U.S. patent application number 09/985793 was filed with the patent office on 2002-07-11 for exhaust gas purifying catalyst and method of producing same.
This patent application is currently assigned to NISSAN MOTOR CO., LTD.. Invention is credited to Nakamura, Masanori, Suga, Katsuo.
Application Number | 20020091064 09/985793 |
Document ID | / |
Family ID | 18816196 |
Filed Date | 2002-07-11 |
United States Patent
Application |
20020091064 |
Kind Code |
A1 |
Nakamura, Masanori ; et
al. |
July 11, 2002 |
Exhaust gas purifying catalyst and method of producing same
Abstract
An exhaust gas purifying catalyst for an internal combustion
engine of an automotive vehicle. The exhaust gas purifying catalyst
comprises a monolithic substrate. A first catalytic layer is formed
on the monolithic substrate. The first catalytic layer contains at
least one noble metal selected from the group consisting of
rhodium, platinum and palladium, compound of at least one metal
selected from the group consisting of alkali metal, alkaline earth
metal and rare earth metal, and alumina. Additionally, a second
catalytic layer is formed on the first catalytic layer and contains
rhodium, at least one noble metal selected from the group
consisting platinum and palladium, compound of at least one metal
selected from the group consisting of alkali metal, alkaline earth
metal and rare earth metal, and alumina. A content of the compound
of the at least one metal in the second catalytic layer is larger
than that in the first catalytic layer.
Inventors: |
Nakamura, Masanori;
(Kanagawa, JP) ; Suga, Katsuo; (Yokohama,
JP) |
Correspondence
Address: |
Richard L. Schwaab
FOLEY & LARDNER
Washington Harbour
3000 K Street, N.W., Suite 500
Washington
DC
20007-8696
US
|
Assignee: |
NISSAN MOTOR CO., LTD.
|
Family ID: |
18816196 |
Appl. No.: |
09/985793 |
Filed: |
November 6, 2001 |
Current U.S.
Class: |
502/302 ;
502/328; 502/330 |
Current CPC
Class: |
Y02T 10/12 20130101;
Y02A 50/2324 20180101; B01D 53/945 20130101; B01D 53/9454 20130101;
Y02T 10/22 20130101 |
Class at
Publication: |
502/302 ;
502/328; 502/330 |
International
Class: |
B01J 023/38 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 9, 2000 |
JP |
2000-341458 |
Claims
What is claimed is:
1. An exhaust gas purifying catalyst comprising: a monolithic
substrate; a first catalytic layer formed on said monolithic
substrate, said first catalytic layer containing at least one noble
metal selected from the group consisting of rhodium, platinum and
palladium, compound of at least one metal selected from the group
consisting of alkali metal, alkaline earth metal and rare earth
metal, and alumina; and a second catalytic layer formed on said
first catalytic layer and containing rhodium, at least one noble
metal selected from the group consisting platinum and palladium,
compound of at least one metal selected from the group consisting
of alkali metal, alkaline earth metal and rare earth metal, and
alumina, a content of the compound of the at least one metal in
said second catalytic layer being larger than that in said first
catalytic layer.
2. An exhaust gas purifying catalyst as claimed in claim 1, wherein
a ratio between the content of the compound in said second
catalytic layer and the content of the compound in said first
catalytic layer is higher than 1:1 and not higher than 3:1.
3. An exhaust gas purifying catalyst as claimed in claim 1, wherein
a content of alumina in each of said first and second catalytic
layers is not less than 100 g per one liter of said monolithic
substrate.
4. An exhaust gas purifying catalyst as claimed in claim 1, where a
content of the at least one noble metal in said first and second
catalytic layers is within a range of from 1.5 to 3.0 g per one
liter of said monolithic substrate.
5. An exhaust gas purifying catalyst as claimed in claim 1, wherein
said the content of the compound in said first and second catalytic
layers is within a range of from 1 to 50 g per one liter of the
monolithic substrate, the content being calculated as oxide.
6. An exhaust gas purifying catalyst as claimed in claim 1, wherein
each of said first and second catalytic layers contains compound of
at least two metals selected from the group consisting of alkali
metal, alkaline earth metal and rare earth metal.
7. An exhaust gas purifying catalyst as claimed in claim 6, wherein
the two metals are barium and magnesium.
8. An exhaust gas purifying catalyst comprising: a monolithic
substrate; a catalytic layer coated on said monolithic substrate,
said catalytic layer containing at least one noble metal selected
from the group consisting of platinum, palladium and rhodium,
compound of at least one metal selected from the group consisting
of alkali metal, alkaline earth metal and rare earth metal, and
alumina, said catalytic layer having a surface section including a
surface of said catalytic layer, and an inner section located
inside relative to the surface section, wherein a difference in
concentration of said compound between the surface section and the
inner section of said catalytic layer is within a range of
.+-.10%.
9. A method of producing an exhaust gas purifying catalyst
including a monolithic substrate; a first catalytic layer formed on
said monolithic substrate, said first catalytic layer containing at
least one noble metal selected from the group consisting of
rhodium, platinum and palladium, compound of at least one metal
selected from the group consisting of alkali metal, alkaline earth
metal and rare earth metal, and alumina; and a second catalytic
layer formed on said first catalytic layer and containing rhodium,
at least one noble metal selected from the group consisting of
platinum and palladium, compound of at least one metal selected
from the group consisting of alkali metal, alkaline earth metal and
rare earth metal, and alumina, a content of the compound of at
least one metal in said second catalytic layer being larger than
that in said first catalytic layer, said method comprising:
preparing first powder by causing the at least one noble metal
selected from the group consisting of rhodium, platinum and
palladium to be carried on alumina; preparing a first aqueous
solution of the compound of at least one metal selected from the
group consisting of alkali metal, alkaline earth metal and rare
earth metal; forming a first mixture of the first powder and the
first aqueous solution; grinding the first mixture to form a first
slurry; coating the first slurry on the monolithic substrate to
form the first catalytic layer; preparing second powder by causing
rhodium to be carried on alumina; preparing third powder by causing
at least one noble metal selected from the group consisting of
platinum and palladium; preparing a second aqueous solution of the
compound of at least metal selected from the group consisting of
alkali metal, alkaline earth metal and rare earth metal; forming a
second mixture of the second powder, the third powder and the
second aqueous solution; grinding the second mixture to form a
second slurry; and coating the second slurry on the first catalytic
layer formed on the monolithic substrate to form the second
catalytic layer.
10. A method as claimed in claim 9, wherein powder materials of
each of the first and second slurries have a median particle
diameter of not larger than 4 .mu.m.
