JPH08120476A - Activation of electrode for producing hydrogen peroxide and production of hydrogen peroxide - Google Patents

Abstract

PURPOSE: To activate a cathode to be used again at high current efficiency by allowing a carbon based cathode used for producing H2 O2 by reducing oxygen in an alkali aq. solution to contact with a gas introduced into a cathode chamber. CONSTITUTION: The cathode used in a method for producing H2 O2 by reducing oxygen in the alkali aq. solution, consisting essentially of carbon deteriorated in activity and composed of a gas diffusion electrode, is brought into contact with the gas introduced into the cathode chamber after the electrolytic solution in the cathode chamber is removed. As a result, since the cathode is activated without taking out from the cathode chamber and is used again at high current efficiency, H2 O2 is continuously produced at high current efficiency over a long period by alternately repeating the reduction and activation of the cathode. Further, the introduced gas is preferably low in humidity and oxygen, and air, nitrogen and the like are exemplified as the gas though the kind is not restricted in particular.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for activating a gas diffusion electrode containing carbon whose activity is lowered as a main component, and a method for producing hydrogen peroxide using this activation method. More specifically, by activating a gas diffusion electrode containing carbon having a reduced activity as a main component, an alkaline aqueous solution containing medium and low concentrations of hydrogen peroxide is continuously produced by an electrolytic method for a long time. Regarding the method.

[0002]

2. Description of the Related Art Hydrogen peroxide has long been useful as a bleaching agent and an oxidizing agent, and most of them are currently produced by hydrogenation and oxidation of anthraquinone. However, the anthraquinone hydrogenation / oxidation method has large equipment and is suitable for mass production. Therefore, hydrogen peroxide produced by the hydrogenation / oxidation method of anthraquinone is generally concentrated to about 50 to 60% and transported to the place of consumption. For this reason, a concentrating facility is also required, and the equipment is becoming larger.

However, when hydrogen peroxide is used, it is not necessary to have a particularly high concentration, but it is usual to dilute it before use. Therefore, if an aqueous solution containing medium- and low-concentration hydrogen peroxide can be simply produced at the place of use at the time of use and can be used as it is, the user has a great advantage. For example, for the paper manufacturing industry that uses a large amount of hydrogen peroxide for bleaching pulp, it is extremely useful if an aqueous solution containing medium and low concentrations of hydrogen peroxide can be easily and inexpensively produced adjacent to a pulp mill. Is.

Therefore, the inventors of the present invention have reexamined the previously known methods for producing hydrogen peroxide from such a viewpoint. As a result, it has been found that the method of electrolytically reducing oxygen in an alkaline aqueous solution has a simple process and does not require a corrosion-resistant material, and thus the apparatus can be simplified.

In the method for producing hydrogen peroxide by electrolysis, the supplied oxygen is electrolyzed on the cathode side separated by the ion exchange membrane to generate an alkaline peroxide anion, and hydrogen is generated on the anode side while generating oxygen. Generates ions. This permeates the cation exchange membrane and produces an alkaline aqueous hydrogen peroxide solution on the cathode side. Therefore, the anode is naturally acidic, and the hydrogen peroxide produced at the cathode is more stable on the acidic side. Therefore, when obtaining hydrogen peroxide for storage, there is an example of operating the anode on the acidic side. Many.

For example, as an example of acidifying the anode side,
U.S. Pat. No. 4,384,931 includes anode / acid aqueous solution / cation exchange membrane / alkali aqueous solution / cathode composed of gas diffusion electrode /
An electrolytic cell is disclosed that is constructed in the order of oxygen. In addition, Kuhn et al., J. Electrochem. Soc. 130, Vol. 117p (1983), consists of anode / acid aqueous solution / cation exchange membrane / acid aqueous solution / anion exchange membrane / alkali aqueous solution / cathode consisting of gas diffusion electrode / oxygen in order. An electrolytic cell is described.

On the other hand, hydrogen peroxide as a bleaching agent for pulp is used on the alkaline side. Therefore, in this case, in order to use the alkaline aqueous hydrogen peroxide solution produced at the cathode as it is, the alkaline electrolyte is often flown to neutralize or operate on the alkaline side.

