Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
1995-06-01
2001-08-14
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S213200, C423S244110
Reexamination Certificate
active
06274107
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a catalyst composition and method for the oxidation of oxidizeable components of diesel engine exhaust, and more specifically to the treatment of such diesel exhaust to reduce the content of particulates and other pollutants discharged to the atmosphere.
2. Background and Related Art
As is well-known, diesel engine exhaust is a heterogeneous material which contains not only gaseous pollutants such as carbon monoxide (“CO”) and unburned hydrocarbons (“HC”), but also soot particles which comprise both a dry, solid carbonaceous fraction and a soluble organic fraction. The soluble organic fraction is sometimes referred to as a volatile organic fraction (“VOF”), which terminology will be used herein. The VOF may exist in diesel exhaust either as a vapor or as an aerosol (fine droplets of liquid condensate) depending on the temperature of the diesel exhaust.
Oxidation catalysts comprising a platinum group metal dispersed on a refractory metal oxide support are known for use in treating the exhaust of diesel engines in order to convert both HC and CO gaseous pollutants and particulates, i.e., soot particles, by catalyzing the oxidation of these pollutants to carbon dioxide and water. One problem faced in the treatment of diesel engine exhaust is presented by the presence of sulfur in diesel fuel. Upon combustion, sulfur forms sulfur dioxide and the oxidation catalyst catalyzes the SO
2
to SO
3
(“sulfates”) with subsequent formation of condensible sulfur compounds, such as sulfuric acid, which condense upon, and thereby add to, the mass of particulates. The sulfates also react with activated alumina supports to form aluminum sulfates, which render activated alumina-containing catalysts inactive. In this regard, see U.S. Pat. No. 4,171,289 at column 1, line 39 et seq. Previous attempts to deal with the sulfation problem include the incorporation of large amounts of sulfate-resistant materials such as vanadium oxide into the support coating, or the use of alternative support materials such as a-alumina (alpha), silica and titania, which are sulfate-resistant materials.
The prior art also shows an awareness of the use of zeolites, including metal-doped zeolites, to treat diesel exhaust. For example, U.S. Pat. No. 4,929,581 discloses a filter for diesel exhaust, in which the exhaust is constrained to flow through the catalyst walls to filter the soot particles. A catalyst comprising a platinum group metal-doped zeolite is dispersed on the walls of the filter to catalyze oxidation of the soot to unplug the filter.
As is well-known in the art, catalysts used to treat the exhaust of internal combustion engines are less effective during periods of relatively low temperature operation, such as the initial cold-start period of engine operation, because the engine exhaust is not at a temperature sufficiently high for efficient catalytic conversion of noxious components in the exhaust. To this end, it is known in the art to include an adsorbent material, which may be a zeolite, as part of a catalytic treatment system in order to adsorb gaseous pollutants, usually hydrocarbons, and retain them during the initial cold-start period. As the exhaust gas temperature increases, the adsorbed hydrocarbons are driven from the adsorbent and subjected to catalytic treatment at the higher temperature. In this regard, see for example U.S. Pat. No. 5,125,231 which discloses (columns 5-6) the use of platinum group metal-doped zeolites as low temperature hydrocarbon adsorbents as well as oxidation catalysts.
SUMMARY OF THE INVENTION
Generally, in accordance with the present invention, there is provided a catalyst composition and a method for oxidizing oxidizeable components of diesel engine exhaust in which at least some of a volatile organic fraction of the diesel exhaust is converted to innocuous materials, and in which gaseous HC and CO pollutants may also be similarly converted. The objectives of the invention are attained by an oxidation catalyst comprising a catalytic material comprising a mixture of high surface area ceria, a zeolite and, optionally, a high surface area alumina. The catalytic material optionally may carry a low loading of platinum catalytic metal dispersed thereon or palladium catalytic metal dispersed thereon. Alternatively, or in addition, the zeolite of the catalyst composition may be doped, e.g., ion-exchanged, with a catalytic moiety such as one or more of hydrogen ion, platinum, copper, nickel, cobalt, iron, etc. The method of-the invention is attained by flowing a diesel engine exhaust, e.g., the exhaust of a diesel-powered automobile or light truck, into contact under oxidation reaction conditions with a catalyst composition as described above.
Specifically, in accordance with the present invention there is provided a catalyst composition for treating a diesel engine exhaust stream containing a volatile organic fraction, which composition comprises a refractory carrier on which is disposed a coating of a catalytic material comprising a catalytically effective amount of ceria and, optionally, a catalytically effective amount of alumina, each having a BET surface area of at least about 10 m
2
/g, preferably a surface area of from about 25 m
2
/g to 200 m
2
/g, and a zeolite, for example, Beta zeolite or a zeolite selected from the group consisting of Y-zeolite, pentasil (e.g., ZSM-5), Mordenite, and mixtures thereof.
In one aspect of the present invention, the zeolite comprises a three-dimensional zeolite characterized by pore openings whose smallest cross-sectional dimension is at least about 5 Angstroms and having a silicon to aluminum atomic ratio (“Si:Al atomic ratio”) of greater than 5, e.g., a Si:Al atomic ratio of from about 5 to 400.
In another aspect of the invention, the zeolite comprises from about 10 to 90, preferably from about 20 to 70, percent by weight, the alumina comprises from about 60 to 5, preferably from about 50 to 20, percent by weight, and the ceria comprises from about 60 to 5, preferably from about 50 to 20, percent by weight, of the combined weight of the zeolite, the alumina and the ceria.
Yet another aspect of the invention provides for the zeolite to be doped with a catalytic moiety, e.g., ion-exchanged or impregnated, with an ion or with a neutral metal-containing species selected from the group consisting of one or more of hydrogen, platinum, rhodium, palladium, ruthenium, osmium, iridium, copper, iron, nickel, chromium and vanadium, preferably, one or both of platinum and iron.
Still another aspect of the invention provides that the refractory carrier has a plurality of parallel exhaust flow passages extending therethrough and defined by passage walls on which the catalytic material is coated, and further comprising either dispersed platinum carried on the catalytic material in an amount of from about 0.1 to about 60, e.g., 0.1 to 15, preferably 0.1 to 5, g/ft
3
or dispersed palladium carried on the catalytic material in a quantity of from about 0.1 to 200, preferably 20 to 120, g/ft
3
.
In accordance with the method aspect of the present invention, there is provided a method for treating a diesel engine exhaust stream containing a volatile organic fraction, the method comprising contacting the stream with any of the catalyst compositions described above under oxidizing conditions including a temperature high enough to catalyze oxidation of at least some of the volatile organic fraction. For example, the temperature of the exhaust stream initially contacted with the catalyst composition may be from about 100° C. to 800° C.
DEFINITIONS
As used herein and in the claims, the following terms shall have the indicated meanings.
The term “BET surface area” has its usual meaning of referring to the Brunauer, Emmett, Teller method for determining surface area by N
2
adsorption. Unless otherwise specifically stated, all references herein to the surface area of a ceria, alumina or other component refer to the BET surface area.
The term “activated alumina” has its usual meaning
Adomaitis John R.
Deeba Michel
Farrauto Robert J.
Voss Kenneth E.
Yavuz Bulent O.
Engelhard Corporation
Griffin Steven P.
Ildebrando Christina
Negin Richard A.
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