BR112021010421A2 - Electrolytic gas evolution electrode - Google Patents

Abstract

ELECTRODE FOR ELECTROLYTIC EVOLUTION OF GAS. The invention relates to an electrode for gas evolution in electrolytic processes comprising a valve metal substrate and a catalytic coating comprising two layers. a first layer comprising valve metal oxides, ruthenium and iridium and a second layer comprising one or more metals chosen from among platinum group elements.

Description

"ELECTRODE FOR ELECTROLYTIC EVOLUTION OF GAS" FIELD OF THE INVENTION

[0001] The invention relates to an electrode for gas evolution in electrolytic processes comprising a valve metal substrate and a catalytic coating comprising two layers. A first layer comprising valve metal, oxides of ruthenium and iridium and a second layer comprising one or more metals chosen from platinum group elements.

BACKGROUND OF THE INVENTION

[0002] The field of the invention relates to the preparation of a catalytic coating for electrodes used in brine electrolysis processes. This coating is applied to a metal substrate, typically titanium or other valve metal.

[0003] Over the years, brine electrolysis technology has been subjected to innovations for an efficient implementation from an energy point of view and from the cost/benefit of the use of resources. In this increasingly challenging context, anode optimization plays a key role. In particular, numerous efforts have been made in order to reduce the anode overvoltage in the generation of chlorine and in order to reduce the oxygen concentration in the chlorine gas generated and therefore produce chlorine gas with a high purity.

[0004] An additional difficulty lies in obtaining an electrode capable of maintaining high performance for a long period of time.

[0005] In general, the processes for electrolysis of brines, for example, brines of alkali chloride such as sodium chloride, for the production of chlorine and caustic soda, are carried out with anodes made of titanium or other valve metal, activated with a surface layer of ruthenium dioxide (RuO2) optionally mixed with tin dioxide (SnO2) and another noble metal, as, for example, described in EP0153586. Consequently, it is possible to obtain a reduction in the overvoltage of the chlorine evolution anodic reaction and therefore in the overall energy consumption.

[0006] The newly described formulation, along with the other tin-containing formulations, however, has the problem of also reducing the overvoltage of the concomitant oxygen development reaction, leading to the production of chlorine gas contaminated with an excessive amount of oxygen. .

[0007] Another partial improvement in performance is obtained by applying to a metal substrate a formulation based on RuO2 and SnO2 combined with a reduced amount of IrO2 as, for example, described in WO2016083319. A similar formulation allows optimal values of cell potential and moderate amounts of oxygen to be obtained.

[0008] Other prior art coatings, such as the formulation described in WO2012081635 comprising two catalytic coatings, the first containing titanium and noble metal oxides and the second containing an alloy of platinum and palladium, also allow ideal values of potential of cell and reduced amounts of oxygen in chlorine gas are obtained; however, they do not provide the electrode with an ideal resistance capable of maintaining superior levels of performance, in relation to catalytic activity and selectivity, for an adequate period of time.

[0009] US 2013/0186750 A1 describes an electrode suitable for chlorine evolution which has alternating layers of two distinct compositions, namely one type of layers comprising iridium, ruthenium and valve metals and another type of layer comprising oxides of iridium, ruthenium and tin.

[0010] US 2013/0334037 A1 describes an electrode for electrolysis including a conductive substrate, a first layer formed on the conductive substrate containing at least one oxide selected from ruthenium oxide, iridium oxide and titanium oxide and a second layer formed on the first layer containing an alloy of platinum and palladium.

[0011] US 4,626,334 describes an anode comprising an electroconductive substrate provided with a coating of solid solution of (Ru-Sn)O2 for electrolysis of brine.

[0012] JP S62243790 describes an electrode having a first coating layer comprising a mixture of platinum and iridium oxide and a second coating layer comprising a mixture of ruthenium oxide and tin oxide.

[0013] Therefore, there is an evident need to identify a new catalytic coating for electrodes for the evolution of gaseous products in electrolytic cells in brine electrolysis processes, characterized by a higher level of catalytic activity and a high resistance capable of sustaining higher levels performance for a longer period of time under common operating conditions over prior art formulations.

