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Student Number 91223005
Author Chen-Fu Mai(麥真富)
Author's Email Address s1223005@cc.ncu.edu.tw
Statistics This thesis had been viewed 2417 times. Download 1690 times.
Department Chemistry
Year 2003
Semester 2
Degree Master
Type of Document Master's Thesis
Language zh-TW.Big5 Chinese
Title On the Adsorption of Formaldehyde at
Pt(111) and Pt(100) Electrodes: as Probed with Voltammetry and Scanning Tunneling Microscopy
Date of Defense 2004-06-17
Page Count 104
Keyword
  • Pt
  • STM
  • Abstract 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 (࡞ ´ ࡞)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 (ࡣ ´ ࡣ)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´࡟), 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.
    Table of Content 目錄
    中文摘要………………………………………………………………..Ⅰ
    英文摘要………………………………………………………………..Ⅴ
    目錄……………………………………………………………………..Ⅹ
    圖、表目錄……………………………………………………………ⅩⅣ
    第一章 緒論1
    1-1前言1
    1-2 燃料電池的簡介1
    1-3 鉑金屬的介紹3
    1-3-1 鉑(100)的探討4
    1-3-2 鉑(100)的重排現象4
    1-3-3 鉑金屬在燃料電池上的應用8
    1-3-4 Pt/Ru金屬在燃料電池上的應用9
    1-4 一氧化碳在鉑(111)電極上的吸附及反應10
    1-5 有機物在電極上反應的簡介11
    1-6 甲醇在燃料電池上的應用13
    1-6-1 甲醇在鉑電極上的吸附及反應機制13
    1-6-2 甲醇在Pt/Ru合金電極上的吸附及反應機制14
    1-7 甲醛的介紹16
    1-7-1 甲醛在鉑電極上的吸附及反應機制17
    1-8 甲酸的介紹17
    1-8-1 甲酸在鉑電極上的吸附及反應機制18
    1-9 電化學界面上 Surfactant 的效應18
    1-9-1 鉛於金屬電極上的應用19
    第二章 實驗部分21
    2-1 藥品部分21
    2-2 氣體部分21
    2-3 金屬部分21
    2-4 儀器設備22
    2-5 實驗步驟23
    第三章 結果與討論26
    3-1 甲醇在鉑(111)電極之研究26
    3-1-1 鉑(111)電極在0.1 M過氯酸溶液中的循環伏安圖26
    3-1-2 鉑(111)電極在含有甲醇的過氯酸溶液中的CV圖26
    3-1-3 鉑(111)電極在含有甲醇的過氯酸溶液中之STM圖像27
    3-2 甲醇在鉑(100)電極之研究34
    3-2-1 鉑(100)電極在0.1 M過氯酸溶液中的CV圖34
    3-2-2 鉑(100)電極在含有甲醇的過氯酸溶液中之CV圖34
    3-2-3 鉑(100)電極在0.1 M過氯酸溶液中的STM圖像35
    3-2-4 鉑(100)電極在含有甲醇的過氯酸溶液中的STM圖像36
    3-3 甲醛在鉑(111)電極之研究41
    3-3-1 鉑(111)電極在含有甲醛的過氯酸溶液中的CV圖41
    3-3-2 鉑(111)電極在含有甲醛的過氯酸溶液中的STM圖像43
    3-3-3 改變電位對甲醛在鉑(111)電極吸附的影響44
    3-4 甲醛在鉑(100)電極之研究47
    3-4-1 鉑(100)電極在含有甲醛的過氯酸溶液中的CV圖47
    3-4-2 鉑(100)電極在含有甲醛的過氯酸溶液中的STM圖像48
    3-4-3 改變電位對甲醛在鉑(100)電極吸附的影響49
    3-4-4 甲醛在鉑(111)和鉑(100)電極之比較50
    討論:51
    3-5 甲酸在鉑(111)電極之研究64
    3-5-1 鉑(111)電極在含有甲酸的過氯酸溶液中的CV圖64
    3-5-2 鉑(111)電極在含有甲酸的過氯酸溶液中的STM圖像64
    3-6 甲酸在鉑(100)電極之研究73
    3-6-1 鉑(100)電極在含有甲酸的過氯酸溶液中的CV圖73
    3-7 鉛在鉑(111)電極上吸附之研究76
    3-7-1 循環伏安圖76
    3-7-2 鉛在鉑(111)電極上吸附之STM圖76
    3-8 一氧化碳吸附在鉑(111)電極之研究84
    3-8-1 一氧化碳吸附在鉑(111)電極的循環伏安圖84
    3-8-2 一氧化碳吸附在鉑(111)電極之STM圖像84
    3-9 鉑(111)電極的氧化研究90
    3-9-1 鉑(111)電極在0.1 M過氯酸之CV圖90
    3-9-2 鉑(111)電極在0.1 M過氯酸之STM圖91
    第四章 結論96
    4-1 甲醇在鉑(111)與鉑(100)電極之研究96
    4-2 甲醛在鉑(111)和鉑(100)電極之研究96
    4-3 甲酸在鉑(111)和鉑(100)電極之研究97
    4-4 鉛吸附在鉑(111)電極之研究98
    4-5 一氧化碳吸附在鉑(111)電極之研究98
    4-6 鉑(111)電極氧化之研究98
    第五章 參考文獻100
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    Advisor
  • Shueh-Lin Yau(姚學麟)
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    Date of Submission 2004-06-29

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