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In this thesis, the ability of silica-supported native transition metals to catalyze H2-dependent CO2 reduction to metabolic carbon compounds under plausible hydrothermal vent conditions as a possible scenario for the origins of life is studied. The effect of modifications of the silica support is investigated and the catalytic performance of different metals including cobalt, iron, and copper is compared with a focus on synergistic effects in bimetallic catalysts. Mechanisms for the formation of relevant prebiotic carbon compounds are suggested.
First, Co-based catalysts on different modified silica supports are investigated for CO2 reduction to prebiotic carbon compounds using a continuous flow setup for catalytic testing. Mesoporous SBA-15 model silica supports with different incorporated cations (Mg, Al, Ca, Ti, and Zr) and controlled physicochemical properties were synthesized to study the effect of varying Co−support interactions with simulated natural mineral surroundings. As products from catalytic CO2 reduction at 180 °C and 2.0 MPa, methane, methanol, carbon monoxide, and hydrocarbons were obtained in the gas phase. The observation of intermediates of the acetyl-coenzyme A (acetyl-CoA) pathway of autotrophic carbon metabolism, formate and acetate, in the condensed product phase supported the relevance of the materials as possible prebiotic catalysts. The catalytic activity and product selectivities varied depending on the support material, which was attributed to different ratios of metallic and partially oxidized Co species resulting from the specific interaction strength with the respective support material. The highest concentrations of formate (3.6 mM) and acetate (1.2 mM) were obtained with the Ti-containing catalyst, but overall the biologically relevant CO2 reduction activity was proven to be robust to different possible natural mineral surroundings.
In the second part, the performance of silica-supported Co‒Fe alloy catalysts with varying metal ratios is compared under the same reaction conditions to elucidate the synergistic effects of both metals, which coexist in hydrothermal vent minerals. A similar range of products was obtained compared to the study in the first part of this thesis. However, the respective selectivities were changed by a decreased hydrogenation activity of the alloyed catalysts compared to monometallic Co and a physical mixture of both metals. This resulted in a maximum formate concentration of 6.0 mM and acetate concentration of 0.8 mM over the CoFe alloy with the 1/1 metal ratio as in the natural mineral wairauite. Based on the different relative reaction rates for the formation of the gas phase products, different predominant reaction pathways were suggested for Co-rich and Fe-rich catalysts. The results indicated a similar role of the Co active site in the Co‒Fe catalysts compared to the enzyme methyl transferase in the acetyl-CoA pathway with the same transition metals in its core structure. This further supported the natural tendency of the metals to catalyze the same reactions in the biological CO2 fixation pathway and geochemical reactions.
Third, Cu‒Fe bimetallic catalysts with different metal ratios are investigated for the synthesis of more complex prebiotic carbon compounds from CO2 based on their ability to catalyze higher alcohol synthesis under technical synthesis gas chemistry conditions. In addition to formate and acetate, the Cu7Fe catalyst also formed propionate and butyrate in mM concentrations at 220 °C and 2.0 MPa. These carbon compounds not only present intermediates of autotrophic CO2 fixation pathways, but also possible carbon substrates for heterotrophic organisms at hydrothermal vents. The difference in the product range of the bimetallic catalysts compared to the monometallic counterparts was attributed to the catalytic activity of interfaces of Cu and Fe particles. Besides the liquid product phase, the selectivities of the gas phase products changed significantly and methanol was formed as the main product over the bimetallic catalysts while monometallic Cu was selective for the synthesis of carbon monoxide. Indications for the reaction mechanisms were derived based on the relative reaction rates for the formation of the gas phase products. Further, in-situ and post-reaction powder X-ray diffraction (XRD) analyses were performed to elucidate the active metal phases under the reaction conditions.
Overall, this thesis contributes to a better understanding of the ability of natural transition metals to have acted as prebiotic catalysts for the synthesis of organic carbon compounds before the existence of enzymes at the origins of life. The effect of porous silica-based mineral matrixes as natural support materials for the metal particles is elucidated and the performance of different mono- and bimetallic catalysts is compared with a focus on synergistic effects. Reaction mechanisms are suggested to explain the different product selectivities of the metals, which enables a more detailed comparison of the catalytic principles in the geochemical and biological pathways.
