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The present invention relates generally to catalysts comprising ruthenium oxide and to processes for catalyzing the oxidation and conversion of sulfur dioxide (SO2) to sulfur trioxide (SO3) using such catalysts. SO2 at low concentrations in process gas streams can be effectively oxidized to SO3 at r

The present invention relates generally to catalysts comprising ruthenium oxide and to processes for catalyzing the oxidation and conversion of sulfur dioxide (SO2) to sulfur trioxide (SO3) using such catalysts. SO2 at low concentrations in process gas streams can be effectively oxidized to SO3 at relatively low temperatures using the ruthenium oxide catalysts of the present invention. In one application, the ruthenium oxide catalysts are used in the final contact stage for conversion of SO2 to SO3 in multiple stage catalytic converters used in sulfuric acid manufacture.

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What is claimed is: 1. A process for the catalytic oxidation of sulfur dioxide to sulfur trioxide, the process comprising contacting a feed gas mixture comprising sulfur dioxide and oxygen with an oxidation catalyst comprising an active phase comprising ruthenium oxide thereby producing a conversio

What is claimed is: 1. A process for the catalytic oxidation of sulfur dioxide to sulfur trioxide, the process comprising contacting a feed gas mixture comprising sulfur dioxide and oxygen with an oxidation catalyst comprising an active phase comprising ruthenium oxide thereby producing a conversion gas comprising sulfur trioxide, wherein the temperature of the gas in contact with the ruthenium oxide oxidation catalyst is no greater than about 400° C. 2. The process as set forth in claim 1 wherein the oxidation catalyst comprises an acid-resistant support having the active phase thereon. 3. The process as set forth in claim 2 wherein the acid-resistant support is in the form of a powder, pellet, ring, sphere or foraminous monolith. 4. The process as set forth in claim 2 wherein the acid-resistant support is in the form of a foraminous monolith. 5. The process as set forth in claim 2 wherein the active phase comprises ruthenium oxide having an average crystallite size of less than about 150 Å. 6. The process as set forth in claim 5 wherein the active phase comprises ruthenium oxide crystallites ranging in size from about 30 Å to about 100 Å. 7. The process as set forth in claim 6 wherein the active phase comprises ruthenium oxide crystallites ranging in size from about 50 Å to about 80 Å. 8. The process as set forth in claim 2 wherein the ruthenium oxide active phase comprises ruthenium oxide and a promoter comprising a further metal oxide having a metal oxidation state of +4 or +3. 9. The process as set forth in claim 2 wherein the sulfur dioxide content of the feed gas mixture is no greater than about 2% by volume. 10. The process as set forth in claim 9 wherein the temperature of the gas in contact with the oxidation catalyst is from about 325° C. to about 400° C. 11. The process as set forth in claim 10 wherein the temperature of the gas in contact with the oxidation catalyst is from about 350° C. to about 375° C. 12. The process as set forth in claim 9 wherein the process is essentially adiabatic and the heat and sulfur dioxide content of the feed gas mixture are such that the exothermic heat of oxidation of sulfur dioxide to sulfur trioxide does not increase the temperature of the gas in contact with the oxidation catalyst above about 400° C. 13. The process as set forth in claim 12 wherein the sulfur dioxide content of the feed gas mixture is no greater than about 1% by volume. 14. The process as set forth in claim 13 wherein the average conversion of sulfur dioxide to sulfur trioxide is at least about 90%. 15. The process as set forth in claim 14 wherein the sulfur dioxide content of the conversion gas is less than about 50 ppmv. 16. A process for making sulfuric acid and/or oleum from a source gas comprising sulfur dioxide, the process comprising: forming a converter feed gas mixture by combining the source gas with an oxygen source; introducing the converter feed gas mixture into a catalytic converter comprising a plurality of catalyst stages in series, each catalyst stage containing an oxidation catalyst effective for oxidizing sulfur dioxide to sulfur trioxide, thereby contacting the converter feed gas mixture with the oxidation catalyst contained in at least the first catalyst stage in said series to form a partial conversion gas comprising sulfur trioxide and residual sulfur dioxide and oxygen; passing the partial conversion gas through at least one further catalyst stage in said series, the oxidation catalyst contained therein comprising an active phase comprising ruthenium oxide, thereby oxidizing residual sulfur dioxide in the partial conversion gas to sulfur trioxide and forming a conversion gas comprising sulfur trioxide and residual sulfur dioxide; and contacting the conversion gas with an aqueous solution comprising sulfuric acid for absorption of sulfur trioxide therefrom in a sulfur trioxide absorption zone to produce additional sulfuric acid and/or oleum and a sulfur trioxide-depleted gas comprising sulfur dioxide. 