Note: Descriptions are shown in the official language in which they were submitted.
CA 02485893 2004-10-25
CATALYST FOR PURIFYING EXHAUST GASES
BACKGROUND OF THE INVENTION
Field of the Invention
X0001] The present invention relates to a catalyst for purifying
exhaust gases, mainly for automotive applications. More
particularly, it relates to a catalyst for purifying exhaust gases
which can suppress the degradation by poisoning.
Description of the Related Art
(0002 Automotive exhaust systems have been equipped with a variety
of catalysts for purifying exhaust gases, such as oxidizing
catalysts, three-way catalysts and NOX sorbing-and-reducing
catalysts, in order to remove HC, CO and NOX in exhaust gases by
oxidation and/or reduction. For example, three-way catalysts have
been produced using a honeycomb-shaped substrate formed of
cordierite or metallic foils in the following manner. A coating
layer is formed on a surface of the cellular passages of the substrate
using alumina and/or ceria. Then, a catalytic ingredient, such as
Pt and Rh, is loaded on the coating layer. The resulting three-way
catalysts have been used in an exhaust atmosphere produced by burning
an air-fuel mixture whose air-fuel (A/F) ratio is controlled around
14.6, the stoichiometric ratio. Thus, the three-way catalysts
purify HC and CO by oxidizing them, and simultaneously purify NOX
by reducing them.
~0003~ When fuels containing additives such as Pb and Mn are used,
exhaust gases contain Pb and Mn components therein. Accordingly,
there arises a drawback that the catalytic-ingredient active points
in catalysts have been covered with the Pb and Mn components to
degrade the activity. Moreover, it has been known that P, Zn and
1
CA 02485893 2004-10-25
Ca contained in engine oils cause similar drawbacks.
0004 ~ Japanese Unexamined Patent Publication (KOKAI) No.
2002-172,329 discloses a catalytic structure comprising a trapping
layerfor collecting catalyst-poisoning componentsin exhaustgases.
The trapping layer is disposed in proximity to the upstream-end
surface of an NOX sorbing-and-reducing catalyst. The catalytic
structure can suppress the NOX sorbing member of the NOX
sorbing-and-reducing catalyst being poisoned bysulfur, because the
trapping layer collects sulfur components.
(0005 The trapping layer disclosed in the patent publication
contains a trapping component which can react with sulfur components
to trap sulfur components, thereby suppressing the sulfur poisoning
of the NOX sorbing member only. However, the patent publication
neither sets forth nor suggests suppressing the poisoning of
catalytic ingredients such as Pt . Moreover, the patent publication
is silent on another poisoning substances, such as Pb and Mn, other
than sulfur.
0006) The present invention has been developed in view of such
circumstances . It is therefore an obj ect of the present invention
to suppress the poisoning degradation of catalytic ingredients by
poisoning substances, such as Pb and Mn, thereby improving the
durability of catalysts for purifying exhaust gases.
~0007~ A catalyst for purifying exhaust gases according to the
present invention comprises:
a substrate comprising an exhaust-gas passage;
a coating layer formed on the exhaust-gas passage; and
a catalytic ingredient loaded on the coating layer,
2
CA 02485893 2005-O1-20
wherein a loading density of the catalytic ingredient on a
downstream area discriminated from an upstream area by a
predetermined length from an upstream end of the substrate is greater
than a loading density of the catalytic ingredient on the ~zpstream
area.
(0008) It is desirable that the upstream area of the exhaust-gas
passage can be free from the catalytic ingredient loaded.
(0009) Moreover, it is preferable that the predetermined length
from an upstream end can fall in a range of less than 30~ of an overall
length of the substrate. In addition, it is desirable that the
predetermined length from an upstream end can fall in a range of
from 5 to 20~ of an overall length of the substrate.
(0010) The present catalyst for purifying exhaust gases can
suppress the poisoning substances such as Pb and Mn poisoning the
catalytic ingredient. Therefore, the present catalyst exhibits
high activities even after it is subjected to a durability test,
because the degradation of the activities of the catalytic
ingredient is suppressed.
(OOliJ In particular, when the length of .the upstream area of the
exhaust-gas passage which is free from the loaded catalytic
ingredient falls in a range of less than 30 0 of the overall length
of the substrate, the present catalyst exhibits activities equal
to or higher than those of conventional catalysts in which catalytic
ingredients are loaded over the entire length.
(0012) A more complete appreciation of the present invention and
many of its advantages will be readily obtained as the same becomes
better understood by reference to the following detailed description
3
CA 02485893 2004-10-25
when considered in connection with the accompanying drawings and
detailed specification, all of which forms a part of the disclosure.
