JPH03188291A - Electrolytic catalyst coating - Google Patents

Abstract

PURPOSE: To make it possible to lessen the generation of oxygen by using an electrode of an electrically conductive metallic base material having a coating of a specific compsn. enhanced in stability under an alkaline condition at the time of electrolyzing a halogen-contg. soln. at low pH.

CONSTITUTION: The coating is formed on the electrode consisting of the electrically conductive metallic base material in order to intensify the stability under the alkaline condition at the time of electrolyzing the halogen-contg. soln. at the low pH. This coating contains ≥15 to <25% iridium oxide, 35 to 50mol% ruthenium oxide and ≥35 to <45mol% titanium dioxide with the oxide existing in the coating as 100mol%. Then, the molar ratio of the titanium oxide of the coating to the total of the iridium oxide and ruthenium oxide is smaller than 1:1 and the molar ratio of the ruthenium oxide to the iridium oxide is larger than 1.5:1 and up to 3:1. The generation of the oxygen is lessened by using such coated electrode.

COPYRIGHT: (C)1991,JPO

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to an electrode that reduces the evolution of oxygen during electrolysis of halogen-containing solutions, particularly at low pH. The present invention relates to an electrode with enhanced alkali stability made of a conductive metal coated with an oxide, a composition for the coating, and a method for manufacturing the electrode.

BACKGROUND OF THE INVENTION Electrodes for electrolytic processes having a base metal or core metal carrying a metal oxide layer or coating are known. The core metal of the electrode is a valve metal such as titanium, tantalum, zirconium, niobium or tungsten. If the coating is a mixed oxide, the oxide of the core metal or base metal can contribute to the mixture. For example, U.S. Patent No.
As taught in No. 711,385, such mixtures can include oxides of the base metal plus oxides of one or more metals such as platinum, iridium, rhodium palladium, ruthenium, and osmium.

Such a mixture, which may be referred to as a noble metal mixed oxide, may be a mixture of ruthenium oxide and iridium oxide. This is taught generally in U.S. Pat. No. 3,632,498, and a specific example in combination with titanium oxide is taught in U.S. Pat. No. 3,948,751. Alkali metal halides, such as those used as coatings on electrodes for the electrolysis of aqueous sodium chloride solutions, are described in U.S. Pat. No. 4,005,004 when further mixed with both titanium oxide and zirconium oxide. teaches that such noble metal mixed oxides are particularly useful.

Such a mixture forms a solid solution coating that directly enhances the practical utility of the electrode for its intended purpose, as taught in that patent.

Furthermore, in recent years, the molar amount of titanium oxide has been increased to be greater than the total molar amount of iridium oxide and ruthenium oxide in order to improve the wear resistance of electrodes, especially when used in electrolysis that produces a combination of oxygen and chlorine. It was suggested that. This is disclosed in US Pat. No. 4,564,434, which also teaches that the molar amount of iridium oxide is greater than the molar amount of ruthenium oxide.

(Problems to be Solved by the Invention) Electrolysis of halogen-containing solutions, for example, chlorine by salt water electrolysis.
It would be desirable to provide an electrocatalytic coating that reduces oxygen evolution in alkali production. It would also be particularly desirable to provide a coating that retards weight loss upon exposure to caustic fluids. In addition to achieving a combination of these characteristics,
It would be most beneficial if this could be achieved without sacrificing other desirable features, such as sacrificing the chlorine evolution potential of the anode. It would also be advantageous to prepare the electrodes using coating compositions that are easy to prepare, have simple formulations, and are easy to use and easy to use.

(Means for Solving the Problems) The present invention is broadly directed to an electrode that reduces the generation of oxygen when a halogen-containing solution is electrolyzed, especially at low pH. It consists of an electrically conductive metal substrate having a coating with enhanced stability under conditions, the coating comprising at least 15 mol % and less than 25 mol % of iridium oxide, based on 100 mol % of the oxide present therein, iridium oxide, 3
5-50 mol% ruthenium oxide and more than 30 mol%4
Contains less than 5 mol% titanium oxide. Therefore, the molar ratio of titanium oxide to iridium oxide plus ruthenium oxide in this coating is less than 1:1, the molar ratio of ruthenium oxide to iridium oxide is greater than 1.5:1, and 3:l.
Must.

Another feature of the invention is a coating composition suitable for providing the mixed metal oxide coating as described above, and a still further feature of the invention is a method of manufacturing the electrode as described above.

This electrode is particularly useful as an anode in a membrane cell used for the electrolysis of salt water having a pH within the range of about 2 to about 4.

