U.S. patent application number 09/201121 was filed with the patent office on 2001-12-06 for burned gas purifying catalyst.
This patent application is currently assigned to MAZDA MOTOR CORPORATION. Invention is credited to IWAKUNI, HIDEHARU, KUROKAWA, TAKAHIRO, KYOGOKU, MAKOTO, MURAKAMI, HIROSHI, OKAMOTO, KENJI, SUMIDA, HIROSUKE, TAKAMI, AKIHIDE, YAMADA, HIROSHI, YAMAMOTO, KENICHI.
Application Number | 20010049337 09/201121 |
Document ID | / |
Family ID | 26418878 |
Filed Date | 2001-12-06 |
United States Patent
Application |
20010049337 |
Kind Code |
A1 |
KUROKAWA, TAKAHIRO ; et
al. |
December 6, 2001 |
BURNED GAS PURIFYING CATALYST
Abstract
An exhaust gas emission control catalyst includes an under
catalyst layer containing at least one of barium and lanthanum and
an over catalyst layer containing an agent for absorbing water in a
gas, at least one of the under catalyst layer and over catalyst
layer containing catalytic metal.
Inventors: |
KUROKAWA, TAKAHIRO;
(HIROSHIMA-SHI, JP) ; TAKAMI, AKIHIDE;
(HIROSHIMA-SHI, JP) ; KYOGOKU, MAKOTO;
(HIROSHIMA-SHI, JP) ; IWAKUNI, HIDEHARU;
(HIROSHIMA-SHI, JP) ; OKAMOTO, KENJI;
(HIROSHIMA-SHI, JP) ; SUMIDA, HIROSUKE;
(HIROSHIMA-SHI, JP) ; YAMAMOTO, KENICHI;
(HIROSHIMA-SHI, JP) ; MURAKAMI, HIROSHI;
(HIROSHIMA-SHI, JP) ; YAMADA, HIROSHI;
(HIROSHIMA-SHI, JP) |
Correspondence
Address: |
STAAS & HALSEY
700 ELEVENTH STREET NW
SUITE 500
WASHINGTON
DC
20001
|
Assignee: |
MAZDA MOTOR CORPORATION
Hiroshima
JP
|
Family ID: |
26418878 |
Appl. No.: |
09/201121 |
Filed: |
November 30, 1998 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09201121 |
Nov 30, 1998 |
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08895880 |
Jul 17, 1997 |
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5958826 |
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08895880 |
Jul 17, 1997 |
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08639507 |
Apr 29, 1996 |
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5677258 |
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Current U.S.
Class: |
502/325 ;
502/339 |
Current CPC
Class: |
B01J 23/58 20130101;
B01J 37/0244 20130101; B01D 2255/902 20130101; F01N 3/0842
20130101; F01N 2250/12 20130101; B01D 53/945 20130101; Y02A 50/20
20180101; F01N 3/0814 20130101; F01N 2570/14 20130101; Y02T 10/12
20130101; B01D 53/261 20130101; F01N 3/0835 20130101; B01J 23/63
20130101 |
Class at
Publication: |
502/325 ;
502/339 |
International
Class: |
B01J 023/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 28, 1995 |
JP |
7-104498 |
Mar 29, 1996 |
JP |
8-077821 |
Claims
What is claimed is:
1. A catalyst construction for purifying gas comprising: a catalyst
carrier; an under catalyst layer coated on said catalyst carrier,
said under catalyst layer containing at least one of barium and
lanthanum; an over catalyst layer coated on said under catalyst
layer, said over catalyst layer containing an agent for absorbing
water in a gags; and a catalytic metal contained in at least one of
said under catalyst layer and said over catalyst layer.
2. A catalyst construction as defined in claim 1, wherein said
water absorbing agent comprises a crystalline metal silicate.
3. A catalyst construction as defined in claim 1, wherein said
catalytic metal comprises at least one selected from noble metals
and is contained in said over catalyst layer.
4. A catalyst construction as defined in claim 2, wherein said
catalytic metal comprises at least one selected from noble metals
and is contained in said over catalyst layer.
5. A catalyst construction as defined in claim 3, wherein said
noble metals are platinum and rhodium.
6. A catalyst construction as defined in claim 4, wherein said
noble metals are platinum and rhodium.
7. A catalyst construction as defined in claim 3, wherein at least
one of said under catalyst layer and said over catalyst layer
contains a cerium oxide.
8. A catalyst construction as defined in claim 4, wherein at least
one of said under catalyst layer and said over catalyst layer
contains a cerium oxide.
9. A catalyst construction as defined in claim 3, wherein said
under catalyst layer contains palladium and alumina.
10. A catalyst construction as defined in claim 4, wherein said
under catalyst layer contains palladium and alumina.
11. A catalyst construction as defined in claim 3, wherein said
over catalyst layer has a weight proportion relative to a total
weight of said over catalyst layer and said under catalyst layer in
a range between {fraction (3/40)} and {fraction (34/40)}.
12. A catalyst construction as defined in claim 4, wherein said
over catalyst layer has a weight proportion relative to a total
weight of said over catalyst layer and said under catalyst layer in
a range between {fraction (3/40)} and {fraction (34/40)}.
13. A catalyst construction as defined in claim 9, wherein said
under catalyst layer further contains platinum.
14. A catalyst construction as defined in claim 10, wherein said
under catalyst layer further contains platinum.
15. A catalyst construction as defined in claim 3, wherein an
amount of said barium is in a range of 7 and 45% by weight of a
total amount of said over catalyst layer and said under catalyst
layer.
16. A catalyst construction as defined in claim 4, wherein said an
amount of said barium is in a range of 7 and 45% by weight of a
total amount of said over catalyst layer and said under catalyst
layer.
