二嗪(C4H4N2)異構物在金(111)之吸附及其對甲酸催化的影響

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題名:	二嗪(C4H4N2)異構物在金(111)之吸附及其對甲酸催化的影響
作者:	許育豪;Hsu, Yu-Hao
貢獻者:	化學學系
關鍵詞:	金(111);循環伏安法;掃描隧道式顯微鏡;二氮雜苯異構物;自主裝;甲酸催化;STM;CV;Pyridazine
日期:	2024-07-19
上傳時間:	2024-10-09 15:22:54 (UTC+8)
出版者:	國立中央大學
摘要:	本研究探討了二氮雜苯異構物Pyridazine(PD)、Pyrimidine(PM)與Pyrazine(PZ)在金(111)電極上的吸附行為，以及對甲酸催化影響。研究結果表明，PD分子在金電極上的吸附是一個模型系統，用於了解有機物在金屬電極上的結構。考慮到 PD 的 pKa 值為 2.2，5 PD分子可能具有質子化和非質子化形式，其在 pH 1 和 pH 3 的硫酸鹽和過氯酸鹽介質中的吸附進行了檢測。此外，PD 具有 4.22 D 8的大偶極矩，可以通過施加的電位在金電極上不同地排列。在正電位下，陰離子和 PD 的吸附在金電極上是耦合的。分子解析STM成像能夠直接觀察 PD 分子隨著電位、pH 和陰離子的變化在有序的 Au(111) 電極上的吸附情況。在 pH 1 的介質中，在較小的正電位下吸附的 PD 可能是質子化的 (PDH+)，通過分子間的氫鍵形成延長的分子鏈。隨著電位移向正值，PD 分子鏈解體成為單獨的分子。在轉變為有序排列和垂直取向之前，PD 在水平或傾斜的取向中以混亂狀態吸附在 0.7 V (相對於 Ag/AgCl) 的電極上。在含有 1 mM PD 的 0.1 M H2SO4 中獲得的伏安圖中，這一轉變以一個明顯的電流尖峰顯示出來。PD 吸附分子之間的分子間相互作用決定了它們在金電極上的方位角取向和空間排列。在 H2SO4 和 HClO4 中，隨著正電位的增加，PD 覆蓋率增加，直到在 1 V 處，陰離子取代了 PD 吸附分子。這一發現表明，除了吸附強度外，在正電位下濃度高出百倍的陰離子比 PD 更為重要。 PM分子在金(111)電極上的吸附行為顯示，超過0.5V後開始吸附，但未觀察到明顯的特徵峰，這可能是PM分子吸附結構的無序性所導致，在負向掃描過程中，0.2V時出現還原峰，表明PM分子從金表面脫附，並確認了PM分子的吸附和脫附特徵峰。 Nernst方程的實驗結果顯示電位負移約120mV，證實了此反應涉及單一電子轉移過。 PZ分子在金(111)電極上的吸附行為顯示，在正電位下CV曲線與背景電流相似，表明其僅微量吸附。在負電位下，PZ分子顯示出顯著的電流峰（A1/C1），根據計算，這對電流峰為擴散波，這意味著其反應主要發生在溶液中。pH值升至3時，特徵峰在更負的電位出現，實驗結果顯示220mV的差異，表明有其他因素影響反應，導致實際測得的電位差異大於Nernst方程預測的理論值。在甲酸催化方面，PD、PM和PZ三種分子在pH1環境下，PZ擁有最強的催化效率，而pH3環境下的催化效率排序為PD > PM > PZ。在pH3環境中三種分子皆已未質子化狀態占多數，而PD分子與金(111)較強的吸附能力，使其表現出最高的催化效率。PM分子的催化效率居中，主要是由於其適中的吸附強度和活性位點。PZ分子受質子化影響最小，因此改變pH值對其影響不顯著。這說明質子化為影響催化效率的關鍵因素，這對理解和優化電催化反應中的分子設計和反應條件提供了重要的見解。 ;This study investigates the adsorption behavior of pyridazine (PD), pyrimidine (PM), and pyrazine (PZ) isomers on a gold (111) electrode, as well as their influence on formic acid catalysis. The results indicate that the adsorption of PD molecules on a gold electrode serves as a model system for understanding the structure of organics on metal electrodes. Considering the pKa value of PD is 2.2, PD can exist in both protonated and non-protonated forms, and its adsorption was examined in sulfate and perchlorate media at pH 1 and pH 3. Additionally, with a large dipole moment of 4.22 D, PD can arrange differently on the gold electrode depending on the applied potential. At positive potentials, the adsorption of anions and PD on the gold electrode is coupled. Molecular-resolution STM imaging allows direct observation of the adsorption of PD molecules on an ordered Au(111) electrode as the potential, pH, and anions vary. In pH 1 media, PD adsorbed at less positive potentials is likely protonated (PDH+), forming extended molecular chains through intermolecular hydrogen bonds. As the potential shifts to positive values, PD chains dissociate into individual molecules. Before transitioning to an ordered arrangement and vertical orientation, PD adsorbs in a disordered state at a horizontal or tilted orientation on the electrode at 0.7 V (vs. Ag/AgCl). This transition is evident as a distinct current peak in the voltammogram obtained in 0.1 M H2SO4 with 1 mM PD. The intermolecular interactions among adsorbed PD molecules determine their azimuthal orientation and spatial arrangement on the gold electrode. In both H2SO4 and HClO4, PD coverage increases with positive potential until anions replace PD adsorbed molecules on the gold electrode at 1 V. This finding suggests that at positive potentials, the much more concentrated anions become more critical than PD, besides the adsorption strength. The adsorption behavior of PM on the gold (111) electrode shows that adsorption begins after 0.5 V, but no distinct characteristic peaks are observed, possibly due to the disordered structure of adsorbed PM molecules. During the negative scan, a reduction peak appears at 0.2 V, indicating PM molecules desorbing from the gold surface and confirming the adsorption and desorption characteristic peaks of PM. Experimental results with the Nernst equation show a negative shift of about 120 mV, confirming that this reaction involves a single electron transfer process. The adsorption behavior of PZ on the gold (111) electrode shows that at positive potentials, the CV curve is similar to the background current, indicating only trace adsorption. At negative potentials, PZ shows significant current peaks (A1/C1), which are identified as diffusion waves, suggesting that the reaction primarily occurs in the solution. When the pH is raised to 3, characteristic peaks appear at more negative potentials, with experimental results showing a difference of 220 mV, indicating that other factors influence the reaction, causing the actual measured potential difference to be greater than the theoretical value predicted by the Nernst equation. Regarding formic acid catalysis, among the three molecules PD, PM, and PZ in a pH 1 environment, PZ shows the highest catalytic efficiency, while in a pH 3 environment, the catalytic efficiency order is PD > PM > PZ. In a pH 3 environment, all three molecules predominantly exist in the non-protonated state. PD exhibits the highest catalytic efficiency due to its stronger adsorption ability on Au(111). The catalytic efficiency of PM is intermediate, mainly due to its moderate adsorption strength and active sites. PZ is least affected by protonation, so changing the pH value does not significantly impact its catalytic efficiency. This indicates that protonation is a key factor affecting catalytic efficiency, providing important insights for understanding and optimizing molecular design and reaction conditions in electrocatalytic reactions.
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