OXIDATION CATALYST AND METHOD OF USE

Abstract: Oxidation catalyst compositions include a catalytic material having a BET surface area of at least about 10 m<2>/g and consisting essentially of a combination of bulk ceria and a bulk second metal oxide which may be one or more of titania, zirconia, ceria-zirconia, silica, alumina-silica and alpha -alumina. The combination may optionally also include activated alumina having a BET surface area of at least about 10 m<2>/g. The ceria, second metal oxide and optional activated alumina may be mixed together or provided as discrete layers. Optionally, one of platinum or palladium metal may be dispersed on the catalytic material provided that the platinum, when used, is used in limited amounts to preclude excessive oxidation of SO2 to SO3. The catalyst compositions may be used for oxidation of oxidizable components in a gas-borne stream, e.g., in a method to treat diesel engine exhaust by contacting the hot exhaust with the catalyst composition to promote the oxidation of the volatile organic fraction. The optional inclusion of platinum or palladium promotes the oxidation of gas phase components, e.g., hydrocarbons and carbon monoxide.

Description: OXIDATION CATALYST AND METHOD OF USE. CROSS-REFERENCE TO RELATED APPLICATIONS This application is a continuation-in-part of appli¬ cation Serial No. 07/798,437, filed November 26, 1991. BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a catalyst composition and method for the c-idation of oxidizeable components of a gas-borne stream, e.g., for treatment of diesel engine ex¬ haust, and more specifically to the treatment of such die¬ sel exhaust to reduce the particulates content thereof. Backqr d and Related Art is well-known, gas-borne streams from industrial procfe .; or engine exhausts often contain oxidizeable pollu.,. its 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 hydroc?rbons ("HC"), but also soot particles which, as described more detail below, comprise both a dry carbonaceous f: , ion and a hydrocarbon liquid which is sometimes refer to as a volatile organic fraction ("VOF"), which terminology will be used herein, or a solu¬ ble organic fraction. Accordingly, although sometimes loosely referred to as an "exhaust gas", the exhaust of a dies 1 engine is actually a heterogeneous material, com¬ prising 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 tem¬ perature 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 particu- lates, i.e., soot particles, by catalyzing the oxidation of these pollutants to carbon dioxide and water. One pro¬ blem 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 S0 2 to S0 3 ("sulfates") with subse¬ quent 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 inac¬ tive. In this regard, see U.S. Patent 4,171,289 at column 1, lir-- " 9 et seq. Previous attempts to deal with the sulfa.- 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 (α-alumina), sili¬ ca 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 sul- fation. Co-Pending Parent Application Co-pending parent application Serial No. 07/798,437 filed November 26, 1991 discloses and claims a catalyst composition and method for the treatment of diesel engine exhaust and co-pending parent application Serial No. r a continuation-in-part of Serial 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 ex¬ haust streams. The catalyst composition comprises a cata- lytic material consisting essentially of bulk ceria and bulk alumina. Optionally, a low loading (0.1 to 15 g/ft 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 ap¬ plications. 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 pollu¬ tants may also be similarly converted. The objectives of the invention are attained by an oxidation catalyst com¬ prising 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 (α-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 rela- tively high loading of palladium. The method of the invention is attained by flowing a gas-borne stream, e.g., a diesel engine exhaust, into con¬ tact with the catalyst composition under reaction condi¬ tions. Essentially, the compositions of the present in- vention differ from those of parent application Serial No. 07/798,437 in that the alumina constituent of the inven¬ tion described in the parent case is entirely or partly replaces by one or more other second metal oxides. Specifically, in accordance with the present inven- tion 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 α-alumina. In accordance with an aspect of the present inven¬ tion, the second metal oxide has a BET surface area of at least about 10 mVg. Another aspect of the invention pro¬ vides that the composition further comprises activated alumina having a BET surface area of at least about 10 mVg. 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 sin¬ gle 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 com- bined 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 dis¬ posed above or below the ceria and second metal oxide lay- ers, 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 plat¬ inum, 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 palladi¬ um 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 comprises an aluminum-stabilized ceria. In accordance with the method of the present inven¬ tion, there is provided a method of oxidizing oxidizeable components of a gas-borne stream, -.g., a method of treat¬ ing diesel engine exhaust containing a volatile organic fraction. The method includes contacting the stream with a catalyst composition as described above at a temperature high enough to catalyze oxidation of at least some of the oxidizeable component, e.g., the volatile organic frac¬ tion, for example, by contacting the stream or diesel ex¬ haust with the catalyst composition at an inlet tempera¬ ture of 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 "gas-borne stream" means a gaseous stream which may contain non-gaseous components such as solid particulates and/or vapors, liquid mist or droplets, and/or solid particulates wetted by a liquid. The term "BET surface area" has its usual meaning of referring to th-. Brunauer, Emmett, Teller method for de¬ termining surface area by N 2 absorption. Unless otherwise specifically stated, all references herein to the surface area of bulk ceria, the second metal oxides, or other components means the BET surface area. The term "activated alumina" has its usual meaninα of a high BET surface area alumina, comprising primarily one or more of gamma-, theta- and delta-aluminas (γ-, θ- and δ-aluminas) . The term "catalytically effective amount" means that the amount of material present is sufficient to affect the rate of reaction of the oxidation of pollutants in the ex¬ haust being treated. The term "inlet temperature" shall mean the tempera¬ ture of the exhaust, test gas or other stream being treat- ed immediately prior to initial contact of the exhaust, test gas or other stream with the catalyst composition. The term "ceria-second metal oxide catalytic mater¬ ial" means a combination of ceria and second metal oxides selected from the class consisting of one or more of tita- nia, zirconia, ceria-zirconia, silica, alumina-silica and α-alumina, the ceria having a BET surface area of at least about 10 m/g, and the average surface area of the combi¬ nation of high surface area ceria and second metal oxide being at least 10 mVg. The term "combination" when used with reference to a combination of (a) bulk ceria and (b) bulk second metal oxide or (c) the foregoing and bulk activated alumina, in¬ cludes combinations attained by mixtures or blends of (a) and (b) and/or (c) and superimposed discrete layers of (a) and (b) and/or (c). The term "aluminum-stabilized ceria" means ceria which has been stabilized against thermal degradation by incorporation therein of an aluminum compound. As is well-known, high surface area refractory oxides such as ceria and activated alumina are subject to loss of surface area (thermal degradation) and consequent reduction in catalytic efficiency upon prolonged exposure to high tem¬ peratures. A suitable ceria-stabilization technique is shown in U.S. Patent 4,714,694 of C.Z. Wan et ' al, issued on December 22, 1991, the disclosure of which is incorpo¬ rated herein. As disclosed in U.S. Patent 4,714,694, ce¬ ria particles are impregnated with e.g., an aqueous solu¬ tion of a soluble aluminum compound such as aluminum ni- trate, aluminum chloride, aluminum oxychloride, aluminum acetate, etc. After drying and calcining the impregnated ceria in air at a temperature of, e.g., from about 300°C to 600°C for a period of 1/2 to 2 hours, the aluminum com- pound impregnated into the ceria particles is converted into an effective thermal stabilizer for the ceria. The term "aluminum-stabilized" ceria is sometimes used al¬ though the aluminum is probably present in the ceria as a compound, presumably alumina, and not as elemental alumi- num. Of course, ceria may be stabilized against thermal degradation by other means, such as by impregnation with lanthana or zirconia. Activated alumina, which is optionally employed in bulk form as a component of t.