U.S. patent application number 16/970519 was filed with the patent office on 2020-12-03 for exhaust gas purification catalyst device.
This patent application is currently assigned to CATALER CORPORATION. The applicant listed for this patent is CATALER CORPORATION, TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Isao CHINZEI, Seiji NAKAHIGASHI, Shunsuke OISHI, Takaya OTA, Yoshinori SAITO, Hiromasa SUZUKI.
Application Number | 20200376467 16/970519 |
Document ID | / |
Family ID | 1000005059049 |
Filed Date | 2020-12-03 |
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United States Patent
Application |
20200376467 |
Kind Code |
A1 |
OISHI; Shunsuke ; et
al. |
December 3, 2020 |
EXHAUST GAS PURIFICATION CATALYST DEVICE
Abstract
This exhaust gas purification catalyst device includes an upper
layer which includes first carrier particles and rhodium, and a
lower layer which includes second carrier particles, and the upper
layer includes a rhodium enriched area in the range a, from the
downstream end in the exhaust gas flow to greater than 50% and up
to 80% of the upper layer length, and a range b from the upper
layer top surface less than 20 .mu.m in the depth direction. The
rhodium enriched area contains at least 50% and less than 100% of
all the rhodium in the upper layer.
Inventors: |
OISHI; Shunsuke;
(Kakegawa-shi, JP) ; OTA; Takaya; (Kakegawa-shi,
JP) ; SAITO; Yoshinori; (Toyota-shi, JP) ;
NAKAHIGASHI; Seiji; (Toyota-shi, JP) ; CHINZEI;
Isao; (Toyota-shi, JP) ; SUZUKI; Hiromasa;
(Toyota-shi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
CATALER CORPORATION
TOYOTA JIDOSHA KABUSHIKI KAISHA |
Kakegawa-shi, Shizuoka
Toyota-shi, Aichi |
|
JP
JP |
|
|
Assignee: |
CATALER CORPORATION
Kakegawa-shi, Shizuoka
JP
TOYOTA JIDOSHA KABUSHIKI KAISHA
Toyota-shi, Aichi
JP
|
Family ID: |
1000005059049 |
Appl. No.: |
16/970519 |
Filed: |
January 29, 2019 |
PCT Filed: |
January 29, 2019 |
PCT NO: |
PCT/JP2019/003022 |
371 Date: |
August 17, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
B01D 2255/1025 20130101;
B01J 35/0006 20130101; B01J 23/63 20130101; F01N 2370/04 20130101;
B01J 37/0244 20130101; B01D 2255/9022 20130101; B01J 37/0228
20130101; B01D 53/9413 20130101; B01D 2255/407 20130101; F01N
3/2803 20130101 |
International
Class: |
B01J 23/63 20060101
B01J023/63; B01J 35/00 20060101 B01J035/00; B01J 37/02 20060101
B01J037/02; B01D 53/94 20060101 B01D053/94; F01N 3/28 20060101
F01N003/28 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 21, 2018 |
JP |
2018-029033 |
Claims
1. An exhaust gas purification catalyst, comprising: an upper layer
containing first carrier particles and rhodium, and a lower layer
containing second carrier particles, wherein the upper layer
includes a rhodium-rich portion in a range of greater than 50% to
80% of the length of the upper layer from a downstream side end of
an exhaust gas flow and of less than 20 .mu.m in the depth
direction from an outermost surface of the upper layer, and the
rhodium-rich portion contains rhodium in an amount of 50% to less
than 100% of the total rhodium included in the upper layer.
2. The exhaust gas purification catalyst device according to claim
1, wherein the range of the rhodium-rich portion in an exhaust gas
flow direction is up to 70% of the length from the upper layer
exhaust stream flow downstream side end.
3. The exhaust gas purification catalyst device according to claim
2, wherein the range of the rhodium-rich portion in the depth
direction is a range of up to 18 .mu.m in the depth direction from
the outermost surface of the upper layer.
4. The exhaust gas purification catalyst device according to claim
1, wherein a content of ceria in the upper layer is greater than 5%
by mass to 25% by mass or less, when the total mass of the first
carrier particles in the upper layer is 100% by mass.
5. The exhaust gas purification catalyst device according to claim
4, wherein the content of ceria in the upper layer is 7% by mass to
15% by mass, when the total mass of the first carrier particles in
the upper layer is 100% by mass.
6. The exhaust gas purification catalyst device according to claim
1, wherein a quantity of rhodium included in the range of the
rhodium-rich portion up to 50% of the length of the rhodium-rich
portion from the downstream side end of the exhaust gas flow is 70%
or less of the quantity of rhodium included in the rhodium-rich
portion.
7. The exhaust gas purification catalyst device according to claim
1, wherein the lower layer and the upper layer are present on a
substrate.
8. The exhaust gas purification catalyst according to claim 1,
wherein the lower layer constitutes a part or the entirety of a
substrate, and the upper layer is present on the lower layer.
9. A method for the production of the exhaust gas purification
catalyst device according to claim 1, comprising the steps of:
providing a coating liquid including a rhodium precursor to an
exhaust gas downstream side end of an exhaust gas purification
catalyst device precursor comprising an upper layer containing
first carrier particles and rhodium and a lower layer containing
second carrier particles, and thereafter suctioning from an exhaust
gas upstream side end of the exhaust gas purification catalyst
device precursor.
Description
FIELD
[0001] The present invention relates to an exhaust gas purification
catalyst device.
BACKGROUND
[0002] Exhaust gas purification catalysts are used to purify, for
example, HC, CO, and NO.sub.x which are contained in the exhaust
gas from internal combustion engines of automobiles. Such exhaust
gas purification catalysts typically comprise a substrate having
formed thereon a catalyst coating layer which contains an inorganic
oxide and a precious metal.
[0003] In such exhaust gas purification catalysts, there has been
proposed a technology in which a catalyst coating has a two-layer
structure comprising a lower layer containing Pd and an upper layer
containing Rh, whereby the catalytic performance thereof is
improved (Patent Literature 1). Furthermore, there has been
proposed a technology in which Pd contained in a lower layer and Rh
contained in an upper layer are unevenly distributed near the
surface of the respective layers, whereby the utilization rate of
the Pd and the Rh is increased (Patent Literature 2).
[0004] Patent Literature 3 is known to disclose a method in which a
precious metal is carried on a substrate using a precious metal
solution.
[0005] Conventionally, the catalyst coating layer in an exhaust gas
purification catalyst device as described above is formed on a
substrate which itself does not have an exhaust gas purification
capability, such as, for example, a cordierite honeycomb substrate.
However, in recent years, there has been proposed an exhaust gas
purification catalyst in which a precious metal is supported on a
substrate composed of inorganic oxide particles (Patent Literature
3).
