PATENT SPECIFICATION
`4.. Mn A-...|:,...-:,._ mug <n91 c ['71
`roux tram: “)0 ha» 1071
`
`(11) 1346 369
`A
`
`PATENTS ACT 1949
`
`SPECIFICATION NO 1346369
`
`The following corrections were allowed under Section 76 on 30 July 1974.
`
`Page 1.
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`line 20, after processes insert
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`,
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`Page 1,
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`line 54,
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`for irridium read iridium
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`Page 1,
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`line 78, after series insert,
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`Page 4,
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`line 48,
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`for zinc chromium read zinc-chromium
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`Page 5,
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`line 70, after resorted insert to
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`Page 6,
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`line 43, after solution delete ammonium carbonate
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`Page 8,
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`line 1, after per insert square
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`Page 8,
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`line 116, for as read are
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`Page 9,
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`line 43,
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`for found read bound
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`Page 9,
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`line 92, after analogous delete,
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`Page 11,
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`line 56,
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`for aluminate read aluminum
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`Page 12,
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`line 75,
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`for feet read foot
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`Page 13,
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`line 27, after surface insert
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`,
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`Attention is also directed to the following printers errors:-
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`Page 2,
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`line 55, after a insert metal having a
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`Page 3,
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`line 53, after iron insert spinel
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`Page 3,
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`line 76, after vanadium— insert magnesium
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`Page 4,
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`line 111, for metal
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`insert metals,
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`Page 5,
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`line 30,
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`for ben read been
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`Page 5,
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`line 31, after to insert for
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`Page 5,
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`line 33
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`for temperature read temperatures
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`Page
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`6,
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`Page 9,
`Page 9,
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`9.
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`Page
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`line 23, after oxalate insert solution
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`line 79,
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`for
`
`(BeRuoé) read (BeRuDB)
`
`line 79 fbr ruthenate read ruthenite
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`line 88,
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`for (MgRuO3) read (MgRuoé)
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`Page 10,
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`line 121,
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`for such read Such
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`Page 13,
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`line 1,
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`for whever read wherever
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`Page 13,
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`line 7,»f0r of read to
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`Page 13,
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`line 102,
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`for comprising read comprises
`
`THE PATENT OFFICE
`23 August 1974
`
`,
`
`R 77160/3
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`

`

`PATENT SPECIFICATION
`
`(11)
`
`1346 369
`
`1346369
`
`(22) Filed 29 Oct. 1971
`(21) Application No. 5 O3 15/71
`(32) Filed 2 Nov. 1970‘
`(31) Convention Application No. 86062
`(32) Filed 8 March 1971 in
`(31) Convention Application No. 121683
`(33) United States of America (US)
`(44) Complete Specification published 6 Feb. 1974
`(5 1) International Classification BOIK 3 /06//C22]? 13/00
`(52) Index at acceptance
`C7B AlC AZCZ AZD A9
`BZE 191 19X 19Y 203 ZOY 348 378 41X 41Y 498 568 62X
`62Y 638 679 708 768 778
`
`C7B 13
`
`
`
`(54) ELECTRODES
`
`We, PPG INDUSTRIES, INC., a
`(71)
`Corporation organised and existing under the
`laws of the Commonwealth of Pennsylvania,
`United States of America, of One Gateway
`Center, Pittsburgh, Commonwealth of Penn-
`sylvania, 15222, United States of America,
`(assignee
`of ALEKSANDRS MARTIN-
`SONS), do hereby declare the invention, for
`which we pray that a patent may be granted
`to us, and the method by which it is to be
`performed, to be particularly described in and
`by the following statement:—
`The present invention relates to electrodes
`suitable for use for electrolytic purposes, for
`example for the electrolysis of aqueous alkali
`metal chloride solutions.
`Chlorine and alkali metal hydroxides, such
`as lithium hydroxide, sodium hydroxide, and
`potassium hydroxide, are produced commer—
`cially by either of two electrolytic processes
`electrolysis in a dipahragm cell or electrolysis
`in a mercury cell. Alkali metal chlorates are
`made in a cell similar in structure to a dia-
`phragm cell but not having a diaphragm.
`The design and operation of mercury cells,
`diaphragm cells, and chlorate cells, are more
`fully treated in Mantel], Electrochemical
`Engineering, McGraw-Hill, New York, NY.
`(1960), and Sconce, Chlorine, Reinhold, New
`York, N.Y. (1962).
