Bulk metal carbides

Because the enthalpies of formation of such molecular metal carbonyl carbide clusters have yet to be measured or calculated accurately, it has not been possible to calculate their metal-carbon bond enthalpies. However, enough thermochemical information is available on some bulk metal carbides to allow the strengths of both their metal-metal and metal-carbon bonds to be assessed, as indicated in the next section. 

MIXED METAL-CARBON CLUSTERS AND METAL CARBIDES 

Detailed discussion of bulk metal carbides would be inappropriate here, but aspects of their structures and thermochemistry are worth noting. Many metal carbides are metallic-type conductors of electricity, and have structures very similar to those of the bulk metals, with similar metal-metal distances, but with carbon atoms occupying interstitial sites (commonly octahedral holes) in the metal lattice. Thermochemical information is available on enough of them to get some insight into the relative strengths of both their metal-metal and metal-carbon bonding. Unfortunately, the metals that would be of most interest (osmium, rhenium, and rhodium) for the purpose of comparison with the molecular metal carbonyl carbides already discussed are not known to form stable binary carbide phases M cCj, and the carbides of the 3d metals in the same groups as these have very complicated structures. We therefore focus below on carbides of early transition metals, about which more is known.  

Bond enthalpies thus calculated for early transition metal carbides MC and M2C are given in Table 4.3. The carbon atoms occupy octahedral holes in the metal lattice in all cases except WC, in which the carbon sites are trigonal 

TABLE 4.3. Interatomic Distances (Angstrom) and Caicniated Bond Enthalpies (kcal moL ) of Some Metal Carbides 

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As far as phenomenological modeling is concerned, an excellent review of earlier thermodynamic approaches to chemisorption and surface reactivity was given by Benziger (156), who also developed some general thermodynamic criteria for dissociative versus nondissociative adsorption of diatomic and polyatomic molecules on transition metal surfaces (137, 156). In particular, for quantitative estimates of QA, A = C, N, or O, Benziger (156) used the heats of formation of bulk metal carbides, nitrides, and oxides. The BOC-MP approach is different, however, not only analytically but also in making direct use of experimental values of QA. 

The formal relationship between interstitial carbide molecular clusters and M-C binary phases with respect to coordination of the carbide atom and M-C bonding interactions has already been discussed in several reviews [10,11,71,72]. We confine ourselves to the latest results obtained in this field. The best example of bulk metal carbide structural behaviour in a large metal cluster is probably represented by the recently characterized [HNi3g(00) 2 51 penta-anion, which has been obtained by reaction of [Ni5(CO)i2j with hexachloropropene [73]. The Ni32Cb inner core of this cluster is closely related to a fragment of the Cr23C binary phase [74]. 

Anyhow, the proton-induced conversion of carbide atoms to give hydrocarbons, and their corresponding reaction with molecular hydrogen under mild conditions, are indicative of a close analogy in the reactivity of molecular and bulk carbides. Analogous chemical behaviour is indeed shown by bulk metal carbides [113]. The above stoichiometric reactions, in conjunction with the remarkable proton-induced reduction of CO discovered by Whitmire and Shriver [110], allow one to mimic Fischer-Tropsch products with clusters. 

Scientists from Politecnico di Milano and Ineos Vinyls UK developed a tubular fixed-bed reactor comprising a metallic monolith [30]. The walls were coated with catalytically active material and the monolith pieces were loaded lengthwise. Corning, the world leader in ceramic structured supports, developed metallic supports with straight channels, zig-zag channels, and wall-flow channels. They were produced by extrusion of metal powders, for example, copper, fin, zinc, aluminum, iron, silver, nickel, and mixtures and alloys [31]. An alternative method is extrusion of softened bulk metal feed, for example, aluminum, copper, and their alloys. The metal surface can be covered with carbon, carbides, and alumina, using a CVD technique [32]. For metal monoliths, it is to be expected that the main resistance lies at the interface between reactor wall and monolith. Corning... 

In these reactions there is a net generation of the gases CO and C02 this leads to a bulk flow of the gas mixture from the reaction zone. There are other types of reactions involving gaseous intermediates, in which there is no net gas generation. An example is the formation of a metal carbide from the metal and carbon ... 

CHX and hydrocarbon wax are, respectively, the active intermediates formed by the hydrogenation of surface carbide and products of FTS formed by chain growth and hydrogenation of CHX intermediates. The hydrocarbon wax can contain molecules with the number of carbon atoms in excess of 100. Bulk carbide refers to a crystalline CoxC structure formed by the diffusion of carbon into bulk metal. Subsurface carbon may be a precursor to these bulk species and is formed when surface carbon diffuses into an octahedral position under the first surface layer of cobalt atoms. 

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. 

Multiatom metal, metal oxide, metal sulfide, metal carbide, metal nitride, or metal phosphide nanoclusters that are so small that they have properties different from those of the corresponding bulk materials... 

CO, and therefore we exclude the presence of these bulk metal carbonyls. The dissociation of CO by the Boudouard reaction (or the decomposition of palladium carbonyls) would lead to carbon deposition (Kung et al., 2000 McCrea et al., 2001) and should produce a feature at 284.0 eV characteristic of graphite or at 284.4 eV characteristic of amorphous carbon. In the case of carbide species, a feature at lower BE (<283.5 eV) would appear. Even if carbon dissolved in the palladium bulk near the surface region, the escape depth of the Cls electrons (about 2 nm) should have been sufficient to allow its detection. The absence of any carbon-related signals indicates that CO does not dissociate at 400 K and approximately 1 mbar, even over the course of several hours. This result is an important argument in the discussion about the possibility of CO dissociation at high pressure. 

The development of low erosion doped graphites has reached a point where chemical erosion at elevated temperatures can be suppressed. The important parameter appears to be the atomic distribution of metal carbide dopants. The future development must go in the direction of the production of doped bulk CFC materials. 

The present calculations for the UC and TiC reproduce fairly well the peak position of the experimental valence X-ray photoemission spectrum, indicating that the cluster models used are appropriate to treat with the bulk properties of the metal carbides. 

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