EP0867527B1 - Electrode with catalytic coating for electrochemical processes and manufacture thereof - Google Patents

Description

OBJECT OF THE INVENTION

The present invention relates to the manufacture of a electrocatalytic overlap electrode and establishing a process for the construction of this, so that the final product has a high resistance to passivation, without preventing good electrochemical behavior and durability satisfactory by chlorine production electrolysis and oxygen.

The designed electrode is formed by a substrate, electrically conductive, made with a material resistant to electrolyte and reaction products, a valve metal, on which a coating is deposited electrocatalytic consisting of a plurality of layers of a ternary mixture of valve metal oxides and platinum group metals (ruthenium dioxide and iridium dioxide). The first layer being in contact with the base metal, has a chemical composition enriched with valve metal oxide (titanium dioxide) against noble metal oxides (ruthenium dioxide and iridium dioxide, while in the following the valve metal oxide content, relative to that of noble metal oxides, decreases gradually in each new layer until in the most the composition is 100% metal oxides noble. The molar ratio of noble metals, dioxide ruthenium to iridum dioxide, remains constant in each of the layers, in the interval indicated in this memory.

The metal substrate consists of a metal of valve, chosen from titanium, tantalum, tungsten, zirconium, niobium, or some of their alloys. The preferred metal is titanium, although other conductive metals can be used which do not are not attacked by electrolytic means.

The invention relates to a specific type of electrode: anodes usable in processes electrolytic, consisting of a surface layer electrocatalytic deposited on a series of layers intermediaries until reaching the internal substrate of valve metal.

The intermediate layers of the active covering, applied according to the process of the present invention between the metal substrate and the external coating (100% noble metal oxides), show a high resistance to corrosion in addition to conductive properties, since the mixture of activating oxides contains on the one hand, titanium dioxide (TiO ₂ ) and on the other hand, metal oxides of the platinum group, present in small quantities acting as doping oxides which penetrate into the crystal lattice of the dioxide of titanium, by modifying its structure and forming mixed oxides or solid solutions allowing the conduction of electricity from the central metallic core to the most superficial layers. It is important to point out that the low concentration of noble metal in the internal layers, or close to the titanium substrate, allows them to have a low catalytic activity, a property resulting advantageous for them, because they undergo minimal wear by l reactive interaction with the electrolyte which can infiltrate through the cracks or pores of the external covering.

Another property of the new cover is the good adhesion of the catalyst to the substrate, as well as different layers that compose it, obtained by means of the gradation of solid mixtures of oxides valve metal and noble metal, from the first layer to the last, so that the covering is enriched with noble metal and depleted in valve metal, thereby causing a better physical interconnection between a layer and the following, following the structural similarity between two adjoining layers.

According to what we exposed in the paragraphs previous, the ultimate function of the intermediate layers is, on the one hand, to exercise as a barrier in protecting from oxidation to the metal substrate and thereby avoiding an early passivation of the electrode, and secondly, given the farm adhesion between the catalytic coating and the metallic substrate, in particular improving the properties mechanical of the electrode, without separation of material active.

These anodes are specially indicated, although not exclusively, for any process of obtaining halogens or compounds derived therefrom by chemical dissolution in liquid absorbents, from aqueous solutions of metal halides, for example: the production of chlorine in memory cells, mercury cathode cells, diaphragm cells; the manufacture of hypoclorite by seawater electrolysis and residual water treatment. They can also be used in other electrolytic processes like making chlorate, electro-recovery of metals, protection cathodic by printed current of submerged installations in corrosive solutions, as well as buried, metal plating, organic electrosynthesis, etc.

This memo describes, a type of activated anode and a simple and economically viable process for the manufacture thereof, particularly indicated for its good electrochemical activity and its high resistance to passivation, for the indicated electrochemical processes previously.

