U.S. patent application number 12/937913 was filed with the patent office on 2011-02-10 for particulate combustion catalyst, particulate filter and exhaust gas purifying apparatus.
This patent application is currently assigned to MITSUI MINING & SMELTING CO., LTD. Invention is credited to Akira Abe, Junji Kawano.
Application Number | 20110030354 12/937913 |
Document ID | / |
Family ID | 41198887 |
Filed Date | 2011-02-10 |
United States Patent
Application |
20110030354 |
Kind Code |
A1 |
Abe; Akira ; et al. |
February 10, 2011 |
PARTICULATE COMBUSTION CATALYST, PARTICULATE FILTER AND EXHAUST GAS
PURIFYING APPARATUS
Abstract
Provided is a particulate combustion catalyst including a
carrier formed of Zr oxide, a carrier formed of a Zr--Ce composite
oxide, or a carrier formed of a composite oxide containing Zr, Ce,
and at least one metal selected from among Nd, La, Fe, Y, Pr, Ba,
Ca, Mg, Sn, and Sr; an oxide of at least one metal selected from
the group consisting of Ba, Ca, Mg, and Sr, the metal oxide being
supported on the carrier in an amount, as reduced to metal, of 0.5
to 30 mass % with respect to the carrier; and metallic Ag or Ag
oxide, which serves as a catalyst component and is supported on the
carrier. Also provided are a particulate filter coated with the
catalyst; and an exhaust gas cleaning apparatus including a
particulate filter coated with the catalyst.
Inventors: |
Abe; Akira; (Saitama,
JP) ; Kawano; Junji; (Saitama, JP) |
Correspondence
Address: |
YOUNG & THOMPSON
209 Madison Street, Suite 500
Alexandria
VA
22314
US
|
Assignee: |
MITSUI MINING & SMELTING CO.,
LTD
Tokyo
JP
|
Family ID: |
41198887 |
Appl. No.: |
12/937913 |
Filed: |
October 23, 2008 |
PCT Filed: |
October 23, 2008 |
PCT NO: |
PCT/JP2008/069242 |
371 Date: |
October 14, 2010 |
Current U.S.
Class: |
60/297 |
Current CPC
Class: |
B01J 2523/00 20130101;
B01D 2239/0478 20130101; B01D 53/944 20130101; B01J 37/0242
20130101; B01D 2255/20715 20130101; B01J 2523/00 20130101; F01N
2510/065 20130101; B01J 2523/00 20130101; B01D 2255/407 20130101;
B01J 2523/00 20130101; B01D 2255/30 20130101; B01J 2523/00
20130101; B01J 23/002 20130101; B01D 2255/204 20130101; B01J
2523/00 20130101; B01D 2258/012 20130101; B01J 2523/00 20130101;
F01N 3/035 20130101; B01D 2255/104 20130101; B01D 2255/2047
20130101; B01D 2255/2042 20130101; B01J 23/66 20130101; B01D
2255/2045 20130101; B01D 2255/206 20130101; B01J 2523/3712
20130101; B01D 39/06 20130101; B01J 2523/3712 20130101; B01J
2523/3725 20130101; B01J 2523/3706 20130101; B01J 2523/3706
20130101; B01J 2523/25 20130101; B01J 2523/48 20130101; B01J
2523/48 20130101; B01J 2523/3712 20130101; B01J 2523/18 20130101;
B01J 2523/18 20130101; B01J 2523/22 20130101; B01J 2523/3706
20130101; B01J 2523/3712 20130101; B01J 2523/18 20130101; B01J
2523/48 20130101; B01J 2523/48 20130101; B01J 2523/18 20130101;
B01J 2523/24 20130101; B01J 2523/23 20130101; B01J 2523/25
20130101; B01J 2523/18 20130101; B01J 2523/3712 20130101; B01J
2523/48 20130101; B01J 2523/3706 20130101; B01J 2523/18 20130101;
B01J 2523/3725 20130101; B01J 2523/25 20130101; B01J 2523/48
20130101; B01J 2523/3725 20130101; B01J 2523/3725 20130101 |
Class at
Publication: |
60/297 |
International
Class: |
F01N 3/035 20060101
F01N003/035 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 14, 2008 |
JP |
2008-105156 |
Claims
1. A particulate combustion catalyst characterized by comprising a
carrier formed of zirconium oxide; an oxide of at least one metal
selected from the group consisting of Ba, Ca, Mg, and Sr, the metal
oxide being supported on the carrier in an amount, as reduced to
metal, of 0.5 to 30 mass % with respect to the carrier; and
metallic Ag or Ag oxide, which serves as a catalyst component and
is supported on the carrier.
