JP2002201001A - Stabilization method for metal complex hydride water solution - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method to stabilize a metal complex hydride water solution and to be able to prevent sufficiently hydrogen generation from the water solution under the state of noncontacting with a catalyst. SOLUTION: The method is to stabilize the metal complex hydride water solution by adding an alkali. Concerning the characteristics of the method, the content of the metal complex hydride in the water solution is 30 wt.% or more and that of the alkali is 2.5 wt.% or more.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to a method for stabilizing an aqueous solution of a complex metal hydride.

[0002]

2. Description of the Related Art In modern society, hydrogen is an important chemical raw material that is used in large quantities in the synthetic chemical industry, petroleum refining, and the like. On the other hand, in order to solve future energy problems and environmental problems, hydrogen utilization technology as clean energy is considered to be important, and the development of fuel cells that store hydrogen and use it as fuel is underway. Have been.

[0003] Such fuel cells are gas operated cells, in which the energy obtained from the reaction of hydrogen and oxygen is directly converted to electrical energy. Because such fuel cells have extremely high efficiency compared to conventional combustion engines, vehicles with fuel cells are not compatible with ZEV (Zero Emissio
n Vehicle).

[0004] On the other hand, as a method of storing hydrogen, a method of compressing and storing in a cylinder, a method of cooling into liquid hydrogen, a method of adsorbing on activated carbon, and a method of using a hydrogen storage alloy have been proposed. Among these methods, it is considered that a hydrogen storage alloy plays a major role in a moving medium such as a fuel cell vehicle. However, hydrogen-absorbing alloys also include weight due to being an alloy (small occlusion amount per unit weight), deterioration due to repeated occlusion and release (pulverization and structural change of alloy), and rare metals. There are many issues to be overcome, such as securing resources.

In recent years, a method of generating hydrogen by hydrolyzing a rock salt type alkali hydride (sodium hydride) proposed by Powerball has attracted attention. Sodium hydride reacts violently when it comes in contact with water to generate hydrogen, so that sodium hydride is coated with a resin film, and hydrogen is generated by cutting this film. However, the amount of hydrogen that can be generated from sodium hydride is a maximum of 8.8% by weight (per gram of sodium hydride), and there is a problem that the energy density is not always sufficient to be used as fuel for a fuel cell vehicle. In addition, rock salt-type alkali hydride reacts violently when it comes into contact with water, which has been a problem in terms of safety.

Against this background, sodium borohydride, which is a water-soluble complex metal hydride, has attracted attention as a new hydrogen generation source. Hydrogen is generated from sodium borohydride according to the following hydrolysis reaction: NaBH ₄ + 2H ₂ O → NaBO ₂ + 4H ₂ . And the amount of hydrogen that can be generated from sodium borohydride is a maximum of 21.3% by weight (per 1 g of sodium borohydride), and the amount of hydrogen generated is twice or more as compared with the method using sodium hydride. The energy density required for a fuel cell vehicle will be satisfied. It is known that such hydrolysis of sodium borohydride is promoted in the presence of a catalyst, "An ultrasafe hydrogen generator: aqueou
s, alkaline borohydride solutions and Ru catalyst ",
SCAmendola et al., Journal of Power Sources, vo
L.85, the p.186-189 (2000), 20% NaBH 4 and 10% NaOH
A method is described in which a ruthenium-based catalyst is brought into contact with an aqueous sodium borohydride solution containing hydrogen to generate hydrogen.

[0007]

However, in such a conventional method for generating hydrogen using an aqueous solution of a complex metal hydride, a small amount of hydrogen is generated even when the aqueous solution is not in contact with a catalyst. As a result, there is a problem in that it is difficult to control the generation of hydrogen depending on the presence or absence of contact with the catalyst.

The present invention has been made in view of the above-mentioned problems of the prior art, and has been made to stabilize an aqueous solution of a complex metal hydride, and to stabilize an aqueous solution of a complex metal hydride (hereinafter, referred to as a catalyst non-contact state in some cases). ) Is intended to provide a method capable of sufficiently preventing generation of hydrogen from the aqueous solution.

