Electrode catalyst for electrolytic ammonia synthesis, method for producing electrode catalyst for electrolytic ammonia synthesis, rare earth oxide, and cell for electrolytic ammonia synthesis

Abstract

To provide: an electrode catalyst for electrolytic ammonia synthesis, which enables ammonia to be electrolytically synthesized with good efficiency; and a method for producing the same.SOLUTION: The present invention provides an electrode catalyst for electrolytic ammonia synthesis, which comprises a carrier and an active metal supported on the carrier. The carrier is a rare earth oxynitride. The present invention also provides a method for producing the same. The carrier is represented by, for example, the general formula: AxByOaNb. In the formula, A denotes a tetravalent lanthanoid, B denotes a trivalent lanthanoid, x denotes the molar proportion of the lanthanoid A among all lanthanoids and satisfies 0.4≤x<1, y denotes the molar proportion of the lanthanoid B among all lanthanoids and satisfies 0<y≤0.6, and x+y=1, a denotes the molar proportion of oxygen atoms in the carrier and satisfies 1.0≤a<2, and b denotes the molar proportion of nitrogen atoms in the carrier and satisfies 0<b≤0.6 and 1.4≤a+b<2.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrode catalyst for the electrolytic synthesis of ammonia, a method for producing an electrode catalyst for the electrolytic synthesis of ammonia, a rare earth oxide, and a cell for the electrolytic synthesis of ammonia. [Background technology]

[0002] In recent years, ammonia has attracted attention as a component that can be used in applications such as food, fertilizer, and hydrogen energy carriers. Conventionally, ammonia has been industrially synthesized using the Haber-Bosch process, which uses an iron-based catalyst. However, iron-based catalysts require the reaction of hydrogen and nitrogen under high-temperature and high-pressure conditions. Therefore, research into various types of ammonia synthesis catalysts is ongoing, with the aim of synthesizing ammonia under milder conditions than those used by the Haber-Bosch process.

[0003] For example, Patent Document 1 discloses "a catalyst for ammonia synthesis, which is used in ammonia synthesis by providing a catalyst between a pair of electrodes and applying a voltage that does not cause discharge between the electrodes in the presence of hydrogen and nitrogen, the catalyst comprising a catalytically active component and a composite oxide containing zirconium." [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2014-171916 Summary of the Invention [Problem to be solved by the invention]

[0005] However, to achieve carbon neutrality, further milder catalytic reaction conditions for ammonia synthesis are required. In particular, electrolytic synthesis, which enables direct ammonia synthesis from water and nitrogen, is attracting attention from the perspectives of reaction conditions, hydrogen procurement, and compatibility with renewable energy.

[0006] The present invention has been made in view of such points, and an object thereof is to provide an electrode catalyst for ammonia electrolytic synthesis capable of efficiently synthesizing ammonia by an electrolytic synthesis method and a method for producing the same.

Means for Solving the Problems

[0007] One aspect of the present invention is an electrode catalyst for ammonia electrolytic synthesis including a carrier and an active metal supported on the carrier, wherein the carrier is a rare earth oxynitride.

[0008] Another aspect of the present invention includes a step of preparing a rare earth oxynitride, and a step of supporting an active metal on the rare earth oxynitride to obtain a catalyst in which the active metal is supported on the rare earth oxynitride. A method for producing an electrode catalyst for ammonia electrolytic synthesis. <00​​​​​​​​​​​​​​​​​​​​​​​​​​​The present invention provides an electrode catalyst for electrolytic synthesis of ammonia that can efficiently synthesize ammonia, and a method for producing the same. Objects, configurations, and effects other than those described above will become apparent from the following description of the embodiments. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram schematically illustrating the effects of an electrode catalyst for electrolytic synthesis of ammonia according to one embodiment of the present invention. [Figure 2A] FIG. 1 is a diagram schematically illustrating the effect of a conventional electrode catalyst for the electrolytic synthesis of ammonia (Ru catalyst). [Figure 2B] FIG. 2 is a diagram schematically illustrating the effect of an electrode catalyst for the electrolytic synthesis of ammonia of a comparative example (Fe catalyst). [Figure 3] FIG. 1 is a diagram showing a production flow for producing a rare earth composite oxynitride from a rare earth composite oxide. [Figure 4A] FIG. 1 shows CO2 desorption spectra during CO2-TPD measurement in Comparative Example 1 and Example 1. [Figure 4B] FIG. 1 is a diagram showing the amount of CO2 desorption during CO2-TPD measurement in Comparative Example 1 and Example 1. [Figure 5] FIG. 1 is a diagram showing XPS spectra in Comparative Example 1 and Example 1. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, examples of embodiments to which the present invention is applied will be described using drawings and the like. In the drawings, the dimensions and shapes of each part are exaggerated for clarity, and the actual dimensions and shapes are not accurately depicted. Therefore, the technical scope of the present invention is not limited to the dimensions and shapes of each part shown in these drawings. Furthermore, the following description shows specific examples of the contents of the present invention, and the present invention is not limited to these descriptions. Various changes and modifications can be made by those skilled in the art within the scope of the technical ideas disclosed in this specification. Furthermore, in the drawings used to explain the present invention, parts having the same functions are designated by the same reference numerals, and repeated description thereof may be omitted.

