Photocatalytic Water Oxidation And Organic Substrates Oxidation By Cobalt Cubane And Ru Complexes

An ideal way for solar energy utilization is photocatalytic water splitting to generate hydrogen or energy-containing compounds by artificial photosynthesis. To date, the4H+/4e-process of water oxidation driven by sunlight remains a major obstacle to overall water splitting. Efficient molecular water oxidation catalysts have attracted much attention. An alternative approach for artificial photosynthesis is the oxidation of organic compounds using water as oxygen source by transition metal complexes. Such a system is thought to greatly benefit the environment without using hazardous chemical oxidants.In this thesis, three cobalt cubane complexes with various aceto-ligand Col-Co3[Co4O4(X-CH2COO)4(Py)4](X=Me, OMe, Br, Py=pyridine) were synthesized and compared with reported catalyst27a [Co4O4(Ac)4(Py)4].Among these catalysts, the redox potential of [Co4O4]+/[CO4O4] increased with the increase of electronegativity of substituent groups on aceto-ligand:E1/2(Col)<E1/2(Co2)<E1/2(27a)<E1/2(Co3). Under visible-light irradiation in0.1M NaHCO3buffer solution using [Ru(bpy)3Cl2] as photosensitizer and Na2S2O8as electron acceptor, Co2shows3times improvement on catalytic oxygen evolution over those of Col, Co3and27a. DPV spectra confirmed that the onset potential for water oxidation shown by Co2is lower than those of Col, Co3and27a by110mV.Photosensitizer-catalyst assembly has been demonstrated to significantly improve the reactivity with respect to the multi-component system containing separated chromophore and catalyst. Cobalt cubane cluster27a [Co4O4(OAc)4(Py)4] was chosen as catalyst unit to incorporate with polypyridine Ru(II) chromophore for the construction of supramolecular. To the end, water oxidation assemblies A1and A2in linear and cyclic configuration, respectively, were prepared by a simple method. In pH7NaHCO3buffer solution, visible light-driven water oxidation was efficiently performed by these assemblies. In particular, the cyclic assembly A2, with high stability, exhibited significantly enhanced activity over the linear A1and the multi-component system.Ruthenium polypyridyl dyes are costly and not sufficiently stable under long-term light irradiation. To overcome this problem, a hybrid photocatalytic system with ruthenium aqua complexes as catalysts, BiVO4as a light absorber,[Co(NH3)5Cl]2+as a sacrificial electron acceptor and water as an oxygen source was developed. The hybrid system showed promising efficiency and selectivity in alcohol and sulfide oxidation. the photogenerated holes in semiconductor are used to oxidize molecular catalysts into the high-valent RuIV=O intermediates, which oxidize the substrate forward.