U.S. patent application number 16/101710 was filed with the patent office on 2018-12-20 for filter for filtering particulate matter from exhaust gas emitted from a compression ignition engine.
The applicant listed for this patent is JOHNSON MATTHEY PUBLIC LIMITED COMPANY. Invention is credited to Louise ARNOLD, Robert BRISLEY, Guy Richard CHANDLER, Andrew Francis CHIFFEY, Keith Anthony FLANAGAN, David GREENWELL, Christopher MORGAN.
Application Number | 20180361364 16/101710 |
Document ID | / |
Family ID | 40565750 |
Filed Date | 2018-12-20 |
United States Patent
Application |
20180361364 |
Kind Code |
A1 |
ARNOLD; Louise ; et
al. |
December 20, 2018 |
FILTER FOR FILTERING PARTICULATE MATTER FROM EXHAUST GAS EMITTED
FROM A COMPRESSION IGNITION ENGINE
Abstract
A filter for filtering particulate matter (PM) from exhaust gas
emitted from a compression ignition engine, which filter comprising
a porous substrate having inlet surfaces and outlet surfaces,
wherein the inlet surfaces are separated from the outlet surfaces
by a porous structure containing pores of a first mean pore size,
wherein the porous substrate is coated with a wash coat comprising
a plurality of solid particles comprising a molecular sieve
promoted with at least one metal wherein the porous structure of
the wash coated porous substrate contains pores of a second mean
pore size, and wherein the second mean pore size is less than the
first mean pore size.
Inventors: |
ARNOLD; Louise; (Wayne,
PA) ; BRISLEY; Robert; (Royston, GB) ;
CHANDLER; Guy Richard; (Royston, GB) ; CHIFFEY;
Andrew Francis; (Royston, GB) ; FLANAGAN; Keith
Anthony; (Royston, GB) ; GREENWELL; David;
(Royston, GB) ; MORGAN; Christopher; (Royston,
GB) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
JOHNSON MATTHEY PUBLIC LIMITED COMPANY |
London |
|
GB |
|
|
Family ID: |
40565750 |
Appl. No.: |
16/101710 |
Filed: |
August 13, 2018 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
15006319 |
Jan 26, 2016 |
|
|
|
16101710 |
|
|
|
|
14107607 |
Dec 16, 2013 |
9261004 |
|
|
15006319 |
|
|
|
|
13203631 |
Jan 6, 2012 |
8608820 |
|
|
PCT/GB2010/050347 |
Feb 26, 2010 |
|
|
|
14107607 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01J 23/745 20130101;
B01J 29/56 20130101; B01D 2275/30 20130101; B01J 23/10 20130101;
B01J 29/46 20130101; F01N 3/035 20130101; B01D 53/9445 20130101;
B01D 2255/908 20130101; B01J 29/7415 20130101; B01J 29/7615
20130101; B01J 23/464 20130101; B01J 29/061 20130101; B01J 29/146
20130101; B01J 29/166 20130101; B01J 29/126 20130101; B01D 53/9477
20130101; B01D 2255/1025 20130101; B01D 2255/9155 20130101; B01J
23/42 20130101; B01J 29/06 20130101; B01J 37/038 20130101; B01J
37/0217 20130101; B01J 35/0006 20130101; F01N 2310/06 20130101;
B01J 23/40 20130101; B01J 37/0036 20130101; F01N 2510/06 20130101;
F01N 3/2803 20130101; B01J 29/405 20130101; B01J 29/44 20130101;
B01D 2255/1021 20130101; B01J 29/7007 20130101; B01D 2255/1023
20130101; Y02T 10/12 20130101; Y10S 55/30 20130101; B01J 29/505
20130101; B01J 35/04 20130101; B01J 35/10 20130101; B01D 2255/50
20130101; B01J 29/088 20130101; B01J 29/7215 20130101; B01J 29/7815
20130101; F01N 2510/068 20130101; B01J 23/72 20130101; B01J 23/44
20130101; Y02A 50/20 20180101; B01J 29/082 20130101; B01J 29/54
20130101; B01J 29/7057 20130101; B01J 35/023 20130101; B01D 2279/30
20130101; B01D 2255/92 20130101; B01J 37/0246 20130101 |
International
Class: |
B01J 29/76 20060101
B01J029/76; B01J 23/44 20060101 B01J023/44; B01J 29/50 20060101
B01J029/50; B01J 23/745 20060101 B01J023/745; F01N 3/28 20060101
F01N003/28; B01J 23/10 20060101 B01J023/10; B01J 23/42 20060101
B01J023/42; B01J 35/00 20060101 B01J035/00; B01J 23/72 20060101
B01J023/72; B01D 53/94 20060101 B01D053/94; B01J 29/78 20060101
B01J029/78; B01J 37/03 20060101 B01J037/03; B01J 29/14 20060101
B01J029/14; B01J 29/70 20060101 B01J029/70; B01J 29/08 20060101
B01J029/08; B01J 29/72 20060101 B01J029/72; B01J 29/40 20060101
B01J029/40; B01J 29/12 20060101 B01J029/12; B01J 29/74 20060101
B01J029/74; B01J 29/16 20060101 B01J029/16; B01J 29/54 20060101
B01J029/54; B01J 37/02 20060101 B01J037/02; B01J 23/40 20060101
B01J023/40; B01J 35/02 20060101 B01J035/02; F01N 3/035 20060101
F01N003/035; B01J 29/06 20060101 B01J029/06; B01J 29/44 20060101
B01J029/44; B01J 35/04 20060101 B01J035/04; B01J 29/46 20060101
B01J029/46; B01J 37/00 20060101 B01J037/00; B01J 35/10 20060101
B01J035/10; B01J 23/46 20060101 B01J023/46; B01J 29/56 20060101
B01J029/56 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 26, 2009 |
GB |
0903262.4 |
Dec 24, 2009 |
GB |
0922612.7 |
Claims
1. A filter for filtering particulate matter (PM) from an exhaust
gas, the filter comprising: a. a wall flow filter having inlet and
outlet surfaces and a porous substrate between the inlet and outlet
surfaces, wherein the porous substrate has pores of a first mean
pore size, b. a first washcoat coated on the inlet and/or outlet
surface of the porous wall flow substrate and within the wall flow
substrate, wherein the first washcoat has a second mean pore size
that is less than the first mean pore size, wherein the filter
further comprises a layer of a second washcoat, wherein the first
washcoat and second washcoat layer have different formulations and
wherein substantially none of the second washcoat enters the wall
flow substrate, and wherein at least one of the first and second
washcoats comprise a metal selected from Cu, Fe, Ce, Pt, Pd, or
Rh.
