Method for producing methanation catalyst, methanation catalyst, method for synthesizing methane, and cell for synthesizing methane

Abstract

A method for producing a methanation catalyst with improved methane selectivity, a methanation catalyst, a methane synthesis method, and a cell for methane synthesis are provided. [Solution] A method for producing a methanation catalyst that produces methane by reducing carbon dioxide includes the steps of: producing an intermediate by reacting a cerium-containing compound and a copper-containing compound with an organic acid containing citric acid; and calcining the intermediate to produce solid solution particles containing cerium and copper. The method for producing a methanation catalyst, the methanation catalyst, the methane synthesis method, and the methane synthesis cell are provided.

Description

[Technical Field]

[0001] The present disclosure relates to a method for producing a methanation catalyst, a methanation catalyst, a methane synthesis method, and a cell for methane synthesis. [Background technology]

[0002] Methods for producing target products such as hydrocarbons by reducing carbon dioxide are known. For example, a carbon dioxide reduction catalyst is known that reduces carbon dioxide by hydrogenating it and produces methanol (Patent Document 1). The carbon dioxide reduction catalyst described in Patent Document 1 includes a carrier containing an oxygen storage-release material and a catalytic metal made of a transition metal that is supported on the carrier. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2021-146258 Summary of the Invention [Problem to be solved by the invention]

[0004] In some cases, a carbon dioxide reduction catalyst produces not only the target compound but also by-products. For example, in the case of Patent Document 1, it is described that not only the target compound, methanol, but also by-products such as carbon monoxide are produced, but the hydrogenation reaction of carbon monoxide also proceeds to produce methanol. In the methanation of carbon dioxide to produce methane using a carbon dioxide reduction catalyst, it is desirable to produce as much of the target methane as possible.

[0005] In view of the above, an object of the present disclosure is to provide a method for producing a methanation catalyst with improved methane selectivity, a methanation catalyst, a methane synthesis method, and a cell for methane synthesis. [Means for solving the problem]

[0006] Specific means for solving the problems include the following aspects. <1> A method for producing a methanation catalyst that produces methane by reducing carbon dioxide, the method comprising the steps of: producing an intermediate by reacting a cerium-containing compound and a copper-containing compound with an organic acid that includes citric acid; and calcining the intermediate to produce solid solution particles that contain cerium and copper. <2> Organic acids also include malic acid <1> A method for producing the methanation catalyst described in <3> The step of producing the intermediate includes reacting an organic acid with a mass containing at least 1.2 times the amount of acid groups required to react with all metal ions contained in the cerium-containing compound and the copper-containing compound. <1> or <2> A method for producing the methanation catalyst described in <4> A methanation catalyst that produces methane by reducing carbon dioxide, comprising a carrier containing cerium (IV) oxide and carrying the catalytic metal copper, and having a particle shape with multiple primary particles arranged on the surface, each primary particle having a spherical portion. <5> X-ray diffraction analysis shows that the crystallite size of cerium(IV) oxide is 3nm to 12nm. <4> The methanation catalyst according to claim 1. <6> The copper content is 1.0% to 5.0% by mass relative to the cerium (IV) oxide content. <4> or <5> The methanation catalyst according to claim 1. <7> A methane synthesis cell for generating methane by a reduction reaction using an electrochemical reaction of carbon dioxide, <4> or <5> 10. A cell for methane synthesis comprising a diffusion layer supporting the methanation catalyst according to claim 1, and an electrolyte, wherein the electrolyte is a solid polymer membrane. <8> The diffusion layer is made of carbon paper with a fibrous surface carrying a methanation catalyst. <7> The methane synthesis cell according to claim 1. <9> <4> or <5> A method for synthesizing methane, comprising reducing carbon dioxide using the methanation catalyst according to claim 1. <10> <7> or <8> 2. A method for synthesizing methane, comprising reducing carbon dioxide using the methane synthesis cell according to claim 1. [Effects of the Invention]

[0007] According to embodiments of the present disclosure, a method for producing a methanation catalyst with improved methane selectivity, a methanation catalyst, a methane synthesis method, and a methane synthesis cell are provided. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is an image obtained by observing a methanation catalyst according to one embodiment of the present disclosure at 10,000x magnification using a scanning electron microscope. [Figure 2] FIG. 2 is an explanatory diagram illustrating the configuration of the methane synthesis cell. [Figure 3] FIG. 3 shows images obtained by observing methanation catalyst 1, which is an embodiment of the present disclosure obtained in the Examples, and methanation catalyst 5 obtained by a conventional method using a scanning electron microscope at magnifications of 5,000x, 10,000x, and 30,000x. [Figure 4] FIG. 4 is an explanatory diagram illustrating the methane synthesis apparatus used in the examples. [Figure 5] FIG. 5 is a graph showing the faradaic efficiency of the methanation catalyst obtained in the examples in the synthesis of methane and the like. [Figure 6] FIG. 6 is a graph showing the faradaic efficiency of the methanation catalyst obtained in the examples in the synthesis of methane, etc. [Figure 7] FIG. 7 is an image obtained by observing a cross section of the diffusion layer obtained in the example cut in the thickness direction with a SEM reflection electron microscope. DETAILED DESCRIPTION OF THE INVENTION

[0009] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments and can be implemented with appropriate modifications within the scope of the purpose of the present disclosure. Components indicated by the same reference numerals in each drawing are the same components. Explanations of duplicated components and reference numerals in each drawing may be omitted. The sizes of the components in each drawing are conceptual, and the relative size relationships between the components are not limited thereto.

