JP2722263B2 - Electrode for electrolysis and method for producing the same - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an electrode for electrolysis and a method for producing the same, and more particularly to an insoluble electrode for electrolysis having excellent anodic oxidation performance used for electrolysis accompanied by an anodic oxidation reaction, and production thereof. About the method.

(Prior art and its problems) As an insoluble electrode used for an electrolytic reaction, a carbon electrode excellent in corrosion resistance and stability has been used for a long time. However, when the carbon electrode is used for an oxygen generation reaction, there is a problem that generated oxygen is combined with carbon as an electrode material and gradually consumed as carbon dioxide. In order to solve this problem, it has been proposed to use metal titanium which is extremely stable in a positively polarized atmosphere and does not cause any consumption, and the surface of which is plated with platinum or the like to form a substantially extremely stable platinum plating layer. It has been used as an electrode for electrolysis. Although the electrode has potential fluctuations at the beginning of electrolysis, it has stability, long life,
It has been widely used from the viewpoint of easy handling, and is still widely used for various applications even today.

On the other hand, in 1965, a so-called dimensionally stable electrode was proposed, in which a coating mainly composed of titanium metal as a base material and a platinum group metal oxide on its surface was formed, and the electrode was mainly composed of a conductive oxide. Due to its extremely low overvoltage and stability, it has been widely used in the field of the electrolytic industry including the soda electrolytic industry, and is now widely used in industrial plating, organic electrolysis and the like.

Lead and lead alloy electrodes are known as inexpensive materials, and are widely used in the field of industrial electrolysis, particularly industrial plating. An electrode mainly composed of lead dioxide has been developed as a more stable material which has solved the disadvantages of the lead electrode, which is somewhat unstable, and the lead dioxide electrode is extremely stable electrolytically and hardly consumed. It has extremely excellent performance for water and water treatment.

Each of the above-mentioned insoluble electrodes for electrolysis is selected and used in consideration of the performance of the electrode according to each application in the field of industrial electrolysis.

For example, the platinum group metal oxide electrode can be said to be an epoch-making electrode having almost perfect stability and catalytic ability in soda electrolysis and extremely small overvoltage for gas generation. This electrode is anodized, for example, trivalent chromium. For anodizing to hexavalent chromic acid and as an electrode for other anodic oxidation processes, its catalytic activity is low, and in aqueous solution electrolysis, oxygen generation by water electrolysis reaction is the main reaction. It is not enough as an electrode. In addition, the carbon electrode itself has insufficient function as an anodic oxidation catalyst.

A titanium electrode with a platinum coating has a high overvoltage as a water electrolysis reaction and can be maintained at a desirable high potential for anodic oxidation reaction, but has poor selectivity of anodic oxidation reaction, for example, in an industrial chrome plating bath as compared with a lead anode. Although the electrolytic potential for the same current density is kept higher, there is a problem that the ability to oxidize trivalent chromium to hexavalent chromic acid is extremely small.

However, while the lead electrode has excellent anodic oxidation performance, the consumption when used as an anode is several mg / AH, and various lead alloys have been studied for stabilization. It consumes about 10 ³ to 10 ⁵ times more than the oxide anode, and has the disadvantage that the eluted lead stays as sludge in the electrolytic solution or, in the case of plating, contaminates the plating layer itself. .

A lead dioxide electrode in which a substrate is coated with lead dioxide has been put into practical use as a lead-based electrode which retains the characteristics of the lead electrode and is less consumed. Although this lead dioxide electrode depends on the manufacturing conditions, etc., the consumption during use is about one-thousandth or less than that of lead and lead alloy electrodes, it has excellent corrosion resistance, and extremely excellent performance as long as it is operated continuously. Show. However, when the lead dioxide electrode is left in an electrolytic bath in which the current is stopped, its immersion potential is about 1.6 Vv
s.NHE has the disadvantage of eluting as PbO ₂ → Pb ⁺⁺ , has the disadvantage that it must always be positively polarized and maintained at about 1.8 V vs. NHE or more, and is surprisingly stable in continuous processes On the other hand, when turning off and on in a short time used for gravure roll chrome plating or the like, there is a disadvantage that the life is greatly shortened.

(Object of the Invention) The present invention solves the drawbacks of the lead dioxide electrode, that is, the drawback that it is easily eluted when the power is turned off, and provides sufficient oxidation catalyst activity and long-term durability and handling that can be used for an anodic oxidation process. An object of the present invention is to provide an insoluble electrode having ease and a method for manufacturing the same.

