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Abstract

Carbon can be used as an alternative to metal based catalysts for oxidative dehydrogenation reaction (ODH) of aromatic hydrocarbons, alkanes or alkenes. The active sites of the catalyst are still unknown, but it should be either one or more of the oxygen functional groups. On the other hand, carbon materials are used as catalytic supports for metal particles. The oxygen groups on the support material are the anchoring sites to immobilize the metal particles. The interaction between oxygen groups and metal particles improves the catalytic activity and selectivity of the catalyst. Therefore, functionalization of carbon materials is a very important method, furthermore it is important to know how many different oxygen groups exist on the carbon surface. There is still a big lack of information about the kind of oxygen groups on carbon.
Highly oriented pyrolytic graphite (HOPG) is a well-defined model system without any metal impurities and pores. Therefore, the oxygen functionalities on HOPG were studied. To study the nature of the induced oxygen groups, combined Temperature Programmed Desorption (TPD) and X-Ray Photoelectron Spectroscopy (XPS) were used. UV photoelectron spectroscopy (UPS) was applied to monitor the electronic structure and the damage of the HOPG surfaces. The surface morphology was characterized by Scanning Electron Microscopy (SEM) and Scanning Tunneling Microscopy (STM).
Since the dissociation rate of oxygen on defect-free HOPG is practically zero, defects were introduced on HOPG by sputtering. Two paths were used: Sputtering with argon in an oxygen- or water-gas atmosphere and sputtering with argon and oxygen itself. Our investigation shows that sputtering with oxygen produces mainly oxygen species in the HOPG matrix. In contrast, sputtering with Ar in oxygen gas atmosphere produces mainly oxygen species in a small amount and these oxygen species are situated on the HOPG surface. The second sputter method is used as the standard method for functionalization of HOPG, with the goal of generating oxygen groups on it.
Even though HOPG is a well-defined material, there are various construction defects in the HOPG matrix. Our investigation shows that these minimal differences of the original HOPG structure can be determined with Ar-TPD spectra. Furthermore, this method can be used to assess the HOPG quality.
In XPS spectra, various oxygen 1s signals occur only in a narrow range of 2.5 eV and overlap in this range. Thus, it is very difficult to identify single oxygen peaks and assign them clearly. In the literature, O1s peaks are fitted with different numbers of O1s components, but without mentioning why a certain number of oxygen groups is used for O1s-XPS fitting. Our investigation shows clearly, at least five O1s components are necessary to fit the O1s XPS spectra properly. The thermal stability and oxidizability of the oxygen components is used to discuss the character of the oxygen components.