-e second metal oxide, may also thermally stabilized by a similar impregnation of the alumina with precursors of rare earth metal oxides such as ceria. However, thermal stabilization of the alu¬ mina is usually not necessary for the temperatures encoun¬ tered in treating diesel engine exhaust. Reference herein or in the claims to ceria, any of the second metal oxides, or alumina as being in "bulk" form means that : he ceria, second metal oxides or alumina are present as σrete particles (which may be, and usu¬ ally are, of veι_ small size, e.g., 10 to 20 microns in diameter or even smaller) as opposed to having been dis¬ persed from solution :. to particles of another component. For example, the thermal stabilization of ceria particles (bulk ceria) with alumina as described above with respect to U.S. Patent 4,714,694 results in the alumina being dis- persed into the ceria particles and does not provide the dispersed alumina in "bulk" form, i.e., as discrete albeit very fine particles of alumina. In the finished product, the bulk particles are consolidated into an adherent coat¬ ing on the refractory carrier. The abbreviation "TGA" stands for thermogravimetric analysis which is measure of the weight change (e.g., loss) of a sample as a function of temperature and/or time. The abbreviation "DTA" stands for differential thermal analysis which is measure of the amount of heat emitted (exother ) or absorbed (endotherm) by a sample as a function of temperature and/or time. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a plot comparing the DTA exotherm areas for combustion of lubricating oil catalyzed by, respec¬ tively, (a) a platinum-containing embodiment of the cata¬ lytic material of the present invention comprising a 1:1 weight ratio mixture of ceria and the indicated second metal oxide, and (b) the second metal oxide alone contain¬ ing platinum; Figure 2 is a plot comparing the TGA onset tempera¬ tures for combustion of lubricating oil catalyzed by, re- spectively, (a) a platinum-containing embodiment of the catalytic material of the present invention comprising a 1:1 weight ratio mixture of ceria and the indicated second metal oxide, and (b) the second metal oxide alone contain¬ ing platinum; and Figure 3 is a plot comparing the DTA onset tempera¬ tures for combustion of lubricating oil catalyzed by, re¬ spectively, (a) a platinum-containing embodiment of the catalytic material of the present invention comprising a 1:1 weight ratio mixture of ceria and the indicated second metal oxide, and (b) the second metal oxide alone contain¬ ing platinum. DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS THEREOF The present invention provides an oxidation catalyst composition which is effective for oxidizing oxidizeable components of a gas-borne stream, for example, for treat¬ ing diesel engine exhaust. In the latter case, the cata¬ lyst composition is particularly effective with regard to reducing the total particulates in the exhaust. The car¬ bonaceous particulates ("soot") component of diesel engine exhaust is, as is well-known, comprised of two major com¬ ponents. One component is relatively dry carbonaceous particles and the other, usually referred to as a volatile organic fraction ("VOF") or soluble organic fraction ("SOF"), is a mixture of high molecular weight hydrocar¬ bons comprised of unburned and partially burned diesel fuel and lubricating oil. The volatile organic fraction is present in the diesel exhaust as either a vapor phase or a liquid phase, or both, depending on the temperature of the exhaust. Generally, it is not feasible to attempt to remove or treat the dry, solid carbonaceous particu- lates component of the total particulates by catalytic treatment, and it is the VOF component which can be most effectively removed by conversion via utilization of an oxidation catalyst. Therefore, in order to reduce the total particulates discharged so as to meet present and impending Government regulations concerning maximum allow¬ able total particulates, the volatile organic fraction, or at least a portion thereof, is oxidized to innocuous C0 2 and H 2 0 by being contacted with an oxidation catalyst under suitable reaction conditions. The required U.S. Government limits for 1991 on HC, CO and nitrogen oxides ("NO χ ") emissions in diesel engine exhaust have been largely met by suitable engine design modifications. For 1994 the HC, CO and NO χ limits remain unchanged from 1991 standards but the upper limit on total particulates will be reduced from the 1991 level of 0.25 grams per horsepow¬ er-hour ("g/HP-hr") to 0.10 g/HP-hr. Although the oxida¬ tion catalysts of the present invention, when employed as a diesel exhaust treatment catalyst, are primarily con¬ cerned with effectuating a reduction in total particu- lates, they are also capable, with the optional addition of platinum (in limited amounts) or palladium, of provid¬ ing the added advantage of also oxidizing a portion of the HC and CO contained in the gaseous component of the diesel engine exhaust. The oxidation catalysts of the present invention also avoid or reduce the unwanted side effect of promoting the oxidation of S0 2 to S0 3 in diesel exhaust which, as noted above, contributes to the particulates problem because the condensation of sulfuric acid and oth- er sulf te condensibles which accumulate on, and add to, the mass of the particulates in the exhaust. However, the oxidation catalysts of the present in¬ vention have utility for uses other than the treatment of diesel engine exhaust, but are useful for oxidation of gas-borne oxidizeable components generally, such as any application in which lubricating oils are discharged, e.g., the exhaust of compressed natural gas engines, etha- nol-fueled engines, compressors, gas turbines, etc. Many alternate-fuel engines such as compressed natural gas en¬ gines are built as diesel engine carcasses and therefore inherently discharge significant quantities of lubricating oils. In accordance with the teachings of the present in- vention it has been found that the beneficial effect of oxidizing pollutants generally, and in particular of re¬ ducing diesel exhaust particulates emissions by oxidation of the volatile organic fraction thereof, can be attained by a c lytic material which is a mixture of one or more selected second metal oxides and a high surface area ceria. For purposes of illustration, the benefits of the present invention will be described in detail below with respect to the treatment of diesel engine exhaust. The basic and novel characteristics of the present invention are believed to reside in the use of the defined combina¬ tion of ceria and second metal oxide as an oxidation cata¬ lyst without the addition of metal catalytic components thereto, except as specifically otherwise defined in cer¬ tain dependent claims. Preferably, but not necessarily, the bulk second metal oxides, like the ceria, will have a surface area of at least about 10 mVg, preferably at least about 20 m 2 /g. For example, the bulk ceria may have a surface area of from about 70 to 150 m 2 /g. The combina¬ tion of ceria and the second metal oxide(s) should have a BET surface area of 10 m 2 /g or higher. Optionally, up to about 90 percent by weight, e.g., from about 5 to 90 per¬ cent by weight, of the total weight of bulk ceria, second metal oxide and activated alumina may be provided by the activated alumina, which may have a BET surface area of 10 m 2 /g or higher, preferably at least about 20 mVg, e.g., a surface area of from about 120 to 180 m/g. When alumina is present, each of the ceria, alumina and other metal ox- ide is preferably present in the amount of at least about 5 percent by weight of the total weight of metal oxides present. A catalyst composition in accordance with the present invention effectively catalyzes the oxidation of the vola- tile organic fraction so as to provide a significant re¬ duction in total particulates in diesel engine exhaust and exhibits good durability, that is, long life. It should be noted that the prior art generally considers refractory base metal oxides used in diesel oxidation catalysts to be merely supports for the dispersal thereon of catalytically active metals such as platinum group metals. In contrast, the present invention that a ceria-second metal oxide catalytic materia- consisting essentially of only bulk ceria and one or more selected bulk second metal oxides which provides a mixture of sufficiently high sur¬ face area, e.g., at least 10 mVg, preferably at least 20 m /g, and dispersed on a suitable carrier, provides a du¬ rable and effective diesel oxidation catalyst. It has further been found that beneficial effects are attained in some circumstances by the optional incorpora¬ tion of platinum or palladium in the catalyst composition, provided that the platinum is present at loadings much lower than those conventionally used in oxidation cata¬ lysts. It has been discovered that, most surprisingly, a limited quantity of platinum in the catalyst composition actually reduces the undesirable oxidation of S0 2 to SO. relative to that encountered by using the ceria-second metal oxide catalytic material alone, while nonetheless promoting some oxidation of CO and HC gaseous components of the diesel exhaust. The suppression of the oxidation of S0 2 to S0 3 by the addition of very low loadings of platinum, e.g., 0.1 g/ft 3 , is a very surprising finding, given the powerful catalytic activity of platinum in pro- moting oxidation reactions generally. Without wishing to be bound by any particular theory, it may be that the presence of a low loading of platinum on the ceria occu¬ pies some catalytic sites on the ceria, thereby moderating the tendency of ceria to promote the oxidation of S0 2 to S0 3 . When palladium is employed, it may be used in much higher loadings than platinum, because palladium even in high loadings, e.g., 50 g/ft 3 or more, does not promote excessive oxidation of S0 2 to S0 3 . If a catalytic metal such as platinum is added to the catalyti- rrmposition, it serves to catalyze the oxidation of gaε - ' --"ze HC and CO pollutants as an added benefit. Howev-- . _;:h catalytic metal is not needed to supplement the =.