High temperature reduction

Metal teUurides for semiconductors are made by direct melting, melting with excess teUurium and volatilizing the excess under reduced pressure, passing teUurium vapor in an inert gas carrier over a heated metal, and high temperature reduction of oxy compounds with hydrogen or ammonia. 

Both processes also use up-graded ilmenite (slags). About 30% of the world s titanium feedstocks are suppHed by titanium slag producers in Canada, South Africa, and Norway. Slags are formed by the high temperature reduction of ilmenites in electric furnaces. Much of the iron oxide content is reduced to metallic iron and separated as a saleable by-product. Magnesium and other impurities may also be incorporated in the following equations. 

Metallic Sr and Ba are best prepared by high-temperature reduction of their oxides with Al in an evacuated retort or by small-scale electrolysis of fused chloride baths. They have limited use as getters, and a Ni-Ba alloy is used for sparkplug wire because of its high emissivity. Annual production of Ba metal is about 20-30 tonnes worldwide and the 1991 price about 80-140/kg depending on quality. 

High-temperature reduction of Na2Ti03 with hydrogen produces nonstoichiometric materials, Na jTi02 (jr = 0.20-0.25), called titanium bronzes by analogy with the better-known tungsten bronzes (p. 1016). They have a blue-black, metallic appearance with high electrical conductivity and are chemically inert (even hydrofluoric acid does not attack them). 

By contrast, ZrCl and ZrBr, also prepared by the high temperature reduction of ZrX4 with the metal, appear to be genuine binaiy halides. They are comprised of hep double layers of metal atoms surrounded by layers of halide ions, leading to metallic conduction in the plane of the layers, and they are thermally more stable than the less reduced phases. Zrl has not been obtained, possibly because of the large size of the iodide ion, and, less surprisingly, attempts to prepare reduced fluorides have been unsuccessful. 

The introduction of a Pt function influences weakly the behavior of the two high-temperature reduction peaks, but markedly decreases the temperature of the minor low-temperature reduction step from 700 K to 350 K (Figure 5b). These data suggest that some reduction of WOx-Zr02 species can occur during n-alkane isomerization reactions (440-500 K). These reducible W species may act as redox sites required for the conversion of H-atoms to H species on WOx-based solid acids. 

The addition of substantial amounts (up to 32%) of aluminium powder to conventional explosives enhances the energy release by up to 100% [1], involving high temperature reduction of liberated carbon dioxide and water by the metal [2]. 

At the end of the seventies, scientists at Exxon discovered that metal particles supported on titania, alumina, ceria and a range of other oxides, lose their ability to chemisorb gases such as H2 or CO after reduction at temperatures of about 500 °C. Electron microscopy revealed that the decreased adsorption capacity was not caused by particle sintering. Oxidation, followed by reduction at moderate temperatures restored the adsorption properties of the metal in full. The suppression of adsorption after high temperature reduction was attributed to a strong metal-support interaction, abbreviated as SMSI [2]. 

A schematic of this high temperature reduction apparatus is shown in Figure 4. 

High-temperature reactions with vacuum microbalance, 5 119 High-temperature reduction, 34 19 effects on titania-supported metals, 36 176-177, 180... 

EIGURE 8.3. High temperature reduction of tungsten oxide. H2O partial pressure is indicative of the rate of reduction. The four reduction steps are distinctly separated, indicating that the material passes through each equilibrium composition as it is reduced. Erom Lessner and Schubert. 

Magnetite is obtained in aqueous, alkaline systems by precipitation from a mixed Fe /Fe solution, by oxidation of Fe solution via green rust or Fe(OH)2, or by interaction of Fe with ferrihydrite. Another pathway involves high temperature reduction of Fe oxides (e. g. with H2). Maghemite forms topotactically by wet or dry oxidation of magnetite or by heating lepidocrocite and by thermal decomposition of various organic Fe-salts (cf chap. 20). 

Barium sulfide may also be made by high temperature reduction of barium sulfate with methane. 

Also, the metal may be obtained by high temperature reduction of ceri-um(lll) chloride with calcium ... 

Erbium metal is produced from rare-earth minerals. Methods of preparation are similar to dysprosium, involving sulfuric acid treatment, ion exchange separation from other lanthanides, roasting, conversion to hahde, and finally high temperature reduction with calcium or sodium, (see Dysprosium). 

Direct or American Process. The direct process is noted for its simplicity, low cost, and excellent thermal efficiency. It consists of an initial high-temperature reduction (1000-1200 °C) of a zinc-containing material (as oxide), the reducing agent being coal. Reduction takes place according to Boudouard s equations ... 

Let us consider the high temperature reduction of oxide solid solutions as discussed in Chapter 9. The overall reaction reads (A, B) O + H2 = A + BO + H20. We conclude from Figure 11-10 a that the receding phase boundary is always morphologically unstable, in accordance with Figure ll-5b (see also [D.P. Whittle (1983)]). There is yet a second kind of instability involved in the oxygen activity change at the... 

Elemental boron can be prepared by the high-temperature reduction of B2O3 with magnesium, but the product is impure and amorphous ... 

In transition metal complexes of suitable geometry the metal may undergo intramolecular oxidative insertion into C-H bonds. Intermediates of Pd-catalyzed C-C bond formation can also undergo such cyclometalations to yield palladacycles . This can give rise to unexpected products or, if the palladacycles are too stable, the catalyst will be consumed and no further reaction will occur. At high temperatures reductive elimination from such complexes can occur to yield cyclic products. 

The high-temperature reduction of barium sulfate with coke yields the water-soluble barium sulfide (BaS), which is subsequently leached out. Treatment of barium sulfide with the relevant chemical yields the desired barium salt. Purification of the product is complicated by the impurities introduced in the coke. Pure barium carbonate and barium sulfate are made by precipitation from solutions of water-soluble barium salts. 

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