WO1992005861A1 - Catalyst composition containing base metal oxide-promoted rhodium - Google Patents

Catalyst composition containing base metal oxide-promoted rhodium Download PDF

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Publication number
WO1992005861A1
WO1992005861A1 PCT/US1991/006940 US9106940W WO9205861A1 WO 1992005861 A1 WO1992005861 A1 WO 1992005861A1 US 9106940 W US9106940 W US 9106940W WO 9205861 A1 WO9205861 A1 WO 9205861A1
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Prior art keywords
ceria
oxide
weight
metal oxide
catalyst composition
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PCT/US1991/006940
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French (fr)
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Samuel J. Tauster
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Engelhard Corporation
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Priority to JP3515927A priority Critical patent/JPH06504940A/en
Publication of WO1992005861A1 publication Critical patent/WO1992005861A1/en

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9445—Simultaneously removing carbon monoxide, hydrocarbons or nitrogen oxides making use of three-way catalysts [TWC] or four-way-catalysts [FWC]
    • B01D53/945—Simultaneously removing carbon monoxide, hydrocarbons or nitrogen oxides making use of three-way catalysts [TWC] or four-way-catalysts [FWC] characterised by a specific catalyst
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
    • B01J23/46—Ruthenium, rhodium, osmium or iridium
    • B01J23/464—Rhodium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/54—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
    • B01J23/56—Platinum group metals
    • B01J23/63—Platinum group metals with rare earths or actinides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/54—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
    • B01J23/56—Platinum group metals
    • B01J23/64—Platinum group metals with arsenic, antimony, bismuth, vanadium, niobium, tantalum, polonium, chromium, molybdenum, tungsten, manganese, technetium or rhenium
    • B01J23/656—Manganese, technetium or rhenium
    • B01J23/6562—Manganese
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/10—Internal combustion engine [ICE] based vehicles
    • Y02T10/12—Improving ICE efficiencies

Definitions

  • the present invention is concerned with catalysts use ⁇ ful for the treatment of gases to reduce contaminants con ⁇ tained therein, such as catalysts of the type generally re ⁇ ferred to as "three-way conversion” or “TWC” catalysts.
  • TWC catalysts are polyfunctional in that they have the capabilitiesi ⁇ ty of substantially simultaneously catalyzing both oxidation and reduction reactions, such as the oxidation of hydrocar ⁇ bons and carbon monoxide and the reduction of nitrogen ox ⁇ ides in a gaseous stream.
  • Such catalysts find utility in a number of fields, including the treatment of the exhaust gases from internal combustion engines, such as automobile and other gasoline-fueled engines.
  • the platinum group metal may comprise platinum or pal ⁇ ladium, preferably including one or more of rhodium, ruthen ⁇ ium and iridium, especially rhodium.
  • the refractory metal oxide support may comprise a high surface area alumina coat ⁇ ing (often referred to as "activated” or "gamma” alumina) carried on a carrier such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, as well known in the art.
  • the carrier may also comprise refractory particles such as spheres or short, extruded segments of a refractory material such as alumina.
  • the catalytically active materials dispersed on the activated alumina may contain, in addition to the platinum group metals, one or more base metal oxides, such as oxides of nickel, cobalt, manganese, iron, rhenium, etc., as shown, for example, in CD. Keith et al U.S. Patent 4,552,723.
  • the activated alumina typically exhibits a BET surface area In excess of 60 square meters per gram ("m /g"), often up to 2 about 200 m /g or more.
  • Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases.
  • the refractory metal oxide supports are subject to thermal degradation from extended exposure to the high tem ⁇ peratures of exhaust gas resulting in a loss of exposed cat ⁇ alyst surface area and a corresponding decrease in catalytic activity. It is a known expedient in the art to stabilize refractory metal oxide supports against such thermal degra- dation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia or stron- tia or, most usually, rare earth metal oxides, for example, ceria, lanthana and mixtures of two or more rare earth metal oxides. For example, see CD. Keith et al U.S. Patent 4,171,288.
  • TWC catalysts are currently formulated with complex washcoat compositions containing stabilized Al p 0_, an oxygen storage component, primarily ceria, and precious metal cata ⁇ lytic components. Such catalysts are designed to be effec- tive over a specific operating range of both lean of, and rich of, stoichiometric conditions.
  • oxygen stor ⁇ age component is used to designate a material which is be- lieved to be capable of being oxidized during oxygen-rich (lean) cycles of the gas being treated, and releasing oxygen during oxygen-poor (rich) cycles.
  • Such TWC catalyst compo ⁇ sitions enable optimization of the conversion of harmful emissions (HC, CO and NO ) to Innocuous substances.
  • rhodium is the most ef ⁇ fective for reducing NO to harmless nitrogen. Unfortunate- ly, rhodium is also the most expensive of these costly mate- rials and, consequently, effective rhodium utilization in automotive exhaust catalysts, such as TWO catalysts, has been extensively studied.
  • U.S. Patent 4,675,308 discloses a meth ⁇ od of effective utilization of rhodium by placing it on alu ⁇ mina which Is segregated from ceria-containing particles, because ceria renders the rhodium less active.
  • Other at ⁇ tempts to segregate rhodium from ceria are disclosed in U.S.
  • Patent 4,806,519 Japanese Patent application 88-326823/46 (J63240947A) of Nissan Motor KK (10.02.87-JP-027383), and in Japanese Patent publication JP63 77,544 (88 77,544).
  • It is known to utilize zirconia as a support for rhodium e.g., U.S. Patent 4,233,189 teaches the use of non-alumina sup ⁇ ports such as zirconia for rhodium.
  • U.S. Patent 4,492,769 discloses palladium and other platinum group met ⁇ als dispersed on a zirconia support together with base met ⁇ als.
  • zir ⁇ conia has certain disadvantages including a lower surface area than gamma alumina and the fact that zirconia itself is not a thermally stable support. Zirconia undergoes a phase transition between its monoclinic crystalline structure and its more stable tetragonal crystalline structure over a wide temperature range; such transition causes drastic sintering of the associated precious metals.
  • the rho ⁇ dium is stated to be supported on the alumina in a high con- centration and in a relatively large rhodium particle size.
  • the zirconia Is stabilized with the ceria (or yttria or cal- cia) by impregnation of zirconia with, for example, a solu ⁇ tion of cerium nitrate, followed by calcination.
  • the rhodi ⁇ um is supported on at least one of the stabilized cerium ox- ide and the refractory inorganic oxide.
  • a catalyst composition comprising a cata- lytic material In which rhodium is dispersed on a ceria- promoted zirconia support, such as a co-formed (as defined below) ceria-zirconia support, with the rhodium being sta- bilized against thermal degradation by a base metal oxide promoter.
  • a catalyst composition comprising a carri- er on which is disposed a washcoat of a catalytic material.
  • the catalytic material comprises a ceria-promoted support, e.g., a co-formed ceria-zirconia support, having dispersed thereon a catalytically effective amount of rhodium and a rhodium-stabilizing amount of a base metal oxide promoter.
  • the ceria content of the ceria-promoted zirconia support (which in all cases described herein optionally may be a co-formed ceria-zirconia support) may comprise, for example, from about 5 to 25% by weight of the weight of the ceria- promoted zirconia support.
  • the base metal oxide promoter dispersed on the ceria-promoted zirconia sup ⁇ port may be present in an amount of from about 1 to 10%, preferably 1 to 5%, by weight of the weight of the ceria- promoted zirconia support, and Is selected from the group consisting of calcium oxide, copper oxide, Iron oxide, lan ⁇ thanum oxide, magnesium oxide, manganese oxide, nickel oxide and tin oxide.
  • the washcoat further comprises a refractory metal ox- ide, e.g., alumina.
  • the base metal oxide comprises one or both of nickel oxide and magne ⁇ sium oxide.
  • the method aspect of the invention provides a method for treating a gas (such as the exhaust gas of a gasoline- fueled automobile engine) containing noxious components comprising one or more of carbon monoxide, hydrocarbons and nitrogen oxides by converting at least some of the noxious components into innocuous substances, the method comprising contacting the gas under conversion conditions, for example, at an Initial temperature of from about 250°C to 500°C, with a catalyst composition comprising a carrier on which is dis- posed a washcoat comprising a catalytic material as de ⁇ scribed above.
  • a gas such as the exhaust gas of a gasoline- fueled automobile engine
  • noxious components comprising one or more of carbon monoxide, hydrocarbons and nitrogen oxides
  • a "rhodium-stabilizing amount" of the base metal oxide promoter means an amount of the promoter which significantly ameliorates the catalytic de-activation which an otherwise identical rhodium-containing catalyst lacking the base metal oxide promoter would sustain under identical conditions of use, due to formation of stable rhodium oxides from the cat ⁇ alytic rhodium metal.
  • innocuous substances refers to the C0 2 and H 0 formed by oxidation of CO and hydrocarbons and the N ? formed by reduction of nitrogen oxides.
