(12) United States Patent
`Twigg et al.
`
`(10) Patent N0.:
`(45) Date of Patent:
`
`US 6,294,141 B1
`"‘Sep. 25, 2001
`
`US006294141B1
`
`(54) EMISSION CONTROL
`
`(75)
`
`Inventors: Martyn Vincent Twigg, Caxton;
`Anthony John Joseph Wilkins,
`waldcn; Nigel Simon Wfll’ Cambridge,
`an OHGB)
`
`4,451,441
`4,516,990
`4,535,588
`4,670,233
`4,759,918 *
`4,828,807
`4,849,274 *
`
`5/1984 Ernest etal. .................... .. 423/213.2
`5/1985 Erdmannsdiirfer et al.
`........... .. 55/96
`8/1985 Sato etal. .................. ..
`60/286
`6/1987 Erdmannsdnerfer et al.
`.. 423/213.2
`7/1988 Homeier et al.
`................ .. 423/213.5
`5/1989 Domesle etal. .
`.. 423/213.7
`7/1989 Cornelison .......
`428/116
`
`
`
`(73) Assignee: Johnson Matthcy Public Limited
`Company, London (GB)
`
`4:902:45” *
`5,746,989 *
`
`2/1999 90°F” 9‘ a'-
`5/1998 Murachi et al.
`
`-- 423/213-5
`............... .. 423/212 R
`
`(*) 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) Appl. No.: 08/949,979
`
`(22) Filed:
`
`Oct. 14, 1997
`
`FOREIGN PATENT DOCUMENTS
`
`5/1985 (DE).
`3337903 A1
`8/1990 (EP).
`382434 *
`10/1962 (GB).
`1014498
`2188 559A 10/1987 (GB).
`Sho. 54-74419
`6/1979 (JP).
`WO 96127078
`9/1996 0470).
`
`OTHER PUBLICATIONS
`
`Abstract of JP09079024 Claiming Priority of 1995.
`
`(30)
`
`Foreign Application Priority Data
`
`* cited by examiner
`
`Oct. 11, 1996
`
`(GB) .................................................. 9621215
`
`(51)
`
`(52) U.S. Cl.
`
`Int. Cl.7 ........................... B0lD 47/00; B01D 50/00;
`BO1J 8/02; F01N 3/00
`................. .. 423/213.7; 423/212; 423/213.2;
`423/213.5; 423/213.7; 423/235; 423/239.1;
`422/169; 422/171, 428/116; 60/272; 60/274;
`60/282; 60/299
`(58) Field of Search ............................... .. 423/212, 213.2,
`423/2135, 213.7, 235, 239.1; 422/169,
`171; 428/116; 60/272, 274, 282, 299
`
`(56)
`
`References Cited
`U.S. PATENT DOCUMENTS
`
`Primary Examiner—SteVen P. Griffin
`Assistant Examiner—Jonas N. Strickland
`
`(74) Attorney, Agent, or Firm—Ratner & Prestia
`
`(57)
`
`ABSTRACT
`
`An emission control system especially for light duty diesel
`engines has a first catalyst, 2, positioned upstream of a
`second catalyst, 3. The first catalyst converts NO to N02 and
`the second catalyst oxidises HC, CO and VOF. The tendency
`of the second catalyst to collect soot particles is reduced by
`combustion of the soot particles by N02 from the first
`catalyst.
`
`4,303,552
`
`12/1981 Ernest et al.
`
`....................... .. 252/465
`
`19 Claims, 4 Drawing Sheets
`
`
`
`JM 1018
`
`1
`
`JM 1018
`
`

`
`U.S. Patent
`
`Sep. 25,2001
`
`Sheet 1 of4
`
`US 6,294,141 B1
`
`
`
`2
`
`

`
`U.S. Patent
`
`Sep. 25,2001
`
`Sheet 2 of4
`
`US 6,294,141 B1
`
`
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`

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`U.S. Patent
`
`Sep. 25,2001
`
`Sheet 3 of4
`
`US 6,294,141 B1
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`

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`U.S. Patent
`
`Sep. 25,2001
`
`Sheet 4 of4
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`US 6,294,141 B1
`
`FIG.