SUMMARY OF THE INVENTION

[0014] Various aspects of the present invention are described in the appended claims.

[0015] The present invention relates to an electrode for evolution of gaseous products in electrolytic cells, for example, for evolution of chlorine in alkaline brine electrolysis cells, comprising a catalytic coating applied on a metal substrate. In the present context, the term catalytic coating denotes two different catalytic layers with different catalytic compositions wherein the first catalytic layer formed on the substrate comprises at least a mixture of iridium, ruthenium, tin and platinum or oxides thereof or combinations thereof and a second catalytic layer formed on the first catalytic layer comprises platinum and tin or oxides thereof or combinations thereof. The tin of the second catalytic layer is present in decreasing concentration from the interface with said first catalytic layer towards the upper surface of the second catalytic layer, that is, the surface opposite the interface with the first catalytic layer, and the platinum of said first catalytic layer is present in decreasing concentration from the interface with said second catalytic layer towards the substrate.

[0016] The present invention also relates to an electrode for evolution of gaseous products in electrolytic cells, for example for evolution of chlorine in alkaline brine electrolysis cells, comprising a valve metal substrate and a coating comprising a first layer catalytic layer formed on said substrate containing a mixture of iridium, ruthenium, tin and platinum or oxides thereof or combinations thereof and a second catalytic layer formed on said first catalytic layer containing platinum and tin or oxides thereof or combinations thereof, wherein said The first layer is obtained from a first platinum-free 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 second tin-free catalytic solution containing platinum, applied to said first catalytic layer and subjected to a treatment the heat. 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 that the tin concentration in the second solution is at least an order of magnitude lower than the average tin concentration in the second layer obtained from the second solution. Preferably, a platinum-free solution contains at most platinum as an impurity and a tin-free solution contains tin at most as an impurity.

[0017] This double layer structure, applied to a metal substrate, typically titanium, titanium alloy or other valve metal, allows energy savings to be combined with excellent purity of chlorine gas produced while maintaining characteristics optimal performance in terms of catalytic activity and selectivity over a long period of time.

[0018] The first catalytic layer, formed on the substrate, preferably comprises ruthenium oxide, iridium oxide, tin oxide and metallic platinum or oxides thereof. RuO2 is widely known for its excellent catalytic activity and its stability in an alkaline medium which is enhanced by the presence of IrO2; the presence of SnO2 guarantees a slower consumption of the noble metals present.

[0019] The second catalytic layer, formed in the first layer, comprises tin or its oxides and one or more metals chosen from among the platinum group elements, especially platinum itself, which are known to increase selectivity and reduce energy consumption. .

[0020] The inventors have observed that an electrode with a similar catalytic coating, wherein said second catalytic layer comprises platinum in a molar percentage with reference to the metallic element in the range between 48 and 96% (or, not taking the tin component in consideration, of 50 and 99.999 %) in the form of the metal or its oxide, can offer the advantage of subsequently reducing the overvoltage of the reaction for chlorine evolution.

[0021] In the context of the present invention, ranges denoted by "from" or "between" include the specified upper and lower limits, respectively.

[0022] In another embodiment, in addition to platinum and tin, said second catalytic layer comprises palladium or rhodium in the form of metals or their oxides, or combinations thereof, in molar percentage with reference to metallic elements in the range between 0 and 24 % (or, not taking the tin component into account, between 0 and 25 %), where the elements are in the form of metals or oxides thereof. This can guarantee a high catalytic activity due to the combined presence of two or more noble metals.

[0023] The second catalytic layer preferably comprises tin or its oxide in an average molar percentage with respect to the metallic element in a range of 4 to 12%. Since the concentration of the tin component varies in a direction perpendicular to the interface between the first and second layers, the tin concentration is an average of the concentration profile across the second catalytic layer.

[0024] Therefore, in a preferred embodiment, in addition to unavoidable impurities, the second catalytic layer consists of platinum and tin, and optionally palladium and/or rhodium, in mole percentage with reference to metallic elements in the ranges of 48 to 96% platinum , from 4 to 12% of tin, from 0 to 24% of palladium and from 0 to 24% of rhodium.

