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

• the invention relates to electrocatalytic anodes which may be used in acidic, neutral or alkaline solutions, for the production of chlorine and chlorine compounds and electrowinning of metals, such as zinc.
• Platinum-iridium coated electrodes of the type described in U.S. Patent 3,177,131 have found widespread use in electrolytic production of chlorine and other chlorine-containing products. These anodes possess three of the four qualities which are particularly desirable in commercial electrodes:, long life, high efficiency and high selectivity. However, the fourth desirable quality, low initial cost, is not obtained, as the initial cost of these anodes is quite high com ⁇ pared to the relatively inexpensive materials which were used for anodes in electrochemical applications early in this century. For instance, graphite anodes were used to produce chlorine and lead anodes were used for electrowinning. Even though these materials had rather limited life and only moderate efficiency, they were extensively used because they were inexpensive and readily available.
• Activation over-voltage is the dif ⁇ ference between electrical potential required to obtain the desired production at the operating current and the standard half-cell potential for the desired reaction.
• the activation over-voltage of precious metal-coated electrodes is typically less than about 100 millivolts over the life of the electrode. This represents about a 4% loss in a typical process running at 3 to 4 volts.
• the power savings obtainable with new electrodes could be as high asabout 8%, which might normally,justify replacement.
• the over-voltage obtained with graphite electrodes typically ranges from about 300 to about 500 millivolts.
• Selectivity of an electrode is the ability of the electrode to preferentially catalyze the reaction which produces the desired products.
• precious metal-coated electrodes typically produce about 1 to 3 /0 (volume percent) oxygen, which is undesirable, and lesser amounts of undesirable chlorine-oxygen compounds.
• graphite electrodes typically produce about 4% oxygen along with chlorinated organics and carbon dioxide. Since the raw material for chlorine is common salt, the major production expenses are the capital cost of the plant and the cost of the electrical energy for the electrolysis. Thus, even though more modern precious metal-coated electrodes initially cost substantially more than obsolescent graphite electrodes, they are ultimately more economical, as they are more efficient.
• the platinum-iridium anode coatings of commerce are normally specified as containing 70% Pt:30% Ir by weight of the metal, even though the metals are thought to form at least a thin surface film of oxide which catalyzes the electrolysis reaction with the iridium being present throughout as the oxide.
• These anodes are prepared by making a solution of salts or resinates of the two metals in an appropriate solvent, such as alcohol, for salts or a mixture of essential oils for resinates, applying multiple thin coats, and firing in air or other oxidizing atmosphere between 350 and 550°C.
• an appropriate solvent such as alcohol
• the cost of iridium is comparable to that of platinum, and both have escalated rapidly in recent years.
• platinum- iridium-coated anodes In chlorine production, platinum- iridium-coated anodes generally evolve less than about 1% oxygen, which is considered very good. However, some of the platinum group metals, while still precious, are considerably less precious than platinum, and further, their prices have not risen as rapidly as that of platinum; so in the 60's, ruthenium oxide-coated anodes became commonly used, while platinum- iridium was used on a smaller scale.
• the commerical ruthenium oxide coatings are thought to be prepared as described in U.S. Patent 3,632,498, by dissolving RuCl_ «l-3H 2 0 and TiCl 3 or TaClj. in alcohol, applying thin coats of the solution to titanium, and firing each coat in air in the temperature range of 350-600°C.
• Precious metal-coated titanium anodes are usually dimensionally stable and have very long life in brine electrolyses, as compared to graphite, but there is a need to obtain alternative coatings incor ⁇ porating smaller amounts of the more expensive precious metals while achieving comparable or even longer life at lower cost, or at least without unduly increasing the cost of the coating. Further, high selectivity must be maintained.
• coating loading in the range of 5-20 grams per square meter in expectation of a life of more than 7 years in a chlorate cell. Multiple coating applications may be needed to obtain this loading range, so cost of application can be a significant part of the overall cost. Longer life could be obtained by increasing the loading further, but as a practical matter, this is not usually done because of precious metal and labor costs.
• An object of the present invention is to provide promoted anode coatings which have efficiency and life which are substantially equivalent to the prior art platinum-iridium composite coatings over a wide range of conditions, but which have reduced platinum- iridium content and can be used for production of chlorine and chlorine-containing compounds.
