Electric field catalyst and method for producing same

Abstract

The purpose of the present invention is to provide an electric field catalyst capable of exhibiting improved catalytic activity and a method for producing the catalyst. The present invention provides a catalyst that allows a chemical reaction to proceed while a direct current is applied. The catalyst comprises a Ce-based oxide and Ru supported on the Ce-based oxide. The Ce-based oxide has a CeO2 crystallite diameter in the range of 3-12 nm.

Description

Electric field catalyst and method for manufacturing the same

[0001] This invention relates to an electric field catalyst and a method for producing the same.

[0002] In recent years, research has been progressing on catalysts that allow chemical reactions to proceed while a direct current is flowing through them. Catalysts that allow chemical reactions to proceed while a direct current is flowing through them are called "electric field catalysts," and chemical reactions that proceed while a direct current is flowing through an electric field catalyst are called "electric field catalytic reactions."

[0003] Non-Patent Document 1 describes an electric field catalyst for synthesizing ammonia from a raw material gas containing nitrogen and hydrogen. In the electric field catalyst described in Non-Patent Document 1, Ru is used as the catalytically active component, and CeO is used as the support for the catalytically active component. ２ This is used. According to the electric field catalyst described in Non-Patent Document 1, the ammonia synthesis reaction can be carried out efficiently even at low temperatures (150-200°C).

[0004] RSC Adv. , 2024, vol. 14, pp. 9869-9877

[0005] In the electric field catalyst described in Non-Patent Document 1, CeO is used during the electric field catalytic reaction. ２ Sintering and crystal growth occur in CeO ２ The specific surface area decreases, and as a result, CeO ２ Ru particles supported on the material combine to form Ru particles with a larger particle size. As a result, the reactive active sites of Ru decrease and CeO ２ This results in a decrease in proton conduction from Ru, leading to a reduction in catalytic activity.

[0006] Therefore, the present invention aims to provide an electric field catalyst capable of exhibiting improved catalytic activity and a method for producing the same.

[0007] The present invention encompasses the following inventions: [1] A catalyst for carrying out a chemical reaction while a direct current is flowing, wherein the catalyst comprises a Ce-based oxide and Ru supported on the Ce-based oxide, and the CeO ２The catalyst with a crystallite diameter of 3 nm or more and 12 nm or less. [2] The catalyst according to [1], wherein the Ce-based oxide is a composite oxide containing Ce and one or more additional elements. [3] The catalyst according to [2], wherein the one or more additional elements are selected from Group 2 elements, Group 13 elements, and first transition elements. [4] The catalyst according to [2] or [3], wherein the one or more additional elements are selected from Al, Mg, and Ti. [5] The catalyst according to any one of [2] to [4], wherein the mass content of the one or more additional elements in terms of oxide in the composite oxide is 0.1% by mass or more and 25.0% by mass or less. [6] The catalyst according to any one of [1] to [5], wherein the specific surface area of the catalyst is 50 m ２ / g or more. [7] The catalyst according to any one of [1] to [6], wherein the lattice volume of the Ce-based oxide is 155 Å ３ or more and 159 Å ３ or less. [8] The catalyst according to any one of [1] to [7], wherein the chemical reaction is a reaction for synthesizing ammonia from a raw material gas containing nitrogen and hydrogen. [9] A method for producing a catalyst for promoting a chemical reaction while flowing a direct current, comprising the following steps: (1A) preparing a plurality of candidate catalysts each containing a Ce-based oxide and Ru supported on the Ce-based oxide; and (1B) selecting a candidate catalyst having a CeO ２ crystallite diameter of 3 nm or more and 12 nm or less from the plurality of candidate catalysts.

[0008] According to the present invention, there is provided an electric field catalyst capable of exhibiting improved catalytic activity and a method for producing the same.

[0009] FIG. 1 is a schematic view of a reaction apparatus according to an embodiment. FIG. 2 is a diagram showing an X-ray diffraction pattern of the catalyst powder of Example 1. FIG. 3 is a diagram showing an X-ray diffraction pattern of the catalyst powder of Example 2. FIG. 4 is a diagram showing an X-ray diffraction pattern of the catalyst powder of Comparative Example 1.

[0010] <<Catalyst>> Hereinafter, embodiments of the catalyst of the present invention (hereinafter referred to as "the present catalyst") will be described.

[0011] This catalyst relates to a catalyst that allows a chemical reaction to proceed while a direct current is applied, in other words, an electric field catalyst.

[0012] This catalyst contains a Ce-based oxide and Ru supported on the Ce-based oxide, and the CeO ２ It is characterized by having a crystallite size of 3 nm or more and 12 nm or less.

[0013] Examples of the catalyst's form include powder, pellets, and layers.

[0014] The chemical reactions catalyzed by this catalyst are not particularly limited, as long as they are chemical reactions that Ru can catalyze. Examples of chemical reactions that Ru can catalyze include the reaction of synthesizing ammonia from a raw material gas containing nitrogen and hydrogen (hereinafter referred to as the "ammonia synthesis reaction"), and the reaction of decomposing ammonia into nitrogen and hydrogen.

[0015] In one embodiment, the chemical reaction carried out by this catalyst is the ammonia synthesis reaction. The ammonia synthesis reaction is represented by the formula: N ２ +3H ２ →2NH ３ It is represented as follows.

[0016] When a direct current is passed through this catalyst and a raw material gas containing nitrogen and hydrogen is brought into contact with it, the catalytic activity of this catalyst causes the ammonia synthesis reaction to proceed, and ammonia is produced from the raw material gas. The mechanism is thought to be as follows: In a typical thermal reaction that does not use an electric field, N adsorbed on the surface of metal Ru ２ It is dissociated into N, and the dissociated N is H ２ Hydrogenated, ammonia (NH ３ ) is produced. In contrast, in the electric field catalytic reaction using this catalyst, CeO ２ H via OH groups present on the surface ＋ N adsorbed onto metal Ru by conduction ２ H ＋ N is supplied, and the reaction intermediate N ２ H ＋ A formation is created. And then, N ２ H ＋ electrons (e － ) is supplied, and ammonia (NH ３) is generated.

[0017] In the electric field catalyst described in Non-Patent Document 1, CeO is used during the electric field catalytic reaction. ２ Sintering and crystal growth occur in CeO ２ The specific surface area decreases, and as a result, CeO ２ Ru particles supported on the material combine to form Ru particles with a larger particle size. As a result, the reactive active sites of Ru decrease and CeO ２ A decrease in proton conduction from to Ru occurs, resulting in a decrease in catalytic activity. In contrast, the CeO of the Ce-based oxide in this catalyst ２ Since the crystallite size is between 3 nm and 12 nm, CeO in Ce-based oxides ２ Even if sintering and crystal growth occur, the specific surface area of ​​the Ce-based oxide does not decrease easily, thereby suppressing the bonding between Ru particles supported on the Ce-based oxide and the resulting formation of Ru particles with larger particle sizes. As a result, the reaction active sites of Ru decrease and CeO ２ The decrease in proton conduction from to Ru is suppressed. Therefore, this catalyst can exhibit improved catalytic activity. This catalyst can exhibit improved catalytic activity (ammonia synthesis activity) in the ammonia synthesis reaction. Furthermore, this catalyst can exhibit improved catalytic activity in chemical reactions other than the ammonia synthesis reaction.

[0018] Ce-based oxides refer to oxides that contain Ce. Ce-based oxides are CeO ２ As long as the crystallite size is between 3 nm and 12 nm, CeO ２ It may also be a composite oxide containing Ce and one or more additional elements.

[0019] Ce-based oxides are used as supports for Ru. From the viewpoint of improving Ru support, it is preferable that the Ce-based oxide is porous.

[0020] The form of the Ce-based oxide is not particularly limited as long as Ru can be supported on the surface of the Ce-based oxide. Examples of the form of the Ce-based oxide include particles, layers, etc. The particles may be primary particles or secondary particles. Secondary particles are aggregated particles formed by the aggregation of primary particles.

[0021] The Ce-based oxide is preferably a composite oxide containing Ce and one or more additional elements. The one or more additional elements in the composite oxide contribute to the CeO content in the composite oxide. ２ Since sintering and crystal growth are suppressed, the decrease in the specific surface area of ​​the composite oxide is suppressed, thereby suppressing the bonding between Ru particles supported on the composite oxide and the formation of Ru particles with larger particle sizes resulting from this. As a result, the reaction active sites of Ru decrease and CeO ２ The decrease in proton conduction from to Ru is suppressed. Therefore, when the Ce oxide is a composite oxide, this catalyst can exhibit improved catalytic activity.

[0022] A composite oxide is a complex containing two or more metal oxides (i.e., an oxide of Ce and an oxide of one or more additional elements). In the complex, the two or more metal oxides are chemically bonded. The complex also includes solid solutions of the two or more metal oxides. In the complex, some of the Ce and / or some of the one or more additional elements may exist in one or more states other than oxides (e.g., one or more states selected from metals, alloys, salts, etc.). The oxide of Ce is CeO ２ This means that. The oxides of additional elements will be discussed later.

[0023] Examples of composite oxides include CeO ２ A composite oxide obtained by modifying the surface of with one or more additional elements or their oxides, CeO ２ Examples include composite oxides obtained by solid-solving one or more additional elements or their oxides.

