Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
2001-03-28
2003-02-25
Wood, Elizabeth D. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S302000, C502S303000
Reexamination Certificate
active
06524992
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to a single layer high performance three-way catalyst (TWC) containing a catalytic coating comprising platinum, rhodium and various oxide materials on an inert carrier body.
Three-way catalysts are used to convert the pollutants carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NO
x
) contained in the exhaust gas of internal combustion engines into harmless substances. Known three-way catalysts with good activity and durability utilize one or more catalytic components from the platinum group metals such as platinum, palladium, rhodium deposited on a high surface area, refractory oxide support, e.g., a high surface area alumina. The support is usually carried in the form of a thin layer or coating on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure.
The ever increasing demand for improved catalyst activity and life has led to complex catalyst designs comprising multiple catalyst layers on carrier structures, each of the layers containing selected support materials and catalytic components as well as so called promoters, stabilizers and oxygen storage compounds.
For applying the different layers onto the carrier structures so-called coating dispersions, coating compositions or washcoat compositions are prepared which comprise the support materials in finely divided form and optionally additional soluble components. The liquid phase of the coating composition is preferably water. This coating composition is used to apply the catalytic coating onto the carrier structures. The techniques for applying the coating are well known to the expert. The fresh coating is then dried and calcined to fix the coating and to transform the optional soluble components of the coating composition into their final insoluble form.
For the production of double or multiple layer catalysts a dedicated coating composition for each layer has to be provided. This increases the production costs. Therefore, it is one object of the present invention to design a single layer catalyst which approximates the catalytic properties of sophisticated multiple layer catalysts.
Modern three-way catalysts make use of the platinum group metals platinum, palladium and rhodium. Platinum and palladium mainly promote the oxidation of hydrocarbons (HC) and carbon monoxide (CO) and may be present in the catalyst simultaneously or alternatively. Rhodium primarily promotes the reduction of nitrogen oxides (NO
x
). While platinum and palladium may replace each other to a certain extent, this is not the case for rhodium. The exhaust gas cleaning efficiencies promulgated by the most recent legal exhaust gas standards can only be met at reasonable cost by using rhodium together with one or both of platinum and palladium.
On the other hand it was observed that rhodium containing three-way catalysts suffer under the so-called fuel-cut ageing. The term fuel-cut ageing describes catalyst performance degradation due to fuel-cut after high load operation of the internal combustion engine. Such a situation occurs frequently during fast driving phases when abrupt deceleration is required. During fast driving phases the engine is operated at air/fuel ratios slightly below the stoichiometric value. The exhaust gases may reach temperatures well above 900° C. resulting in even higher catalyst temperatures due to the exothermic conversion reactions at the catalyst. In case of abrupt deceleration modern motor electronics completely stop fuel supply to the engine with the result that the normalized air/fuel ratio (also called lambda value &lgr;) of the exhaust gas jumps from rich to lean values.
These large excursions of the normalized air/fuel ratio from rich to lean values at high catalyst temperatures degrade catalytic activity. Catalytic activity can at least partly be recovered by prolonged operation under stoichiometric or rich exhaust gas conditions. The faster catalytic activity is regained after fuel-cut ageing the better is the overall catalyst performance. Speeding up recovery of catalytic activity after fuel-cut ageing is therefore mandatory for modern three-way catalysts.
Therefore, it is another object of the present invention to provide a catalyst with higher resistance towards fuel-cut ageing. That is, after high temperature ageing under lean exhaust gas conditions, the catalyst should recover its full three-way efficiency quickly. Reduced fuel-cut ageing will also improve the overall dynamic behaviour of the catalyst.
U.S. Pat. No. 4,965,243 discloses a single layer three-way catalyst comprising, on activated alumina, platinum and rhodium in a weight ratio of 5:1 and further cerium oxide, barium oxide and zirconium oxide. This combination of components is said to be very effective for maintaining an excellent catalyst activity even after the catalyst has been exposed to high temperatures of 900 to 1100° C.
U.S. Pat. No. 5,200,384 describes a single layer three-way catalyst comprising, on activated alumina, platinum and rhodium in a weight ratio of 5:1 and further cerium oxide an a coprecipitated ceria-stabilized zirconia having a weight ratio of ceria to zirconia between 1:99 and 25:75. The addition of the coprecipitated ceria-stabilized zirconia to the three-way catalyst is said to enhance the activity of the catalyst at low temperature after high temperature ageing.
U.S. Pat. No. 5,254,519 discloses a single layer catalyst comprising a combination of a coformed rare earth oxide-zirconia having a rhodium component dispersed thereon and a first activated alumina having a platinum component dispersed thereon. The catalyst may comprise a second rhodium component dispersed on the first alumina support. Alternatively, the second rhodium component may be dispersed on a second alumina component.
During the last years there could be observed a tendency of replacing platinum in three-way catalysts completely with palladium because of its lower price and good oxidation activity. Palladium/rhodium and platinum/palladium/rhodium three-way catalysts had been developed which exhibited excellent catalytic activities at high palladium loads. Meanwhile the high demand for palladium has created a world-wide palladium shortage associated with a large increase of palladium prices. Nowadays palladium is more expensive than platinum. Therefore it is still another object of the present invention to provide a catalyst using platinum and rhodium with less precious metal costs but equivalent catalytic activity compared to palladium and rhodium containing catalysts.
SUMMARY OF THE INVENTION
These and further objects of the invention can be achieved with a single layer high performance catalyst containing on an inert carrier body a catalytic coating comprising platinum, rhodium and various oxide materials.
The catalyst is characterized in that the catalytic coating comprises
a) at least one first support material selected from the group consisting of a first active alumina, ceria rich ceria/zirconia mixed oxide and a zirconia component, said at least one first support material being catalyzed with a first part of the total platinum amount of the catalyst, and
b) a second support material catalyzed with the second part of the total platinum amount and with rhodium said second support material being a second active alumina.
The term “a material is catalyzed with” means that said material holds on its surface catalytically active components in highly dispersed form, such as platinum, rhodium or palladium.
The present invention is based on a co-pending European patent application of the inventors with publication number EP 1 046 423 A2. This application discloses a double layer catalyst with an inner and an outer layer on an inert carrier body comprising noble metals from the platinum group deposited on support materials. In the inner layer platinum is deposited on a first support and on a first oxygen storage component and in the outer layer platinum and rhodium are deposited on a second support and the second layer furt
Kreuzer Thomas
Lindner Dieter
Lox Egbert
Mussmann Lothar
Votsmeier Martin
DMC2 Degussa Metals Catalysts Cerdac AG
Kalow & Springut LLP
Wood Elizabeth D.
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