CITATION LIST
Patent Literature
[0006] [PTL 1] Japanese Unexamined Patent Publication (Kokai) No.
7-6017 [0007] [PTL 2] Japanese Unexamined Patent Publication
(Kokai) No. 2011-255378 [0008] [PTL 3] Japanese Unexamined Patent
Publication (Kokai) No. 2008-302304 [0009] [PTL 4] Japanese
Unexamined Patent Publication (Kokai) No. 2015-85241
SUMMARY
Technical Problem
[0010] In conventionally known exhaust gas purification catalyst
devices, for example, it is not possible to sufficiently meet the
demand for improvement of exhaust gas purification capacity, in
particular, a sufficiently high NO.sub.x purification capacity,
when the mass flow rate (Ga) of the exhaust gas is high, such as
when the accelerator is depressed.
[0011] The object of the present invention is to provide an exhaust
gas purification catalyst device which can exhibit a sufficiently
high exhaust gas purification capacity, in particular, a
sufficiently high NO.sub.x purification capacity, even when the
mass flow rate of exhaust gas is high.
Solution to Problem
[0012] The present invention is as described below.
[0013] <Aspect 1> An exhaust gas purification catalyst,
comprising: [0014] an upper layer containing first carrier
particles and rhodium, and [0015] a lower layer containing second
carrier particles, wherein [0016] the upper layer includes a
rhodium-rich portion in a range from greater than 50% to 80% of the
length of the upper layer from a downstream side end of an exhaust
gas flow and of less than 20 .mu.m in the depth direction from an
outermost surface of the upper layer, and [0017] the rhodium-rich
portion contains rhodium in an amount of 50% to less than 100% of
the total rhodium included in the upper layer.
[0018] <Aspect 2> The exhaust gas purification catalyst
device according to <Aspect 1>, wherein the range of the
rhodium-rich portion in an exhaust gas flow direction is up to 70%
of the length of the upper layer from the upper layer exhaust
stream flow downstream side end.
[0019] <Aspect 3> The exhaust gas purification catalyst
device according to <Aspect 2>, wherein the range of the
rhodium-rich portion in the depth direction is a range of up to 18
.mu.m in the depth direction from the outermost surface of the
upper layer.
[0020] <Aspect 4> The exhaust gas purification catalyst
device according to any one of <Aspect 1> to <Aspect
3>, wherein a content of ceria in the upper layer is greater
than 5% by mass to 25% by mass or less, when the total mass of the
first carrier particles in the upper layer is 100% by mass.
[0021] <Aspect 5> The exhaust gas purification catalyst
device according to <Aspect 4>, wherein the content of ceria
in the upper layer is 7% by mass to 15% by mass, when the total
mass of the first carrier particles in the upper layer is 100% by
mass.
[0022] <Aspect 6> The exhaust gas purification catalyst
device according to any one of <Aspect 1> to <Aspect
5>, wherein a quantity of rhodium included in the range of the
rhodium-rich portion up to 50% of the length of the rhodium-rich
portion from the downstream side end of the exhaust gas flow is 70%
or less of the quantity of rhodium included in the rhodium-rich
portion.
[0023] <Aspect 7> The exhaust gas purification catalyst
device according to any one of <Aspect 1> to <Aspect
6>, wherein the lower layer and the upper layer are present on a
substrate.
[0024] <Aspect 8> The exhaust gas purification catalyst
according to any one of <Aspect 1> to <Aspect 6>,
wherein the lower layer constitutes a part or the entirety of a
substrate, and the upper layer is present on the lower layer.
[0025] <Aspect 9> A method for the production of the exhaust
gas purification catalyst device according to any one of <Aspect
1> to <Aspect 8>, comprising the steps of: [0026]
providing a coating liquid including a rhodium precursor to an
exhaust gas downstream side end of an exhaust gas purification
catalyst device precursor comprising an upper layer containing
first carrier particles and rhodium and a lower layer containing
second carrier particles, and thereafter suctioning from an exhaust
gas upstream side end of the exhaust gas purification catalyst
device precursor.
Advantageous Effects of Invention
[0027] According to the present invention, there is provided an
exhaust gas purification catalyst device which can exhibit a
sufficiently high exhaust gas purification capacity, in particular,
a sufficiently high NO.sub.x purification capacity, even when the
mass flow rate of exhaust gas is high.
BRIEF DESCRIPTION OF DRAWINGS
[0028] FIG. 1 is a schematic cross-sectional view illustrating the
typical structure of an exhaust gas purification catalyst device
according to the present invention.
DESCRIPTION OF EMBODIMENTS
<<Exhaust Gas Purification Catalyst Device>>
[0029] The typical structure of the exhaust gas purification
catalyst device of the present invention is shown in FIG. 1.
[0030] The exhaust gas purification catalyst device (100) of FIG. 1
comprises: [0031] an upper layer (10) containing first carrier
particles and rhodium, and [0032] a lower layer (20) containing
second carrier particles, wherein [0033] the upper layer (10)
includes a rhodium-rich portion (11) in a range of (a) greater than
50% to 80% of the length of the upper layer from a downstream side
end of an exhaust gas flow and of (b) less than 20 .mu.m in the
depth direction from an outermost surface of the upper layer, and
the rhodium-rich portion (11) contains rhodium in an amount of 50%
to less than 100% of the total rhodium included in the upper layer
(10).
[0034] The upper layer (10) of the exhaust gas purification
catalyst device (100) of FIG. 1 is composed of the rhodium-rich
portion (11) near the surface of the upper layer on the downstream
side of the exhaust gas flow and the other portions (12) of the
upper layer.
[0035] The rhodium-rich portion (11) contains rhodium in an amount
of 50% to less than 100% of the total rhodium included in the upper
layer (10).
[0036] In exhaust gas purification catalyst devices, the exhaust
gas purification reaction proceeds from the upstream side of the
exhaust gas flow, and the reaction heat increases the temperature
of the downstream catalyst coating layer which promotes the
downstream exhaust gas purification reaction. By concentrating the
rhodium included in the upper layer on the downstream side of the
exhaust gas flow, the exhaust gas, in particular, NO.sub.R, can be
efficiently purified at the rhodium-rich portion on the downstream
side of the flow.
[0037] Furthermore, by concentrating the rhodium near the surface
of the upper layer, even when the mass flow rate is high, since the
exhaust gas can diffuse up to the depth of the rhodium-rich portion
near the surface, efficient exhaust gas purification becomes
possible.
[0038] The structure of an exhaust gas purification catalyst device
as an example of a preferred embodiment (hereinafter referred to as
"the present embodiment") of the exhaust gas purification catalyst
device of the present invention will be described.
