U.S. patent application number 11/772035 was filed with the patent office on 2008-01-10 for arrangement for reducing nitrogen oxides in exhaust gases.
Invention is credited to Andreas Doring, Florian Walde.
Application Number | 20080008629 11/772035 |
Document ID | / |
Family ID | 38670667 |
Filed Date | 2008-01-10 |
United States Patent
Application |
20080008629 |
Kind Code |
A1 |
Doring; Andreas ; et
al. |
January 10, 2008 |
Arrangement for Reducing Nitrogen Oxides in Exhaust Gases
Abstract
An arrangement and method for reducing the nitrogen oxide
content in the exhaust gas of an internal combustion engine with
the aid of ammonia and/or ammonia-releasing reduction agents,
whereby ammonia and/or ammonia-containing reduction agent is added
to the exhaust gas stream upstream of a catalyst combination
composed of an SCR catalyst and a subsequent NH.sub.3-oxidation
catalyst in such a way that a homogeneous mixture of exhaust gas
and ammonia is present upstream of the SCR catalyst. To optimize
the reaction or conversion of nitrogen oxides, disposed downstream
of the combination of a first SCR catalyst and a first
NH.sub.3-oxidation catalyst is at least one second catalyst having
SCR activity in order in this way to reduce the nitrogen oxides
formed at the first NH.sub.3-oxidation catalyst due to insufficient
selectivity of the catalyst to nitrogen with not yet oxidized
NH.sub.3.
Inventors: |
Doring; Andreas; (Munchen,
DE) ; Walde; Florian; (Dietenhofen, DE) |
Correspondence
Address: |
ROBERT W. BECKER & ASSOCIATES
707 HIGHWAY 333, SUITE B
TIJERAS
NM
87059-7507
US
|
Family ID: |
38670667 |
Appl. No.: |
11/772035 |
Filed: |
June 29, 2007 |
Current U.S.
Class: |
422/171 ;
427/178 |
Current CPC
Class: |
Y02T 10/24 20130101;
F01N 13/0097 20140603; Y02T 10/12 20130101; F01N 13/009 20140601;
F01N 2370/02 20130101; F01N 3/2066 20130101; F01N 2240/40 20130101;
F01N 2610/02 20130101; F01N 3/2882 20130101; F01N 3/106 20130101;
F01N 13/0093 20140601 |
Class at
Publication: |
422/171 ;
427/178 |
International
Class: |
B21C 47/00 20060101
B21C047/00; B01D 51/10 20060101 B01D051/10; B05D 3/12 20060101
B05D003/12 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 8, 2006 |
DE |
10 2006 031 659.2 |
Claims
1. An arrangement for reducing the nitrogen oxide content in the
exhaust gas of an internal combustion engine with the aid of
ammonia and/or ammonia-releasing reduction agents, comprising: a
catalyst combination composed of a first SCR catalyst (5, 5', 5'',
5'''), an NH.sub.3-oxidation catalyst (6, 6', 6'', 6''') disposed
downstream of said first SCR catalyst, and at least one second SCR
catalyst (7, 7', 7'', 7''') disposed downstream of said
NH.sub.3-oxidation catalyst; and means for adding ammonia and/or
ammonia-containing reduction agent to an exhaust gas stream
upstream of said catalyst combination such that a mixture of
exhaust gas and ammonia is present upstream of said first SCR
catalyst (5, 5', 5'', 5''').
2. An arrangement according to claim 1, wherein at least one second
NH.sub.3-oxidation catalyst (8, 8', 8'') is disposed downstream of
said second SCR catalyst (7, 7', 7'', 7''').
3. An arrangement according to claim 2, wherein at least one
further SCR catalyst (9), or further SCR catalyst (9) and
NH.sub.3-oxidation catalyst (10) in alternating sequence, are
disposed downstream of said second NH.sub.3-oxidation catalyst
(8'), and wherein such arrangement ends with an SCR catalyst or an
NH.sub.3-oxidation catalyst.