2. The filter of claim 1, wherein the second washcoat layer is
coated on the outlet surface of the wall flow filter.
3. The filter of claim 1, wherein the second washcoat layer is
coated on the outlet surface of the wall flow filter.
4. The filter of claim 1, wherein either one or both of the inlet
and outlet surfaces of the wall flow filter comprise a plurality of
washcoat layers comprising the first washcoat layer and the second
washcoat layer.
5. The filter of claim 2, wherein the first washcoat is coated on
the inlet surface of the wall flow filter.
6. The filter of claim 1, wherein one of the first or second
washcoats comprise a metal is selected from Cu, Fe, and Ce, and the
other of the first or second washcoats comprise a metal is selected
from Pt, Pd, and Rh.
7. The filter of claim 6, wherein at least one of the first and
second washcoats comprise an aluminosilicate molecular sieve.
8. The filter of claim 6, wherein the second washcoat comprises an
aluminosilicate molecular sieve containing a metal selected from Cu
and Fe.
9. The filter of claim 6, wherein the first washcoat comprises an
aluminosilicate molecular sieve containing a metal selected from Cu
and Fe.
10. The filter according to claim 1, wherein the first washcoat
comprises solid particles having a mean particle size of about 1 to
20 .mu.m.
11. The filter according to claim 1, wherein the washcoat within
the porous wall flow substrate comprises solid particles having a
D90 particle size distribution of 0.1 to 20 .mu.m.
12. The filter according to claim 1, wherein the first washcoat
comprises interparticle pores and/or pores made by formation of gas
on decomposition or combustion.
13. The filter according to claim 1, wherein the pore size of the
first washcoat is 5 nm to 5 .mu.m.
14. The filter according to claim 1, wherein pores at a surface of
the porous substrate comprise a pore opening and the washcoat
causes a narrowing of substantially all the pore openings.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation of U.S. Continuation
patent application Ser. No. 15/006,319, filed on Jan. 26, 2016,
which claims priority to U.S. Divisional Patent application Ser.
No. 14/107,607, filed Dec. 16, 2013, and patented on Feb. 16, 2016,
which claims priority to U.S. National Stage patent application
Ser. No. 13/203,631, filed on Jan. 6, 2012, and patented on Dec.
17, 2013, which claims priority to U.S. National Phase application
of PCT International Application No. PCT/GB2010/050347, filed Feb.
26, 2010, and claims priority of British Patent Application No.
0903262.4, filed Feb. 26, 2009, and British Patent Application No.
0922612.7, filed Dec. 24, 2009, the disclosures of all of which are
incorporated herein by reference in their entirety for all
purposes.
FIELD OF THE INVENTION
[0002] The present invention relates to a filter for use in
treating particulate matter (PM) and oxides of nitrogen derived
from a compression ignition engine.
BACKGROUND OF THE INVENTION
[0003] Compression ignition engines cause combustion of a
hydrocarbon by injecting the hydrocarbon into compressed air and
can be fuelled by diesel fuel, biodiesel fuel, blends of diesel and
biodiesel fuels and compressed natural gas. The purpose of the
present invention is different from the invention claimed in UK
patent application no. 1003244.9 filed on 26 Feb. 2010 entitled
"Filter". The purpose of the invention in that patent application
is a filter for particulate matter in exhaust gas of a positive
ignition engine.
[0004] Ambient PM is divided by most authors into the following
categories based on their aerodynamic diameter (the aerodynamic
diameter is defined as the diameter of a 1 g/cm.sup.3 density
sphere of the same settling velocity in air as the measured
particle): [0005] (i) PM-10--particles of an aerodynamic diameter
of less than 10 .mu.m; [0006] (ii) Fine particles of diameters
below 2.5 .mu.m (PM-2.5); [0007] (iii) Ultrafine particles of
diameters below 0.1 .mu.m (or 100 nm); and [0008] (iv)
Nanoparticles, characterised by diameters of less than 50 nm.
[0009] Since the mid-1990's, particle size distributions of
particulates exhausted from internal combustion engines have
received increasing attention due to possible adverse health
effects of fine and ultrafine particles. Concentrations of PM-10
particulates in ambient air are regulated by law in the USA. A new,
additional ambient air quality standard for PM-2.5 was introduced
in the USA in 1997 as a result of health studies that indicated a
strong correlation between human mortality and the concentration of
fine particles below 2.5 .mu.m.
[0010] Interest has now shifted towards nanoparticles generated by
diesel and gasoline engines because they are understood to
penetrate more deeply into human lungs than particulates of greater
size and consequently they are believed to be more harmful than
larger particles, extrapolated from the findings of studies into
particulates in the 2.5-10.0 .mu.m range.
[0011] Size distributions of diesel particulates have a
well-established bimodal character that corresponds to the particle
nucleation and agglomeration mechanisms, with the corresponding
particle types referred to as the nuclei mode and the accumulation
mode respectively (see FIG. 1). As can be seen from FIG. 1, in the
nuclei mode, diesel PM is composed of numerous small particles
holding very little mass. Nearly all diesel particulates have sizes
of significantly less than 1 .mu.m, i.e. they comprise a mixture of
fine, i.e. falling under the 1997 US law, ultrafine and
nanoparticles.
[0012] Nuclei mode particles are believed to be composed mostly of
volatile condensates hydrocarbons, sulfuric acid, nitric acid etc)
and contain little solid material, such as ash and carbon.
Accumulation mode particles are understood to comprise solids
(carbon, metallic ash etc.) intermixed with condensates and
adsorbed material (heavy hydrocarbons, sulfur species, nitrogen
oxide derivatives etc.). Coarse mode particles are not believed to
be generated in the diesel combustion process and may be formed
through mechanisms such as deposition and subsequent re-entrainment
of particulate material from the walls of an engine cylinder,
exhaust system, or the particulate sampling system. The
relationship between these modes is shown in FIG. 1.