[0014] The "~" described in this specification is used in the sense of a range having the numerical values described before and after it as the lower limit value and the upper limit value. In the numerical ranges described step by step in this specification, the upper limit value or the lower limit value described in one numerical range may be replaced with the upper limit value or the lower limit value described in other step-by-step numerical ranges. The upper limit value or the lower limit value of the numerical range described in this specification may be replaced with the value shown in the examples.

[0015] The electrode catalyst for ammonia electrolytic synthesis, which is one embodiment of the present invention, includes a carrier and an active metal supported on the carrier, and the carrier is a rare earth oxynitride. The carrier is, for example, a rare earth oxynitride containing tetravalent lanthanoids and trivalent lanthanoids. The carrier is, for example, of the general formula A x B y O a N b (where A represents a tetravalent lanthanoid, B represents a trivalent lanthanoid, x represents the molar fraction of lanthanoid A in the total lanthanoids, 0.4 ≦ x < 1, y represents the molar fraction of lanthanoid B in the total lanthanoids, 0 < y ≦ 0.6, x + y = 1, a represents the molar ratio of oxygen atoms in the carrier, 1.0 ≦ a < 2, b represents the molar ratio of nitrogen atoms in the carrier, 0 < b ≦ 0.6, and 1.4 ≦ a + b < 2) and is a rare earth oxynitride represented by.

[0016] The active metal includes, for example, at least one selected from iron (Fe) and molybdenum (Mo), and does not contain ruthenium (Ru) or nickel (Ni) as the main component.

[0017] In the active metal, the content of Fe is usually 50% by weight or more, preferably 75% by weight or more, more preferably 90% by weight or more, based on the total weight of the active metal. Since the active metal may be composed of only Fe, the upper limit value of the content of Fe is not limited.

[0018] In the active metal, the content of Mo is usually 0 wt% or more, preferably 5 wt% or more, more preferably 10 wt% or more, based on the total weight of the active metal. Since the active metal may be composed only of Mo, there is no upper limit to the content of Mo.

[0019] When the active metal contains Fe and / or Mo within the above content range, it is possible to achieve both resistance to hydrogen poisoning (covering of the active metal with hydrogen gas generated by proton reduction on the cathode) on the electrode catalyst and activity for nitrogen activation and dissociation (activation, adsorption, and dissociation of nitrogen).

[0020] In addition to Fe and Mo, the active metal may further include other metals, such as at least one metal species having a low hydrogen evolution potential selected from the group consisting of cobalt (Co), tungsten (W), rhenium (Re), and osmium (Os).

[0021] When the active metal contains the above-mentioned other metals other than Fe and Mo, the content of the other metals is not limited, but is usually 0.5 wt % or less, preferably 0.1 wt % or less, based on the total weight of the active metal. Since the active metal does not need to contain metals other than Fe and Mo, the lower limit of the content of the above-mentioned other metals is not limited.

[0022] If the active metal contains other metals as described above, the catalytic performance may be deteriorated, so it is preferable that the active metal does not contain such other metals.

[0023] Furthermore, the active metal preferably does not contain ruthenium (Ru) or nickel (Ni) as a major component. "Not containing ruthenium (Ru) or nickel (Ni) as a major component" means that it is substantially free of ruthenium (Ru) or nickel (Ni), and the content of such metals is usually 0.5% by weight or less, preferably 0.1% by weight or less.

[0024] The average particle size of the active metal is not limited, but is usually 0.1 nm or more, preferably 0.5 nm or more, and usually 50 nm or less, preferably 25 nm or less, for example, 0.1 nm to 50 nm, preferably 0.5 nm to 25 nm.

[0025] Here, the average particle size of the active metal can be measured by a CO pulse adsorption method.

[0026] When the average particle size of the active metal is within the above range, the number of active sites increases, and the nitrogen decomposition ability and ammonia production ability can be improved.

[0027] The specific surface area of ​​the active metal is not limited, but is usually 0.1 m 2 / g or more, preferably 0.5m 2 / g or more, and usually 100m 2 / g or less, preferably 50m 2 / g or less, for example, 0.1m 2 / g~100m 2 / g, preferably 0.5m 2 / g~50m 2 / g.

[0028] When the specific surface area of ​​the active metal is within the above range, it is possible to increase the number of coating sites of the active metal, that is, the number of catalytic active sites.

