U.S. patent application number 10/547216 was filed with the patent office on 2007-04-26 for exhaust-gas purification system for the selective catalytic reduction of nitrogen oxides in the lean exhaust gas of internal combustion engines and method of exhaust-gas purification.
This patent application is currently assigned to UMICORE AG & CO. KG. Invention is credited to Jurgen Gieshoff, Thomas Kreuzer, Egbert Lox, Markus Pfeifer, Paul Spurk, Barry Van Setten.
Application Number | 20070089403 10/547216 |
Document ID | / |
Family ID | 32863902 |
Filed Date | 2007-04-26 |
United States Patent
Application |
20070089403 |
Kind Code |
A1 |
Pfeifer; Markus ; et
al. |
April 26, 2007 |
Exhaust-gas purification system for the selective catalytic
reduction of nitrogen oxides in the lean exhaust gas of internal
combustion engines and method of exhaust-gas purification
Abstract
The present invention relates to an exhaust-gas purification
system for the selective catalytic reduction of nitrogen oxides.
The system includes at least one catalyst having catalytically
active components for the selective catalytic reduction (SCR
components). An NOx storage catalyst (5) is arranged upstream of
the SCR catalyst (3) in the exhaust-gas purification system. For
performing the selective catalytic reduction, metering means (8)
for supplying a compound decomposable into ammonia is provided
between the NOx storage catalyst and the SCR catalyst (3). At low
exhaust-gas temperatures, the NOx storage catalyst (5) adsorbs the
nitrogen oxides contained in the exhaust gas and desorbs them only
at rising exhaust-gas temperatures, so that they can afterwards be
converted by the SCR catalyst (3) which is active then. This
results in an altogether improved conversion rate for the nitrogen
oxides.
Inventors: |
Pfeifer; Markus; (Solingen,
DE) ; Van Setten; Barry; (Rodenbach, DE) ;
Spurk; Paul; (Weiterstadt, DE) ; Gieshoff;
Jurgen; (Biebergemund, DE) ; Lox; Egbert;
(Hochwaldhausen, DE) ; Kreuzer; Thomas; (Karben,
DE) |
Correspondence
Address: |
KALOW & SPRINGUT LLP
488 MADISON AVENUE
19TH FLOOR
NEW YORK
NY
10022
US
|
Assignee: |
UMICORE AG & CO. KG
Hanau
DE
|
Family ID: |
32863902 |
Appl. No.: |
10/547216 |
Filed: |
February 26, 2004 |
PCT Filed: |
February 26, 2004 |
PCT NO: |
PCT/EP04/01945 |
371 Date: |
May 18, 2006 |
Current U.S.
Class: |
60/286 ; 60/295;
60/297 |
Current CPC
Class: |
Y02T 10/12 20130101;
F01N 2570/14 20130101; Y02A 50/20 20180101; B01D 2255/20723
20130101; F01N 3/0842 20130101; F01N 13/0093 20140601; B01D
2255/20776 20130101; F01N 3/0814 20130101; B01D 2255/20761
20130101; B01D 2255/20738 20130101; F01N 2610/02 20130101; B01D
53/9418 20130101; B01D 2251/2062 20130101; B01D 2255/20707
20130101; F01N 3/2066 20130101; F01N 13/009 20140601; B01D
2255/20769 20130101; B01D 2255/50 20130101; F01N 3/0821 20130101;
F01N 2240/40 20130101; B01D 53/9481 20130101 |
Class at
Publication: |
060/286 ;
060/295; 060/297 |
International
Class: |
F01N 3/00 20060101
F01N003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 26, 2003 |
DE |
103 08 287.5 |
Claims
1. An exhaust-gas purification system for the selective catalytic
reduction of nitrogen oxides, which includes at least one catalyst
having catalytically active components for the selective catalytic
reduction (SCR components) and through which the lean exhaust gas
of an internal combustion engine flows, wherein an NOx storage
catalyst is arranged upstream of the SCR catalyst and metering
means for supplying a precursor compound of ammonia to the exhaust
gas is located between the NOx storage catalyst and the SCR
catalyst.
