CA1048230A - Rare earth metal oxides having perovskite structure for electrodes in electrochemical reactions - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
An electrode for electrochemical reactions comprising a substrate of a film forming or barrier metal covered with a cobaltite of at least two rare earth metals, one of the rare earth metals having a high atomic number and the other having a lower atomic number. The cobaltite has the general formula:
LnXLn'(1-x)CoO3 in which Ln is a rare earth metal having an atomic number of at least 65, Ln' is a rare earth metal having an atomic number of less than 65, and X is between 0.001 and 0.999.

Description

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~ack~rr~uncl n-r rl`he I~v~n~on T~le present lnvention concern~ a new e]ectrode which can be us~d in electrolyt:Lc cells serving for the production of chlorine, caustic soda or chlorates. The cells ~erving for the production o~ chlorine or caustic soda are either diaphragm cells or mercury cells. The chlorates are produced in a cell whose structure is similar to that o~ the diaphragm cells but whlch nevertheless has no diaphragm.
The electr~des previously generally employed as anodes in electro~ytic cslls were ~requently made of graphite.
Their use has always entailed certain disadvanta~es resulting ~rom thelr wear which causes an increase in the voltage neces-sary ~or the proper operatlon of the electrolysls cell as the result of the wear which increases in the distance between anodes and cathodes and the contamination of the electrolyte.
More racently lt has been attempted to develop anodes from a metal havlng good reslstance to corrosi~n by the electro-lyte which metal is covered with an electrochemically actlve precious metal, t~e resulting composite then being sub~ected to a treatment which favors activationO These anodes are dimensionally stable and do not have the above-mentloned draw-backs. For anodes o~ this type it has been proposed to employ a core of zircon1um, zirconium-titanium alloy, tantalum or niobium covered with platinum. m ere has also been propo~ed an anode of titanium covered with platinum. Titaniums like the other core metals mentioned above, being a ~ilm forming or barrier metal capable OI Eorming a :Eilm or barrier layer of oxide in the electrolysis solutions to protect its surface ~rom c~rrosion at the place~ where the platinum is porQus.
Als~, electrodes have been produced of one of these film ~orming or barrier metals or alloys capable of ~Drming a .
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~ I.m or balI~iel~ ~ayer~ covered with an oxl.de o:E preclous metal or w:Lt}l mi.x~ure; of ox.ldes of preciou~ and non-precious metal~.
As an electrode covering or coati.ng there has also been proposed an electrolytic deposit o~ cobalt oxide, the electrocatalytic properties of whlch are very close to those o~ the precious metals, their alloys or the-lr ~ompounds. It is also known that deposits o~ salina oxide, cobalt oxide (Co304), have properties very close to th~se of the precious metals. However, none of these compounds of cobalt can be lQ used in sol~d form or as deposit in indus-trial practice a~ a -...... result of the lack o~ stability o~ their electrocatalytic properties~ As a matter of fact~ these compounds when used as anodes, rapidly become electrlcally insulating and oppose the passage of the current, thus producing a resistance which l.eads to prohibitive ovsrvoltages~
- It has recently been suggested that these drawbacks could be avoided by means of an electrode formed of a substrate or titanium or other similar ~ilm forming or barrier metal covered with a thin film o~ an electroconduct~ve coating, o~
a metal ~f the platinum group, for instance9 on which an outer layer or surface o~ perovskite is applied. This pP.rovskite is an oxygenated compound of tw~ di~ferent metals which is well known in the literature and may be represented by the empirical formula:
. Aa Bb 0 in which A represents one metal ion and B another metal ion.
A and B are related by the equation a ~ b - 6, in which a and b represent the conventional valences or ionic charges o~ the ions A and B, respectively. Among these perovskites there are compounds having the ~ormula ~nCoO~ in whlch I~ is a metal of 3(1 tlle rar~ earth metal ~`amLl~. These rare ealth me~al cobaltites have a characterLstic perovqkit~ structure whlch ls well kno~m from crystallo~raphs ancl can be recognized by a special X-ray diffraction diagram.
These cobaltites have a rela-tively high electric conductivity which varies with the temperature, the rare earth metal playing an lmportant role ln the mechanism of conduction.
The electrocatalytic power O:r these c3baltite com-pounds is not necessarily related to the perDvskite structure~
since there are numerous compounds having th-ls structure, such as, ~or instance, ~aCrO3~ lanthanum chromite, which are without it. However~ it is necessary in the case of the rare earth cobaltites to obtain the perovskite structure which alone seems to withstand corrosion in slightly acid medium. It has been noted that this corrosion is smaller the more acid the character of the rare earth used. The compound LaCoO3~ lanthanum cobal~
tite, for instance, although having remarkable electrocatalytic propèrtie3, is entirely unsuitable to cons~itute an anode *or an electrolysis cell as a result of the ease with which it pa~ses into solution in sllghtly acid chlorinated medium. This defect decreases when the lanthanum is replaced by a rare earth o~ higher atomic n~unber. One succeeds in this way in consider~
- ably improving the resistance to chemical and electrochemical corrosion by using rare earths of higher and higher atomic number~ as will be shown belo~:
The compounds LaCoO~ PrCoO3, NdCoO3 and GdCoO~ are prepared from an intimate mixture of the oxides of the stated elements, which is calcined at 1200 C. for 15 hours. The series of compounds thus prepared is ana~yzed by X-ray di~frac-~0 tion and is ~ound in each case to be solely of the perovskite , . . . . ~i:

