Tungsten furnace, tubular

The products prepared in the above manner are ground as finely as possible and compressed under 2000 kg./cm into rods 3 x 40 or 5 X 40 mm. Successful molding usually requires the addition of 2-5% of metal powder. The rods are embedded in nitride powder (to prevent formation of an oxide coating) and presintered in a small tubular tungsten furnace (cf. Part I, p. 40) at about 2300°C in a stream of Ng the small amount of free metal is converted to nitride in the process. Since the reaction is usually accompanied by considerable shrinkage of the rods and concomitant appearance of porosity, the material must be repulverized, remolded and resintered. This procedure is repeated two to four times, until the presintered rods exhibit some constancy of density. 

Alonso and collaborators have reported the synthesis of CoMo, NiMo and NiW catalysts using tetraalkylammonium thiometallate precursors using either ex situ activation method under flow of H2/H2S or through in situ decomposition under dibenzothiophene hydrodesulfurization conditions (52-55). In situ activation led systematically to more efficient catalysts. However, a balance between structural carbon (see below) and the formation of carbon in excess blocking active sites must be optimized. This situation depends on the reducibility of the initial sulfide phase. Use of tetraalkylammonium precursors is then beneficial for M0S2-based systems, particularly promoted by Co or Ni. The situation is reversed for WS2-based catalysts due to the more difficult reducibility of tungsten sulfide. Alonso and co-workers have more recently developed a modified ex situ activation method, that generates active catalysts (56-58). The method consists of the partial hydrothermal decomposition of the precursor at 523 K followed by complete activation in a tubular furnace at 673 K. 

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