FIELD OF THE INVENTION

The invention relates to an electrode for gas evolution in electrolytic processes comprising a catalytic coating containing oxides of tin, ruthenium, titanium and one or more elements selected from the group consisting of niobium, tantalum and tungsten applied to a metallic substrate, and a method for its preparation.

BACKGROUND OF THE INVENTION

The first commercial production of chlorine through the electrolysis process dates back to the late 19th century; and nowadays, the chlor-alkali industry is one of the main electrochemical processes for the production of chlorine and sodium hydroxide.

There are three different processes for the electrolysis of alkali chloride brines: mercury, diaphragm and membrane.

The diaphragm cell was the first technology developed for brine electrolysis, followed by the mercury process; it is only in the 1970s in which the chlor-alkali industry is revolutionized through the development of a new electrolysis process involving the use of ion-selective membranes.

Nowadays, membrane technology is the most widely used, closely followed by diaphragm technology. Both technologies involve the use of anodes based on titanium or other valve metal activated with a superficial layer of ruthenium dioxide (RuCh) which has the property of lowering the overvoltage of the anodic evolution reaction of chlorine.

A partial improvement in terms of chlorine overvoltage and therefore of process voltage and total energy consumption can be obtained by adding to a formulation based on RuO2, mixed with SnO2, a certain amount of a second noble metal selected between iridium and platinum, for example as described in EP0153586; this, and other tin-containing formulations, however, present the problem of simultaneously lowering the overvoltage of the concurrent oxygen evolution reaction, so that the chlorine produced by the anodic reaction is contaminated with an excessive amount of oxygen.

A further partial improvement in terms of performance can be obtained by applying on a metal substrate a formulation based on RuCh, TiCh and SnCh added with lrC>2, for example, as described in EP19731919. However, these catalytic coatings involve the presence of high quantities of noble metals.

Finally, coatings of the prior art such as, for example, the formulation described in JPS6338592 based on tin oxides and noble metals, are generally prepared starting from tetravalent tin precursors, in particular tin tetrachloride (SnCI4), mixed with the corresponding precursors of the noble metal in aqueous solution. However, the extreme volatility of the precursors thus obtained makes them unfavorable for application in industrial processes.

It is therefore evident the need to have a new catalytic coating for electrodes for the evolution of gaseous products in electrolytic cells, in particular for use in the electrolysis processes of alkaline chloride brines, characterized by a lower noble metal loading compared to prior art formulations which exhibit a good catalytic activity together with a high durability under the usual operating conditions.

SUMMARY OF THE INVENTION

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

The present invention relates to the preparation of a catalytic coating for electrodes used, among other things, in alkali chloride brine electrolysis processes, such a coating is applied to a metal substrate, typically titanium, titanium alloy or other valve metal.

More specifically, the present invention consists in the application on a metal substrate of a formulation based on ruthenium, tin, titanium and one or more elements selected from the group consisting of niobium, tantalum and tungsten; a formulation thus obtained leads to reach excellent performances in terms of catalytic activity for the reaction of chlorine even with low quantities of noble metal.

The increase of noble metals prices has indeed led to a continuous effort to reduce their loading in catalytic coatings and the field of alkali chloride brine electrolysis is no exception.

In a first aspect, the invention relates to an electrode for gas evolution in electrolytic processes comprising a metal substrate and a catalytic coating containing 10-20% tin, 25-45% ruthenium, 20-40% titanium, and 10-20% of one or more elements selected from the group consisting of niobium, tantalum, tungsten in the form of metals or their oxides in percentage by weight referred to the elements. It is evident that the person skilled in the art will select the molar percentages of the single elements in such a way that the total sum of the molar percentages of the components is 100.

The inventors have observed that such catalytic coating formulation allows the application of a lower amount of ruthenium without compromising its diffusion on the metal substrate, on the contrary allowing its uniform distribution.

In a further embodiment said one or more elements selected from the group consisting of niobium, tantalum, tungsten is tantalum.

In one embodiment, the titanium of the catalytic coating is present as an oxide in the rutile form.

The inventors have observed that the presence of tantalum and/or its oxides, even in small amounts, allows the resulting titanium oxide to be organized in the rutile form, further allowing to obtain a homogeneous solid solution of rutile.

It has in fact been observed that electrodes of the prior art with coatings comprising ruthenium, tin and titanium often show, in the analysis of the crystalline structure, an anatase peak for the titanium oxide, which affects their performance in terms of duration. When this occurs, further heat treatment is required to allow a better organization of the oxide material.

According to the present invention, the metal substrate may be any metal suitable for use as an electrode support for electrochemical processes. In particular, as a metal substrate for an anode to be used in chlor-alkali electrolysis processes. In this case the most commonly used metal substrates can be chosen between titanium and titanium alloy.