2. A particulate combustion catalyst characterized by comprising a
carrier formed of a zirconium-cerium composite oxide; an oxide of
at least one metal selected from the group consisting of Ba, Ca,
Mg, and Sr, the metal oxide being supported on the carrier in an
amount, as reduced to metal, of 0.5 to 30 mass % with respect to
the carrier; and metallic Ag or Ag oxide, which serves as a
catalyst component and is supported on the carrier.
3. A particulate combustion catalyst characterized by comprising a
carrier formed of a composite oxide containing zirconium, cerium,
and at least one metal selected from among Nd, La, Fe, Y, Pr, Ba,
Ca, Mg, Sn, and Sr; an oxide of at least one metal selected from
the group consisting of Ba, Ca, Mg, and Sr, the metal oxide being
supported on the carrier in an amount, as reduced to metal, of 0.5
to 30 mass % with respect to the carrier; and metallic Ag or Ag
oxide, which serves as a catalyst component and is supported on the
carrier.
4. A particulate combustion catalyst according to claim 1, wherein
the amount, as reduced to metal, of metallic Ag or Ag oxide, which
serves as a catalyst component and is supported on the carrier, is
0.1 to 25 mass % on the basis of the total mass of the carrier and
an oxide of at least one metal selected from the group consisting
of Ba, Ca, Mg, and Sr.
5. A particulate combustion catalyst according to claim 1, wherein
the surface of the carrier is provided with SiO.sub.2, TiO.sub.2,
ZrO.sub.2, or Al.sub.2O.sub.3, which serves as a binder
component.
6. A particulate filter characterized by being coated with a
particulate combustion catalyst as recited in claim 1.
7. An exhaust gas cleaning apparatus characterized by comprising a
particulate filter coated with a particulate combustion catalyst as
recited in claim 1.
8. A particulate combustion catalyst according to claim 2, wherein
the amount, as reduced to metal, of metallic Ag or Ag oxide, which
serves as a catalyst component and is supported on the carrier, is
0.1 to 25 mass % on the basis of the total mass of the carrier and
an oxide of at least one metal selected from the group consisting
of Ba, Ca, Mg, and Sr.
9. A particulate combustion catalyst according to claim 3, wherein
the amount, as reduced to metal, of metallic Ag or Ag oxide, which
serves as a catalyst component and is supported on the carrier, is
0.1 to 25 mass % on the basis of the total mass of the carrier and
an oxide of at least one metal selected from the group consisting
of Ba, Ca, Mg, and Sr.
10. A particulate combustion catalyst according to claim 2, wherein
the surface of the carrier is provided with SiO.sub.2, TiO.sub.2,
ZrO.sub.2, or Al.sub.2O.sub.2, which serves as a binder
component.
11. A particulate combustion catalyst according to claim 3, wherein
the surface of the carrier is provided with SiO.sub.2, TiO.sub.2,
ZrO.sub.2, or Al.sub.2O.sub.2, which serves as a binder
component.
12. A particulate combustion catalyst according to claim 4, wherein
the surface of the carrier is provided with SiO.sub.2, TiO.sub.2,
ZrO.sub.2, or Al.sub.2O.sub.2, which serves as a binder
component.
13. A particulate combustion catalyst according to claim 8, wherein
the surface of the carrier is provided with SiO.sub.2, TiO.sub.2,
ZrO.sub.2, or Al.sub.2O.sub.2, which serves as a binder
component.
14. A particulate combustion catalyst according to claim 9, wherein
the surface of the carrier is provided with SiO.sub.2, TiO.sub.2,
ZrO.sub.2, or Al.sub.2O.sub.2, which serves as a binder
component.
15. A particulate filter characterized by being coated with a
particulate combustion catalyst as recited in claim 2.
16. A particulate filter characterized by being coated with a
particulate combustion catalyst as recited in claim 3.
