JP5170702B2 - Catalysts for electrochemical oxidation of hydrides - Google Patents

Description

  The present invention relates to a catalyst for electrochemical oxidation of hydride and its use.

  In recent years, hydrogen has been considered promising as a new energy source due to environmental problems and energy problems. For example, the development of hydrogen automobiles that use hydrogen directly as fuel and fuel cells that use hydrogen are being developed. The fuel cell has excellent advantages such as small size, high power generation efficiency, no generation of noise and vibration, and use of waste heat.

  When hydrogen is used as an energy source, it is necessary to supply hydrogen as a fuel safely and stably. A method for supplying hydrogen directly as compressed hydrogen or liquid hydrogen, or a hydrogen storage material such as a hydrogen storage alloy. Various methods have been proposed, such as a method for storing and supplying hydrogen by using hydrogen and a method for supplying hydrogen by steam reforming methanol or hydrocarbon.

  However, these methods can be used for production on a factory scale or generation of hydrogen to the extent that they can be used in laboratories, but the required amount of hydrogen fuel can be continuously supplied, and downsizing is required. It is unsuitable as a hydrogen supply method for automobile fuel cells; portable fuel cells for mobile phones, personal computers, and the like.

On the other hand, attempts have been made to use various metal hydrides such as LiAlH ₄ and NaBH ₄ as a hydrogen generating material. In particular, many current portable hydrogen generators employ a method of generating hydrogen by adding a catalyst containing a noble metal in a metallic state such as platinum-supported carbon to a hydride. However, in the method using a hydride oxidation catalyst composed of a noble metal in a metal state, since the hydride self-decomposition reaction proceeds in an aqueous solution, it is difficult to control the hydrogen generation amount, the hydrogen generation rate, and the like. There is also a problem that the amount of noble metal used is large because noble metal is used in a bulk metal state (see Non-Patent Document 1 below).

  In addition, development of a direct fuel cell that uses metal hydride as a direct fuel without generating hydrogen from these metal hydrides is also underway (see Non-Patent Documents 2 and 3 below). However, since these fuel cells use a noble metal in a metallic state on the anode side, there is a problem that hydrides decompose during non-power generation, and some hydrides decompose without being oxidized electrochemically even during power generation. . There is also a problem that the amount of noble metal used is increased on the anode side.

Fuel Process. Technol. 87 (2006) 811-819 Electrochem. Commun. 10 (2008) 100-102 J. Power Sources 176 (2008) 287-292

  The present invention has been made in view of the current state of the prior art described above, and its main purpose is a catalyst effective for the oxidation reaction of hydride, particularly when hydride is used as a hydrogen generation source. It is to provide a novel catalyst that can solve the problems of conventional noble metal catalysts when generating electricity using hydride as a direct fuel.

  The present inventor has intensively studied to achieve the above-described object. As a result, it was found that a complex containing a specific noble metal atom has an excellent activity as an electrochemical oxidation catalyst for various hydrides, and also has a very low activity for hydride self-decomposition reaction. . The metal complex can exhibit excellent performance when used as an anode catalyst for a hydrogen generator or a direct borohydride fuel cell. For example, an electrochemical oxidation reaction of a hydride using the metal complex In combination with the hydrogen generation reaction at the cathode makes it easy to control the rate and amount of hydrogen generation, and to make effective use of the electrochemical energy generated during the hydrogen generation reaction. The headline and here came to complete the present invention.

That is, the present invention provides the following catalyst for electrochemical oxidation of hydride and use thereof.
1. A catalyst for electrochemical oxidation of a hydride comprising rhodium, ruthenium or iridium as a metal component, and comprising a nitrogen-containing polycyclic compound or a metal complex containing CO as a ligand.
2. Item 2. The catalyst for electrochemical oxidation of a hydride according to Item 1, wherein the metal component is rhodium and the ligand is a porphyrin compound.
3. Item 3. The catalyst for electrochemical oxidation of a hydride according to Item 1 or 2, wherein the metal complex is supported on a conductive support.
4). Item 4. The catalyst for electrochemical oxidation of a hydride according to any one of Items 1 to 3, wherein the hydride is an alkali metal borohydride compound.
5. 5. An anode for a direct fuel cell using a borohydride compound as a fuel, comprising the catalyst according to any one of Items 1 to 4 as an anode electrode catalyst.
6). An anode side electrode part to which a solution containing a hydride and a hydroxide is supplied;
A cathode-side electrode portion that is arranged opposite to the anode-side electrode portion and generates hydrogen by a proton reduction reaction;
A hydrogen generator comprising an ion permeable diaphragm disposed between the anode side electrode part and the cathode side electrode part,
A hydrogen generator comprising the catalyst according to any one of Items 1 to 4 as a catalyst for the anode side electrode.
7). 7. The hydrogen generator according to item 6, wherein the anode electrode catalyst is a catalyst having an anode electrode potential lower than the cathode electrode potential in an open circuit.

  Hereinafter, the catalyst for electrochemical oxidation of hydride of the present invention will be described in detail.

Catalyst for Electrochemical Oxidation of Hydride The catalyst for electrochemical oxidation of hydride of the present invention contains rhodium, ruthenium or iridium as a metal component, and contains a nitrogen-containing polycyclic compound or CO as a ligand. It is a metal complex.

  Among these, examples of the nitrogen-containing polycyclic compound used as the ligand include porphyrin compounds, phthalocyanine compounds, and salen compounds.

  By using these specific metal complexes as catalysts, it becomes possible to oxidize hydrides with a low overvoltage. Further, since the metal complex has low activity for chemical decomposition reaction of hydride, the decomposition reaction of hydride is extremely slow when the electrode provided with the metal complex is not connected to the cathode electrode.

In the present invention, as long as it contains the above-mentioned metal component and the ligand contains a nitrogen-containing polycyclic compound or a noble metal complex containing CO, it can be used without particular limitation. It is not particularly limited.

  In particular, a metal complex containing a porphyrin compound as a ligand is preferable in that the decomposition rate of a hydride without being connected to the cathode electrode is very low and the overvoltage of the hydride electrochemical oxidation reaction is low.

