Catalyst for ammonia synthesis and method for producing same

Abstract

Provided is a technique for enhancing ammonia synthesis activity.　The present invention provides a catalyst for ammonia synthesis, which contains metal particles and a promoter disposed close to the metal particles, wherein the proportion of Co in metal atoms contained in the metal particles is 50 atomic % or more, and the promoter contains one or more elements selected from the group consisting of rare earth elements, Zr, Hf, Ti, V, Ru, Rh, Ca, Sr, Fe and Mn.

Description

Ammonia synthesis catalyst and method for producing the same

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

[0002] Patent Document 1 discloses an ammonia synthesis catalyst in which Co is supported on a composite oxide made of BaMgOx.

[0003] WO2021 / 153738

[0004] Kresse, G.; Furthmu¨ller, Comput. Mater. Sci. 1996, 6, 15-50.Kresse, G.; Furthmu¨ller, Phys. Rev. B 1996, 54 11169-11186.Blo¨chl, PE, Phys. 17953-17979.Kresse, G.; Joubert, D., Phys. Rev. B 1999, 59, 1758-1775.Perdew, JP; Burke, K.; Ernzerhof, M., Phys. Rev. Lett. 1996, 77, 3865-3868.Perdew, JP; Ruzsinszky, A., Phys. Rev. Lett. 2008, 100, 136406. Erratum Phys. Rev. Lett. 2009, 102, 039902.Mills, G.; Jo´nsson, H., Phys. Rev. Lett. 1994, 72, 1124-1127.Henkelman G.; Jo´nsson, H., J. Chem. Phys. 2000, 113, 9901-9904.

[0005] Although the catalyst of Patent Document 1 exhibits excellent ammonia synthesis activity, it is desired to further improve this activity.

[0006] The present invention has been made in view of the above circumstances, and provides a technology for increasing ammonia synthesis activity.

[0007] According to the present invention, the following inventions are provided. [1] A catalyst for ammonia synthesis, comprising metal particles and a promoter disposed in close proximity to the metal particles, wherein the proportion of Co in the metal atoms contained in the metal particles is 50 atomic % or more, and the promoter contains one or more elements selected from the group of elements consisting of rare earth elements, Zr, Hf, Ti, V, Ru, Rh, Ca, Sr, Fe, and Mn. [2] The catalyst according to [1], wherein the promoter further contains Ba. [3] The catalyst according to [1] or [2], wherein the promoter is in the form of an oxide coating the metal particles. [4] The catalyst according to [3], wherein the oxide is a composite oxide containing Ba and at least one element from the group of elements. [5] The catalyst according to [4], wherein the atomic ratio of the total of all elements contained in the group of elements to Ba contained in the promoter is 0.01 to 1.00. [6] The catalyst according to [4] or [5], wherein the oxide is a composite oxide containing Ba, Ce, and Zr. [7] The catalyst according to [6], wherein the atomic ratio of Zr to Ce is 0.1 to 10.0. [8] The catalyst according to any one of [1] to [5], wherein the promoter contains one or more elements selected from the group consisting of rare earth elements other than Ce, Zr, Hf, Ti, V, Ru, Rh, Ca, Sr, Fe, and Mn. [9] The catalyst according to any one of [1] to [8], wherein the promoter contains at least one of rare earth elements, Ti, Zr, and Hf.

[10] The catalyst according to [9], wherein the promoter contains at least one of Ti, Zr, and Hf.

[11] The catalyst according to

[10] , wherein the promoter further contains a rare earth element.

[12] The catalyst according to

[11] , wherein the rare earth element is Ce.

[13] The catalyst according to any one of [1] to

[12] , wherein the content of the metal particles in the catalyst is 20 to 60 mass%, the atomic ratio of the promoter to Co contained in the catalyst is 0.05 to 0.20, the promoter contains Ba, and the atomic ratio of the total of all elements contained in the element group to Ba contained in the promoter is 0.10 to 0.60.

[14] The catalyst according to any one of [1] to

[13] , wherein a plurality of the metal particles are connected by an inorganic binder.

[15] The catalyst according to

[14] , wherein the inorganic binder is composed of an oxide containing Mg or Al.

[16] The catalyst according to [1], wherein the promoter is in the form of an oxide coating the metal particles, the oxide containing at least one of Ce and Zr and Ba, and the average particle size of the metal particles is 20 nm or less.

[17] The catalyst according to

[16] , wherein a plurality of the metal particles are connected by an inorganic binder, the inorganic binder being composed of an oxide containing Mg or Al, and the inorganic binder having an average particle size of 20 nm or less.

[18] The method for producing the catalyst according to any one of [1] to

[17] , comprising a mixing step, a calcination step, and a reduction step, wherein in the mixing step, an oxide constituting the promoter and a precursor of the metal particles are mixed to produce a mixture, in the calcination step, the mixture is calcined to produce a calcined product containing an oxide of the metal particles, and in the reduction step, the oxide of the metal particles in the calcined product is reduced.

[19] A method for producing the catalyst according to any one of [1] to

[17] , comprising a mixing step, a calcination step, and a reduction step, in which a precursor of the promoter and a precursor of the metal particles are mixed to produce a mixture in the mixing step, the mixture is calcined in the calcination step to produce a calcined product containing an oxide of the metal particles, and the oxide of the metal particles in the calcined product is reduced in the reduction step.

[20] The method according to

[19] , wherein in the mixing step, a precursor of the promoter, a precursor of the metal particles, an oxycarboxylic acid, and a glycol are mixed to produce a mixture, and a dehydration-condensation step is provided between the mixing step and the baking step, in which the oxycarboxylic acid and the glycol are dehydration-condensed.

[0008] The present inventors have focused on the fact that the rate-determining step in the ammonia synthesis reaction from nitrogen molecules and hydrogen molecules is the process of dissociation of the triple bond of the nitrogen molecule, and have investigated elements that promote this dissociation. As a result, they have found that the above-listed element group functions as a promoter that promotes the above dissociation more effectively than Ba, which is disclosed in Patent Document 1, and have completed the present invention.

[0009] This shows the ammonia synthesis reaction pathway used in quantum chemical calculations. This is a graph showing the relationship between the adsorption energy Ead and the activation energy Ea of the N-N bond dissociation reaction when there is no promoter (Co) and when the promoter is M (=Ba, Zr, La, Sc) (Co-M). This shows the relationship between the adsorption energy and the N-N bond distance when various elements are used as promoters. This shows the relationship between the amount of charge transfer and the N-N bond distance when monoxides of various elements are used as promoters. This shows the relationship between the amount of charge transfer and the N-N bond distance when oxides of various elements (M) are used as promoters. ２ O ３ ) as a promoter, and oxide (M ２ O ３ 6A shows the relationship between the amount of charge transfer and the N-N bond distance when both CeO and Ba are used as promoters. 6B and 6C show reaction diagrams for the N-N bond dissociation reaction when the promoter is Ba or its oxide. 7A and 7B show reaction diagrams for the N-N bond dissociation reaction when the promoter is Ba or its oxide. ２ or ZrO ２1 shows a reaction diagram of an N-N bond dissociation reaction when the ammonia synthesis catalyst 10 of Example 1 or Comparative Example 1 is used. 2 shows a graph showing the relationship between reaction temperature and ammonia yield when the ammonia synthesis catalyst 10 of Examples 2 to 4 or Comparative Example 2 is used. 3 shows a graph showing the relationship between reaction temperature and ammonia yield when the ammonia synthesis catalyst 10 of Example 5 or Comparative Example 3 is used. 4 shows a graph showing the relationship between space velocity and ammonia yield when the ammonia synthesis catalyst 10 of Example 5 or Comparative Example 3 is used. 5 shows a graph showing the relationship between reaction temperature and ammonia yield when the ammonia synthesis catalyst 10 of Examples 6 to 8 or Comparative Example 4 is used. 6 shows a graph showing the relationship between reaction temperature and ammonia yield when the ammonia synthesis catalyst 10 of Examples 6 to 8 or Comparative Example 4 is used. 7 shows STEM-EDS mapping images of Co and Mg, respectively, of the ammonia synthesis catalyst 10 of Example 2. 8 shows the particle size distribution of Co prepared based on the image of 9, and 9 shows the particle size distribution of MgO prepared based on the image of 10. (a) and (c) are STEM-EDS mapping images of Co and Mg, respectively, of the ammonia synthesis catalyst 10 of Example 3, (b) shows the particle size distribution of Co created based on the image of (a), and (d) shows the particle size distribution of MgO created based on the image of (c). (a) and (c) are STEM-EDS mapping images of Co and Mg, respectively, of the ammonia synthesis catalyst 10 of Example 4, (b) shows the particle size distribution of Co created based on the image of (a), and (d) shows the particle size distribution of MgO created based on the image of (c). (a) and (c) are STEM-EDS mapping images of Co and Mg, respectively, of the ammonia synthesis catalyst 10 of Comparative Example 2, (b) shows the particle size distribution of Co created based on the image of (a), and (d) shows the particle size distribution of MgO created based on the image of (c).

