Hydrogen production method, hydrogen production device, fuel cell and power generation system

Abstract

To provide a hydrogen production method, etc. in which ammonia is reduced.SOLUTION: A first aspect of the invention is a hydrogen production method that produces hydrogen by a hydrolysis reaction of ammonia borane, the hydrogen production method including trapping ammonia generated by the hydrolysis reaction of ammonia borane as ammonium ion with a substance having pH buffer action. A second aspect of the invention lies in the hydrogen production method according to the claim 1 including trapping ammonia into an ammonia borane solution as ammonium ion. A third aspect of the invention lies in the hydrogen production method according to the claim 1 where the substance having pH buffer action is selected from a group consisting of a phosphate buffer agent, an acetate buffer agent, and a citrate buffer agent.SELECTED DRAWING: Figure 6(c)

Description

[Technical Field]

[0001] The present invention relates to a hydrogen production method, a hydrogen production device, a fuel cell, and a power generation system. [Background technology]

[0002] Hydrogen is used in a variety of fields, including energy. For example, in addition to industrial uses such as oil refining and petroleum product manufacturing, semiconductors, food, and metal processing, its applications are increasing in a variety of fields, including fuel cells, automotive fuels, and batteries. In particular, to further realize a hydrogen energy society, it is necessary to establish technologies for producing, storing, transporting, and utilizing hydrogen.

[0003] Because hydrogen is a gas, it is difficult to transport large quantities at once when transporting it from the place where it is produced to various places where it is used, which are far away. Also, when storing hydrogen, high-pressure tanks are required to store large amounts of gas. Storing it as a liquid requires cooling it to ultra-low temperatures, which requires a lot of energy. Also, while transporting it via pipelines is used, this requires the construction of large-scale infrastructure.

[0004] Rather than storing or transporting hydrogen itself, methods are being considered in which liquid or solid compounds that carry hydrogen are produced from gaseous hydrogen and then stored or transported. By changing the form to a gas or solid, the volume can be reduced, making transportation and storage easier. When the hydrogen-carrying compound arrives at its destination, it is stored in a storage container, or the hydrogen is extracted and used at an appropriate time. When separating hydrogen from the hydrogen-carrying compound, various methods are selected, such as using a catalyst. As means of transportation, various facilities are used as needed, including transportation means such as cars, trains, ships, and pipelines.

[0005] In addition to the above-mentioned fields, the hydrogenation of compounds can be applied in various fields. Hydrogen-carrying compounds are very useful and can be used in various fields such as pharmaceuticals, textiles, food, electricity, and machinery, in addition to the above-mentioned applications. Therefore, various methods have been proposed so far. However, it cannot be said that they have been fully developed.

[0006] Compounds that can carry hydrogen and from which hydrogen can be easily extracted include ammonia borane (NH3BH3), ammonia (NH3), and methylcyclohexane (C6H 11 Known examples include organic hydrides such as HCl (CH3), and formic acid (HCOOH).

[0007] In particular, the hydrolysis reaction of ammonia borane can extract hydrogen from stably stored hydrogen storage materials without the need for a heat source, and is therefore expected to be put to practical use as a hydrogen source for portable fuel cells and emergency power sources. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Patent Publication No. 2018-103158 Summary of the Invention [Problem to be solved by the invention]

[0009] However, the ammonia produced along with hydrogen in the hydrolysis reaction of ammonia borane poisons the fuel electrode of the fuel cell, causing irreversible damage (deterioration), so it is required that "the ammonia content in hydrogen for fuel cells be 0.1 ppm or less" (ISO / TS14687-2).

[0010] The present invention has been made in view of the above-mentioned background art, and has an object to provide a hydrogen production method that reduces ammonia. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention employs the configurations set forth in the claims. The present invention will be described in detail below.

[0012] A first aspect of the present invention is A method for producing hydrogen by a hydrolysis reaction of ammonia borane, comprising: A method of producing hydrogen in which ammonia generated by the hydrolysis reaction of ammonia borane is trapped as ammonium ions using a substance with pH buffering properties. is located.

[0013] A second aspect of the present invention is 2. The method for producing hydrogen according to claim 1, wherein ammonia is trapped as ammonium ions in the ammonia borane solution. is located.

[0014] A third aspect of the present invention is 2. The method for producing hydrogen according to claim 1, wherein the substance having a pH buffering effect is selected from the group consisting of a phosphate buffer, an acetate buffer, and a citrate buffer. is located.

[0015] A fourth aspect of the present invention is 2. The method for producing hydrogen according to claim 1, wherein the substance having a pH buffering effect is a phosphate buffer. is located.

[0016] A fifth aspect of the present invention is 5. The method for producing hydrogen according to claim 4, wherein the phosphate buffer is a mixture of sodium dihydrogen phosphate and dipotassium hydrogen phosphate. is located.

[0017] A sixth aspect of the present invention is 5. The method for producing hydrogen according to claim 4, wherein the amount of the phosphate buffer is 1.05 equivalents or more and 1.10 equivalents or less relative to the amount of ammonia borane. is located.

[0018] A seventh aspect of the present invention is 5. The method for producing hydrogen according to claim 4, wherein the phosphate buffer contains potassium dihydrogen phosphate in an amount greater than 1.00 equivalents relative to ammonia borane. is located.

[0019] An eighth aspect of the present invention is 2. The method for producing hydrogen according to claim 1, wherein the release of ammonia is suppressed by contacting the aqueous ammonia borane solution with a solid phosphate buffer. is located.

[0020] A ninth aspect of the present invention is 2. The method for producing hydrogen according to claim 1, wherein the release of ammonia is suppressed by using a phosphate buffer in an amount of 1.00 equivalents or less relative to ammonia borane and an ammonia removal filter. is located.

[0021] A tenth aspect of the present invention is 2. The method for producing hydrogen according to claim 1, wherein after dissolving a substance having a pH buffering effect in an aqueous solution of ammonia borane, a powdered substance having a pH buffering effect is added to the aqueous solution of ammonia borane to initiate a hydrolysis reaction of ammonia borane. is located.