As an example of making the anode side alkaline, Japanese Patent Publication No. 59-15990 discloses an example of anode / alkali aqueous solution / separator / gas diffusion electrode / cathode / oxygen. In addition, Sudoh et al., J. Chem. Eng. Japan, 24, 165.
P (1991) discloses an example of anode / alkali aqueous solution / ion exchange membrane / alkali aqueous solution / cathode consisting of gas diffusion electrode / oxygen.

[0009]

By the way, it is considered that the reaction in the diffusion electrode occurs at the point where the three phases of the gas, the electrolytic solution and the solid conductor come into contact with each other. Therefore, in order to maximize the current efficiency, it is necessary to maximize the area of the three-phase contact area. If the space in the diffusion electrode is filled with the electrolytic solution, the mass transfer rate of the gas to the conductor becomes too slow and the current efficiency deteriorates. On the contrary, the electrolytic reaction hardly occurs because there is no electrolytic solution in which the space in the diffusion electrode is filled with gas.

Therefore, as a method of maintaining good contact between the three phases, many studies have been conducted on the material of the cathode which is the gas diffusion electrode. Efficiently diffuses oxygen in fluidized beds and fixed beds of carbon and metal particles, which are essentially porous conductors, and in conventional electrode materials such as felt and cloth of carbon fibers and metal fibers. Various proposals have been made to improve the current efficiency.

Takenaka et al., DENKI KAGAKU, Volume 57, 11
No. 1073p discusses activated carbon, carbon black and graphite, and further discusses water repellent treatment.
Also, Sudoh et al., J. Chem. Eng. Japan, 24, 165p (19
In 91), a compact of graphite powder and a graphite felt are studied. Furthermore, regarding water repellent treatment, in addition to the above, Noriyoshi Hotta et al., Material, Vol. 40, No. 448, 84
~ 88p discloses a method of treating a solid conductor with polytetrafluoroethylene (PTFE). However, it is technically difficult to uniformly treat the solid conductor with PTFE so as to impart appropriate wettability without impairing the conductivity due to the PTFE coating.

The applicant has already conducted various experiments and studies on the material of the gas electrode for increasing the current efficiency in the electrolysis system. As a result, among the commonly used fluidized beds and fixed beds of metal particles, and among the felts and cloths of carbon fibers and metal fibers, the carbon fibers (cloths) with high packing density maintain good current efficiency for a long time. Was found and filed a patent application first (Japanese Patent Laid-Open No. 6-20038)
No. 9).

Further, in the course of the study, in operating the electrolytic hydrogen peroxide production facility, the current efficiency is gradually decreased when the current efficiency is high at the beginning of the operation, but the current efficiency is gradually decreased when the current operation is continuously performed for a long time. It turned out that there is. The applicant of the present application has proposed in Japanese Patent Laid-Open No. 6-200389 that this problem can be solved by using a porous amorphous carbon fiber material for the cathode. However, as a result of subsequent studies, it was found that even when a porous amorphous carbon fiber material was used for the cathode, the current efficiency decreased with time when the device was scaled up or the supplied current amount was increased. did. Further, even if the current efficiency is lowered, if the electrolysis is continued as it is, the cathode may be deactivated at last.

Therefore, an object of the present invention is to activate a cathode composed of a gas diffusion electrode whose main component is carbon whose current efficiency is lowered as a result of electrolysis so that it can be used again with high current efficiency. To provide. In particular, an object of the present invention is to provide a method of activating a gas diffusion electrode installed in a cathode chamber without taking it out of the cathode chamber. A further object of the present invention is to utilize a method of activating a gas diffusion electrode installed in a cathode chamber without taking it out of the cathode chamber, which makes it unnecessary to assemble an electrolysis apparatus for each activation, and is suitable for continuous operation. It is to provide a manufacturing method of.

