CA1058564A - Electrode coating - Google Patents

Abstract

IMPROVED ELECTRODE COATING

Abstract of the disclosure:

A solid solution of a noble metal oxide, titanium oxide and zirconium oxide, containing 1 to 50 mol % of titanium oxide and zirconium oxide as the total content, is coated on an anti-corrosive substrate, e.g. titanium metal to give an electrode which is excellent in low oxygen content in chlorine gas, low electrode consumption and low chlorine overvoltage when it is used as anode for electrolysis of an aqueous sodium chloride solution.

Description

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This invention relatos to an improved electrode which can be used for electrolysis of an aqueous alkali metal halide ~e.g. sodium chlorido) solutlon and a process for producing the same. More particularly, this invention relates to an electrode comprising an anti corrosive conductor which i5 coated with a solid sol~ltion at least ~hree components of a noble metal oxide and 1 to 50 ~ of titanium oxide and zirconiu~ oxide and to a process for producing the s~me.
1~ ~eretofore, electrodes compri~ing an anti-corrosive conductor such as titanium coated with noble metal have been known. ~owever, they are high in chlorine ove~voltage and thi5 disad~antage increases with the lapse o~ time, when it i~ used as anode. Furthermore, they are liable to be we~ted with sodi~ amalgam, and similar metals.
Additionally they are ~ery expensive and also liabla to peeling. Thus, practical utilization of these electrodes has been difficult.
- ` A num~er of patents have been published concern-- 20 ing electrodes comprising anti-corrosive conducto~s coated with noble metal oxides. Examples of these include Japane~e Patent Publications No. 3409~71, No. 3~54/73, No. 29482/71, ~o. 9402/72, and ~o. 31510~72. ~he electrodes disclosea by these patents are coated with noble met~l ; 25 oxides. It has also been proposed in some of th~se patents ~, .
to coat a mixture of noble metal oxide with a second compo-~ent such as titanium oxide, ~ince it is generally difficult to coat a metal such as titanium fir~ly wi~h a noble metal oxid~. Canadian Patent 932700 disclo~es a multitude of e~ectrode coatin~ compositions containing at least one .
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, f~l ~L~5~569' oxide of a film forming metal, and at least one oxide o~
a platinum group metal. The patent specifically teaches in the specification that the content of said film ~orming metal is preferably more th~n 50 mol percent. For example, no ~avorable result i~ ohtained unless the electrode coating c~ntains 70 mole ~ titanium oxide in a mi~ed cry~tal with platinum metal oxide.
The present invention provides an electrode co~prising an anti-corrosive conductor havinf3 a coating of a solid solution of a noble metal oxide, titanium oxide and zirconium oxide, the total of titanium oxide and zirconium oxide being fr~m 1 to 50 mol ~ in said coating.
The present invention also provides a process for producing an electrode as mentioned above, which comprises lS coating an anti-corrosiYe conductor with a solution contain-ing a noble metal co~oun~, a titanium compound and a zirconium compound and then heating ~he coated product to oxidize the coated cc~pounds.
~ The elec~rode of ~he present inve~tion is specific in that the noble metal .~xide is not present in the coating in pure form but as a mixed c~ystal or non~cry~tall~ne state.
Furthermorc, hy the use of 1 to S0 mol % o both zirconium oxide and titanlum oxide, the electrode is high in oxygen overvo1tage in spite of a long life and low chlorine over-voltage. Thus, when the electxode of the invention isprovided for use as anode in electrolysis of an aqueous sodium chloride solution, the amount of oxygen gas mixed in the halosfen gas such aQ chlorine can be greatly reduced, the anode potentlal can be maintained low, and the electrode will have a long and useful life~
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The anti-corrosive conductor used in the present invention i8 a conductor which is anti-corrosive to elec-trolytes or electrolyzed products which it contacts when used as electsode~ These include for example titanium, zlrconium, tantalu~, niobiwm, alloys thereof, carbon, and the like.
Noble metal oxides which may be employed include oxides of ruthenium, rhodiwm, palladium, osmium, lridium, platinum, and mixtures thereof. Ruthenium oxide is parti-cularly pre~erred because it is relatively less expensiveand low in chlorine overvoltage.
In the solid solution coating according to the present invention, the sum of molar percentages of titaniu~
oxide and zirconium oxide i5 from l to 50 mol %, preerably from 10 to 45 mol %, each or titanium oxide and zirconium oxide can be varied from 0.5 to 49.5 mol ~. Within said range, the percentage of titanium oxide is preferably fr~m - 10 to 45 ~ol % and that of zirconium oxide is preferably from 1 ta 15 ~ol %. If the sum is less than 1 mol ~, the noble metal oXide canno~ be efficiently converted to a solid 5olution and therefore it is not fiYmly aahered to the anti-corrosive conductor. ~s the result, while chlorine overvoltage i9 low, oxygen overvoltage is also low so that the uxygen gas content o' the chlorine gas increases when ~t is used as anode for electrolysis of sodium chlorlde.
On the other hand, if tlle sum is more than 50 mol ~, the amount of noble metal oxide i~ so $mall that rhlorine overvoltage rapidly increases and causes an increase of the electrolysis voltage to the practical d~sadvantage of ~he process. Addit~onally, the oxygen gas _ 4 _ ~ . .
,, ~OS~S64 content in the chlorlne ga~ also ~ncreases. Purthermore, when the amount of the noble metal oxide i~ too ~mall, the electrode is rapidly corroded by the passage o~ current at high current density.
A~ mentioned above, the coating of the present invention contains three e~senti~l components, na~ely a noble metal oxide, titanium oxide and zirconium oxide.
While being not llmited to any theory, the reason for the presence of said essential components will ~o~ be explained in detail, as it is currently understood, by referring to ~he coating wherein ruthenium oxide is used as noble me~ai oxide.
The binary component system of ruthenium oxi~e and titanium oxide can be made into a solid 501utio~.
lS The results of X~ray analysis sho~ that there exists a state where neither pure crystal of ruthenium oxide nor pure crystal of ti~anlum oxide is observed. If the coating by thi~ system is in such a state, it will adhere to titanium metal substrates very well. ~owever, when the coated product

