US3804740A - Electrodes having a delafossite surface - Google Patents

Abstract

ELECTRODES USEFUL FOR ELECTROCHEMICAL REACTIONS ARE DISCLOSED. ALSO DISCLOSED ARE ELECTROLYTIC CELLS UTILIZING SUCH ELECTRODES AND THE USE OF SUCH ELECTRODES IN THE CONDUCT OF ELECTROCHEMICAL REACTIONS. THE ELECTRODES HAVE DELAFOSSITE SURFACES ON SUITABLE ELECTROCONDUCTIVE BASES. DELAFOSSITES ARE ELECTROCONDUCTIVE OXYCOMPOUNDS OF METALS AND INCLUDE PLATINUM COBALT DELAFOSSITE (PTCOO2), PALLADIUM COBALT DELAFOSSITE (PDCOO2), PALLADIUM CHROMINUM DELAFOSSITE (PDCRO2), PALLADIUM RHODIUM DELAFOSSITE (PDRHO2), PALLADIUM RUTHENIUM DELAFOSSITE

(RDRUO2), PALLADIUM LEAD DELAFOSSITE (PDPBO2), THE PALLADIUM LANTHANIDE DELAFOSSITES, SILVER COBALT DELAFOSSITE (AGCOO2), SILVER GADOLINIUM DELAFOSSITE (AGGAO2), SILVER SCANDIUM DELAFOSSITE (AGSCO2), SILVER INDIUM DELAFOSSITE (AGINO2), SILVER THALLIUM DELAFOSSITE (TGTIO2), COPPER COBALT DELAFOSSITE (CUCOO2) AND COPPER IRON DELAFOSSITE (CUFEO2), INCLUDING THE MIMERAL DELAFOSSITE (CUFEO2).

Description

' April 16, 1974 c. N. WELCH 3,804,740

I ELECTRODES HAVING A DELAFOSSITE SURFACE Filed Feb. 1, 1972 4 Sheets-Sheet 1 I 3 .000 2 soo d 2, 000 L500 loo l l 2-500 v d 2.000 [.500

3.000 ZISOO CL 2,000 0 I-ZDIO loo 0 April 16, 1974 c. N. WELCH ELECTRODES HAVING A DELAFOSSITE SURFACE 4 Sheets-Sheet 2 Filed Feb. 1, 1972 N mmmmOmQ April 1974 c. N. WELCH 3 ELECTRODES HAVING A DELAFOSSITE SURFACE Filed Feb. 1, 1972 4 Sheets-Sheet 4 FIG. 8

4e 50 0562559 ze- 3,804,740 ELECTRODES HAVING A DELAFOSSITE SURFACE Cletus N. Welch, Barberton, Ohio, assignor to Nora International Company, Panama, Panama 7 Filed Feb. 1, 1972, Ser. No. 222,501

Int. Cl. B01k 3/06; C01]: 7/06, 11/26 .s.c1. 204-29o R, Claims ABSTRACT OF THE DISCLOSURE copper cobalt delafossite (CuCo and copper iron delafossite (CuFeO includnig the mineral delafossite (CuFeO BACKGROUND Numerous electrochemical reactions, such'as the electrolysis of brines, hydrochloric acid, and sulfates, electroplating, electrowinning, electrolytic production of metal powders, electrolytic cleaning, electrolytic pickling,

.and the electrochemical generation of electric power, in-

volve the use of non-consumable anodes. Previously, graphite anodes have been used, especially in such processes asthe electrolysis of brines and the electrolysis ofhydrochloric acid. More recently, electrodes have been :developedfor such processes utilizing a suitable electroconductive base or substrate and an electrocatalytic coating thereon. Typically, such electrocatalytic coatings ha've beenthe platinum group metals; e.g., platinum, osmium, iridium, ruthenium,-palladium, and rhodium, and their oxides. i

SUMMARY OF THE INVENTION It has nowbeen found that a particularly satisfactory electrode for the conduct of electrochemical reactions may be provided by the use of a delafossite surface on a suitable electroconductive substrate or base member. Delafossites are oxides of high electrical conductivity having the stoichiometric formula:

A is platinum, palladium, silver, or copper. B is typically chromium, iron, cobalt, rhodium, aluminum, gadolinium, scandium, indium, thallium, lead, ruthenium, and the lanthanides. B may, however, be any metal ion having a stable +3 fonmal valence state compatible with the delafossite structure.

Delafossites also include non-stoichiometric com- .pounds isostructural with the stoichiometric delafossites.

A suitable electroconductive substrate is an electroconductive substrate that is substantially not attacked by the electrolyte.

3,804,740 Patented Apr. 16, 1974 DETAILED DESCRIPTION OF THE INVENTION 7 According to this invention, an electrode is provided having a delafossite surface on an electroconductive substrate. Delafossites are metal oxycompounds having the stoichiometric formula:

ABO

where A is platinum, palladium, silver, or copper, and B is chromium, iron, cobalt, rhodium, aluminum, gadolinium, scandium, indium, thallium, lead, ruthenium, and the lanthanides. B may also be any metal ion having a +3 formal valence state compatible with the delafossite structure. Delafossites also include non-stoichiometric compounds having the delafossite-type structure as defined herein. Delafossites have a unique crystallographic structure similar to that of the natural mineral delafossite (CuFeO with the slight differences in crystallographic structure being due to slightly different ionic radii.

Delafossites provide particularly satisfactory electrode materials because they combine high electrical conductivity comparable to that of metals, with high resistance to chemicalattack comparable to that of refractory metal oxides. For example, the bulk electrical conductivity of compressed powders of the platinum and palladium delafossites is on the order of about 10 to 10 (ohm-centimeters)'" Such platinum and palladium delafossites are, however, resistant to attack by strong acids such as aqua regia at C. and by nascent chlorine.

Delafossites are a family of oxides of two or more metals, the oxide having a rhombohedral structure with a hexagonal crystal habit similar to that of the mineral delafossite CuFeO Delafossites may further be characterized in that their crystals have a space group of 166, aShoenflies group of D a Standard Full Symbol of R5 (R, bar 3, two over m) and an International Symbol of R5 (R, bar 3, m)

Such oxides and their crystallography are particularly described in the articles by W. J. Croft et al., Acta Orystallogn, 'vol. 17, p. 313 (1964); W. Gessner, Z. Anorg. Allg. Chem., vol. 352, p. (1966); H. 'Hahn et al., Z. Anorg. Allg. Chem., vol. 279, p. 281 (1955); .W. Dann-hauser et al., J. Amer. Chem. Soc., vol. 77, p. 896 (1955); H. Wiedersich et al., Mineral Mag., vol. 36, p. 643 (1968); A. Krause et al., Z. Anorg. Allg. Chem., vol. 228, p. 352 (1936); A. Pabst, Amer. Mineral, vol. 31, p. 539 (1946); and A. H. Muir et al., J. Phys. Chem. Solids, vol. 28, p. 65; R. D. Shannon et al., Chemistry of Noble Metal Oxides. I. Synthesis and Properties of ABO Delafossite Compounds," Inorganic Chemistry, vol. 10, p. 713 (1971); C. I. Prewitt et al., Chemistry of Noble Metal Oxides. '11. Crystal Structures of PtCoO PdCoO CuFeO and AgFeO Inorganic Chemistry, vol. 10, p. 719 (1971); D. B. Rogers et al., Chemistry of Noble Metal Oxides. III. Electrical Transport Properties and Crystal Chemistry of ABO, Compound With the Delafossite Structure," Inorganic Chemistry, vol. 10, p. 723 (1971); US. Pat. 3.498,931 to D. B. Rogers et al. for Electrically conductive Oxides Containing Palladium and Their Preparatiom US. Pat. 3,514,414 to 3 R. D. Shannon for Electrically Conductive Platinum Cobalt Oxides; and US. Pat. 3,414,371 to D. B. Rogers for Low Pressure Synthesis of Electrically Conductive Platinum Cobalt Oxide From a Platinum Halide and an Oxide Containing Cobalt.

As a general rule, delafossites are further characterized in that the A ions (corresponding to copper in the mineral delafossite) have an effective ionic radius corresponding to two coordination by oxygen; i.e., effective ionic radii within the range of 0.45 to 0.68 angstrom. In the case of the particularly electroconductive platinum and palladium delafossites, the A ions have effective ionic radii within the range of 0.58 to 0.61 angstrom. The B ions (corresponding to iron ions in the mineral delafossite) have an ionic radius corresponding to an octahedral coordination; i.e., effective atomic radii in the range of from about 0.50 to about 1.05 angstrom units. Effective ionic radius as used herein also encompasses the equivalent term crystal ionic radius.