[0022] An eleventh aspect of the present invention is 11. The method for producing hydrogen according to claim 10, wherein the aqueous ammonia borane solution is a saturated ammonia borane solution. is located.

[0023] A twelfth aspect of the present invention is 5. The method for producing hydrogen according to claim 4, wherein a catalyst that promotes the hydrolysis reaction of ammonia borane is brought into contact with an aqueous solution in which ammonia borane and a phosphate buffer are dissolved. is located.

[0024] A thirteenth aspect of the present invention is 5. The method for producing hydrogen according to claim 4, wherein the aqueous solution of ammonia borane is contacted with a solid mixture of a catalyst that promotes the hydrolysis reaction of ammonia borane and a phosphate buffer. is located.

[0025] A fourteenth aspect of the present invention is A method for producing hydrogen in which ammonia generated by the hydrolysis reaction of ammonia borane is trapped as ammonium ions by partially dissolving a phosphate buffer in a saturated solution of ammonia borane. is located.

[0026] A fifteenth aspect of the present invention is A hydrogen production apparatus for producing hydrogen by a hydrolysis reaction of ammonia borane, A hydrogen production device that traps ammonia generated by the hydrolysis reaction of ammonia borane as ammonium ions using a substance with pH buffering properties. is located.

[0027] A sixteenth aspect of the present invention is 16. The fuel cell according to claim 15, comprising the hydrogen production device. is located.

[0028] A seventeenth aspect of the present invention is 16. The power generation system according to claim 15, comprising the hydrogen production device. is located. [Effects of the Invention]

[0029] According to the present invention, a method for producing hydrogen that reduces ammonia can be obtained.

[0030] Further objects, features, and advantages of the present invention will become apparent from the following detailed description of the embodiments of the present invention and the accompanying drawings. [Brief explanation of the drawings]

[0031] [Figure 1(a)] FIG. 1 is a diagram showing the procedure of ammonia borane hydrolysis reaction test. [Figure 1(b)] FIG. 1 is a schematic diagram of a test device. [Figure 2]FIG. 1 shows the hydrogen generation behavior from aqueous ammonia borane solutions with and without the addition of each buffer. [Figure 3(a)] FIG. 1 shows a mass spectrum of the gas produced from the ammonia borane hydrolysis reaction when no buffer agent is added. [Figure 3(b)] FIG. 1 shows a mass spectrometry spectrum of the gas produced from the ammonia borane hydrolysis reaction when a phosphate buffer is added. [Figure 4] Schematic diagram of an ammonia concentration evaluation system that uses pH fluctuations in a dilute hydrochloric acid trap. [Figure 5] FIG. 1 is a graph showing hydrogen generation characteristics for each standing period after dissolving ammonia borane in phosphate buffer solution (or ion-exchanged water) at each pH. [Figure 6(a)] 1 is a schematic diagram of the setup of an aqueous ammonia borane solution and a catalyst in a bifurcated test tube. [Figure 6(b)] This is a schematic diagram of the setup of an aqueous ammonia borane solution, a catalyst, and a buffer in a bifurcated test tube. [Figure 6(c)] This is a schematic diagram of the setup of an aqueous ammonia borane solution, a catalyst, and a buffer in a bifurcated test tube. [Figure 7(a)] FIG. 1 shows a mass spectrometry spectrum of the product gas from the hydrolysis reaction at an ammonia borane concentration of 2.6 mol / L. [Figure 7(b)] FIG. 1 shows a mass spectrometry spectrum of the product gas from the hydrolysis reaction at an ammonia borane concentration of 2.6 mol / L. [Figure 7(c)] FIG. 1 shows a mass spectrometry spectrum of the product gas from the hydrolysis reaction at an ammonia borane concentration of 2.6 mol / L. [Figure 8(a)] FIG. 1 shows a mass spectrometry spectrum of the product gas from the hydrolysis reaction at an ammonia borane concentration of 10 mol / L. [Figure 8(b)] FIG. 1 shows a mass spectrometry spectrum of the product gas from the hydrolysis reaction at an ammonia borane concentration of 10 mol / L. DETAILED DESCRIPTION OF THE INVENTION

[0032] (Background to the conception of this embodiment) Because ammonia is corrosive and toxic, research is being conducted into materials for removing ammonia from gases, such as zirconium phosphate (Patent Publication No. 2020-131088), ammonium hydrogen sulfate mixed with carbon (Patent Publication No. 2016-135722), and citric acid (Patent Publication No. 2011-245398). A system could be envisioned in which the gas produced by the hydrolysis reaction of ammonia borane is supplied to a trap containing these ammonia adsorbents, removing the ammonia from the gas and obtaining highly pure hydrogen.

[0033] However, in the hydrolysis reaction of ammonia borane, theoretically an equimolar amount of ammonia (one-third the molar amount of hydrogen generated) is produced to the ammonia borane that is decomposed, and therefore the ammonia concentration in the gas supplied to the trap is high, necessitating frequent replacement of the ammonia adsorbent and increasing the trap's capacity.When actually operated in combination with a fuel cell, when the trap reaches its ammonia adsorption limit, gas with a high ammonia concentration will be supplied to the fuel cell, which may cause rapid deterioration of the fuel cell.

[0034] There is a need for technology to reduce the concentration of ammonia released from ammonia borane reaction solutions, but research into the hydrolysis reaction of ammonia borane has focused mainly on developing catalysts to increase the rate of hydrogen production and methods to regenerate the boron residue after the reaction into ammonia borane, and very little research has been done on reducing the concentration of ammonia released from aqueous ammonia borane solutions.

[0035] Hydrogen is generated by adding a catalyst to an aqueous solution of ammonia borane, but the generated hydrogen contains ammonia at a concentration of over 1000 ppm. International standards (ISO14687-2, 2012) stipulate that the ammonia concentration in hydrogen supplied to fuel cells must be 0.1 ppm or less, so the ammonia concentration needs to be reduced.

[0036] Therefore, the inventors came up with the idea of ​​reducing the ammonia concentration by using this embodiment. This embodiment reduces the amount of ammonia released from the solution, making it possible to extend the life of a filter for removing ammonia and make it more compact, or even to create a hydrogen supply system that does not require a filter.

