U.S. patent application number 14/845854 was filed with the patent office on 2015-12-31 for oxidation catalyst for a lean burn internal combustion engine.
This patent application is currently assigned to JOHNSON MATTHEY PUBLIC LIMITED COMPANY. The applicant listed for this patent is Johnson Matthey Public Limited Company. Invention is credited to David BERGEAL, Andrew Francis CHIFFEY, Marie FEUERSTEIN, Paul Richard PHILLIPS, Wolfgang STREHLAU, Daniel SWALLOW, James WYLIE.
Application Number | 20150375221 14/845854 |
Document ID | / |
Family ID | 43598924 |
Filed Date | 2015-12-31 |
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United States Patent
Application |
20150375221 |
Kind Code |
A1 |
BERGEAL; David ; et
al. |
December 31, 2015 |
Oxidation Catalyst for a Lean Burn Internal Combustion Engine
Abstract
An apparatus is disclosed. The apparatus comprises a lean-burn
internal combustion engine, engine management means and an exhaust
system for treating exhaust gas of the engine. The exhaust system
comprises a first oxidation catalyst disposed on a first honeycomb
monolith substrate. The first oxidation catalyst comprises platinum
supported on a first metal oxide support comprising at least one
reducible oxide, and is substantially free of alkali metals and
alkaline earth metals. The engine management means is arranged,
when in use, intermittently to modulate the lambda composition of
the exhaust gas entering the first oxidation catalyst to a rich
lambda composition.
Inventors: |
BERGEAL; David; (Ware,
GB) ; CHIFFEY; Andrew Francis; (Ware, GB) ;
FEUERSTEIN; Marie; (Royston, GB) ; PHILLIPS; Paul
Richard; (Royston, GB) ; STREHLAU; Wolfgang;
(Sulzbach, DE) ; SWALLOW; Daniel; (Sandy, GB)
; WYLIE; James; (Royston, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Johnson Matthey Public Limited Company |
London |
|
GB |
|
|
Assignee: |
JOHNSON MATTHEY PUBLIC LIMITED
COMPANY
London
GB
|
Family ID: |
43598924 |
Appl. No.: |
14/845854 |
Filed: |
September 4, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13996575 |
Sep 11, 2013 |
9140167 |
|
|
PCT/GB2011/052547 |
Dec 21, 2011 |
|
|
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14845854 |
|
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61425464 |
Dec 21, 2010 |
|
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Current U.S.
Class: |
502/34 |
Current CPC
Class: |
B01D 53/944 20130101;
B01D 2255/1021 20130101; B01J 35/04 20130101; F01N 3/2803 20130101;
B01D 2255/20761 20130101; B01D 2255/2094 20130101; B01J 23/96
20130101; B01J 37/0244 20130101; B01J 38/04 20130101; B01D
2255/20746 20130101; B01D 2255/2073 20130101; B01J 23/40 20130101;
B01J 29/7415 20130101; B01D 53/9477 20130101; B01D 2258/012
20130101; B01J 23/63 20130101; B01J 29/7007 20130101; B01J 35/0006
20130101; B01J 37/0246 20130101; B01J 29/90 20130101; F02D 41/0275
20130101; B01J 37/0225 20130101; B01D 53/96 20130101; B01D
2255/2065 20130101; B01D 2255/20738 20130101 |
International
Class: |
B01J 38/04 20060101
B01J038/04; B01J 35/04 20060101 B01J035/04; B01J 35/00 20060101
B01J035/00; B01J 29/90 20060101 B01J029/90; B01J 29/74 20060101
B01J029/74 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 23, 2010 |
GB |
1021887.3 |
Claims
1-26. (canceled)
27. A method of recovering an oxidation activity of a first
oxidation catalyst aged in an exhaust gas of a lean-burn internal
combustion engine, which first oxidation catalyst comprising
platinum supported on a first metal oxide support and disposed on a
honeycomb substrate monolith, which method comprising the step of
intermittently contacting the first oxidation catalyst with exhaust
gas modulated to a rich lambda composition, wherein the first metal
oxide support comprises at least one reducible oxide and wherein
the first oxidation catalyst is substantially free of alkali metals
and alkaline earth metals.
28. The method according to claim 27, wherein the at least one
reducible oxide is selected from the group consisting of oxides,
composite oxides and mixed oxides of one or more metal selected
from the group consisting of manganese, iron, tin, copper, cobalt
and cerium, and stabilised homologues thereof.
29. The method according to claim 28, wherein the at least one
reducible oxide comprises at least one of MnO.sub.2,
Mn.sub.2O.sub.3, Fe.sub.2O.sub.3, SnO.sub.2, CuO, CoO and
CeO.sub.2.
30. The method according to claim 28, wherein the stabilised
homologue of CeO.sub.2 comprises zirconia, at least one non-cerium
rare earth oxide or both zirconia and at least one non-cerium rare
earth oxide.
31. The method according to claim 28, wherein the first metal oxide
support consists essentially of bulk at least one reducible oxide
or optionally stabilised homologues thereof.
32. The method according to claim 28, wherein the at least one
reducible oxide or optionally stabilised homologue thereof is
supported on the first metal oxide support with the platinum.
33. The method according to claim 27, wherein the first oxidation
catalyst disposed on the monolith substrate has a platinum group
metal loading of >10 g/ft.sup.3.
34. The method according to claim 27, wherein the first oxidation
catalyst comprises palladium supported on the first metal oxide
support in combination with the platinum.
35. The method according to claim 27, wherein the first oxidation
catalyst comprises at least one molecular sieve.
36. The method according to claim 35, wherein the at least one
molecular sieve comprises at least one precious metal and either
(i) copper, (ii) iron, or (iii) copper and iron.
37. The method according to claim 27, wherein the first oxidation
catalyst is combined with a second oxidation catalyst different
from the first oxidation catalyst, which second oxidation catalyst
comprises at least one precious metal supported on a second metal
oxide support.
38. The method according to claim 27, wherein the first honeycomb
monolith substrate, and, where present, a second honeycomb monolith
substrate, is disposed upstream of a catalysed filter for filtering
particulate matter from the exhaust gas.
39. The method according to claim 27, wherein the first honeycomb
substrate monolith is disposed upstream of a monolith substrate
comprising a catalyst for selectively reducing oxides of nitrogen
using a nitrogenous reductant.
40. The method according to claim 39, wherein the monolith
substrate is a flow-through monolith substrate.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is the U.S. National Phase application of
PCT International Application No. PCT/GB2011/052547, filed Dec. 21,
2011, and claims priority benefit of U.S. Provisional Patent
Application No. 61/425,464, filed Dec. 21, 2010 and Great Britain
Patent Application No. 1021887.3, filed Dec. 23, 2010, the
disclosures of all of which are incorporated herein by reference in
their entireties for all purposes.
FIELD OF THE INVENTION
[0002] The present invention relates to an oxidation catalyst for a
lean-burn internal combustion engine and in particular it relates
to recovering catalytic oxidation activity following ageing in
engine exhaust gas. The oxidation catalyst has particular
application in the treatment of exhaust gas from vehicular internal
combustion engines.