BACKGROUND OF THE INVENTION

In the chemical industry, electrochemical manufacturing process, occupy an important place, one of its advantages the main ones being the low contamination produced environmental point of view. An important part of chemical research and development is dedicated to innovation and improvement of these processes, in which the electrodes play a role paramount. Reason why there is a big investigative interest in obtaining construction of electrodes of high stability, capable to endure corrosive chemical atmospheres with a minimal overvoltage for a specific reaction which may be manufactured with precise quality control and at commercially acceptable cost.

Historically, the materials used for the manufacturing of anodes were graphite, nickel, lead or platinum, but these electrodes have limitations to their applications, due, depending on the case, to dimensional instability, contamination of electrolyte or cathode by deposition, the sensitivity to electrolyte impurities, high overvoltages for the desired reaction or cost Student.

Recently, these materials used as a base metal for electrode construction are what valve metals such as titanium, tantalum, zirconium, niobium and their alloys. All these exhibit great chemical stability in various electrolytes through the passivation process undergoes by them, by means of which they are covered superficially thin and compact film of metal oxide itself, which protects the metal base underlying a subsequent chemical attack. From aforementioned valve metals, titanium is particularly commercially interesting because it presents good qualities of stability and a cost lower than rest.

Titanium, in an oxidizing chemical means, is passivated by forming a thin surface layer of titanium dioxide (TiO ₂ ) which has semiconductor properties. The passivated metal then behaves as a poor conductor of electric current under normal conditions and it cannot be used directly as an anode; but it is valid when the metal / metal oxide assembly is coated with an electrocatalytic material.

Titanium anodes covered with materials electrochemically active (also called electrocatalysts), such as metals from the platinum or oxides of these metals, assumed a very important advance of the state of the art in general and a revolution in the chloro-alkali industry in particular. This type of electrode receives the name dimensionally stable anodes (DSA).

One type of catalytic coating, used successfully in the last two decades for the anodic generation of chlorine by electrolysis of alkali chlorides, consists of a solid mixture or dissolution of ruthenium oxide (RuO ₂ ), as it appears in various published patents (Spanish Patents No. 350337, 400915 among others).

Valve metal anodes having this type of active covering have good electrical conductivity and low anode discharge voltages of chlorine and oxygen, it is due to the excellent electrocatalytic properties of ruthenium dioxide (RuO ₂ ). Titanium dioxide (TiO ₂ ) acts as a catalyst support matrix, but it also provides chemical stability against corrosion of the coating, obtaining industrially acceptable durability in the process of obtaining chlorine by electrolysis of chlorides. alkaline.

Besides the anodic production processes of chlorine by salt water electrolysis, in industry electrochemical there are many processes where a detachment of oxygen in the anode, as well by primary as secondary reaction. As examples of electrolytic operations of this type we can include: organic electrosynthesis, recovery electrolytic metals, cathodic protection, electrolysis of saline solutions diluted like water seawater, electrolytic production of chlorate, etc.

When an electrode like the one described above (with an overlap of RuO ₂ / TiO ₂ ) is used in an electrochemical process with generation of oxygen, the ruthenium dioxide undergoes significant corrosion during the anodic polarization, producing the dissolution of the latter. ci, with the resulting wear of the catalyst and the gradual increase of the overvoltage, finally arriving at the passivation of the electrode in a short period of time.

In order to solve the problems of rapid corrosion of ruthenium oxide in conventional anodes of chemical formulation RuO ₂ / TiO ₂ on Ti, other coatings have been proposed by treating stabilizing ruthenium dioxide, without losing its excellent properties. catalytic.

