CA2034063A1

CA2034063A1 - Catalyst composition containing segregated platinum and rhodium components

Info

Publication number: CA2034063A1
Application number: CA002034063A
Authority: CA
Inventors: Chung-Zong Wan
Original assignee: Engelhard Corp
Current assignee: BASF Catalysts LLC
Priority date: 1990-02-22
Filing date: 1991-01-11
Publication date: 1991-08-23
Also published as: JP3274688B2; EP0443765B1; US5057483A; JPH04219140A; KR0169320B1; DE69133034T2; ZA91431B; ATE218921T1; EP0443765A1; KR910021258A; DE69133034D1

Abstract

ABSTRACT OF TEE DISCLOSURE
A catalyst composition suitable for three-way conver-sion of internal combustion engine, e.g., automobile gaso-line engine, exhaust gases includes a catalytic material disposed in two discrete coats on a carrier. The first coat includes a stabilized alumina support on which a first plat-inum catalytic component is dispersed and bulk ceria, and may also include bulk iron oxide, a metal oxide (such as bulk nickel oxide) which is effective for the suppression of hydrogen sulfide emissions, and one or both of baria and zirconia dispersed throughout the first coat as a thermal stabilizer. The second coat, which may comprise a top coat overlying the first coat, contains a co-formed (e.g., co-precipitated) rare earth oxide-zirconia support on which a first rhodium catalytic component is dispersed, and a second activated alumina support having a second platinum catalytic component dispersed thereon. The second coat may also in-clude a second rhodium catalytic component, and optionally, a third platinum catalytic component, dispersed as an acti-vated alumina support. The present invention also provides a method for treating engine exhaust gases by contacting the gases under conversion conditions with the catalyst compo-sition.

Description

BAC~GRO~ND OF r~ INVE~TIO~
_ . _ Fleld of the Inventlon The present lnventlon is concerned wlth catalysts use-ful for the treatment of gases to reduce contaminants con-tained therein, 3uch as catalysts o~ 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 substantlally slmultaneously catalyzing both oxidatlon and reduction reactions, such as the oxidation of hydrocar-bons and carbon monoxide and the reduction of nitrogen ox-ides ln a gaseou~ stream. Such catalysts flnd utility in a number of fields, including the treatment of the exhaust gases from internal combustion englnes, such as automobile and other gasollne-fueled englnes.
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Background and Related Art In order to meet governmental emisslons standards ~or lnternal combustion engine exhau3ts, so-called catalytlc converters contalning a suitable catalyst such as a TWC cat-alyst, are emplaced ln the exhaust gas llne of lnternal com-bustion engines to promote the oxidatlon o~ unburned hydro-carbons ("HC") and carbon monoxlde ("CO") and the reduction of nitrogen oxides ("NOX") in the exhaust gas. For this purpose, TWC catalysts compri~ing a minor amount of one or more platlnum group metal~ di~tencled upon a high surface area, refractory metal oxide support are well known in the - ar~. The platinum group metal may comprise platinum or pal-ladium, preferably lncluding one or more of rhodium, ruthen-lum and iridium, especlally rhodium. The refractory metal : oxlde support may compri~e a high surface area alumina coat-ing (often referred to as "actlvated" or "gamma" alumina) carried on a carrier such a a monolithic carrier comprislng a refractory ceramlc or metal honeycomb ~tructure, a~ well known in the art. I'he carrier may also comprise refractory particle~ such as spheres or short, extruded segments of a refractory material such as alumina.

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The catalytlcally actlve materlals dlspersed on the actlvated alumina may contain, ln addltion to the platlnum group metals, one or more base metal oxides, such as oxides of nlckel~ cobalt, manganese, iron, rhenlum, etc., as shown, for example, in C.D. Keith et al U.S~ Patent 4,552,732. The actlvated alumina typlcally exhlbits a ~ET surface area in excess of 60 s~uare meters per gram ("m~/g"), o~ten up to about 200 m /g or more. Such actlvated alumina is usually a mlxture of the gamma and delta phases of alumlna, but may also contain substantlal amounts of eta, kappa and theta alumina phases.
The refractory metal oxlde supports are sub~ect to thermal degradation from extended exposure to the high tem-peratures o~ exhaust gas resulting in a loss of exposed cat-alyst sur~ace area and a corresponding decrease in catalytlcactivlty. It is a known expedient in the art to stabilize refractory metal oxide supports against such thermal degrad-atlon by the use o~ materials such as zirconla, titania, al-kaline earth metal oxides such as baria, calcla or strontla or, most usually, rare earth metal oxide~, for example, cer-ia, lanthana and mlxtures of two or more rare earth metal oxides. For example, see C.D. Keith et al U.S. Patent 4,171,288.
TWC catalysts are currently ~ormulated wlth complex washcoat compositlons containing stabllized A1203, an oxygen storage componentJ primarlly cerla, and preclous metal cata-lytic components. Such catalysts are deslgned to be ef~ec-tive over a specific operating range o~ bo~h lean o~, and rich of, stoichlometrlc condltlonq. (The term "oxygen stor-age component" ls used to de~ignate a material which ls be-lleved to be capable of belng oxidlzed during oxygen-rlch (lean) cycles of the gas being treated, and releaslng oxyg~n durlng oxygen-poor (rlch) cycles.) Such TWC catalyst com-posltlons enable optlmlzatlon o~ the conversion o~ harmful emlsslons (HC, C0 and N0x) to lnnocuous substances. or the three preclous metals, platlnum, palladium and rhodlum, con-ventionally used in TWC catalysts, rhodium i~ the most ef-"