(0013) Fig. 1 is a cross-sectional view for roughly illustrating
an arrangement of a catalyst for purifying exhaust gases according
to Example No. 1 of the present invention.
(0014) Fig. 2 is an explanatory diagram for illustrating how to
find a cross conversion.
(0015) Fig. 3 is a graph for illustrating relationships between
CO-NOX cross conversions and proportions of a length of an upstream
area of an exhaust-gas passage with respect to an overall length
of a substrate.
DETAILED DESCRIpTT_ON OF THE PREFERRED EMBODT_MENTS
(0016) Having generally described the present invention, a further
understanding can be obtained by reference to the specific preferred
embodiments which are provided herein for the purpose of
illustration only and not intended to limit the scope of the appended
claims.
(0017) According to studies carried out by the present inventors,
it has been apparent that the poisoning degradation of catalytic
ingredients by poisoning substances such as Pb and Mn occur
intensively on the upstream part of catalysts when catalysts are
brought into contact with exhaust gases containing Pb and Mn.
Accordingly, in the present catalyst for purifying exhaust gases,
the loading density of the catalytic ingredient on the downstream
area is greater than that on the upstream area discriminated by a
predetermined length from the upstream end of the substrate.
Consequently, since the catalytic ingredient is loaded less on the
upstream area in which the catalytic ingredient is likely to be
4
CA 02485893 2004-10-25
degraded due to poisoning by poisoning substances, and is loaded
more on the downstream area of the exhaust-gas passage excepting
the upstream area, it is suppressed that the catalytic ingredient
is poisoned by poisoning substances. Therefore, not only the
present catalyst can utilize the catalytic ingredient effectively,
but also it can suppress the activities degrading after being
subjected to a durability test.
(0018 Poisoning substances adsorb or deposit mainly onto the
upstream area of the exhaust-gas passage. Hence, when the loading
density of the catalytic ingredient on the upstream area is made
less than the loading density of the catalytic ingredient on the
downstream area of the exhaust-gas passage excepting the upstream
area, it is possible to suppress the poisoning degradation of the
catalytic ingredient. For example, the loading density of the
catalytic ingredient on the upstream area can be made less than the
loading density of the catalytic ingredient on the downstream area
by a certain extent. It is preferable, however, to reduce the
loading density of the catalytic ingredient on the upstream area
from large to small stepwise or gradually in the direction
approaching the upstream end of the substrate, because the upstream
area is more likely to be poisoned than the downstream area is.
Moreover, it is preferable as well to reduce the loading density
of the catalytic ingredient at parts of the upstream area at which
the flow rate of exhaust gases is fast, because the degradation of
the catalytic ingredient is more likely to occur deeply at parts
of catalysts at which the flow rate of exhaust gases is fast.
~0019~ It is often difficult, however, to provide the loading
density of the catalytic ingredient with a distribution in view of
CA 02485893 2004-10-25
production processes. Hence, it is preferable to arrange the
present catalyst for purifying exhaust gases so that the upstream
area of the exhaust-gas passage can be free from the loaded catalytic
ingredient . Such a catalyst can be produced with extreme readiness .
Thus, with such an arrangement, it is possible to completely prevent
the upstream area of the exhaust-gas passage from being degraded
by poisoning. Moreover, it is possible to additionally load the
catalytic ingredient, which is to be loaded on the upstream area,
on the downstream area of the exhaust-gas passage so that the present
catalyst can fully show the activities.
~0020~ Note that poisoning substances deposit on the upstream area
of the exhaust-gas passage gradually. However, the deposition of
poisoning substances does not pose any problem, because the
deposited poisoning substances are emitted to the outside together
with exhaust gases when the deposition reaches a certain amount and
the flow rate of exhaust gases enlarges.
~0021~ The length of the upstream area of the exhaust-gas passage
can preferably fall in a range of less than 30 0 of an overall length
of the substrate. According to experiments conducted by the present
inventors, it has been found out that, when the length of area on
which no catalytic ingredient is loaded exceeds 30 0 of the overall
length of the substrate, the activities of the resulting catalysts
degrade after being subjected to a durability test, compared with
those of conventional catalysts in which catalytic ingredients are
loaded over the entire length of the substrate. The reason has not
been clear yet. It is believed, however, as follows. Let the
loading amount of the catalytic ingredient, the absolute value, be
identical in the present catalyst and the conventional catalysts,
6
CA 02485893 2004-10-25
the loading density of the catalytic ingredient increases too much
in the downstream area of the present catalyst. As a result, the
granular growth of the catalytic ingredient occurs after the present
catalyst is subjected to a durability test so that the number of
the active points decreases.