`Common to all three processes has been the
`use of carbon anodes. These carbon anodes
`are a constant source of difficulty. They are
`short lived and subject to uneven wear and
`erosion. In mercury cells, frequent adjustment
`is required in order to maintain a constant gap
`between the cathode and anode and thereby
`maintain a constant voltage drop across the
`electrolyte. In diaphragm cells and chlorate
`cells, no provision is made for varying the
`anode—cathode space, and, accordingly,
`the
`voltage
`increases with time. Additionally,
`organic solvents present in the graphite elec—
`trodes may plug the diaphragm, resulting in a
`further increase in voltage. Reaction of the
`anode products with the graphite anode result
`
`in halogenated hydrocarbons being present in
`the anode product.
`Many attempts have been made to remedy
`these problems. It has long been recognized
`that a superior anode would be one made of
`a solid precious metal, that is, a metal chosen
`from the
`group
`consisting
`of platinum,
`irridium, osmium,
`ruthenium,
`rhodium and
`palladium. However,
`this is neither econo-
`mical nor practical. The art
`shows many
`attempts to» obtain a lower cost electrode hav—
`ing the long life and low overvoltage of a
`solid precious metal electrode, as well as the
`low cost of a graphite electrode. These
`attempts have sought to provide a durable
`coating, usually of a platinum group metal
`or platinum group metal oxide, on an elec—
`tro—conductive base, which base usually is
`composed of a valve metal such as titanium.
`These electrodes are noticeably longer in
`life and operate at lower voltages than carbon
`electrodes. However, platinum group metals
`either as such or as their oxides are expensive.
`It has now been found that a particularly
`satisfactory anode for the electrolysis of brines
`may be provided by an electrode having an
`outer electroconductive surface comprising 2
`spinel
`on
`an
`electroconductive
`substrate
`wherein an intermediate layer comprising an
`oxycompound of a second transition series
`platinum group metal chosen from oxides of
`the second transition series platinum group
`metals, alkaline earth metal (as herein defined)
`oxides of the second transition series platinum
`group metals, and rare earth metal (as herein
`defined) oxides of the second transition series
`platinum group metals is placed between the
`substrate and outer surface. The platinum
`group metals of the second transition series
`consist of ruthenium, rhodium, and palladium.
`The oxycompound of the second transition
`series platinum group metals may be present
`in the substrate and, additionally, may also
`be dispersed through the spinel coating. When
`' the oxycompound is also dispersed through the
`spinel coating, it may be dispersed uniformly
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`[Price 2513]
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`SEE CORRECTIGE‘i
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`SUP ATTACHED
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`1,346,369
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`or non-uniformly. The resulting anode appears
`to ofier economies of construction and opera-
`tion over an anode having a metallic platinum
`coating on an electroconductive substrate.
`As used herein, “ spinel ” means an oxy—
`compound of at
`least 2 types of ion of a
`metal or metals which oxycompound has the
`unique crystal structure and formula charac-
`teristic of spinels. The spinel may be applied
`with a suitable binding agent over a suitably-
`treated metal structural member or substrate.
`Such spinel itself comprises mainly the ions
`of the metal or metals and oxygen in chemical
`combination. However, amounts, usually minor
`amounts, of other materials such as other
`metal oxides, sulfides, fluorides, or even metals
`in the metallic state, may be entrapped in
`or associated with the spinel crystal structure
`without seriously impairing the desirable pro-
`perties of the spinel surface.
`A suitable electroconductive substrate is one
`that
`retains its electroconductive properties
`during the formation of
`the spinel surface
`thereon and in the course of using the result-
`ing electrode for its intended purpose. Such
`a substrate will be resistant to oxidation dur-
`ing fabrication and electrolysis, and will not
`be subject to attack by the gases liberated
`during electrolysis. Preferably,
`the substrate
`should be more electroconductive than the
`spinel itself.
`The resulting electrode is long lived in the
`electrolytic cell environment and has satis-
`factory overvoltage characteristics.
`In the accompanying drawings the sole
`Figure is an X—ray diffraction pattern of an
`electrode produced according to the procedure
`of Example I, below.
`Suitable electroconductive substrates hav-
`ing a spinel outer coating and an inter-
`mediate layer of an oxycompound of a second
`transition series platinum group metal
`(i.e.,
`ruthenium, rhodium, palladium) therebetween
`produce an anode that is dimensionally stable
`in an environment where chlorine is electro-
`lytically produced. Spinels are oxycompounds
`of the two or more types of ion of one or
`more metals characterized by a unique crystal
`structure, stoichiometric relationship, and X-
`ray diffraction pattern.
