| 摘要: | 摘要 利用掃描式電子穿隧顯微鏡(in situ scanning tunneling microscopy , STM)及循環伏安法(cyclic voltammetry , CV)探討鉑(111)和鉑(100)電極在含有甲醇、甲醛和甲酸的過氯酸溶液中的電化學催化反應。 Ⅰ. 甲醇在鉑(111)與鉑(100)電極之研究 甲醇在鉑(111)電極上的吸附力和電位有密切的關係，在較負電位時(E < 0.3 V)，其吸附力弱，因此STM的探針可能會干擾甲醇的吸附，在此環境中，吸附之甲醇分子聚集成島狀物，並未形成規則之結構。在較正電位時，分子之吸附力增強，吸附量也隨之增加，但仍然未形成一規則之結構。 但如將電位固定於0.32 V吸附甲醇於鉑(100)電極上，則STM結果顯示在電極表面上即刻出現一規則排列的結構，其單位晶格為一正方形，邊長為4 Å，且其分子密排方向平行於<011>方向，這些特徵指向一(Ö2 ´ Ö2)R45°結構，覆蓋度(q) = 0.5。 甲醛在鉑(111)與鉑(100)電極之研究 就甲醛在鉑電極上的吸附而言，由於它的吸附力弱，因此在較低濃度時(< 1 mM)，鉑電極表面沒有分子的吸附。然而，伏安圖中甲醛經水合反應所生成的CH2(OH)2 (methylene glycol)與甲醇在兩個鉑電極上的氧化導致在0.4和0.6 V出現兩根特徵峰。當甲醛濃度³ 10 mM時，HCO(formyl)會吸附在鉑電極的表面上。由於甲醛在水溶液內會以CH2(OH)2為主要之物種，因此這些吸附物可能是來自於CH2(OH)2。這現象在鉑(100)電極上特別地顯著，當掃描速率為 10 mV/s時，鉑(100)電極上CH2(OH)2起始氧化電位由0.4 V移動至0.6 V。鉑(100)電極上的CH2(OH)2氧化電流密度大約是鉑(111)的三倍，這結果指出鉑(100)電極對於甲醛的氧化有較高的電催化活性。當電位在0.1 ~ 0.3 V區間時，高解像的STM原子解析圖顯示在鉑(111)和鉑(100)電極上均有高規則度的結構，分別為(Ö7 ´ Ö7)R19.1°和 (Ö2 ´ Ö2)。負電位時(-0.1 V)氫原子吸附的量會佔優勢，進而取代電極面上的兩個規則排列結構。藉由STM可探討電位對這些吸附晶格的影響，觀察發現在低電位時台階邊緣顯現和平台相比有較高反應活性，接著在更正電位時會導致一全面性的氧化反應，這些改變是可逆的並且和電位息息相關，如在較負電位(0.1 V)時規則的排列結構又會出現。 甲酸在鉑(111)與鉑(100)電極之研究 同樣的，甲酸在鉑(111)上的吸附和鉑電極之電位及電解液的組成關係密切，如電解液為0.1 M過氯酸，於0.1 ~ 0.3 V之間，甲酸分子僅局部性的吸附於電極表面上，由於高解像STM顯示甲酸分子為成對的亮點，因此推測其在鉑(111)電極表面上的吸附方式可能經由其酸基上的兩個氧原子，甲酸分子所形成之規則結構為(2 ´ 2)。 隨著掃描速率的增加，在0.3至0.7 V之間不論掃描方向是正或是負，氧化電流都會明顯上升。然而，在0.2 ~ 0.35 V之間氫原子氧化還原的特色，和掃描速率有關，此特徵之電流密度會隨著掃描速率的增加而變大，這現象可能反映出甲酸在鉑(100)電極上的吸附較氫原子慢。 Ⅱ. 鉛在鉑(111)電極上的吸附 鉛原子以成核成長機制吸附，隨著時間的增加，核種逐漸沿平面以二維方式擴大，且平台邊緣也開始發生鉛原子的沈積，十分鐘後，區域邊界、台階線及亮紋明顯增高。小範圍STM圖像發現鉛原子規則排列在鉑(111)電極表面上，並且形成三種不同的domain，量測其亮點間距約為5.5和4.7 Å，此結構為(2 ´ Ö3)。 Ⅲ. 一氧化碳在鉑(111)電極上的吸附 當鉑(111)電極在含有飽和一氧化碳的過氯酸溶液中，藉由STM來觀察鉑電極的穩定性與鉑原子的移動性。電位於0.1 V時，鉑(111)電極表面吸附一(2 ´ 2)之結構，覆蓋度為0.75。藉由一系列隨時間變化所取得之STM圖像發現，一氧化碳的吸附造成靠近台階邊緣的鉑原子遷移至較高穩定的位置，例如位於配位數較高的平台上。STM結果顯現出台階邊緣，不論其確切的原子結構為何，都會變成鋸齒狀，且鉑原子會聚集成單原子高的島狀物。這現象可能是一氧化碳在鉑(111)的吸附大大地減弱鉑原子的鍵結能量或是增加靠近台階邊緣鉑原子的移動性。 Ⅳ. 鉑(111)電極的氧化 將鉑電極的電位操控至1.6 V使其氧化來觀察鉑電極表面的重構現象。實驗中掃描電位區間由0 至1.6 V，之後將電位定於0.1 V，STM觀察發現鉑電極平台和台階結構並沒有太大的改變，但電極表面出現高密度的島狀物和凹洞。高解析的STM圖像發現在鉑(111)電極的平台和島狀物上有規則的原子結構，顯示鉑表面原子在正電位氧化時，造成電極平台上的鉑原子移位。因此藉由目前所得到的STM結果清楚的說明:當E > 1.6 V 時，受電場的影響和氧原子形成位置的互換，並且隨著掃描電位區間由0 V至1.6 V圈數的增加，島狀物和凹洞的數量也會隨之增加且變得更密集。 Abstract This thesis is divided into four parts. First, the adsorption of formaldehyde (HCHO)、methanol(CH3OH) and formic acid (HCOOH) on Pt(111) and Pt(100) electrode surfaces was examined with cyclic voltammetry and in situ scanning tunneling microscope (STM) in 0.1 M HClO4. Ⅰ. Methanol on Pt(111) and Pt(100) The adsorption of methanol on Pt(111) electrode is so weak that experimental parameters such as supporting electrolyte and potential strongly affect the coverage and structures of methanol ad-molecules. For example, the coverage of methanol was less than one tenth of a monolayer within the potential region of 0.1 and 0.3 V in 0.1 M