SYNTHESIS, CHARACTERIZATION AND APPLICATIONS OF NANOSTRUCTURED MATERIALS AS NOVEL CATALYST SUPPORTS IN ETHANOL REFORMING FOR HYDROGEN PRODUCTION WU XUSHENG NATIONAL UNIVERSITY OF SINGAPORE 2010 SYNTHESIS, CHARACTERIZATION AND APPLICATIONS OF NANOSTRUCTURED MATERIALS AS NOVEL CATALYST SUPPORTS IN ETHANOL REFORMING FOR HYDROGEN PRODUCTION WU XUSHENG (B. Eng., Xiamen University) A THESIS SUBMITTED FOR THE DEGREE OF Ph.D. OF ENGINEERING DEPARTMENT OF CHEMICAL & BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2010 Acknowledgements Acknowledgements First of all, I would like to greatly thank to Professor Sibudjing Kawi, my supervisor, who suggested the excellent research directions and who spent a lot of time in revising paper for publication. I also deeply appreciate for his invaluable guidance, patience, and constant encouragement. I have benefited immensely from his brilliant thoughts and profuse knowledge and can’t sufficiently express my thanks for his thoughtful kindness. I am also grateful to Professor Hidajat Kus for his help and support. I also extend my appreciation to Prof. Chung Tai Shung, Prof. Hong Liang, Prof. Kang En Tang, Prof. Tan Thiam Chye, and Prof. Song Lianfa for their instructive teachings. Sincere appreciation goes to Dr. Sun Gebiao and Dr. Yang Jun, my seniors whose patient assistance and help have provided emotional support to me. Special thanks also go to Dr. Song Shiwei, Dr. Yong Siekting, and Dr. Li Peng, whose friendship has given shape to my own intellectual and personal pursuits. Thanks also to Ms. Kesada, Ms. Warrinton, Mr. Usman, Mr. Noom, Ms. Yaso, Mr. Saw, my fellow classmates in Department of CHBE, to Ms Ng Ai Mei and Ms How Yoke Leng, for their patience and professional dedication. Moreover, I appreciate NUS and Department of Chemical & Biomolecular Engineering of NUS for awarding me the research scholarship. Finally, I deeply appreciate my family for their encouragement and support. Especially, I offer my deepest gratitude to my wife, Liu Zengjiao, for her boundless love, wholehearted care and making every effort to help. Without her, it would have been impossible for me to continue my Ph.D. studies. In her inimitable ways, over the last several years, she continues to enrich my efforts with her endless affection. i Table of Contents Table of Contents Acknowledgements i Table of Contents ii Summary vi Nomenclature .ix List of Figures .x List of Tables xiv List of Schemes .xv Chapter 1. Introduction .1 1.1. Research background .1 1.2. Research objectives 1.3. Organizations of thesis Chapter 2. Literature Review 2.1. Development of nanotechnology .9 2.2. Nanotechnology applied in catalysis . 11 2.3. Nanostructured materials .16 2.3.1. Mesoporous materials .17 2.3.1.1. SBA-15 18 2.3.1.2. M-SBA-15 (M=Al, Ce etc.) 19 2.3.1.3. Applications of SBA-15 and M-SBA-15 (M=Al and Ce etc.) .22 2.3.2. Nanotubes .24 2.3.2.1. Properties of nanotubes .25 2.3.2.1.1. Symmetry properties 26 2.3.2.1.2. Electronic properties .26 2.3.2.1.3. Thermodynamic properties .27 2.3.2.1.4. Mechanical properties 28 2.3.2.2. Carbon nanotubes 29 2.3.2.3. Oxide nanotubes 32 2.3.2.3.1 Synthesis methods of oxide nanotubes 32 2.3.2.3.2. Formation mechanism of oxide nanotubes .35 2.3.2.3.3. Applications of oxide nanotubes .37 2.4. Reviews of CO2 reforming (CRE) and steam reforming of ethanol (SRE) .38 2.4.1. Reactions of CRE and thermodynamic study .39 2.4.2. Reactions of SRE and thermodynamic study .40 2.4.3. Catalysts for CRE and SRE 42 2.4.3.1. Oxide-supported metal catalysts for CRE and SRE 43 2.4.3.1.1. Non-noble metal catalysts 43 2.4.3.1.2. Noble-metal catalysts .45 Chapter 3. Experimental .49 ii Table of Contents 3.1. Reaction system .49 3.2. Product analysis .50 3.3. Characterization of nanomaterials and catalysts .51 Chapter 4. Steam Reforming of Ethanol to H2 over Rh/Y2O3: Crucial Roles of Y2O3 Oxidising Ability, Space Velocity and H2/C 55 4. 