Catalyst, method for producing catalyst, method for producing metaborate, hydrogen generator, and fuel cell system

Abstract

[Problem] To provide: a catalyst which has high hydrogen generation efficiency; a method for producing the catalyst; a method for producing a reusable metaborate; a hydrogen generator which is provided with the catalyst; and a fuel cell system which is provided with the hydrogen generator. [Solution] According to one embodiment of the present invention, there is provided a catalyst which is used for the purpose of generating hydrogen from a borohydride salt and water, the catalyst containing a metal oxide as a main component and a metal boride which is supported by the metal oxide.

Description

Catalyst, catalyst manufacturing method, metaborate manufacturing method, hydrogen generation device, and fuel cell system

[0001] The present invention relates to a catalyst, a method for producing a catalyst, a method for producing a metaborate, a hydrogen generating apparatus, and a fuel cell system.

[0002] It has been shown that a layered double hydroxide called Green Rust (GR) can generate hydrogen from a hydrogen storage compound such as ammonia borane (see Patent Document 1). However, Patent Document 1 does not fully consider how to increase the efficiency of generating hydrogen from a hydrogen storage compound, and there is room for further improvement.

[0003] Japanese Patent Application Laid-Open No. 2022-80364

[0004] In view of the above circumstances, the present invention provides a catalyst with excellent hydrogen generation efficiency and a method for producing the same, a method for producing reusable metaborate, a hydrogen generation device including the catalyst, and a fuel cell system including the hydrogen generation device.

[0005] According to one aspect of the present invention, there is provided a catalyst for generating hydrogen from a borohydride salt and water, the catalyst containing a metal oxide as a main component and a metal boride supported on the metal oxide.

[0006] According to this embodiment, hydrogen can be efficiently generated from borohydride salt.

[0007] FIG. 1 is a schematic diagram showing the configuration of a hydrogen generation device of the present embodiment. FIG. 2 is an X-ray diffraction profile of the catalysts obtained in Examples 1 and 2. FIG. 3 is a schematic diagram showing the configuration of an evaluation device for measuring the amount of hydrogen generated. FIG. 4 is a graph showing the change over time in the hydrogen generation rate for the catalysts of Examples 1 and 2 and Comparative Examples 1 to 3. FIG. 5 is a graph showing the change over time in the hydrogen generation rate for the catalysts of Example 3, Comparative Examples 4 to 8 and the Reference Example. FIG. 6 is a diagram showing an NMR spectrum measured on a reaction solution after a hydrogen generation test using the catalyst of Example 1. FIG. 7 is a diagram showing an NMR spectrum measured on a reaction solution after a hydrogen generation test using the catalyst of the Reference Example.

[0008] Hereinafter, embodiments of the present invention will be described. Various features shown in the following embodiments can be combined with each other. The catalyst of this embodiment is a catalyst used to generate hydrogen from a borohydride salt and water. Here, examples of the borohydride salt include sodium borohydride, potassium borohydride, magnesium borohydride, lithium borohydride, and ammonia borane. These borohydride salts may be used alone or in combination of two or more.

[0009] This catalyst contains a metal oxide as a main component and a metal boride supported on the metal oxide. In this specification, the term "main component" refers to the component that is most abundant when multiple components are included, preferably at a concentration of approximately 75% by mass or more, more preferably at a concentration of approximately 85% by mass or more, and even more preferably at a concentration of approximately 90% by mass or more. Components other than the "main component" are referred to as "minor components." Therefore, in this embodiment, the metal boride can be referred to as the minor component. Note that minor components may also include components other than the metal boride. In such catalysts, the crystallite size of the metal oxide is preferably approximately 15 nm to 45 nm, more preferably approximately 20 nm to 40 nm, and even more preferably approximately 25 nm to 35 nm. Metal oxides with such an appropriate crystallite size undergo sufficient grain growth and have excellent stability. Therefore, even when the catalyst is used repeatedly, its alteration, deterioration, damage, etc. can be effectively prevented. The crystallite size of this metal oxide can be measured by X-ray diffraction (XRD).

[0010] The BET specific surface area of ​​the catalyst is 10 m ２ / g or more 65m ２ / g or less, and ２ / g or more 60m ２ / g or less is more preferable, and ２ / g or more 55m ２ / g or less is even more preferable. A catalyst having such a moderate BET specific surface area can increase the contact area with the borohydride salt (boron hydride or its ions), thereby further improving the efficiency of hydrogen generation from the borohydride salt. Note that even if the BET specific surface area of ​​the catalyst is increased beyond the above upper limit, no further increase in effect can be expected. The BET specific surface area of ​​this catalyst can be measured by the BET single-point method using nitrogen gas in accordance with JIS Z 8830:2013 "Method for measuring the specific surface area of ​​powders (solids) by gas adsorption."