[0025] According to a preferred embodiment of the aforementioned electrode, the first catalytic layer comprises metals or metal oxides of iridium, ruthenium, tin in mole percentages of Ru = 24-34 %, Ir = 3-13 %, Sn = 30 -70 % with reference to metallic elements.

[0026] The first catalytic layer preferably comprises platinum or its oxide in an average molar percentage with respect to the metallic element in a range of 3 to 10%. Since the concentration of the platinum component varies in a direction perpendicular to the interface between the first and second layers, the platinum concentration is an average of the concentration profile across the first catalytic layer.

[0027] Obviously those skilled in the art will select the molar percentages of the individual elements in such a way that the sum total of the molar percentages of the components is 100. Especially, if no other metal is present in the first catalytic layer, oxides of Sn or Sn are preferably present in a concentration of 55-70% with reference to the metallic element.

[0028] In another embodiment, said first catalytic layer comprises another valve metal chosen from titanium, tantalum and niobium, in an amount, expressed in molar percentage, in the range between 30 and 40% with reference to the metallic element; in fact, it has been observed how the presence of another valve metal such as titanium allows good catalytic activity to be combined with a substantial increase in electrode resistance in processes that require current reversal.

[0029] In a preferred embodiment, in addition to unavoidable impurities, the first catalytic layer consists of iridium, ruthenium, tin and platinum and optionally titanium, in molar percentage with reference to metallic elements in the ranges of 3 to 13% of iridium, of 24 34% ruthenium, 30 to 70% tin, 3 to 10% platinum and 30 to 40% titanium.

[0030] The inventors observed that, surprisingly, in the catalytic coating described above, an interlayer diffusion phenomenon occurs: tin from the first catalytic layer diffuses into the second layer, while platinum from the second catalytic layer diffuses into the first layer. The diffusion of tin in the second catalytic layer occurs through a concentration gradient, so that the amount of tin in the second catalytic layer is maximum at the interface between the two catalytic layers and decreases towards the outer surface of the second catalytic layer.

[0031] The presence of tin diffused in the second catalytic layer can advantageously delay the consumption of the noble metals present in the second catalytic layer, allowing ideal performance characteristics in terms of catalytic activity and selectivity to be maintained for a longer period of time, without compromising catalytic performance.

[0032] Similarly, 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 maximum at the interface between the two catalytic layers and gradually decreases towards the inner surface of the first. catalytic layer.

[0033] The diffusion of platinum in the first catalytic layer allows the catalytic activity to be enhanced. This additionally allows for better catalytic performance characteristics to be maintained over the lifetime of the electrode, also where prolonged use of the electrode causes wear of the second layer over time. The elements present and the particular structure of the catalytic coating allow better performance characteristics over the prior art to be guaranteed with the added advantage of increasing the operating life of the electrode.

[0034] The electrode according to the invention still surprisingly allows the best performance characteristics in terms of activity and selectivity to be maintained over time.

[0035] The presence of tin has a high impact on selectivity; however, if tin is present in high amounts on the outer surface of the catalytic coating, in combination with the platinum, it attenuates the increase in the catalytic activity of the platinum itself.

[0036] The diffusion of tin from the first catalytic layer to the second produces an element concentration profile between the layers, which allows a high catalytic activity together with an ideal selectivity to be maintained, still allowing the consumption of the noble metals present in the second catalytic layer is decelerated. The tin concentration profile between the two catalytic layers is characterized by a monotonic reduction in the concentration of the element within the second layer in the opposite direction to the first layer.

[0037] In another embodiment, the first catalytic layer has a noble metal specific load in the range between 3 and 8 g/m² and the second catalytic layer has a noble metal specific load in the range between 0.8 and 4 g/m² . The inventors have found that thus reduced loads of noble metal are more than sufficient to impart optimal catalytic activity.

