| 摘要: | 本研究利用循環伏安法(CV)、掃描式電子穿隧顯微鏡(STM)和表面增強紅外光吸收技術(SEIRAS)，探討沉積於鈀(111)表面之鉑薄膜，對於甲醇、甲酸、甲醛的催化活性。鈀(111)電極本身在高溫時會吸入氫氣，同時可能在鈀(111)表面形成氫化物，在接觸鉑鹽後可將鉑離子還原成鉑金屬，以島狀物形式存在於鈀電極表面(以Pt@Pd(111)代表)，鉑島狀物的高度以及大小和浸泡時間有關，但最後呈現穩定的狀態，這一點和甲醇氧化峰往正電位位移的現象有關，由於島狀物的生成使得氧化峰會改變位置，使得產生的一氧化碳需要更多活性位置氧化。循環伏安的結果顯示甲醇、甲酸、甲醛氧化電流在Pt@Pd(111)電極比鉑(111)大20、10及2倍多，這些催化活性的提升可能源自鈀載體改變鉑膜的電子結構。但將修飾層及載體互換時，Pd@Pt(111)電極對有機小分子的催化活性大幅減低，顯示一高活性載體(鉑)不一定會提升修飾層(鈀)的活性。
為了解鈀載體鉑修飾層電子結構的影響，以X射線光電子光譜檢測鉑的4f 5/2和4f 7/2能階電子束縛能的位移，此電子結構的變化同時會影響一氧化碳的吸附，當鉑4f 5/2和4f 7/2能階電子束縛能降低會使得金屬d-band中心下降，導致費米能階附近電荷密度下降，會使鍵結力變弱。紅外光吸收的結果發現一氧化碳分子的C-O拉伸吸收從高往低波數移動，代表C-O之間的振動減弱，同時一氧化碳吸附量降低，因此有機小分子的氧化也許不必經過一氧化碳，或是因為對於一氧化碳的鍵結力下降。
催化活性的提升除了鈀載體改變鉑膜的電子結構，還有一個因素是粗糙度。循環伏安圖顯示催化效果隨著浸泡鉑鹽時間拉長而達到提升，除了和鉑覆蓋表面的量有關，由循環伏安圖可以得知Pt-OH還原峰超過浸泡時間60秒以後有明顯的增加，還有一氧化碳起始氧化電位會有提前現象，這是由於表面的粗糙度隨浸泡時間在60秒後明顯增加。
最後，觀察在Pt@Pd(111)電極上甲醇氧化電流對時間的關係，以瞭解此電極的穩定度(或被毒化的程度)，結果顯示此電極的電流降低的幅度比鉑(111)低，因此Pt@Pd(111)電極比較不容易被毒化失去催化能力。對甲酸氧化也有不錯的穩定性，但是甲醛的氧化電流明顯的下降，顯示這些分子的氧化途徑明顯不同，這是因為甲醛氧化在粗糙面容易生成一氧化碳所致。
;In situ scanning tunneling microscopy (STM) and Surface-Enhanced Infrared Absorption Spectroscopy (SEIRAS) have been used to investigate the catalytic activity of a Pt thin supported by Pd(111) (denoted by Pt@Pd(111)) toward the oxidation of fuel molecules such as methanol, formic acid, and formaldehyde. Annealing a Pd bead with a hydrogen flame results in absorption hydrogen in the Pd bulk and possibly producing Pd hydride. This hydrogen-containing Pd(111) electrode can reduce PtCl62- to metal Pt, producing Pt islands on the Pd(111) electrode. The amount of Pt deposit varied with immersion time and [PtCl62-]. The effect of immersion time on the morphology of the Pt deposit and electrocatalytic activity was investigated. It is consistent with the positive shift of oxidation peak toward methanol oxidation, because the form of Pt islands will change the position of oxidation peak.
Pt@Pd(111) electrode resulted in 20, 10, 2 times higher oxidation currents than Pt(111) in 0.1 M H2SO4 containing 1 M methanol, formic acid, and formaldehyde, respectively. The reason behind these promotions can be traced to the modification of the electronic structure of the Pt adlayer by the Pd substrate. This issue was addressed by x-ray photoelectron spectroscopy (XPS), which shows a downshift of the 4f5/2 and 4f7/2 levels. This change likely reduces the bonding energy of CO molecule, the common poisonous species generated in the oxidation of methanol. On the other hand, the electrode with opposite structure, Pt@Pd(111), is passivated toward oxidation of small organic fuel molecule. The downshift in the Pt 4f binding energy means downshift of its d-band center and lower density of state at the Fermi level, which ultimately weakens the chemisorption bonds. This view is supported by IR results showing C-O vibration shifted from higher wavenumber to the lower wavenumber, implying low C-O vibration. Meanwhile, the intensity of C-O absorption band seen with was lower than that of Pt(111), suggesting less adsorbed CO molecules on the Pd@Pt(111) electrode. This result suggests that the oxidation of these organic molecules might bypass the route to generate CO molecules or the bond strength between CO and Pt is weaker.
The improvement of catalytic activity is twofold. In addition to electronic effect, the roughness and geometric structure of the electrode also contribute. Cyclic voltammetry shows that the catalytic activity would increase with immersing time. As the reduction charge of OH species increases for samples prepared by immersion time longer than 60 s in the dosing Pt salt, the overall area of the electrode increases also. The onset potential of CO oxidation shifts negatively. Thus, roughness of electrode was also important to the catalytic activity.
The durability of this Pt@Pd(111) and Pt(111) electrodes toward oxidation reaction of was studied by using chronoamperometry in methanol containing sulfuric acid. The oxidation current observed at both electrodes decreased with time, revealing poisonous phenomena attributed to the generation of CO species. The extent of poisoning is less at the Pt@Pd(11) electrode than the Pt(111), which results from different oxidation mechanism and the extent of poisonous effect. The reason is when formaldehyde oxidation on ragged surface, it is easily to form CO molecules. |
