U.S. patent application number 12/409212 was filed with the patent office on 2010-04-01 for continuous diesel soot control with minimal back pressure penatly using conventional flow substrates and active direct soot oxidation catalyst disposed thereon.
Invention is credited to John G. Nunan, Barry W. L. Southward.
Application Number | 20100077727 12/409212 |
Document ID | / |
Family ID | 42055931 |
Filed Date | 2010-04-01 |
United States Patent
Application |
20100077727 |
Kind Code |
A1 |
Southward; Barry W. L. ; et
al. |
April 1, 2010 |
CONTINUOUS DIESEL SOOT CONTROL WITH MINIMAL BACK PRESSURE PENATLY
USING CONVENTIONAL FLOW SUBSTRATES AND ACTIVE DIRECT SOOT OXIDATION
CATALYST DISPOSED THEREON
Abstract
There is disclosed high cell density or tortuous/turbulent flow
through monolithic catalyst devices for the direct catalytic, and
(semi) continuous oxidation of diesel particulate matter. The
catalysts relate to OIC/OS materials having a stable cubic crystal
structure, and most especially to promoted OIC/OS wherein the
promotion is achieved by the post-synthetic introduction of
non-precious metals via a basic (alkaline) exchange process. The
catalyst may additionally be promoted by the introduction of
Precious Group Metals.
Inventors: |
Southward; Barry W. L.;
(Frankfurt am main, DE) ; Nunan; John G.; (Tulsa,
OK) |
Correspondence
Address: |
SMITH, GAMBRELL & RUSSELL
SUITE 3100, PROMENADE II, 1230 PEACHTREE STREET, N.E.
ATLANTA
GA
30309-3592
US
|
Family ID: |
42055931 |
Appl. No.: |
12/409212 |
Filed: |
March 23, 2009 |
Related U.S. Patent Documents
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Application
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Filing Date |
Patent Number |
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12240170 |
Sep 29, 2008 |
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12409212 |
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12363310 |
Jan 30, 2009 |
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12240170 |
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12363329 |
Jan 30, 2009 |
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12363310 |
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Current U.S.
Class: |
60/274 ;
60/301 |
Current CPC
Class: |
B01D 2255/206 20130101;
B01J 2523/00 20130101; B01J 2523/00 20130101; B01J 23/63 20130101;
B01J 35/04 20130101; B01J 23/002 20130101; B01J 2523/00 20130101;
B01D 2255/102 20130101; B01J 23/66 20130101; B01D 53/944 20130101;
B01D 2255/407 20130101; B01J 2523/00 20130101; B01J 2523/00
20130101; B01J 2523/00 20130101; B01J 2523/828 20130101; B01J
2523/3712 20130101; B01J 2523/3712 20130101; B01J 2523/3712
20130101; B01J 2523/36 20130101; B01J 2523/3718 20130101; B01J
2523/3712 20130101; B01J 2523/48 20130101; B01J 2523/48 20130101;
B01J 2523/3706 20130101; B01J 2523/3712 20130101; B01J 2523/3718
20130101; B01J 2523/3718 20130101; B01J 2523/3706 20130101; B01J
2523/3706 20130101; B01J 2523/3718 20130101; B01J 2523/49 20130101;
B01J 2523/3706 20130101; B01J 2523/828 20130101; B01J 2523/3706
20130101; B01J 2523/3706 20130101; B01J 2523/3706 20130101; B01J
2523/828 20130101; B01J 2523/18 20130101; B01J 2523/23 20130101;
B01J 2523/23 20130101; B01J 2523/48 20130101; B01J 2523/23
20130101; B01J 2523/48 20130101; B01J 2523/18 20130101; B01J
2523/3712 20130101; B01J 2523/3718 20130101; B01J 2523/48 20130101;
B01J 2523/48 20130101; B01J 2523/49 20130101; B01J 2523/824
20130101; B01J 2523/48 20130101; B01J 2523/00 20130101; B01J
2523/00 20130101; B01J 2523/3712 20130101; B01J 2523/3718
20130101 |
Class at
Publication: |
60/274 ;
60/301 |
International
Class: |
F01N 3/20 20060101
F01N003/20; F01N 3/10 20060101 F01N003/10 |
Claims
1. A catalyst system for the direct catalytic oxidation of
particulate matter in the off-gas of an internal combustion engine
wherein the system comprises a standard flow through monolith
device, upon which is coated an active oxidation catalyst
formulation for the direct, low temperature oxidation of
aforementioned particulate matter, with the active catalyst
containing an active redox oxide disposed therein.
2. The catalyst system of claim 1, wherein the monolith is a flow
through monolith with >900 cells per square inch.
3. The catalyst system of claim 1, wherein the monolith is a flow
through monolith with >600 cells per square inch.
4. The catalyst system of claim 1, wherein the monolith is a flow
through monolith with >400 cells per square inch.
5. The catalyst system of claim 1, wherein the monolith is a metal
monolith capable of introducing turbulent flow in the exhaust
stream.
6. The catalyst system of claim 1, wherein the monolith is a metal
or ceramic foam presenting a flow path of highly tortuous
nature.
7. The catalyst system of claim 1, wherein the catalyst system is a
refractory oxide.
8. The catalyst system of claim 1, wherein the catalyst system
contains cerium.
9. The catalyst system of claim 1, wherein the oxide is a cerium
oxide in the form of a solid solution of cerium and zirconium oxide
(Ce--Zr oxide).
10. The redox active oxide of claim 1, wherein the oxide is a
cerium oxide in the from of a Ce--Zr oxide solid solution that is
substantially phase pure cubic fluorite solid solution (as
determined by conventional XRD method) with oxygen ion conducting
properties and comprises a. up to about 95% zirconium b. up to
about 95% cerium c. up to about 20% of a stabiliser selected from
the group consisting of rare earths, yttrium and mixtures
thereof.
11. The catalyst system of claim 1, wherein the catalyst system is
a substantially phase pure cubic fluorite solid solution
additionally modified by the introduction of one or more base metal
dopant species selected from the group consisting of a transition
metal, an alkali metal, an alkaline earth metal and a group IIIb
metal.
12. The catalyst system of claim 11, wherein the redox oxide is a
base metal doped cerium containing cubic fluorite solid solution
produced by contacting redox active material with a precursor
solution of dissolved cations under conditions of high ph/low
hydronium ion (H.sub.3O.sup.+)/low proton (H.sup.+) content.
13. The catalyst system of claim 12, wherein the base metal is
introduced into the redox active oxide by means of an ammonium
hydroxide/ammoniacal complex of the metal cation.
14. The catalyst system of claim 12, wherein the base metal is
introduced into the redox oxide by means of an organic amine
complex of the metal cation.
15. The catalyst system of claim 12, wherein the base metal is
introduced into the redox oxide by means of a hydroxide compound of
the metal cation.
16. The catalyst system of claim 12, wherein the concentration of
metal species introduced is about 0.01 weight % to about 10 weight
%.
17. The catalyst system of claim 16, wherein the concentration of
metal species introduced is most preferably 0.1 wt % to about 2.5
wt %
18. The catalyst system of claim 12, wherein the base metal doped
solid solution contains metal at high levels of dispersion such
that phase analysis by conventional XRD methods retains a
substantially phase pure cubic fluorite phase (>95%), with bulk
metal oxide dopant phase being recorded at <5% and dopant metal
oxide particle size, as determined by line-broadening/Scherrer
equation method, is about 30 A to about 100 A.
19. The catalyst system of claim 12, wherein the base metal doped
solid solution contains metal at high levels of dispersion such
that phase analysis by XRD reveals the promoted material maintains
at least 95% cubic fluorite phase after hydrothermal oxidising
aging at 1100.degree. C.
20. The catalyst system of claim 12, wherein the base metal doped
solid solution contains metal at high levels of dispersion such
that phase analysis by XRD reveals the promoted material maintains
at least 99% cubic fluorite phase after hydrothermal oxidizing
aging at 1100.degree. C.
21. A device for the direct catalytic oxidation of soot comprising
the catalyst system of claim 1, and a housing wherein the
temperature of required for soot oxidation is about 100 to about
650.degree. C.
22. A device for the direct catalytic oxidation of soot comprising
the catalyst system of claim 1, and a housing wherein the
temperature of required for soot oxidation is about 200 to about
400.degree. C.
23. A device for the direct catalytic oxidation of soot comprising
the catalyst system of claim 1, and a housing wherein continuous
soot oxidation occurs for temperatures of about 100 to about
650.degree. C.
24. A catalytic system for the direct catalytic oxidation of soot
according to claim 1, wherein the catalyst system is free of a
platinum group metal.
25. The catalyst system for the direct catalytic oxidation of soot
according to claim 1, further comprising a platinum group
metal.
26. The catalyst system for the direct catalytic oxidation of soot
according to claim 25, wherein the platinum group metal is selected
from the group consisting of platinum, palladium, rhodium and
mixtures thereof.
27. The catalyst system for the direct catalytic oxidation of soot
according to claim 25, further comprising a catalytically active
washcoat disposed upon the monolith as a single layer washcoat
which additionally contains Al.sub.2O.sub.3, modified
Al.sub.2O.sub.3, SiO.sub.2, ZrO.sub.2, or combinations thereof or
other suitable refractory oxide as an additional support or binding
agent.
28. The catalyst system for the direct catalytic oxidation of soot
according to claim 25, further comprising a catalytically active
washcoat disposed upon the monolith in two or more layers with a
first layer containing substantially Al.sub.2O.sub.3, modified
Al.sub.2O.sub.3, SiO.sub.2, ZrO.sub.2, combinations thereof or
other suitable refractory oxide as a support or binding agent and a
second layer comprising the active oxidation catalyst formulation,
including a base metal doped mixed oxide.
29. A method of treating exhaust gas comprising passing an exhaust
gas over the catalyst system of claim 1.
Description
REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of provisional
application 61/308,879, filed Mar. 27, 2008, and is a
continuation-in-part of application Ser. No. 12/240,170 filed Sep.
29, 2008, and application Ser. Nos. 12/363,310 and 12/363,329, both
filed Jan. 30, 2009, all of which are relied on and incorporated
herein by reference.
INTRODUCTION AND BACKGROUND
[0002] Over the last thirty years increasingly stringent
legislative limits have been introduced to regulate the emissions
from both petrol (gasoline) and diesel internal combustion engines.