The composition of the present invention is widely applicable to any electrically conductive metal substrate that has sufficient electrical conductivity to function as an electrode during the electrolytic process. That is, although the metal of the base material is widely considered, from the viewpoint of applying electrocatalytic coating,
Base metals such as nickel or manganese are more typical, with valve metals exclusively including titanium, tantalum, aluminum, tungsten, zirconium and niobium. Of particular interest is titanium because of its toughness, corrosion resistance, and availability. Suitable metals for the substrate include commonly available elemental metals themselves as well as metal alloys and intermetallic mixtures. For example, titanium is commonly used with nickel, cobalt,
Alloyed with iron, manganese or copper. More specifically, grade 5 titanium contains up to 6.75% aluminum and 4.5% vanadium;
titanium contains up to 6% aluminum and 3% tin, grade 7 up to 0.25% palladium, grade 10 10-13% molybdenum + 4.5% by weight.
7.5% by weight of zirconium, and so on.

The coating composition applied to the coated metal substrate is aqueous and is almost always just water without any additional liquids mixed in. Preferably, deionized or distilled water is used to avoid inorganic impurities.

For reasons of economy in preparation and use, aqueous compositions that can be used are solutions of the precursor components in aqueous media, ie, aqueous solutions of the precursors of the oxides that will be present in the coating. Precursor components used in aqueous solutions are those that can be efficiently and economically dissolved in water, such as those that can be brought into solution without requiring extended boiling conditions. Furthermore, this precursor must be capable of giving the respective metal oxide upon thermal decomposition. If all components are present in the same composition, they must also be compatible. In this regard, these components should be non-reactive with each other, e.g. not reacting to form products that would introduce harmful non-oxide substituents into the coating or lead to precipitation from the coating solution. is advantageous. Typically, each precursor component is a metal salt, most often a halogen salt, and preferably all chloride salts for reasons of solution preparation economy and efficiency; however, other useful salts include: There are also iodides, bromides and ammonium chloride salts such as ammonium hexachloride or ruthenate.

Besides the preferred precursor components, in most cases, no additional solution components are present in the individual or combined solutions, with only one exception. The exception to this is the substantial presence of inorganic acids. For example, a solution of iridium trichloride can further contain a strong acid, which is almost always hydrochloric acid, and is usually present in an amount to provide about 5 to 20 weight percent acid. The typical pH of the individual or combined solutions is less than 1, such as within the range of about 0.2 to 0.8.

When the coating composition is a solution of all precursor components, the composition contains at least 15 mole percent and less than 25 mole percent of the iridium component, based on 100 mole percent of all components.
It contains 50 mole percent ruthenium component and 30 mole percent or more and less than 45 mole percent titanium component. Compositions containing less than 15 mole percent iridium components are unsuitable for providing the best caustic stability electrodes, for example, when the electrodes are used in chlor-alkali cells. On the other hand, less than 25 mole percent iridium precursor is desirable for best low operating potential efficiency of the coating. Regarding ruthenium, the component is about 35% in solution.
Amounts less than mole percent are insufficient to give the resulting coating a low chlorine potential of maximum efficiency, while amounts below 50 mole percent increase coating stability. In order to obtain the best coating properties, the molar ratio of ruthenium oxide to iridium oxide in the resulting coating should be 1.5:
1 is greater than the above, and 3: There is a fight.

With respect to the titanium precursor in the coating composition, amounts of less than 30 mole percent titanium are uneconomical;
If it is more than 5 mole percent, the electrode coating operating in the chlor-alkali cell may have a higher operating voltage. For best economics while still providing the most desirable overall coating properties, the coating solution should provide approximately 18-22 mole percent iridium, 35-40 mole percent ruthenium, and 40-44 mole percent titanium. It is preferable that the various components be contained in appropriate proportions. The molar ratio of titanium oxide to the sum of iridium oxide and ruthenium oxide in the resulting coating is less than 1:1, but is almost always greater than 0.5:1.

It is advantageous to clean the surface of the base metal before applying the coating composition to the base metal. This is done by any process used to clean metal surfaces, including mechanical cleaning. It is also advantageous to use conventional cleaning treatments of chemically or electrolytically degreasing or other chemical cleaning operations. If the preparation of the substrate includes annealing and the metal is grade 1 titanium, the titanium is about 450
Although it is possible to anneal at temperatures above 0.degree. C. for about 15 minutes or more, it is most often and advantageous to anneal at higher temperatures, such as 600-875.degree.

