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| 題名: | 氫氧化亞鐵吸附於鉑(111)電極上的結構及電催化活性 |
| 作者: | 黃詩詠;Huang, Shih-Yung |
| 貢獻者: | 化學學系 |
| 關鍵詞: | 電化學;鐵金屬;一氧化碳氧化;氧氣還原反應;燃料電池 |
| 日期: | 2023-07-26 |
| 上傳時間: | 2024-09-19 14:45:57 (UTC+8) |
| 出版者: | 國立中央大學 |
| 摘要: | 本研究利用循環伏安法 (Cyclic Voltammetry，CV) 和掃描式穿隧電子顯微鏡 (Scanning Tunneling Microscope，STM) 探討三個主題：氫氧化亞鐵的空間結構、陰離子對氫氧化亞鐵的影響及催化活性。第一部分為在 pH3 硫酸溶液(1 mM H2SO4 + 0.1 M K2SO4) 含有10 mM FeSO4中，Pt(111) 電極上吸附 Fe(OH)2 薄膜的空間結構。本實驗採用傳統的退火及冷淬方法處理 Pt(111) 電極，使其表面生成單層的天然氧化物，從而形成有序的 Fe(OH)2 薄膜。在 0.1 ~ -0.1 V (vs. Ag/AgCl) 可以觀察到獨特的車輪狀 (SW) 結構，與真空中觀察到的六邊形 FeO(111) 雙層結構不同，該車輪狀結構是扭曲的六邊形。將電位正調至 0.4 V，車輪狀結構轉變為有序的顆粒狀形貌，為 (6 39) 的傾斜結構；將電位從 -0.1 V 往負電位移動，表面轉變為無序的莫爾圖案，此過程為不可逆的反應，隨著電位移動表面會先長出第二層 Fe(OH)2 以及第三層的金屬鐵，接著金屬鐵會以三維（Volmer-Weber mode）的方式生長。
第二部分著重於陰離子及硼酸共吸附對 Fe(OH)2 在 Pt(111) 的空間結構，同樣的 STM 實驗也在 pH3 鹽酸溶液 (1 mM HCl + 0.1 M KCl + 10 mM FeCl2)中進行，表面會隨電位出現微三角環 (TL) 及類似車輪狀形貌，皆是一個扭曲的六方型晶格，藉由表面不同形貌可以得知，溶液中的陰離子會和鐵離子共吸附於電極表面上。當溶液中含有硼酸時，可以觀察到硼酸在負電位形成獨特梯形狀形貌，得以證實硼酸和鐵離子共吸附於電極表面上。
第三部分探討了鐵氧化物修飾的 Pt(111) 對一氧化碳氧化(CO)、氧氣還原(ORR)、甲酸氧化(FAO)和氫氣析出反應(HER)的催化活性。CO 氧化反應一直被視為一個重要的模型系統，能夠深入理解異相催化反應。電化學結果顯示，有 Fe(OH)x 修飾的電極其 CO 氧化電位比單純 Pt(111) 更負；掃描式穿隧電子顯微鏡(STM)觀察表面有 Fe(OH)x 修飾的 Pt(111) 在含有飽和 CO 的過氯酸中的電極表面狀態，發現 Fe(OH)x 和 CO 以不同的區域吸附，並競爭 Pt 電極表面的活性位點，可能是以 Fe(OH)x 和 CO 區域之間的交界作為 CO 氧化的活性位點。
Fe(OH)x 修飾的 Pt(111) 對 ORR 和 FAO 也表現出很高的活性。在 0.1 M KOH 中，ORR 的半波電位從 -0.29 V 移動到 -0.19 V，氧氣通過 4e- 過程還原成水；FAO 活性通過峰電流增加三倍來體現。這些優點可能來自於氧氣與 Fe(OH)x 修飾的 Pt 電極之間的氧結合強度增加，然而，Fe(OH)x 修飾對 Pt 電極的還原反應並沒有正面影響，像是 Fe(OH)x/Pt(111) 電極無法在鹼性溶液中促進水的分解。
;The current study employed cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) to probe the redox chemistry and spatial structure of an iron oxide thin film adsorbed on an ordered Pt(111) electrode in pH3 sulfate solution (1 mM H2SO4 + 0.1 M K2SO4) containing 10 mM FeSO4. We used the conventional annealing-and-quenching method to treat the Pt(111) electrode. The subsequently produced sub-monolayer native oxide on this Pt sample enabled the formation of an ordered FeO thin film. In situ STM imaging yielded a unique spoke – wheel (SW) structure between 0.1 and -0.1 V (vs. Ag/AgCl) in pH3 sulfate solution. As opposed to the hexagonal FeO(111) bilayer structure observed on Pt(111) in vacuum, the SW structure was distorted hexagonal. While the potential was shifted to -0.4 V, the spoke-wheel (SW) structure transformed into an ordered spot-like morphology with Pt(111) - (6 39) oblique structure. On the other hand, when the potential was shifted from -0.1 V towards negative values, the surface transformed into a disordered moiré pattern, representing an irreversible reaction. As the potential continued to decrease, a second layer of iron oxide and a third layer of metallic iron gradually formed on the surface. The growth of metallic iron followed a three-dimensional mode known as the Volmer-Weber mode.
In order to investigate the possible coadsorption of anion and borate with Fe2+ on the Pt electrode, we conducted similar STM experiment with a Pt(111) electrode in pH3 chloride medium (1 mM HCl + 0.1 M KCl + 10 mM FeCl2). At 0.1 V, the surface exhibited varying sizes of micro triangular loops (TL) morphology, which was consistent with the results in sulfuric acid, showing a distorted hexagonal lattice. Shifting the potential from 0.1 to 0 V resulted in a transformation of the TL to a SW – like structure. It has the same characteristics as the SW pattern seen in pH3 sulfate medium, except it is apparently less ordered. The different surface morphologies indicated the co-adsorption of anions from the solution with iron ions on the electrode surface. In the presence of borate in the solution, a unique trapezoidal morphology was observed at negative potentials, confirming the co-adsorption of borate and iron ions on the electrode surface.
The catalytic activity of Pt(111) modified with iron oxide toward the oxidation of carbon monoxide (CO), oxygen reduction reaction (ORR), oxidation of formic acid (FAO), and hydrogen evolution reaction (HER) are examined. The CO oxidation reaction has been a model system to gain insight of heterogenous catalytic reaction. Voltammetric results show that the Fe(OH)x modifier causes CO admolecule to be oxidized at more negative potential than that of bare Pt(111). In situ STM is used to probe the state of Fe(OH)x – modified Pt(111) electrode immersed in CO – saturated perchloric acid, revealing Fe(OH)x and CO are adsorbed in segregated domains and compete for surface sites on the Pt electrode. It is likely that the domain boundies between Fe(OH)x and CO domains act as the active sites for CO oxidation.
The Fe(OH)x modified Pt(111) also exhibits higher activity toward ORR and FAO. The half wave potential for ORR shifts from -0.29 to -0.19 V and oxygen molecule is reduced to water via the 4e- process in 0.1 M KOH. The FAO activity is manifested by the increase of peak current by three times. These merits can stem from an increase of oxygen binding strength of oxygen entities to the Fe(OH)x modified Pt electrode. However, the Fe(OH)x modifier does not show a positive effect of Pt electrode on the reduction reaction. For example, Fe(OH)x/Pt(111) electrode cannot promote water splitting in alkaline media. |
| 顯示於類別: | [化學研究所] 博碩士論文
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