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

Definitions

• Electrodes are known to be useful in a variety of electrolytic applications such as chlor-alkali electrolysis, electrowinning, cathodic protection, electroplating, electro-organic synthesis and the like.
• these electrodes consist of an electrically conductive substrate, inert to the electrolytic environment, bearing on the surface thereof on electrically conductive electrocatalytically-active coating.
• valve metal titanium, tantalum
• platinum group metal platinum group metal
• mixed oxide coating of valve and platinum metal oxides although many others are known.
• electrically conductive substrate it is intended to refer to the metallic support for the elec trode. It is essential that this substrate be resistant to the electrolytic environment in which it will be employed in order that, upon mechanical failure of the coating, excessive attack will not occur.
• a valve metal such as titanium, tantalum, zirconium or an alloy thereof is employed. However, it is not essential that the entire substrate be of this material. Rather, a core of a more highly conductive metal, such as copper or aluminum, may be employed if clad or coated with the valve metal.
• a valve metal may carry a surface coating designed to alter electrode properties, such as a platinum metal layer.
• the physical form of the substrate is independent of the invention and it may be flat or shaped, continuous or foraminous, as required and desired for the particular end use envisioned.
• binder it is intended to refer to any material, organic or inorganic, capable of adhering the particulate solid solution to the underlying substrate in a manner relatively permanent under the conditions obtaining during use.
• the binder itself need not be, and most often is not, electrically conductive, a sufficient amount of the solid solution being dispersed throughout the binder to provide a path for current from the underlying substrate to the electrolyte.
• a diverse group of materials is contemplated, such as the fluorocarbons, e.g., polyvinylidene fluoride; sodium silicate (subjected to a subsequent acid treatment); an inorganic glaze of refractory materials such as titania or silica glasses, and the like.
• amorphous oxide binders which may be obtained by thermal decomposition of a solution of the salts thereof in the presence of oxygen, especially the amorphous valve metal oxides and particularly tantalum and niobium oxides. These latter two oxides have been noted to produce remarkably uniform and adherent coatings without detracting from the activity of the electrocatalytic material.
• solid solution it is intended to refer to those mixtures of platinum group metal oxides in which atoms of the different platinum metals are randomly distributed throughout the parent metal oxide crystal lattice, thus giving the resultant solid solution properties different from either platinum metaloxide alone or a mere physical combination of same.
• the platinum group metals contemplated are ruthenium, rhodium, palladium, osmium, iridium and platinum, preferably ruthenium, palladium, rhodium and iridium.
• Particularly preferred are solid solutions of ruthenium and iridium oxides.
• at least one percent of the second platinum metal oxide must be present in the parent platinum metal oxide crystal lattice.
• At least one mole percent of ruthenium oxide must be present in an iridium-ruthenium system. It is by manipulation of the relative amounts of platinum metal oxides present that it is possible to control the properties of the electrodes, particularly with respect to the relative overvoltages at which different elements are discharged when employing the electrodes as anodes in aqueous solution.
• a low chlorine overvoltage may be coupled with a high oxygen overvoltage, especially when the solid solution is of 25% RuO,/% lrO on a mole basis, thereby reducing the likelihood of contamination of the chlorine with oxygen and the attendant current inefficiency.
• a low oxygen overvoltage electrode may be designed for use as an electrowinning anode.
• anode employing 92-93% RuOJ7-8% [r0 in an amorphous tantalum binder and on a titanium substrate is particularly suited to use in the electrolysis of dilute aqueous brine to form hypochlorite.
• the amount of binder present will be that sufficient to adhere the particulate solid solution to the underlying substrate in a continuous fashion without being so much as to completely isolate the individual particles. This is particularly important where the binder is not itself electrically-conductive under conditions of use since, otherwise, disruption of electrical continuity will result. In most instances, this means that from 0.0ll .0 part by weight of binder will be used per part of particulate solid solution.
• the electrodes of this invention depend for their versatility upon the formation of the particulate solid solution prior to its application to the electrode. While this preparation may be accomplished in a number of ways known to the art, it is preferred, in order to obtain the material in the desired particle size of less than 0.] micron, that the solid solution be precipitated from an acidic solution of platinum metal salts. Thus, the desired concentrations of the appropriate metals are dissolved in acid solution, either jointly or separately followed by combination. The pH of the solution is then slowly raised, generally to a value in excess of 8, causing precipitation of the material as a hydrated oxide. This precipitate is now filtered, washed and dried at a temperature sufficient to remove the water, e.g., 110 C. The solid solution of platinum metal oxides is then preferably heat-treated at a temperature within the range of 300900 C preferably 450800 C, still more preferably 600-700 C, to crystallize and stabilize same.
• a dispersion of the aforesaid particulate solid solution is formed in a solution containing the desired binder.
