W carbides

The first nonself-consistent APW calulations of the WC monocarbide electronic structure were carried out by Alekseev, Arkhipov and Popova (1982). More correct results for hexagonal WC were obtained later by Mattheiss and Hamann (1984) using the LAPW method. It was shown that the band structure of WC contains much broader (as compared with B1 monocarbides) bands of hybridised C2p-W5d states with a larger energy interval between these bands and their antibonding counterparts. This is evidence of stronger covalency in WC. The Fermi surface of hexagonal WC shows several small hole and electron pockets near the T, K, L points in the Brillouin zone. For WC the total DOS at the Fermi 

As a result, the 5d-orbital populations are considerably higher in WjC than in W and WC. The data allowed the authors to conclude that the catalytic activity of pure W C could be higher than that of WC. There are some indirect justifications of the validity of this proposition. Samsonov and Kharlamov (1975) observed higher catalytic activity of M02C, which is isoelectronic and isostructural to WjC, as compared with 

Before starting the discussion of the Vila and vina d-metal carbides, it is worth noting that as the valency of the metal increases both the C content of the compounds formed and the stability of the compounds decrease. 

Mn forms four cabides with a maximum C/M ratio of about 0.43, but for Fe and Co carbides this quantity does not exceed 0.33. 

Some characteristics of chemical bonding in these phases are given in Table 2.3. As is seen, the M-C bonds are formed, as in the carbides considered above, mainly due to the overlap of the M3d and C2p AOs. C-C bonds are practically absent. The M-M bond populations are bigger than the M-C bond populations, but these quantities prove to be of the same order of magnitude, and metal-carbon bonds cannot be neglected in the description of chemical bonding in these carbides. 

---

Weimer, A. W., Carbide, Nitride and Boride Materials Synthesis and Processing, Chapman Hall, London, 1997. 

Table 14.2 shows that an increase in reaction temperature always leads to an increase in carbon concentration for the Mo and W carbides. 

Synthesis of thin films ofCr, Mo and W carbides and nitrides 274... 

Surface oxides play a paramount role in imparting the electrocatalytic activity to W carbides [92, 426 - 429]. W is known to be covered by a thin oxide layer which... 

Fig. 21. Effect of carbon deficit on hydrogen evolution in 0.5 M H2S04, 20 °C, on W carbides. (1) WC (2) WC0 9. After ref. 92, by permission of Plenum Press.

For hydrogen oxidation bi-component metal doped systems deposited on Raney nickel for AFC Mo and W carbides for AFC prepared by method of precipitation from a gas phase Radicals of following composition -OH, -OSO3H, -COOH, -OPO(OH)3 for PEMFC Some organic catalysts like biologically active [NiFe]-hydrogenase, pyropolymers, etc. ... 

Two carbides are found at RT in this system, the metastable W2C with three crystal forms, and WC. Cubic WC, also exists at high T but cannot be retained on cooling. The preparation of the W carbides is similar to that of the Mo carbides. 

More publications were found related to carbides. First, Suslick s early report [64] that certain carbonyls sonicated in a decalin solvent under argon. For Fe and Co, nanostructured metals are formed for Mo and W, metal carbides (e.g., M02C) are produced. Molybdenum carbide was used later as a catalyst. The selectivity and catalytic activity of the Mo and W carbides was examined in the dehydrogenation of alkanes [140]. Another carbide that has already been mentioned is that of Pd [65], which was prepared by Maeda s group. Iron carbide was a byproduct that served as protective layer in Nikitenko s work on air-stable iron nanoparticles [70]. 

Several catalyst samples of tungsten carbide and W,Mo mixed carbides with different Mo/W atom ratios, have been prepared to test their ability to remove carbon monoxide, nitric oxide and propane from a synthetic exhaust gas simulating automobile emissions. Surface characterization of the catalysts has been performed by X-ray photoelectron spectroscopy (XPS) and selective chemisorption of hydrogen and carbon monoxide. Tungsten carbide exhibits good activity for CO and NO conversion, compared to a standard three-way catalyst based on Pt and Rh. However, this W carbide is ineffective in the oxidation of propane. The Mo,W mixed carbides are markedly different having only a very low activity. 

Ni-Mo and Ni-W carbides phases supported on mesostructured MCM-41 with polymeric carbon were prepared in order to give structural stability to support. Surfaces as well as the structural properties of materials were studied. XRD and TEM analysis showed a similar structure before and after the treatment although geometrical and periodicity arrangements decrease lightly. 

In the second part of this study the carbon-coated materials were used as supports for Ni-Mo and Ni-W carbides synthesis. 

For high-temperature applications, carbides are used as pure-material sintered parts or in a Co/Mo/W/carbide sintered composite. They outperform standard alloys and superalloys in rocket nozzles and jet engine parts where erosion resistance at... 

Samples of quartz crystals were crushed into micron size particles using vibro-set with W-carbid. They did not indicate the condition of the pounding of the crystals although it is well known that crushing of the optically active quartz particularly with vibro-mil can cause twinnings formation, amorphisation of quartz, and its hydrophilisation... 

Colloids and amorphous metals and alloys are further interesting nontraditional catalysts, but there are difficulties in manufacturing them reproducibly. Other development possibilities are represented by transition metal compoimds such as Mo and W carbides... 

Interest in new compositions and new synthetic routes in the context of catalysis is growing, and recent examples of the synthesis and use of non-oxidic, ceramic compositions in catalysis include SiC as a support for Ni or Pt in CO hydrogenation (2), SiC as a support for Co and Mo for thiophene hydrodesulphurisation (3), transition metal (Ti, Ta, Mo or W) carbides for methanol decomposition (4), early transition metal carbides, nitrides or borides for hydrodenitrogenation of quinoline (5), and the synthesis of high surface area molybdenum carbide (6). 

Tri] Trindade, B., Vieira, M.T., Amato, A.M., Cime, J.S., Mechanical Behaviour of Sputtered M-Fe-C (M=Cr, Mo, W) Carbides , Fundamentals of Nanondentation and Nanotribology. Symposium. Mater. Res. Soc., 305-310 (1998) (Crys. Stracture, Morphology, Experimental, Meehan. Prop., 7) cited from abstraet... 

Usually, M02C and WjC coatings are applied onto non-oriented metal substrates by electrolysis of Na2W04-based oxide melts [10, 11]. However, in many instances, it is necessary to obtain continuous cathode deposits with preset properties (structure, orientation, and crystallite size). Therefore, an important role in electrolysis is played by the initial stages of crystal nucleation. In Refs [12-14], the results of such a study for electrodeposited Mo and W are presented. The initial stages of nucleation of Mo and W carbides have not been yet studied. The only known investigation was performed for platinum and glassy carbon electrodes [15]. 

WEI 92] WEIMER A.W., Carbide, nitride and boride materials synthesis and processing. Chapman and Hall, London, 1992. 

---