[0024] CeO contained in complex oxides ２ This is proton conduction to Ru, H ２ Adsorption and dissociation of, formation of reaction intermediates, supported metal species or N ２ It is a component that contributes to proton donation and other processes.

[0025] The oxides of one or more additional elements contained in a composite oxide are the CeO of the composite oxide. ２ It is a component that contributes to the adjustment of crystallite size, etc.

[0026] One or more additional elements may consist of one additional element, or two or more additional elements.

[0027] One or more additional elements are selected from metallic elements other than Ce. "Metallic elements other than Ce" also include metalloid elements such as B.

[0028] From the viewpoint of improving the catalytic activity of this catalyst (especially ammonia synthesis activity), it is preferable that one or more additional elements be selected from Group 2 elements, Group 13 elements, and first transition elements.

[0029] Group 2 elements include Be, Mg, Ca, Sr, Ba, and Ra. The oxide of Be is BeO, the oxide of Mg is MgO, the oxide of Ca is CaO, the oxide of Sr is SrO, the oxide of Ba is BaO, and the oxide of Ra is RaO.

[0030] From the viewpoint of improving the catalytic activity of this catalyst (especially ammonia synthesis activity), the Group 2 element is preferably Mg.

[0031] Group 13 elements include B, Al, Ga, In, and Tl. The oxide of B is B ２ O ３ Al oxides are Al ２ O ３ Ga oxides are Ga ２ O ３ In oxides are In ２ O ３ The oxide of Tl is Tl ２ O ３ It means...

[0032] From the viewpoint of improving the catalytic activity of this catalyst (especially ammonia synthesis activity), the Group 13 element is preferably Al.

[0033] First transition elements include Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn. Sc oxides are Sc ２ O ３ The oxide of Ti is TiO ２ The oxide of V is V ２ O ３ Cr oxides are Cr ２ O ３Mn oxides are Mn ３ O ４ Fe oxides are Fe ２ O ３ Fe ３ O ４ and one or more selected from FeO, and the oxide of Co is Co ３ O ４ Ni oxide is NiO, and Cu oxide is Cu ２ One or two selected from O and CuO, and Zn oxide refers to ZnO.

[0034] From the viewpoint of improving the catalytic activity of this catalyst (especially ammonia synthesis activity), the first transition element is preferably Ti.

[0035] From the viewpoint of improving the catalytic activity of this catalyst (especially ammonia synthesis activity), it is preferable that one or more additional elements be selected from Mg, Al, and Ti.

[0036] When one or more additional elements are selected from Al and Ti, the specific surface area of ​​the catalyst can be improved, thereby improving its catalytic activity. Therefore, it is preferable that the one or more additional elements are selected from Al and Ti. The specific surface area of ​​the catalyst when one or more additional elements are selected from Al and Ti will be described later.

[0037] From the viewpoint of improving the catalytic activity of this catalyst (especially ammonia synthesis activity), the mass content of one or more additional elements in the composite oxide in terms of oxides (hereinafter referred to as "content C1") is preferably 0.1% by mass or more and 25.0% by mass or less, more preferably 0.5% by mass or more and 20.0% by mass or less, even more preferably 1.0% by mass or more and 15.0% by mass or less, even more preferably 2.0% by mass or more and 10.0% by mass or less, and particularly preferably 3.0% by mass or more and 7.0% by mass or less. Each of the above lower limits may be combined with any of the above upper limits. When the composite oxide contains two or more additional elements, content C1 means the total mass content of the two or more additional elements in the composite oxide in terms of oxides.

[0038] From the viewpoint of improving the catalytic activity of this catalyst (especially ammonia synthesis activity), CeO ２ The converted mass content (hereinafter referred to as "content C2") is preferably 75.0% by mass or more and 99.9% by mass or less, more preferably 80.0% by mass or more and 99.5% by mass or less, even more preferably 85.0% by mass or more and 99.0% by mass or less, even more preferably 90.0% by mass or more and 98.0% by mass or less, and particularly preferably 93.0% by mass or more and 97.0% by mass or less. Each of the above lower limits may be combined with any of the above upper limits.

[0039] In one embodiment, the sum of the content of C1 and C2 is 100% by mass.

[0040] The content C1 and C2 are defined by the following formulas: Content C1 = (mass of one or more additional elements in the composite oxide in terms of oxide equivalent) / (mass of the composite oxide) × 100 Content C2 = (CeO ２ (Converted mass) / (Mass of composite oxide) × 100

[0041] In this invention, "mass of a metal element in a composite oxide on an oxide basis" means the mass of the oxide of the metal element, calculated assuming that the metal element exists as an oxide of that metal element. This definition applies to any metal element in a composite oxide. The meaning of "oxide of a metal element" is as described above.

[0042] In this invention, "mass of composite oxide" refers to the total mass of all metal elements in the composite oxide in terms of their oxide equivalents. The significance of metal element oxides is as described above.

[0043] The content ratios C1 and C2 are determined by the following method. The catalyst is analyzed by X-ray fluorescence analysis (XRF) to identify all metal elements contained in the catalyst, and the mass content (mass%) of each identified metal element in terms of oxide is measured. For Fe, the Fe content of Fe ２ O ３ For the converted mass content, the mass content of Fe in terms of oxide is used, and for Cu, the mass content of Cu in terms of CuO is used as the mass content of Cu in terms of oxide. For Ru, the mass content of RuO２ The converted mass content is the mass content of Ru on an oxide basis. XRF can be performed according to the conditions described in the examples. Hereinafter, the mass content (mass%) of each metal element on an oxide basis measured by XRF will be referred to as the "XRF measurement value" for each metal element. The content C1 and C2 are calculated from the following formulas. Content C1 = (XRF measurement value of one or more additional elements) / (sum of XRF measurement values ​​of all metal elements in the composite oxide) × 100 Content C2 = (XRF measurement value of Ce) / (sum of XRF measurement values ​​of all metal elements in the composite oxide) × 100

[0044] From the perspective of improving the catalytic activity of this catalyst (especially ammonia synthesis activity), CeO ２ The crystallite size is 3 nm to 12 nm, preferably 3 nm to 11 nm, more preferably 7 nm to 11 nm, and even more preferably 7 nm to 10 nm. Each of the above lower limits may be combined with any of the above upper limits.

[0045] CeO ２ The method for measuring the crystallite size is as follows: X-ray diffraction (XRD) is performed on a sample obtained from this catalyst using a commercially available X-ray diffractometer. XRD can be performed according to the conditions described in the examples. In the obtained XRD pattern, CeO ２ Among the peaks originating from the source, the peak located at 2θ = 28.6° ± 0.5° is identified, and Scherrer's formula is applied to the identified peak to determine the crystallite size. The Scherrer constant is assumed to be 0.94. The crystallite size obtained from the peak located at 2θ = 28.6° ± 0.5° is given to CeO ２ This is referred to as the crystallite size. The specific method for measuring the crystallite size is as described in the examples.

[0046] Ce-based oxides are CeO ２ If that is the case, CeO ２ CEO ２ The crystallite size is, for example, CeO ２ This can be adjusted by controlling the preparation conditions (e.g., reaction time, reaction temperature, pH, etc.).

[0047] When the Ce-based oxide is a composite oxide, the CeO of the composite oxide ２ The crystallite size can be adjusted, for example, by adjusting the type and / or amount of one or more additional elements contained in the composite oxide, and / or when preparing the composite oxide by the precipitation method, by adjusting the preparation conditions of the precipitate (such as temperature, time, pH during aging, etc.).

[0048] From the viewpoint of improving the catalytic activity (especially ammonia synthesis activity) of the present catalyst, the lattice volume of the Ce-based oxide is preferably 155 Å ３ or more and 159 Å ３ or less, more preferably 155 Å ３ or more and 158 Å ３ or less, even more preferably 155 Å ３ or more and 157.5 Å ３ or less, still even more preferably 155 Å ３ or more and 156.2 Å ３ or less. In particular, when the Ce-based oxide is a composite oxide, the lattice volume of the composite oxide is preferably within the above range.

[0049] The lattice volume of the Ce-based oxide can be measured by the method described in the examples. [[ID=2s]]

[0050] When the Ce-based oxide is a composite oxide, the lattice volume of the composite oxide can be adjusted, for example, by adjusting the type and / or amount of one or more additional elements contained in the composite oxide, and / or when preparing the composite oxide by the precipitation method, by adjusting the preparation conditions of the precipitate (such as temperature, time, pH during aging, etc.).

[0051] Ru is used as a catalytic active component. The form of Ru is not particularly limited as long as Ru can be supported on the surface of the Ce-based oxide. Examples of the form of Ru include particles, layers, etc. The particles may be primary particles or secondary particles. Ru may be supported on a part of the surface of the Ce-based oxide or on the entire surface of the Ce-based oxide. [[ID=]]

[0052] Ru can exist in one or more states selected from, for example, metallic Ru, oxides of Ru, salts of Ru, etc. Ru may exist in two or more states. The oxide of Ru is RuO ２ , RuO ４ and Ru ２ O ３ and means one or more selected therefrom. From the viewpoint of improving the catalytic activity of the present catalyst, at least a part of Ru preferably exists in the state of metallic Ru.

