CA2027329A1

CA2027329A1 - Staged three-way conversion catalyst and method of using the same

Info

Publication number: CA2027329A1
Application number: CA002027329A
Authority: CA
Inventors: Wayne M. Rudy
Original assignee: Engelhard Corp
Current assignee: BASF Catalysts LLC
Priority date: 1989-11-08
Filing date: 1990-10-11
Publication date: 1991-05-09
Also published as: EP0427494A3; EP0427494A2; KR100192190B1; KR910009331A; US5531972A; ZA908658B; US5010051A; JPH03169346A

Abstract

ABSTRACT OF THE DISCLOSURE
A catalyst composition has an upstream stage and a downstream stage, the upstream stage containing a catalytic material which is different from the catalytic material contained on the downstream stage and is characterized by having a low ignition temperature, e.g., 350°C to less than 400°C, for the substantially simultaneous conversion of HC, CO and NOx pollutants contained in, e.g., the exhaust of an automobile engine operating at a substantially stoichio-metric air-to-fuel weight ratio. The downstream catalytic material is characterized by having a higher conversion efficiency for substantially simultaneous conversion of HC, CO and NOx than the upstream catalytic material at elevated operation temperatures which may be, for example, from about 400 to 800°C. The method of the invention includes passing a gaseous exhaust stream containing HC, CO and NOx pollu-tants sequentially through first the upstream and then through the downstream catalytic materials.

Description

BAC~GROUND OF T~3 INVENTION
.
~leld O~ The Inventlon The present invention is concerned with catalysts use-S ful for the treatment of gases to reduce contaminants con-tained therein. More speclflcally, the present 'nventlon ls concerned with lmproved catalysts of the type generally re-ferred to as "three-way conversion" or "TWC" catalysts. TWC
catalysts are polyfunctlonal ln that they have the capabill-ty of substantlally simultaneously catalyzlng both oxidationand reduction reactlons, such as the oxldatlon of hydrocar-bons and carbon monoxide and the reduction of nitrogen ox-ldes. Such catalysts find utllity in a number of flelds9 includlng the treatment of the exhaust gases from internal ~5 combustion engines, such as gasoline-fueled automobile and other spark-ignition engines.

Background and Related Art In order to meet governmental emisslons standards ~or unburned hydrocarbons, carbon monoxide and nitrogen oxlde contamlnants ln vehlcle and other englne exhaust gases, so-called catalytic converters contalnlng suitable catalysts ~re emplaced ln the exhaust gas line of lnternal combustion engines to promote the oxidation of unburned hydrocarbons ("HC") and carbon monoxide ("CO") and the reduction of nl-trogen oxldes ("N0x") in the exhaust gas. ~wo separate cat-aly3t members or beds can be used ln series, for example, the first to promote reductlon of NOX and the second to pro-mote oxldatlon of HC and CO, wlth optional oxygen (air) introductlon between the beds. Alternatively, a single bed TWC catalyst, whlch substantlally simultaneously promotes both oxldation and reduction aq descrlbed above, may be used, provided that the air-to-fuel weight ratio ("A/F
ratio") o~ the engine whose exhaust is being treated is held close to the stoichiometric ratio. For the foregoing pur-pose, catalysts comprlsing one or more platlnum group metals and, optlonally, base metal oxides distended upon a hlgh , .
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sur~ace area, refractory oxide support a.e well `~nown ln the art. The support may comprlse a high surface area alumina coating carrled on any suitable carrler such as a L efractory ceramlc or metal honeycomb structure, as well known ln the art. For example, see C.D. Keith et al U.S. Patent 4,552,732. Such high surface area alumina materlals are generally referred to in the art as "gamma alumlna" (a~
lthough it ls usually a mixture of the gamma and delta phases and may also contain eta, kappa and theta phases) or "actlvated alumina". Such alumina, when fresh, typically exhibits a BET sur~ace area ln excess of 50 square meters per gram ("m /g"), often up to about 200 m /g or more. It is a known expedient to stabllize such actlvated alumlna supports agalnst thermal degradatlon by the use o~ materlals such as zlrconia, tita~la, alkaline earth metal oxides such as baria, calcia or strontia or, most usually, .are earth metal oxides, ~or example, ceria, lanthana and mlxtures of 'wo or more rare earth metal oxldes. ~or example, see C.D.
~elth et al U.S. Patent 4,171,288.
It is known in the art to provide two catalyst members ln series to treat noxious pollutants in an exhaust gas.
~or example, U.S. Patent 3,896,616 of C.D. Keith et al dls-closes an arrangement for treating exhaust gases ~.om ln-ternal combustion englnes in which an initlal catalyst of relatively small volume is placed as close to the exhaust manifold of the englne as possible with a second, larger volume catalyst posltioned Yurther along the exhaust pipe beneath the vehlcle. The small volume of the initial cat alyst and the higher temperature oF the exhaust æases closer to the englne cause the lnitlal catalyst to heat up very qulckly, ~hereby commencing puri~icatlon of the exhaust gas durlng the start-up perlod when the downstream catalyst, because of lts lower operatlng temperature, ls stlll rela-tively lneffectlve. The Patent discloses that the art has contemplated by-passing the initlal catalyst after the sub-sequent or downstream catalyst ls heated to operatlng tem-perature (see column 3, llne~ 4-19). However, the Patent - ' ' : ':

