U.S. patent application number 15/212354 was filed with the patent office on 2016-11-10 for coating a monolith substrate with catalyst component.
The applicant listed for this patent is Johnson Matthey Public Limited Company. Invention is credited to Guy Richard CHANDLER, Keith Anthony FLANAGAN, Paul Richard PHILLIPS, Paul SCHOFIELD, Michael Leonard William Spencer, Hedley Michael STRUTT.
Application Number | 20160325272 15/212354 |
Document ID | / |
Family ID | 41795944 |
Filed Date | 2016-11-10 |
United States Patent
Application |
20160325272 |
Kind Code |
A1 |
CHANDLER; Guy Richard ; et
al. |
November 10, 2016 |
COATING A MONOLITH SUBSTRATE WITH CATALYST COMPONENT
Abstract
A method of coating a honeycomb monolith substrate comprising a
plurality of channels with a liquid comprising a catalyst component
comprises the steps of: (i) holding a honeycomb monolith substrate
substantially vertically; (ii) introducing a pre-determined volume
of the liquid into the substrate via open ends of the channels at a
lower end of the substrate; (iii) sealingly retaining the
introduced liquid within the substrate; (iv) inverting the
substrate containing the retained liquid; and (v) applying a vacuum
to open ends of the channels of the substrate at the inverted,
lower end of the substrate to draw the liquid along the channels of
the substrate.
Inventors: |
CHANDLER; Guy Richard;
(Royston, GB) ; FLANAGAN; Keith Anthony; (Royston,
GB) ; PHILLIPS; Paul Richard; (Royston, GB) ;
SCHOFIELD; Paul; (Royston, GB) ; Spencer; Michael
Leonard William; (Royston, GB) ; STRUTT; Hedley
Michael; (Biggleswade, GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Johnson Matthey Public Limited Company |
London |
|
GB |
|
|
Family ID: |
41795944 |
Appl. No.: |
15/212354 |
Filed: |
July 18, 2016 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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14199323 |
Mar 6, 2014 |
9415365 |
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15212354 |
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12984290 |
Jan 4, 2011 |
8703236 |
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14199323 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 53/9418 20130101;
B01D 2255/9155 20130101; B01J 37/0246 20130101; B05C 9/02 20130101;
A44B 13/0011 20130101; B01J 29/763 20130101; B01J 27/224 20130101;
B05C 9/04 20130101; B01J 21/08 20130101; B01D 2255/91 20130101;
B01J 35/04 20130101; B05C 9/045 20130101; B01J 15/005 20130101 |
International
Class: |
B01J 29/76 20060101
B01J029/76; B01D 53/94 20060101 B01D053/94; B01J 35/04 20060101
B01J035/04; B01J 37/02 20060101 B01J037/02; B01J 21/08 20060101
B01J021/08; B01J 27/224 20060101 B01J027/224 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 4, 2010 |
GB |
1000019.8 |
Claims
1.-17. (canceled)
18. A catalysed wallflow filter substrate monolith to which the
manufacturer of the wallflow filter substrate monolith has
pre-coated to inlet channels thereof a surface membrane layer
comprising finely divided refractory solids, wherein the outlet
channels comprise an axially substantially uniform coating profile
of catalyst washcoat, which catalysed wallflow filter substrate
monolith is obtainable by coating a honeycomb monolith substrate
comprising a plurality of channels with a liquid comprising a
catalyst component with a method comprising the steps of: (a)
holding a honeycomb monolith substrate substantially vertically;
(b) introducing a pre-determined volume of the liquid into the
substrate via open ends of the channels at a lower end of the
substrate; (c) sealingly retaining the introduced liquid within the
substrate; (d) inverting the substrate containing the retained
liquid; and (e) applying a vacuum to open ends of the channels of
the substrate at the inverted, lower end of the substrate to draw
the liquid along the channels of the substrate.
19. (canceled)
20. A catalysed wallflow filter substrate monolith according to
claim 18, wherein the porosity of the wallflow filter substrate
monolith prior to coating is from 40 to 80%.
21. A catalysed wallflow filter substrate monolith according to
claim 18, wherein a mean pore volume of the wallflow filter
substrate monolith prior to coating is from 8 to 45 .mu.m.
22. A catalysed wallflow filter substrate monolith according to
claim 18, wherein the catalyst washcoat is a NO.sub.x trap, a
catalysed soot filter washcoat comprising supported platinum group
metal or a NH.sub.3-SCR catalyst.
23. A catalysed wallflow filter substrate monolith according to
claim 22, wherein the NH.sub.3-SCR catalyst comprises a transition
metal exchanged zeolite.
24. A catalysed wallflow filter substrate monolith according to
claim 23, wherein the transition metal is selected from the group
consisting of copper, iron, cerium and mixtures of any two or more
thereof and the zeolite is selected from the group consisting of
Ferrierite, CHA, BEA and MFI (ZSM-5).
25. A catalysed wallflow filter substrate monolith according to
claim 24, wherein the transition metal exchanged zeolite is
selected from the group consisting of Cu/CHA, Fe/Ferrierite,
Fe/ZSM-5, Fe--Ce/ZSM-5, Fe/BEA and Fe--Ce/BEA.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority of British Patent
Application No. 1000019.8, filed Jan. 4, 2010, the disclosure of
which is incorporated herein by reference in its entirety for all
purposes.
FIELD OF THE INVENTION
[0002] The present invention relates to a method of, and an
apparatus for, coating a honeycomb monolith substrate comprising a
plurality of channels with a liquid comprising a catalyst
component.
BACKGROUND OF THE INVENTION
[0003] "Honeycomb monolith substrate" as defined herein includes
metal and ceramic flow-through monoliths having a plurality of
channels or cells which extend longitudinally along the length of
the substrate structure and wherein the channels are open at both
ends thereof; and metal and ceramic filters including ceramic
wall-flow filters having a plurality of channels or cells which
extend longitudinally along the length of the substrate structure
and wherein channels at a first end of the substrate that are open
are blocked at the opposite end and channels that are open at the
opposite end are blocked at the first end, the arrangement being
such that every other adjacent cell has an open end (or a blocked
end) on the first end of the wall-flow filter and a blocked end (or
an open end) on the opposite end thereof so that when an end of the
wall-flow filter is viewed it resembles a chess board of open and
blocked channels. Fluid communication between the open channels at
the first end of the wall-flow filter and the open channels of the
opposite end thereof is via the porous wall structure of the
wall-flow filter.
[0004] The definition "honeycomb monolith substrate" also includes
metallic so-called "partial filters" such as that which is
disclosed in WO 01/80978 or the substrate disclosed in EP
1057519.