[0037] (Concept of this embodiment) In this embodiment, a pH buffer is added to the ammonia borane aqueous solution to reduce the amount of ammonia released from the aqueous solution. The weak acid contained in the pH buffer provides protons, converting ammonia (NH3) into ammonium ions (NH4 + ) and collected in aqueous solution.

[0038] In this embodiment, the ammonia generated in the hydrolysis reaction of ammonia borane (Equation 1) is trapped as ammonium ions in the reaction solution by adding a pH buffer. NH3BH3+ 3H2O → 3H2+ NH3+ B(OH)3(1)

[0039] Here, a pH buffer is, for example, a solution of a weak acid and its salt or a weak base and its salt, which are responsible for donating and accepting protons, respectively. A pH buffer is, for example, a compound added to a solution to maintain the pH of the solution at a specific value even when other acids or bases are added to the solution to a certain extent. A pH buffer suppresses sudden changes in pH. A pH buffer is, for example, a certain type of weak acid or weak base.

[0040] Ammonia produced by the hydrolysis reaction becomes ammonium ions by receiving protons from the proton donor in the buffer added to the aqueous solution. Alternatively, the hydroxide ions produced by the equilibrium reaction of ammonia in water (Equation 2) are consumed by protons donated by the buffer, shifting the equilibrium to the right and allowing ammonia to be captured in the aqueous solution as ammonium ions. NH3 + H2O ⇔ NH4 + + OH- (2)

[0041] One approach would be to install a filter between the hydrogen generator and the fuel cell to remove ammonia from the gas phase, but because the hydrolysis reaction of ammonia borane releases high concentrations of ammonia exceeding 1000 ppm, this would require a larger capacity filter or frequent filter replacement. Furthermore, if the generated gas is supplied to the fuel cell with a deteriorated filter, the uncaptured high-concentration ammonia may come into contact with the anode, potentially causing irreversible deterioration of the fuel cell.

[0042] This embodiment can constantly reduce the ammonia concentration in hydrogen released from the ammonia borane aqueous solution to about 10 ppm. Furthermore, this embodiment can easily achieve "ammonia concentration of 0.1 ppm or less" by using it in combination with a filter, thereby extending the life of the filter and reducing the risk of fuel cell failure due to filter deterioration.

[0043] (Summary of each example) The outline of each embodiment is as follows. Example 1. Comparison of various buffers using acid and its salt (same anion) combinations Example 2. Comparison of buffers combining organic weak acids and inorganic salts Example 3. Effect of adding phosphate buffer on ammonia release concentration over time Example 4. Effect of pH on phosphate buffer Example 5. Study on the effective amount of phosphate buffer added Example 6. Ammonia suppression effect of phosphate buffer when ammonia borane concentration is increased Example 7. Ammonia release suppression effect without dissolving phosphate buffer Example 8. Ammonia release suppression effect when phosphate buffer is partially dissolved in ammonia borane saturated solution Example 9. Examination of the ammonia release suppression effect of materials used as ammonia removal filters

[0044] In Example 1, we demonstrate that the ammonia concentration in the released hydrogen can be dramatically reduced by adding a pH buffer to the ammonia borane aqueous solution using several pH buffers (phosphate buffer, acetate buffer, and citrate buffer).

[0045] Generally, a buffer is a mixture of a weak acid and its salt, or a weak base and its salt. However, Example 2 demonstrated that a mixture of a weak organic acid and an inorganic salt, such as a mixture of acetic acid and phosphate, can also achieve the same ammonia release suppression effect as Example 1. Examples 1 and 2 demonstrate that any substance with pH buffering properties can be used as an additive that exhibits an ammonia release suppression effect in the hydrolysis reaction of ammonia borane. It was also found that a phosphate buffer (particularly a mixture of sodium dihydrogen phosphate and dipotassium hydrogen phosphate) is a preferred buffer.

[0046] Examples of buffers include acetate buffers (e.g., acetic acid + sodium acetate), phosphate buffers (e.g., phosphoric acid + sodium phosphate), citrate buffers (e.g., citric acid + sodium citrate), citrate-phosphate buffers (e.g., citric acid + sodium phosphate), borate buffers, tartrate buffers, Tris buffers, phosphate-buffered saline, and McIlvaine buffers. More specifically, examples of buffers include disodium hydrogen phosphate, sodium dihydrogen phosphate, potassium phosphate, dipotassium hydrogen phosphate, potassium dihydrogen phosphate, trometamol, sodium carbonate, sodium bicarbonate, meglumine, arginine, maleic acid, formic acid, malic acid, succinic acid, pivalic acid (trimethylacetic acid), pyridine, picolinic acid, amines (e.g., triethanolamine), glycine, and piperazine.

[0047] In Example 3, we analyzed the change over time in the concentration of ammonia released during the hydrolysis reaction. We found that while the ammonia concentration in the hydrogen increases over time without a buffer, adding a buffer keeps the ammonia concentration low and constant. Considering the long-term supply of hydrogen to fuel cells, this discovery is expected to contribute to significantly extending the life of fuel cells and ammonia filters (when used in combination).

[0048] Examples 4 and 5 show findings for determining conditions that are preferable for putting this embodiment into practical use.

[0049] In Example 4, it was shown that the lower the pH in the state in which the buffer agent was dissolved, the faster the rate of hydrogen production during the hydrolysis reaction. However, even under weakly acidic conditions of about pH 6.5 or near-neutral conditions of pH 7.0, the dissolved ammonia borane self-decomposed (reacted with protons) over time, and the longer the storage period in the aqueous solution, the lower the hydrogen yield obtained when the catalyst was added.

[0050] In Example 5, a buffer was added in an amount of 0.54 to 1.2 times the amount of ammonia borane in terms of the amount of substance, and the influence of the phosphate buffer / ammonia borane ratio on the ammonia release suppression effect was clarified. This Example 5 showed that the preferred amount of phosphate buffer added is 1.05 to 1.10 times the amount of ammonia borane in terms of the amount of substance.

