Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D3/00—Electroplating: Baths therefor
  • C25D3/02—Electroplating: Baths therefor from solutions
  • C25D3/50—Electroplating: Baths therefor from solutions of platinum group metals
• • 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
  • C25B11/075—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound
  • C25B11/081—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound the element being a noble metal

Definitions

• This invention relates to anode assemblies comprising a surface of platinum borne on a supporting structure of titanium and a particularly advantageous method of producing such assemblies; it relates further to the use of assemblies prepared in this special manner as anodes in the electrolysis of brine.
• One object of this present invention is the provision of improved anode assemblies comprising a surface of platinum borne on a supporting structure of titanium which have especially useful properties when used as anodes in the electrolysis of aqueous solutions of alkali metal chlorides.
• a second object is the provision of a special method of producing such anode assemblies the advantageous properties whereof arise from and are peculiar to the method of production.
• a further object is the provision of an improved procedure of electrolysing aqueous solutions of alkali metal chlorides characterised in that there is used as an anode in such procedure an assembly comprising a surface of platinum borne on a supporting structure of titanium, the said assembly having been made by the aforesaid special method of production whereby distinct advantages are secured in the electrolysis procedure, particularly in respect of the voltage needed to operate the electrolytic cell, especially at relatively high current density.
• the initial coating with platinum may be brought about by treating the tantalum surface with a solution containing a reducible platinum compound such as chloroplatinic acid together with other conventional adjuvants and subsequently heating at a temperature (approximately 250 C.) at which the chloroplatinic acid is decomposed to form metallic platinum and it is explained that a platinum coating formed simply in this Way is inadequately adherent and if the so coated tantalum is used as an anode in a corrosive electrolyte, for instance in electrolysing sodium chloride solutions to make chlorine or chlorates, the coating is very rapidly stripped off so that the anode becomes useless. It is stated that this defect is only cured by applying the controlled diffusion or alloying step which is the essential object of the invention; this involves a subsequent heating of the coated tantalum at high temperatures in the range 800- 1400" C.
• a reducible platinum compound such as chloroplatinic acid
• anode which is a surface of platinum borne on a supporting structure of titanium or an alloy consisting essentially of titanium is of outstanding value in that it overcomes most of the difiiculties which are met with in designing and constructing an anode for use in cells for the electrolysis of aqueous solutions of alkali metal chlorides to make chlorine and caustic alkali.
• the platinum coating on the titanium support may be provided in a number of Ways, one of them being painting the titanium support with a conventional platinising solution and subsequently heating in the manner practised in the ceramics industry.
• the coating is heated at temperatures above 550C, e.g., at 600 C. in an oxidizing atmosphere' for 5 minutes, the assembly, ifused as ananode in a mercury cell for the electrolysis of brine, requires voltages which for a given current density slowly increase with time over a period of five months, owing to the slow development of overvoltage. It is believed that this. is because during the heating the platinum particles sinter together too much and also diffuse into the titanium. If the coating is heated attemperatures still higher, e.g., 800 C., the electrical contact between the platinum layer, and the titanium is poor and the assembly passes little current if used as an anode in mercury cells.
• the coating of platinum-bearing preparation is heated in an oxidising atmosphere to a temperature below 350 C.,,for.instance,for 15 minutes, the organic matter of the said preparation .is carbonised to some extent and not completely volatilised and burnt.
• the resulting deposit of platinum and carbon has poor adhesion to the underlying titanium, possibly on account of the presence of carbon,.but possibly also because the temperature of heating is too low for the required degree of sintering of the platinum metal particles to one another and to the underlying. titanium.
• the resulting assemblies, if used as anodes in brine electrolysis have a shorter life than those heated at 350 550 C., e.'g., only half the life of those heated at 350 to 500 C. in an oxidising atmosphere.
• a further feature of the invention is a process for making chlorine and caustic alkali by electrolysingaqueous solutions of alkali metal chlorides wherein there is used as the anode of thecell an. assembly comprising a surface coating of platinum on a titanium support which coating has been made by applying to the support a primary coating of a platinum-bearing preparation comprising a thermally reducible platinum compound in an organic vehicle, drying said primary coating and firing the same by heating in anoxidising atmosphere to a temperature in the range 350'550 C.
• titanium includes not only titanium itself but also alloys based 'on titanium and having .anodic polarisation properties comparable with those of titanium.