`Oxycompounds having the spinel structure
`may be represented by the empirical formula
`
`MIIMEIII()41
`
`represents a metal having a
`wherein MII
`valency of plus 2, and MIII
`represents a
`valency of plus 3. The metals may be the
`same metal as in CrHCrLFHO, or different
`metals as in NiCr204. Spinels are more pre—
`cisely represented by the empirical formula
`
`MII<MIIIILMII[1304'
`
`where MII represents a metal having a valency
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`of plus 2, and Mum and MIIIb represent
`metals having a valency of plus 3. Mum and
`Mm" may either be the same metal or
`different metals, and one or both of them
`may represent the trivalent state of the metal
`present in the bivalent state, as
`
`Fen(CumFem)O,,
`
`ions may be ions of
`three metal
`or all
`different metals, as MgAlFeO4.
`and the
`The
`spinel
`crystal
`structure
`methods of identifying it by X-ray techniques
`are described in the literature. For example,
`the spinel structure is discussed in Wyckoff,
`Crystal Structure, Vol.
`3
`(2nd Edition),
`Wiley & Sons, New York (1963) at pages
`75 to 86;
`in Wells, Structural
`Inorganic
`Chemistry, Oxford University Press, New
`York (1950) at pages 379 to 388 ;
`in Evans,
`An Introduction to Crystal Chemistry (2nd
`Edition), Cambridge University Press, New
`York (1966) at pages 171 to 175; and in
`Bragg, Claring—Bull and Taylor, The Crystal-
`line State, Vol.
`4, Crystal Structures of
`Minerals, G. Bell & Sons Ltd., London (1965)
`at pages 102 to 106.
`the spinel
`According to those authorities,
`crystal
`structure may be characterized as
`comprising oxygen ions in an approximately
`cubic
`close-packed relations-hip, with the
`metal
`ions lying in holes of
`the packing.
`Crystal structures having close-packed atoms
`or ions may be conveniently considered, for
`purposes of illustration, as being arranged in
`layers. In the spinel lattice having metal ions
`and layers of close-packed oxygen ions,
`the
`metal ions are smaller than the oxygen ions
`and are found between the layers of oxygen
`ions. Therrelationship within the lattice may
`be shown by imagining that the layers of
`oxygen ions are taken apart,
`leaving asso-
`ciated with each layer of oxygen ions the
`metal
`ions immediately in contact with the
`upper surface of the layer of oxygen ions.
`In this way the spinel
`structure may be
`regarded as built up of two kinds of alter-
`nate layers which layers are superposed one
`on top of the other in an alternating manner.
`The spinel structure may be further charac-
`terized in that one-third of the metal
`ions
`have 4 oxygen neighbors which oxygen
`neighbors are arranged tetrahedrally to the
`metal ion, and that two-thirds of the metal
`ions have 6 oxygen neighbors which oxygen
`neighbors are arranged octahedrally to the
`metal ion.
`close—packed
`the layers of
`In each of
`oxygen ions are diagonal chains of metal
`ions having octahedral geometry. The octa-
`hedral metal
`ions are linked laterally above
`and below the layer of oxygen ions by the
`metal ions having tetrahedral geometry. The
`direction of the chains in any layer is normal
`to the direction of the chains in the adja—
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`1,346,369
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`cent layer. Four layers make up a unit cell.
`The unit cell referred to above is an arbitrary
`parallelopiped which is the smallest repetitive
`unit identifiable as the crystal. The unit cell
`generally, as a matter of convenience, con-
`forms to the symmetry of the system to which
`the crystal belongs. The unit cell is defined
`by the lengths of its edges, and the angles
`included between them. The edges of
`the
`unit cell are termed unit translations in the
`pattern. Starting from any point of origin in
`the lattice and going a distance equal to and
`parallel to: any cell edge, or by any combina—
`tion of such movements, we arrive at a point
`where the whole surrounding structure has
`the same form and orientation as at the point
`of origin. Because of the arbitrary nature of
`the definition of the unit cell, any one ion
`may be entirely within one cell or it may,
`alternatively be divided between two, four, or
`eight unit cells. Additionally,
`the neighbor
`of any one ion may be in the same unit cell
`or in an adjacent unit cell.
`The spinel unit cell contains eight “ formula
`weights”,
`i.e., eight MIIM211104 units. More
`particularly,
`the crystallographic unit cell of
`the spinel structure contains 32 oxygen ions.
`There are equivalent positions in this cell for
`8 metal
`ions surrounded tetrahedrally by 4
`oxygen ions, and for 16 metal
`ions sur-
`rounded octahedrally by 6 oxygen ions.
