JP3949299B2 - Electrode for oxygen reduction and method for producing hydrogen peroxide using the electrode - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to an oxygen reduction electrode for producing hydrogen peroxide subjected to surface modification and a method for producing hydrogen peroxide using the electrode, and more particularly, an organic compound containing a thiol group or a disulfide group. The present invention relates to a modified electrode and a method for producing hydrogen peroxide using the electrode.
[0002]
[Prior art and its problems]
Reactive oxygen species have an important role in vivo in connection with bioactive substance synthesis, bactericidal action, aging phenomenon, and the like. The active oxygen species can be produced on-site by applying energy such as light (ultraviolet rays), electricity, and ultrasonic waves to the raw materials hydrogen peroxide, ozone, oxygen, water, etc. The energy efficiency required is generally low. Recently, the possibility of using novel organic materials as catalysts that generate electrochemically active oxygen (superoxide ions, etc.) has been pointed out (Matsumoto, Tokuda, Osaka, Electroanalysis, vol 8, 648-653 ( 1996)). Although there are α, β-quinoline, pyridine, thiophenol and the like as materials, it is difficult to carry out electrolysis at a high current density, so that it has not been implemented industrially.
Hydrogen peroxide is a useful basic chemical indispensable in the food, pharmaceutical, pulp, fiber and semiconductor industries. Conventionally, hydrogen peroxide is industrially obtained by auto-oxidizing 2-alkylanthraquinol, and at the same time, the anthraquinone obtained at the same time is reduced by hydrogen to return it to the original anthraquinol, and mass synthesis is continuously performed. Yes. The purification requires complicated operations such as repeated rectification, and hydrogen peroxide is unstable and cannot be stored for a long period of time. Therefore, demand for on-site hydrogen peroxide production equipment is increasing.
[0003]
In power plants and factories that use seawater as cooling water, it has traditionally been used to generate hypochlorous acid by directly electrolyzing seawater to prevent the attachment of organisms inside the condenser. However, the use of hypochlorous acid is being regulated from the viewpoint of environmental conservation. In other words, this is to prevent organochlorine compounds from being formed due to the reaction of hypochlorous acid with the organisms and organic substances in the sea water, which causes secondary pollution. On the other hand, it has also been reported that when a small amount of hydrogen peroxide is added to the cooling water, there is a good biofouling prevention effect. However, as described above, there are still problems of safety and pollution countermeasures associated with transportation.
Conventionally, the production of hydrogen peroxide using a reduction reaction of oxygen gas has been proposed. US Pat. No. 3,592,749 has several types of hydrogen peroxide electrolytic production equipment, and US Pat. No. 4,384,931 has an alkaline solution using an ion exchange membrane. A method for producing a hydrogen peroxide solution is disclosed. US Pat. No. 3,969,201 proposes an apparatus for producing hydrogen peroxide comprising a three-dimensional carbon cathode and an ion exchange membrane. However, in these methods, the amount of alkali essential for the production of hydrogen peroxide increases almost in proportion to the produced hydrogen peroxide, so the alkali concentration becomes too high with respect to the concentration of hydrogen peroxide obtained, and the application is limited. End up.
[0004]
Furthermore, Journal of Electrochemical Society, vol.130, 1117- (1983) proposed a method for stably obtaining an acidic hydrogen peroxide solution by using a cation / anion exchange membrane and supplying sulfuric acid to an intermediate chamber. Yes. Furthermore, Electrochemical 57, p1073 (1989) reports a method for improving performance by using a membrane electrode assembly as an anode. However, these methods require power consumption and have a problem in economic efficiency, and an electrolyzer that can be satisfactorily satisfied has not been obtained so far. U.S. Pat. No. 4,631,200 and U.S. Pat. No. 4,921,587 disclose methods using a porous membrane material and a hydrophobic carbon cathode. However, in this method, it is difficult to control the transfer amount and speed of the electrolyte solution from the anode chamber to the cathode chamber, and the operation is complicated.
It is reported that almost neutral hydrogen peroxide can be obtained by electrolyzing a gas diffusion electrode in close contact with the ion exchange membrane and supplying pure water to the anode chamber and oxygen to the cathode chamber. The hydrogen peroxide concentration is low.