  • treat- ment of an automobile engine exhaust gas with a suitable TWC catalyst under conversion conditions will convert at least some of the carbon monoxide ("CO"), nitrogen oxides (“NOx”) and unburned hydrocarbons (“HC”) to innocuous substances, as defined herein.
  • CO carbon monoxide
  • NOx nitrogen oxides
  • HC unburned hydrocarbons
  • ceria-promoted zirconia support means a zir ⁇ conia support material with which ceria is intimately com ⁇ bined, for example, by having a coating of ceria deposited on the zirconia particles, or by impregnating zirconia par ⁇ ticles with a solution or liquid dispersion of a cerium salt or other cerium compound decomposable to the oxide, followed by drying and calcination to convert the cerium compound to ceria.
  • ceria-promoted zirconia support also in ⁇ cludes, as a special case thereof, a co-formed ceria-zircon ⁇ ia support as defined below.
  • co-formed ceria-zirconia support and the term “co-formed” as used with respect to the co-formed ceria-zirconia support material, means that the ceria is distributed substantially throughout the entire matrix of the zirconia particles as will occur, for example, when the cerium oxide and zirconium oxide, or predecessors thereof, are co-precipitated or co-gelled.
  • the defined term is in ⁇ tended to distinguish the material from that obtained in a situation in which ceria Is merely dispersed on or near the surface of the zirconia particles, leaving the core of the particles largely or entirely free of the ceria.
  • references to a component being "dispersed" on a support or re ⁇ ference to "dispersion” or “dispersal” in the same context, means that the component was impregnated onto the support from a solution of the component or of a precursor of the component. This is intended to distinguish the “dispersed” component from the situation in which the component is in ⁇ troduced in "bulk", that is, in fine particulate form.
  • the base metal oxide promoter is nickel oxide
  • it is "dispersed” on the support by impregnating the support with a solution of nickel nitrate and then drying and cal ⁇ cining the support to convert the impregnated nickel salt to nickel oxide.
  • the defined terms thus exclude incorporation of the nickel oxide into the support in the form of fine particles of solid nickel oxide.
  • the present invention provides for the dispersal of a rhodium catalytic component on a zirconia support which Is promoted, e.g., co-formed, with ceria, and therefore is con ⁇ trary to those teachings of the prior art which teach that ceria has a de-activating effect on the performance of rho ⁇ dium when the two are in contact.
  • the invention is accom- pushed by dispersing a base metal oxide promoter together with the dispersed rhodium on a ceria-promoted zirconia sup ⁇ port.
  • the base metal oxide suppresses the rho ⁇ dium sintering which has been known to occur under high tem ⁇ perature oxidizing conditions in the rhodium-containing cat ⁇ alytic materials of the prior art. It is speculated that under such conditions, the base metal oxide forms a complex with the rhodium oxide, thereby “anchoring" the rhodium and preventing the growth of uncomplexed rhodium oxides which destroy the effectiveness of the catalyst.
  • the catalyst compositions of the present in ⁇ vention are free of bulk ceria, at least In amounts which would adversely affect the activity of the rhodium catalyst.
  • the catalyst compositons are substantially free of bulk ceria and, more preferably, are substantially free of ceria either in bulk form (fine particles of ceria) or in dispersed form, except for the ceria used to promote the zirconia support, e.g., the ceria forming part of the co- formed ceria-zirconia support.
  • the promoting ceria e.g., the ceria which is co-formed with the zirconia
  • the present invention embraces ceria-promoted zirconia supports generally, the co-formed ceria-zirconia support is, as indicated above, preferred and the following description will refer thereto. However, it should be understood that except where specifically other ⁇ wise stated the descriptions generally apply to ceria-pro ⁇ moted supports as well.
  • a first catalytic component is prepared by dispersing rhodium metal on a co-formed ceria- zirconia support.
  • the support may be formed by co-precipi- tating or co-gelling zirconia with ceria or by other suit ⁇ able methods to produce a frit in which ceria Is bound in, and distributed substantially throughout, the zirconia com ⁇ ponent.
  • the amount of ceria present in the co-formed sup- port should be limited to between about 1 to 25% by weight of the weight of the co-formed ceria-zirconia support, pre ⁇ ferably from about 5 to 20% by weight, e.g., 12% by weight.
  • the balance of the co-formed support Is substantially or en ⁇ tirely zirconia, which accordingly comprises about 99 to 75% by weight of the co-formed ceria-zirconia support, prefera ⁇ bly about 95 to 80%.
  • cata ⁇ lyst compositions including platinum group metals such as platinum and rhodium dispersed on alumina are admixed with cerium oxide, with the co-formed ceria-zirconia material and optionally with a zirconium compound to form a washcoat slurry.
  • the co-formed ceria-zirconia support of the Japan ⁇ ese application is described as being produced by co-preci ⁇ pitation from a solution of suitable zirconium and cerium compounds.
  • the co-formed ceria-zirconia support produced by coprecipitation is said to retain at high temperatures a quasi-stable cubic crystal structure which is said to have catalytic activity of its own.
  • the co-formed ceria-zir ⁇ conia support is described as having a specific surface area of from about 10 to 150 square meters per gram, preferably from about 50 to 80 square meters per gram.
  • the weight ra ⁇ tio of cerium oxide to zirconium oxide in the co-formed cer ⁇ ia-zirconia support may be from about 1 part ceria to 99 parts zirconia, to about 25 parts ceria to 75 parts zircon- ia. Stated otherwise, the ceria may comprise from about 1% to about 25% by weight of the combined weight of the co- formed ceria-zirconia.
  • rhodium and a base metal cation are dispersed onto the co-formed ceria-zirconia support. This may be accomplished In a conventional manner, for example, by impregnating the co-formed support with a solution of an appropriate rhodium salt, e.g., rhodium nitrate, and with the nitrate salt pre ⁇ cursor of a suitable base metal oxide promoter.
  • a single impregnation solution may contain both a rhodium salt and a base metal salt or separate solutions of a rhodium salt and a base metal salt may be utilized in successive Impregna ⁇ tions.
  • Impregnation is not important and the rhodium salt solution and base metal salt solution may be impregnated onto the co-formed support in any order. How ⁇ ever, it Is more efficient to conduct a single impregnation and single drying and calcining to impregnate both the rho ⁇ dium and base metal precursors, instead of separate impreg ⁇ nation ⁇ ), drying and calcining cycles for each component. To that extent, it is preferred to utilize a single solution containing both a rhodium salt and a base metal salt dis ⁇ solved therein. In any case, the co-formed support, after being impregnated with both the rhodium and base metal oxide precursor salts, is dried and calcined in the conventional manner.
  • Impregnated co-formed support may be dried in air for about 2 to 24 hours at 110°C followed by calcining in air for about 1 to 24 hours at a temperature of about 350 to 550°C Calcining results in the decomposition of the rhodium salt Into rhodium oxide, and of the base met- al salt into the base metal oxide.
  • the resulting calcined composition may then be milled, as well known in the art, so that at least 90% of the particles have a diameter of less than 12 microns.
  • Alumina is preferred as the bulk extender because it also has a binding effect, which helps to secure the washcoat onto the surface of the substrate onto which the washcoat is applied.
  • other extenders in lieu of or in addition to alumina may be used as is known in the art, for example, silica, ti- tania, zirconia and the like.
  • catalytic metals including precious metals such as platinum and/or palladium may be dispersed throughout the washcoat and these, especially platinum or palladium, may conveniently be dispersed on the alumina or other refractory metal oxide em ⁇ ployed as the extender.
  • alumina is preferred and, the alumina used, or the alumina resulting from a final cal ⁇ cining step or from initial use of the catalyst, will pre- ferably be "activated alumina", i.e., a high surface area alumina comprising primarily gamma alumina, although other phases such as theta and eta alumina may be present.
  • the resultant washcoat comprising a mixture of alumina (and/or other extender) and the impregnated co-formed ceria-zirconia particles is coated onto a carrier, such as a cordierite honeycomb, in a manner well known In the art, and is then dried and calcined to provide the finished catalyst.
  • a carrier such as a cordierite honeycomb
  • the co- formed ceria-zirconia support and, optionally, the refracto ⁇ ry metal oxide may be admixed prior to impregnation with the solution or solutions of base metal salts so that both the refractory metal oxide and the co-formed ceria-zirconia sup ⁇ port are impregnated with the base metal compound which is eventually converted into the base metal oxide.
  • an amount of the base metal salt will be used to pro ⁇ vide the desired loading of base metal oxide promoter on the co-formed ceria-zirconia support because, depending on the proportion of refractory metal oxide to co-formed ceria-zir- conia support, a greater or lesser proportion of the base metal solution will be taken up by the refractory metal ox ⁇ ide.
  • the co-formed ceria-zirconia support and, optionally, the refractory metal oxide may be deposited onto a substrate and calcined to form a coating, and the calcined coating is then impregnated with the base metal solution. Conceivably, any of these approaches could also be taken with the rhodium salt solution.
