Supramolecular Systems for Photocatalytic Hydrogen Evolution from Water

Abstract

As human civilization has grown larger, the use of fossil fuels has dramatically increased and severely harmed the earth. In addition to the environmental problems, limited fossil fuel reserves motivated people to search for new energy sources. In this context, there has been growing attention to artificial photosynthesis in order to produce renewable and environmentally-friendly energy sources. Natural photosynthesis in plants transforms solar energy into chemical fuel by synthesizing carbohydrate from water and carbon dioxide. Different from natural photosynthesis, artificial photosynthesis splits water into hydrogen and oxygen to use hydrogen as a chemical fuel (* This thesis limits the scope of artificial photosynthesis to water splitting). Hydrogen does not contain any carbon so that it only produces environmentally-friendly water as a byproduct during combustion. Also, as water produced in this way can be reused as a source of hydrogen, hydrogen energy is renewable. In pursuit of efficient and economical production of hydrogen, numerous approaches have been made. Particularly, there has been a growing attention to supramolecular approach in order to improve the efficiency of photocatalytic system. By using supramolecular approach, we can modulate the photochemical properties of photocatalytic systems, and thus enhance their photocatalytic activity compared with the monomer counterparts. This thesis covers two novel supramolecular systems for photocatalytic hydrogen evolution from water. In Chapter 2, I report on the first self-healing system which spontaneously repairs molecular catalyst during photocatalytic hydrogen evolution. A bipyridine-embedded UiO-type metal-organic framework (MOF), namely Ptn_Ir_BUiO, which incorporates H2-evolving catalyst and photosensitizer was synthesized and subject to photocatalytic hydrogen evolution reaction. Though embedded in MOF, each molecular species could maintain their intrinsic properties because the functionalized MOF system worked as a supramolecular system. Photocatalytic hydrogen evolution with Pt0.1_Ir_BUiO showed very stable molecular photocatalysis without significant decrease in its activity and colloidal formation for 6.5 days at least

in homogeneous counterpart, the molecular catalyst became Pt colloid just after 7.5 h. It was revealed that the arrangement of diimine sites which closely and densely surround the H2-evolving catalyst in the MOF enabled such a highly efficient self-healing. In Chapter 3, an amphiphilic Ir(III) complex was synthesized and shown to form supramolecular assembly in water via hydrophobic collapse. Interestingly, when sodium chloride salt was added to induce the charge screening effect, electrostatic repulsion between self-assembled structures became weaker and the solution became a hydrogel. More importantly, this hydrogel was effective for photocatalytic hydrogen evolution as it could incorporate all necessary components such as water, photosensitizer, catalyst, and sacrificial reducing agent.