U.S. patent application number 13/477305 was filed with the patent office on 2012-11-01 for catalysts for treating transient nox emissions.
This patent application is currently assigned to JOHNSON MATTHEY PUBLIC LIMITED COMPANY. Invention is credited to GUY RICHARD CHANDLER, ALEXANDER NICHOLAS MICHAEL GREEN, JOANNE ELIZABETH MELVILLE, PAUL RICHARD PHILLIPS, STUART DAVID REID.
Application Number | 20120275977 13/477305 |
Document ID | / |
Family ID | 41572892 |
Filed Date | 2012-11-01 |
United States Patent
Application |
20120275977 |
Kind Code |
A1 |
CHANDLER; GUY RICHARD ; et
al. |
November 1, 2012 |
CATALYSTS FOR TREATING TRANSIENT NOx EMISSIONS
Abstract
A heterogeneous catalyst article having at least one combination
of a first molecular sieve having a medium pore, large pore, or
meso-pore crystal structure and optionally containing a first
metal, and a second molecular sieve having a small pore crystal
structure and optionally containing a second metal, and a monolith
substrate onto or within which said catalytic component is
incorporated, wherein the combination of the first and second
molecular sieves is a blend, a plurality of layers, and/or a
plurality of zones.
Inventors: |
CHANDLER; GUY RICHARD;
(CAMBRIDGE, GB) ; GREEN; ALEXANDER NICHOLAS MICHAEL;
(BALDOCK, GB) ; MELVILLE; JOANNE ELIZABETH;
(WITHINGTON, GB) ; PHILLIPS; PAUL RICHARD;
(ROYSTON, GB) ; REID; STUART DAVID; (CAMBOURNE,
GB) |
Assignee: |
JOHNSON MATTHEY PUBLIC LIMITED
COMPANY
LONDON
GB
|
Family ID: |
41572892 |
Appl. No.: |
13/477305 |
Filed: |
May 22, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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PCT/IB2010/003186 |
Nov 30, 2010 |
|
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13477305 |
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Current U.S.
Class: |
423/213.5 ;
502/66; 60/297; 60/299 |
Current CPC
Class: |
B01J 29/0352 20130101;
B01J 2029/062 20130101; B01D 53/9422 20130101; B01D 53/8628
20130101; B01J 37/0009 20130101; F01N 2510/06 20130101; B01J 29/78
20130101; B01J 35/04 20130101; B01J 37/0246 20130101; B01D 53/56
20130101; B01J 29/0358 20130101; B01J 29/72 20130101; B01J 29/7815
20130101; B01J 29/80 20130101; B01J 29/072 20130101; B01J 29/7415
20130101; B01J 37/0244 20130101; B01D 2255/9032 20130101; B01J
29/743 20130101; B01J 29/763 20130101; F01N 3/105 20130101; B01J
29/7215 20130101; B01J 29/7615 20130101; Y02A 50/2325 20180101;
B01D 2255/20761 20130101; Y02A 50/20 20180101; Y02C 20/10 20130101;
Y02T 10/24 20130101; B01J 35/0006 20130101; F01N 3/0842 20130101;
F01N 13/009 20140601; B01J 29/88 20130101; B01J 29/7065 20130101;
B01J 29/74 20130101; B01J 29/005 20130101; B01J 29/0356 20130101;
B01J 29/7049 20130101; B01J 29/76 20130101; B01J 37/0215 20130101;
B01D 2258/012 20130101; B01D 2255/502 20130101; F01N 2370/04
20130101; Y02T 10/12 20130101; F01N 2330/06 20130101; B01J 37/30
20130101; B01J 29/7007 20130101; B01D 53/9418 20130101; F01N 3/2066
20130101; B01J 29/0354 20130101; F01N 3/035 20130101; B01J 29/46
20130101; B01D 53/9413 20130101; B01J 29/70 20130101; B01J 29/723
20130101; B01D 2255/50 20130101; B01J 29/783 20130101; B01J 37/08
20130101 |
Class at
Publication: |
423/213.5 ;
502/66; 60/299; 60/297 |
International
Class: |
B01J 29/80 20060101
B01J029/80; F01N 3/035 20060101 F01N003/035; F01N 3/10 20060101
F01N003/10; B01D 53/94 20060101 B01D053/94; B01J 29/88 20060101
B01J029/88 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 30, 2009 |
GB |
0920927.1 |
Claims
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41. A catalyst for use in selective catalytic reduction (SCR), said
catalyst comprising one or more zeolites of the MFI structure type,
and one or more zeolites of the CHA structure type, wherein at
least part of the one or more zeolites of the MFI structure type
contains iron (Fe), and wherein at least part of the one or more
zeolites of the CHA structure type contains copper (Cu).
42. The catalyst of claim 41, wherein the weight ratio of the one
or more zeolites of the MFI structure type relative to the one or
more zeolites of the CHA structure type ranges from 0.1 to 1.
43. The catalyst of claim 41, wherein one or more of the zeolites
comprise both Al and Si in their respective zeolite frameworks.
44. The catalyst of claim 43, wherein the molar ratio of silica to
alumina (SAR) in the one or more zeolites of the CHA structure type
ranges from 8 to 150.
45. The catalyst of claim 41, wherein the amount of Fe in the one
or more zeolites of the MFI structure type ranges from 0.01 to 20
wt.-% based on the weight of said one or more zeolites.
46. The catalyst of claim 41, wherein the amount of Fe in the one
or more zeolites of the MFI structure type ranges from 0.1 to 10
wt.-% based on the weight of said one or more zeolites.
47. The catalyst of claim 41, wherein the amount of Cu in the one
or more zeolites of the CHA structure type ranges from 0.01 to 20
wt.-% based on the weight of said one or more zeolites.
48. The catalyst of claim 41, wherein the amount of Cu in the one
or more zeolites of the CHA structure type ranges from 0.1 to 10
wt.-% based on the weight of said one or more zeolites.
49. A catalyst article comprising a honeycomb substrate onto which
the catalyst of claim 41 is provided.
50. The catalyst article of claim 49, wherein the substrate is
selected from the group consisting of flow-through substrates and
wall-flow substrates.
51. The catalyst article of claim 49, wherein the catalyst
comprises a washcoat layer provided on the substrate, the zeolites
being contained in one single layer.
52. An exhaust gas treatment system comprising the SCR catalyst of
claim 41, an internal combustion engine, and an exhaust gas conduit
in fluid communication with the internal combustion engine, wherein
said SCR catalyst is present in the exhaust gas conduit, and
wherein the internal combustion engine is one of a lean burn engine
or a diesel engine.
53. The exhaust gas treatment system of claim 52 further comprising
at least one of an oxidation catalyst and a diesel particulate
filter in said conduit and located upstream from the SCR
catalyst.
54. A process for the treatment of a gas stream comprising
NO.sub.x, said process comprising conducting a gas stream over a
catalyst according to claim 41, wherein the gas stream is a diesel
exhaust gas stream.
55. The process for the treatment of a gas stream comprising
NO.sub.x according to claim 54, wherein the gas stream comprises at
least one of ammonia and urea.
56. The process of claim 54, wherein the weight ratio of the one or
more zeolites of the MFI structure type relative to the one or more
zeolites of the CHA structure type ranges from 0.1 to 1.
57. The catalyst of claim 43, wherein the molar ratio of silica to
alumina (SAR) in the one or more zeolites of the CHA structure type
ranges from 25 to 50.
58. The catalyst of claim 41, wherein the amount of Fe in the one
or more zeolites of the MFI structure type ranges from 0.5 to 5.0
wt.-% based on the weight of said one or more zeolites.
59. The catalyst of claim 41, wherein the amount of Cu in the one
or more zeolites of the CHA structure type ranges from 2.0 to 4.0
wt.-% based on the weight of said one or more zeolites.
60. A catalytic combination comprising: a first molecular sieve
having a first framework structure containing a first metal, said
first framework structure selected from the group consisting of
AEL, AFI, AFO, AFR, ATO, iron isomorphous BEA, BEA, GME, HEU, MFI,
MWW, OFF, and a meso-pore crystal structure; and a second molecular
sieve having a second framework structure containing a second
metal, said second framework structure selected from the group
consisting of ACO, AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, ATT,
CDO, CHA, DDR, DFT, EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW,
LEV, KFI, MER, MON, NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV,
THO, TSC, UEI, UFI, VNI, YUG and ZON, wherein said first and second
metals are selected from the group consisting of Cr, Mn, Fe, Co,
Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Pt, Ag, In, Sn, Re, and Ir.
61. The catalytic combination of claim 60, wherein said first
molecular sieve contains about 0.01 to about 20 weight percent of
said first metal, and said second molecular sieve contains about
0.01 to about 20 weight percent of said second metal.
62. The catalytic combination of claim 60, wherein said second
metal is copper, said second molecular sieve has a CHA framework
structure and contains about 0.5 to about 5 weight percent copper,
said first metal is iron, and said first molecular sieve has either
an iron isomorphous BEA framework structure having a
silica-to-ferric oxide ratio of about 50 to about 200 or a pre-aged
BEA framework structure containing 0.5 to 5.0 weight percent
iron.
63. The catalytic combination of claim 62, wherein said second
molecular sieve has a silica-to-alumina ratio of about 8 to about
150.
64. The catalytic combination of claim 62, wherein said second
molecular sieve contains about 2.0 to about 4.0 weight percent
copper.
65. The catalytic combination of claim 62, wherein said second
molecular sieve is SSZ-13.
66. The catalytic combination of claim 65, wherein said catalytic
combination comprises a weight ratio of said first molecular sieve
to said second molecular sieve of about 0.1 to about 1.0.
67. The catalytic combination of claim 66, wherein said first
molecular sieve has an iron isomorphous BEA framework structure and
said weight ratio is about 0.2 to about 0.6.
