JP2004244262A - Hydrogen generation method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stable, efficient hydrogen generation method without increasing the size of an apparatus, and to provide a hydrogen generation method which can be applied to e.g. a fuel cell system. <P>SOLUTION: In the hydrogen generation method, hydrogen is generated by injecting an alkaline solution of a complex metal hydride serving as a hydrogen generator into an aqueous acid solution and reacting them. Specifically, a liquid prepared by dissolving sodium borohydride in an aqueous solution of an alkali metal hydroxide is used as the hydrogen generator. The hydrogen generator is injected into an aqueous solution of citric acid or phosphoric acid at a controlled injection rate or concentration while the pH of a reaction liquid is maintained at ≤4. This enables the control of the amount of hydrogen generated and retains a stable hydrogen-generating rate without requiring a large apparatus. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a hydrogen generation method, and more particularly, to a hydrogen generation method capable of efficiently generating hydrogen while controlling the amount of hydrogen generated.
[0002]
[Prior art]
Heretofore, as a method for producing hydrogen, electrolysis of water, a reaction between a metal and an acid, a reaction between a metal and an alkali, steam reforming of methanol or natural gas, and release from a hydrogen storage alloy have been known.
However, these production methods have problems such as a large amount of energy consumption, a small amount of hydrogen generated for the raw materials used, and a large-scale facility required, and fuels for automobiles and portable electronic devices are used. It was not practical as a hydrogen source for batteries.
[0003]
On the other hand, as shown in the formula (1), a metal hydride complex compound such as sodium borohydride can generate hydrogen by contacting with water, but has an explosive property that generates a large amount of hydrogen at one time. Because of the phenomenon, its handling was difficult.
NaBH ₄ + 2H ₂ O → 4H ₂ + NaBO ₂ (1)
On the other hand, based on the property that the metal hydride complex is stable for a long time in the aqueous alkali solution, the alkali aqueous solution of the metal hydride complex is used as a hydrogen generating agent, and the catalytic activity is converted to the aqueous alkali solution of the metal hydride complex. A method of contacting a metal having the same has been proposed (for example, Patent Document 1).
[0004]
[Problems to be solved by the invention]
According to the above-described method, hydrogen can be safely stored, hydrogen can be generated immediately when the catalyst metal is brought into contact with the hydrogen generator, and hydrogen generation can be easily stopped by stopping the contact. There are advantages.
However, in the case of hydrogen generation by contact with a catalytic metal, it is necessary to increase the contact area in accordance with the rate of hydrogen generation, and if it is necessary to generate hydrogen quickly, a large-scale apparatus is required. For this reason, there has been a problem in terms of practical application as a hydrogen source to be mounted on fuel cells for automobiles and portable electronic devices, for which there is a high demand for miniaturization and high output.
The present invention has been made in view of the above problems, and has as its object to provide a hydrogen generation method for supplying hydrogen used for a fuel cell or the like without increasing the size of the apparatus.
[0005]
[Patent Document 1]
JP 2001-19401 A [Means for Solving the Problems]
According to the present invention, there is provided a method for producing hydrogen, which comprises injecting an alkali solution of a metal hydride complex compound as a hydrogen producing agent into an aqueous acid solution and reacting to produce hydrogen.
[0006]
BEST MODE FOR CARRYING OUT THE INVENTION
The hydrogen generating agent used in the hydrogen generating method of the present invention is an alkaline solution of a metal hydride complex compound.
As the metal hydride complex compound, a compound represented by the general formula (I):
^{_{M a M b H 4-n}} R n (I)
Wherein, M ^a is an alkali metal, M ^b is a Group 3A metal of the Periodic Table of the Elements, R represents an alkyl, alkoxy, aryl, aryloxy or alkanoyloxy group, n represents 0 to 4 integer Is]
Can be used.
In the formula (I), the alkali metal means, for example, lithium, sodium, potassium, rubidium and the like. The metal of Group 3A of the periodic table means, for example, boron, aluminum, gallium and the like.
