Note: Descriptions are shown in the official language in which they were submitted.
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DESCRIPTION
EXHAUST GAS-PURIFYING CATALYST
BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] The present invention relates to an exhaust gas-purifying
catalyst capable of purifying particulate material (hereinafter,
referred to as "PM") , which is contained in a diesel exhaust gas
or the like and mainly contains carbon, from a low-temperature
range. The exhaust gas-purifying catalyst according to the
present invention is particularly useful as a catalyst for
purifying exhaust gas for diesel engines because it can purify
not only PM, but also HC, CO, or NOx.
2. Description of the Related Art
[0002] As to gasoline engines, amounts of noxious ingredients
contained in an exhaust gas have been remarkably reduced by virtue
of the strict regulations for exhaust gases and the advance of
technologies coping with such regulations. On the other hand,
as to diesel engines, it is difficult to purify exhaust gases,
as compared to gasoline engines, due to an unusual circumstance
of diesel engines that noxious ingredients are emitted in the form
of PM (carbon particulates, sulfur-based particulates such as
sulfate particulates, high-molecular hydrocarbon particulates
(soluble organic fraction (SOF) ), or the like).
[0003] Known exhaust gas purifiers for diesel engines, which have
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been developed up to date, are mainly classified into a trap type
(wall flow structure) and an open type (straight flow structure) .
For the trap type exhaust gas purifier, a clogged honeycomb
structure (a diesel PM filter (hereinafter, referred to as a
"DPF")) made of ceramic is known. For example, a DPF is known
which includes a ceramic honeycomb structure with cells clogged
at opposite ends of openings thereof in the form of a checkered
pattern alternately. The DPF includes inlet cells each clogged
at an exhaust gas downstream side thereof, outlet cells each
arranged adjacent to the inlet cells and clogged at an exhaust
gas upstream side thereof, and cell partition walls partitioning
the inlet cells and the outlet cells from each other. In this
DPF, exhaust gas is filtered by pores of the cell partition walls,
which capture PM, so that emission of PM.is suppressed.
[0004] In the above-mentioned DPF, however, an increase in
pressure loss occurs due to accumulation of PM. As a result, it
is necessary to regenerate the DPF by periodically removing the
accumulated PM using a certain means. In accordance with a
.conventional technology, when an increase in pressure loss as
mentioned above occurs, it is possible to regenerate DPF by
burning the accumulated PM using a flow of hot exhaust gas. In
this case, however, an increased amount of the accumulated PM may
cause an increase in temperature during the burning process. For
this reason, the DPF may be melted and damaged, or may be broken
due to thermal stress.
[0005] Therefore, filter catalysts have recently been developed.
For example, Japanese Patent Publication No: 7-106290 discloses
a filter catalyst, the filter catalyst comprises a coating layer
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made of alumina, etc. and formed on surfaces of cell partition
walls of a DPF, and a catalytic metal such as platinum (Pt)
supported on the coating layer. With this filter catalyst,
captured PM is oxidized and burnt in accordance with a catalytic
reaction of the catalytic metal. As the PM is burnt
simultaneously with or successively to the capture thereof, the
filter catalyst can be continuously regenerated. The catalytic
reaction is carried out at a relatively low temperature. Also,
the burning is carried out for a small amount of captured PM. As
a result, the thermal stress applied to the filter catalyst is
low. Thus, there is an advantage in that breakage of the filter
catalyst is prevented.
[0006] Japanese Patent Application Publication No. 9-094434
also discloses a filter catalyst wherein a coating layer
supporting a catalytic metal is formed not only on cell partition
walls, but also on pores of the cell partition walls. Since the
catalytic metal is also supported in the pores of the cell
partition walls, the catalytic metal is likely to contact the PM.
The PM captured by the pores can also be oxidized and burnt.
[0007] Supporting alkali metal or alkaline earth metal on a
coating layer of a filter catalyst, together with noble metal,
is also disclosed in Japanese Patent Application Publication No.
