沉積在金屬氧化物載體上之鉑金屬奈米粒對甲醇氧化反應之電催化特性研究

Abstract

本研究製備多孔性二氧化鈦、氧化鈀奈米薄片及類卡斯特形貌之氧化鎳薄膜做為鉑金屬奈米粒之載體，並探討鉑奈米粒對甲醇氧化反應之電催化特性，此三種氧化物薄膜具有高表面積可提供較多的表面沉積鉑奈米粒子，進而使鉑催化劑有較高的電化學活性表面積(ESA)，循環伏安及一氧化碳脫附實驗指出此三種氧化物載體有效提升鉑奈米粒對甲醇氧化反應之催化活性及抗一氧化碳毒化的能力。 我們利用水熱法製備多孔性二氧化鈦作為鉑金屬奈米粒載體探討甲醇氧化反應之電催化研究，5-7奈米大小的鉑金屬粒以脈衝電鍍法均勻地電鍍在經由600oC真空退火之多孔性二氧化鈦載體，鉑/二氧化鈦電極在循環伏安及一氧化碳脫附實驗中展現對甲醇氧化優異的電催化特性及抗一氧化碳毒化能力，我們歸因此優異的電催化活性為鉑奈米粒與氧化物之間的協同效應所造成，鉑金屬與二氧化鈦載體間電荷交互作用將改變鉑表面的一氧化碳吸附性質，並經由雙官能基機制進而加速吸附在鉑表面的一氧化碳之氧化，並增加鉑催化甲醇氧化的能力。 氧化鈀奈米薄片經由反應性濺鍍沉積在碳布上，接著以脈衝電鍍法沉積鉑奈米粒製備鉑/氧化鈀電極，我們發現在酸性環境中之循環伏安量測，氧化鈀奈米薄片之電化學行為與金屬鈀相似，這是氧化鈀表面在循環伏安量測過程中還原成金屬鈀薄膜之故，因此金屬鈀/氧化鈀的氧化還原行為將影響氧化鈀在循環伏安量測中對甲醇氧化的電催化特性，我們提出一組化學反應機制流程圖用以解釋氧化鈀在酸性溶液中對甲醇氧化反應之現象。此外因為鈀金屬容易溶解於酸性電解液中，因此氧化鈀之奈米薄片形貌在酸性溶液中將會嚴重崩解，但我們發現，將鉑奈米粒沉積於氧化鈀表面將有效減緩氧化鈀之奈米薄片形貌崩解於酸性電解液中。 粗糙表面的鎳薄膜擁有類卡斯特之形貌，可提供高表面積經由脈衝電鍍法讓鉑奈米粒沉積，在鹼性環境下，鉑/鎳電極在循環伏安中在展現優異的甲醇氧化反應及抗一氧化碳毒化特性，循環伏安過程中氧化鎳載體可在表面形成氫氧化鎳，這將有助於鉑金屬經由雙官能基機制提升抗一氧化碳的能力，Langmuir-Hishelwood 及 Eley-Rideal機制分別用來解釋氫氧基吸附於氧化鎳載體及氫氧離子在鹼性溶液中如何增加鉑金屬抗毒化能力，XPS分析指出鉑奈米粒與鎳載體間的電荷轉移，此電荷交互作用將改變鉑表面的一氧化碳吸附性質，有助於一氧化碳被氫氧基氧化之反應，此氫氧基由鎳載體或鹼性電解液中獲得並吸附在鉑奈米粒周圍。 鉑/二氧化鈦，鉑/氧化鈀，鉑/鎳此三種電極有優異的甲醇氧化之電催化特性，我們將此優異之電催化效率歸因於優異的一氧化碳容忍度及高電化學活性表面積，發生在鉑奈米粒與三種金屬氧化物之間的電荷交互作用與雙官能基機制發揮了協同效應，顯著提升鉑奈米粒抗一氧化碳毒化能力，進而增進對甲醇氧化反應之電催化活性。