11. A method of producing an exhaust gas purifying catalyst
including a monolithic substrate; a catalytic layer coated on said
monolithic substrate, said catalytic layer containing at least one
noble metal selected from the group consisting of platinum,
palladium and rhodium, compound of at least one metal selected from
the group consisting of alkali metal, alkaline earth metal and rare
earth metal, and alumina, said catalytic layer having a surface
section including a surface of said catalytic layer, and an inner
section located inside relative to the surface section, wherein a
difference in concentration of said compound between the surface
section and the inner section of said catalytic layer is within a
range of .+-.10%, the method comprising: preparing powder by
causing the at least one noble metal selected from the group
consisting of rhodium, platinum and palladium to be carried on
alumina; preparing an aqueous solution of the compound of at least
one metal selected from the group consisting of alkali metal,
alkaline earth metal and rare earth metal; forming a mixture of the
powder and the aqueous solution; grinding the mixture to form a
slurry; and coating the slurry on the monolithic substrate to form
the first catalytic layer.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to improvements in an exhaust gas
purifying catalyst and a method of producing the catalyst, and more
particularly to the exhaust gas purifying catalyst for removing
hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NOx)
in exhaust gas discharged from an internal combustion engine of an
automotive vehicle, a boiler or the like, particularly for
effectively reducing NOx in exhaust gas in an oxygen-excessive
region or lean region, and the method of producing such a
catalyst.
[0002] Hitherto, automotive vehicles of a low fuel consumption have
been eagerly desired from the view points of exhaustion of
petroleum resource and warming-up phenomena of the earth. Regarding
automotive vehicles with a gasoline-fueled engine, attention has
been paid on development of automotive vehicles provided with a
so-called lean-burn engine. In the automotive vehicles provided
with the lean-burn engine, exhaust gas is in an oxygen-excessive or
lean region (atmosphere) in which an air-fuel (air/fuel) ratio is
leaner or larger than a stoichiometric value. In case that a usual
three-way catalyst is used in the lean region, removing or reducing
NOx can be insufficient under the effect of the excessive oxygen.
Accordingly, development of a catalyst for effectively reducing NOx
even under an oxygen-excessive condition has been desired.
[0003] A variety of catalysts for reducing NOx in such a lean
region have been proposed. One of them is disclosed in Japanese
Patent Provisional Publication No. 5-168860 in which Pt and
lanthanum (NOx adsorbing or trapping agent) are carried on a porous
carrier so that NOx is adsorbed in the lean region and released in
a stochiometric region in which exhaust gas has a stoichiometric
air-fuel (air/fuel) ratio.
[0004] However, fuel and lubricating oil used in the engine
contains sulfur (S) which is to be discharged in the form of oxide
from the engine. Accordingly, the NOx adsorbing agent is subjected
to poisoning with sulfur, and therefore lowering in NOx adsorbing
ability occurs in the NOx adsorbing agent. This is a so-called
sulfur-poisoning.
[0005] In order to protect the NOx adsorbing agent or material from
such sulfur-poisoning, the present inventors have proposed the
following technology in Japanese Patent Provisional Publicaton No.
2000-54825: A catalyst has a catalytic layer which includes an
inner layer, and a surface layer formed on the inner layer. The
surface layer contains a S adsorbing material (such as magnesium
Mg) which can adsorb sulfur but readily decompose sulfur. The inner
layer contains a NOx adsorbing material (such as barium Ba).
[0006] Additionally, Japanese Patent Provisional Publication No.
7-132226 discloses a technology in which the concentration of
alkaline metal or alkaline earth metal is lowered in a direction
from the upstream side to the downstream side relative to flow of
exhaust gas.
SUMMARY OF THE INVENTION
[0007] However, as a result of the present inventors'
investigations on the above conventional or earlier technologies,
it has been confirmed that there is the following rooms for
improvement in the above technologies:
[0008] The technology as described in Japanese Patent Provisional
Publication No. 2000-54825 proposes a catalyst which exhibits a
high performance against the sulfur-poisoning; however the catalyst
is slightly lowered in performance from the viewpoint of adsorption
of NOx.
[0009] Japanese Patent Provisional Publication No. 7-132226 has no
description of sulfur-poisoning in its specification. In regard to
this Publication, the present inventors have found that the effect
on sulfur-poisoning becomes high by locating a plurality of
catalytic layers one upon another.
[0010] Additionally, a catalyst arranged similarly to those in the
above technologies is disclosed in Japanese Patent Provisional
Publication No. 9-57099; however, this catalyst is quite different
in use from the catalysts in the above technologies.
[0011] Therefore, it is an object of the present invention to
provide an improved exhaust gas purifying catalyst and a method of
producing the same catalyst, by which drawbacks encountered in the
conventional and earlier technologies can be effectively
overcome.
[0012] Another object of the present invention is to provide an
improved exhaust gas purifying catalyst and a method of producing
the same catalyst, by which NOx can be effectively adsorbed,
released and reduced (removed) while preventing sulfur-poisoning of
the catalyst.
[0013] A further object of the present invention is to provide an
improved exhaust gas purifying catalyst and a method of producing
the same catalyst, in which compound of alkali metal and/or like is
contained in a larger amount in a surface layer part of a catalytic
layer than that in an inner layer part of the same, while the
compound of alkali metal and/or the like is mixed with a slurry for
forming the catalytic layer in a method of producing the
catalyst.
[0014] An aspect of the present invention resides in an exhaust gas
purifying catalyst comprising a monolithic substrate. A first
catalytic layer (A) is formed on the monolithic substrate. The
first catalytic layer contains at least one noble metal selected
from the group consisting of rhodium, platinum and palladium,
compound of at least one metal selected from the group consisting
of alkali metal, alkaline earth metal and rare earth metal, and
alumina. Additionally, a second catalytic layer (B) is formed on
the first catalytic layer and contains rhodium, at least one noble
metal selected from the group consisting platinum and palladium,
compound of at least one metal selected from the group consisting
of alkali metal, alkaline earth metal and rare earth metal, and
alumina. A content of the compound of the at least one metal in the
second catalytic layer (B) is larger than that in the first
catalytic layer (A).
[0015] Another aspect of the present invention resides in an
exhaust gas purifying catalyst comprising a monolithic substrate. A
catalytic layer is coated on the monolithic substrate. The
catalytic layer contains at least one noble metal selected from the
group consisting of platinum, palladium and rhodium, compound of at
least one metal selected from the group consisting of alkali metal,
alkaline earth metal and rare earth metal, and alumina. The
catalytic layer has a surface section including a surface of the
catalytic layer, and an inner section located inside relative to
the surface section. In this catalytic layer, a difference in
concentration between the surface section and the inner section of
the catalytic layer is within a range of .+-.10%.