BACKGROUND TO THE INVENTION
[0003] A primary purpose of a Diesel Oxidation Catalyst (DOC) is to
oxidise certain components of Diesel engine exhaust gas in order to
meet a relevant emission standard, such as vehicular regulations
including Euro 5. Particularly important reactions include
oxidation of carbon monoxide to carbon dioxide, oxidation of gas
phase hydrocarbons (derived from unburned fuel) to carbon monoxide
and water (H.sub.2O) and--for Diesel exhaust gas--oxidation of the
liquid soluble organic fraction (SOF) of Diesel particulate matter,
which is derived from unburned fuel and lubricating oils.
[0004] A conventional Diesel oxidation catalyst for use in treating
exhaust gas emitted from a vehicle comprises a noble metal, such as
platinum or a mixture of platinum and palladium, supported on an
inert high surface area refractory metal oxide, such as optionally
stabilised alumina.
[0005] While platinum is particularly active amongst precious
metals for promoting oxidation reactions, because it is better able
to remain in its active metallic form, rather than the less active
oxide form, following extended exposure to relatively high
temperature lean-burn internal combustion engine exhaust gases,
such as those encountered in a so-called "close coupled" position
of Diesel-fuelled compression ignition engines, platinum can become
oxidised. (The close coupled position is generally where an inlet
to a monolith substrate carrying a catalyst is at <75 cm, such
as .ltoreq.50 cm downstream of an engine exhaust manifold).
[0006] NO.sub.x absorber catalysts (NACs) are known e.g. from U.S.
Pat. No. 5,473,887 and are designed to adsorb nitrogen oxides
(NO.sub.x) from lean exhaust gas (lambda>1) and to desorb the
NO.sub.x when the oxygen concentration in the exhaust gas is
decreased. According to US '887, desorbed NO.sub.x may be reduced
to N.sub.2 with a suitable reductant, e.g. gasoline fuel, promoted
by a catalyst component, such as rhodium, of the NAC itself or
located downstream of the NAC. In practice, control of oxygen
concentration can be adjusted to a desired redox composition
intermittently in response to a calculated remaining NO.sub.x
adsorption capacity of the NAC, e.g. richer than normal engine
running operation (but still lean of stoichiometric),
stoichiometric (i.e. lambda=1 composition) or rich of
stoichiometric (lambda<1). It is known that oxygen concentration
can be adjusted by a number of means, e.g. throttling, injection of
additional hydrocarbon fuel into an engine cylinder such as during
the exhaust stroke or injecting hydrocarbon fuel directly into
exhaust gas downstream of an engine manifold.
[0007] A typical NAC formulation includes a catalytic oxidation
component, such as platinum, a significant quantity, i.e.
substantially more than is required for use as a promoter such as a
promoter in a TWC, of a NO.sub.x-storage component, such as barium,
and a reduction catalyst, e.g. rhodium. One mechanism commonly
given for NO.sub.x-storage from a lean exhaust gas for this
formulation is:
NO+1/2O.sub.2.fwdarw.NO.sub.2 (1); and
BaO+NO.sub.2+1/2O.sub.2.fwdarw.Ba(NO.sub.3).sub.2 (2),
wherein in reaction (2), the nitric oxide reacts with oxygen on
active oxidation sites on the platinum to form NO.sub.2. Reaction
(3) involves adsorption of the NO.sub.2 by the storage material in
the form of an inorganic nitrate.
[0008] At lower oxygen concentrations and/or at elevated
temperatures, the nitrate species become thermodynamically unstable
and decompose, producing NO or NO.sub.2 according to reaction (3)
below. In the presence of a suitable reductant, these nitrogen
oxides are subsequently reduced by carbon monoxide, hydrogen and
hydrocarbons to N.sub.2, which can take place over the reduction
catalyst (see reaction (4)).
Ba(NO.sub.3).sub.2.fwdarw.BaO+2NO+ 3/2O.sub.2 or
Ba(NO.sub.3).sub.2.fwdarw.BaO+2NO.sub.2+1/2O.sub.2 (3); and
NO+CO.fwdarw.1/2N.sub.2+CO.sub.2 (4);
(Other reactions include
Ba(NO.sub.3).sub.2+8H.sub.2.fwdarw.BaO+2NH.sub.3+5H.sub.2O followed
by NH.sub.3+NO.sub.x.fwdarw.N.sub.2+yH.sub.2O or
2NH.sub.3+2O.sub.2+CO.fwdarw.N.sub.2+3H.sub.2O+CO.sub.2 etc.).
[0009] In the reactions of (1)-(4) above, the reactive barium
species is given as the oxide. However, it is understood that in
the presence of air most of the barium is in the form of the
carbonate or possibly the hydroxide. The skilled person can adapt
the above reaction schemes accordingly for species of barium other
than the oxide and sequence of catalytic coatings in the exhaust
stream.
[0010] Generally, conventional DOCs do not have sufficient activity
at low temperatures for advanced future Diesel engines, such as
HCCI engines. Exhaust gas from future Diesel engines are projected
to have exhaust gas temperatures at least 50.degree. C. lower than
that of Diesel engines found in today's commercially available,
Euro 5-compliant vehicles. Therefore, substantially improved
"light-off" for HC and CO oxidation would be desirable.
Conventional Diesel oxidation catalysts generally use the platinum
group metal (PGM) Pt, or a combination of both Pt and Pd, each
supported on high surface area metal oxide supports such as
alumina, silica-alumina, zirconia, titania, or mixtures
thereof.
[0011] By "light-off" herein we mean the temperature at which a
catalyst catalyses a reaction at a desired conversion activity. For
example, "CO T.sub.50" is a temperature at which the catalyst
catalyses the conversion of carbon monoxide in a feed gas e.g. to
carbon dioxide at least 50% efficiency. Similarly, "HC T.sub.80" is
the temperature at which hydrocarbon, perhaps a particular
hydrocarbon such as octane or propene, is converted e.g. to water
(steam) and carbon dioxide at 80% efficiency or greater.
[0012] A problem with these commercially available catalyst is that
a lower, less active, dispersion of PGM is obtained after exposure
of the catalyst to higher exhaust gas temperatures, e.g.
>300.degree. C. Such lower PGM dispersion, caused by sintering,
causes loss of active catalyst sites for the hydrocarbon and carbon
monoxide oxidation reactions, and therefore typically temperatures
of more than 150.degree. C. are needed to reach complete light-off
in Diesel (compression ignition) applications. The combination of
Pt with Pd, possibly as an alloy, can desirably reduce Pt
sintering. However, this can increase catalyst costs and can reduce
Pt's renowned fresh oxidation activity (Pt is most active in its
metallic state, whereas Pd is more easily oxidised).
[0013] U.S. Pat. No. 5,627,124 discloses a DOC comprising a
relatively low loading (.ltoreq.2 g/ft.sup.3) of platinum supported
on stabilized alumina and ceria in approximately equal proportions.
