Metal carbides mechanism

The mechanisms proposed over the last 50 years for the Fischer-Tropsch synthesis, principally on the basis of studies using heterogeneous catalyst systems, may be divided into three main classes (a) metal-carbide mechanisms (b) hydroxyl carbene, =CH(OH), condensation mechanisms and (c) CO insertion mechanisms. 

The mechanism of the Fischer-Tropsch reactions has been the object of much study (note Eqs. XVI11-55-XV111-57) and the subject of much controversy. Fischer and Tropsch proposed one whose essential feature was that of a metal carbide—patents have been issued on this basis. It is currently believed that a particular form of active adsorbed carbon atoms is involved, which is then methanated through a series of steps such as... 

Titanium Carbide. Carbides of transition metals are known for their hardness, wear resistance and also for their high electrical conductivity, which makes them attractive as a refractory coating material for cutting tools or bearings. Only little work has been done on the electrochemical stability of transition metal carbides with the exception of TiC, where a corrosion and passivation mechanism was suggested by Hintermann et al. [119,120]. This mechanism was confirmed on amorphous TiC produced by metal-... 

The various methods of preparation employed to prepare nanoscale clusters include evaporation in inert-gas atmosphere, laser pyrolysis, sputtering techniques, mechanical grinding, plasma techniques and chemical methods (Hadjipanyas Siegel, 1994). In Table 3.5, we list typical materials prepared by inert-gas evaporation, sputtering and chemical methods. Nanoparticles of oxide materials can be prepared by the oxidation of fine metal particles, by spray techniques, by precipitation methods (involving the adjustment of reaction conditions, pH etc) or by the sol-gel method. Nanomaterials based on carbon nanotubes (see Chapter 1) have been prepared. For example, nanorods of metal carbides can be made by the reaction of volatile oxides or halides with the nanotubes (Dai et al., 1995). 

The revival of interest in Fischer-Tropsch chemistry in the 1970s resulted in new observations that eventually led to the formulation of a modified carbide mechanism, the most widely accepted mechanism at present.202-204,206,214 Most experimental evidence indicates that carbon-carbon bonds are formed through the interaction of oxygen-free, hydrogen-deficient carbon species.206 Ample evidence shows that carbon monoxide undergoes dissociative adsorption on certain metals to form carbon and adsorbed oxygen ... 

The summary provided here illustrates the mechanical properties that characterize the transition metal carbides and nitrides. The materials are hard, strong, and somewhat brittle, and resemble ceramic substances. These properties are reviewed by Santhanam (chapter 2), who also describes the commonly used cobalt-binded cermets. 

In earlier work, it was found for borides, silicides and nitrides that specific activity, expressed as total rate of methane consumption per unit surface area, plummeted with increasing surface area of the catalyst samples.1718 The same relationship was also found for transition metals carbides (Figure 16.4). It should be noted the dependence of specific activity on surface area rather than catalyst composition is unusual for heterogeneous catalytic reactions. In addition, it can be found that the reaction order in the oxidant is perceptibly in excess of 1 (Tables 16.8 and 16.9). Such an order is hard to explain in terms of common mechanism schemes for heterogeneous catalytic oxidative reactions. 

Hogberg, H., Birch, J., Oden, M., Malm, J.O., Hultman, L. and Jansson, U. (2001), Growth, structure and mechanical properties of transition metal carbide superlattices , Journal of Materials Research, 16, 1301-1310. 

Prins summarizes advances in understanding of the reactions in catalytic hydrodenitrogenation (HDN), which is important in hydroprocessing of fossil fuels. Hydroprocessing is the largest application in industrial catalysis based on the amount of material processed. The chapter addresses the structures of the oxide precursors and the active sulfided forms of catalysts such as Ni-promoted Mo or W on alumina as well as the catalytically active sites. Reaction networks, kinetics, and mechanisms (particularly of C-N bond rupture) in HDN of aliphatic, aromatic, and polycyclic compounds are considered, with an evaluation of the effects of competitive adsorption in mixtures. Phosphate and fluorine promotion enhance the HDN activity of catalysts explanations for the effect of phosphate are summarized, but the function of fluorine remains to be understood. An account of HDN on various metal sulfides and on metals, metal carbides, and metal nitrides concludes this chapter. 

