U.S. patent application number 12/991202 was filed with the patent office on 2011-03-10 for method for decreasing nitrogen oxides in hydrocarbon-containing exhaust gases using an scr catalyst based on a molecular sieve.
This patent application is currently assigned to UMICORE AG & CO. KG. Invention is credited to Katja Adelmann, Gerald Jeske, Michael Seyler, Nicola Soeger.
Application Number | 20110056187 12/991202 |
Document ID | / |
Family ID | 39769301 |
Filed Date | 2011-03-10 |
United States Patent
Application |
20110056187 |
Kind Code |
A1 |
Seyler; Michael ; et
al. |
March 10, 2011 |
METHOD FOR DECREASING NITROGEN OXIDES IN HYDROCARBON-CONTAINING
EXHAUST GASES USING AN SCR CATALYST BASED ON A MOLECULAR SIEVE
Abstract
The invention relates to a process for treating diesel engine
exhaust gases comprising nitrogen oxides (NO.sub.x) and
hydrocarbons (HC) by selective catalytic reduction of the nitrogen
oxides with ammonia or a compound decomposable to ammonia as a
reducing agent over an SCR catalyst based on a molecular sieve. The
properties of the catalyst used are such that the hydrocarbons
present in the exhaust gas are kept away from the catalytically
active sites in the catalyst over which the reactions take place by
the molecular sieve-like action of the zeolite present in the
catalyst. This prevents HC-related degradation and aging effects of
the SCR catalyst and achieves a considerable improvement in
nitrogen oxide conversions in HC-containing exhaust gas.
Inventors: |
Seyler; Michael; (Rodenbach,
DE) ; Soeger; Nicola; (Nidderau, DE) ;
Adelmann; Katja; (Darmstadt, DE) ; Jeske; Gerald;
(Neuberg, DE) |
Assignee: |
UMICORE AG & CO. KG
HANAU-WOLDGANG
DE
|
Family ID: |
39769301 |
Appl. No.: |
12/991202 |
Filed: |
April 18, 2009 |
PCT Filed: |
April 18, 2009 |
PCT NO: |
PCT/EP09/02848 |
371 Date: |
November 5, 2010 |
Current U.S.
Class: |
60/274 |
Current CPC
Class: |
B01J 37/0246 20130101;
Y02T 10/12 20130101; B01D 53/945 20130101; F01N 3/103 20130101;
F01N 2370/04 20130101; B01D 53/9418 20130101; F01N 2510/063
20130101; B01D 2255/9205 20130101; F01N 3/2066 20130101; F01N
13/0097 20140603; F01N 3/035 20130101; B01J 29/68 20130101; B01J
35/04 20130101; B01D 2251/2062 20130101; Y02T 10/22 20130101; F01N
2610/02 20130101; B01D 2255/50 20130101; Y02T 10/24 20130101; B01J
29/072 20130101; F01N 3/106 20130101; B01D 2255/20738 20130101;
B01D 2255/20761 20130101 |
Class at
Publication: |
60/274 |
International
Class: |
F01N 3/18 20060101
F01N003/18 |
Foreign Application Data
Date |
Code |
Application Number |
May 7, 2008 |
EP |
EP08008572 |
Claims
1. A process for treating diesel engine exhaust gases comprising
nitrogen oxides (NO.sub.x) and hydrocarbons (HC), comprising: a)
the addition of ammonia (NH.sub.3) as such or in the form of a
compound which gives rise to ammonia under ambient conditions from
a source which does not form part of the exhaust gas line to the
exhaust gas stream comprising nitrogen oxides and hydrocarbons; and
b) the selective reaction of NO.sub.x with the NH.sub.3 added to
the exhaust gas stream over an SCR catalyst comprising a zeolite
exchanged with copper (Cu) and/or iron (Fe), characterized in that
the hydrocarbons present in the exhaust gas are kept away from the
active sites in the catalyst over which the reactions take place by
the molecular sieve-like action of the zeolite.
2. The process as claimed in claim 1, wherein the zeolite present
in the SCR catalyst is selected from the group consisting of
ferrierite, chabazite and erionite.
3. The process as claimed in claim 2, wherein the zeolite is
ferrierite.
4. The process as claimed in claim 1, wherein the hydrocarbon
content in the exhaust gas before the reaction of NO.sub.x over the
SCR catalyst in step b) is at least 50 ppm.
5. The process as claimed in claim 4, wherein the hydrocarbon
content in the exhaust gas before the reaction of NO.sub.x over the
SCR catalyst in step b) has transient peak values of at least 300
ppm.
6. The process as claimed in claim 1, wherein the hydrocarbon
content in the exhaust gas is not significantly reduced after the
reaction of NO.sub.x over the SCR catalyst in step b) at an exhaust
gas temperature at the inlet of the SCR catalyst of up to
180.degree. C., said SCR catalyst comprising a zeolite exchanged
with copper (Cu).
