Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
2000-10-23
2003-11-04
Elve, M. Alexandra (Department: 1725)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S213700, C423S239100, C423S239200, C423S245100, C095S143000, C095S144000, C095S147000, C095S902000
Reexamination Certificate
active
06641788
ABSTRACT:
The present invention relates to an adsorbent for a hydrocarbon. Further, the present invention relates to a catalyst for removing nitrogen oxide in an exhaust gas discharged from an internal combustion engine of e.g. an automobile, and a method for purifying an exhaust gas.
For purification of an exhaust gas containing hydrocarbons discharged from an internal combustion engine of e.g. an automobile, a method has been practically used in which a three way catalyst is contacted with the exhaust gas. However, when the exhaust gas temperature is low at the start up of an engine, the hydrocarbons are likely to be discharged as they are i.e. without being purified by the catalyst.
Therefore, for purification of hydrocarbons from an exhaust gas at a low temperature, JP-A-2-135126 proposes an exhaust gas-purifying apparatus employing an adsorbent for hydrocarbons, having at least one metal supported on a part of a monolithic substrate having Y-type zeolite and mordenite-structured zeolite coated thereon, for the purpose of adsorbing and purifying hydrocarbons. Further, many other adsorbents for hydrocarbons have been proposed wherein zeolites are used as constituting components.
For example, JP-A-5-31359 proposes a zeolite having SiO
2
/Al
2
O
3
molar ratio of at least 40; JP-A-7-213910 proposes at least one zeolite selected from the group consisting of a mordenite, &bgr;-type zeolite and ZSM-5, having a SiO
2
/Al
2
O
3
molar ratio of from 50 to 2000, and USY having a SiO
2
/Al
2
O molar ratio of from 50 to 300; JP-A-8-10566 proposes a zeolite selected from ZSM-5, USY and &bgr;-type zeolite, having a SiO
2
/Al
2
O
3
molar ratio of at least 40; and JP-A-8-224449 proposes a zeolite having a weight ratio of silica to alumina of larger than 300, wherein a preferred zeolite is ZSM-5, zeolite Y, ZSM-11, &bgr;-type zeolite or silicalite.
Further, JP-A-8-229386 discloses a &bgr;-type zeolite prepared by adding seed crystals of &bgr;-type zeolite to a mixed gel, followed by heating at a mild temperature rising rate of from about 28 to 55° C./hr, and JP-A-9-38485 proposes, as a constituting component for an adsorbent for hydrocarbons, a &bgr;-type zeolite which has been subjected to aluminum-removing treatment so that the residual aluminum is less than 20%, whereby the degree of aluminum removal is set so that the specific surface area is maintained at a level of at least 500 m
2
/g
Each of such methods for adsorbing and removing hydrocarbons by means of these adsorbents, is one wherein a hydrocarbon contained in an exhaust gas is once adsorbed on an adsorbent at a low temperature during the start up of an engine and kept adsorbed to a temperature at which an exhaust gas-purifying catalyst will operate, and the hydrocarbon desorbed from the adsorbent in a temperature range higher than said temperature, is purified by the exhaust gas-purifying catalyst.
At present, nitrogen oxide, carbon monoxide and hydrocarbons, which are hazardous to human bodies, in an exhaust gas discharged from a gasoline engine, are removed by a three way catalyst having Pt, Pd or Rh included on a support. In recent years, it has been attempted to disseminate a gasoline engine of a lean-burn system or a direct injection type combustion system or a diesel engine in order to reduce the discharge of carbon dioxide in view of a resource problem or an environmental problem due to warming up of the earth. The exhaust gas from such an engine contains excessive oxygen, and it is accordingly difficult to remove nitrogen oxide by a conventional three way catalyst.
As a method for removing nitrogen oxide from an exhaust gas containing excessive oxygen, there has heretofore been known a method for selective-reduction on V
2
O
5
/TiO
2
wherein ammonia is used as a reducing agent, or a method for absorption into an alkaline solution. However, ammonia is a poisonous substance as designated by law and is highly dangerous, and it is difficult to apply such a method to an automobile which is a mobile source. On the other hand, it is difficult to apply the absorption method to a mobile source such as a common automobile, from the viewpoint of the operation efficiency in e.g. supplementation or treatment of the alkaline solution.
