Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2000-11-15
2003-08-05
Bos, Steven (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S303000, C502S326000, C502S327000, C502S328000, C502S340000, C502S341000, C423S213200, C423S213500
Reexamination Certificate
active
06602820
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to a method for producing a nitrogen oxide storage material that contains at least one storage component in the form of particles of an oxide, carbonate or hydroxide of an element selected from the group consisting of magnesium, strontium, barium, lanthanum and cerium on a support material selected from the group consisting of doped cerium oxide, cerium/zirconium mixed oxide and aluminum oxide or mixtures of these.
In the area of gasoline engines the so-called lean-burning engines, which are operated with lean air/fuel mixtures when operating under a partial load, were developed to reduce fuel consumption. A lean air/fuel mixture contains a higher oxygen content than is necessary for complete combustion of the fuel. The exhaust gas then contains the oxidizing components oxygen (O
2
), and nitrogen oxides (NO
x
) in an excess amount compared to the reducing components carbon monoxide (CO), hydrogen (H
2
) and hydrocarbons (HC). A lean exhaust gas usually contains 3-15 vol % oxygen. However, at load or full load operation, even in the case of lean-running gasoline engines are operated with stoichiometric or even substoichiometric, i.e., rich, air/fuel mixtures, too.
Because of the high oxygen content of the exhaust gas of lean burning motors or diesel motors the nitrogen oxides contained in them may not be reduced to nitrogen by means of the so-called three-way catalysts with simultaneous oxidation of hydrocarbons and carbon monoxide, as is the case with stoichiometrically operated gasoline engines.
For this reason nitrogen oxide storage catalysts that store the nitrogen oxides in lean exhaust gas in the form of nitrates were developed to remove the nitrogen oxides from these exhaust gases.
The operation of nitrogen oxide storage catalysts is described in detail in the SAE publication SAE 950809 which is relied on and incorporated herein by reference. Accordingly, nitrogen oxide storage catalysts consist of a catalyst material that is applied for the most part in the form of a coating onto an inert ceramic or metal honeycomb, a so-called carrier element. The catalyst material contains the nitrogen oxide storage material and a catalytically active component. The nitrogen oxide storage material again consists of the actual nitrogen oxide storage component that is deposited on a support material.
Chiefly, the basic oxides of the alkali metals, alkaline earth metals and rare earth metals, in particular barium oxide, which react with nitrogen dioxide to form the corresponding nitrates, are used as storage components. It is known that these materials in air occur for the most part in the form of carbonates and hydroxides. These compounds are also suitable for storage as nitrogen oxides. For this reason, when the term “basic storage oxides” is used within the scope of the invention, the corresponding carbonates and hydroxides are also included in this term.
The substances usually used as the catalytically active components are the noble metals of the platinum group, which as a rule are deposited onto the support material together with the storage component. Mainly active, high surface area aluminum oxide is used as support material.
The task of the catalytically active components is to convert carbon monoxide and hydrocarbons in the lean exhaust gas to carbon dioxide and water. In addition, they are intended to oxidize nitrogen monoxide contained in the exhaust gas to nitrogen dioxide, so that it can react with the basic storage material to form nitrates. The storage capacity of the storage material drops off with increasing deposition of nitrogen oxides in it and for this reason it has to be regenerated from time to time. To do this, the engine is run with stoichiometric or rich air/fuel mixtures for a short time. Under the reducing conditions in the rich exhaust gas the nitrates that are formed are broken down to nitrogen oxides NO, and, using carbon monoxide, hydrogen and hydrocarbons as reducing agents, reduced to nitrogen, forming water and carbon dioxide. The storage catalyst operates as a three way catalyst during this phase of operation.
Various combinations of storage components and support materials are known from the patent literature. For instance, 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, where at least one part of the barium oxide and the lanthanum oxide forms a mixed oxide. This mixed oxide is intended to suppress the formation of lanthanum aluminate, which would otherwise lead to aging of the catalyst. To make the catalyst, a honeycomb carrier element is first coated with an aluminum oxide dispersion and then dried and calcined. Then the coating is impregnated sequentially or simultaneously with a lanthanum salt solution and a barium salt solution, dried and calcined at 300° C. for a period of 1 h. Then the coating is impregnated with a platinum salt solution, again dried and calcined.
EP 0 653 238 A1 proposes the use of titanium oxide as support material, which contains at least one element from the group consisting of the alkali metals, alkaline earth metals and the rare earth metals in the form of a solid solution. To produce this storage material, titanium dioxide is impregnated with a solution of the precursor compounds of the storage components and then calcined at temperatures over 600° C. to form the solid solution.
EP 0 666 103 A1 describes a catalyst that 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 proposed as support materials. The nitrogen oxide storage component and noble metal are deposited in close association on these support particles. In addition, the catalyst can also contain cerium oxide as an oxygen storage component, with the cerium oxide being kept separate from the noble metal and thus from the nitrogen oxide storage component. To deposit the storage components and the noble metal onto the support material it is impregnated with solutions of precursors of these components and calcined.
WO 97/02886 describes a nitrogen oxide storage catalyst in which metal oxides, metal hydroxides, metal carbonates and metal mixed oxides are used as storage components. The metals can be lithium, sodium, potassium, rubidium, cesium, magnesium, calcium, strontium or barium. The storage components can either be used in powder form or deposited on aluminum oxide by impregnation with precursor compounds of the storage components. The impregnated aluminum oxide powder is calcined for 2 h at 550° C.
DE 197 13 432 A1 describes a catalytic base material for an exhaust gas purification catalyst that is obtained by impregnating cerium oxide powder with a barium-containing solution and calcining the cerium oxide particles at about 400-1100° C. to form and accumulate barium oxides on the surface of the cerium oxide particles. According to this publication, a mixture of barium oxides and cerium oxide particles is heated to a relatively high temperature in order to intentionally form coarse-grained barium oxides on the surface of the cerium oxide particles. Temperatures of 800-1100° C. are effective for this. The cerium oxide particles are preferably calcined at 900° C. for a period of 24 h. Particle sizes of the barium oxide particles between 7 and 14 &mgr;m result. At a calcination temperature of 200° C. the barium oxide particles still have an average particle size of 1.9 &mgr;m.
The catalytic base material in accordance with DE 197 13 432 A1 serves for making a catalyst that is particularly effective when it is used on an engine with lean combustion. This catalyst is thus a so-called lean NO
x
catalyst, which is capable in the presence of sufficient reducing components in the exhaust gas (carbon monoxide and hydrocarbons) of converting nitrogen oxides even in a constantly lean exhaust gas. The catalytic base material increases the temperature
Foerster Martin
Göbel Ulrich
Kiessling Ralph
Ruwisch Lutz Marc
Bos Steven
Degussa-Huls Aktiengesellschaft
Kalow & Springut LLP
Kuhar Anthony
LandOfFree
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