17. An exhaust gas cleaning apparatus characterized by comprising a
particulate filter coated with a particulate combustion catalyst as
recited in claim 2.
18. An exhaust gas cleaning apparatus characterized by comprising a
particulate filter coated with a particulate combustion catalyst as
recited in claim 3.
Description
TECHNICAL FIELD
[0001] The present invention relates to a particulate combustion
catalyst, to a particulate filter, and to an exhaust gas cleaning
apparatus. More particularly, the present invention relates to a
particulate combustion catalyst which realizes removal (through
oxidation) of particulate matter discharged from a diesel internal
combustion engine; to a particulate filter coated with the
particulate combustion catalyst; and to an exhaust gas cleaning
apparatus including the particulate filter coated with the
particulate combustion catalyst.
BACKGROUND ART
[0002] Exhaust gas discharged from diesel engines contains nitrogen
oxides (NO.sub.x) and particulate matter, and release of such
substances into the atmosphere without any treatment is a main
cause of air pollution. Therefore, demand has arisen for strict
regulation for such substances. There has been proposed, as
effective means for removing particulate matter, a flow-through
oxidation catalyst for combustion of soluble organic fractions
(SOFs), and a diesel exhaust gas trapping system employing a diesel
particulate filter for trapping soot. However, for regeneration of
such a particulate filter, particulate matter trapped therein must
be continuously removed through oxidation.
[0003] Hitherto, a variety of continuous regeneration systems have
been proposed, and examples thereof include a system employing a
catalyst including a carrier made of an inorganic oxide (e.g.,
zirconium oxide, vanadium oxide, or cerium oxide), and an expensive
noble metal (e.g., Pt) supported on the carrier (see, for example,
Patent Document 1, 2, or 3); and a continuous regeneration method
involving NO.sub.2 (see, for example, Patent Document 4). This
continuous regeneration method requires provision, upstream of a
particulate filter, of an oxidation catalyst (e.g., Pt) for
oxidizing NO into NO.sub.2, and thus involves high cost. In
addition, reaction involving NO.sub.2 is affected by the ratio of
NO.sub.x to C, and many restrictions are imposed on the employment
of this method.
Patent Document 1: Japanese Patent Application Laid-Open (kokai)
No. H10-047035 Patent Document 2: Japanese Patent Application
Laid-Open (kokai) No. 2003-334443 Patent Document 3: Japanese
Patent Application Laid-Open (kokai) No. 2004-058013
Patent Document 4: Japanese Patent No. 3012249
DISCLOSURE OF THE INVENTION
Problems to be Solved by the Invention
[0004] An object of the present invention is to provide a
particulate combustion catalyst which exhibits excellent heat
resistance, which realizes removal of soot through oxidation at low
temperature without employment of an expensive noble metal, and
which enables oxidation reaction to proceed with the aid of only
oxygen and thus realizes removal of soot through oxidation at low
temperature regardless of the NO.sub.x concentration of exhaust
gas. Another object of the present invention is to provide a
particulate filter coated with the particulate combustion catalyst.
Yet another object of the present invention is to provide an
exhaust gas cleaning apparatus comprising the particulate filter
coated with the particulate combustion catalyst.
Means for Solving the Problems
[0005] In order to achieve the aforementioned objects, the present
inventors have conducted extensive studies, and as a result have
found that the objects can be achieved by employing, as a carrier
of a particulate combustion catalyst, a specific oxide or a
composite oxide having a specific composition; supporting, on the
carrier, an oxide of at least one metal selected from the group
consisting of Ba, Ca, Mg, and Sr; and supporting, on the carrier,
metallic Ag or Ag oxide, which serves as a catalyst component. The
present invention has been accomplished on the basis of this
finding.
[0006] Accordingly, the present invention provides a particulate
combustion catalyst characterized by comprising a carrier formed of
zirconium oxide; an oxide of at least one metal selected from the
group consisting of Ba, Ca, Mg, and Sr, the metal oxide being
supported on the carrier in an amount, as reduced to metal, of 0.5
to 30 mass % with respect to the carrier; and metallic Ag or Ag
oxide, which serves as a catalyst component and is supported on the
carrier.
