Surface area development, carbide reaction

A novel method of preparation of ultrahigh surface area carbides has been developed by the group of Ledoux It involves the reaction of solid carbon with vaporized metal oxide precursors such as Mo03 or W02 [58] The synthesis uses high specific surface area-activated carbons and the final product appears to retain a memory of the porous structure of the starting matenal The carbon acts as a skeleton around which the carbides are formed Catalytically active carbides with surface areas between 100 and 400 m2g 1 are generated. 

Early transition metal carbides with high surface area are active catalysts for various reactions such as hydrodenitrogenation(HDN) [1,2], hydrogenation [3,4], Fischer-Tropsch synthesis [5,6], hydrocarbon isomerization [7]. Synthesis of these materials has attracted great attention over the years and a number of procedures have been reported for the preparation of carbides with high surface areas suitable for catalysts. The most commonly employed method is temperature programmed reaction of metal oxide with a mixture of hydrogen and methane, which was developed by Boudart and co-workers [8-10]. 

Temperature-Programmed Reactions. The Boudart group developed temperature-programmed reaction (TPRe) methods to synthesize carbides and nitrides of molybdenum and tungsten with very high surface areas suitable for... 

The carbides and nitrides of the early transition metals, vanadium, niobium, and molybdenum, are known to possess good catalytic properties. The compounds are synthesized by a temperature programmed reaction (TPR) method where a reactive gas is reacted with a precursor oxide as the temperature is uniformly increased. Results under similar reaction conditions are presented to compare the progress of the reaction, the formation of intermediate phases, and the development of surface areas. The increase in surface area is influenced by the phenomena of pseudomorphism and topotaxy. It is believed that pseudomorphism, found in all of the above syntheses, is associated with the development of internal pores, while topotaxy, found in some of the nitrides, maximizes this process to yield hi surface area products. 

Catalytic materials need high exposures or specific surface areas to be used economically. This has led to development of novel preparative methods, such as temperature programmed reaction (TPR) (d), laser pyrolysis (7), etc., to produce finely divided transition metal carbides and nitrides. Among these, TPR remains a practical method for making large batches with moderate to high surface area products. 

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