Muir et al., op. cit., report that the mineral delafossite (CuFe is rhombohedral with hexagonal planes. The structure consists of stacked hexagonal layers. The iron ions have octahedral coordination while the copper ions have linear two-fold oxygen coordination. The oxygen ions are surrounded by a tetrahedron of three iron ions and one copper ion. Three formulae weights of CuFeO make up one unit cell of the mineral delafossite.

By unit cell is meant an arbitrary parallelepiped which is the smallest repetitive unit identifiable as the crystal. The unit cell, generally as a matter of convenience, confirms 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 patterns. Starting from any point of origin in the lattice and going a distance equal to and paralled to any cell edge, or by any combination of such movements, a point is arrived at where the whole surroundng structure has the same form and orientation as 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 2, 4, or 8 unit cells. Additionally, the neighbor of any one ion may be in the same unit cell or in an adjacent unit cell.

The synthetic platinum cobalt delafossite (PtCoO is reported by C. T. Prewitt, R. D. Shannon, and D. B. Rogers, op. cit., as having the delafossite-type crystallographic structure. The platinum cobalt delafossites, both stoichiometric and non-stoichiometric, are particularly useful in providing the electrodes of this invention.

The platinum cobalt delafossite is reported as having a crystal structure similar to that of the natural mineral delafossite (CUFCO3)- Each oxygen ion is tetrahedrally coordinated by four cations with a platinum ion at one corner of the tetrahedron and a cobalt ion at each of the other corners of the tetrahedron. The platinum ions are linearally coordinated by two oxygen ions. The cobalt ions are octahedrally coordinated by the oxygen ions. The platinum ions are in hexagfional bipyramidal coordination with oxygen ions at the apices and platinum ions at the six equatorial positions allowing for platinum-platinum interactions.

The delafossite crystal structure and the relationships between the platinum cobalt delafossite, the palladium delafossites, and the mineral delafossite (CuFeO may be more clearly illustrated by reference to the figures:

FIG. I is a graphical representation of the X-ray diffraction data reported in the literature for the mineral delafossite (CuFeO and shown in Table 1.

FIG. II is a graphical reprsentation of the X-ray diffraction data reported in the literature for PtCoO platinum cobalt delafossite and shown in Table 2.

FIG. III is a graphical representation of the X-ray diffraction data reported in the literature for PdCoO palladium cobalt delafossite oxide and shown in Table 3.

FIG. IV is a graphical representation of the X-ray diffraction data reported in the literature for PdCrO palladium chromium delafossite and shown in Table 4.

FIG. V is a graphical representation of the X-ray diffraction data reported in the literature for PdRhO palladium rhodium delafossite and shown in Table 5.

FIGS. I-V, graphically representing the data in the literature and shown in Tables 1-5, inclusive, are drawn to the same scale on the abscissa and have a common ordinate. The abscissa indicates I/Io, the reported ratio of the intensity of the reflected ray to the incident ray. The ordinate indicates d, the reported interplanar spacing, calculated by the Bragg equation as described here inafter.

FIG. I was prepared from the data of Saller and Thompson, Phys. Rev., vol. 44, p. 644 (1935), reported in A.S.T.M. X-Ray Powder DiffractionFile' 12-752, and reproduced in Table 1 herein. FIGS. II, III, IV, and V were prepared from the data of R. D. Shannon et al., Inorganic Chemistry, vol. 10, pp. 713, 717, op. cit., and reproduced herein in Tables 2, 3, 4, and 5.

FIG. VI is an X-ray diffraction pattern ofthepalladium cobalt delafossite (PdCoO prepared for use as an electrode material in Example I.

FIG. VII is an X-ray diffraction pattern of the platinum cobalt delafossite (Pt Co O prepared for use as 'an electrode material in Example II.

FIG. VIII is an X-ray diffraction pattern of the platinum cobalt delafossite (Pt Co O prepared for use as an electrode material in Example III.

The platinum and palladium delafossite crystallographic units give a unique family of X-ray diffraction patterns characterized by peaks at d spacings of 5.9072 to 6.0229 angstroms, 2.9728 to 3.0137 angstroms, a doublet with a spacing between peaks of 0.06 to 0.07 angstrom with the first peak at from 2.4260 to 2.3609 angstroms and the second peak of the doublet being at from 2.5884 to 2.5141 angstroms, a peak at 2.1460 to 2.2643 angstroms, and another peak at 1.6483 to 1.7104 angstroms. The data for the mineral delafossite is only reported for degrees 29 in excess of about 30 degrees; that is, for d spacings less than 2.86 angstroms. The mineral delafossite (CuFeO has the doublet at 2.58 and 2.508 angstroms, as well as peaks at 2.238 and 1.658 angstroms. The differences in X-ray diffraction patterns between individual members of the delafossite family are due to slightly different cation radii.

The mineral delafossite (CuFeO has the X-ray diffraction pattern shown in Table 1 and graphically represented in FIG. I. The platinum group metal delafossites have the X-ray diffraction patterns shown in Tables 2, 3, 4, and 5 and graphically represented in FIGS. II, III, IV, and V. It is to be noted that FIGS. I through V, inclusive, are graphical representations of the data reported in the literature and reproduced in Tables 1 through 5, inclusive. Such figures are idealized and show family relationships and trends. FIGS. I through V do not show background noise and randomness normally associated with X-ray diffraction print-outs.

The observations and recordation of X-ray diffraction patterns involves subjecting delafossite powder samples to X-rays from a copper target. The methods for accomplishing this are more particularly described in chapter 5 of Klug and Alexander, X-Ray Diffraction Procedures, John Wiley & Sons, Inc., New York (1954) at pp. 235- 318, and especially at pp. 270-318, and in Newfield, X-Ray Diffraction Methods, John Wiley & Sons, Inc., New York (1966) at pp. 177-207.

As described in Klug and Alexander and in Newfield, X-rays having a wave length of 1.5405 angstrom units are caused to strike a sample of the powder being analyzed. The X-rays are diffracted by the sample. The X-rays so diffracted are particularly intense at certain angles, 6, resulting in peaks on the diffractometer printouts or in lines on photographic diffraction patterns These peaks of high intensity are caused by the X-rays being reflected from parallel planes in the crystal reinforcing each othen'The wave length of the X-rays, the spacing of the planes in the crystal, and the angle, 6, are relatedby Braggs law. Braggs law is:

211 sin 9=nk eral delafossite (CuFeO Particularly to be noted is the doublet at 2.5-8 and 2.508 angstroms and the peaks at 2.238 and 1.658 angstroms, respectively.

Certain generalities may be noted with respect to the powder X-ray dilfraction patterns of the platinum group metal delafossites. First to 'be noted is a particularly strong peak at a d value from about 5.9072 to 6.0229. Next to be noted in FIGS. 11 through V, inclusive, is the existence of a strong doublet where the peaks are separated from each other by from about 0.06 to about 0.07 angstrom with the greater separation of peaks and greater differences of intensities being for the platinum group metal delafossites having the larger unit crystal andthe peaks being closer together and of morev nearly equal intensity for the platinum group metal delafossites having the more compact unit crystal. For the platinum cobalt delafossite (PtCoO the two peaks of the doublet are at 2.4260 and 2.3609.- angstroms. For the. palladium cobalt delafossite (PdCO the two peaks of the doublet are at about 2.4272 and 2.3616 angstroms, while for the palladium chromium delafossite the two peaks are at 2.5065 and 2.4375 angstroms, and for the palladium rhodium delafossite (PdRhO the two peaks are at about 2.5884 and 2.5141 angstroms. Also to be noted is the increasing difference in intensity between the two members of the doublet proceeding fromthe more compact to the less compact unit crystals. The .first peak in doublet represents an interplanar space of from about 2.42 angstroms I I (PtCoO 'to about 2.58 angstroms (PdRhO while the second peak "represents an 'interplanar space of from about 2.46 angstroms (PtCOO to about 2.51 angstroms (PdR-hO Further" tobe noted is a strong peak at from-about 2.1460 'to' about 2.2644 "angstroms. This peak is at about 2.14 angstroms for the more compact unit cell of the platinum cobalt delafossite, at a spacing of 2.1448 angstroms for the palladium cobalt delafossite (PdCoO at a spacing of 2.2088 for palladium chromium delafossite (PdCrO L and at 2.2-6 angstroms for the slightly less "compact unit cell of the platihurnrhodium delafossite (PtRhO Also to be noted is a weak doublet in the range of from 2.0185 and 1.9814 angstroms with intensities of 0.30 and 0.40, respectively, for the more compact platinum cobalt delafossite. As the unit cell increases in size, this particular doublet becomes more spread out and less intense.

