FR2479272A1 - Electrode for electrochemical processes - having active layer contg. platinum gp. metal, oxide, iron and manganese gp. metal oxide(s) and boron oxide - Google Patents

Abstract

Electrode comprises an electrically conductive base topped with an active layer of compsn. (by wt.): 5-45% platinum gp. metal oxide, 19-94.9% oxide of one or more iron and manganese gp. metals, and 0.1-50% boron oxide. The active layer pref. contains Ru and Fe oxides or Co oxide or a mixt. of Mn and Co oxides. The electrode is useful in various electrochemical processes, e.g. as anode in chlor-alkali cells, for chlorate prodn., for electro-organic syntheses, for purificn. of sewage or for regeneration of etching solns. The active layer has a reduced Pt pg. metal oxide content compared with prior art electrodes, but has comparable performance and lifetime.

Description

 The present invention relates to electrocemia and in particular relates to an electrode for electrochemical processes, of the type comprising a conductive electrode support to which an active mass of metal and boron oxides is applied. The electrode is used in particular as an anode in the electrolysis of solutions of chlorides of alkali metals with production of chlorine and caustic soda in electrolysers with diaphragm filtering, as well as in the electrolytic processes of obtaining chlorates, in organic electrode synthesis, for the electrochemical purification of waste water, for the regeneration of chlorine attack solutions.

 In recent times graphite anodes have been widely used in various electrochemical processes. -Graphite anodes have several advantages, in particular that of the use for their manufacture of a non-deficit electrode material, as well as the fact that the anodes are insensitive to short-circuits. On the other hand, graphite anodes have a higher chlorine release potential and, therefore, require the application of a higher voltage to the electrolyser, and are characterized by high wear of the anode material, hence the need for frequent disassembly to replace the anode assemblies.

 The graphite anodes have considerable dimensions and weight, which leads to an unjustified increase in the dimensions of the electrolysers and of the production areas in the electrolysis workshops.

 At present, electrodes are widely used with an electroconductive support to which an active mass is applied. The electroconductive support is made of metal capable of passivation during anodic polarization, for example titanium, tantalum, zirconium, niobium, and alloys of these metals. The electroconductive support may have any shape, for example a perforated or non-perforated flat blade, a rod, a grid, a network, a metalloceramic body.

 An electrode is known in which the active mass contains oxides or a mixture of oxides of metals of the platinum group, for example of ruthenium (Great Britain patent NO 1168558). The active mass is applied in a thin layer of 3 & 10 microns. Metal anodes, in comparison to graphite anodes, have improved electrochemical characteristics, invariable dimensions during prolonged operation, reduced dimensions and weight, high stability of the active mass, a long period of several years between successive replacements of the anode assemblies.

 In this known electrode comprising an electroconductive metal support capable of passivation on which is applied an active mass of oxides or mixtures of oxides of metals of the platinum group (platinum, iridium, ruthenium, rhodium, palladium, osmium), it is possible to introduce into the active mass manganese, lead, cobalt, titanium, tantalum, zirconium or silicon dioxide, the latter being introduced in a proportion less than the weight of the oxides or of the mixture of of platinum group metals (Great Britain patent NO 1168558). For this electrode, the consumption of active mass consisting, for example, of ruthenium oxide, under stationary operating conditions of a diaphragm chlorinator for chlorine , & a current density of 0.2 A / cm2, is 7.5 mg / 1000 ampere-hours. The wear of the active mass is determined by the weight method.

 An electrode is also known comprising a metal support capable of passivation, to which is applied an active mass of oxides of metals of the platinum group and of metals not showing valve properties and chosen from the group of iron, manganese. The proportion of platinum group metal oxide in the active mass is less than 50% of the weight of the oxide or mixture of metal oxides not showing valve properties ("Electrochemistry" review, col. XII , N 5, 1976, Academy of Sciences of the USSR, Moscow, pp. 787-789).

 This electrode, the active mass of which contains 31% by mass of ruthenium dioxide and 69% by mass of iron oxides, used in a diaphragm chlorinator for chlorine operating in stationary regime, at a current density of 0.2 A / cm2, is characterized by wear of the active mass equal to 4.0-5.7 mg / 1000 amp hours (the total amount of electric current flowing during the test being from 787 to 896 amp hours).

