Classifications

• • 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
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
  • C23C4/00—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge
  • C23C4/18—After-treatment
• • 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
  • C25B11/052—Electrodes comprising one or more electrocatalytic coatings on a substrate
  • C25B11/053—Electrodes comprising one or more electrocatalytic coatings on a substrate characterised by multilayer electrocatalytic coatings

Definitions

• Electrodes for use in electrolytic processes have been known which have a base or core metal bearing a layer or coating of metal oxides.
• the core metal of the electrode may be a valve metal such as titanium, tantalum, zirconium, niobium or tungsten.
• the coating is an oxide mixture
• an oxide of the core or substrate metal can contribute to the mixture.
• such mixture can include an oxide of the substrate metal plus at least one oxide of a metal such as platinum, iridium, rhodium, palladium, ruthenium, and osmium.
• such mixture which can be termed a noble metal oxide mixture
• Such have been taught generally in U.S. Patent 3,632,498 and examples shown specifically, when combined with titanium oxide, in U.S. Patent 3,948,751.
• an aqueous alkali-metal halide e.g., sodium chloride
• it has been taught in U.S. Patent 4,005,004 that such noble metal oxide mixture can be particularly serviceable when in further mixture with both titanium oxide and zirconium oxide.
• Such a mixture yields a solid solution coating that ostensibly enhances the practical utilization of the electrodes for their intended use.
• the invention is broadly directed to an electrode having reduced oxygen evolution during electrolysis of halogen-containing solutions particularly at low pH, such electrode comprising an electrically conductive metal substrate having a coating of enhanced stability under alkaline conditions, which coating comprises at least 15, but less than 25, mole percent iridium oxide, 35-50 mole percent ruthenium oxide and at least 30, but less than 45 mole percent titanium oxide basis 100 mole percent of the oxides present in the coating.
• the coating has a molar ratio of titanium oxide to the total of the oxides of iridium and ruthenium of less than 1: 1, and should have a molar ratio of ruthenium oxide to iridium oxide of greater than 1.5: 1 and up to 3: 1.
• the invention is directed to a coating composition adapted for providing the foregoing described mixed metal oxide coating and in a still further aspect is directed to the method of making an electrode which is hereinbefore defined.
• the electrode will be particularly useful as an anode in a membrane cell used for the electrolysis of brine that is at a pH within the range of from about 2 to about 4.
• the coating composition of the present invention is broadly applicable to any electrically conductive metal substrate which will be sufficiently electrically conductive to serve as an electrode in an electrolysis process.
• the metals of the substrate are broadly contemplated, but in view of the application of an electrocatalytic coating, the substrate metals more typically may be such as nickel or manganese, or most always the valve metals, including titanium, tantalum, aluminum, tungsten, zirconium and niobium. Of particular interest for its ruggedness, corrosion resistance and availability is titanium.
• the suitable metals of the substrate can include metal alloys and intermetallic mixtures.
• titanium may generally be alloyed with nickel, cobalt, iron, manganese or copper.
• Grade 5 titanium may include up to 6.75 weight % aluminum and 4.5 weight % vanadium, grade 6 up to 6% aluminum and 3% tin, grade 7 up to 0.25 weight % palladium, grade 10, from 10 to 13 weight % molybdenum plus 4.5 to 7.5 weight % zirconium and so on.
• the coating composition applied to the coated metal substrate will be aqueous, which will most always be simply water without any blending with further liquid.
• aqueous compositions that are serviceable will be solutions of precursor constituents in the aqueous medium, that is, precursors to the oxides that will be present in the coating.
• the precursor constituents utilized in the aqueous solution are those which can be solubilized in water efficiently and economically, e.g., achieve solution without extensive boiling condition.
• the precursors must supply the respective metal oxide on thermodecomposition. Where they are all present in the same composition, they must also be compatible with one another.
• each precursor constituent will be a metal salt that most often is a halide salt and preferably for economy coupled with efficiency of solution preparation such will all be the chloride salt.
• other useful salts include iodides, bromides and ammonium chloro salts such as ammonium hexachloro iridate or ruthenate.
• a solution of iridium trichloride can further contain strong acid, most always hydrochloric acid, which will usually be present in an amount to supply about 5 to 20 weight percent acid.
• the individual or combination solutions will have a pH of less than 1, such as within the range of from about 0.2 to about 0.8.
• the coating composition is a solution of all precursor constituents, such will contain at least 15, but less than 25, mole percent of the iridium constituent, 35-50 mole percent of the ruthenium constituent, and at least 30, but less than 45, mole percent of the titanium constituent, basis 100 mole percent of these constituents.
• a composition containing an iridium constituent in an amount of less than 15 mole percent will be inadequate for providing an electrode coating having the best caustic stability, such as when the electrode is used in a chlor-alkali cell.
• less than 25 mole percent for the iridium precursor will be desirable for best low operating potential efficiency for the coating.
• the ruthenium In regard to the ruthenium, a constituent amount in the solution of less than about 35 mole percent will be insufficient to provide the most efficient low chlorine potential for resulting coatings, while an amount not greater than 50 mole percent enhances coating stability. Also, for best coating characteristics, the molar ratio of ruthenium oxide to iridium oxide in the resulting coating will be from greater than 1.5: 1 up to 3: 1.