[0007]
The alkyl group is a linear or branched alkyl group having 1 to 20 carbon atoms, for example, methyl, ethyl, n-propyl, isopropyl, butyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl , Nonyl, decyl, undecyl, dodecyl, tridecyl, tetradecyl, pentadecyl, hexadecyl, heptadecyl, octadecyl, nonadecyl, icosyl and the like. Among them, those having 1 to 12 carbon atoms, for example, methyl, ethyl, n-propyl, isopropyl, butyl, sec-butyl, tert-butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, undecyl, dodecyl, tridecyl , Tetradecyl, pentadecyl and hexadecyl are preferred.
The alkoxy group is a linear or branched alkoxy group having 1 to 20 carbon atoms, for example, methoxy, ethoxy, n-propoxy, isopropoxy, sec-butoxy, tert-butoxy, pentyloxy, hexyloxy, heptyloxy, It means octyloxy, nonyloxy, decyloxy, undecyloxy, dodecyloxy, tridecyloxy, tetradecyloxy, pentadecyloxy, hexadecyloxy, heptadecyloxy, octadecyloxy, nonadecyloxy, icosyloxy and the like. Among them, those having 1 to 6 carbon atoms, such as methoxy, ethoxy, n-propoxy, isopropoxy, sec-butoxy, tert-butoxy, pentyloxy and hexyloxy are preferred.
[0008]
The aryl group has 6 to 18 carbon atoms and means, for example, phenyl, naphthyl, anthracenyl, pyrenyl and the like. Among them, those having 6 to 12 carbon atoms, for example, phenyl and naphthyl are preferable.
The aryloxy group means an aryloxy group having 6 to 18 carbon atoms, for example, phenoxy, naphthyloxy, anthracenyloxy, pyrenyloxy and the like. Among them, those having 6 to 12 carbon atoms, for example, phenoxy and naphthyloxy are preferable.
[0009]
The alkanoyloxy group means a linear or branched alkylcarbonyl group having 1 to 20 carbon atoms, such as formyl, acetyl, propionyl, butyryl, valeryl, hexanoyl, heptanoyl, stearoyl, isobutyryl, isovaleryl, pivaloyl and the like.
[0010]
Specific examples of the metal hydride complex compound represented by the general formula (I) include, for example, lithium borohydride, sodium borohydride, potassium borohydride, lithium aluminum hydride, sodium trimethoxyborohydride, and triacetoxyhydride. Sodium borohydride, sodium triphenoxyborohydride, lithium triethylborohydride, lithium tri-s-butylborohydride, lithium tributylborohydride, potassium triphenylborohydride, potassium tri-sec-butylborohydride, trimethoxy Examples thereof include lithium aluminum hydride, lithium monoethoxy aluminum hydride, lithium tri-tert-butoxy aluminum hydride, and sodium bis (2-methoxyethoxy) aluminum hydride. Among them, lithium borohydride, sodium borohydride and potassium borohydride are preferable, and sodium borohydride which is more excellent in price and safety is more preferable.
[0011]
The alkali solution may be any one that can dissolve the metal hydride complex compound, and preferably one that can completely dissolve the metal hydride complex compound, but one that can stably maintain the metal hydride complex compound. If present, it may not completely dissolve the metal hydride complex compound. Examples of the alkaline solution include a solution in which a liquid basic substance or a non-liquid basic substance is dissolved in a solution.
[0012]
Examples of the liquid basic substance include triethylamine, diisopropylethylamine, and the like. Non-liquid basic substances include lithium hydroxide, sodium hydroxide, alkali metal hydroxides such as potassium hydroxide, sodium bicarbonate, alkali metal bicarbonates such as potassium bicarbonate, sodium carbonate, potassium carbonate and the like. Quaternary alkylammonium compounds such as tetramethylammonium hydroxide and tetraethylammonium hydroxide, such as alkali metal carbonates, and the like. Among them, alkali metal hydroxides which dissolve in water such as lithium hydroxide, sodium hydroxide and potassium hydroxide, exhibit alkalinity and have high solubility are preferable, and sodium hydroxide is particularly preferable.