[0010] In the present disclosure, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the upper and lower limits. In the numerical ranges described in stages in the present disclosure, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in the numerical ranges described in the present disclosure, the upper or lower limit described in a certain numerical range may be replaced with a value shown in the examples.

[0011] In the present disclosure, "%" with respect to the content means "% by mass" unless otherwise specified.

[0012] In the present disclosure, the term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes as long as the intended purpose of the process is achieved. In the present disclosure, when a plurality of substances corresponding to each component are present in the composition, the amount of each component in the composition means the total amount of the plurality of substances present in the composition, unless otherwise specified. In the present disclosure, a combination of two or more preferred aspects or embodiments is a more preferred aspect or embodiment.

[0013] A method for producing a methanation catalyst according to one embodiment of the present disclosure is a method for producing a methanation catalyst used for methanation of carbon dioxide, and includes the steps of: reacting a cerium-containing compound and a copper-containing compound with an organic acid containing citric acid to produce an intermediate; and calcining the intermediate to produce solid solution particles containing cerium and copper. A methanation catalyst according to one embodiment of the present disclosure is a methanation catalyst that produces methane by reducing carbon dioxide, and comprises a carrier containing cerium (IV) oxide (hereinafter also referred to as ceria) on which a catalytic metal, which is copper, is supported. The catalyst has a shape in which a plurality of primary particles are arranged on the surface, and the primary particles have spherical portions. A methane synthesis cell according to one embodiment of the present disclosure is a methane synthesis cell that produces methane through a reduction reaction using an electrochemical reaction of carbon dioxide, and includes a diffusion layer that supports the methanation catalyst of the present disclosure and an electrolyte, where the electrolyte is a solid polymer membrane. The methanation catalyst according to one embodiment of the present disclosure synthesizes methane by reducing carbon dioxide using the methanation catalyst or methane synthesis cell of the present disclosure.

[0014] The background to the present disclosure will be explained. When methane is produced by a reduction reaction using an electrochemical reaction of carbon dioxide, for example, a method using a solid polymer membrane uses carbon dioxide and water, and the reaction can be carried out at a low temperature. Furthermore, methane can be synthesized in a single step using carbon dioxide and water, which is preferable because it allows for the use of larger facilities and reduces facility costs. However, different routes of reaction are thought to occur during the reduction of carbon dioxide, and a wide variety of products tend to be produced, including methane, ethylene, methanol, and carbon monoxide. If methane is the desired product, products other than methane are by-products. Methanation, which produces methane by the reduction of carbon dioxide, consists of many elementary reactions, and therefore it is conceivable that a wide variety of products such as those described above are produced. Therefore, it is desirable to use a catalyst that improves the ratio of methane produced among the products, that is, methane selectivity.

[0015] The present inventors have focused on methanation catalysts used in producing methane by reducing carbon dioxide, and have found that methane selectivity is improved by using a methanation catalyst that is a solid solution particle containing cerium and copper and is produced by a production method using an organic acid containing citric acid (hereinafter also referred to as the organic acid method).

[0016] According to the method for producing a methanation catalyst, the methanation catalyst, the methane synthesis cell, or the methane synthesis method of one embodiment of the present disclosure, it is possible to improve methane selectivity when producing methane by reducing carbon dioxide.

[0017] The mechanism by which the above-mentioned effects are obtained is not entirely clear. However, the inventors speculate that the methanation catalyst, which is a solid solution particle containing cerium and copper and produced by a production method using an organic acid containing citric acid, allows ceria to function as a carrier having an oxygen storage / release function, maintaining copper, the catalytic metal, in a metallic state having stable reactive active sites, etc., and that the metallic copper having stable reactive active sites is highly dispersed in the ceria carrier, and the methanation catalyst has a particle shape having a plurality of primary particles with spherical portions on the surface, thereby increasing the number of stable reactive active sites for methane synthesis by copper on the surface of the ceria carrier. Here, "highly dispersed" means that the mixture includes a portion that is mixed at the atomic level.

[0018] <Methanation catalyst manufacturing method> A method for producing a methanation catalyst (hereinafter also referred to as a method for producing a methanation catalyst) according to one embodiment of the present disclosure is a method for producing a methanation catalyst used for methanation of carbon dioxide, and includes the steps of: reacting a cerium-containing compound and a copper-containing compound with an organic acid containing citric acid to produce an intermediate; and calcining the intermediate to produce solid solution particles containing cerium and copper.

[0019] (Intermediate generation process) The method for producing a methanation catalyst includes a step of producing an intermediate (hereinafter also referred to as an intermediate production step) by reacting a cerium-containing compound and a copper-containing compound with an organic acid containing citric acid. In the present disclosure, both compounds containing cerium and compounds containing copper are also referred to as metal compounds. A method for producing an oxide by reacting a metal compound with an organic acid is known as an organic acid method. In the present disclosure, an oxide containing cerium and copper may be produced by a conventionally known organic acid method.

[0020] As the metal compound, compounds used in the conventionally known organic acid method, such as carbonates, basic carbonates, hydroxides, nitrates, acetates, etc. Among them, carbonates or hydroxides of the metal contained in the metal compound are preferred.

[0021] The organic acid refers to an organic compound that is an acid. The organic acid preferably contains an acid group. The organic acid preferably contains a carboxylic acid group as the acid group. The organic acid is more preferably a hydroxycarboxylic acid that contains a hydroxyl group and a carboxylic acid group.

