Moderators tungsten

In addition, molybdenum has high resistance to a number of alloys of these metals and also to copper, gold, and silver. Among the molten metals that severely attack molybdenum are tin (at 1000°C), aluminum, nickel, iron, and cobalt. Molybdenum has moderately good resistance to molten zinc, but a molybdenum—30% tungsten alloy is practically completely resistant to molten zinc at temperatures up to 800°C. Molybdenum metal is substantially resistant to many types of molten glass and to most nonferrous slags. It is also resistant to hquid sulfur up to 440°C. 

Syntheses from Dry Metals and Salts. Only metaUic nickel and iron react direcdy with CO at moderate pressure and temperatures to form metal carbonyls. A report has claimed the synthesis of Co2(CO)g in 99% yield from cobalt metal and CO at high temperatures and pressures (91,92). The CO has to be absolutely free of oxygen and carbon dioxide or the yield is drastically reduced. Two patents report the formation of carbonyls from molybdenum and tungsten metal (93,94). Ruthenium and osmium do not react with CO even under drastic conditions (95,96). 

Alloy Compositions and Product Forms. SteUite 21, an early type of cobalt-base high temperature alloy, is used primarily for wear resistance. The use of tungsten rather than molybdenum, moderate nickel contents, lower carbon contents, and rare-earth additions typify cobalt-base high temperature alloys of the 1990s as can be seen from Table 5. 

Molybdenum High melting point less dense than tungsten or tantalum moderately ductile at room temperature Extremely high oxidation rate (volatile oxide)... 

Pt. M, Brissey, Anal. Chem., 24, 1034 (1952), showed that the Lai lines of tantalum and tungsten could be resolved without interference from the second-order niobium Ka lines on the General Electric XRD-3 spectrograph in transmission through a bent mica crystal (4.8). Intensity was only moderate. 

Molybdenum and tungsten are unique in that they are resistant to sulfur, and, in fact, are commonly sulfided before use. The Bureau of Mines tested a variety of molybdenum catalysts (32). They are moderately active but relatively high temperatures are required in order to achieve good conversion, even at low space velocities. Selectivity to methane was 79-94%. Activity is considerably less than that of nickel. Although they are active with sulfur-bearing synthesis gas, the molybdenum and tungsten catalysts are not sufficiently advanced to be considered candidates for commercial use. 

Lithium is used to reduce metallic oxides in metallurgical operations, and the reactions, after initiation at moderate temperatures, are violently exothermic and rapid. Chromium(III) oxide reacts at 185°C, reaching 965° similarly molybdenum trioxide (180 to 1400°), niobium pentoxide (320 to 490°), titanium dioxide (200-400 to 1400°), tungsten trioxide (200 to 1030°), vanadium pentoxide (394 to 768°) also iron(II) sulfide (260 to 945°), and manganese tclluridc (230 to 600°C)... 

In a noteworthy series of studies, Herndon has shown that cyclopropylcarbenes can be used as four-carbon components in molybdenum- and tungsten-mediated [4 + 2 + l]-reactions with alkynes and carbon monoxide (CO). These reactions give cycloheptadienones in moderate yields and with moderate selectivity (Equations (26)—(28)). The mechanism of this reaction is proposed to proceed through a series of steps involving metathesis, GO insertion, ketene formation, cyclopropane cleavage, and finally reductive elimination (Scheme 43).133... 

To prevent the latter mentioned subsequent reactions, the bulky phos-phaalkyne Ph C P as well as tungsten alkoxides of reduced size as, e.g., [W2(ONp)6] were employed in these three-component reactions with no significant success [15]. The crucial steps for the side-product free synthesis of the phosphido complexes 18 are the introduction of a phosphaalkyne possessing a moderate steric bulkiness, which lies between those of f-BuC=P and Ph C P, and resulting from the P-NMR studies (cf. Eq. 5), a reaction temperature mode allowing the complete metathesis reaction to take place at very low temperatures over a long period of time until all the phosphaalkyne has been converted into the metathesis products (about 12 h) only then is the reaction mixture allowed to reach room temperature. We found that MesC=P meets these steric requirements, and the three-component-reaction between MesC=P, [W2(Of-Bu)6] and [M(CO)5(thf)] (M = Cr, W), carried out at -78 °C and warmed up to ambient temperature within 15 h, succeeded in the synthesis and isolation of the phosphido complex 18a,b (Scheme 2) [15]. Furthermore, if t-BuC=P is incorporated into these reactions, the steric requirements of the alkoxide dimer has to be slightly increased. Thus, f-BuC=P reacts with [W2(OPh )6] and [M(CO)5(thf)] (M=Cr, W) under the... 

A good metallic fuel resists air oxidation and moisture, has a high heat output per gram, and is obtainable at moderate cost in fine particle sizes. Aluminum and magnesium are the most widely used materials. Titanium, zirconium, and tungsten are also used, especially in militaiy applications. 

Legzdins et al. have reported the formation of 0x0 alkyls of tungsten and molybdenum in moderate yields (45-60%) according to Eq. 6 [18,19]. 

The electrosynthesis of hydride complexes directly from molecular hydrogen at atmospheric pressure by reduction of Mo(II) and W(II) tertiary phosphine precursors in moderate yield has been described as also the electrosynthesis of trihydride complexes of these metals by reduction of M(IV) dihydride precursors [101,102]. Hydrogen evolution at the active site of molybdenum nitrogenases [103] is intimately linked with biological nitrogen fixation and the electrochemistry of certain well-defined mononuclear molybdenum and tungsten hydrido species has been discussed in this context [104,105]. 

Pentacarbonyl(diphenylmethylene)tungsten(0) is a moderately air-stable soild that is readily soluble in most organic solvents. The resulting solutions are air and light sensitive and decomposed thermally at about 50°. The infrared spectrum of a heptane solution shows bands in the metal carbonyl region at 2070 (m), 1971 (s), and 1963 (s) cm"1, characteristic of a group VI pentacar-bonyl species. The proton NMR spectrum in CS2 or acetone-d6 shows a complex multiplet at 5 7.2 relative to internal tetramethylsilane. 

Furo[2,3- ]pyridines are generated in a cyano [4-1-2] reaction that uses tungsten alkynols and aldehydes which cyclize to 1,3-dienes prior to reaction with nitriles (Scheme 9) <1998JA4520>. The reaction, which is activated by photolysis or with Me3N0-H20, works with unactivated nitriles in both intramolecular and intermolecular reactions to give moderate to good yields of product. 

The selectivity of alcohol depends on the carbide preparation. A maximum in alcohol is achieved for the sample WC/Ti02 (T3) for which the preparation of the carbide combines reduction and carburization steps at moderate temperatures (respectively, 873 K and 1073 K). In this case, anionic vacancies stabilized by mixed oxides are formed, associated with carbon vacancies in mixed carbides resulting in a better interaction of carbidic and oxidic phases. On silica, ceria and zirconia, the extent of carburization is too high and the interaction of the carbide phase with the oxide support is suppressed giving larger isolated particles of tungsten carbide with low dispersion. 

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