Catalytic Water Oxidation By Novel Ruthenium-Based Catalysts

Direct utilization of sunlight to split water into hydrogen and oxygen is an ideal way to convert solar energy into chemical energy and to sovle the energy and environmental problems. Since water oxidation supplies electrons and protons for hydrogen productoin in overall water splitting, it has been considered as the bottleneck in artificial photosynthetic system. Recent years, molecular water oxidation catalysts have attracted much attention due to their superior ability towards water oxidation. Among numerous molecular catalysts, ruthenium-based complexes, due to the high catalytic activity, have been widely concerned.In this thesis, a series of mononuclear Ru complexes [Ru(bda)(4-R-py)2](M1-M4; H2bda=2,2’-bipyridine-6,6’-dicarboxylic acid; py=pyridine) with different substituents on the para position of the pyridine ligand were designed and synthesized. Their catalytic activities towards water oxidation were investigated using a chemical oxidant [Ce(NH4)2(NO3)6] in acidic solution. Complex M1in which the pyridine substituent at the4-position could be protonated giving an intensive electron-withdrawing effect, was found to be the most effective, exhibiting a turnover number (TON) of up to4000. Catalytic water oxidation was also driven by visible light in a three-component system containing [Ru(bpy)3]Cl2as photosensitizer and Na2S2O8as electron sacrificial acceptor. Complex M2bears the most electron-withdrawing trifluoromethyl group under neutral conditions and showed the highest photocatalytic activity with a TON of270over2h. It was concluded that the more electron-withdrawing substituents led to the higher activities towards oxygen evolution for this type of Ru catalysts.Inspired by the bimolecular reaction mechanism in catalytic water oxidation, a series of dinuclear ruthenium catalysts towards water oxidation (D1-D3) were prepared by means of covalently linking monomeric catalysts [Ru(bda)(pic)2](pic=picoline) with flexible spacers. According to this strategy, excellent activities towards water oxidation catalyzed by D1-D3with a TON of up to43000and a turnover frequency (TOF) of up to40s-1were obtained at low catalyst concentration with [Ce(NH4)2(NO3)e] as oxidant which was enhanced much with respect to their monomeric precursor. The kinetic studies on ruthenium dimer pointed to an intramolecular oxygen radical coupling mechanism responsible for the O-O bond formation. Furthermore, by means of introducing a functional group of pyrene, dimeric ruthenium catalysts were integrated with multiwalled carbon nanotubes (MWCNTs) coated on an ITO glass electrode by noncovalent π-π stacking,. In order to develop efficient photocatalytic water oxidation, trinuclear ruthenium supramolecular assemblis Al and A2were perapared by means of covalently linking photosensitizers as axial ligands to the catalytic site [Ru(bda)(pic)2]. Their catalytic activities towards water oxidation were investigated under visible light with Na2S2O8as electron sacrificial acceptor. Assembliy Al exhibited a good photocatalytic activity with a TON of38, which was5times higher than that observed for the three-component system.Above results are valuable to build the applicable solar water splitting system.