[0015]

The present invention is used for a cathode in a method for producing hydrogen peroxide by reducing oxygen in an alkaline aqueous solution, and activates a gas diffusion electrode containing carbon with reduced activity as a main component. A method for activating a cathode for hydrogen peroxide production, characterized in that the electrolytic solution in the cathode chamber is removed, and a gas is introduced into the cathode chamber to bring the gas into contact with the gas diffusion electrode with reduced activity. (First activation method)

The present invention further relates to a method for producing hydrogen peroxide by reducing oxygen in an alkaline aqueous solution at a cathode, which comprises reducing the oxygen and a gas diffusion electrode containing carbon whose activity is lowered as a main component. The activation of a certain cathode is alternately repeated, and the activation is performed by removing the electrolytic solution in the cathode chamber, and introducing gas into the cathode chamber to bring the gas into contact with the gas diffusion electrode with reduced activity. The present invention relates to a characteristic method for producing hydrogen peroxide.

In addition, the present invention is a method for activating a gas diffusion electrode containing carbon having a reduced activity as a main component, which is used as a cathode in a method for producing hydrogen peroxide by reducing oxygen in an alkaline aqueous solution. And a method for activating the cathode for producing hydrogen peroxide (second activation method), characterized in that the gas diffusion electrode with lowered activity taken out from the cathode chamber is heat-treated at a temperature of 40 ° C. or higher. The present invention will be described below.

In the present invention, the cathode to be activated is
A gas diffusion electrode used in a method for producing hydrogen peroxide by reducing oxygen in an alkaline aqueous solution, the active ingredient being carbon as a main component. A method for producing hydrogen peroxide by reducing oxygen in an alkaline aqueous solution is known and used in the known method, and a cathode having a decreased activity is a target for activation in the present invention. Known methods include, for example, the methods described in JP-A-6-88273 and JP-A-6-200389. However, it is not limited to these.

There is no limitation on the gas diffusion electrode containing carbon as a main component to be activated. For example, Japanese Patent Laid-Open No. 6-2
The carbon fiber material having a packing density of 0.3 or more described in No. 00389 can be mentioned. Examples of the carbon fiber material include a knitted carbon fiber, and examples of the knitted carbon fiber include a commercially available graphite cloth. Carbon fiber materials other than graphite cloth may be used.

Further, as the cathode to be activated in the present invention, a porous amorphous carbon material can be exemplified. Porous amorphous carbon materials are amorphous, substantially non-oriented, ie, non-anisotropic, porous carbon materials.
Examples of such a carbon material include a porous amorphous carbon fiber material and a porous amorphous carbon molded body.

The above-mentioned porous carbon fiber material is obtained as follows. When a novolac type phenol resin is melt-spun and heat-treated with formalin, amorphous and non-oriented phenol fibers having slight three-dimensional crosslinks between molecules are obtained. This fiber is flame-proof, heat-resistant, and chemical-resistant, and when it is heated, it is carbonized in its original form and becomes an amorphous carbon fiber having a high carbon content. On this occasion,
Since it does not melt or shrink, if the phenol fiber is processed into felt or cloth and then carbonized, the amorphous carbon fiber felt or cloth can be obtained as it is. Although it does not have a crystal structure like graphite, it is flexible because it is slightly crosslinked, and is optimal for electrodes as amorphous glassy carbon (glassy carbon).

In order to maintain the shape of the electrode, the porous carbon fiber material preferably has a cross shape,
It is preferable that the cloth be filled in the electrode holding frame so that the cloth has a predetermined area and thickness. The packing density has a great influence on the electrolysis efficiency.If the density is low, the contact between the electrode and oxygen or alkali electrolyte is insufficient and the electrolysis efficiency is poor.If the density is too high, the flow of oxygen and alkali electrolyte is disturbed, and Efficiency drops. From such a viewpoint, the packing density is 0.3 or more and 1.
It is suitable to be 5 or less, preferably 0.4 or more and 1.0 or less.

The porous carbon molded body exemplified as the gas diffusion electrode of the present invention can be obtained as follows. The fine spherical spherical phenol resin prepared by the suspension polymerization method is adjusted to have an appropriate porosity after carbonization by controlling the crosslinking density. This fine spherical resin is
A slight cross-linking is performed similarly to the phenol fiber, and then the cross-linked fine spherical spherical phenol resin is sintered and carbonized to obtain a porous glassy carbon molded body. By selecting the particle size of the resin, the molding pressure and the like at this time, it is possible to appropriately control the pore diameter and the porosity.