2~ is used as electrode for electrolysis of sodium chloride, it - is low in chlorine o~ervoltage and the oxygen content of the chlorine gas cannot be decreased. On the other hand, the - binary component system consisting ~f ruthenium oxide and zirconi~m oxide cannot be made into a solid solution. The results of X-ray analysis establish the presence of pure ruthenium oxide crystal. By the use of an electrode having a coating of this two component system for electrolysis of sodlum chloride, the oxygen content in chlorine g~s c~nno~
~igni~icantly be decreased. Furthermore, such an e~ectrode is unsati factory in that the coating adheres poorly to ~ 5 -D

~he titanium metal, and ele~trodc consumption i8 too high.
Whereas, in a ternary component system con~isting of ruthenium oxide and 1 to 50 mol ~ of titanium oxide and zirconium oxide, the component~ are made c~mpletely into a solid solution. The results of ~-ray analysis show no trace of pure crystals o~ r~tthenium oxide, t~tctnium oxide nor zirconium oxide. Due to th~ e~fect o~ zirconium oxide added, an electrode coated with a composition of this system-reduces oxygen content in chlorine gas to a great extent at the ti~e of electrolysis o~ sodi~m chloride. Further, the - electrode is endowed with advantages such as low chlorine overvoltage, excellent adhesiveness to titanium substrate, low electrode consumption, and longer electrode life.
The str~tcture of the coating of the invention, whether it ls in a state of a solid solution or not, can be determined by precise measurement of the lattice con~tants according to conventional method using X-ray.
The coating of the elec~rode is mechanically peeled away and an internal reference substance such as silicon or - - 20 -alumina is added before it is subjected to X-ray a~alysis.
According to values in ~te literature (e.g. AST~
cards), r~ttheniurt oxide belonss to the tetragonal system and has lattice constants of 4.490 A ~a axis) and 3.106 A
~c axis); platinum oxide belongs tc the rhombic system with a axis 4.487 A: b axis 4.536 A: c axis 3.137 A;
iridlt~n oxide belongs to the tetsagonal sy~tem with a axis 4.498 A: c axis 3.154 A; rutile type titanium oxide belongs O O
to tetragonal systent with a axis 4.594 A: c axis 2.958 A;
zirconium oxide belongs to the cubic system and, has a axi8 of 5.07 A; ~irconium ox~de belongs to the monoclinic ~ysten~
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_ ~0~i8564 o o and has a axis of 5.148 A, b ~xis of 5.203 A, c axis of 5.316 A and ~ = 9923'.
Within the precision oP the available measurement technique, variations of lattice constants of 0.01 ~ or more can clearly be confirmed. The term "solid solution" as used in the present disclosure refers to a product which deviates O.Ol A or more from the lattice constant of a pure noble metal oxide crystal. ~ccordingly, the solid solution includes ~ixed crystals ana amorphous state struc~ures~
There are various processes for producing a coat-ing of a solid solution. For example, a substrate is directly coated wi~h a molten mixture. ~lternatively, salts ` dissolved in an aqueous solution or a~ organic so~vent are pyrolyzed and precipitated on a substrate. A~ong them, it is preferred from a practical standpoint to coat an anti-corrosive conductor wi~h an aqueous hydrochloric acid ~ solution o~ a noble metal chloride, titanium chloride and - ~irconium chloride and heat the coated product at a tempera-ture higher than the thermal decomposition temperatures of ~aid chlori~es. A practical temperature is from 300 to ~00C, preferably from 400 to 630C, i~ an oxidative atmosphere in the pres~nce of sufficient oxygen to oxidize , ~ha decomposed metal co~pounds. Air is typically employed.
The time of the heating is at least one minute.
Furthermore, for acceleration of formation of a solid solution, other s~bstances s~ch as silica, alumina, boron oxide, lead oxide can also be added to the coating cc~position, whereby adhesi~enes~ of the coating to the anti-corroRive conductor can be improved. In orde~ to effect formation of a ~olid solution more easily, i~ is , .
.