[0010] Hereinafter, embodiments of the present invention will be described. Various features shown in the following embodiments can be combined with each other. Furthermore, each feature can be an invention independently. Furthermore, in the following embodiments, elements not specified in the claims are optional elements and can be omitted. Any number of "0"s (for example, one or two) may be added to the end of numerical values ​​disclosed in the following description. For example, one or two "0"s may be added after "1.4" to make it "1.40" or "1.400".

[0011] 1. Overview The inventors first focused on the fact that the rate-determining step in the ammonia synthesis reaction from nitrogen molecules and hydrogen molecules is the dissociation process of the triple bond of the nitrogen molecule. Regarding elements that promote this dissociation, they used quantum chemical techniques to extract a group of elements that are more effective at promoting the dissociation than Ba. Next, to demonstrate the rationality of the extraction of the element group using this method, they actually performed ammonia synthesis using a catalyst containing an element from the extracted group of elements, and demonstrated its effectiveness. They also demonstrated that the catalyst of the present invention has higher activity than the iron-based catalyst typically used in the Haber-Bosch process. Each item will be explained in detail below.

[0012] 2. Extraction of Element Groups by Quantum Chemical Calculations For quantum chemical calculations, the first-principles calculation software VASP [Non-Patent Documents 1, 2] based on density functional theory was used. The potential was PAW [Non-Patent Documents 3, 4], and the exchange-correlation interaction was PBE [Non-Patent Document 5] and PBEsol [Non-Patent Document 6]. The k-space integration was performed using a 2x2x1 division. The cutoff energy was 400 eV. The structural model used a Co(0001) slab structure consisting of three layers and 48 atoms, with the bottom layer of Co fixed. The atomic positions of the other atoms were optimized until the force was within 0.04 eV / Å. The activation energy was determined using the CI-NEB method [Non-Patent Documents 7, 8].

[0013] The promoter performance of each element was evaluated assuming the ammonia synthesis reaction pathway shown in Figure 1. This reaction pathway includes stages Ref, a' to m'. A reaction energy diagram is also shown in the upper center of Figure 1. The RDS in the reaction energy diagram indicates the rate-determining step. The reaction formula for the elementary reaction at each stage is shown in the lower center of Figure 1. The promoter performance of each element was evaluated based on changes in activation energy, N-N bond distance, adsorption energy, charge transfer amount, etc. when Ba in this reaction formula is replaced with another element.

[0014] Stage Ref shows a state in which a promoter M (e.g., Ba) is disposed in proximity to the cobalt (Co) surface of the metal particle 1. The cobalt surface refers to a surface formed by a regular arrangement of cobalt atoms. Stage a' shows a state in which nitrogen molecules are adsorbed on the cobalt surface at a position in proximity to the promoter M. Stage b' shows a state after the nitrogen molecules have dissociated on the cobalt surface to become nitrogen atoms. Although not shown in FIG. 1 , a high-energy transition state exists in the process of transitioning from stage a' to b', as shown in FIGS. 6 and 7 , and reducing the activation energy of this transition state is considered important for promoting the ammonia synthesis reaction.

[0015] Stages c' to l' show the elementary reactions of adsorption of hydrogen molecules onto the cobalt surface, dissociation of the hydrogen molecules to form hydrogen atoms, and bonding of hydrogen atoms with nitrogen atoms. Stage m' shows the desorption process of the generated ammonia molecules.

[0016] 2 is a graph showing the relationship between the adsorption energy Ead and the activation energy Ea of the N-N bond dissociation reaction when there is no promoter (Co) and when the promoter is M (=Ba, Zr, La, Sc) (Co-M). This graph shows that the larger the absolute value of the adsorption energy Ead, the smaller the activation energy Ea, which holds true for various promoters. Furthermore, when the promoter is Zr, La, or Sc, the activation energy Ea is smaller than when the promoter is Ba, suggesting that Zr, La, and Sc have excellent performance as promoters.

[0017] FIG. 3 shows the relationship between adsorption energy and N-N bond distance when various elements are used as promoters. The N-N bond distance refers to the distance between the centers of N and N in the state of stage a'. The area at the bottom right of the graph in FIG. 3 indicates that the absolute value of the adsorption energy is large and the N-N bond distance is long. Compared to Ba, the rare earth elements Zr, Hf, V, Ru, Rh, Ca, and Fe have larger absolute values ​​of adsorption energy and / or longer N-N bond distances, suggesting that these elements have excellent performance as promoters.

[0018] Figure 4 shows the relationship between the charge transfer amount and N-N bond distance when monoxides of various elements are used as promoters. The larger the absolute value of the charge transfer amount, the more electrons are donated from the promoter to the nitrogen molecules adsorbed on the cobalt particles, promoting the dissociation of the nitrogen molecules. Those plotted in the lower right region of the graph in Figure 4 have excellent performance as promoters and show the same trend as Figure 3. Furthermore, although Mn is not shown in Figure 3, Figure 4 shows that when the promoter contains Mn, Sr, or Ti, the N-N bond distance is longer than when the promoter contains Ba. This suggests that Mn, Sr, and Ti have excellent performance as promoters.

[0019] FIG. 5 shows the oxides (M ２ O ３ ) as a promoter, and oxide (M ２ O ３5 shows the relationship between the charge transfer amount and the N-N bond distance when both Y and Ba are used as promoters. ２ O ３ and Ce ２ O ３ Compared with the case where M is used alone as a promoter, by using these oxides together with Ba as a promoter (i.e., as a double promoter), the absolute value of the charge transfer amount becomes larger and the N-N bond distance becomes longer. ２ O ３ The combined effect of ) and Ba is shown.

[0020] 6A shows that the activation energy of the N-N bond dissociation reaction is 1.08 eV when there is no promoter, and FIGS. 6B and 6C show that when the promoter is Ba or its oxide, the activation energy of the N-N bond dissociation reaction is 0.48 eV or 0.61 eV, respectively. This result indicates that the addition of Ba reduces the activation energy. Also, FIGS. 7A and 7B show that when the promoter is CeO ２ or ZrO ２ 7A-7B suggest that the addition of Ce and Zr lowers the activation energy.

[0021] From the above results, it was found that the ammonia synthesis reaction is promoted when the promoter contains one or more elements selected from the group consisting of rare earth elements, Zr, Hf, Ti, V, Ru, Rh, Ca, Sr, Fe, and Mn.

[0022] 2. Ammonia Synthesis Catalyst 10 As shown in FIG. 1, an ammonia synthesis catalyst 10 according to one embodiment of the present invention includes metal particles 1 and a promoter M disposed adjacent to the metal particles 1.