  Hereafter, a preferable specific example is shown about the metal complex which is an active ingredient of the catalyst of this invention.

  (1) Chemical formula:

(Wherein R _{1 to} R ₁₂ are the same or different and each represents an alkyl group, an aryl group which may have a substituent, a hydrogen atom or a halogen atom, and M represents rhodium, ruthenium or iridium. The metal porphyrin complex represented by this.

  In the above chemical formula, the central metal element represented by M is rhodium, ruthenium or iridium, and rhodium is particularly preferable in that it has a high activity for the electrochemical oxidation reaction of hydride.

R _{1 to} R ₁₂ are the same or different and each represents an alkyl group, an aryl group that may have a substituent, a hydrogen atom, or a halogen atom. Among these, as an alkyl group, methyl, ethyl, isopropyl, n-propyl, t-butyl, sec-butyl, n-butyl, isobutyl, n-pentyl, etc. A branched lower alkyl group is preferred. Further, as the halogen atom, fluorine, chlorine, bromine and the like are preferable. As the aryl group which may have a substituent, a phenyl group, a phenyl group having a substituent, a pyridyl group, a pyridyl group having a substituent, and the like are preferable.

Among these, when R ₁ , R ₄ , R _7, and R ₁₀ are all aryl groups that may have a substituent, the meso position is chemically protected, so that the oxidation reaction and the like are prevented. Increases stability.

Among the aryl groups having a substituent, examples of the pyridyl group having a substituent include a 1-methylpyridyl group. Examples of the substituent in the phenyl group having a substituent include a lower alkoxy group, a lower alkyl group, a halogen atom, —SO ₃ M ¹ (wherein M ¹ is a hydrogen atom, an alkali metal, or —NH ₄ ), —COOM. ² (wherein M ² is a hydrogen atom, an alkali metal, —NH ₄ or an alkyl group), —OCH ₂ —COOM ³ (wherein M ³ is a hydrogen atom, an alkali metal, —NH ₄ or an alkyl group) And the like.

Among these, as a phenyl group having a lower alkoxy group as a substituent, a para-methoxyphenyl group and the like, and as a phenyl group having a lower alkyl group as a substituent, a para-methylphenyl group, 2,4,6-trimethyl Examples of the phenyl group having a halogen atom as a substituent such as a phenyl group include a pentafluorophenyl group.

Among the substituents of the phenyl group, the _-SO 3 ^{M 1, M 1} is a hydrogen atom, an alkali metal or -NH _4. Among these, examples of the alkali metal include K and Na. Among the substituents of the phenyl group, in —COOM ² , M ² is a hydrogen atom, an alkali metal, —NH ₄ or an alkyl group. Among these, examples of the alkali metal include K and Na. Examples of the alkyl group include the same groups as those described above. Among the substituents of the phenyl group, in —OCH ₂ —COOM ³ , M ³ is a hydrogen atom, an alkali metal, —NH ₄ or an alkyl group. Among these, examples of the alkali metal include K and Na. Examples of the alkyl group include the same groups as those described above.

Substituents such as —SO ₃ M ¹ , —COOM ² and —OCH ₂ —COOM ³ can be substituted at the 4-position of the phenyl group, for example, but are not limited thereto.

As a representative example of the porphyrin complex represented by the above chemical formula, R ₁ , R ₄ , R ₇ and R ₁₀ are phenyl groups that may have a substituent, and R _2, R ₃ , R ₅ , R ₆ , A tetraphenylporphyrin complex in which R ₈ , R ₉ , R ₁₁ and R ₁₂ are hydrogen atoms; R ₁ , R ₄ , R ₇ and R ₁₀ are hydrogen atoms, and R _2, R ₃ , R ₅ , R ₆ , R ₈ , R ₉ , R ₁₁ and R ₁₂ are octaalkylporphyrin complexes in which a lower alkyl group is exemplified.

  Among these, preferred specific examples of the tetraphenylporphyrin complex include rhodium tetraphenylporphyrin complex, and preferable specific examples of the octaalkylporphyrin complex include rhodium octaalkylporphyrin.

    (2) Chemical formula

(Wherein R _{1 to} R ₈ are the same or different and each represents a hydrogen atom, an alkyl group, a hydroxyalkyl group, an alkenyl group, a group: —SO ₃ M ₁ (wherein M ₁ represents a hydrogen atom, an alkali A metal or —NH ₄ ), or a group: —R ₉ —COOM ₂ (wherein R ₉ is a linear or branched alkylene group, and M ₂ is a hydrogen atom, an alkali metal or an alkyl group) Or at least one combination of R ₁ and R ₂ , R ₃ and R ₄ , R ₅ and R ₆ , R ₇ and R ₈ is bonded to each other, and these groups are bonded to each other. An aromatic ring which may have a substituent may be formed together with the carbon atom to be used, M represents rhodium, ruthenium or iridium, provided that at least one of R _{1 to} R ₈ is a group: —R ₉ -COOM ₂ ).

  In the above chemical formula, the central metal element represented by M is rhodium, ruthenium or iridium, and rhodium is particularly preferable in that it has a high activity for the electrochemical oxidation reaction of hydride.

  In the above chemical formula, the alkyl group is linear or branched having about 1 to 5 carbon atoms such as methyl, ethyl, isopropyl, n-propyl, t-butyl, sec-butyl, n-butyl, isobutyl, n-pentyl and the like. A chain-like lower alkyl group is preferred.

  As the alkyl group portion of the hydroxyalkyl group, a straight chain having about 1 to 5 carbon atoms such as methyl, ethyl, isopropyl, n-propyl, t-butyl, sec-butyl, n-butyl, isobutyl, n-pentyl or the like A branched lower alkyl group can be exemplified. The hydroxy group can be substituted on any carbon atom of the alkyl group.

  Examples of the alkenyl group include linear or branched alkenyl groups having 2 to 6 carbon atoms such as vinyl, allyl, 2-butenyl, 3-butenyl, 1-methylallyl, 2-pentenyl, 2-hexenyl and the like. Can do.

In the group: —SO ₃ M ₁ , M ₁ is a hydrogen atom, an alkali metal, or —NH ₄ . Among these, examples of the alkali metal include K and Na.

