Anode for electrolytic evolution of chlorine

Abstract

An electrode suitable for chlorine evolution in electrolysis cells consists of a metal substrate coated with two distinct compositions applied in alternate layers, the former comprising oxides of iridium, ruthenium and valve metals, for instance tantalum, and the latter comprising oxides of iridium, ruthenium and tin. The thus-obtained electrode couples excellent characteristics of anodic potential and selectivity towards the chlorine evolution reaction.

Claims

1. An electrode for evolution of gaseous products in electrolytic cells consisting of a metal substrate coated with a plurality of alternating layers of at least one first catalytic composition and at least a second catalytic composition, said first catalytic composition comprises a mixture of oxides of iridium, of ruthenium and of at least one valve metal and being free of tin, said second catalytic composition comprises a mixture of oxides of iridium, of ruthenium, of tin, niobium and at least one noble metal selected from the group consisting of palladium and platinum, wherein the innermost of said plurality of alternating layers corresponds to said first catalytic composition, wherein the electrode reduces chlorine overvoltage without reduction in oxygen overvoltage, wherein said valve metal of said first catalytic composition is titanium and said oxides of iridium, ruthenium and titanium present in said first catalytic composition are Ru=10-30%, Ir=5-25%, Ti=35-80% atomic percentage referred to the metals, and wherein said second catalytic layer is obtained by applying a solution containing precursors of ruthenium, iridium, tin, niobium, and at least one noble metal selection from the group consisting of palladium and platinum, wherein tin is in the form of tin hydroxyacetochloride such that after thermal treatment said oxides of iridium, of ruthenium and of tin present in said second catalytic composition are Ru=20-60%, Ir=1-20%, Sn=35-65%, atomic percentage referred to the metals, wherein Nb and at least Pd or Pt make up the remainder of the composition's atomic proportions.

2. The electrode according to claim 1 wherein in said first catalytic composition platinum is present in a 0.1-5% atomic percentage referred to the metals.

3. The electrode according to claim 1 wherein in said second catalytic composition platinum or palladium is present in an overall 0.1-10% atomic percentage referred to the metals.

4. The electrode according to claim 1 wherein in said second catalytic composition niobium is present in a 0.1-3% atomic percentage referred to the metals.

5. The electrode according to claim 1 wherein the first catalytic composition comprises oxides of Ru, Ir and Ti and are present in said first catalytic composition in a Ru=16-30%, Ir=9-20%, Ti=50-73% atomic percentage referred to the metals.

6. The electrode according to claim 1 wherein in said second catalytic composition oxides of Ru, Ir, Sn, Nb, and Pd or Pt are present in a Ru=20-30%, Ir=1-10%, Sn=59-65%, and Pd=10% or Pt=5% atomic percentage referred to the metals.

7. An electrolysis cell of alkali chloride solutions comprising the electrode of claim 1 as chlorine-evolving anode.

8. An electrode for evolution of gaseous products in electrolytic cells consisting of a metal substrate coated with a plurality of alternating layers of at least one first catalytic composition and at least a second catalytic composition, said first catalytic composition comprises a mixture of oxides of iridium, of ruthenium, of platinum and of at least one valve metal and being free of tin, said second catalytic composition comprises a mixture of oxides of iridium, of ruthenium, of tin, niobium and at least one noble metal selected from the group consisting of palladium and platinum, wherein the innermost of said plurality of alternating layers corresponds to said first catalytic composition, wherein the electrode reduces chlorine overvoltage without reduction in oxygen overvoltage, wherein said valve metal of said first catalytic composition is titanium and said oxides of iridium, ruthenium, platinum, and titanium present in said first catalytic composition are Ru=10-30%, Ir=5-25%, Ti=35-80% atomic percentage referred to the metals, wherein Pt makes up the remainder of the composition's atomic proportions. and wherein said second catalytic layer is obtained by applying a solution containing precursors of ruthenium, iridium, tin, niobium, and at least one noble metal selection from the group consisting of palladium and platinum, wherein tin is in the form of tin hydroxyacetochloride such that after thermal treatment said oxides of iridium, of ruthenium and of tin present in said second catalytic composition are Ru=20-60%, Ir=1-20%, Sn=35-65%, atomic percentage referred to the metals, wherein Nb and at least Pd or Pt make up the remainder of the composition's atomic proportions.

9. The electrode according to claim 8 wherein in said first catalytic composition platinum is present in a 0.1-5% atomic percentage referred to the metals.

10. The electrode according to claim 8 wherein in said second catalytic composition platinum or palladium is present in an overall 0.1-10% atomic percentage referred to the metals.