2003-049627 or Japanese Patent Application Publication No.
2003-049631. The alkali metal or alkaline earth metal forms a
nitrate or sulfate in an exhaust gas. When the nitrate or sulfate
is decomposed, active oxygen is emitted. With the active oxygen,
it is possible to oxidize the PM.- Thus,- it is possible to
effectively oxidize the PM, and thus, to effectively purify the
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exhaust gas.
[0008] However, the filter catalyst including the coating layer
supporting alkali metal or alkaline earth metal, together with
noble metal, also has a problem in that a sufficient PM oxidation
performance cannot be exhibited in a general operation range of
about 400 C or below.
SUMMARY OF THE INVENTION
[0009] The present invention has been made in view of the
above-mentioned problems, and it is an aspect of the invention
to provide an exhaust gas-purifying catalyst which is capable of
oxidizing PM even in a low-temperature range of 300 C or below
and enhancing PM oxidation performance.
[0010] In one aspect, the present invention provides an exhaust
gas-purifying catalyst comprising: a filter substrate having a
wall flow structure, the filter substrate including inlet cells
each clogged at an exhaust gas downstream side of the inlet cell,
outlet cells each arranged adjacent to the inlet cells and clogged
.at an exhaust gas upstream side of the outlet cell, and porous
cell partition walls partitioning the inlet cells and the outlet
cells from each other and having a plurality of pores; and a
catalyst bed formed on the cell partition walls, wherein the
catalyst bed contains a porous oxide, a noble metal supported on
the porous oxide, and an alkali metal supported on the porous oxide
in an amount of 0.6 mole or more per 1L of the filter substrate,
and oxidizes particulate material(PM), which mainly contains
carbon, and is captured by the filter . substrate, from a
low-temperature range of 300 C or below.
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[0011] The catalyst may further comprise a protection layer
formed between the filter substrate and the catalyst bed, and made
of an oxide reactable with the alkali metal.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The above and other objects, and features of the present
invention will become apparent from the following description of
preferred embodiment, given in conjunction with the accompanying
drawings, in which:
FIG. 1 is an explanation view illustrating a structure of
an exhaust gas-purifying catalyst according to an exemplary
embodiment of the present invention;
FIG. 2 is a graph depicting a PM oxidation initiation
temperature and a PM oxidation peak temperature;
FIG. 3 is a graph depicting a relation between temperature
and differential pressure;
FIG. 4 is a graph depicting a relation between potassium
supporting amount and PM oxidation initiation temperature; and
FIG. 5 is an explanation view illustrating a structure of
an exhaust gas-purifying catalyst according to another exemplary
embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] Various embodiments of the present invention will now be
described in detail with reference to the accompanying drawings.
[0014] The present invention provides an exhaust gas-purifying
catalyst including a filter substrate and a. catalyst bed formed
on cell partition walls of the filter substrate. The filter
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substrate has a wall flow structure similar to a conventional DPF
including inlet cells each clogged at an exhaust gas downstream
side thereof, outlet cells each arranged adjacent to the inlet
cells and clogged at an exhaust gas upstream side thereof, and
porous cell partition walls partitioning the inlet cells and the
outlet cells from each other and having a plurality of pores.
[0015] The filter substrate may be formed of a metal foam or a
heat-resistant non-woven fabric. The filter substrate may also
be made of heat-resistant ceramics such as cordierite or silicon
carbide. For example, where the filter substrate is made of
heat-resistant ceramics, a clayey slurry containing cordierite
powder as a major component thereof is prepared. The prepared
slurry is shaped by extrusion, and is then calcined. In place
of the cordierite powder, a mixture of alumina powder, magnesia
powder and silica powder having the same composition as the
cordierite may be prepared. Openings of the cells at one end of
the filter substrate are clogged in the form of a checkered pattern
by clayey slurries having a shape similar to that of the cell
.openings, respectively. Also, openings of the cells each
arranged adjacent to one of the clogged cells are clogged at the
other end of the filter substrate. Thereafter, the clogging
material is fixed using calcining or the like. Thus, a filter
substrate having a honeycomb structure can be fabricated. The
cross-sectional shapes of the inlet cells and outlet cells may
be triangular, square, hexagonal, circular, etc. Of course, they
are not limited to such shapes.
