(12) United States Patent
`Yavuz et al.
`
`(10) Patent N0.:
`
`(45) Date of Patent:
`
`US 6,248,684 B1
`*Jun. 19, 2001
`
`US006248684B1
`
`(54)
`
`(75)
`
`ZEOLITE-CONTAINING OXIDATION
`CATALYST AND METHOD OF USE
`
`Inventors: Bulent O. Yavuz, Plainfield; Kenneth
`E. Voss, Somerville; Michel Deeba,
`North Brunswick; John R. Adomaitis,
`Old Bridge; Robert J. Farranto,
`Westfield, all of NJ (US)
`
`(73)
`
`Assignee: Englehard Corporation, Iselin, NJ
`(US)
`
`Notice:
`
`This patent issued on a continued pros-
`ecution application filed under 37 CFR
`1.53(d), and is subject to the twenty year
`patent
`term provisions of 35 U.S.C.
`154(a)(2).
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21)
`
`(22)
`
`(63)
`
`(51)
`(52)
`
`(53)
`
`(56)
`
`Appl. N0.: 08/255,289
`
`Filed:
`
`Jun. 7, 1994
`
`Related U.S. Application Data
`
`Continuation of application No. 08/038,378, filed on Mar.
`29, 1993, now abandoned, which is a continuation-in-part of
`application No. 07/973,461, filed on Nov. 19, 1992, now
`abandoned.
`
`Int. Cl.7 .................................................... .. B01J 29/06
`U.S. Cl.
`............................... .. 502/66; 502/64; 502/65;
`502/974
`Field of Search .................................. 502/65, 64, 66,
`
`_ 423/213.2, 244.11
`502/
`
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`OTHER PUBLICATIONS
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`Horiuchi et al, “The Effects of Flow—Through Type Oxida-
`tion Catalysts .
`.
`. ”, SAE paper 900600, Feb., 1990.
`The Effects of Flow—Through Type Oxidation Catalysts on
`the Particulate Reduction of 1990’s Diesel Engines, M.
`Horiuchi et al, SAE Paper 900600, Feb. 1990.
`
`(List continued on next page.)
`
`Primary Examiner—Tom Dunn
`
`(57)
`
`ABSTRACT
`
`Oxidation catalyst compositions for treating diesel exhaust
`include ceria and, optionally, alumina, each having a surface
`area of at least about 10 m2/g, and a zeolite, e.g., Beta
`zeolite. Optionally, platinum may be included in the cata-
`lytic material, preferably in amounts which are suflicient to
`promote some gas-phase oxidation of carbon monoxide
`(“CO”) and hydrocarbons (“HC”) but which are limited to
`preclude excessive oxidation of S02 to S03. Alternatively,
`palladium in any desired amount may be included in the
`catalytic material. The zeolite is optionally doped, e.g.,
`ion-exchanged, with one or more of hydrogen, a platinum
`group metal or other catalytic metals. The catalyst compo-
`sitions may be used in a method to treat diesel engine
`exhaust by contacting the hot exhaust with the catalyst
`composition to promote the oxidation of gas-phase CO and
`HC and of the volatile organic fraction component of
`particulates in the exhaust.
`
`23 Claims, 10 Drawing Sheets
`
`JM 1015
`
`3,161,605
`3,331,190
`3,864,452
`3,993,572
`4,053,556
`4,123,391
`4,134,860
`4,171,288
`4,171,289
`4,189,404
`4,200,552
`4,297,328
`4,407,785
`4,418,046
`4,477,417
`4,492,769
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`4,568,655
`4,581,343
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`4,621,071
`4,624,940
`4,683,214
`4,707,341
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`
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`
`................... .. 502/66
`
`.......................... .. 502/65
`
`1
`
`JM 1015
`
`

`
`US 6,248,684 B1
`Page 2
`
`4/1991 (GB) .
`2236493
`1/1981 (JP) .