2. The exhaust-gas purification system according to claim 1,
wherein the NOx storage catalyst is applied onto a diesel
particulate filter.
3. The exhaust-gas purification system according to claim 2,
wherein the diesel particulate filter is a wall flow filter, a
foamed ceramic filter, a ceramic fiber filter or a wire-mesh
filter.
4. The exhaust-gas purification system according to claim 1,
wherein the NOx storage catalyst includes at least one compound of
elements from the group consisting of alkali metals, alkaline-earth
metals or rare earths and is activated with at least one of the
platinum group metals platinum, palladium, rhodium or iridium.
5. The exhaust-gas purification system according to claim 4,
wherein the NOx storage catalyst additionally includes
catalytically active components on the basis of support oxides from
the group consisting of aluminum oxide, silicon dioxide, cerium
oxide, zirconium oxide, titanium oxide or mixed oxides thereof
which are coated with at least one of the platinum group metals
platinum, palladium, rhodium and iridium.
6. The exhaust-gas purification system according to claim 5,
wherein for storing nitrogen oxides, the NOx storage catalyst
includes storage components on the basis of cerium oxide, which are
coated with platinum, and additionally an oxidizing catalyst on the
basis of aluminum oxide, which is also coated with platinum.
7. The exhaust-gas purification system according to claim 1,
wherein the SCR components include a solid acid system of titanium
dioxide and vanadium oxide.
8. The exhaust-gas purification system according to claim 7,
wherein the solid acid system includes at least one of the
components from the group consisting of tungsten oxide, molybdenum
oxide, silicon dioxide, sulfate and zeolites, wherein the zeolites
may be present in the acid H-form or be exchanged with metal ions
within their exchange capacity.
9. The exhaust-gas purification system according to claim 8,
wherein the SCR components include at least one zeolite, wherein
the zeolites are present in the acid H-form or are exchanged with
metal ions within their exchange capacity.
10. The exhaust-gas purification system according to claim 3,
wherein the diesel particulate filter, in addition to the NOx
storage catalyst, also includes catalytically active components for
lowering the ignition temperature of diesel soot.
11. The exhaust-gas purification system according to claim 1,
wherein an oxidizing catalyst for the oxidation of surplus ammonia
is arranged downstream of the SCR catalyst.
12. The exhaust-gas purification system according to claim 11,
wherein a hydrolysis catalyst for the hydrolysis of the precursor
compound decomposable into ammonia is inserted between the metering
means for ammonia and the SCR catalyst.
13. A method of removing nitrogen oxides from the lean exhaust gas
of an internal combustion engine by selective catalytic reduction
using ammonia, wherein the internal combustion engine is operated
continuously with a lean air/fuel mixture and the resulting lean
exhaust gas is routed first over a NOx storage catalyst and
subsequently over an SCR catalyst for the selective catalytic
reduction, wherein a compound decomposable into ammonia is supplied
to the exhaust gas between the NOx storage catalyst and the SCR
catalyst.
14. The method according to claim 13, wherein the NOx storage
catalyst is based on cerium oxide or a mixed oxide of cerium oxide
and zirconium oxide.
15. The method according to claim 14, wherein the exhaust system
contains in addition a diesel particulate filter which is
regenerated from time to time by increasing the exhaust gas
temperature to the ignition temperature of the diesel soot
collected on the filter and at the same time the NOx storage
catalyst is automatically desulfated.
16. The method according to claim 15, wherein the NOx storage
catalyst is coated on a diesel particulate filter which is
regenerated from time to time by increasing the exhaust gas
temperature to the ignition temperature of the diesel soot
collected on the filter and at the same time the NOx storage
catalyst is automatically desulfated.
Description
[0001] The present invention relates to an exhaust-gas purification
system for the selective catalytic reduction (SCR) of nitrogen
oxides in the lean exhaust gas of internal combustion engines.
