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Abstract

In this thesis, various pathways were investigated in order to facilitate the development and improvement of solid catalysts for the selective direct methane oxidation in the platinum/sulfuric acid system. Various catalytic materials were successfully synthesized and systematically investigated with the focus on stability and catalytic performance under reaction conditions. Combined with an intensive material characterization using state-of-the-art techniques it was accomplished to gain an in-depth understanding of the reaction system and to develop a new solid catalyst with superior catalytic activity in direct methane conversion. The synthesis of Pt-modified Covalent Triazine Framework (CTF) was successfully optimized and the influence of synthesis parameters investigated. The intensive characterization of this solid single-site catalyst provided an in-depth insight into material properties and, especially, the interaction of Pt and the carbon support. In order to acquire reliable data on an atomic scale, suitable techniques were used, i.e. ¹⁹⁵Pt solid-state NMR, X-ray absorption spectroscopy and aberration-corrected electron microscopy. In particular, it was observed that a Pt^II species interacts with the pyridinic nitrogen sites of CTF and generates an organometallic complex using the carbon as a solid ligand. Detailed investigations of local Pt environment and geometry for Pt-CTF are in good agreement with its molecular counterpart Pt(bpym)Cl₂. Comprehensive studies of Pt-modified CTFs with various material properties and of the influence of reaction conditions on catalytic performance showed that no significant improvement of catalytic performance could be achieved with this solid ligand. However, in screening experiments using various nitrogen-doped carbon materials a number of candidates were identified as promising alternative support materials. Simultaneously, an improved reaction set-up was developed allowing the measurement of reaction rates with much higher precision. By using this new method, it was shown that an alternative biomass-derived carbon material achieved higher reaction rates than the molecular Pt(bpym)Cl₂ catalyst. However, recycling experiments revealed a decrease in catalytic activity for the successive runs. Subsequently, this finding was systematically investigated by using polyacrylonitrile-based carbon materials that allow varying material properties in a much broader range than the biomass-derived carbon tested before. In particular, it was shown that the carbonization temperature has a significant influence on the catalytic performance indicating that coordination sites of polyacrylonitrile carbonized at high temperatures, i.e. 1100°C, are highly beneficial and substantially increase the reaction rate. Thus, catalytic activities superior to CTF based catalysts and even to the molecular benchmark catalyst were achieved. Moreover, it was found that material porosity also influenced the reaction rates, indicating that mass transfer processes play an important role under reaction conditions. However, recycling experiments showed a deactivation behavior similar to the biomassderived carbon. The origin of this loss in activity could not be identified so far since complex deactivation pathways have to be investigated in more detail.