`56-2920
`1/1981 (J13) _
`565419
`8/1934 (11)) _
`142851
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`60_147240
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`2187111 *
`5/1939 (JP) _
`1.139144
`5/1939
`JP _
`1.155511
`5/1990 E1133 _
`2.125935
`10/1990 (JP) .............................. .. B01J/29/24
`2251247
`3270732 * 12/1991 (JP) ........................................ 502/65
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`12/1993 (JP) .
`6—126165
`5/1994 (JP).
`6.210153
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`8/19/94 (41)) _
`6—312132
`11/1994 (JP) .
`7402957
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`7-33364
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`7-95173
`4/1995 (JP) _
`-
`3
`.
`3131352
`311333 853 '
`840566
`1/1996 (JP) I
`<9-
`4/1996 (J13) _
`1/1990 (W0) 4
`1/1994
`:0 4
`5/1994 ((2:10;
`10/1994 (V170) _
`5/1995 (W0) _
`12/1996 (VVO) _
`12/1996
`.
`1/1997
`_
`
`............................... F01N/3/20
`
`839033
`WO9000441
`W0 94‘/01925
`WO 94/11623
`W0 94./22564
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`
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`,
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`.
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`.
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`1
`~
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`.Ch‘ga:lter1Slf,1°SJSA‘i3f
`‘
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`1
`1559
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`
`* cited by examiner
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`. . . . .. 1Z5/440
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`.
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`.
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`.
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`................... .. 502/66
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`-
`J
`10/V1991
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`-
`~
`,
`1
`351333’
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`42222.; 0 11:22:
`,/
`..............................
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`,
`2
`/I
`.
`.
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`. §02,65
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`.
`.
`02/79
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`- 502/73
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`. 502/'66
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`................... .. 502/347
`5,538,697
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`.......................... .. 422/171
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`................ .. 423/213.5
`
`
`
`FOREIGN PATENT DOCUMENTS
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`7/1990 (EP).
`382434
`8/1990 (EP) 4
`397411
`11/1990 (E12) _
`399891
`11/1990 (EP) _
`404385
`12/1990 (EP) .
`0404385 * 12/1990 (EP) .
`449423
`10,-1991
`(E15) _
`452593
`12,1 991
`(pp) _
`0459396 * 12/1991
`0508513
`31992
`0 485 179 A2
`5:499}!
`0 438 250
`6/1992
`0 491 360 A1
`6/1992
`499931
`8/1992
`921021614
`8/1992
`0 503 500 A1
`9/1992
`503500
`9/1992
`50g513
`10/1992
`0508513 * 10/1992
`0499931
`2/1993
`0559021
`2/1993
`0 369 576
`6/11994
`32$
`£32
`B1
`0 6'38
`8/1497 (EP)'
`2546770
`42/.1984 (FR) 4"""""""""""""""""
`1060424
`3/1967 (GB) 4
`1071373
`6/1967 (GB) .
`1551348
`12/1976 (GB) .
`
`..... .. 42/2132
`.. B01D/53/36
`
`B01D/531,36
`
`B01D/53/36
`B01D/S3/36
`B01]/29'/06
`‘‘‘" F01N/3/20
`F01N/3/20
`'
`
`2
`
`

`
`U.S. Patent
`
`Jun. 19,2001
`
`Sheet 1 of 10
`
`US 6,248,684 B1
`
`'00 % GAS PHASE HC CONVERSION
`
`80
`
`E-l
`
`60
`
`40
`
`20
`
` O ‘ .
`
`
`I00
`I50
`200 250 300 350
`400 450
`500 550 600
`
`CATALYST INLET TEMP. (DEG C)
`
`,._ SAMPLE c—2. O.5gIft3PI
`_,._ SAMPLE c-3. 2.ogm3Pt
`
`_,.__ SAMPLE C-I. No Pl
`__._ SAMPLE E-I. 2.5g/n3Pt
`
`FIG.