68. A heterogeneous catalyst article comprising: said catalytic
combination of claim 60; and a monolith substrate, wherein said
catalytic combination is incorporated onto or within said monolith
substrate, wherein said catalytic combination is in a form selected
from the group consisting of a blend, a plurality of layers, and a
plurality of zones and wherein said second molecular sieve
comprises a majority of said combination based on the combined
weight of said first and second molecular sieves.
69. A method for treating a diesel exhaust gas stream comprising
contacting a feed exhaust gas stream over or through the
heterogeneous catalyst article of claim 68 in the presence of a
nitrogenous reductant at a temperature of about 150.degree. C. to
about 750.degree. C. to produce a treated exhaust gas stream,
wherein said feed exhaust gas stream has a first concentration of
NO.sub.x, said treated exhaust gas stream has a second
concentration of NO.sub.x and said second concentration of NO.sub.x
is less than said first concentration of NO.sub.x.
70. The method of claim 69, wherein: said first molecular sieve has
either an iron isomorphous BEA framework structure having a
silica-to-ferric oxide ratio of about 50 to about 200 or a pre-aged
BEA framework structure containing 0.5 to 5.0 weight percent iron;
said second molecular sieve has a CHA framework structure having a
silica-to-alumina ratio of about 8 to about 150 and contains about
0.5 to about 5 weight percent copper; said combination comprises
said first molecular sieve and said second molecular sieve in a
weight ratio of about 0.25 to about 0.50; said temperature is about
175.degree. C. to about 400.degree. C.; and said feed exhaust gas
stream contacts said heterogeneous catalyst article at a gas hourly
space velocity of about 10,000/hour to about 200,000/hour.
Description
[0001] The present invention relates to a selective catalytic
reduction catalyst comprising an optionally metal-promoted
molecular sieve component for converting oxides of nitrogen
(NO.sub.x) present in exhaust gas emitted from a mobile source,
such as a vehicular lean-burn internal-combustion engine, in the
presence of a nitrogenous reductant.
[0002] As used herein, the term "selective catalytic reduction"
(SCR) defines the catalytic process of reducing oxides of nitrogen
to dinitrogen (N.sub.2) using a nitrogenous reductant. SCR is known
from treating NO.sub.x emissions from industrial stationary source
applications, such as thermal power plants. More recently the SCR
technique has been developed for treating NO.sub.x emissions from
mobile source applications, such as passenger cars, trucks and
buses. A difficulty in treating NO.sub.x from mobile source
applications is that the quantity of NO.sub.x present in the
exhaust gas is transient, i.e. it varies with driving conditions,
such as acceleration, deceleration and cruising at various speeds.
The transient nature of the NO.sub.x component in the mobile
application exhaust gas presents a number of technical challenges,
including correct metering of nitrogenous reductant to reduce
sufficient NO.sub.x without waste or emission of nitrogenous
reductant to atmosphere.
[0003] In practice, SCR catalysts can adsorb (or store) nitrogenous
reductant, thus providing a buffer to the appropriate supply of
available reductant. Technologists use this phenomenon to calibrate
appropriate nitrogenous reductant injection to exhaust gas.
[0004] So, in summary, SCR catalysts for mobile source applications
broadly perform three functions: (i) convert NO.sub.x using e.g.
ammonia (NH.sub.3) as nitrogenous reductant; (ii) store the
NH.sub.3 when there is excess NH.sub.3 in the gas feed; and (iii)
utilise the stored NH.sub.3 under conditions where there is not
sufficient NH.sub.3 present in the gas feed to achieve the required
conversion.
[0005] For practical applications, like treating NO.sub.x emissions
from a mobile NO.sub.x source, such as a motor vehicle, where the
feed gas conditions are rapidly changing, a desirable SCR catalyst
has sufficient NH.sub.3 storage capacity at a given temperature (to
ensure any excess NH.sub.3 is not "slipped" past the catalyst and
to allow conversion to continue if NH.sub.3 is not present in the
feed) and high activity independent of the fraction of NH.sub.3
fill level (fill level is defined relative to a saturated NH.sub.3
storage capacity). The NH.sub.3 fill level can be expressed as the
amount of NH.sub.3 (for example in grams) present on the complete
catalyst (for example in litres) relative to a maximum fill level
at a given set of conditions. NH.sub.3 adsorption can be determined
according to methods known in the art, such as Langmuir absorption.
It will be understood that the fill level of all SCR catalysts is
not directly proportional to the maximal NO.sub.x conversion
activity of the SCR catalyst, i.e. it does not follow that NO.sub.x
conversion activity increases to a maximum at 100% ammonia fill
level. In fact, specific SCR catalysts can show maximal NO.sub.x
conversion rates at a fill level of <100%, such as <90%,
<80%, <50% or <30%.
[0006] The activity of a SCR catalyst can depend on the amount of
NH.sub.3 to which the entire catalyst monolith has been exposed.
Molecular sieve-based SCR catalysts can store ammonia, and the
amount of storage capacity depends, among others, on the
temperature of the gas stream and the catalyst, the feed gas
composition, the space velocity, particularly the NO:NO.sub.2 ratio
etc. The catalyst activity at the onset of exposure of the catalyst
to NH.sub.3 can be substantially lower than the activity when the
catalyst has a relatively high exposure or saturated exposure to
NH.sub.3. For practical vehicle applications, this means the
catalyst needs to be pre-loaded with an appropriate NH.sub.3
loading to ensure good activity. However, this requirement presents
some significant problems. In particular, for some operating
conditions, it is not possible to achieve the required NH.sub.3
loading; and this pre-loading method has limitations because it is
not possible to know what the engine operating conditions will be
subsequent to pre-loading. For example, if the catalyst is
pre-loaded with NH.sub.3 but the subsequent engine load is at idle,
NH.sub.3 may be slipped to atmosphere. Hence, in practical
applications the amount of NH.sub.3 pre-stored has to be lower than
is optimal to ensure that there is limited slip of NH.sub.3 if the
engine is operated in a high load condition that needs NH.sub.3
pre-loading instead of a lower load condition.
[0007] SCR catalysts for use on mobile applications such as
automotive, are required to operate at low temperature whilst also
being tolerant to hydrocarbons. Low temperature operation usually
means that there is very little NO.sub.2 in the feed gas, which
favours the use of copper-based SCR catalysts. However, iron-based
SCR catalysts are typically very good at treating approximately
50:50 NO:NO.sub.2 gas feeds and are also good under high
temperature conditions that may be experienced should an exhaust
system contain a catalysed soot filter (CSF) and the system is
arranged so that the CSF is regenerated (i.e. collected particulate
matter is combusted) periodically by engineering forced high
temperature conditions.
[0008] WO 2008/132452 discloses a method of converting nitrogen
oxides in a gas, such as an exhaust gas of a vehicular lean-burn
internal combustion engine, to nitrogen by contacting the nitrogen
oxides with a nitrogenous reducing agent in the presence of a
molecular sieve catalyst containing at least one transition metal,
wherein the molecular sieve is a small pore zeolite containing a
maximum ring size of eight tetrahedral atoms, wherein the at least
one metal is selected from the group consisting of Cr, Mn, Fe, Co,
Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Ag, In, Sn, Re, Jr and Pt.
Suitable small pore molecular sieves include (using the
three-letter code recognised by the Structure Commission of the
International Zeolite Association) CHA, including SSZ-13 and
SAPO-34; LEV, such as Nu-3; DDR e.g. Sigma-1; and ERI, including
ZSM-34. Broadly, we have found that SCR catalysts for use in the
method of WO 2008/132452 show a maximum NO.sub.x conversion at
relatively high fill level.
[0009] WO '452 explains certain drawbacks to using ZSM-5 and Beta
zeolites for converting NO.sub.x in exhaust gases emitted by mobile
sources, such as vehicles, including that they are susceptible to
dealumination during high temperature hydrothermal ageing resulting
in a loss of acidity, especially with Cu/Beta and Cu/ZSM-5
catalysts; both Beta- and ZSM-5-based catalysts are also affected
by hydrocarbons which become adsorbed on the catalysts at
relatively low temperatures (known as "coking") which hydrocarbons
can be oxidised subsequently as the temperature of the catalytic
system is raised generating a significant exotherm, which can
thermally damage the catalyst. This problem is particularly acute
in vehicular diesel applications where significant quantities of
hydrocarbon can be adsorbed on the catalyst during cold-start.
Coking can also reduce catalytic activity because active catalyst
sites can become blocked.
[0010] According to WO '452, transition metal-containing small pore
molecular sieve-based SCR catalysts demonstrate significantly
improved NO.sub.x reduction activity than the equivalent transition
metal-containing medium, large or meso-pore molecular sieve
catalysts, transition metal-containing small pore molecular sieve
catalysts, especially at low temperatures. They also exhibit high
selectivity to N.sub.2 (e.g. low N.sub.2O formation) and good
hydrothermal stability. Furthermore, small pore molecular sieves
containing at least one transition metal are more resistant to
hydrocarbon inhibition than larger pore molecular sieves.
[0011] During testing of certain SCR catalysts disclosed in WO
2008/132452 for reducing NO.sub.x with nitrogenous reductants
(urea, an NH.sub.3 precursor) it was discovered that the transient
response of the catalysts was sub-optimal for treating NO.sub.x in
transient vehicular exhaust gas. That is, the ability of the SCR
catalysts to treat NO.sub.x in the transiently changing exhaust gas
composition was less than desirable.
[0012] SAE 2008-01-1185 discloses a selective catalytic reduction
catalyst comprising separate iron zeolite and copper zeolite
catalysts arranged in zones coated one behind the other on a
flow-through substrate monolith with the iron zeolite zone disposed
upstream of the copper zeolite zone. No details are given regarding
the zeolites used. Results for transient response (shown in FIG.
17) for the combined iron zeolite/copper zeolite catalyst compared
unfavourably to the use of copper zeolite alone.
[0013] We have now discovered, very surprisingly, that combinations
of transition metal/molecular sieve, e.g. zeolite, catalysts are
more active for NO.sub.x conversion but also have relatively fast
transient response. We have also found that combinations of iron
molecular sieve catalysts can give good activity as well as being
hydrocarbon tolerant.
