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Highly Oriented Pyrolytic Graphite (HOPG) as a Model Catalyst for the Oxidative Dehydrogenation of Ethylbenzene over Carbon Materials


Aburous,  Samer
Inorganic Chemistry, Fritz Haber Institute, Max Planck Society;

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Aburous, S. (2007). Highly Oriented Pyrolytic Graphite (HOPG) as a Model Catalyst for the Oxidative Dehydrogenation of Ethylbenzene over Carbon Materials. PhD Thesis, Freie Universität, Berlin.

Cite as: http://hdl.handle.net/11858/00-001M-0000-0011-003A-5
Catalytic ethylbenzene conversion to styrene which is one of the largest industrial processes worldwide was the main subject of this study. The surface structure of potassium-promoted iron oxide catalyst, which is the industrial catalyst for the dehydrogenation of ethylbenzene to styrene, was studied. For that purpose ion scattering spectroscopy was used to determine the surface termination of this catalytic phase. The obtained observations, in agreement with previous studies, indicated that this phase is potassium terminated, and confirmed the fact that potassium should be on the surface of the catalyst to have its promoting action. There are several known drawbacks of the normal dehydrogenation process including endothermicity, deactivation and the large demand of preheated steam. Oxidative dehydrogenation process (ODH) of ethylbenzene is a promising candidate with several advantages. The absolutely metal free material Highly Ordered Pyrolytic Graphite (HOPG) was chosen as a model catalyst for the ODH of ethylbenzene over carbon materials whose catalytic activity in this process has attracted a lot of interest. Since oxygen-containing groups are supposed to be the active centres of the ODH reactions, argon- and oxygen-sputtering followed by exposure to oxygen atmosphere was used to create those groups. Only oxygen sputtering proved to be efficient in introducing surface oxygen functionality. The sputtered surface was investigated using thermal programmed desorption spectroscopy, auger spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy and scanning electron microscopy. TPD spectra revealed low-temperature CO2 and high-temperature CO desorption peaks. Using SEM and AFM, and comparing the heavily oxygen-sputtered surface with the smooth surface of the cleaved samples, many pits and pin holes were seen on the surface which indicates a significant damage of the surface by repeated oxygen sputtering. Upon performing reaction experiments on cleaved and oxygen-sputtered HOPG, a percentage yield of styrene ranging from 0.25 – 0.6 % was obtained according to reaction conditions without any obvious difference between the cleaved and the oxygen sputtered samples. The system sensitivity was tested and verified. Additionally, the detected activity was proved to be originating from ODH process by performing oxygen-free experiments. Reaction data was also used to determine the apparent activation energy (Arrhenius) which was found to be in the range of 50 kJ/mol for both the cleaved and the oxygen sputtered samples. The reason for the similar results observed was discussed and different explanations were given in which carbonaceous depositions over the catalyst surface play a major role. The reaction rate over HOPG samples was determined, and compared to the active catalyst (onion-like carbon), this comparison lead to the conclusion of the reasonability of using HOPG as a model catalyst for further studies. As a last step, different techniques including SEM, AFM and temperature programmed analysis were used to study the catalyst after reaction. Temperature programmed desorption of cleaved samples after reaction have shown desorption peak of CO which indicates the presence of carbonaceous depositions on the surface because the cleaved surface is known not to evolve any COx at the used temperature range. Temperature programmed oxidation indicated that the carbonaceous deposits are composed mainly of carbon and oxygen. These deposits did not show any obvious redox behaviour. Upon exploring the surface morphology, the cleaved surface was found to be smooth before and after reaction. This is explained by the building of carbon depositions in one layer. On the other hand, the oxygen-sputtered samples were found to be rough before reaction, after reaction, the surface was still rough. This was explained as the building of carbon depositions taking place extensively on the basal plane of HOPG and not on the whole surface.