1. Field of the Invention
The present invention relates to a needle-shaped ceramic body and a needle-shaped catalyst-ceramic body, and more particularly to a needle-shaped honeycomb catalyst-ceramic body capable of being preferably used at sites exposed to air flow of high temperatures in excess of 800° C. and high-speed such as three-way catalysts to be used in automobile for removing NOx, combustion catalysts to be used in gas turbine, and catalysts to be used for purification of high-temperature gas.
In the field of production technology for oxide-based honeycomb structures for supporting three-way catalysts to be used for automobile, the present invention is based on the strong need for the development of a cordierite porous body having a high specific surface area in which decrease in specific surface area caused by sintering can be suppressed even if exposed to high temperatures for long periods of time, and the present invention is useful for providing a needle-shaped ceramic body having as its base material a cordierite porous body that has a high specific surface area in which decrease in specific surface area caused by sintering can be suppressed even if exposed to temperatures in excess of 800° C. for long periods of time, processes for producing a novel cordierite porous body that enables the production of a honeycomb structure for supporting a catalyst formed directly with the cordierite porous body, and products thereof.
The present invention is characterized in that a porous cordierite composed of a porous structure in which needle-shaped crystals are linked three-dimensionally is used particularly as a honeycomb structure for supporting a catalyst, and thereby, the present invention can realize a needle-shaped ceramic body having a cordierite porous body as its base material, in which decreases in specific surface area caused by sintering can be suppressed, a honeycomb body with a cordierite sintered compact itself can be produced, the conventional process in which inside of a honeycomb structure is coated can be omitted, and also low-cost production processes thereof can be provided.
2. Description of the Related Art
Oxide-based honeycomb structures for supporting a catalyst have already achieved practical application in the sites such as, for example, three-way catalysts to be used for automobile and combustion catalysts to be exposed to high temperatures for long periods of time, and developments aiming the further improvement in their properties are aggressively progressed. In particular, since cordierite has a high melting point on the order of 1400° C., an extremely low coefficient of thermal expansion, and superior thermal shock resistance, its honeycomb structure is used at locations to be subjected to high temperatures in excess of 800° C. as a support for catalysts such as for example, three-way catalysts for automobile, combustion catalysts for gas turbines, and catalysts for high-temperature gas purification.
In this manner, although the usefulness of the cordierite as a catalyst support has been recognized, it was difficult to produce a cordierite porous body having a high specific surface area and thermal stability by the conventional methods for producing cordierite porous body. Consequently, catalyst-ceramic body loaded with a precious metal in which the cell wall surface of the ceramic support composed of a highly thermal shock resistant cordierite honeycomb structure is coated with gamma alumina have been widely used in the prior art as a catalysts for exhaust gas purification. This formation of a coating on the surface is because the specific surface area of the cordierite is too small for loading the cordierite with the required amount of catalyst components. Consequently, materials having a large specific surface area like the gamma alumina are used to increase the surface area of the ceramic support.
However, coating of the cell wall surface of the ceramic support with gamma alumina leads to an increase in thermal capacity due to the increase in weight. Although studies have been conducted in recent years to lower thermal capacity by reducing cell wall thickness of the support in order to achieve early catalyst activation, a formation of a coating on the surface diminishes the effect thereof considerably, and therefore, solution of these problems is requested as an important issue. In addition, the coating the surface with gamma alumina also had caused the problems that pressure loss of the support increases since the opening surface area of each cell is decreased, the coefficient of thermal expansion of the support increases as compared with that in the case of the cordierite only, and gamma alumina shifts to alpha alumina at high temperatures of 1000° C. or higher in which sintering progresses, thereby it is difficult to maintain a high specific surface area thereof.
The inventors of the present invention had previously succeeded in developing a cordierite porous body composed of needle-shaped crystals of cordierite having a submicron diameter, and proposed honeycomb structures using a cordierite porous body directly, and honeycomb structures in which the inner walls of a cordierite porous body was coated as honeycomb structures composed of a cordierite porous body (see Japanese Laid-open Patent Publication Nos. 2003-321280, 2003-212672, 2003-025316, 2002-355511, 2002-119870, 2002-172329, 2001-310128 and H11-171537/1999). In the case of using the cordierite at a site to be exposed to high temperatures, there have been no methods other than coating the inner walls of a honeycomb structure with gamma alumina and the like.
Consequently, various studies have been conducted on ceramic bodies capable of supporting catalyst components without forming a coating layer. For example, a method has been proposed in which the specific surface area of a cordierite itself is improved by an acid treatment and a followed heat treatment (see Japanese Patent Publication No. H05-050338/1993). However, this method was not practical since the strength of the cordierite is decreased due to destruction of the crystal lattice of the cordierite caused by the acid treatment and the heat treatment.
Therefore, the inventors of the present invention previously proposed a ceramic support capable of supporting a required amount of catalyst components without forming a coating layer in order to improve specific surface area (see Japanese Laid-open Patent Publication No. 2003-080080). This ceramic support is that one or more types of elements that constitute the base ceramic with an element other than a constituent element are substituted, and by immersing this ceramic support in a solution of a precious metal compound such as hexachloroplatinic acid, platinum (II) chloride or rhodium chloride and by followed burning thereof, a precious metal catalyst can be loaded directly on the substituent element. Accordingly, this support has higher strength and improved durability as compared with that of supports of the prior art in which vacancies are formed by carrying out the acid treatment and the heat treatment. In addition, a catalyst-ceramic body has been proposed as a ceramic support capable of direct loading of catalyst components, the catalyst-ceramic body has a resistance to thermal deterioration which is produced by loading a primary catalyst first and then loading a promoter in the direct loading of a primary catalyst component and promoter component on the surface of a support (Japanese Laid-open Patent Publication No. 2003-230838).