U.S. patent application number 11/946620 was filed with the patent office on 2008-05-29 for oxidation catalyst and exhaust-gas purification system using the same.
This patent application is currently assigned to ICT CO., LTD.. Invention is credited to Masanori Ikeda, Naohiro KATO.
Application Number | 20080124264 11/946620 |
Document ID | / |
Family ID | 39148612 |
Filed Date | 2008-05-29 |
United States Patent
Application |
20080124264 |
Kind Code |
A1 |
Ikeda; Masanori ; et
al. |
May 29, 2008 |
Oxidation Catalyst and Exhaust-Gas Purification System Using the
Same
Abstract
An oxidation catalyst that efficiently promotes oxidation of NO
to NO.sub.2 even in a low temperature range, and an exhaust-gas
purification system and method that efficiently removes exhaust-gas
components even in a low temperature range are provided. This
invention provides an oxidation catalyst comprising platinum and
palladium as catalytically active components, which promotes
oxidation of nitrogen monoxide to nitrogen dioxide, wherein the
oxidation catalyst comprises 1 to 55% by weight of the palladium
relative to 100% by weight of the platinum.
Inventors: |
Ikeda; Masanori;
(Akashi-shi, JP) ; KATO; Naohiro; (Himeji-shi,
JP) |
Correspondence
Address: |
BUCHANAN, INGERSOLL & ROONEY PC
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
ICT CO., LTD.
Osaka-shi
MI
INTERNATIONAL CATALYST TECHNOLOGY, INC.
Auburn Hills
|
Family ID: |
39148612 |
Appl. No.: |
11/946620 |
Filed: |
November 28, 2007 |
Current U.S.
Class: |
423/213.5 ;
422/180; 502/333; 502/339; 502/74 |
Current CPC
Class: |
B01D 2251/2062 20130101;
B01J 23/745 20130101; B01D 53/944 20130101; B01J 37/0248 20130101;
B01J 23/44 20130101; F01N 2240/40 20130101; F01N 3/206 20130101;
Y02T 10/12 20130101; B01D 53/9477 20130101; B01D 2255/1023
20130101; B01J 35/10 20130101; B01J 37/0246 20130101; B01J 37/0036
20130101; B01J 35/04 20130101; B01D 2255/1021 20130101; B01J
35/0006 20130101; B01D 53/9418 20130101; B01D 2258/01 20130101 |
Class at
Publication: |
423/213.5 ;
502/339; 502/74; 502/333; 422/180 |
International
Class: |
B01D 53/56 20060101
B01D053/56; B01J 23/42 20060101 B01J023/42 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 29, 2006 |
JP |
2006-322490 |
Claims
1. An oxidation catalyst comprising platinum and palladium as
catalytically active components, which promotes oxidation of
nitrogen monoxide to nitrogen dioxide, wherein the oxidation
catalyst comprises 1 to 55% by weight of the palladium relative to
100% by weight of the platinum.
2. The oxidation catalyst according to claim 1, wherein the
catalytically active components are supported on a refractory
inorganic oxide to form a catalytically active component-supporting
inorganic oxide which is further supported on a three-dimensional
structure.
3. The oxidation catalyst according to claim 1, wherein the
refractory inorganic oxide is at least one member selected from the
group consisting of activated alumina, alumina-silica, zirconia,
titania and zeolite.
4. The oxidation catalyst according to claim 1, wherein the
three-dimensional structure is a honeycomb structure.
5. An exhaust-gas purification system, which comprises the
oxidation catalyst set forth in claim 1 in an exhaust-gas passage
of an internal-combustion engine.
6. The exhaust-gas purification system according to claim 5, which
comprises a reducing agent supplying apparatus for supplying a
reducing agent at the latter stage of the oxidation catalyst in the
exhaust-gas passage, and further a NO.sub.X reduction catalyst is
installed at the latter stage of the reducing agent supplying
apparatus in the exhaust-gas passage.
7. The exhaust-gas purification system according to claim 6, which
further comprises at least one member selected from the group
consisting of a diesel particulate filter, an agent of suppressing
slip of a reducing agent, a catalyst for hydrolyzing urea and
another oxidation catalyst at the latter stage of the oxidation
catalyst.
8. The exhaust-gas purification system according to claim 6,
wherein the reducing agent is at least one member selected among
urea, ammonia, light oil, dimethyl ether, methanol, ethanol and
propanol.
9. A process for purifying an exhaust gas discharged from an
internal-combustion engine, comprising (1) a step for converting
nitrogen monoxide in the exhaust gas to nitrogen dioxide by
bringing the above exhaust gas into contact with the oxidation
catalyst set forth in claim 1, and (2) a step for reducing the
nitrogen oxides in the exhaust gas by bringing the exhaust gas
obtained in the step (1) into contact with a NO.sub.X reduction
catalyst in the presence of a reducing agent.
10. The process according to claim 9, further comprising (3) a step
for removing at least a part of particulate matters.
11. A process for purifying an exhaust gas discharged from an
internal-combustion engine, comprising (1) a step for converting
nitrogen monoxide in the exhaust gas to nitrogen dioxide by
bringing the above exhaust gas into contact with the oxidation
catalyst set forth in claim 2, and (2) a step for reducing the
nitrogen oxides in the exhaust gas by bringing the exhaust gas
obtained in the step (1) into contact with a NO.sub.X reduction
catalyst in the presence of a reducing agent.
12. A process for purifying an exhaust gas discharged from an
internal-combustion engine, comprising (1) a step for converting
nitrogen monoxide in the exhaust gas to nitrogen dioxide by
bringing the above exhaust gas into contact with the oxidation
catalyst set forth in claim 3, and (2) a step for reducing the
nitrogen oxides in the exhaust gas by bringing the exhaust gas
obtained in the step (1) into contact with a NO.sub.X reduction
catalyst in the presence of a reducing agent.
13. A process for purifying an exhaust gas discharged from an
internal-combustion engine, comprising (1) a step for converting
nitrogen monoxide in the exhaust gas to nitrogen dioxide by
bringing the above exhaust gas into contact with the oxidation
catalyst set forth in claim 4, and (2) a step for reducing the
nitrogen oxides in the exhaust gas by bringing the exhaust gas
obtained in the step (1) into contact with a NO.sub.X reduction
catalyst in the presence of a reducing agent.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an oxidation catalyst and
an exhaust-gas purification system using the same, and in
particular, to an exhaust-gas purification system having excellent
performance in purifying particularly nitrogen oxides from an
internal combustion engine such as a diesel engine.
[0003] 2. Description of Related Art
[0004] As a method for purifying nitrogen oxides discharged from an
internal combustion engine such as a diesel engine, a method of
installing a reduction catalyst along the flow of an exhaust gas
and purifying NO.sub.X on a reduction catalyst by a reducing agent
supplied from front of the reduction catalyst has drawn attention.
Ammonia (NH.sub.3), urea ((NH.sub.2).sub.2CO), a hydrocarbon such
as light oil, and the like are used as the reducing agent. It is
known that the reaction for purifying NO.sub.X proceeds via
NO.sub.2 using any of the above reducing agents. It is known, for
example, that the reaction for purifying nitrogen oxides using urea
proceeds mostly according to following formulae (1) to (4) and that
formula (2) known as standard-SCR (selective catalytic reduction)
is a slower reaction than formula (3) known as fast-SCR (C. Scott
Sluder, et al., Low Temperature Urea Decomposition and SCR
Performance, SAE paper 2005-01-1858).