[0038] According to another aspect, the present invention relates to a method for obtaining an electrode for evolution of gaseous products in electrolytic cells, for example, for evolution of chlorine in alkaline brine electrolysis cells, comprising the following stages:

[0039] application to a valve metal substrate of a first platinum-free solution comprising a mixture of iridium, ruthenium and tin, subsequent drying at 50-60°C and decomposition of said first solution by heat treatment at 400-650°C °C for a period of 5 to 30 minutes;

[0040] repetition of stage a) until said first catalytic composition is obtained with a desired specific noble metal charge;

[0041] application of a second tin-free catalytic solution containing platinum being subsequently dried at 50-60°C and decomposition of said first solution by heat treatment at 400-650°C for a period of 5 to 30 minutes;

[0042] repetition of stage c) until said first catalytic composition is obtained with a desired specific noble metal charge.

[0043] In one embodiment, the temperature of said thermal decomposition in steps a) and c) is between 480 and 550 °C.

[0044] In one embodiment, said first solution further comprises titanium.

[0045] In another embodiment, said second solution comprises palladium and rhodium alone or in combination with each other.

[0046] In a preferred embodiment of the present 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 °C, preferably at a temperature of about 500 °C, for at least 60 minutes, preferably between 60 and 180 minutes, more preferably between 80 and 120 minutes.

[0047] Preferably, the first solution comprises the iridium, ruthenium and tin compounds and optionally the titanium compounds in the form of organometallic complexes. In one embodiment, the organometallic complexes are aceto-hydrochloride complexes of tin, ruthenium, iridium and optionally titanium, respectively.

[0048] Without wishing to be limited to a particular scientific theory, it is possible that stages a and c for heat treatment or decomposition of the method described above, together with the elements present and the concentrations thereof within said first and said second solutions, since their diffusion coefficient is also temperature dependent, they contribute to the interdiffusion of tin and platinum present respectively from the first catalytic layer to the second catalytic layer and vice versa.

[0049] According to another aspect, the invention relates to a cell for the electrolysis of chlor-alkali solutions comprising an anode compartment and a cathode compartment wherein the anode compartment is equipped with the electrode in one of the ways as per described above, used as an anode for chlorine evolution.

[0050] According to another aspect, the invention relates to an industrial electrolyzer for the production of chlorine and alkali from chloride-alkali solutions, when still lacking polarization protection devices and comprising a modular arrangement of electrolytic cells with the anode and cathode compartments separated by ion exchange membranes or diaphragms, wherein the anode compartment comprises an electrode in one of the forms as described above used as an anode.

[0051] The following examples are included in order to demonstrate particular embodiments of the invention, the practicability of which has been largely verified within the claimed range of values. It will be apparent to those skilled in the art that the compositions and techniques described in the examples below represent compositions and techniques for which the inventors have found good operation of the invention in practice; however, those skilled in the art will further note, in light of the present description, that various modifications can be made to the various described embodiments still yielding identical or similar results without departing from the scope of the invention. EXAMPLE 1

[0052] A titanium mesh measuring 10 cm x 10 cm was washed three times in deionized water at 60 °C, changing the liquid each time. Washing was followed by a heat treatment for 2 hours at 350 °C. The screen was then subjected to a treatment in a 20% HCl solution, boiling for 30 minutes.

[0053] 100 ml of a first acetic solution was then prepared containing tin acetate-hydrochloride complex, ruthenium acetate-hydrochloride complex and iridium acetate-hydrochloride complex and having a molar composition equal to 25% Ru, 11% of Ir and 64% of Sn with reference to metals.

[0054] A second solution was also prepared containing an amount of Pt diamino dinitrate, Pt(NH3)2(NO3)2 corresponding to 40 g of Pt dissolved in 160 ml of glacial acetic acid and then made up to a volume of one liter with 10% by weight acetic acid.

[0055] The first acetic solution was applied to the titanium mesh by painting in 8 coats. After each coating, a drying step at 50-60 °C was performed for about 10 minutes, then a heat treatment for 10 minutes at 500 °C, each time being cooled in air before application of the coating. next coating.

[0056] The procedure was repeated until a charge expressed as the sum of Ir and Ru with reference to metals equal to 7 g/m² was reached.

[0057] Subsequently, the second solution was applied by painting in 4 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0058] The procedure was repeated until a total Pt load equal to 2.5 g/m² was reached.

[0059] A final heat treatment at 500 °C for 100 minutes was performed last.