11. The electrode according to claim 8 wherein in said second catalytic composition niobium is present in a 0.1-3% atomic percentage referred to the metals.

12. A method for manufacturing the electrode according to claim 1 comprising the execution of the following sequential steps on a metal substrate: a. applying a first solution containing the precursors of the components of said first catalytic composition; b. optionally drying at 50-200° C. for a time of 5 to 60 minutes; c. decomposing said first solution by thermal treatment at 400-850° C. for a time not lower than 3 minutes in the presence of air; d. applying a second solution containing the precursors of the components of said second catalytic composition; e. optionally drying at 50-200° C. for a time of 5 to 60 minutes; f. decomposing said second solution by thermal treatment at 400-850° C. for a time not lower than 3 minutes in the presence of air; and g. optionally repeating steps a-c or the whole sequence of steps a-f once or more times.

13. The method according to claim 12 wherein the sequence consisting of steps a-c and the sequence consisting of steps d-f are reversed.

14. The method according to claim 12 wherein the sequence consisting of steps a-c is repeated more than once before step d, and the sequence of steps d-f is repeated more than once before step g.

Description

EXAMPLE 1

(1) A piece of titanium mesh of 10 cm×10 cm size was blasted with corundum, cleaning the residues with a compressed air jet. The piece was then degreased using acetone in an ultrasonic bath for about 10 minutes. After drying, the piece was dipped in an aqueous solution containing 250 g/l of NaOH and 50 g/l of KNO.sub.3 at about 100° c for approximately 1 hour. After the alkaline treatment, the piece was rinsed three times in deionised water at 60° C., changing the liquid each time. The last rinse was carried out adding a small amount of HCl (about 1 ml per litre of solution). An air drying was then effected and the appearance of a brown hue, due to the growth of a thin TiO.sub.x film, was observed.

(2) 100 ml of a first hydroalcoholic solution, containing RuCl.sub.3*3H.sub.2O, H.sub.2IrCl.sub.6*6H.sub.2O, TiCl.sub.3 in a water and 2-propanol mixture acidified with HCl, having a molar composition of 30% Ru, 20% Ir, 50% Ti referred to the metals were prepared.

(3) 100 ml of a second hydroalcoholic solution containing RuCl.sub.3*3H.sub.2O, H.sub.2IrCl.sub.6*6H.sub.2O, NbCl.sub.5, PdCl.sub.2 and tin hydroxyacetochloride obtained in accordance with the procedure disclosed in Example 3 of WO 2005/014885, in a water and ethanol mixture acidified with HCl, having a molar composition of 20% Ru, 10% Ir, 10% Pd, 59% Sn, 1% Nb referred to the metals were also prepared.

(4) The first solution was applied to the titanium mesh piece by brushing in three coats; after each coat, a drying at 100-110° C. for about 10 minutes was carried out, followed by a thermal treatment of 15 minutes at 450° C. The piece was cooled on air each time before applying the subsequent coat.

(5) The second solution was then applied to the titanium mesh by brushing in three coats, drying and final thermal treatment as for the first solution.

(6) At the end of the whole procedure, an overall noble metal loading of 9 g/m.sup.2 was achieved, expressed as the sum of Ru, Ir and Pd referred to the metals.

(7) The thus obtained electrode was identified as sample #1.

EXAMPLE 2

(8) A piece of titanium mesh of 10 cm×10 cm size was blasted with corundum, cleaning the residues with a compressed air jet. The piece was then degreased using acetone in an ultrasonic bath for about 10 minutes. After drying, the piece was dipped in an aqueous solution containing 250 g/l of NaOH and 50 g/l of KNO.sub.3 at about 100° c for approximately 1 hour. After the alkaline treatment, the piece was rinsed three times in deionised water at 60° C., changing the liquid each time. The last rinse was carried out adding a small amount of HCl (about 1 ml per litre of solution). An air drying was then effected and the appearance of a brown hue, due to the growth of a thin TiO.sub.x film, was observed.

(9) 100 ml of a first hydroalcoholic solution, containing RuCl.sub.3*3H.sub.2O, H.sub.2IrCl.sub.6*6H.sub.2O, Ti(III) ortho-butyl titanate, H.sub.2PtCl.sub.6 in a water and 2-propanol mixture acidified with HCl, having a molar composition of 16.5% Ru, 9% Ir, 1.5% Pt, 73% Ti referred to the metals were then prepared. 100 ml of a second hydroalcoholic solution as that of example 1 were also prepared.

