CA1044178A

CA1044178A - Electrodes with multicomponent coatings

Info

Publication number: CA1044178A
Application number: CA197,146A
Authority: CA
Inventors: Charles R. Franks; Barry A. Schenker; James M. Kolb; Kevin J. O'leary
Original assignee: Diamond Shamrock Technologies SA
Current assignee: Diamond Shamrock Technologies SA
Priority date: 1973-04-19
Filing date: 1974-04-09
Publication date: 1978-12-12
Also published as: JPS5026769A; DE2419021B2; DE2419021A1; US3875043A

Abstract

ABSTRACT OF THE DISCLOSURE
Electrodes useful in a wide variety of electrolytic processes comprise a conductive substrate bearing on at least a portion of the surface thereof a four-component coating, said components being the oxides of tin, antimony, at least one platinum group metal, and a valve metal selected from the group titanium and tantalum.

Description

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Recent years have seen a proliferation of dimensionally stable electrodes, i.e., wear resistant conductive substrates bearing on the surface thereof an electrically conductive, electrocatalytically active coating. Among these have been electrodes coated with (1) an antimony oxide doped tin oxide containing a platinum metal oxide as the electrocatalytic agent or (2) mixed crystals (so].id solutions) of a valve metal oxide and a platinum metal oxide~ While such electrodes are far superior to the previously employed graphite, particularly in the area o chlor-alkali electrolysis, understandable efforts have continued to extend the life of these electrodes (that is, reduce the platinum metal wear-rate per unit of product) and/or to reduce the tendency of the coatings to passivate (that is, increase in operating potential to a point at which further -~1 , , operation becomes impractical), especially under oxygen-evolving conditions. Further, owing to inherent limitations relating both to life and passivation tendencies, no single electrode 1 coating system has been found applicable to use in a wide variety I of electrochemical processes.
$~ATEMENT OF THE INVENTION -I Therefore, it is an object of the present invention to 11 ~ provide a coated electrode having a long coating life.
It is a ~urther object of the present invention to provide ;~ an electrode, the coating of which is extremely resistant to ~, passivation~
~3~ It is a st~ll further object of the~present invention to `provide an~electrode, the properties of whiah may be adapted for -u~e~in~a~variety~of elPctro~chemical ~ocesses.
These and further objects of the present invention will become apparent to those skilled in the art from the specification and claims that follow.
~3 There has now been found an electrode comprising an

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;. .: .: i : . : , ., electrically conductive supporting substrate bearing on at least a portion of the surface thereof a coating consisting essentially of from 1.0 to 10.0 percent antimony oxide, from 30 to 90 percent tin dioxide, from 1.0 to S0 percent of at least one platinum group metal oxide, and from 0.5 to 30 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of tin to antimony oxides is between 95:5 and 85:15. Further, within the aforestated ranges, those coatings having high valve metal and platinum metal 10 oxide concentrations are particularly useful as anodes at which oxygen is evolved. On the other hand, those coating compositions having low valve metal oxide concentrations and moderate concen~
trations of platinum metal oxides are particularly useful in chlor-I a~kali electrolysis.

As stated, the invention lies in a combined coating of ~ oxid~s of tin, antimony/ at least one platinum group metal, and - ;~
I a valve metal selected from the group titanium and tantalum on a co~ductive substrate, useful as an eleritrode, especially 20 as an anode, in a variety of electrochemical processes including l electrowinning of metals ~e.g., copper, nickel, and zinc) from I aqueous solution; chlor-alkali electrolysis including chlorine, chlorate, or hypochlorite production; electroplating; oxygen evolution from organic acidic solutions; ozone generation;
cathodic protection; electrodialysis; and the like.
`l~ Suitable substrates incl~de generally any metal of -~
sufficient~electrical conducti~ity and mechanical and chemical ,~ resistance to the cell environment in which it is to be employ-ed~ For example, these materials may include nickel, steel, ~ 30 stainless steel, titanium, niobium, ~irconium, and tantalum. ~
i Especially preferred for most applications are titanium, -I niobium, or tantalum substrates. Of course, those substrates .,, : .
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bearing an exterior coating, such as copper or aluminum-cored titanium or a platinum or other conductive metal layer over a titanium substrate, are contemplated. Generally, prior to , deposition of the coating and in order to provide a base to which the coating may be satisfactorily anchored, an etching or other cleaning operation is employed.
, The configuration of the electrode will vary considerably ; with the application intended but may generally be in the form ~, of a rod or a shee~, either continuous or foraminous, of the 10 ~ppropriate material.
, What may be considered the first of the components in ;, the coating composition is tin dioxide, pEeferably present in the form of crystalline,SnO2 and employed within the range of from 30 to 90 percent by weight of the total coating composition on an oxide basis, especially 30 to 50 percent for oxygen l applications and 60 to 90 percent for chlorine. , :l The antimony oxide component enters into the tin oxide "
, crystal lattice, rendering same more electrically ¢onductive. ,,-Although the antimony is present in an indeterminate oxide form ~ 20 owing to its entrance into the tin oxide crystal lattiçe,,it "~ may be expressed for convenience sake as Sb2O3- Thus, on thisj basis, the antimony oxide is present within the range from 1.0 ~ to 10, preferably 4~0 to 10, percent by weight ,'` Thé foregoing ranges of tin and antimony oxides are further qualifi~d by the proviso that they be present, respect- -i~ely, in the range, on,a mole ratio basis as the oxides, of : ~ .. . .
'3 ~ 95:5 to 85:15, especially 90:10. In this fashion there is -~ obtained the desired doping ef~ect of the antimony on the tin ~, ,~ oxide without the presence of an excess separate phase of 30 antimony oxides. ,, The third component of the coating is at least one "platinum group metal oxide", by which term it i~ intended to _4_ , :~ " , .. .

