金屬氧化物載體上之白金奈米粒尺寸對甲醇氧化反 應電催化活性之影響

白金(Pt)為甲醇燃料電池常用之觸媒材料，用以催化甲醇氧化反應(methanol oxidation reaction, MOR)。一般增加Pt觸媒催化能力的方法為降低Pt粒子尺寸以提高觸媒表面積，並提升基材與白金觸媒之間的電子(electronic)效應與雙官能基(bifunctional)效應。電子效應可以增進Pt對MOR的催化活性，雙官能基效應則可以降低MOR產物一氧化碳(CO)對Pt的毒化效應。本研究製作了兩種Pt奈米粒陽極，一為使用電漿輔助原子氣相沉積(plasma-enhanced atomic layer deposition, PEALD) 在鈦金屬薄膜的自然氧化層(Ti/TiO2)上沉積Pt奈米粒，二為以射頻濺鍍(radio frequency, RF)法在氧化銦錫(ITO)玻璃上製備Pt奈米粒，兩者的平均粒徑分佈皆為2-8 nm，我們探討在兩種氧化物基材上，Pt奈米粒尺寸對MOR電催化活性的影響。依據 X光光電子能譜(x-ray photoelectron spectroscopy, XPS)分析， II 沉積在Ti/TiO2 上的Pt 奈米粒與基材之間缺乏明顯電子效應，雙官能基效應主導了Pt 奈米粒對MOR 催化活性的尺寸效應。由於TiO2 為氧化物半導體，電阻率高，當Pt 尺寸過小，基於PEALD 沉積特性，小尺寸的Pt 奈米粒離散，密度低、因此Pt/TiO2 陽極有較高的歐姆過電壓，對MOR 電催化效能不佳。本實驗發現Pt 奈米粒尺寸在5 nm 左右，Pt/TiO2 電極有較最佳的MOR 電催化活性。 ITO 基材導電性佳，在低電位下表面可產生大量氫氧官能基，有助於增強雙官能基效應，根據 XPS 分析，沉積其上的Pt 奈米粒之Pt 4f 電子束縛能有藍移現象，奈米粒尺寸越小，藍移現象越顯著，顯示 Pt 奈米粒與ITO 載體之間有強烈電子效應。減小Pt 奈米粒尺寸可以提升雙官能基效應，但在CO 脫除實驗中，小尺寸的Pt 奈米粒具有較高的CO 氧化電位，意味CO 與較小的Pt 奈米粒有較強的鍵結，因此抵銷了雙官能基效應。此外小尺寸的Pt 奈米粒易溶解於酸性電解液中，由XPS 得知，平均白金奈米粒粒徑2 nm 的陽極會有80 % 的 Pt 4f 訊號損失。由本研究發現，5-6 nm 大小的Pt 奈米粒有較佳的抗CO 毒化能力與穩定的電催化特性，尺寸小於 2 nm 的Pt 奈米粒不適合用作甲醇燃料電池觸媒。

關鍵字

電漿輔助原子氣相沉積；射頻濺鍍沉積；甲醇氧化反應；，白金奈米粒子；氧化鈦；氧化銦錫；，雙官能基機制；電荷轉移效應；計時電流法量測

並列摘要

Platinum (Pt) is a commonly used catalyst for the methanol oxidation reaction (MOR) in direct methanol fuel cells (DMFC). To improve the catalytic activity of the Pt catalyst toward MOR, the particle size of the Pt catalyst is generally reduced to increase the electrochemical surface area (ECSA) and to enhance the electronic and the bifunctional effects occurring between the substrate and the platinum catalyst. The electronic effect can enhance the catalytic activity of Pt toward MOR, and the bifunctional effect can improve the resistance against the CO poisoning that results from incomplete MOR. In this study, two types of anodes were fabricated. One anode have Pt nanoparticles deposited on the natural oxide layer of a titanium thin film (Ti/TiO2) by plasma-enhanced atomic layer deposition (PEALD). The other has Pt nanoparticles deposited on indium tin oxide (ITO) glass by radio frequency (RF) sputtering deposition. The particle size distribution on the two anodes is in the range of 2-8 nm. We investigated the effect of Pt nanoparticle size on the electrocatalytic activity of the catalyst toward MOR for the two anodes. According to x-ray photoelectron spectroscopy (XPS) analyses, V the electronic effect is trivial on the anode with Pt nanoparticle loaded on the Ti/TiO2 support. The bifunctional mechanism thus becomes the primary factor determining the size effect on the catalytic activity of Pt nanoparticles toward MOR. TiO2 is an oxide semiconductor with a high resistivity. When the Pt nanoparticle size is very small, the Pt/TiO2 anode exhibits a high Ohmic overpotential because Pt nanoparticles are sparsely distributed on the TiO2 substrate as a result of the deposition characteristics of PEALD, leading to a poor electrocatalytic performance in MOR. From the study, the Pt/TiO2 electrode ahs the best MOR electrocatalytic activity when the Pt particle size is around 5 nm. In contrast to the TiO2 substrate, the ITO substrate has a high conductivity, and is with a large number of hydroxyl surface groups in acidic electrolytes at potentials, which may enhance the bifunctional mechanism. According to XPS analysis, the binding energy of Pt 4f electrons emitted from Pt nanoparticles has a blue shift. The smaller the nanoparticle size, the larger the blue shift, indicating that there is a strong electronic effect between the Pt nanoparticles and the ITO support. Although, reducing Pt nanoparticle size can improve the bifunctional effect, it degrades the resistance against CO poisoning according to the CO stripping test, which shows a high overpotential for CO oxidation overpotential, suggesting a stronger adhesion strength of CO on smaller Pt nanoparticles. As a consequence, the benefit of a smaller Pt particle size to electrocatalytic activity due to the bifunctional mechanism is offset by the adverse electronic effect. Moreover, small-sized Pt nanoparticles are easily dissolved in the acidic electrolyte. The Pt/ITO anode with an average Pt VI particle size of 2 nm shows a Pt 4f XPS signal loss of 80% after chronoamperometric measurement for one hour. From this study, it was found that the electrode loaded with Pt nanoparticles with a size of 5-6 nm has a better resistance against CO poisoning and a stable electrocatalytic activity. On the other hand, Pt nanoparticles smaller than 2 nm in size are not a suitable catalyst for DMFC applications.

並列關鍵字

Plasma enhanced atomic layer deposition ； magnetron sputtering ； methanol oxidation reaction ； bifunctional mechanism ； electronic effect ； chronoamperometric measurement