Ammonia synthesis catalyst and method for producing same

Abstract

Provided are: an ammonia synthesis catalyst which enables the highly efficient synthesis of ammonia; and a method for producing the ammonia synthesis catalyst.　The present invention relates to an ammonia synthesis catalyst comprising a support material and an active metal supported on the support material, wherein the active metal comprises at least one metal selected from ruthenium and nickel, iron is not contained as a main component, and the support material is a rare earth oxynitride.

Description

Ammonia synthesis catalyst and method for producing the same

[0001] The present invention relates to an ammonia synthesis catalyst and a method for producing the same.

[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 method for producing ammonia from ammonia synthesis gas by contacting a catalyst comprising ruthenium as a catalytically active material supported on a boron nitride and / or silicon nitride carrier with the ammonia synthesis gas under ammonia-producing conditions.

[0004] Patent Document 2 discloses an ammonia synthesis catalyst for synthesizing ammonia from nitrogen and hydrogen, characterized in that a metal is supported on a carrier made of a semiconductor containing a transition element.

[0005] In Patent Document 3, a composite oxide A containing a metal element represented by the composition of general formula (1) is disclosed. ｎ X ｙ M ｍ (1) (in the general formula (1), A is a lanthanoid characterized by being at least partially or entirely in a valence III state; X is either a Group 2 element of the periodic table selected from the group consisting of Ca, Sr, and Ba, or a lanthanoid, and represents an element different from A; M is either a Group 1 element of the periodic table, a Group 2 element selected from the group consisting of Ca, Sr, and Ba, or a lanthanoid, and represents an element different from A and X; n is 0<n<1; y is 0<y<1; m is 0≦m<1; and n+y+m=1) is disclosed.

[0006] In Patent Document 4, a composite oxide containing cerium (Ce) and a lanthanoid other than Ce is solid-dissolved with at least one additive metal element selected from the group consisting of titanium (Ti), zirconium (Zr), hafnium (Hf), aluminum (Al), gallium (Ga), indium (In), silicon (Si), germanium (Ge), and tin (Sn), and the composite oxide is represented by the following formula: Ce ｘ A １－ｘ－ｙ B ｙ O ｄ (wherein A represents the lanthanoid other than Ce, B represents the added metal element, x represents the molar fraction of Ce, y represents the molar fraction of the added metal element, 1-x-y represents the molar fraction of the lanthanoid other than Ce, x and y satisfy 0.1≦x≦0.9, 0.01≦y≦0.3, 0.11≦x+y≦0.91, and d represents the molar ratio of oxygen atoms and satisfies 1.5≦d≦2), and ruthenium (Ru) supported on the composite oxide support is disclosed.

[0007] JP 2001-149780 A JP 2015-66468 A International Publication No. 2019 / 059190 JP 2022-165370 A

[0008] However, in order to achieve carbon neutrality, further milder catalytic reaction conditions are required for ammonia synthesis.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an ammonia synthesis catalyst capable of synthesizing ammonia efficiently, and a method for producing the same.

[0010] As a result of extensive research, the present inventors have found that an ammonia synthesis catalyst using a rare earth oxynitride as a support improves nitrogen decomposition ability at low temperatures, and as a result, ammonia can be synthesized efficiently, thereby completing the present invention.

[0011] That is, the ammonia synthesis catalyst according to the present invention is characterized by comprising a rare earth oxynitride and a metal catalyst supported on the rare earth oxynitride. This specification incorporates the disclosure of Japanese Patent Application No. 2023-160976, from which the present application claims priority.

[0012] The present invention provides an ammonia synthesis catalyst capable of efficiently synthesizing 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.

[0013] FIG. 1 is a diagram schematically illustrating the effect of the ammonia synthesis catalyst of the present invention. ０．５ La ０．５ O ａ ) to cerium-lanthanum composite oxynitride (Ce ０．５ La ０．５ O ａ N ｂ 1 is a diagram showing a production flow for producing a cerium-lanthanum composite oxynitride according to Example 1; ２ , Ce in Comparative Example 2 ０．５ La ０．５ O ａ , and Ce in Example 1 ０．５ La ０．５ O ａ N ｂ 1 is a graph showing the ammonia production rate converted into surface area for the ammonia synthesis catalysts of Comparative Example 1, Comparative Example 2, and Example 1.

