Carbon surface chemistry

J.S. Mattson and H.B. Mark, Activated Carbon Surface Chemistry and Adsorption from Solution, Marcel Dekker, 1971. 

Nikolla E, Holewinski A, Schwank J, and Linic S. Controlling carbon surface chemistry by alloying carbon tolerant reforming catalyst. J Am Chem Soc 2006 128 11354-11355. 

We already touched on some aspects of carbonate surface chemistry e.g., in Chapter 3.4. We have already illustrated some of the factors that affect surface charge and the point of zero charge, pHpzc, in Chapter 3.5, and have discussed certain elementary aspects of CaC03 nucleation in Chapter 6.5 and of coprecipitation (and solid solution formation) in Chapter 6.7. 

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 ... 

Figure 5.4 summarizes in a schematic way the current state of knowledge of carbon surface chemistry, with an emphasis on the details of graphene edges. As discussed in Section 5.2.1, it shows the dominant surface functionalities, all containing oxygen because of the often inevitable contact of realistic carbon materials with 02 from air. The main features of interest here are (i) the existence of free edge sites and (ii) the notion that the basal plane is not as chemically inert as is often... 

The intrigued reader is left with the essential task—and arguably a rather difficult but very rewarding one—to verify this vague and imprecise reference to carbon surface chemistry. 

An ambitious review of carbon applications in chemical power sources (see also Section 5.3.5) was offered by Fialkov [94], It is disconcerting, however, that the author discusses the influence exerted by... surface properties without citing even one of the well-known—or well-cited or more recent—studies on carbon surface chemistry. And yet, in conjunction with the use of carbon in air (oxygen) electrodes, he speculates that the oxygen electroreduction kinetics depend on... the degree to which side faces of carbon crystallites are developed because base groups are formed there and presumably interact (e.g., with HjOj) in the following manner ... 

Kinoshita did not discuss the possible reasons for these differences, nor has apparently anyone else, at least not convincingly. For example, based on everything known about carbon surface chemistry today, it is not easy to explain why electron transfer would occur on carbon blacks prior to 02 adsorption, and on graphite or GCs subsequent to 02 adsorption. 

FIGURE 5.23 Updated proposal for 02 reduction that is consistent with current knowledge of carbon surface chemistry. 

Despite these principal ambiguities the thermal desorption method is a standard characterization technique in carbon surface chemistry. Various examples and data about desorption profiles for a selection of carbon treatments can be found in the literature [88, 90, 155, 182, 183]. 

C.A.L.Y. Leon. L.R. Radovlc, Interfacial Chemistry and Electrochemistry of Carbon Surfaces, Chemistry and Physics of Carbon 24 (1994) 213. (Review, surface characterization by physical and chemical means, double layer, functional groups.)... 

In addition to pore size distribution, the surface chemistry of the activated carbon can have an important influence on the adsorption of certain compounds. As the adsorptive surface of most activated carbons is hydrophobic, they are best suited for the removal of neutral organic molecules, while polar and ionic compounds show much less affinity for adsorption. For the adsorption of polar compounds such as phenol, research has shown that the carbon surface chemistry is more relevant than the total available adsorption capacity or surface area [72-74]. It has been found that the presence of acidic surface oxides, whose concentration can be increased by oxygen adsorption or chemical treatment, leads to a decrease in adsorptive capacity for compounds such as phenols and increases the base adsorption capacity [75 [. 

It is not our intention to present an exhaustive review of this important subject. Up-to-date reviews are provided elsewhere [38,37]. The classic review by Garten and Weiss [41 ] offers an excellent historical perspective. We do need to summarize here the issues that are essential for understanding the aqueous-phase adsorption phenomena. The main features of carbon surface chemistry are presented first and the con.sequent acid/base behavior of carbons is briefly discus.sed to illustrate their amphoteric character. In Section III it is shown that these phenomena often govern the adsorption of most inorganic compounds. In Section IV we argue that these phenomena can be dominant in the adsorption of organic compounds as well, but they are more often only a part of the whole story. 

FIG. 2 Macroscopic representation of the features of carbon surface chemistry that are thought to be sufficient for understanding aqueous-phase adsorption phenomena. 

Moreno-Castilla and coworkers [139,140] did clarify the relationship between carbon surface chemistry and chromium removal. Table 3 summarizes some of the key results. Upon oxidation of carbon M in nitric acid (sample MO), the surface has become much more hydrophilic and more acidic, and the uptakes increased despite a decrease in total surface area. The enhancement in Cr(III) uptake was attributed to electrostatic attraction between the cations and the negatively charged surface. The enhancement in Cr(VI) uptake (at both levels of salt concentration) was attributed to its partial reduction on the surface of carbon MO (perhaps due to the presence of phenolic or hydroquinone groups), which is favored by the lower pH. The increase in uptake on carbon MO with increasing NaCl concentration is consistent with this explanation, from a straightforward analysis of the Debye-Hvickel and Nemst equations the decrease in uptake on carbon M was attributed to the competition of specifically adsorbed Cl and CrOj- ions on the positively charged surface. 

TABLE 4 Influence of Carbon Surface Chemistry (given as pHip,) on the Adsorption of Mo(Vl) Anions... 

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

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