U.S. patent application number 12/989289 was filed with the patent office on 2011-03-03 for honeycomb structure-type catalyst for purifying exhaust gas exhausted from automobile, method for producing the same and method for purifying exhaust gas using the same catalyst.
This patent application is currently assigned to N.E. CHEMCAT CORPORATION. Invention is credited to Hiroyuki Endo, Takahiro Kurokawa, Hiroki Nakayama.
Application Number | 20110047975 12/989289 |
Document ID | / |
Family ID | 41550235 |
Filed Date | 2011-03-03 |
United States Patent
Application |
20110047975 |
Kind Code |
A1 |
Nakayama; Hiroki ; et
al. |
March 3, 2011 |
HONEYCOMB STRUCTURE-TYPE CATALYST FOR PURIFYING EXHAUST GAS
EXHAUSTED FROM AUTOMOBILE, METHOD FOR PRODUCING THE SAME AND METHOD
FOR PURIFYING EXHAUST GAS USING THE SAME CATALYST
Abstract
A catalyst for purifying exhaust gas, which is a relatively
cheap three-way-catalyst containing noble metal components, and is
capable of suppressing sintering of the noble metals even at high
temperature, and purifying a carbon monoxide (CO), a hydrocarbon
(HC) and a nitrogen oxide (NOx), and is superior, in particular, in
purifying the nitrogen oxide, along with a method for purifying
exhaust gas. This catalyst is provided by a honeycomb
structure-type catalyst having a carrier made of a honeycomb-type
structure, coated with a catalyst composition in two or more
layers, for purifying a carbon monoxide, a hydrocarbon and a
nitrogen oxide contained in exhaust gas, characterized in that; a
catalyst layer (A) at the upper layer side comprises a palladium
component supported with a heat resistant inorganic oxide, an
oxygen storage release material and a barium component, and a
catalyst layer (B) at the lower layer side comprises a rhodium
component supported with a cerium-zirconium-type composite oxide
having a cerium/zirconium ratio by weight of 0.05 to 0.2, as
converted to an oxide, or the like.
Inventors: |
Nakayama; Hiroki; (Shizuoka,
JP) ; Endo; Hiroyuki; (Shizuoka, JP) ;
Kurokawa; Takahiro; (Shizuoka, JP) |
Assignee: |
N.E. CHEMCAT CORPORATION
Tokyo
JP
|
Family ID: |
41550235 |
Appl. No.: |
12/989289 |
Filed: |
April 22, 2009 |
PCT Filed: |
April 22, 2009 |
PCT NO: |
PCT/JP2009/057958 |
371 Date: |
October 22, 2010 |
Current U.S.
Class: |
60/274 ; 29/890;
60/299 |
Current CPC
Class: |
B01D 2258/012 20130101;
B01D 2255/9207 20130101; B01D 2255/908 20130101; B01J 37/038
20130101; B01D 2255/9022 20130101; B01J 37/0248 20130101; F01N
3/2803 20130101; Y02A 50/20 20180101; B01D 2255/91 20130101; B01J
21/066 20130101; Y02T 10/12 20130101; B01J 23/63 20130101; B01D
53/945 20130101; B01D 2255/407 20130101; Y10T 29/49345 20150115;
B01D 2255/2042 20130101; B01D 2258/014 20130101; B01J 23/58
20130101; Y02A 50/2324 20180101; B01J 35/04 20130101; Y02T 10/22
20130101; B01D 2255/1023 20130101; B01D 2255/1025 20130101; B01J
35/0006 20130101 |
Class at
Publication: |
60/274 ; 29/890;
60/299 |
International
Class: |
F01N 3/00 20060101
F01N003/00; B21D 51/16 20060101 B21D051/16; F01N 3/10 20060101
F01N003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 17, 2008 |
JP |
2008-185924 |
Claims
1. A honeycomb structure-type catalyst having a carrier made of a
honeycomb-type structure, coated with a catalyst composition in two
or more layers, for purifying a carbon monoxide, a hydrocarbon and
a nitrogen oxide contained in exhaust gas, characterized in that; a
catalyst layer (A) at the upper layer side comprises a palladium
component supported on a heat resistant inorganic oxide, an oxygen
storage release material and a barium component, and a catalyst
layer (B) at the lower layer side comprises a rhodium component
supported on a cerium-zirconium-type composite oxide having a
cerium/zirconium ratio of weight of 0.05 to 0.2, as converted to an
oxide.
2. The honeycomb structure-type catalyst according to claim 1,
characterized in that the heat resistant inorganic oxide of the
catalyst layer (A) at the upper layer side is selected from
.gamma.-alumina, .gamma.-alumina added with lanthanum, ceria, or
the cerium-zirconium-type composite oxide.
3. The honeycomb structure-type catalyst according to claim 1,
characterized in that content of the palladium component is 0.1 to
30 [g/L] per unit volume of the honeycomb-type structure.
4. The honeycomb structure-type catalyst according to claim 1,
characterized in that content of the barium component is 0.1 to 50
[g/L] per unit volume of the honeycomb-type structure, in an oxide
equivalent.
5. The honeycomb structure-type catalyst according to claim 1,
characterized in that, in the catalyst layer (B) at the lower layer
side, the cerium-zirconium-type composite oxide is contained in 5
to 150 [g/L] per unit volume of the honeycomb-type structure.
6. The honeycomb structure-type catalyst according to claim 1,
characterized in that content of the rhodium component is 0.01 to 5
[g/L] per unit volume of the honeycomb-type structure.
7. The honeycomb structure-type catalyst according to claim 1,
characterized in that the barium component is not substantially
contained in the catalyst layer (B) at the lower layer side, and
the rhodium component is not substantially contained in the
catalyst layer (A) at the upper layer side.
8. A method for producing the honeycomb structure-type catalyst
according to any one of claims 1 to 7, coating the catalyst
composition in two or more layers onto the carrier made of the
honeycomb-type structure by the wash coat method, characterized in
that the catalyst layer (B) at the lower layer side is formed by
coating catalyst composition slurry containing the
cerium-zirconium-type composite oxide (having a cerium/zirconium
ratio by weight of 0.05 to 0.2, as converted to an oxide) supported
with the rhodium component onto the honeycomb-type structure,
subsequently the catalyst layer (A) at the upper layer side is
formed by coating catalyst composition slurry, containing the heat
resistant inorganic oxide supported with the palladium component,
the oxygen storage release material and the barium component, onto
the honeycomb-type structure, followed by firing.
9. The method for producing the honeycomb structure-type catalyst
according to claim 8, characterized in that the barium component in
the catalyst composition slurry is mainly composed of barium
sulfate.
10. A method for purifying exhaust gas, characterized in that a
carbon monoxide, a hydrocarbon and a nitrogen oxide contained in
exhaust gas are purified by contacting exhaust gas exhausted from
an automobile with the honeycomb structure-type catalyst according
to any one of claims 1 to 7.
11. The method for purifying exhaust gas according to claim 10,
characterized in that the automobile uses gasoline as fuel.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a honeycomb structure-type
catalyst for purifying exhaust gas exhausted from an automobile, a
method for producing the same and a method for purifying exhaust
gas using the same, and a catalyst for purifying exhaust gas, which
is a relatively cheap three-way-catalyst containing noble metal
components, and is capable of suppressing sintering of the noble
metals even at high temperature, and purifying a carbon monoxide
(CO), a hydrocarbon (HC) and a nitrogen oxide (NOx), and is
superior, in particular, in purifying the nitrogen oxide, a method
for producing the same, and a method for purifying exhaust gas.
[0003] 2. Description of the Prior Art
[0004] Conventionally, various components have been used as a
catalyst composition, in response to objective thereof, to purify
exhaust gas exhausted from an internal combustion engine of an
automobile or the like. Among these, platinum group metals to be
used as major catalyst components, are supported, in a highly
dispersed state, on a heat resistant inorganic oxide with high
surface area, such as activated alumina, and converted to catalyst
composition slurry together with other catalyst materials to coat
on a honeycomb structure-type carrier (refer to Patent Document
1).
[0005] As a catalyst for purifying exhaust gas, the platinum group
metals such as platinum (Pt), palladium (Pd), rhodium (Rh) are
general. Pt and Pd are converted to oxidative active species in
many cases, and among them, Pt has particularly high activity, and
still exerts high purification performance even after being
poisoned or particle growth. Therefore, it has been used as the
catalyst for purifying exhaust gas exhausted widely from an
internal combustion engine of an automobile or the like. However,
because output of Pt is low and market price thereof has been
rising in recent years. Therefore, it is studied to reduce use
amount thereof, in view of resource protection or a cost
aspect.