[0014] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the dimensions and shapes of each part are exaggerated for clarity and do not accurately depict the actual dimensions and shapes. 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.

[0015] The symbol "to" used herein means a range having the numerical values ​​before and after it as the upper and lower limits. In numerical ranges described in stages in this specification, the upper or lower limit described in one numerical range may be replaced with the upper or lower limit described in another stage. The upper or lower limit of a numerical range described herein may be replaced with a value shown in the examples.

[0016] The present invention provides an ammonia synthesis catalyst comprising a carrier and an active metal supported on the carrier, wherein the active metal comprises at least one selected from ruthenium (Ru) and nickel (Ni), and does not contain iron (Fe) as a main component, and the carrier 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 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, and 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≦0.60, and 1.4≦a+b<2).

[0017] In the active metal, the content of Ru is usually 50 wt % or more, preferably 75 wt % or more, and more preferably 90 wt % or more, based on the total weight of the active metal. Since the active metal may be composed only of Ru, there is no upper limit to the content of Ru.

[0018] In the active metal, the content of Ni 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 Ni, there is no upper limit to the content of Ni.

[0019] When the active metal contains Ru and / or Ni in the above content range, the activity from activation to dissociation of nitrogen (activation, adsorption and dissociation of nitrogen) can be further increased.

[0020] In addition to Ru and Ni, the active metal may further include other metals, such as at least one metal selected from the group consisting of Fe, cobalt (Co), molybdenum (Mo), tungsten (W), rhenium (Re), and osmium (Os).

[0021] When the active metal contains metals other than Ru and Ni, 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 Ru and Ni, the lower limit of the content of the other metals is not limited.

[0022] If the active metal contains other metals, the catalytic performance may be deteriorated, so it is preferable that other metals are not contained.

[0023] 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.

[0024] Here, the average particle size of the active metal can be measured by a CO pulse adsorption method.

[0025] 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.

[0026] 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, 0.1 m ２ / g to 100m ２ / g, preferably 0.5m ２ / g to 50m ２ / g.

[0027] 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.

[0028] The content of the active metal in the ammonia synthesis catalyst is not limited, but is usually 3% by weight or more, preferably 5% by weight or more, and usually 20% by weight or less, preferably 10% by weight or less, for example, 3 to 20% by weight, preferably 5 to 10% by weight, based on the total weight of the ammonia synthesis catalyst.

[0029] When the ammonia synthesis catalyst contains the active metal in the above content range, the activity from activation to dissociation of nitrogen (activation, adsorption and dissociation of nitrogen) can be made higher.

[0030] The carrier 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 among all lanthanoids, and 0.4≦x<1; y represents the molar fraction of lanthanoid B among all lanthanoids, and 0<y≦0.6, and 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≦0.60, and 1.4≦a+b<2), and has a fluorite-type crystal structure.

[0031] The tetravalent lanthanoid selected as A is at least one selected from the group consisting of cerium (Ce), praseodymium (Pr), neodymium (Nd), terbium (Tb), and dysprosium (Dy), but is not limited thereto. The tetravalent lanthanoid selected as A is preferably Ce, which can stably assume a tetravalent state.

[0032] The trivalent lanthanoid selected as B is 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), but is not limited thereto. The trivalent lanthanoid selected as B is preferably inexpensive La, which can stably assume a trivalent state.

[0033] The molar fraction x of A in all lanthanoids is not limited as long as the support can have a fluorite-type crystal structure, but 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.

[0034] The molar fraction y of B in all lanthanoids is not limited as long as the support can have a fluorite-type 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.

[0035] By using a rare earth oxynitride having a fluorite-type crystal structure in which different lanthanides are mixed as a support for the active metal, it is possible to increase the electron donating ability from the support to the active metal, and the N on the active metal can be ２ This can promote the dissociation of

[0036] In the present invention, the support is doped with nitrogen atoms, which are doped by substituting for oxygen atoms in the crystal structure of the support.

