RU2791363C2 - Anode for electrolytic chlorine recovery - Google Patents

Abstract

FIELD: applied chemistry.

SUBSTANCE: electrode for gas emittance in electrolytic processes, including a valve metal substrate and a catalytic coating containing 5-40% tin, 3.6-15% iridium, 18-40% ruthenium and 30-70% titanium in the form of metals or their oxides, in mole percent in terms of these elements, and the mentioned catalytic coating is obtained by thermal decomposition of an acetic solution containing hydroxyacetochloride complexes of iridium, ruthenium, tin and titanium. Options of the electrode production method, as well as to an electrolysis cell and an electrolyser.

EFFECT: marked increase in the resistance of the end electrode in situations of electric current reversal compared to the resistance obtained in the presence of Ti only, and also gives optimal values of the cell potential.

13 cl, 2 dwg, 1 tbl

Description

FIELD OF TECHNOLOGY TO WHICH THE INVENTION RELATES

The present invention relates to a process for obtaining an electrode suitable for use as an anode in electrolytic cells for the production of chlorine. The electrode thus obtained contains a catalytic layer containing oxides of tin, ruthenium, iridium and titanium deposited on a valve metal substrate.

BACKGROUND OF THE INVENTION

The field of the invention relates to the preparation of a catalytic coating for electrodes used in alkali chloride brine electrolysis processes, said coating being deposited on an electrically conductive substrate, typically titanium, titanium alloy or other valve metal.

Electrolysis processes of alkali chloride brines, such as sodium chloride brine for the production of chlorine and caustic soda, are often carried out with titanium or other valve metal anodes activated by a surface layer of ruthenium dioxide (RuO ₂ ), which tends to reduce the overvoltage of the anodic chlorine evolution reaction. .

A partial improvement in terms of chlorine surge and thus in terms of process voltage and overall energy consumption can be obtained by adding to the composition based on RuO ₂ mixed with SnO ₂ some second noble metal selected from iridium and platinum, for example, as described in EP0153586; however, this and other compositions containing tin, such as the composition described in US4513102, lead to the problem of simultaneously reducing the overvoltage of the competing oxygen evolution reaction, and to the chlorine produced by the anodic reaction with excess oxygen.

An additional partial improvement in performance can be obtained by depositing a composition based on RuO ₂ and SnO ₂ with the addition of small amounts of IrO ₂ on a metal substrate, for example, as described in WO2016083319. Such a composition makes it possible to obtain optimal values of the cell potential and moderate amounts of oxygen. However, this last-mentioned composition and the other compositions mentioned above do not provide the electrode with optimal resistance to the reversals of electric current that inevitably occur in the event of a failure in the operation of an industrial plant.

US 2013/186750 A1 describes an electrode for chlorine evolution, consisting of a metal substrate coated with alternating layers of two different compositions, namely, a layer containing oxides of iridium, ruthenium and valve metals, for example, tantalum, and a layer containing oxides of iridium, ruthenium and tin.

Chellamall et al. "Anodic incineration of phthalic anhydride using RuO ₂ -IrO ₂ -SnO ₂ -TiO ₂ coated on Ti anode", Arabian Journal of Chemistry (2012), vol. 9, appendix 2, November 2016, p. S1690-S1699, an anode for electrochemical oxidation of phthalic anhydride is described, consisting of a titanium substrate coated with mixed metal oxides of RuO ₂ , IrO ₂ , SnO ₂ and TiO ₂ prepared by a thermal decomposition method.

In addition, prior art coatings, such as the composition described in JPS6338592, based on oxides of tin and noble metals, are usually prepared from stannous precursors, in particular tin tetrachloride (SnCl ₄ ), mixed with the corresponding noble metal precursors. in aqueous solution. However, the extremely high volatility of the thus obtained precursors makes them inconvenient for use in industrial processes.

Therefore, it is clear that there is a need for a new catalytic coating for electrodes for degassing in electrolytic cells in alkali metal chloride brine electrolysis processes, having greater catalytic activity and greater resistance to electrical current reversals under normal operating conditions relative to prior art compositions.