17. A catalyst construction as defined in claim 11, wherein said an
amount of said barium is in a range of 7 and 45% by weight of a
total amount of said over catalyst layer and said under catalyst
layer.
18. A catalyst construction as defined in claim 12, wherein said an
amount of said barium is in a range of 7 and 45% by weight of a
total amount of said over catalyst layer and said under catalyst
layer.
19. A catalyst construction as defined in claim 3, wherein an
amount of said barium is in a range of 10 and 30% by weight of a
total amount of said over catalyst layer and said under catalyst
layer.
20. A catalyst construction as defined in claim 4, wherein said an
amount of said barium is in a range of 10 and 30% by weight of a
total amount of said over catalyst layer and said under catalyst
layer.
21. A catalyst construction as defined in claim 11, wherein said an
amount of said barium is in a range of 10 and 30% by weight of a
total amount of said over catalyst layer and said under catalyst
layer.
22. A catalyst construction as defined in claim 12, wherein said an
amount of said barium is in a range of 10 and 30% by weight of a
total amount of said over catalyst layer and said under catalyst
layer.
23. A catalyst construction for purifying gas comprising: a
catalyst carrier; an under catalyst layer coated on said catalyst
carrier, said under catalyst layer containing a catalytic metal;
and an over catalyst layer coated on said under catalyst layer,
said over catalyst layer containing a crystalline metal silicate
supporting barium.
24. A catalyst construction for reducing nitrogen oxides (NOx),
hydrocarbons (HC) and carbon monoxide (CO) in an automotive engine
exhaust gas to nitrogen (N.sub.2), hydrogen dioxide (HO.sub.2) and
carbon dioxide (CO.sub.2), respectively, said catalyst construction
comprising: a catalyst carrier; an under catalyst layer coated on
said catalyst carrier, said under catalyst layer containing at
least one of barium and lanthanum; an over catalyst layer coated on
said under catalyst layer, said over catalyst layer comprising an
agent for absorbing water in an exhaust gas; and a catalytic metal
contained in at least one of said under catalyst layer and said
over catalyst layer.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The invention relates to a gas purifying catalyst, and, more
particularly, to a burned gas purifying catalyst for use with an
exhaust system of an automobile engine suitable for emission level
controls of nitrogen oxides (NOx), hydrocarbons (HC) and carbon
monoxide (CO).
[0003] 2. Description of the Related Art
[0004] As one of catalysts installed in an exhaust line of an
automobile engine to purify the exhaust gas or to significantly
lower emission levels of oxides of nitrogen (NOx) as well as
hydrocarbons (HC) and carbon monoxide (CO) in the exhaust gas an
automobile exhaust control catalyst, there has been a monolith type
catalytic convertor which is formed with an under catalyst layer
having active alumina particles and platinum (Pt) and rhodium (Rh)
on a monolith honeycomb carrier and an over catalyst layer, coated
over the under catalyst layer, which comprises barium-fixed ceria
(cerium oxide) particles, active alumina particles and palladium
(Pd). Such a catalyst is known from, for instance, Japanese
Unexamined Patent Publication No. 3-207446. The reason for fixing
barium (Ba) to the ceria particles is to prevent the ceria from
suffering heat deterioration. The barium-fixed ceria particles are
produced in such a manner to dry and solidify a mixture of a barium
nitrate solution with ceria particles as a solid lump of barium
nitrate-adsorbed ceria particles and break it into particles. The
over catalyst layer is coated by dipping a catalyst carrier in a
slurry of a palladium chloride solution with the barium
nitrate-adsorbed ceria particles and, thereafter, dry and burn the
slurry film on the catalyst carrier.
[0005] While barium (Ba) is essentially intended to play as an
agent to prevent heat deterioration of ceria, it is in some cases
used to purify exhaust gases, in particular to reduce nitrogen
oxides (NOx) in exhaust gases as is known from, for instance,
Japanese Unexamined Patent Publication No. 7-108172. The catalyst
described in the Japanese Unexamined Patent Publication No.
7-108172 is a monolith honeycomb type catalytic convertor that
carries an under catalyst layer having barium (Ba) supported by an
alumina support and an over catalyst layer having platinum (Pt) and
rhodium (Rh) supported by an alumina support. This catalyst reduces
nitrogen oxides (NOx) through the steps of oxidizing nitrogen
oxides (NOx) with the barium (Ba) in the over catalyst layer, a
lowering the concentration of oxygen (O.sub.2) in the exhaust gas
so as to produce a reducing atmosphere in which the nitrogen oxides
(NOx) are separated from the barium, and reducing the nitrogen
oxides (NOx) by the catalytic metal in the under catalyst layer
making the utilization of hydrocarbons (HC) and carbon monoxide
(CO) in the exhaust gas as reducing agents.
[0006] A typical problem the exhaust gas emission control catalysts
experience is sulfur (S) poisoning and/or water (H.sub.2O)
poisoning and is significant in particular if they contain large
amounts of ballium which has a strong tendency to be poisoned. It
has been proved by the inventors of this invention that lanthanum
(La) has a tendency of the sulfur (S) poisoning and/or water
(H.sub.2O) poisoning as well. Accordingly, unless the catalyst
containing platinum and rhodium or palladium as main catalytic
metals is kept away from the sulfur (S) poisoning and water
(H.sub.2O) poisoning, it is difficult that the catalyst maintains
its intended emission control efficiency for a long period of
time.
SUMMARY OF THE INVENTION
[0007] It is an object of the present invention to provide a
catalyst construction for purifying gases which prevents lanthanum
and barium from sulfur poisoning and/or water poisoning.
[0008] It is another object of the present invention to provide an
catalyst construction for purifying automobile exhaust gases which
maintains its intended emission control efficiency for a long
period of time.