`
`I
`
`3
`
`

`
`U.S. Patent
`
`Jun. 19,2001
`
`Sheet 2 of 10
`
`US 6,248,684 B1
`
`% CO CONVERSION
`
`IOO
`
`C-3
`
`60
`
`40
`
`20
`
`-20
`
`I00
`
`I50
`
`200 250 300 350 400 450 500 550 600
`
`CATALYST INLET TEMPERATURE (DEG C)
`
`__.___SAMPLE c—3. 2.ogm3P1 _._ SAMPLE E-l. 2.5g/ft3P1
`
`FIG. 2
`
`4
`
`

`
`U.S. Patent
`
`Jun. 19,2001
`
`Sheet 3 of 10
`
`US 6,248,684 B1
`
`% VOF REMOVAL
`
`I00
`
`80
`
`60
`
`40
`
`20
`
`0
`
`I00
`
`I50
`
`200 250 300 350 400 450 500 550 600
`
`CATALYST INLET TEMPERATURE (DEG C)
`
`_._ SAMPLE c—3. 2.OgIft3Pf
`
`_+_SAMPI£ E-I. 2.5gIft3P1
`
`FIG. 3
`
`5
`
`

`
`U.S. Patent
`
`Jun. 19,2001
`
`Sheet 4 of 10
`
`US 6,248,684 B1
`
`% TPM REMOVAL
`
`E-|
`
`C-3
`
`\
`
`E-I
`
`I00
`
`50
`0
`
`-I50
`
`-200
`
`-250
`
`D0 I50
`
`200 250 300 350 400 450 500 550 600
`
`CATALYST INLET TEMPERATURE (DEG C)
`
`__._ SAMPLE c—3. 2.OgIfi3P1
`
`__4_ SAMPLE E—|. 2.5gm3P+
`
`FIG. 4
`
`6
`
`

`
`U.S. Patent
`
`Jun. 19,2001
`
`Sheet 5 of 10
`
`US 6,248,684 B1
`
`S03 PLUS H20 (SULFURIC ACID) EMISSIONS (9/bhp-hr)
`
`0.25
`
`0.2
`
`0.15
`
`0.|
`
`0.05
`
`0
`I00
`
`I50
`
`200 250 300 350 400 450 500 550 600
`
`CATALYST INLET TEMPERATURE (DEG C)
`
`_._BASE LEE - N0 CATALYST
`
`_a_SAMPLE E-l.2.5 gIfl3Pi
`
`_,k_SAMPLE c—3. 2.0 gIft3Pt
`
`FIG. 5
`
`7
`
`

`
`U.S. Patent
`
`J
`
`009429
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`
`U.S. Patent
`
`Jun. 19,2001
`
`Sheet 7 of 10
`
`US 6,248,684 B1
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`U.S. Patent
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`Jun. 19,2001
`
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`U.S. Patent
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`Jun. 19,2001
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`
`US 6,248,684 B1
`
`1
`ZEOLITE-CONTAINING OXIDATION
`CATALYST AND METHOD OF USE
`
`CROSS-REFERENCE TO RELATED
`APPLICATION
`
`This is a continuation of application Ser. No. 08/038,378
`filed Mar. 29, 1993, now abandoned which is a continuation
`in part of Ser. No. 07/973,461 filed Nov. 19, 1992, now
`abandoned.
`
`BACKGROUND OF THE INVENTION
`
`1. Field of the Invention
`
`This invention relates to a catalyst composition and
`method for the oxidation of oxidizeable components of
`diesel engine exhaust, and more specifically to the treatment
`of such diesel exhaust to reduce the content of particulates
`and other pollutants discharged to the atmosphere.