1. Introduction 56 4.2. Experimental .58 4.2.1. Catalyst preparation 58 4.2.2. Reaction system of SRE .59 4.3. Results and discussion .59 4.3.1. Ethanol conversion over Rh-based Catalysts .59 4.3.2. Surface area and dispersion analysis 60 4.3.3. XRD analysis 61 4.3.4. TPR-H2 analysis .62 4.3.5. XPS Analysis 65 4.3.6. Activity test 67 4.3.7. Effect of gas hourly space velocity (GHSV) on product distribution 70 4.3.8. Optimal GHSV over Rh/Y2O3 for steam reforming of ethanol (SRE) .74 4.3.9. A new indicator: H2/C for study of efficiency of converted ethanol 76 4.3.9.1. Effect of Rh loading and temperature on H2/C 78 4.3.9.2. Effect of water/ethanol molar ratio 80 4.4. Conclusions .81 Chapter 5. A Novel Rh/Y2O3-Nanotube Catalyst for Steam Reforming of Ethanol to H2: Effects of Anti-Sintering of Rh Species and Ultra-Low Rh Loading on Catalyst Performance 82 5.1．Introduction .82 5.2. Experimental .85 5.2.1. Catalyst preparation 85 5.2.2. Reaction system of SRE .86 5.3. Results and discussion .86 5.3.1. TEM, Surface area and dispersion analysis 87 5.3.2. TPR analysis .89 5.3.3. XRD analysis 90 5.3.4. XPS analysis .92 5.3.5. Evaluation of Rh-based catalysts .94 5.3.5.1. Effect of catalyst support on ethanol conversion .94 5.3.5.2. Effect of catalyst support on product selectivity .97 5.3.6. Activity test 106 5.3.7. Ultra-low Rh loading over Rh/Y2O3 nanotubes for SRE .111 5.4. Conclusions . 113 Chapter 6. Rh/Ce-SBA-15: Active and Stable Catalyst for CO2 Reforming of Ethanol to H2 . 115 6.1. Introduction . 116 iii Table of Contents 6.2. Experimental .121 6.2.1. Preparation of catalysts 121 6.3. Results and discussion .122 6.3.1. Effect of active metals over SBA-15 supports on hydrogen production rate for CO2 reforming of ethanol .122 6.3.2. Morphology of Ce-SBA-15 supports and 1%Rh/Ce-SBA-15 125 6.3.3. XRD patterns of 1%Rh/Ce-SBA-15 based catalysts 128 6.3.4. Properties of 1%Rh/Ce-SBA-15 catalysts 129 6.3.5. H2-TPR profiles of 1% Rh/Ce-SBA-15 series catalysts .131 6.3.6. XPS analysis of 1%Rh/Ce-SBA-15 based catalysts .134 6.3.7. Activity test 136 6.3.8. Stability test 140 6.4. Conclusions .142 Chapter 7. Synthesis, Growth Mechanism and Properties of Open-Hexagonal and Nanoporous-Wall Ceria Nanotubes Fabricated Via Alkaline Hydrothermal Route 144 7.1. Introduction .145 7.2. Experimental .150 7.3. Results and discussion .151 7.4. Conclusions .171 Chapter 8. A Crucial Role of Oxidation State And Reducibility of Rh Species Over A Novel Rh/CeO2-Nanotube Catalyst for CO2 Reforming of Ethanol to H2 172 8.1. Introduction .173 8.2. Experimental .178 8.2.1. Preparation of catalysts 178 8.2.2. Characterization of catalysts 179 8.2.3. Activity test 180 8.3. Results and discussion .180 8.3.1. Morphology of CeO2 nanotubes and 1% Rh/CeO2 nanotubes .180 8.3.2. XRD patterns of Rh-based catalysts .183 8.3.3. Surface area and dispersion analysis 184 8.3.4. XPS analysis of Rh-based catalysts 185 8.3.5. The reducibility of well-dispersed Rh species on catalyst surface by H2-TPR analysis .189 8.3.6. The mobility of lattice oxygen over Rh-based catalysts by H2-TPR analysis192 8.3.7. Formation process of enhanced lattice oxygen density at crystalline defect sites .194 8.3.8. Activity test 196 8.3.9. Reaction mechanism via redox properties and oxygen vacancies 200 8.4. Conclusions .205 Chapter 9. Conclusions and Recommendations 206 9.1. Conclusions .206 9.1.1. Steam reforming of ethanol (SRE) .207 9.1.1.1. Development of Rh/Y2O3 as SRE catalyst .207 iv Table of Contents 9.1.1.2. Development of Rh/Y2O3-nanotube catalysts as novel SRE catalyst208 9.1.2. CO2 reforming of ethanol (CRE) 208 9.1.2.1. Development of Rh/Ce-SBA-15 as CRE Catalyst 209 9.1.2.2. Synthesis and characterization of Ce(OH)3 and CeO2 nanotubes .209 9.1.2.3. Development of Rh/CeO2-nanotube catalysts for CRE 211 9.2. Future works 211 References .214 Publications .248 v Summary Summary The recent synthesis and applications of oxide nanotubes and mesoporous materials attract intense research interests due to their chemical and physical properties. This thesis reports the synthesis and characterization of Rh supported on nanostructured materials, such as oxide nanotubes and