For the :slightly less compact palladium cobalt delafossite, the doublets appear at d spaces of 2.0165 and 1.9715 angstroms, respectively, and the weaker of the two has an; intensity'of about 10.15. -For the palladium uh??? who hah v .inte s s of less t a 0.05-

6 For the more compact platinum group metal delafossite crystals, a peak exists at about 1.76 to 1.81 angstroms. This peak, at about 1.7655 for the platinum cobalt delafossite (PtCoO has an intensity of about 0.30.

For the palladium cobalt delafossite, the peak appearing at an interplanar' distance of about 1.7616 angstroms has an intensity of about 0.30. For the slightly less compact palladium chromium delafossite (PdCrO the peak appears at about 1.8084 angstroms and has an intensity of less than 0.05.

TABLE 1 X-ray powder. difiraction pattern for copper iron delafossite (CuFeO d1 I/Io 2.86, 35 2.58 18 2.508 100 2.238 25 2.083 6 1.902- 9 1.658 35 TABLE 2 X-ray powder diffraction pattern for platinum cobalt delafossite (PtCoO d: I/Io 5.9450 v 90 2.9728 Q 100 1.3751 p W 1.3.513 a TABLE. 3;

X-ray powder diffraction pattern for palladium'cobalt delafossite (PdCoO 1,

2.0165 15 1.9713 -L. --;f ;.-I .J.- 50 1.7616 I .30 1.6445 85 7 TABLE 4 X-ray powder diffraction pattern for palladium chromium delafossite (PdcrO X-ray powder diffraction pattern for palladium rhodium delafossite (PdRhO d: I/Io 6.0209 nu 5 3.0130 90 2.5884 2.5141 100 2.2644 95 2.1198 5 2.0088 2 1.7104 85 1.5071 1.4874 70 1.5106 85 1.3503 80 1.2945 60 Finally to be noted is a medium intensity peak at from about 1.6453 to about 1.7104 angstroms. This peak has an intensity of from about 0.30 for palladium chromium delafossite to about 0.85 for the palladium cobalt (PdcO2) and palladium rhodium (PbRhO2) delafossite.

It is at 1.6453 angstroms for the platinum cobalt (PtCoO delafossite, at about 1.6445 angstroms for the palladium cobalt delafossite, at about 1.6864 angstroms for the palladium chromium delafossite, and at about 1.7104 angstroms for the palladium rhodium delafossite. Delafossites include metal oxycompounds having the X-ray diffraction 'patterns tabulated in Tables 1 through 5, inclusive, which data are pictorially represented in FIGS. I through V inclusive, and described hereinbefore.

Chemically, delafossite oxides are oxycompounds of two or more metals having the general formula:

ABO

While the general formula ABC; is given, it should be noted that non-stoichiometric compounds having defect structures are possible and are included within the scope of this invention. Such non-stoichiometric, defect structured delafossites have the formula A B O where either x or y or both are less than 1; for example, from about .70 to 100. Such delafossites are within the contemplated scope of this invention. When delafossites having the formula ABO, are referred to herein, such non-stoichiometric delafossites are also contemplated. The metal ion represented by A may be any formally monovalent metal ion having an effective ionic radius of from about 0.45 angstrom unit to about 0.68 angstrom unit. Such monovalent ions are copper (Cuwith an effective ionic radius of 0.47 angstrom, palladium (Pd+) having an effective ionic radius of about 0.59 angstrom, platinum (Pt+) having an effective ionic radius of'about 0.60 angstrom, and silver (Ag having an effective ionic radius of about 0.67 angstrom. Twoor more of such monovalent cations may be present in the same delafossite crystal. The preferred delafossites are those where the monovalent cation has an effective ionic radius of from about 5.58 to 0.61 angstrom, such as palladium (+1) and platinum (+1).

The cation represented by B is most commonly chrw mium, iron, cobalt, rhodium, aluminum, gadolinum, scandium, indium, thallium, lead, ruthenium, and the lanthanides (La, Ce, Pr, Nd, Pm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu and Y).'B is typically a trivalent cation having an effective ionic radius of from about 0.50 angstrom unit to about 1.05 angstrom units, and most frequently from about 0.50 angstrom unit to about 0.89 angstrom unit. The preferred trivalent cations are those having an effective ionic radius of from about 0.50 to about 0.70 angstrom unit and include cobalt (Co having an effective ionic radius of about 0.52 angstrom, chromium (Cr having an effective ionic radiusof 0.62 angstrom, and rhodium (Rh having an effective ionic radius of about 0.70 angstrom. The cations represented by B may, however, be any metal ion having a stable +3 formal valence and an effective ionic radius of from 0.50 to 1.05 angstroms. Such metal ions include the Group IIIa metals, scandium, yttrium, and the lanthanides; titanium in Group IVa; vanadium in Group Va; the Group VIa metals chromium, molybdenum, and tungsten; the iron triol, iron, cobalt, and nickel; the second transition series platinum group metals ruthenium, rhodium, and palladium; the Group HIb metals aluminum, gadolium, indium, and thallium; tin and lead in Group IVb; and antimony and bismuth in Group Vb.

The platinum delafossites include the stoichiometric oxycompound of platinum and cobalt having the delafossite structure (PtCoO as well as non-stoichiometric oxycompounds of platinum and cobalt having the delafossite structure. Such non-stoichiometric delafossites have the formula:

Pt C0 03 T *5 where x is from about 0.70 to 1.00, y is from about 0.70 to 1.00, and z is less than 0.11. Typically, z is from about 0.004 to about 0.11, M is Sc, Ti, V, Cr, Mn, Fe, Co, or Ni. The platinum cobalt delafossite particularly useful as the electrode surface of this invention has a formula:

where n is from about 0.50 to about 0.8.

The palladium delafossites particularly useful in providing an electrode surface of this invention are those delafossites having the formula:

PdMO

where M is cobalt, chromium, rhodium, ruthenium, lead, or a lanthanide (including yttrium), or mixtures thereof. Such palladium delafossites, as palladium cobalt delafossite (PdCoO palladium chromium delafossite (PdCIO palladium rhodium delafossite (PdRhO the delafossite of chromium and rhodium (PdCrO /PdRhO the palladium delafossite of cobalt and rhodium (PdCoO /PdRhO the palladium delafossite of ruthenium ('PdRu'O the palladium 9 palladium delafossite of lead (PdPbO the palladium delafossites of the lanthanides, and the like, are particularly useful as electrode surfaces with the electrodes of this invention.

The silver and copper delafossites have a higher electrical resistivity, that is, a lower electrical conductivity, than do the platinum and palladium delafossites. The silver delafossites are those delafossites having the formula:

AgMO

where M is cobalt, gadolinium, scandium, indium, and thallium. Compressed powders of silver delafossites have a bulk electrical conductivity of from about 10- to about I 10- -(ohm-centimeters)- T he' copper delafossites such as copper iron delafossite (CuFeO natural delafossite (CuFeO copper aluminum delafossite (CuAlO and copper cobalt delafossite '(CuCoO are also useful in providing the electrode suras the platinum and palladium delafossites may be used to provide an electroconductive layer on the electroconductive base or substrate with the delafossite surface having a further exterior coating of a suitable electrocatalytic material such as a spinel, for example, as disclosed in commonly assigned, copending US. application Ser. No. 106,840 of Paul P. Anthony filed Jan. 15, 1971, or a perovskite or perovskite bronze as disclosed in commonly assigned, copending US. application Ser. No. 121,693 of Paul '1. Anthony filed Mar. 8, 1971. Alternatively,'other electrocatalytic materials, such as ruthenates, ruthenites, rhodites, rhodates, and the like may be used to provide an exterior surface on the delafossite coating. Alternatively, thedelafossite surface may have interposed betweenit and the electrolyte a porous layer of a substantially nonreactive material such' as titanium dioxide, vanadium oxide, tantalum oxide, tungsten oxide, niobium oxide, hafnium oxide, zirconium oxide, and the like, as well as silicon dioxide. Such additional exterior coatings serve to provide further mechanical durability to the delafossite. Furthermore, such porous exterior coatings provide surface area for surface catalyzed reactions of the electrode products, or of the electrode reagents. Preferably porous exterior oxide coatings are formed in situ as will be described hereinafter. Various materials may be admixed with the delafossite material, but the surface of the electrode nevertheless consists essentially of the delafossite material. For example, conductive materials, such as perovskites, perovskite bronzes, platinum group metals, oxides of platinum group metals, and mixed carbides, nitrides, oxides and borides resistant to electrolyte may be admixed with the delafossite. Such materials may be present to provide additional conductivity or reactivity. Alternatively, materials such as the oxides of titanium, tantalum, niobium, hafnium, tungsten, aluminum, vanadium, silicon, and other film-forming metals may be admixed with the delafossite to provide additional durability to the surface. Any oxide, formed in situ during the preparation of the surface and substantially non-reactive with the electrolyte, will serve to bind the delafossite particles to the substrate.