• the coating solution will contain constituents in a proportion such as to provide from about 18-22 mole percent iridium, 35-40 mole percent ruthenium, and 40-44 mole percent titanium.
• the resulting coating will furthermore have a molar ratio of titanium oxide to the total of the oxides of iridium ruthenium of less than 1: 1, but most always above 0.5: 1.
• the substrate metal Before applying the coating composition to the substrate metal, the substrate metal advantageously is a cleaned surface. This may be obtained by any of the treatments used to achieve a clean metal surface, including mechanical cleaning. The usual cleaning procedures of degreasing, either chemical or electrolytic, or other chemical cleaning operation may also be used to advantage.
• the substrate preparation includes annealing, and the metal is grade 1 titanium
• the titanium can be annealed at a temperature of at least about 450°C. for a time of at least about 15 minutes, but most often a more elevated annealing temperature, e.g., 600°-875°C. is advantageous.
• etching it will be with an active etch solution.
• Typical etch solutions are acid solutions. These can be provided by hydrochloric, sulfuric, perchloric, nitric, oxalic, tartaric, and phosphoric acids as well as mixtures thereof, e.g., aqua regia.
• Other etchants that may be utilized include caustic etchants such as a solution of potassium hydroxide/hydrogen peroxide in combination, or a melt of potassium hydroxide with potassium nitrate.
• the etch solution is advantageously a strong, or concentrated, aqueous solution, such as an 18-22 weight % solution of hydrochloric acid, or a solution of sulfuric acid.
• the solution is advantageously maintained during etching at elevated temperature such as at 80°C. or more for aqueous solutions, and often at or near boiling condition or greater, e.g., under refluxing condition.
• the etching will prepare a roughened surface, as determined by aided, visual inspection. Following etching, the etched metal surface can then be subjected to rinsing and drying steps to prepare the surface for coating.
• the coating composition can then be applied to the metal substrate by any means for typically applying an aqueous coating composition to a substrate metal.
• Such methods of application include brush, roller, and spray application.
• combination techniques can be utilized, e.g., spray and brush technique.
• Spray application can be either conventional compressed gas or can be electrostatic spray application.
• electrostatic spray application will be used for best wrap around affect of the spray for coating the back side of an article such as a mesh electrode.
• the applied composition will be heated to prepare the resulting mixed oxide coating by thermodecomposition of the precursors present in the coating composition.
• This prepares the mixed oxide coating containing the mixed oxides in the molar proportions as above discussed.
• Such heating for the thermodecomposition will be conducted at a temperature of at least about 440°C. peak metal temperature for a time of at least about 3 minutes. More typically the applied coating will be heated at a more elevated temperature for a slightly longer time, but usually a temperature of greater than about 550°C. is avoided for economy and to avoid detrimental effects on anode potential where the coated metal will serve as an anode. Suitable conditions can include heating in air or oxygen.
• the heated and coated substrate will usually be permitted to cool to at least substantially ambient temperature.
• the resulting finished coating has a smooth appearance to the unaided eye, but under microscopic examination is seen to be non-homogeneous, having embedded crystallites in the field of the coating.
• a coating solution was prepared by combining 157 gms of iridium, using a solution of iridium trichloride in 18% by weight HCl, 144 gms of ruthenium, using a solution of ruthenium trichloride in 18% by weight HCl, 80 gms of titanium, using titanium tetrachloride in 10% by weight HCl solution, 331 gms HCl, using 36 weight % solution, then diluting to 10 liters with deionized water.
• This provided a coating composition having 21 mole % iridium; 36.3 mole % ruthenium, and 42.7 mole % titanium.
• Four liters of 93 grams per liter (gpl) HCl solution were then added to make the final coating solution.
• This solution was applied using a hand roller to a titanium mesh substrate having a diamond-patterned mesh, with each diamond pattern having about 8 millimeters (mms.) long way of design plus about 4 mms. short way of design.
• the titanium mesh had been annealed at 600°C. for 30 minutes and etched in 25 wt % sulfuric acid at 85-90°C., then water rinsed and air dried. The applied coating was air dried then baked at 470°C. Eighteen (18) coats were applied in this manner. After the final coat, the anode was postbaked at 525°C. for 4 hours.
• the average caustic weight loss of 5.27 gm/m2 was especially noteworthy since a comparative coating having 7.8 mole percent iridium oxide, 15 mole percent ruthenium oxide and 77.2 mole percent titanium oxide exhibited such weight loss of 8.9 gm/m2 when tested under the same conditions. Moreover, again comparatively, but as the mole percent changed to more closely approach the invention composition, but still in a comparative coating, the caustic weight loss increased to 19.2 gm/m2

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

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Publications (2)

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• 1990
  • 1990-11-15 CA CA002030092A patent/CA2030092C/fr not_active Expired - Fee Related
  • 1990-11-28 JP JP2328816A patent/JPH03188291A/ja active Pending
  • 1990-11-28 AR AR90318490A patent/AR245508A1/es active
  • 1990-11-30 BR BR909006109A patent/BR9006109A/pt not_active IP Right Cessation
  • 1990-12-04 AT AT90810945T patent/ATE127863T1/de not_active IP Right Cessation
  • 1990-12-04 DE DE69022386T patent/DE69022386T2/de not_active Expired - Fee Related
  • 1990-12-04 EP EP90810945A patent/EP0437178B1/fr not_active Expired - Lifetime
  • 1990-12-04 ES ES90810945T patent/ES2076349T3/es not_active Expired - Lifetime
  • 1990-12-07 KR KR1019900020120A patent/KR910012344A/ko not_active Application Discontinuation
  • 1990-12-07 NO NO905305A patent/NO302858B1/no unknown
  • 1990-12-07 AU AU67858/90A patent/AU640719B2/en not_active Ceased

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