Classifications

• • C25B11/0442—
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/042—Electrodes formed of a single material
  • C25B11/046—Alloys
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/055—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the substrate or carrier material
  • C25B11/057—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the substrate or carrier material consisting of a single element or compound
  • C25B11/061—Metal or alloy
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/091—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds
  • C25B11/093—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds at least one noble metal or noble metal oxide and at least one non-noble metal oxide
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B11/00—Electrodes; Manufacture thereof not otherwise provided for
  • C25B11/04—Electrodes; Manufacture thereof not otherwise provided for characterised by the material
  • C25B11/051—Electrodes formed of electrocatalysts on a substrate or carrier
  • C25B11/073—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material
  • C25B11/091—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds
  • C25B11/097—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of at least one catalytic element and at least one catalytic compound; consisting of two or more catalytic elements or catalytic compounds comprising two or more noble metals or noble metal alloys

Definitions

• the invention relates to an electrode suitable for operating as anode in electrolysis cells, for instance as chlorine-evolving anode in chlor-alkali cells.
• electrodes consisting of a metal substrate equipped with a coating based on noble metals or oxides thereof are for instance utilised as cathodes for hydrogen evolution in water or alkali chloride electrolysis processes, as anodes for oxygen evolution in electrometallurgical processes of various kinds or for chlorine evolution in alkali chloride electrolysis. Electrodes of such kind can be produced via thermal route, i.e. by suitable thermal decomposition of solutions containing the precursors of metals to be deposited; by galvanic electrodeposition from suitable electrolytic baths; by direct metallisation via flame or plasma spraying processes or chemical or physical phase vapour deposition.
• the electrolysis of sodium chloride brine directed to the production of chlorine and caustic soda is often carried out with anodes consisting of a titanium or other valve metal substrate activated with a superficial layer or ruthenium dioxide (RuO 2 ) in order to lower the overvoltage of the anodic chorine evolution reaction.
• anodes consisting of a titanium or other valve metal substrate activated with a superficial layer or ruthenium dioxide (RuO 2 ) in order to lower the overvoltage of the anodic chorine evolution reaction.
• RuO 2 ruthenium dioxide
• catalyst formulations based on mixtures of oxides of ruthenium, iridium and titanium are also known, all capable of lowering the overvoltage of the anodic chorine evolution reaction.
• Electrodes of such kind are generally produced via thermal route.
• Catalytic formulations can be deposited on the substrate by phase vapour deposition techniques, having the advantage of allowing an extremely accurate control of coating deposition parameters.
• these are fundamentally characterised by being batch-type processes, requiring the loading of the substrate in a suitable deposition chamber, which has to undergo a slow depressurisation process, lasting several hours, in order to be able to treat a single piece.
• the remarkable duration of the process severed hours being usually necessary, depending on the required noble metal loading
• the application of high amounts of catalytic coatings leads to coatings having a very limited lifetime.
• the present invention relates to an electrode for evolution of gaseous products in electrolysis cells consisting of a valve metal substrate coated with at least one first catalytic composition and with an outer catalytic composition, said at least one first catalytic composition comprising a mixture of oxides of a valve metal or of tin and of noble metals selected from the group of platinum metals (PM) or oxides thereof taken alone or in admixture, said at least one first catalytic composition obtained by thermal decomposition of precursors, said outer catalytic composition comprising noble metals selected from the group of platinum metals or oxides thereof taken alone or in admixture, said outer catalytic composition being deposited by means of a chemical or physical phase vapour deposition technique, the amount of noble metal on said first catalytic composition being higher than 5 g/m 2 of surface and the amount of noble metal in said outer catalytic composition ranging between 0.1 and 3.0 g/m 2 of surface.
• the inventors have surprisingly found out that the deposition of one last catalytic layer, with the specified characteristics, through chemical or physical phase vapour allows obtaining an electrode with unexpected features both in terms of duration and of potential decrease.
• the first catalytic composition of the electrode according to the invention comprises titanium, iridium, ruthenium in form of metals or oxides.
• the outer catalytic composition comprises ruthenium and/or iridium in form of metals or oxides.
• the specific noble metal loading in the first catalytic composition ranges between 6 and 8 g/m 2 and the specific metal loading in the outer catalytic composition ranges between 1.5 and 2.5 g/m 2 .