[0016] The cell partition walls have.a porous structure allowing
an exhaust gas to pass therethrough. In order to form pores in
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the cell partition walls, combustible powder such as carbon powder,
wood powder, starch, or resin powder is mixed with the slurry.
As the combustible powder is burnt during the calcining process,
pores are formed in the cell partition walls. It is possible to
control the diameter and volume of the pores by adjusting the size
and content of the combustible powder. The inlet cells and outlet
cells are communicated with each other by the pores. Accordingly,
although PM is captured in the pores, gas can flow from the inlet
cells to the outlet cells via the pores.
[0017] Preferably, the cell partition walls have a porosity of
40% to 70%. Also, the pores preferably have an average diameter
of 10 pm to 40pm. Where the cell partition walls have the porosity
and average pore diameter ranging as described above, it is
possible to suppress an increase in pressure loss even when the
catalyst bed is formed to range from 100g/L to 200g/L. It is also
possible to suppress a decrease in strength. Thus, capture of
PM can be more effectively achieved.
[0018] In the exhaust gas-purifying catalyst according to the
present invention, the catalyst bed is provided at the cell
partition walls of the filter substrate. Although the catalyst
bed may be formed only on the surfaces of the cell partition walls,
it is preferred that the catalyst bed be also formed on the surfaces
of the pores in the cell partition walls. The catalyst bed
contains a porous oxide, noble metal supported on the porous oxide,
and alkali metal supported on the porous oxide.
[0019] The porous oxide may include alumina, zirconia, titania,
silica, or ceria conventionally used as a.catalyst support, or
a composite oxide or mixture of at least two of the catalyst
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supports. Among these materials, y-alumina having a large
specific surface area is preferable.
[0020] The noble metal supported on the porous oxide may be
selected from Pt, Pd, Rh, Ir, Ru, etc. Among these elements, it
is preferable to select Pt, which exhibits a high oxidation
activity to PM. Preferably, the supported amount of the noble
metal ranges from 0.1g to 5g per 1L of the filter substrate: When
the supported amount of the noble metal is less than the above
range, it is impractical due to an excessively low activity. On
the other hand, when the supported amount of the noble metal is
more than the above range, saturated activity is exhibited, and
the costs are increased. The supporting of the noble metal may
be achieved by an adsorption supporting method, a impregnating
supporting method, or the like using a solution containing a
nitrate of the noble metal dissolved therein.
[0021] For the alkali metal supported on the porous oxide, Na,
K, Li, Cs, etc. may be used. Among these elements, K is preferable
which exhibits a particularly-high oxidation activity to PM.
Preferably, the supported amount of the alkali metal is 0. 6 mole
or more per 1L of the filter substrate. When the supported amount
of the alkali metal is less than the above range, it is difficult
to initiate oxidation of PM at a temperature of 300 C or below.
Although there is no particular upper limit of the supported
amount of the alkali metal, it is preferred that the supported
amount of the alkali metal have an upper limit of about 2 moles
per 1L of the filter substrate, for purification of exhaust gases
of vehicles. When the supported amount of the alkali metal
exceeds the upper limit, a degradation in the activity of the noble
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metal occurs, thereby degrading the performance capable of
purifying HC, CO, NOX, etc.
[0022] In addition to the noble metal and alkali metal,
transition metals, typical metals, alkaline earth metals, rare
earth elements, etc. may be supported in catalyst bed within a
range giving no adverse effect on the purification performance.
