Catalyst – solid sorbent – or support therefor: product or process – Zeolite or clay – including gallium analogs – And additional al or si containing component
Reexamination Certificate
1999-12-10
2002-08-13
Griffin, Steven P. (Department: 1751)
Catalyst, solid sorbent, or support therefor: product or process
Zeolite or clay, including gallium analogs
And additional al or si containing component
C502S064000, C502S074000, C502S325000, C502S326000, C502S327000, C502S328000, C502S333000, C502S334000, C502S339000, C502S340000, C502S341000, C502S527120
Reexamination Certificate
active
06432859
ABSTRACT:
BACKGROUND OF THE INVENTION
1 Field of the Invention
The present invention relates to an exhaust gas purifying catalyst and a method of producing the exhaust gas purifying catalyst.
2 Description of the Related Art
There have been known exhaust gas purifying catalysts which comprise two catalytic layers, a first one of the two catalytic layers containing at least one noble metal selected from a group of Pt, Rh and Pd which is partly or entirely supported on ceria, Ba and K and a second one of the two catalytic layers containing at least one noble metal selected from the group of Pt, Rh and Pd but not containing K, which are formed in the form of double layers on a substrate. Such an exhaust gas purifying catalyst shows an increased ability to absorb NOx in a lean atmosphere (which refers to the atmosphere containing oxygen in excess as compared with exhaust gas produced with combustion of a stoichiometric air-to-fuel mixture) by virtue of the first catalytic layer and functions as a three-way catalyst in exhaust gas produced from combustion of a stoichiometric air-to-fuel mixture. In the exhaust gas purifying catalyst K and Na work as NOx absorbing component and the noble metal supported on ceria works as a promoter for NOx absorption. One of such exhaust gas purifying catalyst is known from, for example, Japanese Unexamined Patent publication No. 8-281106. Another type of exhaust gas purifying catalyst that is known from, for example, Japanese Unexamined Patent publication No. 9-85093, comprises an alumina coating layer impregnated with a noble metal such as Pt formed on a substrate, the alumina coating layer being further impregnated with K, Na and Li. In the exhaust gas purifying catalyst these K, Na and Li are made apt to produce a compound carbonate which puts the exhaust gas purifying catalyst in an unstable state so as to show improved NOx adsorbing and desorbing characteristics and makes it recover from sulfur-poisoning to gain high NOx conversion efficiency. Still another type of exhaust gas purifying catalyst such as known from, for example, Japanese Unexamined Patent publication No. 10-151357 comprises an alumina under layer formed on a honeycomb substrate and an over layer formed on the under layer. These under and over layers are impregnated with a mixture of a solution of a NOx absorbent such as Ba, K and Na and a solution of a noble metal with an effect of improving the dispersibility of the NOx and the noble metal.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an exhaust gas purifying catalyst which has improved resistance to sulfur-poisoning ability of a NOx absorbent such as Ba adapted to absorb NOx in the lean atmosphere (e.g. the exhaust gas with oxygen concentration higher than 4% for example from combustion of a lean air-fuel mixture at an air-to-fuel ratio ranging from 16 to 100) so as thereby to increase its NOx conversion efficiency.
It is another object of the present invention to provide a method of producing an exhaust gas purifying catalyst which has improved resistance to sulfur-poisoning of a NOx absorbent such as Ba adapted to absorb NOx in the lean atmosphere, e.g. in the exhaust gas from combustion of a lean air-to-fuel mixture with an air-to-fuel ratio ranging from 16 to 100 so as thereby to increase its NOx conversion efficiency.