`2. Background and Related Art
`As is well-known, diesel engine exhaust is a heteroge-
`neous material which contains not only gaseous pollutants
`such as carbon monoxide (“CO”) and unburned hydrocar-
`bons (“HC”), but also soot particles which comprise both a
`dry, solid carbonaceous fraction and a soluble organic frac-
`tion. The soluble organic fraction is sometimes referred to as
`a Volatile organic fraction (“VOF”), which terminology will
`be used herein. The VOF may exist in diesel exhaust either
`as a vapor or as an aerosol
`(fine droplets of liquid
`condensate) depending on the temperature of the diesel
`exhaust.
`
`Oxidation catalysts comprising a platinum group metal
`dispersed on a refractory metal oxide support are known for
`use in treating the exhaust of diesel engines in order to
`convert both HC and CO gaseous pollutants and
`particulates, i.e., soot particles, by catalyzing the oxidation
`of these pollutants to carbon dioxide and water. One problem
`faced in the treatment of diesel engine exhaust is presented
`by the presence of sulfur in diesel fuel. Upon combustion,
`sulfur forms sulfur dioxide and the oxidation catalyst cata-
`lyzes the S0, to S03 (“sulfates”) with subsequent formation
`of condensible sulfur compounds, such as sulfuric acid,
`which condense upon, and thereby add to,
`the mass of
`particulates. The sulfates also react with activated alumina
`supports to form aluminum sulfates, which render activated
`alumina-containing catalysts inactive. In this regard, see
`U.S. Pat. No. 4,171,289 at column 1, line 39 et seq. Previous
`attempts to deal with the sulfation problem include the
`incorporation of large amounts of sulfate-resistant materials
`such as vanadium oxide into the support coating, or the use
`of alternative support materials such as (1-alumina (alpha),
`silica and titania, which are sulfate-resistant materials.
`The prior art also shows an awareness of the use of
`zeolites,
`including metal-doped zeolites,
`to treat diesel
`exhaust. For example, U.S. Pat. No. 4,929,581 discloses a
`filter for diesel exhaust, in which the exhaust is constrained
`to flow through the catalyst walls to filter the soot particles.
`Acatalyst comprising a platinum group metal-doped zeolite
`is dispersed on the walls of the filter to catalyze oxidation of
`the soot to unplug the filter.
`As is well-known in the art, catalysts used to treat the
`exhaust of internal combustion engines are less effective
`during periods of relatively low temperature operation, such
`as the initial cold-start period of engine operation, because
`the engine exhaust is not at a temperature sufficiently high
`for efficient catalytic conversion of noxious components in
`the exhaust. To this end, it is known in the art to include an
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`adsorbent material, which may be a zeolite, as part of a
`catalytic treatment system in order to adsorb gaseous
`pollutants, usually hydrocarbons, and retain them during the
`initial cold-start period. As the exhaust gas temperature
`increases, the adsorbed hydrocarbons are driven from the
`adsorbent and subjected to catalytic treatment at the higher
`temperature. In this regard, see for example U.S. Pat. No.
`5,125,231 which discloses (columns 5-6) the use of plati-
`num group metal-doped zeolites as low temperature hydro-
`carbon adsorbents as well as oxidation catalysts.
`SUMMARY OF THE INVENTION
`
`Generally, in accordance with the present invention, there
`is provided a catalyst composition and a method for oxidiz-
`ing oxidizeable components of diesel engine exhaust in
`which at least some of a volatile organic fraction of the
`diesel exhaust is converted to innocuous materials, and in
`which gaseous IIC and CO pollutants may also be similarly
`converted. The objectives of the invention are attained by an
`oxidation catalyst comprising a catalytic material compris-
`ing a mixture of high surface area ceria, a zeolite and,
`optionally, a high surface area alumina. The catalytic mate-
`rial optionally may carry a low loading of platinum catalytic
`metal dispersed thereon or palladium catalytic metal dis-
`persed thereon. Alternatively, or in addition, the zeolite of
`the catalyst composition may be doped, e.g., ion-exchanged,
`with a catalytic moiety such as one or more of hydrogen ion,
`platinum, copper, nickel, cobalt, iron, etc. The method of the
`invention is attained by flowing a diesel engine exhaust, e.g.,
`the exhaust of a diesel-powered automobile or light truck,
`into Contact under oxidation reaction conditions with a
`
`catalyst composition as described above.