[0053] The present catalyst may be subjected to a reduction treatment or may not be subjected to a reduction treatment. However, from the viewpoint of improving the catalytic activity of the present catalyst, it is preferably subjected to a reduction treatment. Ru usually exists in the state of an oxide of Ru before the reduction treatment and usually exists in the state of metallic Ru after the reduction treatment.

[0054] From the viewpoint of improving the catalytic activity of the present catalyst, the mass content ratio of the Ce-based oxide in the present catalyst (hereinafter referred to as "content ratio C3") is preferably 85.0% by mass or more and 99.9% by mass or less, more preferably 93.0% by mass or more and 99.0% by mass or less, and even more preferably 95.0% by mass or more and 97.0% by mass or less. Any of the above lower limit values may be combined with any of the above upper limit values.

[0055] From the viewpoint of improving the catalytic activity of the present catalyst, the mass content ratio in terms of metal of Ru in the present catalyst (hereinafter referred to as "content ratio C4") is preferably 0.1% by mass or more and 15.0% by mass or less, more preferably 1.0% by mass or more and 7.0% by mass or less, and even more preferably 3.0% by mass or more and 5.0% by mass or less. Any of the above lower limit values may be combined with any of the above upper limit values.

[0056] From the viewpoint of improving the catalytic activity of the present catalyst, the total of content ratios C3 and C4 is preferably 1.0% by mass or more, more preferably 10.0% by mass or more, and even more preferably 50.0% by mass or more. The upper limit value is 100% by mass. In one embodiment, the total of content ratios C3 and C4 is 100% by mass.

[0057] The content C3 and C4 are defined by the following formulas: Content C3 = (Mass of Ce-based oxides in this catalyst) / (Mass of this catalyst) × 100 Content C4 = (Mass of Ru in metal equivalent in this catalyst) / (Mass of this catalyst) × 100

[0058] In this invention, "the metallic equivalent mass of Ru" means the mass of metallic Ru, which is determined by assuming that Ru exists as metallic Ru.

[0059] In this invention, "mass of the catalyst" means the sum of the masses obtained by classifying all metal elements contained in the catalyst into Ru and other metal elements, determining the mass of Ru in terms of metal equivalent, and determining the mass of other metal elements in terms of oxide equivalent.

[0060] The concentrations of C3 and C4 are determined by the following method. The catalyst is analyzed by X-ray fluorescence analysis (XRF) to identify all metal elements contained in the catalyst, and the mass content (mass%) of each identified metal element in terms of oxide is measured. For Fe, the amount of Fe is calculated. ２ O ３ For the converted mass content, the mass content of Fe in terms of oxide is used, and for Cu, the mass content of Cu in terms of CuO is used as the mass content of Cu in terms of oxide. For Ru, the mass content of RuO ２ The converted mass content is the mass content of Ru in terms of oxide. XRF can be performed according to the conditions described in the examples. Hereinafter, the mass content (mass%) of each metal element in terms of oxide measured by XRF will be referred to as the "XRF measurement value" for each metal element. Using a Ru-containing standard sample whose composition is known, RuO ２ XRF measurement value (RuO ２ A calibration curve is created showing the relationship between the converted mass content (mass%) and the ICP measurement value of Ru (mass content of Ru in metallic terms (mass%)). The ICP measurement value is a measurement value obtained using inductively coupled plasma atomic emission spectrometry (hereinafter referred to as "ICP"). In ICP, the mass content (mass%) of each metal element in metallic terms in the molten material obtained by alkali melting the sample is measured. The XRF value of Ru obtained from the above XRF analysis (RuO ２The converted mass content is converted to the mass content (mass%) of Ru in metallic equivalent using the calibration curve described above. The content C3 and C4 are calculated from the following formulas: Content C3 = (Sum of XRF measurements of all metal elements in the Ce-based oxide) / {(Sum of XRF measurements of all metal elements other than Ru in this catalyst) + (Mass content of Ru in metallic equivalent)} × 100 Content C4 = (Mass content of Ru in metallic equivalent) / {(Sum of XRF measurements of all metal elements other than Ru in this catalyst) + (Mass content of Ru in metallic equivalent)} × 100

[0061] From the viewpoint of improving the catalytic activity of this catalyst, the specific surface area of ​​this catalyst is preferably 50 m². ２ / g or more, more preferably 90m ２ The amount is 1 / g or more. The upper limit of the specific surface area of ​​this catalyst is not particularly limited. The specific surface area of ​​this catalyst is, for example, 300 m². ２ It may be less than / g.

[0062] When the catalyst contains one or more additional elements selected from Al and Ti, the specific surface area of ​​the catalyst is preferably 90 m². ２ It is 1 / g or more. The explanation regarding the upper limit of the specific surface area of ​​this catalyst is the same as above.

[0063] When calcination is performed during the production of this catalyst, the specific surface area of ​​the catalyst described above is the value in the state before reduction treatment. When reduction treatment is performed instead of calcination during the production of this catalyst, the specific surface area of ​​the catalyst described above is the value in the state after reduction treatment. The specific surface area can be measured by the method described in the examples.

[0064] ≪Method for Manufacturing This Catalyst≫ The following describes embodiments of the method for manufacturing this catalyst.

[0065] This catalyst can be produced, for example, by a screening method comprising the following steps (1A) and (1B).

[0066] <Process (1A)> Process (1A) is a process for preparing multiple candidate catalysts.

[0067] Each candidate catalyst prepared in step (1A) comprises a Ce-based oxide and Ru supported on the Ce-based oxide. In each candidate catalyst prepared in step (1A), the CeO of the Ce-based oxide ２ Whether the crystallite size is between 3 nm and 12 nm is usually unknown. The above explanation regarding this catalyst refers to CeO ２ This also applies to each candidate catalyst prepared in step (a), except that the crystallite size is between 3 nm and 12 nm.

[0068] The methods for producing each candidate catalyst prepared in step (1A) will be described later.

[0069] <Process (1B)> In process (1B), CeO2, a Ce-based oxide, is selected from the multiple candidate catalysts prepared in process (1A). ２ This is a process for selecting candidate catalysts with a crystallite size of 3 nm or more and 12 nm or less.

[0070] CeO in each candidate catalyst ２ The crystallite size can be measured by the method described in the examples. CeO ２ Based on the crystallite size measurement results, CeO2, a Ce-based oxide, was selected from among several candidate catalysts. ２ Candidate catalysts with a crystallite size of 3 nm to 12 nm can be selected.

[0071] From the perspective of selecting catalysts with improved catalytic activity (particularly catalytic activity in ammonia synthesis reactions), the CeO2 oxide in the selected candidate catalysts is important. ２ The crystallite size is 3 nm to 12 nm, preferably 3 nm to 11 nm, more preferably 7 nm to 11 nm, and even more preferably 7 nm to 10 nm. Each of the above lower limits may be combined with any of the above upper limits.

[0072] ≪Method for Manufacturing Candidate Compositions≫ Each candidate composition prepared in step (1A) can be manufactured by a method comprising the following steps (2A) to (2D).

[0073] <Process (2A)> Process (2A) is the process of preparing the alkaline aqueous solution (A), raw material solution (B), and raw material solution (C).

[0074] An alkaline aqueous solution (A) can be prepared, for example, by dissolving a base in water and then adjusting the pH of the resulting aqueous solution as needed.

[0075] The base used in preparing the alkaline aqueous solution (A) is not particularly limited as long as it dissolves in water or reacts with water to produce hydroxide ions, and can be selected from, for example, carbonates, bicarbonates, hydroxides, etc. One type of base may be used alone, or two or more types of bases may be used in combination.

[0076] Examples of carbonates include alkali metal salts such as sodium carbonate and potassium carbonate, and ammonium salts such as ammonium carbonate and ammonium bicarbonate. Examples of bicarbonates include alkali metal salts such as sodium bicarbonate and potassium bicarbonate, and ammonium bicarbonate. Examples of hydroxides include alkali metal salts such as sodium hydroxide and potassium hydroxide.

[0077] From the viewpoint of promoting precipitate formation, the pH of the alkaline aqueous solution (A) is preferably 8 or higher, more preferably 9 or higher. The upper limit is not particularly limited.

[0078] The pH of the alkaline aqueous solution (A) may be adjusted using an acid. The acid used to adjust the pH can be selected from, for example, nitric acid, hydrochloric acid, hydrobromic acid, sulfuric acid, phosphoric acid, etc.

[0079] The raw material liquid (B) contains Ce-based oxide raw materials.

[0080] The raw material solution (B) can be prepared by dissolving or dispersing the Ce-based oxide raw material in water, and then adjusting the pH of the resulting aqueous solution or dispersion as needed.

[0081] As raw materials for Ce-based oxides, for example, water-soluble salts of Ce and, optionally, water-soluble salts of one or more additional elements can be used. Depending on the type of metal element, the water-soluble salts can be selected from, for example, nitrates, chlorides, acetates, carbonates, etc.

[0082] The raw material liquid (C) contains the raw material Ru.

[0083] The raw material solution (C) can be prepared by dissolving or dispersing the Ru raw material in water, and then adjusting the pH of the resulting aqueous solution or dispersion as needed.

