Oxidation of lanthanides

In 1985 Imamura et al. developed manganese-cerium composite catalyst independently without noticing aforementioned patent data [25]. They started from modifying Co/6i composite oxide which wa.s effective for the wet-oxidation of various carboxylic acids [26]. They tried to see the effect of the addition of various oxides of lanthanide at the beginning they didnT appreciate the extraordinary superior function of CeO - The series of experiments led to the combination of Ce02 and manganese oxide as the best choice. 

Oxides of Lanthanides or Actinides and their Quasi-binaries with Transition... 

Table 17. Summary of mass spectrometric studies of oxides of lanthanides or actinides and their quasi-binaries with transition metals except those listed in Table 18. (The gaseous species are underlined if their enthalpies of formation or of dissociation are given)... 

Based on the above discussion, the function of the wet-oxidation catalysts should be confined to (i) activation of oxygen and (ii) direct electron transfer with the reactants (redox reaction) in the first step of the reaction. CeO seems to effectively contribute to both factors. CeOj behaves quite differently from other oxides of lanthanide and is always a constituent of automobile-exhaust purification catalysts. It stabilizes supports and keeps high surface area [64,65], prevents the sintering of precious metals and, thus, stabilizes their dispersed state [66,67], and acts as an oxygen reservoir [68,72]. When combined with precious metals, it works in various reactions other than the purification of vehicle exhausts e.g., detoxification of NjO, methanol decomposition, methanol synthesis, combustion of formaldehyde, etc [47,73-75]. Precious metals are remarkably activated and behave quite differently on CeO compared with their action on other supports. 

An early classification of the rare earths (the oxides of lanthanides) in relation to their separation from ores, was in the ceritic earths (the oxides from lanthanum to samarium) and yttric earths (from europium to lutetium, but also scandium and yttrium). A further refinement of analytical methods made it possible to split the yttric earths into terbic (europium, gadolinium, terbium), erbic (dysprosium, holmium, erbium, thulium), and ytterbic (ytterbium and lutetium) earths, along with yttrium oxide and scandium oxide. 

Mercury salts, in addition to silver salts, can be used as oxidants in reactions performed in non-ac]ueous solvents. Like the Ag+ ion, the Hg + ion is an effective oxidant. In both cases the reduction products, metallic silver and mercury, can be separated easily from reaction mixtures. The oxidation of lanthanides, such as Sm, Er, and as well as Sc and by HgCl2 in... 

Originally, general methods of separation were based on small differences in the solubilities of their salts, for examples the nitrates, and a laborious series of fractional crystallisations had to be carried out to obtain the pure salts. In a few cases, individual lanthanides could be separated because they yielded oxidation states other than three. Thus the commonest lanthanide, cerium, exhibits oxidation states of h-3 and -t-4 hence oxidation of a mixture of lanthanide salts in alkaline solution with chlorine yields the soluble chlorates(I) of all the -1-3 lanthanides (which are not oxidised) but gives a precipitate of cerium(IV) hydroxide, Ce(OH)4, since this is too weak a base to form a chlorate(I). In some cases also, preferential reduction to the metal by sodium amalgam could be used to separate out individual lanthanides. 

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. 

The chlorides, bromides, nitrates, bromates, and perchlorate salts ate soluble in water and, when the aqueous solutions evaporate, precipitate as hydrated crystalline salts. The acetates, iodates, and iodides ate somewhat less soluble. The sulfates ate sparingly soluble and ate unique in that they have a negative solubitity trend with increasing temperature. The oxides, sulfides, fluorides, carbonates, oxalates, and phosphates ate insoluble in water. The oxalate, which is important in the recovery of lanthanides from solutions, can be calcined directly to the oxide. This procedure is used both in analytical and industrial apptications. 

Selective Oxidation. Cerium, the most abundant lanthanide, can be separated easily after oxidation of Ce(III) to Ce(IV), simplifying the subsequent separation of the less abundant lanthanides. Oxidation occurs when bastnaesite is heated in air at 650°C or when the hydroxides are dried in air... 

