In the reaction chamber 21, the electrolytic solution 13 serves as a reaction solution for producing a carbonyl compound.

[0017] In the electrochemical system 20, the electrolytic solution 13 flows from the second supply port 16Y to the second discharge port 17Y in the anode chamber 10Y, and the electrolytic solution 13 discharged from the second discharge port 17Y is supplied to the reaction chamber 21 from the supply port 21A via the connection path 18A. The electrolytic solution 13 then flows from the supply port 21A to the discharge port 21B in the reaction chamber 21 and is discharged from the discharge port 21B. The electrolytic solution 13 discharged from the discharge port 21B may be supplied to the anode chamber 10Y from the second supply port 16Y via the connection path 18B. In this manner, the electrolytic solution 13 flows by convection F so as to circulate between the anode chamber 10Y and the reaction chamber 21. In other words, the electrolytic solution 13 filled in the anode chamber 10Y has a convection flow that circulates the electrolytic solution 13 from being discharged to the outside of the electrochemical cell 10 and then returning to the anode chamber 10Y (the region on the second electrode side). The circulation of the electrolytic solution 13 may be repeatedly and continuously performed. The electrolytic solution 13 may be caused to flow along the convection F by a pump such as a diaphragm pump, a syringe pump, or a peristaltic pump, but is not particularly limited thereto, or may be caused to flow by known means other than a pump, such as by utilizing gravity to form the convection F. The same applies to the second and subsequent embodiments described later.

[0018] A voltage is applied between the anode 12 and the cathode 11 by a power supply 19. When the voltage is applied, carbon dioxide supplied to the inside of the electrochemical cell 10 is reduced by the first catalyst 14 at the cathode 11 to produce carbon monoxide. The electrochemical reaction that takes place at the cathode 11 is typically as shown in the following formula (A): CO ２ +2H ＋ +2e － →CO+H ２ O (A)

[0019] On the other hand, when a voltage is applied, the redox species are converted from reduced species to oxidants (e.g., redox mediators) at the anode 12. Therefore, the redox species are circulated between the reaction chamber 21 and the anode chamber 10Y while being appropriately converted to oxidants at the anode 12. The reaction that takes place at the anode 12 is a reaction in which hydroquinones are converted to benzoquinones and anthrahydroquinones are converted to anthraquinones, as will be described later. The reaction that takes place at the anode 12 is as shown in formula (B) below. In FIG. 1 and the following description, "Q" represents benzoquinones or anthracequinones, and H ２ Q represents a hydroquinone or an anthrahydroquinone. ２ Q → Q+2H ＋ +2e － (B) In this embodiment, the second catalyst is dispersed or dissolved in the electrolytic solution 13, and the second catalyst also circulates between the cathode chamber 10Y and the reaction chamber 21.

[0020] Carbon monoxide produced in the cathode 11 is supplied to the reaction chamber 21 via the connection path 18A. In the reaction chamber 21, a carbonyl compound is produced from carbon monoxide, an oxidant (redox species), and a reaction substrate by the action of the second catalyst contained in the electrolytic solution 13. The carbonyl compound is at least one of an organic carbonate and an organic oxalate, with organic carbonate being preferred.

[0021] Although not particularly limited, an example of a reaction in which the reaction substrate is methanol and the electrolyte (redox species) is a benzoquinone or an anthracequinone is shown in the following formula (C). When the reaction substrate is methanol and the redox species is a benzoquinone or an anthracequinone, carbon monoxide and methanol react in the reaction chamber 21 to produce dimethyl carbonate (DMC). Furthermore, the benzoquinones or anthracequinones are reduced to hydroquinones or anthrahydroquinones. CO + Q + CH ３ OH → DMC + H ２ Q (C)

[0022] When a carbonyl compound is synthesized, the oxidant (Q) is converted into a reducing species (H ２ The synthesized carbonyl compounds and reduced species (H ２ Q) are returned to the anode chamber 10Y along the convection current F together with the electrolytic solution 13. This circulation is continuously repeated, but for example, when the concentration of carbonyl compounds in the electrolytic solution 13 reaches a certain level or higher, the electrolytic solution may be discharged to the outside through the outlet 21B or the like. In addition, the reduced species (H ２ The carbon monoxide (Q) is re-oxidized in the anode chamber 10Y and converted into the oxidant (Q), which is then reused in the synthesis of carbonyl compounds. In this embodiment, the above reaction cycle is repeated, and carbon monoxide supplied from the cathode chamber 10X is continuously converted into carbonyl compounds. In the electrochemical system 20, the reaction may be carried out batchwise or continuously. When carried out continuously, the circulating electrolyte may be continuously carried out by, for example, partially withdrawing it to the outside through the outlet 21B while newly supplying the electrolyte from the outside through, for example, the supply port 16Y. The temperatures inside the anode chamber 10Y, the cathode chamber 10X, and the reaction chamber 21 in the electrochemical system 20 are not particularly limited, but are preferably near room temperature, for example, about 0 to 60°C, and preferably about 10 to 40°C.

[0023] As described above, carbon monoxide delivered from the cathode chamber 10X is converted to carbonyl compounds in the reaction chamber 21. However, not all of the carbon monoxide is converted, and some of it may be contained in the electrolytic solution 13 and circulate between the reaction chamber 21 and the anode chamber 10Y. Therefore, carbon monoxide may be converted to carbonyl compounds in the presence of an oxidant in a portion other than the reaction chamber 21 (for example, the anode chamber 10Y or the connecting paths 18A and 18B). However, in this embodiment, the synthesis of carbonyl compounds is more efficiently carried out by providing a reaction chamber 21 separate from the anode chamber 10Y where oxidation of redox species occurs.

[0024] Each component used in the electrochemical system 20 will be described in detail below. [Cathode] A cathode 11 is disposed inside the cathode chamber 10X, and the cathode 11 preferably includes a first catalyst 14. The cathode 11 preferably includes an electrode substrate (current collector), and the first catalyst 14 is preferably supported on the electrode substrate that constitutes the cathode 11.

[0025] (First Catalyst) The first catalyst is a reduction catalyst capable of reducing carbon dioxide to carbon monoxide. The first catalyst is not particularly limited as long as it is a catalyst capable of reducing carbon dioxide to a reduced product such as carbon monoxide, and may, for example, contain a metal element. The metal element may be the metal itself or a metal compound. Examples of the metal element include, but are not limited to, V, Cr, Mn, Fe, Co, Ni, Cu, Sn, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, Hf, Ta, W, Re, Ir, Pt, Au, Hg, Al, Si, In, Sn, Tl, Pb, Bi, Sb, Te, U, Sm, Tb, La, Ce, Zn, Os, and Nd. The metal compound may be an inorganic metal compound or an organic metal compound of these metals, and may specifically be a metal salt, specifically a metal halide, metal hydroxide, metal nitrate, metal sulfate, metal acetate, metal phosphate, etc. Furthermore, the metal compound may be a metal oxide, metal carbonyl, metal acetylacetonate, etc.

[0026] The metal element used in the first catalyst is preferably a metal element from Groups 7 to 12. Specific preferred examples include Mn, Fe, Ni, Ru, Co, Rh, Cu, Zn, Ag, Au, Pd, Ir, Pt, and Os, with cobalt (Co) or silver (Ag) being more preferred. The use of these metal elements facilitates increasing the efficiency of carbon dioxide conversion to carbon monoxide, and enhances catalytic activity. The metal elements used in the metal derivative may be used alone or in combination of two or more.

[0027] The first catalyst may contain a carbon compound in addition to the metal or metal compound. The carbon compound is preferably a conductive carbon compound. Furthermore, the carbon compound is preferably porous carbon. More specifically, the carbon compound may include mesoporous carbon, activated carbon, carbon black such as ketjen black and acetylene black, graphite, carbon fiber, graphene, carbon nanotubes, etc., among which carbon black is preferred, and conductive carbon black is more preferred. The carbon compound is preferably in the form of particles. Furthermore, the BET specific surface area of ​​the porous carbon may be, for example, 10 m 2 / g or more 3000m 2 / g or less, preferably 100m 2 / g or more 1500m 2 / g or less. When the surface area of ​​the porous carbon is within the above range, an appropriate amount of active particles is supported on the porous carbon, and the catalyst can have appropriate catalytic activity. The BET specific surface area can be measured by gas adsorption analysis.

[0028] When the first catalyst contains a carbon compound, it is preferable that the carbon compound be used as a carrier and that active particles having a metal element (e.g., a metal or a metal compound) be supported on the carbon compound. The first catalyst is preferably made into a powder or particle form by heat-treating a first catalyst raw material mixture obtained by mixing the carbon compound with a metal derivative such as a complex containing the metal element. The first catalyst, in which active particles having a metal element are supported on a carbon compound, is preferably further supported on an electrode substrate.

[0029] The first catalyst may be a catalyst containing a nitrogen element and a metal element (nitrogen-containing metal catalyst), but in that case, it is preferably a catalyst containing a nitrogen element, a metal element, and a carbon compound. In this case, too, it is preferable that active particles containing a metal element are supported on a carbon compound. The nitrogen element used in the nitrogen-containing metal catalyst is preferably derived from a nitrogen-containing compound described below. Specific examples of metal elements used in the nitrogen-containing metal catalyst are as described above, and the preferred metal elements are also as described above. The use of a nitrogen-containing metal catalyst increases the efficiency of carbon monoxide production.

