Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Silicon containing or process of making
Reexamination Certificate
2001-04-13
2004-08-17
Langel, Wayne A. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Silicon containing or process of making
C502S243000, C502S252000, C502S261000, C502S262000, C502S324000, C502S325000, C502S328000, C502S330000, C502S332000, C502S334000, C502S339000
Reexamination Certificate
active
06777370
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a layered catalyst composite useful for reducing contaminants in exhaust gas streams, especially gaseous streams containing sulfur oxide contaminants. More specifically, the present invention is concerned with improved catalysts of the type generally referred to as “three-way conversion” catalysts. The layered catalysts trap sulfur oxide contaminants which tend to poison three-way conversion catalysts used to abate other pollutants in the stream. The layered catalyst composites of the present invention have a sulfur oxide absorbing layer before or above a nitrogen oxide absorbing layer. The sulfur oxide absorbing layer selectively and reversibly absorbs sulfur oxides over nitrogen oxides and alleviates sulfur oxide poisoning of the nitrogen oxide trap.
2. Related Art
Three-way conversion catalysts (“TWC”) have utility in a number of fields including the abatement of nitrogen oxides (“NO
X
”), carbon monoxide (“CO”), and hydrocarbon (“HC”) pollutants from internal combustion engines, such as automobile and other gasoline-fueled engines. Three-way conversion catalysts are polyfunctional because they have the ability to substantially simultaneously catalyze the oxidation of hydrocarbons and carbon monoxide and the reduction of nitrogen oxides. Emissions standards for nitrogen oxides, carbon monoxide, and unburned hydrocarbon contaminants have been set by various government agencies and must be met by new automobiles. In order to meet such standards, catalytic converters containing a TWC catalyst are located in the exhaust gas line of internal combustion engines. The catalysts promote the oxidation by oxygen in the exhaust gas of the unburned hydrocarbons and carbon monoxide and the reduction of nitrogen oxides to nitrogen. For example, it is known to treat the exhaust of engines with a catalyst/NO
X
sorbent which stores NO
X
during periods of lean (oxygen-rich) operation, and releases the stored NO
X
during the rich (relatively fuel-rich) periods of operation. During periods of rich operation, the catalyst component of the catalyst/NO
X
sorbent promotes the reduction of NO
X
to nitrogen by reaction of NO
X
(including NO
X
released from the NO
X
sorbent) with HC, CO, and/or hydrogen present in the exhaust.
TWC catalysts exhibiting good activity and long life comprise one or more platinum group metals, e.g., platinum, palladium, rhodium, ruthenium, and iridium. These catalysts are employed with a high surface area, refractory oxide support such as a high surface area alumina coating. The support is carried on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material. The supported catalyst is generally used with a NO
X
storage (sorbent) component including alkaline earth metal oxides, such as oxides of Ca, Sr and Ba, alkali metal oxides such as oxides of K, Na, Li and Cs, and rare earth metal oxides such as oxides of Ce, La, Pr and Nd, see U.S. Pat. No. 5,473,887.
Sulfur oxide (“SO
x
”) contaminants present in an exhaust gaseous stream tend to poison and thereby inactivate TWC catalysts. SO
X
is a particular problem because it is generated by the oxidation of sulfur compound impurities often found in gasoline and diesel fuel. IWC catalysts employing NO
X
storage components tend to suffer from loss of long-term activity because of SO
X
poisoning of the NO
X
traps. NO
X
trap components also trap SO
X
and form very stable sulfates which require extreme conditions and a high fuel penalty to regenerate the trapping capacity of the NO
X
storage component. A guard or filter (e.g., alumina) may be placed before the TWC catalyst to protect the catalyst from SO
X
but these guards or filters often become saturated with So
X
. Without valves, these guards require artificial engine cycles to desorb SOx by creating extended rich A/F period at elevated temperature. However, the SOx released under these conditions normally caused high H2S emission with unpleasant odor and to some extent poison the downstream NO
X
absorber.
High surface refractory metal oxides are often employed as a support for many of the catalytic components. For example, high surface area alumina materials, also referred to as “gamma alumina” or “activated alumina” typically exhibit a BET (Brunauer, Emmett, and Teller) surface area in excess of 60 square meters per gram (“m
2
/g”), and often up to about 200 m
2
/g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases. Refractory metal oxides other than activated alumina may be utilized as a support for at least some of the catalytic components in a given catalyst. For example, bulk ceria, zirconia, alpha alumina and other materials are known for such use. Although many of these materials have a lower BET surface area than activated alumina, that disadvantage tends to be offset by the greater durability of the resulting catalyst.
Exhaust gas temperatures can reach 1000° C. in a moving vehicle and such elevated temperatures can cause activated alumina, or other support material, to undergo thermal degradation with accompanying volume shrinkage especially in the presence of steam. During this degradation, the catalytic metal becomes occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. U.S. Pat. No. 4,171,288 discloses a method to stabilize alumina supports against such thermal degradation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia, or strontia, or rare earth metal oxides such as ceria, lanthana, and mixtures of two or more rare earth metal oxides.
U.S. Pat. Nos. 4,714,694, 4.727,052, and 4,708,946 disclose the use of bulk cerium oxide (ceria) to provide a refractory oxide support for platinum group metals other than rhodium. Highly dispersed, small crystallites of platinum on the ceria particles may be formed and stabilized by impregnation with a solution of an aluminum compound followed by calcination.
U.S. Pat. No. 3,993,572 discloses catalysts for promoting selective oxidation and reduction reactions. The catalyst contains platinum group metal, rare earth metal (ceria) and alumina components which may be supported on a relatively inert carrier such as a honeycomb.
U.S. Pat. No. 4,714,694 discloses a method of making a material which includes impregnating bulk ceria or a bulk ceria precursor with an aluminum compound and calcining the impregnated ceria to provide an aluminum stabilized ceria.
U.S. Pat. No. 4,808,564 discloses a catalyst for the purification of exhaust gases having improved durability which comprises a support substrate, a catalyst carrier layer formed on the support substrate and catalyst ingredients carried on the catalyst carrier layer. The catalyst carrier layer comprises oxides of lanthanum and cerium in which the molar fraction of lanthanum atoms to total rare earth atoms is 0.05 to 0.20 and the ratio of the number of the total rare earth atoms to the number of aluminum atoms is 0.05 to 0.25.
U.S. Pat. No. 4,367,162 discloses a three-way catalyst system which comprises a carrier having a substructure of refractory material in the form of a honeycomb structure and a porous layer of a powder formed on the surface thereof selected from the group consisting of a powder of zirconium oxide and a mixed powder of zirconium oxide powder with at least powder selected from the group consisting of alumina, alumina-magnesia spinel and cerium oxide, and a catalyst ingredient supported thereon consisting of cerium oxide and a metal selected from the group consisting of platinum, palladium, and mixtures thereof.
U.S. Pat. No. 4,438,219 discloses an alumina catalyst, stable at high temperatures, for use on a substrate. The stabilizing material is d
Engelhard Corporation
Langel Wayne A.
Negin Richard A.
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