[0009] This invention has been achieved on the basis of the
knowledge obtained from the results of various investigations and
assessment conducted by the inventors of this application that a
combination of a composition of barium and lanthanum and a zeolite
support prevents the barium and lanthanum from sulfur poisoning
and/or water poisoning and maintains the intended activity of the
barium and lanthanum for a long period of time.
[0010] These objects of the present invention are achieved by
providing a gas purifying catalyst construction comprising an under
catalyst layer containing at least one of barium and lanthanum and
an over catalyst layer containing an agent to absorbing water in a
gas. The over catalyst layer prevents the barium and/or the
lanthanum from sulfur poisoning and/or water poisoning. The
catalytic metal may be contained either one or both of the under
and over catalytic layers.
[0011] The water adsorbing agent comprises a crystalline metal
silicate which works to prevent the barium and lanthanum from
sulfur poisoning and/or water poisoning. This is because the metal
silicate, such as MFI-type zeolite, is one of materials that
exhibit excellent water adsorbing performance and prevent water
poisoning, consequently. Further, the metal silicate in the over
catalyst layer prevents the barium and lanthanum in the under
catalyst layer from easily contacting with gases, enhancing the
prevention of sulfur poisoning and/or water poisoning.
[0012] The over catalyst layer contains one or more selected from a
noble metal group of catalytic materials such as platinum (Pt),
rhodium (Rh), palladium (Pd) and iridium (Ir), which works to lower
emission levels of oxides of nitrogen (NOx) as well as hydrocarbons
(HC) and carbon monoxide (CO) in burned gases. In particular, when
the over catalyst layer contains platinum and rhodium as the noble
metal group of catalytic materials, the catalyst exhibits an
excellent NOx emission control efficiency in burned gas resulting
from the combustion of a lean air-fuel mixture through a
synergistic effect of the platinum and rhodium catalytic materials
and additives such as barium or lanthanum in combination.
[0013] If a small amount of palladium is added into the under
catalyst layer, it is preferred to support the palladium by a
cerium oxide or alumina so as to deposit the palladium particles
separately from the rhodium particles. The reason for a decline in
the catalytic activity with a rise in ambient temperature has been
considered to be caused by the absence of intermediate products of
the hydrocarbon combustion contributory to reduction or
decomposition of nitrogen oxides which results from expeditious
combustion of hydrocarbons. Although the reason for the improvement
in high temperature NOx emission control efficiency of the catalyst
resulting from the presence of the cerium oxide has not been
clearly proved, the presence of cerium in the catalyst of the
invention suppresses the combustion of hydrocarbons at high
temperatures and produces easily intermediate products of the
hydrocarbon combustion. When the cerium is contained in the under
catalyst layer, the cerium is prevented by the barium and lanthanum
from experiencing thermal deterioration and exhibits its primary
chemical activity for a long period of time.
[0014] The under catalyst layer may contain an additive of
palladium or alumina with the effect of improving low temperature
catalytic activity of the catalyst. The palladium exhibits its
catalytic activity at temperatures lower as compared with platinum
and rhodium and burns hydrocarbons in low temperature exhaust gases
from an automobile engine which is still cold. Consequently, even
when the exhaust gas is still at low temperatures, the combustion
of hydrocarbons by means of the under catalyst layer causes the
over catalyst layer to rapidly raise its temperature sufficiently
to burn hydrocarbons. The combustion of hydrocarbons is utilized to
reduce or decompose nitrogen oxides in burned gases.
[0015] Contact of the palladium in the under catalyst layer with a
large amount of hydrocarbons causes poisoning, lowering the
catalytic performance. However, the metal silicate contained in the
over catalyst layer absorbs hydrocarbons in burned gases and,
consequently, prevent the palladium in the under catalyst layer
from hydrocarbon poisoning even when insufficient combustion of
hydrocarbons occurs while the engine is still cold.
[0016] Because, unlike platinum and rhodium, palladium is easy to
exhibit its catalytic activity rather after having been oxidized,
the catalyst containing palladium and alumina in the under catalyst
layer exhibits well its catalytic activity since the alumina works
more easily as a source of oxygen supply and promotes oxidization
of the palladium. In this instance, the alumina is prevented by the
barium and lanthanum from thermal deterioration.
[0017] The over catalyst layer preferably has a weight proportion
relative to the total weight of the over and under catalyst layers
in a range between {fraction (3/40)} and {fraction (34/40)}. If the
lower limit is exceeded, it will be difficult for the catalyst to
form the over catalyst completely over the under catalyst layer and
to exhibit an intended NOx emission control efficiency. On the
other hand, if the upper limit is exceeded, the over catalyst layer
arrests the catalytic effect of the barium and lanthanum or
palladium in the over catalyst layer. In this instance, a
significant feature is that, because the barium and lanthanum
prevents thermal deterioration of the catalyst and works as a NOx
absorbing agent to contribute the reduction of nitrogen oxides, the
catalyst maintains a high NOx emission control efficiency even
after the catalyst has been exposed to high temperature burned
gases. Consequently, even when the over catalyst layer has the
weight proportion less than {fraction (3/40)}, the catalyst
exhibits the intended NOx emission control efficiency. If anything,
the barium and lanthanum in the over catalyst layer which has
rather a small weight proportion is more contributory to NOx
emission control efficiency. In view of these facts, the over
catalyst layer is more preferable to have the weight proportion in
a range between {fraction (5/40)} and {fraction (16/40)}.