[0005] Most of the nitrogen oxides in the exhaust gas discharged
from an engine are NO when the engine is running at a low output
(low speed) (that is, when the exhaust gas is at low temperature),
while the equimolar amounts of NO and NO.sub.2 react in formula
(3). It is considered, therefore, that the reaction of formula (3)
is promoted by making the molar ratio of NO.sub.2 to NO close to
1:1 by forming NO.sub.2 when the exhaust gas is at low
temperature.
(NH.sub.2).sub.2CO+H.sub.2O.fwdarw.2NH.sub.3+CO.sub.2 (1)
4NH.sub.3+4NO+O.sub.2.fwdarw.4N.sub.2+6H.sub.2O (2)
2NH.sub.3+NO+NO.sub.2.fwdarw.2N.sub.2+3H.sub.2O (3)
2NO.sub.2+4NH.sub.3+O.sub.2.fwdarw.3N.sub.2+6H.sub.2O (4)
2NO+O.sub.2.fwdarw.2NO.sub.2 (5)
[0006] A method of making use of plasma is known as a technology
for forming NO.sub.2. A method for decreasing particulate matters
in an exhaust gas by using nitrogen dioxide and ozone formed by
means of generating plasma in the exhaust gas is disclosed
(US-A-2004-168429 (JP-A-2004-169643)).
[0007] Another technology for forming NO.sub.2 is a technology of
using an oxidation catalyst to promote the reaction of the above
formula (5). For example, platinum (Pt), palladium (Pd), ruthenium
(Ru), rhodium (Rh) and the like are proposed as a active component
of a catalyst for oxidizing NO in the exhaust gas to NO.sub.2 in
pursuit of removing diesel particulates (particulate matters) in
U.S. Pat. No. 4,902,487 (JP-A-1-318715). Specific data on the
metals other than Pt, however, are not disclosed. In addition, in
"Barry J. Cooper, et al., Role of NO in Diesel Particulate Emission
Control, SAE paper, 890404", it is shown that Pd, Rh, iridium (Ir)
and Ru catalysts have poor performance of NO oxidation and in
particular that Pd supported on Al.sub.2O.sub.3 has a NO oxidation
ratio of 0%.
[0008] On the other hand, as purification method of nitrogen
oxides, a purification method for an exhaust gas in which a part of
NO is oxidized to NO.sub.2 with a Pt catalyst and then the exhaust
gas is introduced to on a reduction catalyst together with ammonia,
noticing that most reduction catalyst has higher activity for
nitrogen dioxide than for nitrogen monoxide, is proposed in
US-A-2002-039550 (JP-A-2002-1067)
BRIEF SUMMARY OF THE INVENTION
[0009] It is necessary for the reactions of the above formulae (1)
to (4) to proceed efficiently in order to remove nitrogen oxides
efficiently. When the temperature of an exhaust gas is high as in
the case of running at high speed, the reaction of formula (2)
proceeds easily, which result in efficient removal of nitrogen
oxides. On the other hand, when the temperature of an exhaust gas
is low as in the case of running in urban area, as described above,
it is desirable to promote the reaction of formula (3) by promoting
the reaction of formula (5) in order to form NO.sub.2. In addition,
when the temperature of exhaust gases is low, the reaction of
formula (2) hardly proceeds as mostly the reaction of formula (3)
proceeds, which makes it an important subject to form NO.sub.2.
However, the reaction of formula (5) is so slow under a low
temperature condition even when the Pt catalyst disclosed in the
above U.S. Pat. No. 4,902,487 or US-A-2002-039550 is used that
sufficient effect for purifying nitrogen oxides and particulate
matters has not been obtained. In addition, when the temperature of
exhaust gases is low, it has been necessary in a conventional
method to elevate the temperature of the exhaust gas, which
sometimes has made a system complicated.
[0010] In addition, since the reaction for NO.sub.2 purification
represented by formula (4) proceeds even at relatively low
temperature, a greater demand for an oxidation catalyst for
converting NO to NO.sub.2 when an exhaust gas is at low temperature
has risen.
[0011] Further, installation of an apparatus for separately forming
NO.sub.2 as described in US-A-2004-168429 requires a
plasma-generating device and its peripheral devices to be
installed, which poses a problem of high equipment cost and power
cost and makes it unsuitable to be used for vehicles due to a
sufficient installation space to be required.
[0012] Therefore an object of the present invention is to provide
an oxidation catalyst for efficiently promoting oxidation of NO to
NO.sub.2 even in a low temperature range.
[0013] Another object of the present invention is to provide an
exhaust-gas purification system and method for efficiently removing
nitrogen oxides or particulate matters even in a low temperature
range.
[0014] The present inventors have found, after having intensively
studied a way to solve the above problem, that an oxidation
catalyst containing platinum and palladium as catalytically active
components which contains 1 to 55% by weight of the palladium
relative to 100% by weight of the platinum, efficiently promotes
oxidation of NO to NO.sub.2 even in a low temperature range, and
completed the present invention.
[0015] In other words, the present invention provides an oxidation
catalyst comprising platinum and palladium as catalytically active
components, which promotes oxidation of nitrogen monoxide to
nitrogen dioxide, wherein the oxidation catalyst comprises 1 to 55%
by weight of the palladium relative to 100% by weight of the
platinum.
[0016] In addition, the present invention provides an exhaust-gas
purification system, which comprises an oxidation catalyst
containing platinum and palladium as catalytically active
components and promoting oxidation of nitrogen monoxide to nitrogen
dioxide which contains 1 to 55% by weight of the palladium relative
to 100% by weight of the platinum, in the exhaust-gas passage of an
internal-combustion engine.
[0017] Further, the present invention provides an exhaust-gas
purification method for purifying the exhaust gas discharged from
an internal-combustion engine, having (1) a step for converting
nitrogen monoxide in the exhaust gas to nitrogen dioxide by
bringing the exhaust gas into contact with an oxidation catalyst
containing platinum and palladium as catalytically active
components which contains 1 to 55% by weight of the palladium
relative to 100% by weight of the platinum, and (2) a step for
reducing the nitrogen oxides in the exhaust gas by bringing the
exhaust gas obtained in the step (1) into contact with a NO.sub.X
reduction catalyst in the presence of a reducing agent.
[0018] Still further, the present invention provides an exhaust-gas
purification method for purifying the exhaust gas discharged from
an internal-combustion engine, having (1) a step for converting
nitrogen monoxide in the exhaust gas to nitrogen dioxide by
bringing the exhaust gas into contact with an oxidation catalyst
containing platinum and palladium as catalytically active
components which contains 1 to 55% by weight of the above palladium
relative to 100% by weight of the platinum, (2) a step for reducing
the nitrogen oxides in the exhaust gas by bringing the exhaust gas
obtained in the step (1) into contact with a NO.sub.X reduction
catalyst in the presence of a reducing agent, and (3) a step for
removing at least a part of particulate matters.
[0019] Since the oxidation catalyst of the present invention can
form NO.sub.2 even when an exhaust gas is at low temperature, it
can efficiently promote a reaction of a substance to be reacted
with NO.sub.2 even when an exhaust gas is at low temperature.
[0020] The above and other objects, features and advantages of the
present invention will become clear from the following description
of the preferred embodiments.
BRIEF DESCRIPTION OF DRAWINGS
[0021] FIG. 1 is a drawing showing the result of Evaluation Example
1.
