Anode for oxygen evolution

Abstract

An electrode for electrochemical processes comprises a substrate of titanium or other valve metal, an intermediate protection layer based on valve metal oxides and a catalytic layer based on oxides of tin and of iridium doped with small amounts of oxides of elements selected between bismuth, antimony, tantalum and niobium. The electrode used in electrometallurgical processes, for example in the electrowinning of metals, as anode for anodic oxygen evolution presents a reduced overvoltage and a higher duration.

Claims

1. Electrode suitable for oxygen evolution in electrolytic processes comprising a valve metal substrate, an external catalytic layer and a protective layer consisting of valve metal oxides interposed between the substrate and the catalytic layer, wherein said valve metal oxides of said protective layer consists of titanium and tantalum oxides in an 80:20 molar ratio; said catalytic layer comprises mixed oxides of iridium, of tin and of a doping element M, wherein M is bismuth, said catalytic layer obtained by applying a solution containing precursors of iridium, tin and said doping element M to the valve metal substrate and subsequently decomposing said solution by a thermal treatment in air at a temperature of 480 to 530° C. to obtain an average crystallite size of said mixed oxides lower than 5 nm; the molar ratio Ir:(Ir+Sn) ranges from 0.25 to 0.55; and the molar ratio M:(Ir+Sn+M) ranges from 0.02 to 0.15.

2. The electrode according to claim 1 wherein said molar ratio M:(Ir+Sn+M) ranges from 0.05 to 0.12.

3. The electrode according to claim 1 wherein said molar ratio Ir:(Ir+Sn) ranges from 0.40 to 0.50.

4. The electrode according to claim 1 wherein the average crystallite size of said mixed oxides is lower than 4 nm.

5. The electrode according to claim 1 wherein said valve metal substrate is a solid, punched or expanded sheet or a mesh of titanium or titanium alloy.

6. Process of cathodic electrodeposition of metals from an aqueous solution comprising causing anodic evolution of oxygen on the surface of an electrode according to claim 1.

Description

EXAMPLE 1

(1) A titanium sheet grade 1 of 200×200×3 mm size was degreased with acetone in a ultrasonic bath for 10 minutes and subjected first to sandblasting with corundum grit until obtaining a value of superficial roughness R.sub.z of 40 to 45 μm, then to annealing for 2 hours at 570° C., then to an etching in 27% by weight H.sub.2SO.sub.4 at a temperature of 85° C. for 105 minutes, checking that the resulting weight loss was comprised between 180 and 250 g/m.sup.2.

(2) After drying, a protective layer based on titanium and tantalum oxides at a 80:20 weight ratio was applied to the sheet, with an overall loading of 0.6 g/m.sup.2 referred to the metals (equivalent to 0.87 g/m.sup.2 referred to the oxides). The application of the protective layer was carried out by painting in three coats of a precursor solution—obtained by addition of an aqueous TaCl.sub.5 solution, acidified with HCl, to an aqueous solution of TiCl.sub.4—and subsequent thermal decomposition at 515° C.

(3) A 1.65 M solution of Sn hydroxyacetochloride complex (SnHAC in the following) was prepared according to the procedure disclosed in WO 2005/014885.

(4) A 0.9 M solution of Ir hydroxyacetochloride complex (IrHAC in the following) was prepared by dissolving IrCl.sub.3 in 10% vol. aqueous acetic acid, evaporating the solvent, adding 10% aqueous acetic acid with subsequent solvent evaporation twice more, finally dissolving the product in 10% aqueous acetic acid again to obtain the specified concentration.

(5) A precursor solution containing 50 g/l of bismuth was prepared by cold dissolution of 7.54 g of BiCl.sub.3 under stirring in a beaker containing 60 ml of 10% wt. HCl. Upon completion of the dissolution, once a clear solution was obtained, the volume was brought to 100 ml with 10% wt. HCl.

(6) 10.15 ml of the 1.65 M SnHAC solution, 10 ml of the 0.9 M IrHAC solution and 7.44 ml of the 50 g/l Bi solution were added to a second beaker kept under stirring. The stirring was protracted for 5 more minutes. 10 ml of 10% wt. acetic acid were then added.

(7) The solution was applied by brushing in 7 coats to the previously treated titanium sheet, carrying out a drying step at 60° C. for 15 minutes after each coat and a subsequent decomposition at high temperature for 15 minutes. The high temperature decomposition step was carried out at 480° C. after the first coat, at 500° C. after the second coat, at 520° C. after the subsequent coats.

(8) In this way, a catalytic layer having an Ir:Sn:Bi molar ratio of 33:61:6 and a specific Ir loading of about 10 g/m.sup.2 was applied.

(9) The electrode was identified with the tag “Ir33Sn61 Bi6”.

EXAMPLE 2

(10) A titanium sheet grade 1 of 200×200×3 mm size was pre-treated and provided with a protective layer based on titanium and tantalum oxides in an 80:20 molar ratio as in the previous example.

(11) A precursor solution containing 50 g/l of antimony was prepared by dissolution of 9.4 g of SbCl.sub.3 at 90° C. under stirring, in a beaker containing 20 ml of 37% wt. HCl. Upon completion of the dissolution, once a clear solution was obtained, 50 ml of 20% HCl were added and the solution was allowed to cool down to ambient temperature. The volume was then finally brought to 100 ml with 20% wt. HCl.

(12) 10.15 ml of the 1.65 M SnHAC solution of the previous example, 10 ml of the 0.9 M IrHAC solution of the previous example and 7.44 ml of the 50 g/l Sb solution were added to a second beaker kept under stirring. The stirring was protracted for 5 more minutes. 10 ml of 10% wt. acetic acid were then added.