 A method of manufacturing an electrode is known, in which an active mass of oxides of platinum group metals and of iron group metals, manganese, is applied to an electroconductive metal support capable of passivation. The application of the active mass to the support is carried out from a solution of thermally decomposable compounds of said metals, and is followed by a heat treatment. The application of the solution of thermally decomposable metal compounds can be repeated several times (review "Electrochemistry", vol. XII, No. 5, 1976, Academy of Sciences of the USSR, Moscow).

 The active mass is applied to the electroconductive support from a solution containing thermally decomposable compounds of a platinum group metal and an iron group metal, manganese, then the support is subjected to a heat treatment. .

 We also know the variant consisting in first applying on the support the solution of thermally decomposable compounds of a platinum group metal, then in carrying out a heat treatment, & applying on said support a solution of thermally decomposable compounds of other constituents, and to subject it to a heat treatment (requests from RFA N s 2110043, 2126840).

 The present invention relates to an electrode whose composition of the active mass would make it possible to reduce the consumption of lethal platinum group during the operation of the electrode.

 This object is achieved by the fact that the electrode for electrochemical processes, of the type consisting of: a support made of electroconductive material to which is applied an active wear consisting of a metal oxide of the platinum group and of an oxide of the group iron, manganese, is characterized, according to the invention, in that the active mass also contains boron oxide in a proportion of 0.1 & 50% is trapped.

 It is advantageous, in order to improve the behavior of the electrode, for the active mass to contain ruthenium oxide and iron oxide or cobalt oxide, or a mixture of manganese and cobalt oxides .

 The invention also relates to a method of manufacturing an electrode for electrochemical processes, of the type providing for the application to a support of an electroconductive material of an active mass, comprising the application to said support of a solution containing thermally decomposable compounds of metals of the platinum group and of the iron or manganese group, followed by heat treatment at an elevated temperature, process in which, according to the invention, a thermally decomposable boron compound is introduced into the solution a proportion of 0.1 & 50X by mass calculated as boron oxide.

 It is advantageous, in order to increase the resistance of the electrode, for said solution to contain a thermally decomposable compound of boron, iron and ruthenium, or boron, cobalt and ruthenium, or boron, cobalt, manganese and of ruthenium.

 The invention further relates to a method of manufacturing an electrode for electrochemical processes, providing for the application to a support of electroconductive material of an active mass, comprising the application to said support of a first solution of thermally decomposable compound of at least one platinum group metal and its heat treatment at an elevated temperature, and the subsequent application to said support of a second solution containing a thermally decomposable compound of at least one iron group metal, manganese or its mixture with a thermally decomposable compound of az minus a platinum group metal, with heat treatment of said support at an elevated temperature, process in which, according to the invention, a thermally decomposable boron compound is introduced into said second solution in a proportion of 0.1 to 50X by mass, calculated as boron oxide.

 It is rational that said first solution contains a thermally decomposable compound of ruthenium, and that said second solution contains a thermally decomposable compound of boron and cobalts or boron, cobalt and manganese, or boron, cobalt and ruthenium.

 The recommended metal oxide contents ensure a high electrochemical activity of the electrode, which does not increase during the increase in the share of these constituents above 50% by mass. The introduction of boron oxide in the active mass increases the resistance of the electrode in comparison with electrodes whose active mass contains only oxides of metals from the platinum group and metals chosen from the group of iron and manganese.

 The authors of the present invention have established that the resistance of the active mass increases when the boron oxide content is not less than 0.1% by mass and greater than 50% by mass. The increase in the boron oxide content above 50% results in a sudden reduction in the electrochemical activity of the electrode.

 In this way, the proposed electrodes manufactured by the process according to the invention ensure an increase in the resistance of the electrode, that is to say of its active mass, of 1.2 d 2 times in comparison with the known electrodes.

 The electrodes proposed, as regards their electrocatalytic properties, are analogous to known electrodes.

The electrocatalytic activity is assessed according to the value of the potential of the anode relative to the normal hydrogen electrode for comparison under the chlorine electrolysis conditions in a diaphragm electrolyser. The electrodes proposed, at a current density of 0.2 A / cm2, 8 the temperature of 900C in a solution containing 300 g / l of NaCl, have potentials, during the anodic polarization, of 1.34 å 1, 37 V compared to the normal hydrogen comparison electrode, except the compositions containing manganese dioxide.