[0022] FIG. 2 is a schematic drawing showing an example of the
exhaust-gas purification system of the present invention.
[0023] FIG. 3 is a schematic drawing showing an example of the
exhaust-gas purification system of the present invention.
[0024] FIG. 4 is a schematic drawing showing an example of the
exhaust-gas purification system of the present invention.
[0025] FIG. 5 is a schematic drawing showing an example of the
exhaust-gas purification system of the present invention.
[0026] FIG. 6 is a schematic drawing showing an example of the
exhaust-gas purification system of the present invention.
[0027] FIG. 7 is a schematic drawing showing an example of the
exhaust-gas purification system of the present invention.
[0028] FIG. 8 is a schematic drawing showing an example of the
exhaust-gas purification system of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0029] The first aspect of the present invention is an oxidation
catalyst containing platinum and palladium as catalytically active
components and promoting oxidation of nitrogen monoxide to nitrogen
dioxide, which is characterized by containing 1 to 55% by weight of
the above palladium relative to 100% by weight of the above
platinum.
[0030] As described above, Pt, Pd, Ru, Rh and the like are
disclosed as a catalytically active component for promoting
oxidation of NO to NO.sub.2, for example, in U.S. Pat. No.
4,902,487. However, only an effect of a catalyst containing Pt
alone as an active component is shown specifically in U.S. Pat. No.
4,902,487. Pt has been usually used as an active component for
forming NO.sub.2, because Pd used alone as a catalytically active
component has much poorer oxidation activity compared with Pt used
alone as described in "Barry J. Cooper, et al., Role of NO in
Diesel Particulate Emission Control, SAE paper, 890404". The
present invention has found that a catalyst made of a combination
of Pt and Pd has a synergistic effect on promotion of nitrogen
dioxide formation and also has found the range where an oxidation
catalyst functions effectively by specifying contents of the Pt and
Pd even under a low temperature condition. In addition, the
oxidation catalyst has an adequate activity after the oxidation
catalyst is used for a long period.
[0031] The ratio of the contents of Pt and Pd by weight is 1 to 55%
by weight of Pd relative to 100% by weight of Pt. Further, it is
preferably that the ratio of the contents of Pt and Pd by weight is
1 to 45% by weight, 1 to 35% by weight, 1 to 20% by weight, 4 to
20% by weight, 4 to 12% by weight, 5 to 12% by weight of Pd
relative to 100% by weight of Pt, in this order. An oxidation
catalyst containing these ranges of Pt and Pd promotes conversion
of NO to NO.sub.2 very efficiently even in a low temperature
range.
[0032] The amount (sum) of Pt and Pd to be used is preferably 0.1
to 20 g, more preferably 0.5 to 10 g per liter of a
three-dimensional structure. This range of the amount gives
excellent catalyst activity in initial and endurance operation.
[0033] The oxidation catalyst of the present invention may contain
other metals and oxides thereof as catalytically active components
as long as the catalyst contains Pt and Pd at the above specified
ratio. The metal includes specifically a metal such as ruthenium,
rhodium, iridium, gold, cobalt, nickel and copper, and an alloy
thereof, and the like. The content of other metals and oxides
thereof is usually 0 to 2000% by weight relative to 100% by weight
of catalytically active components.
[0034] Preferably, the oxidation catalyst of the present invention
has a structure where a catalytically active component is supported
on a refractory inorganic oxide to form a catalytically active
component-supporting inorganic oxide which is further supported on
a three-dimensional structure.
[0035] The catalytically active component which is supported on a
refractory inorganic oxide is not particularly limited, and
preferably is present in the state of catalyst metal particles
having a mean particle diameter of preferably 1 to 50 nm.
[0036] The refractory inorganic oxide to be used in the present
invention is not particularly limited as long as it is usually used
as a catalyst carrier, and includes activated alumina such as
.alpha.-, .gamma.-, .delta.-, .eta.- and .theta.-alumina; zirconia,
titania, zeolite, silica; or a composite oxide of these oxides such
as alumina-silica, alumina-titania, alumina-zirconia and
titania-zirconia. From the standpoint of improving catalyst
performance, activated alumina, alumina-silica, zirconia, titania
and zeolite are preferable and activated alumina is more
preferable. The above inorganic oxide may be used alone or in
combination of two or more. Preferably, the shape of the refractory
inorganic oxide is a powder.
[0037] The amount of the refractory inorganic oxide (excluding
active components) to be used is preferably 10 to 300 g, more
preferably 20 to 150 g per liter of a three-dimensional structure.
This range of the amount gives sufficient dispersion of a noble
metal and satisfactory durability.
[0038] The BET specific surface area of the refractory inorganic
oxide is preferably 50 to 750 m.sup.2/g, more preferably 150 to 750
m.sup.2/g. The mean primary particle diameter of the refractory
inorganic oxide powder is 0.5 to 150 .mu.m, preferably 1 to 100
.mu.m.
[0039] The three-dimensional structure to be used in the present
invention includes a heat-resistant carrier such as a honeycomb
carrier, and particularly, a honeycomb structure of an integrally
molded type is preferable, for example, a monolithic honeycomb
carrier, a metal honeycomb carrier, a plug honeycomb carrier and a
pellet carrier are included.
[0040] The monolithic carrier includes a carrier that is usually
called a ceramic honeycomb carrier, and particularly, a honeycomb
carrier made of material such as cordierite, mullite,
.alpha.-alumina, zirconia, titania, titanium phosphate, aluminum
titanate, betallite, spondumene, aluminosilicate, magnesium
silicate is preferable, and cordierite-base is particularly
preferable among these. In addition, a three-dimensional structure
made of a heat-resistant metal of oxidation resistance such as
stainless steel and a Fe--Cr--Al alloy is used.
[0041] These monolithic carriers are manufactured by an extrusion
molding method, a method of rolling up a sheet-shaped element, and
the like. The shape of a gas passage (cell shape) may be any of a
hexagon, a quadrangle, a triangle and a corrugated shape. The
carrier having a cell density (number of cell/cross-sectional area)
of 100 to 600 cells/square inch can be satisfactorily used, and
preferably 200 to 500 cells/square inch.
[0042] The oxidation catalyst of the present invention may contain
an alkaline metal such as potassium, an alkaline-earth metal such
as magnesium and barium, a rare-earth metal such as lanthanum and
cerium, and an metal oxide thereof, or zeolite, and the content
thereof is usually 0 to 100 g per liter of a three-dimensional
structure.
[0043] The production method for the oxidation catalyst of the
present invention is not particularly limited, and will be
described below with specific examples.
[0044] Firstly, as starting raw materials of Pt and Pd, inorganic
salts such as a chloride (halide), a nitrate, a sulfate, an
ammonium salt, an amine, a carbonate, a bicarbonate, a nitrite and
an oxalate of Pt and Pd; a carboxylate such as a formate, a
hydroxide, an alkoxide, and an oxide of Pt and Pd; preferably a
chloride, a nitrate, an ammonium salt, an amine and a carbonate are
prepared. Specifically, in the case that the active component is
Pt, dinitrodiammine platinate nitrate, chloroplatinic acid
(hexachloroplatinic acid), platinum nitrate and the like are
included, and in the case that the active component is Pd,
palladium nitrate, palladium chloride are included. The above raw
material and a refractory inorganic oxide powder are subjected to
wet milling to prepare an aqueous slurry. A three-dimensional
structure is coated with the slurry, dried usually at 50 to
150.degree. C. for 30 minutes to 8 hours and then calcined at
usually 300 to 800.degree. C., preferably 400 to 600.degree. C. for
15 minutes to 2 hours, preferably 30 minutes to an hour to obtain
the oxidation catalyst of the present invention. Thus obtained
oxidation catalyst having a metal-particle diameter of a
catalytically active component of 1 to 50 nm gives excellent
oxidation performance. The oxidation performance of the oxidation
catalyst having a metal-particle diameter smaller than 1 nm is
undesirably impaired due to interaction with a carrier, whereas the
oxidation catalyst having a metal-particle diameter larger than 50
nm has undesirably less metal surface area resulting in less
numbers of metal atoms to be used for catalytic reaction.
