| 摘要: | 直接乙醇燃料電池已經廣泛的研究和應用在移動型及攜帶型能源轉換器上。乙醇是種毒性低且生物相容性好的燃料，且可大量的由農作物或生質能源製造。此外，直接乙醇燃料電池的副產物為乾淨的水。因此，直接乙醇燃料電池被視為相當具有潛力的綠色能源系統; 然而直接乙醇燃料電池的發展受限於乙醇氧化反應(ethanol oxidation reaction)緩慢的動力學性質以及貴金屬觸媒的高價格。因此，為了增益觸媒之乙醇氧化反應效能，系統性的藉由操控還原溫度、去合金化、熱處理以及添加氧化物等改質方式，探討其活性與結構之關連性。 本研究採用沉澱沉積法(deposition-precipitation)來製備鈀銅 (原子比為 3:1)觸媒，實驗製程使用氫氣為還原劑。而以SnO2修飾的鈀銅觸媒則是利用二步驟還原法來製備。另一方面，去合金化和熱處理的改質方法被應用在改質鈀銅觸媒的合金結構或表面性質，進而提升乙醇氧化反應。本實驗中，所製備觸媒的組成、結構、形貌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光能量散射光譜儀、X光繞射分析儀、高解析穿透式電子顯微鏡、X光電子能譜儀以及電化學循環伏安法做系統性的分析。我們可以發現對鈀銅觸媒而言，抗毒化能力、合金度(degree of alloying)以及晶粒尺寸會隨還原溫度升高而增加，然而，過高的還原溫度導致觸媒粒子的燒結效應，進而阻礙乙醇氧化反應，研究發現570 K的還原溫度下，鈀銅觸媒具有好的合金結構、晶粒尺寸、抗毒化能力以及乙醇氧化效能。 在去合金化改質方面，經由5圈去合金化過程可以增進鈀銅觸媒之電化學活性表面積，但對乙醇氧化能力僅有稍微提升。然而25圈的去合金化過程會劣化鈀銅觸媒的結晶性導致乙醇氧化能力下降。在使用熱處理改質方面，氫氣熱處理會造成鈀銅合金分相、合金度以及表面的PdOx減少，而氧化熱處理會在鈀銅觸媒表面生成過量非晶質PdOx相以及在塊材中結晶PdO相導致乙醇氧化能力下降。然而，一氧化碳熱處理能夠最佳化合金度以及觸媒表面的PdOx進而提升一氧化碳和乙醇氧化的效能。 在SnO2修飾的鈀銅觸媒方面，當Sn的添加含量為10 wt %時，在塊材中適量的PdSn相和表面的SnO2能夠提升一氧化碳、長時間下乙醇氧化能力以及電化學活性表面積。而在Sn的添加含量為20 wt %時，表面過量的SnO2會阻礙觸媒的活性表面積，而降低乙醇氧化能力。因此，合金度、觸媒尺寸大小、表面氧化態以及表面組成對氧化乙醇效能而言扮演重要的角色。 在整個系列樣品中，經一氧化碳熱處理後鈀銅觸媒具有最佳的乙醇氧化反應，相對於剛還原的鈀銅觸媒可增進190 %的乙醇氧化效能，而對SnO2修飾的Pd-Cu/C觸媒來說，當添加Sn含量為10 wt %的觸媒含有PdSn相和SnO2具有較佳的一氧化碳氧化能力及電化學活性表面積以及最佳長時間乙醇氧化反應，相對於剛還原鈀銅觸媒可增進約230 %的長時間乙醇氧化效能。 Direct ethanol fuel cells (DEFCs are considered more and more important on mobile and portable applications. Ethanol is environmental friendly and can be massively produced by biomass or agriculture. Moreover, the byproduct of the chemical reaction in DEFCs is pure water. Therefore, DEFCs are regarded as the potential green technology for future energy systems. However, significant challenges in the development of DEFC technology are limited by the sluggish kinetic of ethanol oxidation reaction (EOR) and high price of noble metal catalysts. Therefore, in order to enhance EOR, development of high performance catalysts by means of manipulation of reducing temperatures, dealloying, heat treatments, and oxide promoter have been elucidated systematically. In this study, Pd-Cu/C (with an atomic ratio of 3:1) electrocatalysts are prepared by the deposition–precipitation (DP) method using H2 as the reducing agent. SnO2 modified Pd-Cu/C catalysts are prepared by the two-steps reduction method. On the other hand, dealloying process and heat treatments have been applied for Pd-Cu/C catalysts to alternate structure and/or surface oxidation states and modify the EOR performance systematically. The activity-structure, morphology, surface species and electroactivity of the Pd-Cu/C and SnO2-modified Pd-Cu/C catalysts are characterized 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), X-ray photoelectron spectroscopy (XPS) measurements, and cyclic voltammetry (CV) measurements, respectively. For the Pd-Cu/C catalysts, it is observed that, If/Ib ratio, degree of alloying (Dalloy), and grain size increases as the reducing temperature increases. However, severe sintering deteriorates the EOR performance. Therefore, 570 K is a suitable reducing temperature for preparation of Pd-Cu/C catalysts with fine alloy structure, small crystalline size, good anti-poisoning ability and excellent EOR performance. During dealloying route, the electrochemical surface area (ECSA) of as-reduced Pd-Cu/C catalysts can be enhanced after 5 cycles, however, their EOR activity is only slightly promoted. After dealloying of 25 cycles, the crystallinity of PdCu alloy becomes severely weak, thus degrading the EOR performance. For the heat-treated Pd-Cu/C catalysts, H2 treatment deconstructs the Pd-Cu alloy phase, and thereby decreasing the Dalloy and surface PdOx composition. On the contrary, O2 treatment causes the significant enrichment of amorphous PdOx on the surface and crystalline PdO in the bulk, which do not benefit EOR. The CO treatment optimizes the Dalloy and surface PdOx composition of the catalysts, thereby enhancing their CO oxidation and EOR performance. For the SnO2 modified Pd-Cu/C catalysts, 10 wt % of Sn addition promotes the chronoamperometric (CA) performance, attributed to the significant amount of PdSn phase in the bulk and SnO2 species on the surface, which enhance their CO oxidation and ECSA. On the other hand, 20 wt % Sn additions may cause a great amount of SnO2 on the surface and block the active site of the catalysts, thus retarding the EOR performance. Consequently, the Dalloy, particle size, surface oxidation state, surface compositions, and structures of catalysts are significant conditions for EOR. In this research, the CO heat-treated Pd-Cu/C catalyst has the best EOR activity which is enhanced about 190 % when compared with the as-reduced sample. In terms of SnO2 modified Pd-Cu/C catalysts, 10 wt % of Sn modified Pd-Cu/C catalysts containing moderate PdSn phase and surface SnO2 display high ECSA, CO-oxidation and chronoamperometric activity, which is enhanced about 230 % when compared with the as-reduced sample. |