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5eaches contlnuing operation of the initial catalyst ~fter the englne start-up perlod ln order to reduce the amount of nitrogen oxldes in the engine exhuast gas. ln order to ~ccomplish thls result, an addltional, extraneous fuel Is lntroduced between the initlal and subsequent catal~sts.
(~olumn 4, line 34 et seq.) The initlal catalyst ~ay also be supplled, durlng the englne start-up period, with elther a fuel or an oxygen contalning gas, depending on operatlng condltlons. (See column 4, lines 3-24.) As disclosed at column 9, llne 22-4~ of the Patent, the lnitlal catalyst may comprise an actlvated alumina support ln which one or more platlnum group metals, prefer-ably including a catalytlcally effectlve amount of palla-dlum, are dlsposed. The subsequent catalyst s dlsclosed as o~ slmllar composillon, comprlslng one or ~ore ?latir.um group metals, especially platlnum or palladium, and other lngredients such as base metals including i.on group metals.
(See column 12, lines 21-35.) German Offenlegungsschrlft 36 08 292 Al, A. Konlg et ~1 discloses a catalytlc converter for treating internal combustlon englne exhaust comprlslng a first converter (5) contalnlng a multlfunctlon catalyst and a second converter (6) wlth a nltrogen oxlde reduclng catalyst, and lncludlng the introductlon of air vla line 7 between the catalyst stages. The Patent discloses a system in which the second converter ls a NOx reduction catalyst, ~or example a zeolite or cokç support contalnlng a base metal oxide, such as tl tania or vanadia, disposed thereon. The up-stream catalyst is a conventional three-way conversion catalyst and, ~n accordance wlth the lnventlon, ~x and ammonia emanatlng from the first catalyst are converted ln the second cata- .
lyst. The publicatlon dlscusses that ln retroflttlng auto-moblles whlch do not contaln oxygen sensor probes and other equlpment necessary to control the air-to-fuel ratio within the narrow range necessary for good conversion rates using a three-way conver~ion catalyst, the dlsclosed arrangement provldes a means for converting the nitrogen oxides as well . :

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as ammonla whlch may emanate from the fl.st eatal~st, -s?e-clally when the alr-to-fuel ratlo ls not closely controlled.
The second catalyst also advantageously contalns an oxygen sto age component, such as a zeollte, so that ammonia Dro-s duced ln the first converter durlng fuel-rlch operatlon can be stored ln the second converter and, other subsequent fuel-lean operatlon, be used for the reductlon of nltrogen oxldes.

SUHMARY OF THE INVENTION
In accordance with the present inventlon, there ls provlded a catalyst compositlon for convertlng HC, CO and NOx ln a gaseous stream flowed therethrough. ~he composl-tlon comprlses an upstream catalyst member and a downstream catalyst member, as sensed ln the dlrect'on of ~low o~ the ~aseous stream through the catalyst composltion. ~he up-stream catalyst member comprlses a flrst catalytic materlal characterl~ed by havlng an lgnltlon temperature for substan-tially slmultaneous converslon o~ HC, CO and NOx whlch ls ]ower than that of a second catalytlc materlal. ?he ~own-stream catalyst member comprlses a second catalytlc materlal characterlzed by havlng a higher converslon efflclency than the flrst catalytlc material for substantlally slmultaneous conversion of HC, CO and NOx at operatlng temperatures above the lgnltion temperature.
In another aspect of the present inventlon, the first catalytlc materlal has, when the gaseous stream ls a sub-stantlally stolchiometrlc exhaust gas mlxture, an ignltlon temperature for substantially simultaneous conver~lon of HC, 50 and NOx which is less than about 400C, ~or example, 375 or 350C, and the downstream catalyst member ~as for the gaseous stream treated by the upstream catalyst member a converslon efflclency of at least about 94% ~or substantial-ly simultaneous converslon of HC, CO and NOx at an operating temperature of at least about 400C.
Yet another aspect of the invention provldes that the operating temperature range starts at about 400C, ~or exam--:;

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~le, the operatlng temperature range may ~e from about 400 to 800C.
In accordance with another aspect of the present in-vention, the first cataly~ic materlal may comprlse a plati-s num catalytic component disposed on a refractory metal oxide support and a rhodium catalytic component disposed on a refractory metal oxide support. The second catalytic ma-terial may comprlse a rhodium catalytic component disposed on a zirconla/dlspersed cerla support and a platinum cata-lytic component dlsposed on a refractory metal oxlde sup-port.
Yet another aspect of the present inventlon provldes a catalyst composltin as descrlbed above wherein the flrst catalytlc materlal comprises a first platinum catalytic component dispersed on a flrst activated alumina support, a rhodlum catalytic component dlspersed on a second activated alumina support, and a second platinum catalytic component dlsposed on a ceria support. The second catalytlc material comprlses a second rhodlum catalytlc component dlspersed on a zlrconla/dlspersed ceria support, a third rhodium cata-lytlc component dlsposed on a thlrd actlvated alumlna sup-port, a thlrd platlnum catalytlc component dlsposed on a fourth activated alumina support, and a fourth platinum catalytlc component dlsposed on a ceria support.
In accordance wlth a method aspect of the present invention, there ls provlded a method of substantlally simultaneously converting HC, C0 and NOX pollutants con-tained in a gaseous stream. The method comprises the ~ollowlng steps. The 2aseous stream is ~lowed through a ~irst catalyst zone and therein the gaseous stream is contacted with a flrst catalyst member comprlsing a first catalytlc materlal havlng an ignltion temperature for sub-stantlally slmultaneou~ conversion of HC, C0 and N0x, when the gaseous stream ls a substantlally stolchlometrlc exhaust gas mixture, which is lower than the correspondlng ignition temperature of a second catalytlc materlal, described below.
~he gaseous stream ls lntroduced lnto the flrst catalyst :

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zone at a temperature at or above the ignition temperature of the flrst catalytlc material hut below an operating tem-perature range, descrlbed below, to convert witnin the first catalyst zone some, but less than all, of ea h of the HC, CO and NOX content of the gaseous stream to innocuous sub-stances. In this way, the temperature of the ~aseous stream ls increased, for example, it may be lncreased to wlthln the operatlng temperature range. The gaseous stream from the first catalyst zone is then flowed to a second catalyst zone and is therein contacted with a second catalyst member which comprises a second catalytic material havlng a higher con-verslon efficiency (for substantlally slmultaneous conver-slon of ~C, CO and NOX) at temperatures wlthin the operatlng temperature range than does the flrst catalytic material.
~he gaseous stream is contacted with the second c~alytic material at a temperature wlthln the operatlng temperature .ange to substantially simultaneously convert at least some of the remainlng HC, CO and NOX to lnnocuous substances, e.g., H2O, CO2 and N2.
Other method aspects of the present inventlon lnclude utllizlng a 400 to 800C operatlng range temperature and utilizlng as the catalyst composition catalyst compositions as descrlbed above.
As used herein and ln the clalms, reference to a "cat-alytic component" means and lncludes catalytlcal'y effectlve fo~rms of the component, or precursors thereof, such as the elemental metal, an oxlde or other compound or a complex of the metal, or an alloy or mixture including the metal, or a comblnatlon of any of the foregolng. ~or example, reference to a "platlnum catalytlc component" means a catal~tlcally effectlve form of platinum or a precursor thereof, such as platinum metal, an oxide or some other platlnum compound or complex, or an alloy or mlxture lncludlng platlnum, or a comblnatlon of any of the foregolng.
As used hereln and ln the clalms, the "lgnitlon tem-perature" of a catalyst ls deflned as the lowest temperature at whlch a stolchlometrlc exhaust gas stream lntroduced into ; . .