`Specifically,
`in accordance with the present invention
`there is provided a catalyst composition for treating a diesel
`engine exhaust stream containing a volatile organic fraction,
`which composition comprises a refractory carrier on which
`is disposed a coating of a catalytic material comprising a
`catalytically effective amount of ceria and, optionally, a
`catalytically effective amount of alumina, each having a
`BET surface area of at least about 10 m2/g, preferably a
`surface area of from about 25 m2/g to 200 m /g, and a zeolite,
`for example, Beta zeolite or a zeolite selected from the group
`consisting of Y-zeolite, pentasil (e.g., ZSM-5), Mordenite,
`and mixtures thereof.
`
`In one aspect of the present invention, the zeolite com-
`prises a three-dimensional zeolite characterized by pore
`openings whose smallest cross-sectional dimension is at
`least about 5 Angstroms and having a silicon to aluminum
`atomic ratio (“Si:Al atomic ratio”) of greater than 5, e.g., a
`Si:Al atomic ratio of from about 5 to 400.
`
`In another aspect of the invention, the zeolite comprises
`from about 10 to 90, preferably from about 20 to 70, percent
`by weight,
`the alumina comprises from about 60 to 5,
`preferably from about 50 to 20, percent by weight, and the
`ceria comprises from about 60 to 5, preferably from about 50
`to 20, percent by weight, of the combined weight of the
`zeolite, the alumina and the ceria.
`Yet another aspect of the invention provides for the zeolite
`to be doped with a catalytic moiety, e.g., ion-exchanged or
`impregnated, with an ion or with a neutral metal-containing
`species selected from the group consisting of one or more of
`hydrogen, platinum, rhodium, palladium,
`ruthenium,
`osmium,
`iridium, copper,
`iron, nickel, chromium and
`vanadium, preferably, one or both of platinum and iron.
`Still another aspect of the invention provides that the
`refractory carrier has a plurality of parallel exhaust llow
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`US 6,248,684 B1
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`3
`passages extending therethrough and defined by passage
`walls on which the catalytic material is coated, and further
`comprising either dispersed platinum carried on the catalytic
`material ir1 an amount of from about 0.1 to about 60, e.g., 0.1
`to 15, preferably 0.1 to 5, g/ft3 or dispersed palladium
`carried on the catalytic material in a quantity of from about
`0.1 to 200, preferably 20 to 120, g/ft3.
`In accordance with the method aspect of the present
`invention, there is provided a method for treating a diesel
`engine exhaust stream containing a volatile organic fraction,
`the method comprising contacting the stream with any of the
`catalyst compositions described above under oxidizing con-
`ditions including a temperature high enough to catalyze
`oxidation of at least some of the volatile organic fraction.
`For example, the temperature of the exhaust stream initially
`contacted with the catalyst composition may be from about
`100° C. to 800° C.
`
`DEFINITIONS
`
`As used herein and in the claims, the following terms shall
`have the indicated meanings.
`The term “BET surface area” has its usual meaning of
`referring to the Brunauer, Emmett, Teller method for deter-
`mining surface area by N2 adsorption. Unless otherwise
`specifically stated, all references herein to the surface area of
`a ceria, alumina or other component refer to the BET surface
`area.
`
`The term “activated alumina” has its usual meaning of a
`high BET surface area alumina, comprising primarily one or
`more of y-, 6- and 5-aluminas (gamma, theta and delta).
`The term “catalytically effective amount” means that the
`amount of material present is suflicient to affect the rate of
`reaction of the oxidation of pollutants in the exhaust being
`treated.