4. An arrangement according to claim 1, wherein the combinations of
the respective SCR catalysts (5, 7 or 5', 7' or 5'', 7'', 9 or
5''', 7''') differ along a direction of flow of the exhaust
gas.
5. An arrangement according to claim 1, wherein the combinations of
the respective NH.sub.3-oxidation catalysts (6', 8 or 6'', 8', 10
or 6''', 8'') differ along a direction of flow of the exhaust
gas.
6. An arrangement according to claim 1, wherein a first combination
of SCR catalyst (5, 5', 5'', 5''') and NH.sub.3-oxidation catalyst
(6, 6', 6'', 6''') is optimized to selectivity by the selective
active catalyst materials, and wherein subsequent further SCR
catalysts (7, 7', 7'', 7''', 9) and NH.sub.3-oxidation catalysts
(8, 8', 8'', 10) are optimized to high conversion rates by the
selected active catalyst materials.
7. An arrangement according to claim 1, wherein at least a portion
of the SCR catalysts (5, 5', 5'', 5''', 7, 7', 7'', 7''', 9)
contain V.sub.2O.sub.5 as active substituents.
8. An arrangement according to claim 1, wherein at least a portion
of the SCR catalysts (5, 5', 5'', 5''', 7, 7', 7'', 7''', 9)
contain iron and/or copper and/or cobalt-containing zeolites.
9. An arrangement according to claim 8, wherein the zeolites are at
least one of the types ZSM-5, OSI, EPI, AEN, MFI, FAU and BEA.
10. An arrangement according to claim 1, wherein the
NH.sub.3-oxidation catalysts (6, 6', 6'', 6''', 8, 8', 8'', 10)
contain at least one of the group consisting of platinum,
palladium, rhodium, iridium, and their oxides as active
components.
11. An arrangement according to claim 1, wherein at least one of
the SCR catalysts (5, 5', 5'', 5''', 7, 7', 7'', 7''', 9) and the
NH.sub.3-oxidation catalysts (6, 6', 6'', 6''', 8, 8', 8'', 10) are
solid catalysts or coated catalysts on metal or ceramic supports or
substrates.
12. An arrangement according to claim 1, wherein the SCR catalysts
(5''', 7''') and the NH.sub.3-oxidation catalysts (6''', 8'') are
applied to a common support or substrate.
13. A method of producing the catalyst arrangement of claim 1,
wherein the various catalyst combinations are applied by immersing
a support or substrate into various solutions containing the
catalysts, are dried, and are subsequently calcined; or wherein the
various catalyst combinations are produced by impregnating a
catalyst layer already applied to a support or substrate or by
impregnating a solid catalyst; or wherein the various catalyst
combinations are produced by using metal foils as supports or
substrates, prior to rolling the individual foils up, coating them
by spraying, subjecting them to a finishing treatment that includes
a drying process, and only then rolling them up to form a honeycomb
body.
Description
[0001] The instant application should be granted the priority date
of Jul. 8, 2006 the filing date of the corresponding German patent
application 10 2006 031 659.2.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to an arrangement and a method
for reducing the nitrogen oxide content in the exhaust gas of an
internal combustion engine with the aid of ammonia and/or
ammonia-releasing reduction agents.
[0003] Nitrogen oxides belong to the limited exhaust gas components
that are produced during combustion processes and the permissible
emissions of which are continuously being lowered. In this
connection, the reduction of the nitrogen oxides generally occurs
with the aid of catalysts. In oxygen-rich exhaust gas, a reduction
agent is additionally necessary in order to raise the selectivity
and the NO.sub.x conversions. These methods have become known under
the general term SCR processes, whereby SCR stands for "Selective
Catalytic Reduction". They have been used for many years in the
power plant industry, and in recent times also with internal
combustion engines. A detailed description of such methods can be
found in DE 34 28 232 A1. V.sub.2O.sub.5-containing mixed oxides,
for example in the form of V.sub.2O.sub.5/WO.sub.3/TiO.sub.2, can
be used as SCR catalysts. Typical Of V.sub.2O.sub.5 percentages are
between 0.2-3%. Ammonia or ammonia-releasing compounds, such as
urea or ammonium formate in solid or dissolved form, are used in
practical applications. For the conversion of one mole of nitric
oxide, one mole of ammonia is necessary.