See Regulation (EC) No 715/2007 of the European Parliament and of
the Council, 20 Jun. 2007, Official Journal of the European Union L
171/1, see also Twigg, Applied Catalysis B, vol. 70 p 2-25 and R.
M. Heck, R. J. Farrauto Applied Catalysis A vol. 221, (2001), p
443-457 and references therein. In the case of diesel/compression
ignition engines this has led to the implementation of the Diesel
Oxidation Catalyst (DOC), Diesel NOx Trap/NOx Storage Catalyst
(DNT/NSC) and Selective Catalytic Reduction catalyst (SCR) to
address gaseous emissions of CO, HC (DOC) and nitrogen oxides
(NOx). However, in addition to the gaseous components, the diesel
exhaust stream also contains entrained solids, commonly referred to
as particulate matter or soot. This carbon-based material is a
byproduct of incomplete combustion and arises due to heterogeneity
of the air-fuel mixture within the cylinder and presents a unique
and specific challenge with regards to its control and conversion
into environmentally benign products. Thus, while previously it has
been possible to meet all legal requirements for exhaust emissions
of particulates via engine-related control measures only (SAE Paper
2007-01-0234, Pfeiffer et al.), the stringent targets embodied in,
for example, Euro 5 or Euro 6 (Regulation (EC) No 715/2007 of the
European Parliament and of the Council, 20 June 2007, Official
Journal of the European Union L 171/1) necessitates the
introduction of the Diesel Particulate Filter (DPF), aka the
`Wall-Flow Filter` to enable specific remediation of soot.
[0003] The DPF typically comprises an inert porous ceramic e.g.
silicon carbide, cordierite etc. monolith substrate which may be
additionally wash-coated with an active catalytic formulation to
facilitate the chemistries required of the device e.g. soot
combustion, (secondary) emission control, NOx abatement, etc. The
wash-coat formulation itself is typically a heterogeneous-phase
catalyst and may contain particles of highly active precious group
metal (PGM) dispersed and stabilized on a refractory oxide support
or supports; e.g. alumina. The DPF may additionally contain an
Oxygen Storage (OS) component to enhance the regeneration function
of the filter.
[0004] The DPF achieves high filtration efficiency of particulates
as a result of the physical filtration achieved by forcing the
exhaust flow through the porous wall of filter. However, over time
this results in a build up of stored material, commonly referred to
as a filter-cake, within the filter which results in an ever
increasing back pressure penalty, arising from the work required to
force the gas flow through an increasingly dense flow restriction.
This flow restriction leads to an unacceptable decrease in engine
performance and hence, the filter-cake must be combusted in order
to `regenerate` the filter to a near pristine condition such that
it is able to again store the carbonaceous particulates with
minimal back pressure penalty. However, at this time a fully
passive and continuous soot regeneration technology has not been
demonstrated on a vehicle and hence the complete regeneration of
the filter requires an "active" or forced regeneration strategy;
see e.g. U.S. Pat. No. 7,441,403; U.S. Pat. No. 7,313,913. These
active strategies are reliant upon the manipulation of the gross
reaction conditions of the exhaust in order to achieve filter
regeneration. Hence, the regeneration of the particulate filter
described above may be achieved by the use of auxiliary devices.
For example, an air fuel nozzle and an ignition device can be used
and operated, when desired, to heat the exhaust gases and the
particulate filter to the temperature required for the homogeneous
combustion of the trapped particulate matter. In this manner, the
trapped soot may be burned from the filter surfaces to permit a
continuous flow of the exhaust gases. Alternatively, an electric
heater may be employed to generate the heat to initiate the
combustion cycle; e.g. U.S. Pat. No. 7,469,532. More commonly
however, the filter is regenerated by a so-called "post-injection"
cycle in which secondary fuel is introduced, either by late
cylinder injection or via dedicated fuel injection unit in the
exhaust train, and the hydrocarbons thus entrained in the exhaust
flow are combusted over an oxidation catalyst situated prior to the
DPF to generate a transient thermal `bloom` within the filter which
initiates the conversion of the soot into environmentally benign
products (CO.sub.2, H.sub.2O); e.g. see SAE paper 2008-01-0481 and
references therein.
[0005] However, the use of wall-flow filters to achieve efficiency
in particulate removal presents some immediate issues. Firstly due
to the wall flow mechanism of filtration, the DPF introduces a
significant back-pressure on the engine. Moreover, the addition of
a washcoat to the bare filter increases this back pressure and the
sieving action employed to trap soot results in yet a further and
continuous increase in back pressure. As indicated previously any
increases in backpressure are at the expense of engine efficiency
and lead to an ever increasing fuel economy penalty, due to the
wasted work pushing exhaust gas through the soot filter cake,
washcoat formulation and filter. Thus, significant efforts have
been expended in the development of mechanically and thermally
robust DPFs with high filtration efficiency but decreased
back-pressure penalty and in the development of active washcoat
formulations capable of high conversions at minimal wash-coat loads
in attempts to minimize the back-pressure/fuel economy issues
otherwise evident.
[0006] Additionally, there remain outstanding questions regarding
the regeneration cycle employed by the DPF. Such traditional
`active` cycles are all energy intensive and result in a
substantial and unattractive fuel penalty; i.e. an additional and
ongoing operational cost. Thus, the use of sacrificial hydrocarbon
species in the active regeneration cycle imposes as high as a 5%
decrease in fuel economy. Moreover, the implementation of an active
emissions control strategy requires complex and accurate engine
management protocols to avoid incomplete regeneration and/or
untreated emissions; e.g. U.S. Pat. No. 7,412,822. In addition,
soot combustion initiated in this manner results in a phenomenon
known as `oil dilution` which can both adversely affect engine
operation and result in ash deposition (inorganic salts) within the
filter which impact the back pressure, soot capacity and catalytic
performance of the filter; e.g. U.S. Pat. No. 7,433,776. Finally,
it should be noted that soot combustion initiated in this manner
proceeds in a more homogeneous; i.e. non-catalytic manner and can
be uncontrolled. This in turn can result in localized exothermic
`hotspots` of extreme temperature (T>1000.degree. C.) which can
easily damage the catalytic efficiency of a formulation (PGM
sintering, PGM de-alloying, surface area and porosity collapse of
the support oxide). In the worst case scenario, catastrophic
uncontrolled combustion of soot can destroy the DPF monolith
through thermal degradation or even melting the monolith.
[0007] Hence, many attempts have been made to address or limit the
extent of the issues related to the active regeneration strategy.
Such efforts are exemplified by attempts to introduce more passive
regeneration strategies based upon the use of the redox chemistry
of advanced OS materials, e.g. US 2005/0282698 A1. In these studies
it was shown that decreases in the temperature required for soot
oxidation may be achieved by utilisation of active oxygen species
derived from a redox active washcoat OS material. The OS materials
used in the DPF are typically based upon CeO.sub.2 or other redox
oxide and are employed to `buffer` the catalyst from local
variations in the air/fuel ratio during regeneration or other
transient event. They do this by `releasing` active oxygen from
their 3-D structure in a rapid and reproducible manner under
oxygen-depleted transients, `regenerating` this lost oxygen by
adsorption from the gaseous phase when oxygen-rich conditions
arise. This activity is attributed to the redox activity of
CeO.sub.2 via the 2Ce.sup.4+.fwdarw.2Ce.sup.3++[O.sup.2-] reaction.
This high availability of oxygen is critical for the promotion of
generic oxidation/reduction chemistries e.g. CO/NO chemistry for
the petrol (gasoline) three-way catalyst, or more recently for the
direct catalytic oxidation of particulate, matter (soot) in the
CDPF e.g. US 2005/0282698 A1, SAE 2008-01-0481 and references
therein. This work is one of many studies examining the chemistry,
synthesis, modification and optimisation of Ce--Zr based OS
materials. For example, the use of Ceria-Zirconia materials doped
with lower valent ions for emission control applications have been
extensively studied e.g. U.S. Pat. No. 6,468,941, U.S. Pat. No.
6,585,944. These studies demonstrate that lower valent dopant ions
such as Rare Earth metals, e.g. Y, La, Nd, Pr, etc., Transition
metals e.g. Fe, Co, Cu etc. or Alkaline Earth metals e.g. Sr, Ca
and Mg can all have a beneficial impact upon oxygen ion
conductivity This is proposed to arise from the formation of oxygen
vacancies within the preferred cubic fluorite lattice of the solid
solution which lowers the energy barrier to oxygen ion transport
from the crystal bulk to the surface thereby enhancing the ability
of the solid solution to buffer the air fuel transients occurring
in the exhaust stream of a typical petrol (gasoline) three-way
catalyst application.
[0008] Additionally it has been shown (U.S. Pat. No. 6,468,941 and
U.S. Pat. No. 6,585,944) that the use of specific examples of the
above dopants can provide full stabilization of the preferred cubic
fluorite lattice structure for ceria-zirconia solid solutions, with
Y being identified as having particular benefit hereto. The
presence of the preferred cubic fluorite structure has been found
to correlate with the most facile redox chemistry for
Ce.sup.4+.fwdarw.Ce.sup.3+, from both the surface and bulk of the
crystal, thus dramatically increasing the oxygen storage and
release capacity, as compared to bulk CeO.sub.2. This benefit is
especially pronounced as the material undergoes crystal
growth/sintering due to the hydrothermal extremes present in
typical exhaust environments. The incorporation of especially Y and
to a lesser extent La and Pr have also been demonstrated to limit
or, in certain cases, circumvent the disproportionation of the
single cubic phase Ceria-Zirconia into a composite consisting of
more Ce-rich cubic phases and more Zr-rich tetragonal phases, a
process which results in marked decrease in redox function, surface
area etc. of the solid solution.
[0009] Finally, U.S. Pat. No. 6,468,941, U.S. Pat. No. 6,585,944,
U.S. patent application Ser. No. 12/363,310 and U.S. patent
application Ser. No. 12/363,329 (both applications being
incorporated herein by reference) teach the potential for employing
base, i.e. non-precious group (Pt, Pd, Rh, Au etc.) dopant metals
into or with the cubic fluorite lattice of the solid solution as an
alternative means to promote the redox chemistry of cerium, with
Fe, Ni, Co, Cu, Ag, Mn, Bi and mixtures of these elements being
identified as of particular interest. Hence while typical
non-promoted OS materials typically exhibit a redox maximum, as
determined by H.sub.2 Temperature Programmed Reduction (TPR), at
ca. 600.degree. C., the inclusion of base metals within the lattice
can decrease this temperature by >200.degree. C. at a fraction
of the cost incurred by the use of precious metals.
