Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
1997-10-14
2001-09-25
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S212000, C423S213200, C423S213500, C423S213700, C423S235000, C423S239100, C422S169000, C422S171000, C428S116000, C060S272000, C060S274000, C060S282000, C060S299000
Reexamination Certificate
active
06294141
ABSTRACT:
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 monoxide (“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 normal 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 Continuously Regenerating Trap (“CRT”). This patent teaches a system involving a filter to remove soot, and combusting the soot using a gas containing NO
2
. Such a gas is produced by fitting a catalyst upstream of the filter, to oxidise nitric oxide present in the exhaust gas to NO
2
. 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 system for internal combustion engines which emit carbonaceous particulates, particularly for diesel, especially light duty diesel, engines, said system comprising a first catalyst effective to oxidise NO to NO
2
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 NO
2
-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 NO
2
oxidation and is suitably a relatively high loading platinum catalyst. Such a catalyst may desirably have from about 50 to about 200 g Pt/ft
3
(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 flexing 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/in
2
(62 cells/cm
2
) or higher, that is to say tend preferably to 600 cells/in
2
(93 cells/cm
2
). Such a monolith may be, for example, 100 or 200 cells/in
2
(15.5 or 31 cells/cm
2
). 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/in
2
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 N
2
, perhaps intermittently by the mechanism of NOx storage on components in the catalyst or by continuous regeneration of a selective catalyst.
The first and/or second catalysts may incorporate trapping 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 NO
2
, but other oxidised nitrogen oxides may be produced. The necessary reaction is that such product NO
2
or oxidised nitrogen oxides contribute to the combustion of the soot particles, and, for ease of reference, the designation NO
2
is used herein. The practical requirement is that sufficient NO
2
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 NO
2
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 during those parts of the engine operating cycle at which the exhaust gas temperature 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 modified by the skilled person without departing from the inventive concepts.
Initial tests confirming the benefits of the present invention 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/in
2
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/in
2
(31 cells/cm
2
) metal honeycomb substrate carrying 70 g/ft
3
(=2.47 g/liter) Pt was placed in front of the soote
Twigg Martyn Vincent
Wilkins Anthony John Joseph
Will Nigel Simon
Griffin Steven P.
Johnson Matthey Public Limited Company
Ratner & Prestia
Strickland Jonas N.
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