Photocatalytic And Electrocatalytic Reduction Of CO₂ By Polypyridyl Iron Complexs

The burning of fossil fuels has caused increasing global warming.One of the ideal ways to mitigate environmental pollution,climate warming and energy problems is to use molecular catalysts to convert carbon dioxide into high value-added chemicals or liquid fuels that are driven by renewable light and electrical energy.At present,many precious metal molecular catalytic systems have been developed,and research based on cheap metal molecular catalytic systems is scarce.From the perspective of economics and durability,in the future,the key is to develop cheap and efficient molecular catalytic systems.Based on the above problems,we use cheap metal iron as the central metal,and the catalyst ligand uses polypyridine which can stabilize low-valence transition metals.In order to improve the catalytic performance,the coordination environment of the central metal was optimized by changing the coordination and structure of the pyridine ligand.A series of studies on the photocatalytic and electrocatalytic properties of polypyridyl-iron complexes have been performed,and the catalytic properties of other inexpensive metals such as cobalt,nickel,and manganese complexes have also been compared.The details are as follows:1.A pendentate quinoline-pyridine ligand（dqtpy = 6,6’’-di（quinolin-8-yl）-2,2’:6’,2’’-terpyridine）and its iron,cobalt and nickel complexes were designed and synthesized.Organic dye purpurin was used as the photosensitizer and BIH was used as the electron donor.Established a photocatalytic system for non-noble metal molecules.The iron complex exhibits excellent photocatalytic activity towards CO₂-to-CO conversion with TON（CO）of 544 with selectivity of 99.3 % using the commercially available organic dye purpurin as photosensitiser and BIH as electron donor.While the catalytic activity of cobalt and nickel complexes is poor,the TON of CO is only 8 and 15 respectively.On the other hand,all the three complexes show good electrocatalytic activity for CO₂ reduction when using 2,2,2-trifluoroethanol as proton source.The lack of activity in photocatalytic CO₂ reduction by cobalt and nickel complexes can be attributed to the redox potential of CoI / Co0+ and NiI / Ni0+ which is significantly more negative than the PP-/ PP2-while in the case of iron complex the FeI / Fe0 redox potential bevomes more positive than those of the PP-/ PP2-couple.2.To further improve the catalytic activity of the pentacoordinated iron pyridine complex,a quinquepyridine ligand（qnpy = 2,2’:6’,2’’:6’’,2’’’:6’’’,2’’’’-quinquepyridine）and its iron,cobalt,nickel and manganese complexs were designed and synthesized by adjusting the structure of the ligand and adding pyridyl basis of the previous chapter.Ru（phen）3Cl2（phen = 1,10-phenanthroline）was used as the photosensitizer and BIH was used as the electron donor.Established a molecular photocatalytic system in acetonitrile（Me CN）/ water（H₂O）solution Among them,iron complex is a highly efficient and robust catalyst for photocatalytic reduction of CO₂ to CO.Addition of water greatly improves the photocatalytic efficiency.A turnover number（TON）for CO as high as 14095 with 98 % selectivity can be achieved with 50 % H₂ O.The photocatalytic performance of the system is improved under aqueous acetonitrile solution.Compared with the photocatalytic activity of quinquepyridine cobalt,nickel,manganese complexes and other pyridine iron complexes under the same conditions,the quinquepyridine iron complexes still have the best catalytic performance.The addition of pyridyl group optimizes the coordination environment of the central metal iron and improves the catalytic efficiency of the system.