[0010] However, while these base metals can be beneficially
incorporated in the CeZrOx lattice and this incorporation can
significantly promote low temperature redox function for fresh
materials, the addition of these elements can also decrease fresh
and aged phase purity and significantly decrease hydrothermal
durability (promote crystal sintering and material densification),
leading to losses in aged performance cf. base compositions without
additional base metal. In addition, during conventional aging
cycles reactions may occur between the gas phase and the CeZr
material which can result in extraction of these additional base
elements from the cubic fluorite lattice. This, in turn, can result
in formation of separate bulk phase(s) with low intrinsic catalytic
activity or in a worst case scenario, phases which directly
interact with the OS or other catalyst component resulting in a
direct or indirect poisoning of the catalyst. Hence, until
recently, particular synthetic care was required to enable the
incorporation of promotant lower valent ions into the cubic
fluorite structure while ensuring both the electrical neutrality
and phase preservation. Thus, as shown in U.S. application Ser. No.
12/363,310, the synthesis of an OS material containing a specific
low valent base metal promoter (Ag) `doped` into a cubic fluorite
structure with ca. 40% Ce resulted in phase disproportionation into
Ce-rich and Ce-poor domains, with a marked decrease in redox
performance. This contrasted with a newly developed basic exchange
process which was able to provide an equivalent composition with
high activity and hydrothermal durability for use in the DPF.
[0011] Unfortunately, despite the large number of attempts to
employ advanced OS materials in either passive or active
regeneration methodologies in vehicular applications these have
previously met with limited success. Extensive studies of the
chemistry occurring in these systems have demonstrated that the
activity of the OS-based catalyst is dependent upon high `Contact
Efficiency` between the OS material and the soot; e.g. see, Applied
Catalysis B. Environmental 8, 57, (1996). Subsequent studies,
described in SAE paper 2008-01-0481 and U.S. application Ser. Nos.
12/363,310 and 12/363,329 have now identified that the loss of
contact efficiency between the OS and soot may arise from specific
chemistries involving the significant NO engine emissions typical
of pre-EuroV legislation engines. This process has been denoted as
`de-coupling` of the OS and soot and is the result of the reaction
of engine out NO over oxidized PGM to produce NO.sub.2 which
combusts the soot in the immediate environment of the catalyst
producing CO+NO. The NO byproduct of this process is further
`recycled` to NO.sub.2 and the soot combustion re-initiated, again
removing only that soot which immediately contacts the catalyst.
This cycle is the basis of U.S. Pat. No. 4,902,487 and was
previously believed to be the major reaction providing low
temperature soot combustion/regeneration. However, this mechanism
appears only effective at removing low concentrations of soot and
indeed only that proportion of soot in direct contact with the
catalyst. Thus, this mechanism effectively `de-couples` the
catalyst and soot and dramatically decreases the effectiveness of
the OS-mediated regeneration method and may in fact be considered
to be a reactive poison which effectively `deactivates` the `true`
OS mediated low temperature, passive, soot regeneration reaction
required for optimum soot emission control. Fortunately, however,
the design of the next generation ion exchanged OS materials has
been found to be effective at both circumventing this `de-coupling`
process and also in promotion of the redox characteristics of the
OS and hence demonstrated robust performance benefits with respect
to soot regeneration on all both the engine dynamometer and in
vehicle trials on wash-coated DPFs (see U.S. application Ser. No.
12/363,329).
[0012] Based upon the aforementioned requirements and challenges,
it is apparent that the conventional approach of using the
wall-flow DPF to ensure highly effective trapping and subsequent
combustion of particulates presents many challenging technical
obstacles. Accordingly, what is needed in the art are improved
materials and/or methods for the control and conversion of
particulate matter whilst offering both a reduction in fuel penalty
for regeneration but also decreased complexity with regards to
initiation and control of any regeneration cycle compared to the
traditional DPF and conventional active regeneration strategy.
Herein we propose the use of an in-line soot combustion device and
catalyst with minimal/significantly decreased back-pressure penalty
for the (semi) continuous and (semi) passive combustion of retained
particulate matter at significantly lower temperatures than those
required for the conventional wall-flow filter.
SUMMARY OF THE INVENTION
[0013] A significant advance in the development of a method and
apparatus for the (semi) continuous, direct catalytic, oxidation of
diesel particulate matter may be realised by the combination of
base metal modified Oxygen Storage (OS) materials with a
conventional flow substrate. The substrate is selected from a range
of ceramic or metallic technologies upon which the active washcoat
is disposed. Such substrates can be metallic parts, ceramic or
metal foams. The substrate is further characterised by presenting a
high number of channels or cells per unit area or by the ability to
introduce turbulent flow due to the construction of its internal
flow channels. The particular combination of the base metal
modified OS direct soot oxidation catalyst with the flow through
monolith provides a synergy which enables high conversion of
particulate matter without the backpressure penalty introduced by
the conventional DPF. Specifically, the synergy is believed to
arise from the ability of the active OS to combust soot at lower
temperatures which in turn is facilitated by the decreased thermal
mass of the conventional substrate, with the latter still providing
sufficient geometric surface area for soot deposition and reaction.
This provides for the large improvements in lower temperature
activity and is in marked contrast to the conventional wall flow
DPF wherein large thermal mass of the substrate, particularly for
SiC DPF, inhibits initiation and especially propagation of soot
combustion. Thus, this combination of technologies provides a means
for the effective conversion of particulate matter under conditions
more typical of the standard driving cycle i.e. soot combustion
without recourse to high temperature active regeneration cycles and
the various penalties and other issues associated thereto.
[0014] The doped OS materials herein are based upon
ZrO.sub.2/CeO.sub.2 solid solutions containing a substantially
phase pure cubic fluorite structure and are produced by the
specific ion exchange of base i.e. non-precious group metals. The
range of appropriate materials and full details regarding execution
of the ion exchange are described in U.S. application Ser. Nos.
12/363,310 and 12/363,329. The mode of ion exchange essentially
involves the introduction of active metal/cations into the solid
solution under chemically basic, i.e. conditions of high pH, that
is say high OH.sup.-/low hydronium (H.sub.3O.sup.+) or proton
(H.sup.+) content. As demonstrated in the aforementioned work, the
resultant materials demonstrate high activity and hydrothermal
durability in contrast to any promotion realized by conventional
impregnation of an acidic metal e.g. metal nitrate, where formation
of bulk oxide phases in fresh materials and rapid sintering of such
oxide phases, with resultant deactivation, is the norm. The
proposed exchange of the H.sup.+ species, present at Ce.sup.3+
defect sites within the Ce--ZrOx lattice, by metal ions enables the
incorporation and stabilization of specific mono-valent e.g.
K.sup.+, di-valent e.g. Cu.sup.2+, tri-valent e.g. Fe.sup.3+ and
higher valence ions at high dispersion within the oxide matrix. The
choice of base metals thus incorporated is based upon oxides known
to be active for reactions of especial interest or catalytic
importance. Metals of specific catalytic significance include Ag,
Cu, Co, Mn, Fe, alkali metals, alkaline earth metals or transitions
metals, or other metal or metalloid known to form a stable nitrate
which can undergo subsequent decomposition and reduction N.sub.2
under conditions within the conventional operational window of the
vehicle exhaust. The term "transition metal" refers to the 38
elements in Groups 3 to 12 of the Periodic Table of Elements.
[0015] The use of high cell density/turbulent flow through
monoliths is also required to provide sufficient interaction and
subsequent reaction between the entrained soot particles within the
exhaust flow and the active catalytic coating. The term high cell
density is consistent with preformed flow through monolith
substrates with a large (.gtoreq.600) number of individual cells of
flow channels per square inch. It is proposed that this high cell
density firstly introduces turbulence at the inlet to maximise
possible soot collisions with the active wash-coated walls of the
monolith. Secondly, the high cell density restricts the flow path
through the monolith, again increasing the potential for
particulate collisions and retention/reaction on the active
wash-coat, but without the large backpressure penalties associated
with the conventional DPF. Moreover the use of the flow-through
substrate removes existing constraints regarding total washcoat
loading, or the use of layered technologies with specific
functionalities, e.g. soot combustion catalyst in one layer
(overcoat) and SCR catalyst in a second layer (undercoat), equally
it enables the use of an undercoat rich in Al.sub.2O.sub.3 to
provide high washcoat adhesion, but with low intrinsic catalytic
function, onto which a second pass containing all required OS, PGM
and NOx trap etc. active components may be dispersed. In this
second example, the overcoat would under normal conditions present
lower adhesion and would conventionally be diluted with binder,
e.g. Al.sub.2O.sub.3, however, the incorporation of binder results
in a decrease in activity due to dilution of the active phase,
hence the layered design is preferred. This layering ensures the
surface coating that would interact/react with the soot as it
passed through the flow-through substrate would exclusively consist
of active material and would therefore maximize catalytic action.
The enabling of higher washcoat loads when using the flow through
monolith also provides the capability of employing higher
concentrations of active materials to be coated on the substrate
thereby further enhancing the performance and hydrothermal
durability of the technology without the catastrophic back pressure
penalty such an approach would present using the conventional DPF.
Hence by use of the flow through substrate washcoat load could be
increased from 10 g/l to 180 g/l or higher concomitantly increasing
the effective geometric surface area for catalyst to soot contact
to again increase in combustion efficiency.
[0016] In addition by use of the flow though monolith the textural
characteristics of the washcoat e.g. particle size, roughness etc.
may be optimised for activity rather than merely to minimize back
pressure penalty. Conventional formulations for DPFs typically
target a D.sub.50 (diameter of particle at 50%) value of 5 microns
or less to enable `in-wall` coating, i.e. coating of the internal
porosity of the substrate without formation of a discrete washcoat
layer on the surface of monolith, in order to minimize backpressure
penalty. Such a particle size distribution is typically achieved by
aggressive milling of the raw materials used in the washcoat.
