Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
EXHAUST EMISSION CONTROL CATALYSTS AND
METHOD FOR CONTROLLING EXHAUST EMISSION
TECHNICAL FIELD
The present invention relates to catalysts and a
method for controlling (purifying) the emission of
exhaust from an internal combustion engine. The present
invention relates more particularly to catalysts and a
method for controlling the emission of exhaust, capable
of reducing injurious (noxious) components, particularly
NOX (nitrogen oxide), contained in the exhaust by
decomposition and of removing particulate matter,
unburned hydrocarbon, and carbon monoxide by combustion.
BACKGROUND ART
NOX in the atmosphere is the cause of photochemical
smog and acid rain. Hence, the discharge of NOX from
moving sources like automobiles incorporating internal
combustion engines, such as gasoline engine and diesel
engine, poses a social problem. The internal combustion
engine is one of the NOX sources. Therefore, there is a
tendency towards tightening up the laws and regulations
concerning the amount of discharge of NOX. Accordingly,
the development of exhaust emission control catalysts is
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being actively carried out.
As a conventional exhaust emission control catalyst
for controlling the emission of exhaust from the gasoline
engine, a so-called three-way catalyst capable of
simultaneously reducing NOX, unburned hydrocarbon, and
carbon monoxide is known. Since the exhaust from a
typical gasoline engine contains very little oxygen, it
is possible to achieve efficient reduction of NOX by
unburned hydrocarbon or carbon monoxide, and decrease NOX.
However, the exhaust from the diesel engine contains
excessive oxygen because of its engine characteristics.
Moreover, stoichiometrically, the exhaust from the diesel
engine contains less hydrocarbon and carbon monoxide
which function as a reducing agent compared to NOX.
Therefore, when a conventional three-way catalyst is used
for the treatment of the exhaust from the diesel engine,
NOx can hardly bedecreased.
Furthermore, since the exhaust from the diesel
engine contains a large amount of particulate matter
formed by carbons, soluble organic fractions (SOF),
sulfate, etc., the amounts of these elements discharged
are restricted by the laws and regulations. Therefore,
when using a typical three-way catalyst for the treatment
of the exhaust from the diesel engine, it is also
required to reduce the particulate matter. However, such
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a catalyst can hardly decrease the particulate matter.
In resent years, a lean burn gasoline engine, and a
cylinder injection of fuel type gasoline engine have been
developed for the purpose of decreasing the fuel
consumption. Since these engines cause lean burn, the
oxygen concentration in the exhaust from these engines is
high. Therefore, when a typical three-way catalyst is
used for the treatment of the exhaust from such gasoline
engines, it is difficult to decrease NOX.
In order to solve this problem, a catalyst
containing a porous carrier like zeolite carrying copper
is proposed as an exhaust emission control catalyst which
effectively removes NOX in exhaust containing a large
amount of oxygen, such as exhaust from the diesel engine
and exhaust from the lean burn gasoline engine. An
example of such a catalyst is disclosed in Japanese
Publication for Unexamined Patent Application No.
100919/1988 (Tokukaisho 63-100919). However, this
catalyst is inferior in heat resistance, and its NOX
removing ability tends to be lowered by sulfur oxides
like SOa contained in the exhaust. Namely, there is a
problem that the catalyst is readily poisoned.
Moreover, Japanese Publication for Unexamined Patent
Application No. 137963/1993 (Tokukaihei 5-137963)
discloses an exhaust emission control catalyst containing
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platinum as a principal component. However, since this
catalyst is highly active to oxidize S02 in the exhaust,
a large amount of sulfates are produced by the oxidation
of SOa, resulting in an increase in the content of
sulfates in the exhaust. Thus, there is a problem that
this catalyst increases the amount of particulate matter
in the exhaust due to the sulfates . In particular, in
the case of the exhaust from a diesel engine which
contains a larger amount of particulate matter compared
to a gasoline engine, it is required to reduce the
discharge of the particulate matter to a lower level.
Hence, there is a more serious problem in respect of the
exhaust from the diesel engine, namely an increased
amount of particulate matter is produced.
Furthermore, Japanese Publication for Unexamined
Patent Application No. 219147/1992 (Tokukaihei 4-219147)
discloses an exhaust emission control catalyst containing
a particular zeolite which carries cobalt, copper and/or
rhodium, and rare earth metal as essential components.
However, only an exhaust emission control catalyst
using lanthanum or cerium as the rare earth metal is
disclosed as an example in this publication. When
lanthanum or cerium is used as the rare earth metal, the
activity of oxidizing SO~ in the exhaust becomes higher.
Therefore, when this catalyst is used for the treatment
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of high temperature exhaust from a diesel engine, a large
amount of sulfates are produced by the oxidation of SOa,
resulting in an increase in the content of the sulfates
in the exhaust. Thus, like the above-mentioned catalyst,
this catalyst causes a problem that the amount of
particulate matter in the exhaust is increased.
Considering the above-mentioned conventional
problems, it is an object of the present invention to
provide exhaust emission control catalysts and a method
for controlling the emission of exhaust, which can
efficiently decrease NOX in exhaust containing a large
amount of oxygen like exhaust from a diesel engine, and
reduce the amount of particulate matter in the exhaust
under high temperature conditions.
DISCLOSURE OF THE INVENTION
In order to achieve the above-mentioned object, the
present inventors eagerly studied exhaust emission
control catalysts, and found that an exhaust emission
control catalyst having a catalytic component containing
copper, praseodymium, and yttrium has an excellent
ability that is not seen in the conventional exhaust
emission control catalysts. The inventors completed the
present invention based on the results of the study.
