Performance improvement of tio2 catalysts supported on adsorbents

PERFORMANCE IMPROVEMENT OF TiO2 CATALYSTS SUPPORTED ON ADSORBENTS LI GANG NATIONAL UNIVERSITY OF SINGAPORE 2007 PERFORMANCE IMPROVEMENT OF TiO2 CATALYSTS SUPPORTED ON ADSORBENTS LI GANG (PhD, NUS) A THESIS SUBMITTED FOR THE DEGREE OF PhD OF ENGINEERING DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2007 Acknowledgement Acknowledgement I would like to convey my deepest appreciation to my supervisor, Assoc Prof Zhao X S., George for his constant encouragement, invaluable guidance, patience and understanding throughout the whole length of my PhD candidature This project had been a tough but enriching experience for me in research I would like to express my heartfelt thanks to A/P Dr Zhao for his guidance on writing scientific papers including PhD thesis I would also like to take this opportunity to acknowledge Prof M B Ray for her kindly advice and guidance during first years of my research work In addition, I want to express my sincerest appreciation to the Department of Chemical and Biomolecular Engineering for offering me the chance to study at NUS with a scholarship It’s my pleasure to work with a group of brilliant, warmhearted and lovely people, Dr Zhou Zuocheng, Dr Su Fabing, Dr Lv Lu, Dr Yan Qingfeng, Dr Zhou Jinkai, Mr Bao Xiaoying, Mr Wang Likui, Mr Bai Peng, Ms Lee Fang Yin, Ms Liu Jiajia, Ms Tian Xiaoning, Ms Zhang Lili, Ms Wu Pingping Particular acknowledgement goes to Mr Chia Phai Ann, Mr Shang Zhenhua, Dr Yuan Zeliang, Mr Mao Ning, Dr Rajarathnam D., Madam Chow Pek Jaslyn, Mdm Fam Hwee Koong Samantha, Ms Lee Chai Keng, Ms Tay Choon Yen, Mr Toh Keng Chee, Mr Chun See Chong, Ms Goh Siew Ping, Ms Ng Ai Mei, Ms Lum Mei Peng Sharon, and Ms How Yoke Leng Doris for their kind supports I thank my wife, Mo Huajuan, my daughter, parents, and brother It is no exaggeration to say that I could not complete the PhD work without their generous help, boundless love, encouragement and support i Table of Contents Table of Contents Summary………………………….………………………………………………… vii Nomenclature ………………………………………………………………… ix List of Tables …….…………………………………………………………… xi List of Figures………………………………………………………….………… xii CHAPTER INTRODUCTION………………………… ……….……….…1 1.1 Background… 1.2 Application of photocatalyst TiO2 in wastewater treatment…………… 1.3 Objectives of the project………………………………………….………….9 1.4 Structure of thesis…… ……………………………………………………10 CHAPTER LITERATURE REVIEW………………….…… ……….….12 2.1 Principles of semiconductor photocatalysis 12 2.1.1 Radiation source … 12 2.1.2 Mechanism of TiO2 photocatalyst………………………….… ……….15 2.1.3 TiO2 surface reactions…………………………………….………… 19 • 2.1.4 Role of OH radicals……….………………….……….……………… 21 2.1.5 Kinetics of TiO2 photocatalysis………….………….…………………22 2.2 Particle dispersion of TiO2……………………………………………… 24 2.2.1 Heterogeneous photocatalysis………………………….………… … 24 2.2.2 Particle aggregation……………………….………………….…….… 26 2.2.3 Methods for avoiding/minimizing particle aggregation … 28 2.3 Roles of support …………………….……………………………… ….…31 2.3.1 TiO2 supported on porous materials……………………………… .32 2.3.2 TiO2 supported on non-porous materials…….………….…….… …41 ii Table of Contents 2.3.3 Adsorbents used as a TiO2 support………………………………… 47 2.3.3.1 β-zeolite… 47 2.3.3.2 Al-pillared Montmorillonite……………….………………… 48 2.3.3.3 MCM-41 … 49 2.3.3.4 SBA-15……………………………………………….………….50 2.4 pH effect on photoreaction……………………………………………….….51 2.5 Enhancement of photocatalytic reaction rate… ………….….……………54 2.5.1 Hydrogen peroxide…………………………….………………………55 2.5.2 Ozone………………………………………………………………….60 2.5.3 Noble metal doping……………………………………………………62 2.6 Catalyst reuse………………………………………………….…………….66 2.6.1 Sedimentation separation……………………….…………………… 67 2.6.2 Magnetic separation……………………………………….………… 69 CHAPTER EXPERIMENTAL SECTION………………………………72 3.1 Reagents and apparatus……………………………………….………… 72 3.2 Synthesis of supported TiO2 photocatalysts……………… …….…….…74 3.2.1 Synthesis of MCM-41……………………………………….