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Computational Studies on Homogeneous Water Oxidation Catalysis for Artificial Photosynthesis

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Title: Computational Studies on Homogeneous Water Oxidation Catalysis for Artificial Photosynthesis
Author(s): Rauhalahti, Markus
Contributor: University of Helsinki, Faculty of Science, Department of Chemistry
Discipline: Physical Chemistry
Language: English
Acceptance year: 2018
The development of new energy sources for the replacement of fossil fuels is an important task in chemistry. Artificial photosynthesis is a viable option for the generation of fuels. In it, water molecules are oxidized and the resulting protons are reduced to hydrogen, or further combined with carbon dioxide to form carbon fuels. The processes are powered by solar energy. Water oxidation is the bottleneck in the process, as the reaction requires the breaking of four O-H bonds, formation of a O-O bond, and the dissociation of the formed oxygen molecule. Research in past decades has resulted in transition metal complexes, mostly with ruthenium and iridium metal centers, which catalyze oxidation of water with moderate turnover frequencies and numbers. In order for the artificial photosynthesis to be a viable source of energy, catalysts using cheaper and more abundant first row transition metals and having better performance are needed. The the theory section the relevant inorganic and quantum chemistry and the used computational methods are presented. In the literature section, biological photosynthesis and the modular artificial photosynthetic system are presented and the most important water oxidation catalysts are highlighted. In the research section, an efficient water oxidation catalyst by the group of Sun is studied computationally. The coordination geometries and spin-state energetics of the catalyst were studied using Ru, Fe, and Os metal centers at different stages of the catalytic cycle. The Fe catalyst was a high-spin complex with weakened metal-ligand bonding due to the occupation of antibonding metal-ligand orbitals. The modification of the ligand framework with substituents was also studied. Substitution did not have a major effect in charge distributions or coordination geometries, implying that the differences in reactivities observed experimentally are due to environmental effects.

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