[0023] The proportion of Co in the metal atoms contained in the metal particles 1 is 50 atomic % or more. This proportion is, for example, 50 to 100 atomic %, and preferably 80 to 100 atomic %. The surface containing Co atoms is an excellent reaction field that promotes the various elementary reactions necessary for ammonia synthesis, so the ammonia synthesis reaction is promoted by using metal particles 1 containing Co as the main component. Specific examples of this proportion are 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 99, and 100 atomic %, and may be in a range between any two of the values ​​exemplified here.

[0024] Even when the metal particle 1 contains metal atoms other than Co, it is preferable that the metal particle 1 has a site where Co is regularly arranged, and it is more preferable that Co is arranged to form a hexagonal close-packed structure. In addition, it is preferable that the metal particle 1 has a surface (e.g., a (0001) surface) where cobalt atoms are regularly arranged. As the metal other than Co contained in the metal particle, a transition metal other than Group 4 is preferable, and as the transition metal, from the viewpoint of high catalytic activity, it is preferable that it is one or more selected from the group consisting of Ru, Fe, Co, Ni, Rh, Pd, Os, Ir, and Pt, and Ru or Fe is more preferable. It is preferable that the metal contained in the metal particle 1 does not form a solid solution with the oxide constituting the promoter M.

[0025] The average particle diameter of the metal particles 1 is preferably 100 nm or less, more preferably 50 nm or less, even more preferably 35 nm or less, and even more preferably 20 nm or less. A smaller particle diameter of the metal particles 1 is advantageous because it increases the number of active sites when used as an ammonia synthesis catalyst. The lower limit of the average particle diameter of the metal particles is not particularly limited, but is, for example, 0.5 nm or more, or 1 nm or more. The average particle diameter of the metal particles can be calculated by randomly selecting 100 to 150 metal particles from a TEM image, measuring the particle diameter of each, and averaging the measured values. The average particle diameter of the metal particles is, for example, 1 to 100 nm, specifically, for example, 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, or 100 nm, and may be between any two of the values ​​exemplified herein, or may be equal to or less than any of the values ​​exemplified herein.

[0026] The standard deviation of the particle diameter of the metal particles 1 is preferably 7.0 nm or less, and more preferably 5.0 nm or less. The smaller this value, the smaller the variation in particle diameter of the metal particles 1. The standard deviation can be calculated based on the particle size distribution created when determining the average particle diameter. This standard deviation is, for example, 1.0 to 7.0 nm, and specifically, for example, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, or 7.0 nm, and may be in a range between any two of the numerical values ​​exemplified here or less than any of them.

[0027] The crystallite diameter of the metal particles 1 is preferably 10.0 nm or less, and more preferably 7.0 nm or less. The smaller this value, the larger the surface area of ​​the metal particles 1, and the higher the reaction efficiency. The crystallite diameter of the metal particles 1 can be determined by the method shown in the examples. Specific examples of this value are 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, and 10.0 nm, and may be in a range between any two of the values ​​exemplified here or less than any of them.

[0028] The Co coordination number of the metal particles 1 is preferably 9.0 or less, and more preferably 8.5 or less. In this case, the particle diameter of the metal particles 1 is small, the surface area is large, and the reaction efficiency is likely to be high. The Co coordination number can be determined by the method shown in the examples. This value is, for example, 7.0 to 9.0, specifically, for example, 7.0, 7.1, 7.2, 7.3, 7.4, 7.5, 7.6, 7.7, 7.8, 7.9, 8.0, 8.1, 8.2, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, or 9.0, and may be in a range between any two of the values ​​exemplified here or less than any of them.

[0029] The content of the metal particles 1 in the ammonia synthesis catalyst 10 is, for example, 20 to 60 mass%, preferably 25 to 55 mass%, and more preferably 30 to 50 mass%. Specific examples of this content include 20, 25, 30, 35, 40, 45, 50, 55, and 60 mass%, and may be in a range between any two of the values ​​exemplified here.

[0030] The promoter M is arranged close to (preferably adjacent to) the metal particles 1 (preferably the cobalt surface in the metal particles 1), thereby further promoting the ammonia synthesis reaction. As shown in Figure 1, the promoter M acts on nitrogen molecules adsorbed on the metal particles 1 to stabilize the transition state when the nitrogen molecules dissociate, thereby promoting the dissociation reaction of the nitrogen molecules. Therefore, the promoter M only needs to be close enough to the metal particles 1 to be able to act on the nitrogen molecules adsorbed on the metal particles 1.

[0031] The promoter M contains one or more elements (preferably two or more) selected from the element group EG consisting of rare earth elements, Zr, Hf, Ti, V, Ru, Rh, Ca, Sr, Fe, and Mn. As described above, when the promoter M contains one or more elements selected from this element group, the ammonia synthesis reaction is further promoted. The rare earth elements are Sc, Y, or lanthanoids. Lanthanoids are elements with atomic numbers 57 to 71, specifically La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. The rare earth elements are, for example, rare earth elements other than Ce (i.e., Sc, Y, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu).

[0032] The promoter M preferably contains a rare earth element, more preferably Y or a lanthanoid, more preferably Y, La, Ce, Pm, Gd, or Lu, and even more preferably Y or Ce. The promoter M preferably contains at least one of rare earth elements, Ti, Zr, and Hf, and even more preferably at least one of Ti, Zr, and Hf (preferably Zr). The promoter M preferably further contains a rare earth element in addition to at least one of Ti, Zr, and Hf. This rare earth element is preferably Ce. The promoter M more preferably contains both Zr and Ce. These elements are elements demonstrated in the examples described below, or are homologous to these elements.

[0033] The atomic ratio of each of Ti, Zr, or Hf to the rare earth element contained in the promoter M, and the atomic ratio of all of these elements combined (for example, the atomic ratio of Zr to Ce) are, for example, 0.1 to 10.0, preferably 0.2 to 5.0, and more preferably 0.5 to 2.0. In this case, the combined effect is particularly effective. Specific examples of this atomic ratio are 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, 7.0, 7.5, 8.0, 8.5, 9.0, 9.5, and 10.0, and may be in a range between any two of the values ​​exemplified here.

[0034] It is preferable that the promoter M further contains Ba. As shown in Fig. 5, by using Ba in combination with an element included in the above-mentioned element group EG to form a double promoter, the ammonia synthesis reaction is further promoted.

[0035] The atomic ratio of promoter M to Co contained in the ammonia synthesis catalyst 10 (or Co contained in the metal particles 1) is, for example, 0.01 to 1.00, preferably 0.02 to 0.50, and more preferably 0.05 to 0.20. If this atomic ratio is too small, the reaction-promoting effect of promoter M is likely to be insufficient, and if this atomic ratio is too large, the number of reaction sites on the Co surface is too small, making it more likely to decrease activity. Specific examples of this atomic ratio are 0.01, 0.02, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, and 1.00, and may be in a range between any two of the values ​​exemplified here. The atomic ratio of the atoms constituting the ammonia synthesis catalyst 10 can be easily measured by fluorescent X-ray analysis. When light elements are contained, the atomic ratio can be measured by a method called inductively coupled plasma emission spectrometry.

[0036] The atomic ratio of each element in the element group EG to Ba in the promoter M and the total atomic ratio of all elements in the element group EG are, for example, 0.01 to 1.00, preferably 0.05 to 0.80, and preferably 0.10 to 0.60. Within these ranges, the effect of the double promoter is particularly pronounced. Specific examples of this atomic ratio include 0.01, 0.05, 0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55, 0.60, 0.65, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, and 1.00, and may be within a range between any two of the values ​​exemplified here.

[0037] In one example, the content of metal particles 1 in catalyst 10 is 20 to 60 mass %, the atomic ratio of promoter M to Co contained in catalyst 1 is 0.05 to 0.20, promoter M contains Ba, and the atomic ratio of the total of all elements contained in element group EG to Ba contained in promoter M is 0.10 to 0.60.

