Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1999-07-29
2001-03-20
Dunn, Tom (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C423S263000, C423S593100, C423S608000, C501S103000, C501S152000, C502S303000
Reexamination Certificate
active
06204219
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to novel compositions based on mixed oxides of cerium oxide and at least one non-noble metal oxide. The novel oxide compositions exhibit a high specific surface area and excellent heat resistance.
In particular, this invention relates to a process for the preparation of the oxide compositions, and to the use thereof as catalyst and/or catalyst supports for the purification and/or conversion of exhaust gases from internal combustion engines.
BACKGROUND OF THE PRIOR ART
Cerium oxide and zirconium oxide are known compounds that are particularly useful constituents, either alone or in combination, in a wide variety of catalyst compositions, e.g., multifunctional catalyst compositions, especially catalysts suited for the treatment or conversion of exhaust gases emanating from internal combustion engines. By “multi-functional” is intended a catalyst capable of effecting not only the oxidation, in particular, of carbon monoxide and of hydrocarbons present in the exhaust gas, but also the reduction of the oxides of nitrogen also present in such gas (“three-way” catalysts).
To meet stringent air emissions regulations, catalysts are being placed closer and closer to the engine thus subjecting them to higher temperatures. In order to maintain their effectiveness, they should not sinter and loose surface area. Consequently, there exists a need for catalysts that maintain high surface area even after prolonged exposure to temperatures of 900° C.
Further, when a catalyst structure consists of a mixture of various catalytic components, such as ceria and zirconia, it follows that a more intimate mixture of the components will result in a more effective catalyst structure.
Attempts to prepare mixed oxides of cerium and zirconium having a high and thermally stable surface area are known. For example, U.S. Pat. No. 5,717,218 discloses a thermally-stable, high surface area ceria-zirconia mixed oxide having a pure monophasic CeO
2
crystalline habit. The mixed oxides are prepared by subjecting a mixture of oxide solutions to thermohydrolysis, preferably in a nitrogen atmosphere and under pressure, to form the desired oxide. This process is undesirable since it is time consuming and requires expensive equipment such as high pressure reactors.
U.S. Pat. No. 5,532,198 also discloses a process of preparing a high surface area cerium/zirconium mixed oxide. The oxides are prepared by admixing a zirconium sol with a cerium sol, spray drying the admixture and calcining the dried material. The process requires that the ratio of the mean size of the zirconium sol particle to the mean size of the cerium sol be within a specified range in order to obtain a product having a sufficiently high surface area.
It has also been proposed in Japanese Patent Application No. (Kokai) 55,315/1992 to prepare fine powders of cerium oxide and zirconium oxide having a high specific surface area and excellent heat resistance by a coprecipitation process. The powders are prepared by mixing a water-soluble zirconium salt with a water soluble salt of cerium (III) or cerium (IV) to form a mixed salt solution, and thereafter treating the salt solution with excess base to precipitate a mixed oxide powder. Where the cerium salt is a cerium (III) salt, the Japanese reference teaches adding hydrogen peroxide to the salt solution simultaneous or subsequently with the precipitation step to oxidize the trivalent cerium to the tetravelant state. The highest surface area reported for cerium/zirconium powders produced in accordance with the process disclosed in this reference was only 26.5 m
2
/g.
SUMMARY OF THE INVENTION
To overcome the deficiencies hereto associated with prior processes and mixed oxide compositions, the present invention provides a simple, economical and novel process for the preparation of compositions based on mixed oxides of cerium and other non-noble metals. The process provides high surface area oxide compositions via a coprecipitation method without the need for expensive high pressure techniques.
The process of the present invention includes the steps of treating a homogeneous, aqueous solution of a cerium IV salt and at least one non-noble metal salt with an aqueous hydrogen peroxide solution. The peroxide treated solution is thereafter coprecipitated with an excess of a base. The precipitate is filtered, washed and spray dried to form a powder. The dried powder is calcined to convert the resulting hydroxide particles to particles of cerium oxide and at least one non-noble metal oxide.
The present invention also provides improved mixed oxide compositions for promoting oxidation formed by the process above. The oxide composition possesses a specific surface area of greater than 100 m
2
/g and maintains an increased surface area after prolonged exposure to thermal conditions. Advantageously, the oxide compositions of the invention are intimately mixed as exhibited by X-ray diffraction techniques.
The process and compositions of the present invention provide a catalyst/catalyst support having outstanding durability under harsh conditions at elevated temperatures. The catalyst/catalyst supports produced in accordance with the present invention have a decreased loss in surface area under thermal conditions when compared to catalyst/catalyst supports prepared using oxide powders produced by earlier processes.
DETAILED DESCRIPTION OF THE INVENTION
The process for the preparation of the mixed oxide composition according to the invention will now be more fully described.
As indicated above, the first stage of the process of the invention entails preparing a mixture comprising a homogeneous, aqueous salt solution of a cerium IV salt and at least one non-noble metal salt. The aqueous solution is obtained by preparing individual solutions of a water-soluble cerium IV salt and the desired non-noble metal salts and then mixing, in any order, said solutions.
Salts useful in the process of the present invention herein include any aqueous soluble salt of cerium IV and the desired non-noble metals. Suitable salts include, but are not limited to, nitrates and sulfates. The nitrates are preferred herein. Aqueous nitrate solutions can be obtained, for example, by reacting nitric acid with the suitable hydrated compound, e.g. cerium (IV) hydroxide.
Cerium (IV) salts useful in the process of the invention may contain, without disadvantage, a small amount of cerium in the cerous state. Preferably, the cerium IV salts contain at least 85% of cerium IV, most preferably greater than 95% of cerium IV.
Suitable non-noble metal salts useful in the invention include, without limitation, salts of transition metals, rare earth metals, and combinations thereof. In a preferred embodiment of the invention, the non-noble metal salt is a salt having a non-noble metal component selected from the group consisting of zirconium and a rare earth metal. In a more preferred embodiment of the invention, the non-noble metal salt is zirconium salt used alone or in combination with a salt of lanthanum, yttrium, praeseodymium, or mixtures thereof.
The amount of cerium, zirconium and rare earth metal salts used to prepare the salt solution useful in the process of the invention will vary depending upon the desired concentration of oxides in the final mixed oxide composition. Generally, the amount of cerium, zirconium and rare earth metal salt will correspond to the stoichiometric proportions of mixed oxides in the final compositions.
In a second stage of the process according to the invention, the salt solution is treated with an aqueous solution of hydrogen peroxide. In general, hydrogen peroxide is added in an amount equal to the weight of oxides desired in the final oxide composition. Preferably the amount of hydrogen peroxide is at least 2.5 moles per molar equivalent of cerium in the salt solution; most preferably the amount of hydrogen peroxide added to the aqueous solution is about 2.5 to about 4.5 moles per molar equivalent of cerium in the salt solution.
The peroxide treated salt solution is thereafter
Brezny Rasto
Koranne Manoj M.
Artale Beverly J.
Dunn Tom
Maggio Robert A.
Nguyen Cam N.
W. R. Grace & Co.,-Conn.
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