[0013] The composition of nucleating particles may change with
engine operating conditions, environmental condition (particularly
temperature and humidity), dilution and sampling system conditions.
Laboratory work and theory have shown that most of the nuclei mode
formation and growth occur in the low dilution ratio range. In this
range, gas to particle conversion of volatile particle precursors,
like heavy hydrocarbons and sulfuric acid, leads to simultaneous
nucleation and growth of the nuclei mode and adsorption onto
existing particles in the accumulation mode. Laboratory tests (see
e.g. SAE 980525 and SAE 2001-01-0201) have shown that nuclei mode
formation increases strongly with decreasing air dilution
temperature but there is conflicting evidence on whether humidity
has an influence.
[0014] Generally, low temperature, low dilution ratios, high
humidity and long residence times favour nanoparticles formation
and growth. Studies have shown that nanoparticles consist mainly of
volatile material like heavy hydrocarbons and sulfuric acid with
evidence of solid fraction only at very high loads.
[0015] Particulate collection of diesel particulates in a diesel
particulate filter is based on the principle of separating
gas-borne particulates from the gas phase using a porous barrier.
Diesel filters can be defined as deep-bed filters and/or
surface-type filters. In deep-bed filters, the mean pore size of
filter media is bigger than the mean diameter of collected
particles. The particles are deposited on the media through a
combination of depth filtration mechanisms, including diffusional
deposition (Brownian motion), inertial deposition (impaction) and
flow-line interception (Brownian motion or inertia).
[0016] In surface-type filters, the pore diameter of the filter
media is less than the diameter of the PM, so PM is separated by
sieving. Separation is done by a build-up of collected diesel PM
itself, which build-up is commonly referred to as "filtration cake"
and the process as "cake filtration".
[0017] It is understood that diesel particulate filters, such as
ceramic wallflow monoliths, may work through a combination of depth
and surface filtration: a filtration cake develops at higher soot
loads when the depth filtration capacity is saturated and a
particulate layer starts covering the filtration surface. Depth
filtration is characterized by somewhat lower filtration efficiency
and lower pressure drop than the cake filtration.
[0018] Selective catalytic reduction (SCR) of NO.sub.x by
nitrogenous compounds, such as ammonia or urea, was first developed
for treating industrial stationary applications. SCR technology was
first used in thermal power plants in Japan in the late 1970s, and
has seen widespread application in Europe since the mid-1980s. In
the USA, SCR systems were introduced for gas turbines in the 1990s
and have been used more recently in coal-fired powerplants. In
addition to coal-fired cogeneration plants and gas turbines, SCR
applications include plant and refinery heaters and boilers in the
chemical processing industry, furnaces, coke ovens, municipal waste
plants and incinerators. More recently, NO.sub.x reduction systems
based on SCR technology are being developed for a number of
vehicular (mobile) applications in Europe, Japan, and the USA, e.g.
for treating diesel exhaust gas.
[0019] Several chemical reactions occur in an NH.sub.3 SCR system,
all of which represent desirable reactions that reduce NO.sub.x to
nitrogen. The dominant reaction is represented by reaction (1).
4NO+4NH.sub.3+O.sub.2.fwdarw.4N.sub.2+6H.sub.2O (1)
[0020] Competing, non-selective reactions with oxygen can produce
secondary emissions or may unproductively consume ammonia. One such
non-selective reaction is the complete oxidation of ammonia, shown
in reaction (2).
4NH.sub.3+5O.sub.2.fwdarw.4NO+6H.sub.2O (2)
[0021] Also, side reactions may lead to undesirable products such
as N.sub.2O, as represented by reaction (3)
4NH.sub.3+5NO+3O.sub.2.fwdarw.4N.sub.2O+6H.sub.2O (3)
[0022] Various catalysts for promoting NH.sub.3-SCR are known
including V.sub.2O.sub.5/WO.sub.3/TiO.sub.2 and transition
metal/zeolites such as Fe/Beta (see U.S. Pat. No. 4,961,917) and
transition metal/small pore zeolites (see WO 2008/132452).
[0023] EP 1663458 discloses an SCR filter, wherein the filter is a
wallflow monolith and wherein an SCR catalyst composition permeates
walls of the wallflow monolith. The specification discloses
generally that the walls of the wallflow filter can contain thereon
or therein (i.e. not both) one or more catalytic materials.
According to the disclosure, "permeate", when used to describe the
dispersion of a catalyst slurry on the wallflow monolith substrate,
means the catalyst composition is dispersed throughout the wall of
the substrate.
[0024] WO 2008/136232 A1 discloses a honeycomb filter having a cell
wall composed of a porous cell wall base material and, provided on
its inflow side only or on its inflow and outflow sides, a surface
layer and satisfying the following requirements (1) to (5) is used
as DPF: (1) the peak pore diameter of the surface layer is
identical with or smaller than the average pore diameter of the
cell wall base material, and the porosity of the surface layer is
larger than that of the cell wall base material; (2) with respect
to the surface layer, the peak pore diameter is from 0.3 to less
than 20 .mu.m, and the porosity is from 60 to less than 95%
(measured by mercury penetration method); (3) the thickness (L1) of
the surface layer is from 0.5 to less than 30% of the thickness
(L2) of the cell wall; (4) the mass of the surface layer per
filtration area is from 0.01 to less than 6 mg/cm.sup.2; and (5)
with respect to the cell wall base material, the average pore
diameter is from 10 to less than 60 11m, and the porosity is from
40 to less than 65%. See also SAE paper 2009-01-0292.
[0025] NOx absorber catalysts (NACs) are known e.g. from U.S. Pat.
No. 5,473,887 and are designed to adsorb nitrogen oxides (NOx) from
lean exhaust gas (lambda >1) and to desorb the NOx when the
oxygen concentration in the exhaust gas is decreased. Desorbed NOx
may be reduced to N.sub.2 with a suitable reductant, e.g. gasoline
fuel, promoted by a catalyst component, such as rhodium, of the NAC
itself or located downstream of the NAC. In practice, control of
oxygen concentration can be adjusted to a desired redox composition
intermittently in response to a calculated remaining NOx adsorption
capacity of the NAC, e.g. richer than normal engine running
operation (but still lean of stoichiometric or lambda=1
composition), stoichiometric or rich of stoichiometric (lambda
<1). The oxygen concentration can be adjusted by a number of
means, e.g. throttling, injection of additional hydrocarbon fuel
into an engine cylinder such as during the exhaust stroke or
injecting hydrocarbon fuel directly into exhaust gas downstream of
an engine manifold.
