國立交通大學 加速器光源科技與應用碩士學位學程 碩士論文. Investigation of Displacement Reaction for Pt on Ru/C. Nanoparticles by X-ray Absorption Spectroscopy 研究生 : 張立忠 - PDF

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國立交通大學 加速器光源科技與應用碩士學位學程 碩士論文 利用 X 光吸收光譜研究鉑原子在釕奈米顆粒的置換反應 Investigation of Displacement Reaction for Pt on Ru/C Nanoparticles by X-ray Absorption Spectroscopy 研究生 : 張立忠 指導教授 : 吳樸偉教授 李志甫博士 中華民國九十九年七月 National

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國立交通大學 加速器光源科技與應用碩士學位學程 碩士論文 利用 X 光吸收光譜研究鉑原子在釕奈米顆粒的置換反應 Investigation of Displacement Reaction for Pt on Ru/C Nanoparticles by X-ray Absorption Spectroscopy 研究生 : 張立忠 指導教授 : 吳樸偉教授 李志甫博士 中華民國九十九年七月 National Chiao Tung University Graduate Program for Science and Technology of Accelerator Light Source Thesis 利用 X 光吸收光譜研究鉑原子在釕奈米顆粒的置換反應 Investigation of Displacement Reaction for Pt on Ru/C Nanoparticles by X-ray Absorption Spectroscopy Student: Li-Chung Chang Advisors: Prof. Pu-Wei Wu Dr. Jyh-Fu Lee July, 利用 X 光吸收光譜研究鉑原子在釕奈米顆粒的置換反應 研究生 : 張立忠 指導教授 : 吳樸偉教授 李志甫博士 Student:Li-Chung Chang Advisor:Prof. Pu-Wei Wu Dr. Jyh-Fu Lee 國立交通大學 加速器光源科技與應用碩士學位學程 碩士論文 A Thesis Submitted to Graduate Program for Science and Technology of Accelerator Light Source College of Engineering National Chiao Tung University in partial Fulfillment of the Requirements for the Degree of Master in Graduate Program for Science and Technology of Accelerator Light Source July 2010 Hsinchu, Taiwan, Republic of China 中華民國九十九年七月 3 利用 X 光吸收光譜研究鉑原子在釕奈米顆粒的置換反應 研究生 : 張立忠 指導教授 : 吳樸偉博士 李志甫博士 國立交通大學加速器光源科技與應用碩士學位學程 摘要置換反應為一個新穎的觸媒製備方式, 對於直接甲醇燃料電池陽極或陰極觸媒都有相關的應用 該方法解決了傳統觸媒製備時遭遇的問題, 例如貴重金屬的乘載量太高和使用率太低 利用置換反應所製備的觸媒可以降低貴重金屬的承載量到單一原子層或甚至更低 可以提升貴重金屬參與觸媒反應的使用率, 減少貴重金屬的浪費便可以降低燃料電池系統單價 然而, 置換反應的反應機制相當複雜, 需要投入更多的研究加以釐清 藉由增加對反應過程的基礎知識, 使得置換反應所製備出來的觸媒得以最佳化 在本研究中, 置換反應中所參與的元素為鉑與釕 選擇這兩個元素做為研究對象的原因, 除了鉑釕合金之間會進行置換反應外, 鉑釕合金也是一個效能優異的直接甲醇燃料電池陽極觸媒 首先, 我們使用化學還原法製備的碳承載之釕奈米顆粒, 緊接著將他們浸泡入鉑的前驅物中進行置換反應後進行分析 並且, 在不同酸鹼值下的鉑前驅物, 對於釕的置換反應中的影響也進行深入的探討 研究方法主要使用 X 光吸收光譜來鑑定置換反應過程中, 鉑的氧化價數和微結構變化情形 同時, 釕的氧化價數和微結構變化也進行分析, 使得我們可以分別用鉑與釕的變化建構一個合理的反應機制 另一方面, 由穿透式電子顯微鏡得知釕奈米顆粒均勻分布在碳的承載物上, 並且置換上鉑之後的顆粒大小並沒有明顯的變化 ;X 光繞射圖顯示釕受到了鉑的影響, 晶格有變大的情形 ; 能量分散 X 光譜儀和感應耦合電漿質譜分析儀指出置換反應中, 鉑的沉積量和釕被置換的量受鉑前驅物酸鹼值有相當的影響 i 總結以上分析結果, 利用置換反應將鉑沉積在釕奈米顆粒的反應過程中 : 鉑離子被釕還 原至較低的氧化價數並沉積在釕奈米顆粒的表面, 而釕氧化成離子並解離入前驅物水溶液當 中 然而, 鉑並非直接與釕形成鍵結, 是藉由氧的嫁接形成鍵結在釕的表面 ii Investigation of Displacement Reaction for Pt on Ru/C Nanoparticles by X-ray Absorption Spectroscopy Student: Li-Chung Chang Advisors: Prof. Pu-Wei Wu Dr. Jyh Fu Lee Graduate Program for Science and Technology of Accelerator Light Source National Chiao Tung University Abstract An investigation to the displacement reaction mechanism for Pt on Ru/C nanoparticles was carried out by analyzing the immersion of Ru/C nanoparticles into hexachloroplatinic acids with various ph values. X-ray diffraction patterns indicated slight Ru lattice expansion of Pt deposition. The exact variation of Pt and Ru during displacement reaction was determined by inductively coupled plasma mass spectrometry, which suggested that the dissolution of Ru was mainly resulted from the displacement by Pt. The Ru/C and PtRu/C were approximately 2 to 3 nm in sizes from TEM images from transmission electron microscope. Energy dispersive x-ray spectroscopy determined the composition of Pt on Ru nanoparticles. X-ray absorption spectroscopy analysis provided a detailed reaction mechanism which indicated that Pt cations were reduced to moderate oxidation states or merely physically adsorbed contingent on the types of complexing ligands. In addition, the Pt was deposited onto Ru through bridged-oxygen instead of forming a direct Pt-Ru bonding during displacement reaction. iii Acknowledgements In the beginning, it is a great honor