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4~8 include the oxides of platinum, palladium, ruthenium, iridium, rhodium, and osmium, preferably ruthenium, iridium, rhodium, and palladium, and especially mixtures o~ ruthenium with iridium, rhodium, or palladium oxides. These platinum group metal oxides are present in their most high-~y oxidized form and within the range of from 1.0 to 50 percent by weight. When the electrode is being fabricated for use as an anode at which ; oxygen is evolved, primarily or as a coproduct, amounts wi~hin the range of from 20 to 40 percent platinum metal oxide are preferred. On the other hand, when a chlor~alkali anode is intended, amounts within the range of 1~0 to 25 percent are preferred.
The final component is a valve metal oxide selected ;, from the group consisting of titanium and tantalum oxides.
While the titanium is yregent in the ~orm of ~iO~ and is generally crystalline (rutile) in nature, when tantalum is employed, an essentially amorphous tantalum oxide results. -~
Therefore, although it is expressed as Ta2O3, it is understood that mixtures of tantalum oxides may in fact be present. The amounts of valve metal oxides employed are generally within the range of from 0.5 to 30 percent by weight, especially 15 to 25, for oxygen-evolving applications and 0.5 to 3.0 or ~I chlor-alkali electr~lysis. Further, for a chlor-alkali ~ application, titanium is preferred as the valve metal whereas '~! in oxygen-evolving applications the preferréd valve metal is tantalum, although they are interchangeable in many instances. ~ ;
~; Generally speaking, the use of small amounts of the ~alve metal -;
oxide acts to extend ~he life of the electrode coating while the incorporation of larger amounts adds resistance to passivation.

~! 30 In summary, an example of a préferred anode for oxygen-evolving applications is a coating of ~rom 30 to 50 percent SnO2-4.0 to-8.0 percent Sb203, 20 to 40 percenh plahinum metal oxide, and 15 to 25 percent valve metal oxide on a titanium, tantalum, ~5-: . . . . ........................................ .
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or niobium substrate.
On the other hand, an example of a preferred chlorine anode is a coating of 60 to 90 percent SnO2, 4 ~o 10~percent Sb2o3, 1.0 to 25 percent platinum metal dioxide, and 0.5 to 3.0 percent titanium or tantalum oxide on a titanium substrate.
While many of the ~ariety of methods known for producing mixed metal oxide coatings may be employed, the preferred method of preparing the multicomponent coating composition on the substrate is by deposition from a solution of the appropriate thermochemically decomposable salts. For example, it is desirable to paint or brush an acidified alcoholic solution of said salts onto the substrate followed by drying at 100-140C ~or from