^ιon of the ceria-second metal oxide catalytic ma- terial in reducing total particulate emissions. The cata¬ lytic metal, be it platinum or palladium, does not appear to play a role in controlling particulates in that the quantity or type of metal (platinum or palladium) utilized in the catalytic material does not significantly affect the rate of particulates conversion. In this regard, see the data presented in parent application Serial No. 07/798,437, incorporated herein. The catalysts of the present invention may take the form of a carrier or substrate, such as a monolithic hon- eycomb structure (a body having a plurality of gas flow passages extending therethrough) , on which is applied a coating of the catalytic material comprising a mixture of high surface area ceria and one or more second metal ox¬ ides and, optionally, activated alumina and, optionally, platinum or palladium. As discussed below, discrete coat¬ ings of the ceria, second metal oxide and alumina may be employed. The Carrier (Substrate) The carrier used in this invention should be rela¬ tively inert with respect to the catalytic composition dispersed thereon. The preferred carriers are comprised of ceramic-like materials such as cordierite, α-alumina, silicon nitride, zirconia, mullite, spodumene, alumina- silica-magnesia or zirconium silicate, or of refractory metals such as stainless steel. The carriers are prefer¬ ably of the type sometimes referred to as honeycomb or monolithic carriers, comprising a unitary cylindrical body having a plurality of fine, substantially parallel gas flow passages extending therethrough and connecting both end-faces of the carrier to provide a "flow-through" type of carrier. Such monolithic carriers may contain up to about 700 or more flow channels ("cells") per square inch of cross section, although far fewer may be used. For example, the carrier may have from about 7 to 600, more usually from about 200 to 400, cells per square inch ("cpsi") . While this discussion and the following examples re¬ late to flow-through type carrier substrates, wall-flow carriers (filters) may also be used. Wall-flow carriers are generally similar in structure to flow-through carri¬ ers, with the distinction that each channel is blocked at one end of the carrier body, with alternate channels blocked at opposite end-faces. Wall-flow carrier sub¬ strates and the support coatings deposited thereon are necessarily porous, as the exhaust must pass through the walls of the carrier in order to exit the carrier struc- ture. The Catalytic Material The ceria-second metal oxide catalytic material may be prepared in the form of an aqueous slurry of ceria particles and second metal oxide particles, the particles optionally being impregnated with the platinum or pal¬ ladium catalytic metal component, if one is to be util¬ ized. The slurry is then applied to the arrier, dried and calcined to form a catalytic material coating ( "wash- coat") thereon. Typically, the ceria and second metal ox¬ ide particles are mixed with water and an acidifier such as acetic acid, nitric acid or sulfuric acid, and ball milled to a desired particle size. The optional catalytic metal component, e.g., plati¬ num or palladium, is, when used, preferably dispersed on the ceria particles or on the second metal oxide parti¬ cles, or on both the ceria and second metal oxide parti- cles. If activated alumina is present as part of the combination of the catalytic material, some or all of the catalytic metal component may be dispersed on it. In such cases, the ceria and/or second metal oxides and/or acti¬ vated alumina act as both as a catalytic material and a support for the optional catalytic metal component. Such incorporation may be carried out after the ceria-second metal oxide catalytic material is coated as a washcoat on¬ to a suitable carrier, by impregnating the coated carrier with a solution of a compound of the metal, followed by drying and calcination. However, preferably, the ceria particles or both the ceria and second metal oxide parti¬ cles are impregnated with a compound of the platinum or palladium catalytic metal before a coating of the ceria- second metal oxide catalytic material is applied to the carrier. In either case, the optional platinum or palla¬ dium metal may be added to the ceria-second metal oxide catalytic material as a solution of a soluble compound of the metal, the solution serving to impregnate the ceria and second metal oxide particles, which may then be dried and the platinum or palladium fixed thereon. Fixing may be carried out by calcination or by treatment with hydro¬ gen sulfide or by other known means, to render the metal in water-insoluble form. Generally, the slurry of ceria and second metal oxide particles, and activated alumina if present, whether or not impregnated with the platinum or palladium metal salt solution, will be deposited upon the carrier substrate and dried and calcined to adhere the catalytic material to the carrier and, when the catalytic metal compound is present, to revert the platinum or palladium compound to the ele¬ mental metal or its oxide. Suitable platinum or palladium compounds for use in the foregoing process include potas¬ sium platinum chloride, ammonium platinum thiocyanate, amine-solubilized platinum hydroxide, chloroplatinic acid, palladium nitrate, and palladium chloride, as is well- known in the art. During calcination, or at least during the initial phase of use of the catalyst, such compounds, if present, are converted into the catalytically active elemental metal or its oxide. In an alternate catalyst composition design, sepa¬ rate, discrete layers of bulk ceria, bulk second metal ox¬ ide and, optionally, bulk activated alumina may be em- ployed. These discrete layers are applied as separate coats superimposed one above the other on the carrier. The order of application of such discrete layers is not important and each layer (of ceria, second metal oxide and, optionally, activated alumina) ma comprise either the first-applied or inner coat or layer, the last-applied or outer coat or layer or, if a third layer is present, the intermediate layer or coat. More than three layers may be used, e.g., a layer of a given material may be re¬ peated or two or more of the second metal oxides may be present as discrete layers of different second metal ox¬ ides. When a catalytic metal is present in a catalyst composition in which the catalytic material is present in two or more discrete layers or coats of materials, the catalytic metal may be dispersed in any one or more of the discrete layers or coats. When the catalytic material is applied as a thin coating or coatings to a suitable carrier, such as de¬ scribed above, the proportions of ingredients are conven¬ tionally expressed as weight of material per unit volume of catalyst, as this measure accommodates the presence of different sizes of catalyst composition voids provided by different carrier wall thicknesses, gas flow passage di¬ mensions, etc. Grams per cubic inch ("g/in 3 ") units are used to express the quantity of relatively plentiful com- ponents such as the ceria-second metal oxide catalytic ma¬ terial, and grams per cubic foot ("g/ft 3 ") units are used to express the quantity of the sparsely used ingredients, such as the platinum or palladium metal. For typical die- sel exhaust applications, the ceria-second metal oxide catalytic material of the present invention generally may comprise from about 0.25 to about 4.0 g/in 3 , preferably from about 0.25 to about 3.0 g/in 3 of the coated carrier substrate, optionally including from about 0 to 25, pref¬ erably from about 0 to 15 g/ft 3 of platinum or 0 to 200 g/ft 3 , preferably from about 0 to 120 g/ft 3 of palladium. Without wishing to be bound by a particular theory, applicants offer the following hypothesis to explain the superior performance of the ceria-second metal oxide cat¬ alytic materials according to this invention when used to treat diesel engine exhaust. It is believed that diesel exhaust contains a significant proportion of gases or vapors which are close to their dew point, i.e., close to condensing to a liquid, and thereby adding to the VOF por¬ tion of the particulates at the conditions obtaining in the exhaust pipe. These "potential particulates" condense in the ceria-second metal oxide catalytic materials, their condensation being enhanced by a capillary condensation effect, a known phenomenon in which a capillary-like ac¬ tion facilitates condensation of oil vapors to liquid phase. The small pore size of the high surface area ce¬ ria-second metal oxide catalytic material is believed to provide such capillary condensation action for the VOF. Generally, the higher the surface area of the ceria and alumina, the smaller is their pore size. As the exhaust temperature increases during increased work loads imposed on the diesel engine, the condensed hydrocarbon liquids (condensed VOF) are desorbed from the ceria-second metal oxide catalytic material and volatilize, at which time the catalytic effect of the ceria-second metal oxide catalytic material, which provides numerous acidic sites, is be¬ lieved to enhance cracking and gas phase oxidation, i.e., combustion, of the desorbed, re-volatilized hydrocarbon (VOF) vapors. Even if a proportion of the vapors re-vola¬ tilized from the condensate is not combusted, the cracking . of heavy VOF components to lighter hydrocarbons reduces the total amount of condensibles, so that the total par- ticulates output from the diesel engine is concomitantly further reduced. In this latter regard, the ceria-second metal oxide catalytic material is believed to act as a trap and a storage medium for condensed or condensible VOF during relatively cool phases of the exhaust, and releases the cracked VOF only upon re-volatilization thereof during relatively hot phases. The porous nature of the ceria- second metal oxide catalytic material is also believed to promote rapid diffusion of the VOF throughout the washcoat structure, there., facilitating relatively low temperature gasif' " ca i an idation of the VOF upon increases in tempc .ture of t. catalyst during higher engine load (and there re increased exhaust gas temperature) cycles. Data on ag " show that the presence of sulfates does not sig- nific -ly adversely affect the capacity of the ceria-sec¬ ond m..