However, the use of this `hyper-milling` to obtain the very small
particles for in-wall coating has been found to be extremely
destructive to the activity, stability and surface areas of the OS
and alumina components employed in typical formulations. As a
result such a process can adversely affect the rate of release and
total Oxygen Storage capacity of the OS. In addition the
hyper-milling can result in cation extraction and phase
disproportionation for the OS with further poisoning of any PGM
function arising from the deposition of extracted cations. In
contrast, the use of a washcoat with high textural/roughness
characteristics has previously been identified as beneficial in
three-way applications (e.g. see SAE 2005-01-1111) and may enhance
initial flow turbulence and thus increase the probability of
catalyst to soot contact. The retention of texture due to the
absence of aggressive milling can also be expected to increase the
probability of primary soot moieties effectively contacting the OS
material. The extent of intimate contact has been shown to
correlate directly with direct soot combustion (see Applied
Catalysis B. Environmental 8, 57, 1996, U.S. application Ser. No.
12/363,329, SAE 2008-01-0481). Moreover, the integrity of the
active formulation with respect to phase, OS function or PGM
functionalities is always of prime importance especially herein
since it has been demonstrated that the energy produced by the
combustion of HC, CO or the SOF (soluble organic fraction) present
in soot matter have been identified as a means of initiation and
propagation of the combustion of the remaining soot (akin to
striking a match or a primer, see SAE 2008-01-0481).
[0017] Benefits and features of the present invention include:
[0018] a) Provision of a hydrothermally robust direct soot catalyst
system, active at temperatures relevant to diesel vehicle operation
for (semi) continuous, direct catalytic, oxidation of soot;
[0019] b) Particulate control system without requirement for DPF
substrate thereby removing associated substrate cost, back pressure
constraints, canning and space requirements and ancillary systems
associated with conventional DPF;
[0020] c) Provision of an active catalyst providing full oxidation
function without recourse to complex conventional active
regeneration cycles with associated fuel penalty, filter cake
formation, potential for catastrophic uncontrolled regeneration,
oil dilution, ash deposition or other issue associated with
conventional DPF;
[0021] d) Flexibility of coated part design with respect to
washcoat load, particle size/texture and hence the ability to
optimize washcoat based upon performance and durability
requirements and not merely backpressure constraints;
[0022] e) Ability to employ multilayer technologies with specific
functionalised layers to providing additional catalytic properties
and functions from a single monolith and to potentially achieve
further chemical synergies and performance advantages previously
impossible when employing the conventional DPF.
[0023] f) Synergistic operation between the active washcoat and
high cell density substrate to facilitate rapid oxidation of soot
and soluble organic fraction to thereby circumvent the potential
for `face plugging`, a phenomenon associated with the use of
conventional high CPSI monoliths with conventional catalyst
formulations.
[0024] This strategy clearly contrasts to those employed in
conventional DPF systems. For the conventional design catalytic
functionality is typically more limited i.e. control of CO, HC from
primary or secondary emissions ([SAE Paper 2007-01-0234, Pfeiffer
et al.), NH.sub.3--SCR of NOx (US 2008/202107-A), etc. Moreover,
the design constraints for conventional formulations are
significant and are typically based upon the primary balance
required between filtration efficiency and maximum system
backpressure.
[0025] In order to meet the design targets for this synergistic
operation of catalyst and monolith there are several key
performance requirements that must be met. Firstly, there is a
requirement for increased ceria reducibility at lower temperatures
than is conventionally obtained with binary, tertiary or even
quaternary Ce--Zr--REOx systems. This reducibility is critical to
achieve the low temperature O-ion donation from the catalyst to the
soot which has been proposed as being a key reaction step (SAE
2008-01-0481; U.S. application Ser. Nos. 12/363,310 and 12/363,329;
Appl. Catal. B vol. 17, 1998, p 205, Appl. Catal. B vol. 75, 2007 p
189, Catal. Today 121, 2007, p 237, Appl. Catal. B vol. 80, 2008, p
248]. Hence examinations of the use of CeOx or CeZrOx containing
oxide solid solutions for soot oxidation have been widespread.
However conventional CeZrOx solid solutions, as typically employed
in three-way catalysts, typically exhibit a redox maximum, as
determined by H.sub.2 Temperature Programmed Reduction (TPR) at ca.
600.degree. C. This imposes the requirement for high exhaust
gas/reaction temperatures in the application in order for the OS
material to provide the maximum "buffering" or oxygen donation
benefit. Moreover, this requirement for high temperature to access
the active lattice oxygen is a barrier to the implementation of
CeZrOx for lower temperature direct soot oxidation. In order to
address this temperature issue OS materials are typically
"promoted" by the addition of a Precious Group Metal (PGM)
component, e.g. Pt, Pd or Rh. However, promotion by these metals
contributes a large additional cost to the price of the emission
control system. Moreover the addition of PGM, especially Pt,
promotes the `classical` chemistry of NOx-mediated soot oxidation
as described in U.S. Pat. No. 4,902,487. However, it has now been
found and clearly demonstrated that NOx mediated soot oxidation is
only effective at removing low concentrations of soot and indeed
only that proportion of soot in direct contact with the catalyst
and may be considered to be an effective catalyst poison for direct
OS mediated soot oxidation (SAE 2008-01-0481, U.S. application Ser.
Nos. 12/363,310 and 12/363,329). Thus, what is required in the art
is a method to promote the oxygen ion conductivity of the
CeOx/CeZrOx-based oxide material, but without use of expensive PGM
and without the undesirable consequence of increasing the NOx
oxidation chemistry of the catalyst.
[0026] A second limitation, again typical OS materials used to
date, is a limitation with regard to their total Oxygen Storage
Capacity, that is to say the amount of available oxygen as measured
by TPR is typically lower than that expected from consideration of
the total Ce IV content of the OS material. Many data available to
date are consistent with as little as only ca. 50% of the total Ce
IV available undergoing reduction. At this time it is uncertain
whether this is due to a fundamental issue, or due to limitations
with the current synthetic method(s) employed in the manufacture of
the OS material leading to a mixed Ce IV/Ce III valency or whether
a combination of additional chemical, structural or textural
limitations are responsible.
[0027] Finally, typical OS materials provide only limited, if any,
additional synergies to the emission control system. As described
elsewhere, ideal material components provide additional integrated
chemical mechanisms to further enhance emissions control, e.g. NOx
scavenging and reduction to N.sub.2.
[0028] Hence, while OS materials are key components in realising
highly active and materials present significant limitations to
development of the next generation of exhaust catalyst that will be
required to comply with newer and ever more stringent emission
targets. What is required is a new class of OS materials that are
active at lower temperatures, especially the Cold Start portion of
vehicular applications to promote catalytic function. These OS
materials should also display high hydrothermal durability and be
tolerant to potential exhaust poisons in order to enable their use
in the wide range of demanding exhaust environments.
[0029] The above-described and other features will be appreciated
and understood by those skilled in the art from the following
detailed description, drawings, and appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] The invention will be further understood with reference to
the accompanying drawings, wherein:
[0031] FIG. 1--shows a schematic of the synthetic gas bench (SGB)
reactor in which the concept trials were executed;
[0032] FIG. 2--shows the impact of reactive gas mixture during
loading on subsequent soot combustion;
[0033] FIG. 3--outlines the various gas compositions employed
during the loading and regeneration trials;
[0034] FIG. 4--compares the back pressure (hereafter B.P.) response
for a 400 CPSI (cells per square inch) flow through monolith during
a three hour (10800 s) soot loading cycle as a function of either
gas environment during loading or temperature and gas
environment;
[0035] FIG. 5--impact of reaction conditions during loading (ex
FIG. 4) on subsequent soot combustion cycle via TPO;
[0036] FIG. 6--compares the O.sub.2 concentrations at the reactor
outlet during the TPO cycles described in FIG. 5;
[0037] FIG. 7--impact of monolith cell density on B.P. response
during soot loading;
[0038] FIG. 8--impact of monolith cell density on combustion of
retained soot;
[0039] FIG. 9--impact of cell density on O.sub.2 consumption for
combustion of retained soot;
[0040] FIG. 10--impact of soot loading temperature for a 900 CPSI
flow through monolith;
[0041] FIG. 11--displays the B.P. response and O.sub.2 consumption
traces associated with TPO cycles described in FIG. 10;
[0042] FIG. 12--shows an example of a temperature programmed soot
loading using a 900 CPSI monolith;
[0043] FIG. 13--shows a temperature programmed reaction experiment
performed after the temperature programmed reaction soot loading in
FIG. 12;
[0044] FIG. 14--shows the TPO results for the 900 CPSI monolith
after a soot loading with reactive gas and temperature ramp, as per
FIG. 12;
[0045] FIG. 15--the performance of the coated 900 CPSI monolith is
examined in a temperature programmed reactive gas soot loading;
[0046] FIG. 16--shows a TPO performed subsequent to the loading
cycle of FIG. 15;
[0047] FIG. 17--the effect of GHSV on B.P., CO.sub.2 evolution and
O.sub.2 consumption during a soot loading in reactive gas with a
simultaneous ramp from 100 to 200.degree. C. for a 900 CPSI
part;
[0048] FIG. 18a--TPO of samples ex FIG. 17--B.P. response and
CO.sub.2 evolution; and
[0049] FIG. 18b--TPO of samples ex FIG. 17--NO and CO.sub.2
evolution and O.sub.2 consumption.
DETAILED DESCRIPTION OF INVENTION
[0050] FIG. 1 shows a schematic of the synthetic gas bench (SGB)
reactor in which the concept trials were executed. Prior to a
normal experiment, a monolith core (using a flow through washcoated
monolith with 400 or 900 cell per square inch-CPSI) and quartz
sleeve packed with quartz wool were placed in the stainless steel
reactor as shown. During the course of the subsequent experiments
the temperature, pressure drop and O.sub.2 content of the reactor
were monitored using the respective probes positioned as shown in
FIG. 1. Representative sampling of the gaseous reaction byproducts
was performed by on-line mass spectrometry with appropriate
corrections for m/z overlaps.
[0051] The test protocol employed in these studies typically
consisted of two phases:
[0052] Phase 1) Soot loading cycle: In this portion of the
experiment Printex U soot analogue (obtained from Evonik Degussa)
is introduced into the reactor via the use of fluidised bed system.
The fluidized bed unit contains the soot material and a flow of
N.sub.2 is based through the base of the bed to establish the fluid
condition and thus entrain suspended solid material with the gas
flow. The N.sub.2/soot flow is then mixed with the reactive gas leg
and passes through the reactor, where soot deposition in the
monolith may occur. The rate of soot delivery is 0.2 g/hour under
typical loading conditions. In order to retain any soot passing
through the flow through monolith, i.e. determine soot `slip` or
low filtration efficiency, a bed of quartz wool was packed in the
outlet position of the reactor.