mesoporous materials, and their applications as highly active and stable catalysts for H2 production in steam reforming of ethanol (SRE) and CO2 reforming of ethanol (CRE). A fundamental understanding of the cause of the high activity and the stability of Rh/oxide-nanotube catalysts has also been studied in this work. SRE is regarded as an effective and important method for H2 production since the hydrogen in steam not only could be transformed to H2 gas but also could minimize coke formation. Among the four Rh-based catalysts investigated, Rh/Y2O3 was found to show excellent catalytic performance for H2 production in SRE. Some related factors have also been investigated to determine the key factor causing the different catalytic performance of the four Rh-based catalysts in SRE. Furthermore, a novel Rh/Y2O3-nanotube catalyst has also been developed and found to have even higher H2 production rate than Rh/Y2O3 in SRE due to the anti-sintering of Rh species on Y2O3 nanotubes. vi Summary Some of the significant findings of this research in SRE are as follows: (1) The strong oxidising ability of Y2O3 is found to be the key factor underlying the high activity and stability of Rh/Y2O3, suggesting a strong relationship between the oxidizing ability of the catalyst support and its catalytic performance; (2) A new indicator, H2/C, has been proposed in this study, for the first time, and it was found to have a strong linkage to the optimal H2 production rate under the lowest C emission in SRE; (3) the anti-sintering property of Y2O3 nanotubes was discovered for supported metal catalyst and this has significant influence on the catalyst’s performance. CRE has been considered as one of the important methods to solve the global warming as CRE involves CO2, which is a greenhouse gas, and ethanol, which is a renewable source. A series of Rh supported on Ce-SBA-15 catalysts, which have unique nanopores, have been synthesized and applied as CRE catalysts. Since Ce is found to promote the low catalytic activity of SBA-15 silica support, CeO2 nanotubes are then developed and applied, for the first time, as the novel catalyst support. A novel Rh/CeO2-nanotube catalyst, synthesized in this study, is found to show an excellent H2 production rate and ethanol conversion in CRE due to the versatile and remarkable redox properties of Rh on CeO2 nanotubes. 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Kawi, Rh Supported on Y2O3 Nanotube: A New Catalyst for Steam Reforming of Ethanol to Hydrogen, 21st National Annual Meeting (North American Catalysis Society), San Francisco, USA. Jun. 6-12, 2009 (Poster Presentation). 9. X. S. Wu and S. Kawi, Production of Hydrogen from Steam Reforming of 248 Publications Ethanol over a Rh/Y2O3 Catalyst, 21st National Annual Meeting (North American Catalysis Society), San Francisco, USA. Jun. 6-12, 2009 (Poster Presentation). 10. X. S. Wu and S. Kawi, Rh/Ce-SBA-15: Active and Stable Catalyst for CO2 Reforming of Ethanol to Hydrogen, 10st International Conference on CO2 Utilization (ICCDU-X), Tianjin, China. May 17-22, 2009 (Oral Presentation). 11. X. S. Wu and S. Kawi, CO2 Reforming of Ethanol for Production of H2/Syngas over Rh/CeO2 Nanotubes, 10st International Conference on CO2 Utilization (ICCDU-X), Tianjin, China. May 17-22, 2009 (Poster Presentation). 12. X. S. Wu and S. Kawi, Synthesis Mechanism of CeO2 Nanotubes under Alkaline Route, MRS Fall Meeting, Boston, USA, Dec. 5-8, 2008 (Poster Presentation). 249 [...]