7. The process as claimed in claim 1, wherein the hydrocarbon
content in the exhaust gas is not significantly reduced after the
reaction of NO.sub.x over the SCR catalyst in step b) at an exhaust
gas temperature at the inlet of the SCR catalyst of up to
280.degree. C., said SCR catalyst comprising a zeolite exchanged
with iron (Fe).
Description
[0001] The invention relates to a process for reducing the level of
nitrogen oxides in the exhaust gas of internal combustion engines
operated predominantly under lean conditions. More particularly,
the invention relates to a process for treating diesel engine
exhaust gases comprising nitrogen oxides and hydrocarbons by
selective catalytic reduction of the nitrogen oxides with ammonia
or a compound decomposable to ammonia as a reducing agent over an
SCR catalyst based on a molecular sieve.
[0002] In addition to the pollutant gases which result from
incomplete combustion of the fuel, these being carbon monoxide (CO)
and hydrocarbons (HC), the exhaust gas of diesel engines comprises
particulate material (PM) and nitrogen oxides (NO.sub.x). In
addition, the exhaust gas of diesel engines contains up to 15% by
volume of oxygen. It is known that the oxidizable pollutant gases,
CO and HC, can be converted to harmless carbon dioxide (CO.sub.2)
by passing them over a suitable oxidation catalyst, and
particulates can be removed by passing the exhaust gas through a
suitable particulate filter. Technologies for removal of nitrogen
oxides from exhaust gases in the presence of oxygen are also well
known in the prior art. One of these "denoxing" processes is the
SCR process (SCR=Selective Catalytic Reduction), i.e. the selective
catalytic reduction of the nitrogen oxides with the reducing agent
ammonia over a catalyst suitable therefor, the SCR catalyst. It is
possible to add ammonia as such to the exhaust gas stream, or in
the form of a precursor compound decomposable to ammonia under
ambient conditions, "ambient conditions" being understood to mean
the environment of the compound decomposable to ammonia in the
exhaust gas stream upstream of the SCR catalyst. To perform the SCR
process, a source for providing the reducing agent, an injection
apparatus for metered addition of the reducing agent as required
into the exhaust gas and an SCR catalyst arranged in the flow path
of the exhaust gas are needed. The totality of reducing agent
source, SCR catalyst and injection apparatus arranged on the inflow
side to the SCR catalyst is referred to as an SCR system.
[0003] To comply with the exhaust gas limits for diesel motor
vehicles which apply in the USA and in Europe, it has been
sufficient to date to remove only some of the pollutants in the
exhaust gas by exhaust gas aftertreatment processes. The formation
of the pollutant gases remaining was reduced by appropriate
calibration of the combustion conditions within the engine to such
an extent that the limits could be complied with without additional
exhaust gas aftertreatment. For example, by selection of
appropriate calibration points in the combustion within the engine,
the emission of nitrogen oxides could be kept so low that no
exhaust gas aftertreatment for removal of nitrogen oxides was
needed. On the other hand, the exhaust gas did contain relatively
large amounts of carbon monoxide (CO), uncombusted hydrocarbons
(HC) and particulates (PM), which were removable, for example, by a
series connection of diesel oxidation catalyst and diesel
particulate filter in the exhaust gas system. This process is still
being used today, especially in automobiles with diesel
engines.
[0004] FIG. 1 shows a schematic of the conflicting aims of
reduction in particulates and NO.sub.x by measures within the
engine, and the achievability of the corresponding EU-IV/EU-V
limits. A reduction in particulate emission as a result of measures
within the engine results in a rise in the nitrogen oxide contents
in the untreated emission, and necessitates downstream denoxing of
the exhaust gas (1). Vice versa, a reduction in nitrogen oxide
emissions by virtue of measures within the engine leads to an
increase in particulate emission and requires the use of a diesel
particulate filter (2) for attainment of the prescribed limits.
[0005] The use of diesel particulate filters in commercial vehicles
is undesirable owing to the unit sizes required because of greater
exhaust gas mass flows and the exhaust gas pressure drops
associated with the installation thereof. Therefore, HC and
particulate emissions in commercial vehicle applications have to
date been reduced within the engine to such an extent that a
specific exhaust gas aftertreatment is not needed to comply with
the prescribed particulate limits. Instead, the nitrogen oxides
emitted to an increased degree are removed by an SCR system, which
may be preceded upstream by a diesel oxidation catalyst to increase
the low-temperature conversion.
[0006] With the even stricter limits that will be prescribed in the
future, measures within the engine will generally no longer be
sufficient to reduce the level of individual pollutant gases.
Exhaust gas aftertreatment to remove all pollutant gases emitted by
the engine will be obligatory in general for diesel vehicles newly
registered from 2010. It will thus be necessary for the present
applications for diesel exhaust gas aftertreatment to combine
diesel oxidation catalyst, diesel particulate filter and SCR
systems, though the combination of these units entails altered
operating conditions for the SCR catalyst in particular. Three
systems of this kind are currently being tested: in the "SCRT.RTM.
system" according to EP 1 054 722, a diesel oxidation catalyst, a
diesel particulate filter and an SCR system are arranged in
succession in the flow direction of the exhaust gas. Alternatively,
the SCR system may be arranged between a close-coupled diesel
oxidation catalyst and a diesel particulate filter in the underbody
of the vehicle (DOC-SCR-DPF) or upstream of a unit composed of
diesel oxidation catalyst and diesel particulate filter
(SCR-DOC-DPF).
