CA2166494C - Metal electrodes for electrochemical processes - Google Patents

Abstract

A metallic electrode for electrochemical processes comprising a metal support and on at least a portion of said support, a conductive coating consisting essentially of a mixed oxide compound of (i) a compound selected from the group consisting of the general formulae aM2?3 and bSb2?y; wherein M is selected from the group consisting of Al, Rh and Cr in the trivalent state; and y is 3 or 5, (ii) cRuO2, (iii) dIrO2 and (iv) eTiO2; wherein the mole fraction of the sum of a+b in the mixed oxide compound is in the range 0.01 to 0.42, the mole fraction c of RuO2 is in the range 0.00 - 0.42, the mole fraction of d of IrO2 is in the range 0.00 to 0.42, provided that the sum of c+d is at least 0.02 and the mole fraction e of TiO2 is in the range of 0.14 to 0.93. The electrodes have low precious metal content, provide improved durability and improved current efficiency-anodic overvoltage performance. They are used in the electrolysis of chloride-containing liquors in the production of, for example, chlorine and more particularly, chlorate.

Description

METAL ELECTRODES FOR
This invention relates to an improved coating intended for constituting the active surface of a metal electrode of use in the electrolysis of alkali metal halides, and, particularly, in the production of sodium chlorate from said electrolysis.
BACKGROUND OF THE INVENTION
In electrolytic cells for the production of chlorine, such as those of the diaphragm and membrane type, an aqueous solution of an alkali metal halide is electrolyzed to produce chlorine at the anode and an alkali hydroxide and hydrogen at the cathode. The products of electrolysis are maintained separate.
In the production of sodium chlorate, the chlorine and alkali hydroxide are allowed to mix at almost neutral pH and the sodium hypochlorite is formed in the above mixing which then is converted to sodium chlorate.
' United States Patent No. 3,849,282 - Deguldre gel., describes a coating for metal electrodes, which coating comprises a compound AB04 being associated with an oxide of the type MOI where M is ruthenium and/or iridium:
The coatings of the electrodes described therein may be used in various ST,331 electrochemical processes such as cathodic protection, desalination or purification of water, electrolysis of water or hydrochloric acid, production of current in a fuel cell, reduction or oxidation of organic compounds far the electrolytic manufacture of per salts, and as anodes in the electrolysis of aqueous solutions of alkali metal halides, particularly sodium chloride, in diaphragm cells, mercury cells, membrane cells and chlorate production cells, where they catalyze the discharge of chloride ions. The castings of the electrodes described therein are stated ca adhere to their metal sup~porc and lx resistant to ele~ochemical attack.
United Staua Patent Na.3,718,5St - Mar<insons, describes an Ip eiectro:.anductiw caring iof metal elei:crodea. ~rhich cc~tiri~ cc~mpiises a n~ixturr of amarEf?tow dtaniutn ~~fioxide acrd a m~rart~er of the grouly consisci.r~~;
of ruthenium and ruthenium dioxide. 'Ifie elr:ccrrades dcscribr~:i theusin arC
characterized by having a law oxygen and chlorine overvoliage, tesistant;e t~~
G~rrosion and de~nil~sician for coatings cc3ncaining less than 50~ 1yy weight of ritanium (as oxide) bas;;~'9 4n tt~c total metal content of the castings.