`Spinels may further be characterized on
`the basis of which metal ions occupy which
`positions
`in the crystal
`structure. Those
`spinels wherein the positions of tetrahedral
`coordination are occupied by the divalent
`metallic ions and the positions of octahedral
`coordination are occupied by the trivalent
`metal
`ions are “regular ”
`spinels. Those
`spinels reported in the literature as being
`“regular” spinels, and their stoichiometric
`formulae:
`include:
`the magnesium-vanadium
`spinel
`(MgV204);
`the zinc-vanadium spinel
`(va,o,); the magnesium-chromium spinel
`(MgCr204);
`the manganese-chromium spinel
`(MnCr204) ;
`the
`iron-chromium spinel
`(FeCr204);
`the
`cobalt—chromium spinel
`(CoCr204);
`the
`nickel-chromium spinel
`(NiCr204;)
`the
`copper-chromium spinel
`(CuCr20,)3
`the
`zinc
`chromium spinel
`(ZnCrgoi);
`the
`zinc—manganese
`spinel
`(ZnMnZOQ;
`the zinc-iron spinel (ZnFezor);
`the cadmium—iron (CdFe204);
`the copper—
`cobalt spinel (CuC0204); the zinc-cobalt spinel
`(ZnCogO4) 3
`the magnesium-rhodium spinel
`(Mth204) ;
`the
`zinc—rhodium
`spinel
`(ZnRh204);
`the magnesium-aluminum spinel
`(MgA1204);
`the manganese—aluminum spinel
`(MnA1204);
`the
`iron-aluminum spinel
`(FeA1204) ;
`the
`cobalt-aluminum spinel
`(C0A1204);
`the
`zinc-aluminum
`spinel
`(ZnA1204) ,
`the
`nickel-aluminum spinel
`(NiA1204); and the calcium-gallium spinel
`(CaGa204).
`Other spinels, wherein the tetrahedral posi—
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`tions are occupied by one-half of the trivalent
`metal
`ions and wherein the remaining one—
`half of the trivalent metal
`ions along with
`all of the divalent metal
`ions are in octa—
`hedral
`positions,
`are
`characterized
`as
`“inverse ” spinels. In “inverse” spinels the
`arrangement of octahedral divalent and octa—
`hedral
`trivalent positions
`is
`substantially
`random. Such spinels, and their
`stoichio—
`metric formulae,
`include:
`the titanium-mag-
`nesium spinel
`(Tng204);
`the vanadium—
`spinel (VMg204), the magnesium—iron spinel
`(MgFe204);
`the
`titanium—iron
`spinel
`(TiFe2.O,); the cobalt-iron spinel (CoFegO4);
`the nickel~iron spinel (NiFeZOQ; the copper—
`iron
`spinel
`(CuFeEOQ;
`the
`titanium-
`zinc
`spinel
`(TiZn204);
`the
`tin—cobalt
`spinel
`(SnC0204);
`the
`tin-zinc
`spinel
`($1an04);
`the magnesium—gallium spinel
`(MgGaZOQ;
`the magnesium-indium spinel
`(MgIn204);
`the manganese—indium spinel
`(MnIn204); the iron-indium spinel (Feln204);
`the coba1t~indium spinel (CoInEO4); and the
`nickel-indium spinel (NiIn204).
`In still other spinels,
`the distribution of
`metal ions is less regular, the spinels exhibit-
`ing both normal and inverse arrangement, as
`is discussed, for example in Bragg, Claring—
`bull, and Taylor, The Crystalline State, Vol.
`4; Crystal Structure of Minerals, G. Bell &
`Sons. Ltd, London (1965) at pages 105 and
`106.
`The spinel crystallographic unit gives a
`unique X-ray diffraction pattern correspond-
`ing to the spacings between the crystallo-
`graphic planes. Typically, the observation of
`this X—ray diffraction pattern involves sub-
`jecting spinel
`samples
`to X-rays from a
`copper target. Methods of accomplishing this
`are more particularly described in chapter 5
`of King and Alexander, X—ray Diffraction
`Procedures, John Wiley and Sons, Inc., New
`York (1954), at pages 235 to 318, and especi—
`ally at pages 270 to 318, and in Ncwfield,
`X-ray Diffraction Methods, john Wiley and
`Sons, Inc., New York, N.Y., (1966), at pages
`177 to 207. As described therein,
`these X-
`rays have a wave length of 1.5405 angstrom
`units. The X-rays diffracted by the sample
`are particularly intense at certain angles, 6,
`resulting in peaks on diffractometer print-
`outs as in the Figure or in lines on photo—
`graphic diffraction patterns. This high inten-
`sity is caused by the X»rays “ reflected ” from
`parallel planes in the crystal reinforcing each
`other. The wave length of the X—rays,
`the
`spacing of the planes in the crystal, and the
`angle, «6’, are related by Bragg’s Law. Bragg’s
`Law is
`
`2d sin 19:11)»
`
`where d is the distance between the planes
`of the crystal,
`:1 is an integer, A is the wave
`length of the X—rays, and 9 is the angle of
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`4|
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`incidence of the X—rays, and also the angle
`of reflection of the X—rays.
