Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1999-06-10
2001-01-16
Dunn, Tom (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S341000, C502S346000, C423S594120, C423S598000, C423S599000, C423S213200, C423S213500, C423S219000, C423S235000, C423S239100
Reexamination Certificate
active
06174835
ABSTRACT:
BACK GROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exhaust gas purifying catalyst for removing nitrogen oxide (NOx) in exhaust gases of a lean-burn internal combustion engine or the like, and particularly to an exhaust gas purifying catalyst which exhibits a high-efficient exhaust gas purifying rate under the surplus oxygen atmosphere and is excellent in durability.
2. Description of the Related Art
The conventional exhaust gas purifying catalysts for purifying nitrogen oxide (NOx) contained in the exhaust gas of vehicles used the catalyst using alumina or zeolite, and the ternary catalyst in which noble metal is carried on a carrier formed of alumina or zeolite. However, these have less effect with respect to the exhaust gas of the Diesel engine with lean burning. Therefore, a catalyst of Perovskite type has been developed to purify the exhaust gas containing a large quantity of oxygen. However, the catalyst poses a problem in that the reduction rate of nitrogen oxide of the exhaust gas is low.
In the catalyst disclosed in Japanese Patent Application Laid-Open No. 5-317652, in the lean-burn state of the Diesel engine, nitrogen oxide (NO) is oxidized into nitrogen dioxide (NO
2
) by noble metal salt and adsorbed on alkaline earth metal salt such as salt of Barium (Ba), Strontium (Sr), Calcium (Ca), Magnesium (Mg), etc. and then the NOx is further reduced and released. However, the catalyst cannot be sufficient in terms of the nitrogen oxide removing rate and the durability.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an exhaust gas purifying catalyst which has notable effect for exhaust gas containing much oxygen and is excellent in durability.
It is another object of the present invention is to provide an exhaust gas purifying apparatus, which is high in catalyst activity, stable in composition and excellent in durability.
According to the present invention, a brown millerite type composite oxide was used as a catalyst for decomposing and absorbing nitrogen oxide (NOx), and the brown millerite was made to coexist with noble metal salt as a reducing catalyst to thereby realize higher nitrogen oxide (NOx) removing rate than a catalyst used an alkaline earth metal or an oxide or salt of the alkaline earth metal, for example, barium oxide (BaO) which represent of the alkaline earth metal as an nitrogen oxide (NOx) absorbent. Particularly, by adjusting the addition amount of the noble metal (salt) as a reducing catalyst, the catalyst enhance the removing ability of nitrogen oxide (NOx) and the durable service life.
The reason is that the catalyst using the alkaline earth metal, the alkaline metal or rare earth metal such as barium oxide (BaO) removes nitrogen oxide (NO) merely by absorption, whereas the brown millerite has the properties to remove, decompose and adsorb or absorb nitrogen oxide (NOx). That is, since nitrogen oxide (NOx) tends to be adsorbed in the constructive strain such as an oxygen defect derived from conversion of construction under a high temperature and the number of ions in construction tends to change, which are constructive features of the brown millerite, nitrogen oxide (NO) is decomposed into nitrogen and oxygen without using the reducing catalyst. Since the adsorbed nitrogen oxide (NO) by brown millerite as described above is reduced by the reducing catalyst such as Platinum (Pt), the oxygen oxide (NOx) removing rate is further improved.
In case the alkaline earth metal is contained in the brown millerite construction, the characteristic of the alkaline earth metal for adsorbing nitrogen oxide (NOx) is added to the characteristic for removing and decomposing nitrogen oxide (NOx) of the brown millerite, and the nitrogen oxide (NOx) removing rate is further improved. While the basic construction of the brown millerite is established even by two components, the constituent elements (sites) A and C in the general formula described hereinafter, the components of the constituent elements B and D are selected, whereby two times or more nitrogen oxide (NOx) removing rate is obtained by using the same weight of barium oxide (BaO).
In the present invention, at least two kinds of fine noble metal particles are dispersed and fixedly mounted on the surface of brown millerite type composite oxide particles expressed by the general formula, A
3
B
4
O
9
, A
2
B
2
O
5
or AB
2
O
4
. The brown millerite type composite oxide includes at least one selected from the group consisting of the following rare earth elements, alkaline earth elements or metal elements, Ce, Nd, Eu, Gd, Tb, Dy, Ho, Er, Ca, Sr, Ba, Ti, Zn, Y, Zr, In and Hf. The noble metal includes at least two selected from the group consisting of Rh, Ru, Au, Pd, Ir and Pt. An average particle diameter of the noble metal particles is less than 25 nm. Two kinds or more of fine noble metal particles as the reducing catalyst are dispersed and fixedly mounted on the surface of the brown millerite type composite oxide particles to thereby widen the active temperature zone as the nitrogen oxide removing catalyst. The noble metal particle itself has the activity and oxidation resistance as an oxidation-reduction catalyst, but is low in reactive activity relative to the constituent element of the brown millerite type composite oxide. Accordingly, the noble metal particles as a catalyst are dispersed on the surface of the brown millerite type composite oxide particles. Further, the noble metal as a catalyst is selected to thereby suppress formation of peroxide and carbonate and suppress decomposition of brown millerite, thus stabilizing the brown millerite to a specific construction of A
3
B
4
O
9
, A
2
B
2
O
5
, AB
2
O
4
) for a longer service life.
In the exhaust gas purifying catalyst according to the present invention, the noble metal particle functions as a catalyst so as to maintain the construction in which the brown millerite type composite oxide particles exhibits high reactive activity with respect to nitrogen oxide (NO) which is stable in high temperature and to lower the activation energy necessary for phase transition.
In the exhaust gas purifying catalyst according to the present invention, the noble metal reacts with nitrogen oxide (NOx) to act as the reducing catalyst for removing nitrogen oxide (NO) and to stabilize the brown millerite type composite oxide of A
3
B
4
O
9
or A
2
B
2
O
5
phase, and as a result, high nitrogen oxide removing rate and durability are obtained.
The reason why the noble metal stabilizes the brown millerite type composite oxide of A
3
B
4
O
9
or A
2
B
2
O
5
phase is contemplated due to the fact that nitrogen oxide is decomposed by the noble metal, that production of nitric acid group is suppressed, that consumption of adsorbed oxygen is accelerated and that activation energy required for transfer to A
3
B
4
O
9
or A
2
B
2
O
5
phase is lowered by the noble metal catalyst.
The reason why the brown millerite type composite oxide particles exhibits high reactive activity is contemplated due to the fact that the brown millerite type composite oxide of A
3
B
4
O
9
or A
2
B
2
O
5
phase is used and that the noble metal particles are remained in a fine situation to react with nitrogen oxide (NOx). since the noble metal particles fixed on the surface of the brown millerite by heat treatment in a relative low temperature.
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Naito Isao
Oosumi Kazuo
Unno Yasuaki
Dunn Tom
Isuzu Ceramics Research Institute Co. Ltd.
Nguyen Cam N.
Staas & Halsey , LLP
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