[0016] A further aspect of the present invention resides in a
method of producing an exhaust gas purifying catalyst including a
monolithic substrate; a first catalytic layer (A) formed on the
monolithic substrate, the first catalytic layer containing at least
one noble metal selected from the group consisting of rhodium,
platinum and palladium, compound of at least one metal selected
from the group consisting of alkali metal, alkaline earth metal and
rare earth metal, and alumina; and a second catalytic layer (B)
formed on the first catalytic layer and containing rhodium, at
least one noble metal selected from the group consisting of
platinum and palladium, compound of at least one metal selected
from the group consisting of alkali metal, alkaline earth metal and
rare earth metal, and alumina, a content of the compound of at
least one metal in the second catalytic layer (B) being larger than
that in the first catalytic layer (A). The producing method
comprises (a) preparing first powder by causing the at least one
noble metal selected from the group consisting of rhodium, platinum
and palladium to be carried on alumina; (b) preparing a first
aqueous solution of the compound of at least one metal selected
from the group consisting of alkali metal, alkaline earth metal and
rare earth metal; (c) forming a first mixture of the first powder
and the first aqueous solution; (d) grinding the first mixture to
form a first slurry; (e) coating the first slurry on the monolithic
substrate to form the first catalytic layer; (f) preparing second
powder by causing rhodium to be carried on alumina; (g) preparing
third powder by causing at least one noble metal selected from the
group consisting of platinum and palladium; (h) preparing a second
aqueous solution of the compound of at least metal selected from
the group consisting of alkali metal, alkaline earth metal and rare
earth metal; (i) forming a second mixture of the second powder, the
third powder and the second aqueous solution; (j) grinding the
second mixture to form a second slurry; and (k) coating the second
slurry on the first catalytic layer formed on the monolithic
substrate to form the second catalytic layer.
[0017] A still further aspect of the present invention reside in a
method of producing an exhaust gas purifying catalyst including a
monolithic substrate; a catalytic layer coated on the monolithic
substrate, the catalytic layer containing at least one noble metal
selected from the group consisting of platinum, palladium and
rhodium, compound of at least one metal selected from the group
consisting of alkali metal, alkaline earth metal and rare earth
metal, and alumina, the catalytic layer having a surface section
including a surface of the catalytic layer, and an inner section
located inside relative to the surface section, wherein a
difference in concentration between the surface section and the
inner section of the catalytic layer is within a range of .+-.10%.
The producing method comprises (a) preparing powder by causing the
at least one noble metal selected from the group consisting of
rhodium, platinum and palladium to be carried on alumina; (b)
preparing an aqueous solution of the compound of at least one metal
selected from the group consisting of alkali metal, alkaline earth
metal and rare earth metal; (c) forming a mixture of the powder and
the aqueous solution; (d) grinding the mixture to form a slurry;
and (e) coating the slurry on the monolithic substrate to form the
first catalytic layer.
BRIEF DESCRIPTION OF THE DRAWING
[0018] FIG. 1 (single figure) is a fragmentary enlarged sectional
view of an example of an exhaust gas purifying catalyst according
to the present invention.
DETAILED DESCRIPTION OF THE INVENTION
[0019] According to a first aspect of the present invention, an
exhaust gas purifying catalyst comprises a monolithic substrate. A
first or inner catalytic layer (A) is formed on the monolithic
substrate. The first catalytic layer contains at least one noble
metal selected from the group consisting of rhodium, platinum and
palladium. The first catalytic layer (A) further contains compound
of at least one metal selected from the group consisting of alkali
metal, alkaline earth metal and rare earth metal (element), and
alumina. Additionally, a second or outer (surface-side) catalytic
layer (B) is formed on the first catalytic layer and contains
rhodium, at least one noble metal selected from the group
consisting platinum and palladium. The second catalytic layer
further contains compound of at least one metal selected from the
group consisting of alkali metal, alkaline earth metal and rare
earth metal, and alumina. A content of the compound of the at least
one metal in the second catalytic layer (B) is larger than that in
the first catalytic layer (A).
[0020] The exhaust gas purifying catalyst is for an internal
combustion engine or a combustion device (burner, furnace or
boiler). In this case, the exhaust gas purifying catalyst is a
so-called NOx adsorption and reduction catalyst adapted to adsorb
nitrogen oxides in exhaust gas which is in a lean region, and
reduces the adsorbed nitrogen oxides in exhaust gas which is in a
range including a rich region and a stoichiometric region. In the
lean region, exhaust gas has an air/fuel ratio larger or leaner (in
fuel) than a stoichiometric level, and therefore corresponds to
so-called lean exhaust gas. In the stoichiometric region, exhaust
gas has an air/fuel ratio around the stoichiometric level. In the
rich region, exhaust gas has an air/fuel ratio smaller or richer
(in fuel) than the stoichiometric level, and therefore corresponds
to so-called rich exhaust gas.
[0021] Exhaust gas discharged from the engine or the combustion
device contains SOx (sulfur oxides) in addition to NOx (nitrogen
oxides), in which NOx is low in adsorptive activity as compared
with SOx. Accordingly, when exhaust gas is in the lean region, NOx
and SOx in exhaust gas is adsorbed in the NOx adsorption and
reduction catalyst, in which NOx reaches not only the second or
outer catalytic layer but also the first or inner catalytic layer.
In contrast, almost all of SOx is adsorbed in the second or outer
catalytic layer and cannot reach the first or inner catalytic
layer. As a result, the catalyst is brought into a condition where
NOx and SOx are adsorbed in the outer catalytic layer while NOx is
adsorbed in the inner catalytic layer.
[0022] When exhaust gas becomes into the rich region or the
stoichiometric region, the adsorbed NOx and SOx are released from
the catalyst. At this time, NOx is released from both the outer and
inner catalytic layers while SOx is released from the upper
catalytic layer and hardly released from the inner catalytic
layer.
[0023] In order that adsorbed SOx is released from the catalyst, a
large amount of reducing agent is required. Examples of the
reducing agent are HC, CO and the like contained in exhaust gas. in
which it is assumed that a slight amount of such reducing agents
can reach the inner layer; and SOx is hardly released from the
inner layer even if it is adsorbed in the inner layer. Accordingly,
if SOx is adsorbed in the outer layer as much as possible without
reaching the inner layer when exhaust gas is in the lean region,
SOx can be readily released from the catalyst when exhaust gas is
in the rich region or the stoichiometric region.
[0024] According to the present invention, the catalyst contains
compound(s) of alkali metal, alkaline earth metal and rare earth
metal, serving as absorbing (trapping) agents for NOx and SOx.
Additionally, the content of the compound(s) in the outer catalytic
layer (B) is larger than that in the inner catalytic layer (A)
thereby effectively adsorbing SOx into the outer catalytic layer
(A). Since adsorbed SOx is readily released from the catalyst,
sulfur-poisoning of the catalyst can be prevented thereby
effectively reducing or removing NOx without lowering a NOx
adsorbing ability even when exhaust gas is the lean region.
[0025] Here, assume that, for example, an upstream-side catalytic
layer having a larger content of the above compound(s) is formed at
an upstream side on the peripheral surface of a single monolithic
substrate whereas a downstream-side catalytic layer having a
smaller content of the above compound(s) is formed at the
downstream side of the peripheral surface of the same monolithic
substrate. In this case, if SOx cannot be sufficiently adsorbed by
the upstream-side catalytic layer, SOx which has not been adsorbed
is unavoidably flown onto the downstream-side catalytic layer.
