Activation, carbon surfaces mechanisms

In the literature it has been found that during mixing aggregate breakdown occurs for a number of carbon blacks in highly viscous rubbers [115— 118]. Recently, the aggregate breakdown was also attributed to classes of specific shapes of individual carbon blacks [108]. The opinion about the mechanical consequences of this process is quite different. On the one side, no obvious relationship to reinforcement is conjectured [116]. On the other side, improvements of the mechanical performance, due to the creation of new, active carbon surface, is assumed, which participates in formation of a strong filler-rubber coupling [118]. 

Finally, and in apparent contradiction with both the results of Yoshida et al. [215] and the report by Huang [97], the efficiency of removal was in all cases lower in acidic than in neutral solutions. This was explained by noting that enhanced reduction is favored at higher pH, as shown in the above equations. More recently. Adams [220] performed electron microscopic analyses of an activated carbon surface that was contacted with HgCb solution and confirmed the presence of both Hg and Cl, thus endorsing the importance of the reduction mechanism. 

In addition to the influence of the basicity of the AC surface, Faria et al. [166], studying the effect of pH on the catalytic decomposition of ozone, postnlated that the electrostatic interactions between activated carbon surface and the solutes involved in the mechanism (snch as OH ions) may also play a role. When the pHpzc of the activated carbon is higher than the pH of the solntion, the surface of the material becomes positively charged, enhancing the attraction of hydroxide ions and, in particnlar, improving the heterogeneous decomposition of ozone. 

Fig. 35. Island mechanism of MM adsorption/oxidation on activated carbon surface...

Several mechanisms have been proposed to explain the activation of carbon surfaces. These have Included the removal of surface contaminants that hinder electron transfer, an Increase In surface area due to ralcro-roughenlng or bulld-up of a thin porous layer, and an Increase In the concentrations of surface functional groups that mediate electron transfer. Electrode deactivation has been correlated with an unintentional Introduction of surface contaminants (15). Improved electrode responses have been observed to follow treatments which Increase the concentration of carbon-oxygen functional groups on the surface (7-8,16). In some cases, the latter were correlated with the presence of electrochemical surface waves (16-17). However, none of the above reports discuss other possible mechanisms of activation which could be responsible for the effects observed. 

The purpose of this paper Is 1) to describe the electrochemistry of ferrl-/ferro-cyanlde and the oxidation of ascorbic at an activated glassy carbon electrode which Is prepared by polishing the surface with alumina and followed only by thorough sonlcatlon 2) to describe experimental criteria used to bench-mark the presence of an activated electrode surface and 3) to present a preliminary description of the mechanism of the activation. The latter results from a synergistic Interpretation of the chemical, electrochemical and surface spectroscopic probes of the activated surface. Although the porous layer may be Important, Its role will be considered elsewhere. 

The adsorption action of activated carbon may be explained in terms of the surface tension (or energy per unit surface area) exhibited by the activated particles whose specific surface area is very large. The molecules on the surface of the particles are subjected to unbalanced forces due to unsatisfied bonds and this is responsible for the attachment of other molecules to the surface. The attractive forces are, however, relatively weak and short range, and are called Van der Waals forces, and the adsorption process under these conditions is termed as a physical adsorption (physisorption) process. In this case, the adsorbed molecules are readily desorbed from the surface. Adsorption resulting from chemical interaction with surface molecules is termed as chemisorption. In contrast to the physical process described for the adsorption on carbon, the chemisorption process is characterized by stronger forces and irreversibility. It may, however, be mentioned that many adsorption phenomena involve both physical and chemical processes. They are, therefore, not easily classified, and the general term, sorption, is used to designate the mechanism of the process. 

Carried out testing confirmed the supposition about the degradation mechanism of electrode in NiOx-carbon ultra capacitors (Figure 1). Obviously, the smaller are the dimensions of particles of conductive additive (colloidal graphite) and the more active its surface is (nickel powder), the faster oxidation processes are taking place on the surface of these particles. 

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