[0011] It is preferable that the metal contained in the metal oxide and the metal boride are the same. In this case, the adhesion between the metal oxide and the metal boride can be increased, and therefore, even when the catalyst is used repeatedly, deterioration and damage of the catalyst due to separation of the metal boride from the metal oxide can be suitably prevented. In addition, in this case, the metal boride can be produced relatively easily by boriding the metal oxide. The metal is preferably a transition metal. Oxides of transition metals tend to grow well and have crystallite sizes within the above range. Furthermore, catalysts containing such transition metal oxides have a larger BET specific surface area, which can increase the efficiency of hydrogen generation from borohydride salts.

[0012] The transition metal is preferably at least one of metals belonging to Groups 8, 9, and 10 of the periodic table, and more preferably at least one of iron (Fe), ruthenium (Ru), cobalt (Co), rhodium (Rh), iridium (Ir), nickel (Ni), palladium (Pd), and platinum (Pt). The use of these transition metals can further enhance the above-mentioned effects. Furthermore, the metal oxides and metal borides may contain only metals with the same valence, or may contain metals with different valences.

[0013] The metal oxide preferably contains a divalent metal and a trivalent metal. In this case, the atomic ratio of the trivalent metal to the divalent metal (M ３＋ / M ２＋) is preferably about 1 or more and 6 or less, more preferably about 1.5 or more and 5 or less, and even more preferably about 2 or more and 4 or less. According to the studies of the present inventors, it has been found that a catalyst containing a large amount of trivalent metals tends to have a large BET specific surface area, and can improve the efficiency of hydrogen generation from borohydride salts. This atomic ratio can be determined by Mössbauer spectroscopy. Catalysts having the above-mentioned properties can be suitably produced by the catalyst production method described below.

[0014] Examples of such metal oxides include cobalt oxide (Co ３ O ４ ), iron oxide, etc. These metal oxides may be used alone or in combination of two or more. Examples of iron oxides include Fe ３ O ４ (Magnetite), Fe ２ O ３ (hematite), γ-Fe ２ O ３ (hegmatite), FeO(OH) nH ２ O (limonite), Fe １－ｙ Among these, the metal oxides include Co ３ O ４ and / or Fe ３ O ４ Preferably, Co ３ O ４ According to the study by the present inventors, Co ３ O ４ and / or Fe ３ O ４ It has been found that a catalyst containing as a main component and having the above crystallite size and BET specific surface area has a particularly high efficiency in generating hydrogen from a borohydride salt. Furthermore, such a catalyst is highly stable even when used repeatedly.

[0015] Such a catalyst contains a metal boride. The crystal structure of such a metal boride is more easily destabilized than the crystal structure of a metal oxide. As a result, a catalyst containing a metal boride has excellent reactivity with a borohydride salt, and the efficiency of hydrogen generation from the borohydride salt is higher. In this case, the content of the metal boride in the catalyst is preferably about 0.1 atom% or more and 10 atom% or less, more preferably about 1 atom% or more and 10 atom% or less, and even more preferably about 3 atom% or more and 10 atom% or less. This prevents the crystal structure of the catalyst as a whole from becoming more unstable than necessary.

[0016] The shape of the catalyst is not particularly limited, and examples thereof include particulate, granular, pellet, tablet, block, sheet, and fibrous shapes. Among these, the catalyst is preferably particulate. A particulate catalyst makes it easy to adjust its BET specific surface area. In this case, the volume average particle diameter of the catalyst (primary particles) is preferably about 0.1 μm or more and 10 μm or less, preferably about 2.5 μm or more and 10 μm or less, and more preferably about 5 μm or more and 10 μm or less. Here, the volume average particle diameter refers to the particle diameter (D ５０ ) refers to

[0017] Such a particulate catalyst preferably has particles (mother particles) composed of a metal oxide and a coating layer composed of a metal boride that covers at least a portion of the particles. With this configuration, many of the highly active metal borides are unevenly distributed near the surface of the catalyst, further increasing the efficiency of hydrogen generation from borohydride. In this case, the coverage of the particle surface with the coating layer is not particularly limited, but is preferably about 50% or more, more preferably 60% or more, even more preferably 70% or more, particularly preferably 80% or more, and most preferably 90% or more, and may be substantially 100%.

[0018] The above catalyst can be produced, for example, by the following catalyst production method. The method for producing the catalyst of this embodiment will be described below. The method for producing the catalyst of this embodiment includes a step (first step) of preparing a metal oxide and a borohydride salt, a step (second step) of reacting the metal oxide with the borohydride salt to convert a portion of the metal oxide into a metal boride and support the metal boride on the metal oxide to obtain a catalyst, and a step (third step) of recovering the catalyst. Each step will be described below in order.

[0019] (First Step) In this step, a metal oxide and a boron hydride salt are prepared. The metal oxide can be produced, for example, as follows. First, a raw material containing a metal compound to be converted into a metal oxide, a boron hydride salt, and a liquid medium providing a reaction field are prepared. Examples of the metal compound include layered double hydroxides (LDHs) containing a metal and having interlayer anions and interlayer water molecules, metal oxides different from the metal oxide contained in the catalyst, divalent metal salts, and trivalent metal salts. These metal compounds may be used alone or in combination. Among these, the metal compound preferably includes at least one selected from the group consisting of layered double hydroxides containing a metal and having interlayer anions and interlayer water molecules, and metal oxides different from the metal oxide contained in the catalyst. Use of these metal compounds allows for efficient production of metal oxides.