[0023] The catalyst bed is formed by preparing a slurry of the
porous oxide powder with a binder ingredient such as an alumina
sol and water, applying the slurry to the cell partition walls,
and calcining the applied slurry, thereby forming a coating layer.
In this case, it is preferable to support the noble metal and alkali
metal on the coating layer. Alternatively, a slurry may be
prepared using catalyst powder prepared by previously supporting
the noble metal on the porous oxide powder. In this case, the
supporting of the alkali metal may be performed after the
formation of the catalyst bed using the prepared slurry. The
application of the slurry to the cell partition walls may be
achieved using a general dipping method. However, it is
preferable to remove a surplus of the slurry filled in the pores,
while forcibly filling the slurry in the pores of the cell
partition walls by air blow or air suction.
[0024] In this case, the formation amount of the coating layer
or catalyst bed preferably ranges from 30g to 200g per 1L of the
filter substrate. When the formation amount of the coating layer
or catalyst bed is less than 30g/L, it is impossible to prevent
a degradation in the durability of the noble metal. On the other
hand, the formation amount of the coating layer or catalyst bed
exceeding 200g/L is impractical due to an excessively high
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pressure loss.
[0025] Preferably, a protection layer made of an oxide reactable
with the alkali metal is formed between the filter substrate and
the catalyst bed. The protection layer functions to suppress the
alkali metal supported in the catalyst bed from migrating to the
filter substrate in a high-temperature atmosphere, and thus, to
suppress a degradation in the strength of the filter substrate.
It is also possible to suppress a degradation in the concentration
of the alkali metal in the catalyst bed caused by the migration
of the alkali metal to the filter substrate. Accordingly, a
degradation in PM oxidation activity can be suppressed.
[0026] Examples of the oxide reactable with the alkali metal may
be Ti02, Si02r A1203, B203, P205, etc. Preferably, the formation
amount of the protection layer corresponds to a thickness of
0.001um to 5pm or ranges from 1g to 50g per 1L of the filter
substrate. When the formation amount of the protection layer is
less than the above range, it is difficult to suppress the
migration of the alkali metal to the filter substrate. On the
other hand, the formation amount of the protection layer exceeding
the above range is impractical due to an excessive increase in
pressure loss.
[0027] That is, in the exhaust gas-purifying catalyst according
to the present invention, alkali metal is supported in an amount
of 0.6 mole or more per 1L of the filter substrate. As a large
amount of alkali metal is supported as described above, it is
possible to achieve an increase in the possibility that the alkali
metal comes into contact with PM. Also, the temperature, at which
PM can be oxidized, is lowered, so that PM can be oxidized at a
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low temperature of 300 C or below.
[0028] Accordingly, the exhaust gas-purifying catalyst
according to the present invention can purify PM by oxidation from
a low-temperature range lower than 300 C, so that the PM oxidation
performance can be considerably enhanced. As a result,
accumulation of PM is suppressed, thereby suppressing an increase
in pressure loss. Thus, continuous regeneration of the catalyst
for PM purification can be stably achieved, so that it is possible
to prevent defects such as cracks caused by forced regeneration.
[0029] Where a protection layer made of an oxide reactable with
the alkali metal is formed between the filter substrate and the
catalyst bed, as described above, it is possible to suppress the
alkali metal from migrating to the filter substrate by the
protection layer. Accordingly, it is possible to suppress a
degradation in the strength of the filter substrate in accordance
with a reaction of the alkali metal with cordierite. It is also
possible to suppress a degradation in PM oxidation performance
because consumption of the alkali metal is suppressed in
.accordance with the reaction.
EXAMPLES
(Example 1)
[0030] FIG. 1 illustrates an exhaust gas-purifying catalyst
according to this example. This catalyst includes: a filter
substrate 1 including inlet cells 10 each clogged at an exhaust
gas downstream side thereof, outlet cells 11 each arranged
adjacent to the inlet cells, and clogged at an exhaust gas upstream
side thereof, and porous cell partition walls 12 partitioning the
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inlet cells 10 and the outlet cells 11 from each other; and a
catalyst bed 2 formed on the surfaces of the cell partition walls
12 and on the surfaces of pores formed in the cell partition walls
12.
