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
  • C25B11/075—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound
  • C25B11/077—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound the compound being a non-noble metal oxide
  • C25B11/0773—Electrodes formed of electrocatalysts on a substrate or carrier characterised by the electrocatalyst material consisting of a single catalytic element or catalytic compound the compound being a non-noble metal oxide of the perovskite type

Definitions

• An electrode for electrochemical reactlons comprising a substrate of a film forming or barrier metal covered with a cobaltite of at least two rare earth metals, one '3 ggiff of the rare earth metals having a high atomic number
• the present invention concerns a new electrode which can be used in electrolytic cells serving for the production of chlorine, caustic soda or chlorates.
• the cells serving for the production of chlorine or caustic soda are either diaphragm cells or mercury cells.
• the chlorates are produced in a cell whose structure is similar to that of the diaphragm cells but which nevertheless has no diaphragm.
• the electrodes previously generally employed as anodes in electrolytic cells were frequently made of graphite. Their use has always entailed certain disadvantages resulting from their wear which causes an increase in the voltage necessary for the proper operation of the electrolysis cell as the result of the wear which increases in the distance between anodes and cathodes and the contamination of the electrolyte.
• anodes More recently it has been attempted to develop anodes from a metal having good resistance to corrosion by the electrolyte which metal is covered with an electrochemically active precious metal, the resulting composite then being subjected to a treatment which favors activation. These anodes are dimensionally stable and do not have the above-mentioned drawbacks.
• anodes of this type it has been proposed to employ a core of zirconium, zirconium-titanium alloy, tantalum or niobium covered with platinum.
• an anode of titanium covered with platinum Titanium, like the other core metals mentioned above, being a film forming or barrier metal capable of forming a film or barrier layer of oxide in the electrolysis solutions to protect its surface from corrosion at the places where the platinum is porous.
• electrodes have been produced of one of these film forming or barrier metals or alloys capable of forming a film or barrier layer, covered with an oxide of precious metal or with mixtures of oxides of precious and non-precious metals.
• perovskite is an oxygenated compound of two different metals which is well known in the literature and may be represented by the empirical formula:
• cobaltites have a relatively high electric conductivity which varies with the temperature, the rare earth metal playing an important role in the mechanism of conduction.
• the electrocatalytic power of these cobaltite compounds is not necessarily related to the perovskite structure, since there are numerous compounds having this structure, such as, for instance, LaCrO lanthanum chromite, which are without it.
• LaCrO lanthanum chromite which are without it.
• the compound LaCoO lanthanum cobaltite for instance, although having remarkable electrocatalytic properties, is entirely unsuitable to constitute an anode for an electrolysis cell as a result of the ease with which it passes into solution in slightly acid chlorinated medium. This defect decreases when the lanthanum is replaced by a rare earth of higher atomic number.
• the compounds LaCoO PrCoO NdCoO and GdCoO are prepared from an intimate mixture of the oxides of the stated. elements, which is calcined at 1200C. for 15 hours.
• the series of compounds thus prepared is analyzed by X-ray diffraction and is found in each case to be solely of the perovskite structure.
• the chemical resistivity in acid medium of these mixed oxides is then measured as follows:
• LaCoO PrCoO NdCoO GdCoO about about about about Time 1 hr. 30 hr. 400 hr. 500 hr.
• the new electrodes in accordance with the invention comprise a substrate of a film forming or barrier metal covered with a cobaltite compound described above which forms the surface of the electrode.
• This compound has the general formula in which Ln represents a rare earth metal of high atomic number, such as at least about 65, Ln a rare 4 earth metal of lower atomic number, such as below about 65, and x is a number between 0.001 and 0.999, and preferably between about 0.05 and 0.3.
• the new cobaltite compounds in accordance with the invention have a substantially higher resistance to acid corrosion than the known rare earth metal cobaltites, while having the same characteristics of conductivity and the same electrocatalytic properties.
• the rare earth metals which can be used are those listed in the Periodic Table of the Elements. Those of high atomic number comprise terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. The rare earths of lower atomic number comprise lanthanum, cerium, praseodymium, neodymium, samarium, europium and gadolinium.
• the substrate, or core, of the electrode is advantageously formed of film forming or barrier metal, that is to say, of metal forming a passivating layer of oxide which permits the passage of current only in the direction towards the cathode.
• film forming metals are well known and include, for example, titanium, tantalum, tungsten, hafnium, zirconium, aluminum, niobium and their alloys.
• Graphite can also be used and is intended to be included in the term film forming metal as used herein.
• the substrates may be solid pieces or thin, non-perforated plates. They may also be of perforated plates or metal gauze. Their shape is desirably that customarily employed for the anodes of electrolysis cells.
• the value of the ionic rays of the component rare earth metals of the cobaltite compound is important, and that it is not possible to combine merely any rare earth metals in any proportion.
• a rare earth having an ionic radius as small as that of erbium, it is necessary to introduce a rather large proportion of a rare earth metal having a rather high ionic radius such as that of neodymium.
• the rare earth cobaltite need not be limited to two rare earths, but may comprise three rare earths or even more, the essential factor being the retention of the perovskite structure from one or more rare earth metals leading to this structure with one or more rare earths not leading to it.
• These new compounds may be prepared like all the other cobaltites or perovskite structure by processes well known to the man skilled in the art. That is to say, thermolyzable organic or inorganic salts, oxides or hydroxides of the different elements are mixed, coprecipitated and cocrystallized. Then after the drying and crushing operations, the powder obtained, whether or not compacted, is calcined at a temperature between about 900 and about 1500C. for a period of time which may vary from 2 hours to 72 hours.
• the perovskite compounds which can be used for the electrodes of the invention may be prepared by any of the processes described in the literature. For example, by the process described in the journal American Mineralogist, Vol. 39 (l), 1954.
• EXAMPLE 1 Compounds are prepared of the general formula Gd e Tb Coo in which x is the quantity of Gd ions in the gadolinium cobaltite which are replaced by terbium ions.
• the cobaltite thus prepared is then deposited on a titanium plate of 10 mm. width by 30 mm. length and 1 mm. thickness which has been previously cleaned by sanding, washed with distilled water. and dried.
• a suspension of the cobaltite is prepared in the following manner: To 1 gram of powder there is added 1 gram of hydrated cobalt nitrate hexahydrate, 1 ml. of water and 1 ml. of isopropyl alcohol. The paste obtained is agitated vigorously until homogeneous suepension is obtained, the agitation being maintained during the production of the deposit. A layer of the suspension of the cobaltite is applied on the surface of the titanium plate by brush. After drying for 5 min. in an oven at 100C, the resulting electrode is kept for 10 min. in a furnace at a temperature of 400C. while it is swept by air. This operation is repeated 20 times. The amount of product deposited is 40 mg./cm The deposit on the electrode consists of cobaltite and 20% cobalt oxide.
• the electrode thus prepared is placed in an electrolysis cell for the manufacture of chlorine and caustic soda, in which the electrolyte is a solution of 300 grams per liter of sodium chloride maintained at 80C. and a pH of 4.
• a current such as to produce an anodic current density of 25 amperes per square decimeter is then passed into the cell; the anodic oxidation voltage of the chloride ions is l millivolts when referred to a saturated calomel electrode. After 1000 hours of electrolysis, the anode potential remains unchanged.
• EXAMPLE 2 In accordance with the procedure of Example 1, compounds are prepared of the general formula Gd ,Dy CoO from gadolinium, dysprosium and cobalt oxides, the quantities of which, as a function of x, are summarized in Table 4, below:
• An electrode is prepared with a surface of Gd Dy C on a titanium plate in accordance with the pro- 1 cedure of Example 1. This electrode is used as electrolysis anode for the manufacture of chlorine. For a brine of 300 grams per liter at 80C. and a pH of 4, there is obtained a strong liberation of chlorine with a current density of 25 amperes per square decimeter, under a voltage of 100 millivolts when referred to a saturated calomel electrode. After a prolonged period of electrolysis, the anode potential remains unchanged.
• An electrode is prepared having a surface of Nd Tb CoO on a plate of titanium by means of an organic or inorganic binder in accordance with a procedure substantially the same as that of Example 1.
• This electrode is used as electrolysis anode for the manufacture of chlorine.
• An electrode for electrochemical reactions comprising a substrate covered with a compound having a perovskite structure, characterized by the fact that the substrate is of a film forming metal and the compound of perovskite structure is a cobaltite of rare earths having the general formula Ln Ln ,CoO in which Ln has an atomic number of at least about and Ln has an atomic number below about 65, wherein x is between 0.001 and 0.999.
• Ln is a member selected from the class consisting of lanthanum, cerium, praseodymium, neodymium, Samarium, europium and gadolinium.
• the film forming metal substrate is a member selected from the class consisting of titanium, tantalum, tungsten, hafnium, zirconium, aluminum, niobium and their alloys.