[0084] As a raw material for Ru, for example, a water-soluble salt of Ru can be used. Depending on the type of metal element, the water-soluble salt can be selected from, for example, nitrates, chlorides, acetates, carbonates, etc.

[0085] The alkaline aqueous solution (A), the raw material solution (B), and the raw material solution (C) may each contain one or more solvents other than water. The one or more solvents other than water can be selected from organic solvents such as alcohols, for example.

[0086] <Process (2B)> Process (2B) is the process of mixing the alkaline aqueous solution (A) and the raw material solution (B).

[0087] When an alkaline aqueous solution (A) and a raw material solution (B) are mixed, a precipitate is formed. Specifically, each metal ion contained in the raw material solution (B) reacts with one or more selected from hydroxide ions, carbonate ions, and bicarbonate ions contained in the alkaline aqueous solution (A), and possibly with one or more other metal ions contained in the raw material solution (B), to form sparingly water-soluble hydroxides, sparingly water-soluble carbonates, sparingly water-soluble bicarbonates, or sparingly water-soluble carbonate hydroxides (hereinafter referred to as "sporadic water-soluble hydroxides, etc.") and precipitate. For example, one metal ion M1 contained in the raw material solution (B) ２＋ This consists of one or more metal ions M2 selected from hydroxide ions, carbonate ions, and bicarbonate ions in the alkaline aqueous solution (A), and optionally one other metal ion M2 contained in the raw material solution (B). ２＋ In response, M1(OH) ２ M1CO ３ [(M1 ｘ M2 １－ｘ ) ２ (OH) ２ (CO ３The precipitate may form (0 < x ≤ 1), etc. The precipitate contains sparingly water-soluble hydroxides of Ce, and optionally one or more additional elements. The sparingly water-soluble hydroxides are, for example, particulate. Two or more particles may form composite particles.

[0088] CeO ２ From the viewpoint of adjusting the crystallite size to a desired range, the pH of the mixture of the alkaline aqueous solution (A) and the raw material solution (B) is preferably 8 to 10, more preferably 8.5 to 9.5. The pH of the mixture refers to the pH of the mixture immediately after mixing. The pH of the mixture may change (for example, increase) over time.

[0089] When mixing the alkaline aqueous solution (A) and the raw material solution (B), the raw material solution (B) may be added to the alkaline aqueous solution (A), or the alkaline aqueous solution (A) may be added to the raw material solution (B).

[0090] From the viewpoint of making the pH of the entire mixture uniform, it is preferable to stir one while adding the other dropwise when mixing the alkaline aqueous solution (A) and the raw material solution (B).

[0091] CeO ２ From the viewpoint of adjusting the crystallite size to a desired range, the temperature of the alkaline aqueous solution (A) when mixing with the raw material solution (B) is preferably 60°C to 80°C, more preferably 65°C to 75°C.

[0092] CeO ２ From the viewpoint of adjusting the crystallite size to a desired range, the temperature of the raw material solution (B) when mixed with the alkaline aqueous solution (A) is preferably 25°C or higher and 80°C or lower.

[0093] CeO ２ From the viewpoint of adjusting the crystallite size to a desired range, it is preferable to age the precipitate while stirring the mixture of the alkaline aqueous solution (A) and the raw material solution (B). From the same viewpoint, the aging time is preferably 0.25 hours or more and 1 hour or less, more preferably 0.5 hours or more and 0.7 hours or less. From the same viewpoint, the aging temperature is preferably 60°C or more and 80°C or less, more preferably 65°C or more and 75°C or less.

[0094] After the precipitate has matured, the resulting suspension may be subjected to hydrothermal treatment. This allows crystals to grow to a certain size.

[0095] Hydrothermal treatment can be performed by placing the suspension in a pressure-resistant sealed container and heating it at a predetermined temperature for a predetermined time. The temperature for hydrothermal treatment is preferably 100°C to 180°C, more preferably 110°C to 150°C, and even more preferably 120°C to 140°C. The duration of hydrothermal treatment is preferably 12 hours to 48 hours, more preferably 15 hours to 25 hours. The pressure inside the container during hydrothermal treatment is not particularly limited as long as the water inside the container can remain in a liquid state, and can be appropriately set according to the hydrothermal treatment temperature. The pressure inside the container during hydrothermal treatment is usually the saturated vapor pressure corresponding to the hydrothermal treatment temperature or a pressure exceeding it. When the hydrothermal treatment temperature is 120°C, the hydrothermal treatment pressure is, for example, 0.15 MPa to 0.25 MPa.

[0096] The amount of alkaline aqueous solution (A) used, the amount of raw material solution (B) used, and the concentration of metal ions contained in the raw material solution (B) can be adjusted as appropriate according to the composition of the catalyst to be produced.

[0097] The precipitate obtained by mixing the alkaline aqueous solution (A) and the raw material solution (B) is preferably washed before being used in the next step. Water can be used as the washing solution. The precipitate can be washed by bringing it into contact with the washing solution (for example, by mixing the precipitate and the washing solution). After bringing the precipitate into contact with the washing solution, solid-liquid separation is performed to obtain the washed precipitate in the form of a cake. The solid-liquid separation method can be selected from, for example, filtration, centrifugation, decantation, etc. The precipitate is preferably washed until the amount of metal elements (for example, alkali metal elements such as Na and K) derived from the alkaline aqueous solution (A) in the filtrate or supernatant obtained by solid-liquid separation after bringing the precipitate into contact with the washing solution is 1 ppm or less.

[0098] The precipitate obtained by mixing the alkaline aqueous solution (A) and the raw material solution (B) is preferably formed into a cake and dried before being used in the next step. If the precipitate is washed, it is preferable to form the washed precipitate into a cake and dry the cake. Drying can be carried out according to conventional methods. The drying temperature is usually 60°C to 150°C, preferably 80°C to 120°C, and the drying time is usually 12 hours to 48 hours, preferably 15 hours to 24 hours.

[0099] The dried cake material may be ground as needed. Grinding can be carried out according to conventional methods. Grinding can be done dry or wet using, for example, a mortar and pestle, hammer mill, ball mill, bead mill, jet mill, roller mill, etc.

[0100] <Process (2C)> Process (2C) is the process of calcining the precipitate obtained in process (2B).

[0101] The calcination of the precipitate can be carried out according to conventional methods. Calcination is usually performed in an atmospheric environment. The calcination temperature is usually between 400°C and 1000°C, preferably between 400°C and 600°C. The calcination time is usually between 2 hours and 8 hours, preferably between 4 hours and 6 hours.

[0102] The calcined precipitate may be ground as needed. Grinding can be carried out according to conventional methods. Grinding can be done dry or wet using, for example, a mortar and pestle, hammer mill, ball mill, bead mill, jet mill, roller mill, etc.

[0103] Ce-based oxides can be obtained by calcining the precipitate.

[0104] <Process (2D)> Process (2D) is a process in which the Ce-based oxide obtained in process (2C) is impregnated with the raw material solution (C).

[0105] Step (2D) can be carried out, for example, as follows: Add the Ce-based oxide obtained in step (2C) to the raw material liquid (C) and stir, dry the resulting dispersion, heat it, and obtain a powder. Dry the obtained powder. Calcine the obtained dried powder. Instead of calcining the dried powder, a reduction treatment of the dried powder may be performed. Drying, calcination, and reduction treatment can be carried out according to conventional methods. The drying temperature is usually 60°C to 150°C, preferably 80°C to 120°C, and the drying time is usually 12 hours to 30 hours, preferably 15 hours to 24 hours. Calcination is usually carried out in an atmospheric environment. The calcination temperature is usually 200°C to 1000°C, preferably 300°C to 800°C. The calcination time is usually 0.5 hours to 120 hours, preferably 1 hour to 24 hours. The reduction treatment is carried out in a reducing gas atmosphere containing hydrogen gas. The reduction treatment temperature is usually between 200°C and 900°C, preferably between 250°C and 600°C. The reduction treatment time is usually between 1 hour and 5 hours, preferably between 2 hours and 4 hours. When nitrates are used as raw materials, at least a portion of the nitrates may remain in the form of nitrates (nitrate oxides).

[0106] In step (2D), Ru can be supported on the Ce-based oxide obtained in step (2C). In this way, a candidate catalyst can be obtained.

[0107] When calcination is performed in process (2D), at least a portion of the Ru supported on the Ce-based oxide becomes Ru oxide (RuO ２ RuO ４ and Ru ２ O ３ It exists in one or more states selected from the following. A portion of Ru may exist in the state of metallic Ru.

[0108] When reduction treatment is performed in process (2D), at least a portion of the Ru supported on the Ce-based oxide exists in the state of metallic Ru. A portion of the Ru is Ru oxide (RuO ２ RuO ４ and Ru ２ O ３ It may exist in one or more states selected from the following.

[0109] <<Reaction Apparatus>> Below, an embodiment of a reaction apparatus using this catalyst will be described based on Figure 1.

[0110] Figure 1 is a schematic diagram of a reaction apparatus 1 according to one embodiment.

[0111] The chemical reactions carried out by reactor 1 are not limited as long as they are chemical reactions that Ru can catalyze. Specific examples of chemical reactions that Ru can catalyze are as described above.