[0020] Examples of interlayer anions include carbonate ions, lactate ions, chloride ions, and sulfate ions. Among these, the interlayer anions preferably include at least one selected from the group consisting of carbonate ions and lactate ions. Such layered double hydroxides are preferably compounds represented by the following general formula (1): [where M ＩＩ is a divalent metal, M ＩＩＩ is a trivalent metal, and A ｎ－ is the interlayer anion, and H ２O is an interlayer water molecule.] Examples of the liquid medium include water (distilled water, ultrapure water, RO water, etc.), alcohols (ethanol, etc.), esters (ethyl acetate, etc.), ketones (acetone, etc.), etc. These liquid media may be used alone or in combination of two or more.

[0021] Next, the raw material is reacted with a borohydride salt in the presence of a liquid medium. This converts the metal compound to a metal oxide. The reaction temperature between the raw material and the borohydride salt is preferably about 5°C to 95°C, more preferably about 50°C to 95°C, and even more preferably about 75°C to 95°C. At such a reaction temperature, the conversion of the metal compound to a metal oxide proceeds rapidly. The reaction time between the raw material and the borohydride salt is preferably about 1 minute to 20 minutes, more preferably about 3 minutes to 15 minutes, and even more preferably about 5 minutes to 10 minutes. Within such a reaction time, the conversion of the metal compound to a metal oxide can proceed sufficiently.

[0022] The reaction between the raw material and the borohydride salt may be carried out while irradiating light. This allows the conversion of the metal compound to the metal oxide to proceed more rapidly. Examples of light include sunlight and light of a specific wavelength. The reaction ratio of the borohydride salt to the metal compound (borohydride salt / metal compound) is preferably approximately 1 to 50 in mass ratio, more preferably approximately 5 to 40, and even more preferably approximately 10 to 30. By reacting the metal compound and the borohydride salt at such a reaction ratio, a sufficient amount of metal oxide can be obtained.

[0023] Next, the liquid medium is removed from the reaction mixture to recover the metal oxide. The liquid medium can be removed by, for example, filtration, suction filtration, decantation, centrifugation, or the like. The recovered catalyst may also be washed with a cleaning solution. Examples of cleaning solutions include water (distilled water, ultrapure water, RO water, etc.), alcohols (ethanol, etc.), esters (ethyl acetate, etc.), and ketones (acetone, etc.). These cleaning solutions may be used alone or in combination. Furthermore, the metal oxide may be dried. The metal oxide can be dried by, for example, natural drying, heat drying, blow drying, vacuum drying, or the like. If necessary, the metal oxide may be pulverized. The pulverization may be performed using, for example, a jet mill, a ball mill, a hammer mill, or the like. The metal oxide may be purchased commercially rather than produced by the user. In this case, the operation in the first step may be omitted.

[0024] (Second Step) In this step, the prepared metal oxide is reacted with a borohydride salt in the presence of a liquid medium. At this time, a portion of the metal oxide is converted to a metal boride, and the metal boride is supported on the metal oxide to obtain a catalyst. The liquid medium is the same as described above. The reaction temperature between the metal oxide and the borohydride salt is preferably about 5°C or higher and 95°C or lower, more preferably about 50°C or higher and 95°C or lower, and even more preferably about 75°C or higher and 95°C or lower. At such a reaction temperature, the conversion of the metal oxide to the metal boride can be sufficiently promoted.

[0025] The reaction time between the metal oxide and the borohydride salt is preferably 1 minute or more and 20 minutes or less, more preferably 3 minutes or more and 15 minutes or less, and even more preferably 5 minutes or more and 10 minutes or less. Such a reaction time allows the conversion of the metal oxide to the metal boride to proceed sufficiently. The reaction between the metal oxide and the borohydride salt may be carried out while irradiating with light similar to that described above. This allows the conversion of the metal oxide to the metal boride to proceed more rapidly. The reaction ratio of the borohydride salt to the metal oxide (borohydride salt / metal oxide) is preferably approximately 1 to 50 in mass ratio, more preferably approximately 5 to 40 in mass, and even more preferably approximately 10 to 30 in mass. By reacting the metal oxide and the borohydride salt at such a reaction ratio, a sufficient amount of catalyst can be obtained.

[0026] Depending on the type of metal compound as a precursor of the metal oxide and the type of borohydride salt, the reactions in the above two steps may occur consecutively. In this case, the operation of recovering the metal oxide in the first step is omitted. In this case, the reaction temperature is preferably about 5°C to 95°C (about 50°C to 95°C, or about 75°C to 95°C). The reaction time is also preferably about 1 minute to 20 minutes (about 3 minutes to 15 minutes, or about 5 minutes to 10 minutes). In this case, the reaction may be performed while irradiating with light similar to that described above. Furthermore, the reaction ratio of the borohydride salt to the metal compound (borohydride salt / metal compound) is preferably about 1 to 50 (about 5 to 40, or about 10 to 30) by mass.

