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

Abstract

[Problem] To provide: a catalyst having excellent hydrogen generation efficiency; a method for producing the same; a hydrogen generator comprising the catalyst; and a fuel cell system comprising the hydrogen generator. [Solution] According to one aspect of the present invention, provided is a catalyst used to generate hydrogen from a borohydride salt. The catalyst contains, as a main component, a layered double hydroxide having interlayer anions and interlayer water molecules and including iron. The BET specific surface area is 40 m2 / g or more.

Description

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

[0001] The present invention relates to a catalyst, a method for producing the catalyst, a hydrogen generating device, 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 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. The catalyst contains, as a main component, an iron-containing layered double hydroxide having an interlayer anion and interlayer water molecules. The BET specific surface area is 40 m. ２ / g or more.

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

[0007] 1 is a process diagram showing an embodiment of a method for producing a catalyst of the present invention. FIG. 2 is a schematic diagram showing the configuration of an embodiment of a hydrogen generation device of the present invention.

[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 the present invention is a catalyst used to generate hydrogen from a borohydride salt. 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, as a main component, a layered double hydroxide (LDH) that has interlayer anions and interlayer water molecules and contains iron. A catalyst having such a composition can efficiently generate hydrogen from a borohydride salt. In this specification, the term "main component" refers to the component that is contained in the largest amount when multiple components are contained. Components other than the "main component" are referred to as "minor components." The catalyst may consist solely of the layered double hydroxide that is the main component, or it may contain minor components different from the main component.

[0010] The BET specific surface area of ​​the catalyst is 40 m ２ / g or more, and ２ / g or more 200m ２ / g or less, and ２ / g or more 180m ２ / g or less is more preferable, and ２ / g or more 160m ２ / g or less is even more preferable. Thus, a catalyst having a high 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, and it takes time and effort to produce a catalyst having such a large BET specific surface area. 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] The crystallite size of the layered double hydroxide is preferably about 20 nm to 70 nm, more preferably about 30 nm to 60 nm, and even more preferably about 35 nm to 50 nm. Layered double hydroxides with such relatively large crystallite sizes undergo sufficient particle growth and are highly stable. Therefore, even if the catalyst is used repeatedly, it can be effectively prevented from altering, deteriorating, or being damaged. The crystallite size of this layered double hydroxide can be measured by X-ray diffraction (XRD).

[0012] The layered double hydroxide contains trivalent iron and divalent iron. The atomic ratio of trivalent iron to the sum of trivalent iron and divalent iron is preferably approximately 0.75 or less, more preferably approximately 0.4 to 0.7, and even more preferably approximately 0.5 to 0.6. According to the inventors' studies, layered double hydroxides containing a relatively large amount of divalent iron form a structure in which fine spherical particles are attached to the surfaces of columnar particles. This increases the BET specific surface area of ​​the catalyst, thereby improving the efficiency of hydrogen generation from borohydride salts. This atomic ratio can be determined by Mössbauer spectroscopy. A catalyst having the above BET specific surface area and atomic ratio can be suitably produced by the catalyst production method described below.

[0013] Examples of interlayer anions include carbonate ions, lactate ions, chloride ions, and sulfate ions. Among these, the interlayer anions preferably include at least one of carbonate ions and lactate ions. These interlayer anions are formed by the reaction of boron hydride (BH) generated from boron hydride salts. ３ : in the case of ammonia borane) or its ion (BH ４ － The layered double hydroxide is easily exchanged with boron hydride or its ions between layers where water molecules exist (e.g., sodium borohydride), and thus boron hydride or its ions can be easily incorporated between layers where water molecules exist. This further increases the efficiency of hydrogen generation through the hydrolysis reaction (reaction with water molecules) of boron hydride or its ions. The layered double hydroxide is preferably a compound represented by the general formula: [Fe ＩＩ １－ｘ Fe ＩＩＩ ｘ(OH) ２ ] ｘ＋ (x / n) [A ｎ－ ] ｘ－ ・m [H ２ O] (where A ｎ－ is the interlayer anion, and H ２ O is an interlayer water molecule.

[0014] The above catalysts are produced by the following catalyst production method. One embodiment of the catalyst production method of the present invention will be described below. FIG. 1 is a process diagram showing one embodiment of the catalyst production method of the present invention. The catalyst production method shown in FIG. 1 includes a step of preparing raw materials containing a divalent iron salt (step S1), a step of preparing a mixed solution by mixing the raw materials (step S2), a step of obtaining a catalyst by hydrothermal synthesis of the raw materials in the mixed solution (step S3), a step of recovering the catalyst by removing the solvent (step S4), a step of washing the catalyst (step S5), and a step of drying the catalyst (step S6). Each step will be described in order below.