(0022 Moreover, it is especially preferable that the length of
the upstream area of the exhaust-gas passage can fall in a range
of from 5 to 200 of the overall length of the substrate. When the
length of the upstream area of the exhaust-gas passage falls in the
range, the activities of the present catalyst for purifying exhaust
gases improve more, compared with those of conventional catalysts
in which catalytic ingredients are loaded over the entire length
of the substrate.
(0023 The substrate is formed as a honeycomb shape or a foamed
shape. It is possible to use the following for making the substrate:
honeycomb structures or foamed structures formed of heat-resistant
ceramicssuch ascordierite; honeycombstructuresformed of metallic
foils; and foamed structures formed of metallic fibers. Note that
the cellular density and porosity of the substrate can be equal to
those of substrates used conventionally.
(0024 The coating layer is formed of a single member selected from
the group consisting of alumina, titania, zirconia, ceria and silica,
or a mixture of the oxides. Alternatively, the coating layer can
be formed of a composite oxide composed of a plurality of the oxides .
It is possible to use conventional raw materials for making the
coating layer. Depending on the types of catalyst, it is preferable
to select an optimum raw material. For example, when the present
catalyst for purifying exhaust gases is turned into a three-way
7
CA 02485893 2004-10-25
catalyst, it is preferable to mix alumina with ceria or a
ceria-zirconia composite oxide having an oxygen sorbing-and-
releasing ability and use the resulting mixture for making the
coating layer.
(0025) Note that, even when no coating layer is formed on the
upstream area of the exhaust-gas passage, the resulting present
catalyst for purifying exhaust gases likewise operates and effects
advantages . Accordingly, when the present catalyst is arranged so
that the upstream area of the exhaust-gas passage is free from the
loaded catalytic ingredient, it is preferable not to form the coating
layer on the upstream area. With such an arrangement, the
ventilation resistance exerted to exhaust gases lowers in the
upstream area so that the pressure loss can be reduced. Moreover,
when the cellular density in the upstream area is increased to make
the pressure loss equal to that of conventional catalysts, the
cellular superficial area contacting with exhaust gases enlarges.
Consequently, the adsorption or deposition of poisoning substances
enlarges in the upstream area so that it is possible to further
suppress the poisoning degradation of the catalytic ingredient in
the downstream area.
(0026) The coating layer can be formed by utilizing a wash coating
method using a slurry and followed by drying and calcining the slurry,
as it has been done conventionally. In accordance with the wash
coating method, it is possible not to form the coating layer on the
upstream area of the exhaust-gas passage with extreme readiness,
because the wash coating method makes it possible not to deposit
the slurry on the upstream area. Note that the forming amount of
the coating layer can be as usual, for example, from 100 to 300 g
8
CA 02485893 2004-10-25
with respect to 1 L of the substrate.
(0027) It is possible to appropriately select and use a noble metal,
such as Pt, Rh, Pd, Ir and Ru, for the catalytic ingredient, depending
on the types and applications of the present catalyst for purifying
exhaust gases. Moreover, in certain applications, it is possible
to use a transition metal, such as Fe, Ni, Co, Cu and W, for the
catalytic ingredient. The loading amount of the catalytic
ingredient can be from 0. 1 to 10 g with respect to 1 L of the substrate,
but can be changed appropriately, depending on the types of and
applications of the present catalyst.
(0028) The catalytic ingredient can be loaded on the coating layer
by a water absorption method or an adsorption method in the same
manner as having been done conventionally. In the water absorption
method, the coating layer is impregnated with a solution in which
a compound of the catalytic ingredient is solved, and is dried and
calcined subsequently. In the adsorption method, the substrate
provided with the coating layer is immersed into and taken up from
a solution in which a compound of the catalytic ingredient is solved,
and is dried and calcined subsequently. Alternatively, the
catalytic ingredient can be loaded on an oxide powder making the
coating layer by the water absorption method or the adsorption method
in advance, and the coating layer can be formed using the resulting
catalytic powder.
(0029) Hereinafter, the present invention will be described in
detail with reference to examples and comparative examples.
