Cathode electrode catalyst for ammonia electrosynthesis, and ammonia electrosynthesis device

Abstract

Provided is a cathode electrode catalyst for ammonia electrosynthesis that contains a perovskite-type metal oxynitride. The perovskite-type metal oxynitride includes, as main metal elements: A, which is at least one element selected from the group consisting of Ba, Sr, and Ca and which contains 50 mol% or more of Ba; and Mo or B, which is at least one element selected from the group consisting of Ta, Nb, and W. The perovskite-type metal oxynitride satisfies a prescribed formula.

Description

Cathode electrode catalyst for electrolytic synthesis of ammonia and ammonia electrolytic synthesis device

[0001] The present disclosure relates to a cathode electrode catalyst for electrolytic synthesis of ammonia and an apparatus for electrolytic synthesis of ammonia.

[0002] Currently, ammonia is produced by the Haber-Bosch process. In the production of ammonia, a catalyst active in ammonia synthesis plays an important role. As disclosed in Patent Document 1, ruthenium (Ru), a noble metal, is known to exhibit excellent catalytic performance in ammonia synthesis.

[0003] The Haber-Bosch process produces CO ２ Recently, as disclosed in Non-Patent Document 1, ２ As a method for producing ammonia free of CO₂, a method for synthesizing ammonia from the reaction between hydrogen and nitrogen obtained by water electrolysis (hereinafter referred to as "ammonia electrolysis") has attracted attention.

[0004] Japanese Patent Application Laid-Open No. 2006-88058

[0005] Kenichi Akika, "Expectations for Energy Carrier LCA: Examples of CO2-free Hydrogen and Ammonia Introduction," Journal of Life Cycle Assessment, Japan, Vol. 12, No. 3, pp. 161-169, July 2016

[0006] In the ammonia electrolysis process, Ru, which exhibits excellent catalytic performance for ammonia synthesis, can be used for the cathode electrode. However, Ru is a noble metal and is expensive, so a new cathode electrode catalyst is needed to replace it.

[0007] The present disclosure has been made in view of the above circumstances, and one of its objectives is to provide a novel cathode electrode catalyst for ammonia electrolysis and an ammonia electrolysis apparatus using the same.

[0008] A first aspect of the present invention is a cathode electrode catalyst for electrolytic synthesis of ammonia, comprising a perovskite-type metal oxynitride, wherein the perovskite-type metal oxynitride comprises, as main metal elements, A, which is at least one element selected from the group consisting of Ba, Sr, and Ca, and contains at least 50 mol% of Ba; and B, which is at least one element selected from the group consisting of Ta, Nb, and W, or Mo; wherein the cathode electrode catalyst for electrolytic synthesis of ammonia satisfies all of the following formulas (1) to (5) when the perovskite-type metal oxynitride contains B, and satisfies all of the following formulas (1′) to (5′) when the perovskite-type metal oxynitride contains Mo: [A]: [B]: [O]: [N] = 1: a: b: c ... (1) 0.90≦a≦1.10 ... (2) 1.00≦b≦2.50 ... (3) 0.50≦c≦2.00 ... (4) 2.50≦(b+c)≦3.10 ... (5) [A]: [Mo]: [O]: [N] = 1: a': b': c' ... (1') 0.90≦a'≦1.10 ... (2') 1.50≦b'≦3.40 ... (3') 0.10≦c'≦1.50 ... (4') 2.50≦(b'+c')≦3.50...(5') Here, [A], [B], [O], [N] and [Mo] represent the contents of A, B, O, N and Mo, respectively, expressed in mol %.

[0009] A second aspect of the present invention is the catalyst according to the first aspect, wherein A is Ba.

[0010] A third aspect of the present invention is the catalyst according to the first or second aspect, wherein the B comprises Nb.

[0011] A fourth aspect of the present invention is an ammonia electrolysis apparatus comprising: a cathode electrode containing a perovskite-type metal oxynitride; an anode electrode; an electrolyte; and a voltage application unit, wherein at least a portion of the electrolyte is disposed between the cathode electrode and the anode electrode; and the voltage application unit is connected to the cathode electrode and the anode electrode so as to apply a voltage between the cathode electrode and the anode electrode; and the perovskite-type metal oxynitride comprises, as main metal elements, A, which is at least one selected from the group consisting of Ba, Sr, and Ca, and contains 50 mol % or more of Ba; and B, which is at least one selected from the group consisting of Ta, Nb, and W, or Mo, wherein when the perovskite-type metal oxynitride contains B, all of the following formulas (1) to (5) are satisfied; and when the perovskite-type metal oxynitride contains Mo, all of the following formulas (1') to (5') are satisfied. [A]: [B]: [O]: [N] = 1: a: b: c ... (1) 0.90≦a≦1.10 ... (2) 1.00≦b≦2.50 ... (3) 0.50≦c≦2.00 ... (4) 2.50≦(b+c)≦3.10 ... (5) [A]: [Mo]: [O]: [N] = 1: a': b': c' ... (1') 0.90≦a'≦1.10 ... (2') 1.50≦b'≦3.40 ... (3') 0.10≦c'≦1.50 ... (4') 2.50≦(b'+c')≦3.50...(5') Here, [A], [B], [O], [N] and [Mo] represent the contents of A, B, O, N and Mo, respectively, expressed in mol %.

[0012] A fifth aspect of the present invention is a cathode electrode catalyst for the electrolytic synthesis of ammonia, comprising a perovskite-type metal oxynitride, wherein the perovskite-type metal oxynitride contains, as main metal elements, A, which is at least one element selected from the group consisting of Ba, Sr, and Ca, and contains 50 mol % or more of Ba, and B, which is at least one element selected from the group consisting of Ta, Nb, Mo, and W, and satisfies all of the following formulas (1) to (5): [A]:[B]:[O]:[N]=1:a:b:c (1) 0.90≦a≦1.10 (2) 1.00≦b≦2.50 (3) 0.50≦c≦2.00 (4) 2.50≦(b+c)≦3.10 (5) Here, [A], [B], [O] and [N] represent the contents of A, B, O and N, respectively, expressed in mol %.