4NO+4NH.sub.3+O.sub.24N.sub.2+6H.sub.2O (1)
[0004] If a platinum-containing NO oxidation catalyst is disposed
upstream of the SCR catalyst to form NO.sub.2,
2NO+O.sub.22NO.sub.2 (2)
[0005] the SCR reaction can be significantly accelerated and the
low temperature activity can be considerably raised.
[0006] The SCR process causes special problems with the reduction
of nitrogen oxides of internal combustion engines, and here in
particular in vehicles, since care must be taken that there is no
emission of unused ammonia. In contrast to the situation with power
plants, in vehicles, no adequately precise and stable exhaust gas
sensors are available for regulating the system and hence for
avoiding NH3 emissions when an overdosing of reduction agent
occurs. In addition, the use of V.sub.2O.sub.5 is problematic since
at temperatures over 650.degree. C. it sublimes, so that in recent
times zeolite having the active substituents iron and/or copper
and/or cobalt have been used.
[0007] In order despite the inadequate sensor technology to avoid
undesired NH.sub.3 emissions, without additional measures the SCR
catalyst must be significantly over dimensioned in order to ensure
adequate reliability against ammonia slippage. The situation can be
improved if an NH.sub.3-oxidation catalyst is provided downstream
of the SCR catalyst. Such an arrangement is shown, for example, in
DE 37 33 501 A1. Furthermore, it is known from EP 410 440 B1 to
provide the SCR catalyst and the NH.sub.3-oxidation catalyst on a
common support or substrate.
[0008] Noble metals of the platinum group, as well as oxides
thereof, can be used as the active material for the
NH.sub.3-oxidation catalyst. The intended task of the
NH.sub.3-oxidation catalyst of oxidizing excess NH.sub.3 to
nitrogen can be realized only inadequately in practice due to the
too low selectivity of the active substituents, for example
platinum-containing substituents, so that the oxidation, as shown
in the following equations, rather than ending at the oxidation
state [0] ends at the oxidation states [+1], [+2] or even only at
[+4] and thus again nitrogen oxides result.
4NH.sub.3+3O.sub.22N.sub.2+6H.sub.2O [0] (3)
2NH.sub.3+2,5O22NO+3H.sub.2O [+2] (4)
2NO+O.sub.22NO.sub.2[+4] (5)
NH.sub.3+NO.sub.22NO+H.sub.2O [+2] (6)
In addition, the platinum metals (platinum, palladium, rhodium,
iridium. osmium, ruthenium) as well as their oxides, that are used
as active material for the NH.sub.3-oxidation catalyst, are very
expensive and rare. For this reason, the NH.sub.3-oxidation
catalysts are generally made very small, which means that they are
often overloaded, so that there is no complete NH.sub.3
conversion.
[0009] It is an object of the present invention to provide an
arrangement that on the one hand reliably presents ammonia
slippage, and on the other hand reduces the residual nitrogen oxide
content in the exhaust gas to a minimum. It is furthermore an
object to provide a method for producing such an arrangement.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] These and other objects and advantages of the present
application will appear more clearly from the following
specification in conjunction with the accompanying schematic
drawings, in which:
[0011] FIG. 1 shows an arrangement for the selective catalytic
reduction of exhaust gases,
[0012] FIG. 2 shows a combination of two SCR/NH.sub.3-oxidation
catalyst arrangements,
[0013] FIG. 3 shows a catalyst arrangement having a plurality of
combinations of SCR catalysts and NH.sub.3-oxidation catalysts,
[0014] FIG. 4 shows an arrangement similar to that of FIG. 2 on a
common support or substrate, and
[0015] FIG. 5 is a table showing the numeric relationships.