In the group: —R ₉ —COOM ₂ , as the alkylene group represented by R ₉ , for example, methylene, ethylene, trimethylene, 2-methyltrimethylene, 2,2-dimethyltrimethylene, 1-methyltrimethylene, methylmethylene C1-C6 linear or branched alkylene groups such as ethylmethylene, tetramethylene, pentamethylene and hexamethylene groups. The alkyl group and alkali metal represented by M ₂ are the same as those described above.

In the metalloporphyrin complex represented by the above chemical formula, a compound in which two or more of R _{1 to} R ₈ are a group: —R ₉ —COOM ₂ has a particularly high activity for an electrochemical oxidation reaction of a hydride. It is what has.

  (3) Chemical formula

(Wherein R _{1 to} R ₁₆ are the same or different and each represents a hydrogen atom, an alkyl group, an alkoxyl group, a halogen atom or a sulfonic acid group, or R ₂ and R ₃ , R ₆ and R _7). , R ₁₀ and R ₁₁ , and each combination of R ₁₄ and R ₁₅ are bonded to each other to form an aromatic ring which may have a substituent together with the carbon atom to which these groups are bonded. M represents a rhodium, ruthenium or iridium) metal phthalocyanine complex.

In the above chemical formula, the central metal element represented by M is rhodium, ruthenium or iridium, and rhodium is particularly preferable in that it has a high activity for the electrochemical oxidation reaction of hydride.

In the above chemical formula, R _{1 to} R ₁₆ are the same or different and each represents a hydrogen atom, an alkyl group, an alkoxyl group, a halogen atom, or a sulfonic acid group. Further, among these, at least one of each combination of R ₂ and R ₃ , R ₆ and R ₇ , R ₁₀ and R ₁₁ , R ₁₄ and R ₁₅ is bonded to each other, and these groups are bonded. You may form the aromatic ring which may have a substituent with a carbon atom.

  Among these, as an alkyl group, methyl, ethyl, isopropyl, n-propyl, t-butyl, sec-butyl, n-butyl, isobutyl, n-pentyl, etc. A branched lower alkyl group is preferred. Examples of the alkoxy group include straight chain or branched chains having 1 to 6 carbon atoms such as methoxy, ethoxy, n-propoxy, isopropoxy, n-butoxy, tert-butoxy, n-pentyloxy, n-hexyloxy group, and the like. An alkoxyl group is preferred. Further, as the halogen atom, fluorine, chlorine, bromine and the like are preferable.

Among the metal phthalocyanine complexes represented by the above chemical formula, for example, a compound in which R _{1 to} R ₁₆ are all hydrogen atoms has high activity for an electrochemical oxidation reaction of a hydride.

Each combination of R ₂ and R ₃ , R ₆ and R ₇ , R ₁₀ and R ₁₁ , R ₁₄ and R ₁₅ is bonded to each other, and has a substituent together with the carbon atom to which these groups are bonded. As an aromatic ring-containing compound, the following chemical formula

The compound represented by these can be illustrated. In the above chemical formula, R _{17 to} R ₃₂ are the same or different and each represents a hydrogen atom, an alkyl group, an alkoxyl group, a halogen atom, or a sulfonic acid group. Specific examples of each of these groups are the same as R _{1 to} R ₁₆ .

  (4) Chemical formula:

(Wherein, M is rhodium, ruthenium or iridium).

  In the above chemical formula, the central metal element represented by M is rhodium, ruthenium or iridium, and rhodium is particularly preferable in that it has a high activity for the electrochemical oxidation reaction of hydride.

  In the salen complex described above, an arbitrary substituent may be present on the salen ring.

  (5) A complex containing one or more molecules of CO as a ligand per metal atom.

In the complex containing CO as a ligand, the number of metal atoms in the complex is preferably about 1 to 10. CO may be coordinated to one metal or may be coordinated to a plurality of metal atoms. When coordinated to a plurality of metal atoms, it is in a state of being crosslinked by CO. There are no particular limitations on the type of ligand other than CO. Examples of such a metal complex containing CO as a ligand include a metal complex represented by the chemical formula: Rh ₂ Cl ₂ (CO) ₄ , a metal complex represented by Rh ₆ (CO) ₁₆ , and the like.

Method for Producing Metal Complex The metal complex described above can be produced , for example, by dissolving a compound serving as a ligand of the target complex and the metal compound in a solvent and heating.

  For example, for the rhodium complex, a compound body containing a monovalent rhodium metal can be used as the rhodium compound. As the solvent, a solvent capable of dissolving the above-described ligand compound and rhodium compound may be used. For example, ethanol may be used. The heating temperature may be, for example, the reflux temperature of the solvent used.

Further, for example, a metal complex containing CO represented by the chemical formula: Rh ₂ Cl ₂ (CO) ₄ as a ligand can be easily obtained as a commercial product.

Supported Catalyst The above-described metal complex can have a high catalytic activity for an electrochemical oxidation reaction of a hydride by being supported on a conductive support.

  The conductive carrier is not particularly limited, and for example, various carriers conventionally used as catalyst carriers for polymer electrolyte fuel cells can be used. Specific examples of such a carrier include carbonaceous materials such as carbon black, activated carbon, and graphite. Among these, carbon black is a particularly preferable substance as a conductive carrier because it is excellent in conductivity and has a large specific surface area.

The shape of the conductive carrier is not particularly limited, and for example, a conductive carrier having an average particle diameter of about 0.1 to 100 μm, preferably about 1 to 10 μm can be used. Further, when carbon black is used, for example, is preferably one in the range BET specific surface area of about 100~800m ² / g, more is intended to be within the range of about 200 to 300 [m ² / g preferable. As a specific example of such a carbon black, those commercially available under the trade name Vulcan XC-72R (Cabot) can be used.

  As a method for supporting the conductive carrier, for example, a known method such as a dissolution drying method or a gas phase method can be applied.