[0051] In Example 6, we investigated how the ammonia release suppression effect changes when the concentration of the ammonia borane aqueous solution is increased. As a result, we found that even when the concentration is increased, the ammonia release concentration can be reduced to about 10 ppm by using a phosphate buffer. However, we found that a phosphate buffer equivalent to 1.05 to 1.10 equivalents does not dissolve in a high-concentration ammonia borane aqueous solution of 3 mol / L or more.

[0052] Therefore, in Example 7, it was demonstrated that the release of ammonia was suppressed by simply contacting a high-concentration aqueous solution of ammonia borane with a phosphate buffer (powder), i.e., even in the hydrolysis reaction without the buffer dissolved. The powder is, for example, a solid substance that has been crushed into very fine particles.

[0053] In Example 8, the ammonia release suppression effect when a phosphate buffer is partially dissolved in an ammonia borane saturated solution will be described.

[0054] In Example 9, the effect of suppressing ammonia release by a material used as an ammonia removal filter will be described.

[0055] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

[0056] Example 1 Comparison of various buffers using acid and its salt (same anion) combinations

[0057] Figure 1(a) shows the procedure for the ammonia borane hydrolysis reaction test, and Figure 1(b) is a schematic diagram of the test equipment.

[0058] Each buffer (Additive A and Additive B) listed in Table 1 was dissolved in 50 ml of ion-exchanged water, followed by 100 mg of ammonia borane. The amount of Additive A, which provides protons, was adjusted to a 1:1 molar ratio relative to the amount of ammonia borane. The amount of Additive B was determined to maintain a pH of approximately 6.5. 50 mg of activated carbon carrying 1 wt% platinum was added as a catalyst to initiate the hydrolysis reaction. The total amount of gas generated during the reaction was collected in a Tedlar bag, and the ammonia concentration in the collected gas was analyzed after the reaction was completed using an ammonia detector. The Tedlar bag was placed in a flask containing water, and the amount of water expelled from the flask due to the expansion of the bag caused by the generated gas was measured to investigate the gas (mainly hydrogen) generation behavior.

[0059] [Table 1]

[0060] FIG. 2 shows the hydrogen generation behavior from aqueous ammonia borane solutions with and without the addition of each buffer.

[0061] When no buffer was used (ammonia borane alone was dissolved in water with the addition of a catalyst), the ammonia concentration in the evolved gas was 100 ppm, whereas when a buffer was added, it was less than 10 ppm in all cases, confirming the effectiveness of the buffer in suppressing ammonia release. A comparison of hydrogen production behavior revealed that the hydrogen production rate was nearly the same when a phosphate buffer was added as when no buffer was used, whereas the hydrogen production rate decreased when acetate and citrate buffers were added (Figure 2). Furthermore, when considering the total amount of buffer added, the amounts of citrate and acetate buffers were significantly larger than those of phosphate buffer, which increased the overall weight of the system and resulted in a decrease in the hydrogen energy density per weight. Therefore, phosphate buffer was found to be relatively preferable.

[0062] Example 2 Comparison of buffers combining organic weak acids and inorganic salts

[0063] Low molecular weight organic weak acids such as formic acid and acetic acid are expected to reduce the weight of the buffer added, but when salts of these organic weak acids (e.g., sodium formate, sodium acetate, tripotassium citrate, etc.) are used as additive B, a large amount must be dissolved to adjust the pH to around 7, and it was found that this significantly reduces the hydrogen energy density per weight. Therefore, in order to find a lightweight buffer that is effective in suppressing ammonia release, a buffer combining organic weak acids (formic acid, acetic acid, citric acid) and inorganic salts (phosphates, carbonates) was added and the hydrolysis reaction of ammonia borane was carried out.

[0064] Table 2 shows the details of each experimental condition, the amount of hydrogen produced, and the ammonia concentration in the hydrogen.

[0065] [Table 2]

[0066] When 0.1 g of ammonia borane was dissolved in 10 ml of ion-exchanged water, approximately 1000 ppm of ammonia was released along with hydrogen. It was found that when formic acid and acetic acid were dissolved with carbonate or phosphate, ammonia release was suppressed in all combinations. However, a decrease in hydrogen production was observed when formic acid was used, suggesting that there may be a buffer more preferable than formic acid. When the molar ratio of additive A, which provides protons, to ammonia borane (additive A / ammonia borane ratio) was taken as equivalent, it was found that the ammonia concentration could be reduced to 70 ppm without reducing the hydrogen yield when 1 equivalent of acetic acid and dipotassium hydrogen phosphate were dissolved. Increasing the amounts of acetic acid and dipotassium hydrogen phosphate added to 2 and 3 equivalents reduced the ammonia release to 18 and 5 ppm, respectively. However, since the organic weak acid-inorganic salt mixed buffer had a heavier total weight than the phosphate buffer (here, potassium dihydrogen phosphate and disodium hydrogen phosphate) used in Experiment 1, which weighed 0.89 g per equivalent, it was determined that the phosphate buffer was a relatively preferable additive.

[0067] Example 3 Effect of adding phosphate buffer on the time course of ammonia release concentration

[0068] As the hydrolysis reaction of ammonia borane progresses, the pH of the aqueous solution increases, the water temperature increases due to the exothermic reaction, and the ammonia concentration in the aqueous solution increases. All three of these factors can promote the release of ammonia into the gas phase. Therefore, the gas released during the hydrolysis reaction was analyzed using a quadrupole mass spectrometer to investigate the time-dependent changes in ammonia release behavior.

[0069] A solution of ammonia borane (10 ml of water and 100 mg of ammonia borane) and a catalyst (50 mg of 1 wt% platinum-supported activated carbon) were placed in a bifurcated test tube. After purging the test tube with argon, the ammonia borane solution was brought into contact with the catalyst to initiate the hydrolysis reaction, and the product gas was analyzed by mass spectrometry. A similar experiment was also performed using an aqueous solution of ammonia borane containing 1.1 equivalents of phosphate buffer (here, 0.4300 g of sodium dihydrogen phosphate and 0.5233 g of dipotassium hydrogen phosphate) to compare the ammonia release behavior.

[0070] Figure 3 shows the mass spectrometry (MS) spectra obtained with and without the addition of a buffer. Figure 3(a) shows the mass spectrometry spectrum of the gas generated from the ammonia borane hydrolysis reaction without the addition of a buffer. Figure 3(b) shows the mass spectrometry spectrum of the gas generated from the ammonia borane hydrolysis reaction with the addition of a phosphate buffer.

