ES2606306T3 - Cathode for hydrogen electrolyte release - Google Patents

Abstract

An electrode suitable for use as a cathode in electrolytic processes comprising a metal substrate equipped with a catalytic coating, said catalytic coating comprising, an internal layer containing platinum directly in contact with the substrate, at least one intermediate layer consisting of a mixture of oxides containing 40-60% by weight of rhodium relative to the elements, and an outer layer of ruthenium oxide.

Description

5

10

fifteen

twenty

25

30

35

40

Four. Five

fifty

DESCRIPTION

Catodo for electrolytic release of hydrogen Field of the invention

The invention relates to an electrode, in particular to a metal electrode for use as a cathode for the release of hydrogen in industrial electrolytic processes and a process for its production.

Background of the invention

The alkali brine electrolysis for the simultaneous production of chlorine and alkali, and the hypochlorite and chlorate generation processes are the most typical examples of industrial electrolytic applications with cathode hydrogen release, although the electrode is not limited to any particular use. In the electrolytic process industry, competitiveness is related to several factors, the main one being the reduction in energy consumption, directly related to the process voltage; This justifies the numerous efforts that have been invested to reduce the various components of the latter, which should include overvoltage. The cathode surges that are obtained naturally by means of electrodes composed of chemical resistant material (for example, carbon steel) without catalytic activity have been considered adequate for a long time. In the present case, the market, however, requires increasing concentrations of caustic products, which makes the use of carbon steel cathodes unfeasible due to corrosion problems; In addition, the increase in energy costs has led to the use of catalysts to facilitate the cathode release of hydrogen. One possible solution is to use nickel substrates, which are more chemically resistant than carbon steel, and catalytic platinum coatings. Cathodes of this type are generally characterized by a correct cathode overvoltage, although it has a limited useful life, probably due to insufficient adhesion of the layer to the substrate. A partial improvement in the adhesion of the catalytic coating to the nickel substrate can be obtained by incorporating rare earths to the formulation of the catalytic layer, optionally as a porous outer layer that performs a protective function against the underlying platinum-based catalytic layer; This type of cathode has a sufficient duration under normal operating conditions, being exposed, however, to suffer serious damages that derive from the occasional current investments that inevitably occur in case of industrial plants malfunction.

A partial improvement in the resistance to current inversions can be obtained by activating the nickel cathode substrate with a coating consisting of two different phases, a first catalytic phase based on platinum and rhodium, and a second phase comprising palladium With a protective function. Said cathode of two different phases is described in WO 2008/043766 A2 of the present applicants. However, this type of formulation requires large loads of platinum and rhodium in the catalytic phase, as to determine a high production cost.

A less expensive catalytic coating, which has a high activity combined with a certain resistance to current investments, is obtained from mixtures of ruthenium and rare earths, for example, praseodymium; The resistance of the electrodes obtained according to said formulation can be increased by interposing a thin platinum-based layer between the cathode substrate and the catalytic coating. Said electrode is described in WO 2012/150307 of the present applicants.

The aforementioned formulations make it possible to obtain electrodes capable of operating for sufficient time intervals in correctly operated industrial electrolysers, according to a common practice in the industry, with polarization devices that act in case of scheduled or sudden plant closures by imposing a small residual tension that serves to protect the cell components from corrosion. Thanks to these devices, the current reversals can only occur during a short period of time that elapses between the closing of the electric charge and the beginning of the residual voltage, during which the cathodes should not undergo any major change. However, the most recent advances in the design of industrial electrolysers, in particular of electrolysers for the production of chlorine and alkali from alkali brines consisting of electrolytic cells with anodic and cathode compartments separated by ion exchange membranes, offer the use of materials and construction techniques that make possible the dispersion with polarization devices, whose installation and management entail an important additional cost. The closure of the plant in an electrolyzer free of polarization device entails, at least in the initial phase, phenomena of cell tension inversion caused by the presence of reaction product residues in two compartments: in these conditions, the cell of Electrolysis can work for a short period as a battery, with the corresponding cathodes being subject to the anodic tension pass. This entails the need to provide cathodes with an increasing tolerance to current investments, compared to the best state-of-the-art formulations.

Summary of the invention

Some aspects of the invention are set forth in the appended claims.

