For tungsten oxide

In the temperature range of400 to 700 °C the values of the equilibrium constants of the first two reactions are larger than the corresponding values for tungsten oxide reduction. Thus, for an equal moisture content in the hydrogen used, the reduction of molybdenum... 

Though they utilized a novel hollow-sphere form of WO3 rather than WO2 72 nanorods, the work of Li s group in researching tungsten oxide as gas sensors can be applied to thenanorodform. Li sporous, hollow spheres were substituted for tungsten oxide polycrystalline films used in the previous studies by Galatsis et al. and Choi et Thus, they overcome the sensitivity and selectivity problems caused by the... 

The reported apparent activation energies for disproportionating propylene are lower for the cobalt molybdate-alumina than for tungsten oxide-silica. With co-... 

Studies related to alumina-supported tungsten oxide metathesis catalysts also continue.Unsupported alumina tungstate, Al2(W04)3, has metathesis activity between 100 and 200° C it has been proposed that this difficult-to-reduce compound is a reasonable precursor to the active metathesis sites for tungsten oxide supported on 7-alumina. However, a controversy exists as to the presence of Al2(W04)3 as a major component on the surface of these catalysts Raman spectra do not show bands that can be attributed to Al2(W04)3. Evidence reported in the literature for the formation of aluminium tungstate on the surface is not valid because of impurities in the reference compounds used. The conclusion that this compound is not a major surface component does not completely rule out the possibility that it is involved in the catalytic active phase,especially since the number of active sites is extremely small, e.g., 10 sites per gram of... 

Figure 8. Model of the transformations observed for tungsten oxide on an alumina film by controlled atmosphere electron microscopy.

Figure 4-15. Structures suggested on the basis of EXAFS spectroscopy and other physical methods for tungsten oxide clusters in the sodalite cages of zeolite NaY. [229] The structures correspond to half loaded and fully loaded cages. Reproduced from Journal of the American Chemical Society with permission of the American Chemical Society.

Early catalysts for acrolein synthesis were based on cuprous oxide and other heavy metal oxides deposited on inert siHca or alumina supports (39). Later, catalysts more selective for the oxidation of propylene to acrolein and acrolein to acryHc acid were prepared from bismuth, cobalt, kon, nickel, tin salts, and molybdic, molybdic phosphoric, and molybdic siHcic acids. Preferred second-stage catalysts generally are complex oxides containing molybdenum and vanadium. Other components, such as tungsten, copper, tellurium, and arsenic oxides, have been incorporated to increase low temperature activity and productivity (39,45,46). 

The first-stage catalysts for the oxidation to methacrolein are based on complex mixed metal oxides of molybdenum, bismuth, and iron, often with the addition of cobalt, nickel, antimony, tungsten, and an alkaU metal. Process optimization continues to be in the form of incremental improvements in catalyst yield and lifetime. Typically, a dilute stream, 5—10% of isobutylene tert-huty alcohol) in steam (10%) and air, is passed over the catalyst at 300—420°C. Conversion is often nearly quantitative, with selectivities to methacrolein ranging from 85% to better than 95% (114—118). Often there is accompanying selectivity to methacrylic acid of an additional 2—5%. A patent by Mitsui Toatsu Chemicals reports selectivity to methacrolein of better than 97% at conversions of 98.7% for a yield of methacrolein of nearly 96% (119). 

Polymers containing 8-hydroxyquinoline appear to be selective adsorbents for tungsten in alkaline brines (95). In the presence of tartrate and citrate, quinaldic acid [93-10-7] allows the separation of zinc from gallium and indium (96). Either of these compounds can selectively separate lead and zinc from oxide ores as complexes (97). It is also possible to separate by extraction micro quantities of rhenium(VII), using quinoline in basic solution (98). The... 

Liquid-Phase Epoxidation with Hydroperoxides. Molybdenum, vanadium, and tungsten have been proposed as Hquid-phase catalysts for the oxidation of the ethylene by hydroperoxides to ethylene oxide (205). tert- uty hydroperoxide is the preferred oxidant. The process is similar to the arsenic-catalyzed route, and iacludes the use of organometaUic complexes. 

This is by far the most stable and best-known oxidation state for chromium and is characterized by thousands of compounds, most of them prepared from aqueous solutions. By contrast, unless stabilized by M-M bonding, molybdenum(III) compounds are sparse and hardly any are known for tungsten(III). Thus Mo, but not W, has an aquo ion [Mo(H20)g] +, which gives rise to complexes [MoXg] " (X = F, Cl, Br, NCS). Direct action of acetylacetone on the hexachloromolybdate(III) ion produces the sublimable (Mo(acac)3] which, however, unlike its chromium analogue, is oxidized by air to Mo products. A black cyanide,... 

The reaction is carried out under an inert atmosphere in an open crucible at approximately 830°C. Figure 1 shows typical equipment used for direct oxide reduction. Vitrified magnesium oxide ceramic is commonly used as a container material, but tungsten and tantalum can also be used(3). If the latter are used, CaF2 is added to lower the temperature needed to liquify... 

New inorganic electrode films based on molybdenum and tungsten oxides have been introduced for aqueous solutions heteropolyanion electrodes... 

The volatility of the metal oxide is a critieal factor, because some metal oxides are too volatile and cannot be used unless an excess is employed, e.g., as for tungsten. ... 

In addition to these different types of alloys, some studies were also devoted to alternatives to platinum as electrocatalysts. Unfortunately, it is clear that even if some catalytic activities were observed, they are far from those obtained with platinum. Nickel tungsten carbides were investigated, but the electrocatalytic activity recorded for methanol oxidation was very low. Tungsten carbide was also considered as a possible alternative owing to its ability to catalyze the electrooxidation of hydrogen. However, it had no activity for the oxidation of methanol and recently some groups showed that a codeposit of Pt and WO3 led to an enhancement of the activity of platinum. ... 

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