A method of synthesizing from oxygen and hydrogen using a palladium catalyst under high pressure and a method of synthesizing in a fuel cell have been proposed, but are impractical because of the high pressure.
Treated water contains impurities such as calcium and magnesium, and when hydrogen peroxide is generated by the reduction reaction of oxygen, alkali is always generated. Therefore, the reaction with these hydroxides or existing carbon dioxide It is known that precipitation occurs and the electrode reaction is inhibited. Carbon is mainly used as the electrode material, but its life is short because it is consumed by hydrogen peroxide. Like carbon, gold capable of proceeding with two-electron reduction is chemically stable and known as a catalyst for hydrogen peroxide production, but its current efficiency is low and impractical.
[0005]
OBJECT OF THE INVENTION
An object of the present invention is to provide an electrode for reducing oxygen to obtain hydrogen peroxide with high efficiency and a method for producing hydrogen peroxide using the electrode.
[0006]
[Means for solving problems]
The present invention is characterized in that hydrogen peroxide is produced by oxygen reduction by modifying the surface of a conductive electrode substrate, at least partially coated with gold, with an organic compound containing a thiol group or a disulfide group, followed by Au-S bonding. An oxygen reduction electrode and a method characterized in that hydrogen peroxide is produced by closely contacting an ion exchange membrane as a diaphragm with the electrode as a cathode, and supplying oxygen-containing gas to the electrode to reduce oxygen. .
[0007]
The present invention will be described in detail below.
The electrode for oxygen reduction of the present invention covers gold on at least a part of the surface of a conductive electrode substrate made of a metal, metal oxide, carbon or the like, which is a porous or flat plate material, and further has a thiol group or The organic compound containing a disulfide group is preferably modified into a thin film (self-assembled monolayer).
The reason why the generation efficiency of hydrogen peroxide is increased by the electrode of the present invention can be estimated as follows.
In general, in hydrogen peroxide generation by oxygen reduction, first, superoxide ions, which are one-electron reduction products of oxygen, are generated according to formula (1), and these superoxide ions are further converted according to formula (2) or formula (3). It is converted into hydrogen peroxide ions, which are two-electron reduction products of oxygen, and proceeds.
[0008]
^{_{O 2 + e - → O 2}} - ▲ 1 ▼
2O ₂ ⁻ + H ₂ O → O ₂ + HO ₂ ⁻ + OH ^{− (} 2)
H0 ₂ ⁻ + H ₂ O + 2e ⁻ → 3OH ^{− (} 3)
^{_{2H0 2 - → 2OH - + O}} 2 ▲ 4 ▼
[0009]
The generated hydrogen peroxide ions are reduced according to the formula (3), and are self-decomposed according to the formula (4) to become hydroxide ions, resulting in a reduction in the generation efficiency.
However, in the present invention, the surface of the conductive electrode substrate is coated with gold and further modified with an organic compound containing a thiol group or a disulfide group to form an Au—S bond on the electrode surface. Superoxide ions are converted to hydrogen peroxide according to the above formula (2). In formula (3), further reduction of hydrogen peroxide ions, and in formula (4), the reaction can be achieved if self-decomposition of hydrogen peroxide ions can be avoided. Does not progress. The electrode surface on which the Au—S bond is formed is hydrophobic, and the reaction of formula (1) occurs advantageously, but the reduction reaction of hydrogen peroxide ions hardly occurs (that is, the overvoltage increases), and the Au electrode By binding the sulfur compound to the surface, the hydrogen peroxide ion resolution of Au itself is poisoned, and as a result, decomposition of hydrogen peroxide ions hardly occurs. On the normal electrode surface, hydrogen peroxide, which is a two-electron reduction product of oxygen, is further reduced or self-decomposed and gradually consumed on the spot. However, as in the electrode according to the present invention, Au—S bonds are present on the electrode surface. When formed, both of these reactions are suppressed, and hydrogen peroxide, which is the final product, remains stably in the solution used in the electrolytic reaction, and high production efficiency is achieved. In addition, the hydrophobicity of the thiol group makes it difficult for adverse impurities to easily enter. Polyaniline or the like is known as a catalyst having electronic conductivity for performing an electrode reaction, but this catalyst only provides a reaction field, which is different from the organic compound containing a thiol group or disulfide group of the present invention. Yes.
