Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Waste gas purifier
Reexamination Certificate
2000-05-23
2004-10-26
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Waste gas purifier
C422S180000, C502S304000
Reexamination Certificate
active
06808687
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a catalytic converter for cleaning the exhaust gas of an internal combustion engine of a car. More specifically, the present invention relates to a catalytic converter for effectively removing nitrogen oxide (NO
x
), carbon monoxide (CO) and hydrocarbons (HC) contained in such exhaust gas.
BACKGROUND ART
Conventionally, a so-called three-way catalytic converter has been used for removing the harmful substances (NO
x
, CO, HC and the like) from exhaust gas. The three-way catalytic converter utilizes, as an active substance, a precious metal or metals such as platinum, palladium and/or rhodium. This three-way catalytic converter works as a reduction catalyst for reducing NO
x
to N
2
while also working as an oxidation catalyst for oxidizing CO and HC to CO
2
and H
2
O. In this way, the three-way catalytic converter is capable of cleaning exhaust gas by removal of the harmful substances such as NO
x
, CO and HC.
In recent years, various researches have been made to improve the performance of a three-way catalytic converter. An example of the three-way catalytic converters which have resulted from such researches utilizes cerium oxide (CeO
2
) provided coexistently with a three-way catalyst. CeO
2
has an oxygen-storing capacity (OSC). The OSC is the ability to occlude excess oxygen in an oxygen-excessive atmosphere and to release the occluded oxygen in an oxygen-deficient atmosphere. In an oxygen-excessive atmosphere, for example, reducing reaction occurs reluctantly. In the presence of CeO
2
, it occludes the excess oxygen to reduce the concentration of oxygen. Thus, when exhaust gas having a high oxygen concentration is supplied to the exhaust gas cleaning catalytic converter, CeO
2
occludes oxygen to lower the oxygen concentration of the exhaust gas, thereby enabling the three-way catalyst to effectively perform reducing reaction. Conversely, when exhaust gas having a low oxygen concentration is supplied to the exhaust gas cleaning catalytic converter, CeO
2
releases oxygen to increase the oxygen concentration of the exhaust gas, thereby enabling the three-way catalyst to effectively perform oxidation.
In cars, there is an increasing demand for dealing with higher restrictions on cold emissions, so that the exhaust gas cleaning catalytic converter is required to effectively clean exhaust gas even at relatively low temperature when the engine is not sufficiently warmed up immediately after its start for example. Thus, the catalytic converter needs to have high catalytic activity at low temperature. For this purpose, the engine tends to be located at the exhaust manifold which is near the engine rather than below the body floor. Due to the location at the manifold, the catalytic converter may be often exposed to high temperature of no less than 900° C. (or sometimes no less than 1,000° C.). Thus, the catalytic converter needs to provide high durability at such a high temperature.
It is, therefore, an object of the present invention to provide a catalytic converter for cleaning exhaust gas which is capable of providing good catalytic activity at low temperature while also providing high resistance against high temperature.
DISCLOSURE OF THE INVENTION
According to the present invention, a catalytic converter for cleaning exhaust gas comprises a first coating layer formed on a heat-resistant support, and a second coating layer formed on the first coating layer. The first coating layer contains alumina which supports palladium. The second coating layer contains Ce—Zr complex oxide which coexistently carries platinum and rhodium, and Zr—Ce complex oxide which differs in composition from the Ce—Zr complex oxide and which coexistently carries platinum and rhodium.
In a preferred embodiment of the present invention, the Ce—Zr complex oxide may be represented by the following general formula.
Ce
1−(x+y)
Zr
x
M
y
O
2−z
(1)
In the formula (1), M represents a rare earth element other than Ce and Zr or an alkaline earth metal. z represents the degree of oxygen deficiency determined by the valence and proportion of the contained element M. Further, 0.25≦1−(x+y)≦1.0, 0≦x≦0.55, and 0≦y≦0.2.
The Zr—Ce complex oxide may be represented by the following general formula.
Zr
i−(a-b)
Ce
a
N
b
O
2-c
(2)
In the formula (2), N represents a rare earth element other than Ce and Zr or an alkaline earth metal. c represents the degree of oxygen deficiency determined by the valence and proportion of the contained element N. Further, 0.55≦1−(a+b)≦1.0, 0≦a≦0.45, and 0≦b≦0.2.
In a preferred embodiment of the present invention, the second coating layer has a surface layer portion which coexistently carries platinum and rhodium.
In a preferred embodiment of the present invention, the second coating layer has a surface layer portion which carries one of platinum and rhodium alone.
In a preferred embodiment of the present invention, the first coating layer additionally contains barium salt of an inorganic acid.
In a preferred embodiment of the present invention, the first coating layer supports 30~100 g of alumina and 0.5~8.0 g of palladium per liter of the heat-resistant support.
In a preferred embodiment of the present invention, the Ce—Zr complex oxide carries a total amount of 0.3~3.0 g of platinum and rhodium per liter of the heat-resistant support, whereas the Zr—Ce complex oxide carries a total amount of 1.0~3.0 g of platinum and rhodium per liter of the heat-resistant support.
In a preferred embodiment of the present invention, the surface layer portion of the second coating layer carries a total amount of 0.05~2.0 g of platinum and rhodium per liter of the heat-resistant support.
In a preferred embodiment of the present invention, the first coating layer additionally contains Ce—Zr complex oxide which does not carry any precious metal.
In a preferred embodiment of the present invention, the second coating layer additionally contains alumina which does not support any precious metal.
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Tan Isao
Tanaka Hirohisa
Uenishi Mari
Daihatsu Motor Co. Ltd.
Tran Hien
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