  • the relative concentrations of rhodium salt and base metal salt in the impregnating solution or solutions are se- lected to provide the desired overall loadings of base metal oxide promoter and rhodium in the finished catalyst composi ⁇ tion.
  • the amount of base metal oxide promoter provided is typically from about 1 to 10% by weight of the weight of the co-formed ceria-zirconia support itself (not counting the weight of the promoter, catalytic metals or other components dispersed on the co-formed support), dry basis, with the base metal oxide promoter measured as the oxide.
  • the catalytic material may be prepared by Introducing both the alumina or other extender or binder material as well as the co-formed ceria- zirconia support material into a solution containing a dis- solved base metal salt so that, in the finished catalyst, the base metal oxide is dispersed throughout substantially the entire catalytic material.
  • the preferred amount of base metal oxide promoter that is, 1 to 10% by weight, prefera ⁇ bly 1 to 5% by weight, of the weight of the co-formed ce- ria-zirconia support, is the amount of base metal oxide pro ⁇ moter dispersed on the co-formed ceria-zirconia support it ⁇ self.
  • the amount, if any, of base metal oxide on the aluml- na or other components in the washcoat is not counted to ⁇ wards the preferred 1 to 10% range. It is believed that an excessive amount of the base metal oxide promoter dispersed on the co-formed ceria-zirconia support itself may prevent the reduction of the "anchored" rhodium oxides to rhodium metal and thus be detrimental to catalytic activity.
  • Example 1 A A series of catalyst compositions according to this invention is prepared as follows.
  • a co-formed ceria-zirco ⁇ nia support is obtained by co-precipitating ceria and zir ⁇ conia from a solution of both zirconium and cerium soluble compounds. The result is a co-formed ceria-zirconia support comprising about 12% by weight ceria and about 88% by weight zirconia.
  • each solution used to prepare a catalyst sample exemplary of the present Invention contained both rhodium nitrate and a base metal salt dissolved therein.
  • the metal salt solutions were employed at concentrations to provide the rhodium and base metal loadings set forth in TABLE I.
  • Each of the metal solution-impregnated portions of co-formed support were dried in air at 100°C for 1 hour and then calcined in air for 2 hours at 450°C
  • the dried and calcined rhodium-impregnated co- formed ceria-zirconia compositions obtained from Parts A and 3 respectively are separately milled so that at least 90% of the resulting particles have a diameter of less than 12 mi- crons.
  • Each milled component is separately combined with milled alumina of approximately the same particle size (about 90% of the alumina particles having a diameter of less than 12 microns) in a 1:1 ratio (dry solids weight basis) to form a washcoat slurry.
  • the slurries were coated onto respective cordierite honeycomb cores which were then dried in air at 110°C and calcined at 450°C for 2 hours.
  • the resulting concentration of rhodium in the washcoat was 0.39% by weight of the co-formed ceria-zirconia support.
  • the total washcoat loading was about 1.2 grams per cubic inch of the carrier.
  • the choice (or exclusion) of base met ⁇ al salt and the corresponding base metal oxide and the amount thereof (or lack thereof) in the catalyst composi ⁇ tion, when present, is shown in TABLE I below.
  • the various samples are identified as Samples IA, IB, 1C, 2A, 2B, etc..
  • Catalyst compositions prepared according to Example 1 were loaded into testing chambers and subjected to a 50 hour aging cycle at 900°C inlet gas temperature.
  • the cycle in- eluded a simulated fuel shut-off for five seconds every min ⁇ ute.
  • the fuel shut-off simulation was attained by introduc ⁇ ing air into the exhaust gas ahead of the catalyst to pro ⁇ vide a lean gas to the catalyst.
  • the engine utilized for the aging burned a commercially available normal hydrocarbon gasoline fuel having a lead content of about 3 milligrams Pb per gallon of fuel.
  • compositions were then tested, first by determining the "light-off" temperature (defined below) of the catalysts In a perturbated flow of engine ex ⁇ haust gases generated by combusting a stoichiometric air to fuel ratio combustion mixture, and then under "sweep" test ⁇ ing conditions at + 0.3 A/F Ratio Units, 2 Hz, 450°C, and a space velocity of 80,000 volumes of gas per volume of cata ⁇ lyst per hour. All catalyst composition samples were tested in the exhaust gas generated by an engine burning a commer ⁇ cial unleaded gasoline containing not more than 5 milligrams per gallon Pb. The performance of the various compositions is shown in TABLE I, below.
  • T ⁇ Q light- off temperatures
  • the temperature of the exhaust gas was in ⁇ creased at a rate of approximately 20°C per minute until the light-off temperature was attained.
  • % Prom means the percent by weight of base metal promoter dispersed on the co-formed ceria-zirconia support of the catalyst composition sample and "T,- Q " IS the light-off temperature.
  • % Conversion means the percentage of the pollutant (HC, CO and NOx) in the untreated exhaust gas which is converted to innocuous substances, i.e., COp, HpO and/or N p .
  • the defini ⁇ tion of "Lean” and “Rich” is based on the definition of the stoichiometric air to fuel weight ratio for gasoline as be ⁇ ing 14.65 parts by weight air to parts by weight gasoline.
  • A/F The air to fuel ratio
  • A/F The air to fuel ratio
  • Sample 2B shows a significant reduction in light-off temperature and substantially increased conversion of NOx under rich operating conditions, and of CO under stoichio- metric operating conditions as compared to Comparative Sam ⁇ ple 2A.
  • Sample 3B shows significantly improved activity for CO reduction at stoichi ⁇ ometric conditions.
  • the incorporation of 5 weight percent MgO in Sample 4B shows, as compared to Comparative Sample 4A, slgnficantly lowered light-off temperature and improved conversion of NOx under rich conditions, improved conversion of all three pol ⁇ lutants under stoichiometric conditions, and of HC and CO under lean conditions.
  • significantly decreased ac ⁇ tivity for NOx under lean conditions was observed.
  • Sample 5B shows signifi ⁇ cantly improved conversion of CO at stoichiometric condi ⁇ tions as compared to Comparative Sample 5A.
  • the inclusion of 4 weight percent NiO in Sample 5C did not show a significant improvement in acti ⁇ vity under these particular test conditions.
  • the efficacy of NiO as a base metal oxide promoter in the cata ⁇ lyst compositions of the present invention is amply demon- strated by Samples 9A-C, discussed below, as well as in Sam ⁇ ples IA and IC, as discussed above.
  • Samples 9B and 9C as compared to Comparative Sample 9A shows that the addition of 2 percent NiO signifi ⁇ cantly improves conversion of CO under lean conditions and marginally improves conversion of CO under stoichiometric conditions, while the addition of 1 percent by weight NiO in Sample 9B had the adverse effect of decreasing the conver ⁇ sion of CO under rich conditions.
  • Two catalyst compositions were prepared using tech ⁇ niques similar to those of Example 1 to produce catalyst compositions in which the washcoat is provided as two dis ⁇ crete bottom and top layers on the substrate.
  • the substrates comprising cordierite honeycomb cores were ini- tially coated with a slurry comprising approximately 67% by weight (dry solids basis) of activated alumina and 33% by weight of bulk ceria, I.e., fine particles of cerium oxide.
  • the platinum was impregnated with a platinum hydroxide meth- ylethanolamine solution so that approximately 2% by weight of the combined impregnated alumina and bulk ceria comprised platinum.
  • the coated cordierite substrate was provided with a top coat by being dipped into a slurry prepared as follows. Rhodium ni- trate was utilized to impregnate activated alumina and the impregnated alumina was mixed with a quantity of co-formed ceria-zirconia support in which ceria comprised about 12% by weight of the weight of the co-formed support. The quantity of zirconyl acetate was then Introduced to the slurry of alumina and co-formed ceria-zirconia support particles so that the zirconyl acetate impregnated both the alumina and the co-formed ceria-zirconia particles.
  • Excess slurry was blown om the twice-coated core which was then dried and calcined to provide an outer or top coat layer comprising approximately 38% by weight alumina, approximately 53% by weight of co-formed ceria-zirconia support, approximately 9% by weight zirconia and approximately 0.42% by weight rhodi ⁇ um.
  • the Inner or bottom coat comprises about 68% by weight of the total weight of washcoat (catalytic material) and the top coat comprised about 32% by weight.
  • nickel nitrate is added to the top coat slurry in an amount sufficient so that, if all of the nickel were to migrate to the co-formed ceria-zirconia support, it would amount to
  • the total loading of precious metal (platinum plus rho ⁇ dium) on the catalyst Samples 12A and 12B Is 40 grams per cubic foot, with the platinum and rhodium being present in a weight ratio of 5 parts platinum to 1 part rhodium.
  • Example 3 The catalyst compositions prepared according to Example 3 were loaded into testing chambers and subjected to an aging cycle similar to that described in Example 2 except that an Inlet gas temperature of 850°C and an actual fuel shut-off was utilized, instead of the simulated fuel shut- off employed in Example 2. The fuel shut-off was continued for about five seconds every minute.
  • the abbreviations of TABLE II have the same meaning as those given above for TABLE I.