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~tructlIre, ~ e chemlcal resLstlvl~y Jn acld medium of ~h~e mixcd oxides 15 then mcasured a~ fo]lows:
To 1 gram of -the po~dcr of~ each compound there are added 200 ml. of O.lN hydrochlorlc acid, The attack is allo~ed to continue for 1 hour in the cold. A~ter filtration, the cobalt and the rare earth present in ~he ~iltra~e are deter mined. 'rhe follo~ing table sets ~orth the corro~lon of the compounds in terms of percentage~ that is to sa~ the ratio o~
the total mass o~ the meta:l elements present ln the so]utlon to the total mass o~ the metal ele~ents present in 1 gram of cobaltite.
CDrrDs lDn LaCoO3 35%
PrCoO3 9.6%
NdCoO~ 5.7%
GdCoO3 4.3%
m0 cobaltites were thereupon deposited on a substrate -such as 7 ~or instance~ a graphite plate~ the powd~r and the substrate being -then subjected to very high pressure to form an electrode in accordance with methods well known to the man skilled in the art. The electrode was -then u~ed as anode in a 300 grams per liter aqueous solution of sodium chloride main^-tained at 80 C, and a pH of 4r The current densi~y was 25 amperes per square decimeter, The following table summarizes the results~
of the lives obtained:
LaCoO~ PrCoO3 NdCoO3 GdC03 Time about 1 hr. about 30 hr, about 400 hr, ~bout 500 hr, mere i 5 thus noted the good correlation between the electroly-sis life and the corrosion in acid medium, However~ one is limited in the use of the heavy rare earth metals as Plectrodes in electrolysis cell~ by the tendency ~4~'~30 which tll~se r~re ec~rths have to give in whole or in part a mix~d solid phase, Co (TR)~2 )3~ which is more or less rich in cobalt and known by crys~llo-graphs ~der the phase designation C-T1203. ~ range of existcnce of the dif-ferent crystalline phases has been established and is described, for instance, on page 10 of the book by F.S. Galasso, "Structure Properties of Perovskite-Type Compo~lds," Pergamon Press, 1969. ~lis limi~ation is very disturbing, since the cobalt oxide phase, rare earth n~tal oxide of the structure C-T1203 being readily soluble in acids, is unsuitable for the desired use in electro-lysis. This particular behavior of ~.he rare eart}ls of high atomic nwnber is explained by crystallographic considerations utilizing ionic rays.
It is, accordingly, an object of the present invention ~o provide electrodes for electrochemical reactlons which do not have the shortcomings of the prior art.
It is also an object of the present invention to provide an electrode for an electrolytic cell which employs a cobaltite of perovskite structure, which electrode has improved properties.
It is a further object of the present invention to provide electrodes for electrolytic cells, which electrodes have excellent resistance to corrosion.
According to the invention, there is provid~d an electrode for :~
electrochemical reactions, comprising a substrate covered with a compound having a perovskite structure, characterized by the fact that the substrate is of a film forming metal and the compound of perovskite structure is a cobaltite of rare earths having the general formula:
LnxLn~ x)co 3 in which x is between 0.001 and 0.999, Ln is a rare earth metal having an atomic number of at least 65 and Ln' is a rare earth metal,other than pro-methium, having an atomic number below 65.
Thus, the new rare earth or rare earth metal cobaltite compounds employed in the electrodes comprise at least two rare earth metals, one or more of these rare earth metals having a high atomic number of at least 65 ,~