  For example, in the dissolution drying method, a metal complex is dissolved in an organic solvent, and a conductive carrier is added to this solution. For example, the metal complex is adsorbed on the carrier by stirring for several hours, and then the organic solvent is dried. Just do it. In addition, when a large amount of metal complex is contained in the organic solvent, the metal complex that is not adsorbed on the conductive support is removed by filtering after adsorbing the metal complex on the conductive support until equilibrium is reached. Thus, only the metal complex interacting with the support can be left on the surface of the support.

  In this method, any organic solvent can be used without particular limitation as long as it can dissolve the metal complex. For example, chlorinated hydrocarbons such as dichloromethane and chloroform and lower alcohols such as ethanol can be preferably used.

  If the dispersion obtained by filtration is further washed with an organic solvent until the washing liquid becomes transparent, the metal complex having a weak interaction with the conductive carrier can be washed away and firmly adsorbed on the conductive carrier. It is possible to obtain a highly active catalyst containing only the metal complex.

  In the case of carrying by a vapor phase method, for example, a known method such as a plasma vapor deposition method, a CVD method, or a heating vapor deposition method can be adopted.

  The amount of the metal complex to be supported on the conductive carrier is not particularly limited. For example, it is preferable to carry about 10 μmol to 1000 μmol of the metal complex with respect to 1 g of the conductive carrier, and about 20 μmol to 100 μmol. Is more preferable.

Use of the catalyst of the present invention Any of the metal complexes described above has excellent activity as a catalyst for electrochemical oxidation reaction of hydride.

The hydride to be subjected to the electrochemical oxidation reaction by the catalyst of the present invention is not particularly limited. For example, a compound represented by the general formula: X (YH _4-n Z _n ) can be used. . In the above general formula, X is an alkali metal, NH ₄ , or M _1/2 (wherein M is an alkaline earth metal), Y is B, Al or Ga, and Z is a halogen atom or a single atom. And n is an integer of 0 to 3. Examples of the alkali metal include Li. Na, K, Rb, Cs, etc., examples of the alkaline earth metal include Mg, Ca, Sr, Ba and the like, and examples of the halogen atom include F, Cl, Br,
I etc. can be illustrated. Examples of the monovalent hydrocarbon group include a linear or branched alkyl group having about 1 to 6 carbon atoms, such as methyl, ethyl, n-propyl, isopropyl, n-butyl, Isobutyl, tert-butyl, sec-butyl, n-pentyl, 1-ethylpropyl, isopentyl, neopentyl, n-hexyl, 1,2,2-trimethylpropyl, 3,3-dimethylbutyl, 2-ethylbutyl, isohexyl, 3 -A methylpentyl group etc. can be illustrated.

Among these hydrides, alkali metal borohydride compounds such as LiBH ₄ , NaBH ₄ and KBH ₄ are particularly preferable.

  By using the catalyst of the present invention, the hydride electrochemical oxidation reaction proceeds at a low overvoltage. For example, when an alkali metal borohydride compound is used, the following reaction proceeds.

BH ₄ ⁻ + 8HO ⁻ → BO ₂ ⁻ + 6H ₂ O + 8e ⁻
The electrochemical oxidation reaction of the hydride can proceed with high selectivity by setting it to a predetermined potential in a state where the hydride is in contact with the catalyst. For example, the electrochemical oxidation reaction can be allowed to proceed by immersing the electrode provided with the catalyst of the present invention in an aqueous solution containing an alkaline hydride having a pH of about 12 or more and setting the electrode at a predetermined potential. The specific potential in this case cannot be defined unconditionally because it varies depending on the type of metal complex used, the type of hydride, the state of the solution, etc. For example, the potential measured at 25 ° C. in an aqueous solution of about pH 13 ( As a reference for AgCl / KCl (saturated) electrode), a rhodium complex having a nitrogen-containing polycyclic compound (porphyrin, phthalocyanine, salen, etc.) as a ligand is preferably about -0.8 V to -0.4 V, In particular, it is more preferably about −0.8 V to −0.6 V.

  The catalyst of the present invention can be used for various applications utilizing the electrical oxidation reaction of hydride utilizing the above-described excellent characteristics. For example, it can be effectively used for applications such as an anode catalyst in a direct borohydride fuel cell using hydride as a fuel and an anode catalyst in a hydrogen generator using hydride as a raw material. Hereinafter, these uses will be described.

(1) Direct-type borohydride fuel cell The catalyst of the present invention can be effectively used as an anode electrode catalyst of a direct-type borohydride fuel cell. The structure of the direct borohydride fuel cell using the catalyst of the present invention is not particularly limited as long as it is a direct fuel cell using a borohydride compound as a fuel. For example, an aqueous solution of a borohydride compound such as NaBH ₄ described in “Electrochem. Commun. 10 (2008) 100-102”, “J. Power Sources, 176 (2008) 287-292”, etc. is used as a fuel. It can be used as an anode electrode catalyst of a direct borohydride fuel cell using an anion exchange membrane or an alkaline solution as an electrolyte and a platinum group metal, gold, silver or the like as a cathode electrode catalyst.

  By using the catalyst of the present invention as an anode electrode catalyst of a direct borohydride fuel cell, it is possible to suppress the decomposition of the fuel during non-power generation and to take out electricity from a high voltage.

(2) Hydrogen generator The catalyst of the present invention uses an ion permeable membrane for an anode reaction including an oxidation reaction of a hydride in a solution containing a hydride and a hydroxide and a cathode reaction including a hydrogen generation reaction by reduction of protons. It can be used as an anode catalyst in a method for generating hydrogen by generating the catalyst.

  A hydrogen generation apparatus using the above-described hydrogen generation method includes, for example, an anode side electrode portion to which a solution containing a hydride and a hydroxide is supplied, and a cathode side electrode that is disposed opposite to the anode side electrode portion and generates hydrogen. And an ion permeable diaphragm disposed between the anode side electrode part and the cathode side electrode part, wherein the catalyst of the present invention is used as an anode catalyst in the anode side electrode part. is there.

  Hereinafter, a hydrogen generation method using the catalyst of the present invention will be described with reference to FIG. 1 showing a schematic configuration of one embodiment of the above-described hydrogen generation apparatus.