Accordingly, adsorbing and releasing SOx cannot be effectively
accomplished thereby unavoidably causing sulfur-poisoning of the
catalyst. Further, it will raise the problem that SOx released from
the upstream-side catalytic layer again adheres onto the
downstream-side catalytic layer.
[0026] In contrast, the catalyst according to the present invention
takes such a multi-layer structure that the catalytic layer larger
in content of the above compound(s) serves as the outer layer,
thereby preventing sulfur-poisoning.
[0027] As discussed above, the content of the compound(s) of the
alkali metal and/or the like is larger in the outer layer (B) than
that in the inner layer (A), in which a ratio (by weight) in
content of the compound(s) of alkali metal and/or the like between
the outer layer (B) and the inner layer (A) is preferably higher
than 1:1 and not higher than 3:1. If the ratio is not higher than
1:1, the effect of preventing sulfur-poisoning and lowering the NOx
adsorbing ability cannot be obtained. If the ratio is higher than
3:1 so that the content of the compound(s) of alkali metal and/or
the like is increased, thermal deterioration of noble metal(s)
contained in the outer catalytic layer (B) becomes excessive.
[0028] Additionally, it is preferable that the content of the above
compound(s) of alkali metal and/or the like is within a range of
from 1 to 50 g (calculated as oxide) per one liter of the
monolithic substrate. If the content is less than 1 g per one liter
of the monolithic substrate, the effects of the above compound(s)
of the alkali metal and/or the like for adsorbing NOx and SOx
cannot be sufficiently obtained. Even if the content is increased
over 50 g per one liter of the monolithic substrate, a meaningful
effect corresponding to the increased content cannot be obtained
while promoting thermal deterioration of Rh, Pt and the like.
[0029] The above-mentioned compound(s) of alkali metal and/or the
like is/are alkali metal, alkaline earth metal and/or rare earth
metal and therefore may be any combination of compound(s) of
metal(s) such as alkali metal, alkaline earth metal and rare earth
metal. Examples of the metal(s) are sodium (Na), potassium (K),
rubidium (Rb), cesium (Cs), magnesium (Mg), calcium (Ca), strontium
(Sr), barium (Ba), lanthanum (La), praseodymium (Pr), neodymium
(Nd) and the like. Examples of the compound(s) are preferably in
the form(s) of carbonate, oxide and hydroxide.
[0030] It is preferable that two kinds of the compounds
respectively containing two metals are contained in each of the
inner and outer catalytic layers (A), (B) thereby improving the
effects of adsorbing and releasing SOx. This is assumed to be
caused by the fact that the two kinds of the compounds of alkali
metal and/or the like form a composite substance or compound, and
therefore the composite substance promotes decomposition of sulfate
compound which has been produced under sulfur-poisoning, thereby
allowing SOx to be readily released. The effects of adsorbing and
releasing SOx are particularly remarkable in case of combining the
compound(s) of barium (Ba) and the compound(s) of magnesium (Mg).
This has been apparent from the fact that a compound
BaMg(CO.sub.3)2 was formed, which was proved by a X-ray
diffraction.
[0031] Each of the inner and outer catalytic layers (A), (B)
contains noble metal(s) such as rhodium (Rh), platinum (Pt) and/or
palladium (Pd), and alumina in addition to the above compound(s) of
alkali metal and/or the like. The noble metal(s) may be any
combination of rhodium, platinum and palladium. The noble metal(s)
such as Rh and/or the like contained in the inner and outer
catalytic layers (A), (B) function(s) as a so-called three-way
catalyst for simultaneously oxidizing HC and CO and reducing NOx,
in which the outer catalytic layer (B) contains Rh thereby
improving the removing effect for NOx and SOx.
[0032] It is preferable that the content of alumina in each of the
inner and outer catalytic layers (A) and (B) is not less than 100 g
per one liter of the monolithic substrate. If the content is less
than 100 g per one liter of the monolithic substrate, the compounds
of the alkali metal and/or the like become into close contact with
each other so as to tend to aggregate, thereby exhibiting
insufficient effects as the NOx and SOx adsorbing agent.
[0033] It is also preferable that the content of the above noble
metal(s) is within a range of from 1.5 to 3.0 g per one liter of
the monolithic substrate. Adsorption and release of SOx can be
effectively accomplished under a condition where such a small
amount of the noble metal(s) is contained in the catalyst. If the
content of the noble metal(s) is less than 1.5 g per one liter of
the monolithic substrate, thermal deterioration of the noble
metal(s) is excessive. If the content of the noble metal(s) exceeds
3.0 g per one liter of the monolithic substrate, SOx in an amount
over a necessary level is adsorbed in the catalyst, and therefore
SOx cannot be adsorbed only by the outer catalytic layer so as to
unavoidably reach the inner catalytic layer, thus making it
impossible to release SOx.
[0034] An example of the exhaust gas purifying catalyst according
to the present invention is shown in FIG. 1. The exhaust gas
purifying catalyst includes a honeycomb type (cordierite ceramic)
monolithic substrate which has a plurality of cells (having a
rectangular cross-section) which straight extends throughout the
length of the monolithic substrate. Each cell is defined by four
flat walls of the monolithic substrate. The catalytic layer is
formed to cover the surfaces of the four flat walls defining the
cell. The catalytic layer may be formed of a plurality of layers
which are formed one upon another and different in structure and/or
function. A gas passage is defined inside the catalytic layer.
Exhaust gas discharged from the engine flows through the gas
passage and purified under the action of catalyst components such
as the noble metal(s), the above compound(s) of alkali metal and/or
the like, and alumina which are contained in the catalytic
layer.
[0035] According to a second aspect of the present invention, an
exhaust gas purifying catalyst comprises a monolithic substrate. A
catalytic layer is coated on the monolithic substrate. The
catalytic layer contains at least one noble metal selected from the
group consisting of platinum, palladium and rhodium. The catalytic
layer further contains compound of at least one metal selected from
the group consisting of alkali metal, alkaline earth metal and rare
earth metal, and alumina. The catalytic layer has a surface section
including a surface of the catalytic layer, and an inner section
located inside relative to the surface section, in which a
difference in concentration between the surface section and the
inner section of the catalytic layer is within a range of .+-.10%.
An example of the exhaust gas purifying catalyst takes a structure
as shown in FIG. 1.