to express my deepest gratitude to two of my dedicated advisors, Prof. Pu-Wei Wu and Dr. Jyh-Fu Lee, for their superior guidance so that this thesis could have been completed. They have given me a lot of advice, inspirations, solutions, and criticisms with their profound knowledge in science and research experiences. I sincerely appreciate their patience and kindness, because they always put high priority on my experiments and thesis writing. Besides, they spend precious hours to discuss with me anytime they are available. I have leant from them a lot not only about conducting scientific research, but also professional ethics. This thesis would not have been possible unless the guidance given by my two advisors. In addition, I am grateful to the senior graduate student, Robert Hsieh, who taught me in establishing all practices and experimental set ups related to the research topic. Ideas and comprehension related to the research were formed after many enthusiastic discussions and brainstorming with him. He has made available his support in a number of ways and I would like to thank Robert for all his help. Special thanks are also given to senior Ph.D students, Jeff Chang, Kiokio Hsieh, Lawrence Lin, Rebecca Chen, Eric Chang, and Eric Hwang, for their consultations and encouragements. Moreover, I am indebted to many of my colleagues, Tiffany Chiu, Hedy Wang, Joyce Chen, Wayne Chen, Edward Chang, Martin Liao, Tim Hwang, Fiona Chen, Feon Chen, Lisa Chou, and Lynn Hwang for their friendship and support. I wish to extend my thanks to the staff of department office, Debbie Fu, for her support for paper works. Thanks are also due to my friend, Evelyn Sung, for every touching moments and caring advice when I was troubled. I owe many thanks to my roommates, Brian Ho and Patrick Lee, for being nice and considerate. At last but not the least, I would like to express my gratitude to my parents and younger sister for taking care of me and supporting me along the way of my study. iv Table of Contents Chinese Abstract... i English Abstract... iii Acknowledgements... iv Table of Contents... v List of Tables... vii List of Figures... viii Chapter 1 Introduction Background Motivation... 2 Chapter 2 Literature Review Anode catalysts for DMFC Platinum monolayer fuel cell electro-catalyst X-ray Absorption Spectroscopy of DMFC Catalysts Chapter 3 Experimental Functionalization of commercial carbon XC-72R Fabrication of carbon supported Ru nanoparticles Samples for displacement reaction XRD, TEM, EDX and ICP-MS measurements XAS measurements EXAFS data analysis Chapter 4 Results and discussion Introduction Results and discussion v Chapter 5 Conclusions Reference vi List of Tables Table 1. Results from material characterizations on group A, the immersion baths of group A and the immersion baths of reference group by ICP-MS Table 2 Results from material characterizations on Ru/C immersed with ph 1, ph 2.2 and ph 8 H 2 PtCl 6 (group A) by EDX Table 3 The fitting results from the analysis of Ru K-edge EXAFS spectra Table 4 The fitting results from the analysis of Pt L 3 -edge EXAFS spectra vii List of Figures Figure 1. The synthesis methods for carbon supported PtRu catalysts: (1) impregnation method, (2) colloidal method, and (3) microemulsion method [6] Figure 2. A sub-monolayer Pt on Ru nanoparticles [19] Figure 3. A model of fabricating monolayer Pt on palladium nanoparticles with Cu UPD [19] Figure 4. The cyclic voltammetric (CV) scans of new class catalyst and commercial PtRu for MOR [19] Figure 5. The CV scans of Au modified Pt/C [19] Figure 6. XAS spectrum of Mo K-edge [25] Figure 7. The experimental set up for XAS analysis. (a) Transmission mode; (b) fluorescence mode [25] Figure 