[0029] The content of the active metal in the ammonia electrosynthesis electrode catalyst is not limited, but is usually 3 wt % or more, preferably 5 wt % or more, and usually 20 wt % or less, preferably 10 wt % or less, for example, 3 wt % to 20 wt %, preferably 5 wt % to 10 wt %, relative to the total weight of the ammonia electrosynthesis electrode catalyst.

[0030] When the electrode catalyst for ammonia electrosynthesis contains an active metal within the above content range, it is possible to achieve both resistance to hydrogen poisoning (covering of the active metal with hydrogen gas generated by proton reduction on the cathode) on the electrode catalyst and activity for nitrogen activation and dissociation (activation, adsorption, and dissociation of nitrogen).

[0031] The support for supporting the active metal is represented by the general formula A x B y O a N b(wherein, A represents a tetravalent lanthanoid, B represents a trivalent lanthanoid, x represents the molar fraction of lanthanoid A in the total lanthanoids, 0.4 ≦ x < 1, y represents the molar fraction of lanthanoid B in the total lanthanoids, 0 < y ≦ 0.6, x + y = 1, a represents the molar ratio of oxygen atoms in the carrier, 1.0 ≦ a < 2, b represents the molar ratio of nitrogen atoms in the carrier, 0 < b ≦ 0.6, and 1.4 ≦ a + b < 2), which is a rare earth oxynitride having a fluorite-type crystal structure.

[0032] The tetravalent lanthanoid selected as A is, but not limited to, at least one selected from the group consisting of cerium (Ce), praseodymium (Pr), neodymium (Nd), terbium (Tb), and dysprosium (Dy). Ce, which can stably adopt a tetravalent state, is preferred as the tetravalent lanthanoid selected as A.

[0033] The trivalent lanthanoid selected as B is, but not limited to, at least one selected from the group consisting of lanthanum (La), Pr, Nd, promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), Tb, Dy, holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu). Inexpensive La, which can stably adopt a trivalent state, is preferred as the trivalent lanthanoid selected as B.

[0034] The molar fraction x of A in the total lanthanoids is not limited as long as the carrier can have a fluorite-type crystal structure, but it is 0.4 or more, preferably 0.45 or more, and less than 1, preferably 0.60 or less, more preferably 0.55 or less. For example, 0.4 ≦ x < 1, preferably 0.45 ≦ x ≦ 0.60, more preferably 0.45 ≦ x ≦ 0.55.

[0035] The molar fraction y of B in the entire lanthanoid is not limited as long as the carrier can have a fluorite crystal structure, but is greater than 0, preferably 0.40 or more, more preferably 0.45 or more, and 0.6 or less, preferably 0.55 or less. For example, 0 < y ≤ 0.6, preferably 0.40 ≤ x ≤ 0.55, more preferably 0.45 ≤ x ≤ 0.55.

[0036] By using a rare earth oxynitride having a fluorite crystal structure in which different lanthanoids are mixed as the carrier of the active metal, the electron donating property from the carrier to the active metal can be enhanced, and the dissociation of N2 on the active metal can be promoted.

[0037] In the present embodiment, the carrier is doped with nitrogen atoms. The nitrogen atoms are doped by replacing oxygen atoms in the crystal structure of the carrier.

[0038] The molar ratio a of oxygen atoms in the carrier varies depending on the molar fractions (x and y) of lanthanoids A and B and the molar ratio (b) of nitrogen atoms. The molar ratio a is not limited as long as the carrier can have a fluorite crystal structure, but is less than 2, preferably 1,8 or less, more preferably 1.6 or less, and 1.0 or more, preferably 1.3 or more, more preferably 1.4 or more. The molar ratio a is, for example, 1.0 ≤ a < 2, preferably 1.3 ≤ a ≤ 1.8, more preferably 1.4 ≤ a ≤ 1.6.

[0039] The molar ratio b of nitrogen atoms in the carrier varies depending on the molar fractions (x and y) of lanthanoids A and B and the molar ratio (a) of oxygen atoms. The molar ratio b is not limited as long as the carrier can have a fluorite crystal structure, but is greater than 0, preferably 0.05 or more, more preferably 0.10 or more, and 0.60 or less, preferably 0.30 or less, more preferably 0.20 or less, even more preferably 0.15 or less. The molar ratio b is, for example, 0 < b ≤ 0.60, preferably 0.05 ≤ b ≤ 0.30, more preferably 0.10 ≤ b ≤ 0.20.

[0040] The sum a+b of the molar ratio a of oxygen atoms and the molar ratio b of nitrogen atoms in the support varies depending on the molar fractions (x and y) of lanthanoids A and B. The sum a+b is not limited as long as the support can have a fluorite-type crystal structure, but is smaller than 2, preferably 1.8 or less, more preferably 1.7 or less, and 1.4 or more, preferably 1.5 or more, and more preferably 1.6 or more. The sum a+b is, for example, 1.4≦a+b<2, preferably 1.5≦a+b≦1.8, and more preferably 1.6≦a+b≦1.7.

