鎳—硫系統於水相光催化產氫之研究

Abstract

能源短缺是全球面臨的最大危機，因此替代能源的開發是非常迫切的課題。近年來，學者認為氫氣是具有潛力替代石化燃料，它能夠藉由太陽能裂解水而得。而太陽能轉換化學能的過程能藉由電子提供者、光敏劑以及催化劑來執行。在自然界中，產氫酶主要進行氫氣和質子的可逆氧化還原反應，根據金屬活性中心的不同，產氫酶可分為三類:鎳-鐵、鐵-鐵以及單鐵產氫酶。而其中鎳-鐵產氫酶的活性中心主要為硫醇架橋單鎳單鐵錯合物所構成，又由鎳-鐵產氫酶之產氫機制可知，鎳原子之氧化態在催化過程中會改變，而鐵原子不會，因此科學家認為鎳原子是鎳-鐵產氫酶主要的催化中心。基於這項理由，許多科學家模仿鎳-鐵產氫酶中，鎳的環境來建構催化劑之模型，並應用於光催化產氫之研究。我們的目標為模擬鎳-鐵產氫酶之金屬活性中心作為光催化產氫之催化劑。在我們的研究中，已成功合成出一系列的鎳硫錯合物，並經由X-光單晶繞射儀進行結構鑑定，其光學和電化學性質分別用紫外/可見光儀和循環伏安法進行分析。此外，鎳硫錯合物應用於水相系統之光催化產氫，並改變參數對光催化活性進行探討，例如:溫度、濃度、pH值以及具有不同推拉電子性質之催化劑對光催化產氫系統之影響。經過參數最佳化，我們得到以鎳硫錯合物作為催化劑，螢光素作為光敏劑，三乙胺作為電子提供者於甲醇和水之混合溶液中以五百瓦氙燈進行照射，其氫氣產生之轉換效率為每小時每莫耳催化劑催化每莫耳氫氣產生的量為205。

Energy shortage is an increasingly large problem for the world. The development of alternative energy sources is a very pressing assignment. Recently, hydrogen has been considered as a promising candidate to replace fossil fuels. It can be generated via water splitting by solar-to-chemical energy conversions, which are performed with an electron donor, a photosensitizer, and a catalyst. In nature, hydrogenases catalyze the reversible inter-conversion between proton and hydrogen. With regards to their active site metal center, hydrogenases can be divided into three types: [NiFe], [FeFe], and [Fe] hydrogenases. The active site of [NiFe]-hydrogenases are comprised of a nickel and an iron connected by bridging thiolates. Based on the hydrogen production mechanism of [NiFe]-hydrogenase proposed in previous studies, the oxidation states of the nickel atom is changed while the oxidation states of iron atom remains the same during the process of hydrogen generation. It is believed that mononuclear nickel provides the catalytic center. Therefore, many scientists mimic the environment surrounding the nickel atom to construct model catalysts, which are applied to photocatalytic hydrogen production. Our aim is to mimic the active site of [NiFe] hydrogenases as catalysts for photocatalytic hydrogen generation. A series of nickel thiolate complexes have been synthesized, purified, and characterized by single crystal X-ray diffraction. Furthermore, optical characteristics and electrochemical properties of the nickel thiolate complexes were analyzed by UV-vis spectroscopy and cyclic voltammetry. We report that synthetic nickel thiolate complexes catalyzes the production of hydrogen in the aqueous phase. In addition, experimental parameters were altered to investigate the effects of environment on the hydrogen generation system. For examples, temperature, concentration, pH values, and electronic properties of the catalyst were tested to investigate their effects on hydrogen production efficiency. In conclusion, our studies show that the highest activity of photocatalytic hydrogen production was obtained for nickel thiolate complex as a catalyst, fluoresceine as a photosensitizer and triethylamine as an electron donor in MeOH/H2O under 500 W Xe lamp light irradiation. This system exhibits a turnover frequency (TOF) of approximately 205 moles H2 per mole of catalyst per hour.