Kinetics of Electrochemical Oxidation on Platinum Single-Crystal Surfaces




Yuan, Chentian

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Understanding the structure and kinetics of electrochemical oxidation of platinum single-crystal surfaces at the atomic level is important to understanding and improving the electrocatalysis of platinum in fuel cells. This thesis concerns the analysis of data from combined electrochemical and surface X-ray diffraction (SXRD) experiments on Pt(111) and Pt(100). In the early stages of oxidation, Pt atoms are extracted from their metal lattice sites and become part of a metal oxide. Their locations have been measured by collaborators using SXRD. The corresponding charges measured electrochemically are determined by integration of cyclic voltammetry, potential step and potential sweep-hold experiments, and used to propose reactions that are occurring in the oxidation. The measured charge consists of the wanted charge that passes in the electrochemical reactions and a capacitive charge associated with charging the electrical double-layer. Different ways of subtracting the double-layer charge were investigated. Reactions were proposed for optimally corrected charges, and for the worst case where no correction is made, in order to determine the reliability of the proposed reactions. From cyclic voltammetry experiments on Pt(111) surfaces, during oxidation 0.5 ML adsorbed oxygen and a few extracted PtO (less than 0.1 ML) are formed. Without baseline correction the data are consistent with 0.5 ML Oads and a few extracted PtO2. After fast scans, the Pt(111) surface is restructured as shown by the X-ray signal, but the oxide peaks in CV are nearly unchanged, which suggests that the electron transfer and Pt extraction do not need to be tightly coupled. From CV experiments on Pt(100) surfaces, during oxidation many more extracted Pt are formed than on Pt(111). These form long chains with PtO2 units (0.25 to 0.39 ML) together with Oads and a total coverage of 0.5 ML. If CV isn't baseline corrected, the data is not consistent with long chains with PtO2 units, but short Pt3O8 chains with equal numbers of independent PtO2 groups. From potential step experiments on Pt(100), there is an expectation from the literature that the charge will grow linearly with log t. However, this relationship was only found during the first second of each step, and then is unchanged after ~1 s. The slopes of the logarithmic plots are linearly related to potential. From sweep hold experiments on Pt(100), both charges and coverage of extracted Pt atoms are not very linear vs log t, and their slopes are also not linearly related to potential but change sharply during the potential range of Pt extraction.



Chemistry, Electrochemistry, Platinum, Science, Oxidation