In order to accomplish the foregoing objects of the present invention, an exhaust gas purifying catalyst contains a combination of a NOx absorbing component such as Ba and a K component or a Na component. The NOx absorbing component comprises at least one kind of element selected from a group of alkaline earth metals, a group of alkaline metals and a group of rare earth elements. In the exhaust gas purifying catalyst the K component or the Na component is distributed so as to be made exposed to an exhaust gas prior to the NOx absorbing component. While it has been known in the art that K and Na have the function of absorbing NOx, noticing the respect that K and Na are able to absorb S, the exhaust gas purifying catalyst is adapted so that the NOx absorbing component is less poisoned with S by causing the K component or the Na component to contact with an exhaust gas prior to contact of the NOx component with the exhaust gas so as thereby to absorb S contains more than 7 ppm in the exhaust gas. Although the K component or the Na component is transformed into the form of a carbonate as a result of calcination during preparing the exhaust gas purifying catalyst, when encountering a sulfuric component in an exhaust gas, it absorbs the sulfuric component and is consequently transformed into a sulfate such as K
2
SO
4
or Na
2
SO
4
. However, when the exhaust gas lowers its oxygen content, e.g. when a stoichiometric air-to-fuel ratio is reached, the sulfate is easily decomposed into carbonate. In particular, in the case of a combination of NOx absorbing component and K, even after the catalyst is exposed to a high temperature exhaust gas, it shows a high NOx conversion efficiency and is therefore favorable to improving its heat resistance. A component selected from a group of alkaline earth metals, a group of alkaline metals and a group of rare earth elements, in particular Ba, is preferable as the NOx absorbing component. Moreover, a combination of Ba and another NOx absorbing component may be employed.
In the case of placing a K component or a Na component in an exhaust gas stream upstream from the NOx absorbing component so as to expose the K component or the Na component to the exhaust gas prior to the NOx absorbing component, the catalyst may be comprised of a catalytic layer containing a K component or a Na component (which is referred to as a second catalytic layer) and a catalytic layer containing NOx absorbing component (which is referred to as a first catalytic layer) which are coated side by side on a substrate and placed in an exhaust gas stream so that the second catalytic layer is positioned upstream from the first catalytic layer. Otherwise the catalyst may be preferably comprised of the first catalytic layer formed as an under catalytic layer on a substrate and the second catalytic layer formed as an over catalytic layer over the first catalytic layer. In such the catalyst the first catalytic layer is preferable to contain a noble metal, such as, Pt or Rh in addition to the NOx absorbing component. The noble metal, on one hand, converts NOx in an exhaust gas while the exhaust gas is at higher oxygen contents and, on the other hand, converts NOx that is discharged from the NOx absorbing component while the exhaust gas is at lower oxygen content. As support of the noble metal alumina, preferably &ggr;-alumina and/or cerium may be employed. Since CeO
2
has oxygen absorption ability (O
2
storage effect), when the engine operates at an approximately stoichiometric air-to-fuel ratio, i.e. an excess air ratio (of approximately one (1), it causes the catalyst to act as a three-way catalyst, so as to effectively lower levels of other emissions, such as, hydrocarbon (HC) and carbon monoxide (CO) in addition to NOx.
The second catalytic layer is preferable to contain zeolite supporting a transition metal component in addition to a K component or a Na component. The transition metal supported on the zeolite activates NO and HC, so as to increase the NOx conversion efficiency of the catalyst. That is, NO in an exhaust gas is oxidized to NO
2
which is easily absorbed by the NOx absorbing component, and HC is activated through partial oxidization and cracking. The activated HC promotes a reduction-decomposition reaction of NOx. Noble metals, particularly Pt is preferable as the transition metal and using Pt together with Rh is more preferable. MFI type is preferable as zeolite.
K component or Na component can be included not only in the second catalytic layer but also in the first catalytic layer. In this case it is preferable that the second catalytic layer has a higher concentration of those components than the first catalytic layer. This is because it can reduce an amount of S to diffuse into th
Iwakuni Hideharu
Kyogoku Makoto
Takami Akihide
Yamada Hiroshi
Griffin Steven P.
Mazda Motor Corporation
Nguyen Cam N.
Nixon & Peabody LLP
Studebaker Donald R.
LandOfFree
Method of producing exhaust gas purifying catalyst and... 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 producing exhaust gas purifying catalyst and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of producing exhaust gas purifying catalyst and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2891298