Phenolic surface groups

Figure 3. Schematic representation of the oxygen functional groups on the carbon surface (a) phenol (b) carbonyl (c) carboxyl (d) quinone (e) lactone 12].

The dependency of maximum rate constants for the flotation of coal on the concentration of carboxyl and phenol surface groups is shown in Figure 6. Each data point corresponds to a different coal sample ( 5 ). At low concentration of the surface oxygen groups, the surface of coal is quite hydrophobic and the flotation rate is high. Increase in these surface groups makes the surface hydrophilic with resultant decrease in flotation rate. 

Figure 6. Flotation rate constant of several coals as a function of their phenolic-OH and carboxylic surface groups.

Direct proof of sidewall thiolation has been claimed recently [33], based on analyses of elemental maps in high-resolution TEM images. The authors also proposed that such sulfur functionalities replace the carboxyl and phenolic surface groups upon treatment of previously oxidized MWCNT with phosphorus pentasulfide, as follows ... 

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. 

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 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). 

The [Au(en)2]3+ cation has also been used to deposit gold onto activated carbon fibres by ion exchange of protons in acidic (phenolic) surface groups 101 reduction in hydrogen gave 2-5 nm particles (see also Section 4.5.4). 

Boehm s procedure [6] was used to titrate carboxyls, lactones, phenols and carbonyls functional surface groups. 

As mentioned in Section IV.A.2, Leng and Pinto [3861 specifically addressed the effect of surface properties on the oxic and anoxic adsorption behavior of phenol, benzoic acid, and o-cresol. Commercial carbons were oxidized in air at 350°C, which is known [37] to introduce both CO- and C02-yielding surface groups nevertheless, from FTIR spectra, they concluded that the main differences are due to relative quantities of surface carboxylic groups. Presumably because the experiments were performed at pFl = 7.0, i.e., below the pK, of phenol, their explanation for the decrease in uptake with increasing surface oxygen was not the electrostatic repulsion but increased water cluster formation as well as increased removal of n electrons from the basal planes [450,674], which results in weaker dispersion interactions with phenol. ... 

Let us call this the donor-acceptor complex proposal, similar to that presented recently for adsorption of substituted nitrobenzenes and nitrophenols on mineral surfaces [739]. The experiments on which this proposal is based are (1) isotherms of phenol, nitrobenzene, and m- and / -nitrophenol on one commercial activated carbon at pH = 2-7 and very low solute concentrations ( <1.5% of the solubility limit of these species [6]) and (2) detailed infrared (internal reflection) spectroscopic analysis of the surface after adsorption of / -nitrophenol. Interestingly, neither in this study, nor in any subsequent study that supports this mechanism, has a similar analysis been performed with carbons containing varying concentrations of carbonyl surface groups. Also of interest is that the authors dismiss the electrostatic explanation of the reported pH effects by assuming that the isoelectric point of the carbon (which was dried at 200°C for 12-24 h) was ca. 2.4. 

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... 

A voluminous literature exists on the acidic surface groups and their characterization. Therefore, this aspect of carbon surface chemistry can be here treated relatively briefly. The acidity of these groups opens a convenient way for their determination by titration with aqueous or alcoholic bases. The total of the Bronsted acids, including phenols, reacts with aqueous sodium hydroxide solutions by neutralization adsorption, while carboxylic acids are neutraflzed already by the weak base sodium hydrogen carbonate carboxylic anhydrides (lactones). Lactones and lactols are opened to the carboxylate form by the stronger base... 

---