HClO4. Methanol ad-molecules were adsorbed randomly, producing island-like aggregations. The coverage of methanol indeed increased with more positive potentials, but no ordered structure was identified by high-resolution STM imaging. In contrast, STM molecular resolution reveals the formation of a highly ordered adlattice of (Ö2 ´ Ö2)R45° at 0.32 V in 0.1 M HClO4 upon the addition of methanol into the STM cell. This square lattice contains equally bright protrusions separated by a nearest neighbor spacing of 4 Å. These protrusions are likely to be methoxy (CH3) produced from dehydrogenation of methanol molecules upon their adsorption on Pt(100). This ordered array was gradually eliminated upon stepping potential positively to 0.5 V. Meanwhile, high resolution STM imaging shows the appearance of Pt(100) substrate lattice, suggesting that all methoxy species were completely oxidized to CO2. Formaldehyde on Pt(111) and Pt(100) In dilute (1 mM) HCHO, no adsorption was noted at both Pt electrodes in 0.1 M HClO4. Electroxidation of the hydrated formaldehyde, methylene glycol, and methanol produced peaks near 0.4 and 0.6 V in the voltammograms for both electrodes. Formyl like ad-species were adsorbed on both electrodes when [HCHO] ³ 10 mM. These adsorbates caused some delays in the electroxidation of methylene glycol, the predominant molecular form in aqueous formaldehyde solutions. This phenomenon is particularly pronounced for Pt(100), where the onset of oxidation shifted from 0.4 to 0.6 V for Pt(100) at a scan rate of 10 mV/s. The peak current due to electroxidation of methylene glycol on Pt(100) was nearly three times higher than that of Pt(111), indicating that the former was a more efficient catalyst for this reaction. High-quality in situ STM molecular resolution revealed highly ordered structures, identified as (Ö7 ´ Ö7)R19.1° and c(2 ´ 2), on Pt(111) and Pt(100), respectively, in the potential region between 0.1 and 0.3 V. The adsorption of hydrogen adatoms predominated to displace these two ordered arrays at negative potentials. The effect of potential on the adlayer was imaged by in situ STM, revealing high activity at step defects at low potential polarization, but a more universal reaction scheme at high polarization. These changes were reversible with respect to potential, i.e. ordered structures emerged again at more negative potentials. Formic acid on Pt(111) and Pt(100) The adsorption of formic acid on Pt(111) electrode surfaces was only partial in 0.1 M HClO4 , as revealed by the formation of islands on terraces. High resolution STM imaging reveals the each molecule appeared as a pair of bright spots, suggesting formic acid molecules were adsorbed via its two oxygen in the carboxylic acid group. The ordered structure is