The yearly worldwide production of lanthanides is nearly 70,000 tons (18) measured as contained Ln oxide. The primary sources are given in Table 2. For finished products the principal supplying companies are Molycorp (United States), Rhc ne-Poulenc (France), several Japanese companies, such as Mitsubishi, Santoku, and Shinetsu, and some Chinese organi2ations. The rise of Chinese lanthanide production during the 1980s has become a significant factor in the global market picture. 

Oxides (Ln Oj), fluorides (LnF ), sulfides (Ln S, LnS), sulfofluorides (LnSF) of lanthanides are bases of different functional materials. Analytical control of such materials must include non-destructive methods for the identification of compound s chemical forms and quantitative detenuination methods which does not require analytical standards. The main difficulties of this analysis by chemical methods are that it is necessary to transform weakly soluble samples in solution. 

The lanthanides comprise the largest naturally-occurring group in the periodic table. Their properties are so similar that from 1794, when J. Gadolin isolated yttria which he thought was the oxide of a single new element, until 1907, when lutetium was discovered, nearly a hundred claims were made for the discovery of elements... 

In 1751 the Swedish mineralogist, A. F. Cronstedt, discovered a heavy mineral from which in 1803 M. H. Klaproth in Germany and, independently, i. i. Berzelius and W. Hisinger in Sweden, isolated what was thought to be a new oxide (or earth ) which was named ceria after the recently discovered asteroid, Ceres. Between 1839 and 1843 this earth, and the previously isolated yttria (p. 944), were shown by the Swedish surgeon C. G. Mosander to be mixtures from which, by 1907, the oxides of Sc, Y, La and the thirteen lanthanides other than Pm were to be isolated. The small village of Ytterby near Stockholm is celebrated in the names of no less than four of these elements (Table 30.1). 

The minerals on which the work was performed during the nineteenth century were indeed rare, and the materials isolated were of no interest outside the laboratory. By 1891, however, the Austrian chemist C. A. von Welsbach had perfected the thoria gas mantle to improve the low luminosity of the coal-gas flames then used for lighting. Woven cotton or artificial silk of the required shape was soaked in an aqueous solution of the nitrates of appropriate metals and the fibre then burned off and the nitrates converted to oxides. A mixture of 99% ThOz and 1% CeOz was used and has not since been bettered. CeOz catalyses the combustion of the gas and apparently, because of the poor thermal conductivity of the ThOz, particles of CeOz become hotter and so brighter than would otherwise be possible. The commercial success of the gas mantle was immense and produced a worldwide search for thorium. Its major ore is monazite, which rarely contains more than 12% ThOz but about 45% LnzOz. Not only did the search reveal that thorium, and hence the lanthanides, are more plentiful than had previously been thought, but the extraction of the thorium produced large amounts of lanthanides for which there was at first little use. 

Table 30.1 The discovery of the oxides of Group 3 and the lanthanide elements ...

Determination of cerium as cerium(IV) iodate and subsequent ignition to cerium(IV) oxide Discussion. Cerium may be determined as cerium(IV) iodate, Ce(I03)4, which is ignited to and weighed as the oxide, Ce02. Thorium (also titanium and zirconium) must, however, be first removed (see Section 11.44) the method is then applicable in the presence of relatively large quantities of lanthanides. Titrimetric methods (see Section 10.104 to Section 10.109) are generally preferred. 

While there have been many non-isothermal studies of the decompositions of lanthanide oxalates, fewer detailed kinetic investigations have been reported. The anhydrous salts are difficult to prepare. La, Pr and Nd oxalates decompose [1097] to the oxide with intervention of a stable oxycarbonate, but no intermediate was detected during decomposition of the other lanthanide oxalates. The product CO disproportionates exten-... 

The previously discussed conformational study of 3-substituted thietane oxides using lanthanide shift reagents185 corroborates the conclusions derived from other NMR studies and suggests that all rrans-3-substituted thietane oxides prefer an equatorial oxygen conformation when the thietane oxides are bound to shift reagents. 

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