Theoretical Studies on Artificial Water Splitting-Water Oxidation and Proton Transfer

Abstract: The present thesis is concerned with the theoretical studies on artificial water splitting process. As the quick development of research on utilizing of solar energy, which is a green, clean, and renewable energy source, many research groups focus their attention on artificial photo-synthesis systems inspired by the photosystem I and II. The overall reaction in these artificial systems is water splitting to oxygen and hydrogen. Artificial water splitting can generally be divided into two half reactions, catalytic water oxidation and catalytic proton reduction. There is an increasing interest and demand to understand the detailed mechanism of these two key parts. Since DFT (density functional theory) in particular, has proven to be a powerful and popular tool in exploring reaction mechanisms, B3LYP and M06 functionals were employed to provide a theoretical explanation of these two important reactions in this thesis.For water oxidation reaction, many efficient Water Oxidation Catalysts (WOCs) based on Ru, Ir, etc., have been reported over the last several years. The discovery of mononuclear ruthenium WOCs carrying anionic ligands is one of the major breakthroughs recently. WOCs bearing anionic ligands are able to efficiently drive catalytic water oxidation with relatively higher Turnover Numbers (TON) and Turnover Frequencies (TOF). Therefore the influence of anionic ligands gained our attention. We decided to carry out a detailed investigation on this effect, and try to propose a full mechanism of this catalytic water oxidation as well. We found that 1) The anionic ligands exert a promoting influence on the ligand exchange between picoline and water, which facilitates the formation of aqua-Ru complex, 2) The anionic ligands facilitate the complex access to higher oxidation states, which is necessary for the OO bond formation, and 3) The work of OO bond formation is in progress.For the proton reduction reaction, the transport or movement of protons is vital and interesting in many biological and chemical processes, including the hydrogen uptake/production, the reduction of CO2 to formate, and the reduction of O2 to water. It is often related to energy storage and utilization. However, the details of these processes are still ambiguous. In most natural hydrogenase enzymes or synthetic catalysts based on iron or nickel, the incorporation of a pendant amine is a frequently occurring feature. This internal amine base seems to facilitate this proton transfer by acting as a proton relay. Our calculated results showed that the internal base allows for a splitting of one high enthalpy-high entropy barrier into two: one with a high enthalpy-low entropy barrier and the other with a low enthalpy-high entropy barrier, resulting in a low free energy of activation for proton transfer. Our results can serve as a guideline in the development of new catalysts, not only for proton reduction catalysts, but also for any process that involves proton transfer from a metal hydride to an external base, such as C-H activation and functionalization catalysts.A thorough understanding on the mechanism of water splitting can help generate a strategy to enhance the catalytic performance on both water oxidation and proton reduction. We can tune or modify the synthetic complex by accelerating the slow step (rate-determining step) in the overall catalytic cycle, and can construct artificial water splitting systems with improved performance.