[0005] Typically, ceramic materials for manufacturing honeycomb
monolith substrates include silicon carbide, aluminium nitride,
silicon nitride, aluminium titanate, sintered metal, alumina,
cordierite, mullite, pollucite, a thermet such as
Al.sub.2O.sub.3/Fe, Al.sub.2O.sub.3/Ni or B.sub.4C/Fe, or
composites comprising segments of any two or more thereof.
[0006] The formulation of liquids comprising catalyst components
for coating honeycomb monolith substrates are known to those
skilled in the art and include: aqueous solutions of platinum group
metal compounds, such as platinum, palladium and rhodium compounds,
aqueous solutions of alkali metal and alkaline earth metal
compounds for depositing compounds for absorbing NO.sub.x on the
substrates, and other components such as compounds of transition
metals e.g. iron, copper, vanadium, cerium and transition metal
catalyst promoter compounds; washcoat slurries including
particulate catalyst support materials such as alumina, ceria,
titania, zirconia, silica-alumina and zeolites, optionally
supporting one or more of the above mentioned platinum group metals
or transition metals; and washcoat slurries containing combinations
of supported metal compounds and aqueous solutions of the above
mentioned metal compounds. Such liquids can also include relevant
acids, organic compounds thickeners etc. to improve the catalyst
activity, chemistry of the formulation to suit the intended purpose
of the resulting catalyst, and/or the viscosity and rheology of the
liquid.
[0007] Apparatus for automatedly coating a honeycomb monolith
substrate is known, for example, from our WO 99/47260 and from U.S.
Pat. No. 5,422,138. The latter reference discloses an apparatus
comprising means for holding a honeycomb monolith substrate
substantially vertically and means for introducing a pre-determined
volume of a liquid into the substrate via open ends of the channels
at a lower end of the substrate, i.e. features (a) and (b) of claim
4 of the present specification.
[0008] EP 1325781 discloses a development of the coating technique
disclosed in U.S. Pat. No. 5,422,138 that can be used to produce
modern "zoned" substrates.
SUMMARY OF THE INVENTION
[0009] The present inventors have investigated known techniques for
coating a honeycomb monolith substrate with a liquid comprising a
catalyst component with particular emphasis on coating filter
substrates and they encountered a number of problems.
[0010] One problem was that if a liquid washcoat is too viscous,
the back pressure in the filter can be too high for practical
application of the filters in exhaust systems of diesel vehicles.
The inventors found that washcoat viscosities of about 50 cps may
be required for coating filters and that such low viscosity
washcoats often resulted in uneven coating across the filter
substrate when using known coating methods.
[0011] In practice, the inventors found that it was useful to match
the water absorption factor of the substrate to the suspending
liquid content in a washcoat to achieve a desired percentage axial
coating depth; if the two were not matched, the coating could lack
stability under drying. By removing the suspending liquid,
typically water, from the washcoat on the substrate, washcoat
components are immobilised.
[0012] Furthermore, the inventors observed that by removing
thickeners from the washcoat formulation, resulting in a lower
viscosity washcoat, the drying time could be reduced.
[0013] An alternative method was investigated that included placing
a wall-flow filter substrate in a bath of aqueous solution and
allowing the solution to impregnate the filter by capillary action,
then drying and calcining the impregnated wall-flow filter
substrate. However, it was found that this method did not lend
itself readily to automation because the impregnation step and the
subsequent drying step were too slow. Moreover, the method was not
flexible enough to allow for customer needs such as catalyst
"zoning" to improve activity and to thrift expensive platinum group
metals.
[0014] The method and apparatus disclosed in U.S. Pat. No.
5,422,138 uses higher viscosity slurries, e.g. 100 to 500 cps and
so would not appear to be of practical use in the field of coating
filter substrate monoliths with the desirable lower viscosity
slurries. Furthermore, the use in this method of excess quantities
of liquid containing expensive platinum group metals to coat
honeycomb monolith substrates can lead to the inefficient loss of
some liquid. In this an, it is also important that a coated
honeycomb monolith substrate complies with a contracted
specification agreed between a coated honeycomb monolith substrate
manufacturer and its customer since excessive coating of expensive
platinum group metals can reduce the manufacturer's profit, whereas
coating the honeycomb monolith substrate with too little platinum
group metal can result in the manufacturer coming into conflict
with its customer.
[0015] The inventors have developed a method of, and apparatus for,
automatedly coating honeycomb monolith substrates, particularly
filters, with lower viscosity liquids comprising catalyst
components that enables more careful and accurate loading of
expensive platinum group metal components and prevents losses in
the manufacturer's factory.
[0016] According to one aspect, the invention provides a method of
coating a honeycomb monolith substrate comprising a plurality of
channels with a liquid comprising a catalyst component, which
method comprising the steps of: (i) holding a honeycomb monolith
substrate substantially vertically; (ii) introducing a
pre-determined volume of the liquid into the substrate via open
ends of the channels at a lower end of the substrate; (iii)
sealingly retaining the introduced liquid within the substrate;
(iv) inverting the substrate containing the retained liquid; and
(v) applying a vacuum to open ends of the channels of the substrate
at the inverted, lower end of the substrate to draw the liquid
along the channels of the substrate.
[0017] In an embodiment, a step of sealing an outer surface of the
substrate from, i.e. sealing from liquid communication with, the
open ends of the channels at the lower end of the substrate is
inserted between steps (i) and (ii).
[0018] In another embodiment, the seal retaining the liquid in step
(v) is removed only after a vacuum has been applied.
[0019] In another embodiment of the method, the substrate is a
filter, as defined herein.
