JP2003165701A - Hydrogen storage material and method of manufacturing it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a hydrogen storage material which releases high-purity hydrogen and in addition, enough quantity of hydrogen at a low temperature. <P>SOLUTION: The hydrogen storage material is characterized by containing at least one kind of catalyst selected from the group consisting of nickel, chromium, molybdenum, cobalt, copper, palladium, platinum, iron, ruthenium, rhodium, iridium, tungsten, titanium, manganese and osmium, and carbon. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a hydrogen storage material and a method for producing the same.

[0002]

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

Such fuel cells are gas-operated cells, in which the energy obtained from the reaction of hydrogen and oxygen is directly converted into electrical energy. Such a fuel cell has an extremely high efficiency as compared with a conventional combustion engine and emits no toxic gas such as NOx, SOx, CO, etc. Therefore, a vehicle having a fuel cell has a ZEV (Zero Emission).
Vehicle).

On the other hand, as a method for 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, a method of utilizing a hydrogen storage material, etc. have been proposed. Among these methods, the method using a hydrogen storage material is considered to play a major role in a moving medium such as a fuel cell vehicle.

Against this background, the use of carbon as a hydrogen storage material has been proposed (Japanese Patent Laid-Open No. 8-50439).
4, JP-A-2000-103612, JP-A-2
001-106516, etc.). In addition, JP-A-200
0-87172 discloses alkaline earth metals (Mg, Ca) that have been studied as hydrogen storage materials.
Etc.) and a hydrogen storage material in which carbon is mixed, and by mixing such an alkaline earth metal with carbon, the hydrogenation rate of the alkaline earth metal is improved. Furthermore, when producing a hydrogen storage material composed of Mg and graphite, a method of mechanically pulverizing in the coexistence of a specific organic compound (THF, etc.) has been proposed for the purpose of improving the characteristics of the obtained hydrogen storage material. (J. Alloys. Compd.
293-295 (1999) p564-568, Chem. Commun. (1999) p227
7-2278 etc.).

[0006]

However, even the above-mentioned conventional hydrogen storage materials do not necessarily have a large hydrogen storage amount per unit weight, and have sufficient hydrogen storage / release capacity in terms of practical use. It is hard to say that they are doing it. Further, it is desired that the hydrogen storage material can store and release hydrogen reversibly at a low temperature (preferably room temperature). However, in order to release hydrogen from the above conventional hydrogen storage material, a high temperature (for example, 500 ° C. or higher) is required. ), The hydrocarbon (HC) is easily mixed in the hydrogen released by such heating, and the purity of hydrogen becomes insufficient.

The present invention has been made in view of the above problems of the prior art, and an object of the present invention is to provide a hydrogen storage material capable of releasing hydrogen of a high purity and a sufficient amount at a low temperature.

[0008]

In order to solve the above problems, the hydrogen storage material of the present invention comprises nickel, chromium, molybdenum, cobalt, copper, palladium, platinum, iron, ruthenium, rhodium, iridium, tungsten and titanium. ,
It is characterized by containing carbon and at least one catalyst selected from the group consisting of manganese and osmium.

In the hydrogen storage material of the present invention, the reversible reaction of storage and release of hydrogen is sufficiently achieved by the synergistic effect of the inherent hydrogen storage / release capacity of carbon and the desorption action of hydrogen by the above-mentioned specific catalyst. Since it is promoted, it becomes possible to release hydrogen of high purity and a sufficient amount at a low temperature.

The hydrogen storage material of the present invention has a wave number of 135 obtained by Raman spectrum measurement as a raw material of carbon.
The full width at half maximum of the Raman peak at 0 cm ^-1 is represented by the following formula (1): d <(376 / La) +19.0 (1) [wherein, d represents the full width at half maximum (cm ^-1 ) of the Raman peak,
La represents a crystal grain size (nm) in a plane including the a-axis and the b-axis of the graphite structure]. When the graphite crystal particles satisfying the above specific conditions are blended in the catalyst, the above synergistic effect is further enhanced by the improvement of the dispersion uniformity of the catalyst and carbon, so that a hydrogen storage material having a better hydrogen storage / release capacity is obtained. Will be realized.