iV~3~3 atld llOt resulting ill a com~ound of perovskite structure wllen combined alone with the cobalt. Another of the rare earth metals has an atomic number below 65. Illis ne~i rare earth cobaltite compound has a special X-ray diffraction pattern ~nd a characteristic perovskite structure. This diffraction pattern and structure are fully described in the literature. For ins~ance, in G~apter 5 of the boo~, "Diffraction Procedures," by Klug and Alexander, John Wiley and Sons ~1954), see pages 235 to 318.
As mentionedJ the new electrodes in accordance with ~he invention comprise a substrate of a film forming or barrier metal covered with a co-baltite compound described above which forms ~he surface of the electrode.
This compound has the general formula LnxLn ~l-x)coo3 in which Ln represents a rare earth metal of high atomic number, at least 65, Ln' a rare earth metal of lower atomic number, ie below 65, and x is a number between 0.001 and 0.999, and preferably between about 0.05 and 0.3.
The new cobaltite compounds in accordance with the invention have a substantially higher resistance to acid corrosion than the known rare earth metal cobaltites, while having the same characteristics of conductivity and the same electrocatalytic properties.

The rare earth metals which can be used are those listed in the Periodic Table of the ~lementsO Those of high atomic number comprise terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium. The rare earths of lcwer atomic number comprise lanthanum/ cerium, praseodymium, neodymium, samarium, europium and gadolinium.

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T~c su~ Lnte~ or cole, o:~ the electrode is ad vnnta~usly :~ormed of :L`ilm :r~rmin~rl or barrier metal, that ls to say~ o~ metal ~orm:l.ng a passlvat:Lng la~er o~ ~xide ~rhich permits the passage o~ current only in the dlrection towards the cathode. These ~ilm forming metals are well kn~n and include, ~or example~ t:Ltanium~ tantalumg tungsten~ ha~nium, zirconium~ aluminum, niobium and the:ir allnysO Graph-Lte can also be used and is intended to be included in ~he te~ "film forming metal" as used herein. The substrates may be solid pieces or thin~ non-perforated plates. They may also be of perforated plates or metal gauzeD Their shape is desirably that customaril~ employed for the anodes of electro~ysis cells.
It has been found that the value of the ionic radii o~ the component rare earth metals of the cobalti~e compound is imp~rtant, and that it is not possible to combine merely any rare earth metals in any proportion~ Thus if one uses a rare earth having an ionic radius as small as that o~ erbium, it is necessary to introduce a rather large proportion of a rare e~rth metal having a rather high ionic radius such as that ~f neod~nium.
0~ course~ the rare earth cobaltite need not be limited to two rare earths, but ma~ comprise three rare earths or even more, the essential factor being the retention of the perovskite structure from one or more rare earth metals leading to this structure with one or more rare earths not leading to it.
The~e new compounds may be prepared like all the other cobaltites or perovskite structure by processes well kno~nn to the man skilled in the art. That is to say, thermolyzable organic or inorganic salts, oxides or hydroxides o~ the dif-~erent elements are mixed, coprecipitated and cocrystallized.