  • the data of TABLE II shows significant reduction in light-off temperature for the nickel oxide promoted catalyst as compared to the unpromoted catalyst and significantly better conversions of the noxious components to innocuous substances attained.
  • the suitability of ceria-promoted zirconia supports other than co-formed ceria-zirconia supports for the pur ⁇ poses of the present invention is Indicated by tests of com ⁇ parable, but different, catalyst compositions containing platinum and rhodium catalytic components dispersed on ceria-promoted zirconia supports obtained by impregnating zirconia particles with a solution of cerium nitrate. The impregnated particles were then calcined to decompose the cerium nitrate to ceria.
  • the catalyst was aged and tested on the exhaust gas of a stock Nissan GLD engine.
  • the ceria- promoted zirconia support showed good durability and the catalyst showed good activity, indicating that ceria-pro ⁇ moted zirconia supports generally, including those which are not co-formed, are suitable for purposes of the present in ⁇ vention. While the invention has been described in detail with respect to specific embodiments thereof, it will be appreci ⁇ ated that variations to the invention may be made which will nonetheless lie within the sphere and scope of the invention and are intended to be embraced by the appended claims.

Abstract

A catalyst composition has a washcoat coated onto a monolithic carrier, the washcoat including a catalytic material having rhodium and a base metal oxide promoter dispersed on a ceria-promoted zirconia support, e.g., a co-formed ceria-zirconia support. A refractory metal oxide such as activated alumina may be included in the washcoat in order to enhance adhesion of it to the carrier. The presence of a rhodium-stabilizing amount, for example, 1 to 5 or 10 % by weight of the co-formed support, of the base metal oxide promoter dispersed with the rhodium on the ceria-promoted zirconia support ameliorates undesirable sintering of the rhodium. The limited amount of ceria, for example, from 1 to 25 % by weight of the weight of, for example, the conformed ceria-zirconia support, ameliorates the deleterious effect on rhodium of being intimately dispersed on a ceria-containing support. A method includes utilizing the catalyst composition for the three-way conversion of automotive engine exhaust gases.

Description

CATALYST COMPOSITION CONTAINING BASE METAL OXIDE-PROMOTED RHODIUM
Cross-Reference to Related Application
This application is a continuation-in-part of co-pend¬ ing parent application Serial Number 07/589,^70, filed Sep¬ tember 27, 1990 in the name of Samuel J. Tauster and en- titled "Catalyst Composition Containing Base Metal Oxide- Promoted Rhodium" .
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention is concerned with catalysts use¬ ful for the treatment of gases to reduce contaminants con¬ tained therein, such as catalysts of the type generally re¬ ferred to as "three-way conversion" or "TWC" catalysts. TWC catalysts are polyfunctional in that they have the capabili¬ ty of substantially simultaneously catalyzing both oxidation and reduction reactions, such as the oxidation of hydrocar¬ bons and carbon monoxide and the reduction of nitrogen ox¬ ides in a gaseous stream. Such catalysts find utility in a number of fields, including the treatment of the exhaust gases from internal combustion engines, such as automobile and other gasoline-fueled engines.
Background and Related Art In order to meet governmental emissions standards for internal combustion engine exhausts, so-called catalytic converters containing a suitable catalyst such as a TWC cat¬ alyst, are emplaced in the exhaust gas line of internal com¬ bustion engines to promote the oxidation of unburned hydro- carbons ("HC") and carbon monoxide ("CO") and the reduction of nitrogen oxides ("NO ") in the exhaust gas. For this purpose, TWC catalysts comprising a minor amount of one or more platinum group metals distended upon a high surface area, refractory metal oxide support are well known in the art. The platinum group metal may comprise platinum or pal¬ ladium, preferably including one or more of rhodium, ruthen¬ ium and iridium, especially rhodium. The refractory metal oxide support may comprise a high surface area alumina coat¬ ing (often referred to as "activated" or "gamma" alumina) carried on a carrier such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, as well known in the art. The carrier may also comprise refractory particles such as spheres or short, extruded segments of a refractory material such as alumina.
The catalytically active materials dispersed on the activated alumina may contain, in addition to the platinum group metals, one or more base metal oxides, such as oxides of nickel, cobalt, manganese, iron, rhenium, etc., as shown, for example, in CD. Keith et al U.S. Patent 4,552,723. The activated alumina typically exhibits a BET surface area In excess of 60 square meters per gram ("m /g"), often up to 2 about 200 m /g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases.
The refractory metal oxide supports are subject to thermal degradation from extended exposure to the high tem¬ peratures of exhaust gas resulting in a loss of exposed cat¬ alyst surface area and a corresponding decrease in catalytic activity. It is a known expedient in the art to stabilize refractory metal oxide supports against such thermal degra- dation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia or stron- tia or, most usually, rare earth metal oxides, for example, ceria, lanthana and mixtures of two or more rare earth metal oxides. For example, see CD. Keith et al U.S. Patent 4,171,288.
TWC catalysts are currently formulated with complex washcoat compositions containing stabilized Alp0_, an oxygen storage component, primarily ceria, and precious metal cata¬ lytic components. Such catalysts are designed to be effec- tive over a specific operating range of both lean of, and rich of, stoichiometric conditions. (The term "oxygen stor¬ age component" is used to designate a material which is be- lieved to be capable of being oxidized during oxygen-rich (lean) cycles of the gas being treated, and releasing oxygen during oxygen-poor (rich) cycles.) Such TWC catalyst compo¬ sitions enable optimization of the conversion of harmful emissions (HC, CO and NO ) to Innocuous substances. Of the three precious metals, platinum, palladium and rhodium, con¬ ventionally used in TWC catalysts, rhodium is the most ef¬ fective for reducing NO to harmless nitrogen. Unfortunate- ly, rhodium is also the most expensive of these costly mate- rials and, consequently, effective rhodium utilization in automotive exhaust catalysts, such as TWO catalysts, has been extensively studied.
One of the problems faced by present-day TWC catalysts is accelerated deterioration of the catalysts caused by the high operating temperatures engendered by smaller automotive engines and high speed highway driving. In addition, at¬ tempts to improve fuel economy by using air to fuel ("A/F") ratios higher than stoichiometric, and/or fuel shut-off fea¬ tures, generate a lean (oxygen-rich) exhaust. High tempera- tures and lean conditions of the exhaust gas accelerate the deterioration of platinum and rhodium catalysts, inasmuch as both platinum and rhodium are more readily sintered, and rhodium more strongly interacts with support materials such as alumina, under such conditions. The art has devoted a great deal of effort in attempts to Improve the efficiency of rhodium-containing TWC composi¬ tions. For example, U.S. Patent 4,675,308 discloses a meth¬ od of effective utilization of rhodium by placing it on alu¬ mina which Is segregated from ceria-containing particles, because ceria renders the rhodium less active. Other at¬ tempts to segregate rhodium from ceria are disclosed in U.S. Patent 4,806,519; Japanese Patent application 88-326823/46 (J63240947A) of Nissan Motor KK (10.02.87-JP-027383), and in Japanese Patent publication JP63 77,544 (88 77,544). It is known to utilize zirconia as a support for rhodium, e.g., U.S. Patent 4,233,189 teaches the use of non-alumina sup¬ ports such as zirconia for rhodium. Similarly, U.S. Patent 4,492,769 discloses palladium and other platinum group met¬ als dispersed on a zirconia support together with base met¬ als. The use of rhodium dispersed on zirconia is also dis¬ closed in U.S. Patents 4,233,189 and 4,405,506, and Japanese Patent 6-1,157,347A, dated December 28, 1984. However, zir¬ conia has certain disadvantages including a lower surface area than gamma alumina and the fact that zirconia itself is not a thermally stable support. Zirconia undergoes a phase transition between its monoclinic crystalline structure and its more stable tetragonal crystalline structure over a wide temperature range; such transition causes drastic sintering of the associated precious metals.
There remains a need for improved stabilization against thermal degradation of rhodium-containing catalysts carried on zirconia supports. An attempt at obtaining such stabili¬ zation is disclosed in Nippon Shokubai Japanese Patent ap¬ plication HEI 1-93045 of Kitaguchi et al, filed April 14, 1989 and claiming priority of applications Showa 63-90310 (April 14, 1988) and Showa 63-90311 (April 14, 1988). This application discloses a catalyst comprising rhodium and, op¬ tionally, platinum and/or palladium, dispersed on a support comprising ceria stabilized with zirconia or yttria or cal- cia, and a refractory metal oxide such as alumina. The rho¬ dium is stated to be supported on the alumina in a high con- centration and in a relatively large rhodium particle size. The zirconia Is stabilized with the ceria (or yttria or cal- cia) by impregnation of zirconia with, for example, a solu¬ tion of cerium nitrate, followed by calcination. The rhodi¬ um is supported on at least one of the stabilized cerium ox- ide and the refractory inorganic oxide.