Neithr~., llniv:.d States Pa.cen: 1~~:~. 3,718,551 nor 3,i~~?,~Y'?. gt~,'es any teaching an the current ef~iciercy of tf~~ ~drx.trc~s for d» okid;:~ic~n of chloride in ayue~ms _:~~lucian. Katdwst.~i an:l l~ass'e, Modern Ci:lor AL:.rli Te~,hnolugy, Volume 3, page ~'~1, Society of Chemical Industry by E. Horwood, New York, 1986, Editor: Kevin Wall comment on the relationship tretween overvoltage and oxygen evolution for the axidatian of agtteous chloride solutions using coatings of the type taught by US 3,718,551 wherein a linear relationship between overpocendal arid Iog oxygen content irt chlorine (increasing one -reducing the other) is given. Mot~over, increasing ruthenium oantent ix stated to result in increased o~tygen evolution and reduced overpotential.
United States Patent No. 5017,27b - Alford , issued May 21, 1991, describes . . ..;cal electrodes provided with a coating consisting essexttially of a mixed oxide compound of (i) a compound of the general formula ABU, having a structure of the nrtile-type, where A i~ an element in the trivalent state seler.~tai from the group consisting of Al, Rh and Cr, and B is an element in the 3p pentavalent state selected from the group consisting of Sb and Ta, (1i) Rub and (iii) TiC3~; wherein the mole fraction of t~80, is between 0.01 and 0.42, the mole fraction of RuOz is between 0.03 and 0.42, and the mole fraction of Ti02 is between 0.55 and 0.96. The electrodes have low precious metal content, provide improved durability and improved current efficiency-anodic overvoltage performance. They are used in the electrolysis of chloride containing liquors.
in the production of, for example, chlorine and more particularly, chlorate.
However, notwithstanding the commercial success of electrodes of the type described and claimed in USP 5017276, there is a desire for an electrocatalyst coating having improved properties whereby although undesired byproduct oxygen co-evolution occurs, such oxygen co-evolution has reduced or minimal effect on the coating.
Furthermore, improved coating stability and selectivity towards chlorine evolution is desired under electrolysis conditions which are prevalent in the membrane chloralkali cell, where the anode is in physical contact with the membrane separator, whose surface is believed to be strongly alkaline and, thus, aggressive towards unmodified Ru02/Ti02 coating compositions.
In the case of coatings comprising Ru4Z, oxidation to RuOd with subsequent volatilization, under upset conditions at low chloride concentrations, results in loss of the ruthenium component from the electrode.
Not all the current passing through an alkali halide-containing electrolyte is utilized in the production of the desired products. In the electrolysis of sodium halides a minor part of the current produces oxygen at the anode rather than chlorine and this decreases the process efficiency. In electrolytic cells for the production of chlorine, the oxygen is present in the chlorine gas leaving the cells. This can lead to costly chlorine treatment processes for downstream operations. In chlorate producing cells, because there is no separator to separately confine the anodic and cathodic products, the oxygen becomes mixed with the hydrogen evolved at the cathode. Because of the danger of forming an explosive mixture, it is not desirable in general to operate chlorate-production cells with greater than 2.5 ~ oxygen in the evolved hydrogen. Thus, the amount of oxygen evolved from an anode used for the electrolysis of halide solutions is important for process efficiency and, additionally for chlorate production, safety reasons.

