Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
2000-08-09
2002-11-05
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S239100, C502S304000
Reexamination Certificate
active
06475452
ABSTRACT:
The present invention relates to a manganese-based composition and to its 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 petrol 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 gases 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, and according to a first embodiment of the invention, the composition according to the invention comprises manganese, cerium oxide or a mixture of cerium oxide and zirconium oxide, and is characterized in that it furthermore contains at least one other element selected from terbium, gadolinium, europium, samarium, neodymium and praseodymium.
According to a second embodiment, the composition according to the invention comprises manganese and potassium, and cerium oxide or a mixture of cerium oxide and zirconium oxide, and is characterized in that it 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.
The invention furthermore relates to a process for treating gases with a view to reducing nitrogen oxide emissions by trapping the said oxides, characterized in that a composition consisting essentially of manganese and of cerium oxide is used.
Lastly, the invention relates to a process of the same type which uses the compositions according to the aforementioned two embodiments.
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 is characterized by the nature of the elements which form it and which have been mentioned above. It will be noted here that, in this composition, the cerium oxide or the mixture of cerium oxide and zirconium oxide may form a support, the other elements forming a supported phase. This means that the cerium oxide or the mixture of cerium oxide and zirconium oxide may form the majority element or elements of the composition, on which element or elements the other elements are deposited. For simplicity, the rest of the description will refer to support and supported phase, but it will be understood that, if an element described as belonging to the supported phase were to be present in the support, for example by being introduced therein during the actual preparation of the support, this would not depart from the scope of the present invention.
In the scope of the first embodiment mentioned above, the composition may comprise a supported phase which is based on manganese in combination with terbium, gadolinium, samarium, neodinium or praseodimium, or alternatively a mixture of manganese and at least two of these elements. According to a variant of this first embodiment, the composition may further comprise an alkali metal which may more particularly be sodium or potassium. This alkali metal element may belong to the supported phase.
In the second embodiment, the composition may comprise a supported phase which is based on manganese in combination with potassium. Furthermore, at least one of the two elements manganese and potassium is supplied at least partially by potassium permanganate during the process of preparing the composition. 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.
The amounts of elements in the supported phase of the composition can vary in wide proportions. The minimum proportion is that below which NOx adsorption activity is no longer observed. The manganese proportions may thus vary between 2 and 50%, more particularly between 5 and 30%. The terbium, gadolinium, samarium, neodinium, praseodymium and/or potassium proportions may vary between 1 and 50%, more particularly between 5 and 30%. These proportions are expressed as atomic % with respect to the sum of the support and the elements relevant to the supported phase. It is to be pointed out here, and for the description as a whole, that the manganese and the other elements are present in oxide form in the compositions described.
Supports based on cerium oxide or a mixture of cerium oxide and zirconium oxide are well known. Mention may more particularly be made, as regards mixtures of cerium oxide and zirconium oxide, of those described in patent applications EP-A-605274 and EP-A-735984, the teaching of which is incorporated here. Use may more particularly be Made of supports based on cerium and zirconium oxides in which these oxides are present in a cerium/zirconium atomic proportion of at least 1. As regards these same supports, those which are in the form of a solid solution may also be used. 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 cubic 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 in the crystal lattice of the cerium oxide, and therefore the fact that a genuine solid solution is obtained.
According to a 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 are based on a cerium oxide and a zirconium oxide in a cerium/zirconium atomic proportion of at least 1 and which have a specific surface after calcining for 6 hours at 900° C. of at least 35 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. 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 injected amounts of oxygen to reoxidize the product. The method employed is referred to as an alternative method.
The carr
Fritz Arno
Hedouin Catherine
Seguelong Thierry
Langel Wayne A.
Medina Maribel
Rhodia Chimie
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