SUMMARY OF THE INVENTION
Generally, in accordance with the present invention, there is provided a catalyst composition comprising a cata- lytic material In which rhodium is dispersed on a ceria- promoted zirconia support, such as a co-formed (as defined below) ceria-zirconia support, with the rhodium being sta- bilized against thermal degradation by a base metal oxide promoter.
Specifically, In accordance with the present invention, there is provided a catalyst composition comprising a carri- er on which is disposed a washcoat of a catalytic material. The catalytic material comprises a ceria-promoted support, e.g., a co-formed ceria-zirconia support, having dispersed thereon a catalytically effective amount of rhodium and a rhodium-stabilizing amount of a base metal oxide promoter. The ceria content of the ceria-promoted zirconia support (which in all cases described herein optionally may be a co-formed ceria-zirconia support) may comprise, for example, from about 5 to 25% by weight of the weight of the ceria- promoted zirconia support. In one aspect of the present Invention, the base metal oxide promoter dispersed on the ceria-promoted zirconia sup¬ port may be present in an amount of from about 1 to 10%, preferably 1 to 5%, by weight of the weight of the ceria- promoted zirconia support, and Is selected from the group consisting of calcium oxide, copper oxide, Iron oxide, lan¬ thanum oxide, magnesium oxide, manganese oxide, nickel oxide and tin oxide.
In accordance with another aspect of the present inven¬ tion, the washcoat further comprises a refractory metal ox- ide, e.g., alumina.
In a specific aspect of the present invention, the base metal oxide comprises one or both of nickel oxide and magne¬ sium oxide.
The method aspect of the invention provides a method for treating a gas (such as the exhaust gas of a gasoline- fueled automobile engine) containing noxious components comprising one or more of carbon monoxide, hydrocarbons and nitrogen oxides by converting at least some of the noxious components into innocuous substances, the method comprising contacting the gas under conversion conditions, for example, at an Initial temperature of from about 250°C to 500°C, with a catalyst composition comprising a carrier on which is dis- posed a washcoat comprising a catalytic material as de¬ scribed above.
As used herein and in the claims, the following terms have the Indicated meanings. A "rhodium-stabilizing amount" of the base metal oxide promoter means an amount of the promoter which significantly ameliorates the catalytic de-activation which an otherwise identical rhodium-containing catalyst lacking the base metal oxide promoter would sustain under identical conditions of use, due to formation of stable rhodium oxides from the cat¬ alytic rhodium metal.
The term "innocuous substances" refers to the C02 and H 0 formed by oxidation of CO and hydrocarbons and the N? formed by reduction of nitrogen oxides. For example, treat- ment of an automobile engine exhaust gas with a suitable TWC catalyst under conversion conditions will convert at least some of the carbon monoxide ("CO"), nitrogen oxides ("NOx") and unburned hydrocarbons ("HC") to innocuous substances, as defined herein. The term "ceria-promoted zirconia support" means a zir¬ conia support material with which ceria is intimately com¬ bined, for example, by having a coating of ceria deposited on the zirconia particles, or by impregnating zirconia par¬ ticles with a solution or liquid dispersion of a cerium salt or other cerium compound decomposable to the oxide, followed by drying and calcination to convert the cerium compound to ceria. The term "ceria-promoted zirconia support" also in¬ cludes, as a special case thereof, a co-formed ceria-zircon¬ ia support as defined below. The term "co-formed ceria-zirconia support" and the term "co-formed" as used with respect to the co-formed ceria-zirconia support material, means that the ceria is distributed substantially throughout the entire matrix of the zirconia particles as will occur, for example, when the cerium oxide and zirconium oxide, or predecessors thereof, are co-precipitated or co-gelled. The defined term is in¬ tended to distinguish the material from that obtained in a situation in which ceria Is merely dispersed on or near the surface of the zirconia particles, leaving the core of the particles largely or entirely free of the ceria. The latter situation would occur, for example, if a solution of a solu- ble cerium salt, e.g., cerium nitrate, were Impregnated onto zirconia particles and the resulting impregnated particles were dried and calcined to convert the cerium nitrate to ceria. The material resulting from such treatment fits the definition of a "ceria-promoted zirconia support" but is not a co-formed ceria-zirconia support.
Reference to a component (such as rhodium or the base metal oxide promoter) being "dispersed" on a support or re¬ ference to "dispersion" or "dispersal" in the same context, means that the component was impregnated onto the support from a solution of the component or of a precursor of the component. This is intended to distinguish the "dispersed" component from the situation in which the component is in¬ troduced in "bulk", that is, in fine particulate form. For example, if the base metal oxide promoter is nickel oxide, it is "dispersed" on the support by impregnating the support with a solution of nickel nitrate and then drying and cal¬ cining the support to convert the impregnated nickel salt to nickel oxide. The defined terms thus exclude incorporation of the nickel oxide into the support in the form of fine particles of solid nickel oxide.
DETAILED DESCRIPTION OF THE INVENTION, INCLUDING PREFERRED EMBODIMENTS THEREOF
The present invention provides for the dispersal of a rhodium catalytic component on a zirconia support which Is promoted, e.g., co-formed, with ceria, and therefore is con¬ trary to those teachings of the prior art which teach that ceria has a de-activating effect on the performance of rho¬ dium when the two are in contact. The invention is accom- pushed by dispersing a base metal oxide promoter together with the dispersed rhodium on a ceria-promoted zirconia sup¬ port. Without wishing to be bound to any particular theory, it is believed that the base metal oxide suppresses the rho¬ dium sintering which has been known to occur under high tem¬ perature oxidizing conditions in the rhodium-containing cat¬ alytic materials of the prior art. It is speculated that under such conditions, the base metal oxide forms a complex with the rhodium oxide, thereby "anchoring" the rhodium and preventing the growth of uncomplexed rhodium oxides which destroy the effectiveness of the catalyst. In a preferred case, it is believed that by co-forming the ceria with the zirconia (as opposed to merely dispersing the ceria onto particles of zirconia), and by limiting the amount of ceria in the co-formed support, all as described below, the unde- sired adverse effect of ceria on rhodium is avoided or fur¬ ther significantly attenuated. Generally, the catalyst compositions of the present in¬ vention are free of bulk ceria, at least In amounts which would adversely affect the activity of the rhodium catalyst. Preferably, the catalyst compositons are substantially free of bulk ceria and, more preferably, are substantially free of ceria either in bulk form (fine particles of ceria) or in dispersed form, except for the ceria used to promote the zirconia support, e.g., the ceria forming part of the co- formed ceria-zirconia support.
It should be understood that the promoting ceria, e.g., the ceria which is co-formed with the zirconia, may (but need not) contain small but significant quantities of other rare earth metal oxides as is the case with commercial grades of ceria. Although the present invention embraces ceria-promoted zirconia supports generally, the co-formed ceria-zirconia support is, as indicated above, preferred and the following description will refer thereto. However, it should be understood that except where specifically other¬ wise stated the descriptions generally apply to ceria-pro¬ moted supports as well. In preparing a catalyst composition according to this invention, a first catalytic component is prepared by dispersing rhodium metal on a co-formed ceria- zirconia support. The support may be formed by co-precipi- tating or co-gelling zirconia with ceria or by other suit¬ able methods to produce a frit in which ceria Is bound in, and distributed substantially throughout, the zirconia com¬ ponent. The amount of ceria present in the co-formed sup- port should be limited to between about 1 to 25% by weight of the weight of the co-formed ceria-zirconia support, pre¬ ferably from about 5 to 20% by weight, e.g., 12% by weight. The balance of the co-formed support Is substantially or en¬ tirely zirconia, which accordingly comprises about 99 to 75% by weight of the co-formed ceria-zirconia support, prefera¬ bly about 95 to 80%.
It should be understood that neither the ceria-promoted zirconia support itself generally nor the co-formed ceria- zirconia support itself specifically form any part of the present invention. For example, the nature of co-formed ceria-zirconia supports and their use in a catalyst composi¬ tion are the subject of a prior and co-pending Japanese Pat¬ ent Application 145491/89 , which describes a catalyst for exhaust gas purification in which platinum group metal components Including rhodium are used in a washcoat contain¬ ing a co-formed ceria-zirconia support. Specifically, cata¬ lyst compositions including platinum group metals such as platinum and rhodium dispersed on alumina are admixed with cerium oxide, with the co-formed ceria-zirconia material and optionally with a zirconium compound to form a washcoat slurry. The co-formed ceria-zirconia support of the Japan¬ ese application is described as being produced by co-preci¬ pitation from a solution of suitable zirconium and cerium compounds. The co-formed ceria-zirconia support produced by coprecipitation is said to retain at high temperatures a quasi-stable cubic crystal structure which is said to have catalytic activity of its own. This is stated to contrast with the thermal degradation of zirconium oxide from a quasi-stable cubic system to a σatalytically inactive mono- clinic crystalline configuration. The co-formed ceria-zir¬ conia support is described as having a specific surface area of from about 10 to 150 square meters per gram, preferably from about 50 to 80 square meters per gram. The weight ra¬ tio of cerium oxide to zirconium oxide in the co-formed cer¬ ia-zirconia support may be from about 1 part ceria to 99 parts zirconia, to about 25 parts ceria to 75 parts zircon- ia. Stated otherwise, the ceria may comprise from about 1% to about 25% by weight of the combined weight of the co- formed ceria-zirconia.