Namely, in order to solve the above-mentioned
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problems, exhaust emission control catalysts of the
present invention are characterized in having the
catalytic component containing copper, praseodymium, and
yttrium.
Moreover, in the exhaust emission control catalysts
of the present invention, at least one kind of element
selected from the group consisting of cobalt, iron,
nickel, lanthanum, cerium, and neodymium is preferably
added to the catalytic component.
Furthermore, a preferred weight ratio of copper,
praseodymium, and yttrium in terms of their oxides is
that praseodymium oxide is 0.2 to 20 weight parts and
yttrium oxide is 0.2 to 20 weight parts based on 1 weight
part of copper oxide.
Additionally, in the exhaust emission control
catalysts of the present invention, it is preferred that
the catalytic component is carried in inorganic oxide
which is formed from at least one of zirconia and
zeolite.
In this structure, the exhaust emission ,control
catalyst can efficiently reduce and decompose NOX in the
exhaust containing a large amount of oxygen. Moreover,
the exhaust emission control catalyst not only burns
injurious components contained in the exhaust, such as
unburned hydrocarbon and carbon monoxide, but also burns
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SOF as particulate matter. Furthermore, since the
exhaust emission control catalyst can suppress the
oxidative reaction of S02 in the exhaust, it is possible
to reduce the amount of particulate matter produced from
sulfate by the oxidation of SOa.
Consequently, such an exhaust emission control
catalyst can efficiently remove NOX in the exhaust
containing a large amount of oxygen, and reduce the
amount of particulate matter in the exhaust under high
temperature conditions.
In order to solve the above-mentioned problems, a
method for controlling the emission of exhaust according
to the present invention is characterized by bringing
exhaust containing hydrocarbon and nitrogen oxide in a
mole ratio (hydrocarbon/nitrogen oxide) of 0.5 to 30, and
more preferably 1 to 20, into contact with the exhaust
emission control catalyst. Moreover, it is preferred for
the method of controlling the emission of exhaust of the
present invention that the exhaust is emitted from a
diesel engine.
With this method, it is possible to efficiently
remove NOX in exhaust, particularly NOx in exhaust from a
diesel engine, and suppress the oxidation of SOz in the
exhaust, thereby decreasing the amount of particulate
matter in the exhaust under high temperature conditions
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as described above.
The following description will explain the present
invention in detail.
The exhaust emission control catalysts of the
present invention have a catalytic component containing
at least copper, praseodymium, and yttrium.
With regard to the weight ratio of copper and
praseodymium contained in the exhaust emission control
catalysts, when the ratio is calculated in terms of their
oxides, the amount of praseodymium oxide is preferably
0.2 to 20 weight parts, and more preferably 0.5 to 10
weight parts based on 1 weight part of copper oxide.
If the weight ratio of praseodymium to copper in
terms of their oxides is such that praseodymium oxide is
less than 0.2 weight parts based on 1 weight part of
copper oxide, the effect of suppressing the oxidation
activity of S02 is reduced. Thus, such a ratio is not
preferred. On the other hand, if the weight ratio of
praseodymium to copper in terms of their oxides is such
that praseodymium oxide is more than 20 weight parts
based on 1 weight part of copper oxide, the effect of
suppressing the oxidation activity of S02 corresponding to
the increase in the amount of praseodymium is not
exhibited any longer, resulting in an economical
disadvantage and a lowering of the NOX decomposition
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activity. Hence, such a ratio is not preferred.
With regard to the weight ratio of copper and
yttrium contained in the exhaust emission control
catalysts, when the ratio-is calculated in terms of their
oxides, the amount of yttrium oxide is preferably 0.2 to
20 weight parts, and more preferably 0.5 to 10 weight
parts based on 1 weight part of copper oxide.
If the weight ratio of yttrium to copper in terms of
their oxides is such that yttrium oxide is less than 0.2
weight parts based on 1 weight part of copper oxide, the
NOX decomposition activity is lowered. Thus, such a ratio
is not preferred. On the other hand, if the weight ratio
of yttrium to copper in terms of their oxides is such
that yttrium oxide is more than 20 weight parts based on
1 weight part of copper oxide, an improvement of the NOX
decomposition activity corresponding to the increase in
the amount of yttrium -is not exhibited any longer,
resulting in an economical disadvantage and an increase
in the SOa oxidation activity. Consequently, sulfates are
generated by the oxidation of 502, and the amount of
particulate matter in the exhaust is increased. Thus,
such a ratio is not preferred.
Namely, the weight ratio of copper, praseodymium,
and yttrium in the exhaust emission control catalysts in
terms of their oxides is preferably arranged such that
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the amount of praseodymium oxide is 0.2 to 20 weight
parts and the amount of yttrium oxide is 0.2 to 20 weight
parts based on 1 weight part of copper oxide. With this
arrangement, the NOX decomposition activity is more
efficiently improved, and the SOZ oxidation activity is
more efficiently suppressed.
The catalytic component of the exhaust emission
control catalysts contains copper, praseodymium, and
yttrium. It is more preferred that the catalytic
component further contains at least one kind of element
(hereinafter referred to as other metal) selected from
the group consisting of iron, cobalt, nickel,
lanthanum, cerium, and neodymium. Namely, it is
preferred to add other metal to th'e catalytic component.
By adding other metal to the catalytic component, it is
possible to improve the efficiency of improving the NOX
decomposition activity and suppressing the S02 oxidation
activity.
The other metal may be one kind or a combination of
more than one kinds of the above-mentioned elements.