(Means for Solving the Problems) The present invention firstly provides a valve metal or valve metal alloy substrate with
An electrolytic electrode comprising a lead dioxide coating layer formed thereon, and a porous platinum protective layer coated on the lead dioxide coating layer;
An electrode for electrolysis comprising a plurality of lead dioxide coating layers and a plurality of porous platinum protective layers alternately formed on the surface of a valve metal or a valve metal alloy substrate. A method for producing an electrode for electrolysis, comprising forming a lead dioxide coating layer by electrodeposition, and further forming a porous platinum protective layer on the surface of the coating layer.

 Hereinafter, the present invention will be described in detail.

The present invention protects the lead dioxide electrode with a platinum protective layer while taking advantage of the feature that the lead dioxide electrode has a high catalytic activity with respect to the anodic oxidation reaction. Is eluted, which leads to a drastic shortening of the service life.

As the base of the electrode of the present invention, a so-called valve metal or an alloy mainly containing a valve metal is used. Among the valve metals or alloys thereof, titanium and titanium alloys are particularly desirable in terms of ease of handling and corrosion resistance, and other valve metals such as niobium and tantalum or alloys thereof are used depending on the application. be able to. The shape of the substrate is appropriately selected from a plate-like expanded mesh, a perforated plate, a blind, and the like in consideration of the application and the structure of the electrolytic cell.
It is a three-dimensional shape having a large surface area of an expanded mesh, a perforated plate or a blind so as to hold a lead dioxide coating layer having a thickness of about m to 1 mm.

After directly or pre-treating this electrode substrate surface,
Form a lead dioxide coating layer.

As a pretreatment method of the substrate, the surface cleaning is performed by enlarging the surface area by plast treatment, activating the surface by pickling, and performing negative polarization in an electrolytic solution such as a sulfuric acid aqueous solution to generate hydrogen gas from the substrate surface. There is a method of activating by hydride partially generated by hydrogen gas.

Further, before coating the substrate surface with an electrode material such as lead dioxide, an underlayer is formed on the substrate surface to improve the adhesion between the lead dioxide coating layer and the substrate or to prevent passivation. It may be. As the base layer, a semiconductor oxide such as a titanium-tantalum composite oxide, a conductive oxide such as tin oxide, a platinum metal simple and ruthenium oxide, a platinum group metal such as iridium oxide or an oxide thereof or platinum or the like. The added semiconductive oxide, and α-type lead dioxide and the like are included. For example, in order to form the titanium-tantalum composite oxide layer containing platinum, a chlorinated chloroplatinic acid is added to a mixed hydrochloric acid aqueous solution of titanium chloride and tantalum chloride in the same amount in a molar ratio, and the pretreated substrate is added. After applying to the surface and drying, baking may be performed at 450 to 600 ° C. Platinum alone, iridium oxide, ruthenium oxide, and tin oxide can be obtained by the same thermal decomposition method or other conventional methods.

The underlying layer to be formed is desirably α-type lead dioxide. When a later-described β-type lead dioxide coating layer is formed on the α-type lead dioxide, the adhesion between the substrate, the underlying layer and the coating layer is further improved. To form the base layer of the α-type lead dioxide on the substrate, for example, a current density of 0.1 to 5 A / dm ² at a temperature of 30 to 50 ° C. in a solution of lead oxide (PbO) dissolved in an aqueous solution of caustic soda. The electrodeposition may be performed by the above method, whereby a coating of α-type lead dioxide having a thickness of about 1 to 200 μm can be formed.

Next, a lead dioxide coating layer is formed on the base or underlayer. There are two types of lead dioxide, α-type lead dioxide and β-type lead dioxide, and β-type lead dioxide is superior in terms of corrosion resistance and stability during electrolysis.

In order to coat the lead dioxide powder on the substrate or the underlayer formed on the substrate, it is preferable to use an electrodeposition method.

The formation of the lead dioxide coating layer by the electrodeposition method may be performed according to a conventional method.For example, in a lead nitrate aqueous solution having a concentration of 200 g / to saturation, it is anodic polarized at a temperature of 40 to 80 ° C. and a current density of 0.2 to 10 A / dm ^2. Can be obtained. The thickness of the coating may be determined according to the application, and is desirably about 10 to 500 μm. If the desired thickness is not obtained in a single operation,
Electrodeposition may be repeated several times.

