Bulk tungsten carbide

Table 18.1 Surface composition of bulk tungsten carbides from XPS experiments...

Figure 26.5(a) shows the AES profile of bulk tungsten carbide taken with 3.5 keV Ar+ ions and 30 s, of sputtering per step. The profile shows a... 

Leclercq, L., Almazouari, A., Dufour, M., and Leclercq, G. 1996. Carbide-oxide interactions in bulk and supported tungsten carbide catalysts for alcohol synthesis. In Chemistry of transition metal carbides and nitrides, ed. S. T. Oyama, 345-61. Glasgow Blackie. 

At the simplest level, nanoparticles of hard substances are useful as polishing powders which are able to give very smooth, defect-free surfaces. Indeed, 50 nm nanoparticles of cobalt tungsten carbide are found to be much harder than the bulk material. Therefore, they can be used to make cutting and drilling tools that will last longer. 

Table 18.3 Bulk and surface compositions of supported tungsten carbides...

The activities in FT reaction (expressed as turnover rates, Vt of CO transformed to hydrocarbons and oxygenates) of bulk and supported tungsten carbides are compared to that of a rhodium catalyst (3.5 wt%) supported on alumina (Table 18.6). Its dispersion (94%) has been measured by hydrogen chemisorption by assuming unity stoichiometry of adsorbed hydrogen on Rh. 

An investigation of bulk (a-WC, a-W2C) and supported tungsten carbide was carried out using the formation of alcohols in Fischer-Tropsch synthesis as a chemical probe. The active phases were characterized by X-ray photoelectron spectroscopy. 

Chemical analysis of the supported tungsten carbides allows quantification of the amount of carbidic and amorphous carbon,8 and in addition XPS measurements permit calculation of the carbide stoichiometry before and after FT reaction. This determination is not very accurate but it allows general trends to be discerned and correlations with catalytic properties. The bulk and surface compositions of supported tungsten carbides are shown in Table 18.3 in addition to their BET surface areas. 

Table 18.6 Activity of bulk and supported tungsten carbides compared to a 3.5% Rh/Al203 catalyst in FT reaction...

The XRD of the catalyst after 24 h on stream showed only the original W2C phase. No bulk tungsten sulfide phases were detected. The similarity in produce distribution to the Co-Mo-S/A1203 suggests that a surface sulfur layer might have formed on the carbide by reaction with the thiophene. This sulfur layer would be responsible for the reaction. 

Tensile and shear forces are not the only types of loads that can result in deformation. Compressive forces may as well. For example, if a body is subjected to hydrostatic pressure, which exists at any place in a body of fluid (e.g. air, water) owing to the weight of the fluid above, the elastic response of the body would be a change in volume, but not shape. This behavior is quantified by the bulk modulus, B, which is the resistance to volume change, or the specific incompressibihty, of a material. A related, but not identical property, is hardness, H, which is defined as the resistance offered by a material to external mechanical action (plastic deformation). A material may have a high bulk modulus but low hardness (tungsten carbide, B = 439 GPa, hardness = 30 GPa). 

Clean tungsten carbides, a-WC and a-W C, form essentially only hydrocarbons from CO—H2 reactions. At 673 K and atmospheric pressure, the main products on WC, W2C, and W are methane, CO2, and H2O (121). Ethane and propane are also formed at lower temperatures. WC was substantially more active than W2C and W. The nature of the products can be modified by oxide promoters, as for the case of Rh or Pt, or by the carbon vacancies at the surface (122). At 573 K and 5 MPa with 2H2/CO, turnover rates (based on sites titrated by CO chemisorption) of 0.25-0.85 s were reported for hydrocarbon synthesis over bulk and Ti02-supported tungsten carbides. In addition, WC and WC/Ti02 produced alcohols and other oxygenates with 20-50% selectivity. However, W2C of more metallic character did not produce any oxygenates. Coexistence of carbidic and oxidic components on the catalyst surface appeared to be responsible for alcohol formation. 

The most important bulk material is tungsten carbide sintered with a metallic binder which is usually cobalt. It is known as cemented carbide or hard metal (see Ch. 6, Sec. 8.0). Many combinations of carbides and binders are possible and it is estimated that 20,000 tons of these materials are produced annually throughout the world. An unusual and beneficial feature of WC is that it maintains its high hardness value at high temperature (see Ch. 6, Sec. 8.0)... 

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