(Example No. 1)
(0030) Fig. 1 roughly illustrates a cross-sectional view of a
9
CA 02485893 2004-10-25
catalyst for purifying exhaust gases according to Example No. 1 of
the present invention. The catalyst is a three-way catalyst, and
comprises a honeycomb-shaped substrate 1, a coating layer 2 formed
on an downstream area of the substrate 1 alone and loaded with
catalytic ingredients, and Pt and Rh loaded on the coating layer
2 as the catalytic ingredients. The substrate 1 has an overall
length of 105 mm. The substrate 1 comprises an upstream area 10
extending from the upstream-end surface to the downstream-end
surface by a length of 5.25 mm (i.e., 5o of the overall length of
the substrate 1). Note that the coating layer 2 is not formed on
the upstream area 10, and that Pt and Rh are not loaded on the upstream
area 10.
(0031) Hereinafter, the production process of the exhaust gas-
purifying catalyst according to Example No. 1 will be described
instead of describing the arrangement of the exhaust gas-purifying
catalyst.
X0032) 100 parts by weight of an A1203 powder, 80 parts by weight
of a Ce02-ZrOz composite oxide powder, 40 parts by weight an alumina
sol, and 130 parts by weight of water were mixed. Note that the
solid content of the alumina sol was 10 o by weight . The resulting
mixture was milled to prepare a slurry.
0033) Subsequently, a honeycomb-shaped substrate 1 was prepared.
The substrate 1 was made of cordierite, and had a diameter of 103
mm, an overa:Ll length of 105 mm and a cellular density of 600
cells/inch2. The substrate 1 was immersed into the slurry over a
range by 95% of the overall length from the downstream-end surface.
The substrate 1 was taken up from the slurry, and the excessive slurry
was blown off from the substrate 1. Thereafter, the substrate 1
CA 02485893 2004-10-25
was dried at 250 °C for 2 hours, and was further calcined at 500
°C
for 2 hours, thereby forming the coating layer 2. Thus, the coating
layer 2 was foamed only on the downstream area excepting the upstream
area 10 which extended from the upstream-end surface over a range
by 5% of the overall length of the substrate 1. The forming amount
of the coating layer 2 was 200 g with respect to 1 L of the substrate
1.
(0034 Then, a platinum dinitrodiammine aqueous solution having
a prescribed concentration was absorbed into the substrate 1
provided with the coating layer 2 in a predetermined amount . After
drying, the substrate 1 was calcined at 500 °C for 1 hour, thereby
loading Pt thereon. Moreover, a rhodium nitrate aqueous solution
having a prescribed concentration was absorbed into the substrate
1 in a predetermined amount. After drying, the substrate 1 was
calcined at 500 °C for 1 hour, thereby loading Rh thereon. Note that
Pt and Rh were loaded in an amount of 1. 3 g and 0. 35 g, respectively,
with respect to one piece of the resulting catalyst. In addition,
no Pt and Rh were loaded on the upstream area 10 on which no coating
layer 2 was formed.
(Example No. 2)
(0035) Except that the length of the upstream area 10 was set at
l00 of the overall length of the substrate 1 (i.e., 10.5 mm), a
catalyst for purifying exhaust gases according to Example No. 2 of
the present invention was produced in the same manner as Example
No. 1. Note that Pt and Rh were loaded likewise in an amount of
1.3 g and 0.35 g, respectively, with respect to one piece of the
resulting catalyst.
(Example No. 3)
11
CA 02485893 2004-10-25
X0036) Except that the length of the upstream area 10 was set at
20% of the overall length of the substrate 1 (i.e., 21.0 mm), a
catalyst for purifying exhaust gases according to Example No. 3 of
the present invention was produced in the same manner as Example
No. 1. Note that Pt and Rh were loaded likewise in an amount of
1.3 g and 0.35 g, respectively, with respect to one piece of the
resulting catalyst.
(Example No. 4)
~0037~ Except that the length of the upstream area 10 was set at
300 of the overall length of the substrate 1 (i.e., 31.5 mm), a
catalyst for purifying exhaust gases according to Example No. 4 of
the present invention was produced in the same manner as Example
No. 1. Note that Pt and Rh were loaded likewise in an amount of
1.3 g and 0.35 g, respectively, with respect to one piece of the
resulting catalyst.
(Example No. 5)
~0038~ Except that the length of the upstream area 10 was set at
500 of the overall length of the substrate 1 (i.e., 52.5 mm), a
catalyst for purifying exhaust gases according to Example No. 5 of
the present invention was produced in the same manner as Example
No. 1. Note that Pt and Rh were loaded likewise in an amount of
1.3 g and 0.35 g, respectively, with respect to one piece of the
resulting catalyst.