Further, in the hydrogen storage material of the present invention,
The carbon content is preferably 40 atomic% or more based on the total amount of the catalyst and carbon. When the carbon content is 40 atomic% or more, the hydrogen storage / release capacity tends to be further improved.

Further, the method for producing a hydrogen storage material of the present invention comprises a pulverization step of obtaining graphite crystal particles by mechanically pulverizing graphite, graphite crystal particles obtained in the pulverization step, nickel,
The catalyst and carbon are mixed with at least one catalyst selected from the group consisting of chromium, molybdenum, cobalt, copper, palladium, platinum, iron, ruthenium, rhodium, iridium, tungsten, titanium, manganese, and osmium. And a mixing step for obtaining a hydrogen storage material to be contained.

According to the manufacturing method of the present invention, graphite crystal particles having a fine and uniform particle size can be obtained by such a pulverizing step. By using such graphite crystal particles as a raw material of carbon and mixing this with the above-mentioned specific catalyst, the hydrogen storage material of the present invention having excellent hydrogen storage / release capacity can be efficiently and reliably obtained.

The production method of the present invention preferably further comprises a hydrogen treatment step of bringing the hydrogen storage material into contact with hydrogen. When the carbon contained in the hydrogen storage material has an active site that causes an irreversible reaction, hydrogen once stored may be difficult to be released at a low temperature. Since the points are terminated with hydrogen and deactivated, the effect of promoting the reversible reaction (storage / release of hydrogen) can be further enhanced.

[0015]

BEST MODE FOR CARRYING OUT THE INVENTION Preferred embodiments of the present invention will be described in detail below.

As described above, the catalyst contained in the hydrogen storage material of the present invention is nickel (Ni), chromium (Cr), molybdenum (Mo), cobalt (Co), copper (Cu), palladium (Pd), platinum. (Pt), iron (Fe), ruthenium (Ru), rhodium (Rh), iridium (I
r), tungsten (W), titanium (Ti), manganese (Mn), or osmium (Os). By using these catalysts, a sufficiently high hydrogen desorption action can be obtained. These catalysts may be used alone or in combination of two or more.

The shape of the catalyst is not particularly limited, but its average particle size is preferably 10 μm or less. By using a catalyst having an average particle size of 10 μm or less, the contact efficiency between the catalyst and carbon is increased, so that the synergistic effect of the hydrogen storage / release capacity inherent in carbon and the dehydrogenation action of the catalyst is further enhanced. You can

The hydrogen storage material of the present invention comprises the above catalyst and carbon. As a raw material of such carbon,
Highly pure natural graphite and highly oriented pyrolytic graphite (HOP
It is preferable to use graphite such as artificial graphite having a high degree of graphitization such as G).

FIG. 1 is an explanatory view schematically showing an example of graphite crystal particles used in the present invention. In FIG. 1, carbon layers 1 having substantially the same shape and area are stacked in a column shape to form crystal particles having a graphite structure. In addition, in FIG.
La [nm] is the crystal grain size in the plane including the a-axis and the b-axis of the graphite structure (crystal grain diameter in the direction horizontal to the surface of the carbon layer 1), and Lc [nm] is the crystal in the c-axis direction of the graphite structure. The particle size (the stacking thickness of the carbon layer 1) is shown.

As the graphite crystal fine particles, those having high crystallinity and crystal particle size uniformity are preferably used. Specifically, the half-value width d of the Raman peak at a wave number of 1350 cm ⁻¹ obtained by Raman spectrum measurement is used. It is preferable to use graphite crystal particles satisfying the following formula (1): d <(376 / La) +19.0 (1). The relationship between d and La represented by the above formula (1) is an index of the crystallinity of carbon and the uniformity of the crystal grain size, and when d and La do not satisfy the above conditions, the crystallinity and The uniformity of the crystal grain size is low and the hydrogen storage / release capacity tends to be insufficient. For the same reason as above, graphite crystal particles having a half width d and a crystal particle size La satisfying the condition represented by the following formula (2): d <(341 / La) +10.5 (2) are used. Is more preferable.

In the graphite crystal particles, the crystal particle size La is preferably 4.0 nm or less. When La is 4.0 nm or less, the number of vacancies generated between the crystal particles is increased and the hydrogen storage / release capacity tends to be further enhanced.

