新型可溶性含碸基共軛高分子應用為二氧化碳還原反應之光觸媒

Abstract

本研究的主要目的在於設計與合成可溶性之共軛高分子，並將其做為光催化劑應用於二氧化碳還原反應以產出高經濟價值之太陽能燃料。 首先，我將碸基結構與噻吩環進行結合，合成出了具備碸基基團的最小共軛單元 thiophene-1,1-dioxide (TO) ，並將其做為單軆，與噻吩進行共聚合反應，得到了以碸基單元做為結構中電子受體的線性共軛聚合物，簡稱為 P3HTO-T ，同時合成了不具碸基基團的聚噻吩 P3HT-T 來進行性質的比較。此外，我在 dibenzothiophene-5,5-dioxide (DBTO) 單元導入具備推電子能力的辛氧基基團，再與噻吩環進行偶合反應而獲得高溶解性之線性共軛高分子，命名為 PDBTOOOc-T ，並將其與 P3HTO-T 進行比較，藉此探討不同含碸基之電子受體，對於二氧化碳還原的催化性能差異。 所有的合成產物均利用核磁共振技術 (NMR) 來鑑定它們的結構；透過凝膠滲透層析法 (GPC) 來分析高分子產物之分子量特徵；以紫外-可見光光譜儀與循環伏安儀來測量材料的吸收光譜、能隙與能階位置。然後，將 P3HT-T 、 P3HTO-T 與 PDBTOOOc-T 分別塗佈於玻璃片或分子篩上，做為二氧化碳光化學反應之催化劑。 GC 圖譜分析顯示三者皆能將 CO2 還原成 CO ，而且產物選擇性幾近100 %。它們在玻璃片上的CO產率分別為11.4、41.4 和 47.9 µmole·g-1·hr-1；在分子篩上的產率則為 27.0、134.2 與 256.3 µmole·g-1·hr-1。使用 C18O2 與 H218O 取代一般的 CO2 與 H2O 做為反應物的同位素實驗，產物之質譜分析證實 CO 確實源自於 CO2 的光還原反應。時間解析螢光光譜實驗測得 PDBTOOOc-T 的激子生命期長達 5.66 奈秒，遠比 P3HTO-T 的 1.83 奈秒與 P3HT-T 的 1.37 奈秒長；電化學阻抗儀分析證明 PDBTOOOc-T 具有較小的介面電阻；光生電流密度量測則顯示產出最高光電流密度的大小順序為 PDBTOOOc-T > P3HTO-T > P3HT-T 。此等順序均與 CO 的產率多寡一致。本研究說明在共軛高分子中引入碸基基團可有效提升其催化活性，而且 DBTO 的效能高於 3HTO 單元。

This work aims to design and synthesize soluble conjugated polymers and use them as catalysts in photocatalytic CO2 reduction reactions to produce solar fuels with high commercial value. Firstly, 2,5-dibromo-3-hexylthiophene was oxidized to form 2,5-dibromo-3-hexylthiophene 1,1-dioxide, which was then copolymerized with 2,5-bis(trimethylstannyl)thiophene to yield a soluble conjugated polymer, abbreviated as P3HTO-T. For comparison, a copolymer comprising 3-hexylthiophene and thiophene units, abbreviated as P3HT-T, was also prepared. Additionally, two electron-donating octyloxy groups were anchored to dibenzothiophene-5,5-dioxide (DBTO) unit, which was then copolymerized with thiophene to obtain a highly soluble linear conjugated polymer, named PDBTOOOc-T. The catalytic activities of these three polymers in photocatalytic CO2 reduction reactions were evaluated and compared. The chemical structures of all synthesized products were characterized by nuclear magnetic resonance (NMR) spectroscopy; gel permeation chromatography (GPC) was used to analyze the molecular weight characteristics of polymer products; UV-visible spectroscopy and cyclic voltammetry were employed to measure the absorption spectrum, band gap and energy levels of the materials. The photochemical reaction of CO2 was carried out in a air-tight chamber in the presence of P3HT-T, P3HTO-T or PDBTOOOc-T, which were coated on top of either glass slide or molecular sieve. GC spectrum analysis shows that all three polymers can reduce CO2 to CO, and the product selectivity is nearly 100%. Their CO yields on glass slide were 11.4, 41.4 and 47.9 µmole·gcat-1·hr-1, respectively; their yields on molecular sieves were 27.0, 134.2 and 256.3 µmole·gcat-1·hr-1. The isotope experiments were performed using C18O2 and H218O instead of the usual CO2 and H2O as reactants, and the mass spectrometry analysis of the product confirmed that CO does indeed originate from the photoreduction reaction of CO2. As measured from the time-resolved fluorescence spectroscopy experiment, PDBTOOOc-T has an exciton lifetime of 5.66 ns, which is much longer than the 1.83 ns of P3HTO-T and the 1.37 ns of P3HT-T. Electrochemical impedance analysis indicated that PDBTOOOc-T has the smallest interfacial resistance; the photocurrent density measurement showed that it decreases in the order of PDBTOOOc-T > P3HTO-T > P3HT-T. All these three results are consistent with the sequence of the CO yields. This study demonstrates that the incorporation of sulfonyl groups into conjugated polymers is an effective way to improve their catalytic activity, and DBTO is more effective than 3HTO in catalyzing CO2 photoreduction reactions.