`
`The term “inlet temperature” shall mean the temperature
`of the exhaust, test gas or other stream being treated imme-
`diately prior to initial contact of the exhaust, test gas or other
`stream with the catalyst composition.
`The term “doped” used to refer to a zeolite being doped
`with a metal or hydrogen, and the terms “dope” or “doping”
`used in the same context, means that the metal or hydrogen
`moiety is incorporated within the pores of the zeolite, as
`distinguished from being dispersed on the surface of the
`zeolite but not to any significant degree within the pores of
`the zeolite. Doping of a zeolite is preferably carried out by
`known ion-exchange techniques in which a zeolite is repeat-
`edly flushed with a solution containing metal cations (or an
`acid to provide hydrogen ions), or the zeolite pores are
`flooded with such solution. However,
`the defined terms
`include any suitable technique for incorporating a catalytic
`moiety, e.g., one or more metals as ions or neutral metal-
`containing species or hydrogen ions, within the pores of the
`zeolite, especially by exchange or replacement of cations of
`the zeolite.
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`the thermal
`form into another component. For example,
`stabilization of ceria particles (bulk ceria) with alumina as
`described in U.S. Pat. No. 4,714,694, results in the alumina
`being dispersed into the ceria particles and does not provide
`the dispersed alumina in “bulk” form,
`i.e., as discrete
`particles of alumina.
`The abbreviation “TGA” stands for thermogravimetric
`analysis, which is a measure of the weight change (e.g.,
`weight loss) of a sample as a function of temperature and/or
`time. The abbreviation “DTA” stands for differential thermal
`analysis, which is a measure of the amount. of heat emitted
`(exotherm) or absorbed (endotherm) by a sample as a
`function of temperature and/or time.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIGS. 1-5 show graphically the results of the engine test
`of the catalysts of Example 1, set forth numerically in
`TABLE I-B: specifically;
`FIG. 1 is a plot of gas-phase HC conversion for sample
`E-I and comparative samples C-1, C-2 and C-3 as a function
`of catalyst inlet temperature;
`FIG. 2 is a plot of carbon monoxide conversion for
`samples E-1 and C-3 as a function of catalyst inlet tempera-
`ture;
`
`FIG. 3 is a plot of VOF removal for samples E-1 and C-3
`as a function of catalyst inlet temperature;
`FIG. 4 is a plot of the reduction of the total mass of
`particulates (TMP) for samples E-1 and C-3 as a function of
`catalyst inlet temperature;
`FIG. Sis a plot of S03 and H20 emissions for samples E-1
`and C-3 and for untreated exhaust as a function of catalyst
`inlet temperature;
`FIGS. 6-9 are plots of hydrocarbon, S02 and CO con-
`version for samples E-2, E-3 and C-4 of Example 2: spe-
`cifically;
`FIG. 6 is a plot of gas-phase hydrocarbon conversion as
`a function of catalyst inlet
`temperature at 50,000 space
`velocity for samples E-2, E-3 and C-4;
`FIG. 7 is a plot of gas-phase hydrocarbon conversion as
`a function of catalyst inlet
`temperature at 90,000 space
`velocity for samples E-2, E-3 and C-4;
`FIG. 8 is a plot of S02 conversion as a function of catalyst
`inlet temperature at 90,000 space velocity for samples E-2,
`E-3 and C-4;
`FIG. 9 is a plot of CO conversion as a function of catalyst
`inlet temperature at 90,000 space velocity for samples E-2,
`E-3 and C-4; and
`FIG. 10 is an exemplary TGA/DTA plot of sample E-8 of
`Example 5.
`DETAILED DESCRIPTION OF THE
`INVENTION AND PREFERRED
`EMBODIMENTS THEREOF
`
`The term “washcoat” refers to a thin, adherent coating of
`a material, such as the catalytic material of the present
`invention, disposed on the walls forming the parallel gas
`flow passages of a carrier, which is typically made of a
`refractory material such as cordierite or other oxide or oxide
`mixture, or a stainless steel.
