JPS6125789B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an insoluble anode used for various electrolysis. Namely, production of chlorine, hypochlorite, chlorate, perchlorate in alkali chloride electrolysis, production of persulfate, electrolytic oxidation of organic compounds, electrowinning, seawater desalination, seawater electrolysis, water electrolysis, etc. This invention relates to an insoluble anode that can be used as an anode. Graphite has traditionally been used as an anode in the alkali chloride electrolysis industry, but it suffers from severe electrolytic consumption and cannot provide sufficient corrosion resistance. In recent years, electrodes (DSA) in which the surface of a valve metal substrate such as titanium is coated with a platinum group metal oxide such as ruthenium oxide have been used, but this is economically difficult as it requires a ruthenium layer with a certain thickness or more. There is. Therefore, at present,
It is desired that an inexpensive insoluble anode with excellent performance be developed to replace these. Additionally, when a thin film adhesion layer of platinum group metal oxide is generally formed on a valve metal substrate, pinholes are formed everywhere, and if exposed to an anode environment in this state, damage to the valve metal substrate will occur within a short period of time. A phenomenon of activation occurs and it cannot be used as an anode. In addition, loss of platinum group metal oxides due to exfoliation, abrasion, and dissolution also promotes passivation of the anode. In order to reuse such a passivated electrode, the passivation film must be removed and then the platinum group metal oxide layer must be recoated. The present invention is intended to solve the above problems, and consists of (a) valve metal base (b) MnOx (x is 1.5
(c) An insoluble anode consisting of a surface layer of a platinum group metal or its oxide. Metals that form a passive film, such as titanium, tantalum, zirconium, niobium, tungsten, molybdenum, and antimony, or their alloys are suitable for the valve metal base in (a), but they are suitable for economic efficiency, electromechanical properties, and processing. From the viewpoint of properties, titanium or its alloy is preferable. Various shapes can be adopted as the electrodes, such as a plate shape, a rod shape, an expanded shape, and a porous shape. The intermediate layer (b) is formed as follows. After the surface of the conductive metal substrate is degreased, it is etched by acid treatment, sand blasting, etc., and then a solution of a manganese compound is applied by brushing, spraying, dipping, or the like. Manganese compounds include manganese nitrate, manganese chloride, manganese sulfate,
manganese phosphate, manganese acetate, manganese formate,
Inorganic or organic manganese compounds such as manganese naphthenate and manganese n-butyrate are used. As a solvent, water, ethyl alcohol, methyl alcohol, propyl alcohol, butyl alcohol,
Benzene, toluene, methyl ether, ethyl ether, etc. are used, and alcohols are particularly preferred. After coating the above solution, it is sufficiently dried at a temperature of 200° C. or lower, preferably 150° C. or lower, to evaporate the solvent. Then in an oxidizing atmosphere such as air or oxygen
The interlayer is deposited by thermal decomposition of these compounds by heating to 170-600°C. By repeating this coating, drying, and thermal decomposition, an intermediate layer with a thickness of several tens of microns is formed. Below the above temperature, oxides are not sufficiently produced, and above 600°C, oxides decompose. The preferred weight of the intermediate layer thus obtained is approximately 5 to 50 g/m ² . The crystal structure of the manganese oxide produced in this case was determined by X-ray analysis.
It is a mixed oxide of Mn ₂ O ₃ and β-MnO ₂ ,
It is expressed by the general formula of MnOx (x is 1.5 or more and less than 2.0), and as a result of analysis, α-Mn ₂ O ₃ is 90% of the total oxide.
% or more. In most cases, x in the above general formula is about 1.5 to 1.6. At the same time, since this manganese oxide is a non-stoichiometric compound, it has many lattice defects, has good conductivity, and also has good adhesion to the substrate. The intermediate layer having many lattice defects can only be formed by thermally decomposing the manganese compound at the baking temperature as described above, and it is not possible to form it by thermally decomposing the manganese compound at the baking temperature. Almost manganese dioxide MnO ₂
only is formed and the purpose cannot be achieved.
However, when this electrode is used as it is as an electrode for chlorine generation, the chlorine overvoltage is high, especially in the case of an electrode with only a manganese oxide layer formed.
Chlorine overvoltage 500~ in the current density range of 100A/ ^dm2
It was found that the voltage was as high as 1000mV. The surface layer (c) is provided to increase catalytic activity and is made of a thin layer of platinum group metal or its oxide. i.e. platinum, ruthenium, rhodium, iridium, palladium,
A coating liquid consisting of an aqueous solution or a non-aqueous solution of one or more metal salts selected from osmium is applied onto the intermediate layer. Preferred among the platinum group metals are platinum, ruthenium, and iridium, and water or alcohols are preferred as the solvent. After coating, dry thoroughly at 200°C or lower, preferably 150°C or lower, to evaporate the solvent, and then dry at 300°C or lower, preferably at 150°C or lower.
The surface layer is coated by heating and baking at ~700°C. The heating and calcination temperature should be less than 300°C, without forming an oxide layer of these metals, and if it exceeds 700°C, separation from the intermediate layer will easily occur and the electrode catalytic ability will be impaired. By repeating this operation, a surface layer of platinum group metal or its oxide with a thickness of several micrometers can be obtained. The surface layer thus obtained has excellent adhesion to the intermediate layer, and no peeling was observed as a result of a peel test. Also, the thickness of the surface layer is 1/ of the thickness of the intermediate layer.
10 is sufficient, and even if it is thicker than this, the effect will not change. The chlorine overvoltage of the thus obtained multilayer electrode was 20 to 100 mV in the current density range of 1 to 100 A/dm ² , and the electrocatalytic activity due to the surface layer was remarkable. It is also believed that the presence of this intermediate layer increases the roughness factor of the surface layer and promotes catalytic performance. Furthermore, since the electrode of the present invention has the multilayer structure as described above, it has the following advantages over an electrode in which a platinum group metal oxide layer is directly deposited on a valve metal base. In other words, even if the electrode is exposed to the anode environment and the surface layer falls off due to detachment, abrasion, dissolution, etc.
The exposed intermediate layer has good corrosion resistance against the anode environment and good adhesion to the substrate, so no peeling occurs. Therefore, in this case as well, the potential of the intermediate layer itself is lower than that of the valve metal substrate. Therefore, if the anode potential shows such an anode potential, it is sufficient to stop the electrolytic operation and recoat the surface layer.Also, even if it is too late to replace the electrode, the voltage will not rise suddenly, so the electrode maintainability can be significantly improved. Furthermore, the fact that the platinum group metal oxide layer forming the surface layer can be an extremely thin layer makes it possible to minimize the economic disadvantage caused by the relatively high cost of these metals. Note that all percentages in the examples shown below indicate weight percentages. Example 1 Four commercially available titanium plates (1 x 10 x 0.1 cm) were degreased with acetone, etched in a mixed aqueous solution of 10% H ₂ O ₂ and 5% HF for 30 minutes, and then manganese nitrate was applied to the surface. Apply 0.5mol/aqueous solution,
Dry at 95°C for 20 minutes. Thereafter, heat treatment was performed for 10 minutes in an electric furnace at a temperature of 200°C. Repeat this operation for each titanium plate, depending on the thickness of the coating film: 10μ, 20μ,
Manganese oxide layers were repeatedly formed until the thickness became 30μ and 40μ. Then apply hydrochloric acid (20%) to these surfaces
Apply an ethyl alcohol solution containing 0.1 mol of acidic ruthenium chloride, dry at 95℃, and heat to 450℃.
Heat treatment was performed in an electric furnace at ℃ for 10 minutes. This operation was repeated to form a surface layer with a thickness of 1 μm. Each electrode thus produced was anodically polarized in saturated saline (NaC 310g/, PH=1.0) at 80°C, and the anode potential was measured. Ohmic loss between the Luggin capillary and the electrode was removed from the waveform appearing on the cathode ray tube oscilloscope using a current interrupter. All of these electrodes exhibited low anodic potentials with low chlorine overpotentials, as shown in Table 1 (curve A in Figure 1), regardless of the thickness of the manganese oxide intermediate layer.