• examples of the latter are titanium-zirconium alloys containing up to 14% of zirconium, alloys. of titanium with up to of a platinum metalsuch as platinum, rhodium or iridium and alloys of titanium with niobium or tantalum containing up to of the alloying constituent.
• Platinum-bearing preparations comprising solutions of a thermally-reducible platinum compound in an organic vehicle are themselves Well known, being conventionally used for purposes, of metallic decoration in the ceramic arts. They are usually made by dissolving a thermally decomposable platinum compound such as chloroplatinic acid or bromoplatinic acid in a volatile solvent and adding thereto a mixture of a resin with terpineol, acetoacetic ester or an essential oil. It is usually heldthatin the solution or during the initial drying of the primary coating resinates or sulpho-resinates of platinum are. formed which are burned off in the subsequent firing step to form the ultimate deposit of platinum metal. Such.
• Such platinum-bearing preparation suitable for use in the method of the. present invention is that sold under the trade. name Hanovia 'Liquid Bright Platinum Grade 05-X.
• This product comprises a heat decomposable platinum compound which decomposes in the range of 350 and 550 C., dissolved in an appropriate volatile organic solvent.
• Other suitable preparations may be made by dissolving in a volatile organic solvent a platinum compound which.
• platinum metal and volatile b y-p roducts for instance, chloroplatinic or bromoplatinic acid
• a mixture of a resinand terpineol, ethyl .acetoacetate or an essential oil for instance oil ofv cloves, oil of turpentine, oil of lavender.
• alcohol may be added to an equal volumeof a mixture in equal parts of abietic, acid and oil of clovesr
• platinum-bearing preparations we have spoken above simply of platinum-bearing preparations but it is to be understood that we do not intend the term to be limited strictly to preparations containing platinum as the sole noble; metal.
• Certain of the commercial preparations contain also minor amounts of other metals of the platinum group, particularly rhodium and/ or iridium.
• preparations can readily be made by the methods described above, adding appropriate amounts of readily decomposablerhodium or iridium salts to'the platinum salt.
• .pla'tinum in this specification and claims includes, where the context permits, also platinum containing minor proportions of rhodium and/ or iridium-one such example is an alloy consisting of 70% latinum, 30% iridium.
• the platinised surface is formedby superimposing a number of said depositsby repeating the operation of coatingv with a platinum-bearing preparation ofthe aforesaid kind, drying each. coating. and subsequen'tl'y firing by heating in an oxidising atmosphere, for
• the titanium support preferably first cleaned by removal of the oxide skin as just indicated, can first be given a surface coating of an electrolytic deposit of platinum before the first coating of a platinumbearing operation is applied.
• an electro-deposit can conveniently be made by conventional methods using the known alkaline hexahydroxyplatinate electroplating bath.
• the parts of the surface of the titanium support to be provided with a platinum coating are degreased, then etched for 4 days in analytical reagent quality concentrated hydrochloric acid, allowed to dry, then painted with a platinum-bearing preparation comprising chloroplatinic acid, ethyl alcohol, oil of cloves and abietic acid in the proportions described above, the resulting coating dried for example by infra-red radiation, and the dry coating then fired, in air, in an oven at a temperature in the range 375 -475 C. for minutes.
• the painting
• drying and firing operations are then repeated as often as required to give a platinum coating of desired thickness.
• 5-15 coatings of the platinum-bearing preparation may suitably be appled,-- with the necessary intervening drying and firing of each coating.
• the number of coatings required to build up a desired thickness of platinum depends on the platinum content of the coating preparation and on the thickness of each applied coating. For a given type of coating preparation the maximum usable platinum content is limited in the main by the viscosity of the mixture. We prefer to employ platinum-bearing preparations containing the equivalentof 2-9% by weight of platinum. Even smaller platinum contents are permissible, but they necessitate the application of an excessive number of coatings.
• the coating of platinum left after firing is finely divided and has a light grey mat fininsh. When operating as an anode it is very dark grey or black. It is presumably this state of subdivision of the platinum coating which permits the assemblies of the present invention to behave in so satisfactory a manner for instance as anodes in a mercury cell for the electrolysis of brine.
