沉積於氧化銦錫載體之鉑奈米粒尺寸對於催化甲醇氧化反應之影響

Abstract

白金為燃料電池常用之觸媒，白金之高催化特性與化學穩定性，在直接甲醇燃料電池中扮演重要的角色，然而在催化甲醇的反應中，因催化反應不完全，容易產生一氧化碳，並且附著在白金表面，使白金反應面積下降，形成一氧化碳毒化現象，導致燃料電池效能不佳；在酸性溶液中，氧化銦錫基材的化學特性穩定且抗腐蝕性強，此外氧化銦錫在低電位下表面擁有豐富的氫氧基有助於觸媒之電催化。本研究中藉由射頻濺鍍製程在室溫下成功在氧化銦錫載體上沉積出一系列不同尺寸的白金奈米粒子，並以電化學量測評估觸媒尺寸對陽極電極在酸性電解液中催化活性及抗毒化能力的影響之研究。 白金奈米粒子的尺寸與射頻濺鍍系統的濺鍍時間呈正相關，本實驗製備白金奈米粒子由濺鍍時間 20 秒到 70 秒，根據穿透式電子顯微鏡的觀察，所製備的白金奈米粒子子平均粒徑介於1.9 奈米到 8.3 奈米之間。根據 X 光光電子能譜的分析結果，白金 4f 電子軌域的訊號往高束縛能偏移，奈米尺寸之白金奈米粒子與氧化銦錫載體之間具有明顯的電荷轉移現象，即為電子效應。在一氧化碳脫除實驗中，小尺寸的白金奈米粒子顯示較高的一氧化碳氧化電位，意味一氧化碳分子在小尺寸的白金奈米粒子上具有較強的鍵結強度。在電化學的循環伏安量測中，當白金奈米粒子尺寸越小時，甲醇氧化電流密度( If )與甲醇氧化不完全之殘留物的氧化電流密度( Ib )比值越大，顯示白金奈米粒子尺寸縮小有助於白金觸媒抗一氧化碳毒化。小尺寸的白金奈米粒子與氧化銦錫載體具有較大比例的緊鄰區域，氧化銦錫上的氫氧基容易轉移到白金奈米粒子表面上，協助氧化吸附在白金表面上的一氧化碳。因此小尺寸白金觸媒的抗一氧化碳毒化能力主要是歸功於雙官能基機制。由交流阻抗分析結果發現越大尺寸的白金奈米粒子在甲醇溶液中的界面電荷轉移阻抗(Rct)越低，白金奈米粒子平均粒徑 8.5 奈米之電極表現最佳的甲醇催化活性。在一小時的計時電流法量測中，觀察到小尺寸的白金觸媒易溶解於酸性電解液中，由X光光電子能譜得知白金奈米粒子平均粒徑小於 2 奈米的電極損失約 93 % 的白金訊號。計時電流法量測也證實平均粒徑大於6.5 奈米的電極在定電壓0.5 V(vs SCE)作用三十分鐘後損失大部分催化活性。因此基於本研究的結果，尺寸低於 2 奈米的白金奈米粒子不適合直接甲醇燃料電池觸媒的應用。尺寸介於 3 奈米到 5 奈米的白金奈米粒子作為陽極觸媒擁有最理想的抗一氧化碳毒化能力與穩定的電催化特性。

We deposited Pt nanoparticles on the indium tin oxide (ITO) glass support by sputtering deposition, and investigated the size effect of Pt nanoparticles on the electrocatalytic activity toward methanol oxidation reaction (MOR) in acidic media. The average Pt particles size ranges from 1.9 nm to 8.5 nm depending on of the Pt deposition time. According to x-ray photoelectron spectroscopy (XPS), the ITO support has a strong electronic interaction with Pt nanoparticles. A positive binding energy shift for the Pt 4f core-level electron indicates that negative charges transfer from the nanometer-scaled Pt particle to the ITO substrate. Because the CO stripping peak potential increases with decreasing the size of Pt nanoparticles, CO adspecies are likely to have a higher bonding strength with Pt nanoparticles of smaller size. However, from cyclic voltammetry, smaller Pt nanoparticles have a larger ratio of the forward anodic current to the backward anodic current, suggesting that smaller Pt nanoparticles have a better CO tolerance in MOR. The beneficial size effect of smaller Pt nanoparticles on the CO tolerance can be ascribed to the prevailing bifunctional mechanism on smaller Pt nanoparticles, which have a larger portion of surface area immediately adjacent to the ITO support. The better CO tolerance of smaller Pt nanoparticles, however, does not warrant a higher electrocatalytic activity toward MOR for the Pt catalyst. Pt nanoparticles with the average size of 8.5 nm demonstrate the lowest peak potential and the highest mass activity for methanol electrooxidation. According to electrochemical impedance spectroscopy (EIS), larger Pt nanoparticles exhibit a lower charge transfer resistance (Rct) in the acidic methanol solution, indicating they have a higher electrocatalytic activity toward MOR. Moreover, Pt nanoparticles of very small size tend to be easily dissolved in the acidic electrolyte. From XPS analysis, the electrode with Pt nanoparticles of 2 nm in size shows a Pt loss of 93% after the chronoamperometric (CA) measurement for one hour. The CA measurement also demonstrates that electrodes with the Pt nanoparticle size larger than 6.5 nm lose greatly the electrocatalytic activity after the polarization at 0.5 V (vs SCE) for 30 minutes. The electrode with Pt nanoparticle of 3-5 nm in size shows the best electrocatalytic performance in terms of CO tolerance and electrochemical stability.