[0030] The nitrogen-containing metal catalyst may be a catalyst obtained by heat-treating a first catalyst raw material mixture containing a metal derivative and a nitrogen-containing compound, and particularly preferably a catalyst obtained by heat-treating a first catalyst raw material mixture containing a metal derivative, a nitrogen-containing compound, and a carbon compound. The heat treatment is typically calcination. The nitrogen-containing metal catalyst may be in powder or particulate form. Being in powder or particulate form facilitates support on an electrode substrate, as described below. Furthermore, the contact area with carbon dioxide is likely to be increased, making it easier to improve the conversion efficiency to carbon monoxide. As described above, the nitrogen-containing metal catalyst obtained by heat treatment contains a metal element and a nitrogen element. The nitrogen-containing metal catalyst may be, for example, a metal oxide or a mixture of a metal oxide and a metal, but is not particularly limited thereto. Furthermore, as described above, the nitrogen-containing metal catalyst preferably contains a component derived from the nitrogen-containing compound by heat treatment, and particularly preferably contains a component derived from a pyridine derivative. The component derived from the nitrogen-containing compound preferably contains a nitrogen-containing aromatic ring structure, and specific examples thereof include a pyridine ring structure, an imidazole ring structure, a pyrazole ring structure, and a triazole ring structure. Of these, a pyridine ring structure is more preferred.

[0031] The metal derivative used in the production of the first catalyst preferably contains a metal ion of the above-mentioned metal element. The metal derivative may be used, for example, in the form of a metal salt. Examples of the metal salt include metal nitrates, metal sulfates, metal chlorides, metal bromides, metal iodides, and metal acetates. Among these, metal nitrates are preferred.

[0032] The nitrogen-containing compound is a compound containing nitrogen, and a component derived from the nitrogen-containing compound may form a bond such as a coordinate bond with a metal element derived from the metal derivative in the nitrogen-containing metal catalyst. Specific examples of the nitrogen-containing compound include compounds containing a nitrogen-containing aromatic ring having a nitrogen element as a constituent element of the aromatic ring. Specific examples of the nitrogen-containing compound include pyridine derivatives, imidazole derivatives, pyrazole derivatives, and triazole derivatives. These compounds may be used alone or in combination of two or more. Among these, from the viewpoint of conversion efficiency, a compound selected from pyridine derivatives, imidazole derivatives, and triazole derivatives is preferred, and a pyridine derivative is particularly preferred. That is, the nitrogen-containing metal catalyst is particularly preferably a catalyst obtained by heat-treating a mixture containing a metal derivative, a pyridine derivative, and a carbon compound.

[0033] The nitrogen-containing metal catalyst is preferably in powder or particulate form. A powder or particulate form makes it easier to support the catalyst on an electrode substrate, which will be described later. Furthermore, the contact area with carbon dioxide is more likely to be increased, which makes it easier to improve the conversion efficiency to carbon monoxide. As described above, the nitrogen-containing metal catalyst obtained by heat treatment contains a metal element and a nitrogen element. For example, the nitrogen-containing metal catalyst may be contained in the form of a metal oxide or a mixture of a metal oxide and a metal, but this is not particularly limited. Furthermore, as described above, the nitrogen-containing metal catalyst preferably contains a component derived from a nitrogen-containing compound by heat treatment, and in particular, a component derived from a pyridine derivative. The component derived from the nitrogen-containing compound preferably contains a nitrogen-containing aromatic ring structure. Specific examples include a pyridine ring structure, an imidazole ring structure, a pyrazole ring structure, and a triazole ring structure. Among these, a pyridine ring structure is more preferred.

[0034] A pyridine derivative is a compound having a pyridine ring. The pyridine derivative may be a compound having one, two, three, or four or more pyridine rings per molecule. Examples of pyridine derivatives include 4-aminopyridine, 2,2'-bipyridine, and 4,4'-diamino-2,2'-bipyridine, as well as polymers having multiple pyridine rings per molecule and a weight-average molecular weight of 1,000 or more. Specific examples of polymer compounds include polypyridines such as poly(2,5-pyridine) and poly(3,5-pyridine), and polyvinylpyridine, which is a polymer of vinylpyridine. Of these, poly(4-vinylpyridine) is even more preferred. The weight-average molecular weight is a value measured by gel permeation chromatography (GPC), and polystyrene is preferably used as a standard.

[0035] The amount of metal derived from the metal derivative in the first catalyst raw material mixture is, for example, 0.1 mass % to 80 mass % relative to the total amount of the first catalyst raw material mixture, preferably 0.5 mass % to 50 mass % and more preferably 1 mass % to 35 mass %. The total amount of the first catalyst raw material mixture refers to the total amount of solids in the first catalyst raw material mixture. If volatile components are added to the first catalyst raw material mixture during its production, this amount excludes the volatile components. When the first catalyst raw material mixture uses a nitrogen-containing compound, the amount of the nitrogen-containing compound in the first catalyst raw material mixture is preferably adjusted so that the molar ratio of the nitrogen-containing aromatic ring of the nitrogen-containing compound to the metal element of the metal derivative (nitrogen-containing aromatic ring / metal element) is 0.01 to 100, more preferably 0.1 to 50, and even more preferably 0.1 to 30. The molar ratio represents the ratio between the number of nitrogen-containing aromatic rings in the nitrogen-containing compound and the number of moles of the metal element in the metal derivative. The content of the carbon compound in the first catalyst raw material mixture is not particularly limited, and is, for example, 5 mass % or more and 90 mass % or less, preferably 15 mass % or more and 80 mass % or less, and more preferably 20 mass % or more and 70 mass % or less, relative to the total amount of the first catalyst raw material mixture.

[0036] The first catalyst raw material mixture is preferably heat-treated by heating to a heat treatment temperature of 150°C or higher and 550°C or lower. The heat treatment temperature is more preferably 180°C or higher and 500°C or lower, and even more preferably 200°C or higher and 470°C or lower. The heat treatment is preferably carried out in an inert gas atmosphere such as argon or nitrogen gas. By heat-treating the first catalyst raw material mixture at the heat treatment temperature, a metal-nitrogen element bond can be formed and components derived from the metal derivative, or the metal derivative and the nitrogen-containing compound, can be supported on the carbon compound. The heat treatment time (heat treatment time) is not particularly limited, but is, for example, 0.5 hours or higher and 10 hours or lower, preferably 1 hour or higher and 8 hours or lower, and more preferably 2 hours or higher and 5 hours or lower.

[0037] The first catalyst raw material mixture to be heat-treated is preferably in powder or particulate form. Being in powder or particulate form allows a powder or particulate catalyst to be obtained by heat-treating the mixture. The first catalyst raw material mixture may be obtained, for example, by preparing a diluted solution of the first catalyst raw material mixture by diluting a metal derivative and a nitrogen-containing compound, a metal derivative and a carbon compound, or a metal derivative, a nitrogen-containing compound, and a carbon compound with a dilution solvent, and then drying the diluted solution. The dilution solvent used to dilute the first catalyst raw material mixture may be water or an organic solvent, with organic solvents being preferred.

[0038] The nitrogen-containing metal catalyst is preferably produced by heat-treating the first catalyst raw material mixture, so that metal-nitrogen element bonds derived from the metal derivative and the nitrogen-containing compound are formed in the catalyst. Furthermore, the carbon compound functions as a support in the catalyst, and components derived from the metal derivative and the nitrogen-containing compound are supported on the carbon compound. Specifically, active particles containing a metal element derived from the metal derivative, or a metal element and a component derived from the nitrogen-containing compound, are preferably supported on the carbon compound. The nitrogen-containing metal catalyst having the above configuration efficiently reduces carbon dioxide to carbon monoxide, resulting in high conversion efficiency.

[0039] In addition to the above, the first catalyst may be a carbon compound containing at least one of a heteroatom such as nitrogen, a metal, or a metal compound. Examples of such carbon compounds include nitrogen-containing graphite, nitrogen-containing carbon nanotubes, nitrogen-containing graphene, Ni- and nitrogen-containing graphite, Ni- and nitrogen-containing carbon nanotubes, Ni- and nitrogen-containing graphene, Cu- and nitrogen-containing graphite, Cu- and nitrogen-containing carbon nanotubes, Cu- and nitrogen-containing graphene, Co- and nitrogen-containing graphite, Co- and nitrogen-containing carbon nanotubes, and Co- and nitrogen-containing graphene.

[0040] Among the above, metal salts and metals, particularly cobalt and silver, or metal salts thereof, are preferred as the first catalyst. Specific preferred examples include cobalt nitrate, silver nitrate, and silver. Among these, cobalt salts (especially Co with a valence of +2) and silver are more preferred. Furthermore, as the first catalyst, catalysts containing active particles having a metal element and a carbon compound supporting the active particles are also preferred. Use of these first catalysts facilitates improving the efficiency of reducing carbon dioxide to carbon monoxide. Among these, catalysts containing active particles having a metal element and a carbon compound supporting the active particles are more preferred, with cobalt or silver being even more preferred as the metal element. Furthermore, catalysts containing nitrogen, a metal element, and a carbon compound are also preferred. In the electrochemical cell 10, the first catalyst may be used alone or in combination.