[0027] (Third Step) In this step, the liquid medium is removed from the post-reaction mixture in the same manner as described above, and the catalyst is recovered. The recovered catalyst may also be washed with a washing liquid. The washing liquid is the same as described above. Furthermore, the catalyst may be dried in the same manner as described above. If necessary, the catalyst may be pulverized in the same manner as described above.

[0028] Next, a method for producing a metaborate salt according to this embodiment will be described. The method for producing a metaborate salt according to this embodiment includes the steps of: preparing a borohydride salt and a catalyst (step S1); reacting the borohydride salt with water in the presence of the catalyst to generate hydrogen and obtain a by-product containing the metaborate salt (step S2); and reacting the by-product with hydrogen and a reducing agent to obtain the borohydride salt (step S3). (Step S1) In step S1, the borohydride salt and the catalyst described above are prepared.

[0029] (Step S2) In this step S2, a borohydride salt is reacted with water in the presence of a catalyst, thereby generating hydrogen and obtaining a by-product containing metaborate, as shown in the following formula (2). [wherein M is a metal or ammonium.]

[0030] The reaction temperature between the borohydride salt and water is preferably about 20°C to 100°C, more preferably about 50°C to 100°C, and even more preferably about 60°C to 90°C. By carrying out the reaction between the borohydride salt and water within this temperature range, hydrogen and metaborate can be efficiently produced. The reaction time between the borohydride salt and water is preferably about 0.5 hours to 8 hours, more preferably about 0.5 hours to 5 hours, and even more preferably about 0.5 hours to 2 hours. By carrying out the reaction between the borohydride salt and water within this time range, sufficient amounts of hydrogen and metaborate can be produced.

[0031] The amount of catalyst used is preferably about 0.1 to 20 parts by mass, more preferably about 0.5 to 15 parts by mass, and even more preferably about 1 to 10 parts by mass, relative to 100 parts by mass of borohydride. Even with such a small amount of catalyst used, a sufficient amount of hydrogen can be efficiently generated from borohydride. In contrast, in conventional methods using boric acid and water, described below, the amount of boric acid used is 0.7 or more in terms of the mass ratio of boric acid to sodium borohydride. Therefore, the method of this embodiment allows a larger amount of borohydride to be treated in a single hydrogen generation operation, making it more efficient.

[0032] (Step S3) In this step S3, the generated by-product is reacted with hydrogen and a reducing agent to obtain a borohydride salt. That is, as shown in the following formula (3), the metaborate is regenerated as a borohydride salt. Therefore, the by-product generated in the hydrogen generation operation can be reused. Examples of the reducing agent include elemental metals such as aluminum, magnesium, sodium, calcium, and silicon, and metal hydrides such as sodium hydride, magnesium hydride, and calcium hydride.

[0033] The formula (3) shows the case where aluminum is used as the reducing agent. [wherein M is a metal.]

[0034] The temperature for the reduction reaction of metaborate is preferably about 350°C or higher and 600°C or lower, and more preferably about 400°C or higher and 550°C or lower. By carrying out the reduction reaction of metaborate within this temperature range, it is possible to efficiently regenerate it into borohydride. The time for the reduction reaction of metaborate is preferably about 1 hour or higher and 3.5 hours or lower, and more preferably about 1.5 hours or higher and 3 hours or lower. By carrying out the reduction reaction of metaborate within this time range, it is possible to reliably regenerate it into borohydride.

[0035] The hydrogen gas pressure maintained in this step is preferably about 0.3 MPa or more and 10 MPa or less, and more preferably about 1 MPa or more and 10 MPa or less. This can increase the conversion efficiency to borohydride salts, and since there is no need to use expensive reaction vessels with excellent pressure resistance, an increase in equipment costs can be suppressed. Furthermore, fluoride may be added during the reduction reaction. By using fluoride, the conversion efficiency to borohydride salts can be further increased. Examples of such fluorides include lithium fluoride (LiF), sodium fluoride (NaF), potassium fluoride (KF), and aluminum fluoride (AlF ３ ), potassium aluminum fluoride (KAIF ４ ), sodium hexafluoroaluminate (Na ３ AlF ６ These may be used alone or in combination of two or more.

