JPH0238669B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to an anode that can be used in various types of alkali chloride electrolysis, particularly an anode for electrolysis that is preferable as an anode for an electrolytic cell for producing chlorine-alkali, an electrolytic cell for producing alkali chlorate, and a seawater electrolytic cell. (Prior Art) As ion-exchange membrane type salt electrolyzers have come into practical use in recent years, there has been a desire for an electrode that exhibits low chlorine overvoltage and high oxygen overvoltage without deteriorating the performance of cation exchange membranes. Japanese Patent Publication No. 46-21884 describes an electrode in which a metal valve base is coated with a mixed crystal material of ruthenium oxide and titanium oxide, which is widely used in industrial-scale electrolytic cells and has a sufficiently satisfactory lifespan. I was wearing a kimono. However, there was a problem that the amount of oxygen generated was relatively large and the efficiency of generating chlorine gas was low. This oxygen generation mechanism is a reaction as shown in the following equation. H ₂ O→2H ⁺ +1/2O ₂ +2e The anolyte becomes acidic due to hydrogen ions. In addition, the cation exchange membrane that has a carboxylic acid group as a functional group becomes R-COOH due to this hydrogen ion, becomes an insulator due to weak dissociation, and increases membrane resistance.
There is a problem in that the cation exchange membrane is damaged and its performance deteriorates, and this tendency is particularly severe when the pH of the anolyte is 2 or less. This is because the mixed crystal material contains more than 30 mol% of ruthenium oxide, and 20
If the amount is reduced to about mol %, the amount of oxygen generated can be slightly reduced, but this has the drawback of increasing the chlorine overvoltage. In addition, known platinum-iridium oxide mixture coated electrodes and platinum-iridium alloy coated electrodes have low chlorine overvoltage and high oxygen overvoltage.
It has good chlorine generation efficiency and is very advantageous for ion-exchange membrane type salt electrolyzers, electrolyzers for alkali chlorate production, and seawater electrolyzers, but it has the problem that the coating tends to peel off during use. , a more durable electrode was desired. In order to solve these problems, the applicant has developed an MnOX layer on the platinum-iridium oxide mixture layer.
The second coating layer is a mixture of 2 to 50% by weight of manganese oxide containing a non-stoichiometric compound represented by (X is 1.5 or more and less than 2.0) and 50 to 98% by weight of titanium oxide having a rutile structure. By providing this, the platinum-iridium oxide catalytic active layer peels off, dissolves,
It has been found that wear can be significantly reduced (Japanese Patent Application Laid-open No. 136790/1983). However, further improvement in durability is desirable. In particular, the anode used in recent ion-exchange membrane salt electrolyzers has a current density of about 30A/ ^dm2 .
It has a chlorine overvoltage of 100 mv, and there is room to further reduce it. Especially in recent years, when electricity costs are high, energy-saving electrolytic cells are desired, and it is important to develop anodes that have low chlorine overvoltage and generate less oxygen. (Structure of the Invention) The present invention is intended to solve the above problems. That is, the present invention provides (a) platinum on the valve metal base.
a first coating layer consisting of a mixture of 20 to 80 mol% of iridium oxide and 20 to 80 mol% of iridium oxide having a rutile structure; (b) 3 to 80 mol% of iridium oxide having a rutile structure;
a second coating layer consisting of a mixture of 15 mol%, 5 to 25 mol% of ruthenium oxide, and 60 to 92 mol% of tin oxide;
This is an anode for alkali chloride electrolysis, characterized in that a plurality of the above unit layers (a) and (b) are provided. The coating layer of platinum-iridium oxide mixture has a low chlorine overvoltage, about 40 m at a current density of 30 A/ ^dm2.
