CN113166956A

CN113166956A - Electrode for the electrolytic evolution of gases

Info

Publication number: CN113166956A
Application number: CN201980079027.5A
Authority: CN
Inventors: A·加尔吉乌洛; T·林田
Original assignee: Industrie de Nora SpA
Current assignee: Industrie de Nora SpA
Priority date: 2018-12-03
Filing date: 2019-12-03
Publication date: 2021-07-23
Also published as: IL283404A; CA3120540A1; AU2019391296A1; ZA202103111B; US20210404076A1; TW202022166A; WO2020115028A1; KR20210096661A; JP2022514211A; BR112021010421A2; IT201800010760A1; EP3891322A1; MX2021006438A

Abstract

The invention relates to an electrode for gas evolution in electrolytic processes, comprising a valve metal substrate and a catalytic coating comprising two layers. The first layer contains an oxide of a valve metal, ruthenium and iridium, and the second layer contains one or more metals selected from platinum group elements.

Description

Electrode for the electrolytic evolution of gases

Technical Field

The invention relates to an electrode for gas evolution in electrolytic processes, comprising a valve metal substrate and a catalytic coating comprising two layers. The first layer comprises an oxide of a valve metal, ruthenium and iridium, and the second layer comprises one or more metals selected from platinum group elements.

Background

The field of the invention relates to the preparation of catalytic coatings for electrodes used in brine electrolysis processes. The coating is applied to a metal substrate, typically titanium or other valve metal.

For many years, brine electrolysis technology has experienced innovation towards efficient implementation from an energy perspective and from a cost/benefit of resource utilization. In this increasingly challenging context, optimization of the anode plays a critical role. In particular, many efforts have been made to reduce the overvoltage of the anode in the generation of chlorine and to lower the oxygen concentration in the generated gaseous chlorine, thereby producing gaseous chlorine having high purity.

Another difficulty is to obtain electrodes that can maintain high performance over long periods of time.

Generally speaking, electrolysis processes for the production of chlorine and caustic soda brines, for example alkaline chloride brines, such as sodium chloride, are carried out with anodes made of titanium or other valve metal, optionally with tin dioxide (SnO), as described in EP0153586₂) Ruthenium dioxide (RuO) mixed with another noble metal₂) The surface layer of (2) is activated. Therefore, the overvoltage of chlorine evolution anode reaction can be reduced, thereby reducing the total energy consumption.

However, the formulation just described, together with other formulations containing tin, also has the problem of reducing the overvoltage of the concurrent oxygen-generating reaction, resulting in the production of chlorine gas contaminated with excess oxygen.

Such as described in WO2016083319, by basing RuO₂And SnO₂Formulation of (A) and reduced IrO₂The combination is applied to a metal substrate to achieve another partial improvement in performance. Similar formulations allow to obtain optimal values of cell potential and appropriate amounts of oxygen.

Other coatings of the prior art formulations, as described in WO2012081635, comprise two catalytic coatings, the first containing titanium and noble metal oxides and the second containing platinum and palladium alloys, also allowing to obtain an optimized cell potential value and a reduced amount of oxygen in chlorine; however, they do not give the electrode an optimized electrical resistance that can maintain a high level of performance with respect to catalytic activity and selectivity for a sufficient time.

US 2013/0186750 a1 describes an electrode suitable for chlorine evolution having alternating layers of two different compositions, one type of layer comprising iridium, ruthenium and a valve metal and the other type of layer comprising an oxide of iridium, ruthenium and tin.

US 2013/0334037 a1 describes an electrode for electrolysis comprising a conductive substrate, a first layer containing at least one oxide selected from the group consisting of ruthenium oxide, iridium oxide and titanium oxide formed on the conductive substrate, and a second layer containing an alloy of platinum and palladium formed on the first layer.

US 4,626,334 describes an anode comprising (Ru-Sn) O with a cathode for brine electrolysis₂A solid solution coated conductive substrate.

JP S62243790 describes an electrode having a first coating comprising a mixture of platinum and iridium oxide and a second coating comprising a mixture of ruthenium oxide and tin oxide.

There is therefore a clear need to identify new catalytic coatings for electrodes intended for the evolution of gaseous products in electrolysis cells in brine electrolysis processes, characterized by a higher level of catalytic activity and a high electrical resistance capable of maintaining a higher level of performance for a long time under normal operating conditions with respect to the formulations of the prior art.