`Typically,
`the X-ray diffraction data is
`obtained from a diflractometer that is direct
`reading in 20, wherein 180 degrees —26 is
`the angle between the incident ray and the
`reflected ray. One way of recording X-ray
`diffraction data is in the form of a graph
`of the intensity of the reflected ray versus
`26. X—ray diffraction data recorded in this
`way is shown in the Figure.
`The Figure is a graph of the intensity of
`the reflective ray versus two theta for an
`electrode prepared according to Example I
`and consisting of a titanium member having
`thereon an outer surface of a cobalt—aluminum
`bimetal spinel and silicon dioxide and an inter—
`mediate layer of palladium oxide between the
`cobalt-aluminum spinel, silicon dioxide outer
`surface and the titanium member. The peak
`at 33.92 degrees two theta is the charac-
`teristic PdO peak reported in the literature.
`Also to be noted are the characteristic cobalt—
`aluminum spinel peaks at (in numerical order)
`31.28 degrees two thetah 36.78 degrees two
`theta, 59.18 degrees two theta, and 64.98
`degrees two theta.
`results have been
`Good electrochemical
`obtained with all spinels which have been
`tested. Spinels,
`including normal and inverse
`spinels, as well as those exhibiting both the
`normal and inverse arrangement are con—
`templated:
`
`including
`The spinels of magnesium,
`titanium—magnesium spinel
`(Tng204),
`vanadium—magnesium spinel (VMgzO4), and
`tin—magnesium spinel (SnMggoé).
`including
`The
`spinels
`of
`vanadium,
`(MgV204),
`magnesium—vanadium spinel
`iron-vanadium spinel (FeV204), and zinc—
`vanadium spinel (ZnV204).
`The
`spinels
`of
`chromium including
`magnesium-chromium spinel
`(MgCr204),
`manganese-chromium spinel
`(MnCr204),
`iron-chromium spinel
`(FeCrgO4),
`cobalt—
`chromium spinel
`(COCT204),
`nickel—
`chromium spinel
`(NiCr204),
`copper—
`chromium spinel (CuCrgOi), zinc chromium
`spinel
`(ZnCrgoi),
`cadmium—chromium
`spinel
`(CdCr204),
`and the chromium—
`chromium spinel (CrHCrngQ.
`are
`results
`satisfactory
`Particularly
`including
`The spinels of manganese,
`spinel.
`the
`bimetal
`obtained
`with
`titanium-manganese spinel (TiMn2O4), and
`zinc—manganese spinel (ZnMn204), and the Bimetal spinels are those spinels having the
`manganese—manganese spinel
`formula MIIM211104 as described hereinabove
`where MII and MIII are ions of two different
`
`(ZnFe204), cadmium-iron
`zinc-iron spinel
`spinel
`(CdFeEOi),
`lead-iron
`spinel
`(PbFe20'4),
`and
`aluminum-iron
`spinel
`(FeAlFeOi).
`including mag—
`The spinels of cobalt
`nesium-cobalt spinel (MgC0204), titanium-
`cobalt
`spinel
`(TiConi),
`copper-cobalt
`spinel
`(CuCozoi),
`zinc-cobalt
`spinel
`(ch0204),
`tin-cobalt
`spinel
`(SnC0204),
`and cobalt-cobalt spinel
`(CoUCoJUOi).
`The spinels of nickel _including iron-
`nickel spinel (FeNigoé), and germanium-
`nickel spinel (GeNi204)
`The spinels of rhodium including mag—
`nesium-rho-dium spinel
`(MthgOQ,
`cad—
`mium—rhodium spinel
`(CdRhgoi), cobalt-
`rhodium spinel (CoRh204), copper—rhodium
`spinel
`(CuRh204), manganese-rhodium 80
`spinel
`(MnRhgoi), nickel—rhodium spinel
`(NiRhgoi),
`and
`zinc-rhodium spinel
`(ZnRhgoi).
`The zinc spinels including titanium—zinc
`spinel
`(Tianoi),
`and
`tin—zinc
`spinel
`(SnZn204).
`The aluminum spinels including mag—
`nesium-aluminum
`spinel
`(MgA1204),
`strontium-aluminum
`spinel
`(SrA1204),
`chromium-aluminum spinel
`(CrA1204),
`molybdenum—aluminum spinel
`(MoAIZOé),
`manganese-aluminum spinel
`(MnA1204),
`iron—aluminum spinel
`(FeAlgot),
`cobalt—
`aluminum spinel
`(CoA1204),
`nickel—
`aluminum spinel
`(NiA1204),
`copper— 95
`aluminum spinel
`(CuAlEO4),
`and zinc—
`aluminum spinel (ZnA1204).