In accordance with the teachings of the present inven¬ tion, rhodium and a base metal cation are dispersed onto the co-formed ceria-zirconia support. This may be accomplished In a conventional manner, for example, by impregnating the co-formed support with a solution of an appropriate rhodium salt, e.g., rhodium nitrate, and with the nitrate salt pre¬ cursor of a suitable base metal oxide promoter. A single impregnation solution may contain both a rhodium salt and a base metal salt or separate solutions of a rhodium salt and a base metal salt may be utilized in successive Impregna¬ tions. The order of Impregnation is not important and the rhodium salt solution and base metal salt solution may be impregnated onto the co-formed support in any order. How¬ ever, it Is more efficient to conduct a single impregnation and single drying and calcining to impregnate both the rho¬ dium and base metal precursors, instead of separate impreg¬ nation^), drying and calcining cycles for each component. To that extent, it is preferred to utilize a single solution containing both a rhodium salt and a base metal salt dis¬ solved therein. In any case, the co-formed support, after being impregnated with both the rhodium and base metal oxide precursor salts, is dried and calcined in the conventional manner. For example, the Impregnated co-formed support may be dried in air for about 2 to 24 hours at 110°C followed by calcining in air for about 1 to 24 hours at a temperature of about 350 to 550°C Calcining results in the decomposition of the rhodium salt Into rhodium oxide, and of the base met- al salt into the base metal oxide. The resulting calcined composition may then be milled, as well known in the art, so that at least 90% of the particles have a diameter of less than 12 microns. The impregnated co-formed support com¬ prises active catalyst components and may be milled with a bulk extender such as alumina of approximately the same par¬ ticle size as the active catalyst components and in a weight proportioned to the catalyst components to produce a "wash¬ coat" having an appropriate rhodium loading. Alumina is preferred as the bulk extender because it also has a binding effect, which helps to secure the washcoat onto the surface of the substrate onto which the washcoat is applied. How- ever, other extenders in lieu of or in addition to alumina may be used as is known in the art, for example, silica, ti- tania, zirconia and the like. In addition, other catalytic metals, including precious metals such as platinum and/or palladium may be dispersed throughout the washcoat and these, especially platinum or palladium, may conveniently be dispersed on the alumina or other refractory metal oxide em¬ ployed as the extender. Here too, alumina is preferred and, the alumina used, or the alumina resulting from a final cal¬ cining step or from initial use of the catalyst, will pre- ferably be "activated alumina", i.e., a high surface area alumina comprising primarily gamma alumina, although other phases such as theta and eta alumina may be present. The resultant washcoat, comprising a mixture of alumina (and/or other extender) and the impregnated co-formed ceria-zirconia particles is coated onto a carrier, such as a cordierite honeycomb, in a manner well known In the art, and is then dried and calcined to provide the finished catalyst.
The specific method of making the catalyst Is not crit¬ ical with respect to the sequence of impregnating the solids with the rhodium and, optionally, other precious metal solu¬ tion^) and with the base metal solution(s). Thus, the co- formed ceria-zirconia support and, optionally, the refracto¬ ry metal oxide may be admixed prior to impregnation with the solution or solutions of base metal salts so that both the refractory metal oxide and the co-formed ceria-zirconia sup¬ port are impregnated with the base metal compound which is eventually converted into the base metal oxide. In such case, an amount of the base metal salt will be used to pro¬ vide the desired loading of base metal oxide promoter on the co-formed ceria-zirconia support because, depending on the proportion of refractory metal oxide to co-formed ceria-zir- conia support, a greater or lesser proportion of the base metal solution will be taken up by the refractory metal ox¬ ide. In another alternative, the co-formed ceria-zirconia support and, optionally, the refractory metal oxide may be deposited onto a substrate and calcined to form a coating, and the calcined coating is then impregnated with the base metal solution. Conceivably, any of these approaches could also be taken with the rhodium salt solution.
The relative concentrations of rhodium salt and base metal salt in the impregnating solution or solutions are se- lected to provide the desired overall loadings of base metal oxide promoter and rhodium in the finished catalyst composi¬ tion. The amount of base metal oxide promoter provided is typically from about 1 to 10% by weight of the weight of the co-formed ceria-zirconia support itself (not counting the weight of the promoter, catalytic metals or other components dispersed on the co-formed support), dry basis, with the base metal oxide promoter measured as the oxide. However, it should be understood that it is within the purview of the present invention to introduce larger amounts of the base metal oxide precursor throughout the entire catalytic mate¬ rial or washcoat. Thus, it may be desired to prepare the catalytic material by Introducing both the alumina or other extender or binder material as well as the co-formed ceria- zirconia support material into a solution containing a dis- solved base metal salt so that, in the finished catalyst, the base metal oxide is dispersed throughout substantially the entire catalytic material. The preferred amount of base metal oxide promoter, that is, 1 to 10% by weight, prefera¬ bly 1 to 5% by weight, of the weight of the co-formed ce- ria-zirconia support, is the amount of base metal oxide pro¬ moter dispersed on the co-formed ceria-zirconia support it¬ self. The amount, if any, of base metal oxide on the aluml- na or other components in the washcoat is not counted to¬ wards the preferred 1 to 10% range. It is believed that an excessive amount of the base metal oxide promoter dispersed on the co-formed ceria-zirconia support itself may prevent the reduction of the "anchored" rhodium oxides to rhodium metal and thus be detrimental to catalytic activity.
All percentages by weight given herein and in the claims are calculated on a dry basis with the component (ce¬ ria, zirconia or the base metal oxide promoter as the case may be) calculated as the oxide. It will be appreciated that some of the base metal oxides may exist in different valence states and that the valence states may change during use of the catalyst compositions. However, for purposes of calculating the weight percentage of components of the cata- lyst compositions, the formulae given in TABLE I below for the base metal oxides are used. Ceria and zirconia are tak¬ en as CeOp and ZrOp, respectively.
Example 1 A. A series of catalyst compositions according to this invention is prepared as follows. A co-formed ceria-zirco¬ nia support is obtained by co-precipitating ceria and zir¬ conia from a solution of both zirconium and cerium soluble compounds. The result is a co-formed ceria-zirconia support comprising about 12% by weight ceria and about 88% by weight zirconia. Separate portions of the co-formed ceria-zirconia support were then impregnated with an aqueous solution of rhodium nitrate and the nitrate salt of respective ones of the base metal cations listed in TABLE I, except in the case of tin for which tin chloride (SnCl^.) was used. That is, each solution used to prepare a catalyst sample exemplary of the present Invention contained both rhodium nitrate and a base metal salt dissolved therein. The metal salt solutions were employed at concentrations to provide the rhodium and base metal loadings set forth in TABLE I. Each of the metal solution-impregnated portions of co-formed support were dried in air at 100°C for 1 hour and then calcined in air for 2 hours at 450°C
B. For comparison, portions of co-formed ceria-zirco¬ nia support were prepared In the same manner as in Part A except that the base metal nitrate salt was excluded from the rhodium nitrate solution.
C The dried and calcined rhodium-impregnated co- formed ceria-zirconia compositions obtained from Parts A and 3 respectively are separately milled so that at least 90% of the resulting particles have a diameter of less than 12 mi- crons. Each milled component is separately combined with milled alumina of approximately the same particle size (about 90% of the alumina particles having a diameter of less than 12 microns) in a 1:1 ratio (dry solids weight basis) to form a washcoat slurry. The slurries were coated onto respective cordierite honeycomb cores which were then dried in air at 110°C and calcined at 450°C for 2 hours. The resulting concentration of rhodium in the washcoat was 0.39% by weight of the co-formed ceria-zirconia support. The total washcoat loading was about 1.2 grams per cubic inch of the carrier. The choice (or exclusion) of base met¬ al salt and the corresponding base metal oxide and the amount thereof (or lack thereof) in the catalyst composi¬ tion, when present, is shown in TABLE I below. The various samples are identified as Samples IA, IB, 1C, 2A, 2B, etc..
Example 2
Catalyst compositions prepared according to Example 1 were loaded into testing chambers and subjected to a 50 hour aging cycle at 900°C inlet gas temperature. The cycle in- eluded a simulated fuel shut-off for five seconds every min¬ ute. The fuel shut-off simulation was attained by introduc¬ ing air into the exhaust gas ahead of the catalyst to pro¬ vide a lean gas to the catalyst. The engine utilized for the aging burned a commercially available normal hydrocarbon gasoline fuel having a lead content of about 3 milligrams Pb per gallon of fuel. The compositions were then tested, first by determining the "light-off" temperature (defined below) of the catalysts In a perturbated flow of engine ex¬ haust gases generated by combusting a stoichiometric air to fuel ratio combustion mixture, and then under "sweep" test¬ ing conditions at + 0.3 A/F Ratio Units, 2 Hz, 450°C, and a space velocity of 80,000 volumes of gas per volume of cata¬ lyst per hour. All catalyst composition samples were tested in the exhaust gas generated by an engine burning a commer¬ cial unleaded gasoline containing not more than 5 milligrams per gallon Pb. The performance of the various compositions is shown in TABLE I, below. All comparisons are between catalysts that have been aged and tested together under the same conditions. The temperatures listed under T^Q (light- off temperatures) are the average temperature in degrees Centigrade for conversion to C02 and/or HpO of 50% of the content of, respectively, hydrocarbons and carbon monoxide
In the test gas. The temperature of the exhaust gas was in¬ creased at a rate of approximately 20°C per minute until the light-off temperature was attained.