[0006] As a reduction measures of such use amount of Pt, there has
been studied to substitute Pt or a part of Pt with Pd. Pt and Pd
both are active species having oxidation function, however, Pd is
significant in activity decrease accompanied with poisoning by
sulfur or the like or particle growth, as compared with Pt. In
addition, as compared with Pt, Pd tends to become an alloy when it
is used in combination with Rh. In addition, Pd tended to decrease
performance caused by particle growth under stringent condition
such as high temperature oxidative atmosphere, or by a undesirable
interaction with co-catalyst components or poisoning components in
exhaust gas. Therefore, Pd has been used in combination with a
component aiming at suppressing poisoning, sintering, particle
growth, or alloying (refer to Patent Document 2, and Patent
Document 3).
[0007] In addition, exhaust gas from an automobile contains various
reactive components, as well as has high heat, therefore tends to
make components of catalyst for purifying exhaust gas sintered and
generate poisoning. In the catalyst for purifying exhaust gas,
because a major part of purification activity thereof is a noble
metal, suppression of poisoning or sintering of the noble metal is
an important problem, and various solution methods have been
studied.
[0008] It has been known that NOx in exhaust gas is an air
pollution substance, as well as N2O is greenhouse effect gas
promoting global warming. Therefore, by a government agency of each
nation, various exhaustion regulations of NOx have been enforced.
In purification of such NOx, Rh is used as a catalytically active
species, however, Rh is also a material whose alloying is worried
in using it together with Pd in the same composition (refer to
Patent Document 2). In addition, when Rh is used together with Pt
and Pd, which are oxidative active species, in the same catalyst
composition, there is also worry of setting off oxidation
performance and reduction performance. Therefore, it has been
studied to coat Rh and Pt, and Pd on the honeycomb structure-type
carrier, as each different catalyst composition.
[0009] In the catalyst for purifying exhaust gas, in addition to
the above active species, a co-catalyst component selected from an
inorganic oxide, such as an oxygen storage component (it may be
referred to OSC as well), a barium component (Ba component), a
zirconia, a silica, a titania, an alumina, a zeolite, is often
used. This OSC absorbs oxygen when oxygen concentration in exhaust
gas is high, and discharges oxygen when oxygen concentration in
exhaust gas is low. By storage and discharge of oxygen, change of
the oxygen concentration in exhaust gas is buffered, and thus the
oxygen concentration can be adjusted to the level suitable to
purification of exhaust gas.
[0010] In a three-way-catalyst (TWC) for purifying exhaust gas
exhausted from a gasoline automobile, the catalyst for purifying
exhaust gas is usually designed to exert high activity when
concentrations of HC, CO, NOx and the like, and concentration of
oxygen are in a specific range (which may also be referred to
"window"). Buffering of change of oxygen concentration shows an
action to maintain such a window region, and serves to purification
of toxic components in exhaust gas in high efficiency.
[0011] In addition, HC or CO in exhaust gas is oxidized by Pt or
Pd, however, when oxygen concentration is low, oxidation promotion
of HC or CO is difficult. In such a case, the OSC supplies oxygen
into exhaust gas and oxidizes HC or CO, so as to act to promote
purification of exhaust gas. Such an action may be said to be a
redox reaction, and use of the OSC having high rate of oxygen
supply and storage tends to provide easily a catalyst superior in
purification capability of HC or CO. As the OSC having high rate of
oxygen storage and discharge, a cerium-zirconium-type composite
oxide has been known (refer to Patent Document 4). Reason for high
rate of oxygen storage and discharge is considered that a crystal
structure of the cerium-zirconium-type composite oxide is stable
thermally or in oxidation and reduction, by which an action of a
cerium oxide, which is a major OSC component, is not obstructed,
and can be utilized for an action as the OSC upto the inside of
particles. It should be noted that, zirconium is a transition metal
and an oxide thereof has oxygen storage and discharge capability,
therefore use of a zirconium oxide as the OSC, may be considered,
however, capability of the zirconium oxide as the OSC is said not
to be expected so well as compared with cerium oxide.
[0012] In the catalyst for purifying exhaust gas, a Ba component
has been used as a co-catalyst component. The Ba component has
function of adsorbing NOx in exhaust gas. That is, in the case
where the Ba component is BaCO.sub.3, when NOx concentration in
exhaust gas becomes high, BaCO.sub.3 reacts with NOx to become Ba
(NO.sub.3).sub.2. Such a reaction with NOx may be called adsorption
of NOx or storage of NOx.
[0013] In general, NOx generates in a large quantity, when fuel
supplied to an engine is relatively less than amount of air. The Ba
component temporary absorbs NOx generating in this way. NOx
absorbed by the Ba component is discharged from the Ba component
when concentration of NOx in exhaust gas decreases, and CO
concentration becomes high. This is derived from a fact that the
above-described Ba (NO.sub.3).sub.2 reacts with CO to become
BaCO.sub.3. NOx discharged from the Ba component reacts with HC or
other reducing components at the surface of the Rh component to be
reductively-purified. Such storage and discharge of NOx by the Ba
component may be said to be due to chemical equilibrium of the Ba
component.
[0014] In addition to such OSC or the Ba component, zirconia or the
like is included as a co-catalyst component. Zirconia is said to
enhance purification performance of NOx by promotion of a steam
reforming reaction. In the TWC, zirconia is considered to promote a
steam reforming reaction as follows by using together with the Rh
component (refer to Patent Literature 5).
HC+H.sub.2O-----------.fwdarw.COx+H.sub.2 (1)
H.sub.2+NOx---------.fwdarw.N.sub.2+H.sub.2O (2)
[0015] Under such circumstances, it has been required to the
catalyst for purifying exhaust gas, to exert superior purification
performance of exhaust gas enabling to correspond to various
regulations, to be cheap, and to be small in reduction of
purification performance, even in a long period of use.
PRIOR DOCUMENTS
Patent Documents
[0016] Patent Document 1: JP-A-5-237390
[0017] Patent Document 2: JP-A-2002-326033, [0003], [0004]
[0018] Patent Document 3: JP-A-2004-223403
[0019] Patent Document 4: JP-B-6-75675
[0020] Patent Document 5: WO-2000/027508, page 14
SUMMARY OF THE INVENTION
[0021] In view of the conventional problems, it is an object of the
present invention to provide a catalyst for purifying exhaust gas,
which is a relatively cheap three-way-catalyst containing noble
metal components, and is capable of suppressing sintering of the
noble metals even at high temperature, and purifying a carbon
monoxide (CO), a hydrocarbon (HC) and a nitrogen oxide (NOx), and
is superior, in particular, in purifying the nitrogen oxide, a
method for producing the same, along with a method for purifying
exhaust gas.
[0022] The present inventors have intensively studied a way to
solve the problems of conventional technology and found that for
the honeycomb-type structure, by arranging the Pd component
together with the Ba component and the OSC in the upper layer, and
arranging the Rh component supported on a specific
cerium-zirconium-type composite oxide in the lower layer, the Pd
component and the Rh component become active species for oxidizing
HC or CO, and for reducing NOx, respectively, thus exert superior
performance as the three way catalyst, and have thus completed the
present invention
[0023] That is, according to a first aspect of the present
invention, there is provided a honeycomb structure-type catalyst
having a carrier made of a honeycomb-type structure, coated with a
catalyst composition in two or more layers, for purifying a carbon
monoxide, a hydrocarbon and a nitrogen oxide contained in exhaust
gas, characterized in that; a catalyst layer (A) at the upper layer
side contains a palladium component supported on a heat resistant
inorganic oxide, an oxygen storage release material and a barium
component, and a catalyst layer (B) at the lower layer side
contains a rhodium component supported on a cerium-zirconium-type
composite oxide having a cerium/zirconium ratio of weight of 0.05
to 0.2, as converted to an oxide
[0024] In addition, according to a second aspect of the present
invention, there is provided, in the first aspect, the honeycomb
structure-type catalyst characterized in that the heat resistant
inorganic oxide of the catalyst layer at the upper layer side (A)
is selected from .gamma.-alumina, .gamma.-alumina added with
lanthanum, ceria, or the cerium-zirconium-type composite oxide. In
addition, according to a third aspect of the present invention,
there is provided, in the first aspect, the honeycomb
structure-type catalyst characterized in that content of the
palladium component is 0.1 to 30 [g/L] per unit volume of the
honeycomb-type structure.