`Reference herein or in the claims to ceria or alumina being
`in “bulk” form means that the ceria or alumina is present as
`discrete particles (which may be, and usually are, of very
`small size, e.g., 10 to 20 microns in diameter or even
`smaller) as opposed to having been dispersed in solution
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`invention provides an oxidation catalyst
`The present
`composition which is effective for treating diesel engine
`exhaust, particularly with regard to reducing the total par-
`ticulates and HC and CO content of the exhaust. The
`
`carbonaceous particulates (“soot”) component of diesel
`engine exhaust is, as discussed above, known to be com-
`prised of relatively dry carbonaceous particulates and a
`volatile organic fraction (“VOF”) comprising high molecu-
`lar weight hydrocarbons resulting from unburned and par-
`tially burned diesel fuel and lubricating oil. The VOF is
`present in the diesel exhaust as either a vapor phase or a
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`US 6,248,684 B1
`
`5
`hquid phase, or both, depending on the temperature of the
`exhaust. Generally, it is not feasible to attempt to remove or
`treat the dry, solid carbonaceous particulates component of
`the total particulates by catalytic treatment, and it is the VOF
`component which can be most effectively removed by
`conversion via utilization of an oxidation catalyst.
`Therefore,
`in order to reduce the total particulates dis-
`charged so as to meet present and impending Government
`regulations concerning maximum allowable total
`particulates, the volatile organic fraction, or at least a portion
`thereof, must be oxidized to innocuous CO2 and H20 by
`being contacted with an oxidation catalyst under suitable
`reaction conditions. The required U.S. Government limits
`for 1991 on HC, CO, nitrogen oxides (“NO,,”) and total
`particulate emissions (“TPM”) in diesel engine exhaust have
`been largely met by suitable engine design modifications.
`For 1994 the HC, CO and NOx limits remain unchanged
`from 1991 standards but the upper limit on TPM will be
`reduced from the 1991 level of 0.25 grams per horsepower-
`hour (“g/I-IP-hr”) to 0.10 g/HP-hr. Although the oxidation
`catalysts of the present invention, when employed as a diesel
`exhaust treatment catalyst, are elfective for eifectuating a
`reduction in total particulates, they are also capable, espe-
`cially with the optional addition of platinum or other cata-
`lytic metals as described below, of providing the added
`advantage of also oxidizing a portion of the HC and CO
`contained in the gaseous component of the diesel engine
`exhaust. When sulfur or sulfur compounds are present in the
`exhaust in significant quantities, platinum is eliminated or
`used in limited amounts so as not to promote the unwanted
`effect of excessive oxidation of S02 to S03.
`Further, the zeolite component of the present invention is
`able to trap hydrocarbon molecules which might otherwise,
`during periods when the exhaust gas is relatively cool,
`escape untreated from the catalyst. It is believed that the
`trapped hydrocarbons are either oxidized within the zeolite
`or released from the zeolite only when the temperature of the
`catalyst composition is high enough to effectively catalyze
`oxidation of the trapped hydrocarbons, or both.
`Abasic and novel characteristic of the present invention
`is believed to reside in a catalyst composition comprising the
`defined combination of ceria, zeolite and, optionally,
`alumina, and one or both of the optional doping of the zeolite
`and dispersal of the catalytic metals, platinum or palladium
`as part of the composition and in the use thereof to treat
`diesel exhaust streams.
`
`As noted above, the bulk ceria and the bulk alumina may
`each have a surface area of at least about 10 m2/g, for
`example, at least about 20 ml/g. Typically, the bulk alumina
`may have a surface area of from about 120 to 180 m2/g and
`the bulk ceria may have a surface area of from about 70 to
`150 m2/g.