[0041] (Electrode substrate) The electrode substrate used in the cathode 11 is not particularly limited as long as it is a current collector conventionally used in carbon dioxide reduction electrodes, and examples thereof include carbon substrates, metal substrates, and metal oxide substrates, and it is preferable that the electrode substrate is conductive. Furthermore, it is preferable that the electrode substrate is a porous material. When the electrode substrate is a porous material, it can form a diffusion electrode. When the electrode substrate is a diffusion electrode, the efficiency of reducing gaseous carbon dioxide to carbon monoxide is improved. The electrode substrate is a substrate that constitutes the electrode, and may be, for example, in the form of a sheet or plate, or may be in the form of a layer laminated on the wall surface of a reaction chamber.

[0042] Examples of carbon substrates include porous carbon such as carbon nonwoven fabric. The carbon nonwoven fabric is not particularly limited, and known carbon nonwoven fabrics can be used. For example, commercially available carbon nonwoven fabrics for fuel cells can be used, such as "TORAYCA" (registered trademark) carbon paper and Toray 060 manufactured by Toray Industries, Inc., "AvCarb 1071HCB" manufactured by New Metals and Chemicals, Inc., the BC series manufactured by SGL, and "H23C8" manufactured by FREUDENBERG. Examples of metal substrates include metal mesh, and metals used include gold, silver, platinum, nickel, titanium, and chromium. Examples of metal oxides used in metal oxide substrates include indium oxide, tin oxide, tin-doped indium oxide, and fluorine-doped tin oxide (FTO).

[0043] The first catalyst may be supported on an electrode substrate. The method for supporting the first catalyst on the electrode substrate is not particularly limited, and the first catalyst may be attached to the electrode substrate. Note that, "attached" here refers to a mode in which the catalyst is physically fixed to the electrode substrate, and the atoms constituting the electrode substrate are not chemically bonded to the atoms constituting the catalyst. Therefore, even if the electrode substrate contains a carbon compound such as porous carbon, the electrode substrate itself does not have the above-mentioned metal-nitrogen element bond or metal-carbon element bond. Note that the nitrogen element and metal referred to here are nitrogen element and metal element derived from the first catalyst. However, the atoms constituting the first catalyst may be chemically bonded to the atoms constituting the electrode substrate. For example, when the electrode substrate contains a carbon compound such as porous carbon, the carbon compound itself may have the above-mentioned metal-nitrogen element bond or metal-carbon element bond.

[0044] The first catalyst may be supported on the electrode substrate together with a catalyst additive or the like. The catalyst additive also functions as a binder when supporting the catalyst on the electrode substrate. Furthermore, it also functions as an ion conductor, improving the efficiency of the electrochemical reaction. The catalyst additive may be, for example, in the form of a powder or particles. Examples of the catalyst additive include cation-conductive compounds, anion-conductive compounds, and fluorine compounds other than cation-conductive compounds and anion-conductive compounds.

[0045] The method for supporting the first catalyst on the electrode substrate is not particularly limited, and examples include a method in which a diluted solution obtained by diluting the first catalyst and components other than the catalyst, such as optional catalyst additives, with a diluting solvent such as water or an organic solvent is applied to the electrode substrate using various coating devices or by spray coating, and then dried; and a method in which the electrode substrate is immersed in the diluted solution and then dried. In the cathode, the first catalyst (or catalyst composition) may be formed as a layer on the surface of the electrode substrate by, for example, applying a coating solution containing the catalyst. The catalyst layer may also be formed in a layered form so that part or all of the catalyst layer impregnates the interior of the electrode substrate over part or all of the thickness direction of the electrode substrate, for example, from the surface to the interior of the electrode substrate. The catalyst layer may also have appropriate voids. In the cathode 11, a catalyst is supported on one surface (main surface) of the electrode substrate to form a catalyst layer, but catalysts may also be supported on both surfaces, and catalyst layers may also be formed on both surfaces. In particular, from the viewpoint of efficiently carrying out the reduction reaction of the cathode 11, it is preferable that the first catalyst 14 is supported on the surface on the isolation layer 25 side (i.e., the joint surface with the isolation layer 25) as shown in Figure 1.

[0046] (Electrolyte) In this embodiment, as described above, the anode chamber 10Y and the reaction chamber 21 are filled with the electrolyte 13. The electrolyte 13 in this embodiment includes a redox species and a second catalyst. The redox species in the electrolyte 13 may be an electrolyte. In the electrolyte 13, the redox species may be dissolved in a reaction substrate, or a mixture of the reaction substrate and a solvent, which will be described later. In addition, in the electrolyte 13, the second catalyst may be dissolved or dispersed in a reaction substrate, or a mixture of the reaction substrate and a solvent, which will be described later.

[0047] (Redox Species) In this embodiment, the redox species is an organic redox species, specifically, at least one of a quinone derivative and an anthraquinone derivative. In this specification, quinone derivatives encompass quinone and its derivatives, and anthraquinone derivatives encompass anthraquinone and its derivatives. In this embodiment, using a quinone derivative or an anthraquinone derivative as the redox species enables the synthesis of a target carbonyl compound with high selectivity. Furthermore, since the electrochemical cell 10 does not require the use of alkali metals or alkaline earth metals, the precipitation of inorganic metal salts based on alkali metals and alkaline earth metals can be prevented. This prevents a decrease in device durability and the generation of side reactions, and enables the efficient reduction of carbon dioxide to carbon monoxide while synthesizing a target carbonate compound from carbon monoxide with high selectivity over a long period of time.

[0048] The quinone derivative includes a compound represented by the following formula (1).

[0049] In addition, in formula (1), R １ ~R ４are each independently a hydrogen atom or a substituent. Examples of the substituent include a halogen atom, a hydrocarbon group, a halogenated hydrocarbon group, a nitro group, a cyano group, a sulfonic acid group, an amino group, a substituted amino group, an alkoxy group, a hydroxyl group, a hydroxyl group-containing hydrocarbon group, a carboxyl group, and a carbonyl halide group. Examples of the halogen atom include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom. Among these, a chlorine atom and a bromine atom are more preferred, and a chlorine atom is even more preferred. Examples of the hydrocarbon group include a hydrocarbon group having 1 to 10 carbon atoms. Examples of the hydrocarbon group include an allyl group, a vinyl group, an alkyl group, and an aromatic hydrocarbon group. Examples of the aromatic hydrocarbon group include an aryl group such as a phenyl group or a naphthyl group, an alkylaryl group such as a tolyl group, and an aralkyl group such as a benzyl group. The hydrocarbon group is preferably an alkyl group having 1 to 10 carbon atoms, and more preferably an alkyl group having 1 to 4 carbon atoms. Preferred specific examples of the alkyl group include methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, and tert-butyl groups. Furthermore, examples of the halogenated hydrocarbon group include halogenated hydrocarbon groups having 1 to 10 carbon atoms. Examples of the halogenated hydrocarbon group include halogenated alkyl groups having 1 to 10 carbon atoms, with halogenated alkyl groups having 1 to 4 carbon atoms being more preferred. The halogen atom used in the halogenated hydrocarbon group is as described above, but is preferably a chlorine atom or a bromine atom, and more preferably a chlorine atom. The substituted amino group refers to an amino group in which one or two hydrogen atoms have been substituted with a hydrocarbon group, an acetone group, or another substituent. The hydrocarbon group used as the substituent is as described above, and specific examples of the substituted amino group include a methylamino group, an ethylamino group, a toluidino group, and an acetamide group. Examples of the alkoxy group include alkoxy groups having approximately 1 to 4 carbon atoms, such as a methoxy group and an ethoxy group. Examples of the hydroxyl group-containing hydrocarbon group include hydrocarbon groups having 1 to 10 carbon atoms in which one or more hydrogen atoms have been substituted with hydroxyl groups, and preferably alkyl groups in which one hydrogen atom has been substituted with a hydroxyl group.Examples of the carbonyl halide group include a carbonyl chloride (-COCl) group and a carbonyl bromide (-COBr) group.

[0050] In the compound represented by formula (1), R １ ~R ４ are preferably each independently a hydrogen atom, an alkyl group, or a halogen atom. １ ~R ４ When any of the groups is an alkyl group, R １ ~R ４ Preferably, any one of R is an alkyl group. １ is preferably an alkyl group. １ ~R ４ When any one of R is an alkyl group, the other three are preferably hydrogen atoms. １ ~R ４ When at least one of R is a halogen atom, １ ~R ４ It is sufficient that one, two, three or four of R １ ~R ４ It is preferred that one or four of R are halogen atoms. １ ~R ４ When one of the groups is a halogen atom, R １ is preferably a halogen atom. １ ~R ４It is also preferred that all of are hydrogen atoms. Specific preferred examples of the compound represented by formula (1) include parabenzoquinone, 2-tert-butyl-1,4-benzoquinone, 2,3,5,6-tetrachloro-1,4-benzoquinone, methyl-p-benzoquinone, methoxybenzoquinone, 2-chloro-5-methyl-1,4-benzoquinone, 2,5-dibromo-3,6-dihydroxy-p-quinone, 2,5-dichloro-1,4-benzoquinone, 2,6-dichloro-1,4-benzoquinone, ... Examples of suitable fluoro-1,4-benzoquinone include fluoranil, 5-isopropyl-2-methyl-1,4-benzoquinone, 2-(10-hydroxydecyl)-5,6-dimethoxy-3-methyl-1,4-benzoquinone, 2-bromo-1,4-benzoquinone, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone, 2,3-dimethoxy-5-methyl-1,4-benzoquinone, and 5-isopropyl-2-methyl-1,4-benzoquinone. Among these, parabenzoquinone, 2-tert-butyl-1,4-benzoquinone, and 2,3,5,6-tetrachloro-1,4-benzoquinone are more preferred.