SUMMARY OF THE INVENTION
[0016] Pursuant to the arrangement of the present application, for
reducing the nitrogen oxide content in the exhaust gas of an
internal combustion engine with the aid of ammonia and/or
ammonia-releasing reduction agents, whereby ammonia and/or
ammonia-containing reduction agent is added to the exhaust gas
stream upstream of a catalyst combination comprised of an SCR
catalyst and a subsequent NH.sub.3-oxidation catalyst in such a way
that a mixture of exhaust gas and ammonia is present upstream of
the SCR catalyst, the problem is advantageously solved in that at
least one second catalyst having SCR activity is disposed
downstream of the NH.sub.3-oxidation catalyst, and thus the
nitrogen oxides formed at the NH.sub.3-oxidation catalyst due to
insufficient selectivity of the catalyst again react with
non-oxidized NH.sub.3 and can hence be reduced.
[0017] A further advantageous improvement of the system is to
provide a second NH.sub.3-oxidation catalyst downstream of the
second SCR catalyst in order to similarly oxidize the NH.sub.3
quantities that still occur downstream of the SCR catalyst. This
alternative arrangement of the SCR catalyst and/or
NH.sub.3-oxidation catalyst can be repeated as often as desired and
advantageously contributes to the stability of the overall
system.
[0018] It can also be advantageous to use different catalyst
combinations for the individual SCR catalyst and NH.sub.3-oxidation
catalyst and to thus optimize the individual catalyst to the
NO.sub.x and NH.sub.3 quantities that are produced in the
respective region as well as to the exhaust gas temperatures. This
means, for example, that the first SCR catalyst and the first
NH.sub.3-oxidation catalyst are optimized to high selectivity with
regard to the conversion of the initial products into N.sub.2,
since here the greatest concentrations of the educts NO.sub.x and
NH.sub.3 are present and hence also the highest concentrations of
undesired byproducts of the reaction, such as NO, N.sub.2O or
NO.sub.2, are to be expected. The subsequent SCR catalyst and
NH.sub.3-oxidation catalyst can, in contrast, be optimized to
conversion and less to selectivity,
[0019] V.sub.2O.sub.5-containing mixed oxides, for example in the
form of V.sub.2O.sub.5/WO.sub.3/TiO.sub.2, can advantageously be
used as SCR catalysts. Typical V.sub.2O.sub.5 percentages are
between 0.2-5%, Furthermore advantageous is the use of
zeolite-based catalysts, which contain iron and/or copper and/or
cobalt and/or oxides thereof as the active substituents.
[0020] The use of the following metals and their oxides are
advantageous for the NH.sub.3-oxidation catalyst: platinum and/or
palladium and/or iridium and/or rhodium and/or copper and/or
nickel. These active substituents can also be embedded in a zeolite
matrix.
[0021] Advantageously usable as zeolite not only for the embedment
of the active substituents having SCR activity but also of the
active substituents having NH.sub.3-oxidation activity are the
types ZSM-5 and/or OSI and/or EPI and/or AEN and/or MFI and/or FAU
and/or BEA.
[0022] In this connection, the catalyst can be not only solid
catalyst but also coated catalyst.
[0023] To reduce the cost of the inventive catalyst arrangement, it
is furthermore advantageous to apply the alternatingly arranged SCR
catalyst and NH.sub.3-oxidation catalyst, with their different
catalyst combinations, on a common support or substrate.
[0024] The method of the present invention for producing the
inventive catalyst combinations provides various ways of
proceeding, depending upon the starting material. For example, the
various catalyst combinations can advantageously be applied by
immersing a support or substrate into various solutions containing
the catalyst, can be dried, and can be subsequently calcined. A
further advantageous manner of proceeding provides for producing
the various catalyst combinations by impregnating a catalyst layer
already applied to a support or substrate or by impregnating a
solid catalyst. If metal foils are used as the support or
substrate, however, it is advantageous, prior to rolling the
individual foils up, to coat them by spraying or brushing, to then
subject them to a finishing treatment that includes a drying
process, and only then to roll them up to form a honeycomb
body.
