CO Oxidation on Au-Ag Alloy Nano-Catalysts and the Kinetic Study under H2-Rich Atmosphere

本研究第一部分是將金、銀鹽類負載於含鋁的中孔洞SBA-15薄膜，形成奈米合金觸媒材料，以進行一氧化碳的氧化催化反應，並探討不同合成條件對催化轉化率的影響。此外，在質子交換膜燃料電池 (Proton Exchange Membrane Fuel Cell, PEMFC)中，以碳氫化合物與水反應產生氫氣時，額外生成的一氧化碳將吸附在白金電極上，造成其效率的下降，因此模擬PEMFC的氣體環境，在反應氣體中引入氫氣，期望此金銀觸媒對二氧化碳有較高的選擇性。 近年來，利用金觸媒催化一氧化碳的氧化反應已被廣泛研究，但其活性中心仍具爭議性，而雙金屬觸媒中，各金屬所扮演的角色與載體固有特性也具相關性。因此本研究的第二部份，是以金銀合金觸媒負載於中孔MCM-41材料上，以進行一氧化碳在富氫環境下的動力學催化研究，由實驗數據計算一氧化碳及氧氣的反應級數。 材料鑑定方面，使用了X光粉末繞射儀來了解結構組成，及穿透式電子顯微鏡以鑑定金屬顆粒大小，金屬的含量及比例是以電漿耦合感應質譜儀鑑定之；此外，以氮氣吸脫附儀鑑定材料的表面積、孔洞及孔體積大小。在判斷合金組成方面，除了紫外光-可見光吸收光譜外，也採用延伸X光吸收精細儀，來了解顆粒的組成與分佈。The first part in this thesis, we demonstrated surface functionalized mesoporous SBA-15 thin film as supporting host to confine the gold and silver alloy nanoparticles. We then show its PROX activity is surprisingly high. Proton exchange membrane fuel cells (PEMFC) have been extensively studied in the past decade. Pure hydrogen source is the ideal fuel for mobile energy systems. However, the Pt anode catalysts would be significantly poisoned by traces of CO in reformed gases. To fulfill the high purification requirement, the preferential oxidation of CO (PROX) appears as the most promising approach and the lowest cost technique. In general, the premise for good PROX performance is to choose a catalyst with more oxidation activity for CO than for H2. Recently we found that Au-Ag alloy catalyst showed a relatively high activity and selectivity. In the second part of this thesis, Au-Ag alloy catalyst supported on MCM-41 was used for kinetic measurements of CO oxidation under H2-rich atmosphere. For this catalyst, it not only performs high CO conversion and CO2 selectivity, but also shows satisfactory stability. Many characterization techniques were used to study the catalyst system, such as nitrogen adsorption, TEM, XRD, EXAFS (X-ray absorption fine structure spectroscopy), and UV-Vis spectroscopy. Especially, EXAFS was employed to determine the coordination number and the degree of alloying formation.Figure Caption List.........................................8 List of Tables.............................................13 Chapter 1 Introduction.....................................15 1. 1 Gold nanoparticle catalysts...........................15 1. 2 Applications of gold nanoparticle catalysts...........18 1.2. 1 Proton Exchange Membrane Fuel Cells, PEMFCs.........18 1.2. 2 Probable Catalysts for Preferential Oxidation.......19 1. 3 Mesoporous support material...........................21 1. 4 Kinetic Study of CO Oxidation under H2-rich atmosphere23 Chapter 2 Experimental Sections............................25 2.1 Chemical Reagent.......................................25 2.2 Catalyst Preparation...................................27 2.2.1 To synthesize the mesoporous aluminosilicate support.27 2.2.2 To prepare surface-modified mesoporous silica........27 2.2.3 To prepare the AuAg-containing mesoporous silica.....27 2.3 Catalyst Characterization..............................29 2.3.1 Powder X-ray Diffraction; XRD........................29 2.3.2 Nitrogen adsorption-desorption isotherms; BET........29 2.3.3 Solid UV-Vis Spectroscopy; UV-vis....................29 2.3.4 Transmission Electron Microscopy; TEM................30 2.3.5 Extended X-ray Absorption Fine Structure; EXAFS......30 2.4 Catalytic activity.....................................31 2.5 Kinetic Measurements...................................32 Chapter 3 Results and Discussion I.........................33 3. 1 Textural properties of functionalized supports........33 3. 2 Effects of Au/ Ag ratio of mesoporous support.........35 3. 3 Effects of Si/ Al ratio of mesoporous support.........41 3. 4 Effect of calcined temperature........................47 3. 5 Extended X-ray Absorption Fine Structure..............53 3.5. 1 Effect of Au/Ag ratios for EXAFS spectra............53 3.5. 2 Effect of Si/Al ratios for EXAFS spectra............59 Chapter 4 Result and Discussion II.........................64 4. 1 Catalytic behavior in CO oxidation under H2-free atmosphere.................................................64 4. 2 Catalytic behavior in CO oxidation under H2-rich atmosphere.................................................68 4. 3 Kinetic study of CO oxidation under H2-rich atmosphere ...........................................................70 References.................................................7