[0013] A sixth aspect of the present invention is a cathode electrode catalyst for the electrolytic synthesis of ammonia, comprising a perovskite-type metal oxynitride, wherein the perovskite-type metal oxynitride comprises, as main metal elements, A, which is at least one element selected from the group consisting of Ba, Sr, and Ca, and contains 50 mol % or more of Ba, and Mo, and satisfies all of the following formulas (1') to (5'): [A]:[Mo]:[O]:[N]=1:a':b':c' (1') 0.90≦a'≦1.10 (2') 1.50≦b'≦3.40 (3') 0.10≦c'≦1.50 (4') 2.50≦(b'+c')≦3.50 (5') Here, [A], [Mo], [O] and [N] respectively represent the contents of A, Mo, O and N expressed in mol%.

[0014] A seventh aspect of the present invention is the catalyst according to the fifth aspect, wherein the B comprises Nb.

[0015] Aspect 8 of the present invention is the catalyst according to any one of aspects 5 to 7, wherein A consists of Ba.

[0016] A ninth aspect of the present invention is an ammonia electrolysis apparatus comprising: a cathode electrode containing a perovskite metal oxynitride; an anode electrode; an electrolyte; and a voltage application unit, wherein at least a portion of the electrolyte is disposed between the cathode electrode and the anode electrode; the voltage application unit is connected to the cathode electrode and the anode electrode so as to apply a voltage between the cathode electrode and the anode electrode; the perovskite metal oxynitride comprises, as main metal elements, A which is at least one selected from the group consisting of Ba, Sr, and Ca, and contains 50 mol % or more of Ba; and B which is at least one selected from the group consisting of Ta, Nb, Mo, and W; and the ammonia electrolysis apparatus satisfies all of the following formulas (1) to (5): [A]:[B]:[O]:[N]=1:a:b:c (1) 0.90≦a≦1.10 (2) 1.00≦b≦2.50 (3) 0.50≦c≦2.00 (4) 2.50≦(b+c)≦3.10 (5) Here, [A], [B], [O] and [N] represent the contents of A, B, O and N, respectively, expressed in mol %.

[0017] A tenth aspect of the present invention is an ammonia electrolysis apparatus comprising: a cathode electrode containing a perovskite-type metal oxynitride; an anode electrode; an electrolyte; and a voltage application unit, wherein at least a portion of the electrolyte is disposed between the cathode electrode and the anode electrode; the voltage application unit is connected to the cathode electrode and the anode electrode so as to apply a voltage between the cathode electrode and the anode electrode; the perovskite-type metal oxynitride contains, as main metal elements, A which is at least one selected from the group consisting of Ba, Sr, and Ca, and contains 50 mol % or more of Ba; and Mo; and the ammonia electrolysis apparatus satisfies all of the following formulas (1') to (5'): [A]:[Mo]:[O]:[N]=1:a':b':c' (1') 0.90≦a'≦1.10 (2') 1.50≦b'≦3.40 (3') 0.10≦c'≦1.50 (4') 2.50≦(b'+c')≦3.50 (5') Here, [A], [Mo], [O] and [N] respectively represent the contents of A, Mo, O and N expressed in mol%.

[0018] According to an embodiment of the present invention, it is possible to provide a novel cathode electrode catalyst for the electrolytic synthesis of ammonia and an ammonia electrolytic synthesis apparatus using the same.

[0019] Figure 1A shows, from the bottom up, the X-ray diffraction patterns of Samples Nos. 1, 2, 4, 5, 6, and 7. Figure 1B shows, from the bottom up, the X-ray diffraction patterns of Samples Nos. 3, 8, and 9.

[0020] The present inventors have conducted research from various angles in order to realize a novel cathode electrode catalyst for the electrolytic synthesis of ammonia.

[0021] In one example of the electrolytic synthesis of ammonia, the reaction represented by the following formula (6) is thought to occur on the anode electrode side, and the reaction represented by the following formula (7) is thought to occur on the cathode electrode side. ２ O → O ２ +4H ＋ +4e － ... (6) N ２ +6H ＋ +6e －→2NH ３ ... (7)

[0022] The present inventors have focused on perovskite-type materials with stable crystal structures so that decomposition is suppressed even under the N reduction reaction conditions of the above formula (7). Among these, perovskite-type oxynitrides (general formula ABO ２ It was thought that when nitrogen vacancies are present, the vacancies can function as nitrogen adsorption sites, thereby promoting the reaction of (7).

[0023] The present inventors have investigated the properties of a general perovskite oxide (general formula ABO ３ ) to form a perovskite-type oxynitride (general formula ABO ２ To achieve this, the inventors considered using metal elements from Groups 5 and 6 with high valences at the B site of the perovskite material. They considered that the crystal structure of the perovskite material would be more stable by using one or more elements selected from the group consisting of Ta, Nb, Mo, and W, which are elements from the fifth and sixth periods. Furthermore, the inventors found that when Mo, which has a large valence change, is used at the B site of the perovskite material, the crystal structure of a perovskite oxynitride with a lower nitrogen content can also be more stable.

[0024] As a result of further intensive research, it was found that by mainly containing Ba in the A site of the perovskite material, electron donation from Ba to the adsorbed nitrogen occurs, further promoting the reaction of the above formula (7). As a result, a new cathode electrode catalyst for the electrolytic synthesis of ammonia was realized. This catalyst can efficiently produce ammonia without using Ru. Note that the technical scope of the present invention is not limited by the above mechanism. Details of each requirement specified in the embodiments of the present invention are shown below.

[0025] <1. Cathode Electrocatalyst for Ammonia Electrosynthesis> A novel cathode electrode catalyst for ammonia electrosynthesis according to an embodiment of the present invention comprises a perovskite-type metal oxynitride, the perovskite-type metal oxynitride containing, as main metal elements, A which is at least one element selected from the group consisting of Ba, Sr, and Ca, and contains 50 mol % or more of Ba, and B which is at least one element selected from the group consisting of Ta, Nb, Mo, and W, and satisfies all of the following formulas (1) to (5): [A]:[B]:[O]:[N]=1:a:b:c (1) 0.90≦a≦1.10 (2) 1.00≦b≦2.50 (3) 0.50≦c≦2.00 (4) 2.50≦(b+c)≦3.10 (5) Here, [A], [B], [O] and [N] represent the contents of A, B, O and N, respectively, expressed in mol %.

