NO302858B1 - Electrode having reduced oxygen evolution during electrolysis of halogen-containing solutions and use thereof - Google Patents

Description

Electrodes for electrolytic processes have been known which have a base or a core metal bearing a layer or coating of metal oxides. The core metal of the electrode may be a valve metal, such as titanium, tantalum, zirconium,

niobium or tungsten. If the coating is an oxide mixture,

an oxide in the core or substrate metal may contribute to the mixture. As stated for example in US patent 3711385, such a mixture may include an oxide of the substrate metal plus at least one oxide of a metal, such as platinum, iridium, rhodium, palladium, ruthenium and osmium.

It has also been known that such a mixture, which can be described as a noble metal oxide mixture, can be a mixture of ruthenium oxide and iridium oxide. One such has been generally described in US patent no. 3632498, and examples shown specifically, in combination with titanium oxide, are set forth in US patent 3948751. Especially for use as a coating

on an electrode used in the electrolysis of an aqueous alkali metal halide, e.g. sodium chloride, it has been stated in US patent 4005004 that such a noble metal oxide mixture can be particularly useful when it is present in a further mixture with both titanium oxide and zirconium oxide. Such a mixture provides, as stated in the patent, a coating of a solid solution which apparently increases the practical utilization of the electrodes for their intended use.

More recently, it has been proposed to increase the wear resistance of an electrode, especially when used in electrolysis for the production of oxygen and chlorine in combination with each other, to ensure that the molar amount of titanium oxide is equal to or greater than the number of moles of the combined oxides of iridium and ruthenium. This has been described in US patent 4564434 in which it is also stated that the molar amount of iridium oxide is provided so that it is approximately the same as, or greater than, the molar amount of ruthenium oxide.

Summary of the invention

However, it would be desirable to provide

an electrocatalytic coating which by electrolysis of halogen

containing solutions, e.g. chlorine-alkali production from salt solution electrolysis will result in reduced oxygen development. It would also have been particularly desirable to provide

such a coating exhibits delayed weight loss when exposed to caustic conditions. It would have been highly beneficial if such characteristics could have been achieved, not only in combination, but without compromising other desirable features, e.g. without exceeding the chlorine development potential for the anode. It would also have been advantageous to produce an electrode using a coating mixture which can be easily produced, has a simple composition and offers easy and safe handling and use.

The invention generally relates to an electrode which has reduced oxygen evolution during electrolysis of halogen-containing solutions at low pH and consists of an electrically conductive metal substrate with a coating which has increased stability under alkaline conditions, being characterized by the fact that the coating consists of at least 15, but less than 25 mol% iridium oxide, 35-50 mol% ruthenium oxide and at least 30, but less than 45, mol% titanium oxide, base 100 mol% of these oxides present in the coating, whereby the coating has a molar ratio between titanium oxide and the oxides of iridium and ruthenium combined of less than 1:1 and the molar ratio between ruthenium oxide and iridium oxide being from over 1.5:1 and up to 3:1.

The invention also relates to the use of an electrode as described above as an anode in a membrane cell used for the electrolysis of salt solution which has a pH within the range of from 2 to 4.

Description of the preferred embodiments

The coating mixture can generally

applied to any electrically conductive metal substrate that will suffice

electrically conductive to be able to serve as an electrode in an electrolysis process. The aim is thus to be able to use a wide spectrum of metals for the substrate, but taking into account the application of an electrocatalytic coating, the substrate metals can more typically be such as nickel or manganese, or almost always the valve metals, including titanium, tantalum, aluminium, tungsten, zirconium and niobium. Titanium is of particular interest due to its robustness, corrosion resistance and availability. The suitable metals for the substrate can, as well as the normally available elemental metals as such, include metal alloys and intermetallic mixtures. For example, titanium may generally be alloyed with nickel, cobalt, iron, manganese or copper. Specifically, grade 5 titanium may include up to 6.75 wt% aluminum and 4.5 wt% vanadium, grade 6 up to 6% aluminum and 3% tin, grade 7 up to 0.25 wt% palladium, and grade 10 from 10 to 13 wt% molybdenum plus 4.5 to 7.5 wt% zirconium, etc.

The coating composition applied to the coated metal substrate will be aqueous, which almost always involves only water without mixing with additional liquid. Deionized or distilled water is preferably used to avoid inorganic contaminants. For reasons of economy in production and use, the aqueous mixtures that are useful will be solutions of precursor constituents in the aqueous medium, i.e. precursors for the oxides that will be present in the coating. The precursor components used in the aqueous solution are those that can be effectively and economically dissolved in water, e.g. while obtaining solutions without extensive boiling conditions. The precursors must also deliver the respective metal oxide by thermal decomposition. If they were all present in the same mixture, they would also have to be compatible with each other. In this respect, they are advantageously not reactive to each other, e.g. they will not react to form products that will lead to harmful non-oxide substituents in the coating or precipitation from the coating solution. As a rule, each precursor component will be a metal salt, which is most often a halide salt, and for economic reasons, together with efficient production of the solution, this will consist exclusively of the chloride salt. However, other useful salts include iodides, bromides and ammonium chloride salts, such as ammonium hexachloriridate or -ruthenate.