Examples of the other metal include iron, cobalt, nickel,
lanthanum, cerium, neodymium, lanthanum-iron, lanthanum-
cobalt, lanthanum-nickel, cerium-iron, cerium-cobalt,
cerium-nickel, neodymium-iron, neodymium-cobalt, and
neodymium-nickel. Among these metals, it is particularly
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preferred to use iron, cobalt, nickel, and a combination
thereof as the other metal. By adding such a metal, the
exhaust emission control catalysts of the present
invention can more efficiently improve the NOX
decomposition activity and suppress the SOa oxidation
activity.
The exhaust emission control catalysts usually
contain a refractory (fire-resistant) inorganic oxide in
addition to the above-mentioned catalytic component. It
is preferred that the catalytic component is carried in
the refractory inorganic oxide. Namely, it is preferred
that the catalytic component is dispersed in the
refractory inorganic oxide. It is more preferred that
the exhaust emission control catalysts contain a
refractory carrier (substrate) for carrying the catalytic
component together with the refractory inorganic oxide.
Namely, it is preferred that the catalytic component is
carried by the refractory carrier in such a state that
the catalytic component is dispersed in the refractory
inorganic oxide. In such a structure, it is possible to
efficiently bring the exhaust into contact with the
catalytic component.
Examples of a copper compound used for causing the
refractory carrier to carry copper include: inorganic
salts of copper, such as copper nitrate, copper sulfate,
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copper phosphate, copper carbonate, copper chloride,
copper bromide, and copper iodide; organic salts of
copper such as copper acetate, and copper citrate; and
copper oxide. The use of copper nitrate, copper sulfate,
copper chloride and copper acetate is particularly
suitable.
With regard to the amount of copper to be carried on
the refractory carrier, copper is preferably in the range
of 1 to 20 grams in terms of copper oxide, and more
preferably in the range of 1 to 10 grams, based on 1
litre of the refractory carrier.
When the amount of copper is less than 1 gram, the
NOX decomposition activity is lowered. Thus, such an
amount is not preferred. On the' other hand, when the
amount of copper is more than 2U grams, an improvement of
the NOX decomposition activity corresponding to the
increase in the amount of copper is not exhibited any
longer, resulting in an economical disadvantage.
Moreover, when the amount of copper is more than 20
grams, the SOz oxidation activity is enhanced, land the
amount of particulatE: matter in the exhaust is increased
due to the generation of sulfates by the oxidation of SOz.
Thus, such an amount is not preferred.
Examples of a praseodymium compound used for causing
the refractory carrier to carry praseodymium include:
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inorganic salts of praseodymium, such as praseodymium
nitrate, and praseodymium fluoride; organic salts of
praseodymium, such as praseodymium acetate, and
praseodymium oxalate; and praseodymium oxide. The use of
praseodymium nitrate, praseodymium fluoride, and
praseodymium acetate is particularly suitable.
With regard to the amount of praseodymium to be
carried in the refractory carrier, praseodymium is
preferably in the range of 1 to 20 grams in terms of
praseodymium oxide, and more preferably in the range of
1 to 10 grams, based on 1 litre of the refractory
carrier.
When the amount of praseodymium is less than 1 gram,
the effect of supprF:ssing the S02 oxidation activity is
lowered. Thus, such an amount is not preferred. On the
other hand, when the amount of praseodymium is more than
20 grams, an improvement of the effect of suppressing the
S02 oxidation activity corresponding to the increase in
the amount of praseodymium is not exhibited any longer,
resulting in an economical disadvantage. Moreover, when
the amount of praseodymium is more than 20 grams, the NOX
decomposition activity is lowered. Thus, such an amount
is not preferred.
Examples of a yttrium compound used for causing the
refractory carrier t=o carry yttrium include: inorganic
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salts, such as yttrium nitrate, yttrium carbonate,
yttrium chloride, and yttrium fluoride; organic salts,
such as yttrium acetate, and yttrium oxalate; and yttrium
oxide. The use of yttrium nitrate, yttrium fluoride,
yttrium acetate, and yttrium carbonate is particularly
suitable.
With regard to the amount of yttrium to be carried
on the refractory carrier, yttrium is preferably in the
range of 1 to 20 grams in terms of yttrium oxide, and
more preferably in the range of 1 to 10 grams, based on
1 litre of the refz~actory carrier.
When the amount of yttrium is less than 1 gram, the
NOX decomposition activity is lowered. Thus, such an
amount is not preferred. On the other hand, when the
amount of yttrium is more than 20 grams, an improvement
of the NOX decomposition activity corresponding to the
increase in the amount of yttrium is not exhibited any
longer, resulting in an economical disadvantage.
Moreover, when the amount of yttrium is more than 20
grams, the SOz oxidation activity is enhanced, 'and the
amount of particulate matter in the exhaust is increased
due to the generation of sulfates by the oxidation of SO2.
Thus, such an amounts is not preferred.
Examples of a metal compound used for causing the
refractory carrier to carry the other metal (hereinafter
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referred to as other metal compound) include: inorganic
salts, such as nitrate, sulfate, phosphate, carbonate,
chloride and fluoride of iron, cobalt, nickel, lanthanum,
cerium and neodymium; organic salts, such as acetate,
oxalate, and citrate; and oxides. More specifically,
examples of the other metal compound include: cobalt
nitrate, cobalt chloride, cobalt acetate; iron nitrate,
iron sulfate, iron chloride, iron citrate, iron oxide;
nickel nitrate, nickel chloride, nickel acetate, nickel
oxide; lanthanum nitrate, lanthanum chloride, lanthanum
acetate, lanthanum carbonate; cerium nitrate, cerium
sulfate, cerium ammonium nitrate, cerium carbonate,
cerium acetate; neod~-mium nitrate, neodymium chloride,
neodymium acetate anc3 neodymium carbonate.