Disclosure of Invention

Various aspects of the invention are described in the appended claims.

The invention relates to an electrode for evolving gaseous products in electrolysis cells, for example for evolving chlorine in alkaline brine electrolysis cells, comprising a catalytic coating applied on a metal substrate. In the present context, the term catalytic coating means two different catalytic layers having different catalytic compositions, wherein a first catalytic layer formed on a substrate comprises at least a mixture of iridium, ruthenium, tin and platinum or their oxides or respective combinations, and a second catalytic layer formed on the first catalytic layer comprises platinum and tin or their oxides or respective combinations. The tin of the second catalytic layer is present in a concentration decreasing from the interface with the first catalytic layer towards the upper surface of the second catalytic layer (i.e. the surface opposite to the interface with the first catalytic layer), and the platinum of the first catalytic layer is present in a concentration decreasing from the interface with the second catalytic layer towards the substrate.

The invention also relates to an electrode for evolving gaseous products in an electrolytic cell, for example for evolving chlorine in an alkaline brine electrolytic cell, comprising a valve metal substrate and a coating comprising a first catalytic layer formed on said substrate comprising iridium, ruthenium, tin and platinum or oxides thereof or mixtures thereof, and a second catalytic layer formed on said first catalytic layer comprising platinum and tin or oxides thereof or combinations thereof, wherein said first catalytic layer is obtained from a platinum-free first precursor solution comprising a mixture of iridium, ruthenium and tin, applied to said substrate and subjected to a heat treatment, and wherein said second catalytic layer is obtained from a tin-free second catalytic solution comprising platinum, applied to said first catalytic layer and subjected to a heat treatment. The terms "platinum-free" and "tin-free" in the sense of the present invention mean that the platinum concentration in the first solution is at least one order of magnitude lower than the average platinum concentration in the first layer obtained from said first solution, and the tin concentration in the second solution is at least one order of magnitude lower than the average tin concentration in the second layer obtained from the second solution. Preferably, the platinum-free solution contains platinum at most as an impurity, and the tin-free solution contains tin at most as an impurity.

This double layer structure applied to the metal substrate, usually titanium, titanium alloys or other valve metals, combines the savings in energy consumption with the excellent purity of the chlorine gas produced, while maintaining the optimum performance characteristics in terms of catalytic activity and selectivity over a long period of time.

The first catalytic layer formed on the substrate preferably contains ruthenium oxide, iridium oxide, tin oxide, and metallic platinum or an oxide thereof. RuO₂Are well known for their excellent catalytic activity and stability in alkaline media, the stability of which is due to IrO₂Is improved by the presence of; SnO₂The presence of (a) ensures a slow consumption of the noble metal present.

The second catalytic layer formed on the first layer contains tin or an oxide thereof and one or more metals selected from platinum group elements, particularly platinum itself, which is known for improving selectivity and reducing energy consumption.

The inventors have observed that an electrode with a similar catalytic coating, in which the second catalytic layer comprises platinum in the form of a metal or of its oxide, in a molar percentage of the metallic element ranging between 48% and 96% (or from 50% to 99.999% when the tin component is not considered), can provide the advantage of reducing the overvoltage of the chlorine evolution reaction subsequently.

In the context of the present invention, the ranges denoted by "from" or "between … …" include the specified upper and lower limits, respectively.

In another embodiment, the second catalytic layer comprises, in addition to platinum and tin, palladium or rhodium in the form of a metal or of an oxide thereof or of a combination thereof, in a molar percentage calculated as metallic element ranging between 0% and 24% (or, when the tin component is not considered, between 0% and 25%), wherein the element is in the form of a metal or of an oxide thereof. This can ensure high catalytic activity by virtue of the presence of a combination of two or more noble metals.

The second catalytic layer preferably contains tin or an oxide thereof, and the average molar percentage on a metal element basis is in the range of 4% to 12%. When the concentration of the tin component varies in a direction perpendicular to the interface between the first layer and the second layer, the tin concentration is an average of the concentration distribution through the second catalytic layer.

Thus, in a preferred embodiment, the second catalytic layer consists, apart from unavoidable impurities, of platinum and tin and optionally palladium and/or rhodium, in molar percentages, calculated as metallic elements, ranging from 48% to 96% of platinum, from 4% to 12% of tin, from 0% to 24% of palladium and from 0% to 24% of rhodium.

