| Abstract: | 隨著工業的進步，二氧化碳排放所造成的問題已經無法再被忽略，如何解決全球暖化及再生能源的研究已經成為當今的熱門議題，而電化學還原二氧化碳反應(carbon dioxide electrochemical reduction reaction, CO2RR)被視為一項關鍵技術，可減少二氧化碳排放經由轉化成高價值化學燃料，然而目前仍舊有許多困難需要克服，像是CO2RR與電解水產氫反應(hydrogen evolution reaction, HER)的競爭關係，CO2RR活性低以及產物選擇性上的控制。目前已經有許多研究致力於研發不同催化材料以改善其選擇性及效能，其中二元金屬奈米觸媒被視為相當有潛力的材料，藉由第二金屬的添加，可以改善其中間產物的化學吸附能，並進一步提升產物的選擇性及活性。
本研究分成兩個部分，第一部分在奈米銅觸媒內添加對於一氧化碳產物具有高選擇性的金屬(例如:金,銀以及鋅)以形成二元奈米觸媒(Cu9Au1/C, Cu9Ag1/C 及 Cu9Zn1/C)，在電化學活性上，低添加量第二金屬進入觸媒表面，可降低一氧化碳與觸媒間化學吸附能，進而提升一氧化碳產物的法拉第效率。其中以Cu9Au1/C的修飾效果最好，在-0.8 V下具有69 %的一氧化碳法拉第效率及最低(-0.5 V)的起始電位。
第二部分則是合成不同比例的碳支撐銅金二元觸媒(Cu9Au1/C, Cu8Au2/C及Cu7Au3/C)，利用高角度環形暗場相掃描電子顯微鏡 (high angle annular dark-field scanning transmission electron microscopy, HAADF-STEM) 及能量散射光譜儀 (energy dispersive spectrometer, EDS) 證實其為銅核/金殼的結構，根據一氧化碳剝離試驗(CO stripping)及X光電子能譜儀 (X-ray photoelectron spectroscopy, XPS)的結果，隨著表面組成中的金增加，對於一氧化碳中間產物的吸附能隨之下降，而在電催化CO2RR的效能表現上，Cu8Au2/C展現最優異的電化學性能，在-0.8 V下達到94 %的法拉第效率及-439 A/gAu-1的一氧化碳質量活性，甚至達到純金奈米觸媒(Au/C)的2.8倍，本研究揭示了藉由形成銅金核殼奈米觸媒可降低貴金屬的添加並且經由電子修飾效應達到高的一氧化碳選擇性與活性。
;With the industrial progress, carbon dioxide emissions cannot be ignored. How to reduce the global warming and develop the renewable energy becomes the hot topic. Electrochemical carbon dioxide reduction reaction (CO2RR) has been seen as the key method to solve the problem, which reduce the CO2 emission and produce value-added chemicals simultaneously. However, the biggest challenges in electrochemical CO2RR are competition between CO2RR and hydrogen evolution reaction (HER), low CO2RR activity and difficult control of products selectivity. Many studies have been dedicated to improve the selectivity of products and enhance the performance for electrochemical CO2RR by different materials. Among them, bimetallic nanocatalysts have attracted a lot of attention, attributed to the binding energy of intermediate changed with the second metal addition, further promoting the selectivity and activity.
This study is divided into two parts. In the first part, the carbon-supported Cu9M1 (M=Au, Ag and Zn) bimetallic catalysts have been prepared. The introduction of metals with high CO selectivity on the surface has demonstrated the good promotion on CO selectivity due to the weakening of the binding energy for intermediates. Cu9Au1/C has shown the highest CO faradaic efficiency (69 % at -0.8 V (vs. RHE)) and the lowest onset potential (-0.5 V (vs. RHE)).
In the second part, CuxAu10-x/C with Cu/Au ratios of 9/1, 8/2, and 7/3 have been prepared. High angle annular dark-field scanning transmission electron microscopy (STEM) and energy dispersive spectrometer (EDS) confirm the formation of Cu core and Au shell structure. According to the CO stripping and X-ray photoelectron spectroscopy (XPS) results, the binding energy of CO is reduced with an increase in the Au content on the surface composition. For the CO2RR, Cu8Au2/C has exhibited the best electrochemical performance both on CO selectivity (CO faradaic efficiency of 94 %) and CO mass activity (-439 A gAu-1) at –0.8 V (vs. RHE) among the various of CuAu/C, which even achieves 2.8 times higher than pure Au/C catalysts. This study reveals that the bimetallic nanocatalysts with core-shell structures like CuAu/C can be used to decrease the noble metals usage and reach high selectivity and activity through electronic modification. |