[0026] A typical NAC formulation includes a catalytic oxidation
component, such as platinum, a significant quantity, i.e.
substantially more than is required for use as a promoter such as a
promoter in a TWC, of a NOx-storage component, such as barium, and
a reduction catalyst, e.g. rhodium. One mechanism commonly given
for NOx-storage from a lean exhaust gas for this formulation
is:
NO+1/2O.sub.2.fwdarw.NO.sub.2 (4); and
BaO+NO.sub.2+1/2O.sub.2.fwdarw.Ba(NO.sub.3).sub.2 (5),
wherein in reaction (4), the nitric oxide reacts with oxygen on
active oxidation sites on the platinum to form NO.sub.2. Reaction
(5) involves adsorption of the NO.sub.2 by the storage material in
the form of an inorganic nitrate.
[0027] At lower oxygen concentrations and/or at elevated
temperatures, the nitrate species become thermodynamically unstable
and decompose, producing NO or NO.sub.2 according to reaction (6)
below. In the presence of a suitable reductant, these nitrogen
oxides are subsequently reduced by carbon monoxide, hydrogen and
hydrocarbons to N2, which can take place over the reduction
catalyst (see reaction (5)).
Ba(NO.sub.3).sub.2.fwdarw.BaO+2NO+3/2O.sub.2 or
Ba(NO.sub.3).sub.2.fwdarw.BaO+2NO.sub.2+1/2O.sub.2 (6); and
NO+CO.fwdarw.1/2N.sub.2CO.sub.2 (7);
(Other reactions include
Ba(NO.sub.3).sub.2+8H.sub.2.fwdarw.BaO+2NH.sub.3+5H.sub.2O followed
by NH.sub.3+NO.sub.x.fwdarw.N.sub.2+yH.sub.2O or
2NH.sub.3+2O.sub.2+CO.fwdarw.N.sub.2+3H.sub.2O+CO.sub.2 etc.).
[0028] In the reactions of (4)-(7) above, the reactive barium
species is given as the oxide. However, it is understood that in
the presence of air most of the barium is in the form of the
carbonate or possibly the hydroxide. The skilled person can adapt
the above reaction schemes accordingly for species of barium other
than the oxide and sequence of catalytic coatings in the exhaust
stream.
[0029] In Europe, since the year 2000 (Euro 3 emission standard)
emissions are tested over the New European Driving Cycle (NEDC).
This consists of four repeats of the previous ECE 15 driving cycle
plus one Extra Urban Driving Cycle (EUDC) with no 40 second warm-up
period before beginning emission sampling. This modified cold start
test is also referred to as the
[0030] "MVEG-B" drive cycle. All emissions are expressed in
g/km.
[0031] The Euro 5/6 implementing legislation introduces a new PM
mass emission measurement method developed by the UN/ECE
Particulate Measurement Programme (PMP) which adjusts the PM mass
emission limits to account for differences in results using old and
the new methods. The Euro 5/6 legislation also introduces a
particle number emission limit (PMP method), in addition to the
mass-based limits.
[0032] Emission legislation in Europe from 1 Sep. 2014 (Euro 6)
requires control of the number of particles emitted from both
diesel and gasoline passenger cars. For diesel EU light duty
vehicles the allowable limits are: 500 mg/km carbon monoxide; 80
mg/km nitrogen oxides (NOx); 170 mg/km total hydrocarbons+NOx; 4.5
g/km particulate matter (PM); and particulate number standard of
6.0.times.10.sup.11 per km. The present specification is based on
the assumption that this number will be adopted in due course.
[0033] A difficulty in coating a filter with a catalyst composition
is to balance a desired catalytic activity, which generally
increases with washcoat loading, with the backpressure that is
caused by the filter in use (increased washcoat loading generally
increases backpressure) and filtration efficiency (backpressure can
be reduced by adopting wider mean pore size and higher porosity
substrates at the expense of filtration efficiency).
SUMMARY OF THE INVENTION
[0034] We have now discovered, very surprisingly, that by coating a
filter substrate monolith on a surface thereof with a washcoat, as
opposed to permeating the filter walls with the washcoat as is
disclosed in EP 1663458 it is possible to achieve a beneficial
balance of backpressure, filtration and catalytic activity.
Moreover, we have found that by appropriate selection of molecular
sieve size it is possible to tune the backpressure of the filter at
a similar catalytic activity, thus increasing design options.
[0035] According to one aspect, the invention provides a filter for
filtering particulate matter (PM) from exhaust gas emitted from a
compression ignition engine, which filter comprising a porous
substrate having inlet surfaces and outlet surfaces, wherein the
inlet surfaces are separated from the outlet surfaces by a porous
structure containing pores of a first mean pore size, wherein the
porous substrate is coated with a washcoat comprising a plurality
of solid particles comprising a molecular sieve promoted with at
least one transition metal wherein the porous structure of the
washcoated porous substrate contains pores of a second mean pore
size, and wherein the second mean pore size is less than the first
mean pore size.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] In order that the invention may be more fully understood,
reference is made to the accompanying drawings wherein:
[0037] FIG. 1 is a graph showing the size distributions of PM in
the exhaust gas of a diesel engine. For comparison, a gasoline size
distribution is shown at FIG. 4 of SAE 1999-01-3530;
[0038] FIGS. 2A and 2B show schematic drawings of three embodiments
of washcoated porous filter substrates according to the
invention;
[0039] FIG. 3 is a schematic graph of mercury porosimetry relating
the pore size distribution of a porous filter substrate, a porous
washcoat layer and a porous filter substrate including a porous
surface washcoat layer;
[0040] FIG. 4 is a graph showing the results of a Soot Loading Back
Pressure study comparing backpressure against soot loading for 5.66
inch.times.6 inch SiC wallflow filters coated with two different
oxidation catalyst washcoat loadings (g/in.sup.3) and a bare filter
(all not according to the invention) with a Fe/beta zeolite
selective catalytic reduction (SCR) catalyst (according to the
invention) at a comparable washcoat loading;
[0041] FIG. 5 is a graph comparing the backpressure in the same
Soot Loading Back Pressure test for a Cu/SSZ-13 zeolite (a small
pore zeolite) catalyst and a Fe/Beta zeolite (a large pore zeolite)
SCR catalyst; and
[0042] FIG. 6 is a bar chart comparing the particulate number
emissions (particulate number per kilometre) from a 2.0 litre Euro
5 compliant light duty diesel vehicle fitted with standard diesel
oxidation catalyst followed by a 3.0 litre SiC filter at 23 .mu.m
nominal mean pore size coated with a Fe/Beta zeolite SCR catalyst
for meeting the Euro 5/6 particle number emission limit of
6.times.10.sup.11 km.sup.-1 (UN/ECE Particulate Measurement
Programme (PMP)) with the same system containing a bare filter.