[0038] The metal atom contained in the promoter M may be in a zero-valent state or in an oxidized state. When this metal atom is in an oxidized state, its oxidation number is, for example, 1 to 6, specifically, for example, 1, 2, 3, 4, 5, or 6, or may be in a range between any two of the values ​​exemplified here. This is because the above-mentioned quantum chemical calculations revealed that the performance of the promoter does not differ significantly between when the metal atom contained in the promoter M is in a monometallic atom state and when it is in an oxidized state. The state of the promoter M is not particularly limited and may be any of a simple substance, a mixture, a compound, a solid solution, an oxide, etc., but an oxide is preferable from the viewpoint of ease of production and activity. Furthermore, when the promoter M is in the form of a hydroxide or carbonate, its activity is relatively low, and therefore it is not preferable. For this reason, if hydroxides or carbonates are generated during the production process of the ammonia synthesis catalyst 10, it is preferable to provide a process for removing these.

[0039] The promoter M is preferably in the form of an oxide that coats the metal particles 1. This oxide is preferably a composite oxide containing Ba and at least one element included in the element group EG, more preferably a composite oxide containing Ba and Ce or Zr, and even more preferably a composite oxide containing Ba, Ce, and Zr. This oxide may completely coat the metal particles 1, but it is preferable to partially coat the metal particles 1 so that the reactant nitrogen molecules and hydrogen molecules are easily adsorbed onto the metal particles 1. The atomic ratio of Zr to Ce is as described above.

[0040] The ammonia synthesis catalyst 10 contains a plurality of metal particles 1, and it is preferable that the plurality of metal particles 1 are connected by an inorganic binder. The inorganic binder in the present invention functions as a so-called carrier, and by connecting the metal particles 1 to the inorganic binder, particle enlargement due to aggregation of the metal particles is suppressed, and high dispersibility is maintained when the particle diameter of the metal particles 1 is small (for example, an average particle diameter of about 20 nm). At the same time, the plurality of metal particles 1 are less likely to break apart, improving handleability. The inorganic binder is composed of an inorganic substance, and is preferably composed of an oxide. The inorganic binder may or may not have the effect of accelerating the ammonia synthesis reaction. It is preferable that the inorganic binder is composed of an oxide containing Mg or Al, and more preferably composed of an oxide of Mg or Al, and it is more preferable that the inorganic binder is composed of an oxide of MgO or Al. ２ O ３ It is more preferable that the wiring board is made up of:

[0041] The oxide constituting the inorganic binder and the oxide constituting the promoter M may be in a phase-separated state without forming a solid solution, or may be in a solid solution. In the former case, the ammonia synthesis catalyst 10 has a configuration in which, for example, metal particles 1 coated with an oxide constituting the promoter M are supported on the oxide constituting the inorganic binder. In the latter case, the ammonia synthesis catalyst 10 has, for example, an oxide containing a metal element (Mg, Al, etc.) contained in the inorganic binder and a metal element contained in the promoter M, which coats the metal particles 1 and also functions as a binder connecting the metal particles 1 to each other. The former configuration can be obtained by producing the ammonia synthesis catalyst 10 by the first method described below, and the latter configuration can be obtained by producing the ammonia synthesis catalyst 10 by the second method described below.

[0042] The atomic ratio of the metal element (e.g., Mg, Al) contained in the inorganic binder to the Co contained in the ammonia synthesis catalyst 10 (preferably Co contained in the metal particles 1) is, for example, 0.1 to 20, and specifically, for example, 0.1, 0.5, 1, 1.5, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20, and may be a range between any two of the values ​​exemplified here. When the ammonia synthesis catalyst 10 is produced by the first method described below, the atomic ratio is preferably 3 to 20, and more preferably 5 to 10. When the ammonia synthesis catalyst 10 is produced by the second method described below, the atomic ratio is preferably 0.1 to 5, and more preferably 0.5 to 2.

[0043] Preferably, the inorganic binder is composed of an oxide containing Mg or Al (preferably MgO), and the average particle size of the inorganic binder is preferably 100 nm or less, more preferably 50 nm or less, even more preferably 35 nm or less, and even more preferably 20 nm or less. In this case, the surface area of ​​the catalyst 10 tends to be large. The average particle size of the inorganic binder can be determined in the same manner as for the metal particles 1. The average particle size of the inorganic binder is, for example, 1 to 100 nm, specifically, for example, 0.5, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, or 100 nm, and may be in a range between any two of the numerical values ​​exemplified here or less than any of them.

[0044] The standard deviation of the particle size of the inorganic binder is preferably 7.0 nm or less, and more preferably 5.0 nm or less. The smaller this value, the smaller the variation in particle size of the inorganic binder. The standard deviation can be determined in the same manner as for metal particles 1. This value is, for example, 1.0 to 7.0 nm, specifically, for example, 1.0, 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, 6.0, 6.5, or 7.0 nm, and may be in a range between any two of the values ​​exemplified here or less than any of them.

[0045] The BET specific surface area of ​​the catalyst 10 is, for example, 15 m ２ / g ｃａｔ That is more than 20m ２ / g ｃａｔ More than 30m is preferable. ２ / g ｃａｔ The above is more preferable. In this case, the reaction efficiency is likely to be high. The BET specific surface area of ​​the catalyst 10 can be determined by the method shown in the examples. This value is, for example, 15 to 100 m ２ / g ｃａｔ Specifically, for example, 15, 20, 25, 30, 35, 40, 50, 100 m ２ / g ｃａｔ and may be a range between any two of the values ​​given here.

[0046] The packing density of the catalyst 10 is, for example, 0.70 g / cm３ or more, and 0.80 g / cm ３ More than 0.90 g / cm ３ The above is more preferable. In this case, it is easy to increase the amount of catalyst 10 packed, which makes it easy to improve reaction efficiency. The packing density can be determined by the method shown in the examples. This value is, for example, 0.70 to 1.50 g / cm ３ Specifically, for example, 0.70, 0.75, 0.80, 0.85, 0.90, 0.95, 1.00, 1.05, 1.10, 1.20, 1.30, 1.40, 1.50 g / cm ３ and may be in a range between any two of the values ​​given here. To increase the packing density, the catalyst 10 is preferably prepared by the second method (sol-gel method).

[0047] 3. Methods for Producing the Ammonia Synthesis Catalyst 10 3-1. First Method (Impregnation Method) The first method for producing the ammonia synthesis catalyst 10 includes a mixing step, a calcination step, and a reduction step.

[0048] Mixing Step In the mixing step, the oxide constituting the promoter M and the precursor of the metal particles 1 are mixed to generate a mixture.

[0049] The oxide constituting the promoter M can be obtained by firing a precursor of the promoter M. Examples of the precursor include nitrates and hydroxides. When the promoter M contains multiple elements, a composite oxide can be obtained by firing a mixture of precursors prepared for each element.

[0050] Furthermore, by mixing the oxide constituting the inorganic binder with a precursor of the promoter M and firing the mixture, the oxide constituting the promoter M can be supported on the oxide constituting the inorganic binder.

[0051] In one example, Ba(OH) ２ A mixed aqueous solution of cerium nitrate and zirconyl nitrate is mixed with MgO powder, stirred, and then fired to obtain a structure in which oxides containing Ba, Ce, and Zr are supported on the MgO powder.

[0052] The calcination to obtain the oxide constituting the promoter M can be carried out under any conditions capable of oxidizing the precursor. For example, the calcination can be carried out at 500°C or higher for 1 hour or longer. The calcination temperature is, for example, 500 to 900°C, preferably 600 to 800°C. Specific examples of this temperature include 500, 550, 600, 650, 700, 750, 800, 850, and 900°C, and may be within any two of the values ​​exemplified herein. The calcination time is, for example, 1 to 10 hours, preferably 2 to 8 hours. Specific examples of this time include 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 hours, and may be within any two of the values ​​exemplified herein. This calcination can be carried out in air or in an oxygen-containing atmosphere, such as a mixed gas of oxygen and an inert gas, at any oxygen concentration.