[0022] In addition to citric acid, organic acids that can be used include, for example, lactic acid, malic acid, citric acid, tartaric acid, glycolic acid, and glycine. Of these, it is preferable to use malic acid in addition to citric acid. Only one organic acid or two or more organic acids may be used.

[0023] In the intermediate production step, a metal compound is reacted with an organic acid containing citric acid. In the intermediate production step, the organic acid is preferably reacted in a solution. The solution may be an aqueous solution or an organic solvent, and the reaction is preferably carried out in an aqueous solution. A glycol such as propylene glycol may also be added to the solution. The temperature at which the metal compound is reacted with the organic acid is preferably within a range of 40° C. to 120° C., more preferably 45° C. to 80° C., and even more preferably 45° C. to 60° C. Within the above range, an oxide in which copper is highly dispersed in ceria can be obtained.

[0024] The order in which the metal compound and the organic acid are reacted is not particularly limited, but the methanation catalyst produced by the method for producing a methanation catalyst preferably comprises mixing a solution of a cerium-containing compound with an organic acid to prepare a mixture, and then adding a copper-containing compound to the mixture. This allows for the production of an oxide in which copper, which provides stable reaction active sites, is dispersed at the atomic level on the ceria support.

[0025] In the intermediate production step, when the required amount of organic acid is defined as the mass of organic acid containing the amount of acid groups necessary for reaction with all metal ions contained in the metal compound, the amount of organic acid used in the intermediate production step, which is the mass of organic acid used, is preferably greater than the required amount of organic acid. In other words, the amount of organic acid used in the intermediate production step is preferably such that the amount of acid groups contained in the organic acid used is in excess of the amount required for reaction with all metal ions.

[0026] Specifically, in the intermediate production step, the amount of organic acid used is preferably at least 1.2 times the amount of acid groups required to react with all metal ions contained in the metal compound, more preferably 1.5 to 5 times, and particularly preferably 2 to 4 times the amount of acid groups. The above range is preferable because by reacting the metal compound with the organic acid, fine solid solution particles with a highly uniform composition can be obtained.

[0027] The acid group contained in the organic acid is preferably a carboxylic acid. When the acid group contained in the organic acid is a carboxylic acid, the required amount of organic acid can be calculated from the number of carboxylic acid groups (-COOH) contained in the organic acid relative to the valence of the metal element contained in the metal compound. For example, when the metal element is cerium, cerium is tetravalent, and 4 moles of carboxylic acid groups are required for 1 mole of cerium. In contrast, when the organic acid is citric acid, citric acid has three carboxylic acid groups, so the required amount of organic acid is 4 / 3 moles of citric acid for 1 mole of cerium. When the amount of organic acid used is in excess of the amount of acid groups required to react with all metal ions contained in the metal compound, the composition uniformity of each metal element contained in the methanation catalyst is improved.

[0028] (Firing process) The method for producing a methanation catalyst preferably includes a step of calcining the intermediate to produce solid solution particles containing cerium and copper (hereinafter also referred to as the calcination step). A solid solution refers to a solid phase in which two or more metals are mixed together at the atomic level, and in the present disclosure, it is considered to be a solid phase in which copper, the catalytic metal, is dispersed at the atomic level in the ceria support.

[0029] The intermediate to be fired is preferably a dried intermediate obtained by drying or dehydrating a solution containing the intermediate in advance. Therefore, the firing step may include a step (drying step) of drying or dehydrating a solution containing the intermediate. It is preferable to perform the firing step after the drying step.

[0030] In the drying step, the drying method is not limited, and examples thereof include a method using a dryer. When a dryer is used, drying is preferably carried out within a range of 50°C to 130°C, more preferably within a range of 105°C to 115°C. It is believed that the drying of the intermediate product produces an organic acid salt containing citrate, in which the metal and citric acid are bonded together. If the content is within the above range, solid solution particles containing fine citrate particles can be obtained in the subsequent process.

[0031] In the firing step, firing conditions are preferably 250°C to 450°C for 2 to 72 hours, more preferably 4 to 24 hours, for low-temperature firing, and 450°C to 700°C for 2 to 72 hours, more preferably 4 to 24 hours, for high-temperature firing. Under the above firing conditions, a fired product is obtained by firing the intermediate. Among the calcination conditions, the primary particle size of the methanation catalyst produced tends to differ between low-temperature calcination and high-temperature calcination. Specifically, the primary particle size tends to be smaller in low-temperature calcination than in high-temperature calcination. Therefore, low-temperature calcination is preferred because it increases the surface area of ​​the solid solution particles.

[0032] Solid solution particles are obtained from the calcined product. The calcined product is preferably pulverized. The pulverization can be carried out by a conventionally known method. By pulverization, fine powder-like solid solution particles containing particles can be obtained. The solid solution particles can be used as a methanation catalyst. Therefore, the methanation catalyst is a particle having a particulate shape.

[0033] <Methanation catalyst> A methanation catalyst (hereinafter also referred to as methanation catalyst) according to one embodiment of the present disclosure is a methanation catalyst that produces methane by reducing carbon dioxide, and comprises a carrier containing cerium (IV) oxide, on which copper, a catalytic metal, is supported, and has a particle shape with a plurality of primary particles arranged on the surface, and the primary particles preferably have spherical portions. That is, the methanation catalyst is a secondary particle formed by aggregation of primary particles, and the secondary particle has a particle shape in which a plurality of primary particles are arranged on the surface. The methanation catalyst has the specific particle shape described above, and its crystal plane structure is thought to provide a large contact area between the catalytic metal copper and carbon dioxide. This structure is thought to significantly improve methane selectivity.