[0036] In response to this, Japanese Patent Application Laid-Open No. 2024-114480 discloses a technique for generating hydrogen by reacting sodium borohydride with boric acid and water, as shown in the following formula (4). In this case, borax is produced as a by-product. Therefore, in order to regenerate borax into sodium borohydride, it is necessary to first produce sodium metaborate according to the following formula (5):

[0037] As described above, the method of this embodiment requires one less step to obtain borohydride (sodium borohydride) from the by-product compared to conventional methods. This is expected to reduce costs in the production and regeneration of borohydride. It is preferable that the by-product further contains carbonate. It is believed that such carbonate is generated by the reaction of a portion of the borohydride with atmospheric carbon dioxide in step S2. If the by-product contains carbonate, the borohydride obtained when the metaborate is regenerated (converted) into borohydride also contains carbonate. Therefore, when this borohydride is used to generate hydrogen again, the interlayer anions that escape from the catalyst (layered double hydroxide) can be replenished with carbonate ions generated from the carbonate, which is believed to prevent a decrease in catalytic activity.

[0038] Next, the hydrogen generation device of this embodiment will be described. This hydrogen generation device includes a catalyst and a borohydride salt. FIG. 1 is a schematic diagram showing the configuration of the hydrogen generation device of this embodiment. The hydrogen generation device 1 shown in FIG. 1 has a cylindrical container 2 with a bottom. A partition wall 3 is provided halfway up the height of the container 2, and the space inside the container 2 is divided into an upper space 2a and a lower space 2b. A solid borohydride salt is accommodated on the partition wall 3 in the upper space 2a. A mixed liquid of a catalyst and water is accommodated in the lower space 2b. A lid 4 is attached to the container 2 so as to close the upper opening, and a piston 5 is disposed above the borohydride salt accommodated in the upper space 2a.

[0039] A compressed spring (biasing member) 6 is disposed between the lid 4 and the piston 5. As a result, the borohydride salt is biased (pressed) downward by the piston 5. The container 2 is provided with an injection port 21 communicating with the upper space 2a and an exhaust port 22 communicating with the lower space 2b. A discharge port 31 communicating with the upper space 2a is provided in the center of the partition wall 3. The injection port 21 and the lower space 2b are connected by a line L, and a filter (not shown) and a pump P are provided in this order along the line L, starting from the lower space 2b.

[0040] With this configuration, when the pump P is operated to supply water from the mixed solution from the lower space 2b to the upper space 2a via the line L and the injection port 21, the borohydride salt dissolves in the water. The water with the dissolved borohydride salt is returned to the lower space 2b via the discharge port 31 and supplied to the mixed solution. As a result, hydrogen is generated from boron hydride or its ions due to the action of the catalyst in the mixed solution. The generated hydrogen is then discharged from the hydrogen generation device 1 via the exhaust gas port 22. The reaction between the borohydride salt and the catalyst is sufficiently promoted by irradiation with sunlight, but a light source that irradiates the mixed solution contained in the lower space 2b with light of a specific frequency wavelength may be provided on the outer periphery of the container 2.

[0041] Furthermore, the reaction between the borohydride salt and the catalyst proceeds sufficiently even at room temperature (25°C), which has the advantage that hydrogen can be generated from the borohydride salt at low cost. A fuel cell system is provided by combining such a hydrogen generator 1 with a fuel cell. That is, the fuel cell system of this embodiment includes the hydrogen generator 1 and a fuel cell that generates electricity using hydrogen generated in the hydrogen generator 1. The catalyst, the method for producing the catalyst, the method for producing the metaborate salt, the hydrogen generator, and the fuel cell system have been described above, but they are not limited to the above embodiments. Furthermore, they may be provided in the following forms.

[0042] (1) A catalyst used to generate hydrogen from a borohydride salt and water, the catalyst containing a metal oxide as a main component and a metal boride supported on the metal oxide.

[0043] (2) The catalyst according to (1) above, wherein the metal contained in the metal oxide and the metal contained in the metal boride are the same.

[0044] (3) The catalyst according to (2) above, wherein the metal is a transition metal.

[0045] (4) The catalyst according to (3) above, wherein the transition metal is at least one of metals belonging to Group 8, Group 9, and Group 10 of the periodic table.

[0046] (5) The catalyst according to any one of (1) to (4) above, wherein the catalyst has particles made of the metal oxide and a coating layer made of the metal boride that coats at least a portion of the particles.

[0047] (6) The catalyst according to any one of (1) to (5) above, wherein the content of the metal boride in the catalyst is 0.1 atom % or more and 10 atom % or less.

[0048] (7) In the catalyst according to any one of (1) to (6), the crystallite size of the metal oxide is 15 nm or more and 45 nm or less, and the BET specific surface area of ​​the catalyst is 10 m ２ / g or more 65m ２ / g or less.

[0049] (8) A method for producing the catalyst according to any one of (1) to (7) above, comprising the steps of: preparing the metal oxide and a borohydride salt; and reacting the metal oxide with the borohydride salt to convert a part of the metal oxide into the metal boride and to support the metal boride on the metal oxide, thereby obtaining the catalyst.

[0050] (9) In the method for producing a catalyst according to (8) above, the reaction temperature between the metal oxide and the borohydride salt is 5°C or higher and 95°C or lower.

[0051] (10) In the method for producing a catalyst according to (8) or (9) above, the reaction time between the metal oxide and the borohydride salt is 1 minute or more and 20 minutes or less.