[0053] Examples of precursors of the metal particles 1 include organometallic compounds, chlorides, nitrates, and nitrosyl nitrates of the metal elements constituting the metal particles 1. When the metal element is Co, acetylacetonatocobalt(II), cobalt nitrate, cobalt chloride, and nitrosyl cobalt nitrate can be used as the precursor, with acetylacetonatocobalt(II) being particularly preferred in terms of its high ammonia synthesis activity. When the metal element is Ru, triruthenium dodecacarbonyl, ruthenium acetylacetonate, ruthenium chloride, and ruthenium nitrosyl nitrate can be used as the precursor.

[0054] When an organometallic compound is used as a precursor for the metal particles 1, it is advantageous to use an organic solvent as the solvent. Examples of organic solvents include tetrahydrofuran (THF), methanol, ethanol, acetone, hexane, and toluene. While these solvents can be used without any pretreatment if they are common commercially available products, it is more preferable to use solvents that have been purified, dehydrated, or the like.

[0055] Calcination Step In the calcination step, the mixture is calcined to produce a calcined product containing an oxide of the metal particles 1.

[0056] If the mixture contains a solvent, it is preferable to remove the solvent prior to calcination. Calcination of the mixture decomposes the precursor of the metal particles 1 to form an oxide. Calcination can be performed, for example, at 300°C or higher for 1 hour or longer. The calcination temperature is, for example, 400 to 800°C, preferably 450 to 700°C. This temperature may be, for example, 300, 350, 400, 450, 500, 550, 600, 650, 700, 750, or 800°C, or may be within any two of the values ​​exemplified here. The calcination time is, for example, 1 to 10 hours, preferably 2 to 8 hours. This time may be, for example, 1, 2, 3, 4, 5, 6, 7, 8, 9, or 10 hours, or may be within any two of the values ​​exemplified here. This calcination is preferably performed under an inert gas atmosphere such as nitrogen or argon. Reduction Step In the reduction step, the oxide of the metal particles 1 in the calcined product is reduced. Through the above steps, the ammonia synthesis catalyst 10 is obtained.

[0057] Since the metal particles 1 have a low catalytic activity in the ammonia synthesis reaction when in an oxide state, in this process, the metal particles 1 in the oxide state are reduced to a metallic state. This reduction can be performed by heating the fired product in an atmosphere of a reducing gas such as hydrogen gas. This heating can be performed, for example, at 500°C or higher for 0.2 hours or longer. The temperature is, for example, 500 to 900°C, and preferably 600 to 800°C. Specific examples of the temperature include 500, 550, 600, 650, 700, 750, 800, 850, and 900°C, and may be within a range between any two of the values ​​exemplified herein. The time is, for example, 0.2 to 5 hours, and preferably 0.5 to 3 hours. Specific examples of the time include 0.2, 0.5, 1.0, 1.5, 2, 3, 4, and 5 hours, and may be within a range between any two of the values ​​exemplified herein.

[0058] During heating in the firing step or reduction step, the oxide constituting the promoter M flows and covers at least a portion of the metal particles 1. When the promoter M contains Ba, the oxide constituting the promoter M flows and is more likely to coat the metal particles 1.

[0059] 3-2. Second Method (Sol-Gel Method) The method for producing the ammonia synthesis catalyst 10 by the second method includes a mixing step, a calcination step, and a reduction step.

[0060] Mixing Step In the mixing step, a precursor of the promoter M and a precursor of the metal particles 1 are mixed to produce a mixture.

[0061] The precursor of promoter M is as described in the first method. In the first method, an oxide constituting promoter M is generated in advance from the precursor of promoter M, and then the oxide constituting promoter M is mixed with the precursor of metal particles 1. However, in the second method, the precursor of promoter M is mixed with the precursor of metal particles 1 without generating the oxide constituting promoter M in advance. The precursor of metal particles 1 is as described in the first method.

[0062] In the mixing step, a mixture may be produced by mixing a precursor of promoter M, a precursor of metal particles 1, an oxycarboxylic acid, and a glycol. In this case, a dehydration condensation step of dehydrating and condensing the oxycarboxylic acid and the glycol is preferably provided between the mixing step and the baking step. The oxycarboxylic acid coordinates with the metals of the precursor of promoter M and the precursor of metal particles 1 to form a complex, and the dehydration condensation is then carried out to produce a complex polymer. In this case, the precursor of metal particles 1 is preferably one that easily forms a complex, and nitrates are particularly preferred. Specific examples of preferred oxycarboxylic acids include citric acid, tartaric acid, malic acid, tartronic acid, glyceric acid, oxybutyric acid, hydroacrylic acid, lactic acid, and glycolic acid. Of these, citric acid is particularly preferred. Preferred glycols include ethylene glycol, propylene glycol, trimethylene glycol, 1,4-butanediol, 1,5-pentanediol, and 1,6-hexanediol. Of these, ethylene glycol is particularly preferred.

[0063] Calcination Step In the calcination step, the mixture is calcined to produce a calcined product containing an oxide of the metal particles 1.

[0064] If the mixture contains a solvent, it is preferable to remove the solvent prior to firing. By firing the mixture, the precursor of the promoter M and the precursor of the metal particles 1 are decomposed and converted into their oxides. The firing conditions are the same as those of the first method. If a complex polymer is formed, the organic matter in the complex polymer is removed by combustion in the firing step. - Reduction step In the reduction step, the oxide of the metal particles 1 in the fired product is reduced. The reduction step can be carried out in the same manner as in the first method.

[0065] During heating in the firing step or reduction step, the oxide constituting the promoter M flows and covers at least a portion of the metal particles 1. When the promoter M contains Ba, the oxide constituting the promoter M flows and is more likely to coat the metal particles 1.

[0066] 4. Production of Ammonia Using the Ammonia Synthesis Catalyst 10 Ammonia can be produced by reacting nitrogen gas and hydrogen gas in the presence of the ammonia synthesis catalyst 10. The method for synthesizing ammonia itself is not particularly limited, but for example, ammonia can be produced by supplying a raw material gas consisting of hydrogen gas and nitrogen gas into a reaction vessel loaded with the catalyst. The reaction temperature is preferably 300 to 550°C, more preferably 300 to 500°C, and even more preferably 300 to 450°C. Specific examples of this temperature include 300, 350, 400, 450, 500, and 550°C, and may be in a range between any two of the values ​​exemplified here.

[0067] The reaction pressure when synthesizing ammonia using the ammonia synthesis catalyst 10 is preferably a low pressure of 0.1 to 20 MPa, more preferably 0.1 to 15 MPa, and even more preferably 0.1 to 10 MPa. Specific examples of this pressure are 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, and 20 MPa, and may be in a range between any two of the numerical values ​​exemplified here.

[0068] Examples of the present invention will be described below. In the following examples, for the sake of simplicity, the term "Ba-loaded Co" will be used. ０．０１５ Ce ０．００４ Zr ０．００４ Mg０．９７７ O ｘ " is used to refer to a metal carrier represented by "Co / Ba ０．０１５ Ce ０．００４ Zr ０．００４ Mg ０．９７７ O ｘ The x representing the proportion of oxygen O in the composite oxide is the number of oxygen atoms required for the composite oxide to maintain electrical neutrality.

