Tantalates Reduction

Tantalum powder is produced by reduction of potassium heptafluoro-tantalate, K2TaF7, dissolved in a molten mixture of alkali halides. The reduction is performed at high temperatures using molten sodium. The process and product performance are very sensitive to the melt composition. There is no doubt that effective process control and development of powders with improved properties require an understanding of the complex fluoride chemistry of the melts. For instance, it is very important to take into account that changes both in the concentration of potassium heptafluorotantalate and in the composition of the background melt (molten alkali halides) can initiate cardinal changes in the complex structure of the melt itself. 

Other contributions to this book have taken a molecular view of parasitic nematodes, yet molecules make only a rather brief appearance here. This chapter has tried to show that parasitic nematodes are fascinatingly and tantalizingly diverse at a phenotypic level. It has focused particularly on diversity in phenotypes that are apparent in response to environmental conditions within or outside a host. The interaction of parasites with within-host factors is a major current research effort. However, helminth immunology is particularly notable for its inattention to diversity, especially when compared with the immunology of parasitic protozoa (Read and Viney, 1996). Observations of the interaction of host immunity with subsequent development in S. ratti show the potential power of such interactions. It is also clear that a principal mechanism of the action of host immune responses is against nematode fecundity (Stear et al., 1997). This is likely to be a molecularly complex interaction. Understanding this interaction, as well as variation in the interaction is interesting, but could also form the basis of control by transmission-reduction rather than eradication per se. 

H. Kato, A. Kudo, Photocatalytic reduction of nitrate ions over tantalate photocatalysts, Phys. Chem. Chem. Phys. 4 (2002) 2833-2838. 

Only one sulphide of tantalum, namely, the disulphide, TaS2, is known, and this is prepared by a dry method. Hydrogen sulphide does not precipitate any sulphides when passed into a solution of a tantal te, nor does reduction take place it is without appreciable action on tantalum pentoxide even at 1200° C.s... 

Likewise, on a molybedenum electrode methanol has been produced in >50 Faradaic efficiency at potentials between -0.7 and -0.8V vs. SCE. A single, but tantalizing report of both methane and ethylene production at copper electrodes has been claimed to occur between ca. -1.35V versus NHE ( 1). The mechanisms by which these reductions occur, although of extreme importance in this field, are unknown. 

One of the more extensively studied and discussed structural variables has been equilibrium out-of-plane distortion of the porphyrin. Out-of-plane porphyrin deformation typically results in a bathochromic (red) shift of the optical absorption spectrum and a shift to more negative reduction potentials (easier to oxidize). These tantalizing correlations, based upon studies of model complexes comprising porphyrin ligands such as octaethyltetraphenylporphyrinate (OETPP ), having sterically crowded peripheries that force large out-of-plane deformations, have driven... 

Alkali metal tantalates (ATaOs, A = Li, Na, and K) have been employed as photocatalysts for water splitting since 2001 [27]. Varying the A site alkali cations from Li" " to results in extension of light absorption (Fig. 4.6a) and red shifting of excitation and emission peaks (Fig. 4.6b). It is also worth noting that a reduction of the Stokes shift is observed following the increase of atomic number of the alkali... 

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