• each anode in each cell of a series of mercury cells is an assembly produced according to the invention, because in such a series it is desirable to be able to short out any one cell temporarily by connecting its anode to its cathode While leaving the rest of the series operating. This method of shorting out a cell causes hydrogen to be momentarily discharged from the anode. It is found that these assemblies are particularly well able to withstand the momentary cathodic polarisation thus generated.
• Assemblies produced according to the method of the invention are particularly well adapted to serve as improved anodes for use in electrolytic cells for the manufacture of chlorine and caustic alka l-i by the electrolysis of aqueous solutions of alkali metal chloride, for instance as an anode of the kind which is broadly described in 00- pending application Serial No. 780,882 (filed December 16, 1958) or as an impermeable barrier provided with an anodic surface coating of platinum on one side thereof in a multi-electrolytic cell as is broadly described in copending application Serial No. 796,856 (filed March 3, 1959).
• Assemblies produced according to the method of the invention may also be fashioned so as to permit them to serve as improved electrodes in electrolytic cells for the manufacture of alkali metal chlorates, for instance as bipolar electrodes in a multi-electrolytic cell 6 as claimed in co-pending application Serial No. 114,354 (filed June 2, 1961).
• Example 1 A comparison at given current densities of the cell voltages of a mercury cell having as anode an electrodeposited platinum coating on an expanded sheet of titanium metal with a mercury cell of the same dimensions having as anode a structure produced according to the method of the present invention consisting of fired coatings of a platinum-bearing preparation on an expanded sheet of titanium was carried out as follows.
• An anode was prepared by electrodepositing 100 grams of platinum per square metre, calculated on the cell cathode area, on an expanded sheet of titanium which had been degreased, etched for 4 days in analytical reagent quality concentrated hydrochloric acid and subsequently dried.
• An anode was also prepared by applying to a similarly degreased, etched and dried expanded sheet of'titanium two coatings of a platinum-bearing preparation of platinum resinate in an essential oil (oil of cloves) containing the equivalent of 8% of platinum and firing each coating in air at 550 C. to give a deposit of platinum of 25 grams er square metre of cathode area.
• A is the intercept on the voltage axis obtained by extrapolating the straight line backwards.
• C current density (lea/m b is the slope of the curve.
• the rate of loss of platinum from the two types of anode when used in a mercury cell for the electrolysis of brine was of the same order, namely, about 1 gram of platinum per ton of chlorine produced when the mercury cell operated at 4 ka./m.
• Example 2 A comparison at given current densities of the cell voltage of a mercury cell having as anode an electrodeposited platinum coating on an expanded sheet of titanium with voltages of mercury cells of the same dimen- An anode was again prepared by electrodepo'siting 100 grams of platinum per square metre, calculated on the cell cathode area, on an expanded sheet of titanium which had been degreased, etched for 4 days in analytical reagent quality concentrated, hydrochloric acid and subsequently dried.-
• anodes prepared by firing the applied platinum bearing preparation at the lower temperatures have an advantage in lower operating voltages for long periods of time over those prepared by firing the applied platinum-bearing preparation at the higher temperatures, e.g., 525 and 550 C.
• the rate of loss of platinum from the various typesof anodes used in this test was approximately the same, namely about 1 gram of platinum per ton of chlorine pro prised when the mercury cells operated at 4 lea/m
• Example 3 A platinum-bearing preparation containing about 3% by weight of platinum was prepared as follows. 5 g. abietic acid were mixed with 5 g. eugenol, heated to 150 C. and cooled to give a light'brown syrup. 1 g. chloroplatinic acid was dissolved in 2 ml. absolute ethyl alcohol and mixed with the syrup.
• Each of the coated sheets of titanium was employed as the anode in an experimental mercury cell immediately after it had been prepared, and the measurement of cell voltage for a range of current densities. exceeding 2 ka./m. showed the following characteristics.
• the deposit of platinum was about 30 g. per square metre, calculated on the cell cathode area.
• a platinum-bearing preparation was made up as follows. 2 g. abietic acid were mixed with 2 g. eugenol. To this was added a solution of 1.216 g. chloroplatinic acid in 1.5 ml. ethyl alcohol. The resulting mixture was too viscous and was therefore diluted with half its volume of cugeuo'l before use, the platinum content then being about 5% by weight.