[0015] (Step S1) In this step S1, a divalent iron salt is prepared as a raw material for the catalyst, as well as a nucleating agent and a reducing solvent. Here, various salts containing divalent iron can be used as the divalent iron salt. Among them, the divalent iron salt is FeCl ２ , Fe(NO ３ ) ２ , FeSO ４ and Fe(CH ３ COO) ２ It is preferable that the divalent iron salt is at least one selected from the group consisting of: The use of such a divalent iron salt facilitates the production of a layered double hydroxide (catalyst) that is stable and suffers little deterioration over time. Furthermore, although the reason is not clear, investigations by the present inventors have shown that the BET specific surface area of ​​the catalyst tends to increase. Furthermore, it is preferable that the divalent iron salt is a hydrate. The use of such a divalent iron salt facilitates the production of a layered double hydroxide (catalyst) having the above-mentioned properties.

[0016] The nucleating agent is not particularly limited, but examples thereof include sodium acetate (CH ３ COONa), potassium acetate (CH ３COOK), sodium acetate trihydrate (CH ３ COONa 3H ２ It is preferable to use at least one selected from the group consisting of ammonium nitrate, ...

[0017] (Step S2) In this step S2, a divalent iron salt, a nucleating agent, and a reducing solvent are mixed substantially simultaneously to prepare a mixed solution. Mixing the divalent iron salt and the nucleating agent separately into a solvent requires preheating before hydrothermal synthesis. However, since the divalent iron salt and the nucleating agent are mixed into the solvent at once, hydrothermal synthesis can be performed without preheating, thereby reducing the number of steps. Because the divalent iron salt and the nucleating agent are not dissolved but remain suspended in the mixed solution, the mixed solution is also referred to as a suspension. (Step S3) In this step S3, raw materials are reacted in the mixed solution by hydrothermal synthesis to obtain a catalyst. Here, the hydrothermal synthesis is preferably performed while the mixed solution is contained in a sealed container. In this case, continuous oxygen supply from the external environment to the mixed solution can be prevented. As a result, the amount of divalent iron oxidized to trivalent iron can be reduced. This makes it possible to make the catalyst particles finer, thereby increasing the BET specific surface area and also increasing the amount of divalent iron contained in the catalyst.

[0018] The temperature in the hydrothermal synthesis is preferably about 50°C to 350°C, more preferably about 150°C to 250°C, and even more preferably about 150°C to 250°C. By carrying out the hydrothermal synthesis within this temperature range, it is possible to efficiently produce a layered double hydroxide (catalyst), and to prevent structural changes in the layered double hydroxide and alteration or deterioration of the solvent, thereby producing a high-quality layered double hydroxide. The time period for the hydrothermal synthesis is preferably about 0.5 hours to 50 hours, more preferably about 1 hour to 40 hours, and even more preferably about 1.5 hours to 30 hours. By carrying out the hydrothermal synthesis within this time period, a sufficient amount of layered double hydroxide (catalyst) can be produced.

[0019] (Step S4) In this step S4, the solvent is removed from the mixed solution after hydrothermal synthesis to recover the catalyst. The solvent can be removed, for example, by filtration, suction filtration, decantation, centrifugation, etc. (Step S5) In this step S5, the recovered catalyst is washed with a washing liquid. Examples of washing liquids include water (distilled water, ultrapure water, RO water, etc.), alcohols (ethanol, etc.), and ketones (acetone, etc.). These may be used alone or in combination of two or more. (Step S6) In this step S6, the catalyst is dried. The catalyst can be suitably dried, for example, by natural drying, heat drying, blow drying, vacuum drying, etc.

[0020] Next, an embodiment of the hydrogen generation device of the present invention will be described. This hydrogen generation device includes a catalyst and a borohydride salt. FIG. 2 is a schematic diagram showing the configuration of an embodiment of the hydrogen generation device of the present invention. The hydrogen generation device 1 shown in FIG. 2 has a cylindrical container 2 with a bottom. A partition wall 3 is provided midway along 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 contained on the partition wall 3 in the upper space 2a. A mixed liquid of a catalyst and water is contained 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 contained in the upper space 2a.

[0021] 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.

[0022] 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.

[0023] 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. The fuel cell system of the present invention is provided by combining such a hydrogen generator 1 with a fuel cell. That is, one embodiment of the fuel cell system of the present invention comprises the hydrogen generator 1 and a fuel cell that generates electricity using hydrogen generated in the hydrogen generator 1. While the catalyst, catalyst manufacturing method, hydrogen generator, and fuel cell system of the present invention have been described above, the catalyst, catalyst manufacturing method, hydrogen generator, and fuel cell system of the present invention are not limited to the above embodiments. Furthermore, the catalyst may be provided in the following forms.