[0013]
Examples of the solution include water or a water-miscible organic solvent such as an alcohol (eg, methanol, ethanol, ethylene glycol, diethylene glycol, and the like), dimethylformamide, dimethylacetamide, ethylene glycol, diethylene glycol, and the like. These may be used alone or as a mixture of two or more. Further, a liquid basic substance may be used as a mixture with the above-mentioned solution. Among them, water is preferred.
The concentration of the alkaline solution is, for example, about 3 to 9 mol / l.
[0014]
The concentration of the metal hydride complex compound in the alkaline solution can be appropriately adjusted according to the amount of generated hydrogen and the generation rate. Therefore, by controlling this concentration, the amount of generated hydrogen can be controlled. In order to generate hydrogen efficiently, the concentration is preferably as high as possible. The upper limit is the saturated concentration of the alkaline substance, but if the concentration is high, the amount of hydrogen generated per volume increases, but if the concentration is too high, the metal hydride complex compound is difficult to dissolve, and the alkaline solution is heated at high and low temperatures. There is a significant difference in the solubility of the metal hydride complex in water. Therefore, depending on a change in ambient temperature, the metal hydrogen complex compound may cause a crystallization phenomenon before reaching the saturation solubility. Thus, for example, the ratio of the metal hydride complex compound to the alkaline solution used is about 9 to 22 moles of the metal hydride complex: alkaline solution: about 6 to 18 moles, and further about 12 to 19 moles: about 8 to 15 moles. Is appropriate.
[0015]
Acids used in the aqueous acid solution include inorganic acids such as sulfuric acid, hydrochloric acid, and organic acids such as citric acid, phosphoric acid, acetic acid, formic acid, and propionic acid. In general, when sodium hydroxide is used as an alkaline aqueous solution and sulfuric acid is used as an acid aqueous solution, the reaction produces hardly soluble sodium sulfate simultaneously with hydrogen. Further, if the reaction continues, the liquid in the reaction vessel is slurried by the sodium sulfate precipitate and the viscosity increases, so that the liquid foams violently when hydrogen is generated, and the acid aqueous solution may be discharged from the hydrogen gas outlet. . In addition, when nitric acid or hydrochloric acid is selected as the acid aqueous solution, toxic gases such as sulfurous acid gas and chlorine gas are generated simultaneously with the generation of hydrogen. May affect the tolerance of Further, an acid having low solubility in water such as oxalic acid may have difficulty in securing a hydrogen ion concentration in a solution in some cases. Therefore, depending on the mode of use, it is preferable to generate a sparingly soluble salt by reacting with the hydrogen generating agent or generate no toxic gas.Furthermore, the solubility in water is high, and a sufficient hydrogen ion concentration can be ensured. Organic acids, which are possible and relatively safe to handle, are preferred. Among them, aqueous citric acid and aqueous phosphoric acid are more preferred. In addition, as a medium of the aqueous acid solution, water or a mixture of water and one or more of the above-described solutions can be used. Among them, water is preferred. The concentration of the acid in the aqueous acid solution is suitably about 20 to 60% by weight (1.2 to 3.8 mol / L), and more preferably about 30 to 50% by weight (1.8 to 3.2 mol / L). Is preferred.
[0016]
In the hydrogen generation method of the present invention, hydrogen is generated by injecting an alkaline solution of a metal hydride complex compound into an aqueous acid solution, and it is preferable to maintain the pH of the aqueous acid solution during hydrogen generation at 4 or less. That is, when the alkaline aqueous solution is injected into the acid solution, the acid solution is neutralized, whereby the hydrogen ion concentration decreases and the pH value increases. Then, the speed of the neutralization reaction decreases so as to correspond to the hydrogen ion concentration. As a result, even if the injection of the hydrogen generating agent is stopped, there occurs a problem that the hydrogen generation does not stop immediately, and the hydrogen generation rate cannot be controlled by the injection speed of the hydrogen generating agent. Therefore, in order to control the generation of hydrogen, it is necessary to keep the pH value of the acid solution low and prevent the neutralization reaction rate from decreasing. For example, it is preferable that the initial acid concentration of the acid aqueous solution is set to be high, and is maintained at pH 4 or less, preferably pH 3.5 or less until the end of hydrogen generation. Further, the acid aqueous solution or the acid itself may be gradually supplied in accordance with the progress of the reaction or the rise of the pH.
