Study Of Complexes Containing Transition Metals Co/Cu/Ni For Catalytic Water Oxidation

Hydrogen energy is attractive since it is clean,renewable and efficient,and therefore it is supposed to an alternative of traditional fossil fuel.The energy source of the earth is solar energy,only one 2,200,000,00th of which can meet the energy demand of all the life.It is a very ideal pathway for us to utilize solar energy and abundant water resources into split water into O₂ and H₂ by artificial photosynthesis.Overall water splitting contains to half reaction:water oxidation and the reduction of protons.The water oxidation reaction is a four-electron transfer process that is coupled to the removal of four protons.What is more,it needs high activation energy.Therefore,water oxidation reaction is considered to be the bottleneck for water splitting.In order to prove the efficiency of water oxidation,it is very necessary to develop stable water oxidation catalysts?WOCs?with high efficiency.WOCs fall into two categories,i.e.,heterogeneous WOC and homogeneous WOC.Heterogeneous WOCs are convenient for recycling and easy to be prepared on large-scale.Homogeneous WOCs,especially the complexes based on organic ligand have more advantages with catalytic efficiency,structure modification and mechanistic understanding.Here,a series of Co,Cu,Ni complexes are researched.Their catalytic performance in photocatalytic or electrocatalytic water oxidation reaction is systematically investigated.The main results are as following:1.The synthesis and characterization of a binuclear??OH,??O₂ bridged Co complex with N4 Schiff base ligand are carried out.We further investigated the catalytic ability of this complex using Na₂S₂O₈,Ru?bpy?₃²⁺and visible light as sacrificial electron acceptor,photosensitizer and light source,respectively.Chelating experiments,Cyclic voltammetry?CV?measurements,control potential electrolysis?CPE?,X-ray photoelectron spectroscopy?XPS?and scanning electron microscope?SEM?confirmed that this complex is inactive for photocatalytic and electrocatalytic water oxidation,although it is stable.In combination with previous reports by others,we believe that dinuclear cobalt complexes containing??OH and??O₂ structure should be carefully examined when it was used catalyst for water oxidation,particularly with regard to the purity,stability and catalytic activity.2.We described the photocatalytic water oxidation performance of a water soluble complex that anion is hexameric ring cobalt-containing macrocycle containing N-?Phosphonomethyl?iminodiacetic acid ligand,K₁₂[Co?H₂O₃PCH₂N?CH₂CO₂H?₂?]₆?2?.In the visible light/Ru?bpy?₃²⁺/Na₂S₂O₈ water oxidation system,the catalytic activity and the influence of its polynuclearity on the catalytic process of water oxidation are further investigated.Complex 2 is capable of catalyzing visible light-driven water oxidation with unprecedented efficiency.Optimal turnover number,turnover frequency,oxygen yield and quantum yield are 4350,162.6 s^-1,62.6%and60.6%,respectively.UV-vis absorption spectra,CV measurements,aging test,chelating experiments,Nanosecond laser flash photolysis,kinetics activity measurements,IR spectrometry and XPS confirmed that complex 2 is a genius homogeneous WOC.The active species is complex 2 rather than Co²⁺or CoO_x.Combining kinetic isotope experiments and structure analysis of complex 2,we present the influence of the polynuclear structure of complex 2 on its catalytic mechanism.3.An octanuclear Cu?II?cluster[Cu₈?dpk·OH?₈?OAc?₄]?ClO₄?₄?3,dpk=di-2-pyridyl ketone?that contains two Cu₄O₄ cubic fragments has synthesized and characterized.The catalytic activity of complex 3 has also been researched.In the[Ru?bpy?₃]²⁺/Na₂S₂O₈ system,its catalytic activity is the best amongst all of the Cu-based photocatalytic WOCs.Collective experiments including UV-vis absorption spectrum,electrochemical tests,filtration experiments and capillary electrophoresis confirmed that complex 3 is a robust homogeneous WOC.Analyzing the IR spectrometry of complex 3,we found that the high activity of complex 3 is not result the electronic effect of the ligand.We supposed that the two OH functional groups of the dpk·OH ligands closest to the five coordinated Cu^II atom may participate in hydrogen bonding with solvent water molecules to pre-organize water molecules,which accelerates the rate of water oxidation by facilitating the nucleophilic attack of water on the five-coordinate Cu^II atom and the oxidation of water.4.Utilizing?-phenylenediamine as precursor,three tetradentate ligands with phenylene oxamate structure has been synthesized.Three Ni based complexes?Me₄N?₂[NiLⁱ]?i=1-3?with square-planar coordination structure were prepared by react Ni?ClO₄?₂·6H₂O with these tetradentate ligands.After characterized,the catalytic activity of electrochemical water oxidation and stability were measured under alkaline condition.CPE,UV-vis absorption spectra and SEM indicated that?Me₄N?₂[NiL¹]is a homogeneous WOC for electrochemical water oxidation.While?Me₄N?₂[NiL²]and?Me₄N?₂[NiL³]are un stable under the same condition,both of them decomposed into NiO_x during water oxidation.The CV measurements of?Me₄N?₂[NiL¹]were carried out under different pH.The pH-potential diagram was obtained according the dependence of redox potential of?Me₄N?₂[NiL¹]on pH value.Combining result of the constant potential electrolysis experiment of?Me₄N?₂[NiL¹]in anhydrous CH₃CN,the number of electrons that transfer in each oxidative step of?Me₄N?₂[NiL¹]was determined.We further proposed the mechanism of water oxidation catalyzed by?Me₄N?₂[NiL¹].The relationship between the coordination structure and stability of?Me₄N?₂[NiLⁱ]is put forward.