[0034] A primary particle is a part of a methanation catalyst that has a particulate form and is visible when the methanation catalyst is observed under an electron microscope. A primary particle may consist of multiple crystallites. Here, a crystallite refers to an even smaller crystal that constitutes a primary particle. When the methanation catalyst has a particulate shape with a plurality of primary particles arranged on the surface, when the methanation catalyst is observed under an electron microscope, it is possible to observe a methanation catalyst having a shape in which a plurality of primary particles, which are usually smaller than the individual methanation catalyst particles, are arranged on the surface of the individual methanation catalyst particles.

[0035] The size of the primary particles is preferably 5 nm to 50 nm, and more preferably 10 nm to 30 nm. The size of the primary particles can be controlled by conditions such as the firing temperature. The particle size of the particulate methanation catalyst is preferably 0.5 μm to 20 μm, and more preferably 1 μm to 10 μm. The size of the primary particles can be controlled by conditions such as the calcination temperature and pulverization.

[0036] Specifically, as shown in Figure 1, when the methanation catalyst is observed with a scanning electron microscope (SEM), it is observed that the catalyst has a particulate shape with multiple primary particles on its surface. Furthermore, the primary particles of the methanation catalyst are observed to be rounded and have spherical portions. Since the primary particles are rounded and have spherical portions, it is thought that the surface area of ​​the methanation catalyst is large, and the contact area between the catalytic metal copper and carbon dioxide is also large. The methanation catalyst is composed of secondary particles formed by agglomeration of primary particles, but the secondary particles may have any shape as long as they are larger than the primary particles and have an independent shape. Therefore, the secondary particles may have various shapes, including plate-like, spherical, etc.

[0037] In the methanation catalyst, the crystallite size of the cerium (IV) oxide as determined by X-ray diffraction analysis is preferably 3 nm to 12 nm. The crystallite size is more preferably 6 nm to 10 nm. When the crystallite size is within the above range, the methanation catalyst has particularly excellent methane selectivity. The crystallite size determined by X-ray diffraction analysis can be calculated using the Scherrer equation for the (111) plane diffraction line of cerium oxide.

[0038] In addition, the specific surface area of ​​the methanation catalyst measured by the BET method is 50 m 2 / g~120m 2 / g, more preferably 55m 2 / g~100m 2 When the specific surface area is within the above range, a methanation catalyst with an excellent methane yield can be obtained. The specific surface area according to the BET method can be measured by a method in accordance with JIS Z 8830:2013.

[0039] In the methanation catalyst, the copper content is preferably 1.0% to 5.0% by mass relative to the ceria content, and more preferably 1.5% to 3.0%. Within this range, the reduction reaction of carbon dioxide proceeds, improving the yield of the methane-containing product itself and the proportion of methane in the product. Therefore, the methane yield is improved. The content of each component in the methanation catalyst can be measured, for example, by ICP (Inductively Coupled Plasma) emission spectrometry.

[0040] A methane synthesis method (hereinafter also referred to as the methane synthesis method) according to one embodiment of the present disclosure is a method for synthesizing methane by reducing carbon dioxide using a methanation catalyst. The methanation catalyst has improved methane selectivity. Therefore, according to the methane synthesis method, methane can be synthesized at a high yield using carbon dioxide and water as raw materials.

[0041] The methanation catalyst or the methane synthesis method can be used in various methane synthesis cell configurations as long as it can promote the electrochemical reaction of carbon dioxide. For example, the methanation catalyst and the methane synthesis method can be used not only in cells using a solid polymer electrolyte but also in cells using a liquid electrolyte.

[0042] <Methane synthesis cell> The methanation catalyst or methane synthesis method is suitable for carbon dioxide reduction. For example, it is suitable for carbon dioxide reduction using a solid polymer as an electrolyte, and is suitable for carbon dioxide reduction by a method called PEM (Polymer Electrolyte Membrane) reduction method. Therefore, the methane synthesis cell can adopt a configuration that is conventionally known for carbon dioxide reduction by the PEM method. The methane synthesis cell is a methane synthesis cell that produces methane through a reduction reaction using an electrochemical reaction of carbon dioxide, and includes a diffusion layer that supports a methanation catalyst and an electrolyte, and the electrolyte is preferably a solid polymer membrane.

[0043] (Cell configuration) The methane synthesis cell of the present disclosure includes an electrolyte and a diffusion layer. The diffusion layer has a catalyst layer supporting a methanation catalyst. 2 , a methane synthesis cell 10 according to the present disclosure is configured by electrically connecting two diffusion layers, each including an anode 13 for generating oxygen and a cathode 12 for synthesizing methane, which are arranged on either side of an electrolyte 11, which is a solid polymer membrane. That is, one diffusion layer serves as the anode 13, and the other diffusion layer serves as the cathode 12. The diffusion layer according to the present disclosure is used as the diffusion layer for the cathode 12. 2, as indicated by the straight line and the arrow, electrons move from the cathode 12 to the anode 13. The movement of hydroxide ions in the electrolyte 11 causes an electrochemical reaction to proceed. Flow path 15 is a flow path through which water and oxygen flow, and oxygen is generated by introducing water into flow path 15 and then discharged. Flow path 14 is a flow path through which carbon dioxide and methane flow, and methane is generated by introducing carbon dioxide into flow path 14 and then discharged. The methane synthesis cell 10 is disposed within a cell structure 16 .

