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Graphene Synthesis, Catalysis with Transition Metals and Their Interactions by Laser Photolysis 69 Fig. 12. An illustration of the Fischer-Tropsch synthesis [www.rccostello.com/copure.html] Recently [24], Fischer-Tropsch Synthesis (FTS) reactions have suggested possibility to obtain carbon and H2O rather than alkanes and H2O through the reaction (2n+1)H2 + nCO → C (graphene) + nH2O when the catalysis and other parameters are varied. For instance, the deactivation of a 20 wt% Co/γ-Al2O3 catalyst during the FTS at 240 °C, 20 bar, and a H2:CO ratio of 2 was studied in a fixed-bed micro-reactor [Fei]. The CO conversion had reduced by 30% after 200 h, and both carbidic and polyaromatic carbon species could be detected on the catalyst using a combination of Temperature- Programmed Hydrogenation (TPH), X-ray Photoelectron Spectroscopy (XPS) and High Resolution Transmission Electron Microscopy (HRTEM). Using Density Functional Theory (DFT), the relative stability of different types of deposited carbon on the Co catalyst was evaluated. Extended layers of graphene were the most stable form, followed by a p4g surface carbide phase initiating from the step edges. Both are more stable than surface CH2 groups by 99 and 79 kJ/mol. Also Swart et al [25] have studied the possible catalyst deactivation mechanisms in the FTS synthesis of graphene overlayer by elucidating the adsorption of graphene on the fcc – Co(111) surface. A chemical interaction between the graphene sheet and the cobalt surface was observed as evidenced by the partial DOS and Bader charge analysis. The adsorption energy was found to be small when normalized per carbon atom, but becoming large for extended graphene sheets. Graphene removal from the surface via lifting or sliding was considered. The energy barrier for sliding a graphene sheet is lower than the barrier for lifting, but the energy barriers become significant when placed into the context of realistic catalytic surfaces in the nano-meter range.PDF Image | GRAPHENE SYNTHESIS CHARACTERIZATION PROPERTIES
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