[0025] In addition, according to a fourth aspect of the present
invention, there is provided, in the first aspect, the honeycomb
structure-type catalyst characterized in that the honeycomb
structure-type catalyst characterized in that content of the barium
component is 0.1 to 50 [g/L] per unit volume of the honeycomb-type
structure, in an oxide equivalent.
[0026] In addition, according to a fifth aspect of the present
invention, there is provided, in the first aspect, the honeycomb
structure-type catalyst characterized in that in the catalyst layer
at the lower layer side (B), the cerium-zirconium-type composite
oxide is contained in 5 to 150 [g/L] per unit volume of the
honeycomb-type structure.
[0027] In addition, according to a sixth aspect of the present
invention, there is provided, in the first aspect, the honeycomb
structure-type catalyst characterized in that content of the
rhodium component is 0.01 to 5 [g/L] per unit volume of the
honeycomb-type structure.
[0028] Still more, according to a seventh aspect of the present
invention, there is provided, in the first aspect, the honeycomb
structure-type catalyst characterized in that the honeycomb
structure-type catalyst characterized in that the barium component
is not substantially contained in the catalyst layer at the lower
layer side (B), and the rhodium component is not substantially
contained in the catalyst layer at the upper layer side (A).
[0029] On the other hand, according to an eighth aspect of the
present invention, there is provided, in any one of the first to
the seventh aspects, the method for producing the honeycomb
structure-type catalyst coating the catalyst composition in two or
more layers onto the carrier made of the honeycomb-type structure
by the wash coat method, characterized in that the catalyst layer
(B) at the lower layer side is formed by coating catalyst
composition slurry containing the cerium-zirconium-type composite
oxide (having a cerium/zirconium ratio by weight of 0.05 to 0.2, as
converted to an oxide) supported with the rhodium component onto
the honeycomb-type structure, subsequently the catalyst layer (A)
at the upper layer side is formed by coating catalyst composition
slurry, containing the heat resistant inorganic oxide supported
with the palladium component, the oxygen storage release material
and the barium component, onto the honeycomb-type structure,
followed by firing.
[0030] Still more, according to a ninth aspect of the present
invention, there is provided, in the eighth aspect, the method for
producing the honeycomb structure-type catalyst, characterized in
that the barium component in the catalyst composition slurry is
mainly composed of barium sulfate.
[0031] On the other hand, according to a tenth aspect of the
present invention, there is provided, in any one of the first to
the seventh aspects, a method for purifying exhaust gas,
characterized in that a carbon monoxide, a hydrocarbon and a
nitrogen oxide contained in exhaust gas are purified by contacting
exhaust gas exhausted from an automobile with the honeycomb
structure-type catalyst.
[0032] In addition, according to an eleventh aspect of the present
invention, there is provided, in the tenth aspect, the method for
purifying exhaust gas, characterized in that the automobile uses
gasoline as fuel.
[0033] The honeycomb structure-type catalyst of the present
invention uses a palladium (the Pd component) in addition to a
rhodium (the Rh component) as catalytically active species,
however, the Pd component is relatively cheap and also abundant in
resource, as compared with Pt, which is expensive and is feared
resource depletion thereof, and combined use with the Ba component
enhances dispersion property of Pd. Therefore, it exerts superior
performance also for purification of exhaust gas exhausted from a
combustion engine using fossil fuel, such as a diesel engine or a
gas turbine in addition to a gasoline engine.
[0034] This catalyst for purifying exhaust gas, when used for the
TWC, is capable of purifying HC or CO in high efficiency not
inferior to a catalyst using Pt, as well as exerting superior
purification performance of NOx and activity at low temperature,
even after being loaded with high temperature by being performed a
durability test under heating so as to promote performance decrease
of a catalyst.
BRIEF DESCRIPTION OF THE DRAWINGS
[0035] FIG. 1 is a graph showing T50 (.degree. C.), when exhaust
gas is processed using the catalysts of the present invention, and
the comparative catalysts.
[0036] FIG. 2 is a graph showing purification rates of CO, HC and
NOx, when exhaust gas is processed using the catalysts of the
present invention, and the comparative catalysts.
[0037] FIG. 3 is a STEM Photograph of Pd after a durability test of
catalyst composition slurry 2 relevant to the present
invention.
[0038] FIG. 4 is a STEM Photograph of Pd after a durability test of
catalyst composition slurry 3 used in producing a comparative
catalyst.
[0039] FIG. 5 is a graph showing results of an evaluation test by a
FTP mode, when exhaust gas is processed using the catalysts of the
present invention, and the comparative catalysts.
[0040] FIG. 6 is a schematic drawing showing basic catalyst
composition of a catalyst of the present invention.
[0041] FIG. 7 is an explanation drawing showing main reactions
supposed in basic catalyst composition of a catalyst of the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0042] Explanation will be given below in detail on the honeycomb
structure-type catalyst of the present invention, mainly on an
automotive honeycomb structure-type catalyst, in particular, the
TWC for purifying exhaust gas exhausted from a gasoline engine.
However, the present invention should not be limited to such TWC
application, and is applicable widely for exhaust gas exhausted
from an engine using fossil fuel, within a range of the gist of the
present invention.
1. Noble Metal Active Species
[0043] The honeycomb structure-type catalyst of the present
invention is composed of two or more layers of catalyst
compositions coated on the honeycomb-type structure, and the Pd
component and the Rh component are used as the noble metal active
species to oxidize HC or CO, and to reduce NOx, respectively
[0044] In the present invention, the Pd component and the Rh
component should form each separate catalyst layer, for the
honeycomb-type structure. Use of the Pd component and the Rh
component in the same composition makes alloying, thus providing
fear that both metals thus alloyed decrease activity by cancelling
each activity. However, it is considered that by arranging the Rh
component and the Pd component in each different layer, each
activity of the Rh component and the Pd component can be promoted.
It is preferable that the Pd component is arranged in the upper
layer and the Rh component is arranged in the lower layer.
(Pd and a Base Material Thereof)
[0045] In the catalyst for purifying exhaust gas, the Pd component
or the Rh component is used by being supported on the base material
of a heat resistant inorganic oxide. In the present invention, the
base material supporting the Pd component is selected as
appropriate from alumina, zirconia, titania, silica, zeolite,
silica-alumina, or the like. Among these, as the base material of
the Pd component, alumina is preferable, and in particular
.gamma.-alumina with high specific surface area is preferable
because of being superior in heat resistance, and maintaining
active species in a highly dispersed state in a long period of use.
In addition, in the base material, the OSC such as the
cerium-zirconium-type composite oxide may be contained. By
containing the OSC, oxidation activity is enhanced in exhaust gas
atmosphere having low concentration of oxygen and high
concentration of HC or CO.
(Rh and a Base Material Thereof (OSC))
[0046] The Rh component is supported on the cerium-zirconium-type
composite oxide containing a large amount of zirconium component.
Ratio of the Rh component and the Pd component is preferably the Rh
component:the Pd component=1:5 to 1:30, and more preferably 1:3 to
1:20, in ratio by weight, as converted to a metal. The honeycomb
structure-type catalyst of the present invention purifies HC, CO
and NOx by actions of both of the Pd component and the Rh
component, and when it is in such a composition range, excellent
purification performance of HC, CO and NOx is exerted.
[0047] In the present invention, the Rh component and the Pd
component are used in a multi-layered catalyst in two or more. In
the multi-layered catalyst, the upper layer has more opportunities
to contact with exhaust gas as compared with the lower layer.
Therefore, the upper layer tends to exert high purification
activity. However, the upper layer is exposed to reactive gas such
as HC, CO, NOx. In addition, the upper layer is exposed to exhaust
gas at high temperature, and may generate sintering or particle
growth of metal components in some cases. The metal components with
particle growth decreases surface area, and decreases activity as a
catalyst. In order to compensate decrease in catalytic activity
caused by such particle growth, it is necessary to use a large
quantity of the metal components so as to have sufficient surface
area, even after particle growth. In this case, if a metal
component is Pd, it can be used in a large quantity because of
being relatively cheap, and by use in a large quantity, a catalyst
layer exerting sufficient purification performance can be obtained,
even when Pd particles grow and thus surface area decreases.
[0048] To such a Pd component, Rh has been known as an active
species with high activity. Therefore, it is capable of exerting
activity even by small amount, as long as particle diameter is
small and a dispersion state is good. However, Rh is scarce in
resource amount as well as expensive as compared with Pd.