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the catalyst wlll undergo converslon of at east ,07, o? each of CO, HC and NOx contalned in the stream. ~s used ln this definltlon, a "stolchlometrlc exhaust gas stream" is t~e ~xhaust gas from a gasollne-fueled spa.k i~nltlon automobile engine operated at a stoichiometric air to fuel .atio.
Still other aspects of the present lnvention are de-scribed in the followlng detailed descrlption of the inven-tion.

BRI~F D~SCRIPTION OF TH~ DRA~ING
The sole Figure of the drawing is a graph plotting on the absclssa (vertlcal axls) converslon efficiency for HC, ~0 and NOx agalnst, on the ordinate (horizontal axis) tem-perature.
~5 DETAILED D~SCRIPTION OF T~ INVENTION
AND SPEC~FI~ EMBODI~ENTS TE~REOF _ The deslgn of a particular catalyst system, l.e., a catalytlc converter, for the treatment of pollutants con-tained in automotive exhaust gases Involves lnherently con-~lictlng requlrements, lnasmuch as both oxidatlon and reduc-tlon reactlons must be promoted a~d a wlde range of oper-atlng condltions must be accommodated. Accordlngly, any glven system necessarlly requlres compromlses ln order to attaln a deslred over-all perform~lnce of the catalytlc con-~erter sy~tem. For example, the catalyst composit~on used should become actlve at as low a temperature as posslble, that ls, it should have a desirably low "light-off" or "ig-nltion" temperature in order tha~ lt become effective as soon as posslble after start-up of a cold engine. ~uring the start-up phase, before the exhaust gas and the catalytlc converter are heated to a sufflclently hlgh operatlng tem-perature, a large quantlty of unconverted pollutants pass lnto the atmosphere through the catalyst. (In thls regard, see the discusslon contalned ln Keith e~ al U.S. Patent 3,896,616, discussed above, especially at column l, llne 43 to column 2, llne 25.) On the other hand, the catalytlc ::

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converter utl1ized must also convert as high a ?ro~ortion of the pollutants as posslble when it attains normal operatlng temperature, that is, it must demonstrate good actlvity withln the operating temperature ran~e. ~t ~ust also demon-5 strate acceptable durability, that is, it must maintain itsactivity and resist thermal degradation for at least 50,000 miles of high temperature operation. Further, a successful catalytic converter must be resistant to poisoning ~rom lead found in small quantities even in unleaded commercially available fuels, as well as phosphorus which emanates from englne lubricatlng oil consumption. In additlon, the ideal catalyst system must be able to demonstrate good activity and an acceptably low ignitlon temperature over a wide range of exhaust gas flow rates and compositlons, the latter being lS determnlned ln large part by the A/~ ratio employed ~nd varylng conditions of load and operatlon. ~hus, the cata-lytic converter system must react quickly to rapid changes ln operating conditions and transient excursions in exhaust gas composition and temperature.
No single catalyst composition can optimize ~ach of the desired attributes of the catalytic converter. As indi-cated by the above-descrlbed German Published Application 36 o8 292 and Kelth et al U.S. Patent 3,896,616, attempts have been made to improve over-all performance by divlding the catalytic system into two stages or beds. In one case, the German reference provides a separate catalyst for oxidation and another catalyst for N0x reduction, and in the other case the Keith et al Patent places a portion of the catalyst in close physlcal proximity to the engine to take advantage of the higher exhaust gas temperatures available there.
Generally, the catalyst compositions of the present lnvention contain two stages, a flrst or "upstream" stage which ls characterlzed by a catalyst having a good, that ls, a low, lgnltion temperature followed by a second or "down-stream" stage characterized by a catalyst having good con-version efflclency at an elevated operation temperature.
Wlth this arrangement, the first stage of the catalyst be-.;
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comes effectlve to convert .~C, C0 ~nd ~Oy ~t the lo~ emper-~tures obtalning before the englne and its exhaust attains an elevated operating temperature. The catalytic reactlon thus commenced at a relatively low temper~ture ln ~he fl.st stage of the catalyst serves to qulckly heat the first stage and then the second stage catalyst to an elevated operating temperature at which the second stage catalyst has a higher actlvlty for converslon of HC, C0 and N0x than does the first stage catalyst.
Generally, the first or "upstream" stage of the cata-lyst compositlon of the present lnventlon may comprlse a suitable catalyst demonstratlng a low ignltion temperature, that is, a catalyst which becomes effective at relatively low temperatures of the gas being treated to substantially simultaneously convert a stated proportion, e.g., at leas~
50%~ of the origlnal content of each of ~C, C0 and Nx in the gas supplled to the catalyst.
A sultable catalyst demonstratlng a low ignition tem-perature comprises a catalytlc material comprising a plati-num catalytic component dlsposed on an actlvated alumlnarefractory support, a platinum catalytic component disposed on an alumlna-stabilized bulk cerla support, and a rhodlum catalytic component dlsposed on an activated alumina sup-port. This upstream catalytlc materlal may be dlsposed on a monollthlc ceramic support, such as a cordlerite support havlng a plurality of ~ine gas flow passages extending therethrough. As descrlbed ln more detall below, this type of catalyst composition displays an excellent low ignitlon temperature ~or substantially slmultaneous converslon of HC, C0 and N0x. Thus, thls type of catalyst has been found to be capable of attalnlng at least 50~ converslon for each of HC, C0 and N0x temperatures of less than 400C when treatlng a stolchiometrlc exhaust gas mlxture. However, it dlsplays lower conversion rates than some other catalyst composltlons for such substantially slmultaneous conversions at higher operating temperatures, for example, withln an operating temperature range of from about 400 to 8000C.