DETAILED DESCRIPTION OF THE INVENTION
[0043] Mean pore size can be determined by mercury porosimetry.
[0044] It will be understood that the benefit of the invention is
substantially independent of the porosity of the substrate.
Porosity is a measure of the percentage of void space in a porous
substrate and is related to backpressure in an exhaust system:
generally, the lower the porosity, the higher the backpressure.
However, the porosity of filters for use in the present invention
are typically >40% or >50% and porosities of 45-75% such as
50-65% or 55-60% can be used with advantage. The mean pore size of
the washcoated porous substrate is important for filtration. So, it
is possible to have a porous substrate of relatively high porosity
that is a poor filter because the mean pore size is also relatively
high.
[0045] The porous substrate can be a metal, such as a sintered
metal, or a ceramic, e.g. silicon carbide, cordierite, aluminium
nitride, silicon nitride, aluminium titanate, alumina, cordierite,
mullite e.g., acicular mullite (see e.g. WO 01/16050), pollucite, a
thermet such as Al.sub.2O.sub.3/Fe, Al.sub.2O.sub.3/Ni or
B.sub.4C/Fe, or composites comprising segments of any two or more
thereof. In a preferred embodiment, the filter is a wallflow filter
comprising a ceramic porous filter substrate having a plurality of
inlet channels and a plurality of outlet channels, wherein each
inlet channel and each outlet channel is defined in part by a
ceramic wall of porous structure, wherein each inlet channel is
separated from an outlet channel by a ceramic wall of porous
structure. This filter arrangement is also disclosed in SAE 810114,
and reference can be made to this document for further details.
Alternatively, the filter can be a foam, or a so-called partial
filter, such as those disclosed in EP 1057519 or WO 01/080978.
[0046] In one embodiment, the first mean pore size e.g. of surface
pores of the porous structure of the porous filter substrate is
from 8 to 45 .mu.m, for example 8 to 25 .mu.m, 10 to 20 .mu.m or 10
to 15 .mu.m. In particular embodiments, the first mean pore size is
>18 .mu.m such as from 15 to 45 .mu.m, 20 to 45 .mu.m e.g. 20 to
30 .mu.m, or 25 to 45 .mu.m.
[0047] In embodiments, the filter has a washcoat loading of
>0.25 g in.sup.-3, such as >0.50 g in.sup.-3 or .gtoreq.0.80
g in.sup.-3, e.g. 0.80 to 3.00 g in.sup.-3. In preferred
embodiments, the washcoat loading is >1.00 g in.sup.-3 such as
.gtoreq.1.2 g in.sup.-3, >1.5 g in.sup.-3, >1.6 g in.sup.-3
or >2.00 g in.sup.-3 or for example 1.6 to 2.4 g in.sup.-3. In
particular combinations of filter mean pore size and washcoat
loading the filter combines a desirable level of particulate
filtration and catalytic activity at acceptable backpressure.
[0048] In a first, preferred embodiment, the filter comprises a
surface washcoat, wherein a washcoat layer substantially covers
surface pores of the porous structure and the pores of the
washcoated porous substrate are defined in part by spaces between
the particles (interparticle pores) in the washcoat. That is,
substantially no washcoat enters the porous structure of the porous
substrate. Methods of making surface coated porous filter
substrates include introducing a polymer, e.g. poly vinyl alcohol
(PVA), into the porous structure, applying a washcoat to the porous
filter substrate including the polymer and drying, then calcining
the coated substrate to burn out the polymer. A schematic
representation of the first embodiment is shown in FIG. 2A.
[0049] Methods of coating porous filter substrates are known to the
skilled person and include, without limitation, the method
disclosed in WO 99/47260, i.e. a method of coating a monolithic
support, comprising the steps of (a) locating a containment means
on top of a support, (b) dosing a pre-determined quantity of a
liquid component into said containment means, either in the order
(a) then (b) or (b) then (a), and (c) by applying pressure or
vacuum, drawing said liquid component into at least a portion of
the support, and retaining substantially all of said quantity
within the support. Such process steps can be repeated from another
end of the monolithic support following drying of the first coating
with optional firing/calcination.
[0050] In this first embodiment, an average interparticle pore size
of the porous washcoat is 5.0 nm to 5.0 .mu.m, such as 0.1-1.0
.mu.m.
[0051] A D90 of solid washcoat particles in this first surface
coating embodiments can be greater than the mean pore size of the
porous filter substrate and can be in the range 10 to 40 .mu.m,
such as 15 to 30 .mu.m or 12 to 25 .mu.m. "D90" as used herein
defines the particle size distribution in a washcoat wherein 90% of
the particles present have a diameter within the range specified.
Alternatively, in embodiments, the mean size of the solid washcoat
particles is in the range 1 to 20 .mu.m. It will be understood that
the broader the range of particle sizes in the washcoat, the more
likely that washcoat may enter the porous structure of the porous
substrate. The term "substantially no washcoat enters the porous
structure of the substrate" should therefore be interpreted
accordingly.