In a further embodiment, the catalytic coating has a specific ruthenium load comprised between 2.2 and 9 g/m ² .

In a further aspect, the present invention relates to an electrode comprising a metal substrate and a catalytic coating wherein said catalytic coating is obtained by thermal decomposition of a solution of tin, ruthenium, titanium, and one or more selected elements in the group consisting of niobium, tantalum and tungsten. Said solution containing 10- 20% of tin, 25-45% of ruthenium, 20-40% of titanium, and 10-20% of one or more elements selected from the group consisting of niobium, tantalum, tungsten in percentage by weight referred to the elements.

In a further aspect, the present invention relates to an electrode comprising a metal substrate and a catalytic coating wherein said catalytic coating is obtained by thermal decomposition of a solution of tin, ruthenium, titanium, wherein said one or more elements chosen from the group consisting of niobium, tantalum and tungsten is tantalum. Said solution containing 10-20% of tin, 25-45% of ruthenium and 20-40% of titanium, and 10- 20% of tantalum in percentage by weight referred to the elements.

In a further aspect, the present invention relates to an electrode comprising a metal substrate and a catalytic coating wherein said catalytic coating is obtained by thermal decomposition of a solution of tin, ruthenium, titanium, wherein said one or more elements chosen from the group consisting of niobium, tantalum and tungsten is niobium. Said solution containing 10-20% of tin, 25-45% of ruthenium and 20-40% of titanium, and 10- 20% of niobium in percentage by weight referred to the elements. In a further aspect, the present invention relates to a process for obtaining an electrode for the evolution of gaseous products in electrolytic cells, for example for the evolution of chlorine in alkaline brine electrolysis cells, comprising the following steps: a) application to a metal substrate of a solution comprising the precursors of ruthenium, tin, titanium and one or more elements selected from the group consisting of niobium, tantalum, tungsten, subsequent drying at 50-60°C and thermal decomposition at 450- 600°C for a time of 5 to 30 minutes; b) repetition of step a) until obtaining a catalytic coating with a specific ruthenium load comprised between 2.2 and 9 g/m ² ; c) thermal treatment at 450-600°C for a time of 50 to 200 minutes.

Ruthenium and titanium precursors and niobium, tantalum, or tungsten precursors are compounds selected from the group consisting of chlorides, nitrates, iodides, bromides, sulfates, butyls, or acetates of the metals and their mixtures thereof.

The tin precursors are generally prepared according to the procedure described in WO 2005/014885.

The use of tin precursors as described in WO 2005/014885 not only allows to overcome the limitation of the prior art, providing an anodic catalytic coating with a well-controlled chemical composition, but also ensures a good distribution of the oxides of the elements of the catalytic coating, formed during the various thermal treatments, over the entire surface of the metal substrate.

The method according to the invention does not exclude the application of additional coating compositions, such as a barrier coating composition applied directly to the metal substrate, prior to step a) and thus prior to the application of the catalytic coating, or a upper coating composition after stage b.

In a further aspect, the invention relates to an electrolysis cell of alkaline chloride solutions and/or an electro-chlorination cell comprising an anodic compartment and a cathodic compartment wherein the anodic compartment is equipped with the electrode in one of the forms as described above, used as an anode for chlorine evolution.

In a further aspect, said anodic compartment and said cathodic compartment are separated by a diaphragm or an ion exchange membrane.

Under a further embodiment, the invention relates to an electrolyser for the production of chlorine and alkali starting from alkali chlorides solutions comprising a modular arrangement of electrolytic cells with the anodic and cathodic compartments separated by ion exchange membranes or by diaphragms, wherein the anodic compartment comprises an electrode in one of the forms as described above used as anode.

The following examples are included in order to demonstrate particular embodiments of the invention, whose practicability has been amply verified within the range of values claimed. It will be evident to those skilled in the art that the compositions and the techniques described in the examples that follow represent compositions and techniques for which the inventors have encountered a good operation of the invention in practice; however, those skilled in the art will furthermore appreciate in the light of the present description, various modifications could be made to the various embodiments described still giving rise to identical or similar results without straying from the scope of the invention. EXAMPLE 1

A titanium mesh of 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 2-hour thermal treatment at 350°C. The mesh was then subjected to a treatment in a 20% HCI solution, boiling for 30 minutes.

100 ml of a solution containing tin, ruthenium, titanium and tantalum and having a composition by weight equal to 40% Ru, 14% Sn, 31 % Ti and 15% Ta were then prepared.