[0010]
The conductive electrode substrate used in the present invention is made of a corrosion-resistant material such as carbon, titanium, niobium, tantalum, nickel, iron or oxides thereof, and is made of a wire mesh, powder sintered body, metal fiber sintered body, etc. It is molded into the form. In order to supply oxygen and improve processing capacity, a porous body is desirable, but a flat plate or the like can also contribute to an increase in production efficiency.
It is also preferable to use a gas diffusion electrode. A conductive electrode substrate may be prepared using carbon powder as a raw material, and a gas diffusion electrode may be prepared using the conductive electrode substrate. In this case, the gas supply layer is formed inside the electrode, and the gas is supplied from the back surface. In order to quickly supply and remove the reaction product gas and liquid, it is desirable to use a hydrophobic or hydrophilic material in a dispersed manner. As an example, there is a porous cathode made of carbon cloth (manufactured by Nippon Carbon Co., Ltd.) in which a gold catalyst is supported on carbon powder and fixed with a fluororesin. In addition, it is desirable to use a so-called semi-hydrophobic gas diffusion electrode having a hydrophilic reaction layer and a water-repellent gas diffusion layer on both sides. In addition, the gas diffusion electrode can be used in close contact with the cation exchange membrane, and the distance between the two can be kept to a minimum, thereby minimizing the electrolysis voltage. However, since the produced catholyte is drawn out to the opposite side of the ion exchange membrane through the gas diffusion electrode, and the gas supply is performed from the liquid withdrawal side, the structure is slightly complicated. The adhesion between the gas diffusion electrode and the cation exchange membrane may be mechanically bonded in advance, or a pressure of about 0.1 to 30 kgf / cm ² may be applied during electrolysis.
[0011]
The conductive electrode substrate is coated with gold as described above. The coating is performed so as to be 10 to 100 g / m ² by a technique such as a thermal decomposition method, a resin fixing method, a vapor deposition method, electroplating, or electroless plating.
Next, an organic compound containing a thiol group or a disulfide group is coated on the surface of the conductive electrode substrate. Examples of the compound include thiophenol, p-thiocresol, 2-mercaptopyrimidine, butanethiol, 2-aminoethanethiol, 2-mercaptoethanol, 3-mercaptopropanol, 3-mercaptopropionic acid, mercaptoacetic acid, 2-mercaptopyridine. , Ethylene disulfide, phenyl disulfide, 2-hydroxy disulfide and the like.
The coating can be easily and selectively formed on the gold surface of the electrode substrate by immersing the electrode substrate in water or an organic solvent (for example, methanol or acetone) in which a thiol component is dissolved. The thiol or disulfide compound remaining on the electrode substrate surface without being fixed is easily removed when immersed in a solution containing only an organic solvent, and further washed with water and dried to obtain an electrode for oxygen reduction. Such modification with a compound having a thiol group or disulfide group is very simple. Even if the electrode for oxygen reduction is consumed after use, it can be easily re-applied by simply immersing it in an organic compound solution containing a thiol group or disulfide group. Can be activated.
[0012]
This oxygen reduction electrode can be used as a cathode of an electrolytic cell for producing hydrogen peroxide. This electrolytic cell is preferably a two-chamber electrolytic cell partitioned by a cation exchange membrane into an anode chamber and a cathode chamber. The cation exchange membrane is used to produce hydrogen peroxide with high efficiency by preventing hydrogen peroxide generated in the cathode chamber from being transferred to the anode chamber and being oxidized and decomposed into water and oxygen, and This is because by maintaining the hydrogen peroxide concentration high and reducing the amount of hydrogen peroxide produced, the electrical conductivity of the electrolyte is kept high, the electrolysis voltage is lowered, and the power consumption is reduced. Although the cation exchange membrane that can be used is not particularly limited, it is desirable to use a fluororesin-based membrane that is durable against an oxidizing agent such as hydrogen peroxide. As a typical cation exchange membrane, There are perfluorosulfonic acid membranes such as Nafion 115, 117, 315, 350 under the trade name.
