Effect of activation on the surface chemistry of carbons from polymer precursors

Carbonized chars prepared from polyacrylonitrile, polyethyleneterephthalate and cellulose were activated in a steam/nitrogen flow to ca. 50% burn-off (b.o.). The surface chemistry of these carbons has been characterized by X-ray photoelectron spectroscopy (XPS), adsorption from benzene-methanol binary liquid mixture, and potentiometric acid-base titration. The activation of the various polymer-based chars, in spite of the treatment similarity (temperature, burn-off) results in different physical and chemical surface properties. It has been confirmed by XPS analysis, that the elevated temperature of the activation not only promotes the relative extension of the graphitic regions by removal of noncarbon atoms and part of the amorphous carbon, but also helps on the further development of graphitic regions. The carbons contain both hydrophobic and hydrophilic sites on their surface already in their carbonized form, but the activated ones show a more hydrophobic character. The carbons exhibit an amphoteric surface, containing both acidic and basic surface groups. The titration curves show a hysteresis when performed upward and downward. Suspensions of the activated samples exhibit a basic pH. Both the increased hydrophobic property and the basicity can be attributed to extended graphitic regions. Part of the oxygen atoms forming basic functional groups may contribute to basicity, as well. (C) 2001 Elsevier Science Ltd. All rights reserved.