(Rhodium-Rich Portion)
[0039] The rhodium-rich portion of the exhaust gas purification
catalyst device of the present embodiment is arranged in the upper
layer containing the first carrier particles and rhodium in a range
of greater than 50% to 80% of the length of the upper layer from a
downstream side end of an exhaust gas flow and of less than 20
.mu.m in the depth direction from an outermost surface of the upper
layer.
[0040] The range of the rhodium-rich portion in the exhaust gas
flow direction is within the range of greater than 50% to 80% of
the length of the upper layer from the downstream side end of the
exhaust gas flow. When the rhodium-rich portion is arranged within
the range of up to 80% of the length of the upper layer from the
downstream side end of the exhaust gas flow, the degree of
concentration of the rhodium becomes sufficiently high, and as a
result, the reaction heat generated on the upstream side of the gas
flow can be effectively used. The range of the rhodium-rich portion
in the exhaust gas flow direction may be within a range of up 75%,
up to 70%, up to 65%, or up to 60% of the length of the upper layer
from the downstream side of the exhaust gas flow.
[0041] Conversely, for efficient exhaust gas purification, it is
desirable that the rhodium-rich portion in which rhodium is
concentrated have a significant length. From this viewpoint, it is
necessary that the range of the rhodium-rich portion in the exhaust
gas flow direction exceed 50% of the length of the upper layer from
the downstream side end of the exhaust gas flow, and it may be
arranged across a range of 55% or more or 60% or more thereof.
[0042] The range of the rhodium-rich portion in the depth direction
of the catalyst coating layer is a range of less than 20 .mu.m in
the depth direction from the outermost surface of the upper layer.
As used herein, "outermost surface of the upper layer" refers to
the surface on the side of the upper layer contacting the exhaust
gas flow, i.e., the surface on the side opposite the surface which
contacts the lower layer. When the rhodium-rich portion is arranged
within a range of less than 20 .mu.m in the depth direction from
the outermost surface of the upper layer, even when the mass flow
rate of the exhaust gas is high, the exhaust gas can diffuse across
the entirety of the depth direction of the rhodium-rich portion,
whereby the rhodium can be efficiently used. The range of the
rhodium-rich portion in the depth direction of the catalyst coating
layer may be up to 19 .mu.m, 18 .mu.m, or 17 .mu.m in the depth
direction from the outermost surface of the upper layer.
[0043] The content of rhodium in the rhodium-rich portion is within
the range of 50% to less than 100% of the total rhodium included in
the upper layer. By setting the rhodium content of the rhodium-rich
layer to 50% or more of the total rhodium of the upper layer, the
effect of concentration of the rhodium in the rhodium-rich portion
can be sufficiently exhibited. Conversely, by restraining the
rhodium content of the rhodium-rich portion to less than 100% and
arranging a small amount of rhodium in the other portions of the
upper layer, oxygen release from an oxygen release material (OSC
material, e.g., ceria) optionally included in the upper layer is
promoted. Thus, under, for example, rich conditions, it becomes
easy to maintain the atmosphere in the catalyst coating layer near
stoichiometry, whereby exhaust gas purification capacity is
maintained at a high level. The content of rhodium in the other
portions of the upper layer other than the rhodium-rich portion
will be described later.
[0044] As described above, in the exhaust gas purification catalyst
device of the present embodiment, by concentrating the rhodium in
the upper layer on the downstream side of the exhaust gas flow and
efficiently utilizing the reaction heat generated on the upstream
side, whereby purification efficiency can be increased when the
mass flow rate of the exhaust gas is high. Thoroughly pursuing this
purpose, by evenly arranging rhodium within the range of the
rhodium-rich portion and increasing the effective length of
rhodium-rich portion which can utilize the heat of the reaction on
the upstream side, catalytic efficiency when the mass flow rate of
exhaust gas is high can be further increased.
[0045] From such a viewpoint, the quantity of rhodium included in
the rhodium-rich portion in the range up to 50% of the length of
the rhodium-rich portion from the downstream side end of the
exhaust gas flow may be 70% or less of the rhodium included in the
rhodium-rich portion. The content of rhodium included in this range
may be 67% or less, 65% or less, 63% or less, or 60% or less and
51% or more, 53% or more, 55% or more, 57% or more, or 60% or more
of the rhodium included in the rhodium-rich portion.
[0046] The rhodium-rich portion constitutes a part of the upper
layer. Thus, regarding the composition of the rhodium-rich portion,
aside from the rhodium content, the descriptions of the upper layer
described below equally correspond thereto. For example, the
rhodium included in the rhodium-rich portion may be carried on the
first carrier particles included in the upper layer.
<Upper Layer>
[0047] The upper layer of the exhaust gas purification catalyst
device of the present embodiment contains the first carrier
particles and rhodium.
(First Carrier Particles)
[0048] The first carrier particles may be particles of an inorganic
oxide.
[0049] The inorganic oxide constituting the first carrier particles
may be one or more oxides such as, for example, aluminum, silicon,
zirconium, titanium, and rare earth elements. The inorganic oxide
is preferably one or more selected from alumina, silica,
silica-alumina, zeolite, titania, zirconia, ceria, and oxides of
rare earth elements other than ceria. It is particularly preferable
that the inorganic oxide contain alumina and zirconia, or contain
alumina and a ceria-zirconia composite oxide (CZ). The case in
which an oxide of a rare earth element other than ceria is
contained together with alumina and zirconia or alumina and CZ is
also a preferable aspect of the present embodiment.
[0050] In the upper layer of the exhaust gas purification catalyst
of the present embodiment, the content of ceria may be in the range
of 25% by mass or less, when the total mass of the first carrier
particles in the upper layer is 100% by mass. It is preferable that
the content of the ceria in the upper layer be limited for the
following reasons.
[0051] Since the catalyst coating layer contains ceria having an
oxygen storage capacity, the catalyst coating layer can be
maintained near stoichiometry even under rich conditions or lean
conditions. Conversely, it is necessary that rhodium be promptly
reduced and activated during warm-up, for example, when the engine
is started. If the amount of ceria in the catalyst coating layer is
excessively large, the amount of oxygen released from ceria will be
excessively large and the reduction of rhodium will be delayed,
which may prevent early activation of the exhaust gas purification
catalyst device. To avoid this, the content of ceria in the upper
layer is preferably limited to a predetermined value or less.