In the individual or combined solutions will

in addition to the suitable precursor component almost always no additional component is present with only one exception. This exception will practically always be the presence of inorganic acid. For example, a solution of iridium trichloride may further contain a strong acid, almost always hydrochloric acid, which will usually be present in an amount which will give 5-20% by weight acid. The individual or combined solutions will typically have a pH of less than 1, as e.g. within the range from 0.2 to 0.8.

When the coating mixture is a solution of all precursor components, this will contain at least 15, but less than 25, mol% of the iridium component, 35-50 mol% of the ruthenium component and at least 30, but less than 45, mol%

of the titanium component, base 100 mol% of these components. A composition containing an iridium component in an amount of less than 15 mol% will be insufficient to provide an electrode coating having the best stability under caustic conditions, such as e.g. when the electrode is used in a chlorine-alkali cell. On the other hand, less than 25 mol% of the iridium precursor would be desirable in order to

reach the best low working potential efficiency for the coating. As regards the ruthenium, a component amount in the solution of less than approx. 35 mol% may be insufficient to provide the most effective low chlorine potential for coatings obtained, while an amount not greater than 50 mol% increases coating stability. In order to achieve the best coating characteristics, the molar ratio between ruthenium oxide and iridium oxide in the obtained coating will also be from over 1.5:1 and up to 3:1.

For the titanium precursor in the coating mixture, an amount providing less than 30 mol% titanium is uneconomical, while 45 mol% titanium or more can lead to a higher working potential for the electrode coatings when working in chlor-alkali cells. In order to achieve the best economy together with the most desired overall coating characteristics, the coating solution will preferably contain components in a quantity ratio such that they provide 18-22 mol% iridium, 35-40 mol% ruthenium and 40-44 mol% titanium. The resulting coating will also have a molar ratio between titanium oxide and the total amount of the oxides of iridium and ruthenium of less than 1:1, but almost always above 0.5:1.

Before the coating mixture is applied to the substrate metal, the substrate metal advantageously has a cleaned surface. This can be achieved using any of the treatments used to achieve a clean metal surface, including mechanical cleaning. The usual cleaning methods that involve deposition, either chemically or electrolytically, or another chemical cleaning operation can also be used with advantage. If the preparation of the substrate includes annealing and the metal is titanium of quality 1, the titanium can be annealed at a temperature of at least 450°C for a time of at least 15 minutes, but most often a more elevated annealing temperature, e.g. 600-875°C, beneficial.

After the above operation, e.g. cleaning or cleaning and annealing, and including any desirable rinsing and drying steps, the metal surface is ready for continued operation. If such an operation is etching, this will be carried out with an active etching solution. Typical etching solutions are acidic solutions. These can be achieved using hydrochloric acid, sulfuric acid, perchloric acid, nitric acid, oxalic acid, tartaric acid and phosphoric acid as well as mixtures thereof, e.g. aqua regia. Other etchants which can be used include caustic etchants, such as a solution of a combination of potassium hydroxide/hydrogen peroxide, or a melt of potassium hydroxide with potassium nitrate. To achieve efficient operation, the etching solution is advantageously a strong, or concentrated, aqueous solution, such as e.g. an 18-22% by weight solution of hydrochloric acid, or a solution of sulfuric acid. Moreover, the solution during the etching is advantageously kept at an elevated temperature, such as e.g. at 80°C or higher for aqueous solutions, and often at or near boiling conditions or higher, e.g. under reflux condition. The etching will preferably produce a roughened surface, as determined by aided visual inspection. After etching, the etched metal surface can be subjected to rinsing and drying steps to make the surface ready for coating.

The coating composition can then be applied to the metal substrate using any method of typically applying an aqueous coating composition to a substrate metal. Such application methods include brush, roller and spray application. In addition, combination methods can be used, e.g. spraying and brushing method. Spray application can either be carried out in the usual way with compressed gas or it can be carried out by electrostatic spray application. Advantageously, electrostatic spray application will be used to achieve the best repacking effect of the shower for coating the backside of an article, such as a grid electrode.