Next, a porous platinum protective layer is formed on the surface of the lead dioxide coating layer. The method for forming the platinum protective layer is not particularly limited, but it can be formed by an electrochemical method such as electroless plating or electroplating or a thermal decomposition method. It is known that when a platinum layer having a thickness of 1 μm or less is formed by an electrochemical method, the platinum layer becomes a porous layer. In order to form a platinum protective layer by electroless plating, for example, a solution containing a reducing agent and a platinum salt can be applied to the surface of the lead dioxide coating layer, and the platinum salt can be reduced on the surface to form a platinum layer. . However, according to this method, it is necessary to be careful in handling since the lead dioxide of the lead dioxide coating layer may be reduced and converted into a lower-order lead oxide depending on the reducing agent used. As the electroplating, there are a cyan method and other methods. Typically, platinum cyanide 5 to 15 g /, potassium cyanide 5 to 15
g / and 10 to 30 g / potassium dihydrogen phosphate are used as an electrolytic solution, and plating is performed while stirring the bath at a current density of about 0.2 to 10 A / dm ² at room temperature to 60 ° C. Usually, in this type of plating, the current efficiency reaches 70 to 800%, but for the purpose of the present invention to produce a porous platinum protective layer, the current density is slightly higher, for example, about 1 to 5 A / dm ^2. In some cases, it is preferable to suppress the current efficiency to about 50% by slightly lowering the platinum content in the electrodeposition solution.

On the other hand, the porous platinum protective layer can be formed by a thermal decomposition method.

The formation of a platinum coating by a thermal decomposition method is a conventional method, but is characterized in that platinum has good adhesion to a substrate and a thin uniform coating can be formed as compared with the electrodeposition method. The thermal decomposition conditions are not particularly limited as long as the lead dioxide of the lead dioxide coating layer formed on the substrate does not decompose, and the temperature is 350 ° C. in order to ensure strong adhesion between the formed platinum protective layer and the lead dioxide coating layer. It is desirable to apply a temperature higher than about. The stable region of lead dioxide is below 290 ° C, and at temperatures higher than that, oxygen may be released and reduced to lower-order lead oxides such as PbO ₂ → PbO and Pb ₂ O _3. For this purpose, it is desirable to further perform a chemical oxidation treatment.
Typically, the substrate on which the lead dioxide coating layer is formed is immersed in an aqueous solution of chloroplatinic acid butanol and deionized water, or the liquid is applied to the substrate, and the temperature is 350 to 600 ° C., preferably depending on the raw materials. It is calcined at a temperature of 450 to 550 ° C and pyrolyzed to produce a coating. The coating is oxidized by immersing the coating in an aqueous solution of a relatively strong oxidizing agent such as sodium hypochlorite and sodium hydrogen phosphate at room temperature for 2 to 30 minutes. This treatment also keeps the platinum in a metallic state without being oxidized,
The lead oxide partially converted into PbO ₂ → PbO and Pb ₂ O ₃ by heating returns to the original lead dioxide.

When a platinum protective layer having a desired thickness cannot be obtained by one operation using the thermal decomposition method, it is preferable to repeat the thermal decomposition operation a plurality of times.

(Example) Next, an example of a method for producing an electrode for electrolysis according to the method of the present invention will be described. However, the example does not limit the present invention.

Example 1 A rolled expanded mesh made of titanium and having a thickness of 1.5 mm was used as a substrate. The surface of the substrate was roughened by grid blasting, followed by pickling with 80% 25% sulfuric acid for 6 hours. TiCl ₄ , TaCl ₅ , H ₂ PtCl containing ₄₀ , 10 and 50 mol% of titanium, tantalum and platinum respectively on the surface of this substrate
_An aqueous solution composed of ₆ and HCl was applied as a coating solution, and baked at 520 ° C. for 15 minutes in a muffle furnace with air flow. Repeat this operation four times and convert to platinum 0.5g / m ²
Forming a coating of titanium-tantalum oxide and platinum of titanium thickness;

This substrate was anodic polarized in a 25% aqueous sodium hydroxide solution saturated with lead oxide at 40 ° C. at a current density of 1 A / dm ^{2 for} 1 hour, whereby a black α-type lead dioxide coating was formed on the front surface of the mesh-like substrate. Formed.

Further, an aqueous solution of 400 g / lead nitrate was used as an electrolyte, a small amount of nitric acid was added to adjust the pH to ≤1, the electrolyte was maintained at 60 to 70 ° C., and anodic polarized at a current density of 4 A / dm ^{2 for} 2 hours. Electrodeposition. As a result, a black-gray β-type lead dioxide coating layer having a metallic luster was obtained.

The substrate was charged with a current density of 2 A / dm ² in a commercially available platinum plating solution.
When the electrodeposition was performed for 3 minutes, the surface became white. The current efficiency is about 30% from the weight change at this time, and about 1 μm.
This means that m of the platinum protective layer has been formed.

The surface was further coated with β-type lead dioxide under the conditions described above, and a platinum protective layer was further formed under the same conditions as described above.

The electrode prepared in this manner was used as an anode for chromium plating in a surge bath, and was continuously operated at a current density of 30 A / dm ² for 30 hours. As a result, trivalent chromium was maintained at 1.7 g / and lead dioxide coating was performed. It was found that there was no substantial difference from the electrodes. When this electrode and the lead dioxide-coated electrode were left in the chromium plating bath for 200 hours without passing an electric current, the lead dioxide electrode was yellowed due to the formation of lead chromate on the surface and the volume expanded, and the lead dioxide electrode was separated from the substrate. In contrast, peeling was hardly observed in the electrode of this example.