[0052] From the viewpoints described above, the content of ceria of
the upper layer may be 25% by mass or less, 23% by mass or less,
20% by mass or less, 18% by mass or less, or 15% by mass or less,
when the total mass of the first carrier particles in the upper
layer is 100% by mass. Conversely, from the viewpoint of
maintaining the state of the catalyst coating layer near
stoichiometry, the upper layer may contain ceria in a range which
does not delay rapid reduction of the rhodium when starting the
engine. Thus, the content of ceria of the upper layer may be a
quantity exceeding 5% by mass, and may be, for example, 6% by mass
or more, 7% by mass or more, 8% by mass or more, or 10% by mass or
more, when the total mass of the first carrier particles in the
upper layer is 100% by mass. The case in which the content of ceria
of the upper layer is in the range of greater than 5% by mass to
25% by mass or less, or 7% to 15% by mass, when the total mass of
the first carrier particles in the upper layer is 100% by mass, is
a preferred aspect of the present embodiment.
[0053] When the cerium atom is a part of the composite oxide (for
example, CZ), the content of ceria means the amount converted into
the composition of CeO.sub.2.
[0054] The first carrier particles may be primary particles or may
be secondary particles in which primary particles are
agglomerated.
[0055] The average particle diameter of the first carrier particles
may be, for example, 1 .mu.m or more, 3 .mu.m or more, 5 .mu.m or
more, or 10 .mu.m or more, and may be, for example, 50 .mu.m or
less, 40 .mu.m or less, 30 .mu.m or less, or 20 .mu.m or less.
(Rhodium)
[0056] The upper layer contains rhodium. The rhodium may be carried
on the first carrier particles in the upper layer.
[0057] The content of rhodium of the upper layer, as a metal
conversion mass of rhodium per unit volume of the substrate, may
be, for example, 0.01 g/L or more, 0.05 g/L or more, 0.10 g/L or
more, or 0.15 g/L or more, and may be, for example, 0.50 g/L or
less, 0.40 g/L or less, 0.35 g/L or less, or 0.30 g/L or less.
[0058] The rhodium may be included in the rhodium-rich portion of
the upper layer and the other portions of the upper layer.
[0059] As described above, when the entirety of the rhodium in the
upper layer is arranged in the rhodium-rich portion and no rhodium
is included in the other portions of the upper layer at all, oxygen
is less likely to be released from the OSC material optionally
included in the upper layer, and it may be difficult to maintain
the atmosphere in the catalyst coating layer near stoichiometry,
resulting in impaired exhaust gas purification capacity. To prevent
this, the portions of the upper layer other than the rhodium-rich
portion may contain, as a metal conversion mass of rhodium per unit
volume of the substrate, for example, 0.01 g/L or more, 0.02 g/L or
more, 0.03 g/L or more, 0.04 g/L or more, or 0.05 g/L or more of
rhodium.
[0060] In order to concentrate the rhodium in the rhodium-rich
portion to develop the effect of the present invention of improving
exhaust gas purification capacity when the mass flow rate of the
exhaust gas is high, the content of rhodium in the portions of the
upper layer other than the rhodium-rich portion may be, as a metal
conversion mass of rhodium per unit volume of the substrate, for
example, 0.12 g/L or less, 0.10 g/L or less, or 0.08 g/L or
less.
(Optional Components of Upper Layer)
[0061] The upper layer may include options components other than
the first carrier particles and rhodium.
[0062] Examples of optional components of the upper layer include
precious metals other than rhodium, alkali metal compounds,
alkaline earth metal compounds, transition metals, and binders.
[0063] The precious metal other than rhodium may be, for example, a
platinum-group metal other than rhodium, and specifically, is one
or more selected from, for example, palladium and platinum.
However, since the exhaust gas purification catalyst device of the
present embodiment can exhibit sufficiently high exhaust gas
purification capacity without the inclusion of a precious metal
other than rhodium in the upper layer, a precious metal other than
rhodium may not be arranged in the upper layer.
[0064] Examples of alkali metal compounds include potassium
compounds and lithium compounds. Examples of alkaline earth metal
compounds include calcium compounds, barium compounds, and
strontium compounds. These have the effect of improving the heat
resistance of the resulting catalyst.
[0065] The binder adheres the first carrier particles to each other
and the first carrier particles to the lower layer, and has a
function of imparting mechanical strength to the catalyst coating
layer. Examples of such a binder include alumina sol, zirconia sol,
silica sol, and titania sol.
(Upper Layer Coating Amount)
[0066] The coating amount of the upper layer, as the mass of the
upper layer per unit volume of the substrate, may be, for example,
50 g/L or more, 60 g/L or more, 70 g/L or more, or 80 g/L or more,
and may be, for example, 300 g/L or less, 250 g/L or less, 200 g/L
or less, or 150 g/L or less.
(Lower Layer)
[0067] The lower layer of the exhaust gas purification catalyst of
the present embodiment contains second carrier particles. The lower
layer may contain, in addition to the second carrier particles, a
precious metal and other optional components.
(Second Carrier Particles)
[0068] The second carrier particles included in the lower layer may
be appropriately selected and used from those exemplified above as
the first carrier particles included in the upper layer.
(Precious Metal)
[0069] The lower layer may contain precious metals. These precious
metals may be carried on the second carrier particles in the lower
layer.
[0070] The precious metal of the lower layer may be a
platinum-group metal, and is one or more selected from, for
example, rhodium, palladium, and platinum. In order to efficiently
exhibit the effect of the exhaust gas purification catalyst device
of the present invention, it is appropriate from rhodium to be
concentrated in the upper layer. From this viewpoint, the content
of rhodium contained in the lower layer may be less than 50%, and
may be 30% or less, 10% or less, 5% or less, or 1% or less, when
the total rhodium contained in the exhaust gas purification
catalyst device of the present invention is 100%. The lower layer
may not contain rhodium at all.
[0071] The lower layer may contain a precious metal other than
rhodium, which is preferable.
[0072] The precious metal other than rhodium of the lower layer may
be, for example, a platinum-group metal other than rhodium, and
specifically, is, for example, one or more selected from palladium
and platinum. The content of the precious metal other than rhodium
of the lower layer, per unit volume of the substrate, may be, for
example, 0.1 g/L or more, 0.2 g/L or more, 0.3 g/L or more, 0.4 g/L
or more, or 0.5 g/L or more, and may be, for example, 1.0 g/L or
less, 0.8 g/L or less, 0.7 g/L or less, or 0.6 g/L or less.
(Optional Components of Lower Layer)
[0073] The lower lay may contain, in addition to the second carrier
particles and the optionally arranged precious metal, optional
components other than these.
[0074] Examples of the optional components of the lower layer
include alkali metal compounds, alkaline earth metal compounds,
transition metals, and binders. These are the same as those
described above as optional components of the upper layer.
(Lower Layer Coating Amount)
[0075] The coating amount of the lower lay, as the mass of the
lower layer per unit volume of the substrate, may be, for example,
50 g/L or more, 60 g/L or more, 70 g/L or more, or 80 g/L or more,
and may be, for example, 300 g/L or less, 250 g/L or less, 200 g/L
or less, or 150 g/L or less.
