Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Waste gas purifier
Reexamination Certificate
2000-06-07
2003-02-11
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Waste gas purifier
C422S105000, C422S180000, C060S285000, C060S299000, C502S325000, C502S302000
Reexamination Certificate
active
06517784
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to methods for removing nitrogen oxides effectively from exhaust gas containing the nitrogen oxides such as exhaust gas from internal combustion engines of automobiles etc, exhaust gas from consumers appliance such as cooking apparatus and the like, and exhaust gas from boilers in factories and thermal power stations.
Further, the present invention relates to catalysts for removing nitrogen oxides from the exhaust gas. The catalysts relating to the present invention are preferable for purifying the exhaust gas released from lean burn engines.
Nitrogen oxides (NOx) are contained in exhaust gas released from internal combustion engines of automobiles etc. The nitrogen oxides are harmful to human bodies, and become an origin to destroy a global environment by acid rain etc. Therefore, various catalysts for removing the nitrogen oxides in exhaust gas have been investigated.
Most of catalysts for purifying exhaust gas from automobiles which are under developing at present have been aimed at treating exhaust gas from automobiles wherein a ratio of fuel to air, that is air/fuel by weight, is set at approximately stoichiometric ratio, that is theoretical air to fuel ratio (air/fuel=14.7 by weight). Combustion under the stoichiometric condition generates and releases hydrocarbons and carbon monoxide in addition to the nitrogen oxides. The hydrocarbons and carbon monoxide are also environment contaminating material. Therefore, three way catalyst which can treat the above three substances simultaneously have been a main object of the development. The three way catalyst is a general name for catalysts which can simultaneously treat nitrogen oxides, hydrocarbons, and carbon monoxide in exhaust gas. Most of the three way catalysts contain noble metals such as rhodium, palladium, and platinum as for main components.
However, currently, lean burn engines which burn fuel under a fuel to air ratio (air/fuel) larger than the theoretical fuel to air ratio are becoming a main current as for automobile engines in view of decreasing fuel consumption. With the lean burning, oxygen content in the exhaust gas increases, and an activity of conventional three way catalyst decreases in the presence of the oxygen. Accordingly, nitrogen oxides in the exhaust gas can not be removed effectively. Therefore, development of catalyst for purifying exhaust gas from lean burn engines becomes necessary.
As the catalyst for purifying exhaust gas from lean burn engines, a catalyst which is composed of copper supported by zeolite (JP-A-1-130735 (1989), Proceeding of 68th. Meeting for discussing catalyst, 3F108, (1991)), and a catalyst which is composed of cobalt and rare earth metals supported by zeolite and further at least one of copper and rhodium is supported by the zeolite (JP-A-4-219147 (1992)) are disclosed.
A catalyst which absorbs NOx under a lean burning condition, desorbs the absorbed NOx under a stoichiometric burning condition, and reduces the NOx is disclosed in JP-A-5-261287 (1993). The catalyst is composed of barium oxides, lanthanum oxides, and platinum, all of which are supported by an alumina supporter.
All the above described conventional catalysts for purifying exhaust gas from lean burn engines have such a problem that the catalysts lack a long durability, because all the catalysts contain zeolite. Under the lean burning condition, water is generated approximately 10% by volume by combining hydrocarbons and oxygen in the exhaust gas. Zeolite has such a property that the zeolite loses zeolite structure when it is heated under a condition existing water. Once the zeolite structure is broken, active components supported by the zeolite coagulates, and the catalytic activity decreases.
The catalyst which is composed of barium oxides, lanthanum oxides, and platinum, all of which are supported by an alumina supporter has such a problem that the barium oxides which is contained by a high concentration are thermally deteriorated.
SUMMARY OF THE INVENTION
Accordingly, one of the objects of the present invention is to provide a method for treating exhaust gas to purify effectively the exhaust gas released from lean burning.
Other one of the objects of the present invention is to provide a catalyst which can purify the exhaust gas released lean burning effectively, and has preferable durability.
The method for treating exhaust gas relating to the present invention comprises the process of contacting the exhaust gas flow containing nitrogen oxides with the catalyst in the presence of at least one of hydrocarbons and carbon monoxide to reduce the nitrogen oxides to nitrogen.
The catalyst comprises a structure wherein active components are supported by inorganic oxide supporters, the active components comprises at least one of noble metals selected from rhodium, platinum, and palladium, at least one of rare earth metals, at least one of alkali earth metals, and magnesium. Concentration of the noble metal is in a range of 0.05~3.5 mol % to the inorganic oxides supporter 100 mol %, the rare earth metal is in a range of 0.7~20 mol %, the alkali earth metal is in a range of 4-16 mol %,.rhodium is less than 1.9 mol %, platinum is less than 2.6 mol %, palladium is less than 2.8 mol %, the noble metal is contained in a form of metal or oxide, the rare earth metal is contained in a form of oxide, and the alkali earth metal is contained in a form of oxide or carbonic acid salt.
As the active component for the catalyst of the present invention, the active component composed of rhodium, platinum, cerium, and magnesium is most preferable, and the catalytic activity is maximum.
The catalyst of the present invention is preferably composed of in a manner that the inorganic oxide supporter supports the rare earth metal, the rare earth metal supports the noble metal, and the noble metal supports the alkali earth metal. The catalyst of the above described structure has a preferable dispersibility of noble metal components and a high catalytic activity.
The noble metal components are composed of rhodium and platinum, and the rhodium is preferably supported on the platinum.
In the catalyst of the present invention, the noble metal gives a reaction field for generating N
2
from nitrogen oxide and hydrocarbon. By containing alkali earth metal and/or rare earth metal, adsorption of nitrogen oxide at surface of the catalyst is enhanced. Further, the alkali earth metal and the rare earth metal have a strong bonding activity with oxygen, and accordingly, the metals can absorb oxygen in NOx and proceed the catalytic reaction to form N
2
even under a condition existing oxygen.
Magnesium in the active components has an effect to enhance the reducing reaction of the nitrogen oxides by increasing crystallinity of the noble metal. The crystallinity of the noble metal can be enhanced by supporting magnesium after supporting the noble metal on the supporter.
When the catalyst is a mixture of particles composed of an inorganic oxide supporter supporting the noble metal, cerium, and magnesium as the active components and particles composed of an inorganic oxide supporter supporting the noble metal, lanthanum, and barium as the active components, the catalyst has a significantly high catalytic activity under both stoichiometric and lean burning operating conditions. The particles composed of an inorganic oxide supporter supporting the noble metal, cerium, and magnesium have a superior performance for purifying the exhaust gas under both stoichiometric and lean burning conditions, and the particles composed of an inorganic oxide supporter supporting the noble metal, lanthanum, and barium have a property to absorb NOx under the lean burning condition. Therefore, the catalytic performance for purifying the exhaust gas can be enhanced by combining the above two kinds of particles, because NOx is absorbed under the lean burning condition and the NOx is released under the stoichiometric burning condition to be reduced to N
2
.
As for the inorganic oxide supporter, porous oxides such as T
Iizuka Hidehiro
Kato Akira
Kitahara Yuichi
Kuroda Osamu
Miyadera Hiroshi
Hitachi , Ltd.
Kenyon & Kenyon
Tran Hien
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