3 to 10, especially 5, minutes and finally by baking in an oxidizing atmosphere, e.g., air, at 450 to 520 C, especially 500C, for from 5 to 10, especially about 7" ~inutes. ~ his procedure may then be repeated any number of times until the desired coating thickness is obtained~ for example, 6 to 10 coats.
The preferred solvents for the thermally decomposable salts are the lower alkanols, such as ethanol, propanol, amyl alcohol, and especially n-butyl alcohol, although other solvents including I water, may be employed, to which there is generally added from ;
O to 50 percent by volume of an acid, such as concentrated hydrochloric acid ~36%). The concentration of the salts from which the coating composition is derived is such as to give a .
metal content in solution within the range of 50 to 200 grams ~-per liter. The salts employed are generally any thexmally ;
decomposable inorganic or organic salt or organic es~er of the : `:
i~ metals in question such as the chlorides, nitrates, alkoxides, -alkoxy h~lides, resinates, amines, and the like. Specific and lllustrative examples include potassium hexachlororuthenate, I hexachloroiridic acid, ruthenium trichloride or tribromide, l ' ' ,' : , ,.. , ~ . . . . . .
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orthobutyl titanate, tantalum pentachloride, antimony trichloride or pentachloxide, and stannic chloride or dibutyl tin dichloride.
It Will be understood by those skilled in the art that it is possible to use a number of combinations of preformed oxides of the various component me~als and salts of the remaining materials, although it is generally believed that preformed valve metal oxides should not be employed nor should separately preformed tin and antimony oxides be used. Further, if thermal decomposition is incomplete, small amounts of salts may remain 10 without detrimental effect in the coating, for example, small -amounts of chloride in the primarily oxide coating.
.
In order that those skilled in the art may more readily ( understand the present invention and certain preferred embodiments 1 ~Y which it may be carried into effeat, the following specific I examples are afforded.

;l ~ EXAMPLE 1 ¦ A series of electrodes is prepared and evaluated as anodes as follows. In each instance, the quantity of thermally -! decomposable salt set forth in Table 1 is dissolved in 45 ml of ~ 20 ethanol with stirring. The resultant solution is brushed onto `1 an expanded kitanium mesh substrate, previously cleaned by ~

I etching for 30 minutes in boiling (18%) aqueous hydrochloric ~ -,. . .
jl~ acid. ~he solution is applied to the mesh by brushing, followed : 1 , , :, .
~l by dryin~ the anode for 3 minutes at 110C and fixing in air at ~;
.: . .. .
500C ~or 7 minutes. T~is brushing, drying, and baking procedure is repeated until a coating ~ntai~gj 1.7 grams of ~ ~ -'~ ruthenium per square foot of anQ~e surfacé is obtained (usually -~¦ 6-10 coats). Fol~owing the final baking, the electrodes are evaluated as anodes in a 150 g/l sulfuric acid solution at 30 3 amperes per square inch opposite a titanium mesh cathode and -, at an electxode gap of 2 inches. The test is continued until :;, . '':

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the anodes have passivated, i.e., a voltage of 8.0 volts or ; greater is obtained. The lifetime of the anode, that is, the number of hours of successful operation until passivation occurs, is reported in the following Table 1. ~:
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From this it is apparent that Anodes 4 and 5, according to the present invention, are greatly superior to either an anode combining the valve metal and platinum group metal (Anode 1) or the platinum metal-antimony-tin system (Anode 2). Further, Anode 3 illustrates that the range of components of the present invention is critical to obtaining an anode having a long lifetime.

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Four electrodes were prepared rom the following solutions:
Anode 6 - 50 m~ n-butanol, 12.5 g SnC14 5H2O, 1 ; 0.91 g SbC13, and 1.1 g RuC13 xH2O
;' (38~ Ru).
-~ Anode 7 - 45 ml ethanol, 5.0 g orthobutyl 3 titanate, 1.1 g SbC13, 15.1 g SnC14-5H2O, and 7.6 g RuC13-xH2O
(38% Ru).
¦ Anode 8 - 50 ml n-butanol, 12.5 g SnC14 SH2O, 0.91 g SbC13l 7.0 g orthobutyl titanate, and 1.1 g RuC13-xH2Q ,~
(38%~!Ru).
3 Anode 9 - 45 ml ethanol, 4.5 g TaC15, 1.1 g -SbC13, 15.1 g SnC14-5H2~, and 7.6 g RuCl3.xH2O (38~ Ru).
Each anode is pre~ared by applying six coats of the solution by brush, with heating in air between each coat first at 110C for 3 minutes followed by 7 minutes at 500C. ;
Thes~ electrodes are eYaluated as anodes in a hori~ontal mercury cell spaced 0.14 inch above and parallel to a mercuxy cathode flowing at a rate of 450 ml/minute. The `l , .~ .
30 electrolyte is a 310 g/l brine s~lution having a pH within the ;
range of 3~6 and a temperature of about 70C. ~o establish the ,~! wear-rate of the anodPs, electrol~sis i~ conducted at 6 amperes -~
',: ~' 1'7~
per square inch for 500 ~ours, the loss being determined by we~ght differential. Results, together with the composition of each anode coating calculated on an oxide basis, appear in Table 2. -~