SUMMARY OF THE INVENTION

Various aspects of the present invention are set forth in the appended claims.

The present invention consists in applying a composition based on RuO ₂ , IrO ₂ , SnO ₂ and TiO ₂ onto a metal substrate; the thus obtained composition leads to a significant improvement in the resistance of the electrode in situations of electric current reversal and, in addition, allows you to increase the selectivity of the reaction of obtaining Cl ₂ /O ₂ in favor of the first of them.

In one aspect, the present invention relates to an electrode for outgassing in electrolytic processes comprising a valve metal substrate and a catalytic coating containing 5-40% tin, 3.6-15% iridium, 18-40% ruthenium and 30-70% titanium. in the form of metals or their oxides, in mole percent in terms of these elements, and the mentioned catalytic coating is obtained by thermal decomposition of an acetic solution containing hydroxyacetochloride complexes of iridium, ruthenium, tin and titanium.

The inventors found that the presence of tin and titanium in the form of metals or their oxides in a catalytic layer containing oxides of ruthenium and iridium, deposited from an acetic solution containing a hydroxyacetochloride complex of these components in the above concentrations, leads to a noticeable increase in the resistance of the final electrode in situations reversal of electric current compared to the resistance obtained in the presence of only Ti.

It was also found that the present composition, obtained from precursors containing hydroxyacetochloride complexes, gives optimal values of the cell potential.

In one embodiment, the present invention relates to an electrode having a catalytic coating that contains a mixture containing 6-30% tin, 3.7-12% iridium, 20-30% ruthenium and 50-70% titanium in the form of metals or their oxides. .

The inventors have found that an electrode with such a catalytic coating containing significant amounts of valve metal allows moderate oxygen concentrations to be obtained, thereby improving the selectivity of the Cl ₂ /O ₂ production reaction in favor of the former.

In a further embodiment, the present invention relates to an electrode in which the catalytic coating comprises a mixture containing 8-18% tin, 4-10% iridium, 18-36% ruthenium and 45-65% titanium in the form of metals or their oxides.

This last-mentioned embodiment has the advantage of increasing the life of the electrode while maintaining optimum selectivity for chlorine and oxygen evolution reactions.

In a further aspect, the present invention relates to an electrode for outgassing in electrolytic processes, comprising a valve metal substrate provided with a catalytic coating containing a mixture containing tin, iridium, ruthenium and titanium in the form of metals or their oxides, in which the molar ratio of Ru:Ir is between 1.5 and 8, and the Ti:Sn molar ratio is between 1 and 6, and the valve metal to noble metal molar ratio is between 1.2 and 5, said catalytic coating consisting of thermally decomposed layers of acetic solution, containing hydroxyacetochloride complexes of the aforementioned metals.

The inventors have found that an electrode with such a catalytic coating having said range of noble metal to valve metal ratios allows high performance in terms of resistance to current reversals while maintaining good catalytic activity for chlorine evolution and good selectivity for the Cl ₂ /O production reaction. ₂ in favor of the first one.

In a further embodiment, the catalytic coating has a specific noble metal content, expressed as the sum of iridium and ruthenium, of 6 to 12 g/m ² , and the preferred valve metal support is a titanium or titanium alloy support.

In a further aspect, the present invention relates to an electrode comprising a valve metal substrate and a catalytic coating, said catalytic coating being obtained by thermal decomposition of an acetic solution containing iridium, ruthenium, tin and titanium hydroxyacetochloride complexes, said solution containing 5-40% tin, 3 .6% iridium, 18-40% ruthenium and 30-70% titanium, in mole percentages based on these elements.

In a further aspect, the present invention relates to a method for producing an electrode for the evolution of gaseous products in electrolytic cells, for example, for the evolution of chlorine in cells for the electrolysis of alkaline brines, comprising the following steps:

a) application of an acetic solution containing hydroxyacetochloride complexes of iridium, ruthenium, tin and titanium on a valve metal substrate, then drying at 50-60°C and thermal decomposition at 450-600°C for 5 to 30 minutes until the specific content of the noble metal, expressed as the sum of iridium and ruthenium, from 0.4 to 1 g/m ² ;

b) repeating step a) until a catalytic coating with a specific noble metal content of 6 to 12 g/m ² is obtained;

c) heat treatment at 450-600°C for 50 to 200 minutes.

