Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1998-06-19
2002-02-19
Griffin, Steven P. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S325000, C502S326000, C502S328000, C502S349000, C502S514000, C502S527120
Reexamination Certificate
active
06348430
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to an exhaust gas treatment catalyst for internal combustion engines with two catalytically active layers on a carrier structure.
Combustion engines emit, as the main pollutants in the exhaust gas, carbon monoxide CO, unburnt hydrocarbons HC and nitrogen oxides NO
x
, a high percentage of which are converted into the harmless components water, carbon dioxide and nitrogen by modern exhaust gas treatment catalysts. Conversion takes place under substantially stoichiometric conditions, that is the oxygen contained in the exhaust gas is controlled using a so-called lambda sensor in such a way that the oxidation of carbon monoxide and hydrocarbons and the reduction of nitrogen oxides to nitrogen can take place almost quantitatively. The catalysts developed for this purpose are called three-way catalytic converters.
Stoichiometric conditions prevail when the normalized air/fuel ratio &lgr; is 1. The normalized air/fuel ratio &lgr; is the air/fuel ratio standardized to stoichiometric conditions. The air/fuel ratio states how many kilograms of air are required for complete combustion of one kilogram of fuel. In the case of conventional gasoline engine fuels, the stoichiometric air/fuel ratio has a value of 14.6. The engine exhaust gas has more or less large, periodic variations in normalized air/fuel ratio depending on the load and the engine speed. To produce better conversion of oxidizable hazardous components under these conditions, oxygen-storing components such as, for example, pure cerium oxide or cerium oxide-containing components are used which bind oxygen when it is present in excess and release it again when there is a deficiency of oxygen in the exhaust gas.
The present invention deals with catalyst coatings on inert, monolithic carrier structures, in particular honeycomb structures with parallel flow channels for the exhaust gas. The number of flow channels per cross-sectional area is called the cell density. Inert carrier structures with cell densities between 10 and 250 cm
−2
are used, depending on the requirements of the application. These may be extruded, ceramic carrier structures made from cordierite, mullite or similar, temperature resistant materials. Alternatively, honeycomb structures made from steel sheeting may be used.
In the context of the present invention, a layer is called catalytically active when it is able to at least partly catalyze the conversion, mentioned above, of hazardous substances contained in the exhaust gas from combustion engines to give harmless components. Included among the harmful substances are in particular carbon monoxide, nitrogen oxides and hydrocarbons, wherein the hydrocarbons also include hydrocarbons present in the condensed form on soot particles in the exhaust gas.
The catalytic coating generally contains, as catalytically active components, several noble metals from the platinum group in the Periodic Table of Elements and high surface area materials and other components such as oxygen-storing materials, promoters and stabilizers. The coating is applied to the internal walls of the flow channels by known coating processes using an aqueous coating dispersion which contains the various components in the catalyst.
The catalyst components may be added to the coating dispersion in a variety of forms:
a) as “finely divided solids”
This is understood to mean powdered materials with particle sizes from 1 to about 50 &mgr;m. In the English language literature, the expressions “bulk material” or “particulate material” are used for these.
b) as “colloidal solids”
These have particle sizes of less than 1 &mgr;m. The particulate structure of finely divided and colloidal solids is retained even in the final catalyst coating.
c) in the form of soluble “precursor compounds”
Precursor compounds are generally deposited as high surface area solids and converted into the actual catalysis-promoting components by thermal treatment in an oxidative or reductive atmosphere and are then present in a highly dispersed form with crystallite sizes of generally less than 10 nm. At extremely high concentration or in the case of very low solubility, precursor substances may also be present in between the finely divided solids and have particle sizes similar to those of the latter.
The finely divided solids in the coating dispersion serve in part as support materials for the highly disperse materials resulting from the precursor compounds. The finely divided solids must have a high specific surface area for this purpose. Materials with high surface areas in the context of this invention are those with a specific surface area, also called the BET surface area, of more than 10 m
2
/g. The specific surface area can be measured using nitrogen absorption isotherms in accordance with DIN 66132.
Examples of solids with high surface areas are the so-called active aluminum oxides. These are finely divided aluminum oxides which have the crystal structure of transition phases of aluminum oxide. Chi, delta, gamma, kappa, theta and eta-aluminum oxide being included here.
Active aluminum oxides have specific surface areas of up to 400 m
2
/g. With increasing temperature, the crystal structures mentioned above are interconverted with a simultaneous reduction in the specific surface area (see Ullmann's Encyclopedia of Industrial Chemistry; 5th edition 1985; vol. A1; pages 557-563). Above 1150° C., only low surface area alpha-aluminum oxide is stable. This process can be slowed down by stabilizing with alkaline earth metal oxides, in particular barium oxide, rare earth oxides, preferably lanthanum oxide, or silicon dioxide. Stabilized, active aluminum oxides usually contain 1 to 10 wt. % of barium oxide, lanthanum oxide or silicon dioxide, with reference to the total weight of stabilized material, for this purpose.
To differentiate high surface area support materials from the inert, monolithic carrier for the coating, the latter is called a carrier structure in the context of this invention, the high surface area support materials, on the other hand, being called supports or support materials.
Pure cerium oxide or mixed oxides of cerium and zirconium are frequently used as oxygen-storing materials. Mixed oxides are obtainable, for example, by coprecipitation of precursor compounds of the two elements. Cerium-rich mixed oxides with more than 50 wt. % of cerium and zirconium-rich mixed oxides with more than 50 wt. % of zirconium are known. Cerium-rich mixed oxides are called cerium/zirconium mixed oxides and zirconium-rich mixed oxides are called zirconium/cerium mixed oxides in the following description.
EP 0 314 057 B1 describes a rhodium-free, three-way catalytic converter which has two catalytically active layers on a carrier structure, wherein the first layer applied to the carrier structure contains platinum and the second, upper layer contains palladium. Active aluminum oxide is used as support material for these components in both cases. The layers also contain cerium oxide which is introduced by means of a cerium salt and/or a solid cerium compound. The layers may also contain zirconium oxide, lanthanum oxide, neodymium oxide, praseodymium oxide and nickel oxide, as separate substances or in a mixture. The noble metals are introduced to the layers by impregnation. In a similar way, EP 0 314 058 B1 describes a platinum-free three-way catalytic converter which consists of two catalytically active layers on a carrier structure. The first layer contains palladium and the second layer contains rhodium. In this case again, active aluminum oxide is used as the support material. Both layers also contain cerium oxide and optionally the same promoters and stabilizers as those in accordance with EP 0 314 057 B1.
U.S. Pat. No. 5,057,483 also describes a catalyst composition comprising two discrete layers on a monolithic carrier structure. The first layer contains a stabilized aluminum oxide as support material for platinum and finely divided cerium oxide. The first layer may also contain finely divided iron oxide
Kreuzer Thomas
Lindner Dieter
Lox Egbert
Mussmann Lothar
Van Yperen Renee
Degussa - AG
Griffin Steven P.
Smith , Gambrell & Russell, LLP
Vanoy Timothy C.
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