Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Silicon containing or process of making
Reexamination Certificate
1996-02-21
2001-07-03
Niebling, John F. (Department: 2812)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Silicon containing or process of making
C502S262000, C502S304000, C423S213500
Reexamination Certificate
active
06255249
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 a gas-borne stream, e.g., for treatment of diesel engine exhaust, and more specifically to the treatment of such diesel exhaust to reduce the particulates content thereof.
2. Background and Related Art
As is well-known, gas-borne streams from industrial processes or engine exhausts often contain oxidizeable pollutants such as unburned fuel and vaporized or con-densed oils. For example, diesel engine exhaust contains not only gaseous pollutants such as carbon monoxide (“CO”) and unburned hydrocarbons (“HC”), but also soot particles which, as described in more detail below, comprise both a dry carbonaceous fraction and a hydrocarbon liquid which is sometimes referred to as a volatile organic fraction (“VOF”), which terminology will be used herein, or a soluble organic fraction. Accordingly, although sometimes loosely referred to as an “exhaust gas”, the exhaust of a diesel engine is actually a heterogeneous material, comprising gaseous, liquid and solid components. 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 alpha-alumina (&agr;-alumina), silica and titania, which are sulfation-resistant materials.
Generally, the prior art has attempted to deal with these problems by dispersing a suitable oxidation catalyst metal, such as one or more platinum group metals, upon a refractory metal oxide support which is resistant to sulfation.
CO-PENDING PARENT APPLICATION
Co-pending parent application Ser. No. 07/798,437 filed Nov. 26, 1991 discloses and claims a catalyst composition and method for the treatment of diesel engine exhaust and co-pending related application Ser. No. 07/973,461, a continuation-in-part of Ser. No. 07/798,437, discloses and claims an oxidation catalyst and method for the oxidative purification of pollutants in gas-borne streams generally, including diesel engine exhaust streams. The catalyst composition comprises a catalytic material consisting essentially of bulk ceria and bulk alumina. Optionally, a low loading (0.1 to 15 g/ft
3
of catalyst volume) of platinum or a conventional loading (0.1 to 200 g/ft
3
of catalyst volume) of palladium may be incorporated into the catalytic material of the parent applications.
SUMMARY OF THE INVENTION
Generally, in accordance with the present invention, there is provided an oxidation catalyst composition and a method for oxidizing oxidizeable components of a gas-borne stream, e.g., treating diesel engine exhaust in which at least the volatile organic fraction component (described below) of the diesel exhaust particulates 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 base metal oxide catalytic material consisting essentially of a mixture of high surface area bulk ceria and one or more of the following bulk second metal oxides:
titania, zirconia, ceria-zirconia, silica, alumina-silica and alpha-alumina (&agr;-alumina). The one or more second metal oxides themselves may have a high surface area and, optionally, the catalytic material may further contain a bulk activated alumina. The catalyst composition of the present invention optionally may have dispersed thereon only one of platinum or palladium catalytic metals, for example, a relatively low loading of platinum or a relatively high loading of palladium.
The method of the invention is attained by flowing a gas-borne stream, e.g., a diesel engine exhaust, into contact with the catalyst composition under reaction conditions. Essentially, the compositions of the present invention differ from those of parent application Ser. No. 07/798,437 in that the alumina constituent of the invention described in the parent case is entirely or partly replaced by one or more other second metal oxides.
Specifically, in accordance with the present invention there is provided an oxidation catalyst composition which comprises a refractory carrier on which is disposed a coating of a catalytic material having a BET surface area of at least about 10 m
2
/g and consisting essentially of a combination of ceria having a BET surface area of at least about 10 m
2
/g, preferably from about 25 m
2
/g to 200 m
2
/g, and a bulk second metal oxide selected from the group consisting of one or more of titania, zirconia, ceria-zirconia, silica, alumina-silica and &agr;-alumina.
In accordance with an aspect of the present invention, the second metal oxide has a BET surface area of at least about 10 m
2
/g. Another aspect of the invention provides that the composition further comprises activated alumina having a BET surface area of at least about 10 m
2
/g.
In one aspect of the invention, the ceria and the second metal oxide each comprises from about 5 to 95 percent, preferably from about 10 to 90 percent, more preferably from about 40 to 60 percent, by weight of the combination. The ceria and the second metal oxide may be combined as a mixture and the mixture deposited as a single layer coating on the refractory carrier, or the ceria and the second metal oxide may be present in respective discrete superimposed layers of ceria and the second metal oxide. The ceria layer may be above or below the second metal oxide layer.
When the activated alumina is present it may be mixed with the ceria and the second metal oxide to form a combined ceria-second metal oxide-alumina coating, or the alumina may be mixed within one or both of discrete ceria and second metal oxide layers. Alternatively, the alumina may be present as a discrete activated alumina layer disposed above or below the ceria and second metal oxide layers, or between them.
Another aspect of the invention provides that the catalyst composition optionally further comprises platinum dispersed on the catalytic material, the platinum being present in the amount of from about 0.1 to 15 g/ft
3
of the composition, preferably from about 0.1 to 5 g/ft
3
of the composition. When the catalyst composition includes platinum, another aspect of the invention provides that at least a catalytically effective amount of the platinum is dispersed on the ceria; further, at least a catalytically effective amount of the platinum may also be dispersed on the second metal oxide component.
Yet another aspect of the invention provides that the catalyst composition optionally further comprises palladium dispersed on the catalytic material, the palladium preferably being present in the amount of from about 0.1 to 200 g/ft
3
of the composition, preferably in the amount of from 20 to 120 g/ft
3
, more preferably in the amount of from about 50 to 120 g/ft
3
, of the composition.
Still another aspect of the invention provides that the ceria co
Farrauto Robert J.
Galligan Michael P.
Voss Kenneth E.
Yavuz Bulent O.
Engelhard Corporation
Ghyka Alexander G.
Negin Richard A.
Niebling John F.
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