[0009]

Means for Solving the Problems The present inventors have conducted intensive studies to achieve the above object, and as a result, the content of the complex metal hydride in the aqueous solution of the complex metal hydride was set to 30% by weight or more, Further, they have found that it is possible to sufficiently stabilize an aqueous solution of a complex metal hydride by adding an alkali so as to have a content of 2.5% by weight or more,
The present invention has been completed.

That is, the method of stabilizing an aqueous solution of a complex metal hydride according to the present invention is a method of stabilizing an aqueous solution of a complex metal hydride by adding an alkali, wherein the content of the complex metal hydride in the aqueous solution is Is 30% by weight or more and the alkali content is 2.5% by weight or more.

In the method for stabilizing an aqueous solution of a complex metal hydride according to the present invention, the aqueous solution of a complex metal hydride is stabilized,
When not in contact with the catalyst, generation of hydrogen from the aqueous solution is sufficiently prevented. The reason why the aqueous solution of the complex metal hydride is stabilized is not clear, but the amount of protons (H ⁺ ) in the aqueous solution of the complex metal hydride is reduced by adding an alkali to the aqueous solution of the complex metal hydride, Furthermore, since the concentration of the complex metal hydride in the aqueous solution is increased and the water content is relatively reduced by adding an alkali, it reacts with H ⁻ in the complex metal hydride in the hydrolysis reaction. The present inventors believe that the amount of protons in water that can become hydrogen is reduced and the generation of hydrogen in a catalyst non-contact state is sufficiently suppressed.

Further, in the method for stabilizing an aqueous solution of a complex metal hydride according to the present invention, the ratio of the content of water to the content of the complex metal hydride in the aqueous solution (water content / content of complex metal hydride). Amount) (weight ratio) is preferably 0.5 to 2.22. If this ratio is less than the above lower limit, when the aqueous solution is brought into contact with the catalyst to generate hydrogen, the amount of water that contributes to hydrolysis is small, and a sufficient amount of generated hydrogen tends not to be achieved. On the other hand, when this ratio exceeds the above upper limit, generation of hydrogen in a state where the aqueous solution is not brought into contact with the catalyst tends to be not sufficiently prevented.

Further, in the method for stabilizing an aqueous solution of a complex metal hydride according to the present invention, the alkali is preferably sodium hydroxide or potassium hydroxide. If such a strong alkali is used, H in the aqueous solution of the complex metal hydride may be used.
^- The amount is more efficiently reduced, and there is a tendency that the generation of hydrogen in the catalytic non-contact state is suppressed more reliably.

[0014]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the method for stabilizing an aqueous solution of a complex metal hydride according to the present invention will be described below in detail.

In the method for stabilizing an aqueous solution of a complex metal hydride according to the present invention, the content of the complex metal hydride in the aqueous solution of the complex metal hydride is set to 30% by weight or more,
Alkali is added so as to have a content of at least% by weight. As a result, the aqueous solution of the complex metal hydride is sufficiently stabilized, and the generation of hydrogen from the aqueous solution is sufficiently prevented when the aqueous solution is not in contact with the catalyst.

Such a complex metal hydride has a high hydrogen content and is efficiently produced when it is brought into contact with a catalyst. Therefore, NaBH ₄ , NaAlH ₄ , LiBH ₄ , LiAlH ₄ , KB
_{H 4, KAlH 4, Mg (} BH 4) 2, Ca (BH 4) 2, Ba (BH 4) 2, Sr (BH 4) 2
And Fe (BH ₄ ) ₂ are preferred. Such complex metal hydrides may be used alone or in combination of two or more. Since NaBH ₄ is inexpensive, has relatively low reactivity with water itself, and has a high theoretical capacity for hydrogen generation of 21.3 wt%, NaBH ₄ is more preferable as the complex metal hydride.

Further, the alkali according to the present invention includes:
Strong alkalis such as sodium hydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxide, sodium metaborate, ammonium hydroxide, and barium hydroxide are preferably used, and among them, sodium hydroxide and potassium hydroxide are more preferable. When such an alkali is used, the amount of H 2 ^{− in} the aqueous solution of the complex metal hydride tends to be reduced more efficiently, and the generation of hydrogen in a catalyst non-contact state tends to be more reliably suppressed. In addition, such an alkali may be used alone or in combination of two or more.