An anode is installed on the anode chamber side of the cation exchange membrane. The anode is not particularly limited and may be determined according to the liquidity of the electrolytic solution.
[0013]
A sheet-like hydrophilic liquid permeable layer made of zirconium oxide or silicon oxide may be provided between the cation exchange membrane and the gas diffusion cathode. This hydrophilic liquid permeable layer inhibits gas supply by taking out the catholyte containing the product to be taken out to the cathode chamber side through the gas diffusion cathode to the peripheral part of the hydrophilic liquid permeable layer. This prevents the catholyte from becoming stagnant in the gas diffusion cathode and performs a smooth gas supply and extraction to achieve a reduction in electrolysis voltage.
The electrode according to the present invention uses a sulfur compound modified on the surface via a metal-S bond to form a pipe to which organisms easily adhere with the electrode of the present invention, and between the separately installed anodes. If electricity is supplied to the pipe and oxygen is supplied, it is possible to prevent the organism from attaching to the pipe.
[0014]
Next, an electrolytic cell for producing hydrogen peroxide using the oxygen reduction electrode of the present invention will be illustrated based on the attached drawings, but the present invention is not limited to this.
FIG. 1 is a longitudinal sectional view showing an example of an electrolytic cell for producing hydrogen peroxide using the oxygen reduction electrode of the present invention.
The electrolytic cell main body 1 is divided into an anode chamber 3 and a cathode chamber 4 by a cation exchange membrane 2, and the anode chamber 3 is slightly separated from the cation exchange membrane 2 and is made of a porous insoluble metal such as an expanded mesh. An anode 5 is installed. A gas diffusion cathode 6 is in close contact with the cathode chamber surface of the cation exchange membrane 2, and a cathode current collector 7 is connected to the opposite surface of the gas diffusion cathode 6 to the cation exchange membrane. The gas diffusion cathode 6 supports a gold catalyst on the surface of a carbon cloth obtained by molding carbon powder using a fluororesin as a binder, and dissolves the carbon cloth in an organic solvent such as a thiol compound such as butanethiol or a disulfide compound such as ethyl disulfide. It is prepared by immersing it in the solution and taking it out, followed by washing and drying.
8 is an anolyte supply port formed on the bottom plate of the anode chamber 3, 9 is an anolyte and generated oxygen gas outlet formed on the top plate of the anode chamber 3, and 10 is a hydrogen peroxide collector formed on the top plate of the cathode chamber 3. An outlet 11 is an oxygen-containing gas supply port formed in the bottom plate of the cathode chamber 4.
[0015]
When a dilute aqueous solution of, for example, sodium hydroxide is supplied from the anolyte supply port 8 of the electrolytic cell main body 1 having such a structure, and an oxygen-containing gas is supplied from the oxygen-containing gas supply port 10, the anode 5 and the cathode 6 are energized. Sodium ions and hydrogen ions are generated in the anode chamber and pass through the cation exchange membrane 2 to reach the cathode chamber 4.
On the other hand, in the cathode chamber, superoxide ions are generated on the cathode surface by oxygen reduction, and hydrogen peroxide ions are further generated. Further electrode reduction and autolysis of hydrogen peroxide ions are suppressed at the cathode surface modified with a thiol compound or disulfide compound. Therefore, since the generated superoxide ion is almost quantitatively converted to hydrogen peroxide, high current efficiency is achieved.
[0016]
【Example】
Next, although the Example of hydrogen peroxide manufacture using the electrode for oxygen reduction by this invention is described, this Example does not limit this invention.
[0017]
[Example 1]
A porous insoluble electrode made of titanium having an electrode area of 0.2 dm ² and supporting an iridium oxide (IrO ₂ ) catalyst was used as an anode.