[0045] The second aspect of the present invention is an exhaust-gas
purification system wherein the oxidation catalyst of the first
aspect of the present invention is installed in the exhaust-gas
passage of an internal-combustion engine. Since the oxidation
catalyst of the first aspect of the present invention efficiently
converts NO to NO.sub.2 even in a low temperature range as
described above, the exhaust-gas purification system of the second
aspect of the present invention can efficiently remove nitrogen
oxides even under a low temperature condition.
[0046] The embodiments of the present invention will be described
hereinafter with reference to the drawings. The technical scope of
the present invention should be defined by the scope of claims and
is not limited to the following embodiments.
[0047] FIG. 2 is a schematic drawing showing an embodiment of the
exhaust-gas purification system of the second aspect of the present
invention. The exhaust-gas purification system shown in FIG. 2 is
equipped with an oxidation catalyst 2 installed in the exhaust-gas
passage of an internal-combustion engine, a apparatus (reducing
agent injection nozzle 8, pump 9, reducing agent tank 10) supplying
a reducing agent to latter stage of the oxidation catalyst, and a
NO.sub.X reduction catalyst 3 in the exhaust-gas passage at latter
stage of the reducing agent supplying apparatus. The
internal-combustion engine 1 and the pump 9 are controlled by a
controller 4. A temperature controller is composed of the
controller 4 and thermocouples 5 to 7.
[0048] The exhaust gas discharged from the internal-combustion
engine 1 is introduced through the exhaust-gas passage to the
oxidation catalyst 2 installed in the exhaust-gas passage. The
exhaust gas contains NO and NO.sub.2 as nitrogen oxides, most of
which is NO under a low temperature condition. By contacting with
the oxidation catalyst 2, the conversion reaction of NO in the
exhaust gas to NO.sub.2 proceeds.
[0049] The reducing agent stored in the reducing agent tank 10 is
then supplied from the reducing agent injection nozzle 8 to the
exhaust-gas passage with actuation of the pump 9.
[0050] Ammonia, urea, a hydrocarbon such as light oil, an alcohol
of 1 to 3 carbon atoms and an ether of 2 to 6 carbon atoms can be
used as the reducing agent, but from the standpoint of handling,
preferably at least one selected from urea, light oil, dimethyl
ether, methane, ethane, propane, hexane, ethylene, propylene,
hexene, gasoline, kerosene, fuel oil A (JIS), fuel oil C (JIS),
methanol, ethanol and propanol, more preferably at least one kind
selected from urea, ammonia, light oil, dimethyl ether, methanol,
ethanol and propanol, still more preferably urea, light oil,
dimethyl ether and ethanol, and particularly preferably urea and
light oil are used. It is preferable in the case of use of urea
that a catalyst for hydrolyzing urea 14 is installed downstream as
shown in FIG. 3 or the NO.sub.X reduction catalyst to be described
later has a function for hydrolyzing urea. In addition, it is
preferable that urea water is injected from the injection nozzle 8
as shown in FIG. 3.
[0051] Subsequently, remaining NO and NO.sub.2 are converted to
N.sub.2 on the NO.sub.X reduction catalyst 3 installed in the
exhaust-gas passage. The nitrogen oxides in the exhaust gas are
efficiently removed even under a low temperature condition by the
system comprising the above plural stages. It should be noted that
a catalyst of suppressing slip of a reducing agent 11 is installed
at the latter stage of the NO.sub.X reduction catalyst in FIG.
3.
[0052] In addition, the temperature controller composed of the
controller 4 and the thermocouples 5 to 7 shown in FIG. 2 can
control the temperature of the exhaust gas at each stage. The
temperature of the exhaust gas may be controlled to be preferably
140.degree. C. or higher, more preferably 160.degree. C. or higher
before the exhaust gas is introduced to the oxidation catalyst
2.
[0053] In addition, in the case that the reducing agent is urea,
the temperature of the exhaust gas is controlled to be preferably
135.degree. C. or higher, more preferably 160.degree. C. or higher
before the exhaust gas is introduced to the reduction catalyst 3.
In such a temperature range, preferably, thermal decomposition of
urea represented by the following formula (6) or hydrolysis of
isocyanic acid, which is generated by the thermal decomposition of
urea, represented by the following formula (7) proceeds easily.
(NH.sub.2).sub.2CO.fwdarw.NH.sub.3+HNCO (6)
HNCO+H.sub.2O.fwdarw.NH.sub.3+CO.sub.2 (7)
[0054] In the case that the reducing agent is a hydrocarbon, the
temperature of the exhaust gas before the exhaust gas is introduced
to the NO.sub.X reduction catalyst 3 is preferably the boiling
point of the hydrocarbon or higher. In the case that the reducing
agent is light oil, the temperature of the exhaust gas is
controlled to be preferably 200.degree. C. or higher, more
preferably 250 to 550.degree. C. In such a temperature range,
preferably, the liquid hydrocarbon hardly sticks to the surface of
the reduction catalyst leading to efficient removal of
NO.sub.X.
[0055] The NO.sub.X reduction catalyst is not particularly limited
as long as it is usually known, and may be selected as appropriate
according to the vehicle weight, the engine displacement and the
like, to be mounted. In the case that the reducing agent is urea, a
zeolite catalyst exchanged by a specific transition metal is
preferably used. In addition, in the case that the reducing agent
is a hydrocarbon such as light oil, an alcohol of 1 to 3 carbon
atoms and an ether of 2 to 6 carbon atoms, a noble metal such as
Pt, Pd and Rh supported on an inorganic carrier such as alumina,
silica and alumina-silica is preferably used in a low temperature
range of 300.degree. C. or lower, whereas a zeolite catalyst
exchanged by a specific transition metal is preferably used in a
high temperature range of 300.degree. C. or higher.
[0056] The zeolite catalyst exchanged by a transition metal is
preferably supported on a three-dimensional structure. As the
three-dimensional structure, the structure described in column of
the above oxidation catalyst can be used.
[0057] The transition metal is not limited, and includes copper,
iron, cerium, vanadium, chromium, nickel and oxides thereof,
preferably copper, iron, vanadium and oxides thereof.
[0058] The amount of the transition metal to be used is preferably
1 to 80 g, more preferably 2 to 40 g per liter of a
three-dimensional structure. This range of the amount gives
excellent catalyst activity in initial and endurance operation. It
should be noted, the zeolite catalyst exchanged by a transition
metal includes not only the zeolite having the form where a
transition metal is ion exchanged, but also the zeolite having both
of the form where a transition metal is ion exchanged and the form
where a transition metal is coated. The above amount of the
transition metal to be used indicates the sum of the ion-exchanged
transition metal and the transition metal coated on the
zeolite.
