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 properties of activated carbons is provided by rationalizing the differences in O₂ adsorption and desorption behavior of hydrogen- and nitrogen-treated activated carbons. The effects of heat treatment on the electrochemical behavior of these carbons were also instrumental in elucidating the nature and the distribution of carbon active sites. Activated carbon surfaces stabilized with hydrogen at 950 °C adsorb very little O₂ at room temperature but the graphene layers are terminated with many free carbon active sites because significant O₂ adsorption does take place at 150 °C. Furthermore, the low-coverage differential heats of adsorption on sites accessible at 150 °C were lower than those on sites accessible at 25 °C. The role of these free carbon sites in determining the basicity of activated carbon was also addressed. On the basis of the important finding that the point of zero charge exhibits a maximum at intermediate heat-treatment temperature, it is proposed that - in addition to the delocalized basal-plane π electrons-the localized π electrons at graphene edge (e.g., inplane divalent sigma pairs) act as Lewis bases that interact with protons in aqueous solution.