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An effective catalyst compositlon for utilization as the second or "downstream" stage of the catalyst composl-tion, is a catalyst of the type disclosed in co-pending patent application of Samuel J. Tauster et al, filed con-currently with this application and entitled "Three-way Conversion Catalyst Including A Ceria-containing Zirconia Support". That application dlscloses that by utillzing in a catalytic composition a rhodium catalytic component dis-?ersed~ upon a ceria-impregnated zirconia support, a cata-lyst of enhanced TWC efficacy is attained. The rhodlum onceria-impregnated zirconia is believed to promote both a water gas shlft reaction, thereby promoting the oxidation of CO to carbon dioxide, as well as a steam reformlng .eaction, thereby promoting the reactlon of saturated hydrocarbons (HC) wlth ~2~ to form hydrogen and carbon oxides.
There are teachlngs ln the art that rhodium should not be utillzed in TWC catalysts or the like in contact with ~are earth metal oxides such as ceria, because of undesir-able reaction between the rhodium and cerla, especially when lean (oxygen rich) conditions exist in the exhaust ~as being treated. For example, see U.S. Patent 4,727,052 of C.Z. Wan et al. As described at column 5, lines 1-36 of this patent, rhodium tends to react with rare earth metal oxides, in-cluding ceria, especially under high temperature conditions, and this has a deleterious ef~ect on catalyst activity. In this regard, see also U.S. Patent 4,678,770 of C.~ an et al, the disclosure of which is hereby incorporated herein.
However, in the case of the downstream stage catalyst used hereln, the amount of ceria lmpregnated into the zirconia used is limited to not more than about 15 percent by weight, preferably not more than about 10 percent by weight ceria, expressed as the comblned welght of the cerla and zlrconla.
3y thus controlling the amount of ceria dlspersed on the zlrconla, it has been found that the adverse reactlon be-tween rhodlum and ceria does not occur to any significantextent. Further, the presence of the ceria is believed to asslst in retainlng the rhodlum crystallites in place on the zlrconla support and ln preventlng or reta.ding sl~terlng of the rhodium to form crystallltes of undeslrably l~rge size.
The controlled amount of cerla dlspersed on the zlrconla ls belleved to stablllze the catalyst by precludlng or .educlng slnterlng of the rhodium and, as lndicated above, ls be-lleved to promote steam reformlng and water gas shlft ~eac-tlons, thereby enhancing e~ficiency of the catalyst. Thus, the rhodium catalytic component is dispersed on a ceria-lmpregnated zlrconla support contalning a limlted amount of cerla to provlde a zlrconla/dlspersed cerla support. Al-though not ~ishlng to be bound thereby, it ls belleved that the rhodlum catalytic component on zirconla/dlspersed ceria support also enhances the efficlency of the consumptlon of both hydrocarbons and carbon monoxlde ln the gaseous stream belng treated.
The downstream stage catalyst composltlons used ln the present lnventlon may alqo contain other catalytlc compo-nents utlllzed for their known properties. ~or example, the downstream stage catalyst composltlon may also contaln a second rhodlum catalytlc component supported conventlonally on an alumina support, ln order to malntaln ~ood actlvlty for N0x reductlon. Further, the downstream sta~e catalyst composltlons may also contaln a platlnum catalytic component dlspersed upon an activated alumlna support. Thls component results ln relatlvely large platinum crystallltes belng dls-~ersed on the surface of the actlvated alumlna and orovldes a catalytic component which is believed to have good effi-clency for the oxidatlon of saturated hydrocarbons. A se-cond platinum catalytic component dispersed on cerla, such as an alumina-stabllized cerlum oxlde support, as dlsclosed in U.S. Patent 4,714,694 of C.Z. Wan et al, the disclosure of whlch ls hereby lncorporated hereln, may also be included in the composition. This platinum catalytlc component is belleved to be dispersed as relatively small crystallltes on the surface of the alumina stablllzed cerla, thereby pro-vldlng a catalytlc component whlch ls belleved to have good efficlency for the oxldatlon of carbon monoxlde and unsat-3 ~ ~

urated hydrocarbons to carbon dioxide and ~2 One or both o~ the up~tream and downstream catalyst compositions of the present invention may also contain a high-porosity refractory metal oxit~ which lncreases the over-all poroslty of the catalytlc materlal, as disclosed in U.S. Patent 4,757,045 of M.E. Turner et al, the disclosure o~ whlch is hereby incorporated hereln. For example, the catalytlc material ("washcoat") may comprlse a support in-cluding zirconia havlng a dispersed ceria phase thereon and an actlvated alumlna support. The washcoat may also contaln a mlnor amount of another refractory metal oxlde of hlgher poroslty than the support materlal, such refractory metal oxlde servlng to increase the parosity of the washcoat. As ~sed herein and in the clalms, a "mlnor" amount of the high-poroslty re~ractory metal oxlde means that the `^.lgh-poroslty refractory metal oxlde comprlses less than flfty percent by welght of the comblned welght of the hlgh-poroslty re~ractory metal oxlde and the refractory metal oxlde supports (excludlng the welght of catalytlc compo-nents, such as p,latlnum and rhodlum) dlspersed thereon.Usually the hlgh-poroslty refractory metal oxlde wlll com-prlse only about l to 20 percent by welghtl e.g., 5 to 10 percent by welght of the comblned refractory metal oxides, on the same basls as above lndlcated. The hlgh-poroslty refractory metal oxide deslrably has an accesslble pore volume o~ greater than about 0.03 cublc centlmeters per gram, a sur~ace area of less than about 25 square meters per gram, and a pore slze range such that at least about 3~iO o~
lts pore volume is provlded by pores havlng a dlameter o~ at least 2000 Angstroms when the second metal oxlde particles helng measured for pore slze are at least 44 mlcrons in dla-meter. Pulverized cordlerlte ls ~ell-sulted ~or the pur-pose.
The catalytlc materlals of both the upstream and down-stream catalysts may be applied to any suitable substrate, for example, to a ceramlc substrate such as a cordlerite substrate comprlslng a plurallty of cells whlch deflne flne, .