[0059] The zeolite to be used is not particularly limited, and
includes BEA type, MFI type, FER type, FAU type and MOR type
zeolite, preferably BEA type and MFI type zeolite.
[0060] The amount of zeolite to be used (excluding active
components) is preferably 10 to 300 g, preferably 50 to 300 g per
liter of a three-dimensional structure. This range of the amount
gives sufficient dispersion of a transition metal and satisfactory
durability. In addition, the BET specific surface area of zeolite
is preferably 50 to 750 m.sup.2/g, more preferably 150 to 750
m.sup.2/g. In addition, the mean primary particle diameter of
zeolite is preferably 0.5 to 150 .mu.m, more preferably 1 to 100
.mu.m.
[0061] The NO.sub.X reduction catalyst to be used in the present
invention may contain an alkaline metal such as potassium, an
alkaline-earth metal such as magnesium and barium, a rare-earth
element such as lanthanum and cerium, and oxides thereof or
zeolite.
[0062] The production method for the NO.sub.X reduction catalyst
that can be used in the present invention is not particularly
limited. As one example, a production method for a zeolite catalyst
exchanged by a specific transition metal will be specifically
described below.
[0063] Firstly, a transition metal salt such as iron nitrate, iron
acetate and iron sulfate is prepared as a raw material of a
transition metal. The above raw material and zeolite are then
subjected to wet milling to prepare an aqueous slurry. A
three-dimensional structure is coated with this slurry, dried
usually at 50 to 150.degree. C. for 30 minutes to 8 hours and then
calcined at 300 to 800.degree. C., preferably 400 to 600.degree. C.
for 15 minutes to 2 hours, preferably 30 minutes to 1 hour to
obtain a NO.sub.X reduction catalyst.
[0064] In addition, the purification system of the present
invention includes various improved embodiments that use the
oxidation catalyst of the first aspect of the present invention,
preferably have a supplying apparatus of a reducing agent at the
latter stage of the oxidation catalyst and further have a NO.sub.X
reduction catalyst at the latter stage of the supplying apparatus
of a reducing agent. For example, an embodiment having a catalyst
of suppressing slip of a reducing agent 11 at the latter stage of
the NO.sub.X reduction catalyst 3 (FIG. 4), an embodiment having a
diesel particulate filter 13 at the latter stage of the NO.sub.X
reduction catalyst 3 (FIG. 8), an embodiment including a diesel
particulate filter 13 at the latter stage of the oxidation catalyst
2 (FIG. 5), an embodiment including a catalyst for hydrolyzing urea
14 (FIG. 3) and a combination (FIG. 6 and FIG. 7) of these
embodiments are included. Thermocouple 12 is installed after the
NO.sub.X reduction catalyst 3 in the embodiment of FIG. 5 or FIG.
6.
[0065] In the case that the reducing agent is urea or ammonia, an
embodiment having a catalyst of suppressing slip of a reducing
agent 11 installed as shown in FIG. 4 is preferable. It should be
noted that, the catalyst of suppressing slip of a reducing agent
indicates a catalyst for preventing discharge of extra ammonia, for
which a conventionally known oxidizing catalyst can be used.
[0066] In addition, a diesel particulate filter can remove at least
a part of particulate matters. In an embodiment including a diesel
particulate filter, a fuel supplying apparatus composed of a fuel
injection nozzle 17, a pump 18 and a fuel tank 19 may be installed
before the diesel particulate filter 13 as shown in FIG. 5 or FIG.
7. In such an embodiment, the filter requires processing for
regeneration when particulates are accumulated. The system of FIG.
7 is described as follows: Fuel is supplied from the fuel supplying
apparatus to the oxidation catalyst 16 through the exhaust-gas
passage and the temperature of the exhaust gas is controlled to be
high enough to burn the particulates. Thermocouple 15 is installed
after the diesel particulate filter 13 in the embodiment of FIG. 7.
The fuel necessary for regeneration of the diesel particulate
filter may be supplied either to the exhaust-gas passage as shown
above or before an exhaust-gas discharge step after fuel combustion
in an engine cylinder. In addition, when a diesel particulate
filter is used, preferably, the temperature of the exhaust gas to
be introduced to the diesel particulate filter is not too low,
specifically 200.degree. C. or higher in view of regeneration of
the diesel particulate filter. Various kinds of diesel particulate
filters are available and a known filter such as a filter made of
cordierite and a filter made of heat-resistant silicon carbide can
be used. In addition, the diesel particulate filter may contain a
catalyst component. The catalyst component is not limited and
includes one that at least one kind of catalytically active
component such as platinum, palladium, rhodium, iridium, gold,
silver, iron, copper and manganese is supported on a refractory
inorganic oxide. The refractory inorganic oxide includes activated
alumina; zirconia, titania, zeolite, silica, ceria, magnesia,
silica-alumina, ceria-zirconia, and the mixture thereof. The amount
of the catalytically active component is preferably 0.1-10 g per
liter of the catalyst, and the amount of the refractory inorganic
oxide is preferably 5-100 g per liter of the catalyst.
[0067] A catalyst of suppressing slip of a reducing agent and a
catalyst for hydrolyzing urea that are conventionally known can be
used.
[0068] The third aspect of the present invention is an exhaust-gas
purification method for purifying an exhaust gas discharged from an
internal-combustion engine. The exhaust-gas purification method is
characterized by comprising (1) a step for converting nitrogen
monoxide in the exhaust gas to nitrogen dioxide by bringing the
exhaust gas into contact with the oxidation catalyst of the first
aspect of the present invention and (2) a step for reducing the
nitrogen oxides in the exhaust gas by bringing the exhaust gas
obtained in the step (1) into contact with a NO.sub.X reduction
catalyst in the presence of a reducing agent. Preferably, the
method comprises further (3) a step for purifying at least a part
of particulate matters. The details of each step are as described
above.
[0069] The oxidation catalyst of the present invention and the
purification system using it can purify nitrogen oxides discharged
from various internal-combustion engines such as a diesel engine, a
gasoline engine and a compressed-natural-gas engine, and are
preferably applied to a diesel engine among the above engines or to
a vehicle driven by a diesel engine.
EXAMPLES
[0070] The embodiments of the present invention will be described
hereinafter in more detail using the examples and comparative
examples. The technical scope of the present invention is not
limited to the following embodiments.
Example 1
[0071] 20.4 g of an aqueous solution of dinitrodiammine platinum
corresponding to 2.32 g of platinum, 1.1 g of an aqueous solution
of palladium nitrate corresponding to 0.15 g of palladium and 80 g
of activated alumina (.gamma.-Al.sub.2O.sub.3, BET specific surface
area: 200 m.sup.2/g, mean primary particle diameter: 6 .mu.m) were
subjected to wet milling by a ballmill to prepare 300 g in total of
an aqueous slurry (A). A cylindrical honeycomb carrier made of
cordierite of 24 mm in diameter and 50 mm in length having 400
cells per square inch of cross-sectional area was coated (wash
coat) with this slurry, dried at 120.degree. C. for 8 hours and
then calcined at 500.degree. C. for an hour to obtain catalyst A so
that the sum of platinum, palladium and alumina is 82.47 q per
liter of the honeycomb carrier.
