Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Patent
1999-03-01
1999-11-30
Bell, Mark L.
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
502 55, 502 38, 502302, 50252714, 50252716, 502439, 4232132, 106415, 501152, B01J 2500, B01J 2034, B01J 2104, B01J 802, C04B 3550
Patent
active
059942606
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a cerium oxide with pores having a lamellar structure, a process for its preparation and its use in catalysis.
Cerium oxide, alone or in the presence of other metal oxides, is mainly used as a catalyst, in particular for treating automobile exhaust gases.
Good catalytic reactivity requires a cerium oxide with a large specific surface area which does not reduce by too great an extent when the oxide is subjected to high temperatures, for example of the order of 800.degree. C.
In addition to the specific surface area, the pore characteristics can be important for the catalytic properties of a product. Thus the shape and distribution of the pores can have an influence on the contact between phases in the catalysis process. Further, a reduction in surface area with temperature is mainly die to gradual blocking of the pores and this phenomenon can be accelerated, depending on the shape of the pores.
The invention aims to provide a cerium oxide with specific pore characteristics.
The invention also aims to provide a process for producing this cerium oxide.
Thus the cerium oxide of the invention is characterized in that it has pores having a lamellar structure.
The invention also provides a process for preparing a cerium oxide with pores having a lamellar structure by heat treatment of a cerium carbonate, characterized in that a compound constituted essentially by an octahydrated cerium carbonate is heat treated.
Further characteristics, details and advantages of the invention will become more clear from the following description and accompanying drawings in which:
FIG. 1 is an electron microscope photograph of a grain of cerium oxide of the invention;
FIG. 2 is an electron microscope photograph at a higher magnification of a portion of a grain of cerium oxide of the invention.
The term "specific surface area" used in the remainder of the description means the BET specific surface area determined by nitrogen adsorption in accordance with the American standard ASTM D 3663-78 established using the BRUNAUER-EMMETT-TELLER method described in "The Journal of the American Society", 60, 309 (1938).
The pore volume and pore size are determined using well known techniques also employing the BET method. Reference should in particular be made to the publication "Adsorption, Surface Area and Porosity", Gregg and Sing, Academic Press, 1967, pp 160-172 and 174-177.
The principal characteristic of the cerium oxide of the invention is the structure of its pores. These are lamellar in structure. They are in the form of slits or channels with a substantially constant diameter. This is an essential difference over prior art cerium oxides, the pores of which are substantially spherical in structure.
A further characteristic of the oxide of the invention is that its pores can have a homogeneous structure. This means that at least the majority of the pores, more particularly substantially the totality thereof, have the same lamellar or channel structure.
In a additional feature of the oxide of the invention, at least a portion of the pores, more particularly substantially all of the pores, or in other words the lamellae or channels constituted by them, are oriented parallel to each other.
In a variation of the invention, at least a portion of the pores extend throughout the grain thickness. The term "grain" here means the cerium oxide particle obtained after deagglomeration of agglomerates. This arrangement of pores allows passage from one oxide grain surface to the other via the pores.
In a further advantageous variation of the invention, the pore distribution is unimodal. More particularly, the ratio .sigma./m (d.sub.84 -d.sub.16)/2d.sub.50 is at most 0.6, d.sub.n (n=84, 16 or 50) being defined as the diameter at which all of the pores with a diameter greater than that diameter constitute n% of the pore volume. More particularly, the ratio .sigma./m can be at most 0.45 and in some cases, it can be at most 0.3.
The characteristics of the pore structure described above can be clearly seen in FIGS. 1 and 2
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Bonneau Lionel
Ferlin Patrick
Zing Christophe
Bell Mark L.
Hailey Patricia L.
Rhodia Chimie
Shedden John A.
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