[0060] The electrode obtained in this way was identified as specimen nº 1. EXAMPLE 2

[0061] A titanium mesh measuring 10 cm x 10 cm was washed three times in deionized water at 60 °C, changing the liquid each time. Washing was followed by a heat treatment for 2 hours at 350 °C. The screen was then subjected to a treatment in a 20% HCl solution, boiling for 30 minutes.

[0062] 100 ml of a first acetic solution were,

then, preparations containing tin acetate-hydrochloride complex, ruthenium acetate-hydrochloride complex and iridium acetate-hydrochloride complex and having a molar composition equal to 26% Ru, 10% Ir and 64% Sn with reference to metals.

[0063] 100 ml of a second acetic solution 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 with reference to the metals.

[0064] The first acetic solution was applied to the titanium mesh by painting in 8 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0065] The procedure was repeated until a charge expressed as the sum of Ir and Ru with reference to metals equal to 6.7 g/m² was reached.

[0066] Subsequently, the second acetic solution was applied by painting in 4 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0067] The procedure was repeated until a total noble metal charge expressed as the sum of Pt and Pd with reference to the metals equal to 2.7 g/m² was reached.

[0068] Finally, a final heat treatment at 500 °C for 100 minutes was performed.

[0069] The electrode obtained in this way was identified as specimen No. 2. EXAMPLE 3

[0070] A titanium mesh measuring 10 cm x 10 cm was washed three times in deionized water at 60 °C, changing the liquid each time. Washing was followed by a heat treatment for 2 hours at 350 °C. The screen was then subjected to a treatment in a 20% HCl solution, boiling for 30 minutes.

[0071] 100 ml of a first acetic solution were then prepared containing tin acetate-hydrochloride complex, ruthenium acetate-hydrochloride complex and iridium acetate-hydrochloride complex and having a molar composition equal to 26% Ru, 10% of Ir and 64% of Sn with reference to metals.

[0072] 100 ml of a second acetic solution was then prepared containing an organometallic complex of platinum, an organometallic complex of palladium and RhCl3 and having a molar composition equal to 86% Pt, 10% Pd and 4% Rh with reference to metals.

[0073] The first acetic solution was applied to the titanium mesh by painting in 8 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0074] The procedure was repeated until a charge expressed as the sum of Ir and Ru with reference to metals equal to 6.7 g/m² was reached.

[0075] Subsequently, the second acetic solution was applied by painting in 4 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0076] The procedure was repeated until a total noble metal charge expressed as the sum of Pt, Pd and Rh with reference to the metals equal to 2.8 g/m² was reached.

[0077] Finally, a final heat treatment at 500 °C for 100 minutes was performed.

[0078] The electrode obtained in this way was identified as specimen nº 3. EXAMPLE 4

[0079] A titanium mesh measuring 10 cm x 10 cm was washed three times in deionized water at 60 °C, changing the liquid each time. Washing was followed by a heat treatment for 2 hours at 350 °C. The screen was then subjected to a treatment in a 20% HCl solution, boiling for 30 minutes.

[0080] 100 ml of a first acetic solution was then prepared containing tin acetate-hydrochloride complex, ruthenium acetate-hydrochloride complex, iridium acetate-hydrochloride complex and titanium acetate-hydrochloride complex and having a molar composition equal to 25% Ru, 10% Ir, 35% Sn and 30% Ti with reference to metals.

[0081] 100 ml of a second acetic solution 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 with reference to the metals.

[0082] The first acetic solution was applied to the titanium mesh by painting in 8 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0083] The procedure was repeated until a charge was reached expressed as the sum of Ir and Ru with reference to the metals equal to 6.7 g/m².

[0084] Subsequently, the second acetic solution was applied by painting in 4 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0085] The procedure was repeated until a total noble metal charge expressed as the sum of Pt and Pd with reference to the metals equal to 2.7 g/m² was reached.

[0086] Finally, a final heat treatment at 500 °C for 100 minutes was performed.