Thus, in anodic oxygen generation reactions, the stability of ruthenium oxide (RuO ₂ ) increases significantly by means of the addition to the active covering of iridium oxide (IrO ₂ ). Iridium dioxide has a slightly weaker catalytic activity than that of ruthenium for the release of oxygen; on the other hand, its chemical stability is much higher, then by mixing the two components in the same catalyst, it is possible to combine the good catalytic and stability properties of the two compounds, resulting in a catalytic coating having improved properties compared to RuO ₂ ( R. Kötz, S. Stucki, J. Electrochem. Soc., 132. 103-107 (1985); R. Kötz, S. Stucki, Electrochimica Acta , 31 , 1311-1316, (1986)

In the production of chloro-alkali by electrolysis of salt water, where the generation of oxygen is an undesired side reaction, both by loss of faradic yield and by the degradation that the oxygen produced in the catalyst of RuO ₂ , the use of anodes with catalytic covering of RuO ₂ / IrO ₂ formulation causes the anodic detachment of oxygen reaction to be disadvantaged compared to that of chlorine, thereby improving the yield of the reaction, as well as the anodic stability . In RuO ₂ / IrO ₂ covered anodes, the corrosion rate of ruthenium then decreases significantly and, consequently, which also reduces the degradation of the electrode, by prolonging the service time of the latter.

• Brevets espagnols n° 366834 et 366987, où l'on décrit la formation d'un recouvrement mixte de RuO₂/IrO₂/TiO₂ où les métaux nobles se mélangent en plusieurs proportions. Le recouvrement est formé au moyen de l'application d'une dissolution d'activation sur le substrat métallique en diverses couches, en ayant toutes une composition constante.
• Brevet espagnol n° 386726, où sur une pellicule de TiO₂ générée par anodisation d'un substrat de titane, on applique un recouvrement électroconducteur consistant à un mélange d'oxydes de ruthénium et d'iridium.
• Brevet espagnol n° 504795/1 , où sur une base de titane, préalablement traitée à l'acide oxalique sur laquelle une pellicule de TiO₂ est formée, on applique un revêtement actif formé par un mélange d'oxydes de ruthénium et d'iridium.
• Brevet européen n° 437178, où l'on décrit un recouvrement composé de mélanges d'oxydes d'iridium, de ruthénium et de titane, dans des proportions bien définies, en obtenant une électrode inhibant le décollement de l'oxygène lors de la production de chlore dans des cellules à membrane, ainsi qu'une faible surtension de décharge de chlore, avec des petites pertes de catalyseur dans des milieux caustiques.

Among the patents which develop an anodic coating of mixtures or solid solutions of ruthenium oxide and iridium, we can mention the following:

• Spanish Patents Nos. 366834 and 366987, in which the formation of a mixed covering of RuO ₂ / IrO ₂ / TiO _{2 is described,} where the noble metals mix in several proportions. The covering is formed by applying an activating solution to the metal substrate in various layers, all having a constant composition.
• Spanish patent n ° 386726, where on a TiO _{2 film} generated by anodizing a titanium substrate, an electroconductive coating consisting of a mixture of ruthenium and iridium oxides is applied.
• Spanish patent n ° 504795/1 , where on a titanium base, previously treated with oxalic acid on which a TiO _{2 film} is formed, an active coating is applied formed by a mixture of ruthenium and iridium oxides .
• European patent n ° 437178, which describes a covering composed of mixtures of iridium, ruthenium and titanium oxides, in well defined proportions, by obtaining an electrode inhibiting the detachment of oxygen during production of chlorine in membrane cells, as well as a low chlorine discharge overvoltage, with small losses of catalyst in caustic media.

Although with catalytic coatings like those described we deal with fighting deactivation early anodes, minimizing one of the contributions in the passive state of an electrode, as it is the case of degradation of the surface layer of the catalyst by loss or dissolution of its components, according to a significant number of authors, the main cause or reason of passivation is the growth of the film valve metal oxide, in the interfaces of metal substrate / base metal reaction coating with oxygen, which is diffused and infiltrated through active cover slots. This phenomenon causes loss of adhesion of the catalytic coating to the substrate metallic, arriving, in some cases, at the separation of the active layer, followed by a fall in the conductivity of the electrode assembly, increased anode overvoltage and final destruction of the anode.