(Comparative Example No. 1)
~0039~ Except that the coating layer 2 with Pt and Rh loaded was
formed over the entire length of the substrate 1 (i.e., 105 mm),
a catalyst for purifying exhaust gases according to Comparative
Example No. 1 was produced in the same manner as Example No. 1. Note
12
CA 02485893 2004-10-25
that Pt and Rh were loaded likewise in an amount of 1.3 g and 0.35
g, respectively, with respect to one piece of the resulting catalyst .
(Test and Evaluation)
(0040) The catalysts according to Example Nos. 1 through 5 and
Comparative Example No. 1 were disposed in an exhaust system of an
engine testing bench equipped with a gasoline engine whose
displacement was 1.8 L, respectively. Then, the gasoline engine
was driven using gasoline which contained Mn in an amount of 35 mg/L.
The gasoline engine was driven while changing the air-fuel ratio
A/F. Under the conditions that the exhaust gases exhibited a
catalyst inlet temperature of 400 °C and a space velocity of 100, 000
hr-1, the respective catalysts were examined for the HC, CO and NOX
conversions. The thus measured conversions were labeled the
initial conversions.
(0041 Subsequently, the catalysts according to Example Nos. 1
through 5 and Comparative Example No. 1 were subjected to a
durability test on the same engine testing bench, respectively. In
the durability test, the gasoline engine was driven using gasoline
which contained Mn in an amount of 35 mg/L similarly, but the
catalysts were held in the exhaust gases whose catalyst inlet
temperature was controlled at 900 °C for 50 hours . Then, in the same
manner as the examination for the initial HC, CO and NOX conversions,
the catalysts, which had been subjected to the durability test, were
examined for the HC, CO and NOX conversions after the durability
test.
(0042 The thus obtained results were summarized as a graph in which
the A/F ratios were plotted against the horizontal axis and the
conversions were plotted against the vertical axis. Moreover, as
13
CA 02485893 2004-10-25
illustrated in Fig. 2, an HC-NOX cross conversion and an CO-NOX cross
conversion, the intersecting points of the A/F ratio-HC conversion
curve and A/F ratio-NOX conversion curve and the A/F ratio-CO
conversion curve and A/F ratio-NOx conversion curve, were determined
for each of the catalysts according to Example Nos. 1 through 5 and
Comparative Example No. 1 as shown in Fig. 2. All of the catalysts
according to Example Nos . 1 through 5 and Comparative Example No .
I exhibited a great correlation between the HC-NOX cross conversion
and the CO-NOX cross conversion. Accordingly, the CO-NOX cross
conversions were selected as a representative characteristic, and
were summarized as a graph in which the CO-NOX cross conversions
were plotted against the vertical axis and the proportions of the
upstream area 10 with respect to the overall length of the substrate
1 were plotted against the horizontal axis. Fig. 3 illustrates the
results.
(0043 It is understood from Fig. 3 that the catalysts according
to Example Nos. 1 through 4 exhibited exhaust-gas purifying
performance equal to or higher than that of the catalyst according
to Comparative Example No. 1 after the durability test, though the
catalyst according to Comparative Example No. 1 exhibited the
highest exhaust-gas purifying performance initially. It is
apparent that this resulted from the fact that no Pt and Rh were
loaded on the upstream area 10 of the catalysts according to Example
Nos. I through 4. It is believed that the catalysts according to
Example Nos. 1 through 4 effected the advantage because it is
suppressed that Pt and Rh are being poisoned by Mn.
(0044 Moreover, the catalysts according to Example Nos . 1 through
3 showed upgraded exhaust-gas purifying performance after the
14
CA 02485893 2004-10-25
durability test, compared with that of the catalyst according to
Comparative Example No. 1. Therefore, it is evident that the length
of the upstream area 10 can particularly desirably fall in a range
of from 5 to 20% of the overall length of the substrate 1.
~0045~ The present invention can be applied to oxidizing catalysts,
three-way catalysts, NOX-selective-reducing catalysts and NOX
sorbing-and-reducing catalysts. Moreover, it can be applied to
filter catalysts as well. For example, filter catalysts comprise
a diesel particulate-material filter (i.e., DPF) whose cellular
passages are clogged alternately at the opposite ends and in which
a coating layer with a catalytic ingredient loaded is formed in the
pores of the cellular walls in the DPF.
~0046~ Having now fully described the present invention, it will
be apparent to one of ordinary skill in the art that many changes
and modifications can be made thereto without departing from the
spirit or scope of the present invention as set forth herein
including the appended claims.