6(A). The XRD patterns for carbon supported Ru (Ru/C), group A ph 1, ph 2.2, ph 8 and pure carbon Figure 6(B). The XRD patterns at 38 degrees for group A ph 1, ph 2.2, and ph Figure 7(A). The XRD patterns for carbon supported Ru (Ru/C), group B ph 1, ph 2.2, ph 8 and 23 Figure 7(B). The XRD patterns at 38 degrees for group B ph 1, ph 2.2, and ph Figure 8(A). The XRD patterns for carbon supported Ru (Ru/C), group C ph 1, ph 2.2, ph 8 and 24 Figure 8(B). The XRD patterns at 38 degrees for group C ph 1, ph 2.2, and ph Figure 9. The TEM images for carbon supported Ru Figure 10. The TEM images for group A, Ru/C immersed with ph 1 H 2 PtCl Figure 11. The TEM images for group B, Ru/C immersed with ph 2.2 H 2 PtCl viii Figure 12. The TEM images for group C, Ru/C immersed with ph 8 H 2 PtCl Figure 13. Ru K-edge XANES spectra of Ru metallic powder, RuO 2, Ru/C immersed with HClO 4, DI water and KOH Figure 14. Ru K-edge XANES spectra of Ru metallic powder, RuO 2, Ru/C immersed with ph 1, ph 2.2 and ph 8 H 2 PtCl 6 (group A) Figure 15. Ru K-edge XANES spectra of Ru metallic powder, RuO 2, hydrogen reduced Ru/C immersed with ph 1, ph 2.2 and ph 8 H 2 PtCl 6 (group B) Figure 16. Ru K-edge XANES spectra of Ru metallic powder, RuO 2, Ru/C immersed with ph 1, ph 2.2 and ph 8 H 2 PtCl 6 and followed by hydrogen reduction (group C) Figure 17. Ru K-edge EXAFS spectra of Ru/C immersed with HClO 4, DI water and KOH Figure 18. Ru K-edge EXAFS spectra of Ru/C immersed with ph 1, ph 2.2 and ph 8 H 2 PtCl 6 (group A) Figure 19. Ru K-edge EXAFS spectra of hydrogen reduced Ru/C immersed with ph 1, ph 2.2 and ph 8 H 2 PtCl 6 (group B) Figure 20. Ru K-edge EXAFS spectra of Ru/C immersed with ph 1, ph 2.2 and ph 8 H 2 PtCl 6 and followed by hydrogen reduction (group C) Figure 21. Pt L 3 -edge XANES spectra of Pt foil, H 2 PtCl 6 solution adjusted to ph 1, ph 2.2 and ph Figure 22. Pt L 3 -edge XANES spectra of Pt foil, Ru/C immersed with ph 1, ph 2.2 and ph 8 H 2 PtCl 6 (group A) Figure 23. Pt L 3 -edge XANES spectra of Pt foil, hydrogen reduced Ru/C immersed with ph 1, ph 2.2 and ph 8 H 2 PtCl 6 (group B) Figure 24. Pt L 3 -edge XANES spectra of Pt foil, Ru/C immersed with ph 1, ph 2.2 and ph 8 H 2 PtCl 6 and followed by hydrogen reduction (group C) Figure 25. Pt L 3 -edge EXAFS spectra of H 2 PtCl 6 solution adjusted to ph 1, ph 2.2 and ph ix Figure 26. Pt L 3 -edge EXAFS spectra of Ru/C immersed with ph 1, ph 2.2 and ph 8 H 2 PtCl 6 (group A) Figure 27. Pt L 3 -edge EXAFS spectra of hydrogen reduced Ru/C immersed with ph 1, ph 2.2 and ph 8 H 2 PtCl 6 (group B) Figure 28. Pt L 3 -edge EXAFS spectra of Ru/C immersed with ph 1, ph 2.2 and ph 8 H 2 PtCl 6 and followed by hydrogen reduction (group C) x Chapter 1 Introduction 1.1 Background The Galvanic displacement reaction has received considerable attention and research effort as one of important methodologies to prepare platinum-based anode and cathode catalysts for direct methanol fuel cells (DMFCs), in order to achieve dramatic reduction of precious metal content and improvement of mass activity. Nano-sized catalysts including mono-layers, clusters and alloys are formed based on displacement reaction, because the replacement of non-noble metals by noble metals provides the manipulation of surface atomic morphology. Similar phenomena observed by Adzic et al and were described as spontaneous deposition or irreversible adsorption. Platinum ions were reduced to metallic state onto ruthenium nanoparticles accompany with the dissolution of ruthenium. However, detailed reaction mechanism such as byproducts, oxidation states and effects caused by various ph values still requires further investigation. A thorough study by Manandhar et al