Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
2001-02-21
2003-09-16
Langel, Wayne A. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S213200, C423S239100, C423S239200, C502S304000
Reexamination Certificate
active
06620392
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a catalyst for purifying an exhaust gas and a method for purifying an exhaust gas with the catalyst.
Three way catalysts, which can purify HC (hydrocarbon), CO and NO
x
simultaneously in an exhaust gas in the vicinity of the theoretical air-fuel ratio very effectively, are known as a catalyst for purifying an exhaust gas from an engine. Also known is a so-called lean NO
x
purifying catalyst, which is as follows. At a lean air-fuel ratio, NO
x
contained in the exhaust gas is stored in a NO
x
storage material such as Ba, and at the theoretical air-fuel or a rich air-fuel ratio, the stored NO
x
is migrated onto a precious metal and is reacted with a reducing gas such as HC, CO and H
2
contained in the exhaust gas to reduce NO
x
to N
2
for purification, and to oxidize and purify the reducing gases at the same time.
Generally, these catalysts contain an oxygen storage material that stores and releases oxygen by changing the oxidation number, and CeO
2
or a CeO
2
•ZrO
2
mixed oxide is commonly used as the oxygen storage material. In the three way catalysts, these oxides serve to correct a deviation from the theoretical air-fuel ratio by storing or releasing oxygen. In the lean NO
x
purifying catalyst, these oxides serve as an oxygen supply source for oxidizing a large amount of NO contained in an exhaust gas to NO
2
, which can easily be stored in the NO
x
storage material.
Japanese Patent Laid-Open Publication NO.6-315634 discloses a catalytic structure for nitrogen-oxygen catalytic reduction comprising a carrier, an inner layer on the carrier and a surface layer on the inner layer, the inner layer comprising a catalytic component expressed by a general formula A
x
B
1-x
CO
3
, (where A is at least one element selected from the group consisting of La, Y, Ce and the like, B is at least one element selected from the group consisting of Na, K, Sr and the like, and C is at least one element selected from the group consisting of Mn, Co, Zr and the like, and 0≦×≦1), the surface layer comprising a catalyst component where an active component comprising an oxide of an element of Group Ib, IIa or IIb of the periodic table is supported by a support such as aluminum oxide, titanium dioxide, and zirconium oxide. In this catalytic structure, hydrocarbon is adsorbed by the surface layer for activation, whereas a nitrogen oxide is adsorbed onto the inner layer for activation, so that the activated hydrocarbon and the activated nitrogen oxide are reacted at the interface therebetween. High activity and selectivity for reduction of the nitrogen oxide are expected from this structure.
However, when a catalyst is exposed to a high temperature atmosphere for a long time, an oxide constituting the oxygen storage material is deteriorated, so that oxygen is not stored or released properly. As a result, the performance of the catalyst for purifying an exhaust gas is deteriorated.
The object of the present invention is to provide a catalyst for purifying an exhaust gas whose performance for purifying an exhaust gas is not significantly deteriorated at exposure to a high temperature atmosphere for a long time and that has excellent heat resistance.
Another object of the present invention is to improve the sulfur poisoning resistance and the regeneration properties from sulfur poisoning.
Furthermore, another object of the present invention is to provide a method for purifying an exhaust gas using such a catalyst.
SUMMARY OF THE INVENTION
The present invention uses a Ce—Zr—Sr mixed oxide containing Ce, Zr and Sr as constituent elements (which may be referred to as a CeO
2
·ZrO
2
·SrO mixed oxide in the following description).
A catalyst for purifying an exhaust gas of the present invention includes a catalytic metal that serves for oxidation of HC and CO in the exhaust gas containing oxygen and reduction of NO
x
in the exhaust gas, and a mixed oxide comprising Ce, Zr and Sr.
According to this embodiment, the mixed oxide that acts as an oxygen storage material contains Sr in addition to Ce and Zr, so that the oxygen storage function of the catalyst is not significantly deteriorated even if the catalyst is exposed to a high temperature atmosphere for a long time. Moreover, a catalyst having an excellent heat resistance can be obtained. The reason for this is not clear, but it may be as follows.
An analysis indicates that when a Ce—Zr mixed oxide (CeO
2
·ZrO
2
mixed oxide) is heated, ZrO
2
is separated. However, in a Ce—Zr—Sr mixed oxide, such separation of ZrO
2
hardly occurs, and the Ce—Zr—Sr mixed oxide is highly crystalline. Therefore, even if it is exposed to a high temperature, it hardly is degraded so that the oxygen storage function is not deteriorated. It seems that Sr contributes to this high crystallinity.
An analysis indicates that since the primary particles of the Ce—Zr—Sr mixed oxide have a small particle size, it is difficult that sintering due to heat proceeds. It seems that Sr contributes to the fact that the particles are fine.
An analysis indicates that since the secondary particles of the Ce—Zr—Sr mixed oxide have a large particle size, mesopore is also large which makes it easy that the exhaust gas is diffused to the inside. This advantageously serves for storage and release of oxygen, and therefore high oxygen storage ability can be exerted at a relatively high temperature as well. Furthermore, it seems that the fact that Sr activates oxygen advantageously serves for storage and release of oxygen.
Therefore, the catalyst of the present invention can be provided at an exhaust pipe in which the temperature of the catalyst is constantly or temporarily at 700° C. or more or in a place where the temperature of the catalyst is at 800° C. or more, or at further higher 900° C. or more, such as a site immediate downstream of an exhaust manihold.
Furthermore, when the present invention is used to a ternary catalyst, even if the catalyst is exposed to a high temperature atmosphere for a long time, the mixed oxide effectively can function as the oxygen storage material that corrects a deviation of the air-fuel ratio from the theoretical air-fuel ratio by storage and release of oxygen. Thus, high HC purification performance can be obtained.
When the present invention is used as a lean NO
x
purifying catalyst, even if the catalyst is exposed to a high temperature atmosphere for a long time, the mixed oxide effectively can function as the supply source that supplies oxygen having high activity for oxidation of NO. Thus, at a lean air-fuel ratio, NO is oxidized to NO
2
, which is readily stored by the NO
x
storage material, so that high lean NO
x
purification performance can be obtained.
NO
x
storage materials have a problem of so-called sulfur poisoning that deprives a NO
x
storage material of its function as a NO
x
storage material, because of its formation of a salt by reacting with a sulfur oxide contained in exhaust gases. However, the Ce—Zr—Sr mixed oxide makes it possible that deterioration of the lean NO
x
purification performance due to sulfur poisoning can be suppressed to a small level, and provides a catalyst having excellent sulfur poisoning resistance. The reason for this is not clear, but it seems that the presence of Sr provides fine particles of the NO
x
storage materials and the surface area of the NO
x
storage materials becomes large, so that the NO
x
storage materials are unsusceptible to sulfur poisoning. In addition, the catalyst can be regenerated by raising the temperature of the sulfur-poisoned catalyst, and in the catalyst of this embodiment, the Ce—Zr—Sr mixed oxide has high heat resistance so that very high regeneration performance can be obtained.
The Ce—Zr—Sr mixed oxide is advantageous for sulfur poisoning resistance when the content of Zr becomes large, and the heat resistance thereof is improved when the content of Ce becomes large. However, the content of Sr is too excessive, the heat resistance is deteriorated.
In the Ce—Zr—Sr mixed oxide, the amount of released oxyge
Miyoshi Seiji
Okamoto Kenji
Takami Akihide
Yamada Hiroshi
Langel Wayne A.
Mazda Motor Corporation
Strickland Jonas N.
Studebaker Donald R.
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