[0052] (11) A method for producing a metaborate, comprising the steps of: preparing a borohydride salt and the catalyst according to any one of (1) to (7) above; and reacting the borohydride salt with water in the presence of the catalyst to generate hydrogen and obtain a by-product containing the metaborate salt.

[0053] (12) The method for producing a metaborate according to (11) above, wherein the by-product further contains a carbonate.

[0054] (13) The method for producing a metaborate according to (11) or (12) above, further comprising a step of reacting the by-product with hydrogen and a reducing agent to obtain the borohydride salt.

[0055] (14) A hydrogen generating device comprising the catalyst according to any one of (1) to (7) above and a boron hydride salt.

[0056] (15) A fuel cell system comprising the hydrogen generation device according to (14) above and a fuel cell that generates electricity using hydrogen generated by the hydrogen generation device. Of course, this is not a limitation.

[0057] Finally, while various embodiments of the present disclosure have been described, they are presented as examples and are not intended to limit the scope of the invention. The novel embodiments may be embodied in various other forms, and various omissions, substitutions, and modifications may be made without departing from the spirit of the invention. Such embodiments and modifications are intended to be included within the scope and spirit of the invention, as well as within the scope of the inventions and their equivalents as defined in the claims.

[0058] The present invention will be described in more detail below using the following examples and comparative examples, but the present invention is not limited to the following examples.

[0059] 1. Preparation of catalyst (Example 1) First, 1 mole of FeCl ３ ・6H ２ 0 (trivalent iron salt), 3 moles of sodium acetate (nucleating agent), and 82 moles of glycerol (reducing solvent) were placed in a glass beaker. Then, the above components were heated at 80°C for 30 minutes in the glass beaker. This resulted in the formation of FeCl ３ ・6H ２ A mixed solution was prepared by mixing O and sodium acetate with glycerol, which was then placed in a sealed polytetrafluoroethylene container and mixed to obtain a mixed solution.

[0060] The sealed container was kept sealed, and the mixed solution was heated at 200°C for 24 hours. ３ ・6H ２ Layered double hydroxide (iron compound: metal compound) was obtained by hydrothermal synthesis of O and sodium acetate. The mixed solution after hydrothermal synthesis was then centrifuged to recover the layered double hydroxide. The recovered layered double hydroxide was then washed twice with 20 mL of ethanol, once with 20 mL of a cleaning solution made by mixing pure water and ethanol in a 1:1 volume ratio, and once with 20 mL of ethanol. The washed layered double hydroxide was then dried at 60°C for 12 hours.

[0061] Next, 10 mg of layered double hydroxide and 0.2 g of sodium borohydride were mixed to prepare a mixture. Water was supplied to this mixture at 0.8 mL / min for 1 minute, and the mixture was heated at 95°C for 10 minutes. Through these steps, a catalyst in which iron boride (metal boride) was supported on iron oxide (metal oxide) was obtained. From the results of XRD measurement, it was found that the layered double hydroxide contained Fe ３ O ４ (Fe ３＋ / Fe ２＋ Furthermore, the results of XPS and Mössbauer spectroscopy showed that Fe ３ O４ It was confirmed that FeB was supported on the surface of the catalyst.

[0062] (Example 2) Iron oxide (metal oxide) ２ O ３ A mixture was prepared by mixing 10 mg of iron boride powder (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., "Wako 1st Grade") with 0.2 g of sodium borohydride. Water was supplied to this mixture at a rate of 0.8 mL / min for 1 minute, and the mixture was heated at 95°C for 10 minutes. Through these steps, a catalyst in which iron boride (metal boride) was supported on iron oxide (metal oxide) was obtained. No diffraction peaks were observed in the XRD measurement. Furthermore, the results of the XPS measurement showed that Fe ２ O ３ It was confirmed that FeB was supported on the surface of the catalyst.

[0063] (Example 3) Cobalt oxide (metal oxide) ３ O ４ A mixture was prepared by mixing 10 mg of cobalt boride powder (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., "Practical Grade") with 0.2 g of sodium borohydride. Water was supplied to this mixture at 0.8 mL / min for 1 minute, and the mixture was heated at 95°C for 10 minutes. Through these steps, a catalyst in which cobalt boride (metal boride) was supported on cobalt oxide (metal oxide) was obtained. No diffraction peaks were observed in the XRD measurement. Furthermore, the results of the XPS measurement showed that Co ３ O ４ It was confirmed that CoB was supported on the surface of the catalyst.

[0064] (Comparative Example 1) Fe powder (manufactured by Kojundo Chemical Laboratory Co., Ltd., "FEE12PB") was prepared as elemental iron (elemental metal). This Fe powder was used as a catalyst. (Comparative Example 2) FeO powder (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., "091-06411") was prepared as iron oxide (metal oxide). This FeO powder was used as a catalyst. (Comparative Example 3) Fe ３ O ４ Powder (manufactured by MERCK, "powder, <5 μm, 95%") was prepared. ３ O ４The powder was used as a catalyst.