[0018] The catalyst may be modified with the result of exhibiting
the intended performance in that the over catalyst layer contains
barium and the under catalyst layer contains a catalytic metal. In
this case, the utilization is made of a crystalline metal silicate
as a support for the barium which prevents the barium from sulfur
poisoning and/or water poisoning. Containing the barium in the over
catalyst layer makes it easy to manufacture the catalyst. In the
case where the catalyst is made through steps of washcoating a
slurry of barium on a carrier and further washcoating a slurry of
the metal silicate mixed with a catalytic metal over the barium
contained coating, if the slurry of the metal silicate mixed with a
catalytic metal is acid, there occurs the problem that the barium
in the under catalyst layer eluates in the form of a barium
hydroxide Ba(HO).sub.2 into the slurry. Because the catalyst of the
invention is, however, manufactured by a step of washcoating a
slurry containing barium after having formed the under catalyst
layer, the problem of the elusion of barium is not encountered.
[0019] The amount of barium in a range of 7 and 45% by weight of
the total amount of the over and under catalyst layers is
preferable for the catalyst to produce an improvement in NOx
emission control efficiency. If the lower limit is exceeded, the
barium is difficult to exhibit sufficiently its effect. On the
other hand, if the upper limit is exceeded, the catalyst
experiences a decline in NOx emission control efficiency. this has
been considered to be caused by an adverse effect of a large amount
of the barium to the performance of other catalytic metals. In view
of this point, an appropriate amount of barium is proved to be in a
range between 10 and 30%.
[0020] The catalyst of the invention is manufactured in various
manners. Specifically, when forming the under catalyst layer with
barium contained, a slurry of alumina, ceria and binder, such as
alumina binder, mixed with an appropriate amount of distilled water
is washcoated on a monolith type honeycomb carrier. The coating is
dried at a temperature between 150 and 300.degree. C. for two to
four hours and burned in the air at a temperature of approximately
500.degree. C. for one to four hours. The barium in the form of a
solid barium compound powder is contained in the slurry. As the
solid barium compound, a barium oxide (BaO), a barium dioxide
(BaO.sub.2) a barium carbonate (BaCO.sub.3) and a barium sulfate
(BaSO.sub.4) can be employed. In order to support the palladium in
the under catalyst layer, the coating after having been burned is
impregnated with a palladium nitrate solution and dried and
burned.
[0021] Alternatively, an under coating is prepared by washcoating a
slurry of alumina, ceria and alumina binder mixed with distilled
water on the honeycomb carrier and drying and burning the slurry.
Thereafter, the coating is impregnated with a palladium solution
and with a barium solution in this order or vise versa. A solution
of barium and palladium may be admitted. As the barium solution, it
is preferred to employ a barium acetate solution and a barium
nitrate solution. The under catalyst layer may be achieved by a
number of times of impregnation a coating with palladium and barium
and drying the coating and a final burning treatment of the
coating.
[0022] When forming the over catalyst layer, a slurry of a powdered
zeolite (crystalline metal silicate) with platinum and rhodium
supported, a ceria powder and binder mixed with distilled water is
prepared. The slurry is washcoated over the under catalyst layer,
dried at a temperature between 150 and 300.degree. C. for two to
four hours and burned in the air at a temperature of approximately
500.degree. C. for one to four hours. The powdered zeolite with
platinum and rhodium supported is prepared by producing a slurry of
a mixture comprising a zeolite powder, a palladium solution and a
rhodium solution and spraydrying and burning the slurry. Otherwise,
the slurry may be solidified by evaporating solution liquids.
Alternatively, the zeolite powder may be impregnated with a
platinum solution and a rhodium solution, and dried and burned. A
dinitrodiamine platinum solution and a rhodium nitrate solution may
be employed as the platinum solution and rhodium solution,
respectively.
[0023] Crystalline metal silicates are a porous material whose pore
has a majority of microscopic pores and includes an aluminum group
of metals as a main metal component of the crystal. Aluminosilicate
silicate, i.e. zeolite, which is typical as an aluminum group
metal, includes Y-type zeolite, moldenite, MFI-type zeolite, and
.beta.-type zeolite. In place of aluminum or together with
aluminum, metal silicates containing gallium (Ga), cerium (Ce),
manganese (Mn) or terbium (Tb) may be employed.
[0024] The ceria as a cerium oxide may be added in various forms.
For example, if the ceria is added into the over catalyst layer,
the ceria may be mixed with the metal silicate as a support for the
platinum and rhodium. Alternatively, the ceria with the platinum
and rhodium supported thereby may be mixed with the metal silicate
with the platinum and rhodium supported thereby. The same forms can
be taken to form the under catalyst layer.
[0025] While the ceria is available as a cerium oxide, it is easy
to experience thermal deterioration. In view of thermal resistance,
a double oxide of cerium and zirconium (Zr) is preferable to be
employed as a cerium oxide. Alumina may be added together with the
cerium oxide.
[0026] When forming the over catalyst layer with barium contained,
the under catalyst layer is formed in advance by washcoating a
slurry of alumina, ceria and binder mixed with an appropriate
amount of distilled water on a monolith type honeycomb carrier. The
coating is dried at a temperature between 150 and 300.degree. C.
for two to four hours and burned in the air at a temperature of
approximately 500.degree. C. for one to four hours. The palladium
is contained in the under catalyst layer by impregnating the
coating with a palladium nitrate solution and drying and burning
the coating.
[0027] Thereafter, the over catalyst layer is formed by washcoating
a slurry of powdered zeolite with platinum and rhodium supported
thereby, ceria and binder mixed with an appropriate amount of
distilled water, drying the coating at a temperature between 150
and 300.degree. C. for two to four hours and burning it in the air
at a temperature of approximately 500.degree. C. for one to four
hours.
[0028] The impregnation with barium can be carried out in various
forms. For instance, a slurry comprised of a mixture of the zeolite
powder (not containing a catalytic metal) and solid barium
particles, a palladium solution and a rhodium solution is
spraydried and burned.