<Substrate>
[0076] In the exhaust gas purification catalyst device of the
present embodiment, the lower layer and the upper layer as
described above may be present, in this order, on a substrate, or
the lower lay may constitute a part or the entirety of the
substrate and the upper layer may be present on the lower layer,
which is a part or the entirety of the substrate. In either case,
the capacity of the substrate may be, for example, approximately 1
L.
[0077] As the substrate in the case in which the lower layer and
the upper layer are present on a substrate, those which are
generally used as the substrates of exhaust gas purification
catalyst devices can be used. For example, the substrate may be
composed of a material such as cordierite, SiC, stainless steel,
and inorganic oxide particles, and may be, for example, a
monolithic honeycomb substrate.
[0078] In the case in which the lower layer constitutes a part or
the entirety of the substrate, the substrate may be composed of
inorganic oxide particles, and a part or the entirety of the
inorganic oxide particles may be of the same type as the second
carrier particles of the lower layer. The substrate in this case
may also be, for example, a monolithic honeycomb substrate.
<<Exhaust Gas Purification Catalyst Device Production
Method>>
[0079] The exhaust gas purification catalyst device of the present
embodiment may be produced by a method comprising, for example,
providing a coating liquid including a rhodium precursor to an
exhaust gas downstream side end of an exhaust gas purification
catalyst device precursor comprising an upper layer containing
first carrier particles and rhodium and a lower layer containing
second carrier particles, and thereafter suctioning from an exhaust
gas upstream side end of the exhaust gas purification catalyst
device precursor.
<Preparation of Exhaust Gas Purification Catalyst Device
Precursor>
[0080] The exhaust gas purification catalyst device precursor may
be produced by, for example: [0081] a method in which a lower layer
containing second carrier particles and an upper layer containing
first carrier particles and rhodium are formed in this order on a
substrate (first method), or [0082] a method comprising forming a
substrate, and forming an upper layer containing first carrier
particles and rhodium on the obtained substrate, wherein a part or
the entirety of the substrate is constituted by a lower layer
containing second carrier particles (second method).
[0083] According to the first method described above, an exhaust
gas purification catalyst device precursor comprising a lower layer
and an upper layer on a substrate is obtained. According to the
second method, the lower layer constitutes a part or the entirety
of the substrate, and an exhaust gas purification catalyst device
precursor comprising an upper layer on the lower layer, which is a
part or the entirety of the substrate, is obtained. The first and
second methods will be described in order below.
<Exhaust Gas Purification Catalyst Device Precursor Preparation
(First Method)>
(Substrate)
[0084] In the first method for preparing the exhaust gas
purification catalyst device precursor, as the substrate, a
predetermined substrate which should be included in the exhaust gas
purification catalyst device may be selected and used. The
substrate is, for example, a monolithic honeycomb substrate
composed of cordierite or inorganic oxide particles, as described
above.
(Lower Layer Formation)
[0085] The lower layer can be formed by applying a lower layer
formation coating liquid containing the material of the lower layer
or a precursor thereof to such a substrate and firing after removal
of the solvent as necessary.
[0086] Since the lower layer of the exhaust gas purification
catalyst device precursor of the present embodiment contains the
second carrier particles, the lower layer formation coating liquid
may also contain second carrier particles suitably selected from
among those described above.
[0087] Like the lower layer of the exhaust gas purification
catalyst device, the lower layer of the exhaust gas purification
catalyst device precursor may contain precious metals, preferably
one or more selected from palladium and platinum. In this case, by
including precursors of the desired precious metals in the lower
layer formation coating liquid, these precious metals can be
contained in the obtained lower layer. The precursors of the
precious metals may be, for example, halides, nitrates, or sulfates
of the desired precious metals.
[0088] In addition to the second carrier particles and the
optionally used precious metal precursors, the optional components
of the lower layer described above may be included in the lower
layer formation coating liquid.
[0089] The solvent of the lower layer formation coating liquid may
typically be water.
[0090] The lower layer can be formed by applying such a lower layer
formation coating liquid onto the surface of the substrate and
firing after removing the solvent as necessary.
[0091] As a coating method, a known method such as a dipping method
or a pouring method can be adopted without limitation. The coating
amount is appropriately selected in accordance with the desired
coating layer amount.
[0092] Firing after coating can be carried out in accordance with a
method of heating at a temperature of, for example, 400 to
800.degree. C., preferably 450 to 600.degree. C., for a time of,
for example, 5 minutes to 4 hours, preferably 30 minutes to 3
hours. The ambient atmosphere during firing may be air.
(Upper Layer Formation)
[0093] The lower layer can be formed by next applying an upper
layer formation coating liquid comprising the material of the upper
layer or a precursor thereof to the lower layer formed as described
above, and firing after removal of the solvent as necessary.
[0094] Since the upper layer of the exhaust gas purification
catalyst device precursor of the present embodiment contains the
first carrier particles, the upper layer formation coating liquid
may also contain first carrier particles appropriately selected
from among those described above.
[0095] Like the upper layer of the exhaust gas purification
catalyst device, the upper layer of the exhaust gas purification
catalyst device precursors contains rhodium. In this case, by
including a rhodium precursor in the upper layer formation coating
liquid, rhodium can be contained in the upper layer to be obtained.
The rhodium precursor may be, for example, rhodium nitrate, rhodium
chloride, sodium rhodium chloride, rhodium chloride pentaamine, or
carbonyl acetyl rhodium.
[0096] In addition to the first carrier particles and the rhodium
precursor, the optional components of the upper layer described
above may be contained in the upper layer formation coating
liquid.
[0097] The solvent of the upper layer coating liquid may typically
be water.
[0098] By applying such an upper layer coating liquid onto the
surface of the lower layer on the substrate and firing after
removing the solvent as necessary, the upper layer can be formed,
whereby the exhaust gas purification catalyst device precursor can
be obtained.
[0099] The application and firing of the upper layer formation
coating liquid may be carried out in the same manner as the
application and firing of the lower layer formation coating
liquid.
<Exhaust Gas Purification Catalyst Device Precursor Preparation
(Second Method)>
(Substrate Formation)
[0100] In the second method, first, the substrate is formed. A part
or the entirety of the substrate is constituted by the lower layer
containing the second carrier particles. When the desired lower
layer of the present embodiment is a part of the substrate, it is
desirable that the lower layer constitute the surface of the
substrate.
[0101] Formation of the substrate in the second method can be
carried out in accordance with, for example, the method described
in Patent Literature 4 using the second carrier particles of the
desired lower layer of the present embodiment as a part or the
entirety of the inorganic oxide particles constituting the
substrate.