According to one embodiment of the above method, said acetic solution contains 5-40% tin, 3.6-15% iridium, 18-40% ruthenium and 30-70% titanium, preferably 6-30% tin, 3.7-12% iridium , 20-30% ruthenium and 50-70% titanium, more preferably 8-18% tin, 4-10% iridium, 18-36% ruthenium and 45-65% titanium, in mole percent based on these elements.

In a further embodiment, the present invention relates to a method for manufacturing an electrode for outgassing in electrolytic processes comprising the steps of:

a) application of an acetic solution containing hydroxyacetochloride complexes of iridium, ruthenium, tin and titanium, containing 5-40% tin, 3.6-15% iridium, 18-40% ruthenium and 30-70% titanium, on a valve metal substrate, c molar percent in terms of these elements; then drying at 50-60° C. and thermal decomposition at 450-600° C. for 5 to 30 minutes until a noble metal specific content, expressed as the sum of iridium and ruthenium, of 0.4 to 1 g/m ² is reached;

b) repeating step a) until a catalytic coating with a specific noble metal content of 6 to 12 g/m ² is obtained;

c) a final heat treatment at 450-600° C. for 50 to 200 minutes.

Said acid solution contains 6-30% tin, 3.7-12% iridium, 20-30% ruthenium and 50-70% titanium and preferably 8-18% tin, 4-10% iridium, 18-36% ruthenium and 45 -65% titanium, in mole percent in terms of these elements.

The use of precursors containing a tin hydroxyacetochloride complex together with iridium, ruthenium and titanium hydroxyacetochloride complexes overcomes the limitations of the prior art by applying an anodic catalytic coating with well controlled chemical composition. The main criticality of the prior art is due to the high volatility of tin tetrachloride, which causes uncontrolled losses of the latter during the various thermal treatments necessary for the deposition of the catalytic layer.

In one embodiment, the thermal decomposition temperature in steps a) and c) is between 480 and 550°C.

In a further aspect, the invention relates to a cell for the electrolysis of alkali chloride solutions, comprising an anode compartment and a cathode compartment, the anode compartment being provided with an electrode in one of the forms described above, used as an anode for chlorine evolution.

In an additional aspect, the invention relates to an industrial cell for the production of chlorine and alkali, based on alkali chloride solutions, even if it does not contain protective polarization devices, and containing a modular arrangement of electrolytic cells with anode and cathode compartments separated by ion exchange membranes or diaphragms, and the anode the compartment contains an electrode in one of the forms described above, used as an anode.

The invention will now be described in more detail in connection with the following examples and the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a scanning electron microscope image of the cross section of the electrode described in Example No. 1;

Fig. 2 is a scanning electron microscope image of the cross section of the electrode described in Counter-Example No. 2C.

EXAMPLES

The following examples are included to demonstrate specific embodiments of the invention, whose feasibility has been fully confirmed in the claimed range of values. It will be apparent to those skilled in the art that the compositions and methods described in the examples below are compositions and methods for which the inventors have found the invention to work well in practice; however, those skilled in the art will also appreciate that various modifications may be made to the various embodiments described in light of the present disclosure, yet yielding identical or similar results while remaining within the scope of the invention.

EXAMPLE 1

A piece of titanium mesh with dimensions of 10 cm × 10 cm was washed three times in deionized water at 60°C, changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350°C. The grid was then treated in a 20% HCl solution, with boiling for 30 minutes.

Then, 100 ml of an acetic solution was prepared containing tin hydroxyacetochloride complex together with ruthenium hydroxyacetochloride complex, iridium hydroxyacetochloride complex and titanium hydroxyacetochloride complex and having a molar composition equivalent to 20% Ru, 7% Ir, 17% Sn and 56% Ti.

