Power plants – Internal combustion engine with treatment or handling of... – By means producing a chemical reaction of a component of the...
Reexamination Certificate
1999-02-08
2003-09-30
Langel, Wayne A. (Department: 1754)
Power plants
Internal combustion engine with treatment or handling of...
By means producing a chemical reaction of a component of the...
C423S213500, C423S239100, C423S245100, C423S247000, C502S328000, C502S332000, C502S333000, C502S334000, C502S339000
Reexamination Certificate
active
06625976
ABSTRACT:
This invention relates to catalysts used to remove undesirable components in the exhaust gas from internal combustion engines. More particularly, the invention is concerned with improved catalysts of the type generally referred to as three-way conversion or TWC catalysts.
The exhaust from internal combustion engines contains hydrocarbons. carbon monoxide and nitrogen oxides which must be removed to levels established by various government regulations. The aforementioned three-way catalysts are poly-functional in that they have the capability of substantially simultaneously catalysing the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides.
Typical three-way catalysts which exhibit good catalytic activity and long life contain one or more platinum group metals (eg Pt, Pd, Rh, Ru and Ir) located upon a high surface area porous refractory oxide support, eg a high surface area alumina coating. The porous refractory oxide support is carried on a suitable non-porous refractory substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure or refractory particles such as spheres, pellets or short extruded segments of a suitable refractory material.
Three-way catalysts are currently formulated with complex washcoat compositions containing stabilised alumina, an oxygen storage component (primarily stabilised ceria) and precious metal catalytic components. The term “oxygen storage component” is used to designate a material which is capable of being oxidised during oxygen-rich (lean) cycles of the exhaust gas being treated and reduced during oxygen-poor (rich) cycles of the exhaust gas being treated.
Three-way catalysts typically have been based on platinum/rhodium catalysts in preference to palladium which suffered from certain disadvantages including the high sensitivity of palladium to poisoning by sulphur and lead. However, with increased use of lead free petrol around the world, palladium is an extremely promising substitute for the traditionally used platinum/rhodium catalysts. Furthermore, the much lower cost of palladium makes it a highly desirable alternative to platinum/rhodium in three-way catalysts, provided the desired catalytic performance can be achieved.
The art has devoted a great deal of effort in attempts to improve the efficiency of palladium containing three-way catalysts. Thus, in an article in Third Int. Cong. Catal. and Auto Poll. Controls, Pre-print Vol. 1, pages 125 to 135, the authors, Dettling et al, describe the inclusion of a low temperature catalyst component (Pd/Al
2
O
3
) and a high temperature catalyst component (Pd/CeO)
2
in the same catalyst composition for high activity under both low and high operating temperatures.
WO 95/00235 (Engelhard Corporation) also describes a palladium containing catalyst composition containing low and high temperature catalyst components structured as two washcoat layers
WO 95/07600 (Allied Signal) describes a palladium containing three-way catalyst as a single layer. However, according to the method of preparation, the finished catalyst only has the high temperature Pd/CeO
2
component.
U.S. Pat. Nos 4,727,052, 5,057,483, 5,008,090 and 5,010,051; GB Patent 1495637; and European Patent Applications 92302928.4 and 0427293A2 also describe three-way conversion catalysts based on platinum group metal catalytic components.
We have found that platinum group metal three-way catalysts containing a high temperature functional component and a low temperature functional component when prepared by the unique methods of the present invention exhibit greatly improved three-way catalytic activity even after extended high temperature aging.
In this specification, by high temperature functional catalytic component is meant a catalytic component which exhibits catalytic activity at higher temperatures (eg above about 500° C.) and by low temperature functional catalytic component is meant a catalytic component which exhibits catalytic activity at lower temperatures (eg in the range 200 to 400° C.)
According to the present invention there is provided a method of making a platinum group metal three-way catalyst composition which contains a high temperature catalytic component and a low temperature catalytic component with each catalytic component being present in the catalyst composition as separate distinct particles in the same washcoat layer, which method comprises:
(a) forming on a non-porous substrate a combined washcoat of a high temperature catalyst support material and a low temperature catalyst support material from a slurry in which each of the catalyst support materials is of sufficiently large particle size so as to prevent each catalyst support material from forming a solution or a sol with the liquid medium of the slurry; and
(b) impregnating a platinum group metal or metals into each catalyst support material either after formation of the washcoat on the non-porous substrate or before forming the washcoat slurry.
Preferably, separate slurries of the high temperature support material and the low temperature support material are prepared and the two slurries are then blended together and coated onto the non-porous substrate.
The non-porous substrate may be a refractory ceramic or metal honeycomb structure or refractory particles such as spheres, pellets or short extruded segments of a suitable refractory material.
Further according to the present invention, the proportions of the high temperature catalytic component and the low temperature catalytic component required in the catalyst composition are determined by the respective water absorption capabilities of each catalyst support material and the respective amounts of each catalyst support material present in the washcoat.
Preferably, the water absorption capabilities of the high temperature catalyst support material and the low temperature catalyst support material are respectively 0.2 to 1.0 ml/g and 0.5 to 2.5 ml/g.
Suitably, the catalyst support materials have a mean particle size of less than 20 microns, preferably between 1 and 20 microns and more preferably about 5 microns.
The platinum group metal is selected from platinum, palladium, rhodium, ruthenium, iridium or any combination thereof.
Preferably, the high temperature catalyst support material is an oxygen storage material.
Suitable oxygen storage materials include ceria, perovskites. NiO, MnO
2
and Pr
2
O
3
with stablised ceria being the preferred material.
Suitable stabilisers for ceria include zirconium, lanthanum, alumina, yttrium, praeseodymium and neodymium with zirconium being preferred.
Suitably, the zirconium stablised ceria contains 2 to 50% ZrO
2
, a preferred composition being about 58% by weight CeO
2
and about 42% by weight ZrO
2
.
Suitable low temperature catalyst support materials are stabilised alumina and unstabilised alumina.
Suitable stabilisers for alumina include lanthanum, barium and zirconium with lanthanum being preferred.
Preferably, the lanthanum stabilised alumina contains 2 to 7% lanthanum oxide.
The method of the invention may utilise a catalyst promoter, preferably selected from Nd, Ba, Ce, La, Pr, Mg, Ca and Sr with Nd and Ba being particularly suitable. The catalyst promoters may be added to the slurry or separately impregnated.
Further preferably, the method of the invention utilises a compound effective for the suppression of hydrogen sulphide emissions from the catalyst composition. Suitable such compounds include NiO, Fe
2
O
3
and BaO with NiO being preferred.
Suitably, the method according to the invention utilises a compound which is effective in preventing preferential absorption of the platinum group metal in one or other of the high temperature or low temperature catalyst support materials. Preferred such compounds include citric acid, acetic acid and oxalic acid.
From another aspect, the present invention is a platinum group metal three-way catalyst composition made by any of the methods described above.
From yet another aspect the present invention is a platinum group metal three-way catalyst compositi
Andersen Paul Joseph
Bennett Christopher John
Cooper Barry John
Shady Phillip
Johnson Matthey Public Limited Company
Langel Wayne A.
RatnerPrestia
Strickland Jonas N.
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