[0036] The compound(s) of the alkali metal and/or the like is
preferably uniformly and highly dispersed in each of the inner and
surface sections, in which specifically a difference in
concentration between the inner section containing a central part
and the surface section containing a surface is within a range of
.+-.10%. This concentration difference can be specified under an
analysis using a X-ray microanalyzer (XMA) or the like. Typically,
in case of the catalytic layer having a thickness of 60 .mu.m, the
concentration of an upper-most surface part (in the surface
section) containing the surface and the concentration of the
central part lower by 30 .mu.m from the surface of the catalytic
layer are measured, thereby obtaining the difference in
concentration. It will be understood that the thickness of the
catalytic layer is a thickness of a flat part of the catalytic
layer as indicated by the character T shown in FIG. 1 and not a
thickness of a corner part of the catalytic layer.
[0037] Thus, the compound(s) of alkali metal and/or the like are
uniformly contained in the catalytic layer, and therefore is/are
highly dispersed in the catalytic layer. As a result, when the
amount of reducing gas (HC, CO and NOx) is decreased relative to
that of oxidizing gas (O.sub.2 and NOx), NOx is adsorbed as
NO.sub.2 into the catalytic layer. Conversely, when the amount of
the reducing gas is increased relative to that of the oxidizing
gas, a reaction for releasing and reducing the adsorbed NO.sub.2 is
made and effectively progresses. Even if S is adhered to the
catalytic layer, S tends to be readily released from the catalytic
layer under the action of the reducing gas. This is assumed to be
caused by the fact that the compound(s) of alkali metal and/or the
like is/are carried in the catalytic layer in a highly dispersed
condition so as to raise a releasing rate of S under the effect of
the reducing agent.
[0038] It will be appreciated that the content, the kind and the
like of the alumina, the noble metal(s) and the compound(s) of
alkali metal and/or the like are not limited to particular ones in
the exhaust gas purifying catalyst of the second aspect; however,
they are preferably selected as same as those in the exhaust gas
purifying catalyst of the first aspect thereby exhibiting a further
high efficiency in adsorption and release of NOx while preventing
sulfur-poisoning.
[0039] As will be understood, the above exhaust gas purifying
catalysts are preferable to be high in heat resistance in view of
the fact that the catalysts are subjected to high temperatures.
Accordingly, it is preferable to add component(s) for improving the
heat resistance of the noble metal(s), alumina and the like, to the
catalytic layer. Examples of the improving component(s) are ceria,
zirconia, lanthanum, barium and the like which have been
conventionally used in the three-way catalyst.
[0040] The monolithic substrate of the exhaust gas purifying
catalysts of the embodiments may be employed upon selecting from
conventional ones which are formed of a refractory inorganic
material. Preferably, the monolithic substrate is a honeycomb-type
monolithic substrate or the like formed of cordierite, stainless
steel or the like, in which powder (pulverized material) of the
catalyst components of the catalytic layer is coated in the form of
slurry on the surface of each wall defining the cell serving as the
gas passage. Otherwise, the powder of the catalyst components
themselves may be formed into a honeycomb structure.
[0041] The exhaust gas purifying catalysts according to the present
invention can effectively reduce NOx preferably when air/fuel ratio
of air/fuel mixture to be supplied to the engine is within a range
of from 20 to 50 and a range of from 10.0 to 14.6.
[0042] Next, production of the above exhaust gas purifying
catalysts according to the present invention will be discussed
hereinafter.
[0043] The exhaust gas purifying catalyst of the first aspect is
produced as follows: First, noble metal(s) such as Rh, Pt and/or Pd
(i.e., Rh, Pt or Pd, or any combination of these noble metals)
is/are carried on alumina powder thereby preparing catalytic
powder. This catalytic powder is mixed with an aqueous solution of
the compound(s) of alkali metal, alkaline earth metal and/or rare
earth metal (i.e., alkali metal, alkaline earth metal or rare earth
metal, or any combination of these metals) to prepare a mixture.
This mixture is ground to prepare a slurry for the inner catalytic
layer (A). This slurry is coated on the surface of the monolithic
substrate thereby forming the inner catalytic layer (A) on the
surface of the monolithic substrate, more specifically on the
surface defining the cell of the monolithic substrate.
[0044] Subsequently, Rh is carried on alumina powder to prepare
Rh-carried alumina powder. Pt and/or Pd are carried on alumina
powder to prepare Pt and/or Pd-carried alumina. These powders are
mixed with an aqueous solution of compound(s) of alkali metal,
alkaline earth metal and/or rare earth metal (i.e., alkali metal,
alkaline earth metal or rare earth metal, or any combination of
these metals) to prepare a mixture. This mixture is ground to
prepare a slurry for the outer catalytic layer (B). This slurry is
coated on the inner catalytic layer (A) thereby forming the outer
catalytic layer (B) on the inner catalytic layer (A).
[0045] It will be understood that each of the above mixtures may
contain other component(s) such as material(s) for improving heat
resistance of the catalyst.
[0046] In conventional technologies, a variety of methods have been
employed to cause compound(s) of alkali metal and/or the like to be
carried on a catalyst component. Typical one of these methods is as
follows: First, a substrate is coated with a slurry which contains
alumina or the like on which noble metal(s) and/or the like is
carried. Thereafter, the thus coated substrate is immersed in an
aqueous solution containing alkali metal(s) or the like so as to be
impregnated with the alkali metal(s) or the like.
[0047] In contrast, according to the production method of the
present invention, alumina carrying noble metal(s) is mixed with
the aqueous solution of the compound(s) of alkali metal and/or the
like to prepare the mixture. Then, the mixture is ground.
Accordingly, particle size (diameter) of the compound(s) of alkali
metal and/or the like is effectively decreased. As a result, even
when sulfate compound is produced under occurrence of
sulfur-poisoning, such sulfate compound tends to be readily
decomposed. Additionally, even if the amount of noble metal(s)
contained in the catalyst is relatively small, adsorption and
release of NOx can be effectively achieved.
[0048] The above has been proved by measuring the particle size
(diameter) of alkali metal(s) and/or the like using a X-ray
diffraction (XRD) as follows: The average particle size (diameter)
of alkali metal(s) and/or the like was 22 nm in the catalyst
prepared by the production method of the present invention. In
contrast, the average particle size (diameter) of alkali metal(s)
and/or the like of the catalyst prepared by the method of the
conventional technology (in which the substrate is impregnated with
the solution of alkali metal and/or the like after it is coated
with the slurry containing noble metal(s) and/or the like) was 27
nm.
[0049] It is preferable that power materials of the above-mentioned
slurry has a median particle diameter of not larger than 4 .mu.m.
This decreases the particle size (diameter) of the alkali metal
and/or the like, thereby further promoting the effect of causing
sulfate compound produced under sulfur-poisoning to tend to be
readily decomposed.