[0029] A solution of platinum, a solution of rhodium and a solution
of barium may be added to the zeolite powder (not containing a
catalytic metal). Otherwise, solid barium powder may be added into
the slurry to be coated for the over catalyst layer. Alternatively,
after impregnating with a barium solution a mixture of the powdered
zeolite with platinum and rhodium supported thereby, powdered ceria
and binder, a slurry may be prepared by adding distilled water to
the mixture.
[0030] In order to add barium in both under and over catalyst
layers, a slurry is prepared by mixing alumina, ceria and binder
with distilled water and washcoated on a monolith type honeycomb
carrier. The coating is dried at a temperature between 150 and
300.degree. C. for two to four hours and burned in the air at a
temperature of approximately 500.degree. C. for one to four hours.
Thereafter, the coating is impregnated with a palladium nitrate,
dried at a temperature between 150 and 300.degree. C. for two to
four hours and burned in the air at a temperature of approximately
500.degree. C. for one to four hours. Subsequently, a slurry is
prepared by mixing powdered zeolite with platinum and rhodium
supported thereby, ceria and binder with an appropriate amount of
distilled water and washcoated over the under catalyst layer. The
coating is dried at a temperature between 150 and 300.degree. C.
for two to four hours and burned in the air at a temperature of
approximately 500.degree. C. for one to four hours. Finally, the
coatings are impregnated with a barium solution, achieving the
under and over catalyst layers.
[0031] With regard to the under catalyst layer, the weight
proportions of the alumina, ceria and binder relative to the
honeycomb carrier are preferred to be 2-20%:less than 20%:1-10% ,
and more suitably to be 4-10%:1-10%:2-5%.
[0032] On the other hand, with regard to the over catalyst layer,
the weight proportions of the zeolite, ceria and binder relative to
the honeycomb carrier are preferred to be 6-35%:less than
35%:2-20%, and more suitably to be 5-25%:1-6%:5-10%.
[0033] The barium solution contains suitably barium between 0.5% by
weight and a saturated concentration and more suitably higher than
1% by weight. The impregnation with the barium solution is suitably
performed at a temperature between 10 and 40.degree. C.
[0034] As the alumina, .gamma.-alumina containing an appropriate
amount, for instance 7.5% by weight, of lanthanum is suitably
employed.
[0035] The amount of palladium is preferred to be between 0.5 and
20 grams, and more suitably between 1 and 7 grams, per one liter of
the volume of the honeycomb carrier.
[0036] The weight proportion between the platinum and rhodium is
preferably 75:1, and the total amount of the platinum and rhodium
is preferably in a range between 0.5 and 6 grams, and more suitably
between 1 and 4 grams, per one liter of the volume of the honeycomb
carrier.
[0037] The catalyst containing at least one of barium and lanthanum
in the under catalyst layer, a water absorbing agent in the over
catalyst layer, and a catalyst metal in at least one of the under
and over catalyst layers exhibits significantly excellent
performance of reducing nitrogen oxides (NOx), hydrocarbons (HC)
and carbon monoxide (CO) in an automotive engine exhaust gas to
nitrogen (N.sub.2), hydrogen dioxide (HO.sub.2) and carbon dioxide
(CO.sub.2), respectively.
BRIEF DESCRIPTION OF THE DRAWINGS
[0038] The above and other objects and features of the present
invention will be clearly understood from the following description
with respect to a preferred embodiment thereof when considered in
conjunction with the accompanying drawings, in which:
[0039] FIG. 1 is a schematic cross-sectional view showing a
structure of a catalyst in accordance with an embodiment of the
invention;
[0040] FIG. 2 is a table describing the result of NOx removal
efficiency measurements for catalysts of Examples I-III and an
evaluation sample catalyst E-I;
[0041] FIG. 3 is a table showing the result of evaluation of the
effect of catalyst manufacturing steps on barium desorption;
[0042] FIG. 4 is a graph showing the effect of the amount of barium
on NOx removal efficiency;
[0043] FIG. 5 is a graph showing the effect of the total amount of
catalyst layers on NOx emission control efficiency; and
[0044] FIG. 6 is a graph showing the temperature dependency of NOx
emission control efficiency.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0045] As shown in FIG. 1, a catalyst 1 of the invention was
comprised under and over catalyst layers 4 and 3 coated on a
cordierite monolith honeycomb carrier 2 having 400 cells per square
inch.
EXAMPLE I
[0046] The under catalyst layer 4 was formed by wash-coating a
slurry of a composition of 60 grams of .gamma.-alumina (a particle
size of less than 4 .mu.m, a purity of higher than 95%), 60 grams
of ceria and 12 grams of alumina as a binder mixed with an
appropriate amount of distilled water on the cordierite monolith
honeycomb carrier 2, drying the coating at for 150.degree. C. for
two hours and burning the coating at 500.degree. C. for two hours.
Further, after having impregnated this coating with a
dinitro-diamine palladium solution and dried at for 150.degree. C.
for two hours and further burned at 500.degree. C. for two hours,
the coating was impregnated with a barium nitrate solution. The
over catalyst layer 3 was formed by washcoating a slurry of 48
grams of a powdered catalyst composition, 63 grams of ceria (a
particle size of less than 4 .mu.m, a purity of higher than 95%)
and 35 grams of alumina as a binder mixed with an appropriate
amount of distilled water over the over catalyst layer 4 of the
cordierite monolith honeycomb carrier 2, and drying the coating at
150.degree. C. for two hours and burning it at 500.degree. C. for
two hours. The catalyst composition was prepared by spraydrying a
mixture of 42 grams of an acid solution of dinitro-diamine platinum
(II) nitrate, 1.0 gram of a rhodium nitrate solution and 144 grams
of H-type zeolite ZSM5 (SiO.sub.2/Al.sub.2O.sub.3=80) as a catalyst
powder, drying the catalyst powder at 200.degree. C. for two hours
and burning it at 500.degree. C. for two hours.
