| 摘要: | 本論文利用循環伏安法(Cyclic Voltammetry，CV)和掃描式電子穿隧顯微鏡(Scanning Tunneling Microscope，STM)，探討在鉑(111)電極上，錫修飾層的結構及對於一氧化碳吸附及氧化的影響，另外探討在酸中，二氧化碳還原為一氧化碳之活性。
STM結果表明，錫鉑載體上先以單層之方式成長，到達約0.6層後，轉為三維島狀的方式成長。錫普遍以團簇吸附於鉑(111)電極表面，在少數情狀下觀察到和載體夾30度的鏈狀結構，但這些結構並不穩定，會隨時間消失後轉變成島狀聚集。藉由電位掃描進行錫電鍍時，於負電位觀察到金屬態的錫原子，於正電位則觀察到氧化錫結構。
錫的修飾對一氧化碳在鉑(111)電極上的氧化檢測，循環伏安圖中出現兩個氧化特徵峰，其中主峰在0.45～0.5 V，此一特徵和鉑(111)相比，向負移動0.1 V，可歸因於錫所提供的氫氧根，其與一氧化碳反應後成二氧化碳離開電極表面，同時在主峰之前，於0.2～0.3 V出現一較小的氧化特徵峰，可能是錫修飾後，一氧化碳的吸附能力變弱使其易被氧化。分子解像STM的結果顯示，在低錫覆蓋鉑電極上，吸附的一氧化碳分子，其結構隨正電位轉變，在最後氧化變亂前，從緊密排列之(2 × 2)轉成較鬆散之(√7 × √7)，但在高覆蓋度的錫鉑電極，吸附一氧化碳形成(√7 × √7)的結構，可能此時鉑電極上未被錫佔據的空間，無法讓一氧化碳分子形成一緊密的結構。在一氧化碳氧化的過程中，觀察到中間產物 - 碳酸氫根離子，以不規則的方式吸附在鉑(111)電極上。
在鉑(111)電極上，一氧化碳和錫競爭吸附，因此會影響彼此的結構，在負電位時，一氧化碳的吸附較強，促使錫會移動並聚集成島狀特徵，在飽和的一氧化碳環境，數小時後，三維的錫轉變成平坦的島狀物，或以磊晶的方式吸附在鉑(111)表面。
二氧化碳在鉑電極的還原反應，目前的結果顯示，其速率會和電解液與鉑電極晶面有關，修飾少量的錫後，可導致形成除一氧化碳外的小分子，其氧化電位和甲醇類似。在鉑(111)電極上，二氧化碳還原一氧化碳是一緩慢的過程，可以利用STM觀察其結構和移動。
;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. |