[0053] Phase 2) Regeneration: The sample is purged with dry N.sub.2
and then heated in N.sub.2/O.sub.2 (as a TPO or Temperature
Programmed Oxidation) or reactive gas mixture (as described in FIG.
3) to 750.degree. C. and the reactor conditions e.g. back pressure,
O.sub.2 content, temperature and off gas monitored. Note that any
soot trapped in the quartz wool is also combusted during the
regeneration but only at high temperatures via the conventional
homogeneous combustion pathway, this turn enables a determination
of soot `slip` through the monolith and thus determine the impact
of cell density on trapping efficiency. An important note is made
herein in that prior to any performance testing the various
monolith cores were stabilised with respect to performance/aged,
this being achieved by an in-situ thermal treatment at 750.degree.
C. for four hours.
[0054] Examples of regeneration tests for a 900 CPSI monolith
coated with the active catalyst described in Examples 1 (method of
manufacture of Ag--OS component) and 2 (process for the production
of the final coated monolith) are shown in FIG. 2. Herein we
examine the impact of the presence of O.sub.2 during soot loading
on the subsequent combustion (catalytic vs homogeneous) of the soot
retained in the catalyst and secondary quartz trap. The performance
is broadly similar and shows the presence of two discrete
combustion events, one at ca. 250.degree. C., ascribed to direct
catalytic soot combustion arising from soot in good contact with
the active washcoat and a second event with two features at between
600-700.degree. C. from filter cake combustion and combustion of
soot retained in the quartz wool filter. This result mirrors those
in SAE 2008-01-0481 and re-confirms the critical importance of
catalyst-soot contact for direct oxidation to occur. A difference
may be seen in O.sub.2 consumption, as determined by the O.sub.2
sensor, which suggests that the presence of O.sub.2 during the
loading is slightly beneficial, possibly through O.sub.2
chemisorption on the surface of the soot during loading.
[0055] FIG. 3 outlines the various gas compositions employed during
the loading and regeneration trials. Hence, in subsequent figures
any reference to Reactive gas, for example, refers to a gas
composition containing N.sub.2, O.sub.2, CO, NO and propene in the
concentrations listed in line 3.
[0056] FIG. 4 compares the back pressure (hereafter B.P.) response
for a 400 CPSI (cells per square inch) flow through monolith during
a three hour (10800 s) soot loading cycle as a function of either
gas environment during loading or temperature and gas environment.
The data reflect a clear difference between soot loading at
200.degree. C. under N.sub.2/O.sub.2 and reactive gas mixture. In
the former case there is a continuous increase in the B.P. response
of the system (monolith plus quartz wool filter bed), consistent
with a systematic deposition and accumulation of soot. In contrast
the loading cycle at 200.degree. C. under reactive gases shows a
markedly lower rate of B.P. increase during the accumulation cycle.
This is consistent with a large decrease in the concentration of
soot matter accumulated within the system over time, from which it
may be inferred that there is consumption, i.e. oxidation of soot
during the loading cycle. Comparison of CO.sub.2 evolution data
during the loading cycle did show significantly higher CO.sub.2 for
the reactive gas loading, although given the simultaneous oxidation
of CO and propene this data is regarded as a partial corroboration
only and subsequent TPO (FIG. 5) is considered more definitive. The
trend of decreased B.P. increase is even more evident for loading
cycles in reactive gas at 250 and 300.degree. C. Thus, at
250.degree. C. there is only a small increase in the B.P. over the
cycle while at 300.degree. C. the B.P. can be seen to actually
decrease after the initial loading period. Again both samples
showed high levels of CO.sub.2 production during loading,
consistent with continuous, direct catalytic, oxidation of
soot.
[0057] Subsequent TPO, shown in FIG. 5, is consistent with the B.P.
response trends seen during soot loading (FIG. 4). Herein TPO after
the loading cycle at 200.degree. C. in N.sub.2/O.sub.2 results in a
CO.sub.2 evolution profile with three features, a small oxidation
feature at between 250-350.degree. C., ascribed to catalytic
combustion of soot and two large CO.sub.2 features at 640.degree.
C., due to filter cake combustion, and at >700.degree. C.
ascribed to the combustion of soot `slip` i.e. soot that passed
through the monolith and was trapped in the quartz wool `filter`
toward the outlet of the reactor. Since this quartz wool is located
outside of the main heated zone of the furnace any soot trapped
herein is only combusted at high temperatures and hence provides a
simple diagnostic as to the extent of soot `slip`. Thus, in this
instance, it can be seen that at lower temperatures, and in the
absence of general combustion chemistry, there is a large `slip` of
soot through the conventional 400 CPSI part. This soot is
accumulated and results in the large B.P. increase seen in FIG. 4.
This response may be contrasted with the reactive gas loading at
200.degree. C. In this instance there are again three main CO.sub.2
evolution features, catalytic combustion at ca, 300.degree. C.,
filter cake combustion at 650.degree. C. and `slip` combustion at
ca. 710.degree. C. However, the total CO.sub.2 production is
decreased to a large extent, especially for the highest temperature
`slip` event, consistent with increased continuous soot oxidation
during the loading. Moreover, the CO.sub.2 due to catalytic
combustion is significantly increased and filter cake CO.sub.2
decreased, reflecting a significant enhancement of catalytic
function under simulated exhaust conditions. These trends are
further evident in the loading cycles at 250 and 300.degree. C.
Both show further decreases in total CO.sub.2 production i.e.
retained soot and especially decreases in CO.sub.2 due to soot
`slip`. Hence, a comparison of the 300.degree. C. reactive gas
loading to the 200.degree. C. N.sub.2/O.sub.2 loading shows a
decrease in CO.sub.2 of >80%, i.e. >80% of soot loaded during
the cycle is combusted via a continuous, direct catalytic, soot
oxidation process.
[0058] FIG. 6 compares the O.sub.2 concentrations at the reactor
outlet during the TPO cycles described in FIG. 5. The data reflects
the same trends noted above with decreased O.sub.2 consumption
being recorded for reactive gas soot loading cycles and for soot
loading cycles at 250 and 300.degree. C. For the latter two cycles,
there is also the appearance of a feature at ca. 475.degree. C.,
which does not correlate to any specific CO.sub.2 evolution
feature. This peak is ascribed to the desorption of NO/NO.sub.2
from the catalyst and will be examined in more detail in later
figures (see FIGS. 9, 11, 13, 14, 15, 16 and 18b). Note, due to the
positioning of the O.sub.2 sensor at the outlet of the monolith
there is no O.sub.2 consumption recorded for the high temperature
soot `slip` event.
[0059] The impact of monolith cell density, 900 CPSI vs 400 CPSI,
on the B.P. response during soot loading is recorded in FIG. 7.
Comparison of the loading cycles at 200.degree. C. show general
similarities for the two substrates, albeit that the 900 CPSI part
shows a slightly higher rate of B.P. increase during the loading
cycle, consistent with the expected large impact of soot
accumulation in the narrower channels of this substrate.
[0060] A comparison of the subsequent TPO reactions after the
loading cycle of FIG. 7 is shown in FIG. 8. The data show a clear
change in the effectiveness of the technology as a function of cell
density. Hence in contrast the previous data for the 400 CPSI part,
the 900 CPSI substrate shows a dramatic improvement in soot
filtration efficiency, with only very small CO.sub.2 evolution
features seen for both filter cake and `slip` combustion events.
Moreover, the sample also exhibits an increased efficiency with the
direct catalytic oxidation feature, hence peak CO.sub.2 production
from direct catalytic oxidation is now observed at ca. 240.degree.
C. versus ca 300-310.degree. C. for the 400 CPSI monolith. Thus by
use of the high cell density monolith and active washcoat it is
possible to achieve high filtration efficiency, >95% based upon
the total CO.sub.2 production at T>500.degree. C. versus the 400
CPSI monolith, and also continuous, low temperature, direct
catalytic soot oxidation.
[0061] Comparable differences in performance with respect to
O.sub.2 consumption are observed in FIG. 9 for the 900 CPSI vs the
400 CPSI monoliths. Ex 900 CPSI O.sub.2 consumption is
predominantly seen for T<300.degree. C., with no significant
O.sub.2 consumption at T>600.degree. C. The converse is seen for
the 400 CPSI with a major O.sub.2 consumption being recorded at ca.
610-620.degree. C., from filter cake oxidation. Interestingly, all
three samples again show an additional feature at ca. 475.degree.
C. as per FIG. 6, associated with NOx evolution from the
washcoat.
[0062] The impact of loading on temperature on subsequent TPO of
accumulated soot is the 900 CPSI monolith result in large decreases
in accumulated soot. Hence, while loading at 100.degree. C. gives a
peak CO.sub.2 yield of ca. 8200 counts/s, there is only 6000 c/s
and 1,000 c/s for loading cycles at 150 and 200.degree. C.
respectively. Moreover, for the loading cycles at 150 and
200.degree. C. there is no evidence for filter cake formation,
based upon the absence of any higher temperature CO.sub.2
production peak. Indeed integration of the total CO.sub.2 evolution
from the ex 200 cycle on the 900 CPSI monolith versus the
200.degree. C. N.sub.2/O.sub.2 cycle on the 400 CPSI monolith
indicates >99% of all soot introduced during the loading cycle
undergoes direct catalytic combustion, thereby offering the
potential for usage of the technology in a `real` life
application.
[0063] FIG. 11 displays the B.P. response and O.sub.2 consumption
traces associated with the TPO cycles described in FIG. 10. In all
cases the data sets are consistent with the observed CO.sub.2
production profiles. Hence, in all cases, CO.sub.2
evolution/residual soot combustion is associated with O.sub.2
consumption and with a net decrease in B.P. as the monolith
channels are cleaned of the restrictive soot particles. The extent
of O.sub.2 consumption follows the net CO.sub.2 production i.e.
100>150>200.degree. C. Again, all samples the secondary NOx
related feature at 475.degree. C. The B.P. responses also appear to
reflect the conditions of soot loading with the `relaxation`
response being sharpest for the 200.degree. C. cycle, then
150.degree. C. and finally 100.degree. C. loading cycle, again
consistent with the residual soot retention for the various
tests.