`
`4
`
`Zl—‘!\JL|_'l-h(J‘lG'J\lI
`sum wt tg)
`
`500 ppm SULPHUR
`
`ZERO SULPHUR
`
`I CONVENTIONAL
`
`ACCORDING TO INVENTION
`
`5
`
`

`
`US 6,294,141 B1
`
`1
`EMISSION CONTROL
`
`The present invention concerns emission control, and
`more especially it concerns the reduction or elimination of
`soot particle emission from internal combustion engine
`exhaust gas, particularly that from diesel
`(compression
`ignition) engines.
`Although diesel engines generally emit considerably less
`gaseous pollutants, ie hydrocarbons (“HC”), carbon mon-
`oxide (“CO”) and volatile organic fractions (“VOF”) than
`gasoline spark ignition engines, it has become necessary to
`incorporate a catalyst in the exhaust gas system to meet
`current emission control regulations in the European Union
`for passenger cars and light vehicles. Such catalysts are
`generally based on metal or ceramic honeycomb substrate
`structures as are well known for oxidation or three-way
`catalysts for gasoline-engined vehicles. During certain nor-
`mal operations of diesel engines, particularly low speed and
`low temperature driving conditions such as city driving, the
`catalysts can become covered with carbonaceous material or
`soot. This may inhibit the gaseous reactions normally taking
`place on the catalyst surface by covering catalyst surfaces,
`and in worst cases may block partially or totally the channels
`in the honeycomb substrate, resulting in high pressure drop
`that can impair engine performance.
`The deliberate trapping and subsequent combustion of
`soot is known in the context of heavy duty diesels (trucks
`and buses) in order to improve the environment, and we
`refer to our U.S. Pat. No. 4,902,487 which describes a
`system which has now been commercialised as the Continu-
`ously Regenerating Trap (“CR ”). This patent teaches a
`system involving a filter to remove soot, and combusting the
`soot using a gas containing N02. Such a gas is produced by
`fitting a catalyst upstream of the filter, to oxidise nitric oxide
`present in the exhaust gas t.o N02. Heavy duty diesels
`provide exhaust gases at relatively high temperature, and
`low sulphur fuel needs to be used.
`We have now discovered that a variation on the concept
`of the CRT may be used to deal with soot inadvertently
`trapped on a catalyst in a light duty diesel. Light duty diesels
`operate at appreciably lower temperatures, especially under
`light load, than heavy duty diesels, which is generally a
`disadvantage for catalytic processes. It is also the case that
`the new generation direct injection gasoline engines can be
`limited in their engine management by avoidance of soot-
`forming conditions. It can increase the engine operating
`envelope, and possibly increase economy under certain
`conditions, if an emission control system which can deal
`with soot, can be developed.
`The present invention provides an emission control sys-
`tem for internal combustion engines which emit carbon-
`aoeous particulates, particularly for diesel, especially light
`duty diesel, engines, said system comprising a first catalyst
`effective to oxidise NO to N02 and a second catalyst,
`effective at least to oxidise HC, CO and VOF, each catalyst
`being supported on honeycomb flow-through monoliths,
`whereby soot. particles trapped on or within said second
`catalyst monolith are combusted in the N02-containing gas
`from said first catalyst, and wherein the monolith used as a
`support for said first catalyst is such as to minimise the
`collection of soot particles.
`Preferably, the first catalyst is formulated to have high
`activity for NO to N02 oxidation and is suitably a relatively
`high loading platinum catalyst. Such a catalyst may desir-
`ably have from about 50 to about 200 g Pt/ft3 (1.77—7.06 g
`Pt/liter of catalyst volume).
