초록 ▼

The present invention relates to a process for inducing homogeneous precipitation of a metal oxide, wherein said metal is capable of existing in at least two cationic oxidation states, which process comprises the steps of: i) providing an aqueous solution of a metal in a lower cationic oxidation sta

The present invention relates to a process for inducing homogeneous precipitation of a metal oxide, wherein said metal is capable of existing in at least two cationic oxidation states, which process comprises the steps of: i) providing an aqueous solution of a metal in a lower cationic oxidation state; and ii) adding an oxidant capable of oxidising said metal to a higher cationic oxidation state under conditions such that the mixing of said aqueous solution and said oxidant is substantially complete before precipitation of an oxide of said metal in its higher oxidation state occurs.

대표청구항 ▼

1. A process for inducing homogeneous precipitation of a metal oxide, wherein said metal is capable of existing in at least two cationic oxidation states, which process comprises the steps of:(i) providing an aqueous solution of a metal in a lower cationic oxidation state, and(ii) adding an oxidant

1. A process for inducing homogeneous precipitation of a metal oxide, wherein said metal is capable of existing in at least two cationic oxidation states, which process comprises the steps of:(i) providing an aqueous solution of a metal in a lower cationic oxidation state, and(ii) adding an oxidant capable of oxidizing said metal to a higher cationic oxidation state under conditions such that the mixing of said aqueous solution and said oxidant is substantially complete before precipitation of an oxide of said metal in its higher oxidation state occurs,wherein the rate of oxidation is reduced by cooling the aqueous solution of said metal in a lower cationic oxidation state and/or the oxidant prior to mixing. 2. A process as claimed in claim 1, wherein the aqueous solution of said metal in a lower cationic oxidation state and the oxidant are cooled to a temperature in the range of from −10 to 10° C. prior to mixing. 3. A process as claimed in claim 2, wherein the aqueous solution of said metal in a lower cationic oxidation state and the oxidant are cooled to a temperature in the range of from 0 to 5° C. prior to mixing. 4. A process as claimed in claim 1, wherein the metal oxide which precipitates out of solution is a product of hydrolysis of the metal in its higher cationic oxidation state. 5. A process as claimed in claim 1, wherein the oxidant is added as an aqueous solution. 6. A process as claimed in claim 1, wherein the metal is selected from Ce or Fe. 7. A process as claimed in claim 1, wherein the aqueous solution of said metal in a lower cationic oxidation state comprises nitrate as a counter-ion. 8. A process as claimed in claim 1, wherein the aqueous solution of said metal in a lower cationic oxidation state is of a concentration in the range of from 0.01 to 1.0 mol/1. 9. A process as claimed in claim 1, wherein the aqueous solution of said metal in a lower cationic oxidation state is of a concentration of approximately 0.1 mol/1. 10. A process as claimed in claim 1, wherein the oxidant comprises hydrogen peroxide. 11. A process as claimed in claim 10, wherein the metal salt or oxide has the general formula M(OH) x−y OOH y wherein X is equal to the oxidation state of the metal cation M and y≧1. 12. A process as claimed in claim 10, wherein the metal in its lower oxidation state is Ce 3+ , the metal in its higher oxidation state is Ce 4+ and the metal oxide which precipitates has the general formula Ce(OH) 4−y OOH y wherein y≧1. 13. A process as claimed in claim 1, comprising the additional step of adding hydroxide ions to the reaction mixture so as to substantially complete the precipitation process. 14. A process as claimed in claim 13, wherein said hydroxide ions are provided by the addition of ammonium hydroxide. 15. A process as claimed in claim 1, comprising the further step of isolating the precipitate. 16. A process as claimed in claim 15, comprising the further step of washing and drying the isolated precipitate. 17. A process for the precipitation of a weakly agglomerated nanocrystalline powder of a metal oxide, which process comprises the steps of:(i) inducing homogeneous precipitation of said metal oxide by a process according to claim 13; and(ii) isolating the precipitate. 18. A process as claimed in claim 17, which further comprises the step of subjecting the precipitate to hydrothermal treatment. 19. A process as claimed in claim 17 comprising the further step of washing and drying the precipitate. 20. A process as claimed in claim 18, wherein said hydrothermal treatment is at a temperature of from 100 to 300° C. 21. A process as claimed in claim 18, wherein said hydrothermal treatment is at a temperature of approximately 180° C. 22. A process as claimed in claim 17, wherein said precipitate comprises a weakly agglomerated nanocrystalline powder of a metal oxide having a mean particle size in the range of from 2 to 10 nm with a standard geom etric deviation in the particle size less than or equal to 1.2. 23. A process as claimed in claim 17, wherein said precipitate comprises a weakly agglomerated nanocrystalline powder of a metal oxide having a mean particle size in the range of from 2 to 5 nm with a standard geometric deviation in the particle size less than or equal to 1.1. 24. A process as claimed in claim 23, wherein said metal oxide comprises cerium oxide.