Carbon surface chemistry effect

N-doping has already been reported for ACF and activated carbon [150,152], It is well known that the uptake pressure and the shape of the H20 isotherm are functions of both micropore size and surface chemical properties. In this case, however, the influence of micropore size can almost be excluded and the observed difference in the uptake pressure be attributed solely to carbon surface chemistry. It is therefore reasonable to conclude that the inner pore surface of the N-doped carbon is more hydrophilic than that of the undoped one. Since the O content of the former carbon is lower than that of the latter, the above results indicate that in this case the presence of N groups is more effective for H20 adsorption. 

The study by Hsieh and Teng [11] can conveniently set the stage for the discussion that follows. It illustrates well how challenging it is to interpret the electrochemical effects in terms of specific changes in carbon surface chemistry. The authors argued that the following three processes may be responsible for these effects in an acidic solution ... 

TABLE 6 Effect of pH and Carbon Surface Chemistry on the Adsorption of Cu(ll) Cations... 

Moreno-Castilla et al. [69] have shown that the adsorption of substituted phenols on activated carbons depends on solution pH. Thus at acidic pH the amount ad.sorbed remained practically con.stant or increa.sed slightly with increasing pH. When the pH increased further, there was a decrease in the amount adsorbed the pH at which this decrease took place depended on the difference between the external and internal surface charge density as measured by electrophoretic and titration measurements, respectively. A sharp turn toward a more substantive discussion of coupled pH and surface chemistry effects thus occurred in the mid-1990s, and these publications are analyzed below. The seeds for such a discussion were planted much earlier, however, and we analyze first how and why it took decades for them to flourish. 

The historically popular concept of hydrolytic adsorption, which in fact obscures the key role of carbon surface chemistry, has often been used to account for the pH effects. Thus, for example, Rosene and Manes [622] used a Polanyi-based model of competitive adsorption between benzoic acid and sodium benzoate. They criticize the approach taken by Ward and Getzen [677], especially with regard to anion interaction with a positively charged surface, and essentially ignore the amphoteric character of the activated carbon. 

TABLE 35 Effects of Carbon Surface Chemistry and Dissolved Oxygen on the Uptake of 2-Chlorophenol... 

Weber and Van Vliet [20] briefly described the Michigan Adsorption Design and Applications Model (MADAM), which includes both equilibrium and kinetic considerations, while Manes [729] advocated the use of the Polanyi adsorption potential theory, even to account for pH effects neither of these approaches includes a description of the role of carbon surface chemistry. 

As abundantly documented in Section IV.B.I, some adsorption systems involving aromatic adsorbates are very much influenced by electrostatic interactions. Clearly, a model that takes into account both electrostatic and dispersion interactions is needed. Such a model has been presented by Muller and coworkers [523-525]. Radovic and coworkers [674] used this model to illustrate the possibly dramatic effects of modifications of carbon surface chemistry on equilibrium uptakes of / -nitrophenol they have also extended it to evaluate the relative importance of electrostatic and dispersive interactions [738]. This approach is summarized next. 

Dr. Turov and Professor Leboda combine their expertise in nuclear magnetic resonance and adsorption phenomena to propose a new tool for a more incisive analysis of adsorbate-adsorbent interactions. Such an analysis is of critical importance in so many applications where it is becoming increasingly clear that adsorbate-carbon interactions are governed by both pore size and surface chemistry effects. These range from the ubiquitous water adsorption to the design of carbon-coated silicas with tailored ratios of hydrophobic to hydrophilic surface sites. 

One factor that may be important, but not systematically investigated, is the influence of the Pt electrocatalyst-support interactions on the electrocatalytic activity for O2 reduction. In Figure 14, an attempt to incorporate the pHzpc as a qualitative measure of the importance of carbon surface chemistry and metal-support interaction on the electrocatalytic activity of Pt is reported. The trend of the data in Figure 14 suggests that the specific activity for oxygen reduction increases as the pHzpc of the surface becomes more basic this effect may be related to the parallel increase of the particle size with the pHzpc of the catalyst. At this stage, one... 

A simple mechanism of adsorption/oxidation of hydrogen sulfide was first proposed by Hedden and coworkers [31]. According to them, dissociation of hydrogen sulfide occurs in the film of adsorbed water at the virgin carbon surface and then hydrogen sulfide ions, HS , are oxidized by oxygen radicals to elemental sulfur. Since then many studies have been done to account for such factors as a role of water [26, 32, 34, 36, 37, 40, 48, 49], role of oxygen [18—27], autocatalysis by sulfur [27, 28], influence of pore sizes [19, 29, 33, 35, 38], role of carbon surface chemistry [41—44], the effects of ash [49, 58—60], and last but not least, speciation of surface oxidation products [41—46]. 

Li, L., QuirJivan, P.A., and Knappe, D.R.U. (2002). Effect of activated carbon surface chemistry and pore structure on the adsorption of organic contaminants from aqueous solution. Carbon, 40, 2085—100. 

Al-Degs, Y. et al.. Effect of carbon surface chemistry on the removal of reactive dyes from textile effluent. Water Res., 34, 927, 2000. 

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