[0014] According to one aspect, the invention provides a
heterogeneous catalyst article comprising (a) a catalytic component
comprising a combination of a first molecular sieve having a medium
pore, large pore, or meso-pore crystal structure and optionally
containing about 0.01 to about 20 weight percent of a first metal,
and a second molecular sieve having a small pore crystal structure
and optionally containing about 0.01 to about 20 weight percent of
a second metal, wherein said first and second metals are exchanged
or free with respect to the molecular sieve's crystalline frame
work and are independently selected from the group consisting of
Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd, Pt, Ag, In, Sn,
Re, and Ir; and (b) a monolith substrate onto or within which said
catalytic component is incorporated, wherein said combination of
the first and second molecular sieves is selected from the group
consisting of a blends, a plurality of layers, and a plurality of
zones.
[0015] Types of combinations of catalysts that are useful in the
present invention include blends of two or more catalysts, a
plurality of layers wherein each layer consisting of a single
catalyst, and a plurality of zones, wherein each zone consists of a
single catalyst. The combinations are characterized by properties
that are not obtainable by any of their constituent parts acting
independently of the combination. Turning to FIGS. 4a-4d, shown are
certain embodiments of different combinations according to the
present invention. FIG. 4a shows of a blend 100 comprising a blend
of two molecular sieves 104 coated on a substrate 102. As used
herein, the term "blend", with respect to molecular sieves means a
volume of two or more molecular sieves having approximately the
same proportions relative to one another throughout the volume.
Also shown is an embodiment of a plurality of zones 110 comprising
a first zone consisting of a first molecular sieve 116 and a second
zone comprising a second molecular sieve 118, wherein the first and
second zones are coated on a substrate 102 and are adjacent to each
other and to said monolith substrate. The direction 114 of exhaust
gas flow is also shown. In FIG. 4c, shown is an embodiment of a
plurality of layers 120 comprising first layer 116 and a second
layer 118, wherein the second layer is adjacent to both said first
layer and the substrate and is between the first layer and the
substrate. FIG. 4d shows two combinations 130, wherein the first
combination is two zones (molecular sieve 116 and blend 104) and
the second combination is blend 104. For embodiments that utilize
two or more combinations, the molecular sieves for each combination
are independently selected. Although not shown in the figures,
other multiple combinations are within the scope of the invention
as well. For example, an arrangement similar to that of FIG. 4d,
but instead of a blend, the second combination could be a plurality
of layers. Other multiple combinations include the use of a blend
as one or more layers; the use of layers as one or more zones; and
the like. When multiple combinations are used, the order of
combinations with respect to exhaust gas flow through the catalyst
component is not particularly limited. However, it is highly
preferred that at least one medium, large, or meso-pore molecular
sieve always be disposed upstream of any small pore molecular
sieves.
[0016] The combinations preferably have a majority of the first
molecular sieve component relative to the second molecular sieve
component. In certain embodiments, combination comprises the first
molecular sieve and the second molecular sieve in a first molecular
sieve: second molecular sieve weight ratio of about 0.1 (i.e.,
1:10) to about 1 (i.e., (1:1). In certain embodiments, the weight
ratio of first molecular sieve to second molecular sieve is about
0.25 to about 0.50. In certain embodiments, the weight ratio of
first molecular sieve to second molecular sieve is about 0.3 to
about 0.4.
[0017] According to another aspect provided is a catalyst for
selectively catalysing the conversion of oxides of nitrogen using a
nitrogenous reductant in a feed gas whose composition, flow rate
and temperature are each changeable temporally, which catalyst
comprising a combination of a first molecular sieve component and a
second molecular sieve component, wherein in a direct comparison
tested on the Federal Test Procedure (FTP) 75 cycle the catalyst
has a higher cumulative conversion of NO.sub.x at equal or lower
NH.sub.3 slip than either molecular sieve component taken
alone.
[0018] In particular, we have observed in at least one embodiment a
synergic relationship between the first molecular sieve component
and the second molecular sieve component which can be used to
improve a transient response to NO.sub.x conversion of a SCR
catalyst comprising molecular sieve, e.g. a small pore molecular
sieve while retaining the advantages of using the small pore
molecular sieve as a component in a SCR catalyst. A "catalyst for
selectively catalysing the reduction of oxides of nitrogen in a
feed gas with a nitrogenous reductant" shall be referred to herein
as a "selective catalytic reduction" (or "SCR") catalyst. For the
avoidance of doubt, it is intended that SCR catalysts containing
combinations of three or more molecular sieves fall within the
scope of the present invention.
[0019] In a preferred embodiment, the catalyst has a higher
cumulative conversion of NO.sub.x at equal or lower NH.sub.3 slip
than either molecular sieve component taken alone where the
cumulative molar NO:NO.sub.2 ratio in feed gas entering said
catalyst is equal to or less than 1. In certain other preferred
embodiments, the NO:NO.sub.2 ratio in feed exhaust gas stream is
about 0.8 to about 1.2. In certain other preferred embodiments, the
NO:NO.sub.2 ratio in feed exhaust gas stream is less than about
0.3, while in other preferred embodiments, the ratio is greater
than about 3.
[0020] In a further preferred embodiment, the catalyst has a higher
cumulative conversion of NO.sub.x to dinitrogen at equal or lower
NH.sub.3 slip than either molecular sieve component taken
alone.
[0021] This invention significantly improves catalyst activity so
that higher activity is obtained at lower NH.sub.3 exposures (low
exposure relative to the saturated storage capacity of the
catalyst) compared to current state-of-the-art SCR catalysts. The
rate of increase of activity from zero ammonia exposure to
saturated ammonia exposure is referred to as the `transient
response`.
[0022] In one embodiment, the first molecular sieve component
achieves the maximum NO.sub.x conversion at a lower NH.sub.3 fill
level for the conditions selected than the second molecular sieve
component. For example, the ammonia fill level of the first
molecular sieve component can be in the range of 10-80%, such as
20-60% or 30-50%.
[0023] The first and second molecular sieves can be selected
independently from zeolites and non-zeolite molecular sieves.
"Zeolite" according to the International Zeolite Association, is
generally considered to be an alumino-silicate, whereas a
"non-zeolite molecular sieve" can be a molecular sieve of the same
Framework Type (or crystal structure) as the corresponding zeolite,
but having one or more non-aluminium/non-silicon cations present in
its crystal lattice, e.g. phosphorus, both cobalt and phosphorus,
copper or iron. So, for example, SSZ-13 is a zeolite of Framework
Type Code CHA, whereas SAPO-34 is a silico-aluminophosphate
non-zeolite molecular sieve sharing the same CHA Framework Type
Code. Particularly preferred are iron-containing aluminosilicate
zeolites (non-zeolite molecular sieves as defined herein) such as
Fe-containing ZSM5, Beta, CHA or FER disclosed for example in
WO2009/023202 and EP2072128A1, which are hydrothermally stable and
have relatively high SCR activity. Advantageously we have also
found that in certain embodiments catalysts comprising these
iron-containing aluminosilicate zeolites produce little or no
ammonium nitrate, and exhibit relatively high selectivity, e.g. low
N.sub.2O. Typical SiO.sub.2/Al.sub.2O.sub.3 mole ratios for such
materials are 30 to 100 and SiO.sub.2/Fe.sub.2O.sub.3 of 20 to 300
such as 20 to 100.
[0024] In preferred embodiments, the first (zeolitic or
non-zeolitic) molecular sieve component can be a small pore
molecular sieve containing a maximum ring size of eight (8)
tetrahedral atoms, optionally selected from any set out in Table 1.
Optionally, the second (zeolitic or non-zeolitic) molecular sieve
component also can be a small pore molecular sieve containing a
maximum ring size of eight (8) tetrahedral atoms and can be
selected independently of the first molecular sieve component from
any set out in Table 1.
TABLE-US-00001 TABLE 1 Details of small pore molecular sieves with
application in the present invention Framework Type material* and
Type (by illustrative isotypic Framework framework Dimension-
Additional Type Code) structures ality Pore size (.ANG.) info ACO
*ACP-1 3D 3.5 .times. 2.8, 3.5 .times. Ring sizes--8, 4 3.5 AEI
*AlPO-18 3D 3.8 .times. 3.8 Ring sizes--8, 6, 4 [Co--Al--P--O]-AEI
SAPO-18 SIZ-8 SSZ-39 AEN *AlPO-EN3 2D 4.3 .times. 3.1, 2.7 .times.
