Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
2002-05-10
2004-09-28
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S213200, C423S213500, C423S592100
Reexamination Certificate
active
06797247
ABSTRACT:
TECHNICAL FIELD
The present invention relates to materials that 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 present 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.
BACKGROUND OF THE INVENTION
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, as is preferable, by catalytic 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-A1-0 540 280 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 that are more stable than 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 catalysed oxidation treatment 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 DeNO
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, hydrothennally 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 that generates channels with a size sufficient to accommodate oxides of nitrogen have been described (Japanese patents JP09075715A and JP09075718A). The solids used in those patents are mixed oxides with the crystallographic structure of hollandite and specific formulations K
3.6
Zn
1.8
Sn
6.2
O
16
and K
1.8
A
1.8
Sn
6.2
O
16
.
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 that is rich in oxygen to a tetragonal structure that 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
x
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.
The materials of the present patent can be found in the natural state or they can be synthesised in the laboratory. EP-A-0 623 556 and EP-A-0 710 622, for example, describe methods for producing certain of such solids. Further, among such solids, materials based on manganese and with channels with sufficient diameters for inserting NO
x
have been described in the Gmelin Handbook (Gmelin Handbook of Inorganic and Organometallic Chemistry, Mn n° 56, A5b1, Springer-Verlag, 1996).
SUMMARY OF THE INVENTION
The invention concerns materials for eliminating oxides of nitrogen NO and NO
2
(NO
x
) in particular those present in exhaust gases, for example internal combustion engines of automotive vehicles operating in a medium which is super-stoichiometric in oxidising agents, said materials being capable of adsorbing NO
x
and which can desorb NO
x
by raising the temperature or by treatment with a mixture that is rich in reducing agents. The materials are mixed oxides the framework of which is constituted by metal cations M each surrounded by 6 oxygen atoms and wherein the octahedra (MO
6
) thus formed are connected together by edges and peaks generating a structure that produces channels in at least one direction in space. Said materials are of the OMS structure type with a hollandite (OMS 2*2), romanechite (OMS 2*3) or todorokite (OMS 3*3) type structure.
The material of the invention can trap oxides of nitrogen at low temperatures and desorb them at the temperature at which a DeNO
x
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 that 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 that substantially stabilises the oxides of nitrogen and oxides of sulphur into the nitrate and sulphate forms respectively. The SO
x
that can be inserted with the NO
x
into the structure of the material of the invention are desorbed in a temperature range that is similar to that of the NO
x
. Preventing the formation of stable sulphates ensures that poisoning of the adsorbing
Becue Thierry
Mabilon Gil
Villeret Philippe
Institut Francais du Pe'trole
Medina Maribel
Millen White Zelano & Branigan P.C.
Silverman Stanley S.
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