(13) The solution was applied by brushing in 8 coats to the previously treated titanium sheet, carrying out a drying step at 60° C. for 15 minutes after each coat and a subsequent decomposition at high temperature for 15 minutes. The high temperature decomposition step was carried out at 480° C. after the first coat, at 500° C. after the second coat, at 520° C. after the subsequent coats.

(14) In this way, a catalytic layer having an Ir:Sn:Sb molar ratio of 31:58:11 and a specific Ir loading of about 10 g/m.sup.2 was applied.

(15) The electrode was identified with the tag “Ir31Sn58Sb11”.

COUNTER EXAMPLE 1

(16) A titanium sheet grade 1 of 200×200×3 mm size was pre-treated and provided with a protective layer based on titanium and tantalum oxides in an 80:20 molar ratio as in the previous examples.

(17) 10.15 ml of the 1.65 M SnHAC solution of the previous examples and 10 ml of the 0.9 M IrHAC solution of the previous examples were added to a beaker kept under stirring.

(18) The solution was applied by brushing in 8 coats to the previously treated titanium sheet, carrying out a drying step at 60° C. for 15 minutes after each coat and a subsequent decomposition at high temperature for 15 minutes. The high temperature decomposition step was carried out at 480° C. after the first coat, at 500° C. after the second coat, at 520° C. after the subsequent coats.

(19) In this way, a catalytic layer having an Ir:Sn molar ratio of 35:65 and a specific Ir loading of about 10 g/m.sup.2 was applied.

(20) The electrode was identified with the tag “Ir35Sn65”.

COUNTER EXAMPLE 2

(21) A titanium sheet grade 1 of 200×200×3 mm size was pre-treated and provided with a protective layer based on titanium and tantalum oxides in an 80:20 molar ratio as in the previous examples. 10.15 ml of 1.65 M SnHAC solution and 10 ml of 0.9 M IrHAC solution were added to a beaker kept under stirring as in the previous example.

(22) The solution was applied by brushing in 8 coats to the previously treated titanium sheet, carrying out a drying step at 60° C. for 15 minutes after each coat and a subsequent decomposition at 480° C. for 15 minutes.

(23) In this way, a catalytic layer having an Ir:Sn molar ratio of 35:65 and a specific Ir loading of about 10 g/m.sup.2 was applied.

(24) The electrode was identified with the tag “Ir35Sn65 LT”.

EXAMPLE 3

(25) Coupons of 20 mm×60 mm size were obtained from the electrodes of the preceding examples and counterexamples and subjected to anodic potential determination under oxygen evolution, measured by means of a Luggin capillary and a platinum probe as known in the art, in a 150 g/l H.sub.2SO.sub.4 aqueous solution at a temperature of 50° C. The data reported in table 1 (SEP) represent the values of potential difference at a current density of 300 A/m.sup.2 with respect to a PbAg reference electrode. Table 1 moreover reports the crystallite average size detected via X-ray diffraction (XRD) technique and the lifetime observed in an accelerated life test in a 150 g/l H.sub.2SO.sub.4 aqueous solution, at a current density of 60 A/m.sup.2 and at a temperature of 50° C.

(26) The results of these tests demonstrate how the addition of doping amounts of bismuth or antimony to a tin and iridium oxide-based coating allows combining an excellent oxygen evolution potential, typical of tin/iridium based formulations obtained at reduced decomposition temperature, with the optimal duration shown by tin/iridium oxide-based formulations obtained at high decomposition temperature.

(27) The tests were repeated, obtaining equivalent results, varying the amount of bismuth and antimony in the molar range 2-15% referred to the metals: the best results were observed, both for bismuth and for antimony or for a combination of the two, in the molar range 5-12% referred to the metals.

(28) Almost equivalent results were obtained by addition of amounts of niobium or tantalum in the same concentration ranges.

(29) TABLE-US-00001 TABLE 1 Average SEP Deactivation time in crystallite (mV vs. PbAg) 150 g/l H.sub.2SO.sub.4 @60 kA/m.sup.2, Electrode size (nm) @300 A/m.sup.2 50° C. Ir33Sn61Bi6 3.5 −460 900 Ir31Sn58Sb11 3.7 −440 870 Ir35Sn65 5.9 −405 880 Ir35Sn65 LT 4.1 −430 340

EXAMPLE 4

(30) The accelerated duration test of the previous table was repeated at the same conditions on equivalent coupons obtained from the same electrodes, upon addition of potassium fluoride (1 mg/l or 5 mg/l di F.sup.−) or of MnCl.sub.2 (20 g/l of Mn.sup.++), giving the results reported in table 2, indicating a tolerance higher than expected for the electrode samples in accordance with the invention.

(31) TABLE-US-00002 TABLE 2 Deactivation Deactivation Deactivation time in time in time in 150 g/l H.sub.2SO.sub.4 + 150 g/l H.sub.2SO.sub.4 + 150 g/l H.sub.2SO.sub.4 + 1 mg/l 5 mg/l 20 g/l Electrode F.sup.− F.sup.− Mn.sup.++ Ir33Sn61Bi6 730 370 860 Ir31Sn58Sb11 645 350 860 Ir35Sn65 650 360 850 Ir35Sn65 LT 265 105 310

(32) The previous description shall not be intended as limiting the invention, which may be used according to different embodiments without departing from the scopes thereof, and whose extent is solely defined by the appended claims.

(33) Throughout the description and claims of the present application, the term “comprise” and variations thereof such as “comprising” and “comprises” are not intended to exclude the presence of other elements, components or additional process steps.