Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
2000-08-03
2002-10-29
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S213500, C423S239100, C502S208000, C502S209000, C502S210000, C502S211000, C502S213000, C502S425000
Reexamination Certificate
active
06471923
ABSTRACT:
TECHNICAL FIELD
The present invention relates to materials which encourage elimination by adsorption of oxides of nitrogen (NO and NO
2
, usually termed NO
x
) present in a gas mixture which may be super-stoichiometric in oxidising compounds, and in particular in oxygen, said materials not being poisoned by the sulphur-containing products encountered in those gases. The invention is applicable to eliminating oxides of nitrogen (NO
x
) present in the exhaust gases from automotive vehicles, in particular from vehicles functioning with diesel fuel.
PRIOR ART
The high toxicity of oxides of nitrogen and their role in the formation of acid rain and tropospheric ozone have led to the instigation of strict regulations limiting the discharge of such compounds. In order to satisfy those regulations, it is generally necessary to eliminate at least a portion of such oxides present in exhaust gases from automotive or stationary engines and from turbines.
The elimination of oxides of nitrogen by thermal decomposition or preferably by catalytic if decomposition can be envisaged, but the high temperatures demanded by this reaction are incompatible with those of the exhaust gases. Only catalytic reduction of oxides of nitrogen to nitrogen is possible using the reducing agents which are present, albeit in small quantities, in the exhaust gases (CO, H
2
, unburned hydrocarbons or where combustion in the engine has been imperfect), and also by injecting a complement to those reducing compounds upstream of the catalyst. Such reducing agents are hydrocarbons, alcohols, ethers or other oxygen-containing compounds, they can also be a liquid or gaseous fuel (under pressure, CNG, or liquefied, LPG) feeding the engine or turbine.
European patent EP-A-0 540 280 describes an apparatus for reducing emissions of oxides of nitrogen in the exhaust gases from internal combustion engines, which comprises a material for adsorbing and desorbing oxides of nitrogen. In that process, the oxides of nitrogen are stored in the form of nitrates when the engine is burning lean, i.e., depleted in hydrocarbons. However the storage capacity of a trap operating using that principle is generally deteriorated by adsorption of sulphur-containing products contained in the exhaust gas which form sulphates which are more stable that the nitrates, poisoning the trap.
Further, following NO
x
trapping, a step for desorbing the oxides of nitrogen must be carried out followed by their reduction. Devices for treatment by catalysed oxidation of carbon monoxide CO and hydrocarbons HC contained in the exhaust gases are known which, for example, use catalysts for reducing oxides of nitrogen, known as NO
x
catalysts, which are active for reducing NO
x
in temperature ranges in the range 200° C. to 350° C. and which comprise, for example, precious metals on oxide supports such as platinum or palladium deposited on an alumina, titanium oxide or zirconium support, or by perovskites, or in temperature ranges in the range 350° C. to 600° C. comprising, for example, hydrothermally stable zeolites (for example Cu-ZSM5). A device for treating exhaust gases from a compression ignition engine comprising a catalyst and an oxides of nitrogen adsorbent placed in the exhaust collector has been described, for example, in patents EP-A1-0 540 280 and EP-A1-0 718 478.
Thus a material behaving as a trap for oxides of nitrogen has to be capable of adsorbing the oxides of nitrogen at low temperatures up to the temperature necessary for the NO
x
reduction catalyst to function, the trap then allowing the oxides of nitrogen coming into contact with the DeNO
x
catalyst to desorb at a temperature sufficient to trigger the NO
x
reduction reaction.
Oxides with a crystallographic structure which generate channels with a size sufficient to accommodate oxides on nitrogen have been described (Japanese patent JP09075715A). The solids used in that Japanese patent are oxides containing Al and Sn or Zn and Sn mixtures, and have the crystallographic structure of hollandite.
French patent FR-A-2 733 924 describes a material with formula YBa
2
Cu
3
O
7−x
which can integrate the oxides of nitrogen in the mixed oxide composing the material. That patent indicates that the material, after being charged with oxides of nitrogen, is transformed by passing from an orthorhombic structure which is rich in oxygen to a tetragonal structure which is depleted in oxygen when the oxygen content of the gas reduces, and that phase transition causes desorption of oxides of nitrogen. According to that process, it is possible to influence adsorption and desorption of the oxides of nitrogen by varying the amounts of oxygen in the exhaust gases. It has recently been demonstrated (K-Y Lee, K. Watanabe, M. Misono, Applied Catalysis B 13, 241 (1997)) that the adsorption of NO in the presence of oxygen on the material YBa
2
Cu
3
O
7−x
leads to the formation of barium nitrate species (Ba(NO
3
)
2
). That same study also showed that that material suffers a dramatic loss of its oxides of nitrogen adsorption properties in the presence of carbon dioxide by forming barium carbonates. Since barium sulphate species are more stable than the nitrate species, it is feared that a compound of the YBa
2
Cu
3
O
7−x
type is also poisoned in the presence of sulphur dioxide by forming sulphate species on the oxides of nitrogen adsorption sites.
A. Clearfield and J. Stynes, J. Inorg. Nucl. Chem., 26 (1964), 117 and G. Alberti and E. Torracca, J. Inorg. Nucl. Chem., 30 (1968), 317 describe materials with a structure composed of phosphate tetrahedra and containing an element with an oxidation number of +4.
SUMMARY OF THE INVENTION
The invention concerns materials for adsorbing and desorbing oxides of nitrogen NO and NO
2
, in particular those present in exhaust gases, for example from the internal combustion engines of automotive vehicles functioning in a medium which is super-stoichiometric in oxidising agents, said materials being able to desorb NO
x
by raising the temperature. The materials are lamellar in structure and comprise phosphate tetrahedra and at least one element A selected from elements from groups IVB, VB, VIB, VIIB and IVA of the periodic table.
SIGNIFICANCE OF THE INVENTION
The adsorption and desorption material of the invention can trap oxides of nitrogen at low temperatures and desorb them at the temperature at which a deNOx catalyst is capable of reducing them. These materials are insensitive to the oxides of sulphur and carbon contained in the exhaust gases, which prevents the materials from being poisoned. The materials adsorb oxides of nitrogen over a wide temperature range while desorption is carried out in a very narrow temperature range which means that thermal regeneration is easy to control. During desorption, the oxides of nitrogen which have been adsorbed are released in bursts with a high NO
x
concentration, which is beneficial to the reaction kinetics for reduction of the desorbed oxides of nitrogen. The kinetics of the reduction of NO
x
by hydrocarbons are positive with respect to the oxides of nitrogen species. Said material does not have a basic oxide phase which substantially stabilises the oxides of nitrogen and oxides of sulphur into the nitrate and sulphate forms respectively. The SO
x
which can be inserted with the NO
x
into the structure of the material of the invention are desorbed in a temperature range which is similar to that of the NO
x
. Preventing the formation of stable sulphates ensures that poisoning of the adsorbing, material is minimal, meaning that the regeneration frequency and the regeneration temperature are lower, and thus the service life of the NO
x
trap is longer.
DESCRIPTION OF THE INVENTION
The present invention concerns materials for adsorbing and desorbing oxides of nitrogen, the structure of which is composed of phosphate tetrahedra containing at least one element (A) selected from elements from groups IVB, VB, VIB, VIIB and IVA of the periodic table. Preferably, this element (A) has an oxidation number of +4. The phos
Becue Thierry
Mabilon Gil
Villeret Philippe
Institut Francais du Pe'trole
Langel Wayne A.
Millen White Zelano & Branigan P.C.
Strickland Jonas N.
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