Example 2
[0072] 19.8 g of an aqueous solution of dinitrodiammine platinum
corresponding to 2.25 g of platinum, 1.57 g of an aqueous solution
of palladium nitrate corresponding to 0.22 g of palladium and 80 g
of activated alumina (.gamma.-Al.sub.2O.sub.3, BET specific surface
area: 200 m.sup.2/g, mean primary particle diameter: 6 .mu.m) were
subjected to wet milling by a ballmill to prepare 300 g in total of
an aqueous slurry (B). A cylindrical honeycomb carrier made of
cordierite of 24 mm in diameter and 50 mm in length having 400
cells per square inch of cross-sectional area was coated (wash
coat) with this slurry, dried at 120.degree. C. for 8 hours and
then calcined at 500.degree. C. for an hour to obtain catalyst B so
that the sum of platinum, palladium and alumina is 82.47 g per
liter of the honeycomb carrier.
Example 3
[0073] 14.4 g of an aqueous solution of dinitrodiammine platinum
corresponding to 1.64 g of platinum, 5.92 g of an aqueous solution
of palladium nitrate corresponding to 0.83 g of palladium and 80 g
of activated alumina (.gamma.-Al.sub.2O.sub.3, BET specific surface
area: 200 m.sup.2/g, mean primary particle diameter: 6 .mu.m) were
subjected to wet milling by a ballmill to prepare 300 g in total of
an aqueous slurry (C). A cylindrical honeycomb carrier made of
cordierite of 24 mm in diameter and 50 mm in length having 400
cells per square inch of cross-sectional area was coated (wash
coat) with this slurry, dried at 120.degree. C. for 8 hours and
then calcined at 500.degree. C. for an hour to obtain catalyst C so
that the sum of platinum, palladium and alumina is 82.47 g per
liter of the honeycomb carrier.
Comparative Example 1
[0074] 21.8 g of an aqueous solution of dinitrodiammine platinum
corresponding to 2.47 g of platinum and 80 g of activated alumina
(.gamma.-Al.sub.2O.sub.3, BET specific surface area: 200 m.sup.2/g,
mean primary particle diameter: 6 .mu.m) were subjected to wet
milling by a ballmill to prepare 300 g in total of an aqueous
slurry (D). A cylindrical honeycomb carrier made of cordierite of
24 mm in diameter and 50 mm in length having 400 cells per square
inch of cross-sectional area was coated (wash coat) with this
slurry, dried at 120.degree. C. for 8 hours and then calcined at
500.degree. C. for an hour to obtain catalyst D so that the sum of
platinum and alumina is 82.47 g per liter of the honeycomb
carrier.
Comparative Example 2
[0075] 17.6 g of an aqueous solution of palladium nitrate
corresponding to 2.47 g of palladium and 80 g of activated alumina
(.gamma.-Al.sub.2O.sub.3, BET specific surface area: 200 m.sup.2/g,
mean primary particle diameter: 6 .mu.m) were subjected to wet
milling by a ballmill to prepare 300 g in total of an aqueous
slurry (E). A cylindrical honeycomb carrier made of cordierite of
24 mm in diameter and 50 mm in length having 400 cells per square
inch of cross-sectional area was coated (wash coat) with this
slurry, dried at 120.degree. C. for 8 hours and then calcined at
500.degree. C. for an hour to obtain catalyst E so that the sum of
palladium and alumina is 82.47 g per liter of the honeycomb
carrier.
Evaluation Example 1
[0076] After catalysts A, B, C, D and E were calcined at
700.degree. C. for 11 hours, a gas, wherein the composition was
NO:300 ppm, CO:300 ppm, O.sub.2:6%, propane:60 ppm (equivalent in
methane), propylene:240 ppm (equivalent in methane), CO.sub.2:6%,
H.sub.2O:6% and nitrogen:balance (volume ratio), was passed over
each catalyst as an exhaust gas at a S.V. (space velocity) of
70,000/hr. The concentration of nitrogen monoxide (NO) and the
concentration of nitrogen dioxide (NO.sub.2) were measured while
the temperature at the catalyst inlet and in the catalyst bed was
raised from 100.degree. C. to 200.degree. C. at a rate of
10.degree. C. per minute. The calcination at 700.degree. C. for 11
hours is a durable treatment for evaluating a catalyst performance
after long-period use. The conversion ratio of NO to NO.sub.2 was
calculated by the following equation and plotted against the
temperature at the catalyst-bed inlet. The results are shown in
FIG. 1.
Conversion Ratio ( % ) = NO 2 concentration after oxidation
catalyst NO concentration before oxidation catalyst .times. 100
##EQU00001##
[0077] It can be seen from the above results that the oxidation
catalyst of the present invention which contains a specific amount
of Pd and Pt as catalytically active components shows a higher
conversion ratio of NO to NO.sub.2 than an oxidation catalyst which
contains only Pt as a catalytically active component.
Example 4
[0078] 26.4 g of an aqueous solution of dinitrodiammine platinum
corresponding to 3 g of platinum, 7.13 g of an aqueous solution of
palladium nitrate corresponding to 1 g of palladium and 120 g of
activated alumina (.gamma.-Al.sub.2O.sub.3, BET specific surface
area: 200 m.sup.2/g, mean primary particle diameter: 6 .mu.m) were
subjected to wet milling by a ballmill to prepare 300 g in total of
an aqueous slurry (F). A cylindrical honeycomb carrier made of
cordierite of 1 inch in diameter and 3 inches in length having 400
cells per square inch of cross-sectional area was coated (wash
coat) with this slurry, dried at 120.degree. C. for 8 hours and
then calcined at 500.degree. C. for an hour to obtain catalyst F so
that the sum of platinum, palladium and alumina is 124 g per liter
of the honeycomb carrier.
Example 5
[0079] 28.2 q of an aqueous solution of dinitrodiammine platinum
corresponding to 3.2 g of platinum, 3.57 g of an aqueous solution
of palladium nitrate corresponding to 0.5 g of palladium and 120 g
of activated alumina (.gamma.-Al.sub.2O.sub.3, BET specific surface
area: 200 m.sup.2/g, mean primary particle diameter: 6 .mu.m) were
subjected to wet milling by a ballmill to prepare 300 g in total of
an aqueous slurry (G). A cylindrical honeycomb carrier made of
cordierite of 1 inch in diameter and 3 inches in length having 400
cells per square inch of cross-sectional area was coated (wash
coat) with this slurry, dried at 120.degree. C. for 8 hours and
then calcined at 500.degree. C. for an hour to obtain catalyst G so
that the sum of platinum, palladium and alumina is 123.7 g per
liter of the honeycomb carrier.
Example 6
[0080] 29.3 g of an aqueous solution of dinitrodiammine platinum
corresponding to 3.32 g of platinum, 1.43 g of an aqueous solution
of palladium nitrate corresponding to 0.2 g of palladium and 120 g
of activated alumina (.gamma.-Al.sub.2O.sub.3, BET specific surface
area: 200 m.sup.2/g, mean primary particle diameter: 6 .mu.m) were
subjected to wet milling by a ballmill to prepare 300 g in total of
an aqueous slurry (H). A cylindrical honeycomb carrier made of
cordierite of 1 inch in diameter and 3 inches in length having 400
cells per square inch of cross-sectional area was coated (wash
coat) with this slurry, dried at 120.degree. C. for 8 hours and
then calcined at 500.degree. C. for an hour to obtain catalyst H so
that the sum of platinum, palladium and alumina is 123.52 g per
liter of the honeycomb carrier.