[0051] The quinone derivative may be a benzoquinone represented by the above formula (1), but when carbon monoxide reacts with a reaction substrate to synthesize a carbonyl compound, the benzoquinone represented by formula (1) is reduced to a hydroquinone represented by the following formula (1-1). Therefore, the quinone derivative may be a hydroquinone represented by the following formula (1-1). In addition, in formula (1-1), R １ ~R ４ is the same as above. Specific examples of suitable hydroquinones include reduction products corresponding to the specific examples of benzoquinones mentioned above.

[0052] Examples of the anthraquinone derivative include compounds represented by the following formula (2). In addition, in formula (2), R １１ ~R １８ are each independently a hydrogen atom or a substituent. Specific examples of the substituent include the above-mentioned R １ ~R ４The detailed description thereof is the same as that given in formula (1), and therefore will not be repeated here.

[0053] In the compound represented by formula (2), R １１ ~R １８ are preferably each independently a hydrogen atom, an alkyl group, or a halogen atom. １１ ~R １８ When any of the groups is an alkyl group, R １１ ~R １８ It is preferable that any one of R is an alkyl group. １２ is more preferably an alkyl group. １１ ~R １８ When any of R is a halogen atom, １１ ~R １８ It is preferable that one of R is a halogen atom, and among them, R １２ is more preferably a halogen atom. １１ ~R １８ When any one of the groups is an alkyl group or a halogen atom, it is more preferable that the others are hydrogen atoms. Preferable specific examples of the compound represented by formula (2) include 2-ethylanthraquinone, 2-chloroanthraquinone, anthraquinone, anthraquinone-2-carboxylic acid, 2-methylanthraquinone, 2-tert-butylanthraquinone, 1,4-dichloroanthraquinone, 1-nitroanthraquinone, 2-phenylanthraquinone, 2-vinylanthraquinone, 2-bromoanthraquinone, 1-bromoanthraquinone, 1,8-dichloroanthraquinone, 1,4,5,8-tetrachloroanthraquinone, 2,6 -dibromoanthraquinone, anthraquinone-2-carbonyl chloride, 1,4-diamino-2,3-dichloroanthraquinone, 1,5-diaminoanthraquinone, 1-amino-2-methylanthraquinone, 1-hydroxy-4-toluidinoanthraquinone, 1,4-bis(methylamino)anthraquinone, 1-acetamido-4-hydroxyanthraquinone, anthraquinone-2,3-dicarboxylic acid, and the like. Among these, 2-ethylanthraquinone and 2-chloroanthraquinone are preferred.

[0054] The anthraquinone derivative may be an anthraquinone represented by the above formula (2), but when carbon monoxide reacts with a reaction substrate to synthesize a carbonyl compound, the anthraquinone represented by formula (2) is reduced to an anthrahydroquinone represented by the following formula (2-1). Therefore, the anthraquinone derivative may be an anthrahydroquinone represented by the following formula (2-1). In addition, in formula (2-1), R １１ ~R １８ The above is the same as above. Specific examples of suitable anthrahydroquinones include reduction products corresponding to the specific examples of anthraquinones described above.

[0055] The concentration of the redox species selected from quinone derivatives and anthraquinone derivatives in the electrolytic solution is not particularly limited, and may be adjusted to a level at which the synthesis of an oxygen-containing organic substance such as a carbonyl compound proceeds appropriately, and is, for example, 0.001 M or more and 5.0 M or less, preferably 0.01 M or more and 1.0 M or less, and more preferably 0.05 M or more and 0.5 M or less.

[0056] Furthermore, in this embodiment, it is preferable that the electrolyte 13 is substantially free of alkali metals and alkaline earth metals. Generally, in the electrosynthesis of carbonyl compounds, alkali metal halide salts and alkaline earth metal halide salts, such as lithium chloride, lithium bromide, sodium chloride, and sodium bromide, are used as redox species. However, in this embodiment, as described above, the use of specific organic redox species allows the synthesis reaction of carbonyl compounds to proceed appropriately. Therefore, even if the electrolyte 13 is substantially free of alkali metals and alkaline earth metals, carbonyl compounds can be synthesized from carbon monoxide with high selectivity. Furthermore, the substantial absence of alkali metals and alkaline earth metals can prevent the precipitation of inorganic metal salts due to redox species, thereby preventing a decrease in the durability of the device and the occurrence of side reactions. Therefore, the target carbonate compound can be synthesized with high selectivity over a long period of time. Furthermore, in this embodiment, since the electrolyte 13 does not substantially contain an inorganic electrolyte, the labor required for separating and purifying the carbonyl compound from the electrolyte 13 can be reduced. The alkali metals include sodium, potassium, rubidium, cesium, and francium, and the alkaline earth metals include magnesium, calcium, strontium, barium, and radium.

[0057] Furthermore, in this specification, the term "electrolyte 13 is substantially free of alkali metals and alkaline earth metals" means that trace amounts of alkali metals and alkaline earth metals may be unavoidably mixed in as long as the effects of the invention are achieved. Specifically, the total concentration of alkali metals and alkaline earth metals may be, for example, less than 0.001 mol / L, preferably less than 0.0001 mol / L, and more preferably less than 0.00001 mol / L, and most preferably, the electrolytic solution 13 is free of alkali metals and alkaline earth metals. Similarly, the electrolytic solution 13 preferably is substantially free of inorganic metal salts other than the second catalyst described below. Specifically, the total concentration of inorganic metal salts other than the second catalyst may be, for example, less than 0.001 mol / L, preferably less than 0.0001 mol / L, and more preferably less than 0.00001 mol / L, and most preferably, the electrolytic solution 13 is free of inorganic metal salts other than the second catalyst. Note that the moles of alkali metals and alkaline earth metals and inorganic metal salts refer to the number of moles of metal atoms. The same applies below.

[0058] (Second Catalyst) The second catalyst contained in the electrolytic solution may be dissolved or dispersed in the electrolytic solution (i.e., the reaction substrate, or a mixture of the reaction substrate and a solvent described below). As described above, the electrolytic solution 13 circulates between the reaction chamber 21 and the anode chamber 10Y, and it is preferable that the second catalyst also circulates between the reaction chamber 21 and the anode chamber 10Y together with the electrolytic solution 13. The second catalyst is a catalyst that synthesizes a carbonyl compound from carbon monoxide, but in this embodiment, it is preferable that the second catalyst is a catalyst that produces a carbonyl compound from carbon monoxide and a reaction substrate in the presence of an oxidant of the redox species.

[0059] The second catalyst contains at least one metal element selected from Groups 8 to 11 elements as a catalytically active species. The use of a Group 8 to 11 element in the second catalyst facilitates the synthesis of a carbonyl compound from carbon monoxide with high selectivity. Specific examples of Group 8 to 11 elements used in the catalyst include Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au. Among these, Co, Ni, Cu, Rh, Pd, Ag, Ir, Au, and Pt are preferred, with Pd, Au, Cu, and Ir being more preferred, Pd and Cu being even more preferred, and Pd being even more preferred. The use of any of the above elements facilitates the synthesis of an organic carbonate, an organic oxalate, or both from carbon monoxide with high selectivity. Furthermore, among the above elements, the use of Pd facilitates the production of an organic carbonate with high selectivity.

[0060] The metal element used in the catalyst may be used alone or in combination of two or more. When two or more types are used in combination, two or more metal elements selected from Groups 8 to 11 may be used in combination, or a metal element of Groups 8 to 11 may be used in combination with a metal element other than Groups 8 to 11. As the metal element other than Groups 8 to 11, a metal element of Period 4 may be preferably used, but a metal element other than Period 4 may also be used.

[0061] (Metal Salt) The metal element constituting the second catalyst may be contained in the electrolytic solution in the form of a metal ion, or may be contained in the electrolytic solution in a form other than a metal ion. In one embodiment, the catalyst is a metal salt, and is preferably blended in the electrolytic solution as a metal salt. The metal salt may be a salt of the metal element, but is preferably a palladium salt or a copper salt. Examples of the metal salt include metal nitrates, metal sulfates, metal chlorides, metal bromides, metal iodides, and metal acetates. Of these, metal chlorides and metal nitrates are preferred, and metal chlorides are more preferred. Specifically, examples of metal nitrates include cobalt nitrate (Co(NO3)2), nickel nitrate (Ni(NO3)2), copper nitrate (Cu(NO3)2), and rhodium nitrate (Rh(NO3) ３ ), palladium nitrate (Pd(NO3) ２), silver nitrate (AgNO ３ ), iridium nitrate (Ir(NO3) ４ ), platinum nitrate (Pt(NO3) ４ ), gold nitrate (AuNO3), ruthenium nitrate (Ru(NO3) ３ ), iron nitrate (Fe(NO ３ ) ３ ), manganese nitrate (Mn(NO ３ ) ２ ), zinc nitrate (Zn(NO ３ ) ２ ), chromium nitrate (Cr(NO ３ ) 　３ ), tin nitrate (Sn(NO ３ ) ４ ) and the like. Specific examples of metal chlorides include PdCl ２ , RuCl ３ , IrCl ３ , PtCl ４ , AuCl ３ , CuCl ２ Also, HAuCl ４ Among the above metal salts, PdCl ２ , HAuCl ４ , Ir(III)Cl ３ , CuCl ２ Preferably, PdCl ２ , CuCl ２ More preferably, PdCl ２ The metal salts may be used alone or in combination of two or more.

