Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1999-10-13
2002-07-02
Griffin, Steven P. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S327000, C502S334000, C502S339000, C502S527120, C502S527150
Reexamination Certificate
active
06413904
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATION
This application is based on German Application DE 198 47 008.8, filed Oct. 13, 1998, which disclosure is incorporated herein by reference.
FIELD OF THE INVENTION
The invention provides a nitrogen oxide storage catalyst which contains at least one finely divided catalyst material and also at least one nitrogen oxide storage component from the group of alkali and alkaline earth metals.
BACKGROUND OF THE INVENTION
Nitrogen oxide storage catalysts are used for the exhaust gas treatment of lean-mix operated gasoline engines (so called lean-mix engines) and diesel engines. These engines are operated with greater than the stoichiometric air to fuel ratio, that is the oxygen content in this mixture is substantially larger than would be required for complete combustion of the fuel. The oxygen excess in the exhaust gas from these engines is also correspondingly high. For this reason the hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO
x
) contained in the exhaust gas cannot be converted with well-known three-way catalytic converters since these require a stoichiometrically composed exhaust gas for the simultaneous conversion of these harmful substances.
However, hydrocarbons and carbon monoxide can easily be converted into carbon dioxide and water by oxidation in the presence of an exhaust gas catalyst due to the high oxygen content of these engines. The conversion of nitrogen oxides on the other hand presents great difficulties. Nitrogen oxide storage catalysts have been suggested for solving this problem. These catalysts contain basic compounds that are able to absorb the nitrogen oxides from the lean-mix exhaust gas or to react with them to give nitrates and they are thus removed from the exhaust gas. Suitable compounds for this purpose are the oxides of alkali and alkaline earth metals. Some of these compounds are present in air in the form of carbonates and hydroxides which are also suitable for the storage of nitrogen oxides.
The mode of operation of nitrogen oxide storage catalysts is described in detail in the SAE-document SAE 950809. In addition to basic storage compounds, these catalysts also contain platinum as a catalytically active component in order to oxidize nitrogen oxides, most of which are present as nitrogen monoxide, to nitrogen dioxide so that they can react with the basic storage compounds in the required manner. The storage capacity of the storage compounds reduces as storage of the nitrogen oxides increases. The storage compounds must therefore be regenerated from time to time. For this purpose, the air to fuel mixture and thus also the exhaust gas is enriched for a short period. Under the reducing exhaust gas conditions which then exist, the stored nitrates are decomposed again to give nitrogen oxides and are converted to nitrogen, water and carbon dioxide with consumption of the reducing constituents in the exhaust gas on the catalytically active component.
Nitrogen oxide storage catalysts are generally deposited in the form of a coating on the walls of the flow channels of inert support structures in a honeycomb shape. These so-called honeycomb structures are generally shaped in the form of a cylinder. They have flow channels for the exhaust gas which are parallel to the axis and these are arranged in a regular array over the cross section of the honeycomb structure. The number of flow channels per cross section of area on the honeycomb structure, also known as the cell density, is between 10 and 200 cm
2
. The amount of catalyst coating on the honeycomb structure, that is the loading of the honeycomb structure with the storage catalyst, is generally quoted as a concentration in grams per liter volume (g/l) of the honeycomb structure.
The basic problem with known nitrogen oxide storage catalysts is their low resistance to aging, that is their storage capacity is irreversibly damaged with increasing operating time due to the high exhaust gas temperatures. The reasons for this damage are many and various and depend on the particular formulation of the storage catalyst.
The storage compounds are generally applied in highly dispersed form to the surface of a support material in order to ensure sufficient interaction of the storage compounds with the exhaust gas. One basic aging mechanism, according to SAE Technical Paper 970746, consists of the storage compound reacting with the support material. Thus, in the case of a storage material consisting of barium oxide on zirconium oxide which has been aged for a period of 24 hours at 750° C., the production of barium zirconate BaZrO
3
has been observed. Barium oxide on titanium oxide leads to the production of barium titanate. In both cases this reaction of the storage compound with the support material was associated with a high loss of nitrogen oxide storage capacity. Zirconium oxide and titanium oxide are thus not suitable as supports for alkali and alkaline earth metal storage compounds due to their high tendency to react with barium oxide if they are subjected to high thermal stresses under the conditions of use. Aluminum oxide behaves slightly better as a support material. However even here the production of barium aluminate takes place at high temperatures with prolonged aging.
Various combinations of storage compounds and support materials which are also intended to solve the aging problem have been disclosed in the patent literature. Thus EP 0 562 516 A1 describes a catalyst of barium oxide, lanthanum oxide and platinum on a support material of aluminum oxide, zeolite, zirconium oxide, aluminum silicate or silicon dioxide, wherein at least some of the barium oxide and the lanthanum oxide form a mixed oxide. Due to this mixed oxide, the production of lanthanum aluminate, which would otherwise lead to aging of the catalyst, is intended to be suppressed. Loading concentrations of 0.05 to 10.0 mol/l are cited for the storage compounds on the honeycomb structure used as a support structure for the catalyst coating. In the case of barium oxide as a storage compound, this means a maximum loading with up to 1534 g/l. The concentrations mentioned in the examples are 0.15 mol barium oxide per liter of honeycomb structure, that is 23 g/l.
To suppress reaction of the storage compound with an aluminum oxide support, EP 0 645 173 A2 suggests dissolving lithium in the support in such a way that a solid solution of aluminum oxide and lithium is produced. 0.3 mol of barium oxide per liter of honeycomb structure is mentioned as the loading concentration in the examples, that is 46 g/l.
EP 0 653 238 A1 suggests titanium oxide, which contains at least one element from the group of alkali metals, alkaline earth metals and rare earth metals in the form of a solid solution, as support material. This document mentions 0.1 mol/l as a loading concentration for the storage compounds and thus lies within the scope of the values mentioned above.
EP 0 657 204 A1 mentions 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 storage 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. The loading concentrations for the storage compounds mentioned in the examples are also 0.3 mol/l.
EP 0 666 103 A1 describes a catalyst which contains a nitrogen oxide storage compound and a noble metal on a porous support material. Aluminum oxide, zeolite, zirconium oxide, aluminum silicate and silicon dioxide are suggested as support materials. The nitrogen oxide storage compound and noble metal are deposited in close association on the same support particles. In addition, the catalyst may also contain cerium oxide as an oxygen storing compound, wherein cerium oxide is kept separate from the noble metal and thus also from the nitrogen oxide storage compound. The loading concentration for the storage compounds in the examples in t
Domesle Rainer
Göbel Ulrich
Strehlau Wolfgang
Griffin Steven P.
Kalow David
Nguyen Cam N.
OMG AG & Co. KG
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