[0047] In the monolith honeycomb type catalyst construction thus
prepared, the under catalyst layer 4 was comprised of a coating of
14% by weight of the cordierite monolith honeycomb carrier 2 and
contained the alumina and ceria of a weight proportion of 1:1. The
palladium contained in the layer was 4 grams relative to one liter
of the volume of the monolith honeycomb carrier 2, and the barium
contained in the layer was 47% by weight of the coating. The over
catalysts layer 3 was comprised of a coating of 28% by weight of
the cordierite monolith honeycomb carrier 2 and contained the
zeolite ZSM5 and ceria of a weight proportion of 100:44. The
platinum and rhodium contained in the layer were 1.8 grams and
0.024 grams, respectively, relative to one litter of the volume of
the monolith honeycomb carrier 2, respectively. Impurities
contained in the catalyst layers 3 and 4 were less than 1%.
EXAMPLE II
[0048] In this Example the over catalyst layer 3 described in
Example I was modified in that barium was impregnated not in the
under catalyst layer but in the over catalyst layer. The over
catalyst layer was formed by washcoating a slurry of a compound
comprising a powdered zeolite ZSN5 support and platinum and rhodium
catalyst particles and a ceria and binder mixture mixed with an
appropriate amount of distilled water. The barium contained in the
over catalyst layer was 23% by weight of the coating.
EXAMPLE III
[0049] In this example, barium was deposited in the whole catalyst
layer by impregnating both over and under catalyst layers 3 and 4
with a barium nitrate solution and drying and burning the coatings
on the same conditions as described in Example I. The barium
contained in the whole catalyst layers was 15% by weight of the
coatings.
[0050] In order to make comparative evaluation of the emission
control efficiency of the catalysts described in Examples I through
III, an evaluation sample catalyst E-I was prepared.
[0051] The evaluation sample catalyst E-I was prepared by forming
only an under catalyst layer of 42% by weight of the monolith
honeycomb carrier 2. The single catalyst layer was impregnated with
ballium in the same manner and on the same conditions as described
in Example I.
EVALUATION
[0052] In order to assess the resistance of Examples I-III and the
comparative sample catalyst E-I against sulfur (S) poisoning,
evaluation tests were conducted by the utilization of a fixed bed
flow-through type reaction system. Evaluation was made from
measurements of NOx removal efficiency of the catalyst which was
set in the fixed bed flow-through type reaction system and exposed
to a pre-heated gas at approximately 300.degree. C. simulated as a
burned gas of an air-fuel mixture of A/F=22 for a predetermined
period of time. Measurements were made for the respective catalysts
before and after sulfur-treatment in which the catalyst is exposed
to a sulfur gas for sulfur poisoning.
[0053] The simulated burned gas had compositions as follows:
[0054] HC 4,000 ppm
[0055] NO 250 ppm
[0056] CO 0.15%
[0057] CO.sub.2 7.7%
[0058] H.sub.2 150 ppm
[0059] O.sub.2 7%
[0060] N.sub.2 the rest
[0061] The result of measurements is shown in FIG. 2.
[0062] As apparent from FIG. 2, the result proves that there is no
significant difference in nitrogen oxide removal efficiency among
the catalysts before the sulfur-treatment and, however, pronounced
differences in nitrogen oxide removal efficiency among the
catalysts after the sulfur-treatment. Specifically, the catalyst of
Example I has the best nitrogen oxide removal efficiency do not
show in nitrogen oxide removal efficiency. This is because the
ballium is concentrated in the under catalyst layer and effectively
protected by the zeolite ZSM5 in the over catalyst layer from
sulfur poisoning, consequently. Of all of them, the catalyst of
Example II is the worst. It is considered that the barium which is
contained in the over catalyst layer only significantly suffers
sulfur poisoning due to direct exposure to the burned gas. Although
the evaluation sample catalyst E-I contains no zeolite ZSM5, it
demonstrates a favorable result. This is considered to result from
the distribution of ballium over the inside of the single catalyst
layer which prevents the direct exposure of the ballium to the
burned gas which causes less sulfur poisoning of the ballium. The
catalyst of Example III, which contains ballium in both under and
over catalyst layers, demonstrates a nitrogen oxide removal
efficiency better than the evaluation sample catalyst E-I. This is
considered that the zeolite ZSM5 in the over catalyst layer
protects the ballium in the under catalyst layer and suppresses
sulfur poisoning of the ballium.
[0063] In order to evaluate the steps of manufacturing the catalyst
of the invention, each catalyst was prepared by coating a catalyst
material containing ballium of 30 grams relative to one liter of
the volume of the cordierite monolith honeycomb carrier 2.
Measurements were made for the actual amount of ballium in the
catalyst and the ratio of ballium desorption. In ballium desorption
tests, the weight of the catalyst was measured after exposing the
fresh catalyst to ultrasonic waves for three hours and drying it at
temperatures between 150.degree. C. and 200.degree. C. The amount
of ballium was investigated by the utilization of inductively
coupled plasma (ICP) method. The result is shown in FIG. 3.
[0064] As clearly understood from FIG. 3, the catalysts of Example
II and III and evaluation sample catalyst E-I have relatively high
actual measurements of ballium. The reason for the lowest actual
measurement of the catalyst of Example I which contains ballium in
the under catalyst layer only is considered that the ballium in the
under catalyst layer is contacted by the weak acid slurry during
coating the over catalyst layer with the effect of being released
as a hydroxide. Accordingly, in view of preventing the ballium from
being released and changed in amount, it is preferred to contain
the ballium in the over catalyst layer.
[0065] With regard to the desorption resistance, of all of the
tested catalysts, the catalyst of Example II, which contains the
ballium in the over catalyst layer only, has the highest desorption
ratio. This is because the over catalyst layer is hardened with the
presence of ballium and yields a large difference in thermal
expansion coefficient relative to the under catalyst layer,
resulting easy separation from the under catalyst layer.
