Transition metals carburizing

The equation for the rate of oxidation of the transition metals at high temperatures, which form a solid solution of oxygen before the oxide appears at the surface has die same form as that derived for die carburizing of die metal, and... 

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. 

The first syntheses of transition metal nitrides and carbides were derived from metallurgical processes,1,3 and consisted in the nitridation or carburization of the metal or of the oxide at severe conditions, in particular at high temperatures (>1500 K). Consequently, the resulting powders generally had low specific surface areas (Sg). 

The XRD pattern (Figure 7.18(a)) indicates that the obtained sample is cubic phase TiC, with cell constant a= 4.34A. TEM imaging (Figure 7.18(b)) reveals that it consists of nanoparticles of size 10-20 nm [67]. The co-reduction carburization method may be used to prepare other transition metal carbides. For example, as shown in Reaction (20), nanocrystalline ZrC, size 10-25 nm was synthesized on heating at 550 °C for 12 h [68]. 

Iron is the industrial Fischer-Tropsch catalyst and is applied in practice. Reduced iron interacts strongly with carbon. Because the activation energy for carbon diffusion into the metallic iron lattice is low (40-65 kJ/mol), the metal converts to iron carbides during reaction. According to Niemantsverdriet et al, the initial rate of the Fischer-Tropsch reaction is low because the carburization process consumes most of the carbon. When the iron particles become saturated, carbon stays at the surface where it is available for the actual Fischer-Tropsch reaction. Molybdenum is also converted to a carbide or an oxide when exposed to synthesis gas in this state it is an active catalyst. However, the early transition metals form stable but unreactive compounds in synthesis gas and are inactive as Fischer-Tropsch catalysts. 

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