[0087] The electrode obtained in this way was identified as specimen nº 4. EXAMPLE 5

[0088] A titanium mesh measuring 10 cm x 10 cm was washed three times in deionized water at 60 °C, changing the liquid each time. Washing was followed by a heat treatment for 2 hours at 350 °C. The screen was then subjected to a treatment in a 20% HCl solution, boiling for 30 minutes.

[0089] 100 ml of a first acetic solution was then prepared containing tin acetate-hydrochloride complex, ruthenium acetate-hydrochloride complex, iridium acetate-hydrochloride complex and titanium acetate-hydrochloride complex and having a molar composition equal to 25% Ru, 10% Ir, 35% Sn and 30% Ti with reference to metals.

[0090] 100 ml of a second acetic solution was further prepared containing an organometallic complex of platinum, an organometallic complex of palladium and RhCl3 and having a molar composition equal to 86% Pt, 10% Pd and 4% Rh with reference to metals.

[0091] The first acetic solution was applied to titanium mesh by painting in 8 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0092] The procedure was repeated until a charge expressed as the sum of Ir and Ru with reference to metals equal to 6.7 g/m² was reached.

[0093] Subsequently, the second solution was applied by painting in 4 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0094] The procedure was repeated until a total noble metal charge expressed as the sum of Pt, Pd and Rh with reference to the metals equal to 2.7 g/m² was reached.

[0095] Finally, a final heat treatment at 500 °C for 100 minutes was performed.

[0096] The electrode obtained in this way was identified as specimen No. 5. COUNTEREXAMPLE 1

[0097] A titanium mesh measuring 10 cm x 10 cm was washed three times in deionized water at 60 °C, changing the liquid each time. Washing was followed by a heat treatment for 2 hours at 350 °C. The screen was then subjected to a treatment in a 20% HCl solution, boiling for 30 minutes.

[0098] 100 ml of a hydroalcoholic solution were then prepared containing RuCl3*3H2O, H2IrCl6*6H2O, TiCl3 in an isopropanol solution, having a molar composition equal to 23% Ru, 22% Ir, 55% Ti .

[0099] The solution was applied to the titanium mesh by painting in 14 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the workpiece was cooled in air before the next coating was applied.

[0100] The procedure was repeated until a total noble metal charge expressed as the sum of Ir and Ru with reference to the metals equal to 11 g/m² was reached. Then, a final heat treatment at 500 °C for 100 minutes was performed.

[0101] The electrode thus obtained was identified as specimen No. 1C. COUNTEREXAMPLE 2

[0102] A titanium mesh measuring 10 cm x 10 cm was washed three times in deionized water at 60 °C, changing the liquid each time. Washing was followed by a heat treatment for 2 hours at 350 °C. The screen was then subjected to a treatment in a 20% solution of HCl, boiling for 30 minutes.

[0103] 100 ml of a first acetic solution were then prepared containing tin acetate-hydrochloride complex, ruthenium acetate-hydrochloride complex and iridium acetate-hydrochloride complex and having a molar composition equal to 26% Ru, 10% of Ir and 64% of Sn with reference to metals.

[0104] 100 ml of a second acetic solution was also prepared containing an organometallic platinum complex and an acetate-tin hydrochloride complex and having a molar composition equal to 87% Pt and 13% Sn with reference to the metals.

[0105] The first acetic solution was applied to the titanium mesh by painting in 6 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0106] The procedure was repeated until a total noble metal charge expressed as the sum of Ir and Ru with reference to the metals equal to 6 g/m² was reached.

[0107] Subsequently, the second acetic solution was applied by painting in 4 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0108] The procedure was repeated until a total noble metal charge expressed as Pt with reference to the metal equal to 2.5 g/m² was reached.

[0109] Finally, a final heat treatment at 500 °C for 100 minutes was performed.

[0110] The electrode thus obtained was identified as specimen No. 2C. COUNTEREXAMPLE 3

[0111] A titanium mesh measuring 10 cm x 10 cm was washed three times in deionized water at 60 °C, changing the liquid each time. Washing was followed by a heat treatment for 2 hours at 350 °C. The screen was then subjected to a treatment in a 20% HCl solution, boiling for 30 minutes.

