Praseodymium oxidation states

In conclusion I should like to consider a few of the chemical investigations which might be accomplished in the rare earth field by Mossbauer spectroscopy. The study of nonstoichiometric oxides has been discussed earlier, but there is the problem of finding an appropriate doping nuclide for the praseodymium oxide system. The element most capable of following the changes in oxidation state of the praseodymium is terbium-159, which does have a Mossbauer state, however, with a rather broad resonance (58,0 k.e.v., = 0.13 nsec.). Nevertheless, with a sufiiciently... 

Since the main oxidation state of praseodymium is +3, most of its stable compounds are built by the Pr ion. A major example follows ... 

Ives et al. (79) tended to reject our hypothesis that brown colours of mixed oxides (and in particular less pure NdaOs) are due to traces of praseodymium. However, these authors noted the interesting effect that such dark colours (also of Pro,oaTho.9802) bleach in the reflection spectrum at higher T. It was noted that mantles of NdaOa alone rapidly hydrate to a pinkish powder (carbonate ) in humid air. It is weU-known that -type sesquioxides are far more reactive, and for instance dissolve almost instantaneously in aqueous acid, than cubic C-type samples. Ives et al. 19) also studied the broad continuous spectrum of the orange light emitted from Thi- 11 0 2+2/ where the oxidation state of uranium is rather uncertain. 

The so-called sesquioxide (PuOi 5 i.75) is a typical mixed oxidation state oxide, similar to those formed by uranium, praseodymium, terbium, titanium, and many other metals. Its composition shows continuous variation with changes in temperature and pressure of oxygen above the oxide. 

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 lanthanides, unlike the transition metals and the actinides, tend not to form compounds over a range of oxidation states. The +3 oxidation state is characteristic of all of the lanthanides, and the oxide fluorides of formula LnOF (Ln = lanthanide metal) are well known. The less stable oxidation states of + 2 and + 4 are known, but the latter is represented only by the dioxides and tetrafluorides of cerium, praseodymium, and terbium, and no tetravalent oxide fluorides have been reported. 

This oxidation state is very important for cerium and of minor, though not negligible, importance for praseodymium and terbium. With the possible exception of a few fluorides, for example, Cs3LnF7 for Nd and Dy, no other Ln compounds are known. 

These elements are usually terpositive, forming salts such as La(N03)g 6H20. Cerium forms also a w ell-defined series of salts in which it is quadripositive. This oxidation state corresponds to its atomic number, 4 greater than that of xenon. Praseodymium, neodymium, and terbium form dioxides, but not quadrivalent salts. 

The mixture of these effects described above is obtained by the doping of rare earths elements with variable oxidation state. For example, incorporation of terbium or praseodymium increases both oxygen desorption at lower temperatures and the creation of oxygen vacancies than those of pure ceria. The former is due to the lower binding energy of a lattice oxygen in the mixed oxides and the latter is to the existence of irivalent terbium and praseodymium ions. In addition, a similar effect is also provided by the ternary oxides Cco6Zro4.iM 02. >2 (M = La, and Ga ), ... 

All the rare-earth elements occur in the HI oxidation state in compounds, and can be separated and determined in this form to provide what is known as the total REE. Samarium, europium, and ytterbium also occur in the unstable n oxidation state, whereas cerium, praseodymium, and terbium can be found in the IV oxidation state. 

The series of 15 elements, lanthanium to lutetium, is known as the lanthanide series. These elements all form trivalent ions in solution quadrivalent oxidation states of cerium, praseodymium, and terbium, and bivalent states of samarium and europium are also obtained. 

For most lanthanides, the 3+ oxidation state is the most stable, and therefore almost all REE-oxides are presented as REE2O3. However, some of the lanthanides may have several valences in one and the same oxide, so formulas are given to express this phenomenon. Praseodymium oxide usually contains 3+ and 4+ praseodymium in a somewhat variable ratio, depending upon the conditions of formation. Its formula is rendered as PreOn. Similarly, Xb407, one of the main commercial terbium compounds, contains some Xb4+ along with the more stable Xb3+. Ce has the 3+ state as most stable oxidation state, and the oxide is represented as 6203. 

McColm et al. (1977) considered that the eomposition ranges seem to be related to the amounts of R(IV) found in the respective nitrides and carbides (Lorrenzelli et al. 1970, Atoji 1962). In the cerium systems, Ce(IV) is present up to 70%, while in the praseodymium case the amount is less and in the lanthanum system the higher oxidation state is absent. This suggests that Ce(IV) and Pr(IV) do assist in preventing the catenation that leads to acetylide ion formation. However, the reviewers suggest that this factor is probably minor while the size factor of the rare earth atom is essential. 

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. 

The chemistry of rare earths is often discussed only in terms of the trivalent ions and indeed, contrary to the actinides, the oxidation states encountered in lanthanide compounds in the solid state and especially in solution are few in number. Standard electrode potentials M(II-III) and M(III-IV) indicate that, besides the trivalent rare earth ions, only Eu (-0.35 V), Yb + ( — 1.15 V), Sm + ( — 1.55 V) and Ce (+1.74 V) are sufficiently stable to exist in aqueous solutions (Nugent, 1975). It has long been known that alkaline conditions and many complexing anions such as nitrate, phosphate and sulfate stabilize Ce(IV) (Jorgensen, 1979) and recently it has been shown that large complex-forming ligands such as heteropolyanions also stabilize to some extent tetravalent praseodymium and terbium (Spitsyn, 1977). 

Cerium is the most abundant element of the rare earths. On average the Earth s crast contains 66 ppm of cerium (=66 g per ton), a value that is very comparable with the abundance of copper (68 ppm) (Emsley, 1991). Eew people know that there are on Earth larger resources of cerium than of other more popular elements like cobalt (29 ppm), lead (13 ppm), tin (2.1 ppm), silver (0.08 ppm) or gold (0.004 ppm). A special property of cerium is that it has a stable tetravalent oxidation state besides the trivalent state which is so common for the rare earths. Although the tetravalent oxidation state is also known for solid state compounds of praseodymium and terbium, cerium is the only rare-earth element that has a stable tetravalent oxidation state in solution. Many of the applications of cerium are based on the one-electron Ce +/Ce + redox couple. 

On the Phase Interval PrOi.50 to PrOui in the Praseodymium Oxide-Oxygen Systems, R. Turcotte, J. Warmkessel, R.J.D. Tilley and L. Eyring, J. Solid State Chem., 3,265-272 (1971). 

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