Carbide catalysts

Carbothermal synthesis of nano-sized tungsten carbide catalyst... 

The application of ly transition metal carbides as effective substitutes for the more expensive noble metals in a variety of reactions has hem demonstrated in several studies [ 1 -2]. Conventional pr aration route via high temperature (>1200K) oxide carburization using methane is, however, poorly understood. This study deals with the synthesis of supported tungsten carbide nanoparticles via the relatively low-tempoatine propane carburization of the precursor metal sulphide, hi order to optimize the carbide catalyst propertira at the molecular level, we have undertaken a detailed examination of hotii solid-state carburization conditions and gas phase kinetics so as to understand the connectivity between plmse kinetic parametera and catalytically-important intrinsic attributes of the nanoparticle catalyst system. 

This complex and structurally related molecules served as a functional homogeneous model system for commercially used heterogeneous catalysts based on chromium (e.g. Cp2Cr on silica - Union Carbide catalyst). The kinetics of the polymerization have been studied to elucidate mechanistic features of the catalysis and in order to characterize the potential energy surface of the catalytic reaction. 

Silica-supported heterogenous Cr systems, such as the Phillips247,248 and Union Carbide catalysts,249,250 are used in the commercial production of polyethylene. The active sites are widely agreed to contain low-valent Cr centers. The relatively ill-defined nature of these catalysts has led to considerable efforts to synthesize well-defined homogeneous Cr-based catalysts. 

The rates of transesterification of triglycerides to methyl esters, efficiently catalyzed by boron carbide (B4C), were, on the other hand, faster under microwave conditions, probably because of superheating of the boron carbide catalyst, which is known to be a very strong absorber of microwaves [40], Scheme 10.3. Yields of methyl ester of up to 98% were achieved. 

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. 

Wang, Y., and Davis, B. H. 1999. Fischer-Tropsch synthesis Conversion of alcohols over iron oxide and iron carbide catalysts. Applied Catalysis A General 180 277-85. 

Carbenium ions, 42 115, 143 acid catalysis, 41 336 chemical shift tensors, 42 124-125 fragments in zeolites, 42 92-93 history, 42 116 superacids, 42 117 Carbide catalysts, 34 37 Carbidic carbon, 37 138, 146-147 Carbidic intermediates, 30 189-190, 194 Fischer-Tropsch synthesis, 30 196-197, 206-212... 

Transition metal carbide catalysts have also been explored as methane partial oxidation catalysts [110] promising results were obtained over M02C systems and enhancements were reported with the addition of transition metal promoters. 

In October, 1910, the author had established the fact that certain magnetites as well as synthetic iron catalysts can be as effective as the uranium carbide catalyst which, in the hands of F. Haber, had proved to be of outstanding activity. At 500°C. and at a pressure of 100 atmospheres and a gas velocity of 50 liters per hour, 5 volume % of ammonia were obtained in the exit gases with 2 g. of a magnetite and 4.5% with a synthetic iron catalyst, the latter being operated at 550°C. 

Ji et al. have shown that nickel-promoted tungsten carbide catalysts can also directly convert cellulose to ethylene glycol in a one step process [52, 53]. These Ni-W2C/AC catalysts exhibited a remarkable higher selectivity for ethylene glycol than Pt/Al203 and Ru/C. Indeed, after 30 min of reaction at 518 K under 60 bar of H2, cellulose was completely converted into water-soluble polyols over a 2% Ni-30% W2C/AC-973 catalyst (61% yield in ethylene glycol). 

The role of complexes and modifiers in the reduction has been discussed in [20-24]. The investigation of the liquid phase catalytic hydrogenation of N03 , N02, and NH2OH at tungsten carbide catalyst furnished interesting information concerning the electrocatalytic properties of the latter [25]. 

THE CHEMISTRY OF TRANSITION METAL CARBIDES AND NITRIDES 16.2 Carbide catalysts and their characterization... 

Also in prior work14 the specific catalytic activity in the H2 +02 reaction was shown to decline in the following order W > WC > W03, i.e. a carbide stands midway between the corresponding metal and oxide. In subsequent experiments a similar result was obtained for other carbide catalysts (Table 16.4). 

The carbide catalysts (WC, W2C) were compared to the precursor W03 in the FT reaction after the same pretreatment in hydrogen at 673 K for 10 h (Table 18.2). The space velocity was maintained constant (4500 h 1)... 

The intrinsic nature of tungsten carbide catalyst in CO-H2 reactions is to form hydrocarbons. This property can be modified by oxidic promoters as for the case of noble metals like Pt or Rh or by the presence of carbon vacancies at the surface. To increase the production of alcohols in the Fischer-Tropsch reaction, the catalyst should be bifunctional, with oxidic and carbidic components as in the case of WC on Ti02. Overcarburization of WC on supports like Si02 or Zr02 where the W-O-metal interaction is weak leads to C/W ratios close to unity and does not result in alcohol formation. 

The ratio of C02 produced to CO converted is 0.5 for this case. The C02/ CO ratios presented in Table 19.2 show the water-gas shift activity increased in the order Fe carbide < precipitated < ultrafine oxide. The ability of the Fe carbide catalyst to suppress the water-gas shift enables it to have a high hydrocarbon production. 

Figure 19.6 XRD results for iron carbide catalyst synthesized by laser pyrolysis at various times of the Fischer-Tropsch Synthesis (1-Fe304, 2-x-Fe5C2, 3-e -Fe2 2C).

Figure 22.1 XRD pattern of the tungsten carbide catalyst after calcination at 750 K.

Dimethylsulfide was added to the n-heptane to give 1000 ppm S in the gas phase. Figure 22.3 shows the effect of sulfur on conversion. The platinum catalyst lost its activity in about 3 h, whereas the tungsten carbide catalyst was very slightly affected during the experiment. Sulfur tolerance is important in petroleum refineries, since it may allow the substitution of the costly noble metals with carbides in streams containing sulfur. 

Figure 24.9 A comparison of catalytic performances of iso-butane dehydrogenation on vanadium and on vanadium carbide catalysts. The reaction was carried out in a circulating batch reactor. The initial partial pressure of isobutane was 13.3 kPa Torr, which was mixed with He for a total pressure of 100 kPa.

The typical BET surface area of the freshly prepared iron carbide catalyst is approximately 70 m2 g 1. The surface area of the precipitated catalyst and ultrafine catalyst before pretreatment was 140 m2 g 1 and 250 m2 g 1, respectively however, following pretreatment with CO the surface areas dropped to 32 m2 g 1, and 64 m2 g-1, respectively. The particle sizes of the iron carbide and precipitated catalysts after 170 h, determined by X-ray line broadening, were 27 nm and 30 nm, respectively. The ultrafine catalyst had an average particle size of 25 nm after 240 h of synthesis. These particle sizes correspond to a surface area of about 40 m2 g-1. 

Figure 19.2 Synthesis gas conversion as a function of time for the iron carbide catalyst synthesized by laser pyrolysis (weight = 12.0 g, Sg = 70 m2 g ). O, CO , H2 O, CO +...

Hydrocarbon production and selectivities at comparable CO conversion are given in Table 19.2. The ultrafine iron oxide catalyst had a very poor C2-C4 olefin selectivity while the olefin selectivity of the precipitated catalyst was slightly higher than the iron carbide catalyst. This is surprising because Rice et al. report higher olefin selectivity for a similar iron carbide catalyst than a conventional Fe/Co catalyst.6 Soled et al. have subsequently reported that the conventional catalyst contains acidic sites which... 

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