Niobium zinc compounds

The most common oxidation state of niobium is +5, although many anhydrous compounds have been made with lower oxidation states, notably +4 and +3, and Nb can be reduced in aqueous solution to Nb by zinc. The aqueous chemistry primarily involves halo- and organic acid anionic complexes. Virtually no cationic chemistry exists because of the irreversible hydrolysis of the cation in dilute solutions. Metal—metal bonding is common. Extensive polymeric anions form. Niobium resembles tantalum and titanium in its chemistry, and separation from these elements is difficult. In the soHd state, niobium has the same atomic radius as tantalum and essentially the same ionic radius as well, ie, Nb Ta = 68 pm. This is the same size as Ti ... 

Tantalum compounds in solution do not give a colour change on bring reduced with zinc, and this test also serves to establish niobium in the presence of titanium, which produces a violet coloration.2 Vanadium, molybdenum, and tungsten solutions, however, behave similarly to niobium, and these metals must, therefore, be previously removed. 

A number of groups have tested a wide range of promoter elements and compounds. Hutchings and Higgins [148] found that chromium, niobium, palladium, antimony, ruthenium, thorium, zinc and zirconium had very little effect on the specific activity of (VO)2P207. A significant increase in surface area was observed with zirconium, zinc and chromium, which could be of use as structural promoters. Iron-, cesium- and silver-doped catalysts showed a decrease in the specific activity, while cobalt and molybdenum were the only promoters found to increase the specific activity. 

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