Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-04-19
2002-12-10
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
Reexamination Certificate
active
06491886
ABSTRACT:
The present invention relates to a process for treating gases to reduce emissions of oxides of nitrogen using a catalytic composition having a support based on silica and titanium oxide.
It is known that reducing emissions of oxides of nitrogen (NO
x
) from the exhaust gases of car engines is carried out in particular using three-way catalysts which make stoichiometric use of the reductive gases present in the mixture. Any excess of oxygen is manifested in a drastic deterioration in the performance of the catalyst.
However, certain engines, such as diesel engines or lean-burn petrol engines (i.e. engines operating with a lean mixture) are economic on fuel but emit exhaust gases which permanently include a large excess of oxygen—at least 5%, for example. A standard three-way catalyst therefore has no effect on the NO
x
emissions of these engines. Nevertheless, limiting NO
x
emissions has become imperative owing to the more vigorous automotive post-combustion standards which now extend to this type of engine.
There is therefore a real need for an effective catalyst to reduce the NO
x
emissions for this type of engine and, more generally, for the treatment of gases containing NO
x
. In addition, the aim is to obtain catalysts which are active at moderate temperature.
With this aim in mind, the process of the invention for treating gases to reduce the emissions of oxides of nitrogen is of the type which uses a catalytic composition comprising a catalytic phase on a support, and is characterized in that the support is based on silica and titanium oxide in an atomic proportion Ti/Ti+Si of between 0.1 and 15%.
Other characteristics, details and advantages of the invention will appear more completely on reading the following description as will various specific but non-limitative examples which are intended to illustrate it.
The principal characteristic of the catalytic composition used in the process of the invention is the support of this composition. This support is based on silica and titanium oxide in the specific proportion given above. In accordance with one embodiment this proportion can be between 1 and 10%.
It is advantageous to use supports which have a higher specific surface area and are thermally stable. It is therefore possible with advantage to use supports having a specific surface area of at least 350 m
2
/g and more particularly of at least 600 m
2
/g after calcination at 750° C. for 6 hours. By specific surface area here is meant the BET specific surface area determined by nitrogen adsorption in accordance with the standard ASTM D 3663-78 based on the method of Brunauer-Emmett-Teller described in the Journal of the American Chemical Society, 60 (1938) 309.
Supports with these surface values are generally of the mesoporous type; in other words, they have the characteristic of having a significant pore volume contributed by mesopores (pores of between 2 and 10 nm in diameter).
The support based on silica and titanium oxide can be prepared by any process capable of giving a support of sufficient specific surface area.
Mention may thus be made more particularly of a process of micellar texturing using, as silica source, organic compounds of silicium, especially alkyl silicates such as tetraethyl orthosilicate and making this silica source react with a titanium compound or source, in the presence of a surfactant. These organic compounds of silicium are generally employed in the form of solutions in alcohols, especially aliphatic alcohols. As titanium compound, organic titanium compound can be used such as alkyl or alkoxy titanates which are generally employed in the form of alcoholic solutions like the silicium compound. Concerning the surfactant, those of the type with active cation such as amides and quaternary ammonium salts may be employed more particularly. The reaction can be carried out by mixing the organic silicium compound and the titanium compound and by heating. The surfactant is subsequently added to the mixture thus heated. The precipitate obtained is separated from the reaction medium. This precipitate is subsequently calcined, generally in air, to give the support, which can subsequently be shaped. Calcination can be carried out in two stages. In the first stage calcination is carried out at a temperature sufficient to remove the surfactant. This temperature may be approximately 650° C. In the second stage, calcination is carried out at a temperature at least equal to that at which the catalyst will be used. This temperature may be approximately 750° C.
The support can be present in various shapes, such as pellets, beads, cylinders or honeycombs of variable dimensions.
By way of additives, the support may comprise one or more rare earth metal oxides. By rare earth metals are meant the elements of the group consisting of yttrium and the elements from the Periodic Table whose atomic number is from 57 to 71. As a rare earth metal more particular mention may be made of lanthanum. The additive content, expressed in atomic % of additive/Ti+Si+additive, can be not more than 20%, especially not more than 10%.
The catalytic composition of the invention may additionally comprise a catalytic phase. This phase can be based on at least one metal chosen from the elements of groups IIIA to IIB of the Periodic Table.
More particularly, the catalytic phase is based on at least one metal chosen from the metals of group VIII of the Periodic Table.
The Periodic Table of the Elements to which reference is made is that published in the supplement to the Bulletin de la Société Chimique de France No. 1 (January 1966).
By way of examples of metals which can be used in the catalytic phase mention may be made of platinum, palladium, rhodium, ruthenium and iridium. Mention may also be made of iron, copper and chromium and of vanadium, niobium, tantalum, molybdenum and tungsten.
The metal content of the catalytic phase and the metal content, especially the platinum content, of the composition can vary within wide proportions. This proportion, expressed as weight of metal relative to weight of support, is commonly between 500 and 40,000 ppm, preferably between 2500 and 20,000 ppm and especially between 5000 and 15,000 ppm.
The catalytic phase can be deposited on the support, preferably the calcined support, by any known technique.
It is therefore possible to use the impregnation technique, by soaking the support in a solution or a sol of the element or elements which makes up or make up the catalytic phase and then removing the excess solution or sol by draining or by passage in a rotary evaporator.
In the particular case of a catalytic phase based on platinum and in accordance with one variant of the invention, the platinum is provided in the form of a sol. The platinum sol will preferably be chosen so that it has a colloidal size of between 2 nm and 10 nm and more particularly between 3 nm and 8 nm. The colloidal size is determined by transmission electron microscopy (TEM).
According to one particular variant, impregnation is carried out “dry”; in other words, the total volume of solution or sol used is approximately equal to the total pore volume developed by the support to be impregnated. As far as the determination of this pore volume is concerned, it can be carried out by the known method of mercury porosimetry or else by measuring the quantity of water absorbed by a sample.
Following impregnation, the support is dried if appropriate and then calcined. Drying is usually carried out in air at a temperature which may vary between 80 and 300° C. and is preferably chosen between 100 and 150° C. Drying is continued until a constant weight is obtained. In general, the duration of drying is between 1 and 24 hours. Calcination of the support with the deposited active or catalytic phase is generally carried out at a temperature of not more than 750° C. and, more particularly, not more than 550° C. in the case where platinum is used in the catalytic phase. The duration of calcination can vary within wide limits and is, for example, between 1 and 24 hours, preferably be
Hedouin Catherine
Macaudiere Pierre
Johnson Edward M.
Rhodia Chimie
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