SnO2 sb23Ru02 TiO2Ta25 Wear-Rate Anode % % % % % g/ton Cl , 6 83.8 8.18.1 - - 0.29 -7 55.4 6.430.6 7.6 - 0.25 ~ 81.9 8.97.9 1.3 - 0.14 -9 47.4 5.22702 - 20.2 0.11 . ... . .
From the table, it is evident that Anode 6, without ~ the added valve metal oxide, exhibits the highesk wear-rate.
i When tantalum is employed (~node 9) or using relatively small amounts of titanium (Anode 8), the best results are obtained. ~-An anode coating solution is prepared from 45 ml ethanol, 4.5 g TaC15, 1.1 g SbC13, 15.1 g SnC14-5H20, and - -7.6 g RuC13-xH20 (38% Ru). An etched titanium mesh substrate ` 20 I is coated by brushing, drying at 110C for 3 minutes, and baking in air at 500C for 7 minutes. The coating procedure is repeated until a coating having a pl~tinum gxoup metal content of l gram per square foot is ob~ained. This is labeled ~
il Anode 10. ~ -.
Anode 11 is prepared in an identical fashion but substituting 0.92 g of IrC13 and 6~54 g RuC13-x~20 for the ',:: . :
ruthenium content of Anode 10. Anode 12 is likewise similar ~ with the exceptLon that 1.28 g o~ RhC13.3H20 and 6.65 g i RuC13-xH20 comprise the platinum group metal content. --When evaluated according to the lifetime test described in Example 1 above, Anodes 10, 11, and 12 have lifetimes, 3 . 2 '. , -11- '.

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respectively, of 185, 250, and 350 hours. This indicates the '~ substantial improvement possible employing a mixture of platinum ~ metal oxides in the coating.
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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed axe defined as follows:

1. An electrode comprising an electrically conductive supporting substrate bearing on at least a portion of the surface thereof a coating consisting essentially of from 1.0 to 10 percent antimony oxide, as Sb2O3, on a weight basis, from 30 to 90 percent SnO2, from 1.0 to 50 percent of at least one platinum group metal oxide, and from 0.5 to 30 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of tin to antimony oxides is between 95:5 and 85:15.

2. An electrode as in Claim 1 wherein the supporting substrate is selected from the group consisting of nickel, steel, stainless steel, titanium, niobium, zirconium, and tantalum.

3. An electrode as in Claim 1 wherein the platinum metal oxide is RuO2.

4. An electrode as in Claim 1 wherein the valve metal oxide is TiO2.

5. An electrode as in Claim 1 wherein the valve metal oxide is amorphous tantalum oxide.

6. An electrode as in Claim 1 wherein the ratio of tin to antimony oxides is about 90:10.

7. An anode for use in oxygen-evolving applications, which anode comprises an electrically conductive supporting substrate bearing on at least a portion of the surface thereof a coating consisting essentially of from 4.0 to 8.0 percent antimony oxide, calculated as Sb2O3, on a weight basis, from 30 to 50 percent SnO2, from 20 to 40 percent of at least one platinum metal oxide, and from 15 to 25 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of tin to antimony oxides is between 95:5 and 85:15.

8. An anode as in Claim 7 wherein the substrate is selected from the group consisting of nickel, steel, stainless steel, titanium, niobium, zirconium, and tantalum.

9. An anode as in Claim 7 wherein the platinum metal oxide is a combination of RuO2 and IrO2.

10. An anode as in Claim 7 wherein the platinum metal oxide is a combination of ruthenium and rhodium oxides.

11. An anode as in Claim 7 wherein the valve metal oxide is amorphous tantalum oxide.

12. An anode for use in chlor-alkali electrolysis, which anode comprises a valve metal substrate selected from the group consisting of titanium, niobium, zirconium, and tantalum bearing on at least a portion of the surface thereof a coating consisting essentially of from 4.0 to 10 percent antimony oxide, calculated as Sb2O3, on a weight basis, from 60 to 90 percent SnO2, from 1.0 to 25 percent of at least one platinum group metal oxide, and from 0.5 to 3.0 percent of a valve metal oxide selected from the group consisting of titanium and tantalum oxides, with the proviso that the mole ratio of tin to antimony oxides is between 95:5 and 85:15.

13. An anode as in Claim 12 wherein the substrate is titanium.

14. An anode as in Claim 12 wherein the valve metal oxide is TiO2.