Carbon surfaces acidity distribution

The physicochemical properties of carbon are highly dependent on its surface structure and chemical composition [66—68], The type and content of surface species, particle shape and size, pore-size distribution, BET surface area and pore-opening are of critical importance in the use of carbons as anode material. These properties have a major influence on (9IR, reversible capacity <2R, and the rate capability and safety of the battery. The surface chemical composition depends on the raw materials (carbon precursors), the production process, and the history of the carbon. Surface groups containing H, O, S, N, P, halogens, and other elements have been identified on carbon blacks [66, 67]. There is also ash on the surface of carbon and this typically contains Ca, Si, Fe, Al, and V. Ash and acidic oxides enhance the adsorption of the more polar compounds and electrolytes [66]. 

Asymmetric induction during the reduction of 4-(48) was observed when a surface-modified carbon cathode was used.70 Optical yields were low but the effect of the chiral amino acid bound to the carbon surface was proved to be a true surface phenomenon. Induction of chirality by homogeneous rather than surface-bound agents has also been studied.71 All the isomeric acetylpyridines (48) were reduced in the presence of three different chiral alkaloids. Both carbinol products 2- and 4-(49) were shown to possess induced chirality, but the 3-carbinol (49) had none under any of the conditions tried. More rapid protonation of the intermediate was proposed to account for the lack of induced chirality. Optimization of optical yields was done.72 The pinacols (50) formed along with 49 were found to have no induced chirality. Optical yields have been as high as 50%.73 The role of electroabsorption was found to be important in the reduction of 2-(48).74 Product distributions were noted as a function of surfactant present in the electrolyte, carbinol 49 being favored... 

Reactions 12-15 describe the standard neutralization reaction for an organic carboxylic acid. The discrepancy in neutralization capacity for the different reagents can be used to estimate the pK distribution [88]. Amounts between 50 and 500 /zeq acid per gram of carbon are characteristic values which indicate the high degree of experimental perfection required to determine such surface acidity data. Reactions 10 and 11 form derivatives of the carboxylic acid and can be used to volumetrically detect their presence by monitoring the gaseous reaction products. 

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 [. 

A porous structure with small pores uniformly distributed can be achieved chemically by evolution of gas with the mixed slurry followed by stabilization of the bubble structure. The gas must be formed by reaction between two evenly and finely dispersed substances. Also, the rate of reaction must be slow, particularly immediately after mixing, so that the gas-producing substances can be incorporated properly before a significant amount of gas has evolved. Otherwise, much of the gas is lost during the mixing step. Mixtures of carbonates with acids such as dolomite and sulfuric acid have been used. Stabilization of the mixture to prevent bubbles from rising to the surface can be... 

Contescu, A., Contescu, C., Putyera, K., and Schwarz, J.A. (1997). Surface acidity of carbons characterized by their continuous pK distribution and Boehm titration. Carbon, 35, 83—94. 

Experimental. Characterizations of a heterogeneous surface by means of surface group titration utilizing visible and ultraviolet chemical indicators to define the titration end point have frequently been employed with white solid catalysts(7-12), (17-20). Aspects of the surface acid group distribution have often correlated with the catalytic activity of the solid(2-9), (21-25). An adaptation of the technique appears to be suitable for studying the interactions between the surface acid groups in mixtures of carbon black and white reference solids. 

Advantages and disadvantages of the titration technique are discussed in detail elsewhere(26) It suffices here to state that the surface acid group distributions on the white reference solids are sufficiently well defined by the experiment to provide a gauge by which to measure the interaction of carbon black with the white solids in binary mixtures. 

The potentiometric titration offers a meaningful description of acidic properties of the carbon surfaces, in terms of their proton affinity distribution, proving qualitative and quantitative information on the number and the strength of the acidic sites, and thus giving complementary information to that given by other techniques. [215],... 

Both series of carbons fitted the same curve well, showing a marked decrease in the PZC value as a result of an increase in the oxidation time, along with an increase in the surface acidity of the carbons. Infonnation on the distribution of surface functionalities is a key factor in many carbon applications, such as the manufacturing of carbon-supported catalysis. The chemistry of the support determines the precursor/support interactions and, hence, the nature of the active species formed. Thus, the subject of the distribution of oxygen-containing... 

Karanfil and Kilduff studied the adsorption of two synthetic organic contaminants, trichloroethylene (TCE) and trichlorobenzene (TCB), on coal-based and wood-based granulated activated carbons. The activated carbon surface was modified by liquid-phase oxidation with nitric acid and by degassing in an inert atmosphere. The activated carbons were characterized by elemental analysis, surface area and pore-size distribution, and acid-base adsorption techniques. The adsorption isotherms were determined by equilibrating a known weight of the carbon sample with different concentrations of TCE and TCB solutions, and analyzing the solution by gas chromatography. 

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