-- 216b494 A further source of oxygen in chlorate-production cells can arise due to catalytic decomposition of the intermediate sodium hypochlorite by metallic contaminants. Unfortunately, the platinum metal oxides used as electrocatalytic coatings for chloride oxidation are also excellent catalysts for hypochlorite decomposition. It is important therefore not only for long uniform performance life of the anode coating but also to minimize catalytic decomposition of the sodium hypochlorite that strongly adhering electrocatalytic coatings should be employed on electrodes for the electrolysis of halide solutions.
Further, electrocatalytic coatings produced solely from platinum group metal compounds can, depending upon the platinum metal used, be expensive. It is desirable therefore, that provided the operating characteristics of low oxygen evolution, low voltage, low wear rate are satisfied, the proportion of platinum group metal in the coating should be as low as possible.
Surprisingly, we have found ~ that notwithstanding the order of magnitude greater economic cost of iridium over ruthenium, economical and improved electrocatalytic iridium-containing coatings can be obtained. We believe that improved coating stability and efficacy at low brine. concentrations in an alkaline environment as found, for example in a commercial chloralkali membrane cell will be obtained.
Thus, we have found that substitution of part of the Ru02 component with IrOz in the electrode composition defined in USP 5017276 provides improved coatings.
SUMMARY OF THE IQWENTION
It is an object of the present invention to provide an electrode having an electrocatalytically active coating which is resistant to corrosion when used in the electrolysis of alkali metal halide solutions. .
It is a further object to provide an electrode for said use having a coating with very low wear rate.
It is a still further object to provide an electrode for said use having a coating which has an improved chlorine to oxygen overpotential and hence reduced electrolytically produced oxygen as a function of chlorine produced in the electrolysis of aqueous halide solutions.
It is a yet still further object to provide an electrode for said use having a coating which has a low anodic overvoltage.
It is a yet further object to provide an electrode for said use having a coating having, improved longevity in operation under oxygen producing conditions.
It is a still yet further objective to provide an electrode having improved coating stability and selectivity towards chlorine evolution under electrolysis conditions prevalent in membrane chloralkali cells where the anode is a physical contact with the separator membrane.
It is a still further object to provide an electrode for said use having an improved oxygen overpotential to operation temperature performance and hence reduced electrolytically produced oxygen as a function of operation temperature increase.
Accordingly, the invention provides in its broadest aspect a metallic electrode for electrochemical processes comprising a metal support and on at least a portion of said support, a conductive coating consisting essentially of a mixed oxide compound of (i) a compound selected from the group consisting of the general formula aM203 and bSb20r; wherein M is selected from the group consisting of Al, Rh and Cr in the trivalent state; and y is 3 or 5, (ii) cRu0l, (iii) dIrOz and (iv) eTi02; wherein the mole fraction of the sum of a+b in the mixed oxide compound is in the range 0.01 to 0.42, the mole fraction c of RuOz is in the range 0.00 - 0.42, the mole fraction of d of Ir41 is in the range 0.00 to 0.42, provided that the sum of c+d is at least 0.02 and the mole fraction a of Ti02 is in the range of 0.14 to 0.93.
In a preferred aspect, the invention provides a metallic electrode as hereinabove defined for electrochemical processes comprising a metal support and on at least a portion of said support, - a conductive coating consisting essentially of a mixed oxide compound of (i) a compound selected from the group consisting of the general formula MSb03 and MSb04, where M is an element in the trivalent state selected from the group consisting Al, Rh, and Cr, (ii) cRu02, (iii) dIrOz and (iv) eTiOz; wherein the mole fraction of MSb03 and/or MSb04 is in the 0.01 to 0.42 range, the mole fraction c of RuOz is in the range 0.00-0.42, the mole fraction d of Ir4l is in the range 0.00 to 0.42, provided that the sum of c+d is at least 0.02 and the mole fraction a of TiOz is in the range of 0.14 to 0.93.
More preferably, the mixed oxide compound has a mole fraction of at least 0.02 Ru02 and 0.02 IrOz and M is aluminium.
The electrodes of the present invention have low precious metal content and provide low wear rates and improved current efficiency-anodic overvoltage performance. They are used in the electrolysis of chloride containing liquors in the production of, for example, chlorine, and, particularly chlorate.
It is preferred to place the conductive coating of use in the present invention on a metal support at least superficially made of titanium or a metal of the titanium group. Advantageously, titanium is clad on a core of a more conductive metal such as copper, aluminum, iron, or alloys of these metals.
Preferably, the coating of use in the present invention consists essentially of the compounds as defined hereinabove in the relative amounts defined; yet more preferably, the coating consists of those compounds as defined.
Thus, the components MSb04 and/or MSb03, Ru02, Ir02 and TiOz must be present together in the coating in the relative amounts defined whether or not a further constituent is present in the coating.
However, it has been found advantageous to maintain certain concentrations within the above defined limits when the conductive coating is intended for the manufacture of metallic anodes for the electrolysis of chloride containing solutions, especially sodium chloride. We have surprisingly found that for particular concentrations of Ru4i and IrOZ, for example, in equimolar 0.05 mole fractions that for certain proportions of MSb04 and Ti02 electrochemical performance superior to that applying for mixtures of Ru02 alone with ABO, as defined in aforesaid U.S. Patent No. 5017276 and TiOz is obtained and, moreover, improved coating stability is indicated for coatings the subject of this invention than admixtures of ABO, and TiOz with RuOz.