We then focus on the Pichler-Schulz CO insertion mechanism (39). This reaction has been much less investigated than the carbide mechanism. We recognize that in homogeneous catalysis, alkene hydroformyla-tion has been investigated extensively it appears that hydroformylation is much more difficult on metallic surfaces than in the presence of mononuclear cationic metal complexes (40). 

Recombination of CH. fragments is an essential step to initiate the chain-growth reaction according to the Sachtler-Biloen carbide mechanism. In a series of elegant papers, Cheng et al. (31-33) reported on the structure dependence as well as on the metal dependence of this class of reactions. Activation energies for CH. —CH recombination on flat and stepped surfaces of cobalt are listed in Table 4. 

The diffusion of carbon in transition metal carbides proceeds via a vacancy mechanism and thus the diffiisivity increases with increasing vacancy concentration in the nonmetal sublattice. However, carbon diffusivities versus composition were measured only for very few types of... 

Single-crystal and poly crystalline transition metal carbides have been investigated with respect to creep, microhardness, plasticity, and shp systems. The fee carbides show slip upon mechanical load within the (111)plane in the 110 direction. The ductile-to-brittle transformation temperature of TiC is about 800 °C and is dependent on the grain size. The yield stress of TiC obeys a Hall Petch type relation, that is, the yield stress is inversely proportional to the square root of the grain size. TiC and ZrC show plastic deformation at surprisingly low temperatures around 1000 °C. 

Carbothermic reactions are sohd-sohd reactions with carbon that apparently take place through intermediate CO and CO2. The reduction of iron oxides has the mechanism Fe Oy-H CO =>xFe-t C02, C02 + C=>2C0. The reduction of hematite by graphite at 907 to 1007°C in the presence of hthium oxide catalyst was correlated by the equation 1 — (1 — x) = kt. The reaction of sohd ihnenite ore and carbon has the mechanism FeH03-l- CO=>Fe -I-Ti02 + C02, CO2 + C = 2CO. A similar case is the preparation of metal carbides from metal and carbon, C -I- 2H2 CH4, Me -I- CH4 MeC -I- 2H2. 

In the steam cracking of hydrocarbons, a small portion of the hydrocarbon feed gases decomposes to produce coke that accumulates on the interior walls of the coils in the radiant zone and on the inner surfaces of the transferline exchanger (TLX). Albright et identified three mechanisms for coke formation. Mechanism 1 involves metal-catalyzed reactions in which metal carbides are intermediate compounds and for which iron and nickel are catalysts. The resulting filamentous coke often contains iron or nickel positioned primarily at the tips of the filaments. This filamenteous coke acts as excellent collection sites for coke formed by mechanisms 2 and 3. Mechanism 2 results in the formation of tar droplets in the gas phase, often from aromatics. These aromatics are often produced by trimerization and other reactions involving acetylene. Some, but not all, of these droplets collect... 

Among Fe, Cu, Ti, Ni, Mo andNb, Fe shows the best catalytic effect on diamond nucleation. Pd has also a positive catalytic effect on diamond nucleation, while Co suppresses diamond nucleation by promoting soot formation. The strong reactivity of these metals with carbon, the formation of metal carbides, the supersaturation of carbon in/on the metals and/or the deformation of graphite sheets by metal atoms to form diamond structure have been proposed as possible mechanisms governing the catalytic effects. 

Thermally, elastically, chemically and electrically they share many of the advantageous attributes of their respective stoichiometric binary transition metal carbides or nitrides they are electrically and thermally conductive, chemically stable. Mechanically they cannot be more different, however they are most readily machinable (Fig. 1.4b) and relatively soft. It is the ability of the basal planes to readily delaminate from each other, instead of fracturing, that renders them unique and why they have been labeled nanolaminates (Fig. 1.4a). 

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