While the delafossite-coated electrodes of this invention may include an electrode that is a bulk delafossite, such electrodes will not normally be utilized for reasons of economy. Preferably, the delafossite electrodes of this invention will be in the form of an electrode having a delafossite surface on a suitable electroconductive substrate. The delafossite surface may be as thin as eight angstroms. But, as a practical matter, the surface will be at least 60 microinches thick, and, preferably, in excess of 150 to 200 microinches thick. The delafossite surface need not, however, be greater than about 250 to 300 microinches in thickness. While a delafossite surface greater than 300 microinches in thickness (for example, as thick as 800 microinches or more) may be used without deleterious effect, no additional advantages are obtained thereby.

By suitable electroconductive substrate or base is meant a substrate having an electrical resistivity within economic limits for its intended use, and being substantially non-reactive with the electrolyte and products of electrolysis. For example, in the electrolysis of brines a suitable electroconductive substrate would be one that is substantially non-reactive with sodium hydroxide or sodium chloride solutions and not attacked by nascent chlorme.

Suitable electroconductive substrate or base materials include the valve metals. The valve metals are those metals which form an oxide film under anodic conditions. The valve metals include titanium, tantalum, niobium, hafnium, tungsten, aluminum, zirconium, vanadium, and alloys thereof. Most commonly, for reasons of cost and availability, titanium or titanium alloys will be used as the substrate or base member of the electrodes of this invention. However, other materials such as graphite or carbon may be used as the electroconductive base or substrate material without deleterious effects. A laminate of a valve metal and a less expensive metal such as iron or steel may be used with a delafossite coating on the valve metal.

Alternatively, titanium hydride or other electroconductive, anodically-resistant hydrides may be used as the electroconductive base or substrate member of the electrode of this invention. Such hydride may be present as the sole base member or it may be in the form of a plate of the hydride on another material. Alternatively, the hydride may be present as a hydride surface of a metal, for example, as a titanium base with a titanium hydride layer Fetween' the titanium metal base and the delafossite surace.

A layer of an electroconductive material, more conductive than the delafossite and also resistant to the electrolyte, may be interposed between the delafossite and the substrate or base member. Such intermediate layer may be a platinum group metal such as metallic ruthenium, rhodium, palladium, osmium, iridium, platinum, or alloys thereof. Particularly satisfactory alloys include platmum-palladium alloys, especially those having from about three to about fifteen weight percent platinum, and platmum-iridium alloys, particularly those having from about two to about fifty weight percent iridium. Alternatively, such intermediate layer may be an oxide of a platinum group metal such as ruthenium oxide (RuO rhodium oxide (Rh 0 palladium oxide (PdO), osmium oxide (OsO iridium oxide (Ir O platinum oxide (PtO or mixtures thereof. Such mixtures may include mixtures of platinum oxide and palladium oxide having from about three to about fifteen weight percent platinum oxide, or platinum oxide and iridium oxide containing from about two to about fifty weight percent iridium oxide. Such intermediate layer may also contain mixtures of a platinum gronp metal and the oxide thereof, or mixtures of one platinum group metal and the oxide of another platinum group metal. Additionally, oxides of other metals such as titanium, zirconium, hafnium, vanadium, niobium, tantalum, tungsten, and aluminum may be present in the intermediate coating with the platinum group metals or the oxides thereof. Such intermediate layer when present is normally from about 20 to about microinches thick, and preferably from about 60 to about 120 microinches thick. It may be thinner, for example, as thin as 5 microinches, if applied uniformly so as to provide a pore-free coating. It may also be thicker, but without any significant effect.

Alternatively, where an electrode is provided having a delafossite surface on a substrate which substrate reacts with the delafossite to form an electrically insulative barrier therebetween after extended periods of electrolysis at high current density, there may be interposed therebetween a layer of a less-reactive material that is more re- .sistant to the electrolyte than the substrate or base member. By less-reactive material is meant a material that does not form an electrically insulative barrier or aid in the formation of such a barrier when interposed between the delafossite and the substrate. Such less-reactive material may be a precious metal or oxide thereof as described hereinabove. Alternatively, such less-reactive material may be platinum cobalt (PtCoO delafossite, palladium chromium (PdCrO delafossite, or palladium rhodium PdRhO delafossite when used to provide a layer between a palladium cobalt delafossite (PdCoO surface and a titanium substrate. Such intermediate coating, when present, is normally from 20 to 120 microinches thick, and preferably from 60 to 120 microinches thick. It may be thinner, for example, as thin as 5 microinches if applied uniformly so as to provide a pore-free coating. It may also be thicker, but without any significant effect.

The delafossite surfaced electrodes of this invention are useful in any electrochemical process where a nonconsumable electrode is used. By non-consumable is meant an electrode that is not dissolved by the electrolyte and redeposited on the opposite electrode. For example, the electrodes of this invention may be used in the electrolysis of brines, sulfates, hydrochloric acid, phosphates, and the like. Alternatively, the electrodes of this invention are useful in those electrolytic processes where cations are deposited on a cathode; for example, in electrowinning, electrorefining, electroplating, electrophoresis, electrolytic cleaning, and electrolytic pickling. Copper, nickel, iron, manganese, brass, bronze, cadmium, gold, indium, silver, tin, zinc, cobalt, chromium, and the like may be electroplated from suitable solutions onto cathodes. Alternatively, metal powders may be prepared by depositing cations out of solution onto a suitable cathode using the electrodes of this invention. The electrodes of this invention may be used for electrolytic cleaning using aqueous solutions of sodium phosphate, sodium carbonate, and the like. Additionally, electrolytic pickling of suitable materials rendered cathodic with respect to the anode of this invention may be used for the electrolytic oxidation of organic compounds. For example, the electrolytic oxidation of propylene to propylene oxide or propylene glycol may be carried out using the electrodes of this invention. Metal structures such as ships hulls may be cathodically protected using the anodes of this invention. In each use of the electrodes of this invention enumerated above, the cell comprises an electrode pair having an anode and a cathode, at least one member of the electrode pair being the delafossite surfaced electrode herein contemplated, an electrode of opposite polarity, and a means to establish an external voltage or electromotive force between the anode and cathode whereby the anode is positively charged with respect to the cathode and an electrical current is caused to pass from one member of said electrode pair to the other member.

Additionally, the electrodes of this invention may be used in fuel cells. When used in fuel cells the delafossite surfaced electrodes will be permeable to the flow of electrolyte. Such electrolyte permeability may be provided by an electroconductive porous substrate having delafossite materials deposited therein, or by a dispersion of delafossite particles in a suitable inert media, or by other methods known in the art. Such fuel cells utilizing the electrodes of this invention include a delafossite surfaced electrode, an electrode of opposite polarity spaced from the delafossite surfaced electrode, apparatus for feeding electrolyte into the space between the electrodes in order 12 to internally generate an electromotive force between said electrodes, and apparatus for recovering the electrical energy generated within the fuel cell.

