Chemistry of inorganic compounds – Carbon or compound thereof – Binary compound
Patent
1990-11-27
1994-05-03
Bell, Mark L.
Chemistry of inorganic compounds
Carbon or compound thereof
Binary compound
423444, 501 87, C01B 3134
Patent
active
053085973
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD OF THE INVENTION
The invention relates to the production of metal carbides with a high specific surface area and stable even at high temperature and the activation of the said surface particularly when they are used as a catalyst in chemical reactions.
STATE OF THE ART
High temperature catalysis is in particular used in the petrochemicals industry (for example reforming . . . ), catalytic exhaust converters or any other high temperature chemical reaction.
On the one hand, it is interesting to be able to conduct catalytic reactions at the highest possible temperatures and on the other to be able economically to regenerate the catalysts with or without their carrier for repeated uses. With such an application in mind, it is necessary for the catalyst and/or its carrier not to be damaged when they are subjected to high temperatures. From this point of view, aluminia with a large surface area is not satisfactory. That is why the Applicants have already perfected a process for obtaining a catalyst carrier consisting of silicon carbide with a high specific surface area, doped or otherwise, on which the catalyst element is deposited (EP 88-420352-2).
Continuing their research they then sought to obtain carbides of metals such as, for example, Mo, W, Ro, V, Nby Ta, Ti . . . of high specific surface area, possibly in excess of 200 sq.m/g. These carbides likewise offer considerable interest as a high temperature catalyst carrier but in contrast to a non-metallic element such as Si, they exhibit catalytic properties as such, or they may exacerbate or modify the properties of the catalytic active phases (for example of Pt) which are deposited thereon. It is quite obvious that carbide powders having such a surface area may likewise be used for other applications such as, for example, the production of sintered parts having unprecedented properties, by means of simplified processes.
A process is already known (Journal of Catalysis Vol. 106, pp. 125-1333 1987) for obtaining Mo.sub.2 C carbide of a specific area which does not exceed 100 sq.m/g on the basis of a solid MoO.sub.3 oxide and a CH.sub.4 :H.sub.2 gaseous mixture according to a temperature programmed reaction which must take into account the value of the H.sub.2 /CH.sub.4 ratio, a reaction which therefore calls for special attention if one is to avoid the formation of polymeric carbon which is a catalyst contaminant and which might possibly have to be destroyed by a special process. Furthermore, this fairly complex method does not lend itself readily to industrial production of a non-contaminated product.
Furthermore, the document Journal of Catalysis Vol. 112, pp. 44-53, 1988, while pointing out the need to have Mo.sub.2 C carbide which has a specific surface area of more than 200 sq.m/g suggests a method of obtaining purely an alpha-MoC.sub.1-x carbide with a specific surface area greater than 200 sq.m/g. This method is also more complex than that previously mentioned since it passes through an intermediate stage of producing a nitride which is then converted to the carbide likewise by a temperature programmed reaction.
OBJECTS OF THE INVENTION
Faced with these difficulties, the Applicants have sought to perfect a simple process which could be used for large scale production of metal carbides of high specific surface area, that is to say always greater than 20 sq.m/g or better than 50 sq.m/g but generally far greater than 100 sq.m/g, such a carbide possibly being in the form of granules or shaped pieces.
Another object of the invention is to provide heavy metal carbides which can be used preferably either as a catalyst carrier or as a catalyst used as such or deposited on a carrier; other applications of these carbides are likewise possible in other fields, for example sintering.
Another object is to obtain mixed carbides of heavy metals of high specific surface area which may offer significant advantages if used in catalysis. Thus it is possible to obtain shaped pieces or grains having a core of a different chemical nature from that of
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patent: 4216034 (1980-08-01), Miyake et al.
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patent: 4784839 (1988-11-01), Bachelard et al.
Guille Jean-Louis
Ledoux Marc-Jacques
Marin Sophie
Pham-Huu Cuong
Bell Mark L.
Pechiney Electrometallurgie
Wright A.
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