  A hydrogen generator 1 shown in FIG. 1 includes an anode side electrode 2, a cathode side electrode 3, and an ion exchangeable diaphragm 4 as main components. A solution containing a hydride and a hydroxide is supplied to the anode side electrode 2 from the anode side raw material inlet 6 as a raw material solution.

As the hydroxide, for example, a compound represented by the general formula: AOH can be used. In the above general formula, A is an alkali metal, NH ₄ , or M _1/2 (wherein M is an alkaline earth metal). Examples of the alkali metal include Li, Na, K, Rb, and Cs, and examples of the alkaline earth metal include Mg, Ca, Sr, and Ba. Of these hydroxides, alkali metal hydroxides such as LiOH, NaOH, and KOH are particularly preferable.

In a solution containing hydride and hydroxide, as a solvent, hydride: X (YH _4-n Z _n ) can be ionically dissociated into X ⁺ and (YH _4-n Z _n ) ⁻ in the solution. Any solvent can be used without particular limitation. As such a solvent, in addition to water, a non-aqueous solvent having a high Lewis basicity, a low Lewis acidity, and a high relative dielectric constant can be used. Examples of such nonaqueous solvents include diethyl ether, dimethoxyethane, diethoxyethane, diglyme, triglyme, tetraglyme, tetrahydrofuran, methyltetrahydrofuran, dioxane, dioxolane, tetramethylpyran, and other ethers, carbon disulfide, methyl acetate, γ-Butyllactone, methyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, acetone, formamide, methylformamide, dimethylformamide, acetamide, methylacetamide, dimethylacetamide, ethylenediamine, methylpyrrolidone, pyridine, aniline, N-methylpyrrolidone, tetra Methylurea, tetramethylguanidine, acetonitrile, nitromethane, dimethylsulfoxide, sulfolane, triethyl phosphate, hexamethylphosphorus Triamide, ammonia and the like. Preferred non-aqueous solvents are tetrahydrofuran, acetonitrile, dimethyl sulfoxide, dimethylacetamide, methylpyrrolidone, pyridine and the like.

  The concentration of hydride in the solution containing hydride and hydroxide is not particularly limited, but is preferably about 0.0001 to 30 mol / L, and more preferably about 0.001 to 10 mol / L.

The hydroxide concentration is preferably about 0.0001 to 20 mol / L, and 0.01 to 4 mol / L.
More preferably, it is about.

By bringing the solution containing the hydride and hydroxide into contact with the anode side electrode, an oxidation reaction of hydride ions existing in the solution proceeds on the surface of the anode side electrode. This produces an oxidation product of hydride ions and water. For example, when a borohydride compound is used as a hydride, an anodic reaction proceeds by the following reaction, and hydride ions: BH ₄ ⁻ are oxidized.

Anode reaction: BH ₄ ⁻ + 8OH ⁻ → BO ₂ ⁻ + 6H ₂ O + 8e ⁻
The reaction temperature of the anodic reaction is not particularly limited, and the reaction can usually proceed at room temperature.

The amount of catalyst supported in the anode, for example, be a 0.1 to 5.0 mg / cm ² or so, it is preferable to 0.1-3.0 mg / cm ² or so.

  Electrons generated by the anode reaction pass through the external circuit 5 and are supplied to the cathode side electrode 3, and the following cathode reaction including hydrogen generation occurs on the surface of the cathode side electrode 3.

The cathode ^{_{reaction: 8H 2 O + 8e - →}} 4H 2 + 8 OH -
The reaction temperature of the cathode reaction is not particularly limited, and the reaction can usually proceed at room temperature.

In the cathode side electrode 3, for example, by supplying water from the cathode side raw material inlet 8, the above-described cathode reaction can proceed. In this case, water may be supplied directly, or a humidified inert gas may be supplied. Even when water is not supplied from the cathode side raw material inlet 8, water contained in the anode raw material or water generated by the anode reaction is
When the ion-permeable diaphragm is moved to reach the cathode side electrode 3, the above-described cathode reaction can proceed. Hydrogen generated on the surface of the cathode side electrode 3 is recovered from, for example, a hydrogen outlet 9 installed in the cathode side electrode portion. Further, hydroxide ions generated at the cathode side electrode 3 pass through the ion permeable diaphragm and move to the anode side raw material solution, or can be used again for the anode reaction by mixing with the raw material solution. . This makes it possible to proceed the reaction without supplying new hydroxide from the outside.

  The cathode side electrode 3 is not particularly limited. For example, a porous electrode in which a catalyst is supported on a carrier can be used.

  For example, the cathode side electrode 3 is provided opposite to the anode side electrode 2 so as to be in contact with the other surface of the ion permeable diaphragm 4.

  The catalyst used in the cathode-side electrode 3 may be any catalyst that has activity for the hydrogen generation reaction by proton reduction. For example, a fourth catalyst such as Ru, Rh, Pd, Ag, Os, Ir, Pt, or Au may be used. Or 5 period group 8-11 metal can be mentioned, Pt black is especially preferable.

Amount of catalyst supported in the cathode 3 is, for example, be a 0.1 to 5.0 mg / cm ² or so, it is preferable to 0.1-3.0 mg / cm ² or so.

As the ion permeable diaphragm 4, an anion permeable or cation permeable diaphragm can be used. When an anion permeable diaphragm is used, OH ⁻ ions generated by the cathode reaction pass through the anion permeable diaphragm 4 and move to the anode side electrode portion.

  In this case, the total reaction combined with the anode reaction described above is as follows.

Total reaction: BH ₄ ⁻ + 2H ₂ O → BO ₂ ⁻ + 4H ₂
As the ion permeable diaphragm 4 disposed between the anode side electrode 2 and the cathode side electrode 3 described above, for example, an anion permeation capable of passing cations dissociated from OH ⁻ ions or hydrides generated by the cathode reaction is allowed. An permeable membrane or a cation permeable membrane can be used.

As the anion permeable diaphragm, it is preferable to use a diaphragm that can permeate OH ⁻ ions generated by the anode reaction but does not permeate the hydride anion supplied to the anode electrode portion. This prevents the hydride anion from being hydrolyzed at the cathode electrode surface. As such an anion permeable membrane, for example, a trade name: Neoceptor AMX (Astom Co., Ltd.) membrane or the like can be used.