[0052] According to a second embodiment, the washcoat can be coated
on inlet and/or outlet surfaces and also within the porous
structure of the porous substrate. We believe that a surface
coating around a pore opening at the inlet and/or outlet surfaces,
thereby narrowing the e.g. surface pore size of a bare filter
substrate, promotes interaction of the gas phase including PM
without substantially restricting the pore volume, so not giving
rise to significant increases in back pressure. That is, the pores
at a surface of the porous structure comprise a pore opening and
the washcoat causes a narrowing of substantially all the pore
openings. A schematic representation of the second embodiment is
shown in FIG. 2B.
[0053] Methods of making a filter according to the second
embodiment can involve appropriate formulation of the washcoat
known to the person skilled in the art including adjusting
viscosity and surface wetting characteristics and application of an
appropriate vacuum following coating of the porous substrate (see
also WO 99/47260).
[0054] In our research and development work we have found that
coated filters according to the first or second embodiments can be
obtained by dip coating in a washcoat composition followed by
draining the coated part, then application of a low vacuum to
remove excess washcoat before drying and calcining. This method
produces a surface coating (as determined by scanning electron
microscope (SEM)) and in this respect distinguishes the coated
filter wherein the SCR catalyst "permeates" the filter walls, as
disclosed in EP 1663458.
[0055] In the first and second embodiments, wherein at least part
of the washcoat is coated on inlet and/or outlet surfaces of the
porous substrate, the washcoat can be coated on the inlet surfaces,
the outlet surfaces or on both the inlet and the outlet surfaces.
Additionally either one or both of the inlet and outlet surfaces
can include a plurality of washcoat layers, wherein each washcoat
layer within the plurality of layers can be the same or different,
e.g. the mean pore size in a first layer can be different from that
of a second layer. In embodiments, washcoat intended for coating on
outlet surfaces is not necessarily the same as for inlet
surfaces.
[0056] Where both inlet and outlet surfaces are coated, the
washcoat formulations can be the same or different. Where both the
inlet and the outlet surfaces are washcoated, the mean pore size of
washcoat on the inlet surfaces can be different from the mean pore
size of washcoat on the outlet surfaces. For example, the mean pore
size of washcoat on the inlet surfaces can be less than the mean
pore size of washcoat on the outlet surfaces. In the latter case, a
mean pore size of washcoat on the outlet surfaces can be greater
than a mean pore size of the porous substrate.
[0057] Whilst it is possible for the mean pore size of a washcoat
applied to inlet surfaces to be greater than the mean pore size of
the porous substrate, it is advantageous to have washcoat having
smaller pores than the porous substrate in washcoat on inlet
surfaces to prevent or reduce any combustion ash or debris entering
the porous structure.
[0058] In the second embodiment, wherein at least part of the
washcoat is in the porous structure, a size, e.g. a mean size, of
the solid washcoat particles can be less than the mean pore size of
the porous filter substrate for example in the range 0.1 to 20
.mu.m, such as 1 to 18 .mu.m, 1 to 16 .mu.m, 2 to 15 .mu.m or 3 to
12 .mu.m. In particular embodiments, the abovementioned size of the
solid washcoat particles is a D90 instead of a mean size.
[0059] In further particular embodiments, the surface porosity of
the washcoat is increased by including voids therein. Exhaust gas
catalysts having such features arc disclosed, e.g. in our WO
2006/040842 and WO 2007/116881.
[0060] By "voids" in the washcoat layer herein, we mean that a
space exists in the layer defined by solid washcoat material. Voids
can include any vacancy, fine pore, tunnel-state (cylinder,
prismatic column), slit etc., and can be introduced by including in
a washcoat composition for coating on the filter substrate a
material that is combusted during calcination of a coated filter
substrate, e.g. chopped cotton or materials to give rise to pores
made by formation of gas on decomposition or combustion.
[0061] The average void ratio of the washcoat can be from 5-80%,
whereas the average diameter of the voids can be from 0.2 to 500
.mu.m, such as 10 to 250 .mu.m.
[0062] Promoter metals can be selected from the group consisting of
at least one of Cu, Hf, La, Au, In, V, lanthanides and Group VIII
transition metals, such as Fe. The molecular sieve for use in the
present invention can be an aluminosilicate zeolite, a
metal-substituted aluminosilicate zeolite or a non-zeolitic
molecular sieve. Metal substituted molecular sieves with
application in the present invention include those having one or
more metals incorporated into a framework of the molecular sieve
e.g. Fe in-framework Beta and Cu in-framework CHA.
[0063] Where the molecular sieve is non-zeolitic molecular sieve,
it can be an aluminophosphate molecular sieve selected from the
group consisting of aluminophosphate (AlPO) molecular sieves, metal
substituted aluminophosphate molecular sieves (MeAlPO) zeolites,
silico-aluminophosphate (SAPO) molecular sieves and metal
substituted silico-aluminophosphate (MeAPSO) molecular sieves.
[0064] In particular, the molecular sieve can be a small, medium or
large pore molecular sieve. By "small pore molecular sieve" herein
we mean a molecular sieve containing a maximum ring size of 8, such
as CHA; by "medium pore molecular sieve" herein we mean a molecular
sieve containing a maximum ring size of 10, such as ZSM-5; and by
"large pore molecular sieve" herein we mean a molecular sieve
having a maximum ring size of 12, such as beta. Small pore
molecular sieves with particular application in the present
invention are any of those listed in Table 1 of WO2008/132452.
[0065] Specific examples of useful molecular sieves arc selected
from the group consisting of AEI, ZSM-5, ZSM-20, ERI, LEV,
mordenite, BEA, Y, CHA, MCM-22 and EU-1.
[0066] The metal substitutent and/or the transition metal promoter
can be selected from the group consisting of groups IB, IIB, IIIA,
IIIB, VB, VIB, VIB and VIII of the periodic table.
[0067] In embodiments, the metal can be selected from the group
consisting of Cr, Co, Cu, Fe, Hf, La, Ce, In, V, Mn, Ni, Zn, Ga and
the precious metals Ag, Au, Pt, Pd and Rh.
[0068] Metals of particular interest for use as transition metal
promoters in so-called NH.sub.3-SCR are selected from the group
consisting of Ce, Fe and Cu. Suitable nitrogenous reductants
include ammonia. Ammonia can be generated in situ e.g. during rich
regeneration of a NAC disposed upstream of the filter (see the
alternatives to reactions (6) and (7) hereinabove). Alternatively,
the nitrogenous reductant or a precursor thereof can be injected
directly into the exhaust gas. Suitable precursors include amrnomum
formate, urea and amrnomum carbamate. Decomposition of the
precursor to ammonia and other by-products can be by hydrothermal
or catalytic hydrolysis.