To prevent the diffusion and infiltration of oxygen through the active layer during the electrolysis and thus avoid the oxidation of the metal substrate below the active coating, various proposals have been made, such as providing the anode with a barrier, between the conductive substrate and the catalytic coating, composed of an alloy of platinum and iridium or of a cobalt, manganese, palladium, lead or platinum oxide (Japanese patent n ° 19429/76). However, an intermediate layer of this type, consisting of a material having considerable electrocatalytic activity, has a high reactivity with the electrolyte, which can infiltrate through the layers of the coating, giving rise to reactions which wear down the barrier and this causing a reduction in physical adhesion of the catalyst to the substrate, an increase in tension, a deterioration of the coating and a passivation of the anode. Then, a barrier of these characteristics is not satisfactory for manufacturing electrodes having an acceptable duration.

Another proposal was made in Japanese Patents Nos. 60-21232 and 60-22074 to provide the electrode with an appropriate intermediate layer, between the substrate and the catalytic coating, in order to prevent the growth of the oxide of titanium in the base metal. However, electrodes of this type are no more satisfactory for electrocatalytic processes in which one works with high current densities, due to the fact that the electrical conductivity of these intermediate layers is generally lower than that of the upper covering.

Another attempt to minimize the disadvantages of the oxidation of the metal substrate consisted in the manufacture of electrodes having an intermediate layer consisting of platinum dispersed in the matrix of a non-precious metal oxide (Japanese Kokai patent n ° 60-184691 ). These electrodes do not show very significant advantages, due to the fact that the platinum does not have a high resistance to corrosion.

Other patents such as French No. 2,599,386 deal with solving the problem of passivation, and then of the durability of the anode, by means of catalytic coatings comprising a first intermediate layer, composed of earth elements. rare, a second intermediate layer of a certain compound of the base metal or oxide thereof and a top coating of electrochemically active material.

In British Patent No. 2,239,260A describes the preparation of an electrode specially suitable for its use in electrolytic processes with oxygen generation, consisting of a base metal (preferably titanium) on which apply a covering formed of two types of layers of mixture of iridium (IrO ₂ ) and tantalum (Ta ₂ O ₅ ) oxides having a different molar ratio of iridium to tantalum in each of the layers and being these applied alternately one on the other, taking into account the fact that the deepest, in contact with the substrate, is the least enriched in noble metal. In addition to the advantages of a low overvoltage for the discharge of oxygen and satisfactory durability, the adhesion of the covering to the titanium substrate is good in this type of anodes. The electrodes described are not used in processes for obtaining chlorine.

An electrode has also been proposed having a laminated coating, comprising an oxide layer and a layer of a platinum group metal or an oxide of said metal ( Japanese Patent No. 48072/74 ). When these electrodes are used by electrolysis with anodic detachment of oxygen, the passivation of these is produced in a short period of time in operation.

All of these publications illustrate the efforts to avoid early passivation of anodes dimensionally stable in various processes electrochemicals where chlorine and / or oxygen. For this, we use overlays catalytic, whose chemical composition is more corrosion resistant, without prejudice to good metallic properties and electrochemical catalyst whose function is to protect the base metal and avoid oxidation thereof, which would lead to the loss of electrical conductivity in the interface of metallic substrate / active covering.

Despite all the tests carried out so far, provide dimensionally stable anodes (DSA) with active coatings, with good properties electrocatalytic and of great durability in corrosive chemical media, is always a problem not definitively resolved and then a subject open to new solutions. Among these, having of electrodes with technical characteristics singular, as are those that this patent presents, which moreover not only have a form simple, but also economical to manufacture.

DESCRIPTION OF THE INVENTION

The electrode that we are describing is provided with an electrically conductive metal substrate, selected from the group of valve metals, such as titanium, tantalum, zirconium, niobium, tungsten, or some of their alloys. For his physical, chemical and economic characteristics, preferably uses titanium and its alloys.