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~ective for reducing NOX to harmless nitrogen. Un~o~tunate-ly, rhodlum ls also the most e~pensive of these costly mate-rlals and, consequently, effective rhodium utilizatlon in automotlve exhaust catalysts, such as TWC catalysts, has ; 5 been extenslvely studied~
One of the problems faced by present-day catalysts is the high operating temperatures engendered by smaller auto-motlve engines and hlgh speed hlghway drl~lng. Not only alumina support materlals as noted above, but oxygen storage components are susceptible to thermal degradatlon at such ; hlgh temperatures. Thermal degradation adversely af`fects the stablllty of the catalyst and ePfectiveness of the pre-clous metals used therein. In addltion, attempts to improve fuel economy by using air to fuel ("A/F") ratlos higher than stoichlometrlc, and/or fuel shut-off features, generate a lean (o~ygen-rich) exhaust. Hlgh exhaust gas temperatures and lean gas conditions accelerate the deterioration of platinu~ and rhodlum catalysts, lnasmuch as platlnum ls more readlly sintered, and rhodlum more strongly lnteracts wlth support materials such as alumlna, at such condltions.
The art has devoted a great deal of effort ln attempts to improve the efficlency of platlnum and rhodlum-based TWC
composltlons~ Thus, U.S. Patent 4,675,308 dlscloses a meth-od o~ effective utillzatlon of rhodlum by placlng lt on alu-;~ 25 mlna whlch ls segregated from cerla-contalnlng partlcles slnce cerla enhances the lnteractlon between rhodium and alumlna, whlch renders the rhodium less actlve.
U.9. Patent 4,806,519 separates the rhodlum component ln a layered structure ln whlch rhodlum is supported on alu-mlna in a second coat whlch ls segregated ~rom the ceria-contalning material in a ~lrst coat. However, in both cases the rhodium is stlll prlmarlly in contact wlth alumina sup-port partlcles so that any thermal degradation occurrlng to the alumlna wlll lnevltably affect the catalytic efflciency o~ the rhodlum.
The u~e of layered coatlngs ln catalyst composltlons ;~ is also shown in two Japanese patent publicationsO Japanese - 2~3~9~
Patent App lcation 88-32682~J46 (J63240-947A) of Nlssan Mot-or KK (10.02.87-JP-027383) dlscloses a catalyst comprislng a support having two dl~ferent alumlna coatings separately loaded thereon. One alumlna coatlng contains ceria-alumina : 5 aild ceria on which platlnum, palladium or rhodium is dis persed, and ls stated to be effective for CO and HC removal.
-The other alumina coating, which ls stated to be effective for NOX removal, is made from lanthana-alumina and zirconium oxlde partially stabilized with Pr and on which palladium or rhodium is di~persed. The catalyst is stated to have TWC
activity.
Nissan Motor Company Ltd. Japanese patent publication JP63 77,544 (88 77,544), 7 April 1988, dlscloses a catalyst comprising a first washcoat containing activated alumina bearing rare earth oxides, and a second washcoat dlsposed over the flrst washcoat and containing activated aluminum bearing rare earth oxides, mainly ceria and zirconia. Pal-;ladium is kept away from polsonous ~ubstances near the wash-coat surface~ and form~ LA-O-Pd ln the flrst washcoat and Rh-O-Zr ln the ~econd wa~hcoat.
Co-pendlng and commonly a~slgned U.S. Patent Appllca-tlon Serial No. 07/234,226 present;s a method to improve thermal stabillty of a TWC catalyst containlng platlnum and rhodium by lncorporating a barlum compound and a zlrconium compound together with cerla ln bulk ~orm. Thls is stated to enhance stabillty of the alumlna washcoat upon exposure to hlgh temperatures~
`In another approachg U.S. Patent 4,233,189 teaches the ; u~Ye of non-alumina supports such as zlrconla for rhodlum, so that rhodium-alumina interaction can be avoided. However, zlrconia has a lower surface area than ~amma alumina and it-self is not a thermally stable support. Zirconia undergoes a phase transltlon between it~ monoclinlc crystalline struc-ture and lts more stable tetragonal crystalline ~tructure over a wlde temperature range. Such transition causes dras-tic slntering Or the a~soclated preclous metal~. Thus, a need still exists for improved stabillzatlon agalnst thermal :.
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' _5_ ~3~3 degradation of precious metals contalning TWC catalysts.

S~M~RY OF TH~ IN~NTION
In accordance with the present invention there is pro-vided a catalyst composltion comprisil,g a carrier on which is disposed a catalytic materlal, th? catalytic materlal comprlsing the ~ollowlng components contained in two coats.
A flrst coat ls car ied on the carrier and comprises a flrst activated alumina support on which is dispersed a catalytic-ally ef~ective amount o~ a ~irst platinum catalytic compo-nent. A catalytlcally ef~ectlve amount o~ ~ulk cerla i5 in-cluded ln the first coat. Optionally, the flrst coat may also include a bul~ iron oxide and a catalytically ef~ective amount o~ metal oxlde effectlve ~or the suppression of se-condary emissions such aA H2S. The metal oxide may com-prise, for example, bulk nickel oxide. The ~irst coat may optionally also lnclude a thermal stabilizer dispersed therein, ~or example, one or both of baria and zlrconia may be dlspersed in the ~irst coat, e.g., on both the alumina and the bulk cerla thereo~, in an amount su~flcient to sta-blllze the alumlna and bulk cerla against thermal degrada-tion. The carrler also carrles a second coat, whlch optlon-ally may comprlse a topcoat overlying the ~lrst coat and comprlslng a co-~ormed rare earth oxide-zlrconla support, a Z5 catalytlcally e~fectlve amount of a ~lrst rhodlum catalytlc component dispersed on the co-formed zlrconla support, a se-cond actlvated alumlna support, and a catalytically effec-tlve amount o~ a ~econd platinum catalytlc component dis-persed on the second alumina support. In additlon to the above-3peci~ied second coat lngredlents, the second coat may - optlonally lnclude a second rhodium catalytic component dis-~ persed either on the second activated alumina support (on `~ the alumina partlcles on which the second platlnum catalytlc component i3 also dispersed) or on a thlrd activated alumina support. Optionally, a thlrd platinum catalytlc component may be dispersed, together ~lth the second rhodlum catalytic component~ on the third activated alumina support. The se-`; :
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-6- 2~3~3 cond coat, like the first coat, may optlonally include a thermal stablllzer dispersed thereln ln an amount sufflclent to stabllize the second activated alumlna support (and the thlrd actlvated alumina support, lf present, agalnst thermal degradatlon). The thermal stabllizer dlspersed ln the se-cond coat may comprlse zirconla.
Other aspects of the present invention provide for the lncluslon of one or more of the following features, singly or in a combinatlon of two or more of them. Thus, the metal oxide effectlve for the suppression of secondary emlsiions such a~ H2S may be one or more of oxides of nlckel, copper, managnese and germanium; the thermal stablllzer ln the flrst coat may be one or more of cerla, barla and zlrconla; and the rare earth oxide of the co-formed rare earth oxlde-zlr-conla support may be one or more of oxldes of cerium, neo-dymium and yttrlum, preferably cerlum oxide.
Another aspect of the pre~ent invention provides that the carrier may comprise a refractory body havlng a plural-lty of substantlally parallel passage~ extending there-through, the pas~ages being deflned by wall~ and the cata-lytlc materlal belng coated on the walls as the aforesald flrst coat and second coat.
In accordance with the pre~ent inventlon there ls also provlded a method for treatlng a gas such as the exhaust of an lnternal combustlon englne, especially a gasollne-fueled englne, which contalns noxlou~ components such as one or ` more of carbon monoxlde, hydrocarbons and nltrogen oxldes.
~ The method comprlses converting at least some of these nox-i lou~ component~ to innocuous ~ubstance~ by contacting the gai~i under conversion conditlons with a catalyst compositlon as described above. As used herein and in the clalm~, ~con-version conditions" means condltions sultable ~or the cata-ly~t compositlon of the lnventlon to catalyze the reaction of one or more (usually all three) of hydrocarbons, carbon monoxlde and nitrogen oxides to "innocuous subqtances", l.e, to water, carbon dioxlde and nitrogen. The catalyst compo-sltlons of the present lnventlon are capable of acting as .
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TWC catalysts, i.e., cat~lyzing substantially simultaneous converslon of C~, HC and N0X.
As used hereln and in the clalms, the term "co~formed' as used with respect to the rare earth oxide-zlrconia sup-port material, means that the rare earth oxide or oxides are dispersed substantially throughout the entlre matrix of the zirconla particles as will occur3 ~or example, when the rare earth oxide(s) and zirconium oxide, or predecessors thereof, are co-precipltated or co-gelledO The defined term is ln-tended to dlstlngulsh from the sltuatlon ln which rare earthoxides are merely dispersed on or near the surface of the zlrconia particles, leaving the core of the partlcles large-ly or entirely ~ree of the rare earth oxide(s).