[0044] (electrolyte) Since the methanation catalyst of the present disclosure is suitable for reducing carbon dioxide, the electrolyte is preferably a solid polymer membrane. As the electrolyte, any known electrolyte for batteries or the like can be used. Specifically, it is preferable to use a solid polymer membrane containing an electrolyte, such as polyethylene oxide.

[0045] (diffusion layer) In the methane synthesis cell of the present disclosure, any diffusion layer can be used as long as it can be used as the diffusion layer of the methane synthesis cell by supporting the methanation catalyst of the present disclosure on the catalyst layer. The diffusion layer can be made of a conventionally known porous material, such as carbon paper.

[0046] In the methane synthesis cell of the present disclosure, the diffusion layer is preferably carbon paper having a fibrous surface. By supporting the methanation catalyst of the present disclosure on the surface of carbon paper and using it as a catalyst layer, the methane yield is improved.

[0047] The methanation catalyst of the present disclosure can be supported on the catalyst layer by a conventionally known method, for example, by applying a dispersion solution prepared by dispersing fine powdery solid solution particles, which are the methanation catalyst of the present disclosure, in a solvent to carbon paper, which is the catalyst layer, and then drying the solvent.

[0048] The methane synthesis cell of the present disclosure includes, in addition to an electrolyte and a diffusion layer, electrodes including an anode and a cathode for promoting an electrochemical reduction reaction of carbon dioxide, a housing, and the like, and these components may each be configured in a manner that is conventionally known. The methane synthesis cell of the present disclosure may be used as a single cell or as a combination of multiple cells connected together.

[0049] The methane synthesis method of the present disclosure is a method for synthesizing methane by reducing carbon dioxide using the methane synthesis cell of the present disclosure. By using the synthesis cell of the present disclosure, methane can be synthesized with improved methane selectivity. Therefore, the methane synthesis cell of the present disclosure can synthesize methane at a high yield using carbon dioxide and water as raw materials. [Example]

[0050] The present disclosure will be described in further detail below based on examples. The materials, amounts used, ratios, processing details, processing procedures, etc. shown in the following examples can be changed as appropriate without departing from the spirit of the present disclosure. Therefore, the scope of the present disclosure should not be construed as being limited by the following examples.

[0051] Example 1 (Synthesis of methanation catalysts using organic acids) 102 g of cerium carbonate with a Ce content of 45.5% by mass, 140 g of citric acid monohydrate, and 134 g of malic acid were added to 1000 g of ion-exchanged water, and the mixture was heated to 50°C with stirring to dissolve. Next, 3.6 g of copper acetate with a Cu content of 31.8% by mass was added and dissolved at 50°C with stirring to prepare a solution, which was then held for 2 hours. The solution was dried in a dryer at 108°C to obtain a Cu-Ce-organic acid intermediate. The Cu-Ce-organic acid intermediate was placed in an alumina sagger and fired in an electric furnace at 400°C for 6 hours. The fired product was pulverized in a mortar to obtain solid solution particles of an oxide containing cerium and copper, in which copper and cerium were dissolved in a solid solution of Cu / CeO2 = 2 mass%. These solid solution particles were designated as methanation catalyst 1. The mass of organic acid used was 2.9 times the required amount. The amount of cerium element is 0.33 moles, and since the valence is tetravalent, the number of carboxylic acids required is 1.33 moles. The amount of copper element is 0.018 moles, and since the valence is divalent, the number of carboxylic acids required is 0.036 moles, so the number of carboxylic acids required for cerium and copper combined is 1.37 moles. There are 0.66 moles of citric acid, so the number of carboxylic acids is 1.99 moles, and there are 1.0 moles of malic acid, so the number of carboxylic acids is 1.99 moles, for a total of 3.98 moles. Dividing by the amount of acid groups required, 2.9 times the mass of organic acid was used based on the amount of acid groups required.

[0052] Example 2 Except for adjusting the amounts of raw materials so that the solid solution particles had a Cu / CeO2 ratio of 1 mass %, solid solution particles were obtained in the same manner as in Example 1. The solid solution particles were designated as methanation catalyst 2.

[0053] Example 3 Solid solution particles were obtained in the same manner as in Example 1, except that the Cu-Ce-organic acid intermediate was placed in an alumina sagger and fired in an electric furnace at 700°C for 6 hours. These solid solution particles were designated as methanation catalyst 3.

[0054] Example 4 Except for adjusting the amounts of raw materials so that the solid solution particles had a Cu / CeO2 ratio of 5 mass%, solid solution particles were obtained in the same manner as in Example 1. The solid solution particles were designated as methanation catalyst 4.

[0055] (Comparative Example 1) (Synthesis of methanation catalyst by hydrothermal method) An aqueous solution of sodium bicarbonate was prepared by dissolving 14.4 g of sodium bicarbonate in 200 g of ion-exchanged water, and an aqueous solution of cerium nitrate was prepared by dissolving 72 g of cerium nitrate hexahydrate in 100 g of ion-exchanged water. The aqueous sodium hydrogen carbonate solution was heated to 40°C, and the aqueous cerium nitrate solution was added dropwise over 30 minutes while stirring, and the mixture was further stirred for 5 hours while maintaining the temperature at 40°C. The resulting precipitate was filtered and washed with ion-exchanged water. The resulting precipitate was pulverized in a ball mill for 8 hours, then transferred to a reaction vessel, and 500 g of ion-exchanged water was added. The mixture was refluxed for 8 hours while stirring, cooled, filtered, and washed. The resulting residue was calcined at 350°C for 6 hours to obtain cerium oxide powder. 18 g of cerium oxide powder ground in a mortar and 18 g of copper acetate aqueous solution with a copper concentration of 2% were added to an evaporating dish, and after 10 minutes of ultrasonic treatment in an ultrasonic cleaner, the mixture was dried at 60°C while mixing appropriately to obtain a dry powder. The dried powder was transferred to a crucible and fired at 400°C for 6 hours. The fired product was pulverized in a mortar to obtain solid solution particles of oxides containing cerium and copper, with Cu / CeO2 = 2% by mass. These solid solution particles were designated methanation catalyst 5.