[0112] In one embodiment, the chemical reaction carried out by the reactor 1 is an ammonia synthesis reaction. When the chemical reaction carried out by the reactor 1 is an ammonia synthesis reaction, the reactor 1 can produce ammonia from a raw material gas containing nitrogen and hydrogen.

[0113] As shown in Figure 1, the reaction apparatus 1 includes a reactor 2.

[0114] As shown in Figure 1, the reactor 2 comprises a reaction chamber 20, a pair of electrodes 3 and 4 provided in the reaction chamber 20, and a catalyst 5 provided between the pair of electrodes 3 and 4.

[0115] As shown in Figure 1, the reactor 2 has a reactor body 21. The reaction chamber 20 is formed inside the reactor body 21. The material of the reactor body 21 is not particularly limited as long as it is chemically inert in the chemical reaction carried out by the reactor 1. Examples of materials for the reactor body 21 include quartz and borosilicate glass. The size of the reactor body 21 is not particularly limited. The inner diameter of the reactor body 21 is, for example, 6 mm or more and 8 mm or less. The length of the reactor body 21 is, for example, 1000 mm or more and 1500 mm or less.

[0116] As shown in Figure 1, the reactor 2 has a gas inlet 22 and a gas outlet 23. The gas inlet 22 and the gas outlet 23 are each provided in the reactor body 21. In one embodiment, the gas inlet 22 is provided on one end of the reactor body 21 in the longitudinal direction, and the gas outlet 23 is provided on the other end of the reactor body 21 in the longitudinal direction.

[0117] As shown in Figure 1, electrodes 3 and 4 are arranged facing each other within the reaction chamber 20 at a predetermined distance apart. Electrode 3 is the electrode on the voltage application side and is electrically connected to the voltage application means 7 (described later). Electrode 4 is the electrode on the ground side and is grounded.

[0118] The materials of electrodes 3 and 4 are not particularly limited, as long as they are chemically inert in the chemical reaction carried out by the reaction apparatus 1 and have conductivity that allows for the formation of an electric field. Examples of materials for electrodes 3 and 4 include stainless steel and titanium alloy.

[0119] The form of electrodes 3 and 4 is not particularly limited as long as they can come into contact with the catalyst 5. Examples of electrode forms for electrodes 3 and 4 include rods, flat plates, meshes, etc. The planar shape of a flat plate is not particularly limited and examples include circles, squares, etc. As shown in Figure 1, electrodes 3 and 4 are, for example, rods that extend in the longitudinal direction of the reactor body 21.

[0120] Both electrodes 3 and 4 are in contact with the catalyst 5. This prevents the generation of plasma discharge through the air in the reaction chamber 20.

[0121] As shown in Figure 1, when electrodes 3 and 4 are in the form of rods, the tips of the rods are in contact with the catalyst 5.

[0122] The distance between electrode 3 and electrode 4 is not particularly limited, as long as both electrodes 3 and 4 can contact the catalyst 5. The distance between electrode 3 and electrode 4 is equal to or less than the thickness of the catalyst 5. The thickness of the catalyst 5 is the length of the reactor body 21 (the direction in which the raw material gas flows).

[0123] This catalyst will be used as catalyst 5. The description of this catalyst is as described above.

[0124] The form of catalyst 5 is not particularly limited as long as it can come into contact with electrodes 3 and 4. Examples of catalyst forms for catalyst 5 include particles, layers, etc.

[0125] As shown in Figure 1, the catalyst 5 exists, for example, as a layer extending approximately perpendicular to the length of the reactor body 21, located approximately in the center of the reactor body 21.

[0126] The amount of catalyst 5 can be adjusted as appropriate according to the supply amount of raw material gas, etc.

[0127] As shown in Figure 1, the reaction apparatus 1 may be equipped with a support means 6 for supporting the catalyst 5. For example, a porous support plate can be used as the support means 6. Examples of materials for the support plate include ceramics.

[0128] As shown in Figure 1, the reaction apparatus 1 may include a voltage application means 7 for applying a voltage between electrodes 3 and 4. The voltage application means 7 is, for example, a power supply.

[0129] As shown in Figure 1, the reaction apparatus 1 may include a voltage control means 8 for controlling the voltage applied between electrodes 3 and 4.

[0130] The reaction apparatus 1 can apply a voltage between electrodes 3 and 4 using a voltage application means 7 and, if necessary, a voltage control means 8, so that a DC current flows through the catalyst 5.

[0131] When the chemical reaction carried out by the reaction apparatus 1 is an ammonia synthesis reaction, the DC current flowing through the catalyst 5 is preferably 1 mA or more and 10 mA or less, from the viewpoint of effectively exhibiting the catalytic activity of the catalyst 5.

[0132] As shown in Figure 1, the reaction apparatus 1 may be equipped with a raw material gas supply means 9.

[0133] As shown in Figure 1, the raw material gas supply means 9 includes a supply pipe 91 that supplies the raw material gas G1 to the reaction chamber 20.

[0134] If the raw material gas G1 is a mixed gas containing a first gas G2 and a second gas G3, the reaction apparatus 1 may include a first gas supply source 92 that supplies the first gas G2 to the supply pipe 91 and a second gas supply source 93 that supplies the second gas G3 to the supply pipe 91, as shown in Figure 1.

[0135] When the chemical reaction carried out by the reactor 1 is an ammonia synthesis reaction, the raw material gas G1 is a mixture of nitrogen and hydrogen gas, the first gas G2 is nitrogen gas, and the second gas G3 is hydrogen gas.

[0136] As shown in Figure 1, one end of the supply pipe 91 is connected to the gas inlet 22 of the reactor 2.

[0137] For example, a first gas supply source 92 can be a first gas-containing cylinder, a first gas generator, or the like.

[0138] As the second gas supply source 93, for example, a second gas-containing cylinder, a second gas generator, etc., can be used.

[0139] As shown in Figure 1, the raw material gas supply means 9 may include a dilution gas supply source 94 that supplies dilution gas G4 to the supply pipe 91. Examples of dilution gas G4 include inert gases such as argon gas. Examples of dilution gas supply sources 94 include dilution gas-containing cylinders and dilution gas generators.

[0140] First gas G2 is supplied from the first gas supply source 92 to the supply pipe 91, second gas G3 is supplied from the second gas supply source 93 to the supply pipe 91, and optionally dilution gas G4 is supplied from the dilution gas supply source 94, thereby preparing a raw material gas G1 containing the first gas G2, second gas G3, and optionally dilution gas G4. The prepared raw material gas G1 is supplied to the reaction chamber 20 through the gas inlet 22 of the reactor 2. The raw material gas G1 supplied to the reaction chamber 20 comes into contact with the catalyst 5 as it passes through it.

[0141] The raw material gas supply means 9 may include a pump. The raw material gas supply means 9 can supply the raw material gas G1 to the reaction chamber 20, for example, by utilizing the suction and discharge forces of the pump.

[0142] As shown in Figure 1, the reactor 1 may be equipped with a temperature control means 10 for controlling the temperature of the environment in which the catalyst 5 is located (the ambient temperature around the catalyst 5, i.e., the ambient temperature inside the reaction chamber 20). The temperature control means 10 may include, for example, a heating furnace surrounding the reactor 2. The heating furnace may be, for example, an electric furnace. The heating furnace can raise the temperature of the environment in which the catalyst 5 is located by heating the reactor 2 inside the heating furnace.

[0143] As shown in Figure 1, the reactor 1 may be equipped with a discharge pipe 11 for discharging the product gas M, which is generated from the raw material gas G1 by the action of the catalyst 5, from the reaction chamber 20, along with unreacted raw material gas G1, etc. If the chemical reaction carried out by the reactor 1 is an ammonia synthesis reaction, the product gas M is ammonia.

[0144] As shown in Figure 1, one end of the discharge pipe 11 is connected to the gas outlet 23 of the reactor 2.

[0145] <Method for Producing Ammonia> The following describes an embodiment of a method for producing ammonia from a raw material gas containing nitrogen and hydrogen using reaction apparatus 1.

[0146] A method for producing ammonia according to one embodiment includes the following steps: (a) preparing a reactor 2 comprising a reaction chamber 20, a pair of electrodes 3 and 4 provided in the reaction chamber 20, and a catalyst 5 provided between the pair of electrodes 3 and 4; (b) supplying a raw material gas G1 containing nitrogen and hydrogen to the reaction chamber 20 and bringing the raw material gas G1 into contact with the catalyst 5; and (c) applying a voltage between the pair of electrodes 3 and 4 so that a direct current flows through the catalyst 5.

[0147] If catalyst 5 has not undergone reduction treatment, reduction treatment of catalyst 5 may be performed before the start of steps (b) and (c). Reduction treatment of catalyst 5 can be performed by supplying a reducing gas containing hydrogen gas to the reaction chamber 20 and bringing the reducing gas into contact with catalyst 5. When performing reduction treatment of catalyst 5, the temperature of the environment in which catalyst 5 is placed (ambient temperature of catalyst 5, i.e., atmospheric temperature in the reaction chamber 20) is, for example, 200°C or more and 900°C or less. The time for contacting catalyst 5 with the reducing gas is, for example, 1 hour or more and 5 hours or less. In addition to hydrogen gas, the reducing gas may also contain a diluent gas. Examples of diluent gases include inert gases such as argon gas. The content ratios of hydrogen gas and diluent gas in the reducing gas can be adjusted as appropriate. The content ratio of hydrogen gas in the reducing gas is, for example, 3% to 100% by volume, based on the volume of the reducing gas. The content ratio of diluent gas in the reducing gas is, for example, 0% to 97% by volume, based on the volume of the reducing gas.