This solution was applied to a piece of titanium mesh with a brush, in 14 layers. After applying each layer, drying was carried out at 50-60°C for about 10 minutes, followed by heat treatment for 10 minutes at 500°C. The piece was air-cooled each time before applying the next layer.

This procedure was repeated until a total specific noble metal content, expressed as the sum of Ir and Ru in terms of metals, was reached, equal to 8 g/m ² . Then, a final heat treatment was carried out at 500° C. for 100 minutes.

The thus obtained electrode was designated as sample No. 1.

EXAMPLE 2

A piece of titanium mesh with dimensions of 10 cm × 10 cm was washed three times in deionized water at 60°C, changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350°C. The grid was then treated in a 20% HCl solution, with boiling for 30 minutes.

Then, 100 ml of an acetic solution containing tin hydroxyacetochloride complex together with ruthenium hydroxyacetochloride complex, iridium hydroxyacetochloride complex and titanium hydroxyacetochloride complex and having a molar composition equivalent to 30% Ru, 4% Ir, 15% Sn and 51% Ti was prepared.

This solution was applied to a piece of titanium mesh with a brush, in 14 layers. After applying each layer, drying was carried out at 50-60°C for about 10 minutes, followed by heat treatment for 10 minutes at 500°C. The piece was air-cooled each time before applying the next layer.

This procedure was repeated until a total specific noble metal content, expressed as the sum of Ir and Ru in terms of metals, was reached, equal to 8 g/m ² . Then, a final heat treatment was carried out at 500° C. for 100 minutes.

The thus obtained electrode was designated as sample No. 2.

EXAMPLE 3

A piece of titanium mesh with dimensions of 10 cm × 10 cm was washed three times in deionized water at 60°C, changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350°C. The grid was then treated in a 20% HCl solution, with boiling for 30 minutes.

Then, 100 ml of an acetic solution was prepared containing the tin hydroxyacetochloride complex together with the ruthenium hydroxyacetochloride complex, the iridium hydroxyacetochloride complex and the titanium hydroxyacetochloride complex and having a molar composition equivalent to 23% Ru, 12% Ir, 19% Sn and 46% Ti.

This solution was applied to a piece of titanium mesh with a brush, in 14 layers. After applying each layer, drying was carried out at 50-60°C for about 10 minutes, followed by heat treatment for 10 minutes at 500°C. The piece was air-cooled each time before applying the next layer.

This procedure was repeated until a total specific noble metal content, expressed as the sum of Ir and Ru in terms of metals, was reached, equal to 8 g/m ² . Then, a final heat treatment was carried out at 500° C. for 100 minutes.

The thus obtained electrode was designated as sample No. 3.

EXAMPLE 4

A piece of titanium mesh with dimensions of 10 cm × 10 cm was washed three times in deionized water at 60°C, changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350°C. The grid was then treated in a 20% HCl solution, with boiling for 30 minutes.

Then, 100 ml of an acetic solution was prepared containing tin hydroxyacetochloride complex together with ruthenium hydroxyacetochloride complex, iridium hydroxyacetochloride complex and titanium hydroxyacetochloride complex and having a molar composition equivalent to 20% Ru, 10% Ir, 16% Sn and 54% Ti.

This solution was applied to a piece of titanium mesh with a brush, in 14 layers. After applying each layer, drying was carried out at 50-60°C for about 10 minutes, followed by heat treatment for 10 minutes at 500°C. The piece was air-cooled each time before applying the next layer.

This procedure was repeated until a total specific noble metal content, expressed as the sum of Ir and Ru in terms of metals, was reached, equal to 12 g/m ² . Then, a final heat treatment was carried out at 500° C. for 100 minutes.

The thus obtained electrode was designated as sample No. 4.

EXAMPLE 5

A piece of titanium mesh with dimensions of 10 cm × 10 cm was washed three times in deionized water at 60°C, changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350°C. The grid was then treated in a 20% HCl solution, with boiling for 30 minutes.

Then, 100 ml of an acetic solution containing tin hydroxyacetochloride complex together with ruthenium hydroxyacetochloride complex, iridium hydroxyacetochloride complex and titanium hydroxyacetochloride complex and having a molar composition equivalent to 21% Ru, 7% Ir, 32% Sn and 40% Ti was prepared.

