Rare earths, oxidation states

The lanthanides, distributed widely in low concentrations throughout the earth s cmst (2), are found as mixtures in many massive rock formations, eg, basalts, granites, gneisses, shales, and siUcate rocks, where they are present in quantities of 10—300 ppm. Lanthanides also occur in some 160 discrete minerals, most of them rare, but in which the rare-earth (RE) content, expressed as oxide, can be as high as 60% rare-earth oxide (REO). Lanthanides do not occur in nature in the elemental state and do not occur in minerals as individual elements, but as mixtures. 

Transition metal oxides, rare earth oxides and various metal complexes deposited on their surface are typical phases of DeNO catalysts that lead to redox properties. For each of these phases, complementary tools exist for a proper characterization of the metal coordination number, oxidation state or nuclearity. Among all the techniques such as EPR [80], UV-vis [81] and IR, Raman, transmission electron microscopy (TEM), X-ray absorption spectroscopy (XAS) and NMR, recently reviewed [82] for their application in the study of supported molecular metal complexes, Raman and IR spectroscopies are the only ones we will focus on. The major advantages offered by these spectroscopic techniques are that (1) they can detect XRD inactive amorphous surface metal oxide phases as well as crystalline nanophases and (2) they are able to collect information under various environmental conditions [83], We will describe their contributions to the study of both the support (oxide) and the deposited phase (metal complex). 

Acid soluble rare earth salt solution after the removal of cerium may be subjected to ion exchange, fractional crystalhzation or solvent extraction processes to separate individual rare earths. Europium is obtained commercially from rare earths mixture by the McCoy process. Solution containing Eu3+ is treated with Zn in the presence of barium and sulfate ions. The triva-lent europium is reduced to divalent state whereby it coprecipitates as europium sulfate, EuS04 with isomorphous barium sulfate, BaS04. Mixed europium(ll) barium sulfate is treated with nitric acid or hydrogen peroxide to oxidize Eu(ll) to Eu(lll) salt which is soluble. This separates Eu3+ from barium. The process is repeated several times to concentrate and upgrade europium content to about 50% of the total rare earth oxides in the mixture. Treatment with concentrated hydrochloric acid precipitates europium(ll) chloride dihydrate, EuCb 2H2O with a yield over 99%. 

The rare earth oxides have a number of distinguishing properties important in catalytic applications. The oxides are basic O) compared to alumina, lanthanum oxide (La203) being the most basic. The oxides also have good thermal stability, a valuable characteristic in most industrial applications. Some rare earths including cerium, praseodymium, and terbium form non-stoichiomet-ric oxides ( ), an important property shared by many good oxidation catalysts. These mixed valence state compounds are typically polymorphic. 

Many of these vapours will break down spontaneously to atoms in the flame. Others, particularly diatomic species such as metal monoxides (e g. alkaline earth and rare earth oxides), are more refractory. Monohydroxides which can form in the flame can also give problems. The high temperature and enthalpy of the flame aid dissociation thermodynamically, as does a reducing environment. The role of flame chemistry is also important. Atoms, both ground state and excited, may be produced by radical reactions in the primary reaction zone. If we take the simplest flame (a hydrogen-oxygen flame), some possible reactions are the following ... 

The rare earth chlorides can be separated through sublimation but a very high temperature and good vacuum are required. Recently [46] Eu2+ has been obtained pure by the distillation of its halides using the fact that Eu2+-halides are less volatile than the halides of trivalent rare earths. Sm, Eu and Yb oxides can be reduced to the divalent state by carbon and volatilized selectively from a mixture with other rare earth oxides [47]. 

The separation of rare earth oxides at 2500° C in a Solar furnace has been attempted [48], and Ce4+-oxide was obtained in a pure state from its mixture with lanthanum oxide. 

The most successful of the anion emitters are based on rare earth oxide matrices, with the rare earth in the +3 oxidation state. Europium oxide, Eu703, is the most successful of these anion emitters and also has the most stable +2 oxidation state of all the rare earths. This feature of Eu203, the stability of the +2 oxidation state, is thought to be responsible for this compound s being the best matrix. Whereas the matrix in the +3 oxidation state allows the migration of the anions away from the counter ion, the stability of the +2 oxidation states allows reactions of the following type to take place ... 