The invention preferably provides an electrode coating comprising the above ingredients in the mole fractions in the following ranges:
MSbO, 0.05 - 0.3 RuOz . 0.0 - 0.3 Ir(~ 0.02 - 0.3 Ti02 0.55 - 0.93; and more preferably, the coating comprises MSb04 0.05 - 0.2 Ru02 0.0 - Ø2 Ir02 0.02 - 0.2 Ti02 0.60 - 0.92; wherein M is as hereinabove defined.
Three particularly useful embodiments are:
RhSb04 . Ru02 . Ir4i . 9TiOZ and .
A1Sb04 . Ru02 . IrOz . 9Ti02 A1Sb04 . 2Ir0z . 9TiOz DESCRIPTION OF PREFERRED EMBODIMENTS
In order that the invention may be better understood preferred embodiments will now be described by way of example only, with reference to the accoiripanying Examples:

This Example illustrates the preparation of an electrode having a coating of the formula:
AlSbO, . RuOI . Ir02 . 9Ti01 A solution x was prepared by dissolving 0.28 gms of A1C13 and 0.26 gms of SbCls in 40 mls of n-butanol, and a solution y was prepared by dissolving 0.51/gms of finely ground RuCl3 . XH20 (40.89°X Ru) and 1.0 gms of H2IrC16 . XH20 (40 ~ Ir) in 40 mls of n-butanol.
Solution x and y were brought together with 6.73 mls [CH3(CH~30],Ti and mixed well. This solution was applied in six layers onto plates of titanium which had previously been hot-degreased in trichloromethylene, Z1b6494 vacu-blasted, and then etched for seven hours at 80 ° C in 10 ~O oxalic acid solution. After each application of the coating mixture, the plates were dried under infra-red lamps and then heated in air for fifteen minutes at 450°C. After the sixth coating -application, the titanium plates, now fully coated, were heated for 1 hour at 450°C. The amount of material thus deposited was about 8 gms/m2:
~nrmnri.~ i This Example illustrates the preparation of an electrode having a coating of the formula:
AlSbO, . 2Ir02 . 9TiOz A solution x was prepared by dissolving 0.28 gms of AICI3 and 0.62 gms of SbCls in 40 mls of n-butanol, and a solution y was prepared by dissolving 2.0 gms of H2IrC16 . XH20 (4096 Ir) in 40 mls of n-butanol.
Solutions x and y were brought together with 6.73 mls [CH3(CH~30J~Ti and mixed well. This solution was applied in six layers onto plates of titanium which had previously been hot-degreased in trichloromethylene, vacu-blasted, and then etched for seven hours at 80°C in 1096 oxalic acid solution. After each application of the coating mixture, the plates were dried with infra-red lamps and then heated in air for fifteen minutes at 450°C.
After the sixth coating application, the titanium plates, now fully coated, were heated for 1 hour at 450°C. The amount of material thus deposited was about 8 gms/m2.

This Example illustrates the preparation of an electrode having a coating of the formula:
RhSbO, . 2IrOi . 9TiOz A solution x was prepared by dissolving 0.50 gms of RhCl3 . XHZO
(42.5 ~ Rh) and 0.62 gms of SbCls in 40 mls of n-butanol, and a solution y was prepared by dissolving 2.0 gms of HZIrCIb . XH20 (409& Ir) in 40 mls of n-butanol.
Solution x and y were brought together with 6.73 mls [CH3(CH~30]~Ti and mixed well. This solution was applied in six layers onto plates of titanium which had previously been hot-degreased in trichloromethylene, vacu-blasted, and then etched for seven hours at 80 ° C in 10 ~&
oxalic; acid solution. After each application of the coating mixture, the plates were dried with infra-red lamps and then heated in air for fifteen minutes at 450°C.
After the sixth coating application, the titanium plates, now fully coated, were heated for 1 hour at 450°C. The amount of material thus deposited was about 8 gms/m2.
The Ru02/Ir02 coatings of the invention prepared as described in Examples 1, 2 and 3 were evaluated in comparative studies against the most preferred coatings defined in aforesaid USP 5017276 and against other coatings of related compositions in both sodium chlorate production cells and chlorine/sodium hydroxide production membrane cells.
The coated titanium plates were submitted to the following types of evaluations.
The first evaluation relates to the electrode performance with regard to oxygen formation when used in a cell producing sodium chlorate under commercial conditions.
The second evaluation relates to the anodic voltage when the electrode is used under typical conditions of commercial sodium chlorate production.
The third evaluation relates to the electrode performance with regard to oxygen formation and anodic voltage when the electrode is used in chlorine/sodium hydroxide production membrane cells under commercial conditions.
The fourth evaluation relates to the performance of the coating under accelerated wear conditions where the anodic product is chlorine but the production conditions are very much more aggressive than those encountered in commercial practice.
In adhesion tests, the coating showed excellent adherence to the titanium substrate, as was shown by stripping test with adhesive tape applied by pressure, both before and after operation in electrolytic cells for the production of sodium chlorate.