Therefore, it is desired a state which enables to maintain a high
dispersion state. In the present invention, the Rh component is
used in a catalyst layer at the lower layer having less opportunity
of direct exposure to reactive components or high temperature
gas.
[0049] That is, in the present invention, cheap Pd is used in a
large quantity in the upper layer which tends to be sintered
easily, so as to exert sufficient activity as the catalyst layer
even when particles thereof grow, while expensive Rh is put in the
lower layer which is difficult to be sintered, so as to maintain a
good dispersion state thereof, prevent particle growth and maintain
activity. This is particularly effective, in the TWC to be used in
the case when exhaust gas temperature is high.
[0050] It should be noted that, in the upper layer of the catalyst
of the present invention, effect can be exerted by using only the
Pd component, however, the Pt component may be added in an
auxiliary object.
[0051] Fundamental catalyst composition of the honeycomb
structure-type catalyst of the present invention can be shown
schematically in FIG. 6. In FIG. 6, "Ba" represents the Ba
component, "OSC" represents the oxygen storage component to be used
in the upper layer, and "Pd" represents the Pd component. And, "Rh"
in the lower layer represents the Rh component, and "Rh" is
supported on the OSC component. The OSC supporting "Rh" is the
cerium-zirconium-type composite oxide, and is one having more
amount of the zirconium component as compared with the cerium
component. "Rh" is supported on both of a cerium oxide and a
zirconium oxide in the OSC. The cerium oxide in the
cerium-zirconium-type composite oxide changes from CeO.sub.2 to
Ce.sub.2O.sub.3 accompanying with storage and discharge of oxygen,
and was represented as "feria" in FIG. 6, and a zirconium oxide in
the cerium-zirconium-type composite oxide was represented as
"ZrO.sub.2".
[0052] It should be noted that, the schematic drawing represents
reactions with fundamental components, and it is without saying
that in a practical catalyst, other co-catalyst material, binder
material and base material may be used together in some cases, as
needed.
2. The Catalyst Layer at the Upper Layer Side (A)
[0053] In the upper layer of the honeycomb structure-type catalyst
of the present invention, the base material of the heat resistant
inorganic oxide supporting the Pd component, the Ba component and
OSC are included.
(The Base Material of the Heat Resistant Inorganic Oxide)
[0054] The base material where the Pd component is supported is not
especially limited as long as it is porous and superior in heat
resistance, and a heat resistant inorganic material such as
alumina, silica, ceria, zirconia, a cerium-zirconium-type composite
oxide, silica-alumina, zeolite can be used widely. Among theses, as
for alumina, .gamma.-alumina is preferable. .gamma.-alumina has
high heat resistance, is porous and has high specific surface area,
and is superior in dispersing property of the Pd component. And, in
the case of .gamma.-alumina, it is preferable that lanthanum is
added still more. .gamma.-alumina added with lanthanum is superior
in heat resistance, and has been known that when a noble metal
component is supported, it enables to maintain high catalytic
activity even at high temperature (JP-A-2004-290827). Specific
surface area value (measured by a BET method; the same hereafter)
of such .gamma.-alumina, or .gamma.-alumina added with lanthanum is
preferably 80 to 250 m.sup.2/g, and further one having 200 to 250
m.sup.2/g is more preferable. When the specific surface area of
.gamma.-alumina is equal to or lower than 250 m.sup.2/g, a catalyst
with high heat resistance can be obtained. In addition, when the
specific surface area is equal to or higher than 80 m.sup.2/g, a
noble metal component can be stabilized in a highly dispersed
state.
[0055] As such a base material, in addition to a porous inorganic
oxide, ceria or the cerium-zirconium-type composite oxide can also
be used. The cerium-zirconium-type composite oxide can also be
combined with a heat resistant inorganic oxide such as
.gamma.-alumina. When .gamma.-alumina and the cerium-zirconium-type
composite oxide are used in combination, heat resistance and high
dispersing property by .gamma.-alumina, and OSC performance of the
cerium-zirconium-type composite oxide can be exerted together, and
activity of the Pd component enhances. In addition, because
.gamma.-alumina exerts effect as a binder as well, when a catalyst
component is coated on the honeycomb-type structure, it can prevent
peeling of the catalyst component as well. When .gamma.-alumina is
used as a binder, .gamma.-alumina may be used singly not as the
base material.
[0056] Amount of the heat resistant inorganic oxide is set 5 to 150
[g/L], and more preferably 20 to 60 [g/L]. The too much amount
results in a thick catalyst layer in the honeycomb-type structure,
and may narrow cross-section of a through-hole of a honeycomb,
which increases back pressure and may incur decrease in output in
some cases. In addition, the too low amount may deteriorate a
dispersion state of the Pd component, and may incur decrease in
catalyst activity.
(The Oxygen Storage Component)
[0057] The oxygen storage component is not especially limited as
long as it is one having function of oxygen storage and discharge,
and includes specifically the cerium-zirconium-type composite
oxide. By containing the OSC, purification performance of HC or CO
enhances. As the cerium-zirconium-type composite oxide, such a
cerium-zirconium-type composite oxide having higher content of
cerium as compared with the lower layer, and cerium/zirconium ratio
of about 1:1 as converted to an oxide, is preferable. By using the
OSC with high cerium amount in this way, it is possible to quickly
perform oxygen storage and discharge in the upper layer, and
promote oxidation of HC or CO in the Pd component. Such a
cerium-zirconium-type composite oxide may be one added with an
alkali metal, an alkaline earth metal, a transition metal, a rare
earth element other than cerium, as needed.
[0058] The cerium-zirconium-type composite oxide is not especially
limited, and a cerium-zirconium-type composite oxide available on
the market can be used widely. In addition, a production method for
the cerium-zirconium-type composite oxide is not especially
limited, and it is obtained, generally, by mixing and firing cerium
raw materials and zirconium raw materials. Such cerium raw
materials are not especially limited, and various cerium salts such
as nitrate, carbonate, sulfate, acetate, chloride, bromide, or
cerium oxide can be used. In addition, zirconium raw materials are
also not especially limited, and various zirconium salts such as
nitrate, carbonate, sulfate, acetate, chloride, bromide, or
zirconium oxide, baddeleiyite, silica-removed-zirconia or the like
may be used.
[0059] When the cerium-zirconium-type composite oxide is used as
the oxygen storage component, amount thereof is preferably 1 to 150
[g/L], and more preferably 5 to 30 [g/L] as content [g/L] per unit
volume of the honeycomb-type structure, in an oxide equivalent. The
too much amount of the cerium-zirconium-type composite oxide
results in generating new NOx at the catalyst surface in some
cases, when temperature of the catalyst is high. Reason for that is
not certain, however, it is considered that the
cerium-zirconium-type composite oxide has the action of supplying
oxygen, and activity of the cerium-zirconium-type composite oxide
increase at high temperature. In addition, the
cerium-zirconium-type composite oxide oxidizes catalytically active
species, in a highly active state, and could result in decreasing
activity thereof as well.
(The Pd Component)
[0060] A noble metal component contained in the upper layer, as
described above, is one containing Pd. The Pd component may be a
metal, however, may be one in which a part thereof is oxidized to
palladium oxide, by being oxidized in firing during a production
process to be described later, or in a purification process of
exhaust gas.
[0061] Such a Pd component may be decreased in activity by
poisoning caused by a poisoning substance such as sulfur, however,
Pd is cheap and can be used in quantity sufficient to compensate
the decrease in activity. In addition, it has no fear of decrease
in activity for exhaust gas from an engine where fuel with low
sulfur content is used, and a cheap catalyst with high performance
can be obtained. Such fuel with low sulfur content is gasoline, and
it is preferable that the catalyst of the present invention is used
as the TWC for a gasoline engine. Amount of the noble metal
component may be set as appropriate, however, for the Pd component,
it is preferable to be 0.1 to 30 [g/L], and more preferably 1 to 7
[g/L] per unit volume of the honeycomb-type structure, in metal Pd
equivalent. The too much amount of the Pd component results in
proceeding of sintering on the base material, while too less amount
cannot exert effect.
(The Ba Component)
[0062] In the catalyst of the present invention, the Ba component
is contained together with the Pd component in the upper layer. The
Ba component, which is an alkaline earth metal, has been known as a
NOx storage component, however, it has also been known that
presence with the Rh component in the same composition decreases
purification performance of NOx (JP-A-2002-326033, [0013]). Reason
for such decrease in purification performance of NOx is not
certain, however, it is considered to be caused by obstruction of
purification performance of NOx in the Rh component, because the
alkaline earth metal component has an action of storing NOx.