In TABLE I, "Smpl." means Sample, "% Prom." means the percent by weight of base metal promoter dispersed on the co-formed ceria-zirconia support of the catalyst composition sample and "T,-Q" IS the light-off temperature. The term "% Conversion" means the percentage of the pollutant (HC, CO and NOx) in the untreated exhaust gas which is converted to innocuous substances, i.e., COp, HpO and/or Np. The defini¬ tion of "Lean" and "Rich" is based on the definition of the stoichiometric air to fuel weight ratio for gasoline as be¬ ing 14.65 parts by weight air to parts by weight gasoline. The air to fuel ratio ("A/F") is referred to in terms of deviation, measured in A/F Units, from the stoichiometric baseline of 14.65. Thus, an A/F of 14.15 (a rich A/F) is 0.5 A/F Units less than stoichiometric (14.65 - 0.5 = 14.15), and is described as -0.5 A/F Unit. Conversely, an A/F of 15.15 (a lean A/F) is described as +0.5 A/F Unit (14.65 + 0.5 = 15-15). As used in TABLE I, "Rich" means
-0.2 A/F Units (or a 14.45 A/F Ratio), "Lean" means +0.2 A/F Units (or a 14.85 A/F Ratio) and "Stoich." means a stoichio- metric air-to-fuel ratio (14.65) of the combustion mixture fed to the engine which generates the gas being treated. The rhodium content of the catalyst composition samples is given in grams of rhodium per cubic foot of catalyst volume ("g/ft3 Rh") .
TABLE I
Rich Stoich. Lean
% g/ftJ % Conversion
Smpl. Prom. Rh T50 HC CO Nox HC CO Nox HC CO Nox
4.2 434 40 18 71 63 63 77 51 85 33 4.4 413 46 25 81 68 70 81 53 89 34
4.1 405 54 32 89 71 74 82 56 92 33
4.0 421 64 43 82 83 74 65 86 98 34
4.0 401 66 44 88 84 78 66 86 98 34
4.4 381 64 39 97 74 91 77 61 97 32
4.5 375 65 38 97 76 96 78 60 97 32
4.2 384 65 29 94 75 82 80 64 94 35
4.2 346 68 32 98 81 93 84 68 98 30
3.7 400 69 36 98 74 84 88 64 93 70 4.0 395 72 38 98 76 90 88 63 95 70
3.4 396 65 37 96 71 82 87 62 89 68
4.0 402 60 37 95 73 84 81 60 93 35
4.1 401 60 38 96 73 83 80 60 9^ 38
4.4 407 51 27 86 68 73 75 54 86 28 4.0 391 54 29 93 75 86 84 56 91 30
Figure imgf000018_0001
TABLE I (CONT.)
Rich Stoich. Lean g/ft" % Conversion
Smpl. Prom. Rh T50 HC CO Nox HC CO Nox HC CO Nox
8A none 4.2 406 52 38 91 72 90 81 58 87 30 8B 1% 4.3 396 52 37 87 71 87 80 57 84 28
MgO
8C 5% 4.2 381 58 41 95 75 94 83 58 89 29
MgO
9A none 4.2 365 53 36 96 79 91 86 63 86 27 9B 1% 3.9 366 51 30 97 80 92 86 64 88 26
NiO
9C 2% 4.0 363 53 36 97 80 94 86 63 90 26
NiO
10A none 4.3 407 48 30 87 70 80 86 58 85 36 10B 0.7% 4.1 385 58 29 96 79 86 94 65 93 36 CuO
11A none 4.0 365 49 38 95 78 96 90 64 91 31 11B 2% 4.2 347 49 39 96 77 96 93 64 92 32 Fe203
HC 5% 4.2 342 54 40 96 80 97 93 64 90 29 CaO
11D 5% 4.2 342 52 38 97 80 96 91 65 94 30 LapO
The comparison of Comparative Sample IA with Samples IB and IC show that the latter two exhibit a significantly low¬ er light-off temperature than does Sample IA, which contains no base metal oxide promoter. A lower light-off temperature of course means that the catalyst Is more active at lower temperatures, which enhances conversion of pollutants during cold start-up periods of engine operation. Further, the presence of 3 weight percent MgO and 2 weight percent NiO in, respectively, Samples IB and IC significantly improved the percentage conversion of all three pollutants (HC, CO and NOx) under both rich and stoichiometric conditions, as compared to Comparative Sample IA.
Sample 2B shows a significant reduction in light-off temperature and substantially increased conversion of NOx under rich operating conditions, and of CO under stoichio- metric operating conditions as compared to Comparative Sam¬ ple 2A.
With respect to Comparative Sample 3A, Sample 3B shows significantly improved activity for CO reduction at stoichi¬ ometric conditions. The incorporation of 5 weight percent MgO in Sample 4B shows, as compared to Comparative Sample 4A, slgnficantly lowered light-off temperature and improved conversion of NOx under rich conditions, improved conversion of all three pol¬ lutants under stoichiometric conditions, and of HC and CO under lean conditions. However, significantly decreased ac¬ tivity for NOx under lean conditions was observed.
Sample 5B, containing 0.4 percent CuO, shows signifi¬ cantly improved conversion of CO at stoichiometric condi¬ tions as compared to Comparative Sample 5A. However, in this particular test, the inclusion of 4 weight percent NiO in Sample 5C did not show a significant improvement in acti¬ vity under these particular test conditions. However, the efficacy of NiO as a base metal oxide promoter in the cata¬ lyst compositions of the present invention is amply demon- strated by Samples 9A-C, discussed below, as well as in Sam¬ ples IA and IC, as discussed above.
The addition of 1 percent by weight MgO in Sample 6B did not show significantly improved activity relative to Comparative Sample 6A. However, the efficacy of MgO in somewhat greater amount Is amply demonstrated by Sample 8C, discussed below and in Samples IA and IB, as discussed above.
A comparison of Comparative Sample 7A with Sample 7B shows that the incorporation of 2 percent by weight SnO? provides a significantly lower light-off temperature and in¬ creased conversion of NOx under rich conditions, increased conversion of all three pollutants under stoichiometric con- ditions, and increased conversion of CO under lean condi¬ tions.
The addition of 5 weight percent MgO in Sample 8C shows significantly reduced light-off temperature and Increased conversion of HC and NOx under rich conditions, and in¬ creased conversion of CO under stoichiometric conditions.
Similarly, Samples 9B and 9C as compared to Comparative Sample 9A shows that the addition of 2 percent NiO signifi¬ cantly improves conversion of CO under lean conditions and marginally improves conversion of CO under stoichiometric conditions, while the addition of 1 percent by weight NiO in Sample 9B had the adverse effect of decreasing the conver¬ sion of CO under rich conditions.
A comparison of Sample 10B to Comparative Sample 10A shows that the addition of 0.7 percent by weight CuO signi¬ ficantly improved, that is, reduced, the light-off tempera¬ ture as well as providing significant increases In conver¬ sion of all pollutants under all three conditions except for CO under rich conditions and NOx under lean conditions. Finally, reference to Samples 11A through 11D shows that the Indicated base metal oxide promoters in Samples 11B, HC and 110 significantly reduced light-off tempera¬ ture. In addition, the 2 percent by weight Fep0^ of Sample IB marginally improved conversion of NOx at stoichiometric conditions, the 5 percent by weight CaO incorporated in Sam¬ ple HC significantly Improved conversion of HC under rich conditions and marginally improved conversion of NOx under stoichiometric conditions, and the 5 percent by weight La?0_ incorporated into Sample 11D marginally improved conversion of HC under rich conditions and CO under lean conditions .