[0026] According to another embodiment of the present invention, there is provided a novel cathode electrode catalyst for the electrolytic synthesis of ammonia, comprising a perovskite-type metal oxynitride, wherein the perovskite-type metal oxynitride contains, as main metal elements, A, which is at least one element selected from the group consisting of Ba, Sr, and Ca and contains 50 mol % or more of Ba, and Mo, and satisfies all of the following formulas (1') to (5'): [A]:[Mo]:[O]:[N]=1:a':b':c' (1') 0.90≦a'≦1.10 (2') 1.50≦b'≦3.40 (3') 0.10≦c'≦1.50 (4') 2.50≦(b'+c')≦3.50 (5') Here, [A], [Mo], [O] and [N] respectively represent the contents of A, Mo, O and N expressed in mol%.

[0027] According to yet another embodiment of the present invention, there is provided a novel cathode electrode catalyst for the electrolytic synthesis of ammonia, comprising a perovskite-type metal oxynitride, wherein the perovskite-type metal oxynitride comprises, as main metal elements, A, which is at least one element selected from the group consisting of Ba, Sr, and Ca, and contains at least 50 mol% of Ba; and B, which is at least one element selected from the group consisting of Ta, Nb, and W, or Mo, wherein the cathode electrode catalyst for the electrolytic synthesis of ammonia satisfies all of the following formulas (1) to (5) when the perovskite-type metal oxynitride contains B, and satisfies all of the following formulas (1') to (5') when the perovskite-type metal oxynitride contains Mo: [A]: [B]: [O]: [N] = 1: a: b: c ... (1) 0.90≦a≦1.10 ... (2) 1.00≦b≦2.50 ... (3) 0.50≦c≦2.00 ... (4) 2.50≦(b+c)≦3.10 ... (5) [A]: [Mo]: [O]: [N] = 1: a': b': c' ... (1') 0.90≦a'≦1.10 ... (2') 1.50≦b'≦3.40 ... (3') 0.10≦c'≦1.50 ... (4') 2.50≦(b'+c')≦3.50...(5') Here, [A], [B], [O], [N] and [Mo] represent the contents of A, B, O, N and Mo, respectively, expressed in mol %.

[0028] The catalyst can efficiently produce ammonia without using Ru. Furthermore, the catalyst can also exhibit the effect of suppressing decomposition even under N reduction reaction conditions because the crystal structure of the perovskite-type metal oxynitride contained therein is stable.

[0029] The cathode electrode catalyst for ammonia electrosynthesis according to an embodiment of the present invention contains a perovskite-type metal oxynitride. The crystal structure of the perovskite-type metal oxynitride may be cubic, tetragonal, monoclinic, hexagonal, orthorhombic, etc. Whether or not a perovskite-type metal oxynitride is contained can be determined by examining an X-ray diffraction pattern using Cu-Kα radiation and finding that a diffraction peak derived from the (001) plane of the cubic crystal, a diffraction peak derived from the (110) plane of the cubic crystal, a diffraction peak derived from the (002) plane of the cubic crystal, and a diffraction peak derived from the (112) plane of the cubic crystal are present within a diffraction angle range of 20°≦2θ≦60°. The term "diffraction peak derived from the (...) plane of the cubic crystal" used in this specification will be explained below. Perovskite metal oxynitrides may have, in addition to cubic crystals, tetragonal crystals, monoclinic crystals, hexagonal crystals, orthorhombic crystals, and other crystal structures whose symmetry is reduced from that of a cubic crystal. For a cubic crystal, the term refers to the diffraction peaks corresponding to the above-mentioned crystal faces ((001), (110), (002), and (112)), and for other crystal structures whose symmetry is reduced from that of a cubic crystal, refers to the diffraction peaks corresponding to the above-mentioned crystal faces when considered as a cubic crystal (although the actual face indices may change due to the reduced symmetry).

[0030] A in the perovskite metal oxynitride can be any one or more elements selected from the group consisting of Ba, Sr, and Ca, which are capable of forming divalent cations. When A contains 50 mol% or more of Ba, electrons are donated from Ba to the adsorbed nitrogen, thereby promoting the reaction of formula (7). It is preferable that A contains 60 mol% or more, 70 mol% or more, 80 mol% or more, or 90 mol% or more of Ba, in that order, and it is most preferable that A consists entirely of Ba.

[0031] The B of the perovskite metal oxynitride may be any one or more selected from the group consisting of Ta, Nb, Mo, and W, which are metals belonging to Period 5 and Period 6 and Group 5 and Group 6. For example, the B of the perovskite metal oxynitride may be any one or more selected from the group consisting of Ta, Nb, and W. The use of such metals can stabilize the crystal structure of the perovskite metal oxynitride. The inventors have also found that when B contains Nb, ammonia can be produced more efficiently. This is thought to be because the inclusion of Nb in B improves the electronic conductivity of the perovskite metal oxynitride. It is preferred that B contains 50 mol % or more, 60 mol % or more, 70 mol % or more, 80 mol % or more, or 90 mol % or more of Nb, in this order. It is most preferred that B consists of Nb.

[0032] The perovskite metal oxynitride contains the above-mentioned A and B as main metal elements. Preferably, the total content of A and B relative to all metal elements is 90 mol % or more, more preferably 95 mol % or more, and most preferably 100 mol %. In addition to A and B, the perovskite metal oxynitride can tolerate impurity metal elements in a total amount of 10 mol % or less relative to all metal elements. From the viewpoint of cost, the perovskite metal oxynitride preferably contains Ru in an amount of 10 mol % or less, more preferably 5 mol % or less, and even more preferably 1 mol % or less, relative to all metal elements, and most preferably does not contain Ru.