• This solution may consist of an organic polymeric binder dissolved in an appropriate solvent, such as polyvinylidene fluoride in l-methyl-2-pyrr0lidinone, or an inorganic salt, such as sodium silicate, in water, in which case a subsequent acid treatment of the electrode may be required to insolubilize the binder.
• the solid solution will be dispersed in a solution of a valve metal salt, such as TaCl or NbCl in an appropriate solvent, such as a combination of water, butyl alcohol and hydrochloric acid.
• the salt used will be one decomposable at elevated temperatures, e.g., within the range of 300-600 C, and in the presence of oxygen to an amorphous valve metal oxide.
• the amount of solvent required to form the dispersion is that sufficient to dissolve the binder, suspend the dispersion and still result in a sufficient viscosity to facilitate application to the underlying substrate. Such amounts will vary depending upon the materials involved, the ratio of binder to solid solution and the like. Amounts within the range sufficient to form a dispersion containing from 5-50 percent by weight solids are exemplary.
• the electrode is formed by applying the dispersion, e.g., by brushing, spraying or roller coating, to the surface of the electrically conductive substrate followed by heating the thus-coated substrate to a temperature within the range of from 300-600 C in the presence of oxygen.
• EXAMPLE 1 An electrocatalytic material is prepared by dissolving 34.5 grams lrCl -xH O (52.9% Ir) in 1 liter of water containing 25 milliliters HCl and 10 ml HNO After boiling for 5 minutes and cooling, there is added a solution of 23.3 g RUClg'XHQG (40.7% Ru) in l liter of water containing 10 ml HCl. The pH of the resultant solution is adjusted to within the range of L5 to 2.5 with concentrated KOH and heated almost to boiling. While hot, a 10 percent aqueous KOH solution is slowly added, with stirring, to precipitate a hydrous oxide.
• the solution When a pH of 10 is obtained, the solution is brought to a boil, cooled and acidified to a pH of 6.5 with dilute aqueous l-lCi. The precipitate is then re moved by filtration and washed several times with water, following which it is dried at about 100 C. Finally, the oxide is ground and heated in air to a temperature of about 660C for about 16 hours.
• the resultant material has a particle size of less than 0.1 micron and contains 50 mole percent each of iridium and ruthenium oxides.
• the X-ray diffraction pattern shows a single rutile phase with peaks intermediate those of the pure oxides, thus indicating the formation of a solid solution. This same procedure is also employed to prepare solid solutions of other platinum metal oxides by simply altering the identity of the metal salts used. Likewise, the relative proportions of platinum metal oxides in the electrocatalytic material are varied by employing differing amounts of the salts in question.
• electrodes are prepared by forming a dispersion of 1.0 g of electrocatalytic material, 0.5 g of anhydrous TaCl and 1 ml HCl (35%) in l0 ml butanol. A dispersion is then applied to a previously degreased and etched titanium sheet in two coats, with air drying between coats, before heating in air at a temperature of 500 C for 5 minutes. This procedure is repeated three times to give the desired electrode for test purposes.
• the electrodes thus prepared are tested as anodes versus a saturated calomel electrode in l a 300 g/l sodium chloride solution for chloride evolution and (2) a l M H 50, solution for oxygen evolution, with the half-cell voltage measured at C and a current density of 3 amperes/square inch.
• the results appear in Table I.
• the material containing HO showing the greatest displacement and therefore being most efficient with respect to the loss of current to oxygen generation in sodium chloride electrolysis.
• EXAMPLE 2 For the purpose of comparison, Anode No. 4 of Ex- EXAMPLE 3 A further series of anodes is prepared by the procedure of Example 1 with varying amounts of lrO as indicated in Table 11 below.
• the substrate is an etched titanium mesh and four coatingheating cycles as described in Example 1.are employed.
• the anodes are used in the production of sodium hypochlorite by the electrolysis of a dilute (30 g/l) sodium chloride solution at a temperature of 25 C, and an anode current density of 1.0 a.s.i., opposite a titanium cathode. Table 11 indicates the current efficiencies obtained.
• the present invention which allows preparation of the electrocatalytic material extra the electrode and hence a wider range of heat treatment, affords a substantial advantage.
• EXAMPLE 4 A further example of the importance of an ability to termined by noting the time necessary for the cell voltage to rise from an original level of about 4.0 volts to a value of 8.0 volts, such a rise indicating passivation (inactivity) of the anode.
• EXAMPLE 5 To illustrate the use of an electrode of the present invention in the area of cathodic protection, Anode No. 3 from Example 1 is employed opposite a titanium metal cathode in synthetic sea water (28 g/l NaCl) having a pH of 9.5 at a temperature of 2126 C and an anode current density of 3.0 a.s.i. After 6,000 hours continuous operation, there is no change in the initially favorable operating voltage.