[0007] The present invention also provides a particulate combustion
catalyst characterized by comprising a carrier formed of a
zirconium-cerium composite oxide; an oxide of at least one metal
selected from the group consisting of Ba, Ca, Mg, and Sr, the metal
oxide being supported on the carrier in an amount, as reduced to
metal, of 0.5 to 30 mass % with respect to the carrier; and
metallic Ag or Ag oxide, which serves as a catalyst component and
is supported on the carrier.
[0008] The present invention also provides a particulate combustion
catalyst characterized by comprising a carrier formed of a
composite oxide containing zirconium, cerium, and at least one
metal selected from among Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and
Sr; an oxide of at least one metal selected from the group
consisting of Ba, Ca, Mg, and Sr, the metal oxide being supported
on the carrier in an amount, as reduced to metal, of 0.5 to 30 mass
% with respect to the carrier; and metallic Ag or Ag oxide, which
serves as a catalyst component and is supported on the carrier.
[0009] The present invention also provides a particulate filter
characterized by being coated with any of the aforementioned
particulate combustion catalysts. The present invention also
provides an exhaust gas cleaning apparatus characterized by
comprising a particulate filter coated with any of the
aforementioned particulate combustion catalysts.
EFFECTS OF THE INVENTION
[0010] The particulate combustion catalyst of the present invention
exhibits excellent heat resistance. Employment of the particulate
combustion catalyst of the present invention realizes removal of
soot through oxidation at low temperature without use of an
expensive noble metal. When the particulate combustion catalyst is
employed, since oxidation reaction proceeds with the aid of only
oxygen, soot can be removed through oxidation at low temperature
regardless of the NO.sub.x concentration of exhaust gas. Even when
a catalyst system including the particulate combustion catalyst is
exposed to a high-temperature atmosphere for a long period of time,
degradation of the system can be suppressed.
BEST MODES FOR CARRYING OUT THE INVENTION
[0011] The particulate combustion catalyst of the present invention
employs a carrier formed of zirconium oxide, a carrier formed of a
zirconium-cerium composite oxide, or a carrier formed of a
composite oxide containing zirconium, cerium, and at least one
metal selected from among Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and
Sr. Among catalysts having the same composition (except for a
carrier), a catalyst including a carrier formed of a
zirconium-cerium composite oxide tends to be higher in performance
than a catalyst including a carrier formed of zirconium oxide, and
a catalyst including a carrier formed of a composite oxide
containing zirconium, cerium, and at least one metal selected from
among Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr tends to be higher
in performance than a catalyst including a carrier formed of a
zirconium-cerium composite oxide.
[0012] When the particulate combustion catalyst of the present
invention employs, as a carrier, a zirconium-cerium composite
oxide, the cerium oxide content of the composite oxide is
preferably 5 to 50 mass %. When the cerium oxide content exceeds 50
mass %, the specific surface area of the carrier is reduced at a
high temperature (e.g., 700.degree. C. or higher), which may
eventually cause thermal degradation of the catalyst. In addition,
when the cerium oxide content exceeds 50 mass %, an active species
may fail to sufficiently exert its performance. In contrast, when
the cerium oxide content is less than 5 mass %, the carrier
exhibits poor heat resistance, which may eventually cause thermal
degradation of the catalyst.
[0013] When the particulate combustion catalyst of the present
invention employs, as a carrier, a composite oxide containing
zirconium, cerium, and at least one metal selected from among Nd,
La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr, since the carrier contains
an oxide of at least one metal selected from among Nd, La, Fe, Y,
Pr, Ba, Ca, Mg, Sn, and Sr, the carrier exhibits improved thermal
stability, and oxidation property at low temperature is improved.
In order to attain such effects, the amount of an oxide of at least
one metal selected from among Nd, La, Fe, Y, Pr, Ba, Ca, Mg, Sn,
and Sr is preferably 1 mass % or more. However, when the amount of
such a metal oxide exceeds 35 mass %, accordingly, the relative
amounts of zirconium oxide and cerium oxide are reduced, and
characteristics of the carrier containing the zirconium-cerium
composite oxide tend to be deteriorated. Therefore, in the
composite oxide contained in the carrier employed, preferably, the
amount of an oxide of at least one metal selected from among Nd,
La, Fe, Y, Pr, Ba, Ca, Mg, Sn, and Sr is 1 to 35 mass % (i.e., when
two or more metal oxides are employed, the total amount of the
oxides is 1 to 35 mass %), and the cerium oxide content is 5 to 50
mass % (zirconium oxide content: balance).