Alumina and ceria may be mixed together to form one layer, or may
be applied as two separate washcoat layers. According to the
specification, the ceria component is active for SOF oxidation. The
Pt oxidises gas phase hydrocarbon and carbon monoxide. Specific
examples in US '124 comprise Pt loaded on a gamma-alumina
underlayer and a mixture of gamma-alumina alumina-stabilized ceria
(2.5% Al.sub.2O.sub.3) in a top layer.
[0014] WO 2004/076829 discloses the use of Pt/ceria or a
Pt/ceria-zirconia mixed oxide as a thermally regenerable NO.sub.x
storage catalyst, i.e. no periodic changing of the air/fuel mixture
fed to the internal combustion engine to rich air/fuel mixtures is
necessary; reaction (4) is not actively used.
[0015] WO 01/19500 discloses regenerating a sulphur poisoned Diesel
catalyst by modulating the air/fuel ratio (lambda) to 0.90 or
richer for a time which is in aggregate sufficient to cause release
of significant quantities of sulphur-containing species from the
catalyst or catalyst components, whereby the catalyst is
regenerated. The regeneration can be carried out using pulses of
air/fuel ratio modulation from 250 milliseconds to 5 seconds in
duration. The specific examples mention a platinum-based oxidation
catalyst at 90 g/ft.sup.3 loading, but there is no disclosure of
any support material on which the catalyst may be supported.
[0016] WO 2004/025093 discloses a compression ignition engine
operable in a first, normal running mode and a second mode
producing exhaust gas comprising an increased level of carbon
monoxide (CO) relative to the first mode and means when in use to
switch engine operation between the two modes, which engine
comprising an exhaust system comprising a supported palladium (Pd)
catalyst associated with at least one base metal promoter and an
optionally supported platinum (Pt) catalyst associated with and/or
downstream of the Pd catalyst wherein CO is oxidised by the
supported Pd catalyst during second mode operation. The only
disclosure of second running mode producing rich exhaust gas, i.e.
lambda<1, is wherein the catalyst comprises a NO.sub.x
absorber.
[0017] S. E. Golunski et al. published an academic paper entitled
"Origins of low-temperature three-way activity in Pt/CeO.sub.2" in
Applied Catalysis B: Environmental 5 (1995) 367-376.
[0018] US 2010/0221161 discloses an device for the purification of
Diesel exhaust gases, which device comprises, in the flow direction
of the exhaust gas, an oxidation catalyst, a Diesel particle filter
with catalytically active coating, and downstream of a device for
introducing a reducing agent from an external reducing agent
source, and SCR catalyst. The oxidation catalyst and the
catalytically active coating of the Diesel particle filter contain
palladium and platinum. The ratio of the noble metals platinum and
palladium in the overall system and on the individual components,
oxidation catalyst and catalytically coated Diesel particle filter,
are coordinated with one another in such a way as to obtain firstly
an optimum NO/NO.sub.2 ratio in the exhaust gas upstream of the
downstream SCR catalyst, and secondly optimum heating and HC
conversion behaviour during an active particle filter
regeneration.
[0019] We have now discovered, very surprisingly that by contacting
an oxidation catalyst comprising platinum and a reducible oxide
intermittently and momentarily with a rich exhaust gas, the
oxidation catalyst can recover oxidation activity caused by the
platinum becoming oxidised at higher temperatures.
SUMMARY OF THE INVENTION
[0020] The invention is an apparatus that comprises a lean-burn
internal combustion engine, engine management means and an exhaust
system for treating exhaust gas of the engine. The exhaust system
comprises a first oxidation catalyst disposed on a first honeycomb
monolith substrate. The first oxidation catalyst comprises platinum
supported on a first metal oxide support comprising at least one
reducible oxide, and is substantially free of alkali metals and
alkaline earth metals. The engine management means is arranged,
when in use, intermittently to modulate the lambda composition of
the exhaust gas entering the first oxidation catalyst to a rich
lambda composition. The invention also includes a method of
recovering the oxidation activity of the first oxidation catalyst
aged in an exhaust gas of a lean-burn internal combustion
engine.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a graph showing the accumulated CO, both
engine-out and post-catalyst, for a Conventional DOC (not according
to the invention) fitted to a bench-mounted 2.0 litre Diesel engine
run over the MVEG-B European emission test cycle, both before and
after activation;
[0022] FIG. 2 is a graph showing accumulated CO, both engine-out
and post-catalyst, for a DOC according to the invention fitted to
the engine used to test the Conventional DOC in FIG. 2 and run over
the same cycle, both before and after activation; and
[0023] FIG. 3 shows a series of graphs displaying the cumulative CO
detected at engine-out (top trace) and post-catalyst (bottom trace)
on a different 2.0 litre Diesel engine from the one used in FIGS. 1
and 2, including the effects of "in-cycle" activation.
DETAILED DESCRIPTION OF THE INVENTION
[0024] According to one aspect, the invention provides an apparatus
comprising a lean-burn internal combustion engine, engine
management means and an exhaust system for treating exhaust gas of
the engine, which exhaust system comprising a first oxidation
catalyst disposed on a first honeycomb monolith substrate, which
first oxidation catalyst comprising platinum supported on a first
metal oxide support comprising at least one reducible oxide,
wherein the first oxidation catalyst is substantially free of
alkali metals and alkaline earth metals, wherein the engine
management means is arranged, when in use, intermittently to
modulate the lambda composition of the exhaust gas contacting the
first oxidation catalyst to a rich lambda composition.
[0025] As used herein the term "bulk" to refer to a reducible oxide
such as ceria (or any other component) means that the ceria is
present as solid particles thereof. These particles are usually
very fine, of the order of at least 90 percent of the particles
being from about 0.5 to 15 microns in diameter. The term "bulk" is
intended to distinguish from the situation in which ceria is
"dispersed" on a refractory support material e.g. by being
impregnated into the support material from a solution e.g. cerium
nitrate or some other liquid dispersion of the component and then
dried and calcined to convert the impregnated cerium nitrate to a
dispersion of ceria particles on a surface of the refractory
support. The resultant ceria is thus "dispersed" onto and, to a
greater or lesser extent, within a surface layer of the refractory
support. The dispersed ceria is not present in bulk form, because
bulk ceria comprises fine, solid particles of ceria. The dispersion
can also take the form of a sol, i.e. finely divided particles of
e.g. ceria on the nanometer scale.
[0026] While it is known from exhaust systems comprising NACs, such
as are disclosed in U.S. Pat. No. 5,473,887 and WO 2004/025093,
intermittently to contact the NAC with rich exhaust gas in order to
regenerate the NAC, i.e. to release and reduce stored NO.sub.x, NAC
technology is excluded from the present invention because the
oxidation catalyst is substantially free of alkali metals and
alkaline earth metals.
[0027] The arrangement of WO 2004/076829 is also excluded because
in that system NO.sub.x is thermally desorbed during lean operation
and a rich exhaust gas is not used to regenerate the NO.sub.x
storage catalyst.