[0007] The combination of diesel particulate filter and SCR system
in an exhaust gas line means that the SCR catalyst is exposed to
significantly higher HC concentrations over long periods at
particular operating points than has been the case in applications
to date. There are several causes of these increased HC
concentrations:
[0008] Firstly, the combustion within the engine is no longer
calibrated with the aim of dispensing with costly exhaust gas
aftertreatment stages, at one of the extreme points on the
combustion map, but from the aspect of optimizing performance, with
particulates and HC, and also nitrogen oxides, being tolerated
equally as emissions (cf. point (3) in FIG. 1). This causes a
certain basic level of HC stress on the exhaust gas aftertreatment
system, the exhaust gas already having significantly higher HC
concentrations than in the applications customary to date, which
were calibrated for the avoidance of particulates (and HC), in
which SCR systems were used. Secondly, the diesel particulate
filter has to be regenerated at regular intervals, which is
accomplished by controlled burnoff of the particulate load among
other ways. For this purpose, the filter has to be heated to a
temperature above the soot ignition temperature. This "heat-up" is
effected by postinjection of fuel into the piston exhaust stroke of
the cylinder or into the exhaust gas line and by catalytic
conversion of the uncombusted hydrocarbons on an oxidizing catalyst
("heat-up catalyst"). Usually, an upstream diesel oxidation
catalyst assumes the function of the "heat-up catalyst". If this is
not present, as in the SCR-DOC-DPF system, it is also
possible--according to the catalyst formulation--for the SCR
catalyst to assume "heat-up" functions. In each case, higher HC
concentrations are present during the filter regeneration upstream
of the SCR catalyst, since the hydrocarbons injected after ignition
are not fully combusted catalytically during the "heat-up". In an
SCRT.RTM. system, in which diesel oxidation catalyst and diesel
particulate filter are upstream of the SCR catalyst, there is
additionally prolonged HC stress on the SCR catalyst after a
certain run time, which is attributable to the hydrothermal aging
of the oxidation functions in the diesel oxidation catalyst and in
the optionally catalytically coated filter.
[0009] Irrespective of a regeneration of the diesel particulate
filter, further heating measures by postinjection of fuel may be
necessary to compensate for cold start delays, and lead to
transient sharp increases in the HC concentration upstream of the
SCR catalyst.
[0010] The effects mentioned have the result that the SCR catalyst
in modern combined emission control systems is exposed to altered
operating conditions, the HC contents present in the exhaust gas
upstream of the SCR catalyst being much higher than in applications
to date. Under these conditions, conventional SCR catalysts
generally exhibit a clear decline in nitrogen oxide conversion
performances.
[0011] For example, the "conventional" zeolite catalysts described
in U.S. Pat. No. 4,961,917 store significant amounts of HC in the
zeolite pores. They exhibit satisfactory nitrogen oxide conversion
rates only when the hydrocarbon emissions have been removed
virtually completely before entry into the SCR catalyst, for
example by means of a suitable upstream oxidation catalyst.
[0012] EP 0 385 164 B1 describes unsupported catalysts for
selective reduction of nitrogen oxides with ammonia, which
comprise, in addition to titanium oxide and at least one oxide of
tungsten, silicon, boron, aluminum, phosphorus, zirconium, barium,
yttrium, lanthanum and cerium, an additional component selected
from the group of the oxides of vanadium, niobium, molybdenum, iron
and copper. Some of these catalysts exhibit significant oxidation
activity toward hydrocarbons. As a result, there is exothermic
combustion of the hydrocarbons present in the exhaust gas over the
SCR catalyst, which can lead to premature thermal damage to the SCR
functionality in the case of high amounts of HC.
[0013] It was an object of the present invention to specify an
apparatus for reducing the NO.sub.x content of a
hydrocarbon-comprising stream of exhaust gases of an internal
combustion engine operated under lean conditions by means of the
ammonia-operated SCR system, which is configured in an improved
manner compared to the prior art. More particularly, the NO.sub.x
reduction performance in hydrocarbon-comprising diesel engine
exhaust gases should be improved by use of an SCR catalyst
advantageous for such applications.
[0014] The object is achieved by a process for treating diesel
engine exhaust gases comprising nitrogen oxides (NO.sub.x) and
hydrocarbons (HC), comprising: a) the addition of ammonia
(NH.sub.3) as such or in the form of a compound which gives rise to
ammonia under ambient conditions from a source which does not form
part of the exhaust gas line to the exhaust gas stream comprising
nitrogen oxides and hydrocarbons; and b) the selective reaction of
NO.sub.x with the NH.sub.3 added to the exhaust gas stream over an
SCR catalyst comprising a zeolite exchanged with copper (Cu) and/or
iron (Fe). To achieve the object, the properties of the zeolite
present in the catalyst must be such that the hydrocarbons present
in the exhaust gas are kept away from the active sites in the
catalyst over which the reactions take place by the molecular
sieve-like action of the zeolite.