[0056] (Evaluation: particle shape) Methanation catalyst 1 obtained in Example 1 and methanation catalyst 5 obtained in Comparative Example 1 were observed under a scanning electron microscope at 5,000x, 10,000x, and 30,000x magnifications. Images obtained by the observations are shown in Figure 3. As shown in Figure 3, methanation catalyst 1, an embodiment of the present disclosure shown in the column labeled "Citric acid synthesis method, 2 wt% Cu-CeO2," had a shape with multiple primary particles arranged on its surface, and the primary particles had spherical portions. In contrast, methanation catalyst 5 obtained by the hydrothermal method in Comparative Example 1 shown in the column labeled "Hydrothermal method, 2 wt% Cu-CeO2" consisted entirely of rod-shaped particles, with no primary particles having spherical portions.

[0057] (specific surface area) The specific surface areas of methanation catalyst 1 obtained in Example 1 and methanation catalyst 5 obtained in Comparative Example 1 were measured by the BET method using a Macsorb Model HM-1208 manufactured by Mountech Co., Ltd. The sample weight was approximately 0.2 g, and pretreatment was carried out at 300°C for 15 minutes. Methanation catalyst 1 had a specific surface area of ​​70.3 m 2 / g, whereas the methanation catalyst 5 obtained by the hydrothermal method in Comparative Example 1 had a specific surface area of ​​47.3 m 2 / g.

[0058] (crystallite diameter) The crystallite diameters of methanation catalyst 1 obtained in Example 1 and methanation catalyst 5 obtained in Comparative Example 1 were calculated from the results of X-ray diffraction. X-ray diffraction was measured using a Rigaku SmartLab SE with a Cu tube under conditions of 40 kV, 40 mA, a step width of 0.01 degrees, and a scan rate of 0.5 degrees / min. Analysis was performed using Rigaku's SmartLab Studio II software, and the crystallite diameter was calculated using the Scherrer equation for the (111) plane diffraction line of cerium (IV) oxide around 2Θ = 28.5 degrees. In the calculation, a Scherrer constant of 0.94 and a value for the half-width were used. The methanation catalyst 1 obtained in Example 1 had a crystallite diameter of 9.0 nm. The methanation catalyst 5 obtained by the hydrothermal method in Comparative Example 1 had a crystallite diameter of 9.8 nm.

[0059] Example 5 Using the methanation catalyst 1 obtained in Example 1, methane synthesis was carried out by a reduction reaction using an electrochemical reaction of carbon dioxide as follows.

[0060] (catalyst coating) 5 mg of methanation catalyst 1 powder was weighed out, and 2 mL of isopropanol and 2 μL of NAFION (registered trademark) were added thereto, followed by ultrasonic dispersion for 5 minutes to prepare a dispersion solution of methanation catalyst 1. A roughly rectangular carbon fiber sintered compact with a thickness of 280 μm was placed on a hot plate set at 100 to 200°C, and a dispersion solution of methanation catalyst 1 was applied to the top surface (the surface opposite to the surface in contact with the hot plate) using a micropipette. The isopropanol solvent was evaporated after leaving it as it was for a while, thereby producing a diffusion layer 1.

[0061] A cross section of the diffusion layer 1 cut in the thickness direction was observed using a SEM (reflection electron microscope). As shown in Figure 7, the image obtained by observation confirmed that copper, the catalytic metal that appears white, was present throughout the diffusion layer 1.

[0062] The materials and equipment used in producing the diffusion layer 1 are as follows: Sintered carbon fiber (manufactured by Toray Industries, Inc.) Model: TGP-H-090, porosity: 78%, density: 0.44g / cm 3 Water-repellent treatment: 5% (the mass percentage of the water-repellent material applied to the total mass of the carbon fiber sintered body) 2-Propanol (Sigma-Aldrich Japan) Model number: 15-2320-5 Nafion (Fujifilm Wako Pure Chemical Industries, Ltd.) Model number: 323-86702 Ultrasonic dispersion device (manufactured by Nippon Seiki Co., Ltd.) Model number: MB840-B Heater (manufactured by AS ONE) Model number: EHP-170N

[0063] (methane synthesis) A methane synthesis device 20 using a gas phase half cell was fabricated, and methane synthesis was carried out by an electrochemical reaction of carbon dioxide using a methanation catalyst 1 via a diffusion layer 1. 4, the methane synthesis apparatus 20 was equipped with a working electrode (WE) 21, a reference electrode (RE) 22, and a counter electrode (CE) 23. The working electrode (WE) 21 was equipped with a diffusion layer 1, the reference electrode (RE) 22 was equipped with an Ag / AgCl electrode, and the counter electrode (CE) 23 was equipped with a carbon rod.

