Catalyst – solid sorbent – or support therefor: product or process – Miscellaneous
Reexamination Certificate
2001-05-10
2004-03-23
Bell, Mark L. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Miscellaneous
C502S527110, C502S527190, C502S527230, C502S527240, C422S122000, C422S180000, C428S116000, C428S402000, C428S403000, C428S688000, C029S890000
Reexamination Certificate
active
06710014
ABSTRACT:
The present invention relates to a honeycomb body made of a ceramic material with improved radial pressure resistance.
Ceramic bodies with a honeycomb configuration are used in large quantities in the field of catalysis—especially in the field of catalytic purification of automobile exhaust gases. They serve as the substrates for the catalytic material, which is usually applied to the substrates in the form of a catalytically active coating. These types of catalysts are called coated catalysts below. The substrates are generally of cylindrical shape and feature two end faces and a cylindrical shell. From one end face to the other they are traversed by axially parallel channels, so-called flow channels, through which the exhaust gas to be purified is conducted. Such substrates are also called honeycomb bodies.
The catalytic material that serves for conversion of the hazardous substances contained in the exhaust gases (mainly hydrocarbons, carbon monoxide and nitrogen oxides) mostly consists of high-surface-area powdered materials onto which the actual active catalytic components are deposited in a highly dispersed form. This catalytic material is applied to the separating walls between the flow channels in the form of a coating. In order to coat the separating or channel walls with the powdered materials, initially a coating suspension is produced. In this process, the powdered materials are usually suspended in water. The total solids content (dry matter of the powdered materials) of the suspension customarily ranges, depending on the application, between 30 and 60 wt %, as compared to the total weight of the coating suspension.
The ceramic honeycomb bodies that currently are chiefly used in exhaust gas catalysis are produced by way of extrusion of ceramic masses. They feature square, rectangular or hexagonal flow channels with cell densities (number of flow channels per cross-sectional surface area) of 62 cm
−2
. The wall thickness of the channels in this design amounts to approximately 0.16 mm. The cylindrical shell that confines a honeycomb body usually has the same thickness as the channel walls. In certain designs, however, the cylindrical shell is made a bit thicker than the separating walls of the channels for better mechanical stability.
The freshly extruded honeycomb bodies are first dried. The unsintered bodies produced in this manner are then converted into the final honeycomb bodies by firing at temperatures of up to 1,500° C. (depending on the material). In order to make the honeycomb bodies mechanically sufficiently stable, the firing temperature, depending on the material used, is selected in such a way that the material sinters to form a stable ceramic material. In this process, the specific surface area of the ceramic material is reduced to less than 10, often even less than 2 m
2
/g. Commonly, these honeycomb bodies are also called inert, since due to their small specific surface area, they are themselves not suited as supports for catalytically active components and participate to only a negligible degree in the catalytic conversion of the harmful substances contained in the exhaust gases.
So-called extruded catalysts must be distinguished from the coated catalysts just described, since they consist, partly or entirely, of catalytically active material and do not have a special catalytically active coating. The material of these extruded catalysts is highly porous and features a high specific surface area. The temperatures used when firing these ceramic bodies range significantly below those used when firing inert honeycomb bodies. For this reason, these honeycomb bodies remain relatively soft even after firing. Due to their low mechanical stability, typical cell densities are limited to 5 cm
−2
in these honeycomb bodies. The thickness of the separating walls between the flow channels is usually about 1 mm.
In order to improve the catalytic conversion rate for harmful substances, inert honeycomb bodies are being developed at present with cell densities of up to 200 cm
−2
and wall thicknesses of only 0.1 mm or less. These high cell density honeycomb bodies provide a significantly larger geometrical surface for catalytic coating and, because of their lower mass, heat up significantly faster to the operating temperature of the catalyst.
The mechanical stability of high cell density inert honeycomb bodies, in particular their radial pressure resistance, is inferior to those of honeycomb bodies with conventional cell density, and this poses handling problems during catalyst production, in particular during installation into the converter housing. In order to increase mechanical stability, it is therefore currently being attempted to increase the wall thickness of the external layers of the flow channels that are adjacent to the cylindrical shell, by comparison to those in the center of the honeycomb body. These types of honeycomb bodies have been described, for example, in DE 199 02 540 A1. In the following, they will be called ‘non-homogeneous honeycomb bodies’, while conventional honeycomb bodies with homogeneous wall thickness—not counting an reinforced cylindrical shell—will be called ‘homogeneous honeycomb bodies’. It is difficult to extrude non-homogeneous honeycomb bodies with differing wall thicknesses.
It is known to reinforce the edges, against which the exhaust gas flows, of inert substrates for exhaust gas cleaning catalysts by depositing inorganic substances in or on them, in order to reduce wear through abrasion caused by particles contained in the exhaust gas (DE 195 47 599 C1 and DE 195 47 597 C1). According to these documents, the reinforcement is implemented across the entire cross section of the honeycomb body, extending from the end face of the honeycomb body against which the exhaust gas flows to a depth that can amount to up to 20 times the effective cell diameter.
It is an object of the present invention to provide ceramic honeycomb bodies for exhaust gas purification catalysts that feature a higher radial pressure resistance. The ceramic honeycomb bodies can be substrates for coated catalysts, already-coated substrates, i.e. coated catalysts, or extruded catalysts.
The above and other objects of the present invention can be achieved by a honeycomb body made of a ceramic material, of cylindrical shape with a first and a second end face and a cylindrical shell, which honeycomb body is traversed from one end face to the other by axially parallel channels that are formed by the channel walls and that are distributed over the cross section of the honeycomb body in a regular grid pattern, with an outer marginal zone of the honeycomb body, several channel diameters thick, that encloses a central area. A feature of the honeycomb body is that the ceramic material of the cylindrical shell and of the channel walls in the outer marginal zone of the honeycomb body is reinforced by depositing one or several inorganic substances on or in them, in order to increase mechanical stability.
With the term ‘channel diameter’ in this context, we refer to the diameter of a circle with the same cross-sectional area as a flow channel.
The invention is based on homogeneous honeycomb bodies, the separation walls between the flow channels of which are of identical thickness. For this reason, the honeycomb bodies can be produced using known techniques, and there are no production problems encountered either when drying or when firing them. The increase of radial pressure resistance is achieved by means of an after-treatment of the outer marginal zones of the honeycomb bodies. However, the invention is not limited to homogeneous honeycomb bodies. Rather, the after-treatment can also be applied to non-homogeneous honeycomb bodies, in which it causes a further reinforcement of their radial pressure resistance.
The honeycomb body is reinforced, for example, by soaking or impregnating the ceramic material of the cylindrical shell and of the channel walls in the exterior marginal zone of the honeycomb body with suitable reinforcing substances or by apply
Domesle Rainer
Kreuzer Thomas
Lox Egbert
Bell Mark L.
DMC2 Degussa Metals Catalysts Cerdec AG
Hailey Patricia L.
Kalow & Springut LLP
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