DETAIL~D D~SCRIPTIO~ OF T~ ENTION
A~D SP~IFIC ~BODIM~TS THER~OF
The catalyst compositlon of the present inventlon pro-vides a flrqt group of selected component~ in a first coat of catalytic materlal, and a second group o~ selected compo-nents in a second coat of catalytlc material, in order tophysically segregate the components of the respective coats.
The flrqt and ~econd coat~ are discrete coats, each havlng lts own composltlon and identity, and are carried on a suit-able carrier by belng adhered to the carrler (or, ln the case of the qecond coat, by belng adhered to the underlying flrst coat) as a thin, adherent coatlng~ The catalyst com-posltlon of the lnventlon usually comprises a carrler of the type o~ten referred to as a honeycomb or monolithic carrier, whlch carrier is a solld body characterlzed by havlng a plu-~ 30 rallty of flne, substantially parallel, contlnuous and open-; ended gas ~low passage~ extendlng therethrough. The cata-lytlc material i~ dlspersed as a coatlng on the carrler, speclrlcally, on the walls of the ga~ ~low passages thereofO
Such carrlers are normally made of a refractory, ceramic-llke material ~uch as cordlerite, mulllte, alumina, or any other suitable refractory materlal; they may also be made of a refractory metal such as stainless steel or other sultable :
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; -8~ 3 corro~lon-resistant~ ircn based alloysO
The dlscrete flrst and second coats of catalytic mate-rlal, conventionally referred to as "washcoats", are coated ;~ onto a suitable carrier with, preferably~ the flrst coat ad-5 hered to the carrier and the second coat overlying and ad-hering to the flrst coat. With this arrangement, the gas being contacted wlth the catalyst~ e.g., being flowed through the Fassageways of the catalytic material-coated carrler, wlll fir~t contact the second or top coat and pass 10 therethrough in order to contact the underlying bottom or first coat. However, in an alternate configuratlon, the ~econd coat need not overlie the flrst coat but may be pro-vlded on an upstream (as sensed ln the dlrection of gas flow through the catalyst composltlon) portion o~ the carrler, 15 wlth the fir~t coat provlded on a downstream portlon Or the carrler. Thus, to apply the washcoat in thls conflguratlon, an upstream longitudlnal segment only of the carrler would be dlpped into a slurry of the flr~t coat catalytic materl-al, and drled, and the undipped downstream longltudlnal seg-20 ment of the carrler would then be dlpped lnto a ~lurry of the second coat catalytlc materlal and dried. Alternatlve-ly, separate carriers may be used, one carrler on which the first coat is deposited and a second carrier on which the second coat ls deposlted, and then the two separate carriers 25 may be posltloned wlthln a canister or other holdlng devlce ; and arranged so that the exhaust gas to be treated ls flowed in serie~ flrst through the catalyst containing the second coat and then through the catalyst contalning the ~irst coat thereon. However, as lndlcated above, lt ls preferred to 30 utllize a catalyst composltlon in which the ~econd coat overlies and adhere~ to the flr~t coat because such conflg-uration ls believed both to simplify production of the cat-aly~t composltion and to enhance its efficacy.
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The ~irst Coat The Pirst coat (the bottom coat ln the preferred over-lylng-coat conflguratlon of the catalyst composltion) may .:

`"`` ~3~63 contain at least a maJor portion of the platinum component of the catalyst. It ls preferred to depo~lt the platlnum component, or at least the ma~or portlon thereof, on an ac-tlvated alumlna support forming part of the first coat of catalytlc materlal. The actlvated alumina on whlch bhe platlnum catalytlc component ls ~ispersed is stabiiized agalnst thermal degradatlon by one or both of baria and zlr-conia, as described below. The pïatinum catalytlc component may be dlspersed onto the actlvated alumina support by any - 10 sultable technlque. U~ually, thls is carrled out by impreg-nating a slurry of flne partlculate alumina with a solutlon or dlsperslon of a soluble platinum compound or complex and then drylng and calclnlng the thus-impregnated alumina sup-port particles to provide the platlnum catal~tlc component dlspersed thereon. (Thls technlque of lmpregnatlng a slurry of flne, partlculate support materlal may also be used for dlspersing the other catalytic components, such as a rhodlum catalytic component, onto their respective supports.) The platlnum catalytlc component thus di~persed on a thermally stablllzed (as descrlbed below) actlvated alumlna support ls belleved to serve9 ln the compoqitlon o~ the present inven-tlon, the usual functlon of quch platinum catalytlc compo-nentq of catalyzlng at leaqt the converslon of CO to CO2 and ` of HC to water and CO2.
The quantlty of ingredients ln the catalyst composi-tlon may be expressed as the welght o~ such ingredlent~ per unit volume o~ the catalyst compo~itlon. Thls ls partlcu-larly useful for a catalyst composltlon in which the carrler is a honeycomb-type body because thls ba~ls of expression ; 30 accounts for the voldq ln the catalyqt compositlon provided by the gas flow passages extendlng therethrough. For ingre-dlent~ other than preclous metals 9 a convenient basls ls ~` grams per cubic inch ("g/ln3~) o~ the catalyst composltlon and for precious metals, grams per cublc foot ("g/ft3") of the catalyst compo~ltlon. On thls basls of measurement, the ; activated alumlna may be present in the first coat in an amount of from about 0.1 to 4.0 g/in3 and the first platlnum !