`that a diesel omdation catalyst composition
`The fact
`which may contain activated alumina as a major component
`thereof has proven to be successful is in itself surprising, in
`View of the consensus of the prior art that alumina, if used
`at all in diesel oxidation catalysts, must be a low surface area
`alumina (ct-alumina) and/or be used in conjunction with
`sulfate-resistant refractory metal oxides such as zirconia,
`titania or silica.
`It has nonetheless been found that
`in
`
`invention,
`accordance with one aspect of the present
`surprisingly, a catalyst composition comprising a combina-
`tion of high surface area ceria, a suitable zeolite, and,
`optionally, high surface area alumina, provides a catalytic
`material which effectively catalyzes the oxidation of the
`volatile organic fraction so as to provide a significant
`reduction in total particulates in diesel engine exhaust and is
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`capable of adsorbing and catalyzing the combustion of
`gaseous hydrocarbons. It should be noted that the prior art
`generally considers refractory base metal oxides used in
`diesel oxidation catalysts to be merely supports for the
`dispersal
`thereon of catalytically active metals such as
`platinum group metals. In contrast, the present invention
`teaches that a catalytic material comprising ceria and,
`optionally, alumina of sufficiently high surface area (10 m2/g
`or higher) and zeolite, dispersed on a suitable carrier,
`provides a durable and effective diesel oxidation catalyst.
`It has further been found that beneficial effects are
`attained by the optional incorporation of platinum or palla-
`dium in the catalyst composition, provided that in the case
`of platinum, the platinum is present at loadings much lower
`than those conventionally used in oxidation catalysts. If the
`catalytic metal platinum or palladium is added to the cata-
`lytic composition, it serves to catalyze the oxidation of
`gas-phase HC and CO pollutants as an added benefit.
`However, such catalytic metal is not needed to supplement
`the action of the ceria-zeolite or oeria-alumina-zeolite cata-
`lytic material in reducing total particulate emissions. Neither
`the platinum or palladium catalytic metal nor the metals or
`hydrogen used to dope the zeolite appear to significantly
`affect the rate of particulates conversion.
`The Zeolite
`The zeolite employed serves both to catalyze the oxida-
`tion of VOF and to crack the larger VOF molecules and,
`during periods of relatively low temperature operation, to
`trap gas-phase hydrocarbons within the zeolite pores. If the
`zeolite has been doped with one or more catalytic metals or
`hydrogen, the trapped gas-phase hydrocarbons are brought
`into intimate contact with the catalytically active cations
`which facilitates oxidation of the hydrocarbons. In any case,
`the zeolite pores also serve to retain some of the gas-phase
`hydrocarbons during start-up or other periods when the
`catalyst
`is relatively cool and therefore less effective in
`catalyzing oxidation reactions, and to release the hydrocar-
`bons only when the catalyst has been heated to higher
`temperatures. The higher
`temperatures impart suflicient
`energy to the trapped hydrocarbon molecules to enable them
`to escape the zeolite pores, but also enhance oxidation of the
`hydrocarbons in Contact with the catalyst. The zeolite there-
`fore serves not only as a catalyst for VOF oxidation, but as
`a hydrocarbon filter which traps hydrocarbons during peri-
`ods of relatively low temperature and concomitant
`low
`catalytic activity and retains them until they can be effi-
`ciently oxidized by the catalyst during periods of relatively
`high temperature.
`The Carrier (Substrate)
`The carrier used in this invention should be relatively
`inert with respect to the catalytic composition dispersed
`thereon. The preferred carriers are comprised of ceramic-
`like materials such as oordierite, ot-alumina, silicon nitride,
`zirconia, mullite, spodumene, alumina-silica-magnesia or
`zirconium silicate, or of refractory metals such as stainless
`steel. The carriers are preferably of the type sometimes
`referred to as honeycomb or monolithic carriers, comprising
`a unitary body, usually cylindrical in configuration, having
`a plurality of fine, substantially parallel gas flow passages
`extending therethrough and connecting both end-faces of the
`carrier to provide