[0033] The ratios of A and B, oxygen (O) and nitrogen (N) in the perovskite metal oxynitride according to the formula (1) above satisfy all of the formulas (2) to (5) above (i.e., 0.90≦a≦1.10, 1.00≦b≦2.50, 0.50≦c≦2.00, 2.50≦(b+c)≦3.10). The general formula of the perovskite metal oxynitride is ABO ２The ratio is represented by N, and as long as it is within the above range, the crystal structure of the perovskite metal oxynitride can be stably maintained. Preferably, 0.92≦a≦1.08, 1.50≦b≦2.30, 0.70≦c≦1.50, and 2.70≦(b+c)≦3.08 are satisfied, and more preferably 0.95≦a≦1.05, 2.00≦b≦2.15, 0.85≦c≦1.00, and 2.85≦(b+c)≦3.05 are satisfied. In particular, when B is one or more elements selected from the group consisting of Ta and Nb, satisfying the above ratio can more stably maintain the crystal structure of the perovskite metal oxynitride. When B is W, satisfying the conditions of 1.00≦b≦1.50, 1.50≦c≦2.00, and 2.70≦(b+c)≦3.08 is also preferable, since the crystal structure of the perovskite metal oxynitride can be stably maintained. When B is W, satisfying the conditions of 1.00≦b≦1.05, 1.90≦c≦2.00, and 2.85≦(b+c)≦3.05 is even more preferable. The ratios of A, Mo, oxygen (O), and nitrogen (N) in the perovskite metal oxynitride according to the above formula (1′) can all satisfy the above formulas (2′) to (5′) (i.e., 0.90≦a′≦1.10, 1.50≦b′≦3.40, 0.10≦c′≦1.50, and 2.50≦(b′+c′)≦3.50). Within the above ratio ranges, the crystal structure of the perovskite metal oxynitride can be stably maintained. Preferably, 0.95≦a′≦1.05, 2.50≦b′≦3.35, 0.10≦c′≦1.00, and 3.00≦(b′+c′)≦3.45 are satisfied. Whether or not the above ratios are satisfied can be determined, for example, by general composition analysis (FE-TEM / EDS, etc.). Note that impurities other than the elements specified above may also be detected in the composition analysis, but as long as the above ratios are satisfied, they are included in the embodiments of the present invention.

[0034] The cathode electrode catalyst for electrolytic synthesis of ammonia according to the embodiment of the present invention preferably contains the perovskite-type metal oxynitride in an amount of 50 mass % or more, 60 mass % or more, 70 mass % or more, 80 mass % or more, or 90 mass % or more, and most preferably consists of the perovskite-type metal oxynitride.

[0035] The cathode catalyst for electrosynthesis of ammonia according to an embodiment of the present invention may be in powder form, and its particle size is not particularly limited. The cathode catalyst for electrosynthesis of ammonia according to an embodiment of the present invention can be produced by a known method, such as the examples described below. The cathode catalyst for electrosynthesis of ammonia according to an embodiment of the present invention is preferably subjected to an acid treatment. This is thought to remove a catalytically inactive layer present on the catalyst surface, thereby improving at least one of the ammonia production amount and the Faraday efficiency. The acid treatment method is not particularly limited and can be carried out by a known method. For example, an acid may be applied to the catalyst surface or the catalyst may be immersed in acid. The acid immersion may preferably be accompanied by ultrasonic treatment, as this shortens the treatment time. Examples of acids used in the acid treatment include nitric acid, hydrochloric acid, and sulfuric acid. The pH of the acid used in the acid treatment is not particularly limited, but is preferably 4 or less, and more preferably 2 or less. The treatment time is also not particularly limited and can be appropriately selected depending on the type of acid, pH, treatment conditions (whether or not ultrasonic treatment is performed), and the like. The cathode catalyst for electrosynthesis of ammonia according to an embodiment of the present invention may be a composition mixed with, for example, a conductive material. The composition contains an ammonia electrode synthesis cathode catalyst according to an embodiment of the present invention and a conductive material, and may further contain a dispersion medium, a proton-conductive material, other additives, and the like, as necessary. The conductive material is not particularly limited as long as it is a material that can be commonly used as a conductive additive, and suitable examples include conductive carbon. In the composition, the mixing ratio of the ammonia electrode synthesis cathode catalyst according to an embodiment of the present invention and the conductive material is not particularly limited and can be appropriately set depending on the application. The dispersion medium is not particularly limited as long as it can disperse the ammonia electrode synthesis cathode catalyst and the conductive material, and can be, for example, a lower alcohol. Known materials can be used as the proton-conductive material. The mixing ratio is not particularly limited and can be appropriately set depending on the application.

[0036] 2. Ammonia Electrosynthesis Apparatus An ammonia electrosynthesis apparatus according to an embodiment of the present invention includes a cathode electrode containing the perovskite-type metal oxynitride described above, an anode electrode, an electrolyte, and a voltage application unit, wherein at least a portion of the electrolyte is disposed between the cathode electrode and the anode electrode, and the voltage application unit is connected to the cathode electrode and the anode electrode so as to apply a voltage between the cathode electrode and the anode electrode. This allows for efficient production of ammonia without using Ru in the cathode electrode. Furthermore, because the crystal structure of the catalyst (perovskite-type metal oxynitride) contained in the cathode electrode is stable, decomposition of the catalyst can be suppressed even under N reduction reaction conditions.

[0037] In an embodiment of the present invention, the cathode electrode includes the perovskite-type metal oxynitride. The cathode electrode preferably includes the perovskite-type metal oxynitride on its surface, thereby enabling efficient ammonia production with a small amount of catalyst. The cathode electrode may include other materials, such as a conductive material, in addition to the perovskite-type metal oxynitride. The conductive material is not particularly limited and may be any material that is commonly used as a conductive additive, with conductive carbon being a preferred example. The cathode electrode can be fabricated by dripping and applying a composition (hereinafter also referred to as "catalyst ink") containing the ammonia electrode synthesis cathode catalyst according to an embodiment of the present invention onto the surface of a conductive material (electrode). When dripping the catalyst ink, it is preferable to agitate the catalyst ink (e.g., ultrasonically disperse it), and then quickly (e.g., within 2 minutes, preferably within 1 minute) between collection from the ink and dripping. This allows the fabrication of a cathode electrode in which the ammonia electrode synthesis cathode catalyst is uniformly distributed on the surface.

