| 摘要: | 直接甲醇燃料電池(DMFC)具有體積小、工作溫度低且易於貯存與處理的優點，但也有諸如較低的功率密度、陽極白金觸媒易遭一氧化碳毒化、甲醇穿透(crossover)與觸媒價格相對高等缺點，因此在產業界與學術界皆有許多關於提升觸媒效能的研究。本研究採用沉澱沉積法(deposition precipitation, DP)製備重量組成比是Pt : Ru : C = 13.4 : 6.6 : 80之鉑釕陽極觸媒(PtRu/C)。結合氧化鈰修飾劑(CeO2)和不同熱處理等改質方式，製備出甲醇氧化效能超越商用材之優良觸媒。 所製備的觸媒，以X光繞射分析儀(X-ray diffraction, XRD)量測其結構，並以程式溫度還原系統(temperature-programmed reduction, TPR)分析其表面組成，而電催化特性則以電化學循環伏安法(cyclic voltammetry, CV)分析。結果顯示；添加10 wt%氧化鈰因增加了PtRu合金的分散度而增加觸媒活性。然則過多的氧化鈰可能包覆觸媒反而降低觸媒反應活性。比較I05 (0.5V vs. NHE)可發現，觸媒經適當氮氣熱處理(520 ~ 570 K)樣品之氧化電流較氧氣熱處理樣品為高。從XRD分析發現，過高的熱處理溫度導致觸媒顆粒變大及PtRu相分離而使活性下降。TPR分析表面組成顯示經氮氣熱處理後PtRu/C觸媒合金化程度增加，表面主要物種為Pt；而氧氣熱處理後，合金相分離表面產生Pt和RuO2。由上述實驗結果可知，觸媒表面組成為Pt而非RuO2，為具有高甲醇氧化活性之重要關鍵。 The direct methanol fuel cell (DMFC) has many advantages, such as the small size, low working temperature, and easy fuel-feeding. However, it also has some disadvantages, for example, low power density, Pt poison by CO, methanol crossover, and high cost for catalysts. Therefore, many researchers have focused on the study of performance enhancement for catalysts. In this study, PtRu/C alloy catalysts with a weight ratio of Pt : Ru : C = 13.4 : 6.6 : 80 have been prepared by the deposition precipitation method (DP). The modification of the PtRu/C catalysts by combination of addition of CeO2 promoter and different heat treatments has been studied to increase its electrochemical activities of methanol oxidation reaction. For the prepared catalysts, the phase structure was analyzed by X-ray diffraction (XRD). The surface species was measured by temperature-programmed reduction (TPR). The electro-oxidation performance was studied by cyclic voltammetry (CV). CV results showed that the the promotion of the catalytic activity by an addition of 10 wt% CeO2 was attributed to the enhancement of dispersion for PtRu crystallites. However, for the PtRu/Ce20C samples, much amount of CeO2 resulted in the decrease of the catalytic activity. The N2- treated samples (520 ~570 K) exhibited better methanol oxidation current at I05 than air heated ones. XRD analysis illustrated that the large particle size and the phase separation of PtRu alloys caused by the severe heat treatment and led to the deterioration of catalytic activity. TPR characterization revealed that the surface species for N2-treated and air-treated catalysts was Pt and Pt+RuO2, respectively. The N2 treatment can increase the degree of alloying for the catalysts while air treatment resulted in the formation of RuO2 and sintering of Pt. It suggested that an alloy surface with a Pt surface species rather than RuO2 are essential to high I05 for methanol oxidation current. |