[0002] During the combustion of fuels in internal combustion
engines, an exhaust gas is formed which contains unburned
hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and
particles (soot) as pollutants. In diesel engines and lean-burn
gasoline engines, removal of the nitrogen oxides and particles
causes special difficulties.
[0003] Reduction of particles and nitrogen oxide emissions from
motor vehicles envisaged by law (EURO V in Europe and LEV II in the
United States, scheduled for 2007) therefore requires novel
exhaust-gas purification systems be provided allowing to meet the
future limits for maximum pollutant emission.
[0004] In order to convert the nitrogen oxides developed during
combustion, two different catalytic methods have proven to be
successful in the past: on the one hand, there is the NOx adsorber
technology in which, during lean operations of the engine, the
nitrogen oxides are adsorbed on a so-called NOx storage catalyst
and, during rich operations, are desorbed and reduced; and on the
other hand, the SCR technology in which the nitrogen oxides
contained in the oxygen-rich exhaust gas are reduced selectively to
nitrogen and water using ammonia (NH.sub.3) or a corresponding
precursor substance convertible into ammonia (SCR=selective
catalytic reduction).
[0005] While in the NOx adsorber technologies the sulfur content in
fuel leads to poisoning of the NOx storage components and long-term
stability also leaves much to be desired, the ammonia SCR method
has in many cases already proved its durability in long-term use
for the removal of nitrogen oxides from power-station exhaust
gases. In addition, it appears that according to today's state of
the art, NOx conversion rates of up to 90% required in future can
only be realized by employing SCR technologies. Especially in
heavy-duty trucks, where a durability of more than 400,000 miles is
required, SCR systems will very likely be employed.
[0006] Due to the high toxicity and volatility of ammonia,
non-poisonous precursor compounds, especially aqueous urea
solutions, are preferably employed in mobile applications for motor
traffic. The urea solution is hydrolyzed to ammonia and carbon
dioxide by means of hydrolysis catalysts or directly on the SCR
catalyst. Using special metering systems upstream of the hydrolysis
and the SCR catalyst, respectively, the urea solution is injected
into the exhaust-gas flow.
[0007] A disadvantage of this type of exhaust-gas aftertreatment is
that both the hydrolysis of urea and the SCR reaction using the
common SCR catalysts start only at temperatures above 160 to
200.degree. C., that is, the light-off temperatures of SCR
catalysts for the low-temperature range is between 160 and
200.degree. C. As used in this invention, the light-off temperature
of a catalyst refers to the temperature upstream of the catalyst at
which the catalyst converts the pollutant in question (here the
nitrogen oxides) just at a 50% rate.
[0008] Thus, during operating states with exhaust-gas temperatures
below this temperature range, the nitrogen oxides generated by the
engine pass the exhaust-gas purification system unchanged and are
released into the environment. In modern diesel vehicles, this
happens not only after the cold start, but also during normal
operation under operating conditions at low load or when idling.
The inventors found that when a diesel engine is idling, only about
65% of the nitrogen oxides contained in its exhaust gas are
converted into nitrogen and water at the SCR catalyst. By contrast,
at temperatures above 300.degree. C., the conversion rate is 90%
and more. When the performance of an exhaust-gas purification
system for diesel engines is assessed, idle operation accounts for
20% of the assessment, so that a considerable potential for
improvement exists here.
[0009] To account for this fact, the German patent application DE
10054877 A1 already described an exhaust-gas purification system
which includes a catalyst having catalytically active components
for the selective catalytic reduction (SCR components) and an
additional storage component for nitrogen oxides.
[0010] The purpose of this additional NOx storage component is to
temporarily store the nitrogen oxides contained in the exhaust gas
at low temperatures below 160 to 200.degree. C. on the SCR catalyst
and to release them at higher temperatures so that they can
afterwards be reduced on the SCR catalyst using surplus ammonia.
This solution, however, has been only partly successful.