[0024] (1) A catalyst used to generate hydrogen from borohydride salts, which has interlayer anions and interlayer water molecules and contains, as a main component, a layered double hydroxide containing iron, and has a BET specific surface area of ​​40 m ２ / g or more.

[0025] (2) The catalyst according to (1) above, wherein the crystallite size of the layered double hydroxide is 20 nm or more and 70 nm or less.

[0026] (3) The catalyst according to (1) or (2) above, wherein the layered double hydroxide contains trivalent iron and divalent iron, and the atomic ratio of the trivalent iron to the sum of the trivalent iron and the divalent iron is 0.75 or less.

[0027] (4) The catalyst according to any one of (1) to (3) above, wherein the interlayer anions include at least one of carbonate ions and lactate ions.

[0028] (5) A method for producing the catalyst according to any one of (1) to (4) above, comprising the steps of preparing raw materials containing a divalent iron salt, preparing a mixed solution by mixing the raw materials, and obtaining the catalyst by hydrothermal synthesis of the raw materials in the mixed solution.

[0029] (6) In the method for producing a catalyst according to (5) above, the hydrothermal synthesis is carried out in a state where the mixed solution is contained in a sealed container.

[0030] (7) The method for producing a catalyst according to (5) or (6) above, wherein the temperature in the hydrothermal synthesis is 50°C or higher and 350°C or lower.

[0031] (8) The method for producing a catalyst according to any one of (5) to (7) above, wherein the time for the hydrothermal synthesis is 0.5 hours or more and 50 hours or less.

[0032] (9) In the method for producing a catalyst according to any one of (5) to (8) above, the divalent iron salt is FeCl ２ , Fe(NO ３ ) ２ , FeSO ４ and Fe(CH ３ COO) ２ A method for producing a catalyst, wherein the catalyst is at least one selected from the group consisting of:

[0033] (10) The method for producing a catalyst according to (9) above, wherein the divalent iron salt is a hydrate.

[0034] (11) A hydrogen generating device comprising the catalyst according to any one of (1) to (4) above and a boron hydride salt.

[0035] (12) A fuel cell system comprising the hydrogen generation device according to (11) above and a fuel cell that generates electricity using hydrogen generated by the hydrogen generation device. Of course, this is not a limitation.

[0036] 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 invention and its equivalents as defined in the claims.

[0037] 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.

[0038] 1. Preparation of catalyst (Example 1) 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 O was ground in a mortar before use.

[0039] The sealed container was kept sealed, and the mixed solution was heated at 200°C for 24 hours. ２ ・4H ２ A layered double hydroxide (catalyst) was obtained by hydrothermal synthesis of 0 and sodium acetate. The mixed solution after hydrothermal synthesis was then 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 at a volume ratio of 1:1, and once with 20 mL of ethanol. Finally, the washed catalyst was dried at 80°C for 24 hours. Through these steps, the catalyst of Example 1 was obtained.

[0040] (Example 2) The catalyst of Example 2 was produced in the same manner as in Example 1, except that the recovered catalyst was washed with 20 mL of ethanol four times and then dried under vacuum. (Example 3) The catalyst of Example 3 was produced in the same manner as in Example 1, except that the washed catalyst was dried under vacuum.

[0041] (Example 4) A catalyst of Example 4 was produced in the same manner as in Example 1, except that the mixed solution was heated at 100°C. (Example 5) A catalyst of Example 5 was produced in the same manner as in Example 1, except that the mixed solution was heated at 150°C.

[0042] Example 6 A catalyst of Example 6 was produced in the same manner as in Example 1, except that the mixed solution was heated for 12 hours. Example 7 A catalyst of Example 7 was produced in the same manner as in Example 1, except that the mixed solution was heated for 6 hours.

[0043] (Comparative Example) First, 3 moles of sodium acetate (nucleating agent) and 82 moles of glycerol (reducing solvent) were placed in a glass beaker. Next, the above components were heated in the glass beaker at 80°C for 30 minutes. This mixed sodium acetate with glycerol to prepare a mixed solution. Next, 0.25 moles of FeCl ３ ・6H ２ O (iron trivalent salt), and 0.75 mol of FeCl ２ ・4H ２ O (divalent iron salt) was mixed into the mixture and heated at 80°C for 90 minutes. ３ ・6H ２ O and FeCl ２ ・4H ２ O was also mixed with glycerol. Thereafter, a catalyst of the comparative example was produced in the same manner as in Example 1.