[0020] According to a second aspect, the invention provides an
apparatus for coating a honeycomb monolith substrate comprising a
plurality of channels with a liquid comprising a catalyst
component, which apparatus comprising: (a) means for holding a
honeycomb monolith substrate substantially vertically; (b) means
for introducing a pre-determined volume of the liquid into the
substrate via the open ends of the channels at a lower end of the
substrate; (c) means for sealingly retaining the introduced liquid
within the substrate; (d) means for inverting the substrate
containing the retained liquid; and (e) means for applying a vacuum
to open ends of the channels of the substrate at the inverted,
lower end of the substrate to draw the liquid along the channels of
the substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] In order that the invention may be more fully understood,
reference may be made to the series of schematic representations of
an embodiment of the apparatus and method steps of the invention
shown in the accompanying drawings, in which:
[0022] FIG. 1 shows the starting position for an apparatus
according to the invention, without a substrate;
[0023] FIG. 2 shows the apparatus of FIG. 1 with a substrate
inserted;
[0024] FIG. 3 shows the arrangement of apparatus and substrate
wherein the substrate is held by actuation of inflatable
collars;
[0025] FIG. 4 shows the arrangement wherein a charge piston is
lowered and a measured dose of washcoat is introduced into a
displacement volume via a dose control valve;
[0026] FIG. 5 represents the arrangement of FIG. 4, wherein the
dose control valve is closed and the piston has driven the washcoat
charge into the substrate;
[0027] FIG. 6 shows the arrangement wherein the apparatus is
inverted and the lower end of the inverted substrate is inserted
into an open end of a funnel featuring an inflatable collar seal
for application of a vacuum;
[0028] FIG. 7 shows the step wherein the inflatable collars of the
(first) holding means disengage from the substrate so that the
substrate is held by the vacuum funnel collar and a vacuum is
applied to draw the washcoat into the substrate monolith.
[0029] FIG. 8 compares x-ray density profiles for three wallflow
filters, a first (the control) "as-received" from a supplier,
wherein the inlet channels were pre-coated with a membrane layer
comprising finely divided inorganic refractory material, a second
"as-received" and additionally coated on the outlet channels with a
SCR catalyst using a dip coating method (Comparative) and a third
"as-received" and additionally coated on the outlet channels with
the same SCR catalyst at a similar washcoat loading to the second
but using the process according to the invention;
[0030] FIG. 9 is a graph comparing the soot loading vs. back
pressure for fresh samples of wallflow filters prepared by a dip
coating method (Comparative) and the method according to the
invention;
[0031] FIG. 10 is a graph comparing the fresh NO.sub.x conversion
activity for filters prepared according to Example 1 and
Comparative Example 2 fitted in the exhaust system of a laboratory
bench-engine and tested according to the experimental protocols
described in Example 5. In this Figure and subsequent Figures, the
product of Example 1 is labelled "AID" and the product of
Comparative Example 2 is labelled "Dip";
[0032] FIG. 11 is a graph comparing the rate of NH.sub.3 slip for
the fresh filters during the NO.sub.x conversion tests shown in
FIG. 10;
[0033] FIG. 12 is a graph comparing the aged NO.sub.x conversion
activity for filters prepared according to Example 1 and
Comparative Example 2 fitted in the exhaust system of a laboratory
bench-engine and tested according to the experimental protocols
described in Example 5; and
[0034] FIG. 13 is a graph comparing the rate of NH.sub.3 slip for
the aged filters during the NO.sub.x conversion tests shown in FIG.
12.
DETAILED DESCRIPTION OF THE INVENTION
[0035] Whilst the substrate can be manually inserted into the
holding means, it is preferred to use a robotic "pick-and-place"
device in order to increase the automation of the method as a
whole.
[0036] In an embodiment, the holding means comprises a housing for
receiving at least the lower end of the substrate. The skilled
engineer will appreciate that not all substrates have a
conventional circular cross-section, but can also take the form of
oval or "race-track", skewed oval or other asymmetric
cross-section. Whatever the cross-section of the substrate, the
skilled engineer can adopt a suitably shaped housing for receiving
the substrate, as appropriate.
[0037] The holding means can comprise any suitable means for
holding the substrate, for example stiff bristles, or a flexible
fin of elastomeric material supported by an internal wall of the
housing, that extend into the interior space of the housing and are
deformed as the substrate is inserted into an opening of the
housing, or three or more equally-spaced feet disposed in a
substantially common axial plane that extend from the internal wall
surface of the housing into the interior of the housing for
gripping an exterior surface of the substrate following insertion
of the substrate into the housing.
[0038] In a particular embodiment, however, the holding means
comprises at least one inflatable collar disposed on an internal
surface of the housing for engaging with an outer surface of the
substrate. Whilst the inflatable collar can be in the form
disclosed in U.S. Pat. No. 5,422,138, FIGS. 8-16 and associated
description, i.e. a collar that contacts the entire axial length of
the exterior surface of the substrate, we prefer to use an
arrangement comprising a first inflatable collar that contacts the
exterior surface at a lower end of the substrate and a second
inflatable collar that contacts the exterior surface of the
substrate above the lower end of the substrate, e.g. at about
midway between the lower and upper ends of the substrate or in an
upper half of the substrate. A reason for preferring at least two
inflatable collars for engaging the substrate is that the inventors
have found that the substrate is held more rigidly and enables
higher precision for the following method steps, particularly the
inversion step, whereas the single collar shown in U.S. Pat. No.
5,422,138 provides more flexibility in the lateral plane, requiring
higher pressures to hold the substrate to the desired level of
rigidity.
[0039] Any suitable liquid introducing means can be used but in a
particular embodiment it comprises a piston that reciprocates
within a cylinder. Although the term "cylinder" implies a circular
cross-section of a piston head and cylinder bore, in embodiments,
the shape of the piston head and cylinder bore is dictated by the
cross-section of the substrate, i.e. where the substrate is oval in
cross-section, the piston head and cylinder bore are also oval in
cross-section. This is because matching the cross-section of piston
head and cylinder bore with the substrate can promote coating of
the substrate to a more even axial washcoat depth. However, it is
not essential to match the cross-section of the substrate with the
cross-section of the piston head and cylinder bore as this avoids
re-tooling the apparatus for coating substrates of differing
cross-sections.
[0040] Generally, the piston reciprocates within the cylinder
between a first position wherein a surface of the piston head abuts
or aligns with a cylinder head and a second position, wherein an
internal wall of the cylinder, cylinder head and piston head define
a displacement volume.
[0041] In one embodiment, the displacement volume is similar, or
identical, to the volume of liquid to be introduced into the
substrate and the piston returns to the first position following
introduction of the liquid into the substrate. This arrangement is
preferred in an embodiment wherein the surface of the piston head
supports or abuts the lower end of the substrate when the piston is
in the first position. This is so that when the substrate is first
inserted into the housing, the substrate can be supported by the
piston head before the holding means, such as the inflatable
collar, is actuated, providing more reliable engagement with the
holding means and/or means for sealingly retaining introduced
liquid within a substrate (the latter scaling means is discussed
hereinbelow).
[0042] In an alternative embodiment, the displacement volume is
sufficient to accommodate multiple doses of the liquid, i.e.
sufficient liquid for introducing single doses into two or more
substrates. Of course, in this arrangement it may be necessary to
adopt a means for sealingly retaining introduced liquid within a
substrate that can retain the liquid in the cylinder in embodiments
wherein the cylinder itself is inverted.
