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

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.

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 Creators:
Aburous, Samer1, Author              
Schlögl, Robert1, Referee              
Christmann, Klaus, Referee
Affiliations:
1Inorganic Chemistry, Fritz Haber Institute, Max Planck Society, ou_24023              

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Free keywords: Styrene, Oxidative dehydrogenation, HOPG, Carbon catalyst, Model catalyst scientific project
 Abstract: 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.

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Language(s): eng - English
 Dates: 2007-07-26
 Publication Status: Accepted / In Press
 Pages: 91
 Publishing info: Berlin : Freie Universität
 Table of Contents: Introduction
  
1  Iron Oxides as dehydrogenation catalysts (Surface Structure)
1.1  Introduction
1.2  Experimental details and methodology
1.2.1  Preparation of iron oxide films
1.2.2  Flux Calibration of iron source
1.2.3  Spectroscopic methods
1.3  Results and Discussion
1.4  Conclusion

2  Literature Highlights and Methodology
2.1  The element carbon and its importance in catalysis
2.2  Carbon materials in the oxidative dehydrogenation process
2.3  Analysis of surface oxides on carbon
2.4  Methodology
  
3  Experimental Methods and Instrumentation
3.1  UHV Setup
3.2  Micro-flow reactor
3.3  Gas mixing system
3.4  Saturators
3.5  Micro GC
3.6  Sample mounting
3.7  Sample cleaning and preparation prior to experiments
3.8  Carbon Nanotube Samples
3.9  Reaction procedure
3.10  Temperature Measurement
3.11  Highly Oriented Pyrolytic Graphite (HOPG) (Sample Preparation)
3.12  Principles of analytical methods
3.13  Materials
  
4  Creation and characterization of surface oxygen functionality
4.1  Thermal treatment in oxygen partial pressure
4.2  Sputtering with argon and exposure to molecular oxygen
4.3  4.3 Sputtering with oxygen
  
5  Oxidative Dehydrogenation experiments
5.1  Oxidative dehydrogenation reaction over cleaved HOPG
5.2  Oxidative dehydrogenation reaction over oxygen-sputtered HOPG
5.3  Oxidative dehydrogenation reaction over CNT samples
5.4  Repeated oxidative dehydrogenation reaction over HOPG sample
5.5  Is it right to consider HOPG as a model catalyst for ODH reactions over carbon materials?
  
6  Post-reaction Characterization
6.1  Post reaction thermal programmed analysis
6.2  Post reaction scanning electron microscope (SEM) images
6.3  Post reaction atomic force microscope (AFM) images
  
7  Conclusions ……………………………………………………………
  List of Figures ………………………………………………………….
  Curriculum vitæ ………………………………………………………..
  Publications ………………………………………………………….
  Summary ………………………………………………………………..
  Zusammenfassung ……………………………………………………
  Acknowlwdgements
 Rev. Type: -
 Identifiers: eDoc: 320325
DOI: 10.17169/refubium-9702
 Degree: PhD

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