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'' ' ~ ' ' ' -lO- 203~3 catdlytlc component may be present in an amount of from about 5 to 100 g/rt3.
In additlon to the platlnum catalytic component, the first coat may optlonally contaln a palladium catalytlc com-ponent dispersed on an activated alumina support. The pal-ladium catalytlc component may be present ln an amount of up to about 100 g/ft3, that ls, from 0 to 100 g/ft3, e.g., from 0.1 to 100 g/~t3. The palladlum catalytlc component, if pre~ent, may be dl3per~ed on the flr~t activated alumina -__support, individual particles of ~hich may contain both platlnum and palladium catalytlc components dlspersed there-on or the palladlum catalytlc component may be dlspersed on lts own thermally stabillzed actlvated alumlna support par-tlcles, and mixed lnto the first coat composltlon.
The first coat or washcoat layer further lncludes bulk rare earth oxides including cerla, preferably bulk cerla of at least 90 percent purlty a~ CeO2, more preferably 95 per-i cent or 99 percent CeO2, wlth the balance substantlally com-prl~lng other rare earth metal oxldes. The utillzatlon of ~uch hlgh purity bulk ceria in a catalyqt compoqition iY
dlsclo~ed, ~or example, in U.S. Patent 4,714,694 o~ C.Z. Wan ; et al in connection with catalysts comprislng an alumina-stabilized bulk cerla. A~ dlsclosed in thls patent, the bulk ceria contains at least about 95, or at least about 99 _or 99.5, percent by welght CeO2, wlth the predomlnant impur-lty compri~ing lanthana with les~er amounts of other rare earth oxlde~. The amount of bulk cerla ln the flr3t coat of he catalyst compositlon may beA~rom about 0.15 to 1.5 g/in3 o~ the finl~hed cataly~t composltion.
The flr~t coat may ~urther lnclude lron oxlde, prefer-ably lntroduced lnto the manu~acturlng procedure o~ the cat-alyst as Fe304 (magnetlte) because ln thls ~orm the lron ox-lde ls lnsoluble ln the mllllng media normally utllized to prepare the washcoats of the inventlon. The iron oxide, which may be lntroduced ln bulk ~orm, ls catalytlcally ef-fectlve for promoting C0 oxldatlon.
The ~irqt coat may also lnclude a metal oxlde, for ex--11- 21~3~63 ample, bulk nickel o-ide, whlch ls effective for the sup-presslon of any H2S which may be present. The H2S suppres-sor may thus be in bulk form and the iron oxlde ls ln bulk form, the bulk form oxides preferably comprlsing particles of at least O.l micron ln diameter. Such bulk metal oxldes are not slsniflcantly solub'e in the wa~hcoat slurry used during preparation o~ the catalyst composition and do not ; readlly react with activated alumina particles to form un-desirable compo~ite or comblned materials whlch reduce the thermal stabllity of the actlvated alumlna.
The lron oxide J 1~ present, ls preferably employed in an amount o~ about 0.05 g/ln3 to 0.3 g/ln3 of lron oxlde.
The iron oxide serves as a promoter for the oxidatlon of CO
to carbon dloxide. Any form of ferrous or ferrlc oxlde ls sultable for use as the CO oxldation promoter, but Fe3O4 ls preferred as lt is lnsoluble in the ballmllllng medium used to prepare the washcoats. It is preferable not to lncorpor-: ate the lron oxlde ln a dlspersed ~orm on the alumlna, l.e., by lmpregnatlon o~ the alumlna w th an lron salt solutlon ;~ 20 and then calclnlng, and con~equently the lron oxlde 19 pro-vlded in bulk form. As wlth the o1;her bulk lngredients, "bulk" means that the lron oxlde l~ added as flne partlcles, ; prefer-ably of at least O.l mlcrons ~lze (dlameter), of solld lron oxlde, r~ther than belng dlspersed into the other ln-gredlents as by belng lmpregnated into the composltlon ln ; the form o~ a soluble iron salt.
The metal oxide effective Por the suppres3ion o~ H2S
emlsslons may be any ~ultable metal oxlde which serves the `71' purpo~e. Nlckel oxide, preferably bulk nickel oxlde, ls a preferred component for thls purpo~e although other oxides such as germanium oxlde, copper oxlde amd manganese oxlde are also known to be sultable for the purpose. The H2S sup-pressor lngredient ls userul because ceria and alumlna tend to trap sulfur compounds on thelr surfaces. The sul~ur com-pounds, which result from the combustion of sulfur containedin gasollnel are converted to hydrogen sul~lde durlng trans-lent fuel-rich operating condltlons such as ldllng and ac-;,, , , :
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-12- 2~ 3 celeration, and provide a characteristlc foul odor to the exhaust gasesO A sultable metal oxide H2S suppressor such as nickel oxide will at least ~emporarily trap any hydrogen Aulfide which is formed, thereby delaylng the discharge of hydrogen sul~lde Prom the catalyst. During transient fuel-lean operation the sulfides are oxidized or otherwlse decom~
posed in the oxygen rlch en~lronment, and the nydrogen sul-~ide is converted to various sulfates. The quantity of the metal oxlde used depends on lts hydrogen sulfide-trapping capacity. Generally, the metal oxlde loading ln the cata-lyst composltion is typlcally ~rom about 0.05 g/in3 to 0.5 ~` g/ln3, measured as the metal oxide, e.g., NiO. When the metal oxide used for suppresslng the release of H2S com-prise~ nickel oxide, it is desirable not to deposit the NlO
in a disperAed form (eOg., from solutlon) on alumlna. Con-sequently, the nickel oxlde is preferably incorporated into the ~ir~t coat as a bulk flne particulate materlal.
; In contrast, the thermal stabllizer used in the first coat, whlch may be baria or zirconla or both, is incorpor-ated into the other ingredlents ln dlspersed ~orm by lmpreg-natlon of the bulk ingredients (alumina, cerla, etc.) with ~olutlon~ or other dispersions o~ soluble compounds or com-plexes of barlum and/or zlrconium salts, followed by drylng and calclnation of the lmpregnatecl bulk materlals. Thls may be accompllshed by uslng an lmpree;natlon technlque similar to that described above with respect to lmpregnatlng the platlnum catalytic component onto the actlvated alumina sup-port materlal. Thus, soluble salts of ~lrconium and/or bar-lum may be dissolved ln an aqueous solutlon and the solutlon used to lmpregnate the washcoat components of the flrst ; coat. The soluble salts, such as nltrates, are decomposed to oxides durlng the calclning o~ the catalyst composltlon and the re~ultant zlrconium and barium oxldes, by belng ln-` corporated into the actlvated alumlna and the other bulk metal oxldes present, serve to stablllze these materlals againqt thermal degradation. The alumina, cerla, etc., are - thus stablllzed against thermal degradatlon. The amount o~

-13- ?03~0~3 such thermal stabilizers, if present, is preferably from about 0.05 g/in3 to 0.5 g/in3, calculated a3 the metal ox-- lde, for each thermal stabllizer utilized.
The content~ o~ the first coat of catalytic material may therefore compri~e platlnum dlspersed on a thermally stabillzed (wlth baria or zirconia or both) activated alu-mlna, bulk cerlum oxide, bulk lron oxide and a metal oxide, whlch may be in bulk form, such as bulk nickel oxide, whlch ls effec~ive for suppressing the emlssion of hydrogen sul-flde. The bulk ceria9 iron oxide and nlckel oxlde may alsobe impregnated with the stabilizing baria and or zirconia, whlch ls pre~erably dl~persed throughout the flrst coat by lmpregnatlng all the solids thereof by the above descrlbed ; technlque of lmpregnatlon with solutions of soluble barlum ; 15 and/or zlrconlum compounds9 followed by calcinatlon.
The fir~t coat may also contain other components use-ful in such catalytic compositlons. For example, as noted above, a palladlum catalytic component may also be dispersed ;;~ on actlvated alumina partlcles. The fir3t coat composltlon may also contain other components known to be useful a~ com-~ ponents of a catalytic washcoat for thl~ type of catalyst, - lncludlng a supplementary refractory metal oxide to enhance wa~hcoat porosity, such as one provlded by crushed cordler-; lte. The lnclu~ion of a high poroslty refractory metal ox-ide such as crushed cordierlte enhance~ the overall poroslty of the first coat, thereby facllitating the passage there-through o~ the gas belng treated by the catalyst compo~i-tion.

The Second Coat The second coat of catalytlc material~ l.e. 9 the top coat ln the overlying-coat preferred embodiment of the pre-sent invention, contains a rhodlum catalytic component which is dispersed on zirconia support particles which are co - 35 formed wlth, and stabili~ed by, one or more rare earth ox-ides, such as cerium oxide, neodymium oxide and yttrium ox-ide, preferably cerium oxide (ceria). The stabilized9 co-~; ~
'~

