Edge sites correlation with activity

In the case of palladium particles supported on magnesium oxide, Heiz and his colleagues have shown,29 in an elegant study, a correlation between the number of palladium atoms in a cluster and the selectivity for the conversion of acetylene to benzene, butadiene and butane, whereas in the industrially significant area of catalytic hydrodesulfurisation, the Aarhus group,33 with support from theory, have pinpointed by STM metallic edge states as the active sites in the MoS2 catalysts. 

In this paper, we presented new information, which should help in optimising disordered carbon materials for anodes of lithium-ion batteries. We clearly proved that the irreversible capacity is essentially due to the presence of active sites at the surface of carbon, which cause the electrolyte decomposition. A perfect linear relationship was shown between the irreversible capacity and the active surface area, i.e. the area corresponding to the sites located at the edge planes. It definitely proves that the BET specific surface area, which represents the surface area of the basal planes, is not a relevant parameter to explain the irreversible capacity, even if some papers showed some correlation with this parameter for rather low BET surface area carbons. The electrolyte may be decomposed by surface functional groups or by dangling bonds. Coating by a thin layer of pyrolytic carbon allows these sites to be efficiently blocked, without reducing the value of reversible capacity. 

To summarize, one can say that the electrochemical performance of CNT electrodes is correlated to the DOS of the CNT electrode with energies close to the redox formal potential of the solution species. The electron transfer and adsorption reactivity of CNT electrodes is remarkably dependent on the density of edge sites/defects that are the more reactive sites for that process, increasing considerably the electron-transfer rate. Additionally, surface oxygen functionalities can exert a big influence on the electrode kinetics. However, not all redox systems respond in the same way to the surface characteristics or can have electrocatalytical activity. This is very dependent on their own redox mechanism. Moreover, the high surface area and the nanometer size are the key factors in the electrochemical performance of the carbon nanotubes. 

The optimum amount of Co defined as the smallest amount of Co required for the maximum or plateau HDS activity of the Co-Mo catalyst was found to increase as the dispersion of the Mo sulfide species. Figure 9 correlates the promotional ratio with the optimum amount of Co as expressed by the Co/Mo atomic ratio. A linear correlation in Fig. 9 may suggest that the Co species decorating the edge sites of the Mo sulfides are responsible for the HDS activity. The optimum Co/Mo atomic ratio reached unity at the highest dispersion of the Mo sulfide species. 

Finally, Wivel and co-workers (112) have correlated the amount of NO adsorbed with EXAFS experiments for numbering edge sites in the M0S2 crystals. They concluded that only 10% of the edge sites are active and that two kinds of sites exist corner sites with high HDS activity and edge sites with low HDS activity. 

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