Depending on the application needs, the shape geometry of the substrate may be different. Through example, it can be in the form of a flat plate, openwork plate, filament, expanded mesh, standard mesh "louver" etc.

Before coating with the catalyst electrochemical, the titanium metal substrate is first subjected to degreasing treatments and wiping. In some occasions, the preparation of substrate requires heat treatment at temperatures high (500-700 ° C) in an inert atmosphere.

After the above operations, the surface of the substrate is developed for activation, in the subjecting to passivation using a process, among the various described for this purpose, consisting of immerse the titanium base in a solution 1M sodium hydroxide and 1M hydrogen peroxide, a temperature between 40 and 50 ° C for a duration not more than 15 hours. Using this process, the base metal is covered with a thin oxide film the same metal, matte gray color and very adherent. The conditions of this chemical attack must not be exceeded because the titanium oxide layer must be thin enough to protect the metal base subsequent oxidation, while promoting the adhesion of the active recovery and all this without increase the electrical resistance of the electrode importantly because that would imply an increase in the voltage needed to exceed it and then a unnecessary energy consumption, which would raise the cost of the process we would be work at that time.

Once the base metal has been properly prepared, the first layer is coated with a mixture of titanium oxide, ruthenium oxide and iridium oxide, a composition enriched with valve metal oxide, on the face with noble metal oxides. The molar percentage of titanium oxide is 90 to 75%, while that of the noble metal oxides (RuO ₂ + IrO ₂ ) is 10 to 25%.

In the second intermediate layer, the content of titanium oxide decreases, while increasing that of ruthenium and iridium oxides. The molar composition can vary between 87 and 70% for titanium oxide and between 13 and 30% for noble metal oxide (RuO ₂ + IrO ₂ ). In the successive intermediate layers, the content of noble metal oxides gradually increases with respect to the valve metal oxide, until arriving at the last layer or external coating, consisting of a solid mixture or dissolution of 70 mol% d ruthenium oxide and approximately 30 mol% of iridium oxide.

The purpose of the middle layers, as we have already explained previously is to act in as a barrier, protecting the base metal from oxidation and thus avoid the growth of the film titanium oxide, which could lead to the interruption of passage of electric current in the interfaces of substrate / covering and then, to an early passivation of the anode.

All these layers constituting the covering active, are formed using the technique of known mixed crystallization. This technique involves co-precipitation of titanium, ruthenium and of iridium, so that the molecular network of one of them is intertwined with the molecular networks of others, thereby forming mixed crystals or solid solutions. For that, in the first place, we prepares a dilute dissolution of titanium compounds, ruthenium and irididum thermoinstable in a solvent suitable and a titanium substrate is applied once passivated, by painting on it with a brush, a roller, a gun or by immersing it directly in the same. Afterwards, part of the solvent evaporates and the residue remaining on the substrate is subjected to a heating stage, at high temperature and in atmosphere oxidative, when the titanium, ruthenium and iridium are transformed into oxides based metallic.

The heat treatment is carried out in a forced air ventilation, atmospheric pressure and a temperature which can oscillate between 375 and 550 ° C. The heat treatment time varies according to which layer it is.

The painting process can be repeated several times, in each of the stages of formation of the different layers of the covering, until finally obtaining the desired massive thickness of noble metal, which must be between 12 and 15 g / m ₂ .

The described method for activating electrodes ensures the preparation of anodes with an overlap electrocatalytic compound for its external part of a mixture of noble metal oxides, having a concentration equal to or close to 100% and for its internal part, mixture of valve metal oxides and metal oxides platinum group, which increases its concentration of valve metal oxides as one advances towards the core titanium.

