Tungsten preparation

Table 2 shows the initial results for the catalytic dry reforming of methane using bulk carbides of niobium, tantalum, molybdenum and tungsten, prepared by CH4 TPR. The conversions and yields obtained over P-M02C and a-WC are very similar to those predicted by thermodynamic considerations, and thus these materials are efficient catalysts for methane dry reforming. At atmospheric pressure, deactivation was observed over both catalysts after about 8 hours on stream. Examination of the post-catalytic samples by powder XRD (Figure 2) revealed that as the reaction proceeded the active P-M02C was oxidised and converted to... 

The active constituent of the catalyst is an oxide of tungsten prepared by partial reduction of WO - Unsupported WOj was found to have activity but better specific activity is obtained by depositing the oxide on a support. A number of grades of alumina and silica were tested. Gamma alumina was found to react with WOj at 400°C to a considerable extent to form aluminium tungstate. This was identified by XPS and tests on AljfWO ) showed it to be inactive as a catalyst for the isomerization reaction. 

Phenylphosphonic acid (1.58 g, 10 mmol) is added to a diluted solution of 40% tetrabutylammonium hydroxide (13 mL, 20 mmol) in water (100 mL) then a solution of H2W2O11 (21 mL, 20 mmol of tungsten), prepared as above, is added dropwise to the clear solution. The flaky solid that forms is collected, rinsed with cold water, and dried under vacuum at 40°C (heating above 60°C may induce decomposition of the peroxo salt). Yield 7.23 g (62%). An analytical sample was recrystallized from a mixture of 1,2-dichloroethane and ethyl acetate. 

Furthermore, considering the results obtained by Vandecasteele et al. (79), it is clear that carbon contents as low as this are observable in industrial molybdenum and tungsten prepared by powder metallurgy. 

In order to prepare the system for 3D-CT, it is not enough to integrate a second detector array. Besides this special attention has to be paid to the computer hardware, the synchronisation between object movement and the data read out as well as to the collimator of the LINAC. The collimator has been built with 4 tungsten blocks which can be moved individually m order to shape different sht sizes for 2D-CT as well as different cone angles for 3D-CT or digital radiography. 

Molybdenum is also recovered as a by-product of copper and tungsten mining operations. The metal is prepared from the powder made by the hydrogen reduction of purified molybdic trioxide or ammonium molybdate. 

It was originally separated from zirconium by repeated recrystallization of the double ammonium or potassium fluorides by von Hevesey and Jantzen. Metallic hafnium was first prepared by van Arkel and deBoer by passing the vapor of the tetraiodide over a heated tungsten filament. Almost all hafnium metal now produced is made by reducing the tetrachloride with magnesium or with sodium (Kroll Process). 

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). 

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 pentafluofide [19357-83-6] prepared by the reduction of the hexafluoride on a hot tungsten filament in almost quantitative yield... 

Tungsten tetrafluofide [13766-47-7] WF, is a nonvolatile, hygroscopic, reddish-brown soHd. It has been prepared in low yields by the reduction of the hexafluoride with phosphoms trifluofide in the presence of Hquid anhydrous hydrogen fluoride at room temperature (6). 

Tungsten oxytetrafluofide [13520-79-17, WOF, mp 110°C, bp 187.5°C, forms colorless plates. It is prepared by the action of an oxygen—fluorine mixture on the metal at elevated temperatures (7). The compound is extremely hygroscopic and decomposes to tungstic acid in the presence of water. 

Tungsten oxydifluofide [14118-73-17 WO2F2, is a white soHd prepared by the hydrolysis of WOF (8). Its chemistry has not been investigated. 

Tungsten pentachlofide [13470-13-8], WCl, mp 243°C, bp 275.6°C, is a black, crystalline, deHquescent soHd. It is only slightly soluble in carbon disulfide and decomposes in water to the blue oxide, 200 2. Magnetic properties suggest that tungsten pentachlofide may contain trinuclear clusters in the soHd state, but this stmcture has not been defined. Tungsten pentachlofide may be prepared by the reduction of the hexachloride with red phosphoms (9). 

Tungsten tetrachloride [13470-14-9], WCl, is obtained as a coarse, crystalline, deHquescent soHd that decomposes upon heating. It is diamagnetic and maybe prepared by the thermal-gradient reduction of WCl with aluminum (10). 

Tungsten dichlofide [13470-12-7], WCI2, is an amorphous powder. It is a cluster compound and maybe prepared by the reduction of the hexachloride with aluminum in a sodium tetrachloroalurninate melt (11). 

Tungsten oxytetrachlofide [13520-78-0], WOCl, mp 211°C, bp 221°(Z, is a red crystalline soHd. It is soluble in carbon disulfide and ben2ene and is decomposed to tungstic acid by water. It may be prepared by refluxing sulfurous oxychloride, SOCI2, on tungsten trioxide (12) and purified after evaporation by sublimation. 

Tungsten oxydichlofide [13520-76-8], WO2CI2, a pale-yeUow crystalline soHd having an mp of 266°C, is soluble in cold water and in alkaline solution, although partly decomposed by hot water. It is prepared by the action of carbon tetrachloride on tungsten dioxide at 250°C in a bomb (13). 

Tungsten oxytrichlofide [14249-98-0], WOCl, a green soHd, is prepared by the aluminum reduction of WOCl in a sealed tube at 100—140°C (14). 

Tungsten tribromide [15163-24-3] WBr, prepared by the action of bromine on WBr2, in a sealed tube at 50°C (17), is a thermally unstable black powder that is insoluble in water. 

Iodides. Tungsten tetraiodide [14055-84-6] WI, is a black powder that is decomposed by air. It is prepared by the action of concentrated hydriodic acid on tungsten hexachlotide at 100°C. 

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