The porous carbon molded body can be molded into an arbitrary electrode shape at the time of carbonization by sintering molding of fine spherical spherical phenol resin. The shape of the electrode can be appropriately determined in consideration of the shape and size in the electrolytic cell, as in the case of the carbon fiber material (cloth). The density of the porous carbon compact (apparent density, not the true density of the carbon compact) is
As in the case of the carbon fiber material, it affects the electrolysis efficiency, and it is suitable that the range is, for example, about 0.9 to 1.4.

The carbon fibers used in JP-A-6-200389 are not pitch-based carbon fibers but pitch-based or acrylic carbon fibers. Generally, pitch-based carbon fibers have a graphite structure (a layered crystal structure generated by heating amorphous carbon to a high temperature), and have large anisotropy and electrical resistance values differ depending on the crystal direction. Acrylic carbon fibers have higher crystallinity than graphite, because fibers having a crystal structure are carbonized as they are.

In the present invention, the "cathode with reduced activity" is a cathode with reduced activity, that is, current efficiency, as compared with the time of start of electrolysis. Usually, in an electrolytic apparatus for producing hydrogen peroxide, the cathode reaction is set to be rate-determining, and a decrease in current efficiency is caused by a decrease in cathode activity. The degree of activity reduction to be activated can be appropriately determined in consideration of the time required for activation and the like. A decrease in current efficiency directly leads to a decrease in productivity, which is not preferable, but too frequent activation treatment adversely affects productivity. The activation treatment has a current efficiency of, for example, 80% (current efficiency at the beginning of use varies depending on the cathode material, but usually about 90%).
%), Or 1 to 3 times a day or 1 to 2 times in 2 to 3 days.

In the first activation method of the present invention, the electrolyte in the cathode chamber is removed, a gas is introduced into the cathode chamber, and the gas is brought into contact with the gas diffusion electrode whose activity has been lowered, thereby activating the cathode. I do. The specific operation flow is as follows. Stop the power supply for hydrogen peroxide production. Stop supplying electrolyte to the cathode and anode. The supply of humidified oxygen to the cathode is stopped. Remove the electrolyte in the cathode chamber. A gas is introduced into the cathode chamber to bring the gas into contact with the cathode whose activity has decreased. The above steps 1 to 3 can be performed sequentially or simultaneously. The removal of the electrolytic solution in the cathode chamber can be performed by providing a discharge port at the bottom of the cathode chamber, and also by introducing the gas to cause the gas to accompany the electrolytic solution and discharge the gas. . Further, if necessary, the cathode chamber can be washed by filling the cathode chamber with water, for example, distilled water, after removing the electrolytic solution and before introducing the gas. Further, in the above process, the supply of the electrolytic solution to the anode may not be stopped, and only the supply of the electrolytic solution to the cathode may be stopped.

The gas introduced into the cathode chamber in
A gas having a low humidity is preferable, for example, a relative humidity is preferably 50% RH or less, and an absolute dry state is more preferable. The degree of activation of the cathode is influenced by the humidity of the introduced gas, and the humidity of the introduced gas is preferably low, so the gas can be dried before the introduction. The type of gas is not particularly limited, and examples thereof include easily available gases such as oxygen, air, and nitrogen. Especially, since oxygen is used for electrolysis, the operation and the device can be simplified by using oxygen as a gas for utilization.

The introduction amount, introduction speed, introduction time and the like of the gas into the cathode chamber are appropriately determined in consideration of the material, structure and amount of the cathode and the degree of activation of the cathode. In practice, the introduction time is preferably shorter from the viewpoint of productivity, for example, 5 to 60 minutes, preferably 10 to 30 minutes. The gas can be introduced into the cathode chamber not only at room temperature but also under heating. The heating can be performed by directly heating the cathode chamber or by introducing heated gas. The heating temperature is not particularly limited, but may be 30 to 70 ° C., for example. However,
From the viewpoint of preventing the deterioration of the ion exchange membrane, 30
It is preferably in the range of -50 ° C.