[0043] The liquid can be supplied into the cylinder suitably via an
aperture in the cylinder head through which liquid is introduced
into the substrate, through a conduit in the piston rod and piston
head or by valve means in a wall of the cylinder housing. In any
event, it is desirable to supply only the pre-determined volume of
liquid to be introduced into the substrate into the displacement
volume in order to prevent wastage of the coating liquid. In one
arrangement, the displacement volume is the same as the volume of
the liquid to be introduced into the substrate so that little or no
dead space exists when the displacement volume is charged with the
liquid. This is so that the cylinder bore is emptied when the
entire volume of the liquid is introduced into the substrate, and
the piston head abuts the lower end of the substrate, for reasons
explained in greater detail below. Of course, as mentioned above,
it is also possible to charge the displacement volume with
sufficient liquid for two or more liquid introduction steps,
wherein the piston advances step-wise within the cylinder
consequently reducing the displacement (and liquid) volume with
each step.
[0044] The means for sealingly retaining the introduced liquid
within the substrate can be any suitable feature, such as a
guillotine, iris or shutter or a material having a one-way
permeability. However, in a preferred arrangement, the means for
sealingly retaining the liquid is a surface of the piston head
itself, which can include a material to enhance the seal
therebetween e.g. an elastomeric material such as a soft silicone
foam or synthetic rubber. Thus, in the above embodiment wherein the
entire volume of liquid is expelled from the cylinder into the
substrate, i.e., the piston has returned to the first position, the
surface of the piston head contacts the lower end of the substrate,
thus forming the seal to retain the liquid introduced into the
substrate.
[0045] In one embodiment, the liquid retaining means is removed
prior to application of the vacuum.
[0046] However, in another embodiment the liquid retaining means
maintains the seal with the end of the substrate until the vacuum
means applies a vacuum to the inverted lower end of the substrate,
i.e. following switch-off of the vacuum a static vacuum remains in
the substrate. Depending on the nature of the sealing means, this
is so that the liquid does not flow between cells of the substrate,
leading to uneven axial coating depth across the substrate
following inversion; or leak from an end of the substrate into
which the liquid was introduced, and thence down the outside walls
of the substrate, before the vacuum can be applied to draw the
liquid along the channels of the substrate--leading to liquid loss
and an undesirably less cosmetic appearance of the outer "skin"
coating of the substrate. In the embodiment wherein the piston head
provides the seal, maintaining sealing engagement with the
substrate until the vacuum is applied also provides the advantage
of cleaning the piston surface ready for the next substrate.
[0047] In a further embodiment, the housing, piston and cylinder
are all inverted as a single unit by the inverting means.
Desirably, such inverting means comprises a robotic device.
[0048] In an embodiment, the apparatus comprises means for sealing
an outer surface of the substrate from (i.e. sealing from liquid
communication with) the open ends of the channels at the lower end
of the substrate. This may be necessary in embodiments wherein the
cross-section of the piston bore is a different shape from the
cross-section of the substrate, e.g. where the substrate is oval
and the piston bore is circular. This is to stop any residual
liquid in peripheral "dead space" areas of the cylinder from
seeping into the substrate during the inversion step.
[0049] Any suitable sealing means for sealing the outer surface of
the substrate from the open ends of the channels at the lower end
of the substrate can be used, such as the flexible fin mentioned
above, but in a particular embodiment the sealing means comprises
the inflatable collar, or where more than one inflatable collar is
used, the inflatable collar associated with the lower end of the
substrate.
[0050] The vacuum means can take any suitable form, but in one
embodiment it comprises a funnel, the wider end of which is for
receiving an inverted end of the substrate.
[0051] A seal between the vacuum means and the inverted end of the
substrate can be achieved by a fin of flexible material extending
into the space defined by the internal surface of the wider end of
the funnel, wherein the fin is deformed as the substrate is
inserted into the wider end of the funnel and the fin engages with
the outer surface of the substrate. In a particular embodiment,
however, the internal surface of the wider end of the funnel
comprises an inflatable collar for sealingly engaging with the
outer surface of the substrate. Since the seal disposed on the
vacuum means also grips the substrate, it can be regarded as a
second holding means.
[0052] The (first) holding means can be disengaged from the coated
substrate during application of the vacuum and re-applied following
the vacuum step. This may be for at least four reasons: [0053] (i)
in the embodiment wherein the piston head comprises the means for
sealingly retaining the introduced liquid, wherein the seal is
maintained following an application and switch-off of the vacuum,
i.e. a static vacuum remains in the substrate, the substrate can
form a hydraulic seal with the piston head. The (second) holding
means on the vacuum means enables the substrate to be pulled from
the piston head; [0054] (ii) to prevent any loss of vacuum in the
substrate channels; [0055] (iii) to prevent or reduce substrate
edge damage and to protect the substrate; and [0056] (iv) to enable
air to access the housing and to enter the substrate via the end of
the substrate into which the liquid was introduced, although this
may also be effected by providing perforations in the wall of the
housing.
[0057] Following the vacuum step, the apparatus and coated
substrate can be returned to its upright position, following which
the coated substrate can be removed for drying and optional
calcining of the coating.
[0058] The method and apparatus of the present invention enables
the manufacture of modern "zoned" substrates. Following drying and
optional calcining of the coated substrate after a first pass, the
same substrate can be coated in a second pass with a different
liquid from the opposite end from which the first coating was
introduced. For example, the dose weight and solids content of the
liquid and the magnitude of vacuum applied, can all be calculated
and optimised to achieve any axial depth of coating that is
required. It is also possible, in a second pass, to coat the
substrate monolith with a different composition from the opposite
end to a first pass coating and to achieve a desired amount of
overlap between the two coatings where they meet, e.g. 5%. Multiple
coatings, e.g. a third pass coating, over the first or second pass
coating, can also be done following drying and optional calcining,
as desired.
[0059] In this way, the present invention enables the manufacture
of a filter substrate such as disclosed in our WO 2004/079167, i.e.
a zoned filter substrate wherein a first catalyst zone comprises a
diesel oxidation catalyst comprising at least one platinum group
metal (PGM) for oxidising carbon monoxide, hydrocarbons and
nitrogen monoxide and wherein at least one downstream catalyst zone
comprises at least one PGM, wherein the total PGM loading in the
first catalyst zone is greater than the total PGM loading in the at
least one downstream catalyst zone.
[0060] In a particular embodiment, the apparatus is controlled by a
suitably programmed computer so as to perform, when in use, the
series of method steps according to the invention.
