Surface groups quinone

Reaction 5.1 is meant to represent a nonspecific electrostatic interaction (presumably responsible for double-layer charge accumulation) Reaction 5.2 symbolizes specific adsorption (e.g., ion/dipole interaction) Reaction 5.3 represents electron transfer across the double layer. Together, these three reactions in fact symbolize the entire field of carbon electrochemistry electric double layer (EDL) formation (see Section 5.3.3), electrosorption (see Section 5.3.4), and oxidation/reduction processes (see Section 5.3.5). The authors did not discuss what exactly >C, represents, and they did not attempt to clarify how and why, for example, the quinone surface groups (represented by >CxO) sometimes engage in proton transfer only and other times in electron transfer as well. In this chapter, the available literature is scrutinized and the current state of knowledge on carbon surface (electrochemistry is assessed in search of answers to such questions. 

Oxygen is the main heteroatom in the carbon matrix, and the occurrence of functional groups, such as carboxyl, carbonyl, phenols, ends, lactones, and quinones, has been suggested [176,181], These surface groups can be produced during the activation procedure and can as well be introduced subsequent to preparation by an oxidation treatment [178] (see Figure 2.32). 

The absence of silver oxidation and/or reduction peaks is evidence for the electrochemical inactivity of the silver deposited on this carbon (in the form of metallic crystallites). The cyclic voltammogram recorded for the D—Ox carbon (Fig. 50, curve 2) exhibits two anodic peaks (fp., = +0.27 V, p,a = +0.77 V) due to the oxidation of adsorbed silver and surface hydroquinone-like groups, respectively. A single cathodic peak (Ep,) = +0.16 V) is due to the reduction of quinone-like surface groups according to Scheme 19. The large cathodic reduction wave confirms the presence of adsorbed silver cations and their reduction... 

The acid/base properties of carbons are of particular interest in the present context, since it will be seen that catalytic activity of carbons may often be related to amounts of oxygen adsorbed or to the acid/base characteristics of carbons. Thus, for example, the carbon catalysed rate of auto-oxidation of stannous chloride in acid was found to be maximal when the carbons were activated at 550 °C (high oxygen adsorption), but the carbon catalysed oxidation of hydroquinone to quinone was maximal for activation at 875 °C (low oxygen adsorption). It would obviously be of considerable interest to relate catalytic activity with specific surface groups, and such cases will be discussed later in this Chapter. However, the difficulty of analysing surface groups does make the correlation difficult to make. 

FTIR spectroscopy. Several absorption bands are mentioned in the literature [9-11] and attributed to vibrational properties of different surface groups. It is important to stress that this method does not make the difference between identical groups of different strength. In this section, we will only focus on the common features of the support and the catalyst. The phenolic groups seem to be more numerous than the others (quinonic, carboxylic, lactonic). The nitric treatment seems to increase the amount of quinonic groups on the support, and the quinonic and phenolic ones on the catalyst. Those results are in accord with these obtained in the Boehm titration. At this moment, we have no direct information on the basic surface groups. 

A deconvolution procedure of the TPD spectra [21] was used to estimate the amount of the carbonyl and quinone surface groups of the different oxidized catalysts, and a linear correlation with the catalytic activity for ODE was obtained [62] ... 

The experimental results were well described by a kinetic model based on a redox mechanism of the Mars-van Krevelen type, where the quinone surface groups are reduced to hydroquinone by adsorbed ethylbenzene, and reoxidized back to quinone by oxygen [46,63,64], as shown in Figure 6.3. A recent... 

Recently, the remarkable properties of carbon nanotubes (CNTs) and related structures, such as carbon nanofibers (CNFs) and onionlike carbons, have attracted an increasing interest from the catalysis community [66], Although these materials are most often used as supports for active phases, some applications as catalysts have been reported, the oxidative dehydrogenation of ethylbenzene to styrene being the most frequently cited example [67-73], These reports basically confirm the mechanism proposed previously, based on a redox cycle involving the quinone surface groups. 

---