[0069] According to a further aspect, the invention provides an
exhaust system for a compression ignition engine, which system
comprising a filter according to the invention. Compression
ignition engines for use in this aspect of the invention can be
fuelled by diesel fuel, biodiesel fuel, blends of diesel and
biodiesel fuels and compressed natural gas.
[0070] In one embodiment, the exhaust system comprises means for
injecting a nitrogenous reductant or a precursor thereof, into
exhaust gas upstream of the filter. In a particular embodiment, the
nitrogenous reductant is a fluid.
[0071] In another aspect, the invention provides a compression
ignition engine comprising an exhaust system according to the
invention.
[0072] In a further aspect, the invention provides a method of
trapping particulate matter (PM) from exhaust gas emitted from a
compression ignition engine by depth filtration, which method
comprising contacting exhaust gas containing the PM with a filter
comprising a porous substrate having inlet and outlet surfaces,
wherein the inlet surfaces are separated from the outlet surfaces
by a porous structure containing pores of a first mean pore size,
wherein the porous substrate is coated with a washcoat comprising a
plurality of solid particles comprising a molecular sieve promoted
with at least one metal wherein the porous structure of the
washcoated porous substrate contains pores of a second mean pore
size, and wherein the second mean pore size is less than the first
mean pore size.
[0073] In a further aspect, the invention provides a method of
adjusting filter backpressure in an exhaust system of a compression
ignition engine by coating the filter with a first transition metal
promoted molecular sieve SCR catalyst, testing the filter
backpressure to determine whether it meets a pre-determined
backpressure requirement and selecting a second transition metal
promoted molecular sieve SCR catalyst in order to reduce the
backpressure in the system containing the filter coated with the
first transition metal promoted molecular sieve SCR catalyst,
wherein the pore size of the second molecular sieve is >the
first molecular sieve.
[0074] FIGS. 2A and 2B show a cross-section through a porous filter
substrate 10 comprising a surface pore 12. FIG. 2A shows a first
embodiment, featuring a porous surface washcoat layer 14 comprised
of solid washcoat particles, the spaces between which particles
define pores (interparticle pores). It can be seen that the
washcoat layer 14 substantially covers the pore 12 of the porous
structure and that a mean pore size of the interparticle pores 16
is less than the mean pore size 12 of the porous filter substrate
10.
[0075] FIG. 2B shows a second embodiment comprising a washcoat that
is coated on an inlet surface 16 and additionally within a porous
structure 12 of the porous substrate 10. It can be seen that the
washcoat layer 14 causes a narrowing of a pore openings of surface
pore 12, such that a mean pore size 18 of the coated porous
substrate is less than the mean pore size 12 of the porous filter
substrate 10.
[0076] FIG. 3 shows an illustration of a graph relating pore size
to pore number for a porous filter substrate 20, a porous washcoat
layer 22 and a porous diesel filter substrate including a surface
washcoat layer 24. It can be seen that the filter substrate has a
mean pore size of the order of about 15 .mu.m. The washcoat layer
has a bimodal distribution comprised of intraparticle pores 22A (at
the nanometre end of the range) and interparticle pores 22B towards
the micrometer end of the scale. It can also be seen that by
coating the porous filter substrate with a washcoat according to
the invention that the pore distribution of the bare filter
substrate is shifted in the direction of the interparticle washcoat
pore size (see arrow).
EXAMPLES
[0077] The following Examples are provided by way of illustration
only. In the Examples, the Soot Loading Back Pressure ("SLBP") test
uses the apparatus and method described in EP 1850068, i.e.: [0078]
(i) an apparatus for generating and collecting particulate matter
derived from combusting a liquid carbon-containing fuel, which
apparatus comprising a fuel burner comprising a nozzle, which
nozzle is housed in a container, which container comprising a gas
inlet and a gas outlet, said gas outlet connecting with a conduit
for transporting gas from the gas outlet to atmosphere, means for
detecting a rate of gas flowing through the gas inlet and means for
forcing an oxidising gas to flow from the gas inlet via the
container, the gas outlet and the conduit to atmosphere, a station
for collecting particulate matter from gas flowing through the
conduit and means for controlling the gas flow-forcing means in
response to a detected gas flow rate at the gas inlet, whereby the
rate of gas flow at the gas inlet is maintained at a desired rate
to provide substoichiometric fuel combustion within the container,
thereby to promote particulate matter formation; and [0079] (ii) a
method of generating and collecting particulate matter derived from
combusting liquid carbon-containing fuel in an oxidising gas, which
method comprising burning the fuel in a substoichiometric quantity
of oxidising gas in a fuel burner, said fuel burner comprising a
nozzle, which nozzle being housed in a container, forcing an
oxidising gas to flow from a gas inlet to the container to
atmosphere via a gas outlet to the container and a conduit
connected to the gas outlet, collecting particulate matter at a
station located within the conduit, detecting a rate of oxidising
gas flow at the gas inlet and controlling the rate of oxidising gas
flow so that a desired rate of oxidising gas flow is maintained at
the gas inlet.
[0080] The filter is inserted in the station for collecting
particulate matter from gas flowing through the conduit. The fresh
filter is first pre-conditioned at an air flow rate 80 kg/hr in a
lean burn combustion stream using low sulphur diesel fuel (10 ppm
S) to raise the filter inlet temperature to 650.degree. C., a
temperature that is typically used on a vehicle to regenerate a
soot-loaded filter. This pre-conditioning step temperature is well
above the soot combustion temperature and is to ensure that the
filter on test is clean at the outset. Pressure sensors disposed
upstream and downstream of the station monitor the backpressure
across the filter. The backpressure against time is plotted in the
accompanying FIGS. 4-6. The SLBP test is carried out at a filter
inlet temperature of 250.degree. C. at air flow rate of 180 kg/hour
combusting low sulphur diesel fuel (10 ppm S).