~i[)~it5~5~4 preferred to increase the number ~f coats ~y adjustln~ the concentration or vlscosity o~ the co~ting composition so ~hat the thickness per coat may be as thin a~ possible, for example, 3 microns or lesfi, desirably 0.5 micron or less. The thickness of the coating is not limited, but it is usually from 1 to 20 microns. If a thick coating i9 desired, coating and heatin~ procedures mly ~e performed a number of ti~es.
It is very surprising that a solid solution is formed, as determined by X-ray analysis, even though the coating is treated at a temperature, e.g. 600C or lower, which is lower than the ~eltin~ points of titanium oxide, ~irconium oxide or of the no~le metal oxide. In the state o~ such a solid solution, the ratio of metal ato~s of each componen~ to oxygen atoms can no lon~er he expressed as a ratio of whole numbers. Thus, the solia solution is di3tinguished from the conventiona~ metal oxides.
The thus prep~red electrode which i8 coated with the solid solution as described above is improved not only in mechanical adhesivene.ss to substra~e but also in reslst-ance to chemical corrosion. ~here~ore, when it is used as - an anode, lt can be employed for a long time with low electrode consumption. Furthermore, the solid solution of the invention ~xhibits very good electric conductivity and shows a very low electrode potential at a current density as high as 100 ampere~dm2 or more. Even after electrolysis operation continued for a year or more, no increase $n voItage is observed.
The electroae of the lnvention can be used as anode for electrolysis of an aqueous halide solutions.

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In particular, it can suitably be used aq anode for production of caustic soda or caustic potash by either the diaphragm or ion-exchange membrane process, as well as production of chlorate and bromine gas. The following non-limiting ex~mples are gi~en by way of illustration only.
Example 1 ~ mesh with openiny ratio of 60 ~ prepared from a plate of titanium metal with thickness of loS mm is polished with a polishing powder and then dipped in 20 wt.%
- a~ueous sulfuric acid solution at 80~C for ~ hours to coarsen ~he surface. On thiR su~strate is coated a solution o~ 0.33 molJl ruthenium trichloride, 0.13 mol/l of zirconium chl~-ride andl 0.13 mol/l titanium tetrachloride dissolved in 20 wt. ~ agueous hydrochloric solution, followed by heating at - 450C in the air for S minutes. This coating operation is repeatea 10 times, followed finally ~y calcination at 500C
in the air for 3 hours, to produce an electrode. The thick-ness of coating applied is on the average about 0O2 microns per cycle.
This coating is mechanically rubbed off ana the powders of the coating are subjected to analysis by fluorescent X-ray to determine that the composition consists of O.68:0.13:0.19 in terms of the molar ratio of ruthenium o:cide:titanium oxide:zirconium oxide. The coating is su~-stantially free from chlorine, i.e. less than 0.1 wt. %.
Then, the powders of the coating are mixed with metallic silicon as the pri~ary reference and also with u alumina in case when there are overlapping peaks as secondary reference to per~it measure~ent of lattice _ 9 _ .

