Chemical Structure of the Carbon Surface

An activated carbon in contact with a metal salt solution is a two-phase system consisting of a solid phase, which is the activated carbon surface, and a liquid phase which is the salt solution. The solution contains varying amounts of different metal ion species and their complexes so that the interface between the two phases will behave as an electrical double layer and determine the adsorption processes taking place in the system. The adsorptive removal capacity of an activated carbon for metal cations from the aqueous solutions generally depends on the physicochemical characteristics of the carbon surface, which include the surface area, pore-size distribution, electrokinetic properties, and the chemical structure of the carbon surface, as well as on the nature of the metal ions in the solution. 

The more important parameters that determine the adsorptive removal of organic compounds from water are the nature and the molecular dimension of the organic compound, the porous and the chemical structure of the carbon surface, and the pH of the aqueous solution. This chapter will present a review of the work carried out on the adsorption of different groups of organic componnds on different activated carbons. The parameters that determine and influence the adsorption will be examined, and the adsorption mechanisms based on the adsorption data will be suggested. 

Thus, it is apparent that the adsorption of natural organic materials and dissolved organic materials present in surface and ground waters is determined both by the porous and the chemical structure of the carbon surface. 

Besides the crystalline and porous structure, an activated carbon surface has a chemical structure. The adsorption capacity of an activated carbon is determined by the physical or porous structure but strongly influenced by the chemical structure of the carbon surface. In graphites that have a highly ordered crystalline structure, the adsorption capacity is determined mainly by the dispersion component of the van der Walls forces. But the random ordering of the aromatic sheets in activated carbons causes a variation in the arrangement of electron clouds in the carbon skeleton and... 

Taking into account all these factors, it has been found that the most appropriate supports for PFMFCs catalysts are carbon blacks of ca. 250 m g BFT surface area, and the most widely used is Vulcan XC-72R commercialized by Cabot. ° ° Due to the importance of the surface chemistry of these supports, and its influence in the supported active metal phase, ° ° different chemical modifications of the support have been also investigated. The chemical nature of the carbon surface produces different electronic interactions between the noble metals and the carbon support, and affects the metal particle morphology, and influences the catalytic activity. Additionally, during the last few years, new alternative materials to carbon blacks have also been used, especially on the basis of their porous structure (nanotubes, mosoporous carbons) or their microstructure (nano- and microfibers, and microspheres). 

The porous structure of active carbons is the defining factor of their adsorption performance The pore diameter distribution determines the adsorption energy and therefore, the slope of adsorption isotherm, whereas in mainly microporous carbons the micropore volume limits the adsorption capacity at the higher end of t.he adsorptive concentration. Furthermore, the chemical compositor of the carbon surface influences the selectivity of adsorption, e.g. the competition of the water adsorption when working in aqueous solution. However, here only the structural properties of carbons are taken into account. 

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]. 

The pore structure and surface area of carbon-based materials determine their physical characteristics, while the surface chemical structure affects interactions with polar and nonpolar molecules due to the presence of chemically reactive fimctional groups. Active sites—edges, dislocations, and discontinuities—determine the reactivity of the carbon surface. As shown in Fig. 1, graphitic materials have at least two distinct types of surface sites, namely, the basal-plane and edge-plane sites [11]. It is generally considered... 

The physical and chemical activation methods are effective in preparing the microporous carbons with high surface area. However, the pore structures of the carbons are not easily controlled by the activation processes and the size of the pores generated by the activation processes is limited to the micropore range only. Under these circumstances, the templating method which will be considered in the following Section II.2 has recently... 

A final important reproducibility specification should be considered which applies specifically to bonded-phase packings. First, the bonded-phase should be specified as being polymeric or monomeric. If polymeric,information on the % organic or % carbon for the packing and the chemical structure of the bonded phase should be provided. However, as shown before, this information is often not sufficient to determine lot-to-lot chromatographic reproducibility. If the bonded phase is monomeric, data on the % organic or % carbon and chemical structure are also useful, but in addition, the surface coverage calculated from these values (6) should also be provided (EQ. 4). 

Because surface functional groups influence the adsorption properties and the reactivity of activated carbons, many methods, including heat treatment, oxidation, animation, and impregnation with various inorganic compounds, have been developed in order to modify activated carbons [183], These modifications can alter the surface reactivity, as well the structural and chemical properties of the carbon, which can be characterized using various methods, as described in detail elsewhere [176],... 

Mechanical properties, electrical properties, thermodynamic stability, surface chemical activity, and other important parameters can all be discussed relative to the structure of the carbon network, composed of both aromatic layers and 3D-arranged (diamond-like) phases. 

The suitability of this adsorption model to characterize quantitative aspects of surface acidic groups gives no indication, however, about the chemical structure of the reactive sites. Only in combination with the chemical probe reactions is it possible to assign the two types of acid sites to carboxylic acid and hydroxy groups, respectively. It is noted that such an approach can also be used to determine ion exchange capacities for metal ion loading required for the generation of dispersed metal-carbon catalyst systems. 

In this paper, we present a model for activated carbon that takes into account the characteristics of the adsorbent that affect the adsorption of both polar and non-polar species. The structure of the carbon is represented by a PSD, obtained from the analysis of pure-ethane adsorption, and chemical heterogeneity is included by placing regularly distributed carbonyl sites on the surface of the pores. The single-pore isotherms for water and ethane are calculated by grand canonical Monte Carlo (GCMC) simulation. 

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