[0064] In order to better mimic driving conditions we performed
soot loading cycles under conditions of dynamic temperature
changes. Hence, FIG. 12 shows an example of a temperature
programmed soot loading using a 900 CPSI monolith. In this test
there was simultaneous soot loading cycle in full reactive gas
mixture with heating of the sample from 100.degree. C. to
200.degree. C. The data shows the expected CO (and propene)
light-off curves, which were again found to be coincident with soot
combustion, as reflected in the peak then decay seen for CO.sub.2
production and O.sub.2 consumption traces. In this experiment, as
in all tests performed during this study, there was no production
of CO during the oxidation of soot (determined by analysis of the
corrected mass spectrometer peak at m/z 12 in which one may account
for the background and dynamic contributions of mass fragmentation
from CO and CO.sub.2). The continuous combustion of soot also helps
to account for the overshoot seen in the bed thermocouple, which
was found to be ca. 245.degree. C. versus the set point of
200.degree. C.
[0065] FIG. 13 shows a temperature programmed reaction experiment
performed after the temperature programmed reaction soot loading in
FIG. 12. The protocol for this test entailed cooling the sample
in-situ to 100.degree. C. in flowing N.sub.2, after the soot
loading cycle was completed, upon stabilisation at 100.degree. C.,
the full reactive gas mixture was then reintroduced, and the sample
heated to 750.degree. C., per standard method. The data shows the
expected light-off of CO (propene also undergoes light-off but the
signal is omitted for clarity with CO, NO and NO.sub.2 traces) as
evidenced by the responses in CO, CO.sub.2 and also the O.sub.2
sensor. Interestingly, there is again a peak of CO.sub.2 production
at ca. 225.degree. C., and then a decrease, this feature is
ascribed to the combustion of the residual soot retained on the
part. During this combustion event there is no significant change
in the B.P. of the sample, suggesting the retained soot is at such
a low level as to not result in any meaningful contribution to
system backpressure. Finally, there is a very small CO.sub.2
evolution at 475.degree. C., this latter feature is coincident with
the apparent O.sub.2 consumption event noted in previous tests (see
FIGS. 9 and 11) but also with a NOx (NO and NO.sub.2) desorption
event. This event is attributed to the intrinsic NOx scavenging and
release properties of the Ag--OS material, as described in SAE
2008-01-0481, application Ser. Nos. 12/363,310 and 12/363,329.
Thus, during the loading cycle and in the subsequent temperature
programmed reaction, any NO.sub.2 that is generated which would
normally result in `de-coupling` of catalyst-soot contact, is
trapped on the highly dispersed Ag centres and retained to high
temperatures where it is released in the plume observed. The plume
of desorbed NOx then may react with any traces of soot remaining on
the part, particularly any species that are spatially distant from
the catalyst surface i.e. with `poor` contact.
[0066] To further examine the impact of temperature during a
reactive soot loading cycle additional tests were performed. FIG.
14 shows the TPO results for the 900 CPSI monolith after a soot
loading with reactive gas and temperature ramp, as per FIG. 12.
Employing the TPO protocol rather than the full reactive gas mix
simplifies the chemistries and result traces. Thus, in the TPO
protocol there are no light-off features but rather a series of
peaks due to the various phenomena occurring over the catalyst
versus temperature. Firstly there is a CO.sub.2 production peak,
attributed to the combustion of residual retained soot. This peak
is centred at 300.degree. C., ca. 75.degree. C. higher than in the
temperature programmed reaction case. This reflects the important
contribution of the exothermicity of CO and HC light-off in
facilitating lower temperature soot oxidation. Hence, in the
reactive gas temperature ramp, as the CO and HC begin to combust
they generate a thermal bloom with the monolith which is sufficient
to overcome the activation energy barrier for the initiation of
soot oxidation. Then once the combustion of soot is initiated, a
further exotherm is generated and the resulting thermal `cascade`
is sufficient to result in very high soot conversion rates, this
process is the related of the method for lower temperature soot
oxidation as described in US 2005/0282698 A1. The soot oxidation
event in this instance is correlated to a very small B.P.
`relaxation`. The sample is also seen to desorb water, this release
being associated with desorption of combustion by-products from HC
oxidation. Finally, at 475.degree. C. one again sees the NOx
desorption/apparent O.sub.2 consumption event associated with the
Ag--OS scavenging function, however in this instance there does not
appear to be any significant associated CO.sub.2 production from
the combustion of trace soot in poor contact. However, what is
clear is that under the loading conditions employed there is
consistently high activity towards direct soot oxidation resulting
in very low levels of residual soot remaining on the 900 CPSI
monolith, again confirming the potential for the approach for
continuous, direct catalytic, soot oxidation.
[0067] In FIG. 15 the performance of the coated 900 CPSI monolith
is examined in a temperature programmed reactive gas soot loading.
In this instance the maximum temperature employed was 500.degree.
C. (ramping from 50.degree. C.). In the case the CO (and HC)
light-off traces are very clearly represented, as is the associated
O.sub.2 consumption. Again the CO.sub.2 evolution trace shows an
increase to a peak at ca. 250.degree. C. before decreasing to a
steady state value. This is again consistent with the active
catalytic combustion of retained soot. Hence, in this and all other
regards the performance trends replicate previous findings,
including the NOx scavenging/apparent O.sub.2 consumption at ca.
475.degree. C.
[0068] FIG. 16 shows a TPO performed subsequent to the loading
cycle of FIG. 15. Herein there are no significant reaction or
desorption events evident. In particular, there is no additional
CO.sub.2 production, no high temperature soot `slip` phenomenon,
i.e. the data is consistent with complete conversion of any soot
loaded during the loading cycle, further confirming the high
effectiveness of the technology.
[0069] Next the impact of Gas Hourly Space Velocity on performance
was examined. Hence, FIG. 17 contrasts reactive gas loading cycles,
with temperature ramp (100-200.degree. C.) under the standard GHSV
of 15000 h.sup.-1 versus a GHSV of 25000 h.sup.-1 (versus monolith
volume). It is emphasised at this point that the soot delivery rate
in both tests as determined by the flow rate through the fluidised
bed was constant in both cases and the increase in GHSV was
achieved by increasing the flow rates of the various gases within
the reactive gas manifold. Analysis of the subsequent data from
both tests show comparable response with response to gas phase
chemistry, with CO (and HC) light-off being unaffected, as
evidenced by the comparable CO.sub.2 responses. There is an offset
in the O.sub.2 sensor values, possibly due to the total increased
flow employed in the high GHSV test, but again the dynamic
responses are identical. After completion of light-off there is
some difference in the CO.sub.2 responses, with the low GHSV test
showing higher net CO.sub.2. Coincident to this, the B.P. response
of the high GHSV test shows a steady increase, this is ascribed to
a combination of the higher net static back pressure observed due
to the higher flow rate but also to an increase in the net rate of
soot accumulation during the test. This raises questions regarding
location of soot, i.e. is the B.P. increase due to soot `slip` or
is the soot still retained on the part, and also what is the
maximum effective rate of soot deposition that may be employed and
still achieve high continuous soot combustion rates.
[0070] In FIG. 18a/b these questions are answered. Herein the
subsequent TPO cycles loading cycles. The data show only CO.sub.2
production at lower temperatures with a peak at ca. 300.degree. C.
Hence even under the conditions of higher flow there was no soot
slip i.e. all soot introduced was retained with the monolith. The
decrease in rate of soot oxidation is thus ascribed to an
exothermal effect with the increasing flow rate through the part
during loading resulting in a net `dilution` of the exotherm
cascade which is believed to be critical for the propagation of
soot burn. However, as indicated the `excess` soot generated due to
this process is merely retained unreacted on the part, thus in the
subsequent TPO the soot oxidation follows the same profile as the
lower GHSV case and there is simply an increase in the net CO.sub.2
production. This is also reflected in the B.P. responses with the
sample loaded under high GHSV showing a more rapid and larger B.P.
`relaxation` than the sample loaded under lower GHSV. Similarly the
NOx evolution response is larger for the high GHSV sample,
reflecting the higher mass fraction of NOx exposure during the
test. This in turn results in the small differences in apparent
O.sub.2 consumption, as recorded by the O.sub.2 sensor. Thus, to
conclude while there is an impact of GHSV on activity, the impact
is not catastrophic and the monolith retains its ability to either
combust or trap all soot at lower temperatures and then to
facilitate its combustion at temperatures still within the normal
operating window of a conventional vehicle, i.e. the system still
provides effective soot filtration and combustion without recourse
to conventional active regeneration strategy.
DETAILED DESCRIPTION OF THE INVENTION
[0071] The present invention relates to a method and apparatus for
the continuous/semi-continuous direct catalytic, oxidation of
diesel particulate matter by the combination of base metal modified
Oxygen Storage (OS) materials in association with turbulent
flow/high cell density flow through monoliths. The particular
combination of the base metal modified OS direct soot oxidation
catalyst with the flow through monolith provides a synergy which
enables high conversion of particulate matter without the
backpressure penalty introduced by the conventional DPF. It is
believed that the synergy arises from the ability of the active OS
material to combust soot at lower temperatures than in conventional
systems, which in turn is facilitated by the decreased thermal mass
of the conventional substrate, with the latter still providing
sufficient geometric surface area for soot deposition and reaction.
Large improvements in lower temperature activity can be obtained in
marked contrast to the conventional wall flow DPF wherein large
thermal mass of the substrate, particularly for SiC DPF, inhibits
initiation and especially propagation of soot combustion.
[0072] The present invention represents a significant advance in
the development of a method and apparatus for the (semi)
continuous, direct catalytic, oxidation of diesel particulate
matter may be realized by the combination of base metal modified
Oxygen Storage (OS) materials with a conventional flow substrate.
The substrate is selected from a range of ceramic or metallic
technologies upon which the active washcoat is disposed. Such
substrates can be metallic parts, ceramic or metal foams.
[0073] More particularly and in a further aspect, the present
invention relates to a synergistic combination of a catalyst and a
substrate for the filtration and continuous, direct catalytic,
oxidation of diesel particulate matter at low temperatures. The
catalyst comprises catalytically active precious metal (Pt, Pd, Rh
or combinations thereof), a host cerium-based solid solution which
is a substantially phase pure cubic fluorite (as determined by
x-ray diffraction method) of the CeZrOx type which is well known in
the art and a refractory oxide support, e.g.
(.gamma.)Al.sub.2O.sub.3, ZrO.sub.2 or other known oxide support.