[0066] The effect of amounts of ballium in the catalyst of Example
1 on NOx removal efficiency was investigated by the utilization of
the same fixed bed flow-through type reaction system as described
previously. A burned gas (of an air-fuel mixture of A/F=22)
simulated as follows was used:
[0067] HC 4,000 ppm
[0068] NO 160 ppm
[0069] CO 0.16%
[0070] CO.sub.2 9.74%
[0071] H.sub.2 650 ppm
[0072] O.sub.27%
[0073] N.sub.2 the rest
[0074] The result of measurements is shown in FIG. 4. The amount of
ballium is shown by a weight percent relative to the total weight
of the catalyst layers.
[0075] As clearly understood from FIG. 4, it is proved that the
catalyst demonstrates the highest NOx removal efficiency for the
amount of ballium in a range from 7 to 45%, in particular from 10
to 30%. The reason for the low NOx removal efficiency of the
catalysts which contain only small amounts of ballium is that the
ballium adsorbs only a small amount of nitrogen oxides. On the
other hand, the reason for a reduction in NOx removal efficiency of
the catalysts which contain excessive amounts of ballium is that
the ballium rather prevents platinum and rhodium from reducing
nitrogen oxides.
[0076] The effect of total amounts of the under and over catalyst
layers on NOx emission control efficiency was investigated.
Together, the effect of the presence of platinum or ballium in the
under layer on NOx emission control efficiency was also
investigated. Three types of test catalysts having a total amount
of 40% by weight of the cordierite monolith honeycomb carrier 2
were prepared.
[0077] Test catalyst I: Both under and over catalyst layers contain
no ballium.
[0078] Test catalyst II: Only the under catalyst layer, which
contains only palladium as a catalytic metal, contains ballium.
[0079] Test catalyst III: Only the under catalyst layer, which
contains both palladium and platinum as catalytic metals, contains
ballium.
[0080] All of these test catalysts I-III were made in the same
manner as previously described for the catalyst of Example I,
excepting the amounts of the under and over catalyst layers.
Specifically, the test catalyst I is formed in the same manner as
that for the catalyst of Example I excepting not impregnating the
catalyst layers with ballium. The test catalyst III is formed in
the same manner as that for the catalyst of Example I excepting
impregnating the under catalyst layer with platinum by the use of a
dinitro-diamine platinum solution.
[0081] Each of the test catalysts II and III contained the barium
of 15% by weight of the total amount of the under and over catalyst
layers and the palladium of 4 grams relative to one liter of the
volume of the cordierite monolith honeycomb carrier 2. The test
catalyst I contained the palladium of 7 grams relative to one liter
of the volume of the cordierite monolith honeycomb carrier 2. The
test catalyst III contained the palladium of 2 grams relative to
one litter of the volume of the cordierite monolith honeycomb
carrier 2 in the under catalyst layer. Each test catalyst I, II,
III contained the platinum and rhodium of 1.1 grams relative to one
litter of the volume of the cordierite monolith honeycomb carrier 2
in the over catalyst layer, the weight proportion between the
platinum and rhodium being 75:1.
[0082] Measurements of emission control efficiency were conducted
for the test catalysts after heat-aging treatment at 900.degree. C.
for 50 hours by the utilization of the same fixed bed flow-through
type reaction system as described previously. A burned gas (of an
air-fuel mixture of A/F=22) used was simulated as follows:
[0083] HC 4,000 ppm
[0084] NO 250 ppm
[0085] CO 0.15%
[0086] CO.sub.2 7.7%
[0087] H.sub.2 150 ppm
[0088] O.sub.2 7%
[0089] N.sub.2 the rest
[0090] The result of measurements is shown in FIG. 5. The NOx
emission control efficiency was measured with regard to maximum
values for the pre-heated simulated gas. Further, the HC emission
control efficiency was measured with the use of the pre-heated
simulated gas at 300.degree. C.
[0091] The result demonstrates that the test catalysts II and III,
which contain barium, show NOx emission control efficiency
considerably higher over the range of amounts of catalyst layers
than the test catalyst I without barium contained. This results
from the contribution of the barium to the reduction of nitrogen
oxides. With regard to the weight proportion between the under and
over catalyst layers, as compared with the test catalyst I, each of
the test catalysts II and III demonstrates high NOx emission
control efficiency even when it comprises only a very small amount
of coating for the over catalyst layer and has a tendency to have
high NOx emission control efficiency when the weight proportion of
the over catalyst layer relative to the under catalyst layer is
small rather than large. That is, an increase in the amount of
coating for the over catalyst layer makes the under catalyst layer
difficult to exhibit the effect of a barium additive. Considering
the effect of the presence of platinum in the under catalyst layer
on NOx emission control efficiency, the test catalyst III
containing platinum shows NOx emission control efficiency higher
over the range than the test catalyst 11 containing no
platinum.
[0092] While the result demonstrates that the presence of barium
does not produce a significant effect on HC emission control
efficiency, nevertheless, there is a tendency to decline the HC
emission control efficiency with a decline in weight proportion of
the under catalyst layer. This is because the heat-aging treatment
causes sintering of the noble metals and deterioration of the
barium in the under catalyst layer. A coating over 5% by weight for
the over catalyst layer diminishes this tendency. The effect of the
presence of platinum in the under catalyst layer on the HC emission
control efficiency is the same as on the NOx emission control
efficiency, and the test catalyst III containing platinum exhibits
a favorable HC emission control efficiency more than the test
catalysts I and II.