[0038] In an embodiment of the present invention, the form of the electrolyte is not particularly limited, and examples thereof include a liquid electrolyte and a solid electrolyte. In an embodiment of the present invention, the electrolyte may be proton-conductive. When the electrolyte is a liquid electrolyte, at least a portion of the cathode electrode and at least a portion of the anode electrode may be immersed in the liquid electrolyte, and the liquid electrolyte may be present around at least a portion of the cathode electrode and at least a portion of the anode electrode (i.e., the liquid electrolyte may be present not only between the cathode electrode and the anode electrode). The liquid electrolyte may be, for example, a neutral or acidic liquid to cause the reaction of formula (6) above. Furthermore, the liquid electrolyte may include a first electrolyte disposed on the cathode electrode side and a second electrolyte disposed on the anode electrode side, and the first electrolyte and the second electrolyte may be separated by a proton exchange membrane. The first electrolyte and the second electrolyte may have the same composition or different compositions. The material of the proton exchange membrane is not particularly limited, and known materials can be used. When the electrolyte is a solid electrolyte, at least a portion or all of the solid electrolyte may be disposed between the cathode electrode and the anode electrode. The solid electrolyte may be a proton-conducting solid electrolyte.

[0039] In an embodiment of the present invention, the anode electrode decomposes water to produce protons (H ＋ The anode electrode is not particularly limited as long as it can generate protons, and may be, for example, an anode electrode of a known water electrolysis device. The anode electrode preferably contains a catalyst that promotes the decomposition of water into protons, and may contain, for example, a metal or alloy containing at least one element selected from the group consisting of Pt, Pd, Ru, Rh, Ir, Ni, Co, Fe, Cu, Ag, Mn, Nb, Ta, La, Ce, Pr, Nd, and Sm, and / or an oxide or sulfide thereof. The anode electrode may also contain other materials, such as a conductive material, in addition to the catalyst.

[0040] In an embodiment of the present invention, the voltage application unit is connected to the cathode electrode and the anode electrode so as to apply a voltage between the cathode electrode and the anode electrode. By applying a voltage between the cathode electrode and the anode electrode using the voltage application unit, it is possible to cause the reactions of the above formulas (6) and (7). From the viewpoint of facilitating the production of ammonia, the applied voltage is preferably in the range of −0.5 to −1.0 V, more preferably in the range of −0.6 to −0.8 V, based on the reversible hydrogen electrode (RHE).

[0041] The ammonia electrolysis apparatus according to the embodiment of the present invention may include other configurations as long as they do not deviate from the object of the present disclosure. For example, a nitrogen supply unit may be provided on the cathode side to promote the reaction of the above formula (7). The nitrogen source on the left side of the above formula (7) may be NO, NO ２ The ammonia electrosynthesis apparatus according to the embodiment of the present invention can react with other molecules containing N, such as NO and NO. ２ If the electrolyte is a solid electrolyte, a hydrogen and / or water vapor supply unit capable of supplying hydrogen and / or water vapor may be provided on the anode electrode side so as to promote proton generation on the anode electrode side.

[0042] The following examples are provided to more specifically describe the embodiments of the present invention. The embodiments of the present invention are not limited to the following examples, and may be modified as appropriate within the scope of the above-described and below-described aims, and all such modifications are within the technical scope of the embodiments of the present invention.

[0043] <Preparation of cathode electrode catalyst for ammonia electrosynthesis (hereinafter also simply referred to as "cathode electrode catalyst")> A cathode electrode catalyst was prepared as follows so as to have the composition shown in Sample No. 1 in Table 1 below. BaCO as a Ba source ３ (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: purity 99.9 mass%) and Ta as a Ta source ２ O ５Ba and Ta were weighed out so that they were equimolar, and wet-mixed with ethanol. The resulting mixed powder was placed on an alumina boat and placed in a tubular furnace. NH ３ While supplying gas (Sumitomo Seika Chemicals: 99.9%) into the tubular passage, the mixture was heated at 930°C for 15 hours, and the heated powder was dry-mixed. This process was repeated three times for a total of 45 hours of heating, yielding a cathode electrode catalyst of Sample No. 1 in Table 1. A cathode electrode catalyst was also prepared as follows, so as to have the composition shown in Sample No. 2 in Table 1 below. BaCO as a Ba source was added. ３ (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: purity 99.9 mass%) and Nb ２ O ５ (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: purity 99.9 mass%) was weighed so that Ba was in excess of Nb by 50 mol %, and wet-mixed with ethanol. The resulting mixed powder was placed on an alumina boat and placed in a tubular furnace. NH ３ The powder was heated at 900°C for 10 hours while gas (Sumitomo Seika Chemicals: 99.9%) was supplied into the tubular passage. The heated powder was washed with an aqueous nitric acid solution to remove excess Ba components, thereby obtaining the cathode electrode catalyst of Sample No. 2 in Table 1 described below. Furthermore, the conditions for preparing Sample No. 1 were appropriately changed to obtain the cathode electrode catalysts of Samples No. 3 to 9 in Table 1, so as to obtain the compositions of Samples No. 3 to 9. The Ba source and Ta source for Samples No. 3, 4, 6, and 8 were the same as those for Sample No. 1, and the Nb source for Samples No. 5 and 9 was Nb. ２ O ５ (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: purity 99.9 mass%) was used, and SrCO was used as the Sr source for samples 4 and 5. ３ (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: purity 99.9 mass%) was used, and CaCO was used as the Ca source for sample No. 6. ３ (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: purity 99.9 mass%) was used, and La was used as the La source for sample No. 7. ２ O ３ (Kanto Chemical Co., Ltd.: purity 99.9% by mass) was used, and TiO was used as the Ti source for sample No. 7. ２(manufactured by Kojundo Chemical Laboratory: purity 99.99% by mass) was used. ３ (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: purity 99.9 mass%) and Nb ２ O ５ Ba and Nb (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: purity 99.9% by mass) were weighed out so that the Ba and Nb were equimolar, and wet-mixed with ethanol. The resulting mixed powder was placed on an alumina boat and placed in a tubular furnace. NH ３ The mixture was heated at 900°C for 10 hours while gas (Sumitomo Seika Chemicals: 99.9%) was supplied to the tubular furnace, to obtain a cathode electrode catalyst of Sample No. 10 in Table 1. Sample No. 10 differs from Sample No. 2 in that it was not subjected to an acid treatment, i.e., washing with an aqueous nitric acid solution.