• Example 2 Two expanded sheets of titanium prepared as in Example 1 by degreasing, etching anddrying were each given thirteen coatings of the above platinum-bearing preparation, each coating being fired in air at 350 C. on one titanium specimen and at 475 C. on the other.- When tested as anodes in experimentalmercury cells as inExarnple 2, the specimens showed the following characteristics at current densities exceeding 2 kaJ/mF.
• the deposit of platinum was about 30 g. per square metre, calculated on the cell cathode area.
• Example 5 Nine platinised'titaniu'm'anodes were fitted in a commercial mercury cell making chlorine and caustic soda by the electrolysis of brine. Each anode was made by degrea'sing, etching and-drying an expanded titanium sheet as described in Example 1 and subsequently applying 20 coatings of a commercial platinum-bearing preparation consisting of a. platinum resinate in an essential oil (oil of cloves) containing the equivalent of 8% of platinum, each coating in turn being dried and fired in air at 475 C. for 10 minutes. The total platinum deposit was 40 g./m. on each faceof the titanium sheet.
• Brine containing 23% w./w'. of sodium chloride was electrolyzed continuously in thecell for nine months at a temperature of approximately 60? C. and at current densities varying from 2.9 to [5.4 ka./m. calculated on the cathode area of the cell.
• the average overvoltage for the nine anodes was initially 0.060'volt and this had.
• Anodes made according to the teachings of this present invention have been directly compared with and shown to be superior to anodes made according to the teachings of Rosenblatt in US. specification No. 2,719,797, referred to above.
• a method for the production of an anode assembly comprising a surface of platinum on a'titanium support which comprises applying to the titanium support at least one coating of a platinum-bearing preparation comprising a thermally reducible platinum compound in an organic vehicle and which on heating to a temperature between 350 and 550 C. produces a deposit consisting essentially of platinum, said platinumcompound being selected from the group consisting of chloroplatinic acid, bromoplatinic acid and platinum resinates, drying each coating and then firing the same by heating in an oxidising atmosphere to a temperature between 350 and 550 C. to form said deposit consisting essentially of platinum.
• a method according to claim 1 wherein the heating to form said deposit is carried out in air and at a temperature between 350 and 500 C.
• a method for the production of an anode assembly comprising a surface of platinum on a titanium support which comprises applying to the titanium support a plurality of coatings of a platinum-bearing preparation containing 2% to 9% by weight of platinum in the form of i 5.
• a method according to claim 4 wherein before coating with the platinum-bearing preparation, the surface of the titanium support has been treated to remove therefrom the surface skin consisting chiefly of oxide.
• a process for the manufacture of chlorine and caustic alkali which comprises applying to a titanium support at least one coating of a platinum-bearing preparation comprising a thermally reducible platinum compound in an organic vehicle and which on heating to a temperature between 350 and 550 C. produces a deposit consisting essentially of platinum, said platinum compound being selected from the group consisting of chloroplatinie acid, brornoplatinic acid and platinum resinates, drying each coating and then firing the same by heating in an oxidising atmosphere to a temperature between 350 and 550 C. toform said deposit consisting essentially of platinum and electrolysing an aqueous solution of an alkali metal chloride in an electrolytic cell using said platinum coated titanium support as the anode.
• a method for the production of an anode assembly comprising a surface of platinum on a titanium support which comprises applying to the titanium support at least one coating of a platinum-bearing preparation comprising a thermally reducible platinum sulphoresinate in an organic vehicle and which on heating to a temperature between 350 and 550 C. produces a deposit consisting essentially of platinum, drying each coating and then fir-- ing the same by heating in an oxidising atmosphere to a temperature between 350 and 550 C. to form said deposit consisting essentially of platinum.

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Citations (11)

• 0
  • NL NL250923D patent/NL250923A/xx unknown
  • BE BE590159D patent/BE590159A/xx unknown
  • NL NL126075D patent/NL126075C/xx active
• 1960
  • 1960-04-19 FI FI0661/60A patent/FI41540B/fi active
  • 1960-04-19 DE DE1421370A patent/DE1421370B2/de active Pending
  • 1960-04-27 CH CH474260A patent/CH399121A/de unknown
• 1962
  • 1962-08-16 GB GB31497/62A patent/GB984973A/en not_active Expired
• 1963
  • 1963-08-12 US US301659A patent/US3177131A/en not_active Expired - Lifetime
  • 1963-08-16 DE DE19631467221 patent/DE1467221B2/de active Pending

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