ON THE MODIFICATION AND CHARACTERIZATION OF CHEMICAL SURFACE-PROPERTIES OF ACTIVATED CARBON - IN THE SEARCH OF CARBONS WITH STABLE BASIC PROPERTIES

Differences between the surface chemical properties of hydrogen- and n itrogen-treated samples of an activated carbon were quantified using s everal complementary techniques. Calorimetric studies conducted at 303 K revealed that the sample treated in N-2 at 1223 K adsorbs a great d eal of oxygen with unusually high differential heats. In fact, both th e quantity and the heat of adsorption increased when the treatment tem perature was raised from 773 to 1223 K. In contrast, samples treated i n H-2 adsorbed less and less O-2 as the temperature of treatment was r aised; after treatment at 1223 K, virtually no O-2 adsorption occurred . At the same time the H/C ratio in the H-2-treated samples decreased with increasing treatment temperature. Point of zero charge measuremen ts revealed that only H-2 treatments at high temperature (> 1073 K) cr eate basic (hydrophobic) surfaces which are stable after prolonged air exposure. These findings are consistent with the notion that the remo val of oxygen in the form of CO and CO2 during high-temperature N-2 tr eatment leaves unsaturated carbon atoms at crystallite edges; these si tes are very active for subsequent oxygen adsorption. In contrast, hig h-temperature H-2 treatment accomplishes three tasks: (a) it also remo ves surface oxygen; (b) it stabilizes some of the (re)active sites by forming stable C-H bonds; (c) it gasifies the most reactive unsaturate d carbon atoms. The relative contributions of these three effects depe nd on the temperature of H-2 treatment. The carbon surface resulting f rom high-temperature H-2 treatment is stable against subsequent O-2 ad sorption in ambient conditions.