According to a preferred embodiment of the above electrode, the first catalytic layer comprises a metal or metal oxide of iridium, ruthenium, tin, in molar percentages, calculated as metal element, Ru 24-34%, Ir 3-13%, Sn 30-70%.

The first catalytic layer preferably contains platinum or an oxide thereof, and the average molar percentage on a metal element basis is in the range of 3% to 10%. Since the concentration of the platinum component varies in a direction perpendicular to the interface between the first layer and the second layer, the platinum concentration is an average of the concentration distribution through the first catalytic layer.

It goes without saying that the person skilled in the art will select the molar percentages of the individual elements in such a way that the sum of the molar percentages of the components is 100. In particular, if no other metal is present in the first catalytic layer, tin or tin oxide is preferably present in a concentration (in terms of metal element) of 55% to 70%.

In another embodiment, said first catalytic layer comprises another valve metal selected from titanium, tantalum and niobium, in an amount ranging between 30% and 40% expressed in molar percentage of the metallic elements; in fact, it has been observed how the presence of another valve metal, such as titanium, combines good catalytic activity with a significant increase in the electrode resistance during the course of the current reversal required.

In a preferred embodiment, the first catalytic layer consists, apart from unavoidable impurities, of iridium, ruthenium, tin and platinum, and optionally titanium, in molar percentages, calculated on the metallic element, ranging from 3% to 13% iridium, from 24% to 34% ruthenium, from 30% to 70% tin, from 3% to 10% platinum and from 30% to 40% titanium.

The inventors have observed that, surprisingly, in the catalytic coating described above, diffusion phenomena between the layers occur: the tin of the first catalytic layer diffuses into the second layer and the platinum of the second catalytic layer diffuses into the first layer. Diffusion of tin into the second catalytic layer occurs across the concentration gradient such that the amount of tin in the second catalytic layer is greatest at the interface between the two catalytic layers and decreases towards the outer surface of the second catalytic layer.

The presence of tin diffused into the second catalytic layer may advantageously slow down the consumption of the noble metals present in the second catalytic layer, so that the optimized performance characteristics in terms of catalytic activity and selectivity can be maintained for a longer time without impairing the catalytic performance.

Likewise, the diffusion of platinum from the second catalytic layer into the first catalytic layer is such that the amount of platinum in the first catalytic layer is greatest at the interface between the two catalytic layers and gradually decreases towards the inner surface of the first catalytic layer.

The diffusion of platinum into the first catalytic layer allows the catalytic activity to be enhanced. This further allows better catalytic performance characteristics to be maintained throughout the life of the electrode in the event that prolonged use of the electrode causes the second layer to wear away over time. The presence of the elements and the particular structure of the catalytic coating allows better performance characteristics to be guaranteed with respect to the prior art and has the further advantage of increasing the working life of the electrode.

Furthermore, surprisingly, the electrode according to the invention allows to maintain better performance characteristics in terms of activity and selectivity over time.

The presence of tin has a great influence on the selectivity; however, if there is a large amount of tin bound to the platinum on the outer surface of the catalytic coating, the increase in catalytic activity of the platinum itself is reduced.

The diffusion of tin from the first catalytic layer to the second catalytic layer produces a concentration distribution of the elements between the layers, which enables a high catalytic activity and an optimal selectivity to be maintained, and a slow consumption of the noble metals present in the second catalytic layer. The tin concentration distribution between the two catalytic layers is characterized in that the concentration of the element in the second layer decreases monotonically in the direction opposite to the first layer.

In another embodiment, the first catalytic layer has a molecular weight of at least 3g/m²And 8g/m²A specific loading of noble metal in the range between, the second catalytic layer having a specific loading of noble metal of 0.8g/m²And 4g/m²A specific loading of noble metal in the range therebetween. The inventors have found that such reduced noble metal loadings are sufficient to impart optimum catalytic activity.

