改善鉑釕觸媒的電化學活性於直接甲醇燃料電池上的應用

Promotion of Platinum-Ruthenium Catalyst for Direct Methanol Fuel Cell Application

黃勝揚(Sheng-Yang Huang)

指導教授 ： 葉君棣

國立清華大學/理學院/化學系/博士(2006年)

https://doi.org/10.6843/NTHU.2006.00497

摘要

本研究使用初濕含浸法（incipient wetness impregnation）與共沉澱法（co-precipitation）來製備10 wt% Pt50Ru50/C（觸媒中鉑與釕的原子比為50：50），製備得到的鉑釕觸媒會先以氫氣還原並加以活化。TEM與XRD鑑定的結果顯示鉑釕合金（dPtRu ~ 2 nm）均勻地分散在還原後的觸媒上。TPR結果則指出還原後觸媒上面會形成一個鉑金屬在外層；釕金屬在內部的核層結構。催化活性測試的部分，使用循環伏特法（cyclic voltammetry，CV）來測試Pt50Ru50/C觸媒催化甲醇氧化的反應，實驗結果發現兩種改質方式可以提高鉑釕觸媒的電化學活性，包括氧化熱處理以及添加第三種元素，即氧化鈰。
使用氧化鈰來修飾Pt-Ru/C觸媒可以有效地提高甲醇氧化反應的電流值。促進效應來自於添加氧化鈰能夠降低鉑釕合金的顆粒尺寸。然而修飾的效果與製備方法以及氧化鈰的添加量有直接的關係，實驗結果發現，若以含浸法製備時鉑釕合金會被含浸到微孔洞（micropore）中，共沉澱法則會造成部分的鉑釕合金被氧化鈰包覆，以上兩種現象都會降低活性位置（active sites）的數目，使得Pt-Ru合金無法參與反應。研究證明活性位置（NPt）、與反應物接觸的能力（Fs）以及鉑釕合金的分散度（dispersion，DPtRu）直接影響觸媒的電化學活性。活性位置數目最多的鉑釕觸媒擁有最佳的甲醇氧化反應活性。
探討氧化熱處理對Pt-Ru/C觸媒的影響時發現，鉑釕合金的尺寸與結構會受到氧化溫度（To）影響。提高氧化溫度時，合金內部的釕原子會往外偏析（segregation）並且氧化形成非晶質的氧化釕（RuaO2）。釕偏析的程度隨著溫度提高而增加，當氧化溫度高於520 K時便會出現氧化釕的晶粒（RucO2）。經過氧化處理後Pt-Ru/C觸媒的電化學活性也有顯著的提升，觸媒催化活性增加歸因於釕的偏析以及產生氧化釕的區塊（domain）。然而當氧化溫度高於600 K時，作為支撐物的活性碳會因為劇烈氧化而使得原本均勻分散的鉑釕合金顆粒燒結（sintering），進而導致觸媒活性的衰退。

關鍵字

直接甲醇燃料電池 ； 鉑釕觸媒 ； 共沉澱法 ； 氧化鈰 ； 層溫還原技術 ； 氧化處理 ； 化學偏析

並列摘要

Alloy catalyst of 10 wt% Pt50Ru50/C (with a Pt:Ru atomic ratio of 50:50) was prepared by methods of incipient wetness impregnation and co-precipitation and then activated by hydrogen reduction. TEM and XRD examinations indicated that bimetallic crystallites were finely dispersed with a diameter of dPtRu ~ 2 nm on the reduced catalyst. TPR characterization suggested that deposited bimetallic crystallites exhibited a cherry-like structure with Pt at shell and Ru in core. Catalytic activity of the prepared catalyst toward electro-oxidation of methanol was examined by cyclic voltammetry (CV). The activity of alloy catalyst was found promoted by two kinds of modifications, incorporation of ceria and oxidation treatment.
The promotion of ceria was attributed to an increase in the dispersion of deposited alloy crystallites. However, the extent of promotion depended heavily on the procedure of catalyst preparation and the loading of ceria. Evidently, portion of the alloy particles did not participate in reaction due to being impregnated into internal pores of carbon support as well as occluded into bulk of CeO2 crystallites codeposited. A promoted catalyst with the highest exposure of Pt-Ru nanoalloys exhibited the best electro-activity to methanol oxidation.
The size and the structure of dispersed crystallites were found changed on increasing the temperature of oxidation treatment (To). On mild oxidation, atoms of Ru in the core were found gradually segregated to the surface of bimetallic crystallites and oxidized to amorphous RuO2. Crystalline RuO2 (RucO2) was formed on extensive segregation at To > 520 K. The promotion of oxidation treatment was therefore attributed to the segregation of Ru and the formation of RucO2. Oxidation treatment at elevated temperatures of To > 600 K, however, caused a deactivation to the electro-activity. The deactivation is interpreted with excessive oxidation of the carbon support and a severe sintering of dispersed crystallites.

並列關鍵字

Platinum-ruthenium catalyst ； co-precipitation ； temperature programmed reduction ； cerium oxide ； oxidation treatment ； methanol electro-oxidation ； chemical segregation

參考文獻

Chapter 1

[1] U.S. Department of Energy, “Fuel Cell Handbook (Sixth Edition)”, Morgantown, West Virginia, 2002, Chapter 1.

[2] J. Larminie, A. Dicks, “Fuel cell systems explained”, John Wiley & Sons, 2002, Chapter 1.

[3] M. P. Hogarth, T. R. Ralph, Platinum Met. Rev. 46 (2002) 146.

[7] J. N. Murray, P. G. Grimes, “Fuel Cells”, CEP Technical Manual, American Institute of Chemical Engineers (Progress Technical Manual), 1963, pp. 57-65.