With regard to r_he amount of the other metal to be
carried in the refr~~ctory carrier, the other metal is
preferably in the range of 1 to 20 grams in terms of its
oxide, and more preferably in the range of 1 to 10 grams,
based on 1 litre of the refractory carrier.
When the amount of the other metal is less than 1
gram, the NOX decomposition activity is lowered. Thus,
such an amount is not. preferred. On the other hand, when
the amount of the other metal is more than 20 grams, an
improvement of the NOX decomposition activity
corresponding to the increase in the amount of the other
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metal is not exhibited any longer, resulting in an
economical disadvantage. Moreover, when the amount of
the other metal is more than 20 grams, the SOa oxidation
activity is enhanced, and the amount of particulate
matter in the exhaust is increased due to the generation
of sulfates by the oxidation of SO2. Thus, such an amount
is not preferred.
Examples of the refractory inorganic oxide for
dispersing the catalytic component include: active
alumina, such as y-alumina, S-alumina, r~-alumina and B-
alumina, cx-alumina, titania, silica, zirconia, garia,
zeolite; and composite oxides thereof, namely, for
example, silica-alumina, alumina-titania, alumina-
zirconia, and titanic zirconia. It is possible to use
one or a combination of these oxides. For the
application of such an oxide to the exhaust from a diesel
engine, a particularly preferred oxide among the above-
mentioned oxides is zirconia which shows excellent
durability against sulfur oxide, or a mixture of zirconia
and zeolite.
Although the form of the refractory inorganic oxide
is not particularly limited, a preferred form of the
refractory inorganic oxide is powder. Moreover, the
Brunauer-Emmett-Teller specific area (hereinafter
referred to as the "BET specific area") of the refractory
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inorganic oxide is preferably within a range of from 5 to
400 mz/g, and more preferably within a range of from 1G to
300 m2/g. The average: particle diameter of the refractory
inorganic oxide is preferably within a range of from 0.1
to 150 Vim, and more preferably within a range of from 0.2
to 100 ~.m.
It is preferred to use the refractory inorganic
oxide in amounts ranging from :L00 to 250 grams based on
1 litre of the refractory carrier. When less than 100
grams of the refractory inorganic oxide is used based on
1 litre of the refractory carrier, a sufficient
catalytic ability is not obtained. Thus, the amount of
the refractory carrier to be used based on 1 litre of
the refractory carrier should be not less than 100 grams .
Moreover, when more than 250 grams of the refractory
inorganic oxide is used based on 1 litre of the
refractory carrier, an improvement of the catalytic
ability corresponding to the amount of the refractory
inorganic oxide ust:d is not exhibited. It is thus
preferred not to use more than 250 grams of the
refractory inorganic oxide based on 1 litre of the
refractory carrier. As the refractory carrier for
carrying the catalytic component, it is possible to use
a pellet carrier and a monolith carrier. The use of the
monolith carrier i:~ more preferred. Examples of the
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monolith carrier include: open flow type ceramic
honeycomb carrier, open flow type metal honeycomb
carrier; wall flow type honeycomb monolith carrier;
ceramic foam, metal foam; and metal mesh. Among these
carriers, the use of the open flow type ceramic honeycomb
carrier, or the open flow type metal honeycomb carrier is
particularly suitable.
Preferred materials for the ceramic honeycomb
carrier include cordierite, mullite, a-alumina, zirconia,
titania, titanium phosphate, aluminum titanate, petalite,
spodumene, aluminosilicate, and magnesium silicate.
Among the honeycomb carriers formed by these materials,
cordierite is particularly preferred. As the metal
honeycomb carrier, the use of carriers made of
antioxidant refractory metals, such as stainless steel,
and Fe-Cr-Al alloy, is particularly suitable.
These monolith carries are produced by extrusion
molding, tightly winding a sheet-like material, or other
method. The (gas flow) cell of the monolith carrier is
not particularly limited in its shape, and may be
hexagon, quadrangle, triangle or corrugation. The cell
density (the number of cells per unit cross sectional
area) of the monolith carrier is within a range of 150 to
600 cells/square inch, and more preferably within a range
of from 200 to 500 cells/square inch.
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The method for producing the exhaust emission
control catalysts of the present invention is not
particularly limited. For example, it is possible to use
method (1) in which after causing the refractory carrier
to carry the refractory inorganic oxide, the refractory
carrier is caused to further carry the catalytic
component, or method (2) in which the refractory carrier
is caused to carry a mixture of the refractory inorganic
oxide and the catalytic component.
More specifically, in method (1), first, the
refractory inorganic oxide in the form of powder is
water-ground to form slurry. Subsequently, the
refractory carrier is dipped in the resultant slurry.
After removing excessive slurry, the refractory carrier
is dried and calcined. As a result, the refractory
carrier carrying the refractory inorganic oxide is
obtained.
The drying temperature is preferably between 80 and
250 °C, and more preferably between 100 and 150 °C. The
calcining temperature is preferably between 300 and 850
°C, and more preferably between 400 and 700 °C. The
calcining time is preferably between 0.5 to 5 hours, and
more preferably between 1 and 2 hours.
Next, in method (1), the refractory carrier carrying
the refractory inorganic oxide is dipped in a solution
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containing a predetermined amount of the catalytic
component. After removing excessive solution, the
refractory carrier is dried and calcined. As a result,
the catalytic component is carried on the refractory
carrier in such a state that the catalytic component is
contained in the refractory inorganic oxide carried on
the refractory carrier. Thus, an exhaust emission
control catalyst of the present invention is obtained.
The drying temperature is preferably between 80 and
250 °C, and more preferably between 100 and 150 °C. The
calcining temperature is preferably between 300 and 850
°C, and more preferably between 400 and 700 °C. The
calcining time is preferably between 0.5 to 5 hours, and
more preferably between 1 and 2 hours.