[0028] The processes of producing an optimum NO/NO.sub.2 ratio in
the exhaust gas upstream of the downstream SCR catalyst, and of
optimum heating and HC conversion behaviour during an active
particle filter regeneration of US 2010/0221161 are done using an
overall lean exhaust gas and not a rich exhaust gas.
[0029] Methods of providing intermittently rich exhaust gas by
adjusting fuel injection timing in one or more engine cylinders are
known from publications disclosing exhaust systems comprising NACs,
such as those disclosed in U.S. Pat. No. 5,473,887. Alternatively,
also as discussed in U.S. Pat. No. 5,473,887, the engine management
means may be adapted to inject hydrocarbon fuel directly into the
exhaust system carrying the exhaust gas, i.e. downstream of the
engine exhaust manifold.
[0030] In embodiments, the proposed new catalyst comprises a
multiple-layer DOC, with at least one layer desirably, though not
essentially, exhibiting SMSI (Strong Metal Support Interaction)
characteristics (see (1) hereinbelow) and at least one layer being
a "conventional" DOC as described hereinabove (see also (3)
hereinbelow). The catalyst may be part of a system configuration
which is configured to provide rich events (see (3) hereinbelow):
[0031] (1) By supporting PGM on a first metal oxide support such as
ceria, the degree of PGM sintering can be significantly reduced and
therefore a large number of catalyst active sites remain after
thermal ageing. However each catalyst site is not very active
because of the oxidic characteristic of the Pt or PtPd catalyst
site. The most active form of the PGM is its metallic state and we
have found, very surprisingly, that for catalysts according to the
invention the active form is substantially regenerated when it is
exposed to rich conditions. [0032] (2) Ceria and ceria-zirconia
have been used in NO.sub.x traps in combination with other metals
such as K, Cs, Ba, Sr for improving NO.sub.x storage functions.
However, in the present invention, such additional materials are
preferably not used because of their inhibiting effect for HC and
CO oxidation. [0033] (3) The conventional layer is used to maintain
acceptable catalytic activity when it is not possible to perform a
rich event when planned. For example, the majority of the oxidation
activity will occur on at least one of the layers after prolonged
exposure to relatively high lean temperature.
[0034] In order to fully use the attributes of the catalyst
according to the invention, the catalyst may be activated by
exposure to rich exhaust gas conditions for a short period of time.
Following the rich activation step, the catalyst remains active in
the normal lean operating conditions for a substantial duration.
Exposure of the catalyst to high temperature lean conditions can
deactivate the catalyst, and therefore a predetermined algorithm is
used to determine when catalyst re-activation is required.
[0035] Illustrative examples of reducible oxides are oxides,
composite oxides and mixed oxides of at least one metal selected
from the group consisting of manganese, iron, tin, copper, cobalt
and cerium, such as at least one of MnO.sub.2, Mn.sub.2O.sub.3,
Fe.sub.2O.sub.3, SnO.sub.2, CuO, CoO and CeO.sub.2. It is believed
that at least one of these reducible oxides exhibit SMSI activity
supporting platinum for oxidation hydrocarbons and carbon monoxide.
The reducible oxide can be dispersed on a suitable support and/or
the support per se can comprise particulate bulk reducible oxide.
An advantage of, e.g. CeO.sub.2, is that it is relatively thermally
stable, but it is susceptible to sulphur poisoning. Manganese
oxides are not as thermally stable, but they are more resistant to
sulphur poisoning. Manganese oxide thermal stability can be
improved by combining it in a composite oxide or mixed oxide with a
stabiliser, such as zirconium. To some extent, ceria can be made
more sulphur tolerant by forming a composite oxide or a mixed oxide
with a suitable stabiliser, such as zirconium and/or a rare earth
metal (other than cerium).
[0036] By "reducible oxide" herein, we mean that an oxide is
present in situ wherein the metal has more than one oxidation
state. In manufacture, the metal can be introduced as a non-oxide
compound and oxidised by calcinations to the reducible oxide.
[0037] "Composite oxide" as defined herein means a largely
amorphous oxide material comprising oxides of at least two elements
which are not true mixed oxides consisting of the at least two
elements.
[0038] Where the reducible oxide comprises, CeO.sub.2 the ceria can
be stabilised with zirconia, at least one non-cerium rare earth
oxide or both zirconia and at least one non-cerium rare earth
oxide. For example, a ceria-containing mixed oxide can include 86%
by weight ceria, 10% by weight zirconia and lanthana balance; or
80%, ceria, 10% zirconia, 3% lanthana, 7% praseodymia; or 65%
ceria, 27% zirconia and 8% praseodymia.
[0039] In a particular embodiment, the first metal oxide support
consists essentially of bulk at least one reducible oxide or
optionally stabilised homologues thereof. Alternatively, the at
least one reducible oxide or optionally stabilised homologue
thereof can be supported, i.e. as a dispersion on the first metal
oxide support with the platinum.
[0040] In addition to platinum, preferably the first oxidation
catalyst can comprise palladium supported on the first metal oxide
support. However, in a one embodiment, the first oxidation catalyst
is substantially free of palladium.
[0041] Platinum group metals, in the first oxidation catalyst can
be loaded on the monolith substrate at any suitable loading, such
as >10 gft.sup.-3. For catalysts where the platinum group metal
consists of platinum, the platinum loading can be 10-120 gft.sup.-3
and is preferably 30-60 gft.sup.-3. In preferred embodiments
comprising Pt and Pd the total platinum group metal loading can be
from 15 to 300 gft.sup.-3, such as from 30 to 150 gft.sup.-3, e.g.
40 to 120 gft.sup.-3.
[0042] The first oxidation catalyst can comprise one or more
molecular sieve, e.g. aluminosilicate zeolites. A duty of the
molecular sieve in the first oxidation catalyst is for improving
hydrocarbon conversion over a duty cycle by storing hydrocarbon
following cold start or during cold phases of a duty cycle and
releasing stored hydrocarbon at higher temperatures when associated
platinum group metal catalyst components are more active for HC
conversion. See for example Applicant/Assignee's EP 0830201.
Molecular sieves are typically used in catalyst compositions
according to the invention for light-duty Diesel vehicles, whereas
they are rarely used in catalyst compositions for heavy duty Diesel
applications because the exhaust gas temperatures in heavy duty
Diesel engines mean that hydrocarbon trapping functionality is
generally not required. Where the first oxidation catalyst
according to the invention comprises two or more layers, it is
highly preferred that at least one of the two or more layers
includes molecular sieve. Most preferably, in a two layer
embodiment, both the first (or bottom) layer and the second (top)
layer include molecular sieve.
