合成與應用光敏物-鍵橋-催化團簇(P-L-C)模型於光催化產氫系統

Abstract

目前世界上有許多發展中的再生性能源，氫氣被我們選擇做為一個具有潛力的研究目標；因為氫氣是一個相當好的能量載體就像電力一般，可以把化學能儲存在氫氣之中並經由燃燒放熱來釋放出能源。此外，氫氣本身擁有的克燃燒熱相當的高，比起我們常用的石化燃料：石油、天然氣、煤炭等都來的佳。另一方面，氫氣在燃燒後放熱後只會形成水，而水可以再次裂解形成氫氣和氧氣進而達到一個乾淨無汙染的能量循環。 不同於蒸氣重組、電解等高耗能的方法來取得氫氣，我們使用存在於大自然之中的一種產氫酵素，氫化酶(Hydrogenase)。我們模仿了氫化酶的催化中心結構，又名氫團簇(H-cluster)的二鐵二硫中心以進行產氫的研究。自然界中，這整個產氫酶的氧化還原反應起始於：光敏感性物質(Photosensitizer)受到光的照射後形成激發狀態並轉移電子至氫團簇，最後藉著這轉移的電子將質子還原成氫氣。根據自然界的這個光敏物與氫團簇的模型，我們試著將光敏物及氫團簇利用共價修飾的方法連接，而形成光敏物-鍵橋-催化團簇(Photosensitizer-linker-cluster)的P-L-C模型。並期望這個模型能提升電子從光敏物轉移至催化中心的效率，進而提生產氫的結果。藉由核磁共振光譜儀(NMR)、質譜儀(MS)、傅立葉轉換紅外線光譜儀(FT-IR)來進行結構的鑑定與分析，另外也使用了氣相層析-熱傳導偵測器(GC-TCD)來做為產氫的依據並計算出氫氣的轉換數(Turnover number)及轉換率(Turnover frequency)。 第一個合成完的光敏物-鍵橋-催化團簇模型是白金–鍵橋–鐵鐵氫團簇(Pt-L-Fe2S2)，也就是白金三聯吡啶(Pt(tpy))作為光敏物、鐵鐵氫團簇(Fe2S2)作為催化中心。第二個合成完的光敏物-鍵橋-催化團簇模型則是將釕釕氫團簇(Ru2S2)取代了鐵鐵氫團簇作為催化中心，而形成白金–鍵橋–釕釕氫團簇(Pt-L-Ru2S2)。合成完的白金–鍵橋–釕釕氫團簇及白金–鍵橋–鐵鐵氫團簇會在不同質子源(水及甲酸)下探討光催化的產氫。此外，不同推拉電子基團的磷配體(P-ligands)將會被添加在光催化產氫反應中來探討。在有機相中，以甲酸作為質子的來源，白金–鍵橋–釕釕氫團簇的產氫表現要優於白金–鍵橋–鐵鐵氫團簇的模型；而磷配體對白金–鍵橋–釕釕氫團簇的影響中，推電子基團的磷配體要優於拉電子基團的磷配體在光催化產氫的結果；至於白金–鍵橋–鐵鐵氫團簇模型上，其推拉電子基團的磷配體所影響的產氫結果則是沒有顯著差別。此外，不同當量的磷配體也可以增加白金–鍵橋–釕釕氫團簇的氫氣產量，三個當量的三(鄰-甲苯基)磷添加最高可產生13057.8 微莫爾的氫氣，相當於13057.8 TON及2673.9 TOF。而尚未連接的其他光敏性物質(釕、銥)和釕釕氫團簇也進行了光催化產氫的比較，用來推論未來連接後的產氫潛力。 在水相中，白金–鍵橋–釕釕氫團簇進行光催化產氫結果表示抗壞血酸(Ascorbic acid)更優於三乙氨(Triethylamine)來作為水相的電子給體。另外，不同磷配體在水相的產氫影響顯示，推拉電子基團影響產氫結果的特性與有機相類似(推電子基團優於拉電子基團)，但實際上並無法提高白金–鍵橋–釕釕氫團簇水相的產氫。這可能表明磷配體在水相中的機制與有機相並不相同。 總結來說，我們產氫效率已經優於先前的研究，但是詳細的光敏物-鍵橋-催化團簇模型機制還需再進一步的討論與研究。在未來，光敏物-鍵橋-催化團簇模型或許是一個有前途的光催化工業生產氫氣模型。

There are many in the development of renewable energy sources, chosen the hydrogen as a potential research topic; causes the hydrogen can be a good energy carrier just likes electron. Otherwise, hydrogen has very higher heat of combustion per gram, more than the commonly used fossil fuels, including natural gas, coal and so on. On the other hand, combustion of hydrogen to produce water, and water could be raw material for hydrogen. This reaction is clean, non-pollution cycle! Different from the steam reforming, electrolysis of water that commonly used high energy-consuming way to get hydrogen, using naturally occurring enzyme: hydrogenases. We mimic the catalytic center of hydrogenases that was called H-cluster which has two iron disulfide structures and completes the proton reduction in here. In the nature, whole of redox- reaction is beginning in the light-absorbing substances, called photosensitizer are excited by light then transferred electron to the H-cluster. And the protons will reduction by this excited electron. According to this photosensitizer and H-cluster model, we tried to use an artificially synthesized bridge (linker) that connected between the potosensitizer and H-cluster. In this Photosensitizer-linker-cluster (P-L-C) model, we expected that it can accelerate transfer excited electron to catalytic center from photosensitizer, which has the better efficiency than before. And we use NMR, MS and FT-IR to identify the structure, use GC-TCD to compare the turnover number (TON) and turnover frequency (TOF) with the hydrogen generation. The first synthesized P-L-C model was Pt-L-Fe2S2 which the Pt(tpy) as photosensitizer and Fe2S2 as the catalyst, the second synthesized P-L-C model was used Ru2S2 to replacement the Fe2S2 and became Pt-L-Ru2S2. The synthesized Pt-L-Fe2S2 and Pt-L-Ru2S2 were investigated using different proton sources, such as water and formic acid in photocatalytic hydrogen production. In addition, we evaluate the effects of a series of electron-donating phosphine ligands additions on the photocatalytic system for hydrogen production to investigated. In the organic phase, with formic acid as a proton source, Pt-L-Ru2S2 showed higher efficiency of hydrogen evolution than Pt-L-Fe2S2. In the presence of P-ligands, the hydrogen yield obtained from the P-ligands with electron donating functional group is higher than the P-ligands with electron withdrawing functional group in Pt-L-Ru2S2 model, but the affect of P-ligands’ influence was not significant in Pt-L-Fe2S2 model. On the other hands, the different equivalents P-ligands gave the Pt-L-Ru2S2 more hydrogen produced which was the 3 equiv of the P(o-Tol)3 with Pt-L-Ru2S2 had 13057.8 μmol hydrogen produced with 13057.8 TON (TOF = 2673.9 h-1). The others photosensitizers (Ru, Ir-based) were added with no-connecting Ru2S2 in photocatalytic hydrogen production to compared the potential of the Ru-L-Ru2S2 and Ir-L-Ru2S2. In aqueous phase of Pt-L-Ru2S2 model, the results showed that AA was better than TEA as the electron donor. Besides, the affect of P-ligands’ electron donating/withdrawing function group in aqueous phase were similar to in organic phase, but P-ligands couldn’t elevated the hydrogen produced. Summary, the efficiency of hydrogen production is better than in previous studies. The detailed catalytic mechanism of these artificial biomimetic P-L-C models might be discussed and studied. In the future, the P-L-C model will be promising catalysts in the light-driven hydrogen production industry.