[0071] In Figure 3, m in m / z represents the mass number, and z represents the charge when ionized in the mass spectrometer. m / z is roughly equal to the mass number of the molecule or the fragment produced by ionization of the molecule. For example, hydrogen (H2) is detected at m / z = 2. Ammonia (NH3) has a mass number of 17, but when ionized in the mass spectrometer, a hydrogen atom is released, resulting in the spectra (fragments) of NH2 (m / z = 16), NH (m / z = 15), and N (m / z = 14) moving in tandem. Similarly, when water (HO, mass number 18) is detected, fragments of HO (m / z = 17) and O (m / z = 16) are also detected. Because of the influence of water vapor from aqueous solutions, it is appropriate to focus on m / z = 15 rather than m / z = 17 or 16 when monitoring the generation of ammonia with a mass spectrometer.

[0072] In the case of ammonia borane and water alone (Figure 3(a)), without the addition of a buffer, increases in the spectra of hydrogen (m / z = 2), water (m / z = 18, 17, 16), and ammonia (m / z = 17, 16, 15, 14) were observed from the start of the reaction. Focusing on m / z = 15 in particular, its gradual increase from the start of the reaction revealed that the ammonia concentration in the evolved gas increased over time. While m / z = 18, which originates from water, remained almost constant, m / z = 16 also showed a similar increase. This increase in m / z = 16, coupled with m / z = 15, is thought to be due to an increase in the ammonia concentration. On the other hand, when 1.1 equivalents of phosphate buffer were added (Figure 3(b)), the increase in m / z = 15 at the start of the reaction was more suppressed than in the case without the addition of a buffer, and the spectrum remained constant without any increase. From the above, it was revealed that the addition of phosphate buffer consistently kept the amount of ammonia released low.

[0073] Considering the case where hydrogen is supplied to a fuel cell over a long period of time, it is believed that the ability of the buffer to constantly keep the ammonia concentration low is a major advantage. If hydrogen is supplied to a fuel cell using only a filter that removes ammonia, there is a risk that high concentrations of ammonia will be supplied to the fuel cell and irreversible damage will occur if the filter deteriorates and its ammonia removal capacity decreases. However, by using the buffer of this embodiment in combination, even if the filter deteriorates, the ammonia concentration can be kept low, making it possible to minimize damage to the fuel cell.

[0074] Example 4 Effect of pH on phosphate buffers

[0075] Figure 4 is a schematic diagram of an ammonia concentration evaluation system that utilizes pH fluctuations in a dilute hydrochloric acid trap.

[0076] Potassium dihydrogen phosphate and disodium hydrogen phosphate were dissolved in 250 ml of ion-exchanged water to prepare buffer solutions with pH values ​​of 6.5, 7.0, and 7.5. 100 mg of ammonia borane was then dissolved in each buffer solution. To evaluate the stability of ammonia borane in the buffer solution, the solution was left standing at 25 °C for a predetermined period of time. A 1 wt% platinum-supported activated carbon catalyst was added to the ammonia borane-dissolved buffer solution under each standing condition, and the hydrolysis reaction was initiated. Figure 4 shows a schematic diagram of the evaluation apparatus. Ammonia in the gas evolved from the ammonia borane aqueous solution during the hydrolysis reaction was collected by bubbling it through a dilute hydrochloric acid trap at pH 5.3. The amount of ammonia collected was calculated from the pH fluctuation of the dilute hydrochloric acid trap before and after the reaction, and the ammonia concentration in the product gas was estimated using the formula: "amount of ammonia collected / (amount of hydrogen produced + amount of ammonia collected)."

[0077] The pH fluctuations and ammonia concentrations in the dilute hydrochloric acid trap for each buffer pH and static condition are shown in Table 3. It was found that the amount of ammonia released was not pH dependent, and that the ammonia concentration could be suppressed to 10 ppm or less under all pH conditions.

[0078] FIG. 5 is a graph showing hydrogen generation characteristics for each standing period after dissolving ammonia borane in phosphate buffer solutions (or ion-exchanged water) of various pH values.

[0079] Focusing on the effect of pH on hydrogen generation characteristics, when the hydrolysis reaction was initiated immediately after dissolving ammonia borane without a standing period (Figure 5, left), hydrogen generation was fastest at pH 6.5, 7.0, and 7.5, indicating that the lower the pH of the buffer solution, the higher the hydrogen generation rate. Lower solution pH indicates a higher proton concentration in the solution, suggesting that protons in the solution promote the reaction of ammonia borane with BH. The hydrogen production yield (hydrogen yield) decreased with increasing standing time at pH 6.5 and 7.0, indicating that ammonia borane reacts with protons or water molecules in the solution and self-decomposes during the standing period (Figure 5, center, right). The deterioration during the standing period was particularly pronounced at pH 6.5, suggesting that the presence of protons is the main factor causing the self-decomposition of ammonia borane. On the other hand, at pH 7.5, no significant decrease in hydrogen yield was observed even after standing for 4 days, suggesting that a pH of around 7.5 is preferable for the buffer added to the ammonia borane solution (Figure 5, right).

[0080] Table 3 shows the pH fluctuations of the dilute hydrochloric acid trap and the ammonia concentration released during the hydrolysis reaction, with the pH of the phosphate buffer and the standing time after dissolving ammonia borane as parameters.

[0081] [Table 3]

[0082] Example 5 Study on the effective amount of phosphate buffer added

[0083] To determine the effective amount of buffer to be added to ammonia borane, potassium dihydrogen phosphate (proton-donating additive) was added in a ratio of 0.54 to 1.20 times the amount of ammonia borane, and dissolved in 10 ml of water together with disodium hydrogen phosphate to adjust the pH to approximately 7.5. After dissolving 100 mg of ammonia borane, 50 mg of 1 wt% platinum-loaded activated carbon was added, and a hydrolysis reaction was carried out using the same procedure and apparatus as in Figure 1, and the ammonia concentration in the product gas was analyzed.

