Tim Herrendorf

• In this work, novel catalyst systems generated via the MOF-mediated synthesis approach were investigated regarding their activity for the CO2-based Fischer-Tropsch synthesis (FTS). MOFMS catalysts feature unique properties due to the presence of metal particles within a carbon matrix. A carbon shell, which prevents oxidation of the metal particles, high metal loadings in combination with comparatively small particle size and an enhanced carbide formation due to the close proximity of metal and carbon source should be particularly interesting for the CO2-FTS process. Hence, the topic of this thesis was the synthesis and catalytic testing of different Fe- and Fe/Co-containing MOFMS catalysts for CO2-FTS applications. First, MIL-53(Fe) precursors were synthesized, characterized and thermally decomposed at 400 °C, 600 °C and 800 °C to generate MOFMS catalysts. These showed reduced iron, iron carbide and oxidized iron phases in different oxidation states embedded into a porous carbon matrix, depending on the used decomposition temperature. Afterwards, the CO2-FTS activity was tested in a time-on-stream experiment over 12 h and a temperature variation experiment between 200 °C and 400 °C. The best performing catalyst was Fe@C_800°C with a C2+ selectivity of 53.9 % and a C5+ selectivity of 14.2 % at a CO2 conversion of 29.1 % at 350 °C. Furthermore, bimetallic Fe/Co-containing MOFMS catalysts were investigated. The synthesized MIL-53(FexCo1-X) precursors with different Fe/Co metal ratios were decomposed at 800 °C. The resulting bimetallic MOFMS catalysts showed reduced Fe and FeCo metal nanoparticles with a bcc structure encapsulated into a carbon matrix. Additionally, the oxidation preventing effect of MOFMS catalysts was investigated via TPR measurements and it was assumed that a thin layer of Fe3C carbide was formed around the metal nanoparticles, which prevents further oxidation while also allowing catalytic activity for the CO2-FTS process. The same catalytic testing experiments as for the Fe-based catalyst series were performed. The highest performing Fe/Co catalysts were the Fe-rich variants, thus, Fe0.80Co0.20@C showed a maximum C2+ selectivity of 68.2 % and a C5+ selectivity of 11.6 % at a CO2 conversion of 38.2 % at 400 °C. The best unpromoted catalysts were modified via the addition of either potassium, manganese or copper as promoter. The catalytic tests showed the highest selectivity towards FTS products for the K-containing catalysts. The Mn-containing catalysts displayed an inferior performance compared to the unpromoted variants, both regarding selectivity and CO2 conversion. The Cu-containing catalysts exhibited the overall highest CO2 conversions but showed higher methane selectivities than the K-containing variants.