…… …74 3.2.2 Synthesis of supported TiO2 photocatalyst by sol-gel method… .…75 3.2.3 Synthesis of Ti-SBA-15 by co-precipitation method 75 3.2.4 Synthesis of Ti-SBA-15 by impregnation method……………….……76 3.2.5 Synthesis of silica microspheres………………….……………………76 3.2.6 Synthesis of SiO2/TiO2 core/shell photocatalysts…………………… 77 3.2.7 Synthesis of SiO2/TiO2-Pt photocatalysts…………………… ………78 3.2.8 Synthesis of TiO2 fiber……………………………………….……… 78 3.3 Characterization ……………………………………………….………… 79 iii Table of Contents 3.3.1 Scanning electron microscopy (SEM) ………………………………79 3.3.2 Energy dispersive X-ray spectrometry (EDX) .79 3.3.3 Diffusive reflectance UV-Vis spectrophotometer (DR-UV)…….….…80 3.3.4 N2 adsorption/desorption……………………………….….……… …80 3.3.5 Magic-angle spinning-nuclear magnetic resonance (MAS-NMR) spectroscopy………………………………………………………………….81 3.3.6 X-ray diffraction (XRD)…………………………….…………………81 3.3.7 X-ray photoelectron spectroscopy (XPS)…………….……………… 82 3.3.8 Zeta potential……………………………………….………………….82 3.3.9 Transmission electron microscopy (TEM)……………….……………83 3.4 Evaluation of photocatalytic activity………………….……………….… 83 CHAPTER REDISPERSION OF TiO2 NANOPARTICLES IN AQUEOUS PHASE BY VARYING SOLUTION pH AND SURFACE MODIFICATION WITH POLYELECTROLYTE………………….… 86 4.1 Introduction………………………………….………….……….……………86 4.2 Results and discussion……………………….…….………………… ……88 4.2.1 Effect of catalyst dosage on its photoactivity………………… ……88 4.2.2 Redispersion of TiO2 nanoparticles in aqueous phase by varying solution pH…………………………………………………………………….92 4.2.3 Redispersion of TiO2 nanoparticles in aqueous phase by surface modification with polyelectrolyte…………….………….… ……………… 98 4.3 Summary…………………………….…………………… ………… ……103 iv Table of Contents CHAPTER ADVANCED OXIDATION OF ORANGE II USING TiO2 SUPPORTED ON POROUS ADSORBENTS: THE ROLE OF pH, H2O2 AND O3………………………………………………….… .104 5.1 Introduction…………………………… ………………………….…….….104 5.2 Results and discussion………………………………………….….…… 105 5.2.1 Characterization of the catalysts …………………….……… …….105 5.2.2 Dark adsorption of orange II ………………………………… … 109 5.2.3 Photocatalytic degradation of orange II…….………………… ….112 5.2.4 Total organic carbon (TOC) degradation ……………………… .114 5.3 Summary… 120 CHAPTER CHARACTERIZATION AND PHOTOCATALYTIC PROPERTIES OF TITANIUM-CONTAINING MESOPOROUS SBA-15……………………………………………….….………………………….121 6.1 Introduction………………………………………….……………… …121 6.2 Results and discussion………………………………………………… 124 6.2.1 Characterization of photocatalysts ……………………………….…124 6.2.2 Photocatalytic properties of titanium-containing SBA-15.………… 137 6.3 Summary…………………………………………………….…………… 142 CHAPTER PREPARATION AND CHARACTERIZATION OF SiO2/TiO2-Pt CORE/SHELL NANOSTRUCTURES AND EVALUATION OF THEIR PHOTOCATALYTIC ACTIVITY… 143 7.1 Introduction…………………………….………………………….……… 143 7.2 Results and discussion……………………….…………………………… 145 7.2.1 Characterization of the SiO2/TiO2 core/shell photocatalysts …… 145 v Table of Contents 7.2.2 Photocatalytic activity of the SiO2/TiO2 core/shell photocatalysts… 152 7.2.3 Reuse of the SiO2/TiO2 core/shell photocatalysts….……….……… 153 7.2.4 Characterization of the SiO2/TiO2-Pt photocatalysts…….… ……….156 7.2.5 Photocatalytic activity of SiO2/TiO2-Pt photocatalysts……….…… 163 7.3 Summary………………………………………………………………… 169 CHAPTER CONCLUSIONS AND RECOMMENDATIONS… .170 8.1 Conclusions……………………………………………………… ……… 170 8.2 Recommendations……………………….…………….…………………….172 REFERENCES……………………………………………………………… … 175 APPENDIX…………………………………………………….…….