`The
`gallium spinels
`including mag-
`nesium—gallium spinel
`(MgGagoa), zinc-
`gallium spinel
`(ZnGagoi), and calcium— 100
`gallium spinel (CaGa204).
`including mag-
`The
`indium spinels
`nesium—indium spinel (MgIngoé), calcium-
`indium spinel
`(CaIn204), manganese-
`indium spinel
`(Mnln204),
`iron—indium 105
`spinel
`(FeIn204),
`cobalt-indium spinel
`(CovInzoé), nickel-indium spinel (NiIn204),
`cadmium-indium spinel
`(CdIn204),
`and
`mercury—indium spinel (HgIngoi).
`three
`The spinels containing ions of
`metal
`such
`as magnesium-aluminum-
`iron
`spinel
`(MgFeAlOi),
`and
`nickel-
`aluminum-iron spinel (NiFeAlOQ.
`
`65
`
`70
`
`75
`
`85
`
`90
`
`110
`
`115
`
`(MnIIMngIIIO4).
`
`metals.
`Better electrolytic results are obtained with 120
`aluminate spinels, that is Where one or both
`The spinels of iron including magnetite
`of the ions present
`in the plus 3 valency
`(FeHFeJHOi), magnesium iron
`spinel
`state is aluminum, as in CuA1204, CoA1204,
`(MgFe20-4), titanium-iron spinel (TiFegoi),
`manganese—iron spinel (MnFegoé), cobalt— FeAlFeOi, and NiAlgoi.
`iron spinel
`(CoFegoi), nickel—iron spinel
`Best results are obtained with the heavy 125
`(Ntigoi), copper-iron spinel (CuFegoi), metal-aluminate spinels,
`that
`is where the
`
`m
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`4O
`
`45
`
`50
`
`55
`
`60
`
`

`

`1,346,369
`
`metal ion present in the plus 2 valency state
`is iron, cobalt, or nickel, as in FeHAIFemO4,
`CoAlZO.” and NiA1204, and with the heavy
`metal ferrite spinels,
`that is, where iron is
`present
`in the plus 3 valency state, as in
`COF6204, MgFE’204, and NiF6204.
`Whenever FeAlFe04 is referred to herein,
`it is understood that this material may actu-
`ally be a mixture of FeHFezmoi, FeAlzor,
`and FeHAlFemOr. This material may be
`characterized by the presence of
`iron in
`both the plus 2 and plus 3 valency states,
`as well as the presence of aluminum in the
`plus
`3 valency state. Additionally, FeO,
`Fe203, and A1203 may also be present.
`Preferably,
`the spinel
`itself should have
`some
`appreciable
`electroconductivity when
`measured in bulk. While good results have
`been obtained with 2. spinel having an electro~
`conductivity as
`low as
`10""
`(ohm—centi—
`meters)‘ , generally the conductivity should
`be at
`least 10‘1 (ohm-centimeters)"1. More—
`over,
`the thin spinel coatings appear
`to
`exhibit greater conductivity as the electrodes
`are used as anodes in the electrolysis of
`aqueous sodium chloride to- produce chlorine
`and sodium hydroxide. Thus, some electro-
`catalytic effect may play a role in the elec—
`trolytic processes herein contemplated.
`The temperatures which have ben resorted
`the preparation
`of
`spinels,
`typically
`to
`ranging from 750°C.
`to 1350°C., are far
`
`above the temperature which decompose and
`volatilize various compounds of
`the oxida-
`tion inhibitors and the binding agents and,
`in a normal atmosphere oxidize the surface
`of the substrate. For this reason when the
`spinel
`is formed in contact with the sub-
`strate, as for example,
`from mixed oxides
`of the metals, the substrate or support mem-
`ber may suffer some degree of oxidation and,
`in such a case,
`the electrode may show a
`much higher voltage than is desirable. But
`when the spinel
`is formed prior to being
`applied to the substrate or support member,
`the highest temperature that the member is
`exposed to is the higher of either the decom-
`position temperature of the oxidation inhibitor,
`the liquid solvent, or of the binding agent
`compound, and the degree of oxidation of the
`member is negligible.