The electrodes of this invention may be prepared by synthesis of the delafossite and the subsequent coating of the delafossite on a suitable electroconductive substrate or base member. Delafossites may be prepared by oxidation at low temperatures and high pressures such as by a hydrothermal reaction. They may also be prepared by oxidation at high temperatures and high pressures by solid state reactions and by oxidation at low temperatures and low pressures in an oxidizing flux. Alternatively, delafossite may be prepared at low pressure and low temperature in an anion exchange reaction. Delafossites useful in the present invention may be prepared and include those prepared by the methods described in: R. D. Shannon et al., Chemistry of Noble Metal Oxides. I. Syntheses and Properties of ABO Delafossite Compounds, Inorganic Chemistry, vol. 10, p. 713 (1971); U8. Pat. 3,498,931 to D. B. Rogers et al. for Electrically Conductive Oxides Containing Palladium and Their Preparation; and US. Pat. 3,514,414 to R. D. Shannon for Electrically Conductive Platinum Cobalt Oxides; and US. Pat. 3,414,371 to D. B. Rogers for Low Pressure Synthesis of Electrically Conductive Platinum Cobalt Oxide From a Platinum Halide and an Oxide Containing Cobalt.

Hydrothermal synthesis of delafossites are conducted at comparatively low temperatures under oxidizing conditions. The hydrothermal method may be used to produce platinum cobalt delafossite, the copper delafossites, and the silver delafossites. The method involves heating the two oxides and water in a sealed inert tube, such as a platinum or gold tube, at a high pressure, for example, about 3000 atmospheres, and a temperature of about 500 C. to 700 C. for about 24 hours.

The platinum, palladium, and copper delafossites may be synthetized at relatively low pressures and low temperatures in a reaction characterized by an exchange of ions and the formation of a solid or fused salt by-product. In this method, the halide of platinum, palladium, or copper and the oxide of the second metal are reacted to form the delafossite and a halide of the second metal. Alternatively, the halide, or the metal and the halide of the first metal, and a lithium oxycompound of the second metal (as lithium chromate, lithium cobaltate, or lithium rhodate) may be reacted to form the delafossite and a lithium halide.

The anion exchange reaction is carried out in an evacuated sealed silica tube at a temperature of from about 500 C. to about 800 C. The by-product halide salts are removed after the formation of the delafossite by leaching, for example, with water. If metallic platinum, palladium, or copper remains or if the noble metal chloride remains, such residue may be removed by leaching with an appropriate solvent such as hydrobromic acid or aqua regia.

Delafossites may be synthetized by the difusion of the finely-powdered mixed oxides at high temperatures. Copper delafossites may be produced by this solid state method of synthesis at atmospheric pressure and at a temperature of about 1000 C. The platinum and palladium delafossites require a temperature in excess of 700 C. and a pressure of approximately 300 atmospheres in order to form the delafossite by solid state fusion.

The silver delafossites may be produced in an oxidizing flux. By this method a flux of silver nitrate and potassium nitrate is prepared and a chromate, rhodate, or cobaltate added to the flux. The flux is sealed and held at an elevated temperature, for example, above about 350 C., for a period in excess of about hours. In this way a solid solution of nitrate and delafossite is obtained. The nitrate may be removed by leaching with water.

Thereafter the delafossite, synthetized by one of the above methods, may either be utilized itself as an electrode or may be applied to a suitable electroconductive base or substrate member to provide an electrode.

Typically, as described hereinabove, the electroconductive base or substrate member will be a valve metal such as titanium. Such base or substrate member may be in the form of a flat, plate-like sheet, or it may be in the form of a perforate sheet or expanded mesh. Alternatively, it may be in the form of fine wires, rods, or other shapes in order to allow the flow of evolved gases away from the volume between the anode and the cathode into spaces behind the anode.

When a titanium member is utilized as the base or substrate member, such titanium member is prepared for use as an electrode substrate by degr'easing and etching prior to deposition of the delafossite surface. Degreasing may be carried out by any of the methods well known in the art such as by the use of detergents, abrasives, or organic degreasing agents. Thereafter, the degreased titanium base or substrate member may be etched in hydrofluoric acid, hydrochloric acid, or the like. The etching serves to remove the naturally-occurring oxide film and substitute therefore a thin hydride film.

The delafossite may be applied to the etched titanium surface in a number of ways. For example, a slurry of the delafossite may be prepared in a solvent such. as ethanol, butanol, benzyl alcohol, phenol, benzene, cumene, or the like. Such slurry may be brushed onto the surface of the titanium followed by heating to decompose or volatilize the solvent after each coat. The slurry may additionally contain a particulate compound of silicon, titanium, zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, tungsten, or other material capable of forming an oxide in situ during the process of volatilib ing or decomposing the solvent. Most frequently, such particulate compound will be a chloride such astitanium trichloride, TiCl or the like.

Alternatively, a slurry of the delafossite, a metal, and an organic solvent such as ethanol, butanol, benzyl, alcohol, phenol, benzene, cumene, polyene, or the like, and a silica compound or metal compound soluble .or dispersible in the organic solvent such as a resinate, may be prepared. Such slurry may be brushed onto the titanium providing, for example, from about 4 to about 8 coats with heating after each coat to decompose or volatilize the organic constituent. Alternatively, methods such as compression bonding or cathodic electrophoresis may be used to apply the coating-of delafossite.

While the above described methods of coating the titanium substrate with delafossite have described the use of a titanium or silicon compound, such compound is not necessary to the functioning of theelectrode. However, the oxidation product of such compound, that is, titanium dioxide or silicon dioxide, does provide additional physical strength and durability to the delafossite surface. Alternatively, oxides of zirconium, hafnium, vanadium, niobium, tantalum, molybdenum, or tungsten may be formed in situ during the formation of the surface to provide a durable electrode surface. The resulting electrode'prepared as described above may be utilized as an electrode for the conduct of electrochemical reactions as hereinbefore described.

The following examples are illustrative.

EXAMPLE I platinum layer between the i They were introduced into a 10 millimeter outside diameter' 5-inch long quartz tube which was evacuated for2 Aslurry was preparedof 60 'balt delafossite (PdCeO l gram-of a 4.5'weight percent and held at 400 C. for 40 minutes hours, heated to a temperature of 110 C. for 1 hour, and thereafter sealed. The sealed quartz tube was heated to a temperature of 850 C. for 15 minutes and then the temperature was decreased to 700 C. Heating continued at about 700 C. for 65 hours. The resulting product was scraped from the quartz wall and ground carefully. It was thereafter leached twice with 200 milliliters of water at 25 C., was washed with acetone, and dried. In this way 2.40 grams of a metallic gray material were obtained. The material had particles in the size range of 0.03 to 0.5 micron and l to 7 microns.

This powder was examined by X-ray difiraction using a Phillips diffractometer. The X-ray tube was operated at 35 kv. and 15 milliamperes, and the detector was operated at 1265 volts. Radiation from a copper target was used. The diffractometer was operated with a 1 divergence slit, a 0.006 inch receiving slit, and a 1 scatter slit. The detector was rotated at 2 two theta per minute with a time constant of 2 seconds. The specimen was rotated at 1 per minute. The X-ray diffraction data shown in Table 6'and FIG. VI were obtained.

TABLE 6 d: I/Io 5.925 40 A titanium coupon 5%- inches by inch by 4; inch was washed with Comet (TM), a household cleanser containing abrasivesand-cleansers,--rinsed in distilled water,

and dipped inone Weight percent hydrofluoric acid for 1 minute. Thereafter the coupon was inserted in 12 normal hydrochloric acid at 27 C. for 23 hours.

'A solution was preparedof 10 grams of Engelhard =O-5X Platinum Resinate (containing 7.5 weight percent platinum calculated as-the metal) and 9 grams of toluene. Three coats of this solution were applied to the etched titanium coupon.- After each of the first two coats the coupon was-heatedat the rate of 50 C. 'per 5 minutes 10 minutes.

0.5 gram of the palladium cotitanium solution (prepared by adding 3.4 grams of titanium trichloride, -TiCl to 20 grams of ethanol), and

1.0 gram ofethanol. Six coats were brushed onto the etched tltaniumcoupon. After each coat the coupon was heated to,l00.C..for 30 minutes. Thereafter, after the last coat, under thepull of a vacuum pump, the coupon was heated to-400 C. at a rate of 100 C. per 10 minutes,

After the final coat the electrode was tested in a beaker chlorate cell. .The beaker chlorate cell was a 500 milli- .1iter beaker having a platinized titanium cathode spaced inch from the anode by a Teflon (TM) spacer and con- ,taining a saturated solution of sodium chloride. The elec- 75 trolysis was' conducted at a current density of 500 amperes per square foot, and acell'voltag'e of 3.50 volts. The

15 chlorine overvoltage was 0.08 volt at 500 amperes per square foot.

