Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
2000-02-16
2001-10-30
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S213500
Reexamination Certificate
active
06309616
ABSTRACT:
The present invention relates to an adsorbent for purifying a hydrocarbon contained in a gas, such as the atmosphere or an exhaust gas discharged from an internal combustion engine, a method for adsorbing and removing a hydrocarbon, and a method for purifying an exhaust gas, and it is applicable, for example, to purifying a hydrocarbon, particularly ethylene, contained in an exhaust gas discharged from an internal combustion engine of e.g. an automobile, and to adsorbing and removing ethylene as a matured or aged hormone formed from e.g. the crops.
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. It is known that the exhaust gas-purifying ability of such a three way catalyst is observed at a temperature of at least 300° C. Accordingly, when the exhaust gas temperature is low at the start up of an engine, not only the concentration of hydrocarbons in the exhaust gas is high, but also the three way catalyst has not yet reached the operable temperature, whereby the hydrocarbons will be discharged without being purified.
With respect to 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, which supports at least one metal on a part of a monolithic substrate having Y-type zeolite and mordenite 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-6-126165 discloses a molecular sieve having Ag supported thereon; JP-A-6-312132 discloses a zeolite containing Ag, or Ag and at least one metal selected from the group consisting of Co, Ni, Cr, Fe, Mn, Ag, Au, Pt, Pd, Ru, Rh and V; JP-A-8-99033 discloses a zeolite ion-exchanged with Ag and a metal of Group IIIB of the Periodic Table; JP-A-6-210165 discloses an adsorbent consisting of Pd and zeolite; JP-A-6-210163 discloses a zeolite containing Cu, or Cu and at least one metal selected from the group consisting of Co, Ni, Cr, Fe, Mn, Ag, Au, Pt, Pd, Ru, Rh and V; JP-A-6-170234 discloses ZSM-5 zeolite ion-exchanged with at least one metal of Cu and Pd; and JP-A-5-31359 proposes a zeolite with a SiO
2
/Al
2
O
3
molar ratio being at least 40.
Further, it is also known that from an exhaust gas at a low temperature, a hydrocarbon is adsorbed by an adsorbent, and the hydrocarbon which desorbs from the adsorbent as the exhaust gas temperature rises, is utilized to improve the performance for removing nitrogen oxide. The following catalysts have heretofore been proposed as exhaust gas-purifying catalysts comprising a combination of a hydrocarbon adsorbent and a nitrogen oxide-removing catalyst.
JP-A-2-56247 proposes an exhaust gas-purifying catalyst having a first catalyst layer composed mainly of zeolite formed on a substrate and a second catalyst layer composed mainly of a noble metal catalyst having a 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 a hydrocarbon in the exhaust gas are purified. JP-A-5-293380 proposes an exhaust gas-purifying catalyst which comprises a catalyst having a catalyst component containing at least Pt supported on a substrate made of a porous material and an aluminosilicate having solid acidity and molecular sieve function, as the main components, and a hydrocarbon adsorbent having at least one metal selected from alkali metals and alkaline earth metals supported thereon.
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 catalyst metal supported on a crystalline metal-containing silicate, which purifies nitrogen oxide in an exhaust gas in the presence of a hydrocarbon, or prepared by laminating such a NO. catalyst layer and a hydrocarbon-adsorbent layer; 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.
Each of such methods for adsorbing and removing hydrocarbons and methods for purifying exhaust gases employing 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 the exhaust gas-purifying catalyst will operate, and the hydrocarbon desorbed from the adsorbent in a temperature range higher than the temperature, is purified by an exhaust gas-purifying catalyst. Namely, adsorption and removal of a hydrocarbon by an adsorbent will function effectively for the first time when the adsorbent has abilities to selectively adsorb a hydrocarbon at a low temperature and to keep it adsorbed.
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. For example, various types of hydrocarbons are present in a mixed state in an exhaust gas discharged from an internal combustion engine of e.g. an automobile, and adsorbents corresponding to the respective types of hydrocarbons, are required. However, in the above-mentioned prior art, a study on the adsorption characteristics of a lower hydrocarbon, particularly ethylene, has been insufficient, and the adsorption characteristics of ethylene have been insufficient by the adsorbents disclosed in the prior art.
In general, 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 pore structure of the zeolite. With respect to the adsorption characteristics of a hydrocarbon having a small carbon number, it can be said that 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 when purification of an exhaust gas is intended, purification tends to be inadequate, since the hydrocarbon tends to desorp at a temperature lower than the temperature at which the catalyst for purifying the hydrocarbon, represented by the three way catalyst, will operate. Further, in purification of an exhaust gas containing excessive oxygen, the utilization factor of the adsorbed hydrocarbon tends to be low. On the other hand, with respect to the adsorption characteristics of a hydrocarbon having a large carbon number, it can be said that a hydrocarbon having a molecular diameter larger than the pore diameter of the zeolite, can hardly diffuse or migrate into the pores, and the adsorption decreases. Thus, the amount of the hydrocarbon adsorbed decreases, and the hydrocarbon will be discharged as it is i.e. without being adequately purified.
Further, the tempe
Itabashi Keiji
Ito Yukio
Nakano Masao
Ogawa Hiroshi
Griffin Steven P.
Nguyen Cam N.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Tosoh Corporation
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