Anode for electrolytic evolution of chlorine

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.