Reactions with Organic Compounds
In the following treatise, the very numerous reactions have been excluded which lead to the formation of organotungsten compounds. These will be treated in full detail in the pertinent Gmelin volumes.
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5
Reactions with Organic Compounds
In the following treatise, the very numerous reactions have been excluded which lead to the formation of organotungsten compounds. These will be treated in full detaiL in the pertinent GmeLin voLumes.
5.1
Hydrocarbons
5.1.1
General
Tungsten reacts with hydrocarbons at high temperatures to form carbides WC2 and/or WC. The temperatures for the start of carbide formation seem to depend strongLy on the kind of tungsten sampLes (powders, foiLs, fiLaments, etc.) and the hydrocarbon pressure used and to a Lesser degree on the nature of the individual hydrocarbons. The lowest temperatures reported are in the range of 800 to 900°C. Not compLeteLy understood are the conditions under which the carbides W2C or WC are produced. More recent versions of the phase diagram of the W-C system wouLd, for example , predict W2C to be unstabLebelow about 1200°C. However, W2C has been reported to form by reaction of W with gasoline vapors at temperatures as Low as 1000°C. This may have kinetic reasons. W2C seems to be at least a reaction intermediate in almost every case. In addition to thermodynamic and kinetic factors inherent in the W-hydrocarbon systems, foreign influences Like the presence of other gases (hydrogen , by its nature, directly intervenes in the react ion course) and impurities in and on the metaL may be operative and affect the appearance and persistence of the reaction products. Carbide production by carburization of tungsten with hydrocarbons has the advantage of much better controL of carbon content and grain size compared with the direct combination of W with eLementaL C. On the other hand, the react ion with smaLL amounts of hydrocarbons on the metaL or in the ambient may Lead to serious difficulties in the process ing of tungsten , because even minute amounts of carbides cause a drastic decrease of the workability by embrittlement. The adsorption of hydrocarbons on tungsten usually results in the formation of gaseous hydrogen, even at slightly increased temperatures. This , in most cases, is a step process Leading to var ious surface intermediates and ending at higher temperatures in the formation of surface and/or buLkcarbides. Desorption of the hydrocarbons and the ir reaction products may be a concomitant process when the adsorbate-Loaded surfaces are heated from Low tempera ; tures or when the pressure is reduced. Other surface react ions reported are the hydrogenation of unsaturated hydrocarbons by added or seLf-reLeased hydrogen and the hydrocracking of hydrocarbons with Longer C chains to smaller C chain fragments. General Reference:
J. C. Bertolini, J . Massardier (Hydrocarbons on MetaLs, in : D. A. King, D. P. Woodruff, The ChemicaL Physics of Solid Surfaces and Heterogeneous CataLysis, VoL. 3, ELsevier, Amsterdam - Oxford - New York 1984, pp.107/36).
5.1.2 5.1.2.1
Methane Carbide Production
At temperatures well above 1000 K tungsten reacts with CH4 to form the monocarb ide or dicarbide, according to W(s) + CH4(gas) ~ WC(s) + 2H2(gas) or 2W(s) + CH4(gas) ~ W2C(
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