dc.description.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 (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. | en_US |