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Work performed for this study demonstrates that the temperature of treatment and
the identity of the treatment gas both strongly impact the surface chemistry of activated carbon. Two commercial activated carbons were treated in either N2 or H2 at different temperatures up to 2600oC. Several techniques - including microcalorimetry, point of zero charge measurements, thermal desorption - were used to provide insight into important aspects of the chemical surface properties. The results show that activated carbons treated at high temperatures (ca. 950oC) in hydrogen will not react with oxygen and water at ambient temperatures; moreover, surfaces created in this fashion have stable properties in ambient conditions for many months. In contrast, the same carbons treated
in an inert gas (e.g., N2) will react strongly with oxygen and water at ambient
temperatures. In the presence of platinum (or any other noble metal), stable basic
carbons, which will not adsorb oxygen in ambient laboratory conditions, can be created
via a relatively low-temperature process. Treatment at higher temperatures (>1500oC)
produced increasingly stable surfaces in either N2 or H2. A structural model is proposed. To wit: Treatment at high temperatures in any gas will lead to the desorption of oxygen surface functionalities in the form of CO and/or CO2. Absent any atom rearrangement, the desorption of these species will leave highly unsaturated carbon atoms (“dangling carbons”) on the surface, which can easily adsorb O2 and H2O. In an inert gas these “dangling carbons” will remain, but hydrogen treatments will remove these species and leave the surface with less energetic sites, which can only adsorb O2 at elevated temperatures. Specifically, hydrogen reacts with any highly unsaturated carbons in the surface to form methane.
At temperatures greater than 1500oC (e.g., 1800oC, 2600oC), structural annealing
takes place and the consequent growth in the size of graphene layers eliminates the highly
energetic dangling carbon sites. The basicity of the surface originates from two types of Lewis base sites: the localized electron pairs at the edges of the graphene layers and the delocalized p electrons on the basal planes. A hydrogen spillover mechanism was proposed here to explain the lowtemperature process for the stable basic carbon. The role played by platinum (or any
noble metal) is to produce atomic hydrogen, which spills over onto the carbon surface.
This atomic hydrogen hydrogasifies the most reactive, unsaturated carbon atoms at far
lower temperatures than molecular hydrogen, thus leading to surface stabilization at
relatively low temperatures. |
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