`The monolithic support used for the first catalyst is
`preferably a metal monolith which desirably provides flex-
`
`10
`
`15
`
`25
`
`30
`
`35
`
`40
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`45
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`60
`
`65
`
`2
`ing and/or vibration of the honeycomb cell walls for the
`purpose of displacing any soot particles captured Within the
`monolith. The monolith may be consciously designed to
`encourage such flexing and/or vibration, possibly using the
`natural vibration modes of the diesel engine.
`Desirably, the monolith is significantly more open than
`the monoliths used for oxidation or three-way catalysts
`especially for gasoline engines, which are, for example
`desirably 400 cells/inf (62 cells/cmz) or higher, that is to say
`tend preferably to 600 cells/inz (93 cells/cmz). Such a
`monolith may be, for example, 100 or 200 cells/inz (15.5 or
`31 cells/cmz). Desirably, the space velocity of gases flowing
`through the first catalyst is greater than that for the second
`catalyst, in order to reduce the opportunity for particulates to
`lodge therein.
`The second catalyst may be a conventionally formulated
`diesel catalyst, for example on a monolith having 400
`cells/'in2 or more. Soot formation from diesel engine
`exhausts has limited or excluded the use of the high cell
`density monoliths, which are desirable from a catalytic
`convertor viewpoint. The second catalyst could also be a
`three-way catalyst, especially of the “lean-NOx” type, in
`which in addition to the oxidation reactions in respect of HC,
`NO and VOF, there is reduction of NOx to N2, perhaps
`intermittently by the mechanism of NOX storage on com-
`ponents in the catalyst or by continuous regeneration of a
`selective catalyst.
`The first and/or second catalysts may incorporate trap-
`ping components, either as discrete traps prior to the catalyst
`or as components of a layered or composite catalyst
`construction, to trap water vapor, sulphur, HC and/or NOX
`until they are released under catalyst operating conditions
`favorable to their conversion or use.
`It is to be understood that the complex and varying gas
`compositions in an operating condition may not provide
`total conversion of NO to N02, but other oxidised nitrogen
`oxides may be produced. The necessary reaction is that such
`product N02 or oxidised nitrogen oxides contribute to the
`combustion of the soot particles, and, for ease of reference,
`the designation N02 is used herein. The practical require-
`ment is that sufficient N02 is produced so that build-up of
`soot is limited to below the levels at which problems arise.
`For this reason also, it may be desirable to have the first and
`second catalysts positioned close to each other, possibly
`within the same canister.
`
`It is to be noted that some diesel fuels have high sulphur
`contents, eg above 500 ppm, and we have found that the
`presence of sulphur compounds can inhibit
`the reaction
`forming N02 over a platinum catalyst. Desirably, therefore,
`low S fuel is used, but the inhibiting effect of sulphur may
`be reduced to some extent by using high catalyst or gas
`temperatures during periods in which gas and/or catalyst
`temperatures would normally be low, for example under
`light load conditions. This can be achieved by positioning
`the catalyst close to the engine. If necessary, supplementary
`electrical heating of at least the upstream face of the catalyst,
`optionally assisted by or replaced by infra-red or visible
`wavelength radiation from a suitable source, may be used to
`ensure combustion of the soot particles d11rir1g those parts of
`the engine operating cycle at which the exhaust gas tem-
`perature and/or the catalyst temperature are below optimum.
`Certain other catalysts, eg zeolite-based catalysts, are not so
`sensitive as platinum-based catalysts to sulphur compound
`inhibition, and may be used if appropriate.
`The present invention as described herein may be modi-
`fied by the skilled person without departing from the inven-
`tive concepts.
`
`6
`
`

`
`US 6,294,141 B1
`
`3
`Initial tests confirming the benefits of the present inven-
`tion were carried out on a modified 1996 Audi 2.5TDI. A
`
`first catalyst which was a high loading (90 g/cu ft=3.18
`g/liter) platinum on a 4in><4 in metal honeycomb substrate of
`200 cells/inz carrying a conventional oxide washcoat, was
`mounted upstream of the standard catalyst. Soot build-up
`under soot-forming conditions was reduced.