Pt nanoparticles were prepared on nanostructured metal oxides as the electrocatalyst for methanol oxidation reaction (MOR) in this study. Pt nanoparticles were pulse-electrodeposited on the metal oxides, which included nanoporous TiO2 thin film, PdO nanoflake thin film and karst-like NiO thin film. Because of the large loading area on these support and the well-dispersed Pt nanoparticles, these Pt-metal oxide electrodes had a large electrochemical active surface area (ESA). Cyclic voltammetry (CV) and CO stripping measurements showed that the Pt/TiO2, Pt/PdO and Pt/karst-Ni electrode had a high electrocatalytic activity toward methanol oxidation and an excellent CO tolerance compared with the blanket Pt electrode. Porous TiO2 thin films were prepared on the Si substrate by hydrothermal method, and used as the Pt electrocatalyst support for methanol oxidation study. Well-dispersed Pt nanoparticles with a size of 5-7 nm were pulse-electrodepotied on the porous TiO2 support, which was mainly composed of the anatase phase after an annealing at 600oC in vacuum. Cyclic voltammetry (CV) and CO stripping measurements showed that the Pt/TiO2 electrode had a high electrocatalytic activity toward methanol oxidation and an excellent CO tolerance. The excellent electrocatalytic performance of the electrode is ascribed to the synergistic effect of Pt nanoparticles and the porous TiO2 support on CO oxidation. The strong electronic interaction between Pt and the TiO2 support may modify CO chemisorption properties on Pt nanoparticles, thereby facilitating CO oxidation on Pt nanoparticles via the bi-functional mechanism and thus improving the electrocatalytic activity of the Pt catalyst toward methanol oxidation. PdO nanoflake thin films on carbon cloths were prepared by reactive sputtering deposition, and pulse-electrodeposited Pt nanoparticles on the PdO thin films. In acidic electrolytes, the PdO nanoflake thin film has a cyclic voltamperometric (CV) behavior similar to a metallic Pd electrode because of the formation of a metallic Pd surface layer on the PdO thin film under the CV experimental condition. The methanol oxidation reaction (MOR) on the PdO thin film exhibits a CV feature that is closely related to the Pd/PdO redox reaction. We proposed a reaction mechanism scheme for the MOR on the PdO electrode in the acidic solution. The nanoflake morphology on the PdO electrode is seriously damaged during the CV test because of anodic dissolution of metal Pd in acid media. However, the nanoflake damage is greatly alleviated when Pt nanoparticles are electrodeposited on the PdO nanoflake thin film. Negative charges transfer from the PdO support to the Pt nanoparticles according to XPS analysis. The rugged Ni thin films has a karst-like morphology, which provides a large surface area for electrodeposited Pt nanoparticles. Cyclic voltammetry measurements showed that the Pt/karst-Ni electrode had a high electrocatalytic activity toward MOR and CO tolerance in the KOH electrolyte. Ni(OH)2 formed on the Ni support during the potential scan can enhance CO tolerance of Pt nanoparticles via the bi-functional mechanism. The Langmuir-Hishelwood and the Eley-Rideal mechanisms are used to elucidate the role of OH surface groups on the Ni support and OH- ions in the electrolyte, respectively, in the enhancement of the CO tolerance. XPS analysis indicates that negative charges transfer from the Ni support to Pt nanoparticles. The electronic interaction may modify adsorption properties of CO adspecies on the Pt catalyst; the modification allows easy CO electro-oxidation by OH species surrounding the Pt nanoparticles, either from the Ni support or from the alkaline solution. The Pt/TiO2, Pt/PdO and Pt/karst-Ni electrodes have a high electrocatalytic activity toward MOR, and the good electrocatalytic performance of the three electrodes are ascribed to the high CO tolerance and the large ESA. The synergistic effect of the bi-functional mechanism and the electronic interaction makes the Pt/TiO2, Pt/PdO and Pt/karst-Ni electrodes a good catalytic electrode for MOR.