In the method for stabilizing an aqueous solution of a complex metal hydride according to the present invention, the content of the complex metal hydride in the aqueous solution of the complex metal hydride must be 30% by weight or more, and 30 to 50% by weight. % Is more preferable. When the content of the complex metal hydride is less than 30% by weight, the generation of hydrogen in a state where the aqueous solution is not in contact with the catalyst is not sufficiently prevented even if the alkali is added, while 50% by weight
If it exceeds 300, when the aqueous solution is brought into contact with the catalyst to generate hydrogen, the amount of water that contributes to hydrolysis is small, and a sufficient amount of generated hydrogen tends not to be achieved. When the solubility of the complex metal hydride to water used is lower than the above upper limit, the content of the complex metal hydride is preferably lower than the solubility.

Further, in the present invention, the alkali content in the aqueous solution of the complex metal hydride must be 2.5% by weight or more, and more preferably 2.5 to 35% by weight. If the alkali content is less than 2.5% by weight, the generation of hydrogen in a state where the aqueous solution is not in contact with the catalyst is not sufficiently prevented. The amount of water contributing to the hydrolysis when generating the hydrogen is small, and a sufficient amount of generated hydrogen tends not to be achieved. When the solubility of the alkali used in water is lower than the above upper limit, the alkali content is preferably lower than the solubility.

Further, in the present invention, the content of water in the aqueous solution of the complex metal hydride is more preferably 35 to 67% by weight. If the water content exceeds 67% by weight, generation of hydrogen in a state where the aqueous solution is not in contact with the catalyst is not sufficiently prevented. On the other hand, if the water content is less than 35% by weight, the aqueous solution is brought into contact with the catalyst to generate hydrogen. During the reaction, the amount of water that contributes to the hydrolysis is small, and a sufficient amount of generated hydrogen tends not to be achieved.

In the present invention, the ratio of the content of water to the content of complex metal hydride in the aqueous solution of complex metal hydride (water content / content of complex metal hydride) (weight ratio) It is preferably 0.5 to 2.22. If this ratio is less than 0.5, the amount of water that contributes to hydrolysis when generating hydrogen by contacting such an aqueous solution with a catalyst tends to be insufficient to achieve a sufficient amount of hydrogen generation.
If it exceeds 22, the generation of hydrogen in a state where the aqueous solution is not brought into contact with the catalyst tends to be not sufficiently prevented.

In the method for stabilizing an aqueous solution of a complex metal hydride according to the present invention, an aqueous solution containing the above amount of complex metal hydride is added with an alkali of the above amount to form an aqueous solution. Stabilized and in a state not in contact with the catalyst, the generation of hydrogen from the aqueous solution is sufficiently prevented, but the temperature of the aqueous solution at that time is 50 ° C.
The following is preferred. When the temperature of the aqueous solution is 50 ° C. or lower, generation of hydrogen from the aqueous solution tends to be more reliably prevented. When the aqueous solution of the complex metal hydride is stably maintained, the aqueous solution of the complex metal hydride and components other than the alkali may be contained. As such a component, a gas inert to the reaction (nitrogen, CO ₂ , Ar, etc.).

According to the stabilizing method of the present invention, it is possible to maintain the aqueous solution of the complex metal hydride stably, that is, in a state where the generation of hydrogen is sufficiently prevented, for a long period of time exceeding 100 hours. The method for generating hydrogen from the complex metal hydride aqueous solution is not particularly limited. For example, the complex metal hydride is hydrolyzed by contacting the above-mentioned complex metal hydride aqueous solution with a catalyst to generate a sufficient amount of hydrogen. Can be.

As such a catalyst, (i) a hydrogen generating catalyst comprising a metal halide and (ii) a hydrogen generating catalyst comprising a noble metal and a metal oxide or a carbonaceous material are preferably used. Metal chlorides such as cobalt chloride, nickel chloride and ruthenium chloride are more preferred as the metal halide.