Therefore, purification of NOx by the Rh component may not be
promoted in some cases, even when the Ba component, which is the
alkaline earth metal component, is contained only in the lower
layer containing the Rh component.
[0063] As the Ba component, barium carbonate or the like is
included. In an oxygen rich (lean) state, NOx is stored by the Ba
component by being converted to barium nitrate, while in an oxygen
poor (rich) state, NOx stored is discharged by being converted from
barium nitrate to barium carbonate. In atmosphere where NOx is
discharged in this way, HC or CO, which is a reducing component, is
contained abundantly in exhaust gas. NOx discharged is purified by
the Rh component, by utilizing HC, CO, or hydrogen generated by the
steam reforming reaction.
[0064] The Ba component is present as barium oxide in the honeycomb
structure-type catalyst in many cases, however, in producing the
catalyst composition slurry, it may be added in a form of other
barium salts such as barium sulfate, barium carbonate, barium
nitrate and the like, or may be a composite oxide containing barium
oxide, barium sulfate, barium carbonate, and barium nitrate. Among
these, use of barium sulfate decreases viscosity of the catalyst
composition slurry and may enhance coating properties in wash coat
in some cases. The honeycomb structure-type carrier has an
assembled form of many through holes, and high viscosity slurry
makes difficult coating of the catalyst composition slurry into
these through holes. The fact that use of barium sulfate decreases
viscosity of the slurry means easiness of the addition of not only
the Ba component but also a large quantity of catalyst components
to the slurry, and makes easy production of the honeycomb
structure-type catalyst with high coating amount of the catalyst
and high activity.
[0065] In the present invention, use of the Ba component and the Pd
component in the same composition enhances activity as the
catalyst. The reason is considered to be suppression of sintering
of the Pd component by the Ba component. By suppression of
sintering of the Pd component, the Pd component can maintain large
surface area, and a catalyst with high activity can be
obtained.
[0066] Amount of the barium component is preferably 0.1 to 50
[g/L], and more preferably 1 to 30 [g/L] per unit volume of the
honeycomb-type structure, in an oxide equivalent. The barium
component exerts effect even in small amount, however, enhancement
of effect comparable to use amount may not be desired, when use
amount increases to equal mole number as Pd. It should be noted
that, the Ba component may be contained also in the lower layer
within a range not to impair effect of the present invention.
3. The Catalyst Layer at the Lower Layer Side (B)
[0067] The honeycomb structure-type catalyst of the present
invention includes the cerium-zirconium-type composite oxide having
a large amount of the zirconium component supported the Rh
component in the lower layer. In this case, the Rh component is
supported mainly at the surface layer part of the
cerium-zirconium-type composite oxide. The Rh component may be
present in a metal Rh form, however, may be one in which a part
thereof is oxidized to rhodium oxide, by being oxidized in firing
during a production process to be described later, or in a
purification process of exhaust gas.
[0068] Amount of the Rh component is 0.01 to 5 [g/L], and
preferably 0.1 to 1 [g/L] per unit volume of the honeycomb-type
structure, in metal Rh equivalent. The too much amount of the Rh
component results in proceeding of sintering on the base material,
while too less amount cannot exert sufficiently the purification
effect of NOX.
[0069] Here, when the Rh component is supported on the zirconium
oxide like Rh/[ZrO.sub.2], the Rh component is present in an active
state as metal Rh. In addition, in Rh/[ZrO.sub.2], purification of
NOx is promoted by the steam reforming reaction. Therefore, it is
desirable that the OSC of the base material of the Rh component is
rich in the zirconium component.
[0070] Here, the cerium-zirconium-type composite oxide supporting
the Rh component is preferably 0.05 to 0.2, and more preferably
0.05 to 0.1 in ratio by weight [CeO.sub.2/ZrO.sub.2], as converted
to cerium oxide and zirconium oxide. Too less amount of CeO.sub.2
may decrease the oxidation activity of HC, CO, and too much amount
may decrease the purification performance of NOX. To this
cerium-zirconium-type composite oxide supporting the Rh component,
an alkali metal, an alkaline earth metal, a rare earth metal, a
transition metal or the like may be added as well in addition to
cerium and zirconium.
[0071] The cerium-zirconium-type composite oxide is not especially
limited, similarly as the upper layer, and a cerium-zirconium-type
composite oxide available on the market may be used widely. In
addition, a production method for the cerium-zirconium-type
composite oxide is not especially limited, and it may be obtained,
generally, by mixing and firing cerium raw materials and zirconium
raw materials. Such cerium raw materials are not especially
limited, and various cerium salts such as nitrate, carbonate,
sulfate, acetate, chloride, bromide, or cerium oxide may be used.
In addition, zirconium raw materials are also not especially
limited, and various zirconium salts such as nitrate, carbonate,
sulfate, acetate, chloride, bromide, or zirconium oxide,
baddeleyite, silica-removed-zirconia or the like may be used.
[0072] Use amount of the cerium-zirconium-type composite oxide
dispersedly-supported with the Rh component and having more amount
of the zirconium component, is 5 to 150 [g/L], and preferably 40 to
80 [g/L] per unit volume of the honeycomb-type structure to be
described later. The too less amount of the cerium-zirconium-type
composite oxide to be used for dispersed supporting makes distance
between Rh particles to be supported near, thus deteriorates a
dispersed state, causes sintering of Rh particles themselves to
increase particle diameter, and decreases surface area of Rh,
resulting in decreasing activity in some cases. On the other hand,
the too much amount thickens a catalyst layer in the honeycomb-type
structure and narrows cross-section of a through hole of a
honeycomb, which increases back pressure and may incur decrease in
output in some cases of an internal combustion engine when it is
used in the internal combustion engine.
[0073] It is desirable that a binder component is contained further
in the lower layer. It is because the component containing only the
OSC as above may weaken bonding with the honeycomb-type structure
to be described later, in some cases.
[0074] As a binder component, various alumina such as
.gamma.-alumina, silica, zirconia, silica-alumina or the like is
included. Among theses, .gamma.-alumina has been known as activated
alumina because of having high specific surface area and also high
heat resistance, and various kinds of materials are available on
the market. In addition, zirconia has been known as a hydrogen
generating material, and in the present invention, still more
purification of NOx is expected as well.
[0075] It should be noted that, such a binder component may be used
not only in the lower layer but also in the upper layer, as
needed.
[0076] The honeycomb structure-type catalyst of the present
invention has the upper layer and the lower layer as the minimal
configuration unit of the catalyst composition, and it is desirable
to adopt such a layer configuration not only in view of work
efficiency but also in view of cost. However, within a range of the
gist of the present invention, in addition to the upper and the
lower two layers, a separate binder layer, a suppression layer to
suppress migration of the catalyst components, a coating layer, as
well as a different catalyst composition layer or the like may be
provided as appropriate, between the honeycomb-type structure and
the lower layer, or between the lower layer and the upper layer, as
well as at still more upper side of the upper layer.
[0077] In the upper layer, the lower layer of the present
invention, or other layers to be provided, as needed, an oxide of a
transition metal such as platinum, silver, copper, nickel,
tungsten, vanadium, zirconium, silicon, titanium, tungsten; a rare
earth metal such as cerium, lanthanum, neodymium; an alkali metal;
an alkaline earth metal such as barium, calcium, other than the
above essential components, may also be used as a simple substance
or a composite oxide. In addition, various types of zeolites may
also be used in combination.
4. The Honeycomb-Type Structure
[0078] The honeycomb-type structure in the present invention has
many through holes extending from one end face towards the other
end face.
[0079] In addition, in the honeycomb-type structure, from
structural characteristics thereof, it has been known a
flow-through-type and a wall-flow-type It has been known that the
wall-flow-type is used for filtering off solid components such as
soot or SOF in exhaust gas, and is used in a diesel particle filter
(DPF). The wall-flow-type has one end of the through holes composed
of a porous wall sealed alternately, and has an action as a filter
for filtering off particulate substances such as soot.
[0080] On the other hand, the flow-through-type has a structure
having many through holes opening from one free end face towards
the other open end face, and has been used widely in an oxidation
catalyst, a reduction catalyst and the three way catalyst (TWC). It
is desirable that the present invention is used in the TWC,
therefore, it is desirable to be the flow-through-type
honeycomb-type structure.