[0061] The inventors found that the method of coating a honeycomb
monolith substrate according to the invention provides particular
advantages when applied to making wallflow filters comprising
catalysts such as: oxidation catalysts comprising one or more
platinum group metals (the resulting coated filter being generally
known as a catalysed soot filter of CSF); and catalysts for
catalysing the selective reduction of oxides of nitrogen with
nitrogenous reductants such as ammonia and ammonia precursors such
as urea. It is also believed that the method according to the
invention may be used for making filters comprising so-called
NO.sub.x absorber catalysts (NACs), also known as Lean NO.sub.x
Traps or simply NO.sub.x traps.
[0062] The method is flexible in that, with appropriate
manipulation of dosing quantities, washcoat solids content and
vacuum strength and duration, some or all of the filter channels
may be coated, different channel coating lengths can be adopted for
inlet and outlet channels and the methods can be used to make zone
coated wallflow filter arrangements, e.g. wherein a first axial 20%
of the inlet channels are coated with a higher concentration of
platinum group metal than the remainder of the inlet channels
downstream thereof.
[0063] Generally, the washcoat solids content selected for a given
washcoat loading is dependent upon the porosity of the part to be
coated and the axial length of the coating to be applied, and the
precise washcoat solids content required can be determined by
routine trial and error. Typically, however, the washcoat solids
content will be in the range of about 8-40% solids. Generally, to
coat the same axial length of a part, the higher porosity the part,
the lower the washcoat solids content to be used. Also, in order to
coat different axial lengths from the same part with the same
washcoat loading, the shorter the axial length the higher the
washcoat solids content. So to coat a typical cordierite or SiC
wall-flow filter with a washcoat at a standard washcoat loading,
one might select a washcoat solids content of 25% to coat the whole
length of the channels. To coat a relatively short zone of a
wallflow filter, e.g. to coat a short inlet zone of a catalysed
soot filter with a relatively high washcoat loading of platinum
group metals, a much higher washcoat solids content can be selected
e.g. 30-40%. The washcoat volume for coating a shorter axial length
of a part to the same washcoat loading will be less than for a
longer axial length of a part.
[0064] The vacuum to be applied will generally be in the order of
-5 kpa to -50 kpa, with durations ranging from about 0.3 seconds to
about 2 seconds, depending on the washcoat solids content (longer
vacuum duration for lower washcoat solids content) and size of the
part (larger volume parts requiring longer duration and higher
vacuum application). However, typically vacuum applications can be
of the order of about 1 second. Larger substrates, e.g. those
intended for heavy-duty Diesel vehicles, may require higher vacuum
application, such as minus 40-50 kpa, whereas light-duty Diesel
vehicle parts may require vacuum application of Larger substrates,
e.g. those intended for heavy-duty Diesel vehicles, may require
higher vacuum application, such as minus 40-50 kpa, whereas
light-duty Diesel vehicle parts may require vacuum application of
10-30 kpa.
[0065] The inventors have found that a better coating profile can
be achieved by applying a vacuum at step (v) following inversion of
the part in at least two steps: a first short, relatively weak
vacuum application (of the order of -5 to -10 kpa) at relatively
low vacuum pressure and without application of any holding means
such as inflatable collars in the vacuum means; followed by a
second longer and stronger vacuum with holding means actuation. It
is believed that the shorter vacuum application serves to clear the
piston surface and allow the washcoat to run along the length of
the channels before the second, higher vacuum withdraws the liquid
washcoat component thus immobilising the washcoat solids on a
surface of the part. The time between the first and second vacuum
applications may be from 5-10 seconds, such as 6-8 seconds.
Heavy-duty Diesel vehicle parts may require a third or subsequent
vacuum application.
[0066] The inventors have found that the methods of the invention
have particular application for making wallflow filters comprising
catalysts for catalysing the selective reduction of oxides of
nitrogen with nitrogenous reductants such as ammonia and ammonia
precursors, such as urea, for vehicular use. Such selective
catalytic reduction (SCR) catalysts include
V.sub.2O.sub.5/WO.sub.3/TiO.sub.2 and transition metal-exchanged
zeolites such as Fe/Beta zeolite or Cu/CHA. A particular difficulty
with making such products is balancing the competing requirements
of retaining catalyst activity at an acceptable backpressure. High
backpressure has a negative impact on power output and fuel
economy. As emission standards, i.e. the quantities of pollutants
it is permissible to emit from a vehicle, e.g. Euro and Euro 6,
become ever tighter they are also including legislated requirements
for in-use on-board diagnostic (OBD) verification of continuing
catalyst efficacy. OBD requirements are particularly relevant to
catalysed filters as vehicle manufacturers typically include
periodic active removal of particulate matter held on the filter in
their vehicle design to maintain efficient engine performance, in
which exhaust gas temperatures are increased using e.g. engine
management of fuel injection and/or fuel is injected into the
exhaust gas downstream of the engine and combusted on a suitable
catalyst. As vehicle manufacturers are demanding catalyst products
capable of whole (vehicle) life endurance, manufacturers of
catalysed filters seek to counteract catalyst deactivation over
time by loading the filter with as much catalyst as possible at the
outset. However, as mentioned previously, increasing catalyst
loading brings an undesirable increase in filter backpressure.
Whilst it is possible to counteract some of the attendant
difficulties with use of higher porosity filter substrates, such
substrates are more fragile and more difficult to handle. An
alternative means of avoiding unacceptable backpressure is to limit
the amount of catalyst coating. However, decreasing the amount of
SCR catalyst results in lower NO.sub.x conversion and NH.sub.3
storage capacity, which is important for lower temperature NO.sub.x
conversion.
[0067] In developing a method of loading a washcoat of SCR catalyst
onto a wallflow filter substrate, the inventors investigated
conventional coating techniques such as that disclosed in WO
2005/016497, in which a wallflow filter substrate is immersed
vertically in a portion of the catalyst slurry such that the top of
the substrate is located just above the surface of the slurry. That
is, washcoat slurry contacts the inlet face of each channel wall,
but is prevented from contacting the outlet face of each wall. The
sample is left in the slurry for about 30 seconds. The substrate is
removed from the slurry, and excess slurry is removed from the
wallflow substrate first by allowing it to drain from the channels,
then by blowing with compressed air (against the direction of
slurry penetration), and then by pulling a vacuum from the
direction of slurry penetration. The WO '497 disclosure claims that
by use of this technique, the catalyst slurry permeates the walls
of the substrate, yet the pores are not occluded to the extent that
undue back pressure will build up in the finished substrate. The
coated substrate is then dried typically at about 100.degree. C.
and calcined (or fired) at a higher temperature, e.g., 300 to
450.degree. C. The process can be repeated to coat the outlet face
of the wallflow filter.
