Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1999-06-03
2001-05-08
Griffin, Steven P. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S303000, C502S302000, C423S239100
Reexamination Certificate
active
06228799
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a composition based on cerium oxide and on zirconium oxide with a high specific surface and with a high oxygen storage capacity, to its process of preparation and to its use in catalysis, in particular for automobile catalysis.
BACKGROUND OF THE INVENTION
So-called multifunctional catalysts are currently used for the treatment of exhaust gases from internal combustion engines (automobile afterburning catalysis). Multifunctional catalyst is understood to mean catalysts capable of carrying out not only oxidation, in particular of carbon monoxide and hydrocarbons present in exhaust gases, but also reduction, in particular of nitrogen oxides also present in these gases (“three-way” catalysts). Zirconium oxide and cerium oxide today appear as two constituents which are particularly important and advantageous for this type of catalyst.
To be effective, these catalysts must first of all exhibit a high specific surface, even at high temperature. In addition, it is known that cerium makes possible a buffering power with respect to variations in the oxygen content of the gas mixture to be treated and thus makes it possible to improve the performance of the catalyst with respect to the three main pollutants, namely CO, HC and NOx. This buffering power is evaluated by the capacity to store oxygen in an oxidizing environment and to restore it in a reducing environment. However, this oxygen storage capacity decreases so much after exposure to high temperatures that the degree of conversion of the abovementioned pollutants may become insufficient.
For this reason, there exists a need for catalysts capable of being able to be used at high temperature and, in order to do so, exhibiting high stability of their specific surface combined, if possible, with stability of their oxygen storage capacity.
SUMMARY OF THE INVENTION
The subject of the invention is thus the development of a catalytic composition which can meet this need.
The composition according to a first embodiment of the invention is based on a cerium oxide and on a zirconium oxide in a cerium/zirconium atomic proportion of at least 1 and it is characterized in that it exhibits, after calcination for 6 hours at 900° C., a specific surface of at least 35 m
2
/g and an oxygen storage capacity, measured at 400° C., of at least 1.5 ml of O
2
/g.
According to a second embodiment of the invention, the composition is based on a cerium oxide, on a zirconium oxide and on an yttrium oxide in a cerium/zirconium atomic proportion of at least 1 and it is characterized in that it exhibits, after calcination for 6 hours at 900° C., a specific surface of at least 35 m
2
/g and an oxygen storage capacity, measured at 400° C., of at least 1.5 ml of O
2
/g.
According to a third embodiment of the invention, the composition is based, on the one hand, on a cerium oxide and on a zirconium oxide in a cerium/zirconium atomic proportion of at least 1 and, on the other hand, on at least one oxide chosen from scandium oxide and rare-earth metal oxides with the exception of cerium oxide and it is characterized in that it exhibits, after calcination for 6 hours at 900° C., a specific surface of at least 35 m
2
/g.
In addition, the process for the preparation of the compositions according to the invention is of the type in which a mixture is prepared in liquid medium containing a cerium compound, a zirconium compound and, if appropriate, an yttrium, scandium or rare-earth metal compound; the said mixture is heated; the precipitate obtained is recovered and this precipitate is calcined, and it is characterized in that the abovementioned mixture is prepared by using a zirconium solution which is such that the amount of base necessary to reach the equivalent point during an acid/base titration of this solution confirms the condition that, as a molar ratio, OH
−
/Zr≦1.65.
Other characteristics, details and advantages of the invention will become still more fully apparent on reading the following description, as well as the various concrete but non-limiting examples intended to illustrate it.
REFERENCES:
patent: 5693299 (1997-12-01), Chopin et al.
patent: 5712218 (1998-01-01), Chopin et al.
patent: 0605274 (1994-07-01), None
patent: 0628515 (1994-12-01), None
patent: 0629438 (1994-12-01), None
Aubert Maryline
Birchem Thierry
Blanchard Gilbert
Burns Doane Swecker & Mathis L.L.P.
Griffin Steven P.
Rhodia Chimie
Strickland Jonas N.
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