循環電化學應用於高選擇性之二氧化碳還原反應

High selectivity of CO2 reduction through electrochemical redox shuttle

邱士芸(Shih-Yun Chiu)

指導教授 ： 陳浩銘

國立臺灣大學/理學院/化學研究所/碩士(2017年)

https://doi.org/10.6342/NTU201702811

摘要

近年來二氧化碳排放量急遽上升，造成溫室效應且對地球產生危害，因此如何有效率的轉換二氧化碳至能源原料為十分重要的議題。而根據論文回顧，電化學法已經被證實為一有效率還原二氧化碳之方式，然而此領域最大的問題為產物選擇性低，必須耗費能源進行純化。此時催化劑的選擇便十分重要，本研究選用可以在較低電位時產生乙醇的氧化銅為催化劑，並且合成出中空結構而增加反應表面積，此外，為了提升此催化劑的選擇率，本研究創新將循環電化學方法應用於二氧化碳還原反應，利用此獨特的電化學方法來穩定氧化態之中空奈米氧化銅催化劑，並且藉由穩定催化材料的方式，在低電壓時能夠高效率的轉換二氧化碳至乙醇。
透過氣相層析–質譜法聯用量測，可得知還原主要產物為乙醇，然而氫氣為其次要的副產物。此研究中發現，相較定電壓之量測，循環電化學法可更有效率的抑制氫氣產生，提高產物的選擇性，因此本研究更進一步利用臨場X光繞射以及臨場X光吸收技術確認循環電化學法對於催化劑之影響，發現相較於一般定電壓法，使用循環電化學法時，催化劑的氧化價數以及其組成可以維持穩定，不會還原成零價銅，由此可證明循環電化學技術不僅能夠幫助穩定催化劑，還能藉此提高其產物選擇性。最後，本篇研究利用量測結果，推論氧化銅催化劑之所以可以在較低電位下得到高選擇性的產物乙醇，是因為氧化銅上的氧參與催化反應，使得乙醇的形成更加容易。
本研究預期此循環式電化學方式可以運用在各種不同的氧化材料上，幫助未來二氧化碳還原之研究，提高其產物選擇性。

關鍵字

二氧化碳還原 ； 選擇性 ； 氧化銅 ； 循環電化學

並列摘要

The amount of CO2 emissions has increased dramatically in the past few decades. Finding a method which can effectively reduce CO2 to fuel can be a promising solution to this huge emission of CO2. Nowadays, electrochemistry has been proven to be a powerful way to reduce CO2. However, the selectivity of CO2 reduction products is low. Extra energy is needed to purify different products. Therefore, choosing the suitable catalyst for CO2 reduction is the priority in this field. In this work, copper oxide (CuOx) was used as our catalyst due to its high production of ethanol at low potential. Along with the morphology of the cage, the active surface area of CO2 reduction increases dramatically. Then, we came up with an electrochemical redox shuttle method to stabilize the oxidation state of our CuOx material. By using this method, high conversion of CO2 into ethanol at low potential was achieved.
With gas chromatography–mass spectrometry analysis, ethanol is the major product in liquid phase, whereas hydrogen is the byproduct. This study shows that hydrogen evolution decreases dramatically when applying the electrochemical redox shuttle method instead of the constant potential method. In addition, the composition of CuOx material was confirmed by in situ XANES and in situ X–ray diffraction measurement. This information proves that the electrochemical redox shuttle method not only stabilizes the catalyst not to form pure copper, but also improves the selectivity of ethanol. Thus, we predict that oxygen on catalyst plays an important role on CO2 reduction, which allows ethanol formation at low potential.
In conclusion, we provided a novel method to prevent the material itself from being reduced and thus maintain its performance, which could lead to potential applications of other oxidative catalysts.

並列關鍵字

CO2 reduction ； selectivity ； copper oxide ； electrochemical redox shuttle

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