The anode was then inserted in a laboratory chlorine diaphragm cell. The cell had an iron mesh cathode spaced inch from the anode, and separated from the anode by asbestos paper. Electrolysis of a brine containing 315 grams per liter of NaCl was conducted at 500 amperes per square foot and a temperature of about 90 C. The initial cell voltage was 3.64, and the initial chlorine overvoltage was 0.08 volt. After 816 hours of electrolysis, the cell voltage was 3.92 volts and the chlorine overvoltage was 0.33 volt.

EXAMPLE II An electrode was prepared havin a Pt Co O delafossite surface on a titanium substrate.

The delafossite was prepared by an anion exchange re action between platinum chloride, cobalt oxide (C00), and cobalt oxide (C0 0 A mixture of 0.9048 gram of C0 0 and 0.1274 gram of CO0 was prepared. The materials were in the form of minus 325 mesh powder and were further ground in a mortar and pestle. The resulting powder was introduced into a 4-inch long, millimeter outside diameter quartz tube. The quartz tube containmg the mixed powders was then evacuated, sealed, and heated to 700 C. for 25 hours. Thereafter a mixed phase of black powder and bright blue crystals was obtained. This material was regrouud and placed in another quartz tube which was evacuated for 3 hours and thereafter heated to 710 C. for 18 hours. After a total of 33 hours of heating between 700 C. and 710 C., the quartz tube was cooled and material recovered. This material was ground in a mortar and pestle and placed in a 150 millil1ter beaker to which were added 100 milliliters of water. The resulting slurry was filtered through a sintered glass filtering funnel at 25 C. The insoluble residue that was obtained was washed twice with 25 milliliter portions of water and then washed with acetone. The resulting mater1al was a gray-black solid. The material was subjected to X-ray diffraction as described in Example I hereinabove, and the X-ray diffraction pattern shown in Table 7 and FIG. VII was obtained.

d TABLE 7 A 5% inch by inch by inch titanium coupon was A slurry was prepared containing 0.25 gram of the platinum cobalt delafrossite (Pto aCOo O minus 325 mesh, prepared above, 0.5 gram of a 4.2 weight percent solution of titanium chloride (TiCl in ethanol and 0.5 gram of ethanol. Five coats of the above slurry were brushed onto the titanium coupon. After each coat was applied the coupon was heated to C. for about V2 hour and then, after the last coat, heated to 500 C. for 30 minutes in an evacuated tube.

. The electrode was first tested in a beaker cell. The beaker cell was a 400 milliliter beaker containing the anode under test and a platinized titanium cathode spaced inch therefrom by a Tefion (TM) spacer. The electrolyte was a saturated solution of sodium chloride containing .315 grams per liter of sodium chloride. After 18 hours in a beaker chlorate cell, the electrode showed a voltage of 3.15 volts at a current density of 250 amperes per square foot and a voltage of 3.80 volts at 500 amperes per square foot. At 500 amperes per square foot the voltage versus a standard calomel electrode was 1.12 volts and the chlorine overvoltage was 0.06 volt.

The electrode was then placed into a laboratory chlorine cell having an iron mesh cathode, separated from the anode under test by asbestos paper. The cathode was spaced inch from the anode. The initial voltage at 500 amperes per square foot and an electrolyte temperature of 90 C. was 3.58 volts and the initial chlorine overvoltage was 0.06 volt. After 104 days of operation the cell voltage was 3.70 volts and the chlorine overvoltage was 0.08 volt.

. EXAMPLE III An electrode was prepared having a platinum cobalt delafossite (PL CQ O surface on a titanium substrate.

Platinum cobalt delafossite was prepared according to the method of anion exchange by first grinding 5.322 grams of platinum chloride (PtCl 1.9622 grams of cobalt oxide (C0 0 and 0.882 gram of cobalt oxide (C00) to minus 325 mesh and then mixing them together. The mixed powder was placed in a 5-inch long by 12 millimeter outside diameter quartz tube which was then evacuated at 25 C. for 2 hours and at C. for 16 hours and then sealed. Thereafter, the tube containing the powder was heated to 700 C. for 70 hours and cooled rapidly to room temperature.

The tube was opened and found to contain a gray metallic solid, blue solid crystals, and a green solid. This product was regrouud to minus 325 mesh, placed in a second 5-inch by 12 millimeter outside diameter quartz tube. The tube was evacuated for 1 hour at 25 C. and 3 hours at C. Thereafter, the quartz tube was sealed and heated to 700 C. for 47 hours. After heating and subsequent cooling to room temperature, the tube was opened and found to contain blue and green crystals in the cool end of the tube and a metallic gray solid material. The metallic gray solid material was ground to minus 325 mesh and leached twice with 25 milliliter portions of distilled water at 25 C. Thereafter the powder was leached twice with acetone and dried at 70 C. The powder was placed under vacuum at 25 C. for 16 hours. The resulting light metallic gray cooled crystalline material was subjected to powder X-ray diffraction as described in Example I hereinabove. The X-ray diffraction pattern is shown in FIG. VEHI. The X-ray diffraction pattern obtained is substantially identical to that previously reported in the literature for Pt Co O delafossite, with only slight differences.

The titanium coupon was etched as described in Example I hereinabove. A slurry of 0.25 gram of the platinum cobalt delafossite, 0.50 gram of a titanium resinate solution containing 4.2 weight percent titanium calculated as the metal, 0:19 gram of toluene, and 0.06 gram of phenol was prepared. Coats 1, 2, 3, and 4 were heated to 425 C. at the rate of 50 C. for 10 minutes. The final coat was heated to 500 C. at the rate of 50 C. for 10 1 minutes and maintained there for 10 minutes.

1 The resulting electrode was tested in a beaker chlorate cell as described in Example I hereinabove. At a current density of 500 amperes per square foot and an electrolyte temperature of 60 C, the chlorine overvoltage was 0.07

volt, and the electrode voltage was 1.125 volts versus a standard calomel electrode.

The anode was then inserted in a laboratory chlorine diaphragm cell of the type described in Example I hereinabove. Electrolysis was conducted as described in Example I hereinabove, at a current density of 500 amperes per square foot. The initial cell voltage was 3.65 volts, and the initial chlorine overvoltage was 0.07 volt. After 744 hours of electrolysis, the cell voltage was 3.85 volts, and the chlorine overvoltage was 0.28 volt.

EXAMPLE IV A palladium rhodium delafossite (PdRhO surface electrode having a titanium substrate was prepared. The delafossite was prepared from lithium rhodate (LiRhO metallic palladium, and palladium chloride (PdClg) by the method of anion exchange.

Lithium rhodate (LiRhOg) was prepared from rhodium oxide (Rh O and lithium carbonate (Li CO The rhodium oxide (Rh was prepared by heating rhodium chloride (RhCl -3H O) at 790 C. in air for 42 hours. The rhodium oxide so prepared was ground. The lithium rhodate was then prepared by mixing 1.0367 grams of rhodium oxide (Rh O and 0.3660 gram of lithium carbonate (Li CO and placing the mixed powder in an alundum boat and heating to 1040 C. for 66 hours. The material so prepared was ground to minus 325 mesh and examined by X-ray diffraction. An X-ray diffraction pattern substantially identical to that reported in the literature for lithium manganate (LiMn0 was obtained.

The delafossite was prepared by first mixing together and grinding 1.0817 grams of lithium rhodate (LiRhOz), 0.4068 gram of metallic palladium, and 0.6672 gram of palladium chloride (PdClg) to minus 325 mesh. This powder was placed in a 9 millimeter outside diameter by 5 inch quartz tube. The tube was evacuated at 25 C. for 1 hour and then at 110 C. for 3 hours.

Thereafter the tube was sealed. The tube containing the mixed powders was heated at 790 C. for 75 hours. The resulting material was reground and placed into another 9 millimeter outside diameter by 5 inch quartz tube. The tube was evacuated and heated at 180 C. for 18 hours and was thereafter sealed. The tube containing the ground material was then heated for 48 hours at 775 C. Thereafter, the resulting product was leached four times in distilled water and then once in acetone.