After said operations, including cleaning or cleaning and annealing steps and any desired rinsing and drying steps, the metal surface may be subjected to continuous operations. When such a continuous operation is an etching, it is carried out using an active etching solution. A typical etching solution is an acid solution. The solutions of Jl et al. can be supplied with hydrochloric, sulfuric, perchloric, nitric, oxalic, tartaric and phosphoric acids and mixtures thereof, such as aqua regia. Other etching agents that may be used include caustic etching agents such as a combined solution of potassium hydroxide/hydrogen peroxide or a melt of potassium hydroxide and potassium nitrate. For operational efficiency, the etching solution should be a strong or concentrated aqueous solution, e.g.
A 22% strength by weight solution of hydrochloric acid or sulfuric acid is preferred. Furthermore,
It is advantageous to maintain this aqueous solution at an elevated temperature of 80 DEG C. or higher during etching, and often at or near or above boiling conditions, for example under reflux conditions. Preferably, the etching produces a rough surface as measured by visual inspection using an auxiliary instrument. After etching, the etched metal surface is subjected to a rinsing and drying process,
The surface for coating can be prepared.

The coating composition can then be applied to the metal substrate using any typical means of applying an aqueous coating composition to the substrate metal.9 Such application methods include brushing, Application methods include roll and spraying. Furthermore, combination techniques such as a combination of a spray method and a brush method can also be used. The spray application method is a conventional compressed gas spray method or an electrostatic spray application method. When coating the back side of mesh electrodes and the like, it is advantageous to use electrostatic spray application methods in order to obtain the best wrap around effect of the spray.

Following application of the coating, the mixed oxide coating is prepared by heating the applied composition and pyrolyzing the precursors present within the coating composition. In this way, a mixed oxide coating containing said molar proportions of oxides is prepared. Such heating for pyrolysis is performed at a maximum temperature of about 440° C. or higher for about 3 minutes or more. Heating the applied coating at a higher temperature and for a slightly longer period of time is a more typical method, both for economical reasons and to avoid deleterious effects on the anodic potential when the coated metal is used as an anode. , temperatures above about 550°C are generally avoided. Preferred conditions are heating in air or oxygen. After this heating and prior to any further application of the coating composition, the heated and coated substrate is
Usually allowed to cool to at least substantially ambient temperature. The resulting finished coating has a smooth appearance to the naked eye, but microscopic examination reveals non-uniformity with embedded microcrystals in the coated bone. Although it is conceivable to apply coating compositions other than those disclosed in this invention, in order for the coated substrate metal to have the best overall performance as an electrode, sequentially applied coatings should be It would be the composition disclosed in the invention.

The following examples illustrate how the invention may be carried out, but are not to be construed as limiting the invention.

Example 1 Using an iridium trichloride solution in 8% by weight HCI
157 grams of iridium was prepared using a solution of ruthenium trichloride in 18% HCI by weight, 144 grams of ruthenium was prepared using a solution of titanium tetrachloride in 10% HCI by weight.
0 grams of titanium was prepared using a 36 wt% HCI solution.
A coating solution was prepared by combining 31 grams of MCI and diluting with 10 liters of deionized water. Give the composition. Four liters of 93 grams per liter (gp I ) HCI solution was then added to give the final coating solution.

This solution was applied to a titanium mesh substrate with a diamond pattern mesh using a hand roller. Each diamond pattern has a design length of approximately 8 mm (l
long way of design) and approx. 4mm+
The design has a short circuit. 61 layers of this titanium mesh at 0℃
After annealing for 3 minutes at 85-90° C. in 25 wt% VfL acid, it was rinsed with water and air-dried. The applied coating was air dried and then baked at 470°C. In this way, 18 layers of coating were applied. After the final coating, the anode was post-fired at 525° C. for 4 hours.

Eight samples of the resulting coated titanium substrate were heated at 12N
, 25 as anode for 4 hours at 92°C in OH.
When used at kA/m2, its average weight loss is 5°27
grams/m2. When the sample is used as an anode in a chlor-alkali membrane cell operating at 3.3 kA/m2, the oxygen evolved in the chlorine cell product when the electrolyte pH is 2.3 and 4, respectively, is 0.3 kA/m2. 0.6, 0.22 and 0.38% by volume. The working potential in the membrane cell of this anode is 1,09
It was a bolt.

The average caustic weight loss was 5.27 g/l112, indicating that a comparative coating of 7.8 mole percent iridium oxide, 15 mole percent ruthenium oxide, and 77.2 mole percent titanium oxide was tested under the same conditions. .9
This was particularly noteworthy as it showed a weight loss of 112 grams/l. Additionally, a comparative experiment was repeated in which the mole percentages were changed to more closely match the composition of the present invention, but with a comparative coating, and the caustic weight loss increased to 19.2 grams/square.

