Terbium oxidation-reduction

A common feature of almost all of the known phases in the cerium, praseodymium and terbium oxide systems is the common a-axis which corresponds to the l/2 llV p vector (see Table 3-3). The odd members of the series, M 02n-2m) even exhibit a common c-axis which is a vector l/2[112]p. However the phase M62O112 has a different orientation for the a-axis of the supercell with respect to fluorite and modelling studies argue that these phase has divacancies like Pr70i2 [9]. The phase can be observed in thermoanalytical runs during oxidation of Pr70i2 but not in reduction on the higher temperature branch of the hysteresis loop. 

Domain Formation and Interfaces in Praseodymium and Terbium Oxides Obtained by Reduction of Pr02 and Tb02 A Comparison with Group Theory Predictions, C. Boulesteix and L. Eyring, J. Solid State Chem., 66, 125-135 (1987). 

Only cerium has a higher state that is stable in solution, Ce4 corresponding lo the removal of all four valence electrons. The +4 state is a powerful oxidant, with a reduction potential to the + 3 state that is very dependent upon the acid in which it is dissolved (e.g. in sulfuric acid the reduction potential is +1.44 V, in nitric(V) acid +1.61 V, and in chloric(VTT) acid + 1.70 V, indicating some ion pair formation in the first two examples). Praseodymium and terbium have +4 oxides, but these dissolve in acidic solution to give the corresponding + 3 states and dioxygen. 

Terbium occurs in apatite and xenotime and is derived from these minerals as a minor coproduct in the processing of yttrium. Processing involves organic ion-exchange or solvent extraction operations. Elemental terbium is produced by calcium reduction of anhydrous TbH in a reactor under an inert atmosphere. Both the oxides and the metal aie available at 99.9% purity. 

The values for the redox potential for the couple M3 + /M2+ have been estimated57 using a simple ionic model and available thermodynamic data. The results (Table 2) correlate closely with the ionization potentials for the M2+ ions, and are in good agreement with both chemical observations and other estimates obtained by spectroscopic correlations. Irreversible oxidation of terbium(m) to terbium(iv) in aqueous K2C03-K0H solutions has been observed electrochemically 58 the discovery of an intermediate of mixed oxidation state explains partly the reduction behaviour of terbium(iv) deposits. Praseodymium(iv) and terbium(iv) have also been detected in nitrate solutions. 

The formation temperatures and the thermal stabilities of the rare earth sulphates, oxides, and oxide sulphates have been investigated. The thermal decomposition of the chromates of several tervalent lanthanides (praseodymium, gadolinium, terbium, dysprosium, holminum, erbium, and ytterbium) has been found to occur with loss of oxygen and reduction of Cr to Cr by a mechanism involving electron transfer from the co-ordinated oxygen to chromium. ... 

The problem is not symmetrical. Oxidation of the reduced state is generally facile and nonactivated. Adsorption of O2 is exothermic and, on most reduced oxides, occurs readily at low temperature. Reduction of the oxidized state, on the other hand, is temperature dependent and much more difficult. It is here that cerium oxide and, perhaps, the oxides of terbium and praseodymium, excel. Cerium oxide, however, has advantages in terms of cost and availability. It was first introduced as an oxygen storage component in 1981 and has been an essential part of the TWC catalyst ever since. 

In-situ Oxidation and Reduction of the Oxides of Cerium, Praseodymium and Terbium by High Voltage Electron Microscopy, E. Schweda, Z.C. Kang and L. Eyring, J. Microscopy, 145,45-54 (1987). 

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