[0015] Studies of the light-off performance of common SCR catalysts
show that nitrogen oxide reduction with ammonia generally sets in
only when hydrocarbons present in the exhaust gas have been
converted completely. This indicates that the hydrocarbons
reversibly block the catalytically active sites required for the
reduction of the nitrogen oxides with ammonia in transient
metal-based, and also in conventional zeolite-based, SCR catalysts,
thus at least delaying the comproportionation of the nitrogen
oxides with ammonia, or even preventing it depending on the
operating temperature.
[0016] It has now been found that, surprisingly, this delaying or
prevention of the comproportionation of nitrogen oxides with
ammonia to nitrogen is not observed in the presence of hydrocarbons
when the reaction is effected over an SCR catalyst based on a Cu-
and/or Fe-exchanged zeolite, said zeolite having a maximum lower
channel width of 2.6 .ANG.-4.2 .ANG.. By passing the diesel engine
exhaust gas comprising nitrogen oxides and hydrocarbons over such
an SCR catalyst after addition of ammonia as such or of a precursor
compound decomposable to ammonia from a source which does not form
part of the exhaust gas line, it is astonishingly possible to
selectively react NO.sub.x with the NH.sub.3 added to the exhaust
gas stream without influencing the SCR activity by the presence of
other molecules, for example hydrocarbons. The molecular sieve-like
action of the Cu- and/or Fe-exchanged zeolite used keeps such
hydrocarbons away from the active sites over which the reactions
take place. This causes a significant rise in (long-term) activity
of the SCR catalysts used in the process according to the
invention.
[0017] Zeolites may have differently structured channels in one and
the same material. The channel widths for the particular orifices
of the channels may therefore have different lower and upper
channel widths (definition: crystallographic free diameters of the
channels in .ANG.) (Ch. Baerlocher, Atlas of Zeolite Framework
Types, 5th revised edition, 2001, ISBN: 0-444-50701-9). In these
cases, several larger lower channel widths arise for the particular
kinds of channels. The same applies when mixtures of such materials
find use. In these cases, the larger lower channel widths are based
on the material with the smaller channel widths in each case. It is
thus sufficient when at least one channel width of the zeolite used
has a channel width within the specified range of 2.6 .ANG.- 4.2
.ANG..
[0018] When catalysts (based on their presence in the wash coat as
a catalytically active component) based on mixtures of zeolites are
used, this means that inventive embodiments predominate when at
least 40% by weight of the catalyst consists of Cu- and/or
Fe-exchanged zeolites in which a maximum lower channel width of 2.6
.ANG.-4.2 .ANG. exists. Further preferably, the catalyst contains
at least 50% by weight, more preferably at least 60% by weight,
especially preferably at least 70% by weight and most preferably at
least 80% by weight of such materials.
[0019] Within the limits specified, the person skilled in the art
is free to select the larger lower channel widths of the zeolites
used. What is important is that the maximum lower channel width of
the zeolite used is selected such that ammonia and nitrogen oxides
still find access to the active sites within the zeolite, but
hydrocarbons are as far as possible prevented from diffusing into
the channels. In the case of zeolites with a plurality of kinds of
channels, the maximum lower channel width is crucial here.
[0020] Zeolites commonly refer to crystalline aluminosilicates with
a porous framework structure composed of corner-linked AlO.sub.4
and SiO.sub.4 tetrahedra (W. M. Meier, Pure & Appl. Chem. 58,
1986, 1323-1328). By their nature, these have a negative excess
charge in the lattice, which must be balanced by intercalation of
positive ions (e.g. H.sup.+, Na.sup.+, NH.sub.4.sup.+). The ions
can be selected freely. In the present case, some Fe or Cu ions are
selected as counterions (see above). How many ions can be
intercalated is also guided by the ratio of aluminum to silicon
atoms in the crystal lattice. For the present invention, it is
advantageous when the molar ratio of SiO.sub.2 to Al.sub.2O.sub.3
in the zeolite is in the range from 5 to 100. A range from 10 to 60
is preferred, and from 15 to 45 is very particularly preferred.
[0021] Zeolites which satisfy the requirements specified are
familiar to those skilled in the art. Zeolites usable in accordance
with the invention can be found, for example, in the literature Ch.
Baerlocher, Atlas of Zeolite Framework Types, 5th revised edition,
2001, ISBN: 0-444-50701-9. The materials in the present case are
Fe- and/or Cu-exchanged zeolites. These and preparation thereof are
described especially in the literature (K. Sugawara, Appl. Catal.
B. 69, 2007, 154-163; W. Arous et al., Top. Catal. 42-43, 2007,
51-54; Ishihare et al., J. Catal. 169, 1997, 93-102). Preference is
given to zeolites selected from the group consisting of ferrierite,
chabazite and erionite. Very particular preference is given to the
use of ferrierite.