After the introduction of the gas, the electrolytic solution and the humidified oxygen are supplied to the cathode chamber, the electrolytic solution is supplied to the anode chamber, and then the supply of the power source for hydrogen peroxide production is restarted, whereby the peroxide is oxidized. Restart hydrogen production.

Next, a method for producing hydrogen peroxide of the present invention using the above-mentioned first activation method of the present invention will be described. In the method for producing hydrogen peroxide of the present invention, the production of hydrogen peroxide by electrochemical reduction of oxygen and the activation of the cathode, which is a gas diffusion electrode mainly composed of carbon with reduced activity, are alternately repeated. , And the activation of the cathode by the first activation method of the present invention. First, the production of hydrogen peroxide by electrochemical reduction of oxygen is a known method, for example, JP-A-6-88273 and JP-A-6-20.
The method described in No. 0389 can be used as it is. Further, as the cathode used for electrolysis, the cathode described in the first activation method can be used.

As the ion exchange membrane, a cation exchange membrane is used because an alkaline hydrogen peroxide aqueous solution is obtained at the cathode. Considering chemical resistance, it is preferable to use a fluororesin type ion exchange membrane. As the alkaline aqueous solution supplied to the cathode chamber and the anode chamber, an aqueous sodium hydroxide solution is usually used, but an aqueous solution of potassium hydroxide or the like can also be used. The oxygen supplied to the cathode may be an oxygen mixed gas such as air containing oxygen in addition to oxygen.

The alkaline aqueous hydrogen peroxide solution obtained at the cathode is preferably used as it is without being diluted or concentrated. Therefore, the operating conditions can be appropriately changed depending on the purpose of use of hydrogen peroxide. it can. For example,
When used for bleaching pulp, the alkali ratio in the aqueous hydrogen peroxide solution is preferably closer to the use conditions for bleaching. Therefore, it is desirable that the concentration of the alkaline aqueous solution supplied to the cathode chamber is not high, and it is preferably lower than the concentration of the alkaline aqueous solution supplied to the anode chamber. Specifically, the alkali concentration in the cathode chamber is 0.6 mol or less, preferably 0.4 mol or less, and the alkali concentration in the anode chamber is 0.6 mol or more.
It is preferably 0.9 mol or more.

The concentration and supply amount of the electrolytic solution, the supply amount of oxygen and the electric current can be appropriately set depending on the scale of the electrolytic cell. The voltage is 1.5 to 2.5 volts, preferably 1.
It can be 7-2.3 volts. Further, the temperature of the electrolytic solution is preferably set so that the temperature of the supply solution does not rise in consideration of heat generation due to electrolysis. In particular, the temperature of the electrolytically produced liquid in the cathode chamber contains hydrogen peroxide, so it is preferable to control the temperature to 40 ° C. or lower.

If hydrogen peroxide is continuously produced under the above conditions, the electrode efficiency will drop to about 80% after 1 to 3 days depending on the type of the cathode. Therefore, the cathode is activated by the first activation method of the present invention, and after activation, the production of hydrogen peroxide is restarted again. As described above, the activation frequency can be set after a certain period of time or when the current efficiency becomes lower than a certain value.

Next, the second activation method of the present invention will be described. By repeating the activation treatment as in the first method of the present invention, the cathode can be continuously used with a high current efficiency for an extremely long period. However, it is expected that the current efficiency will eventually decrease or the current efficiency will be deactivated (for example, current efficiency of 20% or less). Alternatively, the cathode may be deactivated due to a mistake such as a malfunction of the automatic activation operation. The second activation method of the present invention is a method of activating and recovering a deactivated cathode in the above case. Therefore, in the second activation method of the present invention, “a cathode is used in a method of producing hydrogen peroxide by reducing oxygen in an alkaline aqueous solution,
The method of activating a gas diffusion electrode containing carbon having a reduced activity as a main component "is almost the same as that of the first activation method of the present invention, but the degree of activity reduction is the same as that of the first activation method of the present invention. It may be lower than that in the activation method.