[0025] Further specific features and advantages of the present
application will be described in detail subsequently.
Description of Specific Embodiments
[0026] Referring now to the drawings in detail, an arrangement for
the selective catalytic reduction is schematically illustrated in
FIG. 1. The exhaust gases, which are produced by an internal
combustion engine (not illustrated) by the combustion processes,
and which are symbolized by the arrows in FIG. 1, initially pass
into an exhaust gas treatment section 1, in which a reduction agent
is added to the hot exhaust gas as close to the engine as possible.
As is customary with motor vehicles having SCR catalysts, the
reduction agent is an aqueous urea solution; it is of course also
possible to add solid urea, as already described in detail in the
pertinent technical literature. The dosing is effected as a
function of operating parameters of the internal combustion engine,
controlled by an engine control unit (not illustrated), in such a
way that the aqueous urea solution is sprayed into the exhaust gas
stream via a nozzle 2 directly upstream of a hydrolysis catalyst 3.
The task of the hydrolysis catalyst 3 is to convert the aqueous
urea solution as completely as possible into ammonia and water
vapor while avoiding byproducts. Under certain conditions, this
disassociation is also adequately effected without a hydrolysis
catalyst, in which case the latter can be eliminated. Disposed
parallel to the hydrolysis catalyst 3 is an oxidation catalyst 4,
the task of which, pursuant to the reaction previously designated
by (2), is to oxidize a portion of the nitric oxide contained in
the exhaust gas to nitrogen dioxide by means of the excess oxygen
present in the exhaust gas; the nitric oxide is considerably more
reactive in the subsequent SCR reaction. The actual selective
catalytic reduction of the nitrogen oxides is effected in the SCR
catalyst 5 that is disposed downstream of the exhaust gas treatment
section 1 and that is to convert as great a percentage of the
nitrogen oxides (NO.sub.x) present in the exhaust gas as possible
into nitrogen and water vapor without excess ammonia (NH.sub.3)
remaining in the exhaust gas stream. In view of the constantly
changing operating conditions of an internal combustion engine
operated in a vehicle, it is obvious that the desired conversion
can take place only incompletely. In order in such cases of
insufficient conversion to prevent toxic ammonia from being given
off to the atmosphere along with the partially cleaned exhaust gas,
an NH.sub.3 oxidation catalyst 6 is disposed downstream of the SCR
catalyst and is intended to convert the excess NH.sub.3 into
nitrogen and water vapor. However, this oxidation reaction does not
occur selectively enough, so that as previously indicated, again
nitrogen oxides result (in this connection see the reactions (3) to
(6)). In order to prevent this renewed increase of nitrogen oxides,
it has been established to be suitable to dispose a second SCR
converter downstream of the NH.sub.3-oxidation catalyst 6 for
converting the nitrogen oxides again formed in the
NH.sub.3-oxidation catalyst 6, along with the residual ammonia
present in the exhaust gas, into nitrogen and water vapor. It
should be noted that this presumes that the dimension of the
NH.sub.3-oxidation catalyst 6 and the dosing of the aqueous urea
solution are such that a small percentage of ammonia is present in
the exhaust gas even after the NH.sub.3-oxidation catalyst 6.
[0027] To prevent ammonia that might not be completely converted in
the subsequent SCR converter from passing into the atmospheric air,
a second NH.sub.3-oxidation catalyst 6 can be disposed downstream
of the second SCR catalyst. A simplified illustration of such an
arrangement is shown in FIG. 2. The exhaust gas treatment section
is not shown again in this figure nor in the following figures;
rather, in this regard reference is made to what was shown and
described in conjunction with the illustration of FIG. 1. Adjoining
the illustrated combination of a first SCR catalyst 5' and a first
NH.sub.3-oxidation catalyst 6' in the embodiment of FIG. 2 is a
further combination of a second SCR catalyst 7' and a second
NH.sub.3-oxidation catalyst 8. The indicated sequence of two
similar catalyst combinations offers the possibility of further
optimizing the conversion reactions by the selection of different
active materials for the individual catalysts. For example, it is
expedient to design the first catalyst stage for high selectivity.
