Kinetics of Initial Stages of Pt Oxidation from Electrochemistry and Surface X-Ray Diffraction
Authors: Stubb Natalie , Fuchs Timo , Ruge Martin , Magnussen Olaf M. , Drnec Jakub , Harrington David A. 
Published: February 2020

The kinetics of the electroxidation of Pt to Pt "oxide" have been studied for many years, initially on polycrystalline Pt. Early studies showed that the oxide charge increases linearly with log(time) at constant potential, the so-called direct logarithmic growth law. Later, Harrington showed empirically that the underlying differential equation, with a Tafel potential dependence of the rate constant, predicted also the current plateau behavior in cyclic voltammetry [1]. The Tafel dependence is suggestive of electron-transfer kinetics and was also found in the oxidation of Pt(111) [2]. Despite this, the classical Conway model [3] invokes field-driven dipole flipping, and other literature models invoke bulk film models that cannot be applicable for a few-monolayer-thick oxide. An early atomistic model stressed the role of surface diffusion in leading to irreversibility and predicted a three-layer surface restructuring and the direct logarithmic law, but with the wrong slope [4].

Our recent Surface X-ray Diffraction studies [5] have shown that the number of place-exchanged Pt atoms also follows the direct logarithmic law. We here extend this result to an extended data set that systematically steps into the oxide peak in the region 1-1.2 V vs RHE, which is the peak in which the place-exchange initiates. The intensity of the (1,1,1.5) reflection shows that the place-exchange process initiates in the early stages of this peak, earlier than the peak maximum at 1.05 V where it sharply changes in a 50 mV/s intensity voltammogram. There is a systematic change in the slope of the direct logarithmic law over the range of the peak, with the largest slope at the peak maximum.

The observation that both the current and the number of place-exchanged Pt atoms obey the direct logarithmic law suggests a common limiting process, though in principle the place-exchange and electron-transfer processes need not be coupled. The oxidation of adsorbed OH to O has also been suggested to occur in the same peak and there is some evidence for two processes. A careful comparison of the current density with the coverage of place-exchanged atoms is undertaken here to deduce the electron/place-exchange ratio and indirectly the nature of the accompanying species (O or OH), as well as determining how tightly coupled the exchange and oxidation processes are. The issue of double-layer charging correction is a significant issue in this determination.

The place exchange is the initial step of the irreversible nanoscale restructuring of the surface that occurs on potential cycling. The later stages have recently been quantified using Grazing-Incidence Small-Angle X-ray Scattering [6-7]. A careful study of the kinetics is directed toward a detailed atomistic understanding of the evolution of the surface structure.

The authors thank the European Synchrotron Radiation Facility, Deutsche Forschungsgemeinschaft, and the Natural Sciences and Engineering Research Council of Canada for support.

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