Example 1
CSF and SCR Catalyst Coated Filter Backpressure Comparison
[0081] Three commercially available uncoated 5.66 inch.times.6 inch
SiC wallflow filters having 60% porosity and a mean pore size of
20-25 .mu.m were each coated, separately, with a catalyst washcoat
for a catalysed soot filter (CSF) comprising precious metal
supported on an alumina-based metal oxide and an Cu/Beta zeolite
selective catalytic reduction (SCR) catalyst coating. The CSF
coating was obtained according to the method disclosed in WO
99/47260, i.e. a method of coating a monolithic support, comprising
the steps of (a) locating a containment means on top of a support,
(b) dosing a pre-determined quantity of a liquid component into
said containment means, either in the order (a) then (b) or (b)
then (a), and (c) by applying pressure or vacuum, drawing said
liquid component into at least a portion of the support, and
retaining substantially all of said quantity within the support.
The coated product was dried and calcined and then the process
steps were repeated from another end of the wallflow filter. The
SCR coated filter was obtained by dip coating followed by draining,
the application of a low vacuum to remove excess washcoat before
drying and calcining. This method produces a surface coating (as
determined by scanning electron microscope (SEM)) and in this
respect distinguishes the coated filter wherein the SCR catalyst
"permeates" the filter walls, as disclosed in EP 1663458. Two
different CSF washcoat loadings were obtained, at 0.6 g/in.sup.3
and 1.2 g/in.sup.3. The SCR coated filter was washcoated at a
loading of at 1.1 g/in.sup.3.
[0082] The three coated filters were tested using the SLBP test, a
fourth, uncoated filter was used as a control. The results are
shown in FIG. 4, from which it can be seen that the CSF coating at
approximately the same washcoat loading has considerably higher
backpressure compared to the SCR coated filter. We conclude,
therefore, that there is an inherent coating porosity difference
between CSF and SCR coated filter.
Example 2
SCR Catalyst Coated Filter Backpressure Comparison
[0083] Identical commercially available 5.66 inch.times.7.5 inch
SiC wallflow filters having 60% porosity and a mean pore size of
20-25 .mu.m were washcoated to a loading of 1.1 g/in.sup.3 with
Cu/SSZ-13 zeolite and Cu/Beta zeolite SCR catalysts, each catalyst
having the same particle size D90 (90% of particles in washcoat
having a particle size) at between 4.8-5 .mu.m but apart from the
transition metal/zeolite were in all other respects were
substantially identical. The method of manufacture was to dip coat
the part followed by draining, the application of a low vacuum to
remove excess washcoat and then drying and calcining. A SLBP test
was done to compare the finished parts.
[0084] The results are presented in FIG. 5, from which it can be
seen that the filter coated with the Cu/Beta zeolite catalyst has a
lower rate of backpressure increase than the filter coated with the
Cu/SSZ-13 zeolite catalyst. Since the fundamental difference
between the two SCR catalysts is that the pore size of the SSZ-13
zeolite is 3.8.times.3.8 Angstroms and 5.6-7.7 Angstroms for the
Beta zeolite (source: Structure Commission of the International
Zeolite Association), we conclude that it is possible to adjust
backpressure in the exhaust system, thereby increasing design
options, by selecting a molecular sieve-based SCR catalyst having
an appropriate pore size to achieve the desired backpressure
objective and at the same time meeting emission standards for
NOx.
Example 3
Vehicle Testing
[0085] A 3.0 litre capacity SiC filter at 58% porosity and 23 .mu.m
nominal mean pore size Cu/Beta zeolite SCR catalyst coated filter
manufactured by the dip coating method described in Example 1 was
inserted into an exhaust system of a 2.0 litre Euro 5 compliant
light duty diesel vehicle behind a standard diesel oxidation
catalyst. The vehicle containing the fresh (i.e. unaged) catalysed
filter was then driven over the MVEG-B drive cycle, then the EUDC
part of the MVEG-B cycle three times consecutively to pre-condition
the filter.
[0086] In Europe, since the year 2000 (Euro 3 emission standard)
emissions are tested over the New European Driving Cycle (NEDC).
This consists of four repeats of the previous ECE 15 driving cycle
plus one Extra Urban Driving Cycle (EUDC) with no 40 second warm-up
period before beginning emission sampling. This modified cold start
test is also referred to as the "MVEG-B" drive cycle. All emissions
are expressed in g/km.
[0087] The Euro 5/6 implementing legislation introduces a new PM
mass emission measurement method developed by the UN/ECE
Particulate Measurement Programme (PMP) which adjusts the PM mass
emission limits to account for differences in results using old and
the new methods. The Euro 5/6 legislation also introduces a
particle number emission limit (PMP method), in addition to the
mass-based limits. The new Euro 5/6 particle number emission limit
of 6.times.10.sup.11 km.sup.-1 using the PMP protocol allows for
pre-conditioning of the system prior testing the system to
determine whether it meets the emission standard over the MVEG-B
drive cycle.
[0088] Repeated cold MVEG-B cycles were then run using the
pre-conditioned system. The coated filter was exchanged in the
system for an uncoated filter as a control. The results are shown
as a bar chart in FIG. 6 comparing the particulate number emissions
(particulate number per kilometre) from which it can be seen that
despite pre-conditioning, which would be expected to develop a soot
cake providing improved filtration, the uncoated filter initially
failed the particle number emission limit of 6.times.10.sup.-11
km.sup.-1, but with repeated drive cycles the particle number came
down consistently to within the emission standard. By contrast it
can be seen that the coated filter is well within the emission
standard from the first drive cycle following pre-conditioning. We
interpret these data to mean that the coated filter promotes soot
caking that improves diesel particulate filtration and therefore a
more immediate reduction in particle number, yet--as is seen in
Example 2--the Cu/Beta zeolite coated filter provides a lower
backpressure compared with the Cu/SSZ-13 zeolite SCR catalyst or a
CSF coating at a similar washcoat loading (see Example 1).
Accordingly, the surface Cu/Beta SCR catalyst coating takes away
the requirement to have a soot layer on a higher porosity/mean pore
size filter before filtration occurs. Accordingly, the invention
provides benefits for particle number reduction in "real world"
driving conditions, as opposed to the idealised drive cycle
conditions set for meeting emission standards.
[0089] For the avoidance of any doubt, the entire contents of all
prior art documents cited herein is incorporated herein by
reference.
* * * * *