[0041] When the carrier contains oxygen atoms and nitrogen atoms in a molar ratio within the above range, the basicity of the carrier is increased, and further, dissociation of nitrogen bonds at low temperatures is caused by the presence of pseudo nitrogen vacancies or oxygen vacancies as described below, which results in an improvement in the efficiency of ammonia production.

[0042] The average particle size of the carrier is not limited, but is usually 0.1 μm or more, preferably 0.5 μm or more, and usually 100 μm or less, preferably 10 μm or less, for example, 0.1 μm to 100 μm, preferably 0.5 μm to 10 μm.

[0043] Here, the average particle size of the carrier can be measured using a laser diffraction particle size distribution measuring device.

[0044] By ensuring that the average particle size of the carrier is within the above range, it is possible to increase the number of coating sites for the active metal, that is, the number of catalytic active sites.

[0045] The BET specific surface area of ​​the support is not limited, but is usually 1 m 2 / g or more, preferably 5m 2 / g or more, and usually 300m 2 / g or less, preferably 200m 2 / g or less, for example, 1m 2 / g~300m 2 / g, preferably 5m 2 / g~200m 2It is / g.

[0046] Here, the BET specific surface area means the specific surface area calculated by the BET method and is known in the art.

[0047] When the BET specific surface area of the carrier is within the above range, it is possible to increase the coating locations of the active metal, that is, the number of catalytic active sites.

[0048] Next, a method for manufacturing the electrode catalyst for ammonia electrolytic synthesis described above will be explained.

[0049] The electrode catalyst for ammonia electrolytic synthesis of the above-described embodiment can be manufactured by supporting an active metal on a carrier by a method known in the art, except that the rare earth oxynitride described above is used as the carrier.

[0050] The carrier used for manufacturing the above-described electrode catalyst for ammonia electrolytic synthesis can be manufactured, for example, by the following method.

[0051] First, a rare earth oxide before nitrogen is doped in the rare earth oxynitride described above, that is, a rare earth oxide represented by the general formula A x B y O2 (wherein A represents a tetravalent lanthanoid, B represents a trivalent lanthanoid, x represents the molar fraction of lanthanoid A in the total lanthanoids, 0.4 ≦ x < 1, y represents the molar fraction of lanthanoid B in the total lanthanoids, 0 < y ≦ 0.6, and x + y = 1) is manufactured. Here, the rare earth oxide has the same components as those of the rare earth oxynitride described above except that nitrogen is not doped.

[0052] The rare earth oxide can be manufactured by a method known in the art, for example, the coprecipitation method.

[0053] (Coprecipitation method) In one embodiment, a rare earth oxide containing lanthanoid A and lanthanoid B is prepared by coprecipitation using a composite oxide precursor solution containing a salt of lanthanoid A and a salt of lanthanoid B in a predetermined ratio.

[0054] Specifically, a composite oxide precursor solution is prepared by mixing and dissolving a salt of lanthanoid A, such as a Ce salt, a salt of lanthanoid B, such as a La salt, and a solvent, such as water, alcohol, or a mixed solution of water and alcohol, in a predetermined ratio, particularly the molar fraction of each lanthanoid described above.

[0055] The salt of each lanthanoid is not limited as long as it is soluble in the solvent, and examples thereof include nitrates, sulfates, halides such as chlorides, and organic acid salts such as acetates.

[0056] Next, the prepared composite oxide precursor solution is used to form a rare earth oxide containing lanthanoid A and lanthanoid B by coprecipitation. Specifically, the composite oxide precursor solution is mixed with a base solution, such as an aqueous ammonia solution, an aqueous sodium hydroxide solution, or an aqueous potassium hydroxide solution, for example, by dropping the base solution into the composite oxide precursor solution, and a precipitate is formed by a neutralization reaction.

[0057] The resulting precipitate is then filtered, dried, and calcined to obtain a rare earth oxide containing lanthanoid A and lanthanoid B.

[0058] Filtration can be carried out by methods known in the art.

[0059] Following the filtration step, the filtered precipitate is dried under an oxidizing atmosphere (for example, in air) or under an inert gas atmosphere (for example, under a nitrogen or argon atmosphere).

[0060] The temperature for drying the precipitate is usually 50° C. or higher, preferably 80° C. or higher, and usually 150° C. or lower, preferably 120° C. or lower, for example, 50° C. to 150° C., preferably 80° C. to 120° C. By setting the temperature for drying the precipitate within the above range, the moisture in the precipitate can be sufficiently removed.