[0011] The object of the present invention is to provide an
exhaust-gas purification system which has improved activity for the
conversion of nitrogen oxides at low exhaust-gas temperatures
compared to the prior art. Another subject matter of this invention
is the simultaneous reduction of the particulate emission from the
lean-burn internal combustion engines. Moreover, the invention is
to provide a method of exhaust-gas purification with an improved
conversion rate of the nitrogen oxides at low exhaust-gas
temperatures.
[0012] This object is solved by an exhaust-gas purification system
for the selective catalytic reduction of nitrogen oxides which
includes at least one catalyst having catalytically active
components for the selective catalytic reduction (SCR components)
and through which the lean exhaust gas of an internal combustion
engine flows. The exhaust-gas purification system is characterized
in that a NOx storage catalyst is arranged upstream of the SCR
catalyst and metering means for supplying a precursor compound of
ammonia to the exhaust gas is located between the NOx storage
catalyst and the SCR catalyst.
[0013] The NOx catalyst arranged according to the invention
upstream of the SCR catalyst in the exhaust gas system fulfills two
functions.
[0014] First of all, it is capable of adsorbing the nitrogen oxides
contained in the still relatively cold exhaust gas following the
cold start of the engine or during idling. This prevents the
nitrogen oxides from leaving the not yet active SCR catalyst
without being converted into water and nitrogen. Only at an
elevated temperature the nitrogen oxides will be desorbed and can
then be converted at the SCR catalyst.
[0015] Secondly, the NOx storage catalyst increases the ratio of
nitrogen dioxide to nitrogen monoxide in the exhaust gas, thereby
improving the efficiency of the SCR catalyst. That is, the activity
of the SCR catalyst for converting the nitrogen oxides is highest
when nitrogen dioxide and nitrogen monoxide have an approximate
volume ratio of 1:1 in the exhaust gas. Depending on the engine
operating conditions, the raw exhaust gas of a lean-mix engine
contains 65 to 95 vol. % nitrogen monoxide and, accordingly,
differs from the optimum composition.
[0016] Preferably, the NOx storage catalyst is applied in the form
of a coating on an inert carrier. Suitable carriers are the
so-called honeycomb carriers made of ceramic or metal which are
commonly used in the catalysis of automotive exhaust gases. In a
preferred embodiment of the exhaust-gas purification system
according to the invention, the NOx storage catalyst is applied as
a coating on a diesel particulate filter. In this case, this unit
fulfills a third function, i.e. the removal of soot particles from
the exhaust gas. The diesel particulate filter may be designed as a
wall flow filter, foamed ceramic filter, ceramic fiber filter or
wire-mesh filter. These carriers and filters are known to the
person skilled in the art of automotive exhaust-gas catalysis.
Therefore, a detailed description of these carriers will be
omitted.
[0017] The NOx storage catalyst includes at least one alkaline
compound of elements from the group consisting of alkali metals,
alkaline-earth metals or rare earths which are coated or activated
with at least one of the platinum group metals platinum, palladium,
rhodium or iridium.
[0018] The oxidation activity of the catalyst for nitrogen
mon-oxide may be increased further if the NOx storage catalyst
additionally includes catalytically active components on the basis
of support oxides from the group consisting of aluminum oxide,
silicon dioxide, cerium oxide, zirconium oxide, titanium oxide or
mixed oxides thereof which are coated with at least one of the
platinum group metals platinum, palladium, rhodium and iridium.
[0019] It is especially preferred that the NOx storage catalyst
includes storage components on the basis of cerium oxide, which are
coated with platinum, and additionally platinum as an oxidizing
catalyst on a support based on aluminum oxide. The storage
components on the basis of cerium oxide exhibit the lowest
light-off temperatures for the storage of nitrogen oxides and are
therefore particularly suitable for the exhaust-gas purification
system according to the invention. They are capable of storing
nitrogen oxides to a high degree at temperatures as low as about
120.degree. C. and of gradually releasing them at higher
temperatures, so that they can be converted subsequently at the
downstream SCR catalyst.