It has a chlorine overpotential of only v and is a very catalytically active layer for chlorine generation. However, it has the disadvantage that it is likely to peel off or dissolve due to gas generated during electrolysis. The present inventors have discovered that by providing a second coating layer of an iridium oxide-ruthenium oxide-tin oxide mixture on this platinum-iridium oxide coating layer, the platinum-iridium oxide
It has been found that the defects such as peeling, dissolution, and abrasion of the iridium oxide layer can be significantly improved, and the chlorine overvoltage does not increase and the oxygen overvoltage does not decrease. The second coating layer serves as a protective layer for the first coating layer, which is catalytically active for chlorine generation, and this layer itself has a certain degree of catalytic ability for chlorine generation. For example, the chlorine overvoltage of an electrode having a second coating layer made of a mixture of 10 mol % iridium oxide, 15 mol % ruthenium oxide, and 75 mol % tin oxide was measured by current interpolation at 80°C in saturated saline (NaCl 310 g/, PH 1.0). The results measured by the Prater method are shown in Table 1. In addition, the chlorine overvoltage of the electrode having the first coating layer made of a mixture of 70 mol% of catalytically active platinum and 30 mol% of iridium oxide was measured under the same conditions.
It is also shown in the table.

[Table] As is clear from Table 1, the first coating layer and the second coating layer
The chlorine overvoltage of the coating layer has a difference of 100 mV at a current density of 30 A/dm ² . Regarding oxygen overvoltage, electrodes having the same first or second coating layer were measured in a 2N-H ₂ SO ₄ aqueous solution at 80° C. using a current interrupter method, and the results are shown in Table 2.

[Table] From this table, it can be seen that the oxygen overvoltage between the first coating layer and the second coating layer differs by only 10 mV at a current density of 30 A/dm ² , and the second coating layer is lower. The present invention is an anode comprising a plurality of unit layers including a contact active layer as a first coating layer and a catalyst protection layer as a second coating layer, and the chlorine overvoltage thereof exhibits a low value as shown by the catalyst active layer. The oxygen overvoltage does not reduce the high value exhibited by the catalytic active layer, and provides a durable anode that minimizes peeling and dissolution of the platinum-iridium oxide layer. In the anode of the present invention, even if the performance of the catalytic active layer deteriorates, the voltage does not rise suddenly, and the potential is at most the same as that of the catalytic protective layer.When this happens, electrolysis can be stopped and the anode can be recoated. One of the features is that the electrolytic cell can be managed very easily. The first coating layer of the present invention has a composition of 20 to 80 mol% platinum and 20 to 80 mol% iridium oxide. If the iridium oxide content is less than 20 mol %, stability of the electrode potential cannot be obtained and the chlorine overvoltage tends to fluctuate over time. Moreover, when it exceeds 80 mol%, the chlorine generation efficiency tends to decrease. The second coating layer of the present invention includes 3 to 15 mol% of iridium oxide, 5 to 25 mol% of ruthenium oxide, and tin oxide.
The composition is 60-92 mol%. When iridium oxide and ruthenium oxide are less than the above range, the chlorine overvoltage tends to increase, and conversely when it is greater than the above range, the oxygen overvoltage decreases, so both are unfavorable. It has been found that iridium oxide and ruthenium oxide have a low oxygen overvoltage when used alone, but when they are mixed as in the present invention, they exhibit a high oxygen overvoltage. The reason for this is not clear, but it is thought that when iridium oxide and ruthenium oxide are mixed, the crystal structure is distorted, resulting in a change in electrochemical properties. Examples of the metal used in the valve metal base of the present invention include metals that form passive films, such as titanium, tantalum, zirconium, niobium, tungsten, and molybdenum, and/or alloys thereof.
Usually, titanium and/or its alloys are preferably used from the viewpoint of economy, electromechanical properties, workability, etc. Electrodes can be plate-shaped, rod-shaped, expanded,
Various shapes such as porous are possible. The coating layer of the present invention is formed as follows. The first coating layer (a) is obtained by degreasing the surface of the valve metal base, roughening the surface by etching it with acid treatment, blasting, etc., and then brushing the surface with a mixed solution of a platinum compound and an iridium compound. , by roll coating, spraying, dipping, or other means. The platinum compounds include chloroplatinic acid, ammonium chloroplatinate, potassium chloroplatinate, dinitrodiaminoplatinic acid, etc., and the iridium compounds include iridium trichloride, chloroiridic acid, ammonium chloroiridate, sodium chloroiridate, etc. Examples of the catalyst for the mixed solution include water, ethyl alcohol, methyl alcohol, propyl alcohol, butyl alcohol, lavender oil, anise oil, linaloe oil, turpentine oil, toluene, methyl ether, and ethyl ether. After applying this, 150 ~
It is dried at 200°C for several minutes, and then heat-treated at 300-800°C for 10-20 minutes in an electric furnace in an air or oxygen atmosphere. If the heat treatment temperature is less than 300°C, thermal decomposition will not occur completely, and if it exceeds 800°C, oxidation of the metal substrate will progress and the substrate will be damaged. The second coating layer (b) is formed by coating stannous chloride, stannous sulfate, stannous acetate, 2-
It is formed by applying a mixed solution of a tin compound such as ethylhexanetin, a ruthenium compound such as ruthenium chloride, ruthenium chloride, and an iridium compound such as iridium chloride or chloroiridic acid, followed by drying and firing. As the solvent, water, ethyl alcohol, methyl alcohol, propyl alcohol, butyl alcohol, methyl ether, ethyl ether, etc. are used. Among these, alcohols are preferred. After coating, dry at 150-200°C for several minutes to evaporate the solvent, then heat in an electric furnace in air or oxygen atmosphere.