The first test was performed with an electrolyte at 80°C
containing 500 g/ 1 NaC 103, 110 g/ 1 NaC 1 and 5 g/ 1 NaZCr20.~. The electrolyte was circulated past the coated titanium anode produced above at a fixed rate in terms of litres/Amp-hour and the oxygen measured in the cell off gases over a range of current densities between a and 3kA/m2. (See for example, Elements of Chlorate Cell Design, LH. Waren and N. Tam in Modern Chlor-Alkali Technology, Vol.
3, Editor K. Wall. Ellis Harwood Ltd. Publishers, Chichester England (1985).
The second test was performed with the same apparatus as for the first test but with a Luggin capillary probe used to measure the anodic voltage at various current densities before and after prolonged operation. (See, for example, Application of Backside Luggin Capillaries in the Measurement of Non-uniform Polarization, M. Eisenberg, C.N. Tobi,as and C.R. Wilke, J. Electrochem Soc., July 1955, pp. 415-419).
The third test was performed on coated electrodes in a laboratory size chloralkali cell, having 8 cm diameter disc coated titanium and nickel mesh anode and cathode, respectively, separated by a NAFION m 902 membrane.
Current was fed to a titanium or nickel rod welded to the centre of the respective discs.
Sodium chloride (33 gms/ 1) anolyte at a pH of 4 and sodium hydroxide (30-32 ~O W/W) catholyte were used, with the electrolysis temperature maintained essentially at 85°C.
The fourth test was performed using an electrolyte containing 1.85 M HC104 and 0.25 M NaCI. The electrodes were operated in a cholorine production cell at 30°C and at constant cell voltage using a potentiostat. The current under constant voltage was recorded until it changed significantly which indicated the time-to-failure of the test electrode. (See, for example, Electrochemical Behaviour of the Oxide-Coated Metal Anodes, F. Hine, M.
Yasuda, T. Noda, T. Yoshida and J. Okuda., J. Electrochem Soc., September 1979, pp. 1439-1445).
The performance of these coating in tests 1 and 2 above, are illustrated in Table 1, was surprising in relation to the performance of commercial coatings with separate admixtures of A1Sb04, Ru02, and Ti02 as evidenced by data given in Table 2.
A comparison between Table 1 and Table 2 shows that a preferred coating according to the invention, namely A 1 SbO,. Ru02. Ir0Z.9Ti02 (Sample A, Table 1) has an excellent surface potential and oxygen evolution comparable to the commercial coating of Table 2.
In addition, the sample anode prepared in Example 1 was rechecked after running for 103 days under the same operating conditions as in the first test and the result showed no change in anodic voltage.
Anode Coatings prepared in Example 2 and Example 3 (Samples B, C, Table 1), when used in a chlorate cell, gave anodic voltages of 1.15 and 1.16 volts versus SCE respectively, and the oxygen content of the gases exiting the cells were 3.7 - 5.7 % and 10-11 % , respectively.
As can be seen, the coating prepared as illustrated in Example 1, demonstrates a further improvement in voltage than hitherto found and surprisingly well below that expected from earlier teachings. The coating showed excellent coating stability, both before and after operation in electrolytic cells for the production of chlorate.
In the third test, the results as exhibited in Table 3 show the significant superiority in performance of reduced cell oxygen evolution of the embodiment A 1 SbO,. 2Ir02. 9Ti02 according to the invention in a membrane chloralkali cell.
In the fourth test the resistivity of the coating increased significantly after two hours of operation under accelerated wear testing conditions for chlorine production.