[0084] Tables 4 and 5 show the hydrolysis reaction conditions and the amount of ammonia released for each amount of phosphate buffer added. Table 4 shows the hydrolysis reaction conditions and the ammonia released concentration for 0 to 1.00 equivalents of phosphate buffer. Table 5 shows the hydrolysis reaction conditions and the ammonia released concentration for 1.05 to 1.20 equivalents of phosphate buffer.

[0085] [Table 4]

[0086] [Table 5]

[0087] While 1000 ppm of ammonia was released without the addition of buffer (0 equivalents), the addition of approximately 0.5 equivalents of phosphate buffer reduced the ammonia release concentration to approximately one-tenth of that. At 1.00 equivalents, the ammonia detection tube reading slightly exceeded the 30 ppm mark, suggesting that the actual concentration was approximately 30–35 ppm. When the same amount of potassium dihydrogen phosphate (KDP) as ammonia borane was added, the ammonia concentration could be reduced to approximately 30 ppm. To reduce ammonia to concentrations below 10 ppm, a slight excess of KDP over 1.00 equivalents was found to be preferable. As the amount of buffer added increases, the hydrogen weight density of the entire system decreases, and the addition of buffer tends to slow the hydrogen production rate (Figure 5). Therefore, it is recommended that the amount of buffer added be kept to a minimum. In light of the above, it is believed that the effective amount of buffer added to reduce the amount of ammonia released to one hundredth or less is preferably about 1.05 to about 1.10 times the amount of ammonia borane in terms of the amount of substance.

[0088] Example 6 Ammonia suppression effect of phosphate buffer at elevated ammonia borane concentrations

[0089] To take advantage of ammonia borane's high gravimetric hydrogen density, it is desirable to operate the aqueous solution of ammonia borane at a saturated solution concentration (10 mol / L) as much as possible. However, as the concentration increases, the amount of solvent (water) decreases, which raises concerns that the ammonia capture capacity will decrease and the ammonia concentration in the gas phase will increase. Therefore, we investigated the effect of adding phosphate buffer on the ammonia release concentration under conditions of different ammonia borane concentrations.

[0090] Table 6 shows the hydrolysis reaction conditions and the released ammonia concentration for each ammonia borane concentration (ammonia released concentration with and without the addition of phosphate buffer).

[0091] [Table 6]

[0092] Without the addition of a buffer (ammonia borane alone dissolved in ion-exchanged water), approximately 100 ppm of ammonia was released at 0.06 mol / L, but 1000 ppm at 0.3 mol / L. At even higher concentrations of 2.6 mol / L and 10 mol / L, the ammonia concentration exceeded the 1000 ppm limit on the detector tube for several seconds or even instantaneously, making it impossible to measure. On the other hand, at concentrations of 0.06, 0.3, and 2.6 mol / L, when 1.0–1.1 times the amount of phosphate dissolved relative to the amount of ammonia borane dissolved, ammonia release was suppressed to below 10 ppm at all concentrations, demonstrating that the phosphate buffer is effective in suppressing ammonia release even at high concentrations. Note that because disodium hydrogen phosphate, added as additive B to adjust the pH, has low solubility, dipotassium hydrogen phosphate, which has a higher solubility, was used in the higher-concentration experiments. For the same reason, sodium dihydrogen phosphate, which has a higher solubility than potassium dihydrogen phosphate, was used as additive A, which acts as a proton donor.

[0093] In order to prepare a phosphate buffer solution with a concentration higher than 2.6 mol / L, preparations at 10 mol / L and 3.0 mol / L were investigated, but the result was that phosphate did not dissolve at either concentration. Therefore, to suppress ammonia release by adding a buffer according to this embodiment in the hydrolysis reaction of ammonia borane saturated solution (10 mol / L), it is considered preferable to use a different buffer having the same solubility as ammonia borane, or to use an ammonia removal filter in combination with a reduced amount of phosphate buffer (1 equivalent or less).

[0094] However, as will be described in Example 7 below, even if the buffer is not dissolved in water together with ammonia borane, it has the effect of suppressing the release of ammonia. Therefore, it is expected that this embodiment can be applied to a saturated solution of ammonia borane by dissolving a small amount of buffer in the saturated solution of ammonia borane, adding a powdered buffer separately to the solution just before the start of the reaction, and initiating the hydrolysis reaction.

[0095] Example 7 Ammonia release suppression effect without dissolving phosphate buffer

[0096] Figure 6 shows a schematic diagram of the setup of an ammonia borane solution, catalyst, and buffer in a bifurcated test tube. Figure 6(a) shows the setup of an ammonia borane solution and catalyst powder, Figure 6(b) shows the setup of an ammonia borane solution and phosphate buffer solution and catalyst powder, and Figure 6(c) shows the setup of an ammonia borane solution and catalyst-phosphate buffer mixed powder.

[0097] A solution of ammonia borane was placed in one side of a bifurcated test tube, and either the catalyst alone or a mixture of catalyst and phosphate buffer powder was placed in the other side. The test tube was then filled with argon gas. After a steady state was reached, the bifurcated test tube was tilted to bring the solution of ammonia borane into contact with the catalyst (or the mixture of catalyst and buffer powder), initiating the hydrolysis reaction (Figure 6). The product gas was analyzed using a quadrupole mass spectrometer, as in Example 3, to investigate the ammonia release behavior. As a reference experiment, a solution of phosphate buffer and ammonia borane at a concentration of 2.6 mol / L was prepared and placed in one side of the bifurcated test tube, and the same experiment was performed using the other side (Figure 6(b)).

[0098] Figure 7 shows the mass spectrometry spectrum of the product gas from the hydrolysis reaction at an ammonia borane concentration of 2.6 mol / L (*▼ indicates the reaction starting point). Figure 7(a) shows the results for an aqueous solution of ammonia borane and catalyst powder, Figure 7(b) shows the results for an aqueous solution of ammonia borane and phosphate buffer and catalyst powder, and Figure 7(c) shows the results for an aqueous solution of ammonia borane and catalyst-phosphate buffer mixed powder.