A 5% inch by Vs inch by /8 inch titanium coupon was cleaned and etched as described in Example I hereinabove. A slurry of 0.2 gram of the palladium-rhodium delafossite (PdRhO 0.4 gram of a titanium resinate containing 4.2, weight percent titanium calculated as the metal, 0.05 gram of phenol, and 0.15 gram of toluene was prepared. Four coats of the slurry were brushed onto the titanium coupon. After application of each of coats 1 through 3, the coupon was heated to 450 C. at the rate of 50 C. for 10 minutes and maintained at 450 C. for 10 minutes. After the last coat, the coupon was heated to 500 C. at a rate of 50 C. for 10 minutes and maintained thereat for 10 minutes.

The resulting electrode having a palladium-rhodium delafossite (PdRhOz) surface on a titanium substrate was tested as an anode in a beaker chlorate cell as described hereinabove. At a current density of 500 amperes per square foot and an electrolyte temperature of 60 C., the resulting electrode had a chlorine overvoltage of 0.07 volt.

The anode was then inserted. in a laboratory chlorine diaphragm cell of the type described in Example I hereinabove. Electrolysis was conducted as described in Example I hereinabove at a current density of 500 amperes per square foot. The initial cell voltage was 3.69 volts and the initial chlorine overvoltage was 0.07 volt. After 1320 hours of electrolysis, the cell voltage was 3.71 volts and the chlorie overvoltage was 0.15 volt.

18 EXAMPLE v An electrode was prepared having a palladiumchromim delafossite (PdCI'Og) surface on a titanium substrate. The delafossite was prepared by anion exchange from palladium metal, palladium chloride, and lithium chromate.

Lithium chromate (LiCrO was prepared by reacting lithium carbonate (Li CO' with chromium oxide (Cr O A powder was prepared by grinding 3.6945 grams of lithium carbonate (Li CO and 7.6010 grams of chromium oxide (Cr- 0 This powder was placed in an alundum boat and heated to 330 C. for 4 hours and then to 930 C. for 12 hours. It was subsequently reground and reheated to 930 C. for 23 hours. The material was then reground again and heated to 1040 C. for 23 hours. The resulting dark green material was ground and subjected to X-ray diffraction examination as described in Example I hereinabove where it exhibited an X-ray diffraction isomorphous with that of LiMnO A powder was then prepared by grinding 1.8190 grams of lithium chromate (LiCrO prepared above, 1.0670 grams of metallic palladium (Pd), and 1.7761 grams of palladium chloride ('PdCl to minus 325 mesh and mixing them together. The powder was placed into a 5 inch by 10 millimeter outside diameter quartz tube. The tube containing the mixed powder was evacuated at 130 C. for 16 hours and thereafter sealed. The sealed tube containing the mixed powders was heated to 800 C. for 46 hours. The sealed tube was then cooled and the resulting product was reground and placed in a second 5 inch by 10 millimeter outside diameter quartz tube. The quartz tube was evacuated for 5 hours at 130 C. and thereafter sealed. The quartz tube containing the material was then heated to 800 C. for 64 hours, providing a total heating of hours at 800 C.

The resulting product was ground to minus 325 mesh and leached with distilled water at 25 C. Thereafter, the product was leached twice at 25 C. and dried.

A 5% inch by inch by inch titanium coupon was cleaned and etched as described in Example I hereinabove.

A solution was prepared containing 10 grams of Engelhard 0-5X Platinum Resinate (containing 7.5 weight percent of platinum calculated as the metal) and 9 grams of toluene to yield a 3.9 weight percent platinum solution' (calculated as the metal). Four coats of the platinum resinate solution so prepared were applied to the titanium coupon. The coupon was heated to 400 C. at the rate of 50 C. per 5 minutes and maintained thereat for 10 minutes. After the last coat, the coupon was heated to 500 C. and maintained thereat for 10 minutes.

Thereafter a slurry was prepared containing 0.2 gram of palladium-chromium delafossite (PdCrO 0.4 gram of titanium resinate (containing 4.2 weight percent titanium calculated as the metal), 0.05 gram of phenol, and 0.15 gram of tolene. Four coats of the slurry were brushed onto the coupon. After each of coats 1, 2, and 3, the electrode was heated at a rate of 50 C. per 10 minutes to a temperature of 450 C. and maintained thereat for 10 minutes. After the last coat the electrode was heated at the rate of 50 C. for 10 minutes to a temperature of 500 C. and maintained thereat for 10 minutes.

The resulting electrode having a palladium-chromium delafossite (PdCrO surface on a titanium substrate was tested as an anode in a beaker chlorate cell as described in Example I hereinabove. At a current density of 500 amperes per square foot the anode had a chloride overvoltage of 0.05 volt.

EXAMPLE VI An electrode was prepared having a platinum-cobalt delafossite (Pt Co O surface on a titanium base member with a platinum undercoaton the base member and a porous titanium dioxide layer on the delafossite surface.

The platinum-cobalt delafossite was prepared by the reaction of platinum chloride (PtClwith cobalt (+2, +3) oxide (C 0 and cobalt (+2) oxide (C00). A minus 325 mesh powder was prepared containing 7.9842 grams of platinum chloride (Ptclz), 3.6123 grams of cobalt (+2, +3) oxide (C0 0 and 1.1241 grams of cobalt +2) oxide (000). This powder was ground and mixed with a mortar and pestle. The powder was placed in a5 inch by millimeter outside diameter quartz tube, heated at 130 C. under the pull of a vacuum for 18 hours, and then sealed.

The sealed quartz tube was heated for 46 hours at 700 C. The material was then removed from the quartz tube, reground, and placed in another 5 inch by 10 millimeter outside diameter quartz tube. The material was heated under the pull of a vacuum pump for 8 hours at 130 C., and then the tube was sealed. The tube containing the material'was heated to 700 C. for 64 hours.

The material was removed from the tube, leached five times with distilled water at 25 C., leached twice with acetone at 25 C., and then dried in air at 70 C. The material was then ground to minus 325 mesh in a mortar and pestle. The resulting powder was then further ground for one hour in a Mixer Mill" (TM) by continuous impact with a boron carbide piston.

The resulting material was examined by a scanning electron microscope and found to have an overall size range of 0.02 to 7.5 microns. The size range distribution was bimodal, with the bulk of the material being from 0.02 to 0.3 micron, and from 1.5 to 6.0 microns. The median size of the fines was 0.10 micron, while the median size of the larger particles was 2.5 microns.

A 5% inch by inch by inch titanium coupon was cleaned and etched as described in Example I hereinabove. A slurry was prepared containing 0.25 gram of the Pt C0 O delafossite prepared above, 0.12 gram of titanium tetrachloride, TiCl, in butanol, 0.75 gram of butanol, and 0.15 gram of phenol. Eight coats of the slurry were applied to the titanium coupon. The coupon .was heated after each coat to 425 C. at the rate of 50 C. per 5 minutes.

Thereafter, a slurry of titanium tetrachloride (TiCl in butanolwas prepared, containing 2.25 weight percent titanium calculated as the metal. Four coats of this slurry were applied to the delafossite surface of the coupon. After each of coats 1, 2, and 3, the coupon was heated to 110 C. for 20 minutes and thence to 425 C. for 10 minutes. After the last coat the coupon was heated to 110 C. for 20 minutes; thence to 500 C. for minutes.

In this wayan electrode is provided having a platinumcobalt delafossite surface on a-titanium base with an external titanium dioxide coating. The resulting electrode was inserted in a beaker chlorate cell and utilized as an anode as described hereinabove. At a current density of 500 amperes per square foot the anode had an anode voltage of 1.175 volts versus a standard calomel electrode and a chlorine overvoltage of 0.12 volt.

EXAMPLE VII An electrode was prepared having a platinum-cobalt delafossite (Pt Co O surface on a titanium substrate and an exterior TiO coating on the delafossite surface.

A 5% inch by inch by inch titanium coupon was cleaned and etched as described in Example I hereinabove. A slurry was then prepared containing 0.5 gram of the platinum-cobalt delafossite (Pt Co O prepared in Example VI hereinabove, 0.25 gram of a solution containing titanium tetrachloride (TiCl in butanol (having 4.5% titanium in titanium metal), 0.75 gram of butanol, and 0.15 gram of butanol. Six coats of the slurry were applied to the coupon. After each coat the coupon was heated to 110 C. for 30 minutes and then to 400 C. for 15 minutes. A solution of titanium tetrachloride in butanol, containing 2.1 Weight ercent titanium calculated 20 as the metal, was prepared. Four coats of this slurry were applied to the delafossite surface of the electrode. After each of the first three coats, the electrode was heated to C. for 20 minutes, then to 425 C. for 10 minutes. After the fourth coat, the electrode was heated to 110 C. for 20 minutes and then to 500 C. for 15 minutes.