17. The process as set forth in claim 16 wherein the ruthenium oxide oxidation catalyst comprises an acid-resistant support having the active phase thereon. 18. The process as set forth in claim 17 wherein the acid-resistant support is in the form of a powder, pellet, ring, sphere, or foraminous monolith. 19. The process as set forth in claim 17 wherein the acid-resistant support is in the form of a foraminous monolith. 20. The process as set forth in claim 17 wherein the active phase comprises ruthenium oxide having an average crystallite size of less than about 150 Å. 21. The process as set forth in claim 20 wherein the active phase comprises ruthenium oxide crystallites ranging in size from about 30 Å to about 100 Å. 22. The process as set forth in claim 21 wherein the active phase comprises ruthenium oxide crystallites ranging in size from about 50 Å to about 80 Å. 23. The process as set forth in claim 17 wherein the active phase comprises ruthenium oxide and a promoter comprising a further metal oxide having a metal oxidation state of +4 or +3. 24. The process as set forth in claim 17 wherein the sulfur dioxide content of the partial conversion gas is no greater than about 2% by volume and the temperature of the gas in contact with the ruthenium oxide oxidation catalyst is no greater than about 400° C. 25. The process as set forth in claim 24 wherein the temperature of the gas in contact with the ruthenium oxide oxidation catalyst is from about 325° C. to about 400° C. 26. The process as set forth in claim 25 wherein the temperature of the gas in contact with the ruthenium oxide oxidation catalyst is from about 350° C. to about 375° C. 27. The process as set forth in claim 24 wherein the oxidation of sulfur dioxide to sulfur trioxide in the partial conversion gas is essentially adiabatic and the heat and sulfur dioxide content of the partial conversion gas contacting the ruthenium oxide oxidation catalyst are such that the exothermic heat of oxidation of sulfur dioxide to sulfur trioxide does not increase the temperature of the gas in contact with the ruthenium oxide oxidation catalyst above about 400° C. 28. The process as set forth in claim 27 wherein the sulfur dioxide content of the partial conversion gas is no greater than about 1% by volume. 29. The process as set forth in claim 28 wherein the average conversion of sulfur dioxide to sulfur trioxide in the partial conversion gas is at least about 90%. 30. The process as set forth in claim 29 wherein the sulfur dioxide content of the conversion gas is less than about 50 ppmv. 31. The process as set forth in claim 24 wherein before contacting the ruthenium oxide oxidation catalyst, the partial conversion gas is contacted with an aqueous solution comprising sulfuric acid for absorption of sulfur trioxide therefrom in a sulfur trioxide absorption zone to produce additional sulfuric acid and/or oleum and a partial conversion gas depleted in sulfur trioxide content. 32. The process as set forth in claim 31 wherein said partial conversion gas is formed by passing said converter feed gas mixture through at least the first two catalyst stages in said series of catalyst stages. 33. The process as set forth in claim 32 wherein said partial conversion gas is formed by passing said converter feed gas mixture through at least the first three catalyst stages in said series of catalyst stages. 34. The process as set forth in claim 31 wherein the ruthenium oxide oxidation catalyst is contained within the last catalyst stage in said series of catalyst stages. 35. A process for the preparation of an oxidation catalyst, the process comprising: combining a ruthenium salt solution and an acid-resistant support to form a slurry; adding a base to the slurry to form a catalyst precursor comprising a ruthenium oxide coating on the surface of the support; and heat treating the catalyst precursor at a first temperature of from about 200° C. to about 350° C. for from about 0.1 hour to about 5 hours, and at a second temperature from about 50° C. to about 500° C. for from abut 0.1 hour to about 5 hours to convert the catalyst precursor to the oxidation catalyst comprising the support and an active phase thereon comprising ruthenium oxide. 36. The process as set forth in claim 35 wherein the active phase comprises ruthenium oxide having an average crystallite size less than about 150 Å. 37. The process as set forth in claim 35 wherein the base is ammonium hydroxide. 38. The process as set forth in claim 35 wherein the ruthenium salt is ruthenium chloride hydrate, ruthenium nitrosyl nitrate hydrate, or ruthenium nitrate hydrate.