... FESEM image of Ce(OH)3-OH-NT, (b) FESEM image displaying the outer and inner diameters of Ce(OH)3-OH-NT, (c) FE-TEM image of multi-layer crystal lattice of Ce(OH)3-OH-NT and (d) TEM image of CeO2 open-hexagonal nanotubes [CeO2-OH-NT] formed by calcination of Ce(OH)3-OH-NT 154 Figure 7-3 DTA-TGA analysis of Ce(OH)3-OH-NT 155 Figure 7-4 The effect of hydrothermal treatment time of Ce(OH)3-OH-NT... morphology of Ce(OH)3-OH-NT: (a) 0 day, (b) 15 days, (c) 30 days, and (d) 60 days (with the red arrow showing the nanopore diameter of ~ 2.5 nm) 166 Figure 7-10 H2-TPR profiles of CeO2-NW-NT, CeO2-OH-NT and CeO2-NP 170 Chapter 8 A Crucial Role of Oxidation State And Reducibility of Rh Species Over A Novel Rh/CeO2-Nanotube Catalyst for CO2 Reforming of Ethanol to H2 172 Figure 8-1 (a) FESEM image of. .. displaying the outer and inner diameters of Ce(OH)3-OH-NT, (c) FE-TEM image of multi-layer crystal lattice of Ce(OH)3-OH-NT and (d) TEM image of CeO2 open-hexagonal nanotubes [CeO2-OH-NT] formed by calcination of Ce(OH)3-OH-NT 182 Figure 8-2 (a) 1 wt% Rh and (b) 5 wt% Rh nanoparticles filled inside and on the surface of CeO2 nanotubes 183 Figure 8-3 XRD patterns of Rh-based catalysts... .201 xiii List of Tables List of Tables Table 4-1 Textural characterization of catalysts .61 Table 4-2 Comparison of ethanol conversion and GHSV .71 Table 4-3 Comparison of ethanol conversion and LHSV .71 Table 5-1 Textural characterization of catalysts 88 Table 6-1 Surface properties of 1%Rh/Ce-SBA-15 based catalysts 131 Table 6-2 Conversion of CO2 over 1% Rh/Ce-SBA-15... Literature Review characterization and application, promoting this area of study is an integral part of nanotechnology and nanoscience Generally, nanostructured materials include nanotubes, nanoparticles, mesoporous materials, nanowires and nanorods and so on In this review, we focus on the nanotubes and mesoporous materials and on some aspects of their synthesis, formation and application 2.3.1 Mesoporous... nanotube over two kinds of Ce(OH)3 compound bases: (a) vertical growth of Ce(OH)3-OH-NT over the amorphous nature of flat base of Ce(OH)3 compound and (b) multidirectional growth of Ce(OH)3-OH-NT into nanotube flowers over the spherical core base of Ce(OH)3 compound .165 Scheme 8-1 Formation process of enhanced lattice oxygen density at CeO2 crystalline defect sites and formation of oxygen vacancy... literature review, results and discussion, conclusions and references In Chapter 2, background knowledge and the related literature of nanostructured materials and their application in SRE and CRE reactions are provided Furthermore, the advantages and drawbacks of different metal based catalysts applied in these two reactions are discussed In Chapter 3, the experimental and characterization methods used... first to discuss the general aspects of nanotechnology in relevance to the catalysis, the synthesis and fabrication of nanostructure materials, the characterization of nanomaterials, and the applications of nanostructured materials in reforming of alcohols to produce hydrogen gas This chapter is constituted by four main sections In the first section, the development of nanotechnology in recent years was... smaller features in semiconductors and electronics and so on Novel creations in these intersecting fields are indeed remarkable, and a vibrant and intellectually stimulating community is always exploring new methods to fabricate and examine matter that were not available before In this study, we will be reviewing the chemical synthesis, characterization and applications of nanostructured materials, which... Table 7-1 Properties of CeO2-NW-NT, CeO2-OH-NT and CeO2-NP 168 Table 8-1 Textural characterization of catalysts .185 Table 8-2 Rh 3d XPS data over various Rh-based catalysts 188 Table 8-3 Conversion of CO2 over five Rh-based catalysts 198 xiv List of Schemes List of Schemes Scheme 7-1 Schematic illustration of anisotropic growth of Ce(OH)3-OH-NT along the c-axis of hexagonal nanotube . grateful to Professor Hidajat Kus for his help and support. I also extend my appreciation to Prof. Chung Tai Shung, Prof. Hong Liang, Prof. Kang En Tang, Prof. Tan Thiam Chye, and Prof. Song Lianfa. production in steam reforming of ethanol (SRE) and CO 2 reforming of ethanol (CRE). A fundamental understanding of the cause of the high activity and the stability of Rh/oxide-nanotube catalysts. Textural characterization of catalysts 61 Table 4-2 Comparison of ethanol conversion and GHSV 71 Table 4-3 Comparison of ethanol conversion and LHSV 71 Table 5-1. Textural characterization of catalysts