-14- ~3~3 formed rare earth oxide-zlrconia support pre~erably contalns rrom abolt 2 to 30% by welght of rare earth oxides, prefer-ably ceria, balance predominantly or entlrely zirconla2 Other rare earth oxldes may be present ln small or trace amounts. The functlon cf the rare earth oxldes dispersed throughout the zlrconla matrix ls to stabilize the zirconia ~ agalnst thermal degradation. For example, unstabillzed zir-;~ conta Indergoes a phase transition, wlth drastlc loss of surface area, at about 950C, but the co-~ormed rare earth oxide-zirconia support containlng 12 weight percent CeO2 ; exhiblts a tetragonal crystalline structure throughout the temperature range of TWC catalyst use (up to about 1000C) wlthout undergolng slgnlPicant thermal degradatlon.
The co-form~d rare earth oxide-zlrconia support, some-times herein and in the clalms referred to slmply as the"co-formed zirconla support" may be made by any suitable technlque such as co-precipitation, co-gelling or the llke.
One suitable technlque ls lllu~trated ln the artlcle by LucclnlJ E., Merianl, S., and Sbalzero, O. (1989) "Prepara-i 20 tlon of Zlrconla-Ceria Powders by Coprecipltation o~ a Mlxed Zirconium Cerlum Carbonate ln Water Wlth Urea", Int. J. of Materlalq and Product Technology, vol.4, no. 2, pp. 167-175, the dlsclosure o~ whlch 18 hereby lncorporated hereln. As dlsclo~ed startlng at page 169 of the artlcle, a dllute (0.lM) di~tllled water solution of zlrconyl chloride and `~ cerium nltrate ln proportions to promote a ~lnal product o~
Zr2 ~ 10 mol % CeO2 18 prepared with ammonlum nltrate as a buffer, to control pH. The solution was boiled with con-~tant stlrrlng for two hours and complete precipltatlon was attained wlth the pH not exceeding 6.5 at any stage.
Any other suitable technique for preparing the co-formed rare earth oxide-zlrconia may be employed, provlded that the resultant product contains the rare earth oxlde dlspersed substantially throughout the entlre zirconla ma-trlx in the flnished product, and not merely on the surfaceof the zlrconla partlcles or only withln a ~urface layer, thereby leavlng a substantial core of the zlrconia matrlx wlthout rare earth oxide dispersed therein. Thus, the zir-conlum and cerlum (or other rare earth metal ! salts may ln-clude chlorides, sulfates, nltrates, acetates, etc. The ; co-preclpltates may~ after washing, be spray dried or freeze 5 ~? dried to remove water and then calclned ln alr at about 500C to form the co-formed rare earth oxide-zirconla sup-port.
~g - ~Other rare earth ox'de stablllzers suitable ~or being co-formed with the zirconla lnclude magneslum, calclum and ~ 10 yttrium oxide~. oxiaes of these elements, as well as of - cerlum, are known as good zlrconla ~tabllizers in the cer-amlc lndustry. However, ln catalytlc appllcatlons, zirconia not only has to wlthstand hlgh temperature degradatlon due to phase ~ransformation, but also has to possess a suffici-ently high surface area to enable suitable disperslon there-on of the rhodium catalytic component. For this reason, and becau~e the rare earth oxlde ls to be di~persed throughout substantlally the entlre matrix of the zlrconla partlcles, the rare earth oxlde stablllzer 1~ not used ln bulk, l.e., solld partlculate, ~orm for zlrconla stablllzatlon but 1 co-formed wlth the zlrconla as descrlbed above. Further, rhodlum tends to lnteract wlth bulk cerla ln a manner whlch ls deleterlous to catalytlc performance. Accordingly, the ceria 19 co-formed wlth the zlrconla a~ descrlbed above. In addition, since the solubillty of cerla ln zlrconla to form a homogeneou~ ~olld solutlon i8 about 10 mol percent, the amount of rare earth oxlde co-formed with the zlrconla is llmlted to not more than about 30 weight percent rare earth oxide, based on the welght of rare earth oxide plu~ zlrcon-ia, ln order to avoid or mlnlmlze unde~irable lnteractionbetween the rhodium catalytlc component dlspersed into the co-formed zirconia support, and the rare earth oxides, lt being known that such interaction renders the rhodlum less catalytically actlve. It should be noted that lt is not neces~ary, in order to attaln the beneflts of the present lnvent$on~ to have a homogeneou~ solid solutlon of rare earth oxide(s) in zirconla, but ~uch homogeneous solid so-:~

, ~ ......

: 2~3~3 lutlon ls lncluded in the term "co-formed rare earth ox-lde-zlrconla support". The amount o~ co-formed zlrconia support present ln the second coat is pre~erably not less than about 0.05 g/in3, and may range ~rom about 0.05 to 1.0 g~ln3.
A ~irst rhodlum catalytic component ls dlspersed on the co-~ormed zirconla support and may be present in an amount of from about 0.03 to 1.0, preferablv from about 0.1 to 0.6 welght percent rhodium, calculated as rhodium metal ` 10 and based on the ~elght of rhodlum plus the co-formed zir-conia support. Stated otherwlse, the ~irst rhodium cata-lytlc component may be pre~ent in an amount o~ from about 0.1 to 15 g/~t3 of catalyst composltlon. The rhodlum may be dl~persed on the co-formed zlrconia support by an lmpregna-tlon technlque slmllar to that de~cribed above wlth respect to lmpregnatlng the platlnum onto the alumlna support ln the flrst coat. The rhodlum-lmpregnated co~ormed zlrconla sup-port is heated to thermally ~lx the rhodium on the support, typlcally by ~irst drying the rhodium-impregnated 3upport and then heating it in alr at about 450C. The rhodlum, when dlspersed on a particulate, co-~ormed rare earth ox-ide-ælrconla support as descrlbed above~ serves its usual ~unctlon o~ catalyzing the oxldatlon o~ C0 and the reduction Or N0x in the gases being treated, even after hlgh tempera-ture engine aging, and without signi~icant loss of activity.
The ~econd coat include~ a second actlvated alumlna support on which a second platlnum catalytic component ls dl~per~ed. The second platlnum catalytic component provides capaclty ~or catalyzing the converslon o~ C0 to C02 and HC
30 to C02 and H20 in the second coat. This supplements the C0 and HC converslon capablllty o~ the Plr3t coat. The ~econd platlnum catalytlc component may be dispersed into the ~e-` cond activated alumina particles by an impregnation tech-nlque ~imilar to that described above with re~pect to the ~irst platlnum catalytic component o~ the flrst coat. The ~econd platlnum catalytlc component i~ pre~erably thermally ~ixed on the second actlvated alumina be~ore the platinum-, :

3~3 impregnated se~ond activated alumina ls incorporated intothe second coat. The second platlnum catalytic component is present ln an amount o~ from about 0.05 to 5.0 weight per-cent of the combined welght of the second platinum catalytic component (as platinum metal) and the activated alumina sup-port (includlng the welght of any thermal stabillzers, mea-sured a~ metal oxide, impregnated into the support). Stated otherwise, the second platinum catlytlc component ls present ln an amount o~ ~rom about 1 to 50 g/ft3 of cataly~t compo-sitlon. The second actlvated alumina support present ln the~econd coat 19 preferably pre~ent ln an amount of from about ~. ~
0.1 to 2.0 g/ln'.
The second coat may, llke the flrst coat, be stabiliz-ed agalnst thermal degradation. Although a number of ther-mal stabllizers ~uch as alkallne earth metal oxides and rareearth metal oxides, including ceria, are useful to thermally stabilize activated alumina, zlrconia is pre~erred as a ~ta-bilizer ~or the actlvated alumina u~ed ln the ~econd coat.
Thus, the second actlvated alumlna is stabllized with zir-` 20 conla and may be prepared by commlnuting activated alumina partlcles to a desired sl~e range, and then lmpregnatlng the comminuted partlcles with a solution o~ a ~oluble zlrconium salt. A~ter the impregnatlon, the lmpregnated alumlna par ticleA are calcined to convert the lmpregnated zirconium ~alt, e.g., zirconium nltrate, to zlrconla. The amount ofzlrconla uqed to thermally ~tabillze the second actlvated alumina (and any other actlvated alumina used in the ~econd coat, e.g., the thlrd activated alumina de~cribed below) i~
from about 0.02 to 0.5 g/in3 o~ catalyst compositionO
;~ 30 I~ lt ls de~ired to incorporate lnto the second coat a quantity o~ rhodlum in excess of that whlch can be dlspersed on the zlrconia ~upport or; lf lt ls deslred for any reason to dl~perse part of the rhodium catalytlc component content of the second coat on a support other than the co-~ormed rare earth oxlde-zlrconla support, an additional, second rhodlum catalytic component may be dispersed on an actlvated alumina support, whlch may be the ~econd activated alumlna -~8- 2 ~ ~ L~ ~ 5 ~