-τ-...1 oxide catalytic material to reduce particulate emissions. Generally, other ingredient may be added to the cat¬ alyst composition of the present invention such as conven- tional thermal stabilizers for the activated alumina when it is present, e.g., rare earth metal oxides such as cer¬ ia. Thermal stabilization of high surface area ceria and alumina to militate against ph se conversion to less cata¬ lytically effective low surface area forms is well-known in the art although, as noted above, thermal stabilization of alumina is not usually needed for diesel exhaust ser¬ vice. Such thermal stabilizers may be incorporated into the ceria (or activated alumina when it is used) by im¬ pregnating the particles with, e.g., a solution of a sol- uble compound of the stabilizer metal, for example, an aluminum nitrate solution in the case of stabilizing the ceria, and then drying and calcining the impregnated ceria to convert the aluminum nitrate impregnated therein into alumina. In one embodiment, the catalyst composition of the present invention consists essentially only of the high surface area ceria and high surface area second metal . oxide, preferably present in a weight proportion of 1.5:1 to 1:1.5, with or without thermal stabilizers impregnated therein, and, optionally, platinum in a limited amount or palladium. The basic and novel characteristic of this invention in believed to reside in the use of the combined ceria and second metal oxide as a catalyst without neces- sity of the inclusion of precious metal or other catalytic metals except the optional inclusion of platinum or palla¬ dium. Examples and Data Catalyst compositions in accordance with the present invention may optionally contain an alumina undercoat dis¬ posed on the substrate or carrier with the catalyst com¬ position disposed as a coating on the undercoat. However, it is to be understood that the provision of an undercoat on which the catalytic material of the invention is de¬ picted is optional. It is within the purview of the pres¬ ent invention, (as shown by some of the following exam¬ ples) to deposit the catalytic material directly on the substrate. To demonstrate the efficacy of catalyst compositions in accordance with the present invention a series of cata¬ lyst compositions was prepared according to the following examples using bulk cerium oxide mixed with various other bulk second metal oxides. The weight ratio of ceria to the second metal oxide was 50:50. For comparison purposes comparative catalysts were prepared using the various sec¬ ond metal oxides but without bulk cerium oxide. The cata¬ lyst compositions contained the optional precious metal, platinum, at loading levels equivalent to 0.5 g/ft 3 or 2.0 g/ft 3 . The catalyst compositions were prepared in two forms for testing. Aqueous slurries of the catalytic material were made and used to coat carriers comprised of cordier- ite carriers as described below. These samples were used for testing the gas phase activity of the catalyst compo¬ sitions for hydrocarbon (heptane), CO and S0 2 conversion using a laboratory reactor system, discussed below. Sam¬ ples of dried catalytic material powders were also pre- pared and used to test the lube oil combustion performance of the cat? lyst composition as a measure of the VOF con¬ version efficiency of the various samples. Example 1 A platinum/ceria/γ-alumina catalyst was prepared as follows. A. γ-alumina (supplied by Condea, their designation SBa-150, having a surface area of 150 mVg) was impreg¬ nated with platinum by adding an aqueous solution of amine-solubilized platinum hydroxide dropwise to the alu¬ mina powder with slow mixing (using a dough mixer) in an amount sufficient to bring the alumina powder to incipient wetness and give 0.016 weight percent platinum. (In all the Examples, the weight percent of platinum is based on the total weight of the dried and calcined platinum-im¬ pregnated alumina.) This was followed by addition with mixing of about 5 weight percent glacial acetic acid to precipitate and fix the platinum. (In all the Examples, the weight percent of glacial acetic acid is based on the total weight of dried and calcined platinum-impregnated metal oxides. ) The platinum-treated γ-alumina was then mixed with sufficient water to give a slurry of about 50 weight percent solids. This slurry was then ball milled to achieve a particle size of about 90% by weight of the particles being finer than 12 microns, which required about 8.5 hours of milling. A small portion of the milled, platinum-treated γ-alumina slurry, designated as Slurry 1A, was dried at 110°C overnight and then calcined at 450°C for 2 hours to give Catalyst Powder 1A. B. Bulk cerium oxide (supplied by SKK, Japan) which had been stabilized against thermal degradation with 2.5% alumina (based on the weight of alumina expressed as a percentage of the weight of the alumina-impregnated ceria) and having a surface area of 143 m 2 /g was impregnated with platinum by adding an aqueous solution of amine-solubi¬ lized platinum hydroxide dropwise with slow mixing to the alumina-stabilized cerium oxide powder in an amount suffi¬ cient to bring the cerium oxide powder to incipient wet¬ ness and give 0.016 weight percent platinum, based on the total weight of dried platinum-impregnated powder. This was followed by addition with mixing of about 5 weight percent glacial acetic acid to precipitate and fix the platinum. The platinum-treated cerium oxide was then mixed with sufficient water to give a slurry with about 50 weight percent solids. This slurry was then ball milled to achieve a particle size of about 90% finer than 12 mi¬ crons, which required about 2 hours. This milled, plati¬ num-treated cerium oxide slurry was designated as Slurry IB. C. A portion of milled, platinum-treated γ-alumina slurry (Slurry 1A) and a portion of milled, platinum- treated cerium oxide slurry (Slurry IB) were blended in amounts sufficient to give a mixed slurry with a weight ratio of alumina to ceria of about 50:50. A portion of this mixed slurry of about 50 weight percent solids, des- ignated Slurry IC, was dried at 110°C overnight and then calcined at 450°C for 2 hours to give Catalyst Powder IC. Slurry IC was also used to washcoat cordierite honeycomb substrate cores (1.5 inches in diameter X 3 inches long) having 400 cells per square inch ("cpsi"). The members were dried at 110°C overnight and then calcined at 450°C for 2 hours to give a catalyst with a total washcoat load¬ ing of 1.8 g/in 3 of catalyst volume and a platinum loading of 0.5 g/ft 3 of substrate. This catalyst was designated as Catalyst Member IC. Example 2 A platinum/ceria/α-alumina catalyst was prepared as follows. A. High porosity α-alumina (Allulite foamed ct-alumi- na, supplied by Cabot Corporation) having a surface area of 20 m 2 /g was impregnated with platinum by the incipient wetness technique used in Example 1 to give 0.029 weight percent platinum on α-alumina. A portion of the plati¬ num-treated α-alumina powder was dried at 110°C overnight and then calcined at 450°C for 2 hours to give Catalyst Powder 2A. The platinum-treated α-alumina powder was also added, with mixing, to water along with a silica sol (Nal- co 612) binder at 10% by weight of solids to give a slurry with 25 weight percent total solids content. This slurry was mixed with a mechanical stirrer and not ball milled, so as not to degrade the porous structure of the foamed α-alumina. This slurry was designated as Slurry 2A. B. The same alumina-stabilized bulk cerium oxide used in Example 1, Part B, was impregnated with platinum using the same technique as in Example 1, Part B, except the platinum-treated cerium oxide contained 0.029 weight percent platinum on alumina-stabi- 1 ized cerium xide. A portion of this material was dried at 110°C overnigh ϊind then calcined at 450°C for 2 hours to give Cat yst Powder 2B. The platinum-treated cerium oxide with 0.029 w ht percent platinum was also mixed with water and ball .....lied as in Example 1 to produce a slurry designated as Slurry 2B. C. A portion