Graphitizing carbons, mechanisms formation

Fig. 7. Growth mechanism of graphitic carbon nanofibers. The illustration highlights the observation of spontaneous nickel step edge formation at the carbon-nickel interface. The observations in Reference (52) are consistent with a growth mechanism involving surface transport of carbon and nickel atoms along the graphene-nickel interface.

Graphitized carbon blacks, thus undoubtly display reinforcing abilities which become obvious when considering the tensile strength of the unfilled vulcanizate. It follows that the formation of a filler-elastomer chemical bond is not a requirement for reinforcement to occur. It strongly participates, however, in its effectiveness, and determines the good mechanical properties connected with rubber reinforce-... 

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. 

The reactions shown in Eqs. 3a and 3c are endothermic in nature, whereas the water-gas shift (WGS) reaction shown in Eq. 3b is moderately exothermic. If these reactions are carried out externally (external reformaticm), the efficiency and operation of the cell are significantly affected. Hence, internal reforming is preferred however, Ni acts as an excellent coking catalyst. As a consequence, in the presence of carbonaceous fuels (and in the absence of sufficient water vapor), there is always a possibility of deposition of carbon filament on the surface of Ni. The mechanism involves carbon formation on the metal surface followed by dissolution of the carbon into the bulk of the metal and finally precipitation of graphitic carbon at some surface of the metal particles after it becomes supersaturated with carbon [6]. It not only reduces the active sites for reactions mentioned in Eqs. 2c-j and 3a-c but also destroys the whole anode over a period of time. The following three reactions are the most probable catalytic reactions that lead to carbon formation in high-temperature systems ... 

Slde-on attack between cumulenes (left) or polyacetylenes (right), where one of the reactants may be ionic, can lead to carbon-ring formation through cross-bonding. This corresponds to the Inverse of the mechanism proposed for the transformation of graphite at high temperatures (33) which is analogous to the transformation of benzene to acetylene. 

The type of carbon formed depends on the reaction conditions, whereby higher reaction temperatures favour the formation of the more inert-graphitic carbon nanotubes. Based on kinetic and isotopic investigations on the mechanism of DRM reaction over Ni/MgO catalysts, Wei and Iglesia [189] observed a similarity in turnover rates and first-order rate constants with methane decomposition. Hence, they concluded that the dissociation is the kinetically relevant step for the DRM reaction and that the Ni behavior resembles that of supported noble metal catalysts (Rh, Pt, Ir, Ru). 

A different mechanism is involved in the milling with graphite, carbon black, quartz, silica gel and glass, titanium or zinc oxide [196]. Chemical bonds are broken with formation of radicals on the inorganic surfaces. The... 

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