Catalyst – solid sorbent – or support therefor: product or process – Miscellaneous
Reexamination Certificate
1999-06-02
2001-10-16
Wood, Elizabeth D. (Department: 1755)
Catalyst, solid sorbent, or support therefor: product or process
Miscellaneous
C502S002000, C502S527120, C502S527130, C502S527140, C502S527150, C502S527160, C502S527190, C502S527240, C502S305000, C502S319000, C502S320000, C502S325000, C502S326000, C502S327000, C502S332000, C502S333000, C502S336000, C502S338000, C502S339000, C502S355000
Reexamination Certificate
active
06303538
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a metallic fiber board having catalytic functionality and to a process for making the same. Particularly, the invention relates to a process for the production of a catalyst-bearing metallic fiber board for catalyzing the combustion of hydrocarbons and suitable to be used in different hydrocarbon burning devices such as household boilers.
Household boilers produce heat through the combustion of hydrocarbon gases, though only methane has commonly been used in recent years. In traditional boilers, in order to achieve complete combustion of methane to prevent the products of incomplete combustion, such as CO, from being released into the atmosphere, temperatures of at least about 800° C. are required. Such high temperatures, however, allow undesirable nitrogen oxide compounds to form, generally referred to as NO
x
, that cause serious pollution problems when emitted into the atmosphere. Various systems have been studied to lower the necessary temperature for complete combustion The lowest temperatures at which the complete combustion of methane has been obtained have required the use of catalysts, and these catalytic systems have successfully eliminated the formation of NO
x
compounds.
Generally, the catalysts in these systems are provided on a metallic or a ceramic substrate. Such substrates need to have very open structures in order to allow the air/methane mixture to pass easily. For instance, wire meshes, metallic or ceramic bodies provided with holes, and boards or meshes made of metallic or ceramic fibers have all been used. An article entitled “Performances of Premixed-Air Fibrous Burners with a Metallic and Ceramic Matrix” by Accornero et al., published in “CH
4
Energia Metano”, 1995, No.2, pages 20-33, discloses, for example, systems for lowering the methane combustion temperature involving the use of a fiberous catalyst formed into a board. The fibers in this example are produced from a Fe—Cr—Al alloy coated with an aluminum oxide (alumina) layer on which a catalytic layer of lanthanum manganite, LaMnO
3
, is depositted. The article, however, gives no account of how the LaMnO
3
layer is prepared.
The patent application WO 97/02092 discloses another catalytic system obtained from a perforated foil or a metallic grid on which a porous ceramic layer, preferably of alumina or zirconia, is deposited by means of techniques including plasma spraying, flame spraying, and detonation spraying. The ceramic layer is then impregnated with a catalyst precursor solution or suspension. After suitable thermal treatments the final catalytic system is obtained. The production process described by this application, however, is disadvantageous in that only the side of the metallic substrate facing the spraying nozzles can be coated with the ceramic layer. Consequently, coating two opposite sides of a substantially flat body, such as a perforated foil, requires two successive coating phases. Further, the method is unworkable when applied to fibrous boards because the sprayed ceramic tends to adhere to the first contacted surface, namely the most external fibers, and does not effectively coat the interior fibers.
What is desired, therefore, is a board comprised of metallic fibers having catalytic functionality, as well as a process for producing such a board.
SUMMARY OF THE INVENTION
The present invention provides a metallic fiber board having catalytic functionality and a process for making the same.
An embodiment of the present invention is a metallic fiber board comprising a plurality of metallic fibers, wherein the fibers are coated with a first layer, a second layer, and a catalyst. The first layer comprises a dense oxide covering the surfaces of the fibers. The second layer comprises a porous oxide covering the exterior surface of the first layer. The catalyst is a noble metal selected from the group of elements comprising the eighth group of the periodic table and is distributed across the exterior surface of the second layer. In another embodiment of the present invention the second layer is instead impregnated with the catalyst rather than coated with it.
The present invention is advantageous for several reasons. The fiber board itself provides a convenient and commercially available substrate that may readily be formed into shapes to fit within different boilers. The fiber board also provides a large surface area that may support a larger quantity of a catalyst then could be supported, for example, on a perforated sheet. The fiber board further provides a very open structure that allows gases to pass easily. The first layer of a dense oxide is advantageous for providing good refractory properties. In some embodiments the first layer and the second layer are formed of the same oxide, thereby providing better adherence between the two layers. The second layer, being porous, is advantageous for providing even greater surface area to the fiber board for supporting the catalyst.
An embodiment of the process for making a metallic fiber board with catalytic functionality includes providing a metallic fiber board, thermally treating the fiber board to oxidize the metallic fibers and thus produce a first layer of an oxide, forming a second layer of a porous oxide over the first layer, and forming a catalyst on the surface of the second layer. In this embodiment the second layer is formed by preparing a solution of a precursor compound in a solvent, directing an atomized spray of the solution at the fiber board, and thermally treating the fiber board to decompose the precursor to form the desired oxide. Additional embodiments are directed towards an iterative process whereby the spraying of the solution is performed as a succession of brief spray operations separated by pauses.
In some embodiments the catalyst is similarly formed by preparing a solution containing a catalyst precursor, spraying the atomized solution onto the fiber board, and thermally decomposing the precursor to form the desired catalyst. Further embodiments are directed to performing the spraying process in the same iterative manner described for forming the second layer. Still other embodiments involve forming the catalyst by preparing a solution containing a catalyst precursor, dipping the fiber board into the solution, withdrawing the fiber board, and thermally treating the board to decompose the precursor into the catalyst. Yet another embodiment is directed towards preparing a solution containing precursors for both the second layer oxide and the catalyst, spraying the atomized solution onto the fiber board, and thermally treating the board in order to decompose both precursors together to create a second layer impregnated with the catalyst.
These embodiments of the present invention provide advantages over prior art methods for producing fiber boards with catalytic functionality. Techniques of the prior art such as plasma spraying and flame pyrolisis tend only to coat the topmost fibers and only those surfaces that are directly exposed to the spray nozzles. The present invention allows for a much more complete coating of substantially more fibers, thus imparting significantly more catalytic functionality to the fiber board than would otherwise be possible. Further, these embodiments allow for the production of catalytic fiber boards with complex geometries, such as hemispherical caps. The embodiment in which the second layer is impregnated with the catalyst is further advantageous for reducing the total number of processing steps and the total processing time necessary to produce a catalytic fiber board.
Additional advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description of the preferred embodiments, the examples, and the figures.
REFERENCES:
patent: 3920583 (1975-11-01), Pugh
patent: 4196099 (1980-04-01), Hunter et al.
patent: 4601999 (1986-07-01), Retallick et al.
patent: 4891350 (1990-01-01), Chopin et al.
patent: 5232882 (1993-08-01), Yoshimoto et al.
patent: 5401483 (
Bianchi Claudia L.
Boffito Claudio
Ragaini Vittorio
Toia Luca
Vitali Stefania
Oppenheimer Wolff & Donnelly LLP
SAES Getters S.p.A.
Wood Elizabeth D.
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