應用於CO2還原反應之含碸共軛聚合物光催化劑之研發

Abstract

本研究的目的係在於設計及合成可溶性之有機線性高分子做為光催化劑，用以進行二氧化碳還原反應，期能改善溫室效應。 Dibenzothiophene-5,5-dioxide（DBTO）單元具有良好的電子接受能力、共平面結構和親水性，因而被廣泛用於hydrogen evolution reaction (HER) 中，然而，以DBTO為單體聚合形成的均聚合物，擁有較弱的可見光吸收和較大的光帶隙，導致光收集能力較差。本論文的第一部分，我使用擁有平面性佳、較高電子親和力、以及優良電子遷移率的naphthalenediimide (NDI) 單元，與DBTO共聚合而架構成供體-受體的高分子P(NDI-DBTO)，藉以增加分子內之電子轉移，進而提高在可見光區的吸收能力。此外，其結構中也具有高吸附二氧化碳之有機官能基，有助於二氧化碳於催化劑表面進行還原反應。我們透過聚合物PDBTO與P(NDI-DBTO)比較及探討在有機可溶線性高分子對光催化CO2還原之影響。 第二部分，透過第一部份之研究結果，進一步延伸及設計了兩個線性高分子，其一係在NDI與DBTO中間架橋一個TN單元，形成P(NDI-TN-DBTO)，以此來延長共軛主鏈，擴大吸光範圍與分離CO2和H2O的吸附位點; 另一則在DBTO單元上接枝具有推電子能力的醚官能基團，形成P(NDI-DBTOOMe)，增加分子內之電子轉移效能及材料的吸光能力。 本研究所合成之中間體與最終產物，均利用核磁共振技術鑑定它們的結構。紫外-可見光譜儀顯示改良後的P(NDI-DBTO)、P(NDI-TN-DBTO)與P(NDI-DBTOOMe)皆在可見光區呈現寬廣的吸收峰，可有效增加激子產出率，並且能隙相較於PDBTO皆變窄。循環伏安實驗測得P(NDI-DBTO)、P(NDI-TN-DBTO)與P(NDI-DBTOOMe)之LUMO皆高於二氧化碳還原能階，證明它們都能轉移電子給CO2進行還原反應。還原反應在CO2壓力為880 torr之密閉環境，以AM1.5G的太陽光模擬光源及100 mW/cm2光強下進行，氣相層析圖譜顯示皆僅生成CO為單一產物，其中以P(NDI-DBTO)、P(NDI-TN-DBTO)及P(NDI-DBTOOMe)塗布於13X 之分子篩上，在純水下所得到之產率分別為67.2 μmol*g−1*h−1、35.6 μmol*g−1*h−1及81.4 μmol*g−1*h−1，而後加入TEA為犧牲劑所得到之產率分別為694.6、254.5及1359.2 μmol*g−1*h−1。為了進一步了解P(NDI-DBTO)、P(NDI-TN-DBTO)與P(NDI-DBTOOMe)的差異，我們使用時間解析螢光光譜技術、電化學阻抗儀、光電流響應來深入探討結構對二氧化碳還原之活性影響。

This study aims to develop soluble organic linear polymers as promising photocatalysts for carbon dioxide reduction reactions, thereby relieving the global greenhouse effect. Dibenzothiophene-5,5-dioxide (DBTO) has been widely used as a building block to develop polymer catalysts for hydrogen evolution reactions (HER) due to its good electron-withdrawing ability, planar structure, and hydrophilicity. However, most of the published DBTO-based polymers have a weak light absorbance in visible region and a relatively large bandgap, resulting in poor light utilization efficiency. To solve this problem, in the first part of this thesis, I copolymerized DBTO with naphthalenediimide (NDI) monomer, which has good planarity, high electron affinity, adequate electron mobility, and pro-CO2 groups, to yield a copolymer of P(NDI-DBTO). The alternative donor-acceptor conjugated backbone effectively improved the efficiency of intramolecular electron transfer and then enhanced the light-harvesting capability. The optical and electrical properties, and the catalytic activity for photocatalytic CO2 reduction of PDBTO and P(NDI-DBTO) were extensively examined, compared and discussed. In the second part, I adopted two approaches to further increase the light-harvesting ability of P(NDI-DBTO). The first method includes the insertion of a TN unit between NDI and DBTO monomers as a spacer to extend the conjugated length and spatially separate the adsorption sites of CO2 and H2O, yielding a copolymer called P(NDI-TN-DBTO). The second method involves the incorporation of OMe groups, which have good electron-donating ability, to the two para positions of DBTO, forming a copolymer called P(NDI-DBTOOMe) and increasing the efficiency of intramolecular electron transfer. The effect of the molecular structure on the opto-electronical properties and the ability of reducing CO2 under light illumination were thoroughly investigated. The chemical structure of all intermediates and final products synthesized herein were identified using 1H and 13C NMR techniques. The UV-Vis measurements showed that all P(NDI-DBTO), P(NDI-TN-DBTO) and P(NDI-DBTOOMe) exhibited broad absorption spectra in the visible region, effectively increasing exciton production rate and narrowing the energy gap compared to PDBTO. Cyclic voltammetry experiments revealed that the LUMOs of P(NDI-DBTO), P(NDI-TN-DBTO) and P(NDI-DBTOOMe) are higher than the reduction potentials of CO2, demonstrating that they are all capable of transferring electrons to CO2 to carry out the reduction reaction. The photocatalytic CO2 reduction reactions were performed in an air-tight chamber, which was charged with the polymer catalyst and CO2 gas at 880 torr, under continuous illumination with an AM1.5G solar light simulator at a light intensity of 100 mW/cm2. The gas chromatography spectra showed that CO was produced as a single product in all cases. The CO production rate in the catalytic systems of t P(NDI-DBTO), P(NDI-TN-DBTO) and P(NDI-DBTOOMe) in the presence of H2O using 13X molecular sieve as porous matrix were 67.2, 35.6, and 81.4 μmol*g-1*h-1, respectively. Replacing pure H2O with a mixture of H2O/TEA significantly increased the rate to 694.6, 254.5 and 1359.2 μmol*g-1*h-1, respectively. Furthermore, the time-resolved fluorescence spectroscopy, electrochemical impedance, and photocurrent response were systematically evaluated to gain insights into the catalytic systems of P(NDI-DBTO), P(NDI-TN-DBTO), and P(NDI-DBTOOMe) in the photochemical reduction of CO2.