[0037] The molar ratio a of oxygen atoms in the support varies depending on the molar fractions of lanthanoids A and B (x and y) and the molar ratio of nitrogen atoms (b), and is not limited as long as the support can have a fluorite-type crystal structure. However, it is preferably smaller 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; for example, 1.0≦a<2, preferably 1.3≦a≦1.8, more preferably 1.4≦a≦1.6.

[0038] The molar ratio b of nitrogen atoms in the support varies depending on the molar fractions of lanthanoids A and B (x and y) and the molar ratio of oxygen atoms (a), and is not limited as long as the support can have a fluorite-type crystal structure. However, it 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, and even more preferably 0.15 or less; for example, 0<b≦0.60, preferably 0.05≦b≦0.30, and more preferably 0.10≦b≦0.20.

[0039] 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, and is not limited as long as the support can have a fluorite-type crystal structure. However, it is preferably smaller than 2, preferably 1.8 or less, more preferably 1.7 or less, and 1.4 or more, preferably 1.5 or more, more preferably 1.6 or more; for example, 1.4≦a+b<2, preferably 1.5≦a+b≦1.8, more preferably 1.6≦a+b≦1.7.

[0040] 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.

[0041] 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.

[0042] Here, the average particle size of the carrier can be measured using a laser diffraction particle size distribution measuring device.

[0043] 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.

[0044] The BET specific surface area of ​​the carrier 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 to 300m ２ / g, preferably 5m ２ / g to 200m ２ / g.

[0045] Here, the BET specific surface area means a specific surface area calculated by the BET method, which is well known in the art.

[0046] By setting the BET specific surface area of ​​the support 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.

[0047] The BET specific surface area of ​​the catalyst 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 to 300m２ / g, preferably 5m ２ / g to 200m ２ / g.

[0048] When the BET specific surface area of ​​the catalyst 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.

[0049] The present invention also relates to a method for producing the above-described ammonia synthesis catalyst.

[0050] The ammonia synthesis catalyst of the present invention can be produced by supporting an active metal on a support by a method known in the art, except that the above-described rare earth oxynitride is used as the support.

[0051] The carrier of the present invention can be produced, for example, as follows.

[0052] First, in the rare earth oxynitride described above, the rare earth oxide before being doped with nitrogen, i.e., the rare earth oxide represented by the general formula A ｘ B ｙ O ２ (wherein A represents a tetravalent lanthanoid, B represents a trivalent lanthanoid, x represents the mole fraction of lanthanoid A among all lanthanoids, 0.4≦x<1, y represents the mole fraction of lanthanoid B among all lanthanoids, 0<y≦0.6, and x+y=1), where the rare earth oxide has the same components as the rare earth oxynitride described above, except that it is not doped with nitrogen.

[0053] The rare earth oxide can be produced by a method known in the art, such as a coprecipitation method or a complex polymerization method.

[0054] (Co-precipitation method) In one embodiment, a rare earth oxide containing lanthanoid A and lanthanoid B is prepared by co-precipitation using a composite oxide precursor solution containing a salt of lanthanoid A and a salt of lanthanoid B in a predetermined ratio.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] The resulting precipitate is then filtered, dried, and calcined to obtain a rare earth oxide containing lanthanoid A and lanthanoid B.

[0059] Filtration can be carried out by methods known in the art.

[0060] Following the filtration step, the filtered precipitate is dried under an oxidizing atmosphere (for example, in air) or an inert gas atmosphere (for example, under a nitrogen or argon atmosphere).

[0061] The drying temperature for 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 drying temperature for the precipitate within the above range, the moisture in the precipitate can be sufficiently removed.

[0062] 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.

[0063] 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).

[0064] 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 ammonia synthesis catalyst can be increased.

[0065] 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 ammonia synthesis catalyst can be increased.

[0066] (Complex Polymerization Method) In one embodiment, a rare earth oxide containing lanthanoid A and lanthanoid B is prepared by a complex polymerization method using a composite oxide precursor solution containing a salt of lanthanoid A and a salt of lanthanoid B in a predetermined ratio.

[0067] 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.

[0068] 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.