[0099] When only ammonia borane aqueous solution was contacted with the catalyst (Figure 7(a)), the spectra of m / z = 2 (H2), 17 (OH, NH3), 16 (O, NH2), and 15 (NH) increased immediately after the start of the reaction. The spectrum of m / z = 18 (HO) derived from water remained almost constant and was not linked to m / z = 17 and 16, suggesting that the linked behavior of m / z = 17, 16, and 15 is due to the release of ammonia. The spectrum derived from ammonia continued to increase over time from the start of the reaction, a characteristic of buffer-free conditions. When phosphate buffer was dissolved in water together with ammonia borane (Figure 7(b)), a slight increase in m / z = 15 (NH) was observed immediately after the start of the reaction, but the change was not as large as in the buffer-free condition. Furthermore, the lack of an increase in m / z = 15 during the reaction indicated that the release of ammonia was effectively suppressed. This result is consistent with the ammonia release concentration of 2 ppm observed under the same conditions in Example 6. When the buffer powder was mixed with the catalyst and contacted with an aqueous ammonia borane solution (Figure 7(c)), a temporary increase in m / z = 15 (NH), 16 (NH2, O), and 17 (NH3, OH) was observed immediately after the start of the reaction, but the spectrum soon decreased, showing the same behavior as Figure 7(b). The spectrum derived from ammonia did not increase over time from the start of the reaction, but tended to decrease, demonstrating that the phosphate buffer is effective in suppressing ammonia release even when not dissolved in an aqueous ammonia borane solution.

[0100] A similar experiment was conducted with ammonia borane at its saturated concentration of 10 mol / L. A 10 mol / L ammonia borane solution was prepared by dissolving 0.402 g of ammonia borane in 1.26 ml of ion-exchanged water. As in Figure 6a, the 10 mol / L ammonia borane solution was placed on one side of a bifurcated test tube, and 50 mg of 1 wt% platinum-loaded activated carbon was placed on the other side. After purging the test tube with argon, the test tube was tilted to initiate the reaction. A similar experiment was also conducted with a powder mixture of 50 mg of 1 wt% platinum-loaded activated carbon, 1.52 g of sodium dihydrogen phosphate, and 1.86 g of dipotassium hydrogen phosphate, placed on the other side of the 10 mol / L ammonia borane solution, as in Figure 6c.

[0101] Figure 8 shows the mass spectrometry spectrum of the product gas from the hydrolysis reaction at an ammonia borane concentration of 10 mol / L (*▼ indicates the reaction starting point). Figure 8(a) shows the results for the ammonia borane aqueous solution and catalyst powder only, and Figure 8(b) shows the results for the ammonia borane aqueous solution and the catalyst-phosphate buffer mixed powder.

[0102] When only ammonia borane solution and catalyst were used (Figure 8(a)), the spectrum of m / z = 17, 16, and 15, which is attributable to ammonia, increased significantly, suggesting the release of high concentrations of ammonia. The gas generated under the same conditions was collected in a Tedlar bag and analyzed for ammonia concentration, which was 50,000 ppm. When phosphate buffer was added to the catalyst (Figure 8(b)), the spectrum of m / z = 15 (NH) increased immediately after the start of the reaction. However, there was no subsequent rapid increase in the spectrum or linkage with m / z = 17 and 16, suggesting that ammonia release was suppressed. Analysis of the ammonia concentration in the gas generated under the same conditions revealed a value of 220 ppm, demonstrating that ammonia release can be effectively suppressed even at a high concentration of 10 mol / L (the saturated concentration of ammonia borane) without the presence of a dissolved buffer.

[0103] Table 7 shows the conditions for the hydrolysis reaction of ammonia borane saturated aqueous solution (10 mol / L) and the effect of the presence or absence of a buffer on the ammonia release concentration.

[0104] [Table 7]

[0105] To investigate the ammonia release concentration when a catalyst and buffer powder were added from above to a 10 mol / L ammonia borane aqueous solution, a hydrolysis reaction test was conducted using the evaluation system shown in Figure 1b under the conditions shown in Table 7. When only the catalyst was added to the saturated ammonia borane aqueous solution (condition A in Table 7), 40,000 ppm of ammonia was released, whereas when a mixed powder of catalyst and buffer was added to the saturated ammonia borane aqueous solution (condition B in Table 7), the ammonia concentration was 1,000 ppm. The ammonia release concentration measured in the solid-phase buffer was confirmed, but it was higher than the ammonia release concentration measured in the bifurcated test tube (220 ppm). Because the contact area between the ammonia borane aqueous solution and the buffer was limited immediately after the start of the reaction, ammonia generated at the contact interface between the high-concentration ammonia borane aqueous solution and the catalyst was presumably released into the gas phase before being captured in the liquid phase by the effect of the buffer. As described in Example 5, the presence of only a small amount (0.5 equivalents) of phosphate buffer in the aqueous solution relative to ammonia borane suppressed the ammonia release concentration to approximately 1 / 10 of the original concentration. Therefore, by dissolving a small amount of buffer in a 10 mol / L high-concentration aqueous solution in advance and then mixing additional buffer (powder) with the aqueous solution just before starting the reaction, it is thought that the ammonia release concentration can be sufficiently reduced even at 10 mol / L. Therefore, in order to effectively capture ammonia in the liquid phase, it is desirable for the catalyst and buffer to be located at a position farther from the liquid surface (the bottom side of the reactor) than near the liquid surface of the ammonia borane aqueous solution (the surface layer (liquid-gas interface) that comes into contact with the gas phase space not filled with solution at the top of the ammonia borane aqueous solution in the reactor).

[0106] Example 8 Inhibitory effect of ammonia release when phosphate buffer is partially dissolved in saturated ammonia borane solution

[0107] Table 8 shows the conditions for the hydrolysis reaction in which a mixed powder of catalyst and H2NaPO4 was added to an aqueous solution containing K2HPO4 and saturated ammonia borane, and the ammonia concentration released.