[0062] In the present invention, a metal salt is blended into the electrolytic solution as the catalyst, and the electrolytic solution preferably contains one or more ions selected from nitrate ions, sulfate ions, chloride ions, bromide ions, iodide ions, and hydroxide ions, more preferably chloride ions or nitrate ions, and even more preferably chloride ions. The metal salt may also be a hydrate. The second catalyst may consist solely of a metal salt, or may be appropriately supported on a support described below.

[0063] (Active Particle-Containing Catalyst) In one embodiment, the second catalyst is preferably a catalyst containing active particles having the above-mentioned metal element (hereinafter also referred to as "active particle-containing catalyst"). In this case, it is more preferable that the active particle-containing catalyst further contains a support, and the active particles are supported on the support. The support used in the active particle-containing catalyst is not particularly limited, but from the viewpoint of synthesizing carbonyl compounds from carbon monoxide with high selectivity, carbon compounds and metal compounds are mentioned, and among these, carbon compounds are preferred. Therefore, in one embodiment, the catalyst contained in the electrolytic solution is preferably a catalyst having active particles having a metal element and a carbon compound supporting the active particles.

[0064] As described below, the active particle-containing catalyst can be produced by mixing a metal precursor with a support and heat treating the mixture. The metal precursor is converted into active particles by heat treatment, and the active particles are supported on a support. The active particles in the active particle-containing catalyst have catalytic activity that promotes the reaction when carbon monoxide is electrochemically synthesized into an organic carbonate, an organic oxalate, or both. The active particles containing a metal element are not particularly limited, but may be composed of a metal oxide, a metal itself, or both a metal oxide and a metal. The active particles may also be in the form of a metal salt. The active particles may be metal ions with anions derived from the metal salt of the metal precursor described below as counter anions. The active particles may also be a combination of these. The counter anions are not particularly limited, but include chloride ions, nitrate ions, sulfate ions, iodide ions, bromide ions, etc., with chloride ions being preferred. The metal elements used in the active particles are as described above.

[0065] In the active particle-containing catalyst, the active particles are preferably in the form of fine particles, and the fine particle active particles are preferably supported on a support. The active particles are not particularly limited, but are preferably nano-order particles, and preferably have an average particle diameter of 100 nm or less, more preferably 1 nm to 40 nm. By having the above particle diameter and nanostructuring the active particles, the active area increases, and various performance characteristics of the catalyst are likely to be improved. Note that the particle diameter refers to the area-equivalent circle diameter, which is calculated by determining the area of ​​each particle in image observation using TEM-EDX or the like, and then calculating the diameter from the area of ​​each particle when it is assumed to be a circle.

[0066] The carbon compound constituting the support is preferably porous carbon. The porous carbon is not particularly limited as long as it can support active particles, but a conductive carbon compound is preferred. The use of a conductive carbon compound increases the electrical conductivity of the electrode, making it easier to improve reaction efficiency, etc. More specific examples of porous carbon include mesoporous carbon, activated carbon, carbon black such as ketjen black and acetylene black, carbon nanotubes, graphite, graphene, etc. The porous carbon is preferably particulate. Furthermore, the porous carbon is preferably carbon black, and the carbon black is more preferably conductive carbon black. The carbon compound is preferably particulate. Furthermore, the BET specific surface area of ​​the porous carbon is the same as that of the porous carbon described above for the first catalyst.

[0067] The metal element in the metal compound constituting the support also includes metal elements known as semimetals, such as silicon. Examples of the metal element in the metal compound constituting the support include metal elements of Groups 1 to 6 and metal elements of Groups 13 to 14, and specific examples include silicon, zirconium, magnesium, titanium, cerium, niobium, tungsten, and aluminum. Examples of the metal compound include metal oxides, metal nitrides, metal borides, and metals, with metal oxides being preferred.

[0068] Specific examples of metal compounds constituting the support include silica, magnesia, titanium oxide, zirconia, alumina, cerium oxide, niobium oxide, silica-alumina, silica-magnesia, tungstate zirconia, zeolite, sulfated zirconia, and titanosilicate. Among these, alumina, silica, magnesia, titanium oxide, zirconia, cerium oxide, niobium oxide, silica-alumina, silica-magnesia, tungstate zirconia, and zeolite are preferred, with alumina, silica-alumina, and silica-magnesia being particularly preferred. The alumina is not particularly limited, and examples include α-alumina, β-alumina, and γ-alumina, with γ-alumina being preferred. Furthermore, zeolites such as mordenite (MOR), FAU (Y), BEA (beta), and MFI (ZSM-5) can be used, but are not particularly limited. The support may be used alone or in combination of two or more types.

[0069] The metal compound is preferably porous so that it can properly support the active particles. The BET specific surface area of ​​the metal compound is, for example, 10 m 2 / g or more 1000m 2 / g or less, but preferably 20m 2 / g or more 500m 2 / g or less, more preferably 40m 2 / g or more 200m 2 The metal compound is not particularly limited, but may be, for example, in the form of powder or particles.

[0070] (Method for producing second catalyst) Next, a method for producing the second catalyst according to one embodiment will be described. The second catalyst described above can be obtained by mixing a metal precursor and a support, and heat-treating the mixture containing the metal precursor and the support (hereinafter referred to as the "second catalyst raw material mixture").

[0071] The metal precursor is a compound that becomes the above-described active particles upon heat treatment. Therefore, the metal precursor is preferably a precursor containing a metal element selected from the above-described Group 8 to Group 11 elements, and suitable metal elements are also as described above. The metal element used in the metal precursor may be used alone or in combination of two or more. When two or more elements are used in combination, it is preferable to use two or more precursors containing a metal element selected from the above-described Group 8 to Group 11 elements in combination, or to use a precursor containing a metal element selected from the Group 8 to Group 11 elements in combination with a precursor containing a metal element other than Group 8 to Group 11 (e.g., a metal element of the fourth periodic element). When two or more metal precursors are used in combination, the second catalyst is preferably obtained by mixing two or more metal precursors and a metal compound and then heat-treating the mixture. Therefore, it is preferable that the catalyst raw material mixture contains two or more metal precursors.

[0072] The metal precursor preferably contains a metal ion. The metal precursor may be used, for example, in the form of a metal salt. Examples of metal salts include metal nitrates, metal sulfates, metal chlorides, metal bromides, metal iodides, and metal acetates. Among these, metal chlorides and metal nitrates are preferred, and metal nitrates are more preferred from the viewpoint of forming suitable active particles. Specific examples of metal nitrates and metal chlorides used in the metal precursor are the same as the metal salts used as the catalyst, and therefore, further explanation is omitted. The metal salt may also be a hydrate. The metal salt may be used alone or in combination of two or more. The support used as a raw material in this production method is as described above.

[0073] The content of the metal derived from the metal precursor in the second catalyst raw mixture is preferably 0.1 mass % or more and 70 mass % or less, and more preferably 2 mass % or more and 50 mass % or less, relative to the total amount of the catalyst raw mixture. The content of the support in the catalyst raw mixture is not particularly limited, but is, for example, 10 mass % or more and 95 mass % or less, preferably 20 mass % or more and 85 mass % or less, and more preferably 30 mass % or more and 80 mass % or less, relative to the total amount of the catalyst raw mixture.

[0074] The temperature for heat-treating the second catalyst raw material mixture is preferably 150°C or higher and 1000°C or lower. By setting the heat treatment temperature within this range, active particles can be appropriately formed from the metal precursor while suppressing the generation of unnecessary by-products. Furthermore, it is possible to appropriately support the active particles on the support. Furthermore, migration of the active particles can be prevented, thereby reducing the particle size of the active particles and making it easier to increase the surface area of ​​the second catalyst. From the above perspectives, the heat treatment temperature is preferably 180°C or higher and 1000°C or lower, and more preferably 190°C or higher and 500°C or lower. The heat treatment time is not particularly limited, but is, for example, 0.25 hours or higher and 10 hours or lower, preferably 0.5 hours or higher and 8 hours or lower, and more preferably 1 hour or higher and 5 hours or lower. The heat treatment may be performed in air, in an inert gas atmosphere such as argon or nitrogen gas, or in a reducing gas atmosphere such as hydrogen. Furthermore, a reduction treatment may be performed appropriately after the heat treatment. Examples of the reduction treatment include a reduction treatment using a reducing gas such as hydrogen or carbon monoxide, or NaBH ４ Examples of such methods include a method of performing a wet reduction treatment using a reducing compound such as the above. By performing the reduction treatment, it becomes easier to increase the proportion of zero-valent metal elements (i.e., metal itself) in the active catalyst, and it becomes easier to increase the catalytic activity.

[0075] The second catalyst raw material mixture to be heat-treated is preferably in powder or particulate form. If the second catalyst mixture is in powder or particulate form, the catalyst obtained by heat treatment can also be in powder or particulate form. The second catalyst raw material mixture can be obtained, for example, by preparing a diluted solution of the second catalyst raw material mixture and drying the diluted solution. In the diluted solution of the catalyst raw material mixture, each component (metal precursor and support) is preferably dispersed or dissolved in a dilution solvent. By dispersing or dissolving each component in a dilution solvent, a second catalyst raw material mixture in which each component is homogeneously mixed can be obtained.

