Surface area development, carbide

Medium surface area silicon carbide (grains 0.4-1 mm) used as support was synthesized at the CRV-Pechiney, according to the Shape Memory Synthesis developed by Ledoux and co-workers, involving the gas-solid reaetion between SiO vapours and solid carbon under dynamic vacuum around 1200-1300°C [11-13]. A calcination was then performed at 700°C for 3 h to stabilize the textural eharacteristics and to bum of the remaining unreaeted carbon. No microporosity was present in the SiC. 

Nevertheless, currently the development of Pt iL on WC with good Pt uniformity remains a big challenge. To overcome this issue, one can look for a method by which PtML can be deposited chemically on high surface area metal carbides in order to mass produce PtMu/WC catalyst in a cost-effective manner. 

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]. 

Calcines are products obtained by removing the volatile components of the waste, i.e., water and nitrate, at temperatures between 400 and 900° C. The result is a mixture of oxides of fission products, actinides, and corrosion products in particulate form with a specific surface of 0.1 to 5 ra /g. The plain calcine is not very stable chemically because of its large surface area and the chemical properties of some of the oxides, and it is highly friable. To improve the properties of calcines, advanced forms are developed. One such product is the so-called multibarrier waste form, a composite consisting of calcine particles with inert coatings, such as pyrocarbon, silicon carbide, or aluminum, embedded in a metal matrix. Another advanced calcine is the so-called supercalcine. This is essentially a ceramic obtained by adding appropriate chemicals to the HLW to form refractory compounds of fission products and actinides when fired at 1200°C. Supercalcine requires consolidation by embedding in a matrix but does not need to be coated, as the material is supposed to have inherent chemical stability. 

In order to take advantages of the high stability and metal-support interaction properties of metal carbides, it is necessary to develop them with high surface areas... 

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. 

This chapter presents a review of the synthesis of vanadium, niobium, and molybdenum carbides and nitrides by the TPR method. Factors responsible for development of surface area during transformation are discussed. 

In this section the salient features of carbide and nitride synthesis will be presented first. The role of ammonia and methane/hydrogen as reductant in the development of surface area will be discussed next, with references to the significance of pseudomorphic and topotactic phenomena in these transformations. 

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