These compounds are thermally decomposed by heat treatment at 400 to 600°C. Platinum forming the first coating layer produced in this way.
At the above firing temperature, the tin oxide, ruthenium oxide, and iridium oxide that make up the iridium oxide mixture and the second coating layer contain many crystals that have a common rutile structure and have similar unit cell volumes, so the close contact between these dissimilar metals is reduced. It is thought that this increases the adhesive strength between the two layers. In the present invention, it is necessary to alternately deposit the layers (a) and (b) a number of times, and the use of such an anode brings out its effects significantly. For normal purposes of use, it is desirable to apply each of the coating layers (a) and (b) three or more times, and this operation can be done by appropriately adjusting the amount of solvent. The total amount of platinum-iridium oxide in layer (a) is 7 g/
m ² is sufficient, and the use of expensive platinum group metals can be saved. Further, the total amount of titanium oxide, ruthenium oxide, and iridium oxide in layer (b) must be 4 g/m ² or more, and if it is less than that, the ability as a catalyst protective layer is insufficient. (Effects of the Invention) In this way, the platinum-iridium oxide coating layer maintains the low chlorine overvoltage and high oxygen overvoltage, suppresses the dissolution and peeling phenomenon of expensive platinum group metals in electrolytes, and achieves durability. It is possible to obtain an anode for alkali chloride electrolysis. The present invention will be explained in more detail with reference to Examples below. Composition percentages in the examples are on a molar basis unless otherwise specified. Example 1 Comparative Example 1 A commercially available titanium plate (1 x 10 x 0.1 cm) was degreased with acetone, etched in a 10% by weight hot oxalic acid solution, and a solution having the following composition was applied to its surface. H ₂ PtCl ₆・6H ₂ O 1.00g H ₂ IrO ₆・6H ₂ O 0.43〃 Turpentine oil 2.0ml Lavender oil 10.0〃 Ethyl alcohol 20.0〃 Concentrated hydrochloric acid 0.5〃 Dry this at 120℃ for 20 minutes, then dry at 500℃ Pt70% by firing in an electric furnace for 10 minutes
A first coating layer of 30% IrO ₂ was formed. A solution having the following composition was applied to this surface. SnCl ₂ 3.70g RuCl ₃・3H ₂ O 1.00〃 H ₂ IrCl ₆・6H ₂ O 1.30〃 Concentrated hydrochloric acid 1.0ml n-butyl alcohol 30.0〃 Dry this at 120℃ for 20 minutes, then in an electric furnace at 480℃ 75% _SnO2 by baking for 10 minutes
A second coating layer of 15% RuO ₂ and 10% IrO ₂ was formed. By repeating the deposition operation of the first and second coating layers alternately four times, the total amount of the first coating layer is 0.46mg/
cm ² , and the second coating layer had a total amount of 0.50 mg/cm ² . Similarly, the total amount of only the above first coating layer is 0.46 mg/cm ²
Using the coated electrode as a reference electrode, each chlorine overvoltage was measured in saturated saline (NaCl310g/, PH1.0) at 80°C by the current interrupter method. As a result, both electrodes showed a chlorine overvoltage of 30 mV at a current density of 30 A/dm ² . Next, we measured the oxygen overvoltage at 80°C in 1NH ₂ SO ₄ and found that the electrode of the present invention at a current density of 30 A/dm ²
The voltage was 410 mV, and the voltage of the reference electrode was 410 mV, and there was no difference in oxygen overvoltage. Furthermore, both electrodes were heated with HClO ₄ (2 mol/) and NaCl.
