Tungsten hydrogen reactions

P oly 0 xome tall ate s, derived from both isopoly adds and heteropoly adds, are important homogeneous oxidation catalysts. The metals involved are vanadium, niobium, tantalum, molybdenum, and tungsten. The reactions involved are the oxidation of a wide range of organic compounds by hydrogen peroxide or organic hydroperoxide. 

Tungsten sulfide catalyst should accelerate not only the hydrogenation reactions, but also dehydrogenation of hydroaromatics to aromatics. Thermodynamic calculations show that such reactions are probable at about 500°C. at a hydrogen pressure of about 50 atm. Use of the tung-... 

Pawelec, B., Mariscal, R., Fierro, J.L.G., Greenwood, A., and Vasudevan, P.T. Carbon-supported tungsten and nickel catalysts for hydrodesulfurization and hydrogenation reactions. Applied Catalysis. A, General, 2001, 206, 295. 

Furthermore, if a film of nickel or tungsten, which is very active in the exchange reaction (too fast to be measured) is covered with D2 at 78° K and the excess D2 removed, only about 1 % of the preadsorbed D2 will exchange with H2 circulated over the film at 78° K. It follows, therefore, that only this 1 % of the surface is active in the exchange reaction, while this is clearly not so for hydrogenation reactions. I believe similar results to the above have been obtained in Holland by Schuit and his coworkers. 

Where Pr stands for C3H7. Proton NMR experiments at several temperatures have shown that the relative stability of the dihydride increases at higher temperatures. A thermodynamic analysis of the reaction led to A,7/°(59) = 5.0 2.5kJmor and 7 AA °(59)w 1.5 2.6kJmor. 2 As A,//°(59) = D7/ (W-H2)-fi)//°(H-H)-2 DH°)(W-H), the above value of Z)//°(W-H2) yields (Z)7/ ) (W-H) = 236kJ moP. This tungsten-hydrogen mean bond dissociation enthalpy in the W(ll) seven-coordinate complex is about lOOkJmoP lower than DH°(W-H) in W(Cp)(CO)3H, where the metal is in the same oxidation state ... 

Sokolsky DV, Palanker VS, Baybatyrov EN (1975) Electrochemical hydrogen reactions on tungsten carbide. Electrochim Acta 20 71-77... 

Reduction to Solid Metal. Metals having very high melting points caimot be reduced in the Hquid state. Because the separation of a soHd metallic product from a residue is usually difficult, the raw material must be purified before reduction. Tungsten and molybdenum, for instance, are prepared by reduction of a purified oxide (WO, MoO ) or a salt, eg, (NH2 2 G4, using hydrogen. A reaction such as... 

Titanium tetraiodide can be prepared by direct combination of the elements at 150—200°C it can be made by reaction of gaseous hydrogen iodide with a solution of titanium tetrachloride in a suitable solvent and it can be purified by vacuum sublimation at 200°C. In the van Arkel method for the preparation of pure titanium metal, the sublimed tetraiodide is decomposed on a tungsten or titanium filament held at ca 1300°C (152). There are frequent hterature references to its use as a catalyst, eg, for the production of ethylene glycol from acetylene (153). 

Tungsten dioxide [12036-22-5] WO2, is a brown powder formed by the reduction of WO3 with hydrogen at 575—600°C. Generally, this oxide is obtained as an intermediate in the hydrogen reduction of the trioxide to the metal. On reduction, first a blue oxide, then a brown oxide (WO2), is formed. The composition of the blue oxide was in doubt for a long time. However, it has since been resolved that W2Q03g and W are formed as intermediates, which may also be prepared by the reaction of tungsten with WO3. 

In a typical use of this method, a mixture of hydrogen and methane is fed into a reaction chamber at a pressure of about 1.33 kPa (10 torr). The substrate upon which diamond forms is at about 950°C and Hes about 1 cm away from a tungsten wine at 2200°C. Small diamond crystals, 1 mm or so in si2e, nucleate and grow profusely on the substrate at a rate around 0.01 mm /h to form a dark, rough polycrystalline layer with exposed octahedral or cubic faces, depending on the substrate temperature. 

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