Catalytic converter for cleaning exhaust gas

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

1999-05-19

2001-07-17

Bell, Mark L. (Department: 1755)

Catalyst, solid sorbent, or support therefor: product or process

Catalyst or precursor therefor

Sulfur or compound containing same

C502S222000, C502S223000, C502S303000, C502S304000, C502S325000, C502S328000, C502S333000, C502S339000, C502S340000, C502S341000, C502S349000, C502S350000, C423S213500, C423S215500, C029S890000

Reexamination Certificate

active

06261989

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, respectively.
Meanwhile, 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. Due to such a location, however, the catalytic converter may be often exposed to high temperature of no less than 900° C. (or sometimes even higher than 1,000° C. Thus, the catalytic converter needs to provide a high catalytic activity even at such a high temperature. Further, the catalytic converter is also required to provide a high catalytic activity at relatively low temperature before the engine is sufficiently warmed up upon start thereof.
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 organic oxide (e.g. alumina) and particles of an oxygen-storing oxide of a rare earth element. While this prior art catalytic converter aims to provide a high catalytic activity at a high temperature, it does not pay any attention to catalytic activity at relatively low temperature.
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 maintaining a high catalytic activity even at high temperature while also providing an effective catalytic activity at relatively low temperature of e.g. 200~400° C. before the engine is sufficiently warmed up.
Another object of the present invention is to provide a catalytic converter for cleaning exhaust gas which is capable of preventing or reducing catalytic poisoning.
According to one aspect of the present invention, a catalytic converter for cleaning exhaust gas comprises a heat-resistant support, and a catalytic coating formed on the heat-resistant support, wherein the catalytic coating comprises particles of a cerium complex oxide, Pd carried on the cerium complex oxide particles, particles of zirconium complex oxide, a combination of Pt and Rh coexistently carried on the zirconium complex oxide particles, and particles of a heat-resistant inorganic oxide.
As described above, since the catalytic coating contains palladium (Pd) which provides a good catalytic activity at relatively low temperature, the catalytic converter is capable of effectively cleaning exhaust gas, particularly by removal of hydrocarbons (HC), even before the engine is sufficiently warmed up. In this regard, Pd should be selectively carried on the cerium complex oxide particles because the oxygen-storing ability of the cerium complex oxide converts Pd to PdO which provides a higher catalytic activity than Pd. Further, the cerium complex oxide restrains grain growth of Pd which may lead to a surface area decrease (i.e., a decrease of the catalytic activity). Thus, the Pd-carrying cerium complex oxide particles raise the CO—NO
x
removal cross point where the CO removal ratio and the NO
x
removal ratio coincide.
On the other hand, Pt and Rh are added for primarily enhancing the catalytic activity at high temperature. These precious metals should be selectively and 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). Further, Pt may alloy with Pd at high temperature to result in loss or decrease of their respective catalytic activity, so that they should be supported separately.
The catalytic coating may be a single layer. In this case, the single layer of catalytic coating may further contain at least one sulfate which prevents Pd from being poisoned with hydrocarbons contained in the exhaust gas. Since Pd is liable to poisoning with hydrocarbons, the addition of the poisoning-preventive sulfate is advantageous in maintaining the catalytic activity of Pd for a long time. Further, since the sulfate is thermally stable in comparison with carbonates and acetates used as a poisoning-preventive agent, it will not decompose at high temperature of 1,000° C. to form a complex oxide with the other catalytic components, thereby preventing the catalytic converter from deteriorating in its catalytic performance.
Preferably, the sulfate may be selected from the group consisting of barium sulfate, calcium sulfate, strontium sulfate, cesium sulfate, potassium sulfate, magnesium sulfate, yttrium sulfate, and lanthanum sulfate. Of these candidates, barium sulfate thermally decomposes at a high temperature of about 1,200° C., so that it will not decomposes at a temperature of about 1,000° C. to which the catalytic converter may be subjected when mounted at the intake manifold close to the engine. Thus, the catalytic converter containing the sulfate provides an excellent catalytic ability for a long time even under severe operating conditions while effectively preventing Pd from being poisoned with hydrocarbons.
According to a preferred embodiment, the catalytic coating includes a first coating layer which is formed on the heat-resistant support and contains the Pd-carrying cerium complex oxide particles, and a second coating layer which is formed on the first coating layer and contains the Pt- and Rh-carrying zirconium complex oxide particles.
With the multi-layer structure of the catalytic coating described above, since the Pd-carrying cerium complex oxide particles are contained in the first or inner coating layer, Pd is located farther from the poisoning components (hydrocarbons) and is therefore less likely to be poisoned therewith. In this case, the first coating layer may additionally contain a poisoning-preventive sulfate or sulfates, as already described above. Further, the second coating layer may also con

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