[0031] For the filter substrate 1, a commercially-available DPF
made of cordierite is used. This DPF has a test piece size (35cc,
30mm (diameter) x 50mm (length) ), and a porosity of 60% to 67 0,
a pore volume of 0. 58cc/g to 0. 65cc/g, and an average pore diameter
of 25pm to 35pm at the cell partition walls 12. A detailed
description of the structure of the catalyst bed 2 will be given
through a description of a method for manufacturing the catalyst
bed 2.
[0032] A slurry is prepared by mixing catalyst powder previously
supporting Pt with y-A1203 powder (specific surface area of
220m2/g), together with an alumina sol and ion-exchanged water,
such that the mixture has a viscosity of 100cps or less. The
prepared slurry is milled such that solid grains thereof have an
average diameter of 1pm or less. Thereafter, the filter substrate
1 is dipped in the slurry, to allow the slurry to be introduced
into the cells. The slurry is then sucked from the end of the
filter substrate 1 opposite to the dipped end in a state in which
the filter substrate 1 has been upwardly taken out of the slurry,
to remove a surplus of the slurry from the filter substrate 1.
After being dried by ventilation, the filter substrate 1 is
calcined at 500 C for 3 hours. This procedure is performed two
times, in order to adjust the formation of the coating layer such
that the coating layer is formed on the inlet cells 10 and outlet
cells 11 in substantially same amounts, respectively. The
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formation amount of the coating layer is 150g per 1L of the filter
substrate 1. The coating layer is formed on the surfaces of the
inlet cells 10 and outlet cells 11 and on the surfaces of the pores.
The Pt supporting amount of the coating layer is 3g/L.
[0033] In order to support Li in the coating layer in an amount
of 0.6 mole/L, a certain amount of a lithium acetate aqueous
solution having a certain concentration is then impregnated into
the coating layer. After being dried, the coating layer is
calcined at 300 C for 3 hours. Thus, the coating layer 2
supporting Pt and Li is completely formed.
(Comparative Example 1)
[0034] An exhaust gas-purifying catalyst according to
Comparative Example 1 is prepared in the same manner as Example
1, except that the supported amount of Li is 0.3 mole/L.
(Example 2)
[0035] An exhaust gas-purifying catalyst according to Example
2 is prepared in the same manner as Example 1, except that a
potassium acetate aqueous solution is used in place of the lithium
acetate aqueous solution, and K is supported in the coating layer
in an amount of 0.6 mole/L.
(Example 3)
[0036] An exhaust gas-purifying catalyst according to Example
3 is prepared in the same manner as Example 1, except that a
potassium acetate aqueous solution is used in place of the lithium
acetate aqueous solution, and K is supported in the coating layer
in an amount of 1.5 mole/L.
(Comparative Example 2)
[0037] An exhaust gas-purifying catalyst according to
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Comparative Example 2 is prepared in the same manner as Example
1, except that a potassium acetate aqueous solution is used in
place of the lithium acetate aqueous solution, and K is supported
in the coating layer in an amount of 0.3 mole/L.
(Comparative Example 3)
[0038] An exhaust gas-purifying catalyst according to
Comparative Example 3 is prepared in the same manner as Example
1, except that the alkali metal is not supported.
(Comparative Example 4)
[0039] An exhaust gas-purifying catalyst according to
Comparative Example 4 is prepared in the same manner as Example
1, except that a barium acetate aqueous solution is used in place
of the lithium acetate aqueous solution, and Ba is supported in
the coating layer in an amount of 0.3 mole/L.