Ring sizes--8, 6, 4 5.0 AlPO-53(A) AlPO-53(B) [Ga--P--O]-AEN
CFSAPO-1A CoIST-2 IST-2 JDF-2 MCS-1 MnAPO-14 Mu-10 UiO-12-500
UiO-12-as AFN *AlPO-14 3D 1.9 .times. 4.6, 2.1 .times. Ring
sizes--8, 6, 4 4.9, 3.3 .times. 4.0
|(C.sub.3N.sub.2H.sub.12)--|[Mn--Al--P--O]- AFN GaPO-14 AFT
*AlPO-52 3D 3.8 .times. 3.2, 3.8 .times. Ring sizes--8, 6, 4 3.6
AFX *SAPO-56 3D 3.4 .times. 3.6 Ring sizes--8, 6, 4 MAPSO-56, M =
Co, Mn, Zr SSZ-16 ANA *Analcime 3D 4.2 .times. 1.6 Ring sizes--8,
6, 4 AlPO.sub.4-pollucite AlPO-24 Ammonioleucite [Al--Co--P--O]-ANA
[Al--Si--P--O]-ANA |Cs--|[Al--Ge--O]-ANA |Cs--|[Be--Si--O]-ANA
|Cs.sub.16|[Cu.sub.8Si.sub.40O.sub.96]- ANA |Cs--Fe|[Si--O]-ANA
|Cs--Na--(H.sub.2O)|[Ga--Si--O]- ANA [Ga--Ge--O]-ANA
|K--|[B--Si--O]-ANA |K--|[Be--B--P--O]-ANA
|Li--|[Li--Zn--Si--O]-ANA |Li--Na|[Al--Si--O]-ANA
|Na--|[Be--B--P--O]- ANA |(NH.sub.4)--|[Be--B--P--O]- ANA
|(NH.sub.4)--|[Zn--Ga--P--O]- ANA [Zn--As--O]-ANA Ca-D
Hsianghualite Leucite Na--B Pollucite Wairakite APC *AlPO--C 2D 3.7
.times. 3.4, 4.7 .times. Ring sizes--8, 6, 4 2.0 AlPO--H3 CoAPO-H3
APD *AlPO-D 2D 6.0 .times. 2.3, 5.8 .times. Ring sizes--8, 6, 4 1.3
APO-CJ3 ATT *AlPO-12-TAMU 2D 4.6 .times. 4.2, 3.8 .times. Ring
sizes--8, 6, 4 3.8 AlPO-33 RMA-3 CDO *CDS-1 2D 4.7 .times. 3.1, 4.2
.times. Ring sizes--8, 5 2.5 MCM-65 UZM-25 CHA *Chabazite 3D 3.8
.times. 3.8 Ring sizes--8, 6, 4 AlPO-34 [Al--As--O]-CHA
[Al--Co--P--O]-CHA |Co| [Be--P--O]-CHA |Co.sub.3
(C.sub.6N.sub.4H.sub.24).sub.3 (H.sub.2O).sub.9|
[Be.sub.18P.sub.18O.sub.72]- CHA [Co--Al--P--O]-CHA |Li--Na|
[Al--Si--O]- CHA [Mg--Al--P--O]-CHA [Si--O]-CHA [Zn--Al--P--O]-CHA
[Zn--As--O]-CHA CoAPO-44 CoAPO-47 DAF-5 GaPO-34 K-Chabazite Linde D
Linde R LZ-218 MeAPO-47 MeAPSO-47 (Ni(deta).sub.2)-UT-6 Phi SAPO-34
SAPO-47 SSZ-13 UiO-21 Willhendersonite ZK-14 ZYT-6 DDR
*Deca-dodecasil 3R 2D 4.4 .times. 3.6 Ring sizes--8, 6, 5, 4
[B--Si--O]-DDR Sigma-1 ZSM-58 DFT *DAF-2 3D 4.1 .times. 4.1, 4.7
.times. Ring sizes--8, 6, 4 1.8 ACP-3, [Co--Al--P--O]- DFT
[Fe--Zn--P--O]-DFT [Zn--Co--P--O]-DFT UCSB-3GaGe UCSB-3ZnAs UiO-20,
[Mg--P--O]- DFT EAB *TMA-E 2D 5.1 .times. 3.7 Ring sizes--8, 6, 4
Bellbergite EDI *Edingtonite 3D 2.8 .times. 3.8, 3.1 .times. Ring
sizes--8, 4 2.0 |(C.sub.3H.sub.12N.sub.2).sub.2.5|
[Zn.sub.5P.sub.5O.sub.20]-EDI [Co--Al--P--O]-EDI [Co--Ga--P--O]-EDI
|Li--|[Al--Si--O]-EDI |Rb.sub.7 Na (H.sub.2O).sub.3|
[Ga.sub.8Si.sub.12O.sub.40]-EDI [Zn--As--O]-EDI K--F Linde F
Zeolite N EPI *Epistilbite 2D 4.5 .times. 3.7, 3.6 .times. Ring
sizes--8, 4 3.6 ERI *Erionite 3D 3.6 .times. 5.1 Ring sizes--8, 6,
4 AlPO-17 Linde T LZ-220 SAPO-17 ZSM-34 GIS *Gismondine 3D 4.5
.times. 3.1, 4.8 .times. Ring sizes--8, 4 2.8 Amicite
[Al--Co--P--O]-GIS [Al--Ge--O]-GIS [Al--P--O]-GIS [Be--P--O]-GIS
|(C.sub.3H.sub.12N.sub.2).sub.4| [Be.sub.8P.sub.8O.sub.32]-GIS
|(C.sub.3H.sub.12N.sub.2).sub.4| [Zn.sub.8P.sub.8O.sub.32]-GIS
[Co--Al--P--O]-GIS [Co--Ga--P--O]-GIS [Co--P--O]-GIS
|Cs.sub.4|[Zn.sub.4B.sub.4P.sub.8O.sub.32]- GIS [Ga--Si--O]-GIS
[Mg--Al--P--O]-GIS
|(NH.sub.4).sub.4|[Zn.sub.4B.sub.4P.sub.8O.sub.32]- GIS
|Rb.sub.4|[Zn.sub.4B.sub.4P.sub.8O.sub.32]- GIS [Zn--Al--As--O]-GIS
[Zn--Co--B--P--O]-GIS [Zn--Ga--As--O]-GIS [Zn--Ga--P--O]-GIS
Garronite Gobbinsite MAPO-43 MAPSO-43 Na-P1 Na-P2 SAPO-43
TMA-gismondine GOO *Goosecreekite 3D 2.8 .times. 4.0, 2.7 .times.
Ring sizes--8, 6, 4 4.1, 4.7 .times. 2.9 IHW *ITQ-32 2D 3.5 .times.
4.3 Ring sizes--8, 6, 5, 4 ITE *ITQ-3 2D 4.3 .times. 3.8, 2.7
.times. Ring sizes--8, 6, 5, 4 5.8 Mu-14 SSZ-36 ITW *ITQ-12 2D 5.4
.times. 2.4, 3.9 .times. Ring sizes--8, 6, 5, 4 4.2 LEV *Levyne 2D
3.6 .times. 4.8 Ring sizes--8, 6, 4 AlPO-35 CoDAF-4 LZ-132 NU-3
RUB-1 [B--Si--O]-LEV SAPO-35 ZK-20 ZnAPO-35 KFI ZK-5 3D 3.9 .times.
3.9 Ring sizes--8, 6, 4 |18-crown-6|[Al--Si--O]- KFI
[Zn--Ga--As--O]-KFI (Cs,K)-ZK-5 P Q MER *Merlinoite 3D 3.5 .times.
3.1, 3.6 .times. Ring sizes--8, 4 2.7, 5.1 .times. 3.4, 3.3 .times.
3.3 [Al--Co--P--O]-MER |Ba--|[Al--Si--O]-MER |Ba--Cl--|[Al--Si--O]-
MER [Ga--Al--Si--O]-MER |K--|[Al--Si--O]-MER
|NH.sub.4--|[Be--P--O]-MER K-M Linde W Zeolite W MON *Montesommaite
2D 4.4 .times. 3.2, 3.6 .times. Ring sizes--8, 5, 4 3.6
[Al--Ge--O]-MON NSI *Nu-6(2) 2D 2.6 .times. 4.5, 2.4 .times. Ring
sizes--8, 6, 5 4.8 EU-20 OWE *UiO-28 2D 4.0 .times. 3.5, 4.8
.times. Ring sizes--8, 6, 4 3.2 ACP-2 PAU *Paulingite 3D 3.6
.times. 3.6 Ring sizes--8, 6, 4 [Ga--Si--O]-PAU ECR-18 PHI
*Phillipsite 3D 3.8 .times. 3.8, 3.0 .times. Ring sizes--8, 4 4.3,
3.3 .times. 3.2 [Al--Co--P--O]-PHI DAF-8 Harmotome Wellsite ZK-19
RHO *Rho 3D 3.6 .times. 3.6 Ring sizes--8, 6, 4
[Be--As--O]-RHO [Be--P--O]-RHO [Co--Al--P--O]-RHO
|H--|[Al--Si--O]-RHO [Mg--Al--P--O]-RHO [Mn--Al--P--O]-RHO
|Na.sub.16 Cs.sub.8| [Al.sub.24Ge.sub.24O.sub.96]-RHO
|NH.sub.4--|[Al--Si--O]-RHO |Rb--|[Be--As--O]-RHO Gallosilicate
ECR-10 LZ-214 Pahasapaite RTH *RUB-13 2D 4.1 .times. 3.8, 5.6
.times. Ring sizes--8, 6, 5, 4 2.5 SSZ-36 SSZ-50 SAT *STA-2 3D 5.5
.times. 3.0 Ring sizes--8, 6, 4 SAV *Mg-STA-7 3D 3.8 .times. 3.8,
3.9 .times. Ring sizes--8, 6, 4 3.9 Co-STA-7 Zn-STA-7 SBN *UCSB-9
3D TBC Ring sizes--8, 4, 3 SU-46 SIV *SIZ-7 3D 3.5 .times. 3.9, 3.7
.times. Ring sizes--8, 4 3.8, 3.8 .times. 3.9 THO *Thomsonite 3D
2.3 .times. 3.9, 4.0 .times. Ring sizes--8, 4 2.2, 3.0 .times. 2.2
[Al--Co--P--O]-THO [Ga--Co--P--O]-THO
|Rb.sub.20|[Ga.sub.20Ge.sub.20O.sub.80]- THO [Zn--Al--As--O]-THO
[Zn--P--O]-THO [Ga--Si--O]-THO) [Zn--Co--P--O]-THO TSC
*Tschortnerite 3D 4.2 .times. 4.2, 5.6 .times. Ring sizes-- 8, 6, 4
3.1 UEI *Mu-18 2D 3.5 .times. 4.6, 3.6 .times. Ring sizes--8, 6, 4
2.5 UFI *UZM-5 2D 3.6 .times. 4.4, 3.2 .times. Ring sizes--8, 6, 4
3.2 (cage) VNI *VPI-9 3D 3.5 .times. 3.6, 3.1 .times. Ring
sizes--8, 5, 4, 3 4.0 YUG *Yugawaralite 2D 2.8 .times. 3.6, 3.1
.times. Ring sizes--8, 5, 4 5.0 Sr-Q ZON *ZAPO-M1 2D 2.5 .times.