According to another aspect, the invention relates to a method for obtaining an electrode for evolving gaseous products in an electrolytic cell, for example chlorine in an alkaline brine electrolytic cell, comprising the following steps:

applying a platinum-free first solution comprising a mixture of iridium, ruthenium and tin to a valve metal substrate, followed by drying at 50-60 ℃, and decomposing the first solution by heat treatment at 400-650 ℃ for a period of 5 to 30 minutes;

repeating step a) until said first catalytic composition is obtained at the desired specific loading of noble metal;

applying a tin-free second catalytic solution containing platinum, followed by drying at 50-60 ℃, and decomposing the first solution by heat treatment at 400-650 ℃ for a period of 5 to 30 minutes;

repeating step c) until said first catalytic composition is obtained at the desired specific loading of noble metal.

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

In one embodiment, the first solution further comprises titanium.

In another embodiment, the second solution comprises palladium and rhodium, either by themselves or in combination with each other.

In a preferred embodiment of the invention, the two-layer electrode is subjected to a final heat treatment. In one embodiment, the final heat treatment is carried out at a temperature between 400 ℃ and 650 ℃, preferably at a temperature around 500 ℃ for at least 60 minutes, preferably between 60 and 180 minutes, more preferably between 80 and 120 minutes.

Preferably, the first solution comprises iridium, ruthenium and tin compounds, and optionally a titanium compound in the form of an organometallic complex. In one embodiment, the organometallic complexes are acetyl hydroxychloride complexes of tin, ruthenium, iridium, and optionally titanium, respectively.

Without wishing to be bound to a particular scientific theory, the steps a and c of the thermal treatment or decomposition of the above-described method, as well as the elements present in said first and said second solution and their concentrations are possible, since their diffusion coefficients also depend on the temperature, facilitating the mutual diffusion of the tin and platinum, respectively present, from the first catalytic layer to the second catalytic layer and vice versa.

According to another aspect, the invention relates to an electrolytic cell for the electrolysis of alkaline chloride solutions, comprising an anode compartment and a cathode compartment, wherein the anode compartment is equipped with an electrode, such as one of the forms described above, serving as anode for the evolution of chlorine.

According to another aspect, the invention relates to an industrial electrolyser for producing chlorine and alkali metals from alkali chloride solutions, when also lacking a bias protection device and comprising an electrolytic cell with a modular arrangement of an anode compartment and a cathode compartment separated by an ion exchange membrane or diaphragm, wherein the anode compartment comprises an electrode serving as anode in one of the forms described above.

The following examples are included to demonstrate particular embodiments of the invention, the usefulness of which has been fully verified within the claimed range of values. It will be clear to those skilled in the art that the compositions and techniques described in the examples below represent well-worked compositions and techniques of the invention encountered in practice by the inventors; however, those of skill in the art will further appreciate that various modifications may be made to the various embodiments described in light of the present disclosure without departing from the scope of the present disclosure, while still producing the same or similar results.

Example 1

Titanium mesh of dimensions 10cm x 10cm was washed three times in deionized water at 60 ℃ with each liquid change. After washing, the plate was heat-treated at 350 ℃ for 2 hours. The web was then treated in 20% HCl solution and boiled for 30 minutes.

100ml of a first acetic acid solution containing tin complex acetylhydroxychloride, ruthenium complex acetylhydroxychloride and iridium complex acetylhydroxychloride are then prepared, the molar composition, calculated as metal, being equal to 25% Ru, 11% Ir and 64% Sn.

A second solution was also prepared containing an amount of Pt diamino dinitrate Pt (NH)₃)₂(NO₃)₂Corresponding to 40g of Pt dissolved in 160ml of glacial acetic acid, then made up to a volume of 1 litre with 10% by weight of acetic acid.

An 8 coat first acetic acid solution was applied to the titanium mesh by brushing. After each coating, a drying step was performed at 50 ℃ to 60 ℃ for about 10 minutes, followed by heat treatment at 500 ℃ for 10 minutes. The web was cooled in air each time before the next coating was applied.

The procedure was repeated until a loading (in terms of metal) equal to 7g/m, expressed as the sum of Ir and Ru, was reached²。

Subsequently, 4 coats of the second solution were applied by brushing. After each coating, a drying step was performed at 50 ℃ to 60 ℃ for about 10 minutes, followed by heat treatment at 500 ℃ for 10 minutes. The web was cooled in air each time before the next coating was applied.

The procedure was repeated until a total loading of Pt equal to 2.5g/m was reached²。

Finally, a final heat treatment was performed at 500 ℃ for 100 minutes.