[0017]
In the hydrogen generation method of the present invention, as described above, the amount of hydrogen generated is controlled by controlling the injection amount of the hydrogen generating agent into the acid aqueous solution so that the pH of the acid aqueous solution can be maintained at a predetermined value. be able to. Specifically, in the case of the above-described concentrations of the aqueous acid solution and the alkali solution of the metal hydride complex compound, an injection amount of about 0.1 to 30 ml / min is appropriate. Further, adjusting the concentration of the metal hydride complex compound in the hydrogen generating agent or the like may be combined, or adjusting the acid concentration of the aqueous acid solution may be further combined.
[0018]
The hydrogen generation method in the present invention can be realized by a water displacement method or a known method using a cylinder, a hydrogen storage alloy, or the like.
Further, the hydrogen obtained by the present invention can be suitably used for fuel cells and the like.
Hereinafter, embodiments of the hydrogen generation method of the present invention will be described in detail.
[0019]
The hydrogen generation method of the present invention can be performed using, for example, a hydrogen generation apparatus as shown in FIG.
The hydrogen generator contains a first container 1 containing a hydrogen generating agent 2 and a hydrogen generating agent supply unit including a fixed amount liquid feeder 3 for sending the hydrogen generating agent 2 at a constant rate, and an acid aqueous solution 5. , A reaction section composed of a reaction vessel 4 sealed by a rubber stopper 10, a water tank 6 filled with water 8, and a generated hydrogen collection section composed of a second vessel 7 for collecting hydrogen.
The reaction vessel 4 is equipped with a pH meter 11 for measuring the pH of the aqueous acid solution 5, and includes a hydrogen injection pipe 12 for injecting the hydrogen generating agent 2 and an extraction pipe 9 for extracting generated hydrogen. It is connected.
[0020]
Further, the members constituting the hydrogen generator are resistant to the alkali and the aqueous acid solution of the hydrogen generating agent, and the heat of neutralization generated by the reaction between the hydrogen generating agent and the aqueous acid solution and the internal pressure generated by the generated hydrogen. What is necessary is just to be a thing which is resistant to the change of. In particular, the first container 1, the reaction container 4, and the second container 7 are preferably highly sealed, and examples thereof include a glass or polymer beaker and an Ellenmeyer flask. A commercially available pump or the like that can control the amount of liquid to be sent can be used for the fixed amount liquid sending device 3. The second container 4 may be further provided with a stirring function, for example, a stirring blade or a magnetic stirring device. A flow meter, a trap, or the like may be connected to the end of the hydrogen take-out pipe 9.
Using the hydrogen generating apparatus as shown in FIG. 1 described above, the hydrogen generation rate due to the change in the hydrogen generating agent injection rate, the hydrogen generation rate due to the change in the hydrogen generating agent injection rate, and the pH change of the liquid in the reaction vessel. The effect was measured.
[0021]
Example 1
The first container 1 is filled with a hydrogen generating agent composed of 20 wt% of sodium borohydride, 25 wt% of sodium hydroxide, and 55 wt% of water. The reaction vessel 4 is filled with 50 g of a 40 wt% citric acid aqueous solution.
The hydrogen generating agent was injected into the reaction vessel 4 at a rate of 0.75 to 2.25 ml / min by the constant-rate liquid feeder 3, and the hydrogen generation rate was calculated. The result is shown in FIG.
From FIG. 2, it was found that the hydrogen generation rate increased in proportion to the injection rate of the hydrogen generating agent.
[0022]
Example 2
Hydrogen generation was performed under the same conditions except that lithium borohydride was used instead of sodium borohydride in Example 1 and the hydrogen generating agent was injected into the reaction vessel 4 at an injection rate of 0.375 to 1.125 ml / min. The speed was calculated. The result is shown in FIG.
From FIG. 3, it was found that the hydrogen generation rate increased in proportion to the injection rate of the hydrogen generating agent.
[0023]
Example 3
The first container 1 is filled with a hydrogen generating agent composed of 20 wt% of sodium borohydride, 25 wt% of sodium hydroxide, and 55 wt% of water. The reaction vessel 4 is filled with 50 g of a 40 wt% citric acid aqueous solution.
