Potential-Dependent\nand Face Orientation-Dependent\nElectrochemical Oxidative Desorption Behavior of Sulfur Species Adsorbed\non Platinum Single-Crystal Surfaces

We investigated the effect of surface atomic arrangements\nof electrodes\non electrochemical oxidative desorption behavior of sulfur species\nat Pt single-crystal electrodes with face orientations of (111), (110),\nand (100) by electrochemical measurements, X-ray photoelectron spectroscopy\n(XPS), and density functional theory (DFT) calculations. Upon the\nadsorption of elemental sulfur, electrochemical responses characteristic\nto Pt(111), Pt(110), and Pt(100) electrodes in aqueous electrolytes\nsuch as adsorption/desorption of hydrogen and hydroxyl species completely\ndisappeared, and S 2p peaks attributed to the adsorbed sulfur appeared\nin XPS at each electrode. Those surface-structure-dependent electrochemical\nresponses gradually recovered, simultaneously with the decrease of\nS 2p peaks, by cycling to or holding at positive potentials due to\nthe oxidative desorption of adsorbed sulfur. The recovery of the electrochemically\nactive surface area (ECSA) was promoted by keeping the potential more\npositive for a longer period. Among the three different face orientations,\nthe oxidative desorption of sulfur started from the least positive\npotential at the Pt(111) electrode in both experiments, showing that\nthe atomic arrangement of the Pt(111) electrode is most advantageous\nfor the recovery of ECSA from sulfur poisoning. In the potential holding\nexperiment, the oxidative desorption of sulfur occurred at less positive\npotential at the Pt(111), Pt(100), and Pt(110) electrodes in that\norder. One of the mechanistic reasons is explained with the DFT calculations,\nwhich evidenced that the adsorption energies of SO<sub>2</sub> at\nthe Pt(111), Pt(100), and Pt(110) electrodes are in the same order.\nThis correlation suggests that the desorption of SO<sub>2</sub> formed\nby the oxidation of the adsorbed sulfur is an important step. In the\npotential cycling experiment, however, the oxidative desorption of\nsulfur more easily occurred at the Pt(111), Pt(110), and Pt(100) electrodes\nin that order. Once the adsorbed sulfur is oxidized to SO<sub>2</sub>, SO<sub>2</sub> desorbs from the surface or remains at the surface\nto be subsequently reduced to elemental sulfur in the negative potential\nscan. Since the reduction of SO<sub>2</sub> to elemental sulfur more\neasily occurs at Pt(100) than at the other two electrodes, the recovery\nof ECSA at the Pt(100) electrode became slower in the potential cycling\nexperiment. Thus, the fate of SO<sub>2</sub> formed by the oxidation\nof sulfur is one of the important factors affecting the recovery rate\nof ECSA from sulfur poisoning.