The solution was applied to the titanium mesh by brushing. After each coat, drying was carried out at 50-60°C for about 10 minutes, followed by a heat treatment for 10 minutes at 500°C. The mesh was air cooled each time before the application of the next coat. The procedure was repeated until a total Ru load of 6 g/m ² was reached.

The electrode thus obtained was identified as sample #1 .

EXAMPLE 2

A titanium mesh of 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 2-hour thermal treatment at 350°C. The mesh was then subjected to a treatment in a 20% HCI solution, boiling for 30 minutes.

100 ml of a solution containing tin, ruthenium, titanium and niobium and having a composition by weight equal to 40% Ru, 14% Sn, 31 % Ti and 15% Nb were then prepared.

The solution was applied to the titanium mesh by brushing. After each coat, drying was carried out at 50-60°C for about 10 minutes, followed by a heat treatment for 10 minutes at 500°C. The mesh was air cooled each time before the application of the next coat. The procedure was repeated until a total Ru load of 6 g/m ² was reached.

The electrode thus obtained was identified as sample #2.

EXAMPLE 3

A titanium mesh of 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 2-hour thermal treatment at 350°C. The mesh was then subjected to a treatment in a 20% HCI solution, boiling for 30 minutes.

100 ml of a solution containing tin, ruthenium, titanium and tantalum and having a composition by weight equal to 40% Ru, 19% Sn, 31 % Ti and 10% Ta were then prepared.

The solution was applied to the titanium mesh by brushing. After each coat, drying was carried out at 50-60°C for about 10 minutes, followed by a heat treatment for 10 minutes at 500°C. The mesh was air cooled each time before the application of the next coat.

The procedure was repeated until a total Ru load of 6 g/m ² was reached.

A final heat treatment was then carried out at 500°C for 100 minutes.

The electrode thus obtained was identified as sample #3.

COUNTER EXAMPLE 1

A titanium mesh of 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 2-hour thermal treatment at 350°C. The mesh was then subjected to a treatment in a 20% HCI solution, boiling for 30 minutes.

100 ml of a solution containing ruthenium, titanium, tin having a composition by weight equal to 29% Ru, 25% Ti, 46% Sn were then prepared. The solution was applied to the titanium mesh by brushing. After each coat, drying was carried out at 50-60°C for about 10 minutes, followed by a heat treatment for 10 minutes at 500°C. The mesh was air cooled each time before the application of the next.

The procedure was repeated until a total Ru load of 6 g/m ² was reached.

The electrode thus obtained was identified as sample #1 C.

COUNTER EXAMPLE 2

A titanium mesh of 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 2-hour thermal treatment at 350°C. The mesh was then subjected to a treatment in a 20% HCI solution, boiling for 30 minutes.

100 ml of a solution containing ruthenium, titanium, tin having a composition by weight equal to 30% Ru, 60% Ti, 10% Sn were then prepared.

The solution was applied to the titanium mesh by brushing. After each coat, drying was carried out at 50-60°C for about 10 minutes, followed by a heat treatment for 10 minutes at 500°C. The piece was air cooled each time before the next coat was applied.

The procedure was repeated until a total Ru load of 6 g/m ² was reached.

The electrode thus obtained was identified as sample #2C.

COUNTER EXAMPLE 3

A titanium mesh of 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 2-hour thermal treatment at 350°C. The mesh was then subjected to a treatment in a 20% HCI solution, boiling for 30 minutes.

100 ml of a solution containing ruthenium, titanium, tin and tantalum having a composition by weight equal to 30% Ru, 20% Ti, 10% Sn, 40% Ta were then prepared. The solution was applied to the titanium mesh by brushing. After each coat, drying was carried out at 50-60°C for about 10 minutes, followed by a heat treatment for 10 minutes at 500°C. The piece was air cooled each time before the next coat was applied.

The procedure was repeated until a total Ru load of 6 g/m ² was reached. The electrode thus obtained was identified as sample #3C.

The samples from the examples were characterized as anodes for evolution of chlorine in a laboratory cell fed with sulfuric acid at a concentration of 150 g/l, at a temperature of 65°C.

Table 1 shows the lifetime performances (expressed in hours online - HOL) measured at a current density of 4 kA/m ² . The previous description does not intend to limit the invention, which can be used according to different embodiments without thereby deviating from the purposes and whose scope is uniquely defined by the attached claims. In the description and claims of the present application, the term "comprises" and "contains" and their variants such as "comprising" and "containing" are not intended to exclude the presence of other elements, components, or additional process steps. Discussion of documents, records, materials, apparatuses, articles, and the like is included in the text for the sole purpose of providing context to the present invention; However, it is not to be understood that this matter or part of it constituted general knowledge in the field relating to the invention before the priority date of each of the claims attached to this application.