[0064] The diffusion layer 1 was arranged in the methane synthesis apparatus 20 so that the gas 25 contacted the surface coated with the methanation catalyst 1 (hereinafter also referred to as the catalyst layer) and the opposite surface contacted the electrolyte 24. In the methane synthesis apparatus 20, the diffusion layer 1 was installed such that CO - The cathode reaction is carried out by supplying two gases, and the electrolyte 24 is stored on the other surface of the diffusion layer 1 to carry out an anode reaction. The methane synthesis apparatus 20 was configured to include a voltage application device 26 that controls and applies voltage, a flow meter 27 that measures the flow rate of the generated gas, and a gas chromatograph 28 that collects and analyzes the generated gas.

[0065] 5 ccm of CO2 was flowed on the cathode side and 5 ccm of N2 on the anode side, and the cathode and anode gases were replaced for a sufficient period of time. ccm stands for cubic centimeters / min, and indicates the volume (cc) when the flow rate continues for 1 minute under conditions equivalent to 25°C and 1 atmosphere. In the methane synthesis device 20, no reaction occurs on the anode side, but since the gas produced on the cathode side may leak to the anode side, N2 gas was also flowed on the anode side to recover the leaked gas on the anode side.

[0066] The voltage was controlled and applied based on the cathode side Ag / AgCl reference electrode (RE) 22. The applied voltage was −1.7 V. After starting voltage application and confirming that current was flowing, the flow rate of the produced gas and N2 gas was measured with a flow meter connected to the exhaust line of the gas chromatograph 28 for 1.5 to 2 minutes until analysis and measurement by the gas chromatograph 28. The gas flow rate during this period was about 10 ccm. The amounts of methane, carbon monoxide, ethylene, and hydrogen produced were calculated from the flow rate measured by the flowmeter 27 and the generated gas analyzed by the gas chromatograph 28, and converted into electric charges. The methane production efficiency (Faraday efficiency) was calculated from the ratio of the converted electric charge to the actual current value, and used as an index of methane selectivity. The calculated Faradaic efficiency (FE, unit: %) is shown in the graph in Figure 5.

[0067] The conditions and equipment for methanation using the methane synthesis unit 20 are as follows. <Condition> ·Temperature: Room temperature CO2 gas: 99.99% purity ·Electrolyte: 0.1M, KHCO3 solution Applied voltage: The voltage between the working electrode (WE) 21 and the reference electrode (RE) 22 was controlled at −1.7 V (Example 5), −2.0 V (Example 6), and −2.5 V (Example 7). <Device> Gas chromatography (Shimadzu Corporation) Model:GC-2014 Flow meter (Mesa Labs) Model: Defender 530+ (LOW type), Flow range: 5ml / min to 500ml / min

[0068] (Examples 6 and 7) Methane synthesis was carried out in the same manner as in Example 5, except that the applied voltage was −2.0 V (Example 6) or −2.5 V (Example 7). The calculated faradaic efficiency (FE, unit: %) is shown in the graph of FIG.

[0069] (Comparative Example 2) Methane synthesis was carried out in the same manner as in Example 5, except for using the methanation catalyst 5 obtained in Comparative Example 1. The calculated faradaic efficiency (FE, unit: %) is shown in the graph of FIG.

[0070] (Comparative Example 3 and Comparative Example 4) Methane synthesis was carried out in the same manner as in Comparative Example 2, except that the applied voltage was −2.0 V (Comparative Example 3) or −2.5 V (Comparative Example 4). The calculated faradaic efficiency (FE, unit: %) is shown in the graph of FIG.

[0071] Example 8 Methane synthesis was carried out by a reduction reaction using an electrochemical reaction of carbon dioxide in the same manner as in Example 5, except that no N2 gas was flowed to the anode side, the applied voltage was set as shown below, and the carbon fiber sintered body was changed. The Faraday efficiency (FE, unit: %) calculated for each gas composition analyzed by gas chromatograph 28 is shown in Table 1 and the graph in Figure 6. The analyzed gases were four types: H2, CO, CH4, and C2H4. In Table 1, the column "Composition" indicates the Cu loading ratio in the diffusion layer, which indicates the mass of Cu relative to the total mass of CeO2 in the catalyst coated on the diffusion layer. In the graph of Figure 6, the left axis is a relative scale of the faradaic efficiency, and the right axis is the current value, with COFE indicating the proportion of the charge used to produce CO (i.e., carbon monoxide), and CH4FE indicating the proportion of the charge used to produce CH4 (i.e., methane). The applied voltages are as follows: Applied voltage: Control the voltage between the working electrode (WE) 21 and the reference electrode (RE) 22, performed at -1.7V

[0072] The carbon fiber sintered bodies used were as follows: Sintered carbon fiber (manufactured by Toray Industries, Inc.) Model: TGP-H-060, Porosity: 78%, Density: 0.44g / cm 3 Water-repellent treatment: 50% (the mass ratio of the water-repellent material applied to the total mass of the carbon fiber sintered body)

[0073] Example 9 Methane synthesis was carried out by a reduction reaction using an electrochemical reaction of carbon dioxide in the same manner as in Example 5, except that a diffusion layer 2 prepared using the methanation catalyst 2 obtained in Example 2 was used. The Faraday efficiency (FE, unit: %) calculated for each gas composition analyzed by gas chromatograph 28 is shown in Table 1 and the graph in FIG. 6.