[0112] 100 ml of a first acetic solution were then prepared containing tin acetate-hydrochloride complex, ruthenium acetate-hydrochloride complex, iridium acetate-hydrochloride complex and platinum organometallic complex and having a molar composition equal to 25% Ru, 10% Ir, 35% Sn and 30% Pt with reference to metals.

[0113] The acetic solution was applied to the titanium mesh by painting in 10 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0114] The procedure was repeated until a total noble metal charge expressed as the sum of Ir, Ru and Pt with reference to the metals equal to 8 g/m² was reached.

[0115] Finally, a final heat treatment at 500 °C for 100 minutes was performed.

[0116] The electrode thus obtained was identified as specimen No. 3C. COUNTEREXAMPLE 4

[0117] A titanium mesh measuring 10 cm x 10 cm was washed three times in deionized water at 60 °C, changing the liquid each time. Washing was followed by a heat treatment for 2 hours at 350 °C. The screen was then subjected to a treatment in a 20% HCl solution, boiling for 30 minutes.

[0118] 100 ml of a first hydroalcoholic solution were then prepared containing RuCl3*3H2O, H2IrCl6*6H2O, TiOCl2 in a mixture of water and 1-butanol acidified with HCl, having a molar composition equal to 26% Ru, 23 % Ir, 51 % Ti with reference to metals.

[0119] 100 ml of a second hydroalcoholic solution were also prepared containing H2PtCl6 and PdCl2.

[0120] The first acetic solution was applied to the titanium mesh by painting in 8 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0121] The procedure was repeated until a total noble metal charge expressed as the sum of Ir and Ru with reference to the metals equal to 6 g/m² was reached.

[0122] Subsequently, the second acetic solution was applied by painting in 4 coats. After each coating, a drying step at 50-60 °C for about 10 minutes was performed, then a heat treatment for 10 minutes at 500 °C. Each time, the screen was cooled in air before the next coating was applied.

[0123] The procedure was repeated until a total noble metal charge expressed as the sum of Pt + Pd with reference to the metals equal to 3 g/m² was reached.

[0124] Finally, a final heat treatment at 500 °C for 100 minutes was performed.

[0125] The electrode thus obtained was identified as specimen No. 4C.

[0126] The specimens of the examples and the counterexamples were characterized as anodes for evolution of chlorine in a laboratory cell filled with a brine solution of sodium chloride at a concentration of 200 g/l.

[0127] Table 1 reports the chlorine surge measured at a current density of 4 kA/m² and the volume percentage of oxygen in the chlorine produced. Table 1 Specimens O2/Cl2 cell potential (% in vol.) (V) 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

Specimens O2/Cl2 cell potential (% by vol.) (V) 4C 2.76 0.8

[0128] The specimens of the previous examples were also subjected to a test for operation in a beaker. In Table 2, the anode potentials (CISEP) are reported, measured in a sodium chloride solution at a concentration of 200 g/l at a temperature of 80 °C, corrected for the ohmic drop at a current density of 3 kA/ m². Furthermore, in order to evaluate the selectivity for the chlorine reaction, tests were conducted in sulfuric acid at a current density of 3 kA/m²; reported anode potentials (CISEP) were corrected for ohmic drop. The higher the value of anode potentials measured in sulfuric acid, the greater the selectivity for the chlorine reaction. Table 2 Cisep specimens in NaCl vs NHE Cisep in H2SO4 vs NHE 1 1,336 1,872 3 1,336 1,890 4 1,338 1,872 5 1,338 1,890 1C 1,347 1,693 2C 1,336 1,647 4C 1,336 1,872

[0129] Some specimens were eventually subjected to a longevity test. The longevity test in question is the simulation, in a cell divided by industrial electrolysis conditions. Table 3 refers to the cell voltage for the specimens at the beginning of the test and after a simulated period of one year, as an indicator of their catalytic activity for the evolution of chlorine (Cl OV) measured at a current density of 8 kA/ m² and the percentage of residual charge of the second catalytic layer after a simulated period of one year. Table 3 Specimen Test Start Cl O.V. after 1 % loading of Cl O.V residual year 2 0.035 0.035 80 % 1C 0.050 0.050 - 4C 0.037 0.060 50 %

[0130] The foregoing description is not intended to limit the invention, which can be used according to various modalities without, however, deviating from the objectives and whose scope is exclusively defined by the appended claims.

