In Situ FTIR Investigation Of The Adsorption And Electrochemical Oxidation Kinetics Of Small Organic Molecules On Pt Electrodes

Proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC) are typical fuel cells working at relatively low temperature (<100℃). They are suitable power sources for portable devices, electric vehicle and distributed power station with the advantages of low working temperature, rapid start-up and zero (or low) emission. One of the key issues for such low temperature fuel cells is to keep the electrocatalysts with high activity and long lifetime, which is the prerequisite for commercialization schedule of fuel cells. The state of the art anode and cathode electrocatalysts used in these fuel cells are generally Pt-based noble metal catalysts. However, large scale utilization of the Pt-based catalyst is limited due to the carbon monoxide (CO) poisoning problem, the shortage of Pt resource and high cost. Thus, one of the important tasks in electrocatalysis is to understand the CO poisoning mechanism at Pt electrode and improve the CO-tolerance of the fuel cell catalysts.In-situ infrared (IR) spectroscopy is one of the powerful tools for in-situ monitoring electrochemical reactions taking place at the electrode/liquid interfaces, which can provide information about the nature and orientation of the adsorbates. It is widely used to study the mechanism of electrode reactions on a molecular level because of its high sensitivity and simple selection rules.The present work mainly focuses on the desorption and oxidation of CO at platinum electrodes. Thermostatic electrochemical cells which combined the conventional electrochemical methods and in-situ IR spectroscopic techniques were used to systematically investigate the competitive adsorption of COad with heterogeneous molecules, the effects of temperature and electrode potential upon the desorption and oxidation of adsorbed CO (COad) on Pt electrode surfaces. First, CN-was introduced to displace the adsorbed carbon monoxide. The mechanism of displacement was discussed carefully. Second, the thermal desorption of adsorbed carbon monoxide and its kinetic parameters were studied. At last, the oxidative stripping of carbon monoxide with various coverages at elevated temperatures was examined systematically. In addition, the electrochemical oxidation of methanol on platinum at different temperatures was studied.The major results of this thesis are summarized below: 1. COad displacement by CN-at Pt electrodesThe mechanism and kinetics of CN-assisted CO desorption from Pt electrode was investigated systematically. By analyzing the relationship of IR peak frequency and intensities of bands from both COad and CN-stretching vibration recorded during the exchange process, the CN-assisted COad desorption can be divided into three distinct regions with a volcano shape. A slowignition period appears within the first tens of seconds, which is followed with a fast desorption period and a slow decay after reaching the maximum. Furthermore, the desorption energy of COad from Pt electrode was estimated between 19 and 38 kJ/mol using the temperature dependent results according to the Arrhenius equation, which is much lower than that of COad desorbs in vacuum (129-183 kJ/mol).The inverse linear relationship between the band intensities of COad and CN-stretching modes indicates that the energy released during the adsorption plays an important role in promoting of COad desorption in addition to the repulsive interaction between COad.The energy released during adsorption may transfer to the nearest neighbor CO molecules through the substrate, which facilitates the Pt-CO bond breaking.Furthermore, a mechanism similar to the SN2 mechanism in the bimolecular nucleophilic substitution is suggested to operate during the displacement of COad. CN-may adsorb around the COad, may be even on the same platinum atom. The repulsion force between CN- and COad molecules could weaken the adsorption bond of COad upon Pt. Meanwhile, the energy released during CN-adsorption transferred through the surface and break the bond between COad and platinum atoms, as a result the COad desorbs.2. Investigation of the thermal desorption of COad at elevated temperatureThe low temperature fuel cells usually work under 100℃, thus the thermal desorption kinetics of COad in this circumstance are of great significance for evaluating its contribution to CO tolerance. A flow cell in combination with in-situ IR spectroscopy has been employed to study the thermal desorption of COad with initial different coverages in the temperature region between 10 and 80℃upon Pt electrode. With initial COad coverage below 0.39 ML, COad desorption is not appreciable with temperature up to 80℃. When the initial coverage is higher than 0.51 ML, COad desorption is observed above the room temperature. We found that the thermal desorption process of COad is not just simply the reverse process of adsorption by a comparison of the IR spectroscopy of adsorption and desorption. The activation energy for COad desorption from polycrystalline Pt electrode was estimated to be ca.96-113 kJ/mol using the temperature dependent coverage curve according to the first-order Polanyi-Wigner equation, which decreases with the increase of CO coverage followed by a sharper decline near saturation coverage. However, COad coverage is almost constant between 0.1 and 0.35 V until 80℃in the solution saturated with CO. This was attributed to a rapid exchange of CO molecules between the surface and the solution. The vacancy created by COad desorption can be occupied by the incident CO molecules from the solution quickly.3. Electrochemical oxidation of COad at elevated temperatureThe electrooxidation of CO on Pt electrode at elevated temperature from various initial COad coverages was studied systematically. When starting with saturated CO adlayer, electrochemical results reveal that the onset as well as the peak potential for CO oxidation shifted negatively with temperature rise. From the linear dependence of the peak potential with the temperature at all COad coverages allows us to determine the apparent activation energy of the COad oxidation which is found to be 127±10 kJ/mol. This value is slightly higher than that obtained on polycrystalline Pt electrode (117 kJ/mol), but lower than that on the Pt(111) electrode surface (132 kJ/mol). The consistency of the activation energy under all the coverages demonstrates that the oxidation mechanism is independent of the CO coverage on polycrystalline Pt electrode.IR results reveal that the peak frequency of atop COad under saturation coverage shifted to lower wavenumber when the potential was made more positive at elevated temperatures. This was ascribed to the random desorption of COad during the oxidation process at elevated temperatures. In addition, the diffusion rate of COad on Pt electrode surface was also accelerated under elevated temperatures, making the formation of larger island of COad unfeasible. The red-shift of the IR peak frequency of atop COad was not observed at lower CO coverages at all the temperatures examined, which was attributed to the fact that CO is known to be predominantly adsorbed on defects at low coverages and the binding energy is too high to be overcome. The oxidation of methanol at different temperatures upon Pt electrode is of crucial importance in developing new electrocatalysts with excellent activity for direct methanol fuel cell. The thermostatic flow cell in combination with in-situ FTIR spectroscopy was employed to explore the oxidation mechanism of methanol molecules under different potentials and temperatures. As can be seen from the IR spectroscopy, CO is the only stable intermediate under 0.6 V. From the fitted results to the coverage curve, the dehydrogen and CO oxidation current take only a small fraction of the total faraday current at steady state, about 20% at 0.6 V.