[0068] More recently, manufacturers of wallflow filters have
started to offer products pre-coated on an inlet face thereof with
a surface membrane comprising finely divided refractory particles
to improve, among other features, particle filtration. See e.g. NGK
Insulator Ltd.'s EP 2158956 and Society of Automotive Engineers
(SAE) Technical Paper 2008-01-0621 from the 2008 World Congress
held in Detroit, Mich. Apr. 14-17, 2008 by the named inventors of
EP '956. The inventors of the present invention noted particular
difficulties when coating these so-called "membrane filters" using
the conventional coating techniques of WO '497. See also WO
00/01463 and WO2010062794
[0069] In particular, conventional (dip) coating of the filter into
a catalyst slurry leads to build-up of coating in the membrane
layer, which the present inventors believe to be due to high
capillary forces that direct the coating slurry into the membrane
layer. The membrane layer can become blocked (or "blinded") with
coating, and the resultant filter has significantly higher
backpressure. Using this conventional coating technique to coat
both inlet and outlet channels of membrane filters with e.g.
transition metal-exchanged zeolite-based SCR coatings results in
the SCR catalyst blocking the membrane structure and the resultant
SCR-coated filter has high backpressure.
[0070] The inventors reasoned that the high backpressure
encountered by using a conventional dip coating process to coat
both inlet and outlet channels of membrane filters could be reduced
significantly by dip-coating only the outlet channels, i.e. the
inlet filter channels, on which the substrate manufacturer has
pre-coated the membrane surface coating, are not coated with SCR
catalyst. However, when they tried this approach, the inventors
found that dip-coating (via the outlet channels) resulted in a
coating gradient with a higher proportion of the catalyst coating
on the rear of the filter, some of which they determined to be
disposed in the membrane structure, despite the catalyst being
applied to the opposite face of the channel wall from the
pre-coated surface membrane layer.
[0071] Subsequently, the inventors found that by applying the
method according to the invention to coat the outlet channels only
of a wallflow filter substrate having inlet channels coated with a
membrane layer, which method using an appropriate washcoat solids
content and a relatively rapid vacuum application, the outlet
channel can be coated more uniformly, i.e. less or substantially no
SCR catalyst applied to the outlet channels is found in the inlet
membrane layer.
[0072] According to a further aspect, the invention provides a
method of coating outlet channels of a wallflow filter substrate
monolith to which the manufacturer of the wallflow filter substrate
monolith has pre-coated to inlet channels thereof a surface
membrane layer comprising finely divided inorganic solids with an
axially substantially uniform catalyst washcoat, which method
comprising the steps of: [0073] (i) holding a honeycomb monolith
substrate substantially vertically; [0074] (ii) introducing a
pre-determined volume of the liquid into the substrate via open
ends of the channels at a lower end of the substrate; [0075] (iii)
sealingly retaining the introduced liquid within the substrate;
[0076] (iv) inverting the substrate containing the retained liquid;
and [0077] (v) applying a vacuum to open ends of the channels of
the substrate at the inverted, lower end of the substrate to draw
the liquid along the channels of the substrate.
[0078] Advantages of this aspect of the present invention include
that catalyst washcoating (via the outlet channels) decreases the
coating gradient and gives lower soot loaded backpressure, (where
ammonia or an ammonia precursor is used as a reductant) a higher
NH.sub.3 storage and a higher NO.sub.x conversion (fresh and
hydrothermally aged) compared to an identical catalysed substrate
monolith wherein the catalyst washcoat is instead applied to the
outlet channels by a conventional dip-coating technique (as
described in WO '497). The inventors believe that this improved
coating uniformity should also contribute to better flow
distribution over the filter, which is relevant for dosing of
reducing agents such as nitrogenous reducing agents and subsequent
control of NH.sub.3 slip and NO.sub.x conversion.
[0079] As can be seen in Example 1 and FIG. 8, reducing the amount
of catalyst on the rear of a filter is also beneficial for
"real-world" ageing conditions, as this region is generally exposed
to more severe conditions (higher temperatures and greater ash
exposure) which can result in relatively lower catalytic
performance from this region than that of the front part of the
filter. Utilising the coating method according to the present
invention (via the outlet channels instead of dip coating via the
outlet channels) can decrease the proportion of the catalyst coated
on the rear of the filter and should give a real-world performance
benefit.
[0080] A further advantage of the methods of the present invention
generally over conventional dip-coating methods is that selective
adsorption of components from a multi-component catalyst washcoat
at the expense of one or more other components of the washcoat by
the honeycomb substrate monolith can be substantially reduced or
eliminated compared to a conventional dip coating method.
[0081] According to a further aspect, the invention provides a
catalysed wallflow filter substrate monolith to which the
manufacturer of the wallflow filter substrate monolith has
pre-coated to inlet channels thereof a surface membrane layer
comprising finely divided refractory solids, wherein the outlet
channels comprise an axially substantially uniform coating profile
of catalyst washcoat, which catalysed wallflow filter substrate
monolith is obtainable by the method according to the
invention.
[0082] According to a further aspect, the invention provides a
catalysed wallflow filter substrate monolith to which the
manufacturer of the wallflow filter substrate monolith has
pre-coated to inlet channels thereof a surface membrane layer
comprising finely divided refractory solids, wherein the outlet
channels comprise an axially substantially uniform coating profile
of catalyst washcoat, wherein the washcoat loading in an axially
upstream half of the catalysed wallflow filter substrate monolith
is within 10% (preferably within 8%, more preferably within 5%) of
the washcoat loading in the axially downstream half thereof.
[0083] In embodiments, the porosity of the wallflow filter
substrate monolith according to the latter two aspects of the
invention prior to coating is from 40 to 80%. In preferred
embodiments, the porosity of filters for use in the present
invention are typically >40% or >50% and porosities of 45-75%
such as 50-65% or 55-60%.
[0084] In further embodiments, a mean pore volume of the wallflow
filter substrate monolith prior to coating is from 8 to 45 .mu.m,
for example 8 to 25 .mu.m, 10 to 20 .mu.m or 10 to 15 .mu.m. In
particular embodiments, the first mean pore size is >18 .mu.m
such as from 15 to 45 .mu.m, 20 to .mu.m e.g. 20 to 30 .mu.m, or 25
to 45 .mu.m.
[0085] In embodiments, the catalyst washcoat applied to the outlet
channels of the wallflow filter substrate monolith of the invention
is a NO.sub.x trap, a catalysed soot filter washcoat comprising
supported platinum group metal or a NH.sub.3-SCR catalyst,
preferably a NH.sub.3-SCR catalyst.