The CeZrOx is further modified by the incorporation of an active
base metal, e.g. Ag, Cu etc. as disclosed in application Ser. No.
12/363,310. The catalyst further comprises a monolith substrate, of
conventional design, wherein the monolith is an inert ceramic or
metal substrate upon which the active catalyst formulation/washcoat
is disposed. The monolith substrate is further characterised by a
high cell density, i.e. a large number of active channels per unit
area, for effective synergy a value of >600 cells per square
inch. In the case of a metallic substrate the active washcoat may
be applied to the perforated, punched and embossed metal foils
(e.g. TS, LS, PE and MX type systems; see for example U.S. Pat. No.
6,689,327) with beneficial effect.
[0074] The combined active washcoat and monolith system may be
applied to the challenge of particulate emission control catalysts
for diesel (or other fuel lean) or potential gasoline
(stoichiometric) application. The particular example described
herein is for the application of these materials in the area of
continuous, direct catalytic oxidation of diesel particulate matter
upon its interaction with the high cell density substrate. These
benefits arise in this application due to the aforementioned
synergies arising from the high cell density monolith and the new
generation of modified OS materials. The latter has been previously
been demonstrated as having benefits in affecting either lower
temperature regeneration/oxidation of soot or an increased
regeneration efficiency at a lower temperature as compared to
non-modified OS materials (application Ser. Nos. 12/363,310 and
12/363,329). Now in combination with a conventional flow monolith
of appropriate architecture, it becomes possible to realise a
completely passive particulate control catalyst.
[0075] It should be further noted that the terms "first", "second"
and the like herein do not denote any order of importance, but
rather are used to distinguish one element from another, and the
terms "a" and "an" herein do not denote a limitation of quantity,
but rather denote the presence of at least one of the referenced
items. Furthermore, all ranges disclosed herein are inclusive and
combinable (e.g., ranges of "up to about 25 weight percent (wt. %),
with about 5 wt. % to about 20 wt. % desired, and about 10 wt. % to
about 15 wt. % more desired" is inclusive of the endpoints and all
intermediate values of the ranges, e.g. "about 5 wt. % to about 25
wt. %, about 5 wt. % to about 15 wt. %" etc.
[0076] The details regarding the synthesis, characterization and
preferred compositions, structures, dopant levels etc for the
Cerium-containing mixed oxide/solid solution material are detailed
in Ser. Nos. 12/363,310 and 12/363,329. Preferably, the solid
solution contains a cationic lattice with a single-phase, as
determined by standard x-ray diffraction method. More preferably
this single-phase is a cubic structure, with a cubic fluorite
structure being most preferred. Additionally, it is noted that the
doping process may be performed without formation of an additional
bulk phase, as determined by XRD. In various embodiments, the OS
material may include those OS materials disclosed in U.S. Pat. Nos.
6,585,944; 6,468,941; 6,387,338 and 6,605,264 which are herein
incorporated by reference in their entirety. However, the
flexibility of the basic exchange provides for the modification of
all current known cerium oxide and Ce--Zr-based solid solution
materials to be thusly modified and enhanced.
[0077] As indicated, the OS materials modified by the doping method
shall preferably be characterized by a substantially cubic fluorite
structure, as determined by conventional XRD methods. The
percentage of the OS material having the cubic structure, both
prior and post exchange, is preferably greater than about 95%, with
greater than about 99% typical, and essentially 100% cubic
structure generally obtained (i.e. an immeasurable amount of
tetragonal phase based upon current measurement technology). The
exchanged OS material is further characterized in that it possess
large improvements in durable redox activity with respect to facile
oxygen storage and increased release capacity e.g. as determined by
H.sub.2 Temperature Programmed Reduction (TPR) method. Thus, for Cu
exchanged solid solutions, for example, the reduction of Ce+Cu is
observed to occur at a temperature of about 300 to about
350.degree. C. lower than would occur in the absence of the Cu
dopant (see application Ser. No. 12/363,310).
[0078] In an exemplary embodiment, an active soot oxidation
catalyst comprising a precious group metal or metals (Pt, Pd, Rh
and combinations thereof), a base metal doped cerium-oxide
containing solid solution and a refractory oxide carrier all of
which together are employed as a coating, e.g., disposed on/in an
inert substrate or carrier, the substrate or carrier being
characterized by a high number of channels or cells per unit area
or by the its ability to introduce turbulent flow due to the
construction of its internal flow channels. Exhaust gas treatment
devices can generally comprise housing or canister components that
can be easily attached to an exhaust gas conduit and comprise a
substrate for treating exhaust gases. The housing components can
comprise an outer "shell", which can be capped on either end with
funnel-shaped `end-cones` or flat `end-plates`, which can comprise
`snorkels` that allow for easy assembly to an exhaust conduit.
Housing components can be fabricated of any materials capable of
withstanding the temperatures, corrosion, and wear encountered
during the operation of the exhaust gas treatment device, such as,
but not limited to, ferrous metals or terrific stainless steels
(e.g., martensitic, terrific, and austenitic stainless materials,
and the like).
[0079] Disposed within the shell can be a retention material ("mat"
or "matting"), which is capable of supporting a substrate,
insulating the shell from the high operating temperatures of the
substrate, providing substrate retention by applying compressive
radial forces about it, and providing the substrate with impact
protection. The matting is typically concentrically disposed around
the substrate forming a substrate/mat sub-assembly.
[0080] Various materials can be employed for the matting and the
insulation. These materials can exist in the form of a mat, fibres,
preforms, or the like, and comprise materials such as, but not
limited to, intumescent materials (e.g., a material that comprises
vermiculite component, i.e., a component that expands upon the
application of heat), non-intumescent materials, ceramic materials
(e.g., ceramic fibers), organic binders, inorganic binders, and the
like, as well as combinations comprising at least one of the
foregoing materials. Non-intumescent materials include materials
such as those sold under the trademarks "NEXTEL" and "INTERAM
1101HT" by the "3M" Company, Minneapolis, Minn., or those sold
under the trademark, "FIBERFRAX" and "CC-MAX" by the Unifrax Co.,
Niagara Falls, N.Y., and the like. Intumescent materials include
materials sold under the trademark "INTERAM" by the "3M" Company,
Minneapolis, Minn., as well as those intumescent materials which
are also sold under the aforementioned "FIBERFRAX" trademark.
[0081] Housings as described above are well known and understood by
those skilled in the art.
[0082] The substrates or carrier employed in this invention can
comprise any material designed for use in a spark ignition or
diesel engine environment having the following characteristics in
addition to the high cell density/turbulent flow requirement stated
previously: (1) capability of operating at temperatures up to about
600.degree. C. and up to about 1,000.degree. C. for some
applications, depending upon the device's location within the
exhaust system (e.g., manifold mounted, close coupled, or
underfloor) and the type of system (e.g., gasoline or diesel); (2)
capability of withstanding exposure to hydrocarbons, nitrogen
oxides, carbon monoxide, particulate matter e.g. soot and the like,
CO.sub.2, and/or sulfur; and (3) have sufficient surface area and
structural integrity to support a catalyst, if desired. These
materials should be inert under the conditions imposed on them when
in use. Some possible materials include cordierite, silicon
carbide, metal, metal oxides; e.g. alumina and the like, glasses
and the like, and mixtures comprising at least one of the foregoing
materials. Some suitable inert ceramic materials include `Honey
Ceram`, commercially available from NGK-Locke, Inc, Southfield,
Mich., and `Celcor`, commercially available from Corning, Inc.,
Corning, N.Y. These materials can be in the form of foils, perform,
mat, fibrous material, monoliths e.g. a honeycomb structure, and
the like, other porous structures e.g., porous glasses, sponges,
foams, pellets, particles, molecular sieves, and the like
(depending upon the device), and combinations comprising at least
one of the foregoing materials and forms, e.g., metallic foils,
open pore alumina sponges and porous ultra-low expansion glasses.
Furthermore, these substrates can be coated with oxides and/or
hexaaluminates, e.g. stainless steel foil coated with a
hexa-aluminate scale.
[0083] Although the substrate can have any size or geometry, within
the previously defined limits, the size and geometry are preferably
chosen to optimize surface area in the given exhaust gas emission
control device design parameters. Typically, the substrate has a
honeycomb geometry, with the combs through-channel having any
multi-sided or rounded shape, with substantially square,
triangular, pentagonal, hexagonal, heptagonal, or octagonal or
similar geometries preferred due to ease of manufacturing and
increased surface area.
[0084] The exhaust gas treatment devices can be assembled utilizing
various methods. Three such methods are the stuffing, clamshell,
and tourniquet assembly methods. The stuffing method generally
comprises pre-assembling the matting around the substrate and
pushing, or stuffing, the assembly into the shell through a
stuffing cone. The stuffing cone serves as an assembly tool that is
capable of attaching to one end of the shell. Where attached, the
shell and stuffing cone have the same cross-sectional geometry, and
along the stuffing cone's length, the cross-sectional geometry
gradually tapers to a larger cross-sectional geometry. Through this
larger end, the substrate/mat sub-assembly can be advanced which
compresses the matting around the substrate as the assembly
advances through the stuffing cone's taper and is eventually pushed
into the shell.
[0085] Exhaust gas treatment devices comprising the doped solid
solutions can be employed in exhaust gas treatment systems to
provide both an active soot combustion catalyst but also a NOx
adsorption function, and thus specifically reduce a concentration
of undesirable constituents in the exhaust gas stream. For example,
as discussed above, an exemplary catalyst system can be formed
utilizing the doped OS as a catalyst component, wherein the
catalyst system is disposed on a substrate, which is then disposed
within a housing. Disposing the substrate to an exhaust gas stream
can then provide at least a NOx storage function, and desirably
even reduce the concentration of at least one undesirable
constituent contained therein.
[0086] According to one embodiment of the present invention, the
catalyst does not conform the standard architecture of a CDPF or
Diesel NOx Particulate Trap and hence does not comprise a porous
substrate having alternating channels. Rather the preferred
configuration of the catalyst is as a conventional `flow through`
monolith, of high unit cell count per unit area, upon which is
disposed the active catalyst washcoat. The combination of the
active washcoat with the high internal surface area and turbulent
deposition mechanism is sufficient to facilitate retention and
continuous particulate oxidation under conventional operating
temperatures and flows of a diesel/compression ignition
vehicle.