5.1, 3.7 .times. Ring sizes--8, 6, 4 4.4 GaPO-DAB-2 UiO-7
[0025] In one embodiment, the small pore molecular sieves can be
selected from the group of Framework Type Codes consisting of: ACO,
AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT,
EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW, LEV, KFI, MER, MON,
NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV, THO, TSC, UEI, UFI,
VNI, YUG and ZON.
[0026] Small pore molecular sieves with particular application for
treating NO.sub.x in exhaust gases of lean-burn internal combustion
engines, e.g. vehicular exhaust gases are set out in Table 2.
TABLE-US-00002 TABLE 2 Preferred small pore molecular sieves for
use in the SCR catalyst according to the invention. Structure
Molecular Sieve CHA SAPO-34 AlPO-34 SSZ-13 LEV Levynite Nu-3 LZ-132
SAPO-35 ZK-20 ERI Erionite ZSM-34 Linde type T DDR Deca-dodecasil
3R Sigma-1 KFI ZK-5 18-crown-6 [Zn--Ga--As--O]-KFI EAB TMA-E PAU
ECR-18 MER Merlinoite AEI SSZ-39 GOO Goosecreekite YUG Yugawaralite
GIS P1 VNI VPI-9
[0027] In particular embodiments, the second molecular sieve
(either a zeolite or a non-zeolite molecular sieve) component can
be a medium pore, large pore or meso-pore size molecular sieve.
[0028] In particularly preferred embodiments, the first molecular
sieve is a CuCHA material and the second molecular sieve is a
FeBEA, FeFER, FeCHA or FeMFI (e.g. ZSM-5) wherein the Fe is
impregnated, ion-exchanged and/or present within the crystal
lattice of the molecular sieve.
[0029] By "medium pore" herein we mean a molecular sieve containing
a maximum ring size of ten (10) and by "large pore" herein we mean
containing a maximum ring size of twelve (12) tetrahedral atoms.
Meso-pore molecular sieves have a maximum ring size of >12.
[0030] Suitable medium pore molecular sieves for use as second
molecular sieves in the present invention include ZSM-5 (MFI),
MCM-22 (MWW), ALPO-11 and SAPO-11 (AEL), AlPO-41 and SAPO-41 (AFO),
ferrierite (FER), Heulandite or Clinoptilolite (HEU). Large pore
molecular sieves for use in the present invention include zeolite
Y, such as ultrastable-Y (or USY), faujasite or SAPO-37 (FAU),
AlPO-5 and SAPO-5 (AFI), SAPO-40 (AFR), AlPO-31 and SAPO-31 (ATO),
Beta (BEA), Gmelinite (GME), mordenite (MOR) and Offretite
(OFF).
[0031] It will be appreciated from comments in WO 2008/132452 that
the use of certain medium and large pore molecular sieves, such as
ZSM-5 zeolite or Beta zeolite, can result in catalyst coking.
Selection of certain medium and large pore molecular sieve
components may be inappropriate for some applications: essentially
a balance is being struck between improved transient response on
the one hand and coking issues on the other. However, it may be
possible to reduce or avoid such coking problems with appropriate
exhaust system design, e.g. location of an oxidation catalyst
upstream of the SCR catalyst which can convert some hydrocarbons in
the feed gas that could otherwise have coked medium, large or
meso-pore molecular sieve components. It is also possible in
certain embodiments where a small pore molecular sieve is combined
with a medium, large or meso-pore molecular sieve that the presence
of the small pore molecular sieve reduces the coking on the medium,
large or meso-pore molecular sieve. Another benefit of this
arrangement is that a ratio of NO:NO.sub.2 in feed gas contacting
the catalyst can be adjusted to improve total NO.sub.x conversion
on the SCR catalyst.
[0032] Molecular sieves for use in the present invention can be
independently selected from one-dimensional, two-dimensional and
three-dimensional molecular sieves. Molecular sieves showing
three-dimensional dimensionality have a pore structure, which is
interconnected in all three crystallographic dimensions, whereas a
molecular sieve having two-dimensional dimensionality has pores
which are interconnected in two crystallographic dimensions only. A
molecular sieve having one-dimensional dimensionality has no pores
that are interconnected from a second crystallographic
dimension.
[0033] Small pore molecular sieves, particularly aluminosilicate
zeolites, for use in the present invention can have a
silica-to-alumina ratio (SAR) of from 2 to 300, optionally 4 to 200
such as 8 to 150 e.g. 15 to 50 or 25 to 40. It will be appreciated
that higher SARs are preferred to improve thermal stability
(especially high catalytic activity at a low temperature after
hydrothermal ageing) but this may negatively affect transition
metal exchange. Therefore, in selecting preferred materials
consideration can be given to SAR so that a balance may be struck
between these two properties. SAR for iron-in-framework molecular
sieves is discussed elsewhere in this description.
[0034] In preferred embodiments, the first molecular sieve, the
second molecular sieve or both the first and second molecular
sieves contain one or more metal selected independently from the
group consisting of Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh,
Pd, Ag, In, Sn, Re, Jr and Pt. The metal contained in the first
molecular sieve can be the same or different from that of the
second molecular sieve. So for example, the first molecular sieve
can contain copper and the second molecular sieve can contain iron.
In one embodiment, the two molecular sieves can be ion-exchanged
together.
[0035] It will be appreciated, e.g. from Table 1 hereinabove that
by "molecular sieves containing one or more transition metal"
herein we intend to cover molecular sieves wherein elements other
than aluminium and silicon are substituted into the framework of
the molecular sieve. Such molecular sieves are known as
"non-zeolitic molecular sieves" and include "SAPO", "MeAPO",
"FeAPO", "AlPO.sub.4", "TAPO", "ELAPO", "MeAPSO" and "MeAlPO" which
are substituted with one or more metals. Suitable substituent
metals include one or more of, without limitation, As, B, Be, Co,
Fe, Ga, Ge, Li, Mg, Mn, Ti, Zn and Zr. Such non-zeolitic molecular
sieves can in turn be impregnated by suitable metals listed
hereinabove, i.e. Cr, Mn, Fe, Co etc. One or both of the first and
second molecular sieves can contain substituent framework metals.
Where both the first and the second molecular sieves contain
substituent framework metals, the or each substituent metal is
selected independently from the above list.
[0036] In a particular embodiment, the small pore zeolites and
non-zeolite molecular sieves for use in the present invention can
be selected from the group consisting of aluminosilicate zeolites,
metal-substituted aluminosilicate molecular sieves, such as the
preferred iron-containing aluminosilicate zeolites and
aluminophosphate molecular sieves.
[0037] Aluminophosphate molecular sieves with application in the
present invention include aluminophosphate (AlPO.sub.4) molecular
sieves, metal substituted aluminophosphate molecular sieves
(MeAlPO), silico-aluminophosphate (SAPO) molecular sieves and metal
substituted silico-aluminophosphate (MeAPSO) molecular sieves.
[0038] A particularly interesting group of molecular sieve
components for use either as a first or a second molecular sieve
component are iron substituted aluminosilicates, i.e. where iron is
present in the framework of the molecular sieve. In the preferred
application of the SCR catalysts for use in the present invention,
i.e. for treating exhaust gas from a mobile NO.sub.x source,
iron-substituted aluminosilicates are particularly interesting
because they produce relatively low or no N.sub.2O, which is a
powerful "greenhouse" gas.
[0039] In one embodiment, the at least one transition metal is
selected from the group consisting of Cr, Ce, Mn, Fe, Co, Ni and
Cu. In a preferred embodiment, the at least one transition metal is
selected from the group consisting of Cu, Fe and Ce. In a
particular embodiment, the at least one transition metal consists
of Cu. In another particular embodiment, the at least one
transition metal consists of Fe. In a further particular
embodiment, the at least one transition metal is Cu and/or Fe.
[0040] The total of the at least one transition metal that can be
included in the at least one transition metal-containing molecular
sieve can be from 0.01 to 20.00 wt %, based on the total weight of
the molecular sieve catalyst containing at least one transition
metal. In one embodiment, the total of the at least one transition
metal that can be included can be from 0.1 to 10.0 wt %. In a
particular embodiment, the total of the at least one transition
metal that can be included is from 0.5 to 5.0 wt %. In preferred
embodiments, the transition metal loading is from 2.0 to 4.0 wt %
and the SAR is 25 to 50 or >40 or >60 e.g. 40<100,
40<70 or 60<100, provided that for iron-in-framework
molecular sieves the SiO.sub.2/Fe.sub.2O.sub.3 ratio is met (where
present the SiO.sub.2/Fe.sub.2O.sub.3 is 50 to 200, preferably 50
to 100).
[0041] Transition metals may be incorporated into the molecular
sieves for use in the present invention using techniques well known
in the art, including liquid-phase exchange or solid-ion exchange
or by an incipient wetness process. For manufacture of
iron-containing aluminosilicate zeolites see Journal of Catalysis
232(2) 318-334 (2005); EP2072128; and WO2009/023202 and references
and search citations therein.
[0042] In a particularly preferred embodiment, the catalytic
component comprises or consists of a combination of a first
molecular sieve that is a large pore molecular sieve and a second
molecular sieve that is a small pore molecular sieve. In certain
preferred embodiments, the small pore molecular sieve has a CHA
framework, more preferably a SSZ-13 framework, and contains copper.
In certain preferred embodiments, this small pore molecular sieve
is combined with a large pore molecular sieve having a BEA
framework. Preferably, the BEA framework contains either exchanged
or free iron or is an iron isomorphous BEA molecular structure
(also referred to as BEA-type ferrosilicate), with iron isomorphous
BEA molecular structure being particularly preferred.