On the other hand, in method (2), first, the
refractory inorganic oxide is placed in a solution
containing a predetermined amount of the catalytic
component. After impregnating the refractory inorganic
oxide with the catalytic component, the refractory
inorganic oxide is dried and calcined. As a result, a
mixture formed by the refractory inorganic oxide
containing the catalytic component therein is obtained.
The drying temperature is preferably between 80 and
250 °C, and more preferably between 100 and 150 °C. The
calcining temperature is preferably between 300 and 850
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°C, and more preferably between 400 and 700 °C. The
calcining time is preferably between 0.5 to 5 hours, and
more preferably between 1 and 2 hours.
Next, in method (2), the mixture in the form of
powder is water-ground to form slurry. Subsequently, the
refractory carrier is dipped in the resultant slurry.
After removing excessive slurry, the refractory carrier
is dried and calcined. As a result, the mixture is
carried on the refractory carrier, and an exhaust
emission control catalyst of the present invention is
obtained.
The drying temperature is preferably between 80 and
250 °C, and more preferably between 100 and 150 °C. The
calcining temperature is preferably between 300 and 850
°C, and more preferably between 400 and 700 °C. The
calcining time is preferably between 0.5 to 5 hours, and
more preferably between 1 and 2 hours.
The exhaust emission control catalysts of the
present invention can remove exhaust containing various
amounts of hydrocarbons (the mole number in terms of
methane) /NOX (mole number) ratios (hereinafter referred to
as the HC/NOX ratio) by contact. More specifically, the
exhaust emission control catalysts can satisfactorily
remove injurious components of exhaust in which the HC/NOX
ratio is between 0.5 and 30, and more satisfactorily
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remove injurious components of exhaust in which the HC/NOX
ratio is between 1 and 20.
By removing injurious components of exhaust in which-
the HC/NOX ratio is in the above-mentioned range by
bringing the exhaust into contact with the exhaust
emission control catalyst, NOX in the exhaust can be
sufficiently decomposed, and hydrocarbons in the exhaust
can be substantially completely burned.
Moreover, the exhaust emission control catalyst of
the present invention can satisfactorily remove the
injurious components of exhaust from diesel engine, which
contains excessive oxygen and discharges a particularly
large amount of particulate matter in the exhaust in
which the HC/NOX ratio is between 1 and 20. More
specifically, with the removal of the exhaust from the
diesel engine by bringing the exhaust into contact with
the above-mentioned exhaust emission control catalyst, it
is possible to sufficiently decompose NOX in the exhaust
and substantially completely burn hydrocarbons in the
exhaust. Moreover, it is possible to prevent the
oxidation of sulfur oxides such as S02in the exhaust from
proceeding, thereby reducing the amount of the discharge
of particulate matter caused by sulfate produced by the
oxidation.
When the HC/NO,~ ratio is low, since the amount of
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hydrocarbons which function as a NOX reducing agent is
small, there is a possibility of insufficient
decomposition of NOX by the reduction of NOX. In this
case, it is necessary to introduce a reducing agent into
the exhaust before bringing the exhaust into contact with
the exhaust emission control catalyst so that the HC/NOX
ratio at the time of the contact becomes an appropriate
value.
The temperature at the time the reducing agent is
introduced is preferably between 200 and 600 °C, and more
preferably between 300 and 500 °C.
The reducing agent to be supplied to the exhaust is
not particularly limited. For example, it is possible to
use hydrogen, saturated hydrocarbon, unsaturated
aliphatic hydrocarbon, aromatic hydrocarbon, alcohol,
etc.
Examples of the saturated hydrocarbon include:
alkanes having 1 to 20 carbons, such as methane, ethane,
propane, butane, pentane, hexane, octane, nonane, and
decane; and cycloalkane like cyclohexane. The alkanes
may be straight chain or branched chain alkanes.
Examples of the unsaturated aliphatic hydrocarbon
include alkenes having 1 to 20 carbons, such as
methylene, ethylene, propylene, butene, buthadiene,
pentene, pentadiene, hexene, hexadiene, heptene,
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heptadiene, heptatriene, octene, octadiene, and
octatriene. The alkenes may be straight chain or
branched chain alkenes. Examples of the unsaturated
aromatic hydrocarbon include benzene, toluene, xylene,
and trimethyl benzene.
Examples of the alcohol are alcohols having 1 to 20
carbons, such as methanol, ethanol, propanol, butanol,
pentanol, hexanol, heptanol, octanol, nonanol, and
decanol. The alcohols may be straight chain or branched
chain alcohols.
As the reducing agent, it is preferred to use a
compound which has a liquid phase or gaseous phase at
room temperature because such a compound can be easily
supplied to the exhaust. Moreover, the internal
combustion engine is provided with a fuel tank containing
fuel such as gas oil, natural gas, LPG (liquid propane
gas), gasoline, and methanol. By supplying such fuel as
the reducing agent to the exhaust, it is not necessary to
newly provide a tank for storing the reducing agent,
thereby producing an economical advantage. The method
for supplying the reducing agent is not particularly
limited. Suitable methods are, for example, supplying
the reducing agent through a single pipe, and spraying
the reducing agent with air.
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BEST MODE FOR CARRYING OUT THE INVENTION
The following description will explain the present
invention in detail by presenting examples and
comparative examples. However, these examples are
submitted for the purpose of illustration only and are
not to be construed as limiting the scope of the
invention in any way. The exhaust emission control
ability of exhaust emission control catalysts were
evaluated by carrying out the following test. In this
test, a direct injection type diesel turbo engine (four
cylinders, 2800 cc) was used as the internal combustion
engine, and a gas oil containing 0.05 weight percent of
sulfur was used as fuel of the internal combustion
engine.