[0022] In the context of the invention, "catalyst based on a Cu-
and/or Fe-exchanged zeolite" means that the zeolite has Cu and/or
Fe in place of the positive counterions originally present. The
person skilled in the art can adjust the content of Fe and/or Cu
ions in the zeolite according to his or her specialist knowledge.
An advantageous value is 0.1-10% by weight of the ions based on the
weight of the zeolite. The ratio is preferably 1-8% by weight and
most preferably 1.5-6% by weight.
[0023] The use of the zeolites ferrierite, chabazite and erionite
as constituents of SCR catalysts is already known in the prior art.
Like zeolite beta, zeolite Y, zeolite A or mordenite, ferrierite,
chabazite and erionite exhibit a good activity in the selective
catalytic reduction of nitrogen oxides with ammonia in HC-free or
at least in low-HC diesel engine exhaust gas, "low-HC" exhaust gas
referring to one which has an HC content of not more than 30 ppm.
However, as soon as the hydrocarbon content in the exhaust gas
attains or exceeds a lower threshold of 50 ppm, zeolites with a
larger lower channel width of more than 4.2 .ANG. exhibit an ever
greater decline in activity in the comproportionation of NO.sub.x
with NH.sub.3 with increasing catalyst load. This decline in
activity is particularly marked for SCR catalysts based on zeolite
beta, zeolite Y, zeolite A or mordenite. Copper (Cu)- and/or
iron-exchanged zeolites of the ferrierite, chabazite and erionite
structure type surprisingly do not exhibit this decline in activity
in hydrocarbon-containing diesel engine exhaust gases. In contrast
to the aforementioned zeolites beta, A, Y and MOR, their properties
are such that the hydrocarbons present in the exhaust gas, owing to
the molecular sieve-like action of the zeolites, are kept away from
the catalytically active sites in the catalyst over which the
reactions take place. In this way, the reversible blockage of the
catalytically active sites is prevented, and aging phenomena which
can arise as a result of thermal regeneration effects on the
zeolite and which significantly reduce the long-term stability of
conventional SCR catalysts in HC-containing diesel engine exhaust
gases are avoided.
[0024] The advantages of the process according to the invention
become particularly clear when the hydrocarbon content in the
exhaust gas before the reaction of NO.sub.x with the ammonia
NH.sub.3 added to the exhaust gas stream over the SCR catalyst is
at least 50 ppm. The advantage of the invention is very
particularly clear in the case of hydrocarbon contents in the
diesel engine exhaust gas upstream of the SCR catalyst of at least
100 ppm. This is especially true when the hydrocarbon content in
the exhaust gas before the reaction of NO.sub.x with NH.sub.3 over
the SCR catalyst (in step b)) has transient peak values of at least
300 ppm or 500 ppm. It is pointed out that, according to the
application and mode of operation of the vehicle, it is by no means
rare for HC peak values upstream of the SCR catalyst of 1000 ppm or
more to be observed. In heavy goods vehicle applications and in
diesel engine-operated machinery, HC peaks of up to 2% by volume
are observed in the exhaust gas upstream of the SCR catalyst. In
such cases, use of HC-resistant SCR catalysts, as form the basis of
the process according to the invention, is essential since the
"conventional" SCR catalysts based on zeolite beta, A, Y or MOR
become saturated with hydrocarbons under these conditions and no
longer exhibit any NO.sub.x conversion with ammonia whatsoever.
[0025] The inventive catalysts comprising Cu- and/or Fe-exchanged
zeolites, the properties of which are such that they keep the
hydrocarbons present in the exhaust gas away from the active sites
in the catalyst over which the reactions take place by virtue of
their molecular sieve-like action, preferably do not make any
crucial contribution to the reduction in the hydrocarbon content in
the exhaust gas under regular running cycle conditions. In that
case, the process according to the invention is characterized in
that the hydrocarbon content in the exhaust gas is not
significantly reduced after the reaction of NO.sub.x over the SCR
catalyst in step b) at an exhaust gas temperature at the inlet of
the SCR catalyst of up to 180.degree. C., when said SCR catalyst
comprises a zeolite exchanged with copper (Cu). When the inventive
SCR catalyst comprises iron instead of copper as the exchange ion,
the hydrocarbon content in the exhaust gas is not significantly
reduced after the reaction of NO.sub.x over the SCR catalyst in
step b) at an exhaust gas temperature at the inlet of the SCR
catalyst of up to 270.degree. C.
[0026] The process according to the invention can be performed in
an apparatus in which an oxidation catalyst and/or an optionally
coated particulate filter are arranged upstream of the SCR system,
i.e. upstream of the inventive SCR catalyst and the corresponding
injection device for a reducing agent (NH.sub.3 or precursor
compound). Oxidation catalysts suitable for this purpose can be
found in the literature (EP 1 255 918 B1, US 20050201916). Suitable
particulate filters can likewise be found in the literature (EP 1
250 952 A1, WO 2006/021337 A1). The apparatus for performing the
process according to the invention may also comprise, downstream of
the SCR catalyst, an oxidation catalyst which helps to prevent any
NH.sub.3 slippage present (WO 2007/004774 A1). Also conceivable are
exhaust gas apparatuses in which an SCR system is arranged as
described in the exhaust gas line upstream of an oxidation catalyst
and/or an optionally coated particulate filter. Likewise
conceivable is the arrangement in which the inventive SCR system
with a catalyst based on a Cu- and/or Fe-exchanged zeolite is
arranged between the oxidation catalyst and optionally coated
particulate filter.