  Even when a cation permeable membrane is used, the hydride anion supplied to the anode electrode portion does not move to the cathode electrode portion, and the hydride anion is prevented from being hydrolyzed on the cathode electrode surface. . As the cation permeable membrane, a cation exchange type solid polymer electrolyte membrane capable of passing cations dissociated from hydride can be used. Specific examples of such a solid polymer electrolyte membrane include fluorine-based ion exchange membranes such as a perfluorocarbon sulfonic acid membrane and a perfluorocarbon carboxylic acid membrane, a polybenzimidazole membrane impregnated with phosphoric acid, a polystyrene sulfonic acid membrane, and a sulfone. Examples thereof include a styrene oxide / vinylbenzene copolymer film. Among these, a perfluorocarbon sulfonic acid membrane marketed under a trade name such as Nafion is particularly preferable.

A glass filter having an appropriate pore size can also be used as the ion permeable diaphragm.

  In the hydrogen generator 1 described above, an anode-side raw material inlet 6 and an anode-side outlet 7 can be further provided outside the anode-side electrode 2, and a cathode-side raw material inlet 8 is generated outside the cathode-side electrode 3. A hydrogen discharge port 9 or the like can be provided. Further, a current collector can be provided outside the anode side electrode 2 in contact with the anode side electrode 2, and gas diffusion can be performed outside the cathode side electrode 3 in contact with the cathode side electrode 3. A layered current collector or the like can be provided. The structure of the hydrogen generator including these members may be the same as that of a known hydrogen generator. For example, the structure of the hydrogen generator described in JP-A-2005-71645 can be used.

  In the hydrogen generator having the above-described structure, in particular, the anode catalyst includes a catalyst for electrochemical oxidation of the hydride of the present invention, that is, rhodium, ruthenium or iridium as a metal component, and nitrogen as a ligand. It is an important feature to use a polycyclic compound or a metal complex containing CO. By using such a catalyst, the hydride can be electrode-oxidized at a low overvoltage, and efficient hydrogen generation becomes possible.

The catalyst of the present invention has a chemical hydrogen evolution reaction from hydride (BH ₄ ⁻ + 2 for borohydride).
H ₂ O → BO ₂ ⁻ + 4H ₂ reaction) is so low that it hardly promotes the hydride self-decomposition reaction. For this reason, when the external circuit which connects an anode pole and a cathode pole is opened, hydrogen generation by the self-decomposition of hydride can be suppressed. In addition, by closing the external circuit and adjusting the anode potential according to the type of electrolyte used, the type of anode catalyst, the type of cathode catalyst, etc., the rate of hydrogen generation reaction, the amount of hydrogen generation, etc. can be simplified. Can be controlled. That is, in the hydrogen generator having the above structure, when the potential of the anode electrode in the open circuit is lower than the potential of the cathode electrode, the hydrogen generation reaction based on the above-described reaction can be advanced by closing the external circuit. In addition, when the potential of the anode electrode in the open circuit is higher than the potential of the cathode electrode, the hydrogen generation reaction can be advanced by setting the potential of the anode electrode to a higher potential using an external power source. it can. At this time, it is possible to control the hydrogen generation rate, the hydrogen generation amount, and the like by adjusting the potential of the anode electrode.

  In particular, when a catalyst comprising a rhodium porphyrin complex is used as the anode catalyst, it is easy to control the amount of hydrogen generated, the generation speed, etc., because the action of causing the hydride self-decomposition reaction is low.

  When the potential of the anode electrode is lower than the potential of the cathode electrode during the open circuit, the cathode reaction and the anode reaction proceed spontaneously by closing the external circuit. In this case, the potential difference between the cathode potential and the anode potential can be acquired as electric energy simultaneously with the generation of hydrogen. For this reason, it becomes possible to obtain chemical energy and electrical energy with high energy conversion efficiency from the hydride used as a raw material. Further, there is an advantage that an external power source is unnecessary for hydrogen generation, and the structure of the apparatus can be simplified.

  The anode catalyst that can be used in a hydrogen generator having such a power generation action varies depending on the type of electrolyte used, the concentration of hydride, the type of cathode catalyst, etc. The catalyst may be selected such that is lower than the cathode reaction potential.

For example, when using 0.1 mol / L sodium hydroxide aqueous solution as the electrolyte and platinum as the cathode catalyst, rhodium hematoporphyrin, rhodium tetrakiscarboxyphenylporphyrin, rhodium phthalocyanine, Rh ₂ Cl ₂ (CO) ₄ or the like is used as the anode. By using a catalyst, spontaneous power generation becomes possible. In particular, when rhodium hematoporphyrin, rhodium tetrakiscarboxyphenylporphyrin, or the like is used as an anode catalyst, hydride self-decomposition is slow, which is advantageous in that it is easy to control the amount of hydrogen generation, the rate of hydrogen generation, and the like.

  Although there is no limitation in particular about the use of the hydrogen generated using the above-mentioned hydrogen generator, for example, it can be used as a fuel of a fuel cell. In this case, for example, the hydrogen discharge port 9 described above may be joined to a hydrogen supply line, and the generated hydrogen may be supplied to the fuel cell.

  The hydride electrooxidation catalyst of the present invention has the following remarkable effects.

  (1) The catalyst for electrochemical oxidation of hydride of the present invention has high activity for electrochemical oxidation reaction of hydride, and can oxidize hydride with low overvoltage. Further, the catalyst of the present invention has a very low action for promoting the self-decomposition reaction of hydride because of its low activity for chemical hydrogen generation reaction from hydride. Therefore, by using the catalyst of the present invention as an anode catalyst in a hydrogen generator using hydride as a raw material, it is easy to control the hydrogen generation rate, the amount of hydrogen generation, and the like.

  (2) By using the catalyst of the present invention as an anode catalyst of a fuel cell using hydride as a direct fuel, consumption of hydride in reactions other than electrochemical reactions can be suppressed, and power generation efficiency can be improved. It becomes possible.