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Then af ter t;l~e dry:L21~ and cru~hln~ opera-t;ions, I;he powder ob-tai.ned, ~Ihether or not compacted~ is calc~.ned at a temperature between abou~ ~300 and about 1500 C. :~or a per-lod of t-lme which may vary from 2 hours to 7~ hours. In generalg the perovskite compounds ~Ihich can be used for the electrodes of the invention may be prepared by any of th~ processes descrih~d in the ~.itera-ture. For example, by the process described in the ~ournal "American Mineralogist~" Vol. ~9 (1), 1954.
Speci~ic Disclosure n~ The InventLon . ~
lo In order -to disclose more clearly the nature o~ the present invention, the ~ollowing examples illustrating the inventlon are given~ It should be unders~ood, ho~rever, that this is done solely by way o~ example and is intended neither t~ delineate the scope o~ the inventlon nor limit the ambit of the appended claims. In the examples which follow~ and through-out the specification, the quantities of material are expressed in terms o~ parts by weight, unless otherwise specified ~ompounds are prepared of the general ~ormula Gd~l x)TbxCoO~, in which x is the quantity of Gd ions in the gadolinium cobaltite which are replaced b~ terbium ions.
. These compounds are prepared ~rom an intimate mixture - of gadolinium, terbium and cobælt oxides the quantities which, as a ~unçtion o~ x, are summarized in Table 1, below:
. . Table 1 x . Gd 0 Tb~07 Cobalt oxide content (g2a~s~ (grams) 71~ (grams) 0 18.15 0 8.28 0.05 17 ~5 0 934 8.28 0.1 - 16.~ 1.87 ~.28 ~0 0~2 14.50 3 74 8~28

2~ 9 x Gd~0 Tb407 Coba:Lt ~x:Lde conten~
(~,;ra~ls) (~r~ms) ~ rams) 0.3 12.69 5.60 ~.28 0.5 9.06 9.3~ 8,28 1 0 13.60 ~.28 The mlxtures o~ oxides are compress~d under a pressure o~ 10 t~ns into the form ~ pellets and then calcined at 1200 C. ~r 15 hours. The calcined pellets are then crushed int~ ~ine ~orm, The resul~ing series of compounds thus prepared is analyzed by X-ray dif~raction ~or identi~ication o~ the phases.
Table 2~ below~ summarizes the results obtalned:
Table 2 x = 0 -~ perovskite structure x = 0.05 perovskite structu~e x = 0.1 -- perovskite s~ructure ~- x = 0.2 -- perovskite structure ~ very li~tle C-Tl~0~ structure ~ x = 0.3 -- perovskite structure ~ abundant - C-T1203 structure x = 0.5 -- per~vskite structure ~ very - abundant C-T12~ structure x ~ very 1~ ttle perovskite structure ~ C-T1203 structure The chemical resis~ivity in acid medium Or ~hese ~ixed oxides is then measured as described aboveO
Table 3, below, summari~es the results ~btained~
Table 3 Gd(l-x)Tbxc~o~

