Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
1999-10-14
2003-04-29
Silverman, Stanley S. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C423S213500
Reexamination Certificate
active
06555081
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a catalyst for purifying the exhaust gas from a lean-burn gasoline engine utilized as a drive engine mainly for an automobile and a method for the production thereof.
2. Description of the Related Art
In recent years, the feasibility of the introduction of a lean-burn gasoline engine to the drive engine of an automobile has been under study for the purpose of saving the fuel cost. In this case, the gasoline engine during the constant rate operation assumes a state of introducing air in an excess amount relative to the fuel to save the fuel cost. The conventional gasoline engine is operated at an air-fuel ratio (gravimetric ratio of fuel and air: hereinafter referred to as “A/F”) falling in the neighborhood of 14.6. This ratio is called the theoretical air-fuel ratio. Since the oxygen and the flammable compound in the fuel gas are stoichiometrically on the relation of equivalency, the unburned gas and oxygen do not remain in the exhaust gas when the combustion is perfect.
Various catalysts for purifying engine exhaust gases have been studied heretofore. Among such catalysts, the so-called three way catalyst that features simultaneous removal of hydrocarbons, carbon monoxide, and nitrogen oxides in the neighborhood of the theoretical air-fuel ratio has not been given any due consideration concerning the ability to purify the nitrogen oxides in an excessively oxidizing atmosphere. The lean-burn gasoline engine that is fated to be enveloped with an oxygen-excess atmosphere during the constant rate travel purifies the nitrogen oxides only with difficulty.
The diesel engine and the boiler among the internal combustion engines generally adopt the method of using a reducing agent such as ammonia, hydrogen, or carbon monoxide in the removal of nitrogen oxides. This method, however, has the problem of necessitating a special device for the recovery and disposal of the unaltered reducing agent.
Recently, with a view to removing such nitrogen oxides, a method that resorts to a NO
x
decomposition catalyst formed of a copper ion-containing crystalline aluminosilicate has been proposed (JP-A-60-125,250 and U.S. Pat. No. 4,297,328). It is simply demonstrated to be capable of decomposing nitrogen monoxide (NO) into nitrogen (N
2
) and oxygen (O
2
). It encounters difficulty in removing nitrogen oxides effectively under the actual exhaust gas conditions.
JP-A-63-100,919 offers a statement that the treatment of the exhaust gas with a copper-containing catalyst under an oxidizing atmosphere in the presence of a hydrocarbon preferentially promotes the reaction of NO
x
with the hydrocarbon and induces efficient removal of NO
x
. It is said that the hydrocarbon to be used in this method may be the hydrocarbon contained in the exhaust gas or the hydrocarbon optionally added from the external source. As a concrete mode of embodying this treatment, there is disclosed the method which comprises first bringing the exhaust gas into contact with the copper-containing catalyst thereby depriving it of NO
x
and then forwarding the remainder of the exhaust gas into contact with an oxidizing catalyst thereby depriving it of hydrocarbon, carbon monoxide etc. This method, however, requires a high temperature for effecting the removal of nitrogen oxides and produces insufficiently effect at a low temperature.
Further, since the catalyst mentioned above exhibits only poor resistance to heat and, on exposure to a high-temperature exhaust gas, suffers a decline in the ability thereof to decompose NO
x
, a method which comprises disposing the catalysts parallely and, when the exhaust gas reaches an elevated temperature, by-passing the exhaust gas toward the side of the oxidizing catalyst or the three way catalyst has been disclosed as a preventive measure (JP-A-01-171,625).
International Publication WO 94/25143 has disclosed a method for depriving an exhaust gas of nitrogen oxides by using a catalyst comprising a noble metal such as platinum, palladium, or rhodium, a catalytically active component formed of a compound of an alkali metal such as lithium, or sodium, or an alkaline earth metal such as calcium, and a refractory inorganic oxide and intermittently introducing a reducing substance thereto. This method, as disclosed, has only the catalytically active component uniformly mixed with the refractory inorganic oxide instead of locally distributed thereon.
JP-A-08-117,600 has disclosed an exhaust gas purifying catalyst which comprises an alumina carrier, a noble metal deposited carrier in which a catalytic noble metal is deposited on ceria, and a NO
x
absorber formed of an alkali metal, an alkaline earth metal, and a rare earth element and which has the noble metal deposited carrier and the NO
x
absorber uniformly dispersed in the alumina carrier. This catalyst has the noble metal wholly deposited on the ceria.
Though various catalysts have been proposed, a nitrogen oxide decomposing catalyst that efficiently decomposes and removes NO
x
from the exhaust gas of a lean-burn engine and moreover excels in high temperature resistance has not been developed to date.
SUMMARY OF THE INVENTION
An object of this invention is to provide a nitrogen oxide purifying catalyst effecting efficient removal of NO
x
and having excellent high temperature resistance and resistance to poisoning, and a method for the production thereof.
The present inventors have pursued a diligent study with a view to fulfilling the object mentioned above and, as a result, have discovered a catalyst for purifying the exhaust gas from a lean-burn engine.
The object of this invention is therefore achieved by a catalyst for purifying the exhaust gas from a lean-burn engine which comprises i) a refractory inorganic oxide carrying platinum thereon, ii) a cerium-containing oxide carrying palladium thereon, and iii) an alkali metal compound and/or an alkaline earth metal compound.
The object of this invention is achieved by a method for the production of a catalyst for purifying the exhaust gas from a lean-burn engine, characterized by pulverizing and mixing a refractory inorganic oxide carrying platinum partly or wholly thereon, a cerium-containing oxide carrying palladium partly or wholly therein, and an alkali metal compound and/or an alkaline earth metal compound and either forming the resultant mixture in a three-dimensional structure or coating a refractory three-dimensional structure with the resultant mixture.
The object of this invention is further achieved by a method for the production of a catalyst for purifying the exhaust gas from a lean-burn engine, characterized by pulverizing and mixing a refractory inorganic oxide carrying platinum partly or wholly thereof with a cerium-containing oxide carrying palladium partly or wholly thereon, either forming the resultant mixture in a three-dimensional structure or coating a refractory three-dimensional structure with the resultant mixture, and thereafter depositing thereon the salt of an alkali metal compound and/or an alkaline earth metal compound,
The above and other objects, features, and advantages of the present invention will become clear from the following description of the preferred embodiments.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the purification of the exhaust gas from a lean-burn engine, the catalyst of this invention is capable of oxidize HC (hydrocarbon), CO, NO and N
2
O and at the same time adsorbs NO
2
under an oxidizing condition, reduces or decomposes the adsorbed NO
2
with the HC in the exhaust gas as a reducing agent under a reducing condition and, at the same time, oxidizes CO by utilizing the adsorbed oxygen, and consequently accomplishes the purification of the exhaust gas. Here, for the purpose of saving the fuel cost, the engine is generally operated under an oxidizing condition introducing air in an excess amount relative to the fuel, and a short-time reducing atmosphere is repeated to reduce the amount of NO
x
. For example, the reducing condition is preferred to last for an approx
Hori Masao
Horiuchi Makoto
ICT Co., Ltd.
Nguyen Cam N.
Selitto Behr & Kim
Silverman Stanley S.
LandOfFree
Method of the purification of the exhaust gas from a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of the purification of the exhaust gas from a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of the purification of the exhaust gas from a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3111962