[0043] However, molecular sieves such as aluminosilicate zeolites
are not particularly good supports for platinum group metals
because they are mainly silica, particularly relatively higher
silica-to-alumina molecular sieves, which are favoured for their
increased thermal durability: they may thermally degrade during
ageing so that a structure of the molecular sieve may collapse
and/or the PGM may sinter, giving lower dispersion and consequently
lower HC and/or CO conversion activity. Accordingly, in a preferred
embodiment, the first oxidation catalyst comprises a molecular
sieve at .ltoreq.30% by weight (such as .ltoreq.25% by weight,
.ltoreq.20% by weight e.g. .ltoreq.15% by weight) of the individual
washcoat coating layer. In addition to the platinum supported on a
first metal oxide support comprising at least one reducible oxide,
the composition of the first oxidation catalyst may also comprise
at least one metal oxide selected from the group consisting of
optionally stabilised alumina, amorphous silica-alumina, optionally
stabilised zirconia, titania and mixtures of any two or more
thereof.
[0044] Preferred molecular sieves for use as support
materials/hydrocarbon adsorbers are medium pore zeolites,
preferably aluminosilicate zeolites, i.e. those having a maximum
ring size of ten tetrahedral atoms, and large pore zeolites
(maximum ring size of twelve tetrahedral atoms) preferably
aluminosilicate zeolites, include natural or synthetic zeolites
such as faujasite, clinoptilolite, mordenite, silicalite,
ferrierite, zeolite X, zeolite Y, ultrastable zeolite Y, ZSM-5
zeolite, ZSM-12 zeolite, SSZ-3 zeolite, SAPO-5 zeolite, offretite
or a beta zeolite, preferably ZSM-5, beta and Y zeolites. Preferred
zeolite adsorbent materials have a high silica to alumina ratio,
for improved hydrothermal stability. The zeolite may have a
silica/alumina molar ratio of from at least about 25/1, preferably
at least about 50/1, with useful ranges of from about 25/1 to
1000/1, 50/1 to 500/1 as well as about 25/1 to 100/1, 25/1 to
300/1, from about 100/1 to 250/1.
[0045] A recognised problem with the use of ceria or ceria-based
components in oxidation catalysts is that its activity can be
compromised if it becomes sulphated with sulphur-containing species
present in engine fuel or engine lubricant. The present inventors
propose to reduce or prevent this effect by including at least one
component in the first oxidation catalyst which acts as a "sink"
for the sulphur, i.e. a component which disproportionately adsorbs
sulphur from exhaust gas relative to other components of the first
oxidation catalyst thereby reducing or preventing sulphur poisoning
of active components of the first oxidation catalyst. Preferably
this "sulphur sink" is regenerable, i.e. it is possible to release
sulphur species absorbed (or adsorbed) thereon so that the capacity
of the "sulphur sink" is not finite.
[0046] In preferred embodiments, the "sulphur sink" component
comprises a molecular sieve comprising copper and/or iron, which
molecular sieve is preferably an aluminosilicate zeolite. The
copper and/or iron can be impregnated, ion exchanged or present in
a lattice structure of the molecular sieve per se. In a
particularly preferred embodiment, the copper and/or iron
containing molecular sieve also comprises one or more platinum
group metal, preferably palladium, at relatively low loading, e.g.
.ltoreq.30 gft.sup.-3, which one or more platinum group metal may
be impregnated as an aqueous salt thereof onto the copper and/or
iron-containing molecular sieve. It has been found that the
molecular sieve comprising copper and/or iron, optionally also
comprising platinum group metals, are regenerable when in use, for
example when exposed to lean exhaust gas at temperatures of up to
about 650.degree. C. Such conditions may occur when the first
oxidation catalyst is disposed upstream of a catalysed filter and a
filter regeneration event is triggered (see also below for further
explanation). A primary purpose of the filter regeneration event is
to combust particulate matter held on the filter, and for this
purpose the exhaust gas temperature is raised. However, since the
filter regeneration event requires the temperature to be raised, as
a beneficial side-effect, the first oxidation catalyst is exposed
to lean, high temperature exhaust gas which can cause
sulphur-containing species absorbed on the sulphur sink component
of the first oxidation catalyst to be desorbed.
[0047] Where a molecular sieve is included as a "sulphur sink", the
first oxidation catalyst preferably comprises two layers, wherein a
first (or lower) layer comprises the sulfur sink component and a
second (or upper) layer comprises the platinum supported on a first
metal oxide support comprising at least one reducible oxide.
[0048] Preferably, quantities of the copper and/or iron-containing
molecular sieve-based sulfur sink component are present in the
first oxidation catalyst. e.g. the lower layer, at .ltoreq.50% by
weight of the first oxidation catalyst as a whole. This is in
addition to any molecular sieve hydrocarbon trap component,
mentioned hereinabove. So, where the first oxidation catalyst
comprises two layers, with a lower layer of the copper and/or
iron-based molecular sieve-based sulfur sink component at a
washcoat loading of 2.0 gin.sup.-3 and an upper layer comprising
the platinum supported on a first metal oxide support comprising at
least one reducible oxide and a molecular sieve for hydrocarbon
trapping, which upper layer is also at a washcoat loading of 2.0
gin.sup.-3, the total molecular sieve content of the first
oxidation catalyst, i.e. the lower and upper layers of the catalyst
combined, can be .ltoreq.65 wt % (i.e. .ltoreq.50 wt/o sulfur sink
molecular sieve and 515 wt %, hydrocarbon trap molecular sieve (but
.ltoreq.30 wt % hydrocarbon trap molecular sieve in the upper layer
alone). Alternatively, the sulfur sink component and the platinum
supported on a first metal oxide support comprising at least one
reducible oxide and optional hydrocarbon trap molecular sieve
components can be mixed and applied in a single washcoat layer.
[0049] Molecular sieves with particular application in this
"sulphur sink" aspect of the present invention include any of those
mentioned hereinabove and include also small pore molecular sieves,
i.e. those having a maximum ring size of eight tetrahedral atoms.
Molecular sieves for use in embodiments include a beta zeolite, a
faujasite (such as an X-zeolite or a Y-zeolite, including NaY and
USY), a L-zeolite, a ZSM zeolite (e.g., ZSM-5, ZSM-48), a
SSZ-zeolite (e.g., SSZ-13, SSZ-41, SSZ-33), a mordenite, a
chabazite, an offretite, an erionite, a clinoptilolite, a
silicalite, an aluminium phosphate zeolite (including
metalloaluminophosphates such as SAPO-34), a mesoporous zeolite
(e.g., MCM-41, MCM-49, SBA-15), a metal-incorporated zeolite, or
mixtures thereof; more preferably, the zeolites are beta zeolite,
ZSM-5 zeolite, Fe-3 zeolite, or SSZ-33, or Y-zeolite. The zeolite
is most preferably beta zeolite, ZSM-5 zeolite, Fe-3 zeolite, or
SSZ-33. Most preferred is Fe/Beta zeolite, optionally supporting
palladium.
[0050] In a preferred embodiment, the first oxidation catalyst is
combined with a second oxidation catalyst different from the first
oxidation catalyst, which second oxidation catalyst comprises at
least one precious metal supported on a second metal oxide
support.