[0043] In certain preferred embodiments, the iron isomorphous BEA
molecular structure is crystalline silicate having (1) an
iron-containing BEA-framework structure that has a
SiO.sub.2/Fe.sub.2O.sub.3 mol ratio of about 20 to about 300, and
(2) at least 80% of the contained iron is isolated iron ions
Fe.sup.3+ in a fresh state and/or log(SiO.sub.2/Al.sub.2O.sub.3) by
mol is at least about 2. Preferred BEA-type ferrosilicates useful
in the present invention have a composition represented by
following formula:
(x+y)M.sub.(2/n)O.xFe.sub.2O.sub.3.yAl.sub.2O.sub.3.zSiO.sub.2.wH.sub.2O
wherein n is an atomic value of cation M; x, y, and z represent mol
fractions of Fe.sub.2O.sub.3, Al.sub.2O.sub.3 and SiO.sub.2,
respectively; x+y+z=1; w is a number of at least 0; z/x is 20 to
300, y may be 0, and optionally z/y is at least 100.
[0044] Preferably, iron-containing BEA-framework structure that has
a SiO.sub.2/Fe.sub.2O.sub.3 mol ratio of about 25 to about 300,
about 20 to about 150, about 24 to about 150, about 25 to about
100, or about 50 to about 80. The upper limit of
log(SiO.sub.2/Al.sub.2O.sub.3) by mol is not particularly limited,
provided that the log(SiO.sub.2/Al.sub.2O.sub.3) by mol is at least
2 (i.e., the SiO.sub.2/Al.sub.2O.sub.3 ratio by mol is at least
100). The log(SiO.sub.2/Al.sub.2O.sub.3) by mol is preferably at
least 2.5 (i.e., the SiO.sub.2/Al.sub.2O.sub.3 ratio by mol is at
least 310), more preferably at least 3 (i.e., the
SiO.sub.2/Al.sub.2O.sub.3 ratio by mol is at least 1,000). When the
log(SiO.sub.2/Al.sub.2O.sub.3) by mol exceeds 4 (i.e., the
SiO.sub.2/Al.sub.2O.sub.3 ratio by mol becomes at least 10,000),
the performance for nitrogen oxide reduction is constant at the
highest level.
[0045] In certain preferred embodiments, the CHA molecular sieve is
characterized as having a mean crystal size of greater than about 1
microns, preferably about 1 to about 5 microns, with about 2 to
about 4 microns being most preferredIn the BEA-type ferrosilicate,
the iron ingredient most prominently exhibiting a catalytic
activity for the reduction of nitrogen oxides is not agglomerated
as Fe.sub.2O.sub.3 but is dispersed as isolated iron ion Fe.sup.3+
in the framework structure (i.e., isolated and dispersed in the
silicate frame structure or ion exchange sites). The isolated iron
ion Fe3+ can be detected by the electron spin resonance
measurement. The SiO.sub.2/Fe.sub.2O.sub.3 ratio by mol as used for
defining the composition of the BEA-type ferrosilicate is an
expedient expression for defining the whole iron content including
isolated iron ion Fe.sup.+3 in the BEA-type ferrosilicate.
[0046] The use of small/large pore zeolite blends, particularly
copper exchanged SSZ-13/Fe-BEA combinations, can increase the
formation of N.sub.2O as compared to the constituent components.
Accordingly, the use of this combination can be detrimental in
certain applications. However, the use of copper exchanged
SSZ-13/BEA-type ferrosilicate combinations surprisingly overcame
this problem and so offered improved selectivity to nitrogen. Zoned
and layered SCR catalysts offer further improvements, particularly
when the exhaust gas has about a 50/50 ratio of NO to NO.sub.2.
This catalyst reduces the N.sub.2O emissions by approximately 75%
over the blended equivalent whilst retaining excellent transient
response and good conversion in low NO/NO.sub.2 gas mixes.
[0047] Surprisingly, combinations of copper exchanged
SSZ-13/pre-aging Fe-BEA can produce results substantially better
than combinations have conventional Fe-BEA. Accordingly, instead of
the more conventional processing of aging at 500.degree. C. for 1
hour, the Fe-BEA material is preferably aged at 600-900.degree. C.,
preferably 650-850.degree. C., more preferably 700-800.degree. C.,
and even more preferably 725-775.degree. C., for 3-8 hours,
preferably 4-6 hours, more preferably from 4.5-5.5 hours, and even
more preferably from 4.75-5.25 hours. Embodiments using copper
exchanged SSZ-13/pre-aged Fe-BEA combinations are advantageous in
applications in which the formation of N.sub.2O is undesirable.
Included within the scope of this invention are ratios of Cu:SSZ-13
to pre-aged Fe-BEA similar to those of Cu:SSZ-13 to Fe-BEA and
Cu:SSZ-13 to BEA-Ferrosilicate. Also included within the scope of
the invention are combinations of Cu:SSZ-13 to pre-aged Fe-BEA
similar to those of Cu:SSZ-13 to Fe-BEA and Cu:SSZ-13 to
BEA-Ferrosilicate.
[0048] Both the first and second molecular sieves can be present in
the same catalyst coating, i.e. coated in a washcoat onto a
suitable substrate monolith or each of the first and second
molecular sieve components may be separated in washcoat layers one
above the other, with either the first molecular sieve component in
a layer above the second molecular sieve component or vice versa.
Alternatively, both the first and second molecular sieves can be
combined in a composition for forming substrate monoliths of the
extruded-type. Optionally, the extruded monolith can be further
coated with a washcoat containing one or both of the first and
second molecular sieve component(s). Further alternatives include
forming an extruded substrate monolith comprising one, but not
both, of the first and second molecular sieve components and
coating the extruded substrate monolith with a washcoat containing
the other molecular sieve component not present in the extrudate,
or the washcoat can contain both the first and second molecular
sieves. In all of the arrangements combining extruded and coated
substrate monoliths, it will be understood that where a first
and/or a second molecular sieve component is present in both the
extrudate and the catalyst coating, the first molecular sieve
component can be the same in both the extrudate and the coating, or
different. So for example, the washcoat can contain SSZ-13 zeolite
and the extrudate can contain SAPO-34. The same applies for the
second molecular sieve component, e.g. the washcoat can contain
ZSM-5 zeolite and the extrudate can contain Beta zeolite.
[0049] Washcoat compositions containing the molecular sieves for
use in the present invention for coating onto the monolith
substrate or for manufacturing extruded type substrate monoliths
can comprise a binder selected from the group consisting of
alumina, silica, (non zeolite) silica-alumina, naturally occurring
clays, TiO.sub.2, ZrO.sub.2, and SnO.sub.2.
[0050] Suitable substrate monoliths include so-called flow-through
substrate monoliths (i.e. a honeycomb monolithic catalyst support
structure with many small, parallel channels running axially
through the entire part) made of ceramic materials such as
cordierite; or metal substrates made e.g. of fecralloy. Substrate
monoliths can be filters including wall-flow filters made from
cordierite, aluminium titanate, silicon carbide or mullite; ceramic
foams; sintered metal filters or so-called partial filters such as
those disclosed in EP 1057519 or WO 01/080978.
[0051] According to another aspect, the invention provides an
exhaust system for treating a flowing exhaust gas containing oxides
of nitrogen from a mobile source of such exhaust gas, which system
comprising a source of nitrogenous reducing agent arranged upstream
in a flow direction from a SCR catalyst according to the
invention.
[0052] The source of nitrogenous reducing agent can comprise a
suitable injector means operated under control of e.g. a suitably
programmed electronic control unit to deliver an appropriate
quantity of reducing agent or a precursor thereof (held in a
suitable vessel or tank) for converting NO.sub.x to a desired
degree. Liquid or solid ammonia precursor can be urea
((NH.sub.2).sub.2CO), ammonium carbonate, ammonium carbamate,
ammonium hydrogen carbonate or ammonium formate, for example.
Alternatively, ammonia per se or hydrazine can be used.
[0053] In an alternative embodiment, the source of nitrogenous
reducing agent is a NO.sub.x absorber (also known as a NO.sub.x
trap, lean NO.sub.x trap or NO.sub.x absorber catalyst (NAC)) in
combination with an engine that is configured so that at least one
engine cylinder can operate richer than normal operating
conditions, e.g. in the remaining engine cylinders, e.g. to produce
exhaust gas having a stoichiometrically balanced redox composition,
or a rich redox composition and/or a separate hydrocarbon injector
means arranged upstream of the NO.sub.x absorber for injecting
hydrocarbons into a flowing exhaust gas. NO.sub.x absorbed on the
NO.sub.x absorber is reduced to ammonia through contacting adsorbed
NO.sub.x with the reducing environment. By locating the SCR
catalyst according to the invention downstream of the NO.sub.x
absorber, ammonia produced in situ can be utilised for NO.sub.x
reduction on the SCR catalyst when the NO.sub.x absorber is being
regenerated by contacting the NO.sub.x absorber with e.g. richer
exhaust gas generated by the engine.
[0054] Alternatively, the source of nitrogenous reducing agent can
be a separate catalyst e.g. a NO.sub.x trap or a reforming catalyst
located in an exhaust manifold of each of at least one engine
cylinder which is configured to operate, either intermittently or
continuously, richer than normal.
[0055] In a further embodiment, an oxidation catalyst for oxidising
nitrogen monoxide in the exhaust gas to nitrogen dioxide can be
located upstream of a point of metering the nitrogenous reductant
into the exhaust gas and the SCR catalyst.
[0056] The oxidation catalyst can include at least one precious
metal, preferably a platinum group metal (or some combination of
these), such as platinum, palladium or rhodium, coated on a
flow-through monolith substrate. In one embodiment, the at least
one precious metal is platinum, palladium or a combination of both
platinum and palladium or an alloy of Pd--Au, optionally in
combination with Pt--Pd. The precious metal can be supported on a
high surface area washcoat component such as alumina, an
aluminosilicate zeolite, silica, non-zeolite silica alumina, ceria,
zirconia, titania or a mixed or composite oxide containing both
ceria and zirconia.
[0057] In a further embodiment, a suitable filter substrate is
located between the oxidation catalyst and the catalyst according
to the invention. Filter substrates can be selected from any of
those mentioned above, e.g. wall flow filters. Where the filter is
catalysed, e.g. with an oxidation catalyst of the kind discussed
above, preferably the point of metering nitrogenous reductant is
located between the filter and the catalyst according to the
invention. It will be appreciated that this arrangement is
disclosed in WO 99/39809. Alternatively, if the filter is
uncatalysed, the means for metering nitrogenous reductant can be
located between the oxidation catalyst and the filter.