(Useful name)

Claims (1)

[Claims] 1. An electrically conductive electrode that reduces the generation of oxygen when electrolyzing a halogen-containing solution at low pH, the electrode having a coating that increases stability under alkaline conditions. of a metal substrate, the coating containing oxides present in the coating of 1
15 mole percent or more as 00 mole percent25
less than mol percent iridium oxide, 35-50 mol percent ruthenium oxide, and 30 mol percent or more but less than 45 mol percent titanium oxide, such that the molar ratio of titanium oxide to the sum of iridium oxide and ruthenium oxide of the coating is less than 1:1, and the molar ratio of ruthenium oxide to iridium oxide is greater than 1.5:1;
An electrode characterized in that the ratio is up to 3:1. 2. The electrode of claim 1, wherein said conductive metal substrate comprises a metal selected from titanium, tantalum, zirconium, niobium, aluminum, tungsten, and alloys and intermetallic mixtures thereof. 3. The electrode according to claim 1, wherein the conductive metal substrate is made of titanium or an alloy or intermetallic mixture containing titanium. 4. The electrode of claim 3, wherein said conductive metal substrate comprises an annealed and etched titanium substrate. 5. The coating is a non-uniform but smooth coating of mixed oxides and consists essentially of 18-22 mole percent iridium oxide, 35-40 mole percent ruthenium oxide and 40-44 mole percent titanium oxide. The molar ratio of ruthenium oxide to iridium oxide is 1.7.
The electrode according to claim 1, wherein the ratio is 1 to 2.2:1. 6. The electrode of claim 1, wherein said electrode is an anode in a membrane cell used to electrolyze salt water having a pH in the range of about 2 to about 4. 7. A coating composition suitable for applying a mixed metal oxide on a metal substrate, the composition comprising at least 15 mole percent and less than 25 mole percent of iridium, based on 100 mole percent of the total components. an acidic aqueous medium containing soluble components of iridium, ruthenium, and titanium in proportions providing mole percent ruthenium and 30 mole percent or more but less than 45 mole percent titanium, thereby reducing the titanium oxide versus iridium oxide and ruthenium oxide composition of the composition. A coating composition characterized in that the total molar ratio of ruthenium to iridium is less than 1:1 and the molar ratio of ruthenium to iridium is greater than 1.5:1 and up to 3:1. 8. The coating composition of claim 7, wherein said acidic aqueous medium contains a strong inorganic acid in water and has a pH within the range of about 0.2 to about 0.8. 9. The coating composition according to claim 8, wherein the strong inorganic acid is hydrochloric acid. 10. The coating composition of claim 7, wherein said soluble components are compatible and selected from chloride, bromide, iodide and ammonium chloride salts. 11. The coating composition according to claim 10, wherein all of the soluble components are chlorides, and the aqueous medium contains hydrochloric acid. 12. Iridium in which the above proportion is 18-22 mol percent based on 100 mol percent of all components, 35-4
8. A coating composition according to claim 7, providing 0 mole percent ruthenium and 40-44 mole percent titanium, wherein the ruthenium to iridium molar ratio is from 1.7:1 to 2.2:1. 13. The coating composition of claim 7, wherein said composition provides an electrocatalytic coating of said mixed metal oxide on an electrically conductive valve metal substrate. 14. A method for producing an electrode having an electrocatalytic coating that enhances performance in a chlor-alkali battery cell when the coating is present on the electrically conductive metal substrate of the electrode, the method comprising: annealing the material at a temperature of about 450° C. or more for about 0.25 hours or more; etching the resulting annealed surface with a strong etchant to create a rough surface; 15 mole percent or more and less than 25 mole percent iridium oxide, 35-50 mole percent ruthenium oxide, and 3
A mixed oxide coating of the components is applied by thermal decomposition in an amount such that the coating has a mole ratio of titanium oxide to sum of iridium oxide and ruthenium oxide of 1 to 45 mole percent titanium oxide. : is smaller than 1, and the molar ratio of ruthenium oxide to indium oxide is 1
．． applying an aqueous coating composition comprising components that provide a coating of greater than 5:1 up to 3:1; and applying the resulting coated substrate to a temperature of about 440°C or higher; A method for producing an electrode, comprising: heating for a period of 3 minutes or more. 15. The method of claim 14, wherein the annealed substrate is cooled to at least substantially about room temperature and then the resulting cooled substrate is etched. 16. The method of claim 14, wherein the annealed substrate is etched with a strong inorganic acid etchant. 17. The method of claim 14, wherein said etched surface is rinsed with water before applying said coating composition. 18. The method of claim 14, wherein the etched surface is a textured etched surface. 19. The method of claim 14, wherein said coating composition is applied to said titanium substrate by electrostatic spray application. 20. The method of claim 14, wherein the titanium substrate is annealed at a temperature within the range of about 600<0>C to about 875<0>C. 21. An electrode produced by the method of claim 14, which has a coating that increases stability under alkaline conditions and reduces oxygen evolution when electrolyzing low pH halogen-containing solutions.