1. Field of the Invention
The present invention relates to a purification catalyst for the exhaust gas from an internal combustion engine of an automobile and so forth.
2. Description of the Related Art
Catalytic converter systems are used as systems for purifying the exhaust gas emitted from automobile engines and other internal combustion engines. These catalytic converter systems are provided with a carrier, on which is loaded a precious metal such as platinum or rhodium in the form of a catalyst, in the exhaust pipe, that purifies HC, CO, NOx and so forth present in exhaust gas by an oxidation reaction or oxidation-reduction reaction.
This catalytic converter system uses a so-called monolithic carrier, typically comprising a honeycomb-shaped ceramic carrier having a large number of cells (holes), as the base material, and a catalytic layer containing catalyst components is loaded on the inside of the cells of the monolithic carrier.
In addition, in monolithic carriers of the prior art, it was typical to employ a square shape for the shape of the cells. In monolithic carriers having this type of square-shaped cells (square cells), various technologies have been proposed for improving the purification efficiency of exhaust gas.
For example, in the catalysts loading process, since it is common to use an aqueous slurry that contains the catalyst components, it is preferable to increase the moisture absorption of the partition walls. Consequently, the use of a monolithic carrier having increased partition wall porosity and increased moisture absorption is currently being examined.
In addition, much research has also been conducted on the thickness of the catalyst layer and catalyst performance. For example, Japanese Examined Patent Publication (Kokoku) No. 53-149886 proposes that the use of a mean catalyst layer thickness of 50-300 xcexcm yields a catalyst having long life and which is active at low temperatures.
In addition, Japanese Unexamined Patent Publication (Kokai) No. 62-74453 proposes that, when forming two wash coat layers, making the total film thickness 30-70 xcexcm improves purification performance and demonstrates rapid activity at high SV (space velocity) values.
However, all of the above-mentioned prior art involves the case of loading catalysts on a monolithic carrier having square cells.
On the other hand, triangular cells, hexagonal cells and other cell shapes have been proposed instead of square cells. Since hexagon-shaped cells (hexagonal cells) in particular more closely approach a circle than rectangular cells in geometrical terms, loading of the catalyst layer can be made to be more uniform than in the case of square cells.
Consequently, the use of monolithic carriers having the above-mentioned hexagonal cells has attracted attention in use as a purification catalyst able to accommodate increasingly strict emissions regulations in recent years.
However, in the case of using a monolithic carrier having the above-mentioned hexagonal cells, there have yet to be proposals made regarding the optimum loaded state of the catalyst layer.
In consideration of the above-mentioned problems of the prior art, the object of the present invention is to provide a purification catalyst for exhaust gas using a hexagonal cell monolithic carrier that is able to demonstrate purification performance that is superior to the case of using a square cell monolithic carrier.
The present invention is a purification catalyst for exhaust gas of internal combustion engines comprising: a monolithic carrier provided with hexagonal cells of 200 cells/in2 or more in which the porosity of the partition walls that form said cells is 25% or more; and, a catalyst layer arranged on the surface of said partition walls of said monolithic carrier that contains catalyst components for purification of exhaust gas; wherein, the thickness of said catalyst layer is 10-70 xcexcm at its thin portion, and its thick portion has a thickness of no more than 12 times that of said thin portion.