[0069] 1. Evaluation of the ammonia synthesis catalyst 10 produced by the first method (impregnation method) 1-1. Example 1 <Preparation of composite oxide> Ba ０．０１５ Ce ０．００４ Zr ０．００４ Mg ０．９７７ O ｘ The composite oxide was synthesized as follows: Ba(OH) ２ (Fujifilm Wako Pure Chemical), cerium nitrate (Kanto Chemical), and zirconyl nitrate (Fujifilm Wako Pure Chemical) were dissolved in purified water, and Ba(OH) ２ A 200 mL precursor solution containing 0.0009375, 0.00025, and 0.00025 mol of Ba, Ce, and Zr was prepared as a mixed aqueous solution of Ba, cerium nitrate, and zirconyl nitrate. 2.5 g of MgO (Ube Materials) was added to the solution, and the mixture was stirred at 320 rpm using a magnetic stirrer. The stirring was continued for 1 hour at room temperature. The suspension was evaporated to dryness using a rotary evaporator, and the resulting powder was dried overnight in an oven set at 80°C. The dried powder was crushed in a mortar, and the resulting powder was heated in an electric furnace at 700°C for 5 hours in an air atmosphere to obtain Ba. ０．０１５ Ce ０．００４ Zr ０．００４ Mg ０．９７７ O ｘ A carrier composed of the above was obtained.

[0070] <Supporting Co> Co is supported on the carrier Ba by impregnation. ０．０１５ Ce ０．００４ Zr ０．００４ Mg ０．９７７ O ｘCo was supported on the catalyst. A tetrahydrofuran (THF) (Fujifilm Wako Pure Chemical Industries) solution containing acetylacetonatocobalt(II) (Fujifilm Wako Pure Chemical Industries), a Co precursor, was prepared in a 200 mL recovery flask, to which 1 g of carrier was added, and the mixture was stirred at room temperature for 18 hours or more. The amounts of acetylacetonatocobalt(II) and carrier used were appropriately adjusted so that the amount of Co contained in the catalyst after heating under an argon atmosphere described below was 20 mass%. The stirred suspension was dried under reduced pressure at 35°C and 0.3 atm using a rotary evaporator, and then dried in an oven at 80°C for 18 hours. The obtained powder was evaporated at 80 mL min －１ The precursor was heated at 500°C for 5 hours in a tubular electric furnace under a flow of argon, thereby removing the acetylacetonato ligands in the precursor. ０．０１５ Ce ０．００４ Zr ０．００４ Mg ０．９７７ O ｘ A metal support was obtained.

[0071] <Pretreatment for hydrogen reduction> The Co / Ba obtained above ０．０１５ Ce ０．００４ Zr ０．００４ Mg ０．９７７ O ｘ The metal-supported powder was subjected to hydrogen reduction pretreatment (also simply referred to as "pretreatment") by the following method. The metal-supported powder was pressurized at 20 MPa for 5 minutes to produce a disk, which was then crushed in a mortar and classified with a sieve to produce pellets. The pellet size was adjusted to a diameter of 250 to 500 μm. 100 mg of pellets were packed into a 7 mm diameter Inconel (trademark) catalytic reaction tube, and the front and back of the catalyst layer were fixed with quartz wool. This reaction tube was installed in a fixed-bed flow-type reactor for measuring ammonia synthesis activity, and 180 mL min was supplied to the reaction tube packed with pellets. －１ H ２ and 60 mL min －１ N ２ A mixed gas of Co / Ba was passed through the catalyst and heated at 700°C for 1 hour. ０．０１５ Ce ０．００４ Zr ０．００４ Mg ０．９７７ O ｘThe catalyst obtained had an atomic ratio of Co:Ba:Ce:Zr:Mg=12.51:1.31:0.35:0.35:85.48.

[0072] 1-2. Comparative Example 1 In the preparation of the composite oxide, cerium nitrate and zirconyl nitrate were not used, and the amounts of each reagent were adjusted to obtain the desired composition. ０．０１５ Mg ０．９８５ O ｘ _700℃ reduction) was obtained.

[0073] 1-3. Measurement of ammonia synthesis activity The ammonia synthesis activity of the ammonia synthesis catalyst was measured using the fixed-bed flow reactor used in the hydrogen reduction pretreatment. ２ and N ２ The mixture was cooled to 300°C while passing a mixed gas of H ２ and N ２ While supplying the mixed gas, the pressure was increased to 1.0 MPa using the back pressure valve at the outlet of the reaction tube. ２ , N ２ Each of these was 90 mL min －１ , 30 mL min －１ (Space velocity 72L h －１ g －１ ) and transferred to the reaction atmosphere. ３ Depending on the level of synthetic activity, 200 mL of a 1 to 100 mM (1, 5, 10, 25, 100 mM) aqueous sulfuric acid solution was added to a three-necked flask connected to an electrical conductivity meter, and hydrogen (purity 99.995%, manufactured by Taiyo Nippon Sanso), nitrogen (purity 99.995%, manufactured by Taiyo Nippon Sanso), and NH ３ A mixed gas containing NH was bubbled into the sulfuric acid solution. ３The amount of ammonia produced in the outlet gas was quantified by measuring the change in electrical conductivity caused by the reaction of sulfuric acid with the catalyst. Next, the temperature of the catalyst layer was raised to 350°C, 400°C, or 450°C. Once the temperature of the catalyst layer stabilized at 350°C, 400°C, or 450°C, the catalyst was left for 10 minutes, and the amount of ammonia produced was quantified in the same manner as above. The ammonia yield was calculated from the quantified amount of ammonia produced. The results are shown in Figure 8.

[0074] 8, when the catalyst of Example 1 in which the promoter M contains Ce and Zr in addition to Ba was used, the ammonia yield was higher in the range of 350 to 450° C. compared to Comparative Example 1 in which the promoter M contains neither Ce nor Zr. This indicates that the activity of the ammonia synthesis catalyst was increased by the addition of Ce and Zr.

[0075] 2. Evaluation of the ammonia synthesis catalyst 10 produced by the second method (sol-gel method, complex polymerization method) 2-1. Example 2 <Preparation of composite oxide> Co ０．４７７ Ba ０．０３６ Ce ０．００８ Zr ０．００８ Mg ０．４７１ The Ox composite oxide was synthesized as follows: Cobalt nitrate (Fujifilm Wako Pure Chemical), barium nitrate (Fujifilm Wako Pure Chemical), cerium nitrate (Kanto Chemical), zirconyl nitrate (Fujifilm Wako Pure Chemical), magnesium nitrate (Fujifilm Wako Pure Chemical), citric acid, and ethylene glycol were dissolved in purified water to prepare a mixed aqueous solution, and 0.0009375, 0.00025, and 0.00025 mol of Co, Ba, Ce, Zr, and Mg were added, respectively, and the NO in these metal salts was analyzed. ３ － A 300 mL precursor solution containing citric acid in an amount 5 times the total amount of the substances and ethylene glycol in an amount 10 times the total amount was prepared. The solution was stirred at 400 rpm using a mantle heater equipped with a stirrer at 80°C, and a dehydration condensation reaction was carried out while evaporating water. The resulting sol was placed in a baking dish and dried overnight in an oven set at 80°C. The baking dish containing the sol was removed from the dryer and heated in an electric furnace at 300°C for 5 hours, then at 600°C for 5 hours in an air atmosphere to produce Co. ０．４７７ Ba０．０３６ Ce ０．００８ Zr ０．００８ Mg ０．４７１ Ox was obtained.

[0076] <Pretreatment for hydrogen reduction> The Co obtained above ０．４７７ Ba ０．０３６ Ce ０．００８ Zr ０．００８ Mg ０．４７１ Ox was subjected to hydrogen reduction pretreatment (also simply referred to as "pretreatment") by the following method. The powder of the metal support was pressed at 20 MPa for 5 minutes to prepare a disk, and then the disk was crushed in a mortar and classified with a sieve to prepare pellets. The size of the pellets was adjusted to a diameter of 250 to 500 μm. 100 mg of pellets were packed into a 7 mm diameter Inconel (trademark) catalytic reaction tube, and the front and back of the catalyst layer were fixed with quartz wool. This reaction tube was installed in a fixed-bed flow-type reactor for measuring ammonia synthesis activity, and 180 mL min was supplied to the reaction tube packed with pellets. －１ H ２ and 60 mL min －１ N ２ A mixed gas of Co and Co was passed through the reactor and heated at 700°C for 12 hours. ０．４７７ Ba ０．０３６ Ce ０．００８ Zr ０．００８ Mg ０．４７１ Ox (reduced at 700°C) was obtained.