• a further object is to provide compositions which may be substituted for a portion of the platinum-iridium in the exterior coats of anodes prepared according to prior art techniques, while retaining the desirable properties of these prior art electrodes.
• the anodes of the invention are obtained by first coating a valve metal substrate, such as titanium or tantalum, with a thin layer containing thermally decomposable platinum iridium compounds, firing in an oxidizing atmosphere, then overcoating with an admixture consisting essentially of lead ruthenate and a thermally decomposable compound of a film-forming metal along with platinum and iridium
• the composite electrode formed has a titanium substrate covered with a coating having a platinum-iridium rich inner layer, and an outer layer consisting essentially of from about 10 to about 16% lead, from about 5 to about 7.5 w /o ruthenium, from about 15 to about 20 w /o oxygen, from about 40 to about 65 /o tantalum, and from about 8.5 to about 15 w /o platinum- iridium.
• the electrodes of the present invention are formed on a film-forming or valve metal substrate, which for brine electrolysis includes tantalum, zirconium, niobium, molybdenum, tungsten, hafnium and titanium.
• the substrate will actually be a titanium- clad conductive metal, such as copper or aluminum, since the electrical conductivities of copper and aluminum . are so much greater than that of titanium.
• the substrate may be in the form of a plate, screen, mesh, or other convenient shape.
• titanium we mean either the essentially unalloyed metal or any of its film-forming alloys. Of course, all titanium contains some oxygen, but normal amounts can be tolerated.
• other film-forming metals include zirconium and niobium.
• film-forming metal is understood in the context of the proposed electrolyte, thus for electrolysis of fluorine-containing electrolytes, chromium can be a film-forming metal.
• platinum-iridium rich composite is formed on the substrate.
• This ratio of platinum to iridium in this composite can very widely.
• Typical coatings will contain from about 20 to about 40 /o iridium, and from about 80 to about 60 /o platinum, calculated based on the metal. It appears that sub ⁇ stantially all of the iridium is present as the oxide and that at least a dull gray film of platinum oxide is present after firing in an oxidizing atmosphere. In 5 use, a black oxide appears to be present.
• platinum-iridium composite should be understood to include any compounds, mixtures or solid solutions of platinum and iridium, whether in the form of the metal, .
• the thickness or loading is typically expressed in grams of alloy per square meter (g/m z ) of substrate, since the surface of coating substrate is generally rather rough 15 or non-planar. Typical loadings range from about 1 to about 4 g/m 2 . The preferred range of loadings ranges from about 2 to about 3 g/m 2 .
• composition of the outer layers of the electrode may vary widely. As applied, typical compo- sitions will contain from about 65 to about 75 w/o
• the lead ruthenate may range in compo ⁇ sition from Pb 2 Ru 2 O g to Pb 2 u 2 0_, depending upon the
• the preferred thermally decomposable, film-forming metal compound is tantalum chloride.
• the amount of tantalum oxide range from about 65 w to about 70 /o, with the amount of Pt-Ir (calculated based on the metal) ranging -from about 10.5 to about 12.5 w/o, and the balance being lead-ruthenate.
• the term "lead ruthenate-tantalum oxide composite" should be understood to mean a compound, solid solution, mixture or dispersion of lead, ruthenium, tantalum and oxygen, whether or not all of the lead and ruthenium are asso ⁇ ciated in the form of the compound lead ruthenate.
• the term "lead ruthenate-tantalum oxide - Pt-Ir composite” should be understood to mean a compound, solid solution, mixture or dispersion of lead, ruthenium, tantalum, platinum, iridium and oxygen, whether or not all of the lead and ruthenium are associated as lead ruthenate.
• the thickness or loading of this layer is also commonly expressed in g/m 2 . Preferred loadings range from about 10 to about 20 g/m 2 . Heavier loadings can be used, if it is economically desirable to do so.
• Th-e outer layers may be applied by techniques which are essentially similar to those mentioned earlier in reference to applying the Pt-Ir composite layer.
• the preferred method of application is to disperse or slurry lead ruthenate powder in a volatile polar aqueous or organic solvent, such as an alcohol or glycol.
• a volatile polar aqueous or organic solvent such as an alcohol or glycol.
• the appropriate amounts of the chloride salts of tantalum, platinum and iridium may be added to the solvent, and the admixture applied to the substrate by brushing, dipping or spraying, the solvent removed and the anode fired in an oxidizing atmosphere.