[0148] Step (b) can be carried out using the raw material gas supply means 9. Specifically, nitrogen gas is supplied from the first gas supply source 92 to the supply pipe 91, hydrogen gas is supplied from the second gas supply source 93 to the supply pipe 91, and dilution gas is supplied optionally from the dilution gas supply source 94 to prepare a raw material gas G1 containing nitrogen gas, hydrogen gas, and optionally dilution gas. The prepared raw material gas G1 can then be supplied to the reaction chamber 20 through the gas inlet 22 of the reactor 2. The raw material gas G1 supplied to the reaction chamber 20 comes into contact with the catalyst 5 as it passes through it.

[0149] In step (b), it is preferable to continuously supply the raw material gas G1 to the reaction chamber 20. The raw material gas G1 continuously supplied to the reaction chamber 20 passes through the catalyst 5 continuously and comes into continuous contact with the catalyst 5. The time for which the raw material gas G1 is continuously supplied to the reaction chamber 20 is, for example, 5 minutes or more.

[0150] The concentrations of nitrogen gas, hydrogen gas, and diluent gas in the raw material gas G1 can be adjusted as appropriate. The nitrogen gas concentration in the raw material gas G1 is, for example, 0.1% to 99.9% by volume, based on the volume of the raw material gas G1. The hydrogen gas concentration in the raw material gas G1 is, for example, 0.1% to 99.9% by volume, based on the volume of the raw material gas G1. The diluent gas concentration in the raw material gas G1 is, for example, 0% to 99.8% by volume, based on the volume of the raw material gas G1.

[0151] The flow rate of raw material gas G1 can be adjusted as appropriate. For example, the flow rate of raw material gas G1 is between 50 mL / min and 500 mL / min.

[0152] The pressure inside the reaction chamber 20 after the supply of raw material gas G1 can be adjusted as appropriate. For example, the pressure inside the reaction chamber 20 after the supply of raw material gas G1 is between 0.05 MPa and 10 MPa.

[0153] Step (c) can be carried out using the voltage application means 7 and, if necessary, the voltage control means 8. Specifically, the voltage application means 7 and, if necessary, the voltage control means 8 can be used to apply a voltage between the electrodes 3 and 4 so that a DC current flows through the catalyst 5.

[0154] From the viewpoint of effectively exhibiting the catalytic activity of catalyst 5, the DC current flowing through catalyst 5 is preferably 1 mA or more and 10 mA or less.

[0155] In step (c), it is preferable to use the temperature control means 10 to adjust the temperature of the environment in which the catalyst 5 is placed (the ambient temperature of the catalyst 5, i.e., the ambient temperature inside the reaction chamber 20). From the viewpoint of effectively exhibiting the catalytic activity of the catalyst 5, the temperature of the environment in which the catalyst 5 is placed is preferably 100°C or more and 900°C or less, more preferably 150°C or more and 300°C or less, and even more preferably 170°C or more and 210°C or less. Each of the above lower limits may be combined with any of the above upper limits.

[0156] Alternatively, or in conjunction with the use of the temperature control means 10, the temperature of the environment in which the catalyst 5 is located may be adjusted by adjusting the temperature of the raw material gas G1 supplied to the reaction chamber 20. For example, the temperature of the environment in which the catalyst 5 is located can be increased by supplying heated raw material gas G1 to the reaction chamber 20.

[0157] The voltage applied between electrodes 3 and 4 can be adjusted as appropriate depending on the magnitude of the DC current flowing through the catalyst 5, the composition of the catalyst 5, etc., but is, for example, between 30V and 1500V.

[0158] The order in which the temperature of the environment in which the catalyst 5 is placed is adjusted and the voltage applied between electrodes 3 and 4 are not particularly limited.

[0159] In one embodiment, after adjusting the temperature of the environment in which the catalyst 5 is placed to a predetermined temperature, a voltage is applied between electrodes 3 and 4 so that a DC current of a predetermined magnitude flows through the catalyst 5.

[0160] The order in which steps (b) and (c) are carried out is not particularly limited, as long as the raw material gas G1 can come into contact with the catalyst 5 through which a DC current of a predetermined magnitude flows under a predetermined temperature environment.

[0161] In one embodiment, process (c) is started before process (b). In another embodiment, process (c) is started simultaneously with process (b). In yet another embodiment, process (c) is started after process (b) is started but before it is finished. In these embodiments, process (c) may be finished after process (b) is started but before it is finished, or it may be finished after process (b) is finished.

[0162] In yet another embodiment, step (c) is started after the completion of step (b).

[0163] From the viewpoint of improving ammonia synthesis activity, it is preferable that step (c) be started after the start of step (b), and more preferably after the raw material gas G1 has been stably supplied to the reaction chamber 20 (for example, 60 to 600 seconds after the start of step (b)). Cases in which step (c) is started after the start of step (b) include when step (c) is started after the start of step (b) but before the end of step (b), and when step (c) is started after the end of step (b), but it is preferable that step (c) is started after the start of step (b) but before the end of step (b).

[0164] From the viewpoint of improving ammonia synthesis activity, it is preferable in step (c) to continuously apply a voltage between electrodes 3 and 4 so that a DC current of a predetermined magnitude flows continuously through the catalyst 5 under a predetermined temperature environment. The time for which the DC current flows continuously through the catalyst 5 is, for example, 5 minutes or more. In this invention, "predetermined temperature environment" means that the temperature of the environment in which the catalyst 5 is placed (the ambient temperature of the catalyst 5, i.e., the atmospheric temperature inside the reaction chamber 20) is at a predetermined temperature.

[0165] In one embodiment, the raw material gas G1 is continuously supplied to the reaction chamber 20, and while the raw material gas G1 is continuously in contact with the catalyst 5, a voltage is continuously applied between electrodes 3 and 4 so that a predetermined DC current flows continuously through the catalyst 5 under a predetermined temperature environment.

[0166] When the raw material gas G1 comes into contact with a catalyst 5 through which a predetermined direct current of a predetermined magnitude flows under a predetermined temperature environment, the ammonia synthesis reaction proceeds due to the action of the catalyst 5, and ammonia is produced from the raw material gas G1.

[0167] The ammonia M produced from the raw material gas G1 by the action of catalyst 5 is discharged from the reaction chamber 20 through the gas outlet 23 of reactor 2, sometimes together with unreacted raw material gas G1. The ammonia M discharged from the reaction chamber 20 can be recovered through the discharge pipe 11.

[0168] Hereinafter, composite oxides containing Ce and Al are referred to as "Al-doped CeO ２ ", a composite oxide containing Ce and Mg is called "Mg-doped CeO ２ " and a composite oxide containing Ce and Ti is called "Ti-doped CeO ２ " he said.

[0169] <Example 1> (1) Al-doped CeO ２ The preparation step uses a precipitation method to prepare Al-doped CeO ２ The following was prepared. Specifically, it was done as follows: (1-1) 25.744 g of cerium(III) nitrate hexahydrate and 5.912 g of aluminum nitrate nonahydrate were added to 80.822 g of pure water and stirred to obtain raw material solution (B). (1-2) After heating the raw material solution (B) obtained in (1-1) to 70°C, sodium hydroxide aqueous solution (A) (pH 14) was added dropwise until the pH became 9.0 to form a precipitate. Then, the precipitate was aged at 70°C for 30 minutes while stirring to obtain a suspension. (1-3) The suspension obtained in (1-2) was filtered by suction and the precipitate was recovered. (1-4) The precipitate obtained in (1-3) was dispersed in pure water, and the resulting dispersion was filtered by suction and the precipitate was recovered. The precipitate was washed by repeating this process. Washing was repeated until the Na concentration of the filtrate was 2 ppm or less. The Na concentration was measured using an ion concentration meter (LAQUA twin-Na-11, manufactured by Horiba, Ltd.). (1-5) The precipitate after washing was dried overnight in a 100°C drying chamber (at atmospheric pressure). (1-6) The dried precipitate was calcined at 400°C for 4 hours at atmospheric pressure to obtain powdered Al-doped CeO ２ I obtained it.