The metal oxide according to the catalyst (ii) includes noble metal elements (Pt, Pd, Rh, Ru, Au, etc.) and base metal elements (Y, La, Ce, Pr, Nd, Eu, Gd , Tb, Dy, Ho, Er, Tm,
Yb, Lu, Ca, Mg, Al, K, Ti, Cr, Mn, Fe, Co, Ni, Cu,
Ga, Rb, Sr, Zr, Nb, Mo, In, Sn, Cs, Ba, Ta, W
Etc.) and single oxides or composite oxides of metalloid elements (Si, Ge, As, Sb, etc.), among which titanium oxide, nickel oxide, cerium oxide, zeolite, alumina, zirconia, silicon oxide, silicon oxide, iron oxide, and oxide Manganese, cobalt oxide, zinc oxide and copper oxide are preferred. Further, as such a metal oxide, a composite metal oxide containing lithium is more preferable, and lithium cobaltate (LiCo
O ₂ ), lithium nickelate (LiNiO ₂ ), lithium manganate (LiMnO ₂ , LiMn ₂ O ₄ ), lithium vanadate (LiV
Particularly preferred are lithium-containing composite metal oxides such as O ₂ , LiV ₂ O ₄ ) and lithium chromate (LiCrO ₂ ). The carbonaceous materials for the catalyst (ii) include activated carbon, graphite, activated char, coke, hard carbon (hardly graphitized carbon),
Soft carbon (easy graphitizable carbon) is preferred. Furthermore,
Pt, Pd, Rh, Ru, Ir, O
s, Au, Ag, among which platinum group elements (Pt, Pd, Rh, Ru, I,
r, Os) are preferred. By using such a noble metal as a catalyst in the presence of the metal oxide or the carbonaceous material, a complex metal hydride is produced by the synergistic effect of the catalytic action of the noble metal and the catalytic action of the metal oxide or the carbonaceous material. Hydrolysis tends to proceed more efficiently and the amount of generated hydrogen tends to be further improved.

The form of the coexistence of the noble metal and the metal oxide or the carbonaceous material may be such that the noble metal is supported on the carrier using the metal oxide or the carbonaceous material as a carrier,
A mixture of the two may be used, but the former is preferred because the catalytic activity tends to be higher. Among them, a carrier obtained by supporting a noble metal on a carrier comprising a lithium-containing composite metal oxide is particularly preferred. Further, the carrier is particles having an average particle diameter of 1000 μm or less, and the noble metal has an average particle diameter of 100 nm.
The following fine particles are preferable because the catalytic activity tends to be higher. Further, the content of the noble metal in the catalyst is preferably from 0.01 to 20% by weight, more preferably from 0.5 to 5% by weight, based on the total weight of the catalyst. When the content of the noble metal is less than 0.01% by weight, the catalytic action of the noble metal tends to be insufficient.
The shape of the catalyst is not particularly limited, and a shape such as a powder, a pellet, a monolith, a plate, and a fiber can be selected according to the use conditions.

The method of causing the noble metal to coexist with the metal oxide or the carbonaceous material is not particularly limited. For example, a noble metal and / or a noble metal precursor (a noble metal halide,
Noble metal is supported on a carrier made of the above metal oxide or carbonaceous material by a technique such as impregnation method, precipitation method, kneading method or ion exchange method using nitrate, carbonate, acetylacetonate, tetraammine salt, alkoxide, etc. At least it is possible to obtain a catalyst, but international publication number WO9
It is preferable to obtain a catalyst by a supercritical method using a supercritical fluid described in JP-A-9 / 10167. By using a supercritical fluid such as carbon dioxide, the noble metal is dispersed and supported on the carrier as a noble metal simple substance with a fine particle size of 10 nm or less (particularly preferably 1 nm or less).
The catalytic activity tends to be further improved, whereby the hydrogen generation rate and the amount of generated hydrogen tend to be more significantly improved. Further, after the noble metal and / or the noble metal precursor is supported on the carrier as described above, if necessary, a calcination treatment in nitrogen or air, and / or hydrogen or carbon monoxide or a hydrocarbon (methane, It is preferable to perform the reduction treatment in an atmosphere containing acetaldehyde or the like). The conditions of such a calcination treatment and a reduction treatment are not particularly limited.
Conditions such as heating at a temperature of 50 to 1000 ° C. for 1 to 10 hours are employed.

