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 nitrogen-treated samples of an activated carbon were quantified using several complementary techniques. Calorimetric studies conducted at 303 K revealed that the sample treated in N-2 at 1223 K adsorbs a great deal of oxygen with unusually high differential heats. In fact, both the quantity and the heat of adsorption increased when the treatment temperature was raised from 773 to 1223 K. In contrast, samples treated in H-2 adsorbed less and less O-2 as the temperature of treatment was raised; 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 measurements revealed that only H-2 treatments at high temperature (> 1073 K) create basic (hydrophobic) surfaces which are stable after prolonged air exposure. These findings are consistent with the notion that the removal of oxygen in the form of CO and CO2 during high-temperature N-2 treatment leaves unsaturated carbon atoms at crystallite edges; these sites are very active for subsequent oxygen adsorption. In contrast, high-temperature H-2 treatment accomplishes three tasks: (a) it also removes surface oxygen; (b) it stabilizes some of the (re)active sites by forming stable C-H bonds; (c) it gasifies the most reactive unsaturated carbon atoms. The relative contributions of these three effects depend on the temperature of H-2 treatment. The carbon surface resulting from high-temperature H-2 treatment is stable against subsequent O-2 adsorption in ambient conditions.