Compositions: ceramic – Ceramic compositions – Refractory
Reexamination Certificate
2000-03-06
2001-07-03
Wood, Elizabeth D. (Department: 1755)
Compositions: ceramic
Ceramic compositions
Refractory
C502S304000
Reexamination Certificate
active
06255242
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a novel zirconium- and cerium-based mixed oxide and to a method of producing the same. The invention further relates to a catalyst material for exhaust gas purification which comprises the oxide.
BACKGROUND OF THE INVENTION
Cerium oxide, which has oxidation-reduction performance, is used for improving the catalytic activity of three-way catalysts comprising at least one platinum group element such as platinum, rhodium or palladium and capable of simultaneously eliminating hydrocarbons(HC), carbon monoxide(CO) and nitrogen oxides(NOx) in the exhaust gas discharged from internal-combustion engines of automobiles, among others.
It is known, however, that catalysts comprising a platinum element and cerium oxide undergo marked reduction in oxidation-reduction performance of cerium oxide and the performance as catalysts readily deteriorate at high temperatures above 800° C.
For maintaining the oxidation-reduction performance of cerium oxide at high temperatures, methods have been proposed which comprise adding an oxide of a rare earth metal other than cerium and/or zirconium oxide to thereby suppress the crystallization of cerium oxide (e.g. Japanese Unexamined patent Publication Nos.58347/1989, 116741/1988). Methods comprising preparing a mixed oxide comprising cerium oxide and zirconium oxide in advance and using the same (e.g. Japanese Unexamined Patent Publication Nos.168544/1987, 281144/1989, 284875/1992).
Further, for increasing the function as a promoter or support(carrier) of a catalyst for exhaust gas purification, methods of improving the thermal stability of specific surface area and the oxidation-reduction performance of a mixed oxide comprising cerium oxide and zirconium oxide have also been disclosed (e.g. Japanese Unexamined Patent Publication Nos.55315/1992, 286772/1993, 278444/1997, and Japanese Patent No.2698302). Today, when advanced measures are being taken to protect the earth environment, the durability temperature with respect to the thermal stability of specific surface area and the deterioration in oxidation-reduction performance of a promoter or catalyst carrier amounts to about 1,000° C. as a result of efforts toward improvements in response to more and more tightened regulations.
With such conventional cerium oxide-zirconium oxide mixed oxides, however, the thermal stability of specific surface area is such that the specific surface area after heat treatment at 1,000° C. is about 20 m
2
/g, and the oxidation-reduction performance is such that even those mixed oxides which have not yet been subjected to heat treatment have an oxygen storage capacity of about 0.1 mmol O
2
/g.
Under certain conditions of driving of automobiles (e.g. 10-mode test(emission test mode)in Japan), the majority of the total emission of unoxidized hydrocarbons is emitted at temperatures lower than the temperature at which the catalyst begins to function. This temperature at which the catalyst begins to function is called T
50
(temperature at which 50% of the maximum exhaust gas purification is attained). An attempt has been made to shorten the time required to heat the catalyst by disposing the catalyst in a position close to the engine. According to this measure, the durability temperature required of the catalyst exceeds 1,000° C. and, in addition, the catalyst is exposed to repeated heating cycles comprising rapid heating and cooling to room temperature. Therefore, it is necessary to develop a novel catalyst material capable of satisfying such requirements.
On the other hand, a generally known method of producing cerium oxide and zirconium oxide in the form of a homogeneously united form comprises adding a base to a cerium ion- and zirconium ion-containing aqueous solution and recovering the resulting mixed salt precipitate (Japanese Unexamined Patent Publication No.278444/1997).
However, since the mixed salt precipitate formed by the above method is a gel-like bulky hydroxide precipitate with a high water content, an extra step of filtration or solid-liquid separation is required for removing impurities. Therefore, the rate of treatment per run is necessarily reduced and, furthermore, an immense quantity of heat energy is required for thermal conversion to oxides. Thus, the conventional method of production cannot be said to be adequate for industrial scale production.
SUMMARY OF THE INVENTION
The present invention, which has been made in view of such problems of the prior art as mentioned above, has a primary object to provide a zirconium- and cerium-based mixed oxide having good thermal stability and oxidation-reduction performance, in particular.
As a result of intensive investigations made by the present inventors, it was found that the mixed oxide obtained by a specific production method has particular characteristics. Based on such and other findings, the present invention has at last been completed.
The present invention is thus concerned with a zirconium- and cerium-based mixed oxide defined below and a method of producing the same as defined below.
1. A zirconium- and cerium-based mixed oxide comprising zirconium and cerium, characterized in
(1) that cubic phase purity of a zirconia- and ceria-based solid solution is not less than 95% by volume of the crystal phase thereof and (2) that the cubic phase purity is not less than 75% by volume even after two repetitions of the step of subjecting the mixed oxide to heat treatment at 1,000° C. and then cooling the same to room temperature (first aspect of the invention).
2. A zirconium- and cerium-based mixed oxide comprising zirconium and cerium, characterized in
(1) that it has an initial oxygen storage capacity of not less than 800 &mgr;mol O
2
/g CeO
2
and (2) that it has an oxygen storage capacity of not less than 70% of the initial oxygen storage capacity even after two repetitions of the step of subjecting the mixed oxide to heat treatment at 1,000° C. and then cooling the same to room temperature (second aspect of the invention).
3. A method of producing zirconium- and cerium-based mixed oxides which comprises admixing basic zirconium sulfate with a cerium ion-containing solution and then adding a base to the resulting mixed solution to thereby cause formation of a precipitate.
The present invention is further concerned with a catalyst material for exhaust gas purification which comprises the zirconium- and cerium-based mixed oxide as defined above under 1 and 2.
REFERENCES:
patent: 4927799 (1990-05-01), Matsumoto et al.
patent: 5723101 (1998-03-01), Cuif
patent: 5747401 (1998-05-01), Cuif
patent: 5908800 (1999-06-01), Bonneau et al.
patent: 5945369 (1999-08-01), Kimura et al.
patent: 5958827 (1999-09-01), Suda et al.
patent: 0605274 (1994-07-01), None
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patent: 2698302 (1998-01-01), None
patent: WO 95/18068 (1995-07-01), None
patent: 9743214 (1997-11-01), None
patent: 9845212 (1998-10-01), None
Japanese Unexamined Patent Publication No. 58347 (1989), No month available.
Japanese Unexamined Patent Publication No. 116741 (1988), No month available.
Japanese Unexamined Patent Publication No. 168544 (1987), No month available.
Japanese Unexamined Patent Publication No. 281144 (1989), No month available.
Japanese Unexamined Patent Publication No. 284875 (1992), No month available.
Japanese Unexamined Patent Publication No. 55315 (1992), No month available.
Japanese Unexamined Patent Publication No. 286772 (1993), No month available.
Japanese Unexamined Patent Publication No. 278444 (1997), No month available.
Nakatani Toshio
Ouchi Kimio
Umemoto Yoshichika
Baker & Botts L.L.P.
Daiichi Kigenso Kagaku Kogyo Co., Ltd.
Wood Elizabeth D.
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