[0058] In a further embodiment, the SCR catalyst for use in the
present invention is coated on a filter or is in the form of an
extruded-type catalyst located downstream of the oxidation
catalyst. Where the filter includes the SCR catalyst for use in the
present invention, the point of metering the nitrogenous reductant
is preferably located between the oxidation catalyst and the
filter.
[0059] According to a further aspect, the invention provides a
lean-burn internal combustion engine comprising an exhaust system
according to the invention. In certain embodiments, the engine can
be a compression ignition engine or a positive ignition engine.
Positive ignition engines can be fuelled using a variety of fuels
including gasoline fuel, gasoline fuel blended with oxygenates
including methanol and/or ethanol, liquid petroleum gas or
compressed natural gas. Compression ignition engines can be fuelled
using diesel fuel, diesel fuel blended with non-diesel hydrocarbons
including synthetic hydrocarbons produced by gas-to-liquid methods
or bio-derived components.
[0060] In yet another aspect, the invention provides a vehicle
comprising a lean-burn internal combustion engine according to the
invention.
[0061] In a further aspect, the invention provides a method of
converting oxides of nitrogen (NO.sub.x) in an exhaust gas of a
mobile source whose composition, flow rate and temperature of which
exhaust gas are each changeable temporally, which method comprising
the step of by contacting the NO.sub.x with a nitrogenous reducing
agent in the presence of a selective catalytic reduction catalyst
comprising a combination of a first molecular sieve component and a
second molecular sieve component, wherein in a direct comparison
tested on the Federal Test Procedure (FTP) 75 cycle the catalyst
has a higher cumulative conversion of NO.sub.x at equal or lower
NH.sub.3 slip than either molecular sieve component taken
alone.
[0062] In one embodiment, the catalyst has a higher cumulative
conversion of NO.sub.x at equal or lower NH.sub.3 slip than either
molecular sieve component taken alone where the cumulative molar
NO:NO.sub.2 ratio in feed gas entering said catalyst is equal to,
or less than 1.
[0063] In another embodiment, the catalyst has a higher cumulative
conversion of NO.sub.x to dinitrogen at equal or lower NH.sub.3
slip than either molecular sieve component taken alone.
[0064] In a further embodiment, the nitrogen oxides are reduced
with the nitrogenous reducing agent at a temperature of at least
100.degree. C. In another embodiment, the nitrogen oxides are
reduced with the reducing agent at a temperature from about
150.degree. C. to 750.degree. C. The latter embodiment is
particularly useful for treating exhaust gases from heavy and light
duty diesel engines, particularly engines comprising exhaust
systems comprising (optionally catalysed) diesel particulate
filters which are regenerated actively, e.g. by injecting
hydrocarbon into the exhaust system upstream of the filter, wherein
the catalyst according to the present invention is located
downstream of the filter.
[0065] In a particular embodiment, the temperature range is from
175 to 550.degree. C. In another embodiment, the temperature range
is from 175 to 400.degree. C.
[0066] In another embodiment, the nitrogen oxides reduction is
carried out in the presence of oxygen. In an alternative
embodiment, the nitrogen oxides reduction is carried out in the
absence of oxygen.
[0067] The gas containing the nitrogen oxides can contact the
catalyst according to the invention at a gas hourly space velocity
of from 5,000 hr.sup.-1 to 500,000 hr.sup.-1, optionally from
10,000 hr.sup.-1 to 200,000 hr.sup.-1.
[0068] The metering of the nitrogenous reducing agent contacting
the SCR catalyst can be arranged such that 60% to 200% of
theoretical ammonia is present in exhaust gas entering the SCR
catalyst calculated at 1:1 NH.sub.3/NO and 4:3
NH.sub.3/NO.sub.2.
[0069] In a further embodiment, a step of oxidising nitrogen
monoxide in the exhaust gas to nitrogen dioxide can be performed
prior to introduction of any nitrogenous reducing agent. Suitable,
such NO oxidation step can be done using a suitable oxidation
catalyst. In one embodiment, the oxidation catalyst is adapted to
yield a gas stream entering the SCR catalyst having a ratio of NO
to NO.sub.2 of from about 4:1 to about 1:3 by volume, e.g. at an
exhaust gas temperature at oxidation catalyst inlet of 250.degree.
C. to 450.degree. C.
[0070] In order that the invention may be more fully understood,
reference is made to the accompanying drawings, in which:
[0071] FIG. 1 is a schematic drawing of an exhaust system
embodiment according to the invention;
[0072] FIG. 2 is a schematic drawing of a further exhaust system
embodiment according to the invention; and
[0073] FIG. 3 is a graph showing the results of NO.sub.x conversion
activity tests described in Example 3 on fresh catalysts prepared
according to Examples 1, 2 and 3.
[0074] FIGS. 4a-4d shows different types of combinations of a first
molecular sieve and a second molecular sieve on a substrate.
[0075] FIGS. 5, 6a, 6b, 7, and 8a-8c are graphs showing data
associated with certain embodiments of the invention.
[0076] In FIG. 1 is shown an apparatus 10 comprising a light-duty
diesel engine 12 and an exhaust system 14 comprising a conduit for
conveying exhaust gas emitted from the engine to atmosphere 15
disposed in which conduit is a metal substrate monolith coated with
a NO.sub.x Absorber Catalyst ((NAC)) also known as a NO.sub.x trap
or lean NO.sub.x trap) 16 followed in the flow direction by a
wall-flow filter 18 coated with a SCR catalyst according to the
invention (Cu/SSZ-13 blended with an iron-in-zeolite framework BEA
also ion-exchanged with additional ion-exchanged iron). A clean-up
catalyst 24 comprising a relatively low loading of Pt on alumina is
disposed downstream of wall-flow filter 18.
[0077] In use, the engine runs lean of stoichiometric, wherein
NO.sub.x is absorbed in the NAC. Intermittently, the engine is run
rich to desorb and reduce NO.sub.x. During rich running operation,
some NO.sub.x is reduced to NH.sub.3 and is stored on the
downstream SCR catalyst for further NO.sub.x reduction. The SCR
catalyst also treats NO.sub.x during intermittent rich events. NO
oxidised to NO.sub.2 on the NAC is used to combust soot trapped on
the filter 18 passively. The NAC is also used to combust additional
hydrocarbon during occasional forced (active) regenerations of the
filter.
[0078] FIG. 2 shows an alternative apparatus 11 according to the
invention comprising a diesel engine 12 and an exhaust system 13
therefor. Exhaust system 13 comprises a conduit 17 linking
catalytic aftertreatment components, namely a
2Au-0.5Pd/Al.sub.2O.sub.3 catalyst coated onto an inert ceramic
flow-through substrate 19 disposed close to the exhaust manifold of
the engine (the so-called close coupled position). Downstream of
the close-coupled catalyst 19, in the so-called underfloor
position, is an flow-through catalyst 22 of the extruded type
comprising a mixture of an aluminosilicate CHA ion-exchanged with
Cu and FeCHA, having Fe present in the molecular sieve framework
structure. A source of nitrogenous reductant (urea) is provided in
tank 28, which is injected into the exhaust gas conduit 17 between
catalysts 19 and 22.
[0079] In certain embodiments, provided is a catalyst for
selectively catalysing the conversion of oxides of nitrogen using a
nitrogenous reductant in a feed gas whose composition, flow rate
and temperature are each changeable temporally, which catalyst
comprising a combination of a first molecular sieve component and a
second molecular sieve component, wherein in a direct comparison
tested on the Federal Test Procedure (FTP) 75 cycle the catalyst
has a higher cumulative conversion of NO.sub.x at equal or lower
NH.sub.3 slip than either molecular sieve component taken
alone.
[0080] Preferably, the catalyst has a higher cumulative conversion
of NO.sub.x, preferably to elemental nitrogen, at equal or lower
NH.sub.3 slip than either molecular sieve component taken alone
where the cumulative molar NO:NO.sub.2 ratio in feed gas entering
said catalyst is equal to, or less than 1.
[0081] In certain embodiments, the SCR catalyst has the first
molecular sieve component achieves the maximum NO.sub.x conversion
at a lower NH.sub.3 fill level for the conditions selected than the
second molecular sieve component. Preferably, the lower NH.sub.3
fill level of the first molecular sieve component is in the range
of 10-80%.
[0082] In certain embodiments in the SCR catalyst, the first and
second molecular sieves can be selected independently from zeolites
and non-zeolite molecular sieves. Preferably, one of the molecular
sieve components is a small pore molecular sieve containing a
maximum ring size of eight (8) tetrahedral atoms, preferably
selected from the group of Framework Type Codes consisting of: ACO,
AEI, AEN, AFN, AFT, AFX, ANA, APC, APD, ATT, CDO, CHA, DDR, DFT,
EAB, EDI, EPI, ERI, GIS, GOO, IHW, ITE, ITW, LEV, KFI, MER, MON,
NSI, OWE, PAU, PHI, RHO, RTH, SAT, SAV, SIV, THO, TSC, UEI, UFI,
VNI, YUG and ZON, with CHA, LEV, ERI, DDR, KFI, EAB, PAU, MER, AEI,
GOO, YUG, GIS and VNI being particularly preferred. Preferably, the
other molecular sieve component is selected from a small pore,
medium pore, large pore or meso-pore size molecular sieve.
Preferred medium pore molecular sieve include MFI, MWW, AEL, AFO,
FER and HEU. Preferred large pore molecular sieve include FAU, AFI,
AFR, ATO, BEA, GME, MOR and OFF.
