Surface groups pyrone

Figure 2.5 Proposed model for stabilization of carbon surfaces upon heat treatment () in N2 (inert gas) and H2. For simplicity and illustrative purposes, only two types of oxygen surface groups (pyrone and lactone) and three other types of active sites are shown. These three types are in order of increasing stability (1) type I isolated unpaired-electron sites Ar-X (where Ar represents the aromatic (graphene layer) (2) type II, Ar-C=CX and (3) type II, divalent in-plane a pair, Ar-Xs (where s represents a localized n electron). Structures C and D are formed in H2. Structure D is much more abundant in H2-treated carbons because of the higher concentration of CH2 radicals. (From ref. 84, with permission. Copyright 1996 American Chemical Society.)...

FIGURE 2.32 Significant oxygen surface groups on a carbon surface (a) carboxyl, (b) lactone, (c) carbonyl, (d) ether, (e) pyrone, (f) phenol, and (g) carboxyl anhydride. 

The diverse types of oxygen functional groups determine the acidic and basic character of the carbon surfaces [175,182], The acidic character is typically linked with surface complexes like carboxyl, lactone, and phenol, while the basic nature is regularly assigned to surface groups, such as pyrone, ether, and carbonyl [173,178] (see Figure 2.32). 

As in the case of catalysts for oxygen reduction, the surface properties of Pt catalysts for methanol oxidation are governed by both the preparation method and the nature of carbon support. As discussed above, the presence of surface groups such as carboxylic, carbonyl, phenolic, lactone, and pyrone functionalities determine... 

In addition to quinone groups, and to some extent to pyrones (which undoubtedly confer basicity to carbons), the possibility of existence of an oxygen-free, positively charged basic site on the carbon surface was acknowledged in the elegant early studies by Rivin [22] ... 

As mentioned in Section IV.A, Nelson and Yang [494] have proposed a surface complexation model to describe the effect of pH on adsorption equilibria of chlorophenols on activated carbon. To account for the well known suppression of uptakes as pH increases, they introduce the ionization constants of separate acidic (A) (e.g., carboxyl) and basic (B) (e.g., pyrone) functional groups on the carbon surface ... 

Baiker s group employed the 4-methoxypyrone as a standard substrate thereafter. Under noncatalyhc conditions (the use of a stoichiometric amount of CN), a 94% ee and 95% de at an 80% conversion were achieved [118], A stereochemical model of CN adsorption onto the palladium surface, and interaction of the pyrone derivatives with CN, were also proposed on the basis of spectroscopic investigations, product analysis and computational studies [119]. 

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