On the Modification and Characterization of Chemical Surface Properties of Activated Carbon:  In the Search of Carbons with Stable Basic Properties

Abstract

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 N2 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 H2 adsorbed less and less O2 as the temperature of treatment was raised; after treatment at 1223 K, virtually no O2 adsorption occurred. At the same time the H/C ratio in the H2-treated samples decreased with increasing treatment temperature. Point of zero charge measurements revealed that only H2 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 N2 treatment leaves unsaturated carbon atoms at crystallite edges; these sites are very active for subsequent oxygen adsorption. In contrast, high-temperature H2 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 H2 treatment is stable against subsequent O2 adsorption in ambient conditions.

Surface-Treated Activated Carbons as Catalysts for the Dehydration and Dehydrogenation Reactions of Ethanol

“…13 All these effects can affect the surface active sites that are involved in the dehydrogenation and dehydration reactions of ethanol.…”

Removal of heavy metal from industrial wastewater using chitosan coated oil palm shell charcoal

“…The pH pzc value of acid treated OPSC was found to be around 4.7. It can be concluded that oxidization of oil palm shell charcoal with sulphuric acid yielded acidic surface since pH values of point of zero charge for these materials are at a lower pH range compared to the value reported for most chemically untreated commercial activated carbon (Menendez et al 1996). The surface acidity was due to the introduction of several oxygen-containing functional groups (Faria et al 2004 , the fraction of any particular species is dependent upon the chromium concentration and pH (Udaybhaskar et al 1990).…”

Carbon materials and catalytic wet air oxidation of organic pollutants in wastewater

“…Further exposure of oxidised carbons to CO 2 , N 2 or H 2 during temperature rating up to 1177°C can progressively eliminate specific (acid) surface oxide groups to obtain carbons with basic properties [82,100,167,174,177,178]. High temperature N 2 -reduction produced basic but reoxidisable surfaces at room temperature, whereas a reducing H 2 -atmosphere provided stable basic carbon surfaces [177]. Summarising the current knowledge on surface groups, surface oxides, in particular carboxylic, lactone, lactol, and phenolic groups, are mainly responsible for acidic carbon properties.…”

“…Summarising the current knowledge on surface groups, surface oxides, in particular carboxylic, lactone, lactol, and phenolic groups, are mainly responsible for acidic carbon properties. Basic properties are related to chromene and pyrone groups as well as to Lewis type sites in the graphite carbon matrix itself [100,108,[177][178][179].…”