Example 3
Two catalyst compositions were prepared using tech¬ niques similar to those of Example 1 to produce catalyst compositions in which the washcoat is provided as two dis¬ crete bottom and top layers on the substrate. Thus, the substrates comprising cordierite honeycomb cores were ini- tially coated with a slurry comprising approximately 67% by weight (dry solids basis) of activated alumina and 33% by weight of bulk ceria, I.e., fine particles of cerium oxide. The platinum was impregnated with a platinum hydroxide meth- ylethanolamine solution so that approximately 2% by weight of the combined impregnated alumina and bulk ceria comprised platinum. After drying and calcining the bottom coat, the coated cordierite substrate was provided with a top coat by being dipped into a slurry prepared as follows. Rhodium ni- trate was utilized to impregnate activated alumina and the impregnated alumina was mixed with a quantity of co-formed ceria-zirconia support in which ceria comprised about 12% by weight of the weight of the co-formed support. The quantity of zirconyl acetate was then Introduced to the slurry of alumina and co-formed ceria-zirconia support particles so that the zirconyl acetate impregnated both the alumina and the co-formed ceria-zirconia particles. Excess slurry was blown om the twice-coated core which was then dried and calcined to provide an outer or top coat layer comprising approximately 38% by weight alumina, approximately 53% by weight of co-formed ceria-zirconia support, approximately 9% by weight zirconia and approximately 0.42% by weight rhodi¬ um. The Inner or bottom coat comprises about 68% by weight of the total weight of washcoat (catalytic material) and the top coat comprised about 32% by weight. Additionally, in order to produce the base metal oxide promoted version of this catalyst in accordance with the invention (Sample 12B), nickel nitrate is added to the top coat slurry in an amount sufficient so that, if all of the nickel were to migrate to the co-formed ceria-zirconia support, it would amount to
17.8% by weight of the co-formed ceria-zirconia support. Of course, not all of the nickel migrates to the co-formed sup¬ port and some of it will migrate to the alumina particles contained in the top coat slurry. Because of the surface properties of alumina as compared to those of the co-formed ceria-zirconia support and other factors, only about 10 to 25% of nickel oxide can be expected to end up dispersed onto the co-formed ceria-zirconia support. Therefore, in Sample 12B the amount of nickel oxide promoter dispersed on the co- formed ceria-zirconia support is estimated at from about 1.8 to 4.5% by weight of the weight of co-formed ceria-zirconia support.
The total loading of precious metal (platinum plus rho¬ dium) on the catalyst Samples 12A and 12B Is 40 grams per cubic foot, with the platinum and rhodium being present in a weight ratio of 5 parts platinum to 1 part rhodium.
Ex-ample The catalyst compositions prepared according to Example 3 were loaded into testing chambers and subjected to an aging cycle similar to that described in Example 2 except that an Inlet gas temperature of 850°C and an actual fuel shut-off was utilized, instead of the simulated fuel shut- off employed in Example 2. The fuel shut-off was continued for about five seconds every minute. The abbreviations of TABLE II have the same meaning as those given above for TABLE I. Aging and testing of the catalyst samples were the same as those described above in Example 2, except that the gasoline burned to generate the exhaust gas contained 12 milligrams Pb per gallon and the sweep testing conditions were carried out at +0.5 A/F Ratio Units at 350°C and a space velocity of 135,000 volumes of gas (measured at stan¬ dard conditions of temperature and pressure) per volume of catalyst per hour. Data was not taken at lean conditions.
Figure imgf000023_0001
The data of TABLE II shows significant reduction in light-off temperature for the nickel oxide promoted catalyst as compared to the unpromoted catalyst and significantly better conversions of the noxious components to innocuous substances attained. The suitability of ceria-promoted zirconia supports other than co-formed ceria-zirconia supports for the pur¬ poses of the present invention is Indicated by tests of com¬ parable, but different, catalyst compositions containing platinum and rhodium catalytic components dispersed on ceria-promoted zirconia supports obtained by impregnating zirconia particles with a solution of cerium nitrate. The impregnated particles were then calcined to decompose the cerium nitrate to ceria. The catalyst was aged and tested on the exhaust gas of a stock Nissan GLD engine. The ceria- promoted zirconia support showed good durability and the catalyst showed good activity, indicating that ceria-pro¬ moted zirconia supports generally, including those which are not co-formed, are suitable for purposes of the present in¬ vention. While the invention has been described in detail with respect to specific embodiments thereof, it will be appreci¬ ated that variations to the invention may be made which will nonetheless lie within the sphere and scope of the invention and are intended to be embraced by the appended claims.

Claims

THE CLAIMSWhat is claimed is:
1. A catalyst composition comprising a carrier on which is disposed a washcoat of catalytic material compris¬ ing a ceria-promoted zirconia support having dispersed thereon (i) a catalytlcally effective amount of rhodium and (ii) a rhodium-stabilizing amount of a base metal oxide pro¬ moter.
2. The catalyst composition of claim 1 wherein the ceria-promoted zirconia support comprises a co-formed ceria-zirconia support.
3. The catalyst composition of claim 2 wherein the co-formed ceria-zirconia support comprises from about 1 to 25% by weight of ceria.
4. The catalyst composition of claim 1, claim 2 or claim 3 wherein the base metal oxide promoter dispersed on the ceria-promoted zirconia support is present in an amount of from about 1 to 10% by weight of the weight of the ceria-promoted zirconia support.
5. The catalyst composition of claim 4 wherein the ceria-promoted zirconia support comprises a co-formed zir¬ conia support.
6. The catalyst composition of claim 4 wherein the base metal oxide promoter is present in an amount of from about 1 to 5% by weight of the weight of the ceria-promoted zirconia support.
7. The catalyst composition of claim 6 wherein the ceria-promoted zirconia support comprises a co-formed zir¬ conia support.
8. The catalyst composition of claim 1, claim 2 or claim 3 wherein the washcoat further comprises a refractory metal oxide.
9. The catalyst composition of claim 8 wherein the re¬ fractory metal oxide comprises alumina.
10. The catalyst composition of claim 1, claim 2 or claim 3 wherein the base metal oxide is selected from the group consisting of calcium oxide, copper oxide, iron oxide, lanthanum oxide, magnesium oxide, manganese oxide, nickel oxide and tin oxide.
11. The catalyst composition of claim 10 in which the base metal oxide promoter dispersed on the co-formed ceria- zirconia support is present in an amount of from about 1 to 10% by weight of the weight of the co-formed ceria-zirconia support.
12. The catalyst composition of claim 1, claim 2 or claim 3 wherein the base metal oxide comprises one or both of nickel oxide and magnesium oxide.
13. A catalyst composition comprising a carrier on which is disposed a washcoat of a catalytic material com¬ prising a ceria-promoted zirconia support in which ceria comprises from about 1 to 25% by weight of the weight of the support, and the support has dispersed thereon (i) a cata¬ lytlcally effective amount of rhodium and (ii) a base metal oxide present in an amount of from about 1 to 10% by weight of the weight of the ceria-promoted zirconia support.
14. The catalyst composition of claim 13 wherein the ceria-promoted zirconia support comprises a co-formed ceria-zirconia support.
15. The catalyst composition of claim 13 or claim 14 wherein the base metal oxide promoter is present in an amount of from about 1 to 5% by weight of the weight of the co-formed ceria-zirconia support.
16. The catalyst composition of claim 13 or claim 14 wherein the washcoat further comprises a refractory metal oxide.
17. The catalyst composition of claim 16 wherein the refractory metal oxide comprises alumina.
18. The catalyst composition of claim 13 or claim 14 wherein the base metal oxide is selected from the group con¬ sisting of calcium oxide, copper oxide, iron oxide, lanthan¬ um oxide, magnesium oxide, manganese oxide, nickel oxide and tin oxide.
19. The catalyst composition of claim 13 or claim 14 wherein the base metal oxide comprises one or both of nickel oxide and magnesium oxide.
20. The catalyst composition of any one of claims 1, 2, 3, 13 or 14 further including one or both of platinum and palladium dispersed in the washcoat.
21. The catalyst composition of claim 16 further in¬ cluding one or both of platinum and palladium dispersed on the refractory metal oxide.
22. A method for treating a gas containing noxious com¬ ponents comprising one or more of carbon monoxide, hydrocar¬ bons and nitrogen oxides by converting at least some of the noxious components into innocuous substances, the method comprising contacting the gas under conversion conditions with a catalyst composition comprising a carrier on which is disposed a washcoat of a catalytic material, wherein the catalytic material comprises a ceria-promoted zirconia sup- port having dispersed thereon (i) a catalytically effective amount of rhodium and (ii) a rhodium-stabilizing amount of a base metal oxide promoter.
23. The method of claim 22 wherein the ceria-promoted zirconia support is a co-formed ceria-zirconia support.
24. The method of claim 22 or claim 23 wherein the base metal oxide Is selected from the group consisting of calcium oxide, copper oxide, iron oxide, lanthanum oxide, magnesium oxide, manganese oxide, nickel oxide and tin oxide and the base metal oxide promoter dispersed on the ceria-zirconia support Is present in the amount of from about 1 to 10% by weight of the weight of the co-formed ceria-zirconia support and the rare earth metal oxide comprises from about 1 to 25% by weight of the weight of co-formed ceria-zirconia support.
25. The method of claim 22 or claim 23 wherein the gas is at a temperature of from about 250 to 500°C when it is initially contacted with the catalyst.
26. The method of claim 22 or claim 23 wherein the wash¬ coat further comprises a refractory metal oxide and includes one or both of platinum and palladium dispersed therein.
PCT/US1991/006940 1990-09-27 1991-09-24 Catalyst composition containing base metal oxide-promoted rhodium WO1992005861A1 (en)

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