[0051] In one such arrangement, the first and second oxidation
catalysts are each disposed in separate layers. In a particular
embodiment, the second oxidation catalyst is disposed on the first
honeycomb monolith substrate in an underlayer and the first
oxidation catalyst is disposed in a layer overlying the second
oxidation catalyst (either directly on the underlayer or an
intermediate layer(s) is interposed therebetween). However, the
order of the first and second oxidation catalyst can be reversed,
as desired.
[0052] In an alternative arrangement, the metal oxide support of
the first oxidation catalyst and the metal oxide support of the
second oxidation catalyst are combined in a single layer disposed
on the first honeycomb monolith substrate.
[0053] In a further alternative embodiment, the first oxidation
catalyst is located in a first zone disposed on the first honeycomb
monolith substrate and the second oxidation catalyst is located in
a second zone on the first honeycomb monolith substrate, wherein
the first honeycomb monolith substrate is oriented so that exhaust
gas contacts the first zone prior to the second zone. However, the
first honeycomb monolith substrate can be oriented so that exhaust
gas contacts the second zone prior to the first zone as
desired.
[0054] In a further alternative embodiment, the first honeycomb
monolith substrate comprising the first oxidation catalyst is
disposed upstream of a second honeycomb monolith substrate
comprising the second oxidation catalyst. However, the second
monolith substrate can be oriented so that exhaust gas contacts the
second monolith substrate prior to the first monolith substrate as
desired.
[0055] The honeycomb monolith substrate for use in the present
invention can be made from a ceramic material such as cordierite or
silicon carbide, or a metal such as Fecralloy.TM.. The arrangement
is preferably a so-called flow-through configuration, in which a
plurality of channels extend in parallel from an open inlet end to
an open outlet end. However, the honeycombed monolith substrate may
also take the form of a filtering substrate such as a so-called
wall-flow filter or a ceramic foam.
[0056] Wall-flow filters are ceramic porous filter substrates
comprising a plurality of inlet channels arranged in parallel with
a plurality of outlet channels, wherein each inlet channel and each
outlet channel is defined in part by a ceramic wall of porous
structure, wherein each inlet channel is alternatingly separated
from an outlet channel by a ceramic wall of porous structure and
vice versa. In other words, the wall-flow filter is a honeycomb
arrangement defining a plurality of first channels plugged at an
upstream end and a plurality of second channels not plugged at the
upstream end but plugged at a downstream end. Channels vertically
and laterally adjacent to a first channel are plugged at a
downstream end. When viewed from either end, the alternately
plugged and open ends of the channels take on the appearance of a
chessboard.
[0057] In embodiments comprising two or more honeycomb monolith
substrates in series, each monolith substrate may be selected from
flow-through and filtering configurations. Where a first monolith
substrate has a flow-through configuration and a second monolith
substrate has a filtering configuration, it is preferred that the
filtering monolith substrate is disposed downstream of the
flow-through monolith substrate, to obtain the benefits of
combusting particulate matter trapped on the filtering substrates
at less than 400.degree. C. in NO.sub.2 generated from oxidising NO
on the upstream flow-through monolith substrate (as disclosed in EP
341832). However, in one preferred embodiment, the apparatus
comprises a single monolith substrate, i.e. with no downstream
monolith substrate, e.g. filtering substrate.
[0058] In appropriate applications, the first honeycomb monolith
substrate or, where present, the first and second honeycomb
monolith substrates, can be used without any additional exhaust gas
aftertreatment components, i.e. exhaust gas exiting the first
honeycomb monolith substrate or, where present, the second
honeycomb monolith substrate is exhausted directly to atmosphere
without first encountering additional catalyst(s). However, as
desired, oxidation catalysts according to the present invention can
be integrated into more complex exhaust system architecture. In one
embodiment, the first honeycomb monolith substrate, and where
present the second honeycomb monolith substrate, is disposed
upstream of a filter for filtering particulate matter from the
exhaust gas. Optionally, the filter is catalysed.
[0059] In one embodiment, the filter catalyst is for oxidising
carbon monoxide and unburned hydrocarbon in the exhaust gas, i.e. a
so-called catalysed soot filter for treating, e.g. particulate
matter emitted by Diesel (compression ignition) engines.
[0060] Where the exhaust system comprises a filter, a further
embodiment features a NH.sub.3--SCR catalyst disposed downstream of
the filter. Such arrangement can include means for introducing
nitrogenous reductant, including precursors, to upstream of the SCR
catalyst, e.g. nitrogenous reductant can be introduced between the
filter and the SCR catalyst. Alternatively, the NH.sub.3--SCR
catalyst can be coated on the filter.
[0061] Alternatively, and in a preferred embodiment, the filter
catalyst is a catalyst for selectively catalysing the reduction of
oxides of nitrogen using a nitrogenous reductant. As described
above, such an arrangement can include means for introducing
nitrogenous reductant, including precursors, to upstream of the SCR
catalysed filter. As a further alternative, or in addition to the
means for injecting nitrogenous reductant or a precursor thereof,
in another embodiment, engine management means is provided for
enriching exhaust gas such that ammonia gas is generated in situ by
reduction of NO.sub.x on the first oxidation catalyst. In this
embodiment, and in combination with an appropriately designed and
managed engine, enriched exhaust gas, i.e. exhaust gas containing
increased quantities of carbon monoxide and hydrocarbon relative to
normal lean running mode, contacts the catalyst composition of the
first substrate monolith. PGM-promoted ceria or ceria-zirconia can
promote the water-gas shift reaction, i.e.
CO.sub.(g)+H.sub.2O.sub.(v).fwdarw.CO.sub.2(g)+H.sub.2(g) evolving
H.sub.2. From the side reaction footnote to reactions (3) and (4)
set out hereinabove, e.g.
Ba(NO.sub.3).sub.2+8H.sub.2.fwdarw.BaO+2NH.sub.3+5H.sub.2O,
NH.sub.3 can be generated in situ and stored for NO.sub.x reduction
on the downstream SCR catalyst.
[0062] For certain SCR catalysts, the catalytic reduction of oxides
of nitrogen using nitrogenous reductants can be promoted by
adjusting--preferably, as far as is possible passively--the ratio
of NO:NO.sub.2 to approximately 1:1. For this reason the oxidation
catalyst can be optimised for use in embodiments including SCR
catalysts to generate approximately 1:1 NO:NO.sub.2 ratio for the
benefit of NO.sub.x reduction on a downstream SCR catalyst in as
wide a temperature window as possible.
[0063] Catalysts for selectively catalysing the reduction of oxides
of nitrogen using nitrogenous reductant (SCR catalysts) are known
and can be selected from the group consisting of at least one of
Cu, Hf, La, Au, In, V, lanthanides and Group VIII transition
metals, such as Fe, supported on a refractory oxide or molecular
sieve. Suitable refractory oxides include Al.sub.2O.sub.3,
TiO.sub.2, CeO.sub.2. SiO.sub.2, ZrO.sub.2 and mixed oxides
containing two or more thereof. The non-zeolite catalyst can also
include tungsten oxide, e.g. V.sub.2O.sub.5/WO.sub.3/TiO.sub.2.