x Corrosion , ~
0 ~.2 0~05 Q.l 3.5 0,2 ~ 6.~%
~0 0.3 8.9~ .
0.5 . 6.~%

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It :L~ thus noted th~.t thc compounds of~ thc general formula Gd(~ xCoO~ have minimum corrosion for the highest possible quantity of terbium, which leads to the only true perovsklte structure, that is to s~y, for x = 0.1. An electrode compo3ed o~ a deposit o~ gadolinium and terbium ~obaltite on a titanium support is prepared in the follow:lng ~ er:
Gdo ~Tbo lCoO~ ls prepared by crushing together 16.3 grams o~
o~ Gd203~ 1~87 grams of Tb407 and 8~28 grams of cobalt oxide contalning 71% cobalt, The powder obtained is placed in an alumlna cruclble and then calcined at 1200 C. ~or 15 hours in an atmosphere o~ air~ The product is allowed to cool in the .
furnace and is then crushed until the size o~ the grains ls less than 10 mlcrons. The black powder thus obtained has a characteristic X-ray di~*raction pattern o~ the perovskite ~`
structura of the cobaltites.
The cobaltite thus prepared is then depositad on a `~
titanium plate of 10 mm. width by 30 mm. length and 1 mm. thlck-ness which has been previously cleanad by sanding, washed with distilled water and dried.
A suspension o~ the cobaltite is prepared in the `
~allowing manner: To 1 gram of powder there is added 1 gram of hydratad cobalt nitrate hexahydrate, 1 ml. of water and 1 ml~
of isopropyl alcohol. The paste obtained is agitated vigorously until homogeneous suspension is obtalned, the agitation being maintained during the production of the deposit~ A layer of the suspeneion of the cobaltite is applied on the surface of the titanium plate by brush. After drying ~or 5 min. in an O
oven at 100 C., the resulting electrode is kept ~or 10 min, in a furnace at a temperature of 400 C. while lt is swept by alr.
This operation ls repeated 20 times. The amount of product ~ "

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deposited is ll~ mg./cm2. 'l~le depo31t on the electrode con-slsts of ~0~ cobaltlte and 20~ cobalt ox:ide~
The electrode thus prepared is placed in an electro-lysis cell f~r the manufacture of chlorine and caustic soda, in ~Jh~ch the electrolyte is a so1.ution o~ 300 grams per liter of sodium chloride maintained at 80 C. and a pH o~ 4. A
current such as to produce an anodic current density o~
25 amperes per square d~clmeter is then passed ~Lnto the cell;
the anodic oxidation vDltage of the chloride lons ls llO0 milll-volts when re~erred to a saturated calomel electrode. A~t~r lO00 hours o~ electrolysis, the anode potential remalns un-changed.
:E: ample ? ~ :
In accordance with the procedure of Example l, com-pounds are prepared of the general formula Gd(l x)DyxCoO3 from gadolinium, dysprosium and cobalt oxides, the quantities of which, as a ~unction of x, are summarized in Table 4, below:
- ~ Table 4 x Gd203 Dy20~ Cobalt oxide content _ ~ (grams) _ _ _(gra~s) _ 71% (grams) 0 18.2 0 8c7 0.05 17.2 009~ 8.3 O.l . 16,3 1 86 8.3 0.2 1~.5 3.7~ 8.
3 12 68 ~.6 8.3 0.5 g.o~ 9.~2 8.~
l 0 ~8065 ~.3 An analysis o~ the powders by X-ray dif~raction con-~irms the results obtained in ~xample 1 and shows an evolution for x increasing from 0 to l of the perovskite structure to-wards the structure C-Tl203.

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The chernlcal rc~,~st:Lvlty o~ thesc mlxed oxldes in o.lN hydrochlDric acid ~edlum is then rneasuredg the results bein~ summarized 1n Table 5~ belo~:
I'able 5 _ . ~
Gd(l_x)DYxcoo~

x Corrosion 0 4~2 0,05 2~7%
0.1 2.1%
0. 2 1~ 9%
.3 5~3 0.5 7~3%
1 6.1%
It is thus found that the compounds of general ~ormula Gd(l x)Dy CoO~ have a minimum corrosion for the highest pos~ible quantity of dysprosium, ~/hich confers upon the product the onl~
completely perovskite structure3 that i~ to say9 for x equal ~ ~
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0. 1. ~ ' An electrode is prepared with a surface of Gd~ gDyO lCoO3 on a titanium plate in acc~rdance with the pro-cedure ~ Example 1. This electrode i~ used as electrolysis anode for the manufacture of chlorine. For a brine o~ 300 grams :
per liter at 80 C. and a pH of 4, there is obtained a strong liberation of chlorine with a current density of 25 amperes per ~quare decimeter, under a voltage of 100 millivolts when referred to a saturated calomel electrode, After a prol.ong0d period of electrol~sis, the anode potential remains unchanged.
Example 3 .
In accordance with the procedure of Example 1, com~
pounds of the general ~ormula Nd~l_x)TbxCoO3 are prepared from - : :