[0170] (2) Ru loading process Using impregnation loading method, Al-doped CeO ２ Ru is supported on it, and Ru-supported Al-doped CeO ２The following was prepared. Specifically, it was done as follows: (2-1) 2.712 g of ruthenium nitrate solution (manufactured by Tanaka Kikinzoku Co., Ltd., concentration: 3.882% by mass) was dissolved in 20 mL of pure water to obtain raw material solution (C). (2-2) The Al-doped CeO obtained in (1-6) was added to the raw material solution (C) obtained in (2-1) above. ２ Add 2g and stir, then Al-doped CeO ２ A dispersion was obtained. (2-3) The dispersion obtained in (2-2) above was reduced to 70 Pa using an evaporator and heated to 60°C using a water bath to obtain a powder. (2-4) The powder obtained in (2-3) above was dried overnight in a drying oven (atmospheric atmosphere) at 120°C. (2-5) The dried powder obtained in (2-4) above was subjected to reduction treatment. Specifically, N ２ : H ２ A gas in a 1:3 (volume ratio) ratio was supplied to the dry powder at a total flow rate of 240 mL / min, and a reduction treatment was carried out at 450°C for 2 hours. In this way, the powdered Ru-supported Al-doped CeO was obtained. ２ (Hereinafter referred to as "Catalyst Powder of Example 1") was obtained. In Catalyst Powder of Example 1, at least a portion of Ru exists in the state of metallic Ru. A portion of Ru is Ru oxide (RuO ２ RuO ４ and Ru ２ O ３ It may exist in one or more states selected from the following.

[0171] <Example 2> Except that the precipitate was matured at 70°C for 30 minutes while stirring in (1-2) above, and the resulting suspension was placed in an autoclave container and subjected to hydrothermal treatment at 120°C for 24 hours, the powdered Ru-supported Al-doped CeO was prepared in the same manner as in Example 1. ２ (Hereinafter referred to as "Catalyst Powder of Example 2") was obtained. The pressure inside the container during hydrothermal treatment was set to a pressure at which the water inside the container could exist in a liquid state. In the catalyst powder of Example 2, at least a portion of Ru exists in the state of metallic Ru. A portion of Ru is Ru oxide (RuO ２ RuO ４ and Ru ２ O ３ It may exist in one or more states selected from the following.

[0172] <Example 3> Except for the fact that the precipitate was aged at 70°C for 24 hours while stirring in (1-2) above, the process was the same as in Example 1, and a powdered Ru-supported Al-doped CeO was prepared. ２ (Hereinafter referred to as "Catalyst Powder of Example 3") was obtained. In the catalyst powder of Example 3, at least a portion of Ru exists in the state of metallic Ru. A portion of Ru is Ru oxide (RuO ２ RuO ４ and Ru ２ O ３ It may exist in one or more states selected from the following.

[0173] <Example 4> Except for the fact that the precipitate was aged at 70°C for 120 hours while stirring, the same procedure as in Example 1 was used to prepare powdered Ru-supported Al-doped CeO ２ (Hereinafter referred to as "the catalyst powder of Example 4") was obtained. In the catalyst powder of Example 4, at least a portion of Ru exists in the state of metallic Ru. A portion of Ru is Ru oxide (RuO ２ RuO ４ and Ru ２ O ３ It may exist in one or more states selected from the following.

[0174] <Example 5> Except for changing the amount of aluminum nitrate nonahydrate used to 11.824 g as in (1-1) above, the powdered Ru-supported Al-doped CeO was prepared in the same manner as in Example 1. ２ (Hereinafter referred to as "Catalyst Powder of Example 5") was obtained. In the catalyst powder of Example 5, at least a portion of Ru exists in the state of metallic Ru. A portion of Ru is Ru oxide (RuO ２ RuO ４ and Ru ２ O ３ It may exist in one or more states selected from the following.

[0175] <Example 6> Except for changing the amount of aluminum nitrate nonahydrate used to 17.736 g as in (1-1) above, the powdered Ru-supported Al-doped CeO was prepared in the same manner as in Example 1. ２(Hereinafter referred to as "the catalyst powder of Example 6") was obtained. In the catalyst powder of Example 6, at least a portion of Ru exists in the state of metallic Ru. A portion of Ru is Ru oxide (RuO ２ RuO ４ and Ru ２ O ３ It may exist in one or more states selected from the following.

[0176] <Example 7> In the same manner as in Example 1, except that the raw material solution (B) was heated to 70°C and then sodium hydroxide (pH 14) was added dropwise until the pH reached 12.0 to form a precipitate, the powdered Ru-supported Al-doped CeO was prepared. ２ (Hereinafter referred to as "the catalyst powder of Example 7") was obtained. In the catalyst powder of Example 7, at least a portion of Ru exists in the state of metallic Ru. A portion of Ru is Ru oxide (RuO ２ RuO ４ and Ru ２ O ３ It may exist in one or more states selected from the following.

[0177] <Example 8> Except that 4.485 g of magnesium nitrate hexahydrate was used instead of aluminum nitrate nonahydrate as in (1-1) above, the powdered Ru-supported Mg-doped CeO was prepared in the same manner as in Example 1. ２ (Hereinafter referred to as "the catalyst powder of Example 8") was obtained. In the catalyst powder of Example 8, at least a portion of Ru exists in the state of metallic Ru. A portion of Ru is Ru oxide (RuO ２ RuO ４ and Ru ２ O ３ It may exist in one or more states selected from the following.

[0178] <Example 9> Except that 5.081 g of Ti lactate (manufactured by Matsumoto Fine Chemical Co., Ltd.) was used instead of aluminum nitrate nonahydrate as in (1-1) above, the same procedure as in Example 1 was used to produce powdered Ru-supported Ti-doped CeO ２ (Hereinafter referred to as "the catalyst powder of Example 9") was obtained. In the catalyst powder of Example 9, at least a portion of Ru exists in the state of metallic Ru. A portion of Ru is Ru oxide (RuO２ RuO ４ and Ru ２ O ３ It may exist in one or more states selected from the following.

[0179] <Comparative Example 1> Except that aluminum nitrate notahydrate was not used in (1-1) above, the same method as in Example 1 was used to prepare powdered Ru-supported CeO ２ (Hereinafter referred to as "Catalyst Powder of Comparative Example 1") was obtained. In Catalyst Powder of Comparative Example 1, at least a portion of Ru exists in the state of metallic Ru. A portion of Ru is Ru oxide (RuO ２ RuO ４ and Ru ２ O ３ It may exist in one or more states selected from the following.

[0180] <Measurement of Specific Surface Area> The specific surface area of ​​samples obtained from each catalyst powder in Examples 1 to 9 and Comparative Example 1 was measured using a specific surface area and pore distribution analyzer (BELSORP-mini II, manufactured by Microtrac-Bel). The specific surface area was measured using the BET multipoint method (JIS Z8830:2013) with an automated nitrogen gas adsorption / desorption isotherm measurement program. The amount of powder to be measured was approximately 0.05 to 0.1 g, and the pre-degassing conditions were 3 hours at 150°C under atmospheric pressure. The measurement results are shown in Table 1.

[0181] <Composition Analysis> The composition of samples obtained from each catalyst powder in Examples 1-9 and Comparative Example 1 was analyzed using an X-ray fluorescence analyzer (Primus II, manufactured by Rigaku Corporation). The conditions for X-ray excitation, 2θ scanning, and optical system (atenator, slit, spectroscopic crystal, detector) during XRF analysis were as follows: measurement diameter 10 mm, qualitative range F to U, and measurement time standard. Semi-quantitative composition analysis by FP method was performed on the analysis results using the instrument's built-in SQX software. The composition was calculated assuming that each metal element exists as an oxide. For Ru, Ru is RuO ２ It was assumed that they exist as such. Hereafter, the mass content (mass%) of each metal element in terms of oxide, as measured by XRF, will be referred to as the "XRF measurement value" for each metal element.

[0182] For each catalyst powder, Al-doped CeO ２ Mg Dope CeO ２ Or Ti Dope CEO ２ The mass content (mass%) of the doped elements (Al, Mg, or Ti) in oxide form was determined. The results are shown in Table 1. Al-doped CeO ２ Mass content of Al in oxide form = (XRF measurement value of Al) / (Al-doped CeO ２ (Sum of XRF measurements of all metal elements inside) × 100 Mg-doped CeO ２ Mass content of Mg in oxide form = (XRF measurement value of Mg) / (Mg-doped CeO ２ (Sum of XRF measurements of all metal elements inside) × 100 Ti-doped CeO ２ Mass content of Ti in oxide form = (XRF measurement of Ti) / (Ti-doped CeO ２ (Sum of XRF measurements of all metal elements inside) × 100

[0183] [XRF analysis conditions] X-ray source: Rh kα, voltage: 50 kV, current: 50 mA, attenuator: 1 / 1, slit: S2 (CeKα, TiKα, RuKα) / S4 (AlKα, MgKα), spectroscopic crystal: LiF1 (CeKα, TiK α, RuKα)・PET (AlKα)・RX25 (MgKα), Detector: SC (CeKα, TiKα, RuKα)・PC (AlKα, MgKα), Measurement angle: 5-90° (CeKα, CeKβ 1. CeLα, CeLβ1-3, CeLβ6, CeLG1, CeLG2, CeLG4) 140-148° (AlKα) 36.248°-42.248° (MgKα) 5°-90° (TiKα, TiKβ1) 16°-20° (RuKα), Sampling width: 0.02° (CeKα, TiKα, RuKα) 0.05° (AlKα, MgKα), Scan speed: 30° / min

[0184] <Evaluation of Ru loading amount> Using Ru-containing standard samples with known composition, RuO ２ XRF measurement value (RuO ２A calibration curve was created showing the relationship between the converted mass content (mass%) and the ICP measurement value of Ru (mass content of Ru in metallic terms (mass%)). The ICP measurement value is a measurement value obtained using inductively coupled plasma atomic emission spectrometry (hereinafter referred to as "ICP"). In ICP, the mass content (mass%) of each metallic element in the molten material obtained by alkali melting the sample was measured.