[0044] <X-ray Diffraction Patterns of Cathode Electrode Catalysts> X-ray diffraction measurements were performed on the cathode electrode catalysts (powder) of Samples 1 to 9 using an X-ray diffractometer (Rigaku, Ultima IV protectus ADS, gonioradius 185 mm). The X-ray source was Cu-Kα radiation, the X-ray output was 40 kV-40 mA, the divergence slit was 1°, the divergence vertical limiting slit was 10 mm, the scattering slit and receiving slit were open, the detector was D / tex Ultra, the measurement range was 2θ = 20 to 80°, the sampling width was 0.02°, and the scanning rate was 10.0° / min. Figure 1A shows the X-ray diffraction patterns of Samples 1, 2, 4, 5, 6, and 7, from the bottom of the figure, and Figure 1B shows the X-ray diffraction patterns of Samples 3, 8, and 9, from the bottom of the figure. In FIG. 1A , the plane index of the cubic crystal (or when considered as a cubic crystal), such as (001), is written to the right of each diffraction peak. As shown in FIG. 1A , for at least Samples Nos. 1, 2, and 4 to 7, a diffraction peak derived from the cubic (001) plane, a diffraction peak derived from the cubic (110) plane, a diffraction peak derived from the cubic (002) plane, and a diffraction peak derived from the cubic (112) plane were confirmed within the diffraction angle range of 20°≦2θ≦60°. From FIG. 1A , it was found that at least Samples Nos. 1, 2, and 4 to 7 had a single-phase perovskite-type crystal structure. An X-ray diffraction pattern was similarly obtained for Sample No. 10, and it was found to have a single-phase perovskite-type crystal structure.

[0045] <Ammonia Electrosynthesis Test> Each of the catalysts of Samples 1 to 10 and conductive carbon (Carbon Black Vulcan XC 72R, manufactured by ElectroChem, Inc.) in an amount 1 to 2 times the mass of each catalyst were mixed with ethanol at a ratio of 0.27 mL per 1 mg of conductive carbon. Nafion (registered trademark) (Nafion perfluorinated resin solution, 5% by mass, manufactured by Sigma-Aldrich) was then added at a ratio of 0.030 mL per 1 mg of conductive carbon, and the mixture was then ultrasonically dispersed for 10 minutes to obtain catalyst inks containing each of Samples 1 to 10.

[0046] Ammonia electrolytic synthesis tests were carried out using the catalyst ink. The ammonia electrolytic synthesis tests were carried out using an electrochemical measurement device (Toyo Technica: S11287). All experiments were carried out at room temperature and atmospheric pressure, and 0.1 M H ２ SO ４ 75 mL of a solution (manufactured by Kanto Chemical: 96.0 mass%) was used. ２ (Air Water, purity: 99.999%) for 30 minutes to convert the liquid electrolyte into N ２ It reached saturation.

[0047] The catalyst ink was applied in an amount of 8 μL to the working electrode of the device (manufactured by BAS: Rotating Ring Disk Electrode Device Ver. 3.0, GC disk electrode, electrode radius 0.2 cm) to obtain a cathode electrode having a cathode electrode catalyst on its surface. A Pt electrode was used as the anode electrode (counter electrode). A liquid electrolyte (0.1 M H ２ SO ４ The solution was placed on the cathode and anode sides of the H-type cell of the device, and they were separated by a proton exchange membrane. Nafion® 117 membrane was used as the proton exchange membrane. Hg / Hg was used as the reference electrode. ２ SO ４ The H-type cell was placed on the cathode side of the cell. Each electrode was immersed in a liquid electrolyte. A nitrogen supply unit was placed on the cathode side of the H-type cell, and N ２ Linear sweep voltammetry (lsv) measurements were performed at a rotation speed of 1600 rpm and a sweep rate of 5 mV / sec, and in constant voltage measurements, a constant voltage of −0.8 to −1.0 V was applied to the working electrode (cathode electrode) by a voltage application unit, and the current value was measured for 20 hours.

[0048] NH synthesized in liquid electrolyte ３The amount of produced was measured by the indophenol method. 2 mL of liquid electrolyte was collected from the cathode electrode side of the H-type cell after the experiment, and 2 mL of a mixed solution of 1 M sodium hydroxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity 97% by mass), 5 mass% salicylic acid (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity 99.5% by mass), and 5 mass% trisodium citrate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., purity 99.0% by mass) was mixed with 1 mL of a 0.05 M diluted sodium hypochlorite aqueous solution (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., 5.0% by mass), and 0.2 mL of a 5 mass% adjusted nitroglycide aqueous solution (manufactured by Sigma-Aldrich, purity 99% by mass), and then left in a dark room for 2 hours. Thereafter, the absorbance at a wavelength of 655 nm was measured using a spectrophotometer, and the NH ３ The concentration of NH ４ ＋ The standard solution (manufactured by Fujifilm Wako Pure Chemical Industries, 1000 ppm) was diluted to 0, 0.05, 0.1, 0.2, 0.3, 0.5, 1, 3, and 5 ppm, and the absorbance was measured.

[0049] Weight of the cathode catalyst (mg ｃａｔ Ammonia production per unit R ＮＨ３ (μg h －１ ・mg ｃａｔ －１ ) was derived using the following formula (8): ＮＨ３ = (c ＮＨ３ × V) / (17×t×m) (8) In the above formula (8), c ＮＨ３ is the NH calculated from the absorbance ３ Concentration (μg・mL －１ ), V is the volume of the liquid electrolyte (75 mL in this example), t is the reaction time (h), and m is the amount of catalyst (mg) on ​​the cathode electrode (working electrode).