For purification of an exhaust gas in an excessive oxygen state, many exhaust gas-purifying catalysts have been proposed, including an exhaust gas-purifying catalyst consisting of a zeolite ion-exchanged with at least one metal selected from Pt, Pd, Rh, Ir and Ru (JP-A-1-135541). The catalyst proposed in JP-A-1-135541, is one which reduces and removes nitrogen oxide by means of a reducing gas such as hydrogen or hydrocarbons contained in the exhaust gas.
Further, for the purpose of removing nitrogen oxide in an exhaust gas and suppressing discharge of hydrocarbons, it has been proposed that a hydrocarbon in an exhaust gas is adsorbed on an adsorbent at a low temperature, and by utilizing the hydrocarbon which desorbs from the adsorbent as the exhaust gas temperature rises, the performance for the removal of nitrogen oxide is further improved. The following catalysts have heretofore been proposed as exhaust gas-purifying catalysts, wherein an adsorbent for a hydrocarbon and a nitrogen oxide-removing catalyst are used in combination.
JP-A-2-56247 proposes an exhaust gas-purifying catalyst having a first catalyst layer composed mainly of a zeolite formed on a substrate and a second catalyst layer composed mainly of a noble metal catalyst having an redox ability formed thereon, as a catalyst whereby a hydrocarbon is selectively adsorbed on the zeolite in a cold state and in an air/fuel ratio rich state, and the hydrocarbon which is desorbed from the zeolite as the exhaust gas temperature rises, and nitrogen oxide, carbon monoxide and hydrocarbons in the exhaust gas are purified. JP-A-5-293380 proposes an exhaust gas-purifying catalyst comprising a catalyst containing at least Pt on porous material and on aluminosilicate, which has solid acidity and molecular sieve function, containing at least one metal selected from alkali metals and alkaline earth metals.
Further, JP-A-8-24655 proposes an exhaust gas-purifying catalyst prepared by mixing a hydrocarbon adsorbent which adsorbs a hydrocarbon in an exhaust gas and desorbs the adsorbed hydrocarbon at a temperature higher than a certain level, with a NO
x
catalyst having a metal supported on a crystalline metal-containing silicate, which purifies nitrogen oxide in the exhaust gas in the presence of a hydrocarbon; and JP-A-8-164338 proposes an exhaust gas-purifying catalyst, wherein a hydrocarbon adsorbent made of an inorganic crystalline molecular sieve, is supported on a substrate, a first catalyst layer comprising Pd as a catalyst metal is formed on the surface of the hydrocarbon adsorbent particles, a rare earth oxide layer composed mainly of a rare earth oxide, is formed on the first catalyst layer, and a second catalyst layer comprising at least one of Pt and Rh as a catalyst, is formed on the rare earth oxide layer.
Further, JP-A-9-872 proposes an exhaust gas-purifying system wherein an adsorbent having a hydrocarbon-adsorbing ability and a cold ignition catalytic composition comprising a noble metal and a material having electron donative and/or nitrogen dioxide absorptive and desorptive activities, are disposed in an exhaust pipe of an internal combustion engine.
In recent years, an attention has been drawn to a problem of environmental pollution due to discharge of hydrocarbons, and it is desired to improve the technology for removing such hydrocarbons.
When a zeolite is used as an adsorbent, the adsorption characteristics of a hydrocarbon are influenced substantially by the type of the hydrocarbon and the matrix structure of the zeolite. With respect to the adsorption characteristics of a hydrocarbon having a small carbon number, as the molecular diameter is small, its diffusion and migration into zeolite pores are easy, and adsorption is likewise easy. However, due to the easiness in migration of the hydrocarbon, desorption also tends to be easy, and purif
Miura Hiroshi
Nakano Masao
Ogawa Hiroshi
Elve M. Alexandra
Ildebrando Christina
Tosoh Corporation
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