`the
`is desirable that
`For this reason,
`it
`spinel be formed prior to being applied to
`the support member. This can be accom—
`plished by oxidation of the mixed metals,
`or by mixing and subsequent heating of the
`mixed oxides, or by coprecipitation from
`solutions of compounds of
`the metals fol-
`lowed by heating or by thermal decomposi-
`tion of compounds of the mixed metals. The
`preferred compounds are those which decomh
`pose directly to the oxides on heating and
`do not leave a residue as carbonates, form-
`ates, nitrates, and oxalates, e. g.:
`
`A
`
`Coco,.+A1,(co_.)s —> CoO+A1203 +4c021~
`
`A
`
`The resulting product is an intimate mixture
`of the two oxides which can be heated to
`form the spinel.
`Various procedures may be resorted for the
`preparation of spinels to be used as elec—
`trode surfaces. For example, spinels may be
`prepared from the mixed oxides. Cobalt-
`aluminum spinel,
`copper—aluminum spinel,
`and nickel—aluminum spinel were prepared
`from the mixed oxides. The general pro—
`cedure was to grind stoichiometric amounts
`of the oxides to minus 200 mesh (U.S. Stan-
`dard Sieve Series), mix the ground oxides
`together, place the mixed, ground oxides in
`a crucible, and heat the mixed, ground oxides,
`thereby forming the spinel.
`Another mixed oxide procedure may be
`utilized for the iron—aluminum spinel having
`the stoichiometric formula FeH(FemAl)04.
`By this method the iron-aluminum spinel is
`prepared from the mixed oxides Fe203, FeO,
`and A1203, according to the following pro-
`cedure
`described
`in Blue
`and Claassen,
`Journal of
`the American Chemical Society,
`71, 3839 (1949) and Couglin, King and
`
`the American
`of
`Journal
`Bonnickson,
`Chemical Society, 73, 3891 (1951).
`According to this procedure, FeO was first
`prepared by heating metallic iron and FegOg
`in the presence of water vapor. The reactions
`believed to be taking place are:
`
`Fe+H20 ——>~ FeO+Hz
`
`Fe304+H2 —> 3Fe0 +H20
`
`The FeO prepared as described above,
`Page, and A120,, all ground to minus 200
`mesh (US. Standard Sieve Series) were
`mixed and heated under a vacuum overnight
`at
`a
`temperature of
`about 110°C. The
`material was then heated to 1200°C., under
`vacuum, for 24 hours. The resulting product
`was black, magnetic material exhibiting the
`X-ray diffraction pattern reported in the
`literature to be characteristic of the
`
`FeU(FeIIIA1)o.
`
`spinel.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`65
`
`7O
`
`75
`
`80
`
`85
`
`90
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`95
`
`100
`
`105
`
`110
`
`

`

`1,346,369
`
`Spinels may be prepared by precipitation
`from nitrate solution, followed by oxidation
`of the precipitate to yield the spinel. Copper~
`aluminum,
`copper-chromium,
`copper-iron,
`cobalt—aluminum,
`and
`cobalt—chromium
`spinels,
`inter alia, were prepared by pre-
`cipitation from the nitrate solutions. The
`general procedure was to: prepare an aqueous
`solution 0.5 molar in the nitrate salt of the
`divalent metal and 1.0 molar in the nitrate
`salt of
`the trivalent metal. This solution
`was evaporated to dryness by heating the
`solution to a temperature between 125°C.
`and 140°C. The dried product was then
`heated in an air—aspired furnace to drive off
`the nitrogen compounds, yielding thereby a
`mixed oxide. The mixed oxide was then
`ground to a powder which was then heated
`in a furnace to a temperature sufliciently
`high to form the spinel.
`,
`Spinels useful for electrode coatings may
`also be
`prepared. by precipitation from
`oxalate, and subsequent oxidation. A solution
`of MgSO, and FeSO4 in distilled water was
`prepared. The solution was then filtered. The
`filtered solution was heated to boiling and
`ammonium oxalate and oxalic acid were
`added to the solution with stirring. Boiling
`and stirring were continued until a precipit-
`ate appeared. The resulting precipitate was
`filtered and washed. The washed precipitate
`was then dried and the dried product was
`placed in a porcelain crucible and heated to
`a temperature of 500°C, held at
`the tem-
`perature of 500°C. for 10 minutes and then
`slowly cooled to room temperature. The
`resulting product was ground to minus 200
`mesh and then heated to 950°C. and held at
`950°C.
`for 7-;— hours,
`thereby yielding the
`spinel.
`Spinels may also be prepared by ammo-
`nium carbonate
`precipitation
`from the
`chloride
`solution
`ammonium carbonate.