This solution was applied to a piece of titanium mesh with a brush, in 14 layers. After applying each layer, drying was carried out at 50-60°C for about 10 minutes, followed by heat treatment for 10 minutes at 500°C. The piece was air-cooled each time before applying the next layer.

This procedure was repeated until a total specific noble metal content, expressed as the sum of Ir and Ru in terms of metals, was reached, equal to 9 g/m ² . Then, a final heat treatment was carried out at 500° C. for 100 minutes.

The electrode thus obtained was designated Sample No. 5.

EXAMPLE 6

A piece of titanium mesh with dimensions of 10 cm × 10 cm was washed three times in deionized water at 60°C, changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350°C. The grid was then treated in a 20% HCl solution, with boiling for 30 minutes.

Then, 100 ml of an acetic solution was prepared containing the tin hydroxyacetochloride complex together with the ruthenium hydroxyacetochloride complex, the iridium hydroxyacetochloride complex and the titanium hydroxyacetochloride complex and having a molar composition equivalent to 21% Ru, 9% Ir, 29% Sn and 41% Ti.

This solution was applied to a piece of titanium mesh with a brush, in 14 layers. After applying each layer, drying was carried out at 50-60°C for about 10 minutes, followed by heat treatment for 10 minutes at 500°C. The piece was air-cooled each time before applying the next layer.

This procedure was repeated until a total specific noble metal content, expressed as the sum of Ir and Ru in terms of metals, was reached, equal to 9 g/m ² . Then, a final heat treatment was carried out at 500° C. for 100 minutes.

The thus obtained electrode was designated as sample No. 6.

COUNTEREXAMPLE 1

A piece of titanium mesh with dimensions of 10 cm × 10 cm was washed three times in deionized water at 60°C, changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350°C. The grid was then treated in a 20% HCl solution, with boiling for 30 minutes.

Then prepared 100 ml of water-alcohol solution containing RuCl ₃ ·3H ₂ O, H ₂ IrCl ₆ ·6H ₂ O, TiCl ₃ in isopropanol solution and having a molar composition equivalent to 27% Ru, 12% Ir, 61% Ti.

This solution was applied to a piece of titanium mesh with a brush, in 14 layers. After applying each layer, drying was carried out at 50-60°C for about 10 minutes, followed by heat treatment for 10 minutes at 500°C. The piece was air-cooled each time before applying the next layer.

This procedure was repeated until a total specific noble metal content, expressed as the sum of Ir and Ru in terms of metals, was reached, equal to 13 g/m ² . Then, a final heat treatment was carried out at 500° C. for 100 minutes.

The thus obtained electrode was designated as sample No. 1C.

COUNTEREXAMPLE 2

A piece of titanium mesh with dimensions of 10 cm × 10 cm was washed three times in deionized water at 60°C, changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350°C. The grid was then treated in a 20% HCl solution, with boiling for 30 minutes.

Then prepared 100 ml of a water-alcohol solution containing RuCl ₃ 3H ₂ O, H ₂ IrCl ₆ 6H ₂ O, TiCl ₃ , SnCl ₄ in an isopropanol solution and having a molar composition equivalent to 20% Ru, 7% Ir, 17% Sn and 56% Ti.

This solution was applied to a piece of titanium mesh with a brush, in 14 layers. After applying each layer, drying was carried out at 50-60°C for about 10 minutes, followed by heat treatment for 10 minutes at 500°C. The piece was air-cooled each time before applying the next layer.

This procedure was repeated until a total specific noble metal content, expressed as the sum of Ir and Ru in terms of metals, was reached, equal to 8 g/m ² . Then, a final heat treatment was carried out at 500° C. for 100 minutes.

The thus obtained electrode was designated as sample No. 2C.

COUNTEREXAMPLE 3

A piece of titanium mesh with dimensions of 10 cm × 10 cm was washed three times in deionized water at 60°C, changing the liquid each time. Washing was followed by heat treatment for 2 hours at 350°C. The grid was then treated in a 20% HCl solution, with boiling for 30 minutes.