[0020] Here, cerium oxide may be employed as a pure material or as
a mixed oxide, preferably with zirconium oxide. Preferred are
cerium/zirconium mixed oxides having a content of zirconium oxide
between 5 and 30 wt. % referred to the mixed oxide's total weight.
The above-mentioned mixed oxides are particularly resistant to high
temperature stresses which may occur during full-load operation of
the diesel engine or a filter regeneration, for example. A further
increase of temperature resistance may be obtained by doping the
mixed oxide, for example with praseodymium oxide.
[0021] The aluminum oxide used as a support for platinum is an
aluminum oxide of the so-called transition series, which has a high
specific surface area between 10 and 400 m.sup.2/g. Doping this
material with 2 to 10 wt. % lanthanum oxide may also stabilize it
against thermal stresses. An aluminum silicate having a silicon
dioxide content between 1 and 40 wt. % referred to the total weight
of the aluminum silicate is also particularly suitable.
[0022] It is known that all kind of NOx storage materials are
sensitive to sulfur poisoning. This is due to the chemical
similarity of NO.sub.2 and SO.sub.3. A material which is able to
adsorb nitrogen oxides by forming nitrates will do the same with
sulfur oxides by forming sulfates. In principle the affinity to the
latter reaction is higher. This means that formed nitrates may be
substituted by sulfates but this reaction is not reversible. It is
well known that from that reason standard NOx storage catalysts
have to be desulfated from time to time. Such a desulfation can be
performed at high temperatures under rich exhaust gas conditions.
But specially designed NOx storage materials e.g. as the one
described above on the basis of cerium oxide or mixed oxides of
cerium and zirconium oxide can be desulfated also under lean
exhaust gas condition at elevated temperatures of about 600.degree.
C. This is called thermal desulfation in the following.
[0023] Normally a diesel particulate trap has to be heated-up
periodically to combust the accumulated soot. For this combustion a
temperature of about 600-650.degree. C. is needed. If the NOx
storage catalyst described above on the basis of cerium oxide or
mixed oxides of cerium and zirconium oxide is coated on a filter, a
periodical thermal desulfation in parallel to the filter
regeneration can be achieved.
[0024] Furthermore, if the NOx storage catalyst is coated on a
flow-through honeycomb monolith and the entire exhaust system
contains a particulate filter the temperature increase for a filter
regeneration can be used as well to desulfate the NOx storage
catalyst.
[0025] The SCR components of the SCR catalyst preferably include a
solid acid system of titanium dioxide and vanadium oxide. In
addition, this material may include at least one component from the
group consisting of tungsten oxide, molybdenum oxide, silicon
dioxide, sulfate and zeolites, wherein the zeolites may be present
in the acid H-form or be exchanged with metal ions. However, the
SCR catalyst may entirely consist of zeolites, wherein the zeolites
are present in the acid H-form or are exchanged with metal ions, in
particular with iron and copper, within their exchange
capacity.
[0026] If a diesel particulate filter is employed as a carrier for
the NOx storage catalyst, it is preferred to add components to the
catalyst which lower the ignition temperature of the diesel soot in
order to facilitate the regeneration of the filter by burning off
the soot from time to time. Suitable materials are known from the
German patent applications DE 3141713 A1 and DE 3232729 A1, for
example. These are materials such as lithium oxide, copper
chloride, vanadium oxide/alkali oxide combinations, lithium,
sodium, potassium or cerium vanadate or mixtures thereof, for
example. For lowering the ignition temperature, cerium oxide, which
is already to be used as a storage component, is well suited,
too.
[0027] In a preferred embodiment, a so-called wall flow filter is
used as a diesel particulate filter, the inlet side of which is
provided with a coating made of pure cerium oxide to lower the
ignition temperature of the diesel exhaust particulates and the
outlet side of which comprises a coating made of a mixture of
platinum on cerium/zirconium mixed oxide and platinum on y-aluminum
oxide stabilized with 4 wt. % lanthanum oxide.
