Photocatalytic And Electrocatalytic Water Oxidation Based On Salen-iron Complexes

Being a four-electron and four-proton reaction,water oxidation has been considered to be major challenges for hydrogen production from overall water splitting.It is thus desirable to develop cheap and highly efficient water oxidation catalysts.Considering that homogeneous molecular catalysts have the advantages of high metal utilization,adjustable structure and easily accessible mechanism,and on the other hand,iron is the most abundant metal in the earth's crust,a series of mononuclear,homonuclear and heteronuclear iron complexes bearing N,N?-bis-(2,3-dihydroxybenzylidene)-o-phenylenediamine(Salen)ligand were prepared in this thesis,their catalytic activity and kinetics towards photocatalytic and electrocatalytic oxygen evolution were investigated.It was found that NiFe-Salen heteronuclear bimetallic complex is an efficient catalyst for photocatalytic water oxidation with [Ru(bpy)3]Cl2(1 mM)as the photosensitiser and Na2S2O8(10 mM)as the sacrificial electron acceptor in acetonitrille-borate buffer,producing oxygen with a TON(turnover number)of 384 and a TOF(turnover frequency)of 0.94 s-1 under visible light irradiation.To the best of our knowledge,this is the first example of molecular catalyst based on heteronuclear structure for photocatalytic oxygen evolution.Dynamic light scattering(DLS)measurements confirmed that NiFe-Salen complex maintains its molecular nature during photocatalytic cycles.Encouraged by these results,the catalytic properties of iron complexes based on Salen ligands were furhter examined in electrochemical water oxidation.By employing a mixed electrolyte containing propylene carbonate and water(v/v 8%),a mononuclear Salen iron complex was found to be the most active electrocatalysts among all candidates.A catalytic onset potential of 1.50 V(vs.NHE)and a TOF value of 2.87 s-1 at 1.60 V were determined by cyclic valtammetry and kinetic studies,respectively.The electrochemical activity was allowed to be tuned by ligand substitution.For example,functionalization of methoxy group on salen ligand led to higher catalytic activity but with a decreased durability during long-term electrolysis.