Further, it is preferable that the graphite crystal particles have a sufficiently long length in the in-plane direction (the direction horizontal to the plane including the a-axis and the b-axis of the graphite structure). The ratio (La / Lc) is preferably 0.15 or more. Furthermore, the specific surface area of the graphite particles is 100 m ² /
It is preferably at least g. This makes it possible to increase the number of voids generated on the surface of the graphite crystallite.

Further, in the graphite crystal particles, the ratio of the number of atoms of carbon and hydrogen (H / C) is preferably 0.05 or less. When H / C is 0.05 or less, formation of hydroxyl groups and carboxyl groups at the ends of crystal particles is suppressed.

In the hydrogen storage material of the present invention, the dispersion state of the catalyst and carbon is not particularly limited as long as the contact between the catalyst and carbon is sufficient. For example, a state in which the catalyst particles and the graphite crystal particles are finely dispersed, a state in which the catalyst particles are introduced between the carbon layers of the graphite crystal particles to form an intercalation compound, or the catalyst particles are present on the surface of the graphite crystal particles. It may be in any supported state. Further, for example, when graphite crystal particles satisfying the condition represented by the above formula (1) are used as the carbon raw material, the graphite particles are dispersed in the hydrogen storage material in a state not satisfying the condition represented by the formula (1). It may have been done.

The content ratio of the catalyst and carbon is not particularly limited as long as sufficient hydrogen storage / release capacity is obtained, but the content ratio of carbon is 40 atomic% or more based on the total amount of the catalyst and carbon. Is preferred. If the carbon content is less than 40 atomic%, the hydrogen storage / release capacity tends to decrease.

As described above, the hydrogen storage material of the present invention contains the above-mentioned specific catalyst and carbon, and the synergistic effect of the hydrogen storage / release capacity inherent in carbon and the desorption action of hydrogen by the catalyst. The effect sufficiently promotes the reversible reaction of hydrogen occlusion / desorption, so that a sufficient amount of hydrogen can be released. Further, the hydrogen storage material of the present invention is capable of storing and releasing hydrogen at low temperature (for example, room temperature),
The release of hydrogen at such a low temperature prevents hydrocarbon (HC) from being mixed in, so that the purity of the released hydrogen can be maintained at a high level. The hydrogen storage material of the present invention having such excellent hydrogen storage / release capacity can be efficiently and reliably obtained by the production method of the present invention described later.

That is, the production method of the present invention includes a pulverizing step of mechanically pulverizing graphite to obtain graphite crystal particles, and a mixing step of mixing the graphite crystal particles with the above-mentioned specific catalyst. The production method of the present invention also includes a method in which the pulverization step and the mixing step are performed at the same time, that is, the mechanically pulverized graphite and the catalyst are mixed to perform mechanical pulverization.

The crushing step is preferably carried out so that the obtained graphite crystal particles satisfy the condition represented by the above formula (1). Therefore, a crushing device (ball mill or the like) capable of setting the crushing acceleration to 2 G or more is used. It is preferably used. In particular, it is preferable to use a planetary ball mill because a high crushing acceleration of 10 G or more can be obtained and the crushing effect can be further enhanced.

When oxygen is present in the atmosphere during the crushing step, the graphite being crushed is likely to ignite, so that the crushing process is preferably performed in an atmosphere of an inert gas such as argon. Further, when the pulverization process is performed in an inert gas atmosphere, the amount of impurities such as hydrogen and oxygen mixed can be reduced.

The graphite crystal particles obtained in the pulverizing step are mixed with the above-mentioned specific catalyst to obtain the hydrogen storage material of the present invention in which the catalyst and carbon are sufficiently uniformly dispersed (mixing step).

In the mixing step, the catalyst may be mixed with the graphite crystal particles as it is, or a predetermined compound which is a precursor of the catalyst may be mixed with the graphite crystal particles to support the catalyst on the graphite crystal particles. Good. For example, in the case of obtaining a hydrogen storage material in which graphite crystal particles support platinum, platinum acetylacetonate can be used as a catalyst precursor.

