Potential-Dependent and Face Orientation-Dependent Electrochemical Oxidative Desorption Behavior of Sulfur Species Adsorbed on Platinum Single-Crystal Surfaces

We investigated the effect of surface atomic arrangements of electrodes on electrochemical oxidative desorption behavior of sulfur species at Pt single-crystal electrodes with face orientations of (111), (110), and (100) by electrochemical measurements, X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. Upon the adsorption of elemental sulfur, electrochemical responses characteristic to Pt(111), Pt(110), and Pt(100) electrodes in aqueous electrolytes such as adsorption/desorption of hydrogen and hydroxyl species completely disappeared, and S 2p peaks attributed to the adsorbed sulfur appeared in XPS at each electrode. Those surface-structure-dependent electrochemical responses gradually recovered, simultaneously with the decrease of S 2p peaks, by cycling to or holding at positive potentials due to the oxidative desorption of adsorbed sulfur. The recovery of the electrochemically active surface area (ECSA) was promoted by keeping the potential more positive for a longer period. Among the three different face orientations, the oxidative desorption of sulfur started from the least positive potential at the Pt(111) electrode in both experiments, showing that the atomic arrangement of the Pt(111) electrode is most advantageous for the recovery of ECSA from sulfur poisoning. In the potential holding experiment, the oxidative desorption of sulfur occurred at less positive potential at the Pt(111), Pt(100), and Pt(110) electrodes in that order. One of the mechanistic reasons is explained with the DFT calculations, which evidenced that the adsorption energies of SO2 at the Pt(111), Pt(100), and Pt(110) electrodes are in the same order. This correlation suggests that the desorption of SO2 formed by the oxidation of the adsorbed sulfur is an important step. In the potential cycling experiment, however, the oxidative desorption of sulfur more easily occurred at the Pt(111), Pt(110), and Pt(100) electrodes in that order. Once the adsorbed sulfur is oxidized to SO2, SO2 desorbs from the surface or remains at the surface to be subsequently reduced to elemental sulfur in the negative potential scan. Since the reduction of SO2 to elemental sulfur more easily occurs at Pt(100) than at the other two electrodes, the recovery of ECSA at the Pt(100) electrode became slower in the potential cycling experiment. Thus, the fate of SO2 formed by the oxidation of sulfur is one of the important factors affecting the recovery rate of ECSA from sulfur poisoning.