 The electrode according to the invention can be manufactured as follows. A solution of a mixture of platinum group metal compounds and metals selected from the group of iron, manganese, and containing boric acid is applied to a support made of electrically conductive material, for example titanium. heat treatment with a temperature of 360 & 5000C.

 For the manufacture of the electrode, a solution containing thermally decomposable compounds of platinum group metals can be applied to a support made of electroconductive material, with a subsequent heat treatment at a temperature of 360 to 5000 ° C., then a solution of compounds is applied. thermally decomposable from boron and platinum group metals, iron group manganese, with a subsequent heat treatment.

 For the manufacture of the electrode, a solution containing thermally decomposable compounds of platinum group metals can be applied to a support made of electroconductive material, with a subsequent heat treatment at a temperature of 360 h 5000C, then a solution of compounds is applied. thermally decomposable from metals of the boron group and metals of the iron group, manganese, with a subsequent heat treatment.

 To manufacture the electrode, a solution containing thermally decomposable compounds of boron and platinum group metals, of iron group, of manganese can be applied to a support made of electroconductive material, and a temperature of 20 to 1500 ° C. is pre-dried. followed by heat treatment d 360-5000C.

 The application of the solution and the heat treatment can be carried out several times.

 Other characteristics and advantages of the invention will be better understood on reading the description which follows, of several concrete but non-limiting examples of embodiment.

Example 1
An electrode is produced comprising a support made of electroconductive material in the form of a titanium blade of 30 × 40 × 2 mm, to which an active mass is applied containing (% by mass): boron oxide 0.4, ruthenium dioxide 31, oxide of iron 68.6.

 The electrode is manufactured as follows.

The titanium blade undergoes degreasing in a solution of 5% NaOH, at the temperature of 600C for
10 minutes, then it is placed in an attack solution (at 20X HCl) at 1000C. To apply the active mass, a solution is prepared containing 7.8 ml of iron nitrate (1M solution), 1 g of a ruthenium chloride solution at 19.2% by mass, and 0.2 ml of a 0.5M boric acid solution. The solution is applied to the previously prepared titanium surface, it is dried by gradually increasing the temperature from 20 ° C. to 1500 ° C. for 40 minutes, and a heat treatment is carried out at a temperature of 3600 ° C. for 20 minutes. The operation is repeated 6 times. After applying all the layers, the electrode is heated to 4700C for 1 hour. The overall amount of active mass applied to the electrode is 13.2 g per m of surface of the electrode.

 The electrode was tested when obtaining chlorine by electrolysis in a diaphragm electrolyser: the pH of the solution containing 300 g / l of NaCl is 3 to 5, its temperature is 900C, the current density being 0.2 A / cm2.

 The potential of the anode relative to the comparison hydrogen electrode was equal to 1.35 V. During the test, 2724.4 ampere meters were passed. No loss of active mass was observed. Active mass losses were determined by the weight method. The measurement accuracy was + 0.05 ng.

Example 2
An electrode identical to that of Example 1 and by a method analogous to that of Example i is produced.

 The solution is applied to a previously prepared titanium surface, it is dried at 1200C for 15 minutes and a heat treatment at 4700C for 10 minutes, The operation is repeated 8 times. The total amount of active mass applied to the electrode is 17 g per m2 of electrode surface.

 The electrode was tested when obtaining chlorine by electrolysis in an electrolyser & diaphragm in a solution containing 300 g / l of NaCl, at pH 4.5-5.0, at the temperature of 900C, the density of the current being 0.2 A / cm2. The potential of the anode was 1.35 V.

During the test, 2486 ampere-hours were passed. The loss of active mass during the electrolysis was 0.2 mg per 1000 ampere-hours.

Example 3
An electrode similar to that described in Example 1 and by the same method as in Example 1 is produced, but without prior drying. The overall amount of active mass applied to the electrode is II g per m of surface area of the electrode. The electrode was tested when obtaining chlorine by electrolysis at pH 3 to 4 in a solution containing 280 g / l of NaCl, -d 900C; the anodic current density was 0.2 A / cm.

 The potential of the anode was 1.36 V. During the test, 1346.2 ampere-hours were passed. The loss of active mass during electrolysis was 0.9 mg / 1000 ampere-hours.