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~ 3 ?arallel gas flow passages extendlng from one ~ace to the other of a cylindrical monollth member. Such substrates, ~hlch may also be made of refractory metals such as staln-less steel, are sometlmes referred to in the art as "honey-comb" or "monollthlc" substrates. Alternatively, the cata-lyst composltlons of the present lnventlon may lnclude a washcoat contalnlng the above-descrlbed catalytlc materlal dispersed on a partlculate support made of a refractory ceramic materlal, such as pellets, spheres or extrudates of alpha alumina or the like.
When applylng the catalytlc materlals to a monollth substrate, such as a cordlerite substrate, it ls also pre-ferred to provide an lnitlal undercoat of actlvated alumina.
Activated alumlna, when applied and calclned ln the manner as descrlbed in the followlng Examples, provides a tough, adherent coatlng to the cordierlte or other substrate. The catalyst materlals are then applled as a second or topcoat over the undercoat. Thls not only provldes better adherence of the catalytlc materlal to the substrate when, for exam-ple, the washcoat comprlses cerla, but, by supporting thetopcoat on an undercoat consisting essentlally of activated alumlna, the metal catalytlc components such as platlnum and .hodlum are made more accessible to the gas flowing through the cells o~ the substrate. ~hat Ls, the gas can flow through the topcoat layer and lnto the undercoat, thereby orovldlng enhanced passage and lncreased contact o~ the noxlous components with the catalytlc metals dlspersed throughout the topcoat. The actlvated alumlna undercoat may optlonally be stabllized by a suitable thermal stabilizer such as cerla and/or other rare earth oxldes and the topcoat may lnclude a second, porous refractory metal oxide to en-hance gas flow therethrough.
The upstream and do~nstream stages of the catalysts of the present lnventlon may be contalned on a slngle mono-llthlc substrate or a slngle catalyst bed. Thus, a ceramlcmonollthlc substrate may be partially submerged from one end thereof wlthln a slurry contalnlng the low-lgnltlon tempera-.' ` ~ , , ' . .

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ture upstream catalyst materlal of the present inventlon, and subsequently partially submerged from its other end into a slurry of a catalytic materlal containlng the hlgh actl-vlty downstream catalytlc materlal of the present ~nventlon.
Alternatlvely, the upstream and downstream stages may com-prise separate beds or separate, dlscrete monolithic sub-strates. Thus, the upstream catalyst member may comprise a discrete catalyst member separate from the downstream cata~
lyst member and the upstream and downstream catalyst members may be spaced apart and separated from each other by an open gas-flow zone wlthln which, as ls known ln the art, a turbu-lent conditlon may be restored to the flowlng gases between the upstream and downstream stages.
Specific embodiments of the invention and utllization thereo~ are shown ln the following Examples 1-3.
Example 1 A catalyst denomlnated Catalyst I ls a single-coat catalyst supported on a cordierite substrate. Catalyst I is made as follows.
A. An activated alumina powder, calcined at 950C for 2 hours and exhibltlng a surface area of about 120 m2/g, ls placed ln a one-half gallon ball mlll to whlch is ~lso added 240 mllllllters ("ml") of dlstllled water, and the mlxture is mllled ~or 30 mlnutes at 60 revolutions per minute ("rpm"). An aqueous solutlon of the same MEA platlnum hy-droxlde complex as used ln Part B of Example 1 contalnlng 3.675 grams of platlnum, measured as the metal, was added to the ball mlll and mllling was contlnued ~or 60 additional ~inutes. At that tlme 12.5 ml of a 69 percent nltrlc acld solution wa~ added to the ball mill and the milling was con-tinued to reduce the particles to a particle si~e of less than 12 microns in diameter, to provide an aqueous slurry o~
alumina particles containlng, on a dry basls, 1.47 percent by welght platinum, measured as the metal and based on the welght of platlnum plu9 the alumlna support.
B. A rhodlum on alumlna catalytlc materlal is pre-2 ~3 ~
-15~

?ared by placlng 250 grams of the same alumina powder as used in Step A into a half-gallon ball mill and adding 250 ml dlstilled water, then milling the resultant aqueous slur-ry for 30 mlnutes at 60 rpm. An aqueous solution of rhodium nltrate contalnlng l. 64 grams of rhodium, measured as the metal, was then added to the ball mill and mllling continued for 60 addltional minutes. After that time, 12.5 ml of a 69 percent nitrlc acid solutlon was added to the mill, and milling was continued to reduce the partlcles to a particle size of less than 12 microns in diameter to provide a slurry of alumina particles contalnlng on a dry basls 0.656 welght percent rhodlum, measured as the metal and based on the weight of rhodium plus the alumlna support.
5. Into a one-half gallon ball mlll there was placed 250 grams o~ a ceria powder whlch had been stabilized Wlth alumina, the powder containlng on a dry basis 2.5 ?ercent Al203 based on the total welght of ceria plus alumina. To the ball mill was added 240 ml of distilled water and the mlxture wàs milled for 30 minutes at 60 rpm. A portion of the same aqueous MEA platinum hydroxide complex used in Part 3 of Example l and containing 3.675 grams of platinum, meas-ured as the metal, was then added and milling was continued for 60 additional minutes. A~ter that time, ' 2.5 ml of a 99 percent solutlon of acetlc acld was added to the ~ill and milling was continued to reduce the particles to a size of less than 12 microns in diameter. The resultant slurry con-talned alumlna stabilized ceria particles containlng, in a dry basls, 1.47 percent by welght platlnum, measured as the metal and based on the welght o~ platinum plus the cerla 3~ Support.
D. The three slurries obtained ln Steps A, 3 and C
were blended to provide a coating slip containing, on a dry solids basis, 45 gram8 o~ the 0. 656 percent rhodium on alu-mina, 31 grams of the l. 47 percent platlnum on alumina, and 70 grams of ~he 1.47 percent platinum on alumina-stabllized cerla. Cordlerlte substrates were coated wlth the blended coatlng slip. The substrates were a cordlerlte honeycomb .:

