| 摘要(英) |
In this study, cyclic voltammetry (CV) and scanning tunneling microscope (STM) are used to explore the adsorption and oxidation of carbon monoxide (CO) on Sn-modified Pt(111) electrode.
First, in situ STM imaging reveals the intermediate, designated as bicarbonate HCO3-, in the oxidation process of CO, on the Pt(111) electrode. This HCO3- is adsorbed in disarray. The structure of Sn deposited on Pt(111) is also examined by STM, showing that layer type of growth of Sn for the first 0.6 layer, followed by a three-dimensional growth. Most Sn deposit assumes clusters on Pt(111), but occasionally atomic chains are observed, which are aligned in <121> direction of Pt(111). The lifespan of these Sn chains is only 1 hr before they turn into 3D clusters. If Sn is deposited on Pt(111) under potential modulation, STM reveals atomically flat Sn patches at negative potential, by which structure of atomic Sn deposit and Sn oxide structure are observed at negative and positive potentials.
Two oxidation peaks are observed in the stripping voltammogram recorded with CO monolayer adsorbed on Sn-modified Pt(111) electrode. The main peak emerges at 0.45～0.5 V, which is 0.1 V more negative than that of Pt(111). This shift in CO oxidation potential can be attributed to bifunctional effect, where OH- is produced at the Sn deposit and react with nearby CO adsorbed on Pt sites. When Sn is at a low coverage, the structural change of CO is due to the difference in potential. Conversely, when Sn is at a high coverage, the structural change of CO is due to the increased CO content. A minor pre-peak (0.2～0.3 V) is also noted, as the Pt - CO binding can be weakened by Sn, facilitating CO oxidation at more negative potential.
CO and Sn can compete for the Pt sites on Pt(111). At negative potential, CO binds more strongly than Sn, forcing pre-deposit Sn to move to different sites and aggregate into 3D clusters. As adsorbed CO is removed from the Pt(111) electrode, Sn deposit can migrate to Pt sites and occupy the entire Pt surface. However, it is possible to control the structure of Sn deposit. By keeping the Sn/Pt(111) electrode in CO - saturated electrolyte, 3D Sn aggregates can transform into layered type structures. This epitaxial deposition model is manifested in the flat patches on the Pt(111) electrode.
Finally, the reduction of CO2 to CO at bare and Sn-modified Pt(111) electrodes is examined in sulfuric and perchloric acids. By holding the potential at -0.2 V (vs. Ag/AgCl) in CO - saturated 0.1 M HClO4, we observe ordered (2 2) CO adlattice on Pt(111) by STM imaging. The efficiency of this CO2 to CO conversion is affected by the chemical identity of electrolyte, crystal orientation, and composition of Pt electrode. In contrast to the immobile nature of CO adlayer prepared by dosing with CO directly, the reduction of CO2 and the mobile CO molecules on the Pt(111) electrode are observed. |
| 參考文獻 |
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