[0076] The dilution solvent used to dilute the second catalyst raw material mixture may be water or an organic solvent, with water being preferred. Alternatively, a mixed solvent of an organic solvent and water may be used as the dilution solvent. The dilution solvent may also contain an acid component such as hydrochloric acid, nitric acid, sulfuric acid, acetic acid, citric acid, or lauric acid, or a base component such as ammonia. The concentration of the diluted solution of the catalyst raw material mixture is not particularly limited, but the amount of the metal precursor is, for example, 0.01 to 25 g / L, preferably 0.1 to 5 g / L.

[0077] The content of the second catalyst in the electrolytic solution 13 is not particularly limited, but may be, for example, about 0.02 to 200 g / L, and preferably about 0.1 to 100 g / L. The content here refers to the amount of the second catalyst per 1 L of the electrolytic solution contained in the anode chamber 10Y or the reaction chamber 21.

[0078] (Reaction Substrate) The reaction substrate contained in the electrolytic solution 13 is a compound that serves as a raw material for the carbonyl compound. An alcohol-based compound is preferably used as the reaction substrate. By using an alcohol-based compound as the reaction substrate, an organic carbonate, an organic oxalate, or both of them can be easily produced from carbon monoxide in the presence of an oxidant of the redox species and the second catalyst.

[0079] (Alcohol-Based Compound) An alcohol-based compound is a reaction substrate that reacts with carbon monoxide in an electrochemical system to produce an organic carbonate, an organic oxalate, or both. The alcohol-based compound may be solid, liquid, or gaseous in the environment in which the electrochemical reaction occurs in the electrochemical system, but is preferably liquid. A liquid alcohol-based compound can be easily filled into the reaction chamber 21, the anode chamber 10Y, etc., without using a solvent, as described below. An alcohol-based compound is a compound having at least one hydroxyl group, and more specifically, a compound represented by the following general formula (3). In this specification, the term "alcohol-based compound" is a concept that also includes aromatic hydroxy compounds in which a hydroxyl group is directly bonded to an aromatic ring such as a benzene ring, as typified by phenol, as described below.

[0080] ROH (3) (R represents an organic group having 1 to 15 carbon atoms.) The organic group having 1 to 15 carbon atoms represented by R in the above general formula (3) includes a hydrocarbon group having 1 to 15 carbon atoms. Examples of the hydrocarbon group include an alkyl group having 1 to 15 carbon atoms, an alkenyl group having 2 to 15 carbon atoms, and an aryl group having 6 to 15 carbon atoms. Examples of the alkyl group having 1 to 15 carbon atoms include a methyl group, an ethyl group, various propyl groups, various butyl groups, various pentyl groups, various hexyl groups, various heptyl groups, various octyl groups, various nonyl groups, various decyl groups, various dodecyl groups, and various pentadecyl groups. Examples of alkenyl groups having 2 to 15 carbon atoms include vinyl groups, various propynyl groups, various butynyl groups, various pentynyl groups, various hexenyl groups, various heptenyl groups, various octenyl groups, various nonenyl groups, various decenyl groups, various dodecenyl groups, and various pentadecenyl groups. The term "various" refers to various isomers including n-, sec-, tert-, and iso-. The alkyl or alkenyl group may be linear, branched, or cyclic. Examples of aryl groups having 6 to 15 carbon atoms include phenyl and naphthyl groups. The hydrocarbon groups described above may have a substituent, and in such cases, the total carbon number, including the substituent, is 1 to 15.

[0081] Furthermore, the organic group having 1 to 15 carbon atoms in general formula (3) may contain a heteroatom such as a nitrogen atom, an oxygen atom, a sulfur atom, a phosphorus atom, or a halogen atom. Among these, an oxygen atom is preferred. When an oxygen atom is contained, the oxygen atom is preferably either a hydroxyl group or an oxygen atom of an ether bond. Therefore, it is preferable that R is a hydrocarbon group having at least either a hydroxyl group or an ether bond. It is also preferable that R has one hydroxyl group. In other words, the alcohol-based compound may have two hydroxyl groups. More specifically, the alcohol-based compound having two hydroxyl groups is preferably a group represented by the following formula (3-1): HO-R Ａ —OH (3-1) In formula (3-1), R Ａ is a divalent saturated hydrocarbon group having 2 to 15 carbon atoms, and R ＡThe number of carbon atoms is preferably 2 to 4, more preferably 2 to 3.

[0082] As the compound represented by the general formula (3), among the above, R is preferably an alkyl group having 1 to 8 carbon atoms, an alkenyl group having 2 to 8 carbon atoms, or an aryl group having 6 to 8 carbon atoms. Ａ Also preferred are those having 2 to 4 carbon atoms. Among these, compounds in which R is an alkyl group or an aryl group are more preferred, and particularly compounds in which R is an alkyl group are even more preferred. The alkyl group more preferably has 1 to 3 carbon atoms, even more preferably has 1 or 2 carbon atoms, and most preferably has 1 carbon atom. Specifically, from the viewpoints of reactivity and production efficiency, methanol, ethanol, phenol, 1-propanol, ethylene glycol, propylene glycol, etc. are preferred, and among these, methanol is more preferred. The alcohol-based compounds may be used alone or in combination of two or more types.

[0083] When an alcohol-based compound is used as a reaction substrate, a reaction (also referred to as a first reaction) in which an organic carbonate is produced from carbon monoxide and the alcohol-based compound generally occurs in the electrochemical system 20. However, a reaction (also referred to as a second reaction) in which an organic oxalate is produced from carbon monoxide and the alcohol-based compound may also occur, and both the first and second reactions occur, but it is preferable that at least the first reaction occurs.

[0084] The first reaction is a carbonylation reaction in which an organic carbonate is produced. Specifically, the organic carbonate ((RO)) is produced by the reaction represented by the following formula (i): ２ CO) is produced. CO + 2ROH → (RO) ２ CO + 2H ＋ +2e －(i) In (i), R is the same as above, but preferably R is an alkyl group having 1 to 8 carbon atoms, an alkenyl group having 2 to 8 carbon atoms, or an aryl group having 6 to 8 carbon atoms, more preferably an alkyl group or an aryl group, and even more preferably an alkyl group. The alkyl group preferably has 1 to 3 carbon atoms, even more preferably has 1 or 2 carbon atoms, and most preferably has 1 carbon atom. However, as mentioned above, two or more alcohol compounds may be used in combination, and in that case, (RO) ２ Two R's in one molecule of CO may be different from each other.

[0085] When ROH is represented by the general formula (3-1), an organic carbonate is produced by the reaction shown in the following formula (ii). In addition, in formula (ii), R Ａ is the same as above, but R Ａ The number of carbon atoms is preferably 2 to 4, more preferably 2 to 3, and even more preferably 2.

[0086] Specific preferred organic carbonates include one or more selected from dimethyl carbonate, diethyl carbonate, ethylene carbonate, dipropyl carbonate, propylene carbonate, diphenyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, and ethyl propyl carbonate, and among these, dimethyl carbonate is more preferred.

[0087] The second reaction is a reaction in which carbon monoxide and an alcohol compound produce an organic oxalate represented by the following formula (4): Specifically, the organic oxalate represented by formula (4) may be synthesized by the reaction represented by formula (iii) below. (In formula (4), R is the same as above. However, two R in one molecule may be the same or different.) (In formula (iii), R is the same as above.)

[0088] When ROH is represented by the general formula (3-1), an organic oxalate represented by the following formula (4-1) is produced by the reaction represented by the following formula (iv). (In formula (4-1), R Ａ is the same as above.) (In the formula (iv), R Ａ is the same as above.)

[0089] Specific preferred organic oxalates include one or more selected from dimethyl oxalate, diethyl oxalate, ethylene oxalate, dipropyl oxalate, propylene oxalate, diphenyl oxalate, ethyl methyl oxalate, methyl propyl oxalate, and ethyl propyl oxalate. Among these, dimethyl oxalate is more preferred.

[0090] (Solvent) When the above-described reaction substrate is a solid or gas, or when it is necessary to improve the solubility of the redox species, the electrolytic solution 13 may further contain a solvent. In this case, the reaction substrate may be filled into the reaction chamber 21 or the anode chamber 10Y as a mixed solution with the solvent. Of course, even when the reaction substrate is a liquid, it may be filled as a mixed solution with the solvent.

[0091] The solvent can be appropriately selected from solvents commonly used in electrochemical reactions, such as nitrile solvents such as benzonitrile and acetonitrile; carbonate solvents such as ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate; lactone solvents such as γ-butyrolactone; ether solvents such as 1,2-dimethoxyethane, 1-ethoxy-2-methoxyethane, 1,2-diethoxyethane, tetrahydrofuran, and 2-methyltetrahydrofuran; phosphate ester solvents, phosphoric acids, sulfolane solvents, and pyrrolidones. Among these, nitrile solvents are preferred, and benzonitrile is more preferred. These solvents may be used alone or in combination of two or more. When a solvent is used, the concentration of the reaction substrate in the electrolytic solution 13 may be, for example, about 0.1 to 20 M, preferably 1 to 10 M.

[0092] [Anode] In the anode chamber 10Y of this embodiment, the anode 12 oxidizes the organic redox species (Q) to produce an active intermediate species, an oxidant (H ２ In the anode chamber 10Y, the anode 12 is preferably disposed in a position where it contacts the electrolyte solution 13 filled in the anode chamber 10Y. Note that the electrochemical cell 10 (e.g., the anode chamber 11) may be provided with a reference electrode or the like as needed.