After application of the coating, the applied mixture will be heated to provide the mixed oxide coating by thermal decomposition of the precursors present in the coating mixture. Thereby the mixed oxide coating is obtained which contains the mixed oxides in the molar ratio mentioned above. Such heating for the thermal cleavage will be carried out at a temperature of at least 440°C peak metal temperature for a time of at least 3 minutes. The applied coating will more typically be heated at a more elevated temperature for a slightly longer time, but as a rule a temperature of over 550°C is avoided for economic reasons and to

avoid harmful effects on the anode potential if the coated metal is to be used as an anode. Suitable conditions may include heating in air or oxygen. After such heating, and before further coating such as e.g. when a further application of the coating mixture will be made, the heated and coated substrate will generally be allowed to cool to at least substantially the ambient temperature. The finished coating obtained has a smooth appearance to the naked eye, but when examined under a micro-

scope is seen to be inhomogeneous with embedded crystallites in the coating field. Although it is then aimed to apply coating compositions different from those described herein, in order to thereby achieve the best overall performance from the coated substrate metal used as an electrode, subsequently applied coatings will have the compositions according to the invention as herein is described.

The following example shows a way in which the present invention was carried out in practice, but it should not be interpreted as limiting the invention.

Example

A plating solution was prepared by combining 157 g of iridium, using a solution of iridium trichloride in 18% by weight of HCl, 144 g of ruthenium, using a solution of ruthenium trichloride in 18% by weight of HCl, 8 and titanium, using titanium chloride in 10% by weight HCl solution, 331 g of HCl, using a 36% by weight solution, after which dilution to 10 liters was made with deionized water. Thereby a coating mixture was obtained which had

21 mol% iridium, 36.3 mol% ruthenium and 42.7 mol% titanium.

4 liters of 93 g per liters (gpl) of HCl solution were then added to prepare the final coating solution.

This solution was applied using a hand roller to a titanium mesh substrate with a diamond patterned mesh, each diamond pattern having a long dimension design of approx. 8 mm plus a short dimension design of approx. 4 mm. The titanium mesh had been annealed at 600°C for 30 minutes and etched in 25% by weight sulfuric acid at 85-90°C and then rinsed in water and dried in air. The applied coating was air dried and then fired at 470°C. 18 coatings were applied in this way. After the last coating, the anode was post-fired at 525°C for 4 hours.

When eight samples of the obtained coated titanium substrate were used as an anode in 12 normal NaOH at 95°C for 4 hours at 25 kA/m 2 , the average weight loss was 5.27 g/m 2 .

When a sample was used as an anode in a chlorine-alkali membrane cell operating at 3.3 kA/m<2>, 0.06%, 0.22% and 0.38%, based on volume, of oxygen were produced in the chlorine cell product at an electrolyte pH of 3 and 4 respectively. The working potential of this anode in the membrane cell was 1.09 V measured against a standard calomel reference electrode.

The average caustic weight loss of 5.27 g/m<2> was particularly noteworthy when a comparative coating with 7.8 mol% iridium oxide, 15 mol% ruthenium oxide and 7.2 mol% titanium oxide showed such a weight loss of 8.9 g/m m 2 when tested under the same conditions. Also and again relatively, but gradually as the mole percentage was changed to more closely approach the composition according to the invention, but still in a comparative coating, the caustic weight loss increased to 19.2 g/m<2>.

Claims (6)

1. Electrode that has reduced oxygen evolution during electrolysis of halogen-containing solutions at low pH and consists of an electrically conductive metal substrate with a coating that has increased stability under alkaline conditions, characterized in that the coating consists of at least 15, but less than 25, mol% iridium oxide, 35-50 mol% ruthenium oxide and at least 30, but less than 45, mol% titanium oxide, base 100 mol% of these oxides present in the coating, whereby the coating has a molar ratio between titanium oxide and the oxides of iridium and ruthenium combined of less than 1 :1 and as the mole ratio between ruthenium oxide and iridium oxide is from over 1.5:1 and up to 3:1. 2. Electrode according to claim 1, characterized in that the conductive metal substrate comprises a metal selected from the group consisting of titanium, tantalum, zirconium, niobium, aluminium, tungsten and alloys and intermetallic mixtures thereof. 3. Electrode according to claim 1, characterized in that the conductive metal substrate comprises titanium or an alloy or intermetallic mixture containing titanium. 4. Electrode according to claim 3, characterized in that the conductive metal substrate comprises an annealed and etched titanium substrate. 5. Electrode according to claim 1, characterized in that the coating is a non-homogeneous, but smooth, coating of mixed oxides and essentially consists of 18-22 mol% iridium oxide, 35-40 mol% ruthenium oxide and 40-44 mol% titanium oxide, with a molar ratio between ruthenium oxide and iridium oxide off from 1.7:1 to 2.2:1. 6. Use of an electrode according to claim 1 as an anode in a membrane cell used for electrolysis of salt solution which has a pH within the range of from 2 to 4.