Tungsten oxide reaction with

Owing to the slow rates of diffusion of the cations, the direct solid-state reaction of the oxides Cr2 03 and MO at an elevated temperature is not a good preparation of divalent metal chromium(III) oxides. They can be prepared by more elaborate methods, such as controlled reduction of dichromates MCr207,1 reaction of dichromium tungsten oxide Cr2W06 with a molten divalent metal fluoride2 at 1400°, pyrolysis of complexes,3 and pulverization of slurries containing Cr2 03 and a divalent metal salt.4... 

Welles (1946) has analyzed gas separated in a thermal diffusion column for by the 0 (d, n)F reaction. He prepared tungsten oxide targets with the gas, bombarded them with deuterons in a cyclotron, and measured the fluorine-18 formed. 

Investigation of direct conversion of methane to transportation fiiels has been an ongoing effort at PETC for over 10 years. One of our current areas of research is the conversion of methane to methanol, under mild conditions, using li t, water, and a semiconductor photocatalyst. Research in our laboratory is directed toward ad ting the chemistry developed for photolysis of water to that of methane conversion. The reaction sequence of interest uses visible light, a doped tungsten oxide photocatalyst and an electron transfer molecule to produce a hydroxyl i cal. Hydroxyl t cal can then react with a methane molecule to produce a methyl radical. In the preferred reaction pathway, the methyl radical then reacts with an additional wata- molecule to produce methanol and hydrogen. 

Having established structural and electronic analogies between metal oxides and alkoxides of molybdenum and tungsten, the key remaining feature to be examined is the reactivity patterns of the metal-alkoxides. Metal-metal bonds provide both a source and a returning place for electrons in oxidative-addition and reductive elimination reactions. Stepwise transformations of M-M bond order, from 3 to 4 (37,38), 3 to 2 and 1 (39) have now been documented. The alkoxides M2(0R)6 (MiM) are coordinatively unsaturated, as is evident from their facile reversible reactions with donor ligands, eq. 1, and are readily oxidized in addition reactions of the type shown in equations 2 (39) and 3 (39). 

Oxidation of tungsten with water and reduction of tungsten oxides with hydrogen are quite similar, with the high partial pressure of water or hydrogen driving the reaction in a particular direction. The oxidation reaction has been found to follow this sequence of reactions ... 

This reaction with water at 38°C is very slow and increases with increasing temperatures and pressures. Reaction with water vapor between 20 and 500°C leads strictly to the formation of WO3—no other oxides are formed. The rate of this reaction has been found to be dependent on temperature and the ratio of the partial pressure of water to that of hydrogen. However, by adjusting these partial pressures properly, all known oxides can be formed. When both are low, WO2 is formed. As these pressures increase, the more oxidized forms are produced (WO2.72, WO2.9, and finally WO3). Additionally, higher temperatures favor the more oxidized forms. It also must be noted that hydrated oxides can be easily volatilized above about 900°C, with most volatile form being W02(0H)2. Such volatile compounds may play a crucial role in the formation of tungsten oxide nanorods. 

The final aspect of tungsten oxide reduction chemistry that needs to be considered is the kinetics of the reactions. Under most circumstances, the reduction of tungsten oxides is a transport limited process limited by the rate of transport of the water vapor product out of the material. Under such conditions, no shortcuts in the reduction path may be taken, with the WO3 oxide being reduced according to the following path ... 

Reaction with amorphous silicon at 900°C, catalyzed by steam produces cadmium orthosilicate, Cd2Si04. The same product also is obtained by reaction with sdica. Finely divided oxide reacts with dimethyl sulfate forming cadmium sulfate. Cadmium oxide, upon rapid heating with oxides of many other metals, such as iron, molybdenum, tungsten, titanium, tantalum, niobium, antimony, and arsenic, forms mixed oxides. For example, rapid heating with ferric oxide at 750°C produces cadmium ferrite, CdFe204 ... 

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