[0027] The injection apparatuses used can be selected as desired by
the person skilled in the art. Suitable systems can be found in the
literature (T. Mayer, Feststoff-SCR-System auf Basis von
Ammoniumcarbamat [Solid-state SCR system based on ammonium
carbamate], Thesis, Technical University of Kaiserslautern, 2005).
The ammonia can be introduced into the exhaust gas stream via the
injection apparatus as such or in the form of a compound which
gives rise to ammonia under the ambient conditions. Useful
compounds of this kind include aqueous solutions of urea or
ammonium formate, and likewise solid ammonium carbamate. These can
be drawn from a source provided, which is known per se to the
person skilled in the art (FIG. 2), and supplied to the exhaust gas
stream in a suitable manner. The person skilled in the art more
preferably uses injection nozzles (EP 0311758 A1). By means
thereof, the optimal ratio of NH.sub.3/NO.sub.x is established, in
order that the nitrogen oxides can be converted very substantially
to N.sub.2.
[0028] The exhaust gas originating from the combustion operation,
for example once it has passed over an optional oxidation catalyst
and/or an optionally coated particulate filter, is supplied with
ammonia or the precursor compound in appropriate amounts via the
injection apparatus. Subsequently, it is passed over the SCR
catalyst. The temperature over the SCR catalyst should be between
150.degree. C. and 500.degree. C., preferably between 200.degree.
C. and 400.degree. C. or between 180.degree. C. and 380.degree. C.,
in order that the reduction can take place as completely as
possible. Particular preference is given to a temperature range
from 225.degree. C. to 350.degree. C. for the reduction. In
addition, optimal nitrogen oxide conversions are achieved only when
a molar ratio of nitrogen monoxide to nitrogen dioxide is present
(NO/NO.sub.2=1) or the NO.sub.2/NO.sub.x ratio=0.5 (G. Tuenter et
al., Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 633-636). Optimal
conversions beginning with 75% conversion at only 150.degree. C.
with the same optimal selectivity for nitrogen are achieved
according to the stoichiometry of the reaction equation
2 NH.sub.3+NO+NO.sub.2>2 N.sub.2+3 H.sub.2O
only with an NO.sub.2/NO.sub.x ratio of 0.5. This applies not only
to SCR catalysts based on iron-exchanged zeolites but to all
common, i.e. commercially available, SCR catalysts.
[0029] Metal-exchanged beta-zeolites (maximum lower channel width
6.6 .ANG.) as catalyst materials exhibit a high activity for the
reduction of NO.sub.x by means of ammonia in what are known as SCR
systems. In the presence of hydrocarbons in the exhaust gas, they
exhibit, as described, however, a severe decline in activity
(example 3, FIG. 4 vs. FIG. 3). According to the inventors'
findings, this is attributable to a poisoning or storage effect, in
which the intercalated hydrocarbons occupy the active sites of the
catalyst and thus prevent the desired comproportionation reaction.
The process is reversible in principle, but every regeneration of
the beta-zeolite-based catalyst causes a degradation of the SCR
activity owing to thermally induced aging processes.
[0030] For further illustration of the advantageous use of
metal-exchanged zeolites with a maximum lower channel width of 4.2
.ANG. as the SCR catalyst, the test described below, consisting of
five phases, was carried out on a model gas system (example 4).
[0031] The example used is an SCR catalyst based on Fe-ferrierite
(3.5% Fe), and the comparative example an Fe-beta-zeolite (referred
to as Fe-beta--likewise 3.5% Fe). To illustrate the test procedure,
the metering and temperature profile of the test is shown in FIG.
5. The profiles of the NO.sub.x conversions of the two catalysts
discussed below are shown in FIG. 6, and the corresponding profiles
of HC and CO concentrations in the exhaust gas in FIGS. 7 and 8. It
should be pointed out that the wash coat loadings of the two SCR
catalyst specimens are different (165 g/l for Fe-ferrierite vs. 229
g/l for beta-zeolite).
[0032] Phase I: Determination of NO.sub.x conversion without
hydrocarbons (T=300.degree. C., 250 ppm NO, 250 ppm NO.sub.2, 500
ppm NH.sub.3, 5% by vol. O.sub.2, 1.3% by vol. H.sub.2O, balance
N.sub.2, space velocity 50 000 h.sup.-1). An NO.sub.x conversion of
98% is achieved for Fe-ferrierite, and 99% for Fe-beta.
