Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2001-10-24
2003-01-14
Silverman, Stanley S. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S302000, C502S304000, C502S305000, C502S324000, C502S325000, C502S340000, C502S349000, C502S353000, C502S355000
Reexamination Certificate
active
06506705
ABSTRACT:
The present invention relates to a composition based on cerium oxide or on cerium and zirconium oxides, in the extruded form, to a process for the preparation thereof and to the use thereof as catalyst.
Compositions based on cerium oxide or on mixtures of cerium oxide and of zirconium oxide are well known. They are used in particular as catalyst or catalyst support, in particular for the catalysis of automobile afterburning. These compositions are generally employed in a coating technique, that is to say by mixing them with a binding oxide, such as alumina or silica, and by depositing the mixture obtained, in the form of a layer, on a support. However, the binding oxide can result either in a rapid deactivation of the compositions or in a loss in their selectivity.
On account of this disadvantage, it would be advantageous to be able to obtain these compositions directly in the extruded form. To the knowledge of the Applicant Company, these compositions have never been able to be obtained until now in this form.
The object of the invention is thus to provide these compositions in the form of extrudates.
The Applicant Company has discovered that the use of a specific starting material made it possible to solve this problem.
The invention thus relates to a process for the preparation of a composition based on cerium oxide or on cerium and zirconium oxides which is characterized in that a product is extruded which is based on a cerium hydroxide or oxyhydroxide or on cerium and zirconium hydroxides or oxyhydroxides.
The invention also covers a composition based on cerium oxide or on cerium and zirconium oxides, characterized in that it is provided in the extruded form.
Finally, the invention relates to the use of a composition of the above type as catalyst or catalyst support, in particular in the treatment of exhaust gases from internal combustion engines, in the process for the dehydrogenation of ethylbenzene to styrene and in the catalysis of methanation.
Other characteristics, details and advantages of the invention will become even more fully apparent on reading the following description and various concrete but nonlimiting examples intended to illustrate it.
The product of the invention will first of all be described.
The essential characteristic of the compositions of the invention is their shape.
This is because they are provided in the form of extrudates. Extrudate is understood to mean any object obtained by ejection, under pressure, of a paste through nozzles or dies of chosen shapes. The objects thus obtained can exhibit varied shapes. They can, for example, exhibit cylindrical or semi-cylindrical, square or polygonal cross-sections or alternatively cross-sections in the form of lobes, such as trilobes. The objects can be solid or hollow. They can have the form of a monolith, honeycomb or cylinder, for example.
The compositions of the invention are based on cerium oxide or on cerium and zirconium oxides. This is understood to mean that the cerium oxide or the cerium oxide in combination with the zirconium oxide represent at least 50% by weight of the entire composition. They can be composed essentially or solely of cerium and zirconium oxides with, in addition, if appropriate, one or more additives of the type which will be described hereinbelow.
In the case of compositions based on cerium and zirconium oxides, the respective proportions of cerium and of zirconium can vary within wide limits. More particularly, this proportion, expressed by the Zr/Ce atomic ratio, can vary between 1/20 and 20/1, more particularly between 1/9 and 9/1.
The compositions of the invention can comprise, in addition to cerium and zirconium, additives. These additives will be chosen from those known for improving the catalytic properties of cerium or of zirconium. Use may thus be made of additives for stabilizing the specific surface of these compositions or those known for increasing their oxygen storage capacity.
Mention may be made, as additives, of those belonging to the group consisting of aluminium, silicon, thorium, titanium, niobium, tantalum and rare-earth metals.
Rare-earth metal is understood to mean the elements of the group consisting of yttrium and elements of the Periodic Classification with an atomic number of between 57 and 71 inclusive. Mention may more particularly be made, among rare-earth metals, of yttrium, lanthanum, neodymium and praseodymium.
Mention may also be made, by way of additives, of those belonging to the group consisting of magnesium, scandium, hafnium, gallium and boron.
Finally, mention may be made of additives belonging to the group consisting of iron, bismuth, nickel, manganese, tin and chromium.
It is obvious that all the additives cited here can be present in the compositions of the invention alone or in combination, whatever the group to which they belong. In addition, these additives are generally present in the compositions in the form of oxides.
The amounts of additives can vary within wide proportions. The maximum amount is at most 50%, expressed as weight of oxide of additive with respect to the weight of the entire composition. The minimum amount is that necessary in order to obtain the desired effect. Generally, this amount is at least 0.1%. More particularly, the amount of additive can be between 1 and 20% and more particularly still between 1 and 10%.
The compositions of the invention can exhibit high specific surfaces, even after calcination at high temperature. This surface depends on the nature of the constituents of the composition. The highest surfaces will be obtained for the compositions in which zirconium predominates.
More specifically, the compositions of the invention in which cerium is present with zirconium and where cerium predominates, that is to say that the Ce/Zr atomic ratio is greater than 1, can exhibit specific surfaces of at least 20 m
2
/g, more particularly of at least 30 m
2
/g, after calcination at 900° C. for 6 hours. With one or more additives, especially such as scandium and rare-earth metals and in particular lanthanum, praseodymium or neodymium, these surfaces can be at least 35 m
2
/g or alternatively at least 40 m
2
/g and more particularly still at least 45 m
2
/g after calcination under the same conditions. These compositions with additives can also exhibit a surface of at least 20 m
2
/g and more particularly still of at least 30 m
2
/g after calcination at 1000° C. for 6 hours.
Moreover, the compositions of the invention in which cerium is present with zirconium but where zirconium predominates, that is to say that the Ce/Zr atomic ratio is less than 1, can exhibit specific surfaces of at least 30 m
2
/g, more particularly of at least 40 m
2
/g, after calcination at 900° C. for 6 hours. With one or more additives, especially rare-earth metals and in particular lanthanum, praseodymium or neodymium, these surfaces can be at least 50 m
2
/g or alternatively at least 55 m
2
/g and more particularly still at least 60 m
2
/g after calcination under the same conditions. These compositions with additives can also exhibit a surface of at least 30 m
2
/g and more particularly still of at least 40 m
2
/g after calcination at 1000° C. for 6 hours.
Specific surface is understood to mean the B.E.T. specific surface determined by nitrogen adsorption in accordance with ASTM Standard D 3663-78 based on the Brunauer-Emmett-Teller method described in the periodical “The Journal of the American Chemical Society, 60, 309 (1938)”.
The process for the preparation of the extruded compositions of the invention will now be described.
The main characteristic of the process of the invention is to start from a specific product. This product can be defined in two ways.
The product which is subjected to extrusion may first of all be defined as being a product based on a cerium hydroxide or oxyhydroxide or on cerium and zirconium hydroxides or oxyhydroxides. Such a hydroxide can generally be represented by the formula (1) M(OH)
x
(X)
y
.nH
2
O, in which M represents cerium or zirconium and X an anion, x+y being at most equal to 4 and
Blanchard Gilbert
Quemere Eric
Rhodia Chimie
Silverman Stanley S.
Vanoy Timothy C
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