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con~tants of the crystal by means of X-ray analysis apparatu~ using X line of copper (wavelength: 1.54050 ~).
As the result, only peaks of a tetragonal system with a axi3 of 4.562 A and c axis of 3O090 A are detected. As S clearly seen from this result a solid 301ution i5 formed by deviating from the crystal lattice constants of pure ruthenium oxide and titanium oxide, indicating that neither pure ruthenium ox~de ~or pure titanium oxide i5 present.
The absence of peàks corresponding to the rhombic system and cubic system of zirconium oxide proves that zirconium oxide is also converted into a solid solution.
In an electrolytic cell, wherein an electrode of 1.2 m square preparea according to the above procedu~e is u~ed as anode, a mesh electrode made of ir~n is used as cathode and a cation exchange membrane i8 ~sed as diaphrag~, an anolyte comprising an aqueou~ sodium chloride solution, which i~ maintained at pll of 3.5 and at a concentration ~f 2.5 ~, is circulated. As catholyte, S ~ aqueou~ caustic soda solution is circulated. Both electrolytes are main-tained at 90C and elec~olysis is performed at curren~
density of 50 am~ere~dm2, while generating chlorine gas fron anode and hydrogen gas from cathode. Under these conditions, continuous running i6 carried out for 200 days, and no electrode consumption or change in voltage is ob~erved. The content of oxygen gas in the chlorine yas i8 found to be O.B6 0 by volume, which is by far lower than the oxygen content of the chlorine gas in the fol}ow-ing ref erence example 1.
Reference example 1 ~or comparativa purpose, experime~ts are conducted ' .
I

lOSl~64 by usin~ an electrode havin~ only ruthenium oxide coated thereon, an electrode having only ruthenium oxide and zirconium oxide coated thereon and an electrode having only ruthenium oxide and titanium oxide coated thereon.
Each electrode is prepared by repeating the procedure, comprising coating on the same anti-corrosive substrate as used in EXample 1 a solution o chlorides having the composition as shown in Table I ~issolved in 20 wt. ~ of a hydrochloric acid solution and heating the coated product at 450C in the air fox 5 minutes, for 10 ti~es, follot~ed f~nally by calcination at 500C in the air for 3 hours. The average thickness of the coating per cycle is about 0.2 ~icrons.
The co~position of each coating as well as crystal lattice constants thereof are determined in the same manner as in ~x~mple 1. Furthermore, by using each electrode as anode, the same electrolysis as described in Example 1 is repeated r whereby oxygen gas content in chlorine gas is measured. These resul~s are set ~orth in Table 1.

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- Table 1 clearly shows that the lattice constants of the coating con~isting of ruthenium oxide only are approximately the same as the publi hed ~alues, indicating that substantially no solid solution ~s formed (see experi ment No. 1). The powders of the coating consist~n~ of ruthenium oxidc and zirconium oxide include tetragonal ~ystem, cubic system and monoclinical system crystals.
Among them, the lattice constant~ of the tetragonal sy~tem are approximately the ~me a~ those oE the pure ruthenium oxide, indicating that the ruthenium oxide remains unchanged.
It i8 also con~irmed that there exists a mixture of zirconium oxides o which the tetragonal syst~m crystal has a axis of 5~116 A and of which the noclinical syste~ cry-~tal has a axis of 5.187 A, b axis of 5.116 ~ and c axis of 5.527 A
with B = 10018' ~see experiment ~o. 2 and No. 3~. The powders of the coating consisting only of ruthenium oxide and titanium oxide show crystals only of the tetragonal system, but the lattice constants thereof deviate greatly from those of either pure ruthenium oxide or titanium oxide tb indicate that they ar~ convertea into a solid solution.
- ~owever, when the electrod~ having this coating is used as - anode, oxygen content in chlorine gas cannot be lowered sufficiently.
When these result~ are compa~ed with those of Example 1, the amount of oxygen generated is shown to be less in the electrode of the ternary component system of ruthenium oxide, zirconium oxide and titanium oxide than in any o~ the electrodes of ruthenium oxide only, ruthenium oxide and zirconium oxide, and ruthenium oxide and titanium ; 30 - oxideO