(1 mol/) in a mixed solution at 50° C. and a current density of 200 A/dm ² , and the time until the electrode surface became passivated and the potential suddenly increased was measured. As a result, the potential of the electrode of the present invention increased after 310 hours, and the potential of the comparison electrode increased after 56 hours.
It was found that there was a large difference in durability, showing more than double the durability. Example 2 Comparative Example 2 The first coating layer (coated 4 times) was the same as in Example 1, except that the composition ratio of the second coating layer (0.50 mg/cm ² coated 4 times) was changed as shown in Table 5. An electrode was produced in the same manner as in Example 1. The chlorine overvoltage of this electrode was set to 80% in saturated saline (NaCl300g/, PH2.0).
The chlorine overvoltage was measured in the same manner as in Example 1 at a temperature of 30 A/dm ² at a current density of 30 A/dm 2 . Furthermore, the amount of oxygen contained in the generated chlorine was analyzed by gas chromatography, and the results shown in Table 3 were obtained.

[Table] Example 3 Expanded titanium (10 x 10 x 0.12 cm, opening
8.0lw×3.6sw×1.2st, lw is long axis mm, sw is short axis mm,
st is the cutting width (mm) and Pt70 in the same manner as in Example 1.
% and a first coating layer of 30% IrO ₂ and 75% SnO ₂
The second coating layer of 10% IrO ₂ and 15% RuO ₂ was coated alternately six times, and the first coating layer had a total concentration of 0.69 mg/cm ² , and the second coating layer had a total concentration of 0.69 mg/cm 2 .
The total amount of the coating layer was 0.80 mg/cm ² . This electrode is connected to a cation exchange membrane (product name: Nafion).
901, manufactured by DuPont) as the anode of an electrolytic cell equipped with
Electrolysis of saline water was carried out using a nickel expanded metal with approximately the same area as the cathode and an interelectrode distance of 3 mm. The composition of the anolyte is NaCl 280g/, pH 3.0, and the composition of the catholyte is 32% by weight NaOH aqueous solution. The chlorine overvoltage was measured at an electrolysis temperature of 80° C. and a current density of 30 A/dm ² and was found to be 30 mV, which remained approximately constant during the 600 days of operation. Also, during this period, the oxygen gas concentration in chlorine gas is approximately
It remained approximately constant during operation at 0.5% by volume. Example 4 Comparative Example 3 A first coating layer of 40% Pt and 60% IrO ₂ was applied to a commercially available titanium plate (30 x 25 x 0.3 cm) in the same manner as in Example 1.
A second coating layer of 75% SnO ₂ , 15% IrO ₂ and 10% RuO ₂ was coated alternately 6 times each to give a total of 0.69% of the first coating layer.
mg/cm ² , and the total amount of the second coating layer was 0.80 mg/cm ² . Seawater electrolysis was performed using this electrode as an anode and a stainless steel plate (made of SUS304) of approximately the same area as a cathode with a distance between the electrodes of 3 mm. At a current density of 15A/ ^dm2 , the cell voltage is
The voltage was 3.90v, and no increase in cell voltage was observed during the 300 days of operation. Also, the efficiency of chlorine generation is 85%
It was hot. For comparison, a test was conducted in the same manner as in Example 4, except that only the first coating layer was coated six times to give a total of 0.69 mg/cm ² . Initially, the tank voltage was approximately constant at 3.90V, and the chlorine generation efficiency was 85%, but after the start of operation,
The cell voltage gradually increased from day 93 and reached 4.25v after 150 days. In addition, the chlorine generation efficiency had decreased to 76%.

Claims (1)

[Scope of Claims] 1. On a valve metal base, (a) a first coating layer consisting of a mixture of 20 to 80 mol% of platinum and 20 to 80 mol% of iridium oxide having a rutile structure; (b) having a rutile structure; , iridium oxide 3-15
a second coating layer made of a mixture of 5 to 25 mol% of ruthenium oxide and 60 to 92 mol% of tin oxide; anode.