Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2000-11-09
2003-09-09
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C095S129000, C423S213200, C423S213500
Reexamination Certificate
active
06616904
ABSTRACT:
TECHNICAL FIELD
The present invention relates to materials which promote the removal, by adsorption, of the nitrogen oxides (NO and NO
2
, commonly called NO
x
) present in a gaseous mixture which can contain a superstoichiometric proportion of oxidizing compounds, more particularly oxygen, said materials not being poisoned by the sulfur products found in these gases. The invention applies to the removal of the nitrogen oxides (NO
x
) present in the engine exhaust gases from automotive vehicles and very particularly from diesel vehicles.
PRIOR ART
The high toxicity of nitrogen oxides and their role in the formation of acid rain and tropospheric ozone have resulted in the imposition of strict standards limiting the discharges of these compounds. To meet these standards, it is generally necessary to remove at least part of these oxides present in the exhaust gases from automotive or stationary engines and from turbines.
It is possible to envisage removing the nitrogen oxides by thermal or, preferably, catalytic decomposition, but the high temperatures demanded by this reaction are incompatible with those of the exhaust gases. Catalytic reduction of the nitrogen oxides to nitrogen can only be effected by using the reducing agents which are present, albeit in small amounts, in the exhaust gas (CO, H
2
, unburnt hydrocarbons or hydrocarbons imperfectly combusted in the engine) or by injecting complementary reducing compounds upstream of the catalyst. These reducing agents are hydrocarbons, alcohols, ethers or other oxygen compounds; they can also be the liquid or gaseous fuel (pressurized fuel “CNG” or liquefied fuel “LPG”) feeding the engine or turbine.
Patent application EP 0 540 280 A1 describes a device for reducing the emission of nitrogen oxides in the exhaust gases from internal combustion engines, said device comprising a material for the adsorption and desorption of the nitrogen oxides. According to this method, the nitrogen oxides are stored in the form of nitrates while the engine is running on a lean mixture, i.e. a hydrocarbon-lean mixture. However, the storage capacity of a trap operating according to this principle is generally impaired by the adsorption of sulfur products contained in the exhaust gases; these form sulfates, which are more stable than nitrates and poison the trap.
Furthermore, after the No
X
have been trapped, it is necessary to carry out a step for desorption of the nitrogen oxides, followed by their reduction. Treatment devices are known which involve catalyzed oxidation of the carbon monoxide, CO, and the hydrocarbons, HC, contained in the exhaust gases, for example using catalysts for reducing the nitrogen oxides, called deNO
x
catalysts, which are active in NO
x
reduction over temperature ranges between 200 and 350° C., these catalysts comprising e.g. precious metals on oxide supports, such as platinum or palladium deposited on a support of alumina, titanium oxide or zirconium oxide, or by perovskites, or over temperature ranges between 350 and 600° C., these catalysts comprising e.g. hydrothermally stable zeolites (for example Cu-ZSM5). A device for treating the exhaust gases from a compression ignition engine, comprising a catalyst and a nitrogen oxide absorber placed in the exhaust manifold, is described for example in patents EP 0 540 280 A1 and EP 0 718 478 A1.
The material behaving like a nitrogen oxide trap must therefore be capable of adsorbing the nitrogen oxides at low temperatures, up to the temperature required for the NO
x
reduction catalyst to operate, the trap then assuring the desorption of the nitrogen oxides which come into contact with the deNO
x
catalyst at a sufficient temperature to assure the initiation of the NO
x
reduction reaction.
The materials forming the subject of this patent can be found in the natural state or can easily be synthesized in the laboratory [B. Durand and J. M. Paris, Compte-rendu de l'Académie des Sciences, Paris, vol. 281, series C, pp. 539-542 (1975)] [M. I. Baraton et al., Journal of Solid State Chemistry, 112, pp. 9-14 (1994)].
SUMMARY OF THE INVENTION
The invention relates to materials for removing the nitrogen oxides NO and NO
2
which are present particularly in exhaust gases, for example from internal combustion engines of automotive vehicles running in a medium containing a superstoichiometric proportion of oxidizing agents, said materials being capable of adsorbing the nitrogen oxides and of desorbing the NO
x
when the temperature is raised and when the chemical composition of the gases is varied. The materials are mixed oxides whose structure comprises at least one metal cation A and at least one metal cation B selected from the group consisting of the elements of groups IA, IIA, IIIB, IVB, VB, VIB, VIIIB. VIII, IB, IIB, IIIA, IVA and VA, each surrounded by 6 oxygen atoms, the sum of the charges on A and B being equal to about 6, and the ratio of the cationic radius of A to the cationic radius of B being equal to 1±0.4, preferably 1±0.2. The cations A and B are arranged so as to form an ordered structure ABO
3
of the ilmenite type.
VALUE OF THE INVENTION
The material according to the invention makes it possible to trap the nitrogen oxides at low temperatures and to desorb them at the temperature at which a deNO
x
catalyst is capable of reducing them. These materials are insensitive to the sulfur oxides and carbon oxides contained in the exhaust gases, preventing said materials from being poisoned. The materials adsorb the nitrogen oxides over a wide temperature range, whereas desorption takes place over a very narrow temperature window, affording easy management of the thermal regeneration. When desorption occurs, the previously adsorbed nitrogen oxides are emitted in puffs of high NO
x
concentration, which is beneficial for the kinetics of the reduction reaction of the desorbed nitrogen oxides. The kinetics of reduction of the NO
x
by hydrocarbons is actually of a positive order relative to the nitrogen oxide species. This material is also capable of desorbing the nitrogen oxides when the chemical composition of the gases is varied, with or without temperature variation. Said material does not have a basic oxide phase, which strongly stabilizes the nitrogen oxides and sulfur oxides in the form of nitrates and sulfates respectively. The SO
x
which can be adsorbed with the NO
x
on the material forming the subject of the invention are desorbed over a similar temperature range to that of the NO
x
. Prevention of the formation of stable sulfates assures less poisoning of the adsorbent material, a lower regeneration frequency and temperature and hence a longer life of the NO
x
trap and a gain in energy terms. According to one particular mode of carrying out the invention, association of the material claimed by the Applicant with a group VIII metal enables the adsorbed NO
x
to be eliminated by reduction when the gas composition is changed to a rich medium.
DESCRIPTION OF THE INVENTION
The present invention relates to materials for removing nitrogen oxides, said materials being oxides whose metal cations A and B are octahedrally coordinated and form a structure ABO
3
of the ilmenite type. A and B are selected from elements of the periodic table in such a way that the sum of their oxidation states is equal to about +6 and so that the cations A and B are of similar size.
The material has the capability of adsorbing the NO
x
at low temperatures and of desorbing them at a higher temperature. It is also capable of desorbing the NO
x
when the ratio of reducing compounds to oxygen in the gases is increased.
More precisely, the invention relates to a method of removing the nitrogen oxides in exhaust gases, particularly from internal combustion engines of automotive vehicles, in the presence of materials having a structure ABO
3
of the ilmenite type, as defined hereafter.
The adsorbent phase of the materials used in the method according to the invention has a three-dimensional structure of the ilmenite type and a stoichiometry ABO
3
, where:
Becue Thierry
Malefant Karine
Institut Francais du Pe'trole
Langel Wayne A.
Millen White Zelano & Branigan P.C.
Strickland Jonas N.
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