| 摘要: | 由於直接醇類燃料電池具有定置型、移動型及攜帶型能源轉換器的應用，因此已經廣泛的研究和運用。其中，乙醇是個值得被注意的替代燃料，因為它比甲醇毒性低，且可大量的由農作物或生質能源取得。然而在以乙醇做為燃料的應用上仍存在一些挑戰，例如反應過程中C-C鍵不易斷裂及一氧化碳中間產物易毒化表面鉑(Pt)的活性位置，造成乙醇的氧化機制比甲醇困難且複雜。因此，為了增益鉑基觸媒之乙醇氧化反應(ethanol oxidation reaction)效能，實用可行的方法諸如添加修飾劑或者產生含氧物種，都已被研究探討。 本研究採用沉澱沉積法(deposition-precipitation)來製備PtRu/SnxC(x = 0 - 20 wt %)觸媒，實驗製程使用氫氣為還原劑而以Sn為修飾劑。此外，亦藉由氧化處理製程產生含氧物種。所製備觸媒的組成、結構及表面與活性關係分別以熱重分析儀、感應耦合電漿原子發射光譜分析儀、X光能譜散佈分析儀、X光繞射分析儀、高解析穿透式電子顯微鏡、程式溫度還原系統、X射線光電子能譜儀以及電化學循環伏安法做系統性的分析。在剛還原系列樣品中，發現部份Sn與Pt合金化形成Pt-Sn相，並以金屬態存在。然而，過量的Sn則以非晶質SnO或結晶化SnO2存在。由表面分析及電化學量測得知，欲提升乙醇氧化的電化學活性，必要的物種為表面的Ru和非晶質SnO，而非結晶化的SnO2。經過氧氣熱處理，Ru或Sn會從鉑基合金相中擴散到表面並分別形成RuO2、SnO2或Pt3Sn相。這些表面物種除了幫助乙醇分解吸附在鉑表面，也幫助移除吸附在鉑活性位置的一氧化碳和羧基。然而過量的氧化物會阻礙電化學觸媒的表面活性位置，並降低乙醇氧化反應和一氧化碳氧化反應。由此可知，對PtRu/SnxC電化學觸媒的乙醇氧化反應和一氧化碳氧化反應中，非晶質SnO及適量Ru的表面組成是很重要的條件。因此，在整個系列樣品中，剛還原的PtRu/Sn10C電化學觸媒具有最佳的乙醇氧化反應和一氧化碳氧化反應，而對氧化系列樣品來說，含有RuO2相的PtRu/C具有最佳的乙醇氧化反應。 The direct alcohol fuel cells have been studied extensively because of their stationary, mobile and portable applications. Ethanol is an attractive alternative fuel, because it is less toxic than methanol and can be produced in large quantities from agricultural products or biomass. However, the application of ethanol as fuels also exists some challenges, for example, ethanol is more difficult to be oxidized to CO2 and H2O than methanol owing to the difficulties in C–C bond breaking and the formation of CO-intermediates poison the Pt active sites. Therefore, in order to promote the electroactivity of the Pt-based catalysts toward ethanol oxidation reaction (EOR), the addition of some modifiers and the formation of the oxygen containing species have been elucidated as practical methods. In this study, PtRu/SnxC (x = 0 - 20 wt %) electrocatalysts are prepared by the deposition-precipitation (DP) method using H2 as the reducing agent and Sn as the modifier for the EOR. Besides, the oxidation treatment is applied to generate the oxygen-containing species of the catalysts. The composition, structure, and surface-activity relationship of the Sn modified PtRu/C catalysts can be investigated systematically by the thermal gravimetric analysis (TGA), inductively coupled plasma-atomic emission spectrometer (ICP-AES), energy dispersive spectrometer (EDS), X-ray diffraction (XRD), high resolution transmission electron microscope (HRTEM), temperature-programmed reduction (TPR), X-ray photoelectron spectroscopy (XPS) measurements, and cyclic voltammetry (CV) measurements, respectively. For the as-reduced PtSn/C or PtRu/SnxC catalysts, it is observed that, some Sn exist as the metallic state and alloy with Pt to form Pt-Sn phase, and excess Sn exist as the amorphous SnO or crystalline SnO2. Those amorphous SnO instead of the crystalline SnO2 and the surface Ru on the Snx samples are beneficial for the EOR. During the O2-treatment, Ru and/or Sn diffuse out from the Pt-based alloy phase and exist as RuO2, SnO2 and/or Pt3Sn, respectively. These species can not only promote dissociative adsorption of ethanol on Pt surface but remove CO and carboxyl groups adsorbed on the adjacent Pt active sites. However, the excess oxides may block the active sites on the electrocatalyst surface and deteriorate the EOR or CO-oxidation reaction. Consequently, the surface composing of amorphous SnO and appreciate Ru species are significant condition of PtRn/SnxC toward EOR and CO-oxidation, thus the as-reduced PtRu/Sn10C electrocatalyst has the best EOR activity and CO-oxidation reaction among all samples. In terms of oxidized catalysts, PtRu/C containing RuO2 phase displays high EOR activity. |