[0102] When the lower layer constitutes a part or the entirety of
the substrate, the substrate can be obtained by press-forming a
substrate formation composition and carrying out drying and firing.
The substrate formation composition can be obtained by, for
example, mixing the second carrier particles of the desired lower
layer of the present embodiment with other inorganic oxide
particles as necessary, further adding thereto water and a binder,
and kneading.
[0103] When a precious metal other than rhodium is included in the
desired lower layer, a precursor of the desired precious metal,
such as, for example, a halide, nitrate, or sulfate thereof, may be
included in the substrate formation composition, and after
formation of the lower layer in which the precious metal is not
included, formation may be carried out by a method in which the
lower layer is impregnated with a solution of the precursor of the
desired precious metal and fired. According to this method, a
substrate constituted by a lower layer material including the
precious metal other than rhodium can be obtained.
(Upper Layer Formation)
[0104] The upper layer is next formed on the lower layer formed as
descried above, whereby the exhaust gas purification catalyst
device precursor can be obtained. This upper layer formation may be
carried out in the same manner as the upper layer formation of the
first method.
<Rhodium-Rich Portion Formation>
[0105] Next, the desired exhaust gas purification catalyst device
of the present embodiment can be obtained by forming the
rhodium-rich portion on a predetermined range of the upper layer of
the exhaust gas purification catalyst device precursor.
[0106] The rhodium-rich portion may be formed by a method
comprising the steps of arranging a rhodium-rich portion formation
coating liquid containing a rhodium precursor on the exhaust gas
downstream side end of the exhaust gas purification catalyst device
precursor, and thereafter suctioning from the exhaust gas upstream
side end of the exhaust gas purification catalyst device
precursor.
[0107] The rhodium-rich portion formation coating liquid contains a
rhodium precursor, and in some cases, may further contain a
thickener.
[0108] The rhodium precursor may be, for example, rhodium nitrate,
rhodium chloride, sodium rhodium chloride, rhodium chloride
pentaamine, or carbonyl acetyl rhodium.
[0109] The thickener may be appropriately selected and used from
cellulose-based thickeners, water-soluble polymers, and thickeners
derived from natural products.
[0110] The cellulosed-based thickener may be selected from, for
example, methyl cellulose, ethyl cellulose, carboxymethyl
cellulose, carboxyethyl cellulose, hydroxymethyl cellulose,
hydroxyethyl cellulose, and hydroxypropyl methyl, as well as sodium
salts and calcium salts thereof. The water-soluble polymer may be
selected from polyvinyl alcohol and polyvinyl pyrrolidone. The
thickener derived from natural products may be selected from, for
example, xanthan gum and gelatin.
[0111] The solvent of the rhodium-rich portion formation coating
liquid may typically be water.
[0112] The rhodium-rich portion formation coating liquid may be
used by adjusting the viscosity at a shear rate of 4 sec.sup.-1 to
a range of 1,000 mPas to 7,000 mPas.
[0113] In the formation of the rhodium-rich portion, the
rhodium-rich portion formation coating liquid containing the
rhodium precursor is arranged on the exhaust gas downstream side
end of the exhaust gas purification catalyst device precursor, and
thereafter suctioned from the exhaust gas upstream side end of the
exhaust gas purification catalyst device precursor. Thereafter,
firing may be carried out.
[0114] The range of the rhodium-rich portion in the depth direction
of the catalyst coating layer can be adjusted by changing the
content of the thickener in the rhodium-rich portion formation
coating liquid and appropriately adjusting the viscosity of the
coating liquid to the range described above. In other words, since
the coating liquid having a low viscosity can penetrate more
easily, the lower the viscosity of the coating liquid, the wider
(deeper) the range of the rhodium-rich portion in the depth
direction of the catalyst coating layer can be made, and since the
coating liquid having a higher viscosity can penetrate less easily,
the higher the viscosity of the coating liquid, the more narrow
(shallower) the range of the rhodium-rich portion in the depth
direction of the catalyst coating layer can be made.
[0115] Furthermore, the range of the rhodium-rich portion in the
exhaust gas flow direction can be adjusted by changing the amount
of solvent in the rhodium-rich portion formation coating liquid and
adjusting the amount of the coating liquid. In other words, when
the amount of coating liquid arranged on the exhaust gas downstream
side end of the exhaust gas purification catalyst precursor is
high, by suctioning from the upstream side end, the coating liquid
can spread over a wide (long) range toward the upstream side of the
exhaust gas flow of the rhodium-rich portion, and when the amount
of the coating liquid is small, even if the coating liquid is
suctioned, it is consumed in a narrow (short) range toward the
upstream side of the exhaust gas flow and does not reach the
upstream side. Utilizing this fact, the range of rhodium-rich
portion in the exhaust gas flow direction can be adjusted by
changing the amount of solvent in the rhodium-rich portion
formation coating liquid.
[0116] The exhaust gas flow concentration gradient of the
rhodium-rich portion can be adjusted by changing the time of
suctioning from the exhaust gas upstream side end after the
rhodium-rich portion formation coating liquid has been arranged on
the exhaust gas downstream side end of the exhaust gas purification
catalyst device precursor. In other words, near the downstream side
end of the exhaust gas purification catalyst device precursor, a
relatively large amount of coating liquid comes into contact with
the upper layer during the time from the arrangement of the coating
liquid to suction, whereby more rhodium is carried thereon.
Conversely, at positions on the further upstream side from the
downstream side end, after arrangement of the coating liquid, the
remaining coating liquid moves due to suction, whereby a relatively
small amount of coating liquid comes into contact with the upper
layer. Thus, by changing the time from the arrangement of the
coating liquid to suction, a difference in the amount of coating
liquid which comes into contact with the upper layer between the
downstream side end of the exhaust gas purification catalyst device
precursor and an upstream side position on the further upstream
side moved from the downstream side end can be provided. Therefore,
to evenly arrange rhodium in the exhaust gas flow direction of the
rhodium-rich portion, after arrangement of the coating liquid on
the exhaust gas downstream side end of the exhaust gas purification
catalyst device precursor, suction may be carried out from the
upstream end of the exhaust gas in as short a time as possible.
[0117] Firing, which is subsequently optionally carried out, can be
carried out by a method of heating at a temperature of, for
example, 400 to 800.degree. C., and preferably 450 to 600.degree.
C., for a time of, for example, 5 minutes to 4 hours, and
preferably 30 minutes to 3 hours. The ambient atmosphere during
firing may be air.
[0118] According to the foregoing, the exhaust gas purification
catalyst device of the present embodiment can be produced.