The resulting electrode, having a platinum-cobalt delafossite surface in a titanium substrate and a porous titanium dioxide exterior surface, was tested as an anode in a beaker chlorate cell as described in Example I hereinabove. At a current density of 500 amperes per square foot, the anode voltage was 1.160 volts against a calomel electrode, and the chlorine overvoltage was 0.10 volt.

The anode was then inserted in a laboratory diaphragm chlorine cell of the type described in Example I hereinabove. Electrolysis was conducted as described in Example I hereinabove at a current density of 500 amperes per square foot. The initial cell voltage was 3.74 volts, and the initial chlorine overvoltage was 0.10 volt. After 74 days of electrolysis, the cell voltage was 3.94 volts and the chlorine overvoltage was 0.17 volt.

EXAMPLE VIII An electrode was prepared having a platinum-cobalt delafossite (Pt CO O surface on a titanium base, with a ruthenium dioxide (RuO layer between the delafossite surface and the titanium base.

A 5% inch by inch by inch titanium coupon was degreased, cleaned, and etched as described in Example I hereinabove. A solution was prepared containing 1.5 grams of ruthenium trichloride (RuCl -3H O), and 9 grams of ethanol. Five coats of this solution were brushed onto the coupon. After each of the first four coats the couponwas dried in air for 10 minutes and then heated in air to 300 C. for 10 minutes. After the last coat the coupon was dried in air for 10 minutes and then heated in air to 450 C. for 45 minutes.

A slurry was prepared containing 0.5 gram of the platinum cobalt delafossite (Pt C0 O prepared in Example VI hereinabove, 1.0 gram of a solution containing titanium tetrachloride (TiCl in butanol (having 4.2 weight percent titanium calculated as the metal), 0.2 gram of phenol, and one drop of GAF Igepal CO-530 nonyl phenoxy polyoxyethylene ethanol.

Six coats of the slurry were applied to the coupon. After each coat the coupon was heated at 110 C. for 20 minutes; then to 400 C. for 15 minutes. Thereafter, the coupon was coated with a solution of titanium tetrachloride (TiCl in butanol, containing 2.1 weight percent titanium calculated as the metal. One coat of the titanium tetrachloride-butanol solution was applied, and the coupon was heated to 110 C. for 20 minutes; then to 500 C. for 15 minutes. By this procedure an electrode was obtained having a uniform bluish-gray appearance.

The resulting electrode was tested as an anode in a beaker chlorate cell following the procedure described in Example I hereinabove. At a current density of 500 amperes per square foot the anode had an anode voltage of 1.147 volts versus a standard calomel electrode, and a chlorine overvoltage of 0.09 volt.

The anode was then inserted in a laboratory diaphragm chlorine cell of the type described in Example I hereinabove. Electrolysis was conducted as described in Example I hereinabove at a current density of 500 amperes per square foot. The inital cell voltage was 3.60 volts and the initial chlorine overvoltage was 0.09 volt. After 936 hours of electrolysis, the cell voltage was 3.72 volts and the chlorine overvoltage was 0.07 volt.

EXAMPLE IX An anode was prepared having a palladium cobalt delafossite (PdCoO surface on a titanium base member with an intermediate ruthenium dioxide (RuO coating between the base and the delafossite surface.

A 5% inch by /8 inch by /s inch titanium coupon was degreased, cleaned, and etched as described in Example I hereinabove. A solution was then prepared containing 1.5 grams of ruthenium trichloride (RuCl -H O) in 9 grams of ethanol. Four coats of this solution were brushed onto the coupon. After each of the first three coats the coupon was dried in air for 10 minutes and then heated in a furnace, open to the atmosphere, to 300 C. and maintained thereat for 10 minutes. After the fourth coat the coupon was dried in air for 10 minutes and then heated in a furnace, open to the atmosphere, to 450 C. and maintained thereat for 45 minutes.

A slurry was prepared containing 0.20 gram of the palladium-cobalt delafossite (PdCoO prepared in Example I, 0.4 gram of titanium resinate containing 4.2 weight percent titanium calculated as the metal, 0.15 gram of toluene, and 0.05 gram of phenol. Six coats of this slurry were applied to the titanium coupon. After each of the first five coats the coupon was heated to 425 C. at a rate of 50 C. per minutes. After the last coat the coupon was heated to 500 C. at a rate of 50 C. per 5 minutes and maintained thereat for 15 minutes.

In this way, an electrode was prepared having a palladium-cobalt delafossite on a titanium substrate, with a ruthenium dioxide intermediate layer between the delafossite and the titanium. The resulting electrode was utilized as an anode in a beaker chlorate cell according to the procedure described in Example I hereinabove. At a current density of 500 amperes per square foot the anode had an anode voltage of 1.141 volts versus a standard calomel electrode and a chlorine overvoltage of 0.08 volt.

The anode was then inserted in a laboratory diaphragm chlorine cell of the type described in Example I hereinabove. Electrolysis was conducted as described in Example I hereinabove at a current density of 500 amperes per square foot. The initial cell voltage was 3.48 volts and the initial chlorine overvoltage was 0.08 volt. After 576 hours of electrolysis, the cell voltage was 3.49 volts and the chlorine overvoltage 0.07 volt.

EXAMPLE X An electrode is prepared having a palladium delafossite (PdRuO surface on a titanium base.

A 5% inch by inch by A1 inch titanium coupon is degreased, cleaned, and etched as described in Example I hereinabove.

A mixture was prepared containing 0.4055 gram of metallic palladium (Pd), 0.6749 gram of palladium chloride (PdCl), 1.5813 grams of ruthenium trichloride (RuCl) and 4.4528 grams of lanthanum oxide (La O The mixture was thoroughly ground and then placed in a 12 millimeter by 5 inch quartz tube. The tube was heated, under the pull of a vacuum pump. The tube was heated to 25 C. for 4 hours to 70 C. for 16 hours and to 120 C. for 4 hours. The quartz tube was then evacuated and sealed. The sealed quartz tube was then heated to 775 C. and maintained thereat for 70 hours. The tube was then opened, and the material contained therein was removed, reground, and placed in a second quartz tube. The second quartz tube, containing the reground material was evacuated at 25 C. for 3 hours, and at 120 C. for 4 hours.

22 The tube was then sealed. The tube was then heated to 720 C. for 42 hours.

The tube was then allowed to cool, opened, and the material contained therein was removed. The product was leached five times with water, once with 1 normal hydrochloric acid, twice with water, and then once with the acetone. The resulting product is placed in slurry with titanium chloride in butanol and applied to the etched titanium coupon. The resulting electrode has a palladiumruthenium delafossite (PdRuO -titanium dioxide surface on a titanium base.

It is to be understood that although the invention has bene described with specific reference to specific details of particular embodiments thereof, it is not to be so limited since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

1. An anode having a chlorine overvoltage of less than 0.30 volt at a current density of 500 amperes per square foot comprising:

an electroconductive base member chosen from the group consisting of the valve metals and graphite, said base member having a surface thereon consisting essentially of a delafossite chosen from the group consisting of platinum cobalt delafossite, palladium cobalt delafossite, palladium chromium delafossite, and palladium rhodium delafossite.

2. An anode having a chlorine overvoltage of less than 0.30 volt at a current density of 500 amperes per square foot comprising:

an electroconductive base member chosen from the group consisting of the valve metals, said base member having an exterior surface thereon consisting essentially of a delafossite chosen from the group consisting of platinum cobalt delafossite, palladium cobalt delafossite, palladium chromium delafossite and palladium rhodium delafossite; and an intermediate layer between said delafossite surface and said base member consisting essentially of a material chosen from the group comprising ruthenium, rhodium, palladium, platinum, osmium, iridium, ruthenium oxide, rhodium oxide, palladium oxide, platinum oxide, osmium oxide, and iridium oxide.

3. The electrode of claim 1 wherein said base member is titanium.

References Cited UNITED STATES PATENTS 3,502,506 3/1970 Broyde 204290 F 3,414,371 12/1968 Rogers 423-594 3,514,414 5/1970 Shannon 252-518 3,645,862 2/ 1972 Cotton et a1. 204290 F 3,654,188 4/ 1972 Kolb 252520 3,663,414 5/1972 Martinsous et a1. 204290 F FREDERICK C. EDMUNDSON, Primary Examiner US. Cl. X.R.

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