[0069] Next, a rare earth oxide containing lanthanoid A and lanthanoid B is formed by complex polymerization using the prepared composite oxide precursor solution. Specifically, an oxycarboxylic acid or phosphoric acid is added to the composite oxide precursor solution and dissolved therein, and then glycol or glycerin is added and mixed. This forms a composite metal complex containing lanthanoid A and lanthanoid B.

[0070] Examples of hydroxycarboxylic acids include citric acid, malic acid, and lactic acid. The amount of such hydroxycarboxylic acid or phosphoric acid added is usually 0.1 equivalents or more, preferably 1 equivalent or more, and more preferably 2 equivalents or more, relative to the total amount of lanthanoid A and lanthanoid B, and is usually 50 equivalents or less, preferably 20 equivalents or less, for example, 0.1 to 50 equivalents, preferably 1 to 20 equivalents, and more preferably 2 to 20 equivalents. By setting the amount of hydroxycarboxylic acid or phosphoric acid added within the above range, a composite metal complex containing lanthanoid A and lanthanoid B can be sufficiently formed, and a uniform rare earth oxide can be obtained.

[0071] Examples of glycols include ethylene glycol, propylene glycol, and diethylene glycol. The amount of glycol or glycerin added is typically 0.01 equivalents or more, preferably 0.05 equivalents or more, and more preferably 0.1 equivalents or more, relative to the total amount of lanthanoid A and lanthanoid B, and typically 50 equivalents or less, preferably 20 equivalents or less, and more preferably 10 equivalents or less, for example, 0.01 to 50 equivalents, preferably 0.05 to 20 equivalents, and more preferably 0.1 to 10 equivalents. By setting the amount of glycol or glycerin added within the above range, a composite metal complex containing lanthanoid A and lanthanoid B can be sufficiently formed, and a uniform rare earth oxide can be obtained.

[0072] Next, by heating the aqueous solution of the composite metal complex thus obtained, a chain reaction of dehydration esterification between the carboxyl groups of the oxycarboxylic acid and the hydroxyl groups of the glycol proceeds, producing a polyester polymer gel. Since the lanthanoid A and lanthanoid B are uniformly dispersed in the polyester polymer gel thus obtained, by calcining this polyester polymer gel as described below, a rare earth oxide in which the lanthanoid A and lanthanoid B are uniformly dispersed can be obtained.

[0073] The heating temperature of the aqueous solution of the composite metal complex is usually 50° C. or higher, preferably 80° C. or higher, more preferably 100° C. or higher, and usually 500° C. or lower, preferably 480° C. or lower, more preferably 450° C. or lower, for example, 50° C. to 500° C., preferably 80° C. to 480° C., more preferably 100° C. to 450° C. By setting the heating temperature of the aqueous solution of the composite metal complex within the above range, the dehydration esterification reaction can be advanced to sufficiently produce a polyester polymer gel, and a uniform rare earth oxide can be obtained in the firing step described below.

[0074] The heating time of the composite metal complex aqueous solution is usually 30 minutes or more, preferably 60 minutes or more, and more preferably 120 minutes or more. By setting the heating time of the composite metal complex aqueous solution within this range, the dehydration esterification reaction can be advanced to sufficiently produce a polyester polymer gel, and a uniform rare earth oxide can be obtained in the baking step described below. The upper limit of the heating time of the composite metal complex aqueous solution is not limited, although it will lengthen the process time.

[0075] Next, the polyester polymer gel thus obtained is baked in an oxidizing atmosphere (e.g., in the air) or in an inert gas atmosphere (e.g., in a nitrogen or argon atmosphere) to carbonize the organic matter and obtain a composite oxide precursor powder.

[0076] The baking temperature of the polyester polymer gel 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 baking temperature of the polyester polymer gel within the above range, carbonization of the organic matter can be sufficiently advanced, and a uniform rare earth oxide can be obtained.

[0077] The baking time of the polyester polymer gel is usually 30 minutes or more, preferably 60 minutes or more, and more preferably 120 minutes or more. By setting the baking time of the polyester polymer gel within this range, carbonization of the organic matter can be sufficiently advanced, and a uniform rare earth oxide can be obtained. The upper limit of the baking time of the polyester polymer gel is not limited, although it will lengthen the process time.