<Experimental Example 1>
[0040] Each of the above-described catalysts was mounted to an
exhaust system of an engine bench, to which a diesel engine
(displacement volume: 2,000 cc) was mounted. For attachment of
PM to each catalyst, the diesel engine was operated for 2 hours
under the conditions of an engine RPM of 2,000 rpm, a torque of
3.0 kg, and an exhaust gas temperature of 250 C.
[0041] Each PM-attached catalyst was loaded in an evaluation
apparatus, and was then subjected to an increase in temperature
from room temperature to a temperature of 600 C at a rate of
C/min under the condition in which a model gas consisting of
10% of 02, 500 ppm of NO, and the balance of N2 flowed through
the catalyst at a flow rate of 0.03m3/min.
[0042] The concentration of CO2 in a gas emitted from each
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catalyst during the temperature increase was continuously
measured. Based on the results of the measurement, the
temperature, at which emission of CO2 was begun, was recorded as
a PM oxidation initiation temperature, and the temperature, at
which the measured CO2 concentration had a peak value, was recorded
as a PM oxidation peak temperature. FIG. 2 depicts the recorded
results.
<Experimental Example 2>
[0043] For each of the catalysts according to Example 2 and 3
and Comparative Example 2, the pressure difference between the
gas introduced into the catalyst and the gas emitted from the
catalyst during the temperature increase was continuously
measured. FIG. 3 depicts the measured results.
<Evaluation>
[0044] Referring to FIG. 2, it can be seen that the catalysts
of the examples, wherein Li or K is supported in an amount of 0.6
mole/L, exhibit a low PM oxidation initiation temperature and a
low PM oxidation peak temperature, as compared to the catalysts
of Comparative Examples 1 and 2. That is, it can be clearly seen
that the catalysts of the examples can oxidize PM from a
low-temperature range, and exhibit a high PM oxidation activity
in the low-temperature range.
[0045] It can also be seen that the supported amount of K is
preferable to be 1.5g/L, as compared to 0.6g/L, because the
catalyst of Example 3 exhibits a lower temperatures than that of
Example 2. Also, it can be seen that K is more preferable than
Li because the catalyst of Example 2 exhibits-a lower temperatures
than that of Example 1. On the other hand, it can be seen that
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Ba representing the alkaline earth metal of Comparative Example
4 has no effect obtained in a supported state.
[0046] As shown in FIG. 3, in the catalyst of Example 2, the
differential pressure thereof, which has increased, slightly
decreases around 300 C, again increases, and then greatly
decreases around 400 C. In the catalyst of Example 3, the
differential pressure thereof, which has increased, greatly
decreases around 280 C. In the catalyst of Comparative Example
2, however, the differential pressure thereof still exhibits an
increase around 300 C, and initially exhibits a decrease around
400 C.
[0047] That is, the decrease in differential pressure in the
catalyst of Example 2 near 300 C for the moment is due to the
presence of K in a high concentration of 0.6 mole/L. In the
catalyst of Example 3, wherein K is supported in'a high
concentration of 1.5 mole/L, this decrease is predominantly
exhibited. As shown in FIG. 2, effect differences among Example
2, Example 3, and Comparative Example 2 correspond to differences
-of the above-described action, respectively. Accordingly, it
can be seen that it is necessary to support K in an amount of 0. 6
mole/L.
<Experimental Example 3 = Evaluation>
[0048] A plurality of catalysts were prepared in the same manner
as that of Example 2, except that they had different K supporting
amounts within a range of 0 mole/L to 1.5 mole/L, respectively.
For each of the prepared catalysts, a PM oxidation initiation
temperature was measured in accordance with-the above-described
method. FIG. 4 depicts the measured results.
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[0049] Referring to the curve of FIG. 4, it can be seen that there
is an inflection point around a K supporting amount of 0. 5 mole/L,
and a PM oxidation initiation temperature of 300 C or below is
exhibited when the K supporting amount is 0.6 mole/L or more.