of the platinum-treated α-alumina slur¬ ry (Slurry 2A) and a portion of platinum-treated cerium oxide slurry (Slurry 2B) were blended in amounts suffi- cient to give a mixed slurry with a ratio of α-alumina/- silica sol binder to ceria of about 50:50 by weight. This mixed slurry was designated Slurry 2C. A portion of Slur¬ ry 2C was dried at 110°C overnight and then calcined at 450°C for 2 hours to give Catalyst Powder 2C. Slurry 2C was also used to washcoat cordierite honeycomb substrates (1.5 inches in diameter X 3 inches long, 400 cpsi). The members were dried at 110°C overnight and then calcined at 450°C for 2 hours to give a catalyε ., with a total washcoat loading of 1.0 g/in of substrate and a platinum loading of 0.5 g/ft 3 of substrate. This was designated as Cata¬ lyst Member 2C. Example 3 A platinum/ceria/ceria-zirconia catalyst was prepared as follows. A. Ceria-zirconia powder (supplied by Magnesium Elektron Inc., 12 weight percent ceria) having a surface area of 59 m 2 /g was impregnated with platinum by the in¬ cipient wetness technique used in Example 1. It was dried at 100°C overnight and then calcined at 450°C for 2 hours to give a powder containing 0.029 weight percent platinum on ceria-zirconia. This material was designated Catalyst Powder 3A. B. Ceria-zirconia powder was dry-blended with bulk alumina-stabilized cerium oxide powder (supplied by SKK) in an amount sufficient to give a total ceria to zirconia ratio of about 50:50 by weight. This powder blend was im¬ pregnated with platinum by the incipient wetness technique used in Example 1 to give 0.029 weight percent platinum by weight of the mixed solids. The platinum-treated, ceria- zirconia/cerium oxide powder mixture was added to suffi¬ cient water to give a slurry with about 45 weight percent solids. This slurry was ball milled for 1 hour and the product was designated Slurry 3B. A portion of Slurry 3B was dried at 110°C overnight and then calcined at 450°C for 2 hours to give Catalyst Powder 3B. Slurry 3B was also used to washcoat cordierite honeycomb substrate cores (1.5 inches in diameter X 3 inches long, 400 cpsi). The coated substrates were dried at 110°C overnight and cal¬ cined at 450°C to give a catalyst with a total washcoat loading of 1.0 g/in 3 of substrate and a platinum loading of 0.5 g/ft 3 of substrate. This was designated as Cata¬ lyst Member 3B. Example 4 A platinum/ceria/titania catalyst was prepared as follows. A. Titanium powder (supplied by Degussa, designation P-25) having a surface area of 47 m 2 /g was impregnated with platinum by the incipient wetness technique used in Example 1 to give 0.029 weight percent platinum on titan- ia. A portion of this product was dried at 100°C over- 5 night and then calcined at 450°C to give Catalyst Powder 4A. The platinum-treated titania powder was added to water along with a water soluble methyl-cellulose binder (Methocel, supplied by Dow Chemical) in the amount of 0.25% by weight of solids and mixed to give a 40 weight 0 percent solids slurry which was designated Slurry 4A. The Methocel was added in the form of a 10% aqueous solution. The purpose of the Methocel was to improve the adhesion of * the catalyst washcoat to the cordierite substrate. B. Slurry 4A was blended and mechanically mixed with 5 the platinum-treated cerium oxide containing Slurry 2B to give a mixture containing a titania to ceria ratio of about 50:50 by weight end about 43 weight percent solids. This slurry was designated Slurry 4B. A portion of Slurry 4B was dried at 100°C overnight and then calcined at 450°C 0 for 2 hours to give Catalyst Powder 4B. i...urry 4B was also used to washcoat cordierite honeycomb substrate cores (1.5 inches in diameter X 3 inches long, 400 cpsi). The coated substrates were dried at 110°C overnight and then calcined at 450°C for 2 hours to give a catalyst with a 5 total washcoat loading of 1.0 g/in 3 and a platinum loading of 0.5 g/ft 3 of substrate. This was designated as Cata¬ lyst Member 4B. Example 5 C A platinum/ceria/silica-alumina catalyst with a sili¬ ca/alumina weight ratio of 28:72 was prepared as follows. A. A silica-alumina powder was synthesized from sil¬ ica and alumina-containing aqueous sols. An alumina- coated silica sol (Nalco-612) with 6 weight percent alumi¬ na and 24 weight percent silica content was mixed with sufficient alumina sol (Nalco-614) with 10 weight percent alumina to give a silica to alumina weight ratio of 28:72. ' The mixture was stirred at ambient temperature for 24 hours and then heated to over 50°C to cause gelation. The gelled mixture was dried at 110°C overnight and then cal¬ cined at 650°C for 4 hours. This gave a silica-alumina powder having a surface area of 127 m 2 /g. This powder was then impregnated with platinum via the incipient wetness technique used in Example 1 to give 0.029 weight percent platinum on silica-alumina. Water was added to this mix¬ ture to give a slurry with a solids content of about 50 weight percent and the slurry was ball milled to reduce the particle size to less than 12 microns. This slurry was designated as Slurry 5A. A portion of this slurry was dried at 110°C overnight and then calcined at 450°C for 2 hours to give Catalyst Powder 5A. B. Slurry 5A was blended and mechanically mixed with sufficient platinum-treated cerium oxide containing Slurry 2B to give a silica-alumina to ceria weight ratio of about 50:50. This mixed slurry was designated Slurry 5B. A portion of Slurry 5B was dried at 110°C overnight and then calcined at 450°C for 2 hours to give Catalyst Powder 5B. Slurry 5B was also used to washcoat cordierite honeycomb substrate cores (1.5 inches in diameter X 3 inches long, 400 cpsi). The coated substrates were dried overnight at 110°C and then calcined at 450°C for 2 hours to give a catalyst with a total washcoat loading of 1.0 g/in 3 of substrate and a platinum loading of 0.5 g/ft 3 of sub¬ strate. This was designated as Catalyst Member 5B. Example 6 A platinum/ceria/silica-alumina catalyst with a sili¬ ca to alumina weight ratio of 80:20 was prepared as fol¬ lows. A. A silica-alumina powder was synthesized from silica and alumina-containing aqueous sols. An alumina- coated silica sol (Nalco-612) with 6% alumina and 24% sil¬ ica content was mixed with sufficient alumina sol (Nalco- 614) to give a silica to alumina weight ratio of 80:20. This mixture was stirred for 24 hours and then heated to over 50°C to cause gelation of the mixture. The gelled product was dried at 110°C overnight and then calcined at 650°C for 4 hours. This gave a silica-alumina powder hav- ing a surface area of 83 m /g. This powder was impreg¬ nated with platinum via the incipient wetness technique used in Example 2 to give 0.029 weight percent platinum on silica-alumina. The platinum/silica-alumina powder was then mixed with water to give a slurry with solids content of about 50 weight percent. This slurry was ball milled to give a particle size of less than 12 microns. The resultant slurry was designated Slurry 6A. A portion of Slurry 6A was dried at 110°C overnight and then calcined at 450°C to give Catalyst Powder 6A. B. The platinum/silica-alumina containing Slurry 6A was blended and mechanically mixed with sufficient plati- num/ceria containing Slurry 2B from Example 2 to give a mixed slurry with ceria to silica-alumina ratio of about 50:50 by weight. This was designated Slurry 6B. A por- tion of Slurry 6B was drieα at 110°C overnight and then calcined at 450°C to give Catalyst Powder 6B. Slurry 6B was also used to washcoat cordierite honeycomb substrate cores (1.5 inches in diameter X 3 inches long, 400 cpsi). The coated substrates were dried overnight at 110°C and then calcined at 450°C for 2 hours to give catalysts with a total washcoat loading of 1.0 g/in 3 of substrate and a platinum loading of 0.5 g/ft 3 of substrate. This was des¬ ignated as Catalyst Member 6B. Example 7 A platinum/ceria/γ-alumina catalyst with a platinum loading of 2.0 g/ft 3 of substrate was prepared as follows. A. A γ-alumina powder (supplied by Condea, desig- nated SBa-150, having a surface area of 150 m 2 /g) was added to sufficient water to give a slurry with about 50 weight percent solids. The slurry was ball milled for about 0.5 hours. Next, an aqueous solution of amine-solu- bilized platinum hydroxide was added to give about 0.047 weight percent platinum by weight of γ-alumina. The mix¬ ture was then ball milled an additional 5 minutes, follow¬ ing which about 5% glacial acetic acid was added based on the weight of the γ-alumina to precipitate and fix the platinum. The resultant slurry was ball milled to achieve a particle size of about 90% finer than 12 microns. The milled, platinum-treated γ-alumina slurry was designated Slurry 7A. B. Bulk alumina-stabilized cerium oxide powder (sup¬ plied by SKK) was added to sufficient water to give a slurry with about 50 weight percent solids. The slurry was ball milled for about 5 minutes. Next, an aqueous solution of amine-solubilized platinum hydroxide was added to give about 0.047% platinum by weight of cerium oxide. This mixture was milled an additional 2 minutes, following which about 5% glacial acetic acid was added based on the weight of cerium oxide to precipitate and fix the plati¬ num. The resultant slurry was ball milled to achieve a particle size of 90% finer than 12 microns. The milled, platinum-treated cerium oxide slurry was designated Slurry 7B. C. Platinum-treated γ-alumina containing Slurry 