[0086] Preferably the NH.sub.3-SCR catalyst comprises a transition
metal exchanged zeolite and most preferably the transition metal is
selected from the group consisting of copper, iron, cerium and
mixtures of any two or more thereof and the zeolite is selected
from the group consisting of Ferrierite, CHA, BEA and MFI (ZSM-5).
Particularly preferred combinations are Cu/CHA, Fe/Ferrierite, Fe/
or Fe--Ce/ZSM-5 and Fe or Fe--Ce/BEA.
[0087] FIG. 1 shows the starting position for an apparatus 10
according to the invention, wherein 12 is a holding means
comprising a housing 14 for receiving a lower end of a substrate
monolith and a pair of inflatable collars 16a, 16b in the deflated
condition, wherein the charge piston 18 disposed within cylinder 20
is in the extended, or first, position.
[0088] FIG. 2 shows the arrangement wherein a substrate 22 is
inserted into the holding means 12 by e.g. a "pick and place"
robotic arm and is supported by a surface of the piston head 24
comprising an elastometric material.
[0089] FIG. 3 shows the apparatus of FIG. 2, wherein the inflatable
collars 16a, 16b are actuated to engage with an outer surface of
the substrate monolith 22.
[0090] FIG. 4 shows how the charge piston 18 is lowered by a servo
(not shown) to a pre-programmed depth and a measured dose of
washcoat 26 is pumped through a dose control valve 28 by a
volumetric depositor (not shown) via washcoat supply line 30 into
the displacement volume 32 defined, in part, by an internal wall of
the cylinder, the cylinder head and the piston head 18.
[0091] In FIG. 5, with the dose control valve 28 closed, the
washcoat charge 26 is pushed into the base of the substrate 22. The
piston 18 is returned to the first position and the elastomeric
material face seals the dosed lower end face of the substrate 22 in
preparation for inversion.
[0092] In FIG. 6, the substrate 22 is inverted, e.g. rotated
through 180.degree., into position above a vacuum cone 36. The
vacuum cone 36 is raised into position by means of a pneumatic
cylinder (not shown). An inflatable collar 38 in the vacuum cone is
activated and a first vacuum actuation is triggered.
[0093] The inflatable collars 16a, 16b of the holding means 12
disengage and the vacuum cone 36 pulls the substrate 22 downwards
and away from the surface of the piston head 24 (the vacuum cone
collar 38 remains engaged and the pneumatic piston pulls the
substrate downwards). Subsequently, further vacuum actuations are
applied to the substrate 22. There can be any number of vacuum
actuations, but in the illustrated embodiment there are two vacuum
actuations when the substrate has been disengaged from the piston
head. At this stage the final axial coating depth is achieved as
the liquids are removed from the washcoat slurry.
[0094] By means of the pneumatic cylinder, the vacuum cone 36
pushes the substrate 22 upwards and the inflatable collars 16a, 16b
are re-engaged. The vacuum cone collar 38 disengages and then the
vacuum cone 36 is moved downwards. The substrate 22 and apparatus
10 is then rotated back into the first, upright position so that
the coated substrate can be removed vertically, e.g. using a
"pick-and-place" device for subsequent drying. A fresh substrate
can then be inserted into holding means 14 of apparatus 10 and the
routine can be repeated.
EXAMPLES
Example 1 and Comparative Example 2
Application of SCR Catalyst to Outlet Channels of Commercially
Available Wallflow Filter Having Supplier Pre-Coated Inlet Channel
Membrane Layer
[0095] In this Example 1, a commercially available silicon carbide
wallflow filter (NGK Insulators Ltd., Product code: MSC-111), with
circular cross-section (5.66 inch (14.4 cm) diameter) and 6 inches
(15.24 cm)) in axial length, having a cell density of 300 cells per
square inch, channel wall thickness of 0.305 mm, porosity of 52%
and mean pore size of 23 m estimated by mercury porosimetry and
having inlet channels pre-coated by the supplier (i.e. NGK) with a
membrane layer comprising finely divided refractory particles was
used to compare the physical and chemical properties of the filter
having outlet channels coated with a SCR catalyst by the method
according to the invention and a conventional dip coating
method.
[0096] A washcoat comprising a dispersion of copper exchanged (2.5
wt % copper) CHA molecular sieve NH.sub.3-SCR catalyst was applied
to a 100% axial length of the outlet channels only of the MSC-111
product using the apparatus and method according to the invention.
The washcoat solids content of the Cu/zeolite catalyst was 25% and
a silica sol binder at 10% washcoat solids was included. A washcoat
loading of 0.95 gin.sup.-3 was achieved. The coated part was dried
in flowing air at 100.degree. C. and calcined (i.e. fired) at
500.degree. C. for 1 hour.
[0097] For Comparative Example 2, a similar product at identical
washcoat loading was obtained by a dip coating method described in
WO 2005/016497 using an identical washcoat composition, i.e. the
wallflow filter was (1) dipped into the slurry to a depth
sufficient to coat the channels of the substrate along the entire
axial length of the substrate from one direction; (2) vacuumed from
the coated side for approximately 20 seconds; and dried and
calcined as for Example 1.
[0098] Aged catalysed filters were prepared by lean hydrothermally
ageing products of Example 1 and Comparative Example 2 at
800.degree. C. for 16 hours in 10% oxygen (02), 10% water vapour,
nitrogen (N.sub.2) balance.
Example 3
X-Ray Density Analysis of Coated Filters
[0099] Coated wallflow filters prepared according Example 1 and
Comparative Example 2 were analysed using x-ray density analysis
and compared with a MSC-11 filter as received from the supplier
(i.e. having inlet channels pre-coated with a membrane layer, but
without SCR coating applied to the outlet channels). The results
are shown in FIG. 8, wherein the x-ray density trace is overlayed
on the x-ray of the coated or "virgin" part. X-ray density data
points furthest to the left for a given axial location along the
length of the filter indicate relatively high density, e.g. the
end-plugs of the wallflow filter. Contrastingly, data points
furthest to the right for a given axial location along the length
of the filter indicate relatively low density.
[0100] It can be seen from the x-ray density trace for the
"as-received" MSC-111 part that a density gradient already exists
between the inlet end and the outlet end that the inventors surmise
results from the membrane layer applied by the supplier (outlet end
having higher washcoat density than the inlet end). Comparing the
x-ray density trace for the "as received" product with the
Comparative Example 2 product, it can be seen that the coating
profile increases in density towards the outlet end. It can also be
seen that the density actually decreases from the inlet end towards
the middle of the Comparative Example 2 part relative to the "as
received" part.