(Example 4)
[0050] FIG. 5 illustrates an exhaust gas-purifying catalyst
according to this example. The catalyst according to this example
includes: a filter substrate 1 including inlet cells 10 each
clogged at an exhaust gas downstream side thereof, outlet cells
11 each arranged adjacent to the inlet cells and clogged at an
exhaust gas upstream side thereof, and cell partition walls 12
partitioning the inlet cells 10 and the outlet cells 11 from each
other; a protection layer 3 formed on the surfaces of the cell
partition walls 12 and on the surfaces of pores formed in the cell
partition walls 12; and a catalyst bed 2 formed on the=surface
of the protection layer 3. This catalyst is identical to that
of Example 2, except that the catalyst includes the protection
layer 3. Accordingly, a detailed description of the structure
of the catalyst bed 2 will be given through a description of a
method for manufacturing the catalyst bed 2.
[0051] The filter substrate 1 is dipped in a slurry, in which
a silica sol is distributed, to allow the slurry to be introduced
into the cells. The slurry is then sucked from the end of the
filter substrate 1 opposite to the dipped end in a state in which
the filter substrate 1 has been upwardly taken out of the slurry,
to remove a surplus of the slurry from the filter substrate 1.
After being dried by ventilation, the filter substrate 1 is
calcined at 500 C for 3 hours. This procedure is performed two
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times, in order to adjust the formation of the protection layer
such that the protection layer is formed on the inlet cells 10
and outlet cells 11 in substantially same amounts, respectively.
The formation amount of the protection layer is 20g per 1L of the
filter substrate 1(substantially a thickness of 1pm).
Thereafter, the catalyst bed 2 is formed in the same manner as
that of Example 2.
(Example 5)
[0052] The protection layer 3 which is made of Ti02 is formed
in the same manner as that of Example 4, except that a titania
sol is used in place of the silica sol. Thereafter, the catalyst
bed 2 is formed in the same manner as that of Example 2.
(Example 6)
[0053] The protection layer 3 which is made of A1203 is formed
in the same manner as that of Example 4, except that an,alumina
sol is used in place of the silica sol. Thereafter, the catalyst
bed 2 is formed in the same manner as that of Example 2.
<Experimental Example 4 = Evaluation>
.[0054] For each of the catalysts according to Embodiments 2, 4,
5, and 6, and Comparative Example 3, a high-temperature durability
test was carried out by maintaining the catalyst in a heated state
in an electric furnace at 700 C for 10 hours. Thereafter, the
above-described test was carried out to measure a PM oxidation
initiation temperature. For each catalyst subjected to the
high-temperature durability test, the strength of the filter
substrate 1 was measured by Autograph. Based on the measured
results, the catalysts were evaluated to be ."O" when exhibiting
a compressive strength of more than 2 MPa, when exhibiting
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a compressive strength ranging from 1. 5 MPa to 2 MPa, or " x" when
exhibiting a compressive strength of less than 1.5 MPa. Table
1 shows the evaluated results.
[0055] [Table 1]
K
Supporting Protection PM Oxidation Substrate
Amount Layer Initiation Temp. Strength
Example 2 0.6 mole/L - 360 C x
Example 4 0.6 mole/L Si02 316 C 0
Example 5 0.6 mole/L Tio2 319 C 0
Example 6 0.6 mole/L A1203 341 C IL
Comp. Exam. 3 - - 434 C 0
[0056] Referring to Table 1, it can be seen that the catalyst
of Example 2 exhibits a degradation in substrate strength after
the high-temperature durability test. However, such a
degradation in substrate strength can be suppressed by forming
a protection layer, as in Examples 4 to 6. When a protection layer
made of Si02 or Ti02 is formed, results similar to those of
Comparative Example 3 supporting no K are obtained. In this case,
accordingly, it is possible to greatly suppress a degradation in
substrate strength.
[0057] While the invention has been shown and described with
respect to the preferred embodiments, it will be understood by
those skilled in the art that various changes and modifications
may be made without departing from the spirit and scope of the
invention as defined in the following claims.
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