Example 2 The electrode prepared in Example 1 was heated at 300 ° C. for 1 hour in flowing air. This electrode was immersed in a chromium plating solution in a Sargent bath, but no peeling of the coating layer was observed even after 450 hours.

Example 3 A substrate was prepared and pretreated in the same manner as in Example 1,
Furthermore, after forming a coating of α-type lead dioxide and β-type lead dioxide,
In a muffle furnace in which a 10 g / chlorobutanoic acid aqueous solution of butanol in chloroplatinic acid was applied to the surface of the substrate in terms of platinum and air was passed through.
Heated at 450 ° C. for 10 minutes. After repeating this coating and heating operation three times, a β-type lead dioxide coating was further formed, and
A platinum protective layer was formed by thermal decomposition of platinum several times.

The electrode thus fabricated was evaluated under the conditions of chromium plating in a surge bath in the same manner as in Example 1. The concentration of trivalent chromium was 2.4 g / in chromium plating for 30 hours, far exceeding 8 g / in the case of platinum alone. There was no problem in practical use, and no problem occurred even when immersed in the plating solution for 300 hours without passing current.

Example 4 In order to convert PbO or Pb ₂ O ₃ supposed to be partially formed on the surface of the electrode prepared in Example 3 into lead dioxide, treatment was performed at 60 ° C. for 30 minutes in a 12% sodium hypochlorite solution. After washing with water, it was dried. When this electrode was evaluated under the conditions of chromium plating in a Sargent bath, the chromium concentration of trivalent was 1.7 g / in chromium plating for 30 hours, which was no different from that of a lead dioxide electrode, and was immersed in a plating solution for 300 hours. No problem arose.

(Effect of the Invention) The electrode for electrolysis according to the present invention is formed by forming a lead dioxide coating layer on the surface of a valve metal or valve metal alloy substrate, and further coating a porous platinum protective layer on the lead dioxide coating layer. It is an electrode for electrolysis, and the lead dioxide coating layer and the platinum protective layer may be formed by alternately forming a plurality of layers.

Conventional platinum group metal oxide electrodes with excellent stability lack catalytic activity to promote anodic oxidation, whereas conventional lead dioxide electrodes, which can promote anodic oxidation, cannot suppress their elution, especially when power is turned off, and are turned on and off. When used for electrolysis, plating, and the like in which repetition is repeated, it was not possible to avoid a drastic shortening of the life.

On the other hand, in the electrode for electrolysis according to the present invention, the problem that lead dioxide that promotes anodic oxidation elutes when electricity is not supplied is solved by coating the lead dioxide coating layer with a platinum protective layer. According to the present invention, the catalytic activity ability of accelerating the anodic oxidation and the elution resistance of platinum are combined, and according to the present invention, the elution of the lead dioxide is suppressed when the power supply is stopped while maintaining the anodic oxidation ability of the lead dioxide. It is possible to provide an electrode for electrolysis excellent in electrode activity and corrosion resistance.

Further, the electrode for electrolysis described above according to the method of the present invention is also an electrode having excellent electrode activity and corrosion resistance, and can effectively perform an anodic oxidation reaction such as chromium plating.

Further, when an underlayer is formed between the substrate and the lead dioxide coating layer, the adhesion between the substrate and the coating layer is improved, the peeling of the coating layer can be prevented, and an electrode having more excellent corrosion resistance can be provided.

Claims (7)

(57) [Claims] 1. An electrode for electrolysis comprising a lead dioxide coating layer formed on the surface of a valve metal or valve metal alloy substrate, and a porous platinum protective layer coated on the lead dioxide coating layer. 2. The electrode according to claim 1, wherein an underlayer is formed between the substrate and the lead dioxide coating layer. 3. An electrode for electrolysis comprising a plurality of lead dioxide coating layers and a plurality of porous platinum protective layers alternately formed on the surface of a valve metal or valve metal alloy substrate. 4. A method for producing an electrode for electrolysis, comprising: forming a lead dioxide coating layer on the surface of a valve metal or valve metal alloy substrate by electrodeposition; and forming a porous platinum protective layer on the surface of the coating layer. . 5. The method according to claim 4, wherein a porous platinum protective layer is formed on the surface of the lead dioxide coating layer by electrodeposition or electroless plating. 6. The method according to claim 4, wherein a platinum salt-containing solution is applied to the surface of the lead dioxide coating layer, and a platinum protective layer is formed by thermal decomposition. 7. The method according to claim 6, wherein a chemical oxidation treatment is performed after the thermal decomposition.