' -: ;.

support manufactured by Cornlng Glass ~orks and `navln5 1OO
cells per square inch (62 cells per square centimeter) OI' face area extending therethrough, the cells beln~ of sub-stantlally square cross section. The substrate was oval 'n cross section, measured 7.19 centimeters in length and its faces measured 8.4 by 14.8 centimeters. The ceria-stabil-ized alumina was applied by dlpping the substrate into an aqueous slurry of the stabillzed alumina solids. The sub-strates were dipped into the blended slurry and excess slurry was blown from the cells (passageways). The coated substrates were then drled ln air at 100C for 16 hours and then calcined in air at 450C for 1 hour to provlde a sub-strate containing about 1.91 grams per cubic lnch of the ~rled and calcined coating slip (the "washcoat"~. ~he cat-lS alyst therefore contained about o.g grams of c~talytic met-al-containing ceria and l.01 grams of catalytlc metal con-taining alumina per cubic inch of catalyst. The resultant Catalyst I comprlsed platinum supported on alumlna-stabil-ized ceria, platinum supported on alumina, and rhodium sup-ported on alumina. Catalyst I contalns 40 g/4t3 of ?reciousmetal (platinum and rhoAlum) with a Pt to ~h weight r~tio (as metal) of 5Ø

Example 2 A catalyst having an undercoat adhered to a subs~rate and a catalytic topcoat applied thereto is denomlnated Cat-alyst II and is prepared as follows.
I. The Undercoat.
A. The undercoat is prepared by impregnating ~n acti-vated alumina powder with an aqueous solutlon of ^erium nl-trate, then spray-drying the impregnated powder in air ~t 400F (204C) and calcining the drled powder in air at 900C. The resultant material contalned 5% by welght cerium oxlde (as CeO2) as a thermal stabllizer for the alumina and had a surface area of about 130 square meters per gram ("m /g"). This ceria-stabilized, activated alumina frit was then coated onto a monolith substrate of the type used in -17- ~ 2~

'art D of ~xample 1 to give a loadlng o~ 1.5 grams ~er cublc inch ("g/ln3") o~ cerla-stabllized alumina. Excess slurry was blown ~rom the cells wlth compressed ai., ~nd ~he thus coated substrates were drled and then calclned in air at 500C for one hour.
II. The Topcoat B. One component of the topcoat is prepared by lm-pregnating an actlvated alumina powder having a surface area o~ 130 m /g with an aqueous solutlon of a methylethanolamlne complex of Pt+ hydroxide, whose formula may be expressed lnformally as (MEA)2Pt(oH)6, wherein MEA represent methyl-ethanolamine. The complex solutlon ls of a concentratlon to glve the platinum metal loading described below. The platl-~um compound-lmpregnated activated alumlna ls mllled in a 1~ ball mlll ln the presence o~ acetic acid to chemic~lly fix the platlnum on the alumlna support, provldlng a first alu-mlna support having a platlnum catalytic component dlspersed thereon ln the amount of 1.93 welght percent, measured as platlnum metal and based on the weight of the platlnum plus the alumlna support. Thls ~lrst alumlna support is not thermally stablllzed, that ls, lt ls substantlally ~ree of scabllizing ceria and other known thermal stabillzers such as rare earth metaloxldes generally.
C. A second platlnum-contalning component is sup-ported on an alumina-stabilized bulk cerla which ls prepared by impregnating a cerlum oxlde powder wlth an aqueous solu-tlon of alumlnum nitrate. ~he impregnated cerlum oxlde pow-der ls then dried ln air at 125C and then calcined in alr at about 400C to glve a ceria support having 2.5 welght percent alumina (measured as A12O3) on cerla. Thls alumina-stabllized ceria support is then lmpregnated with a platinum catalytic component and ~ixed, using the same solutlon and technique as in Step B, to provide on the ceria support an ldentlcal loadlng of 1.93 weight percent platinum catalytic component, measured as platinum metal and based on the weight of platinum plus the alumlna support.
D. A second alumlna support havlng a rhodium cata-. : ~ .
,., :- . :
, . , ~, .

, :

~ . '; '` , 2~ jt,l~

lDtic component dispersed thereon is prepared by l~pregna-ting an actlvated alumina powder having a surface area of 130 m2/g wlth an aqueous solution of rhodlum nitrate of a concentration to give the ,hodium catalytic component load-ing descrlbed below. The impregnated alumina ls dried inalr at 125C, then calcined in air at 450C to thermally flx the rhodlum on the support and glve an alumina support whlch is substantially free of ceria (or other stabilizing com-pounds) and contains thereon 0.39 weight percent rhodium, measured as the metal and based on the weight of rhodium plus the alumina support.
E. A zirconia/dispersed ceria phase support is pre-pared by impregnatlng a commerclally avallable zlrconia pow-der with an aqueous solution of cerium nitrate, ~e(N03)3 of a concentration to give the ceria loading desc.ibed below.
The impregnated zlrconla support is drled in air at 125C
and then calcined in air at 450C. The resultant zlrconia support material contains 10 percent by weight cerium oxide measured as CeO2 and based on the weight of zirconia plus ceria. Thls support ls lmpregnated wlth an aqueous solution of rhodium nltrate o~ a concentration to give the rhodlum loadlng descrlbed below. The lmpregnated zi.conia support is dried in alr at 125C and the dried support is calcined ln air ~t 450~C to provlde a zlrconla/dlspersed cerla sup-port havlng thereon 0.39 weight percent rhodium, measured asthe metal and based on the weight of rhodium plus the zlr-conla support.
F. A cordierite powder was prepared by commlnutlng cordierlte substrate scrap material.
u. Each of the flve topcoat components of Steps 3 through F are mllled separately in ball mllls in aqueous media to a particle size range such that at least 90~ by welght of the particles are of a dlameter less than 12 mi-crons. The mllled powders slurrles are then blended to-gether in proportions to provide a coating sllp.
H. The cordierite substrates havlng the undercoat afflxed thereto, obtalned ln Step A, are dipped lnto the ', ~ `. ~

.
:

coatlng slip of Step G and excess slurry is ~lown f.om he cells of the substrates with compressed ai~. ~he hus coated subs~rates are drled ln air ~t 125C and then cal-cined in air at 450C ~or one hour to ?rovide a catalyst composltlon containlng a topcoat and an undercoat of dried, adherent catalytic material ("washcoat"~ thereon. ~he fln-lshed catalyst composltion of Example 1 comprises 0.70 g/in3 of platinum-containing alumina-stabllized ceria support, 0.30 g/in3 of platinum-containing alumina support, 0.50 g/in3 of rhodium-contalning alumlna support, 0.50 g/in3 of rhodium-containing zirconia/dispersed ceria support, and 0.20 g/in3 of ground cordierite.