Further, in the mixing step, it is preferable to use a supercritical fluid such as carbon dioxide (CO ₂ ) as a solvent. When the catalyst and carbon are mixed in a supercritical fluid, the homogeneity of dispersion of both is improved and the hydrogen storage / release capacity tends to be further enhanced. At this time, the treatment conditions vary depending on the type of supercritical fluid, but for example, when using carbon dioxide, the treatment temperature is 40 to 200 ° C. and the pressure is 5 to 50 MP.
a, the treatment time is preferably 0.1 to 10 hours.

By the above pulverizing step and mixing step, the catalyst and carbon can be made sufficiently fine and uniform, and the hydrogen storage material of the present invention excellent in hydrogen storage / release capacity can be efficiently and reliably obtained. However, the manufacturing method of the present invention,
It is preferable to further include a hydrogen treatment step of bringing the obtained hydrogen storage material into contact with hydrogen. When the carbon contained in the hydrogen storage material has an active site that causes an irreversible reaction, hydrogen once stored may be difficult to be released at a low temperature. Since the points are terminated with hydrogen and deactivated, the effect of promoting the reversible reaction (storage / release of hydrogen) can be further enhanced.

When carrying out the hydrogen treatment step, the treatment temperature is preferably 20 to 300 ° C. and the hydrogen pressure is preferably 0.1 to 10 MPa. For example, sufficient hydrogen treatment is performed by holding the hydrogen storage material after the mixing step in an atmosphere of 25 ° C. and a hydrogen pressure of 5 MPa.

The hydrogen treatment step may be carried out only once, but by carrying out the hydrogen treatment step a plurality of times, the hydrogen storage / release capacity of the hydrogen storage material can be further enhanced. That is, a predetermined amount of hydrogen is stored in the hydrogen storage material after one hydrogen treatment step, but after releasing this hydrogen by decompression or the like, the hydrogen treatment step is performed again and the operation of storing hydrogen is repeated. By this, hydrogen storage capacity (hydrogen release capacity)
Can be increased.

[0036]

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

Example 1 In a glove box kept in an argon gas atmosphere, 5 g of graphite and stainless steel balls (4 mmφ) in a stainless steel container (internal volume: 8)
0 ml), and use a planetary ball mill at 400 rpm for 1
The mechanical crushing treatment was performed for 2 hours. Thus, the half-width d of the Raman peak of wavenumber 1350 cm ^-1 obtained by Raman spectroscopy is the 61cm ^-1, crystal grain size La in the plane including the a-axis and b-axis of the graphite structure is located at 3.7nm The graphite crystal particles satisfying the conditions represented by the above formula (1) were obtained.

Next, the obtained graphite crystal particles were placed in a stainless steel reaction container (internal volume: 50 ml), 0.5 g of platinum acetylacetonate and 5 ml of acetone were added thereto, and carbon dioxide was sealed therein. Was sealed. This reaction vessel was heated to 150 ° C. in an oil bath and kept for 2 hours. The pressure in the reaction vessel at this time was 30 MPa. Then, the inside of the reaction vessel was degassed to remove acetone and carbon dioxide to obtain a hydrogen storage material. When the obtained hydrogen storage material was subjected to elemental analysis by ICP, the platinum content was 11.2% by weight, and the carbon content based on the total amount of platinum and carbon was 99.2 atomic%. Met.

The contents were transferred to a stainless steel reaction vessel that could withstand vacuum and high pressure. This reaction vessel was connected to an exhaust device, and the inside of the reaction vessel was adjusted to 0.1 Torr by a rotary pump.
The pressure was reduced to about r. Furthermore, in order to completely perform the degassing treatment in the reaction vessel, the reaction vessel was heated to 200 ° C. under reduced pressure and kept for 1 hour. Next, at room temperature (25 ° C., the same applies below), hydrogen (purity: 99.999%, apply below) was introduced into the reaction vessel and the hydrogen pressure was maintained at 5 MPa. Admitted. After confirming that the absorption of hydrogen (reduction of hydrogen pressure) had stopped, the pressure inside the reaction vessel was reduced at room temperature to release hydrogen.

Further, hydrogen absorption at room temperature and hydrogen pressure of 5 MPa and hydrogen release at room temperature and reduced pressure are repeated, and when hydrogen is released, the release amount (storage amount) is obtained based on the hydrogen pressure in the container. It was This absorption / release of hydrogen was repeated until the amount of released hydrogen reached a certain value. As a result, the maximum hydrogen release amount of the hydrogen storage material of this example was 5.6% by weight.