[0028] During the addition of ammonia to the exhaust gas,
over-dosage and, consequently, undesired emission of ammonia to the
environment may occur. To prevent this from happening, a so-called
ammonia guard catalyst, i.e. an oxidation catalyst for oxidizing
the surplus ammonia, may be arranged downstream of the SCR
catalyst.
[0029] The ammonia required for the selective catalytic reduction
is preferably added to the exhaust gas in the form of a urea
solution, but other precursor compounds that are easily
decomposable into ammonia are possible. In order to facilitate the
decomposition of urea into ammonia and carbon dioxide, a so-called
hydrolysis catalyst may be provided upstream of the SCR
catalyst.
[0030] The invention will now be explained in more detail referring
to the accompanying FIGS. 1 to 4 and the Comparative Examples and
one Example, wherein:
[0031] FIG. 1 shows the basic construction of the exhaust-gas
purification system;
[0032] FIG. 2 shows the exhaust-gas purification system with an
additional hydrolysis catalyst and ammonia guard catalyst;
[0033] FIG. 3 shows a test result of the nitrogen oxide emissions
at an exhaust-gas purification system including an SCR catalyst and
an upstream oxidation catalyst; and
[0034] FIG. 4 shows the nitrogen oxide emissions at an exhaust-gas
purification system including an SCR catalyst and an upstream
diesel particulate filter, the latter being coated with an
oxidation catalyst and a NOx storage catalyst.
[0035] FIG. 1 is a view showing the basic construction of the
exhaust-gas purification system 1 according to the invention. The
system includes an SCR catalyst 3 in a converter housing 2.
Upstream of the SCR catalyst, a catalyst 5 in a converter housing 4
is provided. Metering means 8 for the supply of ammonia or a
compound decomposable into ammonia to the exhaust gas is located
between both converter housings. The metering means in FIG. 1 is
shown only as a simple supply tube to exemplify the metering means
known to the person skilled in the art.
[0036] The catalyst 5 includes both an oxidation catalyst and a NOx
storage catalyst as active components. These active components may
be applied in the form of a coating on a usual honeycomb carrier as
well as on a diesel particulate filter.
[0037] FIG. 2 shows another embodiment of the exhaust-gas
purification system according to the invention. To safely prevent
emission of ammonia during accidental overdosage, the SCR catalyst
3 is provided with a downstream ammonia guard catalyst 7. The
latter is a common oxidation catalyst, the active components of
which may be formed by platinum/aluminum oxide (aluminum oxide
coated, i.e. activated, with platinum). In addition, a hydrolysis
catalyst 6 for the hydrolysis of urea into ammonia is arranged
upstream of the SCR catalyst. All three catalysts 6, 3 and 7 are
preferably accommodated in a single converter housing 2.
[0038] In the embodiments of the exhaust gas systems according to
FIGS. 1 and 2 it is most preferred to use a NOx storage catalyst on
the basis of cerium oxide or mixed oxides of cerium oxide and
zirconium oxide as herein described above. The exhaust gas systems
can then be operated as follows.
[0039] The lean exhaust gas of the internal combustion engine is
first routed over the NOx storage catalyst and subsequently over
the SCR catalyst for selective catalytic reduction and wherein a
compound decomposable into ammonia is supplied to the exhaust gas
between the NOx storage catalyst and the SCR catalyst. The internal
combustion engine can be operated continuously with a lean air/fuel
mixture. The nitrogen oxides contained in the exhaust gas will be
stored by the NOx storage catalyst at low exhaust gas temperatures
and will gradually release them with rising exhaust gas
temperature. At temperatures above approximately 300.degree. C.
nearly all stored nitrogen oxides will have been released. The
released nitrogen oxides will then be converted at the downstream
SCR catalyst to nitrogen and water. No periodic changing of the
air/fuel mixture fed to the internal combustion engine to rich
air/fuel mixtures is necessary.