[0078] Next, the composite oxide precursor powder thus obtained is fired in an oxidizing atmosphere (e.g., in the air) or in an inert gas atmosphere (e.g., in a nitrogen or argon atmosphere), whereby the carbon component is desorbed and a rare earth oxide containing lanthanoid A and lanthanoid B is obtained.

[0079] The calcination temperature of the composite oxide precursor powder 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 composite oxide precursor powder within the above range, desorption of carbon components can be sufficiently promoted, and the ammonia synthesis activity of the resulting ammonia synthesis catalyst can be increased.

[0080] The calcination time of the composite oxide precursor powder 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 composite oxide precursor powder within the above range, desorption of the carbon component can be sufficiently progressed, and the ammonia synthesis activity of the resulting ammonia synthesis catalyst can be increased.

[0081] Subsequently, the rare earth oxide obtained by coprecipitation or complex polymerization is doped with nitrogen element.

[0082] 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.

[0083] The flow rate of ammonia during the calcination of rare earth oxides 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. The concentration of ammonia gas during the calcination of rare earth oxides is usually 1 vol% or more, preferably 5 vol% or more, and usually 100 vol% or less, preferably 50 vol% or less, for example, 1 vol% to 100 vol%, preferably 5 vol% to 50 vol%. 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 incorporation of nitrogen element into the rare earth oxide can be sufficiently promoted, and rare earth oxynitride can be obtained.

[0084] 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, it is possible to sufficiently promote the incorporation of nitrogen element into the rare earth oxide, and to obtain a rare earth oxynitride.

[0085] 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.

[0086] In addition, when 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, a nitrogen atmosphere, an argon atmosphere, or an ammonia atmosphere. 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.

[0087] Finally, an active metal is supported on the obtained rare earth oxynitride by, for example, impregnation and evaporation to dryness, to obtain an ammonia synthesis catalyst in which the active metal is supported on the rare earth oxynitride.

[0088] Specifically, first, an active metal precursor is attached to a rare earth oxynitride in the amount described above using a solution containing an active metal salt, for example, a Ru salt, and a solvent, for example, water, tetrahydrofuran (THF), alcohol, or a mixed solution of water and alcohol.

[0089] The salt of the active metal is not limited as long as it dissolves 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). When the active metal is Ru, dodecacarbonyltriruthenium [Ru ３ (CO) １２], ruthenium acetylacetonate, ruthenium nitrosylnitrate, and ruthenium nitrate are preferred.

[0090] The method for attaching an active metal to a rare earth oxynitride is not limited, and examples thereof include a 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 (impregnation method), and a method in which a solution containing a salt of the active metal and a solvent is adsorbed onto the rare earth oxynitride (adsorption method).

[0091] 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 synthesis 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 synthesis catalyst with excellent ammonia synthesis activity is obtained.

[0092] 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 3 hours or longer, preferably 12 hours or longer. There is no upper limit to the drying time, although it will lengthen the process time.

[0093] 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.

[0094] The calcination temperature of the dried rare earth oxynitride having the active metal precursor adhered 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 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 synthesis catalyst can be improved.

[0095] In the method for producing an ammonia synthesis catalyst of the present invention, the ammonia synthesis catalyst thus produced may be formed into various forms by known methods. For example, it may be formed into pellets, or may be coated onto various substrates such as monolithic substrates, pellet substrates, and plate substrates.

[0096] According to the method for producing an ammonia synthesis catalyst of the present invention, it is possible to produce an ammonia synthesis catalyst 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.

[0097] The ammonia synthesis catalyst of the present invention can efficiently synthesize ammonia by contacting it with a mixed gas containing hydrogen and nitrogen. The method for contacting the ammonia synthesis catalyst with the mixed gas containing hydrogen and nitrogen is not limited, and any method for synthesizing ammonia known in the art can be used.