This means in particular that the reactions taking place within the
NH.sub.3-oxidation catalyst 6' should to the extent possible end at
the oxidation state [0]. This can be influenced by the suitable
selection of the catalyst material. For example, iridium has a
higher selectivity than does platinum; in contrast, with platinum
as the catalyst material the conversion rate increases. Thus, if
iridium, iridium oxide, or a material having iridium substituents
is used in the first NH.sub.3-oxidation catalyst 6' as the active
catalyst material, the conversion of the initial products into
nitrogen is optimized since at this location of the arrangement a
high concentration of the educts or reactants NO.sub.x and NH.sub.3
are present and hence also the greatest concentrations of undesired
byproducts of the reaction, such as NO, N.sub.2O or NO.sub.2, are
to be anticipated if the reaction occurred less selectively. The
subsequent second SCR catalysts 7' and the NH.sub.3-oxidation
catalysts 8, can, in contrast, be optimized to the conversion rate
and less to selectivity. In particular, the second
NH.sub.3-oxidation catalyst 8 is designed for a high conversion
rate by the use of platinum, platinum oxide, or a material having
platinum substituents.
[0028] The previously indicated material particulars with regard to
the active components of the NH.sub.3-oxidation catalysts are, of
course, only examples, especially since the actual catalyst
conditions also depend to a large extent on the carrier or
substrate material upon which the active catalyst material is
applied. This of course also applies to the SCR catalysts. The
catalyst 5' can, for example, be a zeolite-based catalyst that
contains iron and/or copper and/or cobalt and/or oxides thereof as
active components. The second SCR catalyst 7' involves
V.sub.2O.sub.5-containing mixed oxides, for example in the form of
V.sub.2O.sub.5/WO.sub.3/TiO.sub.2, as active catalyst materials,
also for the reason that at this location of the arrangement, the
exhaust gas temperature under all operating conditions does not
exceed the temperature of 650.degree. C., above which
V.sub.2O.sub.5 sublimes.
[0029] The possibility, of course, also exists of combining more
than two catalyst combinations to form a catalyst arrangement. A
corresponding example is shown in FIG. 3 by way of a drawing
showing the principle. FIG. 3 illustrates a first catalyst
combination comprised of the SCR catalyst 5'' and the
NH.sub.3-oxidation catalyst 6'', a second catalyst combination
comprised of a second SCR catalyst 7'' and a second
NH.sub.3-oxidation catalyst 8', followed by a third catalyst
combination comprised of a third SCR catalyst 9 and a third
NH.sub.3-oxidation catalyst 10. The previously indicated
alternating sequence of SCR catalysts and NH.sub.3-oxidation
catalysts can, of course, continue still further, in which
connection the arrangement can end not only with an SCR catalyst
but also with an NH.sub.3-oxidation catalyst. An alternating
sequence of catalysts, as described above, has a stabilizing effect
upon the overall system. Also applicable with this arrangement is
the optimizing possibility described in conjunction with FIG. 2 of
the deliberate selection of the active catalyst materials such that
in the downstream direction, the selectivity of the reactions
decreases while the conversion rate increases.
[0030] There is also the possibility of disposing the previously
described catalyst arrangements on a common support or substrate.
For example, FIG. 4 illustrates in a simplified manner a first
combination of an SCR catalyst 5''' and an NH.sub.3-oxidation
catalyst 6''' that together with a second combination of a further
SCR catalyst 7''' and a further NH.sub.3-oxidation catalyst 8'' are
disposed on a common support or substrate, Examples of supports or
substrates include metal foils that are coated with catalyst
components that are active in correspondence to the previously
described sequence.