Then, 100 ml of an acetic solution was prepared containing a tin hydroxyacetochloride complex together with a ruthenium hydroxyacetochloride complex and an iridium hydroxyacetochloride complex and having a molar composition equivalent to 35% Ru, 6% Ir, 59% Sn.

This solution was applied to a piece of titanium mesh with a brush, in 14 layers. After applying each layer, drying was carried out at 50-60°C for about 10 minutes, followed by heat treatment for 10 minutes at 500°C. The piece was air-cooled each time before applying the next layer.

This procedure was repeated until a total specific noble metal content, expressed as the sum of Ir and Ru in terms of metals, was reached, equal to 8 g/m ² . Then, a final heat treatment was carried out at 500° C. for 100 minutes.

The thus obtained electrode was designated as sample No. 3C.

SAMPLE CHARACTERIZATION

Data on the high volatility of tin tetrachloride and the associated uncontrolled loss of the latter during heat treatment were confirmed by SEM-EDX analysis of two samples of the electrode of Example 1 (sample 1) and the electrode of Counterexample 2 (sample 2C), respectively.

The analysis was performed using a scanning electron microscope (a commercial FEI Inspect F 50 SEM/FEG instrument with an EDAX microanalysis system) equipped with an Everhart-Thornley detection system used in backscattering mode; the working distance was set to 10 mm, the accelerating voltage was set to 20 kV, and the magnification was changed between 10,000-fold and 100,000-fold.

Fig. 1 and 2 show cross-sectional images of the catalytic coatings of the prepared electrodes of Example 1 and Counterexample 2, respectively, and the corresponding composition profiles determined using the ZAF correction method ("ZAF" refers to the correction of matrix effects of the sample; in particular: Z refers to the effect of atomic number , A refers to the X-ray absorption effect, and F refers to the X-ray fluorescence effect).

As can be understood from FIG. 1 relating to the electrode of Example 1 (sample 1), tin is present throughout the entire cross section of the catalytic coating. Additionally, the composition of metals detected by EDX analysis is comparable to the molar composition of the initial acetic solution containing hydroxyacetochloride complexes of tin, ruthenium, iridium and titanium, equivalent to 20% Ru, 7% Ir, 17% Sn and 56% Ti of sample 1.

As can be understood from FIG. 2 relating to the electrode of Counterexample 2 (sample 2C), there is only a slight presence of tin in the entire cross section of the catalytic coating. The metal composition detected by EDX analysis confirms the loss of most of the tin compared to the molar composition of the initial water-alcohol solution containing tin, ruthenium, iridium and titanium, equivalent to 20% Ru, 7% Ir, 17% Sn and 56% Ti sample 2C.

CHLORINE RELEASE TEST

Samples from the examples were characterized as anodes for chlorine evolution in a laboratory cell supplied with 200 g/l sodium chloride brine, with pH precisely controlled at 3.

Table 1 shows the overvoltage of chlorine measured at a current density of 3 kA/m ² , the percentage of oxygen by volume in the chlorine produced, and the reversal resistance expressed as the percentage of precious metal lost.

Table 1

Samples ηCl ₂ (mV) O ₂ /Cl ₂ (vol.%) Conversion Resistance (%) 1 thirty 0.2 3 2 40 0.2 2 3 thirty 0.3 3 4 40 0.2 3 5 thirty 0.3 3 6 thirty 0.3 3 1C 50 0.4 4 2C 50 0.4 8 3C 40 0.3 7

The foregoing description is not intended to limit the invention, which may be used according to various embodiments, but without deviating from its purposes, and its scope is unambiguously defined by the appended claims.

In the description and claims of the present application, the terms "comprises", "comprises" and "includes" and variations thereof such as "comprising", "comprising" and "comprising" are not intended to exclude the presence of other additional elements, components or steps of the method.

Discussion of documents, actions, materials, devices, products, etc. included in this description solely for the purpose of providing context for the present invention. It is not intended or claimed that any or all of these considerations would form part of the state of the art or would be common knowledge in the art of the present invention prior to the priority date of each claim of this application.

