Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Waste gas purifier
Reexamination Certificate
1999-08-26
2003-06-10
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Waste gas purifier
C422S171000, C422S180000, C502S304000, C502S326000, C502S332000
Reexamination Certificate
active
06576200
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a catalytic converter for effectively cleaning the exhaust gas of an automotive internal combustion engine by removal of nitrogen oxide (NO
x
), carbon monoxide (CO) and hydrocarbons (HC). The present invention also relates to an oxygen-storing complex oxide which may be advantageously used for such a catalytic converter.
2. Description of the Related Art
As is well known, the exhaust gas of an automotive internal combustion engine inevitably contains harmful substances such as NO
x
, CO and HC. In recent years, particularly, the restrictions on exhaust gas cleaning are increasingly strict for environmental protection.
A so-called three-way catalytic converter has been most widely used for removing the above-described harmful substances. Typically, a three-way catalytic converter includes a honeycomb support made of a heat-resistant material such as cordierite, and a wash-coat formed on the surfaces of the respective cells of the honeycomb support. The wash-coat contains a heat-resistant inorganic oxide such as Al
2
O
3
, a catalytically active substance such as Pt, Pd and/or Rh, and an oxygen-storing oxide such as CeO
2
. The catalytically active substance reducs NO
x
to N
2
while oxidizing CO and HC to CO
2
and H
2
O, respectively.
The oxygen-storing oxide, typically CeO
2
, has an oxygen storing capacity (hereafter abbreviated as “OSC”); that is, the capacity to occlude gaseous oxygen and to release the occluded oxygen. More specifically, CeO
2
is added for adjusting the oxygen concentration of gaseous atmosphere, so that excess oxygen in the gaseous atmosphere is occluded into the crystalline structure of CeO
2
in an oxygen-rich state (i.e., fuel-lean state which may be simply referred to as “lean state”) for assisting the catalytic converter in reducing NO
x
to N
2
while releasing the occluded oxygen into the gaseous atmosphere in a CO- and/or HC-rich state (i.e., fuel-rich state which may be simply referred to as “rich state”) for assisting the catalytic converter in oxidizing CO and HC to CO
2
and H
2
O. Thus, the catalytic activity of the catalytically active substance is enhanced by the addition of CeO
2
.
However, it has been found that grains or particles of CeO
2
grows due to sintering at high temperature. Such growth of CeO
2
results in a decrease of surface area, consequently causing gradual loss of OSC. Particularly, if the catalytic converter is mounted near the engine, it may be frequently subjected to an extremely high temperature of no less than 900° C. (or sometimes even higher than 1,000° C.), which prompts the grain growth of CeO
2
.
Further, CeO
2
provides its intended OSC only under a condition where an oxidizing atmosphere (corresponding to a lean state) and a reducing atmosphere (corresponding to a rich state) are alternately repeated. More specifically, CeO
2
is capable of occluding oxygen only after it has previously undergone a reducing atmosphere for releasing the previously occluded portion of oxygen, whereas it is capable of releasing oxygen only after it has previously undergone an oxidizing state for occluding oxygen. Therefore, the air-fuel mixture supplied to the engine needs to be controlled in a narrow range (referred to as “window”) near the stoichiometric state such that a lean state and a rich state are alternately repeated.
In view of the above problem, an oxygen sensor may be provided for monitoring the oxygen concentration of the exhaust gas, and the output of the oxygen sensor is used for controlling the air-fuel mixture in the narrow window. However, the oxygen sensor may be deteriorated during operation, so that it is possible that the control center point may unexpectedly shift from the stoichiometric state to a lean side. In such a case, CeO
2
in the catalytic converter may be always put in an oxidizing atmosphere and thus continue to occlude oxygen. As a result, CeO
2
becomes fully loaded with oxygen and is incapable of releasing it as long as the air-fuel mixture is held at the stoichiometric state or a lean state.
DISCLOSURE OF THE INVENTION
It is, therefore, an object of the present invention to provide a catalytic converter for cleaning exhaust gas which is capable of retaining a high catalytic activity for a long time even under severe operating conditions above 900° C.
Another object of the present invention is to provide an oxygen-storing oxide which, when incorporated in a catalytic converter, is capable of effectively storing and releasing oxygen even if the control center point for the air-fuel mixture shifts from the stoichiometric state to a lean side.
According to one aspect of the present invention, a catalytic converter for cleaning exhaust gas comprises a heat-resistant support, and a coating formed on the support, the coating including at least one kind of catalytically active substance and at least one kind of oxygen-storing oxide, wherein the oxygen-storing oxide is selected from oxides of Pr and Tb.
The inventors have found that the an oxide of Pr or Tb exhibits a much higher OSC than CeO
2
both before and after performing high-temperature aging. Therefore, a catalytic converter utilizing an oxide of Pr or Tb in place of or in combination with CeO
2
is capable of providing a high catalytic activity over a long period even under a severe high-temperature operating condition.
In a first embodiment of the present invention, the oxygen-storing oxide is Pr
6
O
11
. Such a simple oxide of Pr may be used in combination with CeO
2
or Ce—Zr complex oxide.
In a second embodiment of the present invention, the oxygen-storing oxide is Tb
4
O
7
. Again, such a simple oxide of Tb may be used in combination with CeO
2
or Ce—Zr complex oxide.
In a third embodiment, the oxygen-storing oxide is a complex oxide of the following formula,
Ce
1−(x+y)
R
x
E
y
Oxide
where “R” represents Pr or Tb, “E” represents at least one element selected from a group consisting of Nd, Y, Gd and Zr, 0.1≦x≦0.8, 0≦y≦0.9, and 0.1≦x+y≦0.9. It should be appreciated that the notation “Oxide” is used because the proportion of oxygen in the complex oxide varies depending on the condition of the atmosphere and the valency of the co-existing elements other than Ce.
As previously described, CeO
2
provides an intended OSC only under a condition where an oxidizing atmosphere (corresponding to a lean state) and a reducing atmosphere are alternately repeated, consequently necessitating the air-fuel mixture to be controlled in a narrow window range across the stoichiometric state. On the other hand, the inventors have experimentally found that an oxide of Pr or Tb is capable of releasing oxygen not only in a reducing atmosphere but also in an inert atmosphere (corresponding to the stoichiometric state) after it has occluded oxygen in an oxidizing atmosphere.
According to the third embodiment, CeO
2
is complexed with an oxide of Pr or Tb. Therefore, the CeO
2
portion of the resulting complex oxide provides a good OSC under a condition where an oxidizing atmosphere and a reducing atmosphere are alternately repeated, whereas the Pr or Tb oxide portion of the complex oxide provides a good OSC under a condition where an oxidizing atmosphere and an inert atmosphere are alternately repeated. Thus, even if the control center point of an oxygen sensor shifts from the stoichiometric state to a lean side due to a deterioration, the oxygen-storing complex oxide can still provide a good OSC.
Further, any one of Nd, Y, Gd and Zr which may be added to the oxygen-storing complex oxide restrains grain growth of the complex oxide under high temperature. Thus, the catalytic converter incorporating the oxygen-storing complex oxide is capable of retaining a high catalytic activity for a long time even under a severe high-temperature operating condition. However, it is also possible to dispense with such a grain growth restraining element.
In the case where the “E” in the above formula is selected from a group consisting of Nd, Y and Gd, the co
Tanaka Hirohisa
Yamamoto Mari
Bednarek Michael D.
Daihatsu Motor Co. Ltd.
Shaw Pittman LLP
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