`A further test was carried out by placing samples of
`standard diesel catalyst in the exhaust from a bench diesel
`engine for three hours. Visual inspection showed significant
`soot deposits. A 200 cells/inz (31 cells/cmz) metal honey-
`comb substrate carrying 70 g/ft3 (=2.47 g/liter) Pt was
`placed in front of the sooted catalyst and the engine was run
`again. After a further three hours, the catalyst was recovered.
`Visual inspection showed that the soot deposits were partly
`removed.
`Similar tests were carried out in which the visual differ-
`
`ence was not very marked, but there was a weight reduction
`indicating soot combustion. Activity of the second catalyst
`was improved compared to the sooted catalyst.
`The invention will now be described with reference to
`the accompanying drawings, in which:
`FIG. 1 is a schematic diagram of a catalyst system for a
`diesel engine according to the present invention,
`FIG. 2 is a chart showing N02 formation at different
`temperatures,
`FIG. 3 is a chart showing CO2 formation at different
`temperatures, and
`FIG. 4 is a chart showing weights of soot collected on a
`second catalyst.
`Referring to FIG. 1, a single canister, 1, for fitting in the
`exhaust system of a light-duty diesel engine, encloses a first
`catalyst, 2. Said first catalyst is a 200 cells/in2 (:62 cells/cmz)
`metal substrate catalyst, carrying an alumina washcoat and
`120 g/ft3 (4.24 g/liter) of Pt. There is a 1 cm gap between the
`first catalyst and a second catalyst which is a 400 cells/in2
`(124 cells/cmz) conventional commercial diesel oxidation
`catalyst. The first catalyst is half the length of the second
`catalyst, and this, together with the lower number of cells per
`unit area, means that the space velocity over the first catalyst
`is appreciably greater than over the second catalyst.
`In operation, NO emitted from the diesel engine is
`converted over the first catalyst
`to N02, according the
`equation
`
`2No+o,—>2No,
`
`Over the second catalyst, N02 reacts with carbonaceous soot
`particles trapped on the face of the second catalyst or in the
`cells, according to the equation
`
`ZNO2+2.C—>2CO2+N2
`
`A series of tests were carried out on a SCAT reactor
`
`(Simulated Car Activity Test) experimental rig. Samples of
`metal honeycomb catalyst substrate were washcoated with
`an alumina washcoat and loaded with different amounts of
`platinum by conventional impregnation technology, ranging
`from 70 to 150 g/ft3 (2.47—5.30 g/liter). The results are
`shown in FIG. 2 as % of total NOx converted to N02, plotted
`against temperature. Although, in general, the greater the Pt
`loading, the greater the conversion, the difference between
`the different Pt loadings was not dramatic.
`The SCAT reactor rig was used again to test the eifects of
`passing a synthetic exhaust gas comprising N02 over two
`samples of catalyst. One catalyst was as prepared, the other
`was a core cut from a catalyst which had been used in a
`diesel exhaust and therefore was sooted. CO2 detected
`
`10
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`4
`leaving the reactor is measured in ppm for both samples and
`is plotted in FIG. 3 against catalyst temperature. It can
`readily been seen that the sooted catalyst generates appre-
`ciably more CO2, indicating that the soot is being com-
`busted.
`A further test was carried out using a Volvo diesel engine
`on a rest bench. The diesel exhaust gas was split into two
`parallel streams and passed through a sample conventional
`oxidation catalyst and an identical catalyst sample preceded
`by a first catalyst carrying 120 g/ft3 (4.24 g/liter) of Pt. The
`back pressure on each stream was equalised and the engine
`was run under conditions which were known to generate
`soot, for four hours. The fuel used in the engine contained
`500 ppm of sulphur. The oxidation catalysts were weighed
`before and after the test and the results are shown in FIG. 4.
`The results are an average of three tests, and then the tests
`were repeated using a zero sulphur fuel. It can be seen that
`although the removal of sulphur from the diesel fuel reduces
`the amount of soot deposited on the catalyst, the effect of the
`first catalyst reduces the amount of soot by at least 50% by
`weight for sulphur-containing fuel and shows a greater
`improvement for sulphur-free fuel.