[0077] 2-2. Example 3 In Example 3, the ammonia synthesis catalyst 10 (Co nitrate) was prepared in the same manner as in Example 2, except that cerium nitrate was not used in the preparation of the composite oxide, and the blending amounts of each reagent were adjusted to obtain the desired composition. ０．４７７ Ba ０．０３６ Zr ０．００８ Mg ０．４７９ O ｘ _700℃ reduction) was obtained.

[0078] 2-3. Example 4 In Example 4, the ammonia synthesis catalyst 10 (Co) was prepared in the same manner as in Example 2, except that zirconyl nitrate was not used in the preparation of the composite oxide, and the blending amounts of each reagent were adjusted to obtain the desired composition. ０．４７７ Ba ０．０３６ Ce ０．００８ Mg ０．４７９ Oｘ _700℃ reduction) was obtained.

[0079] 2-4. Comparative Example 2 In Comparative Example 2, the ammonia synthesis catalyst 10 (Co ０．４７７ Ba ０．０３６ Mg ０．４８７ O ｘ _700℃ reduction) was obtained.

[0080] 2-5. Measurement of ammonia synthesis activity The ammonia synthesis activity of the catalysts of Examples 2 to 4 and Comparative Example 2 was measured by the method shown in "1-3. Measurement of ammonia synthesis activity." The results are shown in FIG.

[0081] As shown in Figure 9, when the catalysts of Examples 2 to 4 in which promoter M contains one or both of Ce and Zr in addition to Ba were used, the ammonia yield was higher in the range of 300 to 450°C compared to Comparative Example 2 in which promoter M contains neither Ce nor Zr. This indicates that the addition of Ce and Zr increased the activity of the ammonia synthesis catalyst. Furthermore, in Example 2 in which promoter M contains both Ce and Zr in addition to Ba, the ammonia yield was particularly high.

[0082] 3. Comparison with Haber-Bosch iron-based catalyst 3-1. Example 5 The hydrogen reduction pretreatment of Example 2 was carried out on 2 mL of catalyst at 600°C for 60 hours at 498 mL min －１ H ２ and 166 mL min －１ N ２ The same method as in Example 2 was used except that a mixed gas of ammonia synthesis catalyst 10 (Co ０．４７７ Ba ０．０３６ Ce ０．００８ Zr ０．００８ Mg ０．４７１ Ox (reduced at 600°C) was obtained.

[0083] In Comparative Example 3, a Haber Bosch iron-based catalyst (AmoMax 10RS manufactured by Clariant) was subjected to hydrogen reduction pretreatment under the same conditions as in Example 5 to obtain an ammonia synthesis catalyst of Comparative Example 3 (Haber Bosch iron-based catalyst, reduced at 600°C).

[0084] 3-3. Measurement of ammonia synthesis activity (constant space velocity) Compared to "1-3. Measurement of ammonia synthesis activity," the catalyst amount was set to 2 mL, the pressure inside the reaction tube was set to 5.1 MPa, and the space velocity (GHSV) was set to 14,400 h －１ The ammonia synthesis activity of the catalysts of Example 5 and Comparative Example 3 was measured in the same manner as in "1-3. Measurement of ammonia synthesis activity", except that the temperature of the catalyst layer was changed between 250° C. and 400° C. The results are shown in FIG.

[0085] As shown in Figure 10, the catalyst of Example 5, in which the promoter M contained Ba, Ce, and Zr, had higher activity in the ammonia synthesis reaction than the iron-based catalyst for Haber-Bosch (reduced at 600°C) of Comparative Example 3. The apparent activation energies of the ammonia synthesis reaction using the catalysts of Example 5 and Comparative Example 3 were 55.8 kJ mol−1, respectively. －１ 、 64.6 kJ mol －１ The apparent activation energy was calculated by Arrhenius plot.

[0086] 3-4. Measurement of ammonia synthesis activity (constant temperature) Compared to "1-3. Measurement of ammonia synthesis activity," the catalyst amount was 2 mL, the pressure inside the reaction tube was 5.1 MPa, the temperature of the catalyst layer was 350 °C, and the space velocity (GHSV) was 2000 h －１ ~18000h －１ The ammonia synthesis activity of the catalysts of Example 5 and Comparative Example 3 was measured in the same manner as in "1-3. Measurement of ammonia synthesis activity," except that the temperature was changed between 0.01 and 0.01. The results are shown in FIG.

[0087] As shown in FIG. 11, the catalyst of Example 5 in which the promoter M contained Ba, Ce, and Zr had higher activity for the ammonia synthesis reaction than the iron-based catalyst for Haber-Bosch use in Comparative Example 3, reduced at 600°C.

[0088] 4. Evaluation of Mg-free ammonia synthesis catalyst 10 4-1. Example 6 In Example 6, the ammonia synthesis catalyst 10 (Co nitrate-free) was prepared in the same manner as in Example 2, except that cerium nitrate and magnesium nitrate were not used in the preparation of the composite oxide, and the amounts of the reagents were adjusted to obtain the desired composition, and the heating time in the hydrogen reduction pretreatment was changed to 1 hour. ０．８９ Ba ０．１ Zr ０．０１ O ｘ _700℃ reduction) was obtained.

[0089] 4-2. Example 7 In Example 7, ammonia synthesis catalyst 10 (Co) was prepared in the same manner as in Example 6, except that zirconyl nitrate was replaced with yttrium nitrate (Sigma-Aldrich) in the preparation of the composite oxide. ０．８９ Ba ０．１ Y ０．０１ O ｘ _700℃ reduction) was obtained.

[0090] 4-3. Example 8 In Example 8, the same procedure as in Example 6 was repeated except that zirconyl nitrate was replaced with cerium nitrate (Fujifilm Wako Pure Chemical Industries, Ltd.) in the preparation of the composite oxide. ０．８３ Ba ０．１ Ce ０．０７ O ｘ _700℃ reduction) was obtained.

[0091] 4-4. Comparative Example 4 In Example 4, the same procedure as in Example 6 was repeated except that zirconyl nitrate was not used in the preparation of the composite oxide. ０．９０ Ba ０．１ O ｘ _700℃ reduction) was obtained.

[0092] 4-5. Measurement of ammonia synthesis activity The ammonia synthesis activity of the catalysts of Examples 6 to 8 and Comparative Example 4 was measured by the method shown in "1-3. Measurement of ammonia synthesis activity." The results are shown in FIG.

[0093] 12, even in the catalysts containing composite oxides not containing Mg, Examples 6 to 8 in which the promoter M contains Zr, Y, or Ce have higher ammonia yields than Comparative Example 4 in which the promoter M does not contain any of these. This shows that even in the catalysts containing composite oxides not containing Mg, the activity of the ammonia synthesis catalyst is increased by adding a promoter M containing Zr, Y, or Ce.

[0094] 5. Characteristic Evaluation The catalysts 10 of Examples 2 to 4 and Comparative Example 2 were evaluated for CO ２ (purity 99.95% or more) and then subjected to deactivation treatment, and then characteristic evaluation was carried out. In the following description, the atomic ratio of the composition of each catalyst is omitted.

[0095]

[0096] Measurement of particle size distribution The particle size distributions of Co particles and MgO particles were measured. The particle size distribution of Co particles was measured by randomly extracting 100 to 150 particles from the STEM-EDS mapping image of Co and measuring the particle size of each. The particle size distribution of MgO particles was measured by randomly extracting 100 or more particles from the STEM-EDS mapping image of Mg and measuring the particle size of each.

[0097] STEM-EDS mapping images were acquired using a JEM-ARM300F2 electron microscope (manufactured by JEOL Ltd.) at an accelerating voltage of 200 kV. The sample was dispersed in ethanol at room temperature, dropped onto a carbon-coated copper grid, and dried in vacuum at room temperature for 24 hours.

