Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
1999-08-23
2002-02-26
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S213500, C423S239100, C502S300000, C502S302000, C502S304000, C502S326000, C502S340000, C502S344000, C502S349000, C502S400000, C502S514000
Reexamination Certificate
active
06350421
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to a nitrogen oxide storage material which contains at least one storage component for nitrogen oxides in the form of an oxide, carbonate or hydroxide of an alkaline earth metal selected fro the group consisting of magnesium, calcium, strontium and barium and an alkali metal selected from the group consisting of potassium and cesium on a support material.
In the gasoline engine sector, so-called lean engines have been developed to reduce the consumption of fuel. These engines operate with lean air/fuel mixtures when not operating under full load. A lean air/fuel mixture contains a higher oxygen concentration than is required for complete combustion of the fuel. In the corresponding exhaust gas the oxidizing components oxygen (O
2
) and nitrogen oxides (NO
x
) are then present in an excess as compared with the reducing exhaust gas components carbon monoxide (CO), hydrogen (H
2
) and hydrocarbons (HC). Lean exhaust gas usually contains 3 to 15 vol. % of oxygen. However, when operating under full or relatively full load, even lean operating gasoline engines have a stoichiometric ratio or even less than stoichiometric ratio, that is to say a rich, air/fuel composition.
Diesel engines on the other hand operate under all conditions with well above stoichiometric air to fuel mixtures.
Due to the high oxygen content of exhaust gas from lean engines or diesel engines, the nitrogen oxides contained therein cannot be reduced, as is the case with stoichiometrically operated gasoline engines, by using so-called three way catalytic converters with simultaneous oxidation of hydrocarbons and carbon monoxide to yield nitrogen.
To remove nitrogen oxides from these exhaust gases, therefore, nitrogen oxide storing catalysts have been produced which store the nitrogen oxides contained in lean exhaust gas in the form of nitrates.
The mode of operation of nitrogen oxide storing catalysts is described in detail in SAE 950809 which is incorporated herein by reference. Accordingly, nitrogen oxide storing catalysts consist of a catalyst material which is generally applied in the form of a coating on an inert honeycomb structure made of ceramic or metal, a so-called carrier structure. The catalyst material contains the nitrogen oxide storage material and a catalytically active component. The nitrogen oxide storage material thus consists of the actual nitrogen oxide storage component which is deposited onto a support material in highly dispersed form.
The basic oxides of alkali metals, alkaline earth metals and rare earth metals, preferably barium oxide, are used as storage components which react with nitrogen dioxide to give the corresponding nitrates. It is known that these oxide materials are largely present, in the presence of air, in the form of carbonates and hydroxides. These compounds are also suitable for storing nitrogen oxides. When therefore in the context of the invention the term “basic storage oxides” is referred to, then this expression is intended to also include the corresponding carbonates and hydroxides.
The noble metals from the platinum group of metals from the Periodic Table of Elements are generally used as catalytically active components and these are generally deposited on to the support material together with the storage component. Active, high surface area aluminum oxide is generally used as support material as is well known in this art.
The task of the catalytically active component is to convert carbon monoxide and hydrocarbons in lean exhaust gas to carbon dioxide and water. In addition they should oxidize any nitrogen monoxide in the exhaust gas to nitrogen dioxide so that it can react with the basic storage material to give nitrates. With increasing incorporation of nitrogen oxides in the storage material the storage capacity of the material decreases and it therefore has to be regenerated from time to time. For this purpose, the engine is operated for short periods with a stoichiometric composition or a rich air/fuel mixture. Under the reducing conditions in a rich exhaust gas the nitrates which have been produced decompose to give nitrogen oxides NO
x
and are reduced to nitrogen, with the production of water and carbon dioxide, by using carbon monoxide, hydrogen and hydrocarbons as reducing agents. The storing catalyst operates as a three way catalytic converter during this operating phase.
A substantial problem with storage materials is their inadequate resistance to ageing at high temperatures. As pointed out in SAE Technical Paper 970746, an important ageing mechanism for nitrogen oxide storage materials comprises the actual storage component reacting with the support material. Thus, when a storage material consisting of barium oxide on zirconium oxide was aged for a period of 24 hours at 750° C. the production of barium zirconate BaZrO
3
was observed. Barium oxide on titanium oxide led to the production of barium titanate. In both cases this reaction of storage component with support material was associated with a high loss of nitrogen oxide storage capacity. Zirconium oxide and titanium oxide are thus unsuitable as supports for alkali metal and alkaline earth metal storage components due to their high tendency to react with barium oxide when they are subjected to high temperatures during use. Aluminum oxide behaved somewhat better as a support material, but even here the production of barium aluminate occurred with long term ageing at high temperatures.
Various combinations of storage components and support materials which are also intended to solve this ageing problem have been disclosed in the patent literature. Thus, EP 0 562 516 A1 describes a catalyst consisting of barium oxide, lanthanum oxide and platinum on a support material made of aluminum oxide, zeolite, zirconium oxide, aluminum silicate or silicon dioxide, wherein at least some of the barium oxide and lanthanum oxide form a mixed oxide. By virtue of this mixed oxide the production of lanthanum aluminate, which would otherwise lead to ageing of the catalyst, is intended to be suppressed.
In order to suppress the reaction of storage components with a support consisting of aluminum oxide EP 0 645 173 A2 proposes dissolving lithium in the support in such a way that a solid solution of aluminum oxide and lithium is formed.
EP 0 653 238 A1 suggests as support material, titanium oxide which contains at least one element selected from the group of alkali metals, alkaline earth metals and rare earth metals in the form of a solid solution.
EP 0 657 204 A1 discloses the mixed oxides TiO
2
—Al
2
O
3
, ZrO
2
—Al
2
O
3
and SiO
2
—Al
2
O
3
as support materials for nitrogen oxide storing catalysts. In addition, mixed oxides of TiO
2
, Al
2
O
3
with alkaline earth metals and rare earth metals, in particular TiO
2
—Al
2
O
3
—Sc
2
O
3
, TiO
2
—Al
2
O
3
—Y
2
O
3
, TiO
2
—Al
2
O
3
—La
2
O
3
and TiO
2
—Al
2
O
3
—Nd
2
O
3
are mentioned as support materials.
EP 0 666 103 A1 describes a catalyst which contains a nitrogen oxide storage component and a noble metal on a porous support material. Aluminum oxide, zeolite, zirconium oxide, aluminum silicate and silicon dioxide are suggested as support material. The nitrogen oxide storage component and noble metal are deposited onto these support particles in very close association. In addition, the catalyst may also contain cerium oxide as an oxygen storage component, wherein cerium oxide is kept separate from the noble metal and thus also from the storage component.
EP 0 718 028 A1 discloses a thermally resistant nitrogen oxide storage material. The high thermal resistance is obtained by finely dispersing the nitrogen oxide storage component in the support material. For this purpose, a solution of a compound of at least one alkali metal, one alkaline earth metal and one rare earth metal is mixed with a solution of an oxide sol of at least one metal selected from Groups IIIb, IVa and IVb of the Periodic Table of Elements, and converted into a gel, dried and calcined. The resulting storage material is amorphous. In the
Domesle Rainer
Göbel Ulrich
Mussmann Lothar
Strehlau Wolfgang
dmc
Griffin Steven P.
Smith , Gambrell & Russell, LLP
Vanoy Timothy C
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