[0098] In the method for synthesizing ammonia using the ammonia synthesis catalyst of the present invention, the synthesis conditions are not limited, and the conditions in the method for synthesizing ammonia known in the art can be used as they are. For example, the molar ratio of hydrogen to nitrogen (H ２ / N ２ ) is usually 0.1 / 1 or more, preferably 0.5 / 1 or more, and usually 5 / 1 or less, preferably 3 / 1 or less, for example, 0.1 / 1 to 5 / 1, preferably 0.5 / 1 to 3 / 1. In addition, the mixed gas containing hydrogen and nitrogen may contain an inert gas (argon gas, etc.) as a carrier gas, but from the viewpoint of ammonia production efficiency, a gas consisting of only hydrogen and nitrogen is preferred.

[0099] The reaction temperature is usually 300° C. or higher, preferably 350° C., and usually 500° C. or lower, preferably 450° C. or lower, for example, 300° C. to 500° C., preferably 350° C. to 450° C. The reaction pressure is usually 0.1 MPa or higher, preferably 1 MPa or higher, and usually 10 MPa or lower, preferably 8 MPa or lower, for example, 0.1 MPa to 10 MPa, preferably 1 MPa to 8 MPa.

[0100] By synthesizing ammonia under the conditions within the above ranges using the ammonia synthesis catalyst of the present invention, ammonia can be synthesized efficiently.

[0101] Figure 1 schematically shows the effect of the ammonia synthesis catalyst of the present invention. In ammonia synthesis using the ammonia synthesis catalyst of the present invention, nitrogen molecules and hydrogen molecules, which are raw materials for ammonia synthesis, first attach to the surface of Ru particles, which are an active metal supported on the surface of a rare-earth oxynitride carrier. Next, the Ru catalyzes the dissociation of the bonds between each molecule. Here, electrons are donated from the rare-earth oxynitride to the Ru, and these electrons further promote the decomposition of each molecule on the Ru surface. Finally, the decomposed nitrogen atoms react with hydrogen atoms to synthesize ammonia. In this reaction, the rare-earth oxynitride has energetically active vacancies (pseudo-nitrogen vacancies) on its surface, which serve to capture nitrogen molecules and promote the dissociation of the bonds of the captured nitrogen molecules.

[0102] 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.

[0103] I. Sample Preparation Comparative Example 1 Cerium oxide (CeO ２ , Specific surface area: 114.5m ２ / g) and Ru salt (Ru nitrate) adjusted to 5 wt % of Ru relative to the total weight of the cerium oxide were mixed in pure water as a solvent, dried at 100°C for 12 hours, and then baked at 450°C for 2 hours to obtain Ru / CeO ２ was prepared.

[0104] Comparative Example 2 Cerium-lanthanum composite oxide (Ce) produced by coprecipitation method ０．５ La ０．５ O ａ , specific surface area: 39.6m ２ The mixture was mixed with a Ru salt (Ru nitrate) adjusted to 5 wt % of Ru based on the total weight of the composite oxide in pure water as a solvent, dried at 100°C for 12 hours, and then baked at 450°C for 2 hours to obtain a Ru / Ce composite oxide. ０．５ La ０．５ O ａ was prepared.

[0105] Example 1 According to FIG. 2, a cerium-lanthanum composite oxide (Ce ０．５ La ０．５O 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 that, the furnace was evacuated. Then, 200 sccm of N ２ While flowing N 2 , the temperature was raised to 500°C at a rate of 10°C / min. When the temperature reached 500°C, 200 sccm of N 2 ２ 500 sccm of NH ３ and maintained at 500°C for 4 hours, after which NH ３ The supply of cerium-lanthanum composite oxynitride (Ce ０．５ La ０．５ O ａ N ｂ The resulting cerium-lanthanum composite oxynitride (specific surface area: 9.33 m) ２ The composite oxynitride was mixed with a 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 baked at 450°C for 2 hours to obtain a Ru / Ce composite oxynitride. ０．５ La ０．５ O ａ N ｂ was prepared.

[0106] II. Analysis II-1. XPS Spectrum Figure 3 shows the XPS spectrum of the cerium-lanthanum composite oxynitride in Example 1. From the N 1s fitting of the enlarged view in Figure 3, it was found that the cerium-lanthanum composite oxynitride in Example 1 was doped with nitrogen, and the amount of nitrogen was about 2.4 atomic % relative to the entire composite oxynitride. Therefore, based on the results in Figure 3 and the results of the spectral intensity analysis, it was found that the cerium-lanthanum composite oxynitride in Example 1 was doped with Ce ０．５２ La ０．４８ O １．５５ N ０．１４Furthermore, it was confirmed that oxygen vacancies (or nitrogen vacancies) exist in the cerium-lanthanum composite oxynitride based on this stoichiometric ratio.