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n~od~:lum~ ~rbium and c~balt ox-ld~s th~ ~uantlt:les of ~Ihlch~
a~ a f~mc~;ion o:~ x~ ~re summarized ln Table 6~ below:
Table 6 x Nc120~ Tb407 Cobal.t ox:ide content (grams) ~grams~ 71~ (grams) 0 16.8 o ~ 3 0.05 15.97 0,93 8~3 0.1 15~13 1.~7 ~.3 0.2 ~3.45 ~-74 8.3 3 11 77 5~61 ~.3 1 0 18.6 8~3 An analysis o~ the powders b~ X-ray dif~raction con-~irms the results ob~ained in Examples 1 and 2 and sh3ws an evolution ~or x increaslng from 0 to 1 o~ the perovskite struc-~ ture towards the C-T1203 structure.
; 15 I~e chemical resistivity o~ these mixed oxides i5 then measured in an O.lN h~drochloric acid medium, the results ~f which are summarized in Table 7, below:
Table 7 Nd ( l _X ~ Tb xc - x C~rrosion . . 0 5.7%
0.05 6 0.1 &.1%
0.2 4-5% . :
2~ 0.~ - 4.8%
It is thus found that the compounds of general formula Nd(l X)Tb CoO3 present minimum corrosion for the highest:
possible amount of terbium, which confers upon the product the only completely perovskite structure, that is t~ say, ~or x equal 0,2.

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An electro~le :l,s prepared hav:Ln~ ~ surrace o~
Ndo 8T~o ~CoO~ on a p].at:e of ~;ltanlum by means ~f an organic or inorgani.c binder .Ln accordance wlth a pr~cedure substantl~lly the same as that of Example 1. This electrode ls used as electrolysis anode for the manufacture o~ chlorine. For a brine of 300 gr~ms per llter at 80 C~ and a pH o~ 4, there ~s obtained a strong liberation of chlorine at a current density of 25 amperes per square decimeter under a voltage o~ 1100 millivolts against a saturated calomel electrode. After a prolonged time of electrolysis~ the anode potential remains unchanged~ :
As will be apparent to those skilled in the art from the foregoing disclosure, cobaltites of other rare earth metals ma~ be employed in the ~ore~oing examples.
The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding an~ equivalents of the features shown and descrlbed - or portions thereof, but it is recognized that various modifi-20 . c-tions sr- possible withln the scope of the invention claimed.
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Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An electrode for electrochemical reactions, comprising a substrate covered with a compound having a perovskite structure, characterized by the fact that the substrate is of a film forming metal and the compound of perov-skite structure is a cobaltite of rare earths having the general formula:
LnxLn'(1-x)CoO3 in which x is between 0.001 and 0.999, Ln is a rare earth metal having an atomic number of at least 65 and Ln' is a rare earth metal, other than promethium, having an atomic number below 65.

2. An electrode according to Claim 1, in which x is between about 0.05 and 0.3.

3. An electrode according to Claim 1 or 2, in which Ln comprises a member selected from terbium, dysprosium, holmium, erbium, thulium, ytterbium and lutetium.

4. An electrode according to Claim 1 or 2, in which Ln' is a member selected from lanthanum, cerium, praseodymium, neodymium, samarium, europium and gadolinium.

5. An electrode according to Claim 1 or 2, in which the film forming metal substrate is a member selected from the class consisting of titanium, tantalum, tungsten, hafnium, zirconium, aluminum, niobium and their alloys.

6. A compound of the general formula:
LnxLn'(1-x)CoO3 in which Ln is a rare earth metal having an atomic number of at least 65, Ln' is a rare earth metal having an atomic number of less than 65, and x is between 0.001 and 0.999.

7. A compound of the general formula:
LnxLn'(1-x)CoO3 in which Ln is a rare earth metal having an atomic number of at least 65, Ln' is a rare earth metal having an atomic number of less than 65, and x is between 0.05 and 0.3.