[0050] Additionally, in order to substantially uniformly and highly
dispersing the compound(s) of alkali metal(s) and/or the like in
the catalytic layer in such a manner that the difference in
concentration of the compound(s) of alkali metal(s) and/or the like
between the surface section and the inner section of the catalytic
layer falls within the range of .+-.10% (by weight), the catalyst
is prepared as follows, similarly to the production method as
discussed above: First, noble metal(s) such as Rh, Pt and/or Pd
(i.e., Rh, Pt or Pd, or any combination of these noble metals)
is/are carried on alumina powder thereby preparing catalytic
powder. This catalytic powder is mixed with an aqueous solution of
compound(s) of alkali metal, alkaline earth metal and/or rare earth
metal (i.e., alkali metal, alkaline earth metal or rare earth
metal, or any combination of these metals) to prepare a mixture.
This mixture is ground to prepare a slurry. This slurry is coated
on the surface of the monolithic substrate thereby forming a
catalytic layer on the surface of the monolithic substrate, more
specifically on the surface defining each cell of the monolithic
substrate.
[0051] According to such a production method, the compound(s) of
alkali metal and/or the like can be carried on other catalyst
components in a well dispersed state, and therefore a rate of
releasing S from the catalyst under the action of the reducing gas
can be increased thereby preventing sulfur-poisoning while
maintaining a high efficiency of releasing and reducing NOx.
EXAMPLES
[0052] The present invention will be more readily understood with
reference to the following Examples in comparison with Comparative
Examples; however, these Examples are intended to illustrate the
invention and are not to be construed to limit the scope of the
invention.
Example 1
[0053] First, alumina powder was impregnated with an aqueous
solution of dinitrodiammine Pt and then calcined in air at
400.degree. C. for 1 hour after drying thereby obtaining Pt-carried
alumina powder (Powder a) which had a Pt concentration of 1.0% by
weight.
[0054] Subsequently, a porcelain ball mill was charged with Powder
a, alumina, a solution of Ba acetate and water, upon which mixing
and grinding was made in the ball mill, thereby obtaining a slurry.
Powder materials (or catalyst components) contained in the slurry
had a median particle diameter of 3 .mu.m. This slurry was coated
on a honeycomb-type cordierite ceramic monolithic substrate having
a volume of 1.7 liters and 400 cells per square inch. The cells
were formed extending throughout the length of the monolithic
substrate. The coated monolithic substrate was blown with air to
remove excessive slurry in the cells under the action of air
stream. Thereafter, the coated monolithic substrate was dried at
130.degree. C. and then calcined at 400.degree. C. for 1 hour
thereby forming an inner catalytic layer (A) having a weight of 110
g per one liter of the monolithic substrate.
[0055] Furthermore, alumina powder was impregnated with an aqueous
solution of dinitrodiammine Pt and then calcined in air at
400.degree. C. for 1 hour after drying thereby obtaining Pt-carried
alumina powder (Powder b) which had a Pt concentration of 1.5% by
weight.
[0056] Alumina powder was impregnated with an aqueous solution of
Rh nitrate and then calcined in air at 400.degree. C. for 1 hour
after drying thereby obtaining Rh-carried alumina powder (Powder c)
which had a Rh concentration of 2.0% by weight.
[0057] Subsequently, a porcelain ball mill was charged with Powder
b, Powder c, alumina, a solution of Ba acetate and water, upon
which mixing and grinding was made in the ball mill, thereby
obtaining a slurry. Powder materials (or catalyst components)
contained in the slurry had a median particle diameter of 3 .mu.m.
This slurry was coated on the inner catalytic layer (A) formed on
the honeycomb-type cordierite ceramic monolithic substrate. The
thus coated monolithic substrate was blown with air to remove
excessive slurry in the cells under the action of air stream.
Thereafter, the coated monolithic substrate was dried at
130.degree. C. and then calcined at 400.degree. C. for 1 hour
thereby forming an outer catalytic layer (B) having a weight of 120
g per one liter of the monolithic substrate. Thus, the exhaust gas
purifying catalyst of Example 1 was produced.
[0058] This catalyst contained Pt, Rh and Ba respectively in
amounts of 2.0 g, 0.5 g and 30 g (calculated as oxide) per one
liter of the monolithic substrate. Additionally, a weight ratio
between Ba contained in the outer catalytic layer (B) and Ba
contained in the inner catalytic layer (A) was 2:1.
Example 2
[0059] A procedure for preparation of Example 1 was repeated with
the exception that a slurry for the inner catalytic layer (A) was
obtained by charging a porcelain ball mill with Powder a, alumina,
a solution of Ba acetate, a solution of Mg acetate tetrahydrate and
water, upon which mixing and grinding was made in the ball mill;
and a slurry for the outer catalytic layer (B) was obtained by
charging a porcelain ball mill with Powder b, Powder c, alumina, a
solution of Ba acetate, a solution of Mg acetate tetrahydrate and
water, upon which mixing and grinding was made in the ball mill,
thereby producing the exhaust gas purifying catalyst of Example
2.
[0060] This catalyst contained Pt, Rh, Ba and Mg respectively in
amounts of 2.0 g, 0.5 g, 20 g (calculated as oxide) and 10 g
(calculated as oxide) per one liter of the monolithic substrate.
Additionally, a weight ratio between Ba and Mg contained in the
outer catalytic layer (B) and Ba and Mg contained in the inner
catalytic layer (A) was 2:1.
Comparative Example 1
[0061] A porcelain ball mill was charged with Powder a in Example
1, alumina and water, upon which mixing and grinding was made in
the ball mill, thereby obtaining a slurry. This slurry was coated
on a honeycomb-type cordierite ceramic monolithic substrate having
a volume of 1.7 liters and 400 cells per square inch. The cells
were formed extending throughout the length of the monolithic
substrate. The coated monolithic substrate was blown with air to
remove excessive slurry in the cells under the action of air
stream. Thereafter, the coated monolithic substrate was dried at
130.degree. C. and then calcined at 400.degree. C. for 1 hour
thereby forming a catalytic layer having a weight of 100 g per 1
liter of the monolithic substrate. Thereafter, the monolithic
substrate with the catalytic layer was impregnated with an aqueous
solution of Ba acetate thereby forming an inner catalytic layer (C)
having a weight of 110 g per one liter of the monolithic
substrate.
[0062] Subsequently, a porcelain ball mill was charged with Powder
b in Example 1, Powder c in Example 1, alumina and water, upon
which mixing and grinding was made in the ball mill, thereby
obtaining a slurry. This slurry was coated on the inner catalytic
layer (C) formed on the honeycomb-type cordierite ceramic
monolithic substrate. The thus coated monolithic substrate was
blown with air to remove excessive slurry in the cells under the
action of air stream. Thereafter, the coated monolithic substrate
was dried at 130.degree. C. and then calcined at 400.degree. C. for
1 hour thereby forming a catalytic layer having a weight of 100 g
per one liter of the monolithic substrate. Thereafter, the
monolithic substrate with the catalytic layer was impregnated with
an aqueous solution of Ba acetate thereby forming an outer
catalytic layer (D) having a weight of 120 g per one liter of the
monolithic substrate. Thus, an exhaust gas purifying catalyst of
Comparative Example 1 was produced.