Example 2 A hydrogen storage material was produced in the same manner as in Example 1 except that rhodium acetylacetonate was used instead of platinum acetylacetonate. When the element analysis by ICP was performed on the obtained hydrogen storage material, the content ratio of rhodium was 13.8% by weight, and the content ratio of carbon based on the total amount of rhodium and carbon was 98.2 atomic%. Met.

Next, using the obtained hydrogen storage material, hydrogen was stored and released in the same manner as in Example 1. As a result, the maximum amount of released hydrogen was 6.2% by weight.

[Example 3] 5 g of graphite and 1.0 g of iron powder (20-60 mesh) were placed in a glove box kept in an argon gas atmosphere and stainless steel balls (4 m) were added.
mφ) and put in a stainless steel container (internal volume: 80 ml) and mechanically pulverized at 400 rpm for 12 hours in a planetary ball mill to contain hydrogen storage material (carbon content based on the total amount of iron and carbon). Ratio: 96.5 atomic%).

Next, using the obtained hydrogen storage material, hydrogen was stored and released in the same manner as in Example 1. As a result, the maximum amount of released hydrogen was 5.1% by weight.

Comparative Example 1 A hydrogen storage material was prepared in the same manner as in Example 3 except that magnesium powder (20-70 mesh) was used instead of iron powder.

Next, using the obtained hydrogen storage material, hydrogen was stored and released in the same manner as in Example 1. As a result, the maximum amount of released hydrogen was 0.3% by weight, and it was confirmed that it was difficult to store and release hydrogen at room temperature with the hydrogen storage material of this comparative example.

[0047]

As described above, in the hydrogen storage material of the present invention, due to the synergistic effect of the inherent hydrogen storage / release capacity of carbon and the desorption of hydrogen by the above-mentioned specific catalyst, Since the reversible reaction of release is sufficiently promoted, it becomes possible to release hydrogen of high purity and a sufficient amount at low temperature.

Further, according to the production method of the present invention, graphite crystal particles having a fine and uniform particle size obtained by a predetermined pulverizing step are used as a raw material of carbon, and this is mixed with the above-mentioned specific catalyst to produce hydrogen. It is possible to efficiently and surely obtain the hydrogen storage material of the present invention which is excellent in storage / release capacity.

[Brief description of drawings]

FIG. 1 is an explanatory view schematically showing an example of graphite crystal fine particles according to the present invention.

[Explanation of symbols]

1 ... Carbon layer, La ... In-plane crystal grain size including a-axis and b-axis of graphite structure, Lc ... C-axis crystal grain size of graphite structure.

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Claims (5)

[Claims] 1. At least one catalyst selected from the group consisting of nickel, chromium, molybdenum, cobalt, copper, palladium, platinum, iron, ruthenium, rhodium, iridium, tungsten, titanium, manganese, and osmium, and carbon. A hydrogen storage material characterized by containing. 2. A half value width of a Raman peak at a wave number of 1350 cm ⁻¹ obtained by Raman spectrum measurement as the carbon raw material is represented by the following formula (1): d <(376 / La) +19.0 (1) [wherein , D represents the half-width of Raman peak (cm ⁻¹ ),
La represents a crystal grain size (nm) in a plane that includes the a-axis and the b-axis of the graphite structure], and is mixed with graphite crystal grains that satisfy the condition represented by the following formula. 1. The hydrogen storage material according to 1. 3. The hydrogen storage material according to claim 1, wherein a content ratio of the carbon is 40 atomic% or more based on a total amount of the catalyst and the carbon. 4. A pulverization step of obtaining graphite crystal particles by mechanically pulverizing graphite, graphite crystal particles obtained in the pulverization step, nickel, chromium, molybdenum, cobalt, copper, palladium, platinum, iron, ruthenium and rhodium. And a mixture of at least one catalyst selected from the group consisting of iridium, tungsten, titanium, manganese, and osmium to obtain a hydrogen storage material containing the catalyst and carbon. A method for producing a hydrogen storage material. 5. The method for producing a hydrogen storage material according to claim 4, further comprising a hydrogen treatment step of bringing the hydrogen storage material into contact with hydrogen.