[0185] XRF measurement values ​​of Ru obtained from the above compositional analysis (RuO ２ The converted mass content (mass%) was converted to the mass content (mass%) of Ru in terms of metal equivalent using the calibration curve described above. For each catalyst powder, the mass content (mass%) of Ru in terms of metal equivalent in the catalyst was determined using the following formula. The results are shown in Table 1. Mass content (mass%) of Ru in terms of metal equivalent in the catalyst = (Mass content (mass%) of Ru in terms of metal equivalent) / {(Sum of XRF measurements of all metal elements other than Ru in the catalyst) + (Mass content (mass%) of Ru in terms of metal equivalent)} × 100

[0186] <Evaluation of Crystallite Size> X-ray diffraction (XRD) was performed on samples obtained from each catalyst powder in Examples 1 to 9 and Comparative Example 1 using a powder X-ray diffractometer (RINT-TTRIIII manufactured by Rigaku Corporation) to obtain X-ray diffraction patterns. XRD was performed under the following conditions: X-ray source: CuKα, operating axis: 2θ / θ, measurement method: continuous, counting unit: cps, start angle: 5°, end angle: 80°, sampling width: 0.02°, scan speed: 0.1° / min, voltage: 50kV, current: 300mA. The X-ray diffraction patterns of the samples obtained from the catalyst powders of Example 1, Example 2, and Comparative Example 1 are shown in Figures 2, 3, and 4, respectively.

[0187] In each X-ray diffraction pattern, CeO ２ The diffraction peaks originating from were identified, and Scherrer's formula was applied to the identified peaks to calculate the crystallite size. The calculation results are shown in Table 1. The peak shapes were fitted using a segmented pseudo-Voigt function. The crystallite size obtained from the peaks located at 2θ = 28.6° ± 0.5° was calculated for CeO ２The crystallite size was used. The Scherrer constant was assumed to be 0.94. Scherrer's formula is as follows: D = K × λ / B cos θ [wherein D is the crystallite size (nm), K is the Scherrer constant, λ is the wavelength of the X-ray (nm), B is the diffraction line width broadening (rad), and θ is the Bragg angle (rad).]

[0188] [Evaluation of Lattice Volume] In the same manner as above, X-ray diffraction patterns were obtained from the catalyst powders of each of the examples 1 to 9 and Comparative Example 1. For the data of each X-ray diffraction pattern, optimization was performed 2 to 4 times using PDXL2 (manufactured by Rigaku Corporation) without background refinement. Then, the "Refine background" option was checked and optimization was performed 2 to 4 times. Using width standard data, optimization was performed again 2 to 4 times. The peak shape was fitted using a segmented pseudo-Voigt function. An automatic search was performed, and CeO ２ The card number was confirmed by specifying the card number derived from [the source]. Then, the WPPF method (Full Pattern Fitting method) was selected and the "Method" item was changed to "Intensity Decomposition". The refinement was performed by setting the number of refinement iterations to 20, and the lattice constant (Å) was determined. ３ The following parameters were calculated. The calculation results are shown in Table 1. The refinement parameters were set as follows: Lattice constant: refined, Profile: refined, Orientation: fixed, Crystal structure: fixed, Method: intensity decomposition.

[0189] <Catalyst Activity Test> Catalyst activity tests were conducted to evaluate the catalytic activity of each catalyst powder in Examples 1-9 and Comparative Example 1. The catalyst activity tests were performed using a fixed-bed flow reactor at atmospheric pressure. The reactor configuration was the same as that shown in Figure 1. A quartz tube with an inner diameter of 6.0 mm and an outer diameter of 8.0 mm was used as the reactor body.

[0190] 100 mg of each catalyst powder was packed approximately in the center of the reactor body in the longitudinal direction to form a catalyst layer (approximately 2.5 mm thick) extending approximately perpendicular to the longitudinal direction of the reactor body. The upper surface of the catalyst layer faced the gas inlet side of the reactor, and the lower surface of the catalyst layer faced the gas outlet side of the reactor. Stainless steel electrodes SUS304 (outer diameter 2 mm) were inserted into the upper and lower surfaces of the catalyst layer, respectively, to apply an electric field. The electrode inserted into the upper surface of the catalyst layer was electrically connected to a high-voltage power supply (high-voltage power supply HAR-30N40 manufactured by Matsusada Precision Co., Ltd.), and the electrode inserted into the lower surface of the catalyst layer was grounded.

[0191] The catalyst layer was reduced before the reaction. Specifically, N ２ : H ２ A gas in a 1:3 (volume ratio) ratio was supplied to the catalyst layer at a total flow rate of 240 mL / min, and a reduction treatment was carried out at 450°C for 2 hours. Through this reduction treatment, the Ru oxide in the catalyst layer was reduced to metallic Ru.

[0192] After the reduction treatment of the catalyst layer, a temperature control means (a heating furnace at approximately 200°C) was used to adjust the temperature of the environment in which the catalyst layer was placed (the ambient temperature around the catalyst layer, i.e., the ambient temperature inside the reaction chamber) to 140-200°C.

[0193] After adjusting the temperature of the environment where the catalyst layer is located, N is injected from the gas inlet of the reactor. ２ : H ２ The raw material gases were supplied continuously to the reaction chamber in the reactor at a flow rate of 240 mL / min in a 1:3 (volume ratio). The purity of all gases used was 99.99% or higher.

[0194] After the raw material gas was stably supplied to the reaction chamber (30 seconds after the start of raw material gas supply), a high-voltage power supply was used in CC (constant current) mode to continuously apply voltage to two electrodes inserted on the upper and lower surfaces of the catalyst layer, thereby continuously applying a DC current of 6 mA to the catalyst layer. The response voltage was measured using an oscilloscope (TDS 2001C, Tektronics Inc.).

[0195] As the raw material gas supplied to the reaction chamber passes through the catalyst layer, it comes into contact with the catalyst layer, and the action of the catalyst layer causes nitrogen in the raw material gas to be released. ２ and H ２ From NH３ The generated NH ３ This is unreacted N ２ and / or H ２ It also leaks out of the reactor's gas outlet.

[0196] The gas that flowed out of the reactor's gas outlet was trapped by passing it through distilled water. Gas sampling was performed three times at 10-minute intervals, starting 10 minutes after the start of voltage application to two electrodes inserted on the upper and lower surfaces of the catalyst layer.

[0197] NH ３ The amount of substance (molecule) was determined by trapping the gas that flowed out of the reactor's gas outlet in distilled water and quantifying it using an ion chromatograph (Tosoh Corporation IC-2001).

[0198] NH ３ The generation rate was calculated using the following formula. The calculation results are shown in Table 1. NH ３ Generation rate = Quantified NH ３ Amount of substance (moles) / Mass of catalyst (g) / Measurement time (h)

[0199]

[0200] 1... Reactor, 2... Reactor, 20... Reaction chamber, 21... Reactor body, 22... Gas inlet, 23... Gas outlet, 3... Electrode (voltage application side), 4... Electrode (ground side), 5... Catalyst, 6... Support means, 7... Voltage application means, 8... Voltage control means, 9... Raw material gas supply means, 91... Supply pipe, 92... First gas supply source, 93... Second gas supply source, 94... Dilution gas supply source, 10... Temperature control means, 11... Discharge pipe, G1... Raw material gas, G2... First gas, G3... Second gas, G4... Dilution gas

Claims

1. A catalyst for carrying out a chemical reaction while a direct current is applied, wherein the catalyst comprises a Ce-based oxide and Ru supported on the Ce-based oxide, and the CeO ２ The catalyst having a crystallite size of 3 nm or more and 12 nm or less.

2. The catalyst according to claim 1, wherein the Ce-based oxide is a composite oxide containing Ce and one or more additional elements.

3. The catalyst according to claim 2, wherein the one or more additional elements are selected from Group 2 elements, Group 13 elements, and first transition elements.

4. The catalyst according to claim 2, wherein the one or more additional elements are selected from Al, Mg, and Ti.

5. The catalyst according to claim 2, wherein the mass content of the one or more additional elements in the composite oxide, on an oxide basis, is 0.1% by mass or more and 25.0% by mass or less.

6. The specific surface area of ​​the catalyst is 50 m². ２ The catalyst according to claim 1, wherein the amount is 1 / g or more.

7. The lattice volume of the Ce-based oxide is 155 Å. ３ 159Å or more ３ The catalyst according to claim 1, which is as follows:

8. The catalyst according to any one of claims 1 to 7, wherein the chemical reaction is a reaction that synthesizes ammonia from a raw material gas containing nitrogen and hydrogen.

9. A method for producing a catalyst for carrying out a chemical reaction while applying a direct current, comprising the following steps: (1A) preparing a plurality of candidate catalysts, each comprising a Ce-based oxide and Ru supported on the Ce-based oxide; and (1B) from the plurality of candidate catalysts, the CeO of the Ce-based oxide ２ The method comprising the step of selecting candidate catalysts having a crystallite size of 3 nm or more and 12 nm or less.

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