[Table] Example 2 Three tantalum plates (1 x 10 x 0.1 cm) were prepared in Example 1.
An intermediate layer of manganese oxide with a thickness of 20 μm was formed by the same treatment as above. Hydrochloric acid (10%) acidic (1) chloroplatinic acid 0.1 mol/aqueous solution was applied to these surfaces, respectively.
(2) Rhodium chloride 0.1 mol/aqueous solution (3) A mixed aqueous solution of ruthenium chloride 0.05 mol/ and iridium chloride 0.05 mol/ is applied and dried at 95℃ for 20 minutes, then the temperature
Heat treatment was performed for 10 minutes in an electric furnace at 500°C. Coating, drying, and heat treatment were repeated until the surface oxide layer had a thickness of 2 μm. These anode potentials were determined in Example 1.
Measurements were made at 80°C in saturated saline solution (PH=1.0) in the same manner as in Example 1. (2) and (3) were measured at the same anode potential as in Example 1, and (1) was measured at 80°C in saturated saline (PH = 1.0). B)
The anode potential is expressed as .

[Table] Example 3 Comparative Example After sandblasting the same titanium plate as in Example 1, an intermediate layer of manganese oxide having a thickness of 20 μm was formed in the same manner. A ruthenium oxide surface layer having a thickness of 1 μm was formed on this surface in the same manner as in Example 1. On the other hand, the intermediate layer was omitted from the same titanium plate, and a 1 μm thick ruthenium oxide layer was formed directly on the surface. HeO ₄ using these two types of electrodes as anodes
When a mixed solution of 2 mol/+1 mol of NaC was electrolyzed at a current density of 100 A/ ^dm2 , the former electrode could be used for about 26 hours, but the surface layer of the latter electrode was consumed in about 11 hours. . As described above, the anode of the present invention exhibited corrosion resistance more than twice that of a titanium electrode on which a ruthenium oxide layer was directly deposited.

[Brief explanation of the drawing]

FIG. 1 is a current-potential curve diagram showing the anodic polarization characteristics of the electrodes of the present invention in Examples 1 and 2.

Claims (1)

[Claims] 1 (a) Valve metal substrate (b) Intermediate layer of manganese oxide containing a non-stoichiometric compound represented by MnOx (x is 1.5 or more and less than 2.0) (c) Surface of platinum group metal or its oxide layer, consisting of an insoluble anode.