[0050] The faradaic efficiency FE (%) of ammonia production was calculated using the following formula (9): FE = (3 × F × c ＮＨ３ × V) / (17×Q)×100 (9) In the above formula (9), F is the Faraday constant (96500 C mol －１ ) and c ＮＨ３ is the NH calculated from the absorbance ３Concentration (μg・mL －１ ), V is the volume of the liquid electrolyte (75 mL in this example), and Q is the total charge (C).

[0051] The test results are summarized in Table 1. In Table 1, the column "Amount of catalyst" indicates the mass ratio (i.e., 0.5 or 1.0) of the catalyst mixed with the conductive carbon. In the column "Catalytic activity", "O" indicates that ammonia was generated, and "X" indicates that ammonia was not generated. ＮＨ３ In the "" column, the amount of ammonia produced is ＮＨ３ " is converted from mass to molar amount, and "R ＮＨ３ In the "" column, the amount of ammonia produced is ＮＨ３ " was calculated from the value per weight of the catalyst by subtracting the electrode area (0.2 × 0.2 × π (cm ２ The values ​​converted to values ​​per 1000 kJ / kg are listed.

[0052]

[0053] The following can be seen from Table 1. Samples Nos. 1, 2, and 10, which satisfy all of the requirements of the embodiment of the present invention, had high faradaic efficiencies for ammonia synthesis (e.g., 1.0% or higher). Furthermore, Samples Nos. 2 and 10 exhibited higher faradaic efficiencies by satisfying a preferred requirement of the embodiment of the present invention (B in the perovskite-type oxynitride containing Nb). Furthermore, Sample No. 2 was subjected to an acid treatment, i.e., washing with an aqueous nitric acid solution, and therefore exhibited at least one of the amount of ammonia produced and the faradaic efficiency improved compared to Sample No. 10. On the other hand, Samples Nos. 3 to 9, which do not satisfy the requirements of the embodiment of the present invention, either had low faradaic efficiencies for ammonia synthesis or did not produce ammonia (lacked catalytic activity).

[0054] <Lattice Constant of Perovskite Oxynitride After Ammonia Electrosynthesis Test> The lattice constant of the crystal structure after the ammonia electrosynthesis test was determined by obtaining an X-ray diffraction pattern at room temperature using the BL02B2 beamline (λ = 0.049629 nm) at SPring-8. The sample was sealed in a glass capillary for measurement. As an example, the change in the lattice constant of the perovskite oxynitride before and after the ammonia electrosynthesis test for Sample No. 1 will be described. The lattice constant of the perovskite oxynitride before the ammonia electrosynthesis test, as determined from the X-ray diffraction pattern of Sample No. 1 in FIG. 1A, was 4.11449 Å, whereas the lattice constant after the ammonia electrosynthesis test was 4.11404 Å, indicating almost no change. From the above, it was confirmed that the ammonia electrosynthesis cathode electrode catalyst according to an embodiment of the present invention suppresses decomposition under N reduction reaction conditions.

[0055] Furthermore, the following experiments were conducted to investigate the effect of acid treatment of the cathode electrode catalyst and the effect of immediately performing the process from ultrasonic dispersion of the catalyst ink to collection and dripping during cathode electrode fabrication. Cathode electrode catalysts were fabricated in the same manner as Sample No. 1 in Example 1 so as to have the compositions shown in Samples No. 11 and 12 in Table 2 below. Cathode electrode catalysts were also fabricated in the same manner as Sample No. 10 in Example 1 so as to have the compositions shown in Samples No. 13 and 14 in Table 2 below. Samples No. 12 and 14 were further treated as follows: 30 mL of 6 M HNO ３ The acid-treated powder was separated by centrifugation. The acid-treated powder was washed twice with distilled water and centrifuged, and the washed powder was separated. The washed powder was dried overnight at 60°C to obtain acid-treated cathode electrode catalysts (samples No. 12 and No. 14).

[0056] X-ray diffraction patterns were obtained for the cathode electrode catalysts (powder) of Samples 11 to 14 in the same manner as in Example 1. As a result, a diffraction peak derived from the (001) plane of the cubic crystal, a diffraction peak derived from the (110) plane of the cubic crystal, a diffraction peak derived from the (002) plane of the cubic crystal, and a diffraction peak derived from the (112) plane of the cubic crystal were confirmed within the diffraction angle range of 20°≦2θ≦60°, and it was found that each sample had a single-phase perovskite-type crystal structure.

[0057] Each catalyst of Samples No. 11 to 14, conductive carbon (Carbon Black Vulcan XC 72R, manufactured by ElectroChem, Inc.), and ethanol were mixed in the same ratio as in Example 1. Furthermore, Nafion (registered trademark) (5% by mass, manufactured by Sigma-Aldrich, Nafion perfluorinated resin solution) was added in the same ratio as in Example 1, and the mixture was mixed by ultrasonic dispersion for 10 minutes. Thereafter, unlike in Example 1, mixing by ultrasonic dispersion was continued, and 2 μL was sampled and dripped onto the same working electrode as in Example 1 (Rotating Ring Disk Electrode Apparatus Ver. 3.0, GC Disk Electrode, electrode radius 0.2 cm, manufactured by BAS) over 30 seconds. This was repeated four times (a total of 8 μL was applied), thereby obtaining a cathode electrode having a cathode electrode catalyst on its surface. The rest of the procedure was the same as in Example 1, and an ammonia electrosynthesis test was performed. The results are shown in Table 2.

[0058] The following can be seen from Table 2. Samples Nos. 11 to 14, which satisfied all of the requirements of the embodiment of the present invention, had high Faraday efficiencies for ammonia synthesis. Furthermore, Samples Nos. 12 and 14, by satisfying the preferred requirement of the embodiment of the present invention (acid treatment), exhibited higher values ​​for at least one of the amount of ammonia produced and the Faraday efficiencies compared to the samples without acid treatment (Samples Nos. 11 and 13). Despite having the same composition and the same non-acid treatment, Sample No. 11 of Example 2 had a higher Faraday efficiency than Sample No. 1 of Example 1, and Sample No. 13 of Example 2 had a higher Faraday efficiency than Sample No. 10 of Example 1. This is thought to be due to the fact that, in Samples Nos. 11 and 13 of Example 2, the process of ultrasonically dispersing the catalyst ink during cathode electrode fabrication, followed by collection and dripping, was carried out immediately, resulting in a more uniform cathode electrode.

