ON THE MODIFICATION AND CHARACTERIZATION OF CHEMICAL SURFACE-PROPERTIES OF ACTIVATED CARBON - MICROCALORIMETRIC, ELECTROCHEMICAL, AND THERMAL-DESORPTION PROBES

Further insight into important aspects of the chemical surface propert ies of activated carbons is provided by rationalizing the differences in O-2 adsorption and desorption behavior of hydrogen- and nitrogen-tr eated activated carbons. The effects of heat treatment on the electroc hemical behavior of these carbons were also instrumental in elucidatin g the nature and the distribution of carbon active sites, Activated ca rbon surfaces stabilized with hydrogen at 950 degrees C adsorb very li ttle O-2 L at room temperature but the graphene layers are terminated with many free carbon active sites because significant O-2 adsorption does take place at 150 degrees C. Furthermore, the low-coverage differ ential heats of adsorption on sites accessible at 150 degrees C were l ower than those on sites accessible at 25 degrees C. The role of these free carbon sites in determining the basicity of activated carbons wa s also addressed. On the basis of the important finding that the point of zero charge exhibits a maximum at intermediate heat-treatment temp eratures, it is proposed that-in addition to the delocalized basal-pla ne pi electrons-the localized pi electrons at graphene edges (e.g., in plane divalent sigma pairs) act as Lewis bases that interact with prot ons in aqueous solution.