Catalyst – solid sorbent – or support therefor: product or process – Catalyst or precursor therefor – Metal – metal oxide or metal hydroxide
Reexamination Certificate
1999-01-26
2001-04-24
Griffin, Steven P. (Department: 1754)
Catalyst, solid sorbent, or support therefor: product or process
Catalyst or precursor therefor
Metal, metal oxide or metal hydroxide
C502S304000, C502S324000, C502S328000, C502S330000, C502S332000, C502S338000, C502S339000, C502S341000, C502S344000, C502S350000, C502S355000, C423S239100, C423S239200
Reexamination Certificate
active
06221804
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a catalyst for purifying exhaust gas and manufacturing method thereof.
2. Description of Related Art
Conventional three-way catalysts are not always sufficient to lower a level of NOx in the exhaust and thereby to purify the exhaust gas because of high oxygen concentration of the exhaust gas while an engine operates at lean-burn conditions.
Japanese Unexamined Patent Publication No. 9-38493 discloses a catalyst effective to purify NOx in a high oxygen concentration exhaust gas which comprises a catalytic noble metal and a NOx adsorbent carried in order on a support material of a composite oxide made of alumina combined with lanthanum (La) and/or cobalt (Co). It is said in the publication that while, at high temperatures, alumina reacts with the NOx adsorbent to cause a reduction in its own specific surface area and clogging of its pores, and further to cause sintering of the catalytic noble metal, the composite oxide is prevented from thermal deterioration.
In a manufacturing method of the catalyst, after forming a layer of cobalt alumina complex oxide by impregnating cobalt (Co) into an alumina layer carried on a support and then drying and calcining it, the cobalt-alumina complex oxide layer is impregnated with platinum (Pt) and dried, and is subsequently impregnated with rhodium (Rh), then dried and calcined. Finally, the catalyst is completed by impregnating barium (Ba) into the cobalt-alumina composite oxide layer then drying and calcining it. In the catalyst made by the above method, platinum (Pt) and rhodium (Rh) carried on a single particle of cobalt-alumina composite oxide stay close to or overlap each other.
The catalyst in which two different kinds of noble metal particles are carried on a single particle of support material tends to make an alloy thereof when it is exposed to a high temperature gas, which leads to aggravation of catalytic performance.
SUMMARY OF THE INVENTION
An object of the invention is to provide an improved catalyst having a NOx adsorbent capable of adsorbing NOx in an exhaust gas while the exhaust gas has high oxygen concentration and desorbing NOx into an exhaust gas while the exhaust gas has low oxygen concentration.
Another object of the invention is to provide an improved catalyst containing different kinds of catalytic noble metals for catalyzing reduction of NOx which provides an increased thermal stability of each catalytic noble metal to keep the capability of catalyzing reduction of NOx.
Another object of the invention is to provide a manufacturing method of the improved catalyst.
The foregoing objects of the invention is accomplished by a manufacturing method of a catalyst for purifying an exhaust gas which has a NOx adsorbent capable of adsorbing NOx in the exhaust gas together with two different kinds of catalytic noble metals, namely a first noble metal and a second noble metal, for catalyzing reduction of NOx.
Specifically, the catalyst manufacturing method comprising the steps of forming a powdered first catalyst having the first noble metal carried on a support material but not containing the second noble metal, forming a powdered second catalyst having the second noble metal carried on a support material but not containing the first noble metal, mixing the powdered first catalyst with the powdered second catalyst to produce a mixture, and impregnating a solution of NOx adsorbent into the mixture to carry the NOx adsorbent on the support materials.
The catalyst manufactured by the above method has the first noble metal and the second noble metal on separate support materials, either same or different kinds, which prevents the two noble metals from forming an alloy when the catalyst is exposed to high temperatures. Thus each noble metal cooperates with the NOx adsorbent to catalyze reduction of NOx, thereby purifying the exhaust gas. Furthermore, even when these noble metals cooperate with each other to function as a catalyst, they exhibit sufficiently their own functions. Furthermore, it is a matter of importance of the method in which NOx adsorbent is not separately supported by the catalytic materials but impregnated in the mixture that the NOx adsorbent is partly supported by the support materials such that a particle of the NOx adsorbent is jointly supported by both of a particle of the support material bearing the first noble metal and a particle of the support material bearing the second noble metal, and by both first and second noble metal particles as well. Accordingly, each of the first and second noble metals on a same NOx adsorbent particle cooperates with the NOx adsorbent particle to help the catalyst to lower the level of NOx in the exhaust gas with an effect of causing the catalyst to easily increase its NOx purification efficiency. In particular, in the case where the first and second noble metals play different NOx purifying actions, both first and second noble metal particles achieve their roles on a same NOx adsorbent particle, which is advantageous to NOx purification through the mediation of NOx adsorbent. Further, the second noble metal makes up a reduction in NOx purification efficiency due to sintering of the first noble metal.
An exhaust gas purifying catalyst is obtained by coating the catalyst on a substrate to form a catalyst layer there on. The catalyst layer has a mixture of the support material bearing the first noble metal and the NOx adsorbent but not containing the second noble metal and the support material bearing the second noble metal and the NOx adsorbent but not containing the first noble metal. In the mixture, a part of the NOx adsorbent is jointly supported by both support materials bearing the first and second noble metals, respectively. As the support material for the first noble metal and/or the second noble metal, aluminum oxide (Al
2
O
3
), ceria (CeO
2
) or at least one of oxides of manganese (Mn), cobalt (Co), titanium (Ti) and iron (Fe) may be used. The first and second noble metals are carried separately on support materials which may be either of the same kind or of different kinds. The exhaust gas purifying catalyst keeps high NOx purification efficiency even after it has been exposed to a high temperature exhaust gas.
Carrying the first and second noble metals on different kinds of support materials separately provides improved catalytic performance. Generally, it can be said that the thermal resistance of a catalyst depends on the thermal stability of support material of the catalyst and that the support material having high thermal stability usually has small specific surface area. It also can be said that, in the case of using two different kinds of noble metals, the amount of one noble metal can often be smaller than that of the other. In this case, a reduction in catalytic performance of the catalyst due to thermal deterioration is prevented by bearing the first noble metal on the first support material with a greater specific surface area whose first noble metal content per unit mass of the support material may be high and bearing the second noble metal on the second support material with a smaller specific surface but with a high thermal stability whose noble metal content may be low. This is because the required amount of the second noble metal is kept even if the first noble metal on the first support material is partly lost or buried in the first support material due to sintering of the first support material. Bearing the second noble metal which may be small in amount on a support material having a large specific area but a low thermal resistance together with the first noble metal prevents significant aggravation of catalytic performance due to a reduction in the amount of the second noble metal caused through sintering of the support material.
It was found by the inventor of this invention, for example, that Pt had high NOx purification efficiency at high oxygen concentration of the exhaust gas and showed a tendency to lower its NOx purification efficiency at low oxygen conce
Iwakuni Hideharu
Kyogoku Makoto
Okamoto Kenji
Takami Akihide
Yamada Hiroshi
Griffin Steven P.
Mazda Motor Corporation
Nguyen Cam N.
Nixon & Peabody LLP
Studebaker Donald R.
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