The electrode thus obtained is labeled as sample # 1.

Example 2

100ml of a first acetic acid solution containing tin complex acetylhydroxychloride, ruthenium complex acetylhydroxychloride and iridium complex acetylhydroxychloride with a molar composition, calculated as metal, equal to 26% Ru, 10% Ir and 64% Sn were then prepared.

A second acetic acid solution of 100ml was also prepared, containing an organometallic complex of platinum and an organometallic complex of palladium and having a molar composition equal to 87% Pt and 13% Pd, calculated as metals.

The procedure was repeated until a loading (in terms of metal) equal to 6.7g/m, expressed as the sum of Ir and Ru, was reached²。

Subsequently, 4 coats of the second acetic acid solution were applied by brushing. After each coating, a drying step was performed at 50 ℃ to 60 ℃ for about 10 minutes, followed by heat treatment at 500 ℃ for 10 minutes. The web was cooled in air each time before the next coating was applied.

The procedure was repeated until a total noble metal loading, expressed as the sum of Pt and Pd on a metal basis, equal to 2.7g/m was reached²。

Finally, a final heat treatment was performed at 500 ℃ for 100 minutes.

The electrode thus obtained is labeled as sample # 2.

Example 3

Then, 100ml of a second acetic acid solution containing an organometallic complex of platinum, an organometallic complex of palladium and RhCl was prepared₃And a molar composition, calculated as metal, equal to 86% Pt, 10% Pd and 4% Rh.

The procedure was repeated until a total noble metal loading (in terms of metal) equal to 2.8g/m, expressed as the sum of Pt, Pd and Rh, was reached²。

Finally, a final heat treatment was performed at 500 ℃ for 100 minutes.

The electrode thus obtained is labeled as sample # 3.

Example 4

Then 100ml of a first acetic acid solution was prepared containing tin complex acetylhydroxychloride, ruthenium complex acetylhydroxychloride, iridium complex acetylhydroxychloride and titanium complex acetylhydroxychloride with a molar composition equal to 25% Ru, 10% Ir, 35% Sn and 30% Ti, calculated as metals.

8 layers of the first acetic acid solution were applied to the titanium mesh by brushing. After each coating, a drying step was performed at 50 ℃ to 60 ℃ for about 10 minutes, followed by heat treatment at 500 ℃ for 10 minutes. The web was cooled in air each time before the next coating was applied.

Subsequently, 4 layers of the second acetic acid solution were applied by brushing. After each coating, a drying step was performed at 50 ℃ to 60 ℃ for about 10 minutes, followed by heat treatment at 500 ℃ for 10 minutes. The web was cooled in air each time before the next coating was applied.

The procedure was repeated until a total noble metal loading (in terms of metal) equal to 2.7g/m, expressed as the sum of platinum and palladium, was reached²。

Finally, a final heat treatment was performed at 500 ℃ for 100 minutes.

The electrode thus obtained is labeled as sample # 4.

Example 5

A second acetic acid solution of 100ml containing an organometallic complex of platinum, an organometallic complex of palladium and RhCl was also prepared₃And a molar composition, calculated as metal, equal to 86% Pt, 10% Pd and 4% Rh.

The procedure was repeated until a total noble metal loading (in terms of metal) equal to 2.7g/m, expressed as the sum of Pt, Pd and Rh, was reached²。

Finally, a final heat treatment was performed at 500 ℃ for 100 minutes.

The electrode thus obtained is labeled as sample # 5.

Comparative example 1

100ml of a water-alcohol solution containing RuCl in isopropanol solution was then prepared₃*3H₂O、H₂IrCl₆*6H₂O、TiCl₃The molar composition is equal to 23% Ru, 22% Ir and 55% Ti.

14 layers of this solution were applied to the titanium mesh by brushing. After each coating, a drying step was performed at 50 ℃ to 60 ℃ for about 10 minutes, followed by heat treatment at 500 ℃ for 10 minutes. The workpiece was cooled in air each time before the next coating was applied.

The procedure is repeated until a total noble metal loading, expressed as metal as the sum of Ir and Ru, equal to 11g/m is reached². Then, the final heat treatment was performed at 500 ℃ for 100 minutes.

The electrode thus obtained is labeled as sample # 1C.