BL
Effect of molar content of AlSbO, RhSbO, Rub IrQ,~
on Anodic Voltage and Oxygen Evolution at 2 IcA/m2 and 80°C
SampleCwting AnodicCell Coating Compoaitioa, VolugeGar Stability Mole Oxygen, fncdon !w/v AlSbO,RhS604RuOiIrOiTiOs V va 2lcA/ah2.31uVeni3.OIuVm=BeforeAfter SCE Tent Tent A 0.08 0.0 0.080.080.75 1.135 1.7 1.54 1.45 Good Good B 0.08 0.0 0.0 0.170.75 1.150 5.7-3.7 Good Good C 0.0 0.08 0.0 0.170.75 1.155 10.0-11.0 Good Fair * ----- After 3 days on load of cell operation BL **

Effectof various om sitions Coating C

on AnodicVoltage and n Evolution yge at 2lcA /m2 80C in a SodiumChlorate m and Syste Mole Ratios Anodic Voltage Volts Oxygen Coating in A1Sb04 Ru02 Ti02 v/s SCE . Offgas % V/VStability 0 0.03 0.97 2.12 1.4 Good 0.02 0.03 0.95 1.98 1.2 Good 0.16 0.03 0.08 1.38 0.8 Good 4 0.10 0.90 1.22 1.5 Good 0 0.20 0.80'1.14 ~ 2.1 Good 0.04 0.20 0.76 1.14 1.9 Good 0.8 0.20 0.0 1.32 0.7 Poor 0.01 0:30 0.69 1.14 2.6 Good 0.18 0.30 0.52 1.14 1.4 Fair 0.56 0.30 0.14 1.19 .1.1 Poor 0 0.50 0.50 1.12 4.9 Fair 0.25 ~ 0.50 0.25 1.16 1.1 Fair 0.50 ~ 0.50 0.0 1.13 2.0 Poor ** As presented in USP 5017276 21 b6494 Effect of Various Coating Com sitions on Oxyeen evolution and cell voltage as determined in a membrane chlorallcali cell cvc~te.~m~
Oxygen in C 12 Cel Voltage so~ ~( R& v/v) A1Sb04 . 2Ru02 . 9Ti0i 2.7 - 3.0 3.69 RhSbo4 . 2Ru01 . 9TiOz 1.8 - 2.0 3.72 AlSbO, . 2Ir02 . 9Ti0= 1.5* 3.71 * a significant advance.
This Example illustrates the surprisingly good voltage-current efficiency performance of a coating of the general formula aA1203.bSb205.cRu4Z.eTi02 in relation to a coating of the type x(A1Z03.Sb205).cRu02 and eTi02 as shown in Table

2. Sb is in the +5 oxidation state.
The coatings were prepared as generally described for Example 1 with appropriate concentration of the species required for the desired coating formulation.
The performance of the coatings was determined using the procedures given for Example 1 and the results obtained are given in Table 4.

2i6b494 UN G~C7G7G~C7G~C~C~C~C7C7C7C7C7C7c~

~ V' ~ ~ c'~1 ~ M M
M l~ C
N

~
y i i ~

~ a O O (~j .~ .~ (vj .~ .~ .-r ...r .-r .-, .r ...~ .-~ .
' .-O

w ..