EXAMPLES
[0087] The benefits obtained by the active washcoat employing the
doped Cerium containing oxide and high cell density monolith are
clearly evident in FIGS. 4-18b, wherein the benefit of enhanced
redox performance of the doped OS, in combination with an
appropriate substrate result in high rates of direct soot
combustion under conditions appropriate for vehicular application.
It should be stressed that the redox promotion obtained by base
metal doping is observed for both a range of cationic dopants and a
range of OS compositions, and the data included herein for the
2Ag--OS is merely a representative example.
[0088] The data herein reflect a systematic study of the various
parameters considered relevant to achieve the desired aim of
continuous, low temperature, direct catalytic soot oxidation. The
impacts of these parameters on performance are summarized, with
reference to specific case data as follows:
[0089] a) Reactivity of soot: The reactivity of soot e.g. soluble
organic fragment has been shown to play a large role in determining
the reactivity of soot and thus the effective performance of soot
oxidation catalysts (Atmos Env, vol. 15 (1), 1981, 91-94, SAE paper
2008-01-0481, App Catal B, vol. 75 (1-2), 2007, p 11-16, etc.).
Indeed, comparison of conventional soot TGA shows an increase in
Tmax (temperature for maximum rate of soot combustion) of ca.
50.degree. C. for Printex U cf. `real` diesel soot collected from a
vehicle (SAE 2008-01-0481). Thus, in this study Printex U soot
analogue was employed to specifically remove this variable from any
discussion. Hence, all particulate matter combusted during these
tests may be considered to be equivalent in reactivity and thus
present no inherent bias in any given data set. Moreover, the
oxidation of the Printex material may be considered to be a `worst
case` scenario i.e. its oxidation is representative of combustion
of very `dry`, refractory carbonaceous material high in graphitic
content and low in SOF. Thus, the promising data herein reflect a
true performance advantage of interest to real world
applications.
[0090] b) Gas environment during soot accumulation: There is a
clear impact of reactive gas chemistry on catalyst performance both
during the loading cycle as shown in FIGS. 2, 4, 5 and 6 but also
the nature of the gas atmosphere can be seen to impact the
regeneration, as is evident from contrasting TPO versus temperature
programmed reaction burn out protocols (FIGS. 5, 8, 10, 13, 14, 16
and 18a/b). This impact is attributed to a combination of heat
transfer and catalyst activation. One heat transfer component
arises due to the external heating of the active catalyst arising
from the combustion of the significant levels of fuel components
within the reactive gas mixture, principally CO and HC. This energy
is retained within the washcoat, resulting in the hotter than
expected bed temperatures observed, and thus helping to overcome
the activation energy barrier to catalytic soot oxidation. A second
combined heat transfer and catalyst activation component is arises
from the activation of the redox oxide arising from its
participation in the CO oxidation process. It has been shown that
the doped cerium oxides are effective oxidation catalysts, even in
the absence of PGM, and can facilitate CO oxidation at low
temperatures (DP-316440). In doing so the catalyst O ion transport
function is activated, and energy released at the active site of CO
oxidation. The subsequent re-oxidation of the depleted oxygen
results in a further exotherm, distributed throughout the entire
structure of the OS, in a sense further priming the OS to initiate
soot oxidation. This mechanism forms part of the basis of US
2005/0282698 A1, wherein a fuller explanation may be found.
[0091] c) Static temperature effect during soot accumulation:
Obviously thermal energy/temperature is required to overcome the
activation energy barrier for catalytic soot combustion. Hence,
with increasing inlet temperature, there is a concurrent increase
in rate of catalytic oxidation and hence decreases in soot slip and
mass of retained soot on the monolith irrespective of all other
factors (FIGS. 4-18b).
[0092] d) The role of cell density of the monolith: This is a key
factor for the invention, with the use of higher cell
density/increased cell count per unit area, resulting in a large
enhancement of catalyst performance (FIGS. 7-9). Thus by merely
substituting the 400 CPSI monolith with, the 900 CPSI one results
in a dramatic improvement in soot filtration efficiency (>95%
based upon the total CO.sub.2 at T>500.degree. C. vs 400 CPSI),
the ability to circumvent soot `slip` through the monolith i.e. no
high temperature CO.sub.2 production due to soot passing through
the monolith and being retained in the quartz wool filter, and also
a small decrease in temperature required for soot combustion (which
is attributed to the higher effectiveness of the oxidation
washcoat). The impact of cell density also has a positive synergy
with temperature with larger net performance gains being observed
for the 900 CPSI monolith for higher temperature than for the 400
CPSI system.
[0093] e) The role of NOx on catalyst performance: The ability of
the metal doped OS material to scavenge NOx at low temperatures and
release the retained species at higher temperatures (see FIGS. 13,
14, 15 and 18b) is of particular importance. This capability
effectively disables the `de-coupling` mechanism of related to
NO.sub.2 mediated soot oxidation, which has been shown to destroy
the intimate contact between catalyst and soot required for direct
catalyzed soot oxidation (see SAE paper 2008-01-0481, U.S. patent
application Ser. No. 12/363,329). Indeed by use of the metal doped
OS material it appears that one can, in selected cases, also employ
the NOx desorption plume beneficially to remove trace particulate
matter that is in poor contact i.e. spatially discrete/removed from
the active catalyst e.g. see FIG. 13. However, it must be stressed
this is not the primary catalytic process responsible for the high
activity seen at low temperatures but is rather an additional minor
benefit.
[0094] g) Dynamic temperature effect during soot accumulation:
Comparison of static temperature soot loading and regeneration
cycles (FIGS. 4, 5, 7, 8 and 11) versus loading and regeneration
cycles with dynamic temperature change i.e. temperature ramp (FIGS.
12, 13, 14 and 15) illustrate a further manifestation of energy
with the process. Thus it may be seen that increases in temperature
during the soot accumulation process, combined with the specific
exotherm associated Light-off of CO and HC, result in a further
improvement in performance compared to a simple static temperature
loading. This is ascribed to the combination of exothermal effects,
which we have dubbed thermal bloom propagation, described
previously and explained in more detail in US 2005/0282698 A1.
[0095] h) The effect of GHSV and flow velocity: Clearly, the
residence time of the particulate matter within the monolith is an
important factor. Thus, the longer the particulate reside within
the monolith channels, the greater is the probability of
interaction with the active washcoat coated on the walls and,
hence, the higher probability of retention and reaction. Moreover,
since the particulates are entrained by the flow i.e. derive their
kinetic energy from Brownian collisions, at higher flow rates, the
velocity of the particulates are higher. This both decreases
residence time within the monolith but also provides a force
driving the following regime to be more laminar and less turbulent,
thereby decreasing the possibility of particulate to wall
interaction. These hypotheses are consistent with data in FIGS. 17,
18a and 18b. In addition at higher flow velocities there is an
increase in energy transport out of the monolith, i.e. the local
exotherms are diminished due to increased Brownian collisions
transferring kinetic energy to molecules leaving the flow channel.
Hence, at higher GHSV the rate of soot oxidation is somewhat
decreased and results in increased formation of retained soot
species. However, under the conditions examined, the increased flow
was not enough to result in soot `slip`, prevent extensive direct
catalytic oxidation or indeed prevent complete regeneration in the
subsequent burn out cycle. Finally, it should be noted that during
the burn out cycle the temperatures required for combustion of the
retained soot mass fraction was still significantly lower than the
>600.degree. C. employed in conventional DPF active
regeneration. Indeed the temperatures required were still only of
the order of 300-330.degree. C., i.e. temperatures easily within
the normal operational window of a diesel vehicle. Hence, the
concept of direct catalytic soot oxidation is applicable to
vehicular application.
EXAMPLES
[0096] The procedure for making 100 grams of 2% Ag(NH.sub.3).sub.2
OS, as employed in the test technology is as follows:
[0097] 1. Weigh 100 g of OS, correct for moisture content (ca. 1.5%
water).
[0098] 2. Weigh 3.15 g of silver nitrate crystals. One must
compensate for the percentage of metal in the nitrate salt or
solution used. Silver nitrate is 63.52% silver.
[0099] 3. Dissolve silver nitrate in 50 g deionised water. The
amount of water used is determined by the water adsorption capacity
of the mixed oxide used. This is generally between 0.5 and 0.5 g
water per gram mixed oxide.
[0100] 4. Add concentrated NH.sub.4OHaq (30% ammonia) to the silver
nitrate solution, dropwise, until a clear silver diamine solution
is obtained. Solution will first turn brown-black, then clear upon
excess addition of ammonium hydroxide.
[0101] 5. Add silver diamine solution to mixed oxide powder. Mix
thoroughly to produce homogeneous and even-colored moist
powder.
[0102] 6. Allow powder to rest at room temperature for one
hour.
[0103] 7. Dry in oven at .about.110.degree. C. for ca. two hours or
until dry.
[0104] 8. Calcine in furnace at 540.degree. C. for four hours in
air.
[0105] OS=40% CeO.sub.2; 50% ZrO.sub.2/HfO.sub.2; 5%
La.sub.2O.sub.3; 5% Pr.sub.6O.sub.11
[0106] The procedure for producing the active washcoat and
producing the 400 and 900 CPSI parts tested in this study is as
follows: Slowly add alumina with milling to a d.sub.50 of 7 microns
(.+-.1), d.sub.90=20-25 and 100% pass<60 microns. Adjust pH to
3.0-3.5 and specific gravity to allow one pass coating then coat
monolith in one pass and calcine at temperatures .gtoreq.540 C for
.gtoreq.one hour. Next slurry required 2Ag--OS in DI water, mill at
the natural pH of the material to a d.sub.50 of 2.+-.0.3, d.sub.90
of <10 microns and 100 pass<30 microns. Prevent pH decreasing
below 4 by addition of base. Next pre-mix pt nitrate and pd nitrate
solutions for 15 minutes. To this mixture add dilute sugar solution
and mix for a minimum of 30 minutes; add to Ag--OS slurry within 60
minutes of initial mixing to avoid precipitation of metal. Add PGM
sugar solution dropwise to Ag--OS slurry vortex. Prior to addition
slurry must be at a pH of 5.5-6.0 and during metal addition,
prevent slurry from going to pH values below 4.0 with the judicious
use of base. Stir two hours to allow full chemisorption. Adjust pH
and specific gravity to allow one pass coating then coat monolith
in 1 pass and calcine at temperatures .gtoreq.540.degree. C. for
.gtoreq.one hour.
* * * * *