characterized as (2 ´ 2) with an intermolecular spacing of 5.6. The effect of scan rate on the morphology of the i-E profile was examined to elucidate the kinetics HCOOH electroxidation. Both positive and negative scans produce pronounced anodic current at potentials between 0.05 and 0.9 V. However, increasing scan rates from 50 to 500 mV/s produced marked differences between the profiles between 0.2 and 0.35 V, where protons discharge. Since the typical hydrogen features is observed at a 500 mV/s scan rate but not at 50 mV/s scan rate, it seems that the adsorption of formic acid was slower than that of hydrogen atoms. Ⅱ. Pb electrodeposition on Pt(111) Underpotential deposition of Pb adatoms results in patches of ordered structures, identified as (2´Ö3), on Pt(111) electrode. Deposition of Pb adatoms preferentially occurs at step edges, followed by lateral expansion of nucleation seeds as more Pb adatoms were deposited. However, the structure of Pb adatoms remained unchanged with deposition of Pb. Ⅲ. The adsorption of carbon monoxide on Pt(111) The goal of conducting in situ STM imaging of carbon monoxide on Pt(111) was to examine the stability of Pt electrodes and mobility of Pt atoms in CO-saturated perchloric acid. The potential of Pt(111) was set at 0.1 V, at which an ordered structure, characterized as (2 ´ 2), q = 0.75 ML, was imaged. Time-dependent STM images reveal that the adsorption of CO molecules yielded relocations of Pt atoms from near step ledges to terraces. STM shows that nearly all step ledges, irrespective of their orientation, became greatly zigzag, along with aggregation of Pt atoms into monoatomic high islands. It seems that the adsorption of CO molecules substantially reduced the binding energy, or greatly increased the mobility of Pt atoms located at step ledges. Ⅳ. The electroxidation of Pt(111) In situ STM was used to examine the restructuring of Pt surface induced by anodic oxidation at potentials positive of 1.6 V. This experiment was performed by conducting potential sweeping between 0 and 1.6 V. Topographic STM scans reveal terrace and step structures seen initially at Pt(111) electrode was nearly unchanged, but a high density of pits and islands were produced by the potential sweeping process. High resolution STM imaging was possible to discern an ordered Pt(111) atomic arrays on not only on terraces, but also on islands. It appears that anodic oxidation of Pt electrode caused displacement of Pt atoms from terraces, rather than steps. The present STM results clearly illustrate that the electric field at E > 1.6 V was strong enough to induce place-exchange between Pt and oxygen atoms. The numbers of islands and pits on terraces increased sharply with the numbers of potential cycling between 0 and 1.6 V. |
