Patent Application
20130065754
The present invention relates to an exhaust gas purification catalyst, more particularly relates to an exhaust gas purification catalyst which is provided with an NOx storage reduction type catalyst layer.
In the past, in three-way catalysts, NOx storage reduction type catalysts have suffered from competitive adsorption of HC, CO, and NOx, so it has been difficult to secure sufficient purification performance.
To solve this, Japanese Patent Publication No. 2009-101252 A1 etc. report NOx storage reduction type catalysts which have two-layer coated structures, but both the upper and lower layers contain an NOx storage material (or NOx holding substance), so the problems that competitive adsorption of HC, CO, and NOx occurs, the active points of the NOx storage reduction reaction end up decreasing, and an NOx storage reduction type catalyst cannot be effectively formed went unresolved.
The present invention has as its object the provision of an exhaust gas purification catalyst which has a structure which prevents competitive adsorption of HC, CO, and NOx and enables effective utilization of an NOx storage reduction type catalyst.
To achieve the above object, the exhaust gas purification catalyst of the present invention is characterized by:
having an NOx storage reduction type catalyst layer which contains at least one type of NOx storage material which is selected from an alkali metal or an alkali earth metal and Pt and/or Rh on a substrate and
having an oxidation catalyst layer which carries Pt and/or Pd on the NOx storage reduction type catalyst layer.
In a preferred embodiment, seen in the direction of flow of exhaust gas, the oxidation catalyst layer has a length of 25 to 60% of the length of the NOx storage reduction type catalyst layer.
In a preferred embodiment, the oxidation catalyst layer has a thickness of 20 to 40 μm.
In a conventional two-layer coat structure, both the upper and lower layers are NOx storage reduction type catalyst layers, so, as shown in
As opposed to this, as a characterizing feature of the present invention, as shown in
The present invention can provide an oxidation catalyst layer on an NOx storage reduction type catalyst layer so as to enable effective utilization of the lower layer NOx storage reduction type catalyst layer. That is, the upper layer oxidation catalyst layer through which the exhaust gas first passes selectively oxidizes the HC (hydrocarbons) and CO (carbon monoxide) which inhibit the reaction of the NOx storage reduction type catalyst. Therefore, in the lower layer NOx storage reduction type catalyst layer, competitive adsorption of HC, CO, and NOx substantially does not occur. Storage and reduction of NOx which passes through the upper layer oxidation catalyst layer selectively occurs.
In the exhaust gas purification catalyst of the present invention, by selectively causing action of the upper layer oxidation catalyst layer and the lower layer NOx storage reduction type catalyst layer, it is possible to avoid competitive adsorption of the HC and CO which should be removed by oxidation and NOx which should be removed by reduction, secure sufficient reaction sites, and enable maximum utilization of the inherent functions of the NOx storage reduction type catalyst.
In one preferable embodiment, as shown in
Further, in one preferable embodiment, the thickness of the oxidation catalyst layer is 20 to 40 μm. In this range, the NOx storage rate of the NOx storage reduction type catalyst layer becomes maximum.
After the lower layer NOx storage reduction type catalyst layer is formed, the upper layer oxidation catalyst layer is overcoated on it to form a catalyst of a vertical two-layer coat structure.
The oxidation catalyst layer which is overcoated on the upper layer is raised in oxidation performance by being provided with Pt and/or Pd which has excellent catalyst ability as an oxidation catalyst.
The oxidation catalyst layer does not have Rh inhibiting the catalyst activity in lean combustion gas added to it, while does not have an NOx storage material which affects the previous metal activity added to it either.
The NOx storage reduction type catalyst layer includes Pt and/or Rh and an NOx storage material. In particular, Rh and the NOx storage material are also added to only the lower layer NOx storage reduction type catalyst layer.
Three types of exhaust gas purification catalysts which have the coat specifications which are shown in Table 1 on cordierite substrates were prepared. Below, DOC indicates an oxidation catalyst layer, while NSR indicates a NOx storage reduction type catalyst layer.
Each invention example was prepared by forming an NOx storage reduction type catalyst layer (NSR) on a substrate, then coating an oxidation catalyst layer (DOC) on the same.
The catalyst size was, for laboratory use, a volume of 35 cc (total length 50 mm) and for actual use, a volume of 14 liters (total length 110 mm).
Each catalyst was tested for simple durability in an electric furnace at 700° C.×27 hours.
Three types of catalysts of the coat specifications of Table 1 were used for tests under the following conditions. The images of composition are shown in
<Configuration of Test Catalysts>
The test conditions were as follows:
<Test Conditions>
Test gas conditions: Shown in Table 2.
Test cycle: Shown in
That is, the catalyst was raised from the initial temperature of 50° C. to 600° C. by 40° C./min. At 600° C., rich spike NOx reduction (rich/lean=5 sec/5 sec) was performed, then the catalyst was immediately cooled in an argon atmosphere down to 350° C. where NOx storage was performed in a lean atmosphere.
Evaluation engine: Diesel engine (exhaust amount: 2.2 liters)
Engine conditions: Shown in Table 3.
Evaluation pattern: PM regeneration→saturated NOx storage amount measurement
<Test Results>
According to the present invention, by overcoating DOC on the NSR, the NOx storage speed is remarkably improved compared with the conventional NSR alone and DOC/NSR in tandem.
Further, the NOx storage speed becomes the highest in the range of 25 to 60% of the NSR length of the lower layer of the overcoat ratio.
When the overcoat amount was near 30 g/liter, the NOx storage speed was the highest. The optimum overcoat amount is in the range of 25 to 35 g/liter centered about 30 g/liter. If converting this to the overcoat thickness with respect to an overcoat ratio of 55%, the optimum overcoat thickness is 20 to 40 μm or so.
According to the present invention, there is provided an exhaust gas purification catalyst which has a structure which prevents competitive adsorption of HC, CO, and NOx and enables effective utilization of an NOx storage reduction type catalyst.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2010/055774 | 3/24/2010 | WO | 00 | 11/19/2012 |