First, the exhaust emission control catalyst was
installed in an exhaust pipe connected to the diesel
engine, and the exhaust was caused to flow for 100 hours
under such conditions that the full load engine speed was
2500 rpm and the temperature at an edge of the exhaust
emission control catalyst on the upstream side
(hereinafter referred to as the "catalyst inlet
temperature") was 700 °C.
Next, the torque of the diesel engine was set and
the exhaust was caused to flow so that the engine speed
was 2000 rpm and the catalyst inlet temperature as a test
CA 02245970 1998-08-11
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temperature was 350 °C. The gas oil functioning as the
NOX reducing agent was supplied to the exhaust pipe at a
position on more upstream side than the installation
position of the exhaust emission control catalyst so that
the HC/NOX ratio in the exhaust became 5.
After the catalyst inlet temperature was
sufficiently stabilized at 350 °C, the concentrations
(mol) of NOX, hydrocarbon, carbon monoxide, and SOZ in the
exhaust before the addition of the gas oil were measured
with continuous gas analyzers . More specifically, NOX was
measured with a chemiluminescent detector (CLD).
Hydrocarbon was measured with a flame ionization detector
(FID) . Carbon monoxide and SO~ were respectively measured
with a non-dispersive infrared analyzer (NDIR). As a
result, it was found that the composition of the exhaust
before the addition of gas oil was formed by 470 ppm NOx,
160 ppm hydrocarbon, 200 ppm carbon monoxide, and 10 ppm
S O2 .
A predetermined amount of the exhaust before the
addition of gas oil was sampled, and diluted with air in
a dilution tunnel. Thereafter, the sample was caused to
pass through a commercially available particulate filter
so as to trap particulate matter in the exhaust. After
trapping the particulate matter, the weight of the
particulate filter was measured, and the content of the
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particulate matter in the exhaust was calculated based on
an increase in the weight of the particulate filter, the
volume of the sampled exhaust, and the dilution ratio by
air. The dilution ratio by air was obtained by measuring
the concentration of carbon dioxide in the exhaust.
Moreover, after trapping the particulate matter, the
particulate filter was extracted with dichloromethane.
By measuring a decrease in the weight of the particulate
filter, the content of SOF in the exhaust was calculated.
After bringing the exhaust into contact with the
catalyst, the contents of NOX, hydrocarbon, carbon
monoxide, SOa, particulate matter, and SOF (hereinafter
referred to as the respective components) in the exhaust
were measured.
The degree of the removal (conversion) of the
respective components, i.e., removal of NOX, removal of
particulate matter, conversion of SOa, removal of SOF,
removal of hydrocarbon, and removal of carbon monoxide,
were calculated based on the contents of the respective
components before the addition of gas oil, and the
contents thereof after the contact with the catalyst.
The degree of removal (conversion) is given in percent by
(Xo - Xl) / Xo x 100
where Xo (mol) is the content before the addition of gas
oil, and Xl (mol) is the content after the contact with
CA 02245970 1998-08-11
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the catalyst.
Similarly, the contents of the respective components
were measured at catalyst inlet temperatures of 450 °C
and 550 °C as the test temperatures, and the degree of
removal (conversion) of the respective components were
calculated.
The composition of the exhaust before the addition
of gas oil at a catalyst inlet temperature of 450 °C was
formed by 470 ppm NOX, 158 ppm hydrocarbon, 120 ppm carbon
monoxide, and 12 ppm 502. On the other hand, the
composition of the exhaust before the addition of gas oil
at a catalyst inlet temperature of 550 °C was formed by
400 ppm NOX, 93 ppm hydrocarbon, 80 ppm carbon monoxide,
and 15 ppm S02.
[Example 1]
3000 grams of zirconia powder with a BET specific
area of 110 m2/g was placed as a refractory inorganic
oxide into an aqueous solution containing 120 grams of
copper nitrate, 258 grams of praseodymium nitrate, 337
grams of yttrium nitrate, and 410 grams of cobalt
nitrate, and sufficiently mixed. After drying the
mixture at 150 °C for 2 hours, the mixture was calcined
at 500 °C for 1 hour. As a result, zirconia powder
carrying a catalytic component in a dispersed state was
obtained.
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Next, the resultant zirconia powder was water-ground
to form slurry. An open flow type honeycomb carrier made
of cordierite was dipped as the refractory carrier in the
resultant slurry. The honeycomb carrier had a
cylindrical shape with a diameter of 5.66 inches and a
length of 6.00 inches, and about 400 pieces of gas flow
cells per square inc~i sectional area.
Subsequently, after removing excessive slurry, the
honeycomb carrier dipped in the slurry was dried at 150
°C for 2 hours, and then calcined at 500 °C for 1 hour.
As a result, an exhaust emission control catalyst was
obtained.
In the resultant catalystY 2 grams of copper oxide
(Cu0) , 5 grams of praseodymium oxide (Pr6011) , 5 grams of
yttrium oxide (Y203), 5 grams of cobalt oxide (Co0), and
150 grams of zirconia were carried per litre c.f_ carrier.
The amounts of these oxides carried are shown in Table
1.
CA 02245970 1998-08-11
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Table 1
Amounts
(g)
of
metallic
oxides
carried
per
litter
of
refractory
carrier
Cu Pr Y Co Fe Zr zeolite
Example 1 2 5 5 5 - 150 -
Example 2 2 5 5 - 5 150 -
Example 3 2 5 5 - - 150 -
Example 4 5 5 5 - - 150 -
Example 5 2 5 10 - - 150 -
Example 6 2 5 5 - - 100 50
Comparative - 5 5 - - 150 -
Example 1
Comparative 2 - 5 - - 150 -
Example 2
Comparative 2 5 - - - 150 -
Example 3
Comparative 30 5 5 - - 150 -
Example 4
Each bar shown in Table 1 indicates that the oxide
indicated in the top of the corresponding column was not
added.
