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In this dissertation, Ni and Fe-based transition metal nanoparticles are prepared via the hard-templating method and their functionality for hydrothermal CO2 reduction to value-added molecules with biological significance are investigated to link transition metals, CO2 reduction, and hydrothermal vents in the context of life´s origins. The dissertation is part of a collaborative project with Prof. William F. Martin from the University of Düsseldorf and Prof. Joseph Moran from the University of Strasbourg, which is financially supported by the Volkswagen Foundation and the Deutsche Forschungsgemeinschaft (DFG).
The first part of the dissertation investigates the influence of the composition of synthetic Ni-Fe particles on abiotic CO2 fixation. A series of Ni-Fe nanoparticles with controlled structures and variable compositions are prepared via the nanocasting method by using spent tea leaves as a sustainable carbon template. The catalytic ability and functionality of the prepared materials are investigated for H2-dependent CO2 reduction under mild hydrothermal conditions. Effects of different reaction parameters including temperature, initial CO2 pressure, pH, and reaction time are systematically studied. Mono- and bimetallic Ni-Fe solids convert CO2 and H2 into formate, acetate, and pyruvate in mM concentrations. These compounds are key intermediates of the acetyl-coenzyme A (acetyl-CoA) pathway, which is considered as the most ancient carbon fixation pathway in the context of life’s origin. It is found that water could act as a hydrogen source for CO2 fixation, and catalytic hydrothermal reactions in the absence of synthetic H2 yield similar products, but in lower concentrations. Bimetallic Ni-Fe nanoparticles, which exist in hydrothermal vent minerals and are active centers of hydrogenase enzymes, display better performance than their monometallic counterparts for hydrothermal CO2 fixation. To get more insights into the reaction mechanism, Ni-Fe particles are also characterized after the catalytic reaction, which indicates the structural and surface alteration of catalysts after the CO2 reduction reaction. While all Fe-containing particles show the formation of FeCO3, Ni displays better stability towards carbonate formation. Both post-reaction characterization and CO2 chemisorption results support that Ni and Fe play dissimilar and complementary roles in CO2 reduction, which has similarities to active sites of carbon monoxide dehydrogenase (CODH) enzyme.
In the second part of the dissertation, ambient temperature conversion of C3-compound (pyruvate) to C5-product (citramalate) over Ni-Fe nanoparticles is studied. Pyruvate is an important product since it is a key intermediate for several metabolic pathways. Initially, pyruvate is obtained from CO2 conversion over Ni3Fe at similar conditions to the biological fixation, namely at 25 bar and 25 ˚C. It is also demonstrated that 13C-labelled pyruvate could be further converted to larger compounds including parapyruvate and citramalate over Ni-Fe catalyst under ambient conditions within 1 h. Citramalate is the C5 product of both the acetyl-CoA pathway and pyruvate condensation reaction in microbial carbon metabolism. In biological systems, both CO2 reduction to pyruvate reaction and further conversion of pyruvate to citramalate require a number of enzymes. Overall, it is found that Ni3Fe nanoparticles could act like enzymes that are associated with the acetyl-CoA pathway and citramalic acid cycle.
The third section focuses on the thermocatalytic conversion of CO2 and H2O (in the absence of synthetic H2 and N2) to oxygenates and amides with Ni-Fe nitride heterostructures. Ni-Fe heterostructures are prepared via ammonia treatment of the native Ni-Fe nanoparticles. Nitride phase composition is tuned by varying several parameters during ammonia treatment, such as temperature, time, and flow rate. Later, the functionality of these heterostructures for being catalysts and substrates is investigated for the fixation of CO2 to metabolic intermediates. Effects of reaction parameters including temperature, pH, time, and pressure are systematically studied. The catalytic reactions performed at the pressure range of 5-50 bar and temperature range of 25-100 ˚C yield formic acid and formamide as main products in the liquid phase. Longer reaction times promote the C-C bond formation, which results in the formation of acetate and acetamide as additional products. In the gas phase, methane and ethane are also detected. In the absence of synthetic N2 and H2 gases, water serves as a hydrogen donor, and nitride heterostructures provide nitrogen for the formation of amides. It is shown that Ni-Fe nitrides can perform as both catalysts and substrates for the direct conversion of CO2 to formamide under mild hydrothermal conditions. Formamide is a key building block for the synthesis of organic molecules relevant to pre-metabolic processes, and its role is essential for nitrogen incorporation into early prebiotics.[1]
The last section of this dissertation deals with the preliminary investigation of simultaneous reduction of CO2 and N2 under extreme hydrothermal conditions. Direct reduction of N2 gas is important since dissolved N2 gas is abundant in hydrothermal vents.[2] For this purpose, a customized reactor that can be operated up to 800 bar and 400 ˚C is designed. In order to simulate the environmental conditions of marine hydrothermal vents in the laboratory, CO2 and N2 fixation reactions are carried out over Ni3Fe nanoparticles at a temperature range of 200-380 ˚C and pressures up to 500 bar. Simultaneous CO2 and N2 fixation products, formamide and acetamide, could be detected at the temperature range of 200-300 ˚C. Further increase in reaction temperature results in the decomposition of amides to gas products. In addition, 15N-labelled N2 reduction reactions with very low N2 pressure and synthetic H2 further revealed the formation of 15N-labelled formamide. For a better understanding of prebiotic chemistry that operates only based on chemical energy, it is a crucial step to identify geochemically plausible metal catalysts that activate CO2 and N2 gases to biologically significant organic compounds and small autocatalytic networks.
Overall, it is shown that Fe and Ni-based solid catalysts, which exist in structures of both hydrothermal vents and important enzymes of biological CO2 fixation pathways, are able to yield basic building blocks of life starting from simple inorganic molecules like CO2, H2O, and N2. These findings align well with the metabolism-first hypothesis.