• Appropriate tempera ⁇ tures may be determined by those skilled in the art, based upon the exact composition of the substrate and coating, but temperatures ranging from 350 to 550°C are generally suitable. Temperatures of about 550°C are most preferred.
• the product formed has a platinum and iridium rich layer adjacent to the substrate covered by a layer consisting essentially of from about 10 to about16 /o lead, from about 5 to about 7.5 /o ruthenium, 5 from about 40 to about 65 /o tantalum, from about 10 to about 20 /o oxygen, from about 6 to about 10 /o platinum, and from about 2.5 to about 5 /o iridium.
• the anode may be used as is or a further layer of Pt-Ir composite 10 (a topcoat) may be applied as previously. If the topcoat is to be applied, the amount of Pt-Ir alloy in the under ⁇ coat, and the intermediate coat may be varied accordingly.
• a Pt-Ir topcoat it is preferred that its loading vary from about 1 to about 4 g/m 2 , and the loading 15.of the Pt-Ir undercoat may be about 1 to about 4 g/m 2 . More preferably, the loadings for each will vary from about 2 to about 3 g/m 2 .
• EXAMPLE 1 Lead ruthenate powder was made by the following 20 synthesis.
• Powdered Ru metal (15.5g) was suspended in 30 ml NaOH solution (25g NaOH) in a three neck flask equipped with magnetic stirrer and condenser. 800 ml of hypo- chlorite solution (4-6% NaOCl) was poured in, and the 5 mixture was stirred for three hours at room temperature. The solution was filtered through a medium fritted filter. The pH was adjusted to 7 with concen ⁇ trated nitric acid. 125 ml of lead nitrate solution (51g Pb(NO.- 2 ) was poured in, the mixture was stirred 0 for one hour, then allowed to settle overnight.
• hypo- chlorite solution 4-6% NaOCl
• the precipitate was removed using a #42 Whatman paper on a Buchner funnel, then washed on the filter with 30 1. of water and dried by vacuum.
• the black powder was sifted through' #50, #100, 5 #200 and #325 mesh sieves. Essentially all of the powder passed the #325 sieve, and Coulter analysis gave the following particle size distribution: 10% less than 0.60 micron 50% less than 1.12 micron 90% less than 3.40 micron
• Initial attempts at x-ray diffraction gave only a weak pattern, indicating an extremely small crystallite size.
• a sample of the powder was heated in air at 550°C for one hour. This treatment caused increased crystallite size, and XRD (X-Ray Diffraction) identified the material as consisting of from about 80 to 85 /o lead ruthenate. Attempts were made to suspend the as-prepared powder in n-butanol. However, the powder was so reactive that it ignited the alcohol at room temperature. Powder heated in air to 550°C was less reactive and could be suspended safely in n-butanol.
• EXAMPLE 2 Lead ruthenate powder, substantially free of impurities, was prepared following the procedure of Example 1 on the same scale, except that the pH of the original caustic solution of sodium ruthenate was not adjusted before addition of lead nitrate, but was allowed to remain in the range of 11 to 12; and the dried filter cake was calcined at 550°C for 30 minutes, then lixiviated with hot water to remove the impurities. Hot water washings were repeated until the supernatant liquid contained less than 10 ppm of Na and Pb ions. X-Ray diffraction indicated that this procedure yielded essentially pure lead-ruthenate.
• EXAMPLE 3 The electrodes described herein were prepared on a substrate of a commercially pure low-iron titanium sheet which, prior to electrode preparation, was pre- treated by standard and conventional procedures, that is, degreasing followed by surface roughening by blasting with abrasive grit, washing with a detergent and air drying.
• lead ruthenate powder as prepared in Example 2, was slurried in n-butanol, applied to titanium, then fired in air to determine adherence.
• EXAMPLE 4 Composite films of lead ruthenate-tantalum oxide in varying proportions of 25:75, 50:50, and 75:25 parts by weight were prepared on the three firing cycles mentioned above.
• the coating formulations were prepared by adding TaCl_ powder gradually to n-butanol while stirring, giving a cloudy solution.
• Lead ruthenate powder, as prepared in Example 2 was added to this solu ⁇ tion to give the proper ratio of lead ruthenate to tantalum oxide in the applied films. Coatings containing 75 w /o lead ruthenate-25 w /o tantalum oxide did not adhere well whether fired at 450°, 550° or 650°C.