(10) The first solution was applied to the titanium mesh piece by brushing in three coats; after each coat, a drying at 100-110° C. for about 10 minutes was carried out, followed by a thermal treatment of 15 minutes at 450° C. The piece was cooled on air each time before applying the subsequent coat.

(11) The second solution was then applied to the titanium mesh by brushing in three coats, drying and final thermal treatment as for the first solution.

(12) At the end of the whole procedure, an overall noble metal loading of 9 g/m.sup.2 was achieved, expressed as the sum of Ru, Ir and Pt referred to the metals.

(13) The thus obtained electrode was identified as sample #2.

EXAMPLE 3

(14) A piece of titanium mesh of 10 cm×10 cm size was blasted with corundum, cleaning the residues with a compressed air jet. The piece was then degreased using acetone in an ultrasonic bath for about 10 minutes. After drying, the piece was dipped in an aqueous solution containing 250 g/l of NaOH and 50 g/l of KNO.sub.3 at about 100° c for approximately 1 hour. After the alkaline treatment, the piece was rinsed three times in deionised water at 60° C., changing the liquid each time. The last rinse was carried out adding a small amount of HCl (about 1 ml per litre of solution). An air drying was then effected and the appearance of a brown hue, due to the growth of a thin TiO.sub.x film, was observed.

(15) 100 ml of a first hydroalcoholic solution, containing RuCl.sub.3*3H.sub.2O, H.sub.2IrCl.sub.6*6H.sub.2O, TiOCl.sub.2 in a water and 1-butanol mixture acidified with HCl, having a molar composition of 17% Ru, 10% Ir, 73% Ti referred to the metals were then prepared.

(16) 100 ml of a second hydroalcoholic solution containing RuCl.sub.3*3H.sub.2O, H.sub.2IrCl.sub.6*6H.sub.2O, NbCl.sub.5, H.sub.2PtCl.sub.6 and tin hydroxyacetochloride obtained in accordance with the procedure disclosed in Example 3 of WO 2005/014885, in a water and ethanol mixture acidified with acetic acid, having a molar composition of 30% Ru, 3% Ir, 5% Pt, 59% Sn, 3% Nb referred to the metals were also prepared.

(17) The first solution was applied to the titanium mesh piece by brushing in three coats; after each coat, a drying at 100-110° C. for about 10 minutes was carried out, followed by a thermal treatment of 15 minutes at 450° C. The piece was cooled on air each time before applying the subsequent coat.

(18) The second solution was then applied to the titanium mesh by brushing in three coats, drying and final thermal treatment as for the first solution.

(19) At the end of the whole procedure, an overall noble metal loading of 9 g/m.sup.2 was achieved, expressed as the sum of Ru, Ir and Pt referred to the metals.

(20) The thus obtained electrode was identified as sample #3.

EXAMPLE 4

(21) A piece of titanium mesh of 10 cm×10 cm size was blasted with corundum, cleaning the residues with a compressed air jet. The piece was then degreased using acetone in an ultrasonic bath for about 10 minutes. After drying, the piece was dipped in an aqueous solution containing 250 g/l of NaOH and 50 g/l of KNO.sub.3 at about 100° c for approximately 1 hour. After the alkaline treatment, the piece was rinsed three times in deionised water at 60° C., changing the liquid each time. The last rinse was carried out adding a small amount of HCl (about 1 ml per litre of solution). An air drying was then effected and the appearance of a brown hue, due to the growth of a thin TiO.sub.x film, was observed.

(22) 100 ml of a first hydroalcoholic solution, containing RuCl.sub.3*3H.sub.2O, H.sub.2IrCl.sub.6*6H.sub.2O, H.sub.2PtCl.sub.6 and TiCl.sub.3 in a water and 2-propanol mixture acidified with HCl, having a molar composition of 16.5% Ru, 9% Ir, 1.5% Pt, 73% Ti referred to the metals were then prepared.

(23) 100 ml of a second hydroalcoholic solution containing RuCl.sub.3*3H.sub.2O, H.sub.2IrCl.sub.6*6H.sub.2O, NbCl.sub.5, H.sub.2PtCl.sub.6 and tin hydroxyacetochloride obtained in accordance with the procedure disclosed in Example 3 of WO 2005/014885, in a water and 2-propanol mixture acidified with acetic acid, having a molar composition of 30% Ru, 3% Ir, 5% Pt, 59% Sn, 3% Nb referred to the metals were also prepared.

(24) The first solution was applied to the titanium mesh piece by brushing in two coats; after each coat, a drying at 100-110° C. for about 10 minutes was carried out, followed by a thermal treatment of 15 minutes at 450° C. The piece was cooled on air each time before applying the subsequent coat.