reported for the first time that XPS data shows partially reduced platinum to intermediate oxidation state between solvated species and that of metal, after spontaneously deposited onto ruthenium single crystal without any externally applied potential. For platinum spontaneous deposition at various ph values, clarifications only indicated that platinum complex was adsorbed at the substrate surface with certain oxidation states. In this research, in-situ X-ray Absorption Spectroscopy is performed to observe the reaction process of displacement reaction. Ruthenium nanoparticles are immersed into hexachloroplatinic acid under various ph conditions. For the platinum precursor, hexachloroplatinic acid, used in this research, a fundamental investigation was perform by Spieker et al. Platinum complex has various forms to maintain stability influenced by ph value, platinum concentration and chloride concentration. Clarifications to oxidation states change and neighboring species of platinum and 1 ruthenium atoms during the reaction process can be provided based on XANES and EXAFS analysis. Furthermore, two mechanisms including either replacement between platinum and ruthenium atoms or spontaneous deposition of platinum atoms accompany with the oxidation of ruthenium will be justified. 1.2 Motivation The existence of human beings has been lasting for almost two hundred thousand year and increasing. Highly developed society shows that the progress of civilization has prospered exponentially regarding the field of all kinds. However, the nature that keeps all living creatures alive unfortunately reveals the opposite consequences based on the symptoms including the green house effect, pollution, ozone depletion, and energy crisis. One of the important issues that affect instantaneously to all people on Earth is the energy crisis. Precautious measures have taken with the best of research facilities, governments, and even individuals in order to eliminate the impact brought by the exhaustion of energy. A majority of energy source originates from fossil fuel. The forecast of depleting fossil fuel has emphasized based on its dramatic consumption. The development of alternative energy has received people s attention considerably for the sake that usable energy still exists on the moment when all fossil fuel reaches its end. The fuel cell is one of the promising solutions that provide reliable and clean energy source. Many different kinds of fuel cells such as Polymer Electrolyte Membrane Fuel Cells, Alkaline Fuel Cells, Phosphoric Acid Fuel Cells, Molten Carbonate Fuel Cells, and Solid Oxide Fuel Cells have been developed with the progress of human technologies. A great amount of research is focused on Polymer Electrolyte Membrane Fuel Cells (DMFC) due to its outstanding properties including high energy density, portability, low working temperature, etc. The performance of DMFC relies greatly on its catalysts on the anode for methanol oxidation 2 reaction (MOR) and cathode for oxygen reduction reaction (ORR). PtRu is a bimetallic compound often used as anode catalyst due to its catalytic activity for MOR and outstanding CO-resistance. For cathode materials, Pt-based and non Pt-based materials have been thoroughly investigated elsewhere which are not covered in this study. However, noble metals are major components of catalysts. Because of the cost and rarity, the reduction of noble metal loading and enhancement of catalysis efficiency have become essential issues throughout the research topics. Significant reduction of noble metal loading is widely proposed as a crucial methodology to achieve improvements on electrochemical active materials. Obstacles were found for dramatic