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Catalyst compositions include finely divided nanoscale particles of at least one supported oxide selected from among zirconium oxide, titanium oxide or a mixed zirconium/titanium oxide deposited onto an alumina-based or aluminum-oxyhydroxide-based support, wherein, after calcination for 4 hours at 9

Catalyst compositions include finely divided nanoscale particles of at least one supported oxide selected from among zirconium oxide, titanium oxide or a mixed zirconium/titanium oxide deposited onto an alumina-based or aluminum-oxyhydroxide-based support, wherein, after calcination for 4 hours at 900° C., the at least one support oxide is in the form of nanoscale particles deposited onto the support, the size of said particles being at most 10 nm when the at least one supported oxide is based is zirconium oxide and being at most 15 nm when the at least one supported oxide is titanium oxide or a mixed zirconium/titanium oxide; such catalyst compositions are especially useful for the selective reduction of NOx.

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1. A catalyst composition comprising: (a) an alumina or an aluminum oxyhydroxide support; and(b) finely divided nanoscale particles consisting of supported zirconium oxide or a supported doped zirconium oxide,wherein the nanoscale particles are deposited on the support, andwherein, after calcination

1. A catalyst composition comprising: (a) an alumina or an aluminum oxyhydroxide support; and(b) finely divided nanoscale particles consisting of supported zirconium oxide or a supported doped zirconium oxide,wherein the nanoscale particles are deposited on the support, andwherein, after calcination for 4 hours at 900° C., the nanoscale particles of supported zirconium oxide or supported doped zirconium oxide deposited onto said support have a particle size of which is at most 10 nm. 2. The catalyst composition as defined by claim 1, wherein the alumina or on aluminum oxyhydroxide support is stabilized by at least one stabilizing element selected from the group consisting of rare earths, barium, strontium and silicon. 3. A method of preparing a catalyst composition as defined by claim 2, wherein the support is based on alumina or on aluminum oxyhydroxide stabilized by and/or doped with a stabilizing and/or doping element selected from the group consisting of the rare earths, barium and strontium, comprising the following steps: (a1) providing a liquid mixture comprising: (i) an aluminum compound selected from the group consisting of alumina, aluminum hydroxides and aluminum oxyhydroxides, (ii) a compound of the stabilizing and/or doping element, and (iii) a colloidal dispersion of zirconium oxide, wherein the mixture further comprises an oxide of an element selected from the group consisting of praseodymium, lanthanum, neodymium and yttrium when the finely divided nanoscale particles consist of a supported doped zirconium oxide;(b1) contacting the above mixture with a base, wherein a suspension comprising a precipitate is obtained;(c1) drying the suspension thus obtained; and(d1) calcining the dried product thus obtained. 4. The method as defined by claim 3, wherein the suspension resulting from step (b1) is subjected to a maturation operation before step (c1). 5. A method of preparing a catalyst composition as defined by claim 2, wherein the support is based on alumina or on aluminum oxyhydroxide stabilized by and/or doped with a stabilizing and/or doping element selected from the group consisting of rare earths, barium and strontium, comprising the following steps: (a2) providing a liquid mixture comprising (i) an aluminum compound selected from alumina, aluminum hydroxides and aluminum oxyhydroxides, and (ii) a colloidal dispersion of zirconium oxide, wherein the mixture further comprises (iii) an oxide of an element selected from the group consisting of praseodymium, lanthanum, neodymium and yttrium when the finely divided nanoscale particles consist of a supported doped zirconium oxide;(b2) contacting the above mixture with a base, wherein a suspension comprising a precipitate is obtained;(c2) adding a compound of the stabilizing and/or doping element to the suspension thus obtained;(d2) drying the suspension resulting from the previous step; and(e2) calcining the dried product thus obtained. 6. The method as defined by claim 5, wherein the suspension resulting from step (b2) is subjected to a maturation operation before step (c2). 7. The catalyst composition as defined by claim 1, having a supported oxide content of at most 50% by weight. 8. The catalyst composition as defined by claim 1, wherein the supported doped zirconium oxide consists of zirconium oxide and an oxide of at least one other element selected from the group consisting of praseodymium, lanthanum, neodymium and yttrium. 9. The catalyst composition as defined by claim 1, wherein, after calcination for 4 hours at 900° C., the size of the supported zirconium oxide nanoscale particles is at most 9 nm. 10. The catalyst composition as defined by claim 1, wherein, after calcination for 4 hours at 1,000° C., the size of the supported zirconium oxide nanoscale particles is at most 20 nm. 11. A method of preparing a catalyst composition as defined by claim 1, comprising the following steps: (i) forming a mixture by contacting (a) an aluminum compound selected from the group consisting of alumina, aluminum hydroxides and aluminum oxyhydroxides with (b1) a colloidal dispersion of a zirconium compound, wherein the aluminum compound is also contacted with (b2) a compound of an element selected from the group consisting of praseodymium, lanthanum, neodymium and yttrium, when the finely divided nanoscale particles consist of a supported doped zirconium oxide;(ii) spray-drying the mixture thus formed; and(iii) calcining the dried product thus obtained. 12. A method of preparing a catalyst composition as defined by claim 1, comprising the following steps: (i) providing a liquid mixture comprising: (a) an aluminum compound selected from the group consisting of alumina, aluminum hydroxides and aluminum oxyhydroxides; and (b) at least one zirconium salt, wherein the mixture further comprises (c) a salt of an element selected from the group consisting of praseodymium, lanthanum, neodymium and yttrium, when the finely divided nanoscale particles consist of a supported doped zirconium oxide;(ii) heating the mixture thus formed to a temperature of at least 100° C.;(iii) recovering the precipitate thus obtained; and(iv) calcining said precipitate. 13. A method of preparing a catalyst composition as defined by claim 1, comprising the following steps: (i) providing a liquid mixture comprising: (a) an aluminum compound selected from the group consisting of alumina, aluminum hydroxides and aluminum oxyhydroxides, and (b) a zirconium salt, wherein the mixture further comprises (c) a salt of an element selected from the group consisting of praseodymium, lanthanum, neodymium and yttrium, when the finely divided nanoscale particles consist of a supported doped zirconium oxide;(ii) contacting a base with the above mixture to form a precipitate;(iii) recovering the precipitate thus obtained; and(iv) calcining said precipitate. 14. The method as defined by claim 13, wherein the precipitate obtained after the addition of the base is subjected to a maturation operation. 15. A catalytic system comprising the catalyst composition as defined by claim 1. 16. The catalyst composition as defined by claim 1, comprising at least one supported crystalline oxide.