[0093] From the above discussion, it is proved that, if the
catalyst contains barium in the under catalyst layer, the weight
proportion of the under catalyst layer relative to the total
catalyst layer is preferred to be in a range between {fraction
(3/40)} and {fraction (34/40)}, more desirably between {fraction
(5/40)} and {fraction (16/40)}.
EXAMPLE IV
[0094] In order to evaluate the NOx emission control efficiency of
a fresh catalyst of the type containing barium in the under
catalyst layer, a catalyst of Example IV and an evaluation sample
catalyst E-II were prepared. This catalyst of Example IV was
comprised of an under catalyst layer containing the platinum and
palladium whose amounts were 2 grams and 7 grams, respectively,
relative to one liter of the volume of the cordierite monolith
honeycomb carrier 2 and barium whose amount was 15% by weight of
the total amount of the coatings. For the evaluation sample
catalyst E-II, the evaluation sample catalyst E-I was modified in
that the single catalyst layer is added with platinum. The single
catalyst layer contained the platinum and palladium whose amounts
were 2 grams and 7 grams, respectively, relative to one liter of
the volume of the cordierite monolith honeycomb carrier 2 and the
barium whose amount was 15% by weight of the amount of the
coating.
[0095] The measurement of NOx emission control efficiency
demonstrated 52% for the fresh catalyst of Example IV and 35% for
the fresh evaluation sample catalyst E-I. This result proves that
the under catalyst layer is advantageous to contain platinum as
well as palladium and barium.
[0096] In order to investigate the effect of lanthanum (La) on NOx
emission control efficiency, catalysts of Examples V-VII were
prepared.
EXAMPLE V
[0097] The catalyst of Example V was comprises of an under catalyst
layer 4 formed by wash-coating a slurry of a composition of
.gamma.-alumina powder and an alumina binder mixed with an
appropriate amount of water on a cordierite monolith honeycomb
carrier 2, and burning the coating at 500.degree. C. for two hours.
After having impregnated the coating with a specified concentration
of palladium nitrate solution and dried and burned at 500.degree.
C. for two hours, the coating was further impregnated with a
lanthanum salt solution and dried and burned at 500.degree. C. for
two hours. As the lanthanum salt, a lanthanum nitrate was employed.
The impregnation of palladium and lanthanum may be carried out
simultaneously.
[0098] The over catalyst layer 3 was formed by washcoating a slurry
of a powdered catalyst composition and alumina binder mixed with an
appropriate amount of water over the under catalyst layer and
drying and burning the coating at 500.degree. C. for two hours. The
catalyst composition was prepared by spraydrying a mixture of a
platinum nitrate-phosphorous solution (a solution of
dinitro-diamine platinum (II) nitrate) and a rhodium nitrate
solution mixed with H- and MFI-type powdered zeolite and providing
a platinum and rhodium contained zeolite catalyst powder.
[0099] In the monolith honeycomb type catalyst construction thus
prepared, the under catalyst layer 4 was comprised of a coating of
15% by weight of the cordierite monolith honeycomb carrier 2 and
contained the alumina, excepting alumina binder, of 13.5% by weight
of the cordierite monolith honeycomb carrier 2. The over catalyst
layer 3 was comprised of a coating of 30% by weight of the
cordierite monolith honeycomb carrier 2 and contained the zeolite
of 24% by weight of the cordierite monolith honeycomb carrier 2.
The amount of the palladium in the under catalyst layer 4 was 6.8
grams per one liter of the volume of the cordierite monolith
honeycomb carrier 2. The amount of the lanthanum was 8% by weight
of the aluminum in the base catalyst layer 4, i.e. 8 parts of the
lanthanum relative to 100 parts of the aluminum. The platinum and
rhodium in the over catalyst layer was 3 grams by total weight per
one liter and had a proportion of 75:1.
EXAMPLE VI
[0100] In the Example the catalyst of Example V was modified in
that the over catalyst layer contained lanthanum and barium as well
as palladium and the amounts of these lanthanum and barium were 4%
by weight of the aluminum in the base catalyst layer 4. The
proportion of the over and under catalyst layers 3 and 4 was the
same as that of Example V. A barium nitrate solution was used to
impregnate the under catalyst layer with the barium.
EXAMPLE VII
[0101] In this Example the over catalyst layer 3 of Example I was
modified in that the amount of ceria was 30% by weight of the
coating. The under catalyst layer 4 contained palladium, lanthanum
and barium supported by an alumina and ceria composition. The
amount of ceria was 30% by weight of the coating. The amount of the
palladium was 6.9 grams per one liter, and the amounts of the
lanthanum and barium were 4% by weight of the aluminum in the base
catalyst layer 4. The proportion of the over and under catalyst
layers 3 and 4 was the same as that of Example V. The under
catalyst layer 4 was formed by impregnating a coating of a powered
composition of alumina and ceria with a palladium salt, a lanthanum
salt and a barium salt in this order and burning the coating.
[0102] In order to assess the temperature dependency of NOx
emission control efficiency of Examples V-VII, evaluation tests
were conducted by the utilization of the fixed bed flow-through
type reaction system. Evaluation was made from measurements of NOx
emission control efficiency of the respective catalyst before and
after heat-aging treatment in the air 900.degree. C. for 50 hours.
The result is shown in FIG. 6.
[0103] It is clearly understood from FIG. 6 that each of the
catalysts of Example V-VII, which contain either one or both of the
lanthanum and barium, demonstrates a decline in NOx emission
control efficiency at both low and high temperatures. This fact
proves that the presence of at least one of the lanthanum and
barium produces an improvement in heat-resistance and that the
lanthanum works just like the barium.
[0104] It is to be understood that although the present invention
has been described with regard to preferred embodiments thereof,
various other embodiments and variants may occur to those skilled
in the art, which are within the scope and spirit of the invention,
and such other embodiments and variants are intended to be covered
by the following claims.
* * * * *