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~xample 2 The compositions of the ~oatings of ruthenium oxide, 2irconium oxide and titanium oxide are varied in this Example. Substrate~ of the same titanium metal mesh as used in Example 1 are coated with solutions of chlorides having the compositions as ~hown in Tabl~ 2 dissolved in 20 wt. ~ aqueous hydrochloric acid, respeatively, followed by heating at 490C in the air for 5 minutes. This coating procedure is repeated ten time~ for each sample, followed finally by calcination ~t S00C in the air for 3 hours.
The thickness per coat is about 0.2 microns on the average.
The results of analysis of each ~oating and électrolysis by using ea~h electrode which are performed under the sa~e conditions as in Example 1 are set fort in ~able 2.
A~ seen from Experiments No. 2 to 6, the amount of oxygen gas in the chlorine is smaller with electrodes coated with a solid ~olution of ruthenium oxide with 1 to 50 mol ~ of zirconium oxide and tit~nium oxide than with those coated with a solid solution outside saia range.
~he chlorine overvoltages in the Tables are ~hown in tenms of values relative to the saturated calomel electrode ~S.C.E.) when electrolysis is perPormed in an aqueous sodi~m chloride solution at the current density of 50 ampere~dm2. Experiments No. 1 to 6 clearly show that chlorine overvoltage 1~ too high with electrodes having less than 50 ~ of ruthenium oxide content in the coat~ng to practical disadvantage, while it is approximately ~onstant, i.e. 1.10 V, with the electrode having 50 % or - more of ruthenium oxide content in the coating.30 ~ Corrosion resi~tance te t~ of these electrodes :1~58564 ; i8 conducted using approximately the same electrolytic cell as ~n Example 1 and 5 N aqueous 30dium chloride solution as ~he anolyte and performing electrolysis at 300 ampere/dm2. The amo~nt of consumption is measured, and the percentage by wei~ht of the consumption relative to the amount of coating i5 calculated. The results are shown in the column of consumption degree, which clearly ~hows that with the electrodes of the present invention tExperiments Nos. 2 to 6) there is less el~ctrode consump-tion than with electrode3 whereln only ruthenium oxide iscoated and with electrodes wherein the ruthenium oxide is le~s than 50 mol %.
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:~OS~564 Eurthermore, as seen from Experiment No. 2, by addition of only about 1 % of titanium oxide and zirconium oxide, the a axis lattice constant of the tetragonal system i8 signi~icantly changea to indicate formation of a solid solution.
- The results in Tables l a~d 2 show that the electroaes of the present invention coated with a solid solution comprising 1 to S0 mol % ~f the 5Um of titanium oxide and zirconium oxide and ruthenium oxide are excellent with any respect, i.e. low oxygen content in chlorine gas at the time o~ electrolysis, low electrode consumption and low chlorine overvolta~e.
Example ~
A mesh with opening ratio of 60 ~ prepared from a plate of titanium ~etal with thickness of 1.5 mm is subjected to the same treatment as in Example 1 and used a~ the anti-corrosive conductor. As the noble metals, ruthenium-platinum and ruthenium-rhodium are used. The chlorides of respective metals are dissolved in 20 wt.
2n hydrochloric acid solu~ion to prepare coating liquids.
Each coat~ng liquid is coated, and then heated at 500C
in the air ~or 5 minutes. The procedures are repeated f 10 times, followed finally by calcination at 550C in the air for 3 hours, to produce an electrode. The thickness -of coating applied per cycle is about 0.15 microns.
m e3e electroaes are subjected to measurement of electrode compositions and lattic~ constant~ to give the re~ult3 as set forth in Table 3, which shows that the electrode3 have excellen~ characteristics ~ith co~ings converted into sblid solution3.