[0061] The drying time of the precipitate is usually 30 minutes or more, preferably 60 minutes or more, and more preferably 120 minutes or more. By setting the drying time of the precipitate within the above range, the moisture in the precipitate can be sufficiently removed. The upper limit of the drying time of the precipitate is not limited, although it will lengthen the process time.

[0062] Following the drying step, the dried precipitate is calcined in an oxidizing atmosphere (for example, in air) or in an inert gas atmosphere (for example, in a nitrogen or argon atmosphere).

[0063] The calcination temperature of the precipitate is usually 610° C. or higher, preferably 625° C. or higher, more preferably 650° C. or higher, and usually 900° C. or lower, preferably 800° C. or lower, more preferably 750° C. or lower, for example, 610° C. to 900° C., preferably 625° C. to 800° C., more preferably 650° C. to 750° C. By setting the calcination temperature of the precipitate within the above range, the stability of the rare earth oxide can be improved, and the ammonia synthesis activity of the resulting electrode catalyst for ammonia electrosynthesis can be increased.

[0064] The calcination time of the precipitate is usually 1 hour or more, preferably 2 hours or more, more preferably 3 hours or more, and usually 48 hours or less, preferably 36 hours or less, more preferably 24 hours or less, for example, 1 hour to 48 hours, preferably 2 hours to 36 hours, more preferably 3 hours to 24 hours. By setting the calcination time of the precipitate within the above range, the stability of the rare earth oxide can be improved, and the ammonia synthesis activity of the resulting electrode catalyst for ammonia electrosynthesis can be increased.

[0065] The obtained rare earth oxide is then doped with nitrogen by a method known in the art, for example, by coprecipitation.

[0066] Doping with nitrogen element can be carried out by a method known in the art, for example, by calcining a rare earth oxide in an ammonia atmosphere.

[0067] The flow rate of ammonia during firing of the rare earth oxide is usually 200 sccm or more, preferably 250 sccm or more, more preferably 300 sccm or more, and usually 1000 sccm or less, preferably 800 sccm or less, more preferably 600 sccm or less, for example, 200 sccm to 1000 sccm, preferably 250 sccm to 800 sccm, more preferably 300 sccm to 600 sccm.

[0068] The concentration of ammonia gas during calcination of the rare earth oxide is usually 1% by volume or more, preferably 5% by volume or more, and usually 100% by volume or less, preferably 50% by volume or less, for example, 1% by volume to 100% by volume, preferably 5% by volume to 50% by volume. Examples of dilution gases for ammonia gas include nitrogen gas and argon gas. By setting the flow rate and concentration of ammonia within the above ranges, the introduction of nitrogen element into the rare earth oxide can be sufficiently promoted, and a rare earth oxynitride can be obtained.

[0069] The firing temperature of the rare earth oxide is usually 400° C. or higher, preferably 425° C. or higher, more preferably 450° C. or higher, and usually 600° C. or lower, preferably 575° C. or lower, more preferably 550° C. or lower, for example, 400° C. to 600° C., preferably 425° C. to 575° C., more preferably 450° C. to 550° C. By setting the firing temperature of the rare earth oxide within the above range, the introduction of nitrogen element into the rare earth oxide can be sufficiently promoted, and a rare earth oxynitride can be obtained.

[0070] The calcination time of the rare earth oxide is usually 1 hour or more, preferably 2 hours or more, more preferably 3 hours or more, and usually 12 hours or less, preferably 8 hours or less, more preferably 6 hours or less, for example, 1 hour to 12 hours, preferably 2 hours to 8 hours, more preferably 3 hours to 6 hours. By setting the calcination time of the rare earth oxide within the above range, the introduction of nitrogen element into the rare earth oxide can be sufficiently progressed, and a rare earth oxynitride can be obtained.

[0071] In firing a rare earth oxide in an ammonia atmosphere, the heating rate, the cooling rate after firing, and the atmosphere during heating and cooling after firing are not limited. The heating rate is, for example, usually 5°C / min or more, preferably 10°C / min or more, and usually 20°C / min or less, preferably 15°C / min or less, for example, 5°C / min to 20°C / min, preferably 10°C / min to 15°C / min. The cooling rate after firing is, for example, usually 5°C / min or more, preferably 10°C / min or more, and usually 20°C / min or less, preferably 15°C / min or less, for example, 5°C / min to 20°C / min, preferably 10°C / min to 15°C / min. The cooling after firing may be allowed to stand. The atmosphere during heating and cooling after firing may be air, nitrogen, argon, or ammonia. When the atmosphere during heating and the atmosphere during cooling after firing are a nitrogen atmosphere, an argon atmosphere, or an ammonia atmosphere, the flow rate of each gas is not limited and is usually 100 sccm or more, preferably 150 sccm or more, and usually 300 sccm or less, preferably 250 sccm or less, for example, 100 sccm to 300 sccm, preferably 150 sccm to 250 sccm.

