Niobium species

It should be noted that the silylhydrides 106 109 were the only products of the reaction shown in Eq. (30). The isomeric agostic species, related to the niobium species 64 and 65 were not identified, and all attempts to convert 106-109 into these or other rearranged species at elevated temperatures resulted in decomposition. 

The Raman investigation of niobium species in aqueous solutions of niobium oxalate (Jehng and Wachs, 1991) nicely showed the dependence of their constitution on pH and concentration. The PZC theory was successfully applied to predict the hydrated, molecular structures of multicomponent supported metal oxide species, such as iron-molybdenum, iron-vanadium, molybdenum-vanadium, tungsten-vanadium, and sodium-vanadium oxide species (Vuurman et al., 1991 Wachs et al., 1993). 

The electrodeposition of Al-Nb alloys has been reported in a BPCI-AICI3 ionic liquid containing NbCls or Nb3Clg [60]. A pentavalent niobium species is reduced... 

Niobium occurs in oxidation state (V) in the dissolver solution and does not extract into pure TBP/OK phases to a significant extent. Values of ca. 5x10 have been quoted for Dn6 between 20% TBP/OK and 2 M HNO3. Inextractable polymeric hydroxy complexes of Nb are formed but the presence of silicates or HDBP can lead to some extraction of niobium species. A model for the extraction by HDBP is provided by HELP, which was thought to extract Nb according to equation (159) on the basis of IR studies. HDBP might similarly lead to some synergic extraction of Nb " in TBP/OK but, as mentioned above, free HDBP will not be available in the first process cycle,and in practice problems do not usually arise with niobium. 

The thermodynamic data determined in the present work, together with the enthalpy measurements in progress, will make it possible to get more insight into the stability of niobium species and correlatively to obtain new routes leading to the optimization of electrochemical processes. 

Linear sweep voltammetry following niobium dissolution shows an electroactive species which can be reduced on a tungsten electrode at about l.OV. The exact nature of this cathodic reaction is presently unclear. Although experiments to date suggest that the reduction is that of a niobium species in solution, attempts to produce pure niobium deposits at potentials between 0 and l.OV have been unsuccessful. 

The enhanced performance of the Nb-containing catalyst has been attributed to the presence of the Bi3WiNb9O30 phase in which active bismuth and tungsten sites are isolated from each other by low active niobium species. Poor crystallization has not allowed the relation between behavior and phase composition for the Al-promoted catalyst to be traced. It should be emphasized that the addition of molybdenum and antimony, which are present in many selective oxidation... 

In situ spectroscopy measurements were performed in NaCl-KCl-based melts during electrochemical reduction and oxidation of niobium species. Glassy carbon rod, tungsten wire or niobium plate were used as working electrodes in the spectroelectrochemical experiments. A molybdenum wire dipped into a NaCl-CsCl-PbCl2 melt served as a counter electrode and a silver wire in a NaCl-CsCl-AgCl (1 mol %) melt acted as the reference electrode. 

Reaction of niobocene dichloride with Grignard reagents afforded not the expected organo-niobium species but hydride olefin complexes, probably... 

XPS results indicate the presence of niobium species in the framework of the NbMCM-41 support at +5 oxidation state (BE 208.7 eV), which does not change after the modification with the Sb-V-Ox phase. Vanadium gives many components in the XPS spectrum in the range of 516 - 524 eV, which covers various oxidation states of vanadium species. It is in line with the detection of SbV04, SbVOs and V2O5 phases. 

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