Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2000-07-17
2002-03-19
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S303000, C502S304000, C502S324000, C502S340000, C502S341000, C502S344000, C502S349000, C502S355000, C423S212000, C423S213200
Reexamination Certificate
active
06358880
ABSTRACT:
The present invention relates to a composition containing a support based on a cerium oxide, a zirconium oxide and a scandium or rare-earth oxide and use for exhaust gas treatment.
It is known that the nitrogen oxide (NOx) emissions in exhaust gases from motor vehicles are reduced, in particular, with the aid of three-way catalysts, which stoichiometrically use the reducing gases present in the mixture. Any oxygen excess leads to a pronounced deterioration in the catalyst's performance.
However, certain engines, for example, diesel engines or lean-burn patrol engines, save on fuel but emit exhaust gases which permanently contain a large oxygen excess, of for example at least 5%. A standard three-way catalyst is therefore ineffective for the NOx emissions in this case. Furthermore, it has become imperative to limit NOx emissions owing to the tightening of motor vehicle emission standards which have now have been extended to these engines.
There is therefore a genuine need for an efficient catalyst to reduce NOx emissions for these types of engines and, generally, for treating games containing NOx.
As a type of catalyst which can meet this need, systems referred to as NOx traps have been proposed which are capable of oxidizing NO into NO
2
and then of absorbing the NO
2
thus formed. Under certain conditions, the NO
2
is re-released then reduced to N
2
by reducing species contained in the exhaust gas. These NOx traps are generally based on platinum. However, platinum is an expensive element. It would therefore be beneficial to provide a platinum-free system in order to reduce the costs of the catalysts.
The object of the invention is therefore to develop a catalyst which can be used as an NOx trap without necessarily using platinum.
To this end, the composition of the invention is characterized in that it comprises a support based on a cerium oxide, a zirconium oxide and an oxide of scandium or a rare earth other than cerium, and a supported phase based on manganese and at least one other element selected from the alkali metals, the alkaline-earth metals and the rare earths.
The invention also relates to a process for treating gases with a view to reducing nitrogen oxide emissions which is characterized in that a composition as defined above is used.
Other characteristics, details and advantages of the invention will become yet more fully apparent on reading the following description, as well as the various concrete but non-limiting examples intended to illustrate it.
The composition of the invention comprises a phase supported on a support.
The supported phase may more particularly correspond to two variants.
According to a first variant, this phase is based, further to manganese, on an alkali metal and/or an alkaline-earth metal. The alkali metal may more particularly be sodium or potassium. The alkaline-earth metal may more particularly be barium or strontium.
According to a second variant, the supported phase is based on manganese and at least one element selected from the rare earths.
Here, and for all of the description, the term rare earth is intended to mean the elements in the group consisting of yttrium and the elements in the Periodic Table having an atomic number of between 57 and 71 inclusive.
The rare earth may more particularly be elected from lanthanum, cerium, praseodymium, neodymium, europium, samarium, gadolinium or terbium. As an advantageous embodiment in the scope of this second variant, mention may be made of a supported phase based on manganese and praseodymium.
Lastly, it is entirely possible in the scope of the present invention to have a supported phase based on manganese and at least two other elements, one being a rare earth and the other being selected from the alkali metals and the alkaline-earth metals.
According to a particular embodiment, the composition of the invention can be obtained by a process in which at least one of the two elements manganese and potassium is supplied at least partially by potassium permanganate. It should be noted that a single element may be supplied by the permanganate, and only partially. Conversely, and preferentially, it is also possible to supply the two elements fully by the permanganate route. All of the variants between these two possibilities may be envisaged. This embodiment makes it possible to obtain compositions having high NOx adsorption capacities.
Another important characteristic of the composition of the invention is the nature of the support of the supported phase.
As indicated above, the support is based on a cerium oxide, a zirconium oxide and an oxide of scandium or a rare earth other than cerium. It is to be emphasized here, and for all of the description, that the invention also applies to any support based on cerium oxide, zirconium oxide and, as a third element, a combination of two or more oxides selected from scandium oxide and the rare-earth oxides.
The supports used are preferably those for which the cerium/zirconium atomic proportion is at least 1.
As a rare earth involved in the composition of the support, mention may more particularly be made of lanthanum, neodymium and praseodymium.
Use may also more particularly be made of the supports satisfying the overall formula Ce
x
Zr
y
M
z
O
2
where M represents at least one element selected from the group comprising scandium and the rare earths other than cerium and where x, y and z satisfy the relationships 0<z≦0.3, 1≦x/y≦19 and
x+y+z=
1.
More particularly, x, y and z may satisfy the following relationships 0.02<z<0.2, 1<x/y<9, it being, more particularly still, possible for the last ratio to be between 1.5 and 4, these limits being inclusive.
According to a particular embodiment, the support is in the form of a solid solution. In this case, the X-ray diffraction spectra of the support reveal the existence of a single homogeneous phase within it. As regards supports which are richer in cerium, this phase actually corresponds to that of a crystallized cerium oxide CeO
2
whose lattice parameters are shifted to a greater or lesser extent relative to a pure cerium oxide, thus reflecting the incorporation of zirconium and the other element (scandium and rare earths other than cerium) in the crystal lattice of the cerium oxide, and therefore the fact that a genuine solid solution is obtained.
According to a preferred variant of the invention, supports are used which are characterized by their specific surface at certain temperatures, as well as their oxygen storage capacity.
The term specific surface is intended to mean the BET specific surface determined by nitrogen adsorption according to the standard ASTM D 3663-78 established on the basis of the Brunauer - Emmett - Teller method described in the periodical “The Journal of the American Society, 60, 309 (1938)”.
It is thus possible to use supports which have a specific surface after calcining for 6 hours at 900° C. of at least 35 m
2
/g. This surface may more particularly be at least 40 m
2
/g. lt may, more particularly still, be at least 45 m
2
/g.
These supports may also have surfaces which are still considerable even after calcining for 6 hours at 1000° C. These surfaces may be at least 14 m
2
/g, more particularly at least 20 m
2
/g and more particularly still at least 30 m
2
/g.
Another characteristic of the supports of this variant is their oxygen storage capacity. This capacity, measured at 400° C., is at least 1.5 ml O
2
/g. It may more particularly be at least 1.8 ml O
2
/g and more particularly still at least 2 ml O
2
/g. In the best cases, this capacity my be at least 2.5 ml O
2
/g. This capacity is determined by a test which evaluates the capacity of the support, or of the product, to successively oxidize amounts of carbon monoxide injected with oxygen and to consume infected amounts of oxygen to reoxidize the product. The method employed is referred to as an alternative method.
The carrier gas is pure helium at a flow rate of 10 l/h. The injections are carried out by means of a loop containing 16 ml of gas. The amounts of CO are inj
Hedouin Catherine
Seguelong Thierry
Bell Mark L.
Hailey Patricia L.
Rhodia Chimie
Shedden John A.
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