`Nickel—chromium,
`nickel—aluminum,
`cobalt-
`aluminum,
`and
`copper—chromium spinels,
`inter alia, were prepared by precipitation
`from chloride solutions. The general pro-
`cedure was to prepare a solution containing
`the chloride salts of the di— and tri-valent
`metals. To- this solution was added ammo—
`nium carbonate. The
`solution was
`then
`stirred under a nitrogen blanket and the
`precipitate which formed was separated by
`centrifuging under a nitrogen atmosphere.
`The centrifugate was dried under a nitrogen
`atmosphere and the solid material ground
`and heated under vacuum for 72 hours, there—
`by yielding the spinel.
`Spincls may also be prepared by ammo-
`nium hydroxide
`precipitation
`from the
`chloride solution. In this prOCedure, a solu-
`tion was prepared from stoichiometric quan-
`tities of the two chlorides. To this solution
`was added a concentrated solution of ammo-
`nium hydroxide. The resulting mixture was
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`stirred until a precipitate appeared. The
`resulting precipitate was separated and dried.
`The dried precipitate was ground to minus
`100 mesh (U.S. Standard Sieve Series) and
`placed in a covered crucible which was
`placed in a vacuum furnace under a vacuum
`of 10‘5 millimeters of mercury and heated
`to a temperature of 900°C. or greater and
`maintained thereat
`for 24 hours,
`thereby
`yielding the spinel.
`Depending on the method of forming the
`mixed oxide and the degree of comminution
`thereof,
`it is possible that all of the mixed
`oxide will not necessarily be converted to a
`spinel but
`that
`some will
`remain as
`the
`original oxide. This has no deleterious effect
`on the anode. The less soluble oxides, as
`A1203, will
`remain on the anode without
`deleterious effect, while the more soluble
`oxide, as COO or NiO, may be dissolved by
`the anolyte when the finished electrode is
`employed as an anode.
`reportedly have
`Also FeEO3 and A120,.
`structures which permit significant quantities
`of either or both to be present in the spinel
`lattice without deleterious effects, and with—
`out being readily detectable by X—ray diffrac—
`tion.
`The preferred spinel usually is applied
`together with a binding agent. These agents
`include organometallic compounds which, on
`heating, decompose to the metal or metal
`oxide and volatiles as well as more permanent
`binders.
`the
`Typically, regardless of the substrate,
`spinel must be made to adhere to the oxy—
`compound of
`the second transition series
`platinum group metal. This may be accom-
`plished by providing a lattice or network
`within the spinel, with a suitable permanent
`binding agent, whereby the adherence of the
`spinel to the intermediate layer of said oxy-
`compound is enhanced.
`A suitable permanent binding agent has
`to be impervious to the chlorine environ-
`ment of the electrolytic cell as for instance
`a metal compound, such as an oxide, sulfide,
`nitride, boride, or carbide of titanium,
`tan-
`talum, niobium, aluminum, bismuth, tungsten,
`molybdenum, zirconium, hafnium, vanadium,
`chromium, or silicon. It has been found that
`particularly good binding results are obtained
`by the formation,
`in sitzc with the spinel
`coating, of a metal oxide that is substantially
`non—reactive with the anolyte. The formation
`of this oxide, in situ, must, moreover,
`take
`place at a temperature below the temperature
`at which any appreciable oxidation of the
`structural member occurs or any adverse
`effect on the undercoat occurs. For
`this
`reason,
`the thermal
`decomposition
`of
`a
`readily decomposed compound having vola—
`tile decomposition products as, for instance,
`an oxalate, carbonate, hydroxide, hydrated
`oxide, or
`resinate of
`titanium,
`tantalum,
`
`7O
`
`75
`
`80
`
`90
`
`95
`
`100
`
`105
`
`110
`
`115
`
`120
`
`125
`
`130
`
`m
`
`

`

`
`
` 7 1,346,369 7
`
`
`
`5
`
`10
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`is built up
`the spinel content
`formed until
`bismuth,
`aluminum,
`silicon, molybdenum,
`to the desired thickness, usually the process
`tungsten, niobium, or
`zirconium, hafnium,
`of brushing and heating being repeated by
`vanadium may be used. Generally the more
`permanent binders are inorganic. Titanium 7 to 20 times. It is to be understood that
`compounds are preferred. Whenever titanium satisfactory results may also be obtained
`dioxide is described as a binding agent, it without heating after every coat of spinel so
`will be understood that other binding agents
`long as the resinate is ultimately decomposed.
`as herein described may be used in lieu of The resulting surface, 200 to 400- micro—
`or in addition thereto.
`inches thick, has 0.02 to 0.04- gram of spinel
`the permanent
`Small
`concentrations of
`per square inch of spinel coated anode sur— 75
`binding
`agent
`are
`e