The present invention relates to a cathode which can be used for the electrolysis of aqueous solutions in which a water-reduction reaction takes place.
More particularly, the present invention relates to an activated cathode which can be used for the electrolysis of alkaline aqueous solutions of alkali metal chlorides, and most particularly for the preparation of chlorine and sodium hydroxide.
Thus, chlorine and sodium hydroxide are manufactured industrially in electrolytic cells, each of these cells comprising several mild steel cathodes and several titanium anodes coated with a mixture of titanium oxide and ruthenium oxide. They are generally fed in an electrolytic solution consisting of about 200 to 300 g/l of sodium chloride.
However, these mild steel cathodes have a relatively high overvoltage in absolute value terms as water-reducing cathodes and also have an insufficient resistance to corrosion by dissolved chlorine.
The term xe2x80x9covervoltagexe2x80x9d means the difference between the thermodynamic potential of the redox couple concerned (H2O/H2) relative to a reference cathode and the potential effectively measured in the medium concerned, relative to the same reference electrode. By convention, the term overvoltage will be used to denote the absolute value of the cathode overvoltage.
Many cathodes have been proposed to overcome these drawbacks.
Thus, French patent application FR 2 311 108 discloses a cathode whose substrate is a plate made of titanium, zirconium, niobium or of an alloy essentially consisting of a combination of these metals, and on which is applied a layer of metal oxide, consisting essentially of an oxide of one or more metals chosen from ruthenium, rhodium, palladium, osmium, iridium and platinum and optionally an oxide of one or more metals chosen from calcium, magnesium, strontium, barium, zinc, chromium, molybdenum, tungsten, selenium and tellurium.
U.S. Pat. No. 4,100,049 describes a cathode comprising a substrate made of iron, nickel, cobalt or an alloy of these metals and a coating of palladium oxide and zirconium oxide.
European patent application EP 209 427 discloses a cathode consisting of an electrically conductive substrate made of nickel, stainless steel or mild steel bearing a coating consisting of a plurality of metal oxide layers, the surface layer consisting of an oxide of a valve metal, i.e. a metal chosen from groups 4b, 5b and 6b of the Periodic Table of the Elements, and the intermediate layer consisting of an oxide of a precious metal from group VIII, i.e. ruthenium, rhodium, palladium, osmium, iridium and platinum.
The intermediate and surface layers can consist of the oxide of the single metal concerned or of a mixed oxide of the metal concerned and of the second metal in small proportion.
Although these cathodes have a satisfactory overvoltage, the Applicant has observed, during the evaluation of the said cathodes, a modification of the polarization curve after the first sweep, revealing damage to the surface layer, which is detrimental to correct protection of the substrate and thus results in a limited lifetime of the said electrodes.
A cathode has now been found which can reduce the overvoltage of the water-reducing reaction in alkaline medium, characterized in that it consists of an electrically conductive substrate coated with an intermediate layer of oxides based on titanium and on a precious metal from group VIII of the Periodic Table of the Elements and with an outer layer of metal oxides comprising titanium, zirconium and a precious metal from group VIII of the Periodic Table of the Elements.
The expression xe2x80x9cprecious metal from group VIII of the Periodic Table of the Elementsxe2x80x9d means, in the present text, ruthenium, rhodium, palladium, osmium, iridium or platinum. Ruthenium or iridium will preferably be used, and most particularly ruthenium.
The intermediate layer advantageously contains titanium oxide and ruthenium oxide.
The outer layer of metal oxides preferably contains titanium oxide, zirconium oxide and ruthenium oxide.
Better still, the outer layer consists essentially of ZrTiO4 accompanied by RuO2 and optionally ZrO2 and/or TiO2.
The material of which the substrate is made can be chosen from electrically conductive materials. It will be chosen advantageously from the group consisting of titanium, nickel, tantalum, zirconium, niobium, iron and alloys thereof.
Titanium, nickel, iron or alloys thereof will preferably be chosen.
The precious metal/titanium molar ratio in the intermediate layer is preferably between 0.4 and 2.4.
The zirconium/titanium molar ratio in the outer layer is generally between 0.25 and 9 and preferably between 0.5 and 2.
The precious metal in the outer layer is at least equal to 10 mol %, preferably between 30 mol % and 50 mol %, relative the metals forming part of the composition of this layer.
The cathode according to the present invention can be prepared according to a process which consists in carrying out the following steps:
a) pretreating a substrate to give it surface-roughness properties,
b) coating the pretreated substrate using a solution A essentially containing titanium and a precious metal, followed by drying and then calcination of the substrate thus coated,
c) coating the substrate obtained in b) using a solution B comprising titanium, zirconium and a precious metal, followed by drying and calcination of the substrate thus coated.
The pretreatment generally consists in subjecting the substrate, either to a sanding operation optionally followed by washing with acid, or to a stripping operation using an aqueous solution of oxalic acid, hydrofluoric acid, a mixture of hydrofluoric acid and nitric acid, a mixture of hydrofluoric acid and glycerol, a mixture of hydrofluoric acid, nitric acid and glycerol or a mixture of hydrofluoric acid, nitric acid and hydrogen peroxide, followed by one or more washes with degassed demineralized water.