[0044] 2. Measurement and Evaluation 2-1. BET Specific Surface Area The BET specific surface area of ​​the catalysts obtained in each Example and 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 Tables 1 and 2 below.

[0045] 2-2. XRD Measurement XRD measurement was performed on the catalysts obtained in each Example and Comparative Example. The XRD measurement was 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 this XRD measurement are shown in Tables 1 and 2.

[0046] 2-3. Measurement of iron atomic ratio The catalysts (layered double hydroxides) obtained in each example and comparative example were subjected to Mössbauer spectroscopy. ５７ The Mossbauer spectrum of Fe was measured. The atomic ratio of trivalent iron to the total of trivalent iron and divalent iron estimated by this Mossbauer spectroscopy is shown in Tables 1 and 2.

[0047] 2-4. SEM Photography SEM photography was performed on the catalysts obtained in each of the Examples and Comparative Examples. SEM photography was performed using a scanning electron microscope (manufactured by JEOL, "JSM-7001F / SHL") at a magnification of 30,000 times to obtain SEM images. These SEM images are shown in Tables 1 and 2.

[0048] 2-5. Hydrogen Generation Test First, 15 mg of the catalyst obtained in each Example and Comparative Example, 5 mL of a 20% by mass aqueous glycerol solution, and 20 μmol of ammonia borane were placed in a test tube and mixed. Next, the test tube was left to stand in an environment where the mixture in the test tube was irradiated with simulated sunlight (solar simulator). The amount of hydrogen generated over a 120-minute period after the start of standing was measured. The amount of hydrogen generated was measured by gas chromatography analysis. Furthermore, when no catalyst was used, almost no hydrogen was generated, and the amount of hydrogen generated was essentially 0 μmol. The results are also shown in Tables 1 and 2.

[0049]

[0050]

[0051] As shown in the SEM images in Tables 1 and 2, the catalysts of each example formed a structure in which fine spherical particles were attached to the surfaces of columnar particles. In contrast, the catalysts of the comparative examples formed a structure of columnar particles with no fine spherical particles attached to the surfaces. It was also confirmed that the amount of hydrogen generated changed by adjusting the BET specific surface area.

[0052] 1: Hydrogen generation device 2: Container 2a: Upper space 2b: Lower space 21: Injection port 22: Exhaust gas port 3: Partition wall 31: Discharge port 4: Lid 5: Piston 6: Spring L: Line P: Pump S1: Step S2: Step S3: Step S4: Step S5: Step S6: Step

Claims

1. A catalyst used to generate hydrogen from borohydride salts, which has interlayer anions and interlayer water molecules, contains iron-containing layered double hydroxide as a main component, and has a BET specific surface area of ​​40 m ２ / g or more.

2. The catalyst according to claim 1, wherein the crystallite size of the layered double hydroxide is 20 nm or more and 70 nm or less.

3. A catalyst according to claim 1 or 2, wherein the layered double hydroxide contains trivalent iron and divalent iron, and the atomic ratio of the trivalent iron to the sum of the trivalent iron and the divalent iron is 0.75 or less.

4. The catalyst according to any one of claims 1 to 3, wherein the interlayer anion includes at least one of a carbonate ion and a lactate ion.

5. A method for producing the catalyst according to any one of claims 1 to 4, comprising the steps of: preparing raw materials containing a divalent iron salt; preparing a mixed liquid by mixing the raw materials; and obtaining the catalyst by hydrothermal synthesis of the raw materials in the mixed liquid.

6. The method for producing a catalyst according to claim 5, wherein the hydrothermal synthesis is carried out in a state where the mixed liquid is contained in a sealed container.

7. The method for producing a catalyst according to claim 5 or 6, wherein the temperature in the hydrothermal synthesis is 50°C or higher and 350°C or lower.

8. The method for producing a catalyst according to any one of claims 5 to 7, wherein the time for the hydrothermal synthesis is from 0.5 hours to 50 hours.

9. The method for producing a catalyst according to any one of claims 5 to 8, wherein the divalent iron salt is FeCl ２ , Fe(NO ３ ) ２ , FeSO ４ and Fe(CH ３ COO) ２ The method for producing a catalyst, wherein the catalyst is at least one selected from the group consisting of:

10. The method for producing a catalyst according to claim 9, wherein the divalent iron salt is a hydrate.

11. A hydrogen generating apparatus comprising: a catalyst according to any one of claims 1 to 4; and a borohydride salt.

12. A fuel cell system comprising: the hydrogen generation device according to claim 11; and a fuel cell that generates electricity using hydrogen generated by the hydrogen generation device.

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