Comparative example 2

A second acetic acid solution of 100ml was also prepared containing an organometallic complex of platinum and a tin complex of acetylhydroxychloride and having a molar composition equal to 87% Pt and 13% Sn, calculated as metals.

A 6 coat first acetic acid solution was applied to the titanium mesh by brushing. After each coating, a drying step was performed at 50 ℃ to 60 ℃ for about 10 minutes, followed by heat treatment at 500 ℃ for 10 minutes. The web was cooled in air each time before the next coating was applied.

The procedure is repeated until a total noble metal loading, expressed as metal as the sum of Ir and Ru, equal to 6g/m is reached²。

The procedure was repeated until a total noble metal loading (in terms of metal) equal to 2.5g/m, expressed as Pt, was reached²。

Finally, a final heat treatment was performed at 500 ℃ for 100 minutes.

The electrode thus obtained is labeled as sample # 2C.

Comparative example 3

Then 100ml of a first acetic acid solution was prepared containing tin complex acetylhydroxychloride, ruthenium complex acetylhydroxychloride, iridium complex acetylhydroxychloride and organometallic complex of platinum, with a molar composition equal to 25% Ru, 10% Ir, 35% Sn and 30% Pt, calculated as metals.

The acetic acid solution was applied to the titanium mesh by brushing for 10 coats. After each coating, a drying step was performed at 50 ℃ to 60 ℃ for about 10 minutes, followed by heat treatment at 500 ℃ for 10 minutes. The web was cooled in air each time before the next coating was applied.

The procedure was repeated until a total noble metal loading (in terms of metal) equal to 8g/m, expressed as the sum of Ir, Ru and Pt, was reached²。

Finally, a final heat treatment was performed at 500 ℃ for 100 minutes.

The electrode thus obtained is labeled as sample # 3C.

Comparative example 4

100ml of a first aqueous-alcoholic solution containing RuCl in a mixture of water and 1-butanol acidified with HCl is then prepared₃*3H₂O、H₂IrCl₆*6H₂O、TiOCl₂The molar composition, calculated as metal, is equal to 26% Ru, 23% Ir, 51% Ti.

A second aqueous-alcoholic solution of 100ml is also prepared, containing H₂PtCl₆And PdCl₂。

The procedure was repeated until a total noble metal loading (in terms of metal) equal to 6g/m, expressed as the sum of Ir and Ru, was reached²。

The procedure was repeated until a total noble metal loading (in terms of metal) equal to 3g/m, expressed as the sum of Pt + Pd, was reached²。

Finally, a final heat treatment was performed at 500 ℃ for 100 minutes.

The electrode thus obtained is labeled as sample # 4C.

The samples of the examples and of the comparative examples are characterized as anodes for chlorine evolution in a laboratory cell filled with an aqueous solution of sodium chloride salt at a concentration of 200 g/l.

Table 1 reports the values at 4kA/m²And the volume percentage of oxygen in the generated chlorine.

TABLE 1

Test specimen	Potential of electrolytic cell (V)	O₂/Cl₂(Vol％)
			1	2.76	0.9
2	2.76	0.7
			3	2.76	0.7
4	2.77	0.8
			5	2.77	0.7
1C	2.78	1.2
			2C	2.76	1.0
3C	2.77	1.5
			4C	2.76	0.8

The samples of the previous examples also underwent handling tests in beakers. In Table 2, the anodic potential (CISEP) measured at a temperature of 80 ℃ in a sodium chloride solution with a concentration of 200g/l, corrected for 3kA/m, is reported²Ohmic drop in current density. In addition, in order to evaluate the selectivity of the chlorine reaction,at 3kA/m²At a current density of (a), tests were carried out in sulfuric acid; the reported anode potential (CISEP) has been corrected for ohmic drop. The higher the value of the anodic potential measured in sulphuric acid, the greater the selectivity of the chlorine reaction.

TABLE 2

Finally, some of the samples were subjected to life tests. The life test mentioned is a simulation carried out in an electrolytic cell divided by industrial electrolysis conditions. Table 3 reports the cell voltage of the samples at the start of the test and after one year of simulation as being at 8kA/m²Catalytic activity of evolved chlorine (Cl O.V.) measured at current density and an indicator simulating the percentage of remaining loading of the second catalytic layer after one year.

