Preparation And Photocatalytic Activity For Reduction Of CO₂ Of Manganese(?) And Nickel(?) Metal Complexes

Environmental problems and energy shortage have always been two important issues disturbing the world.In particular,the increasing CO₂ content makes the environmental pressure greater,resulting in global warming,ocean acidification.The photochemical conversion of CO₂ to CO,CH₄ or other liquid fuels using molecular catalysts is a significant way to solve the problem.Researchers have extensively studied transition metal complexes such as Ru,Re,Ir,Fe,Co,Ni,Mn,Cu,Zn and so on,and have fully proved their photocatalytic capability of reduction CO₂.In this paper,four Mn complexes were synthesized by the coordination of Mn(CO)₅Br with 4,4?-dichloro-2,2?-bipyridine,4,4?-dibromo-2,2?-bipyridine,4,4?-diamino-2,2?-bipyridine and 4,4?-diphenyl-2,2?-bipyridine,respectively.Three Ni complexes were synthesized by using bitriphenylphosphine nickel chloride as metal salts with 4,4?-dichloro-2,2?-bipyridine,4,4?-dibromo-2,2?-bipyridine and 4,4?-diphenyl-2,2?-bipyridine,respectively.According to different metals,the seven complexes were divided into two systems to discuss their photocatalytic reduction of CO₂ process.Firstly,the productions of seven complexes were prepared by solvothermal method,and the crystals were analyzed by single crystal.Query from CCDC database showed that all the seven crystals were novel complexes,which five-coordination M1complex had three metal centers,and the rest complexes were six-coordination single metal center with symmetry.The structures of these complexes were confirmed by mass spectrometry,and the purity of the complexes were by XRD.Secondly,M1,M2,M3,M4 were used as catalyst in systems of photocatalytic reduction of CO₂.And the optimal condition is as follows:the concentration of catalyst was 1?M,the concentration of photosensitizer was 0.45 m M,the concentration of sacrifice and 0.029 m M,the solvents were DMF/TEOA(4/1=v/v),and the system was irradiated for 12 h under 460 nm light.Under these conditions,the turn over numbers(TONs)of photocatalytic reduction of CO₂ to CO could reach 1737,1785,2080 and 1583,respectively.The selectivity was 91.5%,87.4%,88.4%and 94.0%,respectively.the maximum turn over frequencies(TOF_max)could reached 149,145,173and 132 h^-1,respectively.These results indicated that M1,M2,M3,M4 have the property of photocatalytic reduction of CO₂to CO.The similar method was used to study Ni-1,Ni-2,Ni-3 catalysts for photocatalytic reduction of CO₂ to CO.In this system,the optimal conditions are determined as catalyst concentration of 1?M,photosensitizer concentration of 0.45 m M,and sacrifice concentration of 0.029 m M,the solvents of Me CN/H₂O/TEOA(4/1/1=v/v/v),irradiated for 12 h under 460 nm light.Under these conditions,the TONs of photocatalytic reduction of CO₂ to CO could reach 1735,2409 and 743,respectively.The selectivity was 80.0%,70.0%and 76.0%,respectively.The TOF_max could reached144,200 and 62 h^-1,respectively.These experimental results indicated that Ni-1,Ni-2,Ni-3 have photocatalytic property to reduce CO₂ to CO.Finally,a reasonable mechanism of photocatalytic reduction CO₂ to CO was prop osed based on the electrochemical behavior of cyclic voltammetry curves,high resolut ion mass spectrometry,density-functional calculation,UV-Vis absorption spectrum an d fluorescence spectrum.