利用鉑、金、銥以及鈀元素修飾鉑釕合金奈米顆粒應用於催化氧氣還原與甲醇氧化反應之研究

Abstract

甲醇氧化反應與氧氣還原反應分別是質子交換膜燃料電池（PEMFC，Proton exchange membrane fuel cell）中陽極與陰極的反應，兩反應均需要有貴金屬做為催化劑加速反應進行。為了降低貴金屬觸媒成本，近年來有越來越多的觸媒合成或修飾方法被提出，利用微量金屬對主要發揮催化效能的Pt 進行修飾以提升效能的方法，在這幾年也因為各種奈米檢測技術逐 漸成熟而被發展出來，置換反應就是這幾年來被提出的一種新穎的製備方式。

本研究利用 Pt、Au、Ir 和Pd 等元素對商用之PtRu 雙元觸媒以置換反應進行修飾，期望將Ru 置換下來，並以修飾元素換上，目的是以簡單快速的方式對觸媒性質能有所改善。我們將商用觸媒泡入各修飾元素之前驅物中，並以迴流（reflow）方式協助置換反應能夠均勻在觸媒表面發生。

特性檢測部份，我們以 XRD 了解Pt 晶格沒有明顯變化和粒徑大小改變的情形推論置換反應為表面的微量修飾，另外再以ICP-MS 確認置換反應的發生並算出各樣品的Pt 承載量。我們也利用電化學量測對修飾過後的PtRu 觸媒效能進行了解，我們以循環伏安法所得到之資訊算出電化學活性表面積，並以此為基礎評估修飾後的觸媒對甲醇氧化與氧氣還原反應的催化效果，以及甲醇氧化系統中的抗CO 毒化能力，發現Pd 和Pt 可以有效增加甲醇氧化反應之催化效能和抗CO 毒化能力，而Pt 還可以增進氧氣還原反應的比活性。

因為 Pt 的修飾效果顯著，我們也以X 光吸收光譜（XAS，X-ray Absorption Spectroscopy）去了解在經過Pt 修飾過後，Pt 和Ru 鍵結與配位數的改變情況，了解置換反應時的機制。

Methanol oxidation and oxygen reduction are the electrochemical reactions occurring at the anode and cathode of direct methanol fuel cells. In general, noble metals such as Pt and PtRu are necessary as electrocatalysts to accelerate the reaction rate for both processes. To date, many synthetic approaches using minute amounts of metals to modify the Pt surface have been developed with the aim to reduce catalyst costs. In this study, commercial PtRu/C catalyst is modified by Pt,Au, Ir, and Pd via a simple displacement reaction in which the oxidative dissolution of Ru leads to a reductive deposition of metal cations on the PtRu surface when the PtRu/C is mixed and refluxed in a proper electrolyte for extended time.

In material characterizations, XRD is conducted to identify possible shift of Pt lattice and variation in the PtRu size, and the results indicate alteration in surface composition but not in bulk form. The extent of displacement reaction and the loading of Pt is verified by ICP-MS. We also perform electrochemical experiments to explore relevant catalytic performances among our samples. The electrochemical surface area (ECSA), CO tolerance, as well as electrocatalytic performance for methanol oxidation and oxygen reduction are determined by cyclic voltammetry in different electrolytes. We conclude that Pd and Pt modification are able to improve the overall catalytic performance for methanol oxidation and CO tolerance. Moreover, Pt modification is even able to improve the specific activity for oxygen reduction reaction. In addition, we adopt X-ray absorption spectroscopy to investigate variation in the bonding and coordination conditions between Pt and Ru to elucidate the responsible steps occurring the displacement reaction.