[0081] Density of the through hole in such a honeycomb-type
structure is expressed by the number of the holes per unit
cross-sectional area, and this is also called a cell density. As
the cell density of the honeycomb-type structure, one widely used
in general may be used, however, about 100 to 900 cell/inch.sup.2
is preferable, and it is desirable to be about 200 to 600
cell/inch.sup.2. The cell density of over 900 cell/inch.sup.2 tends
to generate clogging by catalyst components or solid components in
exhaust gas, while the cell density of below 100 cell/inch.sup.2
decreases geometric surface area, by which effective use rate of
the catalyst is reduced and makes use as an effective catalyst for
purifying exhaust gas difficult.
[0082] In addition, a thickness of a cell wall composing the
honeycomb is preferably 2 to 12 mil (milliinch), and more desirably
4 to 8 mil. The too thin cell wall makes structurally brittle,
while too thick cell wall decreases geometric surface area, by
which effective use rate of the catalyst is reduced and makes use
as an effective catalyst for purifying exhaust gas difficult.
5. Catalyst Preparation (the Wash Coat Method)
[0083] The honeycomb structure-type catalyst of the present
invention is produced by preparing catalyst composition slurry
containing a catalyst component and a carrier composed of the
honeycomb-type structure, and separately coating the predetermined
catalyst component in layer like onto the carrier. That is, in the
present invention, the catalyst composition slurry is coated onto
the honeycomb-type structure by the known wash coat method to
produce the honeycomb structure-type catalyst.
[0084] Firstly, the honeycomb-type structure side catalyst layer
(the lower layer) containing the Rh component, supported on the
cerium-zirconium-type composite oxide having a large quantity of
the zirconium component, is coated, and a layer (the upper layer)
containing a surface layer side catalyst layer containing
palladium, OSC and the barium component, supported on the base
material of the heat resistant inorganic oxide is coated
thereon.
[0085] The wash coat method is one for obtaining the honeycomb
structure-type catalyst coated with the catalyst composition, by
performing drying and firing, after coating the catalyst
composition onto the honeycomb-type structure. The catalyst
composition slurry is used by adjusting to a viscosity suitable for
application. The viscosity is 300 to 2000 CPS, and preferably 500
to 1000 CPS, as a measurement value with a B-type viscometer. The
catalyst composition slurry having the viscosity of over 2000 CPS
cannot perform coating to the whole inside of the honeycomb-type
structure due to high viscosity, in some cases, even when coating
to the honeycomb-type structure is tried by the wash coat method.
In order to decrease the viscosity of such high viscosity slurry,
it is necessary to increase amount of water, however, slurry with
increased amount of water decreases amount of the catalyst which
can be coated per one time by the wash coat method, and requires
multiple times of wash coats to coat amount of the catalyst
necessary to form one kind of the catalyst composition layer. It is
not preferable to perform multiple times of wash coats, in view of
production efficiency.
[0086] In the catalyst preparation relevant to the present
invention, influence due to the Ba component raw materials to be
used on viscosity increase of the catalyst composition slurry is
large. Conventionally used barium acetate and barium hydroxide
tends to increase viscosity of the catalyst composition slurry
easily. On the other hand, use of BaSO.sub.4 decreases viscosity of
the catalyst composition slurry. By using BaSO.sub.4 as the Ba
component, it becomes also easy to use catalyst materials which
tend to increase viscosity easily, which thus widens options also
as for other catalyst materials such as the base material, OSC. In
the present invention, it is desirable to select the Ba component,
in consideration of viscosity change of the catalyst composition
slurry to be used, and coating easiness by the wash coat
method.
[0087] Drying temperature is preferably 100 to 300.degree. C., and
more desirably 100 to 200.degree. C. Firing temperature is
preferably 300 to 700.degree. C., and in particular 400 to
600.degree. C. is desirable. Heating may be performed by a known
heating means such as an electric furnace, a gas furnace.
[0088] When the catalyst is coated in multiple layers, the wash
coat method can be repeated two or more times. Coating before the
drying step may be repeated two or more times, or a procedure till
the drying step may be repeated two or more times.
6. The Purification Method for Exhaust Gas
[0089] The honeycomb structure-type catalyst (catalyst for
purifying exhaust gas) of the present invention is used by
arranging it in flow of automotive exhaust gas. In arrangement in
flow of exhaust gas, the catalyst of the present invention may be
arranged alone, may be used in multiple, or may be used in
combination with a catalyst having a different action. In addition,
when multiple catalysts are used, they may be arranged adjacently,
or may be arranged just under an engine and under a chassis floor.
The catalyst of the present invention may be used just under an
engine and under a chassis floor, or at either thereof.
[0090] The honeycomb structure-type catalyst of the present
invention can be used in any of a gasoline automobile and a diesel
automobile. Exhaust gas exhausted from a gasoline automobile
contains HC, CO and NOx, however, effect of the catalyst of the
present invention should not be influenced by concentration
thereof. Temperature of exhaust gas exhausted from an automotive
diesel engine varies in a wide range, and when it is classified to
a low temperature region covering from about 150 to 250.degree. C.,
and a high temperature region covering from about 300 to
600.degree. C., the catalyst of the present invention can exert
high denitrification performance in a wide temperature range
covering from the low temperature region to the high temperature
region.
[0091] In recent years, further enhancement of fuel efficiency has
become a big problem also in a gasoline automobile. In order to
attain enhancement of fuel efficiency, in a gasoline automobile,
air/fuel ratio (or A/F) of mixed air supplied to a combustion room
is increased, or fuel is cut temporarily in some cases. In this
way, fuel in the gasoline engine results in performing lean
combustion, and thus increasing generation amount of NOx. The
honeycomb structure-type catalyst of the present invention is
capable of purifying HC, CO and NOx in high efficiency, and is
superior, in particular, in purification performance of NOx,
therefore suitable for purification of exhaust gas exhausted from a
gasoline automobile with a low fuel efficiency specification.
[0092] It should be noted that, also in exhaust gas from a diesel
engine or a gas turbine, there may be the case where fuel is
supplied into exhaust gas, or combustion is made temporarily in a
fuel rich state, or a reducing component is supplied into exhaust
gas, for reductive purification of NOx. In such a case, because
concentration of HC or CO increases, even in a diesel engine where
lean combustion is performed, comprehensive purification capability
can be exerted by the honeycomb structure-type catalyst of the
present invention.
[0093] FIG. 7 is shows main reactions a) to c) supposed in a
catalyst schematic drawing (FIG. 6). a) "a lean state" is a state
of exhaust gas exhausted in lean combustion under large air/fuel
ratio in a combustion engine, b) "a stoichiometric state" is a
state of exhaust gas exhausted from an engine operated in a state
of a theoretical air/fuel ratio of 14.7, and c) "a rich state" is a
state of exhaust gas exhausted in rich combustion state under small
air/fuel ratio in a combustion engine.
[0094] When exhaust gas is in a) a lean state, and in the "feria"
supported the Rh component, oxygen storage in the "feria" is
promoted by utilization of adsorption capability of an oxygen atom
by the Rh component. In addition, by such adsorption capability of
oxygen, there may also be possibility of adsorption of NOx onto the
"Ceria". On the other hand, in the b) "stoichiometric state", NOx
is purified to N2 by the Rh component supported on ZrO.sub.2.
Although not shown in a drawing, a reduction reaction to N.sub.2 is
promoted by a hydrogen atom generated by the steam reforming
reaction or ZrO.sub.2. The Rh component supported on ZrO.sub.2 has
not been oxidized and thus tends to maintain active metal surface
easily and exerts high activity as a NOx purification catalyst.
[0095] On the contrary, the Rh component supported on the "feria"
may be oxidized by active oxygen "O*" discharged from the OSC, and
may decrease activity as a NOx purification catalyst in some cases.
When exhaust gas is in the c) "rich state", oxygen stored in the
"feria" is discharged quickly via Rh. Oxygen discharged is active
oxygen "0*", and is supplied to an oxidation reaction of HC or CO
(not shown) in the Pd component. In addition, in the case where NOx
was adsorbed onto the "feria", it is discharged quickly via the Rh
component similarly as oxygen, and supplied to the Rh component
Rh/[ZrO.sub.2], having active noble metal surface and purified.
[0096] In addition, it has been known that the steam reforming
reaction by Rh is promoted under the b) "stoichiometric"
atmosphere. The steam reforming reaction is one for generating
hydrogen by decomposition of H2O in exhaust gas by an action of the
Rh component, and hydrogen generated here promotes reduction of NOx
in exhaust gas and decomposes to nitrogen (N.sub.2) and water
(H.sub.2O). In this case, when a cerium oxide is present in the
catalyst, change of transitional oxygen concentration in the a)
"lean" and c) "rich" states is buffered, and the b) "stoichiometric
state" is maintained for a long period to promote purification of
NOx. In the catalyst of the present invention, purification of HC,
CO and NOx is promoted by such an interaction of the upper layer
and the lower layer.
