Desorption of gases from graphitic and porous carbon surfaces (2004)

Presented is an experimental study of thermal desorption of polyaromatic hydrocarbons (PAHs), hydrogen peroxide (H2O2) and ethylbenzene (C6H5C2H5) from four different graphitic carbon surfaces: highly oriented pyrolytic graphite, single-wall carbon nanotubes, carbon nanofibers and colloidal graphite. These thermal desorption experiments are carried out with the objective of understanding three different aspects about carbon surfaces: First, to determine the interlayer cohesive energy of graphite; second, to study the oxidation of porous carbon surfaces; and finally to determine the catalytic performance of carbon surfaces for oxidative dehydrogenation (ODH) of ethylbenzene. Thermal desorption of the four polyaromatic hydrocarbon molecules: benzene, naphthalene, coronene and ovalene, from the basal planes of graphite are used to obtain their activation energies for desorption, pre-exponential factors and orders of desorption. From the magnitude of the binding energies of the PAHs, and also from the zero-offset in the linear dependence of the PAH binding energies on respective molecular static-polarizability, it is concluded that physisorption is the interaction between PAHs and graphite surface (van der Waals interaction). The pre-exponential frequency factors of desorption are found to be considerably larger in comparison to those of smaller molecules, which qualitatively is concluded to arise from large differences in the vibrational partition functions of the PAH molecules in the adsorbed and transition states. Using the binding energies of the PAH molecules on graphite, the binding energy contribution arising from a single carbon atom of a graphene sheet is derived. This is done by optimizing the PAH binding energies using the carbon-carbon and carbon-hydrogen interactions potentials obtained from molecular mechanics (MM3) force-field calculations. The resulting interlayer cohesive energy 61±5 meV/atom is found to be 42 % larger than the previously reported value. Oxidation of the carbon nanofibers and colloidal graphite is performed using hydrogen peroxide. The thermal desorption of oxygen, carbon monoxide and carbon dioxide from these oxidized carbon surfaces shows that these surfaces undergo an irreversible change upon exposure to the oxidizing agent. The binding energy of oxygen on these carbon surfaces is found to be remarkably large when compared to reported oxygen binding energies on single-wall carbon nanotubes and falls into the range of chemisorption binding energies. From the temperatures corresponding to decarbonylation and decarboxylation of the oxidized carbon surfaces, the surface-oxygen functionalities are found to be predominantly acidic, such as carboxylic acid and acid-anhydride functionalities. Similarly, using thermal desorption of ethylbenzene from the surfaces of graphite, single-wall carbon nanotubes, carbon nanofibers and colloidal graphite, the corresponding activation energies of desorption and pre- exponential frequency factors are also obtained. As with the PAHs, it is concluded that these molecules are held on the surface by van der Waals interaction, due to the magnitude of their binding energies. Thermal desorption of ethylbenzene from the surfaces of oxidized carbon nanofibers and colloidal graphite was performed to gain a better understanding of oxidative dehydrogenation on carbon surfaces. Invoking the Langmuir-Hinshelwood mechanism for the oxidative dehydrogenation, the binding energies and pre- exponential factors of the reactant (ethylbenzene) and the product (styrene) are calculated. They are found to be very close to those of a pristine carbon surface. Owing to low reaction temperatures, and also due to small contact times, the percentage conversion of ethylbenzene on the two above carbon surfaces was found to be much lower that observed in the traditional iron- oxide catalyzed ODH reactions. However, a high selectivity to form styrene indicates that the ODH reaction has proceed to near-completion on porous carbon surfaces. To sum up, we obtained the interlayer cohesive energy of graphite using the thermal desorption from a well-defined model system. We have shown that colloidal graphite and carbon nanofibers are prone to oxidation at low temperatures if a powerful oxidizing agent is used. These oxidized carbon surfaces serve as high-selectivity catalysts for oxidative dehydrogenation of ethylbenzene.