………… 194 vi Summary Summary Due to the rapid urban and industrial development worldwide, large amounts of chemicals are being used and subsequently released into the natural environment Many control technologies, such as physical adsorption, chemical decomposition and bio-degradation, are available to remove these contaminants Among various treatment technologies, considerable attention has been given to photocatalytic degradation of contaminants by semiconductor photocatalysts due to several advantages, such as complete mineralization of organic compounds, minimization of waste disposal problems and cost reduction TiO2 has been proven to be the most suitable photocatalyst for widespread environmental applications due to many merits TiO2 photocatalysis is a well established advanced oxidation process for the purification of contaminated air and wastewater streams and such applications have been well documented over the last two decades However, some disadvantages of bare TiO2 nanoparticles impede its application as a photocatalyst in wastewater treatment process First, due to the aggregation of TiO2 nanoparticles in aqueous phase, the effective surface area and photocatalytic activity decay rapidly Second, the non-porous nature of TiO2 nanoparticles leads to low specific surface area and adsorption capacity for organic contaminants, thus a low photocatalytic activity In addition, the issues of catalyst recovery and possible reuse have largely hindered the commercialization of TiO2 photocatalyst Extensive research has been carried out to enhance the surface area and improve the photocatalytic property of TiO2, whereas many issues still need to be addressed for maximum utilization of the supported catalysts For example, the effect of the support vii Summary morphology on photocatalytic activity of synthesized catalyst and the separation and reuse of TiO2 photocatalysts are rarely investigated In this thesis work, the dispersion of TiO2 nanoparticles in aqueous phase was studied The spontaneous particle aggregation was confirmed and it could result in low photocatalytic reaction rate due to less effective surface area Two strategies, namely varying solution pH and surface modification by polyelectrolyte, were applied to prevent particle agglomeration In addition, four porous materials, zeolite, Al-pillared Montmorillonite clay, mesoporous MCM-41 and SBA-15 silicates were used to synthesize supported TiO2 catalysts in order to obtain large surface area and improved photocatalytic activity The results 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journal (1) Li, G., Zhao, X S Characterization and photocatalytic properties of titaniumcontaining mesoporous SBA-15 Industrial & Engineering Chemistry Research, 2006, 45, 3569-3573 (2) Li, G., Zhao, X S Madhumita B Ray Advanced oxidation of orange II using TiO2 supported on porous adsorbents: The role of pH, H2O2 and O3 Separation and Purification Technology 2007, 55, 91-97 (3) Gang Li, Likui Wang, Lu Lv, and X S Zhao Preparation and characterization of SiO2/TiO2-Pt core/shell nanostructures and evaluation of their photocatalytic activity Accepted by Journal of Nanoscience and Nanotechnology, 2007 (4) Likui Wang, Linhua Xia, Gang Li, Serge Ravaine, and X S Zhao Patterning the surface of submicron spheres using 3D colloidal crystal template and fabrication of various nonspherical particles Accepted by Angewandte Chemie International, 2008 Papers submitted to international referred journal (5) Li, G., Renbi, Bai, and Zhao X S Synthesis of SiO2/TiO2 core/shell nanostructures by ultrasonic-assisted liquid phase deposition method Submitted to Industrial & Engineering Chemistry Research, 2008 (6) Gang Li, Lu Lv, X.S Zhao New insights into the critical factors influencing the photocatalytic activity of TiO2 nanoparticles Ready to submit to Journal of Colloid and Interface Science 2008 194 ... 1997) On the other hand, the concentration of the contaminants typically is at a low concentration (ppm or less), and pre-concentration of these contaminants on the TiO2 surface where photons are... supported on adsorbents, irrespective of the type of adsorbents used Since the concentration of contaminants in wastewater is quite low (usually in the order of ppm), the application of supported catalysts. .. Degradation of orange II and TOC removal as a function of illumination time over 50% TiO2- Montmorillonite Figure 5.7 TOC decay during photodegradation of orange II by TiO2Montmorillonite at different