According to one aspect, the invention relates to an electrode suitable for use as a cathode in electrolytic processes comprising a substrate made of metal, for example, nickel, which is presented with a catalytic coating 5 formed by at least three different layers: an inner layer, in direct contact with the substrate, containing platinum, at least one intermediate layer consisting of a mixture of oxides containing 40-60% by weight of rhodium relative to the elements and, finally, a layer External based on ruthenium oxide.

A platinum in the inner layer is presented mainly in metallic form, especially in operating conditions according to the cathode release of hydrogen, however, it is not ruled out, especially before the first use, that a platinum or a fraction thereof can present in the form of rust.

In one embodiment, the inner layer consists of a single platinum layer.

In one embodiment, the outer layer consists of a layer of ruthenium oxide alone. In the present context, the term "ruthenium oxide" indicates that said element is present, after the preparation of the electrode, mainly in the form of an oxide; it is not ruled out, especially under operating conditions with cathode release of hydrogen, that said oxide can be partially reduced to ruthenium metal.

In one embodiment, the intermediate layer oxide mixture contains, in addition to rhodium, 10-30% by weight of palladium and 20-40% by weight of rare earths; In one embodiment, the rare earth content consists entirely of praseodymium. In the present context, the term oxide mixture indicates that the elements of the relative formulation are present, after preparation of the electrode, mainly in the form of oxides; 20 it is not ruled out especially in operating conditions with cathode hydrogen release, that the fraction of said oxides can be reduced to metals or even form hydrides, as in the case of palladium.

The inventors have surprisingly observed that formulations of this type transmit a resistance to current inversions sometimes greater than the most detailed formulations of the state of the art in the substantially reduced charge of noble metal.

25 In one embodiment, the specific platinum charge in the inner layer ranges between 0.3 and 1.5 g / m2, the sum of the specific charge of rhodium, palladium and rare earths in the intermediate layer ranges between 1 and 3 g / m2 and, finally, the specific loading of ruthenium in the outer layer ranges between 2 and 5 g / m2. In fact, the inventors have discovered that, in the case of the aforementioned formulations, so many reduced noble metal charges are more than sufficient to transmit a high catalytic activity combined with an unprecedented resistance to current investments in the state of the technique.

According to another aspect, the invention relates to a method for preparing an electrode comprising the application in one or more coatings of acetic solution of Pt (NH3) 2 (NO3) 2 (platinum diamino dinitrate) to a metal substrate, with subsequent drying at 80-100 ° C, thermal decomposition at 450-600 ° C and optional cycle repetition until the desired load is achieved (for example, 0.3-1.5 g / m2 of Pt as metal); the application in one or more coatings of an acetic solution containing rhodium nitrate and optionally palladium and rare earth nitrates to the internal catalytic layer obtained in this way, with subsequent drying at 80-100 ° C, thermal decomposition at 450- 600 ° C and optional cycle repetition until the desired load is achieved (for example, 1-3 g / m2 as sum of Rh, Pd and rare earths); the application in one or more coatings of an acetic nitrosyl nitrate solution of Ru to the intermediate catalytic layer obtained in this way, with subsequent drying at 80-100 ° C, thermal decomposition at 40-450-600 ° C and optional repetition of the cycle until the desired load is achieved (for example, 2-5 g / m2 of Pt as

metal);

As is well known, ruthenium nitrosyl nitrate represents a commercially available compound that is expressed with the formula Ru (NO) (NO3) 3, sometimes it is written as Ru (NO) (NO3) x to indicate that The average state of ruthenium oxidation may differ slightly from the value of 3.

The previous application of the solutions can be done by brushing, spraying, dipping or other known technique.

The inventors have observed that the use of specific precursors in the preparation conditions adopted favors the formation of catalysts with a specially ordered crystalline network, with a positive impact in terms of activity, durability and resistance to current investments.

50 The best results were obtained by adjusting the thermal decomposition temperature of the various solutions in the range between 480 and 520 ° C.

5

10

fifteen

twenty

25

30

35

40

Four. Five

The following examples are included to show particular embodiments of the present invention, whose viability has been largely proven in the claimed range of values. Those skilled in the art should appreciate that the compositions and techniques described in the following examples represent compositions and techniques that the inventors have discovered for their proper functioning in the practice of the invention; however, those skilled in the art should appreciate that, in the light of the present disclosure, some changes can be made in the embodiments that have been described and even so! obtain the same or similar result without departing from the scope of the present invention.