reduction the noble metal loading and further improvement the catalytic MOR activity in the meanwhile. In the recent research by Adzic et al., monolayer-level Pt was synthesized with galvanic displacement reaction, which provides enhanced mass activity and durability for MOR compared with commercial carbon supported PtRu. The fabrication method involved with galvanic displacement reaction for Pt and Ru atoms somehow suffer from its opaque mechanism. Possibilities of chemical reactions happening on the Ru surface consist of either the oxidation of Ru to Ru(OH), or a higher oxidation state of Ru, or any other probable reactions. On the other hand, the possible dissolution of Ru, which also contributes the displacement reaction, should be taken into consideration. The acidic aqueous environment in which the displacement reaction occurs is feasible to dissolve Ru. The dissolved Ru and displaced Ru are substantially mixed together and both results are received during the analysis. On the other hand, the whole reaction takes place in aqueous phase and solid phase. Characterizations taken under ultra high vacuum environment lead to the difficulty of objective observations and analysis with instrument as X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy/energy disperse spectrometer (SEM/EDS). X-ray absorption spectroscopy (XAS) has increasingly received attention for its appropriate analytical instrument for PtRu bimetallic alloys. XAS contributes the unique information regarding the oxidation state, 3 coordination environment, coordination number and bond lengths of the absorbing atom. Remarkably, in-situ measurement can be conducted which is close to working condition of as-observed specimens. The investigation of galvanic displacement reaction mechanism is attempted in this work with XAS and other analytical instrument. Based on the information of the reaction mechanism, the optimized fabrication process for catalysts involving Pt and Ru atoms should be accomplished. 4 2.1 Anode catalysts for DMFC Chapter 2 Literature Review Over the decades, researchers have been eagerly searching for alternative energy sources for solving the energy crisis. The urgent need originates from high-energy demands, fossil fuel depletion, environmental pollution, etc. Among manoy technologies under development, fuel cells have become one of the candidates that provide an efficient and clean energy source. There are various sorts of fuel cells. In particular, a promising type known as direct methanol fuel cell (DMFC) has received considerable attention due to its favorable characteristics with respect to fuel usage and feed strategies [1-2]. It is well known that the key materials that greatly affect the performance of a DMFC are the membrane and electro-catalyst. Unfortunately, severe challenges are present for the electro-catalyst such as the mass activity, reliability, durability, and cost reduction. The improvement of anode catalyst for methanol oxidation reaction (MOR) has been accomplished with the exploration of noble and non-noble electrochemical active materials. Compared with non-noble catalysts, noble metal catalysts such as Pt, demonstrates reasonable electrochemical activity and stability. For the objective of cost reduction and performance improvement, the strategy has shifted to identifying alloys with comparable performance. To date, PtRu alloys have emerged as the leading material that seem to be the state-of-the-art binary anode catalyst. Pure Pt atom goes through a series of reactions for MOR [3], including (1) methanol adsorption; (2) C-H bond activation (methanol dissociation); (3) water adsorption; (4) water
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