7A and platinum-treated cerium oxide containing Slurry 7B were blended and mechanically mixed in sufficient amounts to give a γ-alumina to cerium oxide weight ratio of about 50:50. This mixed slurry was designated Slurry 7C. Slur¬ ry 7C was used to washcoat cordierite honeycomb substrate cores (1.5 inches in diameter X 3 inches long, 400 cpsi). The coated substrates were dried at 110°C overnight and then calcined at 450°C for 2 hours to give a catalyst with a total washcoat loading of 2.5 g/in 3 of substrate and a platinum loading of 2.0 g/ft 3 of substrate. This sample was designated as Catalyst Member 7C. Example 8 A platinum/ceria/silica catalyst with a platinum loading of 2.0 g/ft 3 of substrate was prepared as follows. A. A porous, high surface area silica (CS-1022, sup¬ plied by PQ Corporation) was jet milled to break up aggre¬ gates and give a fine powder consistency. The silica pow¬ der was added to sufficient water with mechanical mixing to give a slurry with 21 weight percent solids. To this slurry was added with mixing an aqueous solution of amine- solubilized platinum hydroxide in an amount sufficient to give 0.058% platinum by weight of silica. Next, about 5% glacial acetic acid by weight of the silica was added with mixing to precipitate and fix the platinum. Mechanical mixing was used to prepare this slurry and not ball mill¬ ing so as not to degrade the porous nature of the silica and to avoid gelation of ~he silica which occurs during ball milling. This slurry was designated Slurry 8A. A portion of Slurry 8A was dried at 110°C overnight and then calcined at 450°C for 2 hours to give Catalyst Powder 8A. B. Bulk alumina-stabilized cerium oxide powder (sup¬ plied by SKK) was added to sufficient water to give a slurry with about 50 weight percent solids. To this slur¬ ry was added sufficient amine-solubilized platinum hydrox- ide solution to give 0.058% platinum by weight of cerium oxide. Next, about 5% glacial acetic acid by weight of cerium oxide was added to the slurry with mixing to pre¬ cipitate and fix the platinum. This mixture was ball milled to reduce the particle size to less than 12 mi- crons. The milled, platinum/cerium oxide-containing slur¬ ry was designated Slurry 8B. C. Platinum-treated silica containing Slurry 8A was blended and mechar cally mixed with sufficient platinum- treated cerium oxide containing Slurry 8B to give a silica to cerium oxide ratio of 50:50 by weight. This mixed slurry was designated Slurry 8C. A portion of Slurry 8C was dried at 110°C overnight and then calcined at 450°C for 2 hours to give Catalyst Powder 8C. Slurry 8C was also used to washcoat cordierite honeycomb substrate cores (1.5 inches in diameter X 3 inches long, 400 cpsi). The coated substrates were dried at 110°C overnight and then calcined at 450°C for 2 hours to give a catalyst with a total washcoat loading of 2.0 g/in of substrate and a platinum loading of 2.0 g/ft 3 of substrate. This sample was designated as Catalyst Member 8C. Lube Oil Combustion Test and Test Results In many diesel engines the VOF in the diesel exhaust consists mainly of diesel lube oil which has been swept r from the cylinder walls and comes through valve guides and turbocharger seals. A laboratory test was used to evalu-- ate the relative performance of the catalyst powders pre- pared in Examples 1-8 for burning diesel lube oil as a model of the VOF in diesel engine exhaust. This test al¬ lows for the relative ranking of catalyst materials for their effectiveness of burning lube oil via the interac- «■ tions between the catalyst and lube oil as they are heated together in air. Thus, a catalyst powder sample is mixed uniformly with a measured amount of lube oil. In this case, 10% lube oil (Cummins Premium Blue, SAE 15W40) by weight of . catalyst powder was used. The mixture of catalyst and lube oil (about 10-30 mg) is placed into the quartz sample pan of a simultaneous TGA/DTA instrument (Thermal Sciences STA 1500) and heated in flowing air using a standard heat¬ ing ramp (20°C/min.) from ambient temperature to 1000°C. The collected data, cumulative weight loss (TGA) and~ heat evolution (DTA) as a function of temperature, are - normalized for the weight of catalyst sample and the - amount of lube oil present. The total weight loss (TGA) measured is made up of water loss which occurs at about 100°C or less and lube oil loss either by volatilization or by combustion. The water loss occurs in a discrete step and can be thus differentiated from the lube oil losses. The exotherm (DTA Peak) is a measure of the lube oil loss due to combustion of the lube oil. These data are used to calculate a DTA Peak Area (uV-sec/mg-catalyst sample/mg-lube oil) which is used as the key measure of the catalyst's ability to catalyze the combustion of VOF (lube oil) in this test. Tests have been conducted in which catalyst powders were used to catalyze the combus¬ tion of lubricating oil and the same catalytic powders were used to prepare catalysts supported on a substrate. Tests of such catalysts on diesel engine exhausts showed good correlation between the performance of the catalyst powder in the laboratory simulation and the performance of the corresponding catalyst in the engine test. Other important data resulting from this test are the temperature at which lube-oil weight loss begins (TGA On¬ set Temperature) and the temperature of the initiation of the exotherm from lube oil burning (DTA Onset Tempera¬ ture) . The catalyst powders from Examples 1-8 were mixed in¬ dividually with lube oil and the TGA/DTA lube oil combus¬ tion test was run for each. The results of the tests are set forth in TABLE I for the catalysts which were plati¬ num-treated mixtures of bulk cerium oxide with the other metal oxides. The results of the tests for the catalyst powders which did not contain bulk cerium oxide, i.e., platinum-treated metal oxides alone, are set forth in TABLE II. TABLE I LUBE OIL COMBUST U RESULTS - (TGA/DTA) CATALYTIC POWDERS - CERIA PLUS METAL OXIDE Pt Loading Equiv. (q/ft 3 ) 0.5 0.5 0.5 0.5 TABLE I Cont'd. LUBE OIL COMBUSTION RESULTS - (TGA/DTA) CATALYTIC POWDERS - CERIA PLUS METAL OXIDE Pt Loadin NOTE: K = X 1000 a. Ceria = Al-stabilized Ceria (2.5%Al 2 0 3 ). b. Ceria/Other Oxide(s) Weight Ratio = about 50:50 c. All samples made as washcoat slips, dried and calcined (450°C, 2 hrs.) d. Powdered samples mixed with 10 weight percent Lube Oil for TGA/DTA test Catalyst Powder Bulk ceria plus IC γ-Alumina 2C α-Alumina-Silica (90:10 weight ratio) 3B Ceria-Zirconia (12:88 weight ratio) 4B Titania 5B Alumina-Silica (72:28 weight ratio) 6B Alumina-Silica (20:80 weight ratio) 8C Silica TABLE II LUBE OIL COMBUSTION RESULTS (TGA/DTA) SINGLE METAL OXIDE Pt Loading Equiv. (q/ft 3 ) 0, 0, o. 0, 0, 0. 2. NOTE: K = X 1000 a. All samples made as washcoat slips, dried and calcined (450°C, 2 hrs. ) b. Powdered samples mixed with 10 weight percent Lube Oil for TGA/DTA test. c. Samples in this TABLE for individual Oxide Components impregnated with platinum. Catalyst Powder Oxide 1A γ-Alumina 2A α-Alumina-Silica (90:10 weight ratio) 3A Ceria-Zirconia (12:88 weight ratio) 4A Titania 5A Alumina-Silica (72:28 weight ratio) 6A Alumina-Silica (20:80 weight ratio) 8A Silica TABLES I and II list the three major parameters determined by the TGA/DTA test, DTA onset temperature, TGA onset tem¬ perature and DTA exotherm area. TABLE I shows the results for the Catalytic Powders in accordance with the present invention, whereas TABLE II shows the results for the com¬ parative single-component oxides. Figure 1 compares the DTA exotherm area of the mixed oxides to that determined for the support oxides. Comparison of TABLE I with TABLE II shows that combination of bulk cerium oxide with other metal oxides results in catalyst compositions with sub¬ stantially higher DTA Peak Areas, and therefore VOF burn¬ ing capability, than for catalysts containing those metal oxides alone. This effect is also shown in a graphical comparison in Figure 1. The catalysts containing bulk cerium oxide with either titania, alumina-silica (72:28), alumina-silica (20:80) and silica showed comparable DTA Peak Areas with one another, >20K, (K=1000) and were also comparable with the comparative catalyst of Example 1 containing bulk cerium oxide with γ-alumina (22.4K). Those catalysts con¬ taining bulk cerium oxide with α-alumina or ceria-zirconia exhibited the next highest set of DTA Peak Areas (>15.5K) which were greater than any of the catalysts containing the metal oxides with no bulk cerium oxide. Comparison of TABLE I"with TABLE II also shows that in some cases catalysts containing a combination of bulk cerium oxide with other metal oxides exhibited initial weight losses associated with lube oil (TGA Onset Tempera¬ tures) at temperatures which were lower than for catalysts containing the metal oxides but with no bulk cerium oxide. For silica the difference was 50°C. Titania and ceria- zirconia were exceptions to this behavior. This effect is also shown in a graphical comparison in Figure 2. Comparison of TABLE I with TABLE II also shows that catalysts containing a combination of bulk cerium oxide with other metal oxides exhibited substantially lower tem¬ peratures for the initiation of the exotherm associated with lube oil combustion (DTA Onset Temperature) than for catalysts containing the metal oxides with no bulk cerium oxide. In the case of alumina-silica (20:80 weight ratio) the difference was as great