[0072] Finally, an active metal is supported on the obtained rare earth oxynitride by, for example, impregnation and evaporation to dryness, to obtain an electrode catalyst for ammonia electrolysis in which the active metal is supported on the rare earth oxynitride.

[0073] Specifically, first, an active metal precursor is attached to a rare earth oxynitride in the amount described above using a solution containing a salt of the active metal, for example, a salt of Fe, and a solvent, for example, water, tetrahydrofuran (THF), alcohol, or a mixed solution of water and alcohol.

[0074] The salt of the active metal is not limited as long as it is soluble in the solvent, and examples thereof include nitrates, sulfates, carbonates, halides such as chlorides, organic acid salts such as acetates, citrates, dinitrodiammine salts, and various complexes (e.g., tetraammine complexes, carbonyl complexes).

[0075] The method for attaching an active metal to a rare earth oxynitride is not limited, and examples thereof include a method (impregnation method) in which the rare earth oxynitride is immersed in a solution containing a salt of the active metal and a solvent to impregnate the rare earth oxynitride with the salt of the active metal, and a method (adsorption method) in which a solution containing a salt of the active metal and a solvent is adsorbed onto the rare earth oxynitride.

[0076] Next, the rare earth oxynitride to which the active metal precursor is attached in this manner is dried, and then calcined under a reducing gas atmosphere or an inert gas atmosphere to obtain an ammonia electrosynthesis electrode catalyst in which the active metal is supported on a rare earth oxide containing lanthanoid A and lanthanoid B. In particular, since the calcination is performed under a reducing gas atmosphere or an inert gas atmosphere (preferably under a reducing gas atmosphere), the active metal is supported in a metallic state on the rare earth oxide support, and an ammonia electrosynthesis electrode catalyst with excellent ammonia synthesis activity is obtained.

[0077] The drying temperature of the rare earth oxynitride having the active metal precursor attached thereto is usually 50°C or higher, preferably 75°C or higher, and usually 150°C or lower, preferably 125°C or lower, for example, 50°C to 150°C, preferably 75°C to 125°C. The drying time is usually 1 hour or longer, preferably 3 hours or longer. The upper limit of the drying time is not limited, although it will lengthen the process time.

[0078] The reducing gas atmosphere is an atmosphere containing a reducing gas such as hydrogen gas, carbon monoxide gas, or hydrocarbon gas, and examples thereof include a mixed gas atmosphere of a reducing gas and an inert gas (nitrogen gas, argon gas, or the like). The concentration of the reducing gas in such a mixed gas atmosphere is usually 1% by volume or more, preferably 5% by volume or more, and usually 30% by volume or less, preferably 20% by volume or less, for example, 1% by volume to 30% by volume, preferably 5% by volume to 20% by volume. Examples of dilution gases for the reducing gas include nitrogen gas and argon gas. Examples of inert gas atmospheres include a nitrogen gas atmosphere, an argon gas atmosphere, and a helium gas atmosphere.

[0079] The calcination temperature of the dried rare earth oxynitride having the active metal precursor attached thereto is usually 200°C or higher, preferably 300°C or higher, and usually 500°C or lower, for example, 200°C to 500°C, preferably 300°C to 500°C. The calcination time is usually 0.5 hours or higher, preferably 1 hour or higher, and usually 10 hours or lower, preferably 5 hours or lower, for example, 0.5 hours to 10 hours, preferably 1 hour to 5 hours. By setting the calcination temperature and calcination time within the above ranges, all of the active metals can be uniformly dispersed and supported on the rare earth oxynitride support in a state where they are sufficiently reduced to a metallic state while avoiding sintering between particles, and the activity of the resulting ammonia electrosynthesis electrode catalyst can be improved.

[0080] In the method for producing an electrode catalyst for the electrolytic synthesis of ammonia according to this embodiment, the electrode catalyst for the electrolytic synthesis of ammonia produced in this manner may be formed into various shapes by known methods.

[0081] According to the method for producing an electrode catalyst for electrolytic synthesis of ammonia of this embodiment, it is possible to produce an electrode catalyst for electrolytic synthesis of ammonia in which an active metal is supported on a rare earth oxynitride in which oxygen atoms in a rare earth oxide are substituted with nitrogen atoms.

[0082] The ammonia electrosynthesis electrode catalyst of this embodiment can be used in an ammonia electrosynthesis cell. For example, in an electrolysis cell using a proton conductor or a proton exchange membrane, the ammonia electrosynthesis electrode catalyst can efficiently synthesize ammonia by bringing protons generated by electrolysis of water as a cathode catalyst into contact with nitrogen as a raw material gas.