In this case, the praseodymium oxide contained in
the exhaust emission control catalyst was 2.5 weight
parts based on 1 weight part of the copper oxide. The
yttrium oxide contained in the exhaust emission control
catalyst was 2.5 weight parts based on 1 weight part of
the copper oxide.
The exhaust emission control ability of the
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resultant exhaust emission control catalyst was evaluated
according to the above-mentioned test. Namely, the
degree of removal (conversion) of the respective
components was measured at catalyst inlet temperatures of
350 °C, 450 °C, and 550 °C. The results are shown in
Tables 2 and 3.
Table 2
Removal Removal Conversion
of of of
NOX particulate S02
( matter (
o (o) o
) )
A B C A B C A B C
Example 1 18 42 36 20 23 14 0 2 9
Example 2 18 36 24 16 19 16 0 3 9
Example 3 12 40 45 16 27 13 0 0 5
Example 4 42 44 30 22 16 5 1 4 15
Example 5 22 43 40 21 26 14 0 1 10
Example 6 12 45 50 18 26 18 0 1 8
Comparative 6 9 10 8 18 20 0 0 0
Example 1
Comparative 26 42 43 18 12 -5 1 10 40
Example 2
Comparative 8 25 13 12 26 16 0 2 18
Example 3
Comparative 48 36 14 16 5 -30 10 27 64
Example 4
A = 350 °C, B = 450 °C, C = 550 °C
[Example 2
An exhaust emission control catalyst was prepared in
the same manner as in Example 1 except that 407 grams of
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iron nitrate was used instead of 410 grams of cobalt
nitrate of Example 1.
In the resultant catalyst, 2 grams of copper oxide,
grams of praseodymium oxide, 5 grams of yttrium oxide,
5 grams of iron oxide (Fez03) , and 150 grams of zirconia
were carried per litre of carrier. The amounts of
these oxides carried are shown in Table 1.
In this case, the praseodymium oxide contained in
the exhaust emission control catalyst was 2.5 weight
parts based on 1 weight part of the copper oxide . The
yttrium oxide contained in the exhaust emission control
catalyst was 2.5 weight parts based on 1 weight part of
the copper oxide.
The degree of removal (conversion) of the respective
components was measured at the above-mentioned test
temperatures. The results are shown in Tables 2 and 3.
CA 02245970 2001-11-20
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Table 3
Removal Removal Removal
of of of
SOF hydrocarbon carbon
(%) (%) mono:~ide
A B C A B C A B C
Example 1 49 '77 86 45 84 98 12 66 81
Example 2 41 T3 85 36 76 95 -15 51 77
Example 3 40 '72 83 31 80 93 -10 61 71
Example 4 61 F~0 85 60 90 97 30 72 78
Example 5 52 75 82 48 78 89 15 54 75
Example 6 46 75 84 44 80 91 10 61 76
Comparative 20 45 50 18 44 46 0 -15 -20
Example 1
Comparative 51 81 88 49 88 97 10 70 80
Example 2
Comparative 31 '75 86 25 83 94 -5 60 76
Example 3
I
Comparative 65 81 86 63 88 94 45 70 77
Example 4
A = 350 °C, B = 450 °C, C = 550 °C
[Example 3]
An exhaust erni.ssion control catalyst was prepared in
the same manner ~;s in Example 1 except that cobalt
nitrate described ~.n Example 1 was omitted.
In the resultant catalyst, 2 grams of copper oxide,
5 grams of praseodymium oxide, 5 grams of yttrium oxide,
and 150 grams of zirconia were carried per litre of
carrier. The amounts of these oxides carried are shown
in Table 1.
CA 02245970 2001-11-20
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In this case, the praseodymium oxide contained in
the exhaust emissic>n control catalyst was 2.5 weight
parts based on 1 weight part of the copper oxide. The
yttrium oxide contained in the exhaust emission control
catalyst was 2.5 weight parts based on 1 weight part of
the copper oxide.
The degree of the removal (conversion) of the
respective components was measured at the above-mentioned
test temperatures . '.L'he results are shown in Tables 2 and
3.
[Example 4]
An exhaust emission control catalyst was prepared in
the same manner as in Example 1 except that the amount of
copper nitrate described in Example 1 was changed from
120 grams to 300 grams and cobalt nitrate in Example 1
was omitted.
In the resultant catalyst, 5 grams of copper oxide,
grams of praseodymium oxide, 5 grams of yttrium oxide,
and 150 grams of z:i.rconia were carried per litre of
carrier. The amounts of these oxides carried are shown
in Table 1.
In this case, ~:he praseodymium oxide contained in
the exhaust emission control catalyst was 1 weight part
based on 1 weight part of the copper oxide. The yttrium
oxide contained in tie exhaust emission control catalyst
CA 02245970 2001-11-20
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was 1 weight part based on 1 weight part of the copper
oxide.
The degree of removal (conversion) of the respective
components was measured at the above-mentioned test
temperatures. The results are shown in Tables 2 and 3.
[Example 5]
An exhaust emisw>ion control catalyst was prepared in
the same manner as iia Example 1 except that the amount of
yttrium nitrate described in Example 1 was changed from
337 grams to 674 grams, and cobalt nitrate was omitted.
In the resultan~ catalyst, 2 grams of copper oxide,
grams of praseodyma_um oxide, 10 grams of yttrium oxide,
and 150 grams of z.irconia were carried per litre of
carrier. The amoun~s of these oxides carried are shown
in Table 1.