The substrate can be in the form of a solid plate, a perforated plate, expanded metal or a cathode basket consisting of expanded or perforated metal.
Solution A is generally prepared by placing in contact, at room temperature and with stirring, essentially a mineral salt or organic salt of titanium and of a precious metal with water or in an organic solvent, optionally in the presence of a chelating agent. The temperature can be raised above room temperature to help the salts to dissolve.
Advantageously, a mineral salt or organic salt of titanium and of a precious metal is placed in contact with water or in an organic solvent, optionally in the present of a chelating agent.
The titanium and the precious metal are preferably present in solution A at a concentration of not more than 10 mol/l.
Solution B is generally prepared by placing in contact, at room temperature and with stirring, a mineral salt or organic salt of titanium, of zirconium and of a precious metal with water or in an organic solvent, optionally in the presence of a chelating agent. When the placing in contact is exothermic, a bath of ice is used to cool the reaction medium.
Advantageously, a mineral salt or organic salt of titanium, of zirconium and of a precious metal is placed in contact with water or in an organic solvent, optionally in the presence of a chelating agent.
The preferred titanium and precious metal salts are the chlorides, oxychlorides, nitrates, oxynitrates, sulphates and alkoxides. The chlorides of precious metals, ruthenium chlorides, titanium chlorides and titanium oxychlorides are advantageously used.
Zirconium salts which can be used are the chlorides and sulphates, zirconyl chloride, zirconyl nitrate and alkoxides such as butyl zirconate.
Zirconyl and zirconium chlorides are particularly preferred.
Organic solvents which may be mentioned are light alcohols, preferably isopropanol and ethanol, and better still isopropanol and absolute ethanol.
Although it is possible, indifferently, to use water or an organic solvent to prepare solution B, it is nevertheless preferred to use an organic solvent when the metal salts are solid at room temperature.
Thus, when the metal salt is zirconium chloride, absolute ethanol or absolute isopropanol is used as solvent.
Titanium and zirconium are generally present in solution B at a concentration ranging from 0.5 to 5 mol/l. The concentration of precious metal in solution B is generally between 0.05 and 10 mol/l and preferably between 0.1 and 5 mol/l.
Solution A can be deposited on the pretreated substrate using various techniques such as sol-gel, electroplating, galvanic electroplating, spraying or coating. The pretreated substrate is advantageously coated with solution A, for example using a brush. The substrate thus coated is then air-dried and/or dried in an oven at a temperature below 150xc2x0 C. After drying, the substrate is calcined in air or under inert gases such as nitrogen or argon or alternatively under oxygen-enriched inert gases at a temperature at least equal to 300xc2x0 C. and preferably between 450xc2x0 C. and 550xc2x0 C., for a period ranging from 10 minutes to 2 hours.
For step c) of the process, the same deposition techniques and the same drying and calcination operating conditions as in step b) can be used, except that the deposition is carried out with solution B.
Other techniques such as chemical vapour deposition (CVD), physical vapour deposition (PVD) or plasma spraying are also suitable for coating the pretreated substrate with an intermediate layer and an outer layer.
Solution A can be deposited either on one of the sides of the pretreated substrate or on both sides. Solution B can also be deposited on both sides of the substrate coated with the intermediate layer.
Depending on the desired thickness of the intermediate layer, step b) of the process can be repeated several times. Similarly, step c) of the process can be repeated several times.
In the intermediate layer, the mass of product deposited is at least equal to 2 g/m2, generally between 10 g/m2 and 60 g/m2 and preferably between 20 g/m2 and 35 g/m2, relative to the geometrical area of the substrate.
The concentration of solution A is judiciously chosen such that this preferred deposited mass can be obtained by repeating step b) a reasonable number of times and preferably between 1 and 10 times.
In the outer layer, the mass of product deposited is at least equal to 5 g/m2, generally between 5 g/m2 and 70 g/m2 and preferably between 25 g/m2 and 50 g/m2 relative to the geometrical area of the substrate. Solution B is generally prepared such that its concentration makes it possible to obtain a preferred deposited mass by repeating step (c) at least once and preferably between 2 and 10 times.
The cathode of the present invention is most particularly suitable for the electrolysis of aqueous solutions of alkali metal chlorides and in particular of aqueous NaCl solutions.
The use of the cathode in combination with an anode allows the electrolytic synthesis of chlorine and the alkali metal hydroxide in a high Faraday yield.
Anodes which may be mentioned are DSA anodes (Dimensionally Stable Anodes) consisting of a titanium substrate coated with a layer of titanium oxide and ruthenium oxide. The ruthenium/titanium molar ratio in this layer is advantageously between 0.4 and 2.4.
The cathode of the present invention has the advantage of having a lower overvoltage than the cathodes of the prior art during electrolysis functioning.
In addition, the cathode of the present invention does not undergo any change from the very first characterization cycles and shows greater chemical stability with respect to corrosive alkaline media.