[0101] The inventors speculate that this coating profile may be
because of the higher density solids at the outlet end cause uneven
airflow during vacuum application in the manufacture of Comparative
Example 2 so that the airflow causes a high level of washcoat
clearing from the axially central portion of the Comparative
Example 2 part. It is also possible that this observation may
result from batch-to-batch variation in the "as received" part.
[0102] By contrast, the filter of Example 1 has a density profile
that is substantially similar to the "as-received" part, including
a similar trend of washcoat density from the inlet end to the
outlet end.
Example 4
Soot Loaded Back Pressure Analysis
[0103] The rate of back-pressure increase relative to soot loading
for each of the filters of Example 1 and Comparative Example 2
using Diesel exhaust gas containing particulate matter were tested
using the Diesel particulate generator (DPG) and test cell
disclosed in European Patent 1850068 A1 and manufactured by
Cambustion Ltd. That is, an apparatus for generating and collecting
particulate matter derived from combusting a liquid
carbon-containing fuel, which apparatus comprising a fuel burner
comprising a nozzle, which nozzle is housed in a container, which
container comprising a gas inlet and a gas outlet, said gas outlet
connecting with a conduit for transporting gas from the gas outlet
to atmosphere, means for detecting a rate of gas flowing through
the gas inlet and means for forcing an oxidising gas to flow from
the gas inlet via the container, the gas outlet and the conduit to
atmosphere, a station for collecting particulate matter from gas
flowing through the conduit and means for controlling the gas
flow-forcing means in response to a detected gas flow rate at the
gas inlet, whereby the rate of gas flow at the gas inlet is
maintained at a desired rate to provide sub-stoichiometric fuel
combustion within the container, thereby to promote particulate
matter formation.
[0104] The filters were fitted each in turn in the station with the
inlet channels pre-coated by the supplier with membrane layer
disposed to receive particulate-containing exhaust gas first. The
apparatus was operated with standard forecourt pump Diesel fuel
containing a maximum of 50 ppm sulphur. The DPG unit was operated
with a gas mass flow rate of 250 kg/hour, a particulate generation
rate of 10 g/hr with an inline particulate silicon carbide filter
maintained at about 240.degree. C. During the particulate matter
loading of each filter the back pressure was determined by a
differential pressure sensor and logged on a computer every 10
seconds.
[0105] The results are shown in FIG. 9, from which can be seen the
soot loading vs. back pressure for fresh samples of wallflow
filters prepared by a dip coating method (Comparative) in
comparison to a sample prepared according to an embodiment of the
invention (Example 1).
Example 5
Fresh and Aged Bench Engine Activity Comparison of Coated
Filters
[0106] The filters of Example 1 and Comparative Example 2 were each
fitted in turn to the exhaust gas system of a bench-mounted Euro IV
compliant, 2 litre direct injection, common rail engine (suitable
e.g. for a passenger car) downstream of a 1 litre oxidation
catalyst (95 g/ft.sup.3 with 2:1 weight ratio of platinum and
palladium coated onto a 350 cell per square inch cordierite
monolithic flow-through substrate), with the filter orientated so
that the channels provided with the pre-coated membrane layer were
on the gas inlet side of the filter. Standard Diesel fuel was used
with 50 ppm sulphur content. A urea injector for injecting urea
solution (AdBlue) into exhaust gas was disposed between the
oxidation catalyst and the filter. Diesel fuel of <10 ppm
sulphur was used. Following an initial warm-up phase, the engine
was run at a series of engine loads in order to achieve a desired
filter inlet temperature. The test conditions used were as shown in
Table 1. "Alpha" is defined as the NH.sub.3/NO.sub.x ratio. So for
an "Alpha of 0.7, a theoretic maximum NO conversion is 70%
according to the reactions
4NO+4NH.sub.3+3O.sub.2.fwdarw.4N.sub.2+6H.sub.2O; and
NO+NO.sub.2+2NH.sub.3.fwdarw.2N.sub.2+3H.sub.2O. An Exhaust Gas
Recirculation valve position programmed into the engine control
strategy of the engine was overridden in order to turn EGR off, so
that steps 3-5 inclusive would be concluded in a reasonable period
of time (instead of hours). The entire series of steps 1 through 5
were conducted one immediately following another,
TABLE-US-00001 TABLE 1 Experimental Protocol for NO.sub.x
Conversion and NH.sub.3 Slip Tests Filter inlet Urea injection Step
No. temperature (.degree. C.) strategy Step end point 1 450 Target
0.7 Alpha 2.5 minutes at set (EGR on) evaluation point 2 400 Target
0.7 Alpha 2.5 minutes at set (EGR on) evaluation point 3 300 Target
1.5 Alpha Detection of 20 ppm (EGR off) ammonia slip at filter
outlet 4 250 Target 1.5 Alpha Detection of 20 ppm (EGR off) ammonia
slip at filter outlet 5 220 Target 1.5 Alpha Detection of 20 ppm
(EGR off) ammonia slip at filter outlet
[0107] The results for the "fresh" catalysed filters are shown in
FIGS. 10 and 11, wherein it can be seen that the peak NO.sub.x
conversion activity of the catalysed filter of Example 1 is
significantly better at each of the three temperature data points
shown. For the NH.sub.3 slip test, it can be seen that NH.sub.3 is
slipped later for the Example 1 catalysed filter than the
Comparative Example 2 catalysed filter. This shows that the
catalysed filter of the invention has a greater NH.sub.3 storage
capacity than the Comparative filter, which is important for
promoting NO.sub.x conversion at low temperature.
[0108] The corresponding results for the aged samples are shown in
FIGS. 12 and 13 respectively, wherein the NO.sub.x conversion
activity is significantly better for the Example 1 sample than the
sample of Comparative Example 2. The same delay in ammonia slip is
also seen showing that, post-ageing, the same advantages are
maintained for the relative to the fresh samples.
[0109] From the results of the Examples taken as a whole, it can be
seen that the MSC-111 wallflow filter samples outlet channel-coated
with SCR catalyst according to the present invention show a more
uniform coating profile, have a lower soot-loading backpressure and
greater NH.sub.3-SCR NO.sub.x conversion (both fresh and aged) and
greater NH.sub.3 storage capacity than the same wallflow filter
coated using a conventional, prior art dip-coating technique.
[0110] For the avoidance of any doubt, the entire content of each
patent document referenced in this specification is incorporated
herein by reference in its entirety.
* * * * *