Tungsten oxide hydrate

Timgsten oxides and tungsten metal exhibit an apparent increase in vj or pressure at elevated temperatures in the presence of water vapor [3.27-3.31]. This increase is due to a reaction between the respective oxide or the metal and the Avater vapor, to form a volatile tungsten oxide hydrate. The reactions can be described by the following equations ... 

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. 

Abstract. Nanopowders of nonstoichiometric tungsten oxides were synthesized by method of electric explosion of conductors (EEC). Their electronic and atomic structures were explored by XPS and TEM methods. It was determined that mean size of nanoparticles is d=10-35 nm, their composition corresponds to protonated nonstoichiometric hydrous tungsten oxide W02.9i (OH)o.o9, there is crystalline hydrate phase on the nanoparticles surface. After anneal a content of OH-groups on the surface of nonstoichiometric samples is higher than on the stoichiometric ones. High sensitivity of the hydrogen sensor based on WO2.9r(OH)0.09 at 293 K can be connected with forming of proton conductivity mechanism. 

The surface of the nanocrystalline nonstoichiometric tungsten oxide conditioned on air (sample NN1) is almost completely formed of crystalline hydrate W03 (H20). Thus on the W4f-spectra (Fig. 2-3) the main component is comp, e with Ep=36.1 eV, on the Ols-spectra (Fig. 3-3) along with the 02 -states, the contributions from OH-groups (comp, g) and from H20 (comp, m) are present. 

After the anneal on air of the nonstoichiometric tungsten oxide nanopowder the signal from crystalline hydrate WO3 (H20) (comp, e) in the spectrum of W4f-level (sample NN2, Fig. 2-4) disappeared. The component d from W6+-states of the oxide dominated in the spectrum of W4f-level after anneal on air and the component c from W5+-states (EpW4f7/2=34.8 eV) appeared in the low energy region. 

The models described above assume that the reaction occurs only in the liquid phase. In some cases, such as isomerization of cyclopropane to propylene on a silica-alumina catalyst,43 reduction of crotonaldehyde over a palladium catalyst,45 and hydration of olefins to alcohols over tungsten oxide,58 the reactions could occur in the gas as well as in the liquid phases. 

Complete the solution of the crystal structure and perform Rietveld refinement of the model of tungsten oxide peroxide hydrate, W02(02)(H20), which crystallizes in the space group symmetry P2]/n with a = 12.07, b = 3.865, c = 7.36 A, p = 102.9". The location of W has been found from a Patterson map and it has the coordinates x = 0.680, y = 0.066, z = 0.364. Note that W usually exhibits octahedral or square-pyramidal coordination (with the peroxide group, 0-0, counted as one ligand). The experimental powder diffraction pattern is found on the CD in the file Ch7Pr08 CuKa.raw. 

In principle, tungsten is oxidized by water. The reaction rate is determined by the temperature and the PH2O/PH2 ratio. As a reaction product, die presence of H2 always has to be considered. The reaction rate as well as the 0 W ratio of the oxide formed increases with temperature and PH2O/PH2 ratio. Water increases the volatility of the tungsten oxides by the formation of the volatile oxide hydrate W02(0H)2. For more details, see Sections 3.2 and 3.3. 

Water reacts with tungsten below the boiling point. In principle, water acts as an oxidizing agent. According to ESCA investigations WO2, WO3, and oxide hydrates are formed [3.3] ... 

Tungsten sheet or powder reacts with water vapor between 700 and 1200 °C to form WO2. Above 900 °C, volatilization via the oxide hydrate occurs. In a moist N2-H2 atmosphere, after one hour at 1700 °C, the loss in W is ... 

The presence of the monomere, volatile oxide hydrate [WO3 H2O resp. W02(OH)2] was proven by mass spectroscopy [3.28], and thermodynamic data are available for all of the above phase equilibria [3.10,3.29]. Based on these free energy data as well as on those of the solid oxides [3.23], the equilibrium partial pressure of the volatile compound can be calculated as a function of humidity. The result of such a calculation is shown in Fig. 3.2 for a temperature of 1000 °C [3.32]. In addition, the equilibrium pressures of the other volatile tungsten compoimds are also presented. From these calculations it is evident, that the oxide hydrate is by far the most volatile tungsten compound in the W-O-H system. 

The oxide hydrate is responsible for the chemical vapor transport of tungsten, which occurs throughout the reduction sequence, a"d which decisively codetermines the physical and chemical properties of the metal powder. Its actual partial pressure during reduction will depend on both the temperature and the prevailing humidity. It will be lower toward the end of reduction, due to the steady decrease in the oxygen partial pressure (humidity) of the system as the reduction proceeds. 

Electrolysis [5.30]. The principle of the method is to have the tungsten scnq> as anode in an electrolyte like sodium hydroxide or sodium carbonate aqueous solution. The tungsten atoms are oxidized electrolytically to the hexavalent state. The resulting solution contains sodium tungstate besides insoluble oxides or oxide hydrates of flie other constituents. Rotating drum or disk electrolytic cells are the applied equipment. 

Mace and Bonilla have established a rate equation for the direct hydration of ethylene over supported tungsten oxide catalyst which uuhcates that hydration proceeds by a surface reaction without preferential adsorption of either ethylene or water and without retardation by strong adsoiption of ethyl alcohol. 

The direct hydration of propylene and higher olefins has also been accomplished. The tungsten oxide type of catalyst was the best found, although supported phosphoric acid gave good results. ... 

Scheelite, natural or synthetic, is attacked by hydrochloric add in an autoclave. An almost insoluble residue of WO,-5 1120, contaminated by silicates from the gangue, is obtained. On treatment with ammonia this oxide-hydrate is dissolved to form a colorless solution and purified by filtration. After evaporation a white substance crystallizes, ammonium paratungstate APT (or ammonium parawolframate APW) with the formula (NH ) j H j Wj O g. APT has been an important substance in the worldwide trade in tungsten products. When this compound is ignited pure WO, is formed. 

Both titania (anatase more than rutile) and, even more, zirconia (tetragonal more than monoclinic), when sulfated or covered with tungsten oxide become very active for some hydrocarbon conversion reactions such as -butane skeletal isomerization [263]. For this reason, a discussion began on whether these materials have to be considered superacidic. Spectroscopic studies showed that the sulfate ions [264] as well as the tungstate ions [265,266] on ionic oxides in dry conditions, are tetracoordinated with one short S=0 and W=0 bond (mono-oxo structure) as shown in Scheme 9.3(11). Polymeric forms of tungstate species could also be present [267]. However, in the presence of water the situation changes very much. According to the Lewis acidity of wolframyl species, it is believed that it can react with water and be converted in a hydrated form, as shown in Scheme 9.3. Residual... 

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