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Example 4 Zirconium metal plate is used as the anti-corrosive conductor. After the plate i defatted by poli~hing powders, the ~urface thereo i8 coarsene~ with water resistant number 240 paper. A solution of 0.1 mol of t~tanlum tstrachloride, 0.1 mol of zirCQnium tetrachloride, 0.5 mol of iridium chloride, 100 ml. of 35 wt. % ~onc. hydrochlDric acid and 900 ml. of ethyl alcohol i8 coated on the plate, ~ollowed by heating at 500C in the a~r for 10 minutes.
Thi~ procedure is repeated 20 time~ before the electrode 1~ produced.
When the coating thus produced i6 analyzed by the procedure similar to Example 1, the molar % of iridium oxide, titanium oxide and Zirconium oxide in the powders of the coating is found to be 81 ~, 8 % and 11 %, respectively. The tetragonal crystal system has a axis O O
- of 4.541 ~ and c axis of 3.096 A to indicate that the -three chmponents form a solid ~olution.
Blectrolys~ of saturated aqueous potassium chloride solution is conducted by using this electrode as anode and mercury as cathode under the conditions of cuxrent passage area of 5 cm x 5 cm, current density of 30 ampere~dm~, electrolyte temperature at 7~C and at p~ 2.
- As a result, the oxygen gas content in the chlorine ga~ is ~ 25 found to be less than 0.1 ~
. .
Example 5 ~antalum is used as the anti-corrosive conductor.
After it i5 sub~ected to the same treatment as in Example

4, a solution o~ 0.72 mol ffl rhodium c~loride, 0.14 mol of titanlum hydroxide and 0.14 mol o~ sirconium hydroxide , ~ ~ - 19 - ~
-: .

~L~Q5~3S64 dinsolved in 35 wt. % conc. hydrochloric aaid is coated thereon. It 18 then heated at 450C in the air for 5 minutes. Thi~ procedure is repeated 10 times, followed fiDally by calcination for 3 hours in air to produce an electrode.
When the powders of the coating are subjected to X-ray analys~s, it is found that they are all in zmorphous ~tate with no crystal ormation.
Electrolysis is conducted by using this electrode as anode, an i~on mesh electrode a~ cathode and a cation exchange membrane as diaphra~m at 30 amperetdm2, utili~ing 2 N lithium chloride qolution a~ anolyte at p~ 3.5 to - ~ produce ~ N lithium hydroxide as catholyte. The oxygen content in chlorine gas ganerated ~rom thP anode is 1.O
~ol. %.
Example fi Titanlu~ is used as the anti-corrosive conductor.
It is ~ubjected to surface treatment by dipping in an aqueous oxalic acid solution at 9QC for 4 hou~s. Then~
on this substrate is coated a solution of 0.7 mol~l - ruthenium chloride, 0.1 mol~1 zirconium chloride and 0.2 mol/l titanium chloride, ~ollowed by h~ating at 500C for 10 minute~. This procedure is repeated 20 times before ~he electrode is produced.
- 25 Electrolysis is conducted using this electrode as anode, asbestos as diaphragm and a mesh electrode of iron as cathode at a current den~ity of 20 ampere~dm2.
A saturated ~odium chlGride sol~tion o p~ 4.5 is used as anolyte and an aqueous ~olution compri~ing caustic ~oda and sodlum chloride as catholyte. The oxygen gas content , .
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1~S8564 in the chlor$ne gas obtained i~ 2.0 ~. When electroly~i~
is performed by using an electrode which i~ coated with ruthenium oxide only, the oxy~en content in chlorine gas is 4.0 %.
Example 7 On a titanium alloy rod 3 mm in dlameter i~
~oated a 25 wt. ~ aqueouq hydrochloric ac~d solution containing 0.1 mol ruthenium chloride, 0.05 mol titanium bxomide, 0.025 mol zirconium chloride, 0.01 mol silicon chloride and 0.01 mol sodium borate, followed by heatlng at 450~C. After repeatin~ said procedure, an electrode is produced.
The coating o~ this electrode is subjected to X-ray analy3is to determine that a solid solution of oxides of ruthenium, zircon~um, silicon and boron is formed and that no pure ruthenium oxide is pre~ent in the coating.
Example 8 An electrode is produced by dipping a carbon pla~e 10 mm thick in a s~ol~en salt co~sisting of silica, lead oxide and borax containing 0.1 mol ruthenium oxide, - 0.01 1 iridium oxide, 0.03 mol titanium oxide and 0.01 mol zirconium oxide. When the coating is analyzed ~y X-ray, it is found that a solid solution is formed and no pure ruthenium oxide nor ixidium oxide i9 present.
;

Refexence example 2 Comparative tests are carried out using various electrodeR coated with three component3 consisting of ruthenium oxide, titanium oxide and e~ther tantalum oxide, ; 30 nioblum oxide, bismuth oxide or tungsten oxide.