Claims (19)

1. An electrode for gas evolution in electrolytic processes, including a valve metal substrate and a catalytic coating containing 5-40% tin, 3.6-15% iridium, 18-40% ruthenium and 30-70% titanium in the form of metals or their oxides, in mole percent in terms of these elements, and the said catalytic coating is obtained by thermal decomposition of an acetic solution containing hydroxyacetochloride complexes of iridium, ruthenium, tin and titanium. 2. The electrode according to claim. 1, in which the said catalytic coating contains 6-30% tin, 3.7-12% iridium, 20-30% ruthenium and 50-70% titanium in the form of metals or their oxides, in mole percent in counting these elements. 3. The electrode according to claim 1, in which the said catalytic coating contains 8-18% tin, 4-10% iridium, 18-36% ruthenium and 45-65% titanium in the form of metals or their oxides, in mole percent in terms of these elements. 4. An electrode according to any one of the preceding claims, wherein said catalytic coating has a specific noble metal content, expressed as the sum of iridium and ruthenium, between 6 and 12 g/m ² . 5. An electrode according to one of the preceding claims, wherein said catalytic coating is obtained by thermal decomposition of an acetic solution containing hydroxyacetochloride complexes of iridium, ruthenium, tin and titanium, said solution containing 5-40% tin, 3.6-15% iridium, 18 -40% ruthenium and 30-70% titanium, in mole percent in terms of these elements. 6. A method for producing an electrode as described in one of the preceding paragraphs, comprising the following steps: a) application of an acetic solution containing hydroxyacetochloride complexes of iridium, ruthenium, tin and titanium onto a valve metal substrate, followed by drying at 50-60°C and thermal decomposition at 450-600°C for 5 to 30 minutes until the specific noble metal content, expressed as the sum of iridium and ruthenium, between 0.4 and 1 g/m ² ; b) repeating step a) until a catalytic coating with a specific noble metal content of 6 to 12 g/m ² is obtained, c) heat treatment at 450-600°C for 50 to 200 minutes. 7. Method according to claim 6, wherein said acid solution contains 5-40% tin, 3.6-15% iridium, 18-40% ruthenium and 30-70% titanium, preferably 6-30% tin, 3.7 - 12% iridium, 20-30% ruthenium and 50-70% titanium, and more preferably 8-18% tin, 4-10% iridium, 18-36% ruthenium and 45-65% titanium, in mole percentages calculated on these elements. 8. A method for the production of an electrode for gas evolution in electrolytic processes, including the following steps: a) application of an acetic solution containing hydroxyacetochloride complexes of iridium, ruthenium, tin and titanium, containing 5-40% tin, 3.6-15% iridium, 18-40% ruthenium and 30-70% titanium, on a valve metal substrate, c molar percent in terms of these elements; subsequent drying at 50-60°C and thermal decomposition at 450-600°C for a period of 5 to 30 minutes to reach a specific noble metal content, expressed as the sum of iridium and ruthenium, from 0.4 to 1 g/m ² ; b) repeating step a) until a catalytic coating with a specific noble metal content of 6 to 12 g/m ² is obtained, c) heat treatment at 450-600°C for 50 to 200 minutes. 9. The method according to claim 8, in which said acidic solution contains 6-30% tin, 3.7-12% iridium, 20-30% ruthenium and 50-70% titanium, and preferably 8-18% tin, 4- 10% iridium, 18-36% ruthenium and 45-65% titanium, in mole percentages of these elements. 10. The method according to any one of paragraphs. 6-8, in which the temperature of said thermal decomposition in steps a) and c) is between 480 and 550°C. 11. Cell for the electrolysis of solutions of alkali chlorides, containing the anode compartment and the cathode compartment, and the anode compartment is equipped with an electrode according to any one of paragraphs. 1-5. 12. An electrolysis cell according to claim 11, wherein said anode compartment and said cathode compartment are separated by a diaphragm or ion exchange membrane. 13. An electrolytic cell for the production of chlorine and alkali from solutions of alkali metal chlorides, containing a modular arrangement of cells according to claim 12.

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