[0093] The anode 12 preferably does not contain a catalyst. To incorporate a catalyst into the anode 12, a process is required in which a catalyst-containing coating solution is applied to an electrode substrate or the electrode substrate is immersed in a catalyst-containing immersion solution, thereby supporting the catalyst on the electrode. However, if the anode does not contain a catalyst, these processes are unnecessary. Therefore, a carbonyl compound can be synthesized with a simple configuration. Furthermore, the anode 12 can generate an oxidant even if it does not contain a catalyst.

[0094] The catalyst not contained in the anode 12 is a second catalyst that promotes an electrochemical reaction that synthesizes an organic carbonate, an organic oxalate, or both from carbon monoxide. Therefore, it is preferable that the anode 12 not support the second catalyst. In this embodiment, when the anode 12 comes into contact with the electrolytic solution 13 that contains a catalyst (second catalyst), the catalyst contained in the electrolytic solution 13 may adhere to the anode 12. However, in this specification, an embodiment in which the catalyst contained in the electrolytic solution adheres to the anode 12 is not included in the embodiment in which "the anode (second electrode) contains a catalyst."

[0095] The anode 12 is composed of, for example, an electrode substrate. Examples of the electrode substrate include, but are not limited to, a carbon substrate, a metal substrate, and a metal oxide substrate, and preferably have electrical conductivity. The substrate may also be a porous material. The electrode substrate can be a porous material to form a diffusion electrode. The electrode substrate can be a diffusion electrode, which can improve the efficiency of converting redox species to oxidants and, in the third embodiment described below, can improve the efficiency of synthesizing carbonyl compounds. The electrode substrate is a substrate that constitutes an electrode and may be, for example, in the form of a sheet or plate. Of the above, a carbon substrate is preferred, and porous carbon is more preferred. Details of the electrode substrate used in the anode 12 are the same as those of the electrode substrate used in the cathode 11, and therefore will not be repeated here.

[0096] The anode 12 is preferably laminated and integrated with the separator 25 (ion exchange membrane). Similarly, the cathode 11 is preferably laminated and integrated with the separator 25 (ion exchange membrane). Therefore, the cathode 11, separator 25, and anode 12 may be laminated in this order to form a laminate. The laminate may be a membrane-electrode assembly in which the anode 12 and the cathode 11 are joined, for example, via an ion exchange membrane. Furthermore, since the laminate is a membrane-electrode assembly, the reaction can proceed appropriately even if the reaction chamber 21 is eliminated, and electrolytes and the like can be reduced, as shown in a second embodiment described below. In this embodiment, when the cathode 11 is integrated with the partition wall 25, metal ions generated in the anode chamber 10Y tend to migrate to the cathode 11 side and precipitate as metal salts on the cathode 11. However, as described above, the precipitation of metal salts can be prevented by using specific organic redox species as the redox species and by making the electrolyte solution 13 substantially free of alkali metals and alkaline earth metals. This improves the efficiency of carbon dioxide reduction and also makes it easier to reduce carbon dioxide to carbon monoxide through a gas-phase reaction in the cathode chamber 10X.

[0097] [Separator Membrane] The separator membrane 25 preferably separates the carbon monoxide generated in the first catalyst 14 (cathode 11) from the redox species. The separator membrane 25 may be any membrane that is permeable to ions but not permeable to the reaction substrate, solvent, carbon monoxide, second catalyst, and redox species, and is preferably an ion exchange membrane. As the ion exchange membrane, a solid membrane is used, and examples thereof include a cation exchange membrane that is permeable to cations such as protons and an anion exchange membrane that is permeable to anions such as hydroxide ions, but an anion exchange membrane is preferred from the viewpoints of ionic conductivity and cost.

[0098] Examples of cation exchange membranes include those having at least one of sulfonyl, carboxyl, phosphate, and silicic acid groups as functional groups. Examples of cation exchange membranes having sulfonyl groups as functional groups include hydrocarbon resin-based polysulfonic acids such as polyethylene sulfonic acid and fullerene-crosslinked polysulfonic acid, and fluororesin-based sulfonic acids such as perfluoroethylene sulfonic acid. Examples of perfluoroethylene sulfonic acid include copolymers of tetrafluoroethylene and perfluoro[2-(fluorosulfonylethoxy)propyl vinyl ether], and commercially available products include "Nafion" (a trademark of DuPont). Examples of membranes having carboxyl groups as functional groups include polycarboxylic acids such as polyacrylic acid. Examples of membranes having phosphate or silicic acid groups as functional groups include heteropolyacids such as silicotungstic acid and phosphotungstic acid. Furthermore, cation exchange membranes made of SiO ２ -P ２ O ５ Phosphate glasses such as those listed above, and ceramics such as perovskite oxides can also be used.

[0099] Examples of anion exchange membranes include resins containing quaternary ammonium salts such as poly(styrylmethyltrimethylammonium chloride), polyethers, and polymers containing imidazolium groups. Examples of resins containing ammonium salts include "FAA-3-50" from FuMA-Tech GmbH and "TM1 Durion Grade" from Orion. Examples of polymers containing imidazolium groups include styrene-based polymers containing imidazolium groups, and specific examples include copolymers of styrene and 1-(p-vinylbenzyl)-3-methylimidazolium (PSMIM), copolymers of styrene and 1-(p-vinylbenzyl)-tetramethylimidazolium (PSTMIM), and copolymers of styrene and 1-(p-vinylbenzyl)-2,3-dimethylimidazolium (PSDMIM).

[0100] Second Embodiment Next, an electrochemical system 20A according to a second embodiment of the present invention will be described. Differences between the second embodiment and the first embodiment will be described below, and descriptions of similar components will be omitted. The second embodiment differs from the first embodiment in that the reaction chamber 21 is not provided. Therefore, in the electrochemical system 20A according to the second embodiment, the electrolyte 13 filled in the anode chamber 10Y is not circulated but remains in the anode chamber 10Y. Furthermore, the connection paths 18A and 18B connecting the electrochemical cell 10 and the reaction chamber 21 are not provided, and the first outlet 17X of the cathode chamber 10X is connected to the second supply port 16Y of the anode chamber 10Y via the connection path 18C. Thus, carbon monoxide generated in the anode chamber 10Y is supplied to the cathode chamber 10Y via the connection path 18C.

[0101] In the anode 12, as in the first embodiment, at least one of the quinone derivative and the anthraquinone derivative, which are organic redox species, is converted to an oxidant (Q). Then, in the anode chamber 10Y, carbon monoxide reacts with the reaction substrate in the presence of the second catalyst and the oxidant to be converted to a carbonyl compound. When being converted to a carbonyl compound, the oxidant (Q) of the redox species is converted to a reduced species (H ２Q). The converted reduced species (H ２ The carbon monoxide (Q) is converted back into the oxidant (Q) at the anode 12 and used to synthesize carbonyl compounds. In this embodiment, as in the first embodiment, the above reaction cycle is repeated, and the carbon monoxide supplied to the anode chamber 10Y is continuously converted into carbonyl compounds. In the electrochemical system 20A, the reaction may be carried out batchwise or continuously. When the reaction is carried out continuously, the electrolytic solution 13 may be continuously carried out by, for example, extracting a portion of the electrolytic solution 13 to the outside through the second outlet 17Y and supplying new electrolytic solution from the outside through the supply port 16Y.

[0102] In this embodiment, too, the use of a specific organic redox species as the redox species allows for the synthesis of the target carbonyl compound with high selectivity. Furthermore, since the electrochemical cell 10 is substantially free of alkali metals, alkaline earth metals, and the like, precipitation of inorganic metal salts based on alkali metals and alkaline earth metals can be prevented. This prevents a decrease in device durability and the occurrence of side reactions, and allows the target carbonate compound to be synthesized from carbon monoxide with high selectivity over a long period of time while efficiently reducing carbon dioxide to carbon monoxide. Furthermore, since the electrolytic solution 13 does not substantially contain an inorganic electrolyte, the number of steps required to separate and purify the carbonyl compound from the electrolytic solution 13 can be reduced. In addition, since the second reaction chamber 21 is not required in this embodiment, the configuration of the electrochemical system 20A can be simplified.

[0103] <Modification 1> In the first embodiment ( FIG. 1 ), the second catalyst is dispersed or dissolved in the electrolytic solution 13 and circulates between the reaction chamber 21 and the anode chamber 10Y together with the electrolytic solution 13. However, the second catalyst does not need to circulate between the first reaction chamber 21 and the anode chamber 10Y. For example, the second catalyst may be configured to remain in the reaction chamber 21. In this case, the second catalyst may be contained in the electrolytic solution 13 in the reaction chamber 21 but not contained in the electrolytic solution 13 in the anode chamber 10Y. A filter (not shown) may be attached to the discharge port 21B of the reaction chamber 21 to prevent the second catalyst in the electrolytic solution 13 filled in the reaction chamber 21 from being discharged from the discharge port 21B together with the electrolytic solution 13. A filter may also be provided in the supply port 21A of the reaction chamber 21. The provision of a filter in the supply port 21A can prevent the second catalyst in the reaction chamber 21 from flowing back and becoming mixed into the anode chamber 10Y, etc.