EXAMPLES
[0119] In the following examples, exhaust gas purification catalyst
devices were produced by sequentially forming a lower layer and an
upper layer on a substrate by a wet coating method using a
cordierite honeycomb substrate (cell number: 600 cpsi, cell shape:
hexagonal cross-section, wall thickness: 2 mil, substrate length:
84 mm), and thereafter forming a rhodium-rich portion on a part of
the upper layer by an impregnation method, and thereafter, the
exhaust gas purification catalyst devices were evaluated.
(Preparation of Lower Layer Formation Coating Liquid)
[0120] The lower layer formation coating liquid was prepared
adding, in terms of mass per unit volume of the honeycomb
substrate, 5.5 g/L palladium metal equivalent of palladium nitrate,
50 g/L of lanthana-alumina composite oxide, 30 g/L of ceria
zirconia composite oxide, 5 g/L of barium sulfate, and an alumina
sol binder to pure water, and thereafter stirring.
[0121] The lanthana content in the lanthana-alumina composite oxide
used herein was 5% by mass in terms of La.sub.2O.sub.3. The ceria
zirconia composite oxide contained small amounts of neodymia,
lanthana and yttria, and the ceria content was 10% by mass in terms
of CeO.sub.2.
(Preparation of Upper Layer Formation Coating Liquid)
[0122] The upper layer formation coating liquid was prepared by
adding, in terms of mass per unit volume of the honeycomb
substrate, 0.05 g/L rhodium metal equivalent of rhodium nitrate, 50
g/L of lanthana-alumina composite oxide, 50 g/L of ceria zirconia
composite oxide, and an alumina sol binder to pure water, and
thereafter stirring.
[0123] The lanthana content in the lanthana-alumina composite oxide
used herein was 5% by mass in terms of La.sub.2O.sub.3. The ceria
zirconia composite oxide contained small amounts of neodymia,
lanthana and yttria, and the ceria content was adjusted using
materials having different ceria contents in each of the Examples
and Comparative Examples.
(Preparation of Rhodium-Rich Portion Formation Coating Liquid)
[0124] The rhodium-rich portion formation coating liquid was
prepared by adding to pure water, in terms of mass per unit volume
of the honeycomb substrate, 0.74 g/L rhodium metal equivalent of
rhodium nitrate and a predetermined amount of thickener
(hydroxyethyl cellulose, produced by DaiCel Corporation), depending
on the case, and thereafter stirring.
Example 1
(Production of Exhaust Gas Purification Catalyst Device)
[0125] A lower layer was formed on the substrate by pouring the
lower layer formation coating liquid into the honeycomb substrate,
blowing off unnecessary coating liquid with a blower, thereafter
drying in a 120.degree. C. dryer for 2 hours, and subsequently
carrying out firing in a 500.degree. C. electric furnace for two
hours.
[0126] Next, an upper layer which does not have a rhodium-rich
portion was further formed on the lower layer on the substrate by
pouring the upper layer formation coating liquid into the substrate
after formation of the lower layer, blowing off the unnecessary
coating liquid using a blower, thereafter drying in a 120.degree.
C. dryer for 2 hours, and subsequently carrying out firing in a
500.degree. C. electric furnace for 2 hours, whereby the exhaust
gas purification catalyst device precursor was obtained.
[0127] The rhodium-rich portion formation coating liquid was poured
from the exhaust gas flow downstream side end of the obtained
exhaust gas purification catalyst device precursor, thereafter
suctioned from the exhaust gas flow upstream side end, and
subsequently firing was carried out in a 500.degree. C. electric
furnace for two hours, whereby the exhaust gas purification
catalyst device of Example 1 was produced. Note that the
concentration of the thickener in the rhodium-rich portion
formation coating liquid used in Example 1 was 1% by mass.
(Analysis of Rhodium-Rich Portion)
(Analysis 1--Range in Coating Layer Depth Direction)
[0128] The depth direction range of the rhodium-rich portion was
examined by filling the communication holes of the exhaust gas
purification catalyst device with resin followed by crushing to
obtain a sample and measuring the distribution of each element in
the crushed sample under the following conditions using a field
emission electron probe microanalyzer (FE-EPMA) (model name
"JXA-8530F" produced by JEOL Ltd.). [0129] Field of view
magnification: 400 times [0130] Beam diameter: minimum [0131]
Acceleration voltage: 20 kV [0132] Irradiation current: 100 nA
[0133] Convergence time: 50 seconds [0134] Resolution:
256.times.256
(Analysis 2--Ratio of Rhodium Contained in 50% of Downstream Side
of Rhodium-Rich Portion)
[0135] A sample obtained by cutting out the downstream 50% portion
of the rhodium-rich portion was ground completely to quantify the
amount of rhodium, and the ratio of the amount of rhodium contained
in the portion to the total rhodium contained in the rhodium-rich
portion was calculated.
(Evaluation of Exhaust Gas Purification Catalyst Device)
[0136] A 50 hour durability evaluation was carried out by a method
in which the obtained exhaust gas purification catalyst device was
attached to the exhaust system of a V-8 engine, the temperature of
the catalyst coating layer was set to 950.degree. C., and the
exhaust of gas in a stoichiometric atmosphere and a lean atmosphere
was alternately repeated at fixed intervals. The circulation time
for each repetition of each exhaust gas was 8 seconds for exhaust
gas in stoichiometric atmosphere and 3 seconds for exhaust gas in
lean atmosphere.
[0137] The exhaust gas purification catalyst device after the
durability evaluation was attached to the exhaust system of an L4
engine, and the warm-up property of NO.sub.x purification at the
time of engine start was evaluated. Specifically, exhaust gas at an
air-fuel ratio (A/F) of 14.55 was supplied to the exhaust gas
purification catalyst device under the condition of an exhaust gas
mass flow rate Ga=24 g/sec, and the temperature (50% reaching
temperature) T50 at which the NO.sub.x purification rate was 50%
was examined. The evaluation results are shown in Table 1.
Examples 2 to 7
[0138] Exhaust gas purification catalyst devices were produced and
evaluated in the same manner as Example 1 except that compositions
having specified ceria contents were used as the ceria zirconia
composite oxide contained in the upper layer formation coating
liquid, the content of thickening agent in rhodium-rich portion
formation coating liquid, the concentrations of the coating
liquids, and time from pouring the rhodium-rich portion formation
coating liquid into the exhaust gas purification catalyst device
precursor to suction were appropriately changed, and the length and
depth of rhodium-rich portion and the ratio (concentration
gradient) of rhodium in the range of 50% in the front of the
rhodium-rich portion were changed. The evaluation results are shown
in Table 1.