[0059] A cathode electrode catalyst was prepared so as to have the composition shown in Table 3 below. Specifically, for sample No. 15, the Ta source of sample No. 1 was replaced with WO, which was a W source. ３ Sample No. 16 was prepared in the same manner as Sample No. 1, except that BaCO (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: purity 99.5 mass%) was used as the Ba source. ３ (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.: purity 99.9 mass%) and MoO ３ Ba and Mo were weighed out so that they were equimolar, and then wet-mixed with ethanol. The resulting mixed powder was fired on an alumina boat at 900°C in air for 10 hours to obtain BaMoO ４ Furthermore, 5% H ２ / BaMoO ３ BaMoO ３ The resulting mixture was calcined at 600°C for 36 hours in an ammonia gas flow of 100 mL / min to obtain Sample No. 16. Sample No. 17 was prepared in the same manner as Sample No. 1. Sample No. 18 was prepared in the same manner as Sample No. 12. Sample No. 19 was prepared in the same manner as Sample No. 10. Sample No. 20 was prepared in the same manner as Sample No. 14.

[0060] X-ray diffraction patterns were obtained for the cathode electrode catalysts (powder) of Samples 15 to 20 in the same manner as in Example 1. As a result, a diffraction peak derived from the (001) plane of the cubic crystal, a diffraction peak derived from the (110) plane of the cubic crystal, a diffraction peak derived from the (002) plane of the cubic crystal, and a diffraction peak derived from the (112) plane of the cubic crystal were confirmed within the diffraction angle range of 20°≦2θ≦60°, and it was found that the samples had a single-phase perovskite-type crystal structure.

[0061] Ammonia electrolytic synthesis tests were carried out on Samples No. 15 to 20 as follows. ２ (Air Water, purity: 99.999%) was supplied for 30 minutes to convert the liquid electrolyte into N ２ Before reaching a saturated state, a pretreatment was carried out by passing (bubbling) the solution through an acid and an alkaline aqueous solution. Except for the above pretreatment, an ammonia electrolytic synthesis test was carried out in the same manner as in Example 2. Note that by carrying out the above pretreatment, trace impurity gases (NO, NO) were removed. ２ etc.) can be removed. ２ etc., N ２ It has higher activity and can promote the reaction of the above formula (7). ２ The ammonia electrolytic synthesis test in Example 3, in which the above-mentioned conditions were removed, may be under more severe conditions than the tests in Examples 1 and 2. The results are shown in Table 3.

[0062]

[0063] The following can be seen from Table 3. Samples Nos. 15 to 20, which satisfy all of the requirements of the embodiment of the present invention, had high Faraday efficiencies for ammonia synthesis (e.g., 1.0% or higher), despite the test conditions being more severe than those of Examples 1 and 2. Furthermore, Samples Nos. 18 and 20, which satisfy the preferred requirements of the embodiment of the present invention (acid treatment), exhibited higher values ​​for at least one of the amount of ammonia produced and the Faraday efficiencies than samples that were not acid-treated (Samples Nos. 17 and 19).

[0064] This application claims priority from Japanese Patent Application No. 2023-134117, filed August 21, 2023. Japanese Patent Application No. 2023-134117 is incorporated herein by reference.

Claims

1. A cathode electrode catalyst for electrolytic synthesis of ammonia, comprising a perovskite-type metal oxynitride, the perovskite-type metal oxynitride comprising, as main metal elements, A which is at least one selected from the group consisting of Ba, Sr, and Ca, and contains at least 50 mol% Ba, and B which is at least one selected from the group consisting of Ta, Nb, and W, or Mo, the cathode electrode catalyst for electrolytic synthesis of ammonia satisfying all of the following formulas (1) to (5) when it contains B, and satisfying all of the following formulas (1') to (5') when it contains Mo. [A]: [B]: [O]: [N] = 1: a: b: c ... (1) 0.90≦a≦1.10 ... (2) 1.00≦b≦2.50 ... (3) 0.50≦c≦2.00 ... (4) 2.50≦(b+c)≦3.10 ... (5) [A]: [Mo]: [O]: [N] = 1: a': b': c' ... (1') 0.90≦a'≦1.10 ... (2') 1.50≦b'≦3.40 ... (3') 0.10≦c'≦1.50 ... (4') 2.50≦(b'+c')≦3.50...(5') Here, [A], [B], [O], [N] and [Mo] respectively represent the contents of A, B, O, N and Mo expressed in mol %.

2. The catalyst of claim 1, wherein A consists of Ba.

3. The catalyst according to claim 1 or 2, wherein B includes Nb.

4. An ammonia electrolysis apparatus comprising a cathode electrode containing a perovskite-type metal oxynitride, an anode electrode, an electrolyte, and a voltage application unit, wherein at least a portion of the electrolyte is disposed between the cathode electrode and the anode electrode, and the voltage application unit is connected to the cathode electrode and the anode electrode so as to apply a voltage between the cathode electrode and the anode electrode, and the perovskite-type metal oxynitride contains, as main metallic elements, A which is one or more selected from the group consisting of Ba, Sr, and Ca and contains 50 mol % or more of Ba, and B which is one or more selected from the group consisting of Ta, Nb, and W, or Mo, wherein when the perovskite-type metal oxynitride contains B, all of the following formulas (1) to (5) are satisfied, and when the perovskite-type metal oxynitride contains Mo, all of the following formulas (1') to (5') are satisfied. [A]: [B]: [O]: [N] = 1: a: b: c ... (1) 0.90≦a≦1.10 ... (2) 1.00≦b≦2.50 ... (3) 0.50≦c≦2.00 ... (4) 2.50≦(b+c)≦3.10 ... (5) [A]: [Mo]: [O]: [N] = 1: a': b': c' ... (1') 0.90≦a'≦1.10 ... (2') 1.50≦b'≦3.40 ... (3') 0.10≦c'≦1.50 ... (4') 2.50≦(b'+c')≦3.50...(5') Here, [A], [B], [O], [N] and [Mo] respectively represent the contents of A, B, O and N expressed in mol %.

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