With this distribution of the concentration of noble metal oxides, on the one hand, a layer is obtained highly conductive electrochemically active conductor, but having a low ruthenium corrosion rate, thanks to the chosen chemical combination of ruthenium and iridum oxide and, on the other hand, a series of very adherent intermediate layers, where the good conductive properties of metal oxides combine with the great corrosion resistance that titanium oxide. For this, these intermediate layers avoid passivation of the electrode during protection of the titanium substrate from oxidation of the coating material.

An electrolytic structure of this type, composed of a metal substrate on which a layered overlay with noble metal ratio (ruthenium plus iridium) with valve metal (titanium), which increases as one advances from the interior towards the outside of it, has advantages to be used as anode in electrochemical processes previously indicated because by maintaining good electrochemical activity, it has a durability especially greater than that of an electrode where the overlay also consists of layers of ternary mixtures of titanium, ruthenium and of iridium, but whose molar proportion between metals is constant in all of them. It also assumes a significant improvement over an electrode where even keeping the active overlap layered at increasing concentration of noble metal in the way described in this Memorandum, the chemical formulation of it does not provide proper resistance against corrosion, to be used in various industrial electrochemical processes, and on the other hand the electrode of the invention can be used there.

PREFERENTIAL IMPLEMENTATION OF THE INVENTION

Although in the previous description of the invention all the questions characterizing it are there indicated, those with a preferential nature and which guarantee their success operational.

Valve metal, as shown previously, may be titanium, tantalum, niobium, tungsten, or one of their alloys, but preferably it is made of titanium.

One of the essences of the invention is a structure as layers and the gradual variation of noble metal oxide concentrations, since the first, or most internal, supported by the metal of base, to the outermost. For reasons the number of layers is finite and oscillates between 4 and 8.

dans la première (interne): TiO₂/RuO₂/IrO₂ = 90/7/3

dans la dernière (externe) TiO₂/RuO₂/IrO₂ = 0/70/30

It is also part of the preferred embodiment to maintain in all the layers a constant molar ratio equal to 30:70 of ruthenium oxide and of iridium oxide (RuO ₂ / IrO ₂₎ , which means that the proportion of the sum of these relative to the total of TiO ₂ + RuO ₂ + IrO ₂ , varies from 10% in the first layer, to 100% in the last or outermost layer. In this way the molar ratio in the above-mentioned layers will be:

in the first (internal): TiO ₂ / RuO ₂ / IrO ₂ = 90/7/3

in the last (external) TiO ₂ / RuO ₂ / IrO ₂ = 0/70/30

The proportion of 90% of TiO ₂ in the first, can decrease up to 80%, while increasing the RuO ₂ and IrO ₂ , the latter always in molar proportion 70:30.

It also happens that the temperature and the time of the heat treatment in an oxidizing atmosphere vary from the first (or more internal) to the last (or more external). In the former, the preferred temperature ranges from 375 ° to 450 ° with a duration of 15 to 20 minutes. In the last, that is to say, that which does not contain TiO ₂ , the treatment is carried out between 475 and 550 ° C. for one hour.

Then we present some explanatory examples and not limiting, with their behavior electrochemical.

We prepared four electrodes, in the way that appears in the example, subjecting them to a test electrochemical activity evaluation, using polarization tests, under anodic detachment of chlorine and oxygen and durability by means of accelerated wear tests. Also, during the time that the anode is active, we periodically monitored the increase in potential anodic, the corrosion rate of noble metals component the recovery and overall loss of metal noble.

Titanium plates of dimensions 50x50x3 mm are degreased with acetone and they are subjected to chemical attack by immersing them in a solution 1 M sodium hydroxide and 1 M hydrogen peroxide for 6 hours, keeping the temperature between 40 and 50 ° C. Then we wash them with lots of water distilled to neutral reaction and dried. Of this way the substrate surface is ready for it apply the catalytic coating.