The hydrogen generating agent is injected into the reaction vessel 4 at an injection rate of 1.5 ml / min by the metering liquid feeder 3, and the total amount of hydrogen generated from the start of the injection of the hydrogen generating agent and the amount of the acid aqueous solution in this process The change in pH was measured with a pH meter 11.
[0024]
The result is shown in FIG. In FIG. 4, the solid line indicates the total amount of hydrogen generated from the start of the injection of the hydrogen generating agent, and the dashed line indicates the pH.
From FIG. 4, it was found that the pH of the liquid in the reaction vessel increased with an increase in the amount of the hydrogen generating agent injected, and the slope of the amount of generated hydrogen became smaller when the liquid reached pH 4, resulting in a decrease in the rate of hydrogen generation.
[0025]
Example 4
Except that the concentration of the citric acid aqueous solution was changed to 20 wt%, the total amount of hydrogen generated from the start of the injection of the hydrogen generating agent and the fluctuation of the pH of the acid aqueous solution during this process were measured in the same manner as in Example 3.
The result is shown in FIG. In FIG. 5, the solid line indicates the total amount of hydrogen generated from the start of the injection of the hydrogen generating agent, and the dashed line indicates the pH.
From FIG. 5, although the initial concentration of citric acid is different, as in Example 3, the pH of the liquid in the reaction vessel rises as the injection amount of the hydrogen generating agent increases, and the amount of hydrogen generated when the liquid reaches pH 4 is increased. And the hydrogen generation rate was reduced.
[0026]
Example 5
In the same manner as in Example 3 except that a 40 wt% phosphoric acid aqueous solution was used instead of the citric acid aqueous solution, the total amount of hydrogen generated from the start of injection of the hydrogen generating agent and the pH of the acid aqueous solution in this process were measured. Variation was measured.
FIG. 6 shows the result. In FIG. 6, the solid line indicates the total amount of hydrogen generated from the start of the injection of the hydrogen generating agent, and the dashed line indicates the pH.
FIG. 6 shows that the hydrogen generation rate decreased when the pH reached 4 as in Example 3.
[0027]
Example 6
Except for using a 20 wt% phosphoric acid aqueous solution in place of the 40 wt% phosphoric acid aqueous solution, the same procedure as in Example 3 was carried out, and the total amount of hydrogen produced since the start of injection of the hydrogen generating agent and the pH of the acid aqueous solution in this process Was measured.
FIG. 7 shows the result. In FIG. 7, the solid line indicates the total amount of hydrogen generated from the start of the injection of the hydrogen generating agent, and the dashed line indicates the pH.
As shown in FIG. 7, a decrease in the hydrogen generation rate was observed when the pH reached 4 as in Example 3.
From the above results, it was found that by maintaining the pH of the liquid in the reaction vessel at 4 or less, the hydrogen generation rate in accordance with the amount of the hydrogen generating agent to be injected can be kept constant.
[0028]
【The invention's effect】
According to the present invention, a metal hydride complex compound having the property of reacting violently when contacted with water to generate hydrogen and having stability in an alkaline aqueous solution is used as an alkaline aqueous solution, and an acid aqueous solution is injected into this. By doing so, the alkaline aqueous solution is partially neutralized and the reaction with water is started, whereby the efficiency of hydrogen generation can be increased and further controlled. In other words, an acid aqueous solution is used in place of the solid acid catalyst conventionally used, and a hydrogen generating agent is injected into such an acid aqueous solution, so that the generation of hydrogen by the metal hydride complex compound is more efficient. In addition, the present invention can be realized with good controllability and without requiring a large-scale device. Therefore, it is possible to supply hydrogen used in a fuel cell system or the like with a compact design while controlling the supply amount strictly, and it is possible to provide an inexpensive and useful fuel cell system or the like.
[0029]
In particular, when an aqueous alkali metal hydroxide solution is used as the aqueous alkali solution, an inexpensive, high-concentration aqueous alkali solution can be provided, and the generation of hydrogen can be promoted with better control.