Example 4
An electrode similar to that of Example 1 is produced and according to the same procedure as in Example 1, but without prior drying and with an active mass containing (% by mass): ruthenium dioxide 31, iron oxide 67 , boron oxide 2. The total amount of active mass applied to the electrode is 15.3 g per m2 of surface of the electrode. The electrode was tested when obtaining chlorine by electrolysis in a diaphragm electrolyser at pH 3 & 4 in a solution containing 280 g / l of NaCl, at the temperature of 900C; the anodic current density was 0.2 A / cm2. The potential of the anode was equal to 1.35 V. During the test, 1605.6 ampere-hours were passed. The loss of active mass during the whole period of operation was 0.56 mg / 1000 ampere-hours.

Example 5
An anode electrode similar to that of Example 1 is produced, but with an active mass containing (% by mass): ruthenium dioxide 31, iron oxide 59, boron oxide 10. The total amount of active mass applied to the The electrode was 10.5 g per m2 of surface of the electrode. The electrode was tested when obtaining chlorine by electrolysis in an electrolyser & diaphragm me & pH of 3 & 5 in a solution containing 280 g / l of
NaCl, & the temperature of 900C; the anodic current density was 0.2 A / cm. The potential was equal to 1.37 V. During the test, 1751 amp hours were passed. No loss of active mass was observed.

Example 6
An electrode analogous to that of Example 1 and according to the same procedure is produced, but without prior drying and with an active mass containing (% by mass): ruthenium dioxide 31, iron oxide 19, boron oxide 50. The total amount of active mass applied to the electrode is 13 g per m2 of surface of the electrode. The heat treatment of the electrode is carried out at a temperature of 500 C.

 The electrode was tested when obtaining chlorine by electrolysis in an electrolyser A diaphragm at pH = 3 - 5, d the temperature of 900C, the anodic current density being 0.2 A / cm2, in a solution containing 300 g / l NaCl. The potential was 1.37V. During the electrolysis, 900 ampere-hours were passed.

The wear of the active mass was 1.2 mg / 1000 amp hours.

EXAMPLE 7
An electrode is produced comprising an electroconductive support in the form of a titanium blade of 30 × 40 × 2 mm to which an active mass is applied containing (% by mass): ruthenium dioxide 5, manganese oxide 84.3, cobalt oxide 10, 3, boron oxide 0.4.

 The electrode is manufactured in the following manner.

 The preparation of the titanium support is carried out as in Example 1. For the application of the active mass, solutions containing manganese nitrate (1M solution), cobalt nitrate (1M solution), a solution of boric acid (0.5 M solution), a solution of ruthenium chloride A 19.2% by mass of ruthenium.

A layer of ruthenium chloride solution is first applied to the previously prepared titanium support, with a subsequent heat treatment at the temperature of 3700C for 10 minutes. The amount of metallic ruthenium applied is 1.3 g per m2 of surface.

Then a solution of cobalt nitrate, manganese and boric acid prepared by mixing the solutions mentioned is applied, and a heat treatment is carried out at the temperature of 3800C for 20 minutes. The operation is repeated 10 times. The total amount of active mass applied to the electrode is 35 g per m2 of surface of the electrode.

 The electrode was tested when obtaining chlorine by electrolysis in a diaphragm electrolyser at pH = 3 to 5 in a solution containing 280 g / l of NaCl, at a temperature of 900C, the anodic current density being 0.1 A / cm2. The potential of the anode relative to the comparison hydrogen electrode was 1.5 V.

During electrolysis, 1070 ampere-hours were passed.

The wear of the active mass layer was 3 mg / I000 ampere-hours.

EXAMPLE 8
An electrode is produced comprising an electroconductive support in the form of a titanium blade of 30 × 40 × 2 mm, to which is applied a mass-containing emSsseactiv (ruthenium oxide 5, cobalt oxide 94.9 and boron oxide 0.1). The electrode is manufactured as follows.

The titanium blade is prepared as shown in Example 1.

To apply the active mass, a mixture of cobalt nitrate solutions (1M solution), boric acid solution (0.5M solution) and ruthenium chloride solution is prepared & a concentration of 19.2% by mass of ruthenium. A layer of ruthenium chloride solution is first applied to the previously prepared titanium support, with a subsequent heat treatment at the temperature of 3700C for 10 minutes. The amount of metallic ruthenium applied is 1.3 g of ruthenium per m2 of working surface. Then a solution of cobalt nitrate and boric acid prepared by mixing said solutions is applied, and the heat treatment is carried out at the temperature of 450 ° C. for 20 minutes.

