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The role of step defects on an electrode is a central issue in the modern study of electrocatalysis. Although scanning tunneling microscopy (STM) has been used to characterize the electrified interfaces of low Miller-indexed single-crystal electrodes, there has been little progress in the STM study of stepped Pt electrodes. Herein, the structures of Pt(332) and Pt(997), two vicinal surfaces to the (111) plane, are examined by high-resolution STM under potential control in 0.5 M H2SO4 solution containing iodide or carbon monoxide. These electrodes are annealed by a hydrogen flame and quenched in Millipore water, giving rise to rough step edges with poorly defined atomic structures at 0.1 V (vs reversible hydrogen electrode) in 0.5 M H2SO4. However, step edges are sharpened and aligned in the 110 direction after adsorption of CO on both electrodes. In contrast, seesaw-like step lines are produced by iodine adsorption. Therefore, the step structure and mobility of Pt atoms are markedly influenced by the adsorbate. Pt(997) and Pt(332) electrodes with (111) facets that are 8 and 5 Pt atoms wide, respectively, afford (7 × 7)R19.1°-I and (3 × 3)R30°-I structures at 0.1 V. In comparison, the (2 × 2)-3CO structure, as seen on Pt(111), is found on both Pt electrodes. Also, STM results yield linearly bonded and threefold bonded CO molecules at the peaks and troughs of steps on Pt(332) and Pt(997). The origins for the faster CO oxidation rate at steps than that at terraces are discussed.
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