Catalyst – solid sorbent – or support therefor: product or process – Miscellaneous
Reexamination Certificate
2001-06-04
2003-11-18
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Miscellaneous
C502S527120, C502S527130, C502S527190, C502S527210, C428S116000, C422S122000, C422S177000, C422S180000, C501S001000, C501S094000, C501S123000, C501S133000, C501S152000, C501S154000
Reexamination Certificate
active
06649563
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an exhaust gas purification catalyst for the purification of exhaust gas emitted from internal combustion engines of automobiles and the like and, specifically, it relates to a ceramic carrier which is ideal as a carrier for an exhaust gas purification catalyst in a lean burn engine or diesel engine, and to a ceramic catalyst body comprising it.
2. Description of the Related Art
“Three-way catalysts” have been widely used in the past for simultaneous purification of CO, HC and NO
x
emitted from automobiles. Recent years have brought a further demand for cleaner exhaust gas and reduced CO
2
emissions in order to protect the natural environment, and various “lean burn” systems have come into use to allow reduction in exhaust gas volumes through improved fuel efficiency. However, since conventional three-way catalysts have reduced NO
x
purification performance at the lean end (region of oxygen excess), the inherent performance cannot be exhibited; NO
x
storage reduction catalysts have therefore been developed to compensate for this problem. In addition to the precious metals, such as Pt and Rh, used for common three-way catalysts, NO
x
storage materials which store NO
x
in lean atmosphere conditions and release and purify stored NO
x
under stoichiometric (theoretical air/fuel ratio) to rich atmosphere conditions are added as cocatalysts, and the NO
x
storage materials are also used with highly basic alkali metals such as Na, K and Cs or alkaline earth metals such as Mg, Sr and Ba.
NO
x
storage reduction catalysts are described, for example, in Japanese Unexamined Patent Publication HEI No. 6-31139, which discloses a catalyst prepared by coating a porous body of &ggr;-alumina, etc. onto a honeycomb carrier composed of a ceramic such as cordierite, a low thermal expansion material which has excellent heat resistance, and loading an alkali metal oxide and Pt, thereby allowing reduction in NO
x
emissions under lean conditions. However, since the HC purification performance is lowered if the basicity of the alkali metal used as the NO
x
storage material is too strong, the NO
x
storage material is selected to match the desired performance.
On the other hand, exhaust gas temperatures have also been increasing in recent years, making it important to improve the high temperature durability of exhaust gas purification catalysts. Incidentally, catalysts with alkali metals loaded as NO
x
storage materials on cordierite carriers have been associated with the problem of reduced NO
x
storage capacity and cordierite carrier impairment under higher exhaust gas temperatures. This is attributed to the fact that the alkali metal easily penetrates into the porous coating layer of &ggr;-alumina and reacts with the Si in the cordierite; as a measure against this, Japanese Unexamined Patent Publication HEI No. 10-165817 proposes using a carrier made of a low thermal expansion material containing no Si, instead of a cordierite carrier.
However, of the &agr;-alumina, zirconia, titania, titanium phosphate, aluminum titanate, stainless steel and Fe—Al—Cr alloy mentioned as examples in Japanese Unexamined Patent Publication HEI No. 10-165817, only the very highly dense (heavy) aluminum titanate exhibits a sufficiently low thermal expansion coefficient for practical use. Aluminum titanate, however, is poorly suited given the trend toward lighter weight vehicles and its high cost increases the cost of the metal carrier. Other ceramic materials have high thermal expansion coefficients, and are also impractical from the standpoint of impact resistance. Thus, it is the current situation that no low-cost carrier material with a low thermal expansion coefficient exists as a substitute for cordierite.
Japanese Unexamined Patent Publication HEI No. 10-137590 discloses an exhaust gas purification filter wherein an alkali metal and an alkaline earth metal are carried on a coating layer comprising at least one from among silica, zirconia, titania and silica-alumina provided on a ceramic carrier, and it is stated that the coating layer inhibits diffusion of the catalyst components into the filter. However, research by the present inventors has shown that these coating layer materials produce compounds by reaction with the alkali metals and alkaline earth metals under conditions of approximately 800° C., which is the temperature at which exhaust gas purification catalysts are generally used. That is, under high temperature conditions of 800° C. and above, the alkali metals and alkaline earth metals react with the coating layer, while the excess alkali metals and alkaline earth metals diffuse to the interior reaching the filter surface, and can also react therewith. Thus, under the current situation in which the maximum exhaust gas temperatures can reach up to around 1000° C., it has been difficult to inhibit diffusion of alkali metals and alkaline earth metals into coating layers made of such materials.
SUMMARY OF THE INVENTION
It is an object of the present invention to realize ceramic carriers and ceramic catalyst bodies which have low cost and excellent high temperature durability, while not exhibiting reduced catalytic performance due to reaction with the alkali metals and alkaline earth metals carried as cocatalysts and used as NO
x
storage materials, and which can maintain the necessary catalytic performance over long periods when used as exhaust gas purification catalysts in lean burn engines.
According to a first aspect of the invention there is provided a ceramic carrier prepared by forming a diffusion-inhibiting layer which inhibits diffusion of the carried catalyst components on the surface of a ceramic honeycomb structure, wherein the diffusion-inhibiting layer is composed of a ceramic material which substantially does not react with the catalyst components under the temperature conditions of use and which has a melting point that is higher than the maximum temperature of use.
Since the diffusion-inhibiting layer substantially does not react with the catalyst components at the use temperature of the catalyst, the catalyst components do not diffuse into the diffusion-inhibiting layer. Thus, it is possible to prevent the diffused-catalyst components from reaching the surface of the ceramic honeycomb structure and reacting with it. Also, since the diffusion-inhibiting layer has a melting point higher than the maximum use temperature of the catalyst, there is no loss of diffusion-inhibiting effect by melting. Consequently, the ceramic honeycomb structure can be constructed of inexpensive and high temperature durable cordierite and can maintain its catalyst performance over long periods, so that it is ideal as an exhaust gas purification catalyst for lean burn engines.
According to a second aspect, the ceramic carrier is one with a thermal expansion coefficient of no greater than 1.5×10
−6
/° C. in the direction of flow. This improves the thermal shock resistance and reduces the risk of thermal shock damage even when used as an exhaust gas purification catalyst through which high temperature exhaust gas flows.
According to a third aspect, the melting point of the ceramic material composing the diffusion-inhibiting layer is 1000° C. or higher. Since the maximum temperature never exceeds 1000° C. during use as an exhaust gas purification catalyst, a ceramic material with a melting point of 1000° C. or higher will not exhibit a reduced function due to melting of the diffusion-inhibiting layer.
According to a fourth aspect, the ceramic honeycomb structure is a material with reactivity for the catalyst components, for example, a ceramic material containing Si, according to a fifth aspect. Ceramic materials containing Si readily react with catalyst components such as NO
x
occluders, and in such cases, providing the diffusion-inhibiting layer can prevent deterioration by the reaction. Specifically, by using inexpensive cordierite with a low thermal expansion coefficient as the ceramic honeycomb structure as
Andou Yosiyasu
Nakanishi Tomohiko
Yamada Masanori
Bell Mark L.
Hailey Patricia L.
Nippon Soken Inc.
Nixon & Vanderhye P.C.
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