[0107] II-2. XRD Patterns ２ , Ce in Comparative Example 2 ０．５ La ０．５ O ａ , and Ce in Example 1 ０．５ La ０．５ O ａ N ｂ 4 shows the XRD pattern and a photograph of the appearance of the sample. It was confirmed from Fig. 4 that a peak shift occurs due to the solid solution of lanthanum in cerium oxide, but no peak shift is observed due to the doping of nitrogen into the cerium-lanthanum composite oxide.

[0108] II-3. Evaluation of Ammonia Production Rate Ammonia synthesis was carried out using the ammonia synthesis catalyst of Comparative Example 1, Comparative Example 2, or Example 1. The ammonia synthesis conditions are as follows: (Ammonia synthesis conditions) After placing the ammonia synthesis catalyst in the FT-IR cell, O ２ While 20% / He was passed through at 100 mL / min, the temperature was raised to 300°C and held for 30 minutes. ２ The gas was switched to 3% / He and the mixture was kept at 100 mL / min for 30 minutes. After that, the cell was evacuated and heated to 400°C. ２ 6% / N ２ The gas was sealed at about 10 atmospheres, and FT-IR measurement of the gas portion was carried out for about 60 minutes.

[0109] Table 1 and FIG. 5 show the ammonia production rates converted into surface areas for the ammonia synthesis catalysts of Comparative Example 1, Comparative Example 2 and Example 1.

[0110]

[0111] From FIG. 5, it was found that, assuming that the number of Ru active sites per surface area is the same, doping nitrogen into the cerium-lanthanum composite oxide in the support dramatically improves the catalytic performance.

[0112] The present invention is not limited to the above-described embodiments, but includes various modifications. For example, it is possible to add, delete, or replace part of the configuration of the embodiments with other configurations. All publications, patents, and patent applications cited in this specification are incorporated herein by reference in their entirety.

Claims

1. An ammonia synthesis catalyst comprising a carrier and an active metal supported on the carrier, wherein the active metal comprises at least one selected from ruthenium and nickel, and does not contain iron as a main component, and the carrier 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 a molar fraction of lanthanoid A among all the lanthanoids, 0.4≦x<1, y represents a molar fraction of lanthanoid B among all the lanthanoids, 0<y≦0.6, and x+y=1, a represents a molar ratio of oxygen atoms in the support, 1.0≦a<2, b represents a molar ratio of nitrogen atoms in the support, 0<b≦0.60, and 1.4≦a+b<2), 2. The ammonia synthesis catalyst according to claim 1, wherein 1.3≦a≦1.

8.

3. The ammonia synthesis catalyst according to claim 1, wherein 0.05≦b≦0.

30.

4. The ammonia synthesis catalyst according to claim 1, wherein the active metal is ruthenium.

5. The ammonia synthesis catalyst of claim 1, wherein A is cerium.

6. The ammonia synthesis catalyst of claim 1, wherein B is lanthanum.

7. A method for producing an ammonia synthesis catalyst, comprising: preparing a complex oxide containing lanthanoid A and lanthanoid B by a coprecipitation method using a complex oxide precursor solution containing a salt of tetravalent lanthanoid A and a salt of trivalent lanthanoid B in a ratio that satisfies 0.4≦x<1, 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); preparing a complex oxynitride containing lanthanoid A and lanthanoid B by calcining the complex oxide in an ammonia atmosphere; and supporting an active metal on the complex oxynitride to obtain a catalyst in which the active metal is supported on the complex oxynitride.

8. 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, 0.4≦x<1, y represents the molar fraction of lanthanoid B among all the lanthanoids, 0<y≦0.6, and x+y=1, a represents the molar ratio of oxygen atoms in the support, 1.0≦a<2, and b represents the molar ratio of nitrogen atoms in the support, 0<b≦0.60).

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