EXAMPLES
[0097] Examples and Comparative Examples of the present invention
will be shown below, however, the present invention should not be
construed restrictive to these Examples.
[0098] Catalyst composition slurries 1 to 4 were produced as
follows by preparing raw materials of the catalyst compositions,
and pulverizing and mixing the slurry with a ball mill.
=Raw Materials of Catalyst Composition Slurry-1=
[0099] an aqueous solution of rhodium nitrate (metal conversion: 7%
by weight:)
[0100] cerium-zirconium-type composite oxide [Ce/Zr (A)] (ratio by
weight as converted to an oxide, Ce/Zr=0.15)
[0101] .gamma.-alumina (specific surface area value: 220
m.sup.2/g)
[0102] Water
[0103] Rh was supported by an impregnation method using an aqueous
solution of rhodium nitrate onto Ce/Zr. After drying this, it was
fired at 300.degree. C. for 1 hour to obtain Rh supporting Ce/Zr
(hereafter may be referred to as Rh/[Ce/Zr (A)]).
[0104] Rh/[Ce/Zr (A)] obtained in this way, .gamma.-alumina as a
binder and water were pulverized and mixed using a ball mill to
obtain catalyst composition slurry-1.
=Raw Materials of Catalyst Composition Slurry-2=
[0105] an aqueous solution of palladium nitrate (metal conversion:
20% by weight:)
[0106] .gamma.-alumina (specific surface area value: 220
m.sup.2/g)
[0107] barium sulfate (BaSO4)
[0108] cerium-zirconium-type composite oxide [Ce/Zr (B)] (ratio by
weight as converted to an oxide, Ce/Zr=1)
[0109] Water
[0110] Pd was supported by impregnating an aqueous solution of a Pd
salt into .gamma.-alumina and Ce/Zr (B). After drying this, it was
fired at 300.degree. C. for 1 hour to obtain Pd supporting
[.gamma.-alumina/Ce/Zr (B)] (hereafter may be referred to as
.gamma.-alumina, Ce/Zr (B)).
[0111] Pd/[.gamma.-alumina, Ce/Zr (B)] obtained in this way,
BaSO.sub.4 and water were pulverized and mixed using a ball mill to
obtain catalyst composition slurry-2.
=Raw Materials of Catalyst Composition Slurry-3=
[0112] an aqueous solution of palladium nitrate (metal conversion:
20% by weight:)
[0113] .gamma.-alumina (specific surface area value: 220
m.sup.2/g)
[0114] Ce/Zr (B)
[0115] Water
[0116] Catalyst composition slurry-3 was obtained by similar
operation as in catalyst composition slurry-2, except that
BaSO.sub.4 was excluded, and .gamma.-alumina of a non base material
was added in an amount equivalent to amount of BaSO.sub.4 excluded
to adjust catalyst amount.
=Raw Materials of Catalyst Composition Slurry-4=
[0117] an aqueous solution of rhodium nitrate (metal conversion: 7%
by weight:)
[0118] Ce/Zr (A)
[0119] barium sulfate (BaSO4)
[0120] .gamma.-alumina (specific surface area value: 220
m.sup.2/g)
[0121] Water Catalyst composition slurry-4 was obtained by adding
BaSO.sub.4 to Rh/[Ce/Zr (A)] of catalyst composition slurry-1,
excluding .gamma.-alumina by amount equivalent to BaSO4 added to
adjust amount of the catalyst, and adding water to be pulverized
and mixed using a ball mill.
=Raw Materials of Catalyst Composition Slurry-5=
[0122] an aqueous solution of rhodium nitrate (metal conversion: 7%
by weight:)
[0123] cerium-zirconium-type composite oxide [Ce/Zr (C)] (ratio by
weight as converted to an oxide, Ce/Zr=0.01)
[0124] .gamma.-alumina (specific surface area value: 220
m.sup.2/g)
[0125] Water
[0126] Rh was supported by impregnating an aqueous solution of
rhodium nitrate into Ce/Zr (C). After drying this, it was fired at
300.degree. C. for 1 hour to obtain Rh supporting Ce/Zr (C)
(hereafter may be referred to as Rh/[Ce/Zr (C)]).
[0127] Rh/[Ce/Zr (C)] obtained in this way, .gamma.-alumina and
water were pulverized and mixed using a ball mill to obtain
catalyst composition slurry-5.
=Raw Materials of Catalyst Composition Slurry-6=
[0128] an aqueous solution of rhodium nitrate (metal conversion: 7%
by weight:)
[0129] Ce/Zr (B)
[0130] .gamma.-alumina (specific surface area value: 220
m.sup.2/g)
[0131] Water
[0132] Catalyst composition slurry-6 was obtained by similar
operation as in catalyst composition slurry-1, except that Ce/Zr
(A) was changed to Ce/Zr (B).
[0133] Honeycomb structure-type catalysts with enhanced durability
were obtained by heating under the following durability conditions,
after laminating the catalyst composition slurries 1 to 6 onto the
following honeycomb-type structure by the wash coat method, and
drying-firing under the following conditions. Layer configuration
(layer) of each honeycomb structure-type catalyst and composition
of each component are shown in Table 1. Numerals shown in a
parenthesis in Table 1 represents amount of the component [g/L] per
unit volume of each catalyst component, and amounts of the Pd
component and the Rh component are values as converted to a
metal.
[0134] It should be noted that, in producing the catalyst
composition slurry 2 of the present invention, BaSO4 was used as
the raw material of the Ba component. Measurement value with a
B-type viscometer on this catalyst composition slurry 2 was 700
CPS, and this slurry was coated easily on the honeycomb-type
structure by the wash coat method.
[0135] After that, the catalyst composition slurry 7 was produced
by using barium hydroxide instead of BaSO4 in the above catalyst
composition slurry 2 to compare change of viscosity thereof and
coating easiness by the wash coat method. Viscosity of the catalyst
composition slurry using barium hydroxide was 3000 CPS. Therefore,
although coating on the honeycomb-type structure was tried using
this catalyst composition slurry 7 by the wash coat method, coating
to whole inside of the honeycomb-type structure was impossible due
to high viscosity. Similar result was obtained also when barium
acetate was used instead of BaSO4. In order to decrease viscosity
of such high viscosity slurry, amount of water was increased,
however, it decreased amount of the catalyst which can be coated
per one time by the wash coat, and required multiple times of wash
coats to coat amount of the catalyst necessary.
[0136] In this way, by using BaSO4, viscosity of high viscosity
catalyst composition slurry with high solid content, or slurry
using a material, which results in increasing viscosity, can be
decreased to a large degree. By using the catalyst composition
slurry having high solid content, a large amount of the catalyst
composition can be coated by one time operation of the wash coat
method. The honeycomb structure-type catalyst having high amount of
the catalyst composition tends to enhance purification performance
of exhaust gas, and by using BaSO4 as the Ba component of the
catalyst composition slurry, such a high performance catalyst can
be produced efficiently.
=The Honeycomb-Type Structure=
[0137] Material: made of cordierite
[0138] Size: 25.4 .phi..times.50 [mm] (volume: 25 cc)
[0139] Cell density: 900 cell/inch.sup.2
[0140] Thickness of cell wall: 2.5 mil
=Drying and Firing Conditions=
[0141] Drying temperature: 150.degree. C.
[0142] Firing furnace: a gas furnace
[0143] Firing temperature: 500.degree. C.
[0144] Firing time: 2 hours.
=Durability Conditions=
[0145] Furnace for the durability: an electric furnace
[0146] Temperature: 950.degree. C.
