Synthesis, Characterization And Photocatalytic Hydrogen Production Characteristics Of Non - Noble Metal Molecular Catalysts

Hydrogen has been labeled the clean energy fuel in this century because of its high calorific value and water as the only by-product of combustion. In different hydrogen production routes, the photocatalytic hydrogen generation opens up a new avenue for utilization of solar energy to produce hydrogen by photocatalytic water splitting, which is considered to be the most attractive approach. For this purpose, in the present thesis, we have designed and synthesized one new binuclear cobalt oxime complex, seven [2Fe2S] model compounds, two dithiolene nickel complex and one water-soluble MPA-Cd Se quantum dots( MPA=mercaptopropionic acid). The performance of hydrogen evolution for the target compounds have been evaluated by the constructed photocatalytic system. The full text is divided into five chapters, which is described detailedly as follows: In Chapter 1, the research backgrounds and significances of the subject are explained, the research methods are also introduced, and then the author mainly elaborates the development of the photocatalytic hydrogen production system based solely on non-noble metals in the recent ten years, including synthesis of photocatalyst and photosensitizer, the construction of photocatalytic hydrogen production system, and the performance of hydrogen evolution. At last, the implication, innovation, limitation and directions for future research were discussed. In Chapter 2, a novel binuclear cobalt oxime complex [Co2(dmg H)4(m-4,4’-bpy)Cl2](1) with axial ligand 4,4’-bpy has been synthsized and characterized by IR, 1HNMR, fluorescence spectra and cyclic voltammetry. After that, a three-component photocatalytic hydrogen production system is constructed comprising of target complex as catalyst, eosin Y(EY2-) as photosensitizer(PS) and triethanolamine(TEA) as sacrifice agent. H2 prodution condiations was optimized. The maximum hydrogen evolution of 1488.3±34.5 lmol(160.0±3.7 TON vs. 1) and H2 evolution rate of 744.2±17.3 mol h-1 are recorded in the optimal conditions with 1 of 3.7×10-4 mol ·L-1, EY2- of 4×10-4mol·L-1, TEOA of 20%(v/v) and p H 10 in 2h irradiation. In addition, the deactivation reasons of photocatalytic system and the mechanism of H2 evolution in the homogeneous photolysis system are also briefly discussed. Hydrogen production process is more likely to be homolytic process of hydride intermediates In Chapter 3, three novel [2Fe2S] biomimetic compounds with [Fe Fe] hydrogenase activity sites, namely {(μ-pdte)[Fe(CO)3][Fe(CO)2L], μ-pdte =μ2-S(CH2)2CH [(CH2)3COOCH3]S-μ2, L= CO(2), L =PPh3(3)},(μ-pdte)[Fe(CO)3] [Fe(CO)(phen)](4), have been designed and synthesized using bioactive lipoic acid ligand, which are further characterized by IR, 1HNMR, elemental analysis and X-ray crystallography crystal structure analysis for 2. And then, 2-4 are used as photocatalyst to construct a three-component photocatalytic hydrogen production system comprising of EY2- as PS and TEA as sacrificial agent. The maximum hydrogen evolution is found to be 106.5 μmol(TON of 13.3 vs. 2), 136.2 μmol(TON of 17.0 vs. 3) and 51.2 mmol(TON of 6.6 vs. 4), respectively. The result indicated that the CO displacement by PPh3 in the target sample could significantly improve the photostability and catalytic activity. Furthermore, the mechanism of electron transfer(ET) is also confirmed by CV, fluorescence spectroscopy experiments and transient spectroscopy experiments. The deactivation of system was mainly because of the photodegradation of EY2- and catalyst in photochemical reaction process. Spectroscopic and electrochemical studies indicated that the reduced FeIFe0 species could be formed by three pathways, namely, electron transfer could be occurred from 1*EY2-, 3*EY2- or EY3- to FeIFeI, respectively.In Chapter 4, four new ‘open butterfly’ [2Fe2S] simulate complexes(5-8) had been synthesized and characterized by IR, UV-visible spectroscopy, 1HNMR, fluorescence spectra, CV and single crystal X-ray crystallography(for 6). The H2 evolution of 5-8 was evaluated by the constructed non-noble metal photocatalytic system consisting of 5-8 as catalyst, EBS2- as photosensitizer and TEA as sacrificial reagent in CH3CN/H2 O solution under visible light irradiation. The maximum H2 evolution was recorded to be 205.0μmol(51.4 TON vs. 8),186.5μmol(41.4 TON vs. 5), 94.7μmol(23.8 TON vs. 7) and 18.4μmol(4.6TON vs.6) under the optimal conditions with 5-8 of 2×10-4mol·L-1, EBS2- of 4×10-4mol·L-1, TEA of 10%(v/v) and p H 12 in CH3CN/H2O(v/v, 1/1) in 4h irradiation, respectively. Among them, compound 8 displayed a higher hydrogen production activity in the above optimal conditions, that is, the more proton capture sites contained in the target catalyst the higher activity of hydrogen production. Furthermore, the electron transfer mechanism of photocatalytic experiments demonstrated by fluorescence spectroscopy and CV. The result confirmed that [Fe2S2]-(Fe0FeI) intermediate could be first formed by ET from 1*EBS2- to target catalyst, and then underwent an EECC(for 5, 6 and 8) or ECEC(for 7) process to form the H2-Fe2S2(η2-H2-FeIIFeI) species, which possessed a sufficient hydride character that could be protonated again in order to give H2 and regenerate the Fe2S2(FeIFeI) species.In Chapter 5, two dithiolene-nickel(II) complexes 9-10 and water-soluble MPA-Cd Se quantum dots had been synthesized and characterized. Hydrogen evolution of 9-10 was evaluated by the constructed heterogeneous hydrogen production system consisting of 9-10 as catalyst, MPA-Cd Se QDs as photosensitizer and TEA as sacrificial reagent in pure water under visible light irradiation.The maximum H2 evolution was recorded to be 2243.4μmol(TON 8974 vs. 9) and 1632.7μmol(TON 6532 vs. 10) under the optimal conditions with the concentration of 9-10 of 1 × 10-5mol·L-1, TEA of 5%, p H 12 of water. Furthermore, the mechanism of H2 evolution was explored by fluorescence spectroscopy and CV, the results indicated that the photo-generated electrons could be transferred from excited states quantum dots QDs * to Ni(II) and generated Ni(I), or directly transferred from QDs to Ni(I) generated Ni(0); and then, Ni(I) and Ni(0) can be generated in combination with a proton to form Ni(II)-H and Ni(I)-H, respectively; finally, collision of two Ni(II)-H or one Ni(I)–H and one proton to generate H2 molecules and regenerate the catalyst Ni(II).