Carbon powder (product name: Vulcan XC-72) carrying a gold catalyst at a rate of 100 g / m ² by pyrolysis method, using fluororesin (Mitsui DuPont, 30J) as a binder and carbon cloth (Nippon Carbon Co., Ltd.) It was carried on a porous electrode substrate made by Kogyo Co., Ltd. This substrate was immersed in methanol containing 50 mmol of butanethiol for 1 hour, taken out, and further immersed in methanol alone to remove carbon powder and fluororesin that were not fixed to the carbon cloth and dried to form a gas diffusion cathode. .
This gas diffusion cathode is installed in close contact with Nafion 350 (manufactured by DuPont), which is a cation exchange membrane. On the opposite side of the cation exchange membrane, there is a space for liquid circulation so that the distance between the electrodes is 5 mm. The anode was installed with a gap therebetween to constitute an electrolytic cell as shown in FIG.
When 6 ml of 1% sodium hydroxide aqueous solution is supplied to the anode chamber side and 20 ml of oxygen on the back side of the cathode is supplied to the cathode gas chamber side at a temperature of 30 ° C. and a current of 2 A is applied, 3.2 g A solution containing 1 / liter of hydrogen peroxide was obtained with a current efficiency of 90%.
[0018]
[Example 2]
Except that butanethiol was changed to thiophenol, an electrolytic cell was constructed under the same conditions as in Example 1 and electrolysis was performed. Similarly, a solution containing 3.2 g / liter of hydrogen peroxide was obtained with a current efficiency of 90%. .
[0019]
[Comparative Example 1]
An electrolytic cell was constructed and electrolyzed under the same conditions as in Example 1 except that it was not immersed in methanol in which butanethiol was dissolved and was not further immersed in methanol alone, and it contained 1.8 g / liter of hydrogen peroxide. A solution was obtained with a current efficiency of 50%.
[0020]
【The invention's effect】
The present invention is characterized in that hydrogen peroxide is produced by oxygen reduction by modifying the surface of a conductive electrode substrate, at least partially coated with gold, with an organic compound containing a thiol group or a disulfide group, followed by Au-S bonding. This is an electrode for oxygen reduction.
When the electrode of the present invention is used for the production of hydrogen peroxide, since an Au-S bond exists on the surface of the electrode, the production of intermediate superoxide ions is advantageous, and the hydrogen peroxide ions produced next are wasted. In the end, it remains as hydrogen peroxide, which is the target product, in the electrolytic solution, so that a high generation efficiency that cannot be obtained by the prior art can be obtained. Furthermore, this electrode can be easily modified with a thiol compound or a disulfide compound, and even when the electrode is consumed due to use, it can be easily reactivated simply by immersing the electrode in the raw material solution.
[0021]
The oxygen reduction electrode of the present invention can also be used to produce hydrogen peroxide by being closely attached to an ion exchange membrane, which is a diaphragm, as a cathode, and supplying oxygen-containing gas to the electrode to reduce oxygen. .
This method can also produce hydrogen peroxide with high efficiency because of the high current efficiency related to the production of hydrogen peroxide possessed by the thiol-based material.
[Brief description of the drawings]
FIG. 1 is a longitudinal sectional view showing an example of an electrolytic cell for producing hydrogen peroxide using an oxygen reduction electrode according to the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Electrolyzer main body 2 ... Cation exchange membrane 3 ... Anode chamber 4 ... Cathode chamber 5 ... Insoluble metal anode 6 ... Gas diffusion cathode 7 ... Cathode collector 8 ... Anolyte supply port 9 ... Anolyte and generated oxygen gas outlet 10 ... Hydrogen peroxide outlet 11 ... Oxygen-containing gas supply port

Claims (2)

An electrode for oxygen reduction, characterized in that at least part of the surface of the conductive electrode substrate coated with gold is modified with an organic compound containing a thiol group or a disulfide group and bonded with Au-S to produce hydrogen peroxide by oxygen reduction. . At least a portion of gold was coated on the conductive electrode substrate surface is modified Au-S bond with an organic compound containing a thiol group or a disulfide group is brought into close contact with the oxygen reduction electrode in the ion-exchange membrane is a membrane as a cathode, A method comprising producing hydrogen peroxide by supplying an oxygen-containing gas to the electrode and reducing oxygen.

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