[0065] (Comparative Example 4) MnO powder (manufactured by Kojundo Chemical Laboratory, "MNO01PB") was prepared as manganese oxide (metal oxide). This MnO powder was used as a catalyst. (Comparative Example 5) MnO ２ Powder (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., "135-09685") was prepared. ２ The powder was used as a catalyst.

[0066] (Comparative Example 6) NiO powder (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., "140-01152") was prepared as nickel oxide (metal oxide). This NiO powder was used as a catalyst. (Comparative Example 7) CuO powder (manufactured by Kojundo Chemical Laboratory, "CUO12PB") was prepared as copper oxide (metal oxide). This CuO powder was used as a catalyst. (Comparative Example 8) Fe ２ O ３ Fe powder (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., "Wako 1st Grade") was prepared. ２ O ３ The powder was used as a catalyst.

[0067] (Reference Example) First, 1 mole of FeCl ２ ・4H ２ 0 (divalent iron salt), 3 moles of sodium acetate (nucleating agent), and 82 moles of glycerol (reducing solvent) were prepared. Next, these were charged into a sealed container made of polytetrafluoroethylene and mixed to obtain a mixed solution. ２ ・4H ２ The O was crushed in a mortar before use. The sealed container was sealed, and the mixed solution was heated at 200°C for 24 hours. This resulted in the FeCl ２ ・4H ２ Layered double hydroxide was obtained by hydrothermal synthesis of 0 and sodium acetate.

[0068] In the Reference Example, this layered double hydroxide was used as a catalyst. The mixed solution after hydrothermal synthesis was centrifuged to recover the catalyst. The recovered catalyst was then washed twice with 20 mL of ethanol, once with 20 mL of a cleaning solution made by mixing pure water and ethanol in a 1:1 volume ratio, and once with 20 mL of ethanol. Finally, the washed catalyst was dried at 80°C for 24 hours.

[0069] 2. Measurement and Evaluation 2-1. XRD Measurement XRD measurements were performed on the catalysts of each Example and Comparative Example. The XRD measurements were performed using an XRD measurement device (Rigaku Corporation, "Ulitima IV"). The crystallite sizes calculated based on the full width at half maximum of the peaks observed in the XRD measurements are shown in Table 1. Note that Figure 2 shows the X-ray diffraction profiles of the catalysts obtained in Examples 1 and 2.

[0070] 2-2. BET Specific Surface Area The BET specific surface area of ​​the catalysts of each example and each comparative example was measured. The BET specific surface area was measured in accordance with JIS Z 8830:2013 "Method for measuring the specific surface area of ​​powder (solid) by gas adsorption." The BET specific surface areas are shown in Table 1 below.

[0071] 2-3. Particle size distribution The particle size distribution of the catalysts of each example and each comparative example was measured by a laser diffraction / scattering method. The measurement was carried out in accordance with the method described in JIS Z 8825:2013 "Particle size analysis - Laser diffraction / scattering method." In the particle size distribution (cumulative distribution) shown with the horizontal axis representing particle size [μm] on a logarithmic scale and the vertical axis representing frequency [volume %], the particle size at which the cumulative value from the smallest particle size reached 50% of the total was defined as the volume average particle size of the primary particles (D ５０ The volume average particle diameter is shown in Table 1.

[0072] 2-4. Hydrogen Generation Test First, using the evaluation apparatus shown in FIG. 3, 10 mg of the catalyst of each Example, Comparative Example, and Reference Example and 0.2 g of sodium borohydride (SBH) were placed in a flask (reaction vessel) to prepare a mixture. Next, for the catalysts of Examples 1 and 2 and Comparative Examples 1 to 3, the mixture was heated at 95°C for 5 minutes, after which water was supplied at 0.8 mL / min for 1 minute, and the mixture was heated at 95°C for 10 minutes. On the other hand, for the catalysts of Example 3, Comparative Examples 4 to 8, and the Reference Example, the mixture was heated at 65°C for 5 minutes, after which water was supplied at 0.8 mL / min for 1 minute, and the mixture was heated at 65°C for 10 minutes. The amount of hydrogen generated was then measured over time using the water drainage method. Specifically, hydrogen generated in the flask was supplied to a water tank. Then, water was supplied from the water tank to a beaker placed on a balance according to the amount of hydrogen supplied to the water tank. The amount of water was measured using a balance and analyzed using a PC connected to the balance, thereby determining the amount of hydrogen generated. The results are shown in Figure 4. Figure 4 is a graph showing the change over time in the hydrogen generation rate for the catalysts of Examples 1 and 2 and Comparative Examples 1 to 3. Figure 5 is a graph showing the change over time in the hydrogen generation rate for the catalysts of Example 3, Comparative Examples 4 to 8, and the Reference Example.

[0073] 2-5. XPS Measurement The catalysts of each example and comparative example were subjected to measurement by photoelectron spectroscopy (XPS) using an X-ray photoelectron spectrometer ("K-Alpha" model, manufactured by Thermo Chemical Co., Ltd.). The content of metal boride in the catalysts obtained in Examples 1 and 2 was determined.