[0108] [Table 8]

[0109] An ammonia borane hydrolysis reaction was carried out under the conditions shown in Table 8 using an apparatus similar to that shown in Figure 1(b). A powder mixture of catalyst and proton donor HNaPO was added to a saturated aqueous solution of ammonia borane containing dissolved basic salt KHPO, and the hydrolysis reaction was initiated. The product gas was collected in a Tedlar bag, and the ammonia concentration was analyzed using a detector tube, revealing a value of 26 ppm. Compared to condition A (no buffer) shown in Table 7 of Example 7, the ammonia concentration was reduced to approximately 1 / 1500. Compared to condition B of Example 7 (Table 7), in which the same amount of proton donor HNaPO was added, dissolving basic salt KHPO in the ammonia borane aqueous solution further reduced the ammonia concentration to approximately 1 / 40. Furthermore, even though the amount of KHPO dissolved in the saturated ammonia borane solution was reduced to approximately 1 / 20, excellent ammonia suppression was observed, demonstrating the ability to reduce the amount of buffer added.

[0110] Example 9 Examination of the ammonia emission suppression effect of materials used as ammonia removal filters

[0111] Nickel chloride, which reacts with ammonia molecules to form an ammine complex, is known as an ammonia adsorbent. As shown in Figure 6(c), a saturated aqueous solution of ammonia borane (0.63 ml of water and 0.2005 g of ammonia borane) was placed in one side of a bifurcated test tube, and a mixture of catalyst (0.0256 g of platinum-supported activated carbon) and nickel chloride (0.2513 g) powder was placed in the other. The ammonia borane solution was brought into contact with the catalyst and nickel chloride mixture to initiate the hydrolysis reaction. The evolved gas was collected in a Tedlar bag and analyzed for ammonia concentration using a detector tube. The ammonia concentration was 1% (10,000 ppm). Comparing the results from Example 7 (Table 7) and Example 8 (Table 8), the ammonia release concentration was lower than when no buffer was added, but the ammonia concentration was only a fraction of that observed when a buffer was added, indicating that the inhibitory effect was significantly greater when a buffer was added.

[0112] (Application example of this embodiment) For example, one possible method is to use this embodiment in a power generation system combined with a fuel cell. Because ammonia borane can be stably stored as an aqueous solution, the aqueous ammonia borane solution can be sold as a cartridge and connected to a system incorporating a catalyst unit and a fuel cell, making it possible to use it as a power source for small stationary generators or mobile vehicles. The buffer agent found to be effective in this embodiment can be added to the cartridge of the aqueous ammonia borane solution or mixed with the aqueous ammonia borane solution in the power generation system, thereby exerting the effect of suppressing ammonia release.

[0113] (Interpretation of rights, etc.) The present invention has been described above with reference to specific embodiments. However, it is obvious that those skilled in the art can modify or substitute the embodiments without departing from the spirit of the present invention. In other words, the present invention has been disclosed in the form of examples, and the contents of this specification should not be interpreted as limiting. To determine the spirit of the present invention, the claims set forth at the beginning of this specification should be taken into consideration.

[0114] It is also clear that the illustrative embodiment of the present invention achieves the above-mentioned objectives, but it will be understood that many modifications and other embodiments can be made by those skilled in the art. Elements or components of the claims, specification, drawings, and illustrative embodiments may be employed singly or in combination with other embodiments. The claims are intended to encompass such modifications and other embodiments, which fall within the spirit and scope of the present invention.

Claims

1. A method for producing hydrogen by a hydrolysis reaction of ammonia borane, comprising: A method of producing hydrogen in which ammonia generated by the hydrolysis reaction of ammonia borane is trapped as ammonium ions using a substance with pH buffering properties.

2. 2. The method for producing hydrogen according to claim 1, wherein ammonia is trapped as ammonium ions in the ammonia borane solution.

3. 2. The method for producing hydrogen according to claim 1, wherein the substance having a pH buffering effect is selected from the group consisting of a phosphate buffer, an acetate buffer, and a citrate buffer.

4. 2. The method for producing hydrogen according to claim 1, wherein the substance having a pH buffering effect is a phosphate buffer.

5. 5. The method for producing hydrogen according to claim 4, wherein the phosphate buffer is a mixture of sodium dihydrogen phosphate and dipotassium hydrogen phosphate.

6. 5. The method for producing hydrogen according to claim 4, wherein the amount of the phosphate buffer is 1.05 equivalents or more and 1.10 equivalents or less relative to the amount of ammonia borane.

7. 5. The method for producing hydrogen according to claim 4, wherein the phosphate buffer contains potassium dihydrogen phosphate in an amount greater than 1.00 equivalents relative to ammonia borane.

8. 2. The method for producing hydrogen according to claim 1, wherein the release of ammonia is suppressed by contacting the aqueous ammonia borane solution with a solid phosphate buffer.

9. 2. The method for producing hydrogen according to claim 1, wherein the release of ammonia is suppressed by using a phosphate buffer in an amount of 1.00 equivalents or less relative to ammonia borane and an ammonia removal filter.

10. 2. The method for producing hydrogen according to claim 1, wherein after dissolving a substance having a pH buffering effect in an aqueous solution of ammonia borane, a powdered substance having a pH buffering effect is added to the aqueous solution of ammonia borane to initiate a hydrolysis reaction of ammonia borane.

11. 11. The method for producing hydrogen according to claim 10, wherein the aqueous ammonia borane solution is a saturated ammonia borane solution.

12. 5. The method for producing hydrogen according to claim 4, wherein a catalyst that promotes the hydrolysis reaction of ammonia borane is brought into contact with an aqueous solution in which ammonia borane and a phosphate buffer are dissolved.

13. 5. The method for producing hydrogen according to claim 4, wherein the aqueous solution of ammonia borane is contacted with a solid mixture of a catalyst that promotes the hydrolysis reaction of ammonia borane and a phosphate buffer.

14. A method for producing hydrogen in which ammonia generated by the hydrolysis reaction of ammonia borane is trapped as ammonium ions by partially dissolving a phosphate buffer in a saturated solution of ammonia borane.

15. A hydrogen production apparatus for producing hydrogen by a hydrolysis reaction of ammonia borane, A hydrogen production device that traps ammonia generated by the hydrolysis reaction of ammonia borane as ammonium ions using a substance with pH buffering properties.

16. 16. The fuel cell according to claim 15, comprising the hydrogen production device.

17. 16. The power generation system according to claim 15, comprising the hydrogen production device.

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