[0074] Example 10 Methane synthesis was carried out by a reduction reaction using an electrochemical reaction of carbon dioxide in the same manner as in Example 5, except that a diffusion layer 3 prepared using the methanation catalyst 3 obtained in Example 3 was used. The Faraday efficiency (FE, unit: %) calculated for each gas composition analyzed by gas chromatograph 28 is shown in Table 1 and the graph in FIG. 6.

[0075] Example 11 Methane synthesis was carried out by a reduction reaction using an electrochemical reaction of carbon dioxide in the same manner as in Example 5, except that a diffusion layer 4 prepared using the methanation catalyst 4 obtained in Example 4 was used. The Faraday efficiency (FE, unit: %) calculated for each gas composition analyzed by gas chromatograph 28 is shown in Table 1 and in the graph of FIG. 6.

[0076] (Comparative Example 5) Methane synthesis was carried out by a reduction reaction using an electrochemical reaction of carbon dioxide in the same manner as in Example 5, except that the diffusion layer used was a carbon fiber sintered compact with a Cu loading ratio of 0 wt %, i.e., a diffusion layer 5 coated with a catalyst consisting only of CeO2. The Faraday efficiency (FE, unit: %) calculated for each gas composition analyzed by gas chromatograph 28 is shown in Table 1 and the graph in Figure 6.

[0077] [Table 1]

[0078] (Evaluation: Methane synthesis) As shown in FIG. 5, the methanation catalyst etc. of the present disclosure suppressed the synthesis of the by-product ethylene and had high methane selectivity compared to the methanation catalyst obtained in Comparative Example 1. In particular, in the case of Example 5 where the applied voltage was −1.7 V, a high faradaic efficiency of 57% was exhibited in methane synthesis, and the methane selectivity was particularly high. As shown in Table 1, FIGS. 5 and 6, it was demonstrated that by using the methanation catalyst of the present disclosure, methane synthesis was achieved while suppressing the production of ethylene. In particular, methanation catalyst 1, which is a solid solution particle in which copper and cerium are solid-solved, Cu / CeO2=2 mass%, and which was produced by low-temperature calcination, showed good methane selectivity. Therefore, it was clear that the methanation catalyst etc. disclosed herein are a method for producing a methanation catalyst with improved methane selectivity, a methanation catalyst, a methane synthesis method, and a cell for methane synthesis.

[0079] Example 12 Methane synthesis was carried out by a reduction reaction using an electrochemical reaction of carbon dioxide in the same manner as in Example 5, except that diffusion layer 1 was replaced with diffusion layer 6. Diffusion layer 6 was produced in the same manner as in Example 5, except that the carbon fiber sintered body was replaced with a 28BC model manufactured by Silgraft. The 28BC model manufactured by Silgraft had a particulate mesoporous layer on the surface of the diffusion layer. The diffusion layer 6 was observed in the same manner as in Example 5. As shown in Fig. 7, in the image obtained by observation, it was confirmed that copper, which is a catalytic metal and appears white, was locally present on the electrolysis side of the diffusion layer 6. It was confirmed that, when the diffusion layer 6 was used, ethylene production was suppressed and methane synthesis was carried out, as in Example 5, but the faradaic efficiency (FE, unit: %) in methane synthesis was better in Example 5. [Explanation of symbols]

[0080] 10 Methane synthesis cell 11 Electrolytes 12 cathode 13 Anode 14, 15 Flow path 16 Cell Structure 20 Methane synthesis unit 21 Working electrode (WE) 22 Reference pole (RE) 23 Opposite (CE) 24 Electrolyte 25 Gas 26 Voltage application device 27 Flow meter 28 Gas chromatograph

Claims

1. A method for producing a methanation catalyst for producing methane by a reduction reaction using an electrochemical reaction of carbon dioxide, comprising: a step of reacting a cerium-containing compound and a copper-containing compound with an organic acid containing citric acid to produce an intermediate; calcining the intermediate to produce solid solution particles containing cerium and copper; A method for producing a methanation catalyst comprising:

2. The method for producing a methanation catalyst according to claim 1 , wherein the organic acid further contains malic acid.

3. 2. The method for producing a methanation catalyst according to claim 1, wherein the step of producing the intermediate comprises reacting the organic acid with a mass containing at least 1.2 times the amount of acid groups required to react with all metal ions contained in the cerium-containing compound and the copper-containing compound.

4. A methanation catalyst for producing methane by a reduction reaction using an electrochemical reaction of carbon dioxide, The catalyst comprises a support containing cerium (IV) oxide and copper as a catalytic metal supported thereon; The particle shape has a plurality of primary particles arranged on the surface, The primary particles of the methanation catalyst have spherical portions.

5. 5. The methanation catalyst according to claim 4, wherein the crystallite size of the cerium (IV) oxide is 3 nm to 12 nm as determined by X-ray diffraction analysis.

6. 5. The methanation catalyst according to claim 4, wherein the copper content is 1.0% to 5.0% by mass relative to the cerium (IV) oxide content.

7. A methane synthesis cell for generating methane by a reduction reaction using an electrochemical reaction of carbon dioxide, A fuel cell comprising a diffusion layer supporting the methanation catalyst according to claim 4 and an electrolyte, The electrolyte is a solid polymer membrane.

8. 8. The methane synthesis cell according to claim 7, wherein the diffusion layer is formed by supporting the methanation catalyst on carbon paper having a fibrous surface.

9. A method for synthesizing methane, comprising using the methanation catalyst according to claim 4 to synthesize methane through a reduction reaction using an electrochemical reaction of carbon dioxide.

10. A method for synthesizing methane by reducing carbon dioxide using the methane synthesis cell according to claim 7.

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