as 95°C. Regardless of the DTA Onset Temperature of the catalysts containing the metal oxides alone, the catalysts containing bulk cerium oxide with the metal oxides exhibited lower DTA Onset Tem¬ peratures in the narrow range 146 to 160°C. This effect is also shown in a graphical comparison in Figure 3. Comparison of the TGA Onset Temperatures and the DTA Onset Temperatures in TABLES I and II shows that the exo¬ therm associated with burning of the lube oil is initiated at a lower temperature than that for the initiation of appreciable weight loss associated with lube oil burning. This at first appears to be a contradiction. However, the probable explanation is that initiation of the exotherm is associated with loss/abstraction and burning of hydrogen from the lube oil in contact with the catalyst and that loss of hydrogen with its low mass is not measured as an appreciable weight loss by TGA. This explanation is sup¬ ported by mass spectrum measurements made during TGA/DTA lube oil combustion tests which show evolution of water as a by-product of combustion at the same temperature as the DTA Onset. The TGA/DTA results shown in TABLES I and II and in Figures 1 through 3 demonstrate that catalysts containing bulk cerium oxide mixed with other metal oxides are more effective for burning a greater fraction of lube oil (VOF) and with initiation of the burning at a lower temperature than for catalysts containing those metal oxides alone. Gas Phase Activity and Test Results Catalyst Members 1-8 were tested to assess the rela- tive gas phase oxidation activity for HC, S0 2 and CO as a function of temperature using a simulated diesel exhaust in a quartz-tube type laboratory diagnostic reactor heated by an electri". furnace. Each of Catalyst Members 1-8 was aged before testing in a flowing stream of 500°C air hav- ing a ten percent by volume water-vapor content for ten hours. The aged samples were then each separately evalu¬ ated using a flowing stream of a simulated diesel exhaust having the composition: 10 % water vapor, 28.57 ppm Heptane, 10 % 0 2 , 200 ppm CO, 4.5% C0 2 , 50 ppm S0 2 , 1000 ppm NO, balance nitrogen. (The percents given above are volume percents and "ppm" means parts per million by volume. The 28.57 ppm heptane is equivalent to 200 ppm C χ hydrocarbons.) Conversion of the CO (to C0 2 ) of the S0 2 (to S0 3 ) and of the heptane (to C0 2 and H 2 0) were determined simultaneously as the differ- ences between the concentrations of those components at the inlet and outlet of each catalyst tested. Determina¬ tions were made at four temperature levels, 275°C, 350°C, 425°C and 500°C and at a space velocity of 50,000 volumes of the simulated diesel exhaust (at standard conditions of temperature and pressure) per volume of catalyst per hour. The results of this test are shown in TABLE III, Parts A through C. Reference in TABLE III, or elsewhere in this application, to a percentage conversion of constituents (rendered as "%C" in TABLE III) of the exhaust or test gas, means the percentage of such constituent initially present in the exhaust or test gas being treated which is converted to another species. Thus, if an exhaust con¬ tains 50 parts per million ("ppm") volume S0 2 and treat¬ ment of the exhaust results in oxidizing 10 ppm of the S0 2 to S0 3 , a 20 percent conversion of the S0 2 has been sus¬ tained. Reference in this application to "space velocity" means the flow rate of exhaust or test gas flowed through a catalyst, expressed as volumes of exhaust or test gas per volume of catalyst per hour, calculated with the ex- haust or test gas at standard conditions of temperature and pressure. Catalyst Member Bulk ceria plus: IC γ-Alumina 2C α-Alumina 3B Ceria-Zirconia (12:88 weight ratio) 4B Titania 5B Alumina-Silica (72:28 weight ratio) 6B Alumina-Silica (20:80 weight ratio) 7C γ-Alumina 8C Silica TABLE III, Part A shows the results for S0 2 conver¬ sion as a function of temperature for the catalysts con¬ taining bulk cerium oxide mixed with various other metal oxides. Because oxidation of S0 2 to S0 3 in a diesel ex- haust can add to the exhaust particulate emissions in the form of sulfuric acid mist ( "sulfate-make") , an effective diesel exhaust catalyst should oxidize as little S0 2 as possible, especially at high exhaust temperatures. TABLE III, Part A shows that S0 2 conversion is affected by plat- inum loading (compare 0.5 with 2.0 g/ft 3 ), even for the unconventionally low platinum levels used in accordance with this invention. Catalyst Member 7C (2.0 g/ft 3 Pt) has been shown to exhibit superior control of "sulfate- make" when compared to commercial diesel catalysts having conventional platinum loading levels, e.g., 40-50 g/ft 3 (see Parent Case). Thus, even the highest S0 2 conversion levels shown in TABLE III, Part A are not considered ex¬ cessive. For the catalysts with 0.5 g/ft 3 platinum, the lowest S0 2 conversion levels were exhibited by the sample con¬ taining α-alumina (Catalyst Member 2C) which was roughly comparable to the sample according to the Parent Case con¬ taining γ-alumina (Catalyst Member IC). The samples con¬ taining alumina-silica (20:80 weight ratio. Catalyst .iem- ber 6B), alumina-silica (72:28 weight ratio. Catalyst Mem¬ ber 5B), titania (Catalyst Member 4B), and ceria-zirconia (Catalyst Member 3B) gave the next highest S0 2 conversion levels with the latter two being slightly higher at tern- peratures of 425 and 500°C. The catalysts with 2.0 g/ft 3 platinum exhibited : higher levels of S0 2 conversion, but the sample containing silica (Catalyst Member 8C) exhibited roughly comparable performance with the comparative γ-alumina-containing sam¬ ple (Catalyst Member 7C) of the Parent Case. TABLE III, Part B shows the HC (heptane) conversion, as a function of temperature. Based on the results for catalysts with 0.5 g/ft 3 platinum, the sample containing ceria-zirconia (Catalyst Member 3B) showed the highest level of HC conversion. The next highest levels were ex¬ hibited by the samples containing titania (Catalyst Member 4B) and alumina-silica (72:28 weight ratio. Catalyst Mem¬ ber 5B) which were comparable. The sample containing aiu- mina-silica (20:80 weight ratio. Catalyst Member 6B) ■_ showed a slightly lower HC conversion level. The samples containing γ-alumina (Parent Case) and α-alumina were gen¬ erally comparable and exhibited the lowest HC conversion. For catalysts containing 2.0 g/ft 3 platinum, the sam- pie containing γ-alumina (Catalyst Member 7C) made accord¬ ing to the Parent Case showed overall slightly better HC conversion than the sample containing silica (Catalyst Member 8C) . : The results show that different metal oxides in co - bination with bulk cerium oxide exhibit substantially dif¬ ferent levels of HC conversion. For the catalysts with : 0.5 g/ft 3 platinum, each of the metal oxides, with the ex¬ ception of α-alumina, gave a catalyst exhibiting higher .HC conversion levels than the catalyst of the Parent Case made with γ-alumina (Catalyst Member IC). Further, the., catalyst containing ceria-zirconia showed higher HC con¬ version at 350 and 425°C than the catalyst of the Parent Case (Catalyst Member 7C), inspite of having only 25% as much platinum. TABLE III, Part C shows CO conversion as a function of temperature for the catalysts containing bulk cerium oxide mixed with various other metal oxides. For the cat¬ alysts with 0.5 g/ft 3 platinum, the samples containing ceria-zirconia (Catalyst Member 3B), alumina-silica (72:28 weight ratio, Catalyst Member 5B) and titania (Catalyst Member 4B) were comparable and gave the highest CO conver¬ sion. The sample containing alumina-silica (20:80 weight ratio, Catalyst Member 6B) was next with a slightly lower CO conversion at the lowest temperature tested (i.e., higher "light-off" temperature). The sample containing α-alumina (Catalyst Member 2C) gave the next highest level of CO conversion and the catalyst of t-he Parent Case con- taining γ-alumina (Catalyst Member IC)- exhibited the low¬ est CO conversion of the group, but ultimately attained at least 55% conversion at 350°C and above. For the cata¬ lysts with 2.0 g/ft 3 platinum, the sample made according to the Parent Case and containing γ-alumina (Catalyst Mem- ber 7C) and the sample containing silica (Catalyst Member 8C) were comparable exhibiting over 90-% conversion of CO at all temperatures tested. : The gas phase activity results discussed above demon¬ strate that the type of metal oxide which is mixed with bulk cerium oxide and treated with low- loadings of plati¬ num to make a catalyst according to this invention can have a marked effect on the level of HC and CO conversion. Proper choice of that metal oxide can provide catalysts with better performance for HC and CO activity than that of the Parent Case containing γ-alumina. The effects on S0 2 conversion are of a lesser magnitude because of the lower conversion levels attained which: is due to the unique low platinum loading levels used. The TGA/DTA test results demonstrate that bulk cerium oxide is the key catalytic component when mixed with other metal oxides to achieve the most extensive burning of lube oil (VOF) and with the lowest possible onset temperature. While the invention has been described in detail with respect to specific preferred embodiments thereof it will be appreciated that variations thereto may be made which nonetheless lie within the scope of the invention and the appended claims.