For this we prepare several paintings activation, by dissolving butyl orthotitane, ruthenium trichloride and iridium trichloride in n-propyl alcohol in different proportions molars of the three components. The molar ratio of rutenium to iridium remains constant in all dissolutions, in the interval 72:28 to 69:31, while

the molar ratio of titanium to noble metal (ruthenium plus iridium) varies from 90:10 to 0: 100.

Activation dissolutions are applied to the two faces of the base metal, using a brush, so that after applying each layer of painting, the solvent is evaporated in air and the sample is subjected to a heat treatment in atmosphere between 425 and 450 ° C for 15 minutes for each intermediate layer, while for the last layer heat treatment is carried out between 475 and 550 ° C and the residence time of the electrode in the oven is one hour

The painting, drying and thermal decomposition process of the activation dissolution is repeated the number of times necessary to obtain a thickness of the noble metal in the coating of approximately 12 g / m ² .

In Table 1, the number, the chemical composition and the molar ratio between the chemical species constituting the layers of the covering of the activated samples are collected, as described in the invention. Sample # SAMPLE 1 SAMPLE 2 SAMPLE 3 SAMPLE 4 Chemical composition % Molar Chemical composition % Molar Chemical composition % Molar Chemical composition % Molar 1 TiO ₂ / RuO ₂ / IrO ₂ 90: 7: 3 TiO ₂ / RuO ₂ / IrO ₂ 87: 9: 4 TiO ₂ / RuO ₂ / IrO ₂ 87: 9: 4 TiO ₂ / RuO ₂ / IrO ₂ 83: 12: 5 2 TiO ₂ / RuO ₂ / IrO ₂ 87: 9: 4 TiO ₂ / RuO ₂ / IrO ₂ 83: 12: 5 TiO ₂ / RuO ₂ / IrO ₂ 75: 18: 7 TiO ₂ / RuO ₂ / IrO ₂ 75: 18: 7 3 TiO ₂ / RuO ₂ / IrO ₂ 83: 12: 5 TiO ₂ / RuO ₂ / IrO ₂ 75: 18: 7 TiO ₂ / RuO ₂ / IrO ₂ 75: 18: 7 TiO ₂ / RuO ₂ / IrO ₂ 70: 21: 9 4 TiO ₂ / RuO ₂ / IrO ₂ 75: 18: 7 TiO ₂ / RuO ₂ / IrO ₂ 70: 21: 9 TiO ₂ / RuO ₂ / IrO ₂ 70: 21: 9 TiO ₂ / RuO ₂ / IrO ₂ 50:35:15 5 TiO ₂ / RuO ₂ / IrO ₂ 70: 21: 9 TiO ₂ / RuO ₂ / IrO ₂ 50:35:15 TiO ₂ / RuO ₂ / IrO ₂ 70: 21: 9 RuO ₂ / IrO ₂ 70:30 6 TiO ₂ / RuO ₂ / IrO ₂ 50:35:15 RuO ₂ / IrO ₂ 70:30 RuO ₂ / IrO ₂ 70:30 RuO ₂ / IrO ₂ 70:30 7 RuO ₂ / IrO ₂ 70:30 RuO ₂ / IrO ₂ 70:30 RuO ₂ / IrO ₂ 70:30 RuO ₂ / IrO ₂ 70:30

For comparative effects, we activated two more samples, one of them with equal layers mixtures of titanium oxides, ruthenium and iridium, according to a process recommended both in the bibliography of scientific articles and patents, and the other with layers composed of oxides of titanium and ruthenium oxide, having a increasing concentration of ruthenium from the first layer at the last in the way that we describe in our invention and analogously to Sample 1.

