Surface Chemistry of Carbon Materials

Activated carbons or carbon fibers are the most common materials nsed as adsorbents and catalysts. They are employed widely in both liquid and gaseous phases. This universality is due not only to their high surface area and high volume of pores, but also to the variety of chemical properties of their surfaces. Although for physical adsorption the porous structure is the most important feature, for reactive adsorption and catalysis the chemical environment plays an important role, provided that the structure is developed sufficiently for dispersion of active chemical species and for accommodation of molecules to be adsorbed or to undergo a targeted chemical reaction. 

When activated carbons and carbon fibers were the carbonaceous materials used most extensively and their applications were limited mainly to adsorption with the requirement to remove pollutants to the levels of micrograms or parts per million, the surface chemistry of carbons was not even taken into account. It was a common belief that if adsorbents were used in excess, their purifying action was sufficient. This approach can be considered as history. Nowadays, when the environmental regulations are stricter and stricter and when sophisticated 

Carbon Materials for Catalysis, Edited by PhUippe Serp and Jos6 Luts Figueiredo Copyright 2009 John Wiley Sons, Inc. 

An example of the effects of the stabihty of carbon chemistry is its impact on the electrochemical performance of carbon electrodes, which is altered by the presence of surface groups [1-3]. Now that the use of carbons as supercapacitors for energy storage has begun to attract interest [4-6], some attention has been devoted to the influence of the surface chemistry of these materials on their capacitance. It was found that surface functionality has a tremendous effect on the electrical double-layer properties and the capacity of the latter for energy storage [7-9]. 

Very straightforward effects of surface chemistry of carbons are seen on the adsorption from solutions of aromatics [10-17], dyes [18], heavy metals [19-24], pharmaceuticals [25-27], polar species such as alcohols [28-30], acids or aldehydes [31,32], and even small-molecule gases [33,34]. In those applications the species present on the carbon surface can enhance the specific interactions or even alter the porosity via blocking of pore entrances for molecules to be adsorbed. Specific interactions include hydrogen bonding, acid-base, and complexation. 

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Knowledge of the surface chemistry of carbon materials is of paramount importance as the pltysicochemical properties of carbons are stron y influenced by the presence of chemical species on the surface, and hence many of their applications are conditioned by their chemical characteri.stics. It is a well-known fact that even small amounts of heteroatoms can exert a significant influence on the physicochemical properties of the carbons, and hence, on the desired properties tor the intended application. Besides, these properties are known to change during storage of carbons. 

Since surface functionalities are unlikely to behave exactly as those in simple organic compoimds, the surface chemistry of carbon materials is much more complex (thought at the same time more versatile) than the surface chemical properties of other inorganic solids. Due to this complexity, the carbon surface should be considered as a unique whole entity rather than as a sum of individual functional groups, and caution must be exercised in the interpretation of the results obtained, as already pointed out by Puri [39]. 

Hulicova-Jurcakova, D., E. Fiset, G. Q. M. Lu, and T. J. Bandosz. 2012. Changes in surface chemistry of carbon materials upon electrochemical measurements and their effects on capacitance in acidic and neutral electrolytes. ChemSusChem 5 2188-2199. 

Bandosz TJ (2009) Surface chemistry of carbon materials. In Serp P, Figueiredo JL (eds) Carbon materials for catalysis. Wiley, New Jersey... 

As in the case of pyrones, the contribution to carbon basicity of n-cation interactions in model aromatic systems was investigated by carrying out ab initio calculations [64], The energies for benzene-HjO pyrene-benzene-HjO", and coronene-H30+ interactions were considered in an attempt to understand the basicity contributions of the basal plane. These theoretical results did support the experimental data concerning the ability of basal planes to contribute to carbon basicity and suggested that n-cation interactions may play an important role in the surface chemistry of carbon materials. 

Chemical adsorption. Since clean carbon surfaces are not saturated, they can chemically adsorb various kinds of molecules, especially those containing oxygen atoms. Here only surface oxides are mentioned. Surface oxides greatly influence the surface chemistry of carbon materials, and some examples are highlighted in Figure 7.11. Different surface treatment results in monolayer or multilayer oxides on the carbon surface. 

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