[0014] In the present invention, an oxide of at least one metal
selected from the group consisting of Ba, Ca, Mg, and Sr must be
supported on any of the aforementioned carriers. A conventionally
known technique (e.g., the impregnation method or the sol-gel
method) may be employed for supporting such a metal oxide on the
carrier. In the present invention, when an oxide of at least one
metal selected from the group consisting of Ba, Ca, Mg, and Sr is
supported on the carrier, the resultant particulate combustion
catalyst exhibits improved heat resistance. In order to attain such
effects, the amount (as reduced to metal) of an oxide of at least
one metal selected from among Ba, Ca, Mg, and Sr is preferably 0.5
mass % or more with respect to the carrier. However, when the
amount (as reduced to metal) of such a metal oxide exceeds 30 mass
% with respect to the carrier, accordingly, the relative amounts of
zirconium oxide and cerium oxide are reduced, and characteristics
of the carrier containing the zirconium-cerium composite oxide tend
to be deteriorated. Therefore, preferably, the amount (as recued to
metal) of an oxide of at least one metal selected from among Ba,
Ca, Mg, and Sr is 0.5 to 30 mass % (i.e., when two or more metal
oxides are employed, the total amount (as recued to metal) of the
oxides is 0.5 to 30 mass %) with respect to the carrier (i.e., 0.5
to 30 parts by mass on the basis of 100 parts by mass of the
carrier).
[0015] In the present invention, metallic Ag or Ag oxide must be
supported, as a catalyst component, on the carrier. A
conventionally known technique (e.g., the impregnation method or
the sol-gel method) may be employed for supporting such a catalyst
component on the carrier. Metallic Ag or Ag oxide, which is
employed in the present invention, is less expensive than, for
example, Pt or Pd. In addition, when metallic Ag or Ag oxide is
employed in combination with a specific carrier used in the present
invention, further excellent effects are obtained, as compared with
the case where a Pt or Pd component is employed. In the present
invention, preferably, the amount (as reduced to metal) of metallic
Ag or Ag oxide supported on the carrier is 0.1 to 25 mass % on the
basis of the total mass of the carrier and an oxide of at least one
metal selected from the group consisting of Ba, Ca, Mg, and Sr.
When the amount of the catalyst component is less than 0.1 mass %,
the catalyst component may fail to sufficiently exhibit its
catalytic effects, whereas when the amount of the catalyst
component exceeds 25 mass %, a specific combination employed in the
present invention may fail to sufficiently exhibit a synergistic
effect. Meanwhile, when the amount of the catalyst component is
large, sintering of metal is likely to occur, and the catalyst
component is not expected to exhibit its catalytic effects.
[0016] In consideration that the particulate filter of the present
invention is produced by causing the particulate combustion
catalyst of the present invention to be held on a base, preferably,
the surface of the carrier is provided with a binder component such
as SiO.sub.2, TiO.sub.2, ZrO.sub.2, or Al.sub.2O.sub.3. When such a
binder component is provided on the surface of the carrier,
adhesion between the base and the carrier is enhanced, and the
catalyst exhibits improved durability and heat resistance.
[0017] The particulate filter of the present invention may assume
any known form of particulate filter, but preferably has a
three-dimensional structure. Specific examples of filters having a
three-dimensional structure include a wall-through filter, a
flow-through honeycomb filter, a wire mesh filter, a ceramic fiber
filter, a metallic porous filter, a particle-charged filter, and a
foam filter. Examples of the material of the base include ceramic
materials such as cordierite and SiC; Fe--Cr--Al alloys; and
stainless steel alloys.
[0018] The exhaust gas cleaning apparatus of the present invention,
which includes therein the aforementioned particulate filter of the
present invention, will be readily appreciated by those skilled in
the art.
[0019] Next will be described a method for producing the
particulate filter of the present invention.
