Catalyst for purification of exhaust gas

Details Catalyst for purification of exhaust gas Catalyst for purification of exhaust gas

1999-01-21

2002-03-12

Griffin, Steven P. (Department: 1754)

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

Zeolite or clay, including gallium analogs

Gallium containing

C502S073000, C502S074000, C502S077000, C502S064000, C502S304000

Reexamination Certificate

active

06355590

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a catalyst for purifying exhaust gas.
BACKGROUND OF THE INVENTION
A catalyst for purifying exhaust gas comprising a combination of a zeolite having platinum deposited thereon and CeO
2
is proposed in JP-A-8-131838 as a catalyst improved in the ability to purify NO
x
(nitrogen oxides) (the term “JP-A” as used herein means an “unexamined published Japanese patent application”). The platinum in the above catalyst functions as a catalyst metal and has the ability to oxidize and reduce the exhaust gas. The oxidizing ability of the platinum contributes to oxidation reactions such as HC (hydrocarbon)+O
2
→H
2
O+CO
2
and CO+O
2
→CO
2
. At a theoretical air/fuel ratio, the platinum adsorbs NO and the reducing ability thereof contributes to reduction reactions such as NO→N
2
. In oxygen-excess atmospheres, the platinum contributes to oxidation reactions such as NO+O
2
→NO
2
and reduction reactions such as NO
2
+HC+O
2
→N
2
+CO
2
+H
2
O.
The zeolite functions to adsorb HC in the exhaust gas and feed the concentrated HC to the platinum. Thus, the percentage of NO
x
purification in oxygen-excess atmospheres can be heightened. CeO
2
has the ability to adsorb NO
x
in oxygen-excess atmospheres to thereby heighten the concentration of NO
x
around the platinum. This also can bring about an improvement in the percentage of No
x
purification in oxygen-excess atmospheres. CeO
2
further has the effect of inhibiting the thermal deterioration of the catalyst.
The present inventors made various investigations on the catalyst described above. As a result, they have concluded that the above-described catalyst, upon exposure to a high-temperature environment containing oxygen and steam, i.e., a hydrothermal environment, deteriorates in the ability to purify NO
x
in oxygen-excess atmospheres, and this is attributable to the poor durability of the zeolite in the hydrothermal environment.
The catalyst described above shows the relatively satisfactory ability to purify NO
x
when the temperature of the exhaust gas, i.e., gas temperature, is low, specifically, in the range of from 150 to 300° C. However, in a high-gas-temperature range above 300° C., the catalyst shows the property of releasing NO
x
.
The reasons for the above phenomenon are thought to be as follows. The purification of NO
x
in the low-temperature range is based on the mechanism of adsorption of NO
x
and reduction of the NO
x
. Consequently, when the amount of adsorbed NO
x
is larger than the amount of reduced NO
x
, the NO
x
remaining unreduced is occluded by the catalyst. This occluded NO
x
is released in that high-temperature range.
As a result of the release of occluded NO
x
in the high-temperature range, the exhaust gas comes to contain the released NO
x
besides the NO
x
originally generated by engine running. In addition, the amount of the NO
x
thus released is not constant. Because of these, there has been a problem that the control of NO
x
purification is difficult.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a catalyst for purifying exhaust gas which can retain the high ability to purify NO
x
in oxygen-excess atmospheres even after exposure to a hydrothermal environment such as that described above and does not release NO
x
in the high-temperature range.
In order to accomplish the above object, the present invention provides a catalyst for purifying exhaust gas which comprises a combination of a carrier having platinum deposited thereon and CeO
2
having an average crystallite diameter (D) satisfying D≧13.0 nm, the carrier being made of a gallosilicate having an SiO
2
/Ga
2
O
3
molar ratio (M) satisfying 100≦M≦1,230.


REFERENCES:
patent: 5306684 (1994-04-01), Itoh et al.
patent: 5804526 (1998-09-01), Satoh et al.
patent: 5916839 (1999-06-01), Pak et al.
patent: 5945369 (1999-08-01), Kimura et al.
patent: A 8-131838 (1996-05-01), None

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