TABLE-US-00001 TABLE 1 Eval Results Rh-Rich Portion NO.sub.x
Purification Range Rate 50% Depth from Ratio to Ratio of Rh Amount
of Reaching Length from Outermost Surface Total Rh in Contained in
Ceria in Temperature Downstream of Upper Upper 50% Downstream Upper
Layer T50 at High Side End (%) Layer (.mu.m) Layer (%) Side Range
(%) (mass %) Ga (.degree. C.) Ex 1 80 17 75 60 10 302 Ex 2 70 17 75
60 10 303 Ex 3 80 17 60 60 10 306 Ex 4 80 17 40 60 10 310 Ex 5 50
17 75 60 20 312 Ex 6 50 25 75 80 20 312 Ex 7 100 17 75 80 20
308
[0139] Referring to Table 1, in the exhaust gas purification
catalyst devices of Example 5, in which the rhodium-rich portion
was present only in the range of 50% or less of the length of the
upper layer from the downstream side end of the upper layer,
Example 6, in which the rhodium-rich portion was present in the
ranges of 50% or less of the length of the upper layer from the
downstream side end of the upper layer and a depth of up to 25
.mu.m from the outermost surface of the upper layer, and Example 7,
in which the rhodium-rich portion was present in the entirety of
the length of the upper layer, the 50% reaching temperature T50 was
high. Furthermore, the same was true for the exhaust gas
purification catalyst device of Example 4, in which the ratio of
rhodium contained in the rhodium-rich portion was less than 50% of
the total rhodium contained in the upper layer.
[0140] Conversely, in the exhaust gas purification catalyst devices
of Examples 1 to 3, in which the rhodium-rich portion was present
in the range of greater than 50% to 80% of the length of the upper
layer from the downstream side end of the upper layer and in the
range of less than 20 .mu.m from the outermost surface of the upper
layer, and in which the content of rhodium contained in the
rhodium-rich portion was equal to 50% to less than 100% of the
total rhodium contained in the upper layer, the 50% reaching
temperature T50 was a low value. It is believed that by
centralizing the rhodium of the upper layer in a predetermined
length near the surface layer on the downstream side of the
catalyst coating layer, even in the case in which the mass flow
rate Ga of the exhaust gas is high, efficient contact between the
rhodium and the exhaust gas can be ensured.
Examples 8 and 9
[0141] Exhaust gas purification catalyst devices were produced and
evaluated in the same manner as Example 1 except that compositions
having specified ceria contents were used as the ceria zirconia
composite oxide contained in the upper layer formation coating
liquid, the concentrations of the rhodium-rich portion formation
coating liquid, and the time from pouring the rhodium-rich portion
formation coating liquid into the exhaust gas purification catalyst
device precursor to suction were appropriately changed, and the
length of the rhodium-rich portion and the ratio (concentration
gradient) of rhodium in the range of 50% of the downstream side of
the rhodium-rich portion were changed. The evaluation results are
shown in Table 2 along with the results of Examples 1 and 2.
TABLE-US-00002 TABLE 2 Eval Results Rh-Rich Portion NO.sub.x
Purification Range Rate 50% Depth from Ratio to Ratio of Rh Amount
of Reaching Length from Outermost Surface Total Rh in Contained in
Ceria in Temperature Downstream of Upper Upper 50% Downstream Upper
Layer T50 at High Side End (%) Layer (.mu.m) Layer (%) Side Range
(%) (mass %) Ga (.degree. C.) Ex 1 80 17 75 60 10 302 Ex 2 70 17 75
60 10 303 Ex 8 80 17 75 80 10 308 Ex 9 100 17 75 80 28 310
[0142] By comparing Examples 8 and 9 and Examples 1 and 2 in Table
2, it was confirmed that in the case in which the quantity of
rhodium contained in the range of up to 50% of the length of the
rhodium-rich portion from the downstream side end of the exhaust
gas flow is equal to 70% or less of the amount of rhodium contained
in the rhodium-rich portion in the predetermined exhaust gas
purification catalyst device of the present invention, the 50%
reaching temperature T50 was reduced. It is considered that the
purpose of the present invention was thoroughly achieved by
centralizing the rhodium of the upper layer across a predetermined
length of the catalyst coating layer from the downstream side.
Examples 10 to 14
[0143] Exhaust gas purification catalyst devices were produced and
evaluated in the same manner as Example 1 except that compositions
having specified ceria contents were used as the ceria zirconia
composite oxide contained in the upper layer formation coating
liquid, the concentration of the thickener in the rhodium-rich
portion formation coating liquid, the concentrations of the coating
liquids, and the time from pouring the rhodium-rich portion
formation coating liquid into the exhaust gas purification catalyst
device precursor to suction were appropriately changed, and the
length and depth of the rhodium-rich portion and the ratio
(concentration gradient) of rhodium in the range of 50% of the
downstream side of the rhodium-rich portion were changed. The
evaluation results are shown in Table 3 along with the results of
Examples 1 and 2.
TABLE-US-00003 TABLE 3 Eval Results Rh-Rich Portion NO.sub.x
Purification Range Rate 50% Depth from Ratio to Ratio of Rh Amount
of Reaching Length from Outermost Surface Total Rh in Contained in
Ceria in Temperature Downstream of Upper Upper 50% Downstream Upper
Layer T50 at High Side End (%) Layer (.mu.m) Layer (%) Side Range
(%) (mass %) Ga (.degree. C.) Ex 1 80 17 75 60 10 302 Ex 2 70 17 75
60 10 303 Ex 10 80 18 75 65 7 305 Ex 11 80 19 75 69 15 302 Ex 12 60
17 75 60 22 304 Ex 13 50 13 75 55 5 312 Ex 14 80 17 75 60 30
310
[0144] Referring to Table 3, in the exhaust gas purification
catalyst devices of Example 13, in which the content of ceria in
the upper layer was 5% by mass, and Example 14, in which the
content of ceria in the upper layer exceeded 25% by mass, the 50%
reaching temperature T50 was high.
[0145] Conversely, the 50% reaching temperatures T50 of the exhaust
gas purification catalyst devices of Examples 1, 2, and 10 to 12,
in which the content of ceria in the upper layer was greater than
5% by mass and 25% by mass or less, were low as compared to
Examples 11 to 13. It is believed that this is because in the
predetermined exhaust gas purification catalyst device of the
present invention, the catalyst layer is maintained in an
atmosphere near stoichiometry because the upper layer contains an
appropriate amount of ceria, whereby the exhaust gas purification
efficiency is improved.
REFERENCE SIGNS LIST
[0146] 10 upper layer [0147] 11 rhodium-rich layer [0148] 12 other
portions of the upper layer [0149] 20 lower layer [0150] 100
exhaust gas purification catalyst device [0151] a range of the
rhodium-rich portion in the direction of exhaust gas flow [0152] b
range of the rhodium-rich portion in the depth direction of the
catalyst coating layer
* * * * *