Coke formation dendrites

In catalysis of cyclohexane on platinized spirals of nichrome heated by electric current, the author and Kotelkov observed 327) considerable coke formation. A puzzling feature of this process is that with the accumulation of coke the reaction rate not only does not fall but at first even increases. This was explained by the author with the help of the dendrite hypothesis of coke formation coke grows in the form of dendrites the foundations of which lie on the interface between chromium (or chromia) and nickel (see above). Dendrites do not prevent the molecules of cyclohexane from getting to the surface of nickel, just like trees that do not prevent the rain from getting into the ground in the forest. The initial acceleration of the reaction with the accumulation of coke was explained by the corrosion of metal because of the formation of carbides as the first stage of coke formation and of the decomposition of these carbides. 

The considerations stated fit the assumption of monolayer coke coverage. In agreement with this model, the inner catalyst surface, laying beyond the radial position of intensified formation of coke precursors, appears in fact accessible, and in this sence not useless, although of reduced efficiency. The problems discussed gain in importance when coke deposits grow in the form of dendrites. 

The formation of carbon-mineral adsorbents containing carbon deposits in the form of dendrites, whiskers or carbon black is not advantageous because of the poor mechanical properties of these deposits. The morphology of the coke depends on the mechanism and conditions of its formation on the mineral surface. Two main mechanisms of formation of carbon deposits can be distinguished consecutive reactions and carbide-forming. The latter mode consists in the thermal decomposition of hydrocarbons. 

Balandin et al. (104) found that the chromia catalyst for the dehydrogenation of butene does not diminish its activity for a rather long time, in spite of the formation of coke. Since not dendrites but tar films are formed on the oxides, it was concluded that the molecules of the decomposition products migrate on the surface, setting free the active centers and accumulating on the inactive sites of the catalyst. 

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