[0031] To represent the numerical relationships, in the table shown
in FIG. 5, for prescribed NH.sub.3 concentrations upstream of the
catalyst system, the resulting NO.sub.x and NH.sub.3 concentrations
(in ppm), which occur with an SCR--NH.sub.3--SCR--NH.sub.3-catalyst
system according to FIG. 2 downstream of the respective catalysts,
are given by way of example. Thus, with a below stoichiometric
addition of ammonia (800 ppm) there already results downstream of
the first SCR catalyst 5' an emission of 230 ppm NO.sub.x, and 10
ppm NH.sub.3. This NH.sub.3 can easily be oxidized at the first
NH.sub.3 catalyst 6'; there is no appreciable increase of NO.sub.x.
The subsequent catalysts (second SCR catalyst 7' and second
NH.sub.3-oxidation catalyst 8) in this case provide no further
contribution toward the reduction of the NO.sub.x or NH.sub.3
quantities. If in contrast the NH.sub.3 concentration is raised to
1000 ppm, after the first NH.sub.3-oxidation catalyst 6' the
NO.sub.x concentration increases from 220 ppm to 280 ppm; at the
same time the NH.sub.3 concentration drops from 200 ppm to 20 ppm.
With this 20 ppm ammonia, the NO.sub.Xconcentration at the
subsequent second SCR catalyst 7' can be reduced to 260 ppm. The
subsequent oxidation of 2 ppm NH.sub.3 at the second
NH.sub.3-oxidation catalyst 8 is complete. The results are even
more significant with an over stoichiometric addition (1200 ppm) of
NH.sub.3. It is here possible to reduce the NO.sub.X concentration
downstream of the overall system to 212 ppm and the NH.sub.3
concentration to 0. The example shows that with a slightly over
stoichiometric addition of NH.sub.3, with the inventive catalyst
arrangement optimal results can be achieved with regard to the
selective reduction of nitrogen oxides to nitrogen without having
to fear an ammonia slippage.
[0032] With regard to the manufacturing processes for the
previously described catalyst arrangements, all manufacturing
processes already known in conjunction with the individual
catalytic converters are possible. In this connection, the
catalysts can be not only solid catalysts, but also coated
catalysts.
[0033] V.sub.2O.sub.5-containing mixed oxides, for example in the
form of V.sub.2O.sub.5/WO.sub.3/TiO.sub.2, can advantageously be
used as SCR catalysts. Typical V.sub.2O.sub.5 percentages are
between 0.2-5%. Furthermore possible is the use of zeolite-based
catalysts, which contain iron and/or copper and/or cobalt and/or
the oxides thereof as active substituents.
[0034] The use of the following metals and their oxides is
advantageous for the NH.sub.3-oxidation catalysts: platinum and/or
palladium and/or iridium and/or rhodium and/or copper and/or nickel
and/or all remaining metals of the platinum group. These active
substituents can also be embedded in a zeolite matrix.
[0035] With regard to the previously mentioned zeolites, it should
be noted that for the applications relevant here the types ZSM-5
and/or OSI and/or EPI and/or AEN and/or MFI and/or FAU and/or BEA
are particularly suitable.
[0036] Especially with the arrangement of a plurality of catalysts
on a support or substrate, the various catalyst combinations can be
applied by immersing the support or substrate into various
solutions containing the catalyst, can be dried, and can be
subsequently calcined. Furthermore, it is possible to produce the
various catalyst combinations by impregnating a catalyst layer
already applied to a support or substrate or by impregnating a
solid catalyst. With the use of metal foils as supports or
substrates, it is possible, prior to rolling the individual foils
up, to coat them with the various catalyst materials by partially
spraying or brushing them, and then subjecting them to a finishing
treatment that includes a drying process, and only then rolling
them up to form a honeycomb body.
[0037] The specification incorporates by reference the disclosure
of German priority document 10 2006 031 659.2 filed Jul. 8,
2006.
[0038] The present invention is, of course, in no way restricted to
the specific disclosure of the specification and drawings, but also
encompasses any modifications within the scope of the appended
claims.
* * * * *