[0147] Durability test time: 6 hours
[0148] Durability test atmosphere: 10% H.sub.2O, air atmosphere
TABLE-US-00001 TABLE 1 (Composition Table) Composition of
components Example Upper Catalyst composition slurry-2 1 layer Pd
(5)/[.gamma.-alumina (30)], Ce/Zr (B) (15), BaSO.sub.4 (10) Lower
Catalyst composition slurry-1 layer Rh (0.5)/[Ce/Zr (A) (80)],
.gamma.-alumina (40) Example Upper Catalyst composition slurry-2 2
layer Pd (5)/[.gamma.-alumina (30)], Ce/Zr (B) (15), BaSO4 (10)
Lower Catalyst composition slurry-4 layer Rh (0.5)/[Ce/Zr (A)
(80)], BaSO.sub.4 (10), .gamma.-alumina (30) Example Upper Catalyst
composition slurry-3 3 layer Pd (5)/[.gamma.-alumina (30)], Ce/Zr
(B) (15), BaSO.sub.4 (10) Lower Catalyst composition slurry-4 layer
Rh (0.5)/[Ce/Zr (A) (80)], BaSO.sub.4 (10), .gamma.-alumina (30)
Example Upper Catalyst composition slurry-2 4 layer Pd
(5)/[.gamma.-alumina (30)], Ce/Zr (B) (15), BaSO.sub.4 (10) Lower
Catalyst composition slurry-5 layer Rh (0.5)/[Ce/Zr (C) (80)],
.gamma.-alumina (40) Example Upper Catalyst composition slurry-2 5
layer Pd (5)/[.gamma.-alumina (30)], Ce/Zr (B) (15), BaSO.sub.4
(10) Lower Catalyst composition slurry-6 layer Rh (0.5)/[Ce/Zr (B)
(80)], .gamma.-alumina (40)
Examples 1 and 2
Comparative Examples 1 to 3
[0149] On the honeycomb structure-type catalysts obtained above,
purification performance of HC, CO and NOx was measured on T50 and
purification rate under the following conditions. Example 1 and
Example 2 of Table 1 (Composition Table) correspond to Examples 1
and 2, and Example 3 to Example 5 correspond to Comparative
Examples 1 to 3.
[0150] T50 indicates temperature for attaining 50% of conversion
performance in components for the catalyst to purify, and has been
known widely by those skilled in the art as an index showing a
light-off temperature (performance). The light-off temperature is
temperature at which purification capability of HC, CO and NOx in
the catalyst begins to function. The fact that the light-off
temperature is low, shows that purification capability is exerted
from low temperature, and performance as the catalyst is high; and
the light-off temperature is high, shows that purification
capability as the catalyst is low. In addition, purification rate
is conversion rate of the components to be purified. Results of T50
are shown in Table 1, and results of purification rate are shown in
Table 2.
=Gas Composition=
[0151] CO: 0.5%
[0152] C.sub.3H.sub.6: 1200 ppm
[0153] CH.sub.4: 500 ppm
[0154] NO: 500 ppm
[0155] O.sub.2: 0.5%
[0156] CO.sub.2: 14%
[0157] H.sub.2: 0.17%
[0158] H.sub.2O: 10%
[0159] The balance: N.sub.2
=Evaluation Conditions=
[0160] Test temperature: from room temperature to 400.degree. C.
Temperature increasing rate: 30.degree. C./minute Flow rate: 36.3
L/minute Space velocity (SV): 86000 h.sup.-1
[0161] As is clear from FIG. 1 and FIG. 2, because the catalysts of
Example 1 and Example 2, which are Examples of the present
invention, contain the Pd component along with the Ba component in
the upper layer, comprehensively superior purification performance
for HC, CO and NOx is exerted. It seems that Example 2 is superior
to Example 1 in conversion rate of NOx, however, it is considered
because the Ba component contained in the lower layer is acting in
a auxiliary way for purification of NOx. Example 2, which is a
catalyst having a large amount of the Ba component, is somewhat
inferior to Example 1 in view of T50 (temperature is increased). It
is considered that this is brought about by alloying of the Rh
component by the Ba component.
[0162] Example 3, which is a catalyst for comparison, does not
contain the Ba component in the upper layer, and contains the Ba
component in the lower layer, and is thus inferior in purification
performance of NOx. It is considered that this is brought about by
alloying of the Ba component and the Rh component, and purification
performance of NOx by the Rh component is not exerted sufficiently.
In addition, Example 3 is inferior also in purification performance
of HC or CO, and it is considered that this is brought about by
absence of the Ba component, resulting in sintering of the Pd
component, decreasing effective surface area in the Pd component,
thus decreasing purification performance of HC or CO.
[0163] Example 4, which is a catalyst for comparison, is one where
Ce/Zr (C) of the cerium-zirconium-type composite oxide having
decreased amount of the Ce component is used as the base material
of the Rh component in the lower layer. In this case, it is
considered that decrease in the Ce component decreases capability
for buffering oxygen concentration of exhaust gas at the vicinity
of the Rh component, shortens "stoichiometric" time for promoting
purification of NOx by the Rh component, and decreases purification
performance of NOx. In addition, in Example 4, also purification
performance of HC or CO is decreased. It is considered that this is
brought about by decreased amount of the Ce component in the base
material of the Rh component, resulting in decrease in oxygen
storage and discharge capability by an interaction of the Rh
component and the Ce component. Example 5, which is a catalyst for
comparison, is one where Ce/Zr (C) of the cerium-zirconium-type
composite oxide having increased amount of the Ce component is used
as the base material of the Rh component in the lower layer. In
this case, although purification performance of HC or CO is the
same as in Examples, purification performance of NOx is decreased.
It is considered that this is brought about by increased amount of
the Ce component in the base material of the Rh component,
resulting in not promoting the steam reforming reaction
sufficiently. In addition, because many Rh components are supported
on the "feria" in the OSC, oxygen discharge is promoted, and new
NOx is generated by reaction of nitrogen components in exhaust gas
and active oxygen, or oxygen concentration in exhaust gas
increases, resulting in not making atmosphere suitable for reducing
NOx.
[0164] Explanation will be given next on coarsening of particles of
the Pd component, with reference to FIG. 3 and FIG. 4. On the
catalyst composition slurry 2 and the catalyst composition slurry
3, change of particles of the Pd component after the durability
test was observed. The observation was performed using a STEM
(Scanning Transmission Electron Microscope). A STEM Photograph of
Pd after the durability test of the q7 2 is "FIG. 3", and a STEM
Photograph of Pd after the durability test of the q7 3 is "FIG.
4".
[0165] Both of the catalyst composition slurry 2 and the catalyst
composition slurry 3 have a good dispersion state of the Pd
component before the durability test, and presence of Pd particles
could not be confirmed in the STEM Photograph, however, after the
durability test, in both of the catalyst composition slurry 2 and
the catalyst composition slurry 3, presence of Pd particles was
abled to be confirmed.
[0166] As the Photographs show, in the catalyst composition slurry
3 not containing the Ba component, significant growth of the Pd
particles was observed, as compared with the catalyst composition
slurry 2 containing the Ba component. In the Pd component with the
particles grown to a giant size, surface area decreases. The Pd
particle with decreased surface area in this way exhibits decrease
in active surface area in the catalytic reaction, and this fact
corresponds to difference of purification performance in Examples
and Comparative Examples.
Comparative Example 4
[0167] In addition, a honeycomb structure-type catalyst was
prepared by replacing the upper layer and the lower layer in
Example 1. A production method of the catalyst and conditions of
the durability test were the same as in Example 1. Catalyst
compositions are shown in the following Table 2 (Example 6).
TABLE-US-00002 TABLE 2 Composition of components Example Upper
Catalyst composition slurry-1 6 layer Rh (0.5)/[Ce/Zr (A) (80)],
.gamma.-alumina (40) Lower Catalyst composition slurry-2 layer Pd
(5)/[.gamma.-alumina (30)], Ce/Zr (B) (15), BaSO.sub.4 (10)
[0168] Next, on Comparative Example 4 and Example 1, an evaluation
test by an FTP mode was performed. Exhaustion results of exhaust
gas components are shown in FIG. 5. The vertical-axis in FIG. 5
represents exhaustion amount of each component in exhaust gas per
running distance [g/mile]. In addition, THC of the horizontal-axis
represents "total hydrocarbon" and NMHC represents "non-methane
hydrocarbon". It should be noted that, as for CO (carbon monoxide),
amount was expressed by 1/10 amount. From these results, it is
understood that the honeycomb structure-type catalyst having "the
upper layer/Rh, the lower layer/Pd", as in Comparative Example 4,
shows more exhaustion amount of toxic components as compared with
the honeycomb structure-type catalyst having "the upper layer/Pd,
the lower layer/Rh" of Example 1.
[0169] It should be noted that, in the similar evaluation test by
the FTP mode, it has been confirmed that Comparative Example 4 is
inferior in performance to Comparative Examples 1 to 3.
[0170] The honeycomb structure-type catalyst of the present
invention can be used for purification of exhaust gas exhausted
from a combustion engine using fossil fuel, such as a gasoline
engine, a diesel engine as well as a gas turbine or the like.
* * * * *