Catalytic converter for cleaning exhaust gas

Details Catalytic converter for cleaning exhaust gas Catalytic converter for cleaning exhaust gas

1999-05-07

2002-10-15

Tran, Hien (Department: 1764)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

Waste gas purifier

C422S171000, C422S180000, C502S303000, C502S304000, C502S339000, C502S341000

Reexamination Certificate

active

06464946

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).
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. The three-way catalytic converter utilizes, as an active substance, a precious metal or metals such as Pt, Pd and/or Rh for reducing NO
x
to N
2
and for oxidizing CO and HC to CO
2
and H
2
O. In this way, the three-way catalytic converter works as a catalyst both for oxidation and reduction.
Various researches have been made to improve the performance of a three-way catalytic converter. One of the three-way catalytic converters which have resulted from such researches utilizes cerium oxide (CeO
2
) which has an oxygen-storing capacity (OSC); that is, the capacity to occlude gaseous oxygen in the crystalline structure and to release the occluded oxygen from the crystalline structure. More specifically, CeO
2
is added to a three-way catalytic converter 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 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 for assisting the catalytic converter in oxidizing CO and HC to CO
2
and H
2
O.
Japanese Patent Publication 5-47263 (which is the granted version of JP-A-63-156545) discloses a catalytic converter for cleaning exhaust gas wherein fine particles of zirconia (ZrO
2
) carrying a precious metal (e.g. Pt, Rh) are coated on a heat-resistant honeycomb support together with particles of heat-resistant inorganic oxide (e.g. alumina) and particles of an oxygen-storing oxide of a rare earth element. In such a converter, the heat-resistant inorganic oxide and the rare earth element oxide intervene between the agglomerates of the zirconia particles for preventing the zirconia particle agglomerate from growing due to agglomerate-to-agglomerate sintering in high-temperature oxidizing atmosphere, thereby limiting a decrease of specific surface area which may result in degradation of catalytic activity.
While the prior art catalytic converter described above prevents sintering between the zirconia particle agglomerates, it fails to prevent the zirconia particles themselves from growing due to particle-to-particle sintering. Further, depending on its mounting position, the catalytic converter may be subjected to an extremely high temperature which prompts the grain growth of zirconia.
More specifically, there is an increasing demand for shifting the mounting location of the catalytic converter from below the body floor to the exhaust manifold which is near the engine, whereby the catalyst can be quickly warmed up after starting the engine. However, when the catalytic converter is located near the engine, it may be of ten exposed to a high temperature of no less than 900° C. (or sometimes even higher than 1,000° C.), which may cause grain growth of ZrO
2
due to particle-to-particle sintering. As a result, the specific surface area of ZrO
2
reduces to result in a decrease of the catalytic activity of the precious metal carried on the zirconia particles.
DISCLOSURE OF THE INVENTION
It is, therefore, an object of the present invention to provide a catalytic converter for cleaning exhaust gas which does not result in an excessive decrease of the catalytic activity of a precious metal or metals even under severe operating conditions above 900° C.
According to the present invention , there is provided a1. A catalytic converter for cleaning exhaust gas comprising: a heat-resistant support; particles of a zirconium complex oxide of the following formula,
Zr
1−(x+y)
Ce
x
R
y
O
2−z
where “R” represents at least one element selected from a group consisting of Al and rare earth elements other than Ce, “z” represents the degree of oxygen deficiency determined by the valence and content of the contained Al and/or rare earth element, 0.1≦x+y≦0.5, 0.1≦x≦0.5, and 0≦y≦0.2; a combination of Pt and Rh coexistently carried on the zirconium complex oxide particles; and particles of an oxygen-storing complex oxide of a rare earth element; wherein the zirconium complex oxide particles are coated on the heat-resistant support together with the oxygen-storing complex oxide particles.
The present invention features that a part of zirconium in zirconia (ZrO
2
) is substituted with cerium (Ce), and optionally with aluminum (Al) and/or a rare earth element or elements other than cerium. Such substitution restrains the mass transfer of zirconium at high temperature, thereby preventing the zirconia particles from unduly growing. As a result, Pt and Rh coexistently carried on the zirconia particles can retain their catalytic activity above a predetermined level even at high temperature.
Preferably, at least a part of the zirconium complex oxide may be solid solution. This feature is additionally effective for restraining the mass transfer of Zr, thereby enhancing the durability of the catalytic converter at high temperature.
The precious metals Pt and Rh are coexistently carried on the zirconium complex oxide particles for the following reason. If Pt alone is carried on the zirconium complex oxide particles, the particles of Pt exhibit a tendency to grow due to the mass transfer of Pt at high temperature. By contrast, if Rh coexists, it restrains the mass transfer of Pt to prevent grain growth (presumably due to the formation of a rhodium oxide layer on the Pt particles which restrains the mass transfer of Pt).
In the above formula, the relation “0.1≦x+y≦0.5” needs to be met because if the ratio of substitution of zirconium with cerium and other elements is lower or higher than this range, it becomes difficult to effectively prevent the mass transfer of zirconium. If the substitution ratio is higher than this range, Pt and Rh coexistently carried on the zirconium complex oxide may adversely interact with each other to lower the catalytic activity. The same reasons also apply to “0.1≦x≦0.5” . Preferably, the value of the “x+y” should lie in the range of 0.2~0.3, whereas the value of the “x” should be set in the range of 0.1~0.28. It should be understood that the content of Zr in the zirconium complex oxide may include 1~3% of hafnium (Hf) which is inevitably contained in Zr ores.
Examples of rare earth elements “R” other than Ce include Y, Sc, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. Of these examples, La and Nd are preferred. The reason for partially substituting Zr of the zirconium complex oxide with the rare earth element other than Ce is that such substitution stabilizes the uniform fluorite structure of the zirconium complex oxide at room temperature while also restraining the mass transfer of Zr in cooperation with Ce.
Alternatively or additionally, a part of Zr in the zirconium complex oxide may be substituted with Al alone or in combination with Y.
The value of the “y” in the above formula is 0~0.2. In this way, the value of the “y” may be 0 so that the zirconium complex oxide does not need to contain Al nor a rare earth element other than Ce because Ce alone partially substituting for Zr of the zirconium complex oxide can restrain the grain growth of the complex oxide particles to some extent. This is why the ranges for the “x+y” and the “x” coincide. However, since the inclusion of Al and/or a rare earth element other than Ce better restrains the growth of

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