In this case, the praseodymium oxide contained in
the exhaust emission control catalyst was 2.5 weight
parts based on 1 weight part of the copper oxide . The
yttrium oxide contained in the exhaust emission control
catalyst was 5 weight parts based on 1 weight part of the
copper oxide.
The degree of removal (conversion) of the respective
components was measured at the above-mentioned test
temperatures. The results are shown in Tables 2 and 3.
CExample 6]
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An exhaust emission control catalyst was prepared in
the same manner as in Example 1 except that a mixture of
2000 grams of zirconia powder with a BET specific area of
110 m2/g and 1000 grams of commercially available ZSM-5
zeolite with a BET specific area of 430 m2/g was used as
the refractory inorganic oxide instead of 3000 grams of
zirconia powder with a BET specific area of 110 mz/g
described in Example 1, and cobalt nitrate was omitted.
In the resultant catalyst, 2 grams of copper oxide,
grams of praseodymium oxide, 5 grams of yttrium oxide,
100 grams of zirconia, and 50 grams of zeolite were
carried per litre of carrier. The amounts of these
oxides carried are ~~hown in Table 1.
In this case, the praseodymium oxide contained in
the exhaust emission control catalyst was 2.5 weight
parts based on 1 weight part of the copper oxide. The
yttrium oxide contained in the exhaust emission control
catalyst was 2.5 weight parts based on 1 weight part of
the copper oxide.
The degree of removal (conversion) of the respective
components was measured at the above-mentioned test
temperatures. The results are shown in Tables 2 and 3.
[Comparative Example 1]
An exhaust emi:~sion control catalyst was prepared in
the same manner as in Example 1 except that copper
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nitrate and cobalt nitrate described in Example 1 were
omitted.
In the resultant catalyst, 5 grams of praseodymium
oxide, 5 grams of yttrium oxide, and 150 grams of
zirconia were carried per litre of carrier. The
amounts of these oxides carried are shown in Table 1.
The degree of removal (conversion) of the respective
components was measured at the above-mentioned test
temperatures. The results are shown in Tables 2 and 3.
[Comparative Example 2]
An exhaust emission control catalyst was prepared in
the same manner as in Example 1 except that praseodymium
nitrate and cobalt r~itrate described in Example 1 were
omitted.
In the resultant catalyst, 2 grams of copper oxide,
grams of yttrium oxide, and 150 grams of zirconia were
carried per litre of carrier. The amounts of these
oxides carried are shown in Table 1.
In this case, the yttrium oxide contained in the
exhaust 'emission control catalyst was 2.5 weight parts
based on 1 weight part of the copper oxide.
The degree of removal (conversion) of the respective
components was measured at the above-mentioned test
temperatures. The results are shown in Tables 2 arid 3. -
[Comparative Example 3]
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An exhaust emission control catalyst was prepared in
the same manner as in Example 1 except that yttrium
nitrate and cobalt nitrate described in Example 1 were
omitted.
In the resultant: catalyst, 2 grams of copper oxide,
grams of praseodymium oxide, and 150 grams of zirconia
were carried per litre of carrier. The amounts of
these oxides carried are shown in Table 1.
In this case, the praseodymium oxide contained in
the exhaust emissiorE control catalyst was 2.5 weight
parts based on 1 weight part of the copper oxide.
The degree of removal (conversion) of the respective
components was measured at the above-mentioned test
temperatures. The results are shown in Tables 2 and 3.
[Comparative Example 4]
An exhaust emission control. catalyst was prepared in
the same manner as in Example 1 except that the amount of
copper nitrate described in Example 1 was changed from
120 grams to 1800 grams, and cobalt nitrate was omitted.
In the resultant. catalyst, 30 grams of copper oxide,
5 grams of praseodymium oxide, 5 grams of yttrium oxide,
and 150 grams of zirconia were carried per litre of
carrier. The amounts of these oxides carried are shown
in Table 1.
In this case, ~.he praseodymium oxide contained in
CA 02245970 1998-08-11
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the exhaust emission control catalyst was 0.133 weight
parts based on 1 weight part of the copper oxide. The
yttrium oxide contained in the exhaust emission control
catalyst was 0.133 weight parts based on 1 weight part of
the copper oxide.
The degree of removal (conversion) of the respective
components was measured at the above-mentioned test
temperatures. The results are shown in Tables 2 and 3.
It is clear from Tables 2 and 3 showing the results
of Examples 1 to 6 and Comparative Examples 1 to 4 that
the exhaust emission control catalysts of the examples
are excellent in the exhaust emission control ability,
particularly in the removal of NOX, and in the removal
particulate matter under high temperature conditions.
Additionally, compared to the exhaust emission
control catalysts of Comparative Examples 2 and 4, the
exhaust emission control catalysts of the examples
suppress the conversion of S02 to a great degree, thereby
significantly improving the degree of removal of
particulate matter.
INDUSTRIAL APPLICABILITY
Since the exhaust emission control catalysts of the
present invention can efficiently remove NOX in exhaust
containing a large amount of oxygen and reduce the amount
CA 02245970 1998-08-11
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of particulate matter in the exhaust particularly under
high temperature conditions, the exhaust emission control
catalysts are suitable for use in an internal combustion
engine like a diesel engine.
Moreover, a method for controlling the emission of
exhaust of the present invention can efficiently remove
NOX in exhaust in which the mole ratio of hydrocarbon to
NOX is between 0.5 and 30, and more preferably 1 and 20,
and particularly reduce the discharge of particulate
matter in the exhaust under high temperature conditions.
Thus, this method is suitably used for controlling the
emission of -exhaust from an internal combustion engine
like a diesel engine.