E~ample 3 In order to compare the results obtained using the combination of a low ignltlon temperature catalyst as the upstream catalytlc member with a downstream catalyst having a higher ignition temperature but higher conversion ef~lci-encies at operating temperature than the upstream catalyst, a catalytic converter denominated Converter ~ was assembled, utllizing as the upstream catalyst member Catalyst I o~ Ex-ample 1 and as the downstream catalytic member Catalyst II
of Example 2. The terms "upstrearn" and "downstream" are used as sensed ln the dlrectlon of exhaust gas ~lcw through the catalytlc converter. That is, the gaseous exhaust stream flows in series flrst through the "upstream" catalyst (Catalyst I) and then through the ~downstream~' catalyst (Catalyst II).
A ~lrst comparati~e converter was prepared utillzing as both the upstream and downstream catalyst material Cata-lyst I of Example I; this comparative converter was denomin-ated Converter B. A second comparative converter was pre-pared utilizing as both the upstream and downstream catalyst materlal Catalyst II of Example 2; thls converter was denom-inated Converter C. All three Converters, A, B and C, con-talned the same total amount and type of platinum group metal catalytic components, in the same proportions of ~t ., , ~nd Rh.
All three converters were englne-aged for 24 hours on a standardlzed four-hour aglng cycle ln whlch englne exhaust gas from an englne operating wlth a gasoline contalnlng 12 milligrams Pb per gallon ls flowed through the catalyst.
The engine is operated at idle, high and cruise condltions lncludlng selected air to fuel ratios and varying tempera-tures up to a maximum temperature of about 760 to 815C.
Each was then evaluated on a Volvo 740 GLE automobile equip-ped with a four cylinder engine using the ~ederal Test Pro-cedure as prescrlbed in Part 86 of 40 Code of ~ederal Regu-latlons (40 CFR 86). The Federal Test Procedure ('~TP") .esults, expressed as total weighted grams per mile of CO, hydrocarbon (HC) and NOx emissions escaping from the cata-lyst, are shown below. The ~TP has three test phases, acold transient phase, a cold stabilized phase and a hot transient phase. The "total weighted" results are a weighted average of the three test phases. The results of analysis of the exhaust gas obtalned from each of Converters A, 3 and C is set forth in the following Table.
Table Total ~elghted Grams Per Mile Of Operation Converter HC CO ~lO
!(--A 0.17 0.~4 0.10 B (Comparative) 0.32 1.44 0.24 C (Comparative) 0.31 1.29 0.22 The data of the Table clearly shows the markedly su-perior performance of Converter A, utllizing a catalyst com-posltion in accordance with the present inventlon. The grams of hydrocarbon per mile of operation escaping from Converter A are only slightly more than half of that from Converters B or C. The amount of unconverted CO escaplng from Converter A is only about 2/3 or less than that escap-, .

lng from Converters 3 and C and the amount o~ `Joy escaping ~rom Converter A is less than hal~ that escaping ~.om Con-verters B and C.
The enhanced performance of the catalyst system ln ac-cordance wlth the present invention is well illustrated by the Figure o~ the drawing in which conversion ef~iciency is shown on the abscissa extending from 0% at the origin to 100% and temperature is shown on the ordinate axis, increas-ing in the directlon movlng away from the origin, ~s indi-cated by the arrow parallel to the ordinate axis. ~l showsthe ignition temperature for Catalyst I and I2 shows the ignltion temperature for Catalyst II. It ls noted that Catalyst I demonstrates a better (lower) ignitlon temper-ature than does Catalyst II. However, once oper~ting ,5 temperature (To) is reached, it ls seen ~hat Catalyst II
attains a higher percentage conversion of the noxious pollu-tants than does Catalyst I. The dash-dot curve shows the combined affect attained by utilizing Catalyst I as an up-stream catalyst or Catalyst II as a downstream catalyst.
mhe combined Catalysts I and II are seen to provi~e the senefit of the lower ignition temperature I' ~or start-up ~peratlon and a higher over-all conversion once operating temperature has been attained.
It is of course not necessary ln the practlce of the present inventlon to utllize the speclflc catalysts ~ls-closed in the specific embodiment illust.ated by .xample 3.
Any combination of an upstream catalyst having a signi~l-cantly lower ignition temperature than a downst.eam cata-lyst, which downstream catalyst has a slgni~icantly higher converslon ef~iciency than the upstream catalyst ~lthin a normal operatlng temperature range, e.g., 400 to 300C, may be utlll~ed to attaln the beneflts of the present lnventlon.
Further, it should be understood that stolchiometric operation ls not requlred for successful utilizatlon of the catalyst. Reference to a stoichiometric exhaust gas (an exhaust gas produced by an englne operatlng with a stoichio-metric alr to ~uel ratlo) ln the clalms is for purposes of . :.. - : "
., . . .
-.

deflnition. As those skilled in the art are aware, success-ful TWC treatment of an exhaust gas ls at~iainable within a "window" around stoichiometric operation. Thls window for commercially available gasoline may extend ~1 or, prefer-ably, ~ 0.5, about the stoichiometric NF ratio for gasolineof 14.65.
Whlle the invention has been described in detail wlth respect to speclfic preferred embodiments thereo~9 it will be apparent to those skilled in the art upon a reading and understanding of the foregoing, that variatlons thereto may be made which variations nonetheless lie wlthin the spirit and scope of the inventlon and the appended claims.