The results obtained in the tests for determining the electrochemical activity, durability and wear of the active covering, carried out with these electrodes, are shown in Table 2. Next to the results of voltage, average corrosion rate of ruthenium and loss of noble metal, in brackets, we record the number of hours of operation of the anode, from the start of the durability test, under accelerated wear conditions, until the instant of averaging relevant for the calculation of these parameters. SAMPLE N ° ANODIC POTENTIAL (VOLT., NHE) CORROSION SPEED (g Ru / hA) 10 ^-6 NOBLE METAL LOSS% DURABILITY (HOURS) Initial After electrolysis After electrolysis After electrolysis Ruthenium Iridium 1 1.37 1.38 (1500h) 0.3 (1100h) 20 (1100h) 11 (1100h) > 1500 2 1.38 1.35 (1500h) 0.5 (1010h) 30 (1010h) 22 (1010h) > 1500 3 1.38 1.38 (1500h) 0.3 (1100h) 29 (1100h) 13 (1100h) > 1500 4 1.38 1.35 (1500h) 0.3 (1100h) 25 (1100h) 9 (1100h) > 1500 TiO ₂ / RuO ₂ / TiO ₂ equal layers 1.37 2 (900h) 0.6 (900h) 39 (900h) 36 (900h) 900 TiO ₂ / RuO ₂ different layers 1.36 > 2 (114h) 4.0 (114h) 40 (114h) _ 114

Claims (13)

1. Electrodes with a catalytic coating for electrochemical processes such as: electrolysis of alkali chloride for production of alkali-chlorine, sodium chlorate and sodium hypochlorite; metal electroplating process, electroplating of manganese dioxides, cathodic protection by printed current, organic electrosynthesis or others, characterised in that it comprises an electrical conducting metal substrate, resistant to the electrolyte, on which is a catalytic coating, which consisting of a number of superimposed layers of different compositions, containing a valve metal oxide and oxides of at least two platinum metals, so that the ratio of platinum group oxides to valve metal oxides increases gradually from the first layer onwards, with the molar ratio of platinum group oxides constant to all, so that an outer layer is reached in which there are only oxides of at least two platinum group metals and which is free of valve metals.
2. Electrode as claimed in claim 1, characterised in that the metal substrate is a valve metal from among: titanium, tantalum, tungsten, zirconium, niobium and their alloys.
3. Electrode as claimed in claims 1 and 2 characterised in that the metal substrate is composed of titanium or its alloys.
4. Electrode as claimed in claims 1, 2 and 3 characterised in that the conducting metal substrate or support is coated in a layer of oxide of the same metal, compact and firmly attached.
5. Electrode as in claim 1, characterised in that in the catalytic coating, the valve metal oxide is titanium oxide.
6. Electrode as in claim 1, characterised in that all layers of the catalytic coating of the platinum group metals are ruthenium and iridium.
7. Electrode as in claims 1 and 6, characterised in that the molar ratio of ruthenium oxide to iridium oxide is constant in all layers, with a reference value of 70:30 and which may vary between 72:28 and 69:31.
8. Electrode as in claims 1 to 7, characterised in that the number of different layers which make up the coating varies between 4 and 8.
9. Manufacturing procedure for an electrode according to claims 1 to 8, characterised in that on the degreased and passivated metal substrate are deposited several layers by successive applications of precursor solutions for metal oxides which are transformed into the oxides by thermal decomposition.
10. Manufacturing procedure for an electrode according to above claims, characterised in that the base metal passivation is performed with a 1M aqueous sodium hydroxide solution and 1M hydrogen peroxide solution at a temperature between 40 and 50 °C for a time period between 6 and 15 hours.
11. Manufacturing procedure for an electrode according to claim 9, characterised in that each layer is deposited by applying a solution containing thermally unstable compounds which are transformed into the metal oxides present in each layer by thermal decomposition at a temperature between 300°C and 550°C.
12. Manufacturing procedure for an electrode according to claims 9 and 11, characterised in that the cycle of the solution application and later thermal decomposition can be performed one or more times in order to obtain the desired layer thickness.
13. Manufacturing procedure for an electrode according to claims 9 to 12, characterised in that the thermal decomposition of the final layer or application cycle takes place at a temperature between 475 and 550°C and during at least one hour.