Example

An amount of diamino dinitrate of Pt, Pt (NH3) 2 (NO3) 2 corresponding to 40 g of Pt was dissolved in 60 ml of glacial acetic acid. The solution was stirred for 3 hours while maintaining the temperature at 50 ° C and then brought to the volume of one liter with 10% acetic acid by weight (platinum solution).

An amount of Ru (NO) (NO3) 3 corresponding to 200 g of Ru was dissolved in 600 ml of glacial acetic acid with the addition of a few ml of concentrated nitric acid. The solution was stirred for three hours while maintaining the temperature at 50 ° C. Then the solution was brought to a volume of 1 l with 10% acetic acid by weight (ruthenium solution).

Separately, quantities of Rh (NO3) 3, Pd (NO3) 2 and Pr (NO3) 36H2O corresponding to 4.25 g of Rh, 1.7 g of Pd and 25.5 g of Pr expressed as metals were mixed agitation (rhodium solution).

A nickel 200 mesh of a size of 100 mm x 100 mm x 0.89 mm was subjected to a pickling process with corundum, chemical attack in HCI at 20% at 85 ° C for 2 minutes and thermal annealing at 500 ° C for one hour.

The platinum solution was applied by brushing in a single cycle, performing a drying treatment for 10 minutes at 80-90 ° C and a thermal decomposition for 10 minutes at 500 ° C, obtaining a specific load of 0.8 g / m2 from Pt.

Then, the rhodium solution was applied by brushing on three coatings by performing a drying treatment for 10 minutes at 80-90 ° C and a thermal decomposition for 10 minutes at 500 ° C after each coating, obtaining a specific load of 1, 4 g / m2 of Pt. 0.6 g / m2 of Pd and 0.84 g / m2 of Pr.

Then, the ruthenium solution was applied by brushing on four coatings by performing a drying treatment for 10 minutes at 80-90 ° C and a thermal decomposition for 10 minutes at 500 ° C after each coating, thus obtaining! a specific load of 3 g / m2 of Pt.

The sample was subjected to a performance test, showing an initial cathode potential of ohmic broth correction of -930 mV / NHE at 3 kA / m2 with hydrogen release in 33% NaOH, at a temperature of 90 ° C.

Then, the same sample was subjected to cyclic voltameter in the range of -1 to +0.5 V / NHE at an exploration rate of 10 mV / s; after 25 cycles, the cathode potential was -935 mV / NHE, which indicates an investment of current in the appropriate resistance to operate in industrial electrolysers free of polarization devices.

Counterexample

An amount of diamino dinitrate of Pt, Pt (NH3) 2 (NO3) 2 corresponding to 40 g of Pt was dissolved in 160 ml of glacial acetic acid. The solution was stirred for 3 hours while maintaining the temperature at 50 ° C and then brought to the volume of one liter with 10% acetic acid by weight (platinum solution).

An amount of Ru (NO) (NO3) 3 corresponding to 200 g of Ru was dissolved in 600 ml of glacial acetic acid with the addition of a few ml of concentrated nitric acid. The solution was stirred for three hours while maintaining the temperature at 50 ° C. Then, the solution was brought to a volume of 1 L with 10% acetic acid by weight (ruthenium solution).

Separately, an amount of Pr (NO3) 2 corresponding to 200 g of Pr was dissolved in 600 ml of glacial acetic acid with the addition of a few ml of concentrated nitric acid. The solution was stirred for three hours while maintaining the temperature at 50 ° C. Then the solution was brought to a volume of 1 l with 10% acetic acid by weight (rare earth solution). 480 ml of ruthenium solution was mixed with 120 ml of rare earth solution and left under stirring for five minutes. The solution as! The obtained weight was 1 liter with 10% acetic acid by weight (ruthenium and praseodymium solution).

5

10

fifteen

twenty

25

A nickel 200 mesh of a size of 100 mm x 100 mm x 0.89 mm was subjected to a corundum pickling process, chemical attack in 20% HCI at 85 ° C for 2 minutes and thermal annealing at 500 ° C for one hour.

