Mo carbides

The other large group of nonstoichiometric phases consists of Zr, Nb, Hf, Ta, and Mo carbides. Band structure calculations of ZrC and NbC,  

The APW-CPA method was used also by Klein, Papaconstantopoulos and Boyer (1980) to calculate the band structure of NbC, HfC, and TaC. As an example. Fig. 4.8 shows the total DOSs for NbCj 0 and NbCo.75. The calculations correctly reproduce the lowering of the DOS in the region of hybridised Nb3d-C2p-band with an increase of vacancy concentration. However, the DOS changes monotonically near the Fermi level. 

The ZrC calculations by the APW-CPA method of Ivashchenko, Lisenko and Zhurakovsky (1983) were more refined. Although they do not reproduce the characteristic two-peak fine strueture of the density of vacancy states, they correctly reproduce the increases of the DOSs at the Fermi level and p, as the vacancy concentration increases. 

The KKR-CPA calculations by Marksteiner et al (1986b) for ZrC and NbC yield results similar to those obtained by the same method as for TiCjt and (see Subsection 4.1.1). Fig. 4.9 gives the total DOSs for NbCo.94 and NbCo.75. Fig. 4.10 shows N( p) as a function of x in ZrC and NbC.  

The band structure of nonstoichiometric Mo carbides with NaCl-type structure was studied by Krainik (1988) using the APW-LCAO-CPA method. The results are similar to those from KKR-CPA calculations for NbC as the concentration of nonmetal vacancies in MoC increases, p and N( p) become lower however, in contrast with NbC,, in MoC,  

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The purpose of this chapter is to present systematic experimental procedures to deposit thin films of carbides and nitrides. Strong relationships between synthesis methods and conditions, and product structures, crystallinity, purity and phase were observed. Thin films of W, Cr, and Mo carbides and nitrides were synthesized in two different ways (1) chemical... 

The particle diameter D is related to the full width at half maximum A by the Debye-Scherrer equation D = 0.9 XIA cos0, where 20 is the diffraction angle and X is the X-ray wavelength. Table 27.1 lists the particle size and lattice plane spacing calculated using the strongest (h,k,l) peak for the Fe, W, Mo carbides, nitrides, oxynitrides and oxycarbides. It is important to note that the calculated particle size using the Debye-... 

The oxide, MoOj, reacts with C to give a lower oxide, MoOj at 420-640°C, followed by the metal above 820°C. Finally the carbide forms after the oxide is eliminated. Because the Mo oxides are so volatile, T must be kept low during the initial phase of this reaction. The reaction is accelerated by the presence of H2 or HCl. Although the Mo carbides are easily freed of oxygen, a final high-T (Ca. 2000°C) vacuum anneal should be used if the material is not previously melted. 

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. 

The reaction of ternary carbides with nitrogen has attracted interest because of a novel type of phase separation occurring in double pseudobinary solid solutions of transition-metal multicomponent carbonitrides. Rudy has investigated the system Ti-Mo-C-N and has found that the homogeneous solid solution (Ti, Mo) (C, N), formed at high temperatures, disproportionates into two phases, both of which are isostructural and have nearly the same lattice parameters. One of the two phases is a Ti carbonitride rich in N and poor in Mo, the other is a Ti-Mo carbide rich in Mo hut poor in N. This type of disproportionating is bound to occur in most of the double-pseudobinary carbonitride systems... 

Fig. 6.3 Calculated adsorption energies of carbon monoxide (a), atomic sulfur (b), sulfur dioxide (c) and thiophene (d) on the metal carbides (Ti carbides, V carbides and Mo carbides) as a function of the C/M ratio. The C/M values of 0.5, 1 and 1.5 correspond to bulk M C, bulk MC, and metcar M Cj, respectively. Reprinted with permission from [16]. Copyright 2004 by the American Institute of Physics...

Since Mo carbides are reported to be active for methanation (ref. 11), one might wonder that the enhancement of alcohol synthesis activity during the C0-reaction and also by the CO-treatment is due to the carburization. The initial activity for the hydrocarbon formation was greatly increased by a factor of 3 after carburization by n-butane and k. On the other hand, gradual increase in alcohol production activity was still observed. From these... 

XPS spectra of and Mo. for alumina supported carbides are very different from those of bulk carbides (ref.4). Besides W and Mo carbides, W or Mo oxidized are present in large amounts, probably as W(Mo)+4, Mo+5 and W(Mo)+ ... 

Mo forms a silicide and a carbide. Geib et al [96] reported Auger sputter-profiling results for Mo sputter-deposited on chemically-cleaned P-SiC. Thermodynamic considerations indicate that reaction of Mo with SiC should not be favourable. Other than a thin reacted layer at the immediate interface, there is no indication of reactive intermixing even after an 800 °C anneal. Hara et al [97] reported AES, Rutherford backscattering (RBS), XRD and transmission electron microscopy (TEM) measurements on Mo layers formed either by plasma deposition or by evaporation on chemically-cleaned a- and P-SiC. Prolonged (1 hr) anneals at 1200°C give an outer layer of Mo carbide on top of a layer of Mo silicide. 

Figure 2. X-Ray diffraction pattern of Ni-Mo-Carbides phases supported on MCM41-PAN-P.

TABLE 37.10 Catalytic Properties of Mo Carbide for Fuel Reforming Applications ... 

Then they proposed Ni-Mo carbide catalyst [35] for the low temperature WGS reaction. Among the various catalysts Nio.25Moo.75 catalyst carburized at 873 K was more active than the other catalysts. Usually, Mo carbide catalysts deactivate with time-on-stream. However, Nio.25Moo.75 catalyst carburized at 923 K exhibits stable activity for 300 min of time-on-stream. The Ni contents of 15% and 25% increased the catalytic activity but further Ni content led to a drop in activity. They proposed that the promotion of the WGS reaction activity was due to the formation of Ni-Mo oxycarbide. 

Physical processes may also be used to deposit Pt onto various types of supports. An example of this type of approach is the preparation of Pt-metal monolayers supported on low-cost transition metal carbides, prepared by magnetron sputtering of Pt onto thin films of W and Mo carbides. While Pt monolayers were achieved on the thin-film electrode geometry used in this study, uniform deposition of Pt onto high surface area particulate materials or mesoporous structures by these methods remains challenging [48]. 

Molybdenum carbide was deposited on y-alumina using a chemical vapor deposition (CVD) technique in a gas mixture of M0CI5/C6H6/H2 at a total pressure of 0.12 kPa (53). Bimetallic Pt-Mo carbide supported on EMT zeolite has been synthesized by vapor deposition of Mo(CO)e onto nanometer-sized Pt dispersed on the zeolite, followed by the decomposition of Mo(CO)e under a hydrogen... 

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