[0083] In the ammonia electrosynthesis method using the ammonia electrosynthesis electrode catalyst of this embodiment, the synthesis conditions are not limited, and conditions in ammonia synthesis methods known in the art can be used as they are. For example, the method of contacting nitrogen with protons generated by electrolysis from water, which is a raw material, on the anode side and conducted to the cathode via a proton conductor is not limited, and methods in ammonia synthesis methods known in the art can be used.

[0084] The reaction temperature is usually 400° C. or higher, preferably 400° C., and usually 600° C. or lower. However, the reaction temperature needs to be optimized depending on the proton transport properties of the proton conductor and proton exchange membrane used in the electrolysis cell, and is therefore not limited to the above temperatures.

[0085] By synthesizing ammonia under the conditions within the above ranges using the electrode catalyst for electrolytic synthesis of ammonia of this embodiment, ammonia can be synthesized efficiently.

[0086] In ammonia electrosynthesis using the ammonia electrosynthesis electrode catalyst of this embodiment, water electrolysis first occurs on the anode side, generating protons. The generated protons are then transported to the cathode side via a proton conductor and a proton exchange membrane. Concurrently, bond dissociation of the raw material nitrogen is catalyzed on the surface of Fe particles, which are active metals supported on the surface of the rare earth oxynitride carrier. Here, electrons are donated from the rare earth oxynitride to the Fe, further promoting the decomposition of nitrogen molecules on the Fe surface. Finally, the decomposed nitrogen atoms react with protons, resulting in the electrosynthesis of ammonia. In this reaction, the rare earth oxynitride has energetically active vacancies (pseudo-nitrogen vacancies) on its surface, which capture nitrogen molecules and promote the bond dissociation of the captured nitrogen molecules.

[0087] FIG. 1 schematically shows the effect of the ammonia electrosynthesis electrode catalyst of this embodiment, and FIGS. 2A and 2B schematically show the effect of a comparative ammonia electrosynthesis electrode catalyst. As shown in Figure 2A, Fe has a lower hydrogen generation potential than Ru and other metals, so it is possible to suppress hydrogen poisoning and further improve ammonia production efficiency. However, as shown in Figure 2B, when Fe is selected as the active metal, the bond dissociation of nitrogen molecules is less promoted than in the case of Ru. Therefore, by using the rare earth oxynitride of this embodiment as a support and a metal with a low hydrogen generation potential, such as Fe or Mo, as the active metal, it is possible to achieve both suppression of hydrogen poisoning and high nitrogen dissociation ability, as shown in Figure 1. [Example]

[0088] Hereinafter, several examples of the present invention will be described, but it is not intended that the present invention be limited to those shown in these examples.

[0089] I. Sample Preparation Comparative Example 1 Cerium oxide (CeO2, specific surface area: 114.5m 2The cerium oxide (cerium oxide) was mixed with Ru salt (Ru nitrate) adjusted to 5 wt % of Ru based on the total weight of the cerium oxide in pure water as a solvent, dried at 100°C for 12 hours, and then fired at 450°C for 2 hours to prepare Ru / CeO2.

[0090] Example 1 According to Figure 3, the cerium-lanthanum composite oxide (Ce 0.5 La 0.5 O a ) to cerium-lanthanum composite oxynitride (Ce 0.5 La 0.5 O a N b Specifically, cerium-lanthanum composite oxide (Ce 0.5 La 0.5 O a After the installation, the furnace was evacuated. Then, while flowing 200 sccm of N2, the temperature was raised to 500°C at a rate of 10°C / min. When the temperature reached 500°C, the N2 was replaced with 500 sccm of NH3, and the temperature was maintained at 500°C for 4 hours. After that, the supply of NH3 was stopped and the furnace was allowed to cool, resulting in the production of cerium-lanthanum composite oxynitride (Ce 0.5 La 0.5 O a N b The obtained cerium-lanthanum composite oxynitride was mixed with Ru salt (Ru nitrate) adjusted to 5 wt % of Ru based on the total weight of the composite oxynitride in pure water as a solvent, dried at 100°C for 12 hours, and then fired at 450°C for 2 hours to produce Ru / Ce 0.5 La 0.5 O a N b was prepared.

[0091] II.Analysis II-1.CO2-TPD (CO2 desorption spectrum) Fig. 4A shows the CO2 desorption spectra in Comparative Example 1 and Example 1, and Fig. 4B shows the CO2 desorption amounts in Comparative Example 1 and Example 1. Note that the notation aEn represents a × 10 n For example, 3.E-04 = 3.0 × 10 -44A, the CO2 desorption spectrum obtained by TPD shows that the desorption temperature of Example 1 is higher than that of Comparative Example 1, and the CO2 desorption amount of Example 1 shown in FIG. 4B is greater than that of Comparative Example 1. From the above, it was found that Example 1 has a stronger basic site strength. Therefore, it was confirmed that Example 1 exhibits strong basicity and maintains a high electron donating ability due to the effect of the rare earth oxynitride carrier.