[0020] Any of the aforementioned types of carriers is mixed with a
binder component (e.g., SiO.sub.2 or alumina sol) and water, and
the resultant mixture is finely milled by means of a milling
apparatus (e.g., a ball mill). A particulate filter (e.g., a wire
mesh filter) is coated with the thus-obtained slurry. In general,
the slurry-coated filter is fired at a temperature of about
500.degree. C. to about 700.degree. C. The thus-formed wash-coating
layer is impregnated with, for example, a nitrate of at least one
metal selected from the group consisting of Ba, Ca, Mg, and Sr, and
then drying and firing are carried out. Subsequently, the resultant
product is impregnated with, for example, silver nitrate serving as
a catalyst component, and then drying and firing are carried out.
Alternatively, the wash-coating layer may be impregnated with, for
example, a nitrate of at least one metal selected from the group
consisting of Ba, Ca, Mg, and Sr, together with, for example,
silver nitrate, followed by drying and firing. The total catalyst
coating amount is preferably 10 to 100 g/L (for a wall-flow
particulate filter) or about 50 to about 150 g/L (for a wire mesh
particulate filter). When the total catalyst coating amount is
excessively small, sufficient performance fails to be attained,
whereas when the total catalyst coating amount is excessively
large, back pressure to exhaust gas increases.
EXAMPLES
[0021] The present invention will next be described in detail with
reference to Examples and Comparative Examples. In each of the
Examples and Comparative Examples, a parenthesized numerical value
following each of the oxides constituting a composite oxide
represents the amount (mass %) of the constitutive oxide.
Example 1
[0022] Water (30 g) was added to powder of a composite oxide of
CeO.sub.2(22)ZrO.sub.2(72)La.sub.2O.sub.2(2)Nd.sub.2O.sub.3(4) (20
g), and SiO.sub.2 sol (i.e., a binder component) (5 g, as reduced
to SiO.sub.2) was added thereto, followed by mixing for two hours,
to thereby prepare a slurry. By use of the slurry, a
cordierite-made particulate filter (25.4 mm in diameter.times.60 mm
in length) was coated with the composite oxide. The
composite-oxide-coated filter was dried at 120.degree. C. for three
hours, and then fired in air at 500.degree. C. for one hour. The
composite-oxide-coated filter was found to have a composite oxide
content of 40 g/L. The composite-oxide-coated filter was
impregnated with an aqueous magnesium nitrate solution having a
specific concentration and an aqueous silver nitrate solution
having a specific concentration. The resultant product was dried at
120.degree. C. for three hours, and then finally fired in air at
500.degree. C. for one hour. The finally formed filter was found to
have an Ag content of 5 g/L and an Mg content of 1 g/L. The Ag
content as determined on the basis of the total mass of the
aforementioned composite oxide and magnesium oxide was 12 mass %,
and the Mg content as determined on the basis of the mass of the
aforementioned composite oxide was 2.5 mass %.
Example 2
[0023] The procedure of Example 1 was repeated, except that the
aqueous magnesium nitrate solution was replaced with an aqueous
calcium nitrate solution. The finally formed filter was found to
have an Ag content of 5 g/L and a Ca content of 1 g/L. The Ag
content as determined on the basis of the total mass of the
aforementioned composite oxide and calcium oxide was 12.1 mass %,
and the Ca content as determined on the basis of the mass of the
aforementioned composite oxide was 2.5 mass %.
Example 3
[0024] The procedure of Example 1 was repeated, except that the
aqueous magnesium nitrate solution was replaced with an aqueous
barium nitrate solution. The finally formed filter was found to
have an Ag content of 5 g/L and a Ba content of 1 g/L. The Ag
content as determined on the basis of the total mass of the
aforementioned composite oxide and barium oxide was 12.2 mass %,
and the Ba content as determined on the basis of the mass of the
aforementioned composite oxide was 2.5 mass %.
Example 4
[0025] The procedure of Example 1 was repeated, except that the
aqueous magnesium nitrate solution was replaced with an aqueous
strontium nitrate solution. The finally formed filter was found to
have an Ag content of 5 g/L and an Sr content of 1 g/L. The Ag
content as determined on the basis of the total mass of the
aforementioned composite oxide and strontium oxide was 12.1 mass %,
and the Sr content as determined on the basis of the mass of the
aforementioned composite oxide was 2.5 mass %.