support. Thus, the second actlvated alumlna support may have both platinum and rhodlum catalytlc components dis~
persed therein. Alternatively, the second rhodium catalytic ~` component may be dispersed on activated alumina particles comprlsing a third actlvated alumina support. In such case, the second platinum catalytic component ls supported on one batch o~ actlvated alumlna (the second activated alumina) and the second rhodium catalytic component is supported on a separate batch of activated alumlna (the third activated alumina~. The second rhodium catalytlc component may be dispersed into its activated alumina support particles (the second or thlrd activated a~umina support, as the case may ` be) by lmpregnating the comminuted particles wlth a solution o~ a soluble rhodlum salt such as rhodium chloride, rhodium nitrate, etc. The impregnated particles are then dried and calclned to ~orm an activated rhodlum catalytlc component, uslng technlques well known ln the artO
The total rhodium catalytic component present in the second coat (the qum of the rhodium present as both the ~lrst rhodlum catalytic component dlspersed on the co-~ormed zlrconla support and the second rhodlum catalytlc component dispersed on the actlvated alumlna) may range ~rom about 0.1 to 15 g/ft3.
It ls there~ore seen that the contents of the second coat o~ catalytlc material may therefore comprise rhodlum (the ~lrst rhodium catalytic component) dlspersed on a co-formed rare earth oxlde-zlrconia support and platlnum (the second platlnum catalytic component) dlspersed on an actl-vated alumina support (the second actlvated alumlna sup port). In addltlon, addltional rhodlum, the second rhodlum ~ catalytlc component, may be dispersed on the same alumina ; partlcles as the platlnum or may be dlspersed on a separate batch o~ actlvated alumina partlcles (the thlrd activated alumlna support). The second coat may have a thermal stabi-lizer comprislng zirconla dispersed therethrou~h.

:

-19- ~3~3 Prepar-ation Of The Catalyst " . _ Generally, the catalyst composition of the present in-vention is prepared by coatlng a suitable carrler, such as a cordierlte honeycomb carrier, with a first coat comprising a washcoat containing the first coat ingre~ients described abo~e, e~sentially comprising a platinum catalytic component dispersed on actlvated alumina particles and bulk cerium ox-ide and, optionally, the other describe~ lngredients. The catalyst may be prepared by the known technique of preparing th~ ingredients in an aqueous slurry into which the carrier i8 dipped. Excess slurry is blown by compressed alr from the passages of the carrier, and the coated carrler is then dried and calcined. The resultant first coat-containing carrler is then dipped into an aqueous slurry of the above-de3cribed ingredients of the second coat, that is, lnto anaqueous slurry essentially lncluding co-formed rare earth oxide-zirconia support partlcle~ onto which a rhodium cata-lytic component has been disper~ed, and an activated alumlna on whlch a platlnum catalytic component and, optionally, the other de3cribed ingredients. The thus second-coated carrier is agaln drled and calclned, to provlde the ~inished cata-lyst composltlon.
Certaln embodiments of the Lnvention and the efficacy thereof are demonstrated by the followlng Examples.

E~ample 1 A. The Flrst Coat A quantity of 829 gram~ o~ gamma alumina powder having a sur~ace area of 150 square meters per gram ("150 m2/g'~) was impregnated with an amlne-solubilized aqueous platinum hydroxide (H2Pt(OH)6) solution contalning 10.5 grams of platinum. The platlnum-contalning alumina, 995 grams of bulk ceria (99 weight percent CeO2, having a surface area of 110 m2/g~, zirconium acetate solutlon containing 165.8 grams of ZrO2, and 227.4 grams of barium hydroxide hydrate cry-~tals were ballmilled with water and acetic acid to form a slurry. A quantity of 2000 grams of the slurry (solid~ bas-'' : ' :

, ,~ :

3~3 i8 ) was t~rther mlxed wlth 105.4 gram3 pulverized low sur-face area NiO powder, and 105.4 grams Fe304 powder to form a washcoat coating slurry. A monolith support of cordierite containing about 400 ~low passages per square inch of cross sectlon wa~ dipped into the washcoat slurry. The resultant catalyzed monolith a~ter calcination at 450C contains 16 37 g/ft Pt, 0.75 g/in3 alumina9 0.9 g/ln3 CeO2, 0.1 g/in3 BaO, 0.15 g/in ZrO2, and 0.1 g/in3 Fe203.

B. The Second Coat A quantity of 800 gram~ of alumina powder containlng 0.86 percent by weight of platinum was prepared by lmpreg-nating the alumina powder with the aqueous amine solubilized platlnum salt prevlously described and calcining the impreg-nated alumlna. Subsequent to calcining, the alumina wasballmilled with water and nitric acid to form a slurry. A
quantlty of 436.4 gram~ of cerlum oxide-stabllized zirconia powder (12 welght percent CeO2, having a ~urface area of 55 m /g) was lrnpregnated with rhodium nltrate solutlon contain-ing 1.22 gramq rhodlum. The wet powder waq drled and thencalcined at 450C to obtain a powder containing 0~28 welght percent rhodium, measured as rhodium metal. Thls powder wa~
then mixed with the platinum-contalnlng slurry to form a platlnum- and rhodium-containing slurry. A quantlty of 1200 grams (qolids basl~) of the platlnum-rhodlum slurry waq ~ur~
ther lmpregnated wlth rhodium nitrate ~olutlon contalnlng 0.82 gram~ of rhodium. A zirconlum acetate aqueou~ ~olution contalning 70.6 gram~ ZrO2 was th2n added to the slurry.
The ~lnal coatlng slurry contalned roughly 30 percent by welght ~ollds. The monolith coated with the first coat in ` Part A o~ this Example ~a~ dipped ln the platlnum- and rho-dium-containing slurry. After drying the dipped monollth and then calcining it at 450C, the monolith picked up an addltional 0.9 g/ln3 o~ washGoat containing 2.455 g/~t3 Rh, 35 8.18 g/~t3 Pt, 0.55 g/in3 alumina, 0.3 g/in3 cerlum stabi-llzed zirconia and 0.05 g/in3 ZrO2. The ~inal catalyzed monollth contalned 27 g/ft3 o~ preciou~ metals at 10 Pt/l Rh ~ - '' ~ ' ?1 ~o~a~
r' welght ratlo.

E~ample 2 (Comparutlve ~ample) !' A monolithic catalyst was prepared by lmpregnatlng 784 grams of gamma alumina powder havlng a sur~ace area of 150 square meter~ per gram ("150 m /g") with an amine-solubil-lzed aqueous platlnum hydroxide solution containlng 10.0 grams of platinum. The wet powder was thereafter impreg-nated wlth an aqueous rhodium nltrate solutlon contalnlng 1.0 grams of rhodium. Finally, an aqueous solution contaln-ing 14 ml of acetic acld was dlspersed uniformly lnto the wet alumlna powder.
The platlnum- and rhodium-containlng alumina powder plus 524.ô grams of bulk ceria (99 welght percent purity CeO2, havlng a surface area of 110 m2/g), zlrconium acetate solution contalning 183.3 grams of ZrO2 and 201~6 grams of ; barlum hydroxlde hydrate crystals were ballmilled with water and acetlc acld to form a slurry. 1500 grams of the slurry (sollds basis) was further mlxed with 96 grams of pulverized low surface area NiO powder to form a washcoat coating slur-ry. A cordierite monollth containing 400 flow passages per square lnch of cro~s sectlon was dlpped into the coating ~lurry, dried at 60C and then calclned at 500C. The final catalyst contalned 27 g/ft3 of pre~clous metals at a 10 Pt/l Rh welght ratlo and 0.75 g/ln3 CeO2, 0.14 g/ln3 BaO, 0.26 ; g/ln ZrO2 and 0.15 g/ln3 Nl03 in addltlon to 1.135 g/ln3 alumlna.
;, ~ample 3 (Comparatlve ~xample) A monollthlc catalyst wa~ prepared according to the method descrlbed in Example 1, except that the la~ered ~tructure wa3 replaced wlth one homo~eneous coatlng of the same total composltlon, that ls, the separate ingredients o~
the dl~crete first and second coats were comblned into a single washcoat into which the monollth was dlpped. The flnal catalyzed monolith therefore also contained 27 g~ft3 PM at 10 Pt/l Rh ratio.
:

-22- ~3~fi~

E~ample ~
The catalysts, 85 inJ in volume each, p~oduced accord-ing to Examples 1, 2 and 3 were lndividually loaded in con-verter~ o~ identical shape. The converters were then lndl-vldually aged on a 4.3 liter V-6 engine at a converter inlet temperature o~ 700C for 75 hours u~ing a speclfic aglng cy-cle. The aging cycle consists of ~our phases totalling 60 seconds.
1. Flrst phase lasts 40 seconds and engine operates at stolchiometric ~et point in steady state.
2. Second phase for 6 seconds and engine operates bias rlch which produces 3 percent C0 ln the exhaust.
3. Third phase for 10 ~econds and the englne operates similar to phase 2 except secondary air is ln~ected to generate 3 percent 2 in the exhaust.

4. Fourth phase for 4 seconds and englne operates back to normal stoichiometric setting similar to phase 1 whlle the alr inJection continues~

After the aglng the catalyst~ were evaluated on a V-8 engine dynamometer at an lnlet temperature of 482C and 40,000 hr-l space velocity wherein the alr-to-~uel ratio (A/F~ employed was fluctuated +/- 0.5 A/F units at 1 Hz per-turbations. The results of catalytic efficiencles are sum-marlzed in TABLE 1.

"' -23- 2~3~63 TABL~ I

Conver~lon Efficiency oP Englne Aged Monolithic Catalysts % Conversion at Air_to Fuel Welght Ratio (A/F) Shown AF = 14.45AF = 14.64 AF = 14.85 (AFU = -0.2)(Stoich.) (AFU = ~0.2) CatalystHC C0 N0xHC C0 NOX HC ~0 N0 Example 184 46 9489 89 82 89 100 46 Example 284 44 gl89 71 74 89 93 47 Example 371 43 7390 92 82 89 99 36 ("AFU" = air to fuel ratlo unlts, "Stoich.i' = stoichiometric A/F) By reference to TABLE I, it is immediately apparent : that the catalyst compositlon according to one embodiment o~
the present inventlon (Example 1) outperformed the compara-tive catalyst composltlons oP Examples 2 and 3. The result~
Or Example 1 as compared to Example 2 show the superiorlty 20 oP the catalyst compositlon of Example 1 as compared to the ~ dl~Perent composltion oP Example 2. The two-coat layered `~ structure oP thls lnventlon (Example 1) is seen to exhlblt after aging o~ the catalysts, a wider operating wlndow than the homogeneous, slngle layer structure o~ Example 3, which is of practlcally the same overall composltlon as Example 1.

Whlle the lnventlon has baen descrlbed ln detall wlth respect to ~peclfic prePerred embodlment3 thereo~, lt wlll be apparent that upon a readlng and understandlng of the ~oregolng, varlatlons thereto may occur to those skllled ln `: the art, which varlatlons lie wlthin the scope o~ the ap-pended clalms.
i`;

~ 35 ;~

, ' '. .

Claims

1. A catalyst composition comprising a carrier on which is disposed a catalytic material, the catalytic mate-rial comprising:
a first coat carried on the carrier and comprising a first activated alumina support, a catalytically effective amount of a first platinum catalytic component dispersed on the first alumina support, and a catalytically effective amount of bulk ceria; and a second coat carried by the carrier and comprising a co-formed rare earth oxide-zirconia support, a catalytic-ally effective amount of a first rhodium catalytic component dispersed on the co-formed zirconia support, a second acti-vated alumina support, and a catalytically effective amount of a second platinum catalytic component dispersed on the second alumina support.

2. The catalyst composition of claim 1 wherein the se-cond coat comprises a topcoat overlying the first coat.

3. The catalyst composition of claim 1 or claim 2 wherein the first coat further includes an effective amount of a metal oxide effective for the suppression of H2S emis-sions from the catalyst.

4. The catalyst composition of claim 3 wherein the metal oxide effective for the suppression of H2S is selected from the group consisting of one or more of oxides of nick-el, copper, manganese and germanium.

5. The catalyst composition of claim 1 or claim 2 wherein the first coat further includes bulk iron oxide in a catalytically effective amount for promoting the oxidation of CO.

6. The catalyst composition of claim 5 wherein the bulk iron oxide comprises magnetite (Fe3O4).

7. The catalyst composition of claim 5 wherein the bulk iron oxide is present in an amount of from about 0.05 to 0.3 grams per cubic inch of catalyst composition, calcu-lated as Fe2O3.

8. The catalyst composition of claim 1 or claim 2 wherein the second coat further includes a catalytically ef-fective amount of a second rhodium catalytic component dis-persed on the second activated alumina support.

9. The catalyst composition of claim 1 or claim 2 wherein the second coat further includes a third activated alumina support and a catalytically effective amount of a second rhodium catalytic component dispersed on the third alumina support.

10. The catalyst composition of claim 9 further includ-ing a third platinum catalytic component dispersed on the third alumina support.

11. The catalyst composition of claim 1 or claim 2 wherein the first coat further includes a thermal stabilizer dispersed therein in an amount sufficient to stabilize the first activated alumina and the bulk ceria against thermal degradation.

12. The catalyst composition of claim 11 wherein the thermal stabilizer is selected from the group consisting of one or more of ceria, baria and zirconia.

13. The catalyst composition of claim 11 wherein the thermal stabilizer is selected from the group consisting of baria and zirconia.

14. The catalyst composition of claim 11 wherein the thermal stabilizer comprises both baria and zirconia.

15. The catalyst composition of claim 13 wherein the baria and the zirconia are each present in an amount of from about 0.05 to 0.5 grams per cubic inch of catalyst, composi-tion.

16. The catalyst composition of claim 1 or claim 2 wherein the rare earth oxide of the co-formed rare earth oxide-zirconia support is selected from the group consisting of one or more of oxides of cerium, neodymium and yttrium.

17. The catalyst composition of claim 1 or claim 2 wherein the rare earth oxide of the co-formed rare earth oxide-zirconia support is cerium oxide.

18. The catalyst composition of claim 1 or claim 2 wherein the first rhodium catalytic component is present in an amount of from about 0.03 to 1.0 weight percent of the combined weight of rhodium plus stabilized zirconia support, the weight of rhodium being calculated as the metal.

19. The catalyst composition of claim 1 or claim 2 wherein the bulk ceria in the first coat comprises at least about 0.15 grams per cubic inch of catalyst composition.

20. The catalyst composition of claim 1 or claim 2 wherein the bulk ceria in the first coat is present in an amount of from about 0.15 to 1.5 grams per cubic inch of catalyst composition.

21. The catalyst composition of claim 3 wherein the metal oxide effective for the suppression of H2S is present in an amount of from about 0.05 to 0.5 grams per cubic inch of catalyst composition, calculated as the metal oxide.

22. The catalyst composition of claim 21 wherein the metal oxide comprises bulk nickel oxide and its weight is calculated as NiO.