[0092] II-1. XPS Spectra 5 shows the XPS spectrum of the cerium-lanthanum composite oxynitride in Example 1. From the N 1s fitting of the enlarged view in FIG. 5, it was found that the cerium-lanthanum composite oxynitride in Example 1 was doped with nitrogen. Therefore, based on the results in FIGS. 4A and 4B and the results of the spectral intensity analysis, it was found that the cerium-lanthanum composite oxynitride in Example 1 was doped with Ce. 0.67 La 0.33 O 1.60 N 0.14 Furthermore, it was confirmed that oxygen vacancies (or nitrogen vacancies) exist in the cerium-lanthanum composite oxynitride based on this stoichiometric ratio.

[0093] The present invention is not limited to the above-described embodiment, but includes various modifications. For example, it is possible to add, delete, or replace part of the configuration of the embodiment with another configuration.

Claims

1. An electrode catalyst for electrolytic synthesis of ammonia, comprising a support and an active metal supported on the support. So, The support is a rare earth oxynitride. Electrocatalyst for ammonia electrosynthesis.

2. The electrode catalyst for electrolytic synthesis of ammonia according to claim 1, The electrode catalyst for electrolytic synthesis of ammonia, wherein the support is a rare earth oxynitride containing a tetravalent lanthanoid and a trivalent lanthanoid.

3. The electrode catalyst for electrolytic synthesis of ammonia according to claim 1, The carrier has the general formula: AxByOaNb (Wherein A represents a tetravalent lanthanoid, B represents a trivalent lanthanoid, x represents the mole fraction of lanthanoid A in the total lanthanoids, and 0.4≦x<1; y represents the molar fraction of lanthanoid B in the total lanthanoids, and 0<y≦0.6; x+y=1, a represents the molar ratio of oxygen atoms in the support, and 1.0≦a<2; b represents the molar ratio of nitrogen atoms in the carrier, and is 0<b≦0.6 and 1.4≦a+b<2. The electrode catalyst for electrolytic synthesis of ammonia is a rare earth oxynitride represented by the formula:

4. The electrode catalyst for electrolytic synthesis of ammonia according to claim 1 The active metal includes at least one selected from iron and molybdenum. Electrocatalyst for ammonia electrosynthesis.

5. The electrode catalyst for electrolytic synthesis of ammonia according to claim 1, The active metal does not contain ruthenium and nickel as main components. Electrocatalyst for ammonia electrosynthesis.

6. preparing a rare earth oxynitride; a step of supporting an active metal on the rare earth oxynitride to obtain a catalyst in which the active metal is supported on the rare earth oxynitride; A method for producing an electrode catalyst for electrolytic synthesis of ammonia, comprising:

7. 7. A method for producing the electrode catalyst for electrolytic synthesis of ammonia according to claim 6, In the step of preparing the rare earth oxynitride, Preparing rare earth oxynitrides containing tetravalent lanthanides and trivalent lanthanides; A method for producing an electrode catalyst for electrolytic synthesis of ammonia.

8. 8. A method for producing the electrode catalyst for electrolytic synthesis of ammonia according to claim 7, In the step of preparing the rare earth oxynitride, A composite oxide containing lanthanoid A and lanthanoid B is prepared using a composite oxide precursor solution containing a salt of tetravalent lanthanoid A and a salt of trivalent lanthanoid B in a ratio where x and y satisfy 0.4≦x<1 and 0<y≦0.6, where x is the molar fraction of lanthanoid A and y is the molar fraction of lanthanoid B (x+y=1). A method for producing an electrode catalyst for electrolytic synthesis of ammonia.

9. 8. A method for producing the electrode catalyst for electrolytic synthesis of ammonia according to claim 7, The active metal includes at least one selected from iron and molybdenum. A method for producing an electrode catalyst for electrolytic synthesis of ammonia.

10. 8. A method for producing the electrode catalyst for electrolytic synthesis of ammonia according to claim 7, The active metal does not contain ruthenium and nickel as main components. A method for producing an electrode catalyst for electrolytic synthesis of ammonia.

11. General formula: A ｘ B ｙ O ａ N ｂ (In the formula, A represents a tetravalent lanthanoid, B represents a trivalent lanthanoid, x represents the molar fraction of lanthanoid A among all the lanthanoids, and 0.4≦x<1; y represents the molar fraction of lanthanoid B among all the lanthanoids, and 0<y≦0.6; x+y=1; a represents the molar ratio of oxygen atoms in the support, and 1.0≦a<2; b represents the molar ratio of nitrogen atoms in the support, and 0<b≦0.6; and 1.4≦a+b<2.) A rare earth oxynitride as a support for an electrode catalyst for ammonia electrosynthesis, as shown in

12. An ammonia electrolytic synthesis cell comprising the ammonia electrolytic synthesis electrode catalyst according to claim 1.

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