Example 5
[0026] The procedure of Example 1 was repeated, except that the
composite oxide of
CeO.sub.2(22)ZrO.sub.2(72)La.sub.2O.sub.3(2)Nd.sub.2O.sub.3(4) was
replaced with ZrO.sub.2, and the aqueous magnesium nitrate solution
was replaced with an aqueous barium nitrate solution. The finally
formed filter was found to have an Ag content of 5 g/L and a Ba
content of 1 g/L. The Ag content as determined on the basis of the
total mass of ZrO.sub.2 and barium oxide was 12.2 mass %, and the
Ba content as determined on the basis of the mass of ZrO.sub.2 was
2.5 mass %.
Example 6
[0027] The procedure of Example 1 was repeated, except that the
composite oxide of
CeO.sub.2(22)ZrO.sub.2(72)La.sub.2O.sub.3(2)Nd.sub.2O.sub.3(4) was
replaced with a composite oxide of CeO.sub.2(30)ZrO.sub.2(70), and
the aqueous magnesium nitrate solution was replaced with an aqueous
barium nitrate solution. The finally formed filter was found to
have an Ag content of 5 g/L and a Ba content of 1 g/L. The Ag
content as determined on the basis of the total mass of the
aforementioned composite oxide and barium oxide was 12.2 mass %,
and the Ba content as determined on the basis of the mass of the
aforementioned composite oxide was 2.5 mass %.
Example 7
[0028] Water (700 g) was added to powder of a composite oxide of
CeO.sub.2(22)ZrO.sub.2(72)La.sub.2O.sub.3(2)Nd.sub.2O.sub.3(4) (100
g), and the resultant mixture was milled by means of a ball mill so
as to attain a mean particle size of 1 .mu.m or less. Subsequently,
ZrO.sub.2 sol (i.e., a binder component) (10 g, as reduced to
ZrO.sub.2) was added to the mixture, followed by mixing for two
hours, to thereby prepare a slurry. By use of the slurry, a
cordierite-made particulate filter (25.4 mm in diameter.times.76.2
mm in length) was coated with the composite oxide. The
composite-oxide-coated filter was dried at 120.degree. C. for three
hours, and then fired in air at 500.degree. C. for one hour. The
composite-oxide-coated filter was found to have a composite oxide
content of 40 g/L. The composite-oxide-coated filter was
impregnated with an aqueous barium nitrate solution having a
specific concentration and an aqueous silver nitrate solution
having a specific concentration. The resultant product was dried at
120.degree. C. for three hours, and then finally fired in air at
500.degree. C. for one hour. The finally formed filter was found to
have an Ag content of 2 g/L and a Ba content of 2 g/L. The Ag
content as determined on the basis of the total mass of the
aforementioned composite oxide and barium oxide was 4.7 mass %, and
the Ba content as determined on the basis of the mass of the
aforementioned composite oxide was 5 mass %.
Comparative Example 1
[0029] The procedure of Example 1 was repeated, except that the
aqueous magnesium nitrate solution was not employed. The finally
formed filter was found to have an Ag content of 5 g/L, and the Ag
content as determined on the basis of the mass of the composite
oxide was 12.5 mass %.
Comparative Example 2
[0030] The procedure of Example 7 was repeated, except that the
aqueous barium nitrate solution was not employed. The finally
formed filter was found to have an Ag content of 2 g/L, and the Ag
content as determined on the basis of the mass of the composite
oxide was 5 mass %.
<Evaluation of Catalyst-Coated Particulate Filter in Terms of
Soot Combustion by Use of Simulated Exhaust Gas>
[0031] The Tig (combustion initiation temperature) of soot
corresponding to each of the catalyst-coated particulate filters
produced in Examples 1 to 6 and Comparative Example 1 was measured
through the following method.
[0032] A specific amount of a dispersion prepared by dispersing
carbon (Printex-V (toner carbon), product of Degussa) (20 mg) in
ethyl alcohol was added dropwise to each of the catalyst-coated
particulate filters produced in Examples 1 to 6 and Comparative
Example 1 (25.4 mm in diameter.times.60 mm in length) from above
the filter, followed by drying at 100.degree. C. for 10 minutes.
