光驅動二氧化碳還原反應之有機催化劑研發

Development of Organocatalysts for Light-driven CO2 Reduction Reaction

王士豪(Shih-Hao Wang)

指導教授 ： 鄭如忠

共同指導教授 ： 王立義(Leeyih Wang)

國立臺灣大學/工學院/高分子科學與工程學研究所/博士(2022年)

https://doi.org/10.6342/NTU202200103

摘要

本論文主要設計及合成一系列富勒烯衍生物、共軛小分子和高分子，作為光驅動二氧化碳還原反應之催化劑，並探討分子結構於材料性質、催化活性和催化穩定性之影響。
第一部分，首先以普拉托反應將4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazol le (DTBT)化學接枝至富勒烯吡咯烷上，形成二元體 (dyad)分子，簡稱為DTBT-C60，促使其吸收光譜紅位移，並探討其光學性質和光電化學性質的變化。紫外光-可見光光譜顯示長波長之吸收峰確實因DTBT分子的存在，而增加了400-520 nm區間的吸收範圍，吸收光譜的改善有助於DTBT-C60更好的利用太陽光產生更多的激子，PL以及TRPL的量測結果顯示，引入DTBT能更有效地拆解激子以及減少電荷再結合的機率，DTBT-C60的光電化學性質則分別利用光電壓衰退 (photovoltage decaying measurement)、電化學阻抗分析 (electrochemical impedance spectroscopy)以及光電流響應 (photocurrent response)實驗進行分析。DTBT-C60催化之反應系統在AM1.5G的光譜之太陽模擬光源照射反應24小時後，產出一氧化碳為唯一產物，其產率為144 μmol·gcat-1。同位素的實驗證實水可以有效地作為電子的來源與DTBT-C60反應，且不需要任何犧牲劑添加，更重要地，DTBT-C60維持了非常持久的催化活性，可超越一個禮拜之久。
第二部分，使用萘雙亞醯胺作為電子授體單元，分別與thiophene、thienothiophene、bithiophene 和terthiophene，進行Stille coupling反應，合成一系列的有機共軛小分子，分別簡稱為NDI-2T、NDI-TT、NDI-4T和NDI-6T，並探討其結構上電子供體的能力對於它們的光電性質之影響，其中，由於導入較強推電子能力之電子供體，NDI-6T擁有最紅位移的吸收光譜、最長的激子壽命以及較佳的電子電洞拆解能力，將NDI-4T作為有機催化劑應用於光催化二氧化碳還原反應，可產出168 μmol·gcat-1的一氧化碳產率，相較NDI-TT (111.9 μmol·gcat-1)、 NDI-2T (88.4 μmol·gcat-1)和NDI-6T (40.5 μmol·gcat-1)高出許多，除了NDI-6T外，其一氧化碳產率的趨勢隨著噻吩數量的增加而有所提升。然而，電子供體的推電子能力提升，能有效的縮短能隙，卻同時上移氧化能階，NDI-6T的氧化能階與水的氧化能階相同，降低NDI-6T陽離子氧化水以還原成中性態的驅動力，相對地，NDI-4T具有適當的氧化及還原電位、較低的電荷轉移電阻以及優異的光電流強度，因此，表現出最高的催化活性，而且，反應過程僅需使用水作為電子供體，不需要共催化劑及犧牲劑的加入，此部分之研究不只能展示有機共軛小分子作為催化劑之潛力，也探討了共軛小分子電子供體能力對於二氧化碳還原效率之影響。
第三部分，在共軛高分子的主鏈導入電子授體之萘雙亞醯胺單體，並分別與弱的推電子單元之biphenyl單體或拉電子單元之DTBT單體，利用Stille反應進行聚合形成兩個共軛高分子 (PNDI-BP和PNDI-DTBT)，合成之單體及共軛高分子均有使用核磁共振光譜儀進行結構鑑定，並以凝膠滲透層析測得分子量，並與市售之共軛高分子PNDI-2T針對它們的光學、電化學及結晶性質進行比較，紫外光可見光光譜顯示PNDI-DTBT擁有最寬廣的吸收光譜以及較強的吸收係數，吸收光譜的改善能使PNDI-DTBT更好的利用太陽光產出更多激子，XRD的分析顯示PNDI-2T表現出最好的結晶度，且具有明顯的π-π stacking的訊號。三個高分子催化劑進行二氧化碳光催化還原反應都能在少量水作為電子供體下產出一氧化碳，更重要的是PNDI-BP不只擁有了最高的一氧化碳產率90.0 μmol·gcat-1，也能夠產出甲烷作為產物，產率為1.7 μmol·gcat-1。TRPL及開環電壓衰退之分析，發現萘雙亞醯胺與非共平面的雙苯環單體共聚合之PNDI-BP，可有效地延長電子存活的時間及抑制電荷載子的再結合，電化學阻抗分析以及光電流響應實驗指出PNDI-BP具有最低的電荷傳遞電阻以及較佳的光電流強度，因此，可大幅的提升催化活性以及增加還原反應的產物量，PNDI-BP展現了非常好的可回收性以及卓越的長時間催化穩定性，可穩定產出產物至兩周之久。
綜合以上實驗，我們對於富勒烯衍生物、萘雙亞醯胺之小分子及高分子的結構設計有更進一步的認識與了解，這將有助於未來設計高催化效率及高穩定性的有機催化劑。