Comparative Example 3
[0081] 30.0 g of an aqueous solution of dinitrodiammine platinum
corresponding to 3.4 g of platinum and 120 g of activated alumina
(.gamma.-Al.sub.2O.sub.3, BET specific surface area: 200 m.sup.2/g,
mean primary particle diameter: 6 .mu.m) were subjected to wet
milling by a ballmill to prepare 300 g in total of an aqueous
slurry (I). A cylindrical honeycomb carrier made of cordierite of 1
inch in diameter and 3 inches in length having 400 cells per square
inch of cross-sectional area was coated (wash coat) with this
slurry, dried at 120.degree. C. for 8 hours and then calcined at
500.degree. C. for an hour to obtain catalyst I so that the sum of
platinum and alumina is 123.4 g per liter of the honeycomb
carrier.
Evaluation Example 2
[0082] After catalysts F, G, H and I were calcined at 700.degree.
C. for 16 hours, a gas, whrein the composition was NO:175 ppm,
CO:20 ppm, O.sub.2:13.5%, propane:18 ppm (equivalent in methane),
propylene:72 ppm (equivalent in methane), CO.sub.2:6%,
H.sub.2O:5.1% and nitrogen:balance was passed over each catalyst as
an exhaust gas at a S.V. (space velocity) of 45,000/hr. The
conversion ratio of nitrogen monoxide (NO) to nitrogen dioxide
(NO.sub.2) was measured at 175.degree. C. when the temperature of
the gas was raised at a rate of 10.degree. C. per minute. The
results are shown in Table 1. The calcination at 700.degree. C. for
16 hours is a durable treatment for evaluating a catalyst
performance after long-period use.
TABLE-US-00001 TABLE 1 Catalyst Conversion Ratio of NO to NO.sub.2
(%) F 17.3 G 31.9 H 34.7 I 3.9
[0083] It can be seen from the results of Evaluation Example 2
(Table 1) that even when an amount of the noble metals, a volume of
the honeycomb carrier and exhaust-gas concentration are altered,
the oxidation catalyst of the present invention which contains a
specific amount of Pd and Pt as catalytically active components
shows a higher conversion ratio of NO to NO.sub.2 than an oxidation
catalyst which contains only Pt as a catalytically active
component.
Example 7
[0084] The aqueous slurry (F) was prepared in 15 times as many
amounts by weight as that in Example 4. A cylindrical honeycomb
carrier made of cordierite of 10.5 inches in diameter and 6 inches
in length having 300 cells per square inch of cross-sectional area
was coated (wash coat) with this slurry so that the sum of
platinum, palladium and alumina is 124 g per liter of the honeycomb
carrier, dried at 120.degree. C. for 8 hours and then calcined at
500.degree. C. for an hour to obtain catalyst J.
Comparative Example 4
[0085] The aqueous slurry (I) was prepared in 15 times as many
amounts by weight as that in Comparative Example 3. A cylindrical
honeycomb carrier made of cordierite of 10.5 inches in diameter and
6 inches in length having 300 cells per square inch of
cross-sectional area was coated (wash coat) with this slurry so
that the sum of platinum and alumina is 124 g per liter of the
honeycomb carrier, dried at 120.degree. C. for 8 hours and then
calcined at 500.degree. C. for an hour to obtain catalyst K.
Reference Example 1
[0086] An aqueous solution of iron nitrate of the amount
corresponding to 48 g of iron and 1,600 g of MFI type zeolite (BET
specific surface area: 380 m.sup.2/g, mean primary particle
diameter: 15 .mu.m) were subjected to wet milling by a ballmill to
prepare 4,500 g in total of an aqueous slurry (L). A cylindrical
honeycomb carrier made of cordierite of 10.5 inches in diameter and
6 inches in length having 400 cells per square inch of
cross-sectional area was coated (wash coat) with this slurry so
that the sum of iron and zeolite is 164.8 g per liter of the
honeycomb carrier, dried at 120.degree. C. for 8 hours and then
calcined at 500.degree. C. for an hour to obtain catalyst L.
Example 8
[0087] An exhaust-gas discharge apparatus was fabricated by
installing catalyst J after an engine 1 and reduction catalyst L
after the catalyst as shown in FIG. 2. A direct-injection diesel
engine of 9.8 L was used as the internal-combustion engine.
Comparative Example 5
[0088] An exhaust-gas discharge apparatus was fabricated by
installing catalyst K after an engine 1 and reduction catalyst L
after the catalyst as shown in FIG. 2. A direct-injection diesel
engine of 9.8 L was used as the internal-combustion engine.
Comparative Example 6
[0089] An exhaust-gas discharge apparatus was fabricated in the
same way as Comparative Example 5 except that a cylindrical
honeycomb carrier made of cordierite of 10.5 inches in diameter and
6 inches in length having 300 cells per square inch of
cross-sectional area was installed instead of catalyst K. A
direct-injection diesel engine of 9.8 L was used as the
internal-combustion engine.
Evaluation Example 3
[0090] After catalysts J, K and reduction catalyst L were calcined
at 700.degree. C. for 50 hours, evaluation of the conversion ratios
to NO.sub.2 was carried out using the exhaust-gas discharge
apparatuses of Example 8 and Comparative Example 5. The calcination
at 700.degree. C. for 50 hours is a durable treatment for
evaluating a catalyst performance after long-period use. The engine
was run so that the thermocouple 5 before the oxidation catalyst
(the thermocouple 5 before the reduction catalyst in Comparative
Example 6) indicated 400.degree. C. and the exhaust gas was passed
through the oxidation catalyst for 15 minutes. The engine was then
run so that the thermocouple 5 before the oxidation catalyst
indicates 180.degree. C. on average and 200.degree. C. on average,
and an aqueous solution of urea of 32.5% by weight was added from
the reducing agent injection nozzle 8 at a rate of 180 g per hour
when the thermocouple 5 indicated 180.degree. C. on average and 200
g per hour when the thermocouple 5 indicated 200.degree. C. on
average. The NO.sub.X concentration before the oxidation catalyst
and after the reduction catalyst (before the honeycomb carrier and
after the reduction catalyst in the case of Comparative Example 6)
was measured at each temperature to calculate the NO.sub.X
conversion ratio by the following equation. The results are shown
in Table 2.
NO X Conversion Ratio ( % ) = NO X concentration after reduction
catalyst NO X concentration before oxidation catalyst .times. 100
##EQU00002##
TABLE-US-00002 TABLE 2 NO.sub.x Conversion Ratio [%] Temperature
Comparative Comparative before catalyst Example 8 Example 5 Example
6 180.degree. C. 44.1 42.1 23.0 200.degree. C. 87.3 78.2 26.0
Example 9
[0091] The aqueous slurry (A) was prepared in 5 times as many
amounts by weight as that in Example 1. A cylindrical honeycomb
carrier made of cordierite of 5.7 inches in diameter and 6 inches
in length having 400 cells per square inch of cross-sectional area
was coated (wash coat) with this slurry so that the sum of
platinum, palladium and alumina is 124 g per liter of the honeycomb
carrier, dried at 120.degree. C. for 8 hours and then calcined at
500.degree. C. for an hour to obtain catalyst M.
Comparative Example 7
[0092] The aqueous slurry (D) was prepared in 5 times as many
amounts by weight as that in Comparative Example 1. A cylindrical
honeycomb carrier made of cordierite of 5.7 inches in diameter and
6 inches in length having 400 cells per square inch of
cross-sectional area was coated (wash coat) with this slurry so
that the sum of platinum and alumina is 124 g per liter of the
honeycomb carrier, dried at 120.degree. C. for 8 hours and then
calcined at 500.degree. C. for an hour to obtain catalyst N.