After the application of all the layers, the electrode is brought for 1 hour to 470 C. The total amount of active mass was 30 g per m2 of surface of the electrode.

 The electrode was tested when obtaining chlorine by electrolysis in a 9 diaphragm electrolyser at pH = 4-6 in a solution containing 280 g / l of NaCl, at a temperature of 900C, the anodic current density being 0.2 A / cm2. The potential of the anode was 1.36 V. 2050 amp hours were passed. The wear of the active mass layer was 3 mg / 1000 ampere-hours.

Example 9
An electrode is made from a titanium blade 30 × 40 × 2 mm on which an active mass is applied containing (% by mass): ruthenium oxide 45, iron oxide 53, boron-2 oxide. The electrode is manufactured according to the procedure of Example 1. A ruthenium chloride solution & the 19.2% ruthenium concentration are also used, which is first applied to the previously prepared titanium support ( one layer), with subsequent heat treatment at 3700C for 10 minutes.

The amount of metallic ruthenium applied is 1.3 g / m2 of surface. The total amount of active mass applied to the titanium support is 12.5 g per m 2 of surface of the anode. The electrode was tested during electrolysis in a diaphramame electrolyser at pH = 3-4, in a solution containing 300 g / l of NaCl, at the temperature of 900C, the anodic current density being 0.2 A / cra2. The anode potential was 1.35 V. 1900 ampere-hours were passed. The wear of the active mass during the electrolysis was 1 mg / 1000 ampere-hours.

 Of course, the invention is in no way limited to the embodiments described which have been given only by way of example. In particular, it includes all the means constituting technical equivalents of the means described as well as their combinations if these are executed according to the spirit and implemented in the context of protection as claimed.

Claims (14)

 1.- Electrode for electrochemical processes, of the type consisting of a support made of electroconductive material to which an active mass is applied containing metal oxides of the platinum group and of the iron or manganese group, characterized in that the active mass comprises in addition boron oxide at a rate of 0.1 d 50% by mass.  2. An electrode according to claim 1, Garact6- risée in that the active mass contains ruthenium oxide.  3.- An electrode according to one of claims 1 and 2, characterized in that the active mass contains iron oxide.  4.- An electrode according to one of claims 1 and 2, characterized in that the active mass contains cobalt oxide.  5.- An electrode according to one of claims 1 and 2, characterized in that the active mass contains a mixture of manganese and cobalt oxides.  6.- A method of manufacturing an electrode for electrochemical processes according to claim 1, of the type providing for the application to a support of an electroconductive material of an active mass, said application comprising the application to said support of a solution containing thermally decomposable compounds of metals of platinum, iron, manganese groups, followed by heat treatment at an elevated temperature, characterized in that a thermally decomposable boron compound is introduced into the solution in a proportion of 0 , 1 to 50% by mass, calculated as boron oxide.  7.- Method according to claim 6, characterized in that said solution contains thermally decomposable compounds of boron, iron and ruthenium.  8.- Method according to claim 6, characterized in that said solution contains thermally decomposable compounds of boron, cobalt and ruthenium.  9.- Method according to claim 6, characterized in that said solution contains thermally decomposable compounds of boron, cobalt, manganese, ruthenium.  10.- A method of manufacturing an electrode for electrochemical processes according to claim 1, of the type providing for the application to a support of an electroconductive material of an active mass, said application comprising the application to said support of a first solution of a thermally decomposable compound of at least one platinum group metal and its heat treatment at an elevated temperature, followed by the application on said support of a second solution containing a thermally decomposable compound of at least one metal of the iron group, of manganese or its mixture with a thermally decomposable compound of at least one metal of the platinum group, with heat treatment of said support at a high temperature, characterized in that a compound is introduced into said second solution thermally decomposable from boron in a proportion of 0.1 & 50% by mass, calculated as boron oxide.  11.- Method according to claim 10, characterized in that said first solution contains a thermally decomposable ruthenium compound.  12.- Method according to claim 10, characterized in that said second solution contains a thermally decomposable compound of boron and cobalt.  13.- Method according to claim 10, characterized in that said second solution contains a thermally decomposable compound of boron, cobalt and manganese.  14.- A method according to claim 13, characterized in that said second solution contains a thermally disintegrable compound of boron, cobalt and ruthenium.