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As the anti^corro~i~e conductor, the same mesh with opening ratio of 60 ~ prepared from a titaniwn plate 1.5 mnt thic~ as used in Example 1 i8 u~ed in each s~tpIe.
~he chlorides with ~ompo~itions as shown in Table 4 are aissol~ed in 25 wt. % of hydrochloric acid solutions to prepare the coatiny compo3itions. Each ~lectrode i~
produced by repeating the procedure, which comprises coating each compo~ition and then heating tlle coated product at 450~C in air ~or 5 minutes, for 10 ti~es, followed finally by calcination at 500C in air for 3 hours~
Electroly~is experiments are carxied out using - the~e electrodes and the ~ame electrolytia cell under the sante electrolysis conditions as i~ Example 1. The oxygen contents of chlorine ga~ are measured to give the results ~how~t in Tahle 4.
Table 4 clearly shows that oxygen concentration in ~hlorine gas is not decreased by utilization of the - electrodes of the a~ove comparative experiments.
~0 Example 9 Exantple 1 i5 repeated, but ruthenium oxide i3 replaced b~ a mixture of ruthenium oxide and platinum oxide, a mixture of rutheniunt oxide and palladiunt oxide, a mixture o~ ruthenium oxide and rhodium oxide or a mixture of ru~henium oxide and iridiu~ oxide~ the ratio o ruthenium oxide to other meta} oxiae in each mixture being 50:50 ~by weight). In each case, the resul~ obtained i~ similar to ~tat in Example 1.

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Claims (16)

What we claim is:

1. An electrode comprising an anti-corrosive con-ductor having a coating of a solid solution containing at least one noble metal oxide together with titanium oxide and zirconium oxide, the total amount of titanium oxide plus zirconium oxide being from 1 to 50 mol %; said total amount containing from 0.5 to 49.5 mol % titanium dioxide and 0.5 to 49.5 mol % zirconium oxide.

2. An electrode as claimed in Claim 1, wherein the noble metal oxide is ruthenium oxide.

3. An electrode as claimed in Claim 1, wherein the noble metal oxide is a mixture of ruthenium oxide and platinum oxide.

4. An electrode as claimed in Claim 1, wherein the noble metal oxide is a mixture of ruthenium oxide and palladium oxide.

5. An electrode as claimed in Claim 1, wherein the noble metal oxide is a mixture of ruthenium oxide and rhodium oxide.

6. An electrode as claimed in Claim 1, wherein the noble metal oxide is a mixture of ruthenium oxide and iridium oxide.

7. An electrode as claimed in Claim 1, wherein the total amount of titanium oxide plus zirconium oxide contains from 10 to 45 mol % titanium oxide and 1 to 15 mol % zirconium oxide.

8. An electrode as claimed in Claim 7, wherein the noble metal oxide is ruthenium oxide.

9. An electrode as claimed in Claim 7, wherein the noble metal oxide is a mixture of ruthenium oxide and platinum oxide.

10. An electrode as claimed in Claim 7, wherein the noble metal oxide is a mixture of ruthenium oxide and palladium oxide.

11. An electrode as claimed in Claim 7, wherein the noble metal oxide is a mixture of ruthenium oxide and rhodium oxide.

12. An electrode as claimed in Claim 7, wherein the noble metal oxide is a mixture of ruthenium oxide and iridium oxide.

13. A process for producing an electrode comprising an anti-corrosive conductor having a coating of a solid solution containing at least one noble metal oxide together with titanium oxide and zirconium oxide, the total amount of titanium oxide plus zirconium oxide being from 1 to 50 mol %; said total amount containing from 0.5 to 49.5 mol %
titanium dioxide and 0.5 to 49.5 mol % zirconium oxide;
which process comprises coating said anti-corrosive conductor by precipitating a mixture containing a sufficient amount of a noble metal compound, a titanium compound, and a zirconium compound from a solution containing said compounds to produce the said solid solution when said precipitate is heated at a temperature of from 300 to 700°C
in the presence of oxygen, and thereafter heating said precipitate at a temperature of from 300°C to 700°C in the presence of sufficient oxygen to oxidize the said compounds.

14. A process as claimed in Claim 13, wherein heating is effected at a temperature of from 400 to 600°C.

15. A process as claimed in claim 13, wherein the coating and the heating are repeated in plural cycles.

16. A process as claimed in Claim 15, wherein the thickness of coating per each cycle is controlled to 0.5 micron or less.