[0033] Phase II: Duration 1800 s. A mixture of toluene and dodecane
(1:1 w/w) containing 1000 ppm of Cl is added to the feed gas from
phase I. There is barely any effect on the NO.sub.x conversion of
the Fe-ferrierite (94%), whereas the conversion for Fe-beta
declines significantly to 55%. While the amount of HC in the
exhaust gas rapidly breaks through the feed value of 1000 ppm for
Fe-ferrierite, Fe-beta stores hydrocarbons over the entire duration
of phase II.
[0034] Phase III: Duration 1800 s. The toluene/dodecane mixture is
removed again from the feed gas; the test conditions are thus the
same as in phase I. The NO.sub.x conversion of Fe-ferrierite
remains stable at a high level of 95%, whereas the conversion for
Fe-beta recovers gradually to only 85% over the course of this
phase.
[0035] Phase IV: Duration 200 s. Regeneration of the catalyst at
500.degree. C. Whereas only small amounts of thermally desorbed
hydrocarbons and CO (product of the burnoff of dodecane and
toluene) are detected in the exhaust gas for Fe-ferrierite, it is
found for Fe-beta that the hydrocarbons stored during phase II now
desorb or burn off.
[0036] Phase V: Same conditions as in phase I. For Fe-ferrierite,
the NO.sub.x conversion remains stable at 95%; for Fe-beta, a value
of 93% is determined after the regeneration in phase IV.
[0037] In addition to the decline in SCR activity in the presence
of hydrocarbons, a further undesired effect which occurs in the
case of wide-pore zeolites, particularly in the fresh state, is
significant exothermicity when the hydrocarbons stored on the
catalyst ignite and burn off (see example 5). As a result of the
temperature rise that this causes, the catalyst undergoes aging,
which leads to a reduction in the activity thereof. This is not the
case for zeolites used in accordance with the invention, for
example ferrierite, even in the fresh state. Since only a small
amount of hydrocarbons at most is stored on the catalyst, only a
small temperature rise on the catalyst occurs when they burn off.
The damage to the catalyst is thus also reduced. When, for example,
a Cu-beta-zeolite (Cu-beta) with a maximum lower channel width of
6.6 .ANG. and a Cu-ferrierite with a maximum lower channel width of
4.2 .ANG. are contacted with hydrocarbons, and the hydrocarbons are
ignited by increasing the reactor temperature from 100.degree. C.
to 400.degree. C. in an oxygenous atmosphere, a temperature peak of
more than 700.degree. C. in the exhaust gas occurs in the case of
Cu-beta-zeolite owing to the burnoff of a large stored amount of
intercalated hydrocarbons, whereas the exhaust gas temperature for
Cu-ferrierite follows the reactor temperature without a temperature
peak (FIG. 9).
DESCRIPTION OF THE FIGURES
[0038] FIG. 1: The schematic diagram of the connection between
particulate emission and NO.sub.x emission in the untreated exhaust
gas of an internal combustion engine operated under predominantly
lean conditions and the limits valid according to EU-IV/V; [0039]
(1): Reduction in untreated particulate emission down to below the
given limit by measures within the engine>reduction in NO.sub.x
emission by exhaust gas aftertreatment (denoxing); [0040] (2):
Reduction in untreated NO.sub.x emission down to below the given
limit by measures within the engine>reduction in particulate
emission (diesel particulate filter); [0041] (3): Calibration of
combustion within the engine according to the aspect of optimizing
performance>reduction in particulate and NO.sub.x emission by
exhaust gas aftertreatment measures to achieve the given
limits.
[0042] FIG. 2: The schematic diagram of a preferred embodiment of
the inventive apparatus, comprising (1) the injection apparatus for
addition of ammonia or of an ammonia-generating precursor compound
to the exhaust gas stream (flow direction indicated by ">") from
a source (2) which does not form part of the exhaust gas line, an
SCR catalyst (3) which effectively catalyzes the comproportionation
of the nitrogen oxides with ammonia within a temperature range
between 150.degree. C. and 500.degree. C., and an optional
oxidation catalyst (4) which helps to prevent any NH.sub.3 slippage
present as a result of oxidation of the NH.sub.3 to nitrogen and
water.
[0043] FIG. 3: SCR activity of Cu-ferrierite (maximum lower channel
width 4.2 .ANG.) under 500 ppm of NO, 450 ppm of NH.sub.3, 5%
O.sub.2, 1.3% H.sub.2O and nitrogen (blue) without and (pink) with
200 ppm of propene and 200 ppm of CO.
[0044] FIG. 4: SCR activity of Cu-beta SCR catalyst (maximum lower
channel width 6.6 .ANG.) under 500 ppm of NO, 450 ppm of NH.sub.3,
5% O.sub.2, 1.3% H.sub.2O and nitrogen (blue) without and (pink)
with 200 ppm of propene and 200 ppm of CO (comparative
example).
[0045] FIG. 5: Temperature profile of the model gas test from
example 4, divided into five phases, and metered concentrations of
the NO, NO.sub.2, NH.sub.3 and hydrocarbon components.
[0046] FIG. 6: Profile of the NO.sub.x conversion of the test from
example 4 for an Fe-ferrierite and Fe-beta SCR catalyst according
to example 2.