[0040] The nitrogen oxides storage capacity of the catalyst will
gradually decrease due to sulfur poisoning as already explained
above. The original storage capacity can be re-established from
time to time under lean exhaust gas conditions by increasing the
exhaust gas temperature up to approximately 600.degree. C. The
measures for increasing the exhaust gas temperature of the internal
combustion are well known to the expert. This can be done in such a
way that the exhaust gas composition remains net oxidizing.
[0041] In case the exhaust system contains in addition a diesel
particulate filter then a very beneficial variety of the described
process results. The particulate filter has to be regenerated from
time to time by combusting the diesel soot collected on the filter.
This is done by increasing the exhaust gas temperature to the
ignition temperature of the diesel soot while maintaining net
oxidizing conditions. The ignition temperature is sufficient to
also desulfate the NOx storage catalyst. Thus, filter regeneration
and desulfation of the NOx storage catalyst take place in parallel.
The NOx storage catalyst is automatically desulfated whenever the
particulate filter is regenerated.
[0042] A most preferred process results if the NOx storage catalyst
is coated onto the particulate filter itself.
COMPARATIVE EXAMPLE
[0043] An exhaust-gas purification system according to the prior
art for a 4.2-liter diesel engine was assembled using a
pre-catalyst and an SCR catalyst.
[0044] The pre-catalyst was a diesel oxidizing catalyst as shown in
Example 1 of EP 0920913 A1 (Pt on aluminum silicate mixed with
DAY-zeolite), which was applied in the form of a coating on a
metallic flow-through type honeycomb carrier having a volume of 2
liters and a cell density of 62 cm.sup.2. The coating concentration
was 200 g/l of honeycomb volume, and the platinum concentration was
3.2 g/l (90 g/ft.sup.3) of honeycomb volume.
[0045] The SCR catalyst was an iron-exchanged ZSM5-zeolite catalyst
which was applied in the form of a coating on a metallic honeycomb
carrier having a volume of 4.6 liters and a cell density of 62
cm.sup.2.
[0046] FIG. 3 shows the emission of this system from the diesel
engine and exhaust-gas purification system during the so-called ESC
(European Stationary Cycle) test cycle. This test cycle has been
specifically developed for the emission certification of heavy-duty
diesel engines and includes a total of 13 different load states
with an overall duration of 1680 seconds.
[0047] Curve (a) in FIG. 3 represents the nitrogen-oxide raw
emission of the diesel engine. Curve (b) is the resulting NOx
emission downstream of the SCR catalyst. Over the duration of
77.6%.
EXAMPLE
[0048] The coating of the pre-catalyst of the exhaust-gas
purification system as described in the comparative example was
replaced by a coating made of platinum-activated cerium/zirconium
mixed oxide (80 wt. % cerium oxide, 20 wt. % zirconium oxide). The
coating concentration was 200 g/l and the platinum concentration
was 2.65 g/l (75 g/ft.sup.3) of honeycomb volume. Here, the
cerium/zirconium mixed oxide constituted the NOx storage component
of the storage catalyst.
[0049] FIG. 4 shows the emissions measured on this system during
the ESC test cycle run. As an average over the entire cycle, the
nitrogen-oxide conversion rate was about 86%, being clearly higher
than in the exhaust-gas purification system according to the prior
art.
[0050] A comparison of the two plots of FIG. 3 and FIG. 4 shows
that, in the exhaust-gas purification system according to the prior
art, relatively high nitrogen-oxide emissions occur during the
idling phase at the beginning of the test cycle. By contrast, the
exhaust-gas purification system according to the invention exhibits
only low nitrogen-oxide emissions during the first 240 seconds,
because the NOx storage material used adsorbs the nitrogen oxides
already at relatively low exhaust-gas temperatures. Nevertheless,
with increasing load and after passing the desorption temperature a
relatively strong desorption peak occurs. As an average over the
entire test cycle, however, the exhaust-gas purification system
results in a substantial improvement in the nitrogen-oxide
conversion rate.
* * * * *