. . .
i

Claims

1. A catalyst composition for converting HC, CO and NOx in a gaseous stream flowed therethrough, the composition comprising an upstream catalyst member and a downstream catalyst member, as sensed in the direction of flow of the gaseous stream through the catalyst composition, wherein:
(a) the upstream catalyst member comprises a first catalytic material characterized by having an ignition temperature for substantially simultaneous conversion of HC, CO and NOx which is lower than that of a second catalytic material, defined below; and (b) the downstream catalyst member com-prises a second catalytic material characterized by having a higher conversion efficiency than the first catalytic material for substantially simultaneous conversion of HC, CO
and NOx at operating temperatures above the ignition tem-perature.

2. The catalyst composition of claim 1 wherein the first catalytic material has, when the gaseous stream is a substantially stoichiometric exhaust gas mixture, an igni-tion temperature for substantially simultaneous conversion of HC, CO and NOx which is less than about 400°C, and the downstream catalyst member has, for the gaseous stream treated by the upstream catalyst member, a conversion ef-ficiency of at least about 94% for substantially simultane-ous conversion of HC, CO and NOx at an operating temperature of at least about 400°C.

3. The catalyst composition of claim 2 wherein the ignition temperature is not more than about 350°C.

4. The catalyst composition of claim 2 or claim 3 wherein the operating temperature range is from about 400 to 800°C.

5. The catalyst composition of claim 4 wherein the ignition temperature is not more than about 375°C.

6. The catalyst composition of claim 1, claim 2 or claim 3 wherein (a) the first catalytic material comprises a platinum catalytic component dispersed on a refractory metal oxide support and a rhodium catalytic component dispersed on a refractory metal oxide support, and (b) the second cata-lytic material comprises a rhodium catalytic component dis-posed on a zirconia/dispersed ceria support and a platinum catalytic component disposed on a refractory metal oxide support.

7. The catalyst composition of claim 1, claim 2 or claim 3 wherein (a) the first catalytic material comprises a first platinum catalytic component dispersed on a first ac-tivated alumina support, a rhodium catalytic component dis-persed on a second activated alumina support, and a second platinum catalytic component disposed on a ceria support, and (b) the second catalytic material comprises a second rhodium catalytic component dispersed on a zirconia/dis-persed ceria support, a third rhodium catalytic component disposed on a third activated alumina support, a third platinum catalytic component disposed on a a fourth activ-ated alumina support, and a fourth platinum catalytic component disposed on a ceria support.

8. The catalyst composition of claim 7 further in-cluding in one or both of the first and second catalyst materials a minor amount of a high-porosity refractory metal oxide which is substantially free of catalytic metal compon-ents and increases the porosity of the catalytic material in which it is included.

9. The catalyst composition of claim 7 wherein both the first and second catalytic materials are disposed on a carrier substrate.

10. The catalyst composition of claim 9 wherein at least one of the first and second catalytic materials com-prises a topcoat overlying an undercoat comprising a sta-bilized alumina support.

11. The catalyst composition of claim 1 or claim 2 comprising a discrete upstream catalyst member and a dis-crete downstream catalyst member.

12. The catalyst composition of claim 11 wherein the upstream and downstream catalyst members are spaced one from the other and separated by a gas flow zone.

13. A method of substantially simultaneously convert-ing HC, CO and NOx pollutants contained in a gaseous stream, the method comprising:
(a) flowing the gaseous stream through a first catalyst zone and therein contacting the gaseous stream with a first catalyst member comprising a first catalytic materi-al having an ignition temperature for substantially simul-taneous conversion of HC, CO and NOx, when the gaseous stream is a substantially stoichiometric exhaust gas mix-ture, which is lower than the corresponding ignition tem-perature of a second catalytic material, defined below, the gaseous stream being introduced into the first catalyst zone at a temperature at or above the ignition temperature of the first catalytic material but below an operating temperature range, defined below, to convert within said first catalyst zone some, but less than all, of each of the HC, CO and NOx content of the gaseous stream to innocuous substances and thereby increase the temperature of the gaseous stream, and (b) flowing the gaseous stream from the first cat-alyst zone to a second catalyst zone and therein contacting the gaseous stream with a second catalyst member comprising a second catalytic material having a higher conversion ef-ficiency for substantially simultaneous conversion of HC, CO
and NOx at temperatures within the operating temperature range than does the first catalytic material, the gaseous stream being contacted with the second catalytic material at a temperature within the operating temperature range to sub-stantially simultaneously convert at least some of the re-maining HC, CO and NOx to innocuous substances.

14. The method of claim 13 wherein the ignition tem-perature of the first catalytic material is less than about 400°C and contacting the gaseous stream with the second cat-alyst member within an operating temperature range of from about 400°C to 800°C.

15. The method of claim 14 wherein the ignition tem-perature is not more than about 350°C.

16. The method of claim 13, claim 14 or claim 15 wherein (a) the first catalytic material comprises a plat-inum catalytic component dispersed on a refractory metal oxide support and a rhodium catalytic component dispersed on a refractory metal oxide support, and (b) the second cata-lytic material comprises a rhodium catalytic component dis-posed on a zirconia/dispersed ceria support and a platinum catalytic component disposed on a refractory metal oxide support.

17. The method of claim 13, claim 14 or claim 15 wherein (a) the first catalytic material comprises a first platinum catalytic component dispersed on a first activated alumina support, a rhodium catalytic component dispersed on a second activated alumina support, and a second platinum catalytic component disposed on a ceria support, and (b) the second catalytic material comprises a second rhodium cata-lytic component dispersed on a zirconia/dispersed ceria sup-port, a third rhodium catalytic component disposed on a third activated alumina support, a third platinum catalytic component disposed on a a fourth activated alumina support, and a fourth platinum catalytic component disposed on a cer-ia support.

18. The method of claim 17 wherein there is included in one or both of the first and second catalyst materials a minor amount of a high-porosity refractory metal oxide which is substantially free of catalytic metal components and in-creases the porosity of the catalytic material in which it is included.