(25) The second solution was then applied to the titanium mesh by brushing in three coats, drying and final thermal treatment as for the first solution.

(26) Finally, the first solution was again applied by brushing in two coats, drying and final thermal treatment as above.

(27) At the end of the whole procedure, an overall noble metal loading of 9 g/m.sup.2 was achieved, expressed as the sum of Ru, Ir and Pt referred to the metals.

(28) The thus obtained electrode was identified as sample #4.

COUNTEREXAMPLE 1

(29) A piece of titanium mesh of 10 cm×10 cm size was blasted with corundum, cleaning the residues with a compressed air jet. The piece was then degreased using acetone in an ultrasonic bath for about 10 minutes. After drying, the piece was dipped in an aqueous solution containing 250 g/l of NaOH and 50 g/l of KNO.sub.3 at about 100° c for approximately 1 hour. After the alkaline treatment, the piece was rinsed three times in deionised water at 60° C., changing the liquid each time. The last rinse was carried out adding a small amount of HCl (about 1 ml per litre of solution). An air drying was then effected and the appearance of a brown hue, due to the growth of a thin TiO.sub.x film, was observed.

(30) 100 ml of a first hydroalcoholic solution, containing RuCl.sub.3*3H.sub.2O, H.sub.2IrCl.sub.6*6H.sub.2O, TiCl.sub.3 in a water and 2-propanol mixture acidified with HCl, having a molar composition of 30% Ru, 20% Ir, 50% Ti referred to the metals were prepared.

(31) The solution was applied to the titanium mesh piece by brushing in five coats; after each coat, a drying at 100-110° C. for about 10 minutes was carried out, followed by a thermal treatment of 15 minutes at 450° C. The piece was cooled on air each time before applying the subsequent coat. At the end of the whole procedure, an overall noble metal loading of 9 g/m.sup.2 was achieved, expressed as the sum of Ru and Ir referred to the metals.

(32) The thus obtained electrode was identified as sample #C1.

COUNTEREXAMPLE 2

(33) A piece of titanium mesh of 10 cm×10 cm size was blasted with corundum, cleaning the residues with a compressed air jet. The piece was then degreased using acetone in an ultrasonic bath for about 10 minutes. After drying, the piece was dipped in an aqueous solution containing 250 g/l of NaOH and 50 g/l of KNO.sub.3 at about 100° c for approximately 1 hour. After the alkaline treatment, the piece was rinsed three times in deionised water at 60° C., changing the liquid each time. The last rinse was carried out adding a small amount of HCl (about 1 ml per litre of solution). An air drying was then effected and the appearance of a brown hue, due to the growth of a thin TiO.sub.x film, was observed.

(34) 100 ml of a hydroalcoholic solution containing RuCl.sub.3*3H.sub.2O, H.sub.2IrCl.sub.6*H.sub.2O, NbCl.sub.5, H.sub.2PtCl.sub.6 and tin hydroxyacetochloride obtained in accordance with the procedure disclosed in Example 3 of WO 2005/014885, in a water and 2-propanol mixture acidified with acetic acid, having a molar composition of 30% Ru, 3% Ir, 5% Pt, 59% Sn, 3% Nb referred to the metals were prepared. The solution was applied to the titanium mesh piece by brushing in five coats; after each coat, a drying at 100-110° C. for about 10 minutes was carried out, followed by a thermal treatment of 15 minutes at 450° C. The piece was cooled on air each time before applying the subsequent coat. At the end of the whole procedure, an overall noble metal loading of 9 g/m.sup.2 was achieved, expressed as the sum of Ru, Ir and Pt referred to the metals.

(35) The thus obtained electrode was identified as sample #C2.

EXAMPLE 5

(36) The samples of the previous examples were characterised as anodes for chlorine evolution in a lab cell fed with a sodium chloride brine at 200 g/l concentration, strictly controlling the pH at 3. Table 1 reports chlorine overvoltage measured at a current density of 4 kA/m.sup.2 and the volume percentage of oxygen in product chlorine.

(37) TABLE-US-00001 TABLE 1 Sample ID ηCl.sub.2 (mV) O.sub.2 (%) 1 50 0.25 2 50 0.18 3 49 0.20 4 47 0.17 C1 72 0.25 C2 53 0.80

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

(39) 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 or additives.

(40) The discussion of documents, acts, materials, devices, articles and the like is included in this specification solely for the purpose of providing a context for the present invention. It is not suggested or represented that any or all of these matters formed part of the prior art base or were common general knowledge in the field relevant to the present invention before the priority date of each claim of this application.