Modification of the surface properties of an activated carbon by oxygen plasma treatment

Samples of a commercial activated carbon exposed to an oxygen plasma under identical conditions (150 W power, 0.1 kPa) for different times to attain 3-41 wt% burnoff of carbonaceous matter were characterized with emphasis on texture and surface chemistry. A 100% burnoff sample consisting of the inorganic matter of the starting material was also prepared and characterized to account for its contribution to the surface properties of the plasma-modified carbons. SEM-EDX and XRD showed that the major inorganic constituents of the activated carbons tested were calcium carbonate and silica. The structure of the carbonaceous matter changed little during the plasma treatment, and only a certain selective attack on more disordered material at high burnoff could be inferred. Adsorption of N-2 at 77 K and CO2 at 273 K revealed a moderate decrease in surface area and porosity as a consequence of the plasma treatment (with allowance made for the diluent effect of inorganic matter). Plasma treatment resulted in a more well-defined plateau in the zeta potential-pH curve, appearing at a lower pH than for the untreated carbon, indicating that the negative charge brought about by the plasma treatment was due to dissociation of newly formed acidic groups. The isoelectric point disappeared even at low burnoff and the surface acidity increased up to 10-20 wt% burnoff and then remained constant. The hydrophilicity of plasma-treated carbons, measured by contact angle with water, did not change significantly. It is concluded that the plasma treatment offers potential advantages for the surface modification of activated carbons, as no substantial change in texture takes place whereas the surface chemistry can be modified to tailor specific properties. The oxygen plasma appeared not to reach the smallest micropores of the carbon, indicating that the reaction took place only near the external surfaces of the particles. (C) 1998 Elsevier Science Ltd. All rights reserved.