[0074] 2-6. Mössbauer Spectroscopy Measurement Mössbauer spectroscopy was performed on the catalysts of Examples 1 and 2. In the Mössbauer spectroscopy measurement, approximately 50 mg of each catalyst was kneaded with silicone vacuum grease and sandwiched between high-purity Al foils (area 15 mm x 15 mm), and the Mössbauer spectroscopy was performed at room temperature by a transmission method. ５７ The Fe Mössbauer spectrum was measured.

[0075] 2-7. Confirmation of By-Products NMR measurements were carried out on the reaction liquids after the hydrogen generation tests using the catalysts of Example 1 and Reference Example. １ H-NMR measurement, １３C-NMR measurement, and １１ For the B-NMR measurements, an "AVANCE III 500" manufactured by Bruker BioSpin was used. The results are shown in Figures 6 and 7. Figure 6 shows the NMR spectrum measured on the reaction solution after a hydrogen generation test using the catalyst of Example 1. Figure 7 shows the NMR spectrum measured on the reaction solution after a hydrogen generation test using the catalyst of the Reference Example.

[0076]

[0077] It was confirmed that the activity of the catalysts of each Example was clearly higher than that of the catalysts of each Comparative Example. ２ O ３ It is believed that the activity of the catalysts (powder) increased over time due to the formation of metal borides on the surface during the hydrogen generation test. The catalysts of each example maintained sufficient activity even after the hydrogen generation test was repeated five times. The boron content in the catalysts of each example was 0.1 atom % or more and 10 atom % or less, whereas the boron content in the catalysts of each comparative example was less than 0.1 atom %.

[0078] Furthermore, as shown in Figure 6, it was confirmed that the main component of the by-product in the catalyst of Example 1 was sodium metaborate. Because this by-product contains very few impurities, it can be regenerated into sodium borohydride and reused for hydrogen generation. On the other hand, as shown in Figure 7, although the main component of the by-product in the catalyst of the Reference Example was sodium metaborate, many impurities were confirmed. Because this by-product contains many impurities, it is not suitable for regenerating into sodium borohydride and reused for hydrogen generation.

[0079] 1: Hydrogen generator, 2: Container, 2a: Upper space, 2b: Lower space, 21: Injection port, 22: Exhaust gas port, 3: Partition wall, 31: Exhaust port, 4: Lid, 5: Piston, 6: Spring, L: Line, P: Pump

Claims

1. A catalyst used to generate hydrogen from a borohydride salt and water, the catalyst containing a metal oxide as a main component and a metal boride supported on the metal oxide.

2. The catalyst according to claim 1, wherein the metal contained in the metal oxide and the metal contained in the metal boride are the same.

3. The catalyst according to claim 2, wherein the metal is a transition metal.

4. The catalyst according to claim 3, wherein the transition metal is at least one of metals belonging to Group 8, Group 9, and Group 10 of the periodic table.

5. A catalyst according to any one of claims 1 to 4, comprising particles made of the metal oxide and a coating layer made of the metal boride that coats at least a portion of the particles.

6. The catalyst according to any one of claims 1 to 5, wherein the content of the metal boride in the catalyst is 0.1 atom % or more and 10 atom % or less.

7. The catalyst according to any one of claims 1 to 6, wherein the crystallite size of the metal oxide is 15 nm or more and 45 nm or less, and the BET specific surface area of ​​the catalyst is 10 m ２ / g or more 65m ２ / g or less.

8. A method for producing the catalyst according to any one of claims 1 to 7, comprising the steps of: preparing the metal oxide and a boron hydride salt; and reacting the metal oxide with the boron hydride salt to convert a portion of the metal oxide into the metal boride and to support the metal boride on the metal oxide, thereby obtaining the catalyst.

9. The method for producing a catalyst according to claim 8, wherein the reaction temperature between the metal oxide and the borohydride salt is 5°C or higher and 95°C or lower.

10. The method for producing a catalyst according to claim 8 or 9, wherein the reaction time between the metal oxide and the borohydride salt is 1 minute or more and 20 minutes or less.

11. A method for producing a metaborate, comprising the steps of: preparing a borohydride salt and the catalyst according to any one of claims 1 to 7; and reacting the borohydride salt with water in the presence of the catalyst to generate hydrogen and obtain a by-product containing the metaborate salt.

12. The method for producing metaborate according to claim 11, wherein the by-product further comprises carbonate.

13. The method for producing a metaborate according to claim 11 or 12, further comprising the step of reacting the by-product with hydrogen and a reducing agent to obtain the borohydride salt.

14. A hydrogen generating device comprising: the catalyst according to any one of claims 1 to 7; and a borohydride salt.

15. A fuel cell system comprising: the hydrogen generation device according to claim 14; and a fuel cell that generates electricity using hydrogen generated by the hydrogen generation device.

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