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

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Citations (3)

• 1973
  • 1973-07-20 FR FR7326694A patent/FR2237986B1/fr not_active Expired
• 1974
  • 1974-07-05 US US486051A patent/US3917525A/en not_active Expired - Lifetime
  • 1974-07-17 AT AT591974A patent/AT331820B/de not_active IP Right Cessation
  • 1974-07-17 BR BR5900/74A patent/BR7405900D0/pt unknown
  • 1974-07-17 NL NLAANVRAGE7409649,A patent/NL178612C/xx not_active IP Right Cessation
  • 1974-07-17 NO NO742604A patent/NO138633C/no unknown
  • 1974-07-18 BE BE146678A patent/BE817795A/xx not_active IP Right Cessation
  • 1974-07-18 SU SU2044226A patent/SU557763A3/ru active
  • 1974-07-18 CA CA74205028A patent/CA1048230A/en not_active Expired
  • 1974-07-18 SE SE7409408A patent/SE391743B/xx not_active IP Right Cessation
  • 1974-07-19 IT IT52186/74A patent/IT1016923B/it active
  • 1974-07-19 JP JP8309074A patent/JPS5331478B2/ja not_active Expired
  • 1974-07-19 CH CH1000374A patent/CH587927A5/xx not_active IP Right Cessation
  • 1974-07-19 GB GB3213174A patent/GB1437919A/en not_active Expired

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