Rare earth metal-peroxide

The postulated catalytic cycle of the asymmetric epoxidation reaction is shown in Figure 13.10. A lanthanide metal alkoxide moiety changes to a rare earth metal-peroxide through proton exchange (I). In this step, lanthanide metal alkoxide moiety functions as a Bronsted base. The rare earth metal-BINOL complex also functions as a Lewis acid to activate electron-deficient olefins through monoden-tate coordination (II). Enantioselective 1,4-addition of rare earth metal-peroxide gives intermediate enolate (III), followed by epoxide formation to regenerate the catalyst (IV). 

One method of preparation consists in a modification of the Goldschmidt process. Niobium pentoxide is mixed with an alloy of the rare earths, called mixed metal, obtained in the manufacture of thorium nitrate, and consisting roughly of 45 per cent, of cerium, 20 per cent, of lanthanum, 15 per cent, of didymium, and about 20 per cent, of other rare-earth metals. The reaction is carried out in a magnesia-lined crucible, and is started with a firing mixture of barium peroxide, potassium chlorate, and aluminium powder. Considerable evolution of heat takes place and the reduction is extremely rapid a button of niobium is obtained 4 which, however, is not pure. 

The other pathway leading to the formation ofoxogroups in the coordination sphere of the metal atom is provided by uncontrolled oxidation of the basic alkoxides such as alkali, alkaline earth metal, and quite probably the rare earth metal ones by oxygen dissolved in solvents and present in the atmosphere. The primary oxidation products are peroxides and hydroperoxides — M(OOR)n and M(OOH)n, whose decomposition gives water among the other... 

Separation. — The separation of thorium from the rare earth metals with which it is still mixed may be accomplished by three methods (1) the carbonate separation depends on the fact that thorium carbonate is much more soluble in sodium carbonate than the carbonates of the rare earth metals (2) by the fractional crystallization of the mixed sulfates at 15°-20°, crystals of Th(S04)2 8 H20 are obtained at the insoluble end of the series (3) thorium oxalate forms a soluble double salt with ammonium oxalate, while the rare earth oxalates are almost insoluble in this reagent. Some other methods which have been suggested are fractionation of the chromates,4 of the hydrogen alkyl sulfates,5 of the acetates, by the use of sebacic add 6 and hydrogen peroxide. 

The decomposition of the peroxide to hydroxide and oxygen is a key rate-limiting step in the reaction sequence. To accelerate the reduction of the peroxide species and the overall reaction rate, the air cathode is formulated using catalytic compounds which promote the reaction in step 2. These catalysts are typically metal compounds or complexes such as elemental silver, cobalt oxide, noble metals and their compounds, mixed metal compounds including rare earth metals, and transition metal macrocyclics, spinels, manganese tUoxide, phtalocyanines or perovskites." - ... 

In the context of writing his Principles of Chemistry textbook, Mendeleev formulated his first version of the periodic system of chemical elements in the first two months of 1869. He would spend the next two years elaborating upon this system, expanding the scope and utility of the system in a variety of ways classification of peroxides, the properties of rare earth metals, and, especially, the detailed prediction of properties of three yet-undiscovered chemical elements, which he named eka-aluminum, eka-boron, and eka-silicon. After the publication of these predictions in his most detailed article on the chemistry of the periodic system in 1871 [Mendelejew, 1871], Mendeleev attempted briefly to experimentally discover these elements himself, but quickly abandoned the project by the end of that calendar year. 

Alkali- (and alkaline-earth) metal solutions of liquid ammonia display some bizarre properties that can be accounted for in terms of solvated electrons. The solvated electron is an excellent one-electron reducing agent and can be used to produce peroxides, superoxides, and metals with negative oxidation states. Compounds involving crown ethers and cryptands of the alkali metals in ammonia and related solvents yield the very rare electrides and alkalides. 

This indicates that in order to improve the soot oxidation abilities of ceria efficiently it is important to modify its redox capacity and to enhance the formation of active oxygen species like peroxide or superoxide. This can be conveniendy done either through modification of the structural features of ceria by lattice doping (i.e. with rare-earth elements or zirconium) or by doping with metals able to induce the formation of active oxygen species like silver (see later). 

Sodium peroxide facilitates the separation of uranium and other metals with sodium carbonate. The addition of the peroxide alone to acid solutions of Iron, cobalt, rare earths, titanium, zirconium, hafnium, and thorium causes their precipitation while uranium. If present, remains In solution. 

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