[0131] In the description and claims of the present application, the terms "comprising", "including" and "containing" are not intended to exclude the presence of other elements, components or additional process steps.

[0132] Discussion of documents, items, materials, devices, articles and the like is included in this description solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these topics formed part of the prior art or formed common general knowledge in the field relevant to the present invention prior to the priority date for each claim of this application.

Claims (15)

1. Electrode for gas evolution in electrolytic processes characterized in that it comprises a valve metal substrate and a coating comprising a first catalytic layer formed on said substrate 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 containing platinum and tin or oxides thereof or combinations thereof, wherein said tin of said second catalytic layer is present in decreasing concentration from the interface with said first catalytic layer and in that said platinum from said first catalytic layer is present in decreasing concentration from the interface with said second catalytic layer. 2. Electrode for gas evolution in electrolytic processes characterized in that it comprises a valve metal substrate and a coating comprising a first catalytic layer formed on said substrate 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 containing platinum and tin or oxides thereof or combinations thereof, wherein said first layer is obtained from a first platinum-free 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 second platinum-containing tin-free catalytic composition, applied to said substrate and subjected to a heat treatment. 3. Electrode, according to one of claims 1 or 2, characterized in that said second catalytic layer contains Pt = 48-96% in the form of metal, or its oxides, in molar percentage with reference to the metallic element. 4. Electrode, according to one of claims 1 to 3, characterized in that said second catalytic layer contains Pd = 0-24% or Rh = 0-24%, in the form of metal, or its oxides, or combinations thereof, in the form of metals or their oxides in molar percentage with reference to metallic elements. 5. Electrode, according to one of claims 1 to 4, characterized in that said second catalytic layer contains Sn = 4-12% in the form of metal or its oxides, in average molar percentage with reference to the metallic element. 6. Electrode according to any one of the preceding claims, characterized in that said oxides of iridium, ruthenium and tin of said first catalytic layer are present in molar percentages Ru = 24-34%, Ir = 3-13% , Sn = 30-70 % with reference to metallic elements. 7. Electrode, according to any one of the preceding claims, characterized in that said first catalytic layer also contains titanium oxides in molar percentage of Ti = 30-40% with reference to the metallic element. 8. Electrode, according to any one of the preceding claims, characterized in that said first catalytic layer contains Pt = 3-10% in the form of metal or its oxides, in average molar percentage with reference to the metallic element. 9. Electrode according to any one of the preceding claims, characterized in that the valve metal substrate is selected from the group consisting of titanium, tantalum, zirconium, niobium, tungsten, aluminum, silicon or their alloys. Method for producing an electrode as defined in one of the preceding claims, characterized in that it comprises the following steps: - applying to a valve metal substrate a first platinum-free solution comprising a mixture of iridium, ruthenium and tin, drying subsequent to 50-60°C and decomposition of said first solution by heat treatment at 400-650°C for a time of 5 to 30 minutes; - repeating stage a) until a specific desired noble metal charge is reached; - application of a second tin-free catalytic solution containing platinum and subsequent drying at 50-60°C and decomposition of said second solution by heat treatment at 400-650°C for a time of 5 to 30 minutes; - repeating stage c) until a desired specific noble metal charge is reached. 11. Method according to claim 10, characterized in that the temperature of said thermal decomposition in steps a) and c) is between 480 and 550 °C. 12. Method according to one of claims 10 or 11, characterized in that said first solution contains said iridium, ruthenium and tin in the form of organometallic complexes. 13. Cell for the electrolysis of chloride alkali solutions, characterized in that it comprises an anode compartment and a cathode compartment, wherein the anode compartment is equipped with the electrode as defined in any one of claims 1 to 8. 14. Cell for electrolysis, according to claim 13, characterized in that said anode compartment and said cathode compartment are separated by a diaphragm or an ion exchange membrane. 15. Electrolyzer for the production of chlorine and alkali from chloride alkali solutions, characterized in that it comprises a modular arrangement of cells, in which each cell is the cell as defined in claim 13.