• the invention relates to a method of manufacturing an electrode comprising the deposition of an outer catalytic composition by chemical or physical phase vapour deposition, preferably by reactive sputtering of noble metals selected in the group of platinum metals.
• the invention relates to the reactivation of a used electrode comprising the chemical or physical phase vapour deposition of an outer catalytic composition including noble metals selected from the group of platinum metals or oxides thereof taken alone or in admixture.
• the invention relates to an electrolysis cell of alkali chloride solutions, for instance a sodium chloride brine electrolysis cell directed to producing chlorine and caustic soda, which effects the anodic evolution of chlorine on an electrode as hereinbefore described.
• a sample of titanium mesh of 10 cm ⁇ 10 cm size was blasted with corundum, cleaning the residues with a jet of compressed air.
• the sample was then degreased using acetone in a ultrasonic bath for about 10 minutes. After drying, the sample was dipped into an aqueous solution containing 250 g/l of NaOH and 50 g/l of KNO 3 at about 100° C. for 1 hour. After the alkaline treatment, the sample was rinsed in deionised water at 60° C. for three times, changing the liquid every time. The last rinse was carried out adding a small quantity of HCl (about 1 ml per litre of solution).
• the solution was applied to the sample of titanium mesh by brushing in five coats; after each coat, a drying at 100-110° C. for about 10 minutes was carried out, followed by a thermal treatment of 15 minutes at 450° C. The sample was cooled in air each time prior to applying the subsequent coat.
• a sample of titanium mesh of 10 cm ⁇ 10 cm size was blasted with corundum, cleaning the residues with a jet of compressed air.
• the sample was then degreased using acetone in a ultrasonic bath for about 10 minutes. After drying, the sample was dipped into an aqueous solution containing 250 g/l of NaOH and 50 g/l of KNO 3 at about 100° C. for 1 hour. After the alkaline treatment, the sample was rinsed in deionised water at 60° C. for three times, changing the liquid every time. The last rinse was carried out adding a small quantity of HCl (about 1 ml per litre of solution). An air drying was effected, observing the formation of a brown hue due to the growth of a thin film of TiO x .
• the mesh sample was then introduced into the vacuum chamber of the reactive sputtering equipment.
• the sputtering targets were polarised at the following powers: ruthenium 35 W, iridium 24 W, titanium 250 W.
• the target-electrode substrate gap was about 10 centimetres.
• the process of deposition was carried out, at the same conditions, alternatively on the two sides of the titanium mesh for an overall duration of 220 minutes.
• the thus obtained electrode presented a catalytic coating of about 1 micron and a total noble metal loading of about 9 g/m 2 , expressed as the sum of Ru and Ir referred to the metals.
• a sample of titanium mesh of 10 cm ⁇ 10 cm size was blasted with corundum, cleaning the residues with a jet of compressed air.
• the sample was then degreased using acetone in a ultrasonic bath for about 10 minutes. After drying, the sample was dipped into an aqueous solution containing 250 g/l of NaOH and 50 g/l of KNO 3 at about 100° C. for 1 hour. After the alkaline treatment, the sample was rinsed in deionised water at 60° C. for three times, changing the liquid every time. The last rinse was carried out adding a small quantity of HCl (about 1 ml per litre of solution). An air drying was effected, observing the formation of a brown hue due to the growth of a thin film of TiO x .
• the solution was applied to the sample of titanium mesh by brushing in five coats; after each coat, a drying at 100-110° C. for about 10 minutes was carried out, followed by a thermal treatment of 15 minutes at 450° C. The sample was cooled in air each time prior to applying the subsequent coat.
• the semi-finished electrode was then introduced into the vacuum chamber of the reactive sputtering equipment.
• the sputtering targets were polarised at the following powers: ruthenium 30 W, iridium 35 W.
• the target-electrode substrate gap was about 10 centimetres.
• the substrate was also subjected to a residual polarisation of about 150 V.
• the process of deposition was carried out, at the same conditions, alternatively on the two sides of the electrode for an overall duration of 40 minutes.
• the thus obtained electrode had an outer catalytic coating about 0.1 pm thick and a total noble metal loading of about 9 g/m 2 , expressed as the sum of Ru and Ir referred to the metals.
• the samples of the previous examples were also subjected to a duration test.
• Said duration test is the simulation in a separated cell of industrial electrolysis conditions as regards electrolyte concentration and temperature, but at a current density conveniently increased up to a value 2-3 times higher than the nominal one for the sake of accelerating the experimental response.
• Table 2 reports the precious metal lost per unit current.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• Electrochemistry (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Electrodes For Compound Or Non-Metal Manufacture (AREA)
• Electrolytic Production Of Non-Metals, Compounds, Apparatuses Therefor (AREA)
• Catalysts (AREA)
• Inorganic Chemistry (AREA)
• Electrolytic Production Of Metals (AREA)
• Inert Electrodes (AREA)

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