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Highly Active and Stable Nickel Dimer Complex-Photopump-Sacrificial Donor Systems for Hydrogen Evolution and Effects of Electron Transfer Rates on Photocatatlytic Activity and Stability

초록 (요약문)

Part I: For long time the hydrogen generation from half-cell reaction catalysts has been tested using ethanol as the sacrificial donor. To understand the photocatalytic formation of hydrogen from ethanol, we studied photocatalytic conversion of ethanol with metal-loaded TiO2 as catalyst. The twenty identified reactions take place during vapour-solid photochemical dehydrogenation of ethanol on four metal-doped TiO2 (Mn-TiO2, Mn = nanoparticles od Pd, Pt, Au and Cu) under 1 Sun solar simulated light. We demonstrate the map to show how they are interconnected. The reactions are sensitively affected by the degree of moisture in ethanol and the nature of carrier gas, Ar, CO2, H2, and O2. To construct the map of twenty reactions we also used CH3CHO, CH3CO2H, (C2H5O)2CHCH3, and CH3CO2C2H5, respectively, as an independent starting reagent. Mn-TiO2 serves as cheap yet highly useful universal photocatalysts, which enable all of the above reactions. The reaction is initiated by photoinduced excitation of the charge-transfer (CT) band from ethanol to TiO2 surface, which appears in the UV region by the UV part of the solar light. Part II: Conversion of solar energy to chemical energy has become an important area of research. One of the methodologies is the photocatalytic water splitting into hydrogen and oxygen using solar energy as the energy source. For this purpose, various homogeneous photocatalytic systems consisting of a proton reduction catalyst, a photosensitizer, and a sacrificial donor have been extensively developed as a hydrogen evolution reaction (HER) and their mechanisms have been investigated. The HER consists of two processes; one is harvesting of photon by photopump and efficiently transfer electron to catalyst, two is protonation at active site of complex following the coupling of electro to form hydrogen molecule. In this thesis, the noble complexes possessing the dimeric metal centers are synthesized, namely, bis(μ2-chloro)-dichloro-bis(di-2-pyridyl-disulfide) di-nickel (II), denoted as I and bis(μ2-bromo)-dibromo-bis(di-2-pyridyl-disulfide) di-nickel (II), denoted as II. The orthorhombic (Pbca) structure of the pristine I and II complexes are determined by analyses of single crystal x-ray diffraction data obtained with a synchrotron radiation source with the wavelength of 0.700 Å. Two Ni2 complexes were applied for hydrogen production application and exhibit high efficiency in both electrocatalytic and photocatalytic activity. In the mixture of water and ethanol at pH 7, the cathodic currents of I and II are 1.22 mA, 1.4 mM, respectively. The onset potential of I and II are -0.69 V, -0.69 V, vs Ag/AgCl, respectively. Under the condition of one sun irradiation, we tested their hydrogen production by using fluorescein (FL), triethanol amine (TEOA) in water only, and water and alcohol mixture at optimized pH = 10.6 at RT. System exhibits both highly catalytic acitivity and the highly stable properties. Amount of hydrogen evolves > 40 mL after 120 h using 0.2 mM catalyst. TON reach 4800 and FL photosensitizer keeps intact during reaction of >160 h. In the electron transfer process, the electron transfer rate is characterized by fluorescence quenching measurement. Controlling the electron transfer rate from excited dye to catalyst and the electron transfer rate from sacrificial electron donor to charged dye specie maintain to balance the reaction and keep stability of system. Many factors were investigated involving reactant concentration, solvent, pH, quenching rate, type of donors. Based on these systematical studies to set up the three-component photocatalytic system, the dimer photocatalysts obtain the high hydrogen production and are active from long period of time >120 h in both water and mixture solvent of water and ethanol environment.

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