關鍵字

二氧化碳還原 ； 富勒烯衍生物 ； 有機共軛小分子 ； 共軛高分子 ； 光催化劑 ； 光反應

並列摘要

In this thesis, a series of the fullerene derivatives (PC60BM, PE-C60, DTBT-C60), naphthalene dimide (NDI)-based conjugated molecules (NDI-2T, NDI-TT, NDI-4T and NDI-6T) and conjugated polymers (PNDI-BT, PNDI-DTBT, PNDI-BP) were designed and synthesized as organocatalysts for the photoreduction of CO2. The correlation between chemical structures and properties of organocatalysts and their effect on catalytic performance were investigated.
In the first part, the fulleropyrrolidine functionalized with 4,7-di(thiophen-2-yl)benzo[c][1,2,5]thiadiazole, abbreviated as DTBT-C60, were synthesized by Prato reaction and its optoelectrical, electrochemical properties were investigated. The DTBT-C60 shows a broader absorption spectrum, convering absorption ranging from 400 to 520 nm. The novel C60-chromophore dyad structure makes good use of solar light. The absorption of light by the DTBT chromophore generates exctions, which dissociate into hole and electron by intramolecular charge transfer, as evidenced by the PL and TRPL results. The photovoltage decay, electrochemical impedance spectroscopy (EIS), and transient photocurrent response experiment were carried out to elucidate the photoelectrochemical properties of DTBT-C60. Consequently, the DTBT-C60 exhibits a promising CO yield of 144 μmol·gcat-1 under AM1.5G solar illumination for 24 hours. Moreover, the isotope experiments demonstrate that water molecules can function as an electron source to reactivate DTBT-C60 without the need for other sacrificial electron donors. Impressively, DTBT-C60 exhibits an extremely durable catalytic activity for more than one week under reaction condition.
In the second part, four NDI-derived organic small molecules, NDI-2T, NDI-TT, NDI-4T, and NDI-6T with various number of thiophene units were synthesized and used as metal-free photocatalysts that catalyzed the photochemical reaction of CO2 in the presence of H2O under irradiation by an AM1.5G solar simulator at an one-sun intensity. The structure-property relationship was investigated by exploring the effects of electron-donating ability of donor unit on optical properties, redox potential, electron-hole distribution, and exciton lifetime. Among them, NDI-6T shows not only the most red-shifted absorption, longest exciton lifetime, best electron-hole separation, but also the most upshifted oxidation potential due to the strongest electron-donating ability of donor unit. NDI-4T exhibits the best CO yield of 168.6 μmol·gcat-1, which is much higher than NDI-TT (111.9 μmol·gcat-1), NDI-2T (88.4 μmol·gcat-1), and NDI-6T (40.5 μmol·gcat-1) in the presence of water as electron donor without any sacrificial reagent and co-catalysts. Apart from NDI-6T, the CO yield of NDI-derived small molecules increases as the number of thiophene rings increases. However, the high-lying oxidation potential of NDI-6T results in a weaker driving force for oxidizing water to reactivate the catalysts. Such low regeneration rate may result in high electron-hole recombination rate or the catalysts decompostion, which are unable to maintain the catalytic activity. The enhanced catalytic performance of NDI-4T can be ascribed to its moderate redox potential, long carrier lifetime, superior charge separation efficiency and enhanced photocurrent intensity. This work provides an effective molecular design for optimizing the CO2 photoreduction.
In the third part, we designed and synthesized two NDI-based conjugated polymers, PNDI-BP and PNDI-DTBT through Stille polymerization, which are analogues of the commercial polymer, PNDI-2T. The resulting polymers were characterized using 1H NMR and GPC. The PNDI-DTBT exhibits the most board absorption spectra and the highest absorption coefficient compared with the others. The film morphology of these NDI polymers was investigated by XRD analysis. Among them, the PNDI-2T exhibits the best crystallinity with a noticed π-π stacking feature. They all were able to catalyze the photochemical reaction of CO2 to produce CO in the presence of trace amount of water. More importantly, PNDI-BP not only has the highest CO yield of 90.0 μmol·gcat-1 among the three polymers, but also generates 1.7 μmol·gcat-1 of CH4. In addition, TRPL and OCVD measurment reveal that the copolymerization of NDI with non-coplanar biphenyl monomers could prolong electron lifetime and depress charge-carrier recombination. The EIS analysis and photocurrent response indicated that PNDI-BP possess a reduced interfacial charge transfer resistance and excellent photocurrent density, leading to a boosted catalytic activity and increased yields of reaction products. Impressively, PNDI-BP exhibits excellent recyclability and supreme long-term stability exceeding 2 weeks.
Based on the aboved-mentioned results, this research successfully developed a series of fulleropyrrolidine, NDI-based conjugated small molecules and polymers as organocatalysts for CO2 photoreduction. The achievement of structure-property study will be benefit of developing high-performance organocatalysts.

並列關鍵字

CO2 reduction ； fullerene derivatives ； fulleropyrrolidine ； conjugated small molecules ； conjugated polymers ； photo-catalysts ； photoreaction

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