Example 10
[0093] An exhaust-gas discharge apparatus was fabricated by
installing catalyst M after a direct-injection diesel engine of 2.2
L and a cylindrical diesel particulate filter made of SiC of 5.2
inches in diameter and 9 inches in length having 316 cells per
square inch of cross-sectional area in which 9 g of particulate
matters discharged from the engine accumulated, after the
catalyst.
Comparative Example 8
[0094] An exhaust-gas discharge apparatus was fabricated by
installing catalyst N after a direct-injection diesel engine of 2.2
L and a cylindrical diesel particulate filter made of SiC of 5.2
inches in diameter and 9 inches in length having 316 cells per
square inch of cross-sectional area in which 9 g of particulate
matters discharged from the engine accumulated, after the
catalyst.
Evaluation Example 4
[0095] The removal ratio of particulate matters was evaluated using
the exhaust-gas discharge apparatuses of Example 10 and Comparative
Example 7. The weight change of the diesel particulate filter from
before to after the temperature indicated by the thermocouple
before the oxidation catalyst was raised from 115.degree. C. to
400.degree. C. in 100 minutes was measured and thus the weight of
the particulate matters remaining on the diesel particulate filter
was calculated. The diesel particulate filter was weighed after it
was dried at 150.degree. C. for an hour. The removal ratio of
particulate matters was determined from the weight of the remaining
particulate matters using the following equation. The results are
shown in Table 3.
Removal ratio of particulate matters ( % ) = 100 - Weight of
remaining particulate matters after heated to 400 .degree. C . 9
.times. 100 ##EQU00003##
TABLE-US-00003 TABLE 3 Removal ratio of particulate matters (%)
Example 10 53% Comparative 46% Example 8
[0096] It can be seen from the results of Evaluation Example 4
(Table 3) that the catalyst of the present invention gives also a
high removal ratio of the particulate matters accumulated in a
diesel particulate filter.
Example 11
[0097] 65.7 g of an aqueous solution of dinitrodiammine platinum
corresponding to 7.4 g of platinum, 5.27 g of an aqueous solution
of palladium nitrate corresponding to 0.74 g of palladium and 120 g
of activated alumina (.gamma.-Al.sub.2O.sub.3, BET specific surface
area: 200 m.sup.2/g, mean primary particle diameter: 6 .mu.m) were
subjected to wet milling by a ballmill to prepare 300 g in total of
an aqueous slurry (O). A cylindrical honeycomb carrier made of
cordierite of 24 mm in diameter and 67 mm in length having 400
cells per square inch of cross-sectional area was coated (wash
coat) with this slurry, dried at 120.degree. C. for 8 hours and
then calcined at 500.degree. C. for an hour to obtain catalyst O so
that the sum of platinum, palladium and alumina is 32.04 g per
liter of the honeycomb carrier.
Example 12
[0098] 65.7 g of an aqueous solution of dinitrodiammine platinum
corresponding to 7.4 g of platinum, 2.64 g of an aqueous solution
of palladium nitrate corresponding to 0.37 g of palladium and 120 g
of activated alumina (.gamma.-Al.sub.2O.sub.3, BET specific surface
area: 200 m.sup.2/g, mean primary particle diameter: 6 .mu.m) were
subjected to wet milling by a ballmill to prepare 300 g in total of
an aqueous slurry (P). A cylindrical honeycomb carrier made of
cordierite of 24 mm in diameter and 67 mm in length having 400
cells per square inch of cross-sectional area was coated (wash
coat) with this slurry, dried at 120.degree. C. for 8 hours and
then calcined at 500.degree. C. for an hour to obtain catalyst P so
that the sum of platinum, palladium and alumina is 31.94 g per
liter of the honeycomb carrier.
Example 13
[0099] 65.7 g of an aqueous solution of dinitrodiammine platinum
corresponding to 7.4 g of platinum, 1.32 g of an aqueous solution
of palladium nitrate corresponding to 0.19 g of palladium and 120 g
of activated alumina (.gamma.-Al.sub.2O.sub.3, BET specific surface
area: 200 m.sup.2/g, mean primary particle diameter: 6 .mu.m) were
subjected to wet milling by a ballmill to prepare 300 g in total of
an aqueous slurry (Q). A cylindrical honeycomb carrier made of
cordierite of 24 mm in diameter and 67 mm in length having 400
cells per square inch of cross-sectional area was coated (wash
coat) with this slurry, dried at 120.degree. C. for 8 hours and
then calcined at 500.degree. C. for an hour to obtain catalyst Q so
that the sum of platinum, palladium and alumina is 31.90 g per
liter of the honeycomb carrier.
Example 14
[0100] 65.7 g of an aqueous solution of dinitrodiammine platinum
corresponding to 7.4 g of platinum, 0.66 g of an aqueous solution
of palladium nitrate corresponding to 0.093 g of palladium and 120
g of activated alumina (.gamma.-Al.sub.2O.sub.3, BET specific
surface area: 200 m.sup.2/g, mean primary particle diameter: 6
.mu.m) were subjected to wet milling by a ballmill to prepare 300 g
in total of an aqueous slurry (R). A cylindrical honeycomb carrier
made of cordierite of 24 mm in diameter and 67 mm in length having
400 cells per square inch of cross-sectional area was coated (wash
coat) with this slurry, dried at 120.degree. C. for 8 hours and
then calcined at 500.degree. C. for an hour to obtain catalyst R so
that the sum of platinum, palladium and alumina is 31.87 g per
liter of the honeycomb carrier.
Comparative Example 9
[0101] 65.7 g of an aqueous solution of dinitrodiammine platinum
corresponding to 7.4 g of platinum and 120 g of activated alumina
(.gamma.-Al.sub.2O.sub.3, BET specific surface area: 200 m.sup.2/g,
mean primary particle diameter: 6 .mu.m) were subjected to wet
milling by a ballmill to prepare 300 g in total of an aqueous
slurry (S). A cylindrical honeycomb carrier made of cordierite of
24 mm in diameter and 67 mm in length having 400 cells per square
inch of cross-sectional area was coated (wash coat) with this
slurry, dried at 120.degree. C. for 8 hours and then calcined at
500.degree. C. for an hour to obtain catalyst S so that the sum of
platinum and alumina is 31.85 g per liter of the honeycomb
carrier.
Evaluation Example 5
[0102] After catalysts O, P, Q, R and S were calcined at
700.degree. C. for 50 hours, a gas, whrein the composition was
NO:175 ppm, CO:20 ppm, O.sub.2:13.5%, propane:18 ppm (equivalent in
methane), propylene:72 ppm (equivalent in methane), CO.sub.2:6%,
H.sub.2O:5.1% and nitrogen:balance was passed over each catalyst as
an exhaust gas at a S.V. (space velocity) of 45,000/hr. The
conversion ratio of nitrogen monoxide (NO) to nitrogen dioxide
(NO.sub.2) was measured at 200.degree. C. The results are shown in
Table 4. The calcination at 700.degree. C. for 50 hours is a
durable treatment for evaluating a catalyst performance after
long-period use.
TABLE-US-00004 TABLE 4 Catalyst Conversion Ratio of NO to NO.sub.2
(%) O 22.4 P 21.0 Q 19.7 R 19.2 S 18.0
[0103] The present application is based on JP-Application
Number-2006-322490 filed on Sep. 21, 2006, the entire contents of
which are hereby incorporated by reference.
* * * * *