• An electrode having a prior art 70:30 Pt-Ir composite coating was prepared as described in U.S. Patent 3,177,131 at a loading of 15 g/m 2 on a titanium substrate.
• a second electrode was prepared on a similar titanium substrate, as described, then overcoating, as described in Example 4, with the 75 w /o lead ruthenate- tantalum oxide composite.
• the prior art electrode When tested as an anode in a laboratory chlorate cell containing about 300 g/1, NaCl at a pH of about 6.5 to 7, a temperature of about 60°O at a current density of 1 amp/in 2 (1 amp/6.452 cm 2 or 0.155 amp/cm 2 ) the prior art electrode exhibited an overpotential of approximately 52 mv, while the over- potential of the promoted electrode was approximately 60 mv.
• This example demonstrates that the catalytic activity of. the lead-ruthenate tantalum oxide composite is substantially identical to that of the prior art platinum-iridium composite electrodes.
• EXAMPLE 6 Anodes as set forth in Example 4, containing 75 /o tantalum oxide and 25 w /o lead ruthenate, were also evaluated for durability under conditions which produce sodium hypochlorite, as either a final or an intermediate product.
• the electrolyte contained 63g/l NaCl at ambient temperature, initially and starting pH was about 7.
• the cells were operated until half of the initial salt content was converted to NaOCl, giving a final solution containing 40 g/1 NaOCl, at which time the cells were emptied and refilled with fresh brine.
• Titanium cathodes were used.
• a current density of 5 amps per square inch (5 amps/6.452 cm 2 or 0.775 amp/cm 2 ) was used to produce accelerated wear.
• Samples were prepared in two thicknesses, one approximately 10 g/m 2 obtained by a single application and firing, and the second thickness being about 20 g/m 2 obtained by two applications and firings. These samples passivated very quickly. At constant voltage, some started to pass less current after one or two hours of operation, and all had passivated to the degree that little or no current was passed after 25 to 72 hours. Thus, these anodes would be unsuitable for sodium hypo ⁇ chlorite production. Coating thicknesses were measured by the beta backscatter method. The passivated samples still exhibited the initial coating thicknesses and surface electrical conductivity.
• the electrocatalytic activity of this example demonstrates that the lead ruthenate-tantalum oxide composite coated electrode is substantially equi- valent to that of the platinum-iridium composite coated electrode, the lead ruthenate-tantalum oxide composite coating does not have the durability of the platinum- iridium composite under conditions of extended electrolysis.
• Example 6 these samples operated stably for more than 72 hours. Since catalytic activity had been demonstrated, further life tests were not conducted, as the life of these samples was expected to be so long that life testing would be impractical under these conditions,.
• EXAMPLE 8 The procedure of Example 7 was repeated, except that small amounts of Pt and Ir were added as chloride salts to the alcohol formulation used to produce the topcoat.
• the topcoat had the following nominal compo ⁇ sition after firing:
• ⁇ _- were compared in an accelerated sulfuric acid test to samples prepared with no undercoat, and samples coated only with about 4 g/m 2 of Pt-Ir and no topcoat.
• EXAMPLE 10 A group of 96 electrodes of production size was prepared as described in Example 8, Sample C. The electrodes were installed in a commercial brine electroly- sis plant and operated successfully for 9 months. At the end of that period, an anode was removed and sections were cut from it and prepared for laboratory cell characterization.
• Example 5 When tested according to the procedure set forth in Example 5, one such sample was found to have an over- potential of 62 mv at a current density of 150 /cm 2 , indicating • that high catalytic activity had been retained by the electrode.
• a second section of the electrode operated for 953 hours at 5 ASI (5 amps/6.452 cm 2 or 0.775 amps/cm 2 ) in the sulfuric acid accelarated life test, described in Example 8, indicating that much of the resistance of the electrode to passivation had been retained. While it was difficult to obtain a precise measurement of wear rate due to the small loss of coating, it appears that the life of the anodes will be at least comparable to that of prior art Pt-Ir coated anodes in brine electrolysis.
• EXAMPLE 11 The procedure of- Example 8, Anode B was repeated, except that after application of the two coats of lead ruthenate a further overcoat of 3 g/m 2 of Pt-Ir was applied. Each coat was fired in air at 550°C. When tested in accordance with the accelerated life test described in Example 8, the performance of this anode was substantially equivalent to that of Anode C in Example 8.

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