" H .~ ~
E V ' ' W ~ ~ et eh ~ 00 ~ Vj ~ ~ M M M 00 M ~1 h V1 d ... Wit ..w ~ ~ .--~ .m~ ~ rr ...~ .~ .~ .~
H ~ ..~ ...~ .r ..r i i i i i i i i i i i i i i i W a, , r ~~ > ; ~
r ~
r ~
r .-~ ..
.-r ~
~
~
r ~-N

E 'r V7 t~ N V1 et I~ M O~ O~ N ~ M
~ O M V1 ~D ~C ~ ~ N
N t~ N N N ~O t~ ~D ~O I~ t O ~

(- O O O O O O O O O O O O O O O
O

w I~ l~ O -r ~ N N M M et ~t oo CT Ov Ov O
--~ N N N N N N N N N N N N N

O O O O O O O O O O O O O O O
O

... O

O

C

~ ~'O O ~ l~ M
~~
~ N
~~
~

v N 00 1 t c 1 N c ~ ~ O O O O O ~ O ~ O ~ ~ O ~

Uw ~ 000000000000000 g V ~ ~oooo~o~ ~oooo 00000000 ~ 00000 . c ~z U W ~ ~ ~
~
~ ~

f G
-~' N M ~t V1 ~O I~ 00 O

U

~ ~ C~O~ ~
N M

N N - N N
N

Nh~0~0~

N N .~ .-~
.-i N

v O

E

~
~
~'~

oo o ~tc , -~
-~
N N

G
.
.
.
a oNOOMON~ ~~

oo ...~ ... ..~
,~ .., ..~

a '~ ~ E~ 1 00000 N

a E

~N 00 M ~ 00 O~
M N N N

N

...

O

E, 0 ~ O ~ O

'H ~ O 1 O O O
~, O

U

Uw p N O O O

O

U

O

N
. ~ ~ 1 1 1 1 N
o ~z 0 oN

UA4 at a ..~N~
axo~~.~~ ~

- 16 - ~ SL331 ~ooo~no N M N1 ~D

N N ~ .-i .-i N O~ er1 ~1 e!

y N ~ .-i ~
..i E

..
eE

~n o0 00 0 O~ C~ N h M

~1 ...1 .~ ...~
.-1 ~r _ O
c O

.