Thus, carbon (20 mg) was deposited on one catalyst-coated
particulate filter. The carbon-deposited filter was fixed at a
center portion of a quartz-made simulated exhaust gas reaction
tube. While a circulation gas having the below-described
composition was caused to flow through the quartz reaction tube at
the below-described flow rate, the temperature of the reaction tube
was elevated at the below-described temperature elevation rate by
means of an electric furnace, and CO and CO.sub.2 concentrations
were measured at the outlet of the reaction tube by means of an
infrared analyzer. The temperature as measured at the inlet of the
catalyst-containing reaction tube when CO.sub.2 concentration
reached 400 ppm (i.e., electric furnace control temperature) was
regarded as Tig.
[0033] Gas composition: O.sub.2: 10%, H.sub.2O: 10%, N.sub.2:
balance
[0034] Flow rate: 25 L/min
[0035] Temperature elevation rate: 10 degrees (.degree. C.)/min
[0036] Table 1 shows the thus-measured Tig corresponding to the
respective catalyst-coated particulate filters produced in Examples
1 to 6 and Comparative Example 1, as well as the compositions of
the catalysts.
TABLE-US-00001 TABLE 1 Composition of catalyst Tig Example 1 Ag +
Mg/CeO.sub.2(22)ZrO.sub.2(72)La.sub.2O.sub.3(2)Nd.sub.2O.sub.3(4)
403.degree. C. Example 2 Ag +
Ca/CeO.sub.2(22)ZrO.sub.2(72)La.sub.2O.sub.3(2)Nd.sub.2O.sub.3(4)
409.degree. C. Example 3 Ag +
Ba/CeO.sub.2(22)ZrO.sub.2(72)La.sub.2O.sub.3(2)Nd.sub.2O.sub.3(4)
405.degree. C. Example 4 Ag +
Sr/CeO.sub.2(22)ZrO.sub.2(72)La.sub.2O.sub.3(2)Nd.sub.2O.sub.3(4)
417.degree. C. Example 5 Ag + Ba/ZrO.sub.2 415.degree. C. Example 6
Ag + Ba/CeO.sub.2(30)ZrO.sub.2(70) 407.degree. C. Comparative
Ag/CeO.sub.2(22)ZrO.sub.2(72)La.sub.2O.sub.3(2)Nd.sub.2O.sub.3(4)
452.degree. C. Example 1
<Evaluation of Heat Resistance of Catalyst-Coated Particulate
Filter>
[0037] For evaluation of heat resistance of a catalyst-coated
particulate filter, the balance point temperature of the filter was
measured by use of actual exhaust gas. Each of the catalyst-coated
particulate filters produced in Example 7 and Comparative Example
2, or each of the catalyst-coated particulate filters produced in
Example 7 and Comparative Example 2 and then subjected to thermal
treatment at 700.degree. C. or 800.degree. C. for 20 hours was
placed in a stainless steel holder, and the holder was fixed in a
quartz reaction tube. While a portion of exhaust gas discharged
from a diesel generator engine (engine displacement: 0.2 L)
(rotation speed: 3,000 rpm) was distributed to the quartz reaction
tube at a flow rate of 30.8 L/min, the quartz reaction tube was
heated from outside by means an electric furnace. After the
temperature had reached 300.degree. C., the quartz reaction tube
was heated in a stepwise manner at 20 degrees (.degree. C.)/10 min.
The difference in pressure between the inlet and the outlet of the
reaction tube containing the particulate filter was measured, and
the temperature at which the pressure difference is zero was
determined. The thus-determined temperature was regarded as balance
point temperature. Table 2 shows the balance point temperatures of
the respective catalyst-coated particulate filters. As is clear
from data shown in Table 2, the catalyst-coated particulate filter
of Example 7 (i.e., the particulate filter of the present
invention) exhibits excellent heat resistance (i.e., an increase in
balance point temperature is suppressed even after thermal
treatment at a high temperature), as compared with the case of the
catalyst-coated particulate filter of Comparative Example 2.
TABLE-US-00002 TABLE 2 Thermal treatment Type of catalyst-coated
Balance point temperature, time particulate filter temperature None
Example 7 454.degree. C. Comparative Example 2 454.degree. C.
700.degree. C., 20 hours Example 7 455.degree. C. Comparative
Example 2 488.degree. C. 800.degree. C., 20 hours Example 7
476.degree. C. Comparative Example 2 502.degree. C.
* * * * *