Cobalt-based Catalytic Water Oxidation System

The development of clean and renewable energy can relieve energy crisis and pollution.It is an important research area to realize the sustainable development of human society.The total amount of energy provided by sun every hour is enough for human to consume during the whole year.Utilizing solar energy to split water into 02 and clean energy substance H2 is a promising way for the development of clean and renewable energy.Water splitting process contains two half reaction,one is the water oxidation reaction,the other is the proton reduction reaction.Since water oxidation reaction is related with multistep proton and electron transfer,a large free energy is needed.It is very important to develop active catalysts to realize water oxidation reaction efficiently.This thesis describes the design and synthesize of metal-organic-framework,metal cluster and metal complex based on cobalt and their application in catalytic water oxidation reaction.The main contents of the thesis are listed below:1.Two Kagome Co(II)metal hydroxo layered complexes stabilized by carboxylate ligand have been synthesized and used as heterogeneous catalysts for water oxidation reaction.They show good activities in light-driven water oxidation system.These two compounds show good stability as they can be reused for several times without losing activities.FT-IR spectrum,powder X-ray diffraction patterns together with X-ray photoelectron spectroscopy confirm the layered frameworks and surface compositions survive well after reaction.Cyclic voltammetry(CV)and linear sweep voltammetry(LSV)are used to test their electrocatalytic water oxidation abilities.18O isotope-labeled experiments confirm the evolved O2 is from H2O.2.A disc-like mixed-valent heptanuclear cobalt cluster was synthesized and characterized.It can realize electrochemical catalytic water oxidation.The onset catalytic potential determined to be 1.12 V(vs.NHE),with an overpotential of 421 mV.Under light-driven water oxidation conditions,the highest turnover mnumber is 210,and the corresponding turnover frequency is 0.23 s-1.The water oxidation system catalyzed by cobalt cluster maintain most of its activity after chelating ligand bpy or EDTA added,which indicates free Co ion is not the dominate active species.Electrospray mass spectrometry show that cobalt cluster dissociate in water.The main species are Co?Co24 and Co?Co?3.After reaction,organic ligand of the cluster dissociate and two species[Co2?Co2?(N3)3(OH)8(H20)4]-and[Co3?Co?(N3)3(OH)9(H2O)4]-in higher oxidation state are detected,which may be the active species.3.We design and synthesize metal complex[Cp*(N-N)Co(Cl)]Cl based on Cp*and 1H,1'H-2,2'-biimidazole(N-N),metal complex Cp*Co(N-O)based on Cp*and 2,4-di-tert-butyl-6-(phenylamino)phenol(N-O).The potential abililties of these two compounds acting as water oxidation catalysts are tested under electrochemical and light-driven conditions.Results show that these two compounds act as precatalysts and the free Co ion released during the reaction are the dominate active species.