O. '~ N
M
~
O

O p c,'_, O O
p N
~
e' .a .~ .-r .., .-r ...~

.
. 0 ~ ~ h . E~ 1 0 0 0 0 N
E

O ~ 00 M d' O
M N N N

p .., ~r 00000 C

C p ~ ~ O

,~ ~ O 1 O O O

o .-. H

y.. ,U

E ~

'-' U w ~ 00 N
a 0 a 1 1 1 C C

~

.3~ U O c N ~'1 ~x ~ o ~ ~~ o U a ~G oW

The performance of the coatings as given in Table 2 confirms that coatings of the type RuOZ.Ti02, where the mole fraction of Ru01 is below 0.2 exhibits poor overall performance. It is surprising from the teachings of aforementioned U.S. Pat No. 3,849,282 that coatings of the type A1Sb04.Ru0i show poor coating stability.
It is further surprising that admixtures of A1Sb04 and Ti02 together with Ru02 produce improved performance over admixtures of either when each is taken separately. The reducing overvoltage and oxygen in off gas concentrations for AlSbO, and Ti02 admixtures to Ru02, where Ru02 mole fraction is about 0.03 is particularly surprising in the light of earlier teaching by aforementioned Kotowski and Busse. For Ru02 mole fractions of above 0.17 as illustrated in Tables 2 and 4, the improved performance for a small A1203 or Sb205 content, either taken alone or together in an aA1203 . bSbz05 . eTiO~ admixture over AlSbO, or TiOz alone is of particular note. This is more marked for greater amounts within an optimum range, for higher Ru02 mole fractions.
Table S presents oxygen data measured in the cell off gases as a function of temperature for formulations of the present invention and those for coatings of the type AB04/Ru02 and RuO~/Ti02. It is most surprising that the reduction of off gas oxygen on increasing temperature is significantly lower for coatings of the present invention over those of the ABO,/Ru02 and Ru02/Ti02 types.
Table 6 presents oxygen data measured in the cell off gases as a function of salt (NaCI) content for formulations of the present invention and those for coatings of the type ABO~/Ru02 and Ru02/Ti02. It is surprising that the increase in off gas oxygen on the reduction of salt content is significantly lower for coatings of the present invention over those of the AB04/Ru02 and Ru02/TiOz types. These results indicate that the previous, and current limitations on operating temperature and salt content in the application of electrocatalytic coating for chloride on discharge to produce chlorate would no longer apply when coatings of the present invention are utilized. ~ ' This Example illustrates the surprisingly good voltage-current efficiency performance of coatings of the general formula: aA1z03.bSb203.cRu0l.eTi02, where Sb is in the +3 oxidation state and where a, b, c and d as mole fractions as shown in Table 5. aA1203.bSbzOS.cRuOZ.eTi02, as given in Table 4, where Sb is in the +5 state.
The coatings were prepared as generally described for Example 1 with appropriate concentrations of the species required for the desired coating formulation.
However, in this case, SbCl3 is used as precursor instead of SbCls. The presence of Sb(+3) in these coatings was proven by x-ray diffraction analyses. The performance of the various coating compositions is given in Table 7.
Effect of various CoatinE Compositions using Sb(+3li on Anodic Voltage and Ox~rgen Evolution at 2kA/m2 and 80°C
Coating Composition Anodic Oxygen Mole Fraction Voltage in Off volts gas Coating AI~O~ ~ ~ YGs SCE °o ili 0.015 0.045 0.17 0.77 1.14 1.30 Good 0.03 0.07 0.20 0.70 1.13 1.70 Good 0.02 0.07 0.28 0.63 1.14 1.40 Good Surprisingly, the coatings, as shown in Table 7, exhibited remarkable competitive performance in comparison with similar coatings coated with Sb(+5) as given in Table 4. The coatings demonstrated surprisingly good adherence to the substrate, as was shown by stripping tests with adhesive tape applied by pressure, both before and after operation in electrolytic cells for the production of chlorate.
In addition, the coatings were rechecked after running for 110 days under same operating conditions as in the first test and the result shown no change in both anodic voltage and oxygen content of the exiting cell gases. These coatings confirm the beneficially synergistic effect of the classes of components, the subject of this invention.
Although this disclosure has described and illustrated certain preferred embodiments of the invention, it is to be understood that the invention is not restricted to these particular embodiments. Rather, the invention includes all embodiments which are functional or mechanical equivalents of the specific embodiments and features that have been described and illustrated.

Claims (8)

Claims

1. A metallic electrode for electrochemical processes comprising a metal support and on at least a portion of said support, a conductive coating consisting of a metal oxide component mixture of aM2O3 + bSb2Oy + cRuO2 + dIrO2 + eTiO2 wherein M is Al, Rh or Cr in the trivalent state, y is 3 or 5, and a is less than b in respect of the molar ratios of M2O3 and Sb2Oy, wherein the mole fraction of the sum of a and b in the mixed oxide mixture is in the range of 0.01 to 0.42, the mole fraction c of RuO2 is in the range of 0.00-0.42, the mole fraction of d of IrO2 is in the range of 0.00-0.42, provided that the sum of c and d is at least 0.02, and the mole fraction of c of TiO2 is in the range of 0.14 to 0.93.

2. A metallic electrode according to claim 1, wherein the metal oxide component mixture is Al2O3 + Sb2O5 + RuO2 + TiO2.

3. A metallic electrode according to claim 1, wherein the metal oxide component mixture is Al2O3 + Sb2O3 + RuO2 + TiO2.

4. A metallic electrode as claimed in any one of claims 1 to 3, wherein the mole fraction c of RuO2 is in the range of 0.002-0.42.

5. A metallic electrode as claimed in claim 1, wherein the mole fraction of d of IrO2 is in the range 0.02-0.42.

6. A metallic electrode as claimed in claim 1 wherein y is 5.

7. A metallic electrode as claimed in claim 1 wherein M is Al.

8. A metallic electrode as claimed in claim 1 wherein y is 3.