Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Mixture is exhaust from internal-combustion engine
Reexamination Certificate
1999-09-22
2002-06-04
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Mixture is exhaust from internal-combustion engine
C423S239100
Reexamination Certificate
active
06399035
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for purifying a nitrogen oxide-containing exhaust gas by treating the exhaust gas with a hydrocarbon or an oxygen-containing organic compound as a reducing agent in the presence of a catalyst in an oxidative atmosphere in which an excess amount of oxygen is present to catalytically reduce the nitrogen oxide. It also relates to a catalyst for catalytically reducing such a nitrogen oxide.
BACKGROUND OF THE INVENTION
Nitrogen oxides are included in various exhaust gases, and the nitrogen oxides can be not only the cause for generating photochemical smog and acid rain, but also directly; adversely affect a human body. Therefore, various means have been conventionally proposed for eliminating nitrogen oxides in exhaust gases. In particular, a method wherein a nitrogen oxide-containing exhaust gas is contacted with a catalyst to reduce the nitrogen oxide has been already practically utilized in some field.
Examples of the method include (a) a method for treating nitrogen oxides in an exhaust gas from a gasoline engine of an automobile with a three-way catalyst, and (b) a method of a selectively catalytic reduction of nitrogen oxides in an exhaust gas from large scale facilities such as a boiler with ammonia as a reducing agent. Furthermore, recently there has been proposed (c) a method for treating nitrogen oxide-containing exhaust gases using a hydrocarbon as a reducing agent in the presence of a catalyst comprising a metal such as copper having supported thereon a metallic oxide such as alumina or a catalyst comprising various metals having supported thereon a zeolite (JP-A-63-100929 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”)).
In the above-described method (a), hydrocarbon components and carbon monoxide included in a combustion exhaust gas from a gasoline engine of an automobile are converted to water and carbon dioxide with a catalyst comprising a metal belonging to the platinum group and, at the same time, nitrogen oxides included in the exhaust gas are reduced to nitrogen. An exhaust gas is required to comprise an appropriate amount of oxygen so that nitrogen oxides may be effectively reduced under the reaction conditions. Thus, this method has a problem in that it cannot be used in principle in an atmosphere in which an excess amount of oxygen is present as in a diesel engine.
The method (b) uses ammonia as a reducing agent which is very toxic and often must be handled as a high pressure gas so that it cannot be easily conducted. Furthermore, since the facilities are inevitably have a large size, it is technically difficult to be applied to a small size-exhaust gas generation source, especially a moving generation source. Thus, it is also undesirable in an economical aspect.
In the method (c), as in the above-described method (a), a main subject to be treated is a combustion exhaust gas from a gasoline engine of an automobile. This method is difficult to be applied to a treatment of exhaust gases from a diesel engine and the durability of the catalyst is still insufficient. That is, the above-described catalyst comprising a carrier such as alumina or a zeolite having supported thereon a metal such as copper is readily impaired with a sulfur oxide discharged from a diesel engine and the decrease in activity of the catalyst may occur by the aggregation of metals which are active species.
Accordingly, it is earnestly desired to develop a method capable of purifying an exhaust gas by effectively reducing nitrogen oxides in the exhaust gas in the presence of an excess amount of oxygen and even in a case where the gas contains water vapor or a sulfur-oxide.
SUMMARY OF THE INVENTION
The present invention was made so as to respond to such a requirement.
Namely, an object of the present invention is to provide a method for purifying an exhaust gas by effectively reducing nitrogen oxides in the exhaust gas generated from various facilities, i.e., combustion exhaust gases discharged from a gasoline engine and a diesel engine even in an atmosphere where an excess amount of oxygen is present and, furthermore, in the presence of water vapor or a sulfur oxide.
Another object of the present invention is to provide a catalyst for catalytic reduction of nitrogen oxides used in this method.
These and other objects of the present invention have been attained by a reduction purification method of a nitrogen oxide-containing exhaust gas by contacting said nitrogen oxide-containing exhaust gas with a catalyst in an oxidative atmosphere in which an excess amount of oxygen is present and in the presence of at least one reducing agent selected from the group consisting of a hydrocarbon and an oxygen-containing organic compound, wherein said catalyst comprises an alumina having supported thereon
(a) tin and
(b) at least one selected from the group consisting of ruthenium, palladium, rhodium and indium.
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the term “loading amount” means a percentage of parts by weight of metal species (i.e., tin, ruthenium, palladium, rhodium and indium), in terms of a value converted to a metal, supported on 100 parts by weight of alumina. For example, 2% by weight of a loading amount means that 2 parts by weight, in terms of a value converted to a metal, of an active metal species is supported on 100 parts by weight of alumina. In the present invention, ruthenium, palladium and rhodium are sometimes simply referred to as “the VIII group metal(s)”.
The method for supporting the above-described metal species (tin and the VIII group metal or indium) on an alumina is not particularly limited. Namely, it can be effected according to conventionally known methods.
For example, tin can be supported on an alumina according to an impregnation method. That is, the alumina is impregnated with an aqueous solution of an appropriate tin compound, the impregnated product thus obtained is dried, and the dried product is calcined in air at a temperature of about 200 to 700° C., preferably about 450 to 600° C., for about 1 to 10 hours to support tin on an alumina.
The above-described tin compounds are not particularly limited. Examples thereof include inorganic salts such as tin (II) chloride, tin (I) chloride, tin (II) sulfate, tin (IV) sulfate, tin (II) nitrate and tin (IV) nitrate, and organic salts such as tin (II) oxalate, tin (11) acetate, hexachloroammonium stannate (IV), hexaethylditin (II) and tetraphenyltin. Among these, generally tin (II) chloride is preferably used. In order to increase the solubility of tin (II) chloride in water, an acid aqueous solution of diluted hydrochloric acid, diluted nitric acid or the like can be added to the aqueous solution.
The loading amount of tin in the catalyst for use in the present invention is generally within a range of 0.1 to 15% by weight, preferably 0.5 to 10% by weight, and more preferably 0.5 to 7% by weight, in terms of a value converted to a metal. When the loading amount of tin. is less than 0.1% by weight, effects as a catalyst, as will be described below, cannot be obtained. On the other hand, when the loading amount of tin is more than 15% by weight, there can be a possibility of blocking off the pores of alumina with tin.
Palladium, ruthenium, rhodium and indium can be supported on an alumina as in the case of tin. That is, the alumina is impregnated with an aqueous solution of an appropriate compound of these metals, the impregnated product thus obtained is dried, and the dried product is calcined in air at a temperature of about 400 to 700° C., preferably about 450 to 600° C., for about 1 to 10 hours to support the above-described metals on the alumina.
Examples of the palladium compound include palladium chloride, tetraamminedichloropalladium, tetraamminedinitropalladium and palladiumnitrate. Examples of the ruthenium compound include ruthenium chloride, hexaamminetrichlororuthenium and hexaamminetrinitroruthenium. Examples of the rhodium compound include rhodium chloride, hexaamminetrichlororhodi
Miyamoto Katsumi
Sato Kazuhito
Tabata Mitsunori
Takehara Sadao
Yoshinari Tomohiro
Cosmo Research Institute
Griffin Steven P.
Sughrue & Mion, PLLC
Vanoy Timothy C
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