[0104] The filter may be any filter that does not allow the second catalyst to pass through. For example, when the second catalyst is dispersed in the electrolytic solution 13, a membrane filter, a mesh filter, or the like may be used. On the other hand, when the second catalyst is dissolved in the electrolytic solution 13, a chemical adsorption filter, a physical adsorption filter, or the like may be used. In the configuration of the above-described first modification, while the reaction of carbon monoxide and the reactant to synthesize a carbonyl compound takes place in the reaction chamber 21, the reaction of carbon monoxide and the reactant to synthesize a carbonyl compound does not substantially take place in regions other than the reaction chamber 21. Therefore, inside the anode chamber 10Y, the synthesis of carbonyl compounds does not take place, and the redox species are converted to oxidants, making it even less likely that by-products will be generated.

[0105] <Modification 2> In the first and second embodiments described above, the second catalyst is dispersed or dissolved in the electrolytic solution 13. However, the present invention is not limited to such an embodiment. The second catalyst may be configured to remain in a specific region by being filled inside the reaction chamber or the like. For example, in the first embodiment ( FIG. 1 ), the second catalyst may be filled in the reaction chamber 21. Specifically, the second catalyst may be filled in the form of a layer on a support such as a tray or mesh inside the reaction chamber 21. However, a support does not necessarily need to be provided inside the reaction chamber 21. Furthermore, in the second embodiment, the second catalyst does not need to be dispersed or dissolved in the electrolytic solution 13, 13B, and may be filled in the form of a layer in the anode chamber 10Y, for example.

[0106] The present invention will be explained in more detail by way of examples, but the present invention is not limited to these examples in any way.

[0107] [Example 1] (Preparation of first catalyst) 100 mg of Co(NO ３ ) ２ ・6H ２ 0 (manufactured by Aldrich), 200 mg of poly(4-vinylpyridine) (manufactured by Aldrich), and 100 mg of Ketjen Black (product name "EC-300J" (manufactured by Lion Chemicals), BET specific surface area 800 m ２ The resulting mixture was dispersed in 200 mL of ethanol, dried in an evaporator, and then heat-treated at 350° C. for 2 hours to obtain a first catalyst (Co / C).

[0108] (Preparation of Electrochemical System) A diffusion electrode (cathode) made of carbon paper (product name: H23C8 (manufactured by FREUDENBERG)) coated with 4 mg of the first catalyst (Co / C) was set on the cathode side of a two-compartment membrane electrolysis cell, and a diffusion electrode (anode) made of carbon paper (product name: H23C8 (manufactured by FREUDENBERG)) was set on the anode side, and the anode chamber and cathode chamber were separated by a Nafion ion exchange membrane. The anode, ion exchange membrane, and cathode were joined together to form a membrane-electrode assembly. A separate reaction chamber was prepared, and the outlet of the reaction chamber was connected to the supply inlet of the anode chamber, and the supply inlet of the reaction chamber was connected to the outlet of the anode chamber. An electrochemical system having the same configuration as the electrochemical system according to the first embodiment (see FIG. 1 ) was fabricated, except that the exhaust port of the cathode chamber was not connected to the reaction chamber, and the gas exhausted from the cathode chamber was not supplied to the reaction chamber and was subjected to gas analysis.

[0109] In the cathode chamber, CO ２ A total of 1 atom of HCl and saturated water vapor were supplied at a rate of 20 mL / min. On the other hand, 0.1 M of organic redox species, parabenzoquinone (Tokyo Chemical Industry Co., Ltd.), 4 mg of PdCl ２ The reaction chamber was filled with 50 ml of a benzonitrile solution (electrolyte) containing 5 M methanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), and CO (1 atom) was introduced into the reaction chamber to create a CO2-containing electrolyte. The CO2-containing electrolyte was supplied to the anode chamber using a liquid pump, and the electrolyte was circulated by supplying the electrolyte from the reaction chamber to the anode chamber at a flow rate of 5 mL per minute. A voltage was then applied so that the current value was -10 mA, causing a reaction. 30 minutes after voltage application, the gas components discharged from the cathode chamber were analyzed by gas chromatography to calculate the Faraday efficiency. The Faraday efficiency was calculated for carbon monoxide (CO). The results are shown in Table 1. The components of the electrolyte were analyzed by gas chromatography 30 minutes after voltage application to calculate the selectivity. The selectivity was calculated for organic carbonate. The results are shown in Table 1. Furthermore, after 96 hours of continuous voltage application, deposition of metal salts on the cathode electrode was visually confirmed. The results are shown in Table 1.

[0110] [Examples 2 to 5] The Faraday efficiency of the cathode and the selectivity of the organic carbonate at the anode were calculated in the same manner as in Example 1, except that the type of organic redox species filled in the anode chamber was changed as shown in Table 1. The organic redox species used in Examples 2 to 5 were all manufactured by Tokyo Chemical Industry Co., Ltd.

[0111] [Examples 6 to 10] Examples 6 to 10 were carried out in the same manner as Examples 1 to 5, except that the first catalyst was prepared by the following method. (Preparation of First Catalyst) 100 mg of Ag(NO ３ ) ２ (manufactured by Aldrich Co.), and 100 mg of Ketjen Black (product name "EC-300J" (manufactured by Lion Chemicals Co.), BET specific surface area 800 m ２ The resulting mixture was dispersed in 200 mL of ethanol, dried in an evaporator, and then heat-treated at 350° C. for 2 hours to obtain a first catalyst (Ag / C).

[0112] Comparative Examples 1 and 2 were carried out in the same manner as in Examples 1 and 6, except that, instead of the organic redox species filled in the anode chamber, an inorganic redox species, NaBr (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.), was used, as shown in Table 1.

[0113]

[0114] As shown in Table 1, in each example, in the electrochemical system, by adding a specific organic redox species to the electrolyte on the anode side, carbon monoxide was generated at a high faradaic efficiency on the cathode side, and organic carbonate was produced at a high selectivity from carbon monoxide and an alcohol-based compound on the anode side. In contrast, in each comparative example, a metal electrolyte was used in the electrolyte on the anode side, so carbon monoxide could not be generated at a high faradaic efficiency on the cathode side, and organic carbonate could not be synthesized at a high selectivity from carbon monoxide and an alcohol-based compound on the anode side. Furthermore, in each example, by adding a specific organic redox species to the electrolyte on the anode side without using a metal electrolyte (alkali metal or alkaline earth metal), it was confirmed that metal salts were not precipitated on the cathode, and carbon monoxide could be produced at a high faradaic efficiency on the cathode side for a long period of time.

[0115] REFERENCE SIGNS LIST 10 Electrochemical cell 10Y Anode chamber 10X Cathode chamber 11 Cathode (first electrode) 12 Anode (second electrode) 13, 13B Electrolyte 14 First catalyst 19 Power source 20, 20A, 20B Electrochemical system 21 Reaction chamber 25 Separation layer F Convection

Claims

1. An electrochemical system comprising: an electrochemical cell including a first electrode that reduces carbon dioxide to carbon monoxide, a second electrode, and an electrolyte; and a catalyst that synthesizes at least one carbonyl compound selected from the group consisting of organic carbonates and organic oxalates from carbon monoxide, wherein the electrolyte contains an organic redox species selected from the group consisting of quinone derivatives and anthraquinone derivatives.

2. The electrochemical system of claim 1, wherein the electrolyte is substantially free of alkali metals and alkaline earth metals.

3. The electrochemical system of claim 1 or 2, wherein the organic redox species comprises a quinone derivative.

4. The electrochemical system of claim 1 or 2, wherein the organic redox species comprises an anthraquinone derivative.

5. The electrochemical system according to claim 1 or 2, wherein the catalyst is contained in the electrolyte.

6. The electrochemical system according to claim 1 or 2, wherein the catalyst is not contained in the second electrode.

7. The electrochemical system according to claim 1 or 2, wherein the electrochemical cell comprises an ion exchange membrane that separates an area on the first electrode side from an area on the second electrode side.

8. The electrochemical system according to claim 7, wherein the first electrode is a diffusion electrode, and the first electrode and the ion exchange membrane are integral.

9. The electrochemical system according to claim 7, wherein the second electrode is a diffusion electrode, and the second electrode and the ion exchange membrane are integral with each other.

10. The electrochemical system according to claim 7, wherein carbon monoxide produced in the region on the first electrode side is supplied to the region on the second electrode side.

11. The electrochemical system according to claim 7, wherein the electrolytic solution has a convection current that circulates from the region on the second electrode side to the outside of the electrochemical cell and then returns to the region on the second electrode side.

12. The electrochemical system according to claim 1 or 2, wherein the catalyst is at least one selected from the group consisting of a salt of a metal selected from the group consisting of palladium and copper, and a catalyst comprising active particles having a metal element and a carbon compound supporting the active particles.

13. The electrochemical system according to claim 1 or 2, wherein the first electrode contains a reduction catalyst that reduces carbon dioxide to carbon monoxide, and the reduction catalyst is at least one selected from the group consisting of a metal or metal salt selected from the group consisting of cobalt and silver, and a catalyst containing active particles having a metal element and a carbon compound supporting the active particles.

14. The electrochemical system according to claim 1 or 2, wherein the electrolyte contains a reaction substrate.

15. The electrochemical system according to claim 14, wherein the reaction substrate is an alcohol-based compound.

16. A method for producing a carbonyl compound, comprising synthesizing carbon monoxide from carbon dioxide at the first electrode in the electrochemical system described in claim 1 or 2, and electrochemically synthesizing at least one carbonyl compound selected from the group consisting of organic carbonates and organic oxalates from the carbon monoxide obtained using the catalyst.

Images