The Covalently Bonded Dimolybdenum Complexes For Photocatalytic Hydrogen Evolution Performance Study

The sustainable development of human society requires a safe,clean and renewable energy source to solve the global energy shortage and environmental pollution problems.The solar energy is a limitless renewable energy resources and suitable for replacing the fossil fuels.Photocatalytic hydrogen evolution is the most valuable way to utilize solar energy because hydrogen has high energy density and clean combustion product-water.Photo sensitizer is a key factor for the hydrogen evolution efficiency in photocatalytic hydrogen evolution reaction(HER).To this end,two photocatalytic HER systems using quadruply bonded dimolybdenum complexes which have rich metal-centered photochemistry with a ?2?4?2 electronic configuration for the ground state as photo sensitizers were designed and their HER efficiency and the reaction mechanism of the systems were studied.Chapter 2 introduces a heterogeneous photocatalytic HER system constructed by using K4Mo2(SO4)4 as photo sensitizer.According to the previous publications,88*excited state of the Mo2 complex is excited under visible light with a short lifetime in acidic aqueous solution.It cannot be effectively utilized in the photocatalytic hydrogen production reaction because proton quenching is prone to occur.Thus we constructed a heterogeneous system by introducing TiO2 electron relay,MoS2 co-catalyst and sacrificial electron donor(ED)ascorbic acid(AA)or isopropanol(IPA).In this system,the ? electrons in excited K4Mo2(SO4)4 can be rapidly transferred to the conduction band of TiO2,and the protons are reduced at the active site of the MoS2,which realizes the utilization of the ? electrons,and the illuminating light for excitation of K4Mo2(SO4)4 is extended from the ultraviolet to the visible regions.Due to the coordinative unsaturation of the Mo2 center and the carboxyl group in the AA hydrolysate,AA can easily coordinate with the Mo2 center in the reaction and adsorb on the surface of TiO2 through several hydroxyl groups to bridge the Mo2 center and TiO2,and the metal-to-ligand electron transition(MLCT)absorption band in the UV-Vis-NIR spectra confirms the formation of the Mo2-TiO2 junction.The M02-TiO2 junction effectively improves the electron transfer and charge separation efficiency of the system.With AA added as ED,irradiation of artificial sunlight(AM 1.5)on the reaction in 5 M H2SO4 has produced molecular hydrogen of 13400 ?mol g-1(based on Mo2 complex).In order to further prove the good photosensitized properties of the dimolybdenum complexes,we assembled the zinc porphyrin(ZnP)and dimolybdenum coordination units to obtain a series of single-component photocatalysts ZnP-Mo2,which is introduced in Chapter 3.In addition,we investigated the effect of different substituents of the auxiliary formamidinate ligands(ArNCHNAr)encompassing the Mo2 center on the energy level structure and hydrogen production efficiency and found that the intramolecular electrons transfer and electron transitions are more favorable for the compound with substituent of strong electron-donating and therefore exhibit better catalytic performance.Since the zinc center in ZnP can coordinate axially with triethanolamine(TEOA),the TEOA…ZnP-Mo2 trimer is formed by self-assembly,leading to efficient electron transfer from the electron donor to the photosensitive center.The optimized photocatalytic hydrogen production efficiency(33229 ?mol g-1,108 TON)under AM 1.5 illumination is 90 times higher than that of ZnP under the same conditions(1044?mol g-1,1.2 TON).The single-component complexes display much higher photocatalytic activities for hydrogen generation than most previously published noble-metal-free single-component complexes comprising ZnP.Moreover,there is no hydrogen obtained from the system only containing Mo2 unit without ZnP,due to the short lifetime of ??*excited state in aqueous solution.The ZnP-Mo2 catalysts are charaterized by 1H NMR spectroscopy,electrochemical cyclic voltammetry(CV),UV-vis spectroscopy and emission spectroscopy.Electrons transfer kinetics and reaction mechanism are studied and revealed with the support of DFT calculation.