TABLE 3

The foregoing description is not intended to limit the invention, which may be used according to various embodiments without departing from the objects, and whose scope is defined solely by the appended claims.

In the description and claims of this application, the terms "comprising," "including," and "containing" are not intended to exclude the presence of other additional elements, components, or process steps.

The discussion of documents, items, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.

Claims

1. An electrode for evolving gases in an electrolysis process comprising a valve metal substrate and a coating comprising a first catalytic layer formed on the substrate, the first catalytic layer containing a mixture of iridium, ruthenium, tin and platinum or their oxides or combinations thereof, and a second catalytic layer formed on the first catalytic layer, the second catalytic layer containing platinum and tin or their oxides or combinations thereof, wherein the tin of the second catalytic layer is present at a concentration that is reduced from the interface with the first catalytic layer, and wherein the platinum of the first catalytic layer is present at a concentration that is reduced from the interface with the second catalytic layer.

2. An electrode for evolving gases in an electrolytic process comprising a valve metal substrate and a coating comprising a first catalytic layer formed on said substrate, said first catalytic layer containing a mixture of iridium, ruthenium, tin and platinum or their oxides or combinations thereof, and a second catalytic layer formed on said first catalytic layer, said second catalytic layer containing platinum and tin or their oxides or combinations thereof, wherein said first layer is obtained from a platinum-free first precursor solution comprising a mixture of iridium, ruthenium and tin, applied to said substrate and heat-treated, wherein said second catalytic layer is obtained from a tin-free second catalytic composition containing platinum, applied to said substrate and heat-treated.

3. The electrode according to one of claims 1 or 2, wherein the second catalytic layer contains, in mole percent, 48-96% of Pt as metal or its oxide form, calculated as metallic element.

4. The electrode according to one of claims 1 to 3, wherein the second catalytic layer contains metals or their oxides or a combination thereof in molar percentage, metals or their oxides in the form of Pd 0-24% or Rh 0-24%, calculated as metallic element.

5. The electrode according to one of claims 1 to 4, wherein the second catalytic layer contains, on average, in molar percentage, 4-12% Sn in the form of a metal or an oxide thereof, calculated as metallic element.

6. The electrode according to any one of the preceding claims, wherein the iridium, ruthenium and tin oxides of the first catalytic layer are present in mole percentages Ru-24%, Ir-3-13%, Sn-30-70%, calculated as metallic element.

7. The electrode according to any one of the preceding claims, wherein the first catalytic layer further contains titanium oxide in a molar percentage of Ti-30-40%, calculated as metallic element.

8. The electrode according to any one of the preceding claims, wherein the first catalytic layer contains, on average, in mole percent, Pt-3-10% of the metal or its oxide form, calculated as metallic element.

9. The electrode according to any one of the preceding claims, wherein the valve metal substrate is selected from titanium, tantalum, zirconium, niobium, tungsten, aluminium, silicon or alloys thereof.

10. A method of producing an electrode as defined in one of the preceding claims, comprising the steps of:

-applying a platinum-free first solution comprising a mixture of iridium, ruthenium and tin to a valve metal substrate, followed by drying at 50-60 ℃ and decomposing said first solution by heat treatment at 400-650 ℃ for a time of 5 to 30 minutes;

-repeating step a) until the desired specific noble metal loading is reached;

-applying a second catalytic solution free of tin containing platinum, followed by drying at 50-60 ℃ and decomposition of said second solution by heat treatment at 400-650 ℃ for a time comprised between 5 and 30 minutes;

-repeating step c) until the desired specific noble metal loading is reached.

11. The method of claim 10, wherein the temperature of the thermal decomposition in steps a) and c) is between 480 ℃ and 550 ℃.

12. The method according to one of claims 10 or 11, wherein the first solution contains the iridium, ruthenium and tin in the form of organometallic complexes.

13. An electrolysis cell for the electrolysis of alkaline chloride solutions comprising an anode compartment and a cathode compartment, wherein the anode compartment is provided with an electrode according to any one of claims 1 to 8.

14. An electrolysis cell according to claim 13, wherein the anode and cathode compartments are separated by a diaphragm or an ion exchange membrane.

15. An electrolyzer for producing chlorine and alkali metal from an alkali metal chloride solution comprising modularly arranged electrolysis cells, wherein each electrolysis cell is an electrolysis cell according to claim 13.