This model is based on studies of anion emitters in rare earth oxide matrices in the +3 oxidation state extending over several years. These studies can be summarized as follows ... 

Solid-state reaction between rare-earth oxide and rare-earth fluoride,... 

Fig. 7. The electrical conductivities of binary rare-earth oxide fluorides, Ln-Ln 203F6 measured at 650 C under an oxygen partial pressure of 1.33 x 10 Pa. , more than 1 Sm 1 3 0.1-1 Sm Q, less than 0.1 S m (reproduced with permission from Solid State Ionics, 23 (1989) 99 [19]).

When the binary rare-earth oxide fluorides are utilized in a solid-state fuel battery, they must often withstand at a high temperature of around 1000°C to exhibit sufficient oxide ion mobility, i.e. to supply appropriate electrical current. These... 

The conventional synthesis of rare earth borates, such as solid-state reaction between rare earth oxides and boric acid, flux-aided solid-state reaction at temperatures above 1000 °C, and milling, leads to a poor crystalline integrity and damaged luminescent properties. Other methods like coprecipitation methods through wet process (Boyer et al., 1999) are also studied. 

Rare earth silicates exhibit potential applications as stable luminescent materials for phosphors, scintillators, and detectors. Silica and silicon substrates are frequently used for thin films fabrication, and their nanostructures including monodisperse sphere, NWs are also reliable templates and substrates. However, the composition, structure, and phase of rare earth silicates are rather complex, for example, there are many phases like silicate R2SiOs, disilicate R2Si207 (A-type, tetragonal), hexagonal Rx(Si04)602 oxyapatite, etc. The controlled synthesis of single-phase rare earth silicate nanomateriais can only be reached with precisely controlled experimental conditions. A number of heat treatment based routes, such as solid state reaction of rare earth oxides with silica/silicon substrate, sol-gel methods, and combustion method, as well as physical routes like pulsed laser ablation, have been applied to prepare various rare earth silicate powders and films. The optical properties of rare earth silicate nanocrystalline films and powders have been studied. 

Exclusion of water and oxygen is the primary criterion in rare earth oxysulfide synthesis procedures. This is generally analogous to the case of rare earth sulfides. The industry synthesis technology depends on sulfurizing rare earth oxide powders via solid-state reactions. For instance, the classical sulfide fusion method follows the schematic reaction ... 

Oxides of the lanthanide rare earth elements share some of the properties of transition-metal oxides, at least for cations that can have two stable valence states. (None of the lanthanide rare earth cations have more than two ionic valence states.) Oxides of those elements that can only have a single ionic valence are subject to the limitations imposed on similar non-transition-metal oxides. One actinide rare-earth oxide, UO2, has understandably received quite a bit of attention from surface scientists [1]. Since U can exist in four non-zero valence states, UO2 behaves more like the transition-metal oxides. The electronic properties of rare-earth oxides differ from those of transition-metal oxides, however, because of the presence of partially filled f-electron shells, where the f-electrons are spatially more highly localized than are d-electrons. 

Rare earth oxides, sueh as EuO, can be prepared by reactive evaporation, with the same capability to control the oxidation state as above. ... 

If the Ja quantum number is not well defined, then J2 — J22 = J2X and the unevaluated expectation value of B(R)J2X is implicitly included in the electronic energy. The pair of terms in in Eq. (3.2.19) give rise to off-diagonal matrix elements (A J = 0, AJa = 0, ACl = 1) between different case (c) states, denoted Q.(Ja) or fi(2S+1A) (or Jan, as for the open-core rare earth oxides and halides). The simplicity of HROT in the case (c) basis set exacts a price in the difficulty of evaluating matrix elements of most operators that include Lz, S2, or Sz. 

Among rare earth oxides, sesquioxides show high activity for base-catalyzed reactions. The activities are comparable to those of alkaline earth oxides. The rare earth oxides which are stable at higher oxidation states of metal cations such as Ce02, Tb40 , and PreOn show weakly basic properties. 

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