The platinum solution was applied by brushing in a single cycle, performing a drying treatment for 10 minutes at 80-90 ° C and a thermal decomposition for 10 minutes at 500 ° C, thus obtaining! a specific load of 1 g / m2 of Pt.

The ruthenium and praseodymium solution was applied by brushing in 4 successive coatings, performing a drying treatment for 10 minutes at 80-90 ° C and a thermal decomposition for 10 minutes at 500 ° C after each coating, until the deposition of 4 g / m2 of Ru and 1 g / m2 Pr

The sample was subjected to a performance test, showing an initial cathode potential of ohmic broth correction of -930 mV / NHE at 3 kA / m2 with hydrogen release in 33% NaOH, at a temperature of 90 ° C.

Then, the same sample was subjected to cyclic voltameter in the range of -1 to +0.5 V / NHE at an exploration rate of 10 mV / s; after 25 cycles, the cathode potential was -975 mV / NHE, which indicates a current inversion of the appropriate resistance to operate in industrial electrolyzers free of polarization devices.

The above description is not intended to limit the present invention, which can be used in accordance with different embodiments without departing from the scope thereof, and the scope of which is defined only by the appended claims.

Throughout the description and claims of the present application, the term "understand" and variations thereof, for example, "comprising" and "comprising" are not intended to exclude the presence of other elements, components or additional steps of process.

The discussion of documents, minutes, materials, devices, articles and the like is included in this specification only in order to provide a context for the present invention. It is not suggested or represented that any or all of these subjects form part of the basis of the state of the art or are of general common knowledge in the relevant field for the present invention before the priority date of each revindication of the present application.

Claims (9)

5 10 fifteen twenty 25 30 1. An electrode suitable for use as a cathode in electrolytic processes comprising a metal substrate equipped with a catalytic coating, said catalytic coating comprising, an internal layer containing platinum directly in contact with the substrate, at least one intermediate layer consisting in a mixture of oxides containing 40-60% by weight of rhodium relative to the elements, and an outer layer of ruthenium oxide. 2. The electrode according to claim 1, wherein said metal substrate is made of nickel. 3. The electrode according to claim 1 or 2, wherein said at least one intermediate layer contains 10-30% by weight of palladium and 20-40% by weight of rare earth relative to the elements. 4. The electrode according to claim 3, wherein said rare earths consist of praseodymium. 5. The electrode according to claim 3 or 4, wherein the specific platinum charge in said inner layer is from 0.3 to 1.5 g / m2, the sum of the specific charges of rhodium, palladium and rare earths in said intermediate layer is 1 and 3 g / m2 and the specific loading of ruthenium in said outer layer is 2 and 5 g / m2. 6. A method for manufacturing an electrode according to one of the preceding claims comprising the following steps: a) application of an acetic solution of Pt (NH3) 2 (NO3) 2 to a metal substrate, with subsequent drying at 80-100 ° C and thermal decomposition at 450-600 ° C; b) optional repetition of step a) until obtaining a catalytic internal layer with a specific load of 0.3-1.5 g / m2 of Pt; c) application of an acetic solution containing a rhodium nitrate with optional addition of palladium and rare earth nitrates in said internal catalytic layer, with subsequent drying at 80-100 ° C and thermal decomposition at 450-600 ° C; d) optional repetition of step c) until obtaining a catalytic intermediate layer with a specific load of 1-3 g / m2 as the sum of Rh, Pd and rare earths; e) application of an acetic solution containing nitrosyl nitrate of Ru in said intermediate catalytic layer, with subsequent drying at 80-100 ° C and thermal decomposition at 450-600 ° C; f) optional repetition of step e) until obtaining an internal catalytic layer with a specific load of 2-5 g / m2 of Ru. 7. The method according to claim 6, wherein the temperature of said thermal decomposition of steps a), c) and e) ranges from 480 to 520 ° C. 8. An electrolysis cell comprising an anodic compartment and a cathode compartment separated by an ion exchange membrane, in which the cathode compartment is equipped with an electrode according to any one of claims 1 to 5. 9. An electrolyzer for the production of chlorine and alkali from alkali brine free of protective polarization devices comprising a modular arrangement of cells according to claim 8.