[0098] Based on the obtained particle size distribution, the average particle size and standard deviation were calculated. The results are shown in Table 1 and Figures 13 to 16. In Comparative Example 2, the degree of sintering of MgO was so great that it was difficult to measure the individual particle sizes, so measurement of the particle size distribution was abandoned.

[0099] As is clear from Table 1, both the average Co particle size and the standard deviation of the Co particle size were smaller in Examples 2 to 4 than in Comparative Example 2. This result indicates that the addition of at least one of Zr and Ce refined the Co particles and made the particle size uniform. Furthermore, the average MgO particle size was smaller in Examples 2 to 4 than in Comparative Example 2. This result indicates that the addition of at least one of Zr and Ce refined the MgO particles.

[0100] Furthermore, in catalysts 10 of Examples 2 to 4, the Co particles and MgO particles were each densely arranged like a bunch of grapes due to the reduction in particle size. This structure is thought to be due to the suppression of particle enlargement as a result of the suppression of sintering by the addition of at least one of Zr and Ce.

[0101] Furthermore, when Examples 2 to 4 are compared, in Example 2, in which both Zr and Ce were added, the average Co particle size was smaller than that of Example 3, and the standard deviation of the Co particle size, the average MgO particle size, and the standard deviation of the MgO particle size were smaller than those of Examples 3 and 4.

[0102] X-ray diffraction analysis: X-ray diffraction (XRD) analysis was performed using a MiniFlex 600 (Rigaku Corporation) equipped with a Cu-Kα radiation source (λ = 1.5406 Å) and analyzed using PDXL2 software. Since the Co(110) and MgO peaks overlap, the Co(220) (2θ = 75.8 degrees) peak in the XRD pattern was used to calculate the Co crystallite size using the Scherrer equation. The results are shown in Table 1.

[0103] As shown in Table 1, in Examples 2 to 4 in which at least one of Zr and Ce was added, the Co crystallite diameter was smaller than that in Comparative Example 2. Furthermore, in Examples 2 and 3 in which Zr was added, the crystallite diameter was smaller than that in Example 4 in which Zr was not added.

[0104] BET specific surface area: The BET specific surface area was measured at liquid nitrogen temperature using a BELSORP-mini X (manufactured by Microtrac BEL). Before the measurement, the catalyst was degassed under vacuum at 300°C for 3 hours. The BET specific surface area was measured at a relative pressure P / P ０The results are shown in Table 1.

[0105] As shown in Table 1, Examples 2 to 4, in which at least one of Zr and Ce was added, had a larger BET specific surface area than Comparative Example 2. Furthermore, Example 2, in which both Zr and Ce were added, had an even larger BET specific surface area than Examples 3 and 4, in which only one of Zr and Ce was added. These results indicate an increase in surface area due to the addition of at least one of Zr and Ce.

[0106] Co coordination number The Co coordination number was determined by curve fitting analysis of the EXAFS (Extended X-ray Absorption Fine Structure) spectrum at the Co K absorption edge. The results are shown in Table 1. The higher the proportion of Co present near the surface, the smaller the Co coordination number. Therefore, a small Co coordination number indicates a small Co particle diameter.

[0107] As shown in Table 1, in Example 2 in which Zr and Ce were added, the Co coordination number was smaller than that in Comparative Example 2. This result indicates that the addition of Zr and Ce results in the refinement of Co particles.

[0108] Packing density The packing density of the catalysts of Example 2 and Comparative Example 1 was determined by the following method. First, the catalyst powder was compressed at a pressure of 80 MPa to form a circular disk. Next, the disk was crushed and sieved to obtain pellets with particle sizes in the range of 250 to 500 μm. Next, these pellets were packed into a container, and the apparent density was determined from their mass and volume, which was taken as the packing density. The results are shown in Table 2.

[0109] As shown in Table 2, the catalyst of Example 2 had a higher packing density than the catalyst of Comparative Example 1. Comparative Example 1 differs from Example 2 in that the catalyst was prepared by impregnation and did not contain either Ce or Zr. Therefore, it was found that the packing density could be increased by preparing a catalyst by the sol-gel method and adding Ce and Zr.

[0110]

[0111] 1: Metal particles 10: Ammonia synthesis catalyst M: Promoter

Claims

1. A catalyst for ammonia synthesis comprising metal particles and a promoter disposed in close proximity to the metal particles, wherein the proportion of Co in the metal atoms contained in the metal particles is 50 atomic % or more, and the promoter comprises one or more elements selected from the group consisting of rare earth elements, Zr, Hf, Ti, V, Ru, Rh, Ca, Sr, Fe, and Mn.

2. The catalyst according to claim 1, wherein the promoter further comprises Ba.

3. The catalyst of claim 1, wherein the promoter is in the form of an oxide coating the metal particles.

4. The catalyst according to claim 3, wherein the oxide is a composite oxide containing Ba and at least one element included in the group of elements.

5. The catalyst according to claim 4, wherein the atomic ratio of the total of all elements contained in said element group to Ba contained in said promoter is 0.01 to 1.

00.

6. The catalyst according to claim 4, wherein the oxide is a composite oxide containing Ba, Ce, and Zr.

7. The catalyst according to claim 6, wherein the atomic ratio of Zr to Ce is 0.1 to 10.

0.

8. A catalyst according to claim 1, wherein the promoter contains one or more elements selected from the group consisting of rare earth elements other than Ce, Zr, Hf, Ti, V, Ru, Rh, Ca, Sr, Fe and Mn.

9. The catalyst of claim 1, wherein the promoter comprises at least one of a rare earth element, Ti, Zr, and Hf.

10. The catalyst according to claim 9, wherein the promoter comprises at least one of Ti, Zr and Hf.

11. The catalyst of claim 10, wherein the promoter further comprises a rare earth element.

12. The catalyst according to claim 11, wherein the rare earth element is Ce.

13. A catalyst according to claim 1, wherein the content of said metal particles in said catalyst is 20 to 60 mass %, the atomic ratio of said promoter to Co contained in said catalyst is 0.05 to 0.20, said promoter contains Ba, and the atomic ratio of the total of all elements contained in said element group to Ba contained in said promoter is 0.10 to 0.

60.

14. The catalyst according to claim 1, wherein a plurality of said metal particles are bound together by an inorganic binder.

15. The catalyst according to claim 14, wherein the inorganic binder is composed of an oxide containing Mg or Al.

16. A catalyst according to claim 1, wherein the promoter is in the form of an oxide that coats the metal particles, the oxide containing at least one of Ce and Zr, and Ba, and the metal particles have an average particle size of 20 nm or less.

17. A catalyst according to claim 16, wherein a plurality of said metal particles are connected by an inorganic binder, said inorganic binder is composed of an oxide containing Mg or Al, and the average particle size of said inorganic binder is 20 nm or less.

18. A method for producing a catalyst according to any one of claims 1 to 17, comprising a mixing step, a calcining step, and a reduction step, wherein in the mixing step, an oxide constituting the promoter and a precursor of the metal particles are mixed to produce a mixture, in the calcination step, the mixture is calcined to produce a calcined product containing the oxide of the metal particles, and in the reduction step, the oxide of the metal particles in the calcined product is reduced.

19. A method for producing a catalyst according to any one of claims 1 to 17, comprising a mixing step, a calcining step, and a reduction step, wherein in the mixing step, a precursor of the promoter and a precursor of the metal particles are mixed to produce a mixture, in the calcination step, the mixture is calcined to produce a calcined product containing an oxide of the metal particles, and in the reduction step, the oxide of the metal particles in the calcined product is reduced.

20. The method according to claim 19, wherein the mixing step comprises mixing a precursor of the promoter, a precursor of the metal particles, an oxycarboxylic acid, and a glycol to produce a mixture, and the method further comprises a dehydration condensation step between the mixing step and the baking step in which the oxycarboxylic acid and the glycol are dehydration condensed.

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