  (3) The catalyst of the present invention uses a noble metal element as a complex, and the amount of the noble metal used is small compared to a catalyst using the noble metal in a bulk state, and the cost is low.

  (4) In the hydrogen generator using the catalyst of the present invention, by selecting the catalyst so that the potential of the anode electrode is lower than the potential of the cathode electrode in the open circuit, the cathode reaction and the anode reaction are performed simultaneously with hydrogen generation. The potential difference can be acquired as electric energy.

It is a schematic block diagram which shows one Embodiment of the hydrogen generator using the catalyst of this invention. 3 is an absorbance curve of an ultraviolet / visible spectrum of rhodium hematoporphyrin IX obtained in Production Example 1. 4 is a graph showing measurement results of linear sweep voltammetry in Examples 1 and 2. 3 is a schematic configuration diagram of a hydrogen generator used in Example 30. FIG. 6 is a schematic configuration diagram of a hydrogen generator having a power generation action used in Example 31. FIG. 3 is a graph showing a potential-current curve measured in Example 31. FIG.

  Hereinafter, the present invention will be described in more detail with reference to production examples and examples.

Production Example 1 Production Example of Rhodium Hematoporphyrin IX

A rhodium hematoporphyrin IX represented by the following formula was produced by the following method.

  First, the following chemical formula

30 mg of commercially available hematoporphyrin IX represented by the formula (1) was weighed and dissolved in 100 mL of ethanol, and then 10.6 mg of tetracarbonyldi-μ-chlorodirhodium (I) (Rh ₂ Cl ₂ (CO) ₄ ) was added. The mixture was refluxed with heating at 70 ° C. for 5 hours. The refluxed solution was concentrated with a rotary evaporator, dissolved in ethanol, and ultraviolet / visible spectrum (UV-vis spectrum) was measured to confirm the formation of the target compound. FIG. 2 shows an absorbance curve of an ultraviolet / visible spectrum. In FIG. 2, curve A corresponds to hematoporphyrin IX used as a raw material, and curve B is a spectroscopic curve corresponding to rhodium hematoporphyrin IX, which is the target product.

Examples 1 and 2
Production of rhodium hematoporphyrin IX-supported carbon catalyst The rhodium hematoporphyrin IX obtained in Production Example 1 was dissolved in 10 mL of ethanol to a concentration of 0.7 mM, and carbon black (specific surface area 250 m ² / g , Trade name: Vulcan XC 72R, manufactured by Cabot) was added. After sealing the container, dispersibility was improved by placing it in an ultrasonic cleaner for 1 minute.

  The porphyrin solution in which carbon black was suspended was stirred with a magnetic stirrer for 3 hours, and then the solvent was removed by suction filtration using No. 5C quantitative filter paper manufactured by Toyo Filter Paper Co., Ltd. The carbon black on the filter paper was recovered to obtain a rhodium hematoporphyrin IX-supported carbon catalyst. The amount of rhodium hematoporphyrin IX supported on the obtained rhodium hematoporphyrin IX supported carbon catalyst was 68 μmol / g.

Evaluation of catalytic activity The rhodium hematoporphyrin IX-supported carbon catalyst obtained by the above method was crushed in a mortar, and 5 mg was suspended in 0.5 mL of a mixed solvent (water: ethanol = 1: 1), and then 5 μL 5% Nafion solution (Aldrich) was added. This suspension was placed in an ultrasonic cleaner for 5 minutes to be well dispersed, and then 2 μL was placed on a rotating disk electrode and dried.

Evaluation of the oxidation activity of the catalyst hydride was carried out using a potentiostat (ALS model 711B) manufactured by ALS. The rotational speed is controlled by a rotational speed control device manufactured by BAS Co., Ltd. (
BAS RDE-1). A glassy carbon rotating disk electrode coated with a catalyst was used as a working electrode, a platinum electrode as a counter electrode, and an Ag / AgCl / KCl (sat.) Electrode as a reference electrode.

Using 0.1 M NaOH as an electrolyte, NaBH ₄ was added to 1 mM, and linear sweep voltammetry was measured while rotating the electrode at a speed of 3600 ppm.

  Further, rhodium octaethylporphyrin produced by the same method as in Production Example 1 was supported on carbon black by the same method as the catalyst of Example 2, and linear sweep voltammetry was measured. The measurement results are shown in the graph of FIG.

As is clear from FIG. 3, with respect to the rhodium hematoporphyrin IX-supported carbon catalyst of Example 1, an oxidation current is observed from around −1.0 V with respect to the Ag / AgCl / KCl (sat.) Electrode. Further, with respect to the rhodium octaethylporphyrin-supported carbon catalyst of Example 2, an oxidation current is observed from around −0.85 V, and it can be seen that BH ₄ ⁻ is oxidized even near these potentials. In these catalysts, the oxidation current rapidly increased as the potential became positive. For the catalyst of Example 1, the oxidation current exceeded 1 mA near -0.5 V, and for the catalyst of Example 2, -0.5 In the vicinity of V, the oxidation current exceeded 0.4 mA.

Examples 3-29
In the same manner as in Production Example 1, each metal complex shown in Table 1 below was produced. In Examples 4, 5, 6, 7, 11, 12, 14, 16, 17, 19, 20, 21, 25, 26, and 29, each complex is supported on carbon in the same manner as in Example 1. It was. Examples 3, 8, 9, 1
For 0, 13, 15, 18, 22, 23, 24, 27, and 28, a metal complex was supported on carbon by the following method.

  First, a predetermined amount of metal complex (Examples 3, 15, 18, 23, 24, 27, 28: 0.9 μmol, Example 8: 1.0 μmol, Example 9: 2.35 μmol, Example 10: 30 μmol, Example 13: 1.8 μmol, Example 22: 3 μmol) was dissolved in ethanol or dichloromethane, and 30 mg of carbon black was added. Next, after the carbon black support was well dispersed by an ultrasonic cleaner, each catalyst was prepared by distilling off the solvent by a rotary evaporator.