[0063] This catalyst contained Pt, Rh and Ba respectively in
amounts of 2.0 g, 0.5 g and 30 g (calculated as oxide) per one
liter of the monolithic substrate. Additionally, a weight ratio
between Ba contained in the outer catalytic layer (D) and Ba
contained in the inner catalytic layer (C) was 2:1.
Comparative Example 2
[0064] A porcelain ball mill was charged with Powder b in Example
1, Powder c in Example 1, alumina, a solution of Ba acetate and
water, upon which mixing and grinding was made in the ball mill,
thereby obtaining a slurry. Powder materials (or catalyst
components) contained in the slurry had a median particle diameter
of 3 .mu.m. This slurry was coated on a honeycomb-type cordierite
ceramic monolithic substrate having a volume of 1.7 liters and 400
cells per square inch. The cells were formed extending throughout
the length of the monolithic substrate. The coated monolithic
substrate was blown with air to remove excessive slurry in the
cells under the action of air stream. Thereafter, the coated
monolithic substrate was dried at 130.degree. C. and then calcined
at 400.degree. C. for 1 hour thereby producing a Catalyst 1 which
was provided with a catalytic layer having a weight of 120 g per
one liter of the monolithic substrate.
[0065] This Catalyst 1 contained Pt, Rh and Ba respectively in
amounts of 1.0 g, 0.5 g and 20 g (calculated as oxide) per one
liter of the monolithic substrate.
[0066] A porcelain ball mill was charged with Powder a in Example
1, a solution of Ba acetate and water, upon which mixing and
grinding was made in the ball mill, thereby obtaining a slurry.
Powder materials (or catalyst components) contained in the slurry
had a median particle diameter of 3 .mu.m. This slurry was coated
on a honeycomb-type cordierite ceramic monolithic substrate having
a volume of 1.7 liters and 400 cells per square inch. The cells
were formed extending throughout the length of the monolithic
substrate. The coated monolithic substrate was blown with air to
remove excessive slurry in the cells under the action of air
stream. Thereafter, the coated monolithic substrate was dried at
130.degree. C. and then calcined at 400.degree. C. for 1 hour
thereby producing a Catalyst 2 which was provided with a catalytic
layer having a weight of 110 g per one liter of the monolithic
substrate.
[0067] This Catalyst 2 contained Pt and Ba respectively in amounts
of 1.0 g and 10 g (calculated as oxide) per one liter of the
monolithic substrate.
[0068] The Catalysts 1, 2 were disposed in series and located
respectively on upstream-side and downstream-side relative to flow
of exhaust gas, thus constituting an exhaust gas purifying catalyst
(unit) of Comparative Example 2.
Comparative Example 3
[0069] A procedure of Example 1 was repeated with the exception
that the outer catalytic layer (B) and the inner catalytic layer
(A) were respectively formed to have weights of 90 g and 80 g per
one liter of the monolithic substrate, thereby producing an exhaust
gas purifying catalyst of Comparative Example 3.
Comparative Example 4
[0070] A procedure for preparation of Example 1 was repeated with
the exception that the powder materials (or catalyst components) of
each of the slurries for forming the inner and outer catalytic
layers (A), (B) had a median particle diameter of 5 .mu.m, thereby
producing an exhaust gas purifying catalyst of Comparative Example
4.
Comparative Example 5
[0071] A procedure for preparation of Example 1 was repeated with
the exception that the solution of Ba acetate was used in such an
amount that the resultant catalyst contained 60 g (calculated
oxide) per one liter of the monolithic substrate, thereby producing
an exhaust gas purifying catalyst of Comparative Example 5.
Evaluation of Performance for Exhaust Gas Purifying Catalyst of
Examples 1 and 2 and Comparative Examples 1 to 5
[0072] Evaluation test (for emission performance) was conducted on
the exhaust gas purifying catalysts of Examples 1 and 2 and
Comparative Examples 1 to 5, using an internal combustion engine
having a displacement of 2000 cc, provided with an exhaust system
including an exhaust pipe. For the evaluation test, each exhaust
gas purifying catalyst was disposed inside a casing to constitute a
catalytic converter. The catalytic converter was disposed in the
exhaust pipe of the exhaust system of the engine.
[0073] Prior to the evaluation test, each of the catalysts of
Examples and Comparative Examples underwent a durability test in
which each catalyst was disposed as the catalytic converter in an
exhaust pipe of an exhaust system of an internal combustion having
a displacement of 4400 cc. The durability test was conducted as
follows: The engine was operated for 50 hours using a regular
gasoline (in Japan) as fuel while keeping a temperature at a
converter inlet position immediately upstream of the catalyst at
650.degree. C. After the durability test, the engine was operated
for 5 hours using a gasoline (having a S concentration of 300 ppm)
as fuel while keeping the temperature at the converter inlet
position at 350.degree. C., thereby applying a so-called
sulfur-poisoning treatment on the catalyst. Subsequently, the
engine was operated for 30 minutes using the regular gasoline while
keeping the temperature at the converter inlet position at
650.degree. C., thereby applying a so-called sulfur-releasing
treatment on the catalyst
[0074] In the evaluation test, the engine was operated in a manner
to repeat an operational cycle which included a lean operation at a
lean air-fuel ratio (A/F=20.0) for 10 seconds, a rich operation at
a rich air-fuel ratio (A/F=11.0) for 2 seconds and a stoichiometric
operation at a stoichiometric air-fuel ratio (A/F=14.7) for 5
seconds. During such engine operation, the temperature of exhaust
gas immediately upstream of the catalyst was kept at 350.degree. C.
During the engine operation, concentrations (volume or ppm) of an
exhaust gas component (HC, CO, NOx) was measured respectively at
positions of the exhaust pipe upstream and downstream of the
exhaust gas purifying catalyst. Such measurement was made on the
catalyst both in a first state obtained after a durability test and
a second state obtained after the sulfur-releasing treatment. The
conversion rate (%) was calculated by [(1-the concentration of the
gas component in the exhaust pipe downstream of the catalyst / the
concentration of the gas component in the exhaust pipe upstream of
the catalyst).times.100]. Thus, the convention rate (%) was
determined for the catalyst both in the first and second
states.
[0075] Results (conversion rates) of the evaluation test were shown
in Table 1.
1 TABLE 1 After sulfur-releasing After durability test treatment HC
CO NOx HC CO NOx Example 1 95 95 80 97 96 75 Example 2 97 98 85 98
98 82 Compar. 95 95 78 96 96 63 Example 1 Compar. 95 95 75 95 95 58
Example 2 Compar. 95 94 70 95 95 41 Example 3 Compar. 95 95 78 96
96 68 Example 4 Compar. 93 93 78 95 95 44 Example 5
Example 3
[0076] First, alumina powder was impregnated with an aqueous
solution of dinitrodiammine Pt and then calcined in air at
400.degree. C. for 1 hour after drying thereby obtaining Pt-carried
alumina powder (Powder d) which had a Pt concentration of 1.0% by
weight.