[0047] FIG. 7: Profile of the HC concentrations in the exhaust gas
of the test from example 4 for an Fe-ferrierite and Fe-beta SCR
catalyst according to example 2.
[0048] FIG. 8: Profile of the CO concentrations in the exhaust gas
of the test from example 4 for an Fe-ferrierite and Fe-beta SCR
catalyst (comparative example) according to example 2.
[0049] FIG. 9: Exhaust gas temperature downstream of a Cu-beta SCR
catalyst (comparative example) and a Cu-ferrierite SCR catalyst
after ignition of hydrocarbons stored thereon by increasing the
reaction temperature according to example 5.
EXAMPLES
Example 1
General Preparation of the Catalyst
[0050] A zeolite with a maximum lower channel width of not more
than 4.2 .ANG. is impregnated in a Lodige with copper and/or iron.
After drying, the powder is calcined at 500.degree. C. for 2 hours.
The powder or a mixture of different powders of this kind is
slurried in water and a binder is added (10% by weight of SiO.sub.2
sol, commercially available). Thereafter, the wash coat obtained is
used to coat a monolithic catalyst substrate which is calcined at
500.degree. C. for 2 hours. Drill cores are taken from the monolith
for model gas tests. This procedure was used to prepare the
catalysts for example 3 in FIG. 3 (ferrierite with 5% Cu) and the
comparative example in FIG. 4 (beta with 5% Cu).
Example 2
General Preparation of the Catalyst
[0051] A mixture of silica- and alumina-based binders (SiO.sub.2
sol, commercially available; boehmite, commercially available) is
initially charged in water. The parent zeolite of the SCR catalyst
having a maximum lower channel width of not more than 4.2 .ANG. is
slurried therein. Thereafter, an amount of a suitable iron and/or
copper salt corresponding to the desired metal content is added to
the suspension. After grinding, the wash coat thus obtained is used
to coat a monolithic substrate, and the coated substrate is
calcined. Drill cores are taken from the monolith for model gas
tests. This method was used to prepare the Fe-ferrierites shown in
FIGS. 5 to 8 and to prepare the corresponding comparative example
(Fe-beta, likewise FIGS. 5 to 8).
Example 3
Determination of SCR Activity With and Without Hydrocarbon (FIGS. 3
and 4)
[0052] The drill core to be studied, produced according to example
1, after hydrothermal aging (16 hours at 750.degree. C., 10%
O.sub.2, 10% H.sub.2O, balance N.sub.2, space velocity 2200
h.sup.-1), was studied in a model gas test. For this purpose, in a
descending temperature sequence within the temperature range from
150.degree. C. to 500.degree. C., the NO conversion was determined
under steady-state conditions under 500 ppm of NO, 450 ppm of
NH.sub.3, 1.3% by vol. of H.sub.2O, 5% by vol. of O.sub.2, balance
N.sub.2, space velocity 30 000 h.sup.-1. This test was performed
once in the presence of 200 ppm of propene and 200 ppm of CO in
feed gas, and once in the absence of these substances.
Example 4
Determination of SCR Activity With and Without Hydrocarbon (FIGS. 5
to 8)
[0053] A drill core produced according to example 2, after
hydrothermal aging (48 hours at 650.degree. C., 10% O.sub.2, 10%
H.sub.2O, balance N.sub.2, space velocity 2200 h.sup.-1), was
studied in a model gas test consisting of five phases. [0054] Phase
I: Determination of NO conversion without hydrocarbons
(T=300.degree. C., 250 ppm of NO, 250 ppm of NO.sub.2, 500 ppm of
NH.sub.3, 5% by vol. of O.sub.2, 1.3% by vol. of H.sub.2O, balance
N.sub.2, space velocity 50 000 h.sup.-1). [0055] Phase II: Duration
1800 s. A mixture of toluene and dodecane (1:1 w/w) with 1000 ppm
of Cl is added to the feed gas from phase I. [0056] Phase III:
Duration 1800 s. The toluene/dodecane mixture is removed again from
the feed gas; the test conditions are thus now the same as in phase
I again. [0057] Phase IV: Duration 200 s. Regeneration of the
catalyst by heating the reactor at 500.degree. C. and cooling again
to 300.degree. C. [0058] Phase V: Same conditions as in phase
I.
Example 5
Determination of Exothermicity Resulting from Stored Hydrocarbons
(FIG. 9)
[0059] A drill core, produced according to example 1, of the
catalyst to be studied (diameter 1 inch, length 3 inches) is
contacted with hydrocarbons on an engine test bed at 100.degree. C.
for a period of 60 minutes. Thereafter, the drill core is
preconditioned in a model gas system at reactor temperature
100.degree. C. for 10 minutes (10% O.sub.2, 10% CO.sub.2, 5%
H.sub.2O, balance N.sub.2, total flow rate 4 m.sup.3/h), then the
reactor temperature is raised to 400.degree. C. with the same gas
mixture within 30 seconds. The temperature of the exhaust gas 3
inches beyond the drill core is evaluated as a measure of the
exothermicity which arises.
* * * * *