MODIFICATION OF THE SURFACE-PROPERTIES OF AN ACTIVATED CARBON BY OXYGEN PLASMA TREATMENT

Samples of a commercial activated carbon exposed to an oxygen plasma u nder identical conditions (150 W power, 0.1 kPa) for different times t o attain 3-41 wt% burnoff of carbonaceous matter were characterized wi th emphasis on texture and surface chemistry. A 100% burnoff sample co nsisting of the inorganic matter of the starting material was also pre pared and characterized to account for its contribution to the surface properties of the plasma-modified carbons. SEM-EDX and XRD showed tha t the major inorganic constituents of the activated carbons tested wer e calcium carbonate and silica. The structure of the carbonaceous matt er changed little during the plasma treatment, and only a certain sele ctive attack on more disordered material at high burnoff could be infe rred. Adsorption of N-2 at 77 K and CO2 at 273 K revealed a moderate d ecrease in surface area and porosity as a consequence of the plasma tr eatment (with allowance made for the diluent effect of inorganic matte r). Plasma treatment resulted in a more well-defined plateau in the ze ta potential-pH curve, appearing at a lower pH than for the untreated carbon, indicating that the negative charge brought about by the plasm a treatment was due to dissociation of newly formed acidic groups. The isoelectric point disappeared even at low burnoff and the surface aci dity increased up to 10-20 wt% burnoff and then remained constant. The hydrophilicity of plasma-treated carbons, measured by contact angle w ith water, did not change significantly. It is concluded that the plas ma 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 prope rties. 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.