[0064] In particular embodiments, the SCR catalyst comprises at
least one molecular sieve, such as an aluminosilicate zeolite or a
SAPO. The at least one molecular sieve can be a small, a medium or
a large pore molecular sieve, for example. By "small pore molecular
sieve" herein we mean molecular sieves containing a maximum ring
size of 8 tetrahedral atoms, such as CHA; by "medium pore molecular
sieve" herein we mean a molecular sieve containing a maximum ring
size of 10 tetrahedral atoms, such as ZSM-5; and by "large pore
molecular sieve" herein we mean a molecular sieve having a maximum
ring size of 12 tetrahedral atoms, such as beta. Small pore
molecular sieves are potentially advantageous for use in SCR
catalysts--see for example WO 2008/132452 (the entire contents of
which is incorporated herein by reference).
[0065] Particular molecular sieves with application in the present
invention are selected from the group consisting of AEI, ZSM-5,
ZSM-20, ERI including ZSM-34, mordenite, ferrierite, BEA including
Beta, Y, CHA, LEV including Nu-3, MCM-22 and EU-1. CHA molecular
sieves are currently preferred, particularly in combination with Cu
as promoter, e.g. ion-exchanged and/or impregnated.
[0066] In embodiments, the molecular sieves can be un-metallised or
metallised with at least one metal selected from the group
consisting of groups IB, IIB, IIIA, IIIB, VB, VIB, VIB and VIII of
the periodic table. Where metallised, the metal can be selected
from the group consisting of Cr, Co, Cu, Fe, Hf, La, Ce, In, V, Mn,
Ni, Zn, Ga and the precious metals Ag, Au, Pt, Pd and Rh. Such
metallised molecular sieves can be used in a process for
selectively catalysing the reduction of nitrogen oxides in positive
ignition exhaust gas using a reductant By "metallised" herein we
mean to include molecular sieves including one or more metals
incorporated into a framework of the molecular sieve e.g. Fe
in-framework Beta and Cu in-framework CHA. Molecular sieves can be
ion-exchanged with any of the above metals.
[0067] Metals of particular interest are selected from the group
consisting of Ce, Fe and Cu. Suitable nitrogenous reductants
include ammonia. Alternatively, the nitrogenous reductant or a
precursor thereof can be injected directly into the exhaust gas.
Suitable precursors include ammonium formate, urea and ammonium
carbamate. Decomposition of the precursor to ammonia and other
by-products can be by hydrothermal or catalytic hydrolysis.
[0068] In an alternative arrangement, a flow-through monolith
substrate comprising a SCR catalyst for reducing oxides of nitrogen
using nitrogenous reductant can be located downstream of the first
monolith substrate, and where present the second monolith
substrate, thereby beneficially to adjust the NO:NO.sub.2 ratio in
exhaust gas entering the SCR catalyst.
[0069] In a particularly preferred embodiment, the SCR catalyst is
coated on a filtering substrate monolith, preferably a wall-flow
monolith. It is also possible to make a wall-flow filter from an
extruded SCR catalyst (see Applicant/Assignee's WO 2009/093071 and
WO 2011/092521).
[0070] Engines for use in the apparatus according to the invention
can be a gasoline, spark ignition engine, but has particular
relevance to compression ignition engines, generally known as
Diesel engines, though some compression ignition engines can
operate on other fuels, such as natural gas, bioDiesel or Diesel
fuel blended with bioDiesel and/or Fischer-Tropsch fuels.
[0071] According to a further aspect, the invention provides a
vehicle comprising an apparatus according to any preceding
claim.
[0072] According to a yet further aspect, the invention provides a
method of recovering an oxidation activity of a first oxidation
catalyst aged in an exhaust gas of a lean-burn internal combustion
engine, which first oxidation catalyst comprising platinum
supported on a first metal oxide support and disposed on a
honeycomb monolith substrate, which method comprising the step of
intermittently contacting the first oxidation catalyst with exhaust
gas modulated to a rich lambda composition, wherein the first metal
oxide support comprises at least one reducible oxide and wherein
the first oxidation catalyst is substantially free of alkali metals
and alkaline earth metals.
[0073] In use, the engine management means is configured. e.g. by
programming of an electronic processor, to modulate the lambda
composition of the exhaust gas contacting the first oxidation
catalyst to a rich lambda composition. In practice, it is not
possible to run the engine continuously rich in order to avoid
driveability issues, such as torque shock that can be communicated
through the driver via the steering column and steering wheel.
Preferred configurations include frequently modulating .lamda.
("lean/rich" switching) for a period of time sufficient to recover
oxidation activity to a desired degree. Such lean/rich switching
can set up exotherms in the oxidation catalyst, increasing catalyst
temperature e.g. >550.degree. C. Lean/rich switching
configurations for use in the present invention are known, e.g.
from the art of NO.sub.x trap desulphation regimes. However,
NO.sub.x trap desulfation regimes take place approximately once
about every 2,000-3,000 km that a vehicle has driven. Limits of
enrichment may depend on the apparatus design but may be >0.80,
e.g. .gtoreq.0.90, such as .gtoreq.0.95. However, it is essential
that the exhaust gas composition is at <1 during oxidation
catalyst activity regeneration.
[0074] In a particularly preferred embodiment of the method of the
invention, used in connection with an apparatus comprising a
catalysed filter, e.g. the first oxidation catalyst a SCR coated
filter or a catalysed soot filter (CSF), i.e. a filter comprising
one or more platinum group metals, disposed downstream of a
monolith substrate comprising the first oxidation catalyst, the
method step of recovering an oxidation activity of a first
oxidation catalyst is conducted immediately following a step to
regenerate the filter. Particularly in light-duty Diesel
applications, the system is designed to run a protocol which heats
the exhaust system components, e.g. by engine management means such
as in-cylinder post-ignition injection of hydrocarbon fuel, in
order to combust any soot held on the filter thereby to return the
system to a "clean" state and prevent any issues with soot
build-up, e.g. back pressure problems. Filter temperatures can
reach 600-650.degree. C. in an overall lean exhaust gas (despite
increased hydrocarbon injection). Such filter regeneration
protocols are typically done approximately once every 500 km a
vehicle has driven.
[0075] The method of the present invention can be fuel intensive,
in that a rich exhaust gas is used. The preferred aspect of the
invention takes advantage of the temperature increase developed by
the filter regeneration step so that when the engine is operated by
engine management means to contact the first oxidation catalyst
with rich exhaust (or increased hydrocarbon is injected into the
exhaust gas downstream of the engine) the first oxidation catalyst
is already at a relatively high temperature. This has the advantage
that oxidation activity of the first oxidation catalyst is
recovered more quickly when the first oxidation catalyst is hotter
and at a reduced fuel penalty to the system.