Claims: THE CLAIMS What is claimed is: 1. An oxidation catalyst composition comprises a re¬ fractory carrier on which is disposed a coating of a cata¬ lytic 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 and a bulk second metal oxide selected from the group con¬ sisting of one or more of titania, zirconia, ceria-zirco¬ nia, silica, alumina-silica, and α-alumina. 2. The catalyst composition of claim 1 wherein the ceria and second metal oxide each comprises from about 5 to 95 percent by weight of the combination. 3. The catalyst composition of claim 1 wherein the second metal oxide has a BET surface area of at least about lOmVg and the composition further comprises acti¬ vated alumina having a BET surface area of at least about 10 m /g as part of the combination. 4. The catalyst composition of claim 3 wherein the ceria, the second metal oxide and the activated alumina each comprises from about 10 to 90 percent by weight of the combination. 5. The catalyst composition of claim 1, claim 2 or claim 3 wherein the ceria, the second metal oxide and, when present, the alumina, are each disposed in respective discrete layers one overlying the other. 6. The catalyst composition of claim 1 or claim 3 wherein the ceria comprises an aluminum-stabilized ceria. 7. The catalyst composition of claim 1 or claim 2 wherein the ceria and second metal oxide each comprises from about 40 to 60 percent by weight of the combination. 8. The catalyst composition of claim 1 or claim 3 wherein the ceria has a BET surface area of from about 25 m 2 /g to 200 m 2 /g. 9. The catalyst composition of claim 1 or claim 3 further comprising platinum dispersed on the catalytic ma¬ terial, the platinum being present in the amount of from about 0.1 to 15 g/ft 3 of the composition. 10. The catalyst composition of claim 9 wherein the platinum is present in the amount of from about 0.1 to 5 g/ft 3 of the composition. 11. The catalyst composition of claim 10 wherein at least a catalytically effective amount of the platinum is dispersed on the ceria. 12. The catalyst composition of claim 11 wherein at least a catalytically effective amount of the platinum is dispersed on the second metal oxide. 13. The catalyst composition of claim 1 or claim 3 further comprising palladium dispersed on the catalytic material, the palladium being present in the amount of from about 0.1 to 200 g/ft 3 of the composition. 14. The catalyst composition of claim 13 wherein the palladium is present in the amount of from about 50 to 120 g/ft 3 of the composition. 15. A catalyst composition for purifying diesel en¬ gine exhaust comprising a refractory carrier on which is disposed a coating of a catalytic material having a BET surface area of at least 10 m 2 /g and consisting essential¬ ly of a combination of bulk ceria having a BET surface area of at least about 10 m /g and a bulk second metal oxide selected from the class consisting of one or more of titania, zirconia, ceria-zirconia, silica / alumina-silica, and α-alumina, the ceria and the second metal oxide each comprising from about 5 to 95 percent by weight of the combination. 16. The catalyst composition of claim 15 further including a catalytically effective amount of a metal se¬ lected from the group consisting of platinum present in the amount of from about 0.1 to 15 g/ft 3 of the composi¬ tion and palladium present in the amount of from about 0.1 to 200 g/ft 3 of the composition, the catalytic metal being dispersed on the coating. 17. The catalyst composition of claim 16 wherein the catalytic metal is platinum and at least a catalytically effective amount of the platinum being dispersed on the ceria. 18. The catalyst composition of claim 1, claim 3 or claim 15 wherein the ceria and the second metal oxide are disposed in respective discrete layers, one of which over¬ lies the other. 19. The catalyst composition of claim 15 wherein the second metal oxide has a BET surface area of at least 10 m 2 /g and the composition further comprises alumina hav¬ ing a BET surface area of at least about 10 mVg. 20. The catalyst composition of claim 15 or claim 19 wherein the ceria and second metal oxide each comprises from about 10 to 90 percent by weight of the combination. 21. The catalyst composition of claim 15 or claim 19 wherein the ceria and the second metal oxide each com¬ prises from about 40 to 60 percent by weight of the combi¬ nation. 22. The catalyst composition of claim 15 or claim 19 wherein the platinum group metal comprises platinum and is present in the amount of from about 0.1 to 5 g/ft . 23. The catalyst composition of claim 15 or claim 19 wherein the palladium is present in the amount of from about 20 to 120 g/ft 3 . 24. The catalyst composition of claim 23 wherein the ceria comprises aluminum-stabilized ceria. 25. A method for oxidizing oxidizeable components of a gas-borne stream comprises contacting the stream with a catalyst composition at a temperature high enough to cata¬ lyze oxidation of at least some of the oxidizeable compo¬ nent, the catalyst composition comprising a catalytic ma¬ terial having a B" ** surface area of at least 10 m 2 /g and consisting esser -lly of a combination of bulk ceria hav¬ ing a 'TΞT surfat area of at least about 10 m 2 /g and a bulk second metal oxide selected from the class consisting of one or more of titania, zirconia, ceria-zirconia, sili¬ ca, alumina-silica, and α-alumina. 26. A method for treating a gas-borne stream compris¬ ing a diesel engine exhaust stream containing a volatile organic fraction comprises contacting the stream with a catalyst composition at a temperature high enough to cata¬ lyze oxidation of at least some of the volatile organic fraction, the catalyst composition comprising a catalytic material having a BET surface area of at least 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 and a bulk sec¬ ond metal oxide selected from the class consisting of one or more of titania, zirconia, ceria-zirconia, silica, alu¬ mina-silica and α-alumina. 27. The method of claim 25 or claim 26 wherein the second metal oxide has a BET surface area of at least 10 m 2 /g and the composition further comprises alumina having a BET surface area of at least about 10 m 2 /g. 28. The method of claim 25 or claim 26 wherein the ceria and the second metal oxide each comprises from about 5 to 95 percent by weight of the combination. 29. The method of claim 25 or claim 26 wherein the ceria and the second metal oxide each comprises from about 40 to 60 percent by weight of the combination. 30. The method of claim 25 or claim 26 wherein the temperature of the stream initially contacted with the catalyst composition is from about 10α°C to 800°C. 31. The method of claim 25 or claim 26 wherein the ceria has a BET surface area of from about 25 m 2 /g to 200 m 2 /g. 32. The method of claim 25 or claim 26 wherein the catalyst composition further comprises catalytically ac¬ tive platinum dispersed on the catalytic material, the platinum being present in the amount σf from about 0.1 to 15 g/ft 3 of the composition. 33. The method of claim 32 wherein at least a cata¬ lytically effective amount of the platinum is dispersed on the ceria. 34. The method of claim 32 wherein the platinum is pres;eenntt iinn tt.he amount of from about Oil to 5 g/ft 3 of the composition. 35. The method of claim 34 wherein at least a cata¬ lytically effective amount of the platinum is dispersed on the ceria. 36. The method of claim 25 or claim 26 wherein the catalyst composition further comprises catalytically ac¬ tive palladium dispersed on the catalytic material, the palladium being present in the amount of from about 0.1 to 200 g/ft 3 of the composition. 37. The method of claim 36 wherein the palladium is pres:eenntt iinn tthe amount of from about 20 to 120 g/ft 3 of the composition. 38. The method of claim 25 or claim 26 wherein the ceria comprises aluminum-stabilized ceria. 39. The method of claim 25 or claim 26 wherein the ceria and the second metal oxide are each disposed in re¬ spective discrete layers, one overlying the other. 40. The method of claim 27 wherein the ceria, the second metal oxide and the alumina are disposed in re¬ spective discrete layers, one overlying the other.

Inventors: FARRAUTO ROBERT J, GALLIGAN MICHAEL P, VOSS KENNETH E, YAVUZ BULENT O, FARRAUTO, ROBERT, J., GALLIGAN, MICHAEL, P., VOSS, KENNETH, E., YAVUZ, BULENT, O.
Applicants: ENGELHARD CORP, ENGELHARD CORPORATION
Application year: 1992
Main classification: B01J23
IPC codes: B01D53, B01J23, B01J35, B01J37, F02B3, F24C15