The reaction system for generating hydrogen from the aqueous solution of the complex metal hydride may contain components other than the above aqueous solution of the complex metal hydride and the catalyst, and such a component may be a gas inert to the reaction. (Nitrogen, CO ₂ , Ar, etc.). When generating hydrogen, organic acids such as acetic acid, oxalic acid, carbonic acid, and lactic acid, and inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, sulfurous acid, hydrogen sulfide, and phosphoric acid may be contained. It is preferable to add a larger amount of acid than the alkali contained therein. On the other hand, if oxygen is present, the generated hydrogen tends to burn easily, so it is better to exclude it as much as possible.

The reaction conditions for generating hydrogen from the aqueous solution of the complex metal hydride are not particularly limited.
0 ° C is preferred, and 10 to 80 ° C is more preferred. If the reaction temperature is lower than 0 ° C., water freezes and the hydrogen generation rate tends to decrease, while if it is higher than 200 ° C., even under pressurized conditions, water easily becomes water vapor and the hydrogen generation rate decreases. Tend to.

[0030]

EXAMPLES Hereinafter, the present invention will be described more specifically based on examples and comparative examples, but the present invention is not limited to the following examples.

Examples 1 to 7 and Comparative Examples 1 to 5 5 ml of an aqueous solution of a complex metal hydride having the composition shown in Table 1 was added to a volume of 1
After filling in a 00 ml Erlenmeyer flask, it was kept at the temperature shown in Table 1 for the time shown in Table 1 as well, and hydrogen generation during the measurement was performed based on the water level change of the female burette in Shibata Scientific Gas Analyzer (product code: 6071-4). The amount was determined. Table 1 shows the hydrogen generation amount obtained by the measurement.

[0032]

[Table 1]

As is apparent from the results shown in Table 1, the complex metal hydride aqueous solution of the present invention having a complex metal hydride content of 30% by weight or more and an alkali content of 2.5% by weight or more. It has been confirmed that according to the method for stabilization, generation of hydrogen is reliably prevented for 100 hours even under a temperature condition of 50 ° C.

[0034]

As described above, according to the method for stabilizing an aqueous solution of a complex metal hydride of the present invention, the generation of hydrogen from an aqueous solution of a complex metal hydride is prevented in a state where the aqueous solution of the complex metal hydride is not in contact with a catalyst. It is possible to sufficiently prevent it.
Therefore, the method for stabilizing an aqueous solution of a complex metal hydride of the present invention is very useful in controlling hydrogen generation depending on the presence or absence of contact with a catalyst when using a complex metal hydride as a hydrogen supply source for a fuel cell. is there.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Yasuaki Kawai 41-cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central Research Laboratory Co., Ltd. 41, Yokomichi, Toyota Central Research Laboratory, Inc. (72) Inventor Shinichi Matsumoto 1 Toyota Town, Toyota City, Aichi Prefecture Inside Toyota Motor Corporation (72) Inventor Harumichi Nakanishi 1 Toyota Town, Toyota City, Aichi Prefecture Address Toyota Motor Corporation F term (reference) 5H027 AA02 BA13

Claims (3)

[Claims] 1. A method for stabilizing an aqueous solution of a complex metal hydride by adding an alkali thereto, wherein the content of the complex metal hydride in the aqueous solution is 30% by weight or more and the content of the alkali is reduced. A method for stabilizing an aqueous solution of a complex metal hydride, comprising at least 2.5% by weight. 2. The ratio (weight ratio) of the water content to the complex metal hydride content in the aqueous solution is 0.5 to 2.2.
2. The method for stabilizing an aqueous solution of a complex metal hydride according to claim 1, wherein the method is 2. 3. The method for stabilizing an aqueous solution of a complex metal hydride according to claim 1, wherein the alkali is sodium hydroxide or potassium hydroxide.