`What is claimed is:
`
`1. An emission control system for internal combustion
`engines which emit carbonaceous soot particles, which
`system comprising a first catalyst etfective to oxidize NOX
`to N02 and a second catalyst effective to oxidize
`hydrocarbons, carbon monoxide and volatile organic
`fractions, each catalyst being supported on a honeycomb
`flow-through monolith, whereby soot particles trapped on or
`within said second catalyst are combusted in the N02
`containing gas from said first catalyst, and wherein the first
`catalyst is supported on a flexible metal monolith of up to 31
`cells/cmz, whereby soot particle collection on the first cata-
`lyst support is minimized.
`2. A system according to claim 1, wherein said first
`catalyst contains 1.77 to 7.06 g/liter of catalyst volume, of
`platinum.
`3. A system according to claim 1, wherein said second
`catalyst is selected from oxidation catalysts and lean-NOx
`three-way catalysts.
`4. An internal combustion engine which emits carbon-
`aceous soot particles during at least part of the operating
`cycle, fitted with an emission control system according to
`claim 1.
`5. A light-duty diesel engine, fitted with an emission
`control system according to claim 1.
`6. In a process for the purification of exhaust gases from
`a light duty internal combustion engine which emits car-
`bonaceous soot particles by passing said gases over a
`honeycomb flow-through monolith and an oxidation catalyst
`effective at least to oxidize HC, CO and VOF, the improve-
`ment comprising first passing said gases over a first catalyst
`effective to oxidize NOx to N02 and subsequently passing
`the gas enriched with N02 over said honeycomb flow-
`through monolith and oxidation catalyst in order to enhance
`catalytic oxidation of soot particles trapped on or within said
`oxidation catalyst.
`7. The process of claim 6 wherein the gas containing N02
`also includes water vapor.
`8. The process of claim 6, wherein the oxidation catalyst
`comprises a platinum group metal.
`9. The process of claim 8, wherein the oxidation catalyst
`comprises a platinum group metal on a monolithic honey-
`comb.
`
`10. The process of claim 9, wherein the honeycomb flow
`through catalyst includes a catalyst which facilitates par-
`ticulate catalytic oxidation.
`
`7
`
`

`
`US 6,294,141 B1
`
`5
`11. The process of claim 10, wherein the oxidation
`catalyst is a three way catalyst.
`12. An emission control system for an internal combus-
`tion engine whieh emits carbonaceous soot particles, which
`system comprising a first catalyst elfective to oxidize NOX
`to N02 and a second catalyst etfective to oxidize
`hydrocarbons, carbon monoxide and volatile organic
`fractions, each catalyst being supported on a honeycomb
`flow-through monolith, the cell density and dimensions of
`said respective monoliths being selected to provide a greater
`space velocity over said first catalyst than over said second
`catalyst whereby soot particle collection on the first catalyst
`support is minimized and whereby soot particles trapped on
`or within said second catalyst are combusted in the N02
`containing gas from said first catalyst.
`13. A system according to claim 12, wherein said first
`catalyst contains 1.77 to 7.06 g/liter of catalyst Volume, of
`platinum.
`
`10
`
`6
`14. Asystem according to claim 12, wherein said second
`catalyst is selected from oxidation catalysts and lean-NOX
`three-way catalysts.
`15. An internal combustion engine which emits carbon-
`aceous soot particles during at least part of the operating
`cycle, fitted with an emission control system according to
`claim 12.
`
`16. A light-duty diesel engine, fitted with an emission
`control system according to claim 12.
`17. A system according to claim 12, wherein said first
`catalyst support is a flexible metal monolith.
`18. A system in accordance with claim 17, wherein the
`cell density of said first monolith is up to 200 cells per square
`inch.
`19. A system in accordance with claim 18, wherein the
`cell density of at least 400 cells per square inch.
`*
`*
`*
`*
`*
`
`8