[0083] In certain embodiments, one or both of the first and second
molecular sieves contains a substituent framework metal selected
from the group consisting of As, B, Be, Co, Fe, Ga, Ge, Li, Mg, Mn,
Ti, Zn and Zr. In certain embodiments, one or both of the first
molecular sieve component and the second molecular sieve component
contain one or more metal selected independently from the group
consisting of Cr, Mn, Fe, Co, Ce, Ni, Cu, Zn, Ga, Mo, Ru, Rh, Pd,
Ag, In, Sn, Re, Ir and Pt, preferably Cr, Ce, Mn, Fe, Co, Ni and
Cu.
[0084] In one aspect of the invention, provided is an exhaust
system for treating a flowing exhaust gas containing oxides of
nitrogen from a mobile source of such exhaust gas, which system
comprising a source of nitrogenous reducing agent arranged upstream
in a flow direction from a selective catalytic reduction catalyst
described herein. In certain embodiments, the system further
comprises an oxidation catalyst disposed upstream of the source of
nitrogenous reducing agent and the SCR catalyst. In certain
embodiments, the system further comprises a filter disposed between
the oxidation catalyst and the source of nitrogenous reducing
agent.
[0085] In one aspect of the invention, provided is a lean-burn
internal combustion engine, such as a compression ignition engine
or a positive ignition engine, comprising an exhaust system
described herein. In certain embodiments, the engine comprises a
NO.sub.x absorber which functions, at least in part, as the source
of a nitrogenous reducing agent.
[0086] In one aspect of the invention, provided is a vehicle
comprising a lean-burn internal combustion engine described
herein.
[0087] In one aspect of the invention, provided is a method for
converting oxides of nitrogen (NO.sub.x) in an exhaust gas of a
mobile source the composition, flow rate and temperature of which
exhaust gas are each changeable temporally, which method comprising
the step of contacting the NO.sub.x with a nitrogenous reducing
agent in the presence of a selective catalytic reduction catalyst
comprising a combination of a first molecular sieve component and a
second molecular sieve component, wherein in a direct comparison
tested on the Federal Test Procedure (FTP) 75 cycle the catalyst
has a higher cumulative conversion of NO.sub.x at equal or lower
NH.sub.3 slip than either molecular sieve component taken alone. In
certain embodiments of the method, the catalyst has a higher
cumulative conversion of NO.sub.x, preferably to dinitrogen, at
equal or lower NH.sub.3 slip than either molecular sieve component
taken alone where the cumulative molar NO:NO.sub.2 ratio in feed
gas entering said catalyst is equal to, or less than 1. In certain
embodiments of the method, the NO.sub.x is converted at a
temperature of at least 100.degree. C., preferably from about
150.degree. C. to 750.degree. C. In certain embodiments of the
method, the gas containing the NO.sub.x contacts the SCR catalyst
at a gas hourly space velocity of from 5,000 hr.sup.-1 to 500,000
hr.sup.-1. In certain embodiments, about 60% to about 200% of
theoretical ammonia contacts the SCR catalyst calculated at 1:1
NH.sub.3/NO and 4:3 NH.sub.3/NO.sub.2. In certain embodiments of
the method, the NO:NO.sub.2 ratio in gas contacting the SCR
catalyst is from about 4:1 to about 1:3 by volume.
[0088] The following Examples are provided by way of illustration
only.
EXAMPLES
Example 1
Method of Making Fresh 3 wt % Cu/SSZ-13 (aluminosilicate CHA)
Catalysts
[0089] Commercially available SSZ-13 zeolite (CHA) was
NH.sub.4.sup.+ ion exchanged in a solution of NH.sub.4NO.sub.3,
then filtered. The resulting materials were added to an aqueous
solution of Cu(NO.sub.3).sub.2 with stirring. The slurry was
filtered, then washed and dried. The procedure can be repeated to
achieve a desired metal loading. The final product was calcined.
The materials prepared according to this Example are referred to
herein as "fresh".
Example 2
Method of Making Fresh 5 Wt % Fe/Beta Catalyst
[0090] Commercially available Beta zeolite was NH.sub.4.sup.+ ion
exchanged in a solution of NH.sub.4NO.sub.3, then filtered. The
resulting material was added to an aqueous solution of
Fe(NO.sub.3).sub.3 with stirring. The slurry was filtered, then
washed and dried. The procedure can be repeated to achieve a
desired metal loading. The final product was calcined. The
materials prepared according to this Example are referred to herein
as "fresh".
Example 3
Catalyst Mixtures
[0091] Separate physical blends of fresh and aged 1:3
Fe/Beta:Cu/SSZ-13 by weight were prepared by physical mixture of
samples made according to Examples 1 and 2 Likewise physical blends
of 1:3 BEA-Ferrosilicate:CuSSZ-13 by weight were prepared.
Example 4
NO.sub.x Conversion Activity Tests
[0092] The activity of the fresh powder samples prepared according
to Examples 1, 2 and 3 were tested at 250.degree. C. in a
laboratory apparatus using the following gas mixture: 125 ppm NO,
375 ppm NO.sub.2 750 ppm NH.sub.3, 14% O.sub.2, 4.5% H.sub.2O, 4.5%
CO.sub.2, N.sub.2 balance at a space velocity of 60,000 hr.sup.-1.
The test is stopped when 20 ppm NH.sub.3 is detected downstream of
the sample. The results are shown in FIG. 3.
[0093] From the results it can be seen that the Fe/Beta sample has
a fast transient response, but limited maximum conversion. It also
slips NH.sub.3 early on in the test compared with the Cu/SSZ-13 and
Fe/Beta+Cu/SSZ-13 blend. Transient response is defined as the rate
at which NOx conversion increases as the level of NH.sub.3 fill on
the catalyst increases. The Cu/SSZ-13 has better, higher maximum
conversion but a slower transient response. The combination of
Fe/Beta and Cu/SSZ-13 gives fast transient response, higher maximum
conversion, but also has higher conversion than the individual
components at intermediate NH.sub.3 fill levels, which is evidence
of synergy. Pre-aged 1:3 Fe/Beta:Cu/SSZ-13 will provide improved
results as well.
Example 5
Comparison of Blends, Layers, and Zones Combinations
[0094] Three samples of a 1:3 (by weight)
BEA-Ferrosilicate:CuSSZ-13 combination were prepared and separately
coated on substrates as a blend, zones, and layers. The three
coated substrates were exposed to a test environment similar to
that described in Example 4, except that the NO:NO.sub.2 ration was
about 50:50. The results are shown in FIG. 5.
[0095] From the results it can be seen that zones and blends
achieve higher NOx conversion compared to blends.
Example 6
N.sub.2O Formation
[0096] A samples of a 1:3 (by weight) FeBEA:CuSSZ-13 combination
and three samples of a 1:3 (by weight) BEA-Ferrosilicate:CuSSZ-13
combination were prepared and separately coated on substrates. The
FeBEA:CuSSZ-13 combination was coated as a blend, whereas the
samples of BEA-Ferrosilicate:CuSSZ-13 combination were separately
coated as a blend, zones, and layers. Each of the samples were
exposed to a simulated diesel gas exhaust combined with NH.sub.3
dosing (20 ppm slip). The average N.sub.2O formation during
exposure was recorded and is shown in FIGS. 6a and 6b.
[0097] It is clear that the FeBEA:CuSSZ-13 blend produces
significant N.sub.2O resulting in an apparent reduction in maximum
conversion and `N.sub.2 selective` transient response. This
reduction in conversion also outweighs that observed for the two
components evaluated independently. Surprisingly, the
BEA-Ferrosilicate:CuSSZ-13 blend produces substantially less
N.sub.2O than that observed for any other CuSSZ-13/zeolite blend.
However, layers and zones of BEA-Ferrosilicate:CuSSZ-13 maintain
the low N.sub.2O make observed for the blend, but also show
improved transient response under different NO.sub.2 levels (see
FIG. 7).
Example 7
Effect of NO:NO.sub.2 Ratios
[0098] Four samples of BEA-Ferrosilicate:CuSSZ-13 were prepared and
tested for NOx conversion capacity during exposure to simulated
diesel exhaust gas combined with a NH.sub.3 reductant. Testing was
performed at 250.degree. C. and gas hourly space velocity of about
60,000/hour. The results are provided in the table below. Here, the
reference ("ref.") catalyst is CuSSZ-13, "low fill" refers to an
NH.sub.3 level at less than about 0.5 g/L of exhaust gas, and "high
fill" refers to an NH.sub.3 at greater about 1 g/L of exhaust
gas.
TABLE-US-00003 0% NO.sub.2 50% NO.sub.2 75% NO.sub.2 3:1 BEA-
Better than ref Better than ref Similar at low Ferrosilicate:
fills, better at CuSSZ-13 high fills (BLEND) 1:1 BEA- Poor at low
fills, Better than ref Similar at low Ferrosilicate: better at high
fills, better at CuSSZ-13 fills than ref high fills (BLEND) 1:3
BEA- Similar to ref Much better Better than ref Ferrosilicate: than
ref. Much at low fills, CuSSZ-13 better selectivity similar at high
(ZONE) fill 1:3 BEA- Similar to ref Much better Better than ref
Ferrosilicate than ref. Much at low fills, layered over better
selectivity similar at high CuSSZ-13 fill
Example 8
Multiple Combinations
[0099] Samples of FeBEA, CuSSZ-13, and BEA-Ferrosilicate were
prepared can combined in the indicated combinations and multiple
combinations shown in FIGS. 8a-8c. In the legends, the ratios are
give by weight, blends are shown in parenthesises, and zones are
indicated by "//", with the first named component disposed upstream
with respect to gas flow past the catalyst. Each of the
combinations and multiple combinations were exposed to a simulated
diesel gas exhaust gas stream containing an NH.sub.3 reductant. The
NO:NO.sub.2 ratio in the exhaust gas was varied from only NO, 50:50
NO:NO.sub.2 (by weight), and 75% NO.sub.2 (by weight), to test the
catalyst at different conditions. Each combination or multiple
combination was evaluated for NO.sub.x conversion (corrected for
N.sub.2O formation) as a function of NH.sub.3 fill level.
* * * * *