In particular, when the metal-hydrogen complex compound is the specific compound represented by the general formula (I) described above, the solubility in an aqueous alkali solution is high and the degree of ionization is high, so that an efficient hydrogen generator is supplied. be able to.
[0030]
Further, when sodium borohydride is used as the metal hydride complex compound, not only is it safe, but also because the material cost is low, hydrogen can be produced safely and inexpensively.
Further, when an aqueous solution of an organic acid, particularly an aqueous solution of citric acid or phosphoric acid, is used as the aqueous acid solution, more stable and efficient hydrogen generation can be realized.
In addition, by maintaining the pH of the aqueous acid solution during hydrogen generation at 4 or less, a stable hydrogen generation rate can be maintained, and the amount of hydrogen is controlled while controlling the amount of generated hydrogen without requiring a large-scale apparatus. It becomes possible to supply.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing an example of a hydrogen device used to carry out the hydrogen generation method of the present invention.
FIG. 2 is a diagram showing the relationship between the rate of hydrogen generation and the rate of injection of a hydrogen generating agent when sodium borohydride and an aqueous citric acid solution are used.
FIG. 3 is a diagram showing the relationship between the rate of hydrogen generation and the rate of injection of a hydrogen generating agent when using lithium borohydride and an aqueous solution of citric acid.
FIG. 4 is a graph showing a relationship between a hydrogen generating agent injection amount, a hydrogen generation amount, and a pH in an acid aqueous solution when sodium borohydride and a 40 wt% citric acid aqueous solution are used. It is a figure showing the relation with speed.
FIG. 5 is a diagram showing a relationship between a hydrogen generating agent injection amount, a hydrogen generation amount, and a pH in an acid aqueous solution when sodium borohydride and a 20 wt% citric acid aqueous solution are used. It is a figure showing the relation with speed.
FIG. 6 is a diagram showing the relationship between the amount of hydrogen generating agent injected, the amount of hydrogen generated, and the pH in an acid aqueous solution when sodium borohydride and a 40 wt% phosphoric acid aqueous solution are used. It is a figure showing the relation with speed.
FIG. 7 is a diagram showing the relationship between the amount of hydrogen generating agent injected, the amount of hydrogen generated, and the pH in an acid aqueous solution when sodium borohydride and a 20 wt% phosphoric acid aqueous solution are used. It is a figure showing the relation with speed.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 1st container 2 Hydrogen generating agent 3 Constant-quantity liquid feeder 4 Reaction container 5 Acid aqueous solution 6 Water tank 7 Second container 8 Water 9 Removal pipe 10 Rubber stopper 11 pH meter 12 Hydrogen injection pipe

Claims (8)

A hydrogen generation method comprising: injecting an alkali solution of a metal hydride complex compound as a hydrogen generating agent into an aqueous acid solution and reacting to generate hydrogen. The hydrogen generation method according to claim 1, wherein the alkali solution is an aqueous solution of an alkali metal hydroxide. The metal hydride complex is represented by the general formula (I):
^{_{M a M b H 4-n}} R n (I)
Wherein, M ^a is an alkali metal, M ^b is a Group 3A metal of the Periodic Table of the Elements, R represents an alkyl, alkoxy, aryl, aryloxy or alkanoyloxy group, n represents 0 to 4 integer Is]
The method for producing hydrogen according to claim 1 or 2, which is a compound represented by the formula: The method for producing hydrogen according to claim 3, wherein the metal hydride complex is sodium borohydride. The method for producing hydrogen according to any one of claims 1 to 4, wherein the aqueous acid solution is an aqueous solution of an organic acid. The hydrogen generation method according to claim 5, wherein the aqueous solution of the organic acid is a citric acid aqueous solution or a phosphoric acid aqueous solution. The method for producing hydrogen according to any one of claims 1 to 6, wherein the amount of hydrogen produced is controlled by controlling the amount of the hydrogen producing agent injected into the aqueous acid solution or the concentration of the aqueous alkali metal compound complex. The hydrogen generation method according to any one of claims 1 to 7, wherein the pH of the aqueous acid solution during hydrogen generation is maintained at 4 or less.

Images