In the second activation method of the present invention, the cathode with lowered activity is taken out from the cathode chamber. When taking out, the power supply, the supply of the electrolytic solution, and the supply of humidified oxygen are stopped. The taken-out cathode is washed, if necessary, with water and then heat-treated at a temperature of 40 ° C. or higher. The heat treatment temperature is preferably 45 ° C. or higher. The upper limit is 1
50 ° C., and the preferable heat treatment temperature is 50 to 100 ° C.
Is in between. Further, the heat treatment time can be appropriately determined in consideration of the degree of activation, and can be, for example, 1 to 5 hours. The heating atmosphere is not particularly limited, but is usually air. However, if necessary, it can be performed in a non-oxidizing atmosphere such as a nitrogen atmosphere. The activated cathode can be installed again in the cathode chamber and used for the production of hydrogen peroxide.

[0038]

According to the first activation method of the present invention, it is possible to activate the gas diffusion electrode whose activity has been lowered without disassembling the device and taking out the cathode, and to the same extent as before the start of use. . Further, according to the method for producing hydrogen peroxide of the present invention utilizing the first activation method, hydrogen peroxide can be produced continuously for a long period of time with high current efficiency. Therefore, NaOH / suitable for applications such as pulp bleaching
An alkaline aqueous solution of hydrogen peroxide having a H ₂ O ₂ weight ratio of about 2 can be efficiently produced. Further, according to the second activation method of the present invention, the gas diffusion electrode whose activity has decreased can be activated by a simple treatment, and can be used again for producing hydrogen peroxide with high current efficiency.

[0039]

EXAMPLES The present invention will be further described below with reference to examples. Example 1 Electrolysis conditions A Ni plate was used as the anode, and an alkaline aqueous solution (NaOH concentration 1.00 mol / liter) was used as the electrolyte solution on the anode side.
It was supplied to the anode chamber at 00 ml / min. A cation exchange membrane (Nafion 117: manufactured by DuPont, membrane thickness 0.
3 mm) was used. As the cathode, a graphite cloth (GF-20-P-21E manufactured by Nippon Carbon Co., Ltd.) having a packing density of 0.44 and an area of 50 cm ² was used. Cathode (reaction layer)
Alkaline aqueous solution (NaOH concentration: 0.1 mol / liter) was supplied at 0.8 ml / min. Humidified oxygen to be electrolyzed was supplied to the cathode chamber at 2.5 l / min. The electrolysis was performed by constant current electrolysis with a current value of 3.0A. The capacity of the cathode in the cathode chamber was 5 cm ² .

Activation conditions After electrolysis was carried out for 12 hours under the above conditions, the following treatments were carried out. Turn off the power to stop electrolysis Stop supplying electrolyte to the cathode and anode Stop supplying humidified oxygen to the cathode Replace the liquid in the cathode chamber with distilled water Supply dry oxygen (relative humidity 30% RH) to the cathode , Took 5 minutes. Next, at room temperature, dry oxygen was supplied at 2 liter / min for 20 minutes.
Distilled water in the cathode chamber was completely removed 600 seconds after the start of the supply of dry oxygen. After the completion of the supply of dry oxygen, the supply of the electrolytic solution to the cathode and the anode and the supply of the humidified oxygen to the cathode were restarted, and then the power was turned on to restart the electrolysis. It took 30 minutes from stop to restart. The humidity of the gas discharged from the cathode chamber immediately before the completion of the supply of dry oxygen was about 48%. Furthermore, the activation treatment was performed 24 hours, 48 hours, and 72 hours after the start of electrolysis. Table 1 shows the amount of catholyte-producing liquid, the concentration of hydrogen peroxide in the catholyte, and the current efficiency immediately before each activation treatment.

Example 2 In the same manner as in Example 1, except that the supply of the electrolytic solution to the anode in Example 1 was continued without stopping, and the liquid in the cathode chamber was not replaced with distilled water. Electrolysis and activation were repeated. Table 1 shows the amount of the cathodic liquid produced, the concentration of hydrogen peroxide in the catholyte and the current efficiency immediately before the activation treatment 12 hours, 24 hours, 48 hours and 72 hours after the start of electrolysis. Further, the electrolysis is performed for 24 hours, and then the activation is repeated. Table 1 also shows the amount of the catholyte-producing solution, the hydrogen peroxide concentration of the catholyte and the current efficiency immediately before the activation treatment after 1488 hours and 2496 hours.

