Reduction of Uranyl VI species by low oxidation state niobium ions

3 Reduction of Uranyl(VI) species by low oxidation state niobium ions 

Under certain conditions even higher oxidation state metal ions are capable of reducing uranyl(VI) species. The reduction is possible when, for example the added metal species has higher affinity for oxygen than uranium(VI). In the presence of beryllium(II) chloro-species uranyl ions are unstable in chloride melts due to formation of very stable insoluble beryllium oxide [28]. Since uranium(VI) does not form stable purely chloride species at elevated temperatures, it decomposes to uranium(IV) chloro-species and chlorine. Zirco-nium(IV), hafnium(IV) and niobium(V) chlorides, despite their quite high affinity for oxygen, do not react with uranyl(VI) species in alkali chloride melts even at 750 °C and when their concentration exceed that of uranium by two orders of magnitude. 

The reaction of metallic tellurium, palladium, silver, molybdenum, niobium, zirconium and gaseous hydrogen with alkali chloride melts containing uranyl(VI) chloride leads to its reduction to uranyl(V) chloride and uranium dioxide. In the case of Nb and Zr some uranium(IV) chloride is also formed. Nb(III) and (IV) ions also reduce uranyl(VI) to uranyl(V) and uranium(IV) species and UO2. 

The degree of U(VI) conversion to UO2 increases with temperature and also on going from NaCl-2CsCl to LiCl melt. Results obtained in various alkali chloride based melts indicate that the melts in equilibrium with uranium dioxide contain a mixture of uranyl(VI) and (V) species or at low uranium concentrations essentially only uranyl(V) ions. 

and Steunenberg, R.K. (1964) Formation and spectra of uranyl(V) chloride in molten chloride solvents. Inorg. Chem, 3(7), 989-992. 

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