23. The catalyst composition of claim 1, claim 2 or claim 20 wherein the bulk ceria comprises at least 90 per-cent by weight CeO2, the balance comprising other rare earth oxides.

24. The catalyst composition of claim 1 or claim 2 wherein the second coat further incudes a thermal stabilizer dispersed therein in an amount sufficient to stabilize the second activated alumina support against thermal degrada-tion.

25. The catalyst composition of claim 24 wherein the thermal stabilizer comprises zirconia.

26. The catalyst composition of claim 25 wherein the zirconia for alumina stabilization is present in an amount of about 0.02 to 0.5 grams per cubic inch of catalyst com-position.

27. The catalyst composition of claim l or claim 2 wherein the rare earth oxide of the co-formed rare earth oxide-zirconia support comprises ceria and is present in an amount of from about 2 to 30 percent by weight of the com-bined weight of ceria and zirconia in the co-formed zirconia support.

28. The catalyst composition of claim 1 or claim 2 wherein the first coat further comprises crushed cordierite.

29. The catalyst composition of claim 1 or claim 2 wherein the co-formed rare earth oxide-zirconia support is present in an amount of from about 0.05 to 1.0 grams per cubic inch of the catalyst composition.

30. The catalyst composition of claim 1 or claim 2 wherein the second platinum catalytic component is present in an amount of from about 0.05 to 5.0 weight percent of the combined weight of platinum, measured as the metal, and the activated alumina in the second coat.

31. The catalyst composition of claim 1 or claim 2 wherein the activated alumina is present in the first coat in an amount of from about 0.1 to 4.0 grams per cubic inch of catalyst composition.

32. The catalyst composition of claim 1 or claim 2 wherein the activated alumina is present in the second coat in an amount of from about 0.10 to 2.0 grams per cubic inch of the catalyst composition.

33. The catalyst composition of claim 1 or claim 2 wherein the first platinum catalytic component is present in the first coat in an amount of from about 5 to 100 grams per cubic foot of catalyst composition.

34. The catalyst composition of claim 1 or claim 2 wherein the second platinum catalytic component is present in the second coat in an amount of from about 1 to 50 grams per cubic foot of catalyst composition.

35. The catalyst composition of claim 1 or claim 2 wherein the total amount of rhodium catalytic component pre-sent in the catalyst composition is present in an amount of from about 0.1 to 15 grams per cubic foot of catalyst compo-sition.

36. The catalyst composition of claim 1 or claim 2 further including up to 100 grams per cubic foot of palladi-um catalytic component dispersed on the activated alumina support in the first coat.

37. The catalyst composition of claim 1 or claim 2 wherein the carrier comprises a refractory body having a plurality of substantially parallel passages extending therethrough, the passages being defined by walls and the catalytic material being coated on the walls as said first coat and said second coat.

38. A catalyst composition comprising a carrier on which is disposed a catalytic material, the catalytic mate-rial comprising:
a first coat carried on the carrier and comprising a first activated alumlna support, a catalytically effective amount of a first platinum catalytic component dispersed on the first alumina support, a catalytically effective amount of bulk ceria and of bulk iron oxide, a catalytically effec-tive amount of bulk nickel oxide, and baria and zirconia dispersed throughout the first coat in an amount sufficient to stabilize the alumina and the other bulk metal oxides against thermal degradation; and a second coat comprising a topcoat overlying the first coat and carried by the carrier, and comprising a co-formed rare earth oxide-zirconia support, a catalytically effective amount of a first rhodium catalytic component dispersed on the co-formed zirconia support, and a second activated alumina support having a catalytically effective amount of a second platinum catalytic component dispersed thereon, and zirconia dispersed throughout the second coat in an amount sufficient to stabilize the activated alumina support therein against thermal degradation.

39. The catalytic composition of claim 38 including a second rhodium catalytic component dispersed on the second activated alumina support.

40. The catalyst composition of claim 38 further in-cluding a third activated alumina support on which cata-lytically effective amounts of a second rhodium catalytic component and a third platinum catalytic component are dis-persed.

41. 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 to innocuous substances, the method com-prising contacting the gas under conversion conditions with a catalyst composition comprising a carrier on which is dis-posed a catalytic materials the catalytic material compris-ing (1) a first coat carried on the carrier and comprising a first activated alumina support, a catalytically effective amount of a first platinum catalytic component dispersed on the alumina support, and a catalytically effective amount of bulk ceria and (ii) a second coat carried by the carrier and comprising a co-formed rare earth oxide-zirconia support, a catalytically effective amount of a first rhodium catalytic component dispersed on the co-formed zirconia support, and a catalytically effective amount of a second platinum catalyt-ic component dispersed on the second alumina support.

42. The method of claim 41 wherein the catalyst compo-sition further includes in the first coat a catalytically effective amount of bulk iron oxide, a metal oxide present in an amount sufficient to suppress H2S emissions, and at least one of baria, ceria and zirconia dispersed throughout the first coat in an amount sufficient to stabilize the ac-tivated alumina and other bulk oxides in the first coat against thermal degradation.

43. The method of claim 42 wherein the metal oxide to suppress H2S emissions is nickel oxide.

44. The method of claim 41 wherein the second coat of the catalyst composition further includes a catalytically effective amount of a second rhodium catalytic component dispersed on the second activated alumina support.

45. The method of claim 41 wherein the second coat of the catalyst composition further includes a third activated alumina support and a catalytically effective amount of a second rhodium component dispersed on the third alumina sup-port.

46. The method of claim 45 wherein the second coat of the catalyst composition further includes a third platinum component dispersed on the third alumina support.

47. The method of any one of claims 41 or 42 wherein the catalyst composition comprises a refractory body having a plurality of substantially parallel passages extending therethrough, the passages being defined by walls and the catalytic material being coated on the walls with the second coat comprising a top coat overlying the first coat.

48. 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 to innocuous substances, the method com-prising contacting the gas under conversion conditions with a catalyst composition comprising a carrier on which is dis-posed a catalytic material, the catalytic material compris-ing (1) a first coat carried on the carrier and comprising a first activated alumina support, a catalytically effective amount of a first platinum catalytic component dispersed on the alumina support, a catalytically effective amount of bulk ceria and of bulk iron oxide, a catalytically effective amount of bulk nickel oxide, and baria and zirconia dis-persed throughout the first coat in an amount sufficient to stabilize the alumina and iron oxide against thermal degra-dation; and (ii) a second coat comprising a topcoat overly-ing the first coat and carried by the carrier, and compris-ing a co-formed rare earth oxide-zirconia support, a cata-lytically effective amount of a first rhodium catalytic com-ponent dispersed on the co-formed zirconia support, and a catalytically effective amount of a second platinum cata-lytic component dispersed on the second alumina support.

49. The method of claim 48 wherein the second coat of the catalyst composition further includes a catalyticaly ef-fective amount of a second rhodium catalytic component dis-persed on at least one of the second activated alumina sup-port and an optional third activated alumina support.

50. The method of claim 48 or claim 49 wherein the first coat and second coat each include a thermal stabilizer in an amount effective to stabilize the activated alumina and other bulk metal oxides respectively present therein against thermal degradation.

51. The method of claim 50 wherein the thermal stabi-lizer in the first coat is selected from the group consist-ing of one or more of ceria, baria and zirconia and the thermal stabilizer in the second coat comprises zirconia.

52. The method of claim 50 wherein the thermal stabi-lizer in the first coat is selected from the group consist-ing of one or both of baria and zirconia, and the thermal stabilizer in the second coat comprises zirconia.