[0076] In order that the invention may be more fully understood,
the following Examples are provided by way of illustration only and
with reference to the accompanying drawings, in which:
[0077] FIG. 1 is a graph showing the accumulated CO, both
engine-out and post-catalyst, for a Conventional DOC (not according
to the invention) fitted to a bench-mounted 2.0 litre Diesel engine
run over the MVEG-B European emission test cycle, both before and
after activation;
[0078] FIG. 2 is a graph showing accumulated CO, both engine-out
and post-catalyst, for a DOC according to the invention fitted to
the engine used to test the Conventional DOC in FIG. 2 and run over
the same cycle, both before and after activation; and
[0079] FIG. 3 shows a series of graphs displaying the cumulative CO
detected at engine-out (top trace) and post-catalyst (bottom trace)
on a different 2.0 litre Diesel engine from the one used in FIGS. 1
and 2, including the effects of "in-cycle" activation.
EXAMPLES
Example 1
Manufacture of Fully Formulated DOC
[0080] A honeycomb 400 cells per square inch flow-through
cordierite monolith substrate was coated with a two-layer
Conventional catalyst, wherein a first layer comprised 1.67
gin.sup.-3 Al.sub.2O.sub.3 and 0.33 gin.sup.-3 Beta zeolite (2
gin.sup.-3 washcoat loading in total for the bottom layer) and PtPd
and a second, overlayer comprised 0.833 gin.sup.-3 Al.sub.2O.sub.3
and 0.167 gin.sup.-3 Beta zeolite (1 gin.sup.-3 washcoat loading in
total for the top layer) and Pt only. The first layer was applied
to a virgin monolith substrate as a washcoat containing salts of Pt
and Pd using the methods disclosed in WO 99/47260, i.e. comprising
the steps of (a) locating a containment means on top, first end of
a substrate monolith, (b) dosing a pre-determined quantity of a
first washcoat coating component into said containment means,
either in the order (a) then (b) or (b) then (a), and (c) by
applying pressure or vacuum, drawing said first washcoat coating
component into at least a portion of the substrate monolith, and
retaining substantially all of said quantity within the substrate
monolith. In a first step a coating from a first end of application
can be dried and the dried substrate monolith can be flipped
through 180 degrees and the same procedure can be done to a top,
second end of the substrate monolith, with substantially no overlap
in layers between applications from the first and second ends of
the substrate monolith. The resulting coating product is then
dried, then calcined. The process can be repeated with a second
washcoat coating component, to provide a catalysed (bi-layered)
substrate monolith according to the invention. The resulting coated
monolith substrate was coated with a washcoat of the second
catalyst containing Pt salts followed by same drying and firing
steps. The amounts and concentrations of the Pt and Pd salts used
were calculated to result in a total platinum group metal loading
(for the two layer construct) of 100 gft.sup.-3 at a 2Pt:Pd ratio.
The bottom layer comprised 33 gft.sup.-3 Pd and 63 gft.sup.-3 Pt
and the top layer comprised 3 gft.sup.-3 Pt. The finished catalyst
was lean hydrothermally aged at 800.degree. C. for 16 hours in 10%
H.sub.2O, 10% O.sub.2, balance N.sub.2.
[0081] A two-layer catalyst for use in the present invention was
made using the same techniques used to make the Conventional
two-layer catalyst and comprised a bottom layer comprising 2
gin.sup.-3 of a 60:40 mixture by weight of
CeO.sub.2:Al.sub.2O.sub.3 and 45 gft.sup.-3 platinum group metal in
total at a 2Pt:Pd ratio and a top layer of 2 gin.sup.-3
Al.sub.2O.sub.3 only supporting 45 gft.sup.-3 total platinum group
metal at 2Pt:Pd ratio (90 gft.sup.-3 total platinum group metal
loading). This catalyst was lean hydrothermally aged in a similar
manner to the Conventional catalyst.
Example 2
Engine Testing of Fully Formulated DOC
[0082] The catalyst of Example 1 was fitted in a pre-existing
close-coupled position (i.e. close to the engine exhaust manifold)
to a 2.0 litre Diesel bench-mounted engine having an engine
management system with a NO.sub.x trap engine calibration, i.e. the
vehicle as sold is fitted with a NO.sub.x trap and NO.sub.x trap
engine calibration. In our tests, the NO.sub.x trap was removed and
replaced with a DOC. In a first run, the cumulative CO conversion
of the lean hydrothermally aged DOC was tested over the European
MVEG-B cycle and the exhaust gas composition was monitored both
upstream and downstream of the DOC using an engine dynamometer. No
"rich purge" occurred during the cycle, i.e. the engine management
system did not switch to a condition adapted to remove and reduce
NO.sub.x adsorbed on the catalyst. A "rich purge" is used to
activate catalysts according to the invention and any rich purge
effected during the tests would have made comparison of the results
difficult.
[0083] Following the first run, the DOC was activated off-cycle.
The activation used was done by overriding the engine management
control to produce 4 cycles each of 30 seconds lean followed by 10
seconds rich.
[0084] The results are shown in FIGS. 1 and 2. In FIG. 1 it can be
seen that the cumulative CO conversion for the Conventional DOC is
substantially identical for both the lean hydrothermally aged
("before activation") and "after activation" MVEG-B runs. However,
for the DOC according to the invention, the CO conversion is
substantially improved. These results are also summarized in Table
1.
TABLE-US-00001 TABLE 1 Conventional DOC Active DOC Activation
Before After Before After CO g/km 0.50 0.50 0.63 0.13 Activation
Details 4 .times. (30 s lean/10 rich)
Example 3
In-Cycle Activation of Fully Formulated DOC
[0085] An identical, lean-hydrothermally aged catalyst according to
the invention was prepared as described in Example 1. This catalyst
was fitted in the close-coupled position to a bench-mounted 2.0
litre Diesel engine different from the engine used in Example 2,
but an engine that was also calibrated for NO.sub.x trap
regeneration. The engine was run over the European MVEG-B emission
cycle and the CO content of the exhaust gas both at engine-out and
downstream of the catalyst was monitored using an engine
dynamometer. The cumulative CO content of the exhaust gas is shown
in the graph at FIG. 3, top left, with the top trace showing CO
content in engine-out emissions. CO emissions for this first run
were 1.04 g/km.
[0086] Following the first MVEG-B test cycle, a second test was run
but in this cycle (shown in the graph at FIG. 4, top right) the
engine conducted a NO.sub.x purge (or "activation") event of 8
seconds rich. A third MVEG-B test cycle was performed immediately
after activation (see FIG. 4, bottom left-hand graph). It can be
seen from this third graph that the cumulative CO conversion is
improved over the first and second test cycles. The recorded CO
emissions for the third cycle was 0.43 g/km.
[0087] From this Example 3 it can be seen that the catalyst for use
in the present invention can be activated (and hence repeatedly
reactivated) intermittently in use to maintain CO and hydrocarbon
conversion at limited fuel penalty for each activation.
[0088] For the avoidance of any doubt, the entire contents of every
patent document referenced herein is incorporated herein by
reference.
* * * * *