[0077] Boehmite alumina powder was impregnated with an aqueous
solution of Rh nitrate and then calcined in air at 400.degree. C.
for 1 hour after drying thereby obtaining Rh-carried boehmite
alumina powder (Powder e) which had a Rh concentration of 1.0% by
weight.
[0078] Subsequently, a porcelain ball mill was charged with Powder
d, Power e, alumina powder and a solution of Ba acetate, upon which
mixing and grinding was made in the ball mill, thereby obtaining a
slurry. This slurry was coated on a honeycomb-type cordierite
ceramic monolithic substrate having a volume of 1.7 liters and 400
cells per square inch. The cells were formed extending throughout
the length of the monolithic substrate. The coated monolithic
substrate was blown with air to remove excessive slurry in the
cells under the action of air stream. Thereafter, the coated
monolithic substrate was dried at 130.degree. C. and then calcined
at 400.degree. C. for 1 hour thereby forming a catalytic layer
having a weight of 100 g per one liter of the monolithic substrate.
Then, operation of coating the slurry on the monolithic substrate
was again made thereby adding a catalytic layer (having a weight of
130 g per one liter of the monolithic substrate) on the previously
formed catalytic layer, thereby finally forming a catalytic layer
(on the monolithic substrate) having a weight of 230 g per one
liter of the monolithic substrate. Thus, the exhaust gas purifying
catalyst of Example 3 was produced.
[0079] This catalyst contained Pt, Rh and Ba respectively in
amounts of 2.0 g, 0.5 g and 30 g (calculated as oxide) per one
liter of the monolithic substrate. A distribution condition of Ba
in the catalytic layer was inspected by the X-ray microanalyzer
(XMA). As a result, it was confirmed that the concentration
difference of Ba between the surface section and the inner section
was within the range of .+-.10% both in the previously formed
catalytic layer and the added catalytic layer.
Comparative Example 6
[0080] First, alumina powder was impregnated with an aqueous
solution of dinitrodiammine Pt and then calcined in air at
400.degree. C. for 1 hour after drying thereby obtaining Pt-carried
alumina powder (Powder d) which had a Pt concentration of 1.0% by
weight.
[0081] Boehmite alumina powder was impregnated with an aqueous
solution of Rh nitrate and then calcined in air at 400.degree. C.
for 1 hour after drying thereby obtaining Rh-carried boehmite
alumina powder (Powder e) which had a Rh concentration of 1.0% by
weight.
[0082] Subsequently, a porcelain ball mill was charged with Powder
d, Power e and alumina powder, upon which mixing and grinding was
made in the ball mill, thereby obtaining a slurry. This slurry was
coated on a honeycomb-type cordierite ceramic monolithic substrate
having a volume of 1.7 liters and 400 cells per square inch. The
cells were formed extending throughout the length of the monolithic
substrate. The coated monolithic substrate was blown with air to
remove excessive slurry in the cells under the action of air
stream. Thereafter, the coated monolithic substrate was dried at
130.degree. C. and then calcined at 400.degree. C. for 1 hour
thereby forming a catalytic layer having a weight of 100 g per 1
liter of the monolithic substrate. Then, operation of the slurry
coating and catalytic layer formation on the monolithic substrate
was again made thereby adding a catalytic layer (having a weight of
100 g per one liter of the monolithic substrate) on the previously
formed catalytic layer, thereby forming a catalytic layer (on the
monolithic substrate) having a weight of 200 g per one liter of the
monolithic substrate. Thereafter, the thus formed catalytic layer
was impregnated with an aqueous solution of Ba acetate, thereby
finally forming a catalytic layer (on the monolithic substrate)
having a weight of 230 g per one liter of the substrate. Thus, an
exhaust gas purifying catalyst of Comparative Example 6 was
produced.
[0083] This catalyst contained Pt, Rh and Ba respectively in
amounts of 2.0 g, 0.5 g and 30 g (calculated as oxide) per one
liter of the monolithic substrate. A distribution condition of Ba
in the catalytic layer was inspected by the X-ray microanalyzer
(XMA). As a result, it was confirmed that the concentration
difference of Ba in the surface section relative to that in the
inner section was about .+-.10% in the previously formed catalytic
layer, whereas the concentration difference of Ba in the surface
section relative to that in the inner section was about .+-.15% in
the previously formed catalytic layer.
Example 4
[0084] A procedure of Example 3 was repeated with the exception
that the porcelain ball mill was charged with Mg acetate in
addition to Powder d, Power e, alumina powder and a solution of Ba
acetate, thereby producing an exhaust gas purifying catalyst of
Example 4 provided with a finally formed catalytic layer having a
weight of 230 g per one liter of the monolithic substrate.
[0085] This catalyst contained Pt, Rh, Ba and Mg respectively in
amounts of 2.0 g, 0.5 g, 20 g (calculated as oxide) and 10 g
(calculated as oxide) per one liter of the monolithic substrate. A
distribution condition of Ba and Mg in the catalytic layer was
inspected by the X-ray microanalyzer (XMA). As a result, it was
confirmed that the concentration difference of Ba and Mg between
the surface section and the inner section was within the range of
.+-.10% both in the previously formed catalytic layer and the added
catalytic layer.
Evaluation of Performance for Exhaust Gas Purifying Catalysts of
Examples 3 and 4 and Comparative Example 6
[0086] The same evaluation test (for emission performance)
conducted on Examples 1 and 2 and Comparative Examples 1 to 5 was
also conducted on the exhaust gas purifying catalysts of Examples 3
and 4 and Comparative Example 6 with the exception that no
sulfur-poisoning treatment and no sulfur-releasing treatment were
carried out. Accordingly, in the evaluation test, the conversion
rate (%) was determined only for the exhaust gas purifying catalyst
in the state obtained after the durability test.
2 TABLE 2 After durability test HC CO NOx Example 3 99 90 92
Compar. 98 90 85 Example 6 Example 4 98 91 95
[0087] As appreciated from the above, according to the present
invention, compound of alkali metal and/or like is contained in a
larger amount in a surface layer part of a catalytic layer than
that Ad in an inner layer of the same, while the compound of alkali
metal and/or the like is mixed with a slurry for forming the
catalytic layer in a method of producing the catalyst. Accordingly,
the catalyst can effectively adsorb (trap), release and reduce
(remove) NOx while preventing sulfur-poisoning of the catalyst.
[0088] The entire contents of Japanese Patent Application
P2000-341458 (filed Nov. 9, 2000) are incorporated herein by
reference.
[0089] Although the invention has been described above by reference
to certain embodiments and examples of the invention, the invention
is not limited to the embodiments and examples described above.
Modifications and variations of the embodiments and examples
described above will occur to those skilled in the art, in light of
the above teachings. The scope of the invention is defined with
reference to the following claims.
* * * * *