Comparative Example 1 A method for continuously producing hydrogen peroxide was carried out under the same electrolysis conditions as in Example 1 and without activation. Table 1 shows the amount of the catholyte-producing liquid, the concentration of hydrogen peroxide in the catholyte and the current efficiency at each time.

Comparative Example 2 In Example 1, instead of the activation treatment, electrolysis initiation 12
The following operations were carried out after 24 hours, 48 hours and 72 hours. Turn off the power to stop electrolysis. Stop supplying humidified oxygen to the cathode. Continue supplying electrolyte to the cathode and anode. After 30 minutes, restart electrolysis. Table 1 shows the amount of catholyte-producing solution immediately before each treatment, the concentration of hydrogen peroxide in the catholyte, and the current efficiency.

Comparative Example 3 In Example 1, instead of the activation treatment, electrolysis was started 12
The following operations were carried out after 24 hours, 48 hours and 72 hours. Turn off the power and stop electrolysis. Stop supplying electrolyte to the cathode. Continue flowing anolyte. Continue supplying humidified oxygen to the cathode. After 30 minutes, restart electrolysis. Table 1 shows the amount of catholyte-producing solution immediately before each treatment, the concentration of hydrogen peroxide in the catholyte and the current efficiency.

[0045]

[Table 1]

From the above examples and comparative examples, the following can be seen. From the comparison between Examples 1 and 2, the replacement of the anode with distilled water may or may not be performed during the termination treatment. However, the replacement of distilled water has an advantage of preventing deterioration of the ion exchange membrane. According to the comparison between Example 2 and Comparative Example 3, if the humidified oxygen (80% RH) is kept flowing during the stop treatment, the current efficiency is slightly recovered, but it is insufficient. Comparative examples 1 and 2
Therefore, when neither humidified oxygen nor dry oxygen is supplied at the time of stop treatment, current efficiency is not recovered at all, which is the same as continuous operation.

Example 3 Example 1 except that a phenol-based carbon fired fiber cloth (CC509-20, manufactured by Nippon Kynol Co., Ltd.) was used as the cathode.
Under the conditions shown in (1), continuous operation was carried out for 1200 hours, and the carbon sheet of the cathode whose current efficiency had dropped to about 20% was taken out (current efficiency at the beginning of electrolysis was about 89%). The carbon sheet taken out was washed with distilled water and heat-treated in a dryer (atmosphere: air) at the temperature and time shown in Table 2. After heat treatment,
The carbon sheet was placed in the cathode chamber, and electrolysis was performed again under the same conditions as in Example 1. Table 2 shows the current efficiency at the beginning of electrolysis
Shown in As is clear from Table 2, by the above heat treatment,
The carbon sheet was activated.

[0048]

[Table 2]

Claims (3)

[Claims] 1. A method for activating a gas diffusion electrode containing carbon having a reduced activity as a main component, which is used as a cathode in a method for producing hydrogen peroxide by reducing oxygen in an alkaline aqueous solution, the method comprising: The method for activating a cathode for producing hydrogen peroxide, characterized in that the electrolytic solution is removed, gas is introduced into the cathode chamber, and the gas is brought into contact with the gas diffusion electrode whose activity has decreased. 2. A method for producing hydrogen peroxide by reducing oxygen in an alkaline aqueous solution at the cathode, which comprises reducing the oxygen and reducing the activity of the cathode as a gas diffusion electrode containing carbon as a main component. Alternately repeated activation, and the activation is carried out by removing the electrolyte solution in the cathode chamber, introducing a gas into the cathode chamber and contact the gas to the gas diffusion electrode with reduced activity. Method for producing hydrogen peroxide. 3. A method for activating a gas diffusion electrode, which is used as a cathode in a method of reducing oxygen in an alkaline aqueous solution to produce hydrogen peroxide and has a reduced activity as a main component, the method comprising: A method for activating a cathode for hydrogen peroxide production, characterized in that the gas diffusion electrode with reduced activity taken out from is heated at a temperature of 40 ° C. or higher.