• anode comprising a 2:1 mole ratio solid solution coating of EXAMPLE 6
• a solid solution is formed by dissolving 34.5 g lrCl -x- H O (52.9% 1r) and 25.0 g RhCl 'xH o (39.1% Rh) in one liter of water containing 25 ml HCl.
• the pH of the resulting solution is adjusted to pH 1.5 2.5 with concentrated KOH.
• the hydrous oxides are coprecipitated by slowly adding a 10 percent aqueous KOH solution while stirring until a pH value of 10 is obtained.
• the solution is then warmed to 60-80 C, while maintaining a pH of 10, cooled and filtered. After several washings with water, the precipitate is dried at about 100 C.
• the oxide is ground and heated in air at 600 C for 16 hours.
• the resultant material contains 50 mole percent each of iridium and rhodium oxides and its X-ray diffraction pattern shows a single phase corundum structure with peaks displaced from that of pure R11 0,
• This catalyst when bound to a titanium substrate as in Example 1, results in an anode exhibiting excellent behavior in neutraland basic brine.
• EXAMPLE 7 A solid solution is prepared as in Example 1 using 23.3 g RuCl -xH O (40.7% Ru) and 27.4 g RhCl 'xH- O (39.1% Rh). The coprecipitated hydrous oxides are fired at 660 C for 20 hours. This material, containing 10 mole percent rhodium and mole percent ruthenium oxides, exhibits an X-ray diffraction pattern showing a single rutile phase with the peaks shifted from those of pure RuO An anode prepared from this material has shown excellent behavior in the electrolysis of brine and in the electrolysis of acid solutions producing oxygen.
• EXAMPLE 8 Another catalyst material is prepared by dissolving 23.3 g RuCl and 18.5 g PdCl (anhydrous) in 500 ml water, precipitating the hydrous oxides as in Example and firing at 550 C for 16 hours. The resulting material contains mole percent palladium and 90 mole percent ruthenium oxides. X-ray shows a single rutile phase with peaks shifted from pure RuO An electrode prepared from this electrocatalyst is useful in acidic oxygen-evolving applications.
• EXAMPLE 9 By the same procedures, 10% PdO 90% lrO and 10% PdO 90% R110 solid solutions are prepared.
• EXAMPLE 10 To illustrate the use of an electrode of the invention in aqueous electrowinning, an experiment isi conducted in an electrolytic cell employing a titanium mesh cathode, a porous polyfluorocarbon spacer (NA- FlON, trademark of E. l. du Pont de Nemours and Co., Inc.) and an anode comprising a titanium mesh substrate bearing a coating of 1 part amorphous tantalum oxide and 2 parts of a 90% RuO 10% HO; solid solution electrocatalyst.
• the anolyte is a 200 g/l aqueous slurry of chalcopyrite ore concentrate and ferric chloride in 10% HC].
• Hydrochloric acid (10%) is also the medium for the catholyte which is a leach of cuprous, ferric and ferrous ions from the ore slurry.
• Electrolysis is conducted initially at a temperature of 30 C and an anode current density of 0.3 a.s.i. (electrode gap, 075 inch), the operating cell voltage being 5.15.
• Copper metal (powder) and H are the cathode products while C1 CuCl, FeCl S and H O are all found in the anolyte (the important effect being the dissolution of additional copper values for later reduction on recycle to the catholyte).
• a voltage of 17.7 follows.
• An electrode consisting essentially of an electrically conductive substrate, an electrocatalytically active material and an amorphous valve metal oxide binder for adhering said material to at least a portion of the surface of said substrate.
• the material being a particulate solid solution of iridium and ruthenium oxides having a particle size of less than 0.1 micron, iridium oxide being present within the range of 10-99 mole percent, the balance being ruthenium oxide.
• An electrode as in claim 1 wherein the binder is an amorphous oxide selected from the group consisting of tantalum and niobium oxides.
• An electrode consisting essentially of an electrical ly-conductive substrate, an electrocatalytically-active material and an amorphous valve metal oxide binder for adhering said material to at least a portion of the surface of said substrate, the material consisting essentially of a particulate solid solution of at least two platinum group metal oxides having a particle size of less than 0.1 micron with one oxide being present with the range of l to 99 mole percent and the balance being the second oxide.
• An electrode as in claim 4 wherein the amorphous binder is an oxide selected from the group consisting of tantalum and niobium oxides.

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• 1972
  • 1972-06-23 US US00265690A patent/US3853739A/en not_active Expired - Lifetime
• 1973
  • 1973-04-30 CA CA169,852A patent/CA997298A/en not_active Expired
  • 1973-05-22 ZM ZM88/73*UA patent/ZM8873A1/xx unknown
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