About each catalyst mentioned above, the catalytic activity with respect to the electrochemical oxidation reaction of NaBH ₄ was evaluated by the same method as in Example 1. In addition, about the electrode using each metal complex of Example 3, 9 and 11, in order to increase the mechanical strength of a modified electrode, it heat-processed at 150 degreeC for 1 hour before the measurement.

Tables 1 to 6 show the voltage (mV) at 20 μA obtained from linear sweep voltammetry.
), -0.7V current value (μA) and maximum current value (μA). Of these, 20 μA
The voltage at the time of becoming -0.7V and the current value at -0.7V are both indicators for overvoltage.The more negative the voltage at 20μA, the smaller the overvoltage and the larger the current value at -0.7V. It can be said that the catalyst has high activity. The maximum current value is an index relating to the reaction rate. The higher the value, the faster the reaction rate. The current value is a value obtained by subtracting the current value when sodium borohydride is 0 mM.

Example 30
0.5 mL of a mixed solvent containing 5 μL of Nafion 5% solution (Aldrich) (water: ethanol = 1: 1)
), 0.2 mL of a suspension of 5 mg of the rhodium octaethylporphyrin-supported carbon catalyst used in Example 2 was placed on a glassy carbon electrode (effective surface area = 4.5 cm ² ) and dried to obtain a rhodium complex. A modified electrode was prepared. In addition, a suspension of platinum-supported carbon (40%, Johnson Matthey) was prepared by the same method (however, suspended in 0.5 mL of water), and applied and dried on the glassy carbon electrode by the same method. .

Subsequently, the amount of hydrogen generation was measured using the hydrogen generator shown in FIG. First, 0.1 M NaOH was placed in an H-type cell separated by a glass filter, and sodium borohydride was added to 10 mM on one side. The upper part of the H-type cell is sealed with a silicon stopper. After removing dissolved oxygen by purging both electrolytes with argon, the rhodium complex-modified electrode is immersed in the electrolyte containing sodium borohydride, and platinum is supported on the electrolyte containing no sodium borohydride. A carbon modified electrode was immersed. After standing for 100 minutes, the hydrogen gas in the gas phase was quantified.

Next, after the electrolytic solution was purged with argon again, a voltage of 0.3 V was applied between both electrodes for 100 minutes, and then the hydrogen gas in the gas phase was quantified.

  The measurement results of the hydrogen generation amount are shown in Table 7 below.

  As is clear from the above results, in the open circuit state where no voltage is applied, almost no hydrogen gas is generated, but when a voltage is applied, current flows and hydrogen gas is generated.

The results show that the rhodium octaethylporphyrin-supported carbon catalyst does not decompose BH ₄ ⁻ in the open circuit state, but can electrochemically oxidize BH ₄ ⁻ when a voltage is applied by connecting the circuit. In addition, this apparatus can be said to be a controllable hydrogen generator that can generate hydrogen by applying a little voltage.

Example 31
In the same manner as in Example 30, Example 1 (rhodium hematoporphyrin), Example 6 (rhodium tetrakiscarboxyphenylporphyrin), Example 9 (rhodium phthalocyanine), and Example 10 (tetracarbonyldi-μ-chlorodioxy). Rhodium, Rh ₂ Cl ₂ (CO) ₄
The rhodium complex-modified electrode was prepared using each rhodium complex.

In addition to the electrode modified with the rhodium octaethylporphyrin-supported carbon catalyst used in Example 30 (Example 2), a hydrogen generator having the same structure as the hydrogen generator used in Example 30 was used. For the rhodium complex-modified electrode, a potential-current curve was obtained when immersed in an electrolytic solution containing sodium borohydride. FIG. 5 shows a schematic diagram of a hydrogen generator for measuring the above-described potential-current curve and the reaction at each electrode.

FIG. 6 shows a potential-current curve when each rhodium complex-modified electrode is used. The cell voltage here is the voltage of the platinum catalyst electrode relative to the rhodium complex catalyst electrode. The electrolytic solution containing sodium borohydride (rhodium complex side) was purged with nitrogen gas, and the electrolytic solution not containing sodium borohydride (platinum-supporting carbon side) was purged with hydrogen gas.

As is apparent from the current curve, the hydrogen generating apparatus shown in FIG. 5, in the case of using the rhodium complex described above as an anode electrode catalyst, in addition to be used as a hydrogen generator, BH ₄ ^- - the potential of FIG. 6 It was confirmed that power generation was possible as a battery using an electrochemical oxidation reaction with a rhodium complex as an anode reaction and a hydrogen generation with a platinum catalyst as a cathode reaction.

DESCRIPTION OF SYMBOLS 1 Hydrogen generator 2 Anode side electrode 3 Cathode side electrode 4 Ion-permeable diaphragm 5 External circuit 6 Anode side raw material inlet 7 Anode side outlet 8 Cathode side raw material inlet 9 Hydrogen extraction port

Claims (7)

A catalyst for electrochemical oxidation of a metal hydride comprising rhodium, ruthenium or iridium as a metal component and comprising a nitrogen-containing polycyclic compound or a metal complex containing CO as a ligand. The catalyst for electrochemical oxidation of a metal hydride according to claim 1, wherein the metal component is rhodium and the ligand is a porphyrin compound. The catalyst for electrochemical oxidation of a metal hydride according to claim 1 or 2, wherein the metal complex is supported on a conductive support. The catalyst for electrochemical oxidation of a metal hydride according to any one of claims 1 to 3, wherein the metal hydride is an alkali metal borohydride compound. An anode electrode for a direct fuel cell using a borohydride compound as a fuel, comprising the catalyst according to any one of claims 1 to 4 as an anode electrode catalyst. An anode-side electrode portion to which a solution containing a metal hydride and a hydroxide is supplied;
A cathode-side electrode portion that is arranged opposite to the anode-side electrode portion and generates hydrogen by a proton reduction reaction;
A hydrogen generator comprising an ion permeable diaphragm disposed between the anode side electrode part and the cathode side electrode part,
A hydrogen generator comprising the catalyst according to any one of claims 1 to 4 as a catalyst for the anode side electrode. 7. The hydrogen generator according to claim 6, wherein a catalyst having an anode electrode potential lower than that of the cathode electrode in an open circuit is used as the anode electrode catalyst.

Images